Preparation of Core–Shell-Structured Nanoparticles (with a Noble-Metal or Metal Oxide Core and a Chromia Shell) and Their Application in Water Splitting by Means of Visible Light

摘要

Core–shell-structured nanoparticles, consisting of a noble metal or metal oxide core and a chromia (Cr₂O₃) shell, were studied as promoters for photocatalytic water splitting under visible light. Core nanoparticles were loaded by impregnation, adsorption or photodeposition onto a solid solution of gallium nitride and zinc oxide (abbreviated GaN:ZnO), which is a particulate semiconductor photocatalyst with a band gap of approximately 2.7 eV, and a Cr₂O₃ shell was formed by photodeposition using a K₂CrO₄ precursor. Photodeposition of Cr₂O₃ on GaN:ZnO modified with a noble metal (Rh, Pd and Pt) or metal oxide (NiO_x, RuO₂ and Rh₂O₃) co-catalyst resulted in enhanced photocatalytic activity for overall water splitting under visible light (λ400 nm). This enhancement in activity was primarily due to the suppression of undesirable reverse reactions (H₂–O₂ recombination and/or O₂ photoreduction) and/or protection of the core component from chemical corrosion, depending on the core type. Among the core materials examined, Rh species exhibited relatively high performance for this application. The activity for visible-light water splitting on GaN:ZnO modified with an Rh/Cr₂O₃ core–shell configuration was dependent on both the dispersion of Rh nanoparticles and the valence state. In addition, the morphology of the Cr₂O₃ photodeposits was significantly affected by the valence state of Rh and the pH at which the photoreduction of K₂CrO₄ was conducted. When a sufficient amount of K₂CrO₄ was used as the precursor and the solution pH ranged from 3 to 7.5, Cr₂O₃ was successfully formed with a constant shell thickness (≈2 nm) on metallic Rh nanoparticles, which resulted in an effective promoter for overall water splitting.