In this article, we report the fabrication of gallium oxide (α-Ga2O3) microspheres (GOMs) by a self-assembly process. Gallium nitrate with oxalic acid in a hydrothermal process results in α-GaOOH, which was further converted into gallium oxide by calcinations at 450 °C for 3 h. We first report the formation of various morphological α-GaOOH by using the above-mentioned methodology. The influence of hydrothermal temperature and time on the crystal structure and its morphology was studied, and the results indicated that hydrothermal temperature played an important role in the final morphology of α-GaOOH. The flower-like α-GaOOH formed at 175 °C is converted into rodlike α-Ga2O3 after calcination at 450 °C, and the α-GaOOH microsphere and microrod formed at 200 and 225 °C retained their morphology during the calcination process, respectively. The synthesized α-GaOOH and α-Ga2O3 were characterized using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), and nitrogen adsorption analysis. The XRD patterns indicated that well-crystallized α-GaOOH and α-Ga2O3 were formed in a hydrothermal and calcination process, respectively. The FE-SEM images indicated the formation of well-organized microspheres and microflowers, which were composed of nanoparticles and nanoplates, respectively. The photocatalytic degradation of Acid Orange 7 (AO7) dye and Cr(VI) reduction by using the synthesized GOM under UV light irradiation was investigated. The photocatalytic experiment showed superior photocatalytic activity of GOM having a higher efficiency than TiO2. We propose a plausible mechanism for the formation of various morphologies of α-GaOOH and α-Ga2O3.

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