Surface enhanced Raman spectroscopy cycles

Metal dissolution is the inverse process to the deposition so its principles can be derived from preceding considerations. It should, however, be borne in mind that the preferred sites for deposition need not be the same as those for the dissolution. This is particularly true if the reactions are far from equilibrium. Therefore, rapid cycling of the potential between the deposition and the dissolution region can lead to a substantial roughening of the electrode surface, which can be used in techniques such as surface-enhanced Raman spectroscopy (see Chapter 15 ), which require a large surface area. 

The reaction scheme of Bode [11] was derived by comparison of the X-ray diffraction patterns of the active materials with those for the model compounds. How the 8-Ni(OH)2 in battery electrodes differs from the model compound is discussed in Section 5.3.I.3. In recent years, the arsenal of in situ techniques for electrode characterization has greatly increased. Most of the results confirm Bode s reaction scheme and essentially all the features of the proposed a/y cycle. For instance, recent atomic force microscopy (AFM) of o -Ni(OH)2 shows results consistent with a contraction of the interlayer distance fiom 8.05 to 7.2 A on charge [61-63]. These are the respective interlayer dimensions for the model a-Ni(OH)2 and y-NiOOH compounds. Electrochemical quartz crystal microbalance (ECQM) measurements also confirm the ingress of alkali metal cations into the lattice upon the conversion of a-Ni(OH)2 to y-NiOOH [45,64,65]. However, in situ Raman and surface-enhanced Raman spectroscopy (SERS) results on electrostretching modes that are consistent with a weakening of the O-H bond when compared with results for the model a- and 8-Ni(OH)2 compounds [66]. This has been ascribed to the delocalization of protons by intercalated water and Na ions. Similar effects have been seen in passive films on nickel in borate buffer electrolytes [67]. 

A similar comparison of results of TPR/TPO-Raman spectroscopy with those of quantitative TPR has also been made for alumina-supported vanadia (Kanervo et al., 2003). However, the Raman signal of V205 crystals is at least ten times more intense than that of surface VOx species for excitation in the wavelength range of 514—532 nm, because of resonance enhancement (Xie et al., 2000). Thus, only a minor fraction of the surface VO species on alumina aggregated to form microcrystals during reduction and oxidation cycles. 

Pyridine and pyrazine have received considerable attention in Raman spectroscopy because of their tremendously enhanced Raman scattering cross section when adsorbed on activated Ag surfaces. The activation consisted of an oxidation-reduction cycle in the presence of the Raman-active... 

---