Environmental Engineering Reference
In-Depth Information
24.2.3.4 Nanocrystalline Films and Dye Sensitization
Thin metal oxide films exhibit interesting photochromic,
electrochromic, photocatalytic, and photoelectrochemical (PeCh) properties. Their nanoparticles are in electronic contact,
allowing for electric charge percolation. This charge transport is highly efficient, and the quantum yield is practically unity
[21]. TiO
2
nanotube array films and TiO
2
nanoparticulate films were investigated for the photodegradation of organic
compounds [25]. The degradation efficiency of TiO
2
nanotube array films was much higher as compared with the TiO
2
nanoparticulate films with similar thickness and geometric area due to the higher internal surface area of the nanotube
structure. The photoactivity of the TiO
2
nanotube films was strongly influenced by the thickness and very slightly by the
tube diameter [25b, f, 26]. The effects of the nanotube structure including tube length, tube wall thickness, and crystallinity
on the photocatalytic activity for the degradation of organic compounds were investigated in detail by several researchers
[27]. generally, the reaction rate increased gradually by increasing TiO
2
film thickness till a maximum due to the increment
of the TiO
2
photocatalyst that may participate in the photodegradation. However, a further increase in TiO
2
film thickness
resulted in somewhat decreasing the photoreaction rate. When the film is thicker than the light penetration depth, the active
thickness is practically constant and the bottom film serves only as a support. The photoreactants have a longer diffusion
path in the longer nanotubes; therefore, the photoreaction rate decreases [19b]. Nanostructured films made up of wide band-
gap semiconductors, such as TiO
2
, respond only in the uv region. in this film, the effective surface area can be enhanced
1000-fold; thus, light absorption can be modified with only a dye or a short bandgap semiconductor monolayer adsorbed on
each particle [28]. For example, by successively dipping the ZnO film in Cd
2+
and S
2−
solutions, one can cast a thin film of
CdS nanocrystallites. These heterostructured semiconductor films are photoelectrochemically active and generate photocur-
rent under visible light excitation.
24.2.3.5 Plasmon-Sensitized TiO
2
Nanoparticles
Plasmonic nanoparticles such as silver and gold nanoparticles have been
employed as an attractive approach to promote the performance of photocatalysts due to their surface plasmon resonance,
which can enhance the visible light absorption and consequently increase the overall efficiency of the photocatalysts [9af, 29].
Core-shell nanoparticles have been applied to enhance photocatalytic activity by preventing the electron-hole recombination.
it is expected that, due to the coupling of plasmon resonances in the core with the electron-hole pair generation in the shell,
these hybrid Ag/Au TiO
2
nanoparticles will exhibit photocatalytic activity in the visible spectral range, thereby making more
efficient use of solar energy. Zheng et al. [9af] recently synthesized plasmonic photocatalysts m-TiO
2
(m =Au, Pt, Ag) and
investigated their visible light photoactivities for the oxidation of benzene. Ag/AgCl [30], Ag/AgBr/WO
3
·H
2
O [9ak], and Ag
and Au nanoparticles supported on ZrO
2
and SiO
2
are also efficient visible light active photocatalysts for both oxidative and
reductive reactions [31].
24.2.3.6 Immobilization of TiO
2
in many of the reported photocatalytic applications, nanocatalysts were used as nanopar-
ticle slurry. But using nanoparticles in slurry form has several practical constraints. Particle-particle aggregation, increased
process cost due to additional solid/liquid separation, reduced penetration depth of incident light because of the solution tur-
bidity, and health hazard due to fugitive emissions of nanoparticles during slurry preparation are some of the practical con-
straints [32]. immobilization of the photocatalysts onto a support medium can eliminate most of the limitations linked to the
use of nanoparticles in slurry form. However, this can lead to a decrease in the surface area available for reactions compared to
suspended systems. These problems can be resolved by supporting nanoparticles on large-surface area materials like quartz,
silica gel, alumina, zeolites, clays, activated carbon (AC), or glass spheres [33], which could avoid the formation of macro-
scopic aggregates of photoactive particles [34].
Khatamian et al. have reported the preparation of TiO
2
nanoparticles and metal ion-doped TiO
2
nanoparticles supported on
ZSm-5 zeolite [35]. The degradation efficiency of dye was much higher as compared with bare TiO
2
due to the high adsorption
capacity of the supported TiO
2
that results in the higher concentration of dye molecules around the TiO
2
particles. Furthermore,
the strong electrostatic field present in the zeolite framework can effectively reduce the recombination rate of electron-hole
pairs, thus increasing the photocatalytic activity [35].
Another possible solution to facilitate the separation of catalysts is the use of solid catalysts with magnetic properties, which
can be efficiently separated from the reaction medium applying an external magnetic field. most of the magnetic photocatalysts
developed so far contain a magnetic core (magnetite (Fe
3
O
4
), maghemite (γ-Fe
2
O
3
), or ferrite (e.g., NiFe
2
O
4
)), and a catalyst
shell (TiO
2
) [36]. An intermediate and inert layer is necessary between the TiO
2
shell and magnetic core in order to avoid pho-
todissolution of iron and to prevent the magnetic core from acting as an electron-hole recombination center. Silica is chemically
inert and does not affect any redox reactions at the core surface. Silica is also optically transparent, permitting light energy to
penetrate.
recently, there have been great advances in the development of TiO
2
photocatalytic membranes, since their simultaneous
physical separation and chemical decomposition of water contaminants are promising for practical and full-scale applications [37].
