Hydrothermal temperature-driven structural, morphological and optical evolution of nanostructured LaNiO3 perovskite thick films toward highly selective and rapid CO gas sensing applications

Nanostructured LaNiO 3 thick films were synthesized by a hydrothermal route at 90–180 °C and subsequently fabricated by screen printing for CO gas sensing. X-ray diffraction identified the LaNiO 3 perovskite phase, represented in the rhombohedral R 3 ¯ c structure using the hexagonal setting, with the XRD-derived crystallite size decreasing from 23.26 to 17.47 nm as the hydrothermal synthesis temperature increased. FE-SEM revealed a progressively interconnected porous morphology, while BET analysis showed an increase in measured specific surface area from 4.7690 to 5.6595 m 2 g −1 . UV–Vis diffuse-reflectance measurements exhibited a systematic shift in the Tauc-derived effective optical absorption edge from 2.32 to 2.70 eV. Considering the correlated-metallic character reported for LaNiO 3 , these values are interpreted as comparative effective optical absorption-edge energies rather than definitive intrinsic band-gap values. XPS revealed temperature-dependent changes in the Ni 2p and O 1 s surface chemical environments, while FTIR, TEM, HRTEM and EDS provided complementary information on chemical bonding, morphology, lattice characteristics and elemental composition. The LaNiO 3 @180 thick-film sensor exhibited the highest measured CO response among the investigated samples, reaching 88.76% at 800 ppm CO at an operating temperature of 150 °C, with response and recovery times of 13 and 28 s, respectively. CO sensing was experimentally investigated from 10 to 1200 ppm, with a measurable response of 1.15% at 10 ppm and a linear calibration range of 10–100 ppm (R 2 = 0.99276, slope = 0.3197% ppm −1 ). A formal limit of detection was not determined because the baseline-noise parameters required for statistical LOD analysis were not obtained. The optimized sensor exhibited preferential response toward CO relative to the experimentally investigated CO 2 , HFC-134a and water-vapour interferents, together with good short-term repeatability of 88.76 ± 0.02% over five consecutive cycles and stable sensing performance during the eight-week evaluation. The observed sensing behaviour is correlated with the combined evolution of crystallite size, interconnected porous morphology, measured surface accessibility and temperature-dependent surface chemical environment. These results demonstrate the influence of hydrothermal synthesis temperature on the structural, textural, surface-chemical and CO-sensing characteristics of LaNiO 3 thick films.