Environmental Engineering Reference
In-Depth Information
middle). Synthesis of TiO
2
crystals dominated by high-energy facets needs to conine crystal
growth within the kinetically controlled regime. Under liquid-phase conditions, the nucle-
ation process can be controlled by selecting an appropriate reaction medium and surface
adsorbate, which may reduce the surface energy of speciic facets and thus obtain an anatase
crystal dominated by high-energy surfaces. One typical example is that a luorine ion caused
an exceptional stabilization of the {001} facets of anatase TiO
2
crystals. Lu and coworkers irst
reported the synthesis of micron-sized anatase TiO
2
crystals with 47% {001} facets using HF as
the capping agent and TiF
4
as the precursor.
59
This pioneering work has triggered an intensive
research interest in preparing faceted anatase. Remarkably, TiO
2
nanosheets with exposed
{001} facets up to 89% were synthesized with the involvement of a concentrated HF solution.
60
To reduce or avoid the usage of HF, alcohols or ionic liquids are used in combination with
HF of a low concentration to synthesize anatase TiO
2
crystals with {001} facets. Theoretical
calculation indicates that, under an acidic environment, alcohols tend to dissociate to form
an alkoxy group bound to coordinated unsaturated Ti
4+
cations on {001} and {101} facets.
The higher density of unsaturated Ti
4+
cations on the {001} facet induce more obvious selec-
tive adhesion of the alkoxy group, which retards the growth of the anatase TiO
2
crystal
along the [001] direction.
62
Another beneit of alcohols is that they provide an opportunity
to control the dispersion degree and the particle size. Moreover, diethylenetriamine in
isopropyl alcohol can act as a luorine-free structure-directing agent to stabilize the high-
energy {001} facets of the anatase TiO
2
crystals.
63
Inspired by faceted TiO
2
, crystal facet engineering has been extended to other semicon-
ductor photocatalysts, including metal oxides (e.g., ZnO, SnO
2
, Cu
2
O, WO
3
) and composite
oxides (e.g., BiVO
4
, Ag
3
PO
4
, Zn
2
GeO
4
) (Figure 11.15), such as ZnO nanodisks with domi-
nant {001} facets,
64
SnO
2
octahedral nanocrystals with dominant {221} facets,
65
WO
3
octa-
hedron enclosed with {111} facets, Ag
3
PO
4
rhombic dodecahedrons with only {110} facets,
66
and cubes bounded by {100} facets. However, the capping agents adsorbed on the crystal
surface need to be removed before photocatalytic applications. Recently, monoclinic BiVO
4
nanoplates with exposed {001} facets were synthesized by a straightforward hydrothermal
route without any template or organic surfactant.
67
Nevertheless, it is still a great challenge
to synthesize photocatalysts with highly reactive facets without capping agents.
11.4.2 Photoreactivity on Different Facets
11.4.2.1 Dissociative Adsorption on Crystal Facets
The adsorption of reactant molecules on a photocatalyst's surface is essential for the pho-
tocatalytic reactions to occur. Actually, the adsorption behavior of substrate molecules
varies on different crystal facets. The surface interaction between the water molecule and
TiO
2
plays an important role in the photocatalytic reaction. It is documented that water
can only be molecularly adsorbed on {101} facets, while the chemically dissociated water
molecules are energetically favored on {001} facets involving the bridging O-2c and Ti-5c
atoms,
68
which lead to the formation of two hydroxyls terminally bounded to adjacent Ti
sites and thus facilitate the formation of reactive species (Figure 11.16). Indeed, {001} facets
can not only enhance photoreduction activity in water splitting but also promote photo-
oxidation ability with respect to the decomposition of organic pollutants. It appears that
{001} facets are more effective for dissociated adsorption of substrate molecules than the
thermodynamically stable {101} facets.
69
This process can affect the photocatalytic reaction
mechanism at the molecular level. In other words, {001} facets can mediate the dissociated
adsorption of pollutant molecules by enhanced interfacial charge transfer.
