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The ZnO foam structure assembled from nanoflakes with exposed {001} facets has been successfully prepared by heating aqueous Zn(NO3)2-polyvinyl pyrrolidone mixed solution. Response of the ZnO foam structure towards ethanol, acetone and n-Butylamine was observably increased by increasing the amounts of 3-coordinated Zn (Zn3c) atoms on the ZnO (001) surface through hydrogenation. The unsaturated Zn3c atoms are regarded as the reaction active sites and they contribute to generating free electrons, adsorbing oxygen molecules and catalyzing the reaction of the target gas with chemisorbed oxygen. The sensing mechanism will be instructive for the understanding of the gas sensing and heterogeneous catalytic reactions and the design of high performance sensing materials and catalysts. The hydrogenation optimizing surface-structural engineering may be a general strategy to enhancing sensing and catalytic properties.

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 °C. The NiO foam structures with exposed {1 1 1} facets obtained at 650 °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 °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 O2 molecules, generate free electrons and catalyze the chemical reaction between O2 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.

As a surface optimization strategy, hydrogenation has been successfully used to enhancing the responses of CdO octahedrons sensors to acetone, ethanol and triethylamine by increasing the numbers of 3-coordinate Cd (Cd3c) atoms on the CdO (111) surface. The Cd3c atom with three dangling bonds at Cd-CdO (111) surface is considered to be a reactive center for gas sensing, which has three functions: adsorbing oxygen from air, generating free electrons, and catalyzing the redox reaction of the target gas with adsorbed oxygen. The sensing mechanism of surface Cd3c atom acting as a reactive center can not only deepen understanding the mechanisms of gas sensing and heterogeneous catalytic reactions, but also provide a new insight for developing highly performance sensing and catalytic materials. In addition, hydrogenation may be a general effective strategy for improving sensing and catalytic performances of semiconductor materials.