الفهرس 
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Abstract
Capturing solar energy and converting it into clean chemical energy by photoelectrochemical energy conversion (PEC) systems is considered very promising route to overcome the shortage problem of non-renewable energy globally. Metal oxides and sulfides are considered excellent candidates for this technology due to their good electrical and mechanical properties. Furthermore, the wide band gap for metal oxides and sulfides can be tailored by doping.
We investigated the doping effect on the structure, electronic and optical properties of some metal oxides and sulfides. Zirconia (ZrO2) and Hafnia (HfO2) were chosen to represent metal oxides and La3NbS2O5 was metal sulfide we have chosen due to its mechanical and electrical properties. We selected three dopants, 1st one is Titanium Ti (3d metal), 2nd dopant was Niobium Nb (4d metal) and the 3rd dopant was Tungsten W (5d metal).
Material studio 8 was used to perform all calculations using the spin-polarized DFT calculations based on the GGA as implemented in the CASTEP code with new parameterization (PBEsol) that specifically targeted solids.
This thesis consists of three main chapters
CHAPTER I: (Introduction)
In this chapter different aspects of energy challenge have been discussed, energy transformation challenges, driving forces of clean energy, risk of climate changes, etc. We also presented a general overview on energy forms (chemical energy, mechanical energy, electrical energy, nuclear energy, thermal energy, etc). Finally we presented in details, types of semiconductors, fundamentals of photo catalysis, band gap engineering for metal oxides and metal sulfides which consider the core of this thesis.
CHAPTER II: (Computational Methods and Details)
This chapter revised a brief background about princibles of quantum mechanics. Hartree-Fock (HF) models are addressed firstly, followed by density functional theory (DFT). Dielectric functions were also reported as well as providing information about how the optical properties could be calculated. Furthermore, HF/DFT Hybrid Functionals and different types of basis sets were also presented. In this work we used the spin-polarized density functional theory (DFT) calculations based on the generalized gradient approximation (GGA) with new parameterization that specifically targeted solids PBEsol as implemented in CASTEP code with the plain wave pseudopotential method. We used the Hubbard approach in order to accurately describe the energy band gap, the dopant effect and to avoid the underestimation error of DFT.
CHAPTER III: (Results and Discussion)
This chapter devided to three main parts, 1st part collects the results and discussion of doping effect on band structure and Optical Properties of Monoclinic Zirconia (m-ZrO2). The second part displays results and discussions of our comparative study for the effect of Ti, Nb and W incorporation on the Electronic and Optical Properties of Hafnia (m-HfO2).In the last part of this chapter, we displayed our DFT study for the electronic and structural properties of La3NbS2O5
3.1 Engineering Band structure and Optical Properties of m-ZrO2
In this part of chapter 3 we introduced first principles calculations using Hubbard approach (DFT+U) with PBEsol correlation were carried out to study doping effect on band structure and optical properties of pristine ZrO2. We applied the optimal values of Ud (8 eV) and Up (4.35 eV) in order to reproduce the experimental band gap. For pristine ZrO2, the calculations indicated indirect band gab of 5.79 in good agreement with experiment and direct band gap of 6.1 eV. The introduction of Nb and W in the crystal structure of pristine led to displacement of the bad gap edges.The total density of states and PDOS calculations indicated that VBM of pristine is mainly constructed by O 2p states while, CBM constructed mainly by Zr 4d states. However, doped crystals the CBM are mainly constructed by 4d and 5d states of Nb and W respectively. Furthermore, the optical absorption of doped crystals extended into the visible and near-infrared regions which confirmed by higher dielectric of the imaginary part at zero voltage. For dopant location effect, the band structure and density of states of different ZrO2:W systems with different locations of W atoms reflects non-significant .our promising findings in this study highly promote the studied crystals to be excellent candidates for solar energy conversion devices.
3.2 Comparative Study of Ti, Nb and W incorporation on the Electronic and Optical Properties of Pristine Hafnia (m-HfO2)
In this 2nd part of chapter 3 we introduced First-principles calculations (DFT+U) to do comparative study of the effects of the incorporation of 3d, 4d, and 5d metal atoms on the electronic and optical properties of m-HfO2. Ti, Nb, and W are chosen to represent 3d, 4d, and 5d metals respectively. We applied optimal values of Ud (8e V) and Up (4.35 eV) in order to reproduce the experimental band gap. Our calculations indicated direct band gap of pristine m-HfO2 at 5.24 eV in good agreement with experimental range (2.5-2.8 eV), and other theoretical findings. The incorporation of metal atoms in the crystal structure of HfO2 displaced the band gap edges and downshift CBM which led to band gap tightening as the following 5.24, 3.26, 1.12, and 0.92 eV for HfO2, HfO2:Ti, HfO2:W, HfO2:Nb respectively. For HfO2:W and HfO2:Nb, the band gap reduction was more significant than HfO2:Ti. The total DOS and PDOS calculations illustrated that the VBM of pristine HfO2 is mainly constructed by O 2p states, while the CBM is constructed mainly by Hf 4d sates. For doped crystals the CBM are mainly constructed by 3d, 4d and 5d sates of Ti, Nb, and W respectively. The optical properties calculations showed peaks below zero for real part of dielectric constant of pristine HfO2 and HfO2:Ti which reflects the near metallic behavior of this systems. The absorption of doped systems extended into the visible and near-infrared regions, which good matched with the dielectric of imaginary part at zero voltage. For doped systems, there are clear similarity in the effect of the incorporation of Nb (4d metal ) and W (5d metal) on the electronic and optical properties of HfO2, which differed to large extent than the effect of the incorporation of Ti (3d metal). The absorption of HfO2 is duplicated upon Ti atom insertion (HfO2:Ti) .Based on the results of this study, we would like to emphasize that these results provide a solid theoretical starting point that motivates further experimental studies into the application potential of these doped metal oxide systems.
3.3 Electronic and Structural Chemistry of La3NbS2O5
In the lasr part of chapter3 we present DFT study for the electronic and structural properties of La3NbS2O5 .The crystal structure of La3NbS2O5 belongs to I4/mmm (139) space group, a = 0.40781, c = 2.523 nm, c/a = 6.187, V = 0.4196 nm3, Z = 2. Experimental band gap of La3NbS2O5 in the range of 2.13-2.26 eV. The calculate band gap for optimized structure of La3NbS2O5 was zero which doesn’t match with experimental value. We repeat calculations many times but the result was the same. Based on the results of this study, we would like to emphasize that published crystal structure data of La3NbS2O5 may be not accurate enough and we need to make extra effort to reach right lattice structure data of this promising metal sulfide.