Immobilization of Co, Mn, Ni and Fe oxide co-catalysts on TiO2 for photocatalytic water splitting reactions

Abstract

Here we report a systematic study of a series of non-noble-metal co-catalysts based on Co, Mn, Ni and Fe oxides that were prepared by wet impregnation of the corresponding acetylacetonate precursors onto a model TiO₂ substrate, followed by their oxidative decomposition. We analyze thermal evolution of the impregnated M(acac)_x–TiO₂ composites with a combination of analytical methods and reveal strong differences in the precursor decomposition onsets and the resulting product composition, compared to the case of pure M(acac)_x precursors. Consequent electron microscopy analyses of the resulting MO_x–TiO₂ composites indicate the presence of small (1–5 nm) amorphous MO_x nanoparticles that are homogeneously distributed on the surface of the substrate TiO₂. Complementing Raman and photoluminescence (PL) spectra confirm pronounced effects of MO_x deposition on the state of TiO₂ substrate and suggest strong electronic communication between the components. The composites obtained at 350 °C were further tested towards sacrificial hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) demonstrating the dynamic nature of the NiO_x–TiO₂ photocatalyst whose Ni⁰ active HER sites were generated in situ upon light exposure. In contrast, FeO_x–TiO₂, CoO_x–TiO₂, and NiO_x–TiO₂ were all active towards OER, featuring water oxidation ability in descending order, while XPS data of the samples after reaction indicate that partial oxidation of M species takes place during the course of the photocatalytic experiment. This work provides detailed insights on the wet chemistry-based preparation of MO_x co-catalysts decorating oxide nanopowders including optimization of the thermal treatment, potential substrate effects and synergy as well as further prospects in photocatalysis.