Optical Absorption Bands in the Milled Gallium Oxide-hematite Nanoparticles System

Abstract

Nanoparticles of the gallium oxide-hematite mixed-oxide system were synthesized by mechanochemical activation using high-energy ball milling for durations of 0, 2, 4, 8, and 12 hours.  Optical diffuse reflectance spectroscopy (ODRS) measurements were performed across the full UV-VIS-NIR range.  Optical absorption bands in the spectra of hematite and gallium oxide nanoparticles were identified.  Hematite (Fe₂O₃) exhibits a strong absorption band in the visible range (~2.2 eV), while other prominent absorption bands are located at ~3.1-3.2 eV and ~5.3-5.8 eV.  These features arise from the electronic structure of the iron ions and the crystal lattice, with antiferromagnetic interactions in certain structural configurations enhancing the intensity of the absorption bands. Ga₂O₃ has an intrinsic bandgap of approximately 4.8 eV, rendering it transparent throughout the visible and into the deep-UV region.  Free electrons in doped Ga₂O₃ can absorb light from the IR to UV via transitions between conduction bands, including a direct optical transition that occurs at approximately 3.55 eV (349 nm).  The optical absorption properties of the gallium oxide-hematite system were found to depend on the milling processing parameters.