Elsevier

Applied Clay Science

Volume 194, 1 September 2020, 105695
Applied Clay Science

Research Paper
Montmorillonite: A structural evolution from bulk through unilaminar nanolayers to nanotubes

https://doi.org/10.1016/j.clay.2020.105695Get rights and content

Highlights

  • Montmorillonite is exfoliated into unilaminar nanolayers.

  • A hydrothermal process is developed for the rolling up of exfoliated nanolayers.

  • Montmorillonite nanotubes bring a new member to the nanotube family.

Abstract

Delicate nanostructures, especially tubular nanoarchitectures, have been verified to exhibit amazing physicochemical properties due to the unique atomic arrangements and are applied to various fields including optics, electronics, etc. Herein, montmorillonite nanotubes were prepared for the first time from the exfoliated unilaminar nanolayer counterparts via a facile hydrothermal process using cation-type surfactant cetyltrimethylammonium bromide (CTAB, C16H33(CH3)3NBr) as the directing reagent, realizing a structural evolution from bulk through unilaminar nanolayers to nanotubes, and bringing a new member to the nanotube family. This work offers a new approach to the structural evolution of lamellar layered silicate minerals into nanotubes for more functionalities.

Introduction

Due to the development of synthetic strategies, to date kinds of nanostructures have been explored, such as zero-dimensional (0D) nanoparticles including quantum dots, one-dimensional (1D) nanoribbons and nanotubes, and two-dimensional (2D) nanosheets, etc. (Pu et al., 2017; Liu et al., 2007; Wan et al., 2018a; Chen et al., 2019a; Lin et al., 2020). Among them, 1D nanotubes have attracted wide interest because of their great potential in magnetic, optical, electronic, and mechanical applications (Hillenbrand et al., 2003; Hong and Myung, 2007; Jiang and Guo, 2011). It has been well unveiled that commonly the nanotubes own layered structures and are mainly prepared through two approaches. (i) The nanotubes are synthesized from the corresponding salt precursors with the assistance of soft templates. The typical cases are hydroxide and WS2 nanotubes (Li et al., 2002; Liu et al., 2010). (ii) There exists a long belief that if proper conditions are given, all the lamellar solids can be transformed into tubular structures. Actually, many layered sheets, such as unilaminar graphene, have been reported rolling up to realize the 2D-to-1D transformation, giving rise to tubular nanoarchitectures (Viculis et al., 2003; Ma et al., 2004). For the dimensional evolution into nanotubes, if the host layer of lamellar sheets satisfies any one of the following features, the rolling up process will easily proceed. (i) The host layer is relatively flexible and/or crystallographically ultrathin. The typical sample is single-layer graphene (~0.34 nm in thickness) (Lee et al., 2008). (ii) Along the stacking direction, the host layer (e.g. titania) is geometrically asymmetric, which causes the force to unbalance and thus the successful curling under proper conditions (Schmidt and Eberl, 2001).

Montmorillonite is a layered silicate clay mineral with a chemical formula of Al2−xMgx(Si4O10)(OH)2·(Nax·nH2O) (Kurahatti et al., 2010; Zhu et al., 2016; Varadwaj et al., 2016). As can be seen from Fig. 1a, its host layer comprises an octahedral metal-oxygen sheet which is confined by two adjacent tetrahedral [SiO4] sheets, forming a sandwich-like structure with a thickness of 0.96 nm, while the intercalated substances are Na+ cations and crystal water molecules to balance the negative charge of host layers and can be further exchanged by other cations, e.g. Mg2+ and Ca2+, or even organic tetra-alkyl ammonium groups (Laudelout et al., 1968; Qu et al., 2008). Due to the unique structure and physicochemical properties, montmorillonite has been widely used in drug delivery and sustained release, support for catalysts and/or electrochemical materials, CO2 capture or as raw materials for silicon anodes, etc. (Salcedo et al., 2012; Zeng et al., 2016; Chen et al., 2019b). For the further fine processing, functionalization, and high-value-added utilizations, montmorillonite materials with peculiar structure and morphology are exceedingly desired. Another analogous layered silicate clay mineral, halloysite (Al2Si2O5(OH)4·nH2O) with a tubular morphology, has been prepared via the rolling up of lamellar kaolinite layers in the presence of detergents or a hydrothermal process using Al(OH)3 and SiO2 as the starting materials (Wan et al., 2018b; Kuroda et al., 2011; White et al., 2012). It is noteworthy that as shown in Fig. 1b, in contrast to that of montmorillonite, the host layer of halloysite (i.e. unilaminar kaolinite nanolayer) comprising a tetrahedral [SiO4] sheet and an octahedral [AlO6] sheet is asymmetric and thinner in thickness (Lvov et al., 2016; Yuan et al., 2013). In other words, the rolling up of lamellar montmorillonite layers is more difficult in theory. It is, therefore, a substantial breakout to roll up such a thick sandwich-like layer into a nanotube, and if successful, a new member will be created to the functional nanotube family.

Stimulated by the abovementioned points, a facile hydrothermal process was developed for the rolling up of montmorillonite nanolayers into nanotubes for the first time using cation-type CTAB surfactant as the directing reagent, wherein the unilaminar nanolayers were obtained through the exfoliation of the bulk counterparts. On the basis of the microstructural observations on montmorillonite nanotubes, a plausible rolling-up mechanism was proposed.

Section snippets

Experimental section

Natural montmorillonite (MgO 5.5 wt%, Al2O3 22.1 wt%, SiO2 66.7 wt% without the consideration of H2O content) was purchased from Zhejiang Fenghong Clay Chemical Co., Ltd. and used directly without any further purification. CTAB was of analytic grade from Sinopharm Chemical Reagent Co., Ltd. (Shanghai).

Results and discussions

Montmorillonite nanotubes were prepared from the nanolayer counterparts via a facile hydrothermal process at an elevated temperature of 120 °C using cation-type CTAB surfactant as the directing reagent, as illustrated by Fig. 2. Here, montmorillonite nanolayers were obtained by the mild exfoliation of natural bulk clay mineral in water under continuous stirring.

The phase structure of natural montmorillonite was examined by XRD. Fig. 3a shows the typical XRD pattern. The strong diffraction at

Conclusion

A structural evolution from bulk through unilaminar nanolayers to nanotubes was achieved on layered montmorillonite. Montmorillonite nanotubes were prepared via the rolling up of unilaminar nanolayer counterparts under a hydrothermal condition at 120 °C using CTAB as the directing reagent, bringing a new member to the nanotube family. Both the successful exfoliation and the subsequent preparation of its nanotubes via the rolling behavior of nanolayer counterparts have revealed the layered

Declaration of Competing Interest

There are no conflicts to declare.

Acknowledgements

H. Wan and A. Yan contributed equally to this work. The authors acknowledge the financial support by National Natural Science Foundation of China (51874357, 51872333), Hunan Provincial Natural Science Foundation of China (2019JJ10006). X. L. acknowledges support from Shenghua Scholar Program of Central South University.

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