The sensing reaction on the Ni-NiO (111) surface at atomic and molecule level and migration of electron

NiO octahedrons with capped Cl− ions were synthesized by heating aqueous NiCl2 solution at 400 °C. The as-synthesized cubic phase NiO octahedrons are enclosed by {111} surfaces. For acetone, formaldehyde, triethylamine and ethanol, the NiO octahedrons show higher responses than NiO particles, and their responses are significantly increased by removing the surface Cl− ions and −OH groups. The Sr of the hydrogenated NiO octahedrons is 1.46, 1.50, 2.34 and 1.53 towards 50 ppm of acetone, formaldehyde, triethylamine and ethanol, respectively. The excellent sensing properties are attributed to the increased the quantity of 3-coordinated unsaturated Ni atoms on the Ni-NiO surface through removal of Cl− ions and −OH groups. The 3-coordinated unsaturated Ni atoms are considered to serve as a sensing reaction active site, and a gas sensing reaction mechanism is presented at the atomic and molecule level. The 3-coordinated unsaturated Ni atoms can absorb O2 molecules, generate free electrons and catalyze the reaction between the absorb O2 with the detected gas. The sensing mechanism of 3-coordinated unsaturated Ni atoms serving as the reaction active sites will be useful for the understanding of sensing and other catalytic reaction mechanisms as well as the design and fabrication of advanced gas sensing materials and catalysts.