Experimental and ab initio studies on the structural, magnetic, photocatalytic, and antibacterial properties of Cu-doped ZnO nanoparticles.

Copper-doped ZnO nanoparticles with a dopant concentration varying from 1-7 mol% were synthesized and their structural, magnetic, and photocatalytic properties were studied using XRD, TEM, SQUID magnetometry, EPR, UV-vis spectroscopy, and first-principles methods within the framework of density functional theory (DFT). Structural analysis indicated highly crystalline Cu-doped ZnO nanoparticles with a hexagonal wurtzite structure, irrespective of the dopant concentration. EDX and EPR studies indicated the incorporation of doped Cu2+ ions in the host ZnO lattice. The photocatalytic activities of the Cu-doped ZnO nanoparticles investigated through the degradation of methylene blue demonstrated an enhancement in photocatalytic activity as the degradation rate changed from 9.89 × 10-4 M min-1 to 4.98 × 10-2 M min-1. By the first-principles method, our results indicated that the Cu(3d) orbital was strongly hybridized with the O(2p) state below the valence band maximum (VBM) due to covalent bonding, and the ground states of the Cu-doped ZnO is favorable for the ferromagnetic state by the asymmetry of majority and minority states due to the presence of unpaired electron.

Spinodal Decomposition in the Mg-Al-Fe-O System.

A supersaturated spinel solid solution having a nominal compositional ratio of Mg/Al/Fe = 0.5:1.0:1.5 was prepared using a conventional solid-state reaction at 1573 K in air followed by quenching in ice water. The formula of the resulting spinel structure compound (the spinel) was determined to be (Mg0.50AlFe0.262+Fe1.243+)0.97O4 based on a Rietveld refinement and thermogravimetry, indicating a cation-deficient spinel structure having mixed valences of Fe. This spinel was found to decompose to γ-Fe2O3 and a modified, Fe-poor spinel structure compound via a spinodal decomposition below 855 K. The spinodal temperature was estimated using the sidebands appearing in X-ray diffraction patterns in addition to the temperature dependence of magnetization values. This spinodal decomposition was accompanied by the oxidation of Fe2+ to Fe3+ and produced a unique grid-like microstructure (with a grid width of approximately 25 nm) along with enhancement of the saturated magnetization of the material. A sample cooled to room temperature in a furnace after heating at 1573 K in air had a lamella structure having a width of approximately 0.1 µm and comprised particles with a mixture of γ-Fe2O3 and the Fe-poor spinel compound on their surfaces. Subsequent heating of this same material to 1373 K in air formed ε-Fe2O3 in the particles. The crystallographic relationship between ε-Fe2O3 and the modified spinel structure compound was aε // [112̅]s, bε // [1̅10]s, and cε // [111]s (where ε and s indicate the ε-Fe2O3 and spinel, respectively).

Synthesis of Monophasic Al3BC3 Powder with Hexagonal Plate-like Morphology.

This paper describes the formation mechanism of monophasic Al3BC3 powder with hexagonal plate-like morphology using Al, B4C, and C powders in a stoichiometric ratio. For the synthesis of pure Al3BC3 by the conventional solid-state reaction method, it is important to avoid contamination from the starting powders. Thermogravimetry-differential thermal analysis curves and quantitative analysis by inductively coupled plasma-atomic emission spectroscopy revealed that there was no vaporization of the powder mixture during heating up to 1470 °C in a high-purity argon atmosphere. We found that the phases and Al3BC3 particle morphology after heating at 1800 °C strongly depended on the mixing conditions. In the case of a two-step process (remixing and reheating the sample), particles with random morphology were observed. On the other hand, well-faceted Al3BC3 particles were predominantly formed in a one-step process that involved heating after ball-milling for 24 h.

Studying and Utilizing Traditional Technologies: Microstructure and Formation Mechanism of ε-Fe2O3 on Traditional Japanese Bizen Stoneware.

Traditional Japanese Bizen stoneware is produced by firing a specific type of green clay in a wood-fired kiln at approximately 1200 °C. During this process, single crystalline branched dendrite-like particles of Al-substituted ε-Fe2O3 (ε-Fe1.7Al0.3O3) with widths and lengths of approximately 15 and 30 µm, respectively, are formed on the surface of the ceramic. Composite particles consisting of ε-Fe2O3 epitaxially connected to spinel structure compounds [comprising the Fe-substituted spinel (Mg,Fe)(Al,Fe)2O4 and γ-Fe2O3)] with lengths of approximately 3 µm are also generated. The present work clarified the crystallographic relationship between ε-Fe2O3 and the spinel structure compounds. In addition, brown-colored samples similar to Bizen pottery and with surface Al-substituted ε-Fe2O3 particles were prepared by heating clay with K2CO3 under a 10 vol % CO gas and 90 vol % Ar gas mixture using an electric furnace instead of a firewood kiln. Hence, a traditional method was adapted to achieve the industrial production of ε-Fe2O3 crystals.

Preparation and Characterization of Additional Metallic Element-Containing Tubular Iron Oxides of Bacterial Origin.

Biogenic microtubular iron oxides (BIOXs) derived from Leptothrix spp. are known as promising multifunctional materials for industrial applications such as ceramic pigments and catalyst carriers. Here, we report unprecedented BIOX products with additive depositions of various metallic elements prepared by a newly devised "two-step" method using an artificial culture system of Leptothrix cholodnii strain OUMS1; the method comprises a biotic formation of immature organic sheaths and subsequent abiotic deposition of Fe and intended elements on the sheaths. Chemical composition ratios of the additional elements Al, Zr, and Ti in the respective BIOXs were arbitrarily controllable depending on initial concentrations of metallic salts added to reaction solutions. Raman spectroscopy exemplified an existence of Fe-O-Al linkage in the Al-containing BIOX matrices. Time-course analyses revealed the underlying physiological mechanism for the BIOX formation. These results indicate that our advanced method can contribute greatly to creations of innovative bioderived materials with improved functionalities.

Hydrothermal Synthesis and Crystal Structure of a Mixed-Valence Bismuthate, Na3Bi3O8.

Four types of bismuth oxides, Na3Bi3O8, NaBiO3, α-Bi2O3, and ε-Bi2O3, were obtained by hydrothermal reactions using NaBiO3·nH2O in NaOH solution. The crystal structure of a new phase (Na3 Bi3+)Bi25+O8 ((Na0.75Bi0.25)2BiO4) was determined by using single crystal X-ray diffraction data, and this compound was found to show a Na2MnCl4-related structure with a monoclinic system (space group, Pm) with the following lattice parameters: a = 5.990 (2) Å, b = 3.335 (2) Å, c = 10.108 (2) Å, and ß = 91.08 (3)°. The final R-factors R1 and wR2 were 0.041 and 0.090 (all data), respectively. The new phase was composed of mixed valence states of Bi (Bi3+ and Bi5+, with a mean Bi valence of 4.30) with five distinct Bi sites, where two Bi5+ (Bi1 and Bi2) fully occupied the distorted octahedral sites and three Bi3+ (Bi3, Bi4, and Bi5) were statistically distributed at the split sites with Na+ (Na3, Na4, and Na5). The Na6 site is fully occupied. The distorted Bi5+O6 octahedra formed one-dimensional chains via edge-sharing along the b-axis, with the chains held by Bi3+/Na+ split sites. The structural feature except for the split distribution of Bi3+/Na+ was classified as a Na2MnCl4-type structure. DFT calculations based on a model discounting the split distribution of Bi3+/Na+ indicated that Bi 6s and O 2p orbitals form sp hybridization at the conduction band. This new mixed valence bismuth oxide exhibited photocatalytic activity for phenol degradation under visible light irradiation. In addition to Na3Bi3O8, the hydrothermal reaction using NaBiO3·nH2O in NaOH solution yielded micrometer-sized single crystals of an ilmenite-type NaBiO3 and two polymorphs of bismuth oxides with monoclinic (α-Bi2O3) and orthorhombic (ε-Bi2O3) structures, depending on the reaction temperature and NaOH concentration.

High pressure synthesis of a quasi-one-dimensional GdFeO3-type perovskite PrCuO3 with nearly divalent Cu ions.

A new perovskite-type cuprate PrCuO3 has been synthesized by high-pressure oxygen annealing. Synchrotron X-ray powder diffraction and absorption spectroscopy revealed that PrCuO3 crystallizes in the GdFeO3-type structure with cooperative Jahn-Teller distortion, forming one-dimensional chains of corner-shared CuO4 plaquettes with nearly divalent Cu ions.

Biosorption of metal elements by exopolymer nanofibrils excreted from Leptothrix cells.

Leptothrix species, aquatic Fe-oxidizing bacteria, excrete nano-scaled exopolymer fibrils. Once excreted, the fibrils weave together and coalesce to form extracellular, microtubular, immature sheaths encasing catenulate cells of Leptothrix. The immature sheaths, composed of aggregated nanofibrils with a homogeneous-looking matrix, attract and bind aqueous-phase inorganics, especially Fe, P, and Si, to form seemingly solid, mature sheaths of a hybrid organic-inorganic nature. To verify our assumption that the organic skeleton of the sheaths might sorb a broad range of other metallic and nonmetallic elements, we examined the sorption potential of chemically and enzymatically prepared protein-free organic sheath remnants for 47 available elements. The sheath remnants were found by XRF to sorb each of the 47 elements, although their sorption degree varied among the elements: >35% atomic percentages for Ti, Y, Zr, Ru, Rh, Ag, and Au. Electron microscopy, energy dispersive x-ray spectroscopy, electron and x-ray diffractions, and Fourier transform infrared spectroscopy analyses of sheath remnants that had sorbed Ag, Cu, and Pt revealed that (i) the sheath remnants comprised a 5-10 nm thick aggregation of fibrils, (ii) the test elements were distributed almost homogeneously throughout the fibrillar aggregate, (iii) the nanofibril matrix sorbing the elements was nearly amorphous, and (iv) these elements plausibly were bound to the matrix by ionic binding, especially via OH. The present results show that the constitutive protein-free exopolymer nanofibrils of the sheaths can contribute to creating novel filtering materials for recovering and recycling useful and/or hazardous elements from the environment.

High-Pressure Polymorph of NaBiO3.

A new high-pressure polymorph of NaBiO3 (hereafter ß-NaBiO3) was synthesized under the conditions of 6 GPa and 600 °C. The powder X-ray diffraction pattern of this new phase was indexed with a hexagonal cell of a = 9.968(1) Å and c = 3.2933(4) Å. Crystal structure refinement using synchrotron powder X-ray diffraction data led to RWP = 8.53% and RP = 5.55%, and the crystal structure was closely related with that of Ba2SrY6O12. No photocatalytic activity for phenol decomposition was observed under visible-light irradiation in spite of a good performance for its mother compound, NaBiO3. The optical band-gap energy of ß-NaBiO3 was narrower than that of NaBiO3, which was confirmed with density of states curves simulated by first-principles density functional theory calculation.

Preparation of yellowish-red Al-substituted α-Fe2O3 powders and their thermostability in color.

Inspired by the traditional Japanese pigment Fukiya bengala, nanocomposite materials were synthesized using a polymer complex method, comprising Al-substituted α-Fe2O3 (hematite) particles with diameters ranging from 40 to 100 nm and ultrafine Fe-substituted α-Al2O3 (corundum) particles smaller than 10 nm in diameter. The obtained powders exhibited a vivid yellowish-red color and high thermostability, making them attractive as potential overglaze enamels on porcelain. Quantitative color measurements revealed that, when heated to 700, 800, and 900 °C, samples displayed high lightness (L*) and color-opponent dimensions (a* and b*) at 10 mol % Al. For the same particle size samples, L*, a*, and b* values increased with the Al molar ratio, revealing that Al substitution in the hematite structure intrinsically enhances lightness and chroma in hematite color. These samples mostly retained their color upon reheating at 900 °C, indicating their high thermostability. This thermostability should originate from the Al substitution-induced enhancement in lightness and chroma in hematite color, which should counter color fading caused by particle growth. These composite materials are expected to find application in the porcelain industry, cosmetics, and nanotechnology.

Valence transitions in negative thermal expansion material SrCu3Fe4O12.

The valence states of a negative thermal expansion material, SrCu3Fe4O12, are investigated by X-ray absorption and (57)Fe Mössbauer spectroscopy. Spectroscopic analyses reveal that the appropriate ionic model of this compound at room temperature is Sr(2+)Cu(~2.4+)3Fe(~3.7+)4O12. The valence states continuously transform to Sr(2+)Cu(~2.8+)3Fe(~3.4+)4O12 upon cooling to ~200 K, followed by a charge disproportionation transition into the Sr(2+)Cu(~2.8+)3Fe(3+)(~3.2)Fe(5+)(~0.8)O12 valence state at ~4 K. These observations have established the charge-transfer mechanism in this compound, and the electronic phase transitions in SrCu3Fe4O12 can be distinguished from the first-order charge-transfer phase transitions (3Cu(2+) + 4Fe(3.75+) → 3Cu(3+) + 4Fe(3+)) in Ln(3+)Cu(2+)3Fe(3.75+)4O12 (Ln = trivalent lanthanide ions).

Stability of α''-Fe16N2 in hydrogenous atmospheres.

We revealed the inherent instability of α''-Fe16N2 in hydrogenous atmospheres due to the denitrification toward α-Fe by forming NH3 at the particle surface. Coating the particle surface with SiO2 to suppress the formation of NH3 has proven to be a simple yet powerful method to enhance the stability of α''-Fe16N2 in hydrogenous atmospheres.

Control of bond-strain-induced electronic phase transitions in iron perovskites.

Unusual electronic phase transitions in the A-site ordered perovskites LnCu3Fe4O12 (Ln: trivalent lanthanide ion) are investigated. All LnCu3Fe4O12 compounds are in identical valence states of Ln(3+)Cu(2+)3Fe(3.75+)4O12 at high temperature. LnCu3Fe4O12 with larger Ln ions (Ln = La, Pr, Nd, Sm, Eu, Gd, Tb) show an intersite charge transfer transition (3Cu(2+) + 4Fe(3.75+) → 3Cu(3+) + 4Fe(3+)) in which the transition temperature decreases from 360 to 240 K with decreasing Ln ion size. In contrast, LnCu3Fe4O12 with smaller Ln ions (Ln = Dy, Ho, Er, Tm Yb, Lu) transform into a charge-disproportionated (8Fe(3.75+) → 5Fe(3+) + 3Fe(5+)) and charge-ordered phase below ∼250-260 K. The former series exhibits metal-to-insulator, antiferromagnetic, and isostructural volume expansion transitions simultaneously with intersite charge transfer. The latter shows metal-to-semiconductor, ferrimagnetic, and structural phase transitions simultaneously with charge disproportionation. Bond valence calculation reveals that the metal-oxygen bond strains in these compounds are classified into two types: overbonding or compression stress (underbonding or tensile stress) in the Ln-O (Fe-O) bond is dominant in the former series, while the opposite stresses or bond strains are found in the latter. Intersite charge transfer transition temperatures are strongly dependent upon the global instability indices that represent the structural instability calculated from the bond valence sum, whereas the charge disproportionation occurs at almost identical temperatures, regardless of the magnitude of structural instability. These findings provide a new aspect of the structure-property relationship in transition metal oxides and enable precise control of electronic states by bond strains.

Quantitative understanding of thermal stability of α''-Fe16N2.

The thermal stability of α''-Fe16N2, which attracts much interest because of its superior magnetic properties featuring a large magnetocrystalline anisotropy (Ku ~ 1 × 10(7) erg cm(-3)) and a large saturation magnetization (Ms ~ 234 emu g(-1)), though unfortunately thermally unstable, has been quantitatively studied.

Nano-micrometer-architectural acidic silica prepared from iron oxide of Leptothrix ochracea origin.

We prepared nano-micrometer-architectural acidic silica from a natural amorphous iron oxide with structural silicon which is a product of the iron-oxidizing bacterium Leptothrix ochracea. The starting material was heat-treated at 500 °C in a H2 gas flow leading to segregation of α-Fe crystalline particles and then dissolved in 1 M hydrochloric acid to remove the α-Fe particles, giving a gray-colored precipitate. It was determined to be amorphous silica containing some amount of iron (Si/Fe = ~60). The amorphous silica maintains the nano-microstructure of the starting material-~1-µm-diameter micrometer-tubules consisting of inner globular and outer fibrillar structures several tens of nanometer in size-and has many large pores which are most probably formed as a result of segregation of the α-Fe particles on the micrometer-tubule wall. The smallest particle size of the amorphous silica is ~10 nm, and it has a large surface area of 550 m(2)/g with micropores (0.7 nm). By using pyridine vapor as a probe molecule to evaluate the active sites in the amorphous silica, we found that it has relatively strong Brønsted and Lewis acidic centers that do not desorb pyridine, even upon evacuation at 400 °C. The acidity of this new silica material was confirmed through representative two catalytic reactions: ring-opening reaction and Friedel-Crafts-type reaction, both of which are known to require acid catalysts.

Suppression of intersite charge transfer in charge-disproportionated perovskite YCu3Fe4O12.

A novel iron perovskite YCu3Fe4O12 was synthesized under high pressure and high temperature of 15 GPa and 1273 K. Synchrotron X-ray and electron diffraction measurements have demonstrated that this compound crystallizes in the cubic AA'3B4O12-type perovskite structure (space group Im3, No. 204) with a lattice constant of a = 7.30764(10) Šat room temperature. YCu3Fe4O12 exhibits a charge disproportionation of 8Fe(3.75+) → 3Fe(5+) + 5Fe(3+), a ferrimagnetic ordering, and a metal-semiconductor-like transition simultaneously at 250 K, unlike the known isoelectronic compound LaCu3Fe4O12 that currently shows an intersite charge transfer of 3Cu(2+) + 4Fe(3.75+) → 3Cu(3+) + 4Fe(3+), an antiferromagnetic ordering, and a metal-insulator transition at 393 K. This finding suggests that intersite charge transfer is not the only way of relieving the instability of the Fe(3.75+) state in the A(3+)Cu(2+)3Fe(3.75+)4O12 perovskites. Crystal structure analysis reveals that bond strain, rather than the charge account of the A-site alone, which is enhanced by large A(3+) ions, play an important role in determining which of intersite charge transfer or charge disproportionation is practical.

B-site deficiencies in A-site-ordered perovskite LaCu3Pt(3.75)O12.

An A-site-ordered perovskite LaCu3Pt(3.75)O12 was synthesized by replacing Ca(2+) with La(3+) in a cubic quadruple AA'3B4O12-type perovskite CaCu3Pt4O12 under high-pressure and high-temperature of 15 GPa and 1100 °C. In LaCu3Pt(3.75)O12, 1/16 of B-site cations are vacant to achieve charge balance. The B-site deficiencies were evidenced by crystal structure refinement using synchrotron X-ray powder diffraction, hard X-ray photoemission spectroscopy, and soft X-ray absorption spectroscopy, leading to the ionic model La(3+)Cu(2+)3Pt(4+)(3.75)O(2-)12. Magnetic susceptibility data for this compound indicated a spin-glass-like behavior below T(g) = 3.7 K, which is attributed to disturbance of the antiferromagnetic superexchange interaction by the B-site deficiencies.

Acidic amorphous silica prepared from iron oxide of bacterial origin.

Microporous and mesoporous silica derived from biogenous iron oxide is an attractive catalyst for various organic reactions. Biogenous iron oxide contains structural silicon, and amorphous silica remains after iron oxide is dissolved in concentrated hydrochloric acid. The amorphous silica containing slight amounts of iron (Si/Fe = ∼150) is composed of ∼6-nm-diameter granular particles. The amorphous silica has a large surface area of 540 m(2)/g with micropores (1.4 nm) and mesopores (<3 nm). By using pyridine vapor as a probe molecule to evaluate the active sites in the amorphous silica, it was found that this material has strong Brønsted and Lewis acid sites. When the catalytic performance of this material was evaluated for reactions including the ring opening of epoxides and Friedel-Crafts-type alkylations, which are known to be catalyzed by acid catalysts, this material showed yields higher than those obtained with common silica materials.

An oxyhydride of BaTiO3 exhibiting hydride exchange and electronic conductivity.

In oxides, the substitution of non-oxide anions (F(-),S(2-),N(3-) and so on) for oxide introduces many properties, but the least commonly encountered substitution is where the hydride anion (H(-)) replaces oxygen to form an oxyhydride. Only a handful of oxyhydrides have been reported, mainly with electropositive main group elements or as layered cobalt oxides with unusually low oxidation states. Here, we present an oxyhydride of the perhaps most well-known perovskite, BaTiO(3), as an O(2-)/H(-) solid solution with hydride concentrations up to 20% of the anion sites. BaTiO(3-x)H(x) is electronically conducting, and stable in air and water at ambient conditions. Furthermore, the hydride species is exchangeable with hydrogen gas at 400 °C. Such an exchange implies diffusion of hydride, and interesting diffusion mechanisms specific to hydrogen may be at play. Moreover, such a labile anion in an oxide framework should be useful in further expanding the mixed-anion chemistry of the solid state.

Science in the art of the master Bizen potter.

Bizen stoneware, with the characteristic reddish hidasuki or "fire-marked" pattern, is one of Japan's best known traditional ceramic works of art. The means of creating and controlling the various hues of the hidasuki pattern has remained a mystery to outsiders for about a thousand years; the methods were known only to master potters who served under generations of master potters before them. In this Account, we present the results of 30 years of study in which we investigated the microstructure and color-formation process in Bizen stoneware. We discovered that the hidasuki pattern results from the precipitation of corundum (alpha-Al(2)O(3)) and the subsequent epitaxial growth of hematite (alpha-Fe(2)O(3)) around it in a approximately 50-microm-thick liquid specifically formed in the ceramic surface. The epitaxial composites include hexagonal plate-like alpha-Fe(2)O(3)/alpha-Al(2)O(3)/alpha-Fe(2)O(3) sandwiched particles and also surprisingly beautiful flower-like crystals, centered by hexagonal corundum crystals and decorated by several hexagonal hematite petal crystals. Bizen stoneware is produced from a unique clay that can only be mined from the Bizen area of Okayama Prefecture, Japan. The clay has an unusually high Fe content compared with the traditional porcelain clay, as well as Si, Ca, Mg, and Na. Prior to firing, the Bizen works are wrapped in rice straw that was used originally as a separator to prevent adhesion. The hidasuki pattern only appears where the rice straw is in direct contact with the clay; the rice straw supplies potassium, which reduces the melting point of the ceramic surface, thereby converting the contact area into a site for these reactions to take place. The effect is almost accidental and is produced without the aid of any artificial glazing and enameling. An unexpected variety of substances, including metallic iron coated by graphite, Fe(3)P, and epsilon-Fe(2)O(3), were also found to appear at low oxygen partial pressures. Many of the techniques used by master potters are passed down through an apprenticeship system; an unfortunate consequence is that they are poorly documented. Moreover, the masters of these techniques are often unaware of the underlying chemical reactions that take place. Chemical studies of traditional processes can provide new inspiration to artists, allowing them to control the various factors and thus produce new works, and perhaps new functional materials. We studied the process of creating Bizen stoneware and then mimicked the color-producing process under controlled laboratory conditions, demonstrating the possibilities of the endeavor.