Fine synthesis of photocatalitic thin films based on oxynitrides for water splitting with high efficiency

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The hydrogen production by a photocatalyst from sunlight and water is a potential approach to solve the energy problems essentially. The purpose of this projict is the development of photocatalitic films based on d^0 and/of d^10 group (oxy)nitrides for water splitting under visible light illumination without bias voltage. First, we carried out the development of photoelectrode based on a solid solution of GaN and ZnO. The photoelectrodes were prepared by pasting (Ga_<1x>Zn_x)(N_<1x>O_x) powder onto conducting glass. We intensively surveyed the effects of surface modification. As a result, we found that photocurrent is extremely enhanced by the loading of IrO_2 as a cocatalyst. Moreover, the gas evolved from the photoelectrode was found to be O_2, while the gas evolved from the counter electrode was found to be H_2. The amount of evolved gases is consistent with the photocurrent. Next, we expanded target materials in oxysulfide. Sulfides are known as a photofunctional material. They are, however, generally unstable in water and believed to be unsuitable for a photocatalyst in water. We explore the potential possibilities of oxysulfides as a photocatalyst in water. We found that La_5Ti_2AgS_5O_7 have promising potential for H_2 production from water. Photoelectrodes were prepared by pasting powder and/or by RF magnetron sputtering on conducting substrates. By photoelectrochemical measurements, La_5Ti_2AgS_5O_7 was found to p-type semiconductor. P-type photoelectrodes are expected to evolve H_2 without self-degradation. These photoelectrodes have practical potential to produce H_2 and O_2 from water and sunlight, and more extensive studies are required.