Sunlight-driven water splitting has been studied actively for production of renewable solar hydrogen as a storable and transportable energy carrier [1-3]. Both the efficiency and the scalability of water-splitting systems are important factors for practical utilization of renewable solar hydrogen because of the low areal density of solar energy. Particulate photocatalyst systems do not involve any secure electric circuit and thus can be spread over a wide area by inexpensive processes potentially. In this regard, activation of particulate photocatalysts and development of their reaction systems are important subjects.

A semiconductor photocatalyst can split water into hydrogen and oxygen thermodynamically when the band gap straddles the potentials of the hydrogen evolution reaction (0 V vs. RHE) and the oxygen evolution reaction (+1.23 V vs. RHE). In addition, it is generally necessary to modify photocatalysts with appropriate cocatalysts in order to facilitate charge separation and surface redox reactions. The authors’ group has studied various semiconducting materials including oxides, (oxy)nitrides, and (oxy)chalcogenides for photocatalytic water splitting [1]. Recently, we have found that doping Al into SrTiO₃ boosts the water splitting activity by two orders of magnitude [4]. The resultant Al-doped SrTiO₃ achieved an apparent quantum yield (AQY) of 30% at 360 nm. Through the optimization of the preparation and modification methods of Al-doped SrTiO₃, the AQY of photocatalytic water splitting has been upgraded to 56% and higher at 365 nm. Processing of such particulate photocatalysts into potentially extensible forms is to be presented [5].

Two different photocatalysts can also be combined so that hydrogen and oxygen are generated on the different photocatalysts [1,3]. Recently, the authors’ group has developed particulate photocatalyst sheets consisting of the hydrogen evolution photocatalyst (HEP) and the oxygen evolution photocatalyst (OEP) embedded into conductive layers by particle transfer [6-11]. A photocatalyst sheet consisting of La- and Rh-codoped SrTiO₃ as the HEP and Mo-doped BiVO₄ as the OEP embedded into a carbon conductor exhibits a solar-to-hydrogen energy conversion efficiency of 1.0% at ambient pressure [10]. The photocatalyst sheet shows significantly higher water splitting activity than the corresponding powder suspension systems, because the conductor layer transfers photogenerated electrons between photocatalyst particles effectively. In addition, evolution of hydrogen and oxygen in close proximity allows to prevent generation of pH gradient during the water splitting reaction. Therefore, the photocatalyst sheet is scalable directly without sacrificing the high activity. However, the absorption edge wavelengths of La- and Rh-codoped SrTiO₃ and Mo-doped BiVO₄ are 520 nm at most. It is necessary to utilize photocatalysts with longer absorption edge wavelengths to pursuit higher STH values. We have found that some (oxy)chalcogenides and (oxy)nitrides with narrower band gap energies are applicable as the HEP and the OEP of particulate photocatalyst sheets.

In this talk, recent progress and future challenges in photocatalytic water splitting and system development will be presented.

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