Controlling Cubic versus Octahedral Morphology in Plasmonic Aluminum Nanoparticle Synthesis with Titanocene Catalysts: A Systematic Study

Ti precatalysts containing the titanocene moiety (Cp₂Ti-, Cp = cyclopentadienyl) can, under certain conditions, selectively produce cubic Al nanocrystals through the dehydrocoupling of alane amine adducts such as AlH₃·NMe₂Et. A systematic study of reaction conditions reveals that cubic Al nanoparticle formation occurs at a higher temperature (e.g., 65 °C) and/or higher catalyst-concentration conditions (e.g., 0.5 mol % [Ti]). Kinetic studies reveal that under these conditions nanoparticle formation and alane consumption are much faster, and cubic nanoparticle formation takes place under kinetically controlled conditions. On the other hand, employing a wide suite of TiX₄ (X = anionic ligand)-type precatalysts yielded only octahedral-shaped aluminum nanoparticles regardless of conditions. Finally, we report the synthesis of a hydride-bridged Ti-Al heterobimetallic compound from the reaction of Cp₂TiCl₂ with AlH₃·NMe₂Et and characterized it to show that it is a reaction intermediate in the Ti-catalyzed aluminum nanoparticle synthesis.