Environmental Engineering Reference
In-Depth Information
Photoreduction and photooxidation reactions occurring on or close to the photocatalyst surface are able to degrade organic
and inorganic pollutants and destroy microorganisms. A general expression describing photocatalytic oxidation of a chlorinated
hydrocarbon compound may be written [8, 10]:
xyz
+
(
)
(
yz
)
→+ ++
+
(10.5)
CHCl
+
OOHCl
x
z
z
HO
xy z
2
2
2
4
2
The photonic efficiency ζ is used to quantify photocatalyst performance. It can be determined from the ratio of the initial rate
of photocatalytic degradation to the rate of incident photons [22] and should not be confused with the quantum yield Φ, which
relates the amount of reactant consumed to the number of photons absorbed by the catalyst at a given wavelength λ. The photonic
efficiency has an inverse dependence on the incident photon rate [8]. This is due to the limitation of the reaction rate because of
electron-hole recombination, which competes with the reaction of electrons and holes with acceptor and donor molecules and
results in a nonlinear dependence of the photonic efficiency on the photon reaction rate. Generation of electrons and holes is not
sufficient by itself to ensure that a photocatalytic reaction occurs. The electron-hole recombination rate must, in addition, be low
enough to allow an adequate number of carriers to reach the surface of the photocatalyst and react with adsorbed molecules.
10.3
syNthesis techNiques
A wide range of methods are available for the preparation of nanostructured TiO 2 . These include sol-gel techniques, micelle
and inverse micelle methods, nonhydrolic sol processes, hydrothermal and solvothermal methods, direct oxidation, chemical
vapor deposition, physical vapor deposition, electrodeposition, sonochemical methods, and microwave techniques [23]. many
different types of low-dimensional and three-dimensional nanostructures can be fabricated using these synthesis methods, such
as nanoparticles, nanorods, nanowires, nanotubes, and nanosheets. Size-related changes in mechanical, chemical, electronic,
and optical properties are observed in these nanomaterials; phase transitions may also be affected.
It is possible to control the particle size and crystal structure of TiO 2 nanomaterials by varying the synthesis conditions. The
proportions of rutile and anatase in mixed-phase thin films deposited by pulsed laser deposition are dependent on the irradiation
energy and the oxygen working pressure [24] and can additionally be modified by high-temperature annealing [25]. The crystal
structure of TiO 2 thin films produced by radiofrequency magnetron sputtering is strongly influenced by the deposition temper-
ature, with higher temperatures favoring formation of the rutile phase [26]. Pure anatase films can be obtained by increasing the
substrate to target distance [27]. This results in increased scattering of the sputtered particles, leading to higher energy losses
before they impinge on the substrate and a consequent reduction in the thermal energy transmitted to the growing film. In the
case of films produced by chemical vapor deposition, both the crystal structure and the grain size are highly dependent on the
synthesis temperature [28].
The physical characteristics of nanopowders are related to the preparation method and the synthesis parameters. for the
sol-gel process the chemistry of the precursor is critical in determining both the particle size and the phase content [29], while
the diameter of single-phase anatase nanoparticles prepared by heating amorphous TiO 2 in air increases with increasing tem-
perature [30]. The proportions of anatase and rutile phases present in TiO 2 nanoparticles obtained by thermal plasma synthesis
can be controlled by varying the working pressure. Rutile is the more abundant phase for low-pressure synthesis, while anatase
predominates at higher pressures [31]. Temperature and pressure play similarly important roles in determining particle size and
crystallinity in nanopowders synthesized by direct current reactive magnetron sputtering [32]. The properties of anatase
nanoparticles prepared by the solvothermal technique, on the other hand, can be controlled by varying the temperature, precursor
concentration, and hydrolysis ratio [33].
Nanoparticle size is a critical parameter influencing both the thermal stability [34] and the adsorption of molecules on the
surface [35], which are governed by size-dependent effects on the surface enthalpy and surface free energy. Due to the influence
of particle size on the surface free energy, adsorption increases much more rapidly with decreasing particle size than would be
expected on the basis of the increase in surface area alone. The photocatalytic activity of nanoparticles prepared by the sol-gel
method is dependent on the calcination temperature due to its influence on the particle size, physical properties, and crystal-
linity [36]. Particle shape is the main factor affecting photocatalytic activity in nanoparticles produced by the flame hydrolysis
technique [37-39].
Surface defect chemistry has a considerable influence on the photocatalytic reaction [40]. The principal defects of interest
on the TiO 2 surface are Ti 3+ point defects and oxygen vacancies, which act as traps for charge carriers [41]. Ti 3+ surface defects
have an additional important function in photocatalytic reactions because they act as preferential binding sites for adsorbed
species. The semiconducting behavior of TiO 2 is closely related to the defect chemistry due to its effect on the electron and hole
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