Film-substrate potential, relative

Film electrodes In contrast to the chemical etching methods, the film deposition method adds atoms at the substrate to form a rough film and/or discontinuous islands. This technique is commonly adapted from prior work in surface science and often realized in UHV chambers. However, the deposited film electrode is unlikely to have the same crystalline structure as the bulk metal phase formed by metallurgy that is usually used in electrochemistry. Moreover, film electrodes exhibit relatively poor surface stability and electrochemical re-versibiKty, particularly if the experiment is conducted over a wide potential region. 

The formation of deposits on platinised anodes can cause anode degradationThus dissolved impurities present in water which are liable to oxidation to insoluble oxides, namely Mn, Fe, Pb and Sn, can have a detrimental effect on anode life. In the case of MnOj films it has been stated that MnOj may alter the relative proportions of Cl, and O, produced and thus increase the Pt dissolution rate Fe salts may be incorporated into the TiO, oxide film and decrease the breakdown potential or form thick sludgy deposits. The latter may limit electrolyte access and iead to the development of localised acidity, at concentrations sufficient to attack the underlying substrate . 

Such effects are observed inter alia when a metal is electrochemically deposited on a foreign substrate (e.g. Pb on graphite), a process which requires an additional nucleation overpotential. Thus, in cyclic voltammetry metal is deposited during the reverse scan on an identical metallic surface at thermodynamically favourable potentials, i.e. at positive values relative to the nucleation overpotential. This generates the typical trace-crossing in the current-voltage curve. Hence, Pletcher et al. also view the trace-crossing as proof of the start of the nucleation process of the polymer film, especially as it appears only in experiments with freshly polished electrodes. But this is about as far as we can go with cyclic voltammetry alone. It must be complemented by other techniques the potential step methods and optical spectroscopy have proved suitable. 

A different scenario occurs when the bonding between the substrate and film is relatively weak. For WKl the first minimum in E N)—NfJLp is positive, and the chemical potential is greater than jjlq. In that case... 

Figure 3 Schematic of a nanoporous 2 film in the dark showing the movement of compensating positive ions (circles with + ) through the film that screens a negative potential (electrons shown as - ) applied to the Sn02 substrate electrode, (a) The electric field is screened close to the substrate when the potential is positive of the conduction band, but (b) extends further into the semiconductor for more negative potentials. The potential distribution also depends on the relative rates of interfacial versus interparticle charge transfer (Fig. 2).

In these experiments, the potential distribution was measured under conditions where the interfacial current density was minimized by the use of an inert electrolyte. If the electron-transfer rate across the interface had truly been zero (Ret = °°), the whole 2 film would have eventually charged up to the applied potential it was the unavoidable leakage current across the interface and the relatively short time scale of our experiments that prevented this from happening. These experiments show that even when Rct is maximized, ion motion through the nanoporous film causes the applied potential to drop near the substrate electrode in nonilluminated DSSCs. As we showed earlier, decreasing Rct causes the applied potential to drop even closer to the substrate electrode. 

As another example, oxide films on a vapor-deposited Ag substrate are presented [116]. Detailed XPS investigations show the development of Ag20 already 0.15 V below its equilibrium potential of E = 0.35 V [ 115]. Fig.50a presents the k-weighted Fourier Transform of the reflectivity-EXAFS, FT(ARk). of a 2.5 nm thick oxide film formed in 1 M NaOH at E = 0.40 V at an angle 0 = 0.09° relative to the surface. 

Fig. 2.18. Kelvin probe force microscopy (KPFM) picture of the cross-section of a modulation doped Zni- Mg CtAl/ZnO film on a silicon substrate. The contact potential is given relative to pyrolytic graphite ( = 4.07 eV). The local variation of the contact potential is shown on the left side, while the chemical composition, determined by SIMS is displayed on the right side. Deposition parameters p = 0.2Pa, P = 75Wrf, Tsub = 300°C, single layer thickness dZno = dZni xMgxO Ai = 160 nm...

---