posted on 2024-01-25, 01:03authored byFelix Rauh, Johannes Dittloff, Moritz Thun, Martin Stutzmann, Ian D. Sharp
Attenuated total
reflection surface-enhanced infrared absorption
spectroscopy (ATR-SEIRAS) is a powerful method for probing interfacial
chemical processes. However, SEIRAS-active nanostructured metallic
thin films for the in situ analysis of electrochemical phenomena are
often unstable under biased aqueous conditions. In this work, we present
a surface-enhancing structure based on etched black Si internal reflection
elements with Au-coatings for in situ electrochemical ATR-SEIRAS.
Using electrochemical potential-dependent adsorption and desorption
of 4-methoxypyridine on Au, we demonstrate that black Si-based substrates
offer advantages over commonly used structures, such as electroless-deposited
Au on Si and electrodeposited Au on ITO-coated Si, due to the combination
of high stability, sensitivity, and conductivity. These characteristics
are especially valuable for time-resolved measurements where stable
substrates are required over extended times. Furthermore, the low
sheet resistance of Au layers on black Si reduces the RC time constant
of the electrochemical cell, enabling a significantly higher time
resolution compared to that of traditional substrates. Thus, we employ
black Si-based substrates in conjunction with rapid- and step-scan
Fourier transform infrared (FTIR) spectroscopy to investigate the
adsorption and desorption kinetics of 4-methoxypyridine during in
situ electrochemical potential steps. Adsorption is shown to be diffusion-limited,
which allows for the determination of the mean molecular area in a
fully established monolayer. Moreover, no significant changes in the
peak ratios of vibrational modes with different orientations relative
to the molecular axis are observed, suggesting a single adsorption
mode and no alteration of the average molecular orientation during
the adsorption process. Overall, this study highlights the enhanced
performance of black Si-based substrates for both steady-state and
time-resolved in situ electrochemical ATR-SEIRAS, providing a powerful
platform for kinetic and mechanistic investigations of electrochemical
interfaces.