Chemisorption charge-transfer process

We have thus far talked about the chemisorption of ions at the semiconductor/electrolyte interface and charge transfer in the semiconductor surface layer. The main charge transfer process of interest is the transfer of electrons and holes across the semiconductor/electrolyte interface to the desired electrolyte species resulting in their oxidation or reduction. For any semiconductor, electrode charge transfer can occur with or without illumination and with the junction biased in the forward or reverse direction. 

Chemisorption of hydrogen — Process leading to the formation of strongly bound (chemisorbed) hydrogen atoms on an adsorbent (mostly on metal) either via the dissociative adsorption of molecular hydrogen (H2) or, in the case of electrified interfaces, by charge transfer process occurring, for instance, with H+(H30+) or H20 species... 

Actual electron transfer does occur in oxidation/reduction, or "redox", reactions. In this type of reaction, there is a change in the oxidation state of the adsorbate. A simple example is the chemisorption of an alkali atom, in which it becomes a 1+ ion, transferring its outer electron to empty electron orbitals of the substrate. It is the large electric dipole moment created by this charge transfer process that lowers the work function of surfaces on which alkali atoms are adsorbed (e.g., "cesiation") by up to several eV. This type of bonding is generally strong, and it can also be either molecular or dissociative. 

Chemisorption supposes breaking of chemical bonds in the reactant and formation of bonds with the electrode surface with charge transfer across the interface the nature of this process is pseudo-capacitive [5]. 

Due to the chemical potential difference for species in the electrolyte and the photoelectrode, and by virtue of the fact that the electrode can be run in forward and reverse bias configurations, a number of important processes at the interface can be discerned. In each case, we will be concerned with the energy required for the process under consideration to occur and its resulting effects on photoelectrode performance. We can think of these processes as being of four basic types chemisorption, the desired electron or hole charge transfer, surface decomposition and electrochemical ion injection. In the rest of the paper we will briefly summarize our present understanding of each. 

That surface interactions play such a role clearly demands that some sort of surface state concept be invoked. However, no simple techniques have yielded direct information about the nature of such states. To explain charge transfer, isoenergetic electron or hole processes are normally invoked with a subsequent thermalization of the electron or hole in the semiconductor. This unfortunately necessitates the existence of a surface state at the level of the redox potential. This may of necessity occur when strong chemisorption is present. However, in those cases,... 

If one were to describe the essence of electrode kinetics in one short phrase, it would be the transition from electronic to ionic conduction, and the phenomena associated with and controlling this process. Conduction in the solution is ionic, whereas in the electrodes and the connecting wires it is electronic. The transition from one mode of conduction to the other requires charge transfer across the interfaces. This is a kinetic process. Its rate is controlled by the catalytic properties of the surface, the chemisorption of species, the concentration and the nature of the reacting species and all other parameters that control the rate of heterogeneous chemical reactions. 

When an electrocatalytic reaction involves a primary step of molecular dissociative chemisorption, for example, a c,e mechanism, then the electrocatalysis arises more directly, in the same way as for many regular catalytic processes that involve such a step of dissociative chemisorption. In this type of electrocatalytic reaction, the dissociated adsorbed fragments, for example, adsorbed H in H2 oxidation, become electrochemically ionized or oxidized in one or more charge-transfer steps following the initial dissociation. The rate... 

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