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The effects of crystal orientation and ferroelectric domain structure on the photochemical reactivity of La2Ti2O7 have been measured. Electron backscattered diffraction was used to determine the orientations of the surfaces of crystals in a ceramic specimen, piezo-force microscopy was used to determine the domain structure, and the photocathodic reduction of silver from an aqueous silver nitrate solution was used to evaluate the photochemical reactivity. The reactivity is greatest on (001) surfaces (this is the orientation of the layers in this (110)p layered perovskite structure) while surfaces perpendicular to this orientation have the least reactivity. Complex domain structures were observed within the grains, but they appeared to have no effect on the photocathodic reduction of silver, in contrast to previous observations on other ferroelectrics. La2Ti2O7 is an example of a ferroelectric oxide in which the crystal orientation has a greater influence on the photochemical reactivity than polarization from the internal domain structure.

The photochemical reactivity of the SrTiO3 surface is affected by the pH of the surrounding aqueous solution. Scanning electron microscopy and atomic force microscopy have been used to quantify the...

The photochemical reactivity of the SrTiO

Electrochemical energy devices, such as batteries and fuel cells, contain active electrode components that have highly porous, multiphase microstructures for improved performance. Predictive electrochemical models of solid oxide fuel cell (SOFC) electrode performance based on measured microstructures have been limited to small length scales, a small number of simulations, and/or relatively homogeneous microstructures. To overcome the difficulty in modeling electrochemical activity of inhomogeneous microstructures at considerable length scales, we have developed a high-throughput simulation application that operates on high-performance computing platforms. The open-source application, named Electrochemical Reactions in MIcrostructural NEtworks (ERMINE), is implemented within the MOOSE computational framework, and solves species transport coupled to both three-phase boundary and two-phase boundary electrochemical reactions. As the core component, this application is further incorporated into a high-throughput computational workflow. The main advantages of the workflow include:•Straightforward image-based volumetric meshing that conforms to complex, multi-phased microstructural features•Computation of local electrochemical fields in morphology-resolved microstructures at considerable length scales•Implementation on high performance computing platforms, leading to fast, high-throughput computations, Graphical abstract Image, graphical abstract

The spontaneous polarization of domains in ferroelectric materials has been used to spatially separate photogenerated electrons and holes, reducing recombination and thereby improving the efficiency of photochemical reactions. Here, the influence of the magnitude of the polarization on photochemical reactivity is investigated. The magnitude of the out-of-plane component of the polarization was characterized by scanning Kelvin probe force microscopy (KFM). By examining crystals with orientations that deviate by only a few degrees from (001), two types of domains were identified: those with polarization vectors nearly perpendicular to the surface and those with polarization vectors nearly parallel to the surface. The photochemical reactivity was measured using topographic atomic force microscopy to determine the amount of Ag+ (Pb2+) that was photochemically reduced (oxidized) to Ag (PbO2) on the surface. For the reduction reaction, the reactivities of domains with polarizations nearly perpendicular to the surface were only about 3 times greater than the reactivities of the domains with polarizations nearly parallel to the surface, indicating that, for this reaction, the magnitude of the out-of-plane polarization is less important than its sign. For the oxidation of lead, only the domains with polarizations nearly perpendicular to the surface were reactive, indicating that for this reaction, both the sign and magnitude of the polarization are important.

When particulate photocatalysts promote a reaction, the overall reaction rate is limited by the constraint that the photoreduction and photooxidation reactions must occur on the surface of the same particle and at the same solution potential. The optimized solution potential would be the one where the photoanodic and photocathodic reactions proceed at the same maximum rate. Here, the relative photochemical reactivity of BaTiO3 was investigated as a function of the surrounding aqueous solution pH. Topographic atomic force microscopy (AFM) was used to quantify the amount of metallic silver produced from the photochemical reduction of Ag+ (Ag+→Ag) under UV light illumination. The overall reaction rate was observed to increase with pH. A similar pH-dependence existed for both high concentration (0.1 M) and low concentration (10−4 M) Ag+ solutions. When the pH is changed, the adsorbed charge at the surface is changed and this has the same effect as applying an external electric field. The observations are consistent with the idea that increasing the pH increases the net negative charge adsorption on the surface, increasing the upward band bending and promoting the oxidation half reaction, which is the rate-limiting factor for the overall reaction. As a result, the rate of Ag+ reduction increases with pH. The results show that adjusting the aqueous solution pH has the potential to control the rates of the two half reactions and increase the overall photochemical reaction rate.

In the present study, an efficient approach for the prediction of Warburg-type element is proposed via the analysis of the anode-supported solid oxide fuel cell (SOFC) performance under various working conditions. The details of the performance, polarization curves, impedance behavior, and species distribution profiles within the electrode are investigated via the combination of equivalent circuit model (ECM) analysis and multiphysics numerical simulations. The multiphysics simulation is developed and calibrated with experimental results of SOFC button cells under various working currents. With the complete datasets generated from the calibrated simulations, the trends of the element parameters involved in equivalent circuit model are analyzed. Generalized empirical functions are proposed as well as the procedures of prediction of performance under different conditions. The verification cases show good agreement between the predicted results from proposed model and the reference results. This proposed approach can be utilized to quickly predict the properties for desired performance in the manufacturing processes, and it also has the potential of reducing the computational cost in the simulation of large SOFCs.

The performance of electrochemical devices depends on the three-dimensional (3D) distributions of microstructural features in their electrodes. Several mature methods exist to characterize 3D microstructures over the microscale (tens of microns), which are useful in understanding homogeneous electrodes. However, methods that capture mesoscale (hundreds of microns) volumes at appropriate resolution (tens of nm) are lacking, though they are needed to understand more common, less ideal electrodes. Using serial sectioning with a Xe plasma focused ion beam combined with scanning electron microscopy (Xe PFIB-SEM), two commercial solid oxide fuel cell (SOFC) electrodes are reconstructed over volumes of 126 × 73 × 12.5 and 124 × 110 × 8 μm3 with a resolution on the order of ≈ 503 nm3. The mesoscale distributions of microscale structural features are quantified and both microscale and mesoscale inhomogeneities are found. We analyze the origin of inhomogeneity over different length scales by comparing experimental and synthetic microstructures, generated with different particle size distributions, with such synthetic microstructures capturing well the high-frequency heterogeneity. Effective medium theory models indicate that significant mesoscale variations in local electrochemical activity are expected throughout such electrodes. These methods offer improved understanding of the performance of complex electrodes in energy conversion devices.

Combinatorial substrate epitaxy (CSE) is used to investigate polymorph competition between metastable scrutinyite (α-PbO2) structured (s-) and stable rutile structured (r-) SnO2 during local epitaxial growth across orientation space on polycrystalline columbite (c-) CoNb2O6 substrates. Growth occurs in a grain-over-grain fashion, where individual grains of c-CoNb2O6 support the growth of individual grains of SnO2. Both metastable s-SnO2 and stable r-SnO2 crystals are observed, each growing on specific ranges of substrate orientations and each having a single specific orientation relationship (OR) with substrate grains on which it grew. s-SnO2 adopts the unit-cell over unit-cell OR that can be expressed as the alignment of primary eutactic planes: (100)s*[001]s*∥(100)c*[001]c* (where the * indicates the use of Pcnb setting). s-SnO2 grains grow on a slight majority of orientations and specifically on orientations inclined from the (010) pole of c*-CoNb2O6. r-SnO2 adopts an OR that can be expressed as the al...

Three-dimensional (3D) distributions of microstructural features in electrodes of solid oxide fuel cells (SOFCs) directly influence their electrochemical performance. Commercial grade SOFCs produced in cost-constrained environments can be expected to exhibit variations on the mesoscale (≈ hundreds of µm) of the microscale features (≈ tens of µm) that determine the local electrochemical activity. We are interested in quantitatively determining the mesoscale distributions of microscale properties in SOFC electrodes, especially quantifying the tails of distributions to establish correlations between such outliers and long-term performance. In this talk, we describe a new method that allows for the fast reconstruction of incredibly large volumes while maintaining a resolution on the order of tens of nm, allowing for fine-scale features to be accurately quantified. We use a Xe plasma focused ion beam (FIB) for slicing and a scanning electron microscope (SEM) for imaging. The pFIB allows for an increase in material removal rates by a factor of ≈ 100 compared to Ga ion FIBs, while the SEM resolution remains very high (herein ≈ 50nm), comparable to nano-CTs. In the same time as required for collecting Ga-FIB or nano-CT data sets, we collected a pFIB volume that is ≈ 70-80 times larger than attainable with Ga-FIB-SEM and ≈15-20 times larger than with nano-CT. From this data, we quantified the mesoscale distributions, containing both the mean and the variance, of key microscale features including volume fraction, average particle size, tortuosity, and triple phase boundary density. The commercial SOFC electrodes investigated were found to exhibit significant variations in the mesoscale distributions of their local microstructural features, which will be discussed. Importantly, these mesoscsale variations in microstructural features are expected to result in significant variations in electrochemical activity throughout the electrode. Using an effective medium theory model of polarization resistance, we quantified the distribution of microscale polarization resistance and mapped it spatially. Though the average value of polarization resistance is within range of expectation, the measoscale distribution of the microscale/local value of polarization resistance around the average indicates that several local areas are well outside the average, and potentially operating in regions expected to impact long-term cell performance.

Recent progress in micro-scale three-dimensional (3D) characterization techniques such as focused ion beam – scanning electron microscopy (FIB-SEM) and X-ray nanotomography has brought unprecedented opportunities in linking material microstructure to performance and properties. The link between solid oxide fuel cell (SOFC) electrode microstructure and performance is sometimes established by effective medium theory, where effective performance parameters are calculated from a representative volume element (RVE), an adequate volume that statistically represents global microstructure. However, different from performance, degradation and failure have not been addressed by mean parameters from effective medium theory. With the advent of modern 3D characterization techniques, there are new approaches in linking microstructure and degradation. This work aims to emphasize distributions (and outliers), rather than the mean, in better establishing the link between microstructure and degradation, failure, and performance. Using an open source finite element framework (MOOSE, Idaho National Laboratory) on a high performance computing platform (Joule, National Energy Technology Laboratory), a numerical transport and reaction model is applied to microstructural meshes to compute local distributions of electrochemical parameters. The microstructural meshes are obtained from the plasma sourced FIB-SEM characterization of a commercial SOFC electrode. The RVE size with respect to the computed distributions is determined based on examining the distributions from a series of microstructural meshes with various sizes. To validate the distributions, the averaged numbers from the distributions are compared with the effective medium theories in the literature. Finally, features such as the tails of the distributions and the local gradients of the electrochemical parameters are highlighted and discussed.

The electrodes in solid oxide fuel cells (SOFCs) consist of three phases interconnected in three dimensions. The volume needed to describe quantitatively such microstructures depends on several lengths scales, which are functions of materials properties and fabrication methods. This work focuses on quantifying the volume needed to represent “intermediate frequency” heterogeneities in electrodes of a commercial SOFC using X-ray computed tomography (CT) over two different length scales. Electrode volumes of 150 μm × 150 μm × 9 μm were extracted from a synchrotron-based micro-CT data set, with 13 μm3 voxels. 13.6 μm × 19.8 μm × 19.4 μm of the cathode and 26.3 μm × 24.8 μm × 15.7 μm of the anode were extracted from laboratory nano-CT data sets, both with 653 nm3 voxels. After comparing the variation across sub-regions for the grayscale values from the micro-CT, and for the phase fractions and triple phase boundary densities from the nano-CT, it was found that the sub-region length scales needed to yield statistically similar average values were an order of magnitude larger than those expected to capture the “high frequency” heterogeneity related to the discrete nature of the three phases in electrodes. The challenge of quantifying such electrodes using available experimental methods is discussed.

High-temperature annealing in air is used to produce SrTiO3 (111) surfaces with two types of atomically flat terraces: those that promote photoanodic reactions and those that promote photocathodic reactions. Surface potential measurements show that the photocathodic terraces have a relatively more positive surface potential than the photoanodic terraces. After depositing thin TiO2 films on the surface, from 1 to 13 nm thick, the surface of the film above the photocathodic terraces also has photocathodic properties, similar to those of the bare surface. While a more positive surface potential can be detected on the surface of the thinnest TiO2 films (1 nm thick), it is undetectable for thicker films. The persistence of the localized photocathodic properties on the film surface, even in the absence of a measurable difference in local potential, indicates that the charge associated with specific terraces on the bare SrTiO3 (111) surface remains localized at the TiO2/SrTiO3 interface and that the buried charg...

Metastable polymorphs, many of which have never been fabricated, have been predicted to exhibit interesting and technologically relevant properties. Epitaxial synthesis is a powerful structure-directing method that can produce metastable polymorphs but is typically done in a trial and error fashion. Unfortunately, the relevant thermodynamic terms governing epitaxial synthesis of new materials are unknown. Accurate calculation of the relevant thermodynamic terms and their incorporation into predictive models would accelerate the synthesis of metastable polymorphs by identifying thermodynamically favorable paths. Using density functional theory with three different functionals, we computed several relevant terms for TiO2 anatase (A) and rutile (R) film growth on low-index surfaces of SrTiO3 (STO) and BaTiO3 (BTO) cubic perovskites. After identifying potential coherent epitaxial interfaces based on experimental observations, the volumetric formation, volumetric strain, and areal substrate–film interface ener...

Two of three levels of hierarchical ferroelastic domains of γ-WO3 ceramics are shown to be piezo-responsive (polar) for surfaces of any general orientation. Importantly, the surface activity for the photocathodic reduction of Ag+ and photoanodic oxidation of Pb2+/Mn2+ ions is correlated to those same two levels of ferroelastic domains for all orientations.

SrTiO3(110) orientated crystals have been heated to temperatures between 1000 °C and 1200 °C in air, alone or in the presence of powder reservoirs of TiO2 or Sr3Ti2O7. In these conditions, the surface is terminated by two types of atomically flat terraces. One has a relatively higher surface potential and promotes the photochemical reduction of silver (it is photocathodic) and the other has a relatively lower surface potential and promotes the photochemical oxidation of lead (it is photoanodic). Measurements of the step heights between the terraces indicate that the surfaces with different properties have different terminations. By adjusting the time and temperature of the anneal, and in some cases including reservoirs of TiO2 or Sr3Ti2O7, it is possible to change the surface area fraction from 98% photocathodic to 100% photoanodic. The surface is more photocathodic when the annealing temperature is lower, the duration shorter, and if Sr3Ti2O7 is present. The surface is more photoanodic if the temperature is higher, the annealing duration longer, and if TiO2 is present. The results make it possible to control the surface potential and the ratio of photocathodic to photoanodic area on the SrTiO3(110) surface.

International audience; Using the Combinatorial Substrate Epitaxy (CSE) approach, we report the stabilization of Dy 2 Ti 2 O 7 epitaxial monoclinic, layered-perovskite phase Dy 2 Ti 2 O 7 thin films. To achieve this, the films are deposited on high density, polished La 2 Ti 2 O 7 polycrystalline ceramic substrates, which are stable as monoclinic layered-perovskites, and were prepared by conventional sintering. Microstructural analysis using electron backscatter diffraction (EBSD), electron diffraction (ED), and high-resolution transmission electron microscopy (HRTEM) support this observation. Further, they reveal that the cubic pyrochlore phase is observed far from the interface as films are grown thicker (100 nm), confirming the importance of substrate-induced phase and space group selection. This works reinforces the vast potential of CSE to promote the stabilization of metastable phases, thus giving access to new functional oxide materials, across a range of novel material systems including ferroelectrics.

Direct correlation of SOFC performance with electrode evolution requires compared electrodes to have quantifiably similar initial microstructures. Commercial SOFCs do not satisfy this requirement. Therefore, methods are explored to quantify morphological differences between many micro-scale sub-volumes taken from large meso-scale volumes of commercial anodes exposed to different operating conditions. Three microstructures were reconstructed in 3D using plasma-FIB SEM serial sectioning data (≈1002 μm2 in-plane and 6–9 μm in thickness, t) from two anodes exposed to different operating histories. 50 to 100 rectangular prism sub-volumes (122 μm2 x t μm) from each are compared using ANOVA methods. Variations in the local volume fractions and/or particles sizes render each of the three reconstructions statistically non-overlapping for many of their global averages, mesoscale distributions, and comparisons using ANOVA. However, the YSZ backbones are self-similar and scale adjusted electrodes are shown to have many comparable ANOVA sub-volumes, which allows for a quantification of relative microstructural changes in Ni/pore distributions. Ni coarsening and de-wetting of the YSZ phase are consistent with relative changes observed in the operated anode. Caution is nevertheless required in comparing commercial electrodes because spatial variations in the microstructural distributions are as significant as, or greater than, those from operation.

The optimization of coated ferroelectric photocatalysts represents a promising pathway toward competitive efficiencies. Here, we present a computational model that is used to predict the internal quantum efficiency (IQE) of individual coated ferroelectric domains. Criteria are identified that enable increased efficiencies. The net current favored on a negatively polarized domain of BaTiO3 coated with a 20 nm TiO2 (anatase) film is shown to be oxidation, whereas reduction is favored on positively polarized coated BaTiO3 domains. The results indicate that the IQE of the minority carrier reaction (oxidation) at physical conditions reported in the literature is less than 1%. By increasing the carrier lifetime to approximately 100 ns, increasing the reduction reaction kinetics, and optimizing the potential, complementary electron and hole reactions of equal magnitude can occur separately on positive and negative domains with an IQE of over 90% (45% if operated in a Z-scheme). This analysis can be applied to he...

The orientation dependence of the visible-light stimulated photochemical reduction of aqueous Ag+ on polycrystalline hematite Fe2O3 was determined by observing the relative amount of reduced Ag0 on crystals of known surface orientation over all possible orientations. The results show that surfaces oriented within 20° of the (11¯02) plane are the most active and surfaces close to the (0001) plane are the least active. A strong correlation is observed between the orientation-dependent activity and the orientation-dependent surface potential measured by scanning Kelvin probe force microscopy. Surfaces near the most active (11¯02) plane and the least active (0001) plane are, respectively, at the most positive and the least positive ends of the potential range among all grains. The trends in activity are concluded to arise from a combination of internal fields associated with surface charges and bulk transport effects.

The relative reactivities of monoclinic and tetragonal BiVO4-based ceramics are described, focusing on the contributions of ferroelastic domains and surface orientations. First, the photo-oxidation of Mn2+ on monoclinic BiVO4 is shown to be domain selective, occurring on the set of domains that is not active for the photo-reduction of Ag+. Next, the domain and orientation dependent photochemical reduction of Ag+ is investigated for two samples in the co-doped family (Bi1−0.5xNa0.5x)(V1−xMox)O4: a monoclinic sample (x = 0.05) and a tetragonal sample (x = 0.175). Like BiVO4, the x = 0.05 sample has ferroelastic domains that exhibit contrast in piezoresponse force microscopy (PFM) and exhibit domain specific reactivity. The x = 0.175 sample has no domains, no PFM contrast, and uniform grain reactivity. The orientation dependence of Ag+ reduction is similar for the two samples, with surfaces near (001) being more active than others. For any given orientation, however, the monoclinic sample is quantitatively more active than the same orientation in the tetragonal sample. These collected results are consistent with the hypothesis that monoclinic-structured BiVO4 ceramics have enhanced photochemical activity owing to ferroelastic domain-selective reactivity, which allows for local separation of photogenerated carriers and redox reactions.

The internal quantum efficiency (IQE) of multidomain ferroelectric BaTiO3 coated with thin (10 nm) anatase TiO2 films has been modeled in two dimensions, using physically reasonable property values of each material. The minority carrier current density averaged across the entire surface, composed of equal portions of positively and negatively polarized domains separated by 180° boundaries, is similar to that reported previously for a single negatively polarized domain (modeled in one dimension). This indicates that photogenerated carriers driven away from the surface in one domain can be collected at the surface of the neighboring domain and participate in surface reactivity. For a wide range of physically reasonable domain widths, from approximately 100 to 400 nm, the limiting IQE is more than 90% of the maximum value, and far exceeds that for Z-scheme domain reactivity, where the carriers driven away from the surface would recombine in the bulk. When the potential and the domain width of alternating positively and negatively polarized domains are optimized, the balancing reduction and oxidation currents occur on the surface with a total limiting IQE as high as 90%, implying there exists significant room for improvement of photocatalysts using spatially varying internal fields at or near the reactive surface.

Photocatalysts are being extensively investigated to convert renewable solar energy into chemical energy but suffer from high costs or low efficiencies. We report on the development of highly visible-light photoactive composites of titania (TiO2) and single-wall carbon nanotubes (SWCNTs) that rapidly photodegrade methylene blue dyes under visible-light illumination in the absence of cocatalysts. We fabricated these freestanding porous composites of density ≈36 mg mL−1 (volume fraction ≈ 0.01) by an in situ sol–gel synthesis of titania nanoparticles of diameter ≈9 nm within SWCNT aerogels. The SWCNT aerogels are three-dimensional porous networks of individualized SWCNTs having a density ≈9 mg mL−1 (volume fraction ≈ 0.006), whose large surface area and high porosity enable substantial titania loading and unimpeded dye transport to titania. The TiO2/SWCNT aerogel composites had a surface area of 293 m2 g−1 and pores of diameters between 2–25 nm. X-ray photoelectron spectroscopy showed a strong bonding interaction between titania and SWCNTs (i.e., titanium–carbon and titanium–oxygen–carbon bonds), which possibly rendered these aerogel composites photoactive in visible-light with an absorption edge ≈2.6 eV. In contrast, titania is only active in ultraviolet-light due to its large bandgap (≈3.2 eV). Further, they degraded dyes at a rate of ≈25 μmol g−1 h−1 with a rate constant of ≈0.012 min−1 under visible-light irradiation, values that are more than two times greater than those from other titania-based photocatalysts under visible or ultraviolet illumination. In comparison, titania nanoparticles alone were essentially inactive under similar test conditions. Interestingly, the rate constant for dye degradation decreased with an increase in dye concentration, but the overall rate of degradation remained nearly unchanged. Moreover, the addition of platinum cocatalysts did not improve the photocatalytic performance of the TiO2/SWCNT composites. These observations suggest that the composites efficiently separate visible-light generated electron–hole pairs and that photodegradation was limited by the availability of reactive sites on titania (the anodic reaction). We postulate that further enhancements are plausible through composite design and that our facile fabrication method can be readily adapted to create nearly any freestanding photocatalyst/SWCNT aerogel composites for use in high performance photoelectrochemical cells.

The rates of hydrogen production from four different BaTiO3/TiO2 core/shell photocatalysts and their components were measured as a function of pH using a parallelized and automated photochemical reactor (PAPCR) to determine the optimized structure and pH condition for photochemical hydrogen production. The amount of hydrogen produced by each reactor in a 96-reactor array was quantified by the color change of a hydrogen-sensitive material. For the core/shell photocatalysts annealed at 600 °C, the increase in hydrogen production rate with pH at pH 3–9 is ascribed to the adsorption of more negatively charged species that bend the bands upward, promoting hole transport to the surface and the oxidation half reaction. At intermediate pH, the core/shell catalyst annealed at 600 °C had the highest reactivity, indicating that the generation and transport of charge carriers were improved by the BaTiO3/TiO2 core/shell structure. The results demonstrate that PAPCR is an effective way to compare the performance of hydrogen-producing catalysts with different structures and in different operating conditions.

A high-throughput, high-performance computational approach is used to simulate local electrochemistry in three-dimensions of solid oxide fuel cell electrodes, with the aim of understanding distributions of performance values within microstructures. Simulations are carried out on 47 three-phase cathodes, whose lateral {vertical} dimensions are 22 {15} times their average particle size (0.46 μm). The 47 microstructures are spread across four distinct groups having different standard deviations in their particle size and/or local volume fraction distributions. The average performance simulated compares favorably to two accepted effective medium theory (EMT) models, but a significant discrepancy between the locally-resolved simulations and the EMT models arises from the Ohmic transport terms. This is borne out further by local electrochemical values, specifically distributions of local activation overpotentials and regions of extremely high current densities: values often connected with degradation. The impact of particle size and volume fraction distributions on the distribution of performance values–both within and between microstructural groups—is highlighted throughout. Results from this study indicate that high-performance simulations in a high-throughput, large-data workflow can elucidate performance characteristics that are not captured by continuum level models using EMT. Understanding of these detailed performance characteristics is expected to lead to more durable and reliable electrochemical cells.

The spontaneous polarization of domains in ferroelectric materials has been used to spatially separate photogenerated electrons and holes, reducing recombination and thereby improving the efficiency of photochemical reactions. Here, the influence of the magnitude of the polarization on photochemical reactivity is investigated. The magnitude of the out-of-plane component of the polarization was characterized by scanning Kelvin probe force microscopy (KFM). By examining crystals with orientations that deviate by only a few degrees from (001), two types of domains were identified: those with polarization vectors nearly perpendicular to the surface and those with polarization vectors nearly parallel to the surface. The photochemical reactivity was measured using topographic atomic force microscopy to determine the amount of Ag

BiVO4 surfaces of all possible orientations have been used to photochemically reduce Ag+ and oxidize Pb2+. The surface orientations of grains, measured by electron backscatter diffraction, were correlated to the amounts of reduced and oxidized products on each surface. The results show that the photochemical reduction of Ag+ is strongly favored on BiVO4 (001) surfaces (indexed in group I2/b). However, Ag+ can be reduced on all orientations, with the rate of reduction decreasing with increasing inclination from the [001] direction. The oxidation of Pb2+ is strongly favored on (hk0) surfaces perpendicular to (001). Surfaces within 50° of [001] have a much lower activity for the oxidation of Pb2+. The results also show that reduction and oxidation reactions occur on complementary surfaces of BiVO4 crystals.

It is broadly understood that strontium-doped lanthanum manganate (LSM) cathodes for solid oxide fuel cells (SOFCs) have two pathways for the reduction of oxygen: a surface-mediated pathway culminating in oxygen incorporation into the electrolyte at the triple-phase boundary (TPB), and a bulk-mediated pathway involving oxygen transfer across the electrode-electrolyte interface. Patterned electrode and thin film experiments have shown that both pathways are active in LSM. Porous electrode geometries more commonly found in SOFCs have not been amenable for precise measurement of active electrode width because of the difficulty in precisely measuring the electrode geometry. This study quantitatively compares a reaction-diffusion model for the oxygen reduction reaction in LSM to the impedance spectrum of an experimental LSM porous electrode symmetric button cell on a yttria-stabilized zirconia (YSZ) electrolyte. The porous microstructure was characterized using computed tomography (nano-CT) and Bayesian model-based analysis (BMA) was used to estimate model parameters. BMA produced good fits to the data, with higher than expected values for the interfacial capacitance at the LSM-YSZ interface and vacancy diffusion activation energy; these results may indicate that the active width of the electrode is on a similar scale with that of the space-charge width at the LSM-YSZ interface. The analysis also showed that the active width and proportion of current moving through the bulk pathway is temperature dependent, in accordance with patterned electrode results.

International audience; Sr2FeMoO6 (SFMO) films were deposited on polycrystalline spark plasma synthesized Sr2MgWO6 (SMWO) substrates. Films were grown using pulsed laser deposition at temperatures (T dep) between 720 °C and 820 °C in a vacuum environment of pressure ${{P}_{\text{dep}}}={{10}^{-6}}$ mbar (0.1 mPa); after deposition they were cooled either in a pressure ${{P}_{\text{cool}}}={{P}_{\text{dep}}}$ or ${{P}_{\text{cool}}}={{10}^{-4}}$ mbar (10 mPa) O2. Despite the use of an isostructural substrate, the growth and cooling conditions play the primary role in determining details of the films' structures and properties, similarly to single-crystals. Grazing x-ray and electron back-scatter diffraction indicate that vacuum-cooled films were pure perovskite-structured SFMO exhibiting grain-over-grain growth that aligned the perovskite sub-cells. SrMoO4 impurities were observed in the x-ray patterns for the oxygen-cooled films similarly to single-crystal substrates. Magnetic, electronic and magnetoresistive properties were all a function of growth and cooling environments. The Curie temperature and magnetization of the films increased with T dep up to 800 °C. The vacuum-cooled films had low-resistivities with essentially metallic conductivity (small resistivity increases occurred at low-T), while the conductivity of oxygen-cooled films were consistent with variable range hopping. The oxygen-cooled films had higher low-field magnetoresistance effects at 5 K than the vacuum-cooled films, which seems consistent with SrMoO4 forming at grain boundaries. This work opens the route to tailor the electronic properties by engineering the grain boundaries in thin films.

Photoelectrochemical conversion of solar energy is explored for many diverse applications but suffers from poor efficiencies due to limited solar absorption, inadequate charge carrier separation, redox half-reactions occurring in close proximity, and/or long ion diffusion lengths. We have taken a drastically different approach to the design of photoelectrochemical cells (PECs) to spatially isolate reaction sites at the nanoscale to different materials and flow channels, suppressing carrier recombination and back-reaction of intermediates while shortening ion diffusion paths and, importantly, avoiding mixed product generation. We developed massively parallel nano-PECs composed of an array of open-ended carbon nanotubes (CNTs) with photoanodic reactions occurring on the outer walls, uniformly coated with titanium dioxide (TiO2), and photocathodic reactions occurring on the inner walls, decorated with platinum (Pt). We verified the redox reaction isolation by demonstrating selective photodeposition of mangan...

In this study, in situ electrical biasing was combined with transmission electron microscopy (TEM) in order to study the formation and evolution of Wadsley defects and Magnéli phases during electrical biasing and resistive switching in titanium dioxide (TiO2). Resistive switching devices were fabricated from single-crystal rutile TiO2 substrates through focused ion beam milling and lift-out techniques. Defect evolution and phase transformations in rutile TiO2 were monitored by diffraction contrast imaging inside the TEM during electrical biasing. Reversible bipolar resistive switching behavior was observed in these single-crystal TiO2 devices. Biased induced reduction reactions created increased oxygen vacancy concentrations to such an extent that shear faults (Wadsley defects) and oxygen-deficient phases (Magnéli phases) formed over large volumes within the TiO2 TEM specimen. Nevertheless, the observed reversible formation/dissociation of Wadsley defects does not appear to correlate to resistive switching phenomena at these length scales. These defect zones were found to reversibly reconfigure in a manner consistent with charged oxygen vacancy migration responding to the applied bias polarity.

Monolithic and superlattice films composed of TaN and TiN layers were synthesized using pulsed laser deposition on a variety of substrates, and were characterized using X-ray diffraction, transmission electron microscopy, and nanoindentation. Films deposited on square-symmetry substrate surfaces, e.g., (100)-oriented MgO, TiN, or TaN, were epitaxial (100)-oriented rock-salt ( rs -)polymorphs at both 750 °C and 950 °C. On hexagonal-symmetry substrate surfaces, e.g., Al 2 O 3 (001), both TiN and TaN were epitaxial (111)-oriented rs -polymorphs at 750 °C, as were TiN films deposited at 950 °C. Similar TaN films deposited at 950 °C, however, were (001)-oriented hexagonal ( h -)TaN on Al 2 O 3 (001) and > 2.5 nm away from rs -TiN(111) buffer layers, including in superlattices with thick TaN layers. Superlattices (all with a 3:7 TiN:TaN ratio, > 500 nm thick, which took > 13 h to complete) deposited at 750 °C behaved as expected, exhibiting clear superlattice peaks and a maximum hardness when the superlattice period was between 5 and 10 nm. No superlattice peaks were observed in the (100)-oriented isostructural rs -superlattices deposited at 950 °C, indicating that a significant degree of interdiffusion occurred, but the amount was orientation dependent, as (111)-oriented isostructural rs -layers were similar at the two temperatures. Non-isostructural superlattices deposited at 950 °C that had (111)-oriented rs -layers and (001) h -TaN layers also exhibited little interdiffusion and, importantly, had a secondary hardness maximum near a chemical repeat period of 15 nm, rendering it the hardest superlattice for these longer chemical repeat periods. These composite results indicate that multi-functionality could be built into polycrystalline coatings because the phases, chemical stabilities, and hardness properties of TiN/TaN superlattices can be strongly dependent on local orientation and chemical periodicity.

Grain-boundary plane distributions (GBPDs), grain size distribution (GSDs), and upper tail departure from log-normal GSDs were quantified in dense and porous La0.8Sr0.2MnO3 samples to understand expected microstructures in solid oxide fuel cells. Samples were sintered at 1450°C for 4 h and then annealed between 800°C and 1450°C. The GBPDs and normalized GSDs reached steady state during sintering and little variation occurred during annealing. The GBPDs were nearly isotropic, with the relative areas of {001} planes being slightly higher than random (and the relative areas of {111} planes being less than random). The porous sample had an almost identical GBPD, whereas the almost isotropic pore boundary plane distribution was essentially opposite to the GBPD. The upper tails of the experimental GSDs, and several theoretical distributions, were characterized using peaks-over-threshold analysis. Dense samples, and all normal grain growth models, exhibit lower frequencies of large grains in the upper tail than would a log-normal distribution, and the experimental distributions are similar to the Mullins distribution. Porous samples, however, have an anomalous increased frequency of large grains in the upper tail, as compared to all the model distributions, even though other metrics of the microstructure indicate the dense and porous systems are similar.

The photocatalytic activity of materials for water splitting is limited by the recombination of photogenerated electron-hole pairs as well as the back-reaction of intermediate species. This review concentrates on the use of electric fields within catalyst particles to mitigate the effects of recombination and back-reaction and to increase photochemical reactivity. Internal electric fields in photocatalysts can arise from ferroelectric phenomena, p-n junctions, polar surface terminations, and polymorph junctions. The manipulation of internal fields through the creation of charged interfaces in hierarchically structured materials is a promising strategy for the design of improved photocatalysts.

A semi-empirical model is developed to quantitatively characterize electrode heterogeneities over the micro- and mesoscales, specifically between the relationship of the mean-squared normalized variance in the volume fraction, ( σ Φ V Φ ¯ V ) 2 , and the mean particle size normalized linear dimension, L a ¯ N . The model was developed analyzing data from five large-volume physical reconstructions – collected using Xe-plasma focused ion beam with SEM (PFIB-SEM) – of several commercial cell electrodes and from unique sets of synthetic microstructures designed to have controlled distributions in particle size and local volume fractions. When comparing physical reconstructions from distinct regions of the same electrode, millimeter length scale heterogeneities are also observed, even in microstructures with limited mesoscale variability. While the model is developed using synthetic microstructures, it is used to quantify three different types of heterogeneities in the commercial cells. The potential origins are discussed with respect to variations in particle size distributions in feedstocks and to phase distributions related to fabrication processes; the potential performance impacts are discussed with respect to two effective medium theory models. The characterization and analytical methodologies and model presented can support the design and development of improved electrodes.

The grain-by-grain orientation relationships between an Fe2O3 film, grown using pulsed laser deposition, and a polycrystalline SrTiO3 substrate were determined using electron backscatter diffraction. This high-throughput investigation, we call combinatorial substrate epitaxy, enables the characterization of film growth on all grain orientations in a single experiment, allowing the determination of the preferred epitaxial orientation (PEO) of this non-isostructural film/substrate pair. Heavily-twinned rhombohedral α-Fe2O3 (hematite) grew epitaxially over the entire orientation space of the cubic perovskite substrate. Over 500 local orientation relationships (ORs) were investigated and more than 90% of these ORs, regardless of the interface plane normal, could be described using a single epitaxial OR: 0001 10 1 ¯ 0 Fe 2 O 3 111 1 1 ¯ 0 SrTiO 3 . This OR aligns the eutactic (nearly close-packed) planes and directions between these dissimilar crystal structures. Importantly, the growth of Fe2O3 on a single crystalline (100)-SrTiO3 results in several different orientation relationships. These results suggest that growth on high Miller-index (low-symmetry) surfaces provides more general information about the PEO than growth on low Miller-index (high-symmetry) surfaces. The epitaxial film growth on high Miller-index surfaces and the overwhelming observation of the eutaxial OR support the hypothesis that a very small number of simple crystallographic descriptors guide epitaxial film growth over all of orientation space, even for non-isostructural film/substrate pairs.

To better understand the response of oxygen vacancy concentration to applied potential, the lattice parameter of pulsed laser deposited La0.6Sr0.4Co1-xFexO3-δ thin films was monitored using in situ X-ray diffraction. We demonstrate that the chemical expansion under applied potential depends on the cathode morphology, which determines the contribution of different reaction pathways. We investigated applied potential dependent lattice expansion on La0.6Sr0.4Co1-xFexO3-δ with 3 different Co:Fe ratios in an attempt to connect bulk chemical expansion data to thin films. We find that the chemical expansion trends in thin films are different than expected from bulk data.

A new technique, combinatorial substrate epitaxy, has been used to study the polymorphic stability and orientation relationships (ORs) for TiO2 thin films grown by pulsed laser deposition on polycrystalline BiFeO3 at 600 °C. Electron backscatter diffraction data from 150 substrate/film pairs were analyzed to determine that anatase (A) grew with the OR (1 1 2)A || (1 1 1)BFO and [ 1 1 ¯ 0 ] A | | [ 1 1 ¯ 0 ] BFO on BiFeO3 (BFO) substrates oriented within 35° of [1 0 0]. Rutile (R) was found on all other substrate orientations with (1 0 0)R || (1 1 1)BFO. The in-plane orientation was primarily [0 0 1]R || [ 1 1 ¯ 0 ] BFO , but some films near the anatase/rutile phase boundary were rotated by 30° so that [0 0 1]R || [ 1 ¯ 2 1 ¯ ] BFO . Because these substrate film pairs have high-index interface planes, conventional epitaxy arguments based on two-dimensional lattice mismatch in low-index planes are considered to be limiting cases of a more general model involving the three-dimensional alignment of closest packed planes and directions, regardless of the interface plane.

LaMnO3 and (La,Sr)MnO3 were prepared as mesoporous powders using sol-gel methods that incorporated evaporation-induced self-assembly of surfactants as structure-directing agents. The precursor and surfactant concentrations, surfactant character (mainly molecular weight), aging humidity, and annealing temperature were all found to influence the specific surface area of the powders, which were maximized at 50 and 46 m2/g for LaMnO3 and (La,Sr)MnO3, respectively. Increased metal nitrate contents in the initial sol composition and increased relative humidity during aging led to products having increased specific surface areas owing to increased mesoporosity. The lower molecular weight surfactant CTAB was found to generate higher specific surface area powders than those generated using the larger molecular weight nonionic surfactants P-123 and F-127. On the other hand, the relative thermal stability of the mesoporous powders, annealed at 750°C for 100 h, increased with the molecular weight of surfactant. Mesoporous powders having promising thermal stability and surface areas in the 20–25 m2/g range were easily obtainable using P-123 and F-127, indicating that high surface area electrocatalysts with mesoporous microstructures can be prepared for high temperature applications.

Substrate- and thickness-related effects on the oxygen surface exchange of La(0.7)Sr(0.3)MnO(3) (LSM) thin films were investigated to understand better cathode reactivity in solid oxide fuel cells. Epitaxial (100)-oriented LSM films were fabricated on (100)-SrTiO(3) and (110)-NdGaO(3) substrates and were characterized using electrical conductivity relaxation. A strong substrate effect on the chemical surface exchange coefficient (k(chem)) was observed, with a higher k(chem) found for films on SrTiO(3) than those on NdGaO(3). Two distinct activation energies (E(a)) were observed for k(chem), which were assigned to two parallel exchange processes; the relative contributions from each depended on the substrate, film thickness, and temperature. For films coherently strained to the substrates, k(chem) values differed by almost an order of magnitude, whereas E(a) was ∼1.5 (± 0.1) eV on both substrates. For relaxed films, k(chem) values differed only by a factor of 2, and E(a) was ∼0.75 (± 0.1) eV on both substrates. We discuss the strain effect relative to the native surface exchange and the thickness effect relative to the extended defect populations in the films. The outcome of this study sheds light on how microstructural features affect surface chemistry in modified cathodes.