Interfacial Electric Field Distribution Of Ionic Liquids/ Electrodes System Probed By In-situ EC-SERS

As a novel type of green solvent, room temperature ionic liquids(RTILs) have exhibited wide applications in the electrochemistry owing to the distinct advantages such as low vapor pressure, good conductivity, and wide electrochemical window. The structure of electric double layer in RTILs is rather different from the traditional electrolytic solutions and non-aqueous solution due to their high density of mobile ions. The achievement on well understanding electric double layer structure is still remained significant challenge. Recent developments on the spectroelectrochemistry have offered the rich spectroscopic information for obtaining the interfacial structure at molecular level. However, the lack of the spatial information of those techniques leads to the difficulties in the determination of the interfacial electric distribution. Hence, analyzing the spatial information of the interfacial electric field distribution at “RTILs/ electrode” interface is of great significance to understand the structure and properties of electrical double layer of ILs.The surface enhanced Raman spectroscopy(SERS) combined with electrochemistry measurements is performed to detect the Stark effect of molecular vibrational frequency. Owing to the sensitivity of the triple bond to the electric field, we have detected the spatial information of the electric field at ILs interface utilizing probes with different structures and lengths. The study will provide evidences to establish a new electric double layer theory at the “ILs/electrode” interface. The main results are outlined as follows:1、The probe molecules having ring and triple bond such as 4-cyanopyridines(PCP),4-aminobenzonitrile(PABN), and 4-(4-Aminophenyl)benzonitrile(APBN) were employed to resolve the adsorption behavior by EC-SERS to analyze the Stark effect of C≡N stretching mode on the Au electrode. The results showed that in the positive potential region the molecules were adsorbed on the electrode with the nitrogen of pyridine or amino. The Stark shifts of nitrile were negligible owing to the electronic buffer action of rings. In the negative potential region, the molecules tended to lie flat on the surface resulting into a measureable Stark effect.2、The molecules with similar structure but different lengths were selected as probes, the adsorption behavior and the Stark effects of which have been studied. With the probes as rulers, comparing the response to electric field of cyanide ion(CN-), thiocynate(SCN-), 3-Aminopropanenitrile(3-APN), and 6-Aminohexanenitrile(6-AHN) we have measured the electric field distribution with high spatial resolution. The results revealed that the Stark tuning rates became smaller as the molecular length increased. The overall potential dropped dramatically in the first layer(Helmholtz layer), in the range about 5~6 ? from the Au electrode interface to the bulk solution. This method provided a technique to measure the electric field distribution with high resolution.3、The influences of IL composition and trace water on the electrical double layer thickness were investigated with 6-AHN as a probe. It was found that the thickness of the electric double layer became thicker as the ion size became larger resulting into more measurable Stark tuning rate. Moreover, the cation size had more profound influence on the thickness of the electric double layer than aion. Trace water would affect the interfacial electric field distribution, adversely changing the Stark tuning rates of 6-AHN significantly.