Single-micelle-templated synthesis of hollow silica nanospheres with tunable pore structures

Hollow silica nanospheres (HSNs) were synthesized by using swollen micelles of Pluronic F108 (EO132PO50EO132) block copolymer surfactant as soft templates at low silica-precursor/ surfactant ratios. An unprecedented tunability of the pore size was achieved in this single-micelle-templating synthesis through the initial synthesis temperature control in the F108/ toluene system, for which the pore diameter gradually increased from 16 to 44 nm upon decreasing the synthesis temperature from 25 to 14 degrees C. An additional pore size adjustment coupled with the change in the size of the entrances to the nanospheres can be achieved by selecting the hydrothermal treatment temperature, as shown for the synthesis at 25 degrees C. The inner diameter of the hollow nanospheres can also be modified by changing the silica alkoxide precursor used or its ratio to the surfactant. Moreover, the formation of hollow nanospheres, their inner void size and the temperature adjustability depend on the swelling agent used. In the cases of xylene and ethylbenzene swelling agents, the pore size adjustment was complicated by the morphology change from the loose aggregates of hollow spheres to the consolidated particles with multiple mesopores as the temperature was lowered. On the other hand, these swelling agents afforded nanospheres of particularly large diameter (22-26 nm) at 25 degrees C. Less potent swelling agents (1,3,5-triisopropylbenzene and cyclohexane) afforded largely disordered consolidated structures instead of single-micelle-templated nanospheres. The work demonstrated the potential of Pluronic F108 combined with an appropriate swelling agent to template silica nanospheres with hollow interiors of size and accessibility adjustable over a wide range through simple temperature control.