Effect of Composition on the Photoelectrochemical Behavior of Anodic Oxides on Binary Aluminum Alloys

Abstract

The photoelectrochemical behavior of anodic films on Al alloys, containing titanium, tantalum, and tungsten ͑valve metals͒, has been studied as a function of alloy composition and anodizing conditions. Photocurrent spectroscopy has been used to get information on bandgap and the flatband potential values of different mixed oxides. Both insulator-like and semiconducting behavior has been observed for anodic oxides grown on Al-W and Al-Ti alloys dependent on alloy initial composition. Anodic films on valve metals alloys are of interest for investigating mechanisms of oxide growth; different works 1-6 have mainly focused their attention toward their morphology, structure, and composition to get insights on mechanism of anodic oxidation and ionic transport in "mixed oxides." Because these films are also of interest for possible application in electronics, for instance in metal-oxide semiconductor junctions [7][8][9][10][11][12] or in dye-sensitized photoelectrochemical and solid-state solar cells, 13,14 the possibility of controlled modification of the solid-state properties, such as bandgap, flatband potential, and dielectric constant, by use of "mixed oxides," is appealing from practical and theoretical viewpoints. This aspect becomes more important if we consider the prominent role of the solid-state properties of passive layers in controlling also the pitting behavior of metals and alloys. [15][16][17] Notably, it has been proposed that the bandgaps of crystalline binary oxides correlate with the electronegativities of their constituents, thus suggesting the possibility to predict the bandgaps of ternary oxides, 18-20 using an average electronegativity parameter for the cationic group and hence, tailoring of oxide properties. The correlation can apply also to amorphous oxides if account is taken of the influence of disorder on the optical bandgap value. 20,21 In the present work, a study of the solid-state properties of anodic films on several Al alloys was carried out by using photocurrent spectroscopy ͑PCS͒ to measure their bandgap and flatband potential. The validity of the above mentioned correlation was tested to get information about its applicability and limits. ExperimentalAl-23 atom % Ti, Al-53 atom % Ti, and Al-34 atom % Ta alloys, prepared by physical vapor deposition, and Al-W alloys, containing 15, 23, and 77 atom % W, prepared by dc magnetron sputtering, were anodized either potentiodynamically, at 100 mV s −1 , or galvanostatically, at 5 mA cm −2 in either 0.1/0.01 M ammonium pentaborate electrolyte ͑ABE͒, of pH 8.5, or 0.01 N H 2 SO 4 solution at room temperature. A three-electrode cell, with a saturated calomel electrode ͑SCE͒, and a two-electrode cell with a platinum cathode, were used in potentiodynamic and galvanostatic experiments, respectively.The composition of Al-W alloys and anodic oxides grown on them were analyzed by Rutherford backscattering spectroscopy ͑RBS͒, using 1.83 MeV He + ions supplied by the Van de Graaff accelerator of the University of Paris. The scattered ions were detect...

Physicochemical Characterization of Passive Films and Corrosion Layers by Differential Admittance and Photocurrent Spectroscopy

“…23 where it is reported the fitting of the band gap values as a function of the compositional parameter χ c obtained for different amorphous ternary oxides, containing both sp-metal (i. e. Al 3+ ) and d-metal cations, grown by anodizing sputter-deposited or physical vapour deposited metal alloys. 143 In both cases a ΔE am value in agreement with the theoretical expectation was derived from the fitting of experimental data and, significantly, larger ΔE am value were measured for anodic films made by oxides having a tendency to grow in truly amorphous state. Further evidence in favour of the proposed corre-lation for anodic films grown on Mo-Ta, Ti-Zr, Hf-W metallic alloys can be found in Refs.…”

“…Due to the limited number of systems investigated it remains unsolved the limits of applicability of such correlation to the sp,d-metal mixed oxides. Very recent results both for crystalline bulk Mg-Zn 142 oxides and amorphous anodic oxide on Al-W alloys 143 alloys seem to suggest that when the atomic fraction of d-metal in the ternary oxides reaches values lower than 20% the experimental E g data are better interpolated by the sp-metal oxide correlation. These aspects need further investigations aimed to better define the limits of validity of the proposed semiempirical correlation.…”

“…We have to mention that apart the nature of the oxide other experimental parameters can affect the lattice disorder degree of passive films and then the ΔE am value in anodic films. 127,143,146 …”

Photocurrent spectroscopy applied to the characterization of passive films on sputter-deposited Ti–Zr alloys

“…1c). As already found for several anodic films on valve metals and valve metal alloys [13][14][15][16], a possible origin of such dependence is the variation of the light absorption coefficient according to the following law:…”

Enhanced Capacitance of Composite Anodic ZrO2 Films Comprising High Permittivity Oxide Nanocrystals and Highly Resistive Amorphous Oxide Matrix

“…However, there is a positive empirical correlation between  ox and  (Figure 1). Valve metals such as aluminum, [7][8][9][10][11][12][13] niobium, [14][15][16][17][18] and titanium [19][20][21][22][23][24][25] were alloyed in order to control the properties of anodic oxide films and improve their dielectric properties. The general trend in amorphous anodic oxide films is that their properties on binary valve metal alloys-including the relative permittivity and formation ratio-are the compositional averages of the respective alloy-constituting metals.…”