Halide vapor phase epitaxial (HVPE) Ga 2 O 3 films were grown on c-plane sapphire and diamond substrates at temperatures up to 550 °C without the use of a barrier dielectric layer to protect the diamond surface. Corundum phase α-Ga 2 O 3 was grown on the sapphire substrates, whereas the growth on diamond resulted in regions of nanocrystalline β-Ga 2 O 3 (nc-β-Ga 2 O 3 ) when oxygen was present in the HVPE reactor only during film growth. X-ray diffraction confirmed the growth of α-Ga 2 O 3 on sapphire but failed to detect any β-Ga 2 O 3 reflections from the films grown on diamond. These films were further characterized via Raman spectroscopy, which revealed the β-Ga 2 O 3 phase of these films. Transmission electron microscopy demonstrated the nanocrystalline character of these films. From cathodoluminescence spectra, three emission bands, UVL′, UVL, and BL, were observed for both the α-Ga 2 O 3 /sapphire and nc-Ga 2 O 3 /diamond, and these bands were centered at approximately 3.7, 3.2, and 2.7 eV.
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Oxygen Reduction Electrocatalysis with Epitaxially Grown Spinel MnFe 2 O 4 and Fe 3 O 4
Nanocrystalline MnFe2O4 has shown promise as a catalyst for the oxygen reduction reaction (ORR) in alkaline solutions, but the material has been sparingly studied as highly ordered thin-film catalysts. To examine the role of surface termination and Mn and Fe site occupancy, epitaxial MnFe2O4 and Fe3O4 spinel oxide films were grown on (001)- and (111)-oriented Nb:SrTiO3 perovskite substrates using molecular beam epitaxy and studied as electrocatalysts for the oxygen reduction reaction (ORR). High-resolution X-ray diffraction (HRXRD) and X-ray photoelectron spectroscopy (XPS) show the synthesis of pure phase materials, while scanning transmission electron microscopy (STEM) and reflection high-energy electron diffraction (RHEED) analysis demonstrate island-like growth of (111) surface-terminated pyramids on both (001)- and (111)-oriented substrates, consistent with the literature and attributed to the lattice mismatch between the spinel films and the perovskite substrate. Cyclic voltammograms under a N2 atmosphere revealed distinct redox features for Mn and Fe surface termination based on comparison of MnFe2O4 and Fe3O4. Under an O2 atmosphere, electrocatalytic reduction of oxygen was observed at both Mn and Fe redox features; however, a diffusion-limited current was only achieved at potentials consistent with Fe reduction. This result contrasts with that of nanocrystalline MnFe2O4 reported in the literature where the diffusion-limited more »
- Publication Date:
- NSF-PAR ID:
- 10318625
- Journal Name:
- ACS Catalysis
- Volume:
- 12
- ISSN:
- 2155-5435
- Sponsoring Org:
- National Science Foundation
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