Use of Chemistry as a Powerful Means for Synthesis and Processing of Nano Particles

Abstract

The chemical synthesis of small particulate systems has become an interesting area of scientific activities since the potential of nano-scale systems has been demonstrated by the early work of Gleiter [1] and Henglein [2]. Colloidal systems in liquid phases with particle sizes in the lower nano range, however, have been known for centuries, and commercial applications of SiO₂ sols are known since the middle of the last century. It is a matter of fact that during each precipitation process from solution a growth reaction takes place and a nano size range is passed through. If the precipitation process is carried out far away from the point of zero charge (p. z. c.), the particles will be charged and prevented from agglomeration as long as the conditions for stability (e.g. critical distance, particle size versus precipitation) are not violated. This leads to stable sols, the concentration of which has to be kept rather low, as described in [3] and the references cited therein. Approaching the p. z. c., the interaction mechanisms get activated and gels (strong interaction) or precipitates (slow interaction) are formed. In fig. 1 the differences of charge stabilized and sterically stabilized systems during upconcentration are shown. In the charge stabilized systems, due to a strong interaction of surface groupings which are necessary to build up surface charges, in general, aggregates to be formed, are rather uncontrolled, and irregularly structured gels or precipitates are formed. As a consequence, in gels the green density is rather low (≤20% by volume). If the surface of the small particles, however, is covered by unreactive groupings, only weak van der Waal’s forces are active and it should be possible to obtain more regularly packed solids. Typical interaction mechanisms are hydrogen bonds (in solution), dipole-dipole interactions, hydrophobic interactions, van der Waal’s forces or even the formation of chemical bonds.