Oxidation kinetics of nitrogen doped TiO2−δ thin films

Abstract

The oxidation kinetics of nitrogen doped, oxygen deficient titanium dioxide thin films has been studied in atmospheres of pure oxygen or nitrogen at 500 °C, 550 °C, and 600 °C, respectively, by means of in situ optical spectroscopy. The thin films show high electronic absorbance in the visible and NIR region, accompanied by a red shift of the absorption edge of about 0.4 eV, e.g., from about 2.9 to 2.5 eV at 600 °C. The time dependent decrease of absorbance due to oxidation is found to follow a parabolic rate law. An activation energy of about 1.96 eV can be obtained from the temperature dependence of the parabolic oxidation rate constant. In the framework of a microscopic oxidation model, this energy barrier is attributed to the diffusion of titanium interstitials in the re-oxidized part of the thin films as a rate-determining process. In addition, an attempt is made to evaluate the kinetics of nitrogen release from the time dependent blue shift of the absorption edge during re-oxidation.

Controlling the Electrical Properties of Undoped and Ta‐Doped TiO2 Polycrystalline Films via Ultra‐Fast‐Annealing Treatments

“…This would further reduce the concentration of electrons, in addition to the effect of the oxygen partial pressure which has been shown to be the ruling mechanism. [42] Finally, it is noteworthy how fast the crystallization process proceeds leading to the formation of anatase grains with a typical lateral size of several micrometers within only few minutes. This is due to the characteristic 'explosive' crystallization of TiO 2 .…”

Defect Chemistry of Pr Doped Ceria Thin Films Investigated by in Situ Optical and Impedance Measurements

“…18 Optical absorption has also been used to probe the oxidation state of multivalent cations and/or to investigate redox kinetics in oxide single crystals 19−24 and thin films. 25,26 Optical characterization techniques benefit from the ability to readily achieve in situ and contact-free measurements, and with high spatial resolution, due to the local nature of the interaction of electromagnetic radiation with solid matter. Our group demonstrated that this optical approach can be applied to PCO thin films for in situ recording of redox kinetics as well as the equilibrium defect concentrations ([Pr 4+ ] and consequently δ), by monitoring the change in transmission spectra upon oxygen activity and/or temperature change.…”

“…We and others recently have demonstrated that the chemical capacitance ( C chem ), obtained from in situ electrochemical impedance spectroscopy (EIS) measurements, can be used to quantify δ in dense oxide thin films over wide limits with high precision. − Furthermore, we earlier demonstrated the ability to both control δ by applying a DC bias across an electrochemical cell and to study its defect chemistry by C chem measurements even to high oxygen activities (up to 280 atm), not readily accessible by conventional means . Optical absorption has also been used to probe the oxidation state of multivalent cations and/or to investigate redox kinetics in oxide single crystals − and thin films. , Optical characterization techniques benefit from the ability to readily achieve in situ and contact-free measurements, and with high spatial resolution, due to the local nature of the interaction of electromagnetic radiation with solid matter. Our group demonstrated that this optical approach can be applied to PCO thin films for in situ recording of redox kinetics as well as the equilibrium defect concentrations ([Pr 4+ ] and consequently δ), by monitoring the change in transmission spectra upon oxygen activity and/or temperature change. , Moreover, with the novel arrangement of simultaneous in situ optical and EIS measurement on the one same film, our preliminary studies at one temperature (600 °C) demonstrated that both defect thermodynamics and surface exchange kinetics in PCO thin films can be characterized and modeled. , …”

Investigation of Nonstoichiometry in Oxide Thin Films by Simultaneousin SituOptical Absorption and Chemical Capacitance Measurements: Pr-Doped Ceria, a Case Study

“…While other researchers have previously used optical absorption to probe the oxidation state of multivalent cations and/or to investigate redox kinetics in oxide single crystal − and thin films, in this work, 10PCO thin films were simultaneously examined, for the first time , by in situ optical absorption and EIS measurement as a function of temperature and pO 2 , thereby enabling investigation of nonstoichiometry, on the same film, by both chemical capacitance and absorption change. This both confirms that optical absorption can serve as a viable in situ means for monitoring δ in thin films and provides the means for extracting the magnitude of δ as a function of T and pO 2 .…”