Electrochemical Tip Enhanced Raman Spectroscopy

Electrochemical techniques have been widely used in various fields of modern industry.Lithium-ion batteries,metal corrosion and protection,electroplating,and metallurgy all involve electrochemical processes.Electrochemical processes are dynamic,the adsorption and desorption,orientation,reaction behaviors of monolayer or sub-monolayer interfacial species change with potentials.Therefore,to gain a deep understanding of the interfacial electrochemical processes,it is extremely necessary to develop characterization techniques with high detection sensitivity and the ability to in-situ detect interfacial dynamics from molecular level.Electrochemical tip-enhanced Raman spectroscopy(EC-TERS)combines the electrochemical technology with the tip-enhanced Raman spectroscopy(TERS)technology,which can obtain the real-time in-situ morphology information of sample and molecular fingerprint information of interfacial species under potential control.Although this technology has a good application prospect,the current electrochemical systems in which EC-TERS can be applied are very limited.Preparing EC-TERS tips with high activity and stability in electrochemical environment is of great significance to make the application of EC-TERS techniques versatile.The main contents and conclusions of this paper arc as follows:(1)By using a method which combines chemical method and atomic layer deposition,the Au/Ag@SiO2 TERS tips were successfully fabricated.A monolayer of 3-Mcrcaptopropyltriethoxysilane molecules assembled on the surface of bare Au/Ag tips.After the 3-MPS molecules hydrolyzed in acid solution,the initial silica shell formed.Then ALD was used to deposit silica on the surface of tips.The Faraday current of the Ag@SiO2 tips in the liquid phase is remarkably reduced compared with that of bare Ag tips and the tip can scan stably in an electrochemical environment and obtain the signals of the sample.(2)Au/Ag@HfO2 tips were fabricated successfully by using atomic layer deposition only.The cyclic voltammetry test was carried out to observe the influence of the shell thickness on the current of the tip in liquid phase.The thickness of HfO2 was characterized by atomic force microscopy,and the relationship between the number of deposition cycles and the thickness of HfO2 layer was obtained.In the following work,the intensities of TERS signal of 4-PBT molecules on Au(111)obtained by using Ag tips were compared before and after depositing HfO2 with different number of cycles,and it was found that the intensities were slightly enhanced when the number of cycles was less than 35.We predicted that the reason for this phenomenon is that the HfO2 layer enables LSPR of Ag tips to move to the area of excitation laser,thus enhancing the intensity of signals.(3)Electrochemical redox reaction of anthraquinone derivatives was studied.By means of cyclic voltammetry,STM imaging,EC-TERS and DFT,the adsorption state of molecules on the surface of Au(111)and the corresponding relationship between molecular structure and potential were studied.It was found that the reaction was irreversible under negative potential and laser illumination.By analyzing the results of EC-TERS,it was concluded that light and negative potential induced irreversible process.SPR induced by laser was not necessary factor.In the following work,we hypothesized and verified the mechanism of the reaction.