Microstructure and properties of halloysite nanotubes and modification methods: A comprehensive review

Halloysite nanotubes (HNT) have received widespread interest due to their unique microstructure and properties. They have been widely used as nano-containers or supporting materials to prepare advanced functional materials in many fields, spanning from life science and medical uses to energy and environmental applications. It is well-known that the performance of HNT is highly influenced by their surface area and pore volume, depending on the detailed structure properties, such as lumen space, wall thickness and length of HNT. Particularly, the loading ability of HNT is related to the detailed microstructures, such as morphology parameters (the inner/outer diameter and length), wall structure, active sites, interlayer space, as well as surface modification by organic groups and inorganic nanoparticles. As a special one-dimension nanoporous clay mineral with tubular morphology, the fundamental knowledge on the microstructure of HNT is of key importance to their specific applications. This review summarized several key factors related to the microstructures of halloysite, such as the morphology and origin of HNT, the coexisting impurities and the dispersion degrees of HNT, then rationalize the relations between the microstructures and physicochemical properties of halloysite, followed by the recent progress on microstructures evolution of HNT under various modification methods such as calcination, acid/alkali treatment, pseudomorphic transformations, and surface modifications as well as functional loading, physical expansion and intercalation.