Nanotechnostructured Catalysts TiO Nanoparticles for Water Purification - Nanotechnologies for Water Environment Applications - page 57

reduces the transfer rate of charge carriers, the increase in photon absorption efficiency
or generation rate of the charge carrier usually can compensate for the negative effect
and greatly enhance the overall photoreactivity. The dopants can be generally
categorized by type of the ions (cation and anion) and will be discussed in the
following sections.
3.3.3.1 Cation Doping
Metal ions have been extensively used for cation doping. The dopants including
primary metal ions, transition metal ions, and lanthanide ions (e.g., Nd 3+ , Os 3+ , Re 5+ )
have been reported. A dopant with its valence being lower or higher than that of Ti 4+ is
known as n- or p-type doping, respectively. A very comprehensive discussion on
photoreactivity of metal ion doped TiO 2 and its charge carrier recombination dynamics
was reported by Choi et al. (1994). The transition metal ions are the most commonly
studied cation dopants due to their d-electron configuration that is similar to Ti 4+ . In
addition, the d-electron configuration of the transition metals is often responsible for the
high catalytic reactivity. Serpone et al. (1994a) systematically studied the photoactivity
of TiO 2 colloids doped with Fe 3+ , Cr 3+ and V 5+ ions. They reported that all doped
samples showed extended absorption threshold in the visible light range and the Cr 3+
doped sample had the largest red shift with absorption edge around 600 nm. By the ion
implantation method, Anpo et al. (1987) noticed that V 4+ , Cr 3+ and several other
transition metal ions remarkably shifted the absorption edge of TiO 2 to the visible light
region. However, the bandgap reduction observed optically does not guarantee an
improvement in catalytic reactivity. There are several facts to be considered: (a) the
binding site of the dopant. If the dopant is not substitutionally doped, dissolution of the
transitional metal due to corrosion can drop the catalytic performance in repeated
operation; (b) the energy position of the impurity states. The impurity states have to be
very close to the valance or the conduction band edges to avoid the charge carrier
recombination; and (c) the red shifts in optical absorption tests can be contributed to any
inter-band states transition in the forbidden band, and does not necessarily reflect the
“improved photoactivity. Therefore, it is very important to theoretically understand the
electronic structure of doped TiO 2 in order to design the photocatalyst as well as
predicting its photocatalytic performance. Combining the super-cell approach and the
full-potential linearized-augmented-plane-wave (FLAPW) method, Umebayashi et al.
(2002) reported theoretical calculations on electronic structure of rutile-phase TiO 2
doped with several dopants (e.g., V, Cr, Mn, Fe, Co and Ni). Results of DOSs
calculations indicate that the electronic states introduced by 3d metal are due to the t 2g
states from the dopant. They also concluded that with an increase in atomic number of
3 d metals, the localized states shifted to the lower energy level (donor level). In other
words, the lower atomic number of 3 d metal dopant can preserve larger oxidation
potential, whereas the high atomic number of 3 d metals preserves the reduction potential
for redox reactions. The mid range atomic number 3 d metals contribute to mid gaps