Optical properties of nanoscopic gold particles adsorbed at electrode surfaces: The effect of applied potential on plasmon resonance absorption.

We have examined the UV/Visible spectra of nanoscopic gold particles adsorbed on antimony-doped tin oxide transparent electrodes in contact with aqueous electrolytes of varying type and concentration. For the four electrolytes KPF6, NaClO4, KCI, and KBr, we observe a positive shift in the Au-particle plasmon resonance absorption maximum lambda(max) as the electrode potential is increased to more positive values. The degree of the potential-induced shift depends on the electrolyte type and concentration and on the size of the Au panicles. For all electrolytes, the shift in peak absorption is largest for smallest Au particles (Delta lambda(max)/Delta E = 40 nm/V). Ar constant particle size, the potential-induced shift is largest in the presence of Cl- and Br- and smallest in the presence of PF6- and ClO4-. We discuss these results in the context of models which consider: 1) electrode potential effects on free electron concentration (Drude Theory); and 2) anion adsorption processes which affect the plasmon resonance condition via changes in the Au surface layer refractive index. (C) 1997 Acta Metallurgica Inc.