Chemistry Reference
In-Depth Information
To decrease the large band gap (E
g
) of ZnO NT for developing the ap-
plication in photocatalysis, Song et al.
108
modulated E
g
by adjusting the
doping Cu concentration and the concentration gradient. It was shown
that E
g
decreases from the perfect ZnO NTs of 4.5 eV to 1.95 eV of Cu doped
NTs. When the Cu concentration is fixed, the band gap decreases as
concentration gradient decreases. Similarly, with the fixed concentration
gradient, the band gap decreases as the concentration decreases.
Moradian et al.
109
examined Cd and Mg doped (10, 0) ZnO NTs. It was
found that the Mg doped NTs are more stable than pristine and Cd doped
NTs. Due to ionic bonding of Mg-O bond in the Mg doped NT, the
semiconducting energy gap is increased while in the Cd doped case the
Cd-O bond is less ionic, hence the energy gap decreased. Therefore, a
blue shift for the case of Mg doped and a red shift for the case of Cd
doped is observed.
4.1.3 Oxygen vancancy creation. At elevated temperatures oxygen va-
cancies may occur in ZnO NTs. When an oxygen vacancy (V
O
)iscreated,
the defect ZnO NTs behave different geometrical and electronic properties
from the pristine ones. An et al.
88
performed prototype zigzag (6,0) ZnO
NTs with different fraction of V
O
defects on the sidewall. The oxygen va-
cancies induces certain strain within the nanotube near the V
O
sites. Some
impurity states are introduced near the energy band edges, showing n-type
semiconducting. Moreover, a small gap occurs near the conduction band
edge. The Fermi level is lifted slightly as oxygen vacancies increase. The
authors suggested that oxygen vacancies enhanced major carrier transfer
from the valence band to conduction band. Thus, the electronic properties
of the ZnO NTs can be tuned by introducing some V
O
defects. The for-
mation energy of 1V
O
,2V
O
,and3V
O
defect (defined as the energy differ-
ence between the perfect tube and defective tube plus oxygen (number of
vacancy) per supercell) was 6.10, 11.9, and 17.83 eV, respectively, indicating
an endothermic process. The Mulliken charge analysis shows that the local
net charge resulting from one Vo is about 0.76 e.
4.2 Adsorption on ZnO nanotubes
The adsorption of molecules on ZnO NTs has included H
2
,O
2
, CO, NO
2
,
NH
3
and CPs/CPRs in recent theoretical reports. The main properties that
have been calculated and reported include the structural changes, ad-
sorption energy, adsorption site and geometry, charge transfer and
electronic properties, as also summarized in the review.
15
4.2.1 Adsorption of H
2
. Our group
104
studied H
2
adsorption on ZnO
(5, 5) NTs. We found that the adsorption energies of H
2
are nearly the
same on the outside and inside of the NTs (-0.11 vs. -0.12 eV) with dis-
tances of 2.83 and 3.98 Å between one of the H atoms and a surface O
atom, respectively. When a Pd atom adsorbed on the NT wall (Pd/ZnO
NTs), the adsorption energies were a little reduced to -1.02 eV and
-1.03 eV, respectively. An et al.
88
also examined the adsorption of H
2
on
ZnO (6, 0) NTs. They found that H
2
is physisorbed with adsorption energy
of -0.031 eV and a binding distance of 2.529 Å between one of the H
