Chemistry Reference
In-Depth Information
and (18, 0) NTAs both with oxygen atom down in the molecule, respect-
ively (Fig. 19h). NO
2
prefers to point its oxygen atoms rather than the
nitrogen atom towards the in-wall surface with adsorption energies of
-0.192 eV and -0.220 eV, charge transfer of 0.031 and 0.014 e, binding
distances of 3.1 and 2.9 Å on (12, 0) and (18, 0) NTAs, respectively, making
an inverted ''V''-shape configuration perpendicular to the radial plane
(Fig. 19g). NH
3
chemisorbed on (12, 0) and (18, 0) NTAs with adsorption
energies of -0.968 eV and -1.124 eV, charge transfer of 0.352 and 0.334 e,
with the N sitting 2.27 and 2.25 Å away from the Ti
5c
atom, respectively
(Fig. 19i). Notably, the hydrogen bond and stronger ion-dipole inter-
actions are partly responsible for the enhanced adsorption of NH
3
. It can
be assumed that TiO
2
NTAs can act as ecient catalysts for dissociating
NH
3
. However, TiO
2
NTAs may not be a good candidate to sense NH
3
because the larger adsorption energy probably makes the recovery time
slow. Overall, the adsorption strength of gas on (12, 0) and (18, 0) NTAs in
their most configurations are in the orders: NH
3
cNOWNO
2
WN
2
OW
CO
2
WCOWCH
4
E
NO
2
WCO
2
WCH
4
WO
2
,
respectively. The electronic structures of both NTAs upon adsorption
suggest that O
2
, NO and NO
2
adsorptions give rise to impurity states in
the original band gaps of pure NTAs, thus maybe enhancing the transport
properties and electronic conductivity of NTAs. However, CO, CO
2
,N
2
O,
CH
4
and NH
3
have little effect on modifying the electronic properties
of NTAs.
O
2
and NH
3
cNOWCOWN
2
O
E
3.3 Reactions on TiO
2
nanotubes
3.3.1 Water splitting on pristine TiO
2
nanotubes arrays. Photo-
catalytic splitting water into hydrogen and oxygen by utilizing solar en-
ergy is regarded as an effective strategy to solve oil crisis. It is well-known
that water can be splitted into hydrogen and oxygen at the cathode and
anode, respectively. TiO
2
NTs or NTAs have demonstrated ecient pho-
toanodes because of the reduced bulk electron/hole recombination and
the enhanced quantum yield. Our group has developed the theoretical
framework of water splitting on TiO
2
NTAs.
80,82,83
We focus on a simple
one-electron transfer mechanism from a thermodynamics perspective,
which assumes that the driving force for the photo-oxidation reaction at
the anode is provided by the photoinduced hole at the upper edge of the
VB.
90
The reaction landscape of water-splitting is put forth as follows:
Anode:
HO*
þ
H
þ
þ
e
H
2
O
þ
*
-
(11)
O*
þ
H
þ
þ
e
HO*
-
(12)
HOO*
þ
H
þ
þ
e
O*
þ
H
2
O
-
(13)
O
2
þ
*
þ
H
þ
þ
e
HOO*
-
(14)
Cathode:
4H
þ
þ
4e
-
2H
2
(15)
