Nanostructured materials for photocatalytic hydrogen production

Current Opinion in Colloid & Interface Science 14 (2009) 260–269

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Current Opinion in Colloid & Interface Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c o c i s

Nanostructured materials for photocatalytic hydrogen production Jiefang Zhu ⁎, Michael Zäch ⁎ Department of Applied Physics, Chalmers University of Technology, 412 96 Göteborg, Sweden

a r t i c l e

i n f o

Article history: Received 23 March 2009 Received in revised form 1 May 2009 Accepted 6 May 2009 Available online 14 May 2009 Keywords: Nanostructured materials Nanomaterials Photocatalysis Photocatalyst H2 Hydrogen Semiconductor TiO2 Light harvesting Water splitting Solar light Visible light

a b s t r a c t Photocatalytic hydrogen (H2) production represents a very promising but challenging contribution to a clean, sustainable and renewable energy system. The photocatalyst material plays a key role in photocatalytic H2 production, and it has proven difficult to obtain corrosion resistant, chemically stable, visible light harvesting and highly efficient photocatalysts, which have their band edges matching the O2 and H2 production levels. Nanoscience and nanotechnology are opening a new vista in the development of highly active, nanostructured photocatalysts with large surface areas for optimized light absorption, minimized distances (or times) for charge-carrier transport, and further favorable properties. Our focus here is on recently developed nanostructured photocatalysts. In particular, the particle size, chemical composition (including dopants), microstructure, crystal phase, morphology, surface modification, bandgap and flat-band potential of the nanophotocatalysts have shown a visible effect on photocatalytic H2 production rates, which may be further increased by adding sensitizers, cocatalysts or scavengers. Finally, potential directions required to push this research field a step further are highlighted. © 2009 Elsevier Ltd. All rights reserved.

1. Introduction Growing environmental concerns related to the extensive use of non-sustainable fossil fuels (oil, natural gas and coal) and a constantly increasing energy demand will force mankind, sooner or later, to tap into clean and sustainable sources of energy. Photocatalysis is expected to make a great contribution to both environmental treatment (emission cleaning and water purification) and renewable energy. Hydrogen (H2) is widely considered to be the future clean energy carrier in many applications, such as environmentally friendly vehicles, domestic heating, and stationary power generation. Photocatalytic H2 production from water is one of the most promising ways to realize a hydrogen economy for three reasons. (1) This technology is based on photon (or solar) energy, which is a clean, perpetual source of energy, and mainly water, which is a renewable resource; (2) it is an environmentally safe technology without undesirable by-products and pollutants; and (3) the photochemical conversion of solar energy into a storable form of energy, i.e. hydrogen, allows one to deal with the

⁎ Corresponding authors. Zhu is to be contacted at Present address: Department of Materials Chemistry, The Ångström Laboratory, Uppsala University, Box 538, 751 21 Uppsala, Sweden. Tel.: +46 18 471 3700; fax: +46 18 51 3548. Zäch, Tel.: +46 31 772 3368. E-mail addresses: [email protected], [email protected] (J. Zhu), [email protected] (M. Zäch). 1359-0294/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.cocis.2009.05.003

intermittent character and seasonal variation of the solar influx. However, it has proven difficult to find an ideal photocatalyst, which meets all the requirements (chemical stability, corrosion resistance, visible light harvesting and suitable band edges) that would render photocatalytic H2 production a viable alternative. Fortunately, nanoscience and nanotechnology have boosted the modification of existing photocatalysts and the discovery and development of new candidate materials [1], as shown in Fig. 1. The rapidly increasing number of scientific publications constitutes clear bibliographical evidence for the significance of this hot topic. Since 2004, the number of publications on nanophotocatalytic H2 production has increased by a factor of about 1.5 times every year. Many papers studied the impact of different nanostructures and nanomaterials on the performance of photocatalysts, since their energy conversion efficiency is principally determined by nanoscale properties. The structural and electronic properties of semiconductor photocatalysts largely determine the process of photocatalytic H2 production, including basic steps such as the absorption of photons, charge separation and migration, and surface reactions. A semiconductor consists of a valence band (VB) and a conduction band (CB), which are separated from one another by a bandgap (Eg), as shown in Fig. 2(c). In the ground state, all electrons exist in the VB. Under irradiation by photons with energy equivalent to or larger than Eg, some of the electrons are excited from the VB to the CB, leaving empty states, socalled holes, in the VB. These photogenerated electrons and holes may

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Fig. 1. The number of publications on (nano)photocatalytic H2 production sorted by year. The inset shows the fraction of publications on photocatalytic H2 production, which deal with nano-aspects. Data were collected from the “Web of Science”, and entries until Mar 20, 2009, have been considered.

recombine in the bulk or on the surface of the semiconductor on a time scale which is slower than the time required for their formation (Fig. 2(b) and (c)). Electrons and holes that travel to the surface of the semiconductor before they recombine can cause reduction (H2 formation) and oxidation (O2 formation) reactions, respectively. For H2 production to occur, the CB bottom-edge must be more negative than the reduction potential of H+ to H2 (EH+/H2 = 0 V vs NHE at pH = 0), while the VB top-edge should be more positive than the oxidation potential of H2O to O2 (EO2/H2O = 1.23 V vs NHE at pH = 0) for O2 formation from water to occur. Apart from the band edge requirements outlined above, there are many other requirements for an ideal semiconductor photocatalyst, of which bandgap and corrosion resistance are most important. From Fig. 2(c), the minimum bandgap of semiconductor photocatalysts for water splitting appears to be 1.23 eV, corresponding to a wavelength of 1008 nm. Note that the gap of 1.23 V between the reduction and oxidation potentials of water splitting is relatively fixed, although the reduction and oxidation potentials simultaneously shift with solution pH (−59 mV per pH unit). However, if we take into account thermodynamic losses and overpotentials that are necessary at various steps in the photocatalytic process to ensure a reasonable reaction rate, effective photocatalysts have been shown to exhibit bandgaps larger than 2 eV, corresponding to a wavelength of b620 nm [2,3]. For example, a considerable activation barrier in the charge transfer between photocatalysts and water has to be overcome by an over-

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potential. The bandgap requirement is further restricted if solar light is to be used for water splitting, which is the goal for the future. Since the intensity of sunlight is small for wavelengths below 400 nm (3.1 eV), visible light responsive photocatalysts should have bandgaps between 2 and 3.1 eV. As mentioned above, photocatalytic H2 evolution may take place only when the conduction band edge of the photocatalyst is above (i.e. more negative than) the potential of the H+/H2 redox couple. Otherwise, the flat-band potential should be adjusted to the desired level by surface chemical modification. Besides thermodynamic requirements, an ideal photocatalyst should also have kinetic advantages, i.e. high photocatalytic activity. There are a few non-oxide semiconductors (GaAs, CdS, CuInS2, etc.) that meet all of the above requirements. However, they are unstable and/or corrode easily, either dissolving or forming a thin film, which prevents electrons from transferring across the semiconductor/liquid interface. It has thus proven difficult to find a simple and cost-effective photocatalyst meeting all requirements. It seems that photocatalysts based on oxide semiconductors, which are less efficient but corrosion resistant, will be the commercial or practical alternatives. Nanostructured photocatalysts bear the potential to improve many of the weaknesses outlined above. For instance, electron-hole recombination may be reduced in nanosized photocatalysts due to short charge transfer distances, and reactant adsorption and product desorption can be enhanced due to the high surface area offered by nanostructures. This review summarizes the recent development of nanophotocatalysts for H2 production. It excludes, however, work on photoanodes used for photoelectrochemical (PEC) H2 generation, which has been reviewed elsewhere [2–5]. Considering the mechanism of the redox reactions, the principle of photocatalytic H2 production is similar to that of photoelectrochemical H2 generation. The main difference between the two approaches lies in the location of the redox reactions. In the photocatalytic process, both oxidation and reduction reactions occur on the surface of a photocatalyst, which functions as both photoanode and photocathode, and as a result, a mixture of H2 and oxidized gas (mostly O2) is evolved (Fig. 2(a) and (b)). On the other hand, oxidation and reduction take place at spatially separated photoanode and photocathode, respectively, in the photoelectrochemical process, resulting in H2 and oxidized gas (mostly O2) being evolved separately. It should be noted that the efficiency of photocatalysts is normally lower than that of photoanodes in H2 generation since hydrogen and oxygen have a tendency to react back to water if they are evolved in the same location. However, compared to photoanodes, photocatalysts do not need a conductive substrate for charge collection, so that a much broader selection of synthetic methods, such as solid-state high temperature synthesis, can be adopted. This allows photocatalysts to be prepared with relative ease

Fig. 2. (a) A suspension of photocatalyst particles in water under irradiation. (b) The processes involved in water splitting by a photocatalyst particle are photon absorption, electronhole generation (and recombination), charge transport, and oxidation/reduction reactions on the semiconductor surface. (c) The principle and energy diagram for photocatalytic water splitting on a semiconductor.

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and at a competitive cost. In a photocatalytic system, cocatalysts can be easily introduced by firing and mixing. Furthermore, studies focusing on photocatalysts may provide a convenient screening approach for the selection of suitable photoanodes. Given these advantages, the quest towards finding valuable photocatalysts for water splitting has involved a large diversity of materials and strategies. The literature distribution of different nanomaterials, which have been reported to be capable of photocatalytic H2 production, is shown in Fig. 3. Most photocatalysts are semiconductors or based on semiconductors (composites or mixtures). Among them, the classic photocatalyst TiO2 (including modified TiO2) has been the most attractive since the initial work of Fujishima and Honda [6], due to its corrosion resistance, non-toxicity and low price. In recent years, the work on nanostructured TiO2 has focused on novel synthesis methods yielding TiO2 with special properties, and the modification of TiO2 by dopants, sensitizers, cocatalysts, scavengers and other semiconductors in order to enhance the visible light response and efficiency. In addition, some novel, alternative materials, e.g. oxynitrides and oxysulfides synthesized mainly by the Domen group [7•], have also shown exciting results in this field. 2. Nanophotocatalysts for H2 production Photocatalysts are typically made of metal oxides (see Sections 2.1–2.3 below), metal sulfides (Section 2.4), oxysulfides (Section 2.5), oxynitrides (Section 2.5) and composites thereof (Section 2.6). In most cases, metal cations with the highest oxidative states in photocatalysts have d0 (red area in Fig. 4) or d10 (green area) electronic configuration, while O, S and N (blue area) show their most negative states. The bottom of the conduction band consists of the d and sp orbitals of the metal cations, while the top of the valence band in metal oxides is composed of O 2p orbitals, which are normally located at ca. + 3 V (vs NHE) or higher [7*,8•]. The valence bands of metal oxysulfides and oxynitrides are formed by S 3p (and O 2p) and N 2p (and O 2p), respectively. Some alkali (Li, Na, K, Rb or Cs), alkaline earth (Mg, Ca, Sr and Ba) and transition-metal ions (Y, La or Gd) can construct the crystal structure of layered perovskite and cubic pyrochlore compounds, but do not contribute to the energy band structure of these compounds [8•]. 2.1. Nanostructured TiO2 Compared to other photocatalysts for H2 production, TiO2 has received more attention because it is stable, corrosion-resistant, non-

Fig. 3. Statistical distribution of scientific publications focusing on nanomaterials for photocatalytic H2 production. The total number of published papers focusing on photocatalytic nanomaterials until Oct 31, 2008 is 138, as obtained from the “Web of Science”. Papers were grouped into several categories based on materials chemistry, where TiO2, non-TiO2 oxides, oxysulfides and oxynitrides include both the pure material and modifications thereof (e.g. noble metal deposition), respectively. The category labeled “composites and mixtures” mainly includes couples of different semiconductors or photocatalysts.

toxic, abundant and cheap. Currently, there are two factors, which limit its practical and economical application, however. On the one hand, the photon-to-hydrogen efficiency is too low due to rapid recombination of photogenerated electrons and holes as well as the fast back-reaction of H2 and O2 to H2O. On the other hand, it is inactive under visible light irradiation due to its large bandgap, which impedes the use of TiO2 as a solar energy harvesting photocatalyst. In order to overcome these deficiencies, many bulk, surface and operating environment modifications have been conducted. Nanostructured mesoporous TiO2 has been studied recently due to its high surface area and special porous structure, which is beneficial to both the absorption of reactants and desorption of products. Nanocrystalline mesoporous TiO2 was prepared by a sol–gel process with surfactants [9,10]. The photocatalytic activity of mesoporous TiO2 calcined at 600 °C for H2 evolution was considerably higher than that of commercial TiO2, Ishihara ST-01 and Degussa P25. Pt nanoparticles with 1–2 nm diameter can easily be introduced into mesoporous TiO2 during the gelation [11,12]. Mesoporous TiO2 with smaller TiO2 crystallite size and higher surface area can be prepared by a hydrothermal method [13]. To obtain high photocatalytic H2 production activity, it is crucial to control the thermal treatment conditions leading to high crystallinity of the anatase phase without the formation of rutile. However, Peng et al. have reported that hydrothermally synthesized TiO2 nanoparticles without calcination, having a large specific surface area (438 m2/g) and small crystallites (2.3 nm) dispersed among amorphous mesoporous domains, exhibited the best photocatalytic activity for H2 production compared with samples calcined at different temperatures and the commercial photocatalyst P25 [14]. A new synthesis route was carried out within a day through a template-free and non-hydrothermal route, in which adding a KCl electrolyte controlled the electrostatic repulsive force between TiO2 nanoparticles towards the formation of a mesoporous structure [15•]. The photocatalytic activity for H2 evolution of the thus prepared meso-TiO2 was the highest compared to nonporous colloidal-TiO2 and commercial Degussa P25 and Hombikat UV-100 (HBK) samples. The photocatalytic reduction of metal cations (M= Ni2+, Co2+, Cu2+, Cd2+, Zn2+, Fe2+, Ag+, Pb2+) on the surface of mesoporous TiO2 (specific surface area 130–140 m2/g, pore diameter 5–9 nm and anatase content 70–90%), synthesized by the sol–gel technique, was found to result in the formation of nanostructured metal-semiconductor composites (TiO2/M) [16]. It was shown that the photocatalytic activity of TiO2/ M samples increases with increasing anatase content in the original mesoporous titania, which is in agreement with results from the references mentioned above [9,10,13]. These metal–TiO2 nanostructures exhibit a remarkable photocatalytic activity for hydrogen evolution from water–alcohol mixtures, their efficiency being 50–60% greater than that of the metal-containing nanocomposites based on Degussa P25. The anatase content and pore size were the basic parameters determining the photoreaction rate. The rate of the photocatalytic hydrogen formation in water–ethanol mixtures was found to also depend strongly on the metal nature, increasing from silver to nickel to copper. This dependence was interpreted in terms of different electronic interactions between metal nanoparticles and TiO2 surface. The morphology of TiO2 plays a very important role in the efficiency of photocatalysis for H2 production. One-dimensional TiO2 (nanowires, nanorods, nanotubes and nanofibers) has attracted more and more attention. Compared to spherical particles, one-dimensional TiO2 nanostructures could provide a high surface area and a high interfacial charge transfer rate. The carriers are free to move throughout the length of these nanostructures, which is expected to reduce the e−/h+ recombination probability. TiO2 nanotubes promoted with Pt metal were prepared and found to be a photocatalytic dehydrogenation catalyst in neat ethanol for producing H2 gas [17]. The prepared TiO2 nanotubes consist of small anatase grains, and have high surface areas (BET surface area of 250–300 m2/g) and large one-dimensional

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Fig. 4. Elements constructing photocatalysts for water splitting.

mesopores/macropores (total pore volume of 0.88 ml/g). These properties make them a suitable candidate to be utilized as a photocatalyst. Self-organized TiO2 nanotube-layers were formed by electrochemical anodization of Ti in a HF electrolyte [18]. Pt was deposited on the TiO2 nanotube layer by plasma sputtering. The Pt–TiO2 nanotube photocatalyst generated H2 successfully from an alkaline water solution. Ptionized TiO2 nanotubes were reported for the stoichiometric production of H2 and O2 by water splitting under visible light with hydrogen evolution rates of 14.6 and 2.3 µmol/h in aqueous methanol and pure water, respectively [19]. Nanostructured TiO2 films with controlled morphology and thickness were synthesized in an ambient-pressure, single-step flame aerosol reactor (FLAR) for use in water splitting photocells and dye-sensitized solar cells [20]. Two different morphologies were studied: a granular morphology and a highly crystalline columnar morphology. Due to differences in electron transport and lifetime in the TiO2 film, the columnar morphology outperformed the granular morphology for both applications, achieving a UV light to hydrogen conversion efficiency of 11% for water splitting, and a visible light to electricity conversion efficiency of 6.0% for the dye-sensitized solar cell. The columnar morphology consisted of single-crystal structures, approximately 85 nm in width and oriented normal to the substrate. It was found that the conversion efficiency for water splitting was maximized for an optimum thickness of 1.5 μm, which was a tradeoff between light absorption and electron transport losses. TiO2 nanowires (TiO2 NWs) were synthesized through a one-step hydrothermal process in 10 M NaOH (aq.) at 150 °C for 72 h and post heat treatment at 300–1000 °C for 2 h [21]. As the temperature of the post heat treatment increased, as-synthesized TiO2 NWs were transformed into TiO2 (B), anatase and rutile gradually while preserving 1D morphology. Among all the crystal structures, anatase TiO2 NWs exhibited the highest photocatalytic H2 evolution rate, which was also higher than that of the starting TiO2 powder (Degussa P25). However, Lin et al. suggested that a bi-crystalline structure consisting of TiO2 (B) nanotubes (or nanofibers) and anatase nanoparticles might act as an active, H2-producing photocatalyst [22,23]. The synergetic effect of bi-crystalline TiO2 (B) and anatase has also been observed in photocatalytic degradation of dyes [24,25]. Fabrication and design of TiO2 thin film photocatalysts play an important role in H2 production, since TiO2 thin films have many advantages for practical application in photocatalytic decomposition of water, compared to TiO2 powder. Anpo's group has reported on the separate evolution of H2 and O2 using a visible light responsive TiO2 (Vis-TiO2) thin film photocatalyst prepared by a radio frequency magnetron sputtering (RF-MS) deposition method [26•]. The RF-MS

deposition method uses a TiO2 plate as the sputtering target and Ar as the sputtering gas. The precise control of the substrate temperature (Ts) is a major factor in the synthesis of these Vis-TiO2 thin film photocatalysts. It was shown that the film prepared at Ts = 873 K consisted of rutile, and had the highest photocatalytic activity under both UV and visible light irradiation. Compared to anatase (bandgap = 3.2 eV), rutile has a narrower bandgap (3.0 eV), which, to some extent, benefits visible light absorption. Additionally, the unique declining O/Ti ratio from the surface to the bulk of the prepared Vis-TiO2 thin films was suggested to cause a significant perturbation of the electronic structure of TiO2, enabling the absorption of visible light for stable photocatalytic reactions. Visible light was considered to be absorbed by the inside bulk of the film rather than the surface, while UV light is absorbed in the surface regions because Vis-TiO2 is covered with a stoichiometric TiO2 phase. A similar appearance of visible light activity due to newly formed oxygen vacancy states between the valence and the conduction bands in the TiO2 band structure has been reported in [27]. The RF-MS VisTiO2 films were prepared on one side of a metal (e.g. Ti) foil, while the opposite side was covered with Pt. The prepared photocatalyst assembly was then mounted on an H-type glass container, as shown in Fig. 5, separating two aqueous solutions. The TiO2 side of the photocatalyst was immersed in 1.0 M NaOH aqueous solution, and the Pt side was immersed in 0.5 M H2SO4 solution in order to add a small chemical bias. Even under visible light, water could be decomposed into separated H2 and O2 with a good linearity against the irradiation time. It was also reported that the greater the pH difference between the two compartments of the H-type container, the higher the yield of H2 and O2. This indicates that the pH difference provides a small chemical bias as an electrical driving force for electron transfer from the TiO2 to the Pt side through the Ti foil substrate. This unique design applies the advantages of a photoelectrochemical cell in a photocatalytic system for H2 production without any sacrificial reagents. When these novel Vis-TiO2 thin film photocatalysts were prepared on various metal substrates such as Al, Fe, Pd, Pt, Ti and Zr, the photocatalytic H2 evolution rate was revealed to increase with a decrease in the work function of the substrate (work function: Pt N Pd N Fe N Ti N Zr N Al), suggesting that the work function of the substrates plays a significant role in the electron transfer from the TiO2 moiety to the Pt side through the metal substrate [28]. Furthermore, the effect of chemical etching by HF solution on the photoelectrochemical and photocatalytic performances of Vis-TiO2 thin films has been investigated [29]. The Vis-TiO2 thin films treated with HF solution (HF-Vis-TiO2) exhibited a larger surface area and higher

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Fig. 5. (Left) H-type glass container for the separate evolution of H2 and O2 using a TiO2 thin film photocatalyst (TiO2 side: 1.0 M NaOH aq; Pt side: 0.5 M H2SO4 aq). (Right) TiO2 thin film photocatalytic system prepared on a metal substrate with an oxidation site (TiO2) on the irradiated side and a reduction site (Pt nanoparticles) on the back side for the decomposition of H2O into O2 and H2. Reprinted with permission from [26•], M. Anpo et al., Top Catal 2008; 49: 4–17. © Springer Science+Business Media, LLC 2008.

donor density than Vis-TiO2, indicating that the remarkable increase in the photocatalytic water splitting rate and photocurrent in HF-VisTiO2 may be due to the short diffusion length required for holes to reach the solid–liquid interface as well as to the high conductivity. 2.2. Other binary metal oxides In addition to TiO2, there are some other traditional metal oxides, which have also been investigated extensively due to their specific advantages. Among them, ZnO, α-Fe2O3 and WO3 are representative. However, they all have their inherent drawbacks in photocatalytic H2 production. ZnO is easily photo-corroded under bandgap irradiation by photogenerated holes. WO3 is a stable photocatalyst for O2 evolution under visible light irradiation, but no H2 production is observed due to its low conduction band level. α-Fe2O3 has the same problem as WO3 and, moreover, is not very stable in acidic solutions. Photocatalytic decomposition of liquid water into H2 and O2 by Ta2O5 has been studied since 1994 [30]. A nanocrystalline mesoporous Ta2O5 photocatalyst for H2 production was recently synthesized through a combined sol–gel process with a surfactant-assisted templating mechanism [31]. The effects of NiO cocatalyst loading and Fe doping have also been studied [31,32]. Nanostructured VO2 photocatalysts for hydrogen production were reported by Zhang et al. [33•]. The monoclinic and tetragonal phases known for bulk VO2 are seldom used as photocatalysts due to the narrow bandgap of 0.7 eV. However, the prepared nanorod VO2, which has a body-centered-cubic (bcc) structure and a large optical bandgap of 2.7 eV, shows excellent photocatalytic activity for hydrogen production. Using films of aligned VO2 nanorods, the hydrogen production rate reaches a high rate of 800 mmol/m2/h from a mixture of water and ethanol under UV light irradiation at a power density of 27 mW/cm2, thus constituting a good example of how crystal structure and morphology may affect photocatalytic activity.

separated by the photocatalytic niobate sheet [8•]. Furthermore, an electric field gradient originating from the uneven K+ distribution on opposite sides of the niobate sheets assists electron-hole separation. Potassium hexaniobate nanoscrolls (NS-K4Nb6O17) were studied as building blocks for visible light-driven H2 production from water using tris(2,2′-bipyridyl)ruthenium(II) (Ru(bpy)2+ 3 ) chloride as a sensitizer and ethylenediaminetetraacetic acid (EDTA) as an electron donor [35]. Interestingly, H2K2Nb6O17 nanosheets and H4Nb6O17 nanoscrolls obtained from K4Nb6O17 can preserve the photocatalytic activity for H2 evolution observed for the parent compound [36]. A Dion–Jacobson type layered perovskite (ACa2Nb3O10, A = H or K) has been studied as a photocatalyst for H2 production using methanol as a sacrificial donor [37–40]. Both Pt and Rh2O3 can act as cocatalysts to speed up H2 evolution. Photogenerated charge carriers relaxed with second order kinetics on a sub-nanosecond time scale depending on the nanosheet size. A series of H2 production photocatalysts based on K4Ce2M10O30 (M = Ta, Nb) and their solid solution K4Ce2Ta10 − XNbXO30 (X = 0–10) was prepared [41•,42] and shown to have an appropriate bandgap energy of ca. 1.8–2.3 eV (corresponding to an absorption edge of 540– 690 nm). Density functional theory (DFT) calculations indicated that the valence bands of these photocatalysts are composed of hybridization with O 2p + Ta 5d (or Nb 4d) and occupied Ce 4f orbitals, while the conduction bands are mainly contributed by the Ta 5d (or Nb 4d) orbitals (Fig. 7). K4Ce2M10O30 (M = Ta, Nb) have a parallelepiped (tunnel) surface structure. This is beneficial to the formation of “nanonests”, to which the cocatalysts, nanoparticles of Pt, RuO2 and NiOX, can be strongly associated, thus avoiding aggregation and improving photocatalytic H2 generation greatly. This is a classic example of the effect of surface nanostructures on photocatalytic performance.

2.3. Ternary and quaternary metal oxides Investigations of more complex oxides with three and four elements are significant. K4Nb6O17 was reported as a novel niobate photocatalyst for H2 evolution in 1986 [34]. It has a layered structure with two kinds of interlayers. H2 is evolved from one interlayer, in which cocatalysts are introduced by ion exchange or interlayer reaction, while O2 production occurs in the other interlayer, as shown in Fig. 6. In this case, the sites for H2 and O2 evolution are

Fig. 6. Water splitting over K4Nb6O17 photocatalyst with layered structure. Reprinted with permission from [8•], A. Kudo et al., Chem Soc Rev 2009; 38: 253–78. © RSC Publishing 2009.

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Fig. 7. Bandgap structure of K4Ce2M10O30 (M = Ta, Nb) and comparison with redox couples for photocatalytic production of H2 and O2 from water. Reprinted with permission from [41•], WF Shangguan, Sci Tech Adv Mater 2007; 8: 76–81. © NIMS and Elsevier Ltd. 2006.

Highly efficient photocatalytic H2 generation can also be obtained with different nanostructured titanates, such as cubic SrTiO3 [43], Ruddlesden–Popper type layered perovskite Sr3Ti2O7 [44], (110)layered perovskite La2Ti2O7 [45], Na2Ti2O4(OH)2 [46] and perovskite K2La2Ti3O10 [47]. Tantalates of the ATaO3 (A: Li, Na, and K) type consist of cornersharing TaO6 octahedra with perovskite-like structures. NaTaO3 shows the highest photocatalytic activity among the ATaO3 (A: Li, Na, and K) type photocatalysts when a NiO cocatalyst is loaded, which is due to the suitable conduction band level consisting of Ta 5d and energy delocalization caused by the small distortion of TaO6 connections [8•]. Furthermore, the photocatalytic activity of NiO-loaded NaTaO3 doped with lanthanum was 9 times higher than that of nondoped NiOloaded NaTaO3 [48]. The small particle size and the ordered nanostep surface structure of the NiO/NaTaO3:La photocatalyst resulting from doping contributed to highly efficient water splitting. H2 evolution proceeded on ultrafine NiO cocatalyst particles, which were loaded on the edges of the nanostep structure of NaTaO3:La nanoparticles, while O2 evolved in the grooves of the nanostep structure. In this way, the reaction sites for H2 and O2 evolution were separated, avoiding the back-reaction to water. This result underlines the importance of the nanostructured surface morphology of photocatalysts. 2.4. Metal sulfides Metal sulfides are normally considered attractive candidates for visible light responsive photocatalysis. The valence band of metal sulfides normally consist of 3p orbitals of S, which result in a more negative valence band and narrower bandgap as compared to metal oxides. Recent studies have focused on CdS, ZnS and their solid solutions. CdS has a suitable bandgap (2.4 eV) and good band positions for visible light assisted water splitting. However, S2− in CdS is easily oxidized by photogenerated holes, which is accompanied by the elution of Cd2+ into the solution, similar to ZnO. Such photocorrosion is in fact a common problem to most metal sulfide photocatalysts. CdS can yet be an outstanding photocatalyst for H2 production under visible light irradiation in the presence of a hole scavenger (S2− or SO2− 3 ). ZnS is another good photocatalyst for H2 production, but its 3.6 eV bandgap corresponds to UV light. Recent developments aiming at improving CdS and ZnS photocatalysts can be divided into four directions: (1) synthesis of onedimensional and porous CdS; (2) doping and formation of solid solutions of CdS and ZnS; (3) addition of cocatalysts on CdS; and (4) development of support and matrix structures for CdS. (1) A solvothermal method was applied to synthesize CdS nanorods [49] and nanowires [50], which have high photocatalytic activity for H2 production. Mesoporous CdS nanoparticles with an average pore size of 54 Å and a particle size of 4–6 nm have been prepared by template-free, ultrasonic-mediated precipitation at room temperature [51]. Pt-loaded mesoporous CdS shows a hydrogen production rate of 1415 µmol/h/0.1 g catalyst. Nanoporous CdS, including nanosheets and hollow nanorods, has been prepared by a two-step aqueous route, which consists of

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an initial precipitation of nanoporous Cd(OH)2 intermediates and a subsequent S2−/OH− ion exchange [52]. The obtained CdS nanostructures contain pores with 3 nm diameter and exhibit a very high photocatalytic H2 yield if they are loaded with monodispersed, 3–5 nm large Pt nanocrystals. (2) As mentioned above, ZnS has too large a bandgap to respond to visible light. Doping and the formation of solid solutions of ZnS and narrow bandgap semiconductors can enhance the visible light utilization of ZnS. (AgIn)XZn2(1 − X)S2 solid solutions between ZnS and AgInS2 with a narrow bandgap showed photocatalytic activities for H2 evolution from aqueous solutions containing sacrificial reagents, S2− and SO2− 3 , under visible light irradiation (wavelength larger than or equal to 420 nm) [53•] The absorption of the solid solutions shifted monotonically to longer wavelengths as the ratio of AgInS2 to ZnS increased. The dependence of the photophysical and photocatalytic properties upon the composition was mainly due to a change in band position caused by the contribution of the Ag 4d and S 3p, and Zn 4s4p and In 5s5p orbitals to the valence and conduction bands, respectively. ZnS and CdS have similar crystal structures, which make them form solid solutions easily. ZnS–CdS solid solutions were reported to be active for H2 production under visible light irradiation [54–57]. (3) Novel cocatalysts besides noble metals have been found matching CdS. The rate of H2 evolution on CdS is increased by up to 36 times when loaded with only 0.2 wt.% of MoS2, with the activity of MoS2/CdS being even higher than those of CdS photocatalysts loaded with different noble metals, such as Pt, Ru, Rh, Pd, and Au [58•]. The WC/CdS nanocomposite photocatalyst presented in [59] also exhibited a high rate of H2 production, comparable to that of Pt/CdS, under visible light irradiation from water containing sulfide and sulfite ions as hole scavengers. Like the Pt cocatalyst loaded on the surface of CdS, WC provides active sites for proton reduction and causes fast diffusion of photogenerated electrons from CdS towards WC, leading to efficient charge separation and high photocatalytic activity for H2 production. (4) The photocatalytic H2 production by nanosized CdS was enhanced by immobilization of CdS on various supports, such as aluminum-substituted mesoporous silica molecular sieve (Al-HMS) [60], microporous and mesoporous silicas [61], porous and rough polyethylene terephthalate fibers (PET) [62], a designated glass [63], modified (cyclized) polyphenylene sulfide (PPS) [64] and ETS-4 zeolite [65]. 2.5. Nitrides, oxynitrides and oxysulfides Nitride, oxynitride and oxysulfide photocatalysts have been systematically investigated by Domen's group. Here, the oxynitride and oxysulfide photocatalysts differ from photocatalysts doped with N or S. For oxynitrides and oxysulfides with metals of d0 electronic configuration, like TaON and Sm2Ti2S2O5, the valence band mainly consists of hybridized N 2p (S 3p) and O 2p orbitals, while the conduction band is mostly composed of the empty d orbitals of the corresponding metal. In this case, oxynitrides or oxysulfides thus contain N or S as constituent elements to form the valence band. Thus, photogenerated holes can migrate smoothly in the valence band of the photocatalysts, which is particularly advantageous for water oxidation involving a 4-electron transfer. In contrast, the dopant levels in the forbidden bandgap are usually discrete, which is unfavorable to the migration of photogenerated holes. From the viewpoint of the electronic band structure, d10 metal (like Ge or Ga) based semiconductors are advantageous over the d0 configurations in that the bottom of the conduction band is composed of hybridized sp orbitals of metals. The hybridized sp orbitals possess large dispersion, leading

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to increased mobility of photogenerated electrons in the conduction band and thus high photocatalytic activity [7•]. As a d10 electronic configuration metal nitride, β-Ge3N4 was synthesized from GeO2 powder by nitridation under a flow of NH3 at 1153–1173 K for 10 h [66•]. When modified with RuO2 nanoparticles, the material performed photocatalytic stoichiometric evolution of H2 and O2 from pure water. DFTcalculations revealed that the tops of the valence band are formed by N 2p orbitals, while the bottoms of the conduction band consist of hybridized 4 s and 4p orbitals of Ge. An oxynitride with d10 electronic configuration, (Ga1 − XZnX)(N1 − XOX) which can decompose water under visible light, was devised in [67•]. As both GaN and ZnO have wurtzite structures with similar lattice parameters, a solid solution between them could be synthesized. Despite the large bandgaps of plain GaN and ZnO (N3 eV), the p–d (i.e., N2p–Zn3d) repulsion in the GaN– ZnO solid solution shifts the top of the valence band formed by N 2p orbitals upward, resulting in a narrower bandgap. The bandgaps of the solid solutions were estimated to be 2.4–2.8 eV by diffuse reflectance spectra. DFT calculations indicate that the visible light response of the solid solutions originates from the contribution of Zn 3d orbitals to the valence band. Like β-Ge3N4 modified with RuO2 nanoparticles as H2 evolution sites, (Ga1 − XZnX)(N1 − XOX) catalyzed observable, steady and stoichiometric H2 and O2 evolution under visible light irradiation (λ N 400 nm). The activity of (Ga1 − XZnX)(N1 − XOX) modified with various transition-metal oxides as cocatalysts was further improved by co-loading Cr, which is due to the formation of suitable reaction sites by intimate interaction between Cr and the paired metal component [68]. Noble-metal/Cr2O3 core/shell nanoparticles prepared by in situ photodeposition were also developed as a new type of H2 evolution cocatalyst [69•]. In this system, the Rh nanoparticles forming the core induced the migration of photogenerated electrons from the (Ga1 − XZnX)(N1 − XOX) bulk to the surface, while the Cr2O3 shell provided a H2 evolution site at the external surface, which blocked the back-reaction of H2 and O2 to H2O on Rh. Post-calcination at moderate temperature resulted in an improved performance of (Ga1 − XZnX)(N1 − XOX) in photocatalytic water splitting under visible light [70]. The improvement is attributable to the reduction of zinc- and/or oxygen-related defects, which act as recombination centers.

In a conventional system for photocatalytic water splitting, semiconductor photocatalysts should possess both a CB above (i.e. more negative than) the reduction potential of H+ to H2 and a VB below (i.e. more positive than) the oxidation potential of H2O to O2, as shown in Fig. 8(a). This constraint seriously limits photocatalyst selection and visible light utilization. A Z-scheme photocatalytic water splitting system, which involved two-step photo-excitation under visible light irradiation, has been developed by mimicking the natural photosynthesis of green plants [80]. The Z-scheme system consisted of a H2evolution photocatalyst (PS1 in Fig. 8(b)), an O2-evolution photocatalyst (PS2 in Fig. 8(b)), and a reversible redox mediator (Ox/Red), which acted separately as electron donor (PS1) and acceptor (PS2) for the respective half reaction, and which was different from irreversible sacrificial reagents used in conventional systems. Photocatalysts, which alone are only effective in carrying out one of the two half reactions in water splitting, are capable of running both half reactions when arranged in a Z-scheme, which is the merit of this two-photon system. SrTiO3, TaON, CaTaO2N and BaTaO2N can work as H2-evolution photocatalysts, while WO3, BiVO4 and Bi2MoO6 can act as O2-evolution photocatalysts. The IO3−/I− and Fe3+/Fe2+ redox couples normally form reversible electron mediators [80–83]. An all-solid-state Z-scheme, based on a CdS–Au–TiO2 three-component nanojunction, was recently reported, where PS1(CdS), PS2(TiO2) and the electron-transfer system (Au) were spatially fixed [84•]. The vectorial electron transfer of TiO2 → Au → CdS occurs as a result of excitation of both TiO2 and CdS. The electron supply from TiO2 to CdS via Au restricts the selfdecomposition of CdS due to the oxidation of surface S2− ions by the photogenerated holes in CdS. In a Z-scheme system, H2 and O2 are evolved separately from two different photocatalysts, which, to some extent, restrains the back-reaction of water decomposition. The key factors in the design of a Z-scheme system are to find a pair of photocatalysts for separate H2 and O2 production with high efficiency, and a reversible electron mediator, the redox potential of which can meet the requirements of being electron donor and acceptor in the respective unit reactions. The energy levels and charge transfer should also be considered when designing both nanocomposites and Z-scheme systems. 3. Summary and outlook

2.6. Nanocomposites and Z-scheme systems In order to separate photogenerated electrons and holes and/or utilize visible light, nanocomposites have been extensively studied in photocatalytic H2 production. The intercalation of TiO2 , CdS, Cd0.8Zn0.2S and Fe2O3 nanoparticles into layered compounds such as H2Ti4O9, H4Nb6O17, K2Ti3.9Nb0.1O9, HNbWO6, HTaWO6, HTiNbO5 and HTiTaO5 has been reported [71–75], where intercalation suppresses the growth of the nanoparticles. When these intercalated nanoparticles are excited by bandgap irradiation, the photogenerated electrons can be quickly transferred to the matrix layered compounds. In the LaMnO3/CdS nanocomposite, the holes photogenerated by visible light in the valence band of CdS can move to the valence band of LaMnO3 and react with electron donors (Na2S and Na2SO3), while photogenerated electrons remain in the conduction band of CdS and react with water to produce H2. This charge–carrier separation at the nanoscale is responsible for the improved photocatalytic activity [76]. Photocatalytic H2 production via water splitting was also reported on Ni/NiO/KNbO3/CdS nanocomposites using visible light irradiation at wavelengths longer than 400 nm in the presence of isopropanol [77]. The observed high photocatalytic activity is due to effective charge separation of photogenerated electrons and holes in CdS, which is achieved by electron injection into the conduction band of KNbO3. From the above examples, it is clear that the band positions of different constituents in nanocomposites must match well. Furthermore, the preparation method and the cocatalyst location are also important factors in nanocomposites. This was recently demonstrated in CdS/TiO2/Pt [78,79].

Photocatalytic H2 production offers unique opportunities to develop an alternative and sustainable energy system and to reduce

Fig. 8. Reaction mechanism of photocatalytic water splitting into H2 and O2 for: (a) a conventional one-step photo-excitation system, and (b) a system mimicking the Zscheme of photosynthesis (two-step photo-excitation system). Reprinted with permission from [80], Arakawa H et al., J Photochem Photobio A: Chem 2002; 148: 71–7. © 2002 Elsevier Science B.V.

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Fig. 9. Existing band engineering approaches: (a) cation doping, which creates a discrete impurity energy level (DL) within the forbidden bandgap; (b) valence band modification, which forms a new valence band with higher top; and (c) solid solution formation, producing a new couple of valence and conduct bands, whose bandgap is between those of the component semiconductors.

emission of greenhouse gases. In photocatalytic H2 production, photocatalyst materials play a crucial role. Novel photocatalysts are expected to be developed, while the existing materials should be modified and optimized. Nanostructured photocatalysts are expected to be a future trend, since nanosized photocatalysts have shown much better performance than their bulk counterparts. It is difficult to say what kind of material is perfect or best, since there is no existing photocatalyst that meets all the requirements, and all candidates have their specific drawbacks. Metal oxide photocatalysts are definitely considered promising materials. Although most metal oxides show low photonic efficiency, these cheap materials possess good corrosion resistance, and hence can exhibit stable performance for a long time. In the light of the preceding literatures, TiO2 has been and will be one of the most important photocatalysts for H2 production, owing to its availability, low price, non-toxicity, high photoactivity and stability. The biggest disadvantage of TiO2 is the low utilization of visible light, as a consequence of its wide bandgap. Multi-component metal oxides with perovskite structures like ATaO3 (A = Li, Na, K), Sr2M2O7 (M = Nb, Ta), AmBmO3m + 2 (m = 4, 5; A = Ca, Sr, La; B = Nb, Ti), RbLnTa2O7 (Ln = La, Pr, Nb, Sm), pyrochlore structures like Bi2MNbO7 (M = Al, Ga, In), RbWMO6 (M = Nb, Ta), A2Ta2O6 (A = Na, K), orthorhombic structures like ATa2O6 (A = Ca, Sr, Ba), tungsten bronze structures like K4Ce2M10O30 (M = Ta, Nb), columbite structures like NiM2O6 (M = Nb, Ta), tunnel structures like MIn2O4 (M = Ca, Sr) and scheelite structures like AMO4 (A = Ag, Bi; M = Mo, W) have been scrutinized in detail. Some of these complex oxides have suitable band structures for visible light water splitting, since more metal elements can contribute to the construction of valence and/or conduction band. Some display specific “nest” nanostructures, which are suitable for nanosized cocatalyst loading. Metal sulfides can be applied in practical photocatalytic H2 production systems if sulfur by-products such as hydrogen sulfide from the hydrogenated desulfurization process in petrochemical plants and mining industries are available and plentiful. Solid solutions from two or more semiconductors are ideal photocatalytic systems driven by visible light, the optical and photocatalytic properties of which can be adjusted gradually by suitable choices of the constituents and their contents. Oxynitrides and oxysulfides have been found to be stable photocatalysts for water reduction and oxidation under visible irradiation. This finding provides good clues to fine-tuning the valence band position. The ability to adjust and fine-tune bandgap and band positions is instrumental in developing visible light responsive photocatalysts for H2 production. Three approaches towards band engineering have been reported, namely (i) cation doping, (ii) valence band modification and (iii) solid solution formation [85•], as shown in Fig. 9. Valence band modification (like TaON and Sm2Ti2S2O5 mentioned in Section 2.5) and solid solutions (like (AgIn)XZn2(1 − X)S2 mentioned in Section 2.4) are promising and effective due to the formation of new bands,

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while discrete donor levels are introduced by cation doping. These discrete levels are inconvenient for the migration of photogenerated holes, and the donor levels easily act as recombination centers. Appropriate band construction is necessary, but not sufficient. In order to obtain efficient H2 production, combining different techniques and approaches is indispensable. First of all, a database of existing nanophotocatalysts has to be established, since plenty of nanostructured materials have already been used in photocatalytic H2 production. The accumulation of a large amount of empirical/experimental and theoretical data is important in the quest to foster a deep understanding of the preparation, properties and performance of photocatalysts and their optimization for water splitting. This database should include information regarding the nanomaterials used, preparation methods, modifications, photocatalytic reaction environments and activity, etc. Using this database will avoid repeated, unnecessary work and direct the development of new nanophotocatalysts. With the aid of crystallography, physics and chemistry handbooks, and model calculations, new nanophotocatalysts will be designed and screened, and their band structures may be obtained. Novel synthesis methods for nanophotocatalysts should be developed, since synthetic methods strongly affect material performance. Low-dimensional nanostructures with high aspect ratio and porous nanostructures will certainly attract more attention, as they favor charge and mass transport, respectively. There is a requirement to understand the interfacial and local properties of nanophotocatalysts, such as the interfaces between photocatalyst, cocatalyst and water, and the location of grain edges and boundaries. The charge and mass transport processes in these areas, which are predominant in nanostructures, are different from those in the bulk phase. In most cases, photocatalytic H2 production can be increased dramatically by modifying the catalyst's nanostructure with cocatalysts. Hence, the design of “nano-nest” structures on photocatalysts is important in order to host nanosized cocatalysts in well-defined and tailored positions. The reaction kinetics in photocatalytic H2 production has to be emphasized to a greater extent if maximum efficiency is expected. Theoretical and modelling work is useful and imperative in better understanding of the process and mechanism of nanophotocatalytic H2 production as well as designing new nanophotocatalysts and miniphotoreactors. Calculations provide a molecular picture of hydrogen production on catalytic surfaces, which may allow us to design such catalytic surfaces on the basis of insight [86]. Multitechnology integration will provide a bright prospect for photocatalytic H2 production by nanomaterials. Acknowledgment The authors gratefully acknowledge the Foundation for Strategic Environmental Research (Mistra; Dnr 2004-118) for financial support. References and recommended reading [1] Zäch M, Hägglund C, Chakarov D, Kasemo B. Nanoscience and nanotechnology for advanced energy systems. Curr Opin Solid State Mater Sci 2006;10:132–43. [2] Alexander BD, Kulesza PJ, Rutkowska L, Solarska R, Augustynski J. Metal oxide photoanodes for solar hydrogen production. J Mater Chem 2008;18:2298–303. [3] Bak T, Nowotny J, Rekas M, Sorrell CC. Photo-electrochemical hydrogen generation from water using solar energy. Int J Hydrogen Energy 2002;27:991–1022. [4] Currao A. Photoelectrochemical water splitting. Chimia 2007;61:815–9. [5] Aroutiounian VM, Arakelyan VM, Shahnazaryan GE. Metal oxide photoelectrodes for hydrogen generation using solar radiation-driven water splitting. Sol Energy 2005;78:581–92. [6] Fujishima A, Honda K. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972;238:37–8. [7] Maeda K, Domen K. New non-oxide photocatalysts designed for overall water • splitting under visible light. J Phys Chem C 2007;111:7851–61 [This article presents recent research progress in the development of visible light-driven photocatalysts,

• ••

Of special interest. Of outstanding interest.

268

[8] •

[9]

[10]

[11]

[12]

[13]

[14]

[15] •

[16]

[17]

[18] [19]

[20]

[21] [22]

[23]

[24]

[25]

[26] •

[27]

[28]

[29]

[30]

[31]

[32]

J. Zhu, M. Zäch / Current Opinion in Colloid & Interface Science 14 (2009) 260–269 focusing on the refinement of non-oxide-type photocatalysts such as oxynitrides and oxysulfides.]. Kudo A, Miseki Y. Heterogeneous photocatalyst materials for water splitting. Chem Soc Rev 2009;38:253–78 [This review shows the basics of photocatalytic water splitting and experimental points, and surveys heterogeneous photocatalysts for water splitting into H2 and O2, and H2 or O2 evolution from an aqueous solution containing a sacrificial reagent.]. Sreethawong T, Suzuki Y, Yoshikawa S. Synthesis, characterization, and photocatalytic activity for hydrogen evolution of nanocrystalline mesoporous titania prepared by surfactant-assisted templating sol–gel process. J Solid State Chem 2005;178: 329–38. Sreethawong T, Suzuki Y, Yoshikawa S. Photocatalytic evolution of hydrogen over nanocrystalline mesoporous titania prepared by surfactant-assisted templating sol–gel process. Catal Commun 2005;6:119–24. Sreethawong T, Puangpetch T, Chavadej S, Yoshikawa S. Quantifying influence of operational parameters on photocatalytic H2 evolution over Pt-loaded nanocrystalline mesoporous TiO2 prepared by single-step sol–gel process with surfactant template. J Power Sources 2007;165:861–9. Sreethawong T, Yoshikawa S. Enhanced photocatalytic hydrogen evolution over Pt supported on mesoporous TiO2 prepared by single-step sol–gel process with surfactant template. Int J Hydrogen Energy 2006;31:786–96. Jitputti J, Pavasupree S, Suzuki Y, Yoshikawa S. Synthesis and photocatalytic activity for water-splitting reaction of nanocrystalline mesoporous titania prepared by hydrothermal method. J Solid State Chem 2007;180:1743–9. Yi H, Peng T, Ke D, Ke D, Zan L, Yan C. Photocatalytic H2 production from methanol aqueous solution over titania nanoparticles with mesostructures. Int J Hydrogen Energy 2008;33:672–8. Lakshminarasimhan N, Bae E, Choi W. Enhanced photocatalytic production of H2 on mesoporous TiO2 prepared by template-free method: Role of interparticle charge transfer. J Phys Chem C 2007;111:15244–50 [This paper reports a meso-TiO2 exhibiting an enhanced photocatalytic activity for H2 evolution synthesized by a simple and facile, template-free non-hydrothermal method. The enhanced photocatalytic activity of meso-TiO2 is ascribed to the compact and dense packing of TiO2 nanoparticles forming a uniform agglomerate, which enables efficient charge separation through interparticle charge transfer.]. Korzhak A, Ermokhina N, Stroyuk A, Bukhtiyarov V, Raevskaya A, Litvin V, et al. Photocatalytic hydrogen evolution over mesoporous TiO2/metal nanocomposites. J Photochem Photobio A-Chem 2008;198:126–34. Lin CH, Lee CH, Chao JH, Kuo CY, Cheng YC, Huang WN, et al. Photocatalytic generation of H2 gas from neat ethanol over Pt/TiO2 nanotube catalysts. Catal Lett 2004;98:61–6. Nam W, Han GY. Preparation and characterization of anodized Pt-TiO2 nanotube arrays for water splitting. J Chem Eng Jpn 2007;40:266–9. Khan MA, Akhtar MS, Woo SI, Yang OB. Enhanced photoresponse under visible light in Pt ionized TiO2 nanotube for the photocatalytic splitting of water. Catal Commun 2008;10:1–5. Thimsen E, Rastgar N, Biswas P. Nanostructured TiO2 films with controlled morphology synthesized in a single step process: performance of dye-sensitized solar cells and photo watersplitting. J Phys Chem C 2008;112:4134–40. Jitputti J, Suzuki Y, Yoshikawa S. Synthesis of TiO2 nanowires and their photocatalytic activity for hydrogen evolution. Catal Commun 2008;9:1265–71. Kuo HL, Kuo CY, Liu CH, Chao JH, Lin CH. A highly active bi-crystalline photocatalyst consisting of TiO2 (B) nanotube and anatase particle for producing H2 gas from neat ethanol. Catal Lett 2007;113:7–12. Lin CH, Chao JH, Liu CH, Chang JC, Wang FC. Effect of calcination temperature on the structure of a Pt/TiO2 (B) nanofiber and its photocatalytic activity in generating H2. Langmuir 2008;24:9907–15. Zhu J, Zhang J, Chen F, Anpo M. Preparation of high photocatalytic activity TiO2 with a bicrystalline phase containing anatase and TiO2 (B). Mater Lett 2005;59: 3378–81. Zhu J, Zhang J, Chen F, Iino K, Anpo M. High photocatalytic activity TiO2 prepared by modified sol–gel method: characterization and their photocatalytic activity for degradation of XRG and X-GL. Top Catal 2005;35:261–8. Kitano M, Tsujimaru K, Anpo M. Hydrogen production using highly active titanium oxide-based photocatalysts. Top Catal 2008;49:4–17 [This paper reviews the development of photocatalytic water splitting by TiO2, and reports a unique TiO2 thin film system for the separate evolution of H2 and O2 from water under sunlight irradiation.]. Nakamura I, Negishi N, Kutsuna S, Ihara T, Sugihara S, Takeuchi K. Role of oxygen vacancy in the plasma-treated TiO2 photocatalyst with visible light activity for NO removal. J Mol Catal A: Chem 2000;161:205–12. Kitano M, Tsujimaru K, Anpo M. Decomposition of water in the separate evolution of hydrogen and oxygen using visible light-responsive TiO2 thin film photocatalysts: Effect of the work function of the substrates on the yield of the reaction. Appl Catal A-Gen 2006;314:179–83. Kitano M, Iyatani K, Tsujimaru K, Matsuoka M, Takeuchi M, Ueshima M, et al. The effect of chemical etching by HF solution on the photocatalytic activity of visible light-responsive TiO2 thin films for solar water splitting. Top Catal 2008;49:24–31. Sayama K, Arakawa H. Effect of Na2CO3 addition on photocatalytic decomposition of liquid water over various semiconductor catalysts. J Photochem Photobio A: Chem 1994;77:243–7. Sreethawong T, Ngamsinlapasathian S, Suzuki Y, Yoshikawa S. Nanocrystalline mesoporous Ta2O5-based photocatalysts prepared by surfactant-assisted templating sol–gel process for photocatalytic H2 evolution. J Mol Catal A-Chem 2005;235: 1–11. Jing DW, Guo LJ. Hydrogen production over Fe-doped tantalum oxide from an aqueous methanol solution under the light irradiation. J Phys Chem Solids 2007;68: 2363–9.

[33] Wang YQ, Zhang ZJ, Zhu Y, Li ZC, Vajtai R, Ci LJ, et al. Nanostructured VO2 • photocatalysts for hydrogen production. Acs Nano 2008;2:1492–6 [This paper reports a new crystal structure for nanostructured VO2, with bodycentered-cubic (bcc) structure and a large optical band gap of 2.7 eV, which shows excellent photocatalytic activity in hydrogen production.]. [34] Domen K, Kudo A, Shinozaki A, Tanaka A, Maruya K, Onishi T. Photodecomposition of water and hydrogen evolution from aqueous methanol solution over novel niobate photocatalysts. J Chem Soc Chem Commun 1986:356–7. [35] Maeda K, Eguchi M, Youngblood WJ, Mallouk TE. Niobium oxide nanoscrolls as building blocks for dye-sensitized hydrogen production from water under visible light irradiation. Chem Mater 2008;20:6770–8. [36] Sarahan MC, Carroll EC, Allen M, Larsen DS, Browning ND, Osterloh FE. K4Nb6O17derived photocatalysts for hydrogen evolution from water: nanoscrolls versus nanosheets. J Solid State Chem 2008;181:1678–83. [37] Compton OC, Carroll EC, Kim JY, Larsen DS, Osterloh FE. Calcium niobate semiconductor nanosheets as catalysts for photochemical hydrogen evolution from water. J Phys Chem C 2007;111:14589–92. [38] Carroll EC, Compton OC, Madsen D, Osterloh FE, Larsen DS. Ultrafast carrier dynamics in exfoliated and functionalized calcium niobate nanosheets in water and methanol. J Phys Chem C 2008;112:2394–403. [39] Compton OC, Mullet CH, Chiang S, Osterloh FE. A building block approach to photochemical water-splitting catalysts based on layered niobate nanosheets. J Phys Chem C 2008;112:6202–8. [40] Hata H, Kobayashi Y, Bojan V, Youngblood WJ, Mallouk TE. Direct deposition of trivalent rhodium hydroxide nanoparticles onto a semiconducting layered calcium niobate for photocatalytic hydrogen evolution. Nano Lett 2008;8:794–9. [41] Shangguan WF. Hydrogen evolution from water splitting on nanocomposite • photocatalysts. Sci Tech Adv Mater 2007;8:76–81 [This paper provides an overview of the promoting effects of nanosized modifications in the interlayer and surface of photocatalysts for hydrogen evolution with visible light.]. [42] Tian MK, Shangguan WF, Yuan J, Wang SJ, Ouyang ZY. Promotion effect of nanosized Pt, RuO2 and NiOX loading on visible light-driven photocatalysts K4Ce2M10O30 (M = Ta, Nb) for hydrogen evolution from water decomposition. Sci Tech Adv Mater 2007;8:82–8. [43] Liu Y, Xie L, Li Y, Yang R, Qu JL, Li YQ, et al. Synthesis and high photocatalytic hydrogen production of SrTiO3 nanoparticles from water splitting under UV irradiation. J Power Sources 2008;183:701–7. [44] Jeong H, Kim T, Kim D, Kim K. Hydrogen production by the photocatalytic overall water splitting on NiO/Sr3Ti2O7: effect of preparation method. Int J Hydrogen Energy 2006;31:1142–6. [45] Arney D, Porter B, Greve B, Maggard PA. New molten-salt synthesis and photocatalytic properties of La2Ti2O7 particles. J Photochem Photobio A-Chem 2008;199:230–5. [46] Li QY, Lu GX. Visible-light driven photocatalytic hydrogen generation on eosin Ysensitized Pt-loaded nanotube Na2Ti2O4(OH)2. J Mol Catal A-Chem 2007;266: 75–9. [47] Tai YW, Chen JS, Yang CC, Wan BZ. Preparation of nano-gold on K2La2Ti3O10 for producing hydrogen from photo-catalytic water splitting. Catal Today 2004;97: 95–101. [48] Kato H, Asakura K, Kudo A. Highly efficient water splitting into H2 and O2 over lanthanum-doped NaTaO3 photocatalysts with high crystallinity and surface nanostructure. J Am Chem Soc 2003;125:3082–9. [49] Janet CM, Viswanath RP. Large scale synthesis of CdS nanorods and its utilization in photo-catalytic H2 production. Nanotech 2006;17:5271–7. [50] Jang JS, Joshi UA, Lee JS. Solvothermal synthesis of CdS nanowires for photocatalytic hydrogen and electricity production. J Phys Chem C 2007;111: 13280–7. [51] Sathish A, Viswanath RP. Photocatalytic generation of hydrogen over mesoporous CdS nanoparticle: effect of particle size, noble metal and support. Catal Today 2007;129:421–7. [52] Bao NZ, Shen LM, Takata T, Domen K. Self-templated synthesis of nanoporous CdS nanostructures for highly efficient photocatalytic hydrogen production under visible light. Chem Mater 2008;20:110–7. [53] Tsuji I, Kato H, Kobayashi H, Kudo A. Photocatalytic H2 evolution reaction from • aqueous solutions over band structure-controlled (AgIn)XZn2(1 − X)S2 solid solution photocatalysts with visible-light response and their surface nanostructures. J Am Chem Soc 2004;126:13406–13 [This paper indicates that the control of band structure by making a solid solution is a useful strategy for band engineering to develop a visible light-driven photocatalyst.]. [54] Zhang K, Jing DW, Xing CJ, Guo LJ. Significantly improved photocatalytic hydrogen production activity over Cd1 − XZnXS photocatalysts prepared by a novel thermal sulfuration method. Inter J Hydrogen Energy 2007;32:4685–91. [55] Liu GJ, Zhao L, Ma LJ, Guo LJ. Photocatalytic H2 evolution under visible light irradiation on a novel CdXCuYZn1 − X − YS catalyst. Catal Commun 2008;9:126–30. [56] Zhang XH, Jing DW, Liu MC, Guo LJ. Efficient photocatalytic H2 production under visible light irradiation over Ni doped Cd1 − XZnXS microsphere photocatalysts. Catal Commun 2008;9:1720–4. [57] Zhang W, Zhong ZY, Wang YS, Xu R. Doped solid solution: (Zn0.95Cu0.05)1 − XCdXS nanocrystals with high activity for H2 evolution from aqueous solutions under visible light. J Phys Chem C 2008;112:17635–42. [58] Zong X, Yan HJ, Wu GP, Ma GJ, Wen FY, Wang L, et al. Enhancement of • photocatalytic H2 evolution on CdS by loading MoS2 as cocatalyst under visible light irradiation. J Am Chem Soc 2008;130:7176–7 [This work presents not only a possibility for the use of MoS2 as a cocatalyst substitute for noble metals in the photocatalytic H2 production but also an important concept that the proper junction structure between cocatalyst and semiconductor is crucial for high photocatalytic activity.].

J. Zhu, M. Zäch / Current Opinion in Colloid & Interface Science 14 (2009) 260–269 [59] Jang JS, Ham DJ, Lakshminarasimhan N, Choi WY, Lee JS. Role of platinum-like tungsten carbide as cocatalyst of CdS photocatalyst for hydrogen production under visible light irradiation. Appl Catal A-Gen 2008;346:149–54. [60] Zhang YJ, Zhang L. Synthesis of composite material CdS/Al-HMS and hydrogen production by photocatalytic pollutant degradation under visible light irradiation. J Inorg Mater 2008;23:66–70. [61] Ryu SY, Balcerski W, Lee TK, Hoffmann MR. Photocatalytic production of hydrogen from water with visible light using hybrid catalysts of CdS attached to microporous and mesoporous silicas. J Phys Chem C 2007;111:18195–203. [62] Lunawat PS, Senapati S, Kumar R, Gupta NM. Visible light-induced splitting of water using CdS nanocrystallites immobilized over water-repellant polymeric surface. Inter J Hydro Ener 2007;32:2784–90. [63] Kale BB, Baeg JO, Apte SK, Sonawane RS, Naik SD, Patil KR. Confinement of nano CdS in designated glass: a novel functionality of quantum dot-glass nanosystems in solar hydrogen production. J Mater Chem 2007;17:4297–303. [64] Kanade KG, Baeg JO, Mulik UP, Amalnerkar DP, Kale BB. Nano-CdS by polymerinorganic solid-state reaction: visible light pristine photocatalyst for hydrogen generation. Mater Res Bull 2006;41:2219–25. [65] Guan GQ, Kida T, Kusakabe K, Kimura K, Fang XM, Ma TL, et al. Photocatalytic H2 evolution under visible light irradiation on CdS/ETS-4 composite. Chem Phys Lett 2004;385:319–22. [66] Sato J, Saito N, Yamada Y, Maeda K, Takata T, Kondo JN, et al. RuO2-loaded β-Ge3N4 • as a non-oxide photocatalyst for overall water splitting. J Am Chem Soc 2005;127:4150–1 [This paper reports β-Ge3N4, a nitride of a typical metal with d10 electronic configuration, to photocatalyze water decomposition into hydrogen and oxygen when combined with RuO2 nanoparticles, which is the first successful example of a non-oxide photocatalyst for overall water splitting.]. [67] Maeda K, Teramura K, Lu DL, Takata T, Saito N, Inoue Y, et al. Photocatalyst releasing • hydrogen from water. Nature 2006;440:295 [This paper describes the overall splitting of water under visible light by a solid solution of gallium and zinc nitrogen oxide, (Ga1 − XZnX)(N1 − XOX), modified with nanoparticles of a mixed oxide of rhodium and chromium.]. [68] Maeda K, Teramura K, Saito N, Inoue Y, Domen K. Improvement of photocatalytic activity of (Ga1 − XZnX)(N1 − XOX) solid solution for overall water splitting by coloading Cr and another transition metal. J Catal 2006;243:303–8. [69] Maeda K, Teramura K, Lu D, Saito N, Inoue Y, Domen K. Noble-metal/Cr2O3 core/ • shell nanoparticles as a cocatalyst for photocatalytic overall water splitting. Angew Chem Int Ed 2006;45:7806–9 [This paper describes a new cocatalyst consisting of a noble-metal core and a Cr2O3 shell prepared for use with the (Ga1 − XZnX)(N1 − XOX) solid solution, and this catalytic system is demonstrated to be effective for visible light-driven overall water splitting.]. [70] Maeda K, Teramura K, Domen K. Effect of post-calcination on photocatalytic activity of (Ga1 − XZnX)(N1 − XOX) solid solution for overall water splitting under visible light. J Catal 2008;254:198–204. [71] Uchida S, Yamamoto Y, Fujishiro Y, Watanabe A, Ito O, Sato T. Intercalation of titanium oxide in layered H2Ti4O9 and H4Nb6O17 and photocatalytic water cleavage with H2Ti4O9/(TiO2, Pt) and H4Nb6O17/(TiO2, Pt) nanocomposites. J Chem SocFaraday Trans 1997;93:3229–34.

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[72] Shangguan WF, Yoshida A. Photocatalytic hydrogen evolution from water on nanocomposites incorporating cadmium sulfide into the interlayer. J Phys Chem B 2002;106:12227–30. [73] Yin S, Maeda D, Ishitsuka M, Wu J, Sato T. Synthesis of HTaWO6/(Pt,TiO2) nanocomposite with high photocatalytic activities for hydrogen evolution and nitrogen monoxide destruction. Solid State Ionics 2002;151:377–83. [74] Wu JH, Lin JM, Yin S, Sato T. Synthesis and photocatalytic properties of layered HNbWO6/(Pt, Cd0.8Zn0.2S) nanocomposites. J Mater Chem 2001;11:3343–7. [75] Jang JS, Kim HG, Reddy VR, Bae SW, Ji SM, Lee JS. Photocatalytic water splitting over iron oxide nanoparticles intercalated in HTiNb(Ta)O5 layered compounds. J Catal 2005;231:213–22. [76] Kida T, Guan GQ, Minami Y, Ma TL, Yoshida A. Photocatalytic hydrogen production from water over a LaMnO3/CdS nanocomposite prepared by the reverse micelle method. J Mater Chem 2003;13:1186–91. [77] Choi J, Ryu SY, Balcerski W, Lee TK, Hoffmann MR. Photocatalytic production of hydrogen on Ni/NiO/KNbO3/CdS nanocomposites using visible light. J Mater Chem 2008;18:2371–8. [78] Park H, Choi W, Hoffmann MR. Effects of the preparation method of the ternary CdS/TiO2/Pt hybrid photocatalysts on visible light-induced hydrogen production. J Mater Chem 2008;18:2379–85. [79] Jang JS, Choi SH, Kim HG, Lee JS. Location and state of Pt in platinized CdS/TiO2 photocatalysts for hydrogen production from water under visible light. J Phys Chem C 2008;112:17200–5. [80] Sayama K, Mukasa K, Abe R, Abe Y, Arakawa H. A new photocatalytic water splitting system under visible light irradiation mimicking a Z-scheme mechanism in photosynthesis. J Photochem Photobio A: Chem 2002;148:71–7. [81] Higashi M, Abe R, Teramura K, Takata T, Ohtani B, Domen K. Two step water splitting into H2 and O2 under visible light by ATaO2N (A = Ca, Sr, Ba) and WO3 with shuttle redox mediator. Chem Phys Lett 2008;452:120–3. [82] Abe R, Takata T, Sugihara H, Domen K. Photocatalytic overall water splitting under visible light by TaON and WO3 with an IO3−/I− shuttle redox mediator. Chem Commun 2005:3829–31. [83] Kato H, Hori M, Konta R, Shimodaira Y, Kudo A. Construction of Z-scheme type heterogeneous photocatalysis systems for water splitting into H2 and O2 under visible light irradiation. Chem Lett 2004;33:1348–9. [84] Tada H, Mitsui T, Kiyonaga T, Akita T, Tanaka K. All-solid-state Z-scheme in CdS– • Au–TiO2 three-component nanojunction system. Nat Mater 2006;5:782–6 [This paper reports an anisotropic CdS–Au–TiO2 nanojunction, in which PS1(CdS), PS2 (TiO2) and the electron-transfer system (Au) are spatially fixed. This threecomponent system exhibits a higher photocatalytic activity than those of the single- and two-component systems, due to vectorial electron transfer driven by the two-step excitation of TiO2 and CdS.]. [85] Kudo A. Recent progress in the development of visible light-driven powdered • photocatalysts for water splitting. Int J Hydrogen Energy 2007;32:2673–8 [This paper reviews the recent progress in the development of visible light-driven photocatalysts for water splitting]. [86] Nørskov JK, Christensen CH. Toward efficient hydrogen production at surfaces. Science 2006;312:1322–3.