Characterization studies of heat-treated halloysite nanotubes

The halloysite nanotubes (HNTs), subjected to different temperatures in the range 250-1000 degrees C, were investigated at room temperature. The materials have been prepared in various times (2, 4, 6 h) at different temperatures (250, 450, 600, and 1000 degrees C) and heating rates (5, 10, 20 degrees C/min). Morphology, structural and physical properties of raw, and thermally treated HNTs were systematically analyzed by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), the nitrogen adsorption-desorption isotherm (BET), as well as Fourier transform infrared (FT-IR) spectroscopy. The XRD patterns indicate that the crystal structure of halloysite is stable up to 450 degrees C. The scanning electron micrograph images of the Halloysite nanotubes showed that the unheated and heated halloysite nanotubes possessed typical tubular shapes. Tubular shapes were observed even at 1000 degrees C. Fourier transform infrared spectroscopy was also used to examine the variations of chemical structures. The Fourier transform infrared spectra of unheated and thermally treated halloysite nanotubes showed the presence of hydroxyl stretching and bending vibration frequencies in the samples. In this study, the structural properties of natural halloysite nanotubes are comprehensively reviewed in terms of given temperature, heating rate, and time. It is thought that the obtained results will guide the design and preparation of halloysite-based new structural and functional materials which can be applicable in various fields.