Environmental Engineering Reference
In-Depth Information
concentrations and charge carrier mobility. control of the defect structures during processing of semiconductor materials thus
allows significant modification of their photocatalytic characteristics [42]. Hydrogen thermal treatment is effective in gener-
ating oxygen vacancies and Ti
3+
point defects in commercially available TiO
2
nanopowders [41]. The surface density of the Ti
3+
defects may be increased by increasing crystallite size in anatase nanoparticles produced by solvothermal synthesis [43, 44].
Plasma treatment can create new oxygen vacancy states in nanocrystalline TiO
2
that change its light absorption properties [45].
10.4
strategies for iNcreasiNg Photoreactivity
Because of the relatively poor efficiency of the photocatalytic reaction, a great deal of the current research in this field is
directed toward development of improved photocatalysts by chemical or structural modification [46]. A photocatalyst immobi-
lized on a substrate has a smaller effective surface area compared to nanoparticles in aqueous suspension. The specific surface
area is greatly increased by fabricating highly ordered arrays of TiO
2
nanotubes by anodic oxidation of a titanium substrate.
These nanotube arrays display enhanced photocatalytic activities compared to thin films [47]. The effect of varying the nano-
tube diameter is comparatively modest, however, because the increase in the active surface area of the photocatalyst as the
diameter is decreased is offset by a reduction in light transmission.
Doping with low concentrations of transition metal ions can increase the photocatalytic activity of TiO
2
by creating charge
carrier traps that prevent recombination [14, 15]. for relatively large particles, volume recombination is the primary mecha-
nism, while for extremely small particles the reduction of the transport distance of electrons and holes to the surface increases
the charge carrier transfer rate. Below a critical diameter, surface recombination becomes dominant, reducing the photocatalytic
efficiency [48]. A systematic investigation of TiO
2
doped with fe
3+
, V
4+
, Re
5+
, mo
5+
, Ru
3+
, mn
3+
, and Rh
3+
ions [14] indicated
that the photoreactivity with chloroform and carbon tetrachloride increased exponentially until the solubility limit for the metal
was exceeded, after which it decreased again, due to the effect of the excess dopant present on the surface. Another study [15]
found that doping with Nd
3+
and Pd
2+
ions was effective in increasing the degradation rate for 2-chlorophenol, while Pt
4+
resulted in only a marginal improvement and fe
3+
was detrimental. The position of the dopant in the crystal lattice is determined
by its size relative to the Ti
4+
ion; interstitial dopants are more effective carrier traps than substitutional ones because they cause
greater perturbation of the localized energy levels. experimental evidence [49] indicates that the effect on photocatalytic activity
is greatest when the ionic radius of the dopant is approximately 0.8 nm.
The photoreactivity of metal ion-doped TiO
2
depends in a complex manner on the concentration and distribution of the
dopants, their energy levels within the lattice, and the d electron configuration [14]. Powders doped with different transition
metal ions by a wet impregnation method generally showed higher photocatalytic activity for degradation of 4-nitrophenol
than the undoped photocatalyst [50]; however, no simple correlation could be made between photoreactivity and the physico-
chemical properties of the photocatalysts. The effect of nontransition metals as dopants has also been investigated. Sb-doped
nanocrystalline TiO
2
synthesized by a coprecipitation method was found to be more effective in degrading methylene blue
than similar material in the undoped state [51]. TiO
2
nanoparticles prepared by the sol-gel process and codoped with fe
3+
and
eu
3+
ions that functioned as traps for holes and electrons, respectively, showed significantly enhanced photodegradation
efficiency for chloroform as a result of the synergistic effect of the two different dopants [52].
Surface loading with noble metal clusters can increase the photoreactivity of TiO
2
by shifting the energy levels in the band
gap [12] or by injecting electrons into the conduction band [13]. for very small clusters of atoms, the physical, chemical, and
electronic properties may additionally be modified by quantum confinement effects. These result in a metal to insulator
transition in Au, Pd, and Ag when the particle diameter is below approximately 3 nm [53, 54]. The peak catalytic activity occurs
in the same size range, indicating a strong correlation with the electronic properties of the metal cluster and in particular the
band gap [55]. figure 10.2 shows a conceptual model of the evolution of the morphology with cluster size and the resultant
effect on the photocatalytic activity. for a diameter of approximately 3 nm, the cluster has a bilayer configuration and the turn-
over frequency (TOf) is a maximum.
comparison may be made between this idealized structure and the appearance of an approximately 5-nm-diameter Au cluster,
seen in figure 10.3, deposited on the surface of mixed-phase TiO
2
from an aqueous solution of HAucl
4
by a precipitation
method [56]. loading TiO
2
with Au in this way has been shown to increase the photodegradation efficiency for 4-chlorophenol
[56] and methyl tertiary-butyl ether [57]. corresponding improvements have also been reported in the photocatalytic activity
toward methyl orange using Ag-modified TiO
2
films prepared by radiofrequency (Rf) magnetron sputtering [58]. The presence
of nanosized Pt deposits on the surface of TiO
2
has furthermore been shown to affect the kinetics and mechanisms of the
photocatalytic degradation of trichloroacetate [59].
UV light constitutes only about 5% of the solar radiation spectrum compared to approximately 46% for the visible wave-
lengths. This has motivated attempts to realize visible light photocatalysts by doping TiO
2
with anions or cations to narrow the
