Electrochemically-synthesized tungstate nanocomposites γ-WO3/CuWO4 and γ-WO3/NiWO4 thin films with improved band gap and photoactivity for solar-driven photoelectrochemical water oxidation

The main aim of this study was to synthesize and characterise tungstate (WO3) nanocomposites with its metal-based nanostructures, such as copper (II) tungstate (CuWO4) and nickel tungsten oxide (NiWO4), as visible-light active thin film photoanodes for solar-driven photoelectrochemical (PEC) water oxidation. FE-SEM and AFM results showed that the bare as-deposited WO3 films were transformed into polycrystalline WO3 structure with highly agglomerated surfaces and roughness during the annealing-induced crystallisation process. XRD results suggested that the bare as-deposited WO3 films undergone phase transformation process from amorphous to the photoactive monoclinic-I (gamma-WO3) at 550 degrees C. XPS results indicated the existence of WO42-, Ni2+ and Cu2+ ions at 35.58 eV, 856 eV and 932.4 eV, respectively. Through the formation of WO3 nanocomposites, the energy band gap was effectively lowered from 2.7 eV (gamma-WO3) -> 2.3 eV (gamma-WO3/CuWO4) -> 2.1 eV (gamma-WO3/NiWO4) as estimated from the UV-Vis spectra. Finally, the corresponding photoactivity of WO3 nanocomposites was estimated by measuring the photocurrent density and gamma-WO3/NiWO4 nanocomposite structure was found to give the highest photocurrent density of 400 mu A/cm(2) at 1.5 V vs Ag/AgCl (4 M KCl). (C) 2018 Elsevier B.V. All rights reserved.