Spontaneous scrolling of Al2Si2O5(OH)4 nanosheets into halloysite nanotubes stimulated by structural doping with GeO2

Al₂Si₂O₅(OH)₄ (halloysite) multiwalled nanotubes (MWNTs) were synthesized for the first time using a simple hydrothermal preparation technique involving the reaction SiO₂ + Al(OH)₃ in the presence of 0.1-0.2 mole fraction GeO₂ substituted for SiO2. The inclusion of small amounts of Ge(IV) into the Al₂Si₂O₅(OH)₄ structure together with the creation of Si(IV) vacancies stimulates curving of layered nanosheets, such that they scroll to form spheroids, and finally tubes. At 0.5 GeO₂ substitution, small single-walled imogolite-type nanotubes (SWNTs) with the Al₂Si_xGe_1-xO₃(OH)₄ structure are formed instead alongside x-ray amorphous particles. Thus the substitution of GeO₂ for SiO₂ provides a direct method to control the morphology of nanostructured aluminosilicates.