第一性原理研究Mn和Cu掺杂六钛酸钾(K_2Ti_6O_(13))的电子结构和光学性质

六钛酸钾(K_2Ti_6O_(13))是宽带隙半导体光催化材料,只能响应波长较短的紫外光.为了使K_2Ti_6O_(13)对可见光响应,本文采用第一性原理方法,研究过渡金属Mn和Cu掺杂改性后K_2Ti_6O_(13)的电子结构和光学性质.计算结果表明:Mn,Cu掺杂后K_2Ti_6O_(13)禁带中出现了杂质能级,这些杂质能级由O 2p和Ti 3d与Mn 3d或Cu 3d态杂化而成.对于Mn掺杂的K_2Ti_6O_(13),其带隙值变小,位于能带中间的杂质能级可作为电子跃迁的桥梁,从而实现了对可见光的吸收.对于Cu掺杂的K_2Ti_6O_(13),其带隙值虽略有增大,但是若考虑将与价带相连的杂质能级,带隙值将大大减小,且此杂质能级可抑制光生载流子的复合,使得掺杂后K_2Ti_6O_(13)吸收带边红移至可见光区并在可见光范围内吸收强度明显增强.总的而言,Mn,Cu的掺杂实现了钛酸钾对可见光的吸收,同时Cu掺杂的效果要优于Mn掺杂的效果.研究结果对K_2Ti_6O_(13)在光催化领域上的应用具有重要的意义.

First-principles study of electronic structures and optical properties of Mn and Cu doped potassium hexatitanate (K2Ti6O13)

Potassium hexatitanate (K₂Ti₆O₁₃) is a kind of wide band-gap semiconductor material with potential applications in photocatalysis. Unfortunately, it only responds to the short wavelengths of ultraviolet light, which seriously limits the utilization efficiency of solar energy. To extend its response to visible light, a promising strategy is to partly substitute some other transition metals for the Ti element. In this work, the electronic structures and optical properties of Mn-and Cu-doped K₂Ti₆O₁₃ are systematically investigated by the first-principles calculations with the aid of the CASTEP module in the Materials Studio software package. The PW91 exchange-correlation functional is used with a plane wave basis set up to a 340 eV cutoff. The computational results show that the Mn-and Cu-doped K₂Ti₆O₁₃ have impurity bands mainly stemming from the mix of Mn or Cu 3d states with Ti 3d states and O 2p states. Compared with the band gap of pristine K₂Ti₆O₁₃ (2.834 eV), the band gap of Mn-doped one becomes narrow (2.724 eV), and its impurity energy level in the middle of the band gap can be used as a bridge for electronic transitions to facilitate the absorption of visible light. Although the band gap of Cu-doped K₂Ti₆O₁₃ slightly increases (2.873 eV), it could be greatly narrowed (1.886 eV) when taking into consideration the impurity energy levels closely connected to the valence band. In addition, the impurity energy levels may form a shallow acceptor and suppress the carrier recombination in the Cu-doped K₂Ti₆O₁₃. As usual, the calculated imaginary part of dielectric function as a function of photon energy shows that the ε₂(ω) value is nearly zero for pure K₂Ti₆O₁₃ when the photon energy is less than 3.5 eV, whereas there are finite values and also some peaks for the Mn-and Cu-doped ones. These peaks may originate from the impurity energy levels, whose occurrence makes the electron excitation occur readily by low photon energy. Thus, the absorption edges in the doped ones can red-shift to the visible-light region with enhancing absorption intensity. Finally, the simulated absorption spectra of the pristine and doped K₂Ti₆O₁₃ are consistent with their electronic structures, which further confirms the above analysis. All the results show that the Cu-doped K₂Ti₆O₁₃ exhibits higher visible-light photocatalytic efficiency than the Mn-doped one. The current work demonstrates that the absorption of visible light can be realized by the Mn or Cu doped potassium hexatitanate, with the effect of the latter being better than that of the former. The obtained conclusions are of great significance for understanding and further developing the potential applications of K₂Ti₆O₁₃ in the field of photocatalysis.