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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]

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]

• The {001} faced PbTiO 3 nanosheets have higher sensitivity than PbTiO 3 powder. • The sensitivity of PbTiO 3 nanosheets is further enhanced by hydrogenation. • The exposed Pb-PbTiO 3 (001) plane was found to be gas sensing active surface. • The surface 8-coordinated Pb atom is evidenced to act as a sensing active site. • It adsorbs oxygen, generates free electrons and catalyzes the gas sensing reaction. PbTiO 3 (PT) nanosheets exposed with {001} crystal faces have prepared by using TiO 2 nanosheets having {001} facets as a Ti source. The PT nanosheets have higher response to acetone, formaldehyde, ethanol and triethylamine than the commercial PT particles. Their response can further be increased by increasing density of the unsaturated 8-coordinated Pb atoms at Pb-terminated PT (001) surface through hydrogenating. The unsaturated 8-coordinated Pb atoms on the (001) surface of PT are evidenced to act the sensing active sites, and an sensing mechanism at atomic level is presented. As an active atom, the 8-coordinated unsaturated Pb atom can adsorb oxygen from air, create electron and catalyze the gas sensing reaction. This mechanism can deepen comprehending of the gas sensing nature of metallic oxides. The surface unsaturated active metallic atom engineering may be a very effective general strategy to enhancing performances of metallic oxide sensing materials. [ABSTRACT FROM AUTHOR]

• Sensitivity of {111} faced MnO octahedron is signally increased by hydrogenation. • This results from an increase in the density of unsaturated Mn atoms on the surface. • The unsaturated Mn atoms are evidenced to serve as an active site for gas sensing. • They can absorb oxygen, generate free electrons and catalyze the sensing reaction. MnO octahedrons with exposed {111} crystal faces have been synthesized in a solvothermal system of Mn(Ac) 2 and C 2 H 5 OH at 200 °C for 48 h. Formation of the MnO octahedron results from the selective adsorption of -OCH 2 CH 3 and -OH groups on the (111) crystal plane. Responses of the MnO octahedrons towards acetone, triethylamine, ethanol and n-Butylamine are enhanced by increasing numbers of the unsaturated 2, 3 and 5-coordinated Mn atoms on the (111) surface by removing surface C 2 H 5 O- and -OH groups through hydrogenation. Thus, the exposed Mn-MnO (111) face is found to be a gas sensing active surface, the unsaturated 2, 3 and 5-coordinated Mn atoms on the (111) surface are evidenced to be the sensing reactive atoms and a detailed sensing mechanism is presented. The unsaturated active Mn atoms produce electrons, adsorb oxygen and catalyze the sensing reactions. The mechanism can deepen the understanding of the nature of oxide semiconductor gas sensing and is helpful to the design of high performance gas sensing materials. [ABSTRACT FROM AUTHOR]