掺杂二氧化钛的稳定性、电子结构及相关性质的第一性原理研究
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摘要
二氧化钛(TiO2)是一种非常重要的宽禁带半导体材料,其中锐钛矿(anatase)和金红石(rutile)是两种比较常见的晶体结构。TiO2具有催化活性高、稳定性好、对人体无毒、成本低等特点,是一种非常理想的半导体光催化剂,因而被广泛用于再生能源以及环境保护等领域。然而TiO2的禁带宽度较大(-3.2eV),只有波长等于或小于387nm的紫外光才能激发TiO2中的价带电子跃迁到导带产生光生电子-空穴对,这只占到了太阳光能的5%,而占太阳光能约43%的可见光得不到充分利用,严重制约了TiO2光催化剂在可见光波段的应用。因此为提高TiO2在整个太阳光波段的光谱响应和催化活性,科研工作者进行了大量的研究并取得了一定程度的进展。特别是近年来,人们开始尝试采用非金属元素掺杂方式在TiO2禁带中引入杂质能级,调节能带结构,促进可见光的吸收,并在一定程度上提高了TiO2在可见光辐射下的光催化效率。2001年,Asahi等人在《科学》杂志上首次报导了氮(N)掺杂可以有效地提高TiO2的可见光光催化活性。此后,人们又陆续合成了一系列非金属元素(如B、C、Si、S、P、F、Cl、Br、I)掺杂的TiO2,并分别研究了这些掺杂元素对TiO2可见光的光吸收效率以及光催化活性等相关性质的影响。
在本文中,我们重点系统地研究了这一系列非金属元素掺杂TiO2的稳定性、电子结构及其相关的光谱吸收等性质,并针对实验和理论上具有广泛争议的一些问题给出了合理的解释。本论文共分为五章。第一章简要介绍了密度泛函理论并对文中所采用的第一性原理计算软件包做了说明。第二章介绍了TiO2材料的基本结构以及TiO2作为光催化材料的相关研究背景和进展。第三章分节阐述了一系列非金属元素(如N、B、S、Si、P、C、F、Cl、Br、I)以及金属元素Cr掺杂TiO2的电子结构及相关的光学、磁学等性质,并讨论了含有点缺陷的非掺杂TiO2的自旋极化及磁耦合特性。第四章揭示了2p轻元素杂质在Ⅱ-Ⅵ族和Ⅲ-Ⅴ族半导体中替换阴离子掺杂时产生d0磁性的本质原因及其规律。第五章对本文的研究内容做了总结并指出了在TiO2光催化研究领域仍有待解决的问题和进一步的研究方向。主要的研究工作和内容如下所述：
(1)对N阴离子掺杂TiO2的电子结构计算表明N掺杂浓度对体系的电了结构特性有重要影响：低浓度(<-2.1 atom%)掺杂,N杂质会在TiO2禁带中引入孤立的充当过渡能级的杂质态；高浓度(≥～4.2 atom%)掺杂,由N杂质引入的N 2p态与价带混合使禁带宽度变窄,缩减了电子的跃迁能。此外,N掺杂可引起Ti02的电子结构产生自旋极化并生成1.0μB的局域自旋磁矩,并且当两个杂质N与同一个Ti成键且其夹角∠N-Ti-N大于1020时,体系可呈现较为稳定的铁磁基态。
(2)首次对实验上发现的B掺杂TiO2的光谱吸收边的蓝移和红移现象给出了合理的理论解释：当B替换TiO2中的O时,B杂质会在禁带中引入部分杂质态,光谱吸收能变小,这与实验上发现的光谱吸收边的红移现象一致；当B在TiO2中以间隙原子存在时,由于莫斯-布尔斯坦效应,光谱吸收能增加了大约0.2-0.3eV,这解释了实验上发现的蓝移现象。
(3)通过对S(P、Si)阴离子和阳离子掺杂TiO2的形成能和电子结构分析,得到了以下结论：(i)对S阴离子掺杂TiO2,与N掺杂TiO2类似,掺杂浓度的改变会导致电子结构特性的差异。对S阳离子掺杂TiO2,S杂质会在禁带中引入由S 3s态和O 2p态组成的定域杂质能级。(ii)对P阴离子掺杂TiO2,P杂质在禁带中引入部分P 3p态；对P阳离了掺杂TiO2,P杂质既未在禁带中引入杂质能级也未引起带隙变化。(iii)对Si替位Ti掺杂TiO2,其带隙减少了大约0.2eV；对Si替位O掺杂Ti02,anatase相结构的光谱吸收能有所减小而rutile相结构的光谱吸收能几乎没有变化。(iv)S(P)在TiO2中的掺杂位置(亦即替换O或者Ti)取决于样品的制备方法和生长条件。在富氧条件下,倾向形成S(P)替换Ti的掺杂结构；而在富钛条件下,倾向形成S(P)替换O的掺杂结构。对Si掺杂TiO2,无论是在富钛还是在富氧条件下,总是倾向于形成Si替位Ti结构。
(4)基于自旋极化的GGA+U密度泛函理论研究了C阴离子和C阳离了掺杂TiO2的几何结构和电子性质。C阴离子掺杂TiO2的禁带中有数条较为分散的能级,并且电子在价带、导带和隙态之间的跃迁具有不同的光谱吸收能。C阳离子掺杂anatase和rutile相TiO2的光学带隙与未掺杂时相比分别减少了大约0.18和0.3eV。这解释了实验上观察到的C阴离子掺杂TiO2具有不同的光谱吸收阂值以及在C阳离子掺杂样品具有较低的光谱吸收能现象。对C阴离子掺杂TiO2的电子自旋极化及其磁耦合特性的研究表明当两个C杂质原子替换位于TiO6八面体两个相反的顶角O原子并形成略微弯曲的C-Ti-C结构时,体系会呈现强烈的铁磁耦合基态。
(5)对卤族元素掺杂TiO2的研究得到了以下结论：(i)在富氧条件下,Br和I替位Ti结构的形成优先于替位O结构,而F和Cl替位O结构的形成优先于替位Ti结构。在富钛条件下,所有的卤族原子替位O结构的形成都优先于替位Ti结构。(ii)对I替位Ti掺杂的TiO2,I杂质以I5+形式存在,并在禁带中引入了主要由I5s轨道组成的双重占据的隙态。对X(X=F、Cl、Br)替位Ti掺杂TiO2, F、Cl和Br杂质分别以F3+、Cl4+和Br4+离子形式存在。并且Cl和Br杂质分别在禁带中引入了由C13p和Br4p轨道组成的单占据的隙态。(iii)对X (X=F、Cl、Br)替位O掺杂Ti02,价带顶和导带底的相对位置与实验结果基本一致,并且TiO2禁带中间不存在隙态只是禁带宽度略有不同程度的减小。对I替位O掺杂的TiO2,I杂质在禁带中引入了主要由I 5p轨道组成的高于价带顶大约0.6eV的隙态,导致了光谱吸收能的减小。
(6)对Cr掺杂的anatase相TiO2的GGA+U计算成功地描述了Cr 3d态的劈裂行为,这与实验上发现的绝缘特性一致；并且电子在价带、导带和隙态之间的跃迁可以分别解释实验上发现的较大和较小的光谱吸收峰。同时研究表明替位Ti掺杂的Cr原子是以Cr4+(3d24s0)阳离子形式存在而并非普遍认为的Cr3+。
(7)对含有点缺陷的非掺杂TiO2的电子结构以及磁学性质的研究表明在anatase相TiO2中,由氧空位引入的多余电子把两个Ti4+离子还原为两个Ti3+离子,并且两个Ti3+产生的局域自旋磁矩形成了稳定的反铁磁耦合。在含有氧空位的rutile相TiO2中也存在类似的反铁磁态。这与实验上在氧缺乏的Ti-O系统中发现的反铁磁特征一致。与之对比,在含有钛空位的TiO2体系中,钛空位使周围的氧产生了自旋磁矩,并且两个钛空位周围氧产生的自旋磁矩可以形成稳定的铁磁耦合。我们推测类似的铁磁性也会出现在其他含有阳离子空位的半导体中。
(8)对掺杂Ⅱ-Ⅵ族和Ⅲ-Ⅴ族半导体的电子结构和磁学性质的研究表明：2p轻元素替换阴离子掺杂可以导致体系产生自发的自旋极化,然而掺杂元素的2p轨道在禁带中必须足够定域化才能产生稳定的磁性基态,并且体系能否产生稳定的磁性基态主要取决于半导体阴离子和掺杂元素间的相对电负性强度。
这些系统的理论研究工作揭示了非金属元素在TiO2中的掺杂位置与样品的生长条件以及掺杂剂本身的电负性之间的内在联系,有助于人们更加清楚地了解掺杂TiO2的电子结构与光谱吸收以及可见光的光催化活性等性质的关系和作用规律,并对实验上合成具有较高可见光光催化效率的TiO2体系提供有益的指导。此外,对2p轻元素掺杂Ⅱ-Ⅵ、Ⅲ-Ⅴ族半导体和TiO2体系的自旋极化和磁耦合特性研究也为实验上寻找“d0”磁性提供了理论基础。
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.
在本文中,我们重点系统地研究了这一系列非金属元素掺杂TiO2的稳定性、电子结构及其相关的光谱吸收等性质,并针对实验和理论上具有广泛争议的一些问题给出了合理的解释。本论文共分为五章。第一章简要介绍了密度泛函理论并对文中所采用的第一性原理计算软件包做了说明。第二章介绍了TiO2材料的基本结构以及TiO2作为光催化材料的相关研究背景和进展。第三章分节阐述了一系列非金属元素(如N、B、S、Si、P、C、F、Cl、Br、I)以及金属元素Cr掺杂TiO2的电子结构及相关的光学、磁学等性质,并讨论了含有点缺陷的非掺杂TiO2的自旋极化及磁耦合特性。第四章揭示了2p轻元素杂质在Ⅱ-Ⅵ族和Ⅲ-Ⅴ族半导体中替换阴离子掺杂时产生d0磁性的本质原因及其规律。第五章对本文的研究内容做了总结并指出了在TiO2光催化研究领域仍有待解决的问题和进一步的研究方向。主要的研究工作和内容如下所述：
(1)对N阴离子掺杂TiO2的电子结构计算表明N掺杂浓度对体系的电了结构特性有重要影响：低浓度(<-2.1 atom%)掺杂,N杂质会在TiO2禁带中引入孤立的充当过渡能级的杂质态；高浓度(≥～4.2 atom%)掺杂,由N杂质引入的N 2p态与价带混合使禁带宽度变窄,缩减了电子的跃迁能。此外,N掺杂可引起Ti02的电子结构产生自旋极化并生成1.0μB的局域自旋磁矩,并且当两个杂质N与同一个Ti成键且其夹角∠N-Ti-N大于1020时,体系可呈现较为稳定的铁磁基态。
(2)首次对实验上发现的B掺杂TiO2的光谱吸收边的蓝移和红移现象给出了合理的理论解释：当B替换TiO2中的O时,B杂质会在禁带中引入部分杂质态,光谱吸收能变小,这与实验上发现的光谱吸收边的红移现象一致；当B在TiO2中以间隙原子存在时,由于莫斯-布尔斯坦效应,光谱吸收能增加了大约0.2-0.3eV,这解释了实验上发现的蓝移现象。
(3)通过对S(P、Si)阴离子和阳离子掺杂TiO2的形成能和电子结构分析,得到了以下结论：(i)对S阴离子掺杂TiO2,与N掺杂TiO2类似,掺杂浓度的改变会导致电子结构特性的差异。对S阳离子掺杂TiO2,S杂质会在禁带中引入由S 3s态和O 2p态组成的定域杂质能级。(ii)对P阴离子掺杂TiO2,P杂质在禁带中引入部分P 3p态；对P阳离了掺杂TiO2,P杂质既未在禁带中引入杂质能级也未引起带隙变化。(iii)对Si替位Ti掺杂TiO2,其带隙减少了大约0.2eV；对Si替位O掺杂Ti02,anatase相结构的光谱吸收能有所减小而rutile相结构的光谱吸收能几乎没有变化。(iv)S(P)在TiO2中的掺杂位置(亦即替换O或者Ti)取决于样品的制备方法和生长条件。在富氧条件下,倾向形成S(P)替换Ti的掺杂结构；而在富钛条件下,倾向形成S(P)替换O的掺杂结构。对Si掺杂TiO2,无论是在富钛还是在富氧条件下,总是倾向于形成Si替位Ti结构。
(4)基于自旋极化的GGA+U密度泛函理论研究了C阴离子和C阳离了掺杂TiO2的几何结构和电子性质。C阴离子掺杂TiO2的禁带中有数条较为分散的能级,并且电子在价带、导带和隙态之间的跃迁具有不同的光谱吸收能。C阳离子掺杂anatase和rutile相TiO2的光学带隙与未掺杂时相比分别减少了大约0.18和0.3eV。这解释了实验上观察到的C阴离子掺杂TiO2具有不同的光谱吸收阂值以及在C阳离子掺杂样品具有较低的光谱吸收能现象。对C阴离子掺杂TiO2的电子自旋极化及其磁耦合特性的研究表明当两个C杂质原子替换位于TiO6八面体两个相反的顶角O原子并形成略微弯曲的C-Ti-C结构时,体系会呈现强烈的铁磁耦合基态。
(5)对卤族元素掺杂TiO2的研究得到了以下结论：(i)在富氧条件下,Br和I替位Ti结构的形成优先于替位O结构,而F和Cl替位O结构的形成优先于替位Ti结构。在富钛条件下,所有的卤族原子替位O结构的形成都优先于替位Ti结构。(ii)对I替位Ti掺杂的TiO2,I杂质以I5+形式存在,并在禁带中引入了主要由I5s轨道组成的双重占据的隙态。对X(X=F、Cl、Br)替位Ti掺杂TiO2, F、Cl和Br杂质分别以F3+、Cl4+和Br4+离子形式存在。并且Cl和Br杂质分别在禁带中引入了由C13p和Br4p轨道组成的单占据的隙态。(iii)对X (X=F、Cl、Br)替位O掺杂Ti02,价带顶和导带底的相对位置与实验结果基本一致,并且TiO2禁带中间不存在隙态只是禁带宽度略有不同程度的减小。对I替位O掺杂的TiO2,I杂质在禁带中引入了主要由I 5p轨道组成的高于价带顶大约0.6eV的隙态,导致了光谱吸收能的减小。
(6)对Cr掺杂的anatase相TiO2的GGA+U计算成功地描述了Cr 3d态的劈裂行为,这与实验上发现的绝缘特性一致；并且电子在价带、导带和隙态之间的跃迁可以分别解释实验上发现的较大和较小的光谱吸收峰。同时研究表明替位Ti掺杂的Cr原子是以Cr4+(3d24s0)阳离子形式存在而并非普遍认为的Cr3+。
(7)对含有点缺陷的非掺杂TiO2的电子结构以及磁学性质的研究表明在anatase相TiO2中,由氧空位引入的多余电子把两个Ti4+离子还原为两个Ti3+离子,并且两个Ti3+产生的局域自旋磁矩形成了稳定的反铁磁耦合。在含有氧空位的rutile相TiO2中也存在类似的反铁磁态。这与实验上在氧缺乏的Ti-O系统中发现的反铁磁特征一致。与之对比,在含有钛空位的TiO2体系中,钛空位使周围的氧产生了自旋磁矩,并且两个钛空位周围氧产生的自旋磁矩可以形成稳定的铁磁耦合。我们推测类似的铁磁性也会出现在其他含有阳离子空位的半导体中。
(8)对掺杂Ⅱ-Ⅵ族和Ⅲ-Ⅴ族半导体的电子结构和磁学性质的研究表明：2p轻元素替换阴离子掺杂可以导致体系产生自发的自旋极化,然而掺杂元素的2p轨道在禁带中必须足够定域化才能产生稳定的磁性基态,并且体系能否产生稳定的磁性基态主要取决于半导体阴离子和掺杂元素间的相对电负性强度。
这些系统的理论研究工作揭示了非金属元素在TiO2中的掺杂位置与样品的生长条件以及掺杂剂本身的电负性之间的内在联系,有助于人们更加清楚地了解掺杂TiO2的电子结构与光谱吸收以及可见光的光催化活性等性质的关系和作用规律,并对实验上合成具有较高可见光光催化效率的TiO2体系提供有益的指导。此外,对2p轻元素掺杂Ⅱ-Ⅵ、Ⅲ-Ⅴ族半导体和TiO2体系的自旋极化和磁耦合特性研究也为实验上寻找“d0”磁性提供了理论基础。
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.
引文
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