Self-assembly of titania supraparticles

Abstract

Titania is one of the most abundant and biocompatible compounds on our planet while also possessing strong optical properties. These optical properties can be further enhanced and manipulated by altering size, shape and structure of the titania. In this project, monodisperse amorphous titania colloids with a polydispersity of 6% and an average diameter of 438 nm were successfully synthesized through a sol-gel method. In this form the colloids displayed iridescent colours, after self-assembly (SA) into colloidal crystals. After silica coating of these titania colloids, stable titania-silica composites were formed with a polydispersity of only 3% and an average diameter of 421 nm, still exhibiting iridescence after SA. These composites were used to successfully form onion shaped and icosahedral photonic supraparticles with diameters ranging between 3 and 30 µm through evaporation induced self-assembly. Especially the icosahedral supraparticles showed bright and patterned colouring upon illumination. The titania-silica composites were also functionalised with octadecyltrimethoxysilane (OTMOS) and dispersed in nonpolar solvents. These colloids had a polydispersity of 4% and an average diameter of 506 nm, they did not show iridescence and formed only partially crystalline supraparticles with diameters ranging between 4 and 20 µm.

The strong display of colours in our synthesized photonic supraparticles shows great promise for future applications in nanophotonics, photocatalyis and structural colouring. However, many parameters used in the experiments are not yet optimized, such as the speed of SA, or fully understood, decreasing reproducibility of the experiments and control over the self-assembly of the supraparicles. Future research should focus on these parameters so that consistent photonic supraparticles can be synthesized.