Sculpting at the Nanoscale: Colloidal Rods with Graded Chemistry

Abstract

Silica is an abundant material on earth. It can be found in the soil of the earth in the form of sand, predominantly in our oceans and on the beaches but also for instance in the desert. Silicon dioxide knows many different forms: it can be found in crystalline (quartz) form but also in amorphous form. Recently, a new synthesis method has been introduced that makes it possible to prepare rod-shaped colloids made from silica. These particles grow from an emulsion droplet that mainly consists of water and ethanol. The droplets are stabilized by the citrate ion and the polymer polyvinylpyrrolidone. The precursor that reacts to form silica can only react from the droplet and not from the oil, because without the presence of water, hydrolysis of the precursor, and the subsequent condensation, the actual formation of silica, cannot take place. In the early stage of the synthesis a thin layer of silica forms at the interface of the droplet. This process turns out to be amenable to a range of modifications that allow the final particle shape to be fine-tuned. In the work described in this PhD thesis, the synthesis and phase behavior of these particles have been studied extensively. It has led to a large number of new nanoscale building blocks. The silica rods were found to have a gradient in structure that varies from the tip to the tail. In the tip of the rod the degree of condensation is low, whereas in the tail of the particles the degree of condensation is high. In between the two ends, the degree of condensation seemed to change gradually from one to the other. In a basic solution these rods transformed into a cone-shape. The composition and structure of the rods were modified by means of the temperature but also by the concentration and composition of the silica precursor. These parameters were used to create new shapes of silica colloids. To further understand the mechanism of growth we studied the synthesis in detail. This insight allowed us to extend the range of particle shapes this synthesis can produce. The difference between de ends of the silica rods can be used to nucleate an oil droplet on the flat side of a silica rod. The nucleation is purely driven by geometry. Finally, we introduce a new solvent in which particles consisting of poly-NIPAM particles, a temperature sensitive polymer, show a temperature dependent diameter.This will allow us to fine tune the depletion force between the attractive rods, which are index matched by the solvent. In the presence of small silica spheres these rods were found to assemble side to side.