Study Of Stability, Electronic Structures And Related Properties Of Doped TiO₂ From First-principles Calculations

Titanium dioxide (TiO2) is one kind of important wide-gap semiconductors, in which anatase and rutile phases are two commonly used structures. TiO2 is one ideal semiconductor photocatalyst because of its many characteristics such as high activity, good stability, nontoxicity and low cost, and thus it has been widely used in the fields of renewable energy and environmental protections. However, the band gap of TiO2 is larger (-3.2eV), and this leads to that only the light with the wavelength equal to or smaller than the 387nm could excite the electrons from valence band to the conduction band and create the electron-hole pairs, which only corresponds to 4% of the solar energy, and the visible-light range accounting for 43% of the whole solar spectra can not be fully used. This heavily restricts the applications of TiO2 in the visible-light range. Therefore, to improve the spectra response and photocatalytic activity of TiO2 in the whole solar spectra range, numerous efforts have been devoted and a certain amount of progress is obtained. Especially in recent years, some attempts have been made to improve the visible-light absorption and promote the photocatalytic activity of TiO2 to some degree by nonmetal doping which introduces some impurity states in the band gap and adjusts its electronic band structure. In 2001, Asahi et al. firstly reported that nitrogen doping could effectively improve the visible-light photocatalytic activity of TiO2 in Science. After that, a series of nonmetal (for example, B, C, Si, S, P, F, Cl, Br, and I) doped TiO2 have been synthesized, and the doping effects of these nonmetals on optical absorption and photocatalytic activity of TiO2 have been studied.In this dissertation, we mainly discussed the stability, electronic structures and optical absorption properties of these nonmetal-doped TiO2, and gave some reasonable explanations for some controversial problems in the experiments and theories. The dissertation is divided into five chapters. In the first chapter, we introduced the density functional theory, and gave a brief description for the first-principles software packages. In the second chapter, we presented the basic structures of TiO2 as well as the research background and progress of TiO2 in the photocatalytic filed. In the third chapter, we discussed the electronic structures and related optical, magnetic properties of nonmetal-doped and Cr-doped TiO2 as well as the spin polarization and magnetic coupling characteristics of undoped TiO2 part by part. In the fourth chapter, we further explored the origin of d0 magnetism inⅡ-ⅥandⅢ-Ⅴsemiconductors by 2p-light-element substitutional doping at anion sites on the basis of the theoretical studies on magnetic properties of C (N)-doped TiO2. In the fifth chapter, we summarized the research contents in this dissertation and pointed out some theoretical problems that need to be solved urgently as well as the further research directions. The main research work and contents are listed as follows:(1) Our electronic structure calculations for N-anion doped TiO2 suggested that the doped systems show different electronic characteristics at different doping levels: at low doping concentration (≤～2.1 atom %), N doping introduces an isolated impurity state in the band gap, which acts as a transition level; at high doping concentration (≥～4.2 atom %), N 2p states mix with the valence band and narrow the band gap, thus reducing the electron transition energy. Our research for the magnetic property of N-anion doped TiO2 indicated that each N dopant induces a local spin magnetic moment of 1.0μB and the system shows a stable ferromagnetic ground state when the two N atoms are coordinated to a common Ti atom and the∠N-Ti-N angle is greater than～1020.(2) We firstly gave a reasonable explanation for experimentally observed blueshift and redshift of optical absorption edge in B-doped TiO2:when B substitutes O in TiO2, B introduces some impurity states in the band gap, thus reducing the optical absorption energy, which is consistent with the experimental redshift; when B exists in TiO2 at an interstitial site, the optical absorption energy increases about 0.2～0.3eV due to the Moss-Burstein effect, which explains the experimental blueshift. (3) We studied the formation energies and electronic structures of S (P, Si) anion and cation doped TiO2 respectively, and obtained the following conclusions:(i) For S-anion doped TiO2, as in the case of N-doped TiO2, the change of doping level leads to the discrepancy of electronic structure characteristics. For S-cation doped TiO2, S dopants introduce some impurity states consisting of S 3s and O 2p states in the band gap, though the band gap has little changes. (ii) For P-anion doped TiO2, the band gap has no changes but some P 3p states lie in the band gap; for P-cation doped TiO2, the band gap narrows little and no impurity states locate in the band gap. (ⅲ) For substitutional Si to Ti doped TIO2, the band gap reduces about 0.2eV; for substitutional Si to O doped TiO2, the optical absorption energy of anatase phase reduces while that of rutile phase has no changes. (ⅳ)The doping sites of S (P, Si) (i.e., at O or Ti site) depend on the preparing method and growth condition. Under O-rich condition, S (P) prefers to form substitutional Ti doped structure while S (P) prefers to form substitutional O doped structure under Ti-rich condition. For Si-doped TiO2, it is always preferred to form substitutional Si doped Ti structure under both O-rich and Ti-rich conditions.(4) We studied the geometrical and electronic properties of C anion and cation doped TiO2 on the basis of spin-polarized GGA+U calculations. For C-anion doped TiO2, C dopant introduces some disperse states in the band gap, and thus the electron transitions among the conduction band, valence band and gap states should have different optical absorption energies. For C-cation doped anatase and rutile TiO2, optical band gap reduces about 0.18eV and 0.3ev with repect to undoped TiO2, respectively. These results explained the experimentally observed different optical absorption thresholds in C-anion doped TiO2 and the low optical absorption energy in C-cation doped TiO2. Meanwhile, we also studied the magnetic property of C-anion doped TiO2, and the calculated results show that a strong ferromagnetic coupling occurs when the two C atoms form a slightly bent C-Ti-C unit by replacing two oxygen atoms at the opposite vertices of a TiO6 octahedron.(5) For halogen-doped TiO2, we obtained following conclusions. (ⅰ) Under O-rich condition, it is energetically more favorable for Br and I to substitute Ti than O, while it is energetically more favorable for F and Cl to substitute O than Ti. Under Ti-rich growth condition, it is energetically more favorable for all halogen atoms to substitute O than Ti. (ⅱ) For substitutional I to Ti doped TiO2,I dopants exist as I5+ions and introduce a doubly occupied band gap state made up of I 5s orbitals. For substitutional X(X=F, Cl, Br) to Ti doped TiO2, the F, Cl and Br atoms exist as F3+, Cl4+and Br4+ions respectively, and the Cl and Br dopants introduce a singly occupied band gap state made up of the Cl 3p and Br 4p orbitals, respectively. (ⅲ) For substitutional X (X=F, Cl, Br) to O doped TiO2, the calculated valence band maximum and conduction band minimum is consistent with the experimental values, and the band gaps of these systems have different degress of narrowing and no gap states are introduced. For substitutional I to O doped TiO2,I dopant introduce some gap states made up of I 5s orbitals above the valence band maximum about 0.6ev, thus leading to the reduction of optical absorption energy.(6) Our GGA+U calculations for Cr-doped anatase TiO2 depicted the splitting behavior of Cr 3d states successfully, which is consistent with the experimentally observed insulating characteristic, and the electron transitions among the valence band, conduction band and gap states could explain the experimental major and minor optical absorption bands. Our results also indicated that the substitutional Cr dopant should exist as Cr4+(3d24s0) instead of the generally believed Cr3+(7) For oxygen-deficient TiO2, our calculated results indicated that in anatase phase, two additional electrons introduced by an oxygen vacancy reduced two Ti4+ions into two Ti3+, and these two Ti3+ions form a stable antiferromagnetic coupling. Similar antiferromagnetic coupling is also found in the oxygen-deficient rutile phase. This is consistent with the experimental antiferromagnetic behavior. On the contrary, in titanium-deficient TiO2, titanium vacancy produces spin magnetic moments on the adjacent oxygen ions, and they form a stable ferromagnetic coupling. We conclude that similar ferromagnetism may also appear in some other semiconductors with cation vacancies.(8) The study of electronic structures and magnetic properties for II-VI and III-V semiconductors indicate that substitutional doping at anion sites by 2p light elements results in a spontaneous spin polarization. However, the 2p orbitals of the dopant must be sufficiently localized in the band gap of the host semiconductors to have a stable magnetic ground state, and the generated spin magnetic moment is sensitive to the relative strength of electronegativities of the dopant and the anion in the host semiconductors.These systemic theoretical studies reveal the intrinsic relationship between the doping sites of nonmetals in TiO2 and the growth condition of samples as well as the electronegativities of dopants. It can help us understand the relationship among the electronic structures, optical absorption and visible-light photocatalytic activity of doped TiO2 more clearly, and provide some beneficial guidance in synthesizing TiO2 with high photocatalytic efficiency. In addition, our studies for spin-polarization and magnetic coupling characteristics of 2p-light-element dopedⅡ-Ⅵ,Ⅲ-Ⅴsemiconductors and TiO2 also provide the theoretical foundation for searching for "d0" magnetism.