Effect of deposition temperature on topography and electrochemical water oxidation of NiO thin films

Abstract

The deposition temperature plays a significant role in controlling the crystallinity, morphology, and texture of thin films. Herein, we report the fabrication of nickel oxide thin films on a fluorine-doped tin oxide-coated glass substrate via a single-step aerosol-assisted chemical vapor deposition technique. Variable temperatures from 450 to 525 °C were used for the fabrication of these NiO thin films. The crystalline structure of nickel oxide thin films was determined by X-ray diffraction which showed cubic structure having space group Fm3m, and unit cell lattice parameters of a = b = c = 4.17 Å that crystallize preferentially along (200) plane. While the Raman spectroscopic study revealed the presence of peaks corresponding to nickel oxide structure. The surface chemical composition and oxidation states were investigated by X-ray photoelectron spectroscopy. Moreover, field-emission scanning electron microscopy aided in examining the temperature-dependent topography and microstructure of films. It was observed that when deposition temperature is increased from 450 to 525 °C, the bandgap energy decreased from 3.59 to 3.41 eV. The electrocatalytic activity for the oxygen evolution reaction was performed under alkaline conditions, using linear sweep voltammetry and cyclic voltammetry at different scan rates. Moreover, ohmic drop and charge transfer resistance are estimated by electrochemical impedance spectroscopy. The nickel oxide deposited at 500 °C showed the highest current density and the earliest onset overpotential as compared with other samples at different temperatures. Morphology-dependent catalytic activity is attributed to substrate temperature which significantly affects the growth of nickel oxide. Cyclic voltammetry reveals the Ni redox feature due to the active phase of nickel (oxy)hydroxide (NiOOH) and there was no significant current until 1.5 VRHE. Measurements of activity as a function of substrate temperature were consistent with the hypothesis that different morphology of the electrode exerts different charge transfer activation on nickel oxide. These results suggested that electrocatalysts coupled with controlled morphology can be tuned in terms of high performance, commercial applicability, and water-splitting devices.