Environmental Engineering Reference
In-Depth Information
plasmonic nanostructure may be different. This requires mastering not only the control-
lable synthesis of metal building blocks but also the design of plasmon-exciton coupling
assemblies to achieve the desired performance. For example, owing to the plasmon-exciton
coupling interactions, an array of Au nanoprisms on WO
3
nanocrystalline ilms harvested
photons in the visible to infrared region and enhanced the intrinsic absorption of WO
3
.
57
11.4 Crystal Facet Engineering for High Photocatalytic Reactivity
11.4.1 Crystal Facet Engineering
Considering the photocatalytic reactions on a semiconductor surface, the exposed crystal
facets and the surface atomic coniguration play a critical role in determining the sur-
face adsorption, photocatalytic reactivity, and selectivity of the catalyst. Generally, the
high-energy facets diminish quickly during the crystal growth process to minimize the
total surface free energy. It is therefore necessary to develop strategies for facet-controlled
growth of micro- and nanocrystallites.
Crystalline TiO
2
is chosen as the representative example to demonstrate crystal facet
engineering for photocatalysts. Theoretical calculation has demonstrated that the order of
the surface energy for anatase TiO
2
is 1.09 J m
−2
for {110} > 0.90 J m
−2
for {001} > 0.53 J m
−2
for
{100} > 0.44 J m
−2
for {101}.
58
Although TiO
2
nanostructures with dominant {001} high-energy
facets is expected to enhance surface properties, the most available anatase crystals are
dominated by the less-reactive {101} facets. Therefore, controllable synthesis and assembly
of anatase TiO
2
with dominant {001} facets is scientiically and technologically signiicant.
According to the Wulff construction, the slightly truncated octahedral bipyramid with
eight {101} and two {001} facets is the common shape of the anatase crystal (Figure 11.14a,
(a)
{001}
{101}
{001}
{001}
B
A
{101}
{101}
Compressed
Elongated
(b)
(c)
(d)
68.3
º
68.3
º
111.7
º
1 µm
1 µm
1 µm
FIGURE 11.14
Morphology of anatase TiO
2
crystals. (a) Schematic of anatase TiO
2
with different percentages of {101}, {001}, and
{010} facets. (b through d) SEM images of anatase crystals dominated by {001}, {101}, and {010} facets, respectively.
(Adapted with permission from Pan, J., Liu, G., Lu, G. Q., Cheng, H.-M.,
Angew. Chem. Int. Ed
., 50, 2133. Copyright
2011, Wiley-VCH.)
