MoOx-based high-density nanoarrays on a substrate via smart anodizing as novel 3D electrodes for nano-energy applications

Identifier:  imarina:9454151

Handle: https://hdl.handle.net/20.500.11797/imarina9454151

Authors:  Mozalev, A; Bendova, M; Kalina, L; Prasek, J; Gispert-Guirado, F; Llobet, E

Abstract:
Nanostructured molybdenum oxide (MoOx) has many exciting properties that are highly dependent on the synthesis procedure. MoOx nanostructures should be aligned on a substrate for nanoelectronic on-chip applications, which has been challenging. Here, for the first time, arrays of MoOx-based nanoprotrusions of various morphologies (nanogoblets and nanorods), dimensions (20-500 nm), and surface densities (up to 1011 cm-2), spatially separated and vertically aligned on a Si wafer, were synthesized via self-organized porous-anodic-alumina (PAA)-assisted anodization of a Mo underlayer covered with a few nm thick Nb interlayer. This creative anodization approach enabled sustainable growth of fully amorphous MoOx within and under the PAA nanopores in several aqueous electrolytes, which other reported methods cannot accomplish. The nanoarrays grow via the outward migration of Mon+ cations enabled by the thin niobium-oxide interlayer, followed by the concurrent migration of Mon+ (n = 4-6) and Nb5+ cations in the PAA barrier layer and along the pore walls, competing with the migration of Mon+ through the anodic molybdenum oxide that grows within the 'empty' pores. The nanorods derived after selective PAA dissolution feature a core/shell heterostructure: The shells are composed of MoO3, several molybdenum suboxides (Mo5+, Mo4+), stoichiometric Nb2O5, and Al2O3, all mixed at the molecular level, whereas the cores are slightly hydrated and reduced MoO3, as revealed by X-ray photoelectron spectroscopy. The annealing in air or vacuum at 550 degrees C increases the oxidation state of Mon+ cations in the shells and causes the formation of monoclinic MoO2 and orthorhombic Nb2O5 nanocrystallites in the bottom oxide. Mott-Schottky analysis disclosed n-type semiconductor properties of the cores, with the charge carrier density reaching 1 x 1022 cm-3, whereas the shells seem more dielectric. The cyclic voltammetry and galvanostatic charge-discharge measurements featured characteristic reversible redox reactions, intensive electron transport, intercalation pseudocapacitive behavior, competitive charge-storage performance, and good rate capability of the rod's cores, which means the potential for nano-energy applications.