Why do Hydrogen and Oxygen Yields from Semiconductor-Based

Why do Hydrogen and Oxygen Yields from Semiconductor-Based Photocatalyzed Water Splitting Remain Disappointingly Low? Intrinsic and Extrinsic Factors Impacting Surface Redox Reactions Nick Serpone,*,† Alexei V. Emeline,‡,§ Vladimir K. Ryabchuk,§ Vyacheslav N. Kuznetsov,§ Yurii M. Artem’ev,‡ and Satoshi Horikoshi∥ †

PhotoGreen Laboratory, Dipartimento di Chimica, Universita di Pavia, Via Taramelli 12, Pavia 27100, Italia Laboratory of Photoactive Nanocomposite Materials, Saint-Petersburg State University, Ulyanovskaya Str. 1, Petergof, Saint-Petersburg 198504, Russian Federation § Faculty of Physics, Saint-Petersburg State University, Ulyanovskaya Str. 1, Petergof, Saint-Petersburg, 198504 Russian Federation ∥ Department of Materials and Life Sciences, Faculty of Science and Technology, Sophia University, 7-1 Kioicho, Chiyodaku, Tokyo 102-8554, Japan ‡

ABSTRACT: Water splitting occurring on a semiconductor photocatalyst has become the Holy Grail process to produce a solar fuel, hydrogen, on irradiation with sunlight (or simulated sunlight) in heterogeneous media. Authors often claim highly efficient evolution of hydrogen and oxygen from water through water splitting or efficient hydrogen evolution in the presence of some sacrificial electron donor, whether photocatalytically or photoelectrochemically. Perusal of the scientific and patent literature reveals that yields of hydrogen are disappointingly low even after decades of remarkable advances in materials science and in strategies to achieve significant progress in water splitting. This Review identifies and discusses intrinsic and extrinsic factors (e.g., Φhν = fn{β, kr, S, D, d, s, τ, αhν}; photostability; back reactions) that impact redox reactions in general and water splitting in particular. The lack of control and handling of these various factors present a challenging, if not an impossible task in improving process efficiencies to achieve significant practical evolution of hydrogen from water splitting. of water was in fact reported for the first time (1967) by Kotel’nikov and Terenin2 for water adsorbed on dispersed γAl2O3, followed later by the 1970s reports by Basov and coworkers on the photodecomposition of water taking place on some metal oxides, alkali halides, and other solids,3,4 including HfO25 and BeO6 particulates.

I

n their 1972 seminal paper in the journal Nature, Fujishima and Honda1 showed that water could be photolyzed electrochemically at a TiO2 semiconductor photoelectrode (water oxidation) with Pt black as the counter electrode for water reduction. Although neither H2 nor O2 evolution was demonstrated at the time, three inferences were made under which water could be decomposed in the absence of any external electrical power: (i) O2 evolution occurs at a potential more negative than that for the H2 evolution (under normal conditions). (ii) H2 evolution occurs at a potential more positive than that for O2 evolution (under normal conditions). (iii) The potential for O2 evolution must be more negative and that for the H2 evolution must be more positive until the former is more negative than the latter. They further proposed that the photodecomposition of water, but more appropriately water splitting, could occur according to reactions 1−4. Although this caught the attention of many, the photosplitting © 2016 American Chemical Society

TiO2 (s) + 2hν → 2e− + 2h+

2h+ + H 2O(l) →

photoexcitation

1 O2 (g) + 2H+(aq) 2

(1)

0 Eox = −1.23eV (pH 0)

(2) Received: August 29, 2016 Accepted: October 7, 2016 Published: October 7, 2016 931

DOI: 10.1021/acsenergylett.6b00391 ACS Energy Lett. 2016, 1, 931−948

Review

http://pubs.acs.org/journal/aelccp

ACS Energy Letters 2e− + 2H+(aq) → H 2(g)

Review 0 Ered = 0.0 eV

Overall: H 2O(l) + 2hν → H 2(g) +

1 O2 (g) 2

two reducing equivalents for hydrogen formation and four oxidizing equivalents for oxygen formation with the energy being provided by an electrical source.29 A genuine watersplitting process involving rutile TiO2 powder was recently reported by Maeda.30,31 Even though proof of concept of the water-splitting process has been demonstrated, the currently available technology is still in the very early stages even after a nearly four-decade period, inasmuch as the reported yields of photocatalytic hydrogen can be said to be somewhat disappointing at best (see Tables 1 and 2).32

(3) Ewater = − 1.23 eV

(4)

A subsequent study by Nozik on the photoelectrolysis of water showed the process needed an oxygen overpotential of 1.0 V for H2 and O2 to evolve owing to the kinetic limitations on the rate of hole injection from the TiO2 space charge layer to the electrolyte.7 The use of water as a source of a fuel produced photoelectrochemically or otherwise was not a novel idea, however. In fact, the French author Jules Verne, in his 1874 book L’Ile Mystérieuse (The Mysterious Island), stated “that water will one day be employed as a fuel, that hydrogen and oxygen that constitute it, used singly or together, will furnish an inexhaustible source of heat and light, of an intensity of which coal is not capable. Someday the coal rooms of steamers and the tenders of locomotives will, instead of coal, be stored with these two condensed gases, which will burn in the furnaces with enormous caloric power.... I believe, that when the deposits of coal are exhausted, we shall heat and warm ourselves with water. Water will be the coal of the future.”

Table 1. Some Systems Used for UV-Light-Induced Photocatalytic Water Splittinga activity (mmol h−1 g−1) photocatalyst

H2

O2

year

NiO NiO NiOx

H2O H2O H2O

3.39 19.8 3.09

1.58 9.7 1.5

2001 2003 2012

NiOx Pt

H2O H2O

0.85 0.16

0.46 0.066

2014 2013

core−shell Rh/ Cr2O3

H2O

4.0

2.0

2014

artificial TiO2 leaf C/TiO2/CNTs

Pt

14.0

b

2010

37.6

b

2014

CNTs-Ta2O5

Pt

20% MeOH 50% EtOH 50% MeOH

32.0

b

2014

NaTaO3 NaTaO3/La α-Ga2O3/βGa2O3 SrKTa5O15 rutile TiO2 powder GaN nanowire

Water may well be the fuel of the 21st century. The report by Fujishima and Honda1 (referred to by some as the Fujishima−Honda effect) and the 1973 oil crisis spurred considerable interest toward renewable energy sources and provided the impetus toward sustained research on the road to achieving water splitting (see for example the work by Balzani and co-workers,8 among others, reported in the journal Science in 1975) because the production of hydrogen, a convenient energy carrier, seemed a reasonable and an environmentally friendly objective in exploiting the abundant and inexhaustible sunlight as the energy to drive the process. There was a general consensus at the time that photoassisted water splitting might indeed offer an alternative energy resource and an attractive opportunity to achieve this target under eco-friendly conditions. Related to this, Bolton later argued that water splitting by visible light was the most attractive option for the photochemical conversion and storage of solar energy and described a photochemical system that could produce H2 in homogeneous aqueous media.9 Unfortunately, that early euphoria and the promise of a quick solution to the 1970s energy crises (another one occurred in 1979) to produce significant quantities of hydrogen in homogeneous aqueous media failed to materialize. In subsequent related studies in the late 1970s and early 1980s, Bard and co-workers examined the possible formation of hydrogen from water in heterogeneous semiconductor aqueous media.10,11 Since then, research activity has continued at a sustained pace as demonstrated by the large number of more recent publications.12−28 In this context, far too many claims have been made in the literature that hydrogen produced photocatalytically can be generated through the process referred to as water splitting. The Nature of Water Splitting. Water splitting by photochemical or photocatalytic means is equivalent to the electrolysis of water whereby the reduction of water occurs at the cathode (H2 evolution), while oxidation of water occurs at the anode (O2 evolution) with the half-cell reactions involving

cocatalysts

none

solution

a Data selected from the work of Li et al.32 bNo oxygen evolved or detected.

The standard free energy for splitting water into H2 and O2 is 1.23 eV. When the energies of the CB and VB of the semiconductor photocatalyst (e.g., TiO2) are such that the redox potentials of both the reductive and oxidative steps are positioned within the band gap of the semiconductor, then water splitting becomes thermodynamically feasible (Figure 1).29 Past experience has shown that when this photocatalyst is naked TiO2, neither hydrogen nor oxygen is evolved in the absence of sacrificial electron donors and acceptors and cocatalysts. According to Gerischer,33 although there are a number of metals that can act as good catalysts for the hydrogen evolution reaction (HER), there are no similar catalytic materials of comparable efficiency for the oxygen evolution reaction (OER) from water. Even though RuO2 has been used for the OER process, it consumes considerable overpotential in the oxidation of water; thus, he suggested that a semiconductor photocatalyst for the water-splitting process must generate a potential much greater than 1.23 eV, in addition to displaying no less than excellent catalytic properties toward anodic or cathodic reactions; not least, the quasi-Fermi levels at the contact point of the semiconductor-electrolyte junction must, under illumination, be so positioned as to effect the oxidation and reduction of water. Such requirements preclude semiconductors with band gaps smaller than 2.3−2.4 eV.33 In semiconductor-based photocatalysis, optical excitation of the semiconductor nanoparticles with energy equal to or greater than its corresponding band gap energy by a light 932


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Table 2. Some Systems Used for the Visible-Light-Induced Photocatalytic Water Splittinga activity (mmol h−1 g−1)

a

photocatalyst

cocatalyst

solution

(Ga0.82Zn0.18)/N0.82O0.18 CoO

Rh2−xCrxO3 none

H2SO4 pH 4.0 water pH 7

ZnAg3SbO4 CdSe/CdS nanorods CdS CdS Ni h-CdSe/CdS CdS−ZnS core shell CdS QDs−ZnIn2S4

none MoS3 Pt Pt, PdS NiCl2 none RGO/Pt

NaOH, H2S triethanolamine Na2SO3, Na2S H2S-diethanolamine 50% i-PrOH Na2SO3, Na2S Na2SO3, Na2S

H2

O2

year

3.09 71.4

1.53 35.4

2008 2014

b b b b b b b

2013 2011 2007 2008 2013 2014 2013

10.2 100.0 27.3 94.0 153.0 0.79 27.0

Data selected from the work of Li et al.32 bNo oxygen evolved or detected.

Hydrogen is often generated from water/photocatalyst dispersions in the presence of a sacrificial electron donor (e.g., alcohols or similar electron donor systems) which causes the photodecomposition of water. In this case, the process cannot be claimed to be water splitting, because the decomposition of water in the presence of sacrificial electron acceptors or donors is not synonymous with water splitting! However, a large number of studies reported continue to claim water splitting in the presence of sacrificial agents. For instance, Tables 1 and 2 report some systems exposed to either UV light or visible light, respectively, to effect the so-called watersplitting process; some of the systems do indeed lead to water splitting and produced the solar fuel H2 as well as O2 in the appropriate stoichiometric quantities, while others required the presence of sacrificial electron donors.32 Disappointingly, an examination of the literature32 and the patent literature29 indicates that the yields of hydrogen (and in some cases of oxygen) from various sources are very low, even though authors have claimed high efficiencies. A recent review by Suib and co-workers43 quoted some results of hydrogen evolving from water with CdS nanorods/ZnS nanoparticles that produced 239 mmol g −1 hr −1 of hydrogen 44 and 647 mmol g−1 hr−1 of H2 with a Au/TiO2 system45 in contrast with the 3.5 mmol g−1 hr−1 of H2 reported initially by us,46 albeit not from the photodecomposition of water but from the photocleavage of hydrogen sulfide with the coupled semiconductors {CdS + TiO2/RuO2} in alkaline aqueous media (see below). Even though the more recent studies report hydrogen evolution from aqueous media in the few hundreds of mmol g−1 hr−1,32,43 the process cannot be viewed as being efficient as too often claimed in the literature.37,47 We suggest that the term ef f icient water splitting be avoided in the future and replaced by enhanced water splitting when making comparisons; after all, the efficiency of a process is best described in terms of the quantum yield (Φ) of the process,48,49 in the present context being hydrogen evolution.

Figure 1. Cartoon illustrating the thermodynamic feasibility of carrying out the water-splitting process upon illumination of a semiconductor photocatalyst with artificial ultraviolet (UV)/visible light or solar light. Here, the anode is the valence band and the cathode is the conduction band when the catalyst particle is photoactivated.

source (preferably sunlight) leads to formation of charge carriers, namely, photoholes in the valence band (VB) and photoelectrons in the conduction band (CB). These charge carriers can, in part, subsequently recombine and in part migrate to the photocatalyst surface, where they react with adsorbed molecules, such as water, and initiate oxidative and reductive chemical reactions, respectively. Photocurrent density versus potential experimental results in photoelectrochemical studies are often used to infer that water splitting can occur,34−38 yet this needs to be demonstrated by detecting the formation of both hydrogen and oxygen39,40 as noninsignificant factors, such as the need for efficient anodic photoelectrodes that are capable of overcoming the high overpotential for the oxygen evolution reaction can impact significantly on the water-splitting process. The OER process requires no less than four oxidizing equivalents and is thus responsible for the otherwise slow kinetics in photoelectrochemical devices.41 In addition, one-electron events that involve formation of intermediate species such as H· and ·OH radicals from water are energetically unfavorable toward achieving the water-splitting process whether driven by sunlight or by any other light source.42

Unless supported by the quantum yield (not quantum efficiency), the term ef f icient water splitting should be avoided and replaced by the more appropriate term enhanced water splitting. 933


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be decreased as the number of such defects impact significantly on quantum efficiencies, (ii) that the n-type semiconductivity be watered-down, (iii) that the kinetics for the OER process be enhanced, and (iv) that an asymmetric electric field be generated within the lattice so as to enhance charge carrier separation.19 Items i−iii were validated experimentally for the gallium−zinc oxynitride (Ga1−xZnx)(N1−xOx) and for SrTiO3 whose band gap energies are 2.7 and 3.2 eV, respectively. According to their asymmetric model, a decrease in the number of defects from 1018 to 1016 cm−3 increases significantly the quantum efficiency of water splitting from ca. 5% to ∼90% through an improvement in the crystallinity of photocatalysts and through a decrease in the density of defect states.19 Along similar lines, Acar et al.51 pointed out that surface morphology, crystal structure, and particle size are some key issues affecting the photocatalyst activity in water splitting under solar irradiation. These factors impact on surface energy and chemisorption properties of the particulates that determine the characteristics of redox reactions on the photocatalyst surface (e.g., selectivity, kinetics, and overpotential, among others), interfacial electron transfer, and resistance to photocorrosion. Apparently, more efficient photocatalytic water splitting could be achieved through band gap engineering.51 Suib and co-workers,43 on the other hand, propose that the current outlook for efficient water splitting should rely on innovative designs of photocatalytic materials and that theoretical and computational models should help our understanding of the electronic density of states and band structure that would point us toward more rational designs of photocatalysts. Lifetimes and Recombination of Photogenerated Charge Carriers. Generation and recombination of carriers determine the stationary concentration of photogenerated carriers in wide band gap solids (Figure 3).52 Both electrons and holes produced on absorption of photons with energy greater than the band gap energy, Ebg, in wide band gap solids initially transit

To devise a system that is efficient, many factors responsible for the low yields of solar fuels have to be identified. The principal goal of this Review is to point out some of the features of semiconductor photocatalysts when exposed to light irradiation and those factors that (may) impact the efficiency of the water-splitting process in particular and surface redox reactions in general. Germane to our discussion, Martinez Suarez and coworkers41 noted three fundamental requirements for developing an optimal visible-light-active photocatalyst for overall water splitting: (1) The conduction band (CB) and valence band (VB) edge potentials must be suitable for overall water splitting (Figure 2).50 (2) The band gap energy of the photocatalyst

Figure 2. Cartoon illustrating the three principal limitations associated with the development of visible-light driven photocatalysts. Reprinted from ref 50. Copyright 2007 American Chemical Society.

must be lower than 3 eV for optimal visible-light harvesting. (3) The photocatalyst must be physicochemically stable during the photocatalytic reaction. In the latter case, both CdS and CdSe are chemically unstable unless either sulfide ions or selenide ions are present in the overall system in alkaline media. By comparison, WO3 has an unsuitable band position to effect the reduction of water, while TiO2, SrTiO3, and NaTaO3 materials possess large band gap energies requiring UV light to activate them. In addition, with regard to item 2, it is evident that a compromise must be struck between, on the one hand, small band gap materials so as to harvest as much of the visible light (or sunlight) as possible while, on the other hand, as noted by Gerischer,33 the band gap energy of the semiconductor photocatalyst must be greater than 2.3−2.4 eV to overcome the overpotential needed for the OER reaction in water splitting. This places severe restrictions on the choice of a suitable photocatalyst. Domen’s group19 also examined the overall water-splitting process using the oxynitride (Ga1−xZn x)(N 1−xO x ) and Zn:Ga2O3 photocatalyst materials. Weak isotope effects and low apparent activation energies led these authors to deduce that most of the charge carriers had recombined before they could contribute in any significant way to redox reactions occurring at the photocatalyst’s surface (see also below for TiO2). The low reactivity of the minority carriers (holes) for the oxygen evolution reaction (OER) impacted significantly on the kinetics of the process of water splitting, which was ascribed to the short lifetimes of the carriers and to an insufficient driving force for the OER process. They further deduced that systems with small band gap energies, such as TaON, Ta3N5, and LaTiO2N, are unlikely to achieve water splitting19 in accord with Gerischer’s propositions.33 To overcome such issues they proposed (i) that the density of defect states in photocatalysts

Figure 3. Scheme illustrating the processes of recombination of photocarriers and events leading to the discharge of the defect (for distinctness for defect with captured electron). Step 1: band-toband optical transition with generation of hot electrons and hot holes. The corresponding initial levels of the excited electron and hole are above the bottom of the conduction band (Ecb) and below the ceiling of the valence band (Evb). Steps 2 and 2′: thermal relaxation of hot carriers. Steps 3 and 3′: radiative and nonradiative band-to-band recombination. Steps 4 and 4′: radiative and nonradiative recombination via defect R acting as a recombination center. 934


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importantly on the rate of charge carrier recombination. Our picosecond pulsed laser spectroscopic examination performed over three decades ago (1983) on the most often used metaloxide photocatalyst TiO2 (∼12 nm colloidal particles) revealed that the lifetime of electron−hole pairs was ca. 30 ns (Figure 4),53 far too short for a slower surface reaction to compete

into Franck−Condon states at energies higher than the bottom of the conduction and the top of the valence band, respectively (step 1 in Figure 3). Thermal equilibrium between the crystalline semiconductor lattice and the photoproduced carriers is established as a result of relaxation of both energy and momentum of the charge carriers within a time scale of around 100 ps (i.e., 10−10 s; steps 2 and 2′). Subsequent lowering of the energy of the carriers occurs via recombination and trapping. In ideal, defect-free solids only radiative and nonradiative band-to-band recombination occurs (steps 3 and 3′). Nonradiative electron transitions from the CB band to the VB band in wide band gap solids (i.e., with Ebg ≥ 3 eV) have low probability because the participation of several phonons with energy around ca. 0.1 eV is required in the process. By contrast, the probability of radiative transitions with emission of photons is greater the larger the band gap energy. Such transitions manifest themselves as interband or otherwise as edge luminescence in solids. Concomitantly, the probability of radiative band-to-band transitions is also low because the momentum conservation law requires that the condition similar to that for band-to-band direct transitions be satisfied (eq 5), that is Δk = k1 − k 2 ≈ 0

(5)

Figure 4. Transient absorption spectra observed at various time intervals after picosecond excitation of 12 nm colloidal TiO2 particulates; [TiO2] = 17 g L−1, pH 2.7, solution deaerated with Ar, optical path length 0.2 cm. Average number of electron−hole pairs present initially in one TiO2 particle is 67. Adapted from ref 53. Copyright 1985 American Chemical Society.

where k1 and k2 are the wave vectors of electrons in the states between which the transition occurs and Δk ≈ 0 indicates momentum is conserved. Typically, band-to-band luminescence in wide band gap solids with photon energy close to Ebg (step 3, Figure 3) is observed at moderate intensities of the exciting light in nearly perfect crystals, wherein competitive radiative recombination of carriers through recombination centers R (defects) is suppressed, whereas under intense photoexcitation of the crystalline solid, luminescence is observed at high concentration of the photocarriers. Recombination of photocarriers via recombination centers is the main pathway of carrier recombination in defective wide band gap solids.52 In this case, a given recombination center R captures a free electron and a free hole (or vice versa) in a single recombination cycle (steps 4 and 4′ in Figure 3). Recombination via a defect involves a consequent two-step transition of an electron from the CB band to a vacant state of the defect and from the defect to a vacant state of the VB band, or otherwise from the defect to the VB band and then from the CB band to a vacant state of the recombination center R. The excess energy of the electron is dissipated during both recombination steps. Usually, dissipation of excess energy at one of the two stages occurs via a nonradiative pathway with the assistance of existing phonons so that the probability of nonradiative transitions increases with a decrease in the number of emitted phonons. Consequently, the energy levels corresponding to efficient recombination centers lie (typically) near the middle of the energy gap in wide band gap solids. Charge carrier trapping and recombination processes determine the stationary concentration of the charge carriers at the surface. Considering that surface defects with trapped charge carriers can act as surface-active centers that initiate surface chemical sequences, the lifetime of the trapped charge carriers on such surface defects corresponds to the lifetime of the chemically active states of the surface-active centers. An important factor that impacts process efficiency in water splitting and in the formation of hydrogen, whenever a sacrificial electron donor is present, are the lifetimes of the photogenerated electron−hole pairs that depend most

effectively against charge carrier recombination, even though this metal oxide is an indirect semiconductor. Trapping of conduction band electrons was a very rapid process occurring within the leading edge of the 30 ps laser pulse, while hole trapping was much slower. Recombination of trapped electrons with free holes was about 10 times faster than hole trapping, a significant disadvantage for the photocleavage of water; thus, there is a need for the removal of free holes by addition of sacrificial electron donors.53 A later subnanosecond study that examined the size dependence of the carrier recombination process revealed that by 10 ns 100% of the carriers had recombined for a 2.1 nm TiO2 particulate, whereas 93% had recombined for a 26.7 nm particle.54 Charge Carrier Separation. It is clear that electron−hole recombination has to be slowed considerably for water splitting, or for that matter any redox reaction taking place at the irradiated photocatalyst surface, to compete effectively, or otherwise for hydrogen to be produced in reasonable quantities even in the presence of sacrificial electron donors (i.e., hole scavengers). Light-induced charge separation constitutes an important step in the conversion of photons into chemical energy by natural photosynthesis or by artificial photosynthetic systems. Compared to the photocleavage of water (water splitting; reaction 6), the photocleavage of its cousin, hydrogen sulfide, is a more facile system to photocleave thermodynamically (reaction 7) in the presence of a suitable photocatalyst (e.g., CdS): 2H 2O(l) + hv → 2H 2(g) + O2 (g)

E 0 = −1.23 eV (6)

H 2S(aq) + hv → H 2(g) + S(s) 935

E 0 = −0.14 eV

(7)


ACS Energy Letters

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transfer across interfaces can occur to activate an otherwise wide band gap photocatalyst with lower energy photons from visible light (Figure 7).

Our earliest studies aimed at producing significant evolution of hydrogen from the photocleavage of H2S in alkaline aqueous media were undertaken using CdS and TiO2 photocatalysts, either alone or in a mixed combination of CdS and TiO2 particulates.55 Results displayed in Figure 5 showed a 40-fold

Figure 5. (a) Quantities of hydrogen formed in sulfide alkaline aqueous media in the presence of CdS alone, TiO2 loaded with 0.5 wt % RuO2, and in the presence of both CdS and TiO2/0.5 wt % RuO2 under UV/vis irradiation at wavelengths longer than 330 nm; conditions: 0.1 M Na2S, 1.0 M NaOH, irradiation at λ > 330 nm; 0.050 g of photocatalysts. Adapted with permission from data reported by Serpone and co-workers in ref 46. Copyright 1988 The Electrochemical Society.

Figure 7. Illustration of a ternary semiconductor system in which two low band gap semiconductors (A and B) are irradiated with visible light to activate a wide band gap semiconductor (C).

The first step in such a ternary nanostructure involves a VB → CB electronic transition in solid A to generate free electrons and holes, and to the extent that the VB position of solid A is lower than the VB band of solid C, hole transfer occurs from A to C, resulting in charge separation in the A−C heterojunction. Photoexcitation of solid B also generates e and h in solid B, and because the CB position of solid B is higher in energy than the CB of solid C, electron transfer occurs from B to C, thereby causing charge separation at the B−C heterojunction. Positioning the CB of solid A higher in energy than the VB of solid B causes electron transfer from A to B and recombination of electrons in A with holes generated in B, thus completing the photoexcitation cycle in which the excited state of solid C has been achieved via two low-energy photon processes. Accordingly, the photoactivity of the wide band gap photocatalyst is then otherwise similar to a high-energy one-photon process. Photoexcitation of components A and B is very efficient because these two solids are activated through their fundamental absorption band. In this way, visible light activation of low band gaps A and B would achieve the same stored energy in the excited state of the wide band gap C as that obtained under UV irradiation. However, low band gap materials tend to be photochemically unstable, so that they may have to be encapsulated to form the heterojunction ABC with a wider band gap but stable photoactive materials to enhance charge separation in the heterostructure (see discussion of photostability below). An example of a thirdgeneration photocatalyst system implicating a visible-lightactive multifunctional ternary composite based on a TiO2− In2O3 heterojunction of nanocrystals decorating porous graphitic carbon nitride for the photocatalytic treatment of hazardous pollutants (or biomass) and evolution of hydrogen was illustrated recently by Jiang and co-workers.58 Redox reactions occur at the surface of a photocatalyst, so that the nature and the integrity of such a surface is a significant factor to consider because it can impact significantly process efficiencies. This was particularly evidenced in the photocleavage of H2S by various batches of in-house synthesized CdS that were pretreated by etching the surface with various acids. The mode of preparation of the photocatalysts also played an important role in the photochemical cleavage of H2S, as demonstrated by Serpone and co-workers over three decades

increase in the quantity of hydrogen produced from the sulfide aqueous media (0.1 M Na2S; 1 M NaOH) at a rate of 3.5 mmol g−1 hr−1 H2 in the presence of both CdS and TiO2/RuO2 relative to conditions in which only CdS was irradiated with UV/visible light (λ > 330 nm) that yielded ca. 0.09 mmol g−1 hr−1.46 Irradiating the TiO2/RuO2 system in the absence of CdS produced only 0.004 mmol g−1 hr−1 of hydrogen under otherwise similar conditions. Although the quantities of hydrogen evolved in the presence of CdS and TiO2 alone were rather small, when both semiconductors were combined the quantity of hydrogen produced increased dramatically under UV/visible irradiation. This was attributed to an interparticle electron transfer process whereby photogenerated conduction band electrons from CdS are vectorially transferred onto the conduction band of TiO2, while the valence band holes from TiO2 are transferred onto the valence band of CdS (Figure 6).55 Since our first suggestion,55 this and similar strategies of charge carrier separation by a combination of various semiconductors are now well rooted in the literature. One such strategy proposed by us56,57 involves a ternary semiconductor system (dubbed third-generation materials) in which their band positions are suitably positioned such that electron

Figure 6. Cartoon illustrating the vectorial displacement of charge carriers between two semiconductors together with the evolution of hydrogen from the photocleavage of hydrogen sulfide. Adapted from ref 57. Copyright 2012 American Chemical Society. 936


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ago.46,59−61 Chemical etching of CdS particles with acids altered their macroscopic structure.46 The photocatalytic activity toward hydrogen evolution from the photocleavage of H2S by acid-etched samples varied in the order HNO3 > HCl > H2SO4 > CH3COOH (see Figure 8), which correlated with the activity of CdS prepared from various Cd2+ salts containing the anions NO3− > Cl− > SO42−.46

This permitted an estimate of the turnover numbers for the photooxidation of hydrogen (TON > 14.5) in the presence of oxygen and the photoreduction of oxygen (TON > 6.6) in the presence of hydrogen, which inferred that the photoreaction occurring on the surface of the metal-oxide photocatalyst was truly photocatalytic. Monitoring the photoinduced coloration of a metal oxide (i.e., formation of F- and V-type color centers), such as ZrO2, can also reveal whether a photoinduced process is photocatalytic, for example, such processes as the photoreduction of O2, the photooxidation of H2, the photooxidation of H2 in the presence of adsorbed oxygen, and the photoinduced transformation of NH3 and CO2 over illuminated ZrO2.65 Photoreactions involving NH3 and CO2 were not photocatalytic, whereas the photooxidation of hydrogen in the presence of oxygen was indeed a photocatalytic process. The process involving ammonia was simply a stoichiometric surface photochemical oxidation of ammonia, while that involving carbon dioxide represented a surface photooxidation reaction, that is, photooxidation of the CO2−· radical followed by desorption of CO2. These results demonstrate that for a true photocatalytic process, two conditions must be met: (i) The rates of consumption of charge carriers in the reduction and oxidation half-reactions must be identical. (ii) The photocoloration of the metal oxide must achieve the limits of accumulation of hole (V type) and electron (F type) color centers when the number of electrons trapped by the defects in the solid is the same as the number of trapped holes. This last condition corresponds to the requirement that at the completion of the reaction cycle the photocatalyst retains its original state. Any deviation from these two conditions points to a noncatalytic nature of the overall surface photochemical process. Clearly, a further description is required as to the conditions under which processes can be said to be photocatalytic.65 In the present context, a solid photocatalyst typically changes its state during photoexcitation. Thermodynamically, this corresponds to the creation of quasi-Fermi levels for the photoinduced defects that generally differ from the quasi-Fermi levels of photogenerated free charge carriers (electrons and holes). Hence, with the photoinduced formation of new defects during illumination, a photocatalyst will change its thermodynamic state and so will not possess the same state as the original state during or after irradiation. This is rather typical in heterogeneous catalysis when the stationary state(s) of surface structure and composition of the catalyst during the catalytic process differs from the initial state. Stated more rigorously, the photocatalyst will retain its original state after photoexcitation only in the case of complete relaxation to its original ground state.65 An example of the relaxation of a photocatalyst is displayed in Figure 9a.65 Photoexcitation of the solid semiconductor generates free charge carriers (CB electrons and VB holes) as a result of band-to-band transitions. Relaxation of the solid then occurs either through band-to-band recombination of the charge carriers or their recombination through specific recombination centers (R; defects in the solid). Complete relaxation through recombination restores the photocatalyst to its original state at the end of photoexcitation. If incomplete relaxation through recombination occurs, then the original state of the photocatalyst will not be regenerated (Figure 9b). In this case, the first step of relaxation would involve trapping of the charge carriers by the solid’s point defects (e.g., by impurity

Figure 8. Plots showing the volume of hydrogen evolved against irradiation time for various CdS systems (0.010 g) in alkaline aqueous suspensions containing 0.1 M Na2S and 1.0 M NaOH; irradiation wavelength >400 nm. The CdS had been pretreated by etching with various acids. After 2 h, the rate of formation of H2 was 0.55 mmol g−1 hr−1 for the CdS (HNO3) system. Adapted with permission from data reported by Serpone and co-workers in ref 46. Copyright 1988 The Electrochemical Society.

There is no question that modifications of the particle surface were responsible for more efficient photoredox processes, which ultimately yielded hydrogen owing to formation of surface defects, thus separation of charge carriers at the particle surface, and to a decreased electron−hole pair recombination at the surface. Related to our earlier findings,46 chemically etched Vis-TiO2/Ti thin films by HF enhanced the formation of hydrogen and oxygen from water, which Kitano and coworkers62 ascribed to a shorter diffusion length of the photogenerated holes reaching the solid−liquid interface being shorter than that of untreated Vis-TiO2/Ti and to the higher conductivity of the fluorine-etched titania sample. Is the Photocatalytic Water-Splitting Process Catalytic? In heterogeneous photocatalysis, the term photocatalytic is more of a historical term used rather casually whenever a semiconductor or a heterostructure of two or more semiconductors is used to effect either the photodegradation of hazardous pollutants or the formation and evolution of hydrogen and/or oxygen from aqueous media with or without the presence of sacrificial electron acceptors/donors. Whether a photocatalyst remains unchanged at the completion of the photocatalytic reaction cycle(s), as required by the definition of catalysis, has remained elusive; the difficulty is the elusive knowledge of the number of surface active sites on the semiconductor photocatalyst. In fact, seldom, if ever, have so-called photocatalyzed processes been demonstrated to be catalytic on the basis of turnover numbers (TONs). To our knowledge, only in two studies has a process or processes been demonstrated to be catalytic as reported by Subbotina et al.63 over silica-supported molybdenum oxide catalysts, and by Emeline and co-workers over ZrO2.64 In the latter study, the number of surface-active sites on the ZrO2 particle surface (ca. 1 × 1016 centers) were determined quantitatively using no less than three different approaches. 937


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from water splitting is related to the quantum yield of the process, we need to consider those factors that affect the quantum yield (Φ) directly. Recall that the quantum yield of a photocatalytic process is defined in the same manner as the quantum yield of a photochemical process, i.e., as the number of molecules of a given product formed (Nmolec; or of a given reactant disappearance) per photon of light (Nph) absorbed by the photocatalyst at a given wavelength.48 Two features of this definition are of special importance. For the case of suspensions or powders, an estimate of the number of absorbed photons has been a daunting task because of scattering effects. The problem can be resolved, however, with the use of appropriate blackbody type reactors for gas−solid systems67 and otherwise for liquid− solid systems in heterogeneous aqueous media.68 The surface concentrations of charge carriers is one of several key factors that determine the efficiency and the selectivity of interfacial chemical reactions.69−73 Such factors are dictated by electronic processes occurring in the solid photocatalyst, for example the photogeneration and recombination of charge carriers and their diffusion (and/or drift) toward the surface, and by physical and chemical decay of charge carriers on the surface. In the absence of an electric field (i.e., E = 0), diffusion is the major pathway for carrier migration toward (or away from) the surface, an assumption applicable to very small particles of photocatalysts and to particles that possess a neutral surface with respect to the bulk.69 Generally, however, a surface charge and a corresponding electric field in the subsurface space charge region of the solid cannot be neglected because it would favor the drift of a charge carrier to the surface (e.g., electrons) and prevent the appearance of the other carrier (e.g., holes) on that same surface. The role and the consequences of subsurface electric fields also affect the activity and the selectivity of photocatalysts in photoinduced surface chemical processes. A theoretical equation that showed the various parameters that affect the quantum yield was developed by solving the continuity equation for a one-dimensional model of a semiconductor that included the subsurface electric field in the space charge region of the semiconductor photocatalyst.74 The solution to this continuity equation yielded a very complex expression that incorporated various factors that affect the quantum yield Φ: (i) the absorption coefficient, α; (ii) the electric field, E; (iii) the diffusion length of carriers, L; (iv) the depth of the subsurface electric field, δ; and (v) their various combinations (see Appendix). However, setting the electric field E → 0 resulted in a more manageable expression (eq 8)74 otherwise identical to the one developed earlier for a model with no electric field (E = 0).69

Figure 9. Cartoons illustrating some of the conditions through which a photocatalyzed process can be considered to be truly catalytic. Adapted from ref 65. Copyright 2005 American Chemical Society.

cations and anions and/or by anion and cation vacancies) in a manner otherwise identical with the first step of recombination through recombination centers (Figure 9a) leading to formation of optically observable color centers (e.g., F-type and V-type defects). The subsequent step of recombination of a trapped charge carrier (e.g., electron) with its free counterpart (e.g., hole) is much less effective (dashed arrows, Figure 9b), which leads to (1) an accumulation of trapped charge carriers, (2) to incomplete relaxation of the solid, and (3) a new state of the photocatalyst that is different from the original state. A truly photocatalytic cycle is illustrated in Figure 9c, which shows that the photocatalyst returns to its original ground state in the same manner as in the case of internal charge carrier recombination.65 However, relaxation now takes place through external surface chemical reactions that generally need not be closed-loop processes. It suffices only that the number of electrons consumed by the electron acceptor, A, be the same as the number of electrons transferred to the catalyst by the electron donor, D, as long as the reaction products are not strongly bonded to the surface of the catalyst so as not to change the chemical composition of the metal-oxide surface. The latter is also true for a photocatalytic process consisting of a closed-loop reaction cycle. In other words, the condition for true photocatalysis is given by d[A]/dt = d[D]/dt or, stated differently, the rate of the surface reduction reaction must be identical to the rate of the oxidation reaction. Therefore, because oxidation of water necessitates four oxidizing equivalents and is a thermodynamically uphill process, it follows that in this case d[A]/dt ≠ d[D]/dt and water splitting cannot be a photocatalytic process. In the extreme case depicted in Figure 9d, when only half a reaction takes place on the surface, the photochemical reaction is stoichiometric, not photocatalytic. An example of such a photochemical reaction is the photoinduced chemisorption of molecules on the surface of metal-oxide solids; for instance, the photooxidation of hydrogen in the absence of oxygen and the photoreduction of oxygen in the absence of hydrogen. A more complete discussion of the criteria and conditions for processes to be photocatalytic is given elsewhere by Emeline and co-workers.66 Quantum Yield of Photoassisted Surface Redox Reactions. Inasmuch as the quantity of hydrogen and oxygen evolving

Φ=

χkrSαL2 D(1 − α 2L2)

d

×

tanh L coth αd − αL tanh

d L

+

sL D

(8)

Predictions from that complex equation (see Appendix) found experimental verification from the experimental quantum yields of photoadsorption of dioxygen (i.e., photoreduction of O2) and dihydrogen (i.e., photooxidation of H2) on TiO2 particles.74 Derivations of such expressions from the continuity equation are useful in that they reveal which factors and how such factors may affect the yields of photoassisted reactions occurring on a semiconductor surface. Considerations of the spatial nonuniformity of photogeneration of charge carriers in the bulk of solids and their limited probability of diffusion toward the photocatalyst surface 938


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led us74 to deduce that the former depends on whether the solid photocatalyst is photoexcited in the intrinsic or extrinsic spectral regions, or whether only the surface is photoexcited. The essential factors to observe similarities between the spectral dependencies of Φ of surface processes and the absorption spectrum are (i) the relative values of the absorption coefficient α (reciprocal centimeters) and (ii) the complexity of the absorption spectrum. Inf luence of the Surface Potential (Charge) Us on Φ and Spectral Dependence of Φ. The quantum yield (Φ) of a surface photochemical reaction depends on the magnitude of the absorption coefficient (α) and, in particular, on the spectral variation of α that leads to the spectral variation of Φ. Figure 10 Figure 11. Theoretical spectral variation of the quantum yields of a surface photochemical reaction with electrons at different surface potentials (Us) and as a function of the spectral variation of the absorption coefficient (α). Reprinted from ref 74. Copyright 2003 American Chemical Society.

depending on the value and direction of the electric field (i.e., on the value of the surface potential or charge). Indeed, for a given range of surface potentials, the apparent shift of the threshold of the spectral dependence of the quantum yield occurs at a photon energy less than (or greater than) the band gap energy of the solid as attested experimentally in the spectral dependence of Φ of the photoreduction of molecular oxygen on Ga2O3 whose band gap is 4.4 eV (Figure 12).67 Figure 10. Theoretical dependencies of the quantum yield of a surface photochemical reaction involving electrons (photoreduction) on the absorption coefficient (α) for different surface potentials (Us). Reprinted from ref 74. Copyright 2003 American Chemical Society.

illustrates the dependence of Φ on different values of α and for different values of the surface potential (charge) Us. Using the most reasonable values of the parameters and the complex expression74 revealed that the quantum yield (Φ) is constant for low values of α, whereas for larger values, Φ increases with increase in α. Physically, this means that the greater the value of α, the closer are the charge carriers generated near the surface so that a larger fraction of photogenerated charge carriers reach the surface and participate in surface chemical processes, notwithstanding the diffusion and drift migration of the carriers. The surface potential (Us) increases (or decreases) the quantum yields depending on the sign of the surface charge and on the sign of the corresponding charge carrier. A steplike absorption band mimics the threshold of the fundamental absorption of a solid photocatalyst (such as TiO2) for which the absorption coefficient (α) can reach very high values. Figure 11 demonstrates that the spectral dependence of Φ parallels the behavior of the absorption feature(s).74 Note how the magnitude of the quantum yield for a hypothetical photoreduction process changes with the surface potential (Us), i.e., if the surface charge is negative, then fewer electrons reach the surface and consequently Φ for the photoreduction will be small, whereas for positive values of Us, a greater number of electrons reach the photocatalyst surface at the expense of photogenerated holes resulting in an increase of the quantum yield of the photoreductive process. In addition, the existence of an electric field leads to a spectral shift of the threshold of the spectral dependence of Φ,

Figure 12. Experimental spectral dependence of the quantum yield of photoreduction of molecular oxygen on the wide band gap semiconductor Ga2O3 (band gap energy, 4.4 eV). Reprinted from ref 67. Copyright 2000 American Chemical Society.

Selectivity of a Photocatalyst. The ability of a photocatalyst to turn a reaction pathway toward a certain product describes the selectivity of the photocatalyst, another significant feature of the photochemical behavior of photocatalysts in heterogeneous media. The selectivity (Si) of the photocatalyst toward the ith reaction product is characterized by the quantum yield (eq 9)74 Si =

dNi /dt Φ = i dNr /dt Φr

(9)

where dNi/dt and dNr/dt are the rates of formation of the ith product and decomposition of the reagent, respectively; Φi and Φr are the corresponding quantum yields. Variation of the selectivity of a photocatalyst is governed by the ratio γ between the surface concentrations of electrons and holes: γ = [es]/[hs]. Where only two reaction pathways are possible (e.g., oxidation and reduction), the selectivities toward 939


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the absorption coefficient (α) and on the ratio of the diffusion lengths, Le/Lh, at the edge of the intrinsic absorption band where α changes by a few orders of magnitude; this can alter the overall reaction pathway. Such an alteration found experimental evidence in the photoinduced reaction of CH4 on TiO 2 particulates under photoexcitation near the fundamental absorption edge of the TiO2 photocatalyst.75 Variations in the wavelength of the incident actinic light leads to the spectral selectivity of a photocatalyst because of the spectral variation of γ (= [es]/[hs]). For very weak (extrinsic) or very strong (intrinsic) light absorption by the semiconductor photocatalyst, the ratio of the surface concentrations of carriers is constant, causing the selectivity of the photocatalyst to remain spectrally invariant.74 The theoretical spectral dependencies of photocatalyst selectivity on α are illustrated in Figure 14 for various surface

certain reaction products can be expressed by eq 10 for the reductive pathway and by eq 11 for the oxidative product. γ Sred = k γ + k ox (10) red

Sox =

kox k red

γ+

kox k red

(11)

It is relevant to point out that the ratio γ (= [es]/[hs]) is governed by the mobilities of the different carriers, their lifetimes, the rates of surface recombination of the charge carriers, and the electric field, the latter being one of the most significant factors to affect the value of γ. The magnitude and direction of the electric field in the subsurface space charge region determine the drift of the carriers either toward the surface or away from the surface depending on the charge of the carrier, thereby increasing or decreasing the ratio γ. Figure 13 illustrates some common dependencies of the selectivity of a

Figure 14. Theoretical dependencies of the spectral selectivity of the photocatalyst toward a reductive pathway on the absorption coefficient (α) at different surface potentials (Us). Reprinted from ref 74. Copyright 2003 American Chemical Society.

Figure 13. Theoretical dependencies of the ratio between the surface concentrations of electron and holes (γ) (1) and the selectivities toward reductive (2) and oxidative (3) reaction pathways as a function of the surface potential (Us). Reprinted from ref 74. Copyright 2003 American Chemical Society.

potentials Us (from −0.1 V to +0.5 V). For a reductive process the selectivity, Sred, increases with increase in α and with increase in the surface potential Us (e.g., from −0.1 V to +0.1 V); for Us = +0.2 V, the selectivity increases at first for small values of α and then decreases for higher values of the absorption coefficient, whereas at higher values of Us (e.g., at +0.5 V) Sred decreases gradually with increasing α. In the absence of an electric field, when migration of charge carriers occurs only through diffusion, the spectral selectivity is also due to differences in the mobilities and lifetimes of the charge carriers and thus on their diffusion lengths.69 However, when the mobilities and lifetimes of electrons and holes are identical, no spectral dependence of the selectivity is expected under any conditions. The presence of an electric field in the near-surface space charge region is responsible for the spectral dependence of the selectivity of the photocatalyst even in such a case, because the electric field creates different effective diffusion lengths for the electrons and holes to migrate toward the surface.74 Back Reaction of Products f rom Water Splitting: Back Reations. A factor often neglected is the possibility that the products from water splitting (H2 and O2), or from some other redox reaction, undergo a back reaction to reproduce the original reactant (water). This is explicitly illustrated in Figure 9c, which shows that subsequent to the formation of the reduced electron acceptor, A−, and the formation of the oxidized electron donor,

photocatalyst on the surface potential (Us) and therefore on the subsurface electric field.74 Clearly, if the surface charge is positive, the ratio [es]/[hs] increases and a photoreduction surface reaction will prevail, whereas a negative surface charge will favor a photooxidation process at the surface. Variations in the surface potential originate from such factors as (a) the reconstruction of the surface occurring during heating or other prior physical treatments of the photocatalyst; (b) specific adsorption of ions on the surface (or in the bulk) of the photocatalyst; and in particular, (c) the variation in the pH of the dispersion (particularly relevant for metal oxides), as well as variations of the surface structure of the photocatalyst resulting from different synthetic methods. For an experimental verification of these factors, note the earlier discussion on the photocleavage of H2S in alkaline media in the presence of CdS. Spectral Selectivity of a Photocatalyst. Theoretically the spectral selectivity of a photocatalyst depends on the different probabilities that electrons and holes reach the photocatalyst surface, as a result of differences in their mobilities and lifetimes (note that the diffusion lengths of carriers are generally different because of differences in their respective mobilities).69 Also the greatest change in the ratio of the surface concentration of electrons and holes, [es]/[hs], depends on 940


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Figure 15. Kinetics of accumulation of the photoinduced formation of (a) F-type color centers, (b) V-type color centers, and (c) Zr3+ color centers during irradiation of reduced zirconia in vacuo (curve 1), in the presence of oxygen (curve 2), in the presence of hydrogen (curve 3), and during the photooxidation of hydrogen in the presence of oxygen (curve 4) recorded at three different wavelengths (620, 360, and 280 nm). Reprinted from ref 65. Copyright 2005 American Chemical Society.

D+, each of which can undergo further reaction to yield their consequent products, they can also react with each other to produce AD. Experimental manifestation of this back reaction is illustrated in Figure 15a−c, which displays the kinetics of accumulation of photoinduced color centers in ZrO2 in vacuo (curve 1) and in the presence of oxygen (curve 2) and hydrogen (curve 3) at three selected wavelengths (620, 360, and 280 nm) that correspond to absorption by F-type (Figure 15a) and V-type (Figure 15b) color centers and by Zr3+ defects (Figure 15c), respectively.65 Also shown are the kinetics of accumulation of photogenerated color centers during the photooxidation of H2 in the presence of molecular oxygen (curve 4). In spite of some variations in the kinetic behavior between oxidized and reduced states of ZrO2, in both cases the effects of photostimulated adsorption of gases on the photocoloration of this wide band gap metal oxide are rather similar. For both states, the photoadsorption (i.e., photooxidation) of H2 caused an increase in the number of Zr3+ centers (λ = 280 nm) and electron F-type color centers (λ = 620 nm): note curve 3 in Figure 15a,c relative to those in vacuo (curve 1). Concomitantly, absorption at 360 nm for reduced zirconia, corresponding mostly to hole V-type color centers (curve 3, Figure 15b), decreased relative to that in vacuo (curve 1). In other words, photoadsorption (i.e., photoreduction) of O2 led to increased absorption by hole color centers (curve 2, Figure 15b) and to decreased absorption by electron color centers (λ = 280 and 620 nm; curves 2 and 4, Figure 15a,c). The effect of photoadsorption on the photocoloration of ZrO2, in particular, and metal oxides, in general, is a good reference benchmark to evaluate whether or not surface photoreactions are catalytic or noncatalytic.65

The photoinduced coadsorption of H2 and O2 gases led to a different behavior of the photocoloration of zirconia compared to the photoadsorption of each gas separately. This signifies that most of the additional color centers, formed as a result of photoadsorption of these gases, are located on the metal-oxide surface. Hence, coadsorption of another gas resulted in the disappearance of the absorption of the corresponding surface color centers and in an increase of the rate of adsorption of the coadsorbate. That is, photoreduction of O2 increased the rate of photooxidation of H2, and the photooxidation of H2 increased the concomitant photoreduction of O2. After a certain period of irradiation, however, the role of photoadsorption decreased, and the photocatalytic oxidation of H2 in the presence of O2 reached a stationary state.65 The concomitant photooxidation and photoreduction of hydrogen and oxygen, respectively, on irradiated zirconia (and other semiconductor photocatalysts alone or in some heterostructure) ultimately leads to the formation of water at the point where the extent of photocoloration of the photocatalyst(s) reaches a steady-state level. Accordingly, in the water-splitting process, once the quantities of hydrogen and oxygen have reached some sort of an equilibrium state, they can react with each other to reform water. This may yet be another reason as to why the observed experimental yields of hydrogen and oxygen are so low. Back and Side Reactions: Effect of Light Intensity. Although the factors mentioned above are applicable to all photochemical and photocatalytic processes, the quantum yield of some photocatalyzed reactions, such as the photodegradation of organic substrates, can be around 0.10−0.20 and as high as unity,68,73 while the quantum yield of water splitting is a few orders of magnitude lower. The reason for this significant difference stems from the fact that the photodegradation of 941


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ca. 10−5−10−7 s (i.e., before the generation of the second hydrogen atom on the particle surface) without the advent of back or side reactions such as reactions 14 and 15. That is certainly not the case. Obviously, the four-electron formation of molecular oxygen becomes even more problematic. Clearly, it is nearly impossible to expect water splitting to be an effective process taking place on a bare semiconductor particle surface irradiated at typical moderate light intensities.

organic molecules is typically a multielectron loss from a sequential process that involves the formation of intermediate products (without effective back reactions), whereas the effective formation of molecular hydrogen or oxygen from the water-splitting process requires parallel formation of primary intermediates such as H· atoms or ·OH radicals taking place via parallel multielectron transfer events. Thus, the photodecomposition of organic substrates (RH) occurs as per eq 12

Cocatalysts may be necessary components in semiconductor heterostructures for effective charge carrier separation and accumulation.

RH(aq) − e → Int1(aq) − e → Int 2(aq) − e → ... → Int i(aq) → ... → CO2 (g) + H 2O(l)

(12)

that is, via a multielectron sequential process for which the order does not typically depend on the temperature of photoexcitation. By contrast, the formation of molecular hydrogen is a parallel multielectron process (eq 13).

One possible way to increase the efficiency of parallel multielectron transfer events is modification of the photocatalyst surface with metals or semiconductor cocatalysts that would provide efficient charge separation and accumulation of charge owing to the Schottky barrier. However, construction of such heterostructures cannot prevent process 14, and to prevent the back reaction 15 from occurring they must be spatially well-ordered to separate reduction and oxidation halfreactions. In this regard, an interesting example was reported by Kudo and co-workers47 on water splitting yielding H2 and O2 over La-doped NaTaO3 photocatalysts with high crystallinity and surface nanostructure. Finally, charge accumulation in cocatalysts, which would favor parallel multielectron transfer, will also cause a decrease of the barrier thickness that in turn would favor back recombination through tunneling (compensation effect) and decrease the efficiency of charge separation and accumulation, thereby leading to a less efficient watersplitting process. Photostability of Photocatalytic Materials. We noted earlier that the photocatalyst must be physically and chemically stable during the photocatalyzed reaction. Too often, however, the efficiency of photocatalytic water splitting attains a maximal value for a few minutes and then suffers a sharp decrease owing to the decomposition of the semiconductor photocatalyst. The concept of photostability of semiconductor materials has been well-documented when used as electrodes in photoelectrochemical cells. The problem of semiconductor material decomposition through photocorrosion processes was reported by Maruska and Ghosh79 and subsequently taken up by Gerischer80 who noted that the photoinduced decomposition of photoelectrodes is a common phenomenon intrinsic to all semiconducting electrodes in photoelectrochemical cells. This represents a serious obstacle in the use of semiconductor photoelectrodes in liquid junction solar cells. Similar photoinduced decomposition of photocatalysts can also occur in semiconductor-based photocatalytic processes.

Accordingly, hydrogen formation can proceed effectively only if there are an excess number of electrons to compete with a back reaction such as reaction 14, i.e., back electron transfer (trapping) from the hydrogen atom H· to the conduction band of the metal-oxide photocatalyst MOx,76−78 or with the side reaction (eq 15) which represents an external electron−hole (radical) recombination process. H·(surf) + MOx (s) → H+(surf) + e(MOx )

(14)

Both processes (14 and 15) can proceed with much greater efficiency than process 13. Indeed, as demonstrated by Emeline and co-workers77 and by Andreev et al.78, the limiting step for process 14 is generation of atomic hydrogen, meaning that back electron transfer is a much faster process. It is worth noting that process 15 is a single-photon process involving a single electron−hole pair, while process 13 is a two-photon process requiring the generation of two electrons. Therefore, at moderate light intensities, back reactions will be more efficient compared to the production of hydrogen; the same is true for the oxygen evolution during water splitting. Light intensities of sunlight or artificial light used in most laboratories are typically in the range ρ = 10 15 −10 17 photon s−1 cm−2. Taking the absorption coefficient to be as high as α = 105 cm−1 and the lifetimes of the charge carriers in a photocatalyst to be as long as τ = 1−10 ns, we estimate the average concentration of photogenerated charge carriers (n = αρτ) to be in the range of 1011−1014 cm−3. For a single nanoparticle with an average size ca. 10 nm, this means that there will be, on average, 10−10−10−7 charge carriers per particle. In other words, provided that all the incident light is absorbed by the photocatalyst particle with a cross section of ∼10−10 cm2, the time period between two sequential photons is then ca. 10−5−10−7 s. Thus, there will be only a single or nonphotogenerated electron−hole pair per particle between absorption of two sequential photons. In turn, that would make practically impossible an effective parallel multielectron transfer process. Moreover, to be more effective, the formation of molecular hydrogen requires that the hydrogen atom generated in the first single-electron transfer process should live at least

The photostability of semiconductor photoelectrodes and photocatalysts and back reactions are two nonnegligible factors in water splitting. Detailed mechanisms of dissolution of solid crystals are very complex even under dark conditions.81,82 The decomposition of semiconductor photocatalysts or photoelectrodes in contact with aqueous solutions depends on the type of reactive species 942


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Figure 16. Cartoons illustrating the increasing photostability of the (a) CdS semiconductor upon (b) loading the particle surface with TiO2 and (c) subsequently with NiOx. Adapted from ref 90. Copyright 2016 American Chemical Society.

where ηHER is the overpotential for the hydrogen evolution reaction; ηOER the overpotential for the oxygen evolution reaction; and IR the resistance loss through the solution, of great importance in photoelectrocatalytic cells. It is estimated that the required value of Ewater splitting cannot be less than 2.3− 2.4 V.33 Accordingly, the redox potential(s) of electrons (and/ or holes) required for water decomposition may be sufficient to cause the decomposition of the semiconductor. In this case, the kinetics of decomposition may prevent the decomposition of a metal-oxide photocatalyst, particularly in aqueous media; for example, the anodic decomposition of TiO2 is favored thermodynamically under illumination at λ < 390 nm but is minimized or otherwise suppressed kinetically. The problem of semiconductor redox decomposition can be resolved in various ways, keeping in mind, however, that there are no semiconducting materials that are absolutely photostable. Nonetheless, it is possible to synthesize a material which is stable toward photoanodic and/or phocathodic decomposition and thus used as a photocatalyst or as a photoelectrode. Second, a protective coating can be applied onto semiconductor materials (Figure 16).90 Such coating or coatings must, however, satisfy some important conditions: (i) it must be highly conductive electrically, (ii) must be transparent and possess a low reflectance toward the working wavelength range, (iii) must be chemically inert relative to the semiconductor protected, and (iv) the coating material must be an effective catalyst for the oxygen (or hydrogen) evolution reaction. Third, a sacrificial redox pair may be added to the solution that would scavenge one of the photogenerated charge carriers responsible for the decomposition. Thin-film materials for the protection of semiconducting photoelectrodes in solar-fuel generators have been reviewed recently by Hu and co-workers.91 Protection of inorganic semiconductors for sustained efficient photoelectrochemical water oxidation has also been considered by Lichterman et al.92 Among the several materials that have proven useful as protective coatings is NiOx, an optically transparent oxide in the visible region and one that provides nearly optimal antireflective properties on a variety of semiconductor surfaces, for example, on Si, InP, amorphous hydrogenated Si (α-Si:H), and CdTe. Nickel oxide is chemically stable at high pH values and upon activation forms a surface layer that is catalytic toward the oxygen evolution reaction.93,94 Under anodic operation in 1.0 M aqueous KOH (pH 14) in simulated sunlight, NiOx films stabilized all of these high-efficiency semiconducting photoelectrodes for over 100 h of sustained,

present in the electrolytes. Under light irradiation, oxidative decomposition of photoelectrodes is caused by reactions with photogenerated holes and reductive decomposition by photogenerated electrons;82 the combination of both processes defines the electrochemical redox mechanism for photocorrosion. Acidic and alkaline solutions can also decompose semiconductors (e.g., CdS, CdSe) by an acid−base mechanism. Complexing agents can also decompose solids, even those metal oxides that are very stable under other conditions (e.g., Ta2O5).84 In this regard, Scaife85 showed that the photoanodic decomposition potential of inorganic oxides (see below) depends on the solution pH; e.g., in the case of Fe2O3, the potentials varied within a wide range from 0.55 to 1.7 V (vs SHE) at various pHs. In his pioneering work, Gerischer80,83,86−88 established that all semiconducting materials in contact with electrolyte solutions undergo some form of decomposition through a redox mechanism and emphasized that in such processes the electronic energy states at the surface of the semiconductor play a primary role because absorption of light by the semiconductor photoelectrodes modifies the electrode surface. The criteria for the photostability of semiconductor photoelectrodes or photocatalysts have been developed using a quasithermodynamic approach80,88,89 that involves standard decomposition potentials to describe reversible reactions of anodic and cathodic decomposition reactions. When the decomposition potentials of a semiconductor are known, it is possible to predict its stability from the free energy, that is, from the redox potentials of photogenerated electrons and holes in the material. If the redox potential of photoholes is more positive than the anodic decomposition potential, then decomposition will occur, otherwise it will not. If the redox potential of photoelectrons is more negative than the cathodic decomposition potential of the electrode, then decomposition becomes possible. These thermodynamic conditions for decomposition, however, do not imply that decomposition will actually occur when these limits are exceeded because the corresponding kinetics of decomposition will dictate the factual fate of the semiconductor materials. Although only a potential of 1.23 V (Ewater splitting) is required to overcome the thermodynamic barrier for the water-splitting process, in practice higher potentials (eq 16) are necessary to overcome solution losses and kinetic barriers: Ewater splitting = 1.23 V + ηHER + ηOER + IR

(16) 943


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generation of carriers in the whole bulk of the solid and the spatial nonuniformity of photogeneration of carriers in that part of the bulk where diffusion is the only pathway for migration of the charge carriers; (xii) the concentration of surface-active sites (defects) or concentration of surface-adsorbed molecules; (xiii) the ratio between surface recombination and bulk recombination of the charge carriers; and (xiv) the ability of charge carriers photogenerated in that part of the solid where charge carriers reach that part of the solid bulk subjected to the near-surface electric field through diffusion. Domen and co-workers19 have identified other factors such as (a) the need for a decrease in the density of defect states in the photocatalyst; (b) a decrease in the (n-type) semiconductivity of the photocatalyst; (c) the enhancement of the kinetics of the oxygen evolution reaction, likely through a suitable choice of cocatalysts with low overpotentials as described by Gerischer;33 and (d) the generation of an asymmetric electric field to enhance charge carrier separation. Acar et al.,51 on the other hand, identified surface morphology, crystal structure, and particle size as some of the key factors that affect the photocatalyst activity in the water-splitting process under solar irradiation, because they influence the surface energy and chemisorption properties of the particulates that impinge on redox reactions on the photocatalyst surface, on interfacial electron transfer, and on resistance to photocorrosion. To summarize, the photogeneration of charge carriers, charge carrier separation (especially at the particle surface; an important step in the conversion of photons into chemical energy), charge carrier trapping, diffusion and drift of charge carriers toward the surface, the rate of charge carrier recombination and their lifetimes that govern the stationary concentration of charge carriers at the surface, together with the effect of photocatalyst particle size on charge carrier recombination are some of the factors that impact significantly on the efficiency (Φhν = fn{β, kr, S, D, d, s, τ, αhν}) and selectivity (Si) of interfacial chemical reactions. Moreover, the surface charge (surface potential) and the corresponding electric field in the subsurface space charge region of the solid are not to be neglected because they favor the drift of a given charge carrier (e.g., electrons) to the surface and prevent the appearance of the other carrier (e.g., holes) on that same surface. The electric field is one of the key factors that affects the relative surface concentration of electrons and holes ([es]/ [hs]), which depends on the ratio of the respective diffusion lengths (Le/Lh) at the edge of the intrinsic absorption band where the absorption coefficient changes by a few orders of magnitude. In the absence of an electric field, however, migration of charge carriers toward the surface of the photocatalyst occurs only through diffusion, so that the spectral selectivity of the photocatalyst (and hence the yields of products) depends on the differences in mobilities and lifetimes of electrons and holes and thus in the diffusion lengths of the charge carriers. Not least are the nature and integrity of the photocatalyst surface (e.g., as may be altered by chemical etching) and the likelihood that the products from the redox reaction (e.g., hydrogen and oxygen from water splitting) undergo a back reaction. Clearly, numerous factors impinge on the overall efficiencies of processes taking place on the surface of a photocatalyst, not least of which is its photostability. Accordingly, the absence of being able to control and manage these various factors presents a challenging, if not an impossible task to achieve significant

quantitative solar-driven oxidation of water to molecular oxygen.94 Titanium dioxide thin films deposited by atomic layer deposition (ALD) have also been used as coatings on such n-type semiconductors as Si, GaAs, GaP, and CdTe. Addition of Ni in the form of a thin film or deposits on a TiO2 particle surface produced efficient, stable photoanodes for water oxidation at pH 14.95 Promising experiments with TiO2 coatings were also implemented for CdS, a visible-light photocatalytic semiconductor often used to produce hydrogen from aqueous media; in naked form, however, CdS is chemically unstable and is easily photocorroded under illumination. As witnessed in Figure 16, loading the rather unstable and low-photoactive CdS semiconductor with a TiO2 coating resulted in increased photostability, and subsequent deposition of NiOx yielded a more photoactive and more stable photocatalytic material toward the photodecomposition of water to produce hydrogen (but no oxygen) from aqueous sulfide/sulphite (S2−/SO32−) media.90 Concluding Remarks. This Review has identified and described some of the factors that impact and may be responsible for the disappointingly low yields of hydrogen and oxygen from the photocatalyzed water-splitting process and, for that matter, any redox product(s) from reactions taking place on the surface of semiconductor photocatalysts. We have argued that the yields of redox products produced in a photocatalyzed process depend on the photocatalytic activity of photocatalysts, described as the number of entities (product) formed in a photocatalytic process divided by the number of photons absorbed by the photocatalyst at a single wavelength or within a well-defined wavelength range.49 This description defines the quantum yield (Φ) of a redox process occurring at the surface of a photocatalyst. That is, the factors that affect the quantum yield are the same as those that affect the chemical yields of hydrogen and oxygen in the water-splitting process. These factors have been identified in part by solving the continuity equation;69,74 they are, among others, (i) the extent of light absorption by the photocatalyst as portrayed by the absorption coefficient; (ii) the probability of charge carrier generation caused by absorption of a single photon given by the quantum yield of internal photoeffects; (iii) the depth of light penetration into the bulk of the photocatalyst material and the distance from the surface where the electric field plays a significant role, and thus the degree of the spatial nonuniformity of the photogeneration of carriers in the bulk subjected to the near-surface electric field; (iv) the presence of an electric field and the corresponding depth of the space charge region in the semiconductor particles; (v) the diffusion length of the charge carriers that depends on the diffusion coefficient and on the lifetimes of the charge carriers, {L = (Dτ)1/2}; (vi) the kinetics of charge transfer to (or from) adsorbed molecules and/or charge carrier trapping by surface defects to form surface-active centers; (vii) the rate of recombination of charge carriers; (viii) the mobility of charge carriers; (ix) the size of the photocatalyst’s particles; (x) the magnitude of the surface charge potential (Us; i.e., the surface charge) which causes significant changes in the selectivity of a photocatalyst owing to the reconstruction of the surface that occurs during heating or other prior physical treatments of the photocatalysts, and/or from specific adsorption of ions on the surface (or present in the bulk as dopants) of the photocatalysts, and/or from the variation in the pH of the dispersion, as well as variations of the surface structure of the photocatalyst resulting from different synthetic methods; (xi) the spatial nonuniformity of photo944


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practical evolution of hydrogen and oxygen from the watersplitting process. On a positive note, perhaps more innovative designs of photocatalytic materials and related strategies, together with the aid of theoretical and computational models to further our understanding of the electronic density of states and the band structure of semiconductor photocatalysts43 might make it be possible to achieve significant efficiencies in the sought-after production of solar fuels, whether through water splitting or through the photoconversion of carbon dioxide to, for example, methanol and methane. The search for the Holy Grail photocatalyst continues.

Review

APPENDIX

A complex equation describing the quantum yield (Φ) of a process occurring on a semiconductor surface was developed on solving the continuity equation.74 Clearly, Φ is a complex function of several parameters such as the absorption coefficient (α), electric field (E), diffusion length (L), depth of the subsurface electric field (δ), and their various combinations. These parameters characterize the physical behavior of solids under photoexcitation, and the parameters are defined in the following list.

• Φ: quantum yield of surface photochemical reaction • kr: rate constant of either charge transfer to (or from) adsorbed molecules or charge carrier trapping by surface defects to form surface-active centers • χ: quantum yield of internal photoeffects (i.e., probability of charge carrier generation caused by absorption of single photon) • α: absorption coefficient • E: electric field • L: diffusion length of carriers = (Dτ)1/2 = 1/β • τ: lifetime of charge carriers • δ: depth of subsurface electric field, i.e., depth of space charge region • s: rate of recombination of charge carriers • d: half-thickness of plate in one-dimensional model • S: concentration of surface-active defects or of adsorbed molecules • D: diffusion coefficient • μ: mobility of charge carriers • Us: surface charge potential • αL: ratio between part of the solid bulk where photogeneration of charge carriers occurs and part of the bulk from which charge carriers reach the surface by diffusion; also reflects part of charge carriers (generated in the bulk within distance 1/α) that appear on the surface because of diffusion • αε: relationship between bulk region at which photocarriers are generated and region of bulk from which charge carriers migrate to surface because of diffusion and drift • αδ: correlates depth of light penetration into bulk of solid and distance from surface where electric field plays a most significant role; also points to degree of spatial nonuniformity of photogeneration of carriers in bulk of crystal subjected to near-surface electric field • αd: spatial nonuniformity of photogeneration of carriers in whole bulk of solid • α(d − δ): spatial nonuniformity of photogeneration of carriers in part of the bulk where diffusion is the only pathway for migration of carriers

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• ε, β, and λ1,2: characterize migration of carriers in the bulk and reflect relation between efficiency of diffusion and drift migration. • β(d − δ): ability of charge carriers photogenerated in part of solid where diffusion is the only migration pathway of carriers to reach part of the solid bulk subjected to near-surface electric field • (λ1,2 − s/D): proportional to ratio between surface recombination and bulk recombination of charge carriers • d/L: in one-dimensional model, scales with ratio between volume of bulk crystal and volume of bulk space near surface from which photogenerated charge carriers reach surface through diffusion

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel./fax: (+1) 514-489-9551. Notes

The authors declare no competing financial interest. Biographies Nick Serpone, currently Visiting Professor at University of Pavia (Italy), is a Fellow of the European Academy of Sciences and Head of its Materials Science Division. He has published over 400 articles in photochemistry, heterogeneous photocatalysis, and microwave chemistry and has contributed several chapters and coedited and/or coauthored several books. Alexei V. Emeline is Professor of Physics and vice-Director of the Resource Center “Nanophotonics” at Saint-Petersburg State University. His research interests are in experimental and theoretical studies of factors affecting activity and selectivity of photocatalysts, spectral sensitization of photoactive materials, superhydrophilic properties of metal oxides, self-cleaning and bactericidal photoactive coatings, and photoluminescent chemical sensors. Vladimir K. Ryabchuk is currently Professor of Physics in the Department of Photonics at Saint-Petersburg State University. His research interests are in photophysics and photochemistry of gas/solid interfaces, heterogeneous photocatalysis, and photostimulated processes in solids. He has published over 100 articles and has coauthored three textbooks. 945


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(13) Maeda, K. Photocatalytic water splitting using semiconductor particles: History and recent developments Photocatalytic water splitting using semiconductor particles: History and recent developments. J. Photochem. Photobiol., C 2011, 12, 237−268. (14) Nakata, K.; Ochiai, T.; Murakami, T.; Fujishima, A. Photoenergy conversion with TiO2 photocatalysis: New materials and recent applications. Electrochim. Acta 2012, 84, 103−111. (15) Liao, C. H.; Huang, C. W.; Wu, J. C. S. Hydrogen production from semiconductor-based photocatalysis via water splitting. Catalysts 2012, 2, 490−516. (16) Hu, Y. H. A highly efficient photocatalyst - hydrogenated black TiO2 for the photo-catalytic splitting of water. Angew. Chem., Int. Ed. 2012, 51, 12410−12412. (17) Li, Y.; Liu, H.; Wang, C. Nanostructured sulfides: synthesis and applications in hydrogen generation nanostructured sulfides: synthesis and applications in hydrogen generation. Curr. Inorg. Chem. 2012, 2, 168−183. (18) Takanabe, K.; Domen, K. Preparation of inorganic photocatalytic materials for overall water splitting. ChemCatChem 2012, 4, 1485−1497. (19) Hisatomi, T.; Minegishi, T.; Domen, K. Kinetic assessment and numerical modeling of photocatalytic water splitting toward efficient solar hydrogen production. Bull. Chem. Soc. Jpn. 2012, 85, 647−655. (20) Nguyen, V. H.; Nguyen, B. H. Visible light responsive titaniabased nanostructures for photocatalytic, photovoltaic and photoelectrochemical applications. Adv. Nat. Sci.: Nanosci. Nanotechnol. 2012, 3, 023001. (21) Smestad, G. P.; Steinfeld, A. Review: Photochemical and thermochemical production of solar fuels from H2O and CO2 using metal oxide catalysts. Ind. Eng. Chem. Res. 2012, 51, 11828−11840. (22) Tran, P. D.; Wong, L. H.; Barber, J.; Loo, J. S. C. Recent advances in hybrid photo-catalysts for solar fuel production. Energy Environ. Sci. 2012, 5, 5902−5918. (23) Maeda, K. Z-scheme water splitting using two different semiconductor photocatalysts. ACS Catal. 2013, 3, 1486−1503. (24) Xing, J.; Fang, W. Q.; Zhao, H. J.; Yang, H. G. Inorganic photocatalysts for overall water splitting. Chem. - Asian J. 2012, 7, 642−657. (25) Navarro Yerga, R. M.; Á lvarez Galván, M. C.; del Valle, F.; Villoria de la Mano, V.; Fierro, F. L. G. Water splitting on semiconductor catalysts under visible-light irradiation. ChemSusChem 2009, 2, 471−485. (26) Leung, D. Y. C.; Fu, X.; Wang, C.; Ni, M.; Leung, M. K. H.; Wang, X.; Fu, X. Hydrogen production over titania-based photocatalysts. ChemSusChem 2010, 3, 681−694. (27) Hisatomi, T.; Maeda, K.; Lu, D.; Domen, K. The effects of starting Materials in the synthesis of (Ga1−xZnx)(N1−xOx) solid solution on its photocatalytic activity for overall water splitting under visible light. ChemSusChem 2009, 2, 336−343. (28) Maeda, K.; Sakamoto, N.; Ikeda, T.; Ohtsuka, H.; Xiong, A.; Lu, D.; Kanehara, M.; Teranishi, T.; Domen, K. Preparation of core−shellstructured nanoparticles (with a noble-metal or metal oxide core and a chromia shell) and their application in water splitting by means of visible light. Chem. - Eur. J. 2010, 16, 7750−7759. (29) Protti, S.; Albini, A.; Serpone, N. Photocatalytic generation of solar fuels from the reduction of H2O and CO2: a look at the patent literature. Phys. Chem. Chem. Phys. 2014, 16, 19790−19827. (30) Maeda, K. Photocatalytic properties of rutile TiO2 powder for overall water splitting. Catal. Sci. Technol. 2014, 4, 1949−1953. (31) Maeda, K. Direct splitting of pure water into hydrogen and oxygen using rutile titania powder as a photocatalyst. Chem. Commun. 2013, 49, 8404−8406. (32) Li, X.; Yu, J.; Low, J.; Fang, Y.; Xiao, J.; Chen, X. Engineering heterogeneous semiconductors for solar water splitting. J. Mater. Chem. A 2015, 3, 2485−2534. (33) Gerischer, H. Semiconductor electrodes and the interaction with light. In Photoelectrochemistry, Photocatalysis, and Photoreractors − Fundamentals and Developments; Schiavello, M., Ed.; NATO ASI

Vyacheslav N. Kuznetsov obtained his Ph.D. in Molecular Physics from Leningrad State University in 1986. He is currently a senior scientific researcher at Saint-Petersburg State University. He has coauthored over 50 articles and four textbooks devoted to photostimulated physical and chemical processes in photoactive metal oxides. Yurii M. Artem’ev is Associate Professor in the Department of Physical Chemistry and a researcher in the Laboratory of Photoactive Nanocomposite Materials of Saint-Petersburg State University. He obtained his Ph.D. in Physical Chemistry from Leningrad (SaintPetersburg) State University in 1984; his research interests have encompassed the photodissociation of water. Satoshi Horikoshi is currently Associate Professor at Sophia University (Tokyo). His research interests involve new material synthesis, novel plant cultivation, formation of sustainable hydrogen energy, and environmental protection using microwave- and/or photoenergy. He has coauthored over 170 scientific publications and has contributed to and edited or coedited 20 books.

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ACKNOWLEDGMENTS N.S. is grateful to Prof. Angelo Albini for his gracious continued hospitality at the PhotoGreen Laboratory of the University of Pavia (Italy). We are also grateful to the Russian Federation for a grant (No. 14.Z50.31.0016) in support of the project “Establishment of the Laboratory of Photoactive Nanocomposite Materials” and to Saint-Petersburg State University for support within the Fundamental Project Support Program (Grant No. 11.38.207.2014).

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ACS Energy Letters

Review

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