Quantitative Analysis of Oxide Growth During Ti Galvanostatic Anodization

Abstract

The mechanism of TiO 2 nanotubes has attracted increasing attention. However, relationships between the anodizing parameters and nanotube size (or oxide volume) have been rarely studied. The traditional field-assisted dissolution theory can only qualitatively explain the cause of the pore formation but cannot quantitatively explain the relationship between the growth height of nanotubes and the anodizing current. Here, the growing processes of TiO 2 nanotubes in four current at three different NH 4 F concentrations were carefully studied. Experimental results contradicting the traditional dissolution theory were found. Considering that the height of the nanotubes does not fully reflect oxide growth, the volume of growing oxide was studied for the first time. The results show that the oxide volume decreases while the pore volume increases with the increase of the concentration of NH 4 F in the process of constant current anodization. However, the sum of the two volumes was approximately a fixed value under the same current. The sum of the oxide volume and the pore volume increases with the increase of the anodizing current, regardless of the concentration of ammonium fluoride. It means that the growth height of nanotubes has nothing to do with the concentration of NH 4 F, which is against the field-assisted dissolution theory.

Unraveling the six stages of the current–time curve and the bilayer nanotubes obtained by one-step anodization of Zr

“…31 Zhang and Yu et al proved that there is no dissolution equilibrium in the anodizing process of titanium. 34,[41][42][43] The growth rate of titanium oxide nanotubes reaches 200 nm min À1 , which is much higher than the dissolution rate of titanium oxide in electrolyte (about 1 nm min À1 ). 62 Many references above have negated the FAD dissolution equilibrium, and the three stages of the current-time curve in Fig.…”

“…32,33 Yu et al showed that the growth rate of porous alumina channels was 150 nm min À1 . 20 Even though many facts deny the FAD theory and dissolution equilibrium theory, [32][33][34][35][36][37][38][39][40][41][42][43] many researchers still ambiguously explain various experimental phenomena by applying the FAD and dissolution equilibrium theories. [44][45][46][47] The second model is an oxygen bubble model based on the viscous ow model [48][49][50] and the electronic current theory.…”

“…[45][46][47][48][49][50][51] The physical meanings of FAD and FAF are always unclear. [15][16][17][18][19][20] Aer discussing the relationship between the volume (or length) of the porous oxide and the quantity of electric charge (the product of electric current and time), 24,36,43 the growth kinetics of the oxygen bubble model becomes clearer. That is, the ionic current causes barrier oxide growth, and the electronic current causes oxygen evolution.…”

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

“…[18][19][20][21][22][23] Zhou et al reported that the growth rate of nanotubes in electrolytes with 2 wt% H 2 O was signicantly faster than that in electrolytes with 10 wt% H 2 O. 24 Zhang et al suggested that the growth rate of nanotubes had nothing to do with the concentration of NH 4 F. 25 In addition to studying the effects of water and uoride ions, researchers have also explored many other factors that inuence the growth of anodic TiO 2 nanotubes, such as voltage and temperature. [26][27][28] Mohan et al reported that higher temperature caused a rise in the dissolution rate of oxides.…”

Evidence of oxygen bubbles forming nanotube embryos in porous anodic oxides

“…Many researchers also demonstrated that the dissolution of titanium oxide by fluoride ion is also weak. [22][23][24][25] Why is the formation mechanism of porous anodic oxides still controversial up to now? We believe that the main reason is that the relationship between the above field-assisted dissolution reactions and anodizing current has not been clearly understood.…”