Cubic colloids : Synthesis, functionalization and applications

Abstract

This thesis is a study on cubic colloids: micron-sized cubic particles with rounded corners (cubic superballs). Owing to their shape, particle packing for cubes is more efficient than for spheres and results in fascinating phase and packing behavior. For our cubes, the particle volume fraction when randomly packed is ~0.74, whereas it is only ~0.64 for spheres. Here, we show that cubic colloids are not only relevant for fundamental research, but also for potential applications in materials science, such as separation membranes and coatings. The research has two focus points: synthesis of cubic colloids cubes, and the functionalization and applications of cubes for materials science.

Cubic colloids can be prepared from different inorganic materials using iron oxide (hematite, α-Fe2O3­­) cubes as shape template. The size of the hematite cubes is tunable between 500 nm and 1500 nm.

We established synthesis procedures to coat the hematite templates with either silica (SiO­2) or titania (TiO2) to obtain core-shell cubes. Subsequent dissolution of the hematite core with acid results in hollow silica and titania cubes.

Moreover, two methods to increase the porosity of the silica cubes have been developed: surface-protected silica etching and surfactant pore templating. With the first method, pores in a wide diameter range were created by enlargement of the micropores (pore diameter < 2 nm) intrinsically present in the silica coating, while preserving the cubic shape. In the second method, cylindrical micelles of cetyltrimethylammonium bromide (CTAB) molecules are incorporated into the silica coating. Removal of the micelles yields well-defined mesopores with an average diameter of 2.5 nm.

In view of cubic colloids as functional materials, we have investigated two methods to add functionality to hollow silica cubes.

As a first example, we used core-shell hematite/silica cubes and exploited the catalytic properties of the core. In the presence of hydrogen peroxide, the hematite core enhances the degradation of organic dyes (modified Fenton reaction), while the silica shell preserves the cubic shape. Further enhancement was achieved by illumination with visible or UV light. We demonstrated that enhanced degradation is neither hampered by the silica shell nor by densely packing the cubes on a substrate.

For our second example, we employed hollow mesoporous silica cubes. These hollow cubes are filled with silver, gold or polypyrrole by in situ synthesis. Reactants are inserted inside the cubes in separate steps. There they react and the resulting product remains confined in the cubes. In this way, aggregation of the functional substances is prevented.

Finally, we delivered proof-of-principle of Cubicle Membranes by employing cubic colloids as building blocks of inorganic separation membranes. To this end, we investigated the formation and liquid permeability of dense random cube packings. Our experiments with solid hematite cubes and hollow microporous silica cubes showed that liquid flows through inter-cube pores rather than through the micropores of the silica cubes. Moreover, by relating our measurements to the Kozeny-Carman relation, we conclude that the relation is applicable to any dense particle packing and is independent of particle shape.