A protocol of step-by-step Raman and IR spectroelectrochemical analysis is presented.
In the presented work, two spectroelectrochemical techniques are discussed as tools for the analysis of the structural changes occurring in the molecule on the vibrational level of energy. Raman and IR spectroelectrochemistry can be used for advanced characterization of the structural changes in the organic electroactive compounds. Here, the step-by-step analysis by means of Raman and IR spectroelectrochemistry is shown. Raman and IR spectroelectrochemical techniques provide complementary information about structural changes occurring during an electrochemical process, i.e. allows for the investigation of redox processes and their products. The examples of IR and Raman spectroelectrochemical analysis are presented, in which the products of the redox reactions, both in solution and solid state, are identified.
The combination of electrochemical and spectroscopic techniques allows for the possibility of tracking the structural changes in molecules present at the electrode surface or in the solution, thus investigating the mechanism of the electrochemical processes. Spectroelectrochemical methods are typically used for the in situ study of the mechanism of the reaction. The undoubted advantage over ex situ measurements is the possibility of observing the signal arising for the intermediate products of processes or investigating the processes, in which products cannot be separated1. Among all spectroscopies, the Raman and infrared spectroscopies are the most powerful for analysis of electrochemical processes due to equipment availability and the often non-destructive nature of the measurements.
Infrared and Raman spectroscopies provide information about the vibrational structure of the species and thus the existing chemical bonds. Since the nature of the signals observed in both techniques is different, some vibrations may be active only in IR or Raman spectra, making them complementary to each other2. This should be taken into account, when planning spectroelectrochemical analysis and, if possible, the vibrational structure of an analyte should be examined using both IR and Raman spectroscopies. The best results are obtained when the changes in the structure are the result of the electrochemical process involving groups active in the certain technique. For example, the infrared spectroscopy would be ideal for processes involving -CO, -CN -NO or -NH groups' formation or breakage3. It is always recommended to register differential spectra of the spectroelectrochemical investigation. Also, such spectra disclose changes in the signals with lower intensity allowing the tracking of changes in the structure of the aromatic systems. Additionally, differential spectra are always less complex as only changes are registered, which makes the interpretation of the spectra much easier.
IR spectroelectrochemical experiments are mainly used for the monitoring of the soluble products, intermediates and reactants of the electrochemical reactions; such tests may be run on various systems, including organic, inorganic, or biochemical systems3,4,5,6,7,8. One should always remember that in the case of IR spectroscopy, solvents in which hydrogen bonding occurs, like water, should be avoided.
There are several ways to proceed with IR and Raman measurements. In the case of IR spectroscopy, measurements can be done in the transmission mode, in which conventional IR cuvettes for liquids can be used. The optically transparent electrodes (e.g., boron-doped diamond electrode) or perforated electrodes (metal gauze working electrode) made of fine metal (Pt or Au) are usually used as the working electrodes in such transmission cells4,9. An example of the transmission spectroelectrochemical cell is presented in the Figure 1.
In the second technique, instead of transmission, the reflectance mode is applied, thanks to the ATR (Attenuated Total Reflection) attachment10. This method allows analyzing both solutions and solid-state materials. Typically when using the method of external reflection absorption spectroscopy, in principle, any working electrode can be used, but only dissolved species can be investigated. However, in some cases, the ATR technique allows also for the investigation of processes in the solid state, using the internal reflection method5,8. A special cell is required for this technique, in which the fine metal sputtered on the ATR crystal acts as a working electrode (Figure 2). In some cases, even the ATR Ge crystal itself can act as an electrode (at least for not too high currents)5.
The second technique is Raman spectroelectrochemistry; a technique combining both electrochemistry and Raman spectroscopy, commonly used in the investigation of the potentially-induced structural changes in the deposited layer of conjugated polymers11, like polyaniline12, polypyrroles13, polycarbazole14 or PEDOT15. Additionally to polymeric films, monolayers can be also tested19,20,21, though in this case metallic substrates, like gold or platinum, are preferred. The procedure of Raman spectroelectrochemical studies is analogical to other spectroelectrochemical techniques, i.e., a spectrometer must be coupled with a potentiostat and the spectra of the film are acquired in the potentiostatic conditions under various potentials' applied18. Typically, the three-electrode spectroelectrochemical cell can be constructed based on the classical quartz cuvette with electrodes mounted in a Teflon holder (Figure 3). The acquisition parameters, like the type of the laser, grating, etc., depend on the properties of the investigated layer. Selection of some parameters can be quite difficult, e.g., one has to remember that various excitation wavelengths can result in different spectra. Usually, the higher energy of incident light the more details are visible on the spectrum, but also the higher risk of fluorescence phenomena that hinders the analysis. Generally, it is very useful to obtain the UV-Vis-NIR spectra of the analyte at first, in order to select the Raman excitation laser. The tunable lasers can be adjusted so that the excitation wavelength induces coincidence with an electronic transition of the molecule, resulting in the resonance Raman scattering. In this case, the increasing Raman scattering intensity in chosen regions of the spectra or even formation of new signals is observed that would not be registered typically. The analysis of the structural changes consists in the assignment of recorded Raman bands, which can be done based on the literature data or DFT simulations23.
Both IR and Raman techniques are recommended for the investigation of the structural changes occurring under applied potential and for the investigation of the products of the redox reaction. However, from the practical point of view, Raman spectroscopy is handier as an analytical tool in such experiments. Raman spectroelectrochemistry gives more possibilities, as it can be also applied to samples with nonpolar bonds. It has been therefore successfully used for the investigation of carbon materials, polymers, batteries, etc.29,30,31,32,33 Since the scattered light is measured substantially in Raman spectroscopy, there are generally no limits in the working electrode material or construction. Additionally, as used herein, incident light (UV-Vis-NIR) is poorly absorbed by the glass, which allows for the use of a standard electrochemical cell. The great advantage is also the possibility of conducting measurements outside the spectrometer through fiber optics. In order to register a Raman spectrum, the incident light needs to be properly focused on the sample. By focusing the light beam at different locations of the measuring cell, it can be decided if the changes in chemical composition occurring in the solution, e.g. near the electrode, or in the species adsorbed on the electrode surface are followed.
The use of Raman spectroscopy with an appropriate resolution also allows for the study of the profile of the solid samples, either on the surface or in its depths, also in the multi-layer structures.34,35,36,37 One can, therefore, get information about the surface topography, the distribution of different chemical species at the surface or in cross-section. Raman spectroelectrochemistry permits in situ tracking of the changes of all these features during redox processes and thus estimate the quality of the individual layers, the durability of the system during multiple oxidation/reduction cycles, or studying the diffusion in multilayer structures. The versatility of Raman spectroelectrochemistry lies in the fact that it can be used to examine both the electrochemical processes in a solution or solid state in a typical experimental cell or even test multilayer solid structures like LEDs, batteries, OPVs, etc.
The undoubted disadvantage of Raman spectroscopy and, thus also spectroelectrochemistry, is its limitation due to observed fluorescence, which often makes it impossible to analyze the spectrum. This phenomenon can be in some cases eliminated by changing the excitation wavelength or preliminary illumination – photo-bleaching.
The authors have nothing to disclose.
The research leading to these results has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 674990 (EXCILIGHT). We thank the networking action funded from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 691684.
Potentiostat | Metrohm | Autolab PGSTAT100 | |
Raman microscope | Renishaw | inVia | |
FT-IR Spectrometer | PerkinElmer | Spectrum Two | |
Bu4NBF4 | Sigma-Aldrich | 86896 | |
DCM | Sigma-Aldrich | 443484 | |
Isopropanol | Sigma-Aldrich | 675431 | |
Acetone | Sigma-Aldrich | 439126 |