Development And Application Of Electrochemical Tip-Enhanced Raman Spectroscopy

Electrochemical interfice is the place where the electron transfer,energy conversion and storage,and mass transfer occur.It determines the performance of an electrochemical system.It is desirable to have a highly sensitive in situ technique to provide molecular fingerprint information for the characterization of electrochemical interfacial process to optimize the electrochemical systems.In this regard,electrochemical tip-enhanced Raman spectroscopy(EC-TERS),which integrates the nanometer spatial resolution,single-molecule sensitivity,rich vibrational information and electrochemical control,shows unique advantages for the molecular level and nanoscale analysis of the electrochemical interface process.However,since its discovery in 2015,the development of electrochemical TERS(EC-TERS)has been relatively slow.There are only 6 papers on EC-TERS published up to now.The key problems that limit the development of EC-TERS,include how to optimize the fabricate high-quality TERS tips and how to improve the sensitivity and stability of EC-TERS instruments.To address the above key problems,we systematically optimized the tip fabrication,improved the EC-TERS instrument and studied the electrochemical and photoelectrochemical reactions with the improved EC-TERS setup.The main results and conclusions of this thesis are listed as follows:(1)We used the pulse etching Imethod to reduce the over-etching time at the final stage of the tip etching and successfully fabricated Ag tips with high TERS enhancement.The effect of the storage conditions on the lifetime of the Ag tips was also studied.We found that a high environmental humility significantly shortened the lifetime of the tips.Additionally,we developed a simply method of using NaBH4 solution to reduce the tarnished silver tip to recover the TERS enhancement,which enables the reuse of silver tips and prolongs the lifetime.(2)We developed a new EC-TERS setup to couple with a water immersion objective to avoid the optical distortion resulting from the mismatch of refractive index.By designing the special spectroelectrochemical cell and customizing the STM head,a water immersion objective with a short working distance and a high numerical aperture could be coupled with the EC-TERS system to improve the excitation and collection efficiency.The sensitivity of the new setup has been significantly improved by 5 times higher than that of our first-generation EC-TERS setup.We mounted the objective on a piezoelectric stage to precisely adjust the coupling between the focused laser spot and tip to minimize the interference of manual adjustment on the SPM system.Additionally,a drift conpensation system was built into the TERS instrument to significantly decrease the drift of the instrument.(3)We in-situ monitored the potential and laser power dependent plasmon-driven decarboxylation reaction by EC-TERS.The plasmonic photocatalytic decarboxylation could be synergistically manipulated by changing the laser power and potential.We visualized the nanoscale reaction region beneath the tip with TERS imaging.On considering the reaction profile driven by the hot carriers was a convolution of the localized plasmonic electric field(determining the generation of the hot carriers)and the exponential decay function(resulting from the carriers transport).We obtained the mean free path of the plasmonic hot carriers in the real space(?20nm)from the experimentally obtained reaction profile.(4)We studied the electrochemical hydrogenation process of 4-nitrophenyl isocyanide(4-NPIC)on Pd/Au(111)surface at different pHs by combining the cyclic voltammetry,EC-TERS and theoretical calculation.Benefitted from the well-defined surface structure of EC-TERS,we obtained the spectrum from the negatively charged intermediate structure during the stepwise electron transfer and protonation process.Such an intermediate cannot be clearly identified in EC-SERS with a complicated interfacial structure.This work shows that EC-TERS is advantageous to capture the fine structure of immediate species during the electrochemical reaction.