The Effect Of V Doping On The Electric Structure And Photocatalytic Activity Of Anatase TiO₂by First Principle Study

In this thesis, we investigated the electronic properties of V-doped TiO₂with differentdoping concentrations, and the effects of doping on the photoabsorption and photocatalyticactivity of TiO₂using the generalized gradient approximation （GGA） density functionaltheory, and meanwhile electric structure of O vacancy-doped TiO₂with similar band structureto V-doped case has also been investigated as well as simple comparision in the bonding ofimpurity states. The main contents are presented as follows:Band structure, DOS and contour surface of orbitals of V-doped TiO₂were calculatedwith different doping concentrations. The Effect of doping on the photocatalytic activity ofTiO₂was considered from the view of carrier combination influenced by impurity states. Theresults showed that the energy of VBM declined by0.04eV under the condition of lowconcentration （x=6.25%） doping. The doping induced unoccupied and occupied states in theband gap. The dopant states are mainly composed of V3d states, and contain a small amountof O2p and Ti3d states. The unoccupied and occupied states can capture electrons and holes,and change V4+ions to V5+and V3+ions respectively, which promoted separation of carriersand suppressed recombination of carriers. Meanwhile the energy gap of0.91eV betweenoccupied and unoccupied states provided driving force for the carriers to detrape andsuppressed recombination. Due to the presence of dopant states, the transition of electronbetween the VBM and the V t2glevels can be achieved. Meanwhile the overlapping andinteraction between β states （spin-down states） of Ti t2gand the counterparts of V t2gdyzanddxzleads to the decline of CBM, which explains the experimentally observed phenomenon ofabsorption edge extending to the visible region. The promoted separation of carriers andnarrowing of band gap induced by V doping can improve the visible light photocatalytic activity. The energy of VBM increased by0.06eV under the condition of high concentration（x=12.5%） doping. The original occupied and unoccupied bands in the band gap overlappedwith each other and the semi-occupied states formed at the bottom of CBs. With heavy doping,the separation of energy gap disappeared. Therefore, there was no driving force to detraptrapped carriers, which leads to the great combination and inhabited photocatalytic activity,and it is in good accordance with observed decreased ptotocatalytic activity.Electric structure of O vacancy-doped TiO₂has also been investigated as well as simplecomparision in the bonding of impurity states with V-doped TiO₂. It is found that O vacancydoping induces Morin phase transition under the CBM of TiO₂, and meanwhile impuritystates at bottom of conduction bands are formed. After comparing of both on the bonding ofimpurity states, the doped states induced by O vacancy can be ascribed to weak σ stylebonging effect of3dyzof first neighbour Ti ions with second neighbour Ti ion3dz2states. Forthe case of V doping, there are found no effective V-V interactions in the crystal and bondingof V-O-Ti develop with π*bonding of V-O and π bonding of O-Ti.The unoccupied and the occupied states can be induced in the band gap by the V doping,and then they can trap electrons and holes, respectively. In addition, the energy gap betweenthem is large enough for trapped carriers to detrape, and therefore the ‘double trapping’ modewould form which will promote the photocatalytic activity.