Microstructural, optical, luminescent and humidity sensing properties of NiO thin films prepared by sol-gel spin coating technique

The sol-gel spin coating method was used to prepare NiO thin films on glass substrates at different rotational speeds (3000–4000 rpm). The NiO thin films prepared at 3000 rpm show uneven surface morphology. On the other hand, NiO films prepared at 3500 and 4000 rpm are more even, with smaller nanoparticles and a better composition ratio. Structural analysis shows that the film prepared at 3000 rpm is poorly crystalline, and 3500 and 4000 rpm show a cubic structure. Films deposited at higher rpm show preferential crystallinity along the (200) plane. Crystallite size reduces from 57 to 25 nm upon the increase of spin speed. Optical measurements show that the deposited films are highly transparent with a maximum transmittance value of 98%. The direct optical band gap increases slightly from 3.65 to 3.69 eV, the indirect band gap increases from 3.10 to 3.34 eV, and the Urbach energy decreases from 0.36 to 0.22 eV as the spin speed increases. Hall effect measurements reveal the p-type conductivity of the films with the carrier concentration in the order of 10 18 cm −3 . NiO thin film prepared at 4000 rpm showed maximum carrier concentration and conductivity with the lowest carrier mobility and Hall coefficient. Photoluminescence study shows strong near-band-edge emission and visible emissions related to intrinsic defects. Humidity sensing study reveals that the film prepared at 4000 rpm exhibits the highest sensitivity and the least response and recovery times at room temperature.