Cyanide adsorption on gold electrodes: a combined surface enhanced Raman spectroscopy and density functional theory study

A combined SERS and DFT study of cyanide adsorption on a gold electrode is presented. From our analysis, the high-frequency mode at ∼2100 cm−1 is ascribed to the C–N stretching frequency at (1 0 0) and (1 1 0) sites. The lower frequency modes at ∼370 and ∼300 cm−1 are ascribed to the Au–CN stretching and bending modes, respectively. The Stark tuning slopes of these modes agree well with the DFT computations. The bending mode at 300 cm−1 has a very small Stark tuning slope of ∼4 cm−1 V−1, experimentally, compared to ∼1 cm−1 V−1, computationally. The Au–CN stretching frequency has a Stark tuning slope of ∼15 cm−1 V−1, experimentally, compared to 16–22 cm−1 V−1, computationally. The positive Stark tuning slopes suggest that cyanide adsorbs as an anion, with a bond polarity between that of the Au–Cl and Au–Br surface bonds. The anionic character of the Au–CN bond is also confirmed by different charge analyses based on the DFT computations. The ordering in binding strength on the three different surfaces is the same as the ordering in bond ionicity. The C–N stretching frequency has two different Stark tuning slopes in the SERS experiments: ∼14 cm−1 V−1 in the more negative potential region corresponding to cyanide adsorption, and ∼35 cm−1 V−1 in the more positive potential region corresponding to gold dissolution. The computational result of ∼8 cm−1 V−1 agrees well with the lower Stark tuning slope. The higher Stark tuning slope is presumably related to the formation of gold–cyanide complexes in the double layer.