Spectroelectrochemistry

Spectroscopy and Electrochemistry | Spectroelectrochemistry Set Up | UV-Vis Absorption | Photoluminescence | Infrared | Raman | X-Ray | NMR | ESP / EPR

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Spectroelectrochemistry (SEC) refers to a range of experimental techniques that combine electrochemistry and spectroscopy using a potentiostat and a spectrometer.

While electrochemical experiments provide information on macroscopic properties, spectroscopic techniques give information on a molecular level. The combination of the two therefore provides a comprehensive picture of a given material.

Spectroelectrochemical techniques are either classified based on the type of spectroscopy used (UV-vis, NIR or IR, Raman spectroscopy, EPR, NMR, XAS, or luminescence) or by the electrochemical approach chosen (chronopotentiometry, chronoamperometry, linear sweep voltammetry, cyclic voltammetry, differential pulse voltammetry, stripping voltammetry, or pulse techniques).

However, it is more common for spectroelectrochemical techniques to be classified by their spectroscopic methods.

Why Combine Spectroscopy and Electrochemistry?

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Electrochemical methods provide a broad picture a reactions progress but cannot reveal molecular-level detail about the intermediate species formed or events occurring at each stage. Spectroscopy can fill in some of these gaps, by examining these intermediate stages, investigating short-lived products and assessing the reversibility of a reaction.

Spectroelectrochemistry techniques provide an in-situ method for further investigating the events occurring during an electrochemical process. The spectroscopic response is typically observed in situ (while the electrochemical reaction is happening) so allows molecular level changes to be tracked in real time.

Furthermore, spectroscopy is a non-invasive measurement technique, so can probe the chemical species at electrode surfaces, without interfering with the reaction itself.

Spectroelectrochemistry Set Up

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Spectroelectrochemistry experiments can be conducted using standard spectroscopy and electrochemistry equipment (e.g. for UV-vis spectroelectrochemistry, a standard USB spectrometer and potentiostat can be used with appropriate connections). Spectroelectrochemistry is usually facilitated by specialist spectroelectrochemical cells – this cell is the most important component of a spectroelectrochemical set up.

The design of the cell is determined by the requirements of the spectroscopy technique and electrodes. The electrochemical cell and electrodes must be optically transparent enough to be able to observe the analyte, at least through a designated optical path. The electrolytes the must also be optically transparent. Measurements in the UV and IR regions are often limited by the optical absorption properties of the cell windows or electrolyte components (e.g. the solvent and conducting salt). In particular, the strong absorption bands that are exhibited by many solvents will limit their use in the infrared region. It is relatively straightforward choose which solvent to use for UV-vis measurements, as most solvents commonly used in electrochemistry are suitable.

Another important consideration is the location and number of optical windows. This again will depend on the type of spectroscopy you are using. Absorbance measurements require two optical windows, to facilitate transmission through the electrode/electrolyte. However, for measurements like Raman spectroscopy, only one transmission window is needed. The cell should also have the correct ports for the electrodes (often working, reference, counter) and gas control needed. It is also important that light is focussed on or the active region of the electrode of interest.

A further practical consideration when designing a spectroelectrochemical experiment is response time. Spectroelectrochemistry measurements have response times that range from 10 seconds to several minutes. This is a bit longer than the typical sweep rate in cyclic voltammetry (CV) which is about 100 mVs^-1. Therefore, reactions labelled as fully reversible in cyclic voltammetry become less reversible. This is due to the slow reaction of the electrogenerated products. This should be accounted for when selecting experimental parameters such as scan rate and potential window.

Types of Electrochemistry

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UV-Vis Spectroelectrochemistry

UV-vis spectroelectrochemistry (UV-vis-SEC) can be used to probe molecules that absorb light in the ultraviolet and visible regions of the electromagnetic spectrum during electrochemical reactions. This can provide essential information about the ground state of products and analytes.

Both the wavelengths at which absorption occurs, and the degree of this absorption are measured by a USB spectrometer. By comparing spectra recorded before and during the applied potential, the absorptions caused by electrochemically generated species can be isolated from background contributions.

With optically transparent electrodes, UV-vis SEC can study optical materials, polymer films, or other modifying layers that are deposited directly onto the electrode surface, or in present in the solution immediately adjacent to it.

Photoluminescence Spectroelectrochemistry

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Photoluminescence spectroelectrochemistry (PL-SEC) uses photoluminescence combined with electrochemistry techniques to selectively investigate the excited states of generated chromophores. Unlike UV-vis SEC, which monitors ground state absorptions, PL-SEC is uniquely sensitive to excited state behaviour, allowing characterization of the reactivity of generated products.

In one example of a PL-SEC experiment, techniques like controlled-potential electrolysis is used to generate a new redox species within an optically transparent thin-layer electrochemical cell. If the electrogenerated species is luminescent, this excitation produces an emission signal that can be monitored in real time as the electrochemical reaction proceeds. This luminescence triggered by a redox-reaction makes PL-SEC a useful technique for developing sensors and in environmental monitoring applications.

One example of this is the use of PL-SEC to monitor the one-electron oxidation of rhenium or technetium complexes , tracking characteristic absorbance and emission peaks to measure concentration in real time. This methodology has potential applications in environmental monitoring, for example in measuring the concentration of radioactive contaminants in water sources.

Infrared Spectroelectrochemistry

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Infrared spectroelectrochemistry (IR-SEC) uses infrared light to probe a sample, rather than visible light which is used in UV-Vis SEC. IR light can probe the vibrational energy levels of a molecule (bond stretching, bending), giving rich information about its molecular structure, its dynamics within the molecule itself, and its interactions with other components (such as with the electrode surface).

IR-SEC can be used to investigate adsorption of a species onto an electrode surface, as well as to identify and unravel molecular mechanisms of biological electron transfer. It has applications in the study of conducting polymers, redox-active organic molecules in solution, inorganic or organometallic complexes, as well as being often used in mechanistic studies and electrocatalysis.

It is becoming increasingly common to combine IR-SEC with other physical techniques. An example of this would be to combine IR-SEC with scanning electrochemical microscopy to gain a more complete analysis of the whole sample.

Raman Spectroelectrochemistry

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Like IR-SEC, in Raman spectroelectrochemistry (Raman SEC), vibrational information is used to gauge the strength of chemical bonds within molecules, between molecules, and at electrode surfaces. Instead of measuring the absorbance of light like IR-SEC, Raman spectroelectrochemistry measures the inelastic scattering of high energy monochromatic light. They have different selection rules, so will still produce different spectra. This makes them complementary techniques. Ideally, you would examine a material using both techniques, but the selection rules and limitations of both methods mean one of these techniques is likely more suitable than the other.

Raman spectroelectrochemistry is often the method of choice for analyzing the structure of materials. It can be used for 2D materials (including carbon materials such as graphite or graphene), nanostructures such as carbon nanotubes, and fullerenes. It can also be used to probe information about crystalline or amorphous structures, and you can obtain crystallite size from the shift and shape of vibration modes. In battery research, Raman SEC can be used to determine the state and quality of nanostructured battery materials and lithium content.

If you are analyzing self-assembled monolayers, you can also use Raman spectroelectrochemistry to obtain information on the orientation of molecules on the surface of electrodes. .

X-Ray Spectroelectrochemistry

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You can use X-ray spectroelectrochemistry (X-ray SEC) to provide information about the basic considerations and symmetry of the electronic structure of active atomic orbitals of different materials. For example, you can use it to determine bond lengths and angles, the identities of neighbouring atoms, and oxidation states. X-ray SEC is also useful for spectroelectrochemical studies on corrosion processes, electrodeposition, ionic adsorption, and yields of fuel cell anodes.

Nuclear Magnetic Resonance Spectroelectrochemistry

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Nuclear magnetic resonance spectroelectrochemistry (NMR-SEC) provides an interesting approach for monitoring and understanding electrochemical and electrocatalysis processes at a molecular level.

Examples of measurements you can perform using NMR-SEC include, the redox behavior of quinone-based systems , the electrooxidation of dopamine at various voltages, PH levels when using Au nanoparticles modified electrode , and the electrocatalysis of hydroquinone by nano-polyaniline (nano-PAn) films.

Electron Paramagnetic Resonance Spectroelectrochemistry

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Electron spin/paramagnetic resonance spectroelectrochemistry (ESP/EPR-SEC) can be used to detect short-lived radicals. You can use ESP/EPR to provide valuable information about the oxidation state of an atom or atoms within a (macro) molecule by using impaired electrons as probes. You can also use it to understand the environment of these atoms. Because of this, ESP/EPR-SEC can be used to investigate paramagnetic species such as organic radicals, proteins, polymers, and transition-metal complexes .

In-situ, ESR/EPR spectroscopy can also be combined with UV-vis spectroscopy.

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