Selective focused-ion-beam sculpting of TiO2nanotubes and mechanism understanding

Abstract

Anodic TiO₂ nanotubes with different structures, doping agents, and decorations have been studied in order to improve energy conversion and storage efficiencies such as in dye sensitized solar cells, solar fuels, and electrochemical supercapacitors. However, the top surface modification of TiO₂ nanotubes has never been addressed. In this study, anodic TiO₂ nanotubes have been selectively closed by high energy focused ion beams and re-opened by low energy focused ion beams. Under a 30 kV Ga⁺ beam, TiO₂ nanotubes are closed with a 65 nm shield layer covering the top entrance when the ion dose is larger than 1.2 × 10¹⁷ ions per cm²; under a 5 kV Ga⁺ beam, the shield layer is removed and the closed tubes are re-opened. An ion-induced viscous flow model has been proposed to explain the influence of Ga⁺ ion beam flux, substrate temperature, initial tube diameter, ion beam dwell time, and the incidence angle of the ion beam.