Insights into the formation mechanism of imogolite from a full-range observation of its sol-gel growth

Imogolite (Imo) was prepared via a sol-gel method. The time-dependent changes in its morphology, structure and texture during the whole synthesis process (from amorphous precursors to the final products through nanoscale intermediates) were monitored by atomic force microscopy (AFM), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR) and N2 physisorption. The results showed that a shape transition from a spherical open imogolite local structure (ImoLS) to a tubular open ImoLS occurred in the process of proto-imogolite (proto-Imo) formation. Based on the overall structure of the obtained solid products and the occurring main reactions, Imo synthesis process was described in five steps: i) formation of amorphous precursors by hydrolysis and condensation of Al and Si; ii) formation of proto-Imo with an open ImoLS at the expense of the amorphous precursors; iii) open ImoLS dominates; it grows and assembles into the first closed ImoLS (tubes and spheres); iv) closed ImoLS dominates and continues to form (mainly tubes) at the expense of open ImoLS; and v) no more open ImoLS; further growth of already-formed Imo via oriented aggregation. These findings provide new insights into the formation mechanism and structure of proto-Imo and Imo, which helps to clarify the Imo synthesis procedure.