SEIRAS-optimized ATR Wafers for Interfacial Spectroelectrochemistry

We show that a microgrooved ATR-SEIRAS optimized Si ATR wafer designed to allow high angles of incidence at the electrode-solution interface provides improved signal-to-noise in electrochemical ATR-SEIRAS.

Microgrooved Si ATR wafers (Figure 2 inset) are excellent internal reflection elements for electrochemical ATR-SEIRAS owing to their low cost and high spectral throughput (1). A microgrooved substrate is formed by selective etching of a standard Si wafer to form an array of individual prisms. In this application note, the spectroelectrochemical performance of a specialized SEIRAS-optimized ATR wafer, with a grooved face angle (GFA) of 55°, was compared to a universal Si wafer ATR element (GFA of 35°).

Experimental Conditions

Identical 30 nm Au films, which serve as the working electrode in an ATR spectroelectrochemistry (SEC) cell, were sputtered on each wafer element (IRUBIS GmbH; Figure 2 inset). The wafer was loaded into a Jackfish J1W SEC Cell designed to interface with Si ATR wafers, and mounted on a VeeMAX III variable angle ATR accessory (PIKE Technologies; Figure 1). A Au coil counter electrode and a Ag/AgCl (saturated KCl) reference electrode were used. The cell was filled with 0.1 M pH 6 acetate buffer, and the Au thin film working electrode was activated by voltammetric cycling (2). FT-IR sample (E = +0.6 V, acetate adsorbed) and reference (E = -0.1 V, no adsorbed acetate) spectra were collected by coadding 128 scans at 4 cm^-1 resolution, and potential difference absorbance spectra were calculated.

Figure 1: Jackfish J1W SEC Cell mounted on a VeeMAX III variable angle ATR accessory from PIKE Technologies.

Results

The angle of incidence (AOI) was varied between 30 to 65 ° to determine optimal response. This range is compressed by refraction to a narrow range at the electrode-solution interface defined by the GFA. For the specialized wafer, the optimal angle at the electrode-solution interface for maximum signal to noise (SNR) was near 50 °, and for the universal wafer it was 40 ° as determined by the acetate v_COO- band strength. Figure 2 shows the potential difference absorbance spectra at peak SNR for the specialized and universal crystals. The upward going band at 1400 cm^-1 is the acetate v_COO- stretch. The weak band near 1200 cm^-1 is due to potential dependent Si phonon modes. The broad negative band near 1650 cm^-1 is due to loss of adsorbed interfacial water upon replacement with acetate. Both the higher signal and lower noise of the v_COO- stretch at 1400 cm^-1 are apparent in the specialized spectrum.

Figure 2: The potential difference absorbance spectra of adsorbed acetate at peak SNR for specialized and universal wafers. Inset: microgrooved wafer ATR element.

Conclusions

The higher throughput and greater accessible AOI at the electrode-solution interface allows the specialized ATR-SEIRAS wafer to achieve a 35% greater SNR than that of the universal wafer in an electrochemical ATR-SEIRAS experiment.

References

(1) T.A. Morhart, B. Unni, M.J. Lardner, and I. Burgess, J. Anal. Chem. 89(21), 11818–11824 (2017).

(2) PIKE Technologies. Jackfish Spectroelectrochemistry Cells for Surface-Sensitive Electrochemical ATR-SEIRAS. Retrieved from https://www.piketech.com/Jackfish-SEC-Spectoelectrochemical-Cell.html.

PIKE Technologies, Inc.
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