Maria Bendova

Metal-substrate-supported tungsten-oxide nanoarrays via porous-alumina-assisted anodization: from nanocolumns to nanocapsules and nanotubes

An array of highly aligned tungsten-oxide (TO) nanorods, ∼80 nm wide, up to 900 nm long, spatially separated at their bottoms by tungsten metal on a substrate is synthesized via the self-localized anodization of aluminum followed by the porous-alumina-assisted re-anodization of tungsten in a sputter-deposited Al/W bilayer. Moreover, the pore-directed TO nanocapsules may grow, which can be electrochemically top-opened in alumina nanopores and transformed to TO nanotubes, representing unique architectures built up on tungsten substrates to date. The as-grown nanorods are composed of amorphous WO3 mixed with minor amounts of WO2 and Al2O3 in the outer layer and oxide–hydroxide compound (WO3·nH2O) with aluminum tungstate (2Al2O3·5WO3), mainly present inside the rods. Once the growing oxide fills up the pores, it comes out as an array of exotic protuberances of highly hydrated TO, with no analogues among the other valve-metal oxides. Vacuum or air annealing at 550 °C increases the portion of non-stoichiometric oxides ‘doped’ with OH-groups and gives monoclinic WO2.9 or a mixture of WO3 and WO2.9 nanocrystalline phases, respectively. The nanorods show n-type semiconductor behavior when examined by Mott–Schottky analysis, with a high carrier density of 7 × 1019 or 3 × 1019 cm−3 for the air- or vacuum-annealed samples, associated with a charge depletion layer of about 8 or 10 nm, respectively. A model for the growth of the metal-substrate-separated TO nanocapsules and tubes is proposed and experimentally justified. The findings suggest that the new TO nanoarrays with well-defined nano-channels for carriers may form the basic elements for photoanodes or emerging 3-D micro- and nano-sensors.

Resistive switching in TiO2 nanocolumn arrays electrochemically grown

Resistive switching in metal oxides, especially in TiO2, has been intensively investigated for potential application in non-volatile memory microdevices. As one of the working mechanisms, a conducting filament consisting of a substoichiometric oxide phase is created within the oxide layer. With the aim of investigating the filament formation in spatially confined elements, we fabricate arrays of self-ordered TiO2 nanocolumns by porous-anodic-alumina (PAA)-assisted anodizing, incorporate them into solid-state microdevices, study their electron transport properties, and reveal that this anodizing approach is suitable for growing TiO2 nanostructures exhibiting resistive switching. The electrical properties and resistive switching behavior are both dependent on the electrolytic formation conditions, influencing the concentration and distribution of oxygen vacancies in the nanocolumn material during the film growth. Therefore, the PAA-assisted TiO2 nanocolumn arrays can be considered as a platform for investigating various phenomena related to resistive switching in valve metal oxides at the nanoscale.

3-D nanostructured tungsten-oxide gas-sensing film via anodizing sputter-deposited Al/W metal layers

The making of 3-D nanostructured metal oxide films is an active and competitive area of research, aiming at novel materials with enhanced properties and sensing devices with improved performances. Here we present the preparation procedure and gas sensing behavior of a novel self-assembled 3-D WO3 nanofilm that effectively combines the advantages of inorganic materials with the simplicity and universality offered by electrochemistry-based formation techniques. The film is formed mainly by electrochemical anodizing and is composed of an array of spatially-ordered upright-standing WO3 nanorods, assembled between the two noble metal patterned electrodes, which serve as direct semiconducting pathways for chemisorption reactions in a gas atmosphere. A test microsensor employing the nanofilm and assembled on a standard TO-8 Metal Can Package showed the fast and intensive response to H2, leaving more opportunities for further improvement of the active film configuration and sensor performance based on the computer-aided modelling and simulation results.