Electroreflectance Study of Gold Nanoparticles Immobilized on an Aminoalkanethiol Monolayer Coated on a Polycrystalline Gold Electrode Surface

Electrochemical and optical properties of two dimensional electrostatic assembly of Au nanocrystals

The spectroscopic and electrochemical properties of two-dimensional electrostatic assembly of Au nanocrystals are examined on poly-L-lysine (pLys) modified gold electrodes. The surface preparation for the nanoparticle deposition involved the self-assembly of a monolayer of 11-mercaptoundecanoic acid on the electrode surface, followed by the electrostatic deposition of pLys from aqueous solution. The polyelectrolyte layer acts as the electrostatic anchor for the Au particles. Electrostatically stabilised Au particles were prepared by homogeneous reduction in the presence of citrate, yielding monodispersed colloidal suspension with an average diameter of 18 +/- 2 nm. After 4 h of deposition, the citrate-stabilised particles reach a maximum surface density of (8.2 +/- 0.1) x 10(10) particles cm(-2), with an average edge-to-edge distance of 25 nm. The particle surface density was estimated from scanning electron micrographs. Kelvin probe measurements were employed for examining changes in surface dipole introduced by the 2D array of nanocrystals. From simple electrostatic arguments, the apparent static dipole moment per particle was estimated of the order of 2700 D. The strong interaction between the nanocrystals and the pLys layer is responsible for the surface charge displacement, leading to changes in the surface dipole of 0.35 eV. These electrostatic interactions also manifest itself by the red shift of the plasmon resonance of the assembly with respect to the aqueous colloidal suspension. Analysis of the spectral broadening was attempted within the framework of the so-called coherent-potential approximation. Finally, electrochemical studies in 1,2-dichloroethane show a large electronic overlap between the nanocrystals and the metal substrate. Results obtained from electrochemical impedance spectroscopy strongly suggest that the electrostatic assembly of nanocrystal behaves like a 2D array of randomly distributed spherical nanoelectrodes.

Electrochemical infrared characterization of CO comains on ruthenium-decorated platinum nanoparticles

Spectra obtained by electrochemical infrared reflection absorption spectroscopy (EC-IRAS) for carbon monoxide (CO) adlayers formed by partial CO dosing on various ruthenium-decorated platinum nanoparticle films are reported. The need to achieve a well distributed rather than aggregated metal nanoparticle array is demonstrated, given that such nanoparticle aggregates induce complex dielectric behavior. The strategy here is to use an "organic glue matrix" (short chain SAMs) between the nanoparticles and the gold substrates. The observed promotion in CO electrooxidation by the existence of a Ru island on Pt nanoparticles, of interest to fuel-cell catalysis, showed a strong relationship with Ru surface concentrations, consistent with previous studies on single crystal or polycrystalline bimetallic surfaces. Two distinctive CO infrared bands, one for the Pt-CO and one for Ru-CO domain were found after the dipole coupling of CO within the two CO domains was minimized. Interestingly, those two CO bands showed independent electrooxidation behavior with electrode potential changes. Also, it is shown that the electrooxidation of CO on large Ru islands is less facile than on small Ru islands. In addition, the activity of commercial Pt/Ru alloy nanoparticles to CO stripping was tested and IRAS spectra were reported as a comparison to our Ru-decorated Pt nanoparticles.