Synthesis, characterization, and photocatalytic properties of Au nanoparticle-anchored TiO2 and CaCO3 core–shell composite particles

Abstract

TiO₂-based photocatalysts have garnered considerable attention, yet a deeper understanding of their photoreduction mechanism is essential for improved material design and application. In this study, core–shell composites were synthesised by coating CaCO₃, with TiO₂ via a sol-gel method using titanium tetraisopropoxide. Gold nanoparticles (AuNPs) were subsequently anchored onto the composite surface. The CaCO₃@TiO₂ composite was prepared by precipitating CaCO₃ on TiO₂ using a carbonation method in which CO₂ was passed through a saturated aqueous solution of Ca(OH)₂ containing TiO₂. The AuNPs were anchored to the surfaces of the core–shell particles via precipitation using HAuCl₄ · 4H₂O as the precursor. Various physicochemical characterizations were performed on the synthesised photocatalysts. The photocatalytically produced amounts of CH₄ and CO were evaluated. As compared to AuNP/TiO₂@CaCO₃, AuNP/CaCO₃@TiO₂ produced 17 times more CH₄ and 1.4 times more CO via CO₂ reduction. The obtained results suggest that multi-electron reduction in AuNP/CaCO₃@TiO₂ to produce CH₄ occurs near CaCO₃, which is an insulator with CO₂ adsorption ability. Furthermore, the amount of CH₄ produced using AuNP/CaCO₃@TiO₂ was 1.1 times higher than that produced by a reported catalyst, which was prepared by physically mixing TiO₂ and CaCO₃ in the same ratio as that in AuNP/CaCO₃@TiO₂; the dissimilarity in the chemical structures of the physically mixed and precipitated photocatalysts was held responsible for the observed differences in their photocatalytic efficiencies. This study advances the existing knowledge about the mechanism of photocatalytic reduction reactions using TiO₂-based materials.