Environmental Engineering Reference
In-Depth Information
(b)
(001)
(c)
(001)
(001)
(101)
(a)
(001)
(101)
(001)
(d)
(101)
(100)
(e)
figure 10.6
Anatase nanocrystal shapes: (a) truncated tetragonal bipyramid; (b) truncated tetragonal bipyramid with a large percentage
area of {001} facets; (c) square sheets with dominant {001} facets; (d) elongated truncated tetragonal bipyramid with a large percentage of
lateral {100} facets; (e) tetragonal cuboid enclosed by {001} and {100} facets. Reproduced with permission from Ref. [100]. © 2011,
Wiley—VcH Verlag GmbH & co.
pyramidal, elongated rhombic, spherical, dog-bone, and bar-shaped, from a titanium butoxide precursor [98, 99]. Water vapor
was utilized as the hydrolysis agent and oleic acid and oleylamine as capping surfactants to control crystal growth. The concen-
trations of the surfactants played a crucial role in determining the nanocrystal shape: higher amounts of oleic acid acted to
suppress growth in the [011] direction by selectively binding to the (001) surface.
A great variety of nanocrystal shapes can be obtained using these synthesis methods [74, 100]. Some of the possible
modifications of the parent equilibrium crystal form are illustrated in figure 10.6. Theoretical modeling provides insights
on the influence of surface chemistry on the preferential growth of low-energy facets and the crystal morphology. An increase
in the photocatalytic efficiency requires both reduction in particle size and control of crystal shape. However, the percentage
of high-energy facets that can be achieved in nanosized crystals is ultimately limited by thermodynamic considerations. One
way of overcoming this obstacle is through the use of hierarchical structures with a high surface area of reactive facets. A low-
temperature hydrothermal process has been described [101] that uses Ti powders in an aqueous Hf solution to produce flower-
like nanostructures consisting of highly truncated tetragonal pyramidal anatase with {001} exposed facets and {101} sides.
Additional improvements in the photoreactivity of TiO
2
may be possible by combining methods for controlled nanocrystal
growth with anion or cation doping. N-doped anatase sheets with predominant {001} facets, synthesized from TiN and Hf by
a hydrothermal technique, strongly absorb visible light [102]. Nanocrystals with a high percentage of {001} facets, prepared
from a Tif
4
precursor by a solvothermal process and codoped with N and S by adding cH
4
N
2
S prior to calcination, show visible
light photocatalytic activity [103]. Selective deposition of low amounts of Pt nanoparticles on the {101} facets of anatase nano-
crystals improves the efficiency of photoreduction and photooxidation processes by specific surface-induced separation of
electrons and holes [104]. The ratio of {001} to {101} facets, and hence the crystal shape, is critical to achieving the optimal
balance between recombination and redox reaction rates.
10.7
aPPlicatioN of Photocatalysis iN Water treatmeNt
Photocatalysis is effective for the destruction of a wide range of organic and inorganic pollutants and the deactivation of
bacteria and viruses. The classes of organic compounds that can be treated include organic acids, aromatic and chlorinated
hydrocarbons, alkanes, haloalkanes, alcohols, surfactants, herbicides, pesticides, and dyes [9, 10, 21]. Hazardous inorganic
substances comprise bromate, chlorate, azide, halides, nitric oxide, platinum, palladium and rhodium species, silver and sulfur
species, metal ions and salts, cyanide, organometallics, thiocyanate, ammonia, nitrates, and nitrites [10, 21]. Pathogens suscep-
tible to photocatalytic disinfection include
Escherichia coli
[105-107],
Salmonella enterica
and
Pseudomonas aeruginoas
[107], and
Clostridium perifringens
[108]. Photocatalysis is a suitable alternative to chemical disinfection of drinking water and
is also effective against chlorine-resistant organisms.
The chemical reactions governing photocatalytic degradation of specific compounds are well known [10]. In other cases the
degradation pathways are less well understood and there is a potential risk of generation of toxic intermediates [18]. chlorine
and bromine species may be formed by hydrolization, and ozone may be produced as a result of the oxidation of adsorbed
hydroxide ions, giving rise to oxygen species that can react with molecular oxygen to form ozone [19]. While the presence of
these compounds may assist disinfection processes, reactions between chlorine, bromine, ozone, and hydroxyl radicals can lead
to the formation of chlorinated or brominated by-products. Photocatalytic degradation of pesticides and herbicides may result
in the production of other toxic compounds [109-111] that, particularly during the initial phase of treatment phase, can be more
