Subjecting phosphotungstic acid solutions to low pH in combination with introduction of polyvalent cations led to the formation of nanostructured microspheres of approximately 2 µm in size, as shown by scanning electron microscopy, which were almost insoluble and resistant to degradation at neutral and high pH. These microspheres were composed of secondary nanospheres with diameters around 20 nm as revealed by transmission electron microscopy and atomic force microscopy. Investigations of the crystal structure of a potential intermediate of this process, namely, acidic lanthanum phosphotungstate, [La(H2O)9](H3O)3[PW12O40]2(H2O)19, showed a tight network of hydrogen bonding, permitting closer packing of phosphotungstic acid anions, thereby confirming the mechanism of the observed self-assembly process. The new material demonstrated promising electrochemical properties in oxygen evolution reactions with the high stability of the obtained electrode material.
Aurivillius oxides have been a research focus due to their ferroelectric properties, but by replacing oxide ions by fluoride, divalent magnetic cations can be introduced, giving Bi2 MO2F4 (M = Fe, Co, and Ni). Our combined experimental and computational study on Bi2CoO2F4 indicates a low-temperature polar structure of P21 ab symmetry (analogous to ferroelectric Bi2WO6) and a ferrimagnetic ground state. These results highlight the potential of Aurivillius oxide-fluorides for multiferroic properties. Our research has also revealed some challenges associated with the reduced tendency for polar displacements in the more ionic fluoride-based systems.
Glycerol electrolysis affords a green and energetically favorable route for the production of value-added chemicals at the anode and H2 production in parallel at the cathode. Here, a facile method for trapping Pt nanoparticles at oxygen vacancies of molybdenum oxide (MoOx ) nanosheets, yielding a high-performance MoOx /Pt composite electrocatalyst for both the glycerol oxidation reaction (GOR) and the hydrogen evolution reaction (HER) in alkaline electrolytes, is reported. Combined electrochemical experiments and theoretical calculations reveal the important role of MoOx nanosheets for the adsorption of glycerol molecules in GOR and the dissociation of water molecules in HER, as well as the strong electronic interaction with Pt. The MoOx /Pt composite thus significantly enhances the specific mass activity of Pt and the kinetics for both reactions. With MoOx /Pt electrodes serving as both cathode and anode, two-electrode glycerol electrolysis is achieved at a cell voltage of 0.70 V to reach a current density of 10 mA cm-2 , which is 0.90 V less than that required for water electrolysis.
Glycerol , Hydrogen , Catalysis , Electrodes , Electrolysis
Uranium carbide (UC) is a candidate fuel material for future Generation IV nuclear reactors. As part of a general safety assessment, it is important to understand how fuel materials behave in aqueous systems in the event of accidents or upon complete barrier failure in a geological repository for spent nuclear fuel. As irradiated nuclear fuel is radioactive, it is important to consider radiolysis of water as a process where strongly oxidizing species can be produced. These species may display high reactivity toward the fuel itself and thereby influence its integrity. The most important radiolytic oxidant under repository conditions has been shown to be H2O2. In this work, we have studied the dissolution of uranium upon exposure of UC powder to aqueous solutions containing HCO3 - and H2O2, separately and in combination. The experiments show that UC dissolves quite readily in aqueous solution containing 10 mM HCO3 - and that the presence of H2O2 increases the dissolution further. UC also dissolves in pure water after the addition of H2O2, but more slowly than in solutions containing both HCO3 - and H2O2. The experimental results are discussed in view of possible mechanisms.
Lignin is an aromatic polymer that constitutes up to 30 wt% of woody biomass and is considered the largest source of renewable aromatics. Valorization of the lignin stream is pivotal for making biorefining sustainable. Monomeric units in lignin are bound via C-O and C-C bonds. The majority of existing methods for the production of valuable compounds from lignin are based on the depolymerization of lignin via cleavage of relatively labile C-O bonds within lignin structure, which leads to yields of only 36-40 wt%. The remaining fraction (60 wt%) is a complex mixture of high-molecular-weight lignin, generally left unvalorized. Here we present a method to produce additional valuable monomers from the high-molecular-weight lignin fraction through oxidative C-C bond cleavage. This oxidation reaction proceeds with a high selectivity to give 2,6-dimethoxybenzoquinone (DMBQ) from high-molecular-weight lignin in 18 wt% yield, thus increasing the yield of monomers by 32%. This is an important step to make biorefining competitive with petroleum-based refineries.
Carbon/chemistry , Lignin/chemistry , Biomass , Catalysis , Molecular Weight , Oxidation-Reduction
Scalable synthetic strategies for high-quality and reproducible thermoelectric (TE) materials is an essential step for advancing the TE technology. We present here very rapid and effective methods for the synthesis of nanostructured bismuth telluride materials with promising TE performance. The methodology is based on an effective volume heating using microwaves, leading to highly crystalline nanostructured powders, in a reaction duration of two minutes. As the solvents, we demonstrate that water with a high dielectric constant is as good a solvent as ethylene glycol (EG) for the synthetic process, providing a greener reaction media. Crystal structure, crystallinity, morphology, microstructure and surface chemistry of these materials were evaluated using XRD, SEM/TEM, XPS and zeta potential characterization techniques. Nanostructured particles with hexagonal platelet morphology were observed in both systems. Surfaces show various degrees of oxidation, and signatures of the precursors used. Thermoelectric transport properties were evaluated using electrical conductivity, Seebeck coefficient and thermal conductivity measurements to estimate the TE figure-of-merit, ZT. Low thermal conductivity values were obtained, mainly due to the increased density of boundaries via materials nanostructuring. The estimated ZT values of 0.8-0.9 was reached in the 300-375 K temperature range for the hydrothermally synthesized sample, while 0.9-1 was reached in the 425-525 K temperature range for the polyol (EG) sample. Considering the energy and time efficiency of the synthetic processes developed in this work, these are rather promising ZT values paving the way for a wider impact of these strategic materials with a minimum environmental impact.
Reduced energy consumption and environmentally friendly, abundant constituents are gaining more attention for the synthesis of energy materials. A rapid, highly scalable, and process-temperature-sensitive solution synthesis route is demonstrated for the fabrication of thermoelectric Cu2-xSe. The process relies on readily available precursors and microwave-assisted thermolysis, which is sensitive to reaction conditions; yielding Cu1.8Se at 200 °C and Cu2Se at 250 °C within 6-8 min reaction time. Transmission electron microscopy (TEM) revealed crystalline nature of as-made particles with irregular truncated morphology, which exhibit a high phase purity as identified by X-ray powder diffraction (XRPD) analysis. Temperature-dependent transport properties were characterized via electrical conductivity, Seebeck coefficient, and thermal diffusivity measurements. Subsequent to spark plasma sintering, pure Cu1.8Se exhibited highly compacted and oriented grains that were similar in size in comparison to Cu2Se, which led to its high electrical and low thermal conductivity, reaching a very high power-factor (24 µW/K-2cm-1). Density-of-states (DOS) calculations confirm the observed trends in electronic properties of the material, where Cu-deficient phase exhibits metallic character. The TE figure of merit (ZT) was estimated for the materials, demonstrating an unprecedentedly high ZT at 875 K of 2.1 for Cu1.8Se sample, followed by 1.9 for Cu2Se. Synthetic and processing methods presented in this work enable large-scale production of TE materials and components for niche applications.
Transparent single crystals of two new iodates K3Sc(IO3)6 and KSc(IO3)3Cl have been synthesized hydrothermally. Single-crystal X-ray diffraction was used to determine their crystal structures. Both compounds crystallize in non-centrosymmetric space groups. The compound K3Sc(IO3)6 crystallizes in the orthorhombic space group Fdd2. The crystal structure is made up of [ScO6] octahedra, [IO3] trigonal pyramids, and [KO8] distorted cubes. The compound KSc(IO3)3Cl crystallizes in the trigonal space group R3. The building blocks are [ScO6] octahedra, [KO12] polyhedra, and [IO3] trigonal pyramids. The Cl- ions act as counter ions and reside in tunnels in the crystal structure. The second harmonic generation (SHG) measurements at room temperature, using 1064 nm radiation, on polycrystalline samples show that the SHG intensities of K3Sc(IO3)6 and KSc(IO3)3Cl are around 2.8 and 2.5 times that of KH2PO4 (KDP), respectively. In addition, K3Sc(IO3)6 and KSc(IO3)3Cl are phase-matchable at the fundamental wavelength of 1064 nm. The large anharmonicity in the optical response of both compounds is further supported by an anomalous temperature dependence of optical phonon frequencies as well as their enlarged intensities in Raman scattering. The latter corresponds to a very large electronic polarizability.
The new quaternary iodate KCu(IO3)3 has been prepared by hydrothermal synthesis. KCu(IO3)3 crystallizes in the monoclinic space group P21/n with unit cell parameters a = 9.8143(4) Å, b = 8.2265(4) Å, c = 10.8584(5) Å, ß = 91.077(2)°, and z = 4. The crystals are light blue and translucent. There are three main building units making up the crystal structure: [KO10] irregular polyhedra, [CuO6] distorted octahedra, and [IO3] trigonal pyramids. The Jahn-Teller elongated [CuO6] octahedra connect to each other via corner sharing to form [CuO5]∞ zigzag chains along [010]; the other building blocks separate these chains. The Raman modes can be divided into four groups; the lower two groups into mainly lattice modes involving K and Cu displacements and the upper two groups into mainly bending and stretching modes of [IO3E], where E represents a lone pair of electron. At low temperatures, the magnetic susceptibility is characterized by a broad maximum centered at â¼5.4 K, characteristic for antiferromagnetic short-range ordering. Long-range magnetic ordering at T C = 1.32 K is clearly evidenced by a sharp anomaly in the heat capacity. The magnetic susceptibility can be very well described by a spin S = 1/2 antiferromagnetic Heisenberg chain with a nearest-neighbor spin exchange of â¼8.9 K.
The new CoBi2O2F4 compound was synthesized by a hydrothermal method at 230 °C. Single-crystal X-ray diffraction data were used to determine the crystal structure. The compound is layered and belongs to the Aurivillius family of compounds. The present compound is the first oxo-fluoride Aurivillius phase containing Co2+. Inclusion of a d-block cation with such a low oxidation state as 2+ was achieved by partially replacing O2- with F- ions. The crystal structure is best described in the tetragonal noncentrosymmetric space group I4Ì with unit-cell parameters a = 3.843(2) Å and c = 16.341(8) Å. The crystal structure consists of two main building units: [BiO4F4] distorted cubes and [CoF6] octahedra. Interestingly, since the octahedra [CoF6] tilt between four equivalent positions, the F atoms occupy a 4-fold split position at room temperature. For the investigation of the structural disorder, Raman scattering data were collected in the range from 10 K to room temperature. As the temperature decreases, sharper phonon peaks appear and several modes clearly appear, which indicates a reduction of the disorder. Magnetic susceptibility and heat capacity measurements evidence long-range antiferromagnetic ordering below the Néel temperature of â¼50 K. The magnetic susceptibility is in agreement with the Curie-Weiss law above 75 K with a Curie-Weiss temperature of θCW = -142(2) K.
Single crystals of the new compound Cu2(SeO3)F2 were successfully synthesized via a hydrothermal method, and the crystal structure was determined from single-crystal X-ray diffraction data. The compound crystallizes in the orthorhombic space group Pnma with the unit cell parameters a = 7.066(4) Å, b = 9.590(4) Å, and c = 5.563(3) Å. Cu2(SeO3)F2 is isostructural with the previously described compounds Co2TeO3F2 and CoSeO3F2. The crystal structure comprises a framework of corner- and edge-sharing distorted [CuO3F3] octahedra, within which [SeO3] trigonal pyramids are present in voids and are connected to [CuO3F3] octahedra by corner sharing. The presence of a single local environment in both the 19F and 77Se solid-state MAS NMR spectra supports the hypothesis that O and F do not mix at the same crystallographic positions. Also the specific phonon modes observed with Raman scattering support the coordination around the cations. At high temperatures the magnetic susceptibility follows the Curie-Weiss law with Curie temperature of Θ = -173(2) K and an effective magnetic moment of µeff â¼ 2.2 µB. Antiferromagnetic ordering below â¼44 K is indicated by a peak in the magnetic susceptibility. A second though smaller peak at â¼16 K is tentatively ascribed to a magnetic reorientation transition. Both transitions are also confirmed by heat capacity measurements. Raman scattering experiments propose a structural phase instability in the temperature range 6-50 K based on phonon anomalies. Further changes in the Raman shift of modes at â¼46 K and â¼16 K arise from transitions of the magnetic lattice in accordance with the susceptibility and heat capacity measurements.
The new oxofluoride compound FeSbO2F2 was synthesized by hydrothermal techniques at 230 °C. Its crystal structure was determined from single-crystal X-ray diffraction data. The compound crystallizes in the monoclinic space group C2/c with one crystallographic site for Fe3+ and Sb3+, respectively. The crystal structure is made of [FeO2F4] octahedra and seesaw [SbO4] building blocks. These are connected to form [FeO2F2]n layers and [SbO2]n chains that bond together via the oxygen atoms to form the three-dimensional framework structure. Magnetic susceptibility and heat capacity measurements indicate long-range anti-ferromagnetic ordering below a Néel temperature of â¼175 K. Two-dimensional anti-ferromagnetic short-range order in the square planar net of the Fe3+ cations extends to temperatures far above the Néel temperature.
Two electrodes for anodic water oxidation made by direct synthesis of inorganic catalysts onto conductive carbon fibre sheets are evaluated. As catalysts two Co- and Sb-containing phases were tested, that is, Co3 Sb4 O6 F6 and the new compound CoSbO4 . The compounds express large differences in their morphology: CoSbO4 grows as thin needles whereas Co3 Sb4 O6 F6 grows as larger facetted crystals. Despite the smaller surface area the latter compound shows a better catalytic performance. When the compound Co3 Sb4 O6 F6 was used it gave a low increase of +0.028â mV h-1 at an overpotential of η=472â mV after 10â h and a stability of +0.48â mV h-1 at an overpotential of η=488â mV after 60â h. The leakages of Co and Sb were negligible and only <0.001â at % Co and approximately 0.02â at % Sb were detected in the electrolyte.
The new quaternary layered oxide CoMo2Sb2O10 was synthesized by hydrothermal synthesis techniques, and its structure was determined from single-crystal X-ray diffraction data. CoMo2Sb2O10 crystallizes in the monoclinic space group C2/c with one Sb3+, Mo6+, and Co2+ atom site per unit cell, respectively. The crystal structure contains building units consisting of [Co2O8]n, [Mo2O8]n, and [SbO2]n chains. These are connected through corner sharing to form charge-neutral [CoMo2Sb2O10]n layers. Thermal decomposition of CoMo2Sb2O10 starts at 550 °C. The magnetic susceptibility follows a Curie-Weiss law above 50 K with a Curie constant of C = 3.46 emu·K·mol-1 corresponding to an effective moment of µeff = 5.26 µB per cobalt atom and a Curie-Weiss temperature θ = -13.2 K. Short-range anti-ferromagnetic ordering dominates below 5 K. Magnetic susceptibility and heat capacity data can be successfully modeled by the predictions from an Ising linear chain with an intrachain spin exchange of ca. -7.8 K.
The single crystals of the new isostructural compounds Sb3O4F and Y0.5Sb2.5O4F and the two previously known compounds M-SbOF and α-Sb3O2F5 were successfully grown by a hydrothermal technique at 230 °C. The new compound Sb3O4F crystallizes in the monoclinic space group P21/c; a = 5.6107(5) Å, b = 4.6847(5) Å, c = 20.2256(18) Å, ß = 94.145(8)°, z = 4. The replacing part of Sb with Y means a slight increase in the unit cell dimensions. The compounds M-SbOF and α-Sb3O2F5 have not been grown as single crystals before and it can be concluded that hydrothermal synthesis has proved to be a suitable technique for growing single crystals of antimony oxofluorides because of the relatively low solubility of such compounds compared to other antimony oxohalides that most often have been synthesised at high temperatures by solid state reactions or gas-solid reactions.
Low temperature thermoelectric (TE) materials are in demand for more efficient cooling and power generation applications. Iron antimonide (FeSb2) draws great attention over the past few years because of its enhanced power factor values. Polycrystalline bulk FeSb2 nanopowder was prepared via a low-temperature molten salts approach followed by subsequent thermal treatment in synthetic air and hydrogen gas for calcination and reduction reactions, respectively. Structural analysis confirms the desired final phase with submicrometer grain size and high compaction density after consolidation using spark plasma sintering (SPS). TE transport properties revealed that the material is n-type below 150 K and p-type above this temperature; this suggests antimony vacancies in FeSb2. The electrical conductivity increased significantly, and the highest conductivity achieved was 6000 S/cm at 100 K. The maximum figure-of-merit, ZT, of 0.04 is achieved at 500 K, which is about 6 times higher than the earlier reported state-of-the art ZT value for the same material.
The new copper oxo-antimony sulphate CuSb6O8(SO4)2 crystallizes in the triclinic space group P1[combining macron] with the unit cell parameters a = 5.5342(4) Å, b = 7.6706(6) Å, c = 9.2374(7) Å, α = 96.505(5)°, ß = 93.818(4)°, γ = 109.733(4)° and Z = 1. The crystal structure is made up of layers stacking along [001]. The layers are charge neutral and are connected to each other by only weak interactions. The copper atoms adopt a square planar [CuO4] coordination and such units are well separated from each other by corner and edge sharing to [SbO4] building blocks. The latter polymerize to form sheets with the formula [Sb3O8]∞. Sulphate groups connect to the antimony oxide sheets by corner sharing and are located at the interface of the layers. Above â¼10 K the magnetic susceptibility follows very well a Curie-Weiss law whereas below 10 K increasing deviations indicate the onset of antiferromagnetic correlations. Fitting the data in the range 10-50 K yields a Curie-Weiss temperature θ of -2.25(5) K. A sharp anomaly centered at TC = 0.67 K in the heat capacity data indicates long-range magnetic ordering. Short range antiferromagnetic correlations well above TC are seen in the magnetic contribution to the heat capacity and the magnetic susceptibility. These can be well described by the magnetism of a spin S = 1/2 Heisenberg chain with the nearest neighbor antiferromagnetic spin exchange interaction of Jintra = â¼2.8 K.
The application of the recently discovered oxofluoride solid solution (Cox Ni1-x )3 Sb4 O6 F6 as a catalyst for water oxidation is demonstrated. The phase exhibits a cubic arrangement of the active metal that forms oxo bridges to the metalloid with possible catalytic participation. The Co3 Sb4 O6 F6 compound proved to be capable of catalyzing 2H2 OâO2 +4H(+) +4e(-) at 0.33â V electrochemical and ≤0.39â V chemical overpotential with a TOF of 4.4â 10(-3) , whereas Ni3 Sb4 O6 F6 needs a higher overpotential. Relatively large crystal cubes (0.3-0.5â mm) are easily synthesized and readily handled as they demonstrate both chemical resistance to wear after repeated inâ situ tests under experimental conditions, and have a mechanical hardness of 270â V0.1 using Vickers indentation. The combined properties of this compound offer a potential technical advantage for incorporation to a catalytic interface in future sustainable fuel production.
Transparent, needle-like single crystals of the isostructural compounds [Sb4O7+3δX4][Zn3]1+δ (X = Cl, Br, I) δ ≈ 0.2 were obtained from chemical reactions in evacuated and sealed silica tubes. First, the average structure was solved in P21/n but the model refined poorly and a lowering of the symmetry to the 3 + 1 dimensional space group P21(α0γ)0 gave a significantly better fit to the data. This model used second order positional modulations for all the atoms. Whereas Sb, Cl (Br, I) and most O positions were well behaved, there was a mismatch with Zn that was better described in a sub-cell, thus yielding a composite structure. The composite nature of the structure leads to a charge imbalance that is compensated by oxygen vacancies.
