1.College of Materials and Chemical Engineering, Hainan University, Haikou 570228, China 2.College of Materials Science and Engineering, Hunan University, Changsha 410082, China
Fund Project:Project supported by the Natural Science Foundation of Hainan Province, China (Grant Nos. 117085, 20165193).
Received Date:01 November 2018
Accepted Date:11 December 2018
Available Online:01 February 2019
Published Online:05 February 2019
Abstract: The crystal structures, defect formation energy, electronic structures and optical properties of oxygen vacancy and/or Ce-(co)doped anatase TiO2 are investigated by using density functional theory plus U calculations. The calculated results indicate that lattice distortion induces the enhanced octahedral dipole moment in Ce doped TiO2 crystal when introducing oxygen vacancy into the lattice of the TiO2 crystal, which is effective for separating the photo-excited electron-hole pairs; meanwhile, compared with the valence band of pure TiO2 and TiO2 mono-doped separately with Ce and oxygen vacancy, the valence band of TiO2 co-doped with Ce and oxygen vacancy broadens drastically, which is mainly contributed from the electronic states of Ce 5d, Ti 4s and O 2p in the valence band shifting toward the lower energy direction. As a result, Ce doped TiO2 with oxygen vacancy is beneficial to the mobility of photo-generated carriers in TiO2. Similarly, the anti-bonding states also move toward the lower band energy direction, which are formed by the mixture of Ce 4f, Ce 5d, Ti 3d, and O 2p orbits in the conduction band. Due to these shifts, the energy gap of Ce and oxygen vacancy codoped TiO2 is narrowed to 2.67 eV with the emerge of the occupied impurity energy levels near Fermi level. Because of the above-mentioned excellence features, the absorption spectra for doped systems exhibit remarkable red-shift, especially, the intensity of optical absorption of TiO2 co-doped with Ce and oxygen vacancy in the visible region and the infra-red region are obviously stronger than those of the Ce mono-doped TiO2. When introducing oxygen vacancy into the Ce-doped system, the calculated conduction band energy edge position changes from ?0.27 eV to ?0.32 eV, which implies that the reducing power of the conduction band edge of TiO2 is remarkably enhanced. More fascinatingly, the calculated band energy edges for the Ce and oxygen vacancy codoped TiO2 can satisfy the basic requirement for water splitting under visible light irradiation. In conclusion, Ce and oxygen vacancy co-doped system can effectively strengthen the photo-catalytic activity of TiO2 and improve the utilization of the solar light; and our calculated results provide a powerful theoretical basis for the applications of the Ce and oxygen vacancy co-doped anatase TiO2 in visible-light-driven water splitting in the future research. Keywords:Ce doping/ oxygen vacancy/ anatase TiO2/ electronic structure/ first-principles
表1采用GGA, GGA + U, LDA及LDA + U方法计算得到的纯锐钛矿相TiO2的晶格参数和带隙宽度 Table1.Lattice parameters and band gap width of pure anatase TiO2 calculated at the GGA, GGA + U, LDA and LDA + U levels of theory.
表3Ce/OV掺杂锐钛矿相TiO2的缺陷形成能(Ef)、平均净电荷(q)和平均偶极矩(p) Table3.Defect formation energy, average net charges and average dipole moment of Ce/OV doped TiO2 supercell after the structure relaxation.
其中, Edoped与Epure分别代表掺杂(缺陷)模型与纯TiO2超胞模型的总能量; ${\mu _{{\rm{Ce}}}}$, ${\mu _{{\rm{O}}}}$, ${\mu _{{\rm{Ti}}}}$分别表示Ce原子、O原子、Ti原子的化学势; m, n, l分别表示掺杂进入TiO2的Ce原子的个数, 以及从TiO2中移除的O原子和Ti原子的个数. 对于Ce-TiO2模型, m = 1, n = 0, l = 1; 类似地, OV-TiO2模型, m = 0, n = 1, l = 0; Ce/OV-TiO2模型, m = 1, n = 1, l = 1. 众所周知, 材料的缺陷形成能与其生长制备条件密切相关. 因此, 本文只计算富氧(O-rich)和富钛(Ti-rich)条件下的形成能. 在富氧条件下, 显然, ${\mu _{\rm{O}}} = {{{\mu _{{{\rm{O}}_2}}}} / 2}$(${{\mu _{{{\rm{O}}_2}}}}$为氧气分子的总能); 此时, ${\mu _{{\rm{Ti}}}}$的大小则由关系式${\mu _{{\rm{Ti}}{{\rm{O}}_{\rm{2}}}}} = $${\mu _{{\rm{Ti}}}} + 2{\mu _{\rm{O}}}$得出. 在富钛条件下, ${\mu _{{\rm{Ti}}}}$的大小取自密排六方相(hcp) $\alpha$-Ti金属中单个原子的能量; 此时, ${\mu _{{\rm{O}}}}$由 $2{\mu _{\rm{O}}} = {\mu _{{\rm{Ti}}{{\rm{O}}_{\rm{2}}}}} - {\mu _{{\rm{Ti}}}}$决定. Ce的化学势由关系式${\mu _{{\rm{Ce}}}} = {\mu _{{\rm{Ce}}{{\rm{O}}_{\rm{2}}}}} - {\mu _{{{\rm{O}}_{\rm{2}}}}}$获得. 各掺杂体系在富氧和富钛条件下的形成能的计算结果列于表3. 结果表明, OV-TiO2在富氧和富钛条件下的形成能的大小分别与Wu等[33]在富氧和富钛条件下的计算值1.00和5.30 eV极其接近. 比较共掺杂与单掺杂体系的形成能大小可知: Ce/OV共掺杂TiO2的缺陷形成能显著低于单掺杂TiO2的形成能, 负的形成能说明Ce/OV共掺杂体系具有良好的稳定性. 23.3.能带结构和态密度 -->
3.3.能带结构和态密度
基于DFT + U的计算方法, 对优化后的纯锐钛矿相TiO2和掺杂与缺陷体系的能带结构和态密度进行了计算. 为方便比较, 各体系的能带图均选取费米能级在能量零点来观察TiO2晶体沿布里渊区高对称方向的能带结构(如图2所示). TiO2掺杂前后各体系对应的总态密度(total density of states, TDOS)和分态密度(partial density of states, PDOS)图(如图3所示)中的费米能级也取为能量零点. 图 2 纯TiO2和Ce/OV掺杂TiO2的能带结构图(图中虚线代表费米能级EF) (a) Pure TiO2; (b) Ce-TiO2; (c) OV-TiO2; (d) Ce/OV-TiO2 Figure2. Band structure of anatase TiO2 before and after doping (the dashed lines represent the Fermi level, EF): (a) Pure TiO2; (b) Ce-doped TiO2; (c) OV-TiO2; (d) Ce/OV-TiO2.
图 3 纯TiO2和Ce/OV掺杂TiO2的态密度图(图中虚线代表费米能级EF) (a) Pure TiO2; (b) Ce-TiO2; (c) OV-TiO2; (d) Ce/OV-TiO2 Figure3. Comparison of partial density of states of anatase TiO2 before and after doping (the dashed lines represent the Fermi level, EF): (a) Pure TiO2; (b) Ce-doped TiO2; (c) OV-TiO2; (d) Ce/OV-TiO2.
(2)和(3)式中, 导带边的还原电势用ECB表示, 组成体系各原子的电负性的几何平均值用X表示, 自由电子的电势值Ee以标准氢电极(normal hydrogen electrode, NHE)为参照(~4.5 eV). 经计算纯TiO2的导带边和价带边的电势分别为?0.31和2.93 eV, 与实验观测值[39]极为相近. 图4为Ce/OV掺杂锐钛矿相TiO2的能带带边位置的示意图. 图 4 Ce/OV掺杂锐钛矿相TiO2的能带带边位置示意图(图中各带边位置的电位均以NHE的电位为参考零点, 粗黑线代表忽略杂质能级时的带边位置, 细红线代表考虑杂质能级时的带边位置) Figure4. Calculated band energy positions of pure TiO2, Ce-TiO2, OV-TiO2, Ce/OV-TiO2 (the energy scale is indicated by the normal hydrogen electrode in electron volts as a reference; the short thick lines and fine red lines represent the band energy positions of TiO2 by neglecting and considering the impurity levels, respectively).
式中C, V, BZ和K分别表示导带、价带、第一布里渊区和倒格矢; ${\left| {{M_ {\rm{CV}}}({{K}})} \right|^2}$为动量跃迁矩阵元; A为与允许直接跃迁吸收系数有关的常数; $\omega $为声子频率; EkC和EkV分别为导带和价带上的本征能级. 进一步由Kramers-Kroning变换关系[43], 可导出介电函数的实部${\varepsilon _1}$. 最终便可计算出材料的光吸收谱. 图5为Ce/OV掺杂锐钛矿TiO2的光吸收谱. 图 5 Ce/OV掺杂锐钛矿TiO2的光吸收谱(插图为可见光区光吸收谱的放大图) Figure5. Absorption spectra of pure and different doping TiO2 models (inset is the expanded absorption spectra in the visible region).