Publication date: 15th December 2014
Z-scheme water splitting, involving two different photocatalysts for hydrogen and oxygen evolution, is a promising approach to harvesting solar energy as chemical energy, because narrow band gap photocatalysts that are active either for water reduction or oxidation can be applied. In our previous study, it was found that La- and Rh-codoped SrTiO3 (SrTiO3:La,Rh) exhibited higher activity than SrTiO3:Rh for the sacrificial hydrogen evolution reaction from an aqueous methanol solution. Moreover, SrTiO3:La,Rh was applicable as a hydrogen evolution photocatalyst (HEP) in redox-mediator-free Z-scheme water splitting based on interparticle electron transfer in combination with Ta3N5 as an oxygen evolution photocatalyst (OEP). In this system, the interparticle electron transfer occurrs because of the physical contact induced by electrostatic attractive force. However, the efficiency of the interparticle electron transfer based on the physical contact is considered to be low. Moreover, the kinds of the photocatalysts applicable to redox-mediator-free Z-scheme water splitting are limited because HEP and OEP must have surface potentials with opposite signs, which is not always possible. The challenge of developing an efficient redox-mediator-free Z-scheme water splitting system lies in ensuring efficient transfer of electrons between HEP and OEP without deteriorating their intrinsic photocatalytic properties.Herein, we report a Z-scheme system consisting of HEP/metal layer (M)/OEP, which utilizesthe metal layer for electron transport, taking SrTiO3:La,Rh/Au/BiVO4 as a prototype. SrTiO3:La,Rh and BiVO4 were prepared according to the previously reported methods. SrTiO3:La,Rh/Au/BiVO4 systems were fabricated by the particle transfer method, which offered an intimate mechanical and electrical contact between the semiconductor particles and the metal layers. SrTiO3:La,Rh/Au/BiVO4 systems exhibited photocatalytic activities for overall water splitting that are 6 and 20 times higher than powder suspensions and SrTiO3:La,Rh/BiVO4 systems without metal layers, respectively. The SrTiO3:La,Rh/Au/BiVO4 systems achieved an apparent quantum yield of 5.9% under 418-nm monochromatic light irradiation and a solar-to-hydrogen conversion efficiency of 0.2%. The high performance of this system is due to the presence of the Au layer that effectively transfers photogenerated electrons from BiVO4 to SrTiO3:La,Rh. The activity of SrTiO3:La,Rh/Au/BiVO4 was virtually independent of the pH of reaction solutions, which was distinct from the conventional powder suspension Z-scheme water splitting systems.Our study offers a new design concept of HEP/M/OEP solid-state device to overcome limitations of the earlier Z-scheme systems and enables efficient photocatalytic water splitting.
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