| CdSe hexagonal nanoflakes with exposed {002} facets,CdTe cubic nanoflakes with exposed {111} facets,Co3O4 three-Dimensional network and Al doped Co3O4 network structures with exposed {111} facets were synthesized through colloid chemical method and water solution combustion routes.Morphologies and structures of these products were characterized by means of X-ray diffraction,scanning electron microscopy,field emission scanning electron microscopy,transmission electron microscopy,X-ray photoelectron spectra,absorption-desorption porosimetry,IR spectrum,UV-vis spectrum and fluorescence spectrum.The reason of CdSe,CdTe and Co3O4 with exposed different crystal facets was studied and the possible formation mechanisms were proposed.The photocatalyisis degradation organic compounds properties of as-obtained CdSe and CdTe nanoplates were studied,and compared to CdSe and CdTe with exposed other facets.In addition,the gas sensing properties of Co3O4 and Al doped Co3O4 network was explored,and the relations between exposed facets and photocatalytic activity,sensing performance were investigated.Photocatalysis and gas sensing mechanism is proposed to provide theoretical guidance for design of high performance photocatalyst and high sensitive sensor material The specific research results as follows:(1)CdSe hexagonal nanoflakes have been synthesized via a reaction of CdCl2-oleylamine with Se-tri-n-octylphosphine at 300 ? for 30 min.The CdSe hexagonal nanoflakes with the typical side lengths of 9.5 to 12.3 nm were single-crystalline wurtzite structure and were enclosed by {001} top and down surfaces.The formation of CdSe hexagonal nanoflakes results from the selective adsorption of oleylamine on the(001)prismatic facet.CdSe nanocrystals with different size is achieved by adjusting reaction temperature and time,The absorption and photoluminescence properties of the CdSe nanocrystals with different sizes was studied,and the size-dependent absorption and photoluminescence properties leaded quantum confined effect were observed.The phtocatalytic activity of the nanocrystals for degradation of malachite green was studied,it is found that the exposed CdSe {001}facets are reactive facets.(2)The search for active semiconductor photocatalysts that can utilize adequately solar-energy remains to be an open issue.CdTe nanoflakes and nanowires with exposed different crystal facets have been synthesized.Photocatalytic activities of the CdTe nanocrystals in degradation of malachite green,methyl orange,acid red 88 were studied,The results found that photocatalytic activities can be enhanced by increasing the texture coefficients of(111)and reducing texture coefficient of(220).The exposed polar {111} facets were considered to be the active facets,the atomic arrangement of CdTe in[111]direction,as well as the Cd and Te atomic charge are studied using periodic density functional theory(DFT)and a charge separation model between polar Cd-CdTe(111)and Te-CdTe((?))surfaces is proposed.An internal electric field is created between polar Cd-CdTe(111)and Te-CdTe((?))surfaces by the spontaneous polarization,and the internal electric field drives separation of the photo induced electrons and holes.Therefore,the CdTe nanoflakes with exposed {111} facets show superior photocatalytic activity,compared with CdTe nanowires with exposed {110}facets.(3)The three-dimensional(3D)Co3O4 network structures were successfully synthesized by a rapid combustion of water solution of Co(NO3)2 containing polyvinylpyrrolidone(PVP)at 450,550 and 650 ?.The 3D Co3O4 network frameworks are assembled from Co3O4 nanosheets.The crystalline,crystallite size and specific surface area of the Co3O4 network structures can be adjusted by changing the reaction temperature.The gas-sensing performance of the as-prepared Co3O4 network structures was investigated toward acetone,ethanol,butylamine and formaldehyde vapor.It was found that Co3O4 network structures obtained at 650? shows superior gas sensing performances,compared with samples obtained at 450 and 550 ?,Co3O4 commercial powder.The enhanced sensing performance is attributed to the unsaturated Co atoms with dangling bonds serving as active sites,and the related sensing mechanism was put forward.(4)The Al-doped Co3O4 3D network structures with exposed {111} facets were successfully synthesized by a facile water solution combustion method at 650 ?,and the affect of different Al-doping concentrations on the gas sensing properties of the Co3O4 network structures was investigated systematically.The results indicated that the 6 at%Al-doped Co3O4 network showed the highest gas sensitivity toward acetone and ethanol.The effect of surface adsorption oxygen content,exposed active crystal face,specific surface area,crystalline and crystallite size on gas-sensing property of Co3O4 samples was investigated.The incorporation of Al ions with Co3O4 nanocrystals adjusts the carrier concentration,and induces the change of the oxygen deficiency and chemisorbed oxygen of Co3O4 network.Thus,the doping of Al3+ into Co3O4 network structures should be a promising method for designing and fabricating the high performance gas sensor,and found that Al doping leads to Co3O4 with exposed {111}crystal faces,it plays a key role in the improvement of the performance of the sensor.The atomic structure of Co3O4(111)facets was studied using periodic density functional theory.Based on the crystal structure of Co3O4 {111} facets,the concept of the surface unsaturated coordination Co atoms with dangling bonds serving as active sites was proposed,and the related sensing mechanism was put forward. |
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