Nanoporous and crystal evolution in nickel oxide nanosheets for enhanced gas-sensing performance

•Nanoporous and crystal evolution in nickel oxide nanosheets was investigated.•Correlation between the porous size, crystal size, defect level and gas sensing properties was clarified.•Effect of carrier gases on sensing characteristics was studied to determine the sensing mechanism.•The nanoporous NiO nanosheets are promising for H2 and H2S monitoring. Understanding the formation mechanism and control of crystal evolution of nanoporous materials is important in the synthesis of nanoporous materials for enhanced gas-sensing performance. Herein, single crystal nickel hydroxide nanosheets were prepared by hydrothermal reaction between nickel chlorine and ammonium hydroxide. Then, the products were subsequently calcined at 400, 500, 600, and 700 °C in air to form nanoporous NiO nanosheets. Evolution of the nanopores and crystal size of the nanoporous NiO nanosheets were investigated X-ray diffraction, thermogravimetric and differential thermal analysis, scanning electron microscopy, transmission electron microscopy, nitrogen adsorption/desorption isotherm and photoluminescence excitation spectroscopy, towards gas-sensing applications. Results showed that the calcination temperature altered the crystallinity, morphology, specific surface area, and the porous structure of the NiO nanosheets. Gas-sensing properties of the synthesized NiO nanosheets towards H2 and H2S were investigated to clarify the effect of material characteristics on gas-sensing performance. The NiO nanosheets calcined at 700 °C for 2 h exhibited the highest response to H2 despite having the largest crystal size and the lowest specific surface area. The response of the porous NiO nanosheet device to H2S gas in air and in nitrogen as carrier was also studied to determine the sensing mechanism.