Subject interfacial electronic

An important subject in this chapter on Electron transfer at electrodes and interfaces is to draw an analogy between electrochemical and interfacial electron transfer between two solid phases. Any theory dealing with electron transfer has a thermodynamic and a kinetic basis. In Section 4.2, it was shown that electrons flow or tunnel in the direction of decreasing electrochemical potential the gradient of the electrochemical potential is the driving force behind a directed flow of electrons,... 

As it gives a nice and relatively simple illustration of the use of various characterization methods, we will discuss the subject of electron-hole recombination dynamics in the next sections, taking the n-GaAs photoanode as the main example. More complicated topics - related to interfacial transfer of photogenerated charge carriers - are discussed in Section 2.1.3.3. 

BODIPY dyes were subjected to modifications in their meso position as the previous fluorescein dyes. Aromatic moieties at this position are perpendicular to the molecular plane but, nevertheless, affect the fluorescence properties of the core [99]. Subtle manipulation of the redox potential can be also read out by changes of the fluorescence lifetime as exemplified in a substrate for phosphoester cleavage [100]. Depending on of the redox properties of the substituents, fluorescence can be turned on and off [101]. When the electrochemical potential of the substituent is altered during a reaction, a fluorogenic probe for this particular reaction is tailored. This was exploited for studying interfacial electron transfer [61]. More applications appear feasible. 

Another area that is subject to significant interest concerns substrate variations, for example comparisons of lET processes for Ti02 and ZnO substrates, which have been carried out both experimentally and theoretically. Calculations suggest that the interfacial electronic coupling can also be sufficiently strong to mediate ultrafast lET for ZnO, but there may be other differences that are important experimentally for the ZnO performance, such as surface stability and screening of injected charges. ... 

Interfacial electrochemistry is about electric charges at interfaces between phases, one of which is an electron conductor and the other an ion conductor. The kinetic part of the subject is about the rate at which these charges transfer across the interphase. However, this definition clearly embraces two limiting cases. 

The recombination of photogenerated electrons and holes is the bane of all solar cells and a major reason for their less than ideal efficiencies. Excitonic solar cells, in which the electrons and holes exist in separate chemical phases, are subject primarily to interfacial recombination. There is, as yet, no theoretical model to accurately describe interfacial recombination processes, and this is an important area for future research. Wang and Suna [91] have laid a possible foundation for such a model by combining Marcus theory with Onsager theory. 

The interfacial structure and charge-transfer mechanism of two immiscible electrolyte solutions, as revealed by the kinetics of the charge-transfer processes, is the subject of Chapter 5 by Z. Samec and T. Kakiuchi. Theoretical and experimental advances made over the last 10 to 15 years in the study of ion- and electron transfer are systematically and critically reviewed. 

Understanding chemical reactivity at liquid interfaces is important because in many systems the interesting and relevant chemistry occurs at the interface between two immiscible liquids, at the liquid/solid interface and at the free liquid (liquid/vapor) interface. Examples are reactions of atmospheric pollutants at the surface of water droplets[6], phase transfer catalysis[7] at the organic liquid/water interface, electrochemical electron and ion transfer reactions at liquidAiquid interfaces[8] and liquid/metal and liquid/semiconductor Interfaces. Interfacial chemical reactions give rise to changes in the concentration of surface species, but so do adsorption and desorption. Thus, understanding the dynamics and thermodynamics of adsorption and desorption is an important subject as well. 

The third part of this text focuses on several important dynamical processes in condensed phase molecular systems. These are vibrational relaxation (Chapter 13), Chemical reactions in the barrier controlled and diffusion controlled regimes (Chapter 14), solvation dynamics in dielectric environments (Chapter 15), electron transfer in bulk (Chapter 16), and interfacial (Chapter 17) systems and spectroscopy (Chapter 18). These subjects pertain to theoretical and experimental developments of the last half century some such as single molecule spectroscopy and molecular conduction—of the last decade. 

It is worth noting that, as far as they are less than several nanometers thick, the passive films are subject to the quantum mechanical tunneling of electrons. Electron transfer at passive metal electrodes, hence, easily occurs no matter whether the passive film is an insulator or a semiconductor. By contrast, no ionic tunneling is expected to occur across the passive film even if it is extremely thin. The thin passive film is thus a barrier to the ionic transfer but not to the electronic transfer. Redox reactions involving only electron transfer are therefore allowed to occur at passive film-covered metal electrodes just like at metal electrodes with no surface film. It is also noticed, as mentioned earlier, that the interface between the passive film and the solution is equivalent to the interface between the solid metal oxide and the solution, and hence that the interfacial potential is independent of the electrode potential of the passive metal as long as the interface is in the state of band edge pinning. 

Interfacial reactions are becoming an increasingly important subject for studies with wide spread applications such as catalysis (7), electronics (2), chemical sensing 3,4), and many other applications (5,6). Understanding the rules that govern these surface reactions provides important information for fundamental studies in chemistry and biochemistry 7,8). Also the availability of numerous analytical techniques capable of detecting chemical changes in films that are few nanometers thick (P), have made studies of interfacial reactions a viable and important area of modem science. 

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