2.4.4 Doping of TiO 2
Another method to limit the e
À /h
+ recombination and thus to enhance the
photocatalytic efficiency consists in doping titania (Xu et al. 2005; Chen et al.
2008; Zaleska 2008; Devi et al. 2010; Zhang et al. 2012). The objective of doping
is also to shift the band gap of titania in the visible region. In fact, the photocatalytic
efficiency of TiO 2 depends partially upon the relative degree of branching of the
reactive electron-hole pairs into interfacial charge transfer reactions (Hoffmann et al.
1995). Because of the shift of the band gap of the photocatalyst, the presence of the
dopant, such as a metal ion, in the TiO 2 crystalline structure can change both the
charge carrier recombination rates and interfacial electron-transfer rates. The dopant
can act either as an electron trap or a hole trap. Lee et al. proceeded to doping of TiO 2
with Fe and W and demonstrated that the photocatalytic activity of metal-doped
TiO 2 decreased as the concentration of dispersed W or Fe ion in the lattice of TiO 2
was increased (Lee et al. 2001a, b). Xu et al. were the first group preparing zinc ion
surface-doped TiO 2 nanoparticles and nanotubes through ligand exchange reaction,
proving that the surface-doped materials exhibit a higher photoactivity than the pure
ones, by decreasing the electron/hole pair recombination rate (Xu et al. 2004, 2005).
Ku et al. demonstrated that an excessive ZnO on the surface of TiO 2 might decrease
the photocatalytic activity of the material by serving as recombination centers of
electron-hole pairs (Ku et al. 2011). This diminution of the photocatalytic performance at large dopant concentrations (low band gap values) can be explained in
terms of charge recombination. For a random distribution of substitutional acceptors/
donors of charge dopants, a Gaussian-like density of states appears at the upper/
lower part of the valence/conduction band. The corresponding density of states is
roughly proportional to the dopant concentration. Nevertheless, high doping concentration (low band gap) may have detrimental effects because tunneling between
trapped charges carriers makes a significant contribution to charge (electron hole)
recombination. Consequently, the photocatalytic rate constant typically decreases
with the shortening of the electron-hole pair distance due to a growing level of
dopant (Moser et al. 1987). This trend can be also interpreted in terms of band
positions (top of valence band and bottom of conduction band, (ii)). The positions of
valence band and conduction band of semiconductor reflect the oxidation ability of
hole and reduction ability of electron, respectively. From the thermodynamic point
of view, if the conduction band is more negative than the O 2 /O
ÁÀ
2 couple, the
photogenerated electrons could reduce O 2 to produce O
Á
2 . Meanwhile, if the position
of the valence band is more positive than the OH
À /OH
Á couple, the photogenerated
holes could oxide OH
À or H 2 O to form OH
Á . When the band gap decreases, i.e., for
high dopant content titania, the conduction band edge position and the valence band
edge position shift downward and upward, respectively. It is now well established
that the low conduction band position of doped titania would be the main reason for
the low activity in photooxidation of organic molecule for low band gap. The
oxidizing potential of the valence band holes remains more positive than the
standard potential of OH
À /OH
Á couple, and the reduction potential of the conduction
band electrons becomes less negative than the potential of O 2 /O
Á
2 . This indicates that
2 Dyes Depollution of Water Using Porous TiO 2 -Based Photocatalysts
61
Another method to limit the e
À /h
+ recombination and thus to enhance the
photocatalytic efficiency consists in doping titania (Xu et al. 2005; Chen et al.
2008; Zaleska 2008; Devi et al. 2010; Zhang et al. 2012). The objective of doping
is also to shift the band gap of titania in the visible region. In fact, the photocatalytic
efficiency of TiO 2 depends partially upon the relative degree of branching of the
reactive electron-hole pairs into interfacial charge transfer reactions (Hoffmann et al.
1995). Because of the shift of the band gap of the photocatalyst, the presence of the
dopant, such as a metal ion, in the TiO 2 crystalline structure can change both the
charge carrier recombination rates and interfacial electron-transfer rates. The dopant
can act either as an electron trap or a hole trap. Lee et al. proceeded to doping of TiO 2
with Fe and W and demonstrated that the photocatalytic activity of metal-doped
TiO 2 decreased as the concentration of dispersed W or Fe ion in the lattice of TiO 2
was increased (Lee et al. 2001a, b). Xu et al. were the first group preparing zinc ion
surface-doped TiO 2 nanoparticles and nanotubes through ligand exchange reaction,
proving that the surface-doped materials exhibit a higher photoactivity than the pure
ones, by decreasing the electron/hole pair recombination rate (Xu et al. 2004, 2005).
Ku et al. demonstrated that an excessive ZnO on the surface of TiO 2 might decrease
the photocatalytic activity of the material by serving as recombination centers of
electron-hole pairs (Ku et al. 2011). This diminution of the photocatalytic performance at large dopant concentrations (low band gap values) can be explained in
terms of charge recombination. For a random distribution of substitutional acceptors/
donors of charge dopants, a Gaussian-like density of states appears at the upper/
lower part of the valence/conduction band. The corresponding density of states is
roughly proportional to the dopant concentration. Nevertheless, high doping concentration (low band gap) may have detrimental effects because tunneling between
trapped charges carriers makes a significant contribution to charge (electron hole)
recombination. Consequently, the photocatalytic rate constant typically decreases
with the shortening of the electron-hole pair distance due to a growing level of
dopant (Moser et al. 1987). This trend can be also interpreted in terms of band
positions (top of valence band and bottom of conduction band, (ii)). The positions of
valence band and conduction band of semiconductor reflect the oxidation ability of
hole and reduction ability of electron, respectively. From the thermodynamic point
of view, if the conduction band is more negative than the O 2 /O
ÁÀ
2 couple, the
photogenerated electrons could reduce O 2 to produce O
Á
2 . Meanwhile, if the position
of the valence band is more positive than the OH
À /OH
Á couple, the photogenerated
holes could oxide OH
À or H 2 O to form OH
Á . When the band gap decreases, i.e., for
high dopant content titania, the conduction band edge position and the valence band
edge position shift downward and upward, respectively. It is now well established
that the low conduction band position of doped titania would be the main reason for
the low activity in photooxidation of organic molecule for low band gap. The
oxidizing potential of the valence band holes remains more positive than the
standard potential of OH
À /OH
Á couple, and the reduction potential of the conduction
band electrons becomes less negative than the potential of O 2 /O
Á
2 . This indicates that
2 Dyes Depollution of Water Using Porous TiO 2 -Based Photocatalysts
61