Starting from the optimized anatase TiO2 unit cell, we
construct the TiOS model with the outermost and second layers terminated
by twofold coordinated O atoms and fivefold coordinated Ti atoms,
respectively. The doping concentration of TiOS doped with TM atoms is
1.37 at.% and six typical doping manners in Fig. 1b are considered. A
TM atom can be vertically adsorbed on a twofold coordinated O atom
(O1-Ad), threefold coordinated O atoms (O2-Ad, O3-Ad), a fivefold
coordinated Ti atom (Ti1-Ad), and a sixfold coordinated Ti atom
(Ti2-Ad). In addition, TM atoms are only allowed to enter the largest
and outermost caves (Cav) because of their large atomic radii. Table 2
shows the impurity formation energies Ef for ten
different 4d TM atom doped models of the TiOS in six different doping
manners. We can see only Ag-Cav and
Cd-Cav dopings have positive Ef , indicating these
two doping processes correspond to endothermic reactions, and are
difficult to occur. This should be reasonable since the structures with
Ag or Cd atoms doped into the caves are unstable owing to the large atom
radii of Ag and Cd. However, all the other doped cases studied here have
negative Ef , which indicates the corresponding
doping processes are exothermic reactions that can occur more easily.
Furthermore, the optimal doping manners for all TM atoms can be readily
found according to the lowest Ef , and listed in
the following: O1-Ad doping for Ru, O2-Ad doping for Pd and Ag, O3-Ad
doping for Y and Rh, Ti1-Ad doping for Tc and Cd, and Cav doping for Zr,
Nb, and Mo. Obviously, different TM atoms have different optimal doping
manners. In fact, even in the same doping manner, the impurity atoms may
have various positions after being optimized. In particular, Tc, Pd, Ag,
and Cd atoms, having stable full or half-full filling outermost
orbitals, prefer the O2-Ad or Ti1-Ad doping, while Y, Zr, Nb, Mo, and Rh
atoms with unstable outermost orbitals and spin multiplicity, prefer the
O3-Ad or Cav doping. The different optimal doping manners are closely
dependent on the 4d electron configuration, the atomic radius, and the
electronegativity of the TM atom. Thus, the optimal doping manners of
all 4d TM atoms have been determined.
Table 2 The formation energies (eV) of TiOS doped with 4d TM
atoms.