Article pubs.acs.org/JPCC
How the Diameter and Structure of (OH)3Al2O3SixGe1−xOH Imogolite Nanotubes Are Controlled by an Adhesion versus Curvature Competition Antoine Thill,*,† Béatrice Guiose,† Maria Bacia-Verloop,‡,§,∥ Valérie Geertsen,† and Luc Belloni† †
Laboratoire Interdisciplinaire sur l’Organisation Nanométrique et Supramoléculaire, IRAMIS, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France ‡ Institut de Biologie Structurale, UMR 5075, CNRS, 41, rue Jules Horowitz, 38027 Grenoble, France § CEA, DSV, Grenoble, France ∥ Université Joseph Fourier, Grenoble 1, France S Supporting Information *
ABSTRACT: Imogolites are natural aluminosilicate nanotubes displaying an astonishing monodispersity in diameter. The diameter is controlled by the structure and composition of the nanotube wall and can be tuned by several chemical manipulations. It has recently been discovered that the structure of imogolite nanotubes can change from singlewalled (SW) to double-walled (DW) when Si is replaced by Ge during synthesis. Starting from the pure Ge composition, we show that the transition between DW and SW structures can be induced by the incorporation of a small quantity of Si in the synthesis. At that point, the suspension contains a mixture of structures with a nearly constant average diameter. In particular, we found evidence for the presence of a few nanoscrolls. Above 25% Si, SW nanotubes become more stable and present a continuously decreasing diameter with increasing Si. A model is proposed to explain the stability of these different nanotubes and, more generally, the structures of other organic or inorganic nanotubes as a balance between rigidity, surface tension, and adhesion competitive energies.
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structure.10−14 Several research groups have shown that the imogolite strain energy displays a clear minimum at a particular radius.11,13,15−17 The substitution of Ge for Si leads to a larger outer diameter (3.3 nm)2,11,18 and a reduced tube length (around 20 nm instead of several hundred nanometers). The change in diameter is explained by the larger Ge−O equilibrium distance (0.173 instead of 0.159 nm).11 The larger size of the Ge tetrahedra thus induces less stress on the gibbsite layer. In the case of pure-Ge imogolite synthesis, Levard et al. demonstrated that an increase in the reagent concentration at the decimolar level is possible with similar growth kinetics (5 days) for Geimogolites.19 This concentration increase has advanced understanding of nanotube formation and structural control.20−22 In particular, using small-angle X-ray scattering (SAXS) and cryoTEM, it was discovered that, when Ge fully replaces Si, one can obtain not only single-walled (SW) nanotubes but also doublewalled (DW) nanotubes.21 Thill et al.23 demonstrated that the formation of the SW or DW structure can be controlled either by the hydrolysis ratio R = OH/Al or by a modification of the
INTRODUCTION Imogolites [(OH)3Al2O3SiOH] are natural aluminosilicate nanotubes formed in volcanic soils. These natural nanotubes have a diameter of 2 nm and lengths up to micrometers. X-ray diffraction and transmission electron microscopy have shown that the radius of imogolites takes a nearly perfectly fixed value. Synthesis protocols to produce imogolites were developed, and the first synthetic imogolites were prepared in very low concentrations.1,2 After 5 days of growth at 100 °C, a tubular material having a diameter of 2.6−2.8 nm was obtained. Wada et al. showed that long-term synthesis produces an intermediate size between natural and synthetic imogolites.3 Synthetic nanotubes also present an astonishing monodispersity in radius. Control of the radius is explained by the imogolite structure which has been characterized by different techniques such as solid-state nuclear magnetic resonance (NMR) spectroscopy,4,5 infrared (IR) spectroscopy,6 and transmission electron microscopy (TEM).7−9 The structure proposed by Cradwick et al.8 is commonly accepted and used in experimental and theoretical studies of imogolites. It consists of a gibbsite sheet curved by the adsorption of orthosilicate tetrahedra into the vacancies of the aluminum dioctaedral layer. The size monodispersity of these nanotubes is mainly attributed to the stretching of the Al−O−Si distances in the imogolite © 2012 American Chemical Society
Received: October 24, 2012 Revised: November 30, 2012 Published: December 1, 2012 26841
dx.doi.org/10.1021/jp310547k | J. Phys. Chem. C 2012, 116, 26841−26849
The Journal of Physical Chemistry C
Article
Figure 1. SAXS curves of the (OH)3Al2O3SixGe1−xOH samples. For clarity, the SAXS curves are shifted by a factor proportional to x. x values are displayed in the figure.
obtained products were freeze-dried, whereas the remaining suspensions were stored at ambient temperature for further analysis. Small-Angle X-ray Scattering (SAXS). The experimental setup includes a rotating anode and collimating optics providing a monochromatic beam of 1 × 1 mm2 at the sample position with a total incident flux of 100 × 106 photons/s. In these experiments, the samples were exposed to an X-ray beam with an incident wavelength of λ = 0.709 Å. The transmitted flux was measured continuously with a photodiode placed on the beam stop. A Marresearch X-ray-sensitive 345-mm plate detector was placed behind the output window of the vacuum chamber at a distance of 720 mm from the sample. The scattering vector q, defined as the difference between the incident and scattered wave vectors, has a modulus q = 4π/λ sin(θ), where 2θ is the scattering angle. The range of scattering vectors extends from qmin = 0.03 Å−1 to qmax = 3.4 Å−1. The samples were introduced in glass capillaries of 2.8-mm diameter and 50-μm wall thickness, and measurements were performed at room temperature with an exposure time of 3600 s. The signal was corrected for background, and standard procedures were applied to obtain the scattered intensity in cm−1 as a function of the scattering vector q.24 Cryo-TEM. Cryo-TEM experiments were undertaken using an FEI Tecnai G2 Polara device operated at 300 kV. The experiments were performed under extremely low electron illumination conditions (low dose,
