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