Categorizing Different Inorganic Compounds Forming Nanotubular Structures

The driving force for the growth of inorganic nanotubes has been briefly mentioned in the previous section. Layered (2D) compounds are known to have fully satisfied chemical bonds on their van der Waals (basal) planes and consequently their (0001) surfaces are generally very inert. In contrast, the atoms on the prismatic (1010) and (1120) faces are not fully bonded and they are therefore chemically very reactive. When nanoclusters of a 2D compound are formed, the prismatic edges are decorated by atoms with dangling bonds, which store enough chemical energy to destabilize the planar structure. One way to saturate these dangling bonds is through a reaction with the environment, e.g., reaction with ambient water or oxygen molecules. However, in the absence of reactive chemical species, an alternative mechanism for the annihilation of the peripheral dangling bonds may be provoked, leading to the formation of hollow closed nanoclusters. For this process to take place, sufficient thermal energy is required in order to overcome the activation barrier associated with the bending of the layers (elastic strain energy). In this case, completely seamless and stable hollow nanoparticles are obtained in the form of either polyhedral structures or elongated nanotubes.

However, this is not the sole mechanism which can lead to the formation of nanotubular or microtubular structures. One mechanism, which was already proposed by Pauling [14], involves 2D compounds with a non-symmetric unit cell along the c-axis, like that of kaolinite. The structure of this compound is made by the stacking of layers consisting of SiO2 tetrahedra and AlO6 octahedra, the latter having a larger b parameter. To compensate for this geometric mismatch, hollow whiskers are formed, in which the AlO6 octahedra are on the outer perimeter and the SiO2 tetrahedra are in the inner perimeter of the layer. In this geometry, all the chemical bonds are satisfied with relatively little strain. Consequently, the chemical and structural integrity of the compound is maintained.

Nanotubes and microtubes of a semi-crystalline nature can be formed by almost any compound, using a template growth mechanism. Amphiphilic molecules with a hydrophilic head group, like carboxylate or an -OH group, and a hydrophobic carbon-based chain are known to form very complex phase diagrams, when these molecules are mixed with, e.g., water and an aprotic (non-aqueous) solvent [15]. Structures with a tubular shape are typical for at least one of the phases in this diagram. This mode of packing can be exploited for the templated growth of inorganic nanotubes, by chemically attaching a metal atom to the hydrophilic part of the molecule. Once the tubular phase has been formed, the template for the tubular structure can be removed, e.g., by calcination. In this way, stable metal-oxide nanotubes, can be obtained from various oxide precursors [16]. Nonetheless, the crystallinity of these phases is far from being perfect, which is clearly reflected by their X-Ray Diffraction (XRD) and electron diffraction patterns. Thus, whereas a net of sharp diffraction spots is observed in the electron diffraction patterns of nanotubes from 2D (layered) compounds, the electron diffraction of nanotubes from 3D (isotropic) materials, appears as a set of diffuse diffraction rings, which alludes to their imperfect crystallinity. Also, the sharpness of the diffraction pattern in the latter case may vary from point to point, alluding to the variation in crystallinity of the different domains on the nanotube.

Interestingly, the stability of inorganic nanotubes and nanoparticles can be quite high. Figure 3 shows theoretical results for the strain energy needed to form a nanotube of a given diameter for both BN nanotubes and C nanotubes [17]. The closed circles represent the strain energy needed to form a BN tube relative to a sheet of hexagonal BN, while the open circles indicate the energy of a carbon nanotube relative to graphite. Clearly both organic and inorganic nanotubes are high energy metastable structures, but compared to their respective sheet materials, BN nanotubes are energetically even more favorable than carbon nanotubes.

Fig. 3. Strain energy versus diameter for the formation of BN and carbon nanotubes relative to their sheet structures. Closed and open circles indicate the energy for BN and carbon nanotubes, respectively. (Courtesy of X. Blase) [17]

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