The NiO foam structures assembled from NiO nanoflakes with exposed {1 1 1} and {0 2 2} facets were successfully prepared by heating aqueous Ni(NO3)(2) solution containing polyvinyl pyrrolidone (PVP) at 450-650 degrees C. The NiO foam structures with exposed {1 1 1} facets obtained at 650 degrees C show significantly higher response values to ethanol, formaldehyde and triethylamine than NiO particles and NiO foam structures with exposed {0 2 2} facets obtained at 450 and 550 degrees C. The excellent sensing performances can be ascribed to the exposed Ni-NiO (1 1 1) crystal facets terminated with Ni atoms. We consider that the surface 3-coordinated Ni atoms function as the active sites for gas sensing reaction and present a new sensing mechanism. The 3-coordinated unsaturated Ni atoms can absorb O-2 molecules, generate free electrons and catalyze the chemical reaction between O-2 with test gas. The sensing mechanism of the 3-coordinated unsaturated Ni atoms functioning as the reaction active sites can deepen understanding of the sensing and other catalytic reactions and offers a guidance to design sensing and catalytic materials with excellent performances.