Heterogeneous photocatalyst materials for water splitting

Abstract

This critical review shows the basis of photocatalytic water splitting and experimental points, and surveys heterogeneous photocatalyst materials for water splitting into H₂ and O₂, and H₂ or O₂ evolution from an aqueous solution containing a sacrificial reagent. Many oxides consisting of metal cations with d⁰ and d¹⁰ configurations, metal (oxy)sulfide and metal (oxy)nitride photocatalysts have been reported, especially during the latest decade. The fruitful photocatalyst library gives important information on factors affecting photocatalytic performances and design of new materials. Photocatalytic water splitting and H₂ evolution using abundant compounds as electron donors are expected to contribute to construction of a clean and simple system for solar hydrogen production, and a solution of global energy and environmental issues in the future (361 references).

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Akihiko Kudo

Akihiko Kudo was born in Tokyo. He received his early education at Tokyo University of Science obtaining a BS degree in 1983 and his PhD degree in 1988 from Tokyo Institute of Technology. After one and half years as a post-doctoral fellow at the University of Texas at Austin he became a Research Associate at the Tokyo Institute of Technology until 1995. He then joined the Tokyo University of Science as a Lecturer before he became Associate Professor in 1998 and Full Professor in 2003. His research interests include photocatalysts for water splitting, electrocatalysis and luminescence materials.

Yugo Miseki

Yugo Miseki was born in Tokyo. He received BS and MS degrees from Tokyo University of Science in 2004 and 2006, respectively. He is currently on a doctorate course in Tokyo University of Science under the supervision of Professor Akihiko Kudo. His research interests include the development of novel photocatalysts for water splitting.

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1. Introduction

Energy and environmental issues at a global level are important topics. It is indispensable to construct clean energy systems in order to solve the issues. Hydrogen will play an important role in the system because it is an ultimate clean energy and it can be used in fuel cells. Moreover, hydrogen is used in chemical industries. For example, a large amount of hydrogen is consumed in industrial ammonia synthesis. At present, hydrogen is mainly produced from fossil fuels such as natural gas by steam reforming.

CH₄ + H₂O → CO + 3H₂ (1)

CO + H₂O → CO₂ + H₂ (2)

In this process, fossil fuels are consumed and CO₂ is emitted. Hydrogen has to be produced from water using natural energies such as sunlight if one thinks of energy and environmental issues. Therefore, achievement of solar hydrogen production from water has been urged. There are several ways for solar hydrogen production.

(i) Electrolysis of water using a solar cell, a hydroelectric power generation, etc.

(ii) Reforming of biomass.

(iii) Photocatalytic or photoelectrochemical water splitting (artificial photosynthesis).

 

The characteristic point of water splitting using a powdered photocatalyst is the simplicity as shown in Fig. 1. Sun shines at photocatalyst powders dispersed in a pool with water, and then hydrogen is readily obtained. The necessity of separation of H₂ evolved from O₂ is disadvantageous toward the photocatalytic water splitting process. However, the problem will be able to be overcome using a Z-scheme photocatalyst system. Moreover, powdered photocatalyst systems will be advantageous for large-scale application of solar water splitting because of the simplicity. So, photocatalytic water splitting is an attractive reaction and will contribute to an ultimate green sustainable chemistry and solving energy and environmental issues resulting in bringing an energy revolution.

Fig. 1 Solar hydrogen production from water using a powdered photocatalyst.

The photon energy is converted to chemical energy accompanied with a largely positive change in the Gibbs free energy through water splitting as shown in Fig. 2. This reaction is similar to photosynthesis by green plants because these are uphill reactions. Therefore, photocatalytic water splitting is regarded as an artificial photosynthesis and is an attractive and challenging theme in chemistry. From the viewpoint of the Gibbs free energy change, photocatalytic water splitting is distinguished from photocatalytic degradation reactions such as photo-oxidation of organic compounds using oxygen molecules that are generally downhill reactions. This downhill-type reaction is regarded as a photoinduced reaction and has been extensively studied using TiO₂ photocatalysts.^1,2

Fig. 2 Photosynthesis by green plants and photocatalytic water splitting as an artificial photosynthesis.

The Honda–Fujishima effect of water splitting using a TiO₂ electrode was reported in the early 1970s. When TiO₂ is irradiated with UV light electrons and holes are generated as shown in Fig. 3.³ The photogenerated electrons reduce water to form H₂ on a Pt counter electrode while holes oxidize water to form O₂ on the TiO₂ electrode with some external bias by a power supply or pH difference between a catholyte and an anolyte. Numerous researchers had extensively studied water splitting using semiconductor photoelectrodes and photocatalysts since the finding. However, efficient materials for water splitting into H₂ and O₂ under visible light irradiation had not been found. Accordingly, the photon energy conversion by water splitting using photocatalysts had been considered to be pessimistic, and its research activity had been sluggish. However, new photocatalyst materials for water splitting have recently been discovered one after another. Although the photon energy conversion using powdered photocatalysts is not at the stage of practical use, the research in photocatalytic water splitting is being advanced. The photocatalytic water splitting is still a challenging reaction even if the research history is long.

Fig. 3 Honda–Fujishima effect-water splitting using a TiO₂ photoelectrode.³

Many reviews and books for photocatalytic water splitting have been published.^4–30 In the present review, we focus on heterogeneous photocatalyst materials of metal oxides, metal (oxy)sulfides and metal (oxy)nitrides for water splitting into H₂ and O₂ in stoichiometric amount, and H₂ or O₂ evolution from an aqueous solution containing a sacrificial reagent. After the bases of photocatalytic water splitting are interpreted heterogeneous photocatalyst materials are surveyed. Factors affecting photocatalytic performances and strategies of photocatalyst design are discussed through the general viewpoint. Some applications of newly developed photocatalysts to other photocatalytic reactions such as degradation of organic compounds are also introduced.

2. Bases of photocatalytic water splitting

2.1 Processes in photocatalytic water splitting

Fig. 4 shows the main processes in a photocatalytic reaction.

Fig. 4 Main processes in photocatalytic water splitting.

The first step (i) is absorption of photons to form electron–hole pairs. Many heterogeneous photocatalysts have semiconductor properties. Photocatalytic reactions proceed on semiconductor materials as schematically shown in Fig. 5. Semiconductors have a band structure in which the conduction band is separated from the valence band by a band gap with a suitable width. When the energy of incident light is larger than that of a band gap, electrons and holes are generated in the conduction and valence bands, respectively. The photogenerated electrons and holes cause redox reactions similarly to electrolysis. Water molecules are reduced by the electrons to form H₂ and are oxidized by the holes to form O₂ for overall water splitting. Important points in the semiconductor photocatalyst materials are the width of the band gap and levels of the conduction and valence bands. The bottom level of the conduction band has to be more negative than the redox potential of H⁺/H₂ (0 V vs. NHE), while the top level of the valence band be more positive than the redox potential of O₂/H₂O (1.23 V). Therefore, the theoretical minimum band gap for water splitting is 1.23 eV that corresponds to light of about 1100 nm.

Band gap (eV) = 1240/λ (nm) (3)

Band levels of various semiconductor materials are shown in Fig. 6. The band levels usually shift with a change in pH (−0.059 V/pH) for oxide materials.^4,29,30 ZrO₂, KTaO₃, SrTiO₃ and TiO₂ possess suitable band structures for water splitting. These materials are active for water splitting when they are suitably modified with co-catalysts. Although CdS seems to have a suitable band position and a band gap with visible light response it is not active for water splitting into H₂ and O₂. S²⁻ in CdS rather than H₂O is oxidized by photogenerated holes accompanied with elution of Cd²⁺ according to the eqn (4).³⁰

CdS + 2h⁺→ Cd²⁺ + S (4)

This reaction is called photocorrosion and is often a demerit of a metal sulfide photocatalyst. ZnO is also photocorroded under band gap excitation even if it is an oxide photocatalyst.

ZnO + 2h⁺→ Zn²⁺ + 1/2O₂ (5)

However, CdS is an excellent photocatalyst for H₂ evolution under visible light irradiation if a hole scavenger exists as mentioned in section 2.2. On the other hand, WO₃ is a good photocatalyst for O₂ evolution under visible light irradiation in the presence of an electron acceptor such as Ag⁺ and Fe³⁺ but is not active for H₂ evolution because of its low conduction band level. The band structure is just a thermodynamic requirement but not a sufficient condition. The band gap of a visible-light-driven photocatalyst should be narrower than 3.0 eV (λ > 415 nm). Therefore, suitable band engineering is necessary for the design of photocatalysts with visible light response as mentioned in section 7.1.1.

Fig. 5 Principle of water splitting using semiconductor photocatalysts.

Fig. 6 Relationship between band structure of semiconductor and redox potentials of water splitting.⁵

The second step (ii) in Fig. 4 consists of charge separation and migration of photogenerated carriers. Crystal structure, crystallinity and particle size strongly affect the step as shown in Fig. 7. The higher the crystalline quality is, the smaller the amount of defects is. The defects operate as trapping and recombination centers between photogenerated electrons and holes, resulting in a decrease in the photocatalytic activity. If the particle size becomes small, the distance that photogenerated electrons and holes have to migrate to reaction sites on the surface becomes short and this results in a decrease in the recombination probability.

Fig. 7 Effects of particle size and boundary on photocatalytic activity.

The final step (iii) in Fig. 4 involves the surface chemical reactions. The important points for this step are surface character (active sites) and quantity (surface area). Even if the photogenerated electrons and holes possess thermodynamically sufficient potentials for water splitting, they will have to recombine with each other if the active sites for redox reactions do not exist on the surface. Co-catalysts such as Pt, NiO and RuO₂ are usually loaded to introduce active sites for H₂ evolution because the conduction band levels of many oxide photocatalysts are not high enough to reduce water to produce H₂ without catalytic assistance. Active sites for 4-electron oxidation of water are required for O₂ evolution. Co-catalysts are usually unnecessary for oxide photocatalysts because the valence band is deep enough to oxidize water to form O₂ as mentioned in section 7.1.1. This is the characteristic point of heterogeneous photocatalysts being different from homogeneous photocatalysts for which O₂ evolution with 4-electron oxidation of H₂O is a challenging reaction. Back reactions to form water by reactions between evolved H₂, O₂, and intermediates easily proceed because of an uphill reaction. Therefore, poor properties for the back reactions are required for the surface of co-catalyst and photocatalyst.

Fig. 8 shows how the processes indicated in Fig. 4 are affected by conditions of a photocatalyst in the case of TiO₂. The TiO₂ photocatalyst is prepared by several methods. For example, amorphous TiO₂ that may be denoted as TiO₂·nH₂O is obtained by hydrolysis of titanium tetra-isopropoxide. When the amorphous TiO₂ is calcined some factors are simultaneously changed. Anatase and rutile are obtained through phase transition. The band gap of anatase is 3.2 eV while that of rutile is 3.0 eV indicating that the crystal structure determines the band gap even if the composition is the same. The difference in the band gap between anatase and rutile is mainly due to the difference in the conduction band level. The conduction band level of anatase is higher than that of rutile leading the difference in photocatalytic abilities between anatase and rutile (brookite TiO₂ is selectively prepared by a hydrothermal method).³¹ Crystallinity is increased by calcination: that is a positive factor as shown in Fig. 7. The crystallinity is confirmed from half-widths of peaks of XRD patterns and also observation by electron microscopes. On the other hand, the surface area (as determined by BET measurement) is decreased with an increase in particle size through sintering: that is a negative factor. Small particle size sometimes gives a quantum size effect as seen in colloidal particles resulting in widening of band gap and blue shift in the absorption spectrum. The resultant photocatalytic activity is dominated by the balance among these factors. A high degree of crystallinity is often required rather than a high surface area for water splitting because recombination between photogenerated electrons and holes is especially a serious problem for uphill reactions. In contrast, high surface area is necessary for photocatalytic degradation of organic compounds because adsorption of the organic compound is the important process. Concentration of surface hydroxyl groups may also affect photocatalytic activity.³²

Fig. 8 Conditions affecting photocatalytic activity of TiO₂.

Many photocatalysts are also materials for solar cells, phosphors and dielectrics. However, the significant difference between the photocatalyst and the other materials is that chemical reactions are involved in the photocatalytic process, but not in the other physical properties. Only when three steps shown in Fig. 4 are simultaneously completed photocatalytic activities can be obtained. Thus, suitable bulk and surface properties, and energy structure are required for photocatalysts. So, it is understandable that photocatalysts should be highly functional materials.

2.2 Photocatalytic H₂ or O₂ evolution in sacrificial systems

Sacrificial reagents are often employed to evaluate the photocatalytic activity for water splitting as shown in Fig. 9, because overall water splitting is a tough reaction. When the photocatalytic reaction is carried out in an aqueous solution including a reducing reagent, in other words, electron donors or hole scavengers, such as alcohol and a sulfide ion, photogenerated holes irreversibly oxidize the reducing reagent instead of water. It enriches electrons in a photocatalyst and an H₂ evolution reaction is enhanced as shown in Fig. 9(a). This reaction will be meaningful for realistic hydrogen production if biomass and abundant compounds in nature and industries are used as the reducing reagents.^33–38 On the other hand, photogenerated electrons in the conduction band are consumed by oxidizing reagents (electron acceptors or electron scavengers) such as Ag⁺ and Fe³⁺ resulting in that an O₂ evolution reaction is enhanced as shown in Fig. 9(b). These reactions using sacrificial reagents are studied to evaluate if a certain photocatalyst satisfies the thermodynamic and kinetic potentials for H₂ and O₂ evolution. These reactions are regarded as half reactions of water splitting and are often employed as test reactions of photocatalytic H₂ or O₂ evolution as mentioned in sections 6 and 7. Even if a photocatalyst is active for these half reactions the results do not guarantee a photocatalyst to be active for overall water splitting into H₂ and O₂ in the absence of sacrificial reagents. From this point, the term of “water splitting” should be distinguishably used for H₂ or O₂ evolution from aqueous solutions in the presence of sacrificial reagents. Water splitting means to split water into H₂ and O₂ in a stoichiometric amount in the absence of sacrificial reagents.

Fig. 9 H₂ or O₂ evolution reaction in the presence of sacrificial reagents—Half reactions of water splitting.

3. Experimental method for water splitting

3.1 Points that should be paid attention

The points that should be paid attention to evaluate photocatalytic water splitting are shown in Fig. 10.⁷

Fig. 10 Important points for evaluation of data for photocatalytic water splitting.

(i) Stoichiometry of H₂ and O₂ evolution. In water splitting, both H₂ and O₂ should form with a stoichiometric amount, 2:1, in the absence of a sacrificial reagent. Often H₂ evolution is observed with a lack of O₂ evolution. In this case, the amount of H₂ evolution is usually small compared with an amount of a photocatalyst. It is not clear if such a reaction is photocatalytic water splitting and it is important to clarify that it is not a sacrificial reaction.

(ii) Time course. Amounts of H₂ and O₂ evolved should increase with irradiation time. To check not only the value of activity or a gas evolution rate but also the time course is important. Repeated experiment is also informative.

(iii) Turnover number (TON). Amounts of H₂ and O₂ should overwhelm an amount of a photocatalyst. If the amounts are much less than the amount of photocatalyst we do not know if the reaction proceeds photocatalytically because the reaction might be due to some stoichiometric reactions. Turnover number (TON) is usually defined by the number of reacted molecules to that of an active site (eqn (6)).However, it is often difficult to determine the number of active sites for photocatalysts. Therefore, the number of reacted electrons to the number of atoms in a photocatalyst (eqn (7)) or on the surface of a photocatalyst (eqn (8)) is employed as the TON.

The number of reacted electrons is calculated from the amount of evolved H₂. The TONs (7) and (8) are smaller than the real TON (6) because the number of atoms is more than that of active sites. Normalization of photocatalytic activity by weight of used photocatalyst (for example, μmol h⁻¹ g⁻¹) is not acceptable because the photocatalytic activity is not usually proportional to the weight of photocatalyst if an amount of photocatalyst is enough for a certain experimental condition. The amount of photocatalyst should be optimized for an each experimental setup. In this case, photocatalytic activity usually depends on the number of photons absorbed by a photocatalyst unless the light intensity is too strong.

(iv) Quantum yield. The rate of gas evolution is usually indicated with a unit, for example μmol h⁻¹. Because the photocatalytic activity depends on the experimental conditions such as a light source and a type of a reaction cell, the activities cannot be compared with each other if the reaction conditions are different from each other. Therefore, determination of a quantum yield is important. The number of incident photons can be measured using a thermopile or Si photodiode. However, it is hard to determine the real amount of photons absorbed by a photocatalyst in a dispersed system because of scattering. So, the obtained quantum yield is an apparent quantum yield (9). The apparent quantum yield is estimated to be smaller than the real quantum yield because the number of absorbed photons is usually smaller than that of incident light.It should be noteworthy that the quantum yield is different from the solar energy conversion efficiency that is usually used for evaluation of solar cells.The number of photocatalysts that can give good solar energy conversion efficiency is limited at the present stage because of insufficient activities for the measurement. However, the solar energy conversion efficiency should finally be used to evaluate the photocatalytic water splitting if solar hydrogen production is considered.

(v) Photoresponse. When a photocatalyst is irradiated with light of energy larger than the band gap, water splitting should proceed. An action spectrum is indispensable to see the photoresponse, especially for a photocatalyst with visible light response (band-path and interference filters are usually employed to obtain monochromatic light for the action spectrum measurement). Even if a material absorbs visible light it does not always show a photocatalytic activity by the excitation of the visible light absorption band. Cut-off filters are sometimes used to see the photoresponse. In this case the onset of the photoresponse can be measured. Water splitting by mechanocatalysis proceeds on some metal oxides under stirring and dark condition.^8,39 Some control experiments such as no photocatalysts or non-irradiation have to be carried out to confirm the photocatalytic reaction and neglect the possibility of the mechanocatalytic water splitting.

There are many other points that researchers have to pay attention. The details of experiments for general photocatalysis are described in the literature by Ohtani.⁴⁰

3.2 Experimental setup

There are several types of apparatus for water splitting. The present authors have usually used a gas-closed circulation system equipped with a vacuum line, a reaction cell and a gas sampling port that is directly connected to a gas chromatograph as shown in Fig. 11. If a photocatalytic activity is too high to use a gas chromatograph a volumetric method is employed for determination of evolved gases. The apparatus should be air-free because the detection of O₂ is very important for evaluation of photocatalytic water splitting. There are several reaction cells. In general, efficient irradiation is conducted when an inner irradiation reaction cell is used. A high-pressure mercury lamp is often used with a quartz cell for photocatalysts with wide band gaps when intensive UV light with wavelength shorter than about 300 nm is especially needed. When visible light irradiation is necessary a Xe-lamp with a cut-off filter is usually employed. It is important to know the spectrum of the incident light. It depends on a light source, a material of a reaction cell, an optical filter, a mirror, etc. A solar simulator that is a standard light source for evaluation of solar cells should ideally be used if solar hydrogen production is considered. A solar simulator with an air-mass 1.5 filter (AM-1.5) irradiates 100 mW cm⁻² of power.

Fig. 11 An example of the experimental setup for photocatalytic water splitting.

4. General view of elements constructing heterogeneous photocatalyst materials

Fig. 12 shows elements constructing heterogeneous photocatalyst materials. The elements are classified into four groups; (i) to construct crystal structure and energy structure, (ii) to construct crystal structure but not energy structure, (iii) to form impurity levels as dopants and (iv) to be used as co-catalysts.

Fig. 12 Elements constructing heterogeneous photocatalysts.

Most metal oxide, sulfide and nitride photocatalysts consist of metal cations with d⁰ and d¹⁰ configurations. Their conduction bands for the d⁰ and d¹⁰ metal oxide photocatalysts are usually composed of d and sp orbitals, respectively, while their valence bands consist of O 2p orbitals. Valence bands of metal sulfide and nitride photocatalysts are usually composed of S 3p and N 2p orbitals, respectively. Orbitals of Cu 3d in Cu⁺, Ag 4d in Ag⁺, Pb 6s in Pb²⁺, Bi 6s in Bi³⁺, and Sn 5s in Sn²⁺ can also form valence bands in some metal oxide and sulfide photocatalysts as mentioned in sections 7.1.4 and 9.3.

Alkali, alkaline earth and some lanthanide ions do not directly contribute to the band formation and simply construct the crystal structure as A site cations in perovskite compounds.

Some transition metal cations with partially filled d orbitals such as Cr³⁺, Ni²⁺ and Rh³⁺ form some impurity levels in band gaps when they are doped or substituted for native metal cations. Although they often work as recombination centres between photogenerated electrons and holes they sometimes play an important role for visible light response as mentioned in sections 7.1.3 and 9.2.

Some transition metals and the oxides such as noble metals (Pt,^41,42 Rh^42,43 and Au^44,45), NiO⁴⁶ and RuO₂^47,48 function as co-catalysts for H₂ evolution. In water splitting, a back reaction to form water from evolved H₂ and O₂ has to be suppressed because of an uphill reaction. Au, NiO and RuO₂ are suitable co-catalysts on which the back reaction hardly proceeds. A Cr–Rh oxide has recently been found as an excellent co-catalyst for H₂ evolution by oxynitride photocatalysts.^49,50 IrO₂ colloids works as an O₂ evolution co-catalyst.^51–53

5. Wide band gap metal oxide photocatalysts for water splitting under UV irradiation

5.1 Oxide photocatalysts consisting of d⁰ metal cations^{42–43,45,46,54–151}

Table 1 shows photocatalyst materials consisting of d⁰ metal cations (Ti⁴⁺, Zr⁴⁺, Nb⁵⁺, Ta⁵⁺ and W⁶⁺) for water splitting with reasonable activities. The activities are not directly compared with each other because experimental conditions such as light sources, reaction cells, and the scale of the reaction are different from each other. But the values of activities would make sense as to how high the activities of photocatalysts are.

Table 1 Oxide photocatalysts based on d⁰ metal ions for water splitting under UV irradiation

Photocatalyst	Crystal structure	BG/eV	Co-catalyst		Reactant solution	Activity/μmol h^−1		QY (%)	Ref. (Year)
				Light source^a		H2	O2
a Hg–Q: combination of 400–450 W Hg lamp with a quartz cell, Hg–P: combination of 400–450 W Hg lamp with a Pyrex cell, Xe–Q: combination of 300–500 W Xe lamp with a quartz cell, Hg–Xe–P: combination of 1000 W Hg–Xe lamp with a Pyrex cell, Hg–Xe–Q: combination of 200 W Hg–Xe lamp with a quartz cell.
Ti photocatalysts
TiO2	Anatase	3.2	Rh	Hg–Q	Water vapor	449		29	43 (1985)
TiO2	Anatase	3.2	NiOx	Hg–P	3 M NaOH	6	2		54 (1987)
TiO2	Anatase	3.2	Pt	Hg–Q	2.2 M Na2CO3	568	287		55 (1997)
TiO2	Anatase	3.2	Pt	Hg–Q	Pure water	106	53		56 (1995)
B/Ti oxide	Anatase	3.2	Pt	Hg–Q	Pure water	22	11		57 (1998)
CaTiO3	Perovskite	3.5	NiOx	Hg–Q	0.2 M NaOH	30	17		58 (2002)
SrTiO3	Perovskite	3.2	NiOx	Hg–P	5 M NaOH	40	19		46, 59–63 (1980)
SrTiO3	Perovskite	3.2	Rh	Hg–Xe–P	Pure water	27	14		42, 43, 64 (1980)
Sr3Ti2O7	Layered perovskite	3.2	NiOx	Hg–Q	Pure water	144	72		65 (2006)
Sr4Ti3O10	Layered perovskite	3.2	NiOx	Hg–Q	Pure water	170		4.5 (at 360 nm)	66 (2002)
K2La2Ti3O10	Layered perovskite	3.4–3.5	NiOx	Hg–Q	0.1 M KOH	2186	1131		67, 68 (1997)
Rb2La2Ti3O10	Layered perovskite	3.4–3.5	NiOx	Hg–Q	0.1 M RbOH	869	430	5 (at 330 nm)	67 (1997)
Cs2La2Ti3O10	Layered perovskite	3.4–3.5	NiOx	Hg–Q	Pure water	700	340		67 (1997)
CsLa2Ti2NbO10	Layered perovskite	3.4–3.5	NiOx	Hg–Q	Pure water	115	50		67 (1997)
La2TiO5	Layered perovskite		NiOx	Hg–Q	Pure water	442			69 (2005)
La2Ti3O9	Layered perovskite		NiOx	Hg–Q	Pure water	386			69 (2005)
La2Ti2O7	Layered perovskite	3.8	NiOx	Hg–Q	Pure water	441		12 (<360 nm)	69–78 (1999)
La2Ti2O7:Ba	Layered perovskite		NiOx	Hg–Q	Pure water	5000		50	69 (2005)
KaLaZr0.3Ti0.7O4	Layered perovskite	3.91	NiOx	Hg–Q	Pure water	230	116	12.5	79 (2003)
La4CaTi5O17	Layered perovskite	3.8	NiOx	Hg–Q	Pure water	499		20 (<320 nm)	70 (1999)
KTiNbO5	Layered structure	3.6	NiOx	Hg–Q	Pure water	30	10		80 (1999)
Na2Ti6O13	Tunnel structure		RuO2	Xe–Q	Pure water	7.3	3.5		81–84 (1990)
BaTi4O9	Tunnel structure		RuO2	Xe–Q	Pure water	33	16		84–90 (1992)
Gd2Ti2O7	Cubic pyrochlore	3.5	NiOx	Hg–Q	Pure water	400	198		76 (2006)
Y2Ti2O7	Cubic pyrochlore	3.5	NiOx	Hg–Q	Pure water	850	420	6 (at 313 nm)	76, 91, 92 (2004)
ZrO2		5.0	None	Hg–Q	Pure water	72	36		93–97 (1993)
Nb photocatalysts
K4Nb6O17	Layered structure	3.4	NiOx	Hg–Q	Pure water	1837	850	5 (at 330 nm)	45, 98–108 (1986)
Rb4Nb6O17	Layered structure	3.4	NiOx	Hg–Q	Pure water	936	451	10 (at 330 nm)	105 (1997)
Ca2Nb2O7	Layered perovskite	4.3	NiOx	Hg–Q	Pure water	101		7 (<288 nm)	70 (1999)
Sr2Nb2O7	Layered perovskite	4.0	NiOx	Hg–Q	Pure water	217	97		70, 109–111 (1999)
Ba5Nb4O15	Layered perovskite	3.85	NiOx	Hg–Q	Pure water	2366	1139	7 (at 270 nm)	112 (2006)
NaCa2Nb3O10	Layered perovskite		RuO2	Hg–Q	Pure water	118	56		113 (2005)
ZnNb2O6	Columbite	4.0	NiOx	Hg–Q	Pure water	54	21		114 (1999)
Cs2Nb4O11	Pyrochlore like	3.7	NiOx	Hg–Q	Pure water	1700	800	3 (at 270 nm)	115 (2005)
La3NbO7	Cubic fluorite	3.9	NiOx	Hg–Q	Pure water	35	17		76, 116 (2004)
Ta photocatalysts
Ta2O5		4.0	NiOx	Hg–Q	Pure water	1154	529		94, 105, 117, 118 (1994)
K2PrTa5O15	Tungsten bronze	3.8	NiO	Hg–Q	Pure water	1550	830		12, 119 (2000)
K3Ta3Si2O13	Tungsten bronze	4.1	NiO	Hg–Q	Pure water	390	200		120 (1997)
K3Ta3B2O12	Tungsten bronze	4.0	None	Hg–Q	Pure water	2390	1210	6.5 (at 254 nm)	121 (2006)
LiTaO3	Ilumenite	4.7	None	Hg–Q	Pure water	430	220		117, 122 (1998)
NaTaO3	Perovskite	4.0	NiO	Hg–Q	Pure water	2180	1100	20 (at 270 nm)	117, 122–124 (1998)
KTaO3	Perovskite	3.6	Ni	Hg–Q	Pure water	6	2		105, 117, 122 (1996)
AgTaO3	Perovskite	3.4	NiOx	Hg–Q	Pure water	21	10		125 (2002)
KTaO3:Zr	Perovskite	3.6	NiOx	Xe–Q	Pure water	9.4	4.2		126, 127 (1999)
NaTaO3:La	Perovskite	4.1	NiO	Hg–Q	Pure water	19 800	9700	56 (at 270 nm)	128, 129 (2000)
NaTaO3:Sr	Perovskite	4.1	NiO	Hg–Q	Pure water	9500	4700		130 (2004)
Na2Ta2O6	Pyrochlore	4.6	NiO	Hg–Q	Pure water	391	195		131 (2006)
K2Ta2O6	Pyrochlore	4.5	NiO	Hg–Q	Pure water	437	226		131, 132 (2004)
CaTa2O6	CaTa2O6 (orth.)	4.0	NiO	Hg–Q	Pure water	72	32		133 (1999)
SrTa2O6	CaTa2O6 (orth.)	4.4	NiO	Hg–Q	Pure water	960	490	7 (at 270 nm)	133 (1999)
BaTa2O6	CaTa2O6 (orth.)	4.1	NiO	Hg–Q	Pure water	629	303		117, 133 (1998)
NiTa2O6		3.7	None	Hg–Q	Pure water	11	4		117 (1998)
Rb4Ta6O17	Layered structure	4.2	NiO	Hg–Q	Pure water	92	46		105 (1996)
Ca2Ta2O7	Layered perovskite	4.4	NiO	Hg–Q	Pure water	170	83		131 (2006)
Sr2Ta2O7	Layered perovskite	4.6	NiO	Hg–Q	Pure water	1000	480	12 (at 270 nm)	109–111, 134 (2000)
K2SrTa2O7	Layered perovskite	3.9	None	Hg–Q	Pure water	374	192		135 (2004)
RbNdTa2O7	Layered perovskite	3.9	NiOx	Hg–Q	Pure water	117	59		136–139 (1999)
H2La2/3Ta2O7	Layered perovskite	4.0	NiOx	Hg–Q	Pure water	940	459		140 (2005)
K2Sr1.5Ta3O10	Layered perovskite	4.1	RuO2	Hg–Q	Pure water	100	39.4	2 (at 252.5 nm)	141 (2007)
LiCa2Ta3O10	Layered perovskite	4.2–4.3	NiOx	Hg–Q	Pure water	708	333		142 (2008)
KBa2Ta3O10	Layered perovskite	3.5	NiOx	Hg–Q	Pure water	170		8 (<350 nm)	70 (1999)
Sr5Ta4O15	Layered perovskite	4.75	NiO	Hg–Q	Pure water	1194	722		134 (2005)
Ba5Ta4O15	Layered perovskite		NiO	Hg–Q	Pure water	2080	910		143 (2005)
H1.8Sr0.81Bi0.19Ta2O7	Layered perovskite	3.88	None	Hg–Q	Pure water	250	110		144 (2008)
Mg–Ta Oxide	Mesoporous		NiO	Hg–Q	Pure water	102	51		145 (2004)
LaTaO4	Fergusonite	3.9	NiOx	Hg–Q	Pure water	116	52		146 (2001)
La3TaO7	Cubic fluorite	4.6	NiOx	Hg–Q	Pure water	164	80		76, 116 (2004)
Other photocatalysts
PbWO4	Scheelite	3.9	RuO2	Hg–Xe–Q	Pure water	24	12		147, 148 (2004)
RbWNbO6	Pyrochlore	3.6	NiOx	Hg–Q	1M RbOH	11.4	4.3		149 (2004)
RbWTaO6	Pyrochlore	3.8	NiOx	Hg–Q	1M RbOH	69.7	34.5		149 (2004)
CeO2:Sr	Fluorite		RuO2	Hg–Q	Pure water	110	55		150 (2007)
BaCeO3	Perovskite	3.2	RuO2	Hg–Q	Pure water	59	26		151 (2008)

---

Valence bands of these photocatalysts, except for AgTaO₃, consist of O 2p orbitals of which the potential is about 3 eV vs. NHE while conduction band levels are more negative than 0 eV. It results in that these materials respond to only UV. An Ag 4d orbital forms a valence band of AgTaO₃ with a O 2p orbital.¹²⁵

These metal mixed oxides are usually prepared by a solid-state reaction. Metal oxides and/or alkali and alkaline earth carbonates of starting materials are calcined at high temperature in air.

A polymerizable complex method¹⁵² is sometimes used for preparation of photocatalysts.^{68,74,80,111,112,134,143} This preparation method gives fine and well-crystalline powders with a high surface area at relatively low calcination temperature and short calcination time compared with a conventional solid state method. The example of an Sr₂Ta₂O₇ photocatalyst is shown in Fig. 13.¹¹¹ SrCO₃ and TaCl₅ are dissolved in an ethylene glycol (EG) and methanol mixed solution containing anhydrous citric acid (CA) of a chelating agent to stabilize metal cations. The transparent colourless solution is heated at 403 K with stirring to promote polymerization between CA and EG. The solution becomes more viscous with time, and a brown resin-like gel is obtained without any visible precipitation after several hours. The brown gel is heated at 723 K for several hours to remove residual solvents and to burn out unnecessary organics. The powder obtained is referred to as powder precursors for Sr₂Ta₂O₇. The powder precursor is calcined at temperatures between 973 and 1273 K for 5–100 h in air. Some metal oxide photocatalysts that are hardly prepared by solid-state reactions can be obtained by the polymerizable complex method.¹⁴³ Aqueous processes such as hydrothermal synthesis^31,131 are also employed for the preparation of photocatalysts. Photocatalysts prepared by these soft processes sometimes show higher activities than those prepared by solid state reaction because they have small particle size and good crystallinity. Next, let us see each photocatalyst.

Fig. 13 Polymerizable complex method for preparation of Sr₂Ta₂O₇ photocatalyst.¹¹¹

5.1.1 Group 4 elements^{42,43,46,54–97}. TiO₂ has extensively been studied for a long time. Although water splitting was firstly demonstrated using a TiO₂ photoelectrode with some external bias as shown in Fig. 3 a powdered TiO₂ photocatalyst can not split water without any modifications such as loading co-catalysts. At the initial stage of the research, it was questionable that a platinized TiO₂ photocatalyst could split water because the activity was usually low and no O₂ evolution was often observed. After that, NaOH-coating⁴³ and additions of alkali carbonates⁵⁵ have been found to be effective for water splitting on the Pt/TiO₂ photocatalyst.

SrTiO₃^29,153 and KTaO₃^29,154 photoelectrodes with perovskite structure can split water without an external bias being different from TiO₂ because of their high conduction band levels as shown in Fig. 6. These materials can be used as powdered photocatalysts. Domen and co-workers have reported that NiO-loaded SrTiO₃ powder can decompose pure water into H₂ and O₂.^46,59–63 The NiO co-catalyst for H₂ evolution is usually activated by H₂ reduction and subsequent O₂ oxidation to form a NiO/Ni double layer structure that is convenient for electron migration from a photocatalyst substrate to a co-catalyst.⁶¹ The pretreated NiO co-catalyst is often denoted as NiO_x in literature. It is important that the NiO co-catalyst does not cause the back reaction between H₂ and O₂, being different from Pt. The excellent NiO co-catalyst has often been employed for many photocatalysts for water splitting as seen in Table 1. Rh is also a suitable co-catalyst for the SrTiO₃ photocatalyst.⁴²

TiO₂ and SrTiO₃ photocatalysts are also active for reduction of NO₃⁻ using water as an electron donor.^155–157

K₂La₂Ti₃O₁₀ that possesses a layered perovskite structure is a unique photocatalyst. H₂ evolution proceeds on a pretreated NiO_x co-catalyst while O₂ evolves at the hydrated interlayer. Many titanate, niobate and tantalate photocatalysts with layered perovskite structure have been reported since the K₂La₂Ti₃O₁₀ photocatalyst was found. Sr₃Ti₂O₇ and Sr₄Ti₃O₁₀ photocatalysts have perovskite slabs of SrTiO₃. La₂Ti₂O₇, La₂Ti₂O₇:Ba, KLaZr_0.3Ti_0.7O₄ and La₄CaTi₅O₁₇ photocatalysts with layered perovskite structure give high quantum yields.

Na₂Ti₆O₁₃ and BaTi₄O₉ with tunnel structure are also unique titanate photocatalysts. KTiNbO₅ shows activity when it is prepared by a polymerizable complex method. Gd₂Ti₂O₇ and Y₂Ti₂O₇ with pyrochlore structure are also active.

ZrO₂ is active without co-catalyst because of its high conduction band level. This photocatalyst is also active for CO₂ reduction to CO accompanied with O₂ evolution by oxidation of water without any sacrificial reagents.⁹³

5.1.2 Group 5 elements^{45,70,76,98–146}. K₄Nb₆O₁₇ and Rb₄Nb₆O₁₇ with layered structure as seen in mica show high activities. These photocatalysts possess two kinds of interlayers in which ion-exchangeable potassium cations exist as shown in Fig. 14.¹⁰⁰ H₂ evolution proceeds in one interlayer with a nickel co-catalyst while O₂ evolution occurs in another interlayer. It is the characteristic of the K₄Nb₆O₁₇ photocatalyst that H₂ evolution sites are separated from O₂ evolution sites by the photoactive niobate sheet.¹⁰¹ This photocatalyst is active for water splitting and H₂ evolution from an aqueous methanol solution even without co-catalyst. Moreover, the activity for the sacrificial H₂ evolution is much enhanced by H⁺-exchange.⁹⁹

Fig. 14 Water splitting over K₄Nb₆O₁₇ photocatalyst with layered structure.¹⁰⁰

Ca₂Nb₂O₇, Sr₂Nb₂O₇ and Ba₅Nb₄O₁₅ with layered perovskite structure show high activity. NaCa₂Nb₃O₁₀ and KCa₂Nb₃O₁₀ stacked with RuO₂ colloids from exfoliated nano-sheets are active for water splitting although the native NaCa₂Nb₃O₁₀ and KCa₂Nb₃O₁₀ are active just for half reactions in the presence of sacrificial reagents.¹¹³

ZnNb₂O₆ photocatalyst with d¹⁰ and d⁰ metal ions produce H₂ and O₂ from pure water.

Ta₂O₅ shows high activity. K₃Ta₃Si₂O₁₃ and K₃Ta₃Bi₂O₁₂ with pillared structure in which three linear chains of corner-shared TaO₆ are connected with each other are active for water splitting without any co-catalyst.^120,121 The activity of K₃Ta₃Si₂O₁₃ drastically increased with loading a small amount of a NiO co-catalyst while naked K₃Ta₃B₂O₁₂ shows high activity. Alkali and alkaline earth tantalates show photocatalytic activities for water splitting into H₂ and O₂. These tantalate photocatalysts are also active for reduction of NO₃⁻ to N₂ using water as an electron donor.¹⁵⁷

Ishihara and co-workers have reported that photocatalytic activity of KTaO₃ is improved by doping of Zr, Ti and Hf. Moreover, modification of the KTaO₃:Zr photocatalyst by some metal complexes such as vitamin B12 improves the photocatalytic activity through a dye sensitized two-photon process.¹⁵⁸ On the other hand, many tantalates with layered perovskite structure are also active. The photocatalytic activity of K₂LnTa₅O₁₅ with tungsten bronze structure depends on Ln as well as RbLnTa₂O₇ with layered perovskite structure.^119,139

Among tantalates, NiO/NaTaO₃ is highly active. The photocatalytic activity of NiO/NaTaO₃ increased remarkably with doping of lanthanide ions.^128,129 An optimized NiO (0.2 wt%)/NaTaO₃:La (2%) photocatalyst shows high activity with an apparent quantum yield of 56% for water splitting. The activity is stable for more than 400 h under irradiation of light from a 400-W high pressure mercury lamp. Bubbles of H₂ and O₂ evolved can be observed when the photocatalyst is irradiated with UV from a 200 W Xe–Hg lamp as shown in Fig. 15. Only light, water and photocatalyst powder exist in the system. It is amazing that reduction and oxidation of water, completely opposite reactions, simultaneously proceed on the same surface of a nano-particle. The NaTaO₃:La photocatalyst is also active for methane coupling.¹⁵⁹

Fig. 15 Water splitting using NiO/NaTaO₃:La photocatalyst.

A series of A₃MO₃ (A = Bi, Al, Ga and In, M = Nb and Ta) has been reported.^160,161

5.1.3 Group 6 and other d⁰ elements^147–151. PbWO₄ with scheelite structure and RbWMO₆ (M = Nb and Ta) with pyrochlore structure are active. The f-block metal oxide, CeO₂ doped with Sr is active for water splitting, though nondoped CeO₂ is not active. The reasonable photocatalytic activity of the CeO₂:Sr is obtained using a quartz rather than a Pyrex cell even though the absorption edge of the material is around 400 nm, suggesting that excitation at higher energy than the minimum band gap excitation leads to the activity.¹⁵⁰

5.2 Factors affecting photocatalytic ability of d⁰ metal oxides

Many photocatalysts for water splitting have been found as shown in Table 1. It is important to make such a photocatalyst library because the relationship between the nature of materials and the photocatalytic abilities can be considered. For example, systematic comparison of photocatalytic activities between niobates and tantalates with similar structure give some information on factors that affect photocatalytic ability as mentioned below.

5.2.1 Effect of conduction band level consisting of Nb 4d and Ta 5d orbitals on photocatalytic performances¹⁰⁹. Sr₂Nb₂O₇ possesses a layered perovskite structure that is the same as Sr₂Ta₂O₇ though the distortion of the framework is slightly different as shown in Fig. 16.^162,163 These crystal structures are composed of NbO₆ and TaO₆ octahedra. Moreover, the ionic radius of Nb⁵⁺ is almost the same as that of Ta⁵⁺. However, the photocatalytic activity of Sr₂Ta₂O₇ is higher than that of Sr₂Nb₂O₇ as shown in Table 1 even if the number of absorbed photons of Sr₂Ta₂O₇ should be smaller than that of Sr₂Nb₂O₇ under the same experimental condition, because the band gap of the former is wider than that of the latter. The difference in photocatalytic activities between niobates and tantalates is mainly due to the conduction band level. The conduction band of Sr₂Ta₂O₇ consists of Ta 5d while that of Sr₂Nb₂O₇ is Nb 4d. The valence band potential of Sr₂Ta₂O₇ should be similar to that of Sr₂Nb₂O₇ because the valence bands consist of O 2p orbitals and six oxygen anions coordinate to Ta⁵⁺ or Nb⁵⁺ with the same ionic radius. Eqn (11), as reported by Scaife for oxides not containing metal cations with partly filled d orbital can be applied to the present system for the approximate determination of the flat band potential,¹⁶⁴ where V_fb and E_g represent a flat band potential and a band gap, respectively.

V_fb(NHE) = 2.94 −E_g (11)

Fig. 16 Layered perovskite structures of Sr₂M₂O₇ (M = Nb and Ta).^162,163

The band structures of Sr₂Ta₂O₇ and Sr₂Nb₂O₇ can be roughly described as shown in Fig. 17.¹⁰⁹ Band structure of NiO is also shown.¹⁶⁵ O₂ evolution on Sr₂Ta₂O₇ is as easy as that on Sr₂Nb₂O₇ because the potentials of their valence bands are deep enough to oxidize water into O₂. Therefore, it is due to the difference in the conduction band level that the photocatalytic activity of Sr₂Ta₂O₇ is higher than that of Sr₂Nb₂O₇. The high conduction band level causes the driving force for reduction of water to form H₂. The reason why Sr₂Ta₂O₇ is able to decompose pure water without co-catalysts is also due to its high conduction band.

Fig. 17 Band structures of Sr₂M₂O₇ (M = Nb and Ta) photocatalysts and NiO co-catalyst.¹⁰⁹

In general, pretreatment of H₂ reduction and subsequent O₂ oxidation is indispensable for obtaining high activities for NiO-loaded photocatalysts as mentioned in section 5.1.1. The pretreatment is indispensable for NiO/Sr₂Nb₂O₇ as usual whereas it is not for NiO/Sr₂Ta₂O₇. In the case of the nontreated NiO/Sr₂Ta₂O₇ photocatalyst, it is possible that the photogenerated electrons in a conduction band of Sr₂Ta₂O₇ can transfer to a conduction band of NiO because of the suitable potential difference as shown in Fig. 17. In contrast, this appears to be difficult for Sr₂Nb₂O₇ because the potential difference in the conduction band between Sr₂Nb₂O₇ and NiO is negligible. In such a case, pretreatment would be necessary as well as in the case of NiO/SrTiO₃. Therefore, the effect of the conduction band level dominates the photocatalytic activities of the Sr₂M₂O₇ system. Systematic investigation of solid solutions also gives some information on factors affecting photocatalytic activities.^110,111

In this section, main factors were just discussed from the viewpoint of band structure as shown in Fig. 5. The nature of connection of MO₆ octahedra in crystal structure should be suitable for migration of photogenerated electrons and holes, and surface reactions of the carriers with water as indicated by steps (ii) and (iii) in Fig. 4 as mentioned in the next section.

5.2.2 Effect of distortion of framework of crystal structure on energy structure^109,122. All of ATaO₃ (A: Li, Na, and K) consist of corner-sharing TaO₆ octahedra with perovskite-like structures as shown in Fig. 18.¹²² The photocatalytic activities of ATaO₃ depends on the A site cation of perovskite-like structure. The bond angles of Ta–O–Ta are 143° (LiTaO₃), 163° (NaTaO₃) and 180° (KTaO₃) in octahedral connection. Wiegel and co-workers have reported the relationship between crystal structures and energy delocalization for alkali tantalates ATaO₃ (A: Li, Na, and K).¹⁶⁶ As the bond angle approaches 180°, excited energy or electron–hole pairs in the crystal migrate more easily and the band gap becomes narrower. Therefore, the order of the delocalization of excited energy or electron–hole pairs is LiTaO₃ < NaTaO₃ < KTaO₃, while that of the band gap is reversed in the order. The degree of localization affects the step (ii) in Fig. 4. This discussion is applied to other photocatalyst systems.^109,125,167 NaTaO₃ shows the highest photocatalytic activity among the ATaO₃ (A: Li, Na, and K) photocatalysts when a NiO co-catalyst is loaded. In the case of NaTaO₃, excess sodium in the starting material is indispensable for showing the high activity indicating that preparation conditions are important.¹²³ The conduction band level of the NaTaO₃ photocatalyst is higher than that of NiO¹⁶⁵ as shown in Fig. 19.¹²² Moreover, the excited energy is delocalized in the NaTaO₃ crystal. Therefore, the photogenerated electrons in the conduction band of the NaTaO₃ photocatalyst are able to transfer to the conduction band of the NiO co-catalyst. Therefore, NiO-loading is effective for the NaTaO₃ photocatalyst even without special pretreatment as observed for a NiO/Sr₂Ta₂O₇ photocatalyst. Thus, the high activity of NiO/NaTaO₃ is due to the suitable conduction band level consisting of Ta 5d and energy delocalization caused by the small distortion of TaO₆ connections.

Fig. 18 Crystal and energy structures of alkali tantalate photocatalysts.¹²²

Fig. 19 Band structures of alkali tantalate photocatalysts and NiO co-catalyst.¹²²

Charge separation of photogenerated electrons and holes is required in the case of a water splitting reaction into H₂ and O₂ more strongly than in the case of photocatalytic reactions in the presence of sacrificial reagents in order to prevent recombination. Sr₂Nb₂O₇ has a dipole moment along perovskite layers, the c axis, due to the distortion of the framework of perovskite layers as shown in Fig. 16. The charge separation may be enhanced by the dipole moment.¹⁰⁹ Inoue has proposed the effects of local distortion of polyhedra consisting of the crystal structure of photocatalysts on the charge separation.¹⁸ These factors affect charge separation of the step (ii) in Fig. 4.

5.2.3 Effect of morphology on creation of active sites^128–130. The photocatalytic activity of NiO/NaTaO₃ increases remarkably with doping of La ions.^128,129 The reaction scheme for the water splitting on the NiO/NaTaO₃:La photocatalyst is shown in Fig. 20.¹²⁹ The particle size of the NaTaO₃:La crystal (0.1–0.7 μm) is smaller than that of the nondoped NaTaO₃ crystal (2–3 μm) and ordered surface nano-steps are created by lanthanum doping. The small particle size with high crystallinity is advantageous in terms of increasing the probability of the reactions of photogenerated electrons and holes with water molecules, rather than recombination as shown in Fig. 7. The H₂ evolution site of the edge is effectively separated from the O₂ evolution site of the groove at the surface nanostep structure. This separation is advantageous, especially for water splitting in order to avoid the back reaction. Doping of Ca, Sr and Ba also gives the same effect as the La doping on the formation of the characteristic morphology of NaTaO₃ and the improvement of photocatalytic activity.¹³⁰ Thus, the change in surface morphology affects the step (iii) in Fig. 4.

Fig. 20 Mechanism of highly efficient water splitting over NiO/NaTaO₃:La photocatalyst.¹²⁹

Time-resolved IR measurements reveal that the La doping prolongs the lifetime of photogenerated electrons in a conduction band or a shallow trap level as shown in Fig. 21.¹⁶⁸ The absorption is due to the electrons photogenerated by band gap excitation at 266 nm. The increase in the lifetime is also one of the factors for the improvement of photocatalytic ability. This factor affects the step (ii) in Fig. 4.

Fig. 21 Decay curves of photogenerated electrons in La-doped NaTaO₃.¹⁶⁸

5.3 Oxide photocatalysts consisting of d¹⁰ metal cations^169–180

d¹⁰ metal oxides such as ZnO and In₂O₃ are well-known photocatalysts for a long time. However, they are not active for water splitting because of photocorrosion according to eqn (5) and the low conduction band level, respectively.⁷ In contrast, Inoue’s group has found various mixed oxide photocatalysts consisting of d¹⁰ metal cations, Ga³⁺, In³⁺, Ge⁴⁺, Sn⁴⁺ and Sb⁵⁺, for water splitting as shown in Table 2. A RuO₂ co-catalyst is indispensable for these photocatalysts except for a Ga₂O₃ photocatalyst. The RuO₂ co-catalyst is loaded using Ru₃(CO)₁₂ by an impregnation method. Conduction bands of these photocatalysts consist of sp orbitals of d¹⁰ metal cations. These bands are dispersed well resulting in high mobility of photogenerated electrons. Inoue has proposed that the dipole moment formed by distortions of MO₄ tetrahedra and MO₆ octahedra enhances charge separation of photogenerated carriers.^170,176,177 The CaIn₂O₄ photocatalyst is also used for degradation of Methylene Blue.^181–184 Sakata and co-workers have reported a highly efficient Zn-doped β-Ga₂O₃ photocatalyst with a Ni co-catalyst.¹⁸⁰

Table 2 Oxide photocatalysts based on d¹⁰ metal ions for water splitting under UV irradiation^a

Photocatalyst	Crystal structure	BG^d/eV	Activity/μmol h^−1		Ref. (Year)
			H2	O2
a Co-catalyst: RuO2, reactant solution: pure water, light source: 200 W Hg–Xe lamp and a quartz cell.b Co-catalyst: NiO, light source: 450 W Hg lamp equipped in a quartz cell.c Co-catalyst: Ni, light source: 450 W Hg lamp equipped in a quartz cell.d Band gaps not mentioned in papers were determined from DRS.
NaInO2	Layered structure	3.9	0.9	0.3	169, 170 (2003)
CaIn2O4	Tunnel structure		21	10	169–172 (2001)
SrIn2O4	Tunnel structure	3.6	7	3	169–173 (2001)
LaInO3		4.1	1	0.5	172 (2003)
YxIn2−xO3		4.3	8	4	174 (2008)
NaSbO3	Ilmenite	3.6	1.7	0.8	171, 175 (2001)
CaSb2O6	Layered structure	3.6	1.5	0.2	175 (2002)
Ca2Sb2O7	Weberite	3.9	3	1	175 (2002)
Sr2Sb2O7	Weberite	4.0	8	3	175 (2002)
Sr2SnO4			5	2.5	171 (2001)
ZnGa2O4		4.2	10	4	176 (2002)
Zn2GeO4	Willemite	4.6	22	10	177 (2004)
LiInGeO4		4.4	26	13	178 (2005)
Ga2O3^b		4.6	46	23	179 (2004)
Ga2O3:Zn^c		4.6	4100	2200	180 (2008)

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6. Wide band gap metal oxide photocatalysts for H₂ or O₂ evolution from an aqueous solution containing a sacrificial reagent under UV irradiation^185–197

Many metal oxide photocatalysts for water splitting without any sacrificial reagents have been developed as shown in Tables 1 and 2. Therefore, it may appear meaningless to develop wide band gap metal oxide photocatalysts not for water splitting but for H₂ or O₂ evolution from an aqueous solution containing a sacrificial reagent under UV irradiation. However, their development is still important to get information on factors affecting photocatalytic activity.

Table 3 shows wide band gap metal oxide photocatalysts that are active for H₂ or O₂ evolution from an aqueous solution containing a sacrificial reagent under UV irradiation.

Many layered titanates are active for H₂ evolution. H⁺-exchange often gives higher activity for H₂ evolution than the native materials even in the absence of co-catalysts such as Pt. It means that these protonated layered metal oxides possess excellent active sites for H₂ evolution. K₂Ti₄O₉ and HCa₂Nb₃O₁₀ with SiO₂ pillars at the interlayer show high activities.¹⁸⁸ These layered metal oxides are attractive materials for preparing nano-sheets. Sasaki’s group has extensively studied nano-sheets of layered oxide materials,¹⁹⁸ and the term “nano-sheet” was probably first used by Sasaki’s group. Titanate nano-sheets show photocatalytic activity for self-cleaning. Osterloh and co-workers have reported photocatalytic reactions using Pt/HCa₂Nb₃O₁₀ nano-sheets.^199,200

Table 3 Oxide photocatalysts for H₂ or O₂ evolution from aqueous solutions in the presence of sacrificial reagents under UV irradiation

Photocatalyst	Crystal structure	BG/eV		H2 evolution^b		O2 evolution^c	Ref. (Year)
			Light source^a	Co-catalyst	Activity/μmol h^−1	Activity/μmol h^−1
a Hg–Q: combination of 400–450 W Hg lamp with a quartz cell, Hg–P: combination of 400–450 W Hg lamp with a Pyrex cell, Xe–Q: combination of 300 W Xe lamp with a quartz cell, Xe–P: combination of 300 W Xe lamp with a quartz cell.b Sacrificial reagent: CH3OH aq.c Sacrificial reagent: AgNO3 aq.d ex-Ca2Nb3O10/K^+ means that nanosheet was flocculated with K^+.
Na2Ti3O7	Layered structure		Xe–P	Pt	19	—	185 (1987)
K2Ti2O5	Layered structure		Xe–P	Pt	34.7	—	185 (1987)
K2Ti4O9	Layered structure		Xe–P	Pt	4.8	—	185 (1987)
Cs2Ti2O5	Layered structure	4.4	Hg–Q	None	500	—	186 (1997)
H^+-Cs2Ti2O5	Layered structure		Hg–Q	None	852	—	186 (1997)
Cs2Ti5O11	Layered structure	3.75	Hg–Q	None	90	—	186 (1997)
Cs2Ti6O13	Layered structure	3.7	Hg–Q	None	38	—	186 (1997)
H^+-CsTiNbO5	Layered structure	3.0	Hg–P	Pt	87	—	187 (1990)
H^+-CsTi2NbO7	Layered structure	3.2	Hg–P	Pt	320	—	187 (1990)
SiO2-pillared K2Ti4O9	Layered structure	3.17	Hg–P	Pt	560	—	188 (2000)
SiO2-pillared K2Ti2.7Mn0.3O7	Layered structure		Hg–P	None	320	—	188 (2000)
Na2W4O13	Layered structure	3.1	Hg–P	Pt	21	9	189 (1997)
H^+-KLaNb2O7	Layered perovskite		Hg–Q	Pt	3800	46	8 (2000)
H^+-RbLaNb2O7	Layered perovskite		Hg–Q	Pt	2600	2	8 (2000)
H^+-CsLaNb2O7	Layered perovskite		Hg–Q	Pt	2200	3	8 (2000)
H^+-KCa2Nb3O10	Layered perovskite		Hg–Q	Pt	19 000	8	8 (2000)
SiO2-pillared KCa2Nb3O10	Layered perovskite		Hg–P	Pt	10 800		190, 191 (1993)
ex-Ca2Nb3O10/K^+ nanosheet^4)	Layered perovskite		Xe–P	Pt	550		192 (2002)
Restacked ex-Ca2Nb3O10/Na^+	Layered perovskite		Xe–P	Pt	880		192 (2002)
H^+-RbCa2Nb3O10	Layered perovskite		Hg–Q	Pt	17 000	16	8 (2000)
H^+-CsCa2Nb3O10	Layered perovskite		Hg–Q	Pt	8300	10	8 (2000)
H^+-KSr2Nb3O10	Layered perovskite		Hg–Q	Pt	43 000	30	8 (2000)
H^+-KCa2NaNb4O13	Layered perovskite		Hg–Q	Pt	18 000	39	8 (2000)
Bi2W2O9	Aurivillius like	3.0	Hg–P	Pt	18	281	193 (1999)
Bi2Mo2O9	Aurivillius like	3.1	Xe–P	—	—	1.8	193 (1999)
Bi4Ti3O12	Aurivillius	3.1	Hg–P	Pt	0.6	3	193 (1999)
BaBi4Ti4O15	Aurivillius	3.1	Hg–P	Pt	8.2	3.7	193 (1999)
Bi3TiNbO9	Aurivillius	3.1	Hg–P	Pt	33	31	193 (1999)
PbMoO4	Scheelite	3.31	Xe–P	Pt	1.9	12.8	194 (1990)
(NaBi)0.5MoO4	Scheelite	3.1	Xe–P	Pt	0.6	58	195 (2004)
(AgBi)0.5MoO4	Scheelite	3.0	Xe–P	Pt	0	10.7	195 (2004)
(NaBi)0.5WO4	Scheelite	3.5	Xe–P	Pt	7	1.3	195 (2004)
(AgBi)0.5WO4	Scheelite	3.5	Xe–P	Pt	0.1	5.8	195 (2004)
Ga1.14In0.86O3		3.7	Hg–P	Pt	30	30	196 (1998)
β–Ga2O3		4.6	Xe–Q	Pt	50	7	196 (1998)
Ti1.5Zr1.5(PO4)4		3.8	Xe–Q	Pt	11.8	—	197 (2005)

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Bi₂W₂O₉, BaBi₄Ti₄O₁₅ and Bi₃TiNbO₉ consisting of a layered structure with perovskite slabs are active not only for O₂ but also H₂ evolution in the presence of sacrificial reagent. Na₂W₄O₁₃ photocatalyst with layered structure is also active for H₂ or O₂ evolution from aqueous solutions in the presence of sacrificial reagents although WO₃ is inactive for H₂ evolution. Homogeneous photocatalysts of tungsten-polyacids are also active for H₂ evolution.²⁰¹

PbMoO₄ with scheelite structure shows activities for H₂ and O₂ evolution in the presence of sacrificial reagents under UV irradiation. The substituted compounds, Na_0.5Bi_0.5MoO₄, Ag_0.5Bi_0.5MoO₄, Na_0.5Bi_0.5WO₄ and Ag_0.5Bi_0.5WO₄, are also active for O₂ evolution. In these photocatalysts, although these molybdates and tungstates respond to only UV, Pb, Bi and Ag play an important role for making the valence bands as mentioned in section 7.1.4.

Solid solutions of β-Ga₂O₃ and In₂O₃ consisting of d¹⁰ cations have been systematically studied for photocatalytic activities for H₂ or O₂ evolution from aqueous solutions in the presence of sacrificial reagents.¹⁹⁶ In this photocatalyst system, the band gap and luminescent energy decrease as the ratio of indium increases.

7. Photocatalysts with visible light response for H₂ or O₂ evolution from an aqueous solution containing a sacrificial reagent

Development of photocatalysts that work only for half reactions of water splitting in the presence of sacrificial reagents might seem meaningless but this view is incorrect. These photocatalysts can be used to construct Z-scheme systems that are active for water splitting under visible light irradiation as mentioned in section 8.3. Moreover, some of them will be able to produce H₂ using biomass and abundant compounds.^33–38Tables 4 and 5 list photocatalysts for H₂ or O₂ evolution from aqueous solutions containing sacrificial reagents under visible light irradiation.

Table 4 Oxide photocatalysts for H₂ or O₂ evolution from aqueous solutions in the presence of sacrificial reagents under visible light irradiation

Photocatalyst	BG (EG)/eV		Activity/μmol h^−1		Ref. (Year)
		Light source^a	H2^b	O2^c
a Xe–L42: 300–500 W Xe lamp with a cut-off filter (L42), Hg–L42: 500 W lamp with a cut-off filter (L42), Xe–L44: 300 W Xe lamp with a cut-off filter (L44), W–L42: 450 W lamp with a cut-off filter (L42).b Co-catalyst: Pt, sacrificial reagent: CH3OH aq.c Sacrificial reagent: AgNO3 aq.d Co-catalyst: IrO2.
WO3	2.8	Xe–L42	—	65	4, 5, 202–204 (1962)
Bi2WO6	2.8	Xe–L42	—	3	193 (1999)
Bi2MoO6	2.7	Xe–L42	—	55	205 (2006)
Bi2Mo3O12	2.88	Xe–L42	—	8	205 (2006)
Zn3V2O8	2.92	Xe–L42	—	10.2	206 (2005)
Na0.5Bi1.5VMoO8	2.5	Xe–L42	—	74	207 (2008)
In2O3(ZnO)3	2.6	Xe–L42	1.1	1.3	208 (1998)
SrTiO3:Cr/Sb	2.4	Xe–L42	78	0.9	209 (2002)
SrTiO3:Ni/Ta	2.8	Xe–L42	2.4	0.5	210 (2005)
SrTiO3:Cr/Ta	2.3	Xe–L42	70	—	211 (2004)
SrTiO3:Rh	2.3	Xe–L42	117	0	212 (2004)
CaTiO3:Rh		Xe–L42	8.5	0	213 (2006)
La2Ti2O7:Cr	2.2	Hg–L42	15	—	214, 215 (2004)
La2Ti2O7:Fe	2.6	Hg–L42	10	—	214, 215 (2004)
TiO2:Cr/Sb	2.2	Xe–L42	0.06	31.5	209 (2002)
TiO2:Ni/Nb	2.6	Xe–L44	0	7.6	210 (2005)
TiO2:Rh/Sb	2.13	Xe–L44	—	16.9	216 (2007)
PbMoO4:Cr	2.26	Xe–L42	—	71.5	217 (2007)
RbPb2Nb3O10	2.5	Xe–L42	4	1.1	218 (1993)
PbBi2Nb2O9	2.88	W–L42	3.2	520	219, 220 (2004)
BiVO4	2.4	Xe–L42	—	421	221–224 (1998)
BiCu2VO6	2.1	Xe–L42	—	2.3	225 (2005)
BiZn2VO6	2.4	Xe–L42	—	6	226 (2006)
SnNb2O6	2.3	Xe–L42	14.4	62.8^d	227–229 (2004)
AgNbO3	2.86	Xe–L42	8.2	37	125 (2002)
Ag3VO4	2.0	Xe–L42	—	17	230 (2003)
AgLi1/3Ti2/3O2	2.7	Xe–L42	—	24	231 (2008)
AgLi1/3Sn2/3O2	2.7	Xe–L42	—	53	231 (2008)

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Table 5 Dye sensitized photocatalysts for H₂ evolution from aqueous solutions in the presence of sacrificial reagents under visible light irradiation

Photocatalyst	Sensitizer	Sacrificial reagent	Light source	Incident light/nm	H2 evolution/μmol h^−1	Ref. (Year)
a Turnover number.
TiO2	Ru(bpy)3^2+	Water–MeOH vapor	500 W Xe	>440	0.9	270, 271 (1982)
Pt/ZnO	Erythrosine	Triethanolamine + I^−	500 W Xe	>420	80.4	272 (1985)
H2K2Nb6O17	Ru(bpy)3^2+	I^−	500 W Hg–Xe	>400	0.4	273 (1993)
Pt/TiO2	Zn-porphyrin	EDTA	1000 W Xe	>520	182 (9h)^a	274 (1995)
Pt/TiO2	NK-2405	Acetonitrile + I	300 W Xe	>410	210	275, 276 (2003)
Pt/TiO2	C-343	Acetonitrile + I	300 W Xe	>410	156	275, 276 (2003)
Pt(in)/H4Nb6O17	NK-2405	Acetonitrile + I	300 W Xe	>410	94	275, 276 (2003)
Ni/K4Nb6O17	CdS	K2SO3	300 W Xe	>420	56	277, 278 (1988)
H4Nb6O17	CdS	Na2S	100 W Hg	>400	220	279 (2001)
H2Ti4O9	CdS	Na2S	100 W Hg	>400	560	279 (2001)

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7.1 Oxide photocatalysts^{4–5,125,193,202–231}

7.1.1 Design of oxide photocatalysts with visible light response. Suitable band engineering is required in order to develop new photocatalysts for water splitting under visible light irradiation as shown in Fig. 22. In general, the conduction bands of stable oxide semiconductor photocatalysts are composed of empty orbitals (LUMOs) of metal cations with d⁰ and d¹⁰ configurations. Although the valence band level depends on crystal structure and bond character between metal and oxygen, the level of the valence band consisting of O 2p orbitals is usually ca. 3.0 eV.¹⁶⁴ Accordingly, a new valence band or an electron donor level (DL) must be formed with orbitals of elements other than O 2p to make the band gap (BG) or the energy gap (EG) narrower because the conduction band level should not be lowered. Not only the thermodynamic potential but also kinetic ability for 4-electron oxidation of water are required for the newly formed valence band. Strategies for the band engineering are shown in Fig. 23. The electron donor level is created above a valence band by doping some elements into conventional photocatalysts with wide band gaps such as TiO₂ and SrTiO₃. It results in the formation of energy gap. On the other hand, some metal cations and anions can contribute to valence band formations above the valence band consisting of O 2p orbitals. Here, band gap is distinguished from energy gap. The energy gap is formed by the impurity level that does not form a complete band. Making a solid solution is also a useful band engineering procedure. Such band engineering is related to the step (i) in Fig. 4.

Fig. 22 Band structure control to develop visible light-driven-photocatalysts for water splitting.

Fig. 23 Strategies of band engineering for design of visible-light-driven photocatalysts.

Oxide photocatalysts for H₂ or O₂ evolution from aqueous solutions in the presence of sacrificial reagents under visible light irradiation are summarized in Table 4.

7.1.2 Native visible-light driven photocatalysts^{193,202–208}. WO₃ is one of the most well known photocatalysts with visible light response for O₂ evolution in the presence of sacrificial reagents such as Ag⁺ and Fe³⁺. Abe and co-workers recently found that Pt/WO₃ is active for degradation of acetic acid, CH₃CHO and IPA under visible light irradiation.²³²

Bi₂WO₆ and Bi₂MoO₆ with the Aurivillius structure are active for an O₂ evolution reaction under visible light irradiation. These tungstate and molybdate photocatalysts are not active for H₂ evolution because of the low conduction band level. These photocatalysts are also used for degradation of HCHO,²³³ CH₃OH,²³⁴ CH₃COOH,^235,236 Rhodamine B^237–249 and Methylene Blue.^240,244,247

7.1.3 Doped photocatalysts^209–217. Doping has often been attempted to prepare visible light-driven photocatalysts (Fig. 23(a)). Here, doping often means replacement with a foreign element at a crystal lattice point of the host material. A TiO₂ photocatalyst is usually employed as a host material for the doping. However, although the white powder becomes colored with doping of transition metal cations, in general, the photocatalytic activity drastically decreases because of formation of recombination centres between photogenerated electrons and holes, even under band gap excitation. However, doping of transition metals is a good strategy to develop visible light responsive photocatalysts if a suitable dopant is chosen as mentioned below.

Co-doping of Cr³⁺/Ta⁵⁺, Cr³⁺/Sb⁵⁺, Ni²⁺/Ta⁵⁺ and doping of Rh cations is effective in sensitization of SrTiO₃ to visible light. These doped SrTiO₃ powders with Pt co-catalysts show photocatalytic activities for H₂ evolution from aqueous methanol solutions under visible light irradiation. Cr and Fe are effective dopants for H₂ evolution over a La₂Ti₂O₇ photocatalyst. Rh-doped SrTiO₃ is one of the rare oxide photocatalysts that can efficiently produce H₂ under visible light irradiation. This Rh doping is also effective for CaTiO₃. The SrTiO₃:Rh photocatalyst plays an important role on a Z-scheme photocatalyst system for water splitting under visible light irradiation as mentioned in section 8.3. On the other hand, TiO₂ (rutile) co-doped with Cr³⁺/Sb⁵⁺, Rh³⁺/Sb⁵⁺ and Ni²⁺/Nb⁵⁺ is active for O₂ evolution from aqueous silver nitrate solutions. In these doped photocatalysts, the dopants form electron donor levels in the band gap of the TiO₂ and SrTiO₃ host materials, resulting in visible light response. When Ti⁴⁺ is replaced with Cr³⁺ or Ni²⁺, the charge becomes unbalanced. This may result in the formation of recombination centres. Co-doped metal cations such as Nb⁵⁺, Ta⁵⁺ and Sb⁵⁺ compensate the charge imbalance, resulting in the suppression of the formation of the recombination centres and maintaining the property of visible light absorption. When about 1% of Cr was doped into TiO₂ without any co-dopant, activities were never obtained as a rule. Thus, transition metal doping into photocatalysts with wide band gaps is effective for the development of visible light responsive photocatalysts if a suitable combination of dopant–co-dopant is chosen. Next, the co-doping effect is discussed using the TiO₂:Rh/Sb and TiO₂:Cr/Sb photocatalysts in more detail.

Fig. 24 shows dependence of photocatalytic O₂ evolution from an aqueous silver nitrate solution on TiO₂:Rh/Sb upon the ratio of doped Sb to Rh.²¹⁶ When only Rh is doped into TiO₂ no activity is obtained and the colour of the photocatalyst is black. When the ratio of Sb/Rh is equal to or larger than the unity O₂ evolution activity is observed accompanied with a colour change from black to orange. TiO₂:Rh without co-doping of Sb contains Rh⁴⁺ because Rh is doped at a Ti⁴⁺ site. The Rh⁴⁺ species predominantly works as a recombination site. As the ratio of co-doped Sb increases, the formation of Rh⁴⁺ is suppressed. Co-doping with Sb⁵⁺ produces Rh³⁺ forming an electron donor level, due to keeping of the charge balance, and results in showing of photocatalytic activities. The same dependency is observed for a TiO₂:Cr/Sb photocatalyst in which formation of Cr⁶⁺ is suppressed by the Sb⁵⁺ co-doping.²⁰⁹ It is confirmed by IR transient absorption spectroscopy for the TiO₂:Cr/Sb photocatalyst that the Sb⁵⁺ co-doping prolongs a lifetime of photogenerated electrons as shown in Fig. 25.²⁵¹ TiO₂:Cr/Sb with 1.0–2.0 of optimum ratios gives slowest decay when it is excited by 532 nm. It is interesting that the lifetime of photogenerated electrons for the optimized TiO₂:Cr/Sb is longer than that for nondoped TiO₂ even by the band gap excitation. The decay is too fast to measure in this time scale by pumping of both wavelengths for inactive TiO₂:Cr/Sb with smaller ratios than unity. The visible light responses are due to the transitions from electron donor levels consisting of Rh³⁺ and Cr³⁺ to the conduction band of the TiO₂ host. The TiO₂:Rh/Sb and TiO₂:Cr/Sb photocatalysts can use visible light up to 600 nm, of relatively long wavelength for O₂ evolution photocatalysts.

Fig. 24 Effect of co-doping of Sb to TiO₂:Rh (1.3%) on photocatalytic activity under visible light irradiation.²¹⁶

Fig. 25 Decay curves of photogenerated electrons in TiO₂:Sb/Cr photocatalyst.²⁵¹

PbMoO₄ shows activities for H₂ and O₂ evolution in the presence of sacrificial reagents under UV irradiation as shown in Table 3. When Cr⁶⁺ is partly replaced for Mo⁶⁺ in this host material Cr⁶⁺ forms an electron acceptor level resulting in a visible light response.²¹⁷ DFT calculation revealed that this visible light response is due to the transition from the valence band consisting of Pb 6s and O 2p to the electron acceptor level composed of Cr 3d empty orbitals. Formation of such an acceptor level is also useful for sensitization of wide band gap photocatalysts to visible light if the potential for H₂ evolution is not required.

Anion doping such as nitrogen to a TiO₂ photocatalyst has been studied for oxidation of organic compounds.²⁵⁰

7.1.4 Valence band-controlled photocatalysts^218–231. In the doped photocatalysts mentioned above, the formation of recombination sites by the dopant is more or less inevitable. Moreover, the level formed by the dopant is usually discrete and thus inconvenient for the migration of holes formed there. Therefore, the formation of a valence band by orbitals not associated with O 2p but with other elements is indispensable for oxide photocatalysts in order to design visible light-driven photocatalysts (Fig. 23(b)).

Orbitals of Pb 6s in Pb²⁺, Bi 6s in Bi³⁺, Sn 5s in Sn²⁺ and Ag 4d in Ag⁺ can form valence bands above the valence band consisting of O 2p orbitals in metal oxide photocatalysts. The degree of the contribution of these metal cations to the valence band formation depends on the crystal structure and the ratio of the metal cations contained.

RbPb₂Nb₃O₁₀ and PbBi₂Nb₂O₉ with layered perovskite structure show activity for H₂ or O₂ evolution.

BiVO₄ with a monoclinic scheelite structure shows photocatalytic activities for O₂ evolution from aqueous silver nitrate solutions under visible light irradiation. BiVO₄ can be prepared by an aqueous process at ambient temperature and pressure^222,223 in an environmentally friendly process. The photocatalytic activity of BiVO₄ prepared by the aqueous process is much higher than that of BiVO₄ prepared by a conventional solid state reaction. The difference in the photocatalytic activity between BiVO₄ obtained by the different methods is due to the crystallinity and defects. The aqueous process is especially advantageous for the preparation of materials in which defects are easily formed by volatilization at high temperature calcination. The valence band formation by Bi 6s orbitals is confirmed by the band structure and density of states obtained by DFT calculation as shown in Fig. 26. The conduction band is composed of V 3d as in other d⁰ oxide photocatalysts. Although BiVO₄ does not show activity for H₂ evolution due to the low conduction level, it is noteworthy that the valence band formed with Bi 6s orbitals possesses the potential for water oxidation to form O₂ accompanied by 4-electron oxidation. BiVO₄ is also used for the decomposition of endocrine disruptors such as nonylphenol²⁵² and degradation of Methylene Blue,^253,254 Methyl Orange,^255–258 Rhodamine B,^259,260 4-n-alkylphenol,^261,262 4-n-nonylphenol,^261,262 aromatic hydrocarbons,²⁶³ and benzopyrene.²⁶⁴ OH radicals that are often an active species for photocatalytic oxidation of organic compounds are not involved with the degradation in the case of the BiVO₄ photocatalyst.²⁶⁵

Fig. 26 Band structure of BiVO₄ calculated by DFT.

SnNb₂O₆ shows activity for H₂ or O₂ evolution when suitable co-catalysts are loaded. Especially, IrO₂/SnNb₂O₆ shows relatively high activity for O₂ evolution.²²⁹ Although SnNb₂O₆ is active for half reactions of water splitting under visible light irradiation overall water splitting is as yet not successful. Sn 5s orbitals in Sn²⁺ form a valence band as seen in SnNb₂O₆ while Sn 5s5p orbitals in Sn⁴⁺ form a conduction band as observed for Sr₂SnO₄ (Table 2).

AgNbO₃ with a perovskite structure and Ag₃VO₄ are active for O₂ evolution. AgLi_1/3Ti_2/3O₂ and AgLi_1/3Sn_2/3O₂ with delafossite structure are synthesized by treating layered compounds Li₂TiO₃ and Li₂SnO₃ with molten AgNO₃ through ion exchange of Li⁺ for Ag⁺ and show activities for O₂ evolution from an aqueous silver nitrate solution under visible light irradiation. The visible light responses of AgNbO₃, AgLi_1/3Ti_2/3O₂ and AgLi_1/3Sn_2/3O₂ are due to the band gap excitation between conduction bands consisting of either Nb 4d or Ti 3d or Sn 5s5p orbitals and valence bands consisting of Ag 4d orbitals.^125,231 AgNbO₃ is also active for the decomposition of endocrine disruptors such as nonylphenol.²⁶⁶ Moreover, band engineering using the Ag 4d orbital is applied to develop solid solution photocatalysts of AgNbO₃–SrTiO₃ for degradations of 2-propanol^267,268 and CH₃CHO.²⁶⁹

7.1.5 Oxide photocatalysts with visible light response by sensitization^270–279. Photocatalytic reactions under visible light irradiation by sensitization of wide band gap semiconductor photocatalysts have been studied as shown in Table 5. TiO₂ and K₄Nb₆O₁₇ loaded with various metal complexes and dyes respond to visible light for H₂ evolution according to a scheme as shown in Fig. 27. After an electron is excited from the HOMO to LUMO of a dye by visible light the electron is injected to a conduction band. H₂ evolves on the wide band gap photocatalyst. This sensitization is applied to a Ru(bpy)₃²⁺/K₄Nb₆O₁₇ thin film electrode that gives a photocurrent responding to visible light.²⁸⁰ Layered metal oxide photocatalysts intercalated with CdS are also active for H₂ evolution in the presence of sacrificial reagents. The layered metal oxides serve as H₂ evolution sites.

Fig. 27 Scheme of sensitized-type photocatalyst.

7.2 (Oxy)nitride and oxysulfide photocatalysts^281–296

Domen and co-workers have reported (oxy)nitrides and oxysulfides as new types of visible light-driven photocatalysts as shown in Table 6. The valence bands of these photocatalysts consist of N 2p and S 3p orbitals, in addition to O 2p, resulting in the formation of narrow band gaps. These materials can utilize up to 500–600 nm visible light.

Table 6 (Oxy)nitride and oxysulfide photocatalysts for H₂ or O₂ evolution from aqueous solutions in the presence of sacrificial reagents under visible light irradiation^a

Photocatalyst	BG/eV	H2 evolution^b			O2 evolution^c			Ref. (Year)
		Co-catal.	Activity/μmol h^−1	QY (%)	Co-catal.	Activity/μmol h^−1	QY (%)
a Light source: 300 W Xe lamp with a cut-off filter (L42).b Sacrificial reagent: CH3OH aq.c Sacrificial reagent: AgNO3 aq. La2O3 or La(NO)3 was added as a buffer for pH.
LaTiO2N	2.1	Pt	30	0.15	IrO2	41	1.5	281 (2002)
Ca0.25La0.75TiO2.25N0.75	2.0	Pt	5.5		IrO2	230	5	281 (2002)
TaON	2.5	Ru	120	0.2		380	34	282–286 (2002)
Ta3N5	2.1	Pt	10	0.1 (420–600 nm)		420	10 (420–600 nm)	284, 286–288 (2002)
CaNbO2N	1.9	Pt	1.5	—		46	—	289 (2002)
CaTaO2N	2.5	Pt	15	—		0	—	290 (2004)
SrTaO2N	2.1	Pt	20	—		0	—	290 (2004)
BaTaO2N	2.0	Pt	15	—		0	—	290 (2004)
LaTaO2N	2.0	Pt	20	—		0	—	289 (2002)
Y2Ta2O5N2	2.2	Pt-Ru	250	—		140	—	291 (2004)
TiNxOyFz	2.2	—	—	—		30	—	292, 293 (2003)
Sm2Ti2O5S2	2.0	Pt	22	0.3		30	0.6	53, 294, 295(2002)
La–In oxisulfide	2.6	Pt	10	0.2	IrO2	7	0.1	296 (2007)

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Oxynitride photocatalysts consisting of metal cations of Ti⁴⁺, Nb⁵⁺ and Ta⁵⁺ with d⁰ configuration are active for H₂ or O₂ evolution in the presence of sacrificial reagents. TaON and Ta₃N₅ give high quantum yields for O₂ evolution. However, they are not active for water splitting into H₂ and O₂ without sacrificial reagents at the present stage. These materials can also be applied to photoelectrochemical cells.^297–299 Although metal sulfides such as CdS cannot evolve O₂ because of photocorrosion, Sm₂Ti₂O₅S₂, an oxysulfide with layered perovskite structure is active for the O₂ evolution.

8. Photocatalyst systems for water splitting under visible light irradiation

There are two types of photocatalyst systems for water splitting under visible light irradiation as shown in Fig. 28. Band engineering is indispensable to develop the single photocatalyst system as shown in Fig. 22. Some oxynitride photocatalysts are active for water splitting as mentioned in the next section. Two-photon systems, as seen in photosynthesis by green plants (Z-scheme), is another way to achieve overall water splitting as mentioned in section 8.3. The Z-scheme is composed of an H₂-evolution photocatalyst, an O₂-evolution photocatalyst, and an electron mediator. Photocatalysts that are active only for half reactions of water splitting as shown in Fig. 9 can be employed for the construction of the Z scheme: that is the merit of the Z scheme. Some photocatalysts listed in Table 4 are actually used for Z-scheme systems.

Fig. 28 Single- and two-photon photocatalyst systems for water splitting.

8.1 d¹⁰ metal nitrides^300–312

Nitrides consisting of d¹⁰ metal cations are active for water splitting as shown in Table 7, in contrast to d⁰ metal (oxy)nitrides. Ge₃N₄ shows activity under UV irradiation. This is the first example of a non-oxide powdered photocatalyst for water splitting.³⁰⁰ GaN is the well known semiconductor that is used for a blue light emitting diode.³¹³ Native GaN powder is not active whereas GaN loaded with Rh_2−xCr_xO₃ co-catalyst and Mg-doped GaN powders are active under UV irradiation. In contrast, GaN:ZnO solid solutions are active under visible light irradiation. The solid solutions are prepared by NH₃-treatment of a mixture of Ga₂O₃ and ZnO at 1123–1223 K for 5–30 h. Although native GaN and ZnO possess only UV absorption bands, the solid solutions have visible light absorption bands depending on the composition as shown in Fig. 29.³⁰⁷ The visible light absorption is due to a Zn-related acceptor level and/or p–d repulsion between Zn 3d and N 2p + O 2p in addition to the contribution of N 2p to valence band formation.^312,314,315 Optimized GaN:ZnO with Rh_2−xCr_xO₃ co-catalyst gives 5.9% of quantum yield.³¹¹ Ge₃N₄:ZnO is also active under visible light irradiation.

Table 7 (Oxy)nitride photocatalysts for water splitting^a

Photocatalyst	BG/eV	Co-catalyst	Incident light/nm	Reactant solution	Activity/μmol h^−1		QY (%)	Ref. (Year)
					H2	O2
a Light source: 450 W high pressure mercury lamp, reaction cell: inner irradiation cell.b Made of quartz.c Made of Pyrex.d Made of Pyrex filled with aqueous NaNO2 solution as a filter.
Ge3N4	3.6	RuO2	>200^b	Pure water	1400	700	9 (at 300 nm)	300–303 (2005)
GaN	3.4	Rh2−xCrxO3	>300^c	H2SO4 (pH 4.5)	19	9.5	0.7 (300–340 nm)	304 (2007)
GaN:Mg	3.4	RuO2	>300^c	Pure water	730	290		305, 306 (2006)
(Ga0.88Zn0.12)(N0.88O0.12)	2.6	Rh2−xCrxO3	>400^d	H2SO4 (pH 4.5)	800	400	5.9 (420–440 nm)	50, 307–311 (2005)
Zn1.44GeN2.08O0.38	2.7	RuO2	>400^d	Pure water	14.2	7.4		312 (2007)

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Fig. 29 Diffuse reflection spectra of (Ga_1−xZn_x)(N_1−xO_x) photocatalysts.³⁰⁷

8.2 d⁰ metal oxides

InTaO₄^316,317 and YBiWO₆³¹⁸ have been reported for water splitting as single photocatalyst systems under visible light irradiation.

8.3 Z-Scheme systems (two-photon process)^319–325

Table 8 summarizes Z-scheme systems that work under visible light irradiation. Combined systems with Fe ion-WO₃,³²⁶ Pt/TiO₂(anatase)–TiO₂(rutile)–IO₃⁻/I^−321,327 and Pt/TiO₂(anatase)–Pt/WO₃–IO₃⁻/I⁻³²¹ are active for water splitting through a two-photon process under UV irradiation because an iron ion and TiO₂ respond to only UV. Combined systems with Pt/SrTiO₃:Cr/Ta for the H₂ evolution photocatalyst and Pt/WO₃ for the O₂ evolution photocatalyst can split water into H₂ and O₂ in stoichiometric amounts under visible light irradiation in the presence of an IO₃⁻/I⁻ redox couple. Oxynitride photocatalysts, TaON, CaTa₂O₂N and BaTa₂O₂N can be used as H₂ evolution photocatalysts with a Pt/WO₃ of O₂ evolution photocatalyst. These photocatalyst systems respond to about 450-nm light, which is limited by the band gap of WO₃. The system of Pt/TaON with RuO₂/TaON is a unique combination and is active up to 500 nm. The Z-scheme system consisting of Pt/SrTiO₃:Rh and BiVO₄ or Bi₂MoO₆ is also active in the presence of an Fe³⁺/Fe²⁺ redox couple. The system of Pt/SrTiO₃:Rh and BiVO₄ responds to 520-nm light, which corresponds to the energy and band gaps of SrTiO₃:Rh and BiVO₄ as shown in Fig. 30. Although the efficiency is low, solar hydrogen production from water has been accomplished using the Z-scheme system with powdered photocatalysts as shown in Fig. 31. It is a simple system: the sun is allowed to shine on the powders dispersed in aqueous solutions of iron ions and Co complexes which causes water splitting to form H₂ and O₂.

Table 8 Z-Scheme type photocatalysts for water splitting under visible light irradiation^a

H2 photocatalyst	O2 photocatalyst	Mediator	Activity/μmol h^−1		QY (%)	Ref. (Year)
			H2	O2
a Light source: 300 W Xe lamp with a cut-off filter (L42).
Pt/SrTiO3:Cr,Ta	Pt/WO3	IO^3−/I^−	16	8	1 (at 420 nm)	319–321 (2001)
Pt/TaON	RuO2/TaON	IO^3−/I^−	3	1.5	0.1–0.2	322 (2008)
Pt/CaTaO2N	Pt/WO3	IO^3−/I^−	6.6	3.3	—	323 (2008)
Pt/BaTaO2N	Pt/WO3	IO^3−/I^−	4	2	—	323 (2008)
Pt/TaON	Pt/WO3	IO^3−/I^−	24	12	0.4 (at 420 nm)	324 (2005)
Pt/SrTiO3:Rh	BiVO4	Fe^3+^/2+	15	7.2	0.3 (at 440 nm)	325 (2004)
Pt/SrTiO3:Rh	Bi2MoO6	Fe^3+^/2+	19	8.9	0.2 (at 440 nm)	325 (2004)
Pt/SrTiO3:Rh	WO3	Fe^3+^/2+	7.8	4.0	0.2 (at 440 nm)	325 (2004)

---

Fig. 30 Action spectrum for water splitting using (Ru/SrTiO₃:Rh)–(BVO₄)–FeCl₃.³²⁵

Fig. 31 Solar water splitting by Z-scheme photocatalyst system with nano-oxides.

9. Metal sulfide photocatalysts with visible light response for H₂ evolution from an aqueous solution containing a sacrificial reagent^328–349

Metal sulfides are attractive materials as candidates of visible-light-driven photocatalysts. The valence band usually consists of S 3p orbitals the level of which is more negative than O 2p as shown in Fig. 22. Although instability is a drawback of metal sulfide photocatalysts the photocorrosion is suppressed by hole scavenger such as S²⁻ and SO₃²⁻.

Many metal sulfide photocatalysts have been reported for H₂ evolution in the presence of sacrificial reagents as shown in Table 9.

Table 9 Sulfide photocatalysts for H₂ evolution from aqueous solutions in the presence of sacrificial reagents

Photocatalyst	BG/eV	Incident light/nm	Light source	Reactant solution	H2 evolution/μmol h^−1	QY (%)	Ref. (Year)
Pt/CdS	2.4	>390	500 W Hg	Na2SO3	40	35 (at 436 nm)	328–331 (1983)
ZnS	3.1	>200	200 W Hg	Na2S + H3PO2 + NaOH	13 000	90 (at 313 nm)	332, 333 (1984)
CuInS2		>300	400 W Xe	Na2SO3	0.3		334 (1992)
CuIn5S8		>300	400 W Xe	Na2SO3	1.8	0.02 (at 460 nm)	334 (1992)
Rh/AgGaS2	2.6	>420	300 W Xe	Na2S + K2SO3	1340	25 (at 440 nm)	17 (2006)
Pt/AgIn5S8	1.8	>420	400 W Xe	Na2S + K2SO3	60	5.3 (at 411.2 nm)	335 (2007)
Pt/NaInS2	2.3	>420	300 W Xe	K2SO3	470	6 (at 440 nm)	336 (2002)
Pt/ZnIn2S4	2.3	>420	300 W Xe	Na2S + Na2SO3	77		337 (2003)
Na10In16Cu4S35	2.0	>420	300 W Xe	Na2S	9	3.7 (at 420 nm)	338 (2005)
In10S18^6−: APE		>300	300 W Xe	Na2SO3	20		338 (2005)
[Na5(H2O)6]^5+[SIn4(SIn4)6/2]^5−	3.2	>300	300 W Xe	Na2SO3	2.4		339 (2005)
ZnS:Cu	2.5	>420	300 W Xe	K2SO3	450	3.7 (at 420 nm)	340 (1999)
ZnS:Ni	2.3	>420	300 W Xe	Na2S + K2SO3	280		341 (2000)
ZnS:Pb, Cl	2.3	>420	300 W Xe	Na2S + K2SO3	40		342 (2003)
Pt/CdS:Ag	2.35	>300	900 W Xe	Na2S + Na2SO3	11 440	25 (at 450 nm)	329 (1986)
CdS–ZnS	2.35	>400	300 W Hg	Na2S + Na2SO3	250	0.60	329–331 (2006)
Pt/AgInZn7S9	2.4	>420	300 W Xe	Na2S + K2SO3	940	20 (at 420 nm)	343,344 (2004)
Pt/Cu0.09In0.09Zn1.82S2	2.35	>420	300 W Xe	Na2S + K2SO3	1200	12.5 (at 420 nm)	345 (2005)
Ru/Cu0.25Ag0.25In0.5ZnS2	2.0	>420	300 W Xe	Na2S + K2SO3	2300	7.4 (at 520 nm)	346,347
Pt/AgGa0.9In0.1S2	2.4	>420	450 W Hg	Na2S + Na2SO3	350		348 (2008)
Pt/[In(OH)ySz]:Zn	2.2	>420	300 W Xe	Na2S + Na2SO3	67	0.59 (at 420 nm)	349 (2004)

---

9.1 Native visible-light driven photocatalysts^328–339

CdS with a 2.4 eV-band gap is a well known metal sulfide photocatalyst that can produce H₂ under visible light irradiation in the presence of a sacrificial reagent.^328–331 CdS has been studied for a long time. ZnS with 3.6 eV-band gap is also a well-known photocatalyst for H₂ evolution though it responds to only UV. It shows high activity without any assistance of co-catalysts such as Pt. Therefore, ZnS is an attractive host photocatalyst for doping and preparing solid solutions as mentioned below. Photocatalytic H₂ evolution on CuInS₂, CuIn₅S₈, AgGaS₂ and AgIn₅S₈ has been reported in the presence of sacrificial reagents. These metal sulfides consist of elements of groups 11 and 13. NaInS₂ with layered structure and ZnIn₂S₄ with spinel structure are active. Feng and co-workers have reported unique photocatalysts of indium sulfide compounds with open-framework structure.^338,339

9.2 Doped photocatalysts^{330,340–342}

Fig. 32 shows diffuse reflection spectra of ZnS doped with various metal cations. Visible light absorption band tails are observed in addition to the band gap absorption band of the ZnS host. These spectra have typical shapes of doped photocatalysts being different from those of band gap transitions. These metal cation-doped ZnS photocatalysts show activities for H₂ evolution from aqueous solutions containing S²⁻ and/or SO₃²⁻ as electron donors. Loading of co-catalysts such as Pt is not necessary for the H₂ evolution, indicating that the high conduction band of the ZnS host is maintained after the doping of metal cations. Ag doping is also effective for a CdS photocatalyst.

Fig. 32 Diffuse reflection spectra of metal ion-doped ZnS photocatalysts.

9.3 Solid solution photocatalysts^{330–332,343–348}

CdS and ZnS possess the same crystal structure indicating that they can form solid solutions. The CdS–ZnS solid solution is active for H₂ evolution.

Solid solutions of AgInS₂–ZnS, CuInS₂–ZnS and CuInS₂–AgInS₂–ZnS that are designed according to the strategy as shown in Fig. 23(c) show high photocatalytic activities for H₂ evolution from aqueous sulfide and sulfite solutions under visible light irradiation. A solid solution photocatalyst of AgGa_0.9In_0.1S₂ is also active. The solid solution formation is usually confirmed by X-ray diffraction. Peaks of XRD shift with the composition of the solid solution according to the difference in ionic radii between metal cations. The diffuse reflectance spectra of AgInS₂–CuInS₂–ZnS solid solutions shift monotonically with the composition of the solid solution as shown in Fig. 33. DFT calculation indicates that the levels of the conduction band consisting of Zn 4s4p and In 5s5p, and of the valence band consisting of Cu 3d, Ag 4d and S 3p, shift with the varying composition. Ru/Cu_0.25Ag_0.25In_0.5ZnS₂ shows an excellent activity for H₂ evolution with a solar simulator (AM-1.5). These sulfide solid solution photocatalysts can utilize visible light of wavelengths up to about 700 nm. Moreover, solid solutions of AgInS₂-CuInS₂ are black photocatalysts with about 1.5 eV band gap for H₂ evolution. The black photocatalysts can utilize near-infrared radiation up to 820 nm. The authors have demonstrated solar hydrogen products from an aqueous Na₂S + K₂SO₃ solution using the AgInS₂–CuInS₂–ZnS solid solution photocatalyst and a reactor of 1 m². H₂ evolution at a rate of about 2 L/m² h was observed in November in Tokyo. This photocatalytic H₂ evolution will be important if abundant sulfur compounds in chemical industries or nature can be used as electron donors as shown in Fig. 34. Ideally, this reaction produces H₂ at ambient temperature and pressure but does not consume fossil fuels and does not emit CO₂. It should be noted that the photocatalytic H₂ evolution is not a solar energy conversion because the change in the Gibbs free energy is not so positive. Toji’s group have studied CdS and ZnS photocatalysts with shell structure in the presence of an electron donor aiming at solar hydrogen production.³⁵⁰

Fig. 33 Diffuse reflection spectra of (CuAg)_xIn_2xZn_2(1−2x)S₂ solid solution.³⁴⁶

Fig. 34 Solar H₂ production using abundant sulfur compounds and metal sulfide photocatalysts.

AgInS₂–ZnS and CuInS₂–ZnS solid solution materials are applied to unique luminescent materials of which emission wavelengths are tuneable with the ratio of the solid solutions.^351–353

10. Conclusions

Energy and environment issues are discussed in literature.^354–359 Solar water splitting including photocatalytic processes is focused on as a candidate of the science and technology for solving the issues in the future.^354,358,359 The number of photocatalysts for water splitting was very limited about twenty years ago. Furthermore, the only well-known visible light driven photocatalysts were CdS and WO₃ for H₂ and O₂ evolution, respectively, even in the presence of sacrificial reagents. Now, many photocatalyst materials have been developed as introduced in the present review paper. So, we are sure that this research area is progressing. For example, a highly efficient water splitting was achieved using a powdered photocatalyst of NiO/NaTaO₃:La under UV irradiation. The finding has proven that highly efficient water splitting is actually possible using powered photocatalysts. New powdered photocatalyst systems of oxynitrides such as Cr_xRh_2−xO₃/GaN:ZnO and Z-scheme systems such as Ru/SrTiO₃:Rh-BiVO₄ have been developed for overall water splitting under visible light irradiation after about 35 years from the report of the Honda–Fujishima effect. Solar water splitting is confirmed using the Ru/SrTiO₃:Rh–BiVO₄ photocatalyst system. Moreover, in the presence of sulfur compounds as electron donors, the sulfide solid solution photocatalysts AgInS₂–CuInS₂–ZnS are highly active for H₂ evolution under solar light irradiation. H₂ is thus realistically obtained under sunlight irradiation. Thus, the library of photocatalyst materials has become plentiful. The photocatalyst library will give information on factors affecting photocatalytic abilities and the further development of new photocatalysts. The science for understanding photocatalytic processes is also developed.^{40,168,251,360,361}

The target for efficiency for water splitting into H₂ and O₂ is 30% in terms of a quantum yield at 600 nm in this research field. This efficiency gives about 5% of solar energy conversion. The Cr_xRh_2−xO₃/GaN:ZnO and Ru/SrTiO₃:Rh-BiVO₄ photocatalysts respond to about 500 nm for overall water splitting so approaching this target but the quantum yield is still low. So, surveying photocatalyst materials are still important. It will be also important to construct the operating system for photocatalytic hydrogen production. Such an achievement will contribute to global energy and environmental issues in the future resulting in bringing about an energy revolution.

Acknowledgements

This work was supported by the Core Research for Evolutional Science and Technology (CREST) program of the Japan Science and Technology (JST) Agency, and a Grant-in-Aid for Priority Area Research from the Ministry of Education, Culture, Science, and Technology. The authors thank Dr Kato, Dr Tsuji, Prof. Domen, Prof. Kakihana, Prof. Kobayashi, Prof. Kohtani, Prof. Onishi and Prof. Torimoto for their collaborations and valuable discussions.

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Footnote

† Part of the renewable energy theme issue.

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