Study On Growth, Structure And Properties Of Anodic Oxides On Titanium

Titanium oxides prepared by electrochemical anodization methods possess highcorrosion resistance, excellent photocatalytic activities, good biomedical compatibility,low-cost, non-toxicity and easy-controllable, therefore have been widely used in the areas ofcorrosion protection, pollutants decomposition, water splitting, dye-sensitized solar cells,sensors, batteries and biomedicine. The preparaed anodic oxides can be either compact barrierfilms or porous nanotubes depending on the used electrolytes. However, for the growth andcrystalline mechanisms of titanium oxide films in different anodization conditions, it is stillnot very clear. Moreover, the TiO2nanotubes (NTs) formed in F contained electrolyte havetheir limitations due to the highly aggressive of F for the formed titanium oxides. In thisthesis, the effects of various anodization parameters on the growth and crystallization oftitanium oxide films in H2SO4solution are investigated in details, and the growth process andcrystalline mechanisms of titanium oxide films in different anodization conditions are alsodiscussed. In addition, a new gentle HBF4contained electrolyte is employed for thefabrication of anodic TiO2NTs. The details are presented as below:The influences of anodization modes on growth process and crystallizing mechanisms ofanodic oxide films on titanium are first studied. The results show that the potentiostaticallygrown film is thicker, rougher and more crystalline than the film formed at potential-sweepand combined modes. For titanium oxide film formed in the potentiostatic mode,“flower-like”crystalline grains are developed due to very high local current density at the local defect sitesof titanium surface. In the case of potential-sweep and combined anodization modes, thecrystallization of titanium oxide film is homogeneous, which is mainly caused by the internalcompressive stresses formed during the growth of anodic oxides.Comparisons of the structure and properties of the anodic oxides films formed on themechanically abraded bulk titanium and the sputter-deposited titanium substrates are alsopresented. The titanium substrates can largely influence the properties of the formed anodicfilms. A more smooth and compact titanium oxide film could grow on the sputter-depositedtitanium substrate, which is unfavorable to the ionic migration through the film and thendelays the film growth and crystallization. The influence of anodizing time on formation and crystallization of the potentiostaticallyformed titanium oxide films is studied both at low and high applied potentials. It is revealedthat prolonging the anodizing time is beneficial for the growth and crystallization of titaniaanodic films. The titanium anodization process follows two distinct stages. In the filmformation stage, the film thickness and crystallinity increase fast. While in the film agingstage, the thickness and crystallinity of titanium oxide films only slightly change withanodizing time due to the enhancement of film dissolution rate.A new BF4contained electrolyte is introduced for the preparation of anodic TiO2NTs.The obtained TiO2NTs are highly ordered and extremely smooth, have unique hierarchicalupper-nanoporous and lower-nanotubular structure, and show enhanced photocatalyticactivities than the TiO2NTs formed in common used F contained electrolyte. It is consideredthat the decomposition of BF4into F under high electric field is the key for the formation ofanodic TiO2NTs in BF4contained electrolyte.Three different Ti substrates, the rough abraded Ti foil, the smooth electropolished Ti andthe two-step Ti plate, are employed for the preparation of TiO2NTs. It is revealed that the Tisubstrate morphology can largely influence the formation of TiO2NTs. The two-step Tisubstrate has ordered hexagonally distributed dimples which play the role of template for theformation of new TiO2NTs. As a result, the TiO2NTs grown via a two-step anodizationapproach have highly ordered structure and extremely smooth sidewalls, hence possess thebest photoelectrochemical performance among the three kind of substrates.The morphology and properties of anodic TiO2NTs are also improved by adding lacticacid or gluconic acid to the unique BF4contained electrolyte. It is shown that with specificadditives to the solution, the geometrical morphology and application properties of theobtained TiO2NTs can be significantly enhanced. We ascribe this improvement to thedecrease of surface OH groups content on TiO2NTs, which is related to the chelating effect ofthe added weak organic acids.In general, the compact titanium oxides barrier layers are first prepared in0.1mol·L-1H2SO4solution. It is shown that the growth and crystallization of titanium oxide films aredetermined by the anodizing potential, time, anodization mode and titanium substrate. Theabove results are beneficial for the understanding of the formation, growth and crystallization mechanisms of anodic films on titanium or other metals. Moreover, ordered anodic TiO2NTsare successfully prepared in a new BF4contained electrolyte. The obtained TiO2NTs haveunique nanopore/nanotube hierarchical structure and show enhanced photocatalytic properties.The research provides an alternative way for the preparation of TiO2NTs with F freeelectrolyte.