Light-Induced Synthesis of Heterojunctioned Nanoparticles on a Semiconductor as Durable Cocatalysts for Hydrogen Evolution

This work attempted to synthesize heterojunctioned nanoparticles consisting of a transition metal and Cr on powdered SrTiO3, an n-type semiconductor exhibiting photocatalytic activity for overall water splitting. This was performed via band gap irradiation of SrTiO3 (λ > 300 nm) in an aqueous methanol solution containing a transition metal precursor and K2CrO4. The resulting multicomponent nanoparticles were examined as promoters for photocatalytic overall water splitting. Among the transition metals examined, Au and Pd became effective promoters for overall water splitting upon codeposition of Cr. In the case of Au, which is stable in its metallic state, the resulting (Au+Cr) nanoparticles had a core/shell structure consisting of metallic Au (the core) and amorphous Cr2O3 (the shell), similar to Au/Cr2O3 prepared by a stepwise photodeposition method. However, when using a core transition metal with a tendency to form an oxide, such as Pd, the nanoparticles had different morphologies and electronic states, depending on the proportion of Cr. In the case of a combination of Pd and Cr, the photocatalytic activity for overall water splitting was strongly dependent on the structure and electronic state of the (Pd+Cr) multicomponent cocatalyst. Increasing the proportion of Cr was found to suppress the reverse reaction (that is, H2–O2 recombination), an effect that is not realized when employing a conventional impregnation method.