Verlag des Forschungszentrums Jülich
JUEL-4236
Han, Bo
Interfacial Electrochemistry and in situ SEIRAS investigations of Self-Assembled Organic Monolayers on Au/Electrolyte Interfaces
158 S., 2006
Electrified solid/liquid interfaces offer unique opportunities for the fabrication
and characterization of 2D supramolecular nanostructures. The combination of
macroscopic electrochemical methods with structure sensitive techniques, such as
scanning probe microscopy (SPM) or vibrational spectroscopy enables the
development of a detailed knowledge on structure and functionality of the created
molecular assemblies. In the present dissertation, the steady state and dynamic
properties of self-assembled adlayers of trimesic acid and thioalkylviologens were
investigated on gold/electrolyte interfaces employing surface enhanced IR absorbance
spectroscopy (SEIRAS) in combination with various electrochemical methods.
The double layer properties of the Au/aqueous electrolyte interface were
investigated first for massive Au(111) single crystal electrodes and Au(111-20 nm)
film electrodes in contact with 0.1 M H2SO4 and 0.1 M HClO4. Cyclic voltammetry
revealed that the response of the film electrodes resembles that of a massive Au (111)
electrode with a low step density. The potential-dependent adsorption of anions and
interfacial water was investigated in detail by in situ SEIRAS.
As the second topic, the adsorption and phase formation of trimesic acid (TMA)
on Au(111) and Au(111-20 nm) electrodes were comprehensively investigated by
electrochemical techniques, scanning tunneling microscopy and SEIRAS. Depending
on the applied electrode potential, TMA molecules form several highly ordered 2D
adlayers: two physisorbed phases, one characterized by a hexagonal honeycomb
pattern and the other by close-packed linear-dimers of planar oriented molecules; and
one stripe-like chemisorbed adlayer of perpendicularly oriented molecules. Weakly
hydrogen-bonded, strongly hydrogen-bonded and isolated water species are coadsorbed.
The potential-induced formation of chemisorbed TMA starts with a fast
orientation change from planar, physisorbed molecules to tilted and/or perpendicular
in a disordered chemisorbed phase. Subsequently, the chemisorbed TMA molecules
align slowly into the highly ordered stripe-like structure. Both substrate/adsorbate and
intermolecular hydrogen-bonding interactions are strongly dependent on the electrode
potential.
The comparison with benzoic acid (BA) and isophthalic acid (IA) shows that
each TMA molecule is chemisorbed via one carboxylate group in an gatoph
configuration, while the two other -COOH side groups are directed towards the
electrolyte and therefore available for further hydrogen-bonding between neighboring
TMA molecules. The orientation of BA and IA molecules in the respective adlayers
showed a similar potential dependence as TMA.
The third topic of the dissertation is focused on the self-assembly and interfacial
reactivity of redox-active thioalkylviologens immobilized on gold/electrolyte
interfaces. Voltammetric experiments revealed two one-electron redox processes of
the surface confined adlayer, the reduction of the viologen dication V2+ to the radical
cation V.+, and the reduction of the radical cation to the neutral species V0. The first
redox process V2+ ↔ V.+ is reversible, while the second redox process V.+ ↔ V0 is
only quasi reversible. A complete monolayer of V2+ exhibits a sandwich-like structure:
the van der Waals interaction between the alkyl chains leads to two well-ordered
hydrophobic layers, with the redox-active bipyridinium layer enclosed in between.
The alkyl chains are aligned in an all-trans configuration in a tilted orientation with
respect to the surface normal. However, they exhibit conformational disorder up to
some extent. The long axis of the central bipyridinium unit is also tilted. The
alignment of V2+ is stabilized by inter-planar π - π and coulombic interactions. The
SAM of V.+ is composed of coexisting monomers V.+ and dimers (V.+)2. The
alignment of V.+ is stabilized by a strong vibronic coupling within the dimers as well
as the π - π stacking between the monomers. The dimerization and the alignment are
both favored by the more tilted bipyridinium units with an increase of the alkyl chain
lengths.
Attempts to investigate the kinetic behavior of the redox electron-transfer
process V2+ → V.+ lead to qualitative conclusions. The overall redox process starts
with an interfacial electron transfer (tunneling mechanism) from the electrode to the
V2+ moiety through the alkyl chains. In a subsequent slow process, the dimerization of
V.+, the delocalization of the radical electrons, and the migration of ClO4
- take place simultaneously. The overall rate of the redox reaction is determined by the
dissociation of the ion-pair complex and the migration of the ClO4
- anions.
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