Group 13 Lewis acid catalyzed synthesis of metal oxide nanocrystals via hydroxide transmetallation

Abstract

A new transmetallation approach is described for the synthesis of metal oxide nanocrystals (NCs). Typically, the synthesis of metal oxide NCs in oleyl alcohol is driven by metal-based esterification catalysis with oleic acid to produce oleyl oleate ester and M-OH monomers, which then condense to form M_xO_y solids. Here we show that the synthesis of Cu₂O NCs by this method is limited by the catalytic ability of copper to drive esterification and thus produce Cu⁺-OH monomers. However, inclusion of 1–15 mol% of a group 13 cation (Al³⁺, Ga³⁺, or In³⁺) results in efficient synthesis of Cu₂O NCs and exhibits size/morphology control based on the nature of M³⁺. Using a continuous-injection procedure where the copper precursor (Cu²⁺-oleate) and catalyst (M³⁺-oleate) are injected into oleyl alcohol at a controlled rate, we are able to monitor the reactivity of the precursor and M³⁺ catalyst using UV-visible and FTIR absorbance spectroscopies. These time-dependent measurements clearly show that M³⁺ catalysts drive esterification to produce M³⁺-OH species, which then undergo transmetallation of hydroxide ligands to generate Cu⁺-OH monomers required for Cu₂O condensation. Ga³⁺ is found to be the “goldilocks” catalyst, producing NCs with the smallest size and a distinct cubic morphology not observed for any other group 13 metal. This is believed to be due to rapid transmetallation kinetics between Ga³⁺-OH and Cu⁺-oleate. These studies introduce a new mechanism for the synthesis of metal oxides where inherent catalysis by the parent metal (i.e. copper) can be circumvented with the use of a secondary catalyst to generate hydroxide ligands.