Surface-enhanced Raman spectroscopy adsorption

Recent developments in the mechanisms of corrosion inhibition have been discussed in reviews dealing with acid solutions " and neutral solu-tions - . Novel and improved experimental techniques, e.g. surface enhanced Raman spectroscopy , infrared spectroscopy. Auger electron spectroscopyX-ray photoelectron spectroscopyand a.c. impedance analysis have been used to study the adsorption, interaction and reaction of inhibitors at metal surfaces. 

Cation radicals of several metallated TPP and OEP complexes were reported to have unique IR active ring modes (21. These absorptions were observed in the region 1250-1290 cm for TPP complexes and 1520-1570 cm for OEP complexes. IR and raman active modes have also been observed for several different reduced oxygen species adsorbed on various metal substrates. Observations of the adsorption of oxygen on silver by surface enhanced raman spectroscopy (SERS) 22), show superoxide molecules (O2) have a frequency of 1053 cm, and peroxide (0 ) of 697 cm. This same study reported additional peaks were observed at 815 cm and 1286... 

An investigation of the adsorption of pyrazine and pyridine on nickel electrodes by in situ surface-enhanced Raman spectroscopy was reported in [44]. The result suggests that both pyrazine and pyridine were strongly adsorbed onto the substrates. It also implies that pyridine was adsorbed perpendicularly onto the substrate, while pyrazine adsorbed onto the substrate in a slightly tilted vertical configuration. 

The mechanism of C02 reduction to methane at Cu electrodes has been proposed by various groups [72-74], most of which involved the splitting of adsorbed CO followed by the hydrogenation of surface C atoms. When DeWulf et al. used X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy to study the reaction [72], they observed surface-bound carbenes (Cu CH2) as an intermediate in the system. Likewise, others used both in situ infrared (IR) reflection absorption spectroscopy and surface-enhanced Raman spectroscopy to observe the initial product of C02 reduction on Cu [74]. Typically, two different linearly bound CO species were identified and attributed to adsorption on either surface defect sites or terraces. 

Sanchez-Cortes, S., Francioso, O., Ciavatta, C., Garcia-Ramos J. V., and Gessa, C. (1998). pH-dependent adsorption of fractionated peat humic substances on different silver colloids studied by surface-enhanced Raman spectroscopy. J. Colloid Interface Sci. 198(2), 308-318. 

A related technique is based on the fact that signals from adsorbed species are much larger than from the same species in solution (surface enhanced Raman spectroscopy, SERS)23,24. The phenomenon was first noted in a study of the adsorption of pyridine on silver electrodes25, and has been extended to the investigation of the adsorption of many species such as, for example, porphyrins. 

Spectroscopic techniques such as surface enhanced Raman spectroscopy (SERS) have also been used to investigate the adsorption of DNA onto the electrode surfaces [76, 77]. 

El Amri C, Baron M-H, Maurel M-C (2004) Adenine adsorption on and release from meteorite specimens assessed by surface-enhanced Raman spectroscopy. J Raman Spectrosc 35 170-177... 

Adsorption and reaction at electrochemical interfaces as probed by surface-enhanced Raman spectroscopy. Annual Review of Physical Chemistry, 55, 197-229. 

Leung, L.-W.H. and Weaver, M.J. (1987) Extending surface-enhanced Raman spectroscopy to transition-metal surfaces carbon monoxide adsorption and electrooxidation on platinum- and palladium-coated gold electrodes. Journal of the American Chemical Society, 109, 5113-5119. 

Wilke, T., Gao, X., Takoudis, C.G., and Weaver, M.J. (1991) Surface-enhanced Raman spectroscopy as a probe of adsorption at transition metal-high-pressure gas interfaces NO, CO, and oxygen on platinum-, rhodium-, and ruthenium-coated gold. Langmuir, 7, 714-721. 

Beyond intramolecular modes of species present in the electrolyte solution or interacting with an electrode surface, modes caused by adsorptive interaction between the adsorbed species and the electrode surface can be studied (e.g. the silver-halide stretching mode with surface enhanced Raman spectroscopy (SERS) for details. 

Surface-enhanced Raman spectroscopy was intensively used to study organic additives on silver, copper, and gold." An example is an investigation of the orientation of nicotinic acid on silver. The Raman spectra were measured at different pH values. This is shown in Figure 7.29a. Preferential adsorption of the nicotinic acid anion was observed. The dissociation reactions are shown in Figure 7.29b. 

---