A Spectroelectrochemical Analysis of Polymeric Films

Intrinsically conducting polymers (ICPs) have gained considerable attention thanks to their unique properties, including excellent chemical, oxidative, and thermal stability, catalytic abilities, adjustable electrical properties, and optical and mechanical features.

ICPs are employed in a range of applications, including in sensors, light-emitting diodes, antistatic coatings, flexible devices, transistors, and electrochromic devices such as smart windows (which regulate the amount of light passing through).

Poly(3,4-ethylenedioxythiophene), or PEDOT, is among the most promising ICPs currently available due to its high conductivity, catalytic properties, electrochemical stability, high insolubility in most common solvents, and interesting electrochromic properties. For example, it is colored in the neutral state but transparent in the doped state.

In the study discussed in this article, PEDOT film is evaluated using spectroelectrochemical techniques.

Instrumentation and Software

A Raman characterization investigation was performed using a SPELEC RAMAN (785 nm laser) instrument (shown in Figure 1a), a Raman probe consistent with the laser wavelength, and a Raman spectroelectrochemical cell for SPEs (screen-printed electrodes).

UV-Vis spectroelectrochemical measurements were acquired using a SPELEC instrument (Figure 1b), a reflection probe for spectral range, and a reflection cell for SPEs.

Figure 1. (a) SPELEC RAMAN and (b) SPELEC instruments used in the study of PEDOT film. Image Credit: Metrohm Middle East FZC

Gold SPEs (220AT) modified with a PEDOT film were used, allowing users to obtain clear, precise, and succinct information about the performance of PEDOT on the electrode surface.

The SPELEC and SPELEC RAMAN instruments were controlled via DropView SPELEC software. DropView SPELEC provided spectroelectrochemical information using tools to perform treatment and analysis of the collected data. Table 1 displays all software and hardware used for this study.

Table 1. Hardware and software equipment overview. Source: Metrohm Middle East FZC

Characterization of PEDOT

Raman spectroelectrochemistry (SEC) was utilized for the fingerprint characterization of the different oxidation states (neutral and doped) of PEDOT deposited on the Au SPE.

The spectrum of the neutral state was attained at -0.40 V (blue line in Figure 2) and p-doped PEDOT at +0.50 V (red line in Figure 2) in a 0.1 mol/L lithium perchlorate (LiClO₄) aqueous solution.

Figure 2. Raman spectra of neutral (blue line) and p-doped (red line) PEDOT. Image Credit: Metrohm Middle East FZC

Table 2 displays the vibrational modes assignments for each Raman band. The characteristic vibrational modes depend on the polymer oxidation state, principally those in the Raman shift region (1100 to 600 cm^-1).

Some Raman bands of PEDOT were up-shifted in the doped state. It should be noted that although the C_α-C_α’ inter-ring stretching vibrational mode was not detected in neutral PEDOT, it was observed at 1293 cm^-1 in the doped state.

Table 2. Vibrational assignment of neutral and doped PEDOT [1–3]. Source: Metrohm Middle East FZC

Valuable qualitative information provided by UV-Vis SEC facilitated the full characterization of the PEDOT film (previously deposited on the gold working electrode).

Spectroelectrochemical experiments were carried out in a 0.1 mol/L LiClO₄ aqueous solution, scanning the potential from 0.00 V to +0.70 V and back to -0.40 V at 0.05 V/second for two cycles.

UV-Vis spectra were acquired in reflection configuration (300 millisecond integration time), causing almost 300 spectra to be collected during the experiment. The synchronization of the electrochemical and spectroscopic responses is guaranteed by the SPELEC instrument.

Cyclic voltammetry (see Figure 3a) did not display any notable electrochemical peaks related to the change in the oxidation state of PEDOT. However, as shown in Figure 3b, a UV-Vis band centered at 525 nm can be observed clearly in the simultaneously recorded spectra.

Figure 3. (a) Cyclic voltammogram and (b) 3D plot of the UV-Vis spectra obtained from PEDOT deposited on the 220AT SPE in 0.1 mol/L lithium perchlorate by scanning the potential from 0.00 V to +0.70 V and back to -0.40 V at 0.05 V/s for two cycles. Image Credit: Metrohm Middle East FZC

The development of the absorption band at 525 nm with shifting potential is shown in Figure 4. At first, the absorbance decreased from 0.00 V to +0.70 V. In the backward scan, absorbance grew to -0.40 V and fell to 0.00 V, where it reached a similar value to the start of the experiment.

In the second scan, the spectroscopic signal displayed equal spectroelectrochemical behavior. Absorbance at 525 nm at -0.40 V achieved the same value for both scans, representing the stability of this film for at least two cycles.

Figure 4. Evolution of the UV-Vis band at 525 nm with varying potential. Image Credit: Metrohm Middle East FZC

The evolution of this absorbance band with potential was in line with the electrochromic properties of PEDOT: it was colored in the neutral state at negative potentials yet colorless in the doped state at positive potentials.

Figure 5 shows the relevant derivative voltabsorptogram (dAbs/dt versus potential) at 525 nm. The derivative curve is only connected to the faradaic section of the concomitant current flow. As Figure 5 shows, this derivative curve demonstrates the polymer doping and de-doping progressions through its reversible behavior.

Figure 5. Derivative voltabsorptogram at 525 nm. Image Credit: Metrohm Middle East FZC

Conclusions

SEC is a multi-response method that delivers excellent results in classifying electrochromic materials, such as the PEDOT polymer. Raman SEC provides diagnostic insights by distinguishing between the neutral and doped states of the sample, as the position of Raman bands shifts with the oxidation state.

UV-Vis SEC, on the other hand, reveals an absorption band in the visible region, allowing for spectral monitoring of PEDOT's electrochemical classification. In the neutral state, absorbance increases at negative potentials, while in the doped state, it decreases at positive potentials.

Understanding the stability of the PEDOT coating under varying potentials and gaining a complete understanding of its optical properties are crucial in the development of new applications.

References and Further Reading

1. Feng, Z.-Q., Wu, J., Cho, W., et al. (2013). Highly Aligned Poly(3,4-Ethylene Dioxythiophene) (PEDOT) Nano- and Microscale Fibers and Tubes. Polymer, 54(2), pp.702–708. https://doi.org/10.1016/j.polymer.2012.10.057.
2. Garreau, S., Louarn, G., Froyer, G., et al. (2001). Spectroelectrochemical Studies of the C₁₄-Alkyl Derivative of Poly(3,4-Ethylenedioxythiophene) (PEDT). Electrochimica Acta, 46(8), pp.1207–1214. https://doi.org/10.1016/S0013-4686(00)00693-9.
3. Tran-Van, F., Garreau, S.; Louarn, G., et al. (2001). Fully Undoped and Soluble Oligo(3,4-Ethylenedioxythiophene)s: Spectroscopic Study and Electrochemical Characterization. J. Mater. Chem., 11(5), pp.1378-1382. https://doi.org/10.1039/b100033k.

This information has been sourced, reviewed and adapted from materials provided by Metrohm Middle East FZC.

For more information on this source, please visit Metrohm Middle East FZC.