Epitaxial Stabilization and Persistent Nucleation of the 3C Polymorph of Ba0.6Sr0.4MnO3.

Ba-rich compositions in the BaxSr1-xMnO3 (BSMO) cubic perovskite (3C) system are magnetic ferroelectrics and are of interest for their strong magnetoelectric coupling. Beyond x = 0.5, they only form in hexagonal polymorphs. Here, the 3C phase boundary is pushed to Ba0.6Sr0.4MnO3 for the thin films. Using regular pulsed laser deposition (rPLD), 3C Ba0.6Sr0.4MnO3 could be epitaxially stabilized on DyScO3 (101)o substrates by using a 0.1% O2/99.9% N2 gas mixture. However, the 3C phase was mixed with the 4H polymorph for films 24 nm thick and above, and the films were relatively rough. To improve flatness and phase purity, changes in growth kinetics were investigated and interval PLD (iPLD) was especially effective. In iPLD, deposition is interrupted after completion of approximately one monolayer, and the deposit is annealed for a specific period of time before repeating. Both film flatness and, more importantly, the volume of the 3C polymorph improved with iPLD, resulting in 40 nm single-phase films. The results imply that iPLD improves the persistent nucleation of highly metastable phases and offers a new approach to epitaxial stabilization of novel materials, including more Ba-rich BSMO compositions in the 3C structure.

Influence of pH and Surface Orientation on the Photochemical Reactivity of SrTiO3.

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 amount of silver that is photochemically reduced on the surfaces of (100), (110), and (111) oriented crystals as a function of pH. For all orientations, the reactivity increases from pH 3, reaches a maximum, and then decreases at higher pH. The pH associated with the maximum reactivity depends on the crystallographic orientation of the surface. The results indicate that the solution pH influences the charge on the SrTiO3 surface. The amount of surface charge influences band bending within SrTiO3, and the maximum reactivity is achieved at a surface charge where neither the photocathodic nor the photoanodic reaction limit the overall reaction rate.

High performance modeling of heterogeneous SOFC electrode microstructures using the MOOSE framework: ERMINE (Electrochemical Reactions in MIcrostructural NEtworks).

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.

Influence of the Magnitude of Ferroelectric Domain Polarization on the Photochemical Reactivity of BaTiO3.

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.

Quantitative interpretation of impedance spectroscopy data on porous LSM electrodes using X-ray computed tomography and Bayesian model-based analysis.

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.

Controlling the termination and photochemical reactivity of the SrTiO3(110) surface.

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.

Buried Charge at the TiO2/SrTiO3 (111) Interface and Its Effect on Photochemical Reactivity.

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 charge influences the motion of photogenerated carriers.

Nano-Photoelectrochemical Cell Arrays with Spatially Isolated Oxidation and Reduction Channels.

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 manganese oxide on the outside and silver on the inside of the TiO2/CNT/Pt nanotubes. Further, the nano-PECs exhibit improved solar absorption and efficient charge transfer of photogenerated carriers to their respective redox sites, leading to a 1.8% photon-to-current conversion efficiency (a current density of 4.2 mA/cm2) under white-light irradiation. The design principles demonstrated can be readily adapted to myriads of photocatalysts for cost-effective solar utilization.

In situ TEM imaging of defect dynamics under electrical bias in resistive switching rutile-TiO2.

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.

Identifying potential BO2 oxide polymorphs for epitaxial growth candidates.

Transition metal dioxides (BO2) exhibit a number of polymorphic structures with distinct properties, but the isolation of different polymorphs for a given composition is carried out using trial and error experimentation. We present computational studies of the relative stabilities and equations of state for six polymorphs (anatase, brookite, rutile, columbite, pyrite, and fluorite) of five different BO2 dioxides (B = Ti, V, Ru, Ir, and Sn). These properties were computed in a consistent fashion using several exchange correlation functionals within the density functional theory formalism, and the effects of the different functionals are discussed relative to their impact on predictive synthesis. We compare the computational results to prior observations of high-pressure synthesis and epitaxial film growth and then use this discussion to predict new accessible polymorphs in the context of epitaxial stabilization using isostructural substrates. For example, the relative stabilities of the columbite polymorph for VO2 and RuO2 are similar to those of TiO2 and SnO2, the latter two of which have been previously stabilized as epitaxial films.

Photocatalysts with internal electric fields.

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.

Visible-light photochemical activity of heterostructured core-shell materials composed of selected ternary titanates and ferrites coated by tiO2.

Heterostructured photocatalysts comprised of microcrystalline (mc-) cores and nanostructured (ns-) shells were prepared by the sol-gel method. The ability of titania-coated ATiO3 (A = Fe, Pb) and AFeO3 (A = Bi, La, Y) catalysts to degrade methylene blue in visible light (λ > 420 nm) was compared. The catalysts with the titanate cores had enhanced photocatalytic activities for methylene blue degradation compared to their components alone, whereas the catalysts with ferrite cores did not. The temperature at which the ns-titania shell is crystallized influences the photocatalytic dye degradation. mc-FeTiO3/ns-TiO2 annealed at 500 °C shows the highest reaction rate. Fe-doped TiO2, which absorbs visible light, did not show enhanced photocatalytic activity for methylene blue degradation. This result indicates that iron contamination is not a decisive factor in the reduced reactivity of the titania coated ferrite catalysts. The higher reactivity of materials with the titanate cores suggests that photogenerated charge carriers are more easily transported across the titanate-titanate interface than the ferrite-titanate interface and this provides guidance for materials selection in composite catalyst design.

Substrate and thickness effects on the oxygen surface exchange of La(0.7)Sr(0.3)MnO3 thin films.

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.

Enhanced photochemical activity of α-Fe2O3 films supported on SrTiO3 substrates under visible light illumination.

The visible light photochemical reactivity of a 50 nm thick α-Fe(2)O(3)(0001) (hematite) film on a SrTiO(3)(111) substrate is compared to the reactivities of bulk hematite and the same film supported by α-Al(2)O(3)(0001). The hematite film supported by SrTiO(3)(111) is far more reactive then the other two cases.

Effect of crystal and domain orientation on the visible-light photochemical reduction of Ag on BiFeO3.

The reduction of silver from an aqueous solution on BiFeO3 surfaces, activated by visible light, was investigated as a function of crystal and ferroelectric domain orientation. When excited by light with energy between 2.53 and 2.70 eV, BiFeO3 photochemically reduces silver cations from solution in patterns corresponding to the underlying ferroelectric domain structure. Silver is preferentially reduced on domains with a positive polarization directed toward the surface. The amount of reduced silver depends on whether the component of the domain polarization normal to the surface is positive or negative, but is relatively insensitive to the crystal orientation. It is concluded that the ferroelectric polarization decreases electron drift to the surface in domains with a negative polarization, causing spatially selective photochemical behavior, and that the direction of the polarization is more important than the amplitude.