Photoconductivity analyzed in the frequency domain - an introductory case study of strontium titanate

Strontium titanate (STO, SrTiO3) has been used for many applications in solid state electrochemistry and is considered a standard and model material. Its characteristics, and those of its derivatives such as STF (SrTi0.65Fe0.35O3-x), have been characterized by many groups on various aspects, such as electronic/ionic conductivity, oxygen exchange kinetics and the impact of doping. Recently, the interaction of light with STO/STF has been of increased interest. A persistent photoconductivity has been observed [1] and enhanced oxygen exchange kinetics have been detected, opening up new fields of application, such as a light-driven fuel cell [2]. The reasons behind these effects remain subject to discussion or even speculation as the relation to the relatively large bandgap and the photoresponse at long wavelengths remains unclear. What makes the analysis of these effects difficult is the interplay of many electrochemical and photoelectrochemical processes that contribute to the photoresponse including the electronic and ionic conductivity, the number and nature of charge carriers, charge traps, phonon related effects, and surface reactions. With electrochemical impedance spectroscopy (EIS), one can distinguish diverse processes on the basis of their time constants and how they evolve as a function of operating conditions, such as temperature, atmosphere (leading to stoichiometry changes) and illumination. However, the impact of light can only be characterized implicitly as a change in other processes that also prevail in the dark. Intensity modulated photocurrent/-voltage spectroscopy (IMPS/IMVS) have been shown to reveal valuable information about charge carrier dynamics for photoelectrodes and photovoltaic cells [3]. To the best of our knowledge, these techniques have never been applied to devices or materials that are not photoactive, or in other words, that do not show a photovoltage, such as a symmetrical model cells based on STO or STF. However, with the small signal light perturbation that is the key element of IMPS and IMVS, we can trigger the light effect directly and analyze the system response by its current and voltage signals. In this contribution, we will begin with a brief introduction into IMPS and IMVS and show how these techniques can be applied to model electrodes consisting of STO and STF. The results are compared to EIS under different illumination and we will show how to extract the relevant information about the photoresponse. By evaluating the activation energies of the different electrochemical and photoelectrochemical processes, we can attribute those to physical effects and clarify some of the previously unknown processes that lead to anomalies observed in STO/STF under illumination. The capacity of IMPS and IMVS have been underestimated so far and in this contribution, we will conclude with an outlook for their potential to other fields of application, such as ionic motion in perovskite solar cells that are thought to be responsible for their accelerated degradation under illumination. This work was supported by JSPS Core-to-Core Program, A. Advanced Research Networks: “Solid Oxide Interfaces for Faster Ion Transport”. References [1] M. C. Tarun et al., Phys. Rev. Lett. 111, 187403, 2013. [2] G. C. Bunauer, Adv. Funct. Mater. 26, 120, 2016. [3] D. Klotz et al., Phys. Chem. Chem. Phys. 18, 23438, 2016

Silica: ubiquitous poison of metal oxide interfaces

In this review, we consider the detrimental effects of Si-contamination on electrochemical applications, broadly conceived, in which both ions and electrons play key roles in device operation and where exchange of oxygen between the gas and solid phase is likewise essential for operation

Impact of Oxygen Non‐Stoichiometry on Near‐Ambient Temperature Ionic Mobility in Polaronic Mixed‐Ionic‐Electronic Conducting Thin Films

Enhanced ionic mobility in mixed ionic and electronic conducting solids contributes to improved performance of memristive memory, energy storage and conversion, and catalytic devices. Ionic mobility can be significantly depressed at reduced temperatures, for example, due to defect association and therefore needs to be monitored. Measurements of ionic transport in mixed conductors, however, proves to be difficult due to dominant electronic conductivity. This study examines the impact of different levels of quenched-in oxygen deficiency on the oxygen vacancy mobility near room temperature. A praseodymium doped ceria (Pr0.1Ce0.9O2–δ ) film is grown by pulsed laser deposition and annealed in various oxygen partial pressures to modify its oxygen vacancy concentration. Changes in film non-stoichiometry are monitored by tracking the optical absorption related to the oxidation state of Pr ions. A 13-fold increase in ionic mobility at 60 °C for increases in oxygen non-stoichiometry from 0.032 to 0.042 is detected with negligible changes in migration enthalpy and large changes in pre-factor. Several factors potentially contributing to the large pre-factor changes are examined and discussed. Insights into how ionic defect concentration can markedly impact ionic mobility should help in elucidating the origins of variations seen in nanoionic devices

Nanostructured Pr-doped Ceria (PCO) thin films as sensing electrodes in solid-electrolyte type gas sensors with enhanced toluene sensitivity

Pr0.1Ce0.9O2 (PCO) films, with columnar structure containing voids, were fabricated by pulsed laser deposition (PLD) and integrated into YSZ-based potentiometric gas sensor as a sensing electrodes (SEs). Sensors with PCO film thicknesses of 550, 1100 and 2200 nm, fabricated by controlling the number of PLD shots of 750, 1500 and 3000 respectively, were examined over the temperature range of 450?600 °C. The sensor output voltage, E, of all sensors showed increasing negative change with increasing toluene concentration. The sensor with the thickest PCO SE film (thickness: 2200 nm) showed a much larger toluene response at 450 °C than the others (PCO-SE thickness: 550, 1100 nm). Because PCO is a mixed ionic and electronic conductor and the morphology of SE was porous, the electrochemical oxidation reaction is viewed as occurring at the double phase boundaries ? DPBs (interfaces between PCO/gas) as well as triple phase boundaries ? TPBs (interfaces of PCO/YSZ/gas). Because a fraction of the toluene is catalytically oxidized in part during gas diffusion within the SE films, especially at lower operating temperatures, the much larger toluene response can be explained by the electrochemical oxidation reaction of the partially oxidized products together with toluene at the TPBs and DPBs

Acidity of surface-infiltrated binary oxides as a sensitive descriptor of oxygen exchange kinetics in mixed conducting oxides

Improving the kinetics of O-2 reduction on oxide surfaces is critical in many energy and fuel conversion technologies. Here we show that the acidity scale for binary oxides is a powerful descriptor for tuning and predicting oxygen surface exchange kinetics on mixed conducting oxides. By infiltrating a selection of binary oxides from strongly basic (Li2O) to strongly acidic (SiO2) onto the surface of Pr(0.1)Ce(0.9)O(2-delta)samples, it was possible to vary the chemical surface exchange coefficient k(chem) by 6 orders of magnitude, with basic oxides such as Li2O increasing k(chem) by nearly 1,000 times, with surface concentrations as low as 50 ppm impacting k(chem). Strikingly, although the pre-exponential ofk(chem)scales linearly with the acidity of the infiltrated binary oxide, there is nearly no change in the activation energy. The origin of these dramatic changes is proposed to arise from the systematic increase in electron concentration at the Pr0.1Ce0.9O2-delta surface with the decreasing acidity of the infiltrated binary oxide

Thin-film chemical expansion of ceria based solid solutions: laser vibrometry study

Abstract The chemical expansion of Pr0.1Ce0.9O2–δ (PCO) and CeO2–δ thin films is investigated in the temperature range between 600 °C and 800 °C by laser Doppler vibrometry (LDV). It enables non-contact determination of nanometer scale changes in film thickness at high temperatures. The present study is the first systematic and detailed investigation of chemical expansion of doped and undoped ceria thin films at temperatures above 650 °C. The thin films were deposited on yttria stabilized zirconia substrates (YSZ), operated as an electrochemical oxygen pump, to periodically adjust the oxygen activity in the films, leading to reversible expansion and contraction of the film. This further leads to stresses in the underlying YSZ substrates, accompanied by bending of the overall devices. Film thickness changes and sample bending are found to reach up to 10 and several hundred nanometers, respectively, at excitation frequencies from 0.1 to 10 Hz and applied voltages from 0–0.75 V for PCO and 0–1 V for ceria. At low frequencies, equilibrium conditions are approached. As a consequence maximum thin-film expansion of PCO is expected due to full reduction of the Pr ions. The lower detection limit for displacements is found to be in the subnanometer range. At 800 °C and an excitation frequency of 1 Hz, the LDV shows a remarkable resolution of 0.3 nm which allows, for example, the characterization of materials with small levels of expansion, such as undoped ceria at high oxygen partial pressure. As the correlation between film expansion and sample bending is obtained through this study, a dimensional change of a free body consisting of the same material can be calculated using the high resolution characteristics of this system. A minimum detectable dimensional change of 5 pm is estimated even under challenging high-temperature conditions at 800 °C opening up opportunities to investigate electro-chemo-mechanical phenomena heretofore impossible to investigate. The expansion data are correlated with previous results on the oxygen nonstoichiometry of PCO thin films, and a defect model for bulk ceria solid solutions is adopted to calculate the cation and anion radii changes in the constrained films during chemical expansion. The constrained films exhibit anisotropic volume expansion with displacements perpendicular to the substrate plane nearly double that of bulk samples. The PCO films used here generate high total displacements of several 100 nm’s with high reproducibility. Consequently, PCO films are identified to be a potential core component of high-temperature actuators. They benefit not only from high displacements at temperatures where most piezoelectric materials no longer operate while exhibiting, low voltage operation and low energy consumption.</jats:p

Atomic Resolution Imaging of Nanoscale Chemical Expansion in Pr_<i>x</i>Ce_1–<i>x</i>O_2−δ during <i>In Situ</i> Heating

Thin film nonstoichiometric 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 nonstoichiometric oxides can differ from bulk counterparts in terms of <i>operando</i> defect concentrations, transport properties, and mechanical properties. Here, we present an <i>in situ</i> investigation of atomic-scale chemical expansion in Pr_<i>x</i>Ce_1–<i>x</i>O_2−δ (PCO), a mixed ionic–electronic conducting oxide relevant to electrochemical energy conversion and high-temperature actuation. Through a combination of electron energy loss spectroscopy and transmission electron microscopy with <i>in situ</i> 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 nonstoichiometric oxide films that clarify distinctions between films and bulk counterparts and that hold several implications for <i>operando</i> chemical expansion or “breathing” of such oxide films