Formation and Crystallization of Anodic Oxide Films on Sputter-Deposited Titanium in Potentiostatic and Potential-Sweep Modes

Abstract

Sputter-deposited titanium substrates were anodically treated in sulfuric acid solution both in potentiostatic and potential-sweep modes. The morphology, crystallization, chemical compositions and electrochemical properties of anodic titanium oxide films were detected by AFM, SE, Raman spectra, XPS and EIS. The formed anodic films are smooth and homogeneously crystallized, and that the potentiostatically grown film is slightly thicker and less crystalline than the potentiodynamically formed film. Moreover, a comparison of the structure and properties of the anodic oxides films formed on the mechanical-chemical polished bulk titanium and the sputter-deposited titanium substrates is also presented. The titanium substrates can largely influence the properties of the formed anodic films. A more smooth and compact titanium oxide film could grow on the sputter-deposited titanium substrate, which is unfavorable to the ionic migration through the film and delays the film growth and crystallization.Thin anodic oxide films on titanium are well-known for their high corrosion resistance, 1 good biomedical compatibility 2,3 and excellent photocatalytic activity. 4,5 The crystalline structure, surface morphology, chemical compositions and electrochemical properties of anodic films on titanium are influenced by various anodization parameters. [6][7][8] In general, the growth and crystallization of anodic titanium oxide films can be promoted by raising the applied voltage, 7,9,10 prolonging the anodizing time, 10-13 enhancing the solution temperature, 7,14 increasing the electrolyte concentration 12,13 or decreasing the film growth rate. 15,16 With the incorporation of impurity ions (anions from the electrolytic solution 17 or cations from the titanium alloy) 18-20 in the anodic titanium oxide layers, the film crystallization is suppressed to relatively high voltages. Besides, as compared to the slow-grown anodic titanium oxide films (in potential-sweep mode or galvanostatic mode), the fast-grown films (in potentiostatic mode) are much thicker and more crystalline. 8,[21][22][23] The preparation and surface pretreatment of titanium substrates can also influence the formation and crystallizing process of anodic titanium oxide films. Generally speaking, commercial pure titanium sheets or highly pure titanium plates are the most common specimens used for titanium anodization, and the samples are usually pretreated by mechanical polishing and (or) chemical polishing before electrochemical treatments. As compared to the anodic films produced on unpolished titanium substrates, the films grown on mechanical polished titanium surface are smoother and more compact and have a better corrosion resistance. 11 The chemical polishing of titanium substrates prior to anodization also plays an important role in the anodic film formation. For instance, Kozlowski et al. revealed that crystalline titanium oxides could be only obtained after the naturally formed oxide film on the titanium surface (with the thickness of about 5 nm) was removed by che...

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