g-C₃N₄/dendritic fibrous nanosilica doped with potassium for photocatalytic CO₂ reduction

Abstract

Photocatalytic CO₂ conversion is a promising process for the reduction of CO₂ into useful chemicals and fuels using solar energy. In this work, we have synthesized potassium-doped g-C₃N₄ coated over dendritic fibrous nanosilica (K-CN/DFNS) and studied the photocatalytic CO₂ conversion under visible light illumination. K-CN/DFNS was synthesized by heating KOH-treated melamine in a vacuum-sealed quartz tube by varying the amounts of potassium salt. The scanning electron microscopy images confirmed the coating of potassium-doped g-C₃N₄ over DFNS, while the absorption spectra show extended absorption in the visible region and redshift in the bandgap as compared to the undoped CN/DFNS. XPS analysis further confirms the interaction of potassium ions with g-C₃N₄. During the photocatalytic CO₂ conversion, the selectivity towards the formation of hydrogen (from water splitting) was the highest for the g-C₃N₄ and CN/DFNS catalysts, whereas no hydrogen generation was observed using the potassium-doped catalysts. The highest methane yield observed was 1.7 μmol g⁻¹ for g-C₃N₄ in 4 h using K-CN/DFNS-2 (with 6 wt% of potassium ions). The high activity and improved selectivity of K-CN/DFNS were due to the extended absorption in the visible region, better separation of charge carriers, shifting of the conduction band and valence band positions towards positive potentials, and moderate adsorption of CO₂ molecules on its surface.