Multifunctional NiO/Ti3+–TiO2 for concurrent water reduction and glycerol oxidation to value added products by sunlight driven photocatalysis

Abstract

The present work describes the synthesis of bifunctional-mesoporous-self-doped Ti³⁺ containing NiO/TiO₂ photocatalysts for concurrent utilization of e⁻ and h⁺ to produce H₂ and value-added products (VAPs), respectively, from aqueous glycerol. UV-vis diffuse reflectance results and band gap analysis revealed an improved light absorption due to integration of Ni²⁺ with Ti³⁺/TiO₂. Various electrochemical, PL and TRPL spectral analyses demonstrate p–n heterojunction formation between NiO and Ti³⁺–TiO₂, which enhances charge separation and helps in achieving improved activity. HRTEM analysis of NiO/Ti³⁺–TiO₂ nanocomposites revealed that NiO is highly dispersed on TiO₂ with interfacial heterojunctions between them. XPS results demonstrate the partial reduction of Ti⁴⁺ to Ti³⁺ and Ni–Ti synergetic interaction in NiO/TiO₂ to form NiO/Ti³⁺–TiO₂ nanocomposites. EXAFS studies show that the Ni–O bond distance is similar to that of NiTiO₃ suggesting electronic integration of components of the photocatalyst by forming a Ni²⁺–O–Ti³⁺/Ti⁴⁺ lattice network. Ni²⁺/Ti³⁺–TiO₂ nanocomposites as a bifunctional photocatalyst exhibited significantly enhanced activity in H₂ production and conversion of glycerol to VAPs, namely, glycolaldehyde, 1,3-dihydroxyacetone, and formic acid; formation of these products highlights not only oxidation, but also C–C cleavage of glycerol. The NiO/Ti³⁺–TiO₂ photocatalysts fabricated in thin film form displayed higher photocatalytic efficiency than their powder counterpart. Among NiO/Ti³⁺–TiO₂ nanocomposites NiT-3 exhibits the highest H₂ yield at 15.62 mmol h⁻¹ g⁻¹, which is 38 times higher than that of bare TiO₂. The enhanced photocatalytic activity is ascribed to the high charge carrier density, the synergistic interaction between Ni²⁺ and Ti³⁺–TiO₂, formation of a p–n heterojunction at the interface between NiO and Ti³⁺–TiO₂ and effective utilization of charge carriers for redox reactions.