Papers by Keyword: Hexagonal Phase

Abstract: The evolution regularity of the optical properties and electronic structure for ZnO hexagonal phase is studied under high-pressure, by using first-principles plane wave pseudo-potential method. The method is based on density functional theory (DFT) within the generalized gradient approximation (GGA) approaches. The calculation reveals that the valence band and the conduction band move to the direction of low energy and the direction of high energy by an increase of pressure, respectively, and thus the band gap is enlarged. The results of calculate also show that the energy band structure of ZnO hexagonal phase changes and the band gap is broadened with the increasing of pressure, which results in the move of the optical absorption margin and the optical absorption peak towards the aspect of the high energy.

Abstract: Hexagonal and cubic mesoporous TiO₂ materials were synthesized via a sol-gel route modified by evaporation–induced self–assembly process with different templates. XRD and TEM results confirmed that the mesoporous films were highly organized. Furthermore, the hexagonal mesoporous TiO₂ films were formed in the P123-templated films, whereas the cubic structure ones were found in the F127-templated films. Both the hexagonal and cubic mesoporous films showed photocatalytic activity in decomposing methyl orange solution under UV region, among which the cubic mesostructure presented superior potocatalytic activity because of larger surface area, more open framework and less obstructed diffusion paths of guest molecules.

Abstract: Phase transformation and crystal growth behavior of Ge2Sb2Te5 were investigated systematically by means of in situ heating (from room temperature to 500 oC) of amorphous Ge2Sb2Te5 alloy in a high voltage electron microscope with real-time monitoring. Large-scale crystallization occurred to amorphous Ge2Sb2Te5 around 200 oC. Large crystal growth developed on heating from 200 oC to 400 oC, and single crystalline grains grew up to 150 nm. Eventually the onset of partial melting of thin Ge2Sb2Te5 foil was at 500 oC and liquid Ge2Sb2Te5 was observed for the first time by high-resolution transmission electron microscopy. Hexagonal Ge2Sb2Te5 phase remains after a subsequent cooling.

Abstract: Characteristic 60° dislocations occurred in hexagonal phase of Ge2Sb2Te5 thin foil cooled from 500°C to room temperature in a high voltage transmission electron microscope. The Burgers vector of dislocation was identified as 1/ 24 < 9902 > which is the edge component of 1 3 < 2110 > projected on the (1120) lattice plane. The dislocation resulted from the cooling-induced stress/strain in the Ge2Sb2Te5 alloy.

Abstract: Hexagonal and cubic mesoporous TiO2 films were prepared by using triblock copolymer-templated sol-gel method via evaporation-induced self-assembly (EISA) process. The mesophase of TiO2 film was controlled by spin-speed during the spin-coating process. The hexagonal mesoporous structure was formed at a high spin-speed around 2000 rpm, whereas the cubic mesostructure was formed at a low spin-speed around 600 rpm. XRD and TEM results indicate that those mesostructures are highly organized with a pore diameter of 7 nm. The prepared cubic and hexagonal mesoporous films were tested as photocatalysts for the decomposition of 2-propanol in gas phase. Both films presented considerably higher photocatalytic activity than a nonporous TiO2 films prepared by a typical sol-gel process without addition of triblock copolymer. Notably, we found that the cubic mesoporous films showed a relatively higher photocatalytic activity than the hexagonal mesostructured film. We believe this is due to the orientation of pore channels open on the surface of mesoporous films.