Spontaneous Formation of Ordered Nanobubbles in Anodic Tungsten Oxide during Anodization

Abstract

Anodization of tungsten in NaOH leads to the growth of a porous tungsten oxide layer with either irregular trenches or isolated nanobubbles. For anodic potentials between 30 to 40 V and NaOH concentrations of 2–3 M, nanobubbles with diameters of ∼40 nm form spontaneously and self-organize into hexagonal close-packed ordered arrays with a period of ∼60 nm. The nanobubbles are likely to be initiated by the electrostatic stress on the oxide film that could become more plastic under the influence of high-density anodic current. The ordered arrangement of the nanobubbles manifests a delicate balance of the stress on their surrounding oxide walls.