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

• Arrays of MoO x -based nanorods synthesized by self-organized anodizing of Mo-Nb alloys. • The record-high Mo content in the oxide achieved with this technology is 92 at.%. • Annealing at 550 °C promotes an amorphous-to-crystalline transition in MoO 2 . • The entire surface of the MoO x nanorods reveals n -type semiconductor properties. • Confirmed applications in electrochemical super(pseudo)capacitors and gas sensors. The fabrication of inorganic semiconducting nanomaterials with a high content of molybdenum oxide (MoO x ), self-organized in arrays on a substrate, has long been a challenge, limiting the utilization of MoO x nanostructures in on-chip micro- and nanodevices. Here, for the first time, arrays of MoO x nanostructures, such as bulges, columns, and rods, of high densities (10 8 – 10 10 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 x with various Mo n + cations ( n = 6 to 3) mixed at the atomic level with a minor amount of Nb 2 O 5 and NbO 2 suboxide, which are ‘doped’ at the surface with Al 2 O 3 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 2 nanocrystals dispersed within the amorphous MoO x -Nb 2 O 5 matrix and an increased oxidation state of MoO x 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 × 10 21 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 2 H 5 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.