Materials and mechanisms of artificial photosynthesis

Since the discovery of the Honda-Fujishima effect of water splitting[1] using a titanium dioxide (TiO2) electrode in the early 1970s, numerous studies have been conducted and are still growing on photocatalytic reactions of TiO2. Titanium dioxide serves as a model case for the mechanistic studies of photocatalysis, but it has a serious drawback as a photocatalyst for water splitting: the band gap is too large for absorbing visible photons in sunlight. Obviously it is necessary to synthesize photocatalysts workable with sunlight. One of the authors (AK) has synthesized and tested various heterogeneous photocatalyst materials for water splitting, including metal oxides, metal (oxy)sulfides, and metal (oxy)nitrides.[2] Although these new sets of photocatalysts are promising, their performances have to be further improved for practical use. Obstacles for improving the efficiency of water splitting with those materials stem from lack of understanding of the mechanisms for the photo-induced heterogeneous reactions, including charge splitting dynamics, active sites for redox reactions, reaction intermediates, etc. We need to explore the fundamentals of photocatalytic reactions with those materials to meet our goal. In this talk, we describe the current status of those photocatalyst materials and preliminary results of charge dynamics probed by transient absorption.