Tailoring structure and surface chemistry of hollow allophane nanospheres for optimization of aggregation by facile methyl modification

Allophane, an earth-abundant and easy-to-be-synthesized hollow nanospherical material, readily loses its unique pore structure via irreversible aggregation of particles upon drying, which mainly results from capillary stress in the unsaturated inner cavity. To tackle this problem, we develop a strategy for tailoring the capillary stress and thus the aggregation state of allophane by introducing methyl moieties onto the inner surface during preparation. Combined spectroscopic results verified the formation of methyl-allophane with methyl groups only on its inner surface. The presence of a reflection at approximately 33 angstrom in the X-ray diffraction pattern, ascribed to the interference between particles, indicated an increased structural order in methyl-allophane. The thermal analysis data revealed a decrease of the inner-surface hydrophilcity. The Brunauer-Emmett-Teller (BET) specific surface area increased from 269 to 523 m(2)/g after methyl modification. An aggregation model, in contrast with that of allophane, was proposed based on the microscopic and small-angle X-ray scattering results to explain these observed changes. This work exhibited that substitution of silanol by methyl on the inner surface of allophane leads to improvement of structural order by eliminating the presence of oligomeric silicates and decreases the hydrophilicity, resulting in the reduction of the capillary stress in the inner cavity and thus the inhibition of irreversible aggregation of particles during drying. The insight into the mechanisms underneath the above mentioned changes upon methyl modification unraveled in this work is helpful for addressing the common aggregation issue of other nanomaterials.