Recent progress in oxynitride photocatalysts for visible-light-driven water splitting

Abstract

Photocatalytic water splitting into hydrogen and oxygen is a method to directly convert light energy into storable chemical energy, and has received considerable attention for use in large-scale solar energy utilization. Particulate semiconductors are generally used as photocatalysts, and semiconductor properties such as bandgap, band positions, and photocarrier mobility can heavily impact photocatalytic performance. The design of active photocatalysts has been performed with the consideration of such semiconductor properties. Photocatalysts have a catalytic aspect in addition to a semiconductor one. The ability to control surface redox reactions in order to efficiently produce targeted reactants is also important for photocatalysts. Over the past few decades, various photocatalysts for water splitting have been developed, and a recent main concern has been the development of visible-light sensitive photocatalysts for water splitting. This review introduces the study of water-splitting photocatalysts, with a focus on recent progress in visible-light induced overall water splitting on oxynitride photocatalysts. Various strategies for designing efficient photocatalysts for water splitting are also discussed herein.

Keywords: cocatalyst; oxynitride; photocatalyst; semiconductor; visible light; water splitting.

Figure 1.

Schematic models of (a) photoelectrochemical and (b) photocatalytic water splitting.

Figure 2.

Basic principles of water splitting on a semiconductor photocatalyst.

Figure 3.

Energy diagram of photocatalytic water splitting.

Figure 4.

Schematic of the apparatus for photocatalytic water splitting.

Figure 5.

Strategy for designing visible-light-sensitive photocatalysts for water splitting.

Figure 6.

UV-visible diffuse reflectance spectrum of β-Ge₃N₄.

Figure 7.

Time course of H₂ and O₂ evolution on RuO₂-loaded β-Ge₃N₄ under UV light irradiation. Catalyst 0.5 g, aqueous H₂SO₄ solution (pH = 0) 390 ml, inner-irradiation-type quartz reaction cell, high-pressure mercury lamp (450 W).

Figure 8.

XRD patterns for GaN:ZnO solid solutions with various compositions. (a) GaN (ref), (b) GaN:ZnO (Zn 3.4 at%), (c) GaN:ZnO (Zn 6.4 at%), (d) GaN:ZnO (Zn 13.3 at%), and (e) ZnO. (Reproduced with permission from [56]. Copyright 2005 American Chemical Society).

Figure 9.

UV-visible diffuse reflectance spectra of GaN:ZnO solid solutions with various compositions. (a) GaN (ref), (b) GaN:ZnO (Zn 3.4 at%), (c) GaN:ZnO (Zn 6.4 at%), (d) GaN:ZnO (Zn 13.3 at%), and (e) ZnO. (Reproduced with permission from [56]. Copyright 2005 American Chemical Society).

Figure 10.

Time courses of H₂ and O₂ evolution on RuO₂-loaded GaN:ZnO under (a) UV light irradiation and (b) visible light irradiation. Catalyst 0.3 g, aqueous H₂SO₄ solution (pH = 3) 390 ml, inner-irradiation-type Pyrex reaction cell, with and without an aqueous NaNO₂ solution filter, high-pressure mercury lamp (450 W). (●) H₂, (○) O₂, (△) N₂. (Reproduced with permission from [56]. Copyright 2005 American Chemical Society).

Figure 11.

Linear-sweep voltammograms for (a) bare and (b) Cr₂O₃-coated Rh electrodes in 0.5 M Na₂SO₄ aqueous solution adjusted to pH 3.6 with H₂SO₄ under Ar (dashed line) and O₂ bubbling (solid line) (scan rate, 5 mV s⁻¹). (Reproduced with permission from [65]. Copyright 2009 American Chemical Society).

Figure 12.

Schematic model of the function of metal-core/Cr₂O₃-shell cocatalyst for the promotion of overall water splitting.

Figure 13.

Crystal structures of TaON (a), Ta₃N₅ (b), and perovskite oxynitride (c).

Figure 14.

Basic principles of photocatalytic reactions in the presence of sacrificial reagents.

Figure 15.

Crystal structures of LaTaON₂, LaMg_2/3Ta_1/3O₃, and LaMg_xTa_1−xO_1+3xN_2−3x.

Figure 16.

XRD patterns (a) and UV-visible diffuse reflectance spectra (b) for LaMg_xTa_1−xO_1+3xN_2−3x.

Figure 17.

Schematic of band engineering by compositional tuning.

Figure 18.

Gas evolution during water splitting on the Rh_yCr_2−yO₃/LaTaON₂ (a) and on Rh_yCr_2−yO₃/LaMg_1/3Ta_2/3O₂N (b). Catalyst 0.2 g, pure water 400 ml, inner-irradiation-type Pyrex reaction cell, high-pressure mercury lamp (450 W). (●) H₂, (○) O₂, (▲) N₂.

Figure 19.

Gas evolution during water splitting on TiO₂/Rh_yCr_2−yO₃/ LaMg_1/3Ta_2/3O₂N(a) and on TiO₂/SiO₂/Rh_yCr_2−yO₃/LaMg_1/3Ta_2/3O₂N(b) under UV+visible light irradiation and visible light irradiation alone. Catalyst 0.2 g, pure water 400 ml, inner-irradiation-type Pyrex reaction cell, with and without an aqueous NaNO₂ solution filter, high-pressure mercury lamp (450 W). (●) H₂, (○) O₂, (▲) N₂.

Figure 20.

Schematic of the strategy for the surface coating of photocatalyst to achieve overall water splitting.