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ホーム研究成果一覧化学・材料科学/情報・電気・電子/機械・土木・建築・その他工学>Enhanced water splitting activity of surface-modified TaON photocatalyst

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理工連携による化学イノベーション

Enhanced water splitting activity of surface-modified TaON photocatalyst

2012.12.6

前田 和彦 工学系研究科化学システム工学専攻助教
現在、東京工業大学大学院理工学研究科化学専攻准教授(写真)

堂免 一成 工学系研究科化学システム工学専攻教授

Water splitting using particulate photocatalysts has attracted attention as a means of producing clean hydrogen from water and sunlight. To efficiently convert solar energy into chemical energy in the form of hydrogen, it is required to develop a photocatalytic system that works under visible light, the main component of solar spectrum. Tantalum oxynitride (TaON) is one of the most studied compounds as a visible-light-driven photocatalyst, but has remained a challenge in terms of the suppression of lattice defects that work as recombination centers between photogenerated electrons and holes.

The authors have developed a surface modification technique that effectively suppresses the generation of defects in TaON. TaON can be readily prepared by heating Ta2O5 powder under a flow of NH3 at high temperatures (1123 K~). During this process, however, part of surface Ta5+ cations are inevitably reduced, thereby generating anionic defects to keep the charge balance of the crystal. In contrast, when ZrO2 nanoparticles (10–30 nm in size) are loaded on the surface of Ta2O5, such undesirable reduction of Ta5+ (in other words, the generation of anionic defects) is effectively suppressed even after high-temperature NH3 treatment (Figure 1).

The as-prepared ZrO2/TaON exhibits an enhanced activity for two-step water splitting in combination with WO3 as an oxygen evolution photocatalyst (Figure 2) in the presence of an iodate/iodide shuttle redox mediator, giving an apparent quantum yield of 6.3% at 420 nm. This is the highest value among visible-light-driven water splitting systems reported so far. It is expected that the photocatalytic activity of other (oxy)nitrides would be improved by employing a similar surface modification, as most of them have the same drawback (high density of defects). This possibility is currently under investigation.

imageNitridation of ZrO2/Ta2O5 composite to produce ZrO2/TaON while suppressing the production of reduced tantalum species (defect sites) near the surface of the material.imageTime courses of H2 and O2 evolution from aqueous NaI solution over a mixture of WO3 and ZrO2/TaON (or TaON) under monochromatic light irradiation (λ = 420 nm). Pt species were loaded on both photocatalysts as promoters for reduction process.

Notes

1. Kazuhiko Maeda, Hiroaki Terashima, Kentaro Kase, Masanobu Higashi, Masashi Tabata, Kazunari Domen, “Surface Modification of TaON with Monoclinic-ZrO2 to Produce a Composite Photocatalyst with Enhanced Hydrogen Evolution Activity under Visible Light” Bulletin of the Chemical Society of Japan, 2008, 81 (8), 927–937.

2. Kazuhiko Maeda, Masanobu Higashi, Daling Lu, Ryu Abe, Kazunari Domen, “Efficient Nonsacrificial Water Splitting through Two-Step Photoexcitation by Visible Light using a Modified Oxynitride as a Hydrogen Evolution Photocatalyst” Journal of the American Chemical Society, 2010, 132 (16), 5858–5868.

3. Su Su Khine Ma, Kazuhiko Maeda, Kazunari Domen, "Modification of TaON with ZrO2 to Improve Photocatalytic Hydrogen Evolution Activity under Visible Light: Influence of Preparation Conditions on Activity" Catalysis Science & Technology, 2012, 2 (4), 818–823.

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