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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.

Cyclic voltammogram of 4-NTP adsorbed onto a roughened Au surface in 0.05 mol/L sulfuric acid.
Figure 1. Cyclic voltammogram of 4-NTP adsorbed onto a roughened Au surface in 0.05 mol/L sulfuric acid.

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 of the 4-NTP monolayer recorded during the potential Step experiment in 0.05 mol/L sulfuric acid.
Figure 2. The electrochemical response of the 4-NTP monolayer recorded during the potential Step experiment in 0.05 mol/L sulfuric acid.

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.

Raman spectra acquired at the first and last potential step during the potential step experiment (Figure 2).
Figure 3. Raman spectra acquired at the first and last potential step during the potential step experiment (Figure 2).

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.

Table 1. Raman shifts and associated vibration modes [3,4] of 4-NTP and 4-ATP as measured during this experiment.
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.

  1. 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.
  2. 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.
  3. 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.
  4. 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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