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A 532 nm excitation source is almost five times more efficient than a 785 nm source. Having a higher excitation efficiency is relevant to EC-Raman applications because the concentrations or coverages of the molecules of interest are usually low. Fluorescence from the sample can interfere with the Raman signal. The fluorescence signal differs from Raman scattered light because it has a fixed wavelength that does not shift with the wavelength of the excitation source. Fluorescence can interfere within visible and near-infrared wavelength ranges. This is problematic for Raman spectroscopy because fluorescence is a much more efficient process compared to Raman scattering, and can obscure Raman peaks. Since there is a choice of laser wavelengths, and because the wavelength of Raman scattered light is dictated by the energy of the incident light and the resulting photon, it is possible to position the Raman spectrum (in terms of Raman shift) in a wavelength range that is less affected by fluorescence. Managing fluorescence can be a challenge for the use of Raman spectroscopy in some applications, but it is generally less problematic in EC-Raman experiments because these typically concern small molecules that are less prone to fluorescence. All things considered, the 532 nm excitation source is often the best choice for hyphenated EC- Raman applications.
There is no truly ideal material that can be used in all EC-Raman applications. The choice of window material is usually based on the expected strength and positions of the Raman peaks from the target molecules relative to any interference from the window material, as well as the stability of the window material in the experimental conditions.
The distance between the working electrode and the spectral window requires some consideration as well. The working electrode is positioned close to the spectral window so that the laser can be focused on the surface of the electrode, however, some space is needed so that the diffusion of molecules in the electrolyte is not hindered. Researchers who have experience with combined EC and IR spectroscopy are used to working with the electrode in contact with the spectral window, leaving only a thin film of electrolyte. This configuration is needed because the water in aqueous electrolytes strongly interferes in the infrared spectrum, so the optical pathlength through the electrolyte needs to be minimised. This is one of the situations where Raman is superior in contrast to IR spectroscopy. The molecular vibrations of water are mostly Raman inactive, except for the stretching modes which give rise to a broad band at Raman shift of around 3400 cm-1
which is generally not problematic
for EC-Raman experiments. Therefore, it is possible to leave more space between the working electrode and the spectral window, opening up the possibility of EC-Raman measurements in a flow cell as well as studying gas-forming reactions [16].
Figure 6. The main components of an EC-Raman setup.
The potentiostat drives the electrochemical measurement and provides a variety of techniques to study electrochemical reactions. A full discussion of the options and modules available in modern potentiostats is outside the scope of this article. Instrumentation needs are highly dependent on the type of research and so a flexible approach to instrumentation is often best, meaning that a system that can be upgraded with additional functionality in the future can continue to serve the researcher if their experimental needs change.
There is one aspect of the potentiostat that is crucial for hyphenated EC-Raman experiments - the ability to synchronise Raman spectra collection with the electrochemical measurement. Transient electrochemical techniques like linear sweep voltammetry (LSV) involve changing the potential over time with a fixed rate (the scan rate). A typical EC-Raman experiment could involve sweeping the potential and collecting Raman spectra at fixed potential intervals (i.e., every 20 mV). This experiment yields two types of information: the LSV plot which illustrates current as a function of potential, and a series of Raman spectra collected at various potential values.
In such an experiment, it is important to the quality of the results (and very convenient) if the electrochemical instrumentation has the possibility to communicate with the Raman spectrometer to precisely inform it when to collect each spectrum. Metrohm Autolab potentiostats are equipped with a triggering mechanism that can precisely and automatically synchronise with the Raman spectrometer. The NOVA 2 software controls the triggering mechanism and gives the researcher complete flexibility to program the collection of Raman spectra at any point in their electrochemical measurement. Figure 7 illustrates this triggering mechanism.
To make an elegant bridge between the electrochemical cell and the Raman instrumentation, an optical microscope can be used as a sampling aid to help position the sample and to focus the laser light on its surface. In this setup, the path of the incident light is directed through the microscope lens and to a specific area of the sample. The scattered light is collected by a probe above the sample and carried through the optics to the analyser. Any position of the working electrode that can be brought into focus with the optical microscope can be selectively sampled for Raman spectroscopy. This configuration is important in the case of inhomogeneous samples where different reactions are expected to take place at different locations on the sample (e.g., in corrosion studies). There is an inverse relationship between the laser spot size and the magnification of the microscope lens. For example, by increasing the magnification from 20 times to 50 times, the laser spot size is approximately halved. Typical laser spot sizes range from approximately 200 to 10 μm, depending on the type of laser and magnification through the microscope. This can be particularly important to reduce the contribution from the solution when Raman-active solvents and salts are used as supporting electrolytes. In other words, the use of a simple optical microscope allows for greater spatial selectivity in the XY plane and can help increase surface sensitivity when Raman-active solvents are used.
The experimental setup for hyphenated EC-Raman measurements may seem complex because it combines two independent analytical areas. Both techniques have their complexities and special quirks. Thankfully, experts from both sectors are constantly working to improve these techniques, extend their applications, and share their knowledge to make this powerful combination more accessible. Metrohm Autolab offers hyphenated EC-Raman setups that are tailored to electrocatalysis, battery, and corrosion research. Dedicated electrochemistry and Raman specialists are always available to discuss your experimental needs and help in the selection of appropriate instrumentation.
Conclusions and outlook
The combination of electrochemical and spectroscopic techniques is undoubtedly powerful. In this article, we have provided a few illustrative examples of the insights gained through the application of hyphenated EC-Raman to electrochemical systems. Hundreds of thousands of studies have been published in the past two decades as EC-Raman instrumentation and techniques have become more well-known and accessible, and also more reliable and user-friendly. Presently, the EC-Raman approach is in a growth stage where the number of applications is expanding
rapidly. Scientists from various domains are experimenting with this technique to explore cutting-edge research topics, and also to revisit and confirm (or challenge) present knowledge obtained with more primitive studies. It is reasonable to expect that this trend will continue and that we can anticipate a great deal of insight and discovery from hyphenated EC-Raman studies in the future.
References
1. Vandenabeele, P. Practical Raman Spectroscopy: An Introduction; Wiley: The Atrium, Southern Gate, Chichester, West Sussex, United Kingdom, 2013. DOI: 10.1002/9781119961284
2. Smith, E.; Dent, G. Modern Raman Spectroscopy: A Practical Approach; J. Wiley: Hoboken, NJ, 2005. DOI: 10.1002/0470011831
Figure 7. Triggering the collection of Raman spectra through the NOVA 2 software.
The electrochemical cell design is tailored to the experiment. There are a few things to consider when designing the electrochemical cell. The working electrode must be accessible to the incident light and the sampling optics of the Raman spectrometer. This can be accomplished by placing the working electrode slightly below a special spectroscopic window that has minimal interference with the optical signals. Single crystal sapphire (Al2
can be used as a high-quality window material for EC-Raman applications. Some setups make use of fused quartz, optical glass, and other transparent materials for the window [14,15]. As discussed in the previous section on lattice vibrations, these materials are expected to contribute some background to the Raman spectra.
O3
3. Durickovic, I. Using Raman Spectroscopy for Characterization of Aqueous Media and Quantification of Species in Aqueous Solution. In Applications of Molecular Spectroscopy to Current Research in the Chemical and Biological Sciences; IntechOpen, 2016. DOI:10.5772/64550
4. Mankad, V.; Gupta, S. K.; Jha, P. K.; et al. Low-Frequency Raman Scattering from Si/Ge Nanocrystals in Different Matrixes Caused by Acoustic Phonon Quantization. Journal of Applied Physics 2012, 112 (5), 054318. DOI:10.1063/1.4747933
) of a precise orientation that is optically polished on both sides
5. Colomban, P.; Tournié, A. On-Site Raman Identification and Dating of Ancient/Modern Stained Glasses at the Sainte-Chapelle, Paris. Journal of Cultural Heritage 2007, 8 (3), 242–256. DOI:10.1016/
j.culher.2007.04.002
6. Lipovka, A.; Fatkullin, M.; Averkiev, A.; et al. Surface-Enhanced Raman Spectroscopy and Electrochemistry: The Ultimate Chemical Sensing and Manipulation Combination. Critical Reviews in Analytical Chemistry 2022, 1–25.
DOI:10.1080/10408347.2022.2063683
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