Ligand-Dependent Colloidal Stability Controls the Growth of Aluminum Nanocrystals

The precise size- and shape-controlled synthesis of monodisperse Al nanocrystals remains an open challenge, limiting their utility for numerous applications that would take advantage of their size and shape-dependent optical properties. Here we pursue a molecular-level understanding of the formation of Al nanocrystals by titanium(IV) isopropoxide-catalyzed decomposition of AlH3 in Lewis base solvents. As determined by electron paramagnetic resonance spectroscopy of intermediates, the reaction begins with the formation of Ti3+-AlH3 complexes. Proton nuclear magnetic resonance spectroscopy indicates isopropoxy ligands are removed from Ti by Al, producing aluminum(III) isopropoxide and low-valent Ti3+ catalysts. These Ti3+ species catalyze elimination of H2 from AlH3 inducing the polymerization of AlH3 into colloidally unstable low-valent aluminum hydride clusters. These clusters coalesce and grow while expelling H2 to form colloidally stable Al nanocrystals. The colloidal stability of the Al nanocrystals an...