Nanotechnostructured Catalysts TiO Nanoparticles for Water Purification - Nanotechnologies for Water Environment Applications - page 56

than deep traps. The longer charge carrier lifetime is normally desired for photocatalysis
applications. In contrast, a short charge carrier lifetime of deep traps usually impedes
the photoactivity of the photocatalysts.
The position of dopants can be on the surface or in the lattice. When the surface
concentration of dopants is high, it is considered as surface modifications. The dopants
within the lattice can be substitutional, interstitial, and both. The effect of dopant on the
photoactivity depends critically on its physical position. For instance, the substitutional
dopant ions contribute changes in the electronic structure, which could have great
influence on TiO 2 optoelectronic properties. The interstitial dopants usually contribute
only marginal effects to TiO 2 electronic structure and no great improvement on
photoactivity is expected. The maximum dopant concentration is not only limited by the
dopants' physic nature (i.e., ionic radii) but also the synthesis procedures. When the
dopant ions have comparable ionic radii to that of host ions, it is easier to occupy the
host sites, as opposed to those having large differences in ionic radii. The different
synthesis methods can vary the dopants positions. In some cases, dopant ions may
partially be absorbed on the surface of particles during the initial hydrolysis step in sol-
gel method and partially incorporate in the substitutional and/or interstitial sites of TiO 2
or form separate dopants-related phases during calcinations (Howe 1998). High-energy
physical processes, such as ion implantation, sputtering, and pulsed laser deposition
(PLD), usually provide sufficient energy to ionize the dopants for the substitutional
doping. The plasma enhanced chemical vapor deposition process (PECVD) ionizes the
dopant precursor in the radio frequency (rf) induced magnetic field; PECVD is proved to
be efficient for the synthesis of substitutionally doped TiO 2 (Buzby et al., 2006).
Dopants can enhance the photo-reactivity only within a certain range of concentration.
When the dopant concentration is too high, negative effects such as the loss of the
structure stability and the great increase in charge carrier transfer rate could occur.
Consequently, the photoactivity can be decreased. Therefore, there exists an optimal
dopant concentration. By studying the degradation efficiency of CHCl 3 with Fe 3+ doped
TiO 2 nanoparticles, Zhang et al. (1998) have found that the optimal Fe 3+ doping level
decreases with increasing particle size (e.g., it decreases from 0.2 % at 6 nm to 0.05% at
11 nm of particle size). They have reasoned that, for the same dopant concentration, the
larger particles have a higher possibility of multiple trappings of charge carriers because
the average path length becomes longer, and multiple trappings will lead to a high
volume recombination rate. The limit of the dopant concentration can be material
dependent. When the concentration of dopant is too high, the lattice structure of TiO 2
can be destroyed due to serious lattice mismatching and the loss of stoichiometry. It is
worthy of notice that the optimal concentration for quantum efficiency might not be the
one with the lowest bandgap.
The impurity doping induces visible light absorption of extreme interest.
Although the bandgap narrowing can cause lower photoredox potential energy that