Evolution of hydrogen gas sensing properties of sol–gel derived nickel oxide thin film

Hydrogen sensing properties of nanocrystalline NiO thin films are investigated. NiO thin films were fabricated by a sol–gel method with special efforts to change the porosity of coated films by a multi-step coating and annealing processes. The experimental results indicated that the multi-step annealed NiO thin films had a higher porosity than films fabricated by a typical sol–gel process. When the thicknesses of NiO films were increased, the grain size increased and the sheet resistance of the film decreased. The highest gas response was detected at 175 °C for two layered samples. NiO films fabricated by multi-step annealing showed a better response to hydrogen than NiO fabricated by a typical sol–gel process. The H2 gas response was decreased with increasing the thickness and decreasing the porosity of NiO films. A high response value of 68% was obtained for 3000 ppm of H2 at 175 °C for optimized sample by a multiple annealing process. The effect of humidity on the gas sensing performance of the sensor was studied and the devices were also tested for several cycles to study the repeatability of the sensors. The cross sensitivity measurements showed that the sensor could be used to monitor hydrogen in a likelihood mixture of reducing gases.