Synthesis and characterisation of colloidal metal and semiconductor particles prepared in microemulsions

Abstract

The preparation of colloidal particles of platinum ‘in situ’ in water-in-oil microemulsions formed by the aerosol-OT (AOT)–water–heptane system and the hexadecyltrimethylammonium bromide (CTAB)–water–heptane–chloroform system is described. In both systems, particle growth is controlled and particle sizes of the order of a nm or so are obtained. In the AOT system, there is only a weak correlation between the particle size and droplet size. To obtain further insight into the mechanism of particle growth, the kinetics of the process were investigated. Nucleation particles (Pt°) are produced in an initial step by the reduction of PtCl^2–₆(Pt^IV) using hydrazine as reducing agent. The kinetics of this stage were found to be erratic and this was attributed to a reaction of AOT with hydrazine. This also had the effect of flocculating the microemulsion system (forming networks) which drastically perturbed the size measurements made using the sedimentation velocity (ultracentrifuge) method. Fractal growth was also indicated by light-scattering measurements.

In CTAB microemulsions, the hydrazine was stable but a gradual replacement of the platinum complex ion Pt^IVCl^2–₆ by Br^– was indicated. The kinetics of nucleation were affected by internal partitioning within droplets, but for both systems small particles in the 1–2 nm radius size range were obtained. The sizes were dependent on droplet size but again the particle sizes did not equate with the droplet sizes.

The catalytic ability of these particles in microemulsions has been demonstrated.