Impact of thermal treatment on halloysite nanotubes: A combined experimental-computational approach

Abstract

Halloysite nanotubes (HNTs) are naturally occurring aluminosilicate minerals, known for their unique tubular structure, which have garnered significant interest for a wide range of applications. This study explores the morphological changes of HNTs when subjected to thermal treatment ranging from 25 °C to 1100 °C using a combination of experimental characterization techniques and molecular dynamics simulations. Techniques such as solid-state NMR (SSNMR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller (BET) surface area measurements, and Fourier Transform Infrared Spectroscopy (FT-IR) were employed to analyse the structural evolution. The results reveal two major transitions: the first occurring between 400 and 500 °C, corresponding to the release of intercalated water and partial distortion of the HNT structure, and the second occurring between 900 and 1000 °C, marked by the collapse of the tubular structure and the exposure of alumina on the surface. These findings provide significant insights into the thermal stability of HNTs, informing future applications, especially in high-temperature environments.

Keywords: 27Al NMR; 29Si NMR; Calcination; halloysite; molecular dynamics.

Graphical abstract

Fig. 1

XRD patterns of the calcined HNT samples at different temperatures.

Fig. 2

FT-IR spectra of the six HNT samples corresponding to the temperatures of the most relevant transitions, stretching vibration of Al-OH (red), O–Si–O and Al–O–OH (yellow), apical Si–O (purple), and Al–O–S (blue). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 3

TEM images of the five representative HNT samples at RT, 500, 600, 800, and 1100 °C.

Fig. 4

The evolution of Al2p and Si2p as a function of temperature.

Fig. 5

The atomic ratio of Aluminum (a), Silicon (b), Oxygen (c) and the Al/Si (d), respectively.

Fig. 6

Solid-state ²⁹Si NMR spectra at 10 kHz MAS rate of the calcined HNT samples at various temperatures.

Fig. 7

View of the HNT structures from the cylindrical axis at different temperatures, at 2.5 ps (top panels) and 11 ps (lower panels). Atom color code: Al (cyan), Si (yellow), O(red), and H (white). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 8

Panel (a) and (b), zoom of HNT structures from Fig. 7 at 11 ps at 500 °C and 800 °C, respectively. Panel c) a zoom on the structure showing the coordination of Al with 4 hydroxyl group and two -O-Si-. Atom color code Al (cyan), Si (yellow), O(red), and H (white). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 9

The root mean square displacement for the Si and Al atom coordinates from the initial conformation as a function of time in the HNT structures at different temperatures.