Nanotechnostructured Catalysts TiO Nanoparticles for Water Purification - Nanotechnologies for Water Environment Applications - page 55

The exciton Bohr radius (a B , cm) is expressed as:

ε
α= μ

!

2

(Eq. 3.21)

B

e

2

The Bohr radius, α B , (cm), is defined by the distance between the e - -h + pair. Note
that this effective mass approximation (EMA) is only valid for nanocrystals containing
less than 100 atoms. Efros and Rosen (1998) have suggested that there are two types of
confinement depending on the nanocrystal size. For nanocrystal with a radius R much
larger than the effective Bohr radius α B , the “weak confinement regime” is considered,
and the E(R) is proportional to R -1 . For crystal with a radius R much smaller than the
Bohr radius, the “strong confinement regime” is considered, and E(R) is reversely
proportional to R 2 . Brus (1984, 1986) suggested that, in the “strong confinement
regime,” the second term in Eq. 3.19 is the quantized energy due to quantum
confinement within the potential well. The third term is dominant when the particle size
is in the “weak confinement regime.” It considers the Coulomb interaction between the
e - -h + pair to account for the energy loss due to the recombination process induced by
shallow states along the conduction and valance band edges. The size quantization
effect has been studied on various semiconductors including CdS (Haase et al., 1988),
HgSe, PbSe, CdSe (Nedeljkovic et al., 1986), ZnO (Koch et al., 1985), Cd 3 S 2 (Fojtik et
al., 1985), and TiO 2 (Anpo et al., 1987; Kormann et al., 1988; Kavan et al., 1993). The
reported Q-size effect of semiconductor clusters generally is between 1 and 12 nm.

3.3.3 Impurity Doping

Doping foreign elements in TiO 2 is a very common way to modify its electronic
structure in terms of controlling the majority charge carrier concentration for particular
application. There are several techniques to introduce dopants into the semiconductor
such as ion implantation, thermal diffusion, sputtering, and chemical vapor deposition.
The dopant sources can be either ionic compounds or neutral atoms ionized during the
synthesis process and interact with the host lattice to form chemical bonds. Dopants
change the lattice thermal dynamics, electronic structures, lattice structure, optical
property and photoactivity. With regard to the photoactivity of TiO 2 , ideal dopants
should: (a) increase charge separation efficiency by charge carrier trapping; (b) improve
light absorption by inserting electronic states and narrowing the bandgap; (c) have
similar ionic radii with Ti or O atoms for cation and anion doping, respectively; and (d)
have stable chemical and thermodynamic properties under wide operational conditions.

Depends on the position of the impurity electronic states inserted in TiO 2 band
structure, the dopants can create shallow traps (which have states very close to the
conduction or valance band edge) and deep traps (which have states created at or near
the middle of the forbidden band). Shallow traps give a longer charge carrier lifetime