Environmental Engineering Reference
In-Depth Information
function as sensitizers for light-induced redox-reactions and have the unique
electronic structure of a filled valence band, and an empty conduction band. On
irradiation (Fig.
2
b), electrons in the valence band of semiconductors are excited
to their conduction band, thus leaving holes behind. The resulting electron-hole
pairs can recombine or react separately with other molecules. The holes may
migrate to the surface and react either with electron donors in the solution or with
hydroxide ions to produce powerful oxidizing species like hydroxyl or superoxide
radicals. Meanwhile, the conduction band electrons can reduce an electron
acceptor [
32
]. Consequently, semiconductor materials can act as either an electron
donor or as an electron acceptor for molecules in the surrounding medium,
depending on the charge transfer to the adsorbed species [
32
].
Semiconductor nanomaterials are promising options for inexpensive and
environmentally friendly decontamination systems in which the correlated
chemical reagents, energy source, and catalysts are abundant, inexpensive, non-
toxic, and produce no secondary pollution byproducts [
33
]. Compared to other
semiconductors (e.g., ZnO, Fe
2
O
3
, CdS, and ZnS), TiO
2
is the most widely used
semiconductor catalyst in photocatalysis due to its chemical and biological
inertness, photostability, relative ease of manufacture and utilization, reaction
catalysis efficiency, low cost, and nontoxicity. It does however have the disad-
vantage of solely ultraviolet (UV) activation and not visible [
32
].
2 Fabrication of Cost-Effective Nanostructured
TiO
2
Materials
As mentioned above, TiO
2
is one of the most promising photovoltaic materials
because of its appropriate electronic band structure, photostability, chemical
inertness, and commercial availability [
34
]. TiO
2
exists in nature in three different
polymorphs, namely, rutile, anatase, and brookite. In addition, other synthetic
phases, for example, TiO
2
(B), TiO
2
(H), and TiO
2
(R) as well as several high-
pressure polymorphs have also been reported. Each phase shows different physical
and chemical properties for different functionalities [
34
].
Of these phases, rutile and anatase are the most practically important crystal
structures for energy applications (Table
1
). In general, both anatase and rutile-
type TiO
2
have a tetragonal crystal structure. The difference is that anatase TiO
2
follows a bipyramidal habit, while rutile TiO
2
obeys a prismatic habit [
33
]. These
two tetragonal structures can be constructed by the chains of TiO
6
octahedra,
where each Ti
4+
ion is close to six O
2-
ions. Anatase and rutile crystal structures
are different in the distortion of each octahedron along with the assembly pattern
of the octahedron chains. Specifically, the octahedron in rutile exhibits a slight
orthorhombic distortion, while the octahedron in anatase is largely distorted,
leading to lower symmetry compared to that of orthorhombic. Also, anatase has
larger Ti-Ti distances but shorter Ti-O distances than those in rutile. Finally, the
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