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.