Advanced anodic molybdenum-oxide nanomaterials derived from Mo-Nb alloys

Identifier:  imarina:9632982

Handle: https://hdl.handle.net/20.500.11797/imarina9632982

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

Abstract:
The fabrication of inorganic semiconducting nanomaterials with a high content of molybdenum oxide (MoO <inf>x</inf> ), self-organized in arrays on a substrate, has long been a challenge, limiting the utilization of MoO <inf>x</inf> nanostructures in on-chip micro- and nanodevices. Here, for the first time, arrays of MoO <inf>x</inf> nanostructures, such as bulges, columns, and rods, of high densities (10⁸ – 10¹⁰ cm⁻²), self-standing on a substrate, are synthesized via the in situ anodization of sputter-deposited Mo-Nb alloy layers with variable Mo content, ranging from 5 to 95 at. %, covered with a thin Al layer. The approach involves the growth of a porous anodic alumina (PAA) film that enables the formation of fully amorphous molybdenum-niobium mixed-oxide nanostructures within and under the PAA nanopores. By combining SEM, XPS, and XRD analyses, it is shown that, after selective PAA dissolution, the free-standing nanostructures are composed of a dominating amount of MoO <inf>x</inf> with various Mo ⁿ⁺ cations ( n = 6 to 3) mixed at the atomic level with a minor amount of Nb<inf>2</inf>O<inf>5</inf> and NbO<inf>2</inf> suboxide, which are ‘doped’ at the surface with Al<inf>2</inf>O<inf>3</inf> originating from the PAA cell walls and ‘serving’ as a shape stabilizer. The record-high Mo content achieved with this technology is 92 at. %. The accomplishment is due to the enhanced migration of Mo ⁿ⁺ cations within the mixed oxide inside the PAA nanopores and along the pore walls. Annealing at 550°C induces a unique phase separation, resulting in MoO<inf>2</inf> nanocrystals dispersed within the amorphous MoO <inf>x</inf> -Nb<inf>2</inf>O<inf>5</inf> matrix and an increased oxidation state of MoO <inf>x</inf> at the film surface. The cyclic voltammetry and electrochemical impedance spectroscopy examinations confirm that the entire surface of the oxide nanorods is an n -type semiconductor with a charge carrier density of 2 × 10²¹ cm⁻³. The highly competitive pseudocapacitive properties of the nanoarray derived from Mo-19at. %Nb alloy are disclosed, yielding a capacitance of ∼10 mF cm⁻², rendering the film promising as a 3D semiconductor nanoelectrode for emerging on-chip energy-storage microdevices. Moreover, the film efficiently serves as a gas-sensing layer for rapidly and selectively detecting low concentrations of C<inf>2</inf>H<inf>5</inf>OH (10 ppm) and CO (100 ppm). More potential applications include antibacterial coatings, self-cleaning surfaces, and electrochromic films, where the rod-like nanomorphology and the large number of Mo-containing reactive sites are highly desirable.