Abstract:We explore the redox-dependent
electronic and structural changes
of ferrocene-terminated self-assembled monolayers (Fc SAMs) immersed
in aqueous solution. By exploiting X-ray and ultraviolet photoelectron
spectroscopy combined with an electrochemical cell (EC-XPS/UPS), we
can electrochemically control the Fc SAMs and spectroscopically probe
the induced changes with the ferrocene/ferrocenium (Fc/Fc+) redox center (Fe oxidation state), formation of 1:1 Fc+–ClO4
– ion pairs, molecular orientation,
and monolayer t… Show more
“…Based on the geometry of the cyclopentadienyl (Cp) ligands, Fc has the electronic configuration (a 1 ′) 2 (e 2 ′) 4 and (a 1g ) 2 (e 2g ) 4 for the eclipsed (D 5h ) and staggered (D 5d ) conformations, respectively 53 , 54 . The XPS Fe 2p (core level) and UPS valence spectra can be tracked together because the Fc HOMO is localised to the Fe heteroatom causing them to be strongly correlated 55 , 56 . Fig.…”
Altering electrochemical interfaces by using electrolyte effects or so-called “electrolyte engineering” provides a versatile means to modulate the electrochemical response. However, the long-standing challenge is going “beyond cyclic voltammetry” where electrolyte effects are interrogated from the standpoint of the interfacial properties of the electrode/electrolyte interface. Here, we employ ferrocene-terminated self-assembled monolayers as a molecular probe and investigate how the anion-dictated electrochemical responses are translated in terms of the electronic and structural properties of the electrode/monolayer/electrolyte interface. We utilise a photoelectron-based spectroelectrochemical approach that is capable of capturing “snapshots” into (1) anion dependencies of the ferrocene/ferrocenium (Fc/Fc
+
) redox process including ion-pairing with counter anions (Fc
+
–anion) caused by differences in Fc
+
–anion interactions and steric constraints, and (2) interfacial energetics concerning the electrostatic potential across the electrode/monolayer/electrolyte interface. Our work can be extended to provide electrolyte-related structure-property relationships in redox-active polymers and functionalised electrodes for pseudocapacitive energy storage.
“…Based on the geometry of the cyclopentadienyl (Cp) ligands, Fc has the electronic configuration (a 1 ′) 2 (e 2 ′) 4 and (a 1g ) 2 (e 2g ) 4 for the eclipsed (D 5h ) and staggered (D 5d ) conformations, respectively 53 , 54 . The XPS Fe 2p (core level) and UPS valence spectra can be tracked together because the Fc HOMO is localised to the Fe heteroatom causing them to be strongly correlated 55 , 56 . Fig.…”
Altering electrochemical interfaces by using electrolyte effects or so-called “electrolyte engineering” provides a versatile means to modulate the electrochemical response. However, the long-standing challenge is going “beyond cyclic voltammetry” where electrolyte effects are interrogated from the standpoint of the interfacial properties of the electrode/electrolyte interface. Here, we employ ferrocene-terminated self-assembled monolayers as a molecular probe and investigate how the anion-dictated electrochemical responses are translated in terms of the electronic and structural properties of the electrode/monolayer/electrolyte interface. We utilise a photoelectron-based spectroelectrochemical approach that is capable of capturing “snapshots” into (1) anion dependencies of the ferrocene/ferrocenium (Fc/Fc
+
) redox process including ion-pairing with counter anions (Fc
+
–anion) caused by differences in Fc
+
–anion interactions and steric constraints, and (2) interfacial energetics concerning the electrostatic potential across the electrode/monolayer/electrolyte interface. Our work can be extended to provide electrolyte-related structure-property relationships in redox-active polymers and functionalised electrodes for pseudocapacitive energy storage.
“…For example, a recent study by Wong et al . show that Fc-alkanethiol SAMs form strong ion pairs in the oxidized state which increase the chain length by more than 1 nm as probed by combining electrochemical and XPS measurements 30 . However, techniques to assemble the ferrocene derivative on the different substrates may vary depending on the functional group available.…”
Electroactive self-assembled monolayers (SAMs) bearing a ferrocene (Fc) redox couple were chemically assembled on H-terminated semiconducting degenerate-doped
n
-type Si(111) substrate. This allows to create a Si(111)|organic-spacer|Fc hybrid interface, where the ferrocene moiety is covalently immobilized on the silicon,
via
two alkyl molecular spacers of different length. Organic monolayer formation was probed by Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) and X-ray photoelectron spectroscopy (XPS) measurements, which were also used to estimate thickness and surface assembled monolayer (SAM) surface coverage. Atomic force microscopy (AFM) measurements allowed to ascertain surface morphology and roughness. The single electron transfer process, between the ferrocene redox probe and the Si electrode surface, was probed by cyclic voltammetry (CV) measurements. CVs recorded at different scan rates, in the 10 to 500 mV s
−1
range, allowed to determine peak-to-peak separation, half-wave potential, and charge-transfer rate constant (
K
ET
). The experimental findings suggest that the electron transfer is a one electron quasi-reversible process. The present demonstration of surface engineering of functional redox-active organometallic molecule can be efficient in the field of molecular electronics, surface-base redox chemistry, opto-electronic applications.
“…[1,2,4] As an offshoot of them, ion-surface molecular interactions at SAM were revealed to altering dipolar moment of the surface molecules or modulating the intrinsic potential emanating from the SAM, thereby controlling electron energetics of the solid electrode or an electrostatic potential profiles at electrical double layer. [5][6][7][8][9][10] These phenomena were based on various specific interactions, originated from physicochemical natures of associating species [11][12][13][14][15] (e.g., coordination bonding, coulombic pairing, and hydrogen bonding), and made the surface molecule redistribute its charges by building new bond dipoles or structural changes, thereby altering the dipolar characteristic of SAM. [5][6][7]9] Although these interfacial molecular effects play a substantial role in various electronic and electrochemical systems, comprehensive molecular pictures in these interactions are still limited due to interfacing intricate electrolytes and observing conditions.…”
Section: Introductionmentioning
confidence: 99%
“…[5][6][7][8][9][10] These phenomena were based on various specific interactions, originated from physicochemical natures of associating species [11][12][13][14][15] (e.g., coordination bonding, coulombic pairing, and hydrogen bonding), and made the surface molecule redistribute its charges by building new bond dipoles or structural changes, thereby altering the dipolar characteristic of SAM. [5][6][7]9] Although these interfacial molecular effects play a substantial role in various electronic and electrochemical systems, comprehensive molecular pictures in these interactions are still limited due to interfacing intricate electrolytes and observing conditions. [11,15,16] As a discerning method of solid/liquid interfacial states, an ionovoltaic generation, converting the interfacial potential effects on the electrolyte/dielectric (SAM and insulating layer)/electrode interface to electric signals, has been proposed, and correlative studies were conducted for investigating its mechanism.…”
A surficial molecular dipole effect depending on ion‐molecular interactions has been crucial issues regarding to an interfacial potential, which can modulate solid electronic and electrochemical systems. Their properties near the interfacial region can be dictated by specific interactions between surface and adsorbates, but understandings of the corresponding details remain at interesting issues. Here, intuitive observations of an ionic pair formation‐induced interfacial potential shifts are presented with an ionovoltaic system, and corresponding output signal variations are analyzed in terms of the surficial dipole changes on self‐assembled monolayer. With aiding of photoelectron spectroscopies and density function theory simulation, the ionic pair formation‐induced potential shifts are revealed to strongly rely on a paired molecular structure and a binding affinity of the paired ionic moieties. Chemical contributions to the binding event are interrogated in terms of polarizability in each ionic group and consistent with chaotropic/kosmotropic character of the ionic groups. Based on these findings, the ionovoltaic output changes are theoretically correlated with an adsorption isotherm reflecting the molecular dipole effect, thereby demonstrating as an efficient interfacial molecular probing method under electrolyte interfacing conditions.
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