The effective utilization of conventional non-metallic minerals and the advancement of high-performance functional mineral materials are pivotal for the transformation and upgrading of non-metallic mineral industry. Halloysite nanotubes (HNT) as natural silico-aluminate minerals with a unique tubular structure have exceptional biocompatibility, robust adsorption capabilities, and distinct surface properties. These features make them ideal for a wide range of applications, including drug delivery, adsorption, catalysis and fire resistance. The hollow tubular structure of HNT has attracted interests, as its lumen of HNT can encapsulate a variety of functional substances, imparting diverse properties and functionalities to composite materials. Moreover, the differential properties between their inner and outer surfaces have ample opportunities for targeted modifications in subsequent applications. However, raw HNT suffers from several drawbacks, such as small specific surface area, uneven size distribution, and susceptibility to surface hydrogen bonding leading to aggregation. To expand their application scope and enhance performance across diverse fields, the structural and surface modifications of HNT are emerged as critical research areas.

Some methods such as calcination, acid/alkaline etching, and ultrasound are reported to modulate the tubular structure of HNT. HNT undergoes the phase and crystal structural transformations during calcination. The dehydroxylation and phase segregation of HNT occur at >450 ℃, leading to the formation of amorphous SiO₂ and Al₂O₃. The temperature of >1 000 ℃ causes the disruption of tubular structure, while HNT undergoes a transformation into mullite at 1 200 ℃. Acid/alkaline etching modifies the tubular diameter, surface morphology, and chemical composition of HNT via etching aluminum/silicon on the tube walls. Acids preferentially attack the aluminum octahedral structures within HNT, causing aluminum ions to leach out and form amorphous silica spheres in-situ. Conversely, alkaline preferentially attacks the silicon tetrahedral structures, leading to preferential leaching of silicon and formation of aluminum hydroxide nanosheets. Some previous studies indicated that leaching HNT in phase-segregated state could allow for more thorough removal of aluminum or silicon from the HNT structure. Moreover, HNT with specific length ranges can be obtained via combining ultrasound fragmentation with single viscosity centrifugation processes.

Selective surface modifications on the inner, outer, and interlayer surfaces of HNT facilitate the design of functional surfaces. Silanization as a common method for modifying both the inner and outer surfaces of HNT involves the condensation of silane coupling agents with the hydroxyl groups on the HNT surface. Grafting silane coupling agents with different structures enables the specific anchoring of functional groups on the HNT surface. Electrostatic assembly utilizes the surface charge of HNT and the electrostatic attraction between materials carrying opposite charges to load desired substances onto HNT surfaces, thereby enabling the introduction of various functional substances. Intercalation methods allow for the incorporation of organic or inorganic compounds into the interlayer spaces of HNTs without disrupting their interlayer structure, thus expanding interlayer spacing or loading functional substances onto interlayer surfaces.

HNT exhibits a significant potential across various domains due to its unique structure and diverse chemical properties. The lumen and excellent biocompatibility make it an effective carrier for drugs and controlled release systems, improving drug stability and availability in biomedical applications. The high specific surface area and dual-surface nature of HNT contribute to exceptional adsorption properties, making it suitable for environmental applications such as water treatment, wastewater management, and heavy metal removal. Surface modifications or doping with transition metals enable HNT to act as catalysts, enhancing chemical reactions and controlling selectivity. With remarkable mechanical, thermal, and chemical characteristics, coupled with non-toxicity and high biocompatibility, HNT serves as nanofillers in food packaging to bolster mechanical strength, thermal stability, and extends the shelf life of perishable goods. Moreover, the heat resistance and flame-retardant properties also render it valuable in the development of high-performance fireproof coatings and composite materials. HNT has a promising potential for applications in emerging materials, energy, environmental protection, pharmaceuticals, and beyond.