Lattice-Plane-Dependent Distribution of Ce3+ at Pt and CeO2 Interfaces for Pt/CeO2 Catalysts.

The interaction between a metal and a support, which is known as the metal-support interaction, often plays a determining role in the catalytic properties of supported metal catalysts. Herein, we have developed model Pt/CeO2 catalysts, which enabled us to investigate the interface atomic and electronic structures between Pt and the {001}, {011}, and {111} planes of CeO2 using scanning transmission electron microscopy and electron energy-loss spectroscopy. We found that the number of Ce3+ ions around the Pt nanoparticles followed the order {001} > {011} > {111}, which was the opposite order of the generally accepted stability of low index surfaces of CeO2. Systematic first-principles calculations revealed that the presence of Pt nanoparticles facilitated the formation of oxygen vacancies and that the appearance of the Ptδ+ state was preferred when Pt nanoparticles were in contact with CeO2 {001} planes due to direct charge transfer from Pt to CeO2. These results provide important insights into the nature of the metal-support interaction for a comprehensive understanding of the properties of supported metal catalysts.

Direct identification of the charge state in a single platinum nanoparticle on titanium oxide.

A goal in the characterization of supported metal catalysts is to achieve particle-by-particle analysis of the charge state strongly correlated with the catalytic activity. Here, we demonstrate the direct identification of the charge state of individual platinum nanoparticles (NPs) supported on titanium dioxide using ultrahigh sensitivity and precision electron holography. Sophisticated phase-shift analysis for the part of the NPs protruding into the vacuum visualized slight potential changes around individual platinum NPs. The analysis revealed the number (only one to six electrons) and sense (positive or negative) of the charge per platinum NP. The underlying mechanism of platinum charging is explained by the work function differences between platinum and titanium dioxide (depending on the orientation relationship and lattice distortion) and by first-principles calculations in terms of the charge transfer processes.

Enhanced catalytic performance of spinel-type Cu-Mn oxides for benzene oxidation under microwave irradiation.

Microwave-assisted heterogeneous catalytic oxidation of benzene was investigated over Cu-Mn spinel oxides. The spinel oxides were synthesized by a coprecipitation method from metal nitrate hydrolysis in a solution using tetramethylammonium hydroxide (TMAH) as a precipitation reagent. The catalysts were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, X-ray absorption fine structure, scanning electron microscopy, transmission electron microscope and H2-temperature-programmed reduction studies. Microwave absorption by the Cu-Mn spinel oxide is mainly driven by dielectric losses (dielectric heating). Cu-Mn spinel oxide with a Cu/Mn ratio of 1 exhibited superior activity to single oxides under microwave heating, demonstrating lower apparent activation energy than that obtained under conventional heating. Microwave irradiation lowered the reaction temperature required for benzene oxidation compared with conventional heating. Transient tests were used to investigate the reactivity of oxygen species in the catalytic reaction, and the high reactivity of Cu-Mn spinel oxides was related to the high reactivity of lattice oxygen on the catalyst surface. The reactivity of the oxygen species was enhanced under microwave heating, leading to an enhanced benzene oxidation reaction. The combination of adsorption and catalytic oxidation processes using Cu-Mn spinel oxides and zeolites efficiently decomposed benzene at low concentrations.

Atomic and Electronic Structure of Pt/TiO2 Catalysts and Their Relationship to Catalytic Activity.

Understanding the nature of the interaction between a metal and support, which is known as the metal-support interaction, in supported metal catalysts is crucial to design catalysts with desired properties. Here, we have developed model Pt/TiO2 catalysts based on the deposition of colloidal Pt nanoparticles and studied their atomic and electronic structures before and after a postdeposition treatment that induces catalytic activity using aberration-corrected scanning transmission electron microscopy, electron energy loss spectroscopy, and first-principles calculations. Direct contact between Pt nanoparticles and TiO2 is realized after the postdeposition treatment, which is accompanied by the formation of a Ti3+ state on the TiO2 surface close to the Pt nanoparticles and a Ptδ+ state on the Pt nanoparticles. The origin of these two states and their effect on the catalytic properties are discussed. These findings pave the way for a comprehensive understanding of metal-support interactions in supported metal catalysts.

Observation of novel charge ordering and spin reorientation in perovskite oxide PbFeO3.

PbMO3 (M = 3d transition metals) family shows systematic variations in charge distribution and intriguing physical properties due to its delicate energy balance between Pb 6s and transition metal 3d orbitals. However, the detailed structure and physical properties of PbFeO3 remain unclear. Herein, we reveal that PbFeO3 crystallizes into an unusual 2ap × 6ap × 2ap orthorhombic perovskite super unit cell with space group Cmcm. The distinctive crystal construction and valence distribution of Pb2+0.5Pb4+0.5FeO3 lead to a long range charge ordering of the -A-B-B- type of the layers with two different oxidation states of Pb (Pb2+ and Pb4+) in them. A weak ferromagnetic transition with canted antiferromagnetic spins along the a-axis is found to occur at 600 K. In addition, decreasing the temperature causes a spin reorientation transition towards a collinear antiferromagnetic structure with spin moments along the b-axis near 418 K. Our theoretical investigations reveal that the peculiar charge ordering of Pb generates two Fe3+ magnetic sublattices with competing anisotropic energies, giving rise to the spin reorientation at such a high critical temperature.

Polarization Rotation at Morphotropic Phase Boundary in New Lead-Free Na1/2Bi1/2V1-xTixO3 Piezoceramics.

In this work, we show that polarization rotation enhances the piezoresponse in a high-performance lead-free piezoelectric material, Na1/2Bi1/2V1-xTixO3, a solid solution between tetragonal Na1/2Bi1/2VO3 and rhombohedral Na1/2Bi1/2TiO3, obtained by high-pressure synthesis. The system forms a pure perovskite structure with a favorable morphotropic phase boundary (MPB) located around x = 0.90, which separates the tetragonal and rhombohedral phases. In addition, a distinct monoclinic phase with polarization rotation as functions of composition and temperature is observed. XRD measurements revealed the moderately high Curie temperature of 523 K at x = 0.95 in the MPB. The piezoelectric coefficient d33 of the monoclinic x = 0.95 sample, 42 pC/N, is higher than those of the tetragonal and rhombohedral phases. Even though the present lead-free Na1/2Bi1/2V1-xTixO3 ceramics feature smaller d33 values compared to many currently available lead-free piezoelectric materials as a result of insufficient poling and low density, we expect our findings open up opportunities for exploring promising lead-free piezoelectric materials in Na1/2Bi1/2VO3-based perovskites.

Modulating the Structure and Magnetic Properties of ε-Fe2O3 Nanoparticles via Electrochemical Li+ Insertion.

ε-Fe2O3, a metastable phase of iron oxide, is widely known as a room-temperature multiferroic material or as a superhard magnet. Element substitution into ε-Fe2O3 has been reported in the literature; however, the substituted ions have a strong site preference depending on their ionic radii and valence. In this study, in order to characterize the crystal structure and magnetic properties of ε-Fe2O3 in the Fe2+/Fe3+ coexisting states, Li+ was electrochemically inserted into ε-Fe2O3 to reduce Fe3+. The discharge and charge of Li+ into/from ε-Fe2O3 revealed that Li+ insertion was successful. X-ray magnetic circular dichroism results indicated that the reduced Fe did not exhibit site preference. Increasing the Li+ content in ε-Fe2O3 resulted in decreased saturation magnetization and irregular variation of the coercive field. We present a comprehensive discussion of how magnetic properties are modified with increasing Li+ content using transmission electron microscopy images and considering the Li+ diffusion coefficient. The results suggest that inserting Li+ into crystalline ε-Fe2O3 is a useful tool for characterizing crystal structure, lithiation limit, and magnetic properties in the coexistence of Fe2+/Fe3+.

Enhanced Negative Thermal Expansion Induced by Simultaneous Charge Transfer and Polar-Nonpolar Transitions.

Negative thermal expansion (NTE) induced by simultaneous mechanisms, that is, charge transfer and polar-nonpolar transitions, was observed for the first time in BiNi1-xFexO3 (0.25 ≤ x ≤ 0.5). The low-temperature phase was found to have a polar structure (space group of R3c) with a Bi3+0.5(1+x)Bi5+0.5(1-x)Ni2+1-xFe3+xO3 charge distribution and short-range ordering of Bi3+ and Bi5+. The volume reduction upon heating that was induced by charge transfer between Bi5+ and Ni2+ decreased with increasing x because of the reduction in the amount of Ni2+. Simultaneous polar-nonpolar transition also contributed to NTE, and a composition-independent enhanced volume reduction of ∼2% was observed.

Removal of benzene by non-thermal plasma catalysis over manganese oxides through a facile synthesis method.

Three manganese oxide catalysts (MnOx) were synthesized via a simple method, and then they were introduced into the non-thermal plasma (NTP) system for benzene removal. The XRD and EXAFS results showed the MnOx were mainly in the Mn3O4 phase, and from the analysis of N2 adsorption/desorption isotherms, we knew the MnOx calcined at 250 °C (Mn250) had the largest surface area of 274.5 m2 g-1. Besides, Mn250 also exerted higher benzene adsorption capacity (0.430 mmol g-1) according to C6H6-TPD. O2-TPD indicated that Mn250 showed better oxygen mobility than Mn300. Moreover, by analyzing XPS results, it revealed that Mn250 exhibited rich abundant of surface adsorbed oxygen species (Oads) and moderate ratio of Mn4+/Mn3+, and the reducibility temperature was also the lowest among all the MnOx catalysts drawn by H2-TPR profiles. As a result, Mn250 combined with NTP could remove 96.9% of benzene at a low input power of 3 W (benzene concentration 200 ppm, and GHSV 60,000 mL gcat.-1 h-1), performing the best catalytic activity among the three catalysts and plasma only. Furthermore, the "NTP + Mn250" system also produced the highest CO2 concentration and lowest CO concentration in downstream, and the residual O3 after catalytic reaction was also the lowest, that is to say, the synergistic effect between NTP and Mn250 was more effective than other catalysts in benzene removal. Graphical abstract.

Sodium titanium oxide bronze nanoparticles synthesized via concurrent reduction and Na+-doping into TiO2(B).

A mixed valence compound, sodium titanium oxide bronze (NaxTiO2-B), combines intriguing properties of high electric conductivity and good chemical stability together with a unique one-dimensional tunnel crystal structure available for cation storage. However, this compound has not been studied for a long period because of the strongly reductive condition at high temperature required for its preparation, which limits the morphological control such as the preparation of nanocrystals. For the first time in this paper, the topotactic synthesis of nano-sized NaxTiO2-B with high specific surface area (>130 m2 g-1) from TiO2(B) nanoparticles has been demonstrated. The reaction of metastable TiO2(B) with NaBH4 allows carrier electrons to be doped simultaneously with incorporation of Na+ ions into the interstitial sites of the host Ti-O lattice at relatively low temperature. An electrochemical investigation of Li+- and Na+-ion storage behaviors suggests that the incorporated Na+ ions are mainly placed in the 6-fold coordination sites of bronze. In addition, optical measurements including time-resolved transient spectroscopy revealed that the doped electrons in the NaxTiO2-B nanoparticles are predominantly in the Ti3+ state and behave as a small polaron. The pelletized NaxTiO2-B nanoparticles shows a good electronic conductivity of 1.4 × 10-2 S cm-1 at 30 °C with an activation energy of 0.17 eV, which is attributable to the thermal barrier for the polaron hopping.

Direct Observation of Magnetization Reversal by Electric Field at Room Temperature in Co-Substituted Bismuth Ferrite Thin Film.

Using the electric field to manipulate the magnetization of materials is a potential way of making low-power-consumption nonvolatile magnetic memory devices. Despite concentrated effort in the last 15 years on magnetic multilayers and magnetoelectric multiferroic thin films, there has been no report on the reversal of out-of-plane magnetization by an electric field at room temperature without the aid of an electric current. Here, we report direct observation of out-of-plane magnetization reversal at room temperature by magnetic force microscopy after electric polarization switching of cobalt-substituted bismuth ferrite thin film grown on (110)o-oriented GdScO3 substrate. A striped pattern of ferroelectric and weakly ferromagnetic domains was preserved after reversal of the out-of-plane electric polarization.

Large Negative Thermal Expansion Induced by Synergistic Effects of Ferroelectrostriction and Spin Crossover in PbTiO3-Based Perovskites.

The discovery of unusual negative thermal expansion (NTE) provides the opportunity to control the common but much desired property of thermal expansion, which is valuable not only in scientific interests but also in practical applications. However, most of the available NTE materials are limited to a narrow temperature range, and the NTE effect is generally weakened by various modifications. Here, we report an enhanced NTE effect that occurs over a wide temperature range α‾V=-5.24×10-5∘C-1,25-575∘C, and this NTE effect is accompanied by an abnormal enhanced tetragonality, a large spontaneous polarization, and a G-type antiferromagnetic ordering in the present perovskite-type ferroelectric of (1-x)PbTiO3-xBiCoO3. Specifically, for the composition of 0.5PbTiO3-0.5BiCoO3, an extensive volumetric contraction of ~4.8 % has been observed near the Curie temperature of 700 °C, which represents the highest level in PbTiO3-based ferroelectrics. According to our experimental and theoretical results, the large NTE originates from a synergistic effect of the ferroelectrostriction and spin crossover of cobalt on the crystal lattice. The actual NTE mechanism is contrasted with previous functional NTE materials, in which the NTE is simply coupled with one ordering such as electronic, magnetic, or ferroelectric ordering. The present study sheds light on the understanding of NTE mechanisms, and it attests that NTE could be simultaneously coupled with different orderings, which will pave a new way toward the design of large NTE materials.

Development of Bismuth Ferrite as a Piezoelectric and Multiferroic Material by Cobalt Substitution.

Bismuth ferrite (BiFeO3 ) is the most widely studied multiferroic material with robust ferroelectricity and antiferromagnetic ordering at room temperature. One of the possible device applications of this material is one that utilizes the ferroelectric/piezoelectric property itself such as ferroelectric memory components, actuators, and so on. Other applications are more challenging and make full use of its multiferroic property to realize novel spintronics and magnetic memory devices, which can be addressed electrically as well as magnetically. This progress report summarizes the recent attempt to control the piezoelectric and magnetic properties of BiFeO3 by cobalt substitution.

Systematic charge distribution changes in Bi- and Pb-3d transition metal perovskites.

Charge distribution changes in Bi- and Pb-3d transition metal perovskite type oxides were examined by comprehensive precise structural analysis, spectroscopy, and theoretical investigations. The change in the depth of the d level of the transition metal caused the intermetallic charge transfer. A temperature-induced charge-transfer transition in chemically modified BiNiO3 results in technologically important negative thermal expansion.

Realization of Large Electric Polarization and Strong Magnetoelectric Coupling in BiMn3 Cr4 O12.

Magnetoelectric multiferroics have received much attention in the past decade due to their interesting physics and promising multifunctional performance. For practical applications, simultaneous large ferroelectric polarization and strong magnetoelectric coupling are preferred. However, these two properties have not been found to be compatible in the single-phase multiferroic materials discovered as yet. Here, it is shown that superior multiferroic properties exist in the A-site ordered perovskite BiMn3 Cr4 O12 synthesized under high-pressure and high-temperature conditions. The compound experiences a ferroelectric phase transition ascribed to the 6s2 lone-pair effects of Bi3+ at around 135 K, and a long-range antiferromagnetic order related to the Cr3+ spins around 125 K, leading to the presence of a type-I multiferroic phase with huge electric polarization. On further cooling to 48 K, a type-II multiferroic phase induced by the special spin structure composed of both Mn- and Cr-sublattices emerges, accompanied by considerable magnetoelectric coupling. BiMn3 Cr4 O12 thus provides a rare example of joint multiferroicity, where two different types of multiferroic phases develop subsequently so that both large polarization and significant magnetoelectric effect are achieved in a single-phase multiferroic material.

High-Pressure Synthesis of the Cobalt Pyrochlore Oxide Pb2Co2O7 with Large Cation Mixed Occupancy.

The novel A2B2O7-type compound Pb2Co2O7 was synthesized at 8 GPa and 1673 K. Synchrotron X-ray diffraction shows a cubic pyrochlore structure with space group Fd3̅m. Rietveld structural analysis reveals a large cation mixed occupancy at both A and B sites by about 40%, the greatest value found in the pyrochlore family. In combination with the X-ray absorption spectroscopy results, the specific chemical composition and charge states are determined to be (Co0.6Pb0.4)3+2(Pb0.6Co0.4)4+2O7, in which both the A-site Co3+ and the B-site Co4+ are low-spin. Due to the tetrahedral geometric frustration effects as well as the random Co4+ and Pb4+ distribution at the B site, spin glassy behavior is well observed following the conventional critical slowing down feature in Pb2Co2O7.

A-Site and B-Site Charge Orderings in an s-d Level Controlled Perovskite Oxide PbCoO3.

Perovskite PbCoO3 synthesized at 12 GPa was found to have an unusual charge distribution of Pb2+Pb4+3Co2+2Co3+2O12 with charge orderings in both the A and B sites of perovskite ABO3. Comprehensive studies using density functional theory (DFT) calculation, electron diffraction (ED), synchrotron X-ray diffraction (SXRD), neutron powder diffraction (NPD), hard X-ray photoemission spectroscopy (HAXPES), soft X-ray absorption spectroscopy (XAS), and measurements of specific heat as well as magnetic and electrical properties provide evidence of lead ion and cobalt ion charge ordering leading to Pb2+Pb4+3Co2+2Co3+2O12 quadruple perovskite structure. It is shown that the average valence distribution of Pb3.5+Co2.5+O3 between Pb3+Cr3+O3 and Pb4+Ni2+O3 can be stabilized by tuning the energy levels of Pb 6s and transition metal 3d orbitals.

Ferromagnetism at Room Temperature Induced by Spin Structure Change in BiFe1-x Cox O3 Thin Films.

The coexistence and coupling of ferromagnetic and ferroelectric orders in a single material is crucial for realizing next-generation multifunctional applications. The coexistence of such orders is confirmed at room temperature in epitaxial thin films of BiFe1-x Cox O3 (x ≤ 0.15), which manifests a spin structure change from a low-temperature cycloidal one to a high-temperature collinear one with canted ferromagnetism.

Enhanced Piezoelectric Response due to Polarization Rotation in Cobalt-Substituted BiFeO3 Epitaxial Thin Films.

Polarization rotation induced by an external electric field in piezoelectric materials such as PbZr1-x Tix O3 is generally regarded as the origin of their large piezoelectric responses. Here, the piezoelectric responses of high-quality cobalt-substituted BiFeO3 epitaxial thin films with monoclinic distortions are systematically examined. It is demonstrated that polarization rotation plays a crucial role in improving the piezoelectric responses in this material.

High-Temperature Monoclinic Cc Phase with Reduced c/a Ratio in Bi-based Perovskite Compound Bi2ZnTi1-xMnxO6.

Monoclinic phases with Cm, Pm, and Cc space groups are indispensable to understand the high performance of electromechanical properties at the morphotropic phase boundary (MPB) of lead-based perovskite oxides Pb(ZrxTi1-x)O3 (PZT), [Pb(Mg1/3Nb2/3)O3]1-x-(PbTiO3)x (PMN-PT), and [Pb(Zn1/3Nb2/3)O3]1-x-(PbTiO3)x (PZN-PT). Here, a nearly single monoclinic phase with space group Cc was observed in the Bi-based lead-free perovskite compound Bi2ZnTi1-xMnxO6 at x = 0.4. This phase was the same as the low-temperature phase of the MPB composition of PZT but existed at a much higher temperature. Despite the reduced pseudo c/a ratio of 1.065, which is the same as that of PbTiO3 at room temperature, ionic model calculation based on the Rietveld refinement data indicated the polarization of Bi2ZnTi0.6Mn0.4O6 is 95.8 µC/cm(2). The tilting and significant anisotropic distortion of the octahedron were found to cause the c/a ratio to reduce. Accordingly, the effective piezoelectric constant d33 of Bi2ZnTi0.6Mn0.4O6 thin film was found to be 12 pm/V.