Structural, optical and electrical characterization of hydrothermally synthesized V2O5-NiO nanocomposite thick films for NO2 sensing

Abstract

In this work V2O5–NiO nanocomposites were synthesized by a hydrothermal method with varying V2O5 concentrations (0.1, 0.3, 0.5 and 0.7 N) and fabricated into thick films using screen printing. Structural analysis by XRD confirmed the cubic phase of NiO and orthorhombic V2O5, with crystallite size decreasing from 9.19 to 5.99 nm as V2O5 content increased. SEM micrographs revealed a transition from loosely packed to densely aggregated particles with enhanced porosity, while EDS verified progressive vanadium incorporation and oxygen deficiency at higher doping levels. HR-TEM analysis confirmed nanocrystalline features with distinct lattice fringes, and SAED patterns indicated polycrystalline nature. FTIR spectra showed characteristic Ni–O and V = O vibrations, whereas UV–Vis absorption revealed a red shift with increased doping, accompanied by band gap narrowing from 5.47 to 4.81 eV. XPS confirmed the presence of Ni, V, and O elements, successful incorporation of V into the NiO lattice, and an increased concentration of oxygen vacancies and surface-adsorbed oxygen species, providing abundant active sites for gas adsorption. Electrical measurements indicated semiconducting behavior with negative TCR values, higher resistivity at increased doping, and reduced activation energy (0.0779 eV at HTR and 0.1209 eV at LTR for 0.7 N films). Gas sensing evaluation demonstrated that V2O5 doping improved NO2 detection performance, with the 0.7 N film exhibiting maximum sensitivity of 88.79% (600 ppm at 150 °C), rapid response (10 s), recovery (57 s), excellent selectivity, and reproducibility over six weeks. These results establish the correlation between structural modifications, defect states, and enhanced gas sensing efficiency in V2O5–NiO nanocomposites.