A Dual Catalyst Strategy for Controlling Aluminum Nanocrystal Growth

Abstract

The synthesis of Al nanocrystals (Al NCs) is a rapidly expanding field, but there are few strategies for size and morphology control. Here we introduce a dual catalyst approach for the synthesis of Al NCs to control both NC size and shape. By using one catalyst that nucleates growth more rapidly than a second catalyst whose ligands affect NC morphology during growth, one can obtain both size and shape control of the resulting Al NCs. The combination of the two catalysts (1) titanium isopropoxide (TIP), for rapid nucleation, and (2) Tebbe’s reagent, for specific facet-promoting growth, yields {100}-faceted Al NCs with tunable diameters between 35 and 65 nm. This dual-catalyst strategy could dramatically expand the possible outcomes for Al NC growth, opening the door to new controlled morphologies and a deeper understanding of earth-abundant plasmonic nanocrystal synthesis.

Plate-Like Colloidal Metal Nanoparticles

“…However, similarly to other highly reductive materials, the wet-chemical synthesis of well-defined anisotropic structures has proven to be a difficult challenge. Several research groups have made significant progress, managing to tune the syntheses of spherical, cubic, bipyramidal, and even rod-shaped particles. − In all cases, the key and unique feature of Al nanocrystal growth is the formation of a 2–3 nm oxide layer surrounding the nanoparticles. − For example, the selective oxidation of different crystallographic facets in growing Al nanocrystals was exploited by Luo et al for the preparation of sub-2 nm Al nanosheets, which remains the only report to date . The authors injected a mixture of oxygen and nitrogen gas in the growth solution to regulate the concentration of oxygen during the reduction of AlCl 3 precursor by LiAlH 4 at 140 °C.…”

Reduced-Dimensionality Al Nanocrystals: Nanowires, Nanobars, and Nanomoustaches

“…Al nanowires, nanobars, and nanomoustaches were grown under Ar atmosphere in a glovebox using a vanadium complex, Cp 2 VBr, which decomposed alane catalytically at a slower rate than reported for Al nanocrystal growth using Ti-based catalysts (Figure a). , The catalytic behavior of Cp 2 VBr was observed through electron paramagnetic resonance (EPR) spectroscopy, and the V 3+ center lost all EPR signal upon addition of AlH 3 (Figure S1). The observation of V 3+ X-band EPR signals in Cp 2 VBr suggests very small zero-field splitting between the two 3d electrons, and the elimination of the signals upon its reaction with AlH 3 indicates that a strong electronic bond is formed between the two molecules.…”

“…The slower growth rate was also observed through inductively coupled plasma mass spectrometry (ICP-MS), where the quantification of atomic Al from time-stopped experiments demonstrated a greatly reduced rate of Al particle formation (Figure S2). Previously studied Al NC syntheses incorporating a Ti-centered catalyst ,, typically completed within hours, while the V-catalyzed Al nanowire reaction typically occurred over nominally 24 h.…”

Facet Tunability of Aluminum Nanocrystals

“…27 When Tebbe's reagent was introduced as the catalyst, it resulted in the synthesis of singlecrystal {100} Al nanocubes of highly uniform size. 35,36 Clearly the choice of catalyst had a profound effect on the resulting nanocrystal morphology; however, its precise role has not yet been understood.…”

“…The initial demonstration of this approach utilized the catalyst titanium­(IV) isopropoxide and produced primarily cuboctahedral and other thermodynamically favorable morphologies . When Tebbe’s reagent was introduced as the catalyst, it resulted in the synthesis of single-crystal {100} Al nanocubes of highly uniform size. , Clearly the choice of catalyst had a profound effect on the resulting nanocrystal morphology; however, its precise role has not yet been understood.…”