Colloidal metals: past, present and future

Publisher Website
Google Scholar
Abstract
A critical assessment of the various X-ray and electron-based methods of determining the structure of ultrafine metallic particles of colloidal dimension is given. In particular, examples of the use and potential of high-resolution electron microscopy of high-angle Rutherford scattering and of scanning tunneling microscopic procedures are illustrated. Several growth points of current and possible future uses of colloidal metals are identified; and the role of a variety of colloidal metals in (gas-solid) catalysis, in photocatalysis and in the photo- generation of organic materials are discussed. The prospects of fine- tuning colloidal catalysts are also briefly touched upon. THE PAST Colloidal metals are of ancient lineage. Several methods of preparing them were evolved well over a hundred years ago by Selmi, Faraday and Graham. They were the subject of lively studies by these and other pioneers including Tyndall, Rayleigh, Ostwald, Mie and Bredig and more recently by Rideal, who was among the first to probe their catalytic properties for selective conversions such as the hydrogenation of organic molecules. In numerous other respects, colloidal metals have played a prominent part in man's broader cultural activities: they figure eminently in stained glass windows and other decorative-artistic features, as well as in alchemical and medicinal contexts. In medieval times colloidal dispersions of gold, for example, were reputed to possess remarkable curative properties. Faraday (ref. 1) prepared colloidal dispersions of gold by reducing an aqueous solution of a gold salt, such as sodium chloroaurate, with a solution of phosphorous in carbon disulphide. The reduction proceeds rapidly at room temperature and the bright yellow colour of the chloroaurate is replaced by the ruby colouration characteristic of colloidal gold (of appropriate average particle dimension) within a few minutes of mixing. A more convenient, and certainly less unpleasant solvent than CS2 is diethylether. In a typical preparation (ref. 2), NaAuCl4 (20 mg) is dissolved in distilled water (100 ml) and treated with 2 ml of a saturated solution of yellow phosphorous in diethylether. Reduction of chloroaurate is rapid and quantitative, as demonstrated by absorption spectroscopy; and the concentration of the final solution is 5 x peak centred at 522 nm which is thought to be attributable to a plasmon transition. (It is of interest to note in passing - and Kerker has amplified (ref. 3) this point - that

This publication has 0 references indexed in Scilit: