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

Abstract

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.