Anodizing of Self-Passivating W_xTi_1–x Precursors for W_xTi_1–xO_n Oxide Alloys with Tailored Stability

Abstract

TiO2 and WO3 are two of the most important, industrially relevant earth-abundant oxides. Although both materials show complementary functionality and are promising candidates for similar types of applications such as catalysis, sensor technology, and energy conversion, their chemical stability in reactive environments differs remarkably. In this study, anodic barrier oxides are grown on solid-solution WxTi1-x alloy precursors covering a wide compositional range (0 x 1) with the goal of creating functional oxides with tailored stability. A strong Ti-cation enrichment in the surface region of the grown WxTi1-xOn layer is observed, which can be controlled by both the anodizing conditions and precursor composition. For Ti concentrations above 50 at. %, a continuous nanometer-thick TiO2 protective coating is achieved on top of a homogeneous WxTi1-xOn film as evidenced by X-ray photoelectron spectroscopy and transmission electron microscopy analyses. A comprehensive electrochemical assessment demonstrates a very stable passivation of the surface in both acidic and alkaline environments. This increase in chemical stability correlates directly with the presence of this protective TiO2 film. The results of this work provide insights into the oxidation behavior of W1-xTix alloys, but more importantly demonstrate how controlled oxidation of self-passivating alloys can lead to oxide alloys with thin, protective surface layers that otherwise would require more sophisticated deposition methods.

Effect of atomic structure on preferential oxidation of alloys: amorphous versus crystalline Cu-Zr

“…Moreover, the preferential oxidation of Zr is accompanied by an inward migration of the reaction front, which result in a Cu enrichment in the parent alloy below the reaction front, especially for high oxidation rates (as observed in Ref. [27] ).…”

A combinatorial guide to phase formation and surface passivation of tungsten titanium oxide prepared by thermal oxidation

“…anodizing vs. thermal oxidation). However, for Ti alloying contents below this critical Ti concentration, the barrier anodizing process produces a much more pronounced Ti-surface enrichment in the formed oxide phase ( Figure 7B) 48 . During barrier anodizing a very high external electric field is applied to accelerate oxide formation on the W 1-x Ti x alloy, far from (local) thermodynamic equilibrium conditions, which apparently results in a relatively high Ti concentration at the developing oxide surface.…”

“…Figure 7B summarizes the effect of preferential oxidation on the evolving oxide surface composition as a function of alloying concentration and oxidation temperature. We recently reported on the formation of TiO 2 -passivated W 1-x Ti x O n oxide phases, prepared via barrier anodizing of alloy precursors 48 . Strikingly, the critical Ti alloying content required for the formation of a continuous and protective TiO 2 surface layer is similar for both oxidation processes (i.e.…”

“…Lower oxidation temperatures provide better passivation. *Ti-enrichment upon barrier anodizing from our previous work is displayed for comparison48 .…”

Concepts for chemical state analysis at constant probing depth by lab‐based XPS/HAXPES combining soft and hard X‐ray sources

“…Depth‐resolved analysis by HAXPES at the synchrotron or by DS‐XPS in the lab also provides extended capabilities for studying the electronic properties of energy conversion materials, for example, of band bending effects due to the presence of interface or surface states 19–22 and/or gradients in doping, which can cause a shift in the Fermi level (and thus of the photoelectron lines) as function of depth below the surface (see Figure 1B). Provided that the angular lens opening (or segment) for electron detection is not too large (presumably less than or equal to ±10°), depth‐dependent gradients in valence states and/or chemical composition 18,23–25 could also be nondestructively assessed in the laboratory by combining DS‐XPS with angle‐resolved analysis.…”