Green growth of mixed valence manganese oxides on quasi-freestanding bilayer epitaxial graphene-silicon carbide substrates

Nanostructured manganese oxides (MnO_x) have shown incredible promise in constructing next-generation energy storage and catalytic systems. However, it has proven challenging to integrate with other low-dimensional materials due to harsh deposition conditions and poor structural stability. Here, we report the deposition of layered manganese dioxide (δ-MnO₂) on bilayer epitaxial graphene (QEG) using a simple three-step electrochemical process involving no harsh chemicals. Using this process we can synthesize a 50 nm thick H–MnO₂ film in 1.25s. This synthetic birnessite is inherently water-stabilized, the first reported in the literature. We also confirm that this process does not cause structural damage to the QEG, as evidenced by the lack of D peak formation. This QEG heterostructure enhanced MnO₂'s redox active gas sensing, enabling room temperature detection of NH₃ and NO₂. We also report on transforming this δ-MnO₂ to other MnO_x compounds, Mn₂O₃ and Mn₃O₄, via mild annealing. This is confirmed by Raman spectroscopy of the films, which also confirms limited damage to the QEG substrate. To our knowledge, this is the first synthesis of Mn₂O₃ and Mn₃O₄ on pristine graphene substrates. Both methods demonstrate the potential of depositing and transforming multifunctional oxides on single-crystal graphene using QEG substrates, allowing for the formation of nanostructured heterostructures previously unseen. Additionally, the electrochemical nature of the deposition presents the ability to scale the process to the QEG wafer and adjust the solution to produce other powerful multifunctional oxides.