Bulk Oxide Materials Research Articles

Graphene oxide bulk material reinforced by heterophase platelets with multiscale interface crosslinking.

Graphene oxide (GO) and reduced GO possess robust mechanical, electrical and chemical properties. Their nanocomposites have been extensively explored for applications in diverse fields. However, due to the high flexibility and weak interlayer interactions of GO nanosheets, the flexural mechanical properties of GO-based composites, especially in bulk materials, are largely constrained, which hinders their performance in practical applications. Here, inspired by the amorphous/crystalline feature of the heterophase within nacreous platelets, we present a centimetre-sized, GO-based bulk material consisting of building blocks of GO and amorphous/crystalline leaf-like MnO2 hexagon nanosheets adhered together with polymer-based crosslinkers. These building blocks are stacked and hot-pressed with further crosslinking between the layers to form a GO/MnO2-based layered (GML) bulk material. The resultant GML bulk material exhibits a flexural strength of 231.2 MPa. Moreover, the material exhibits sufficient fracture toughness and strong impact resistance while being light in weight. Experimental and numerical analyses indicate that the ordered heterophase structure and synergetic crosslinking interactions across multiscale interfaces lead to the superior mechanical properties of the material. These results are expected to provide insights into the design of structural materials and potential applications of high-performance GO-based bulk materials in aerospace, biomedicine and electronics.

Preparation and property analysis of solid carbonate-oxide composite materials for an electrolyte used in low-temperature solid oxide fuel cell

The oxide-carbonate composite electrolyte material with high ionic conductivity at low temperature has been thought that it can be used to develop LT-SOFC. However, the carbonate composite electrolyte is not easy to make it dense, especially mixing and packing oxide and carbonate to fabricate the composite electrolyte simply. In this article, rare-earth-doped CeO2 (RDC) (R = La, Sm, Gd, and Gd + Y) series samples were prepared by wet ball-milling, then sintered into fully dense and porous oxide bulk at 1500–1600 °C and 1000 °C. Melted carbonate LNCO, composed of Li2CO3 and Na2CO3 at a molar ratio of 1:1, was combined with porous oxide bulk materials using a bath method at 500 °C for 10 h to prepare a dense carbonate-oxide composite electrolyte. The dense oxide-carbonate composite electrolyte always obtains by this fabrication process. Boiling water was used to remove carbonate from these composites. Lattice parameters were obtained through Rietveld refinement, and a calculation procedure for quantifying the composite density was proposed. The quantified composite density results were verified through scanning electron microscopy microstructure observations. The Ce valence in the RDC oxides and RDC-carbonate composite was analyzed by X-ray absorption near edge structure spectroscopy to observe the effects of heat treatment temperature and carbonate on the Ce4+/Ce3+ mixed-valence state in doped CeO2.

Luminescent colloidal ceria nanoparticles doped with RE3+ ions (RE = Eu, Tb) via cation exchange mechanism

High anionic conductivity in cerium oxide bulk materials and nanocrystals makes these materials perspective for development of solid oxide fuel cells. At the same time, the cation mobility in ceria is considered to be sufficiently less compared to anionic one. In spite of this, using the methods of luminescence spectroscopy we have observed an effective cation exchange between Ce3+ ions in ceria nanoparticles and RE3+ (RE = Eu, Tb) ions in colloidal solutions. This cation exchange leads to formation of RE3+-doped nanoceria with red (Eu3+) and green (Tb3+) luminescence, which intensity gradually increases during the cation exchange process. The dynamics of increase of RE3+ luminescence intensity at different conditions (UV irradiation, H2O2 addition) reveals a key role of Ce3+-Vo-Ce3+ complexes in the processes of incorporation of RE3+ ions into ceria nanoparticles.

Advances in Template Prepared Nano-Oxides and their Applications: Polluted Water Treatment, Energy, Sensing and Biomedical Drug Delivery.

The nano-oxide materials with special structures prepared by template methods have a good dispersion, regular structures and high specific surface areas. Therefore, in some areas, improved properties are observed than conventional bulk oxide materials. For example, in the treatment of dye wastewater, the treatment efficiency of adsorbents and catalytic materials prepared by template method was about 30 % or even higher than that of conventional samples. This review mainly focuses on the progress of inorganic, organic and biological templates in the preparation of micro- and nano- oxide materials with special morphologies, and the roles of the prepared materials as adsorbents and photocatalysts in dye wastewater treatment. The characteristics and advantages of inorganic, organic and biological template are also summarized. In addition, the applications of template method prepared oxides in the field of sensors, drug carrier, energy materials and other fields are briefly discussed with detailed examples.

Optical second harmonic generation from LaAlO3/SrTiO3 interfaces with different in-plane anisotropies

Oxide growth with semiconductor-like accuracy allows the fabrication of atomically precise thin films and interfaces displaying a wide range of phases and functionalities that are absent in the corresponding oxide bulk materials. Among the other properties it was found that a two-dimensional electronic gas is formed under some circumstances at the LaAlO3/SrTiO3(0 0 1) interface separating two typical insulating perovskite crystals. The origin of this conducting state has been discussed at length, since different doping mechanisms can act in these material systems. Many experimental results point to the so-called polar catastrophe scenario as the principal mechanism driving the formation of the two-dimensional electronic gas. According to this mechanism, the existence of an interfacial polar discontinuity is the key ingredient to drive an electronic reconstruction at the LaAlO3/SrTiO3(0 0 1) interface and the consequent formation of a two-dimensional electron gas. This simple picture has been often questioned by the existence of material systems whose interface are predicted being non-polar according to the simplistic ‘ionic’ limit but that display an electrical behavior analogous to that of LaAlO3/SrTiO3(0 0 1) interfaces. This is the case of the LaAlO3/SrTiO3(1 1 0), i.e., a LaAlO3/SrTiO3 interface with a different in-plane orientation. It is evident that to solve such kind of controversies a detailed investigation of the polar or non-polar state of these interfaces is needed, although this is not simple for the lack of experimental tools that are specifically sensitive to interfacial polarity. Here we apply Optical Second Harmonic Generation to investigate LaAlO3/SrTiO3 interfaces with different in-plane orientations to bridge this gap. By comparing our results with recent theoretical findings, we will arrive to the conclusion that the real LaAlO3/SrTiO3(1 1 0) interface is strongly polar.

Ultrafast modification of the polarity at LaAlO3/SrTiO3 interfaces

Oxide growth with semiconductor-like accuracy has led to atomically precise thin films and interfaces that exhibit a plethora of phases and functionalities not found in the oxide bulk material. This yielded spectacular discoveries such as the conducting, magnetic or even superconducting LaAlO$_3$/SrTiO$_3$ interfaces separating two prototypical insulating perovskite materials. All these investigations, however, consider the static state at the interface, although studies on fast oxide interface dynamics would introduce a powerful degree of freedom to understanding the nature of the LaAlO$_3$/SrTiO$_3$ interface state. Here we show that the polarization state at the LaAlO$_3$/SrTiO$_3$ interface can be optically enhanced or attenuated within picoseconds. Our observations are explained by a model based on charge propagation effects in the interfacial vicinity and transient polarization buildup at the interface.

Understanding of Capping Effects on the Tip Shape Evolution and on the Atom Probe Data of Bulk LaAlO3 Using Transmission Electron Microscopy.

Two challenges exist in laser-assisted atom probe tomography (APT). First, a drastic decline in mass-resolving power is caused, not only by laser-induced thermal effects on the APT tips of bulk oxide materials, but also the associated asymmetric evaporation behavior; second, the field evaporation mechanisms of bulk oxide tips under laser illumination are still unclear due to the complex relations between laser pulse and oxide materials. In this study, both phenomena were investigated by depositing Ni- and Co-capping layers onto the bulk LaAlO3 tips, and using stepwise APT analysis with transmission electron microscopy (TEM) observation of the tip shapes. By employing the metallic capping, the heating at the surface of the oxide tips during APT analysis became more symmetrical, thereby enabling a high mass-resolving power in the mass spectrum. In addition, the stepwise microscopy technique visualized tip shape evolution during APT analysis, thereby accounting for evaporation sequences at the tip surface. The combination of "capping" and "stepwise APT with TEM," is applicable to any nonconductors; it provides a direct observation of tip shape evolution, allows determination of the field evaporation strength of oxides, and facilitates understanding of the effects of ultrafast laser illumination on an oxide tip.

Engineering the strongly correlated properties of bulk Ruddlesden-Popper transition metal oxides via self-doping.

We demonstrate via first-principles calculations a novel method of tuning the electron-electron interactions in bulk oxide materials via controlling the cationic layer arrangement. Using the Ruddlesden-Popper oxides LaSrMnO4 and LaSrTiO4 as examples, our study demonstrates that a self-doping effect can be induced by changing the stacking of the neutral and charged cationic layers. It is believed that such a phenomenon is associated with different movements of apical oxygen atoms, resulting in diverse bandgaps, magnetism and orbital degrees of freedom in the same stoichiometric strongly-correlated material. This finding may open up a new direction to engineer the transition metal oxides for practical applications requiring tunable electronic properties without external doping.

Low‐Temperature Cationic Rearrangement in a Bulk Metal Oxide

AbstractCationic rearrangement is a compelling strategy for producing desirable physical properties by atomic‐scale manipulation. However, activating ionic diffusion typically requires high temperature, and in some cases also high pressure in bulk oxide materials. Herein, we present the cationic rearrangement in bulk Mn2FeMoO6 at unparalleled low temperatures of 150–300 oC. The irreversible ionic motion at ambient pressure, as evidenced by real‐time powder synchrotron X‐ray and neutron diffraction, and second harmonic generation, leads to a transition from a Ni3TeO6–type to an ordered‐ilmenite structure, and dramatic changes of the electrical and magnetic properties. This work demonstrates a remarkable cationic rearrangement, with corresponding large changes in the physical properties in a bulk oxide at unprecedented low temperatures.

Low-Temperature Cationic Rearrangement in a Bulk Metal Oxide.

Cationic rearrangement is a compelling strategy for producing desirable physical properties by atomic-scale manipulation. However, activating ionic diffusion typically requires high temperature, and in some cases also high pressure in bulk oxide materials. Herein, we present the cationic rearrangement in bulk Mn2 FeMoO6 at unparalleled low temperatures of 150-300 (o) C. The irreversible ionic motion at ambient pressure, as evidenced by real-time powder synchrotron X-ray and neutron diffraction, and second harmonic generation, leads to a transition from a Ni3 TeO6 -type to an ordered-ilmenite structure, and dramatic changes of the electrical and magnetic properties. This work demonstrates a remarkable cationic rearrangement, with corresponding large changes in the physical properties in a bulk oxide at unprecedented low temperatures.

Defect calculations in semiconductors through a dielectric-dependent hybrid DFT functional: The case of oxygen vacancies in metal oxides.

We investigate the behavior of oxygen vacancies in three different metal-oxide semiconductors (rutile and anatase TiO2, monoclinic WO3, and tetragonal ZrO2) using a recently proposed hybrid density-functional method in which the fraction of exact exchange is material-dependent but obtained ab initio in a self-consistent scheme. In particular, we calculate charge-transition levels relative to the oxygen-vacancy defect and compare computed optical and thermal excitation/emission energies with the available experimental results, shedding light on the underlying excitation mechanisms and related materials properties. We find that this novel approach is able to reproduce not only ground-state properties and band structures of perfect bulk oxide materials but also provides results consistent with the optical and electrical behavior observed in the corresponding substoichiometric defective systems.

Zinc Oxide Nanoparticles Suppress LPS-Induced NF-κB Activation by Inducing A20, a Negative Regulator of NF-κB, in RAW 264.7 Macrophages

Zinc contained in solar salt and bamboo salt plays a critical role in various immune responses. Zinc oxide is a source of zinc, and recently it has been reported that zinc oxide nanoparticles (ZO-NP) more effectively decrease allergic inflammatory reactions than zinc oxide bulk material. The aim of this work was to investigate the regulatory effect of ZO-NP on interferon (IFN)-γ plus lipopolysaccharide (LPS)-stimulated RAW 264.7 macrophages. ZO-NP (0.1-10 μg/mL) did not affect cell viability but toxicity was evident at a ZO-NP concentration of 100 μg/mL. ZO-NP (10 μg/mL) inhibited the IFN-γ plus LPS-induced production of nitric oxide and the protein expressions of inducible nitric oxide synthase and cyclooxygenase-2. The productions of inflammatory cytokines, such as, interleukin (IL)-1β and tumor necrosis factor (TNF)-α were increased by IFN-γ plus LPS but down-regulated by ZO-NP treatment. Furthermore, the up-regulations of IL-1β and TNF-α mRNAs by IFN-γ plus LPS were reduced by ZO-NP at low (0.1 μg/mL) and high (10 μg/mL) concentrations. ZO-NP (0.1, 1, and 10 μg/mL) inhibited the nuclear translocation of nuclear factor-κB by blocking IκBα phosphorylation and degradation. In addition, ZO-NP induced the expression of A20, a zinc finger protein and negative regulator of NF-κB. In conclusion, the present study demonstrated that ZO-NP offer a potential means of treating inflammatory diseases.

Thin Oxide Film Reduction Via the Polyol Method: An Electrochemical Study of a Polyol Reduction Process

Polyols are non-aqueous hydrogen bonded liquids, analogous to alcohols, which have the ability to dissolve both organic and inorganic solids. Polyol synthesis [1] has been widely employed for the preparation of metallic nanoparticles using the temperature-dependent reducing ability of polyols in the presence/absence of capping agents, such as polyvinylpyrrolidone (PVP). In a typical polyol synthesis process, ethylene glycol (EG) is used as a solvent to dissolve metal precursors and as a mild reducing agent. By heating the mixture, the reducing power of the EG increases and ultimately reduces the metal precursors, forming metal nanoparticles with narrow-size distribution. In a modified-polyol method [2], an external reducing agent that is stronger than EG is used. This is usually practiced when the metal precursor can’t be reduced by the ethylene glycol alone. In a polyol or a modified polyol method, the metal precursor can be oxides, hydroxides, nitrates, and acetates that are either dissolved or suspended in the solvent. In case of oxide or hydroxide precursors, they have to be at least slightly soluble in EG at high temperature, otherwise, the addition of an acid is needed. In the work we are presenting here, we are investigating the possibility of using a (modified-)polyol method to reduce thin oxide films of solid surfaces and surface oxide of bulk oxide materials. We investigated the reduction of Sn oxide thin film [3] on a Sn metal substrate as well as the surface reduction of Sn oxide particles to produce meta-stable SnO2@Sn particles that spontaneously form stable SnO2@SnO particles once in contact with air. The reduction of the Sn oxide thin film on a Sn metal substrate was indicated by the sudden change in the Sn electrode potential when measured vs. a reference electrode (Ag/AgCl). Fig. 1 shows a typical OCP (open circuit potential) measured during heating and cooling cycles of hydrazine-containing ethylene glycol. The sudden change of the OCP at ~110°C is attributed to the reduction of the Sn oxide film as the interface changes from SnO2/EG to Sn/EG. More details about the origin of this OCP change will be provided in the presentation with the help of information obtained from quartz crystal microbalance (QCM) and/or impedance spectroscopy (EIS). The surface reduction of bulk Sn oxide in the form of SnO2 particles (50-100 nm) was also investigated. It was found that NaBH4 can reduce the surface oxide resulting in surface SnO. Raman spectroscopy showed that a very small amount of SnO was formed, which is reasonable if it was a surface oxide. Auger and XPS analyses did not show any SnO formation on the surface. The two pieces of information together suggest that SnO has formed on top of the SnO2, but due to the exposure to the atmosphere, the SnO on the top surface oxidized to SnO2 and therefore protected the remaining SnO underneath it and so it was detectable by the Raman and only SnO2 was found on the outermost surface. Further details and supporting results about the oxide reduction in both cases will be provided in the presentation. Acknowledgements The research leading to these results has received funding from the European Community's Seventh Framework Programme (FP7/2013-2016) for the Fuel Cell and Hydrogen Joint Technology Initiative under grant agreement CATAPULT n°.325268. Special thanks to Sebastian Schwaminger for the assistance with XPS and Raman analyses, Katia Rodewald for the superb SEM pictures and analyses and Marianne Hanzlik for the TEM analysis.

Structural changes and conductance thresholds in metal-free intrinsic SiOx resistive random access memory

We present an investigation of structural changes in silicon-rich silicon oxide metal-insulator-metal resistive RAM devices. The observed unipolar switching, which is intrinsic to the bulk oxide material and does not involve movement of metal ions, correlates with changes in the structure of the oxide. We use atomic force microscopy, conductive atomic force microscopy, x-ray photoelectron spectroscopy, and secondary ion mass spectroscopy to examine the structural changes occurring as a result of switching. We confirm that protrusions formed at the surface of samples during switching are bubbles, which are likely to be related to the outdiffusion of oxygen. This supports existing models for valence-change based resistive switching in oxides. In addition, we describe parallel linear and nonlinear conduction pathways and suggest that the conductance quantum, G0, is a natural boundary between the high and low resistance states of our devices.

Uptake of CeO2 nanoparticles and its effect on growth of Medicago arborea In vitro plantlets.

The present study analyzes some effects of nano-CeO2 particles on the growth of in vitro plantlets of Medicago arborea when the nanoceria was added to the culture medium. Various concentrations of nano-CeO2 and bulk ceric oxide particles in suspension form were introduced to the agar culture medium to compare the effects of nanoceria versus ceric oxide bulk material. Germination rate and shoot dry weight were not affected by the addition of ceric oxide to the culture media. Furthermore, no effects were observed on chlorophyll content (single-photon avalanche diode (SPAD) measurements) due to the presence of either nano- or micro-CeO2 in the culture medium. When low concentrations of nanoceria were added to the medium, the number of trifoliate leaves and the root length increased but the root dry weight decreased. Also the values of maximum photochemical efficiency of PSII (F(v)/F m) showed a significant decrease. Dark-adapted minimum fluorescence (F 0) significantly increased in the presence of 200 mg L(-1) nanoceria and 400 mg L(-1) bulk material. Root tissues were more sensitive to nanoceria than were the shoots at lower concentrations of nanoceria. A stress effect was observed on M. arborea plantlets due to cerium uptake.

Tungstate formation in a model scandate thermionic cathode

Crystalline compounds found at the surface of model Ba-Sc-O-W thermionic cathodes (“scandate”) are uniquely identified using Raman spectroscopy. Thin films of sputtered BaO and Sc2O3 on W have been observed in thermionic emission microscopy, field emission scanning electron microscopy, optical microscopy, and Raman Spectroscopy. While the best thermionic electron emission is observed from areas that at the end of the cathode life are completely devoid of thin film BaO, Sc2O3 or observable bulk oxide or tungstate material, the poor emission areas are characterized by BaWO4, Ba2WO5 and long chain linear tungstates (νas = 860 cm−1) that are related to Scx-WOy components. There is no evidence from Raman spectroscopy that tetrahedral Sc2(WO4)3 is present or forms on the surface of the model cathode, or for the presence of Ba3WO6.

Microstructural and Optical Characterization of TiO<SUB>2</SUB> Doped with Ytterbium Synthesized by Sol–Gel and Solar Physical Vapor Deposition Process

Pure and ytterbium doped TiO2 nanopowders in anatase phase have been prepared by sol-gel method (SGM) and Solar Physical Vapour Deposition process (SPVD). The physico-chemical parameters of the nanopowders have been described based on the results of micro-structural characterization performed by X-ray diffractometry, scanning electron microscopy, atomic force microscopy, and nitrogen sorption measurements. Thus, final micro-structural properties of SGM and SPVD titania nanopowders have been compared in detail revealing significant changes in the structure and morphology of these two types of materials. Addition of ytterbium had no significant effect on above-mentioned properties, although it modifies significantly the optical properties of the investigated materials. The luminescent properties of developed material were found to be comparable to bulk oxide materials and better than these reported earlier for ytterbium doped titania. In particular it has been shown that the luminescence of SPVD nanopowders is significantly stronger than this of SGM samples.

Enhanced boundary-scattering of electrons and phonons in nanograined zinc oxide

Nanoscale constituents in bulk materials can promote enhanced boundary-scattering in the transport of phonons as well as electrons, which is considered a key design factor for enhancing thermoelectric properties. Here, we demonstrate a method for synthesizing zinc oxide bulk materials from nanoparticles without significant grain growth by means of pressure-induced deformation at 200 °C. This allows us to comprehensively analyze the grain size dependence of thermoelectric properties in the nanoscale range above 30 nm, the size of a nanoparticle. Grain size was found to largely influence thermal conductivity as well as electrical conductivity. The observed thermal conductivity agreed with the Callaway model, indicating that enhanced phonon boundary-scattering was responsible for the variation. On the contrary, Seebeck coefficient was mostly governed by effective mass and carrier concentration, and was independent of the grain size. The dimensionless figure of merit systematically increased with grain size, which challenged the effect of nanograin on this system.

Metal-insulator transition in thin films of vanadium dioxide: The problem of dimensional effects

Vanadium dioxide has various potential applications in electronics due to the metal-insulator transition (MIT). It is known that oxide structures with nanometric dimensions exhibit properties different from bulk oxide materials because of the spatial confinement and the proximity of the substrate. However, in order to integrate VO 2 into the thriving nano-scale electronics, it is necessary to explore the MIT in this material in thin film nano-structures. In this work, it is shown that there is a fundamental dimensional restriction for the transition to occur even for pure epitaxial VO 2 nano-films and nano-wires. This is associated with the fact that any phase transition turns out to be impossible when the system size is decreased below a certain characteristic length d c. This dimension is estimated to be d c ∼ ξ (where ξ is the correlation length, ∼ 2 nm for VO 2), and, on the basis of the available experimental data, it is shown that the transition temperature falls as the characteristic size (film thickness or nano-wire radius) diminishes, though the predicted theoretical limit of 2 nm is not still experimentally achieved by now. Experimental results concerning the dependence of the threshold voltage on the film thickness at MIT-induced switching in VO 2 based sandwich structures are presented. Finally, the comparison of the authors' experimental data with the literature data, as well as with the analogous features of superconducting phase transitions, is carried out.

Nano-indentation measurement of oxide layers formed in LBE on F/M steels

Ferritic/martensitic (F/M) steels (T91, HT-9, EP 823) are candidate materials for future liquid lead or lead bismuth eutectic (LBE) cooled nuclear reactors. To understand the corrosion of these materials in LBE, samples of each material were exposed at 535 °C for 600 h and 200 h at an oxygen content of 10 −6 wt%. After the corrosion tests, the samples were analyzed using SEM, WDX and nano-indentation in cross section. Multi-layered oxide scales were found on the sample surfaces. The compositions of these oxide layers are not entirely in agreement with the literature. The nano-indentation results showed that the E-modulus and hardness of the oxide layers are significantly lower than the values for dense bulk oxide materials. It is assumed that the low values stem from high porosity in the oxide layers. Comparison with in-air oxidized steels show that the E-modulus decreases with increasing oxide layer thickness.