Simultaneous photodeposition of rhodium–chromiumnanoparticles on a semiconductor powder: structural characterization and application to photocatalytic overall water splitting

Simultaneous photodeposition of rhodium (Rh) and chromium (Cr) nanoparticles on a semiconductor powder was examined as a means of constructing active sites for hydrogen evolution in photocatalytic overall water splitting. A solid solution of gallium nitride and zinc oxide (GaN:ZnO) powder that catalyzes overall water splitting with visible light was employed as a semiconductor support. The photodeposition was carried out in aqueous suspension containing GaN:ZnO, (NH4)3RhCl6, and K2CrO4, and used H2O as an electron donor. With increasing concentration of K2CrO4, the valence state of the codeposited Rh species varied from metallic to trivalent, while that of Cr remained trivalent. At intermediate concentrations of K2CrO4, the photodeposits were core/shell-like crystalline nanoparticles consisting of a metallic Rh core and an Rh(III)–Cr(III) mixed-oxide shell. The photocatalytic activity for visible-light-driven overall water splitting (λ > 400 nm) was strongly dependent on the structure of the photodeposits. Comparative experiments using an analogue, modified with core/shell-structured Rh/Cr2O3 nanoparticles, revealed that core/shell-structured nanoparticles consisting of a metallic Rh core were better than Rh–Cr trivalent mixed oxides for enhancing the photocatalytic activity.