Measurements of the potential of zero charge in room temperature ionic liquids at Ag electrode by surface-enhanced Raman spectroscopy

• In situ SERS was developed to probe electric double layer and pzc of RTILs.
• The minimum of camel-shaped capacitance curves was assigned to pzc.
• SERS was well correlated with the capacitance–potential curves.
• The pzc was depended on the length of alkyl chain of imidazolium based RTILs.

The electrochemical differential capacitance and in situ surface enhanced Raman spectroscopy (SERS) have been combined to investigate the structure of electric double layer (EDL) at a series of imidazolium based ionic liquids/Ag electrode interface. By associating with the change in spectral feature of the imidazolium ring, the potential at local minimum between the two maxima in the camel-shaped differential capacitance curves was assigned to the potential of zero charge (pzc). The pzc shifted positively with increasing the length of the alkyl chain substituted to the imidazolium ring, and the approximate linear relation was observed between the pzc values and the carbon numbers of the alkyl chain. The values of the minimum capacitance at the pzc decreased with increasing the alkyl chain length, which was mainly contributed by the smaller values of dielectric constant (ε) for the RTILs with long alkyl chain. By employing in-situ SERS, the potential dependent spectral feature could be served as the criteria for resolving the adsorption behavior of the cations. With the movement of the potential from relative positive to negative potential, the cation adsorption configuration changed from vertical to nearly flat on. The reorientation could be considered as the evidence for estimating the pzc values. The spectroscopic investigation revealed that the onset potentials for the reorientation were shifted to positive direction as increasing the length of the alkyl chain. The tendency in changing the potential at the minimum capacitance or the reorientation was contributed to the π-electron interaction of imidazolium ring with the Ag electrode, in which the adsorption of cation acted like the behavior of a specific adsorbed anion. The weaker the interaction (long alkyl chain) was, the more the pzc shifted to positive potential direction. The above facts indicated that the classical differential capacitance curves and in situ SERS technique were well correlated for obtaining a deeper insight into the interfacial structure of RTILs at Ag electrodes.