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Co3O4 octahedrons enclosed by {111} facets were synthesized using Cl− ions as a crystal facet control agent. The Co3O4 octahedrons show higher responses than Co3O4 commercial particles to ethanol, methanol, acetone and triethylamine, and the responses are further enhanced by increasing the numbers of 3-coordinated Co atoms (Co3c) with one dangling bond at Co–Co3O4 (111) plane by eliminating surface Cl− ions and OH groups. Accordingly, we deem that the Co3c atom at (111) plane acts as an active center for sensing reaction and present an atomic and molecular level sensing mechanism. The Co3c atoms can produce free electrons, absorb O2 molecules and catalyze the gas-sensing reactions. The surface engineering may be used to bolster gas-sensing properties of other metallic oxides. The concept of Co3c atom as an active center will contribute to understand the true nature of the gas sensing and provide guidance on designing highly performance sensing materials. [ABSTRACT FROM AUTHOR]

The hollow microspheres assembled with ZnO nanoflakes exposing {001} facets have been successfully synthesized by annealing ZnS nanospheres at 550 °C for 2 h. Response of the hollow microspheres assembled from ZnO nanoflakes for ethanol, acetone, or triethylamine is significantly higher than hollow microspheres assembled from ZnO nanorod with dominant {100} facets. The exposed Zn-terminated (001) face is considered to be an active surface for gas sensing. Threefold coordinated Zn (Zn3c) atom with a dangling bond on the Zn-terminated (001) surface serves as a gas-sensing active atom, and thus a gas-sensing mechanism at atomic and molecular level is proposed. The concept of unsaturated Zn3c atoms as a gas-sensing active atoms will be useful for designing of high-performance sensing materials. [ABSTRACT FROM AUTHOR]

We demonstrated a new approach to increasing response of Fe 2 O 3 nanoparticles and sea urchin-like nanostructures towards methanol, ethanol, formaldehyde, acetone, triethlamine and n-Butylamine by increasing number of surface Fe atoms having dangling bonds through hydrogenation. The surface Fe atoms having dangling bonds are believed to serve as the reactive atoms for sensing reaction, thus, a gas sensing mechanism at atomic and molecular level is presented for the first time. The surface unsaturated active Fe atoms can adsorb oxygen molecules, produce electrons and catalyze the chemical reaction of the adsorbed oxygen with target gas. Hydrogenation may be a universal approach to increasing gas sensing performances of metal oxides. The gas sensing mechanism can deepen comprehending electron transfer during the gas sensing process and offer guidance to design the gas sensing materials with higher performances. • Hydrogenation is successfully used to improve the sensitivity of α-Fe 2 O 3 gas sensors. • Density of unsaturated Fe atoms at the surface is increased by hydrogenation. • The unsaturated Fe atoms are regarded to act as an active site for sensing reaction. • They generate free electrons, adsorb oxygen and catalyze the gas sensing reaction. • Hydrogenation may be used to improve gas sensing properties of other metal oxides. [ABSTRACT FROM AUTHOR]

Abstract The ZnO foam structure assembled from nanoflakes with exposed {001} facets has been successfully prepared by heating aqueous Zn(NO 3) 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 (Zn 3c) atoms on the ZnO (001) surface through hydrogenation. The unsaturated Zn 3c 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. Highlights • ZnO foam structure with exposed {001} facets were synthesized. • The enhanced gas-sensing property is attributed to the exposed Zn-ZnO (001) facet. • The unsaturated Zn atom at the (001) facet serves as a sensing reaction active site. • A sensing reaction mechanism at molecule level is proposed. [ABSTRACT FROM AUTHOR]

• Sensitivity of {111} faced Cu 2 O octahedron is signally enhanced by hydrogenation. • Density of 1-coordinated Cu atoms at (111) surface is increased by hydrogenation. • The 1-coordinated Cu atoms are regarded to function as the sensing reactive centers. • They can adsorb O 2 , generate free electrons and catalyze the sensing reaction. • The hydrogenation may be a universal strategy for improving sensing properties. Sensitivity of the Cu 2 O octahedral structures towards ethanol, acetone, triethylamine and formaldehyde has been significantly enhanced by increasing the numbers of 1-coordinated Cu (Cu 1c) atoms on the (111) facet of Cu 2 O octahedrons through hydrogenation. The Cu 1c atom with one dangling bond is considered to be a reaction active center. The active center has three kinds of functions: adsorbing oxygen, creating free electrons and catalyzing the redox reaction between the adsorbed oxygen and the detected gas. The findings can deepen comprehending gas sensing mechanisms and give guidance to design high performance sensing material. In addition, the hydrogenation surface-engineering strategy can be employed to enhance sensing performances of other materials. [ABSTRACT FROM AUTHOR]