Characterization of Surface-Modified Halloysite Nanotubes by Thermal Treatment Under Reducing Atmosphere

In recent years, the halloysite (Al₂Si₂O₅(OH)₄ · 2H₂O) has been highlighted owing to its naturally occurring one-dimensionalmicrostructure that enables versatile applications. Due to the demand for enhancing surface interaction, several types of research such as acid/base treatments have been conducted on the halloysite nanotubes. The objective of this study is to investigate the structural and surface properties of thermally treated halloysites under reducing atmosphere. The heat treatment is carried out in a gas-flow furnace at 400-800 °C under various atmosphere, e.g., ambient air, 4% H₂-balanced Ar, and 99.99% H₂. The thermal treatment of halloysites under reducing atmosphere show a similar phase transition around 500 °C as the heating under air. However, the halloysite reduced in pure hydrogen shows a significant increase of the zeta-potential, -36.7 mV for a 600 °C-treated sample, compared to the other samples. The mechanisms of the zeta-potential increase for the halloysite was also explored.