Preparation Of ZnO / ZnS Nanometer Materials Exposed To {001} High Energy Surface And Its Enhanced Photocatalytic And Sensitive Properties

ZnO nanocones, platform-shaped nanoarchitectures, variable thickness nanosheets, porous hexagonal sheet-like structures, porous ZnO nanospheres assembled with nanosheets and ZnS nanospheres were synthesized through solvothermal or hydrothermal routes. Morphologies and structures of these products were characterized by means of scanning electron microscopy, X-ray diffraction, transmission electron microscopy and IR spectrum. The processes of growth of ZnO and ZnS nanostructures in various geometrical morphologies were studied, and the possible formation mechanisms of ZnO and ZnS nanostructures were proposed. The photocatalytic behaviors and gas-sensing properties of as-obtained ZnO and ZnS nanostructures with various morphologies and exposed facets were studied, and the relations between morphology, size, exposed facets and properties were investigated. These research results would provide a base for further research on synthesis of ZnO and ZnS nanostructures, the physical and chemical properties of ZnO and ZnS nanostructures and their applications in photonic and electronic devices, solar sensor and photocatalyst.(1) ZnO nanocrystals with various (002) orientations were synthesized via a solvothermal reaction of zinc acetate with n-butylamine and tetrahydrofuran at140℃for12h by varying the molar ratio of n-butylamine to Zn(II). The formation of ZnO nanocrystals with various (002) orientations and shapes results from the selective adsorption of different amount of n-butylamine molecules on ZnO (001) surface. Photocatalytic activity of the as-prepared ZnO nanocrystals with various (002) orientations in degradation of methyl orange was studied, it was found that the exposed {001} facets are reactive facets. The structure and atomic charge distribution of the {001} facets were studied by periodic density functional theory calculations. Based on polar structure of the exposed{001} surfaces, a charge separation model between polar ZnO{001} surfaces was proposed. There is an internal electric field between positive Zn-ZnO (001) and negative O-ZnO (001) planes due to the spontaneous polarization. The internal electric field provides the driving force for charge separation. The reduction and oxidation reactions take place on the positive (001) and negative (001) polar planes, respectively. The charge separation model can deepen understanding of the{001} facets of ZnO exhibit enhanced photocatalytic property.(2) Porous ZnO nanosheets with exposed{001} facets coated on Zn foils have been synthesized by annealing sheet-like precursor Zns (OH)8Cl2·H2O-Zn20Cl2-2H20grown on Zn foils at400℃for60min. The precursors were synthesized by heating ZnCl2solution on zinc foil at340℃for60min. Photocatalytic activity of porous ZnO nanosheets in degradation of methyl orange was studied, and the results indicate that the porous ZnO nanosheets show superior photoreactivity, compared to ZnO nanorods as a benchmarking material. The superior photocatalytic activity is attributed to the exposed {001} facets and a high specific surface area. The exposed{001} facets were found to be polar surfaces by periodic density functional theory calculations, and thus a charge separation model between polar ZnO{001} surfaces was proposed. There is an internal electric field between positive Zn-ZnO (001) and negative O-ZnO (001) surfaces due to the spontaneous polarization. The internal electric field provides the driving force for charge separation. The reduction and oxidation reactions take place on the positive (001) and negative (001) polar planes, respectively. The charge separation model can deepen understanding of charge transfer in the semiconductor nanocrystal with highly photocatalytic activities and offer guidance to design more effective photocatalysts as well as new type of solar cells, photoelectrodes or photoelectric devices.(3) ZnS nanocrystals with various (002) orientations were synthesized via a hydrothermal reaction of zinc nitrate-thiourea-poly (N-vinyl-2-pyrrolidone)-ethylene glycol-H2O by varying reaction temperature. The formation of ZnS nanospheres with various (002) orientations results from the selective adsorption of PVP molecules on ZnS (001) surface. Photocatalytic activity of the as-prepared ZnS nanocrystals with various (002) orientations in degradation of methyl orange was studied, it was found that the exposed{001} facets are reactive facets. Based on polar structure of the exposed{001} surfaces, a charge separation model between polar ZnS{001} surfaces was proposed. There is an internal electric field between positive Zn-ZnS (001) and negative S-ZnS (001) planes due to the spontaneous polarization. The internal electric field provides the driving force for charge separation and decrease recombination of photogenerated electrons-holes pairs.(4) Hexagonal porous ZnO nanospheres assembled with nanosheets of exposed {001} facets were synthesized by calcining wurtzite ZnS nanospheres at550℃for2h. The gas sensing properties of porous ZnO nanospheres assembled with nanosheets sensors toward ethanol, triethylamine and acetone vapor with different concentration were tested. The sensor response of porous ZnO nanospheres assembled with nanosheets toward105ppm ethanol vapor,9ppm triethylamine vapor and85ppm acetone vapor at the bestworking temperature are13.6,14.9and9.8, respectively. Photocatalytic activity of porous ZnO nanospheres assembled with nanosheets in degradation of methyl orange was studied at room temperature. The results indicate that the{001} facets of ZnO exhibit enhanced photocatalytic property and porous ZnO nanospheres exhibit much higher intrinsic potocatalytic activities than ZnO hollow microspheres assembled with nanorods, which can be attributed to the higher chemical reaction activity of the exposed high-energy{001} facets.