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The effect of surface functionalization on the structural and magnetic properties of catechol-functionalized iron oxide magnetic (γ-Fe2O3) nanocrystals was investigated. γ-Fe2O3 nanocrystals (NCs) were synthesized from iron acetyl acetonate in phenyl ether with 1,2-tetradecanediol, oleic acid, and oleylamine. X-ray powder diffraction in combination with Mössbauer spectroscopy revealed the presence of γ-Fe2O3 (maghemite) particles only. Replacement of oleic acid (OA) with catechol-type 3,4-dihydroxyhydrocinnamic acid (DHCA) or polydentate polydopamine acrylate (PDAm) surface ligands leads to a pronounced change of the magnetic behavior of the γ-Fe2O3 nanocrystals and separated them into two distinctive magnetic entities. XPS and Mössbauer spectroscopy revealed the shell to be reduced with a magnetite (Fe3O4) contribution of up to 33% of the total mass while the core remained maghemite (γ-Fe2O3). The magnetic interaction between the maghemite core and the magnetite shell strongly reduced the anisotropy constant of the nanocrystals and the effective magnetization. Our experiments show that the surface chemistry strongly affects the phase distribution and the macroscopic magnetic properties of iron oxide nanopowders. [ABSTRACT FROM AUTHOR]

Thermoelectric materials are believed to play a fundamental role in the energy field over the next years thanks to their ability of directly converting heat into usable electric energy. To increase their integration in the commercial markets, improvements of the efficiencies are needed. At the same time, cheap and non-toxic materials are required along with easily upscalable production cycles. Compounds of the tetragonal tungsten bronze (TTB) series Nb8−xW9+xO47 fulfill all these requirements and are promising materials. Their adaptive structure ensures glass-like values of the thermal conductivity, and the substitution on the cation side allows a controlled manipulation of the electronic properties. In this contribution we report the stability study of the two highly substituted samples of the series, Nb5W12O47 (x = 3) and Nb4W13O47 (x = 4), when subjected to thermal cycling. Moreover, we show the results of the thermoelectric characterization of these samples. The two compounds have not been affected by the thermal treatment and showed an improvement of the thermoelectric performances up to a zT = 0.2 above 1000 K. [ABSTRACT FROM AUTHOR]

Magnetic nanocrystals are of great interest for a fundamental understanding of nanomagnetism and for their technological applications. CuxFe3−xO4nanocrystals (x 0.32) with sizes ranging between 5 and 7 nm were synthesized starting from Cu(HCOO)2and Fe(CO)5using oleic acid and oleylamine as surfactants. The nanocrystals were characterized by high-resolution transmission electron microscopy (HRTEM), electron diffraction (ED), magnetization studies and Mössbauer spectroscopy. The CuxFe3−xO4particles are superparamagnetic at room temperature 300 K with a saturation magnetization of 30.5 emu g−1. Below their blocking temperature of 60 K, they become ferrimagnetic, and at 5 K they show a coercive field of 122 Oe and a saturation magnetization of 36.1 emu g−1. The CuxFe3−xO4nanoparticles were functionalized using a hydrophilic multifunctional polymeric ligand containing PEG(800) groups and a fluorophore. By virtue of their magnetic properties these nanoparticles may serve as contrast enhancing agents for magnetic resonance imaging (MRI). [ABSTRACT FROM AUTHOR]

For getting more information about the behavior of hydrogen atoms in partially occupied sites in δ-KIO3·HIO3, we have measured temperature-dependent infrared spectra of this crystal at 14–300 K in the spectral range of OH stretching and bending vibrations. It was found that ν(OH) band (centered at ∼2800 cm−1) has complicated profile which, at temperatures below 220 K, includes weak low-frequency satellites at its low-frequency slope. At 14 K, we have main peak ν(OH) at 2766, satellites at 2606, 2485, and 2387 cm−1, and strong 2δ(OH) overtone peak at 2274 cm−1 enhanced due to Fermi resonance. At the same time, the low-frequency slope of δ(OH) band (centered at 1189 cm−1) shows equidistant satellites separated with period of approximately 25 cm−1. The observed features suggest anharmonic coupling between OH high-frequency vibrations and various low-frequency motions of the crystal lattice. [Copyright &y& Elsevier]

We report an in-depth study of the thermodynamic and magnetocaloric properties of a strongly frustrated magnet, Cs 2 Fe 2 (MoO 4 ) 3 . The underlying structure belongs to the double trillium lattice, which consists of two Fe II ( S = 2) sites with easy-axis and easy-plane single-ion anisotropy. Detailed 57 Fe Mössbauer spectroscopic investigations along with ligand-field calculations support the existence of disparate ground states. The antiferromagnetic ordered structure is presented by the propagation vector k = (0,0,0) with noncollinear magnetic moments of 2.97 μ B (Fe1) and 0.17 μ B (Fe2), respectively. Strong and disordered magnetic correlations exist in the temperature regime between T N ≈ 1.0 K and |θ CW | ≈ 22 K. The large degeneracy of the ground state is investigated in terms of its magnetocaloric response. Magnetization and specific heat measurements indicate a significant magnetocaloric cooling efficiency, making this rare-earth-free compound a promising candidate for cryogenic magnetic refrigeration applications, with refrigeration capacity of 79 J kg -1 for Δ(μ 0 H ) = 8 T., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

Mixed-valence tungsten bronzes A x WO 3 (A = alkali metal, NH 4 + , etc .) are a series of compounds with adaptive structural and compositional features that make them a hot research topic in thermoelectrics, electrochromics, catalysis or energy applications in battery electrodes. The mixed hexagonal lithium ammonium bronze Li x (NH 4 ) y WO 3 is a new member of this structural family whose properties are compared to those of the pure hexagonal tungsten bronze (NH 4 ) x WO 3 . Surface and structural (nanoconfined) Li + cations were characterized by 7 Li single pulse excitation and 1 H- 7 Li cross-polarization (CP) NMR experiments. CP build-up curves and two-dimensional heteronuclear correlation solid-state NMR techniques provide information about the spatial connectivity between different proton and Li + species. At 500 °C the bronze structurally transforms from the hexagonal to a monoclinic phase, and defects are formed that are characterized through the Li + environment. 7 Li exchange spectroscopy (EXSY) NMR experiments provide information about the chemical exchange between the lithium species. The measured 7 Li T 1 and T 2 relaxation time constants and the T 1 / T 2 ratio allow characterizing the local strength of Li + binding.

The reactivity of FeMoO 4 in CsCl fluxes has been investigated by thermal analysis and chemical reactions in evacuated silica ampules. The products have been characterized by ex situ X-ray diffraction methods. Metathesis reactions involving CsCl lead to the formation of Cs 2 Fe 2 (MoO 4 ) 3 and the salt adduct Cs 2 FeCl 4 ·CsCl. A side reaction has been observed, which is associated with a decomposition of [MoO 4 ] 2- in CsCl fluxes yielding Cs 2 Mo 2 O 7 ·CsCl, which contains the rare pyromolybdate anion, [Mo 2 O 7 ] 2- , located in the center of a ∞ 2 [CsCl] hetero-honeycomb arrangement. This salt-inclusion type of compound has been studied further in terms of its formation starting from Cs 2 MoO 4 , MoO 3 , and CsCl. The intermediate adduct phase, Cs 2 MoO 4 ·MoO 3 , contains uncharged ∞ 1 [MoO 2 O 2/2 ] chains that react with CsCl at elevated temperatures to Cs 2 Mo 2 O 7 ·CsCl. Furthermore, the site preference for alkaline-metal cations (K + , Rb + , and Cs + ) has been evaluated for a mixed substitution series. In accordance with the Pearson concept, the polarizability of the respect cation outweighs any size differences for the occupancy of the salt-intergrowth motif, the honeycomb part of the structure.

Preventing bacteria from adhering to material surfaces is an important technical problem and a major cause of infection. One of nature's defense strategies against bacterial colonization is based on the biohalogenation of signal substances that interfere with bacterial communication. Biohalogenation is catalyzed by haloperoxidases, a class of metal-dependent enzymes whose activity can be mimicked by ceria nanoparticles. Transparent CeO 2 /polycarbonate surfaces that prevent adhesion, proliferation, and spread of Pseudomonas aeruginosa PA14 were manufactured. Large amounts of monodisperse CeO 2 nanoparticles were synthesized in segmented flow using a high-throughput microfluidic benchtop system using water/benzyl alcohol mixtures and oleylamine as capping agent. This reduced the reaction time for nanoceria by more than one order of magnitude compared to conventional batch methods. Ceria nanoparticles prepared by segmented flow showed high catalytic activity in halogenation reactions, which makes them highly efficient functional mimics of haloperoxidase enzymes. Haloperoxidases are used in nature by macroalgae to prevent formation of biofilms via halogenation of signaling compounds that interfere with bacterial cell-cell communication ("quorum sensing"). CeO 2 /polycarbonate nanocomposites were prepared by dip-coating plasma-treated polycarbonate panels in CeO 2 dispersions. These showed a reduction in bacterial biofilm formation of up to 85% using P. aeruginosa PA14 as model organism. Besides biofilm formation, also the production of the virulence factor pyocyanin in is under control of the entire quorum sensing systems P. aeruginosa. CeO 2 /PC showed a decrease of up to 55% in pyocyanin production, whereas no effect on bacterial growth in liquid culture was observed. This indicates that CeO 2 nanoparticles affect quorum sensing and inhibit biofilm formation in a non-biocidal manner., (© 2022. The Author(s).)

In order to expand and exploit the useful properties of d 6 -iron(II) and d 5 -iron(III) complexes in potential magnetic, photophysical, or magnetooptical applications, crucial ligand-controlled parameters are the ligand field strength in a given coordination mode and the availability of suitable metal and ligand frontier orbitals for charge-transfer processes. The push-pull ligand 2,6-diguanidylpyridine (dgpy) features low-energy π* orbitals at the pyridine site and strongly electron-donating guanidinyl donors combined with the ability to form six-membered chelate rings for optimal metal-ligand orbital overlap. The electronic ground states of the pseudo-octahedral d 6 - and d 5 -complexes mer -[Fe(dgpy) 2 ] 2+ , cis-fac -[Fe(dgpy) 2 ] 2+ , and mer -[Fe(dgpy) 2 ] 3+ as well as their charge-transfer (CT) and metal-centered (MC) excited states are probed by variable temperature UV/vis absorption, NMR, EPR, and Mössbauer spectroscopy, magnetic susceptibility measurements at variable temperature as well as quantum chemical calculations.

Selective oxidation of thioethers is an important reaction to obtain sulfoxides as synthetic intermediates for applications in the chemical industry, medicinal chemistry and biology or the destruction of warfare agents. The reduced Magneli-type tungsten oxide WO 3- x possesses a unique oxidase-like activity which facilitates the oxidation of thioethers to the corresponding sulfoxides. More than 90% of the model system methylphenylsulfide could be converted to the sulfoxide with a selectivity of 98% at room temperature within 30 minutes, whereas oxidation to the corresponding sulfone was on a time scale of days. The concentration of the catalyst had a significant impact on the reaction rate. Reasonable catalytic effects were also observed for the selective oxidation of various organic sulfides with different substituents. The WO 3- x nanocatalysts could be recycled at least 5 times without decrease in activity. We propose a metal oxide-catalyzed route based on the clean oxidant hydrogen peroxide. Compared to other molecular or enzyme catalysts the WO 3- x system is a more robust redox-nanocatalyst, which is not susceptible to decomposition or denaturation under standard conditions. The unique oxidase-like activity of WO 3- x can be used for a wide range of applications in synthetic, environmental or medicinal chemistry.

Solid state reactions are slow because the diffusion of atoms or ions through the reactant, intermediate and crystalline product phases is the rate-limiting step. This requires days or even weeks of high temperature treatment, and consumption of large amounts of energy. We employed spark-plasma sintering, an engineering technique that is used for high-speed consolidation of powders with a pulsed electric current passing through the sample to carry out the fluorination of niobium oxide in minute intervals. The approach saves time and large amounts of waste energy. Moreover, it allows the preparation of fluorinated niobium oxides on a gram scale using poly(tetrafluoroethylene) (®Teflon) scrap and without toxic chemicals. The synthesis can be upscaled easily to the kg range with appropriate sintering equipment. Finally, NbO2F and Nb3O7F prepared by spark plasma sintering show significant photoelectrocatalytic (PEC) oxygen evolution from water in terms of photocurrent density and incident photon-to-current efficiency (% IPCE), whereas NbO2F and Nb3O7F prepared by conventional high temperature chemistry show little to no PEC response. Our study is a proof of concept for the quick, clean and energy saving production of valuable photocatalysts from plastic waste.

Localized surface plasmon resonance properties in unconventional materials like metal oxides or chalcogenide semiconductors have been studied for use in signal detection and analysis in biomedicine and photocatalysis. We devised a selective synthesis of the tungsten oxides WO 3-x and (NH 4 ) x WO 3 with tunable plasmonic properties. We selectively synthesized WO 3-x nanorods with different aspect ratios and hexagonal tungsten bronzes (NH 4 ) x WO 3 as truncated nanocubes starting from ammonium metatungstate (NH 4 ) 6 H 2 W 12 O 40 ·xH 2 O. Both particles form from the same nuclei at temperatures >200 °C; monomer concentration and surfactant ratio are essential variables for phase selection. (NH 4 ) x WO 3 was the preferred reaction product only for fast heating rates (25 K min -1 ), slow stirring speeds (∼150 rpm) and high precursor concentrations. A proton nuclear magnetic resonance ( 1 H-NMR) spectroscopic study of the reaction mechanism revealed that oleyl oleamide, formed from oleic acid and oleylamine upon heating, is a key factor for the selective formation of WO 3-x nanorods. Since oleic acid and oleylamine are standard surfactants for the wet chemical synthesis of many metal and oxide nanoparticles, the finding that oleyl oleamide acts as a chemically active reagent above 250 °C may have implications for many nanoparticle syntheses. Oriented attachment of polyoxotungstate anions is proposed as a model to rationalize phase selectivity. Magic angle spinning (MAS) 1 H-NMR and powder X-ray diffraction (PXRD) studies of the bronze after annealing under (non)inert conditions revealed an oxidative phase transition. WO 3-x and (NH 4 ) x WO 3 show a strong plasmon absorption for near infra-red light between 800 and 3300 nm. The maxima of the plasmon bands shift systematically with the nanocrystal aspect ratio.

A general method to carry out the fluorination of metal oxides with poly(tetrafluoroethylene) (PTFE, Teflon) waste by spark plasma sintering (SPS) on a minute scale with Teflon is reported. The potential of this new approach is highlighted by the following results. i) The tantalum oxyfluorides Ta 3 O 7 F and TaO 2 F are obtained from plastic scrap without using toxic or caustic chemicals for fluorination. ii) Short reaction times (minutes rather than days) reduce the process time the energy costs by almost three orders of magnitude. iii) The oxyfluorides Ta 3 O 7 F and TaO 2 F are produced in gram amounts of nanoparticles. Their synthesis can be upscaled to the kg range with industrial sintering equipment. iv) SPS processing changes the catalytic properties: while conventionally prepared Ta 3 O 7 F and TaO 2 F show little catalytic activity, SPS-prepared Ta 3 O 7 F and TaO 2 F exhibit high activity for photocatalytic oxygen evolution, reaching photoconversion efficiencies up to 24.7% and applied bias to photoconversion values of 0.86%. This study shows that the materials properties are dictated by the processing which poses new challenges to understand and predict the underlying factors., (© 2021 The Authors. Advanced Materials published by Wiley-VCH GmbH.)

Thermal decomposition is a promising route for the synthesis of metal oxide nanoparticles because size and morphology can be tuned by minute control of the reaction variables. We synthesized CoO nanooctahedra with diameters of ∼48 nm and a narrow size distribution. Full control over nanoparticle size and morphology could be obtained by controlling the reaction time, surfactant ratio, and reactant concentrations. We show that the particle size does not increase monotonically with time or surfactant concentration but passes through minima or maxima. We unravel the critical role of the surfactants in nucleation and growth and rationalize the observed experimental trends in accordance with simulation experiments. The as-synthesized CoO nanooctahedra exhibit superior electrocatalytic activity with long-term stability during oxygen evolution. The morphology of the CoO particles controls the electrocatalytic reaction through the distinct surface sites involved in the oxygen evolution reaction.