Spontaneous Scrolling of Kaolinite Nanosheets into Halloysite Nanotubes in an Aqueous Suspension in the Presence of GeO2

Despite the natural occurrence of aluminosilicate nanotubes as mineral halloysite, suitable hydrothermal methods for their preparation are still undeveloped due to their lower thermodynamic stability under hydrothermal conditions. The effect of addition of GeO 2 to the reaction mixture of SiO 2 and Al(OH) 3 on the distribution of the morphologies of nanostructured aluminosilicates obtained under hydrothermal treatment at pH 2 and 220 °C has been systematically studied. It was found that the formation of multilayered nanotubes of halloysite (Al 2 Si 2 O 5 (OH) 4 ) occurs at 0.2 Ge/(Si + Ge) molar fraction after 7 days of hydrothermal treatment. The as-obtained halloysite nanotubes are characterized by a high degree of crystallinity, a multilayered (25–40 layers) wall structure, and a typical inner diameter of 5–15 nm. A further increase in Ge/(Si + Ge) molar fraction to 0.5 results in precipitation of single-walled nanotubes analogous to germanium-containing imogolite (Al 2 Si x Ge 1– x O 3 (OH) 4, x ≈ 0.65) characterized with an average inner diameter of 0.9–1.2 nm. At a lower molar ratio (0.05, 0.1), partially curved, multilayer nanosheets of kaolinite are produced. The complete analysis of structure and morphology of all products was performed using TEM, EDX, SAED, FTIR, and nitrogen adsorption. The mechanism governing the precipitation of Al 2 Si 2 O 5 (OH) 4 nanotubes in the presence of Ge(IV) is discussed.