Optical properties of anatase and rutile titanium dioxide: Ab initio calculations for pure and anion-doped material

The optical properties of rutile and anatase titanium dioxide (TiO₂) are calculated from the imaginary part of the dielectric function using pseudopotential density functional method within its generalized gradient approximation (GGA) and a scissors approximation. The fundamental absorption edges calculated for the unit cell of both rutile and anatase are consistent with experimentally reported results of single crystal rutile and anatase TiO₂ and with previous theoretical calculations. A significant optical anisotropy is observed in the anatase structure which holds promise for investigating the band gap modification with better visible-light response and provides a reliable foundation for addressing the effect of impurities on the fundamental absorption edge/band gap of anatase TiO₂. Further calculations on the electronic structure and the optical properties of C-, N-, and S-doped anatase TiO₂ are performed. The results are analyzed and discussed in terms of optical anisotropy and scissors approximations.     

Electronic structures and optical properties of TiO₂:Improved density-functional-theory investigation

TiO2 has been recently used to realize high-temperature ferromagnetic semiconductors.In fact,it has been widely used for a long time as white pigment and sunscreen because of its whiteness,high refractive index,and excellent optical properties.However,its electronic structures and the related properties have not been satisfactorily understood.Here,we use Tran and Blaha’s modified Becke-Johnson(TB-mBJ) exchange potential(plus a local density approximation correlation potential) within the density functional theory to investigate electronic structures and optical properties of rutile and anatase TiO2.Our comparative calculations show that the energy gaps obtained from mBJ method agree better with the experimental results than that obtained from local density approximation(LDA) and generalized gradient approximation(GGA),in contrast with substantially overestimated values from many-body perturbation(GW) calculations.As for optical dielectric functions(both real and imaginary parts),refractive index,and extinction coefficients as functions of photon energy,our mBJ calculated results are in excellent agreement with the experimental curves.Our further analysis reveals that these excellent improvements are achieved because mBJ potential describes accurately the energy levels of Ti 3d states.These results should be helpful to understand the high temperature ferromagnetism in doped TiO2.This approach can be used as a standard to understand electronic structures and the related properties of such materials as TiO2.     

低温拉曼光谱研究二氧化钛纳米晶的相变

本文通过对二氧化钛纳米晶在-190℃温度的低温拉曼光谱的研究,得到了在二氧化钛纳米晶聚集体中,可以发生锐钛矿到板钛矿然后到金红石和/或直接到金红石的相变;也可发生板钛矿到锐钛矿然后到金红石和/或直接到金红石的相变。     

Calculations of physical properties of pure and doped crystals: Ab initio and semi-empirical methods in application to YAlO3:Ce and TiO2

In this paper we demonstrate that two independent methods of calculations (DFT based ab initio and semi-empirical crystal field theory) can be used to form a complementary picture of the optical and electronic properties of the doped host and impurity ion. The crystals considered in the present paper are: (i) YAlO₃:Ce³⁺ and (ii) two dominant phases of TiO₂—rutile and anatase. As an example, detailed calculations of the band structure and crystal field energy level scheme of YAlO₃:Ce³⁺ are reported. From the analysis of the band structure and density of states, the character of the YAlO₃ energetic bands and positions of the Ce impurity energy levels were established. It was also shown how the ab initio methods can be used for calculations of the structural properties of solids under elevated pressure. Taking the two dominant phases of TiO₂ as an example, it was demonstrated how the elastic properties can be extracted from the calculated unit cell’s volume at different pressures. Particular attention was paid to the microscopic effects of crystal field, which were evidenced by the pressure-induced changes of the structure and shape of distribution of the Ti 3d electrons density of states. It was demonstrated how the difference in crystal structure of the anatase and rutile phases leads to remarkable difference in microscopic crystal field effects, which was explained by different Ti-O distances in both phases. In addition, the pressure dependence of the band gaps for anatase and rutile was investigated. It was shown that the hydrostatic pressure leads to the band gap narrowing in anatase and band gap widening in rutile, with pressure coefficients +0.00681 eV/GPa for rutile and −0.0088 eV/GPa for anatase.     

Optical properties of anatase and rutile TiO_2 studied by GGA + U

The optical properties of thermally annealed TiO_2 samples depend on their preparation process, and the TiO_2 thin films usually exist in the form of anatase or rutile or a mixture of the two phases. The electronic structures and optical properties of anatase and rutile TiO_2 are calculated by means of a first-principles generalized gradient approximation(GGA) +U approach. By introducing the Coulomb interactions on 3d orbitals of Ti atom(U~d) and 2p orbitals of O atom(U~p), we can reproduce the experimental values of the band gap. The optical properties of anatase and rutile TiO_2 are obtained by means of the GGA+U method, and the results are in good agreement with experiments and other theoretical data. Further, we present the comparison of the electronic structure, birefringence, and anisotropy between the two phases of TiO_2. Finally,the adaptability of the GGA+U approach has been discussed.     

锐钛矿相和金红石相TiO2:Nb的光电性能研究

采用基于密度泛函理论的第一性原理计算了锐钛矿相和金红石相TiO₂:Nb的晶体结构、电子结构和光学性质. 结果表明, 在相等的摩尔掺杂浓度下(6.25%), 锐钛矿相TiO₂:Nb的导带底电子有效质量小于金红石相TiO₂:Nb, 且前者室温载流子浓度是后者的两倍左右, 即具有更大的施主杂质电离率, 从而解释了锐钛矿相TiO₂:Nb比金红石相TiO₂:Nb具有更优异电学性能的实验现象. 光学计算也表明锐钛矿相在可见光区有更大的透过率, 从而在理论上解释了锐钛矿相TiO₂:Nb比金红石相TiO₂:Nb更适于做透明导电材料的原因. 计算结果与实验数据能较好符合. 关键词： 2:Nb')" href="#">TiO₂:Nb 第一性原理 电子结构 光学性能     

TiO_2纳米粒子的Ram an光谱表征和紫外可见光吸收特性

用 Sol- Gel方法制备粒径约 1 0 - 50 nm的 Ti O2 微粒。Raman光谱测量显示 ,水解时 p H值不同 ,在同样热处理温度下得到的 Ti O2 微粒的晶体结构不尽相同。在低于 673K热处理温度下得到的 Ti O2 微粒是板钛矿和锐钛矿的混合晶体。用谱峰拟合技术可以将样品 A1的1 57cm-1 Raman谱峰分解成分属于板钛矿和锐钛矿的两个振动峰。 UV- Vis测量表明 ,锐钛矿型 Ti O2 微粒的紫外光吸收性能最好。而粒度约 2 0 μm的市售锐钛矿型 Ti O2 的紫外光吸收性能甚至比板钛矿型和金红石型 Ti O2 纳米粒子的还略低些 ,表明小尺寸效应和表面缺陷对紫外光吸收亦有贡献     

Optical properties of germanium dioxide in the rutile structure

We present first-principles calculations for the optical properties of germanium dioxide in the rutile structure. The electronic band structure has been calculated self-consistently within the local density approximation using the full-potential linearized augmented plane wave method. The electronic band structure shows that the fundamental energy gap is direct at the center of the Brillouin zone. The determinant role of a band structure computation with respect to the analysis of the optical properties is discussed.     

锐钛矿和金红石二氧化钛中光生载流子的超快动力学：电声相互作用研究

本文结合玻尔兹曼输运方程和电声散射速率计算研究锐钛矿和金红石二氧化钛中光生载流子的超快动力学过程. 其中,动力学模拟所需的结构参数均通过第一性原理计算获得. 结果表明,由于存在强Fr?hlich型电声耦合,纵光学声子模对两个晶相的能量弛豫过程均有十分显著的影响,但是两个晶相的弛豫机理却表现出明显的差异. 对于单条导带内的弛豫过程,锐钛矿和金红石的能量弛豫时间分别为24.0 fs和11.8 fs,前者约为后者的二倍. 这一差异来源于两个晶相中不同的电子扩散分布以及不同的声学模散射贡献. 对于涉及多条导带的弛豫过程,预测的锐钛矿和金红石的总体弛豫时间分别为47 fs和57 fs,其相对大小与单条导带的情况相反. 分析表明金红石相弛豫较慢是因为存在多个速率控制步骤. 这些发现为调控电子动力学以及设计高效的二氧化钛器件提供了有价值的信息.     

N、C共掺杂锐钛矿相TiO2电子结构和光学性质的第一性原理研究

采用基于密度泛函理论的平面波超软赝势方法,研究了N和C原子分别单掺杂和双掺杂锐钛矿TiO2的形成能、晶体结构、电子结构和光学性质。计算结果表明：原子替位掺杂后体系晶格发生畸变;C替位掺杂更倾向于替代Ti位,而非O位;替位掺杂使TiO2光吸收带边发生了明显红移,且在可见光区域的吸收效率明显增加,大大提高了光催化效率。与单掺杂比较发现,N、C双掺杂红移现象更加明显,为较好的掺杂改性方式。     

纳米板钛矿基二氧化钛中的晶格畸变

借助精确的Ｘ射线衍射测量,结合独立衍射峰对不同晶粒度纳米板钛矿基二氧化钛样品中板钛矿和金红石相的微结构参量进行了较细致的研究,给出了标明物相、晶面指数（ｈｋｌ）和晶面间距ｄｈｋｌ值详细分析结果的系列ＸＲＤ谱,并得到了晶面间距、晶格常数、轴比、晶胞体积等参数数值及其相对常规粗晶的变化情况,从而初步揭示出其晶粒微结构的尺寸效应。首次发现,纳米板钛矿基二氧化钛样品中板钛矿和金红石相晶粒均存在着非单调的各     

N、C共掺杂锐钛矿相TiO2电子结构和光学性质的第一性原理研究

采用基于密度泛函理论的平面波超软赝势方法,研究了N和C原子分别单掺杂和双掺杂锐钛矿TiO2的形成能、晶体结构、电子结构和光学性质。计算结果表明：原子替位掺杂后体系晶格发生畸变;C替位掺杂更倾向于替代Ti位,而非O位;替位掺杂使TiO2光吸收带边发生了明显红移,且在可见光区域的吸收效率明显增加,大大提高了光催化效率。与单掺杂比较发现,N、C双掺杂红移现象更加明显,为较好的掺杂改性方式。     

Thermally stimulated transformations in brookite-containing TiO<Subscript>2</Subscript> nanopowders produced by the hydrolysis of TiCl<Subscript>4</Subscript>

TiO₂ nanopowder is produced by the low-temperature hydrolysis of TiCl₄. The phase composition of the sample is found to form at a hydrolysis temperature of 30–38°C. Low-temperature annealing (up to 440°C) increases the degree of crystallinity of the phases present in the sample (anatase, brookite) and only weakly affects their ratio. At 500°C, the sample consists of three phases: rutile is detected apart from anatase and brookite. Brookite begins to fail at 600°C; at 700°C, crystalline brookite fails completely.     

La-N共掺杂锐钛矿相TiO_2的电子结构及光学性质的第一性原理研究

利用基于密度泛函理论的第一性原理平面波超软赝势方法对锐钛矿相TiO_2、La单掺杂及La-N共掺杂锐钛矿相TiO_2的电子结构进行计算,分析La单掺杂及La-N共掺杂对锐钛矿相TiO_2的晶体结构、能带、态密度、差分电荷密度和光吸收性质的影响.结果表明,掺杂后TiO_2的晶格发生畸变,原子间键长的变化使晶格发生膨胀;掺杂后TiO_2的禁带宽度减小,并在禁带中引入杂质能级,导致TiO_2的吸收图谱产生红移现象;与La单掺杂相比,La-N共掺杂锐钛矿相TiO_2的红移程度增强.     

La-N共掺杂锐钛矿相TiO2的电子结构及光学性质的第一性原理研究

利用基于密度泛函理论的第一性原理平面波超软赝势方法对锐钛矿相TiO2、La单掺杂及La-N共掺杂锐钛矿相TiO2的电子结构进行计算,分析La单掺杂及La-N共掺杂对锐钛矿相TiO2的晶体结构、能带、态密度、差分电荷密度和光吸收性质的影响. 结果表明,掺杂后TiO2的晶格发生畸变,原子间键长的变化使晶格发生膨胀;掺杂后TiO2的禁带宽度减小,并在禁带中引入杂质能级,导致TiO2的吸收图谱产生红移现象;与La单掺杂相比,La-N共掺杂锐钛矿相TiO2的红移程度增强.     

Sb掺杂纳米TiO2光催化剂的晶体结构与光催化性能

用溶胶-凝胶法将纳米TiO2:Sb薄膜沉积在玻璃基板上．通过XRD、Raman光谱研究了Sb掺杂量对薄膜的晶体结构和晶相转变的影响．结果表明:纯TiO2薄膜中,TiO2不仅以无定型态存在,而且还以板钛矿和锐钛矿的形式存在．掺入适量的Sb后,由于Sb替代了TiO2的部分Ti形成Sb-O-Ti结构,改变了TiO2的晶格结构,改善了薄膜的结晶效率,使锐钛矿结构的TiO2:Sb含量明显提高．掺杂0．2％Sb时,薄膜的结晶效率最高．254 nm光源照射时,掺杂0．2％Sb的电极的阳极光电流密度可达42．49μA／cm2,是用同种方法制备的纯TiO2薄膜电极的近11倍;对亚甲基蓝具有最高分解性能,其一级反应速率常数为0．171 h／cm2,是未掺杂的纯TiO2薄膜的近2倍．     

First-principle study on optical properties of N-La-codoped anatase TiO

The electronic structures, deformation charge density, dipole moment, and optical properties of N-La-codoped anatase titanium dioxide (TiO2) are studied using the plane-wave ultrasoft pseudopotential method based on the density functional theory (DFT). The optical properties of two-ion-doped TiO2 are analyzed via electronic structures, deformation charge density, and dipole moment. For the model of N-La-doped TiO2 , a smaller atom fraction of N and La atoms induces better optical properties. The absorption edges of two doped TiO2 models redshift to the visible-light region.     

二氧化钒的电子结构及光学性质计算

本文使用基于量子力学第一性原理的CASTEP程序包,计算了单斜结构和金红石结构的二氧化钒(VO2)的电子结构、介电常数、吸收系数、折射率等光学性质。介电函数的虚部、吸收光谱、折射率等它们的峰值位置存在一一对应关系,这表明了它们之间存在着内在的联系,它们都与电子从价带到导带的跃迁吸收有关,这为从物理本质上理解二氧化钒的光学性质提供了重要的依据。计算结果与实验结果符合得很好。     

Electronic structures and optical properties of TiO2:Improved density-functional-theory investigation

TiO2 has been recently used to realize high-temperature ferromagnetic semiconductors.In fact,it has been widely used for a long time as white pigment and sunscreen because of its whiteness,high refractive index,and excellent optical properties.However,its electronic structures and the related properties have not been satisfactorily understood.Here,we use Tran and Blaha＇s modified Becke-Johnson（TB-mBJ） exchange potential（plus a local density approximation correlation potential） within the density functional theory to investigate electronic structures and optical properties of rutile and anatase TiO2.Our comparative calculations show that the energy gaps obtained from mBJ method agree better with the experimental results than that obtained from local density approximation（LDA） and generalized gradient approximation（GGA）,in contrast with substantially overestimated values from many-body perturbation（GW） calculations.As for optical dielectric functions（both real and imaginary parts）,refractive index,and extinction coefficients as functions of photon energy,our mBJ calculated results are in excellent agreement with the experimental curves.Our further analysis reveals that these excellent improvements are achieved because mBJ potential describes accurately the energy levels of Ti 3d states.These results should be helpful to understand the high temperature ferromagnetism in doped TiO2.This approach can be used as a standard to understand electronic structures and the related properties of such materials as TiO2.     

Si掺杂锐钛矿TiO2的第一性原理研究

为了研究Si掺杂对锐钛矿TiO2的电子蛄构和光催化性能的影响,利用基于第一性原理的密度泛函理论计算了纯TiO2及Si掺杂TiO2的杂质形成能、能带结构及态密度.研究蛄果表明,Si的掺杂位置与制备条件有关,富钛和富氧条件下,Si最容易代替TiO2中Ti的位置.几何优化后Si掺杂TiO2超晶胞的晶格参数和晶胞体积都发生一定...