OPTICAL CHARACTERIZATION OF ANODIC ALUMINUM-OXIDE FILMS CONTAINING ELECTROCHEMICALLY DEPOSITED METAL PARTICLES .1. GOLD IN PHOSPHORIC-ACID ANODIC ALUMINUM-OXIDE FILMS

In a previous paper (Preston, C. K.; Moskovits, M. J. Phys. Chem. 1988, 92, 2957) we presented a technique, based on the width of the surface plasmon absorption measured by reflectance spectroscopy, for determining the average size of noble metal particles electrochemically deposited in anodic aluminum oxide films. This method is expanded, here, to take into account the optical anisotropy and nonuniformity of the film which arises from nonspherical nature of the particles themselves as well as from their distribution. Equations describing the reflectance spectrum expected for such films, assumed to consist of an alumina overlayer, a metal-filled oxide layer, and an aluminum substrate, are developed. Experimentally measured s- and p-polarized visible reflectance spectra of gold-filled anodic alumina films prepared in phosphoric acid are found to fit these expressions very well, yielding, as adjustable parameters, the optical thickness of the film, the average size and shape of the gold metal particles, and the equivalent-mass thickness of deposited gold. Particle sizes are found to be more reliably determined from the p-polarized spectra, while the thickness of the alumina overlayer was more accurately derived from the s-polarized spectra. ac electrodeposition was used to introduce gold into the uniform pores of the anodic oxide films. The quantity of metal deposited and the average size of the particles and their eccentricity were found to increase with increasing deposition time. However, these parameters did not vary significantly with the frequency of the ac deposition voltage over the range 50-500 Hz. Several of the parameters determined by the optical method were corroborated by transmission electron microscopy of sections of the films and of isolated gold metal particles and by atomic absorption measurements of the total gold metal content in the films.