Template-Induced Structuring and Tunable Polymorphism of Three-Dimensionally Ordered Mesoporous (3DOm) Metal Oxides.

Convectively assembled colloidal crystal templates, composed of size-tunable (ca. 15-50 nm) silica (SiO2 ) nanoparticles, enable versatile sacrificial templating of three-dimensionally ordered mesoporous (3DOm) metal oxides (MOx ) at both mesoscopic and microscopic size scales. Specifically, we show for titania (TiO2 ) and zirconia (ZrO2 ) how this approach not only enables the engineering of the mesopore size, pore volume, and surface area but can also be leveraged to tune the crystallite polymorphism of the resulting 3DOm metal oxides. Template-mediated volumetric (i.e., interstitial) effects and interfacial factors are shown to preserve the metastable crystalline polymorphs of each corresponding 3DOm oxide (i.e., anatase TiO2 (A-TiO2 ) and tetragonal ZrO2 (t-ZrO2 )) during high-temperature calcination. Mechanistic investigations suggest that this polymorph stabilization is derived from the combined effects of the template-replica (MOx /SiO2 ) interface and simultaneous interstitial confinement that limit the degree of coarsening during high-temperature calcination of the template-replica composite. The result is the identification of a facile yet versatile templating strategy for realizing 3DOm oxides with (i) surface areas that are more than an order of magnitude larger than untemplated control samples, (ii) pore diameters and volumes that can be tuned across a continuum of size scales, and (iii) selectable polymorphism.