Contrasting thermally-induced structural and microstructural evolution of alumino-silicates with tubular and planar arrangements: Case study of halloysite and kaolinite

Materials with the similar chemical structures but different morphological organization exhibit considerable differences in thermally induced chemo-morphological evolution. With recent advancements in multi-scale X-ray scattering measurements, it is now possible to non-invasively probe the structural and microstructural evolution of such materials. In this study, we investigate the chemo-morphological evolution of halloysite and contrast the results with those for kaolinite. Halloysite and kaolinite have tubular and planar morphologies, respectively. Both materials are alumino-silicates comprised of silica tetrahedral (T) and alumina octahedral (O) sheets organized in a TO structure. Four distinct stages in the structural evolution were identified. Stages I, II, III, and IV correspond to temperature ranges of 25-125 degrees C, 125-400 degrees C, 400-625 degrees C, and 625-875 degrees C. Major structural changes correspond to the removal of interlayer/adsorbed water in stage I, the existence of the halloysite structure without interlayer water in stage II, dehydroxylation of halloysite in stage III, and the conversion of ordered halloysite to amorphous meta-halloysite in stage IV. Additionally, heating halloysite up to 875 degrees C resulted in slight widening of the nanotubes, as the average pore radius increased from 6.4 nm to 6.6 nm. Heating also resulted in an increase of wall thickness of the nanotubes from similar to 120 nm (25 degrees C) to 161 nm (875 degrees C). The increase in the halloysite nanotube diameter was attributed to the expansion of the structure and an increase in the surface roughness. The pore sizes in halloysite nanotubes were also confirmed using N-2 adsorption-desorption and nano-X-ray computed tomography (nano-XCT) measurements. The interlayer basal spacing in halloysite changed from 9.8 angstrom to 7.2 angstrom after the removal of interlayer water. At temperatures in the range of 625-875 degrees C, heating halloysite causes a small widening of nanotube pores and a minor increase in the surface area. In contrast, the interlayer spacing in kaolinite collapses on heating which reduces the nanoscale porosity. These studies demonstrate the differences in the chemo-morphological evolution of alumino-silicates with tubular and planar morphologies.

Automated summary

Materials with similar chemical structures but different morphological organizations exhibit significant differences in thermally induced chemo-morphological evolution. Recent advancements in multi-scale X-ray scattering measurements allow for non-invasive probing of the structural and microstructural evolution of such materials. This study investigates the chemo-morphological evolution of halloysite compared to kaolinite, identifying distinct stages in the structural evolution and showing differences in pore size, wall thickness, and surface area changes between the two materials.