Mixed valence manganese oxides (MnOx) have attracted significant research interest in recent years due to the easily reversible redox reactions between manganese oxidation states (Mn+2, Mn+3, and Mn+4)1 which enable applications in catalysis2, energy storage3, and gas sensing4. Of the manganese oxides manganese dioxide (MnO2) has been of particular interest due to its wide variety of synthesized structural polymorphs ((β)1x1 tunnel5, (α)1x2 tunnel5 (γ) spinel5, (δ) layered5) which allow for enhanced control over the available surface area and reactive properties of MnO2. Among these structural polymorphs, the α and δ phases exhibit mixed-valence character with Mn+3 defects being found throughout the crystal structure5. Mn+3 defects have been found to increase the catalytic activity of MnO2 making its presence desirable6. Hexagonal δ-MnO2, in particular, contains a large number of Mn+3 ions as the interlayer contains Mn+2/+3 ions to neutralize the layer charge from Mn+3 lattice defects7. However, the controlled synthesis of hexagonal δ-MnO2 has proven challenging and often relies on the use of harsh chemicals1, high temperatures1, and long reaction times1. These methods also rely on permanganate salts1 as the manganese precursor, which leads to the formation of monoclinic δ-MnO2 and significantly reduces the number of Mn+3 defects in the crystal lattice. In this work, we present the green facile synthesis of high Mn+3 content hexagonal δ-MnO2 via electrodeposition on both silicon(Si) and epitaxial graphene/silicon carbide(EG/SiC) substrates. The electrodeposition was carried out in a three-electrode electrochemical cell with an Ag/AgCl reference electrode, a Pt counter electrode, and the substrate (Si or EG/SiC) as the working electrode utilizing a 100mM manganese acetate solution. The electrodeposition process contained three steps, an initial pulse at the oxidation potential to seed the surface with oxygen, a second pulse at the reduction potential to deposit manganese on the surface, and a third pulse at the oxidation potential to oxidize the deposited manganese, with the potentials determined by cyclic voltammetry. The resulting δ-MnO2 thin films were then characterized through Raman spectroscopy, X-Ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and atomic force microscopy(AFM) to determine its crystalline phase and surface morphology. Raman spectroscopy(Fig. 1) confirmed the formation of MnO2 on both substrates due to the presence of Raman peaks between 650-640cm-1,8 575-585cm-1,8, and 490-510cm-1,8, which are associated with the Mn-O symmetric stretching bond8, the Mn-O stretching bond in the basal plane8, and Mn-O stretching bond in the [MnO6] octehedra8. The formation of the δ phase is confirmed by the presence of a weak peak at 130cm-1,2 consistent with the literature. The observed 9cm-1 redshift of the 490-510cm-1,6 between Si and SiC substrates is associated with the weakening Mn-O stretching bond on Si, which combined with the strengthening of the peak at 600-620cm-1 indicates that the growth on Si contains a higher rate of Mn+3defects2,8. XPS corroborated this result and indicated that the manganese in the δ-MnO2 deposited on EG/SiC had an average oxidation state of 3.3. The AFM images indicated that the surface was made up of nanofibrous nanoparticles, with the deposition on Si exhibiting significantly larger nanofibers (Fig 2.). SEM confirmed this (Fig. 3) and indicated that the deposition on EG/SiC was made up of microplates ~40umx40um in size, which was not observed in the deposition on Si. We believe this variation in surface morphology is due to interactions between SiC and MnO2, which are not present when δ-MnO2 is grown on Si. The reactivity of the deposited δ-MnO2 was then tested by depositing 4 Ti/Au (30nm/120nm) contacts on the δ-MnO2/EG/SiC heterostructures to form a simple gas sensor, which was tested against 5ppm NO2, 5ppm NH3, 1000ppm IPA, and 1000ppm methanol. The sensor displayed a remarkably fast response/ recovery time of 3.8s/2.2s to NH3 and 3.4/6.0s to NO2(Fig. 4), demonstrating the high reactive potential of the δ-MnO2 thin films. This reactivity demonstrates that our electrodeposited δ-MnO2 provides a good candidate for use in other applications of MnO2 such as energy storage and catalysis.
References: [1]L. Spinelle et al.
Sensors 17(7), 1520(2017)[2] F. Cheng et al. Chem. Mater 22(3), 898-905(2010)(3) Y.J. Huang et al. Electrochim Acta 99, 161-165(2013)[4]N. Joshi et al. Microchim. Acta 185, 213(2018)[5]Z. Chang et al. Proc Natl Acad Sci USA 115(23), 5261-5268(2018) [6] Julien et al. Spectrochim Acta A 60, 3(2004) [7] Drits et al. Am Mineralogist 82, 9-10(1997) [8] Julien et al. Solid State Ionics 159, 4(2003)
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