Particle arrangement control and mechanical robustness enhancement of structurally coloured photonic balls composed of SiO2 particles for environmentally benign inorganic colourants†

Abstract

Structurally coloured materials composed of monodisperse particles have attracted considerable attention as environmentally benign colourants. However, these materials exhibit poor mechanical robustness and colour degradation. Although efforts have been made to enhance the mechanical properties of structurally coloured materials, research specifically focusing on their mechanical robustness remains limited. This study investigates the preparation and mechanical robustness of structurally coloured photonic balls (PBs) composed of SiO₂ particles, emphasising the effects of preparation parameters on the particle arrangement and structural colour. We systematically varied the pH and ionic strength of SiO₂ dispersions, alongside the preparation temperatures, to examine their effects on the particle arrangement within PBs. The results indicated that at pH values near the isoelectric point, PBs adopted a colloidal amorphous-type structure, whereas higher pH levels led to colloidal crystalline-type arrangements characterised by vivid structural colours. Notably, increasing the ionic strength transitioned the particle arrangement towards a more disordered state, further emphasising the critical role of surface charge dynamics. In terms of mechanical robustness, the colloidal crystalline-type structures were stronger than their colloidal amorphous-type counterparts. The incorporation of a water-soluble silane compound as a binder in PBs significantly enhanced the interparticle necking formation during heat treatment, thereby improving the mechanical robustness. This study highlights the potential of optimising the preparation conditions for structurally coloured PBs with enhanced mechanical properties, offering a promising alternative to traditional inorganic pigments.