Surface Characterization of Mechanochemically Modified Exfoliated Halloysite Nanoscrolls

Abstract

The surface modifications fundamentally influence the morphology of kaolinite nanostructures as a function of crystallinity and presence of contaminants. Beside morphology, the catalytic properties of 1:1-type exfoliated aluminosilicates are also influenced by the presence of defect sites that can be generated in a controlled manner by mechanochemical activation. In this work, we investigated exfoliated halloysite nanoparticles with quasi-homogeneous, scroll-type secondary structure toward developing structure/function relationships for composition, atomic structure, and morphology. The surface properties of thin-walled nanoscrolls were studied as a function of mechanochemical activation expressed by the duration of dry-grinding. The surface characterizations were carried out by N2, NH3, and CO2 adsorption measurements. The effects of grinding on the nanohalloysite structure were followed by thermoanalytical (TG/DTG) and infrared spectroscopic (FTIR/ATR) techniques. Grinding results in partial dehydroxylation with similar changes as observed for heat treatment above 300ºC. The employed mechanochemical activation shows decrease in dehydroxylation mass loss and the DTG peak temperature, decrease in specific surface area and number of mesopores, increase of surface acidity, blue-shift of surface hydroxide bands, and decrease in intensity in FTIR/ATR bands as a function of grinding time. The experimental observations were used to guide atomic-scale structural and energetic simulations using realistic molecular cluster models for a nanohalloysite particle. A full potential energy surface description was developed for mechanochemical activation and/or heating toward nanometahalloysite formation that aid the interpretation of experimental results. The calculated differences upon dehydroxylation show remarkable agreement with the mass-loss values from DTG measurements.