Parameterization and Extension of a Model of Oxide Growth by a Multi-Method Approach

Abstract

The technology to form anodic oxides on valve metals will strongly benefit from optimization of process parameters based on deterministic modelling. The relatively large number of adjustable parameters precludes the unambiguous interpretation of steady-state and transient electrochemical data in terms of a unique kinetic model. An approach to overcome this challenge by parameterization of a model of film growth and dissolution using a combination of in situ electrochemical impedance spectroscopy (EIS) in a large frequency range, dynamic high-frequency impedance and photocurrent measurements, with ex situ characterization of the oxides by X-ray photoelectron spectroscopy (XPS) is presented. Additional mechanistic information is derived from density functional theory modelling of adsorption of water and fluoride on TiO2. An extension of the model allowing for a multistep transpassive dissolution reaction at the film/solution interface is also parameterized using voltametric, EIS and XPS data for the anodic oxidation of Mo in concentrated H3PO4.

Anodic Oxidation of Tungsten under Illumination-Multi-Method Characterization and Modeling at the Molecular Level

“…The main assumption of the proposed model, based on the experimental results obtained is that non-stoichiometric WO 3 , containing significant amounts of W(V) in the cation sublattice is formed on W, whereas an appreciable concentration of oxygen vacancies is present in the anion sublattice. The formation of the oxide proceeds at the metal/film interface with oxidation of the metal and generation of oxygen vacancies [22]:…”

“…In addition, recombination of cation and oxygen vacancies re-creates the perfect lattice via the inverse Schottky reaction [17][18][19][20][21][22]:…”

“…β being the surface concentration of cation positions. Accordingly, the steady-state current is given by the expression [22]:…”

“…The impedance of ion migration in the oxide is a parallel combination of the impedances due to oxygen and cation transport via vacancy mechanism. The faradic impedance due to migration of anion vacancies is derived as [22]:…”

“…More than a decade ago, a systematic study of the initial stages of anodic oxidation of valve metals such as W, Ti, and Mo was started in our group, aiming at the development and parameterization of a deterministic model of the process [17][18][19][20][21][22]. The proposed model was successfully validated by quantitative comparison to electrochemical, photoelectrochemical, and X-ray photoelectron spectroscopic (XPS) data and key parameter values pertinent to oxide growth, restructuring, and dissolution were estimated.…”