Nanostructured NiO Thin Film for Ammonia Sensing at Elevated Temperatures.

Nanostructured NiO thin film was prepared by the sol–gel spin-coating technique and used for low-concentration ammonia (NH₃) detection at elevated temperatures. The x-ray diffraction (XRD) pattern confirms the cubic phase with a polycrystalline nature. Field-emission scanning electron microscopy (FESEM) shows uniform spherical-shaped grains and substantiates the evident porosity of the film. The film shows 94% of transmittance in the visible region and has an optical band gap of 3.64 eV. Hall measurement confirms the p-type conductivity at room temperature. X-ray photoelectron spectroscopy (XPS) reveals the presence and electronic states of nickel (Ni 2p_3/2, 2p_½) and oxygen (O 1s). Gas-sensing studies on the NiO film reveal that the response was comparatively deprived at low- (< 250°C), and high-temperature (> 250°C) regimes, demonstrating that the reaction directions are spontaneous, along with the rate of adsorption (and diffusion) being extremely slow, with the desorption process dominating the adsorption, resulting in a positive value of Gibbs free energy ( Δ G ). Especially, at a working temperature of 250°C, the film exhibits the limit of detection (LOD) at 0.2 ppm with a response of 1.91 × 10²%, and a maximum response of 4.01 × 10 3 % toward 5 ppm of NH₃ concentration. This remarkable response at low concentration is attributed to the smaller crystallite size and porosity. In addition, and more significantly, the working temperature reduces the depletion region between the grains. As a consequence, this lower the potential barrier and accelerates the diffusion rate. Hence, increasing the response. [ABSTRACT FROM AUTHOR]