Chemistry Reference
In-Depth Information
Overall:
2H
2
O
-
O
2
þ
H
2
(16)
The asterisk represents an active site on various (6, 6) TiO
2
NTAs, such
as the bridge site between Ti and Ti (O or dopant), or the top site of the
dopant or Ti atoms not covered by O. The interactions within sur-
rounding water molecules, direct recombination of oxygen atoms to form
O
2
and activation barriers between the reaction steps are not taken into
account in the calculations.
90-92
In the acidic environment hydrogen-
evolution by direct association between H* occurs much faster than
oxygen-evolution via a HOO* intermediate. Hence, the photoelec-
trochemical oxidation of water on the various doped NTAs is explicitly
concentrated. According to Nørskov et al.'s developed methodology,
90-94
the free-energy difference of the reaction intermediates is used to de-
termine whether the elementary step is thermodynamically permitted,
which is a necessary but not sucient criterion for the reaction to pro-
ceed. The change in free energy for the reaction mechanism (eqs (11-14))
is calculated via DFT and expressed by eq (17):
DG
=
DE
þ
DZPE - TDS
þ
DG
pH
þ
DG
U
(17)
Where DE is the reaction energy, obtained from DFT calculations; the
differences in zero-point energies, DZPE, are calculated using DFT vi-
brational frequencies analysis; TDS, the change in entropy using stand-
ard tables for gas-phase molecules; DG
pH
and DG
U
, the free energy
contributions due to the variation of H
þ
concentration and electrode
potential. The influence of pH on the Gibbs free energies is not con-
sidered by fixing the pH (pH
=
0). The chemical potential for H
þ
þ
e
is
related to 1/2H
2
in the gas phase since the reference potential is the
standard hydrogen electrode (NHE).
In this section, the water oxidation reaction on the TiO
2
NTs and NTAs
meanwhile the corresponding free energy diagram under different ap-
plied potential U are presented. The reaction intermediates of H
2
O
splitting on TiO
2
NTs and NTAs are shown in Fig. 20. Due to the similar
configurations of these reaction intermediates on the inside of TiO
2
NTs
and NTAs, only the geometries on the TiO
2
NTAs are shown. The hydroxyl
(OH
*
) via the dissociation of first H
2
O adsorbs the top site of Ti (eq 11).
Next, the oxygen (O*) from dissociated OH
*
adsorbs either on the top site
of Ti or the bridge site of Ti-O (eq 12). Furthermore, the HOO* is formed
by the oxygen (O*) and the second dissociated H
2
O (eq 13). Finally, O
2
is
released from the NTs and NTAs (eq 14). It can be seen that the geom-
etries of these intermediates on the outside of TiO
2
NTs and the inside
of TiO
2
NTAs are different. The adsorbed species can induce structure
deformation of TiO
2
NTs. However, the structure of TiO
2
NTAs keeps very
well.
According to the formula (17), the Gibbs free energy of the elementary
steps of H
2
O splitting on the outside and the inside of TiO
2
NTs and NTAs
are shown in Fig. 21. For each system, three different potentials have
