Co-precipitation synthesis, humidity sensing and photoluminescence properties of nanocrystalline Co2+ substituted zinc(II)molybdate (Zn1-xCoxMoO4; x = 0, 0.3, 0.5, 0.7, 1)

Zinc-cobalt molybdate composites (Zn_1-xCo_xMoO₄; x = 0, 0.3, 0.5, 0.7, 1) were synthesised by a simple co-precipitation method and characterised by thermogravimetric/differential thermal analysis (TG/DTA), Fourier transform-infrared (FT-IR), Fourier transform Raman (FT-Raman) spectroscopy, X-ray diffraction spectroscopy (XRD), scanning electron microscopy (SEM/EDAX) and transmission electron microscopy (TEM). The surface area was calculated by BET analysis in the adsorption/desorption isotherm. The humidity sensing properties of zinc-cobalt molybdates were tested by dc electrical measurements at different relative humidity environments (RH = 5-98%). The electrical resistance of the composites linearly decreases and the maximum sensitivity of 3672 ± 110 was observed for the Zn_0.3Co_0.7MoO₄ (ZnCM-4) composite towards humidity, which is calculated by the relation S_f = R_5%/R_98%, where the response time is 200 s and the recovery time is 100 s. Photoluminescence (PL) measurement at the room temperature of ZnM-1 composite exhibited a blue emission peak at 475 nm (λ_em) when excited at a wavelength (λ_ex) of 430 nm. During Co²⁺ substitution in Zn²⁺ matrix, a green and red emission peak was observed when excited at a wavelength (λ_ex) of 520 nm.