A novel double-layer nanotube structure fabricated in high concentration H3PO4 and fluoride-containing mixed electrolyte without annealing

•A novel double-layer structure was fabricated in 12 wt% H3PO4 mixed electrolyte.•Nanotube diameter in mixed electrolyte is ∼2.84 times larger than that in classical electrolyte.•The large diameter of the double-layer nanotube is ∼392 nm.•The double-layer structure was synthesized during the anodization process.