| Summary: | Gamma alumina (γ-Al<sub>2</sub>O<sub>3</sub>) is widely used as a catalyst and catalytic support due to its high specific surface area and porosity. However, synthesis of γ-Al<sub>2</sub>O<sub>3</sub> nanocrystals is often a complicated process requiring high temperatures or additional post-synthetic steps. Here, we report a single-step synthesis of size-controlled and monodisperse, facetted γ-Al<sub>2</sub>O<sub>3</sub> nanocrystals in an inductively coupled nonthermal plasma reactor using trimethylaluminum and oxygen as precursors. Under optimized conditions, we observed phase-pure, cuboctahedral γ-Al<sub>2</sub>O<sub>3</sub> nanocrystals with defined surface facets. Nuclear magnetic resonance studies revealed that nanocrystal surfaces are populated with AlO<sub>6</sub>, AlO<sub>5</sub> and AlO<sub>4</sub> units with clusters of hydroxyl groups. Nanocrystal size tuning was achieved by varying the total reactor pressure yielding particles as small as 3.5 nm, below the predicted thermodynamic stability limit for γ-Al<sub>2</sub>O<sub>3</sub>.
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