Anodizations of Al and Ti in NH₄F or H₃PO₄ Solutions and Formation of Porous Anodic Alumina with Special Morphology

Abstract

Porous anodic alumina (PAA) and anodic TiO 2 nanotubes (ATNTs) have been widely investigated for decades. However, their formation mechanism and growth kinetics remain unclear. Here two unconventional and two conventional anodizations of aluminum and titanium are contrasted to overcome this challenge. PAA with special morphology was fabricated in NH 4 F electrolyte for the first time, which cannot be explained by the popular field-assisted theory. Combining the oxygen bubble mold and the oxide flow model with the dissolution model, a new explanation for the special morphology is presented. In addition, anodic titanium oxide films with plenty of cavities were obtained in H 3 PO 4 electrolyte, which absolutely differ from the general compact films. The cavities in anodic titanium oxide films also result from the oxygen bubbles within the oxide films. The anions in electrolyte (e.g., F − , OH − , PO 4 3− ) accumulate, and the anion-contaminated layer (ACL) forms due to the electric field. The ACL plays a decisive role in the generation of the electronic current (J e ) and the formation of oxygen bubble mold. Regular ATNTs were obtained as titanium was anodized in NH 4 F electrolyte. The ACL forms and appropriate J e generates because the dissolution from F − on TiO 2 is limited. However, PAA with special morphology was obtained as aluminum was anodized in NH 4 F electrolyte. Thicker ACL results in the impulse peak of the J e because the dissolution and corrosion from F − on alumina is severe.

A review: research progress on the formation mechanism of porous anodic oxides

“…23,[36][37][38]89 According to the electronic current and ionic current theory, the total current in the anodizing process is composed of ionic current and electronic current. 46,69,74,85,88,[90][91][92][93][94][95][96][97] Among them, the ionic current dominates the growth of oxides (main reaction), and the…”

“…The anion-rich area is called the anion contaminated layer (ACL) in the oxygen bubble mould effect theory. 11,45,87,90 The above process is shown in Fig. 4a and Fig.…”

Evidence of oxygen bubbles forming nanotube embryos in porous anodic oxides

“…The electronic current in the anodization of Al is larger than that of Ti in the same phosphoric acid electrolyte. 64 The expansion of oxygen bubbles in the porous anodic alumina causes the disappearance of the gap between nanotubes, thus forming the porous anodic alumina.…”

Growth Model of the Tin Anodizing Process and the Capacitive Performance of Porous Tin Oxides

“…The simple electrochemical method is also very convenient for wide applications. 5−12 It is universally acknowledged that simple anodization can enable fabrication of nanostructured oxide layers on the surface of various metals and alloys, such as Al, 6,7 Ti, 8,9 Pb, Co, Zr, 10,11 Ti-2Al-1.5Mn alloy, 12 etc. However, the formation mechanisms of these porous oxides are still controversial.…”

“…Contemporarily, nanostructured SnO 2 materials have potential application value in the fields of dye-sensitized solar cells, gas sensing, lithium-ion batteries, photoelectric catalysis, etc. The simple electrochemical method is also very convenient for wide applications. − It is universally acknowledged that simple anodization can enable fabrication of nanostructured oxide layers on the surface of various metals and alloys, such as Al, , Ti, , Pb, Co, Zr, , Ti-2Al-1.5Mn alloy, etc. However, the formation mechanisms of these porous oxides are still controversial. − In the same way, a simple one-step anodization carried out in oxalic acid − and alkaline (e.g., NaOH or NH 3 ) or sulfide- and fluoride-based electrolytes can also result in the formation of nanostructured SnO 2 . − In an alkaline electrolyte, a crack-free tin oxide layer can be fabricated via simple one-step anodization in NaOH at low applied potentials .…”