Hierarchically-ordered nanorods of Ga₂O₃ derived from microwave-assisted hydrothermal approach: Investigation of calcination-induced structural evolution and optical behavior

Hierarchically-ordered nanorods of gallium oxide (Ga₂O₃) were synthesized via a simple microwave-assisted hydrothermal method, and their structural, morphological, and optical properties were investigated in detail. The as-synthesized powders crystallized in orthorhombic gallium oxyhydroxide α-GaOOH phase as confirmed by X-ray diffraction (XRD). Through calcining at 406 ℃ and 597 ℃, the α-GaOOH powders were transformed into rhombohedral α-Ga₂O₃ and monoclinic β-Ga₂O₃ phases, respectively. The scanning electron microscopy (SEM) micrographs showed that the synthesized powders consisted of hierarchically-ordered nanorods linked by side-by-side arrangement. This morphology was attributed to a combinatorial effect of using microwaves and the chosen solution pH. The average length and width of each bunch of rods were ∼7 µm and 0.8 µm respectively, while the average width of each rod was 200 nm. The optical bandgap energies, bond vibrational characteristics, photoluminescence (PL) properties and surface defects of the synthesized structures were examined using UV-Visible diffuse reflectance spectroscopy, Raman spectroscopy, PL spectroscopy and X-ray photoelectron spectroscopy (XPS) respectively. The data obtained indicated changes in the structural phases, particle morphologies and surface defects because of variations in the calcination temperature. The β-Ga₂O₃ rods showed intense room-temperature blue emission attributed to the presence of oxygen vacancy (V_O) defects making this β-Ga₂O₃ with unique particle morphology a potential candidate in gas sensing or optoelectronic device applications.