Front. Chem. China 2009, 4(4): 343–351 DOI 10.1007/s11458-009-0100-1
FEATURE ARTICLE
An overview on water splitting photocatalysts Yuzun FAN, Dongmei LI, Minghui DENG, Yanhong LUO and Qingbo MENG (✉) Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
Photocatalytic splitting of water into H2 is one of the most promising ways for converting solar energy into chemical energy. In the present paper, the basic physical and chemical principles of photocatalysts for hydrogen generating from aqueous solutions are outlined. Strategies for designing visible-light-induced photocatalysts have been reviewed. Besides, ultraviolet-light-induced photocatalysts, dye sensitization, photoelectrochemical water splitting and Z-scheme systems to effectively increase the light harvest are also discussed. Finally, the existing problems, possible solutions and prospects are presented. Keywords water splitting, photocatalysts, hydrogen, solar energy
1 Introduction In 1972, Fujishima and Honda first reported the photo-assisted splitting water into H2 and O2 on TiO2, providing the possibility for the human being to convert solar energy into chemical/electrical energy in a renewable and cheap way [1– 6]. Since then, considerable efforts have been devoted into relevant research fields, such as photocatalysis, dye-sensitized solar cells, and so on [7–16]. More recently, the importance of hydrogen energy has been recognized again due to the imperative requirements of solving the global energy issues and environmental problems associated with fossil fuels [17– 19]. Hydrogen from water splitting by using solar energy is even called “solar-hydrogen”, representing a kind of clean and low-cost fuel [20]. Unfortunately, a satisfactory system under visible light irradiation has not been obtained yet. Thus, the development of stable and efficient photosensitive materials becomes a key to the future of “solar-hydrogen”. For this purpose, a thorough understanding of the photocatalytic processes is necessary. The processes include the separation, the mobility and the lifetime of photogenerated electrons and holes, as well as the comprehensive insights into the correlation between the structure of photocatalysts and their photocatalytic performances. In fact, people often fall into an awkward situation, which is, the materials that are stable in water and can split water into H2 and O2 do not effectively absorb visible light, whereas Received June 1, 2009; accepted September 28, 2009 E-mail:
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those materials that can effectively absorb visible light can not induce water-splitting. This limitation mainly derives from the interaction between the optical, electronic and chemical properties of the light-absorbing materials. In spite of this limitation, however, a remarkable improvement in this field has been achieved through the efforts of the scientists of different disciplines around the world. Various transition metal oxides with d0 electronic configuration (i.e. Ti4+, Zr4+, Nb5+, Ta5+ or W6+) [21–27], metal oxides containing d10 electronic configuration metal ions (i.e. Ga3+, In3+, Ge4+, Sn4+ or Sb5+) [28–35] as well as metal nitrides with d10 electronic configuration [36–39] have been prepared as active ultraviolet (UV)-light-induced photocatalysts for water splitting. Doping [40–44] or dye-sensitization [45–48] on the photocatalysts is revealed beneficial for the occurrence of visible-light-induced photocatalytic processes. Heyduk et al. reported the use of a two-electron mixed-valence dirhodium compound to produce H2 from hydrohalic acid under visible light irradiation [49]. Zou et al. reported a series of nickel-doped indium-tantalumoxide, In1 – xNixTaO4 (x = 0 – 0.2) for the overall water splitting under visible light irradiation [50]. In 2002, Khan et al. synthesized chemically modified n-type TiO2 by controllable combustion of Ti metal in a natural gas flame, which can perform the water splitting at an applied potential of 0.3 V [51]. In 2004 and 2005, solid solutions of (AgIn)xZn2(1 – x)S2 and (Ga1 – xZnx)(N1 – xOx) were developed to emit H2 from aqueous solutions containing sacrificial reagents under visible light irradiation [52,53]. In 2007, titanium disilicide (TiSi2) was used for water splitting with
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reversible storage of O2 and H2 [54]. In China, some research groups from Dalian Institute of Chemical Physics, Nanjing University, Lanzhou Institute of Chemical Physics, Institute of Metal Research, etc. have made much more effort in this field, and fruitful achievements have been obtained [55–73]. This paper reviews the recent development of versatile photocatalysts working under ultraviolet light irradiation and visible-light-induced photocatalysts. The existing problems and possible solutions are proposed.
2 The basic principle of water splitting by photocatalysts The dissociation of H2O occurs as the following reaction, of which the standard Gibbs free energy change (ΔGθ) is 238 kJ$mol–1 or 1.23 eV. H2 OðlÞ ! H2 ðgÞ þ 1=2 O2 ðgÞ, ΔGθ ¼ 238 kJ$mol – 1 (1) Direct thermal dissociation requires a temperature of over 2000°C, or over 900°C with the assistance of a Pt/Ru catalyst [74]. Any semiconductor photocatalyst used for water splitting should possess a band gap energy (Eg) over 1.23 eV. Moreover, in order to achieve overall water splitting, its conduction band (CB) has to be located at a more negative potential than the reduction potential of H+/H2 (0 V vs. NHE), while the valence band (VB) be located at a more positive position than the oxidation potential of O2/H2O (1.23 V vs. NHE). Under illumination with photon energy equal to or higher than the Eg of a semiconductor photocatalyst, electrons are excited from the VB to the CB, leaving holes in the VB. Then, the electrons reduce water to H2 and the holes simultaneously oxidize water to O2, leading to the overall water splitting as shown in Figure 1. The requirements for UV-light-driven photocatalysts are 1) suitable band location and 2) good stability in the photocatalytic reactions, whereas visible-light-driven photocatalysts have to meet the third qualification –– band gap energy less than ca. 3.0 eV. The apparent quantum efficiency can be estimated by the following equation [75]: no: of reacted electrons 100 no: of incident photons no: of evolved H2 molecules 2 100, ¼ no: of incident photons
Figure 1 Energy diagram for H2 production from the photocatalytic water decomposition.
Generally, sacrificial reagents are often used in water oxidation/reduction half-reaction, which can obviously improve the quantum efficiency for water splitting. In the water reduction reaction, the photogenerated holes irreversibly oxidize the electron donors such as alcohol, S2–, and SO23 – , and the H2 evolution reaction is thus enhanced (Figure 2(a)). When waste organic compounds act as electron donors, two objectives are supposed to be achieved: 1) photocatalytic production of hydrogen and 2) degradation of pollutants [62]. Thus, combining photocatalytic water splitting with the photodegradation of waste organic compound is an interesting route for hydrogen production. In the water oxidation reaction, the photogenerated electrons irreversibly reduce
Φð%Þ ¼
(2)
where Φ is the apparent quantum efficiency and it is assumed that all incident photons are absorbed by the reaction system. In order to achieve a higher quantum yield, the photogenerated electrons and holes should be separated as soon as possible to reduce their recombination probability. In addition, the backward reaction between H2 and O2 should be also inhibited.
Figure 2 Scheme of water reduction or oxidation half-reaction: (a) H2 evolution in the presence of electron donor; (b) O2 evolution in the presence of electron acceptor.
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An overview on water splitting photocatalysts
the electron acceptors such as Ag+ and Fe3+, and the O2 evolution reaction will be enhanced (Figure 2(b)) [76]. The overall water splitting can also be achieved by combining the two half reactions (Z-scheme) using a redox couple (Ox/Red) such as IO3– =I – , Fe3+/Fe2 + [77–79]. However, the disadvantage is that this mobile Ox/Red couple will compete with water reduction/oxidation reactions in the solutions, and thus will probably reduce the reaction efficiency. Very recently, Cheng et al. succeeded in combining ZnO/CdS coupled heterostructure with Z-Scheme to enhance the photocatalytic hydrogen evolution efficiency [66].
3 Design of photocatalysts for water splitting under ultraviolet light irradiation As we know, the ultraviolet light accounts for only about 4% of the solar spectrum at the earth’s surface, while visible light accounts for about 43%. Initially, the research on photoinduced water cleavage started from the photocatalysts responsive to UV light, such as TiO2 and other TiO2-based oxide materials. Since 1990s, different kinds of active photocatalysts, i.e. ZrO2, tantalates, have been continuously reported [21]. 3.1
Highly efficient TiO2 semiconductor photocatalysts
TiO2 has been widely studied in photocatalysis due to its high photocatalytic activity, chemical inertness, non-photocorrosion, non-toxicity and low cost. Degussa P25 TiO2 is often used as a standard photocatalyst. For the photoelectrochemical evolution of H2 from water, anatase TiO2 is superior to rutile TiO2, since its flat band potential is more negative (ca. 0.1 eV) [80]. Fujishima and Honda discovered photo-induced water splitting in the photoelectrochemical (PEC) cell with a singlecrystalline TiO2 (rutile) photoanode and a Pt counter electrode: Pt electrode : 2Hþ þ 2e ! H2 TiO2 electrode : H2 O þ 2hþ !
1 O þ 2Hþ 2 2
1 The overall reaction : H2 O ! H2 þ O2 2
(3) (4)
(5)
A metallized, powdered semiconductor as a kind of microPEC cell, has been used as a typical photocatalyst [81,82]. Of particular interest is Pt-loaded TiO2 photocatalyst, which could reduce the overpotential for H2 production. Besides, Pt as a co-catalyst was found to remarkably accelerate the H2 evolution from water. However, Pt/TiO2 could not split pure water in a simple aqueous suspension due to the strong
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backward reaction on Pt [83]. Arakawa et al. discovered that Na2CO3 addition to a Pt/TiO2 aqueous suspension was significantly effective in promoting stoichiometric photodecomposition of water [84,85]. Carbonate salts were supposed to suppress the back reaction over Pt and the afforded peroxycarbonates could assist the O2 evolution, hence, the quantum efficiency was enhanced. Li et al. demonstrated that the photocatalytic activity of TiO2 nanoparticles was remarkably related to the phase junction between the surface anatase and rutile particles with the aid of UV Raman spectroscopy and high-resolution TEM [86]. Suitable phase junction could greatly enhance the photocatalytic H2 production, up to four times increasing. Their work provides a possibility to design a new kind of photocatalysts with surface-phase junctions. 3.2 Other transition metal oxide photocatalysts with d0 electronic configuration 3.2.1
Titanate photocatalysts with tunnel structures
BaTi 4 O9 with pentagonal prism tunnel structure and M2Ti6O13 (M = Na, Ka, Rb) with rectangular tunnel structure are photoactive for water splitting. In titanates with tunnel structures, there are strongly distorted TiO6 octahedra. The deviation of Ti ions from the center of gravity of the surrounding oxygen ions can generate large dipole moments, which correspond to the presence of internal fields in TiO6 octahedra. The internal fields of the distorted TiO6 octahedra can facilitate the separation of photoexcited electrons and holes, and thus lead to a higher quantum yield [80]. 3.2.2
Tantalate photocatalysts
Kudo et al. first reported a series of alkali and alkaline earth tantalate photocatalysts for water splitting [87,88]. The CB and VB of these photocatalysts consist of Ta5d and O2p orbitals, respectively. Alkali and alkaline metal ions could also influence the band gaps of these photocatalysts. Further investigation revealed that the photocatalytic activity can be remarkably increased when NiO was loaded on these photocatalysts, therefore, NiO is supposed to be the catalytic center. The CB of NiO located between the reduction potential of water and the CB of these photocatalysts facilitates the electron transfer from photocatalyst to NiO [89]. Doped NaTaO3 photocatalysts can efficiently split water into H2 and O2. Here, La-doped NaTaO3 photocatalysts are taken as an example. The optimized NiO (0.2 wt%)/ NaTaO3: La (2%) photocatalyst showed the highest activity in the ultraviolet region, with H2 and O2 evolution rates of 440 and 220 mL$h–1, respectively. H2 and O2 evolution sites were efficiently
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separated at the nanosteps created at the surface of the photocatalyst. In comparison with recombination in the bulk, the reaction probability between photogenerated electrons/ holes and water molecules was simultaneously increased, thus resulting in the high activity [90]. 3.2.3
Layered-structure photocatalysts
Layered compounds provide unique electron transfer processes due to their low dimensionality [91]. A4Nb6O17 (A = K, Rb) are typical layered oxide photocatalysts. They have a layered structure with [Nb6O17]4– macropolyanion sheets interleaved by K+ ions. Two types of interlayer regions, referred to as interlayer I and II, are found. The K+ ions in interlayer I can be replaced by various mono- and multivalent metal cations such as Li+, Na+, and Ni2+, whereas the K+ ions in interlayer II can be only replaced by monovalent metal cations. Interlayer I and II have a spontaneous hydration, which is beneficial for water splitting. Under band gap irradiation, unmodified K4Nb6O17 splitted water with very low rate. Ni2+ was introduced to replace some K+ ions in interlayer I by an impregnation method. After reduction (773 K)/oxidation (473 K) treatment, the photocatalytic activity for water splitting was improved and photogenerated electrons can be transferred to the nickel particles, where H2 evolution took place and O2 evolved in interlayer II [80]. Layered perovskite structure photocatalysts A2 – xLa2Ti3 – xNbxO10 (A = K, Rb, Cs; x = 0–1.0) exhibit good photocatalytic activity for water decomposition after being modified with Ni (NiO). H2 evolved on Ni particles on the external surface of the photocatalysts and O2 at the interlayer space. The quantum efficiency of Ni (4%)/ Rb2La2Ti3O10 could reach 30% at around 330 nm [22,92]. 3.3
Photocatalysts with d10 electronic configuration
Photocatalysts with d10 electronic configuration metal cations, such as Ga3+, In3+, Ge4+, Sn4+, and Sb5+, have been developed [29,31,32,93–95]. Generally, these metal oxides with wide band gaps can not efficiently split water. The research on the metal oxide photocatalysts has subsequently expanded to metal oxynitrides and nitrides. β-Ge3N4 was the first photocatalytically active d10-metal nitride [36]. When GaN/RuO2 was directly used in photocatalytic water splitting, a dissatisfactory result was presented. Its activity was increased when doped with divalent metal ion (Mg2+, Zn2+ and Be2+) [37]. Photocatalysts with d10s2 electronic configuration metal cations (e.g. PbWO4) were also reported [96]. Compared with the conventional photocatalysts, the p orbits in these P block elements comprise the electronic structures for the conduction and valence bands, which paves a new way
for the design of efficient photocatalysts in the overall splitting of water. Although the quantity of photocatalysts for water splitting is growing fast and species are versatile, to the most photocatalysts, their wide band gaps only respond to UV light. This limitation may be partly changed by making full use of UV light. Enhancing the utilization ratio of UV light, the quantum yield of ultraviolet-light-induced photocatalysts can be further improved. Due to the larger share of visible light in solar spectrum, the development of visible-lightdriven photocatalysts has become urgent in order to approach the goal of efficient light-to-chemical energy conversion (hydrogen fuel).
4 Design and development of photocatalysts for water splitting under visible light irradiation Unlike the UV-light-driven photocatalysts, the study on photocatalysts for water decomposition under solar light started early 1990s due to the difficulty of beneficial modification on band gap of semiconductors. The control of band structure is crucial to the development of new visiblelight-driven photocatalysts. As shown in Figure 3, four methods are usually adopted: (a) doping with some transition metals to give a donor level above the VB; (b) substitution some oxygen atoms with nitrogen, sulfur or carbon atoms to elevate the original VB; (c) combination of different band structures to afford solid solution [52]; (d) dye-sensitization to increase visible light harvesting. At the beginning, only a few chalcogenides and metal oxides (e.g. CdS and WO3) were demonstrated to response to visible light. So far, four major kinds of photocatalysts have been developed. CdS, CdTe and InP, whose band gaps are better matched up to the solar spectrum, can produce H2 from water under visible light irradiation. Especially, CdS semiconductor nanoparticles have greater advantages in photocatalytic hydrogen production [97]. The photocatalytic hydrogen evolution was further improved by loading cocatalysts (noble metal or their oxides). Li et al. found that the intimate junction between the cocatalyst MoS2 and CdS accounted for the higher hydrogen evolution compared with Pt/CdS [98]. So a proper junction between the cocatalyst and semiconductor is important for high photocatalytic activity. The highest quantum efficiency (93%) at 420 nm was reported by Li et al. with the Pt-PdS/CdS photocatalyst [99]. The band gap of CdS is tunable as a function of particle size, thus can be easily controlled by the precipitation rate. However, the disadvantage of easy photocorrosion in aqueous solution limits their applications as photocatalysts [100]. In CdS aqueous solution, four reactions will occur:
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single- and double-component systems [102]. This is mainly derived from vectorial electron transfer (TiO2 ! Au ! CdS) driven by the two-step excitation of TiO2 and CdS. It could spread a new kind of all-solid-state Z-schemes as efficient visible-light-induced photocatalysts by the modification of various starting materials and the dimensional control. 4.1 Transition metal-doped photocatalysts with wide band gaps
Figure 3 Band engineering for development of visible-lightdriven photocatalysts: (a) doping with transition metal element; (b) substitution with nitrogen or sulfur or carbon element; (c) preparation of solid solution; (d) dye-sensitization. ED: donor energy.
CdS þ h ! hþ þ e – hþ þ OH – !
1 O þ Hþ 2 2
e – þ Hþ !
1 H 2 2
2hþ þ CdS ! Cd2þ þ S
(6) (7)
Doping transition metal ions into active photocatalysts with wide band gaps is a conventional method to prepare the visible-light-driven photocatalysts. The role of doping agents is to form donor levels in forbidden bands. However, these donor levels are sometimes the recombination centers for the photogenerated electrons and holes. In 2001, Zou et al. developed visible-light-induced photocatalysts of InMO4 (M = Ta, Nb, V) [50,103]. When modified with NiO, these photocatalysts presented good ability to split water while the quantum yield at 402 nm reached 0.66%. Besides, Cr-Ta, CrSb co-doped and Rh doped SrTiO3 were also active for H2 evolution from aqueous-methanol solutions under visible light irradiation [104–106]. Cr-Sb, Ni-Nb co-doped TiO2 showed photocatalytic activities for O2 evolution from aqueous silver nitrate solutions [104,107]. Metal ion-implantation with various transition metal ions, such as V, Cr, Mn, Fe and Ni, is a promising modification technique for the red shift of TiO2, while no impurity energy level as recombination centers forms. The order of effectiveness in the red shift is as follows: V > Cr > Mn > Fe > Ni and the transition metal ions are in place of the Ti ions incorporated in the lattice positions of TiO2. Thus the transition metal ion-implanted TiO2 catalysts can absorb and operate effectively not only under UV light but also under visible light irradiation [3,108,109].
(8) 4.2 (9)
Reaction (9) is responsible for the photocorrosion of CdS. Some methods have already been developed to reduce its photocorrosion, such as electron donors, photo-platinization, doping, etc [2,101]. Remarkably, a kind of (Cd0.8Zn0.2)S single crystal nanorods reported by Cheng et al. has been used for the photocatalytic watersplitting and the H2 evolution efficiency was significantly improved in comparison with pure CdS [67]. Recently, an all-solid-state Z-scheme, an anisotropic CdS-Au-TiO2 nanojunction has been constructed by spatially fixing the electron transfer system (Au). This three-component system exhibited a high photocatalytic activity under visible light irradiation, superior to the
(Oxy)nitride and oxysulfide photocatalysts
Substitution of partial oxygen atoms with nitrogen, sulfur or carbon atoms can enhance the valence band position, resulting in visible light response. Domen et al. obtained some (oxy) nitrides photocatalysts containing Ta5 +, such as TaON and Ta3N5, by nitriding Ta2O5 under NH3 flow at high temperature [110–113]. The VBs top of Ta2O5, TaON, and Ta3N5 mainly consists of O2p, O2p + N2p, and N2p orbits, respectively, while the CBs bottom consist of Ta5d orbits. The band gap could shift from 3.9 eV of Ta2O5 to 2.4 eV of TaON to 2.1 eV of Ta3N5 as a result of mixed O2p with N2p orbits. Under visible light irradiation, TaON and Ta3N5 could reduce H+ into H2 or oxidize water into O2 in the presence of a sacrificial electron donor or acceptor, and the maximum quantum yield was 10%.
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Oxysulfide Sm2Ti2S2O5 was obtained by heating a mixture of Sm2S3, Sm2O3 and TiO2 under vacuum or by sulfurizing Sm2Ti2O7 under H2S flow at high temperature [43,114]. The hybridization of S3p and O2p orbits results in lower band gap of Sm2Ti2S2O5 (1.9 or 2.1 eV). Accordingly, Sm2Ti2S2O5 is a stable photocatalyst for H2 or O2 evolution in a sacrificial reagent-containing aqueous solution under visible light irradiation. 4.3
Solid solution photocatalysts
Solid solution photocatalysts can be obtained by the combination of wide/narrow band gap semiconductor materials. Domen et al. prepared (Ga1 – xZnx)(N1 – xOx) solid solution by heating a mixture of Ga2O3 (3.4 eV) and ZnO (3.2 eV) powders under NH3 flow at high temperature [53,75,115]. The VB top of (Ga1 – xZnx)(N1 – xOx) mainly consists of Ga4s and Ga4p orbits, while the CB bottom consists of N2p, Zn3d and O2p orbits. The p-d repulsion between Zn3d and N2p electrons shifts the VB maximum upward without affecting the CB minimum, resulting in visible light response. Thus, (Ga1 – xZnx)(N1 – xOx) solid solution with x = 0.05 – 0.22 acted as a photocatalyst for overall water splitting. The highest activity was obtained by further optimization with x = 0.12 and nitridation for 15 h. Other solid solutions, such as (AgIn)0.22Zn1.56S2, (CuIn)0.09Zn1.82S2 and (CuAg)0.15In0.3 Zn1.4S2, also exhibited photocatalytic activities for H2 evolution from aqueous solutions containing sacrificial reagents (S2–,and SO23 – ) under visible light irradiation [52,116,117]. 4.4
Dye-sensitized photocatalysts
Dye-sensitized wide band gap semiconductors have been studied for effective visible light harvest [118,119]. Abe et al. reported H2 evolution from a water-acetonitrile mixed solution containing iodide electron donor by the assistance of dye-sensitized Pt/TiO2 photocatalysts under visible light irradiation [120]. Its working principle is briefly shown in Figure 3(d). The dye molecules absorb the photons to be excited from ground state to excited state. Then the electrons are injected into the CB of TiO2 semiconductor photocatalyst while the excited dye molecules turn into oxidized dye molecules. These electrons in the CB will reduce water to H2 on Pt. The original state of the dye is restored by the electrons donated by I–. This dye-sensitization principle can easily bring the dye-sensitized solar cells (DSCs) in mind, which is a kind of low-cost new solar cells invented by Grätzel [119]. The stability of dye in aqueous solution needs to be considered, which may be improved by using an organic solvent-water mixture or by nafion coating incorporating dye.
4.5
Photoelectrochemical water splitting
Photoelectrochemical water splitting can be achieved in a tandem cell configuration with the additional bias potential supplied by the tandem cell [121,122]. Bard et al. has already reported direct water decomposition to yield H2 and O2 with bipolar WO3/Pt and dye-sensitized TiO2/Pt semiconductor panels [123]. O2 evolved at the WO3/Pt electrode panel while H2 at the dye-sensitized TiO2/Pt semiconductor panel. The hydrogen production efficiency of this tandem cell was about 1.9% and the maximum hydrogen production efficiency could reach ~2.5% when 0.2 V positive bias was applied. This new system is based on two types of photosystems (solar-toelectrical, electrical-to-chemical) connected in series [124]. Reducing the fabrication cost of tandem cell as well as maintaining a reasonable conversion efficiency may make this solar energy conversion system more promising. Besides the above photocatalysts or photocatalytic systems, some novel compounds have also been selected as photocatalysts. Titanium disilicide (TiSi2) as a promising new class of silicide semiconductors [125], has been used for water splitting. This nonoxidic semiconducting material is inexpensive and abundant. The most important feature is that it shows a wide band-gap spread from 3.4 eV to 1.5 eV, and its VB and CB location meet the requirements for water splitting [54]. High oxygen storage and moderate hydrogen storage were observed at temperatures below 100°C and desorption of oxygen occurred at temperature≥100°C in the dark. Thus, the generated oxygen and hydrogen could be easily separated. However, its disadvantage is poor stability (in particular of TiSi2 in water). It is expected that this problem might be solved by the passivation of TiSi2 through partial replacement of oxide. Inspiringly, a kind of novel nitrogen doped {001} dominant anatase TiO2 sheets has been reported recently to exhibit attractive photocatalytic activity for H2 evolution by Cheng et al. [68]. The electric near-field in the vicinity of metal nanoparticles (Au or Ag) can be enhanced due to the localized surface plasmon [126,127]. This enhanced near-filed could boost the excitation of electron-hole paris in photocatalysts by matching the peak wavelength of the plasmon resonance of nanoparticles with the excitation wavelength of photocatalysts. Taking this concept into photocatalytic splitting of water, this may open up a new way for hydrogen production and may give a different mechanism of the effect of cocatalysts (metal nanoparticle) in photocatalysis.
5 Conclusions and prospects In this paper, the correlation between the photocatalytic activity and the structure of photocatalysts has been discussed.
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In order to efficiently realize solar-to-chemical process, band engineering is the key to developing new photocatalysts. Various methods, including forming donor levels in the forbidden band, enhancing the valence band position, preparing solid solution, have been proven to be effective for the design of visible-light-induced photocatalysts. The quantum yield could also be improved by dye sensitization, photoelectrochemical or Z-scheme systems. Combining different band engineering techniques may be essential for the development of visible-light-driven photocatalysts, for example, co-doping two different metal ions or metal ions and non-metal ions, plasmon-assisted photocatalysts, heterostructure photocatalysts, photocatalysts with given facets, coloading suitable dual cocatalysts. For practical applications, photocatalysts must exhibit high photocatalytic activity and very stable photocatalytic performance. In the meantime, they can be easily attained and reused. Molecular mechanism of water splitting and simulation model for hydrogen production need to be further investigated for hydrogen utilization. These areas create many opportunities for us both in the basic and applied research field. With the development of science and technology as well as the untiring effort of the scientists from all over the world, the efficiency of photocatalytic water splitting will be further improved. And if this large scale hydrogen production from photocatalysts is successfully achieved, the world would benefit from the Hydrogen Economy. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos. 20725311, 20673141, 20873178, 20703063 and 20721140647) and the Ministry of Science and Technology of China (973 Project, Grant No. 2006CB202606 and 863 Project, Grant No. 2006AA03Z341).
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