Effect of sodium doping in NiO/Vermiculite composite on photocatalytic hydrogen production from methanol-water decomposition

Abstract

This study investigates the efficiency of sodium-doped NiO/vermiculite (Vm) composites as photocatalysts for hydrogen production via methanol-water decomposition under UV irradiation. Using natural vermiculite as a support, NiO was introduced as a p-type semiconductor, and sodium doping was achieved using sodium hydroxide (NaOH) or sodium nitrate (NaNO₃). Three synthesis methods - dry synthesis by milling, capillary impregnation, and wet impregnation were investigated for their influence on the structural, textural, optical, and electrical properties and the photocatalytic activity. Characterization techniques, including X-ray fluorescence, X-ray diffraction, atomic absorption spectrometry, photoluminescence, scanning electron microscopy with energy dispersive X-ray spectroscopy confirmed successful incorporation of NiO and sodium into the vermiculite matrix. Photocatalytic tests demonstrated that sodium doping enhances the stability and activity of the photocatalysts by reducing electron-hole recombination rates, with NaOH proving to be a more effective sodium source than NaNO₃. Among the samples, those synthesized via capillary impregnation (NiO(OH)/Vm-C) and dry synthesis (NiO(OH)/Vm-M) showed the highest hydrogen yields (550 and 540 μmol/g cat., respectively) due to optimal crystallite size (∼22–23 nm) and defect-induced charge transfer efficiency. This is the first study to systematically investigate the role of sodium doping in NiO/clay-based photocatalysts and to reveal clear structure–property–activity correlations based on synthesis method and dopant type. The findings highlight the potential of Na-doped NiO/Vm composites as cost-effective and scalable photocatalysts for hydrogen production. The insights gained here lay the foundation for further development of layered, clay-supported photocatalysts beyond conventional oxide systems.