Tracer Investigation of Pore Formation in Anodic Titania

Abstract

Using a sputtering-deposited titanium substrate, incorporating six equally spaced nanolayers of Ti-W alloy, the volume and composition changes accompanying the formation of porous anodic films on titanium in 0.5 wt % NH 4 F in glycerol are investigated. The findings reveal amorphous films with nanotubes of TiO 2 , containing fluoride ions and possibly glycerol derivatives. Tungsten and titanium species are lost to the electrolyte at differing rates during anodizing, leading to an enrichment of tungsten in the film relative to the composition of the substrate. The spacing of tungsten-containing bands in the film is ϳ2.3 that of the original alloy layers during growth of the major pores. The generation of the nanotubes can be explained either by field-assisted flow of film material within the barrier layer to the pore walls, with cation and anion transport numbers of anodic titania in the barrier layer region similar to those of barrier films and with field-assisted ejection of Ti 4+ ions to the electrolyte, or by fieldassisted dissolution, but with a reduction in cation transport number.

Formation Mechanism of Gaps and Ribs Around Anodic TiO2Nanotubes and Method to Avoid Formation of Ribs

“…However, so far, the distinct three stages of the current oscillation or voltage oscillation have not been elucidated via the field-assisted dissolution and the 'plastic flow' models. [7][8][9][12][13][14][15][16][17][18] For example, the increase of current in Figure 8d and the drop of voltage (decrease of current efficiency) in Figure 8e are both controversial. 18,30 Diggle et al 30 indicated that the mechanism of oxide dissolution is chemical in nature.…”

“…15 Until 2006, an alternative mechanism has been proposed by the Manchester group, 16 which is termed as a field-assisted 'plastic flow' model, where the barrier oxide at the metal/oxide (M/O) interface is constantly displaced upwards to form the nanotube walls. [16][17][18] In fact, the 'plastic flow' model is contrary to expectations of the field-assisted dissolution model of ATNTs. 16 Previous studies have shown that fluorine ions played a significant role in the initiation and growth of TiO 2 nanotubes.…”

“…29 They suggested oxygen evolution plays a primary role on the formation of ribs around the ATNTs. 29 Since 2008, based on the characteristics of ionic conductivity and electronic conductivity, 30,31 the oxide breakdown theory 32,33 and the 'plastic flow' model, [16][17][18] we have also introduced a growth model of oxygen bubble mould (OBM), which emphasizes that the columnar pore or the nanotubes result from the oxide flow around the oxygen bubble remained within the pore bottom. [34][35][36] The model and conclusion of the papers above were reinforced by our subsequent works.…”

Empirical kinetics for the growth of titania nanotube arrays by potentiostatic anodization in ethylene glycol

“…LeClere et al [31], who suggest that discrepancies in tube length from the predicted values using the 'known' density of amorphous titanium dioxide can be due to the inclusion of impurities that change the density of the oxide. Whether this is the reason or the empirical, simplified nature of this model, its accuracy for a wide range of conditions has been verified.…”

Excited state properties of anodic TiO2 nanotubes

“…The latter is in agreement with the characteristic conversion of titanium to porous oxides under our specific electrochemical conditions. 19 Moreover, the current density for two samples is illustrated in Fig. 1(e).…”