Oriented Crystallization of Ammonium Sulfate from Hexagonal Boron Nitride/Sulfuric Acid Intercalation Compounds

Controlling crystallization is a long-standing issue both from a fundamental and an applied perspective. In particular, our understanding of the influence of confinement on crystallization is far from complete. In this work, we utilize the confined interlayer spaces of hexagonal boron nitride (h-BN), which is one of the typical two-dimensional layered materials with atomically flat BN sheets, not only to prepare h-BN/H₂SO₄ intercalation compounds but also to initiate crystallization via the acid/base reaction between the intercalant (H₂SO₄) and gaseous ammonia. We report that the present acid/base reaction leads to the formation of highly oriented ammonium sulfate crystals along specific crystallographic directions. This oriented crystal growth originates from the 2D crystal nucleated in the confined interlayer spaces of h-BN.