Environmental Engineering Reference
In-Depth Information
2.0
Extraterrestrial
AM 1.5, (Global)
AM 1.5, (Direct)
1.5
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Wavelength, [nm]
Figure 3.3 Standard Solar Spectra for space and terrestrial use. NREL: National
Renewable Energy Laboratory, Renewable Resource Data Center.
3.2
Photocatalytic Mechanism, General Pathways and Kinetics
3.2.1 Conceptual Physical Models of the Photocatalytic Process
The photocatalytic reaction requires a minimum photon energy that exceeds the
band gap of the material in order to trigger the interband transition of electrons between
the lowest unoccupied molecule orbital (LUMO) and the highest occupied molecule
orbital (HOMO). Upon irradiation, the photoexcited excitons have several de-excitation
pathways (Figure 3.4): (A) the photo-excited electrons recombine with holes on the
semiconductor surfaces; (B) the photo-excited electrons recombine with holes in the
bulk volume; (C) the photo-excited electrons are transferred to the absorbed
organic/trapped or inorganic redox species (typically oxygen molecules in the aerated
solution); and (D) migration of the holes to the surfaces and receive electrons from the
absorbed redox species via oxidation reactions (Fox and Dulay, 1993; Hoffmann et al.,
1995).
An ideal photocatalyst should have a minimum recombination rate (i.e.,
pathways A and B) and long lifetime for charge carriers as to maximize catalytic
reactivity at catalyst surfaces (i.e., pathways C and D). Due to thermodynamical
instability, the excited electrons and holes have a strong tendency to recombine and
become degenerated to lower energy states. Recombination of the excitons can be
radiative (i.e., emitting photons) and non-radiative (i.e., emitting heat and photons).
Surface and bulk recombinations are electronic decay processes in nanoparticles. The
recombination strongly depends on the particle size. For particles with size larger than
 
 
 
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