Morphological Controlled Growth of Nanosized Boehmite with Enhanced Aspect Ratios in an Organic Additive-Free Cationic-Anionic Double Hydrolysis Method

Well-crystallized boehmite nanoparticles showing different sizes and morphologies were fabricated in an organic additive-free cationic-anionic double hydrolysis method using inorganic aluminum chloride salt and sodium aluminate as dual aluminum sources. By adjusting the molar ratios of Al³⁺/AlO₂^- in the synthesis recipe, the boehmite particles' shapes could be controllably tuned from two-dimensional flakes to one-dimensional (1D) rods, needles, and even fibers with enhanced particles' aspect ratios. Through X-ray diffraction and high resolution transmission electron microscopy measurements, details of the microstructural features for boehmite particles were gained, and thus the growth habits are discussed, where in strong alkaline synthesis medium with a low Al³⁺/AlO₂^- molar ratio, dispersed nanoflakes grew with (010) and (101) faces as basal and lateral surfaces, while 1D nanoparticles, i.e. nanorods, nanoneedles, and nanofibers preferentially grew along the [100] direction with (100) and (101) faces unexposed when Al³⁺/AlO₂^- molar ratios were gradually raised. Additionally, the impacts of pH values and Cl^- ions in the suspensions to the particles shapes are also discussed. The evolution of boehmite morphologies and the resultant enlargement of aspect ratios led to increased total surface charges (-potential) and higher isoelectric points of boehmite samples which would benefit the stabilization of particles' suspensions and improvement of surface functionalization abilities.