Hybrid Titania Microspheres of Novel Superstructures Templated by Block Copolymers

Abstract

Two new titania-based nanostructures—namely, hexagonally packed titania hoops (HTHs) and modified large compound vesicles (LCVs)—were created in polystyrene-b-poly(ethylene oxide) (PS-b-PEO)/titania microspheres by coupling self-assembly of the asymmetric amphiphilic block copolymers in selective solvents with the sol–gel process of titania. The formation of these nanostructures was induced by solvent evaporation, where the evaporation rate difference between tetrahydrofuran and water, under certain ambient humidity, strongly affects the nanostructure resulted by changing the force balance, principally involving the stretching of the PS blocks, the surface tension between the PS and the surrounding solvents, and the repulsive interactions among the PEO chains. When the relative humidity of the evaporation process was controlled at 67%, a modified LCVs structure was formed involving a morphology evolution initially from lamellae to large polydisperse vesicles and then to modified LCVs. As the relative humidity decreased from 67% to 30%, the modified LCVs transferred to the HTHs structure, which is ascribed to the reduction of the effective volume fraction of the PEO blocks associated with the decrease in humidity. Stable anatase microspheres of either HTHs or modified LCVs morphology were obtained upon calcination at 500 °C.