Time dependent spectral change upon potential step perturbation for Au nanoparticles immobilized on an organic monolayer-modified ITO electrode

Time dependent spectral change of Au nanoparticles immobilized on an ITO electrode upon the change of electrode potential was investigated. Au nanoparticles of a diameter of 11 nm were immobilized on a monolayer of alkylsiloxane with an amine or thiol end group on an ITO electrode. In addition to the previously clarified charging–discharging process of the particles in response to the sine-wave potential modulation at frequencies higher than 8 Hz, very slow relaxation of visible light absorption after a potential step was found in the present work. The transient of absorbance in response to the potential step was a sum of a fast component within 1 s and a slow single exponential component with a half-life of the order of 30–150 s. The half-life of the slow component and the magnitude ratio of the two components depended on the incident light wavelength and the direction of the potential step. These results suggested that the absorption spectrum of the Au nanoparticles after the potential change is time dependent. The relative magnitude of the slow relaxation in the total spectral change was enhanced in the presence of adsorptive electrolyte anions such as Cl− and Br−, especially at positive potentials, at the expense of the fast component. Implications of these results were discussed, and the possibility of the presence of a slow surface process on the Au nanoparticle surface in the presence of adsorptive anions was invoked.