Studying on the Point-Defect-Conductive Property of the Semiconducting Anodic Oxide Films on Titanium

Abstract

By using electrochemical measurement techniques and based on the point-defect model ͑PDM͒, this work studied the point-defectconductive property of the n-type semiconducting anodic oxide film formed on titanium under steady-state condition within the passive potential region of ca. 0-7 V in 1.0 M HClO 4 solution. The point-defect transport property was characterized by the steady-state current density ͑i ss ͒ through the film and the diffusion coefficient ͑D O ͒ of the point defects in the film. It was demonstrated that with the increase of the film formation potentials, i ss ͑and D O ͒ show an exponentially dependent increase on the potential. This behavior of the oxide films formed on titanium is to some extent different from that of some other n-type anodic oxide films. The influence of potential on the changes in structure and crystallization of the oxide film on titanium was proposed to be taken as a reason for the potential-dependence of i ss ͑and D O ͒ observed in this work. It was also shown that within the passive potential region most of any change in the applied potential does appear predominantly as a change in the potential drop within the oxide film ͑only ϳ6% of that drops across the film/solution interface͒. Additionally, a question about the values of the polarizability of the oxide film/solution interface ͑␣͒ used and estimated within the frames of the PDM was raised and discussed at the end of this paper.The titanium/TiO 2 film/electrolyte interface system has received more and more attention because it has a wide variety of scientific interest and technological applications, including corrosion inhibition, 1,2 dye-sensitized solar energy conversion cells, 3 electrocatalysis ͑or in terms of dimensionally stable anodes͒, 4,5 and the use in photoelectrocatalytical degradation of certain categories of organic pollutants in wastewater, 5-7 etc. In close relation with these applications of titanium-based electrodes, the charge transfer through the oxide film formed on titanium is the most fundamental and essential issue, which has been intensively studied, but has not yet been completely understood. Although there have been considerable experimental study reports about the electron conductive property of the oxide film on titanium, 5,8-12 only very few papers have dealt with the ion-conductive property of the oxide films on titanium. 5 The point-defect model ͑PDM͒ ͑which was developed by Macdonald and Macdonald, 13 Macdonald et al.,14 and Macdonald 15 ͒ and the related treatments such as the mixed-conduction model and the surface charge approach ͑which were developed by Bojinov, 16 Metikos-Hukovic and Grubac, 17 and Bojinov et al. 18 ͒, provide a microscopic description of the migration of the Schottky point defects under the influence of the electrostatic field in the passive oxide film during the film growth under steady-state conditions. Although the validity of the PDM was tested and verified by experimental results for passive films on many metals such as Fe

Effect of ultrasonic cavitation on the diffusivity of a point defect in the passive film on formed Nb in 0.5 M HCl solution

“…The average field strength in the case of static state, e, can be obtained from the slope of the E ff -L ss linear plot (see Fig. 6a), and the fitted slope is approximately 8.310 Â 10 6 V/cm, which is close to the field strength previously reported [31][32][33]. By analogy, the e value obtained in the case of ultrasonic cavitation with the powers of 180 W, 270 W and 450 W are taken to be 6.201 Â 10 6 V/cm, 7.331 Â 10 6 V/cm, 2.239 Â 10 6 V/cm, respectively.…”

“…(3) to calculate the diffusivity of the point defect in the passive film, which is presented in Table 2. It shows that the order of magnitude of D 0 in the case of static state ranges from 10 À17 cm 2 s À1 to 10 À18 cm 2 s À1 , which is comparable to those of point defects in passive films on Fe in a pH 8.5 buffer solution [45], and similar to those of point defects in the oxide film on Ti in a 1 M HClO 4 solution [31]. However, D 0 rapidly arises with the increment of the ultrasonic cavitation power, and D 0 increases three to four orders of magnitude by applying the ultrasonic cavitation power of 450 W. The variations of D 0 and the film formation potential/ultrasonic cavitation power are list in Table 2, it can be seen that D 0 increases with the increment of the formation potential and sharply increases with increasing ultrasonic cavitation power.…”

“…Fig. 7a shows that the steady state current density (i ss ) increases with an increase in potential, which is related to the changing of the film microstructure with formation potential [31,43].…”

“…Refs. [31][32][33] report that the field strength in the passive film can reach a magnitude of approximately 10 6 V/cm. In the case of such high field strength, the calculation of diffusivity (D 0 ) can be written as following [34]:…”

A critical assessment of the Mott-Schottky analysis for the characterisation of passive film-electrolyte junctions

“…therein) and very recently it has been observed also on amorphous TiO 2 film grown on dif ferent single grains of pure (99.9%) titanium [29]. The frequency dependence of the capacitance is reflected in Mott-Schottky plots usually showing both donor density and flat band potential values frequency dependent [4,[29][30][31][32]. When this occurs for crystalline semiconductor/electrolyte junctions several physical explanations have been suggested in the literature as possible source of this dependence [33][34][35][36] [5,[16][17][18]37] that frequency dependent differential admittance val ues are theoretically expected for amorphous semi conductor passive film/electrolyte junction in agree ment with the theory of amorphous semiconductor Schottky barriers.…”