Facile preparation and green synthesis of Ni/BiVO₄ sensor for room temperature NH₃ sensing

Ammonia (NH₃), a toxic gas, requires highly sensitive detection even at low concentrations. In this study, nickel oxide nanoparticles (NiO NPs) were synthesized via a green chemistry approach using Tagetes patula leaf extract, with an optimal precursor-to-extract ratio of 1:4 determined by UV-Vis spectroscopy. Sunlight was identified as the most effective synthesis condition. Bismuth vanadate (BiVO₄) NPs were prepared via a solvothermal method, while the BiVO₄–NiO nanocomposite was fabricated using a hydrothermal approach. The materials were thoroughly characterized using energy-dispersive X-ray spectroscopy (EDX), UV-Vis spectroscopy, Fourier-transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and X-ray diffraction (XRD). The NiO–BiVO₄-based gas sensor exhibited superior sensitivity toward NH₃ compared to acetone and ethanol, with an exceptional response of 97% at 136 ppm. The sensor demonstrated rapid response and recovery times (20 s and 80 s, respectively), a low detection limit of 8 ppm, and outstanding repeatability, long-term stability, and selectivity. These results highlight the potential of the BiVO₄–NiO nanocomposite for advanced gas-sensing applications.