External reflectance IR spectroscopy

Most of the solvents used in electrochemistry, and particularly water, present strong absorption in the mid-IR range. Therefore the use of external reflection IR spectroscopy for the in-situ observation of electrode processes requires a considerable reduction in the solution thickness in the path of the IR beam. Only a very thin layer of electrolyte between electrode and IR window is allowed in order to have enough energy reaching the electrode surface. Typically, the thickness of the solution layer produced by a well-positioned, flat-polished electrode is of the order of 1 - 5 pm. Within this cavity, which has been described by Yeager et al. as diffusionally decoupled, migration is the predominant form of mass transport [26]. 

R. A. Dluhy, S.M. Stephens, S. Widayatl and A.D. Williams, Vibrational Spectroscopy of Biophysical Monolayers. Applications of IR and Raman Spectroscopy to Biomembrane Model Systems at Interfaces, Spectrochim. Acta Part A51 (1995) 1413. (Review on biomembrane model systems studied by surface-sensitive vibrational spectroscopic methods. In particular the following methods are surveyed external reflectance IR spectroscopy, wave-guide Raman spectroscopy cmd SERS.)... 

Ever since the first reports of optical studies of electrochemical systems, efforts have been made to obtain infrared spectra of reaction intermediates and adsorbates. The earliest studies were based on total internal reflection using an n-type germanium electrode (transparent to IR radiation), and OTTLE systems using gold minigrids sandwiched between NaCl plates. These were not particularly successful, however, and it is only recently that these configurations have again been used, this time for Fourier Transform spectroscopy [29,30]. Undoubtedly the most successful technique has been potential modulated external reflectance IR spectroscopy [31]. 

Eor IR-ERAS (external reflection absorption spectroscopy), steel substrates (1mm thick) are cut to 25x25 mm size. Spin-coating of EPl (directly after mixing, 10000 rpm, 10 s) produces thick ca. 5 pm layers on these substrates. Eor DSC, 7 mm steel disks (0.7 mm thick) are cut and spin-coating provides epoxy layers (ca. 10 pm) on the disks. All samples are first cured at RT for 48 h and subsequently post-cured at Tc=40°C for 24 h (PC40). 

The setup for external reflection absorption spectroscopy is shown in Fig. 1. A PPy-covered platinum disc electrode is pressed against a ZnSe window, yielding a distance of some m between the electrode and the window. The electrochemical current of the oxidation process has to pass the thin electrolyte layer. Since only parallel-polarized light interacts with substances near a reflecting metal surface, the nt light was polarized in this way. The IR beam permeates the window, the electrolyte and the polymer and is reflected at the Pt surface. Only this part of the radiation (a in Fig. 1) contains information on the polymer absorption. Reflections at the window and the polymer surface b, c and d in Mg. 1), which also reach the detector, can lead to disturbing spectral features and have to be eliminated or corrected. ... 

In addition to the polymer bands, spectral features at 2259, 1452 and 1376 cm " can be seen. Using external reflection absorption spectroscopy, absorption bands of the electrolyte cannot be compensated completely, leading to these disturbing effects. The bands at 1110 and 624 cm are bands of the C104 -ion. The electrochemical current of the oxidation process causes the migration of these ions from the surrounding bulk electrolyte into the thin electrolyte layer, resulting in an increase in the concentation and in IR absorption of this species. 

Reflectance. Both internal and external reflectance spectroscopy are relatively simple experiments to perform. Commercially available attachments for standard UV-visible spectrometers can be used. For films with strong electronic transitions reasonable spectra can be obtained. The theory for external and internal reflectance is the same as that for the IR and can be found elsewhere (2, 37). The techniques have not been very popular in their applications to surface analysis. The major reason appears to be... 

Modern methods of vibrational analysis have shown themselves to be unexpectedly powerful tools to study two-dimensional monomolecular films at gas/liquid interfaces. In particular, current work with external reflection-absorbance infrared spectroscopy has been able to derive detailed conformational and orientational information concerning the nature of the monolayer film. The LE-LC first order phase transition as seen by IR involves a conformational gauche-trans isomerization of the hydrocarbon chains a second transition in the acyl chains is seen at low molecular areas that may be related to a solid-solid type hydrocarbon phase change. Orientations and tilt angles of the hydrocarbon chains are able to be calculated from the polarized external reflectance spectra. These calculations find that the lipid acyl chains are relatively unoriented (or possibly randomly oriented) at low-to-intermediate surface pressures, while the orientation at high surface pressures is similar to that of the solid (gel phase) bulk lipid. 

Polyimide surface modification by a wet chemical process is described. Poly(pyromellitic dianhydride-oxydianiline) (PMDA-ODA) and poly(bisphenyl dianhydride-para-phenylenediamine) (BPDA-PDA) polyimide film surfaces are initially modified with KOH aqueous solution. These modified surfaces are further treated with aqueous HC1 solution to protonate the ionic molecules. Modified surfaces are identified with X-ray photoelectron spectroscopy (XPS), external reflectance infrared (ER IR) spectroscopy, gravimetric analysis, contact angle and thickness measurement. Initial reaction with KOH transforms the polyimide surface to a potassium polyamate surface. The reaction of the polyamate surface with HC1 yields a polyamic acid surface. Upon curing the modified surface, the starting polyimide surface is produced. The depth of modification, which is measured by a method using an absorbance-thickness relationship established with ellipsometry and ER IR, is controlled by the KOH reaction temperature and the reaction time. Surface topography and film thickness can be maintained while a strong polyimide-polyimide adhesion is achieved. Relationship between surface structure and adhesion is discussed. 

The increasing application of spectroscopic methods in electrochemistry has characterized the last decade and marked the beginning of new developments in electrochemical science [1]. Among these methods, in-situ infrared spectroscopy provides a very useful tool for characterizing the electrode-solution interface at a molecular level. First in-situ infrared (IR) electrochemical measurements were performed in 1966 [2] using the internal reflection form [3]. However, problems in obtaining very thin metal layers on the surface of the prisms used as IR windows, delayed the extensive application of in-situ IR spectroscopy until 1980, when the method was applied in the external reflection form [4]. The importance of this step does not need to be emphasized today. 

---