Recent Advances in Heterogeneous Photocatalytic CO 2 Conversion

Review pubs.acs.org/acscatalysis

Recent Advances in Heterogeneous Photocatalytic CO2 Conversion to Solar Fuels Kan Li,‡ Bosi Peng,‡ and Tianyou Peng* College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, P. R. China ABSTRACT: As a promising approach to achieving two objectives with one strategy, photocatalytic CO2 conversion for C1/C2 “solar fuels” production can provide a package solution to the current global warming and growing energy demand by using inexhaustible solar energy and increasing atmospheric CO2. Although numerous efforts have been made to enhance the CO2 conversion efficiency through developing photocatalysts and CO2 reduction systems in recent years, some challenges still remain in improving the activity and selectivity of the CO2 photoreduction reactions. This review gives an overview of fundamental aspects and recent research advances of heterogeneous photocatalytic CO2 conversion systems in the last 3 years, and the catalysts are categorized as one-step excitation semiconductor systems, one-step excitation photosensitized semiconductor systems, and two-step excitation hybrid systems such as semiconductor heterojunction and Z-scheme systems. Also, some suggestions are given for further confirming that the carbon-containing “solar fuels” are derived from CO2 rather than from the possible carbonaceous impurities in the photocatalytic system, because most of the papers cited in this review have not demonstrated that CO2 is the actual carbon source for photoreduction through 13CO2 labeling or other techniques. Lastly, a short perspective on the challenges and new directions in this field is proposed, which would be of great interest for the further improvements of activity and selectivity of the CO2 reduction reactions. KEYWORDS: photocatalytic CO2 conversion, photocatalyst, photoactivity, selectivity, solar fuel production

1. INTRODUCTION

uphill reaction processes (ΔG > 0) that need to input high energy due to its ultrahigh stability.1,8 As an approach to potentially solve the above-mentioned problems, photocatalytic CO2 conversion into renewable solar fuels is a promising technique (the so-called artificial photosynthesis) with many advantages, including the following: (1) this reaction can be driven by inexhaustible solar energy; 2) the initial reactants can be untreated water and CO2 released from human activities; (3) it is mostly carried out in relatively mild conditions such as low temperature and normal pressure; (4) the hydrocarbon fuels converted from CO2 directly can remit the current energy demand; and (5) this technology can fulfill the recycling of renewable carbon fuels through in situ conversion of CO2 into “solar fuels” without secondary pollution. In brief, photocatalytic CO2 conversion technology can not only reduce the atmospheric CO2 level and produce “solar fuels” but also solve the CO2 storage and global warming problems, and thus, it could be an effective supplement of the CCS strategy even though its corresponding mechanism needs to be better understood and optimized from either a scientific or an economic viewpoint.9 Since the pioneering work on CO2 photoreduction to HCOOH and CH3OH over semiconductors under light

As one of the main greenhouse gases that are present in the atmosphere, carbon dioxide (CO2) levels in the atmosphere have been increasing steadily over the past century as a result of human activities.1−6 Recently, the International Panel on Climate Change (IPCC) predicted that CO2 levels in the atmosphere could reach up to 590 ppm by 2100, and the global mean temperature would rise by 1.9 °C, which may cause disastrous consequences such as ice melting at the Earth’s pole, fast rising sea level, and increasing precipitation across the globe.7,8 Therefore, large reductions of CO2 emissions as soon as possible is required for the sustainable development of human beings. In addition to CO2 capture and storage (CCS) technique, which is considered one of the most promising strategies to reduce CO2 emissions while enabling the continued use of fossil fuels and without compromising the security of energy supply,9 CO2 conversion into simple C1/C2 fuels such as CO, CH4, HCOOH, HCHO, CH3OH, C2H5OH, and other hydrocarbon compounds has consistently drawn attention for more than 30 years.1,7 For example, various solardriven CO2 conversion strategies, such as thermochemical conversion,10 photoelectrochemical conversion,11,12 and photocatalytic conversion,1,7,12 have been developed to chemically reduce CO2. However, the conversion efficiencies for the production of hydrocarbon fuels through the reactions of CO2 reduction with H2O are not yet satisfactory because they are © XXXX American Chemical Society

Received: July 25, 2016 Revised: September 27, 2016

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DOI: 10.1021/acscatal.6b02089 ACS Catal. 2016, 6, 7485−7527

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ACS Catalysis irradiation was reported in 1979,12 numerous efforts have been directed to photocatalytic CO2 conversion for production of “solar fuels”.1−8 In the last 3 years, the studies on photocatalytic CO2 conversion have increased, and significant efforts are made in order to further improve the photoactivity and the selectivity through developing novel photocatalysts and CO2 reduction systems, and there are also numerous and informative reviews and perspectives on the photocatalytic CO2 conversion, including the possibilities and challenges on CO2 photoreduction for solar fuel production,1−5,13,14 various heterogeneous photocatalysts including visible-light responsive and UVlight-responsive ones,1,2,4−9,14−33 as well as CO2 reduction systems and design stratgies for reactors.3,5,7,21,34−37 This review will focus on the most studied photocatalysts and its corresponding systems for the heterogeneous photocatalytic CO2 conversion since 2013, and the categories of photocatalysts are one-step excitation semiconductor systems, onestep excitation photosensitized semiconductor systems, and two-step excitation hybrid systems such as semiconductor heterojunction and Z-scheme system. Since activity and selectivity are mainly determined by the suitable band structures and surface states of photocatalysts as well as the photoreaction conditions, the second section of this review provides a snapshot of the corresponding theoretical foundation of photocatalytic CO2 conversion, such as the thermodynamic requirements, CO2 adsorption and activation, light harvesting and charge-separation design, and performance evaluation, among other factors. Currently, there is no standard protocol for evaluating photocatalytic CO2 conversion performance, and we will not compare the efficiencies of the different photocatalysts and CO2 reduction systems developed so far. Instead, more attention is focused on the mechanism of CO2 reduction and strategies for the design and synthesis of more efficient photocatalysts. Lastly, a short perspective on the challenges and new directions in this field is proposed.

CO2 + 4e− + 4H+ = HCHO + H 2O E 2 = −0.48V (3)

(1)

CO2 + 2e− + 2H+ = CO + H 2O E3 = − 0.53V

(2)

+

−

+

CO2 + 6e + 6H = CH3OH + H 2O E4 = − 0.38V (4)

CO2 + 8e + 8H = CH4 + 2H 2O E5 = − 0.24V

(5)

2H+ + 2e− = H 2 E6 = −0.41V

(6)

2H 2O + 4h+ = O2 + 4H+ E 7 = +0.82V

(7)

It is well-known that a semiconductor has a band structure in which conduction band (CB) is separated from valence band (VB), leaving a bandgap with no electron configuration.6,7,26 The band structure, including the bandgap and the CB/VB positions, is one of the important properties because it determines the abilities of light absorption and photoreaction thermodynamics of a semiconductor. Generally, a slightly higher CB level than the redox potential of CO2 and its reduced products would drive the photogenerated electrons to transfer from the photocatalyst to the surface-adsorbed CO2 species and then convert it into solar fuels step by step, as shown in Figure 1. Although some particular surface sites of a semiconductor

2. THEORETICAL FOUNDATION OF CO2 PHOTOREDUCTION 2.1. Basic Principles of CO2 Photoreduction. Since CO2 is a linear molecule with relatively high thermodynamical stability due to the much higher bond energy of CO (750 kJ mol−1) compared with that of C−C (336 kJ mol−1), C−O (327 kJ mol−1), and C−H (411 kJ mol−1), the photocatalytic CO2 conversion usually demands numerous energy inputs to break the CO bonds.6,18 Although inexhaustible solar energy hitting the earth (1.3 × 105 TW) is a suitable energy source for this conversion,18 CO2 is optically inert at either visible or UVradiation in the wavelengths of 200−900 nm.6 It needs photocatalysts with a suitable band structure, which can be excited by sunlight and then migrate the photogenerated electrons to CO2 to accomplish the CO2 reduction processes. Nevertheless, one-electron reduction of CO2 is highly unfavorable thermodynamically due to the high negative redox potential of CO2/CO2·‑ (−1.90 V vs NHE, at pH 7.00).6,38 In contrast, the proton-assisted multielectron reduction steps of CO2 reduction are much more favorable by considering the relatively lower redox potential (vs NHE, at pH 7.00), as shown in eqs 1−5, which is already practiced in many CO2 photoreduction systems.1,13,21,24 CO2 + 2e− + 2H+ = HCOOH E1 = − 0.61V

−

Figure 1. Schematic illustration of probable mechanism of photocatalytic CO2 conversion over a semiconducting photocatalyst for solar fuels production mediated by suitable redox cocatalysts.

can act as active centers themselves (especially for the oxidation reaction on an oxide semiconductor), efficient photocatalytic reactions proceed only after loading suitable redox cocatalysts in most cases. Accordingly, a suitable photocatalyst for CO2 reduction must fulfill the following several demands: (1) multiple electrons must easily migrate from photocatalyst to CO2; (2) photocatalyst’s CB bottom level must be more negative than the redox potentials of CO2 and its reduced-products; (3) reactants such as H2O, CO2 or carbonates species should be adsorbed on the catalyst, and the product molecules could desorb and diffuse into the system after the CO2 reduction process; (4) the photogenerated holes on the VB of a semiconductor should be consumed by oxide species such as the additional sacrificing reagents or H2O, as shown in eq 7. Otherwise, the accumulated holes could be annihilated by the photogenerated electrons or force the chemical reactions that consume the reduced products of CO2. Therefore, it can be expected that the activity of photocatalytic CO2 conversion can be improved through the optimization of CO2 adsorption, light harvesting, charge separation, and/or their synergistic effects. 7486


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on photocatalysts. Clearly, more information about the photocatalytic centers and insights into the reaction mechanism is necessary to design more effective photocatalysts for CO2 reduction. Although the use of H2O as reducing agent raises a dilemma on how to reduce efficiently CO2 without performing the H2O reduction, some studies on CO2 reduction conjugated with H2O oxidation using semiconductors were reported.43,44 For example, Ag-doped BaLa4Ti4O15 can act as a CO2 reduction photocatalyst in aqueous solution under UV-light irradiation, with a ratio of [CO+H2]/O2 of 2.0, indicating a stoichiometric reaction.43 It indicated that H2O was consumed as a reducing agent for the overall reaction to reduce CO2 with the semiconductor. However, the different reaction conditions used for CO2 reduction and H2O oxidation make their conjugation a difficult challenge.43−47 To enhance the selectivity of CO2 reduction by using H2O as electron donor, a simple artificial photosynthesis(Z-scheme) system was developed by combining a photocatalyst for CO2 reduction with a photocatalyst for H2O oxidation.3,45,46 In such a system, a two-step photoexcitation was conducted to allow CB electrons in the H2O oxidation photocatalyst to compensate for holes in the photocatlyst in a photoexcited state.3 More recently, a similer Z-scheme system that combined photoactive semiconductors with metal-complex catalysts was developed to achieve highly selective CO2 photoreduction, in which the transfer of photoexcited electrons from the CB of the semiconductor to the metal-complex catalyst is essential to promote selective CO2 reduction on the metal complex.47 In the above Z-scheme systems, CO2 photoreduction and H2O photooxidation proceeded simultaneously within one system under sunlight irradiation and thus shows promise for future progress in this field.3 Nevertheless, functionally conjugating CO2 reduction with H2O oxidation using hybrid photocatalysts is difficult because of the different reaction conditions required for CO2 reduction and H2O oxidation, as mentioned above. Moreover, the reoxidation of the CO2 reduction products and the rereduction of the generated O2 would prevent the continuous reaction of H2O oxidation and CO2 reduction. In addition to the photocatalyst’s architecture, reasonable reactor design that can separate CO2 reduction products and O2, similar to water photosplitting systems, is also useful for preventing the back reactions of the CO 2 photoreduction processes.3 Furthermore, an appropriate spatial arrangement of the photocatalyst for CO2 reduction and a photocatalyst for H2O oxidation in Z-systems can also separate CO2 reduction products and O2, similar to the photoelectrochemical systems. On the other hand, it could be worth using other compounds such as alcohols and amines as sacrificial electron donors from the viewpoint of testing the photocatalytic activity of semiconductors. Because these compounds are more difficult to be reduced than water, it can be expected that they will not compete with CO2 in the trapping of the electrons. For a complete reaction of both CO2 reduction and H2O oxidation, hybrid materials such as semiconductor/semiconductor, semiconductor/metal-complex, cocatalyst/semiconductor with appropriate spatial architecture, energy band alignments, photoreaction conditions, and reactor design for separating the products are necessary. For example, Sato’s group combined a InP/[MCE1 + 2-A] hybrid material that can reduce CO2 to formate selectively in H2O under visible light irradiation with a semiconductor photocatalyst capable of H2O

Generally, H2O should be an ideal electron donor and hydrogen source for the photocatalytic CO2 reduction. However, there is a dilemma in the use of H2O as electron donor because it can compete favorably for the electrons and produce the preferential generation of H2 as shown in eq 6.2,3 Under the conditions of CO2 photoreduction, H2O can undergo not only oxidation to O2 by trapping positive holes in the VB of the semiconductor, as shown in eq 7, but also reduction to H2 by using electrons in the CB of the semiconductor. Due to this dual behavior, H2O added to provide hydrogen atoms for CO2 photoreduction can also act as a competing reagent in a CO2 photoreduction system, and thus, the current efficiencies of CO2 photoreduction are several orders of magnitude lower than that of the analogous photocatalytic H2 production from water reduction, which can be explained from the viewpoint of both thermodynamics and kinetics.2,3 Thermodynamics indicate that CO2 reduction requires 6−8 electrons with more-negative reduction potential than two-electron water reduction reaction for H2 generation, while the more complicated reaction mechanisms for CO2 reduction must have several steps in which electrons and protons have to be transferred to CO2 consecutively or simultaneously. Therefore, a photocatalyst must solve this thermodynamic and kinetic contradiction by possessing spatially separated centers for electrons and protons to avoid their collapse for H2 production before being transferred to CO2.2 Generally, an ideal photocatalyst should have photocatalytic centers, in which electron-transferring sites are close enough to other ones acting as acids and transferring at least one proton for the solar fuel production in CO2 reduction systems.2 It means that appropriate architecture of the photocatalytic sites that accommodates light harvesting, efficient charge separation/ trapping, adsorption sites, and acid centers is necessary. Also, the adsorption sites for CO2/H2O should be addressed in the design of a photocatalyst so as to increase the concentration of this substrate near the reactive sites and/or decrease the activation energy of the process.2,3 Therefore, the current tendencies for increasing the selectivity of CO2 reduction products over H2 generation are aimed at designing the photocatalytic centers with adequate adsorption capacity, electron/proton relay units, and cocatalysts.2,39−42 Among which, the loading of cocatalysts (such as complexes, metal, or metal oxide nanoparticles) to drive oxidation or reduction selectivity are one of the most popular approaches.39−42 For example, Miyauchi’s group reported that a CuxO nanocluster cocatalyst can drive the multielectron reduction reactions of both oxygen to produce hydrogen peroxide and CO2 to generate CO,41 and CuxO loaded on SrTiO3 (STO) also exhibited 20% improvement of selectivity for CO2 reduction.42 Similarly, Ag cluster-loaded Ga2O3 exhibited high selectivity for CO2 reduction.40 In which, the bidentate formate species acting as the reaction intermediate was generated from the monodentate bicarbonate (and/or the bidentate carbonate) species on the Ga2O3 surface but not from the monodentate carbonate species on the large Ag particles. The formation of the bidentate formate species could take place at the perimeters of the Ag clusters on the Ga2O3 surface due to such small Ag clusters enhancing the band bending of Ga2O3 and thus promoting the selectivity for CO2 reduction.40 The above results indicate that the selectivity of CO2 reduction products over H2 generation is related not only to the adsorption capability of CO2/H2O but also to the CO2 species adsorbed 7487


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Figure 2. Possible surface structures of CO2 on TiO2 surfaces.57 Figure reproduced with permission from ref 57. Copyright 2014 American Chemical Society.

and the higher reactivity of this site is related to its stronger (Lewis and Brønsted) basicity.57 Also, the activation of the adsorbed CO2 molecules is one of the most challenging steps for CO2 reduction, and surface chemistry of CO2 demonstrated that adsorption of CO2 molecules on the surface of metal or oxide is usually accompanied by activation processes.58−60 Compared with the normal molecule, CO2 in the chemisorption state (mainly carbonate and/or CO2− anion) has a bent O−C−O bond angle and a decreased LUMO, which will favor the charge transfer from the photoexcited semiconductors to the surface-adsorbed CO2 molecules,60 and thus, the photocatalytic CO2 (an acidic molecule) reaction rate can be enhanced by coloading of basic metal oxides as mentioned above. In addition, the energy barrier to form bicarbonate and CH4 can be effectively decreased by 0.05−0.25 eV in aqueous solution compared to the gas−solid interface.58 In the presence of oxygen vacancy (VO) defects, the penta-coordinated Ti atom (Ti5c)-bound CO2 is found to tilt preferentially away from the VO-containing row.59 If another CO2 molecule is present on the neighboring Ti5c row, both CO2 molecules tilt toward the common bridging oxygen (Ob) row that separates them, as proved by scanning tunneling microscope (STM).59 The above investigations indicate that the crystal facet and/or surface engineering of photocatalyst and the reasonable design of their photoreaction system can enhance the surface adsorption and activation of CO2 and adjust the corresponding active sites on photocatalyst, which would be beneficial for improving the activity and selectivity of photocatalytic CO2 conversion significantly. Usually, some important information about the reaction mechanism for CO2 photoreaction can be gained from the above-mentioned investigations on CO2 adsorption and activation processes in combination with some characterization techniques.38,39,56,60 For example, in situ FTIR spectroscopy was used to gain understanding on the mechanism and reaction intermediates in a CO2 photoreduction system using Au−Cu nanoalloys supported on TiO2 as photocatalysts.39 In which, CO2•−, Cu−CO, and elemental C were detected on the photocatalyst surface, and the spectroscopic detection of Ti4+− CO2•− was assigned to the activation of CO2 by specific Ti3+ sites possessing an excess of electron density respect to Ti4+. Therefore, a mechanism in which the role of Au is to respond under visible light and Cu binds to CO and directs the reduction pathway was proposed.39 In 2014, Ye’s group proved

oxidation to fabricate a tandem configuration (including photoanode and photocathode) to reproduce artificial photosynthesis, which allows solar-driven CO2 reduction using H2O as both an electron and a proton source through a Z-scheme system.3 Obviously, developing conceptual frameworks leading to the fabrication of new photocatalysts and their photoreaction systems is extremely urgent in this field.2,3 2.2. CO2 Adsorption and Activation Aspects. It is wellknown that the CO2 adsorption amount and its states on a photocatalyst significantly affect the CO2 reduction processes. Generally, a larger surface area of a photocatalyst makes more active sites for the surface CO2 adsorption, and a high CO2 adsorption level would promote the CO2 reduction in both chemical kinetics and thermodynamics. Therefore, various nanostructured materials such as nanoparticles (0D),48 nanorods/nanotubes/nanowires (1D),49−52 nanosheets (2D),53 or their hierarchical micro/nanostructures,54 were gained so as to create large surface area. Furthermore, some porous materials with high surface areas are also applied as photocatalyst or its support for the photocatalytic CO2 conversion.55,56 Recent studies on CO 2 capture and photochemical conversion promote the understanding of the mechanisms on the photogenerated charge migration and CO2 conversion over TiO2 photocatalysts.1,57−59 Figure 2 shows all possible structures of CO2 on the TiO2 surface,57 which can be grouped in (1) linear structures (L) that include parallel and perpendicular configurations by considering the orientation of the molecule with respect to the surface; (2) monodentated carbonates (MC) that can be conceived of two types, a carbonate coordinated to a surface Ti center or a sort of “surface” carbonate in which one oxygen of the −CO3 moiety is itself a surface oxygen kept nearly fixed at its lattice position; (3) bidentated carbonates (BC) that can be divided into bridging BC, where the carbonate is bound to two adjacent Ti centers, and chelating BC, where the bidentated bond is with a single Ti center; (4) monodentated and bidentated bicarbonates (MB and BB) derived from the corresponding carbonate structures.57 Density functional theory (DFT) and Fourier transform infrared spectroscopy (FTIR) evaluations suggest that the strong coupling with the 2sp states of surface oxygen’s and bent CO2 leads to electron transfer from anatase TiO2 to the surface-adsorbed CO2 molecules.57 Moreover, the formation of carbonate and bicarbonate species occurs preferentially on the surface oxygen sites of anatase TiO2, 7488


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product, but CO formation was drastically reduced as compared with that on the pretreated TiO2. The experimental data including isotope labeling, GC-MS, and FTIR spectra indicated that CO2 and CO32− are the main carbon sources of the CH4 production, which proceeds on the Pd sites of Pd/ TiO2.64 Miyauchi’s group41 investigated the photocatalytic activity and reaction pathway of Cu(II) grafted Nb3O8− nanosheets by using electron spin resonance (ESR) analysis and isotopelabeled molecules (H218O and 13CO2). The labeling experiment results demonstrated that electrons are extracted from water to produce oxygen (18O2) and then reduce CO2 to produce 13CO under UV irradiation. Additionally, the ESR results confirmed that excited holes in the VB of Nb3O8− nanosheets react with water, while the excited electrons in the CB of Nb3O8− nanosheets are injected into the Cu(II) nanoclusters through the interface and are involved in the reduction of CO2 into CO. The same group also conducted isotope tracer analyses (13C and 18O) on the optimized CuxO-SrTiO3 (STO) film, and they confirmed that the CO generated by the CuxO-STO film originated from CO2 and that O2 molecules were produced through water oxidation.42 Namely, their CuxO-STO films can produce fuel from stable molecules like CO2 and water in an uphill reaction under photon irradiation,42 which is similar to the photosystems I and II in natural plants. The above results clearly show that removal of organic adsorbates from photocatalysts and careful analyses of the origin of those carbon-containing products are essential for estimating the performance of the photocatalytic CO 2 reduction.41,42,62−64 Especially, the state-of-the-art production rates of photocatalytic CO2 reduction are still very low,7 and thus, the effects of carbon-containing impurities on the photocatalyst surface need to be taken into account when carbon-doped or high-surface-area photocatalysts synthesized at low temperature without post annealing treatments are used. Because an overestimation of the CO2 photoreduction rate or even false positive results may be obtained due to such species contributing to the overall product yield, isotopic 13CO2 labeling would be one way to demonstrate that the reaction products arise from CO2 reduction and not from other carbon sources, and it has been employed in a few studies so far.16,41,49,56,60,63−66 In addition to 13CO2-labeling experiments, careful analyses of the evolved oxygen and carbon-containg products by using other techniques such as ESR, GC-MS, DRIFT, and FTIR spectra are also critical to accurately quantify CO2 photoreduction perforamnce and understand the underlying reaction pathways.16,41,42 Also, a contrast test of activity in the absence of CO2 but in the presence of H2O would be one cost-effective approach to confirm whether the carbon residues on photocatalysts participate in the CO2 reduction for the formation of solar fuels.63 2.4. Light Harvesting and Charge-Separation Designing. Generally, effective light harvesting could guarantee the photoinduced charge pairs generation thermodynamically, and the charge migrations from bulk to surface and subsequent redox reactions affect the overall efficiency of the solar-to-fuels conversion kinetically. Sunlight (AM1.5G) consists of three main components in terms of wavelengths: ultraviolet rays (λ < 400 nm), visible light (λ = 400−800 nm), and infrared rays (λ > 800 nm), accounting for 4%, 53%, and 43% of the total solar energy, respectively.67 Up until now, most of those photocatalysts have no suitable bandgap energy and/or structures to effectively use the sunlight or fulfill the corresponding redox

that the initial step of the CO2 reduction is the activation of CO2 by a one-electron transfer and formation of surface-bound CO2•− through in situ ESR technique, and then a series of elementary steps including breaking C−O bonds and creating new C−H bonds were investigated by in situ FTIR.56,60 According to in situ ESR and in situ FTIR results, the CO2 photoreduction mechanism that meets a glyoxal pathway was proposed.56 Also, investigations on the interactions of CO2 molecules with surface oxygen vacancies (VO) can gain understanding on the mechanism and reaction intermediates in photocatalysis.38,50,59 For example, STM studies confirmed that CO2 preferentially adsorbs on VO sites of TiO2, and the energy barriers for CO2 hopping into and out of a VO were determined to be 0.09 and 0.16 eV, respectively.59 The electron-induced dissociation of CO2 adsorbed at the VO defect on the TiO2 (110) surface was investigated at the single-molecule level using STM in an attempt to mimic the initial step of CO2 activation on a photocatalyst surface, and the detected transient negative ion (CO2−) due to the injected electron was considered to be the key process in CO2 dissociation on TiO2 (110) surface.38 2.3. A Critical Issue on Carbon Sources for CO2 Reduction. Since carbonaceous residues on the photocatalyst surface, such as organics involved in the synthesis processes, carbon residues, and even the adsorbed organic solvents (e.g., methanol, ethanol, and acetone), exist in the laboratory atmosphere and can participate in the formation of primary products during the CO2 photoreduction conditions and contribute to the overall product yield, it is thus critical to confirm that the carbon-containing products are derived from CO2 rather than from carbonaceous impurities.16,41,42,49,56,60−66 For example, Frei and co-workers have already observed that carbon residues in mesoporous materials can be involved in the production of primary products in the photocatalytic CO2 reduction.61,62 After removing the residual carbon through ozonation process, a conclusive evidence that CO is formed in the absence of carbon residues is provided in a high vacuum IR cell in the case of isolated Ti sites in mesoporous materials, and the onward CO2 reduction conducted under 13CO2 and H2O exhibited merely 13CO production, indicating a true artificial photosynthesis.62 In 2010, Mul’s group has carefully investigated the mechanism of the photocatalytic conversion of CO2 and H2O over copper-oxide-promoted TiO2 (Cu(I)/TiO2) by the combined use of in situ diffuse reflectance IR (DRIFT) spectroscopy and 13CO2, and they proved that organic contaminations on the photocatalyst surface are involved in reactions with predominantly photocatalytically activated surface-adsorbed water.63 Ibusuki and Ishitani’s group reported the effects of adsorbed organic molecules on TiO2 particles on the photocatalytic CO2 reduction.64 From an aqueous suspension of untreated TiO2 (P25) particles, a considerable number of organic molecules (such as acetic acid, methanol, and formic acid) were adsorbed, and a significant amount of CH4 as a major product was produced. However, it was strongly suggested that the CH4 formation mainly proceeded via the photo-Kolbe reaction of acetic acid. Using TiO2 treated by calcination and washing procedures for removal of the organic adsorbates, CO was photocatalytically generated as a major product, along with a very small amount of CH4, from an aqueous suspension under a CO2 at mosphere.64 In contrast, by using Pd (>0.5 wt %) deposited on TiO2 (Pd/TiO2) on which organic adsorbates were not detected, CH4 was the main 7489


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Figure 3. Schematic illustration of four common charge separation mechanisms for photocatalytic CO2 conversion based on one-step excitation semiconductor (a), photosensitized semiconductor (b), two-step excitation heterojunction (c), and two-step excitation Z-scheme (d,e) systems.

and electron donor. Typically, the excitation of dye molecules by the incident light is followed by the photogenerated electron transition from their highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). Subsequent steps include the injection of photogenerated electrons from the dye’s LUMO to the semiconductor’s CB, further migration of the CB electrons to the adsorbed CO2 molecules on the semiconductor for the production of solar fuels; consequently, the oxidized dye molecules can be regenerated for cyclic utilization through sacrificing the electron donor or oxidizing the water. Generally, two-step excitation CO2 reduction processes can be grouped into heterojunctions and Z-scheme systems containing at least different semiconducting components, which 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 respective CBs and VBs. The directions of charge transfer would depend on the relative positions of CBs and VBs of the two semiconductors.15,18 By choosing semiconductors with appropriate band structures, the obtained heterojunctions and Z-scheme can not only gain a wider spectral responsive region but also improve the spatial separation efficiency of the photogenerated carriers.18 In semiconductor heterojunction systems, the electrons can be principlly injected to the material with more positive CB position, while the holes transfer to the material with more negative VB position, as shown in Figure 3c. Usually, the electrons and holes transfer to the different semiconductors in opposite directions and bring a spatial charge separation, which is beneficial for the following CO2 reduction processes.18 In direct Z-scheme systems, the photogenerated electrons can inject into VB of semiconductor I from CB of semiconductor II directly or through conductive intermediate, which are so-called all-solid-states Z-scheme (Figure 3d).70 Typically, the band structures of semiconductors in Z-scheme are almost the same as type II heterojunctions (Figure 3c), and thus, it is necessary to classify their actual electron flow directions. Z-schemes in photocatalytic CO2 conversions are easy to be determined when the CB of semiconductor II was lower than the CO2 redox potential.70 Alternatively, the two semiconductors can be connected in series with reversible redox shuttles (electron donor/acceptor pairs) or conductive

reactions for CO2 reduction. Therefore, many strategies, such as element doping, exploiting narrow bandgap semiconductors or solid solutions, composites with narrow bandgap semiconductors, and photosensitization, have been developed to extend the spectral absorption region of photocatalysts.6,8,15−18 What’s more, charge-separation design for an efficient spatial separation of the photogenerated carriers is another important issue for improving the activity of photocatalytic CO 2 conversion because the charge recombination process (∼10−9 s) is considerably faster than the surface redox processes (10−8 to 10−1 s) in order of magnitudes.21 According to the light harvesting and the photogenerated charge-transfer mechanisms, the currently developed photocatalytic CO2 conversion systems can be divided into four basic forms as shown in Figure 3, that is, (1) one-step excitation semiconductor, (2) one-step excitation photosensitized semiconductor, (3) two-step excitation semiconductor heterojunction and, (4) two-step excitation Z-scheme systems. In one-step excitation systems, the semiconductor takes the task of light harvesting, and the photogenerated carrier separation is actualized by the loaded cocatalysts such as noble metal nanoparticles or Ru/Re complexes (Figure 3a). A noble metal as cocatalyst can act as an electron sink because the Fermi level of the nanosized-metal is normally below the CB edge of semiconductor, which allows the electron transfer from the semiconductor to the loaded noble metal thermodynamically,1 and thus causes a prolonged electron mobility and lifetime, which is beneficial for the CO2 reduction process.48,55 Similarly, the photogenerated electrons of semiconductor can migrate to the loaded Ru/Recomplex that acts as cocatalyst, which is followed by the CO2 reduction process.68,69 The CO2 reduction procedure commonly needs the loading of cocatalysts, which can act as active sites, thereby creating a barrier as an efficient electron trap for accelerating the charge separation. In photosensitized semiconductor systems, the semiconductor acts as an electron acceptor and carrier of the lightharvesting unit (the loaded sensitizer), such as organic dyes, metal complexes, or inorganic quantum dots as shown in Figure 3b.26,66,67 There are four main components: dye molecules (adsorbed on the semiconductor), a semiconductor (as the electron relay and reaction matrix), CO2 reduction cocatalyst (such as Ag, Pt, commonly coloaded on the semiconductor), 7490


Review

ACS Catalysis

that of gas-phase system, it is hard to determine the initial reactant of CO2 species, due to the complex ionization balance of carbonate (H2CO3, HCO3−, CO32−).8,15,16,26 Furthermore, liquid H2O might compete with CO2 for capturing the photogenerated electrons from the semiconductor to produce H2 simultaneously, which may result in low activity and/or selectivity of the photocatalytic CO2 conversion.6 Therefore, the gas-phase photoreaction system is more widely applied to avoid the above-mentioned issue, in which the concentrations of gaseous CO2/H2O and the microstructures of photocatalysts are very critical influencing factors to the activity and selectivity of the gas-phase system.6,7 Moreover, the mixture of CO2 and H2O vapor can be facilely generated from either introducing CO2 though a water bubbler or in situ neutral reaction of carbonate/bicarbonate, and the concentration of CO2 can be determined by the gas pressure.6,7 On the other hand, there is diversity and complexity of solar fuels produced from different photocatalytic CO2 conversion systems.6 For instance, CO, CH4, and other hydrocarbon substances in gas species and carbohydrates (CH3OH, HCHO, HCOOH, C2H5OH, and so on) in liquid phase are usually tested from aqueous CO2 reduction systems, whereas CO, CH4, and other hydrocarbon substances in gas species are the main products for gas-phase systems. Namely, the overall product yield usually depends on various experimental conditions such as the quantity of catalyst, light source, light intensity and lighting area, CO2 concentration, CO2 existing state, reactor construction, and photocatalyst separation, among others, and thus, the CO2 reduction efficiency can also be measured by apparent quantum efficiency (AQE), which is calculated by using the product amount and the incident photon number as shown in eq 8.6,8,15−18

medium to form a Z-scheme (Figure 3e). In this case, the CO2 reduction to solar fuels and oxidation of reduced redox mediators occur on one photocatalyst concurrently with the reduction of oxidized redox mediators or oxidation of water to O2 on the other photocatalyst to simulate the natural photosynthesis of green plants (Figure 3e).70 Nowadays, most efforts are given to the reductive halfreaction of the photocatalytic CO2 conversion, and minimal effort is given to the oxidative one.6,8,15−18 Thermodynamically, accumulation of holes would result in higher charge recombination rate and further equilibrium shifting to consume the produced solar fuels. RuOx as cocatalyst can promote the oxidative half-reaction and then the CO2 reduction for solar fuel production.71,72 Similarly, Tanaka’s group73,74 reported Agmodified La2Ti2O7 and ZnGa2O4 can produce H2/CO and O2 stoichiometrically with high activity of CO2 reduction. It implies that it is necessary to balance the surface charge with respect to the stoichiometric production of solar fuels and O2 so as to consume the photogenerated electrons and holes at the same time. Meanwhile, sacrificial reagents such as triethanolamine (TEOA) or triethylanmine (TEA) are usually used to consume the photogenerated holes during the photocatalytic CO2 conversion, which considers structural stability of photocatalysts.75−80 The above investigations indicate that consumption of the photogenerated holes is as important as that of the CO2 reduction processes using the photogenerated electrons during the photocatalytic CO2 conversion, and balancing the consumptions of electrons and holes is beneficial for improving the activity and stability of the photocatalytic processes. 2.5. Performance Evaluation of Photocatalytic CO2 Conversion. Besides the above basic principles, light harvesting, and charge-transfer aspects, some other important technical parameters, such as photoactivity, selectivity, long-term stability, apparent quantum yield (AQY), and turnover number (TON), are also used to evaluate the performance of various photocatalytic CO2 conversion systems.6,8,15−18 Among which, activity can be directly reflected by the solar fuel production rate under special experimental conditions and commonly denoted as μmol h−1 (or μmol h−1 g catalyst−1). As a stability index, the time course for solar fuel production of a photocatalytic system is also important. Because the activity and selectivity of CO2 reduction reaction are mainly dependent on two factors (CO2 adsorption on catalyst and photogenerated electron transfer toward CO2) in addition to the necessary characteristics of a photocatalytic system as mentioned above,6 the activity indicated as μmol h−1 g catalyst−1 is usually not very meaningful for the performance comparison among different research groups because there are significant differences in the experimental conditions and the produced solar fuels’ types in the field of photocatalytic CO2 conversion. On one hand, the current photocatalytic CO2 conversion systems can be divided into two main completely different kinds (aqueous photoreaction system and gas-phase photoreaction system) as mentioned in our previous review.6 In an aqueous photoreaction system, a certain amount of photocatalyst is dispersed in water under CO2 atmosphere. As the solubility of CO2 in water is relatively low, alkali (NaOH, Na2CO3, or NaHCO3) always acts as solute to enhance the CO2 solubility. Although the concentrations of CO2 species can be enhanced significantly after introducing alkali solution and the contacts of photocatalysts and CO2 species are easier than

AQE (%) =

number of reacted electrons × 100% number of incident photons

(8)

What’s more, when the products from the CO2 reduction system are complex (which is normal in photocatalytic CO2 conversion), the number of reacted electrons refers to the sum of the reacted electrons to generate each product. Therefore, it is better to estimate the CO2 reduction efficiency by using total consumed electron number (TCEN) per unit reaction time and unit mass of photocatalyst, which can be calculated by using the product amount and the incident photon number as shown in eq 9. TCEN =

∑ (c product × nelectrons) × Vreactor mcat. × t irr.

(9)

where TCEN is the total consumed electron number for the photocatalytic CO2 conversion, Vreactor is the reactor volume, tirr. is the irradiation time, mcat. is the catalyst mass, cproduct is the concentration of a certain product of CO2 conversion, and nelectrons is the corresponding consumed electrons per mole of the certain product. In addition, H2 is not considered to calculate the TCEN in the following chapters because it is not converted from CO2 directly. Another important evaluating index of the CO2 photoreduction performance is the long-term stability (durability) that was usually ignored in most of the research reports even though decrease of the photocatalytic CO2 reduction performance was frequently observed with prolonged reaction time.2,7,37,64 Usually, the gradually deteriorated CO2 reduction performance over the reaction time can be mainly ascribed to 7491


V,N-co-doped anatase TiO2 nanotube arrays anatase TiO2 nanotubes

300 W Hg-lamp

7492

100 mg of catalyst, CO2 and H2O vapor 100 mg of catalyst, CO2 and H2O vapor

anatase TiO2 with exposed special facets anatase TiO2 nanoplates with coexposed (101) and (001) facets cubic anatase TiO2 mixed-phase TiO2, Zn(II) porphyrin brookite TiO2 bicrystalline TiO2 with anatase−rutile bicrystalline TiO2 with anatase−brookite bicrystalline TiO2 microspheres with anatase−brookite hydrogenerated TiO2 (HTiO2) mixed-phase TiO2 oxygen-rich TiO2 (P25)

300 W Xe-lamp

300 W Xe-lamp (AM 1.5G) 15 W energy-saving bulbs

500 W Xe-lamp (AM 1.5G)

solar simulator (λ = 200−900 nm)

solar simulator (λ = 200−1000 nm)

300 W Xe-lamp solar simulator (λ = 200−1000 nm)

300 W Xe-lamp 300 W Xe-lamp (λ > 420 nm)

0.2 wt % Mg

30 mg of catalyst, CO2/ H2O/CH3OH ratio of 97.1/2.3/0.6

Ag0

150 mg of catalyst, CO2 and H2O vapor 100 mg of catalyst, 5 mL min−1 of CO2 saturated with H2O vapor

90 mg of catalyst, CO2 and H2O vapor

150 mg of catalyst, CO2 and H2O vapor 50 mg of catalyst, 2.0 mL min−1 CO2 and H2O vapor 100 mg of catalyst, CO2 and H2O vapor

50 mg of catalyst, CO2 and H2O vapor 60 mg of catalyst, CO2 and H2O vapor


0.5 wt %Ag 1.0 wt %Cu

1.0 wt %Pt

300 W Xe-lamp

300 W Xe-lamp

100 mg of catalyst in H2O/TEA (4:1), CO2 bubbled 30 min

TiO2@SiO2 @Fe3O4, [Ru (bpy)2phene-NH2](PF6)2CoPc anatase TiO2

20 W LED (λ > 400 nm)

50 mg of catalyst in water (pH 3.00), CO2 bubbled 30 min 100 mg of catalyst, CO2 and H2O vapor

40 mg of catalyst, CO2 and H2O vapor 200 mg of catalyst, 100 μL of H2O and 50 psi CO2 100 mg of catalyst, CO2 and H2O vapor 80 mg of catalyst, CO2 and H2O vapor

1.25 cm2 catalyst film (0.6 mg), CO2 and H2O vapor 100 mg of catalyst, CO2 bubbled 60 min 5 mg of catalyst, 5 mL of CO2-saturated 0.2 M KHCO3 4 cm2 nanotube film in CO2-saturated 0.1 M KHCO3 8 cm2 nanotubes film in CO2 saturated with 0.1 M KHCO3 100 mg of catalyst, CO2 and H2O vapor

experimental condition

3D hierarchically porous TiO2 1.7 wt %Cu, 0.9 wt %Pt

0.6 wt %Pt

Pt and 0.1 mol % MgO

Pt0/Pt2+ 0.05 wt % Au or 0.4 wt %Ag

Au/Cu (2:1)

co-catalyst

300 W Xe-lamp

halogen lamp

300 W Xe-lamp 300 W Xe-lamp

300 W Xe-lamp 40 W Hg-lamp (λ = 254 nm)

300 W Xe-lamp

mesoporous anatase TiO2 nanofibers anatase TiO2 nanosheet anatase TiO2 hierarchical microspheres ordered mesoporous TiO2 3.0 wt % NaOH modified anatase TiO2 TiO2/Cu(II) phthalocyanine

anatase TiO2 brookite TiO2 nanorod

300 W Xe-lamp (λ = 420−780 nm) 0.3 mW cm−2 LED (λ = 365 nm)

300 W Hg-lamp

TiO2 (P25)

photocatalyst

1000 W Xe-lamp (AM 1.5)

light source

Table 1. Summary of CO2 Photoreduction Systems Containing TiO2 or TiO2-Based Photocatalystsa ref

0.18 ∼7.04

0.019 (CH4)/0.014 (CO) μmol h−1 ∼0.88 (CH4)/∼0.8 (H2) μmol h−1

18.13 1.23 0.01

0.15 (CH4)/0.015 (CO) /0.045 (H2) μmol h−1 0.012 (CH4) μmol h−1

9.90

400 nm) 16 W Hg-lamp and 300 W Xe-lamp

0.5 wt %Cu (CuI) 3 wt %CuO

anatase TiO2 microflower film anatase TiO2 hollow sphere anatase TiO2 nanofibers

500 W Xe-lamp

0.05 mol % Au, 0.16 mol % Pt

1.0 wt %Pd

anatase TiO2

Ag 2.5 wt %Ag

1.0 wt %Au 0.7 wt %Ag

1.0 wt % CuI/Pd 0.5 wt %Pt and 1 wt % MgO 1.0 wt % Pt 0.1 wt % Au

co-catalyst

Two 90 mW cm−2 UV-LED (λ = 365 ± 38 nm) 16 W Hg-lamp and 300 W Xe-lamp 20 W Hg-lamp (λ = 254 nm)

300 W Xe-lamp (λ > 420 nm) 500 W Xe-lamp (λ > 400 nm)

500 W Hg-lamp (λ = 365 nm)

8W UVA lamp (λ = 365 nm)

8 W Hg-lamp (λ = 254 nm)


300 W Xe-lamp (λ > 400 nm) 100 W Xe-lamp (λ = 320−780 nm)

TiO2/5.0 wt % Cu-doped TiO2 films TiO2/1.0 wt % Fe-doped TiO2 films co-doped order mesoporous anatase TiO2 1.5 wt % V-doped, 0.5 wt % Cr-doped, or 1 wt % Codoped anatase TiO2

monoethanolamine functioned TiO2 N-doped rutile TiO2 nanorod arrays 3.0 wt % Cu-doped anatase TiO2 0.5 wt % Cu-doped anatase TiO2

photocatalyst

0.1 wt % Ni-doped anatase TiO2 1.0 wt % Ce-doped anatase TiO2 0.28 mol % Ce- doped anatase TiO2 3 wt % Mo-doped anatase TiO2 nanotubes 10 wt % In-doped anatase TiO2 Ti3+-self-doped rutile TiO2 TiO2 (P25)

18 W cm−2 UV-lamp (λ = 365 nm)

500 W halogen lamp (λ = 380−1100 nm)

300 W Xe-lamp (λ > 420 nm)

6 W cm−2 Hg-lamp (λ = 365 nm)


200 W Hg-lamp or 500 W halogen lamp

125 W Hg-lamp (λ = 365 nm)

300 W visible light sources (UV < 5%)

Xe-lamp

light source

Table 1. continued

100 mg of catalyst, CO2 and H2O vapor 400 mg of catalyst in 400 mL of water, CO2 bubbled 0.5 h before irradiation 2.2 g of catalyst under He flow, containing 1 vol % CO2 and 4 vol % H2O, heating at 140 °C 8.82 mm−2 catalyst film in CO2-saturated water 10 mg of catalyst in 0.2 mL of H2O under 50 psi CO2 5 mg of catalyst, CO2 and H2O vapor

20 mg of catalyst, 4 bar CO2 and H2O vapor 500 mg of catalyst in CO2-saturated water, Na2SO3 as hole scavenger 100 mg of catalyst, CO2 and H2O vapor (7.25:1) 8.82 mm−2 catalyst film in CO2-saturated water

100 mg of catalyst, 0.1 L of CO2-saturated 0.2 M NaOH 100 mg of catalyst, 0.2 M MEA solution, CO2 bubbled 1 h 250 mg of catalyst, 1.2 bar CO2, 2 bar He and H2O vapor 100 mg of catalyst, 1.0 atm water-saturated CO2 20 mg of catalyst, CO2 and H2O vapor

500 mg of catalyst, CO2-saturated 0.2 M NaOH

500 mg of catalyst in 1 L of CO2-saturated water


44 mm2 catalyst film under CO2/He with CO2:H2O = 1:2 40 mm2 catalyst film under CO2/He with CO2:H2O = 1:2 100 mg of catalyst, CO2 and H2O vapor

100 mg of catalyst in water solution, 50 mL min−1 CO2 bubbled 30 min 25 mg of catalyst in 25 mL of solution, CO2 pressure 1.4 bar catalyst-coated ceramic honeycomb monoliths with 177 channels with CO2 inlet



ref

1126.4

0.46 ∼4.76

1.8 (CH3OH) μmol cm−2 h−1 0.021 (CH4)/0.14 (CO)/ 0.028 (H2) μmol h−1 0.57 (CH4)/∼0.10 (CO) μmol h−1

6.0 71.76

0.14

140.8 (CH4) μmol h−1

358.4 (H2)/258.4 (CH4) /∼12 (C2H6) ppm 4.25 (CH4)/1.25 (H2)/ 23.5 (liquid phase) μmol h−1 (anatase) 0.017 (CH4)/0.002 (CO) /0.017 (H2) μmol h−1 3 (CH3OH) μmol cm−2 h−1 (λ > 420 nm); 8.3 (CH3OH) μmol cm−2 h−1 (UV−vis) 0.75 (CH4)/0.15 (O2) μmol h−1 11.96 (CH3OH) μmol h−1

1.07 1.76

293.50

1.41

0.17 (CH4)/0.009 (CO) μmol h−1 20.25 (CH4)/61 (CO)/0.015 (C2H4)/0.70 (C2H6)/ 0.005 (C3H6)/ 0.005 (C3H8) μmol h−1 0.13 (CH4) μmol h−1 0.44 (H2)/0.44 (CH4)/ 0.0006 (CO) μmol g−1 h−1

109

∼0.71

123

121 122

120*

118 119

116* 117

114 115

112 113

111*

110

108

107

106

105*

104

103

102

101*

99

96

95.7

29.6 (V-doped); 14 (Crdoped); 31.4 (Co- doped) 56.0

0.20

0.075 (solution); 1.43 (Na2S)

TCEN (μmol h−1)

0.55 (CH4)/11.75 (CH3OH)/5.2 (HCHO) μmol h−1 ∼0.089 (CH4)/∼4.17 (H2) μmol h−1

7.0 (CH4) μmol h−1

0.005 (CH4)/0.079 (CO) /0.19 (H2)/0.012 (O2) μmol h−1 3.70 (CH4)/9.58 (H2) (V-doped); 5.62 (CH3OH)/ 2.34 (H2) (Cr-doped); 5.22 (CH3OH)/5.38 (H2) (Co-doped) μmol h−1

∼95(CH4) μmol g−1 h−1

10.4 (CH4)/31.2 (CO) ppm h−1 (by N2H4); 8.45 (CH4)/25.35 (CO) ppm h−1 (by NH3) 0.008 (CH4)/0.008 (CO) μmol h−1 (solution); 0.007 (CO)/0.64 (HCOOH)/ 5.2 (H2) μmol h−1 (Na2S) ∼13 (H2)/4 (CH3OH)/3 (C2H5OH)/3 (CH3CHO) μmol g−1 h−1 (UVA);∼3.8 (H2)/∼0.2 (CH3OH) /∼0.2 (C2H5OH) /∼0.2 (CH3 CHO) μmol g−1 h−1 (vis) ∼20 (CH4) μmol g−1 h−1

66.75 (CO)/8.61 (CH4) ppm h−1

main products and highest yield

ACS Catalysis Review


TiO2, K4[Re6S8(CN)6] anatase TiO2 nanotube arrays TiO2 yolk@shell hierarchical microspheres mesoporous anatase TiO2 10 wt % montmorillonite loaded TiO2 TiO2−SBA-15 Cu-TiO2-ZSM-5 20 wt % TiO2−KIT-6

20 W LED (λ > 400 nm)


200 W Hg-lamp

200 W Xe/Hg-lamp

14 W UV-lamp (λ = 254 nm)

300 W UV lamp

7494

Au

Cu(OH)2 cluster

0.5 wt % Pt/1 wt % MgO 5 wt %MgO

co-catalyst

continuous monolith reactor, 20 mL min−1 CO2/ CH4 (1:1) gas catalyst in 1.5 bar 6000 ppm of CO2 and H2O vapor 500 mg of catalyst in 500 mL of CO2-saturated 0.1 M NaHCO3 200 mg of catalyst, 50 mL min−1 flow of 20% CO2 in He and H2O vapor


30 mg of catalyst, CO2 and H2O vapor, 150 °C 50 mg of catalyst, CO2 and H2O vapor, 200 °C 100 mg of catalyst, CO2 and H2O vapor (150 °C) 10 mg of catalyst with 0.1 μmol RePH and 1.5 μmol MOD in 3 mL of DMF solution containing 0.1 M BIH, CO2 bubbled 30 min before irradiation 250 mg of catalyst in 250 mL of water, 10 mL min−1 CO2 catalyst in CO2/CH4/He mix gas (40 psi) 100 mg of catalyst in H2O/TEA/DMF (1:1:3), CO2 bubbled 30 min 100 mg of catalyst in H2O/TEA/DMF (2:1:7), CO2 bubbled 30 min 4 cm2 nanotubes film in CO2-saturated water 100 mg of catalyst, CO2 and H2O vapor

20 mg of catalyst, 2 MPa CO2 and 1 mL of H2O



ref

135 136

10 (CH3OH) /9 (C2H5OH) nmol h−1 cm−2 0.21 (CH3OH) μmol h−1

1.78 (CH4)/4.82 (CO)/ 0.04 (CH3OH)/0.36 (H2) μmol h−1

24.12

142

141

25.0 (CH3OH) μmol h−1

150.2

138, 139 140

1.32 (CH4)/3.44 (CO) /2.74 (CH3OH)/2.88 (H2) μmol h−1 237.5 (CO)/55.7 (C2H6) /0.557 (CH3OH) μmol g−1 h−1 1.13 × 10−3% (AQY)

137

133*

149 (CH3OH) μmol in 24 h

33.75

131 132*

130

126 127 128 129

125

124

0.12 (CH4)/0.03 (C2H4) /0.04(C2H6)/36.25 (H2) μmol in 6 h 18% (CH4) and 14% (CO2) conversion rate 187.6 (CH3OH)/236.4 (O2) μmol in 24 h

1.26

0.80

1.20

1.76

0.0006 (H2)/0.22 (CH4) μmol h−1 0.6 (CO) μmol h−1 produced 15.3% CO 0.40 (CO) μmol h−1 (90 wt % TiO2) TONCO ≥ 570 in 30 h

0.44

TCEN (μmol h−1)

0.025 (CH4)/0.017 (C2H6) μmol h−1


a

Investigations providing evidence that CO2 was the actual carbon source for the carbon-containing products through 13CO2 labeling or other techniques were marked with “*” in the last column of the table.

300 W UV lamp

TiO2, Cu(II) phthalocyanine TiO2, Ru(bpy)3Cl2

125 W Hg-lamp 20 W LED (λ > 400 nm)

Xe-lamp Hg-lamp (λ < 390 nm) Xe-lamp (λ > 400 nm) Xe-lamp (λ > 420 nm)

Fluoresecent bulbs (λ = 400 −800 nm)

W W W W

Au@TiO2 yolk−shell hollow spheres anatase TiO2

photocatalyst

anatase/rutile TiO2 Mg−Al LDO/TiO2 Mg−Al LDO/TiO2 TiO2, 5′-(4-[bis(4- methoxymethylphenyl)amino]phenyl-2,2′-dithiophen-5-yl)cyanoacrylic acid N-TiO2, 0.5% chlorophyll

400 100 450 450

300 W Xe-lamp

300 W Xe-lamp

light source

Table 1. continued



Review

ACS Catalysis

related poor light absorption remain as the main drawbacks of TiO2-based photocatalysts. Non-TiO2 photocatalysts including Ge, Ga, W, and Ni-based materials, graphitic carbon nitride (gC3N4), Ag-AgX plasmonic systems, photoactive metal−organic frameworks (MOFs), and graphene-based systems usually offer improved properties such as smaller bandgap, better matched VB and/or CB levels, more favorable surface chemistry and structures, and future improvements in activity and selectivity of CO2 reduction reactions can be attained by using the same strategies developed for TiO2. Nevertheless, further development and investigations of novel photocatalysts and CO2 reduction systems should be imperative by considering the material’s stability, cyclability, and other features necessary for a practical application. The current CO2 reduction systems developed so far can be roughly divided into one-step excitation and two-step excitation photocatalytic processes as shown in Figure 3. Generally, an effective light harvesting of photocatalyst could guarantee the photogenerated charge generation thermodynamically, and the charge migrations from bulk to surface as well as the subsequent redox reactions would affect the overall efficiency of the solar-to-fuels conversion kinetically. Also, the bandgap structures of semiconductors determine the feasibility of the CO2 reduction reaction, and its surface state and area, morphology, crystal facet, and phase effects must be considered in the design and fabrication of CO2 reduction system.6,7 Therefore, this section will focus on the development in the photocatalysts for one-step excitation CO2 reduction systems in the last 3 years. 3.1. TiO2 and TiO2 Engineering. Among the various semiconducting materials applied in CO2 photoreduction to solar fuel, TiO2 as UV-light-responsive photocatalyst is the most widely studied because of its high chemical stability, nontoxic nature, low cost, and easy availability. However, the energy conversion efficiency of TiO2 for CO2 reduction is still low, mainly due to its wide bandgap (Eg ≈ 3.0 eV), fast photogenerated charge recombination, and reverse reactions (i.e., reoxidation of the reduced products or intermediates), and thus, many strategies, such as crystal phase engineering, crystal facet engineering, surface engineering, bandgap engineering, micro/nanostructure constructing, nanocomposite fabricating, and surface photosensitization, among others, have been attempted to improve the photoactivity of TiO2.8,15,16 Those fundamental understandings gained through the above strategies to modify TiO2 might be helpful guidance for the development of more efficient photocatalysts for CO 2 reduction. Therefore, TiO2 as a model photocatalyst is discussed in the first place to show the effects of the modification methods (crystal phase, crystal facet, and surface engineering) on the photocatalytic mechanism, activity, and selectivity of CO2 reduction reactions. Their corresponding experimental conditions, activities, and selectivity are listed in Table 1. It must be noted that the other strategies, such as bandgap engineering, micro/nanostructure constructing, nanocomposite fabricating, cocatalyst, and dye loading, are not included here, which will be discussed in the following sections. 3.1.1. Crystal Phase Engineering and Phase Heterojunction. Among the three polymorphs (rutile, anatase, and brookite) of TiO2 that naturally exist, rutile is less active mainly due to its fast charge recombination, whereas anatase is demonstrated to be highly active and is widely researched in the field of photocatalytic CO2 conversion.53,81−85 Brookite is rarely studied in photocatalysis likely due to the past difficulties

the photocatalyst deactivation, the CO2 reduction products’ reoxidization, and/or poisoning effects. Among which, the main photocatalyst deactivation mechanisms can be due to the deposition of poisoning species (i.e., adsorbed organic compounds and/or unreactive species on the photocatalyst reactive sites) and material instability such as oxidation and/or photocorrosion, which usually raises a serious issue about the photocatalyst stability. For example, a long-term reaction of CO2 photoreduction with H2O could lead to the deactivation of most p-type semiconductors such as ZnTe, Cu2O, and GaP due to their photocorrosion and poor stability in aqueous electrolytes.37 Also, the extensively used doped TiO2 photocatalysts undergo photocorrosion during the operation of the photocatalytic cycle, leading to the leaching of the dopant metal and gradual deactivation of the catalyst.2 Since the dopant element d orbital is continuously populated and participating in the visible light excitation, the dopant metal undergoes consecutive redox turnovers during operation of the photocatalyst that should lead finally to the migration of the ion outside the lattice (photocorrosion) and deactivation.2 In addition, it was reported that Pd on the surface of TiO2 partly oxidizes to form PdO during the progression of CH4 formation, which deactivated the photocatalytic behavior of Pd/TiO2.64 Occasionally, the CO2 reduction products can act as poisons or occupy the reactive sites on the photocatalyst surface to stop the photocatalytic reduction or cause apparent deactivation of photoctalysts. Moreover, some CO2 reduction products such as methanol with high reactivity in the photocatalytic conditions could decompose (back reaction) especially when the hole accumulation is serious, which causes a stationary low concentration.2 Although continuous flow operation could alleviate the CO2 reduction product decomposition and photocatalyst deactivation, provided that conversion values are sufficiently increased, the recovery of this substrates in highly diluted aqueous solutions would still be an important challenge.2 Clearly, the stability of photocatalysts under reaction conditions (notably resistance to photocorrosion) and routes to their incorporation into reactor designs which facilitate efficient light absorption and mass transport are key considerations for improving the photocatalytic performance of the semiconductors for CO2 reduction.

3. PHOTOCATALYSTS FOR ONE-STEP EXCITATION SYSTEMS Since the first report on photocatalytic CO2 conversion over TiO2 in the 1970s,12 numerous efforts have been focused on the photocatalytic mechanism, activity, and selectivity of the CO2 reduction by using various semiconducting materials.1−9,13−30 An ideal photocatalyst for CO2 conversion should possess: (1) narrow bandgap and good light-harvesting properties, (2) proper CB and VB edges positioned to drive both CO2 reduction and H2O oxidation, (3) efficient spatial charge separation, (4) possibly large surface area with favorable chemistry and energetics, (5) well-developed suitable porosity with good mass transfer properties, (6) high purity and crystallinity, and (6) proper morphology with a short bulk-tosurface distance. Additionally, the experimental conditions will also affect the activity and selectivity of CO2 reduction reactions. For many years, TiO2 is the most widely studied photocatalyst for CO2 conversion, and it is often coupled with other metal oxides, metals, and carbon nanostructured materials for improving the activity. However, the wide bandgap and the 7495


Review

ACS Catalysis

{001} facets.57 The surface Ti sites exposed by the anatase {101} and {001} facets exhibit significantly different Lewis acidity and polarizing power, possibly resulting in different reactivity toward CO2, and the higher reactivity of {001} facets is related to its stronger (Lewis and Brønsted) basicity of the surface oxygen sites as compared with {101} facets.57 On the contrary, the present authors found that the activity order for gaseous-phase CO2 reduction to CH4 is {001} < {101} < {010}.82 On one hand, the experimentally determined order of CB value ({001} < {101} < {010}) might create the above activity order because a higher CB level would more effectively promote the electron transfer to CO2.82 On the other hand, the attenuated total reflectance-Fourier transform infrared (ATRFTIR) spectra indicate that the {010} facets are more active and effective than the {101} facets for the CO2 adsorption in the presence of H2O vapor, and the {001} facets have the worst performance, which also results in the lowest activity of CO2 reduction to CH4 over {001} facets.82 Moreover, the present authors also found that a reversed activity order for the gasous phase CO2 reduction to CH4 when Pt was deposited onto the exposed {001} and {010} facets of anatase.81 Without Ptloading, anatase {010} facets shows a higher activity for CO2 reduction than the {001} facets due to the higher CO2 adsorption capability of {010} facets and longer photogenerated charge lifetime, which is consistent with the above results.82 After 1.0 wt % Pt loading, the small Pt nanoparticles on {010} facets can more efficiently promote the charge separation compared to that of the agglomerated Pt nanoparticles on {001} facets, and thus, {001} facets showed a higher activity than {010} facets.81 Photocatalytic CO2 reduction activity and/or selectivity of TiO2 can also be enhanced by coexposed different facets.83,84,87 Yu and co-workers reported CO2 reduction over anatase TiO2 with coexposed {001} and {101} facets and the effect of the ratio of these facets on the CO2 reduction processes.83 DFT electronic structure calculations (Figure 4) indicate that the

in synthesizing high-purity brookite because of its metastable property.49,86,87 In 2012, Li’s group reported that pure brookite has a higher activity of CO2 reduction than rutile, and the surface-defective brookite is even more active than anatase due to the lower formation energy of VO on brookite.86 More recently, it was found that the CO2 reduction activity of an exposed-crystal-facet-controlled brookite nanorods is dependent on its aspect ratio because the {210} facets work as reduction sites, and Au or Ag on the reduction facets of the brookite nanorods can increase the CH3OH production.49 Our recent study86 also indicated that brookite TiO2 quasi nanocubes (mean size of ∼50 nm), mainly surrounded by four {210} and two {001} exposed facets, show an activity for CO2 reduction. The above investigations indicate that brookite would be active in CO2 reduction to solar fuel production, which can be further adjusted through proper microstructure and/or crystal facet modification.86,87 The feasibility of CO2 reduction using different kinds of mixed-phase TiO2 (brookite, anatase, and rutile) has also been evaluated.88−90 It is believed that the type-II heterojunction (cf. Figure 3c) between different phases of TiO2 play a significant role in improving the charge separation.89,90 For example, Li’s group reported that the anatase/brookite interfaces of bicrystalline TiO2 promote the charge transfer and separation and thus cause higher activity of CO2 reduction to fuels (CO/CH4) than single-phase anatase or brookite.89,90 Moreover, the anataserich anatase/brookite mixture (75% anatase and 25% brookite) is far more active for CO2 reduction than the commercial anatase-rich anatase/rutile TiO2 (P25) with a similar anatase fraction.89,90 This result was ascribed to the following reasons: (1) brookite itself is more active than rutile as a photocatalyst, (2) brookite has a higher CB edge than rutile, which can promote the CO2 reduction reaction with H2O that has a high reduction potential, and (3) the anatase/brookite heterojunction may lead to enhanced charge separation because the excited electrons on brookite CB may transfer to anatase CB due to a slightly higher CB edge of brookite.89 These findings suggest a new direction for the development of efficient TiO2based mixed-phase photocatalysts for CO2 reduction. Further investigations on the interactions of the different phases during the CO2 reduction reactions are necessary for understanding the enhancement mechanism of the mixed-phase TiO2’s activity. 3.1.2. Crystal Facet Engineering and Surface Heterojunction. Crystal facet engineering of TiO2 is one of the most effective approaches to deeply understand the CO2 reaction mechanism over various exposed special facets.53,59,81−84 As for anatase TiO2, the theoretically determined surface energy order of the low index facets is {101} (0.44 J m−2) < {010} (0.53 J m−2) < {001} (0.90 J m−2).15 Among which, {101} facets consist of 50% penta-coordinated Ti atoms (Ti5c) and 50% hexa-coordinated Ti atoms (Ti6c), while {010} and {001} contain 100% Ti5c, which is regarded as the active site for the heterogeneous photocatalytic reactions.91 It means that the most stable {101} facets has a low reactivity for the CO2 reduction, and thus, various approaches to facet engineering are applied to expose the desired high reactivity facets such as {001} and {010}.81−84 Recent investigation by combining FTIR results with periodic DFT calculations indicated that the most stable anatase {101} facets can only weakly adsorb CO2 that mainly retains its molecular properties, while the formation of carbonate and bicarbonate species occurs preferentially on the

Figure 4. (a) Density of states (DOS) plots for {101} and {001} surface of anatase TiO2. O 2p, Ti 3d, and TDOS are partial DOS of O 2p, partial DOS of Ti 3d, and total DOS, respectively. (b) {001} and {101} surface homojunction.83 Figure reproduced with permission from ref 83. Copyright 2014 American Chemical Society.

{101} and {001} facets of anatase TiO2 exhibit different band structures and band edge positions, which then leads to the formation of a surface heterojunction between the coexposed {101} and {001} facets within a single anatase particle.83 It is beneficial for the transfer of photogenerated electrons and holes to {101} and {001} facets, respectively. Therefore, the 7496


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the surface engineering is also used to improving the CO2 reduction activity.57,60,92−95 Surface VO and Ti3+ sites on TiO2 are suggested to be the active sites, where CO2 molecules are absorbed with oxygen atoms bridging with the defected sites, and the attraction between VO and CO2 molecule may lower the reactive barrier due to the generation of an unexpected affinity interaction as proved by DFT calculation.57 Moreover, Ti3+ sites are surface electron centers for the formation of the negatively charged CO2•− species, in which the overlap between the (C−O) π* antibonding orbital of CO2 and Ti 3d orbital is favorable for electron transfer toward CO2.60 In brief, surface engineering is achieved not only by increasing defect sites but also by creating a surface state on the TiO2 surface.60,93,94 Recently, it was found that the oxygen vacancies (Ti3+ sites) initially form on TiO2 surface under H2 plasma treatment at 150 °C and preferentially diffuse into the bulk with further hydrogenation.93 The slightly hydrogenated TiO2 (s-H-TiO2) exhibits enhanced activity for CO2 reduction compared with the pristine TiO2 nanoparticles, while the highly hydrogenated H-TiO2 (h-H-TiO2) displays much worse activity.93 Further investigations reveal that the higher ratio of trapped holes (O− centers) and a lower recombination rate induced by the increase of surface defects might be the critical factors for the high activity of s-H-TiO2; however, h-H-TiO2 possesses a high concentration of bulk defects, leading to a significantly decreased amount of O− centers and enhanced nonradiative recombination, which strongly inhibit the activity.93 Similarly, in situ generation of oxygen through the thermal decomposition of the peroxo-TiO2 complex is also applied to modify the surface of TiO2,73 in which the excrescent Ti−O−O bond in oxygen-rich TiO2 narrows the bandgap to 2.95 eV along with its enhanced visible light absorption intensity, and serves as oxygen defects and electron scavenger to reduce the recombination rate, which is beneficial for participating in the CO2 reduction processes.95 Surface engineering of TiO2 can also be conducted by loading certain non-noble metal species,88,94 inorganic base,60 or organic amine.96 Those introducing species can induce changes in both electronic structure and surface states, and promote the CO2 adsorption. Andreu and Morante’s group have investigated that the role played in the activity of CO2 reduction to CO/CH4 by using the surface layers of bare and Mg-loaded mixed-phase TiO2 (anatase/rutile/brookite) nanoparticles, and they found that a small amount of Mg-loading promotes an increase of the Ti3+ (acting as electron trap) sites and the modified oxygen states (OS, acting as hole trap) concentration, resulting in a low charge recombination rate according to the considerably enhanced activity (3-fold with 0.2 wt % Mg-loading).94 Moreover, a straightforward correlation between the total activity obtained from the photocatalytic process and the density of Ti3+ and OS is stated, revealing the outstanding significance of the surface chemical structure.94 Similarly, Li’s group found that Cu/TiO2 (P25) nanoparticles, prepared by a simple precipitation and calcination method, are dominated by Cu2+ species, and thermal pretreatment in H2 atmosphere results in the transition to a surface dominated by mixed Cu+/Cu0 and the formation of defect sites such as VO and Ti3+.88 Compared with the unpretreated TiO2, the H2pretreated Cu/TiO2 demonstrates a 10-fold and 189-fold enhancement in the production of CO and CH4, respectively. This significant enhancement is mainly attributed to the synergy of the following two factors: (1) the formation of

coexposed {101} and {001} facets promote the charge separation in space and results in the activity enhancement of CO2 reduction to CH4.83 More recently, the same group prepared single-crystalline anatase TiO2 nanocubes with coexposed {100} and {001} facets, which display especially high activity of CO2 reduction to CH3OH/CH4, due to the synergistic effects of better crystallization, a more negative CB position, and coexposed {100} and {001} facets.84 Moreover, the activities of CO2 reduction can be enhanced significantly after removing the capping F− ions, regardless of the relatively lower surface area than P25 (∼20 vs ∼50 m−2 g−1). This enhancement can be ascribed to its slightly higher CB level (∼0.20 V over P25), which results in more reductive electrons, better charge separation, and consequently better charge transfer to TiO2 surface.84 Except for the enhanced activity, the selectivity of CO2 reduction is also influenced by different properties of the coexposed facets of TiO2.87 More recently, brookite TiO2 quasi nanocubes (mean size of ∼50 nm), mainly surrounded by four {210} and two {001} exposed crystal facets, were used as photocatalyst for CO2 reduction. The in situ diffuse reflectance infrared-Fourier transform spectra (DRIFTS) demonstrate that {210} facets of brookite had adsorbed carbonate previously, resulting in the selective CO2 conversion into CO, whereas bicarbonates with protons are preferentially adsorbed on {001} facets, which then enhance the selectivity of CO2 conversion into CH4.87 Furthermore, the Ag nanoparticle size and its distribution on the exposed facets of the brookite nanocubes are significantly affected by the Ag-loading levels (Figure 5), which then cause obvious differences in the activity and selectivity of CO2 reduction to CO/CH4.87

Figure 5. Possible influencing mechanism of the Ag-loading level on the activity and selectivity of CO2 reduction to CO/CH4.87 Figure reproduced with permission from ref 87. Copyright 2016 Elsevier B. V.

The above interesting findings on a single facet and/or coexposed ones not only show the importance of the facet engineering, such as the types/ratio for the optimization of exposed facets, suitable cocatalyst loading, and effective surface heterojunction fabricating, on the enhancement of performance for CO2 reduction over TiO2, but also provide new insights into the design and fabrication of advanced photocatalytic materials for CO2 reduction. In addition to the different activity and selectivity of the exposed facets, the distribution and morphology of cocatalysts on different exposed facets and its effects on the photogenerated charge separation, CO2, and its reduced products adsorption properties as well as light harvesting might be also improtant factors governing the activity and selectivity for CO2 reduction. Further investigations on those aspects would achieve more advanced properties for environmental and energy applications. 3.1.3. Surface Engineering. Since the surface layers of photocatalysts can determine the surface-state density, CO2/ H2O adsorption/activation, and charge recombination kinetics, 7497


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methods such as bandgap engineering, cocatalyst loading, dyeloading, and nanocomposite construction, which would influence the light harvesting and photogenerated charge transfer/separation of CO2 reduction reactions. Their corresponding experimental conditions, activities, and selectivity are also listed in Table 1. 3.2.1. Bandgap Engineering. Since TiO2 is a wide bandgap semiconductor which only absorbs lights in UV region, the band structure engineering via element doping is always applied to narrow the bandgap and extend the absorption spectrum into visible region.7,15,26,97,98 In the last 3 years, TiO2 doped with nonmetal and/or metal ions is still extensively used to enhance the spectral responsive region and activity of CO2 reduction.50,99−102 As reported previously, cation-doping can affect the CB position of TiO2 in the first place, and aniondoping can more significantly change the VB position of TiO2.88 Usually, F, N, C, S, or P anions instead of O element in anatase TiO2 lattice would affect the band structures by the hybrid orbital with O 2s.15,98 Among which, C and N are suitable doping elements as its p orbital contribute to VB by mixing with O 2p orbital.15,98−100 Recently, it was reported that the C and N co-doped sodium titanate nanotubes (C,N-TNT) with an intermediate doping concentration yields the maximum CH4 yield of 9.75 μmol g−1 h−1, 2.7 times higher than that of the bare TNT (3.7 μmol g−1 h−1).100 The improved activity is attributed to the enhanced light absorption, the formation of anatase phase, the significant surface areas, and favorable competition between the dual role Na+ ions play in (desired) CO2 adsorption and (unwanted) charge recombination.100 Except for the VB upward shift that promotes the light harvesting, significant selectivity of the dominant reduced products from CO2 reduction under visible light illumination has been exhibited for N-doped TiO2 (N-TiO2) prepared by using N2H4 and NH3 as N-sources; that is, CO is the main product with NH3-doped N-TiO2, while CH4 is the main product with N2H4-doped N-TiO2 as shown in Figure 6.99 The

surface defect sites promoting CO2 adsorption and subsequent charge transfer to CO2; (2) the existence of Cu+/Cu0 facilitates the photogenerated electron and hole trapping at different sites.88 Surface chemistry of CO2 demonstrated that adsorption of CO2 molecules on metal or oxide surfaces is usually accompanied by activation processes.60,96 Compared with the normal molecule, CO2 in the chemisorption state (mainly carbonate and/or CO2− anion) has a bent O−C−O bond angle and a decreased LUMO, which will favor the charge transfer from the photoexcited semiconductors to the surface-adsorbed CO2 molecules, and thus, the photocatalytic CO2 (an acidic molecule) reaction rate can be enhanced by coloading of basic substances.60,96 For instance, Ye’s group60 reported that NaOH-treated anatase TiO 2 can enhance the surface hydroxylation, which is an effective way to promote chemisorption of CO2 molecules through a meaningful activation step, and leads to significantly improved activity of CO2 reduction to CH4 through a direct proton coupled electron transfer (PCET) in TiO2/NaOH interface without cocatalyst loading.60 Similarly, Xue’s group prepared aminefunctionalized TiO2 by using monoethanolamine (MEA) that possesses a hydroxyl (−OH) group for covalent attachment on TiO2 and a primary amine (−NH2) group to endow an amineterminated surface, and they found that the amine groups on TiO2 surfaces can effectively facilitate the capture of CO2 for photoreduction to hydrocarbon fuels.96 The amine functional groups enable “C−N” bonding with CO2 to form carbamate, which acts as an activation process of CO2 because carbomate is believed to have higher reactivity than the linear CO2 molecule. These activated CO2 molecules are very close to the TiO2 surface and have a higher chance to establish direct interactions with surface Ti cations (Ti3+ or Ti4+), which allows electrons from the photo-excited TiO2 to implement the reduction reactions to produce CO or CH4.96 The above TiO2 engineering can significantly change the chemical structure of the outermost layers of TiO2, which might promote the CO2 capture/activation and the charge separation synergistically, thereby causing improved activity and selectivity of the photocatalytic CO2 conversion. Nevertheless, there are disputable issues such as (1) how to play a role in the interfaces between difference crystal phase/facets for the enhanced CO2 reduction activity;83−87 (2) determining the main function of those non-noble metal species loaded on TiO2 (as surface active sites for CO2 adsorption and activation, or cocatalyst for promoting charge transfer).88,94 Also, the effects of these defects (TiO2 itself and induced ones due to the facet/ surface engineering) or their potential synergies between the loaded metal species and the defects are still poorly understood.88 Further investigation and understanding of the CO2 adsorption/activation and the exact charge-transfer mechanisms in CO2 reduction on metal-loaded TiO2 by decoupling the effects of loaded metal valence from TiO2 defect sites (i.e., VO/Ti3+) and exploring the synergies between them have very critical significance in both theory and practice. 3.2. TiO2 Modification. Generally, the above-mentioned TiO2 engineering (such as crystal phase engineering, crystal facet engineering, and surface engineering) can only promote the charge separation and/or CO2 surface adsorption, and thus the activity of CO2 reduction; however, it cannot significantly alter its band structures, which is not beneficial for effectively utilizing the solar light due to the wide bandgap of TiO2.8,15,16 Therefore, this section will focus on the other modification

Figure 6. Schematic drawing of the product selectivity of CO2 reduction over N-doped TiO2 fabricated by using N2H4 or NH3 as N sources.99 Figure reproduced with permission from ref 99. Copyright 2015 Elsevier B.V.

existence of the reducing N−N groups in N2H4-doped N-TiO2 surfaces provides a reducing environment, leading to CH4 acting as the dominant reduced product.99 It implies that by selecting N-source with the appropriate chemical bond form, control over the desirable reduced products of CO2 reduction under visible light illumination is possible. Introduction of some metal cations (such as Cu,101−104 Co,105,106 Ni,107 Ce,108,109 Mo,110 In,111 Ti,112 V,106 and Cr106) into the host framework not only modifies the crystallinity of TiO2 but also influences its light absorption properties. Also, the enhanced red-shifting from the UV region to longer wavelengths in the visible region can be observed with enhancing cation-doping levels.101−112 For example, Cudoped TiO2 shows a slightly but steadily decreased bandgap 7498


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separation.50 Nevertheless, the CO2 reduction would be hindered by high element-doping levels for TiO2 photocatalysts doped with Cu,101−104 Co,105,106 Ni,107 Ce,109 and In,111 likely due to the extra cation species acting as multiple trap sites and thus facilitating the charge recombination. 3.2.2. Metal Co-Catalyst Loading. Generally speaking, metals and/or its oxidized species with Fermi level below the semiconductor’s CB bottom can be applied as cocatalyst to form Schottky barrier at semiconductor/cocatalyst interface, and promote the charge separation, the multielectron transfer process, and the reaction rate during photocatalytic CO2 conversion process.60 Therefore, noble metals (such as Pt,48,54,55,81,113,114 Au,49,115,116 Ag,49,86,117−119 and Pd112,120), non-noble metals (such as Cu121,122), and/or their alloys species (Au/Cu,39 Cu/Pt,55 Cu/Pd,112 and Au/Pt123) are widely used as cocatalysts for the photocatalytic CO 2 conversion. Although Pt is the most extensively used cocatalyst for photocatalytic H2 production, it is not the best one for the selectivity of CO2 reduction reactions because it always increases the H2 production rate at the same time by consuming the photogenerated electrons, which would compete with the CO2 reduction reactions.113,117,118 For instance, Zhang and Wang’s group has reported that the activity of CH4 production increases in the sequence of Ag < Rh < Au < Pd < Pt (corresponding well to the increase in the efficiency of the charge separation of those noble metal-loaded TiO2), but the reduction of H2O to H2 is accelerated more than the reduction of CO2 in the presence of Pt, leading to a lower selectivity for CO2 reduction and limited increases in CH4 production activity.113 Usually, Pt can enhance the activity of CO2 reduction to CH4 but not to CO because the produced CO can be tightly bound to the Pt cocatalyst, which results in a poisoning effect. Since the binding energy of Ag/CO is much weaker than that of Pt/CO, which is beneficial for CO easily removing from Ag cocatalyst surface and renewing for the subsequent CO2 reduction reactions, Ag is widely employed as cocatalyst of TiO2 for CO2 reduction and shows excellent selectivity in CO production.49,86,117−119 Similar to the above single metal and its oxide cocatalyst, codecorated species (nanoalloys) such as Au/Cu,39 Cu/Pt,55 Cu/Pd,112 and Au/ Pt123 can more effectively promote the charge separation by changing the distribution of electrons of TiO2. Also, the valence states of the loaded metal species as cocatalyst influence the activity and selectivity of CO2 reduction reactions.48,90,120 For instance, Zhao and Zhang’s group has found that TiO2 nanoparticles modified by Pt2+ ions and Pt nanoparticles (Pt2+-Pt0/TiO2), in which Pt2+ ions are doped into TiO2 lattice and Pt nanoparticles are deposited on TiO2 surface.48 The introduction of Pt0 nanoparticles and Pt2+ ions with appropriate ratio effectively enhanced the yields of H2 and CH4 but showed no significant effect on CO production.48 The enhanced activity of Pt2+-Pt0/TiO2 is due to the low charge recombination caused by Pt nanoparticles and the strong visible light absorption as well as the high surface area induced by Pt2+.48 Similarly, Ag nanoparticles deposited on porous TiO2 microspheres with controllable Ag valence were prepared through a sequential hydrothermal, ultrasonic spray pyrolysis, and in situ photoreduction process.90 It is found that Ag0incorporated TiO2 is more active than Ag+-incorporated one, indicating the importance of ensuring the right Ag valence on the catalyst surface. Ag0 serving as a sink can accumulate the photogenerated electron transfer via the Schottky barrier at the Ag0-TiO2 interface and then promote the H2 production from

upon enhancing Cu(II) concentration from 0 to 3 mol % and has the potential to enhance the formation of HCOOH and CO in the presence of sulfide as an electron donor.101 Ola and Maroto-Valer have reported that the absorption spectra of Cubased TiO2 showed an increased shift in the visible light with enhancing Cu loading concentration in comparison with pure TiO2.102 This change in light absorption is attributed to the incorporation of Cu+ ions into TiO2 crystal lattice via the substitution of Ti4+ by Cu+, as well as these Cu+ species serving as electron traps to suppress the charge recombination and facilitate the multielectron reactions, which improved the CO2 reduction activity.102 The decline in the production rate observed upon further increasing Cu+ concentration is probably due to the coverage of TiO2 surface with excess metal particles, which inhibits the interfacial charge transfer due to the insufficient light energy available for activation of the photocatalyst.102 Similarly, Kang’s group reported that the absorption bands of x mol % M-TiO2 (M = Cu, Fe) powders, in which M ions wellinserted into the anatase TiO2 framework, are shifted to higher wavelengths with enhancing M ion concentration.103,104 The prepared powders were fabricated as the TiO2/M-TiO2 doublelayered films for CO2 reduction, and the CH4 generated from the photoreduction of CO2 with H2O increases remarkably over the double-layered film. In particular, CH4 is mostly evolved over the TiO2/5.0 mol % Cu-TiO2103 and TiO2/1.0 mol % Fe-TiO2104 doubled-layered film with 2 and 7 times higher level of production compared to that on TiO2(bottom)/ TiO2(top) double-layered film, respectively. This study shows that the activity over TiO2/M-TiO2 double-layered films can be enhanced by effective charge separation and inhibited charge recombination due to interfacial transfer between TiO2 and MTiO2.103,104 Interestingly, an improved activity of Ce-doped TiO2 for CO2 reduction occurs because of the decrease in the bandgap energy, which can shift the absorption spectrum into visible region,108,109 and also because the Ce3+/Ce4+ mixture retards the charge recombination and promotes the yielding of oxygen radicals with high reduction capacity. 108,109 A similar phenomenon can also be observed from Mo-doped titanate nanotubes (TNTs), in which Mo structure and VO sites are the key factors controlling the activity of CO2 reduction in the presence of monoethanolamine (MEA).108 For the Mo-doped TNTs calcined at 500 °C, the partial corruption of TNTs into anatase particles causes the reduction of Mo species from Mo6+ to Mo5+ and produced VO, which results in the highest CO2 reduction ability.108 More recently, Ti3+-self-doped rutile TiO2 was synthesized through hydrothermal and hydrochloric acid treatment of rutile TiO2.112 It was found that the incorporation of Ti3+ into TiO2 can narrow the bandgap (2.90 eV), leading to significantly enhanced activity of CO2 reduction to CH4 under visible light irradiation.112 Moreover, the activity for CH4 production can be further improved by introducing 1.0 wt % CuI/Pd on the Ti3+-TiO2 surface as cocatalyst.112 Co-doping of nanostructured photocatalysts with anion and/ or cation is also an excellent pathway for improving textural and photocatalytic properties for the respective application domain.50,100 For example, V−N codoped TiO2 nanotube arrays (TANs) were fabricated by a simple two-step method, and XPS data reveal that N is found in the forms of Ti−N−O and V incorporates into the TiO2 lattice in V−N codoped TNAs.50 V and N codoping results in remarkably enhanced activity for CO2 reduction to CH4 due to the effective charge 7499


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charge recombination due to the electron scavenging ability of Au nanoparticles. The physical separation of charge carriers results in a 7-fold longer-lived state compared to bare TiO2 films, which is crucial in order to perform the slow and multielectron demanding CO2 reduction reaction.116 Compared with Ag or Au plasmonic nanoparticles, nonnoble metal nanoparticles are rarely reported to exhibit the SPR effect for visible light harvesting. Recently, Fan’s group reported plasmonic Cu nanoparticles modified TiO2 nanoflower films for CO2 reduction to CH3OH.121 The investigations indicated that Cu nanoparticles and TiO2 film both exhibit visible lightharvesting properties based on the SPR effect of Cu nanoparticles and unique nanostructures of TiO2 film, and Cu/TiO2 films exhibit better CO2 reduction activity due to the charge-transfer property and SPR effect of Cu nanoparticles.121 The CH3OH production activity of 0.5 Cu/TiO2 film is 6.0 times higher than that of pure TiO2 film.121 Except for single metal nanoparticles, some bimetal or metal alloy species such as Au/Cu,39 and Au/Pt,123 are also used as cocatalysts and plasmonic nanoparticles for the photocatalytic CO2 conversion. Xue’s group presents the fabrication of TiO2 nanofibers codecorated with well-dispersed Au and Pt nanoparticles through facile electro-spinning, which shows remarkably enhanced activity on both H2 generation and CO2 reduction.123 The great enhancement is attributed to the synergy of electron-sink function of Pt and SPR effect of Au nanoparticles, which significantly improves the charge separation of TiO2 as shown in Figure 7.123 Similarly, P25

H2O splitting and the multielectron process for CH4 formation; however, the Ag/TiO2 composites containing Ag+/Ag0 shows a simultaneous production of H2, CO, and CH4 from the CO2 reduction system by using methanol as a hole scavenger under UV−vis irradiation.90 Except for Cu nanoparticles loaded on TiO2 as cocatalyst,99 CuO as cocatalyst can significantly enhance the activity of a TiO2 hollow microsphere for CO2 reduction to CO/CH4.100 Furthermore, the CuO incorporation provides the TiO2 with higher selectivity in CH4 production. In addition, after heat treatment at 250 °C in Ar or H2, the asobtained Cu2O-TiO2 and Cu-TiO2 demonstrate a further enhancement in photocatalytic CO2 reduction to CH4.122 3.2.3. Surface Plasmon Resonance Metal Loading. In the last 3 years, there are also increasing reports on noble metal cocatalysts (such as Ag, Au) with localized surface plasma resonance (SPR) effects, which could improve the visible-light harvesting of the photocatalytic systems.39,49,86,87,115−119,121−124 Among various plasmonic nanoparticles, Ag nanoparticles deposited on various materials are the most investigated ones in the field of CO2 reduction.117−119 For example, Ag nanoparticles deposited on TiO2 nanowire films (Ag/TiO2 NWFs) are successfully fabricated by the combination of an ethylene-glycol-assisted hydrothermal route and a microwaveassisted chemical reduction process.117 The investigations indicate that Ag nanoparticles can be well-dispersed on the anatase/rutile mixed-phase TiO2 film and the composites exhibit visible-light-harvesting properties based on the SPR effect of Ag and unique nanowire structure of TiO2 film. Furthermore, the activity of CO2 reduction to CH3OH can be greatly enhanced due to the charge-transfer property of Ag nanoparticles and the efficient light utilization based on the overlapped visible-light-harvesting of Ag nanoparticles and TiO2 NWFs.117 Additionally, a synergetic photocatalytic process is proposed to understand the mechanism of plasmon enhanced activity based on the experimental results under different conditions.117,118 Zou and co-workers synthesized double-shelled hollow hybrid spheres consisting of plasmonic Ag and TiO2 nanoparticles through a simple reaction process, and they found that Ag nanoparticles are dispersed uniformly in the TiO2 nanoparticle shell.118 The plasmonic Ag-TiO2 hollow sphere proves to greatly enhance the activity of CO2 reduction to CH4 in the presence of water vapor under visible light irradiation.118 Also, various Au-supported TiO2 photocatalysts are used in CO2 reduction under visible light irridiation.116,124 Zou’s group fabricated mesoporous Au@TiO2 yolk−shell hollow spheres with highly crystalline and thin TiO2 shells almost effectively separating the Au cores and TiO2 shells.124 The SPR-mediated local electromagnetic field nearby Au nanoparticles can not only enhance the local generation and subsequent separation of electron−hole pairs in TiO2 shells to improve the CO2 reduction activity but also facilitate the chemical reactions involving multiple e−/H+ transfer processes to allow the formation of high-grade carbon species (C2H6), which is rarely observed in precedent CO2 reduction systems.124 Similarly, Au/TiO2 shows visible-light-driven CO2 reduction activity even though yields are significantly lower than under UV irradiation.116 Furthermore, Au nanoparticles also influence the selectivity of CO2 reduction.116 For example, small Au nanoparticles onto TiO2 are found to quantitatively enhance the reduction of CO2 mainly to CH4 and C2+ hydrocarbons, whereas CO and H2 are detected as major products in bare TiO2. This behavior is explained in terms of a decrease of

Figure 7. Schematic diagram of photocatalytic process for H2 production and CO2 reduction on the Au/Pt/TiO2 nanofibers.123 Figure reproduced with permission from ref 123. Copyright 2013 American Chemical Society.

nanoparticles modified by Au/Cu alloy nanoparticles exhibit activity of CO2 reduction to CH4 under sun-simulated light, and the selectivity of CB electrons for CH4 formation is almost complete (about 97%).39 This photocatalytic behavior is completely different from that measured for Au/P25 (H2 evolution) and Cu/P25 (lower activity, but similar CH4 selectivity). Accordingly, a mechanism in which the role of Au is to respond under visible light and Cu binds to CO and directs the reduction pathway is proposed.39 The above studies demonstrate that through rational design of composite nanostructures, one can harvest visible light through SPR effect to enhance the activities of semiconductors initiated by UV light to the more effectively utilized solar spectrum for energy conversion. 3.2.4. Metal-Oxide-Loaded Nanocomposite Photocatalysts. Except for acting as electron sinks or surface modification 7500


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ACS Catalysis

adsorption, but an excess amount of MgO acting as an insulating layer may retard the photoinduced electron transfer to the catalyst surface. Another important finding is that the different surface morphology of MgO-TiO2 microspheres due to different synthesis methods has a significant effect on electron transfer and CO2 reduction activity.126 The same group127,128 integrates CO2 capture and photocatalytic conversion by using hybrid adsorbent/photocatalysts (TiO2 nanoparticles supported on MgAl layered double oxides, i.e., MgAl(LDO)/TiO2). It was found that MgAl(LDO)/TiO2 not only has a much higher CO2 capture capacity than bare MgAl(LDO) and TiO2 but also shows a higher activity of CO2 reduction to CO with water vapor under UV light irradiation at a moderately elevated reaction temperature. The photoinduced electrons on TiO2 may migrate to the MgAl-LDO/TiO2 interfacial sites to promote CO2 reduction.127,128 The above findings may lead to a new area of hybrid adsorbent/ photocatalyst materials that are capable of sequential CO2 capture and photocatalytic conversion. 3.2.5. Dye Loading. CO2 photoreduction can also be driven with the assistance of organic or metal-complex dyes loaded on semiconductors. In such a system, the photoinduced electrons can migrate from dye to semiconductor or reverse pathway, determined by the relative positions of semiconductor’s CBs and the HOMO/LUMO levels of dyes. In a typical photosensitization system, as shown in Figure 3b, the semiconductor acts as electron acceptor and dyes as electron donor.67 By tailoring the molecular structures of dyes, the photocatalytic system can be easily modified into a visible-light-responsive one. Currently, the sensitizers used as light-harvesting units can be divided into organic dyes, metal complexes, inorganic quantum dots, or clusters. Organic dye is scarcely applied in the photosensitization of TiO2 for the photocatalytic CO2 conversion in the last 3 years even though it has been extensively used in photovoltaic conversion. By using 5′-(4-[bis(4- methoxymethyl-phenyl)amino]phenyl-2,2′-dithio-phen-5-yl)cyanoacrylic acid as sensitizer and (4,4′-bis(methyl phosphonic acid)-2,2′-bipyridine)ReI(CO)3Cl as cocatalyst, both of which are anchored on TiO2 particles, a highly robust hybrid system (sensitizer/TiO2/ cocatalyst) was fabricated for visible-light-driven CO2 reduction to CO.129 Remarkable enhancements in CO2 reduction activity of the hybrid system can be achieved by addition of water or other additives such as Li+, Na+, and TEOA. The CO2 reduction activity can be enhanced by ∼300% upon addition of 3% (v/v) H2O, giving a TON ≥ 570 for 30 h. A series of Mott−Schottky (MS) analyses on TiO2 films demonstrate that the flat-band potential (Vfb) of TiO2 in dry DMF is substantially negative but positively shifts to considerable degrees in the presence of water or Li+, indicating that the enhancement effects of the additives on the activity should mainly arise from optimal alignment of the TiO2 Vfb with respect to the excitedstate oxidation potential of the sensitizer and the reduction potential of the cocatalyst. This result confirms that TiO2 in the hybrid system is an essential component that can effectively work as an electron reservoir and as an electron transporting mediator to play essential roles in the persistent activity of selective reduction of CO2 to CO.129 Typically, the LUMO of sensitizer, CB of semiconductor, and LUMO of carbonyl metal−bipyridyl complexes formed a step-like shape, and thus, photoinduced electrons can migrate oriental to carbonyl metal−bipyridyl complexes step by step, as shown in Figure 9.129

species, some basic metal oxides loaded on TiO2 photocatalysts to form nanocomposites can also act as CO2 adsorption sites for the photocatalytic CO2 converison.51,113,125−127 Recently, it was reported that MgO as a basic site is beneficial for the CO2 and H2O adsorption and the formation of carbonate on the catalyst surfaces, which then cause an enhanced activity for the photocatalytic CO2 conversion.51,113,125,126 Moreover, MgO also alters the selectivity of CO2 reduction reactions.113 Loading MgO on Pt-TiO2, the yield of H2 and CO decreases significantly, while the yield of CH4 increases in contrast.113 The selectivity can be ascribed to CO2 adsorption enhancement of MgO and electron density enhancement of Pt synergistically. Nevertheless, excessive MgO act as an insulating layer to retard the electron transfer to the catalyst surfaces.126 The functioning mechanism of MgO and Pt cocatalysts for accelerating the formation of CH4 can be ascribed to the fact that CO2 is first chemisorbed on the MgO layer of TiO2 crystallites.51,113,125 The chemisorbed CO2 molecules become destabilized, and their reactivity is believed to be higher than that of the linear CO2 ones.113,125 On the other hand, the photogenerated electrons on TiO2 can be easily trapped by the Pt nanoparticles because of the lower Fermi energy level. It is known that the formations of CO and CH4 require two and eight electrons, respectively. The enriched electron density on Pt nanoparticles would favor CH4 formation, which is thermodynamically more feasible than CO formation.125 Moreover, the synergistic effect between MgO and Pt, which enhances the density of destabilized CO2 molecules on the catalyst surface, and Pt nanoparticles with enriched electron density can further accelerate the reduction of CO2 to CH4. Clearly, such a synergistic effect requires the intimate contact of Pt between both TiO2 and MgO, strengthening the key role of the interface, as shown in Figure 8.125 Moreover, the coverage

Figure 8. Proposed synergistic mechanism of the intimate contact of Pt nanoparticles between both TiO2 and MgO layer for CO2 photoreduction in the presence of H2O.125 Figure reproduced with permission from ref 125. Copyright 2013 Royal Society of Chemistry.

of TiO2 surfaces with MgO might hinder the direct contact of CO or CH4 with TiO2 surfaces, reducing the reoxidation possibilities for CO and CH4. The functions of MgO are proposed to be mainly enhancing the density of CO2 on the catalyst surface and destabilizing CO2 molecules, which are subsequently reduced by the electrons enriched on the nearby Pt nanoparticles from TiO2.113,125 Li’s group demonstrated that the surface MgO dispersion and CO2 adsorption−desorption dynamics may govern the photocatalytic performance of porous MgO-TiO2 microspheres for solar fuel production using CO2 and H2O as the feedstock.126 Addition of MgO can enhance CO2 and H2O 7501


Review

ACS Catalysis

acid, probably due to its favorable reduction potential.70 Besides, a simple mixture of chlorophyll in Spirulina and NTiO2 also exhibits higher visible-light-responsive activity of CO2 reduction to solar fuels (H2, CH4, C2H4, and C2H6) compared with N-TiO2.130 The outstanding activity can be due to the enhancement in visible light harvesting, the surface VO sites facilitating the reactants adsorption and dissociation, and the synergistic effect between N-TiO 2 and chlorophyll in Spirulina.130 Ru-bipyridine complexes are also employed as sensitizer owing to its stable physicochemical property, suitable energy level, and high extinction coefficient.80,132 Moreover, mixedpolynuclear sensitizers such as tetrasulfonated Co phthalocyanine (CoPcS) coupled with [Ru(bpy)2phene-NH2] (denoted as Ru) is sensitized on TiO2@SiO2@Fe3O4, in which one or two sulfonated groups of CoPcS are combined with the amino functional silicone on the TiO2@SiO2@Fe3O4 surface by forming an ester linkage; the rest of sulfonated groups are combined with Ru in the same manner.80 It is found that the electron transfer channel from Ru to CoPcS and finally to TiO2 core can form, and thus, the Ru-CoPcS@TiO2@SiO2@Fe3O4 system shows visible-light-driven CO2 reduction activity that is quite stable during five runs of 48 h.80 In addition, the introduced Fe3O4 provides a very convenient approach for removing and recycling magnetic catalysts through magnetic separation.80 In addition to dyes, some visible-light-responsive metal clusters such as K4[Re6S8(CN)6] or Cs2Mo6Br8iBrax are also functional as sensitizer in some CO2 reduction systems.133,134 Jian’s group has reported that octahedral hexacyano Re {K4[Re6S8(CN)6]} cluster grafted onto the Cu(OH)2 clustermodified TiO2 {Cu(OH)2/TiO2} support acts as a sensitizer to significantly improve the activity of CO2 reduction to CH3OH/ H2 under visible light irradiation, and Cu is an effective electron trapper able to prohibit charge recombination, as shown in Figure 10.133 CH3OH yield after 24 h irradiation is much higher than Cu(OH)2/TiO2 and an equimolar Re cluster in the presence of TEOA as sacrificial donor.133 Usually, the above-mentioned photosensitized semiconductor hybrid systems show efficient photocatalytic CO 2

Figure 9. Illustration of electron flow in visible-light-induced CO production on MOD/TiO2/RePH.129 Figure reproduced with permission from ref 129. Copyright 2015 American Chemical Society.

Among metal complexes used for photocatalytic CO2 conversion, porphyrins,66,85,130 phthalocyanine, 70,131 and metal−bipyridine complexes (especially for Ru)80,132 are the most popular ones. Among which, porphyrins and phthalocyanines having multi-π electrons conjugated structure, which can be tailored by both center metal ions or surrounding radicals, are suitable dyes for constructing visible-light-driven dyesensitized semiconductors for CO2 reduction.70,85,131 For instance, asymmetric zinc porphyrin with three pyridine and one carboxyl groups on meso-position are synthesized to sensitize on anatase/brookite mixed-phase TiO2 nanoparticles.85 DFT calculation suggested that the photoinduced electrons of asymmetric zinc porphyrin can migrate from the pyridine group to the zinc center and then to the carboxyl group, lastly injecting into VB of TiO2 step by step.85 With a 1.0 wt % loading amount, a photocatalyst without cocatalyst shows the highest activity of CO2 reduction to CO/CH4.85 More intentions are focused on phthalocyanines containing center metals such as Zn(II), Cu(II), Co(II), Ni(II) as sensitizers of semiconductors such as TiO2,70,131 for photocatalytic CO2 conversion. A comparison between Cu(II) phthalocyanine and Cu(II) porphyrin indicates that the Cu(II) phthalocyanine is more efficient in CO2 reduction to formic

Figure 10. Possible mechanism of CO2 reduction by Recluster@Cu(OH)2/TiO2 under visible light irridiation.133 Figure reproduced with permission from ref 133. Copyright 2015 Elsevier B.V. 7502


Review

ACS Catalysis

facilitates the trapping of photogenerated holes and the formation of H+ species, and consequently, it leads to higher alcohol production activity.135 Meanwhile, some 2D nanostructures of TiO2 are fabricated and exhibit effective activity for CO 2 photoreduction reactions.53,82,83 For instance, Ye’s group synthesized ultrathin anatase TiO2 single crystal nanosheets (2 nm in thickness) with 95% of exposed {100} facets, which shows ∼5 times higher activity in both H2 production and CO2 reduction to CH4 than that of the TiO2 cuboids with 53% of exposed {100} facets.53 The high activity of the TiO2 with 95% of exposed {100} facets can be ascribed to a higher percentage of exposed {100} reactive facets, larger surface area offering more surface active sites, and the superior electronic band structure resulting from the higher percentage of {100} facets.53 Moreover, the anatase TiO2 nanosheets have stronger CO2 adsorption and slightly higher CB level than the bulk one, and the enhanced activity of CO2 reduction to CH4 can be ascribed to the synergistic effect of CO2-adsorbing effort and bandgap modification.53 3.3.2. Hierarchical Structures. Many studies have shown that 3D hierarchical architectures, typically possessing high surface areas and diverse pore-interconnectivities, offer enhanced light absorption and catalytic activity relative to their bulk, nanoparticle, or monomodal porous counterparts.16 To date, semiconductors with diverse 3D hierarchical structures are generally created through the self-assembly of nanoscale building blocks, such as nanoparticles, nanosheets, and nanorods, via advanced inorganic synthetic routes.16 In some instances, pore networks created within hierarchical networks not only increase the total surface area but also serve to accelerate reactant/product diffusion to/from active sites. In the last 3 years, a number of promising 3D TiO2 hierarchical structures have been created as effective photocatalysts for CO2 conversion.54,90,117,118,121,124,136 Wilkinson’s group fabricated hierarchical TiO2 microspheres (several hundreds to more than 1000 μm) by using P25 nanoparticles as the building units.54 The as-prepared TiO2 microspheres have a hierarchical porosity composed of macropores, larger mesopores (ca. 12.4 nm), and smaller mesopores (ca. 2.3 nm), in which the interconnected macropores and larger mesopores can increase mass transport and multistep light scattering, which causes a significantly enhanced activity of CO2 reduction to CH4 compared with P25.54 Zou’s group fabricated double-shelled hollow hybrid spheres consisting of plasmonic Ag and TiO2 nanoparticles through a simple reaction process, and they found that Ag nanoparticles are dispersed uniformly in TiO2 nanoparticle shell.118 Compared with the typical plasmon peak of Ag nanoparticles with similar size generally at ∼400 nm, the absorption peak of Ag-TiO2 composite shows a large redshift due to the high refractive index of TiO2 as the surrounding medium. The Ag-TiO2 hollow sphere proves to greatly enhance the activity of CO2 reduction to CH4 in the presence of water vapor under λ > 420 nm light irradiation.118 Similarly, anatase TiO2 microflower film constructed with nanosheets shows photoactivity in CO2 reduction to CH3OH.121 Interestingly, the pure TiO2 microflower film exhibits a visible light-harvesting property, and the absorption in the visible region is substantially enhanced as the wavelength of light increased, which differs from the absorption of P25. The visible light harvesting of TiO2 film is closely related to its nanostructures due to the light scattering caused by pores or cracks in the film.121 Also, hierarchical amine-functionalized

conversion in both sensitization and charge-transfer ways, but the photocatalytic reactions have always occurred in organic solutions with TEA or TEOA as electron donors.66,80,129,130,132 In general, they are less efficient and provide less environmental protection from the viewpoint of practical application, and thus, exploring the hybrid systems with respect to organic-solventfree conditions and without sacrificial reagents will be more attractive in the future. 3.3. TiO2 Microstructure Construction. Nanostructured materials for photocatalytic applications, commonly synthesized in the form of nanoparticles (0D), nanofibers/nanorods/ nanotubes (1D), nanoplates/nanosheets (2D) in addition to more exotic topologies such as nanospheres and nanoflowers, can offer high densities of photoactive surface sites and thinwalled structures able to facilitate rapid interfacial charge transport to adsorbates.16 Recently, many synthetic methods afford precise manipulation of exposed crystal facets, morphologies, structural periodicity, and hierarchical networks, porous architectures, and hybrid photoactive nanomaterials (e.g., plasmonic nanostructures) in addition to the coassembly of sensitizers or cocatalysts and promoters for CO2 reduction.16 Therefore, this section is focused on the design and construction of micro/nanostructures including low-dimension nanomaterials, hierarchical structures, porous structures, and their hybrid structures of photocatalysts for efficient light harvesting, charge separation, and CO2 reduction. Their corresponding experimental conditions, activities, and selectivity are also listed in Table 1. 3.3.1. Low-Dimensional Nanostructures. In the last 3 years, low-dimensional nanostructured TiO2 were extensively applied in the photocatalytic CO2 conversion. Among which, TiO2 nanoparticles are still the most popular photocatalysts, 48,60,81−84,87−89,92−95,101,106−109,111,113−117,120 while 1D49−52,82,99,110,107,123,135 and 2D53,82,83 nanomaterials are generally more attractive than their bulk and nanoparticle counterparts because they facilitate more efficient electron transport and exhibit higher surface-area-to-bulk-volume ratios. For instance, brookite TiO2 nanorods with controllable aspect ratio were synthesized by using titanium ethoxide as Ti source.49 It is found that the oxidative site is on the ends of the nanorod, while the reductive site is along the side of nanorod, and the enhanced activity of CO2 reduction to CH3OH can be related to the effective holes’ migrations which prevent the reverse reaction.49 Coincidently, similar advantageous charge separation can also be observed in TiO2 nanofibers, which can promote CO2 reduction to CO/CH4 in the gas-phase photoreaction system.52,117,123 TiO2 nanotubes (TNTs) also exhibit extraordinary photocatalytic ability in CO2 reduction.97,135 For example, selforganized TiO2 nanotube arrays (TNAs), prepared by electrochemical anodization of Ti foils, was used to the photocatalytic conversion of CO2 and H2O to alcohols.135 After thermal treatment at 450 °C in air for 3 h, the as-prepared amorphous TNAs can be transformed to anatase phase. Compared to the commercial TiO2 nanoparticles (P25), the bandgap of the annealed TNAs decreased by ∼0.2 eV, indicating the red-shifted absorption edge.135 The TNAs with 1D nanotubular structure exhibit much better activity and stability for CO2 reduction to alcohols than P25, which is ascribed to more rapid migration of the photogenerated charge to the surface in TNAs than that in TiO2 nanoparticles. Moreover, water molecules could be easily intercalated into the tubular space of TNAs through hydrogen bonding; this 7503


Review

ACS Catalysis titanate nanosheets based yolk@shell microspheres (Figure 11) were synthesized via one-pot organic-amine-mediated anhy-

Figure 12. FESEM images of the prepared 3D interconnectively macro/mesoporous TiO2 sponges template by 10 wt % LRS at different magnifications.55 Figure reproduced with permission from ref 55. Copyright 2014 The Royal Society of Chemistry.

Figure 11. FESEM (a,b) and TEM (c,d) images of the typical yolk@ shell microspheres.136 Figure reproduced with permission from ref 136. Copyright 2015 American Chemical Society.

drous alcoholysis of Ti(IV) butoxide.136 The as-prepared yolk@shell microspheres contain micropores (50 nm) resulted from the hierarchical assembly of the basic nanostructured building blocks including both tiny 0D nanoparticles and 2D nanosheets. This unique feature is beneficial for enhancing molecular adsorption and diffusion during the photocatalytic reactions. As a result of synergetic tuning of the multifunctional modules of the photocatalytic system, especially light-harvesting centers and CO2 adsorption sites, the designed amine-functionalized titanate yolk@shell microspheres show efficient CO2 reduction to dominant CH3OH without cocatalyst.136 3.3.3. Porous Structures. Micro/meso-/macro-porous photocatalysts with relatively large surface areas usually provide plenty of CO2 adsorption sites, surface active centers, and readily accessed channels, and thus, they exhibit activity superior to their nonporous analogues.16,52,54−56,90,105,137 Moreover, macropores incorporation into meso-/microporous architectures can confer additional benefits via increased light scattering and faster in-pore mass transport of reagents for application in CO2 photoreduction. Therefore, creating porous TiO2 or combining TiO2 with porous supporting matrix are extensively applied to improve the CO2 reduction activity. Ye’s group synthesized well-crystallized ordered mesoporous TiO2 (denoted as OMT) by combining the evaporationinduced self-assembly process with two-step calcination processes.56 The surface area of OMT is over 200 m−2 g−1, which is ∼4-fold of P25. Moreover, the OMT shows that the CH4 adsorption (0.74 cm3 g−1 STP) is more difficult than CO2 adsorption (24.0 cm3 g−1 STP) on the surface. This selectivity of adsorption is beneficial for the CO2 reduction to CH4, and thus, the activity of CH4 production is 71 times and 53 times higher than that of P25 and disordered mesoporous TiO2, respectively. The remarkable performance of OMT for CO2 reduction probably benefits from the confined space effect of ordered mesoporous structure.56 Similarly, ordered mesoporous Co-doped TiO2 is also successfully synthesized by a multicomponent self-assembly process and used as an efficient photocatalyst for CO2 reduction to CH4.105 Interestingly, Zou’s group prepared 3D macro/mesoporous TiO2 sponges (Figure 12), consisting of macroporous frame-

work with interconnected mesoporous channels, through a cogelation of lotus root starch (LRS) with TiO2 precursor, followed by lyophilization and subsequent calcination.55 The synthesized TiO2 has a relatively higher surface area (31.4 m−2 g−1) and CO2 adsorption quantity (6.77 mg g−1) than the referred TiO2 formed in the absence of the LRS, and the macroporous architecture of TiO2 leads to more easy gas diffusion of the reactants and the products and more efficient light harvesting by the multiple reflection effect occurring inside the interior macro-cavities, which causes about 2.60-fold improvement in CO2 photoreduction activity (CH4: 5.13 ppm h−1) compared to the referred TiO2 (1.97 ppm h−1).55 Similarly, KIT-6, a kind of mesoporous silica, was used as template to synthesize mesoporous TiO2 (Meso. TiO2) with higher surface area (190 m2/g), mesoporosity (4 nm pores), and adsorption capacity than the Aeroxide TiO2 (P25).137 The obtained Meso. TiO2 shows more hydrocarbons (CH4, CH3OH) and a higher syngas production together with better reaction kinetics and stability due to its better characteristics than the commercial P25.137 Additionally, isolated TiOx-species loaded on different matrix are proved to be active materials for CO2 reduction.138−142 Supporting matrices could be natural montmorillonite (MMT)138,139 or synthetic materials such as mesoporous silica (SBA-15,140 KIT-6142) and molecular sieve (ZSM-5141). Amin’s group prepared MMT modified TiO2 nanocomposites by sol− gel method, in which anatase TiO2 with reduced crystal size can be produced.138,139 It is revealed that the vacant d-orbitals of MMT transition metal ions have an obvious effect on the activity of TiO2 for CO2 reduction.138,139 Russo’s group synthesized TiO2/KIT-6 with different incorporated TiO2 concentration, in which nanostructured TiO2 is present in both the silica framework and on the surface of KIT-6, where it produced large-surface-area photocatalysts with enhanced adsorption capability of the reactants to photocatalytically convert into CH4, CH3OH (hydrocarbons) and CO, H2 (similar to syngas).142 The formed products are affected directly by the dispersed TiO2 concentration as well as by the calcination temperature. Hydrocarbon and CO formation as well as the reaction kinetics improve as TiO2 concentration are increased from 1 to 20 wt %; however, further increase in TiO2 loadings (to 90 wt %) decreases the hydrocarbon and CO, and 7504


CeO2, Sn(IV)Cl2- phthalocyanine

N-doped graphene, [Cu (bpy)2(H2O)2]Cl2·2H2O C, N-co-doped Na2TiO3

graphene oxide, Cs2Mo6Br8iBrax

20 W LED (λ > 400 nm)

20 W LED (λ > 400 nm)

solar simulator (AM 1.5G)

20 W LED (λ > 400 nm)

N-doped ZnO

Zn-doped Ga2O3

In2O3−x(OH)y

ordered meso-porous Fe-doped CeO2 CeO2 homojunction Ni−Al LDH, Ni−Al 4:1 Ni−Al LDH, Ni−Al 3:1

Zn3Ga/CO3 LDH 3D hierarchical SrTiO3 CaTiO3

SrTiO3

KNbO3

CaTiO3 SrTi0.98Co0.02O3 SrNb2O6 plate

KCaSrTa5O15

8 W fluorescent tube (λ = 369 nm)

400 W Hg-lamp

300 W Xe-lamp

300 W Xe-lamp

300 W Xe-lamp 400 W Hg-lamp 400 W Hg-lamp

7505

500 W Xe-lamp (λ > 420 nm) 300 W Xe-lamp 300 W Xe-lamp



6W UV-lamp (λ = 365 nm) 300 W Xe-lamp (λ > 420 nm) 100 W Xe-lamp (λ = 300−780 nm)

400 W Hg-lamp

0.5 wt %Ag

Pt

1.0 wt %Ag 1.0 wt %Au 0.5 wt %Ag

0.5 wt %Pt/0.5 wt % MnOx

1.5 wt %Cu

MCo2O4 (M = Ni, Zn or Mn), [Ru(bpy)3] Cl2·6H2O

300 W Xe-lamp (λ > 420 nm)

Xe-lamp (λ > 350 nm) Xe-lamp (λ > 350 nm) Hg-lamp Hg-lamp RuOx 0.25 wt % RuOx 1.0 wt %Ag 1.0 wt %Ag

W W W W

g-C3N4, trans(Cl) -[Re(2,2′-bpy-4, 4′-bis-phosphonic acid) (CO)2 Cl2]·(PF6)2 Cu2O Cu2O La2Ti2O7 ZnGa2O4

400 W Hg-lamp (λ > 400 nm)

150 150 400 400

0.1 wt %Ag 10 wt % CuII cluster

co-catalyst

Ga2O3 Nb3O8 nanosheets

photocatalyst

300 W Xe-lamp Xe−Hg lamp (λ = 240−300 nm)

light source

0.5 g of in 350 mL of water with 30 mL min−1 CO2

0.2 g of catalyst in 11.30 mg of CO2-saturated water 100 mg of catalyst, CO2 and H2O vapor 20 mg of catalyst, H2O vapor and 0.2 MPa CO2

100 mg of catalyst in H2O with 400 ppm of CO2

0.10 g of catalyst, 2.3 kPa CO2 and 21.7 kPa H2 50 mg of catalyst in CO2 and H2O vapor 0.2 g of catalyst in water saturated with NaHCO3 under flow of CO2 100 mg of catalyst in water with CO2 (400 ppm)

100 mg of catalyst, CO2 and H2O vapor 0.50 g of catalyst in CO2-saturated 0.1 M NaCl 0.50 g of catalyst in CO2-saturated 0.1 M NaCl

20 mg of catalyst in packed bed reactor with H2/ CO2/ He gas flow 100 mg of catalyst, CO2 and H2O vapor

1.0 g of catalyst in 0.1 M NaHCO3 with 30 mL min−1 CO2

100 mg of catalyst in H2O/TEA/DMF (1:2:3), CO2 bubbled 30 min before irradiation 100 mg of catalyst in H2O/DMF (1:9), CO2 bubbled 30 min 100 mg of catalyst in H2O vapor and CO2 gas (1000 ppm in He) 100 mg of catalyst in H2O/DMF (1:4), CO2 bubbled 30 min 10 mg of catalyst, CO2 and H2O vapor

4 μmol NiCo2O4 and 10 μmol Ru complex in TEOA/ H2O/MeCN (1:2:3) under 1 atm CO2

0.50 g of catalyst in CO2-saturated 0.7 M Na2SO3 10 mg of catalyst, CO2 and H2O vapor 1.0 g of catalyst in water with 30 mL min−1 CO2 1.0 g of catalyst in 0.1 M NaHCO3 with 30 mL min−1 CO2

0.2 g of catalyst in CO2-saturated 1.1 M NaHCO3 0.1 mg of catalyst, KHCO3 solution, CO2 bubbled at 100 mL min−1 for 30 min 8 mg of catalyst in 4 mL of CO2-saturated DMA/TEOA (v/v 4/1)


Table 2. Summary of CO2 Photoreduction Systems Containing Single Photocatalysts Other than TiO2a

0.086 (CH4)/0.112 (O2) μmol h−1 9.3/56.4 (CO)/9.3 (H2) μmol h−1 1.2 (CH4)/21.0 (CO)/ 12.3 (H2) μmol h−1 0.01 (CO)/0.012 (CH3OH) μmol h−1 11.55 (CH4)/1.70 (CO) nmol h−1 0.33 (CO)/0.63 (H2)/ 0.36 (O2) μmol h−1 5.35 (CH4)/1.28 (CO)/20.07 (H2)/ 5.28 (O2) ppm h−1 8.68 (CH4)/12.53 (H2)/ 3.13 (O2) ppm h−1 0.49 (CH4) μmol h−1 6.36 (CH4) ppm h−1 0.0066 (CH4)/0.033 (CO)/0.13 (H2) μmol h−1 1.4 (CO)/2.75 (H2)/ 0.275 (O2) μmol h−1

0.29 (CH4)/1.24 (CO) nmol h

−1

0.0002 (CH4)/0.003 (H2)/0.005 (CO)/ 0.0002 (CH3OH) μmol h−1 117 (CO)/16.7 (H2)/ 70.1 (O2) μmol h−1 3 (CO) μmol h−1

164.4 (CH3OH) μmol in 24 h

160.0 (CH3OH)/1.85 (H2) μmol in 24 h 0.975 (CH4) μmol h−1

∼0.45 (CO) μmol h−1 ∼10/0 ppm (CO) h−1 4.9 (CO)/5.3 (H2)/5.2 (O2) μmol h−1 155 (CO)/8.5 (H2)/74.3 (O2) μmol h−1 21 (CO)/4 (H2) (Ni); 25.1 (CO)/8.7 (H2) (Zn); 27 (CO)/8 (H2) (Mn) μmol in 3 h 84.0 (CH3OH) μmol in 24 h

8.8 (HCOOH) μmol h−1; TON = 141

2.0 (CO)/7.4 (H2)/4.6 (O2) μmol h−1 ∼0.6 (CO) μmol g−1 h−1


2.80

0.12

3.88

0.66

0.092

0.688 112.8 51.6

0.0048

6

234.0

1.25

7.80

9.8 310

∼0.9

4.0 ∼1.2

TCEN (μmol h−1)

ref

159

156 157 158

155

155

152 153 154

147 150 151*

146

145

144*

143

134

100

79

78

75*− 77

71 72 73* 74*

68, 69

40 41*



7506

ZnIn2S4

CuS/Cu2S nanorod and nanowall arrays CoTe ZnTe GaN nanowires array GaN nanowire

250 W Hg-lamp (λ = 365 nm)


carbon dots carbon dots

g-C3N4

S-doped g-C3N4 2D hydroxyl-rich g-C3N4

425−720 nm photoirradiated 1 kW Xe-lamp (λ = 405−720 nm)

300 W Xe-lamp (λ > 420 nm)


300 W halogen lamp 300 W Xe-lamp

graphene oxide graphene oxide

CdIn2S4

250 W Hg-lamp (λ = 365 nm)

Xe-lamp (λ > 420 nm) Xe-lamp (λ > 420 nm) Xe-lamp UV- enhanced Xe-lamp

Bi2S3

250 W Hg-lamp (λ = 365 nm)

W W W W

3.1 wt % ZnS/montmorillonite

8 W Hg-lamp

300 300 300 300

LaPO4 ZnS ZnS

125 W Hg-lamp 355 nm laser radiations 150 W XBO arc lamp (λ > 320 nm)

NaTaO3 BaZrO3 ultrathin ZnGa2O4 Zn2GeO4 nanorods Na2V6O16·xH2O nanoribbons InVO4 ball-flower-like Bi2WO6

NaBiO3 flowerlike hierarchical BiOI BiOCl nanoplates with VO

Hg−Xe arc lamp Xe-lamp Xe-lamp Xe-lamp Xe-lamp (λ > 420 nm) halogen lamp (λ > 400 nm) Xe-lamp (λ = 420−620 nm)

500 W Xe-lamp (λ > 400 nm) 300 W Xe-lamp 500W Xe-lamp (λ = 200−1000 nm)

W W W W W W W

NaTaO3

300 W Xe-lamp (λ > 200 nm)

200 300 300 300 300 500 300

KTaO3 nanoflake

photocatalyst

300 W Xe-lamp

light source

Table 2. continued

1.0 wt %Pt

Ag or Au Au

10 wt %Cu

0.5 wt %Pt 0.5 wt %Rh

Ru0

1.0 wt %Ag

120 mg of catalyst, CO2 and H2O vapor 10 mg of catalyst in 80 mL of CO2-saturated water

200 mg of catalyst in CO2-saturated 1.0 M NaOH

catalyst in CO2-saturated solution catalyst solution under 750 psi CO2 atmosphere

200 mg of catalyst, CO2 and H2O vapor 100 mg of catalyst, 4 μL min−1 wet CO2 vapor

catalyst in CO2 and H2O vapor 10 mg of catalyst, CO2 and H2O vapor ∼3.5 cm−2 catalyst film, CO2 (80 kPa) and H2O vapor ∼2 cm2 nanowire film in CO2 and H2O vapor

4 cm2 array film in CO2 and H2O vapor

10 mg of catalyst in CO2-saturated methanol



50 mg of catalyst, CO2 and H2O vapor 100 mg of catalyst, CO2 and H2O vapor 100 mg of catalyst, CO2 and H2O vapor 100 mg of catalyst, CO2 and H2O vapor 100 mg of catalyst, CO2 and H2O vapor 0.14 g of catalyst in CO2-saturated 0.2 M KHCO3 65 mg of catalyst, 2.0 mL min−1 10% CO2/N2 and H2O vapor 50 mg of catalyst in CO2-saturated 0.08 M NaHCO3 100 mg of catalyst, CO2 and H2O vapor 100 mg of catalyst in 100 mL of water with 400 ppm of CO2 50 mg of catalyst in 70 mL of water with1 atm CO2 100 mg of catalyst in 100 mL of CO2-saturated water 1 g L−1 catalyst in 5 mL of CO2-saturated 10% isopropanol-water 100 mg of catalyst in CO2-saturated 0.2 mM NaOH

1.0 wt %Au 0.3 wt %Ag

1 wt %Pt, 1 wt %RuO2 1.0 wt % NiO

70 mg of catalyst in H2 (40 kPa), CO2 (40 kPa) and H2O vapor



0.5 wt %Pt or Ru

0.01 wt % Ag

co-catalyst

4.97 (CH4) μmol g−1 h−1 0.01 (CH4) μmol h−1 ∼14.8 (CH4) μmol g−1 h−1 994 (CH4)/1084.6 (CO) μmol g−1 h−1 cm−2 6.4 × 10−4 (CH3OH) μmol m−2 h−1 0.294 (CH3OH)/0.388 (CH3CHO) μmol h−1 AQY 0.01−0.03% (HCOOH) 1.2 (HCOOH)/0.06 (CH3COOH) mmol g−1 h−1 1.26 (CH3OH)/0.90 (C2H5OH)/4.27 (O2) μmol h−1 0.044 (CH3OH) μmol h−1 0.0093 (CH4) μmol h−1

0.026 (CH3OH) μmol h−1 0.02 (CH4) μmol h−1 ∼0.15 (CH4)/∼0.10 (CO) μmol h−1, O2 much faster than solar fuels 0.65 (CH4)/0.23 (H2) μmol h−1 ∼90 (CH3OH) μmol h−1 ∼250 (CO)/500 (HCOOH) μmol L−1 (5 h irradiation) 0.117 (CH4)/0.013 (CO) /∼1.33 (H2) μmol g−1 h−1 >7.0 (HCOOH) μmol (4 h irradiation) 29.68 (dimethoxymethane)/28.57 (HCOOH) μmol h−1 7.62 (HCOOH) μmol (4 h irradiation) 39.42 (CH4) μmol m−2 h−1

1.8 (CH4)/8.65 (CO) nmol h−1 ∼0.06 (CH4) μmol h−1 0.69 (CH4) ppm h−1 0.28 (CH4)/ ∼ 10.0 (O2) μmol h−1 ∼0.02 (CH4) μmol h−1 0.21 (CH3OH) μmol h−1 ∼0.036 (CO) μmol h−1

15.26 (CO)/113.44 (H2)/ 49.99 (O2) ppm h−1 3.63 (CH4)/0.15 (CO) (Ru); 0.098 (CH4)/9.74 (CO) (Pt) μmol h−1


0.27 0.074

14.76

5.64

0.084

0.96

5.185

0.16 0.16 ∼1.40

2.24 ∼0.16 1.28 ∼0.072

∼0.54

29.32 (Ru); 20.05 (Pt)

30.52

TCEN (μmol h−1)

ref

189 190

188

186* 187*

184* 185

180 181 182 183

179

178

177

176

175

172 173 174

169 170 171

162 163 164 165 166 167 168

161*

160*



7507

Ti−Cr-TUD-1

Ca(10 wt %)Ti35Si65 -MCM41

NH2−UiO-66(Zr) NH2-MIL-125(Fe) NH2-MIL-125(Ti)

UiO-66(Zr/Ti)- NH2 PCN-222

Cu porphyrin-based MOFs

Ru-MOF (C34N6 O11H26RuCd)

Ru-MOF (C36N6 O12H18RuCd2)

Zn2GeO4/ZIF-8

CdS/Co-ZIF-9

MOF-253, Ru(bpy) (CO)2Cl2

UiO-67, Mn(bpy) (CO)3Br

120 W Hg-lamp (λ = 280−650 nm)


300 W Xe-lamp (λ = 420−800 nm) 300 W Xe-lamp (λ = 420−800 nm) 300 W Xe-lamp (λ = 420−800 nm)

300 W Xe-lamp (λ = 420−800 nm) 500 W Xe-lamp (λ = 420−800 nm)

300 W Xe-lamp (λ > 420 nm)

500 W Xe-lamp (λ = 420−800 nm)

500 W Xe-lamp (λ = 420−800 nm)

500 W Xe-lamp

300 W Xe-lamp (λ > 420 nm)

Xe-lamp (λ= 420−800 nm)

LED (λ = 470 nm)

Ag/AgIO3 graphene, BODIPY

Ti-MCM-41 Ti-KIT-6

9 W UVC lamp (λ = 254 nm) 300 W UV-lamp

500 W Xe-lamp (λ > 400 nm) 450 W Xe-lamp (λ > 420 nm)

SiC

300 W Xe-lamp

AgCl0.75Br0.25

g-C3N4

300 W Xe-lamp (λ > 420 nm)

500 W Xe-lamp (λ > 420 nm)

g-C3N4

300 W Xe-lamp

Ag/AgCl

g-C3N4 helical g-C3N4

15 W energy-saving daylight bulb 300 W Xe-lamp (λ > 420 nm)

500 W Xe-lamp (λ > 420 nm)

g-C3N4

photocatalyst

300 W Xe-lamp (λ > 420 nm)

light source

Table 2. continued

1.0 wt %Pt

1.0 wt %Au

1.0 wt %Pt or 1.0 wt % Au

CoOx

1.0 wt %Pt

2.0 wt %Pt CoOx

CoOx

co-catalyst

12 mg of catalyst in 0.1 M NaHCO3 solution with high CO2 concentration 500 mg of catalyst, CO2 and H2O vapor 0.5 mg of catalyst, 0.5 mL min−1 CO2, pH7.0 buffer solution containing TEOA, β-NAD+, Rh complex, 3 unit formate dehydrogenase

5 mg of MOFs in CO2-saturated 4:1 v/v MeCN/TEOA 50 mg of MOFs in 60 mL of CO2-saturated 10:1 v/v MeCN/TEOA 30 mg of MOFs in water containing 1 mL of TEA, CO2 bubbled at the rate of 0.2 mL min−1 40 mg of MOFs in 60 mL of CO2-saturated 20:1 v/v MeCN/TEOA 40 mg of MOFs in 60 mL of CO2-saturated 20:1 v/v MeCN/TEOA 200 mg of catalyst in CO2-saturated water containing 0.10 mol L−1 Na2SO3 20 mg of CdS, 1 mg of Co- ZIF- 9, 10 mg of bpy in MeCN/H2O/TEOA (v/v 3:2:1) at 1 atm CO2 5 mg of catalyst in CO2-saturated MeCN/TEOA(v/v 10/1) 0.5 mM catalyst in CO2-saturated DMF/TEOA (v/v 4/1) containing [Ru(dmb)3](PF6)2 and BNAH 15 mg of catalyst in CO2-saturated 0.1 M NaHCO3

200 mg of catalyst in CO2 and H2O vapor (CO2:H2O = 1:2) 50 mg of MOFs in CO2-saturated 5:1 v/v MeCN/TEOA 50 mg of MOFs in CO2-saturated 5:1 v/v MeCN/TEOA 50 mg of MOFs in CO2-saturated 5:1 v/v MeCN/TEOA

catalysts in 50 mL reactor with 38 μmol CO2 and 76 μmol H2 O

100 mg of catalyst in CO2-saturated solution with MEA 200 mg of catalyst, CO2 and H2O vapor

50 mg of catalyst, 1 μmol CoCl2, 2,2-bipyridine, TEOA and MeCN under 1 atm CO2 300 mg of catalyst in NaOH solution, CO2 bubbled 30 min

30 mg of catalyst, 1 μmol CoCl2, 2,2-bipyridine, TEOA and MeCN under 1 atm CO2 catalyst in CO2 and H2O vapor 30 mg of catalyst, 1 μmol CoCl2, 2,2-bipyridine, TEOA and MeCN under 1 atm CO2 100 mg of catalyst, CO2 and H2O vapor


TON = 7.3 (CO)/35.8 (HCOOH)/ 11.9 (H2) TON = 1.5 (CO)/50 (HCOOH)/ 0.41 (H2), in 4 h 0.44 (CH3OH)/0.67 (C2H5OH) μmol h−1 0.43 (CH3OH)/0.89 (C2H5OH) μmol h−1 3.05 (CH4)/0.38 (CO) μmol h−1 144.22 (HCOOH) μmol in 2 h; yield of NADPH 54.02%

25.2

13.03

10.66

215 217

214

213

212

211*

210*

209

0.044 (CH3OH) μmol g−1 h−1 50.4 (CO)/11.1 (H2) μmol

208

24.7 (HCOOH) μmol in 8 h 0.26

207*

17.2 (HCOOH) μmol in 6 h

204* 205*

201 202* 203

200

199

197 198

196

195*

193

192 193

206

120.0

52.8

131.2

6.94 19.96

10.14

ref 191*

7.88 (CH3OH) ppm h−1

26.4 (HCOOH) μmol g−1 h−1 20.7 (HCOOH) μmol in 10 h 10.75 (HCOOH)/235 (H2) (Pt); 9.06 (HCOOH) /40.2 (H2) (Au) μmol in 8h TON = 6.27 (HCOOH) 30.0 (HCOOH) μmol in 10 h

3.60 (HCOOH)/0.37 (CH3COOH) μmol h−1 0.78 (CH4)/0.35 (CO) μmol h−1 1.44 (CH4)/0.04 (CH3OH)/4.10 (CO)/ 7.82 (H2) μmol h−1 1.02 (CH4)/80.3 (C2H6)/ 5.9 (C2H4)/640 (C3H8)/ 125.8 (C3H6) × 10−3 ppm min−1 16.4 (CH4) μmol h−1

0.6

∼0.41

17.8

13.02 (CH4) μmol g−1 h−1 8.9 (CO)/0.3 (H2) μmol h−1 ∼0.03 (CH4)/∼0.023 (CH3OH) /∼0.008 (HCHO) μmol h−1 0.3 (CO)/1.85 (H2) μmol h−1

1.87

TCEN (μmol h−1)

0.933 (CO)/∼0.12 (H2) μmol h−1




graphene oxide, [Ru(bpy)2-2-thiophenylbenzimidazole](PF6)2 graphene oxide, [Ru (bpy)2bpm]2−RuCl2·(PF6)4 CaTaO2N, Ru(cat)-Ru(bpy) (CO)2 (Cl)2-type CaTaO2N, Ru(cat)-Ru(bpy) (CO)2 (Cl)2-type anatase TiO2

TiO2(P25)

SiO2 supported Pd-bpy complex

mesoporous organosilica (MeAcdPMO)

20 W LED (λ > 400 nm)

solar light simulator (AM 1.5G)

200 W Hg-lamp

500 W Hg-lamp (λ = 360−440 nm)

500 W Hg-lamp (λ = 405 nm)

[(dmb)(L1) Ru(BL)Re (CO)3Br] (PF6)2

[ReBr(CO)3 (2,2′-bpy-4,4′-bisphosphonic acid)] ·(PF6)2 trans-[Co (cyclam)Cl2]·Cl

1.0 wt % Ag

1.0 wt % Ag

co-catalyst

100 mg of catalyst in H2O/DMF (1:4), CO2 bubbled 30 min 100 mg of catalyst in H2O/TEA/DMF (1:1:3), CO2 bubbled 30 min 4 mg of catalyst in 4 mL of CO2-saturated DMA/TEOA (v/v 1/4) 8 mg of catalyst in 4 mL of CH3OH, CO2 bubbled 20 min before irradiation catalyst in DMF/TEOA (v/v 5/1), bubbling though CO2 containing 2% CH4 before irradiation 1 mg of catalyst in CO2-saturated MeCN/TEOA (v/v 3/1) 20 mg of catalyst in 10 mL of DMF and 2 mL of TEA at 1 MPa CO2 1 mg of catalyst in DMF/TEOA (v/v 5/1) containing 0.1 M BIH, CO2 bubbled 30 min

100 mg of catalyst in H2O/DMF (1:4), CO2 bubbled 30 min catalyst in CO2/CH4/He mix gas (30 psi)


ref

228

227

TOF = 65.8 (CO) and 0.5 (H2) h−1 64 (HCOOH) μmol g−1; TON = 632 (CO)/ 10 (H2)

226

225

223*

222*

TON = 11.2 (CO)

TON = 969 (HCOOH)/ 68 (CO), and 678 (H2) nmol in 24 h TON = 59 (CO)

320 (HCOOH) nmol in 15 h

221*

397.8 (CH3OH) μmol in 48 h

219

218

220*

TCEN (μmol h−1)

13% (CH4) and 15% (CO2) conversion rate 205.0 (CH3OH) μmol in 24 h

3981.9 (CH3OH) μmol g−1 in 48 h


a Investigations providing evidence that CO2 was the actual carbon source for the carbon-containing products through 13CO2 labeling or other techniques were marked with “*” in the last column of the table.

500 W Hg-lamp (λ > 400 nm)

400 W Hg-lamp

20 W LED (λ > 400 nm)

125 W Hg-lamp

graphene oxide, Co(II) phthalocyanine ZnO, Cu(II) phthalocyanine

photocatalyst

20 W LED (λ > 400 nm)

light source

Table 2. continued


7508


Review

ACS Catalysis

can form a homojuction, which promote the charge separation, and CH4 is released on {111} facets of CeO2 with Pt as cocatalyst, while O2 is released on {100} with MnOx.147 Furthermore, some layer double hydroxides (LDHs, [M2+1−xM3+x(OH)2]x+[Ax/n]n−·mH2O), widely used as basic materials for CO2 capture and storage,148,149 are also applied in photocatalytic CO2 conversion.150−152 Their structures are based on edge-sharing octahedral units centered around a metal ion, which is coordinated to OH groups (M2+(OH)6), in which the divalent metal cations are partially substituted by trivalent ones.150 The interlayers of LDHs have effective capability of retaining various kinds of anions to neutralize the positive charge of the hydroxide sheets. Teramura and Tanaka’s group reported that Ni−Al LDH can produce stoichiometric solar fuels (H2/CO/CH4) and Cl2 in water with the existence of Cl−.146,147 Moreover, cations with d10 configuration such as Zn2+ and Ga3+ are also functional in LDHs for CO2 reduction to CO/CH3OH, and the photoinduceed electrons by SPR of Ag nanoparticles can migrate to the CB of Zn3Ga/CO3 LDH, resulting in visible-light-responsive activity.152 3.4.2. Oxysalts. Among various oxysalts used as photocatalysts in CO2 reduction systems, titanates are the most investigated one in the last 3 years. In addition to the abovementioned Ag-modified La2Ti2O7,73 titanate nanotubes100,109 and hierarchical titanate yolk@shell microspheres,136 ATiO3 (A = Na, Sr, Ca, or Pb) are also explored to act as photocatalysts for CO2 reduction.153−157 Similar to TiO2, titanates containing TiO 6 octahedron show promising applications in CO 2 reduction even though most of titanates are still UV-light responsive. Similarly, nanostucture controlling136,153,156 and bandgap engineering100,157 are also popularly used to modify those titanates for getting better light harvesting and CO2 adsorption properties. For example, Ye’s group has reported that the synthesized ATiO3 (A = Sr, Ca, or Pb) with cherry blossom leafs via modified sol−gel or a polymerizable complex method could simulate natural photosynthetic systems (NPS) from the macro/micro/nano region.153 As shown in Figure 13,

increases H2 production. The highest optimization toward hydrocarbon selectivity is shown by 20 wt % TiO2, while 90 wt % TiO2 loading is more selective for H2 production. This result is due to the uniform dispersion and stabilization of 20 wt % anatase TiO2 on KIT-6, which in turn allows more CO2 adsorption and a better light penetration than 90 wt % TiO2/KIT-6.142 Notably, the above-mentioned micro/nanostructure construction of TiO2 can create higher surface area and active sites, offer more CO2 adsorption, better light harvesting, accelerated reactant/product diffusion, and more efficient charge transport/ separation, which enables enhanced activity and selectivity of CO 2 reduction. Especially, hierarchical structured TiO 2 materials usually composed of porous structures are useful in photocatalytic CO 2 conversion for the enhanced CO 2 adsorption, cocatalyst loading and dispersity, charge separation, and light diffusion synergistically and thus bring new insights into the design and fabrication of highly effective photocatalysts for CO2 reduction. 3.4. Non-TiO2 Photocatalysts. In addition to the most popular TiO2-based photocatalysts, many other semiconductors responsive in the UV or UV−vis region are also designed and fabricated as photocatalysts for CO2 reduction. To date, these non-TiO2 photocatalysts through one-step excitation mechanism for CO2 reduction can be grouped as metal oxides/ hydroxides, oxysalts, metal chalcogenides and nitrides, nonmetal-based photocatalysts, mesoporous materials, and metal− organic-frameworks (MOFs). Their modification methods for enhancing the photocatalytic performance of CO2 conversion based on a one-step excitation mechanism are similar to the TiO2 discussed above. The corresponding experimental conditions, activities, and selectivity are listed in Table 2. 3.4.1. Metal Oxides/Hydroxides. In the last 3 years, the other metal oxides or oxide/hydroxide nanocomposites containing d0 or d10 configuration cations such as Cu(I),71,72 Zn(II),143 Ga(III),40,144 In(III),145 and Ce(IV),146,147 were reported as photocatalysts for CO2 conversion. Among which, Cu2O as a p-type semiconductor is also a visible-lightresponsive catalyst in addition to its cocatalytic effect.55,88,112,121 In 2013, Tang’s group reported that the photocatalytic reduction preference shifts from H2 (water splitting) to CO (CO2 reduction) by controlling the exposed facets of Cu2O, and its {001} facets exhibit higher activity for CO2 reduction than the high index facets.72 Moreover, an improved CO production activity can be achieved by coupling the Cu2O with RuOx to form a heterojunction which can slow down the fast charge recombination and stabilize the Cu2O catalyst.71,72 For instance, the deposition of RuOx nanoparticles on Cu2O results in a 2-fold increased yield of long-lived electrons due to the reduced charge recombination and an approximately 6-fold increase in CO production activity.71 Similarly, Ga2O3 as photocatalysts can produce solar fuels (H2 /CO) with stoichiometric O2, and 0.1 wt % Ag-loading on Ga2O3 leads to a significantly enhanced adsorption of the bidentate carbonate species, which is regarded as an important reaction intermediate of CO2 conversion,40,144 and then a ∼5-fold activity enhancement as compared with the pure Ga2O3.40 Also, CeO2 can complete CO2 reduction and H2O oxidation at same time,146,147 and Fe(III) doping can extend the spectral response region from UV to visible region and enhance the surface Ce3+ concentrations as well as chemisorbed oxygen species of the ordered mesoporous CeO2, which results in an enhanced CO/ CH4 production activity.146 Moreover, {100} and {111} facets

Figure 13. Morphology comparisons between natural leaf (A) and leaf-architectured SrTiO3 (B) from the cross section of venation architecture axially; scale bar 10 μm.153 Figure reproduced with permission from ref 153. Copyright 2013 Nature Publishing Group.

the macro/micro/nanostructures of natural leaf are maintained, and light harvesting and CO2/H2O adsorption are carried out though the 3D architectures at multiscaled levels, and the mass transfer could go through the veins, which separate the reactants and products.153 Among the prepared ATiO3 (A = Sr, Ca, or Pb), SrTiO3 shows the highest CO2 reduction activity, and photodeposited Au could enhance activity ∼4-fold 7509


Review

ACS Catalysis compared to that of pristine SrTiO3.153 In addition, bandgap engineering of SrTiO3 doped with cations such as Co, Ni, Fe, or anions such as C and N can enhance the light harvesting in the visible region.100,157 For example, C and N codoping can result in ∼100 nm redshift of band-edge of optical absorption of Na2TiO3 because the C 2p and N 2p orbital can form an impure energy level between the CB and VB of Na2TiO3, and thus C, N-co-doped Na2TiO3 can produce CH4 under AM 1.5G solar simulator.100 Similarly, cations (Co, Ni, Fe)-doped SrTiO3 also shows enhanced light harvesting in the region of 400−500 nm.157 Among which, Co-doping can promote the CO2 adsorb capacity by ∼3-fold compared to pure SrTiO3, and Pt cocatalyzed 2.0 mol % Co-doped SrTiO3 shows a CH4 production activity of 63.6 ppm h−1 under visible light irradiation, which is ∼3 times that of N-doped TiO2.157 Furthermore, some titanates also show activity in photocatalytic H2O oxidation at the same time.73,153,155 For instance, Agmodified La2Ti2O7 could produce stoichiometric solar fuels (H2/CO) and O2 in the CO2-saturated suspension system.73 The other oxysalts containing d0 or d10 configuration cations, such as Nb,155,158 Ta,158−172 Zr,173 Ga,73,164 Ge,165 V,166,167 W,168 and Bi,169−171 are also applied as photocatalysts for CO2 reduction. Ye’s group has reported a generic method to build a series of MTaO3 (M = Li, Na, K) with hierarchical anatomy from macro- to nanoscales using activated carbonized tree trunks as templates, and artificial photosynthesis is carried out on MTaO3 series using only artificial sunlight, water, and CO2 as inputs to produce CO and CH4 as the main outputs.162 CO2 photofixation performance can be enhanced by introducing a macropore network, which mainly enhances light transfer and accelerates gas diffusion; this results in ∼4-fold and ∼2-fold enhancement in CO2 reduction activity of the bulk NaTaO3 and the nanosized ones,162 respectively. Similarly, morphology and cocatalyst effects on the activity and selectivity of CO2 reduction are observed in the KTaO3, and KTaO3 nanoflakes show the best activity for both H2 production and CO2 reduction under UV light irradiation, achieving 20 times (H2) and 7 times (CO) higher activity than the cubic sample prepared by solid-state reaction.162 Pt loading is found to have a positive effect on H2 production but suppresses CO2 reduction, whereas the Ag cocatalyst is beneficial for CO2 reduction and the CO yield doubles with decreased H2 yield.162 Gallate and germinate (especially ZnGa2O4 and Zn2GeO4) with d10 configuration cations are also applied in CO2 reduction systems due to its relatively high CB position.74,164,165 The three-dimensional hierarchical structure of ZnGa2O4 consisting of ultrathin nanosheets is reported to have ∼35% enhanced CO2 conversion activity compared to that of the mesoZnGa2O4, even though its surface area is only half that of mesoZnGa2O4.164 The activity enhancement could be related to the fact that the ultrathin nanosheets provided plenty space and channel for photogenerated electrons from the interior to the surface active sites; additionally, a single exposed facet hinders the charge recombination in the boundaries. Moreover, light scattering in the 3D hierarchical architectures is in favor of enhancing the light absorption.164 Similar to TiO2 photocatalysts, the crystal facet effects are also observed from exposed {110} facets of Zn2GeO4 nanorods with nanorods having different aspect ratios derived from the ion exchange reaction between Na2GeO3 colloidal solution and various zinc salt solutions.165 Zn2GeO4 nanorods derived from ZnCl2 solution show the highest activity of CO2 reduction to CH4 due to the highest surface area, aspect ratio, and exposed ratio of {110}

facets, which is proved to have more surface oxygen vacancies benefiting to the water oxidation half-reaction and CO2 adsorption.165 Except for the above oxometalates, LaPO4 nanorods with hexagonal phase also shows superior performance for CO2 reduction with H2O under mild conditions, and the main reduction products are CH4 and H2.172 The deposited Pt cocatalyst improves the CH4 production activity, and the selectivity for CH4 formation achieved 100% as Pt content increased to 3 wt %. The enhanced activity and selectivity of Ptloaded LaPO4 is attributed to the outstanding ability of the Pt nanoparticles as an excellent electron transfer mediator and absorber for CO2.172 Different from the above wide bandgap oxysalts that only respond in the UV region, 153−164 some oxometalates containing d0 or d10 configuration cations, such as V,166,167 W, 168 and Bi, 169−171 show visible-light-responsive CO 2 reduction activity. For example, InVO4 has a good visiblelight-responsive activity, and the presence of empty donor states in the depletion layer is believed to be responsible for the sub-bandgap absorption of InVO4.167 After loading with NiO, the bandgap of NiO/InVO4 is lower than that of InVO4 due to the formation of sub-band in VB by creating more defect sites on InVO4 surface. Both NiO/InVO4 and InVO4 can reduce CO2 to CH3OH under visible light irradiation, and the activity increased with enhancing NiO loading, which is ascribed to the distribution of photoinduced electrons by loading NiO and the extensive active kinks offered by the pin-holes on NiO/ InVO4.167 Similarly, Na2V6O16·xH2O nanoribbons with exposed {010} and {001} facets synthesized by electrochemical method show visible-light-driven CO2 reduction activity.166 The bandgap is −0.41 eV, which locates between CO and CH4 redox potential (−0.48 and −0.24 eV), ensuring the CO2 reduction selectivity of CH4 in gas phase.166 Furthermore, oxometalates containing W and Bi cations such as ball-flowerlike Bi2WO6,168 NaBiO3,169 and bismuth oxyhalogen (especially for BiIO)170,171 are also used as visible-light-responsive catalysts for CO2 reduction. 3.4.3. Metal Chalcogenides and Nitrides. In the last 3 years, metal chalcogenides including sulfides, such as ZnS,173−175 Bi2S3,176 CdIn2S4,177 ZnIn2S4,178 and CuS/Cu2S,179 and tellurides such as CoTe180 and ZnTe,181 are also applied in photocatalytic CO2 conversion. Just like metal oxides, crystal phase, crystal facet, and morphology effects are also observed from those metal chalcogenides.173−181 Macyk’s group has reported that ZnS nanoparticles derived from different procedures show differences in the electronic band structure, morphology, as well as in the photoactivity for CO 2 conversion.174 Among which, nanocrystalline ZnS-A (3.3 nm) shows a slightly higher activity toward CO2 reduction than ZnS-B (20.6 nm). The particle size influences many other fundamental properties of photocatalysts such as surface area, potentials of bands edges, spectral range of absorbed light, or even charge lifetime. Ru particles deposited at ZnS surface can act as catalyst (lowering the activation energy and improving CO2 adsorption), or as an electron sink (increasing the charge lifetime).174 Furthermore, ZnS nanoparticles stabilized by cetyltrimethylammonium bromide (CTAB) are deposited on montmorillonite (MMT) in order to investigate the performance of ZnS/MMT in the CO2 reduction.175 Although the pristine MMT does not possess any photocatalytic performance, all prepared ZnS/MMT nanocomposites with different ZnS loading amount exhibited a higher activity of CO2 reduction to H2/CH4 in comparison with P25, and the 7510


Review

ACS Catalysis

3.4.4. Non-Metal-Based Photocatalysts. Nonmetal-based photocatalysts especially carbon,23,24,184−187 carbon nitride,17,188−195 and silicon carbide196 are also hot photocatalysts for solar fuel production. In addition to the informative and specialized reviews on this issues,17,23,24 many carbon-based photocatalysts such as carbon quantum dots (CQDs), graphene (G), and/or its oxides (GO) are still used as photocatalysts or its supports in the last 3 years.184−187 For instance, it was reported that GO is an effective, low-cost photocatalyst for simultaneous CO2 reduction and solar energy harvesting.184 By modulating the oxygenated functional groups, the CH3OH conversion rate can be achieved up to 0.172 μmol h−1 g cat−1 on GO under 300 W halogen lamp irradiation, much higher than that of TiO2.184 Moreover, the 2D monolayered structure of GO is also a highly effective substrate of cocatalysts because of its ultrathin structure, suitable energy gap, and various oxygenated functional groups on the basal plane and peripheries, which is beneficial for metal nanoparticles with controlled size distribution incorporating with GO to form a heterogeneous catalyst to enhance CO2 reduction activity, as shown in Figure 14.185 The positive Cu-work function (Figure

increasing ZnS amount affects the degree of ZnS nanoparticles agglomeration on MMT. This agglomeration influences the properties/behavior within the electronic structure of ZnS, correlating with the photocatalytic performance of ZnS/MMT for CO2 reduction.175 Xin and Yin’s group synthesized a series of Bi 2 S 3 nanostructures with different morphologies, such as nanoparticles, urchin-like spheres, microspheres, and thin urchin-like spheres, via a simple template-free solvothermal process.176 It is found that the photocatalytic performances of the obtained samples on the reduction of CO2 to methyl formate in CH3OH are greatly dependent on the morphologies of the samples, and the hierarchical architectures exhibit superior activity due to their better permeability and high light-harvesting capacity compared with Bi2S3 nanoparticles.176 The same group also prepared a series of microspheres of CdIn2S4 by hydrothermal process.177 Among them, the CdIn2S4 synthesized from Lcysteine possesses the most perfect spherical morphology, larger surface area, and stronger visible light absorption ability, which collaborated to the high activity for CO2 reduction to dimethoxymethane and methyl formate in CH3OH.177 Also, crystal phase and morphology effects are observed from ZnIn2S4,178 that is, CO2 reduction activity over hexagonal ZnIn2S4 nanosheets are better than that over cubic ones, and both hexagonal and cubic ZnIn2S4 nanosheets displayed much higher activity due to their large exposed surface area and thin thickness as compared with ZnIn2S4 microspheres.178 Kar and Shankar’s group synthesized a heterostructure of CuS/Cu2S nanorod and nanowall arrays with vertical orientation and leaflike or branched structure by electrochemical anodization of copper foil and copper-coated Kapton substrates.179 The p-type nature of the anodic Cu2S and n-type nature of anodically formed CuS nanostructures led to an excellent activity of CO2 reduction to CH4 under AM 1.5G solar simulator without any cocatalyst or promoter.179 Furthermore, hierarchical structure combined with nanosized CoTe180 and ZnTe181 also show higher activities of CO2 reduction to CH4 in gas atmosphere under visible light irradiation as compared with most of single photocatalysts reported earlier.182 The above results illustrate the crucial roles of particular crystal phase of metal chalcogenides with well-defined shapes and/or morphologies on the CO2 reduction activity.176−179 Moreover, some metal sulfides with narrow bandgap are usually accompanied by rapid charge recombination and metastable surface sulfide ions in an oxidizing environment, and thus, alcohols (methanol or isopropanol) are used as sacrificing reagents to neutralize the photogenerated holes to enhance the stability of metal sulfides.164,176−178 Metal nitrides such as GaN nanowire films and arrays grown on n-type Si(111) substrate also exhibit efficient CO 2 conversion into CH4 and CO.182,183 Rh and Pt nanoparticles with work functions in the range of GaN bandgap act as cocatalysts for photocatalytic CO2 conversion, resulting in changes of production selectivity.183,183 Rh is found to promote the formation of formate ions and then converse to CH4 without CO intermediate, and thus, the CO yield decreases by 1 order of magnitude and CH4 increases 3-fold compared to bare GaN. Moreover, Pt with relatively lower work function can act as electron sink, active CO2 molecular and effective revolute and adsorb H2, which further accelerates CH4 formation.182,183 Moreover, Rh/Cr2O3 core/shell nanoparticles show an enhancement of the reaction rate and selectivity toward CH4 over CO.182

Figure 14. (a) UV photoelectron spectroscopy (UPS)-determined work functions of GO and Cu/GO hybrids and (b) band-edge positions of pristine GO and Cu/GO hybrids compared with CO2/ CH3OH and CO2/CH3CHO formation potential. (c) Schematic photocatalytic reaction mechanism.185 Figure reproduced with permission from ref 185. Copyright 2014 American Chemical Society.

14a) could promote the photoinduced electrons oriented migrating from GO-substrate to the loaded Cu nanoparticles, and with enhancing Cu-loading amounts, the bandgap narrowed from both VB and CB directions (Figure 14b), which can enhance light harvesting of the visible region.185 Among which, 10 wt % Cu-loading on GO, the band structure is suitable for CO2 reduction to special products, and the Cu particle size (4.15 nm) also benefits from the charge transfer, thus resulting in the highest visible-light-driven activity of solar fuel production (6.84 μmol h−1 gcat−1), which is 60 and 240 times higher than that by the pristine GO and the commercial P25,185 respectively. Recently, carbon quantum dots (CQDs) with bright fluorescence emissions, were also researched as new watersoluble photocatalysts for CO2 reduction because their excellent photoinduced redox properties resemble those 7511


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ACS Catalysis found in conventional semiconductor nanostructures.186,187 Bunker and Sun’s group has demonstrated that aqueous suspended bare CQDs can be excited with visible light to convert CO2 into formic acid with the performance competitive to that of widely used semiconductor nanoparticles.186 The surface-doping of the CQDs with a metal can substantially improve the CO2 reduction activity.186,187 Moreover, Au-doped carbon dots show visible-light-driven CO2 reduction to small organic acids, including acetic acid (for which the reduction requires many more electrons than that for formic acid) and, more interestingly, demonstrate significantly enhanced activity with higher CO2 pressures over aqueous suspended photocatalysts.187 As another famous 2D mono/multilayered materials, graphitic carbon nitrides (g-C3N4) are also extensively applied as visible-light-driven catalyst for CO2 reduction due to its narrow bandgap of ∼2.70 eV with suitable CB/VB levels (ca. −1.10/ 1.60 eV).17,188−195 g-C3N4 is easily synthesized by simple calcination of urea, 1 8 8 , 1 9 1 , 1 9 2 thiourea, 1 8 9 , 1 9 4 melamine,188−190,195 cyanamide,193 and other compounds consisting of carbon and nirtride.191 These g-C3N4 photocatalysts synthesized by different sources show different activity due to their differences in porous distribution, surface area, and surface state.188,189,191 For example, g-C3N4 synthesized from urea (UCN) shows thinner mesoporous flake-like structures and larger surface area than that synthesized from melamine (MCN), which results in ∼2.5 times enhancement of AQY (from 0.08% up to 0.18%) in CO2-saturated NaOH solution under visible light.188 Moreover, g-C3N4 from thiourea (TCN) might leave S atom in the supercell of g-C3N4 and thus cause a slightly lower CB bottom than MCN due to the hybridation among C 2p, N 2p, and S 3p orbital.189 The lower CB bottom of S-doped g-C3N4 (TCN) shows better light harvesting and charge separation, thereby resulting in a higher activity in CO2 reduction to CH3OH even though the surface area of MCN was larger than that of TCN.189 Nanostructure and surface engineering are also used to improve the photocatalytic performance of g-C3N4.190,191,193 Ultrasonic exfoliation of bulk MCN in water could result monolayered surface hydroxyl functional g-C3N4 (G-CN).190 Due to the monolayered structure and surface hydroxyl, the surface area enhanced ∼3 times and the CB/VB lifted ∼0.80 eV, resulting in significant increased activity of CO2 reduction to CH4.190 Moreover, mixing with other organic blocks as the comonomer can also promote the activity of g-C3N4. For instance, barbituric acid (BA), 2-aminothiophene-3-carbonitrile (ATCN), 2-aminobenzonitrile (ABN), and diaminomaleonitrile (DAMN) are mixed to fabricate modified CNU.191 As shown in Figure 15, more porous and framework structure could be observed in CNU-BA, resulting in improved optical absorption, reduced charge recombination, and enhanced charge transfers which endows them with ∼15 times better activity of CO2 reduction under visible light.191 Among which, S-contained CNU-ATCN shows the highest activity.189,191 Additionally, helical nanorod-like g-C3N4 (HR-CN) based on chiral mesoporous silica (CMS) as the template and cyanamide (CY) as the precursor was also synthesized.193 HR-CN with surface area of ca. 56 m2 g−1, which is ∼14-fold that of MCN, also displays a deeper color due to multiple reflections of incident light in the nanostructure and thus shows ∼22-fold enhancement compared to MCN in CO2 reduction to CO with decisive selectivity under visible light irradiation.193

Figure 15. SEM and TEM images of CNU (a,c) and CNU−BA (b,d).191 Figure reproduced with permission from ref 191. Copyright 2015 Elsevier B.V.

3.4.5. Mesoporous and MOF Materials. Similar to TiO2based photocatalysts, some porous materials are also used as photocatalyst or its supporting matrix as mentioned in the recent reviews specialized on the application of mesoporous and MOF materials in the field of solar fuel production.22,27−29 Usually, the precise control over the pore structure and surface chemistry of those porous materials could gain good accessibility, higher surface area, enhanced activity, and selectivity of CO2 reduction.196−210 In addition to the above-mentioned isolated TiOx-species loaded on porous materials,140−142 mesoporous network structures modified by incorporating Ti-sites in MCM-41, SBA-15, KIT-6, and TDU-1 silica matrix, are proved to enhance the adsorption of the reactive species and utilization of the incident light, and thereby to improve the photoactivity.197−200 Bai and Wu’s group reported that the incorporated Ti-species in Ti-MCM-41 exist as tetrahedral Ti4+ sites combined with octahedral Ti4+ ones, which may be derived from the conversion of tetrahedral Ti4+ located at the material surface to octahedrally coordinated Ti4+ by reaction with H2O(g) in the atmosphere as well as to segregation of TiO2 species at the material surface.197 The absorption edge of Ti-MCM-41 can be shifted appreciably to longer wavelength with enhancing Ti/Si molar ratios even though MCM-41 shows no significant UV absorption.197 Among various Ti-MCM-41 with different Si/Ti molar ratios, Ti-MCM-41 with Si/Ti molar ratio of 50 shows the best activity of CH4 production (62.42 μmol gcat.−1) in the CO2 reduction systems containing monoethanolamine (MEA) solution, much higher than that of commercial P25 after 8 h of UV illumination.197 Similarly, SBA-15 and KIT-6 incorporated with highly dispersed isolated Ti-sites are also active photocatalysts for CO2 reduction with H2O vapor to obtain fuel products (CH4, CH3OH, CO and H2).198 Among which, the more isolated Ti-species which are uniformly dispersed on 3D KIT-6 mesoporous silica without collapsing the mesoporous structure, have boosted the higher activity, which is even higher than the commercial P25.198 Mul’s group has proved that relatively high Ti-loadings can be achieved in TUD-1 mesoporous silica without losing Ti-dispersion.199 The addition of ZnO nanoparticles and visible-light-responsive CrOx to TiTUD-1 would result in significantly enhanced rates in the 7512


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ACS Catalysis

enhanced activity for formate formation, whereas Au has a negative effect on this reaction.203 The above investigations indicate that both metal clusters and organic linkers could be modified to improve the activity. Kang and Cohen’s group reported MOFs containing mixed metals (Zr/Ti) and mixed ligands show highly efficient and robust CO2 reduction to HCOOH under visible light irradiation.204 The postsynthetic exchange (PSE) of Ti(IV) into Zr(IV)-based MOFs (UIO-66) enable UIO-66 showing activity by lowering the electron-accepting levels of the Zr secondary-building units, and introduction of diamine-substituted ligands greatly enhances the photocatalytic ability by introducing new energy levels for additional light absorption and charge transfer.204 In addition, it was reported that a porphyrin-involved MOF (PCN-222) can selectively capture and further photoreduce CO2 with high efficiency under visible light irradiation.205 The presence of a deep electron trap state in PCN-222 effectively inhibits the charge recombination, and thus, PCN-222 can significantly enhance the activity of CO2 reduction to formate compared to the porphyrin ligand itself.205 Also, Cu-porphyrin-based MOFs show an effective CO2 capture and activation, and lead to the CH3OH production activity improved as high as 7 times compared with the sample without Cu2+.206 In situ FT-IR results suggest that CO2 chemical adsorption and activation over Cu2+ played an important role in improving the efficiency.206 To further overcome the obstacle of stability in the practical applications of MOFs, Luo and co-workers synthesized Rupolypyridine-based MOFs with noninterpenetrated and interpenetrated structures, and they found that both of which show CO2 reduction activity in MeCN solution.207,208 The interpenetrated Ru-MOF possesses good photocatalytic durability and recyclability, and it shows much higher thermal and photic stability in comparison with its noninterpenetrated counterpart.207 Moreover, they fabricated a kind of hierarchical nanoflowers based on a bifunctional Ru-MOF, which can act as stable and effective catalyst for visible-light-driven CO2 reduction.208 The unique flower-like 3D hierarchical nanostructure not only highly improves the photostability of RuMOF but also remarkably enhances the activity of this MOF material.208 In particular, Zn2GeO4/ZIF-8 hybrid nanorods, synthesized by growing ZIF-8 nanoparticles on Zn2GeO4 nanorods, inherit both high CO2 adsorption capacity of ZIF-8 nanoparticles and high crystallinity of Zn2GeO4 nanorods,209 and Zn2GeO4/ZIF8 hybrid nanorods containing 25 wt % ZIF-8 exhibit 3.8 times higher dissolved CO2 adsorption capacity than the bare Zn2GeO4 nanorods, which results in a 62% enhancement in activity of CO2 reduction to CH3OH.209 Similarly, Wang’s group established a CO2 reduction system by employing CdS and Co-ZIF-9 to act as photocatalyst and cocatalyst, respectively.210 This CdS/Co-ZIF-9 system cooperating with bipyridine and TEOA exhibits high activity in CO2 reduction to CO under visible light irradiation due to the enhanced electron transfers by Co-ZIF-9.210 The strategies reported above provide important insights into the efforts in both metal/organic linker designs and microstructure constructing of MOF-based materials for efficient CO2 capture, activation, and photoreduction, which are necessary to further promote the activity and water-stability of MOFs for CO2 reduction in future.204−208 Moreover, semiconductor loaded on mesoporous or MOFs are promising for developing more active and stable photocatalysts for

backward reactions of intermediates, such as formaldehyde, as well as of the produced hydrocarbons (in particular ethylene), demonstrating the importance of evaluating hydrocarbon conversion property over the photocatalysts.199 On the other hand, Kang’s group reported that the activity of CO2 reduction to CH4 can be improved remarkably by introducing Ca on TixMCM-41 (x = 5, 15, 35, and 50 mol %) surface, and Ca(10 wt %)/Ti35-MCM-41 has the best CH4 production activity due to the effective charge separation and the inhibited charge recombination.200 MOFs, as a class of newly inorganic−organic hybrid porous materials, have gained ever-increasing research interesting for its diverse and easily tailored structures.27−29 Nevertheless, most MOFs applied in CO2 reduction systems are not watertolerant due to the weak connection between metals and organic linkers, which must react in CO2-soluted organic solutions.201−208 Moreover, poor stability has long been a major obstacle to the practical applications of MOFs even though it can be partially overcome by using sacrificing reagents such as TEA, TEOA to reverse the electron deficiency for better stability.201−208 In MOFs, the metal clusters are always regarded as reactive centers of CO2 reduction, while the organic linkers are excited by irradiation and then provide channels for excited electrons migrating to the metal centers.201−208 Li’s group reported that NH2−UIO-66(Zr) exhibits CO2 reduction activity in the presence of TEOA as sacrificial agent under visible light irradiation.201 The photoinduced electron of the excited 2-aminoterephthalate (ATA) can transfer to Zr-oxo clusters in NH2−UIO-66(Zr), and the generation of ZrIII and its involvement in CO2 reduction is confirmed by ESR analysis. Moreover, NH2−UIO-66(Zr) with mixed ATA and 2,5diaminoterephthalate (DTA) ligands exhibit better performance due to its enhanced light absorption and increased CO2 adsorption.201 They also found that amine-functionalized Fecontaining MOFs (NH2-MIL-101 (Fe), NH2-MIL-53(Fe) and NH2-MIL-88B(Fe)) show enhanced activity of CO2 reduction to formate under visible light irradiation as compared with the unfunctionalized one, due to the existence of dual excitation pathways as shown in Figure 16: that is, excitation of an NH2

Figure 16. Proposed mechanism for photocatalytic CO2 reduction over NH2-MIL-101(Fe) under visible-light irradiation.202 Figure reproduced with permission from ref 202. Copyright 2014 American Chemical Society.

functionality followed by an electron transfer to the Fe center, in addition to the direct excitation of Fe−O clusters inducing the electron transfer from O2− to Fe3+ to form Fe2+, which is responsible for the CO2 reduction of the unfunctionalized Fecontaining MOFs.202 Furthermore, the same group showed that NH2-MIL-125(Ti) can catalyze CO2 to produce formate with TEOA as sacrificial agent under visible light.203 Compared with pure NH2-MIL-125(Ti), Pt/NH2-MIL-125(Ti) shows an 7513


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In a typical coexcitation dye/semiconductor system, dye molecules absorb visible light and inject the excited electrons into semiconductor’s CB, and then the active sites or the loaded cocatalysts on the semiconductor collect these injected electrons in addition to the ones of the photoexcited semiconductor itself to accomplish the CO2 reduction processes, as shown in Figure 17.218,220,221 Jian’s group has

improving CO2 conversion efficiency by taking advantage of excellent adsorption and/or cocatalysts property of MOFs in aqueous media; such composite materials may bring new insights into the design and applications of highly effective photocatalysts.209,210 Also, MOFs can act both as a solid ligand for building a supported molecular catalyst and as a platform for assembly of several photoactive moieties into one composite system to achieve complicated functions for CO2 reduction owing to its easy tailoring linkers.211,212 For instance, MOF-253 supported Ru-carbonyl complex (Ru(CO)2Cl2) as molecular cocatalyst exhibits visible-light-responsive CO2 reduction activity, which can be further improved by immobilizing Ru(bpy)2Cl2 as sensitizer because Ru(bpy)2Cl2 can react with the surface N,Nchelated sites to form MOF-253 supported [Ru(bpy)2(X2bpy)2+].211 Similarly, Zr(IV)-based MOF incorporated with Mn-bipyridine complex {Mn(bpydc)-(CO)3Br (bpydc = 5,5′-dicarboxylate-2,2′-bipyridine)} also shows visible-light-responsive CO2 reduction activity by immobilizing [Ru(dmb)3]2+ (dmb = 4,4′-dimethyl- 2,2′-bipyridine) as sensitizer and using 1-benzyl-1,4-dihydronicotinamide (BNAH) as a sacrificial reductant.212 3.4.6. Photosensitized Non-TiO2 Materials. Similar to TiO2, photosensitization by loading dyes or SPR metal nanoparticles on non-TiO2 catalysts are also used to enhance the visible light harvesting.68,69,75−79,134,213−223 For instance, Baeg’s group has reported a graphene-based visible-light-active catalyst (CCG− BODIPY) which is chemically converted graphene (CCG) covalently bonded to a light-harvesting BODIPY molecule (1picolylamine-2-aminophenyl-3-oxy-phenyl-4,4′-difluoro-1,3,5,7tetramethyl-2,6-diethyl-4-bora-3a,4a-diaza-s- indacene-triazine).217 The fabricated photocatalyst−biocatalyst coupled system using CCG−BODIPY as photocatalyst leads to high NADH regeneration (54.02%), followed by its consumption in exclusive formic acid production (144.2 μmol) from CO2.217 In an investigation on the phthalocyanines containing center metals such as Cu(II), Co(II), Ni(II) as sensitizers of ZnO for photocatalytic CO2 conversion, it was found that Cu(II) phthalocyanine shows the highest activity without change of the surface state and crystal properties of ZnO substrate.219 In addition, hexa-coordinated Sn(IV)Cl2-phthalocyanine with two chlorine atoms along axial could sensitize on CeO2 by forming Sn−O coordinate bonds,78 and Sn(IV)Cl2-phthalocyanine sensitized CeO2 shows stable activity of CO2 reduction to CH3OH with TEA as electron donor under 24 h visible light irradiation.78 Similarly, Ru-bipyridine complexes are also employed as sensitizer owing to its stable physicochemical property, suitable energy level, and high extinction coefficient.75−77,79,80,132,220 The above-mentioned dye-sensitized CO2 reduction systems are usually conducted through one-step excitation mechanism, in which sensitizers act as visible light harvester to inject the excited electrons into semiconductor for the following CO2 reduction and H2O (or sacrificial reagent oxidation). Nevertheless, some dye/semiconductor systems can also conduct through the two-step excitation mechanism, in which both semiconductor (especially for visible-light-responsive catalyst) and sensitizer can be simultaneously excited by different wavelength light irradiation. According to the electron migrating direction of two-step excitation model, the dye/ semiconductor systems through two-step excitation can be divided into two basic types: coexcitation (similar to Figure 3c) and Z-scheme (similar to Figure 3d) systems.

Figure 17. Schematic diagram of possible mechanistic pathway of CO2 photoreduction on the CoPc-GO.221 Figure reproduced with permission from ref 221. Copyright 2014 The Royal Society of Chemistry.

demonstrated that GO-immobilized sensitizers, such as trinuclear-Ru-polyazine complex,221 Co phthalocyanine,218 and Ru complex,220 are more effective, recyclable, and costeffective photocatalysts for CO2 reduction to CH3OH under visible light irradiation as compared to GO. Moreover, attachment of sensitizer molecules to GO can promote the excited electron transfer to the CBs of GO and prohibit the charge recombination, thus causing a stable activity of CO2 reduction to CH3OH.218,220,221 On the contrary, in a typical dye/semiconductor Z-scheme system, the excited electrons in the LUMO of dye molecules can directly transfer to cocatalyst to reduce CO2 (but not inject into CBs of semiconductor), and the CB electrons of the photoexcited semiconductor transfer to the HOMO of the oxidized dye molecules to reactivate the dye as shown in Figure 18.222 Ishitani’s group reported that Z-scheme visible-lightresponsive CO2 reduction systems consisting of Ag-loaded CaTaO2N or TaON serving as a building block and CH3OH oxidization units, with the binuclear Ru(II) complex as sensitizer unit and CO2 reduction sites.222,223 This hybrid material shows activity for CO2 reduction to HCOOH with high selectivity under visible light according to a two-step excitation of semiconductors and sensitizer, where Ag nanoparticles loaded on semiconductor with optimal distribution mediated the interfacial electron transfer due to reductive equenching.222,223 3.4.7. Co-Catalyst Loading. Co-catalyst effects are often first-considered solutions to promote the charge separation of a photocatalytic system. Similar to the above-mentioned metal cocatalysts of TiO2, most of the non-TiO2 photocatalysts mentioned at this section are also loaded metal cocatalysts as the photogenerated electron sinks, such as P t , 1 5 7 , 1 6 1 , 1 8 2 , 1 8 9 , 1 9 2 , 1 9 4 , 2 0 3 A u, 1 5 3 , 1 6 2 , 1 8 6 , 1 8 7 , 2 0 3 , 2 0 4 Ag,40,152,154,159,160,172 Cu,41,165 Rh,183 Ru,161,174,179 for promoting the charge separation. Recently, DFT calculations indicated 7514


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the electronic interactions between the two units and using a suitable solvent.68,69 Similarly, the light-harvesting property and activity of homogeneous molecular catalysts such as Re, Ru, Co, Pd, or Ru−Re binuclear complexes can be improved through immobilization on metal oxide.225−229 To strengthen the electronic interactions between the two units, 4,4′-position of bipyridyl of the Re complex is functional with phosphate group to form an ester linkage with the surface hydroxyl of photocatalysts.68,69,225 A similar tight linkage between the two units can provide an electron transfer gallery from a semiconductor to metal complexes and enhance the CO2 reduction activity.226,227 Recent nonadiabatic molecular dynamics simulations to Ru complexes assembled on N−Ta2O5 surface indicate that the electron transfer is faster in complexes with COOH anchors than in complexes with PO3H2 groups, due to larger nonadiabatic coupling, whereas quantum coherence counteracts this effect to a small extent.229 The COOH anchor promotes the transfer with significantly higher frequency modes than PO3H2, due to both lighter atoms (C vs P) and stronger bonds (double vs single), and the acceptor state delocalizes onto COOH (but not PO3H2), further favoring the electron transfer in the COOH system. At the same time, the COOH anchor is prone to decomposition (in contrast to PO3H2), making the former show smaller TON in some cases.229 These theoretical predictions are consistent with the corresponding experimental results and legitimates the proposed mechanism of the electron transfer.229

Figure 18. Z-Scheme CO2 photoreduction under visible light using a hybrid that consists of a semiconductor and a binuclear Ru complex.222 Figure reproduced with permission from ref 222. Copyright 2014 American Chemical Society.

that deposition of atom (i.e., Pd or Pt) cocatalysts on g-C3N4 can also significantly enhance the visible-light absorption, rendering them ideal for visible-light-driven CO2 photoreduction. Moreover, the individual metal atoms function as the active sites during CO2 photoreduction, and g-C3N4 provides the source of hydrogen (H*) from the hydrogen evolution reaction.224 In addition, certain metal oxides such as RuOx,71,72,166 NiOx,167 CoOx,191,193,195 and MnOx147 as cocatalysts effectively gather the photoinduced holes of photocatalysts including TiO2. For instance, RuOx loading on Cu2O can promote the migration of the photogenerated-holes, resulting in a significant increase in the yield of long-lived Cu2O electrons and reducing the charge recombination, which is in favor of photocatalytic CO2 conversion.71 Some metal (Re, Ru, Co) complexes can also act as molecular cocatalysts of semiconductors for promoting the CO2 reduction reactions.3,19,68,69,225−229 In this system, the photogenerated electrons of semiconductor can migrate to the loaded Ru/Re complexes, which is followed by the CO2 reduction process, as shown in Figure 3a. It is well-known that molecular catalysts, especially carbonyl metal (Re, Ru, Co, Mn) bipyridyl complexes, are already employed in homogeneous CO2 reduction systems with high quantum efficiency, and the CO2 conversion mechanism over center metals is also understood.3,19 However, one disadvantage of this system is the unavailability of H2O as electron donor under CO2 photoreduction conditions. In contrast, a semiconducting photocatalyst can use H2O as an electron donor under the conditions used for photocatalytic CO2 reduction.3,19 Also, those molecular catalysts can be immobilized on solid materials such as g-C3N4, TiO2, N−Ta2O5, or even organosilica as light harvester to improve the performance of CO2 reduction.68,69,225−229 In this heterogeneous CO2 reduction system, the LUMO position is more positive than the CB level of the semiconductor, and therefore, the photoexcited electrons from the CB of the semiconductor can transfer to the metal-complex cocatalyst to promote the selective CO2 reduction on the metal complex. For example, Meda’s group has reported such heterogeneous system consisting of Ru complex and g-C3N4, which acts as the catalytic and light-harvesting units, respectively, for CO2 reduction to CHOOH.68,69 Its photocatalytic performance can be improved by promoting the injection of electrons from g-C3N4 into Ru-unit, strengthening

4. HYBRID PHOTOCATALYSTS FOR TWO-STEP EXCITATION CO2 REDUCTION SYSTEMS Although the activity and/or selectivity of semiconductor can be improved by various approaches such as facet/surface engineering, bandgap engineering, microstructure constructing, and crystal phase controlling, the essential properties (such as fast charge recombination, limited light absorption, and low quantum efficiency) of a single semiconductor retard the further improvement of overall solar energy conversion efficiency. To promote the charge separation in space, most efforts are paid on the construction of hybrid photocatalysts, such as cocatalyst loading, semiconductor photosensitization, semiconductor heterojunction, Z-scheme fabricating, and so on. Since some of these hybrid systems that catalyze the CO2 photoreduction processes through a one-step excitation mechanism are mentioned above, this section will mainly focus on those hybrid photocatalysts fabricated with different semiconductors, including solid heterostructures (cf. Figure 3c) and solid Z-scheme (cf. Figure 3d) systems that can catalyze the CO2 photoreduction processes through two-step excitation. Their corresponding photocatalytic experimental conditions, activities, and selectivity are listed in Table 3. It worth noting that (1) many solid hetertojunctions mentioned in this section are also conducted through onestep excitation, which will be specially mentioned; (2) some other two-step excitation systems (for instance, the dye/ semiconductor photosensitization systems mentioned above) are not included in the section. Furthermore, this section mainly discuses the corresponding development of this issue since 2013. For further comprehending the scope of this area before 2013, readers are encouraged to consult the recent informative reviews specialized on solid hybrid materials for solar fuel conversion.21,30 7515


Cu2O/TiO2 nanotube array Cu2O/TiO2 hollow nanospheres Cu2O/TiO2 ZnO/TiO2

CeO2/TiO2

Au3Cu-SrTiO3/TiO2

Pt−Bi2WO3/TiO2 (P25)

CdS/TiO2

g-C3N4/TiO2 CeF3/TiO2

Cu3(BTC)2@TiO2

500 W Xe−Hg lamp 300 W Xe-lamp (λ > 420 nm)

300 W Xe-lamp

300 W Xe-lamp

90 W cm−2 UV-LED (λ = 365 nm)

300 W Xe-lamp (λ = 300−800 nm)


300 W Xe-lamp (λ > 400 nm)

7516

ZnTe/SrTiO3

ZnTe/ZnO

ZnO/CuO nanowires

Ni−Ni3(BO3)2/NiO

CdS/Cu−NaxH2−xTi3O7 nanotubes carbon nanotube@ Ni-TiO2

Ag-MWCNT@TiO2 MWCNT/TiO2

Ag@AgBr/CNT

300 W Xe-lamp (λ > 420 nm)

300 W Xe-lamp (λ > 420 nm)

400 W Xe-lamp (λ = 250−810 nm)


450 W Xe-lamp (λ > 420 nm)

15 W energy-saving daylight lamp 15 W energy-saving daylight lamp

150 W Xe-lamp (λ > 420 nm)

graphene/TiO2 graphene/TiO2

graphene/N-TiO2 graphene/TiO2 hybrid nanosheet

300 W Xe-lamp 14 W low- energy light (λ = 254 nm)

15 W energy-saving daylight lamp 300 W Xe-lamp

75 W visible daylight lamp

Pt-PANI/TiO2

200 W Xe-lamp (λ = 320−780 nm)

300 W Xe-lamp 8 W Hg-lamp (λ = 254 nm)

ZnO/g-C3N4 Ag3PO4/g-C3N4

photocatalyst


light source

6.0

1.42 (CH4) ppm h−1 (16.7 wt % Cu2O) 0.75 (CH4)/∼0.067(H2) μmol h−1 (50 mol % TiO2)

0.30 29.92

256 257

254 255

252, 253

∼186 ∼4.82

248 249−251

247

246*

245

244

243

242

241

240

238 239

237

236

235

234

232 233

230 231

70 216

ref

0.79 0.112

5.08

3.57

0.29

4.0

6.36

2.46 129.84

0.054

∼5.04

0.064

0.36 ∼1.50

TCEN (μmol h−1)

0.06 (CH3OH) μmol h 0.01 (CH4)/∼0.41 (CO)/0.08 (CH3OH)/0.01 (C2H5OH) μmol h−1 55.15 (CH3OH) μmol in 6 h 0.008 (CH4) μmol h−1 (7 wt % Cu2O)

−1


0.31 (CH4)/∼1.28 (CO) μmol h−1 (50 mol % CeO2) ∼2.5 cm2 catalyst film over CO2-saturated N2H4, 421 (CH4) μmol g−1 h−1 (0.4 wt % Au3Cu; 25 mol % and then inject 4 mmol CO2 and 8 mmol He SrTiO3) 2.2 g of catalyst mixed with glass spheres, H2O- 14.74 (CH4) μmol h−1 (0.5 wt % Pt; 50 wt % TiO2) saturated He with 1 vol % CO2, 150 °C 20 mg of catalyst, CO2 (0.2 MPa) and H2O 0.0045 (CH4)/0.05 (CO) μmol h−1 (30 wt % vapor graphene) 100 mg of catalyst, CO2 and H2O vapor 1.23 (CO) μmol h−1, (urea/Ti(OH)4 mass ratio 6:4) 400 mg of catalyst in 400 mL of CO2-saturated 21.64 (CH3OH) μmol h−1 (20 wt % CeF3) water 300 mg of catalyst, 0.15 MPa CO2 and H2O 0.79 (CH4) μmol h−1 (33 wt % TiO2) vapor, 40 °C 10 mg of catalyst in CO2 (0.2 MPa) and H2O 0.50 (CH4)/3.2 (H2) μmol h−1 (0.2 wt % Pt; 0.85 wt vapor % PANI) 20 mg of catalyst in CO2 (+25 kPa) and H2O 0.05 (CH3OH) μmol h−1 (25 mol % ZnTe) vapor 10 mg of catalyst in 80 mL of CO2-saturated 0.45 (CH4) μmol h−1 (ZnTe 14.3 mol %) water 1 cm−2 catalyst glass wool supported film in 3 1.9 (CO) mmol g−1 h−1 mL/min wet CO2 flow 125 mg of catalyst in 125 mL of CO2-saturated 0.41 (CH4)/0.90 (CO) μmol h−1 0.2 M NaOH 50 mg of catalyst in 25 mL of water, CO2 ∼1.35 (CH4)/0.85 (C2H6)/0.5 (C3H8)/0.005 (C2H4)/0.035 (C3H6) μL h−1 bubbled 1 h before irradiation catalyst in wet CO2 (humidity 60%) 0.145 (CH4) μmol g−1 h−1 (14.2 wt % CNT; 10 wt % Ni) 100 mg of catalyst, CO2 and H2O vapor 0.091 (CH4)/0.0048 (C2H4) μmol h−1 (2 wt % Ag) 100 mg of catalyst, CO2 and H2O vapor 0.014 (CH4) μmol h−1 (TiO2/ MWCNT mass ratio 1:0.24) 500 mg of catalyst in 100 mL of 0.2 M KHCO3 13 (CH4)/∼3.5(CO)/8.5 (CH3OH)/2 (C2H5OH) μmol h−1 solution under 7.5 MPa CO2 (pH 8.50) 100 mg of catalyst, CO2 and H2O vapor 0.23 (CH4)/∼1.49 (CO) μmol g−1 h−1 0.4 g L−1 catalyst in CO2-saturated solution ∼95 (HCOOH)/∼75 (CH3OH) μmol g−1 h−1 (40 wt % TiO2) (AEF= 0.069%) 100 mg of catalyst, CO2 and H2O vapor 0.037 (CH4) μmol h−1 (G/TBOT mass ratio 1:4) 100 mg of catalyst, CO2 and H2O vapor 0.80 (CH4)/1.68 (C2H6) μmol h−1 (2 wt % graphene)

100 mg of catalyst, CO2 and H2O vapor 10 mg of catalyst, CO2 (0.4 MPa) and H2O vapor, 80 °C catalyst in 100 mL of water with 50 psi CO2 50 mg of catalyst in 100 mL of water under 1.2 atm CO2 40 mg of catalyst, CO2 and H2O vapor 100 mg of catalyst in 100 mL of CO2-saturated water 100 mg of catalyst, CO2 and H2O vapor (4.5%)


Table 3. Summary of CO2 Photoreduction Systems Containing Solid Hybrid Semiconductorsa



7517

g-C3N4/Co-ZIF-9

GaP/TiO2

In(OH)3/Ta-based pyrochlore CdS/TiO2

300 W Xe-lamp (λ > 420 nm)

1500 W Xe-lamp

500 W Xe−Hg-lamp

3 mg of catalyst in 5 mL of CO2-saturated water 100 mg of catalyst, CO2 and H2O vapor

Si/TiO2

SnO2−x/g-C3N4

Au-g-C3N4/WO3

g-C3N4/Bi2WO6

500 W Xe-lamp

3.0 W cm−2 LED (λ = 435 nm)

300 W Xe-lamp (λ > 420 nm)

20 mg of catalyst in cyclohexanol, CO2 bubbled 30 min before irradiation 60 mg of catalyst in 100 mL of water with 50 psi CO2 20 mg of catalyst, CO2 (0.3 MPa) and H2O vapor, 80 °C

78.84

1.04

0.4 (cyclohexanone)/0.4 (cyclohexyl formate) μmol h−1 (88.9 mol % CdS) 19.14 (CH3OH) μmol h−1 (50 mol % Si) ∼0.08 (CH4)/∼0.02 (CH3OH) ∼ 0.36 (CO) μmol h−1, total CO2 conversion, 0.45 μmol h−1 (42.2 wt % SnO2−x) ∼0.099 (CH3OH) μmol h−1 (0.5 wt % Au; 33 wt % g-C3N4) 0.52 (CO) μmol h−1 (13 wt % g-C3N4)

2.84

28.32

2.56

0.87 (AgBr); 0.68 (AgCl)

0.11 11.76

287

286*

285

284

283

281

280

279*

278

277

274, 275 276

273

271 272

270 3.09

0.52 (CH3OH) μmol h−1 (40 wt % rGO)

268 269*

2.49

0.42 (CH3OH) μmol h−1 (∼60 wt % rGO)

264 265 266 267

260, 261 262 263

259

258

ref

46 (CO) ppm h−1 (0.5 wt % rGO)

0.23 0.23 2.01 52.62

1.11 0.88

8.80

0.28

TCEN (μmol h−1)

0.029 (CH4) μmol h−1 (5 wt % rGO) 0.0283 (CH4) μmol h−1 (2 wt % Pt; 5 wt % rGO) 0.25 (CH4) μmol h−1 (0.5 wt % rGO) 8.77 (CH3OH) μmol h−1 (10 wt % rGO)

0.035 (CH4) μmol h (molar ratio Cd/Ti 1:3; 1 wt % Pt) 1.1 (CH4)/0.8 (H2) μmol h−1 (3 wt % NiOx; 1 wt % graphene) 0.14 (CH4) μmol h−1 (15 wt % rGO) 0.11(CH4)/∼0.34 (O2) μmol h−1 >700 (CO) μmol g−1 h−1 (5 wt % TiO2)

−1


0.0135 (CH4) μmol h−1 1.96 (CH3OH) μmol h−1 (∼0.3 wt % carbonloading) 0.11 (CH4) μmol h−1 (30 wt % AgBr); 0.085 (CH4) μmol h−1 (30 wt % AgCl) 50 mg of catalyst, CO2 and H2O vapor 0.32 (CH4) μmol h−1 (0.5 wt % Pt; 25 wt % KNbO3) 20 mg of catalyst, CO2 and H2O vapor 159.2 (CH4) ppm in 4 h (0.5 wt % Pt; 10 wt % In2O3) 0.05 (CH4)/∼0.3 (CO)/0.1 (CH3OH)/0.01 10 mg of catalyst, CO2 (0.4 MPa) and H2O vapor, 80 °C (C2H5OH) μmol h−1, total CO2 conversion 0.46 μmol h−1 (6 wt % ZnO) catalyst in MeCN/TEOA (4:1) solution (80 kPa 9.9 (CO) μmol gCN−1 h−1, (10 wt % g-C3N4) CO2) 20 mg of g-C3N4 and 1 mg of Co-ZIF-9 in 6 mL 20.8 (CO)/3.3 (H2) μmol in 2h of MeCN/H2O/TEOA (3:2:1) solution under 1 bar CO2 300 mg of catalyst in 42 mmol CO2 and 1.7 3.54 (CH4) μmol h−1 (9 wt % GaP) mmol H2O vapor, 70 °C 100 mg of catalyst CO2 and H2O vapor 1.42 (CO)/0.71 (O2) μmol h−1

20 mg of catalyst in 8 mL of CO2-saturated NaHCO3 100 mg of catalyst, CO2 and H2O vapor 100 mg of catalyst, CO2 and H2O vapor catalyst in 3 mL min−1 CO2 and water vapor flow 100 mg of catalyst, CO2 and H2O vapor 100 mg of catalyst, CO2 and H2O vapor 100 mg of catalyst, CO2 and H2O vapor 100 mg of catalyst in 1.0 M NaOH, CO2 bubbled 30 min 100 mg of catalyst in 100 mL of CO2-saturated 0.1 M NaOH solution 0.5 g of catalyst in 3 mL of H2O and 120 mL of CO2 with excess 0.7 M Na2S 100 mg of catalyst in 100 mL of CO2-saturated 0.1 M NaOH 100 mg of catalyst, CO2 and H2O vapor 35 mg of catalyst in 20 mL of H2O, added 1 g dry ice 100 mg of catalyst, CO2 and H2O vapor

20 mg of catalyst, CO2 (0.2 bar) and H2O vapor


355 nm laser 40 mJ per pulse

250 W Hg-lamp

g-C3N4 nanosheet/UiO-66

rGO/TiO2 carbon quantum dot/Cu2O

15 W energy-saving daylight lamp 300 W Xe-lamp

300 W Xe-lamp (λ = 400−800 nm)

rGO/BiVO4

500 W Xe-lamp

ZnO/g-C3N4

rGO/Cu2O

150 W Xe-lamp

500 W Xe-lamp (λ > 420 nm)

rGO/Cu2O

500 W Xe-lamp

Pt−C3N4/KNbO3 Pt-g-C3N4/In2O3

GO/oxygen-rich TiO2 Pt-GO/TiO2 GO/CdS rGO/ZnO

15 W energy-saving daylight lamp 15 W energy-saving daylight lamp 300 W Xe-lamp (λ > 420 nm) 300 W Xe-lamp

300 W Xe-lamp 500 W Xe-lamp

rGO/g-C3N4 graphene/WO3 GO/TiO2

15 W energy-saving daylight lamp 300 W Xe-lamp (λ > 400 nm) 450 W Xe-lamp

AgX(X = Cl or Br)/g-C3N4

NiOx-Ta2O5/graphene

400W metal halide lamp

15 W energy-saving daylight lamp

Pt-rGO/CdS/TiO2

photocatalyst


light source

Table 3. continued



Review 291*, 292*

293* 294

5.20 (Pt); 6.68 (Au)

0.41 0.02 25 mg of catalyst, CO2 and H2O vapor 25 mg of catalyst, CO2 and H2O vapor

(Pt or Au)@CdS/TiO2

Fe2V4O13/rGO/CdS CdS/rGO/TiO2

300 W Xe-lamp (λ = 320−780 nm)

300 W Xe-lamp (λ > 420 nm) 300 W Xe-lamp

4.1. Semiconductor Solid Heterostructures. Generally, solid heterostructures fabricated with two or more semiconductors can enhance the charge separation and CO2 reduction photoreduction efficienciesms. Depending on their band structures of the coupled semiconductors, solid heterojunctions can be divided into straddling gap (type I), staggered gap (type II), and broken gap (type III), as shown in Figure 19.21 Among which, type II heterosturctures are

a Investigations providing evidence that CO2 was the actual carbon source for the carbon-containing products through 13CO2 labeling or other techniques were marked with “*” in the last column of the table.

288 289 290

0.50 (CH3OH) μmol h (molar ratio Fe/Cu 1:1) 0.1 (CH4) μmol g−1 h−1 (5 mol % CdS) 0.21 (cyclohexanone)/0.22 (cyclohexyl formate) μmol h−1 (9.78 wt % ZnFe2O4) Pt: 0.74 (CH4)/0.014 (CO)/ 0.32 (H2)/1.97 (O2); Au: 0.83 (CH4)/0.012 (CO)/0.36 (H2)/ 2.19 (O2) μmol h−1 0.05 (CH4)/ ∼ 0.1(O2) μmol h−1 0.0023 (CH4) μmol h−1 100 mg of catalyst, CO2 and H2O vapor 100 mg of catalyst film, CO2 and H2O vapor 10 mg of catalyst in cyclohexanol, CO2 bubbled 30 min before irradiation 20 mg of catalyst, CO2 and H2O vapor

α-Fe2O3/Cu2O CdS/WO3 ZnFe2O4/TiO2

−1

300 W Xe-lamp (λ > 400 nm) 300 W Xe-lamp (λ > 420 nm) 250 W Hg-lamp

light source

Table 3. continued

photocatalyst



TCEN (μmol h−1)

ref 3.0 0.82

ACS Catalysis

Figure 19. Energy diagrams for three different types of solid heterojunctions.21 Figure reproduced with permission from ref 21. Copyright 2014 The Royal Society of Chemistry.

promising in promoting charge separation in space by transferring the electrons to the lower CB and holes to the higher VB, respectively. Moreover, introducing second semiconductor often provides additional benefits, such as promoted light harvesting, advantageous surface states, and structural properties. Except for the above cocatalyst loaded metal/ semiconductor, oxide/semiconductor, and dye/semiconductor, which can also be considered as heterojunctions that promote the charge separation, although they actualize the photocatalytic processes through one-step excitation, recently reported solid heterojunctions fabricated with two or more semiconductors can be divided into TiO2-based heterostructures, other oxides or oxysalts-based heterostructures, carbon nanostructure-based heterostructures, g-C3N4-based heterostructures, and so on. 4.1.1. TiO2-Based Heterostructures. TiO2-based heterostructures are the most widely studied photocatalysts in the field of CO2 reduction, the other component can be varied among oxides,230−234 oxometalates,235,236 sulfides,237 nitrides,238 halides,239 MOFs (Cu3(BTC) 2),240 and even PANI,241 among others. Also, carbon nanostructured materials such as CNTs, G, GO, and rGO are used to combine with TiO2 to fabricate carbon nanostructure-based heterojunctions, which will be discussed in the following section. In Cu2O/TiO2 composites, as an efficient type II p−n heterojunction, the photogenerated electrons in CB of Cu2O can go downward to the CB of TiO2 and holes in VB of TiO2 upward to the VB of Cu2O under UV−vis light irradiation, as shown in Figure 20,230 causing an efficient spatial charge separation, and then a higher activity and stability of the CO2 reduction system. Under visible light irradiation, the Cu2O/

Figure 20. Schematic of the charge separation in Cu2O/TiO2 heterojunction catalysts under UV−vis light irradiation.230 Figure reproduced with permission from ref 230. Copyright 2014 Elsevier B.V. 7518


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TiO2 can enhance the visible-light-responsive capability and shield the UV region slightly.247−253 For instance, the bandgap of CNTs/TiO2 heterostructure is calculated to ∼2.22 eV, which is much narrower than that of pristine TiO2 (∼3.32 eV), and the overall activity of CO2 reduction to CH4 can be enhanced by ∼4 times after the incorporation of CNTs.247 Due to the unique 2D structures with high surface area and favorable electronic properties, various graphene species (such as G, GO and rGO) with high work function and good electric conductivity can act as an electron acceptor/transporter in hybrid heterojunctions to induce the electrons transfer from semiconductor’s CBs, leading to an efficient charge separation.24,25 Among which, graphene-based TiO2 hybrid heterostructures are the most widely researched photocatalysts.254−258,263−265,271 Recently, 2D sandwich-like G/TiO2 hybrid nanosheets (TiO2/G/TiO2, denoted as G-TiO2) were synthesized by the in situ simultaneous reduction hydrolysis technique (SRH), in which TiO2 nanocrystals uniformly dispersed and tightly packed on both sides of graphene through chemical bonds (Ti−O−C bond), hindering the collapse and restacking of exfoliated sheets of graphene during the CO2 reduction process.257 In contrast with previous GTiO2 nanocomposites, abundant Ti3+ sites are detected on the TiO2 surface in the hybrid materials. The surface Ti3+ sites can trap photogenerated electrons to prevent the charge recombination, and the synergistic effect of the surface Ti3+ sites and graphene favors the selective generation of C2H6, and the yield of the C2H6 increases with the content of incorporated graphene.257 Meanwhile, some investigations also demonstrate that 2D graphene species can also enhance the photocatalytic stability of some metastable semiconductors by efficient charge separation.259,266,268−270 Also, graphene can elevate the CB of WO3 toward photocatalytic reduction of CO2 into CH4 under visible light irradiation, and the activity of G/WO3 is higher than that of GO, WO3, and P25.262 Usually, the photogenerated charge separation of some carbon nanostructure-based heterojunctions can be further improved by loading various cocatalysts such as Au, Ag, Pb, Pt, Ni, NiOx, and so on.247,248,251,252,258,259,265 For instance, a series of ternary composites of noble metal (Pt, Pd, Ag, Au)-doped GO/TiO2 nanocomposites are found to possess an enhanced performance of CO2 reduction to CH4 under visible light illumination.265 Among the noble metals studied, the Pt-doped ternary composite (with 2.0 wt % Pt) demonstrates the best activity due to its highest work function (5.65 eV), which facilitates the charge separation/transfer and expands the absorption band into visible region.265 Moreover, metallic Ni, or Ni/NiO as cocatalysts on carbon nanstructure-based heterojunctions can also promote the performance of CO2 reduction to CH3OH/H2 or CH4.247,259 4.1.4. g-C3N4-Based Heterostructures. As a metal-free visible-light-responsive photocatalyst, g-C3N4 has received incessant interest due to its earth-abundant nature and high chemical stability, but the photocatalytic efficiency is still low to make the system viable for practical applications owing to the high charge recombination rate in the individual semiconductor. Among various modifications to enhance the activity, the construction of semiconductor heterojunctions by coupling g-C3N4 with another semiconductor with appropriate VB and CB is regarded as an effective strategy to reduce the charge recombination.273−279 By matching their band structures, g-C3N4 can form type II heterojunctions with rGO,260,261 AgX (X = Cl, Br),273 MNbO3 (M = K, Na),294,275 In2O3,276

TiO2 heterostructure is always regarded as a sensitization system, whereby Cu2O adsorbed visible light and migrated the photogenerated electrons to TiO2’s CB for CO2 reduction. 230−232 Similarly, ordered mesoporous CeO2 −TiO 2 composites exhibit higher activity of CO2 reduction with H2O to CO/CH4 under simulated solar irradiation than MesoTiO2 as well as Meso-CeO2.233 The introduction of CeO2 species in composites can effectively extend the spectral response from UV to visible region owing to the photosensitization of CeO2. In the meantime, the photogenerated electrons in the TiO2 can drift to the CeO2 under the inner electric field of CeO2/TiO2 heterojunctions because of the energy band bending in space charge region, which is more helpful for the charge separation in TiO2, which results in an improved activity under solar irradiation.234 Also, the existence of mixture of Ce3+/Ce4+ states can prevent the charge combination because Ce3+ can capture holes, resulting in a higher quantum efficiency.234 Interestingly, a novel CeF3/TiO2 composite, synthesized by using a upconversion luminescent material (CeF3), exhibits higher activity of CO2 reduction to CH3OH under visible light (λ > 515 nm) than pure TiO2 due to CeF3 upconverting visible light (420−600 nm) into ultraviolet light (285−380 nm).239 4.1.2. Other Oxides or Oxysalts-Based Heterostructures. Other metal oxides and oxysalts-based nanocomposites such as CuO/KNb3O8,41 ZnTe/SrTiO3,242 ZnTe/ZnO,243 ZnO/ CuO,244 Ni−Ni3 (BO3) 2 /NiO,245 CdS/Cu−Na xH 2−xTi 3O 7 nanotubes (Cu-TNTs),246 are also proved to be a kind of solid heterojunction for efficient charge separation. Most of this kind of heterojunction can form p−n junctions by using p- and n-type semiconductors with suitable band structures. For example, ZnTe as a p-type semiconductor with a direct bandgap of ∼2.26 eV at 300 K, can combine with n-type semiconductors such as SrTiO3 and ZnO to form a p−n junction.242,243 These p−n junctions show good visible-lightdriven capability of CO2 reduction to CH4 even with ∼3.35% ZnTe in terms of atomic percentage, and the combination of ZnTe with SrTiO3 or ZnO visibly increases the formation of CH4 by efficiently promoting electron transfer from the CB of ZnTe to that of SrTiO3 or ZnO under visible light irradiation.242,243 Similarly, p-type CuO nanowires are surface engineered with dense n-type ZnO islands to form heterojunctions using a few pulsed cycles of atomic layer deposition (ALD).244 It was found that the availability of both CuO and ZnO surfaces is necessary for the oxidation and reduction halfreactions to occur during the CO2 reduction to CO, and thus, a two-sep excitation mechanism of CuO and ZnO (similar to Figure 20) whereby simultaneous H2O oxidation and CO2 reduction occurred in the active perimeter region between CuO nanowires and ZnO islands is proposed.244 4.1.3. Carbon Nanostructure-Based Heterostructures. Carbon nanostructured materials are also extensively employed to enhance the activities of hybrid photocatalysts for CO2 reduction. Such carbon nanostructures, including CNTs,247−253 G,254−262 GO,263−266 rGO,267−271 and even CQDs,272 are always acted as cocatalysts in the solid heterostructures owing to its good conductivity. The other component can be varied among oxides, oxometalates, sulfides, nitrides, halides, and so on. Usually, CNTs as 1D carbon nanostructured materials in hybrid photocatalysts can be applied also as cocatalysts to separate the photoinduced charge pairs in space through the d−π overlapping and then migrate along axial CNTs by the π conjugation.247−253 Moreover, the incorporation of CNTs into 7519


Review

ACS Catalysis ZnO,277 and even some MOFs such as UiO-66278 and Co-ZIF9.279 Typically, the bandgap of g-C3N4 lay in the bandgap of AgCl, forming type I heterojunctions, then resulting in no overall improvement in charge separation, while g-C3N4 and AgBr can form a staggered gap (type II heterojunction) due to their suitable band structures.273 In such type II heterojunction, the SPR-excited electrons from Ag nanoparticles, and the photogenerated electrons from the CB of g-C3N4 can be injected to the CB of AgBr. However, the holes are transported from the VB of AgBr to that of g-C3N4, and thus, the overall activity of CO2 reduction to CH4 can be enhanced ∼3 times compared with pure g-C3N4 because of the synergistic effects of more efficient charge separation over type II heterojunctions and SPR effects of AgBr.273 Similar charge-transfer/separation mechanisms through two-step excitation can also be observed for othe g-C3N4-based heterojunctions coupled with MNbO3 (M = K, Na),274 In2O3,276 and ZnO.277 Without exception, the photogenerated electrons from the CB of g-C3N4 can be injected to the CB of those semiconductors to catalyze the CO2 reduction, whereas the holes are transported from the VB of those semiconductors to that of g-C3N4 to promote the oxidation of water or sacrificial reagent. Interestingly, metal-free rGO/g-C3N4 heterostructures with intimate contact are reported to show remarkable enhancement in activity of CO2 reduction to CH4 compared with pure gC3N4.260,261 The efficient charge separation could be related to the synergistic effects of the initimate contact (even through C−O−C covalent link) and π−π overlapping between g-C3N4 and rGO.260,261 Similarly, some MOFs, such as UiO-66278 and Co-ZIF-9,279 can couple with g-C3N4 to form herterojunctions, which exhibit a much higher activity of CO2 reduction to CO than that of bare g-C3N4. It is found that electrons from the photoexcited g-C3N4 can transfer to MOFs, which can act as cocatalyst and substantially suppress the charge recombination in g-C3N4, as well as supply long-lived electrons for the reduction of CO2 molecules adsorbed in MOFs.278,279 Different from the other g-C3N4-based heterojunctions, the above two kinds of heterojunctions are conducted through one-step excitation, and the photogenerated electrons from the CB of g-C3N4 are injected to the CB of rGO or MOs; additionally, rGO or MOFs act as electron acceptor, transfer support, and cocatalyst to promote the CO2 reduction. 4.1.5. An Overview on Microstructures and ChargeTransfer Aspects of Solid Heterostructures. Similar to single semiconductor systems, micro/nano-structured construction of solid heterojunctions are also very important for improving the activity, selectivity, and stability of CO2 reduction systems. Also, hybrid heterostructures with various micro/nano-structures such as nanoparticles (0D),41,128,232,233,236,239,242,243,245,280 nanorods/nanofibers/nanotubes (1D),220,234,235,244,246−253 and nanoplates/nanosheets (2D),242−261,263−271 and porous structures234,278,279 are extensively applied in the photocatalytic CO2 conversion. As mentioned above, some solid heterojunctions can also be conducted through one-step excitation. In this case, heterojunctions can promote the spatial charge separation at interface and/or the redox reactions of CO2 reduction system. On the one hand, the vast majority of carbon nanostructures in those heterojunctions just act as electron acceptor, transport/disperse support, and/or cocatalyst to promote the charge separation and inhibit the charge recombination of the light-harvesting units (semiconductors) because their LUMO levels are usually

lower than the CBs of those semiconductors as a result of its good conductivity.247−261,263−271 Moreover, the components that cannot be excited in the heterojunctions can also act as CO2 adsorption and cocatalyst of CO2 reduction. For example, PANI in TiO2/PANI composite can not only facilitate the chemisorption of CO2 but also accept the photogenerated electrons of TiO2 to promote the charge separation due to the LUMO −0.15 V (vs RHE) of PANI in CO2 atmosphere lower than TiO2’s CB (−0.18 V vs RHE).241 On the other occasions, the components that cannot be excited in the heterojunctions can also act as oxidized cocatalyst to capture holes to oxidize water or sacrificial reagent and thus also promote the charge separation.272,281 For example, in the heterojunction formed between In(OH)3 and Ta-based pyrochlore (TP) nanoparticles system, the electrons generated upon UV irradiation in the CB of TP nanoparticles reduce water or CO2 to produce H2 or CO, whereas the holes are transferred to the VB of In(OH)3 for water oxidation to produce O2.281 Similarly, in the heterojunction formed between CQDs and Cu2O system, the Cu2O is excited by visible light and produces electron−hole pairs; the electrons are consumed in CO2 reduction to CH3OH on the Cu2O surface, whereas the holes transfer to the CQDs surface, where they oxidize H2O to O2.272 Namely, CQDs acting in this way as a photogenerated hole-acceptor but not contributing an additional photoexcitation has been observed, even though CQDs can harvest the light in the region of 400−2200 nm, because there is no direct evidence in support of photoexcited CQDs contributing to the CO2 reduction as mentioned by the authors.272 Of course, semiconductor solid heterojunctions can also conduct as part of another routine, such as a Z-scheme, depending on their band structures of their semiconducting components, which will be discussed in the following section. 4.2. Solid Z-Scheme Systems. Natural photosynthetic systems (NPS) are typical Z-schemes with two-step excitation and two-step sequential charge separation.21,30,282 The chargeseparation processes in NPS inspired the design of artificial photosynthetic systems (APS) consisting of photosystem II (PS II) and photosystem I (PS I), and these Z-scheme systems are driven by a two-step excitation using two different semiconductors and a redox mediator.282 The principal advantage of the Z-scheme lies in the availability of the strongly reductive electrons of one photocatalyst and the strongly oxidative holes of the other, and the electron mediator plays an important role in charge transfer between an oxidation-evolving photocatalyst to a reduction-evolving photocatalyst in Z-scheme systems, as shown in Figure 3e.282 Usually, the electron mediators are redox ionic couples, such as Fe3+/Fe2+, IO3−/I−, and [Co(bpy)3]3+/2+/[Co(phen)3]3+/2+. Nevertheless, this kind of Zscheme system suffers from several negative effects, such as back reactions, strongly visible light absorption of these redox mediators, and operatability only in solution systems. Therefore, all-solid-state Z-scheme systems (cf. Figure 3d), where the photogenerated electrons injecting into the VB of semiconductor I from CB of semiconductor II directly,70,280,283−290 290 or through a conductive intermediate,216,291−295 have been constructed and widely utilized for CO2 reduction. 4.2.1. Direct Z-Scheme Systems. Typically, the band structures of semiconductors in Z-scheme are almost the same as type II heterostructures (cf. Figure 3c) mentioned above. Their main difference is the transfer directions of those photogenerated carriers between the two kinds of semiconductors under light irradiation, which then results in 7520


Review

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with the responsive single semiconductors can validate the Zscheme charge separation processes. 4.2.2. Z-Scheme Systems with Conductive Intermediate. Besides the direct contact of semiconductors in Z-scheme, the charge separation can also go through conductive intermediate such as metals216,291,292 or graphene species.293−295 In the last 3 years, Ag, Pt, and Au nanoparticles dispersed on the interfaces of the semiconductor components are used as electron transfer intermediate of some Z-scheme systems such as Ag3PO4/gC3N4,216 Pt@CdS/TiO2,291 and Au@CdS/IO-TiO2,292 where a vectorial electron transfer of semiconductor I-M-semiconductor II should occur as a result of excitation of both components under the CO2 reduction conditions as shown in Figure 21.291

different redox abilities. In Z-scheme, the excited electrons from the CB of semiconductor I with lower energy can combine with the photogenerated holes in the VB of semiconductor II, thus preserving the electrons with stronger reducibility in the CB of semiconductor II and holes with stronger oxidizability in the VB of semiconductor I as shown in Figure 3d. Finally, the electrons from the CB of semiconductor II can react with CO2 to produce solar fuels. Obviously, the interfacial electron transfer actually occurs in opposite directions for the type II heterojunctions (Figure 3c) and Z-scheme (Figure 3d) systems, even though the two kinds of solid hybrid photocatalysts usually exhibit higher CO2 reduction activity than their respective single photocatalyst systems due to the efficient charge separation. In this regard, it is necessary to prove the photogenerated charge flow directions for confirming an actual Z-scheme system. Of course, different structural features of the solid hybrid photocatalysts and the photoreaction conditions could also lead to different charge-transfer directions and reaction products.70,287 Using ZnO/g-C3N4 as an example, which is a typical solid hybrid photocatalyst, both type II heterojunction (see Figure 3c) and Z-scheme (see Figure 3d) charge-transfer mechanisms can be used to explain the observed improvement of CO2 reduction activity. Recently, the PL analyses of the hydroxyl radicals (•OH) produced on the surfaces of ZnO/g-C3N4, and pure g-C3N4 as well as ZnO upon light irradiation are carried out by using terephthalic acid as a probe molecule.70 It was found that the reduction potential (−0.30 V vs NHE) of superoxide anion radicals (•O2−) is over the CB of ZnO and below the CB of g-C3N4; meanwhile, the oxidation potential (2.80 V vs NHE) of hydroxyl radicals (•OH) is below the VB of g-C3N4 and over the VB of ZnO. In ZnO/g-C3N4 hybrid system, •O2− and •OH are detected simultaneously, proving the photoinduced electrons and holes are on the CB of g-C3N4 and VB of ZnO, respectively. Therefore, g-C3N4/ZnO system is more likely to follow a direct Z-scheme mechanism, rather than the conventional heterojunction-type one.70 A similar Z-scheme mechanism is also proposed in CdS/TiO2,283 g-C3N4/ Bi2WO6,287 ZnFe2O4/TiO2,290 Pt@CdS/TiO2,291 Au@CdS/ IO-TiO2,292 Fe2V4O13/rGO/CdS,293 and TiO2/rGO/CdS.294 In the above Z-scheme systems, the energy potentials of CB of semiconductor components are both more negative than the standard redox potential of CO2 reduction, whereas the energy potentials of VB of semiconductor components are both more positive than the standard redox potential of the water oxidation. In such Z-scheme, both of the semiconductors are active in photocatalytic CO2 conversions, and the chargeseparating methods and photoreaction conditions are complex; thus, intermediate products should be detected to ensure the charge-separation process.70,283,285,290−292 In other Z-scheme systems, the charge-transfer mechanism for enhanced CO2 photoreduction performance are easy to be determined when the energy potentials of CB of semiconductor I (see Figure 3d) are both more positive than the standard redox potential of CO2 reduction because the enriched electrons on this semiconductor component cannot reduce CO2. Such systems include Ag3PO4/g-C3N4,216 SnO2−x/gC3N4,285 g-C3N4/WO3,286 α-Fe2O3/Cu2O,288 CdS/WO3.289 Although the energy potentials of CB of semiconductors such as Ag3PO4,216 SnO2−x,285 WO3,286,289 α-Fe2O3,288 are more positive than the standard redox potential of CO2 reduction, the obviously enhanced activity of CO2 reduction compared

Figure 21. Schematic of the charge separation mechanism for the photoreduction of CO2 with H2O over 3DOM Pt@CdS/TiO2 heterojunction catalysts.291 Figure reproduced with permission from ref 291. Copyright 2015 The Royal Society of Chemistry.

For example, in this all-solid-state Z-scheme system (3DOM Pt@CdS/TiO2 heterojunction), 3D ordered structure of macroporous TiO2 enhances the light harvesting significantly by inner scattering, and Pt nanoparticles are a suitable conductive intermediate as an expedite bridge linked up the CB of TiO2 and the VB of CdS. Owing to the Z-scheme charge separation processes, the yield of solar fuels and O2 is almost by stoichiometric ratio at the optimized conditions, and the activity of CH4 production is ∼25-fold that of CdS/TiO2 without Pt linking.291 Similarly, Zou’s group used rGO dispersed between the semiconductor components as electron transfer intermediate of Z-scheme systems, such as Fe2V4O13/rGO/CdS293 and TiO2/ rGO/CdS,294 where rGO acts as charge-transfer intermediate due to its large plane π-conjugate structures, and a stoichiometric ratio of CH4 and O2 is also detected for the efficient charge separations. 293 Recently, Kudo’s group demonstrated that the visible-light-responsive BiVO4 can be used as an O2 evolving photocatalyst upon CoOx loading for a Z-scheme water-splitting system with metal sulfides as an H2evolving photocatalyst bridged by RGO electron mediator, and they found that the CoOx played an important role in the Zscheme system (BiVO4/rGO/CuGaS2) to promote water oxidation on BiVO4, resulting in the enhancement of the electron injection from BiVO4 to CuGaS2 through RGO.295 The above results indicate that Z-scheme constructions can not only favor the photogenerated charge separations in space but also promote the redox reactions during a CO2 photoreduction processes and therefore would be a promising development direction of photocatalytic CO2 conversion in future. 7521


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5. CONCLUSIONS AND OUTLOOK Although photocatalytic CO2 conversion for solar fuel production over semiconductor can not only alleviate the problem of CO2 emissions in atmosphere but also provide clean chemical energies from incessant solar energy in principle, there is still very long way to go for having the scientific and technological capability to make this artificial photosynthesis practical to use due to the unsatisfactroy light harvesting, low conversion efficiency, lack of selectivity to specified products, and complex photocatalytic mechanisms. First of all, exploring more active and selective photocatalysts that can efficiently retard the charge recombination and capture the solar spectrum is of paramount importance in the field of CO2 photoreduction. Bandgap engineering through transition metal and p-block dopants of wide bandgap semiconductors, such as TiO2 photocatalysts that are only able to utilize a small fraction of the solar spectrum due to its wide intrinsic bandgaps, can provide a simple route to bandgap narrowing, but the improvements on the overall activity and selectivity of TiO2 is still limited. Alternatively, integrated sensitizers or narrow bandgap semiconductors to fabricate hybrid or composite materials should be a promising strategy, whereby both UV and visible light harvesting and efficient charge separation can be obtained through preferential transfer of photoexcited electrons into one material and of holes into the other. On the other hand, non-TiO2 photocatalysts usually possessing superior properties, such as smaller bandgap, more favorable surface chemistry and structures, can be improved in the future by using the same strategies developed for TiO2. Currently, several non-TiO2 photocatalysts such as copper oxides, graphitic carbon nitride, metal oxynitrides, and carbon nanostructures show better light-harvesting properties than TiO2, but it is imperative to develop new photocatalysts having good light harvesting and activity in addition to necessary material’s stability and cyclability for the practical application. In this regard, photoactive material architectures such as crystal facet engineering, porous, hierarchical micro/nano-structures can deliver the requisite optical, material, and catalytic properties necessary to harness sunlight for the rapid and selective CO2 reduction to solar fuels by affording rapid molecular transport to/from active sites, enhanced light harvesting, and superior charge transport and recombination characteristics. New synthetic routes to photocatalyst architectures possessing well-defined physichochemical properties and the elucidation of associated structure−function relations should be particularly investigated in the future. An additional challenge is the optimization of CO2 and H2O adsorption over photocatalyst surfaces because their adsorbing state and amount would affect the activity and selectivity of CO2 conversion in both kinetics and thermodynamics. Usually, high surface area and porous materials increase the density of surface active sites available to chemically bind and activate CO2. Surface/facet engineering and porous construction, combined with large surface area materials, usually result in better CO2 adsorption capacity and more active reactive sites for CO2 conversion. In this regard, advanced synthetic methods afford precise manipulation of exposed crystal facets, surface, morphologies, structural periodicity, and hierarchical pore networks, in addition to integration of photoactive sensitizer or cocatalyst and promoter. These features can offer new routes to maximizing charge carrier lifetimes and consequent electron transfer to CO2. Comprehensive mechanistic insights into the

surface chemistry, the catalytic cycle, and the nature of the surface sites on photocatalysts via thoroughly understanding the atomic structure of the photocatalyst interface and both adsorption and activation processes of surface adsorbates are extremely vital since such considerations should influence the ongoing fundamental research efforts. Another important difficulty in the field of CO2 photoreduction is that there is no standard protocol for evaluating photocatalytic performance, or suitable parameter that enables quantitative benchmarking of CO2 conversion efficiencies to specific fuels to date. One of the reasons is that the final activity and selectivity of CO2 conversion is influenced by various conditions such as intensity and spectrum of light irradiation, reaction temperature, humidity, CO2 concentration, pH and components of the photoreaction solution, mass and distribution of photocatalysts, and even the shape of reaction vessel. To address this deficiency, reasonable performance evaluation systems for photocatalytic CO2 conversion are necessary so as to provide a strong rationale for future development of CO2 conversion. Furthermore, accumulation of holes would result in higher charge recombination and further equilibrium shifting to consume the produced solar fuels, and thus, it is necessary to balance the surface charge that the stoichiometrical production of solar fuels and O2 so as to consume the photogenerated electrons and holes at the same time. However, few studies have focused on the oxidative halfreaction of the photocatalytic CO2 conversion up until now, which will retard the development of an overall artificial photosynthesis. Although some composite systems show efficient photocatalytic CO2 conversion by promoting charge transfer, the photocatalytic reactions have always occurred in sacrificial reagent solutions, which is less efficient and less environmentally benign from the viewpoint of practical application. Therefore, exploring more active and selective hybrid photocatalytic systems with additional properties such as earth abundance, low cost, nontoxicity, and scalability will be more attractive in the future. A critical issue that should not be ignored in this field is to confirm that CO2 is the actual substrate for the carboncontaining products since the state-of-the-art activities of CO2 photoreduction are still very low. Although most of the papers cited in this review have not demonstrated that CO2 is the actual carbon source for photoreduction through 13CO2 labeling or other techniques, some investigations indicated that the carbonaceous residues, such as organics involved in the synthesis processes, carbon residues, and even the adsorbed organic solvents (e.g., methanol, ethanol, and acetone) existing in the laboratory atmosphere, can participate in the formation of primary products during the CO2 photoreduction conditions and contribute to the overall product yield, which causes overestimation (or even false positive results) of the photocatalytic CO2 activity. Those investigations provided evidence that CO2 was the actual carbon source for the carboncontaining products through 13 CO 2 labeling or other techniques were marked with “*” in the last column of Tables 1−3. Among which, isotopic 13CO2 labeling was proven to be an efficient technique for demonstrating the carbon-containing products arising from CO2 photoreduction, and careful analyses of the carbon-containg products and evolved oxygen by using other techniques such as ESR, GC-MS, DRIFT, and FTIR spectra can also accurately quantify CO2 photoreduction perforamnce. Moreover, a contrast test of activity in the absence of CO2 but in the presence of H2O would be one of 7522


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(12) Inoue, T.; Fujishima, A.; Konishi, S.; Honda, K. Nature 1979, 277, 637−638. (13) Yuan, L.; Xu, Y. J. Appl. Surf. Sci. 2015, 342, 154−167. (14) Kondratenko, E. V.; Mul, G.; Baltrusaitis, J.; Larrazábal, G. O.; Pérez-Ramírez, J. Energy Environ. Sci. 2013, 6, 3112−3135. (15) Ma, Y.; Wang, X.; Jia, Y.; Chen, X.; Han, H.; Li, C. Chem. Rev. 2014, 114, 9987−10043. (16) Chen, D.; Zhang, X. G.; Lee, A. F. J. Mater. Chem. A 2015, 3, 14487−14516. (17) Cao, S. W.; Low, J. X.; Yu, J. G.; Jaroniec, M. Adv. Mater. 2015, 27, 2150−2176. (18) Habisreutinger, S. N.; Schmidt-Mende, L.; Stolarczyk, J. K. Angew. Chem., Int. Ed. 2013, 52, 7372−7408. (19) Sahara, G.; Ishitani, O. Inorg. Chem. 2015, 54, 5096−5104. (20) Handoko, A. D.; Li, K. F.; Tang, J. W. Curr. Opin. Chem. Eng. 2013, 2, 200−206. (21) Yuan, Y. P.; Ruan, L. W.; Barber, J.; Loo, S. C. J.; Xue, C. Energy Environ. Sci. 2014, 7, 3934−3951. (22) Linares, N.; Silvestre-Albero, A. M.; Serrano, E.; SilvestreAlbero, J.; Garcia-Martinez, J. Chem. Soc. Rev. 2014, 43, 7681−7717. (23) Sun, H. Q.; Wang, S. B. Energy Fuels 2014, 28, 22−36. (24) Xiang, Q. J.; Cheng, B.; Yu, J. G. Angew. Chem., Int. Ed. 2015, 54, 11350−11366. (25) Najafabadi, A. T. Renewable Sustainable Energy Rev. 2015, 41, 1515−1545. (26) Das, S.; Daud, W. RSC Adv. 2014, 4, 20856−20893. (27) Wang, C. C.; Zhang, Y. Q.; Li, J.; Wang, P. J. Mol. Struct. 2015, 1083, 127−136. (28) Zhang, T.; Lin, W. B. Chem. Soc. Rev. 2014, 43, 5982−5993. (29) Kumar, S.; Wani, M. Y.; Arranja, C. T.; Silva, J. d. A. e.; Avula, B.; Sobral, A. J. Mater. Chem. A 2015, 3, 19615−19637. (30) Zhou, P.; Yu, J. G.; Jaroniec, M. Adv. Mater. 2014, 26, 4920− 4935. (31) Ye, S.; Wang, R.; Wu, M. Z.; Yuan, Y. P. Appl. Surf. Sci. 2015, 358, 15−27. (32) Liu, G. H.; Hoivik, N.; Wang, K. Y.; Jakobsen, H. Sol. Energy Mater. Sol. Cells 2012, 105, 53−68. (33) Tahir, M.; Amin, N. S. Energy Convers. Manage. 2013, 76, 194− 214. (34) de_Richter, R. K.; Ming, T. Z.; Caillol, S. Renewable Sustainable Energy Rev. 2013, 19, 82−106. (35) Das, S.; Wan Daud, W. M. A. Renewable Sustainable Energy Rev. 2014, 39, 765−805. (36) Ola, O.; Maroto-Valer, M. M. J. Photochem. Photobiol., C 2015, 24, 16−42. (37) Chang, X. X.; Wang, T.; Gong, J. L. Energy Environ. Sci. 2016, 9, 2177−2196. (38) Lee, J.; Sorescu, D. C.; Deng, X. J. Am. Chem. Soc. 2011, 133, 10066−10069. (39) Neatu, S.; Macia-Agullo, J. A.; Concepcion, P.; Garcia, H. J. Am. Chem. Soc. 2014, 136, 15969−15976. (40) Yamamoto, M.; Yoshida, T.; Yamamoto, N.; Nomoto, T.; Yamamoto, Y.; Yagi, S.; Yoshida, H. J. Mater. Chem. A 2015, 3, 16810− 16816. (41) Yin, G.; Nishikawa, M.; Nosaka, Y.; Srinivasan, N.; Atarashi, D.; Sakai, E.; Miyauchi, M. ACS Nano 2015, 9, 2111−2119. (42) Shoji, S.; Yin, G.; Nishikawa, M.; Atarashi, D.; Sakai, E.; Miyauchi, M. Chem. Phys. Lett. 2016, 658, 309−314. (43) Iizuka, K.; Wato, T.; Miseki, Y.; Saito, K.; Kudo, A. J. Am. Chem. Soc. 2011, 133, 20863−20868. (44) Teramura, K.; Iguchi, S.; Mizuno, Y.; Shishido, T.; Tanaka, T. Angew. Chem., Int. Ed. 2012, 51, 8008−8011. (45) Iwase, A.; Ng, Y. H.; Ishiguro, Y.; Kudo, A.; Amal, R. J. Am. Chem. Soc. 2011, 133, 11054−11057. (46) Ma, S. S. K.; Maeda, K.; Hisatomi, T.; Tabata, M.; Kudo, A.; Domen, K. Chem. - Eur. J. 2013, 19, 7480−7486. (47) Sato, S.; Morikawa, T.; Saeki, S.; Kajino, T.; Motohiro, T. Angew. Chem., Int. Ed. 2010, 49, 5101−5015.

the cost-effective approaches to confirm whether the carbon residues on photocatalysts participate the CO2 reduction. Conclusive proof that CO2 is the actual substrate for the photoreduction reaction is essential for estimating the performance of the photocatalytic CO2 reduction. Finally, deeper understanding of photocatalytic CO 2 conversion reaction and deactivation mechanisms is necessary for optimizing the surface, facets, phases and other material properties, reaction conditions, as well as other undiscovered aspects. As we know, photocatalytic CO2 conversions to solar fuels are usually complex multielectron processes with the assistance of H2O, and the activity and selectivity of special products are in competition with other products. A combination of experimental and theoretical results from fundamental studies and computer simulations should be preferred as it is the best approach to advance the current state of knowledge on CO2 photoreduction mechanisms. In general, photocatalytic CO2 conversion to solar fuels is an approach that achieves two objectives with one method, which can provide a package solution to the current global warming and energy demand by using inexhaustible solar energy and increasing atmospheric CO2. Therefore, the development and design of CO2 photoreduction systems with high stability, cyclability, nontoxicity, high efficiency, and wide spectral response under sunlight irradiation has very critical significance in both theory and practice.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone/Fax: 86-27-68752237. Author Contributions ‡

K.L. and B.P. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present work is supported by the Natural Science Foundation of China (21573166, 21271146, 20973128, and 20871096), the Funds for Creative Research Groups of Hubei Province (2014CFA007), Natural Science Foundation of Jiangsu Province (SBK2015020824), and Fundamental Research Funds for the Central Universities (2042014kf0228) of China.

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ACS Catalysis

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