posted on 2023-03-10, 20:13authored byNathanael
C. Ramos, J. Will Medlin, Adam Holewinski
Self-assembled monolayers (SAMs) of thiolates have increasingly
been used for modification of metal surfaces in electrochemical applications
including selective catalysis (e.g., CO2 reduction, nitrogen
reduction) and chemical sensing. Here, the stable electrochemical
potential window of thiolate SAMs on Au, Pt, and Cu electrodes is
systematically studied for a variety of thiols in aqueous electrolyte
systems. For fixed tail-group functionality, the reductive stability
of thiolate SAMs is found to follow the trend Au < Pt < Cu;
this can be understood by considering the combined influences of the
binding strength of sulfur and competitive adsorption of hydrogen.
The oxidative stability of thiolate SAMs is found to follow the order:
Cu < Pt < Au, consistent with each surface’s propensity
toward surface oxide formation. The stable reductive and oxidative
potential limits are both found to vary linearly with pH, except for
reduction above pH ∼10, which is independent of pH for most
thiol compositions. The electrochemical stability across different
functionalized thiols is then revealed to depend on many different
factors including SAM defects (accessible surface metal atom sites
decrease stability), intermolecular interactions (hydrophilic groups
reduce the stability), and SAM thickness (stability increases with
alkanethiol carbon chain length) as well as factors such as SAM-induced
surface reconstruction and the ability to directly oxidize or reduce
the non-sulfur part of the SAM molecule.