Growth and physical investigations on NiWO4 thin films as a potential for NO2 sensing

Abstract

In this work, nickel tungstate oxide (NiWO4) based thin films of 200 nm as thickness have been successfully deposited onto conductive Fluorine doped tin oxide (FTO) substrates using a simple and low cost sol gel method via spin coating with interests in gas sensor applications. The elaborated NiWO4 thin films have been investigated in terms of its structural, morphological and optical properties by using X-ray diffractometer, Raman scattering spectroscopy, scanning electron microscopy (SEM) and UV–VIS spectrophotometer. X-ray diffraction (XRD) analysis shows a polycrystalline nature of these films with a crystallite size close to 80 nm. The polycrystalline nature of these films was confirmed by Raman spectroscopy measurement, which depicts the presence of wolframite NiWO4. In addition, the surfaces topographies displayed uniform surfaces in shape and randomly disturbed by particles with sizes of the order of 50 nm. The optical properties of the NiWO4 depicting the presence of a direct band gap of 2.77 eV. Meantime, Charge transfer kinetic processes at the NiWO4/electrolyte interface were identified by electrochemical impedance spectroscopy at different bias values. This interface adopt an equivalent circuit consisting of a parallel charge transfer resistance RCt and a constant phase element (CPE) in series with an electrolyte resistance RS at an applied bias of 1.2 V. The electron lifetime(τe) was calculated to be 2.4 ms. Finally, Nitrogen dioxide (NO2) sensing performance of the films has been carried out and discussed at low operating temperatures varying from 170 °C to 260 °C.