Unlike the synthesis of most metallic nanoparticles, the growth of high purity single-crystal aluminum nanoparticles (nanocrystals) employs a transition metal catalyst, typically titanium(IV) isopropoxide. Recently, monodisperse Al nanocrystals with controlled sizes were synthesized by this approach for sustainable plasmonics, giving rise to applications including selective photocatalysis with antenna-reactor nanostructures. To further tune and enhance the plasmonic response of Al nanocrystals, precise control of nanocrystal shape is desired. However, the underlying mechanisms for the synthesis of Al nanocrystals, particularly the catalytic role of titanium(IV) isopropoxide, remain unknown. Here, we capitalize on reduction of Ti4+ compounds into d1 Ti3+ species, by complexation with AlH3 in coordinating solvents to investigate the mechanism of Al nanocrystal formation using EPR spectroscopy. Results indicate that Ti3+-AlH3 complexes react with excess AlH3 to generate low-valent AlxHy intermediates containing Al2+ radicals. The EPR spectra of the Al2+ intermediates depend on the catalyst and coordinating solvent used to synthesize Al nanocrystals. Using these findings, we propose a mechanism for the observed size-control of Al nanocrystals produced in ethereal solvent mixtures of THF and 1,4-dioxane. The proposed mechanism of nanocrystal formation is corroborated by monitoring the growth of Al nanocrystals by tracking the evolution of their time-dependent plasmonic response using UV-visible spectrophotometry.

Elucidating the mechanism of aluminum nanocrystal formation using EPR spectroscopy

255th National Meeting (2018)

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