Z-scheme photocatalyst systems for water splitting under visible light irradiation
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Introduction Energy and environmental issues on a global level are important topics. It is indispensable to construct clean energy systems in order to solve these issues. Solar H2 production from water is one of the most promising processes. Semiconductor electrodes1–5 and photocatalysts4–11 will be useful for solar water splitting. Solar water splitting using a powdered photocatalyst is a strong candidate for practical large-scale use for a H2 production system because of its simplicity of fabrication. Photon energy is converted to chemical energy accompanied by a largely positive change in the Gibbs free energy through water splitting. This reaction is similar to photosynthesis by green plants from the viewpoint of an uphill reaction with a positive change in the Gibbs free energy. Therefore, photocatalytic water splitting into H2 and O2 in a stoichiometric ratio is regarded as artificial photosynthesis. Artificial photosynthesis is an attractive and challenging theme in chemistry. Although many oxide photocatalysts have been reported for water splitting, they only respond to UV irradiation.4–11 The number of photocatalysts that are active for water splitting under visible light irradiation is very limited. Therefore, it is important to develop improved visiblelight-driven photocatalyst materials for solar water splitting. There are two types of photocatalyst systems for water splitting under visible light irradiation, as shown in Figure 1.
A single-particulate photocatalyst system is one example. Important points in the single-particulate semiconductor photocatalyst system are the width of the bandgap 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 versus NHE [normal hydrogen electrode]), while the top level of the valence band needs to be more positive than the redox potential of O2/H2O (1.23 V). The bandgap of a visible-light-driven photocatalyst should be narrower than 3.0 eV (λ > 415 nm). Therefore, suitable band engineering is necessary for tuning the band positions of photocatalyst materials for water splitting under visible light irradiation. Some oxynitride photocatalysts such as GaN-ZnO and ZnGeN2-ZnO solid solutions are active for water splitting as a single-particulate photocatalyst.12,13 See the Maeda and Domen article in this issue. Two-photon systems, as seen in photosynthesis by green plants (z-scheme), are another way to achieve overall water splitting. The z-scheme is composed of a H2-evolving photocatalyst, an O2-evolving photocatalyst, and an electron mediator, as shown in Figure 1. Photocatalysts that are active only for half reactions of water splitting (sacrificial H2 and O2 evolution reactions) can be employed for the construction of the z-scheme: that is the merit of the z-scheme. There
Akihiko Kudo, Tokyo University of Science, Japan; [email protected] DOI: 10.1557/mrs.2010.3
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MRS BULLETIN • VOLUME 36 • JANUARY 2011 • www.mrs.org/bulletin
© 2011 Materials Research Society
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