3.6. Photocatalytic activities
In order to analyze the effect of the 4d TM atom doping on the
photocatalytic activity of TiOS, we resort to the effective masses and
the transfer rates of the photogenerated
electrons
and holes. For the pure TiOS, the effective masses are found to bemh* (p) = 17.348m0 andme* (p) = 2.387m0 with m0 being the
electron mass, which implies a large rhe (p) =
7.268. This large difference betweenmh* (p) andme* (p) will further result in
the large difference between the transfer rates of electrons and holes,
reducing the recombination of electrons and holes. Therefore, TiOS can
be used as a promising photocatalytic material. When TiOS is doped by 4d
TM atoms, the effective masses of electrons and holes are denoted bymh* (d) andme* (d) along one certain
direction, respectively. We can see in Fig. 6 that the relative
variation ratios rh =mh* (d)/mh* (p)
are all greater than 1, indicating the doping can seriously increase the
effective mass of the photogenerated holes. However, the relative
variation ratios re =me* (d)/me* (p)
are all close to 1, indicating only tiny variations ofme* are observed. Therefore the
doping will make stronger influences on the effective mass of the holes
than the electrons. For clarity, we also plot the ratior’he =rhe (d)/rhe (p) to clearly
reflect the overall doping induced variations of the hole and electron
effective masses with rhe (d) andrhe (p) being the effective mass ratios
corresponding to the doped and pure TiOS. We can see allr’he are greater than 1, indicating all the
dopings can enlarge the effective mass difference between holes and
electrons. This phenomenon is more evident for Y, Zr, Nb, Mo, and Ag
than Tc, Ru, Rh, Pd, and Cd. Furthermore, this demonstrates that the 4d
TM atom doping will further suppress the recombination of the
photogenerated electrons and holes, which can improve the photocatalytic
activity of TiOS in different degrees. Moreover, the dopings of Y and Ag
can lead to larger r’he , indicating both Y and Ag
dopings can be viewed as the better schemes used to enhance
photocatalytic activity of TiOS.
Fig. 6 (Color online) The relative variation ratiosrh , re , andr’he for different doping models.
Furthermore, as discussed before, based on the energy bands of the 4d TM
atom doped TiOS in Fig. 4 we can see that the couplings of the doping
atoms and the TiOS can produce new impurity energy bands in the original
band gap and fill the original CB with electrons. The appearing of the
impurity energy bands will provide new channels through which the
electrons in impurity bands can be pumped to the CB by absorbing the
visible-light photons. Therefore, this procedure can strengthen the
visible-light induced photocatalytic activities of TiOS. Moreover, the
visible-light absorption will also pump the electron below Fermi energy
level in the CB into the empty energy levels above Fermi energy level,
inducing more electron to appear at high energy levels. This also
increases the photocatalytic activities of the TiOS. Thus we can draw
that the 4d TM atom dopings are beneficial for increasing the
visible-light utilization efficiency and improving the photocatalytic
activities of TiOS.