Self-Assembly of Au/TiO2 Sphere-Rod Mixtures into Binary Supraparticles

Self-assembly of rod-sphere nanoparticle mixtures enables the design of advanced materials with anisotropic properties. While the phase behavior of rod-sphere mixtures has been explored in 2D and 3D bulk systems, it has remained unexplored in smaller 3D assemblies. Especially, when combining plasmonic and semiconductor nanoparticles, assemblies with defined particle arrangements offer unique optical and (photo)catalytic properties. In this work, we achieved the assembly of binary 3D assemblies tunable in size and phase behavior. Specifically, we assembled semiconductor TiO2 nanorods and plasmonic Au nanospheres under spherical confinement and explored their phase behavior in experiments and simulations. Through evaporation-driven assembly of emulsion droplets, binary supraparticles were prepared with a phase tunable from colloidal membranes, consisting of single rod layers, to binary smectic phases, composed of alternating rod-sphere smectic layers. Colloidal membranes were obtained for a sphere-to-rod number ratio (NAu/NTiO2) of 5, while lowering this ratio to 0.6 resulted in a binary smectic ordering, as confirmed with advanced electron microscopy. Through numerical simulations, we show that the binary smectic phase is stable for 0 < NAu/NTiO2 ≤ 1 in agreement with experiments. Our binary supraparticle platform provides structures with multi-length scale control for applications in photocatalysis, energy storage, and sensing.