Coupled effects of pH and directing agents on the anisotropic growth of boehmite nanocrystals

Boehmite nanocrystals (BNCs) with high aspect ratios are important building blocks for porous membranes, aerogels, and other functional materials. However, the factors governing the anisotropic growth of BNCs during hydrothermal synthesis remain insufficiently understood, particularly the interplay between solution acidity and directing agents (DAs). In this work, the morphology evolution of BNCs was systematically investigated by tuning the pH value and the type of DAs during hydrothermal synthesis. The results reveal that the aspect ratio of BNCs is strongly influenced by the combined effects of pH and DAs, indicating a pronounced pH–DAs coupling effect in the anisotropic growth process. To clarify the underlying mechanism, a first-principles-based computational model was developed to describe the adsorption of ionic species on crystal facets and the resulting growth rate differences. The calculated anisotropic growth ratios agree well with the experimentally measured aspect ratios of BNCs. This study provides mechanistic insight into the pH–DAs coupling effect and offers theoretical guidance for the controllable synthesis of one-dimensional oxide nanomaterials.