<i>In situ</i> surface-enhanced infrared absorption spectroscopy (SEIRAS) in attenuated total reflection (ATR) configuration has been extended to a Co electrode fabricated by potentiostatic deposition of a 50-nm-thick Co overlayer onto a Au underlayer chemically preformed on the reflecting plane of an ATR Si hemi-cylindrical prism. The as-prepared Co-on-Au film was characterized with atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The AFM images of the films before and after Co coating revealed island structures facilitating the SEIRA effect with Co nanoparticles much smaller than the underlying Au ones. The XPS spectrum did not contain any characteristic peaks related to Au, suggestive of a virtually pinhole-free nature of the Co overlayer. The voltammetric response of the as-prepared films in phosphate buffer solution (PBS, pH 6.9) was characteristic of a polycrystalline bulk Co electrode. Normally directed unipolar bands were found for surface probe CO molecules on Co surfaces in the PBS with their major band (CO<sub>L</sub>) intensity being one order of magnitude higher than that obtained with conventional IR reflection–absorption spectroscopy (IRRAS). By taking advantage of the higher detection sensitivity, the bands for linearly bonded CO (CO<sub>L</sub>) at 1965–2005 cm<sup>−1</sup> and the multi-bonded (CO<sub>M</sub>) band at 1845–1875 cm<sup>−1</sup> were clearly detected with their Stark tuning rates being 59 and 63 cm<sup>−1</sup>·V<sup>−1</sup>, respectively, which would be otherwise unobtainable with the conventional IRRAS in the neutral solution.
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