Chemically modified charge transport

A discussion of the charge transfer reaction on the polymer-modified electrode should consider not only the interaction of the mediator with the electrode and a solution species (as with chemically modified electrodes), but also the transport processes across the film. Let us assume that a solution species S reacts with the mediator Red/Ox couple as depicted in Fig. 5.32. Besides the simple charge transfer reaction with the mediator at the interface film/solution, we have also to include diffusion of species S in the polymer film (the diffusion coefficient DSp, which is usually much lower than in solution), and also charge propagation via immobilized redox centres in the film. This can formally be described by a diffusion coefficient Dp which is dependent on the concentration of the redox sites and their mutual distance (cf. Eq. (2.6.33). 

Having debated the mechanism of charge transport within the polymer film, it is now useful to consider a few examples of chemical applications of polymer modified electrodes. Electrodes coated with [Ru(bipy)2Cl(PVP)]Cl or [Ru(bipy)2(py)(PVP)]Cl2 show strong catalytic effects for the reduction of cerium(IV) and the oxidation of iron(II).52... 

Various kinds of conducting polymers can work as coimter electrode in DSSC. Poly(3,4-ethylenedioxythiophene) (PEDOT), polypyrrole (PPy), PANI are most investigated polymer materials. With the development of nanomaterial, the nanostructure has been introduced into previously mentioned polymers. These polymers are readily deposited by either chemical or electrochemical polymerization process. And they are readily modified into aligned nanostructure that might enhance charge transport and surface area simultaneously (Sun et al., 2015a). 

Figure 8,6 Monomer molecular orbital contour plots of the HOMO and LUMO orbitals of FTF, TFTFT and TFT. Reprinted with permission from S. E. Koh, B. Delley, J. E. Medvedeva, A. Facchetti, A. J. Freeman, T. J. Marks and M. A. Ratner, Quantum chemical analysis of electronic structure and n- and p-type charge transport in perfluoroarene-modified oligothiophene semiconductors, J. Phys. Chem. B, 110, 24361 24370 (2006), Copyright 2006 American Chemical Society...

Refs. [i] Bard A], Faulkner LR (2001) Electrochemical methods. 2" edn. Wiley, New York, pp 156-180 [ii] Inzelt G (2002) Kinetics of electrochemical reactions. In Scholz F (ed) Electroanalytical methods. Springer, Berlin, pp 29-48,137-148 [Hi] Oldham HB, Myland JC (1994) Fundamentals of electrochemical science. Academic Press, San Diego [iv] Rieger PH (1987) Electrochemistry. Prentice Hall, Oxford, pp 151-165 [v] Murray RW (1984) Chemically modified electrodes. In Bard AJ (ed) Electroanalytical chemistry, vol. 13. Marcel Dekker, New York, pp 191-368 [vi] Inzelt G (1994) Mechanism cf charge transport in polymer-modified electrodes. In Bard AJ (ed) Electroanalytical chemistry, vol. 18. Marcel Dekker, New York, pp 89-241 [vii] Harrison JA, Thirsk HR (1971) The fundamentals of metal deposition. In Bard AJ (ed) Electroanalytical chemistry, vol. 5. Marcel Dekker, New York, pp 67-148 [viii] Vargas T, Varma R (1991) In Varma R, Selman JR (eds) Techniques for characterization cf electrodes and electrochemical processes. Wiley, New York, pp 707-760 [ix] Bockris J OM, Khan SUM (1993) Surface electrochemistry. Plenum Press, New York, pp 350-376... 

Chemisorption is a process in which chemical bond forms and the valence electrons transport among the bonding constituent atoms. Processes of charge transport modify the valence band structure and introduce additional DOS features. The derived DOS features are detectable using STS (around Ep), PES (E < Ep), and IPES (inverse PES, E > Ep). 

The nature of the chemical bond bridges the structures and properties of crystals and molecules [1]. Interatomic interaction and electronic distribution in the valence band are the keys to engineering materials. The spontaneous bond contraction enhances the binding energy of the remaining bonds of the lower coordinated atom. Chemical reaction modifies directly the occupied valence DOS by charge transportation or polarization. Bond relaxation and valence band modulation change the properties of a solid. 

---