【作者】 赵冬秋；

【导师】 李蕴才；

【作者基本信息】 河南大学 ， 凝聚态物理， 2011， 博士

【摘要】 随着计算机科学的迅猛发展和量子化学算法的不断改进,材料计算在物理研究中占有越来越重要的位置。材料计算不仅可以用来解释实验现象,更为重要的是还可以预测材料的性能以开发出新型功能材料。近年来,N掺杂TiO₂材料由于可显著提高可见光催化活性而备受关注;然而目前人们对其可见光催化活性机理的认识尚不统一,其微观电子结构性质尚需进一步借助于当前的计算模拟技术进行研究。为此本论文采用第一性原理方法计算了N掺杂TiO₂各体系的电子结构,在此基础上对其电子结构变化的原因和可见光催化活性进行了深入的分析和讨论。主要内容如下:第一章主要介绍了有关二氧化钛的一些基本情况。二氧化钛因具有较好的稳定性、无毒、廉价和光催化活性高等优点而成为当前实验和理论的一个重要研究对象。我们首先简单介绍了TiO₂半导体的光催化基本原理和主要用途;接着简述了实验和理论两方面对TiO₂进行掺杂改性研究的一些方法;然后着重介绍了N掺杂TiO₂光催化剂的研究进展和目前存在的主要问题。最后针对目前存在的主要问题,提出本论文的研究意义和主要研究内容。第二章在简单回顾材料计算科学和量子化学发展的基础上,依次详细介绍了密度泛函理论方法的理论基础和几种常用的交换关联能泛函;最后针对本文所使用的Castep模拟软件进行了简单的介绍。第三章采用基于密度泛函理论的自旋极化平面波计算方法,研究了锐钛矿TiO₂及N掺杂TiO₂的电子结构,分析了其可见光催化活性提高的原因。本文的计算数据表明,与纯TiO₂的相比,N掺杂并未改变体系导带底的位置而使其价带顶升高了0.272 eV,且在略高于价带顶的带隙处引进由N 2p、O 2p和Ti 3d态组成的带间隙态。这三个态的共同作用使得受主态能级展宽增大和带间隙电子态在实空间分布上表现出既有局域性又有离域性的特点。Mulliken集居数分析和差电子密度分析揭示了N掺杂TiO₂电子结构变化的原因:一方面由于N比O少一个电子使得单一N掺杂TiO₂超胞体系少一个电子,在体系中引进受主态;另一方面N取代晶格O掺杂后,N、O电负性差导致在体系中形成相对较强的N-Ti共价键,受此相对较强共价电子云的影响体系电子结构发生了变化。价带的上移和扩展带隙态提高了可见光催化活性,较好的解释了实验现象。第四章通过对三类模型的计算,系统研究了氧空位对N掺杂TiO₂体系电子结构及光催化活性的影响。考虑到氧空位和N杂质间的相互作用,我们分别采用两种方法对这三类体系的电子结构性质进行计算:即对含施主态和不含施主态的体系分别采用DFT+U方法和标准DFT方法。含氧空位TiO₂的计算结果显示:氧空位在带隙中引进了深施主能级,该深施主电子态在实空间分布上具有较强的局域特性。一个氧空位与一个近邻N共存体系的计算结果表明,在带隙中出现两个局域态密度峰,其中一个峰位于价带顶附近,另一个则位于距价带顶约1 eV处。氧空位与两个近邻N共存体系的计算数据显示,当两个NTi₃单元不在同一平面时,价带顶向高能端移动了0.18 eV,当两个NTi₃单元在同一平面时,在近邻价带顶之上出现局域态密度峰。基于以上各体系的电子结构性质,分别对每个体系电子结构改变的原因展开了较深入的分析;最后对各体系的可见光光催化活性进行了简单的讨论。第五章应用超软赝势自旋极化平面波方法,研究了H对N掺杂TiO₂体系的影响。计算结果显示H与N结合可以改善N掺杂TiO₂体系的稳定性,提高体系的含N量;在某种条件下,含N量增加一倍的H、N共存体系比单一N掺杂体系还稳定。此外,各体系的电子结构数据表明,N/H共掺杂TiO₂的价带顶向高能端发生微小的偏移,带隙态消失;这说明在N掺杂浓度相同的情况下,该体系在提高可见光活性上并不优于单一N掺杂的TiO₂体系。然而,对于含N量增加一倍的N、H共掺杂TiO₂体系,价带被抬高了0.54 eV,且价带之上的受主态与价带电子态在某种程度上发生一定的混杂;与单一N掺杂TiO₂的计算结果相比,费米能级附近的电子态离域性增强;这些因素都易于促进体系可见光催化活性的提高。各体系电子结构变化的主要原因是由于N-Ti键的电子能级相对较高所致。更多还原

【Abstract】 Along with the swift development of computer technology and the constant improvement of computational methods of quantum chemistry, material computational technology has become more and more important in modern physics. This is because it has been used to interpret the experimental phenomena, and the special important role is that it can be used to predict and develop the new functional materials. On the other hand, recent researches have reported that N-doped TiO₂ can remarkably improve the photocatalytic activity under visible light irradiation, and it has received extensive attention. However, the uniform unambiguous understanding on the mechanism of visible-light photocatalysis of N-doped TiO₂ is still lacking, and its electronic structural properties still need to be deeply investigated by modern computational simulation techniques. So the first principles calculations are employed to investigate some systems about N-doped TiO₂ in this paper. Based on the calculated results, we further discuss the reasons of the electronic structural change and the visible-light photocatalytic activity of all the doped systems. The main contents are presented as the followings:In the first chapter, some main basic information about TiO₂ is introduced. Titanium dioxide, as one of the most promising photocatalyst, has exceptional properties such as nontoxicity, low cost, and long-term stability against chemical corrosion; and has attracted worldwide research concerns. Firstly, we simply introduce the basic principle of photocatalyic oxidation and the major applications of the TiO₂. Secondly, some common methods used to improve the visible-light photoactivity of TiO₂ by doping are narrated. Then, we highlight the progresses and some unresolved problems in research on N-doped TiO₂. Finally, based on the present problems, we describe the purpose and meaning of our research on N-doped TiO₂。In the second chapter, we firstly give a brief review on the development of computational material science and quantum chemistry. Then, we focus on describing the theoretical basis of the density-function methods and several common exchange-correlation functional approaches in details. At last, we simply introduce the simulation package Castep used in this work.In the third chapter, we investigate the anatase TiO₂ doped with N by using spin-polarized plane-wave method based on density functional theory. The calculated results show that in comparision with pure TiO₂, the conduction band minimum is almost unchanged. However, its valence band maximum shifts to high energy by 0.272 eV, and the band-gap states composed of N 2p, O 2p and Ti 3d states are formed through the three states entering into the gap. The combinations of the three states make the band-gap states expanded and delocalized to some extent. The origin of the electronic structural changes for N-doped TiO₂ is revealed by the electron density difference and the population analysis. It is because that the N-doped TiO₂ supercell has one electron less than pure TiO₂ one and the comparatively stronger N-Ti covalent bonds are formed due to the effect of electronegative difference between nitrogen and oxygen. The lifted VBM and the wide band-gap states enhance the visible-light photocatalytic performance of N-doped TiO₂, providing a good interpretation for the experiment phenomena.In the fourth chapter, we systematically investigate the effect of oxygen vacancy on the electronic structure and the photocatalytic performance of N-doped TiO₂ by calculating three kinds of models. Considering the interactions between the oxygen vacancy and the N dopant, we employ two different methods to calculate the electronic structural properties of the three systems, respectively. The DFT+U and the standard DFT methods are used to study the systems with and without donor states, respectively. The calculated results of titania containing oxygen vacancy show that the oxygen vacancy introduces the deep donor states in band gap, and the distributions of the band-gap states have highly localized character in real space. The calculations of TiO₂ supercell including one oxygen vacancy next neighboring to a nitrogen indicate that there exist two localized density of state peaks in band gap. One peak just locates above the valence band maximum and the other one sites at about 1 eV above the valence band maximum. The obtained data by calculating the TiO₂ supercells with one oxygen vacancy next neighboring to two N, describe that the valence band maximum is shifted to high energy by 0.18 eV as the two NTi₃ units lie in different plane, and that there localizes one density of state peak just above the valence band maximum as the two NTi₃ units place in the same plane. According to the data, we further analyze the reason of electronic structural change for every system. At last, the visible-light photocatalytic activities of all systems are simply discussed.In the fifth chapter, the ultrasolft-pseudopotential spin-polarized plane-wave method is used to investigate the influence of H on the N-doped TiO₂. The calculated results show that the H bonding to the N in N-doped TiO₂ can improve the stability of systems, and hence increases the contents of N dopant. Under certain conditions, the TiO₂ containing N bonded with H is more stable than the TiO₂ doped with single N even if the N concentration in former system is twice as high as that in the latter one. In addition, the electronic structure calculations for all systems show that the valence band maximum of N/H-codoped TiO₂ slightly shifts to high energy and the band-gap states of TiO₂ doped with single N disappear. This means that with the same N contents, the single N-doped TiO₂ has advantage over the N/H-codoped TiO₂ in improvement of visible-light photocatalytic performance. However, for the TiO₂ supercell including one nitrogen as well as a N bonded to a H, the valence band maximum is upraised by about 0.54 eV, the acceptor states weakly mix with valence band, and the distribution of the electronic states near the Fermi level has highly delocalized nature compared to the single N-doped TiO₂. All these factors can promote the increase of visible-light photocatalytic activity. The reasons of electronic structural variations for all N-doped TiO₂ with H, are mainly attributed to the relatively higher electron energy in N-Ti bond.更多还原