Enhancement of the capacitance in TiO2 nanotubes through controlled introduction of oxygen vacancies

Abstract

The many applications of high energy storage devices have forged an increasing interest in research areas related to electrochemical capacitors. Here, in this work, we present a facile method for the fabrication of self-organized titania nanotubes grown by anodic oxidation of titanium foil with different subsequent heat-treatment regimes for use as binder-free working electrodes in supercapacitor applications. The capacitance of these highly ordered titania nanotubes, when exposed to a reductive atmosphere during annealing, was determined to be well above 900 mF cm À2, confirming that the capacitance contribution was pseudocapacitive in nature. The behaviour of oxygen depleted titania in the anatase to rutile (A / R) phase transformation and also in electrochemical charge storage has been studied in detail. It was found that upon the reduction of Ti 4+ to Ti 3+, with oxygen depletion of the structure, the A / R phase transformation was promoted. In addition, the fabricated electrodes showed highly reversible charge-discharge stability.

Enhancement of the electrochemical capacitance of TiO2 nanotube arrays through controlled phase transformation of anatase to rutile

“…First, oxygen vacancies were generated due to the reduction of Ti +4 to Ti +3 at the surface of titania nanotubes. Second, comparing with the previous results for the samples annealed under an Ar atmosphere for 2 h, 27 increasing the annealing time to 5 h led to the formation of lower amount of oxygen vacancies based on the lower rutile Raman downshift. In order to attain a more clear understanding of the structural evolution during the annealing process, X-ray photoelectron spectroscopy (XPS) analysis was applied.…”

“…17 Raman spectra serve as a further proof that increasing the annealing temperature induces the transition from the anatase to rutile phase. Our previous study 27 addressed the behaviour of titania nanotubes in detail upon annealing under different atmospheres, which confirms the existence of oxygen vacancies inside self-organized TiO 2 nanotubes after annealing under an argon atmosphere. Curve fitting of TiO 2 Raman spectra was performed using Origin software employing a Gaussian-Lorentzian peak shape after performing a background correction.…”

“…26,27 The electrolyte consisted of 0.2 wt% NH 4 F dissolved in a mixture of glycerol and de-ionized water (9 : 1 in volume). The synthesis process was conducted in a thermostatic water bath to keep the temperature of the electrolyte constant.…”

“…On the other hand, the existence of such a non-stoichiometric structure can reduce the activation energy of the transformation and accelerate it. 27 Mor et al 30 suggested that, at any temperature range in which both anatase and rutile phases can coexist, the rutile formation occurs at the nanotube-metal interface sites, whereas the anatase crystals located on the nanotube walls cannot be transformed into the rutile phase due to the constraints imposed by the walls.…”

“…30 A comparison between the TNTA annealed at 550 1C in air and under argon atmospheres supports the idea that the transformation rate of the anatase to rutile phase can be much enhanced upon the introduction of a reductive atmosphere. 26,27 Based on grain growth mechanisms, faceting should occur, which results in both high and low grain-boundary motilities. 31 Therefore, certain crystallographically favoured grain boundaries decompose into flat facets that lie along low-energy inclinations.…”