Morphological features of halloysite nanotubes (HNTs) as revealed by various microscopies

Nine halloysites (HNTs) have been examined by SEM, AFM and (cross sectional) TEM to evaluate details of their external and internal morphologies. The samples span morphologies within the cylindrical to prismatic-polygonal framework proposed by Hillier et al., (2016). The ‘carpet role’ model assumed in the conceptualization of most technological applications of HNTs is shown to be far too simplistic. Both cylindrical and prismatic forms have abundant edge steps traversing their surfaces which by analogy with plates of kaolinite correspond to prism faces. The mean value for the diameter of the central lumen of the tubes is 12 nm. Numerous slit-like nano pores, with diameters up to 18 nm also occur between packets of layers, particularly in prismatic forms at the junction between a central cylindrical core and outer packets of planar layers. These pores expose aluminol and silanol surfaces, but unlike the lumen, which is assumed only to expose an aluminol surface, they do not extend along the entire length of the nanotube. Edge steps seen most clearly by AFM correspond in height to the packets of layers seen in TEM. TEM cross sections suggest that tube growth occurs by accretion of a spiralled thickening wedge of layers evolving from cylindrical to polygonal form and reveal that planar sectors may be joined by either abrupt angular junctions or by short sections of curved layers. A more realistic model of the internal and external morphologies of HNTs is proposed to assist with understanding of the behaviour of HNTs in technological applications.