RESUMO
Enabling light-controlled ionic devices requires insight into photoionic responses in technologically relevant materials. Mixed-conducting perovskites containing nondilute Feâserving as electrodes, catalysts, and sensorsâcan support large, electronically accommodated excursions in oxygen content, typically controlled by temperature, bias, and gas atmosphere. Instead, we investigated the ability of low-fluence, above-bandgap illumination to adjust oxygen stoichiometry and drive oxygen fluxes in nondilute Sr(Ti1-xFex)O3-x/2+δ (x = 0.07, 0.35) thin films with high baseline hole concentrations. Films' optical transmission at 2.8 eV was used as a probe of oxygen stoichiometry in the range â¼100-500 °C. We compared pO2-step-driven and UV (3.4 eV)-step-driven visible optical transmission relaxations in films, finding that the time constants and activation energies of the relaxations were consistent with each other and thus with oxygen-surface-exchange-limited kinetics. Blocking oxygen exchange at the solid-gas interface with a UV-transparent capping layer resulted in no UV-induced optical relaxations. These results demonstrate that above-bandgap illumination can increase oxygen content in nondilute compositions through oxygen flux into the solid from the gas. First-principles simulations of defect formation enthalpies indicate that oxygen vacancies are energetically less favorable under steady-state illumination owing to shifts in quasi-Fermi levels. A larger 2.8 eV-optical response to UV illumination in x = 0.07 vs x = 0.35 samples was further investigated through ultrafast transient spectroscopy, where it was found that the x = 0.07 sample exhibits a slower carrier recombination. Together, these results suggest potential design principles for materials supporting large stoichiometry changes under above-gap illumination: (1) long excited carrier lifetimes and (2) highly charged, rather than neutral, defects/associates.
RESUMO
Solid-state heterointerfaces are of interest for emergent local behavior that is distinct from either bulk parent compound. One technologically relevant example is the case of mixed ionic/electronic conductor (MIEC)-metal interfaces, which play an important role in electrochemistry. Metal-MIEC composite electrodes can demonstrate improved catalytic activity vs single-phase MIECs, improving fuel cell efficiency. Similarly, MIEC surface reaction kinetics are often evaluated using techniques that place metal current collectors in contact with the surface under evaluation, potentially altering the response vs the native surface. Techniques enabling direct and local in situ observation of the behavior at and around such heterointerfaces are needed. Here, we develop a spatially resolved optical transmission relaxation (2D-OTR) method providing continuous evaluation of local, high-temperature, controlled atmosphere defect kinetics across a â¼1 cm2 sample area simultaneously in a contact-free manner. We apply it to observe the spatial variance of oxygen incorporation and evolution rates at â¼525-620 °C, in response to step changes in oxygen partial pressure, on MIEC SrTi0.65Fe0.35O3-x films as a function of distance from porous Pt and Au layers. Using this model geometry, we find significant enhancements in kinetics adjacent to the metals that decay over a few millimeter distance. To extract kinetic parameters, we fit the short-term optical data (initial portion of relaxations) with an exponential decay function appropriate for surface-exchange-limited kinetics, yielding apparent surface exchange coefficients (kchem) with spatial resolution, decreasing with distance from the metal. To understand the kinetic processes governing the complete (long-term) optical relaxations, we performed COMSOL simulations, which demonstrated that a combination of laterally varying kchem and in-plane diffusion controls the observed kinetics over the full time range. Further support for spatially varying kchem comes from demonstrations of changing surface and bulk chemistry vs distance from the metal-MIEC interface, by X-ray photoelectron and optical absorption spectroscopies, respectively. Although microporous Pt and Au are not excellent electrodes in isolation, both metals exert a synergistic effect on the oxygen surface exchange rate in the presence of the mixed conducting film.
RESUMO
The oxygen surface exchange coefficient (k) dictates the efficiency and response time of many mixed conductors, so its accurate, continuous measurement in realistic conditions, enabling rational tailoring, is necessary. However, recent results showed that k values determined by a thin-film optical transmission relaxation (OTR) method were orders of magnitude lower than those extracted from the cross-cell AC-impedance spectroscopy (AC-IS) approach, and similar discrepancies among methods exist in the literature. OTR has also detected dramatic increases in k in situ during crystallization. Therefore, in this work, we sought to establish whether k values from OTR are reliable, and to gain further insight into crystallization-induced changes, via comparison to the electrical conductivity relaxation (ECR) method. We performed simultaneous OTR and ECR measurements on the same region of an as-grown amorphous SrTi0.65Fe0.35O2.825+δ (STF) film, prepared by pulsed laser deposition and characterized by Rutherford backscattering spectrometry, during thermal treatment to induce crystallization and a large increase in k. We also compared cross-cell AC-IS vs OTR on an as-grown amorphous film during crystallization and OTR vs ECR on a crystalline-grown film. Simultaneous measurements eliminate variability in k between samples or due to different thermal/gas history. OTR and ECR methods yielded the same k values, and the same crystallization temperature, within error. Both isothermal optical absorption and electrical conductivity changes are proportional to the hole and oxygen concentration changes under the conditions of this study. However, while OTR was able to measure optical absorption changes under all of the conditions tested, ECR was not viable in the high-resistance regime. Cross-cell AC-IS k values were elevated vs OTR values, were less stable over time, and were only accessible in limited conditions. We discuss the potential impacts of current collectors and oxygen exchange driving force on k values determined by cross-cell AC-IS vs ECR vs OTR.