الفهرس 
chapter1Only 14 pages are availabe for public view
 |  | from | 152 |  |  |
| | | from | 152 | | |
Abstract
The main aim of our work is:to predict modified anatase TiO2 based materials with an enhanced photocatalytic activity over pureTiO2 material. Therefore, cationic mono-doping with Cr4+, Co2+, Ni2+ and Se4+ and anionic mono-doping with Se2- and B2- in addition to their co-doping (Cr/B, Co/B, Ni/B, Se4+/B, Ni/Se4+ and Ni/Se2-) are investigated in our study. The effect of mono- and co-doping with the previous ions into the anatase lattice is indicated through studying the changes accompanying such modifications on the crystal structure, electronic and optical properties. In addition, the formation energy of each system is calculated to clarify the stability on such modified systems.
The desired study can be performed in the framework of ab-initio first principle DFT-based calculations. In our calculations, we used two levels of calculations: PBE functional and HSE06 hybrid functional. To reach our goal the following steps were maintained:
- Volume optimization and formation energy calculations of pure, single doped and codoped anatase TiO2 took place using GGA-PBE.
- GGA-PBE optimized volume for each system will be used for the forward calculations based on HSE06.
- HSE06 results will be plotted as DOS and PDOS for each system with comparing to the pristine one and previously reported available results.
- Recording band gaps, introduced states positions with respect to VBM and CBM.
-The optical properties will be also calculated to show possible transitions on absorption of light photons.
Density functional theory-based calculations are adopted for clarifying the validity of defect modified anatase TiO2. For mono-doped systems, (Co, Cr, Ni or Se4+) is substituted for Ti4+ ion from the host lattice as cationic dopants while (Se2- and B) are substituted at oxygen site. In seek more enhancement, (Co/B, Cr/B, Ni/B Se/B, Ni/Se4+ and Ni/Se2-), and oxygen deficiency are also studied and their effect on the photocatalytic activity of anatase TiO2 is examined. The volume optimization calculations are carried out with respect to PBE in order obtain the optimized volume and optimized lattice parameters from which we can follow up our calculations. The crystal structure, electronic, and optical properties are calculated in the framework of HSE06 hybrid functional.
Due to the different electron configuration of dopants from host atoms, new impurity states appear in the band gap region, thus changing the electronic structures. For all single doped and co-doped systems, visible light response is observed. Whereas the absorptions edge is red shift for all systems except for Co, Se4+ mono-doped and oxygen vacancy anatase corresponding to main gap values of 3.32, 3.29 and 3.63 eV, respectively. The deep impurity states on mono-doping, except for Ni where the impurity states are overlapped with the VBM, could affect the recombination of photo-excited electron-hole pairs reducing their lifetime. For all systems, the lowest narrowed band gap is obtained on Ni/Se2- and Ni/B co-doping (2.44 and 2.77 eV, respectively) however the introduction of shallow states is observed for Ni/B co-doping just below the CBM. These shallow states appear also on Cr/B co-doping. Thus, the charge separation efficiency could be enhanced on such co-doping with the previous materials, however, Ni/B co-doping could be considered the best among all due the reduced band gap together with the exitance of shallow state able to enhance the charge separation.
The calculated optical properties emphasize the enhanced absorption efficiency under visible light irradiation for all systems. For cationic doped system, the absorption strength is maximum at 2.18 eV (570 nm) for Se4+ mono-doping while B mono-doping shows higher absorption strength over Co, Cr single doping and Ni/B, Se/B, Ni/Se2- and Ni/Se4+ co-doping. Nonetheless, Ni/B co-doped system show IR absorption activity in accordance with the transitions allowed from the shallow occupied states below CBM regarding the extended visible light absorption. Moreover, the band gap of Ni/B is red shift to 450 nm. Thus, the extended absorption edge to the visible light on Ni/B co-doping together with higher charge separation efficiency could suggest the Ni/B co-doped anatase TiO2 as a good photocatalyst with longer life of the photogenerated charge carriers.