3.2. Optical absorption
Now we study the optical absorption properties of TiOS doped with 4d TM atoms, as shown in Fig. 2. For comparison, the optical absorption properties of the pure TiOS are also plotted by the dash-dotted curves. The scissor operator, namely, the difference 1.03 eV between the experimental band gap (3.23 eV) and the calculated band gap (2.20 eV) is applied to correct the optical properties [32]. For the pure TiOS, the absorption edge is located at around 392 nm as shown in Fig. 2, in agreement with the experimental observation [37] and the theoretical result [38]. This indicates that the optical response is mainly localized in the UV-light region, inducing that the utilization ratio of visible light (approximately 380–780 nm) will be quite low. Therefore, it is necessary to modify the physicochemical properties of TiOS to achieve a better visible-light response. By comparing the solid curve with the dash-dotted one in each pannel in Fig. 2, we can find all 4d TM atoms are able to improve the visible-light response of TiOS to different degrees. Totally speaking, their optical absorption coefficients can be classified into two types: (1) Strong improvement case. In Figs. 2a-2e, we can find that high and wide absorption peaks can be formed in the visible-light region, clearly indicating that the visible-light absorption coefficients are significantly increased for Ti24YO48, Ti24ZrO48, Ti24NbO48, Ti24MoO48, and Ti24AgO48. Notably, the Nb doped TiOS shows the best visible-light response. These results imply that the 4d TM dopants with a half-full or less than half-full outer electron configuration can obviously improve the visible-light response of TiOS. (2) Weak improvement case. In Figs. 2f-2j, the visible-light absorption coefficients are only slightly increased for Ti24TcO48, Ti24RuO48, Ti24RhO48, Ti24PdO48, and Ti24CdO48, implying there exists a weak improvement in the visible-light response. Moreover, we can find that the visible-light response intensity will decrease as the atomic number of the dopants increases from Y, Zr, Nb, Mo to Tc, or from Ru, Rh, Pd to Cd. Yet, very surprisingly, Ag with a large atomic number will show unexpected strong visible-light response, different from its neighboring elements. Furthermore, we can find that the 4d TM atom doping induced absorption improvement can appear in a wide area with the wavelength much longer than the visible-light wavelength. This phenomenon is especially outstanding for the strong improvement case in relative to the weak improvement one. Therefore, we can conclude that, among the investigated 4d TM dopings, the TiOS doped with Y, Zr, Nb, Mo, and Ag can remarkably improve the utilization efficiency of visible light.
Fig. 2 (Color online) The optical absorption coefficients of pure (dash-dotted curve) and doped (solid curve) TiOS. The region between two vertical dotted lines represent the visible-light region.