Anodic Process Of Titanium And Forming Mechanisms Of Anodic Titanium Oxide Nanostructures

Anodic TiO₂ nanotubes have attracted considerable attention due to unique nanostructure.Due to high specific surface area and length-diameter ratio,anodic TiO₂ nanotubes have various applications,including supercapacitors,photocatalysis,dye-sensitized solar cells,etc.However,the formation mechanism of anodic TiO₂ nanotubes remains unclear till now.Systematic study of the whole anodization process under different conditions and the formation mechanism of anodic oxide are necessary.Based on oxygen bubble mould combined with electronic current and ionic current,the anodization process was explored in detail.The formation processes of some novel structures during anodization were explained reasonably.First of all,the one-step anodization and two-step anodization were applied.There is a linear relationship between tube length and anodizing current during two-step anodization.The TiO₂ nanotubes obtained by two-step were highly ordered,as the remained hemispherical concaves left by the first anodization provide quick access for electrolytic transmission and oxygen evolution.Secondly,anodization was conducted based on Ti foils with a compact oxide layer to explore the formation process of nanotube embryos.After 30-second anodization,nanotube embryos were formed,which looked like some converse micro-parachutes.After long-time anodization,TiO₂ nanotubes were obtained with compact oxide layer above.The compact oxide layer was not open,which put the field-assisted dissolution into question.The reasonable explanation was provided based on the combination of anion incorporation,oxygen bubble mould and plastic flow model,as below:At stage ?,an initial exponential decay of the anodizing current is due to the decrease of ionic current jion = A exp?BU/d?with the increase of oxide thickness.Meanwhile,electrolyte species incorporate into the film and anionic incorporation layer is formed near the electrolyte/oxide interface.At stage ?,the increase of the anodizing current is due to the generation of electronic current je?j0exp??d?and its steep increase at the critical thickness dc.The je and oxygen evolution are associated with the anionic incorporation or amorphous-to-crystalline transition.The nanotube embryo results from the oxide flow around the oxygen bubble remained within the film.The pressure in oxygen bubble is the precondition of the oxide flow from the pore base to the pore wall.At stage ?,the stable anodizing current results from the constant jion and constant je,because the corresponding barrier oxide maintains the critical thickness dc.The jion and the je are interdependent and independent,and they are a couple of driving forces during the TiO₂ nanotube forming process.The dissolution reaction may exist in the anodizing process,but it does not contribute to the anodizing current.Then,TiO₂ nanotubes with ribs were fabricated by anodization under various conditions,which means that the formation of ribs does not depend on anodizing parameters.When the entire surface oxide layer was destroyed,the ribs around nanotubes can be formed.The cracked surface oxide layer can provide access between electrolytes and Ti foils.When the infiltration of electrolyte into the gaps of the nanotubes was inhibited or the electrolytes around gaps run out,the ribs around nanotubes cannot be formed.Further,multilayer TiO₂ nanotubes were formed by anodization in HBF4 electrolyte with A-shaped sidewalls.Compared with NH4F electrolyte,anion incorporation takes more time in HBF4 electrolyte,which contributes to thicker barrier oxide.The thicker barrier oxide blocks the oxygen evolution,which results in the A-shaped nanotubes.The formation of multilayer TiO₂ nanotubes is caused by unique escape way of oxygen evolution in the HBF4 electrolytes.The influence of H₂O content on the morphology of nanotubes was explored in detail in H₂SiF6 electrolyte.TiO₂ nanosponge structure was obtained under high H₂O content,as high H₂O content contributes to high ionic current and low electronic current.Little oxygen evolution along with fast oxide growth contributes to the TiO₂ nanosponge structure.Finally,anodizations under different applied current were conducted.High anodizing current leads to nanoribbon structure.The fitting results showed that the electronic current under anodizing current of 12.5,25 and 37.5 mA/cm2 is 6.3,15.2 and 33.3 mA/cm2,respectively.High electronic current contributes to intense oxygen evolution,while the exchange between oxygen evolution and electrolyte entry brings great shear effects on the walls of nanotubes.Due to the presence of the shear effects,the nanotube structure changes into nanoribbon structure.