Environmental Engineering Reference
In-Depth Information
Doping of TiO
2
with transition metals such as iron, cobalt, nickel, manganese, chromium,
vanadium, copper, molybdenum, zirconium, silver, and zinc was tried extensively. Such
metal creates a “new” electron state inside the TiO
2
forbidden band, which can capture
the excited electrons from the TiO
2
valance band and consequently maintain the holes. It
can also allow the light absorption to be widened into the visible region to various extents
depending on the type of dopant and its concentration. Therefore, photocatalysis on TiO
2
can be promoted using visible light. The photodegradation of various organic substrates
depends on the nature of the dopant, concentration, and the microstructural characteris-
tics of the catalyst. Usually there is a critical doping concentration at which any further
increase in dopant concentration results in the charge carrier recombination, thus lower-
ing the photoactivity of the prepared doped TiO
2
. Excess of dopant on the particle surface
of TiO
2
lessens the speciic area of TiO
2
and hinders the adsorption of reactant, and thus
inhibits the photocatalytic activity.
Takeuchi (2003) has investigated the electron spin resonance signals to investigate elec-
tron transfer from TiO
2
to Pt particles. It was found that Ti 3p signals increased with
irradiation time and the loading of Pt reduced the amount of Ti 3p. This observation
indicates the occurrence of electron transfer from TiO
2
to Pt particles. As electrons accu-
mulate on the metal particles, their Fermi levels shift closer to the conduction band of
TiO
2
(Jakob et al., 2003; Subramanian et al., 2003, 2004), resulting in more negative energy
levels. Thermal instability, tendency to form charge carrier recombination centers, as well
as the expensive ion implantation facilities make metal-doped TiO
2
impractical, at many
times.
25.7.3 Doping with Nonmetallic Elements
TiO
2
absorption toward the visible light with nonmetallic elements such as nitrogen (N),
sulfur (S), carbon (C), and phosphorus (P) has been known. Asahi et al. (2001) were the irst
to show an absorption increase in the visible region upon nitrogen doping. This opened
the way to study TiO
2
doping with nonmetallic elements. The insertion of N or S atoms
on TiO
2
produces localized states within the band gap just above the valence band. Thus,
when N- or S-doped TiO
2
is exposed to visible light, electrons are promoted from these
localized states to the conduction band. The substitutional (N-Ti-O) doping of N is most
effective compared with other nonmetal dopants (S, P, and C) because its p states con-
tribute to the band-gap narrowing by mixing with O 2p states. Although doping with S
shows a similar band-gap narrowing, it requires larger formation energy for the substitu-
tion than that required for the substitution of N. Moreover, the ionic radius of S is larger
than O; thus, it is dificult to it it into the TiO
2
crystal.
The mechanism on enhancement of nitrogen doping is that N-doping narrowed the
gap between the valence band and conduction band of N-doped TiO
2
. Asahi et al. (2001)
proposed that the presence of nitrogen introduced new occupied orbitals in between the
valence band and the conduction band. These N-2p orbitals acted as a step up for the elec-
trons in the O-2p orbital, which once populated had now a much smaller jump to make,
to be promoted into the conduction band. Once this process occurs, electrons from the
original valence band can migrate into the mid-band-gap energy level, leaving a hole in
the valence band, which reacts as described before. The reaction mechanism of nonmetal-
doped TiO
2
is given in Figure 25.3.
