Semi-conductor electrodes

Diffusion length in typical semi conductor electrodes, 492... 

The dye sensitised semi-conductor electrode is a transparent conducting sheet of glass coated (5 pm) with nanocrystalline TiOj (diameter 20 nm) doped with a ruthenium bipyridyl complex. The dye absorbs light, becomes excited and injects electrons into the TiOj electrode. The electrons travel into the transparent WO3 hhn and then, to balance the charge, lithium ions from the electrolyte solution insert into the WO3 and in so doing create the coloured species as described above. If the light source is removed then the cell is bleached back to its original colour. However, if the... 

It will be seen that although normal (or thermal ) electrochemical reactions can be sustai ned at low current densities using semiconductors (Le., they act as electron-poor metals), they real ly do not come onto center stage until their photoelectrochemistry is studied (see Chapter 10). Thus, semi conductor electrodes are responsive to light when metals are almost unreactive to it. 

One very interesting class of ET reactions at soft interfaces are those that are photoini-tiated. Following the pioneering studies of the Russian school, including those of Volkov [6] and Kuzmin [7], it has been shown that photosensitizers soluble in one phase are often adsorbed at the interface and can be quenched by electron donor or acceptors. This class of reaction offers interesting perspectives to design biomimetic approaches to artificial photosynthesis. Photoelectrochemistry at the interfaces between two immiscible electrolyte solutions (ITIES) is rather analogous to photoelectrochemistry at a semi-conductor electrode, where the potential drop within the semi-conductor should be considered. 

With a semi-conductor electrode the separation of photogenerated holes and electrons is achieved with the help of the field in the space charge layer. The counterpart of the combination of B and Y on an unselective electrode is the surface recombination of holes and electrons. Thus in the photogalvanic cell the selective electrode both separates B and Y and prevents their recombination. That is why it is a vital part of the cell. 

Since for any system B and Y are energetic species, and furthermore, to be formed, they have to undergo rapid electron transfer reactions one will need a rather special sort of electrode to prevent the recombination. Semi-conductor electrodes (n type Sn02) have been used with limited success for the iron thionine cell. 

The transfer of electrons between a metal or a semi-conductor electrode and a dissolved or surface-bound reactant is no different in kind from the homogenous processes, previously described. In the model proposed by Marcus the electrochemical rate constant, is given by... 

The mechanism operating within a polymer LED involves injection, via a metal electrode, of electrons into the conduction band and holes into the valence band of the polymeric semi-conductor. The electrons and holes diffuse towards each other and then combine to form an exciton, which can move along the polymer chain. These excited states then decay to the ground state with a characteristic fluorescence. 

The photoactive component in these cells is a dye adsorbed chemically onto the surface of the semi-conductor. When light hits this surface, the dye (S) absorbs a photon and becomes excited (S ) in this state it transfers an electron into the TiOj semi-conductor (injection). The positively charged dye (S+) then passes its positive charge to a redox mediator in the bulk electrolyte. The oxidised mediator is attracted to the counter electrode where it is reduced back by electron transfer, thus completing the circuit. 

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... 

The electrode may be a metal or any electron-conducting material, for example a semi-conductor which acts as a source or sink for electrons. 

The aryl-aryl bond formation is the most important reaction in the synthesis of various polyaryl materials which possess valuable conducting properties. The polyaryls such as poly-p-phenylene (PPP), which by various doping procedures reach a conductive state, are in focus as a potential for use as electrode materials in light-weight rechargeable batteries, electrochemical cells, semi-conductor devices, solar cells, and in the several other possible electrochemical uses. Very comprehensive polyarene-chemistry and its practical applications are excellently rewieved by Takakazu Yamamoto [6]. 

J. Jamnik [2003] Impedance Spectroscopy of Mixed Conductors with Semi-Blocking Electrodes, Solid State Physics 157, 19-28. 

The energy-level distribution factor is now recognized as a fundamental factor in the quantum-mechanical representation of electron-transfer rates both in heterogeneous redox reactions(2 ) at electrode surfaces and in homogeneous ones in bulk solution, as well as at semi-conductors(18). 

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