Photosensitization, of semiconductors

Let us now briefly outline the structure of this review. The next section contains information concerning the fundamentals of the electrochemistry of semiconductors. Part III considers the theory of processes based on the effect of photoexcitation of the electron ensemble in a semiconductor, and Parts IV and V deal with the phenomena of photocorrosion and light-sensitive etching caused by those processes. Photoexcitation of reactants in a solution and the related photosensitization of semiconductors are the subjects of Part VI. Finally, Part VII considers in brief some important photoelectrochemical phenomena, such as photoelectron emission, electrogenerated luminescence, and electroreflection. Thus, our main objective is to reveal various photo-electrochemical effects occurring in semiconductors and to establish relationships among them. 

One other approach to the photoelectrolysis of water that has been adopted involves the photosensitization of semiconductor electrodes such as Ti02,362,364,365 SrTi03365,366 or Sn02 265,365,367 by, for example, [Ru(bipy)3]2+. The photochemically excited state of the chromophore injects an electron into the conduction bond of a semi-conductor this is then passed via an external circuit to a platinum electrode for H2 production. The oxidized form of the quencher then forms 02 apparently in an uncatalyzed reaction. Unfortunately, all such systems... 

One other approach to the photoelectrolysis of water that has been adopted involves the photosensitization of semiconductor electrodes such as SrTi03or... 

Photosensitization.—The photosensitization of semiconductor electrodes is still the subject of a number of papers every year. Although the chance is small that single-crystal systems will benefit appreciably from photosensitization, the possibilities for high surface area systems, such as dispersions, are more exciting. Key references on photosensitization have been included in this review, although they do little to change the currently accepted view of the mechanism or efficiency of photosensitization. 

Bearing in mind the semiconductive properties of PCSs one might expect that these substances, being p-type semiconductors in air, possess photosensitizing activity. We, indeed, have demonstrated40 that PCSs, such as poly(schiff base)s, salts of poly(propynoic acid), or polyquinoline, are active photosensitizers of... 

Nasr C, Hotchandani S, Kim WY, Schmehl RH, Kamat PV (1997) Photoelectrochemistry of composite semiconductor thin films. Photosensitization of Sn02/CdS coupled nanocrystal-Utes with a ruthenium polypyridyl complex. J Phys Chem B 101 7480-7487... 

Surfactant vesicles constitute a very flexible medium for the support of semiconductors. Semiconductor particles can be localized at the outer, the inner, or at both surfaces of single-bilayer vesicles (Fig. 102). Each of these arrangements has certain advantages. Semiconductor particles on outer vesicle surfaces are more accessible to reagents and can, therefore, undergo photosensitized electron transfer more rapidly. Smaller and more monodispersed CdS particles can be prepared and maintained for longer periods of time in the interior of vesicles than in any other arrangement... 

Title Polybenzoxazole Precursor, Photosensitive Resin Composition Using the Same, and Manufacturing Method of Semiconductor Device... 

The corrosion behavior of semiconductors can, in principle, be described within the framework of the same concepts as for metals (see, for example, Wagner and Traud, 1938), but with due account for specific features in the electrochemical behavior of a solid caused by its semiconducting nature (Gerischer, 1970). One of the main features is photosensitivity related to a change in the free-carrier concentration under illumination. Photosensitivity underlies the phenomenon of photocorrosion. 

Recently, the electron-transfer kinetics in the DSSC, shown as a schematic diagram in Fig. 10, have been under intensive investigation. Time-resolved laser spectroscopy measurements are used to study one of the most important primary processes—electron injection from dye photosensitizers into the conduction band of semiconductors [30-47]. The electron-transfer rate from the dye photosensitizer into the semiconductor depends on the configuration of the adsorbed dye photosensitizers on the semiconductor surface and the energy gap between the LUMO level of the dye photosensitizers and the conduction-band level of the semiconductor. For example, the rate constant for electron injection, kini, is given by Fermi s golden rule expression ... 

Most of the modern theories of the photoconductivity sensitization consider that local electron levels play the decisive role in filling up the energy deficit The photogeneration of the charge carriers from these local levels is an essential part of the energy transfer model. Regeneration of the ionized sensitizer molecule due to the use of the carriers on the local levels takes place in the electron transfer model. The existence of the local levels have now been proved for practically all sensitized photoconductors. The nature of these levels has to be established in any particular material. A photosensitivity of up to 1400 nm may be obtained for the known polymer semiconductors. There are a lot of sensitization models for different types of photoconductors and these will be examined in the corresponding sections. 

The diverse properties of organic molecular materials, of which the polymers are amongst the primary ones, will, without doubt, be intensively developed in the future. Photosensitive polymer semiconductors with pre-given properties and a broad spectrum of application will be created for various optoelectronic devices. 

Gregg et at.si) examined photosensitization of perylene pigments (Dye 15-17) on a porous Sn02 thin film instead of Ti02 film as DSC, in view of energy matching with conduction band of semiconductors and LUMO of the sensitizers. When perylene-3,4-dicarboxylic acid-9,10-(5-phenanthroline) carboximide (Dye 15) was used, Jsc of 3.26 mA-cm 2, of 0.45 V, and a photoelectric conversion efficiency of 0.89% were observed under AM 1.5 irradiation. IPCE achieves close to 40% at 460 nm. 

This approach can be further extended to photoelectrochemical reactions at modified semiconductor electrodes. In such cases the immobilized substance(s) may serve several functions mediation of the redox process, photosensitization of the semiconductor, and photocorrosion protection. 

Colloids of semiconductors are also quite interesting for the transmembrane PET, as they possess both the properties of photosensitizers and electron conductors. Fendler and co-workers [246-250] have shown that it is possible to fix the cadmium sulfide colloid particles onto the membranes of surfactant vesicles and have investigated the photochemical and photocatalytic reactions of the fixed CdS in the presence of various electron donors and acceptors. Note, that there is no vectorial transmembrane PET in these systems. The vesicle serves only as the carrier of CdS particles which are selectively fixed either on the inner or on the outer vesicle surface and are partly embedded into the membrane. However, the size of the CdS particle is 20-50 A, i.e. this particle can perhaps span across the notable part of the membrane wall. Therefore it seems attractive to use the photoconductivity of CdS for the transmembrane PET. Recently Tricot and Manassen [86] have reported the observation of PET across CdS-containing membranes (see System 32 of Table 1), but the mechanism of this process has not been elucidated. Note, that metal sulfide semiconductor photosensitizers can be deposited also onto planar BLMs [251],... 

Since the surface properties of the colloid have a strong influence on the photoreduction kinetics, it seemed interesting to elucidate the effect of the added surface-active substances on the kinetic regularities of the reactions photosensitized by semiconductor colloid. The surfactant molecules are known to concentrate near the surface of the colloidal particle, so they may affect strongly the kinetics of photocatalytic reactions proceeding at the particles surface [52,53]. Fig. 2.30 presents the temperature dependencies of the initial rate of the MV photoreduction over colloidal CdS prepared at the excess of the sulfide ions. These dependencies were obtained at the addition of different amounts of PAA. One may see that both the initial rate and the observed activation energy of the methylviologen photoreduction do not depend, within the experimental error, on the concentration of... 

Patrick, B. Kamat, P. V. Photoelectrochemistry in semiconductor particulate systems. Part 17. Photosensitization of large-bandgap semiconductors. Charge injection from triplet excited thionine into ZnO colloids, J. Phys. Chem. 1992, 96, 1423. 

Kamat, P. V. Chauvet, J. P. Fessenden, R. W. Photoelectrochemistry in particulate systems. 4. Photosensitization of a Ti02 semiconductor with a chlorophyll analogue, J. Phys. Chem. 1986, 90, 1389. 

MoIV (CN)8 and WIV (CN)8. This results in visible light photosensitization of Ti02, due to electron injection from the excited state of the complexes to the conduction band of the semiconductor. Photoelectrochemical systems with photoelectrodes of polycrystalline Ti02 derivatized with the above complexes give quantum yields of up to 37% upon illumination at the absorption peaks of the complexes, around 420 nm. The photoresponse is extended up to 700 nm. 

Gold nanoparticles are virtually not luminescent, but silver nanoparticles show plasmon emissions with reasonable quantum yields. Furthermore, the non-radiative decay, resulting in electron-hole pair generation, may be used for photosensitization of wide bandgap semiconductors (see Figure 7.5) [16,17]. Similar effects may also be observed as direct photoinduced electron transfer between metal surfaces and surface-bound molecules [18]. 

Figure 18.1 Selected processes occurring during the photocatalytic inactivation of E. coli in the presence of semiconductor photosensitizer (SOD- superoxide dismutase eCB, hVB+ - photogenerated electrons and holes in conduction and valence band, respectively). Adapted from Mitoraj et al. [39], Szacitowski et al. [40] and Sunada et al. [23]...

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