Evidence of oxygen bubbles forming nanotube embryos in porous anodic oxides

Abstract

Anodic TiO2 nanotubes have been studied widely for two decades because of its regular tubular structures and extensive applications. However, the forming mechanism of anodic TiO2 nanotubes remains unclear, because...

Debunking the essential effect of temperature and voltage on the current curve and the nanotube morphology

“… 22 In stage (III), oxygen bubbles evolve from the surface of the nanotubes and the electrolyte reaches the bottom of the nanotubes. 9,20,56 Thus, the thickness of the barrier oxide layer remains the same as well as the ionic current. The ionic current and the electronic current run side by side.…”

“…According to the ‘oxygen bubble model’ and the ionic current and electronic current theories, the growth rate of nanotubes in the same sample is inhomogeneous and some oxygen bubbles have not evolved from the inside of nanotubes entirely. 9 Thus, some nanotubes hide under the compact oxide layer and cannot be seen from the surface, which gives a convincing explanation for the disordered distribution of deep pores. It indicates that the formation of shallow pits is a result of the dissolution of the electrolyte, which makes almost no contribution to the growth of nanotubes.…”

The Anodization of Thin Titania Layers as a Facile Process towards Semitransparent and Ordered Electrode Material

“…As mentioned above according to the field assisted dissolution theory, the tubular structure is generated due to the dissolution of the oxide layer at TiO 2 /electrolyte interface by the fluorine ions, driven by the electric field. Hence, the presence of fluorine ions in the electrolyte is crucial to initiate the formation of the porous texture and achieve a tubular structure [ 43 ]. However, Fahim et al [ 44 ] achieved a bundles tubular structure of a length of 500 nm in aqueous solution containing only sulphuric acid (fluorine free electrolyte).…”

“…The barrier oxide layer grows up to the critical thickness and the total current drops dramatically. Anions are inevitably incorporated into TiO 2 during its formation and this leads to an anion-contaminated layer near the electrolyte [ 43 , 52 , 56 ].…”

“…In the second step, the electronic current increases with the increase of the barrier oxide thickness according to the Equations (9) and (10) proposed by Gong et al [ 43 ] for the electronic and ionic conduction. where J ions is the ionic current density, J e is the electronic current density and J total is the total current density, J 0 the primary electronic current density during the anodization, α is the impact ionization coefficient, U is the applied voltage and d is the thickness of the barrier oxide layer.…”