Fabrication of controllable ultrathin hollow shells by layer-by-layer assembly of exfoliated titania nanosheets on polymer templates

Core-shell composites and titania hollow shells have been fabricated via the layer-by-layer self-assembly approach using exfoliated unilamellar titania nanosheets Ti1-upsilonO24upsilon- (delta similar to 0.09) as inorganic shell building blocks. Polystyrene and poly(methyl methacrylate) spheres with different diameters were employed as colloidal templates, and polyethylenimine was utilized to modify the surface charge of polymers. The ultrathin nature (0.75 nm) of two-dimensional nanosheets enabled tailoring the shell thickness in a nanometer-scale range by varying the coating cycles. SEM revealed the evolution of shell structures on templating cores. An X-ray diffraction peak with a periodicity of ca. 1.6 nm and its enhancement with increasing coating cycles indicated the regular growth of the shell of nanostructured Ti1-upsilonO24upsilon--/PEI repeating pairs. Calcination at 500 degreesC or UV-irradiation of the core-shell composites removed the polymer cores, yielding titania hollow shells. They are characterized by their smooth curvature and extremely small thickness (as small as 5 nm). The titania hollow shells exhibited various optical properties, depending on the treatments.