Visible-light photocatalytic activity of nitrogen-doped NiTiO3 thin films prepared by a co-sputtering process

Abstract

Nickel titanate (NiTiO₃) thin films were grown by a radio frequency magnetron co-sputtering process using metal (Ni and Ti) targets on fused quartz substrates at a substrate temperature of 400 °C. Annealing of as-deposited (amorphous) films was performed at 1100 °C for 2 hours to realize a stable crystalline phase. The effect of the Ti target power (200 and 250 W) and nitrogen doping on the structural, morphological and optical properties of post-annealed NiTiO₃ thin films were investigated besides photocatalytic activity under visible light irradiation. X-ray diffraction measurement on the films revealed a pure ilmenite phase at 250 W Ti power. Preferential orientation changed from [104] to [110] as Ti power increased from 200 to 250 W. Raman studies on NiTiO₃ thin films showed almost all the active modes (5A_g + 5E_g) of a crystalline structure. Two different microstructures were observed by scanning electron microscopy, films showed rounded (250 nm) grains at 200 W Ti target power while facet forms (500 nm) develop in the films deposited at 250 W. Chemical bonding and valence states of the involved ions such as Ni 2p, Ti 2p and O 1s were investigated by X-ray photoelectron spectroscopy. Nitrogen doping modifies the rms roughness from 12 nm to 17 nm as demonstrated on 200 W grown films and contributes also to modify the indirect optical band gap from 2.50 to 2.43 eV in films obtained at 250 W Ti target power. As a crucial role of nitrogen doping, photocatalytic activity in a broad visible light range was observed with a good efficiency for the degradation of methylene blue by nitrogen doped NiTiO₃ thin films.