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

Abstract

Simultaneous in situ optical absorption and electrochemical impedance spectroscopy measurements were performed, for the first time, at elevated temperature on a metal oxide thin film exhibiting oxygen nonstoichiometry, utilizing Pr0.1Ce0.9O2−δ (10PCO) as a model system. Chemical capacitance measurements, capable of providing explicit values of δ, were used to determine the optically absorbing center (Pr4+) concentration and thereby the optical extinction coefficient for Pr4+. The absorption coefficient was found to exhibit a linear dependence on Pr4+ concentration, validating the use of optical absorption to examine defect concentration trends and allowing derivation of the extinction coefficient εPr4+ = 5.01 ± 0.14 × 10–18 cm2. Values of Pr4+ concentration derived from the chemical capacitance and corresponding trends in optical absorption were found to be self-consistent, validating the thin film defect model for 10PCO, thereby confirming that the oxygen reduction enthalpy in thin film 10PCO is lower than that in the bulk. The non-contact optical absorption technique thus provides an additional in situ method for investigating the defect equilibria of thin films and is expected to aid in confirming whether and under what conditions the defect thermodynamics of films differ from that of their bulk counterparts.

Praseodymium Doped Ceria: The Role of 4f-Electrons of Pr Dopant in Doped Ceria

“…Also, almost identical geometrical properties are observed between the two models as shown in Table I in terms of cationanion bond-length at local dopant-vacancy configurational shell. (13,14). reveals that the reduction of Pr which involves the removal of oxygen atoms from the lattice requires about 1.4 eV is in agreement with the calculated E vac = 1.458 eV for U eff = 4.5 eV.…”

Improving the Si Impurity Tolerance of Pr0.1Ce0.9O2−δ SOFC Electrodes with Reactive Surface Additives

“…The OTR profiles following a step change in oxygen partial pressure (pO 2 ) from 4% O 2 to 21% O 2 at 600 °C for fresh and aged (114 hours aging at 600 o C) films are shown in Figure 1. The decrease in transmitted intensity corresponds to an increase in the concentration of optically absorbing Pr 4+ centers 26 and therefore an increase in oxygen content (Equation 1) as described in the introduction. It is noted that upon exposure to the opposite step change in pO 2 , this behavior is nearly completely reproducible.…”

“…Part of the incident beam was reflected and its intensity continuously compared to the transmitted intensity using lockin amplifiers, in order to remove ambient contributions. 26 Oxygen partial pressure was controlled by the use of N 2 /O 2 gas mixtures monitored by mass flow controllers. The time for gas to flush the whole tube is about 10 seconds.…”

“…Electrons can be optically excited from the valence band into the empty Pr 4+ levels, resulting in a reddish coloration, and a consequent decrease in light transmission through the film with oxidation as more Pr 4+ is formed. 26 By following the change in optical transmission with time upon a step change in pO 2 , i.e. optical transmission relaxation (OTR), the oxygen exchange kinetics can be evaluated.…”

Roles of Bulk and Surface Chemistry in the Oxygen Exchange Kinetics and Related Properties of Mixed Conducting Perovskite Oxide Electrodes

“…Point defect chemistry of bulk electrodes can be inferred from fitting thermogravimetric measurements of oxygen content [32], electrical conductivities [33,34], and/or lattice parameters [35] as a function of temperature and oxygen partial pressure with interrelated mass action expressions of defect generating equations. In thin film electrodes, additional techniques include chemical capacitance and optical absorption measurements [36], which can both indirectly provide information about oxygen content.…”