Photocatalysis band structure

The different types of quinones active in photosynthesis are being used as electron acceptors in solar cells. The compounds such as Fd and NADP could also be used as electron/proton acceptors in the photoelectrochemical cells. Several researchers have attempted the same approach with a combination of two or more solid-state junctions or semiconductor-electrolyte junctions using bulk materials and powders. Here, the semiconductors can be chosen to carry out either oxygen- or hydrogen-evolving photocatalysis based on the semiconductor electronic band structure. 

This review will proceed as follows. After a description of the primary processes common to all colloidal semiconductor applications, the thermodynamic requirements of those processes will be discussed, with special emphasis on those relevant to photocatalysis. This will be followed by a short section on how the colloidal state affects the semiconductor band structure and photogenerated charge carrier recombination dynamics, leading into a longer section on the dynamics of interfacial charge transfer... 

Keywords Photocatalysis and catalysis Photocatalytic activity Band structure and excitation Energy conversion Langmuir-Hinshelwood mechanism Electron-hole recombination Quantum efficiency Physical property-activity correlation Synergetic effect. 

The principle of photocatalysis is often explained with an illustration like Fig. 2, a schematic representation of the electronic structures of semiconducting materials, a band model. An electron in an electron-filled valence band (VB) is excited by photoirradiation to a vacant conduction band (CB), which is separated by a forbidden band, a band gap, from the VB, leaving a positive hole in the VB (Section III.B). These electrons and positive holes drive reduction and oxidation, respectively, of compounds adsorbed on the surface of a photocatalyst. Such an interpretation accounts for the photocatalytic reactions of semiconducting and insulating materials absorbing photons by the bulk of materials. In the definition of photocatalysis given above, however, no such limitation based on the electronic structure of a photocatalyst is included. For example, isolated... 

A possible interpretation for better understanding for e -h+ location is that there are sites trapping e or h+ in the crystal lattice and that e and h are trapped by these sites immediately after the band-to-band transition, that is, photoabsorption (9). Location of e and h in the initial stage of photocatalysis as well as the rate should be controlled by the density and spatial distribution of these traps in a photocatalyst. However, there is little information on the density and spatial distribution of traps, since the structure of traps has not been fully clarified (10). 

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