Vol. 101 No. 9 (2025)
Photocatalytic water splitting has been studied as a technology that may produce green hydrogen on a large scale at an affordable cost. Since the discovery in the 1980s that powdered oxide semiconductors such as TiO2 and SrTiO3 function as photocatalysts for splitting water into hydrogen and oxygen, numerous photocatalytic materials have been developed. However, the efficiencies of materials including TiO2 and SrTiO3 were too low at that time. It is still challenging to develop an efficient photocatalyst that produces hydrogen and oxygen from water using sunlight. As a result, until recently, no researchers had attempted to use photocatalysts on a large scale to produce green hydrogen, with the exception of Professor Kazunari Domen, the author of this review in this issue (pp. 564–586).
Systematic research into water-splitting photocatalysts has continued in Japan. Professor Domen has played a central role in developing photocatalytic materials for water splitting. His contribution has led to the discovery of universal principles that remain relevant today.
The photocatalyst used for the photocatalyst panel system on the cover illustration is a highly efficient, durable SrTiO3 photocatalyst developed by Professor Domen. High performance is successfully obtained by flux-treatment with SrCl2 in the presence of Al2O3 and loading of suitable cocatalysts to the SrTiO3 powder. This photocatalyst can split water into hydrogen and oxygen with nearly 100% internal quantum efficiency in the near-ultraviolet light region. It is the first photocatalyst capable of driving a photon-to-chemical conversion reaction with an efficiency comparable to natural photosynthesis by plants, resulting in a significant impact on researchers worldwide.
The cover illustration shows a 100-square-meter photocatalyst panel system developed by Professor Domen. This system contains photocatalyst sheets made by coating glass substrates with the aforementioned SrTiO3 photocatalyst. This panel system uses sunlight to split water outdoors, producing hydrogen and oxygen. It is possible to separate and recover the hydrogen from these bubbles using separation membranes. This is the first example of green hydrogen production using a water-splitting photocatalyst on such a large scale, highlighting the potential of the photocatalytic green hydrogen production process.
The most pressing issue currently is developing a photocatalyst that can efficiently split water in response to visible light. This is essential for producing green hydrogen at a practical level of efficiency. Professor Domen has developed various visible-light-responsive oxynitride and oxysulfide photocatalysts for water splitting, as shown in the cover illustration. Another challenge is reducing the construction and operating costs of reaction systems to supply green hydrogen at a low cost. Professor Domen continues to research and develop photocatalytic green hydrogen production processes from various aspects, and I am confident that he will achieve astonishing results.
Akihiko Kudo
Professor, Faculty of Science, Tokyo University of Science
Table of Contents
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Conformational constraint in natural product synthesis
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Particulate photocatalysts for water splitting to produce green hydrogen on a large scale
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Unconstrained deep learning-based sleep stage classification using cardiorespiratory and body movement activities in adults with suspected sleep apneaSeiichi MOROKUMA , Toshinari HAYASHI, Naoyuki MOTOMURA, Masatomo KANEGAE, Yoshihiko MIZUKAMI, Shinji ASANO, Ichiro KIMURA, Kenji FUJITA, Yutaka KOHDA, Hiroshi IMAI, Yuji TATEIZUMI, Hitoshi UENO, Subaru IKEDA and Kyuichi NIIZEKIVolume 101 Issue 9 Pages 587-603