Structural, optical and humidity sensing studies of nickel oxide (NiO) nanoparticles: effect of calcination temperature

Herein, nickel oxide (NiO) nanoparticles have been synthesized through a simple co-precipitation route. The obtained samples were calcinated at two different temperatures (500 °C and 900 °C) using a muffle furnace. The structural, morphological, and optical properties of nanoparticles were examined by X-ray diffraction, Scanning electron microscopy, Energy dispersive X-ray, Fourier infrared spectroscopy and UV–Vis absorption spectroscopy. The resistive type humidity sensing behavior was studied through a locally fabricated experimental setup. The XRD pattern of the samples revealed that the as-prepared sample appeared in the β-Ni(OH)₂ hexagonal phase, and the samples calcinated at 500 °C and 900 °C, are crystallized in the cubic structure of NiO phase. The formation of highly agglomerated and irregular shaped particles was noticed form SEM images. FTIR analysis confirmed the formation of β-Ni(OH)₂ and pure NiO phases, which is supported by EDX analysis. UV–Vis absorption studies showed a red shift of absorption edge and a varying band gap of NiO nanoparticles with calcination temperature due to the crystal growth and increase in crystallinity of samples. The sample calcinated at 900 °C has shown a humidity sensitivity of 0.3 K-cm/% RH, a limit of detection of 7% RH and good linearity with a correlation co-efficient of 0.995.