The surface adsorption of 2,2′-bipyridine and benzoin on Cu electrode interface probed by surface-enhanced Raman spectroscopy

• In situ SERS was developed to probe surface coordination chemistry.
• The competitive coadsorption was observed between 22BPY and benzil.
• The interfacial configurations of 22BPY, BA and benzoin were proposed.

The surface configurations and properties are recognized of importance to well understand the surface processes of the heterogeneous reaction. Consequently, the surface sensitive technique is highly desired for obtaining the distinct interfacial pictures. The in situ electrochemical surface-enhanced Raman spectroscopy (SERS) was developed to investigate the adsorption behavior of 2,2′-bipyridine (22BPY) in the presence and absence of benzoin (benzoin = 2-hydroxy-2-phenyl acetophenone) and benzoic acid (BA) on Cu electrode surface in nonaqueous solution. Based on the changes in the spectral feature, the surface coordination processes between the Cu surface and 22BPY with BA, benzoin were deduced. In a wide potential range (−1.2 V to 0.1 V), 22BPY adsorbed vertically on the surface through the two nitrogen atoms of cis-configuration molecules. The introduction of benzoin and BA caused the dramatic change in the surface coordination processes of 22BPY to the Cu electrode. With the positive movement of potential, the benzoin was oxidized to benzil (benzil = 1,2-diphenyl-ethane-1,2-dione) accomplishing with the electrochemical oxidation of Cu electrode. The coadsorption of 22BPY, benzil and BA was observed around −0.2 V, while the anion of ClO4− was induced to coadsorb on the electrode surface. The results revealed that the cis-configuration of 22BPY and the deprotonated BA were contributed to the coadsorption behavior, and benzil participated into the coadsorption layer in a neutral formation. The induced coadsorption was transformed to the competitive adsorption between BA at about 0 V. The surface spectral feature was compared to that of the complex [(22BPY)2Cu(BA)]ClO4⋅(benzil) synthesized by direct electrochemical oxidation of Cu electrode in a similar condition. It suggested the surface adsorption and coordination processes were well agreement with the structure of the surface complex. The interfacial configurations of 22BPY, BA and benzoin in different potential regions were proposed.