Photo-excited electrons

Another interesting applications area for fullerenes is based on materials that can be fabricated using fullerene-doped polymers. Polyvinylcarbazole (PVK) and other selected polymers, such as poly(paraphcnylene-vinylene) (PPV) and phenylmethylpolysilane (PMPS), doped with a mixture of Cgo and C70 have been reported to exhibit exceptionally good photoconductive properties [206, 207, 208] which may lead to the development of future polymeric photoconductive materials. Small concentrations of fullerenes (e.g., by weight) lead to charge transfer of the photo-excited electrons in the polymer to the fullerenes, thereby promoting the conduction of mobile holes in the polymer [209]. Fullerene-doped polymers also have significant potential for use in applications, such as photo-diodes, photo-voltaic devices and as photo-refractive materials. 

Similar to the molecular photosensitizers described above, solid semiconductor materials can absorb photons and convert light into electrical energy capable of reducing C02. In solution, a semiconductor will absorb light, and the electric field created at the solid-liquid interface effects the separation of photo-excited electron-hole pairs. The electrons can then carry out an interfacial reduction reaction at one site, while the holes can perform an interfacial oxidation at a separate site. In the following sections, details will be provided of the reduction of C02 at both bulk semiconductor electrodes that resemble their metal electrode counterparts, and semiconductor powders and colloids that approach the molecular length scale. Further information on semiconductor systems for C02 reduction is available in several excellent reviews [8, 44, 104, 105],... 

It was found that zinc oxide catalyses the formation and decomposition of peroxides under the influence of UV radiation [Refs. 117, 395, 396, 543, 640]. The most likely mechanism involves the capture of a photo-excited electron by a molecule of absorbed oxygen, as suggested by... 

When light is absorbed by the zinc oxide, the initial reaction may be a transfer of an electron from zinc to O2 absorbed on the surface, and the resulting -02 is a radical ion. Organic additives undergo oxidation, either by transfer of electrons to the photopositive surface of the zinc oxide after loss of a photo-excited electron by the crystal to O2 or by hydrogen abstraction by the peroxide radical ion to form HO2 . 

However, since most of the photo-excited electrons are most likely transferred to V sites, they should be transferred to the H sites, minimizing any possible source of loss, particularly during the proto-reduction process. This might be realized, for instance, by a co-catalyst. However, this goes beyond the scope of the present study and will be taken on in a forthcoming research project. 

K. Helenelund, S. Hedman, L. Asphmd, U. Gelius, K. Siegbahn. P. Froelich and O. Goscinski Post-Collision Interaction in the Photo-Excited Electron Spectrum of Selenium (Preprint 1986). 

Excitons in semiconductors have a finite lifetime due to a recombination of the photo-excited electron-hole pair. In QDs, the energy released upon exdton annflii-... 

Figure 1. Energy diagrams for FeS, ZnS, and MnS as potential donors of photo-excited electrons (left column) and for the biologically relevant electron acceptors (right column). The Highest Occupied Molecular Orbital (HOMO) level in the ence bands of each semiconductor is shown by a darker color than the respective Lowest Unoccupied Molecular Orbital (LUMO) level in the conduction band. The picture is based on data from references [72,99,122,262,264].

D A D + A- electron transfer (a) from a photo-excited electron donor and (b) to a... 

These experimental results proved both functionally and structurally that the photo-excited electron flow from PS I reaction center can be linked to the electron transport chains and phosphorylation mechanism of the crista membranes in the assembled system. The preparation of more purified PS I particles will be carried out and the transport pathways of the excited electron flow from PS I reaction center will be studied in future. 

Figure 27.11 Schematics ULustrating the relationship between the distribution of nascent photo-excited electrons and the state density in a metal (see also Figure 27.3). The excited electrons are classified as sub-vacuum electrons (fj <0eV) and photoelectrons (f(->0eV). Adapted from Zhou et at, in Laser Spectroscopy and Photo-Chemistry on Metal Surfaces, II, 1995, with permission of World Scientific Publishing Co...

Optical Electron Transfer.—Piering and Malin report another example of electron transfer induced photochemically. The complex (3), previously reported by Toma, is stable with respect to internal electron transfer, but reacts on exposure to visible light. Complex (4) undergoes simultaneous thermal and photo-excited electron... 

Antibacterial, deodorizing and self-cleaning functions of photocatalyst derive from the organic compound decomposition ability of active oxygen, generated from photocatalytic reaction. For the purpose of improving oxidative degradation activity, it is essential to reduce the recombination of photo-excited electron-hole pair. 

To facilitate both the reduction and oxidation of H2O by photo-excited electrons and holes, the match of the band gap and the potentials of the conduction and valence bands are important. Both the reduction and oxidation potentials of water should lie within the band gap of the photocatalyst. The bottom level of the conduction band has to be more negative than the redox potential of H+/H2 (OV vs. NHE), while the top level of the valence band is more positive than the redox potential of O2/H2O (1.23 V). Therefore, the band gap energy (Eg) of the photocatalyst should be >1.23eV to achieve water sphtting. The band gap energy (Eg) can be calculated using equation [228] ... 

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