Nanocrystalline composites of transition metal molybdate (Ni 1–x Co x MoO 4 ; x = 0, 0.3, 0.5, 0.7, 1) synthesized by a co-precipitation method as humidity sensors and their photoluminescence properties

Abstract In this study, nanocrystalline transition metal nickel-cobalt molybdate (Ni 1–x Co x MoO 4 , NiCM; x  = 0, 0.3, 0.5, 0.7, 1) composites were prepared using a simple co-precipitation method. The composites were characterized by thermogravimetric/differential thermal analysis, Fourier transform-infrared spectroscopy, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The NiCM composites were studied to determine their possible use as humidity sensors, and photoluminescence (PL) measurements were obtained. The sensing study was performed in environments with different relative humidity levels (5–98%). The maximum sensitivity of 18624 ± 168 was observed with the Ni 0.7 Co 0.3 MoO 4 composite where the humidity could be calculated according to the relationship: S f  = R 5% /R 98% , where R 5% and R 98% are the dc resistances at 5 and 98% RH, respectively. The photoluminescence measurements acquired at room temperature for the NiCMs included green and red emission peaks when excited at a wavelength ( λ ex ) of 520 nm.