Environmental Engineering Reference
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several tenth of one nm, bulk and volume recombination are dominant. Volume
recombination is the direct or indirect interaction between electrons in the conduction
band and holes in the valance band. Direct recombination of charges within the bands
could occur and it is often radiative with intense band edge luminescence if there are
only none or only a few electronic states within the band gap (Zhang, 2000). With the
increase of band gap states, volume recombination is dominated by direct exciton-
exciton annihilation rather than recombination of trapped charge carriers. When there
are electronic states induced by surface defects and dopants in the bandgap, part of the
conduction band electrons and/or valance band holes can be trapped during their return
to lower energy state. The trapped charge carriers then undergo a radiative or non-
radiative recombination. This recombination process can happen between conduction
band electrons and trapped holes, valance band holes and trapped electrons, and trapped
electrons and trapped holes (Zhang, 2000). However, when the particle size decreases to
several nanometers, it creates high density of charge carriers on the surface; then surface
recombination occurs accordingly. By definition, the surface recombination is referred
to the decay between surface trapped electrons and holes, surface non-trapped electrons
and holes, or surface trapped and non-trapped charge carriers. Therefore, it is possible
to establish a threshold particle size at which there is low rate of charge carrier decay to
yield the maximum, photocatalytic activity.
Figure 3.4 The schematic photocatalytic process in a semiconductor solid and its
potential charge carrier paths. Modified from Hoffmann et al. (1995).
3.2.2 General Reaction Pathways
The semiconductor photocatalysts, especially TiO 2 , have been used successfully
in photocatalytic removal of a wide range of pollutants such as chlorinated compounds,
aromatic compounds, dyes, pesticides, pharmaceuticals and personal care products
 
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