Tip-enhanced Raman spectroscopy (TERS) is a powerful tool for surface analysis that can provide chemical information with high spatial resolution by integrating Raman spectroscopy with scanning tunneling microscopy (STM). By exploiting the fundamental merits of TERS, single-molecule sensitivity and nanometer spatial resolution have been successfully achieved in TERS studies under ultrahigh vacuum (UHV) conditions. In recent studies, TERS has also been used to reveal critical chemical identification in liquid and electrochemical (EC) conditions. However, molecularly-resolved images have not been achieved in these studies, and the lack of clear topographic information could complicate the interpretation of the acquired signal.<br/><br/>Herein, EC-TERS experiment with three excitation wavelengths in combination with in situ EC-STM and absorption measurements has been employed to provide molecular-level insights into the electrochemical processes of cobalt phthalocyanine (CoPc) at the solid/liquid interface supported on a Au(111) substrate. Our molecular resolved EC-STM imaging provide the direct insight of the adsorption behavior of CoPc on Au(111) surface. As the substrate potential becomes more negative, CoPc molecules form a highly ordered monolayer on the Au(111) surface (&gt; −0.4 V) until the order-to-diffusing phase transition is triggered (&lt; −0.4 V). CoPc molecules in the ordered phase are gradually reduced during cathodic scanning, which leads to a redshift in the resonance condition and gives rise to distinct EC-TERS behaviors which depend on excitation wavelengths. The ordered-to-diffusing phase transition of CoPc molecules results in the disappearance of the EC-TERS signal. The catalytic activity of CoPc for the oxygen reduction reaction (ORR) was not visible in EC-STM and has negligible effect on the EC-TERS measurements because the ORR kinetics are much faster than the image and spectral acquisition times. The comprehensive evidence from EC-TERS, EC-STM, and absorption spectroelectrochemistry clearly demonstrates that partially reduced CoPc molecules are the dominant species under steady state measurements during oxygen reduction.

Investigation of cobalt phthalocyanine at the solid/liquid interface by electrochemical tip-enhanced Raman spectroscopy

2019 Spring National Meeting

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