Substrate‐Enabled Room‐Temperature Electrochemical Deposition of Crystalline ZnMnO₃

Abstract

Mixed transition metal oxides have emerged as promising electrode materials for electrochemical energy storage and conversion. To optimize the functional electrode properties, synthesis approaches allowing for a systematic tailoring of the materials’ composition, crystal structure and morphology are urgently needed. Here we report on the room‐temperature electrodeposition of a ternary oxide based on earth‐abundant metals, specifically, the defective cubic spinel ZnMnO3. In this unprecedented approach, ZnO surfaces act as (i) electron source for the interfacial reduction of MnO4− in aqueous solution, (ii) as substrate for epitaxial growth of the deposit and (iii) as Zn precursor for the formation of ZnMnO3. Epitaxial growth of ZnMnO3 on the lateral facets of ZnO nanowires assures effective electronic communication between the electroactive material and the conducting scaffold and gives rise to a pronounced 2‐dimensional morphology of the electrodeposit forming – after partial delamination from the substrate – twisted nanosheets. The synthesis strategy shows promise for the direct growth of different mixed transition metal oxides as electroactive phase onto conductive substrates and thus for the fabrication of binder‐free nanocomposite electrodes.

Conformal Coverage of ZnO Nanowire Arrays by ZnMnO3: Room‐temperature Photodeposition from Aqueous Solution

“…It remained unclear, however, whether the Zn 2+ ions necessary for the reaction are provided by ZnO dissolution or whether ion diffusion from the buried ZnMnO 3 /ZnO interface to the reactive ZnMnO 3 /electrolyte interface takes place. Irrespective of this, a surface‐enabled epitaxial process was identified as the fundamental reason for the observed crystallization of ZnMnO 3 at room temperature [31] …”

“…Electrodeposited ZnO nanowires (Figures 1a and 2a) are hexagonal prisms with an average length of 700±200 nm and a diameter of 50±25 nm [31] and feature the hexagonal wurtzite structure [31] . Nanowires are elongated along the [0001] direction and are single crystals [31] .…”

“…We have reported only recently that highly crystalline ZnMnO 3 nanostructures featuring the defective cubic spinel phase and a two‐dimensional morphology grow on ZnO nanowires from identical precursor solutions upon reductive electrodeposition in the dark [31] . For these crystalline ZnMnO 3 nanosheets we estimated (upon supposition of an indirect semiconductor) a band gap energy of E BG =2.4±0.1 eV [31] . In contrast to observations on these electrodeposited ZnMnO 3 nanostructures, [31] diffuse reflectance (DR) UV/Vis spectra of the photodeposits point to the presence of at least two types of electronic transition in the wavenumber range 400 nm< λ <550 nm (Figure 5a).…”

“…These electrons will thus not contribute to the modification of the quasi‐Fermi level. Indeed, much more negative potentials are required to electrodeposit significant amounts of ZnMnO 3 [31] . However, even after optimized electrodeposition at E Ag/AgCl =0.000 V [31] only a low capacitance of C ≤20 F g −1 has been obtained.…”

“…A nitrogen (N 2 5.0)–purged 0.175 mM KMnO 4 (Sigma Aldrich, purity≥99 %) aqueous solution ( V =80 mL) was used as the precursor for ZnMnO 3 deposition. In a previous study, electrodeposition was performed in pure 0.175 mM KMnO 4 aqueous solution (pH 5.3±0.2) at a deposition potential E Ag/AgCl =0.000 V [31] . In this study, we adjusted the pH value of the precursor solution (to pH 4, pH 7 and pH 10, respectively) by adding KOH (Merck, EMSURE® for analysis) or HCl (Merck, 32 %).…”