A copper and chromium based nanoparticulate oxide as a noble-metal-free cocatalyst for photocatalytic water splitting

Abstract

Nanoparticulate oxides consisting of copper (Cu) and chromium (Cr) were studied as noble-metal-free cocatalysts for photocatalytic water splitting. The structure of the Cu–Cr mixed oxide dispersed on a solid solution of GaN and ZnO (referred to as GaN : ZnO hereafter) was characterized by high-resolution transmission electron microscopy (HR-TEM), X-ray absorption fine-structure (XAFS) spectroscopy, and electrochemical measurements. The mixed-oxide nanoparticle was an effective promoter of photocatalytic overall water splitting on GaN : ZnO, and was loaded by impregnation from an aqueous solution containing Cu(NO₃)₂·3H₂O and Cr(NO₃)₃·9H₂O followed by calcination in air. Impregnation of GaN : ZnO with 1.5 wt% Cu and 2.0 wt% Cr followed by calcination at 623 K for 1 h provided the highest photocatalytic activity, while catalysts modified with either Cu-oxide or Cr-oxide showed little activity. The activity of this photocatalyst was shown to be strongly dependent on the generation of Cu(II)–Cr(III) mixed-oxide nanoparticles with optimal composition and coverage. The results of electrochemical measurements and photocatalytic reactions also indicated that Cu(II)–Cr(III) mixed-oxide nanoparticles on GaN : ZnO are resistant to both the photoreduction of O₂ and water formation from H₂ and O₂, which are undesirable reverse reactions in overall water splitting.