Investigation of Cobalt Phthalocyanine at the Solid/Liquid Interface by Electrochemical Tip-Enhanced Raman Spectroscopy

Electrochemical tip-enhanced Raman spectroscopy (EC-TERS) with three excitation wavelengths in combination with in situ electrochemical scanning tunneling microscopy (EC-STM) and absorption measurements has been employed to provide comprehensive insights into the electrochemical processes of cobalt phthalocyanine (CoPc) at the solid/liquid interface supported on a Au(111) substrate. As the substrate potential becomes more negative, CoPc molecules form a highly ordered monolayer on the Au(111) surface (>0.1 V) until the ordered-to-diffusing phase transition is triggered (<0.1 V). CoPc molecules in the ordered phase are 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 the EC-STM and has negligible effect on the EC-TERS measurements. 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 the oxygen reduction reaction.