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Cu0.27Co2.73O4 nanooctahedrons enclosed by polar {111} planes have been prepared through the selective adsorption of Cl-. Hydrogenation has been successfully used to enhance the responses of the Cu0.27Co2.73O4 nanooctahedron sensors to acetone, ethanol, and n-butylamine. The enhancement of the response results from the increase in the number of 3-coordinated Co/Cu atoms (Co3c/Cu3c) at the (111) plane of Cu0.27Co2.73O4 through removing O-H groups and Cl- ions at the surface by hydrogenation. The Co3c/Cu3c atoms on the (111) plane of Cu0.27Co2.73O4 are considered to function as the gas response active centers. These Co3c/Cu3c active atoms have three functions: generating electrons, adsorbing oxygen from air, and catalyzing the sensing reactions. The hydrogenation polar surface approach can be applied to improve the performances of other sensing materials. Such sensing mechanisms of the Co3c/Cu3c unsaturated atoms as the active centers can be conducive to understanding the gas-sensing essence and the development of sensing materials with high performances.

Network structures assembled from α-Fe2O3 nanosheets with exposed {104} facets were successfully prepared by heating Fe(NO3)3 solution containing polyvinylpyrrolidone (PVP) in air. The α-Fe2O3 nanosheet-based network structures demonstrate significantly higher response to ethanol and triethylamine than α-Fe2O3 commercial powders. The excellent sensing performances can be ascribed to the exposed (104) facet terminated with Fe atoms. A concept of the unsaturated Fe atoms serving as the sensing reaction active sites is thus proposed, and the sensing reaction mechanism is described at the atomic and molecular level for the first time in detail. The concept of the surface metal atoms with dangling bonds serving as active sites can deepen understanding of the sensing and other catalytic reaction mechanisms and provides new insight into the design and fabrication of highly efficient sensing materials, catalysts, and photoelectronic devices.

Hydrogenation is successfully employed to improve sensing performances of the gas sensors based on TiO2 nanosheets with exposed {001} facets for the first time. The hydrogenated TiO2 nanosheets show a significantly higher response toward ethanol, acetone, triethylamine, or formaldehyde than the samples without hydrogenation, and the response further increases with an increase of the hydrogenation temperature. The excellent sensing performances are ascribed to an increase of the density of unsaturated Ti5c atoms on the {001} surface resulting from the hydrogenation process. The unsaturated Ti5c atoms are considered to serve as sensing reaction active sites. They can generate noncontributing (free) electrons and adsorb oxygen molecules, and the detailed sensing mechanism is described at atomic and molecule level. The hydrogenated strategy may be employed to enhance the sensing performances of other metal oxide sensors and catalytic reaction activities of catalyst. The concept of the surface unsaturated meta...