Semiconductor sensitization

In most studies, heterogeneous photocatalysis refers to semiconductor photocatalysis or semiconductor-sensitized photoreactions, especially if there is no evidence of a marked loss in semiconductor photoactivity with extended use. It is meant here that the initial photoexcitation takes place in the semiconductor catalyst substrate and the photoexcited catalyst then interacts with the ground state adsorbate molecule [209]. 

Nazeeruddin, M. K. Graetzel, M. Dyes for Semiconductor Sensitization. In Encyclopedia of Electrochemistry Semiconductor Electrodes and Photoelectrochemistry, Vol. 6 Licht, S., Ed. Wiley-VCH 2002 Darmstadt, pp 407-431. 

Mora-Sero, I. Bisquert, J., Breakthroughs in the Development of Semiconductor-Sensitized Solar Cells Full./. Phys. Chem. Lett. 2010,1 3046-3052. 

Ashokkumar, M., Kudo, A., Saito, N., Sakata, T. 1994. Semiconductor sensitization by RuSj colloids on TiOj electrodes. Chem Phys Lett 229 383-388. 

The semiconductor-sensitized reaction is presumed to occur according to the sequence shown in Scheme 2. Band gap excitation creates an electron-hole pair at the semiconductor surface. 

At low light flux, the semiconductor sensitization is constrained to one electron routes, since the valence band hole is annihilated by a single electron transfer. Presumably after decarboxylation the resulting alkyl radical can be reduced to the observed monodecarboxylate more rapidly than it can transfer a second electron to form the alkene. In a conventional electrochemical cell, in contrast, the initially formed radical is held at an electrode poised at the potential of the first oxidation so that two-electron products cannot be avoided and alkene is isolated in fair chemical yield. Other contrasting reactivity can be expected for systems in which the usual electrochemistry follows multiple electron paths. 

Percherancier, J., Chapelon, B., and Pouyet, B., Semiconductor-sensitized photocatalysis of pesticides in water the case of carbetamide, J. Photochem. Photobiol. A, 87, 261, 1995. 

An entirely different consequence of electron (or hole) transfer to addends is provided by semiconductor-sensitized decomposition of electron acceptors A-X (or electron donors, i.e., hole acceptors, D-Y) [17]. If organic molecules whose redox states (anion radical or cation radical) are unstable with respect to scission into a free radical and an ion [Eqs. (5,6)] are adsorbed onto a wide band gap semiconductor such as Ti02, etc., then back electron (hole) transfer can be inhibited, if the scission process is rapid. 

D + Y F S- 6- Semiconductor sensitization of radical formation. A Light absorption and charge trapping by surface adsorbates to give (B) ion radicals. C Free radical formation by ion radical fragmentation. (Adapted from Ref. 17b)... 

Figure 8 presents the variation of the minimum electronic excitation frequency, Vmin with otheai, determined from Pei using the values of Pxmax summarized in Fig. 7. A semiconductor sensitizer is constrained not to utilize incident energy below the bandgap. As seen in Figure 8 by the intersection of the solid line with Vmin, over one third of insolation power occurs at Vmin < 1.43 eV (867 nm), equivalent to the IR not absorbed by GaAs or wider bandgap materials. The calculations include both the AMO and AMI.5 spectra. In the relevant visible and IR range from 0.5 to 3.1 eV (+0.03 eV) for both the AMO and AMI.insolation spectra, Vmin(otheat) in the figure are well represented (i >A).999) by polynomial fits.

Figure 14. Dependence upon excitation photon energy hv of electron injection quantum yield 4>i obtained for various oxide semiconductors sensitized by cw-[Ru dcbpy)2(NCS)2].

Ashokkumar M., Kudo A., Saito N. and Sakata T. (1994), Semiconductor sensitization by RuSi colloids on TiOi electrodes , Chem. Phys. Lett. 229, 383-388. 

Zaban A., Ferrere S. and Gregg B. A. (1998), Relative energetics at the semiconductor sensitizing dye electrolyte interface , J. Phys. Chem. B 102, 452-460. 

Heimer T. A., d Arcangelis S. T., Farzad F., Stipkala J. M. and Meyer G. J. (1996), An acetylacetonate-based semiconductor-sensitizer linkage , Inorg. Chem. 35, 5319-5324. 

Figure 18. Schematic diagram of the illumination of a surface in laser light-Semiconductor sensitive etching.

The most commonly reported organic semiconductor material is phthalocyanine (Pc), although many embodiments of this material are technically classified as polymers and will not be discussed in this chapter. One sensor of this type is a p-type semiconductor sensitive to CI2 and NO2 at a 150 °C operating temperature, and has the advantage of very low sensitivity to H2, H2S, NH3, CH4, SO2, and CO [20]. Mg, Co, Ni, Cu, or Zn phthalocyanine films have also demonstrated some sensitivity to NO2 [28]. In another study, lead phthalocyanine (PbPc) was configured as an n-type semiconductor when doped with RUO2 and Pd and was shown to be sensitive to H2 and CO gases [21]. 

Al-Sayyed, G., D Oliveira, J. C. and Pichat, P, 1991, Semiconductor - sensitized phot-degradation of 4-chlorophenol in water, J. Photochem. Photobiol. A Chem., 58 99-114. 

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