A High-Performance NO ₂ Sensor Based on NiMoO ₄ Nanorods: Influence of Oxygen Vacancies and Ni ²⁺ /Ni ³⁺ Redox Dynamics

Modulating oxygen vacancies is a highly effective strategy for enhancing the gas-sensing properties of metal oxides. In this study, NiMoO 4 nanorods were synthesized via a facile hydrothermal technique by varying the reaction time. Structural and morphological analyses confirmed the phase formation of NiMoO 4 with a nanorod-like morphology. XPS analysis revealed the coexistence of Ni 2+ and Ni 3+ oxidation states along with the presence of oxygen vacancies, both of which play a pivotal role in tuning the redox activity and conductivity of the fabricated sensor. Among the synthesized samples, the NMo-6 sensor exhibited a good sensing response of 244% to 10 ppm NO 2 at 100 °C. This superior performance is attributed to the higher concentration of oxygen vacancies which enhance surface reactivity and promote efficient charge transfer between the sensor surface and the adsorbed gas molecules. The sensor also demonstrated excellent selectivity, repeatability, and long-term stability over 60 days, with a low detection limit of 0.67 ppm. These findings emphasize that precise optimization of oxygen vacancy concentration and tailoring the oxidation states in NiMoO 4 can substantially enhance gas-sensing efficiency, making it a promising candidate for high-performance NO 2 detection.