Group 13 Precursor Structures and Their Effect on Oxide Nanocrystal Formation

Precursor structures (PSs) in solution are expected to influence both nanocrystal formation mechanisms, as well as crystallization of specific polymorphs. Herein, Group 13 PS structures determined by pair distribution function and extended X-ray absorption fine structure analysis are reported. Corner-sharing octahedral dimers form from the metal nitrates dissolved in either water, isopropanol, or ethanol at room temperature contradicting previous studies that suggested monomers or larger Keggin clusters. Because all crystalline indium oxides have octahedral coordination, crystals can easily nucleate from the observed PSs. Similarly, MOOH (M=Al and Ga) with octahedral M coordination is expected to form readily from the PSs, whereas formation of gamma-M2O3 requires a partial conversion to tetrahedral M coordination. This explains the long-standing observation of initial AlOOH formation as a bottleneck for gamma-Al2O3 synthesis. Different indium polymorphs crystallize from the various solvents, and thus there is no obvious link between the PSs and observed polymorphism.