Fig. 4

SEM images of advanced TiO₂ nanotube morphologies. a Bamboo-type reinforced TiO₂ nanotubes are fabricated under specific alternating-voltage (AV) conditions in ethylene glycol consisting of 0.2 mol/L HF, with a sequence of 1 min at 120 V and 5 min at 40 V. Reproduced from ref. [38]. b Transition from smooth to bamboo-like TiO₂ nanotubes can be induced by anodization with controlled water addition (water contents:1 to 8%) to a 0.135 M NH₄F/ethylene glycol electrolyte

reproduced from ref. [39]. c The 2D nanolace structures are obtained under voltage cycling carried out for an extended period of time in the fluoride containing electrolyte, with a sequence of 50 s at 120 V and 600 s at 0 V. Reproduced from ref. [38]. d The double-walled TiO₂ nanotubes are grown by anodization of Ti in a fluoride containing ethylene glycol electrolyte at 120 V after annealing at 500 °C with a heating rate of 1 °C s⁻¹. Reproduced from ref. [40]. e The branched nanotubes can be observed by voltage stepping, first at 120 V (6 h) and then at 40 V (2 h). Reproduced from ref. [38]. f The double-layer nanotubes with equal or two different tube diameters can be seen. Reproduced from ref. [38]