1.Public Health and Management School, Hubei University of Medicine, Shiyan 442000,?China 2.Hubei Biomedical Detection Sharing Platform in Water Source Area of South to North Water Diversion Project, Shiyan 442000, China
Abstract:By means of state-of-the-art density functional theory (DFT) computations, We designed a new two-dimensional material TiO2. We further investigated the stability, electronic structure, carrier mobility, and optical properties of monolayer TiO2. Our results show that monolayer TiO2 has good kinetic, thermodynamic and mechanical stability and can exist stably at room temperature. The results were demonstrated using the binding energy, phonon spectrum, molecular dynamics simulation, and elastic constant calculation. The band structure indicates that the monolayer TiO2 is an indirect bandgap semiconductor with energy gaps of 1.19 eV (GGA+PBE) and 2.76 eV (HSE06), respectively. The results of state density show that the Ti-3d state electrons constitute the top of the valence band and Ti-4s state electrons constitute the bottom of the conduction band. The electron states of O atoms contribute very little near the Fermi energy level and are mainly distributed in the deep energy level. In addition, the carrier mobility of monolayer TiO2 is smaller than monolayer MoS2, and the electron and hole mobility can reach 31.09 cm2·V–1·s–1 and 36.29 cm2·V–1·s–1, respectively. Due to the anisotropy of hole mobility and electron mobility, the composite rate of electrons and holes is relatively low. This ensures longer service life and better photocatalytic activity of monolayer TiO2. Furthermore, under the condition of uniaxial strain and biaxial strain, the energy gap of monolayer TiO2 has a clear response. The energy gap is more sensitive to biaxial strain than uniaxial strain, indicating that monolayer TiO2 can be applied to various semiconductor devices. The band-edge potential and optical properties of semiconductors indicate that two-dimensional TiO2 is capable of photo-splitting water production, H2 at –5~2% single/biaxial strain, and O2, H2O2, O3, etc. at –5~5% single/biaxial strain. Moreover, the monolayer TiO2 has a high absorption coefficient for visible and ultraviolet light. In conclusion, the monolayer TiO2 has a potential application prospect in the field of optoelectronic devices and photocatalytic materials in the future. Keywords:two-dimensional TiO2/ first principles/ electronic structures/ optical properties
表2二维TiO2有效质量${m^ * }$, 形变势常数$E_{_{\rm{d}}}^i$, 弹性常数${C^{{\rm{2 D}}}}$和载流子迁移率${\mu _{{\rm{2 D}}}}$ Table2.Calculated effective mass ${m^ * }$, deformation potential constant $E_{_{\rm{d}}}^i$, elastic modulus ${C^{{\rm{2 D}}}}$, and carrier mobility ${\mu _{{\rm{2 D}}}}$ for monolayer TiO2 along the a ($\zeta \to K$) and b ($\zeta \to G$) directions, where $\zeta $ represents the position of the valence band top and the conduction band bottom.
图 7 (a) 二维TiO2沿a/b方向的总能量与应变量$\Delta l/l$的关系, 采用二次数据拟合二维结构的平面刚度, 黑色和红色曲线表示沿a和b方向的面内刚度; (b), (c)单层TiO2的VBM和CBM随应变量相对真空能级的变化, 采取线性拟合计算形变势 Figure7. (a) The relation between total energy and the applied strain $\Delta l/l$ along the a/b directions of monolayer TiO2. The quadratic data fitting gives the in-plane stiffness of 2D structures. Black and red curves show the in-plane stiffness along the a and b directions of monolayer TiO2. The shift of VBMs and CBMs for (b-c) monolayer TiO2 with respect to the vacuum energy, as a function of the applied strain along either the a and b direction. The linear fit of the data yields the deformation potential constant.
从图7可以看出, 二维TiO2沿a和b两个方向上的总能量、CBM和VBM变化趋势非常相似, 因而两个方向上的平面刚度(21.27, 21.28 N·m–1)和形变势(3.42, 3.38 eV)差异很小, 如表2所列. 计算载流子有效质量发现, 二维TiO2空穴质量明显大于电子质量, 这说明价带顶附近能级较导带底附近能级更加平坦, 对照图5也证实了这一点. 根据(4)式可知, 正是有效质量的不同导致载流子迁移率表现出明显的各向异性, 沿a, b方向上的电子迁移率为19.92和31.09 cm2·V–1·s–1, 空穴迁移率分别为30.75和36.29 cm2·V–1·s–1, 这比单层MoS2的迁移率(200 cm2·V–1·s–1)[62]要小. 这说明二维TiO2载流子迁移率相对较小, 并且电子和空穴的迁移率表现出明显差异, 即两者的分离效率和异步化程度较高, 导致电子和空穴的复合率较低[63], 这样的半导体更耐用, 光催化活性更好. 为进一步研究弹性形变对二维TiO2电子结构的影响, 分别计算了在–5%—5%的面内压缩/拉伸应变下TiO2的能隙变化, 计算选取HSE06函数, 结果如图8所示. 很明显, 二维TiO2的能隙对弹性应变有着明显响应, 其能隙随压缩/拉伸应变单调增大/减小, 并且在沿a/b轴双轴方向应变下能隙变化得更快, 这说明二维TiO2的能隙可以通过形变进行调控, 以适用于各种电子器件的需求. 图 8 单/双轴应变下能隙变化 Figure8. Band gap of monolayer TiO2 under uniaxial/biaxial strain, calculated using the HSE06 functional.