Electrochemical Deposition of Mn⁺³ rich δ-MnO₂ on Epitaxial Graphene-Silicon Carbide Substrates for Selective Gas Sensing

Mixed valence manganese oxides (MnO_x) have attracted significant research interest due to the easily reversible redox reactions between manganese oxidation states (Mn⁺², Mn⁺³, Mn⁺⁴)¹. Of particular interest among these compounds is the layered(δ) phase of MnO₂ due to its tunable interlayer spacing and high Mn⁺³ content². These Mn⁺³ sites are desirable as they are highly reactive and improve the catalytic activity of MnO₂ structures³. However, the speed of these redox reactions is sharply limited by the low electronic conductivity of MnO₂. To counteract this, we electrodeposited δ-MnO₂ on quasi-freestanding bilayer epitaxial graphene (EG)-silicon carbide substrates. The resulting junction between the EG and the δ-MnO₂ layers leads to improved charge transfer between the materials, improving the reactive speed of δ-MnO₂. We confirmed this by constructing a four-contact gas sensor from the resulting heterostructure, which demonstrated a high selectivity towards nitrogen-based gases and an enhanced response/recovery time. We also confirmed the Mn⁺³ rich structure through Raman spectroscopy and X-ray photoelectron spectroscopy.
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