An in situ SFG and SERS investigation into the electrodeposition of Au from Au(CN)2- and Au(CN)4- solutions

In this paper we report an in situ SFG and SERS study of CN− adsorbed on gold from KAu(CN)2 and KAu(CN)4 baths during potentiostatic electrodeposition. The understanding of the potential-dependent spectroelectrochemical behaviour of adsorbed CN− during metal plating provides useful information for the rationalisation of technological processes. Quantitative analysis of SFG and SERS spectra with single-resonance and Gaussian models, respectively, yielded the potential-dependent parameters of the CN− stretching band: peak position, full width at half maximum and band intensity. Parameters related to the electronic structure of the interface have also been derived from the non-resonant part of the SFG spectra and compared to surface-enhancement rate measured by SERS. Specific aspects of the state of interfacial CN− in the case of Au reduction from Au(I) and Au(III) cyanocomplexes have been highlighted. Furthermore, we found that, in general, for the same bath and under the same electrochemical conditions, the SFG and SERS spectroelectrochemical behaviour is different owing to the different types of surface-sensitivity which characterise these two in situ spectroscopies: strictly interfacial for SFG and related to active sites for SERS. This work proves the complementarity of the two approaches for an insightful understanding of the reaction interface during electroplating processes implying adsorbed intermediates and ligands.