251
range. TiO 2 nanotubes were found to be more efficient than TiO 2 nanoparticles in
the decomposition of organic compounds [201]. The higher photocatalytic activity
was attributed to the shorter carrier-diffusion paths in the tube walls and faster mass
transfer of reactants toward the nanotube surface.
Noble metal doping can reduce the e
−
/h
+
recombination because the photoexcited
electrons tend to migrate to the noble metals with lower Fermi levels while the holes
stay in TiO 2 [244]. The photocatalytic activity of TiO 2 can also be promoted by
creating highly reactive crystallographic facets. Because high-energy {001} facets
diminish quickly during crystal growth, anatase TiO 2 is usually dominated by the
low-energy {101} facets. Using specific capping agent (usually fluoride), the percentage of {001} facets can be increased from less than 10% to up to 89%, substantially enhancing hydroxyl radical production and organic compound decomposition
[117, 237]. The enhanced activity stems from the strong adsorption of reactants on
high-energy facets and the spatial separation of electrons and holes on specific crystal facets [192, 237]. The optimal percentage of {001} facets for photocatalysis is
still debated [192]. Improving contaminant adsorption by modifying photocatalyst
surface is another way to enhance photocatalytic activity due to the short lifetime of
ROS. However, little has been done in this area.
Another actively pursued research area is to extend the excitation spectrum of
TiO 2 to include visible light. The general strategy is doping metal impurities, dye
sensitizers, narrow bandgap semiconductors, or anions into nano-TiO 2 to form
hybrid nanoparticles or nanocomposites [99, 245]. Metals and anions create impurity energy levels or narrow the bandgap; upon visible light excitation, dye sensitizers and narrow bandgap semiconductors inject electrons into TiO 2 to initiate the
catalytic reactions. Among these methods, anion (especially nitrogen) doping was
considered the most cost effective and feasible for industrial applications, although
their stability and long-term efficacy have not been tested. Decreased nitrogen concentration during photocatalysis has been reported [99, 156].
Other than TiO 2 , WO 3 and some fullerene derivatives also have the potential to
be used in photocatalytic water treatment. WO 3 has a narrower bandgap than TiO 2 ,
allowing it to be activated by visible light (<450 nm) [159]. Pt doping further
enhances WO 3 reactivity by facilitating multielectron reduction of O 2 and improving e
−
/h
+
separation [154]. Aminofullerenes generate
1
O 2 under visible light irradiation (<550 nm) and have been known to degrade pharmaceutical compounds and
inactivate viruses [172, 195]. Fullerol and C 60 encapsulated with poly(Nvinylpyrrolidone) can produce
1
O 2 and superoxide under UVA light [35].
Aminofullerenes are more amenable to immobilization than fullerol and are more
effective for disinfection purposes due to their positive charge.
1
O 2 has lower oxidation potential than hydroxyl radicals produced by TiO 2 , while it is a more selective
ROS and consequently less susceptible to quenching by nontarget background
organic matter. Fullerenes are currently much more expensive and not as readily
available as TiO 2 .
Current and Potential Applications for Water and Wastewater Treatment
range. TiO 2 nanotubes were found to be more efficient than TiO 2 nanoparticles in
the decomposition of organic compounds [201]. The higher photocatalytic activity
was attributed to the shorter carrier-diffusion paths in the tube walls and faster mass
transfer of reactants toward the nanotube surface.
Noble metal doping can reduce the e
−
/h
+
recombination because the photoexcited
electrons tend to migrate to the noble metals with lower Fermi levels while the holes
stay in TiO 2 [244]. The photocatalytic activity of TiO 2 can also be promoted by
creating highly reactive crystallographic facets. Because high-energy {001} facets
diminish quickly during crystal growth, anatase TiO 2 is usually dominated by the
low-energy {101} facets. Using specific capping agent (usually fluoride), the percentage of {001} facets can be increased from less than 10% to up to 89%, substantially enhancing hydroxyl radical production and organic compound decomposition
[117, 237]. The enhanced activity stems from the strong adsorption of reactants on
high-energy facets and the spatial separation of electrons and holes on specific crystal facets [192, 237]. The optimal percentage of {001} facets for photocatalysis is
still debated [192]. Improving contaminant adsorption by modifying photocatalyst
surface is another way to enhance photocatalytic activity due to the short lifetime of
ROS. However, little has been done in this area.
Another actively pursued research area is to extend the excitation spectrum of
TiO 2 to include visible light. The general strategy is doping metal impurities, dye
sensitizers, narrow bandgap semiconductors, or anions into nano-TiO 2 to form
hybrid nanoparticles or nanocomposites [99, 245]. Metals and anions create impurity energy levels or narrow the bandgap; upon visible light excitation, dye sensitizers and narrow bandgap semiconductors inject electrons into TiO 2 to initiate the
catalytic reactions. Among these methods, anion (especially nitrogen) doping was
considered the most cost effective and feasible for industrial applications, although
their stability and long-term efficacy have not been tested. Decreased nitrogen concentration during photocatalysis has been reported [99, 156].
Other than TiO 2 , WO 3 and some fullerene derivatives also have the potential to
be used in photocatalytic water treatment. WO 3 has a narrower bandgap than TiO 2 ,
allowing it to be activated by visible light (<450 nm) [159]. Pt doping further
enhances WO 3 reactivity by facilitating multielectron reduction of O 2 and improving e
−
/h
+
separation [154]. Aminofullerenes generate
1
O 2 under visible light irradiation (<550 nm) and have been known to degrade pharmaceutical compounds and
inactivate viruses [172, 195]. Fullerol and C 60 encapsulated with poly(Nvinylpyrrolidone) can produce
1
O 2 and superoxide under UVA light [35].
Aminofullerenes are more amenable to immobilization than fullerol and are more
effective for disinfection purposes due to their positive charge.
1
O 2 has lower oxidation potential than hydroxyl radicals produced by TiO 2 , while it is a more selective
ROS and consequently less susceptible to quenching by nontarget background
organic matter. Fullerenes are currently much more expensive and not as readily
available as TiO 2 .
Current and Potential Applications for Water and Wastewater Treatment