Impact of Surface Faceting on Gas Sensing Selectivity of NiO: Revealing the Adsorption Sites of Organic Vapors on the {111} Facet

Air pollution by volatile organic compounds (VOCs) has been responsible for the decline of human health and environmental quality. NiO-based materials hold great promise as high-performance sensing materials to monitor the VOC concentration in air. The present research focuses on the design of NiO nanocrystals and the investigation of the effect of surface-terminated facets on their VOC gas sensing selectivity. NiO nanoparticles (NPs) with no specific surface facet exhibited a poor responsivity and selectivity toward VOC detection at all observed working temperatures, while the octahedral morphology of NiO exposed to the {111} facet demonstrated a better VOC sensing responsivity (Rg/Ra = 6.2) and high selectivity toward toluene at 250 °C. Owing to the terminated Ni metal as the highly reactive adsorption site, the octahedral morphology of NiO with the {111} facet exhibited an improved sensing and selectivity performance. Based on the experimental evidence, an ab initio DFT calculation is performed to investigate the analyte adsorption on the {111} facet of NiO and explain the high toluene selectivity on this facet. It is revealed that the {111} facet is energetically favorable for toluene adsorption, with an energy adsorption (Eads) of −3.927 eV at the hollow HCP site. This study has a significant potentiality to be extended to many surface phenomena such as sensors, catalysis, and energy storage.