Low-temperature crystallization of anodized TiO2 nanotubes at the solid–gas interface and their photoelectrochemical properties

Abstract

TiO₂ nanotubular arrays formed by electrochemical anodization have attracted significant attention for photoelectrochemical applications that utilize solar energy. However, the as-anodized TiO₂ nanotubes are amorphous, and need to be crystallized by high-temperature thermal annealing. Herein, we describe a low-temperature hydrothermal solid–gas route to crystallize TiO₂ nanotubes. In this process, the as-anodized TiO₂ hydroxo nanotubes are dehydrated to yield anatase phase via solid–gas interface reaction in an autoclave at a temperature of less than 180 °C. The solid–gas interface reaction alleviates the collapse of as-anodized TiO₂ nanotubes during hydrothermal process efficiently. Compared with the common thermal annealing at the same temperature but at atmospheric pressure, the hydrothermal route improves the photocurrent density of TiO₂ nanotubes by ∼10 times in KOH electrolyte. The duration of the hydrothermal reaction has a substantial effect on the photoelectrochemical properties of TiO₂ nanotubes, which is ascribed to the synergetic effect between the crystallization and structural evolution. Electron donors can further suppress the charge recombination in the low-temperature crystallized TiO₂ nanotubes and boost the photocurrent density by ∼120%.