Scholarly article on topic 'ABO3-based photocatalysts for water splitting'

ABO3-based photocatalysts for water splitting Academic research paper on "Materials engineering"

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Abstract of research paper on Materials engineering, author of scientific article — Jinwen Shi, Liejin Guo

Abstract ABO3-based photocatalysts for water splitting were systematically reviewed in this manuscript. Crystal structure and chemical composition characteristics of ABO3 materials were briefly introduced to guide the modification of ABO3-based photocatalysts. The ABO3-based photocatalysts were then reviewed in detail and divided into four groups based on the employed modification strategies, i.e., chemical component adjustment, micro-/nano-structure adjustment, local lattice structure adjustment, and application of the modification strategy of ABO3 photocatalysts in designing A x B y O z photocatalysts. In this section, the recent research works on ABO3-based photocatalysts in our group were presented. Finally, application of ABO3 photocatalysts in Z-scheme systems for overall water splitting was introduced. This review summarized the development of ABO3-based photocatalysts and showed the values and possible direction of future research, thereby offering a guide for photocatalytic water splitting.

Academic research paper on topic "ABO3-based photocatalysts for water splitting"

Progress in Natural Science: Materials International 2012;22(6):592-615

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Progress in Natural Science

Chinese Materials Research Society

Progress in Natural Science: Materials International

www.elsevier.com/locate/pnsmi www.sciencedirect.com

REVIEW

ABO3-based photocatalysts for water splitting

Jinwen Shi, Liejin Guo*

International Research Center for Renewable Energy, State Key Laboratory of Multiphase Flow in Power Engineering (MFPE), Xi'an Jiaotong University (XJTU), 28 West Xianning Road, Xi'an, Shaanxi 710049, China

Received 22 July 2012; accepted 8 November 2012 Available online 18 January 2013

KEYWORDS

Perovskite; Band structures; Micro-/nano-structures; Local lattice structures; Energy conversion; Photochemistry

Abstract ABO3-based photocatalysts for water splitting were systematically reviewed in this manuscript. Crystal structure and chemical composition characteristics of ABO3 materials were briefly introduced to guide the modification of ABO3-based photocatalysts. The ABO3-based photocatalysts were then reviewed in detail and divided into four groups based on the employed modification strategies, i.e., chemical component adjustment, micro-/nano-structure adjustment, local lattice structure adjustment, and application of the modification strategy of ABO3 photocatalysts in designing AxByOz photocatalysts. In this section, the recent research works on ABO3-based photocatalysts in our group were presented. Finally, application of ABO3 photocatalysts in Z-scheme systems for overall water splitting was introduced. This review summarized the development of ABO3-based photocatalysts and showed the values and possible direction of future research, thereby offering a guide for photocatalytic water splitting.

© 2013 Chinese Materials Research Society. Production and hosting by Elsevier B.V. All rights reserved.

1. Introduction

Energy shortage and environmental pollution have become two urgent problems that restrict social development and endanger human survival. In order to resolve the two issues and to realize the sustainable development of society, the most effective route is

■"Corresponding author. Tel.: +86 29 8266 0996; fax: +86 29 8266 9033.

E-mail address: lj-guo@mail.xjtu.edu.cn (L. Guo). Peer review under responsibility of Chinese Materials Research Society.

the active development and utilization of clean and renewable energy sources, among which solar energy is the most promising candidate due to its abundant reservation and wide distribution. However, the large scale utilization of solar energy is still restricted at present due to its low energy density, high geographical dispersion, high time instability, and inconvenience for storage. H2 is a kind of superior energy carrier and ideal clean and renewable energy due to its high energy density, no pollution from the combustion products, convenience for storage and transportation. Consequently, exploring and realizing the conversion and storage of solar energy as hydrogen energy is a powerful route to develop and utilize renewable energy [1]. Photocatalytic H2 evolution under solar light irradiation has shown good prospect for such a conversion, and photocatalytic overall water splitting to form both H2 and O2 stoichiometrically under solar light irradiation is the ultimate goal to realize such a conversion by

1002-0071 © 2013 Chinese Materials Research Society. Production and hosting by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.pnsc.2012.12.002

photocatalysis [2]. Furthermore, in order to develop the system for photocatalytic overall water splitting under solar light irradiation, it is a key task to exploit photocatalysts with high stability, high efficiency, low cost for visible-light-driven H2 and O2 evolution.

The developed photocatalytic materials for H2 and O2 evolution mainly contain inorganic semiconductors [3-7], metal organic complexes [8-10], organic semiconductors [11,12], and recently reported conductors [13], in which inorganic semiconductor photocatalysts showed great promising for future applications [14-17].

2. Semiconductor photocatalysis for water splitting

The fundamental principle and processes for semiconductor-based photocatalytic water splitting have been clearly presented in many excellent review papers [14-16,18-28] and raised the basic requirement for water-splitting photocatalysts as follows to guide the exploration of such photocatalytic materials.

Considering the fundamental principle, photocatalyst should have bandgap larger than the standard Gibbs free energy change (1.23 eV) for water splitting into H2 and O2, and should have conduction band with potential more negative than that for water reduction and valence band with potential more positive than that for water oxidation. Furthermore, based on the processes, firstly, photocatalyst should have bandgap smaller than the energies of incident photons to ensure the excitation of photogenerated charge carriers. Visible light accounts for around 43% of the incoming solar energy, and therefore should be fully used by photocatalysts with excellent visible-light response. Secondly, photocatalyst should have good ability to promote the separation and transfer of photogenerated charge carriers and thus to restrain their recombination. Thirdly, photocatalyst should provide enough reactive sites to accelerate redox reaction and to restrain the back-reaction. In a word, in order to improve the photocatalytic reaction, photocatalyst should enhance the generation, transfer, and reaction of photogenerated charge carriers during the whole photocatalytic process.

According to the types of anions, inorganic semiconductor photocatalysts were mainly classified as S-containing, N-contain-ing, and oxide photocatalysts. In contrast, oxide photocatalysts generally showed unparalleled stability that is the most basic and important requirement of photocatalysts for practical application, and therefore are of great research value. However, the ability of oxide photocatalysts to utilize visible light was often intrinsically restrained by the wide bandgaps, which were usually caused by the low valence bands that consisted of O 2p orbitals with a potential of around 3.0 eV versus NHE [20]. Moreover, the activities of developed visible-light-driven oxide photocatalysts were still low [3]. Accordingly, the research focus on oxide photocatalysts should be the extension of light-response ranges and the enhancement of visible-light-driven photocatalytic activities [29].

In this review paper, we focus on ABO3-based photocatalysts in view of the following characteristics of ABO3 materials.

3. Characteristics of ABO3 materials

3.1. Crystal structure characteristics of ABO3 materials

ABO3 represents a class of typical binary metal oxides. A series of ABO3 with crystal structures could be obtained by using different elements and preparation conditions. By taking the

prevalent perovskite ABO3 as an example, the crystal structure characteristics were introduced as follows so as to indicate the advantages and prospects of ABO3 materials for the application to photocatalytic water splitting under solar irradiation.

ABO3 with ideal cubic perovskite structure (space group, Pm 3m) is shown in Fig. 1. Metal cations in A and B sites are located in the 12- and 6-coordinated environments with O anions, respectively, and the radii of A-site cations are generally larger than those of B-site cations. Eight BO6 octahedra with shared corners formed the three-dimensional framework of cubic cell, the center of which is occupied by A-site cation. The radii of cations varied due to different types and valence states of cations. Consequently, compared with the ideal cubic perovskite ABO3, the real perovskite ABO3 exhibited lattice distortion to varying degrees, thereby resulting in the transformation of crystal phases in the following sequences: orthogonal, rhombohedral, tetragonal, monoclinic, and triclinic phase. As far as photocatalysis is concerned, lattice distortion has important impact on crystal field and thus changes the dipole and electronic band structures, thereby influencing the behaviors of photogenerated charge carriers, including excitation, transfer, and redox reaction, in whole photocatalytic process [30-32].

3.2. Chemical composition characteristics of ABO3 materials

Most metal elements in periodic table are known to be stably located in ABO3 structures, and multicomponent ABO3 materials can be easily synthesized by partial substitution of cations in A and B sites. In terms of photocatalysts, control of electronic band structures by adjusting elemental compositions was accepted as the effective method to extend light-response range. For this reason, ABO3 structures provide the basic framework and extensive space for the combination of elemental compositions and the construction of electronic band structures in photocatalysts [33-38].

Moreover, owing to the requirement of charge balance for in A and B sites, the mixed oxidation states or unusual

Fig. 1 Ideal cubic perovskite structure for ABO3 (cyan, BO6 units; yellow, A atoms). Reprinted with permission from Ref. [27].

oxidation states of metal cations were expected to be maintained in ABO3 structures. With regard to photocatalysts, valence states of elements generally play key roles in determining the ability of light response and the lifetime of photogenerated charge carriers. Accordingly, ABO3 structures can be employed to guarantee the valence-state control of metal cations in photocatalysts [39,40].

Besides, the easy preparation and controllable physico-chemical properties make ABO3 materials excellent model photocatalysts for the investigation of structure-activity relationships during photocatalytic reaction.

4. Development of ABO3 photocatalysts

Based on the crystal structure and chemical component characteristics, different modification strategies (such as chemical component adjustment, micro-/nano-structure adjustment, and local lattice structure adjustment) were employed to develop ABO3 photocatalysts with improved performance for water splitting (see Table 1).

4.1. Chemical component adjustment

The approaches to chemical component adjustment of ABO3 photocatalysts are mainly divided into ion doping, ion replacement, multi-component solid solution, multi-component composite.

4.1.1. Ion doping

Ion doping includes interstitial doping and lattice substitution. Ions are generally doped into ABO3 materials by means of lattice substitution. ABO3 (such as SrTiO3 [81,122-128] and NaTaO3 [50]) showed unparalleled stability and high activity for photocatalytic H2 and/or O2 evolution especially for overall water splitting under ultraviolet (UV) irradiation [50,81,122-128], but the ability to utilize visible light was often intrinsically restrained by wide bandgaps [20]. Accordingly, metal cations were usually doped into ABO3 (such as SrTiO3 [39,49,55,56,58,73,102] and NaTaO3 [120]) to tailor the electronic structures and thus to extend the light response range [20].

Konta et al. found that Ru-, Rh-, and Ir-doped SrTiO3 possessed intense absorption bands in the visible light region due to excitation from the discontinuous levels formed by the dopants to the conduction band of the SrTiO3 host. Mn- and Ru-doped SrTiO3 showed photocatalytic activities for O2 evolution from an aqueous silver nitrate solution while Ru-, Rh-, and Ir-doped SrTiO3 loaded with Pt cocata-lysts produced H2 from an aqueous methanol solution under visible light irradiation. Pt/Rh-doped SrTiO3 photocatalyst gave an AQY of 5.2% at 420 nm for the H2 evolution reaction, and the reducible Rh species contributed to the formation of visible light absorption band and the surface reaction sites [55]. Bae et al. investigated noble metal (Ru, Rh, Ir, Pt, and Pd)-doped SrTiO3 as well, and ascribed the photocatalytic activity of Pt/Rh-doped SrTiO3 to its suitable band energetics, and the induced hybridized Ti/Rh orbitals in the bandgap of SrTiO3 [73]. Chen et al. developed copper doped ABO3 (CaTiO3 [90] and LaNiO3 [91,94]) photocatalysts for visible-light-driven H2 evolution and attributed the enhancement of photocatalytic activity to the smaller bandgap and the lowered probability for recombination of electron-hole pairs.

Furthermore, in order to control the valence states of doped metal cations and to reduce the lattice defects in ABO3 photocatalysts, the strategy of codoping was generally adopted to maintain the charge balance. Kudo et al. developed a series of codoped SrTiO3 photocatalysts, such as (Cr,Sb)-codoped SrTiO3 [49], (Cr,Ta)-codoped SrTiO3 [56], and (Ni,Ta)-codoped SrTiO3 [58], and applied to visible-light-driven photocatalytic H2 evolution in methanol solution. (Cr,Sb)-codoped SrTiO3 showed intense absorption bands in the visible light region and possessed 2.4 eV of energy gaps, respectively. The charge balance was kept by codoping of Sb5+ and Cr3+ ions, resulting in the suppression of formation of Cr6+ ions and oxygen defects in the lattice which should work as effectively nonradiative recombination centers between photogenerated electrons and holes [49]. Compared with Cr-doped SrTiO3, (Cr,Ta)-codoped SrTiO3 had a shorter induction period and showed higher photocatalytic activity. Codoping of tantalum ions suppressed the formation of Cr6+ ions and oxygen defects which would work as nonradiative recombination centers between photogenerated electrons and holes, resulting in the shortening of induction periods and the improvement of photocatalytic activities [56]. As for (Ni,Ta)-codoped SrTiO3, the visible-light responses were due to the charge-transfer transition from the electron donor levels formed by the 3d orbitals of doped Ni2+ to the conduction bands of the host materials. (Ni,Ta)-codoped SrTiO3 showed higher activity than SrTiO3 doped with only nickel, because codoped Ta5+ ions suppressed the formation of Ni3+ ions that are expected to trap photogenerated electrons [58].

In addition, ion doping could be employed not only to extend the light response but also to enhance the photocata-lytic activity. Ishihara et al. found for the first time that controlling the charge density in oxide semiconductors with an acceptor was effective for improving the activity for overall water splitting. Although the photodecomposition activity of NiO/KTaO3 was negligible, doping small amount of acceptor (Zr4+) to form NiO/Zr-doped KTaO3 increased the formation rate of H2 and O2 [42]. Furthermore, Takata et al. examined the doping of aliovalent metal cations to SrTiO3 photocatalyst for overall water splitting. It was found that doping of a cation (Na+) with valence lower than that of the parent cation (Sr2+) effectively enhanced photocatalytic activity but the doping of a cation (Ta5+) with valence higher than that of the parent cation (Ti4+) suppressed photocatalytic activity, because the doping of a lower valence cation introduced oxygen vacancies and decreased Ti3+. The concept of defect engineering by aliovalent doping provides an example of the design of active photocatalysts [81].

Based on the above results, it could be found that both the types and valence states of doped transition metals play important roles in enabling the visible-light response and improving the photocatalytic activities, and ABO3 hosts provide excellent platforms for the modification of ion doping. The basic requirement of charge balance in ABO3 hosts could be flexibly utilized to intentionally control the valence states of doped transition metals and the lattice defects, thereby optimizing the photocatalytic reaction.

4.1.2. Ion replacement

Kato et al. developed AgMO3 (M = Ta and Nb) photocatalysts by replacing Na+ in NaMO3 (M = Ta and Nb) with Ag+.

Photocatalyst Preparation Lamp (filter)3 Reactant solutionb

(mass) strategy

Ga2_.vln.v0, Solid-state 450 W Hg

reaction

Zr-doped KTaO, Solid-state 500 W Xe

(0.1)° reaction

La-doped NaTaO, Solid-state 400 W Hg

(1)° reaction

NaSbO, (0.25)" Solid-state 200 W Hg-Xe

reaction

NaTaO, (l)e Solid-state 400 W Hg

reaction

LiTa03 (l)e Solid-state 400 W Hg

reaction

KTa03 (l)e Solid-state 400 W Hg

reaction

NaTaO, (1) Solid-state 450 W Hg

reaction

AgTa03 (0.15f Solid-state 300 W Xe

reaction (1>300 nm)

AgNbO, Solid-state 300 W Xe

reaction (1>300 nm)

(Cr.Sb)-codoped Solid-state 300 W Xe

S1T1O3 (0.5) reaction (A >420 nm)

La-doped NaTa03e Solid-state 400 W Hg

reaction

LalnO, (0.25)" Solid-state 200 W Hg-Xe

reaction

Baln0 ,Nb0,0, Solid-state 400 W Hg

(0.5)e reaction

Srln0 ,Nb0,0, Solid-state 400 W Hg

(0.5)e reaction

Caln0 ,Nb0,0, Solid-state 400 W Hg

(0.5)e reaction

CaCo1/,Nb-,/,0, Solid-state 300 W Xe

(0.5f reaction (A >420 nm)

SrCo^Nb-^O, Solid-state 300 W Xe

(0.5)e reaction (A >420 nm)

BaCo1/,Nbo/,0, Solid-state 300 W Xe

(0.5f reaction (A >420 nm)

BaZn^^bo^O, Solid-state 400 W Hg

(0.5)" reaction

400 W Hg

5.4 vol% CH,OH/0.05 M AgNO, Pure water 1 mM NaOH Pure water

0.1 mM NaOH (pH= = 10.5)

Pure water

Pure water

0.1 M KNO,

Pure water

Mixture of H20 vapor (20 Torr) and CH4 (70 Torr) with Ar carrier/0.05 M AgN03 8 vol % CH3OH/O.O5 M AgNO,

1 mM NaOH

Pure water

Pure water

Pure water

Pure water

Pure water

Pure water

Pure water

13.5 vol% CH3OH

13.5 vol% CH,OH

Activity/ nmol h_1 gii0

(wavelength/nm)d

Reference (year)

Cocatalyst/H2

Cocatalyst/02

1 wt% Pt/30f

1.5 wt% NiO/ 122.3

0.2 wt% NiO/ 15000f

—/25f

1.5 wt% NiO/57.4

Stoichiometric ratio

-50 (270)

1 wt% Ru02/~6f 1 wt% Ru02/~2f

0.05 wt% NiO/

—/430

—/29

1 wt% Ni/300

0.24 wt% NiO/ 20.7f

0.1 wt% Pt/38f

0.05 wt% NiO/ 20 (270) 1580

—/220

1 wt% Ni/480

0.24 wt% NiO/ 9.5f

—/240f

0.3 wt% Pt/78f —/0.9f

0.2 wt% NiO/ 19800

1 wt% Ru02/ ~1.25f

0.2 wt% NiO/ 9660

1 wt% RuOV ~0.5f

56 (270)

1 wt% NiO_v/27,Of 1 wt% NiO_v/12.3f

1 wt% NiO_v/13.8f -

1 wt% NiO_v/14.8f -

1 wt% Ni0.v/0.86f -

1 wt% Ni0.v/0.86f -

1 wt% NiO_v/1.37f -

0.5 wt% Pt/708.21f -

0.5 wt% Pt/194.84f -

(1998)

(1999)

(2000) [44.45] (2001. 2002) [46] (2001) [46] (2001)

[46] (2001)

[47] (2002)

[48] (2002)

[48] (2002)

[49] (2002)

[50] (2003)

(2003)

[52] (2003) [52] (2003)

[52] (2003)

[53] (2003) [53] (2003)

(2003)

(2004)

Photocatalyst Preparation Lamp (filter)3 Reactant solutionb

(mass) strategy

BaZni/,Ta^,0, Solid-state

(0.5) reaction

Rh-doped SrTiO, Solid-state 300 W Xe 10 vol% CH_,OH

(0.3) reaction (A >440 nm)

(Cr.Ta)-codoped Solid-state 300 W Xe 6.5 vol%

SrTiO, (1) reaction (A >440 nm )

CH3OH 1 wt % Pt/70f - -

BaNi^Nb-^O, Solid-state 400 W Hg 13.5 vol% CH3OH

(0.5)" reaction

BaNi^Ta-^O, Solid-state 400 W Hg 13.5 vol% CH3OH

(0.5)" reaction

BaZni/,Nbi/,0, Solid-state 400 W Hg Pure water

(0.5)= reaction

(Ni.Ta)-codoped Solid-state 300 W Xe 10 vol% CH,OH/0.05 M AgNO,

S1T1O3 (0.3) reaction (A >420 nm)

Cyanocobalamin/ Solid-state 500 W Xe KOH (pH= = 10.5)

K-0.95Ta0.92Zr0 08 reaction

O3 (0.1)"

Fe20,/SrTiO, Hydrothermal 250 W Oriel Arc 8% CH3OH

reaction Lamp (A>420 nm)

BiFeO,/SrTiO, Hydrothermal 250 W Oriel Arc 8% CH3OH

reaction Lamp (A>420 nm)

NaTaO, (l)e Sol-gel reaction 400 W Hg Pure water

BaCo1/,Nbo/,0, Sol-gel reaction 300 W Xe 18.5 vol% CH3OH

(0.2)" (A >420 nm)

LiNbO, (0.1) Wet-chemical 400 W Hg 12.5 vol% CH3OH

method

Cr-doped SrTiO, Solid-state 300 W Hg 13.5 vol% CH3OH

(0.25) reaction (A >420 nm)

SrSnO, (0.2) Hydrothermal 400 W Hg 13.5 vol% CH3OH /0.0135 M Ag:

reaction

NaTaO, (l)e Sol-gel reaction 400 W Hg Pure water

NaNbO, (0.1) Hydrothermal 350 W Hg 5 vol% CH3OH

synthesis

KNbO, (0.1) Hydrothermal 350 W Hg 5 vol% CH3OH

synthesis

NaTaO, (0.1) Hydrothermal 350 W Hg 5 vol% CH3OH

synthesis

KTaO, (0.1) Hydrothermal 350 W Hg 5 vol% CH3OH

synthesis

Sr-doped BaSnO, Polymerized 400 W Hg Pure water

(0.2) complex method

Activity/ nmol h_1 g^0

AQY/% Reference

(wavelength/nm)d (year)

Cocatalyst/H2 Cocatalyst/02

(2004)

0.1 wt% Pt/117f - 5.2 (420) [55]

(2004)

[56] (2004)

0.5 wt% Pt/68.22f - - [54]

(2004)

0.5 wt% Pt/42.16f - - [54]

(2004)

1.0 wt% Ni0.v/ 1.0 wt% Ni0.v/ - [57]

291.22f 145.61f (2004)

0.1 wt Pt/2.4f 0.1 wt% Pt/0.5f - [58]

(2005)

0.2 wt% Pt/575.0 0.2 wt% Pt/280.4 12.2 (300) [59.60]

(2006. 2009)

—/85 - - [61]

(2006)

—/129 - - [61]

(2006)

—/1940 Stoichiometric - [30]

ratio (2006)

0.5 wt% Pt/18.1 - - [62]

(2006)

0.5 wt% Pt/4910f - - [63]

(2006)

0.4 wt% Pt/21f - 0.86 (420.4) [39]

(2006)

0.5 wt% Pt/8.2 —/2.5 4.6 (300)g [64]

(2007)

—/2050 Stoichiometric - [65]

ratio (2007)

—/950 - - [66]

(2007)

—/100 - - [66]

(2007)

—/36750 - - [66]

(2007)

—/1040 - - [66]

(2007)

1.0 wt % Ru02/44f - - [67]

(2007)

NaTaO,-LaCoO, Solid-state 300 W Xe 18.5 vol% CH,OH

(0.5) reaction (A >420 nm)

KNb03/CdS (0.2) Solid-state 500 W Hg-Xe lamp 30 vol% isopropanol

reaction (A >400 nm)

BiFeO, (0.1) Microwave 500 W Hg 4 mM FeCl,

hydrothermal (A >420 nm)

reaction

BaZr03 (0.2)e Sol-gel reaction 400 W Hg Pure water

Ti02/SrTi03 (0.2) Solid-state 150 W Hg 4 M HCOOH

reaction

Ti02/CaTi03 (0.2) Solid-state 150 W Hg 3 M isopropanol

reaction

Ti02/BaTi03 (0.2) Solid-state 150 W Hg 3 M isopropanol

reaction

SrTi03 (0.06) Sol-gel reaction 500 W Hg 41.7 vol% CH3OH

SrTivMi_v03 Hydrothermal 450 W Hg 10 vol% CH3OH

(M = Ru, Rh. ir. reaction (A >420 nm)

Pt. Pd) (0.2)

FeGaO, (0.5) Solid-state 450 W Xe 0.5 M KOH with 2.5 mL min"1

reaction (A >420 nm) H2S flow

KNbO, (0.2) Hydrothermal 400 W Hg 11.9 vol% CH,OH

reaction

SrTi0,-LaTi02N NH3 nitridation 300 W Xe 18.5 vol% CH,OH solution/0.01

(0.15) (A >420 nm) M AgN03

AgSb03 (0.5) Solid-state 300 W Xe 0.0185 M AgNO,

reaction (A >400 nm)

Sr-doped NaTaO, Solid-state 400 W Hg Pure water

(0.5)= reaction

Ba-doped NaTaO, Solid-state 400 W Hg Pure water

(0.5)= reaction

Ca-doped NaTaO, Solid-state 400 W Hg Pure water

(0.5)= reaction

NaTa03 (1)= Sol-gel reaction 400 W Hg Pure water

Cu-doped AgNbO, Liquid-solid 300 W Xe 0.005 M AgNO,

(0.1) method (A >420 nm)

AgNbO,-SrTiO, Solid-state 300 W Xe 0.0185 M AgNO,

(0.5) reaction (A>410 nm)

Na-doped SrTiO, Solid-state 450 W Hg Pure water

(0.3)= reaction

Sr-doped NaTaO, Solid-state 400 W Hg Pure water

(0.5) reaction

Yln03 (0.35) Parallel solution 300 W Xe 0.25 M Na2S+0.25 M Na2SO,

combustion synthesis

0.2 wt% Pt/~4.34f - - [34]

(2007)

0.1 wt% (Ni/NiO)/ - 4.4 (À>400) [68]

150 (2008)

—/~0.75f - [69]

(2008)

—/104.5f —/26.2f 3.7 (200) [70]

(2008)

—/43.78f - - [71]

(2008)

—/13.02f - - [71]

(2008)

—/5.87f - - [71]

(2008)

0.32 wt% Pt/3200 - - [72]

(2008)

1 wt% Pt/15.6f - - [73]

(2008)

1 wt% NiO_v/2495f - 9.3 (550) [74]

(2008)

0.5 wt% Pt/5170 - - [75]

(2008)

3 wt% Pt/10f 3 wt% Pt/8f - [76]

(2008)

—/74f - [77]

(2008)

0.1 wt% NiO/ 0.1 wt% NiO/ - [78]

13600f 6690f (2009)

0.2 wt% Ni0/9780f 0.2 wt% NiO/ - [78]

4780f (2009)

0.2 wt% Ni0/4930f 0.2 wt% NiO/ - [78]

2600f (2009)

—/-2600 —/—1300 - [79]

(2009)

—/9.9f - [80]

(2009)

—/162f 16.4 (420.4) [35.36]

(2008. 2009)

0.5 wt% 0.5 wt% - [81]

RI12O3+O.5 wt% Rh203+0.5 wt% (2009)

Cr203/~6.6f Cr20,/~3.3f

—/1090f - - [82]

(2009)

0.5 wt% Pt/18.88 - 0.65 (420) [37]

(2009)

Photocatalyst Preparation Lamp (filter)3 Reactant solutionb

(mass) strategy

Na(Bi.vTai_.v)03 Hydrothermal 350 W Hg 5 vol% CH3OH

(0.1) reaction (A >400 nm)

NaTaO,-LaCrO, Solid-state 300 W Xe 18.5 vol% CH3OH

(0.5) reaction (A >420 nm)

NaNb03 (0.3)" Complex-based 400 W Hg Pure water

synthesis

BaZi'i_vSnv03 Polymerized 400 W Hg Pure water

(0.2)= complex method

CaTi03 (2.5) Solid-state 300 Xe Mixture of H20 vapor (1.5%) and CE

reaction (50%) with Ar carrier

Ti02/SrTi03 Hydrothermal Low-pressure UVP 11.1 vol% CH3OH

reaction Pen-Ray Hg

NaTaO, (l)e Sol-gel reaction 400 W Hg Pure water

Sr2AlNbOs (0.5) Solid-state 100 W Hg 10 vol% CH3OH/0.01 M AgF

reaction

CaTi!_vCuv03 (0.1) Sol-gel reaction 350 W Xe 5 vol% CH3OH

(A >400 nm)

LaNi0 7Cu0 ,0, Sol-gel reaction 125 W Xe 12.5 vol% HCHO

(0.1) (A >400 nm)

Eosin Y/SrTiO, Sol-gel reaction 300 W Xe 15 vol% DEA+0.5 mM E.Y.

(0.2) (A >400 nm)

SrTiO, (0.2) Sol-gel reaction 300 W Xe 50 vol% CH3OH

(A >400 nm)

LaNi!_vCuv03 Polymerized 125 W Xe 12.5 vol% CH3OH

(0.1)" complex method (A >400 nm)

La-doped NaTaO, Solid-state 300 W Xe Mixture of H20 vapor (1.5%) and CE

(1) reaction (50%) with Ar carrier

In-,0,/NaNb0, Coprecipitation 400 W Hg 18.5 vol% CH3OH

(0.3) method

ZnFe204/SrTi0, Solid-state 200 W tungsten 0.025 M Na2S203

(0.25) reaction

LaFeO, (0.05) Sol-gel reaction 125 W Hg 10 vol% CH3OH/0.05 M AgNO,

(A >420 nm)

BaZrO,-BaTaO,N Polymerized 300 W Xe 4 mM Nal

(0.5) complex method <420<A<800 nm)

(Na.K)Ta03 (0.1)" Molten-salt 350 W Hg Pure water

method

Hf-doped Molten-salt 350 W Hg Pure water

(Na.K)TaO, method

(0.1)°

Sol-gel reaction 400 W Hg Pure water

Activity/ AQY/% Reference

nmol h_1 g^0 (wavelength/nm)d (year)

Cocatalyst/H2 Cocatalyst/02

0.1 wt% NiO/ - - [83]

59.48 (2009)

0.2 wt% Pt/4.5f - 0.5 (420) [38]

0.5% Ru02/~21f 0.5% Ru02/~10f (2009)

-6.6 (340) [84]

—/138f —/37f (2010)

- [85]

(2010)

0.04 wt %/ [86]

0.48 |imol min-1 (2010)

—/386.6f - 0.854 (254) [87]

(2010)

3 wt% Ni0/9000 3 wt% №0/4500 6.4 (310) [88]

1 wt% Ru02/26.7f (2010)

1 wt % Ru02// - [89]

21.6f (2010)

—/22.7 - - [90]

(2010)

—/583 - 2.98 [91]

0.5 wt% Pt/60.3f (2010)

- - [92]

1 wt% Au/200f (2010)

- - [93]

(2010)

—/1180 - - [94]

(2010)

0.03 wt% Pt/ - 30 (240-270) [95]

4.5 |imol min-1 (2010)

0.8 wt% Pt/~42.9 - 1.45 (420) [96]

(2010)

9.2 cm3 h-1 mgi - - [97]

—/430f —/213f (2010)

8.07 (visible light) [98]

0.3 wt% Pt/~0.9f (2010)

- - [99]

—/2510f —/340f (2011)

- [100]

—/4960f —/2460f (2011)

- [100] oa EJ*

(2011)

—/1057f —/530f 2.8 (310) 0

Na!_vKvTaO, (0.1)°

Na0 3La0 5Ti03- Hydrothermal 300 W Xe

LaCr03 (0.3) method (A >420 nm)

Cr-doped SrTiO, Sol-gel 300 W Xe

(0.25) hydrothermal (A >420 nm)

synthesis

Nai_vLavTaO, Sol-gel reaction 400 W Hg

(0.5)° (A >250 nm)

NaTa03 (0.3)° Confined

space synthesis 450 W Hg Pure water

LiNb03 (0.3)° Complex-based 400 W Hg

synthesis

AgNbO,-NaNbO, Molten salt 300 W Xe

(0.1)° treatment (A >400 nm)

La-doped NaTaO, Sol-gel reaction 400 W Hg

(0.5) (A >250 nm)

C3N4/SrTi03 (0.2) Coprecipitation 250 W iron-doped

method metal halide

(A >420 nm)

(Cr.Ta)-codoped Spray pyrolysis 300 W Xe

SrTi03 (A >415 nm)

Ag-doped SrTiO, Hydrothermal 350 W Hg

method

GaFeO, (0.05) Sol-gel reaction 300 W Xe

(A >395 nm)

Sr.vNbO, Solid-state 250 W iron-doped

method metal halide

(A >420 nm)

AgSb03 (0.3) Hydrothermal 300 W Xe

reaction (A >420 nm)

(La.Cr)-codoped Sol-gel 300 W Xe

SrTiO, (0.3) hydrothermal (A >400 nm)

reaction

(Cr.Ta)-codoped Hydrothermal 300 W Xe

SrTiO, (0.3) reaction (A >420 nm)

LaCoO, Sol-gel reaction Xe (A >420 nm)

(0.25 g L"1)

Er-doped SrTiO, Polymerized 300 W Xe

(0.3) complex method (A >420 nm)

CaTaOoN-CaZrO, Polymerized 300 W Xe

(0.1) complex method (A >420 nm)

(Ni.Ta)-codoped Spray pyrolysis 300 W Xe

SrTiO, (0.1) process (A >415 nm)

NaBi.vTai_.v03 (0.1) Spray pyrolysis 300 W Xe

process (A >415 nm)

500 W Xe

18.5 vol% CH3OH 18.5 vol% CH3OH

Pure water

1 wt% Ni0/2000f Pure water

10 vol% CH3OH/O.O2 M AgN03 10 vol% CH3OH 0.025 M oxalic acid

20 vol% CH3OH 10 vol% CH3OH Pure water

0.02 M oxalic acid/0.005 M AgNO,

0.0185 M AgN03

18.5 vol% CH,OH+5 M NaOH

18.5 vol% CH3OH

5.0 mM Na2S208+0.25 mM

[Ru(bpy)3](C104)+50 mM phosphate buffer

solution (pH= =7.0)

0.35 M Na2S+0.25 M Na2SO,/0.0185 M

11.1 vol% HCOOH 20 vol% CH3OH 20 vol% CH3OH 20 vol% CH3OH

(2011)

1 wt% Pt/8.2 - - [101]

0.6 wt% Pt/82.6f (2011)

- 2.95 (410) [102]

(2011)

0.3 wt% (Ni/NiO)/ Stoichiometric - [103]

3450 ratio (2011)

1 wt% Ni0/1000f - [104] (2011)

0.5 wt% Ru02/31f 0.5 wt% Ru02/17f 0.7 (254) [105]

0.3 wt% Pt/~0.2f —/~4.6f (2011)

- [106]

(2011)

—/1430е - - [107]

(2011)

1 wt% Pt/440 - - [108]

(2011)

1 wt% Pt/423.6f - - [109]

(2011)

6.61 mmol m-2 h_1 - - [110]

—/36.lf (2011)

- -0.13 (450) [111]

—/~2.5f (2012)

1 wt%/44.8 - [13]

(2012)

- —/138 - [112]

—/326.8f (2012)

- 25.6 (425) [113]

(2012)

0.5 wt% Pt/~2f - - [114]

(2012)

- 11 |imol s_1 g_1 0.16 (450) [40]

(2012)

1 wt% Pt/46.23 —/44.23 0.03/0.04 (>420) for [115]

H2/02 evolution. (2012)

1 wt% Pt/5.24f respectively

- - [116]

(2012)

1 wt% Pt/561.2 - - [117]

(2012)

0.2 wt% NiO/1355 - - [118]

(2012)

3 wt% Pt/369.8 - 1.45 (346)

O (N O (N O (N O M -H M n CN -H M

^ Si a % < -It

Si x -3

1 £ ^ '

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Ph O ffi

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25 « 25 H

<si td 3

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The band gaps of AgTaO3 and AgNbO3 were 3.4 and 2.8 eV, respectively, being 0.6 eV smaller than the band gaps of NaTaO3 and NaNbO3, both of which only had UV-light response. It was found that a hybrid orbital of Ag 4d and O 2p formed a valence band at a more negative level than O 2p orbitals. AgNbO3 has arisen as a new visible-light-driven photocatalyst possessing the ability to evolve H2 or O2 from water in the presence of sacrificial reagents [48].

Sato et al. reported the ability of NaSbO3 photocatalyst for overall water splitting. However, this material was only active under UV-light irradiation due to the wide bandgaps (about 4.4 eV) [44,45]. Based on the electronic configuration of Ag+, Kako et al. prepared AgSbO3 photocatalyst with an absorption edge up to about 480 nm and successfully applied for visible-light-driven O2 evolution from an aqueous silver nitrate solution [77]. Band-structure calculation revealed that the conduction-band minimum (CBM) and valence-band maximum (VBM) of NaSbO3 were composed of Sb 5s and O 2p orbitals, respectively [44,129], whereas for AgSbO3, the CBM consisted of the hybridized Ag 4d and O 2p orbitals, and VBM mainly consisted of the Ag 5s and the Sb 5s orbitals. Consequently, The Ag+ largely contributed to the bandgap narrowing by lifting up the valance band.

Ion replacement was proved as an effective method to construct new energy bands in ABO3 photocatalysts and thus to extend the light response range.

4.1.3. Multi-component solid solution Designing solid solutions between end-member materials with analogous crystal structures but different electronic band structures has greatly promoted the development of photo-catalysts in recent years [4,41]. On account of the highly susceptible substitution of different cations at both A and B sites of ABO3-type perovskite oxides [39,130], the derived solid solutions with versatile properties are noteworthy [34-36,38,83,106].

Due to the excellent photocatalytic performance of SrTiO3 and NaTaO3 for overall water splitting under UV irradiation [50,123] and the inability of the two materials for harnessing visible light, a series of SrTiO3- and NaTaO3-based solid-solution photocatalysts, such as SrTiO3-AgNbO3 [35,36], NaTaO3-LaCoO3 [34], NaTaO3-LaCrO3 [38], and NaTaO3-NaBiO3 [83], were developed and successfully applied to photocatalytic H2 or O2 evolution under visible-light irradiation. A novel series of solid-solution semiconductors SrTiO3-AgNbO3 have been developed by Wang et al. as highly visible-light-active photocatalysts for efficient O2 evolution. The mixed valent perovskites (AgN-bO3)1_x(SrTiO3)x possess a modulated energy band structure. The conduction band is composed of the hybrid (Ti 3d+Nb 4d) orbitals and the valence band is constructed by the hybrid (O 2p+Ag 4d) orbitals. The modulation of band structure (band gap energy, band edge positions, etc.) depends on the extent of orbital hybridization. As a result of competition between the absorption ability to visible-light and the reductive/oxidative abilities, the highest visible-light activities for O2 evolution are realized over (AgNbO3)0.75(SrTiO3)0.25. It was proved that making solid-solution oxide semiconductors with tunable electronic structures is a feasible band engineering approach for the development of highly visible-light-active photocatalysts.

Cubic perovskite Na0.5La0.5TiO3 has too wide a bandgap to utilize visible light [131]. The analogous perovskite LaCrO3 has a bandgap narrow enough to absorb visible light [132,133] but was reported to show trace amounts of H2 evolved by visible-light-driven photocatalysis [38] since the conduction band level is relatively low and thus the driving force for reducing H2O to H2 is weakened. In our recent work, novel cubic perovskite structure photocatalyst of Na05La05TiO3-LaCrO3 solid-solution single-crystal nanocubes were prepared by means of facile, surfactant-free hydrothermal reactions at 180 °C for 48 h for the first time, and successfully applied to visible-light-driven photocatalytic H2 evolution. The introduction of LaCrO3 component into Na05La05TiO3 rendered the further distortion of BO6 octahedra in ABO3, thus beneficial for the excitation, separation and migration of photogenerated charge carriers. The bandgap was dramatically narrowed by Cr3+ from 3.19 (Na05La05TiO3, i.e., NaLaTi) to 2.25 eV ((Nao.5Lao.5TiO3)1.oo(LaCrO3)o.o8, i.e., NaLaTi-0.08Cr), thereby enabling the utilization of visible light (<550nm). The narrowing of the bandgap was reasonably explained by the band-structure calculations (see Fig. 2). The new CBM was composed of Cr 3d eg orbitals and Ti 3d orbitals, and the new VBM was formed by Cr 3d t2g orbitals rather than O 2p orbitals (see Fig. 3). The solid-solution configuration maintained the charge balance to preserve the valence of Cr3+ rather than Cr6+, and accommodated Cr3+ with high content to form new energy bands instead of localized impurity levels. The hydrothermal preparation strategy ensured the formation of single crystals with high purity, few defects, and regulated morphology; it also guaranteed the valences of Ti4+ and Cr3+ in the solid solution. Consequently, owing to the solid-solution configuration and hydrothermal preparation strategy, the recombination of photogenerated carriers could be effectively suppressed to benefit photocatalytic H2 evolution. (Na0.5La0.5-TiO3)1.00(LaCrO3)0.08 solid-solution single-crystal nanocubes showed stable visible-light-driven photocatalytic activity for H2 evolution (rate, 8.2 mmol h_1 g^t) in methanol aqueous solution (see Fig. 4) [101].

In addition to solid solutions between two ABO3 end-member materials, solid solutions between end-member materials of one ABO3 and one ABX3 perovskite oxynitride, such as SrTiO3-LaTiO2N [76], BaZrO3-BaTaO2N [99], and CaZrO3-CaTaO2N [116], were also designed by different research groups, aiming to lift up valence band and thus to narrow the bandgaps of ABO3 materials by incorporating N to take part in the formation of valence band. SrTiO3-LaTiO2N showed photocatalytic activities for H2 and O2 evolution in corresponding sacrificial reagents under visible light irradiation. The band gaps of (SrTiO3)!_x(LaTiO2N)x reduced from 3.18 to 2.04 eV with increasing x from 0 to 0.30. Moreover, the calculated results suggested that the band gap narrowing came from the hybridization of N 2p and O 2p orbital. Matoba et al. reported a solid solution between BaTaO2N and BaZrO3. Compared with that of BaTaO2N, the photocatalytic activity of BaZrO3-BaTaO2N was

Potential/ eV vs. NHE

Ti3d+Cr3d ea Ti3d

\Cr3d eg _>,.---------

pÜd t2g 02p

02/H,0

LaCr03 Na05La0 5TiO3-LaCrO3 Na05LaDSTiO3

Fig. 3 Schematic band structures of LaCrO3, Na0.5La0.5TiO3-LaCrO3 solid solution, and Na0.5La0.5TiO3. Reprinted with permission from Ref. [101].

Fig. 2 Electronic band structures and total density of states (TDOS) near the Fermi levels for LaCrO3, Na0.5La0.5TiO3-LaCrO3 solid solution, and Na0.5La0.5TiO3 calculated by density functional theory (dark and red lines: results from two different spin polarization direction; blue lines: Fermi levels). Reprinted with permission from Ref. [101].

Fig. 4 Time courses of visible-light-driven H2 evolution on photocatalysts of as-prepared NaLaTi (solid squares) and NaLaTi-0.08Cr (open circles). Reprinted with permission from Ref. [101].

improved six- to nine-fold for H2 evolution in aqueous NaI solution under visible-light irradiation due to an increased driving force for surface redox reactions and fewer defects [99]. The analogous solid solution of CaZrO3-CaTaO2N was prepared almost at the same time, and exhibit photocatalytic activity for H2 evolution from formic acid aqueous solution under visible-light irradiation. The narrow band gap, sufficient crystallization, enough pore volume and large specific surface area resulted in the best photocatalytic activity of (CaZ-rO3)0.2(CaTaO2N)0.8 [116].

Consequently, the solid-solution construction was demonstrated to be a powerful strategy to design ABO3-based photocatalysts, and exhibited the following merits. The band structures of photocatalysts could be optimized to mediate the competition between the absorption ability to visible-light and the reductive/oxidative abilities. The unfavorable valence states of multivalent metal cations and the lattice defects could be depressed to reduce the recombination of photogenerated charge carriers.

4.1.4. Multi-component composites

Synergistic effect on photocatalytic performance could be achieved when different materials were combined to form multi-component composites [19,25,134,135]. The ABO3-based multi-component composites mainly include metal-ABO3 composites [103,110,136], semiconductor-ABO3 composites [61,87,96,97,108,121] and dye-ABO3 composites [59,60,92]. The other components couple with ABO3 generally functioned as cocatalysts or photosensitizers.

Domen et al. designed SrTiO3 photocatalyst loaded with the classical NiO/Ni double-layer cocatalyst by reduction/oxidation pretreatment for overall water splitting. H2 evolution occurs over NiO while O2 evolution takes place over SrTiO3, thus announced the separation of reactive sites for H2 and O2 evolution, respectively. Furthermore, the existence of metallic Ni between the interface of NiO and SrTiO3 facilitated the transfer of electrons between both materials, and therefore enhanced the photocatalytic activity [136]. Moreover, Hu and Teng investigated the interaction between NiO cocatalyst with NaTaO3 photocatalyst for overall water splitting under UV illumination. Activity increased

significantly with NiO loading and reached a maximum at first, and then decreased with further loading. It was found that the interdiffusion of Na+ and Ni2+ cations created a solid-solution transition zone (NaxNi2i2xNiX+O and NixNa!_xTaO3) on the outer sphere of NaTaO3 (see Fig. 5). For less NiO contents, no NiO clusters appeared on the NaTaO3 surface, and the reduction/ oxidation pretreatment did not enhance photocatalytic activity. The high activity resulting from a low NiO loading suggests that the interdiffusion of cations heavily doped the p-type NiO and n-type NaTaO3, reducing the depletion widths and facilitating charge transfers through the interface barrier [88]. By comparing the above results, It could be found that NiO was an excellent cocatalyst loaded on ABO3 photocatalysts for overall water splitting, but the mechanisms for activity enhancement were different due to the different structural features, which were considerably affected by the interaction between cocatalyst and photocatalyst host. Further research on Ni/NiO cocatalyst loaded on ABO3 photocatalysts (such as Ni/NiO/Nat_xLaxTaO3 [103] and Ni/NiO/KNbO3/CdS [68]) for H2 evolution was also performed in other groups. In addition to NiO, other cocatalysts, such as metallic Ag [110], Au, Pt, Ag, Ni, Ce, and Fe [93] were also employed to collect photogenerated electrons and thus to promote photocatalytic H2 evolution.

Heterojunctions could be formed by coupling other band structure-matched semiconductors with ABO3 photocatalysts to accelerate the separation of photogenerated charge carriers.

Fig. 5 Schematic energy-level diagrams of the NiO and NaTaO3 contact with interdiffusion of Ni2+ and Na+ to form p-doped NiO (i.e., NaxNi?i2xNiX+O) and n-doped NaTaO3 (i.e., Nix-Naj_xTaO3) at the interface. Steeper band bending and reduced depletion widths appeared at the heterojunction interface. Ec, Ev, and Ef represent the energy positions of the conduction band edge, valence band edge, and Fermi level, respectively. Reprinted with permission from Ref. [88].

TiO2/SrTiO3 heterostructures were prepared by different groups [87,121]. Ng et al. reported that in comparison to the respective pristine semiconductor photocatalysts, the hetero-structured TiO2/SrTiO3 film showed the highest efficiency in photocatalytic splitting of water to produce H2, 4.9 times that of TiO2 and 2.1 times that of SrTiO3. The enhanced photo-catalytic efficiency is largely attributed to the efficient separation of photogenerated charges at heterojunctions of the two dissimilar semiconductors, as well as a negative redox potential shift in the Fermi level. Furthermore, the coupled semiconductors with ABO3 photocatalysts could act as photo-sensitizers to harness low-energy photons that could not be utilized by the ABO3 host due to the wide bandgaps [61,96,97,108]. A series of SrTiO3-based heterostructural photocatalysts (such as SrTiO3/Fe2O3 SrTiO3/BiFeO3 [61], ZnFe2O4/SrTiO3 [97], and C3N4/SrTiO3 [108]) were developed and successfully applied to visible-light-driven H2 evolution. Luo and Maggard reported that both SrTiO3/Fe2O3 and SrTiO3/BiFeO3 systems exhibited visible-light photocatalytic activity, while pure SrTiO3, Fe2O3, and BiFeO3materials exhibit little visible-light activity. These results demonstrate that composite systems offer a unique and competitive approach to the visible-light activation of SrTiO3 [61]. In addition to semiconductors, dyes (such as Eosin Y [92] and porphyrinoids [59,60]) were also employed as sensitizers to enhance the light response [92] and the separation of photogenerated charge carriers [59,60] on ABO3 photocatalysts.

The above results demonstrated that constructing of ABO3-based composites could not only accelerate the separation of photogenerated charge carriers by matching band structures in heterostructures, but also improve the utilization of incident light by photosensitization.

4.2. Micro-/nano-structure adjustment

Micro-/nano-structure adjustment of materials are generally achieved by employing appropriate preparation strategies [16,27,28,137-139]. As far as ABO3-based photocatalysts are concerned, various preparation strategies were developed to adjust their micro-/nano-structures, aiming to optimize the photocatalytic process.

4.2.1. Solid-state reaction

Solid-state reaction is the most frequently used method to prepare ABO3-based materials, but the high-temperature process generally resulted in as-prepared materials with defects and large particle sizes, which prompted the recombination of

photogenerated carriers and thus led to low activity [64]. However, researchers from Kudo' group successfully prepared NaTaO3-based photocatalysts with nanostep structure by solid-state reaction [43,50,78,82]. Doping of metal cations (including lanthanides [43,50] and alkaline earth metal ions [78,82]) played the key role in constructing such a unique micro-/nano-structure. Among La, Pr, Nd, Sm, Gd, Tb and Dy, La was the most active dopant to improve the photo-catalytic activity for overall water splitting under UV irradiation [43,82]. NiO/La-doped NaTaO3 (i.e., NiO/NaTaO3:La) showed photocatalytic activity 9 times higher than nondoped NiO/NaTaO3. The maximum AQY was 56% at 270 nm, thus setting the highest record for overall water splitting under UV irradiation. It was found that the particle sizes of La-doped NaTaO3 crystals were smaller than that of the nondoped NaTaO3 crystal and that the ordered surface nanostructure with characteristic steps was created by the lanthanum doping. The small particle size with a high crystallinity promoted the reaction of photogenerated electrons and holes with water molecules. NiO cocatalysts were loaded on the edge of the nanostep structure of La-doped NaTaO3 photocatalysts as ultrafine particles. The H2 evolution proceeded on the ultrafine NiO particles loaded on the edge while the O2 evolution occurred at the groove of the nanostep structure. Consequently, the reaction sites for H2 evolution were separated from those of O2 evolution over the ordered nanostep structure. Both the small particle size and the ordered surface nanostep structure of the NiO/La-doped NaTaO3 photocata-lyst contributed to the highly efficient overall water splitting (see Fig. 6) [43].

4.2.2. Hydrothermal/solvothermal reaction Hydrothermal/solvothermal synthesis was widely acknowledged as a powerful tool to generate products with high crystallinity and purity, few defects, and novel morphologies. The technological efficiency in developing bigger, purer, and dislocation-free single crystals is of much importance [140].

Chen and Ye successfully prepared dumbbell-like and rodlike SrSnO3 nanostructures by a facile hydrothermal process and subsequent heat-treatment process. Compared with the sample from solid-state reaction, the as-prepared SrSnO3 nanostructures showed higher activity for H2 and O2 evolution in corresponding sacrificial reagents under UV light irradiation due to the higher surface area and fewer defects [64]. Ding et al. synthesized KNbO3 single-crystal nanowires by a hydrothermal method. The prepared nanowires achieved the highest photocatalytic activity in all reported KNbO3

Fig. 6 Mechanism of highly efficient photocatalytic water splitting over NiO/NaTaO3:La photocatalysts. Reprinted with permission from Ref. [50].

materials for H2 evolution from aqueous CH3OH solutions because of the high surface area and high crystallinity [75]. Yu et al. from the same group synthesized Cr-doped SrTiO3 with high specific surface areas (19.3-65.4 m2 g-1), and good crystallinity, and small crystalline size (20-30 nm) by using a sol-gel hydrothermal method. The as-prepared Pt/Cr-doped SrTiO3 photocatalyst exhibited H2 evolution rate 3 times higher than that of the sample from solid-state reaction in methanol solution under visible light irradiation due to the high specific surface area and good crystallinity [102]. Joshi et al. synthesized BiFeO3 single-crystal nanocubes via simple microwave-hydrothermal procedure in sizes of 50-200 nm and successfully applied to photocatalytic O2 evolution in FeCl2 solution. The microwave condition largely shortened the reaction time (only 30 min) [69]. The hydrothermal conditions have important effects on the physicochemical properties and photocatalytic performance of ABO3 materials. Quaternary perovskite oxide Na0 5Bi0 5TiO3 hierarchical microspheres was synthesized by Wang and Wang via a low-temperature hydrothermal route. The alkaline concentration played crucial roles in the physicochemical properties and the photocatalytic activities. Na0 5Bi0 5TiO3 microspheres prepared with 9 M NaOH exhibited the highest rate of H2 evolution under UV and visible light, because the highest conduction band edge and the smallest particle sizes led to the suppression of electron-hole recombination [119]. Using a three-dimensional mesoporous carbon as template to provide the confined space, Yokoi et al. prepared a colloidal array of NaTaO3 nanoparticles in size of 20 nm and with a surface area of 34m2g_1 by a hydrothermal reaction. The colloidal array of NiO/NaTaO3 nanoparticles showed photocatalytic activity for overall water splitting under UV irradiation more than three times as high as non-structured bulk NaTaO3 particles [104].

The above results confirmed the advantages of hydrother-mal/solvothermal reaction in preparing ABO3-based photo-catalysts with high crystallinity, few defects, small sizes, large surface areas, and special micro-/nano-structures.

4.2.3. Spray pyrolysis process

Spray pyrolysis process is an aerosol process to produce powders of highly uniform in morphology and composition because of short residence time and droplets of several micrometers in which precursors are distributed uniformly before evaporation and decomposition. Park et al. prepared a series of ABO3-based photocatalysts (such as (Cr,Ta)-codoped SrTiO3 [109], (Ni,M)-codoped SrTiO3 (M = La or Ta) [117], and NaBixTa1_xO3 [118]) by this method. Compared with (Cr,Ta)-codoped SrTiO3 photocatalyst prepared by solid-state reaction, the photocatalyst prepared by spray pyrolysis showed a higher (about 100 times) H2 evolution rate and shorter (8 times) induction period in aqueous methanol solution under visible-light irradiation because of the increased surface roughness, compositional uniformity, and surface area, and the smaller amount of Cr6+ [109]. Another two photocatalysts, (Ni,La)-codoped SrTiO3 and (Ni,Ta)-codoped SrTiO3, were also prepared by continuous spray pyrolysis. Furthermore, solid-solution NaBixTa1_xO3 photo-catalysts were prepared by spray pyrolysis process. Compared with that of photocatalyst prepared by the hydrothermal method, the H2 evolution rate of NaBixTa1_xO3

photocatalysts from spray pyrolysis was enhanced almost 20 times in aqueous methanol solution under visible-light irradiation, because of the compositional uniformity and unique surface morphology with large BET surface area of the photocatalyst. The composition of the photocatalyst was optimized at NaBi0 07Ta0 93O3.

4.2.4. Multi-step reaction

In our group, aiming to enhance the visible-light-driven photocatalytic activity, ABO3 with novel micro-/nano-struc-tures were designed and prepared, and successfully applied to visible-light-driven photocatalytic H2 or O2 evolution [112,114]. Hexagonal single-crystal nanosheet-based NaSbO3 hierarchical cuboidal microclusters (i.e., HCMCs) with exposed {001} facets were prepared by means of a facile, surfactant-free hydrothermal reaction at 250 °C for 48 h for the first time. Their growth mechanism was investigated by systematically examining the effect of hydrothermal treatment time and temperature on the crystal structures and particle morphologies of products. Furthermore, based on the similar crystal structure (rhombohedral-phase ilmenite) between NaSbO3 and AgSbO3 to facilitate topotactic transformation, hexagonal single-crystal nanosheet-based AgSbO3 hierarchical cuboidal microclusters with exposed {001} facets were prepared by performing an ion exchange of Ag+ on the sacrificial template of as-prepared hierarchical NaSbO3 precursor at 220 °C for 24 h (see Fig. 7). The as-prepared hierarchical AgSbO3 (i.e., AgSb-HT) showed higher (1.8 times) photo-catalytic activity for visible-light-driven O2 evolution (initial rate, 460 pmolh-1 g"!) than AgSbO3 formed by performing an ion exchange of Ag+ on NaSbO3 precursor prepared by a solid-state reaction (i.e., AgSb-SSR—36 h) (see Fig. 8). The unique structure of hierarchical cuboidal microclusters assembled from single-crystal nanosheets with exposed {001} facets simultaneously optimized all three main processes of the photocatalytic reaction (i.e, photon absorption, separation and migration of photogenerated charge carriers, and redox reaction of photogenerated charge carriers), and therefore enhanced the photocatalytic activity [112]. In addition, the sacrificial templates of microtubular (Cr,Ta)-codoped TiO2 were prepared by solvothermal reaction and calcination post-treatment, and then were successfully transformed to nanocrystal-based microtubular (Cr,Ta)-codoped SrTiO3 by hydrothermal reaction. The as-prepared hierarchical (Cr,Ta)-codoped SrTiO3 also showed higher photocatalytic activity for visible-light-driven H2 evolution than (Cr,Ta)-codoped SrTiO3 formed by a solid-state reaction [114]. Consequently, the preparation strategy based on design and transformation of sacrificial templates is an effective method to construct special micro-/nano-structures; and the special micro-/nano-structure design is an effective route to enhance photocatalytic performance.

Besides, other preparation strategies (such as polymerized complex method and sol-gel route) were employed to obtain ABO3 photocatalysts with special micro-/nano-structures (such as SrTiO3 nanocrystals [72], La-doped NaTaO3 nano-crystals [107], LiNbO3 and NaNbO3 nanowires [84,105]) and with enhanced photocatalytic activities.

Based on the above results, it was found that the micro-/ nano-structures of ABO3 photocatalysts could be adjusted by different preparation strategies, aiming to control the

Fig. 7 Schematic illustration of the growth mechanisms of NaSbO3 and AgSbO3 HCMCs during hydrothermal treatment in a drying oven at 250 °C, showing dissolution-redeposition-recrystallization and dehydration processes, followed by an ion-exchange reaction at 220 °C, which results in a topotactic transformation process. Coordinates are nonlinear. Reprinted with permission from Ref. [112].

0 12 3

Time/h

Fig. 8 Time profiles of visible-light-driven O2 evolution catalyzed by AgSb-SSR—36 h (О; initial rate: 78 mmol h"1) and AgSb-HT (■; initial rate: 138 pmolh"1). Reprinted with permission from Ref. [112].

physicochemical properties, such as crystallinity, crystal size, crystal orientation, surface area, and architecture, and therefore to optimize the whole photocatalytic process.

4.3. Local lattice structure adjustment

Local lattice structure is one of the important physicochemical properties of materials, and hence played a pivotal role in determining the photocatalytic performance. Correspondingly, the local lattice structure adjustment of ABO3 photocatalysts will be introduced and the structure-activity relationship will be discussed in the following section.

4.3.1. Crystal structures

Crystal structures determined the symmetrical characteristics of crystals, and thus inevitably affected the local lattice structures and photocatalytic performance of materials [30-32,46,53,54,65,67,79].

The relationships between Ta-O-Ta bond angles and photocatalytic activities of ABO3-based materials were

comprehensively investigated by different research groups. Kato et al. prepared a series of alkali tantalate ATaO3 (A=Li, Na, and K) photocatalysts that showed activities for overall water splitting under UV irradiation. The interaction between corner-sharing TaO6 octahedra in ilmenite LiTaO3 is so weak that the excited energy is localized because of a bond angle of Ta-O-Ta far from 180o. In contrast, the excited energy is delocalized in perovskite NaTaO3 and KTaO3 in which the bond angle are close to 180o. The high activity of NiO/ NaTaO3 photocatalyst was in part due to the delocalization of excited energy which were dominated by the slightly distorted connection between TaO6 octahedra [46]. Teng et al. investigated the effects of crystal structures, especially the Ta-O-Ta bond angles, in NaTaO3 on the photocatalytic activities in detail [30,65,79]. NaTaO3 with monoclinic and orthorhombic phases were prepared by sol-gel and solid-state reaction, respectively. Compared with the orthorhombic NaTaO3, the monoclinic NaTaO3 showed higher photocatalytic activity for H2 evolution due to the larger surface area and the advantageous features in the electronic and crystal structures. In terms of electronic structure, the monoclinic and orthorhombic phases had indirect and direct bandgaps, respectively. The recombination rate for the photogenerated charge carriers in the monoclinic phase would be much smaller because phonons are involved in the gap transition. In addition, the monoclinic phase was found to have a larger number of effective states available for the photogenerated charge carriers. As for crystal structure, the bond angle of Ta-O-Ta for orthorhombic phase was about 163o while that of the monoclinic phase was close to 180o, which was favorable for delocalization of the excited energy in NaTaO3 [65]. The decrease of photoluminescence intensity with the increase of Ta-O-Ta bond angle directly evidenced that the Ta-O-Ta bond angle affects the separation of photogenerated charge carriers [79]. Furthermore, they improved the photocatalytic activity of NaTaO3 by replacing some Na ions with larger K ions to produce Na!_xKxTaO3 photocatalysts. K-doping at x = 0.05 rectified the distorted perovskite NaTaO3 to a pseudo-cubic phase. The 180o bond angle of Ta-O-Ta in the pseudo-cubic phase may facilitate the separation of photogenerated charges for effective water splitting. Further K-doping (with x>0.05) leads to impurity formation, which bends the Ta-O-Ta linkage and creates

defect states, lowering the photocatalytic activity of Na!_xKx. TaO3 [31].

Besides, the correlation between phonon modes and photo-catalytic activities of ABO3-based BaM1/3N2/3O3 (M = Ni, Zn; N=Nb, Ta) materials were suggested by Yin et al. The Nb-containing photocatalyst in each pair shows higher photo-catalytic activity in evolving H2 from the methanol solution under UV irradiation than the Ta-containing photocatalyst. Raman spectra of these pairs of photocatalysts suggest that some phonon modes in the bending branches for the Ta-containing photocatalyst show an obvious red shift in their frequency, compared with those for the corresponding Nb-containing photocatalyst. Consequently, phonon modes may play an important role in the migration of the photogenerated charge carriers, thus affecting the photocatalytic activity [54].

4.3.2. Site occupancy

Owing to the different local lattice environments of A and B sites, as for cations doped ABO3 photocatalysts, the substitu-tional sites of doped cations were expected to have important influences on the photocatalytic activities.

Wang et al. prepared two Cr-doped SrTiO3 photocatalysts, (Sr095Cr005)TiO3 with A-site substitution and Sr(Ti0 95-Cr0.05)O3 with B-site substitution, and found that the H2 evolution rate over (Sr0 95Cr0 05)TiO3 was 100 times more than that over Sr(Ti0 95Cr005)O3 from aqueous methanol solution under visible-light irradiation. Concerning the charge compensation, the Cr cations located at the Sr2+ site were Cr3+, while those doped at the Ti4+ site were Cr3+ and Cr6+. Cr6+ in Sr(Ti0.95Cr0.05)O3 behaved as recombination centers of photogenerated charge carriers and therefore reduces the photo-catalytic activity. It was thus suggested that the valence states of doped multivalent cations in ABO3 photocatalysts could be controlled by taking advantages of the charge balance and valence difference at A and B sites [39].

Kanhere et al. prepared a series of Bi-doped NaTaO3 photo-catalysts with different substitutional sites for Bi cations. On the one hand, the optical properties of Bi-doped NaTaO3 were tuned by changing the substitutional sites, and Bi ions located at both Na and Ta sites resulted in the narrowest bandgap (2.64 eV). On the other hand, Bi ions at both Na and Ta sites helps maintain the ionic charge balance and thus minimize the native defects (such as oxygen vacancy and cation vacancy). Accordingly, Pt/Bi-doped

NaTaO3 with equal A- and B-site substitution was more active than those with either A- or B-site substitution for visible-light-driven H2 evolution because of the optimized electronic structures for bandgap narrowing to absorb visible-light absorption and the favorable crystal structures for depressing lattice defects to reduce the recombination of photogenerated charge carriers [120].

In our recent work, aiming to mediate the contradiction between the utilization of low-energy photons (visible light) and the preservation of the redox abilities of photogenerated charge carriers and thus to provide a new route to develop single photocatalyst for overall water splitting under visible-light irradiation, upconversion luminescent agent doped ABO3 were designed and prepared, and successfully applied to visible-light-driven photocatalytic H2 and O2 evolution. Furthermore, in order to promote the excitation of photogenerated charge carriers, site-selected doping was adopted to optimize the local lattice environment. A series of upconversion luminescent Er3+ doped SrTiO3 photocatalysts with different initial molar ratios of Sr/Ti (i.e., SrxTiy-Er, Sr/Ti=x/y) have been prepared by a facile polymerized complex method. Er3+ ions, which were gradually transferred from the A to the B-site with increasing Sr/Ti, enabled the absorption of visible light by intraband transition and the generation of high-energy excited states by upconversion processes. Compared with those with A-site occupancy, Er3+ ions with B-site occupancy are located in distorted BO6 octahe-dra with dipole moments and thus subject to much stronger local internal fields, which promotes energy transfer from the high-energy excited states of Er3+ to the host SrTiO3 and facilitates the generation of charge carriers in the valence band and conduction band of the host SrTiO3 (see Fig. 9). Consequently, Er-doped SrTiO3 with B-site occupancy instead of A-site occupancy are validated as novel, stable, and efficient photo-catalysts for visible-light-driven H2 (rate, 46.23 mmolh_1gcat) and O2 evolution (initial rate, 44.23 mmolh"1 g^) (see Fig. 10). The results generally suggest that the introduction of upconver-sion luminescent agent into host semiconductor is a promising approach to simultaneously harness low-energy photons and maintain redox ability for photocatalytic H2 and O2 evolution and that the local lattice environment of doped element greatly determines the photocatalytic activity [115].

The above results demonstrated that site-selected doping of cations into ABO3 host is an effective route to improve the photocatalytic performance. However, it was noted that the substitutional sites should be appropriately selected depending

Fig. 9 Schematic illustration of the main processes of the photocatalytic reaction on SrxTiy-Er samples. Step I: Generation of high-energy excited states in Er3+ by the absorption of low-energy photons. Step II: Band-to-band excitation of SrTiO3 by energy transferred from high-energy excited states in Er3+. Step III: Redox reactions (see the detailed reaction mechanisms in the Supporting Information). Reprinted with permission from Ref. [115].

Fig. 10 Visible-light-driven photocatalytic activity for H2 and O2 evolution on different samples. The ''Amount of O2 evolved'' value corresponds to the amount of O2 evolved during the first 3 h of the photocatalytic reaction and g^t represents ''per gram photocatalyst''. Reprinted with permission from Ref. [115].

on the specific objectives, such as valence-state control, band-structure adjustment, or energy-transfer enhancement.

The effect of cation ordering in ABO3-based Sr-Al-Nb-O double perovskite (SAN) on the photocatalytic activity was revealed by Iwakura et al. for the first time. SAN particles had a domain structure of completely B-site ordered (Sr2AlNbO6) and disordered (SrAl0.5Nb0.5O3) phases. It was found that photocatalytic activities of SAN for the evolution of H2 and O2 respectively from aqueous solutions of methanol and AgF decreased with increasing the fraction of the ordered phase. These results suggested that the photocatalytic activity of ordered Sr2AlNbO6 should be lower than that of disordered SrAl0.5Nb0.5O3, although the reason need to be clarified in future studies [89].

Wu et al. investigated the relationship between cation-filling and photocatalytic activity of AxBO3-based tunnel-structure materials for the first time. The catalytic efficiency decreases quasi-linearly with the A-site cation filling of the deficient perovskite AxNbO3 (A=K, Na, Sr, Ba) and the ideal perovskite ATiO3 (A=Mg, Ca, Sr, Ba). A lower cation filling in the A-site correlates with higher mobility and improved the separation and transport of photogenerated charge carriers, and therefore resulted in higher photocatalytic activity. Moreover, the linear correlation also holds for the reported tunnel photocatalysts of Bi2RNbO7 (R = Y, rare earth element), MIn2O4 (M = Ca, Sr, Ba), ATaO3 (A = Li, Na, K), etc. This study not only demonstrates that it is important to take cation filling into account when ranking activity of tunnel-structure photocatalysts, but also discloses some insights for developing new photocatalysts from crystal chemistry [141].

In addition to the control of crystal structures and site occupancy, other strategies such as surface modification were also employed to adjust the local lattice structures of ABO3-based photocatalysts. Recently, Ouyang et al. successfully improved the activity of photocatalytic H2 evolution over SrTiO3-based photocatalysts by surface alkalinization. It was found that surface alkalization could significantly shift surface conduction band of SrTiO3 photocatalyst to a more negative level, which supplies a strong potential for H2O reduction and consequently promote the photocatalytic efficiency of H2 evolution. This mechanism is also applicable for the visible-

light-sensitive (La,Cr)-codoped SrTiO3. Hence this photoca-talyst could achieve a high apparent quantum efficiency of 25.6% at around 425 nm for H2 evolution in methanol aqueous solution containing 5 M NaOH [113].

4.4. Application of the modification strategy of ABO3 photocatalysts in designing AxByOz photocatalysts

In our recent work, aiming to enhance the visible-light-driven photocatalytic activity and apply the modification strategies of ABO3 to AxByOz, AxByOz with continuously adjusted chemical components and micro-/nano-structures were designed and prepared for visible-light-driven photocatalytic H2 evolution. A series of cubic-phase pyrochlore structure tin(II) antimonate photocatalysts with continuously adjusted chemical components and micro-/nano-structures (i.e., SnSb-T °Cth) were successfully prepared by facile ion-exchange processes with varied ion-exchange time (t) and temperature (T) on the antimonic acid precursor (HSb). With the increase of Sn2+ contents in as-prepared tin(II) antimonate photocatalysts, the extended light-response range due to the narrowed bandgaps enhanced the absorption of photons and the excitation of photogenerated charge carriers (see Fig. 11). The micro-/nano-structures were gradually transformed from regular octahedra around 100 nm to nanocrystallines with smaller sizes, and such nanostructural transformation gradually increased the BET surface areas and pore volumes. The decreased crystal sizes shortened the migration distance of photogenerated charge carriers to the reactive sites for redox reactions on the surfaces and thus reduced the recombination of photogenerated charge carriers; the increased BET surface areas enriched the reactive sites, and also enhanced the adsorption of reactants, while the enlarged pore volumes facilitated the accumulation of reactants as well, thereby accelerating the redox reactions. The approaching stoichiometric compositions (molar ratio of Sn/Sb equal to 1) reduced the crystal lattice defects to restrain the recombination of photogenerated charge carriers. Accordingly, the activities of visible-light-driven photocatalytic H2 evolution were gradually improved and amounted to the highest rate of 40.10 mmol h_1 g^t for stoichiometric Sn2Sb2O7 (see Fig. 12). However, with the further increase of Sn2+ content, the activity sharply decreased due to the nonstoichiometric compositions that increased the crystal lattice defects to accelerate the recombination of photogenerated charge carriers, although the bandgaps and micro-/nano-structures seemed to be further optimized (see Fig. 12). As a result, the as-prepared tin(II) antimonates—especially Sn2Sb2O7 were validated to be novel, stable photocatalysts for visible-light-driven H2 evolution. The control of physicochemical properties based on continuous adjustment of chemical components and micro-/nano-structures is an effective approach to optimize all the processes of photocatalytic reaction and finally enhance photocatalytic activity. It is validated to apply the modification strategies of ABO3 to AxByOz based on the similarity between ABO3 and other AxByOz [142,143].

5. Application of ABO3 photocatalysts in Z-scheme systems for overall water splitting

It is very difficult for a single-photocatalyst system to fulfill the requirements of a narrow bandgap for visible-light absorption

Fig. 11 UV/vis spectra of HSb and SnSb-T °Cth samples. Along the direction of the arrow: HSb, SnSb—30 °C12h, SnSb—30 ° C24 h, SnSb—30 °C36 h, SnSb—60 °C12 h, and SnSb—90 °C12 h. F(Rn) was the Kubelka-Munk function. Reprinted with permission from Refs. [142,143].

Fig. 12 Time courses of visible-light-driven photocatalytic H2 evolution on SnSb-T °Cth samples. Reprinted with permission from Refs. [142,143].

and suitable band-edge positions for redox reactions simultaneously. Consequently, single photocatalysts for overall water splitting under visible-light irradiation are rarely reported to date. Z-scheme system mimics the photosynthesis process of green plants, and is composed of a photocatalyst for H2 evolution (i.e., H2-photocatalyst), a photocatalyst for O2 evolution (i.e., O2-photocatalyst), and electron mediator (not indispensable). Based on a two-photon process in the Z-scheme system, photocatalysts with solely ability to evolve H2 or O2 from the corresponding sacrificial reagents under visible-light irradiation could be coupled effectively, thus realizing visible-light-driven overall water splitting [24].

As discussed above, a variety of ABO3 structure-based photocatalysts for half-reaction of H2 or O2 evolution under visible-light irradiation were developed by means of different modification strategies. Accordingly, ABO3 photocatalyst-based

Z-scheme systems could be constructed to realize visible-light-driven overall water splitting (see Table 2). The electron mediators in the Z-scheme system play important roles in coupling the two H2- and O2-photocatalysts, respectively, and in shuttling the photogenerated carriers. The currently developed Z-scheme systems could be classified as those with ionic electron mediators, with solid electron mediators, and without electron mediators.

5.1. Z-scheme systems with ionic electron mediators

Ionic electron mediators in Z-scheme systems included IO3~/I~ [99,144,146,150,153], Fe3+/Fe2+ [147,149,151], etc. Sayama et al. reported the first Z-scheme system (Pt/(Cr,Ta)-codoped SrTiO3|Pt/WO3) for visible-light-driven overall water splitting by using Pt/(Cr,Ta)-codoped SrTiO3 and Pt/WO3 as H2 and O2-photocatalysts, respectively, and by using IO3~/I~ as electron mediator. The AQY was determined to be ca. 1% at 420 nm [144-146]. Hara and Irie prepared two types of SrTiO3-based photocatalysts, (Bi,Ga)-codoped SrTiO3 and (In,V)-codoped SrTiO3, both of which formed isolated minibands in the forbidden band and thus could absorb visible light. Furthermore, they established a Z-scheme system (Pt/ (Bi,Ga)-codoped SrTiO3 |Pt/(In,V)-codoped SrTiO3) by using the as-prepared SrTiO3-based photocatalysts and IO3~/I~ electron mediator. It was found that visible light contributed to overall water splitting and enhanced the photocatalytic activity [153]. In addition to doped ABO3 photocatalysts, ABO3-based solid-solution photocatalysts were employed to construct Z-scheme system as well [99,150]. Matoba et al. prepared BaZrO3-BaTaO2N solid solution, which was coupled with WO3 to form a Z-scheme system (Pt/BaZrO3-BaTaO2N solid solution/Pt/WO3) in IO37I~ solution. The AQY for overall water splitting was calculated to be about 0.6% at 420-440 nm [99].

In addition to IO37r, Fe3+/Fe2+ was also widely employed to construct Z-scheme systems. Sasaki et al. built a Z-scheme system consisting of Rh-doped SrTiO3 for H2 evolution, BiVO4 for O2 evolution, and Fe3+/Fe2+ for an electron mediator, and investigated the effect of cocatalysts loaded on Rh-doped SrTiO3 on photocatalytic reaction. It was found that compared with the system using a Pt cocatalyst (Pt/ Rh-doped SrTiO31 BiVO4), the system using a Ru cocatalyst (Ru/Rh-doped SrTiO31 BiVO4) showed comparable activity in the initial stage but exhibited no deactivation process for visible-light-driven overall water splitting. The back-reactions, such as, water formation from H2 and O2, reduction of Fe3+ by H2, and oxidation of Fe2+ by O2 were significantly suppressed in the system of Ru/Rh-doped SrTiO31 BiVO4, which gave an AQY of 0.3% [147]. Alternatively, in order to realize the separation of evolved H2 and O2 and thus to prevent back-reactions in the Z-scheme system of Pt/Rh-doped SrTiO31 BiVO4, Yu et al. designed a twin reactor with a Nafion membrane to divide H2- and O2-photocatalysts in two compartments, respectively. It was found that the watersplitting reaction in the twin reactor did not delayed by the membrane through which the electron mediator of Fe3+/Fe2+ was diffused, and the deactivation of Pt/Rh-doped SrTiO3 could be minimized due to the suppression of Fe(OH)3 formation on the photocatalyst surface [151].

Table 2 ABO3-based Z-scheme system for overall water splitting.

H2-photocatalyst (mass/ Lamp (filter)3 Reactant solution (aqueous) Activity/mmol ■ h 1 ■ gca1 AQY/% Reference

g) | O2-photocatalyst --(wavelength/ (year)

(mass/g) Cocatalyst/ Cocatalyst/ nm)

(Cr,Ta)-codoped SrTiO3 300 W Xe 100 mM Nal (pH ==7.0) 0.3 wt% 1 wt% Pt/ 0.1% (420.7) [141,145]

(0.2)|WO3 (0.2) (1 > 420 nm) Pt/1.8b 0.9b (2001)

(Cr,Ta)-codoped 300 W Xe 5 mM Nal 0.3 wt% 0.5 wt% Pt/ 1% (420) [146]

SrTiO3|WO3 Pt/16b 8b (2005)

Rh-doped SrTiO3 300 W Xe 2 mM FeCl3+H2SO4 (pH ==2.4) 0.7 wt% —/-9b 0.3% (420) [147]

(0.05)|BiVO4 (0.05) (1 > 420 nm) Ru/-18.5b —/19b (2008)

Rh-doped SrTiO3 300 W Xe H2SO4 (pH ==3.5) 1 wt% Ru/ 1.7% (420) [148]

(0.1)|BiVO4 (0.1) (1 > 420 nm) 40b (2009)

Rh-doped SrTiO3 450 W Hg 10 mM FeCl2 0.5 wt% 0.7 wt% Pt/ - [149]

(0.2)|WO3 (0.2) (1 > 420 nm) Pt/2.7b 1.1b (2009)

ATaO2N (A = Ca, Sr, Ba) 300 W Xe 5 mM Nal 0.3 wt% 0.5 wt% Pt/ 0.1% (420- [150]

(0.1)|WO3(0.1) (1 > 420 nm) Pt/-6.4b -3.0b 440) (2009)

BaZrO3-BaTaO2N 300 W Xe 1 mM Nal 0.3 wt% 0.3 wt% Pt/ 0.6% (420- [99]

(0.05)|WO3 (0.1) (420 < 1 < 800 nm) Pt/-5.5b ~3b 440) (2011)

Rh-doped SrTiO3 300 W Xe 5 mM FeCl2+H2SO4 0.8 wt% —/0.41 - [151]

(0.4)|BiVO4 (0.4) (pH == 2.4) 15 mM FeCl3+H2SO4 Pt/0.84 (2011)

(pH ==2.4) —/5.5b

Rh-doped SrTiO3 300 W Xe H2SO4 (pH ==3.5) 0.7 wt% 1.03% (420) [152]

(0.03)|BiVO4 (0.03) (1 > 420 nm) Ru/11b (2011)

Sr(V0.005Ti0.99Ïn0.005)O3 Xe 0.01 M NaI 0.1 wt% 0.1 wt% Pt/ - [153]

(0.03)|(Bi0.1Sre.9)(Ti0.9 Pt/-0.8 -0.3 (2012)

Gao.i)O3 (0.03)

aThe lamp (with filter if necessary) was used as light source. bThe rate unit of gas evolution is mmol h_1.

5.2. Z-scheme systems with solid electron mediators

Although ionic electron mediators performed efficiently in relaying electrons, a solid electron mediator is more favorable in terms of recovery of the photocatalyst and reclamation of clean water. Tada et al. designed the first all-solid-state Z-scheme system of CdS/Au/TiO2 by using metallic Au as a solid electron mediator [154]. Using photoreduced graphene oxide (PRGO) as solid electron mediator, Iwase et al. coupled the photocatalysts of Rh-doped SrTiO3 (SrTiO3:Rh) for H2 evolution and BiVO4 for O2 evolution to construct a novel Z-scheme system of Ru/Rh-doped SrTiO3|BiVO4 [152]. It was demonstrated that PRGO could shuttle photogenerated electrons from BiVO4 to Rh-doped SrTiO3 (see Fig. 13). The PRGO prepared by BiVO4 possesses a high electron conductivity and a low degree of hydrophobicity, thus guaranteeing the efficient electron transfer and tripling the water splitting reaction under visible-light irradiation. An AQY of 1.03% at 420 nm was achieved in the Z-scheme system for overall water splitting.

5.3. Z-scheme systems without electron mediators

to shield incident light. In order to exclude the negative effects by electron mediators and to simplify the design, Sasaki et al. fabricated the first Z-scheme system (Ru/Rh-doped SrTiO3|-BiVO4) driven by the interparticle electron transfer between two photocatalysts without an electron mediator for visible-light-driven overall water splitting (see Fig. 14). It was found that the photocatalytic activities were dependent on pH and the highest activity was obtained at pH 3.5, under which the two photocatalysts of Ru/Rh-doped SrTiO3 and BiVO4 were aggregated with suitable contact. Such intimate contact and the Rh species with reversible redox properties in Rh-doped SrTiO3 facilitated the interparticle electron transfer from the conduction band of BiVO4 to impurity level of Rh-doped SrTiO3, and therefore improved the photocatalytic activity. The AQY of the Ru/Rh-doped SrTiO3| BiVO4 system was 1.7% at 420 nm [148].

The above results announced that the Z-scheme system paved an alternative way to visible-light-driven overall water splitting by using ABO3 structure-based photocatalysts and verified the values of the ABO3 structure-based photocatalysts that could only evolve H2 or O2 in corresponding sacrificial reagents.

Generally, an electron mediator is required to construct a Z-scheme system. However, the affinity and energy-level matching between photocatalysts and electron mediators actually limit such a combination to several systems. Moreover, the electron mediators may give undesirable effects such as back-reactions to form water from H2 and O2 evolved, or their intermediates

6. Summary and perspectives

In summary, major progress on the development of ABO3-based photocatalysts for water splitting has been achieved by means of different modification strategies based on the fundamental principle and process of photocatalysis and on

Fig. 13 (a) Schematic image of a suspension of Ru/SrTiO3 and PRGO/BiVO4 in water at pH 3.5. (b) Mechanism of water splitting in a Z-scheme photocatalysis system consisting of Ru/ SrTiO3:Rh and PRGO/BiVO4 under visible-light irradiation. Reprinted with permission from Ref. [152].

the crystal structure and chemical component characteristics of ABO3 materials. The results demonstrated the encouraging prospects of ABO3-based photocatalysts and offered a guide for future research work.

ABO3 photocatalysts with typical crystal structures and chemical components, such as SrTiO3 and NaTaO3, have important research values as model photocatalysts to reveal the structure-activity relationships. Early in 1976, SrTiO3 was proved to be an excellent photocatalyst for overall water splitting under UV irradiation due to its high stability and suitable band-edge positions [155], and, since then, has been extensively investigated by optimizing, for example, the reac-tant solution [123], cocatalyst [125,126,128,156,157], preparation method [127], and defects [81]. The reported results provided abundant data for future research on photocatalytic mechanism.

Modification of ABO3 photocatalysts by combining different modification strategies is favorable for optimizing the whole process (i.e., absorption of photons and excitation of the photogenerated charge carriers, separation and migration of the photogenerated charge carriers, and redox reactions) during photocatalytic reaction, and finally improved the photocatalytic performance.

Development of ABO3-based photocatalysts for overall water splitting under visible-light irradiation remains a great challenge but is still promising if appropriate strategies, such as the incorporation of upconversion luminescent agents (Er, Ho, Eu, Nd, Tm, and so on) and the construction of Z-scheme system, is employed. As for Er-doped SrTiO3 [115], both abilities to produce H2 and O2 in corresponding sacrificial reagents under visible-light irradiation testified to the full-filment of thermodynamic conditions for visible-light-driven overall water splitting. Further systematic and detailed work on the improvement of dynamic conditions (such as separation and migration of photogenerated charge carriers, and redox reactions), by optimizing reactant solution and

Fig. 14 Mechanism of water splitting using the Z-scheme photocatalysis system driven by electron transfer between H2-and O2-photocatalysts. (a) Suspension of Ru/SrTiO3:Rh and BiVO4 at neutral and acidic conditions. (b) Scheme of photo-catalytic water splitting. Reprinted with permission from Ref. [148].

cocatalyst, is in progress in our group in order to realize overall water splitting under visible-light irradiation.

Preparation of ABO3 film photocatalysts is favorable for photoelectrochemical characterization and performance evaluation so that more physicochemical properties can be obtained for more comprehensive understanding of ABO3 powder photocatalysts. Furthermore, novel preparation strategies need to be developed due to the challenge of multi-component metal oxide films.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (NSFC) (Nos. 50821064 and 51121092) and the National Basic Research Program of China (973 Program) (No. 2009CB220000). Jinwen Shi thanks Bing Wang for assisting the literature classification and Table preparation, and the anonymous reviewers for valuable comments and suggestions on an earlier draft of this paper.

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Prof. Dr. Liejin Guo is the director of the State Key Laboratory of Multiphase Flow in Power Engineering in Xi'an Jiaotong University, China. He obtained his PhD Degree in Power Engineering and Engineering Ther-mophysics from Xi'an Jiaotong University in 1989. His research interest includes multiphase flow, heat and mass transfer, and renewable energy technologies. He serves in the editorial boards of several international journals such as International Journal of Hydrogen Energy, International Journal of multiphase flow and Applied Thermal Engineering. He has published more than 400 papers in journals and conferences, given over 30 invited or keynote lectures in international conferences. He has been awarded with the Second Class Prize of National Natural Science Award in 2007 and the Second Class Prize of National Technology Innovation Award in 2009.