Environmental Engineering Reference
In-Depth Information
reaction time. It provides a convenient and effective reaction environment for the
formation of nanocrystalline TiO
2
with high purity, good dispersion, and well-
controlled crystallinity [
116
]. By employing this method, the calcination process,
which is essential to the TiO
2
transformation from amorphous phase to crystalline
phase, can be eliminated. Similarly, by tuning the hydrothermal conditions (such
as temperature, pH, reactant concentration, molar ratio, and additives), crystalline
products with different compositions, sizes, and morphologies can be achieved
[
117
]. Many high-performance TiO
2
nanostructures, including novel nanocrystals
[
114
], nanosheets (Fig.
5
a) [
22
], 1D morphologies (e.g., nanotubes, nanowires,
nanorods (Fig.
5
b)) [
118
-
122
], mesoporous structures (Fig.
5
c) [
116
,
123
], and
especially 3D hierarchical architectures (Fig.
5
d) [
29
,
115
,
124
,
125
] have been
extensively developed via cost-effective hydrothermal/solvothermal methods and
widely applied in DSSCs, H
2
generation and photocatalysis.
Recently, anatase TiO
2
single-crystalline nanosheets (NSs) with a high percentage
of reactive (001) facets have attracted much attention as a dominant source of active
sites for various applications such as photovoltaic cells, photodegradation of organic
molecules, and photocatalytic water splitting [
112
,
126
,
127
]. Both theoretical and
experimental studies indicate that the anatase TiO
2
(001) facets in particular are more
reactive than (101) facets, which could be mainly attributed to different coordination
numbers of Ti in (001) and (101) facets. In the (001) facets, each Ti atom coordinates
with five oxygen atoms (i.e., with 100 % five-coordinate Ti (Ti
5c
) atoms). While in
the (101) facet, each Ti atom is coordinated with either five or six oxygen atoms with a
50 % probability for either case. Thus, the (001) facets display an enhanced number of
oxygen vacancies in comparison with the (101) facets owing to the low coordination
number of Ti with oxygen [
128
,
129
]. However, anatase TiO
2
crystals are usually
dominated by (101) facets, which are thermodynamically stable due to their lower
surface energy than that of (001) facets (i.e., surface energy: c(001) = 0.90
J/m
2
[ c(100) = 0.53 J/m
2
[ c(101) = 0.44 J/m
2
). Consequently (101) facets
account for more than 94 % of the total exposed surface according to the Wolff
construction [
4
].
An important breakthrough in the preparation of anatase TiO
2
sheets with
exposed (001) facets was achieved by Yang et al. in 2008. The synthesis of anatase
TiO
2
single crystals with 47 % exposed (001) facets was realized by using hydro-
fluoric acid (HF) as a shape controlling agent to stabilize the (001) facets (Fig.
6
a-c)
[
130
]. Thereafter, numerous studies have been conducted toward the preparation
and application of different shaped anatase TiO
2
micro- or nanocrystals with
exposed (001) facets [
131
-
133
]. For example, Zheng et al. demonstrated a simple
solvothermal synthesis, where tetrabutyltitanate, Ti(OBu)
4
, was mixed with abso-
lute ethanol and 40 % hydrofluoric acid solution, to produce TiO
2
microspheres
assembled of anatase TiO
2
nanosheets with 83 % dominant (001) facets (Fig.
6
d).
These TiO
2
microspheres exhibited excellent photocatalytic activity for the deg-
radation of methyl orange (MO) as shown in Fig.
6
e[
134
]. Yang et al. demonstrated
controllable hydrothermal synthesis of ultra-thin TiO
2
NSs with a thickness of only
1.6-2.7 nm and up to 82 % (001) facet coverage. It is found that the concentration of
HF used as a capping agent significantly affected the thickness and side length of the
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