Towards a better understanding of gold electroless deposition in track-etched templates

The template method for preparing nanomaterials entails synthesis of tubular or fibrillar nanostructures within the pores of a microporous membrane. If the holes are completely filled, solid nanowires result, while a partial filling with a continuous coating gives rise to nanotubes. This paper focused on the study of the parameters controlling electroless gold deposition in track-etched polycarbonate membranes. The structure and morphology of the obtained nanomaterials are examined and compared by a variety of electron microscopies and atomic force microscopy and examined in relation to electrochemical and spectrophotometric results. As far as nanotubes are concerned, problems with obtaining such a nanostructure are observed, in particular when membranes with pore diameters equal to or smaller than 30 nm are used. In the case of nanowires, defects related to fabrication problems are evidenced when they are used to prepare nanoelectrode ensembles (NEEs), because defects dramatically influence their voltammetric behavior. The role of deposition time and pH on gold nucleation and growth is studied in detail. We demonstrate that these parameters indeed determine the formation of nanowires versus nanotubes. In all cases the deposit is started by the formation of gold nuclei on the walls of the previously activated membrane. Then the nuclei grow, until they are bound together to produce the final nanomaterial. A significantly better control of the deposition is achieved by separating the nuclei formation step from the growth process. At pH 10 the nuclei growth is more regular but slower, whereas at pH 12 the process is faster but their size distribution is wider. At pH 10 the formation of nanotubes is better controlled, whereas at pH 12 continuous nanofibers are more quickly formed.