First principles study on intrinsic defects and Al dopant in TiO2

다국어 초록 (Multilingual Abstract)

Titanium dioxide (TiO¬2¬), a widely used nanomaterial in many applications, has two most abundant phases: anatase and rutile. Anatase is the favored phase in photocatalytic application, formed easily ¬at several nanometers size due to its low surface energy. However, some applications definitely need rutile phase such as in paint, pigment, or mixed-phase even in photocatalysis, raising the importance of controlling the phase formation of TiO¬2¬ nanomaterials, especially in effectively transforming anatase to rutile phase at desired conditions. Globally, million tons of TiO¬2¬ nanomaterials produced annually are utilizing AlCl¬3 as a promoter for the anatase to rutile transformation (ART) while there are experimental results showing that Al is an inhibitor. Obviously, role of Al in phase transformation of TiO¬2¬ is still a matter of debate. Further, when using Al as a dopant in TiO¬2¬, it is so far unclear where Al would incorporate into the materials substitutionally or interstitially, or even on the surfaces. The net effect of intrinsic defects in TiO2 and/or relevant impurities with Al dopant is also worth considered.
In this study, by using DFT calculation in combination with thermodynamic simulation, we have investigated role of Al dopant and the net effect of relevant defects to ART at various simulated thermodynamic condition¬s. The results show that Al-doping under substitutional form Al¬Ti energetically promotes ART by stabilizing rutile-host defects more than the anatase counterparts in both bulk and surface situations, especially at reducing environment. Substitutional dopant Al¬Ti could be compensated by oxygen vancancy V¬O, titanium interstitial Ti¬i, or Al interstitially doped Al¬i. Also, Cl substitution for O dopant and surface effect is expected to generate cooperative effects in favor of rutile phase depending on specific condition.

목차 (Table of Contents)

• Contents
• Chapter 1 – INTRODUCTION 9
• 1.1 TiO2 nanomaterials 9
• 1.2 Synthesis of TiO2 nanoparticles 11
• 1.3 Anatase to Rutile Transformation (ART) phenomenon 13
• 1.4 Aim of this study 17
• Chapter 2 – METHODOLOGY 19
• 2.1 Electronic Structure Methods 19
• 2.1.1 Introduction 19
• 2.1.2 Hohenberg-Kohn theorems 21
• 2.1.3 The Kohn-Sham equations 24
• 2.1.4 The exchange-correlation approximations 25
• 2.2 Computational Methods of DFT 30
• 2.2.1 Pseudopotential 30
• 2.2.2 Plane waves and periodic systems 31
• 2.3 DFT-Thermodynamics Formalism 34
• 2.4 Computational Details 36
• 2.5 Correction scheme to formation energy 42
• Chapter 3 – RESULTS AND DISCUSSION 45
• 3.1 Intrinsic defects in TiO2 45
• 3.1.1 Anatase and rutile primitive unit cells 45
• 3.1.2 Dominant intrinsic defects in TiO2 47
• 3.2 Effect of aluminum doping in TiO2 to ART 55
• 3.2.1 Effect of aluminum doping and complex defect to ART 55
• 3.2.2 Effect of Al-Cl co-doping 66
• 3.3 Surface effect on ART 70
• Chapter 4 – CONCLUSION 73
• ACKNOWLEDGEMENT 74
• REFERENCES 75