Harrington GF, Kalaev D, Yildiz B, Sasaki K, Perry NH, and Tuller HL
ACS Applied Materials & Interfaces [ACS Appl Mater Interfaces] 2019 Sep 25; Vol. 11 (38), pp. 34841-34853. Date of Electronic Publication: 2019 Sep 11.
The oxygen deficiency or excess, as reflected in the nonstoichiometry of oxide films, plays a crucial role in their functional properties for applications such as micro solid oxide fuel cells, catalysis, sensors, ferroelectrics, and memristors. High concentrations of oxygen vacancies may be beneficial or detrimental according to the application, and hence there is interest in controlling the oxygen content of films without resorting to compositional changes. Here, we demonstrate that substantial changes in the nonstoichiometry of Pr0.1Ce0.9O2-δ (PCO), a model mixed ionic electronic conductor, can be achieved by fabricating multilayers with an inert material, SrTiO3 (STO). We fabricated heterostructures using pulsed laser deposition, keeping the total thickness of PCO and STO constant while varying the number of layers and thickness of each individual layer, to probe the effects of the PCO/STO interfaces. Conductivity measurements as a function of oxygen partial pressure (PO2) and temperature showed a significant weakening of the PO2 dependence compared to bulk PCO, which scaled with the density of interfaces. We confirmed that this change was due to variations in nonstoichiometry, by optical transmission measurements, and show that the lower oxygen content is consistent with a decrease in the effective oxygen reduction enthalpy of PCO. These results exemplify the dramatic differences in properties between films and their bulk counterparts, achievable by interface engineering, and provide generalized insight into tailoring the properties of mixed ionic electronic conductors at the nanoscale.
Chen T, Harrington GF, Masood J, Sasaki K, and Perry NH
ACS Applied Materials & Interfaces [ACS Appl Mater Interfaces] 2019 Mar 06; Vol. 11 (9), pp. 9102-9116. Date of Electronic Publication: 2019 Feb 20.
The oxygen surface exchange kinetics of mixed ionic and electronic conducting oxides (MIECs) play a critical role in the efficiency of intermediate-to-high-temperature electrochemical devices. Although there is increasing interest in low-temperature preparation of MIEC thin films, the impact of the resultant varied degrees of crystallinity on the surface exchange kinetics has not been widely investigated. Here, we probe the effect of crystallization on oxygen surface exchange kinetics in situ, by applying an optical transmission relaxation (OTR) approach during annealing of amorphous films. OTR enables contact-free, in situ, and continuous quantification of the oxygen surface exchange coefficient ( kchem); we previously applied it to Pr xCe1- xO2-δ and SrTi1- xFe xO3-δ thin films. In this work, the OTR approach was successfully extended to other mixed conducting thin film compositions for the first time (i.e., perovskite SrTi0.65Co0.35O3-δ and Ruddlesden-Popper Sr2Ti0.65Fe0.35O4±δ), as well as to Pr0.1Ce0.9O2-δ, enabling quantification of the kchem of their native surfaces and comparison of the behavior of films with different final crystal structures. All thin films were prepared by pulsed laser deposition at 25 or 700-800 °C and subject to subsequent thermal treatments with simultaneous OTR monitoring of kchem. The surface roughness, grain size, and crystallinity were evaluated by scanning probe microscopy, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Fluorite Pr0.1Ce0.9O2-δ films grown at 25 °C did not exhibit an increase in kchem after annealing, as they were already crystalline as grown at 25 °C. For all other compositions, OTR enabled in situ observation of both the crystallization process and the emergence of rapid surface exchange kinetics immediately upon crystallization. Perovskite SrTi0.65Co0.35O3-δ and Ruddlesden-Popper Sr2Ti0.65Fe0.35O4±δ thin films grown at 25 °C exhibited at least 1-2 orders of magnitude enhanced kchem after annealing compared with highly crystalline thin films grown at 800 °C, indicating the benefits of in situ crystallization.