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Atomic Resolution Imaging of Nanoscale Chemical Expansion in PrCe O During In Situ Heating x
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Jessica G. Swallow, Ja Kyung Lee, Thomas Defferriere, Gareth Martin Hughes, Shilpa N. Raja, Harry L. Tuller, Jamie H. Warner, and Krystyn J Van Vliet ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b07732 • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 17, 2018
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ACS Nano
Atomic Resolution Imaging of Nanoscale Chemical Expansion in PrxCe1−xO2−δ During In Situ Heating Jessica G. Swallow,† Ja Kyung Lee,‡ Thomas Defferriere,† Gareth Martin Hughes,‡ Shilpa N. Raja,† Harry L. Tuller,† Jamie H. Warner,∗,‡,¶ and Krystyn J. Van Vliet∗,†,§ †Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA, 02139 ‡Department of Materials, University of Oxford, Oxford, OX1 3PH, UK ¶Current address: Department of Materials, University of Oxford, 16 Parks Rd., OX1 3PH, UK §Current address: 77 Massachusetts Avenue, 8-237, Cambridge, MA, 02139, USA E-mail:
[email protected];
[email protected] Abstract Thin film non-stoichiometric oxides enable many high temperature applications including solid oxide fuel cells, actuators, and catalysis. Large concentrations of point defects (particularly, oxygen vacancies) enable fast ionic conductivity or gas exchange kinetics in these materials, but also manifest as coupling between lattice volume and chemical composition. This chemical expansion may be either detrimental or useful, especially in thin film devices that may exhibit enhanced performance through strain engineering or decreased operating temperatures. However, thin film non-stoichiometric
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oxides can differ from bulk counterparts in terms of operando defect concentrations, transport properties, and mechanical properties. Here we present an in situ investigation of atomic-scale chemical expansion in Prx Ce1−x O2−δ (PCO), a mixed ionicelectronic conducting oxide relevant to electrochemical energy conversion and high temperature actuation. Through a combination of electron energy loss spectroscopy and transmission electron microscopy with in situ heating, we characterized chemical strains and changes in oxidation state in cross-sections of PCO films grown on yttria stabilized zirconia (YSZ) at temperatures reaching 650◦ C. We quantified, both statically and dynamically, the nanoscale chemical expansion induced by changes in PCO redox state as a function of position and direction relative to the film-substrate interface. Additionally, we observed dislocations at the film-substrate interface, as well as reduced cation localization to threading defects within PCO films. These results illustrate several key aspects of atomic-scale structure and mechanical deformation in non-stoichiometric oxide films that clarify distinctions between films and bulk counterparts, and that hold several implications for operando chemical expansion or "breathing" of such oxide films.
Keywords chemical expansion, non-stoichiometric oxide, in situ transmission electron microscopy, chemomechanics, thin film, defects Non-stoichiometric oxides that support very large point defect concentrations are active materials in diverse applications including solid oxide fuel cells (SOFCs), electrolyzers, high temperature oxide actuators, catalysts, and gas sensors. 1–4 Generally, the point defects that provide the non-stoichiometry in these materials are oxygen vacancies that foster important functional properties including fast ionic conductivity and/or oxygen exchange kinetics. Formation, annihilation, and migration of these defects also tend to be coupled to material volume, manifest as chemical expansion or lattice volume coupled to defect content. Chemical expansion is a well-known feature of many non-stoichiometric oxides including fluorites, per-
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ACS Nano
ovskites, and Ruddlesden-Popper phases. 5–7 It is often cited as a source of mechanical failure because it can produce undesired stress or strain in layered systems in situ. 8 Such chemomechanical coupling need not always be a negative effect, however. In fact, electrochemically stimulated chemical expansion in such oxides can be used for high temperature actuation in extreme environments. 2 Further, in some cases this volume coupled to chemistry may be used to improve device performance, for example by using tensile strain to decrease oxygen vacancy formation energies and thereby enhance exchange kinetics or diffusivity. 9–11 This concept, known as strain engineering, has the potential to decrease the operating temperature of thin film SOFCs or related devices without sacrificing energy conversion efficiency. 12 In fact, the use of thin film non-stoichiometric oxides extends beyond electrochemical energy conversion and storage applications; resistive switches and gas separation membranes may also benefit from miniaturized dimensions. 4,13 Leveraging or engineering such chemical expansion effects in situ, particularly for applications including strain-engineered SOFC cathodes or high-temperature oxide actuators, requires detailed understanding of how stress and strain develop near mechanically-constrained interfaces in situ. Both the uniformity and magnitude of such effects are important, especially at length scales
