First-principle calculations on optical properties of C-N-doped and C-N-codoped anatase TiO2

The electronic structures, dipole moment and optical properties of C-N-doped and C-N-codoped anatase titanium dioxide (TiO₂) are studied using the plane-wave ultrasoft pseudopotential method of density functional theory (DFT). The results revealed that the absorption coefficients of pure TiO₂ and N-doped TiO₂ are consistent with experimental values in the visible-light region. The bands originating from C/N-2p states lie in the band gap of doped TiO₂. A visible-light absorption edge red-shift can be observed. The atomic charges have changed, resulting in devation of the center of gravity of the negative electric charge from the positive electric charge in the super-cell, and their dipole moment would not be zero. The dipole moment has large influence on the optical responses in the visible region of TiO₂. Because of the small distance (0.531 nm) between C and N atoms, the covalent bond component was easily enhanced between C atom and adjacent O atom, the covalent bonds making it more difficult for the carrier transfer. Moreover, its optical absorption coefficient is going to reduce in the visible-light region. Under the condition of the larger distance (0.691 nm) between C and N atoms, their interaction can be reduced, which is beneficial to electrons transition; as a result, a significant improvement of the photocatalytic activity of TiO₂ has been found under the visible-light irradiation.     

First-principles study of electronic structures and optical properties of Cu, Ag, and Au-doped anatase TiO2

We perform first-principles calculations to investigate the band structure, density of states, optical absorption, and the imaginary part of dielectric function of Cu, Ag, and Au-doped anatase TiO₂ in 72 atoms systems. The electronic structure results show that the Cu incorporation can lead to the enhancement of d states near the uppermost of valence band, while the Ag and Au doping cause some new electronic states in band gap of TiO₂. Meanwhile, it is found that the visible optical absorptions of Cu, Ag, and Au-doped TiO₂, are observed by analyzing the results of optical properties, which locate in the region of 400-1000 nm. The absorption band edges of Cu, Ag, and Au-doped TiO₂ shift to the long wavelength region compared with the pure TiO₂. Furthermore, according to the calculated results, we propose the optical transition mechanisms of Cu, Ag, and Au-doped TiO₂. Our results show that the visible light response of TiO₂ can be modulated by substitutional doping of Cu, Ag, and Au.     

过渡金属(Fe,Co,Ni,Zn)掺杂金红石TiO2的电子结构和光学性质

采用基于密度泛函理论的第一性原理方法对未掺杂以及不同浓度过渡金属Fe,Co,Ni,Zn掺杂金红石TiO₂的超晶胞体系进行了几何优化,并讨论了其晶格常数,电子能带结构和光学性质.研究结果表明:掺杂前后的晶格参数与实验值偏差在3.6%以下;适量的过渡金属掺杂不但影响体系能带结构,拓宽光吸收范围,而且扮演着俘获电子的重要角色,有利于光生电子-空穴对的有效分离以及增强光吸收能力;Fe,Co,Ni,Zn最佳理论掺杂体系分别为Ti_0.75Fe_0.25O₂,Ti_0.75Co_0.25O₂,Ti_0.75Ni_0.25O₂,Ti_0.83Zn_0.17O₂;Fe,Co,Ni3d态分裂为t_2g和e_g态,分别贡献于价带高能级和导带低能级部分,促进了电子-空穴对的生成,从而可提高TiO₂的光催化性能;Zn3d态电子成对填满轨道,不易被激发,故光催化活性无明显提高.     

Concentration-dependent electronic structure and optical absorption properties of B-doped anatase TiO2

The concentration-dependent electronic structures and optical properties of B-doped anatase TiO₂ have been calculated using the density functional theory. The calculated results indicate that the electronic structures of B-doped TiO₂ have changed compared with those of pure TiO₂, which is mainly due to the new midgap states induced by B doping. As to the optical properties, we calculate the imaginary part of dielectric function ε₂(ω) and optical absorption spectra of pure and B-doped TiO₂. Two transitions E₁ and E₂ emerged after B doping. The intensity of absorption is enhanced by B doping both in the UV and visible regions. According to the results of imaginary part of dielectric function ε₂(ω) and DOS, it can be concluded that the two optical transitions correspond to the transitions from the O 2p states in the top of valence band to the midgap states and from the midgap states to the Ti 3d states in the bottom of conduction band, respectively. These results have important implications for the further development of photocatalytic materials.     

Effects of native defects on the electronic structure and photocatalytic activity in anatase TiO2 by first principles calculations

The crystal structure, electronic structure, optical properties and photocatalytic activity of the native defects in anatase TiO₂ were investigated based on the density-functional theory (DFT). The results show that oxygen vacancies (V_O) have the lowest formation energy, and thus are easiest to form in the bulk structure. The conduction and valence band moves to the high or low energy region, and the energy gap becomes narrower for the native point defect models. In particular, oxygen interstitials (O_i) have a direct band gap, and new gap states appear in the band gap, which can be responsible for the high photocatalytic efficiency in anatase TiO₂. The phenomenon of “impurity compensation” takes place for the oxygen and titanium interstitials. Ti vacancy (V_Ti) can promote the utilization of solar light by analyzing the absorption spectra. All the calculated results show that O_i and V_Ti are beneficial in improving the photocatalytic activity of TiO₂ in the UV–visible light range.     

锐钛矿二氧化钛体相缺陷的准粒子能带结构

Quasiparticle band structures of the defective anatase TiO₂ bulk with O vacancy, Ti interstitial and H interstitial are investigated by the GW method within many-body Green''s function theory. The computed direct band gap of the perfect anatase bulk is 4.3 eV, far larger than the experimental optical absorption edge (3.2 eV). We found that this can be ascribed to the inherent defects in anatase which drag the conduction band (CB) edge down. The occupied band-gap states induced by these defects locate close to the CB edge, excluding the possible contribution of these bulk defects to the deep band-gap state below CB as observed in experiments.     

Photocatalytic reduction of oxygen molecules at the (1 0 0) TiO2 anatase surface

The adsorption, photoreduction and chemical activity of oxygen molecules on the (1 0 0) anatase surface have been investigated here together with the effects that surface oxygen vacancies (V_O) can have on these O₂-related processes. We use an original approach by treating molecules on the TiO₂ surface like surface defects in the same framework successfully used for defects in semiconductors. The achieved results: (i) give the first theoretical evidence of an acceptor behaviour of an adsorbed O₂ molecule, which is at the origin of its photoreduction; (ii) show that the V_O donor character is strongly affected by the interaction with O₂; and (iii) suggest that the release of radicals as well as the formation of O₂-related radicals may be favoured by photogenerated electrons in presence of surface V_O’s.     

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     

Influence of dopant level on structural, optical and magnetic properties of Co-doped anatase TiO2 nanoparticles

Cobalt-doped TiO₂ nanoparticles were synthesized by sol-gel method. The associated structural, optical, compositional and magnetic properties of the nanoparticles as a function of cobalt concentration have been systematically studied. The X-ray powder diffraction reveals that all samples have pure anatase phase tetragonal system and the lattice parameter analysis indicated that Co ions may substitute into the lattice of TiO₂. The average particle size is 15 nm, when found through transmission electron microscope. Optical spectroscopy measurement showed that the bandgap value decreases upon increasing Co concentration. The magnetic measurements revealed that the enhanced room temperature ferromagnetism (RTFM) strongly depends on the doping content.     

Different effects of cerium ions doping on properties of anatase and rutile TiO2

Pure and Ce⁴⁺ doped anatase and rutile TiO₂ were prepared by hydrothermal methods and characterized by XRD, TEM, UV-vis diffusion spectroscopy, and XPS measurements. The photocatalytic reactivity of the catalysts was evaluated by the photodegradation of Rhodamine B (RB) under ultraviolet irradiation. The photocatalytic efficiency of the rutile sample doped with an appropriate amount of Ce⁴⁺ was enhanced while all Ce⁴⁺ doped anatase samples showed a much lower activity than pure anatase. The reasons were discussed     

Electronic and optical properties of pure and Mo doped anatase TiO2 using GGA and GGA+U calculations

Comparative GGA and GGA+U calculations for pure and Mo doped anatase TiO₂ are performed based on first principle theory, whose results show that GGA+U calculation provide more reliable results as compared to the experimental findings. The direct band gap nature of the anatase TiO₂ is confirmed, both by using GGA and GGA+U calculations. Mo doping in anatase TiO₂ narrows the band gap of TiO₂ by introducing Mo 4d states below the conduction band minimum. Significant reduction of the band gap of anatase TiO₂ is found with increasing Mo doping concentration due to the introduction of widely distributed Mo 4d states below the conduction band minimum. The increase in the width of the conduction band with increasing doping concentration shows enhancement in the conductivity which may be helpful in increasing electron–hole pairs separation and consequently decreases the carrier recombination. The Mo doped anatase TiO₂ exhibits the n-type characteristic due to the shifting of Fermi level from the top of the valence band to the bottom of the conduction band. Furthermore, a shift in the absorption edge towards visible light region is apparent from the absorption spectrum which will enhance its photocatalytic activity. All the doped models have depicted visible light absorption and the absorption peaks shift towards higher energies in the visible region with increasing doping concentration. Our results describe the way to tailor the band gap of anatase TiO₂ by changing Mo doping concentration. The Mo doped anatase TiO₂ will be a very useful photocatalyst with enhanced visible light photocatalytic activity.     

N,Co共掺杂锐钛矿相YiO2光催化剂的第一性原理研究

采用第一性原理平面波超软赝势方法研究了N,Co共掺杂锐钛矿相TiO2的微观结构和光学性质.结果表明:N,Co共掺杂后TiO2晶格中产生的偶极矩使光生电子-空穴对更有效地分离;在TiO2导带和价带之间形成了新的杂质能级,一方面使吸收带边红移到可见光区,光吸收性能明显增强,另一方面有利于光生电子-空穴对的分离,提高TiO2的光量子效率;与纯TiO2相比,N,Co共掺杂锐钛矿相TiO2带边的氧化还原势只有微小的变化,共掺杂后TiO2的强氧化还原能力得以保持.     

N掺杂锐钛矿TiO2光学性能的第一性原理研究

用平面波赝势方法(PWP)计算了N掺杂锐钛矿型TiO₂前后的光学特性,即介电函数虚部ε₂(ω)，光学吸收系数I(ω)和反射率R(ω). 并从能带结构上解释了为什么掺N后锐钛矿型TiO₂的光学谱在2.93，3.56和3.97eV处相对掺杂前会出现3个峰值的原因. 从光谱图上分析得出，掺杂后TiO₂要发生红移现象，实验现象证实了这一结果. 关键词： N掺杂 2')" href="#">锐钛矿型TiO₂ 光学性能 第一性原理     

Mn,N共掺杂对锐钛矿相TiO2微观结构和性能的影响

采用第一性原理平面波超软赝势方法,系统研究了Mn,N共掺杂对锐钛矿相TiO₂的晶体结构、缺陷形成能、电子结构、光学性质以及氧化还原能力的影响.研究表明:Mn,N共掺杂锐钛矿相TiO₂后,TiO₂晶格发生了畸变,导致晶体八面体偶极矩增加,有利于光生电子-空穴对的有效分离;在TiO₂带隙中出现了杂质能级,使锐钛矿相TiO₂的光学吸收带边红移,可见光区的吸收系数明显增大,有利于光催化效率的提高;在不考虑 关键词： 2')" href="#">锐钛矿相TiO₂ 第一性原理 Mn和N共掺杂 光催化性能     

Sm-N共掺杂对锐钛矿相TiO2的电子结构和吸收光谱影响的第一性原理研究

采用基于密度泛函理论框架下的第一性原理平面波超软赝势方法, 建立了N, Sm分别单掺杂以及Sm-N共掺杂的锐钛矿TiO₂超胞模型, 对其态密度、能带结构和吸收光谱进行了计算. 结果表明: N单掺杂的锐钛矿TiO₂的红移效果最强, 但Sm-N共掺杂锐钛矿TiO₂的载流子寿命更长, 且共掺杂形成的体系更加稳定.     

Optical and hydrophilic properties of Cr doped TiO2-SiO2 nanostructure thin film

Cr doped TiO₂-SiO₂ nanostructure thin film on glass substrates was prepared by a sol-gel dip coating process. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to characterize the structural and chemical properties of the films. A UV-vis spectrophotometer was used to measure the transmittance spectra of the thin film. The hydrophilicity of the thin film during irradiation and storage in a dark place was measured by a contact angle analyzer. The results indicated that Cr doping has a significant effect on the transmittance and super-hydrophilicity of TiO₂-SiO₂ thin film.     

Microstructural, optical and photocatalytic properties of CdS doped TiO2 thin films

CdS doped TiO₂ thin films (with CdS content=0, 3, 6, 9 and 12 at%) were grown on glass substrates. The X-ray diffraction analysis revealed that the films are polycrystalline of monoclinic TiO₂ structure. The microstructure parameters of the films such as crystallite size (D_ν) and microstrain (e) are calculated. Both the crystallites size and the microstrain are decreased with increasing CdS content. The optical constants have been determined in terms of Murmann's exact equations. The refractive index and extinction coefficient are increased with increasing CdS content. The optical band gap is calculated in the strong absorption region. The possible optical transition in these films is found to be an allowed direct transition. The values of E_g^opt are found to decrease as the CdS content increased. The films with 3 at% CdS content have better decomposition efficiency than undoped TiO₂. The films with 6 at% and 9 at% CdS content have decomposition efficiency comparable to that of undoped TiO₂, although they have lower band gap. The CdS doped TiO₂ could have a better impact on the decomposing of organic wastes.     

Electronic structures and optical properties of rutile TiO2 with different point defects from DFT + U calculations

The electronic states and formation energies of four types of lattice point defects in rutile TiO₂ are studied using the first-principles calculations. The existence of oxygen vacancy leads to a deep donor defect level in the forbidden band, while the Ti interstitial forms two local states. It is predicted that oxygen vacancy prefers to combine with Ti-interstitial to form V_O–Ti_i dimer by a partial 3d electron transfer from the Ti_i to its neighboring V_O. The charge distribution between a Ti interstitial and its neighboring Ti ions partially shields the Coulomb interactions. Lastly, optical properties of these defective lattices are discussed.     

S掺杂对锐钛矿相TiO2电子结构与光催化性能的影响

采用基于第一性原理的平面波超软赝势方法研究了掺杂不同价态S的锐钛矿相TiO₂的晶体结构、杂质形成能、电子结构及光学性质.计算结果表明硫在掺杂体系中的存在形态与实验中的制备条件有关;掺杂后晶格发生畸变、原子间的键长及原子的电荷量也发生了变化,导致晶体中的八面体偶极矩增大; S 3p态与O 2p态、Ti 3d态杂化而使导带位置下移、价带位置上移及价带宽化,从而导致TiO₂的禁带宽度变窄、光吸收曲线红移到可见光区.这些结果很好地解释了S掺杂锐钛矿相TiO₂在可见光下具有优良的光催化性能的内在原因.根据计算结果分析比较了硫以不同离子价态掺杂对锐钛矿相TiO₂电子结构和光催化性能影响的差别. 关键词： 2')" href="#">锐钛矿相TiO₂ S掺杂 第一性原理 光催化性能     

氧空位对共掺型锐钛矿TiO2能带结构的影响

基于电子密度泛函理论计算,系统研究了氧空位对V-C(p型)、Cr-N(n型)、V-N和Cr-C(电中性)四种共掺型锐钛矿TiO₂的几何和电子结构的影响．结果表明在最稳定构型中氧空位处在与金属杂质原子相连的位置．由于氧空位的出现,共掺TiO₂禁带中一些未占据的杂质态被填充满电子,掺杂体系的导带带边位置几乎不变,这有利于减少光生载流子复合,且不影响光解水过程中的产氢反应,在厌氧条件下很容易在V-N共掺TiO₂中引入氧空位．