Chemistry Reference
In-Depth Information
diameters in armchair- and zigzag-like forms. The V-O covalent bonds
are the strongest interactions in V
2
O
5
NTs, whereas V-V bonds proved to
be much weaker. The total energies (per V
2
O
5
unit) of SW V
2
O
5
NTs de-
pendence follows a
1/D
2
(D, the tube diameter) behavior, indicating a
decrease in stability with decreasing D. Particularly, zigzag-like V
2
O
5
NTs
are more stable when DW1 nm. All the studied nanotubes are uniformly
semiconducting, and the band gap trends to vanish as the tube diameters
decrease. Particularly, zigzag-like NTs are somewhat higher than those
for armchair-like NTs. Subsequently, they examined the atomic models of
infinite-long scroll-like V
2
O
5
NTs.
127
The scroll-like NTs are more stable
than the ideal cylindrical armchair- and zigzag-like ones. It was estab-
lished that the band gap of the scroll-like tubes, different from those of
armchair- and zigzag-like NTs (2.5-2.9 eV
126
), trends to vanish (up to
B
B
0.1 eV) depending on the atomic configurations of the tubes and the
inter-wall distances. Enyashin et al.
128
further carried out tight-binding
band structure calculations of the Mo doping effect on the electronic
structure and local chemical bonding indices of infinite-long V
2
O
5
NTs
with cylindrical and scroll-like morphologies. It was found that the sta-
bility of V
2
x
Mo
x
O
5
NTs decreased with an increase in the content of Mo.
All uniformly semi-conducting V
2
O
5
nanostructures transform into a
metallic state in addition to an increase of the Fermi energy into the
conduction band because of the repacement of V by Mo. Therefore,
doping of metal oxide tubes may lead to electronic properties that are
more controlled by chemical factors (i.e., the amount of doping) than the
specific geometry of NTs.
Our group
129
has also contributed partly to the V
2
O
5
NTs. The ortho-
rhombic crystal V
2
O
5
(space group Pmmn) is built up by stacking 2D-like
layers along (010) and composed of distorted VO
5
pyramids. There are
three structurally nonequivalent types of oxygen centers in the crystal
V
2
O
5
: single coordinated vanadyl O(1), double coordinated O(2) and
bridging oxygen O(3) triply coordinated to vanadium atoms (Fig. 26a).
Fig. 26 Optimized geometries of V
2
O
5
NTs with different diameters. (a) Top and side
view of bulk V
2
O
5
. (b-g) Cross section of armchair (n, n) (n
=
38) V
2
O
5
NTs.
129
