Probing the Redox-Dependent Electronic and Interfacial Structures in Ferrocene-Terminated Self-Assembled Monolayers with Photoelectron Spectroscopy
Authors: Wong Raymond A , Yokota Yasuyuki , Wakisaka Mitsuru , Inukai Junji , Kim Yousoo 
Published: February 2020

Central to electrochemistry is the electric double layer (EDL) at the electrode-electrolyte interface where important interfacial phenomena including adsorption and charge transfer occur.1 Understanding the nature of the electric double layer regarding the electronic and structural characteristics has implications relating to bio(chemical) sensors, micro-mechanical actuators and energy storage/conversion devices. Amongst model systems, ferrocene-terminated alkanethiol self-assembled monolayers (Fc SAM) on Au are an attractive model electrochemical interface due to their practical applicability and versatility with structural modifications.2 Within the Fc SAM, a change in the ferrocene/ferrocenium (Fc/Fc+) redox state leads to notable changes to the interfacial structure with respect to the EDL, interactions with the electrolyte, and work function. While existing studies exist on probing the redox-induced Fc SAM structural changes using techniques such as surface plasmon resonance (SPR), Raman, IR and quartz crystal microbalance (EQCM),2 experimental efforts aimed at probing the changes in electronic structure are still limited.

Here, we employ X-ray and ultraviolet photoelectron spectroscopy combined with an electrochemical cell (EC-XPS/UPS) which allows us to electrochemically control the Fc SAM redox state and spectroscopically probe the redox-induced electronic and structural changes.3 To perform our experiments, electrochemical (EC) measurements are performed in a separate EC chamber in Ar atmosphere followed by evacuation and then transfer to the XPS/UPS analysis chamber.4 The EC chamber contains a retractable PTFE (polytetrafluorethylene) cell where electrochemical measurements are performed under Ar atmosphere. The cell utilizes a hanging meniscus setup connected via PFA (perfluoroalkoxy alkane) tubing to a syringe containing the electrolyte. This controlled setup permits us to reliably probe the oxidized Fc+ moieties and prevent its decomposition upon exposure to the ambient.

Using this approach, following EC and electrode immersion, we can probe changes to the ferrocene/ferrocenium (Fc/Fc+) redox center (Fe oxidation state), deduce the occurrence of ion-pairing, changes in molecular orientation and monolayer thickness (Figure 1). Oxidation to Fc+ is corroborated by UPS, which shows that a shift of the highest occupied molecular orbital (HOMO) towards higher binding energy along with an increase in work function. The reversibility of our observations is confirmed by reproducibly switching between the Fc/Fc+ states. More broadly, our approach is potentially applicable to a variety of electrochemically active systems to assist in the elucidation of redox-related structure-function relationships. Specific details will be provided during the presentation.

Figure 1. (a) Schematic and photographs of hanging meniscus inside the EC chamber during EC measurements and following removal from the electrolyte. (b) Molecular structure of Fc SAM used in study and cyclic voltammagram performed in 0.1 M NaClO4. (c) Corresponding XPS Fe 2p, Cl 2p and O 1s spectra of the pristine, oxidized Fc+ and neutral Fc states.

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Figure 1