Atomic-Scale Insight into the Structure of Metastable γ-Ga<sub>2</sub>O<sub>3</sub> Nanocrystals and their Thermally-Driven Transformation to β-Ga<sub>2</sub>O<sub>3</sub>
Pedro Castro-Fernández, María Valeria Blanco, René Verel, Elena Willinger, Alexey Fedorov, Paula M. Abdala, Christoph R. Müller
Abstract
Metastable γ-Ga2O3 nanocrystals have gained growing interest for a broad range of technological applications. However, a precise description of their atomic structure and changes thereof during thermally induced transformations that is required to understand and fully exploit their physical and chemical properties is still lacking. In this work, we investigate the structure of γ-Ga2O3 nanocrystals (2.5 nm in diameter) obtained via a colloidal synthesis route and their evolution with thermal treatment. To this end, we have applied synchrotron X-ray atomic pair distribution function (PDF) analysis, complemented by 71Ga solid-state magic-angle spinning nuclear magnetic resonance (MAS NMR), X-ray absorption spectroscopy (XAS), and electron microscopy. The local structure of the γ-Ga2O3 nanocrystals deviates from the average cubic spinel-type structure, revealing a high degree of structural disorder. The average structure of the γ-Ga2O3 nanocrystals is described as a defective spinel with gallium sites in tetrahedral and octahedral (GaIV and GaVI) coordination with oxygen atoms. The modeling of the local structure revealed a low-symmetry distortion of the polyhedra, which are disorderly oriented. The surface structure of the γ-Ga2O3 nanocrystals is different from their bulk, whereby GaVI sites at the outermost layers of the nanocrystals are found in a nonperiodical stacking arrangement with a higher occupancy than in the core, as revealed by high-angle annular dark field imaging scanning transmission electron microscopy (HAADF-STEM). The structural evolution of γ-Ga2O3 nanocrystals upon thermal treatment in air was probed by in situ time-resolved PDF. A gradual transformation of the γ-Ga2O3 nanocrystals toward the thermodynamically stable β-Ga2O3 polymorph occurs at different structural domains at different temperatures. At ca. 300 °C, changes in the local structure showed an increased distortion of the polyhedral units and revealed the appearance of small β-Ga2O3 domains (ca. <1 nm), while the bulk phase transformation took place between 550 and 750 °C and was associated with an increase in the coherence length of the β-Ga2O3 phase.