The combination of Raman spectroscopy and electrochemistry, known as hyphenated EC-Raman, is a powerful tool for researchers. More information can be extracted from this combination than can be gained from either technique alone.
4-nitrothiophenol (4-NTP) is a strongly Raman-active molecule that is also electrochemically active. At mildly cathodic potentials, 4-NTP undergoes a six-electron proton-coupled reduction to 4-aminothiophenol (4-ATP). Therefore, the reduction can easily be monitored using traditional electrochemical techniques (e.g., cyclic voltammetry) but also through the changes to the Raman spectrum. In the past, 4-NTP has served as a model system for studying the SERS (surface-enhanced Raman spectroscopy) effect and has relevance for example in corrosion [1] and electrocatalysis [2] applications.
This Application Note presents a walkthrough of a hyphenated EC-Raman experiment on 4-NTP.
A Metrohm EC-Raman solution was used consisting of a VIONIC powered by INTELLO potentiostat and an i-Raman Plus 532H system (B&W TEK). A specialized cell for EC-Raman was employed (RAMAN ECFC, RedoxMe). The cell consists of three electrodes: an Ag/AgCl reference electrode, Pt wire counter electrode, and an Au disk working electrode.
A SERS substrate was prepared in-situ by electrochemically roughening the Au disk electrode. The 4-NTP was immobilized in a surface monolayer, prepared by drop casting onto an electrochemically roughened Au SERS substrate. The surface was thoroughly rinsed with ethanol before use. The cell was then filled with 0.05 mol/L H2SO4.
All Raman spectra were acquired with an i-Raman Plus 532H controlled by BWSpec software. A potential step experiment was performed where the potential was stepped from 0.2 V to -0.55 V in 0.05 V, 40-second steps. Raman spectra were acquired at each potential step. The Raman spectra were acquired at 100% laser power with a 10 s integration time and averaged three times using the BWSpec Timeline plugin.
The cyclic voltammogram (CV) of the 4-NTP monolayer is shown in Figure 1. This provides information about which potentials will be needed later in the potential step experiment.
The CV also reveals a single, irreversible cathodic peak at approximately -0.3 V vs Ag/AgCl. This peak corresponds to the complete reduction of 4-NTP to 4-ATP (Figure 1, insert).
The electrochemical response recorded during the potential step experiment is shown in Figure 2. VIONIC sends TTL pulses to the i-Raman plus system which triggers the measurement of a new spectrum at the beginning of each step.
For clarity purposes, only the first and last recorded Raman spectrum (corresponding to 0.2 V and -0.55 V, respectively) are shown in Figure 3.
The transformation of 4-NTP to 4-ATP is most easily recognized by the loss of the NO2 stretching mode at 1337 cm-1. The C-C stretching mode at 1572 cm-1 in 4-NTP also shifts to higher wavenumbers in 4-ATP (1578 cm-1). A full assignment of the observed bands is provided in Table 1.
| Compound | Raman Shift (cm-1) | Vibration Mode |
|---|---|---|
| 4-NTP | 1078 | C-H bending |
| 1105 | C-H bending | |
| 1337 | NO2 stretching | |
| 1572 | C-C stretching | |
| 4-ATP | 1078 | C-H bending |
| 1578 | C-C stretching |
A walkthrough of a model experiment for EC-Raman was shown using the example of 4-nitrothiophenol. While the molecule itself is useful in testing new materials for the SERS effect, EC-Raman presents researchers with a convenient way to track the reduction of the molecule.
In general, hyphenated EC-Raman gives excellent molecular insights into electron-transfer reactions occurring in organic molecules.
- Morávková, Z.; Dmitrieva, E. Structural Changes in Polyaniline near the Middle Oxidation Peak Studied by in Situ Raman Spectroelectrochemistry. Journal of Raman Spectroscopy 2017, 48 (9), 1229–1234. https://doi.org/10.1002/jrs.5197.
- Dong, J.-C.; Zhang, X.-G.; Briega-Martos, V.; et al. In Situ Raman Spectroscopic Evidence for Oxygen Reduction Reaction Intermediates at Platinum Single-Crystal Surfaces. Nat Energy 2019, 4 (1), 60–67. https://doi.org/10.1038/s41560-018-0292-z.
- Lopez-Ramirez, M. R.; Aranda Ruiz, D.; Avila Ferrer, F. J.; et al. Analysis of the Potential Dependent Surface-Enhanced Raman Scattering of p-Aminothiophenol on the Basis of MS-CASPT2 Calculations. J. Phys. Chem. C 2016, 120 (34), 19322–19328. https://doi.org/10.1021/acs.jpcc.6b05891.
- Tabatabaei, M.; Sangar, A.; Kazemi-Zanjani, N.; et al. Optical Properties of Silver and Gold Tetrahedral Nanopyramid Arrays Prepared by Nanosphere Lithography. J. Phys. Chem. C 2013, 117 (28), 14778–14786. https://doi.org/10.1021/jp405125c.
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