Crystallization via Oriented Attachment of Nanoclusters with Short-Range Order in Solution

Many mineral crystallization processes in aqueous solutions involve formation of nanoclusters with short-range order. Their transformation into crystalline products is not well understood. Here we investigate the formation of long-range crystalline order within networks of cobalt-based nanoclusters. High-resolution cryogenic transmission electron microscopy (cryoTEM) together with NMR and FTIR spectroscopies shows the formation of ∼0.8 nm sized (Co)(NH3)5CO3 complexes at the initial stage. By ligand exchange, those complexes become bridged by CO32-/OH- ligands and form ∼2 nm sized clusters, which subsequently aggregate into sheetlike networks due to the structural heterogeneity of the clusters. By further ligand change and adjustment in cluster orientations, long-range order is established, which leads to the nucleation of ammonium cobalt kambaldaite nanocrystals. Our observations demonstrate that nanoclusters with short-range order can form crystals via an oriented-attachment pathway, which provides new insights into multistep crystallization processes.