Hydrogen Production over TitaniaBased Photocatalysts

DOI: 10.1002/cssc.201000014

Hydrogen Production over Titania-Based Photocatalysts Dennis Y. C. Leung,[a] Xianliang Fu,*[a] Cuifang Wang,[a] Meng Ni,[a] Michael K. H. Leung,[a] Xuxu Wang,[b] and Xianzhi Fu[b] Because of their relatively high efficiency, high photostability, abundance, low cost, and nontoxic qualities, titania-based photocatalysts are still the most extensively studied materials for the photocatalytic production of hydrogen from water. The effects of the chemical and physical properties of titania, including crystal phase, crystallinity, particle size, and surface area, on its photoactivity towards hydrogen generation have been identified by various investigations. The high overpotential for hydrogen generation, rapid recombination of photogenerated electrons and holes, rapid reverse reaction of molecular hydro-

gen and oxygen, and inability to absorb visible light are considered the most important factors that restrict the photoactivity of titania, and strategies to overcome these barriers have been developed. These issues and strategies are carefully reviewed and summarized in this Minireview. We aim to provide a critical, up-to-date overview of the development of titaniabased photocatalysts for hydrogen production, as well as a comprehensive background source and guide for future research.

1. Introduction Although the hydrogen economy is not a new idea,[1] the depletion of fossil fuel reserves and the environmental harm caused by their overconsumption makes hydrogen more and more appealing. A future energy system based on hydrogen has been proposed as an ideal solution to these two problems. Currently, about 95 % of all hydrogen produced is derived from natural gas through steam–methane reforming [SMR; Equations (1) and (2)], and only about 5 % of hydrogen is produced from renewable resources, mainly via water electrolysis.[2] CH4 þ H2 O ! CO þ 3 H2

ð1Þ

CO þ H2 O ! CO2 þ H2

ð2Þ

Producing hydrogen from natural gas does not help to solve energy-related problems because the supply of fossil fuels is limited and because the SMR process produces massive amounts of carbon dioxide; a main contributor to global warming. The development of new methods to produce hydrogen from sustainable materials, such as biomass and water, will become a hot topic of research in the coming decades.[3–6] Starting from biomass, these methods include (1) hydrocarbon reforming processes, such as steam reforming,[7] catalytic partial oxidation,[8] and autothermal reforming;[6, 9] (2) gasification process, such as thermal pyrolysis[10] and supercritical water gasification;[11] and (3) biological conversion, such as direct photolysis[12] and dark fermentation.[13] Starting from water, the approaches include (1) electrolysis,[14, 15] (2) thermochemical water splitting,[16] and (3) photoelectrolysis and photocatalyzed water splitting.[17–19] These methods and technologies have been described in a series of excellent Reviews (see Refs. [5] and [14, 18–21] for biomass and water, respectively). Among these methods, the photocatalytic production of hydrogen from water is the most attractive and rewarding work because water is abundant and renewable, and because the process ChemSusChem 2010, 3, 681 – 694

can occur at ambient conditions using only sunlight and a semiconductor photocatalyst.[22] Since the pioneering work of Honda and Fujishima on the decomposition of H2O into H2 and O2 with a photoelectrochemical cell comprising Pt and TiO2 electrodes under a small electric bias,[23] H2 production using photocatalysis has been extensively studied and many photocatalysts have been developed. Over 130 semiconductor materials,[18, 21] including oxides, sulfides, nitrides, and hydroxides, have been shown to be able to split water into hydrogen. In terms of quantum efficiency (QE) the record holder is NiO/NaTaO3, with a QE of 56 %.[24] Unfortunately, the material can only be activated under deep-UV irradiation, below 270 nm, and gradually deactivates with increasing reaction time.[25] Significant progress with the visiblelight photocatalyst Cr/Rh(1x)GaN:x ZnO has recently been reported by the Domen group, with a QE of 5.9 %.[26] However, as a metal nitride this material suffers from poor photostability and only works properly in an acid solution .[3, 27] Even so, it may also undergo decomposition and then deactivation with prolonged reaction time. Compared to some photocatalysts, the photoactivity of TiO2 is still low. However, in essence TiO2 is the most promising material for hydrogen generation because it is photostable, environmentally friendly, cheap, and readily available. TiO2-based

[a] Prof. D. Y. C. Leung, Dr. X. L. Fu, C. F. Wang, Dr. M. Ni, Dr. M. K. H. Leung Department of Mechanical Engineering The University of Hong Kong Pokfulam Road, Hong Kong (PR China) Fax: + 852 28585415 E-mail: [email protected] [b] Prof. X. X. Wang, Prof. X. Z. Fu Photocatalytic Research Institute Fuzhou University Fuzhou 350002 (PR China)

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X. L. Fu et al. photocatalysts are still the most extensively studied materials for the photocatalytic production of hydrogen from water. Herein we briefly summarize studies on TiO2-based materials, especially the recent progress. In the following sections, the mechanism of photocatalytic water splitting, the effects of crystal structure and the chemical and physical properties of TiO2 on hydrogen production, the drawbacks of TiO2, and the strategies to overcome these barriers and improve the photoactivity will be reviewed and discussed. Our goal is to give comprehensive background information and provide a guide for future research.

2. Mechanism of Photocatalytic Water Splitting Equation (3) in Figure 1 demonstrates that overall, the watersplitting reaction is thermodynamically “uphill,” with a Gibbs free energy change of 237.2 kJ mol1. This means that during

Figure 1. Basic principle of the overall water-splitting reaction on a semiconductor photocatalyst.

the photocatalytic water-splitting process photon energy is converted into chemical energy via the photocatalyst (i.e., the semiconductor material). Unlike conductors, which have a continuum of electronic states, semiconductors possess a void region in which no energy levels are available. As shown in Figure 1, this region, the so-called band gap (Eg), is located between the highest occupied energy band, called valence band (VB), and the lowest empty band, called conduction band (CB). When the energy of an irradiated photon is larger than Eg, electrons (e) can be excited from the VB into the CB, leaving holes (h + ) in the VB. Whether or not the water-splitting reaction then occurs is strongly determined by the electronic band structure of the photocatalyst. For water reduction, the CB potential must be more negative than the H + reduction potential of H + /H2 [0.0 V vs. the normal hydrogen electrode (NHE); pH 0]. To facilitate the oxidation of water by h + , the VB edge must be more positive than the oxidation potential of O2/H2O (1.23 V vs. NHE; pH 0). Under these conditions the theoretical minimal band gap for water splitting is 1.23 eV, which corresponds to a wavelength of about 1010 nm. Hence, about 70 % of solar photons can theoretically be used for water splitting.[28] However, in practice, considering energy losses during the conversion process, the minimal values are as high as 2.0–2.2 eV.[29]

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The generation of e and h + is just the beginning of the photocatalytic reaction. The steps that follow are charge separation (e and h + ), migration from the bulk to the surface, and the actual water-splitting reaction on the surface. The chargetransport efficiency is strongly affected by the crystal structure, crystallinity, and particle size of the sample. A highly crystalline sample that contains only a small quantity of defects can suppress the recombination of e and h + and, consequently, increase their survival time. Small particle sizes mean that the charges can quickly transfer to the surface after a small migration distance, which can also decrease the recombination probability. The Eg and other physical and chemical properties may also change when decreasing the particle size, and/or by varying the nanostructured particle’s shape. These efforts have been summarized in an excellent Review by Chen and Mao.[19] The final surface reaction, that is, the water-splitting reaction, depends on the photostability of the sample’s surface and the quantity of active sites at which the reaction can proceed. The surface must be strong enough to withstand attack by the photoinduced charges and also provides enough active sites for hydrogen generation. In conclusion, not only the right electronic band structure but also the right bulk and surface properties are required for a photocatalyst to show activity for water splitting. As for TiO2, the influence of these properties on the photoactivity towards hydrogen production will be discussed in the next sections. It should be emphasized that the term “water splitting” as it appears in the following sections does not always refer to the overall water-splitting reaction (i.e., evolution of both H2 and O2). Sometimes it may refer to only half of the water-splitting reaction (mostly H2 evolution) because sacrificial reagents (electron donors) are added to the reaction system to deplete the photogenerated h + .

3. Crystal Structure and Properties of TiO2 TiO2 was the first material to serve as photochemical watersplitting catalyst.[30] Although it can crystallize in three different structures (anatase, rutile, and brookite), the anatase and rutile phases have most often been used as photocatalysts for hydrogen production.[31] For bulk TiO2, the band gaps of anatase and rutile are 3.2 and 3.0 eV,[32] corresponding to absorption thresholds at 390 and 415 nm, respectively. Thus, the rutile form can absorb a more extensive range of light and should, theoretically, show more photoactivity for hydrogen generation, but this is not the case. In most investigations anatase has been found to be more active than rutile.[22, 33, 34] This activity difference can be explained by considering their different electronic band structures. The location of the VBs (mainly originating from O 2p) of these two phases is almost the same (situated at ca. 3.0 V),[32] but their CBs (mainly comprising Ti 3d) are slightly different (Figure 2). The CB potential of rutile coincides almost with the NHE potential, whereas that of anatase is shifted cathodically by almost 0.2 V. Hence, the driving force for water reduction is very small for rutile while the reduction takes place more easily (relatively) in the anatase form. In addition, when comparing rutile to anatase TiO2 the


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Hydrogen Production over Titania-Based Photocatalysts such as modifying TiO2 with noble metals, doping TiO2 with other ions, coupling with other semiconductors, sensitizing with dyes, and adding sacrificial reagents to the reaction solution. Section 5 will be devoted to addressing these strategies

5. Strategies to Overcome the Barriers 5.1 Decreasing the overpotential for hydrogen evolution

Figure 2. Band structures of anatase and rutile TiO2.

poor light absorption capability of rutile near the UV region may also decrease the photocatalytic activity for water splitting,[22, 34] as may its lower mobility for photogenerated carriers.[35] Therefore, to obtain high-activity TiO2 it is crucial to control the fabrication conditions in such a manner that these lead to the anatase phase, without formation of the rutile phase. This can be achieved by controlling the calcination temperature of the sol–gel process[36] and the hydrothermal or solvothermal reaction conditions such as the reaction temperature[37] and the solution pH.[38] Furthermore, many studies have indicated that anatase TiO2 with a higher crystallinity, smaller particle size, and larger surface area is favorable for hydrogen production.[22, 36, 39, 40] Among these factors, the hydrogen evolution activity is more susceptible to the effects of the crystallinity and crystal structure than surface area. As a whole, the photocatalytic activity of TiO2 for hydrogen evolution can be substantially improved by proper selection of the structure, the crystallinity, the particle size, and the surface area of TiO2.

4. Drawbacks of TiO2 for Photocatalytic Hydrogen Evolution Although the photoactivity of TiO2 can be enhanced by optimizing its crystal structure and chemical and physical properties, there are still four main drawbacks for photocatalytic water splitting. (1) The large overpotential required for the evolution of H2 and O2, making TiO2 alone inactive for hydrogen generation.[41] (2) The too-rapid recombination rate of photoinduced e and h + before migrating to the surface to split water. Time-resolved spectroscopy has indicated that about 90 % of photogenerated carriers recombine after excitation.[42] (3) The fast thermal back reaction to produce H2O from H2 and O2.[43] The decomposition of water into H2 and O2 is an energyincreasing process, and thus the backward reaction proceeds easily. (4) The inability to make use of visible light. The band gap of TiO2 is about 3.2 eV and only ca. 4–5 % of the solar light can be used for hydrogen production, while the other ca. 40 %[28] of visible light (400 < l < 700 nm) can not be utilized. These drawbacks can not be overcome by only optimizing the material itself as described in Section 3. Various other strategies to cope with these problems have also been developed, ChemSusChem 2010, 3, 681 – 694

Owing to a large overpotential for the evolution of H2 on the surface of TiO2, the material alone becomes inactive. Usually, this problem can be solved by loading TiO2 with co-catalysts of noble metals, such as Pt, Pd, Au, Rh, and Ag. Loading these metals, which have a low overpotential for hydrogen, can make it easier to generate hydrogen. The metals can thus serve as active sites for hydrogen production on the TiO2 surface (as shown in Figure 3). The lower the overpotential of the

Figure 3. TiO2 loaded with Pt and RuO2, and the reaction process of photocatalytic water splitting.

metal, the higher the activity it shows.[44] Among all these metals, Pt is the most widely used because it has the lowest overpotential and commonly shows the highest activity for hydrogen generation.[45–47] Except for noble metals, NiO[48] and RuO2[49] have also been used to decrease the overpotential for hydrogen and oxygen evolution, respectively. Owing to the high cost of these noble metals, it is imperative to develop inexpensive co-catalysts for replacement. Unfortunately, this work has been largely ignored. Recently, Zong et al.[50] found that the rate of H2 evolution on CdS is significantly enhanced by loading MoS2 as a co-catalyst. The activity was even better than a Pt-loaded sample. Although the photocatalyst used here was CdS, the co-catalyst may also be applicable to TiO2. Some other materials, such as the alloys of Ni and Mo,[51] Mo2C,[52] MoS2,[53] and W2C[54] that have either shown a low overpotential for hydrogen evolution or demonstrated high activities in heterogeneous catalysis reactions involving H2, can also be used for this purpose and require further investigation for verification.


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X. L. Fu et al. 5.2 Suppressing the recombination of photoinduced carriers The recombination of photoinduced e and h + can occur during the following two processes: charge transfer from the bulk to an active site, and interfacial charge transfer from an active sites to a water molecule. The first process occurs on a timescale of pico- to nanoseconds, while the second process occurs in micro- to milliseconds.[55] An increase of either the transportation rate from the bulk to the active surface or of the charges’ survival time on the surface is expected to result in higher activity for water splitting. The transportation rate can be enhanced by increasing the crystallinity of TiO2 or by loading TiO2 with noble metals. The survival time of the charges on the surface can be prolonged by coupling TiO2 with other semiconductors or by adding sacrificial reagents to the reaction solution. 5.2.1 Increasing the crystallinity of TiO2 Extensive works have revealed that the crystallinity of TiO2 plays a very significant role in the photocatalytic H2 production.[22, 36, 39, 56] Amorphous-phase TiO2, which usually comprises numerous defects such as impurities, dangling bonds, and microvoids that can behave as recombination centers for the photoinduced e and h + pairs, results in a decrease of the photocatalytic activity.[57] The crystallinity and crystal structure, rather than the surface area, chiefly govern the photocatalytic H2 evolution activity of a TiO2 photocatalyst, because a highsurface-area sample usually contains a large fraction of amorphous phase. Hence, by taking into account the balance of these two conflicting intrinsic properties (i.e., crystallinity and surface area) it is crucial to control the thermal treatment conditions leading to high crystallinity of anatase phase TiO2 without formation of the rutile phase. It is preferred that anatase TiO2 can be synthesized with a high surface area, on the premise of high crystallinity. 5.2.2 Loading TiO2 with noble metals Besides decreasing the hydrogen evolution overpotential, loading TiO2 with noble metals can also suppress the recombination of e and h + . The work functions of the noble metals Pt, Pd, Au, Rh, Ru, and Ag are 5.65, 5.55, 5.10, 4.98, 4.71, and 4.64 eV, respectively; larger than the work function of TiO2 (4.2 eV).[58] When TiO2 is loaded with these metals, a Schottky barrier can be formed at the metal/TiO2 interface.[41] The barrier will drive electron migrate from bulk TiO2 to the noble metal until a thermodynamic equilibrium is reached, that is, until their Fermi levels (EF) are aligned. When the sample is further exposed to UV light, the photoinduced electrons can cause a shift of the EF of TiO2 to form a new quasi-Fermi level (EF*).[59] Meanwhile, the previous thermodynamic equilibrium state for electron transfer is destroyed, and consequently the e continuously migrate from the TiO2 to the noble metal and finally react with water. The photoinduced h + is then free to diffuse to the TiO2 surface. Femtosecond diffuse reflectance spectroscopy experiments have been used to investigate the dynamics

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of Pt-loaded TiO2 and found that charge separation is responsible for the enhanced photocatalytic activity of Pt-loaded TiO2.[60] Anpo et al.[61] have used electron spin resonance (ESR) signals to investigate the enhancement mechanism of Ptloaded TiO2, and found that the photogenerated electrons quickly transferred from TiO2 to Pt particles while the holes remained on the TiO2. Apparently, the Schottky barrier present at the noble metal/TiO2 interface decreases the recombination rate of electron–hole pairs. A noble metal with a larger work function results in a stronger Schottky barrier effect, and therefore shows a better activity for hydrogen evolution.[45, 47] This is another reason why Pt-loaded TiO2 always shows the highest activity. The loading method, loading amount, distribution, size, and chemical state of the metal(s) are all crucial factors in improving the photocatalytic activity of TiO2 for hydrogen production. Here, Pt-loaded TiO2 is taken as an example to address these issues because it is the most widely used metal for loading on TiO2. (a) Pt loading method. There are two traditional methods to deposit Pt on TiO2 : the impregnation method and the photodeposition method. The impregnation method includes several steps: impregnation of TiO2 with the Pt precursor solution, drying, and finally reduction. The reduction processes are usually performed at higher temperatures by H2 reduction[62, 63] or in suspended solution with NaBH4 as reducing agent.[47, 64] In the photodeposition method, the Pt precursors are reduced by photogenerated electrons and deposited on TiO2 in aqueous solutions.[47, 65–67] The reaction is performed in a deaerated solution, and methanol, alcohols, acetate, and formaldehyde are usually added to the solution to deplete the photoinduced holes.[68] A simpler way to use this method is to directly add the Pt precursors into the reaction system (called in situ photodeposition[45, 69]). In most cases the photodeposition method yields more-active TiO2. The disadvantages of these methods are that the impregnation method usually involves many steps and that it is not easy to control the Pt particle size and dispersion because of the high-temperature treatment, while the photodeposition method requires long-term, high-intensity UV irradiation. To further improve the activity of Pt/TiO2, many investigators have focused on modifying traditional methods or employing novel methods. Plasma-enhanced impregnation has been used to improve the dispersion of Pt, resulting in enhanced photoactivity for hydrogen generation.[63, 70] Xie et al.[71] have developed a novel and simple method to load noble metals on TiO2 in situ, through a redox reaction between the reductive TiIII oxide support and metal salt precursors. The metal was highly dispersed and its size could be well-controlled. Miznkoshi et al.[72] reported that Pt/TiO2 can be prepared via a sonochemical reduction method, with the assistance of prolonged sonication. The photocatalyst showed a higher photoactivity than one synthesized by the conventional impregnation method. (b) Pt loading amount. The hydrogen evolution rate could be significantly increased by increasing the Pt loading amount until reaching a maximum value, after which it gradually decreased with further increasing of the loading amount.[64, 73]


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Hydrogen Production over Titania-Based Photocatalysts This means that the loading amount has an optimal value, which was found to be ca. 1.0 %.[66, 73] Loading too much Pt (too many particles) will cause three problems. Firstly, too many Pt nanoclusters shield the photosensitive TiO2 surface, decreasing opportunity for the absorption of UV light by TiO2.[62, 65] Second, excessive numbers of Pt particles can deform the potential field of TiO2, drawing a part of holes near to the Pt/TiO2 junction, which increases the e and h + recombination rate.[62, 74] Third, overloading facilitates the thermal back reaction, producing H2O from H2 and O2.[43, 75, 76] (c) Pt size and dispersion. When the Pt content is fixed, theoretically speaking, the smaller the Pt size, the larger the number of active sites for hydrogen production. However, the problem is that the variation of Pt particle sizes is not apparent with small loading amounts. A TEM investigation has shown that the size of Pt particles remains almost the same over a range of 0.5–10 % Pt on a TiO2 surface.[77] Although the effect of the Pt particles size on the photoactivity of TiO2 has been largely overlooked, the importance of sufficient electrical contact between Pt and TiO2 for TiO2 loaded with smaller Pt particles has been recognized.[70, 78] Compared with the Pt size, more attention has been paid to the dispersion of Pt. A better Pt dispersion provides a higher metal coverage and a larger metal/support interface, which can produce the same effect as increasing the amount of metal.[70] Furthermore, well-dispersed Pt particles can promote the separation of e and h + and therefore show higher photoactivities.[71, 79] (d) Pt chemical state. For Pt-loaded TiO2, most studies assume that Pt exists as Pt metal (Pt0). In fact, Pt may exist as Pt0, PtII, PtIV, and ionic Pt under different preparation procedures and conditions.[80] A few works have been performed to clarify the state of the active component in photocatalytic degradation reactions, such as degradation of chlorinated organic compounds, carboxylic acid, alcohol, phenolics, and nitric oxide,[80–83] but no general conclusion could be drawn. Lee and Choi[81] found that the photocatalytic activity of Pt/TiO2 in degradation chlorinated organic compounds followed the order Pt0 > Pt(II,IV)Ox/TiO2 > bare TiO2. Wang et al.[82] found that only Pt that exists as PtOx can effectively separate e and h + , consequently improving the photocatalytic oxidation efficiency towards NO. For the water-splitting reaction, Pichat et al.[77] reported a lower photocatalytic activity towards H2 evolution from aliphatic alcohols over Pt0/P25 compared to Pt(II,IV)Ox/P25. Jang et al.[47] found that electron-deficient Pt favored the diffusion of photoelectrons, accounting for its higher photoactivity. No other works are available on this topic. A more careful and systematic study is needed to resolve the question of which chemical state of Pt is more favorable for hydrogen production. 5.2.3 Coupling TiO2 with other semiconductors Coupling TiO2 with other semiconductors (SCs) that possess different redox energy levels, for their corresponding conduction and valence bands, provides another attractive approach to achieving more efficient charge separation. When TiO2 and the SC are activated by light simultaneously (or only one of ChemSusChem 2010, 3, 681 – 694

Table 1. Band positions of selected semiconductors, vs. NHE.[103, 106] Group

Semiconductor

VCB [V]

VVB [V]

Eg [V]

– (a)

TiO2 Cu2O SiC CdS ZnO CuO Ta2O5 SnO2 V2 O 5 Bi2O3 WO3 Ru2S Fe2O3

0.29 1.13 1.1 0.52 0.31 0.2 0.17 0.0 0.2 0.33 0.74 0.19 0.28

2.91 1.07 1.9 1.88 2.89 1.5 3.83 3.5 3.0 3.13 3.44 2.21 2.48

3.2 2.2 3.0 2.4 3.2 1.7 4.0 3.5 2.8 2.8 2.7 2.4 2.2

(b)

(c)

them), photoinduced e would be injected from the semiconductor with a more negative CB level to the positive one, while h + would be transferred from the semiconductor with a more positive VB level to the negative one. Thus, a wide separation of photoinduced charges is achieved, which consequently enhances their lifetime as well as the efficiency of the interfacial charge transfer to water. There are many studies on TiO2 coupled with other semiconductors, such as CdS,[45, 47, 84–86] ZnO,[87] SiC,[88] Cu2O,[89] CuO,[90–92] SnO2,[93, 94] WO3,[95, 96] V2O5,[97] Bi2O3,[98] Fe2O3,[99] ZrO2,[100, 101] SiO2,[100, 102, 103] Ta2O5,[104] MoO3,[96, 105] and others. The band gap, CB, and VB band positions of these SCs are listed in Table 1.[103, 106] According to the relative position of the CB and VB, these composite photocatalysts can be divided into three groups, as shown in Figure 4. (a) CBSC < CBTiO2 and VBSC < VBTiO2. In this case, e injects from SC to TiO2 and h + injects from TiO2 to SC. (b) CBSC > CBTiO2 and VBSC > VBTiO2. Sc: SnO2, WO3, V2O5, Bi2O3 and Ta2O5. In this case, e injects from TiO2 to Sc and h + injects from SC to TiO2. (c) CBSC > CBTiO2 and VBSC < VBTiO2. In this case, e and h + both inject from TiO2 to SC. Among these semiconductors, only TiO2 coupled with Cu2O, SiC, CdS, ZnO, CuO, and Ta2O5 have the potential to evolve hydrogen from water because the e-occupied CB level is more negative than EH2/H2O = 0 V (vs. NHE, pH 0). Their capabilities have been verified by results reported on CdS/TiO2,[45, 47, 85, 107] and CuO/TiO2.[45, 91] Other composite photocatalysts are commonly used to degrade pollutants. It should be pointed out that the reason for the enhanced activity of composite photocatalysts is not only related to the wide separation of photoinduced charges, but also to some other factors such as the enhanced surface acidity[100, 108] or alkalinity.[108] This modification of the surface can promote the absorption of alkalic or acidic substrates to TiO2 and then result in improved photocatalytic activity. 5.2.4 Adding sacrificial reagents to the reaction solution Sacrificial reagents are electron acceptors or donors. Compared to water, they are more easily reduced or oxidized. As illustrat-


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Figure 5. Scheme of a) photocatalytic O2 evolution and b) photocatalytic H2 evolution, in the presence of sacrificial reagents.

Figure 4. a–c) The band structure of composite TiO2 photocatalysts and the migration of photogenerated carriers.

ed in Figure 5, when added to the reaction solution, they can react irreversibly with the photogenerated e (Figure 5 a) or h + (Figure 5 b) instead of water. Thus, the remaining h + or e have more opportunities to react with water. Direct combination of e and h + can be avoided. Electron acceptors such as Ag + , Fe3 + , and Ce4 + [109] are used to improve the O2 evolution rate or to confirm that the photo-

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catalyst has the ability to oxidize water to O2. Because there is no practical significance for hydrogen evolution using electron acceptors, no more details will be presented here. For hydrogen production, that is, e react with H + , the sacrificial reagents are electron donors. Organic compounds such as methanol, ethanol, lactic acid, formaldehyde, CN , and EDTA[33, 92, 94, 110–113] are widely used as electron donors, as are some biomass-derived carbohydrates.[64, 114] Adding these compounds has been proved to be an effective way of enhancing the hydrogen production rate. Different electron donors result in different improvements. The enhancement capabilities have been found to be: EDTA > methanol > ethanol > lactic acid.[115] For carbohydrates, the hydrogen evolution rate decreases with increasing molecular weight.[64] It should be noted that the decomposition of these organic compounds may also contribute to a higher hydrogen yield, because hydrogen is one of their decomposition products. Some other sulfide ions, such as S2 (as H2S or Na2S, K2S) and SO32 (as Na2SO3 or K2SO3) are also used as sacrificial reagents for hydrogen generation. They are commonly used in chalcogenide photocatalyst systems, such as CdS,[45, 116] ZnIn2S4,[117] CdIn2S4,[118] and ZnS,[119] because S2 can react with 2 h + to form S0. Therefore, the corrosion of chalcogenide itself can be suppressed. Added aqueous SO32 can dissolve S0 into S2O32 to prevent any detrimental deposition of S0 onto the photocatalyst surface. It can also be involved in trapping h + . Thus, when a composite of TiO2 and a chalcogenide photocatalyst is used, S2 and SO32 are commonly used as sacrificial reagents for hydrogen evolution.[45, 120] Because electron donors are consumed with hydrogen production, the continuous addition of electron donors is required to sustain that production. To overcome this drawback, a dual photocatalyst system employing a reversible redox couple (denoted as A and R) was developed (also called Z-Scheme system), as shown in Figure 6. The two photocatalysts have different band gaps and band positions: one (PC1) for O2 evolution and another (PC2) for H2 evolution. Because the H2 and O2 are formed in different photocatalysts, the back reaction of H2 and O2 can also be suppressed. The electron acceptor A is first reduced to R by the photogenerated electrons on PC1, and then the electron donor R is oxidized back into A by the photoinduced holes on PC2. In this system, photocatalytic water splitting produces both H2 and O2 without consumption of the sacrificial reagents (the redox couple). Under the mediation of


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Hydrogen Production over Titania-Based Photocatalysts

Figure 6. Diagram of a dual photocatalyst system employing a reversible redox couple.

I/IO3 pairs, the combination of anatase and rutile TiO2 has shown simultaneous H2 and O2 production rates of 180 and 90 mmol h1, respectively.[121] In addition to I/IO3 pairs,[84, 122] some other redox pairs have also used for this purpose, such as Ce4 + /Ce3 + ,[123] Fe3 + /Fe2 + ,[124] Br2/Br ,[125] and NO3/NO2 .[69] Because many pollutants are organic compounds, photocatalytic decomposition of pollutants and photocatalytic production of hydrogen can take place simultaneously when they are used as electron donors. Li et al reported enhanced hydrogen production in a mixed system of pollutants including oxalic acid, formic acid, and formaldehyde.[110] This approach is a win–win solution both for photocatalytic degradation of pollutants and for hydrogen production. Because biomass is a sustainable and carbon-neutral precursor, using biomass-derived products such as bioethanol, glycerol, glucose, and starch as feedstock of sacrificial agents for hydrogen production has excellent prospects.[64, 114] The process combines both photocatalytic splitting of water and light-induced oxidation of the feedstock of carbohydrates (finally oxidized to CO2), which is able to produce hydrogen at room temperature and atmospheric pressure. 5.3 Preventing the back reaction between H2 and O2 Although noble metals can dramatically increase the hydrogen production rate via decreasing the hydrogen evolution overpotential and enhancing the charge separation efficiency, they also enhance the thermal back reaction that produces H2O from H2 and O2.[43] Several compounds have been found to suppress the back reaction when added to the water-splitting process. Yamaguchi et al.[43] found that the hydrogen evolution rate of Pt/TiO2 can be significantly increased by coating with NaOH during the photocatalytic splitting of gas-phase water. They suggested that, in the presence of gas-phase water, a thin NaOH liquid electrolyte layer forms on the catalyst surface and, consequently, prevents the thermal back reaction from occurring on the Pt surface. Sayama et al.[126] reported that a stoichiometric formation of H2 and O2 under UV irradiation can be found by adding Na2CO3 to water, while only trace amounts ChemSusChem 2010, 3, 681 – 694

of H2 could be observed from pure water. By FT-IR investigation, they revealed that the Pt/TiO2 was covered with several types of carbonate species. The thermal back reaction on Pt was found to be inhibited by these species.[127] Additionally, it has been proved by Moon et al that B2O3 can suppress the rapid thermal back reaction.[128] Physically separating the H2 and O2 evolution sites is a thorough way of avoiding the back reaction of H2 and O2. This was achieved by combining two photocatalytic reactions on suspended TiO2 powders using a two-compartment cell, which was equipped with platinum electrodes and a cation-exchange membrane.[125] When the compartments were illuminated by UV light, H2 evolved on the Pt-loaded anatase TiO2 in the Br2/ Br redox mediator solution in one compartment, while O2 was produced on the rutile TiO2 suspended in the Fe3 + /Fe2 + redox mediator solution in the other. Kitano et al.[75] found that a TiO2/Ti/Pt thin film photocatalyst could be applied for the separate evolution of H2 and O2 from water in an H-type glass container, consisting of two water phases separated by a thin film and proton-exchange membrane. No redox mediator was used. In addition, a multiphoton system, as already discussed at the end of Section 5.2.4, can also used for this purpose. 5.4 Tuning the photoactivity of TiO2 into the visible-light region The wide band gap of TiO2 (ca. 3.0–3.2 eV) limits its application to solar light, because the UV component of solar light amounts to less than 4 %. One of the goals for enhancing the performance of TiO2 is to tune the light absorption ability from the UV region to the visible region. There are two ways to achieve this goal. The first is band gap engineering, that is, doping TiO2 with other cations or anions. The second is sensitizing TiO2 with small-band-gap semiconductors, organic dyes, or metal nanoparticles.

5.4.1 Band gap engineering 5.4.1.1 Doping TiO2 with cations Over the past decades, metal-ion-doped TiO2 has been extensively investigated for enhancing its photocatalytic performance on the degradation of various organic pollutants under visible irradiation. The resulting nanomaterials have been regarded as the second-generation photocatalysts.[129, 130] The doped metal ions are mainly transition metals and rare earth metal ions, including V,[131, 132] Cr,[133, 134] Fe,[134, 135] Co,[136] Mo,[137] and In,[129] and lanthanide metals such as La, Ce, Sm, and others. These can be doped into the TiO2 lattice by three kinds of methods: wet chemistry, high-temperature treatment, and ionbeam technique. The wet-chemistry method usually involves the simultaneous hydrolysis of a titanium precursor and doped-metal precursor to form a sol solution. A xerogel then can be obtained by evaporating the solvent. To facilitate the penetration of metal ions into the TiO2 lattice, a further hightemperature or hydrothermal treatment is necessary. A few


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X. L. Fu et al. cation-doped TiO2 materials can be synthesized directly by high-temperature treatment methods. It is commonly used as the post-treatment of a wet-chemistry method. Recently, flamspray pyrolysis[138] and plasma oxidation pyrolysis[139] high-temperature treatment methods were used to prepare Fe-doped TiO2. The ion-beam technique, which was developed by Anpo et al.,[61, 75, 140–142] is an important and effective way to prepare cation-doped TiO2. The technique includes the metal-ion-implantation (MII) and radio-frequency magnetron sputtering (RFMS) methods. For MII, TiO2 is bombarded with transition metal ions that are accelerated by a high voltage and have a high energy. The metal ions are implanted into the TiO2 lattice, substituting for Ti4 + ions. With the RF-MS method, a raw material is formed from a mixed target of TiO2 and metal by sputtering of Ar + and then depositing onto a hot substrate to form a metal-doped TiO2 film.[75, 134, 140, 143] Obviously, both MII and RFMS are physical implantation methods. Compared to chemical doping process, the doped metal ions are implanted deeply into the TiO2 rather than near the surface.[144] This can avoid them acting as recombination centers and thus makes these methods more effective in modifying the band structure of TiO2.[142] Anpo et al.[76] reported that the band absorption of TiO2 could be effectively shifted to visible light by doping with Cr, V, Fe, and Ni, especially for V and Cr ions. The replacement of cations in the crystal lattice of TiO2 (substitute for Ti4 + sites[132]) can create impurity energy levels within the band gap of TiO2, which account for the absorption in the visible range. However, the photoactivity did not always improve consequently. The doped metal ions not only act as visible light absorption centers, but also can act as recombination sites for photogenerated carriers. The photoactivity of cation-doped TiO2 appeared to be a complex function of the type of dopant metal, the dopant concentration and distribution, and the configuration of their d electrons.[145] It has been reported that Cu, Mn, and Fe ions can trap both e and h + , thus doping TiO2 with those metals shows a better activity for acetic acid oxidation than doping with Cr, Co, and Ni cations, which can only trap e .[2] Dholam et al.[134] also found that Fedoped TiO2 showed a higher photoactivity for hydrogen production than Cr-doped TiO2. Choi et al.[145] found that doping TiO2 with Fe3 + , Mo5 + , Ru3 + , Os3 + Re5 + , V4 + , and Rh3 + at 0.1– 0.5 at % significantly increased the photoreactivity for both oxidation and reduction reactions while Co3 + and Al3 + doping decreased the activity. Karakitsou et al.[34] indicated that incorporation of cations of valence higher than that of the parent cation Ti4 + , such as W6 + , Ta5 + , Nb5 + , into the crystal matrix of TiO2 results in enhanced rates of water splitting, while the opposite is observed upon doping with cations of lower valence (In3 + , Zn2 + , Li + ). In addition, the position of the impurity energy levels of dopant, that is, the extension of the absorption range of TiO2, is also decided by the type of dopant metal. Fe, V, and Cr doping proved more effective to extend the absorption threshold to visible light. The dopant concentration of metals has an optimal value, which is commonly lower than ca. 1 wt %.[134, 146, 147] Heavy doping is detrimental to the photoactivity. In order to facilitate the transfer of photoinduced carriers, the metal ions should be injected near the surface. Doping

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too deep or too shallow (such as just at the surface) would make them behave as recombination centers.[145, 148]

5.4.1.2 Doping TiO2 with anions In recent years, anion doping has proved itself as another effective way to enhance the visible light response of TiO2.[149] The anions, such as N, C, S, F, and B, are used to substitute for the oxygen atoms in TiO2 lattice. As shown in Figure 7, the top

Figure 7. Band structure of anion-doped TiO2.

of the valence band of the resulting sample is a composite of O 2p and the p states of the doped anions.[149] The mixing band energy level shifts the VB upward and narrows the band gap of TiO2, while the CB remains unchanged.[150] This means the oxidation ability of TiO2 is decreased after anion doping, but the reduction ability remains almost unchanged. This is a critical point for the water-splitting reaction because the CB level of TiO2 is only slightly higher than the reducing potential of water, but the VB level of TiO2 is far lower than the oxidizing potential of water. Compared to cation doping, anion doping usually forms fewer recombination centers and no there is significant d state formation within the band gap,[151] which is more effective for enhancing the photocatalytic activity. Similar to cation-doped TiO2, anion-doped TiO2 can also be prepared by wet chemistry,[152–154] high-temperature methods,[155, 156] and ion beam techniques.[149, 157] Urea, NH3, NH4F, HF, CS2, thiourea, N2 + , F + , and S2 ion flux, amongst others, are commonly used as the sources of anions. In addition, unlike cation-doped TiO2, anion-doped TiO2 can be prepared directly by oxidation annealing of precursors that contain the doped anions, such as TiN,[158] TiC,[159] TiS2,[160] and H2TiF6.[161] More recently, co-doping of two kinds of atoms into TiO2 has attracted considerable interest, because compared with single-element doping this method can be used to further tailor the band structure of TiO2 or modify the surface structure, and consequently result in a higher photoactivity. Wang et al.[162] found that co-doping with F can be used to tune the band structure of both the VB and the CB of N-doped TiO2,


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Hydrogen Production over Titania-Based Photocatalysts owing to synergistic interactions between N and F. Li et al.[163] and Huang et al.[154] found that, for N–F co-doped TiO2, the doped N atoms improved the visible-light absorption and the doped F atoms led to enhancements of the surface acidity and reactant adsorption capacity. The co-doped sample showed a higher visible-light photocatalytic activity than TiO2 doped solely with N or F. Similar phenomena have been also observed for C–N, B–N, S–N, and Br–Cl, co-doped TiO2.[153, 164, 165] Most of these co-doped atoms are anions. Co-doping of cations and anions into TiO2 has been rarely explored.[166] Recently, based on first-principles band structure calculations and analysis of the band-edge wave function characters, Gai et al.[151] proposed that Mo and C Co-doped TiO2 is a strong candidate material for photoelectrochemical (PEC) hydrogen production through water splitting, because it reduces the band gap to the ideal visible-light region but does not affect much of the position of CB level. However, this assumption has not been supported by experimental results yet. Some of the doped TiO2 materials used for photocatalytic hydrogen production, together with their preparation methods, are listed in Table 2.

5.4.2 Sensitizing 5.4.2.1 Sensitizing with small-band-gap semiconductors Coupling TiO2 with small-band-gap semiconductors (SC1-TiO2) can also extend the energy excitation range of TiO2 to the visible light region. As discussed in Section 5.2.3 (Figure 4 a), when a small-band-gap semiconductor is irradiated with visible light, photogenerated e will be injected from the semiconductor into TiO2 because its CB level is more negative than the latter, while h + remains on the semiconductor. The visible-light-induced e that accumulate in the CB of TiO2 can further react

with protons to produce H2. In order to favor the transfer of e , the CB difference of TiO2 and SC1 must be large enough to overcome their interfacial resistance. The sooner the injection process takes place, the better. Strictly speaking, the visible activity of such a composite photocatalyst does not originate from TiO2, but from SC1. The role of TiO2 is just to separate the photogenerated e , similar to a Pt co-catalyst. TiO2 coupled with CdS, RuS2, Bi2S3, WS2, and AgGaS has been reported to produce hydrogen under visible-light irradiation. Their photoactivities are summarized in Table 3. Among them, CdS is the most extensively studied sample. Most of these small-band-gap semiconductors are chalcogenides. They will be oxidized by the remaining h + during the hydrogen evolution reaction. S2 is commonly used as the sacrificial reagent to prevent photocorrosion. 5.4.2.2 Sensitizing with dyes Organic dyes, which can perform the same function as the small-band-gap semiconductors, have also been widely used as sensitizers for TiO2 energy conversion in visible region.[177, 178] The process is similar to small-band-gap semiconductor sensitization. As shown in Figure 8, under visible light illumination the excited e are injected from the dye into TiO2 to initiate the photocatalytic reaction, while h + in the dyes are reduced by the sacrificial agent to regenerate the dyes and sustain the whole reaction. For this composite photocatalyst, the prerequisite for hydrogen evolution is similar to SC1-TiO2. That is, e must be transported from the dyes to TiO2 as quickly as possible, and the dyes should be stable enough to resist the attack of h + . In order to facilitate e transportation, dyes are linked to the surface of TiO2 nanoparticles via functional groups by various interactions between the dyes and TiO2, such as cova-

Table 2. Doped TiO2 catalysts for photocatalytic hydrogen production. Catalyst

Preparation

Reaction conditions and results

Ref.

Pt/Fe-TiO2

sol–gel & heat treatment hydrothermal & heat treatment RF-MS

cat. 2 mg, Pt 3 wt %, Fe 0.5 at %; 8 W 2 lamp, 254 nm; 1 mL water/ethanol (3:1); compared with P25 the photoactivity was improved by a factor of 614. cat. 500 mg, Fe 1.0 wt %; 300 W Xe lamp, l > 400 nm; 200 mL 20 % ethanol aqueous solution; rH2 = 0.0125 mmol h1 photocatalysts are films, weight unclear; 250 W tungsten halogen lamp, borosilicate glass used as a UV filter; rH2 = 0.015 mmol h1 (1.1 at % Fe), rH2 = 0.005 mmol h1 (1.2 at % Cr) cat. 2 mg; lamp power unclear, l > 420 nm; 70 mL water/ethanol (4:1); rH2 = 0.348 mL h1 oxygen sites in the TiO2 lattice were substituted by carbon atoms. Used as a photoanode in PEC system for hydrogen production cat. 200 mg, 1.3 wt % Pt; 300 W Xe lamp, l > 400 nm; 200 mL water/methanol (3:1); rH2 = 0.026 mL h1 cat. 300 mg, 0.3 wt % Pt; 400 W high pressure lamp; 400 mL water +92.3 g Na2CO3 ; rH2 = 0.140 mmol h1 cat. 65 mg, 0.2 wt % Pt; 400 W halogen lamp, l > 380 nm; 150 mL water/ethanol (2:3); rH2 = 0.079 mmol h1 cat. 100 mg, 0.5 wt % Pt; 400 W high-pressure lamp, l > 420 nm; 80 mL 0.79 m TEOA solution; rH2 = 0.08 mmol h1 cat. 200 mg, calcination at 250 8C; 300 W Xe arc lamp, l > 400 nm; 200 mL water/methanol (3:1); rH2 = 2.6 mL h1 cat. 500 mg; 300 W Xe lamp; 50 g 5 wt % glycerol solution; rH2 = 0.42 mmol h1

[147]

Fe-TiO2 Fe-TiO2 Cr-TiO2 Bi-TiO2 C-TiO2 film Pt/N-TiO2 Pt/Br-Cl-TiO2 Pt/N-TiO2 Eosin Y-Pt/N-TiO2 N-TiO2 Pt/B-N-TiO2

sol–gel & heat treatment pulsed laser deposition sol-gel & heat treatment (urea/TiO2 = 1:1, mol ratio) hydrothermal synthesis, TiCl4 + HBr microemulsion & heat treatment sol–gel & heat treatment heat treatment (urea/TiO2 = 1:1, mol ratio) hydrothermal & heat treatment

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[146] [134] [167] [168] [155] [153] [112] [169] [170] [165]

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X. L. Fu et al. boxylate groups because of the stronger anchoring ability. MalinCatalyst Hydrogen evolution rate Reaction conditions Ref. ka et al.[183] investigated the hydrogen production of zinc porCdS/TiO2 560 mmol h1 0.1 g cat.,2.0 wt % Pt; Hg arc lamp 500 W, [47] phyrin sensitized Pt/TiO2 and l 420 nm;0.1 m Na2S+0.02 m Na2SO3 CuO (7 wt %)/TiO2 890 mmol h1 0.2 g cat.; 125 W medium pressure [91] found that the rate of hydrogen Hg lamp; methanol/water (5:95) production depended on the CdS/TiO2 625 mmol h1 0.1 g cat.,1 wt % Pt; Hg arc lamp 500 W, [171] surface concentrations of platil 420 nm; 0.1 m Na2S+0.02 m Na2SO3 num, zinc porphyrins, and elecCdS/TiO2 (1:2, mol. ratio) 640 mmol h1 0.1 g cat., 0.75 wt % Pt; Hg arc lamp [45] 350 W; l 420 nm, 0.1 m Na2S+0.02 m Na2SO3 tron donors on titania. Astuti CdS/TiO2 (4:1, mol. ratio) 10.6 mmol h1 0.1 g cat., 1 wt % Pt; Hg arc lamp 450 W, [172] et al.[177] investigated the zincl 420 nm; 100 mL 0.1 m Na2S+0.02 m Na2SO3 substituted cytochrome c as a CdS/TiO2/Pt 6720 mmol h1 g1 0.0125 g cat., 0.3 wt % Pt; Xe arc lamp 450 W, [85] sensitizer for hydrogen producl 420 nm; 25 mL 0.04 m Na2S+0.04 m Na2SO3 CdS/TiO2/Pt 285 mL g1 0.1 g cat.; Xe arc lamp 350 W, [173] tion over TiO2. A long-lived l 420 nm; Na2S+M Na2SO3 charge separation was observed, 1 RuS2 (1 wt %)/TiO2-SiO2 25.9 mmol h 1.5 g cat.; 450 W high pressure Hg [103] which resulted in a quantum eflamp; 700 mL water, pH 9 ficiency of 10 5 % for hydrogen RuS2 (1 wt %)/TiO2 29.36 mmol h1 0.4 g cat.; 400 W high pressure Hg lamp, [120] l 400 nm; 2.7 mm Na2S+5.5 mm Na2SO3 evolution in the presence of AgGaS2/TiO2 420 mmol h1 0.1 g cat.; 1 wt % Pt, Hg arc lamp 450 W, l 420 nm; [174] EDTA as a sacrificial electron 100 mL, 0.1 m Na2S+0.02 m Na2SO3 donor. Recently, Du et al.[177, 184] Bi2S3/TiO2 2.9 mmol h1 g1 3 200 W tungsten lamps; other [175] found that some platinum terreaction conditions not mentioned [176] WS2 (3.1 wt %)/TiO2 2130 mmol h1 g1 0.2 g cat., 1 wt % Pt; Xe lamp, 350 W, pyridyl complexes can be used l 430 nm; 200 mL Na2S directly or as chromophores of TiO2 for hydrogen generation from water under visible-light irradiation. In addition, xanthenes (mostly Eosin Y),[113, 169, 185] merocyanine,[186] and heteropoly blue[187] have also been applied for hydrogen production.

Table 3. Some composite TiO2 photocatalysts for photocatalytic hydrogen production

5.4.2.3 Sensitizing with metal nanoparticles

Figure 8. Photocatalytic hydrogen production with dye-sensitized TiO2.

lent bonds, electrostatic interactions, hydrogen bonding, and van der Waals forces.[19] Dhanalakshmi et al.[179] found that only dye molecules absorbed on the surface of TiO2 can effectively inject electrons into TiO2 for water reduction. The dyes are usually transition metal complexes with lowlying excited states, such as polypyridine complexes, phthalocyanines, and metalloporphyrins. The center metals are usually Ru, Zn, Mg, Fe, Pt, and Al. At present most of these dye-sensitized TiO2 are used in solar cells.[178, 180] For hydrogen production, ruthenium bipyridyl complexes are the most extensively used sensitizers.[179, 181, 182] Bae et al.[181] systematically investigated the effects of the anchoring groups in ruthenium bipyridyl complexes on the sensitized production of hydrogen over TiO2 and found that phosphonate anchoring groups showed a much better sensitizing effect for hydrogen evolution than car-

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Nanoparticles of Au and Ag not only can act as an electron sinks for separation of photogenerated charges on TiO2 but also can act as a photosensitizers, based on surface plasmon absorption (SPA).[188] Tian et al.[189] have reported that Au nanoparticles can be photoexcited due to plasmon resonance under visible light irradiation (l > 420 nm). The generated electrons can then transfer from the Au particles into the TiO2 CB with simultaneous transfer of compensating electrons from a donor (methanol, ethanol, and propanol) in the solution to the Au particles. The result indicated that the Au/TiO2 system is potentially applicable as a visible-light-sensitive photocatalyst. A similar reaction process was proposed recently on Ag/AgCl/ TiO2 photocatalyst during degradation of methyl orange by Yu et al.[190] The resonance wavelength strongly depended on the size and shape of the nanoparticles. For example, small Au nanoparticles of < 5 nm diameter did not show any plasmon absorption, but when the size was increased to 5–50 nm a sharp absorption band in the 520–530 nm region could be observed. As the particles grew bigger, the absorption band broadened and covered the visible range. In the case of Ag nanoparticles, a shift in the plasmon absorption band from 400 to 670 nm could be observed as the particle shape changed from spherical to triangular prisms during visible light irradiation.[191] Compared to the above-mentioned approaches, sensitizing TiO2 with noble metal nanoparticles has obvious advantages.


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Hydrogen Production over Titania-Based Photocatalysts First, it is just a surface modification process that does not produce recombination centers in the crystalline lattice, as the doping process does. Second, noble-metal deposits are more stable than small-band-gap semiconductors and dyes, which are often susceptible to photodegradation during the reaction process. Despite the limited work on using noble-metal sensitizing for TiO2 as a visible photocatalyst, the encouraging preliminary results show the promise of this approach for developing new visible photocatalysts.

6. Conclusion Since the discovery of Honda and Fujishima that H2O could be decomposed into H2 and O2 with a PEC cell consisting of Pt and TiO2 electrodes under a small electric bias, photocatalytic generation of H2 over TiO2 based photocatalyst has been extensively studied, because of its relatively high efficiency, high photostability, abundance, low cost, and nontoxic nature. The photocatalytic activity of TiO2 for hydrogen evolution can be substantially improved by proper selection of the crystal structure, crystallinity, particle size, and surface area of TiO2. Among the crystal phases anatase, rutile, and brookite, only anatase and rutile TiO2 are mostly used for hydrogen production, and the anatase phase has shown a considerably higher activity than rutile because of the difference in their band structures. TiO2 samples with high crystallinity, small particle sizes, and large surface areas commonly show high activities for hydrogen production, facilitating the migration of photoinduced charges and providing more active sites. The high overpotential for hydrogen generation, the rapid recombination of photoinduced e and h , the rapid reverse reaction of H2 and O2, and the inability to absorb visible light are generally considered as the main problems that restrict the photoactivity of TiO2 for hydrogen production. The high hydrogen evolution overpotential of TiO2 makes the material itself inactive for hydrogen formation. Loading noble metals with low hydrogen overpotentials can facilitate hydrogen evolution and can also suppress the recombination of photoinduced e and h + because of the formation of Schottky barriers. The most frequently used metal is Pt because it has the lowest overpotential and largest work function. The metals’ loading methods, loading amount, distribution, size, and chemical state are all crucial factors in improving the photocatalytic activity of the TiO2. In addition to loading with noble metals, coupling with other semiconductors and adding sacrificial reagents to the reaction solution are two other approaches to suppress the recombination of photoinduced charge carriers. A wide separation of e and h + can be obtained by coupling of TiO2 with other proper photocatalysts. The added sacrificial reagents are electron donors which can react irreversibly with h + , avoiding the direct combination of e and h + . In order to prevent the thermodynamically favorable back reaction of H2 and O2, several compounds, such as NaOH, Na2CO3, and B2O3, have been added to water to act as suppressors during the water-splitting reaction process. Some reaction equipment that can physically separate the H2 and O2 evolution sites has also been developed. ChemSusChem 2010, 3, 681 – 694

Owing to the wide band gap of TiO2, its application to solar light is limited. This triggers the works to turn the light absorption ability from the UV region to the visible region. There are two ways to achieve this goal. The first route is band-gap engineering, that is, doping TiO2 with other cations, anions, or codoping with both. The doping methods and doping content are the two main factors that influence the photoactivity because the doped ions not only act as visible-light absorption centers but also as recombination sites for photogenerated carriers. The second route is sensitizing TiO2 with small-bandgap semiconductors, organic dyes, or metal nanoparticles. For small-band-gap semiconductor or organic dye sensitizers, photogenerated e must transfer from the sensitizer to TiO2 as quickly as possible, and the sensitizer itself must be stable enough to resist attack by h + . When sensitizing with metal nanoparticles, no such problems are encountered. Although the hydrogen production efficiency of TiO2 is still lower than the 10 % required for practical applications,[28] the efficiency has been considerably improved by the continuing breakthroughs in the synthesis, modification, and tailoring of the electronic structure of TiO2. In addition to developing new photocatalytic materials, research to improve the photocatalytic performance of TiO2 will continue to be the focus of photocatalytic studies, and is highly expected to play an important role in the future hydrogen economy as well as the protection of the environment.

Acknowledgements This work is financially supported by a grant from the Research Grant Council of Hong Kong, PR China (HKU 7150/05E), the ICEE of the University of Hong Kong, the National Natural Science Foundation of PR China (grant Nos. 20873022), the National High Tech R&D Program of China (863 Program, 2008AA06Z326), and the Science and Technology Program of Fujian Province (2006Y0021). Keywords: catalysis · hydrogen · photophysics · titania · water splitting [1] J. Verne, http://www.online-literature.com/verne/mysteriousisland/ 1874. [2] M. Ni, M. K. H. Leung, D. Y. C. Leung, K. Sumathy, Renewable Sustainable Energy Rev. 2007, 11, 401 – 425. [3] K. Maeda, K. Teramura, D. Lu, T. Takata, N. Saito, Y. Inoue, K. Domen, Nature 2006, 440, 295 – 295. [4] a) R. D. Cortright, R. R. Davda, J. A. Dumesic, Nature 2002, 418, 964 – 967; b) Z. Zou, J. Ye, K. Sayama, H. Arakawa, Nature 2001, 414, 625 – 627; c) J. A. Turner, Science 2004, 305, 972 – 974. [5] a) R. M. Navarro, M. A. PeÇa, J. L. G. Fierro, Chem. Rev. 2007, 107, 3952 – 3991; b) J. D. Holladay, J. Hu, D. L. King, Y. Wang, Catal. Today 2009, 139, 244 – 260; c) R. M. Navarro, M. C. Snchez-Snchez, M. C. AlvarezGalvan, F. Valle, J. L. G. Fierro, Energy Environ. Sci. 2009, 2, 35 – 54. [6] G. A. Deluga, J. R. Salge, L. D. Schmidt, X. E. Verykios, Science 2004, 303, 993 – 997. [7] a) D. Wang, S. Czernik, E. Chornet, Energy Fuels 1998, 12, 19 – 24; b) C. Rioche, S. Kulkarni, F. C. Meunier, J. P. Breen, R. Burch, Appl. Catal. B 2005, 61, 130 – 139; c) A. N. Fatsikostas, D. I. Kondarides, X. E. Verykios, Chem. Commun. 2001, 851 – 852.


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Received: January 23, 2010 Published online on April 29, 2010


ChemSusChem 2010, 3, 681 – 694