Titanium Dioxide-Based Nanomaterials for Photocatalytic Fuel

Review pubs.acs.org/CR

Titanium Dioxide-Based Nanomaterials for Photocatalytic Fuel Generations Yi Ma,† Xiuli Wang,† Yushuai Jia,† Xiaobo Chen,‡ Hongxian Han,*,† and Can Li*,† †

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences and Dalian National Laboratory for Clean Energy, 457 Zhongshan Road, Dalian 116023, China ‡ Department of Chemistry, College of Arts and Sciences, University of Missouri-Kansas City, 5100 Rockhill Road, Kansas City, Missouri 64110, United States S Supporting Information *

3.5.4. Mechanistic Studies on Photocatalytic Reactions of Water or Methanol for Hydrogen Production 3.6. Isotope Labeling Dynamic and Kinetic Studies of Photocatalytic Reactions on the Surface of TiO2 3.6.1. Oxidation Reactions on the Surface of TiO2 3.6.2. Hydrogen Production Reactions on the Surface of TiO2 4. Photoreduction of CO2 to Solar Fuels on TiO2Based Photocatalysts 4.1. Mixed-Phase TiO 2 Composites for CO 2 Photoreduction 4.2. Crystal Facet Engineering of TiO2 for CO2 Photoreduction 4.3. TiO2-Based Nanocomposite Photocatalysts for CO2 Photoreduction 4.3.1. Metal Loaded TiO2-Based Nanocomposite Photocatalysts 4.3.2. Metal Oxide Loaded TiO2-Based Nanocomposite Photocatalysts 4.3.3. TiO2-Based Multinary Nanocomposite Photocatalysts 4.4. TiO2-Based Photocatalysts on Various Supports for CO2 Photoreduction 4.4.1. Silica Materials as Supports 4.4.2. Carbon-Based Materials as Supports 4.5. Visible Light-Responsive TiO2-Based Photocatalysts and Photocatalytic Systems for CO2 Photoreduction 4.5.1. Anion Doped TiO2-Based Photocatalysts 4.5.2. Dyes and Molecular Complexes-Sensitized TiO2-Based Photocatalysts 4.5.3. Quantum Dots-Sensitized TiO2-Based Photocatalysts 4.6. Impact of Reaction Conditions on CO2 Photoreduction 4.7. Mechanism of CO2 Photoreduction on TiO2Based Photocatalysts

CONTENTS 1. Introduction 2. Basic Properties of TiO2 Semiconductor Photocatalysts 2.1. Photocatalytic Reactions on TiO2 2.2. Different Phase Structures of TiO2 3. Hydrogen Generation on TiO2-Based Photocatalysts 3.1. Light Harvesting 3.1.1. Bandgap Engineering 3.1.2. Surface Sensitization 3.2. Photogenerated Charge Separation 3.2.1. Fabrication of Heterojunction 3.2.2. Fabrication of Phase Junction 3.2.3. Fabrication of Schottky Junction 3.3. Loading Cocatalysts on TiO2 3.3.1. Metal Cocatalysts 3.3.2. Metal Oxide/Hydroxide/Sulfide Cocatalysts 3.3.3. Molecular Complex Cocatalysts 3.4. From Overall Water Splitting to Biomass Reforming 3.4.1. Photocatalytic Overall Water Splitting on TiO2-Based Photocatalysts 3.4.2. Hydrogen Generation from Biomass and Their Derivatives on TiO2-Based Photocatalysts 3.5. Photocatalytic Reaction Mechanisms on TiO2-Based Photocatalysts 3.5.1. Photoluminescence Study on Carrier Dynamics of TiO2 3.5.2. Time-Resolved Spectroscopic Study on the Effect of Modifications on TiO2 3.5.3. Role of Cocatalyst Pt on TiO2 © 2014 American Chemical Society

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10006 Special Issue: 2014 Titanium Dioxide Nanomaterials

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Received: January 4, 2014 Published: August 7, 2014 9987

dx.doi.org/10.1021/cr500008u | Chem. Rev. 2014, 114, 9987−10043

Chemical Reviews 5. Conclusions, Perspectives, and Remarks Associated Content Supporting Information Author Information Corresponding Authors Author Contributions Notes Biographies Acknowledgments References

Review

fore, the number of publications on TiO2 has increased exponentially in recent decades. Many excellent reviews on TiO2 describing surface chemistry, synthesis method, properties, applications, and so on have emerged since the 1990s.10−14 There are also some comprehensive reviews on photocatalysts, metal oxides, and semiconductors that contain a large volume description on TiO2.15−17 As a versatile material, TiO2 is extensively applied in various fields. Nevertheless, as a photocatalyst, the most challenging and meaningful application of TiO2 should be the photocatalytic fuel generation reactions, mainly including H2 production and CO2 reduction. This Review will focus on the recent research progress in solar fuel generation on TiO2-based photocatalysts, especially for production of hydrogen by biomass conversion and water splitting as well as generation of carbon-based chemical fuels by photoreduction of CO2. Great emphasis will be paid to the emerging strategies for the activity improvement of TiO2-based photocatalysts and better understanding the fundamental mechanisms. The key factors determining the efficiencies of photocatalytic processes for solar fuel generation, including light absorption, charge separation, and surface reactions, will also be addressed. Finally, the perspectives of TiO2-based photocatalysts for solar fuel generation and the future research directions on solar fuels will be presented and discussed.

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1. INTRODUCTION Nowadays, with the rapid depletion of fossil fuels (coal, natural gas, and petroleum oil), the development of renewable energies based on sufficient energy sources with sustainable supply for the long-term in a large quantity is in high demand. Ever increasing attention has been paid to exploring novel methods for developing renewable energy technologies. Conversion of solar energy into chemical energy in the form of so-called “solar fuels”, such as H2, methanol, methane, etc., is considered one of the most perspective strategies to solve the energy and environmental problems in the future. H2 is an excellent energy carrier for the development of lowcarbon emission economy. First, it has high calorific value per mass unit, which is triple that of gasoline and quadruple that of natural gas. Moreover, no greenhouse gases are produced during the combustion process of H2. Therefore, storage of solar energy in the form of H2 is proposed to be one of the most ideal routes for developing clean and sustainable energy in the future. Second, hydrogen is the most abundant element that can be obtained from a broad range of substances. Third, H2 is also an important chemical reagent in the chemical industry. To develop hydrogen economy, extensive research has been carried out on photocatalytic or photoelectrochemical (PEC) splitting of water into H2 and O2 since the first report of the photoelectrochemical water splitting reaction on TiO2 electrode by Fujishima and Honda et al.1 Another meaningful reaction for solar-to-chemical energy conversion is photocatalytic or photoelectrochemical reduction of CO2 to chemical fuels. According to recent reports, global CO2 emission currently is ca. 37 Gt with 30.4 Gt related to the utilization of fossil fuels and predicted to increase up to 36−43 Gt by 2035.2 The atmospheric level of CO2 rose from 270 ppm in the preindustrial era3 to nearly 395 ppm in 2012,4 which far exceeds its natural fluctuation (d = 180−300 ppm) over the past millions of years.5 With the greenhouse effect of CO2 causing serious climate and environmental problems, CO2 emission is now a great global concern. One of the ideal solutions to solve the CO2 problem is to develop solar fuels by artificial photosynthesis, that is, storage of solar energy in chemical forms by photoreduction of CO2 to high-energy compounds such as carbon monoxide, methanol or methane, or even high-carbon based compounds. Such kind of artificial photosynthesis for solar fuels generation is a carbon neutral cycle process because no additional carbon source other than atmosphere CO2 is used. As one of the earliest studied n-type semiconductor photocatalysts, TiO2 has been widely used in environmental purification,6 self-cleaning, H2 production, photosynthesis, CO2 reduction,7 organic synthesis,8 solar cells, etc.9 Being cheap, stable, nontoxic, and environmentally friendly, TiO2 is an idea model of semiconductor photocatalyst to investigate. There-

2. BASIC PROPERTIES OF TiO2 SEMICONDUCTOR PHOTOCATALYSTS 2.1. Photocatalytic Reactions on TiO2

Semiconductor is a kind of material with electrical conductivity between conductor (such as metals) and insulator (such as ceramic). The conductivity of a semiconductor usually increases with the increase of the temperature, which is opposite to that of a metal.18 The unique electronic property of a semiconductor is characterized by its valence band (VB) and conduction band (CB). The VB of a semiconductor is formed by the interaction of the highest occupied molecular orbital (HOMO), while the CB is formed by the interaction of the lowest unoccupied molecular orbital (LUMO). There is no electron state between the top of the VB and the bottom of CB. The energy range between CB and VB is called forbidden bandgap (also called energy gap or bandgap), which is usually denoted as Eg. The band structure, including the bandgap and the positions of VB and CB, is one of the important properties for a semiconductor photocatalyst, because it determines the light absorption property as well as the redox capability of a semiconductor. As shown in Figure 1, the photocatalytic reaction initiates from the generation of electron−hole pairs upon light irradiation. When a semiconductor photocatalyst absorbs photons with energy equal to or greater than its Eg, the electrons in VB will be excited to CB, leaving the holes in VB. This electron−hole pair generation process in TiO2 can be expressed as follows: TiO2 + hν → e−(TiO2 ) + h+(TiO2 )

These photogenerated electron−hole pairs may further be involved in the following three possible processes: (i) successfully migrate to the surface of semiconductor; (ii) be captured by the defect sites in bulk and/or on the surface region of semiconductor; and (iii) recombine and release the energy in the form of heat or photon. The last two processes are generally viewed as deactivation processes because the 9988


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some particular surface sites of a semiconductor can act as the active centers themselves, especially for oxidation reaction on the surface of metal oxide semiconductors. However, in most cases, efficient photocatalytic reactions proceed only after loading noble metal and oxide cocatalysts on semiconductors. 2.2. Different Phase Structures of TiO2

Generally, the physical/chemical properties of a material may be quite different for their various phase structures. The phase structure of TiO2 is one of the important factors determining its photocatalytic performance. TiO2 has mainly four polymorphs in nature, including anatase, rutile, brookite, and TiO2(B). The structural parameters of these polymorphs are listed in Table 1. All four types of TiO2 consist of TiO6 octahedra, but differ in the distortion of the octahedron units and share edges and corners in different manners (Figure 2). For anatase, octahedra Figure 1. Proposed mechanism of photocatalytic reactions on semiconductor photocatalyst.

photogenerated electrons and holes do not contribute to the photocatalytic reaction. Only the photogenerated charges that reached to the surface of semiconductor could be available for photocatalytic reactions. The defect sites in the bulk and on the surface of semiconductor may serve as the recombination centers for the photogenerated electrons and holes, which will decrease the efficiency of the photocatalytic reaction. Efficient charge separation is the most important factor that determines the photocatalytic activities. Various strategies could be applied for improving charge separation efficiency. For example, preparation of semiconductor photocatalysts at high temperatures may lead to high crystallinity that diminishes the formation of charge recombination defect sites. Construction of various kinds of nanostructures such as nanowires (belts)19−24 and nanosheets25−31 may also facilitate charge transportation and promote charge separation efficiency. As compared to zerodimensional nanoparticle, one-dimensional nanostructures exhibit better photocatalytic activity because they have better charge mobility and can reduce the charge recombination. Furthermore, creation of “junctions” with built-in electric fields or chemical potential differences is also an effective strategy for improving charge separation efficiency. The surface catalytic reaction is a successive step of charge separation. In principle, a photocatalytic reaction consists of two half reactions, reduction reaction and oxidation reaction. The electrons in CB may initiate the reduction reaction, and the reduction capability is determined by the position of CB; the holes in the VB involve the oxidation reaction, and the oxidation capability is determined by the position of VB. For the water splitting reaction, the position of CB of a semiconductor photocatalyst should be more negative than the redox potential of H+/H2 (0 V vs NHE, pH = 7), while the energy level of VB should be more positive than the redox potential of O2/H2O (1.23 V vs NHE, pH = 7). Sometimes,

Figure 2. Crystalline structures of TiO2 in different phases: (a) anatase, (b) rutile, (c) brookite, and (d) TiO2(B).

arranging in zigzag chains along {221} share four edges; in rutile, octahedra share only two edges and connect in linear chains parallel to {001};32 while in brookite both corners and edges are connected.13 TiO2(B) is mainly derived from the layered titanates. Therefore, the structure of TiO2(B) is similar to that of the layered precursor, which is composed of corrugated sheets consisting of both edges and corners shared TiO6 octahedra.33 These differences in lattice structures cause different mass densities and electronic band structures in different phase forms of TiO2. Thermodynamically, rutile is the most stable phase, while anatase, brookite, and TiO2(B) are the metastable ones. Rutile can be normally obtained after annealing the other three polymorphs at elevated temperatures. The typical phase transformation process from anatase to rutile has been investigated in detail by Li and co-workers.37−39 The phase

Table 1. Four Main Polymorphs of TiO2 and Their Structural Parameters unit cell parameters crystal form

crystal system

space group

a/nm

anatase rutile brookite TiO2(B)

tetragonal tetragonal orthorhombic monoclinic

I41/amd P42/mnm Pbca C2/m

0.379 0.459 0.918 1.216 9989

b/nm

c/nm

0.545 0.374

0.951 0.296 0.515 0.651

β/deg

refs

107.3

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bandgap semiconductor photocatalyst (Eg ≈ 3.0 eV), TiO2 can only absorb the light in the UV region, which is one of the biggest drawbacks of TiO2.59 Much effort has been devoted to extending the light absorption of TiO2 to the visible region. The two most efficient strategies include: (1) bandgap engineering, which refers to narrowing the bandgap of TiO2 to make it absorb visible light by introducing other elements into TiO2 or through a special preparation method; and (2) surface sensitization, which refers to the application of other visible light active materials as a light harvester to sensitize TiO2. 3.1.1. Bandgap Engineering. 3.1.1.1. Doping with Cations. In the band structure of TiO2, O 2p orbitals contribute to the filled VB, while Ti 3d, 4s, 4p orbitals contribute to the unoccupied CB. The lower position of CB is dominated by Ti 3d orbitals.60−65 Upon doping with other cations in replacement of Ti, an impurity level could be introduced in the forbidden band. This intermediate energy level can act as either an electron acceptor or a donor, which allows TiO2 to absorb visible light. Until now, many investigations have been made in the preparation of visible light-responsive TiO 2 by cation doping.66−71 Early in 1982, Borgarello et al. reported the photocatalytic H2O splitting reaction on Cr3+-doped TiO2 nanoparticles in the visible light region (400−550 nm). The visible light absorption was attributed to the photoexcited transition of Cr3+ 3d electrons into the CB of TiO2.72 Khan et al. prepared Fe3+-doped titania by hydrothermal treatment,73 and claimed that water can be split into a 2:1 stoichiometric ratio of H2 and O2 under visible light irradiation. The authors further used an ion exchange method to introduce Pt, Ir, or Co into titania nanotube (TiNT) and succeeded in effective photocatalytic overall water splitting under visible light irradiation.74,75 Dholam et al. found that the H2 production rate was higher for Fe-doped TiO2 (15.5 μmol h−1) than that for Cr-doped TiO2 (5.3 μmol h−1) under visible light irradiation.76 This is because Fe ions can efficiently avoid the charge recombination by trapping both electrons and holes, while Cr can only trap one type of charge carrier. Besides Fe, Cu and Mn can also trap both electrons and holes and were recommended to be the better doping candidates than Cr, Co, and Ni ions, as the latter ones can only trap one type of charge carrier.77 Later, Dholam and co-workers prepared Cr-dopedTiO2 multilayer thin film with optical absorption energy as low as 2.1 eV.78 The photocurrent of the films was increased as a function of the number of the bilayers, and the H2 production activity was twice higher than that of the undoped TiO2. It was found that codoping with two cations exhibits higher activity than single cation doping. Niishiro et al. reported that TiO2 doped with Ni2+ can produce H2 from aqueous methanol solution under visible light irradiation (λ > 420 nm). When codoped with Nb5+, the absorption intensity of TiO2 can be obviously increased in the visible region. It is proposed that codoping of two cations with different charges can increase the stability of the photocatalyst due to the charge balancing effect.79 Sun et al. showed that the Fe and Ni codoped TiO2 nanoparticles prepared through alcohol-thermal method80 can give a H2 evolution rate of 361.64 μmol h−1 gcat−1 in ethanol aqueous solution under visible light (λ > 400 nm). The high H2 evolution activity was ascribed to the efficient separation of photoinduced electrons and holes by the codoping method, as evidenced by the photoluminescence spectroscopy.

transformation of TiO2 anatase nanoparticles actually starts from the interfaces between the agglomerated anatase particles, leading to a bulk phase transformation. Therefore, this process is usually accompanied by the growth of the particle size.40−42 Furthermore, the defect sites on the surface of anatase particles are proposed to play an important role in the phase transformation process. If the defect sites are blocked by some additives, the phase transformation can be efficiently retarded.37,43 In addition, the mechanism of anatase to rutile phase transformation largely depends on the particle size of the initial anatase. The different mechanisms of rutile nucleation were proposed as follows. The phase transformation from brookite to rutile undergoes brookite → anatase → rutile transition, and a quasi-H2Ti3O7 structure can be observed in the phase transformation from brookite to anatase by UV Raman spectroscopy.44,45 The transition process from TiO2(B) to rutile is similar to that of brookite to rutile, which undergoes the TiO2(B) → anatase → rutile route.46,47 The four polymorphs of TiO2 have been applied in various fields on the basis of their different physical/chemical properties.48−58 The most extensively conducted research on TiO2 still lies in using it as solar energy conversion materials, which is mainly investigated on anatase and rutile. Up to date, various kinds of research works have been carried out using TiO2 as a model semiconductor photocatalyst to produce H2 from water splitting, biomass reforming, industrial waste reforming, and to produce carbon-based solar fuels via CO2 photoreduction. TiO2 only absorbs UV light up to 380 nm, which is an intrinsic limitation for the TiO2-based photocatalysts to efficient light harvesting. However, TiO2 may serve as a good model semiconductor photocatalyst for exploring and understanding the processes and mechanisms of photocatalysis. These fundamental understandings gained through investigation of TiO2-based photocatalysts might be helpful guidance for the development of more efficient photocatalysts for water splitting and CO2 reduction. The photocatalytic activities of these different forms of TiO2 are quite different. Taking anatase and rutile of the most frequently studied photocatalysts of TiO2 as examples, the differences in lattice structures of anatase and rutile TiO2 cause different densities and electronic band structures, leading to different band gaps (for bulk materials: anatase 3.20 eV corresponding to 384 nm and rutile 3.02 eV corresponding to 410 nm). This makes anatase have a slightly higher redox driving force than rutile, although the range of the light absorption by the former is slightly less than that by the latter. Anatase also has much higher surface area than rutile, leading to enhanced adsorption capability and even generation of much more active sites (such as oxygen vacancies). Furthermore, although rutile has better charge carrier mobility due to its higher crystallinity than anatase, the latter can generate more efficient charge separation due to the existence of more oxygen vacancies. Because of these advantages of anatase, it usually shows much higher photocatalytic activity than rutile as shown in the following sections.

3. HYDROGEN GENERATION ON TiO2-BASED PHOTOCATALYSTS 3.1. Light Harvesting

Photocatalytic reaction is initiated from light absorption. Thus, the photocatalytic activity is first restricted by the number and energy of photons absorbed by the photocatalyst. As a wide 9990


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Figure 3. Schematic representation of photoexcitation process in metal-doped TiO2 under visible light irradiation: (a) Ti1−xVxO2; (b) Ti1−xFexO2; and (c) Ti1−xCrxO2. Reprinted with permission from ref 81. Copyright 2002 Elsevier.

significantly enhance the photocatalytic activity of TiO2 for H2 production due to the improvement of the particle dispersion as well as the increase of the surface area instead of the bandgap of TiO2 narrowed.86−90 Huang et al. also reported that doping with Nd3+ can increase the H2 production activity of TiO2, but the effect of light absorption on activity was negligible.91 Wu et al. reported that a Co2+-doped TiO2 had a long-term stability for H2 production, and the presence of CoOx species on the surface of TiO2 was crucial for the stability.92 The enhancement of H2 production activity on Bidoped TiO2 was ascribed to its lower overpotential as compared to the undoped TiO2.93 3.1.1.2. Doping with Anions. Anion doping is another approach to extend the light absorption of TiO2 into the visible region. Unlike cation doping, anion doping could hardly affect the CB band of TiO2, which is made up of Ti 3d, 4s, and 4p orbitals. It usually reconstructs the VB and shifts it upward to narrow the bandgap of TiO2. Asahi and co-workers preferred anion doping rather than cation doping94 by considering that cation doping would lead to instability of the material and require an expensive ionimplantation facility. Furthermore, cation doping usually leads to quite localized d states deep in the bandgap of TiO2 acting as the recombination centers of charge carriers. Figure 4 shows the densities of the states of substitution doping of some anions including C, N, F, P, or S for O in anatase TiO2. It can be seen that N is the most suitable doping element as its p orbitals contribute to VB by mixing with O 2p orbitals, which can

The electronic structures of 3d transition metal-doped TiO2 were investigated by Umebayashi et al. using ab initio calculations.81 It was found that the localized level of t2g state of the doping element lies in the middle of the bandgap when V, Cr, Mn, or Fe is doped and at the top of the VB when Co is doped. For Ni, it contributes to the formation of VB with O2p. Therefore, t2g level is a critical factor determining the final band structure of doped TiO2. The charge transition process on transition metal-doped TiO2 can be better understood by the study of the photoelectrochemical processes,82,83 where the holes leaving in the VB can generate an anodic photocurrent and the electrons excited into the CB can generate a cathodic photocurrent. Some typical transition models are illustrated in Figure 3. It can be seen that the different positions of the t2g level could construct different models of electron excitation, which may lead to quite different photoelectrochemical behavior of the metal-doped TiO2. For example, Matsumoto et al. observed an anodic photocurrent for V-doped TiO2, while both anodic and cathodic photocurrent can be observed for Cr, Mn, or Fe-doped TiO2, revealing different excitation models resulted from the different cation doping.84 Nishikawa et al. studied the relationship between the radius of transition metal cation doping and the changes of the induced bandgap and concluded that Ni3+ and V5+ can reduce the bandgap more efficiently.85 It should be noted that not all of the activity enhancement can be ascribed to the visible light absorption of cation-doped TiO2. For example, doping with rare earth cations can 9991


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was 1.8 times higher than that of Degussa P25. The authors proposed that the improved H2 production activity was due to enhanced light absorption caused by the incorporation of C.102 Krengvirat et al. also prepared C-incorporated TiO2 photoelectrodes with different structural features via rapid-anodic oxidation under different electrical potentials and exposure times. Photoelectrodes with regular nanotubular structures exhibited the H2 evolution rate up to 508.3 mL min−1 cm−2 and conversion efficiency η of ∼2.3% under a bias voltage of 0.3 V, which is superior to those with irregular structures.103 Earlier in 1986, Sato reported NOx impurity in TiO2 could contribute to a shoulder absorption around 450 nm.104 After that, N-doping is widely applied for the extension of light absorption of TiO2.105−115 Yuan et al. synthesized a N-doped TiO2 with high specific surface area by heating a mixture of urea and TiO2. N-doped TiO2 was active in H2 production in Na2SO3 aqueous solution under visible light irradiation (λ > 400 nm).116 The absorption spectrum of this material was shifted up to 600 nm, which was ascribed to both the molecularly chemisorbed N2 and the substitutional N. Sreethawong et al. also used urea as N resource to prepare N-doped TiO2 with mesoporous structure. The optimal preparation condition was at a urea:TiO2 molar ratio of 1:1 and a calcination temperature of 250 °C.117 The similar Ndoped TiO2 with mesoporous structure can be prepared by evaporation-induced self-assembly method, in which pluronic F127, titanium isopropoxide, and urea were used as templating agent, titanium, and nitrogen sources, respectively.115 By annealing in the gaseous ammonia and nitrogen atmosphere, Li and co-workers prepared a N-doped TiO2, which shows obvious activity as evidenced by H2 production under visible light irradiation (λ > 400 nm).118 Lin et al. used a kind of twomicroemulsion technique to synthesize the N-doped TiO2.119 This microemulsion can be used as a nanosize reactor to produce high-purity nanoparticles with uniform size and low aggregation degree. The microwave-assisted hydrothermal method was also applied to synthesize the N-doped TiO2 using titanium sulfate as Ti precursor and urea as N source. The catalyst showed very high H2 production activity under visible light irradiation, which was 15 times higher than that of P25.120 Babu et al. synthesized a N-doped TiO2 with a rice grain-like nanostructure by electrospinning method followed by annealing at high temperatures, which showed superior H2 evolution capability in photoelectrochemical (PEC) reaction.121 Other anions, such as S, P, and B, were also employed for TiO2 doping.122−129 Nishijima et al. found that S-doped TiO2 was superior to N-doped TiO2 for H2 evolution under visible light irradiation, as the bandgap of S-doped TiO2 estimated from the diffuse reflectance spectra was smaller than that of Ndoped TiO2.130 Pal and co-workers investigated the photocatalytic H2 production reactions over C-, N-, and S- doped TiO2 and found the activity decreases in the order: C-TiO2 > STiO2 > N-TiO2.131 The S-doped TiO2 microcube was synthesized by Wen and co-workers using titanium oxydifluoride (TiOF2) as a solid precursor, which showed higher H2 evolution activity as compared to undoped TiO2.132 Liu et al. carefully investigated the effect of spatial distribution of interstitial Bσ+ in TiO2 for H2 and O2 evolution reactions. They found that the presence of interstitial Bσ+ in the shell of TiO2 would result in a downward shift of the band edges.133 Jin and co-workers found that B4O72− anion solution was not only a good system for the preparation of B-doped TiO2, but also an excellent system for H2 evolution for B-doped TiO2.134 The P-

Figure 4. (A) Total DOSs (densities of states) of doped TiO2 and (B) the projected DOSs into the doped anion sites, calculated by FLAPW (full-potential linearized augmented plane wave). The dopants F, N, C, S, and P were located at a substitutional site for an O atom in the anatase TiO2 crystal (the eight TiO2 units per cell). The results for N doping at an interstitial site (Ni-doped) and that at both substitutional and interstitial sites (Nii+s-doped) are also shown. The energy is measured from the top of the VBs of TiO2, and the DOSs for doped TiO2 are shifted so that the peaks of the O 2s states (at the farthest site from the dopant) are aligned with each other. Arb. unit, arbitrary units. Reprinted with permission from ref 94. Copyright 2001 AAAS.

narrow the bandgap of TiO2 by shifting the VB upward. However, there is also an argument that extension of the light absorption into the visible region by N doping is not due to bandgap narrowing. Instead, N doping may introduce local states and oxygen vacancies inside the band gap of TiO2, which is the cause of the visible light absorption.95 Controversial remarks on the origin of the visible light absorption upon cation or anion doping always exist, which need to be further studied. Other elements such as C and P could also introduce some midstates, which however are not well overlapping with the band states of TiO 2. S with a large ionic radius is technologically difficult to be doped into TiO2, although it could generate a smaller bandgap. X-ray photoelectron spectroscopy (XPS) revealed that C, N, or S doping may create some additional electronic states above the VB edge of TiO2, which are accountable for the red shift of these aniondoped TiO2.96 Braun and co-workers also observed an additional eg resonance in the VB of N-doped TiO2 by oxygen near-edge X-ray absorption spectroscopy,97 and proposed that this extra resonance was responsible for the photocatalytic performance of N-doped TiO2 upon excitation at visible light wavelengths. Early in 2002, Khan et al. synthesized a carbon-doped TiO2 by controlled combustion of Ti metal in a natural gas flame. Replacement of some lattice O with C atoms resulted in the extension of light absorption to ca. 535 nm. The photoanode made with this material showed the total conversion efficiency of water splitting reaction for this material was 11% under an applied potential of 0.3 V.98 C-doped TiO2 films and TiO2 nanotube array films were also fabricated by spray pyrolysis of glucose containing TiCl4 and anodization of Ti metal sheets in fluoride solution, respectively.99−101 It was suggested that Cdoping can extend the light absorption threshold of n-TiO2 to 2.84 eV by introducing an additional intragap band of C in the gap at 1.30 eV above the VB. Zhang and co-workers synthesized a microspherical C-incorporated titania using a flame-assisted hydrolysis method. The maximum photocatalytic H2 production rate of this material with sacrificial reagents Na2S/Na2SO3 aqueous solution was up to 8.1 μmol h−1, which 9992


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doped TiO2 was synthesized by Tian and co-workers,125 which showed H2 evolution activity in glycerol solution. It was also found that two anions codoped TiO2 showed higher photocatalytic activity than that of the single anion doped TiO2. Luo and co-workers investigated the Br and Cl codoped TiO2.135 They found that the absorption edge of codoped TiO2 shifted to a lower energy region and led to a higher activity in H2 production as compared to that of single anion doped TiO2. Fang et al. prepared N and S codoped TiO2 via calcination of (NH4)2TiO(SO4)2 at 600 °C, which exhibited high activities in both photocatalytic H2 production (λ > 400 nm) and acid catalysis.136 Bai and co-workers showed that S and C codoped TiO2 had a high photocatalytic H2 production activity when methanol was used as the sacrificial reagent.137 The C and N codoped TiO2 synthesized with TiCN as Ti precursor showed photocatalytic H2 evolution activity at a rate ca. 41.1 μmol h−1.138 B and N codoped TiO2 has been extensively studied for photocatalytic reactions.139−145 Li et al. reported that B−N codoped TiO2 might enhance the H2 production activity under visible light irradiation as compared to that of merely N doped TiO2, which is due to the lifetime of electrons and holes prolonged by B doping.146 Liu and coworkers suggested that a new O−Ti−B−N structure on the surface of B−N codoped TiO2 was responsible for the enhancement of surface charge separation and transfer, leading to improved photocatalytic activity.147,148 3.1.1.3. Codoping with Cations and Anions. Codoping with both cation and anion can also extend the visible light absorption of TiO2. Gai et al. found that codoping with chargecompensated donor−acceptor pairs, such as Mo and C, can significantly raise the VB edge of TiO2 while leaving the CB edge almost unchanged.149 Codoping of N with another metal has also been extensively studied. For example, Sasikala and coworkers found a clear red shift of the absorption edge for In and N codoped TiO2, which showed a better H2 production activity as compared to that of merely In or N doped TiO2 or pristine TiO2.150 A more distinct red shift of TiO2 was found in Ce and N codoped TiO2, which showed 20 times higher H2 evolution activity as compared to that of undoped TiO2.151 It was proposed that codoping with Ce and N can enhance the lattice distortion, and the formation of N−Ti and N−Ti−O bonds was responsible for the narrowing of the bandgap. Selcuk and co-workers systematically investigated N and metal (including Fe, Cr, Ni, and Pt) codoped TiO2 and found that Ni and N codoped TiO2 showed the highest H2 production activity under visible light irradiation.152 Li et al. found that Ga and N codoped TiO 2 exhibited intense visible light absorption.153 In addition, on the basis of the extensive DFT band structure calculations, Yin and co-workers predicted that (Ta, N) and (Nb, N) pairs are the best combination in the high concentration doping regime, while (Mo, 2N) and (W, 2N) were the optimal pairs in the low concentration doping regime.154 It should be noted here that, although many of the above reviewed publications attributed the enhancement of the photocatalytic activity upon doping mainly to the extension of the light absorption, such conclusions lack in-depth investigation of the other vital factors that also influence or determine the photocatalytic activities. For example, the catalytic activity was not well normalized by the specific surface area, which may vary greatly with the doping. Another very important effect of doping is that it might also create catalytic active sites, which may promote either the oxidation of

sacrificial reagents or the reduction of protons, hence enhancing the overall photocatalytic activity. Furthermore, some controversial conclusions were also seen in the doping effect, such as the effect of S doping, which might have resulted from the different experimental conditions. 3.1.1.4. Doping Free and Self-Doped Visible LightResponsive TiO2. Although the introduction of the impurity energy levels by doping with various ions could extend the absorption of TiO2 to the visible light region, it has been debated that doped TiO2 suffers from instability and the impurity would also act as recombination centers leading to the decrease of the photocatalytic activity.67 Therefore, doping free TiO2 with visible light response has attracted increasing attention in recent years.155−157 A radio frequency magnetron sputtering deposition method was introduced to the synthesis of visible light-responsive TiO2 thin films.158,159 This film was found to produce H2 or O2 using sacrificial reagents under visible light (λ ≥ 420 nm) irradiation. In addition, the declined O/Ti composition or the oxygen vacancies created in the TiO2 film was responsible for the visible light response. Sasikala et al. employed solvothermal, sonochemical, and polyol methods to synthesize the self (Ti3+) doped TiO2.160 All of the samples showed the onset of absorption at ∼440 nm, which was due to the defect levels that originated from the oxygen vacancies. Zuo and co-workers developed a combustion method to synthesize TiO2−x, which exhibited a high stability in air and water under light irradiation. The presence of Ti3+ in the bulk, evidenced by the electron paramagnetic resonance (EPR) spectra, was proposed to be responsible for the visible light absorption of TiO2−x.161 Xing and co-workers introduced the air-stable Ti3+ into TiO2 by lowtemperature vacuum activated method.162 They further developed a solvothermal method to prepare the self-doped Ti3+-enhanced TiO2 with NaBH4 as reductant.156 Conventionally, the oxygen vacancies are regarded to be responsible for the Ti 3d defect state in the bandgap. Yet this comment has been challenged recently by Wandt and co-workers.163 The argument was based on the result of high-resolution scanning tunneling microscopy and photoelectron spectroscopy measurements. It was proposed that the Ti interstitials in the nearsurface region may generate the defect state in the bandgap, which is responsible for the visible light absorption. Creating a distortion or new structure by surface modification of TiO2 may also lead to visible light response. Chen and co-workers synthesized a disorder-engineered TiO2 with black color by hydrogenation, which showed substantial activity and stability in photocatalytic H2 production under sunlight.164 This material was consisting of a crystalline TiO2 quantum dot or nanocrystal core and a highly disordered surface layer (Figure 5). The structure retained the benefits of crystalline TiO 2 quantum structures for photocatalytic processes as well as the enhanced visible and infrared absorptions from the structural disorders. The authors indicated that large amounts of lattice disorder in semiconductor could yield midgap states, which are different from that of a single defect. Throughout the photocatalytic testing cycles, the disorder-engineered black TiO2 nanocrystals exhibited high stability. Tao et al. reported a new TiO2 phase with two dimension forms on the surface of rutile TiO2(011) single crystals by low-pressure oxygen annealing, resulting in a narrowed bandgap of only ∼2.1 eV.165 The authors pointed out that this method may also be feasible for the synthesis of 9993


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excited by absorbing visible light to generate excited states, which can inject electrons into the CB of the semiconductor. In a dye-sensitized system, the reduction reaction takes place on the surface of the semiconductor, while the exited dye molecules can be regenerated by reacting with the sacrificial reagent. It should be noted that H2 can be produced from the reaction mixture of dye and EDTA in the absence of semiconductor under visible light irradiation,166 but the H2 production rate is usually quite low, demonstrating that the majority of H2 is evolved on the surface of TiO2. A research on dye sensitization of semiconductor was initiated early in the 1970s.167−169 After the great progress made by O’Regan and Grätzel in 1991,170 dye-sensitized solar cell171−180 and dye-sensitized photocatalytic reactions have attracted ever increasing attention.181−189 In this section, we will introduce recent progress in TiO2 sensitization by transition-metal complex dyes and metal free organic dyes. 3.1.2.2. Transition-Metal Complex Dye-Sensitized TiO2. Ruthenium-based dye-sensitized TiO2 has been widely investigated for photocatalytic H2 production reaction.190−194 Early in the 1980s, Grätzel and co-workers found that H2O could be decomposed under visible light irradiation on TiO2 when Ru(bpy)32+ was used as a sensitizer.195,196 The authors further found a RuL32+2C1− (L = diisopropyl 2,2′-bipyridine4,4′-dicarboxylate) complex could shift the absorption onset to beyond 600 nm by its chemical fixation on TiO2 via Ru−O−Ti bonds.197 The photocatalytic H2 production reaction can be achieved under the light irradiation with wavelength even up to 590 nm. Hirano et al. investigated the photocatalytic performances of Ru(bpy)32+, tris(bipirimidine)Ru(II) (Ru(bpym)3), and porphines-sensitized TiO2.198 They found that Ru(bpym)32+ was the best sensitizer for H2 production under visible light using EDTA as an electron donor (sacrificial reagent). It was suggested that the high affinity of Ru complex on TiO2 was important for the H2 evolution activity, which may facilitate the electron injection process from the excited dye molecule to the semiconductor. Therefore, the property of ligand in Ru complex, which is responsible for the linkage between dye molecule and semiconductor, is undoubtedly important. Bae and Choi synthesized a series of Ru-based complexes with different anchoring groups and investigated the effect of anchoring groups on H2 production activity.199 They found that phosphonate group can be effectively adsorbed on the surface of TiO2 and hardly be hampered by the electron donor (EDTA), while a carboxylate group could be seriously inhibited by EDTA and resulted in a poor activity. However, Peng and co-workers found that the tightly linked dye molecule showed better durability but lower H2 evolution rate, while higher activity was obtained with the loosely attached dye molecule.200 This demonstrates that the dynamic equilibrium between the adsorption of ground-state dye molecule and the release of the oxidized dye molecule on the surface of TiO2 is a crucial factor determining the reaction rate. Charge recombination and stability of Ru-based dye molecules are usually fatal problems for dye-sensitized photocatalytic systems. To overcome these problems, various kinds of strategies have been explored. Kim et al. designed a dye-sensitized TiO2 on a thin layer of Al2O3,201 which can efficiently retard the charge recombination between the electrons injected to the CB of TiO2 and the oxidized dye molecules. Kruth and co-workers introduced a thin polyallylamine layer to stabilize the dye molecule that adsorbed on the surface of TiO2.202 The encapsulated nanostructure can confine

Figure 5. (A) Schematic illustration of the structure and electronic DOS of a semiconductor in a disorder-engineered nanocrystal form with dopant incorporation. Dopants are depicted as black dots, and disorder is represented in the outer layer of the nanocrystal. The conduction and valence levels of a bulk semiconductor, EC and EV, respectively, are also shown, and the bands of the nanocrystals are shown at the left. The effect of disorder, which creases broadened tails of states extending into the otherwise forbidden bandgap, is shown at the right. (B) An image comparing between unmodified white and disorder-engineered black TiO2 nanocrystals. (C and D) HRTEM images of TiO2 nanocrystals before and after hydrogenation, respectively. In (D), a short-dashed curve is applied to outline a portion of the interface between the crystalline core and the disordered outer layer (marked by white arrows) of black TiO2. Reprinted with permission from ref 164. Copyright 2011 AAAS.

powder photocatalysts to realize the visible light responded photocatalytic reactions. The so-called water splitting on TiO2-based photocatalysts is not true for many published works because the H2 was not essentially produced from water splitting but from the reforming of sacrificial reagents, which are thermodynamically feasible. Therefore, the effect of the doping on the activity is usually sensitive because the reforming reaction itself is easily affected by the surface property. 3.1.2. Surface Sensitization. 3.1.2.1. Mechanisms of Dye Sensitization. Dye sensitization is another strategy to extend light absorption of TiO2 to the visible region. Different from bandgap engineering, light is absorbed initially by the dye molecule instead of TiO2. The proposed mechanism of dyesensitized semiconductor is shown in Figure 6. The dye molecules adsorbed on the surface of a semiconductor are

Figure 6. Mechanism of photocatalytic reaction on dye-sensitized TiO2 semiconductor. 9994


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Figure 7. Structure of several organic dyes with different hydrophilic properties and their photocatalytic H2 evolution activity on dye-sensitized Pt/ TiO2. Reprinted with permission from ref 221. Copyright 2010 American Chemical Society.

Perylene dye-sensitized Pt/TiO2 was investigated by Liu and co-workers, which showed H2 production activity under visible light irradiation.220 Lee et al. synthesized a series of dyes with different hydrophilicity (hydrophobic D−H, slightly hydrophilic DMOM, and hydrophilic DEO1−DEO3) and found that the moderately hydrophilic dyes have the highest H 2 production activity under visible light irradiation (shown in Figure 7).221 Han and co-workers also found that the hydrophilic dyes showed higher activity than the hydrophobic dyes in the dye-sensitized TiO2 catalysts.222 Choi et al. reported that the carboxylate anchoring groups of organic dyes were extremely important, because they can affect the binding states between dyes and TiO2 as well as the HOMO−LUMO position of the dye.223 Abe and co-workers investigated the H2 production reaction on merocyanine and coumarin dyesensitized TiO2 using iodide as an electron donor in a water−acetonitrile mixed solution.224−227 They found that acetonitrile was actually a good solvent for both I− and merocyanine dye. With increasing amount of water in the mixed solution, the H2 production activity decreased, which was ascribed to the drop in energy gap between the redox potential of I3−/I− and the HOMO level of the dye. Because of the excellent visible light absorption properties and relative stability, Eosin Y was widely used in the dyesensitized TiO2.228−235 Abe and co-workers constructed a stable dye-sensitized TiO2 by a chemical fixation method via a silane-coupling reagent between Eosin Y and TiO2. This system can produce H2 from aqueous triethanolamine solution under visible light irradiation for a long time.236 Lu and co-workers systematically investigated the Eosin Y-sensitized TiO2237,238 and expanded this system to Ti-MCM-41239 and N-doped TiO2.240 They found that the addition of CuO can improve the absorption efficiency of Eosin Y on the surface of TiO2 and finally received an apparent quantum yield of 5.1% for H2 generation.241 Later, a much higher H2 evolution activity was achieved for multilayer-Eosin Y-sensitized TiO2 with maximal apparent quantum yield of 19.1%.238 It can be seen from the above review that dye sensitization can favorably extend the absorption of light into visible region. However, the dye-sensitized TiO2 photocatalytic systems usually suffer activity loss due to the instability of dye molecules in the photocatalytic reaction environment. Furthermore, injection of charges via interfacial charge transfer from dye molecules to TiO2 is an inefficient charge transfer process, which also limits the efficiency of photogenerated charges. These problems are solved in certain level by tailor designing the ligands of dyes, covalent anchoring of dye molecules on the surface of TiO2, or encapsulation of dye

the dye molecules and improve the stability for a period of over several days. Zhang et al. investigated a binuclear RuII-bipyridyl dye-sensitized TiO2, which can produce H2 without noble metal cocatalyst.203 Besides Ru complexes, zinc porphyrin complexes have been studied as another category of sensitizers by several research groups204,205 and were successfully used in visible light responded solar cell.206−208 Malinka and co-workers found that the H2 production reaction can be performed on zinc porphyrin-sensitized TiO2 under visible light (λ > 520 nm).209 Zakharenko et al.210 and Zhang et al.211 found that platium(II) complex-sensitized TiO2 can promote H2 evolution reaction under visible light irradiation. Besides, two other platinum(II) complexes, [Pt(tpy-phen-COOH)(CC−C6H5)]Cl and [Pt(tpy-COOH)(CC−C6H5)]Cl, were synthesized by Jarosz and co-workers, and used as sensitizers for platinized TiO2 in photocatalytic H2 production under visible light irradiation.212 Nada and co-workers compared three different sensitizers on the TiO2/RuO2-MV2+ system, including copper phthalocyanine, ruthenium bipyridyl, and Eosin Y.213 It was found that copper phthalocyanine exhibited the best performance in H2 production reaction, and the activity was higher in acidic medium than basic medium. 3.1.2.3. Organic Dye-Sensitized TiO2. As described above, metal complex dyes have been extensively investigated in the dye-sensitized TiO2. However, the high cost and the environmental toxicity of these dye molecules limit their wide applications. Therefore, organic dyes with low cost and diverse forms are highly desired.214,215 Early in 1983, Grätzel et al. modified TiO2 particles by surface complexation with 8-hydroxyquinoline.216 This catalyst showed an excellent photocatalytic H2 production activity with EDTA as a sacrificial reagent under visible light irradiation. Shimidzu and co-workers found that heavy-halogenated xanthenes dyes exhibited the high quantum yields of H2 generation but were prone to photodehalogenate, while nonhalogenated xanthenes dyes could resist the photodeterioration yet with comparatively lower activities.217 Gurunathan et al. compared dye-sensitized TiO2 with other modification methods, including Cu(II) ion doping and loading with CdS.218 The authors found that the dye sensitization showed higher H2 production activity than other methods of sensitization, and Rhodamine B and Ru(bpy)32+ were the best choice among the dye sensitizers studied.218 Ikeda and coworkers found that surface modification of TiO2 powders with 1,1′-binaphthalene-2,2′-diol (bn(OH)2) can extend the light absorption of TiO2 to 550−600 nm.219 The sensitized TiO2 showed a H2 evolution activity in triethanolamine solutions even under irradiation with wavelength longer than 540 nm. 9995


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molecules in some porous materials. Yet the problems still exist, and strategies that are more efficient need to be explored. 3.2. Photogenerated Charge Separation

Absorption of photons by semiconductor photocatalyst induces the photogenerated electrons and holes. The photogenerated charge carriers are separated or recombined on the way to the surface reaction sites. The charge separation is a crucial factor determining the light to fuels conversion efficiency. Therefore, much attention has been paid to increasing the charge separation efficiency. Fabrication of junction structure has been recognized as an effective strategy to avoid charge recombination in semiconductors. In the following sections, we will discuss charge separation by junction approach with particular focus on heterojunction, phase junction, and Schottky junction. 3.2.1. Fabrication of Heterojunction. Semiconductorbased photocatalysts with heterojunction can be considered as a coupled system of two semiconductors (e.g., TiO2 coupled with another semiconductor), in which both semiconductors are excited by photons to generate electrons and holes in their CB and VB, respectively. The directions of charge transfer would depend on the relative positions of CB and VB of the two semiconductors. Generally, there are three ways for coupling TiO2 with another semiconductor into heterojunction photocatalyst (types I, II, and III shown in Figure 8a, b, and c, respectively).242 In principle, the electrons can be injected to the material with more positive CB position, while the holes transfer to the material with more negative VB position. For types I and II (shown in Figure 8a and b), the electrons and holes transfer to the different semiconductors in opposite directions and bring a spatial charge separation, which is also called the “direct Z-scheme”. Nevertheless, for type III as shown in Figure 8c, both the electrons and the holes will transfer from one material to the other without spatial charge separation. Most of the composites for H2 production are in type I, because the electron-occupied CB in type II may not satisfy the potential for H2 production. 3.2.1.1. CdS/TiO2 Heterojunction. CdS is a visible lightresponsive n-type semiconductor, which is one of the mostly investigated semiconductors for coupling with TiO2.243−260 Various kinds of CdS/TiO2 composite structures have been studied, which can be classified into two types: CdS nanoparticles on the surface of TiO2 or TiO2 nanoparticles on the surface of CdS. Although different morphologies, phase structures, and preparation methods of TiO2 and CdS were applied, the most common conclusion drawn for the enhancement of the photocatalytic activity in CdS/TiO2 composite structure is attributed to the efficient charge transfer and separation between CdS and TiO2 driven by the internal electric field or potential difference created by the heterojunctions between CdS and TiO2 as shown in Figure 9. In 2001, Hirai et al. used mercaptoacetic acid (MAA) to immobilize CdS nanoparticles onto the surface of TiO2. They found that a great enhancement of H2 production activity of CdS−MAA−TiO2 can be obtained with respect to CdS−MAA under visible light irradiation (λ > 400 nm).261 Jang and coworkers systematically investigated the bulk CdS decorated with nanosized TiO2, which showed an unprecedented high rate of H2 production activity under visible light irradiation (λ > 420 nm) in Na2S/Na2SO3 aqueous solution or H2S-containing alkaline solution.262−264 The authors proposed that the nanobulk composite of CdS(bulk)/TiO2 could provide an

Figure 8. Band structures and migration of the charges in three types of combination systems of TiO2 with other semiconductors. Reprinted with permission from ref 242. Copyright 2010 Wiley-VCH.

Figure 9. Energy level diagram of CdS/TiO2 nanocomposite. Reprinted with permission from ref 721. Copyright 2011 Elsevier.

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MWNT can act as photosensitizer and accelerate the H2 evolution rate under visible light irradiation.309 Wei and coworkers concluded that the formation of chemical bonding between the modified MWNTs and TiO2 is crucial for the catalytic properties.310 Cui et al. observed that the interface integration between TiO2 and MWCNTs results in a slight blue shift and weak symmetry in the Raman spectrum, indicating that there is strong interaction between TiO 2 and MWCNTs.311 Dombi et al. synthesized and systematically investigated TiO2/WO3/Au/MWCNT composite photocatalyst for H2 evolution. It was found that crystal phase, particle size of TiO2, and the MWCNT contents significantly affect the H2 evolution reaction.312 Miyuki and co-workers investigated the composite of single walled carbon nanotubes (SWNTs) and TiO2. They found that the H2 evolution rate could be up to 15.7 μmol h−1 in aqueous oxalic acid solution, while only a negligible amount of H2 was produced in the absence of SWNTs, demonstrating the crucial role of SWNTs in photocatalytic activity.313 Zhang and co-workers reported the photocatalytic H2 evolution on TiO2/graphene sheets (GSs) composite synthesized by a sol−gel method.314,315 A kindly shift to the visible light was observed for TiO2/GSs with optimized 5 wt % GSs content, which showed the highest H2 evolution activity. They also investigated the effect of calcination atmosphere on the activity of TiO2/GSs composite and claimed that the calcination in nitrogen atmosphere was better than that in air. Following this work, a one-step hydrothermal method was introduced to synthesize well-dispersed TiO2 nanoparticles on the surface of GSs.316 The enhancement of photocatalytic H2 production on TiO2/GSs as compared to that of TiO2 was attributed to the excellent electron conductivity of GSs and the chemical bonding formed between TiO2 and GSs. Fan and coworkers dispersed P25 nanoparticles on reduced graphene oxide (RGO) by several techniques.317 The intimate contact between P25 nanoparticles and RGO can accelerate transfer of photogenerated electrons and suppress the recombination of charge carriers. The authors also found that TiO2-RGO performed higher activity than TiO2-CNTs, which was ascribed to the better dispersion of TiO2 nanoparticles on RGO than that on CNTs. The enhancement of visible light absorption and the efficient charge separation abilities of graphene-modified TiO2 were also convinced by other researchers.318,319 Xiang and co-workers reported that graphene as cocatalyst on TiO2 could act as an electron acceptor due to the lower potential of graphene/graphene− as compared to the CB of TiO2.320 They further combined the graphene with layered MoS2 as the hybrid cocatalysts on TiO2, which resulted in an apparent quantum efficiency up to 9.7% at 365 nm light irradiation in the photocatalytic H2 production reaction even without a noble-metal cocatalyst (Figure 10).321 The junctions including heterojunction may be helpful for the charge separation, but this concept is very confused especially in TiO2 related publications. The activity enhancement may be due to the junction effect, but could be due to some other factors. In the reviewed publications, there is much less study made on the charge separation in the TiO2 involved junctions (or composite systems), and there is lack of direct evidence on charge separation. So it is not reliable to simply contribute the catalytic activity to the enhanced charge separation efficiency. 3.2.2. Fabrication of Phase Junction. 3.2.2.1. Anatase− Rutile Phase Junction. As described in section 2, anatase and

efficient charge separation structure to enhance H2 production activity. Furthermore, they found that the cocatalyst Pt deposited on TiO2 showed higher activity than that on CdS, indicating that it was electron transfer from excited CdS to TiO2 that promoted the H2 production.265 Park et al. also indicated that the location of Pt on the CdS−TiO2 system is very important, as the sample with Pt totally deposited on TiO2 (CdS/(Pt−TiO2)) showed a higher H2 production activity by a factor of 3−30-fold than that of Pt−(CdS/TiO2).266 Besides TiO2 particles,267−272 TiO2 nanotubes (NTs) and nanosheets modified with CdS were also widely investigated.273−276 Yin and co-workers used an electrodeposition method to synthesize a core/sheath heterostructure CdS/TiO2 nanotube array.277 Zhang et al. prepared the CdS/TiO2 NTs consisting of uniform anatase nanotubes loaded homogeneously with hexagonal phase CdS nanoparticles. It showed a high H2 production activity in a Na2S/Na2SO3 aqueous solution under visible light irradiation (λ > 400 nm).278 Li and co-workers used ion-change followed by sulfurization method to fabricate CdS nanoparticles inside TiO2 NTs, which showed an apparent quantum yield for H2 production up to 43.4%.279 Liu et al. prepared a highly stable CdS/TiO2 NTs electrode for H2 production under visible light irradiation via sonoelectrochemical anodization and sonoelectrochemical deposition method. They found that the sonoelectrochemical deposition methods can improve both the connectivity between TiO2 NTs and CdS and the dispersion of CdS nanoparticles.280 CdS-modified Pt/TiO2 nanosheets with exposed {001} facets were synthesized by Qi and co-workers. 281 The TiO 2 nanosheets showed the highest H2 production activity as compared to that of TiO2 nanoparticles and commercial P25, indicating the morphology of TiO2 particles causes a significant influence on H2 production activity. Kim et al. introduced the CdS quantum dots (QDs) to the layered titanate nanosheets, which exhibited very high H2 production activity without any cocatalysts. The resulting hybrid-type photocatalysts exhibited visible light absorption ability and a remarkable depression of charge recombination, which are accountable for the high H2 evolution activity.282 Multicomponent photocatalysts containing CdS and TiO2 with junction structures have also been prepared to increase the H2 production activity.283−288 Yet the composite structures are too complex to single out the role of junctions. Following a similar strategy, in addition to CdS, TiO2 was also coupled with many other semiconductors.289−302 3.2.1.2. TiO2/Carbon-Based Materials. In recent years, the composite photocatalyst coupling of TiO2 with carbon-based materials, especially with carbon nanotubes (CNTs) and graphenes, has attracted increasing attention. CNTs with defined electronic properties can be chemically bonded with TiO2.303 This material also shows large surface area (>150 m2 g−1) and excellent mechanical properties.304 Besides, a much higher theoretical specific surface area (∼2600 m2 g−1) was discovered on graphene, a sp2-hybridized two-dimensional carbon nanosheet. This material exhibits the high mobility of charge carriers as well as good mechanical strength.305−307 Therefore, these carbon-based materials are promising candidates to facilitate charge transfer and inhibit the charge recombination process when combined with TiO2-based photocatalysts.308 Liao et al. synthesized a multiwalled carbon nanotube, TiO2, and Ni composite catalysts (MWNT-TiO2:Ni) by a modified chemical vapor deposition method. The authors claimed that 9997


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Figure 10. A structural model of charge transfer in TiO2/MoS2/ graphene composites. Reprinted with permission from ref 321. Copyright 2012 American Chemical Society.

rutile are the most widely used crystal forms of TiO2. Anatase usually exhibits higher activity than rutile in photocatalytic reactions.47,322−325 However, the mixed-phase structure of TiO2 (e.g., P25, Degussa TiO2) containing both anatase and rutile has received much attention, because this mixed-phase structure exhibits higher photocatalytic activity than either anatase or rutile alone.326−331 Li and co-workers have investigated the effect of mixed-phase structure of TiO2 on the photocatalytic H2 evolution reaction in detail. It was found that the highest H2 evolution activity obtained for TiO2 with mixed-phase structure was irrelevant to the surface area.332,333 TiO2 with phase junction was prepared by calcination of anatase TiO2 particles. The phase transformation from anatase to rutile takes place on the surface skin of integrated anatase particles. The higher photocatalytic activity was found for the TiO2 with mixed phases on the surface. These results were further confirmed by depositing smaller anatase particles on rutile, and the corresponding activity was increased with increase of the deposited anatase particles on rutile; the activity is closely related to the surfacephase junction formed between anatase and rutile (Figure 11). The strategy of the surface-phase junction construction was used to improve the photocatalytic activity of P25-based photocatalyst. Through elaborately controlling the crystal phase structure of P25 by calcination at different temperatures, the H2 evolution activity can be greatly increased.334 Relatively higher H2 production activity was also obtained in the mixed-phase anatase/rutile photocatalyst, the composition of which was tuned by different inorganic anions.43,335 Kho and co-workers suggested that the synergistic effect in the mixed-phase structure of TiO2 was due to efficient charge separation across the phase junction.336 The mechanisms of enhanced activity for mixed-phase structure of TiO2 have been widely investigated. The most accepted explanation is the charge separation concept. It is proposed that the driving force for promoting the separation of photogenerated electrons and holes at the phase junction between anatase and rutile is larger than that either of the individual phases. The phase junction’s photocatalytic performance and charge separation was further studied in detail for the α/β phase Ga2O3.337 It was found that the phase junction formed between α and β phase can significantly enhance the photocatalytic activity in overall water splitting on Ga2O3-based

Figure 11. (a) The photocatalytic activities for H2 evolution per surface area of TiO2 samples with increasing amount of anatase TiO2 on the surface of rutile TiO2. (b) Surface-phase junction formed between anatase and rutile. Reprinted with permission from ref 332. Copyright 2008 Wiley-VCH.

photocatalyst. A detailed study using time-resolved IR spectroscopy and ultrafast transient absorption spectroscopy reveals that the ultrafast electron transfer across the α/β phase junction is much faster than the recombination process (fluorescence process), which is the essential role of the phase junction in charge separation. Nakajima et al. investigated the charge transportation process between anatase and rutile nanoparticles using the photoluminescence excitation spectroscopy.338 They proposed that the electrons should transfer from anatase to rutile, because the CB position of anatase is higher than that of rutile. Kawahara and co-workers also observed that, in the interface between anatase and rutile, Ag particles were apt to be deposited on rutile rather than anatase, indicating the electron was transferred from anatase to rutile.339 By observing similar experimental results, Miyagi and co-workers also concluded that the rutile nanoparticles on the anatase film acted as the reduction sites for photocatalytic reactions.340 On the other hand, Leytner et al. observed the electron trapping sites in anatase using time-resolved photoacoustic spectroscopy.341 The trapping sites located on average ca. 0.8 eV below the CB edge of anatase may facilitate electrons transfer from the CB of rutile to the trapping sites of anatase. Hurum and co-workers supported the later opinion by EPR.342−344 Very recently, combining theory and experiment, Scanlon and co-workers demonstrated that the electron affinity of anatase is higher than that of rutile, which would favor the photogenerated electrons transfer from rutile to anatase.345 Although the direction of charge transfer between anatase and rutile has controversial conclusions in the literature (Figure 12), the effect of efficient charge separation by anatase/rutile phase junction is commonly agreed on in both debates. Phase junction approach is a 9998


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Figure 12. Diagram of two possible charge transfer pathways between rutile and anatase: (a) from anatase to rutile; (b) from rutile to anatase.

fore, construction of Schottky junction is also an effective strategy for promoting photocatalytic H2 production reactions.

promising strategy for enhancing charge separation in TiO2 and other polymorph semiconductor-based photocatalysts. The driving force of the potential difference generated by the phase junction of anatase and rutile is determined to be less than ca. 0.2 eV, which is much smaller as compared to the electric fields generated by heterojunction. Although the driving force of phase junction is small, it has been already demonstrated to be effective in driving efficient charge separation between anatase and rutile. Phase junction approach can be regarded as a supplementary strategy for efficient charge separation in addition to heterojunction. 3.2.2.2. TiO2(B)−Anatase Phase Junction. Besides the anatase−rutile phase junction structure, many researchers found that the interface between TiO2(B) and anatase could also facilitate the separation of photogenerated charges and improve the photocatalytic activity.346−351 Lin and co-workers synthesized a bicrystalline structure consisting of TiO2(B) nanotubes and anatase nanoparticles, which exhibited a much higher H2 production activity as compared to P25.352,353 The authors indicated that the close contact between TiO2(B) and anatase is beneficial for the improvement of the photocatalytic activity due to efficient charge separation. Li et al. synthesized an anatase/TiO2(B) core−shell nanofiber and found that this catalyst performed better than either the individual anatase or the TiO2(B).354 In the investigation of TiO2(B) nanofibers with a shell of anatase nanocrystals, Yang and co-workers found that {001} planes in anatase and {100} planes of TiO2(B) could form a well matched and stable interface.355 They proposed that the differences in the band edges of TiO2(B) and anatase could allow the migration of photogenerated holes from anatase to TiO2(B).356 3.2.3. Fabrication of Schottky Junction. Normally, the electron trapping ability of a noble metal is mainly determined by its work function, which is usually larger than that of TiO2. Therefore, electrons formed on the CB of TiO2 can transfer to metal at M/SC interface. Schottky barrier (or Schottky junction) formed in the M/SC junction can lead to efficient charge separation due to the internal electric field.357 In most cases, photocatalytic reactions can proceed in virtue of loading of noble metal and oxide cocatalysts separately. For example, it is hard to produce H2 without metal cocatalysts, especially noble metal cocatalysts, even in the presence of sacrificial reagent. It is because the noble metal not only provides the reduction sites but also serves as the electron sinks, leading to efficient charge separation for photocatalytic reaction. There-

3.3. Loading Cocatalysts on TiO2

Because of fast recombination of photogenerated electrons and holes or lack of appropriate reaction sites, single semiconductor-based photocatalysts usually do not show high efficiency in photocatalysis. To improve the photocatalytic efficiencies, it is necessary to develop composite photocatalysts by loading proper oxidation and/or reduction cocatalysts on semiconductor photocatalysts. In such a composite photocatalyst, the roles of cocatalysts include:358 (1) to provide trapping sites for the photogenerated charges and promote the charge separation, thus enhancing the quantum efficiency; (2) to improve the photostability of the catalysts by timely consuming the photogenerated electrons and holes; and (3) to catalyze the reactions by lowering the activation energy. Various kinds of cocatalysts have been applied to TiO2 to improve the photocatalytic activity of H2 evolution reactions, including metal cocatalysts, metal oxide/sulfide cocatalysts, and hydrogenase-mimic cocatalysts. 3.3.1. Metal Cocatalysts. As early in 1983, Honda et al. provided experimental evidence that the H2 production site was located on Pt in Pt/TiO2 catalyst.359 Since then, Pt loaded TiO2 has been widely investigated.188,360−363 Because the work function of a noble metal is usually larger than that of most semiconductors, electron transfer from CB of semiconductor to metal readily happens. The higher is the work function, the stronger is the electron trapping ability of the metal. Platinum has the largest work function (5.12−5.93 eV)364 among the noble metals. Furthermore, Pt shows the lowest activation energy for proton reduction as shown in Trasatti’s volcano relationship between the exchange current for H2 production and the metal−hydrogen bond strength.365 Therefore, in view of these two essential points, Pt is usually regarded as the best proton reduction cocatalyst in photocatalytic H2 production reactions. The photocatalytic activity of Pt/TiO2 also largely depends on the way Pt is loaded on the surface of TiO2. Cratian and coworkers found that the activity of Pt/TiO2 was less sensitive to the preparation method as compared to Au/TiO2.366 However, Ikuma et al. examined the effect of Pt deposition methods on the H2 production rate and found that a formaldehyde approach could result in the highest activity among the investigated methods.367 Zou et al. developed a cold plasma method for the preparation of Pt/TiO2, which significantly enhanced the H2 production activity as compared to that 9999


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Figure 13. High-resolution TEM images of (left) Pt/TiO2-P (plasma method) and (right) Pt/TiO2-C (impregnation method) with speculated metal−support interface. Reprinted with permission from ref 368. Copyright 2007 Elsevier.

prepared by the impregnation method.368 The benefit of using the cold plasma method for Pt deposition is that it can allow Pt atoms to contact with Ti4+ and O2− ions more effectively due to the larger cross-angle between Pt(111) planes and TiO2(101) planes (Figure 13). Kandiel and co-workers found that the TiO2 with mesoporous structure was beneficial for minimizing the amount of Pt loading.369 A synergistic effect between Pt nanoparticles and polypyrrole on TiO2 was reported by Kandiel and co-workers.370 They proposed that Pt could trap the photogenerated electrons while polypyrrole could channel the photogenerated holes, leading to efficient charge separation and hence a higher H2 production activity. Au is also an efficient H2 evolution cocatalyst for TiO2,371−374 although it has been reported that the H2 evolution activity of Au/TiO2 is about 30% lower than that of Pt/TiO2 under similar experimental conditions.366 Murdoch and co-workers investigated the effect of Au loading amount and Au particle size on photocatalytic H2 production activity of Au/TiO2. They found that the Au particle size in the range between 3−30 nm was quite active for H2 evolution, and the H2 evolution rate increases with the decrease of the Au particle size. However, when the Au nanoparticles were varied in the range of 3−12 nm, the H2 evolution rate did not change significantly, suggesting that the optimal size of Au nanoparticles for H2 evolutions is less than 10 nm.375 Furthermore, the plasmonic effect has also been investigated on Au/TiO2, which is quite different from being a cocatalyst.376,377 Besides Pt and Au, other metal cocatalysts including Pd, Rh, Ru, Ir, Ag, Ni, Co, etc., were also investigated for H2 production on TiO2-based photocatalysts.378−387 Especially, considering the availability and cost-effectiveness, the development of nonnoble metal cocatalysts for the replacement of precious noble metal cocatalysts has been drawing great attention in recent years. For example, Tran and co-workers reported that the earth abundant elements, such as Ni and Co nanoclusters, can serve as attractive alternatives to noble metal cocatalysts for photocatalytic H2 production. At the optimal conditions, Ni and Co cocatalysts displayed ca. 3 times lower activity as compared to Pt.388 Cu-based cocatalyst has also been reported to be a promising cocatalyst for H2 production.241,389−392 Also,

it should be noted that much experimental data demonstrated that Cu species on TiO2 was apt to contain several valence states regardless of its initial state.393−399 In other words, Cu species can self-regulate to the proper valence states for the optimum activity in photocatalytic reaction processes.400 At present, although the activity of non-noble metal loaded TiO2based photocatalysts is relatively low as compared to that loaded with noble metals, current research results already show that the development of earth-abundant non-noble cocatalysts is a viable means because the cost-effectiveness should be seriously considered for the future practical applications. 3.3.2. Metal Oxide/Hydroxide/Sulfide Cocatalysts. Besides metal cocatalysts, metal oxides and metal sulfides can also serve as cocatalysts for H2 evolution. Ni oxides/hydroxides are one of the most studied effective metal oxide cocatalysts for H2 evolution. A different NiO loading method may lead to different photocatalytic activities. Sreethawong et al. reported that a NiO/TiO2 photocatalyst prepared by a single-step sol− gel method showed higher H2 production activity as compared to the conventional incipient wetness impregnation method.401 The mechanism of NiO as H2 evolution cocatalyst is still in debate. Chen and co-workers402 proposed that the higher photocatalytic activity for NiO/TiO2 catalyst was due to a p−n junction formed between NiO and TiO2, because NiO is a ptype semiconductor and TiO2 is an n-type semiconductor. Such kind of p−n junction can create an electric field that promotes charge separation between TiO2 and NiO. However, it was also proposed that Ni/NiO (or NiOx) is a better H2 evolution cocatalyst than NiO.403 Ni hydroxide and hydrate were also reported to be efficient cocatalysts for H2 evolution. Yu and coworkers404 reported that Ni(OH)2 cluster loaded TiO2 exhibited a H2 production rate of 3056 μmol h−1 g−1 and quantum efficiency of 12.4%, indicating that the active species for H2 evolution may be due to formation of Ni(OH)2. Jang et al. 4 0 5 reported that a hydrated Ni complex of [NixII(OH)2x−1(OH2)]+ can also serve as cocatalyst on titanate nanotubes for photocatalytic H2 production from water− methanol solution. They found that the nickel species could provide active sites for proton reduction, leading to fast diffusion of photoelectrons from the titanate layers to the nickel 10000


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Figure 14. Schematic diagram of a biohybrid (enzyme−TiO2) nanoparticle system for H2 production from sunlight. The system consists of a hydrogenase (Db [NiFeSe]−H) as catalyst and the complex (RuP). Reprinted with permission from ref 193. Copyright 2009 American Chemical Society.

facilitate charge separation but also can help us to understand the mechanisms of H2 evolution at the molecular level; and (3) the rate of photoexcited electron transfer from the semiconductor to the complex is fast. Reisner and co-workers193 coupled [NiFeSe]-hydrogenase on a Ru-complex-sensitized TiO2. Such a hybrid system could produce H2 under ambient conditions in the presence of TEOA in aqueous solution with a turnover frequency of ca. 50 (mol H2) s−1 (mol total hydrogenase)−1. The hydrogenases and the Ru complex photosensitizers coattached to TiO2 are responsible for H2 evolution and light harvest, respectively (Figure 14). Lakadamyali and co-workers reported a hybrid system consisting of a cobaloxime on dye-sensitized TiO2 nanoparticles could efficiently produce H2.190 Introduction of a phosphonate anchor group to the cobaloxime complexes can act as a linkage to the metal oxide. Moreover, this cobaloxime complex could be readily repaired by the addition of fresh ligand. Importantly, it was proved that the active species for H2 evolution in their work was indeed the molecular Co complex rather than metallic cobalt or a cobalt oxide.191 These works demonstrate that biomimetic molecular complexes consisting of earth abundant elements can be used as H2 evolution cocatalyst for TiO2 with high efficiency. Although there is no base for comparison of molecular complexes with the noble metal cocatalysts such as Pt because the active species of the former is a molecule and that of the latter is the active sites of nanoparticles, the present experimental results already show that the biomimetic molecular complexes can serve as efficient cocatalysts for the replacement of the noble-metal cocatalysts.

sites. However, if this hydrated nickel complex was partially converted to NiO, the activity decreased, indicating that the hydroxide form of nickel was more active than NiO in this system. Analogous to Ni(OH)2 loaded TiO2, Cu(OH)2 clustermodified TiO2 was also demonstrated to be efficient photocatalyst for H2 evolution, giving a H2 evolution rate of 3418 μmol h−1 g−1 and a quantum efficiency (QE) of 13.9%.406 Dang et al.407 loaded Cu(OH)2 on TiO2 nanotubes by a hydrothermal precipitation method. The efficient photocatalytic activity of Cu(OH)2/TiO2 was ascribed to the presence of more active sites provided by the large specific surface area of the catalyst. Other oxides, such as boron oxide,408 CoOx,409 and RuO2,410 were also reported to be effective cocatalysts for H2 evolution once they are loaded on the surface of TiO2. However, some part of these cocatalysts might function as oxidation cocatalysts, which can synergistically promote the reduction part of the H2 evolution reaction by efficiently taking out of the photogenerated holes. These metal oxide cocatalysts are partially reduced to metals under photocatalytic reactions because the photogenerated electrons are able to reduce these reducible metal oxides. Therefore, both metal and metal oxides may coexist on the surface of TiO2 during photocatalytic reactions even if the initially loaded cocatalysts are metal oxides. Metal sulfides are usually loaded on sulfide or oxysulfide semiconductor as efficient cocatalysts.411−414 However, there are still several examples used on TiO2 for H2 production. Zhang and co-workers reported that the NiS/TiO2 catalyst prepared by solvothermal synthesis method exhibited 30 times higher H2 evolution activity as compared to that of the bare TiO2.415 Xiang et al. modified TiO2 with MoS2 and graphene for photocatalytic H2 production, and found that the synergetic effect of MoS2 and graphene as cocatalysts promotes H2 evolution activity.321 A fatal problem for the metal sulfides acting as the H2 evolution catalyst is that they are usually readily decomposed and suffer a severe stability problem in aqueous solution. 3.3.3. Molecular Complex Cocatalysts. Recently, complex/semiconductor hybrid system for photocatalytic fuel generation has attracted increasing attention. Biomimetic molecular complexes that possess the same function as the natural enzymes are promising “cocatalysts” for coupling with semiconductor photocatalysts for H2 evolution. The advantages of the complex/semiconductor hybrid photocatalyst are as follows:416 (1) as a light harvester, a semiconductor is relatively stable, and the spectral absorption range is broad; (2) using a noble-metal free biomimetic complex as cocatalyst not only can

3.4. From Overall Water Splitting to Biomass Reforming

The main motivation of the photocatalytic research is to develop efficient photocatalysts to drive the thermodynamically uphill reactions by overcoming the energy barrier/overpotential through a relatively low energy reaction pathway. Also, because the processes of a photocatalytic reaction on a semiconductor photocatalyst involves light absorption, charge separation, carrier migration, and surface catalytic reactions, etc., photocatalytic research in practice is also about engineering of a photocatalyst aiming at improving the efficiencies of these key processes. This is also inevitable in the study of TiO2-based photocatalysts. As shown in the previous sections, extensive research efforts have been made to improve the photocatalytic properties of TiO2, including extending light absorption, improving charge separation efficiency, and designing more active sites for H2 production. Water splitting is an energetically 10001


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Figure 15. Plausible water splitting mechanisms over Pt−TiO2 (a) in water and (b) in carbonate salt aqueous solution. Reprinted with permission from ref 420. Copyright 1997 Royal Society of Chemistry.

problems such as long exposure time and low photonic efficiencies can be resolved using a high photon flux source.419 Few examples were reported for overall water splitting on TiO2-based photocatalysts. Most of the research on TiO2 can only obtain H2 but no O2 during the test of overall water splitting reaction. Sayama and co-workers420−422 reported that the addition of carbonate ions is essential for overall water splitting on Pt/TiO2. Only after the addition of carbonate ions can H2 and O2 be produced simultaneously in this system under UV light irradiation. However, if no any ions were added, only H2 but no O2 can be achieved. The authors also compared the effects of different kinds of anions, such as OH−, Cl−, SO42−, and PO43−, but none of them can involve both H2 and O2 simultaneously. The function of carbonate species in improving the overall water splitting is that it can facilitate the desorption of O2 from Pt/TiO2 surface by the formation of peroxocarbonates (Figure 15),420 which may lead to a different process for surface catalytic reaction of water oxidation. It was also reported that addition of iodide could promote the water splitting reaction on Pt/TiO2 due to the fact that the presence of iodide can suppress the back reaction for H2 and O2 to produce H2O.423 Because almost no O2 was detected on TiO2 in most cases without additives, it is necessary to investigate the O2 evolution mechanism on TiO2. Some researchers speculated the O2 evolution mechanisms based on their experimental results on TiO2. Unfortunately, the mechanisms for water oxidation are quite controversial in the literature.10,424−427 Wilson et al. reported that the surface state may act as a possible intermediate of the O2 evolution process.427 Salvador et al. pointed out that the surface OH radicals in [TiOH]+ may serve as the initiation step of the O2 evolution reaction by using photogenerated holes.425,426,428 It was also reported that water oxidation reaction on TiO2 surface produced adsorbed OH radicals as the intermediate.429 On the contrary, Howe et al. reported that photoexcited holes were trapped at the lattice O atoms and no OH radicals were produced during water oxidation.430,431 Fan et al. concluded that OH radicals do not

uphill reaction (requires at least 1.23 eV energy input) involving four electron transfer processes. The H2 production by overall water splitting using solar energy, that is, photocatalytically split water into H2 and O2 in 2:1 stoichiometrical ratio, is the true “solar fuel” because the net solar energy to chemical energy conversion by this reaction is achieved. The H2 produced by photocatalytic biomass reforming is also considered as a kind of “solar fuel” in the literature. Yet, according to a strict definition of solar fuel, the H2 energy produced by biomass reforming can only be considered as “half solar fuel” or “partial solar fuel”. This is because a large portion of the solar energy has been already stored in the chemical bonds of biomass, and the solar energy used during H2 production in biomass reforming is mainly for overcoming the activation energy of the reaction because it is a thermodynamically favorable reaction. The advantage of H2 production by biomass conversion is energy-saving and environmental friendly as compared to other routes, such as H2 production by pyrolysis of biomass. 3.4.1. Photocatalytic Overall Water Splitting on TiO2Based Photocatalysts. It should be emphasized here that lots of papers on H2 evolution work using sacrificial reagents were also claimed to be “successful” water splitting. Yet strictly, these are not water splitting reactions because sacrificial reagents were used and no O2 evolution reaction occurs. Grätzel and co-workers intensively investigated a bifunctional colloidal TiO2 loaded with Pt and RuO2.195,417,418 The H2 generation quantum yield in the overall water splitting reaction was up to ∼30% at 310 nm.195 In this system, Pt is the H2 evolution site, while RuO2 is the O2 evolution site. By using Ru(bpy)32+ or rhodamine B as a sensitizer, overall water splitting reaction was realized under visible light irradiation. Some modified TiO2, such as chromium-doped TiO272 and boron oxides modified TiO2,408 were also found to exhibit overall water splitting activity but with rather low efficiencies. Hameed et al. reported that overall water splitting reaction can be achieved over NiO and TiO2 using 355 nm laser as the radiation source. It was demonstrated that some common 10002


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solution were also investigated, and it was found that Pt/ TiO2(anatase)+Pt/WO3 can also well drive the overall water splitting reactions.447,448 Interestingly, different types of water splitting reactors have emerged in recent years. An H-type glass container for the separate evolution of H2 and O2 was designed by Anpo et al.449,450 As shown in Figure 17, using a TiO2/Ti/Pt

play a significant role in photoinduced oxidation of trichloroethylene on P25.432 Nakamura et al. concluded from the results of their photoluminescence and in situ FT-IR (Fourier transform infrared spectroscopy) spectroscopic study that O2 evolution is not initiated by [TiOH]+ but by a nucleophilic attack of H2O molecule to a surface trapped hole at a surface lattice O site to form [Ti−OOH−Ti] intermediate.433−435 In addition, some other water oxidation mechanisms on TiO2 were also proposed, which are quite controversial with each other.436−442 The possible reasons for the debate of water oxidation mechanism may be that different phases of TiO2 samples were used in these research works. Especially the composition of the commercial P25, which is composed of anatase and rutile mixed phases, may vary on the source of purchasing channels. Recently, Meada443 reported that different phases of TiO2 can lead to different performances for overall water splitting. A series of commercial anatase and rutile from different supply corporations were used for comparison, and he concluded that anatase TiO2 can only evolve H2 but rutile TiO2 can split water into H2 and O2 simultaneously with a H2/O2 ratio close to the stoichiometric ratio of 2. In recent years, biomimetic Z-scheme systems have been developed for the overall water splitting. Fujihara and coworkers constructed a Z-scheme system using TiO2 and two redox mediators (Br2/Br− and Fe3+/Fe2+). The Br− ions can reduce proton to H2, while Fe3+ can oxidize water into O2. These two processes were achieved in a two-compartment cell.444 Kozlova et al. reported a new shuttle charge transfer system of Pt/TiO2 with Ce3+/Ce4+, which can produce H2 and O2 in Ce3+ and Ce4+ solution, respectively. However, the result of combination of these two reactions was not satisfied.445 Abe and co-workers designed a two-step photoexcitation system (or so-called Z-scheme system) for overall water splitting reaction using IO3−/I− as shuttle redox mediator.446 As shown in Figure 16, H2 can be produced with the oxidation of I− to IO3− on

Figure 17. H-type glass container for the separate evolution of H2 and O2. Reprinted with permission from ref 449. Copyright 2006 Elsevier.

photocatalyst system, H2 can be produced on Pt, while O2 can be produced on TiO2 thin film. Similarly, Sun and coworkers used a two-compartment photoelectrochemical cell to produce H2 and O2 separated on highly ordered TiO2 nanotube arrays.451 These investigations provided a possible scalable technology to produce H2 separated from O2. 3.4.2. Hydrogen Generation from Biomass and Their Derivatives on TiO2-Based Photocatalysts. Half reactions of photocatalytic water splitting in the presence of sacrificial reagents have been extensively studied to evaluate the H2 production possibility of photocatalysts. Until now, several typical systems for photocatalytic H2 production have been developed, including S 2− /SO 3 2− , 4 52−45 7 Fe 2+ , 45 8−46 0 Ce3+,445,461−463 I−,224−227,464,465 Br−,444 and CN−.466 These inorganic anions can be easily oxidized by the photogenerated holes and promote the H2 production reaction. For example, the highest quantum yield of ∼93% at 420 nm for H2 production was achieved with S2−/SO32− as the sacrificial reagent using Pt−PdS/CdS as the photocatalyst.413 Adapting the concept of these H2 production reactions in the presence of sacrificial reagents, the biomass can also be regarded as a kind of sacrificial reagent in the photocatalytic reaction of biomass reforming in addition to functioning as the H carrier/supplier. With the ultimate goal to convert naturally abundant cellulose and lignin to H2, organic compounds, such as alcohols, aldehydes, and organic acids, can be the model of biomass compounds in the laboratory scale exploration of photocatalytic biomass reforming to produce H2. The traditional catalytic technologies for H2 production from biomass usually proceed under harsh conditions, such as thermal pyrolysis, steam/oxygen gasification, and supercritical water gasification.467 Photocatalytic reforming of biomass and its derivatives can produce H2 under a mild condition. In addition, the emitted CO2 during biomass reforming can be recycled to produce biomass again via photosynthesis. No extra CO2

Figure 16. Proposed mechanism for overall photocatalytic water splitting using IO3−/I− shuttle redox mediator and a mixture of Pt− TiO2−anatase and TiO2−rutile photocatalysts. Reprinted with permission from ref 446. Copyright 2001 Elsevier.

TiO2−anatase, and O2 can be obtained from H2O with the reduction of IO3− to I− on TiO2−rutile. The simultaneous evolution of H2 and O2 with a stoichiometric ratio was realized in a basic NaI aqueous solution. In such a two-step photoexcitation system, back reaction can be dramatically reduced because H2 and O2 were evolved on different catalysts. Yet the efficiency of the overall water splitting depends on not only the catalysts themselves, but also the diffusion rate of the IO3−/I− and the competition of these two anions with the water oxidation and proton reduction. Various combinations of TiO2 with other semiconductor photocatalysts in NaI aqueous 10003


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the terminal group of glucose molecule, hydroxyl group, as well as aldehyde group will first be oxidized by the photogenerated holes to produce carboxyl group and protons (eqs 1 and 2). The carboxyl group then will be available for decarboxylation via a photo-Kolbe reaction (eq 3), and simultaneously protons will be reduced by the electrons to produce H2 (eq 4).

emission will occur if biomass reforming technology is combined with the photosynthesis or artificial photosynthesis. Therefore, photocatalytic H2 production from biomass has attracted ever increasing attention.468,469 3.4.2.1. Hydrogen Production from Biomass and Other Organic Compounds. In the early 1980s, the Japanese scientists Kawai and Sakata reported photocatalytic H2 production from various kinds of biomass and organic compounds on TiO2 coloaded with Pt and RuO2. The reactants included active carbon,470 methanol,471 ethanol,472 sugar,473 protein,472 polymer,472 and original biomass materials (such as algae, dead insects, and excrement, etc.).472 The corresponding reactions showed a relatively high amount of H2 production, demonstrating that H2 production can be realized via photocatalytic reforming of biomass. Sato and White reported that photocatalytic H2 production reaction can be realized in water with CO,474 ethylene,475 and carbon.476 Harada and co-workers investigated the reaction in lactic acid solution on Pt/TiO2 and Pt/CdS.477 For Pt/TiO2, the main products were H2, CO2, and aldehyde, while for Pt/CdS, H2 and pyruvic acid were mainly produced. The authors indicated that the different reaction pathways for the two catalysts were due to their different CB positions. Kondarides et al. investigated several H2 production reactions over Pt/TiO2; the reactants included azo-dyes,478 alcohol,479 organic acid,479 and other carbohydrates.469 They indicated that in these reactions, H2 and CO2 can be produced in a stoichiometric ratio under mild reaction conditions. Badawy and co-workers reported a nanostructured mesoporous titania and employed it as photocatalyst in photocatalytic H 2 production from an olive mill wastewater.480 It successfully combined the pollutant degradation and H2 production in one reaction. Glycerol and other biomass-derived compounds such as glucose and sucrose were also used as the model compounds for H2 production on doped TiO2 by Luo et al., which showed apparently enhanced activity as compared to that of the overall water splitting reaction.481 The authors suggested that photocatalytic production of H2 from biomass-derived compounds is an alternative strategy for H2 production from renewable and sustainable resources. 3.4.2.2. Hydrogen Production from Glucose. Saccharides are the most abundant form of biomass, which occupy about 75% composition in the biomass produced.482 Glucose is one of the most important saccharides, which can be produced directly by hydrolysis of cellulose. Therefore, many researchers focused on the photocatalytic H2 production from glucose.483−488 Fu and co-workers systematically investigated this reaction on TiO2 loaded with noble metals (Pd, Pt, Au, Rh, Ag, or Ru).489 They found that the maximum rate of H2 generation was about 0.285 mmol h−1. The loading amount of cocatalyst, the initial concentration of glucose, and the pH value of the reaction solution can affect the final activity. Colmenares et al. reported a strong metal support interaction effect (SMSI) of Pt/TiO2 and Pd/TiO2 in H2 production reactions from glucose solution.490 The greater the SMSI performs, the higher is the H2 production activity. The effect of epimerization of D-glucose was also investigated for the photocatalytic H2 generation over Pt/TiO2 by Zhou et al.491 The authors found that α-D-glucose exhibited better performance for H2 evolution than β-D-glucose under neutral condition, while no significant difference was observed under basic or acidic conditions. In the earlier time, John and co-workers proposed the mechanism of photocatalytic H2 production from glucose.492 They considered that

R−CHO + H 2O + 2h+ → R−COOH + 2H+

(1)

R′−CH 2OH + H 2O + 4h+ → R′−COOH + 4H+

(2)

R−COOH → RH + CO2

(3)

2H+ + 2e− → H 2

(4)

3.4.2.3. Hydrogen Production from Glycerol. As we know, the main functional groups of saccharides contain hydroxyl and aldehyde groups. Therefore, besides original biomass material and glucose, some alcohols, such as glycerol, ethanol, and methanol, have been selected as the model compounds of biomass for the investigation of H2 production. Besides, glycerol is one of the byproducts in the biodiesel industry and is now considered as a waste because of its limited requirement in the market.493 Therefore, taking glycerol as feedstock for photocatalytic production of H2 is also an interesting research topic.494−498 Bowker and co-workers reported photocatalytic reforming of glycerol on Pt/TiO2 and Pd/TiO2.499 They found that the corresponding H2 production rates of glycerol reforming were higher than methanol reforming on both catalysts. Daskalaki et al. systematically investigated the reaction conditions of glycerol reforming on Pt/TiO2, including the amount of photocatalyst in suspension, loading amount of Pt, concentration of glycerol, pH of the solution, and the reaction temperature.493 Some combination photocatalysts, such as CuOx−TiO2,394,395,500 Pt− CdS−TiO2,270 as well as doped TiO2,125,501,502 were also investigated for the reaction of glycerol reforming. Panagiotopoulou and co-workers investigated the mechanism of glycerol photo-oxidation and photoreforming reactions on Pt/TiO2.503 They concluded that, if the reaction takes place in the absence of O2, glycerol will be finally photoreformed toward CO2 and H2 with various reaction intermediates, including glycoaldehyde, 2-oxopropanol, acetaldehyde, acetone, ethanol, and methanol. 3.4.2.4. Hydrogen Production from Ethanol. Ethanol can be produced on a mass scale by fermentation process from raw biomass such as starch and sugars. The studies on the photocatalytic H2 production reaction via ethanol reforming are mainly focused on the modification of photocatalyst80,268,352 and the deposition of cocatalyst.366,375,384,504−507 For example, Yoo and co-workers have developed an efficient method for fabricating single Au particles in TiO2 cavities for photocatalytic ethanol reforming.508 They employed this strategy to introduce different sizes of Au particles into TiO2 cavities and found that the highest H2 production rate was obtained when the Au size was 2 nm. Other researchers used the photoelectrochemical 10004


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reforming reaction.511 The surface species such as CH2O(a), CH2OO(a), and HCOO(a) were observed on Pt/TiO2, indicating that the H2 production was coupled with gradual oxidation of methanol (Figure 18). Other researchers found

method to generate H2 from ethanol with a bias, in which the H2 was produced on Pt cathode compartment while the commercial nanocrystalline TiO2 deposited on a conductive transparent electrode was used as photoanode.509 Kaiwa et al. investigated the photocatalytic H2 production reaction on TiO2 at room temperature.510 A series of products were observed, including H2, CH4, CH3CHO, and CH3COOH. Therefore, the authors suggested the reaction pathway as follows: C2H5OH

hν CH3CHO + H 2 catal

CH3CHO + H 2O CH3COOH

(5)

hν CH3COOH + H 2 catal

(6)

hν CH4 + CO2 catal

(7)

After a long-time reaction, the molar ratio of CH4/H2 in the products was around 0.1, which was different from the value estimated from eqs 5−7. Therefore, it was proposed that CH3CHO can also react with hydroxide radical to produce CO2 and H2O directly (eq 8). CH3CHO + H 2O

hν 2CO2 + 5H 2 catal

Figure 18. Proposed mechanism for photocatalytic production of H2 from methanol−water solution. Reprinted with permission from ref 511. Copyright 2007 American Chemical Society.

that the oxidation reaction of methanol proceeds on the surface of TiO2 and the intermediates would be finally oxidized into CO2, as no HCOH or HCOOH species was detected in liquid phase.512 Furthermore, besides the dominant intermediates of HCOH and HCOOH, CH4, CO, and some two-carbon species were also detected.379,513 3.4.2.6. Depression of CO Formation in Photocatalytic Reforming of Biomass. The photocatalytic reforming of biomass to produce H2 is a more complicated reaction. It is usually involved with several intermediates. Some byproducts, for example, CO, are not expected. Even trace CO (5−100 ppm) present in the H2 stream would poison the catalyst in the proton exchange membrane fuel cells.514,515 Therefore, ultralow CO formation is highly desired for the H2 production from photocatalytic reforming of biomass. Wu and co-workers reported that the addition of a small amount of inorganic anions, such as SO42− and H2PO4−, to the TiO2-based photocatalyst system can greatly reduce the CO formation (less than 10 ppm) in the reaction of photocatalytic methanol reforming.516 The CO formation depression was ascribed to the adsorption of the inorganic ions at defect sites of TiO2 where CO was formed. It was also found that the cocatalyst loaded on TiO2 not only increases the H2 production activity but also suppresses the CO production. They investigated the H2 production reaction from glucose reforming on metal/TiO2 and found that both the H2 production activity and the CO selectivity were dependent on the type of cocatalyst. Rh/TiO2 exhibited the highest H2 production activity and the lowest CO selectivity.380 Mohamed and coworkers also proved that the deposition of cocatalyst Ni on TiO2−SiO2 photocatalyst can promote the H2 production activity as well as suppress the CO formation.488 In addition, the size effect of Au was found to be on the CO production.373 The smaller is the particle size, the lower the CO concentration is in the product H2. The authors proposed that the smaller Au particle is in favor of the deep oxidation of HCOOH to CO2, and CO could be oxidized on the perimeter interface between Au particle and TiO2. Xu and co-workers proposed that the surface acidity of TiO2 can affect the CO production in photocatalytic H2 production from different biomass derivatives.43,334,335 The higher is the surface acidity of TiO2, the

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3.4.2.5. Hydrogen Production from Methanol. Methanol with one hydroxide function group and one carbon atom is the simplest alcohol. The research on photocatalytic methanol reforming is regarded as the model reaction for biomass reforming, which can provide some basic understandings on the mechanism of photocatalytic biomass reforming. In the photocatalytic H2 production system, methanol was usually used as a sacrificial reagent to promote the activity. The early study on methanol reforming was reported by Kawai et al.471 A mechanical mixture of TiO2, Pt, and RuO2 was used as photocatalyst, and the relatively high quantum yield was obtained (Table 2). The authors confirmed the formation of Table 2. Photocatalytic H2 Production Rate and Quantum Yield from Methanol Aqueous Solution Using Various Supported TiO2 Photocatalystsa

a

photocatalyst

H2 (mmol (10 h)−1)

quantum yield at 380 nm (%)

TiO2 RuO2−TiO2−Pt TiO2−Pt TiO2−Pd RuO2−TiO2

0.27 5.2 4.6 2.2 0.037

3.2 44.0 40.0 19.0 4.0

Data from ref 471.

HCHO and HCO2H as two important intermediates in the reaction. Therefore, the mechanism of methanol reforming was proposed as follows: CH3OH

hν HCHO + H 2 catal

HCHO + H 2O HCOOH

hν HCOOH + H 2 catal

hν CO2 + H 2 catal

(9) (10) (11)

Chen and co-workers used in situ Fourier transform FT-IR spectroscopy to investigate the mechanism of methanol 10005


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Figure 19. Schematic illustration of relaxation processes of the photogenerated carriers in anatase (a) and rutile TiO2 (b). (A) Electrons quenched to the bottom of the CB, (B) carriers captured by traps, (C) thermal excitation from traps, (D) electrons captured directly by deep traps, (E) carriers relax to luminescent sites, and (F) nonradiative recombination process.

of photocatalysis, because it can supply meaningful information about the relationship between the nature of the defect sites, such as surface oxygen or metal vacancies, and the efficiencies of charge carrier trapping, migration, and transfer. As an indirect semiconductor, band edge luminescence of TiO2 is difficult to observe.525 PL properties of TiO2 highly depend on crystal structure, particle size, defect, dopant, and preparation parameter of TiO2. The main PL features of TiO2 are broad and featureless visible or near-infrared luminescence bands.526−529 Anatase TiO2 always shows a broad visible emission, which were attributed to self-trapped exciton,530,531 oxygen vacancies and defect sites,532−534 impurities or reduced metal ions,535 etc. Rutile TiO2 displays a near-infrared luminescence band peaked at about 830 nm. The luminescence centers of this near-infrared band were identified as Cr3+ impurities,536,537 interstitial Ti3+ ions,535,538,539 intermediate species generated during the photooxidation of water,540−542or intrinsic defects in rutile TiO2.543 Shi et al. studied PL properties of TiO2 in the progress of phase transformation from anatase to rutile. They demonstrated that the visible luminescence band at about 505 nm is related to anatase structure, while the near-infrared (NIR) luminescence band at about 835 nm is associated with rutile TiO2.544 The visible and NIR luminescence are ascribed to oxygen vacancies associated with Ti3+ in anatase TiO2 and intrinsic defects in rutile TiO2, respectively. Therefore, different crystalline structures of TiO 2 result in the different luminescence centers in anatase and rutile. Oxygen vacancies related to visible emission prevailed in anatase, while the defect states related to near-infrared emission are predominant in rutile. To understand the role of different defects in different phase TiO2, Wang et al. investigated anatase and rutile TiO2 with time-resolved PL under weak excitation condition, where trap states play a vital role in carrier dynamics.517 Both the visible emission in anatase and the NIR emission in rutile with lifetimes up to millisecond range essentially originated from the trap state emission. The visible emission band in anatase TiO2 was assigned to donor−acceptor recombination as shown in Figure 19. Oxygen vacancies and hydroxyl groups are donor and acceptor sites, respectively. The NIR luminescence in rutile TiO2 originated from the recombination of trapped electrons with free holes (Figure 19), while the trapped electrons were formed through two paths, direct trapping or trap-to-trap hopping. The trapped carriers in anatase TiO2 may participate in the photocatalytic reactions, and the slow decay processes may be beneficial for the photocatalytic performance, because

higher the CO concentration is in the produced H2. This phenomenon is similar to the thermal catalytic process, in which the HCOOH prefers to dehydrogenation into H2 and CO2 on basic oxide rather than H2O and CO on acidic oxides. 3.5. Photocatalytic Reaction Mechanisms on TiO2-Based Photocatalysts

Although a number of methods and strategies have been developed to improve the photocatalytic performances of TiO2 photocatalyst as shown above, photocatalytic efficiency of H2 production is still very low. Low photocatalytic efficiencies are partly caused by poor charge-transportation properties, rapid charge recombination, and slow charge transfer kinetics at the semiconductor-liquid junction. Charge separation, recombination, and transfer processes are fast or ultrafast processes, which can be followed by ultrafast time-resolved techniques. Timeresolved spectroscopy has proven to be a powerful technique to study the carrier dynamics of photocatalyst, especially the photocatalytic processes. Time-resolved photoluminescence (PL),517 time-resolved infrared (IR),518 and transient absorption spectroscopy (TAS)519,520 are three main time-resolved techniques used in photocatalytic mechanism studies. PL detects the radiation recombination of photogenerated electrons and holes, while TAS gives the absorption signal of photoinduced electrons and holes separately. Moreover, free electrons and shallowly trapped electrons absorb IR light. Thus, time-resolved IR has been utilized to study excited electron dynamics. The excitation intensity should be low enough in the measurement of charge dynamics to avoid Auger recombination and bimolecular recombination (radiative or trap-assisted Auger), which dominate the carrier dynamics under high excitation condition demonstrated in the transient absorption spectroscopic measurements.521−524 By combining results from these different time-resolved spectroscopic techniques, we can determine the key steps in photocatalytic processes, and then help to design new photocatalysts with high solar energy conversion efficiency. 3.5.1. Photoluminescence Study on Carrier Dynamics of TiO2. Among the four main polymorphs of TiO2 in nature, anatase and rutile TiO2 are the two main polymorph photocatalysts extensively studied so far. Anatase usually exhibits higher activity than rutile in photocatalytic reactions. This is due to the different physical/chemical properties of TiO2, especially the different trap states and carrier dynamics. PL spectroscopy is widely utilized in the investigation of the photophysical and photochemical properties of solid semiconductors. Particularly, PL is a powerful technique in the study 10006


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decay following visible excitation is believed to be a key factor for the low efficiency of N-TiO2.547 Pesci et al. examined the oxygen-deficient H:TiO2 nanotube arrays as a model of black TiO2.548 The transient absorption mechanistic study provides strong evidence that the improved electrical properties of oxygen-deficient TiO2 enable remarkably efficient spatial separation of electron−hole pairs on the submicrosecond time scale at moderate applied bias. This results in effective suppression of microsecond to seconds charge carrier recombination. These are the primary factors behind the dramatically improved photocatalytic activity. 3.5.2.2. Effect of Sensitization. Dye sensitization is the main sensitization strategy to extend light absorption of TiO2 to visible region. The mechanism of dye-sensitized photocatalytic reaction is proposed in Figure 6. Considerable efforts are still needed to understand the detailed mechanisms of photo processes occurring at the interface due to the complex nature of kinetic processes following light excitation. Using time-resolved photoluminescence spectroscopy, electron transfer from the diester formed between fluorescein and anthracene-9-carboxylic acid (FL-AN) to colloidal TiO2 particles has been examined by He et al.549 They found that exciting FL-AN with both visible light (around 470 nm) and UV light (about 355 nm) induces electron injection from fluorescein singlet onto the CB of TiO2 (ket = 2.30 × 108 s−1). Meanwhile, the formation of radical cation FL·+-AN on electron injection was confirmed by electron spin resonance and laser flash spectroscopy. Time-resolved infrared (IR) spectroscopy is another powerful technique to study the charge transfer process in TiO2−dye system studies.550 Transient IR absorption of injected electrons in TiO2 in the 1900−2000 cm−1 region was studied by femtosecond IR spectroscopy by Ghosh et al.551 The electron injection from sensitizers to TiO2 and the subsequent backtransfer and relaxation dynamics of the injected electrons correspond to the rise and decay part of the transient IR signal. Using this technique, they determined that the injection time for coumarin 343 to TiO2 in D2O is 125 ± 25 fs, and the decay dynamics of the injected electrons in TiO2 are different from CB electrons of a bulk TiO2 crystal. Electron injection and recombination dynamics of fluorescein 27−TiO2 system were studied by femtosecond transient absorption spectroscopy.552 While the transient absorption signal in the interval 410−600 nm originates primarily from the dye molecule, the signal in the range 600−950 nm is assigned to the electron injected into TiO2. The electron injection was reported to occur with a characteristic time constant of 300 fs, but the recombination processes occur with multiphasic recombination times, ranging from ∼10 ps up to nano- or even microseconds. A great number of research has been devoted to understanding the complicated kinetics in TiO2−dye systems. Kelly et al. reviewed literature reports of excited-state processes in TiO2−dye materials and attempted to clarify the underlying mechanisms responsible for the complex kinetic behavior typically observed.553 Up to now, more research is still needed to understand the kinetics under real working conditions where electrolyte, ions, cocatalysts, and/or applied bias exist. Besides dye sensitization, it has been demonstrated recently that plasmonic nanostructures of noble metals (mainly silver and gold) also show significant promise to extend the absorption of TiO2 photocatalyst to the visible region.554 In 2007, Furube et al. claimed that the plasmon induced electron

PL decay dynamics of anatase are sensitively influenced by heat treatment. In contrast, the trapped electrons in rutile TiO2 may be inhibited from participating in photocatalytic reaction processes because the trap states in rutile TiO2 are very deep. PL results reveal that the trap states of anatase and rutile TiO2 are different obviously, resulting in the different carrier dynamics, which will strongly influence the photocatalytic process of TiO2. To understand the photocatalytic mechanism more properly, it is indispensable to investigate the carrier dynamics of anatase and rutile TiO2 under actual photocatalytic conditions with the presence of cocatalysts and scavengers. 3.5.2. Time-Resolved Spectroscopic Study on the Effect of Modifications on TiO2. As discussed in sections 3.1 and 3.2, various modification methods of TiO2 photocatalysts have been developed and investigated to enable the absorption of visible light, or to increase the photogenerated charge separation to improve the photocatalytic performance of TiO2. In this regard, bandgap engineering and surface sensitization have proven to be the two most efficient strategies to extend the light absorption of TiO2 to the visible region (section 3.1). On the other hand, the fabrication of a heterojunction like CdS/TiO2 and the fabrication of a phase junction like anatase−rutile phase junction are believed to improve the photogenerated charge separation (section 3.2). Different modification methods influence the photocatalytic activity and mechanism in different ways, especially in the kinetics aspects. It is indispensible to understand the exact role of each modification on TiO2 in the photocatalysis process and how they improve the photocatalytic performances of TiO2 kinetically, to design and synthesize more efficient photocatalyst systems. 3.5.2.1. Effect of Doping. Cation doping, anion doping, codoping, and self-doping have been widely used to prepare visible light-responsive TiO2. Time-resolved spectroscopic methods have been applied to investigate the photocatalytic mechanism of doped TiO2, in the light of charge carrier generation−separation−transfer−recombination dynamics. Tojo et al.545 prepared iodine-doped TiO2 powders (I-TiO2) having absorption in the region of ultraviolet (UV) and visible light and studied the transient behavior of the photogenerated charge carriers using time-resolved diffuse reflectance (TDR) spectroscopy. Upon the laser excitation of I-TiO2 powders, long-lived photogenerated holes were formed, while no trapped electrons were observed. It is suggested that the recombination of electron−hole pairs is inhibited because the doping I sites act as trapping site to capture the photogenerated electrons. Moreover, the trapped holes generated in I-TiO2 have no significant oxidation reactivity toward substrates adsorbed on the surface under both UV- and visible-light irradiation. Charge separation and trapping processes in N-doped TiO2 photocatalysts (N-TiO2) were investigated by means of timeresolved microwave conductivity (TRMC).546 The trapping rate increased in N-TiO2 relative to nondoped catalysts due to the increase in the oxygen vacancy induced by N-doping. Moreover, the electron behaviors generated by UV excitation were similar to that generated by visible excitation in N-TiO2, demonstrating that the final state of the optical transition induced by both UV- and visible-light was the CB. The charge separation efficiency under visible light excitation (450 nm) was found to be one-third as high as that under UV excitation in NTiO2. Tang et al. studied N-TiO2 photocatalyst by the use of transient absorption spectroscopy. The rapid electron hole 10007


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TiO2 using time-resolved mid-IR spectroscopy.562 The electron transfer from anatase to rutile across the anatase−rutile interface is proposed. The charge carrier dynamics in mixedphase TiO2 still need further investigation due to the complicated energy levels and kinds of trapped states in mixed-phase TiO2. 3.5.3. Role of Cocatalyst Pt on TiO2. It has been demonstrated that cocatalyst is indispensable for TiO 2 photocatalytic H2 evolution, as discussed in the previous sections. Among all reported cocatalysts, including noble metal, transition metal, metal oxide, and other kinds of cocatalysts, Pt is the most extensively studied H2 evolution cocatalysts. When Pt is loaded on TiO2, excited electrons migrate from TiO2 to Pt once the two Fermi levels are aligned.10 The Schottky barrier formed at Pt/TiO2 interface can serve as an efficient electron trap preventing electron−hole recombination in photocatalysis.10 Nakajima et al. reported that PL intensity of TiO2 decreases upon Pt loading.563,564 The decrease of PL intensity was ascribed to the electron transfer from TiO2 to Pt nanoparticles. The electron transfer improves electron/hole charge separation by confining excited electrons in Pt while holes in VB of TiO2. This will result in the decrease of PL intensity of TiO2. Shi et al. found that the visible luminescence of anatase TiO2 was obviously quenched upon Pt loading, while the NIR luminescence of rutile TiO2 only slightly declined. The obvious quenching of the visible emission and the negligible change of the NIR emission indicate that Pt loading inhibits electron− hole recombination at oxygen vacancies in anatase, while it has little effect on electron−hole recombination at the intrinsic defects in rutile. This may be one of the reasons why the photocatalytic activity of Pt loaded anatase TiO2 is usually higher than that of the rutile TiO2.544 Pt-loaded TiO2 powder photocatalysts were investigated by Furube et al. with femtosecond diffuse reflectance spectroscopy, and the dynamics of photogenerated electrons in TiO2 were discussed.565 The Pt-loaded TiO2 showed a new decay component of a few picoseconds under 390 nm excitation in addition to normal charge recombination kinetics in TiO2. This fast decay profile was attributed to electron transfer from TiO2 to Pt. The lifetimes of photogenerated holes were prolonged to 500 μs by the increased charge separation.517 These results clearly indicate that charge separation is responsible for the well-known enhancement of the catalytic activity of Pt-loaded TiO2. As discussed in section 3.3, various kinds of cocatalysts have been applied to TiO2 to improve the photocatalytic production of H2. They always show different improvement in photocatalytic performances of TiO2, due to their different structure, Fermi level, etc. To study the different cocatalysts, loaded TiO2 with time-resolved spectroscopy can obtain the effect of cocatalyst on charge carriers, and reveal the kinetic reasons for their performance in photocatalytic reactions as cocatalyst. The understanding of cocatalyst in photocatalyst from kinetic aspects will help to select a better cocatalyst for a specific photocatalytic reaction. Many experiments demonstrated that bare TiO2 cannot produce H2 even under UV light irradiation, mainly because the proton reduction and molecular H2 production are difficult on TiO2 surfaces. The cocatalyst such as Pt could not only promote the proton−electron coupling reaction, but also catalyze the evolution of molecular H2, as Pt and many other noble metal catalysts can dissociate molecular H2 with lower

transfer from 10 nm gold nanodots to TiO2 nanoparticles, by using femtosecond transient absorption spectroscopy with an IR probe.555 The transfer time was within 240 fs, and the yield was about 40%. With the fast development of plasmonic nanostructure photocatalyst, the study on the mechanism during the photocatalytic process is highly demanded. Besides the electron transfer mechanism, Wu’s group proposed a plasmonic energytransfer mechanism from the plasmonic metal to semiconductors in Au@SiO2@Cu2O sandwich nanostructures by transient-absorption and photocatalysis action spectrum measurement.556 They also studied the Ag@Cu2O core−shell nanoparticles and claimed that plasmon induced resonant energy transfer (PIRET) and direct electron transfer (DET) simultaneously, generating electron−hole pairs in the semiconductor.557 The photoanode of the sandwich-structured CdS−Au−TiO2 nanorod array was also investigated.523 The gold nanoparticles, sandwiched between the TiO2 nanorod and the CdS quantum dot (QD) layer, not only serve as an electron relay, but also act as a plasmonic photosensitizer. They identified the plasmonic energy-transfer mechanism as direct transfer of the plasmonic hot carriers, and the interfacial Schottky barrier height modulates the plasmonic hot electron transfer and back transfer. 3.5.2.3. Effect of Heterojunction and Phase Junction. Taking advantage of junction structure has been recognized as a strategy to inhibit charge recombination in semiconductors. Fabrication of heterojunction and phase junction has been applied in TiO2 photocatalyst. Semiconductor quantum dots (QD), such as CdS, CdSe, PbS, and PbSe with their tunable band gaps, offer new opportunities for harvesting light energy in the visible and infrared regions of solar light and improving charge separation.558 Robel et al. demonstrated the possibility to modulate the interparticle electron transfer rate from excited CdSe QD into TiO2 by varying the QD particle size with femtosecond transient absorption measurements. The fastest electron transfer was ∼1.2 × 1010 s−1 observed with 2.4 nm CdSe QDs, reflecting an average lifetime of 83 ps. Harris et al. further found that mediation of the electron transfer through TiO2 nanoparticles is achieved by coupling CdSe and TiO2 nanoparticles.559 The coupling increases the quantum yield of MV2+ reduction by a factor of 2, and the electron transfer rate constant is enhanced by an order of magnitude by the presence of both TiO2 and MV2+. Sambur studied the utilization of multiple exciton generation using PbS QD chemically bound to TiO2 single crystals. They demonstrated the collection of photocurrents with quantum yields greater than one electron per photon. The favorable energy level alignment and strong electronic coupling between TiO2 and PbS QD facilitate extraction of multiple excitons more quickly than they recombine, as well as collection of hot electrons from higher quantum dot excited states.560 Fabrication of anatase−rutile phase junction is reported to be an effective method to enhance photocatalytic performances of TiO2. There have been lots of publications studying the role of anatase−rutile phase junction in photocatalysis.339,342 Carneiro et al. studied the optoelectronic properties of mixed-phase TiO2 by time-resolved microwave conductance measurements (TRMC).561 They claimed that the presence of rutile improves the charge separation efficiency by trapping of positive charges at the rutile surface. Shen et al. further investigated the photoinduced electron dynamics of anatase−rutile mixed-phase 10008


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the reason for the higher photoactivity of anatase is due to its easier thermal-recombination of H atoms. The kinetic difference is the main factor for the photocatalytic performances. In future research, photocatalytic mechanism studies in actual photocatalytic conditions are highly needed to reveal the ratedetermining step, because the presence of cocatalyst, scavenger, or applied bias can totally change the reaction kinetics.

activation energy barrier. So it is necessary to use cocatalyst like Pt to produce H2 when using TiO2-based photocatalysts. 3.5.4. Mechanistic Studies on Photocatalytic Reactions of Water or Methanol for Hydrogen Production. Time-resolved spectroscopy can monitor the dynamics of photogenerated carriers. Electron- and hole-consuming reactions of adsorbates always control the decay kinetics of the carriers competing with the electron−hole recombination. Time-resolved spectroscopy is widely used to study not only the mechanism of photocatalytic water splitting, but also photocatalytic reforming of biomass, where biomass always serves as hole scavengers. Yamakata et al. studied the kinetics of the photocatalytic water-splitting reaction on TiO2 by time-resolved IR absorption spectroscopy.518,566 They reported that when TiO2 was exposed to water, holes reacted with surface hydroxyls within 2 μs, and the recombination decay of the electrons was obstructed. When Pt/TiO2 was exposed to water, the oxidation and reduction steps of water splitting reaction affected the electron decay kinetics in different time domains. Photogenerated holes oxidized water within 2 μs, whereas photoinduced electrons reduced water (or proton) in 10−900 μs. They also observed a transient response of O−H stretching band of an adsorbed hydroxyl species at 3677 cm−1, which thermally shifted to low-wavenumber side, when TiO2 catalyst was irradiated by UV-pulse. Yamakata et al. also investigated the mechanism of photocatalytic reaction of methanol on TiO2.567,568 They demonstrated that electron decay was suspended when TiO2 was in pure methanol vapor due to an effective hole-consuming reaction by adsorbed methoxy species. The suspension was released as partial water pressure increasing, indicating that the holes reacted with the methoxy species without the aid of adsorbed water or hydroxyl species. The excess long-lived electrons were consumed by adsorbed water in a time domain of 0.1 s. Tamaki et al. studied hole transfer processes from TiO2 to different alcohols with transient absorption spectroscopy.569 Methanol showed the fastest hole transfer process in comparison with ethanol, 2-propanol, or air. Chen et al. further studied photocatalytic reforming of methanol on Pt/TiO2 with in situ and time-resolved IR.570 For Pt/TiO2 catalysts reduced at high temperature, the capacity of methanol adsorption decreases with the increase of Pt loading, indicating that Pt nanoparticles occupy some of the active sites on TiO2 for methanol adsorption. Surface species CH2O(a), CH2OO(a), and HCOO(a) are detected from the photocatalytic reaction of methanol on Pt/TiO2. Decay rate of longlived electrons correlates well with H2 production activity. Amazingly, the increase of gas-phase methanol or water improves H2 production activity. These results show that long-lived electrons in TiO2 contribute to H2 generation, and decays of the long-lived electrons on millisecond to second time scale in Pt/TiO2 could be ascribed to the reaction for H2 evolution. The molecularly adsorbed methanol or water is found to mediate the proton transfer on the TiO2 surface (Figure 18). The mechanism of photocatalytic reaction of water or methanol for hydrogen production was also widely studied by other characterization techniques, such as in situ IR, and temperature-programmed desorption (TPD). Xu et al.571,572 showed that the reaction paths for photocatalytic H2 formation from methanol on the anatase (101) surface and the rutile (110) surface were quite similar. Moreover, they claimed that

3.6. Isotope Labeling Dynamic and Kinetic Studies of Photocatalytic Reactions on the Surface of TiO2

Better understanding the kinetics and dynamics of photocatalysis on the surface of semiconductor photocatalysts in molecular level is also equally important for the design and synthesis of efficient photocatalysts. Isotope labeling experiments combined with various kinds of spectroscopic techniques are quite powerful tools for the identification of the sources of the elements in the products, especially for those photocatalytic hydrogen production reactions in the presence of sacrificial reagents. 3.6.1. Oxidation Reactions on the Surface of TiO2. Ryuhei Nakamura et al.433 studied the primary intermediates of the water photooxidation reaction at the TiO2 (rutile)/aqueous solution interface. Isotope exchange IR experiment (H216O → H218O) showed 838 and 812 cm−1 bands, which can be assigned to the O−O stretching mode of surface TiOOH and TiOOTi, respectively. This result supports that the oxygen photoevolution is initiated by a nucleophilic attack of a H2O molecule on a photogenerated hole at a surface lattice O site, not by oxidation of surface OH group by the hole. The catalytic role of surface oxygens in photocatalytic oxidation reactions was further explored by José Peral et al. with 18O labeled Ti18O2 under anaerobic photocatalytic oxidation reactions of benzene.573 The incorporation of 18O derived from Ti18O2 surface lattice oxygen ions into final products of CO2 molecules was observed. The photogenerated TiO2 (−OHs•/−Os•−) surface radicals were proposed to be the primary oxidizing species. This indicates that the surface O species are indeed involved in the photocatalytic reactions. Michio Matsumura et al.574 studied water oxidation to O2 on photoirradiated TiO2 nanoparticles using 18O labeled H218O. They found that O2 was photocatalytically produced using O atoms supplied solely from water even in the presence of O2 in the system, indicating that the water oxidation reaction can be proceed in the oxygen-rich environment, although presence of O2 may affect the reaction rate. Furthermore, Yoshio Nosaka et al. carefully studied OH radical diffusion for remote oxidation using D2O vapors over the TiO2 powder photocatalysts. They found that the exchangeable water at the TiO2 surface is the origin of the diffused OH radicals575 in the gas phase, and the quantum yield of OH radicals diffused from the TiO2 surface was estimated to be about 5 × 10−5.576 These isotope experiments demonstrate that water oxidation is actually a quite complicated reaction. It might largely depend on the experimental conditions. More detailed experiments are necessary for the identification of active oxidation sites of TiO2 and the dynamics and kinetics of oxidation reactions on the surface of TiO2. 3.6.2. Hydrogen Production Reactions on the Surface of TiO2. As compared to the lack of dynamics and kinetics data for the oxidation reactions on the surface of TiO2, reduction reactions (especially photocatalytic production of H2) on the 10009


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croscopy (STM), and density functional theory calculations (DFT). Briefly, an excited resonance signal around 5.5 eV, which is associated with dissociated methanol species at Ti5c sites according to the DFT calculations, was detected in the 2PPE spectra upon 400 nm light illumination on the methanol/ TiO2(110) surface (Figure 20). STM images further confirmed

surface of TiO2 have been extensively explored, including isotope labeled D2O experiments and dynamic and kinetic studies. Jae Sung Lee et al.263 studied the source of H in photocatalytic production of H2 in the presence of H2S as a sacrificial reagent over bulk CdS decorated with TiO 2 nanoparticles. Their D2O experiments demonstrated that the element of hydrogen in the produced H2 originated from both H2S and H2O. Methanol has been frequently used as a sacrificial reagent for the evaluation of a photocatalyst’s ability for hydrogen production. There are three possible parallel methanol oxidation paths:513 (i) an indirect OH− radical-mediated path; (ii) a direct path, that is, reaction of valence band holes with adsorbed methanol at the titania−noble metal interface; and (iii) a water-assisted direct path, that is, reaction of valence band holes with methanol molecules adsorbed far from the titania−noble metal interface. H2O/D2O isotope experiments showed that water plays a double role in vapor-phase photocatalytic methanol reforming on noble metal (Pt or Au) modified TiO2: (a) as an oxygen donor in the formaldehyde to formic acid oxidation step and (b) as a proton diffusion medium from surface oxidation sites to noble metal nanoparticles. Chuang et al.577 studied the contribution of CH3OH to the photocatalytic evolution of H2 from aqueous D2O/CH3OH solutions over a Cu/S-TiO2 catalyst under UV illumination. The low rate of H2 formation from D2O/CH3OH solutions indicates that the reaction of hole with CH3OH does not proceed to an appreciable extent. They proposed that the functional role of CH3OH was to react with the ·OH radical produced from the reaction of photogenerated h+ with H2O. Such reaction may generate ·CH2OH electron donor, which injects electrons to the CB of TiO2, increasing the potential for the evolution of H2. However, due to the existence of fast isotope exchange between D2O, CH3OH, and surface OH groups even in the dark prior to the experimental test, special cautions on the conclusions from D−H exchange experiments should be paid. Some more specific transient spectroscopic studies might be needed to further confirm the originality of the H atoms in the evolved H2 when sacrificial reagents such as CH3OH are used. Water/TiO2, methanol/TiO2, ethanol/TiO2, and formaldehyde/TiO2 are some typical prototypical systems in both surface science and photocatalysis of TiO2-based photocatalysts. Only recently, by combining various spectroscopic techniques have the dynamics and kinetics of H2 evolution fundamental reaction steps at these interfaces been better understood at the molecular level. Usually, two possible mechanisms have been proposed for the photocatalytic oxidation of methanol on the surface of TiO2: direct oxidation by photogenerated holes or indirect oxidation by photogenerated ·OH radicals produced by oxidation of surface −OH groups or adsorbed water by the photogenerated holes. However, it is still a challenge to distinguish between the two mechanisms in practice due to the lack of suitable probe techniques.578 Isotope labeling techniques are prone to be a powerful technique for the exploration of the dynamics and kinetics studies of these photocatalytic processes. Recently, Zhou and co-workers579 reported the site-specific photocatalyzed splitting of methanol on TiO2(110) facet using two-photon photoemission (2PPE), scanning tunneling mi-

Figure 20. Time-dependent 2PPE spectra measured for the fresh CH3OH/TiO2(110) surface after it had been exposed to 400 nm light for different time durations. Reprinted with permission from ref 579. Copyright 2010 Royal Society of Chemistry.

that the dissociation of methanol occurred at Ti5c sites. The kinetic monitor of the excited resonance signal (Figure 20) clearly suggested that the dissociation of methanol is photoinduced rather than spontaneous. By comparing the reaction rate of the photocatalyzed dissociation of methanol on TiO2(110) surfaces with different defect density, Zhou et al. found that the defects could accelerate this photocatalytic process, and proposed that the defects may reduce the reaction barrier for photocatalytic dissociation of methanol.580 In addition, the effects of the localized excitation of the defect states lying in the bandgap were also well studied.581 To identify the photochemical products, Guo and coworkers582 performed temperature-programmed desorption (TPD) mass spectroscopy measurements with an extremely low background at high vacuum with a pressure of about 1.5 × 10−12 Torr.583 Under 400 nm irradiation, CD3OH on TiO2(110) can be photocatalytically dissociated to CD2O on the Ti5c sites, leaving H and D atoms on the bridge-bonded oxygen (BBO) sites nearby. The measurement of the kinetics of water desorption from recombination of these bridging hydroxyls (Figure 21) suggested that the cleavage of the O− H bond precedes that of the C−D bond, indicating the photocatalyzed dissociation of methanol takes place in a stepwise manner: hν ,TiO2 (110)

CD3OH(Ti5c) ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ CD3O(Ti5c) + HBBO hν ,TiO2 (110)

CD3O(Ti5c) ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ CD2 O(Ti5c) + DBBO

(1a) (2a)

DFT calculations performed on the ground state of the system showed that the hydroxyl hydrogen dissociation of methanol has a rather small barrier, while methyl hydrogen dissociation of methanol has a considerably larger one (Figure 22). The high reverse barrier to C−D recombination and the facile desorption of CD2O make photocatalytic methanol dissociation on TiO2(110) proceed efficiently. In the case of 10010


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CH 2O(Ti5c) + CH3O(Ti5c) hν ,TiO2 (110)

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ CH3OCHO(Ti5c) + HBBO

(3a)

It is interesting to know whether hydrogen molecules can be released from the BBO sites of TiO2(110) after methanol dissociation. Recent results reported by Xu et al. suggest that less than 7% of the dissociated H atoms (D atoms) from methanol photocatalysis recombines to form molecular hydrogen by heat, rather than by photolysis, while most of the dissociated H atoms (D atoms) will combine with oxygen atoms on the BBO rows to form water that desorbs around 500 K, leaving behind an oxygen vacancy (BBOv) (Figure 23).571

Figure 21. Integrated TPD signals for the water isotopologues desorbed from recombination of the bridging hydroxyls as a function of laser irradiation time. The normalized signals in the inset graph suggested the formation of bridging OH is faster than that of OD. Reprinted with permission from ref 582. Copyright 2012 American Chemical Society.

Figure 23. Mechanism of molecular water and hydrogen (deuterium) production from bridging hydroxyls on TiO2(110) surface. Reprinted with permission from ref 571. Copyright 2013 American Chemical Society.

This demonstrates that photocatalytic hydrogen production from methanol occurs via several elementary reactions; while methanol dissociation is photocatalytically driven, the formation of molecular hydrogen is a thermally driven process. Although the amount of molecular hydrogen desorbed from the bridging hydroxyls on rutile TiO2(110) is relatively low (7% of the bridging hydroxyls), Xu and co-workers demonstrated that on anatase TiO2(101), the formation of molecular H2 and H2O is comparable.572 This might explain why anatase TiO2 is more active than rutile. Studies of these elementary photocatalytic processes allow them to build a detailed picture of the hydrogen production from methanol photocatalysis on TiO2 as depicted in the scheme. From the electron−hole photocatalysis model, photocatalytic dissociation is driven by electrons or holes on the band edges through relaxation. This implies that as long as the photon energy is above the band energy, photocatalysis will happen and should not be strongly dependent on photon energy. However, recent measurements of the photocatalytic dissociation rate of methanol on TiO2(110) at different wavelengths by Xu and coworkers showed that the splitting of methanol at 266 nm is about 2 orders of magnitude faster than that at 355 nm (Figure

Figure 22. Calculated energetic of the two-step dissociation of (A) methanol and (B) water on the TiO2(110) surface. Reprinted with permission from ref 582. Copyright 2012 American Chemical Society.

water, however, the much lower reverse barrier of the second O−H dissociation step as compared to that of C−D in methanol makes recombination facile, thus preventing efficient water splitting. Further coadsorption experiments carried out by Xu et al. suggested neither water nor methanol could affect the photochemistry of the other one.584 In addition to the formation of formaldehyde from methanol photocatalysis on TiO2(110), Guo et al. also found further photocatalytic oxidation can take place.585 TPD results reveal that the production of methyl formate occurs through crosscoupling reaction between formaldehyde and methyl radical on the surface (eq 3a), suggesting that the photooxidation of methanol on TiO2(110) occurs in a multielementary-step way. 10011


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4. PHOTOREDUCTION OF CO2 TO SOLAR FUELS ON TiO2-BASED PHOTOCATALYSTS One of the most important common features of photocatalytic splitting of water and reduction of CO2 for solar fuel generation is that both reactions should ideally use water as the electron source. Water oxidation to generate free electrons is the key step for both of these two reactions. However, photocatalytic reduction of CO2 is much more challenging than water splitting because reduction of CO2 is much more difficult than reduction of protons. This is because of the following reasons: (1) Reduction of CO2 requires high energy input for breaking the OCO double bonds. (2) Water splitting is a four-electron transfer processes, and CO2 reductions to methanol and methane are six- and eight-electron transfer processes, respectively. (3) Reduction of CO2 to chemical fuels such as methanol involves not only carbon reduction, but also proton transfer and hydrogenation, which makes CO2 reduction more complicated than water splitting. (4) If one needs to make high carbon content fuels, like ethanol, reaction goes the more complicated way, because it requires C1 to C2 chemistry. (5) In aqueous solution, competition between reduction of CO2 and reduction of proton exists. Design and synthesis of CO2 reduction photocatalysts with high selectivity for CO2 reduction and inhibiting proton reduction reactions is necessary. (6) Analysis of CO2 reduction reaction is much more difficult due to that a variety of CO2 reduction products might be generated. In 1979, Fujishima and Honda et al. further reported in Nature589 the photoelectrocatalytic reduction of CO2 into organic compounds, such as formaldehyde, methyl alcohol, and methane, in the presence of semiconductor photocatalysts suspended in water. The catalysts invested include not only TiO2, but also WO3, ZnO, CdS, GaP, and SiC. The proposed mechanism for photoreduction of CO2 is as follows:

24), indicating that photocatalytic dissociation of methanol on TiO2(110) is strongly dependent on photon energy.586 Because

Figure 24. Evolution of the H2O (recombination from bridging hydroxyls) TPD yield following 355 and 266 nm irradiation of a 0.5 ML CH3OH covered TiO2(110) surface. The unfilled squares indicate the rise times at 90% of the asymptotic values of the fits for both 355 and 266 nm photocatalysis. Reprinted with permission from ref 586. Copyright 2013 American Chemical Society.

of the energy mismatch, transfer of photogenerated holes to the HOMO of methanol adsorbed on TiO2 is unlikely. In addition to the transfer of photogenerated electrons to the LUMO of methanol, coupling of the phonon energy, which is the result of recombination of electron−hole pairs to the highly vibrationally excited states of methanol, has also been proposed to explain the strong photon-energy dependent phenomena. In the latter case, methanol is clearly dissociated in the ground electronic state, which is much different from the current photocatalysis model where photoexcited charges are directly involved in the reaction. The importance of the electron−hole pair energy demonstrated in this work calls for the development of a more sophisticated surface photocatalysis model that incorporates the effect of photon energy. Such development is expected to enhance our understanding of fundamental processes in photocatalysis and guide the development of more efficient photocatalysts in the future. The photocatalytic chemistry of ethanol587 and formaldehyde588 on TiO2(110) has also been studied by Ma et al. and Xu et al., respectively. From the fundamental research, the active sites and elementary reaction steps have been identified. In addition, the study of the kinetics of the photocatalytic processes on TiO2 has led to the reconsideration of the mechanism of photocatalysis. Although these dynamics and kinetics studies of photocatalysis on the surface of TiO2 are primary results, they strongly indicate that the photocatalysis is a rather complicated process. To efficiently catalyze a photocatalytic reaction, one need to not only consider the processes proceed inside the semiconductor photocatalysts, but also should carefully examine the reaction processes on the surface of semiconductor photocatalysts. Overall, the efficiency of a photocatalytic reaction would be determined by the combination efficiencies of the processes inside the semiconductor photocatalyst (light absorption, charge separation, charge mobility to the surface), but also the complicated surface reaction mechanisms.

catalyst + hν → catalyst*(e−cond + p+ val )

(12)

For an oxidation reaction: H 2O + 2p+ val → 1/2O2 + 2H+

(13)

Reduction reactions: CO2 (aq) + 2H+ + 2e−cond → HCOOH

(14)

HCOOH + 2H+ + 2e−cond → HCHO + H 2O

(15)

HCHO + 2H+ + 2e−cond → CH3OH

(16)

CH3OH + 2H+ + 2e−cond → CH4 + H 2O

(17)

Photogenerated holes in the VB of semiconductor oxidize water to O2 and release H+, and photogenerated electrons in CB reduce CO2 by a sequence of reactions to produce HCOOH, HCHO, CH3OH, and CH4, etc. Also, it was suggested that the charge transfer rates between photogenerated carriers and the solution species depend on the correlation of the energy levels between the semiconductor and the redox agents in solution; that is, the solution species with redox potential more positive with respect to the CB level of a semiconductor is more easily reduced (Figure 25). Following this pioneering work, various research works have been done using TiO2 as a model semiconductor photocatalyst, aiming at increasing the CO2 conversion efficiency and more importantly better understanding the mechanisms of CO2 photoreduction. Although the highest rates of product 10012


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the fraction of bulk material having a nanostructured arrangement of interwoven anatase−rutile crystallites where (i) rutile extends photoresponse close to the visible light region because it has smaller bandgap (3.0 eV) than rutile (3.2 eV), (ii) charge recombination is hindered by charge transfer and separation across the interface/junction of different phases, and (iii) unique interfacial trapping sites are possibly photocatalytic “hot spots”. CO2 photoreduction using different kinds of mixed-phase TiO2 (brookite, anatase, and rutile) has been evaluated in recent years. Gray et al.613 investigated in detail an anatase−rutile nanocomposite synthesized using a simple hydrothermal method followed by sintering at 773 K. The mixed-phase nanocomposite showed higher activity than Degussa P25 in both oxidation and reduction reactions, especially in photoreduction of CO2 to CH3OH and CH4. The effective charge separation between anatase and rutile phases, which is a key factor leading to the superior photoactivity of the synthesized mixed-phase TiO2 nanocomposite, was supported by EPR study at 4 K (Figure 26). The EPR signals with g⊥ = 2.014,

Figure 25. A schematic illustration of the energy correlation between semiconductor catalysts and redox couples. Potentials are against the normal hydrogen electrode. Reprinted with permission from ref 589. Copyright 1979 Nature Publishing Group.

formation did not exceed tens of mmol h−1 gcat−1 (e.g., methane) and no great breakthrough in terms of CO 2 conversion efficiency until present, some useful insights have been gained through this extensive research on TiO2, which might be helpful for further designing and synthesizing more efficient photocatalysts for CO2 reduction. Numerous reviews, perspectives, and accounts have been published in recent years regarding photoreduction of CO2. For example, we include possibilities and challenges on photoreduction of CO2 for fuel production,590 photoreduction of CO2 on Ti-based heterogeneous catalysts,591−598 molecular approaches to photoreduction of CO2,599−601 advanced semiconductors, nanoarchitectures, and heterogeneous systems for photoreduction of CO2 to valuable chemicals,602−604 charge separation on TiO2 for CO2 reduction,605 and new reaction systems and reactors for CO2 reduction.606,607 However, most of these are either broad and general in concept or too specific by focusing on a particular research area. The scope of this Review for photoreduction of CO2 is set to focus on merely the most widely studied TiO2-based photocatalysts, composites, and systems since 1979. Through a thorough review of the advances of CO2 photoreduction on the benchmark model photocatalyst TiO2, we hope to gain further insight into the complicated processes of CO2 photoreduction, identify the key scientific problems hindering improvement of CO2 photoreduction efficiency, find the feasible solution for the design and synthesis of novel photocatalytic systems, and finally achieve efficient conversion of CO2 into solar fuels. On the basis of these considerations, the following feature contents compromise the scope of this part of the Review. (1) Particular focus will be paid to photoreduction of CO2 with H2O as the electron source rather than using other sacrificial reagents. (2) Because there is no photocatalyst that is superior so far to reduce CO2 with high efficiency, we are not keen on comparing the efficiencies of the different photocatalysts developed so far. Instead, we would rather gain more information on the mechanism of CO2 reduction and strategies for the design and synthesis of more efficient photocatalysts. (3) New research directions and possible solutions on developing efficient photocatalysts or systems will be discussed.

Figure 26. Difference EPR spectra of (a) rutile 773 K, (b) the commercial anatase, (c) anatase−rutile 373 K, (d) anatase−rutile 773 K, and (e) Degussa P25. Background spectra in dark have been subtracted from the corresponding spectra under UV/visible light illumination. Electrons trapped (Ti3+) in both anatase and rutile and trapped holes (h+) are labeled. Reprinted with permission from ref 613. Copyright 2008 Elsevier.

1.975, and 1.990 were assigned to the holes (h+) trapped in rutile lattice, electrons (Ti3+) trapped in rutile lattice, and electrons (Ti3+) trapped in anatase lattice, respectively. Both signals of anatase and rutile can be seen in the EPR spectra of mixed-phase materials. Under continuous UV/visible light irradiation, the EPR spectrum of anatase−rutile 373 K, which has no well contact between different phases, exhibited no significant improvement in photocatalytic activity, while the anatase−rutile 773 K, which has intimate interfaces between two phases, exhibited superior photocatalytic activity. This demonstrates the well contact between two different phases at the interfaces; forming the so-called “surface phase junction”332 is vitally important in the assembly of mixed-phase TiO2 nanocomposite photocatalyst for CO2 photoreduction.

4.1. Mixed-Phase TiO2 Composites for CO2 Photoreduction

As shown in the first section of water splitting, the commercial Degussa P25 has been regarded as a benchmark photocatalyst for both oxidation and reduction reactions.338,339,608−612 Its high photocatalytic activity is usually considered to be due to 10013


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Films with bundles of anatase−rutile nanocolumns possessing high densities of two kinds of interfaces (those among the bundles and those between the columns) were fabricated by the magnetic sputtering method.614 The film sputtered at low angle displayed visible light absorption with absorption edge at ca. 500 nm, which exhibited enhanced CO2 to CH4 photoreduction activity under UV light irradiation. The visible light absorption was explained by the creation of nonstoichiometric titania films, which may contribute to the enhancement of the photocatalytic activity. Bai et al.615 modified the surface of anatase TiO2 nanorods with exposed {010} facets using rutile TiO2 nanoparticles by a one-pot synthesis method. After 8 h of continuous irradiation, the total yield of CH4 on mixed rutile-anatase photocatalyst was 18.9 μmol g−1, which is much higher than that on the pure anatase (10.3 μmol g−1) without rutile modification. The enhancement of the photocatalytic efficiency of the mixedphase structure was ascribed to the efficient charge transfer and separation between the different phases. However, the statement of electron transfer from the CB of anatase to that of the rutile is an ambiguous conclusion, which needs to be further studied as discussed in the H2 production sections. Photoreduction of CO2 with water vapor has been studied on three TiO2 nanocrystal polymorphs (anatase, rutile, and brookite) that were engineered with defect-free and oxygendeficient surfaces, respectively.616 Defects of oxygen vacancies (VO) and Ti3+ sites were created by helium pretreatment of the as-prepared TiO2 at a moderate temperature on anatase and brookite but not on rutile. In situ diffuse reflectance infrared FT-IR spectroscopy (DRIFTS) analyses suggested that (1) defect-free TiO2 was not active for CO2 photoreduction because no CO2− was generated, and (2) CO2 photoreduction to CO possibly underwent different reaction pathways on oxygen-deficient anatase and brookite via different intermediates (e.g., CO2− on anatase; CO2− and HCOOH on brookite). The production of CO and CH4 was remarkably enhanced on defective anatase and brookite TiO2 (up to 10-fold enhancement) as compared to the defect-free samples. Remarkably, the defects containing brookite were the most active photocatalysts among the investigated TiO2 polymorphs (Figure 27). Possible reasons for the superior activity of defective brookite include formation of oxygen vacancies, faster reaction rate of CO2− with adsorbed H2O or surface OH groups, and an additional reaction route involving an HCOOH intermediate. As for the deactivation of the TiO2 photocatalysts, formation of hydroxyl groups by the hydrolysis of Ti−O−Ti of TiO 2 was postulated.617 Mixed-phase anatase−brookite (A−B) TiO2 nanomaterials were also synthesized by a hydrothermal method and tested for photoreduction of CO2 to CO and CH4.618 The anatase− brookite phase content was controlled by adjusting the concentration of urea in the precursor solution. The photocatalytic reactions were carried out in the presence of water vapor. By in situ DRIFTS analysis, CO2− and HCO3− were found to be the active reaction intermediates for CO2 reduction. It was further demonstrated that the anatase-rich mixed-phase TiO2 was more active than the brookite-rich one due to higher specific surface area and the smaller bandgaps of the anatase than that of the brookite. Also, the optimal photocatalytic activity was obtained for a bicrystalline mixture with a composition of 75% anatase and 25% brookite. Formation of well-defined interface junctions between anatase and brookite was visualized by the HRTEM images as shown in

Figure 27. Production of CO and CH4 on unpretreated and He pretreated three TiO2 polymorphs for a period of 6 h photoillumination. Inset shows the corresponding HRTEM images of the synthesized anatase (A), rutile (R), and brookite (B). Reprinted with permission from ref 616. Copyright 2012 American Chemical Society.

Figure 28. Analogous to the anatase−rutile mixed-phase TiO2, mixed-phase anatase−brookite photocatalyst may also lead to efficient charge transfer and separation, and even be superior to anatase−rutile. This might be due to the following reasons: (1) brookite itself is more active than rutile in CO2 photoreduction; (2) brookite has a higher CB edge than rutile, which may promote CO2 reduction with H2O; and (3) the excited electrons on brookite CB may transfer to anatase CB due to a slightly higher CB edge of brookite, leading to more efficient charge separation. This work demonstrates that proper designing and synthesis of semiconductor interfaces/junctions is crucial in improving the photocatalytic activity of a polymorph semiconductor photocatalyst. Recently, Ag nanoparticles deposited on porous TiO2 microspheres of pure anatase (A), anatase-rich anatase/ brookite mixture (AB), and brookite-rich anatase/brookite mixture (BA) were controllably synthesized through a sequential hydrothermal, ultrasonic spray pyrolysis, and in situ photoreduction process.619 The XPS analysis revealed that the as-prepared Ag/TiO2 was dominated by the Ag(I) species, while Ag(0) was the major Ag species upon in situ photoreduction of the as-prepared Ag(I)/TiO2. CO2 photoreduction reactions were performed over these different kinds of Ag/TiO2 composites in aqueous solution of methanol under UV−vis irradiation. It was found that CH4 and H2 production rates by Ag(0)/TiO2 were always higher than those of the corresponding as-prepared Ag(I)/TiO2, indicating that Ag(0) is the active cocatalyst for CO2 reduction. Also, interestingly, the anatase-rich anatase/brookite (AB) mixed-phase Ag(0)/TiO2 (AB) showed the best photocatalytic activity among the various combinations. The superb activity of the Ag(0)/Ti(AB) catalysts was ascribed to their large surface area, small and well-dispersed Ag(0) nanoparticles, and an enhanced interfacial charge transfer between the anatase and brookite nanocrystals. 10014


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Figure 28. TEM images of A75B25 (a), and HRTEM images of A75B25 (b and c). A and B represent anatase and brookite, respectively. Reprinted with permission from ref 618. Copyright 2013 Royal Society of Chemistry.

4.2. Crystal Facet Engineering of TiO2 for CO2 Photoreduction

The facet engineering of semiconductors is one of the most effective strategies to achieve advanced properties over photofunctional materials for solar energy conversion. As for the most studied form of anatase TiO2, the {010} is reported to be the most reactive facet among the low index facets of {001}, {010}, and {101}, although the theoretically determined surface energy of {010} (0.53 J m−2) is slightly higher than that of {101} (0.44 J m−2) and much lower than that of {001} (0.90 J m−2).620,621 Because of the difficulty in control and preservation of high energy {101} facets, anatase TiO2 rods with dominant reactive {010} facets were synthesized only recently by Liu et al.622 through hydrothermal treatment of Cs0.68Ti1.83O4/ H0.68Ti1.83O4 particles. Upon Pt loading (1%), excellent photocatalytic activity in converting CO2 into CH4 was observed, which was believed to be due to the synergistic effects of the unique surface atomic structure and higher CB minimum of {010} facet exposed anatase TO2. The high photocatalytic activity of the anatase TiO2 rods with dominant reactive {010} facets was tentatively assigned to the following factors: efficient adsorption of H2O molecules on the (010) surface with 100% Ti5c atoms, strong interaction of CO2 on the (010), and, most importantly, the photoexcited electrons in a more negative CB have a greater ability to reduce CO2 (Figure 29). On the contrary, reversed photocatalytic activity order for conversion of CO2 to CH4 was reported when Pt was deposited onto the {001} and {010}) facets of anatase TiO2. Without Ptloading, the higher CO2 photoreduction activity of anatase TiO2 with {010} facets than that with {001} facets was observed, which was attributed to the higher CO2 adsorption capability of {010} facets and longer separated charge lifetime. After Pt loading (1 wt %), the small Pt nanoparticles loaded on the TiO2 with {010} facets can more efficiently enhance the photogenerated charge separation as compared to that of the agglomerated Pt nanoparticles loaded on the TiO2 with {001} facets. Therefore, TiO2 with {001} facets showed a higher photoactivity than TiO2 with {010} facets.623 Ye et al.624 synthesized ultrathin anatase TiO2 single crystal nanosheets (2 nm in thickness) with 95% of exposed {100} facet, which showed about 5 times higher photocatalytic activity in both H2 evolution and CO2 reduction to CH4 than that of the TiO2 cuboids with 53% of exposed {100} facet. The high photocatalytic activity of the TiO2 with 95% of exposed {100} facet was ascribed to a higher percentage of exposed {100} reactive facets, larger surface area offering more surface active

Figure 29. (a) Diffuse reflectance UV−visible absorption spectra; (b) XPS valence band spectra; and (c) schematic of electronic band alignments of cesium-free microsized (i) and nanosized (ii) anatase rods. The insets in (a) are plots of the transformed Kubelka−Munk function versus the energy of light. Reprinted with permission from ref 622. Copyright 2011 Royal Society of Chemistry.

sites, and the superior electronic band structure resulted from the higher percentage of {100} facet. In addition to the different reactivity of the exposed facets, the distribution of photogenerated charges on the different facets might be also a factor governing the catalytic performance. Yet less attention has been paid to this aspect. 4.3. TiO2-Based Nanocomposite Photocatalysts for CO2 Photoreduction

4.3.1. Metal Loaded TiO2-Based Nanocomposite Photocatalysts. Deposition of Pt nanoparticles on the surface of various nanostructured TiO2 was found to be an effective strategy for photocatalytic conversion of CO2 to CH4. For example, low-dimensional TiO2 nanocomposites deposited with Pt-metal,625 TiO2 nanotube deposited with ultrafine Pt nanoparticles by a rapid microwave-assisted solvothermal method,626 and one-dimensional (1D) nanostructured TiO2 single crystals films coated with ultrafine Pt nanoparticles (NPs, 0.5−2 nm) all exhibited photocatalytic activity for conversion of CO2 to CH4.627 Generally speaking, the formation of CH4 (Eredox/SCE = −0.48 V) is thermodynamically more feasible than the formation of CO (Eredox/SCE = −0.77 V) if the supply of protons and electrons is high enough. Production of CO in 10015


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Figure 30. Schematic diagram of CO2 photoreduction mechanism by using Pt−TiO2 nanostructured films. The magnified circle (center) shows that the photogenerated electrons can move fast inside the highly oriented TiO2 single crystals and flow to the Pt deposits, where the redox reaction occurs to convert CO2 into CO and CH4. The right side of the figure illustrates the energy levels of the Pt−TiO2−CO2 system. CB and VB are abbreviations of conduction band and valence band, respectively. H+ and h+ indicate proton and hole, respectively. Reprinted with permission from ref 627. Copyright 2012 American Chemical Society.

origin of transient activity to these surface species formed under dark condition. Full recovery of the transient catalytic activity after a long dark time of the oxidative treatment implies that the oxidative pretreatment may consume reactive surface OH groups, whereas reductive pretreatment may increase the amount of OH groups, thus restoring the product precursors at the surface more quickly after the reductive treatment. This implies that oxidative and reductive pretreatment greatly influence catalytic performance, and the reduction pretreatment is crucial to create more reactive surface. Au nanoparticle deposited Au/TiO2 photocatalysts were systematically studied for CO2 photoreduction in the presence of water vapor.630 When the Au/TiO2 photocatalyst was irradiated with 532 nm light, the wavelength of which matches with the photon energy of the plasmon resonance of Au nanoparticles, a 24-fold enhancement in the photocatalytic activity was observed. The intense local electric fields created by the surface plasmon resonance of the deposited Au nanoparticles excite the electron−hole pairs locally in TiO2 at a much higher rate than the incident light. This may enhance the sub-bandgap absorption in TiO2, thereby enhancing the photocatalytic activity in the visible range. It was suggested that the plasmon-excited electrons in Au nanoparticles cannot transfer directly from the Au to the TiO2. Only when the photon energy is high enough to excite the d band electrons of Au to the CB of TiO2 may the direct charge transfer between Au and TiO2 may occur. On the other hand, when the photon energy (254 nm) is high enough to excite d band electronic transitions in Au that lies above the redox potentials of CO2/ C2H6, a different mechanism occurs resulting in the production of additional reaction products, including C2H6, CH3OH, and HCHO. Ag also has been demonstrated to be an efficient cocatalyst for CO2 reduction. When Ag nanoparticles were deposited on TiO2 nanorods by electrochemical method, Ag−TiO2 nanocomposites exhibited higher photocatalytic activity for conversion of CO2 to CH4 than that of pure TiO2.631 The advantages of Ag deposition on TiO2 were speculated to be (i) modification of the surface morphologies and structures of TiO2, (ii) improvement of the electron−hole separation by performing as electron traps, and (iii) increase of the surface electron activity by localized surface plasma resonance. Koci et al. examined the particle size effect of pure anatase TiO2 on the photoreduction of CO2 to CH3OH and CH4.632 The anatase particles with crystallite diameters ranging from 4.5

many studies is possibly due to the paucity of electrons. However, if rich amounts of electrons and H+ were generated during the photocatalytic courses, CH4 would be then the main product in the Pt−TiO2 system. The carrier transfer dynamics study by femtosecond time-resolved TA spectroscopy measurements showed that the charge recombination process is much slower for Pt−TiO2 than that for pure TiO2, which is direct evidence for Pt driven suppression of the charge recombination process in the Pt−TiO2 sample (Figure 30). Overall, fast electron-transfer rate in TiO2 single crystals and the efficient electron−hole separation by the Pt NPs are the main reasons for the enhancement of the photocatalytic activity, while the size of the Pt NPs and the unique 1D structure of TiO2 single crystals may also play important roles in delivering high photocatalytic CO2 conversion efficiency. The mechanism of photoreduction of CO2 with H2O over Pt−TiO2 films was investigated.628 It was claimed that the ratedetermining step for photoreduction of CO2 with H2O is the water oxidation reaction, as evidenced by the observation of a significant amount of carbonaceous intermediates accumulated on the surface of TiO2. When gas-phase H2 was allowed in the system, the carbonaceous intermediates were converted to methane, even in the dark, albeit at lower rates. Also, spilledover hydrogen was responsible for the final hydrogenation step. The dynamic nature of continuous photoreduction of gaseous CO2 with H2O on Pt/TiO2 was systematically studied under UV irradiation at 353 and 423 K. This study gives further insights into the origin of the CO2 photoreduction on TiO2based materials.629 The steady-state activity for H2 production and transient activity for CH4 production were identified. The transient activity occurs at the initial phase of the reaction with a maximum activity followed by rapid deactivation after 10−15 min. The steady-state activity only happens for H2 production at T < 373 K. Because of the better surface wetting, both transient and steady-state activities for H2 production by watersplitting were enhanced at T < 373 K. In contrast, the transient activity for CH4 production was favored at higher temperatures. It is noteworthy that these two different activities were also present for pure TiO2 and not specific to platinum-promoted TiO2. Furthermore, the in situ DRIFTS study identified the formation of reactive OH groups on TiO2 surface and the consumption of carbonates and bicarbonates under UV irradiation. The consumption of the OH groups, as well as the recovery and even increase of the surface carbonate and bicarbonate species under dark conditions, unraveled that the 10016


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order: CO > CH4 > C2H6 > C2H4 > C3H6, with CO and CH4 as the major products. The high yields of CO and CH4 were likely due to the significant increase in the number of electron− hole pairs and inhibited charge recombination over In/TiO2 supported microchannels. The performance of the photoreactor for CO production was in the order of In/TiO2monolith > TiO2-monolith > TiO2−SS cell (stainless steel cell). The high efficiency of the catalysts and reactors may be attributed to the following: (1) large illuminated surface area, higher light utilization, efficient adsorption−desorption process, and higher catalyst interparticle mesoporosity; (2) control of the crystal growth, increase of the mesoporosity and the surface area, and reduction of the particle size by In doping; and (3) indium particles on the surface of TiO2 may efficiently trap electrons and hinder the undesired recombination of electrons−hole pairs. 4.3.2. Metal Oxide Loaded TiO2-Based Nanocomposite Photocatalysts. Wu et al.640 studied photoreduction of CO2 to methanol by Cu/TiO2 in an aqueous solution under UV irradiation (254 nm). Cu-loaded titania samples with grain size of ca. 20 nm were synthesized by an improved sol−gel method. XPS analysis revealed that Cu2O is the primary species deposited on TiO2. The highest photocatalytic activity for CO2 reduction to methanol was achieved when the amount of copper loading was 2.0 wt %. The catalytic activity can be further improved if NaOH caustic solution was used as the reaction media. The highest quantum and energy efficiencies were 10% and 2.5%, respectively. O2 was detected as a product of CO2 reduction using O2 sensor, indicating that H2O might the electron source for CO2 reduction. The time dependency of O2 generation approximately matches with the methanol yield. Adsorption of O2 on the surface of TiO2 or consumption by methanol reoxidation could not be ruled out, but the observed methanol to O2 ratio was near constant. The mechanism of CO2 reduction on Cu/TiO2 was also proposed. Molecules of CO2 (or HCO3−) and H2O interact with trapped electrons on the Cu cluster. The reduction of CO2 and the decomposition of H2O proceed competitively. The · OH radicals may be produced via oxidation of OH− by photogenerated holes on the TiO2 surface, which react with the carbon species generated from CO2 to produce methanol. Free O2 is also generated from the oxidation of OH− by holes, which can be partially consumed by the reoxidation of methanol. The redistribution of the electric charge and the Schottky barrier between Cu and TiO2 facilitates charge transfer and separation, hence enhancing the photocatalytic activity. Distribution of Cu species (Cu0 and Cu+) affects the photocatalytic activity of Cu/TiO2 dramatically.641,642 Cu/TiO2 has much higher photocatalytic activity than Ag/TiO2, which is due to the strong affinity between Ag clusters resulting in the photoelectrons reducing the mobility of the latter. The values of redox potential for Cu+ and Cu2+ are Cu2+ + 2e− → Cu0, E0 = 0.34 V; Cu2+ + e− → Cu+, E0 = 0.17 V; Cu+ + e− → Cu0, E0 = 0.52 V. It was found that the photoactivity of Cu/TiO2 decreases when Cu(I) changes to Cu(0) or aggregates after reduction by H2. The photocatalytic activity attains maximum when ca. 25% of the total Cu loading was located on the outermost surface of a TiO2 particle. These demonstrate that the chemical states and the location of Cu on TiO2 play important roles in photoreduction of CO2. Photocatalytic reduction of CO2 to MeOH has been investigated in more detail on various valence state copper doped Cu0/TiO2, CuI/TiO2, and CuII/TiO2 photocatalysts

to 29 nm were prepared by precipitation and sol−gel method. The optimum particle size corresponding to the highest yields of CH3OH and CH4 was ca. 14 nm. They also prepared Agdoped TiO2 catalysts by a sol−gel method and tested their activity for CO2 photoreduction in an aqueous solution under 254 nm UV irradiation.633,634 H2, CH4, CH3OH, and CO were found to be the major products. The enhancement of CO2 photoreduction activity by Ag loading (>wt 5%) was interpreted by spatial separation of photogenerated electron and holes and increase of their lifetimes due to the formation of Schottky barrier at the metal−semiconductor interface. Li et al.635 explored the feasibility of syngas (mixture of H2 and CO, which is a valuable feedstock in producing synthetic petroleum through Fischer−Tropsch process) production from a gas mixture of CO2, H2O, and CH3OH through a photocatalytic reduction process on Ag/TiO2 nanocomposite catalysts under solar light irradiation. The Ag/TiO2 catalyst was prepared using a simple ultrasonic spray pyrolysis (USP) process. The production selectivity of H2 and CO2 could be tuned in the ratio range between 2 and 10 by varying the reaction gas composition. Their control experiments strongly indicated that production of CO is mainly from CO2 but not from the hole scavenger CH3OH, while H2 could come from both H2O and CH3OH. Wang et al. 636 examined the relations between CO 2 photoreduction activities of the Pd and RuO2-modified TiO2 and the surface photovoltage (SPV) response. After deposition of noble metals (Pd and Ru) on the surface of synthesized TiO2, surface state population transition was observed. Corresponding to this SPV response, the catalytic activity of CO2 reduction to formate was improved. Moreover, the stronger was SPV response was, the higher was the catalytic activity of photoreduction of CO2 was. The authors proposed that the Pd cocatalyst not only provides active sites for CO2 reduction, but also acts as reduction species reacting with positive holes on the semiconductor site, thus promoting charge separation and enhancing the photocatalytic activity. Particularly, when Pd and RuO2 were codeposited on TiO2, photogenerated holes and electrons tend to diffuse to RuO2 and Pd, respectively, leading to much higher photocatalytic activity as compared to the TiO2 deposited with single Pd or RuO2 cocatalyst. Also, it should mention that the Pd deposited TiO2 favors formate production, because hydrogen atoms or hydrides bind so tightly on Pd to release H2. Ishitani et al.637 studied the effects of organic adsorbates on CO2 photoreduction products in an aqueous suspension of TiO2. Irradiation of an aqueous suspension of TiO2 resulted in the formation of CH4 as the major product, and its formation was suggested to proceed mainly via the photo-Kolbe reaction of acetic acid.638 After calcination and washing treatment to remove the organic adsorbates, CO along with a very small amount of CH4 was photocatalytically produced. In contrast, Pd (>0.5 wt %) deposited Pd/TiO2 without organic adsorbates on the surface photocatalytically produce CH4 as the main product with the CO formation was drastically diminished. These results clearly show that removal of organic adsorbates from semiconductors is essential for evaluating the photocatalytic processes of CO2 reduction. Recently, it was reported that indium-doped In/TiO2 nanoparticles coated in a microchannel monolith photoreactor were effective for photocatalytic reduction of CO2 to various kind of products under UV irradiation with 200 W mercury lamp.639 Also, the yield of the products is in the following 10017


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100 °C. The activation energy (Ea) for Degussa-P25 and 3% CuO/TiO2 is ca. +26 and +12 kJ/mol, respectively. These Ea values suggest that desorption of products might be the ratelimiting step in the formation of methanol from CO2 with H2O at the catalysts surfaces. Also, in contrast with the results of the others, the lower Ea of 3% CuO/TiO2 suggests that CuO dopant species might be the catalytic sites in enhancing the methanol production. Li et al. found that MgO-patched TiO2 microsphere showed 10 times higher activity for photoreduction of CO2 to CO in the presence of H2O vapor. The effects of MgO were attributed not only to the enhancement of the CO2 adsorption but also to the increase of the stability of the TiO2 photocatalyst. Increasing the reaction temperature from 50 to 150 °C could further improve the CO2 reduction yield due to easier desorption of the reaction intermediates and products. This demonstrates that the dynamic equilibrium of reactants adsorption−intermediates desorption is also a very important factor governing the catalytic performance.646 Wang et al.647 further found that MgO and Pt coloaded Pt−MgO/TiO2 can significantly improve the efficiency of photocatalytic reduction of CO2 to CH4 in the presence of H2O. They proposed that the enriched electron density on Pt nanoparticles would favor the formation of CH4 rather than CO, because formation of CH4 is thermodynamically more feasible than that of CO. The synergistic effect between MgO, which enhances the density of destabilized CO2 molecules on the catalyst surface, and the Pt nanoparticles with enriched electron density can further accelerate the reduction of CO2 to CH4 (Figure 31). The

prepared by an improved impregnation method via the reduction−oxidation steps.643 It was found that CuO is the most active dopant as compared to the other species. It was argued that Cu2O dopant should effectively act as an electron trapper prohibiting electron−hole recombination because it has the highest positive redox potential value of Cu+. However, due to the relatively strong interaction between TiO2 and the dopant particle implanted in the vacant sites of TiO2, the dopant with more positive redox potential exceedingly catches electron from CB edge. Consequently, the dopant-trapped electrons are more difficult to transform to the adsorbed species on catalyst surface, and hence it may play a role as an electron− hole recombination centers. In this case, Cu2+ with the lower redox potential is a more promising dopant species. The electron−hole recombination rate could be effectively reduced due to recycling of Cu2+ and Cu+ according to the following redox equation: e−

O2 /H+

Cu IIO → Cu IO ⎯⎯⎯⎯⎯⎯→ Cu IIO

(18)

The effect of Cu valence state in Cu/TiO2 (P25) nanoparticle for CO2 photocatalytic reduction was also studied by Li et al. They prepared a series of Cu/TiO2 samples by a simple precipitation and calcination method. The as-prepared sample was dominated by Cu2+ species, and thermal pretreatment of the as-prepared samples in He and H2 atmosphere resulted in the samples dominated by Cu+ and mixed Cu+/Cu0, respectively. Photocatalytic activities of these samples in the presence of water vapor were in the order as-prepared (unpretreated) < He-pretreated < H2-pretreated. Especially, as compared to unpretreated TiO2(P25), the H2-pretreated Cu/TiO2 demonstrated a 10-fold and 189-fold activity enhancement in the production of CO and CH4, respectively. Such significant activity enhancements were mainly attributed to the synergy of the following two factors: (1) the formation of surface defect sites, such as oxygen vacancies and Ti3+ sites, which can promote CO2 adsorption and the subsequent charge transfer to the adsorbed CO2; (2) the existence of Cu+/Cu0 couples facilitates electron and hole trapping at different sites; and (3) the significantly promoted CH4 selectivity on H2reduced Cu/TiO2 samples is probably related to interstitial H atoms as revealed by the in situ DRIFTS study.644 These different conclusions on the active species of Cu/TiO2 for CO2 photoreduction might be due to the different synthesis procedures/methods, and probably it is not suitable to use one conclusion to judge the others. However, regardless of the initial valence state of Cu, it seems that the low valence state of Cu+ is the key active species in Cu/TiO2 for CO2 reduction. The effect of Cu loading amount was studied for Cu/TiO2 catalysts synthesized using a refined sol−gel method.645 The photocatalytic activity of CO2 reduction with H2O to CH4 exhibited a strong volcano dependence on Cu loading, reflecting the transition from two-dimensional CuOx nanostructures to three-dimensional crystallites. The optimum CH4 production was observed for 0.03 wt % Cu/TiO2, which is 10fold higher than the analogue of pure anatase synthesized using the same procedure. It should also be noted that the activity promotion by Cu is strongly linked to the morphology nature of the dispersed copper oxide. The two-dimensional islands may act as the trapping centers for anatase photoexcited charge carriers and the primary active sites for CO2 photoreduction. The temperature dependence of CO 2 to methanol conversion was investigated at temperature between 43 and

Figure 31. Proposed mechanism of the MgO layer and Pt nanoparticles over TiO2 for photoreduction of CO2 in the presence of H2O. Reprinted with permission from ref 647. Copyright 2013 Royal Society of Chemistry.

authors proposed that MgO layers might have dual effects on CO2 reduction: (1) Capture the holes and facilitate the separation of photogenerated electrons and holes, similar to the role of Al2O3 thin layers coated on Fe2O3 surfaces. (2) Hinder the direct contact of CO or CH4 with TiO2 surfaces, reducing the reoxidation possibilities for CO and CH4. To achieve such a synergistic effect, tailor design and synthesis of the composite photocatalysts with intimate contact/interface of Pt between both TiO2 and MgO is a crucial requirement. Ye et al.648 synthesized six types of hybrid TiO2 mesoporous “french fries” (MFFs) (TiO2/ZnO, TiO2/Fe2O3, TiO2/CuO, TiO2/NiO, TiO2/Cr2O3, and TiO2/CeO2) by a facile furfural alcohol-derived polymerization−oxidation route (FAPO). The homogeneously hybrid TiO2/ZnO MFFs exhibited a much higher photocatalytic activity for CO2 reduction with H2O to 10018


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CH3OH instead of CH4 under UV light irradiation.653 It was suggested that doped Ce atoms activate H2O and CO2 molecules, while Cu atoms act as the channel of photoelectrons in real time and prevent the recombination of photogenerated electrons and holes. However, due to lack of data provided, the mechanism of CH3OH formation is not clear. Manganese- and copper-doped TiO2 nanocomposites (15− 25 nm in size) prepared via the sol−gel route showed photocatalytic activity for CO2 reduction to methanol.654 Because of Mn and Cu doping, the synthesized nanocomposites showed bandgaps lower than 3 eV. Among the synthesized nanocomposites, the Mn0.22−Cu0.78/TiO2 was found to yield a maximum of 238.6 μmol-MeOH/gcat with the highest energy (18.4%) and quantum (26.5%) efficiencies. The improvement of the photocatalytic activity was ascribed to the rapid transportation of the excited electrons to the metal dopants. Wang et al.655 loaded binary cocatalysts Pt and Cu on TiO2 (Degussa P25) by a stepwise photodeposition technique and found that formation of a core−shell-structured Pt@Cu2O cocatalyst can significantly promote the photoreduction of CO2 to CH4 and CO in the presence of H2O. They proposed that the Cu2O shell provides sites for the preferential activation and conversion of CO2 molecules, while the Pt core extracts the photogenerated electrons from TiO2. In the meantime, the Cu2O shell on Pt can markedly suppress the competitive reduction reaction of H2O to H2, giving 85% selectivity for CO2 production. The mechanism study showed that CH4 formation via the hydrogenation of CO2 or CO adsorbed or formed on the Cu site by the atomic H species generated on the Pt site was unlikely. Cu and Ga codoped Cux−Ga1−x/TiO2 nanocomposites with particle sizes in the range of 25−35 nm were prepared by the sol−gel technique.656 The photocatalytic activities for CO2 reduction to formic acid were evaluated by comparison with that of the commercial TiO2 (P25). The synthesized photocatalysts have specific crystalline phases of anatase, β-Ga2O3, and Cu2O. The bandgaps were higher than 3 eV but with a considerable shift of the optical absorption edge to the visible light region. The optimally doped Cu0.78−Ga0.22/TiO2 exhibited the highest formic acid yield (394 μmol gcat−1) as well as superior quantum efficiency (49%) and selectivity (0.84). Although the authors speculated that doping Ga and Cu into TiO2 may ultimately avoid the surface recombination of electron−hole pairs and thereby enhance the photoactivity, no direct experimental or theoretical calculations were provided. Copper and iodine comodified TiO2 nanoparticles (Cu−I− TiO2) were reported to be effective in photocatalytic reduction of CO2 to CO in the presence of water vapor.657 In this composite photocatalyst, the doped iodine was responsible for visible light activity of the catalyst, while the Cu(I) species facilitates charge transfer and reduction of CO2. Under UV−vis irradiation, the activity of the comodified catalyst (Cu−I− TiO2) was higher than that of the single ion modified catalysts (Cu−TiO 2 or I−TiO 2 ). However, under visible light irradiation, the addition of Cu to I−TiO2 did not lead to significant improvements in CO2 reduction. Unfortunately, no further comments were made on this particularly different phenomenon. Binary CdS or Bi2S3/TiO2 and ternary CdS/Bi2S3/TiO2 nanotube photocatalysts were also prepared for the test of the CO2 reduction to CH3OH under visible light irradiation.658

CH4 as compared to commercial P25 nanopowders. The enhancement of the activity of the hybrid TiO2/ZnO MFFs was ascribed to the following three factors: (1) faster diffusion transport of photogenerated electrons in hybrid TiO2/ZnO than those in pure P25 due to hybridization of low electron mobility TiO2 (0.1−1 cm2/VS) with high electron mobility ZnO (205−300 cm2/VS);649 (2) efficient charge transfer and separation across the TiO2/ZnO interface (heterojunction); and (3) higher specific surface area of TiO2 MFF (225 m2 g−1) ensuring efficient adsorption of CO2 and H2O vapor and providing more active sites in general. A composite of FeTiO3/TiO2 (FTC) heterojunction photocatalyst650 showed photocatalytic activity on selective conversion of CO2 to CH3OH under both visible (λ > 400 nm) and UV−vis (λ > 300 nm) light irradiation. The selective production of CH3OH was interpreted by the analysis of the electronic band structure of FTC. The CB edge of FeTiO3 is about 0.0 V (vs NHE), which is higher than E0 (CO2/CH4) but lower than E 0 (CO2 /CH 3 OH). Therefore, upon light irradiation, photoexcited electrons could reduce the carbonate species generated by the reduction of CO2 to form HCOOH intermediate, followed by further reduction to HCHO and CH3OH. The unique band structure and the junction effect of the two semiconductors as well as the narrow bandgap of FeTiO3 were postulated to be responsible for its high efficiency and selectivity for CH3OH production. Ordered 2D hexagonal mesoporous CeO2−TiO2 composites with high surface area and hierarchical porosity were synthesized through a nanocasting route using ordered mesoporous SBA-15 as the template. Excellent photocatalytic activity in the reduction of CO2 with H2O under simulated solar irradiation was observed.651 It was found by XPS analysis that the addition of CeO2 can greatly enhance the surface chemical adsorption of oxygen species to yield surface oxygen radicals with excellent reduction capability. Furthermore, Ce3+ can interact with holes and prevent the recombination of photogenerated electrons and holes, resulting in a higher quantum efficiency of photocatalytic reaction. 4.3.3. TiO2-Based Multinary Nanocomposite Photocatalysts. To improve the activity for CO2 photoreduction, TiO2-based multinary nanocomposite photocatalysts have also been developed. These includes binary nanocomposites of TiO2 coupled with another semiconductor, ternary nanocomposites of TiO2 with dual cocatalysts loading, and TiO2 modified by doping and loading of two different elements. TiO2 nanocomposite arrays composed of periodically modulated titania nanotube (PMTiNT) platforms with Cu− Pt bimetallic shell coating, which were fabricated by pulse anodization of pre-electropolished titanium substrates in a twoelectrode anodization setup, were reported to be efficient in high-rate sunlight-driven conversion of diluted CO2 and H2O to light hydrocarbons, such as CH4, C2H4, and C2H6.652 The optimized bimetallic composition was found to be Cu0.33− Pt0.67. The high photocatalytic CO2 conversion activity of the Cu−Pt/PMTiNT was believed to be due to increased surface area and improved light absorption by the unique features of the periodic modulated TiO2 nanotube arrays and the bimetallic coating. Because the high purity of the reactant gases is not necessary in this photocatalytic reaction system, the authors claimed that it could potentially be used to convert CO2 captured directly from air or from flue gas. Copper and cerium codoped Cu/Ce−TiO2 was demonstrated to be effective in photoreduction of CO2 with H2O to 10019


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Figure 32. Schematic illustration of the mechanism of photoreduction of CO2 with H2O on the anchored TiO2 proposed by Anpo et al. Reprinted with permission from ref 659. Copyright 1995 Elsevier.

The modification of TiO2 with Bi2S3 or CdS not only can enhance the visible light absorbance, but also form a heterostructure with TiO2 by formation of heterojunctions, which may facilitate transfer and separation of photogenerated charges. It was also shown that the ternary component system has better photocatalytic activity than the binary systems. Artificial photosynthesis via photocatalytic reduction of CO2 using TiO2-based photocatalysts occurs provided that the reaction is only using CO2 and H2O. However, some CO2 reduction reactions reported so far were not conducted with water as the only the electron source but with sacrificial reagents, which are not really the artificial photosynthesis. A criterion of artificial photosynthesis is the evolution of O2 along with the CO2 reduction. It should be pointed out that the evolution of O2 is direct evidence for the solar fuel production from CO2 + H2O reaction.

Ti oxide anchored Y-zeolite prepared by the ion-exchange method showed high selectivity for the formation of CH3OH in the gas phase. XANES and FT-EXAFS study showed that tetrahedral coordinated titanium oxide species were highly dispersed within the Y-zeolite cavities, which might be the reason for its high photocatalytic activity and selectivity for the production of CH3OH. Addition of Pt can largely promote the photocatalytic activity of CH4 formation while decreasing the yield of CH3OH. In comparison, photoreduction of CO2 with H2O by aggregated octahedrally coordinated TiO2 synthesized by impregnation method showed a high selectivity toward production of CH4, being similar to reactions on the powdered TiO2 catalysts. The observed experimental results were interpreted as follows: on the isolated tetrahedral titanium oxide species, the charge transfer excited complexes (Ti3+− O−)* are formed via ligand to metal charge transfer (LMCT) under UV irradiation. Electron transfer from excited Ti3+ to H+ or CO2 generates H atoms or CO and finally carbon radicals; and, simultaneously, electron transfer from the OH− to the trapped hole center of O− will produce OH• radicals. The reaction between these radicals leads to the formation of CH3OH as well as CH4. On the other hand, with the aggregated or bulk TiO2 catalysts, the photogenerated electrons and holes were spatially separated, preventing the reaction between the carbon radicals and OH· radicals on the same active sites, resulting in the preferential formation of CH4 by the reaction of the H atoms and carbon radicals at the electrontrapping center. In the case of Pt loading, the efficient electron transfer from the electron-trapping center Ti3+ to the Pt metals promotes the charge separation, which results in promoting the reaction between the carbon radicals and H atoms on the Pt metals to form CH4 while preventing the reaction between carbon radicals and OH· radicals formed on the different sites to form CH3OH. Similar results were also obtained when zeolites were changed into Ti-MCM-41 and Ti-MCM-48 mesoporous materials.661 Highly dispersed titanium oxide species incorporated in the framework of silica exhibited high and unique photocatalytic reactivity for the reduction of CO2 with H 2 O to produce CH 4 and CH 3 OH under UV irradiation.662 It should be noted that metal doped anatase supported on silica has a detrimental effect on the reduction of CO2 to methanol. Ru doped TiO2 particles have higher photocatalytic activity than undoped TiO2 due to efficient separation of charge carriers by the formation of Schottky barriers between Ru and TiO2. However, the yield increased more notably when TiO2

4.4. TiO2-Based Photocatalysts on Various Supports for CO2 Photoreduction

4.4.1. Silica Materials as Supports. Anpo et al.659 found that the efficiency of CO2 conversion on TiO2 photocatalysts largely depends on the kinds of catalysts, the ratio of CO2 to H2O, and the reaction temperature. Photoreduction of CO2 with H2O on highly dispersed TiO2 nanoparticles anchored on the surface of Vycor glass, finely powdered TiO2, and rutile TiO2 with (100) and (110) exposed facets were subject to investigation under UV light irradiation. Analyses of the gasphase products photocatalytically produced at 275 K showed that CH4, CH3OH, and CO were formed on the anchored TiO2, while only CH4 was found for the powdered TiO2. The powdered anatase-type TiO2 with large bandgap and rich surface −OH groups showed preferential formation of CH4. However, loading of Cu+ on it generated CH3OH instead of CH4, indicating that modification of TiO2 could lead to different CO2 reduction products. Rutile TiO2 with exposed (100) facets exhibited high efficiency for the formation of CH4 and CH 3 OH than TiO 2 with exposed (110) facets. Furthermore, it was found that the tetrahedrally coordinated Ti species was associated with the higher CO2 reduction efficiency on the highly dispersed TiO2 anchored on Vycor glass. On the basis of the detection of Ti3+, ·H, ·C, and ·CH3 radicals by EPR spectroscopy at 77 K, the mechanism of photoreduction of CO2 on anchored TiO2 photocatalysts was proposed as shown in Figure 32. Anpo et al. further studied photoreduction of CO2 with H2O at 328 K using Ti anchored Y-zeolites prepared by ionexchange method and impregnation methods, respectively.660 10020


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Figure 33. Electronic state change in the titanium oxide photocatalysts from semiconducting bulk TiO2 to isolated Ti−oxide molecular species. Reprinted with permission from ref 595. Copyright 2012 Royal Society of Chemistry.

was supported on SiO2.663 This suggests that the efficiency of charge separation in TiO2/SiO2 is more pronounced than Rudoping, which might be due to the strong interaction between silica framework and anatase, probably by the formation of Ti− O−Si bridging bonds as evidenced by FT-IR and Raman spectroscopy. On the contrary, Ru−TiO2/SiO2 has no significant improvement in activity over TiO2/SiO2 except that it shows nearly quadruple times higher activity for the formation of methanol than Ru/TiO2, which can be attributed to the metal inhibiting the formation of the Ti−O−Si bond. Cerium-doped titanium oxide (Ce−TiO2) nanoparticles were also found to be efficient photocatalysts for conversion of CO2 with H2O to CO and CH4 under UV−visible light illumination.664 As compared to pristine TiO2, TiO2 doped by 1% or 3% Ce improved the production of CO by 4 times. The enhancement of the photocatalytic activity was ascribed to the facilitated charge transfer induced by the doped Ce ions, the higher surface area of the catalyst, and the stabilization of anatase phase by Ce doping. Supporting of Ce−TiO2 on the mesoporous silica of SBA-15 can further enhance the CO2 reduction rate. Ce−TiO2/SBA-15 with a Ti:Si ratio of 1:4 demonstrated 8-fold enhancement in CO production and 115fold enhancement in CH4 production. By contrast, amorphous silica as the substrate was much inferior to SBA-15, demonstrating the superior advantage of the mesoporous structure as the support. Cu/TiO2 nanocomposites supported on mesoporous silica were also investigated for photoreduction of CO2 in the presence of water vapor under irradiation with a Xe lamp.665 It was found that CO was the primary product of CO2 reduction for TiO2−SiO2 catalysts without Cu. Addition of Cu [Cu(I) per XPS analysis] led to the increase of the overall CO2 conversion efficiency with CH4 as the main product. At the optimal experimental conditions, the 0.5%Cu/TiO2−SiO2 composite showed a quantum yield of 1.41%. The enhancement of the CO2 photoreduction rate was claimed to be due to the synergistic combination of Cu deposition and high surface area of the SiO2 support (>300 m2/g). Importantly, it was found that O2 will compete with CO2 as electron acceptors, mitigating the CO2 reduction efficiency, evidenced by the observation of no CO2 photoreduction product when the reactor influent gas was changed to a mixture of O2/CO2 (1:1 v/v). Obviously, CO2 photoreduction is not favorable in an O2-rich environment.

The enhancement of the photocatalytic activity of the titanium oxide on silica supports can be understood by the electronic structural changes, from an extended-semiconducting structure to nanosized, extremely small nanoparticles and molecular-sized titanium oxide species (Figure 33).595 In the case of the bulk TiO2 materials, photochemical excitation leads to charge separation with electrons in the CB and holes left in the VB. The generated holes in the VB and the electrons in the CB can react with electron donors and acceptors adsorbed on the surface of titanium oxide, respectively. On the contrary, the Ti oxide moieties are implanted and isolated as “single-site photocatalysts” at the atomic level in the silica matrixes of microporous zeolite and mesoporous silica. These isolated single-site Ti atoms can replace the Si atoms and tetrahedrally coordinate with the surrounding oxygen atoms. UV irradiation of such single-sites brings about LMCT from the oxygen (O22−) to Ti4+ ions, resulting in the formation of pairs of trapped hole centers (O2−) and electron centers (Ti3+). Such a charge transfer excited state, that is, the excited electron−hole pair state, which localizes quite near to each other rather than the electron and hole produced in semiconducting materials, plays a significant role in the enhancement of the photocatalytic activity of the silica supported photocatalysts. Under UV-light irradiation, the reactions on TiO2 powders in the presence of gaseous CO2 and H2O usually produce CH4 as the major product, while on the highly dispersed titanium oxide anchored on porous glass or ordered porous silicas, CH3OH as well as CH4 are usually the major products. First-principles calculations were applied to better understand the role of tetrahedrally coordinated Ti (Ti4) and the quantum confinement (QC) in the CO2 reduction. 666 Calculation results showed that n-type semiconductor of bulk TiO2 is not ideal because excess electrons are thermodynamically driven away from the surface due to upward band bending even if Ti4 atoms are present. However, under-coordinated titanium atoms can significantly facilitate reduction of CO2 to CO and make CO2 anion formation favorable at the surface. Small TiO2 nanoclusters or alternatively localized Ti sites embedded in host materials (such as MCM-41) would be better photocatalysts for CO2 reduction. One may argue that straightforward usage of such TiO2 nanomaterials might not be practical because smaller nanoparticles usually have larger bandgap, which is inefficient for utilizing solar photons. This 10021


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exfoliated graphene sheets during the reduction process. Such assembled graphene−TiO2 hybrid nanosheet system shows efficient photocatalytic activity for the conversion of CO2 to CH4 and C2H6 in the presence of water vapor. In contrast with the previous graphene−Ti0.91O2 nanocomposites, abundant Ti3+ was detected on the surface of TiO2 in the hybrid nanosheet system, caused by reducing agent En. The Ti3+ on the surface can serve as sites for trapping photogenerated electrons and prevent recombination of electron−hole pairs. The authors concluded that the synergistic effect of the surfaceTi3+ sites and graphene favors generation of C2H6, and the yield of the C2H6 increases with the increase of the amount of the incorporated graphene. The longer lifetime and mean free path of electrons on graphene imply that the energetic electrons will cover a larger area of the graphene surface, thereby increasing the likelihood of interaction with adsorbed reactants (H2O and CO2). Abundance of Ti3+ sites on the surface facilitate the coupling of the photoformed ·CH3 radicals into C2H6. For the graphene−TiO2 systems, ·CH3 radicals may be adsorbed on the surface of graphene via π-conjugation between the unpaired electron of the radical and aromatic regions of the graphene. The electron-rich graphene may help to stabilize the ·CH3 species, which restrains the combination of ·CH3 with H+ and e− into CH4. Meanwhile, subsequent increased accumulation of ·CH3 on the graphene raises the opportunity for the C2H6 formation by coupling with another ·CH3 instead of H+. Many other materials were also employed as supports for TiO2-based photocatalysts,676−680 and the photocatalytic conversion of CO2 varies with the supports. Different supports might result in different morphologies and particle size of TiO2, or different interactions between TiO2 and the supports, leading to varied activity.

could be overcome by other strategies, such as doping the nanomaterials to reduce the energy gap significantly. 4.4.2. Carbon-Based Materials as Supports. Wellordered mesoporous graphitic carbons (OMGC) with high specific surface area synthesized using natural seed fat as a carbon precursor were used as the support of TiO2.667 The TiO2/OMGC photocatalyst showed better performance in conversion of CO2 with H2O to CO and CH4 as compared to the other photocatalysts, such as pure TiO2 and TiO2/CMK-3 (amorphous mesoporous carbon). The excellent photocatalytic activity of TiO2/OMGC was attributed to the high electron conductivity in the ordered graphitic carbon materials. Recently, it was also shown that a hierarchically mesostructured TiO2/graphitic carbon composite (meso-TiO2/GC) is an effective photocatalyst for photoreduction of CO2 with H2O into CH4 and CO under simulated solar irradiation.668 The photocatalytic performance of the meso-TiO2/GC is much higher than that of the TiO2/SiO2 composite and commercial Degussa P25 titania. The high specific surface area and surface concentration of hydroxyl groups were accountable for the better performance of the meso-TiO2/GC. Multiwalled carbon nanotubes (MWCNTs) supported with anatase and rutile were synthesized by sol−gel and hydrothermal method, respectively. The main product of photoreduction of CO2 with H2O under UV light irradiation was quite different on these two composite photocatalysts. Anatase/ MWCNT composite catalysts mainly led to the formation of C2H5OH, while rutile/MWCNT led to the formation of HCOOH. The advantage of using MWCNT as support is that it can mitigate the agglomeration of the TiO2 particles and more efficiently transport the photogenerated electron−hole pairs along the tubes, so as to decrease the recombination rate of the e−/h+ pairs and thus improve the photocatalytic activity.669 Graphene, a 2D honeycomb-like network of carbon atoms, has also been used as a support for TiO2 to increase the photocatalytic activity. Graphene is widely recognized as an efficient electron collector and transporter to prolong the lifetime of the photogenerated charge carriers.670,671 Particularly, the conducting band of TiO2 is ca. −4.21 eV,672 and the work function of graphene is ca. 4.42 eV,673 which may lead to efficient charge separation by transferring photogenerated electrons from TiO2 to graphene if they are properly coupled. Zou et al. reported that graphene−Ti0.91O2 hollow spheres consisting of molecular-scale alternating titania and graphene nanosheets exhibited 9 times higher photocatalytic activity for CO2 reduction to CO and CH4 relative to the commercial P25.674 The enhancement of the photocatalytic activity was ascribed to the following three factors. (1) The ultrathin nature of Ti0.91O2 nanosheets allows charge carriers to move rapidly onto the surface to participate in the surface reduction reaction. (2) The sufficiently compact stacking of ultrathin Ti0.91O2 nanosheets with graphene nanosheets allows fast transfer of the photogenerated electrons from Ti0.91O2 nanosheets to graphene, which may enhance the lifetime of charge carriers. (3) Efficient light collection by the hollow structured hybrid system that serves as photon trap allows multiscattering of the incident light. Zou et al. also demonstrated a novel in situ simultaneous reduction-hydrolysis technique (SRH) for fabrication of 2D sandwich-like graphene−TiO2 hybrid nanosheets in a binary ethylenediamine (En)/H2O solvent.675 In this procedure, the dispersion of TiO2 hinders the collapse and restacking of

4.5. Visible Light-Responsive TiO2-Based Photocatalysts and Photocatalytic Systems for CO2 Photoreduction

4.5.1. Anion Doped TiO2-Based Photocatalysts. Several anion doped TiO2-based photocatalysts were found to be effective photocatalysts for CO2 photoreduction. Li et al.681 synthesized 5%, 10%, and 15% iodine doped anatase−brookite TiO2 composite. XPS analysis revealed that I5+ substituted Ti4+ in the lattice, and, as a result, Ti3+ was generated to balance the charge. The 10% I-doped TiO2 composite showed the highest CO2 photoreduction rate under visible light, while the 5% Idoped composite showed the best photocatalytic activity under UV−vis irradiation. Too high iodine doping may generate recombination centers and thus lower the photocatalytic activity. The enhancement of the photocatalytic activity of anatase−brookite composites as compared to those of the individual anatase and brookite was ascribed to the improved visible light absorption caused by the iodine doping and the efficient charge transfer and separation across the anatase− brookite junctions. Zhao et al.682 prepared N-doped TiO2 nanotubes (N-TNTs) by hydrothermal method. The as-synthesized nitrogen doped N-TiO2 powder was a mixture of anatase and rutile, while the N-TNTs that were subject to postheat treatment at 500 °C were completely anatase. Because of the substitution of lattice oxygen by N atoms, N-doped TiO2 showed efficient visible light absorption, and markedly improved visible light photocatalytic activities for CO2 reduction to yield HCOOH, HCHO, CH3OH, etc. Ling et al.683 synthesized visible light absorbing N and C doped TiO2 nanoparticles with controlled crystalline structure 10022


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A dye molecule can capture the energy of visible light to form singlet or triplet states (dye*). Electron(s) from these excited states of the dye can be injected into the CB of TiO2 and then transfer to CO2 adsorbed on TiO2 surface to be reduced. Charge injection to TiO2 promotes the separation of electrons and holes and enhances the efficiency of CO2 photoreduction.684 Tailor design and synthesis of low-cost, environmentally benign, and earth-abundant metals complexes for CO2 photoreduction is highly desirable. Cu, Fe, and Cu/Fe doped TiO2 catalysts sensitized with a full light absorption Ru II (2,2′-bipyridyl-4,4′-dicarboxylate) 2 (NCS)2 dye (N3 dye) were reported to be able to photoreduce CO2 with H2O under concentrated natural sunlight in an optical-fiber photoreactor.685 Production rates of methane and ethylene were similar over both Cu(0.5 wt %)−Fe(0.5 wt %)/TiO2 and N3-dye−Cu(0.5 wt %)−Fe(0.5 wt %)/TiO2 catalysts coated on optical fibers under artificial light irradiation. This implies that N3 dye is not an effective dye for the activity enhancement when the catalysts are coated on the glass fiber. Interestingly, N3 dye was observed to increase the photoactivity of Cu(0.5 wt %)−Fe(0.5 wt %)/TiO2 catalyst toward CH4 production under concentrated natural sunlight, demonstrating that the intensity of the light also greatly matters in the photocatalytic activity and selectivity of the products. RuBpy (tris-(2,2′-bipyridyl) ruthenium(II) chloride), BrGly (1,7-dibromo-N,N′-(carboxymethyl)-3,4:9,10-perylene-diimide), and BrAsp (1,7-dibromo-N,N′-(1S,2-dicarboxyethyl)3,4:9,10-perylenediimide) were reported to be efficient visible light absorbing dyes that can drive photoreduction of CO2 to CH4 in the presence of water if they were coupled with Pt loaded TiO2 thin or thick films.686 In addition to methane production, H2 evolution was also observed. The performances of the dyes for CO2 photoreduction were in the following order: RuBpy > BrAsp > BrGly. Various kinds of supramolecular complexes of zinc(II)/ copper(II)/cobalt(II) porphyrin-ruthenium(II) polypyridly (Figure 35)687 were also used to sensitize TiO2 nanotubes (TNT) for photoreduction of CO2 in aqueous solution under UV−visible light irradiation. It was found that the nature of the

and morphology by a facile hydrothermal method using titanium oxalate complex as Ti precursor. The synthesized anatase and rutile catalysts showed visible light absorption in the range of 400−600 nm, whereas the anatase−brookite composite displayed strong absorption up to 700 nm. The absorption intensity in the visible light region increases in the order anatase < rutile < anatase−brookite. Although all of the synthesized samples showed photocatalytic activity for CO2 reduction to CH3OH under both UV−vis and visible light irradiation, the bicrystalline anatase−brookite composites afforded the maximum CH3OH yield. The mechanism of the enhancement of the photocatalytic activity is depicted in Figure 34. The junction between two crystallites is crucial for efficient charge transfer and separation.

Figure 34. Schematic diagram illustrating the charge transfer between two crystalline phases for efficient CO2 reduction. Reprinted with permission from ref 683. Copyright 2012 Elsevier.

4.5.2. Dyes and Molecular Complexes-Sensitized TiO2-Based Photocatalysts. Because of the limited availability of the visible light absorbing semiconductors with suitable band structures for CO2 reduction, using dyes as light harvester to sensitize semiconductor is another option to drive photocatalytic reactions with visible light. Furthermore, dye photosensitization has been proved to be an inexpensive and effective way to extend the light absorption spectrum of TiO2.

Figure 35. Molecular structure of the supramolecular species constituted by meso-tetrapyridylporphyrin zinc(II)/copper(II)/cobalt(II) coordinated to four [Ru(bipy)2Cl]+ (bipy = 2,2′-bipyridine)/four [Ru(phen)2Cl]+ (phen = 1,10-phenanthroline)/four [Ru(bidypy)2Cl]+ (bidypy = 2,2′bipyridyl-4,4′-dicarboxylate). Reprinted with permission from ref 687. Copyright 2012 Elsevier. 10023


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rather than CO2−HCOO−/HCOOH−CH3OH−CH4 processes, because addition of the key intermediates CH3OH or HCHO in the reaction system did not show any influence on the yield of CH4. Zou et al.690 synthesized a Cu(I) complex dye (Figure 36) for TiO2 sensitization. Upon visible light irradiation

coordinated central metals in porphyrin and peripheral polypyridly ligands has a great influence on the efficiency of hybrid photocatalytic systems. The photocatalytic activity, which was determined according to the methanol yield, decreased in the following order for ZnPyP-Ru(II)polypyridly TNT photocatalysts: ZnPyP-RuBiDipy-TNT > ZnPyP-RuPhen-TNT > ZnPyP-RuBiPy-TNT. Cu(II) or Co(II) porphyrin-Ru(II) polypyridly TNT showed the same photoreduction rule. The different photocatalytic activities of these sensitizers are mainly due to the difference of the peripheral Ru(II) polypyridly ligands. Because of the better conjugated structure and electron flowing capability, phenanthroline complex is more readily excited than the bipyridyl complex. The higher intramolecular energy and electron transfer efficiency of Zn/ Cu/CoPyP-RuPhen-TNT than that of Zn/Cu/CoPyP-RuBiPyTNT led to higher photocatalytic activity of the former. Also, it was found that Zn/Cu/CoPyP-RuBiDipy is a more effective photosensitizer than Zn/Cu/CoPyP-RuBiPy and Zn/Cu/ CoPyP-RuPhen, probably due to the formation of chemical bonding between the photosensitizer and TiO2 in the former, while the latters were simply physically adsorbed. The difference of the coordinated metal ions in the porphyrin ring also influences the photoreduction efficiency of the sensitizedTNT catalysts because the photocatalytic activity decreased in the following order for Zn(II)/Cu(II)/Co(II) PyP-Ru(II)BiDipy-TNT photocatalysts: ZnPyP-RuBiDipy-TNT > CuPyPRuBi Dipy-TNT > CoPyPRuBiDipy-TNT. No sensitizing effects were observed for Fe(III)/Mn(III) porphyrin-Ru(II) polypyridly, indicating that selection of suitable redox-active metal center is also vitally important. Zou et al. synthesized a zwitterionic cyclometalated iridium(III) complex [Ir(4-CF3bt)2(Hbpdc) and used it to sensitize TiO2 for CO2 photoreduction in the presence of water under visible-light irradiation (λ > 420 nm).688 It was found that the complex itself can give effective impetus for selective reduction of CO2 to CO due to that the excited state of the complex can donate electrons to the bound CO2 through a two-electron transfer process. However, when it was coupled with P25, products of CO and CH4 with a ratio of about 1:2 were detected. Direct CO2 reduction to CH4 is usually difficult due to the requirement of eight electron transfer, although the reduction potential of CO2/CH4 (−0.24 V vs NHE) is less negative than that of CO2/CO (−0.46 V vs NHE). It was proposed that TiO2 plays an important role in increasing the probability of a multielectron reaction through carrier-bound intermediates for the formation of CH4. Loading of Pt (ca. 3 wt %) can further improve the yield of CO and CH4 with a ratio of about 1:20, implying that the product selectivity shifts from CO to CH4. The enhancement of the CO2 photoreduction activity with preference of CH4 formation was ascribed due to efficient charge separation by the Pt particles acting as charge-carrier traps. Cobalt phthalocyanine (CoPc) loaded titania (ca. 72% of anatase and 28% of rutile) with uniform and average diameters (less than 20 nm) was found highly active toward photoreduction of CO2 in NaOH solution under tungsten−halogen lamp irradiation.689 The favorable products include formic acid, formaldehyde, methanol, methane, and carbon monoxide. High efficiency of CoPc-TiO2 for CO2 reduction was attributed to the charge injection from photoconductive CoPc to TiO2 under irradiation, which greatly inhibits the recombination of photogenerated electron−hole pairs. It was suggested that the formation of methane was through a ·C generation mechanism

Figure 36. Chemical structure of the cationic copper(I) complex. Reprinted with permission from ref 690. Copyright 2012 Royal Society of Chemistry.

(λ > 420 nm), photoreduction of CO2 to CH4 was observed in the presence of water vapor. Although the detailed mechanism was not fully postulated, the superior performance of the copper complexes was believed to be due to the modification of the key structural and electronic features of the complex that sustain efficient CO2 reduction. Zinc phthalocyanine ZnPc/TiO2 hybrid system prepared by microwave-assisted method was demonstrated to be effective in CO2 reduction to methanol in aqueous solution under simulated solar light irradiation.691 The speculated mechanism of the dye-sensitized TiO2 for CO2 photoreduction is depicted as shown in Figure 37. The phthalocyanines in the solid state

Figure 37. Schematic illustration of CO2 photoreduction with ZnPc/ TiO2 catalyst. Reprinted with permission from ref 691. Copyright 2012 Elsevier.

behave as p-type semiconductors with a “bandgap” of ca. 2 eV. Under visible light irradiation, the photogenerated electrons (e−) and holes (h+) are produced in ZnPc/TiO2. The excited phthalocyanine can generate 1O2 through energy transfer. At the same time, the excited electrons on phthalocyanine can be rapidly injected into the CB of TiO2, generating efficient charge separation by transiently localizing electrons and holes on different components. The excited electrons then may further transfer to the active sites of TiO2 where CO2 is reduced to CH3OH. In the early literature, photoreduction of CO2 on the surface of TiO2 was proposed to proceed typically via a thermodynami10024


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cally uphill one-electron transfer to form high energy CO2•− intermediate (E = −1.90 V vs SHE in water, pH 7)692 at the initial reaction stage. Armstrong et al. reported that TiO2 nanoparticles (NPs) modified with photosensitizer and CO2reducing enzyme CODH I (carbon monoxide dehydrogenase, which is from the anaerobic microbe carboxydothermus hydrogenoformans (Ch)) exhibited an extraordinary catalytic activity for CO2 photoreduction.693 CODH I is one of the five CODH complexes with an unusual [Ni4Fe−4S] active site that catalyzes the reversible oxidation of CO to CO2. Yet in nature, it is a highly active catalyst in both directions (oxidation and reduction).694 This enzyme can bypass the thermodynamically uphill one-electron radical pathway, and CO2 reduction can proceed in a controlled, two-electron reduction path to give CO (E = −0.46 V vs SHE at pH 6) as a clean product. The results indicate that activation of CO2 by a two-electron pathway is energetically more preferable for CO2 reduction. Armstrong et al. also reported photoreduction of CO2 using Ni-containing CODH and [NiFeSe]-hydrogenase, which were attached to TiO2.695 Because the CB potential of anatase (ca. −0.52 V vs SHE, pH 6) is more negative than the equilibrium potential of H+/H2 (−0.36 V vs SHE) but close to that of CO2/ CO (−0.52 V vs SHE, pH = 7), the dye-sensitized TiO2 modified by these enzymes showed excellent activity for H+ and CO2 reduction at applied potentials from −0.45 to −0.55 V vs SHE under neutral aqueous conditions. The CODH and hydrogenase are all reversible electrocatalysts, which may strongly bias the direction of electrocatalysis of CO2/CO and H+/H2 interconversions. Upon being attached to graphite electrodes, these enzymes showed high activities for both oxidation and reduction. However, when they were attached to n-type TiO2 or CdS semiconductor electrodes, a marked shift in bias in favor of CO2 or H+ reduction was observed. Because a reversible electrocatalytic reaction changes direction across a small potential region, the effect of a change in carrier density across this region should be particularly influential. Therefore, the authors further compared the reversibility of H+/H2 and CO2/CO interconversion by hydrogenase and CODH at a metallic-like graphite electrode and at TiO2 and CdS electrodes (n-type semiconductors). It was demonstrated that the semiconducting electrodes can be used to impose directionality on reversible catalysts that operate in the region of the flatband potential, and the catalytic direction can be shifted in favor of reduction over oxidation. As shown in Figure 38, a kind of accumulation layer can be formed at the surface of CdS or TiO2 by the increased electron density at the surface when Eappl is lowered relative to EFB. The increased electron density and the subsequent downward band bending may facilitate electron transfer to the enzyme active site via FeS clusters to catalyze the H2 production or the CO2 reduction. As suggested by the authors, this principle of rectifying catalysis may provide further design criteria for artificial systems that can efficiently convert light energy into storable chemical energy.696 Photoelectrochemical (PEC) method seems to be another ideal option for CO2 reduction because reduction of CO2 and oxidation of water to evolve O2 can take place on different catalysts/solutions separated by special H+ permeable membrane. Such a redox separated system can also efficiently avoid CO2 reduction products from being reoxidized as in the powdered photocatalyst system. Furthermore, the yields of reduction products can be enhanced remarkably by applying the external electrical power. Using a bifunctionalized TiO2 film containing a dye-sensitized zone and a catalysis zone under

Figure 38. Schematic representation of the formation of an accumulation layer at the surface of CdS or TiO2 due to the increased electron density at the surface when Eappl is lowered relative to EFB. The increased electron density and subsequent downward band bending facilitate efficient electron transfer to the enzyme active site via FeS clusters to catalyze H2 production or CO2 reduction. The term EF is the Fermi energy level of the semiconductor. Reprinted with permission from ref 696. Copyright 2013 American Chemical Society.

visible light irradiation, Xue et al.697 realized efficient conversion of CO2 to formic acid, formaldehyde, and methanol. The dye used was bis(2,2-bipyridyl-4,4-dicarboxylato)ruthenium(II) (N719), which is one of the best photosensitizers used in mesoporous TiO2-based dye-sensitized solar cells (DSCs). This work demonstrates that the PEC method may be considered as a highly promising strategy for conversion of CO2 to solar fuels. Concurrent photoelectrochemical (PEC) reduction of CO2 and oxidation of methyl Orange (MO) were reported on a nitrogen-doped TiO2 thin film (NTTF) photoanode.698 Under a 100 W xenon lamp illumination, the onset potential of the total current was approximately 1.5 V (vs SCE), and the maximum total current was around 0.65 mA at 2 V (vs SCE). On the Cu counter electrode in KHCO3 electrolyte, the CO2 reduction products were detected to be formic acid, formaldehyde, methanol, and methane with corresponding maximum Faradaic efficiencies of 5.01%, 1.04%, 5.41%, and 7.83%, respectively. Furthermore, it was found that the methanol in electrolyte can enhance CO2 solubility, thereby suppressing H2 generation and favoring CO2 reduction. Sato et al.699 systematically studied photoelectrochemical conversion of CO2 to HCOO− (formate) using a series of metal complex electrocatalysts (MCE) functionalized semiconductors (SC) as hybrid CO2 reduction electrodes. The semiconductors studied include p-type gallium phosphide (GaP), indium phosphide (InP), and n-type nitrogen-doped tantalum pentoxide (N-Ta2O5). The best combination was screened using a three-electrode configuration system at applied potential of −0.4 V (vs Ag/AgCl) and under a xenon light irradiation (λ > 400 nm). The combination of InP/ [MCE2-A+MCE4] (MCE2-A and MCE4 represent [Ru(4,4′diphosphate ethyl-2,2′-bipyridine)(CO)2Cl2] and [Ru{4,4′di(1H-pyrrolyl-3-propyl carbonate)-2,2′-bipyridine}(CO)(MeCN)Cl2], respectively) showed the best performance by giving the highest formate generation rate (4.71 HCOO−/μmol cm−2) and formate generation efficiency (ca. 80%). A kind of Zscheme system for CO2 photoreduction was also constructed by coupling of platinum-loaded anatase TiO2 on conducting glass (TiO2/Pt) with InP/[MCE2-A+MCE4] (Figure 39). Because of the −0.5 V potential difference between ECBM of 10025


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Figure 39. Z-scheme system for CO2 reduction. Reprinted with permission from ref 699. Copyright 2011 American Chemical Society.

TiO2 and EVBM of InP, efficient electron transfer between these TiO2 and InP occurred without applying external electrical bias. A two-electrode configuration was used in a two-compartment Pyrex cell separated with a proton exchange membrane. Under solar simulator light irradiation for 24 h, formate formation with a turnover number of 17 and efficiency of 70% was achieved. The calculated total solar energy conversion efficiency was ca. 0.04%. The study of charge transfer between adsorbed molecules (such as dyes or other molecular sensitizers, etc.) and semiconductor is crucial for better understanding CO 2 reduction. Two mechanisms are usually considered for charge injection from an adsorbed molecule into the CB of a semiconductor: (i) excitation of electrons from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) of the molecule with subsequent injection into the CB of the coupled semiconductor, and (ii) direct excitation of electrons from the HOMO to the CB, that is, excitation of a charge transfer (CT) state. Both routes can lead to efficient electron and hole separation for the following catalytic reactions, although the latter has not been well studied. Chemical participation of carbon dioxide in a CT state was successfully induced by Mujica et al.700 They synthesized TiO2 nanoparticles with 3-aminosalicylic acid (3ASA). In such a system, the hydroxyl oxygens coordinate in a bidentate form to undercoordinated Ti atoms at the surface, and the CO2 molecules bind on Ti sites in a carbonate form. Identification of a surface Ti-centered radical in the vicinity of CO2 by EPR (Figure 40) suggests the occurrence of charge transfer from the sensitizer to the neighboring site of CO2. This demonstrates that activation of CO2 is through chemical involvement of CO2 with 3-aminosalycilic acid (3ASA) on the surface of TiO2 nanoparticles via CT state. Photoreduction of CO2 with H2O to CH4 was also observed on AgBr/TiO2 photocatalyst under visible light (λ > 420 nm) irradiation.701 The high efficiency of AgBr/TiO2 in CO2 photoreduction was attributed to its strong absorption of visible light. In addition, efficient transfer of photoexcited electrons from the CB of AgBr to that of TiO2 is also accountable for the enhancement of the photocatalytic activity, as supported by no observation of the metallic Ag formation on the catalyst after irradiation. 4.5.3. Quantum Dots-Sensitized TiO2-Based Photocatalysts. Quantum dot (QD) sensitization of TiO2 proved to be another alternative method for efficient utilization of solar energy. It was reported that PbS QDs-sensitized TiO2 can lead to QD size dependent photoreduction of CO2 to CH4 under a

Figure 40. CW-EPR of TiO2−3ASA with and without CO2 irradiated at 77 K for 40 min (top). Binding geometry and electronic levels of the states involved in the system (bottom). Reprinted with permission from ref 700. Copyright 2013 American Chemical Society.

broad band light illumination (420−610 nm). The CO2 photoreduction rate with PbS QDs-sensitized TiO2 was ca. 5 time higher than that of the unsensitized TiO2.702 The advantage of using PbS QDs as TiO2 photosensitizer lies in various ways, including small near IR bandgaps of PbS QDs, efficient carrier multiplication, fast electron injection rates (∼1 ns to TiO2),703 and multiple exciton collection facilitated by the strong electronic coupling between PbS and TiO2.704 These features of TiO2 QD sensitization may lead to the enhancement of CO2 reduction activity. Furthermore, the appropriate band alignment between small PbS QDs and TiO2 (Figure 41) may favor charge separation across the interface, thereby eliminating inefficiencies associated with direct carrier recombination. CdSe quantum dot (QD)-sensitized TiO2 heterostructures were reported to be capable of catalyzing the photoreduction of CO2 in the presence of water under visible light illumination (λ > 420 nm).705 The primary reaction product was CH4, with CH3OH, H2, and CO as the secondary products. Although the CB of bulk CdSe is only slightly above that of TiO2, the quantum confinement shifts the CB of CdSe QDs to higher energies, which facilitates charge injection into TiO2 (Figure 42). Application of QDs to sensitize TiO2 can give CO2 photoreduction activity under visible light irradiation. Yet the QDs are usually not stable. They usually deactivate after prolonged photoexcitation. If the stability issue could be overcome using the existence of emerging nanoscience technology, the QDs sensitization might be an attractive approach for CO2 photoreduction. 4.6. Impact of Reaction Conditions on CO2 Photoreduction

Effects of pressure on CO2 photoreduction in aqueous solution were studied by Mizuno et al.706 Increasing the CO2 pressure can significantly accelerate CO2 reduction rates both in water and in caustic solution. The CO2 reduction products were mainly acetic acid and alcohols in the liquid phase, and methane, ethane, and ethylene as the minor products in the gas phase. The acceleration of the reaction rate toward production 10026


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reactions were dominated by CO2 reductions. The majority of the protons derived from water were either oxidized by the photogenerated holes or consumed by the transient intermediates of CO2 to produce hydrocarbons or carbohydrates. Photoreduction of liquid CO2 was also studied by Kaneco et al. in a pressure range of 5.0−8.0 MPa and temperature range of 273−298 K.707 Formic acid was found to be the exclusive product, which was proposed to be formed by the protonation of the CO2•− radicals. In the presence of isopropyl alcohol as a hole scavenger, photoreduction of high pressure CO2 (up to 2.8 MPa) resulted in the formation of methane (CO2 methanation) as the main product with energy conversion efficiency of ca. 0.0065%. The authors suggested that high CO2 pressure may accelerate the formation of ·C radicals during photoirradiation, leading to the preferential formation of methane in the presence of isopropyl alcohol in solution.708 The photoreduction of supercritical fluid CO2 was also investigated using TiO2 powder photocatalyst in a stainless steel vessel at 9.0 MPa and 35 °C.709 Again, formic acid was the only product in aqueous solution, while no gaseous reduction products were observed. Addition of acid favors the formic acid formation, which was supposed to be generated through the protonation of reaction intermediates on TiO2 powders in solutions. Solvent effects on photoreduction of CO 2 on TiO 2 nanocrystals embedded in SiO2 matrices were studied by Yoneyama et al.710 It was found that the major CO2 reduction products were formate and carbon monoxide, and the yields increased with the increase of the CO2 concentration. Interestingly, the ratio of formate and carbon monoxide was greatly influenced by the kind of the solvent used. Formation of formate was preferred in the solvent of high dielectric constant. A similar trend was observed on the TiO2 without SiO2 matrixes support, although the total yields of the products were relatively low. The solvent effect was illustrated using the adsorption capability of CO2•− anion radicals on the surface of TiO2. In low polar solvents such as CCl4 and CH2Cl2, the photoreduced CO2•− anion radicals may be strongly adsorbed on Ti sites because the anion radicals are not highly solvated in low polarity solvents. This causes the negative charges of one oxygen atom of CO2•− anion radicals to increase, leading to ready formation of CO by removal of the oxygen atom by a proton. On the other hand, if solvents of high dielectric constant such as water and propylene carbonate are used, the CO2•− anion radicals may be greatly stabilized by the solvents instead of strong interaction with TiO2, leading to its direct reaction with a proton to give formate (Figure 43).

Figure 41. (a) Band alignment between TiO2 and PbS QDs. The position of TiO2 CB (−0.5 V) is displayed vs NHE at pH = 7. (b) High-resolution TEM image of 4 nm PbS QD on a TiO2 particle in PbS/Cu/TiO2 photocatalysts. (c) Diffuse reflectance UV−vis spectra of Cu loaded TiO2 catalysts and three different sized PbS QDsensitized Cu/TiO2 catalysts. The spectra are shifted vertically for clarity. Reprinted with permission from ref 702. Copyright 2011 Royal Society of Chemistry.

Figure 42. Band alignment of bulk CdSe and 2.5 nm CdSe QDs with TiO2 and relevant redox potentials of CO2 and H2O. Reprinted with permission from ref 705. Copyright 2010 American Chemical Society.

of the liquid fuels upon increasing the CO2 pressure was ascribed to the increased CO2 availability on TiO2 surface. A similar pressure effect was found when CO2 photoreduction reactions were carried out in NaOH caustic media. Under pressurized CO2 conditions (up to 2.5 MPa), the exclusive yields of methane in the gas phase and ethanol and acetaldehyde in the liquid phase were observed. The enhancement of the CO2 photoreduction activity to both gaseous and liquid products was ascribed to the enrichment of CO2 in the caustic solution. Furthermore, the competing H2O reduction reaction generating H2 was largely suppressed under high pressure. Photochemical reduction of water proceeded only in the early stage of photoirradiation, and then the reduction

Figure 43. Proposed solvents effects on the photoreduction of CO2. Reprinted with permission from ref 710. Copyright 1997 Elsevier. 10027


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utive electron/proton transfer may lead to the formation of methoxyl radical, depending on the experimental conditions (Figure 44). The roles of water include: (i) stabilization of

Overall, various kinds of strategies have been applied for CO2 photoreduction on TiO. (1) Mixed-phase TiO2 composites of anatase/rutile, anatase/brookite showed improved photocatalytic activities due to the formation of phase junctions, which can facilitate charge separation between different phases. (2) Crystal facet engineering of TiO2, especially tuning the {010}, {001}, {100}, {101} facets of anatase, has been proved to be an effective strategy for creating active sites for CO2 photoreduction with or without cocatalysts. (3) Noble metals such as Pt, Au, Ag, Pd, and In have been applied as CO2 reduction cocatalyst on the surface of TiO2. (4) Binary TiO2based nanocomposites loaded with metal oxides such as Cu2O, MgO, ZnO, Fe2O3, NiO, Cr2O3, CeO2 have been proved to increase the charge separation efficiency and enhance the activity of CO2 photoreduction. (5) More complicated multinary TiO2 nanocomposites, such as Pt@Ga2O/TiO2, CdS/Bi2S3/TiO2, Cu−Pt/TiO2. Cu−Ce/TiO2, Mn−Cu/TiO2, Cu−Ga/TiO2, Cu−I/TiO2, have been constructed for CO2 photoreduction. (6) TiO2 also has been dispersed on various kinds of supports for CO2 photoreduction. These supports include Vycor glass, Y-zeolites, Ti-MCM-41, Ti-MCM-48, SBA15, mesoporous graphite carbon, multiwalled carbon nanotubes, and graphene. (7) To use visible light for photocatalytic reduction of CO2, cation (Fe, Cu) or anion (I, N, and N−C) doping, dye and molecular complexes sensitization, and quantum dot (PbS and CdSe) sensitization have been applied. Although all of these strategies showed some improvement of the CO2 photoreduction activity as compared to the bare TiO2, the energy conversion efficiency and selectivity of CO2 reduction are still quite limited. Novel strategies for the development of efficient CO2 photoreduction catalyst are still in high demand.

Figure 44. Proposed mechanism of photocatalytic transformation of CO2 to methoxyl radical over TiO2 in the presence of dissociated/ bound water. Reprinted with permission from ref 711. Copyright 2011 American Chemical Society.

charges (preventing electron−hole recombination), (ii) acting as an electron donor (reaction of water with photogenerated holes to give OH radicals), and (iii) acting as an electron acceptor (formation of H atoms in a reaction of photogenerated electrons with protons on the surface, −OH2+). Dissolved CO2 in the form of carbonates/bicarbonates competes with water for scavenging the photogenerated holes. Direct observation of reaction intermediates, such as H atoms, ·OCH3 radicals, and ·CH3 radicals, suggests a concerted two-electron, one-proton transfer to adsorbed carbon dioxide molecules. The reaction pathways of the three products (formic acid, formaldehyde, and methanol) involved in the generally postulated stepwise reduction of CO2 to CH4 on the surface of anatase TiO2 nanoparticles were explored using EPR and transient absorption spectroscopy.712 The EPR results suggest that formaldehyde and methanol may undergo photooxidation but not one-electron photoreduction reaction, indicating that they mainly act as hole scavengers. However, formic acid can serve as both the hole and the electron acceptor, yielding the protonated radical anions (OC·OH) and formyl radicals, respectively. This indicates that formic acid could be the potential initial intermediate product for the subsequent methane formation. Furthermore, the ultrafast dynamics of hole scavenging was found to be an order of magnitude faster on the surface of TiO2 than that in the corresponding homogeneous systems. Additionally, the equilibrium constant for the reaction of photogenerated electrons in TiO2 with adsorbed CO2 was estimated to be less than 3.2 M−1, the value of which is irrelevant to the presence of the hole scavengers and the product molecules. This demonstrates that the first step of CO2 reduction sequence, that is, the reduction of the adsorbed CO2 to CO2•−, does not depend on the nature of the sacrificial hole scavenger; that is, the efficiency of CO2 reduction is independent of the hole scavenger. Because of the small driving force of the electron transfer to CO2 (ΔG0 ≥ 0.69 kcal/mol, or the driving force of one electron reaction η ≤ 30 mV), back electron transfer process is usually the favorable reaction in the initial stage of CO2 reduction on titania, which is the main reaction for low efficiency of TiO2 photocatalyst. Formation of CO2− on the surface of TiO2 is believed to be the first key step in CO2 reduction,713 as confirmed experimentally714 by the observation of the CO2− radical formation on the irradiated undoped TiO2 using in situ IR spectroscopy and theoretically715 in determining the binding configurations of CO2 on the surface of anatase TiO2. To further understand the superior photocatalytic activity of brookite on CO2 reduction, density functional theory (DFT) calculations had been performed on the brookite (210) and

4.7. Mechanism of CO2 Photoreduction on TiO2-Based Photocatalysts

The mechanism involved in the formation of negatively charged CO2 species on TiO2 surfaces is crucial for us to better understand the complicated CO2 photoreduction processes, which may subsequently guide us to design more effective and selective photocatalysts for efficient CO2 conversion. Here, we give a brief review on the study of reduction mechanisms that appeared after 2009, because a very comprehensive review of much earlier work has been already published.592 The reduction of CO2 to CH4 is a multistep eight-electron transfer process. In the photocatalytic reaction of converting CO2 with H2O to CH4 on TiO2, the electrons are provided by photoexcitation of TiO2 (CB electrons), while water acts as both a proton donor and an electron donor. In this process, water and photogenerated carbonates play important roles. By means of electron paramagnetic resonance (EPR) technique and first-principles calculations, the multiple roles of water and carbonates in the overall photoreduction of CO2 to CH4 on titania nanoparticles were elucidated by Dimitrijevic et al.711 Simultaneous generation of H atoms and CH3 radicals was observed upon illumination of P25 in the presence of CO2 and H2O in a 1:1 molar ratio. This implies that both of the competitive processes, that is, electron transfer to adsorbed/ bound carbon dioxide and to adsorbed/bound protons on the surface of TiO2, occurred. The initial electron transfer is postulated to be the breach of the OCO double bond of CO2 and attachment of a H atom, resulting in the formation of formate, CO2 + 2e− + H+ → HCOO−. This postulation was further supported by the first-principles calculations. Consec10028


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anatase (101) surfaces.716 It turned out that both the anatase (101) and the brookite (210) surfaces have blocks consisting of exposed 5- and 6-coordinated Ti atoms and 2- and 3coordinated O atoms. The brookite surface has slightly shorter interatomic distances and a different block arrangement, resulting in the stronger CO2 adsorption ability of brookite (210) surface to create active sites for CO2 reduction. The interactions of CO2 with the {210} surface of brookite TiO2 were further studied using first-principle calculations on cluster and periodic slab systems.717 Calculation results showed that the brookite {210} surface can provide CO2 interactions energetically similar to those of the anatase {101} surface. In other words, brookite has negligible charge transfer to the CO2 molecule, indicating that the unmodified brookite is not a suitable catalyst for CO2 reduction. However, modification of the brookite surface through the creation of oxygen vacancies may lead to efficient electron transfer to CO2 to generate CO2•− radical and hence efficient CO2 reduction. The presence of oxygen vacancies within the brookite {210} surface plays a crucial role in CO2 reduction. First-principles calculations were also applied to explore the reaction mechanisms of the 2e reduction of CO2 to HCOOH or CO in photochemical reactions catalyzed by the anatase {101} surface.718 The anatase {101} surface plays a critical role in adsorbing CO2 and facilitating the electron- and protontransfer from the surface to CO2 in the process of photochemical reduction of CO2. The 2e reduction of CO2 can lead to the production of either HCOOH or CO. Two energetically competitive reaction pathways to HCOOH were identified, which involve initial 1e (via B1) and 2e (via A1) reduction processes, respectively (Figure 45). In the pathway involving 1e reduction of CO2 to CO2•− radical in a bidentate binding configuration (B1) on the surface, an electron is transferred from the TiO2 surface to the CO2 adsorbate, generating the activated CO2− anion radical with an activation barrier of 0.87 eV. The following steps of one-electron and twoproton transfer with a barrier of 0.46 eV result in HCOOH. Therefore, the activation of CO2 to CO2•− appears to be the rate-limiting step along this reaction pathway to HCOOH. It also suggests that surface doping of Ti cation sites can substantially lower the reaction barrier to activate CO2 through the 1e process. The stability of CO2•− (B1) on these M-doped anatase (101) surfaces is correlated with the M−O bond strength and ionic charge of the dopant. Doping of anatase (101) surface with Zr, Hf, or Sn is proposed to enhance the photochemical reduction of CO2. Reduction of CO2 at specific sites of the rutile TiO2(110)1×1 surface was systematically studied using in situ STM (scanning tunneling microscopy) at 80 K.719 It was found that CO2 adsorbed on the top of the bridge bonded oxygen vacancy (BBOV) at low coverage and dissociated by one-electron attachment process from the tip with an energy threshold of 1.8 eV, which is above the Fermi level of TiO2 (1.4 eV above the TiO2 CB onset). On the other hand, the lowest unoccupied molecular orbital (LUMO) of the adsorbed CO2 is located at 2.3 eV with respect to the Fermi level. The observed dependence of the dissociation rate on the tunneling current suggests that the reduction of CO2 induced by the electron attachment is a single electron process. Kubicki et al. presented a rather different mechanism for initial CO2 activation.720 They performed excited-state ab initio calculations of CO2 adsorbed on clusters of the {010}, {101}, and {001} anatase surface planes. Both post-Hartree−Fock

Figure 45. Illustration of reaction pathways: (a) Route I, via the B configuration to form HCOOH; (b) Route II, via the A1 configuration to form HCOOH (concerted mechanism). The sum of energies of the CO2 and H2 molecules is the zero reference for energy. The C−O bond lengths (Å) and the ∠OCO bond angle (deg) of CO2, as well as the bond lengths between CO2 and the interacting surface atoms of the TiO2 surface are shown. Atoms: Ti in green, O in red, C in silver, and H in white. The hydrogen-bond lengths are also labeled. The symbols in brackets label the states: g, gas phase; a, adsorbed species on the anatase (101) surface; A1, B1, two adsorption configurations of CO2/CO2− on surface; A1H, B1H, adsorption configurations of HCOO− on surface; A1H2, B1H2, adsorption configurations of HCOOH on surface. The sign of “+” indicates noninteracting species (e.g., +H+ + e− means that these H+ and e− are treated in a separate supercell), while “...” indicates two species in proximity. Reprinted with permission from ref 718. Copyright 2012 Royal Society of Chemistry.

calculations on small model surface clusters as well as the density-functional theory (DFT) calculations on larger clusters indicated that the CB electrons in irradiated, stoichiometric TiO2 surfaces may not be transferred to CO2 directly. Using the values for the energy of the TiO2 CB and the standard reduction potential of the HCOOH/H2CO3 couple at pH = 5, Xu and Schoonen672 also pointed out that the electrons in the CB of (bulk) TiO2 may not be transferred to CO2. This is 10029


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papers reporting “achievement” of H2 production via “water splitting” in the presence of various kinds of sacrificial reagents, such as methanol, amine, amide, sulfide, etc. However, it should seriously be pointed out here that such kind of H2 production reaction cannot be considered as a genuine “water splitting” reaction. This is because of the following reasons. (1) The definition of photocatalytic water splitting is that water is simultaneously split into H2 and O2 in a stoichiometric ratio of 2:1 under light irradiation in the presence of photocatalyst. This means that the electrons used for the half reactions of proton reduction must be originally provided by the water oxidation half reaction. In other words, O2 evolution from water oxidation is inevitable for a genuine water splitting reaction. The reactions with no observation of oxygen evolution are not true water splitting. Strictly speaking, they can only be called as “H2 production reaction”, “H2 evolution reaction”, or “half reaction of water splitting”. (2) H2 production reactions using sacrificial reagents are thermodynamically favorable. In such reactions, H2 is produced with no net solar energy conversion into the form of chemical energy, although it has been wrongly called solar fuel. However, as shown in this Review, study on hydrogen production from aqueous solution in the presence of sacrificial reagents may give some meaningful insights into the mechanism of photocatalytic reactions, which may be helpful for the development of efficient photocatalysts/photocatalytic systems for overall water splitting and CO2 photoreduction. The efficiencies of water splitting and CO2 photoreduction on TiO2 are still rather low and far from the requirement of industrial applications. This is due to the intrinsic limitations of TiO2, which can absorb only UV light that is ca. 4% in the solar spectrum. However, with emerging new ideas, new methods, new theories, and new materials, there could be some progress in solving the key scientific problems of solar energy conversion on TiO2-based photocatalysts. From recent advances on photocatalytic production of H2 and photoreduction of CO2 with H2O on TiO2-based photocatalysts, much better understanding on the complicated photocatalytic processes of H2 production, water splitting, and CO2 reduction could be obtained. Because the solar energy conversion efficiency (η) is largely determined by the corresponding individual efficiencies of sequential photocatalysis courses, that is, light harvesting (ηLH), charge separation (ηCS), charge migration and transport (ηCMT), and charge utilization (ηCU) for solar fuels generation, it is important to consider improving the efficiency of all of these processes simultaneously in an integrated system. On the basis of this consideration, the following future research directions for TiO2 could be identified. (1) First is the development of novel strategies or methods to extend the light absorption property of TiO2 into the longer wavelength region. Emerging strategies include bandgap engineering by doping of TiO2 with metal cations and anions, surface sensitization of TiO2 with visible light absorbing dyes, molecular complexes, quantum dots, etc. Although these strategies are promising for extending the light absorption of TiO2 into the visible region, they still suffer from some fatal problems. For example, doping of TiO2 usually also creates charge recombination defect sites, and the dyes or molecular complexes are usually unstable, especially in the highly oxidizing environment of overall water splitting and CO2 reduction with H2O. Novel strategies for enhancing the light absorption of TiO2 are highly desired.

contrary to the widely accepted mechanism for CO2 photoreduction; that is, CO2 is reduced to CO2•− by the electrons from the CB of TiO2. However, considering the electronic interaction between the TiO2 surface and the adsorbate (CO2) could facilitate electron transfer by lowering of the LUMO of the adsorbate (for example, by forming a surface state in the bandgap), the active sites for CO2 reduction were suggested to be the oxygen vacancies.

5. CONCLUSIONS, PERSPECTIVES, AND REMARKS In this Review, following a brief introduction of the solar fuels generation with semiconductor-based photocatalysis (section 1) and the basic properties of TiO2 (section 2), recent advances in the strategies for the development of solar fuels have been reviewed. Section 3 mainly focused on the strategies for H2 production on TiO2-based photocatalysts, including the improvement of the light-harvesting properties of TiO2 by bandgap engineering and surface sensitization, and improvement of the photogenerated charge separation efficiency by fabrication of heterojunction, phase junction, and Schottky junction. Some typical examples of water splitting (although there are only limited numbers of publications on overall water splitting due to intrinsic limitations of TiO2 itself) and biomass reforming for H2 production are given. Recent advances in the study of the dynamic properties of TiO2 are also summarized for a better understanding of the charge carrier properties of different phase TiO2. In section 4, we mainly focused on the recent progress in photoreduction of CO2 with H2O to solar fuels on TiO2-based photocatalysts, including improvement of CO2 photoreduction activity and selectivity by TiO2-based photocatalysts via phase composite engineering, crystal facet engineering, assembly of nanocomposites and nanocomposites on various supports, sensitization of TiO2 by molecular complexes or quantum dots, and control of the reaction conditions. As CO2 reduction is a complicated reaction, the mechanisms of CO2 photoreduction on various kinds of TiO2based photocatalysts were also discussed. TiO2-based photocatalysts, composites, and systems investigated so far for overall water splitting and photoreduction of CO2 with H2O are summarized in the tables of the Supporting Information. Conversion of solar energy into solar fuels by means of photocatalytic or photoelectrocatalytic water splitting and reduction of CO2 with H2O is considered as a viable means to solve the energy and environment problems by the replacement of the fossil fuels with renewable energy. Being a benchmark photocatalyst, TiO2 is by far one of the most widely studied semiconductor photocatalysts. Although tremendous research has been done on TiO2-based photocatalysts, the majority of the work still lies in the development of environmental catalysis for the degradation of pollutants, such as dye and organic wastes. Only during recent decades has research on photocatalytic water splitting and CO2 reduction started to draw more attention. To distinguish these two different research areas, here we use the terms “environmental photocatalysis” and “energy photocatalysis” to describe these two photocatalytic research areas. This Review mainly focused on the latter. Although there are some common concepts and strategies shared in the development of “environmental photocatalysts” and “energy photocatalysts”, the latter is much more difficult, because photocatalytic reactions of water splitting and CO2 reduction are thermodynamically uphill and require multielectron transfer. It should be noted that there are numerous 10030


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investigated. Better understanding the mechanisms of the

(2) Second is the development of new strategies for efficient separation of photoinduced electron and holes in TiO2-based photocatalysts. Efficient charge separation is the vital step for improving the entire efficiency of solar energy conversion. As shown in this Review, single TiO2 semiconductor photocatalyst cannot efficiently split water or reduce CO2 partially due to low charge separation efficiency. The emerging strategies for charge separation include fabrication of phase junctions by tuning the composition of different polymorphs of TiO2, fabrication of heterojunctions by coupling of TiO2 with other semiconductors or cocatalysts, fabrication of Schottky junctions by loading proper metal cocatalysts, and coupling of TiO2 with molecular complexes. It seems assembly of multicomponent integrated photocatalytic systems by combination of these different strategies would be an ideal approach for achieving efficient charge separation. To assemble such kind of system, tailor design and manipulating of the interfaces/junctions between the redox matching functional components with proper built-in electric field/potential difference are extremely important. In terms of semiconductor approach, the junctions and cocatalysts may play crucial roles in enhancing the photocatalytic activity. (3) Next is the development of novel water oxidation and CO2 reduction cocatalysts for TiO2. As shown in this Review, both anatase and rutile have the capabilities for water oxidation, and, more importantly, various kinds of strategies have been developed for CO2 reduction with H2O as the electron sources. Although the efficiencies of both reactions are still quite low, existing research results have already demonstrated that it is possible to photocatalytically split water and reduce CO2 to solar fuels. Because both reactions need to oxidize water, the development of efficient water oxidation cocatalyst suitable for coupling with TiO2 is in urgent need. Considering the stability issue, the core of the water oxidation catalyst should be inorganic as the oxygen evolution inorganic cluster of Mn4CaO5 in PSII of natural photosynthesis and many other effective photocatalytic systems. The proton reduction and CO2 reduction catalysts could be inorganic or organic complexes, depending on whether the proton reduction and CO 2 reduction reactions are separated from the water oxidation reaction. If the proton reduction or CO2 reduction reactions are ideally designed to perform in the dark other than simply under the light illumination together with the water oxidation reaction in one pot as most of the ways reported in the literature, the choice for proton reduction and CO2 reduction catalyst should be broader. Screening of proton reduction and CO2 reduction cocatalyst from a wide range of organic and inorganic materials might give more opportunity to find suitable cocatalysts for efficient proton reduction and CO2 reduction. (4) The way for the integration of water oxidation and CO2 reduction reactions using TiO2 semiconductor photocatalysts should be seriously reconsidered. Following the principles of the photosynthesis in nature, these two reactions should be performed separately, that is, water oxidation in the light irradiation and the CO2 reduction reaction in the dark. In this way, severe backward oxidation of intermediates and products derived from the CO2 reduction can be avoided, and the efficiency of CO2 conversion could be largely improved. To this end, the photoelectrochemical (PEC) approach, which can separate the oxidation and reduction reactions using proper membranes, turns out to be a wise option for CO2 reduction as well as water splitting. (5) Finally, the mechanism of photocatalytic water splitting and CO2 photoreduction on TiO2 should be more carefully

photocatalytic reaction processes, including light harvesting, carrier migration and transport, and the elementary reactions of water splitting and CO2 reduction in atomic or molecular level, is necessary for the improvement of the solar energy conversion efficiency. Only with a comprehensive understanding of all of these processes as guidance, efficient TiO2-based photocatalytic systems may be developed. Combining time-resolved and space-resolved spectroscopic techniques with computational studies, more in-depth knowledge on charge separation in TiO2-based photocatalysts could be obtained. It should be pointed out that, due to the intrinsic limitations, TiO2 may not be the promising photocatalyst for solar fuels generation by means of photocatalytic splitting of water and CO2 photoreduction. It is not expected that a great breakthrough in solar fuels production might occur on TiO2based photocatalysts. It did not happen since the publication of Fujishima and Honda’s work, and it would not happen even in the near future. However, as shown in this Review, TiO2 might be an ideal model of semiconductor-based photocatalyst for investigating the courses and mechanisms of the photocatalytic reactions, which might be a great help in the design and synthesis of more efficient photocatalysts and systems. In view of this point, research on TiO2 for solar fuels generation may continue.

ASSOCIATED CONTENT S Supporting Information *

Tables containing summaries of photocatalysts, composites, and systems. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors

*Phone: 86-411-84379070. Fax: 86-411-84694447. E-mail: [email protected]. *Phone: 86-411-84379760. Fax: 86-411-84694447. E-mail: [email protected]. Author Contributions

C.L. generated a detailed outline of the manuscript, Y.M. drafted sections 1−3 (X.W. drafted section 3.5), H.H. drafted section 3.6 and sections 4 and 5, Y.J. drafted tables and some figures, X.C. provided some materials for the manuscript, and finally H.H. and C.L. revised and edited the entire manuscript for submission. Notes

The authors declare no competing financial interest. 10031


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Biographies

Yushuai Jia currently is a senior Ph.D. student at State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences & Solar Energy Division of Dalian National Laboratory for Clean Energy, China, under the supervision of Prof. Hongxian Han and Prof. Can Li. His research interest is in the development of efficient composite photocatalysts/photocatalytic systems for overall water splitting.

Yi Ma received her Ph.D. degree in Physical Chemistry from Dalian Institute of Chemical Physics & Dalian National Laboratory for Clean Energy in 2013, under the supervision of Prof. Can Li. From 2013 to 2014, she worked as a program researcher/postdoctoral fellow at Eindhoven University of Technology. Her research interests include photocatalysis and photoelectrochemical properties of semiconductor materials. Dr. Chen is an Assistant Professor of Chemistry at the University of Missouri − Kansas City (UMKC). He obtained his Ph.D. degree in 2005 from Case Western Reserve University in the United States. He has published around 60 articles with over 16 000 citations. His research interests include nanomaterials, (photo)catalysis, battery, energy, and environment.

Xiuli Wang received a B.Sc. degree in Chemistry from Nankai University in 2004 and a Ph.D. degree in Physical Chemistry from Dalian Institute of Chemical Physics (DICP), Chinese Academy of Sciences (CAS) in 2011, under the supervising of Prof. Can Li. She has been working as an assistant professor in DICP since 2011. She did postdoctoral research at Imperial College-London from 2012 to 2013 with Prof. James Durrunt. Her research interest lies in the timeresolved spectroscopic study of photo(electro)catalytic process and mechanism.

Hongxian Han is a professor and a group leader in the Division of Solar Energy, Dalian National Laboratory for Clean Energy (DNL) & State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. He obtained his Ph.D. degree from the University of New South Wales (UNSW), Sydney, Australia, in 2003. He was a postdoctoral fellow/project research scientist at the Physical Biosciences Division, Lawrence Berkeley National Laboratory, U.S. (2003−2009). He joined the Dalian National Laboratory for Clean Energy in 2009 and received the “CAS Hundred Talents Program” priority support in 2010. Now his research interest is in the development of photocatalytic systems/devices for water splitting, CO2 reduction, and biomass reforming. 10032


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Can Li received his Ph.D. degree in Physical Chemistry from Dalian Institute of Chemical Physics, Chinese Academy of Sciences, in 1989, and he joined the same institute and was promoted to full professor in 1993. He did postdoctoral research on catalysis and UV Raman spectroscopy at Northwestern University and was visiting professor at Lehigh University, the University of Liverpool, and The Queensland University, and he was awarded the JSPS Professor at Waseda University, Tokyo University of Technology, and Hokkaido University. He was an invited professor at Université Pierre et Marie Curie, Paris VI. He was the President of the International Association of Catalysis Societies (2008−2012). Currently, he is the director of the State Key Laboratory of Catalysis, and the director of the Dalian National Laboratory for Clean Energy (DNL). His research interests include (1) UV Raman spectroscopy and ultrafast spectroscopy; (2) environmental catalysis and green catalysis; (3) heterogeneous asymmetric catalysis; and (4) solar energy utilization based on photocatalysis, photoelectrocatalysis, and photovoltaic cells. He has published more than 500 peer-reviewed papers with over 10 000 citations.

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant nos. 21090341 and 21361140346), and the National Basic Research Program (973 Program) of the Ministry of Science and Technology of China (Grant no. 2014CB239403). H.H. also would like to thank the priority support from the “Hundred Talents Program” of the Chinese Academy of Sciences. REFERENCES (1) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (2) Maginn, E. J. J. Phys. Chem. Lett. 2010, 1, 3478. (3) Aresta, M.; Dibenedetto, A. Dalton Trans. 2007, 2975. (4) Pearson, P. N.; Palmer, M. R. Nature 2000, 406, 695. (5) Smol, J. P. Nature 2012, 483, S12. (6) Frank, S. N.; Bard, A. J. J. Am. Chem. Soc. 1977, 99, 303. (7) Koci, K.; Obalova, L.; Lacny, Z. Chem. Pap. 2008, 62, 1. (8) Kraeutler, B.; Bard, A. J. J. Am. Chem. Soc. 1978, 100, 4317. (9) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Nature 1997, 388, 431. (10) Linsebigler, A. L.; Lu, G. Q.; Yates, J. T. Chem. Rev. 1995, 95, 735. (11) Thompson, T. L.; Yates, J. T. Chem. Rev. 2006, 106, 4428. (12) Diebold, U. Surf. Sci. Rep. 2003, 48, 53. (13) Carp, O.; Huisman, C. L.; Reller, A. Prog. Solid State Chem. 2004, 32, 33. (14) Chen, X.; Mao, S. S. Chem. Rev. 2007, 107, 2891. (15) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69. 10033


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