Probing double layer structure at Au/[BMIm]BF4 interface by molecular length-dependent SERS Stark effect

• The vibrational Stark effect of νCN was determined based on molecule ruler strategy.
• EC-SERS was developed to distinguish the orientation and measure potential dependent Stark effect.
• The interfacial electric field completely decayed before reaching the nitrile group location of longer probe.
• The potential drop occurred over a range of 5–6 Å away from electrode surface in RTILs.

Room temperature ionic liquids (RTILs) played an important role in the electrochemistry because of the advantageous properties. Due to the different electrochemical behavior from the dilute solution, it was still remained a significant challenge to obtain the insight into the interfacial structure. Although the spectroelectrochemistry offered the rich spectroscopic information for obtaining the interfacial structure at molecular level, the lack of the spatial information of those techniques at nano/subnanometer scale led the difficulties in the determination of the spatial structure of double layer. Here, the surface enhanced Raman spectroscopy (SERS) Stark tuning rates of CN stretching frequency were employed to determine the electric double layer structure on Au electrode, and probes with different lengths, involving cyanide ion (CN−), thiocyanate (SCN−), 3-aminopropanenitrile (3-APN) and 6-aminohexanenitrile (6-AHN), were used as rulers to measure the spatial structure of double layer. In this approach, SERS combined with electrochemical control was performed to distinguish the orientation of surface species and detect the vibrational frequency of the nitrile stretching mode. For shorter probes (such as CN− and SCN−), the nitrile group were located in the electric double layer region, resulting in measureable Stark tuning rates (dνCN/dE), while for longer probe molecules (such as 3-APN and 6-AHN), the interfacial potential completely decayed before reaching the nitrile group location, resulting in the negligible Stark tuning rates. The vibrational Stark tuning rates suggested that the potential drop occurred over a range of 5–6 Å from the metal surface into the bulk phase of ionic liquids. The results demonstrated that the overall potential mostly dropped across the first layer and only a small fraction across the second layer. The preliminary results allowed us to obtain an insight into the physical picture of the interfacial structure of RTILs/metal system.