Calcium Sulfate Nanoparticles with Unusual Dispersibility in Organic Solvents for Transparent Film Processing

Calcium sulfate is one of the most important construction materials. Today it is employed as high-performance compound in medical applications and cement mixtures. We report a synthesis for calcium sulfate nanoparticles with outstanding dispersibility properties in organic solvents without further functionalization. The nanoparticles (amorphous with small γ-anhydrite crystallites, 5–50 nm particle size) form long-term stable dispersions in acetone without any sign of precipitation. ¹H NMR spectroscopic techniques and Fourier-transform infrared spectroscopy (FTIR) reveal absorbed 2-propanol on the particle surfaces that induce the unusual dispersibility. Adding water to the nanoparticle dispersion leads to immediate precipitation. A phase transformation to gypsum via bassanite was monitored by an in situ kinetic FT-IR spectroscopic study and transmission electron microscopy (TEM). The dispersibility in a volatile organic solvent and the crystallization upon contact with water open a broad field of applications for the CaSO₄ nanoparticles, e.g., as nanogypsum for coatings or the fabrication of hybrid composites

<i>Screw</i>-<i>Type</i> Motion and Its Impact on Cooperativity in BaNa₂Fe[VO₄]₂

BaNa₂Fe­[VO₄]₂ contains a Jahn–Teller active ion (Fe^II, 3d⁶, high-spin) in an octahedral coordination. On the basis of a combination of temperature-dependent X-ray diffraction and Mössbauer and Raman spectroscopies, we demonstrate the coupling of lattice dynamics with the electronic ground state of Fe^II. We identify three lattice modes combined to an effective canted <i>screw</i>-<i>type</i> motion that drives the structural transition around room temperature from the high-temperature (<i>P</i>3̅) via intermediate phases to the low-temperature phase (<i>C</i>2/<i>c</i>). The dynamics of the electronic ground state of Fe­(II) are evident from Mössbauer data with signatures of a motion-narrowed doublet above 320 K, a gradual evolution of the ⁵E_g electronic state below 293 K, and finally the signature of the thermodynamically preferred orbitally nondegenerate ground state (⁵A_g) of Fe­(II) below 100 K. The continuous nature of the transition is associated with the temperature-dependent phonon parameters derived from Raman spectroscopy, which point out the presence of strong electron–phonon coupling in this compound. We present a microscopic mechanism and evaluate the collective component leading to the structural phase transition

Role of Water During Crystallization of Amorphous Cobalt Phosphate Nanoparticles

The transformation of amorphous precursors into crystalline solids and the associated mechanisms are still poorly understood. We illuminate the formation and reactivity of an amorphous cobalt phosphate hydrate precursor and the role of water for its crystallization process. Amorphous cobalt phosphate hydrate nanoparticles (ACP) with diameters of ∼20 nm were prepared in the absence of additives from aqueous solutions at low concentrations and with short reaction times. To avoid the kinetically controlled transformation of metastable ACP into crystalline Co₃(PO₄)₂ × 8 H₂O (CPO) its separation must be fast. The crystallinity of ACP could be controlled through the temperature during precipitation. A second amorphous phase (HT-ACP) containing less water and anhydrous Co₃(PO₄)₂ was formed at higher temperature by the release of coordinating water. ACP contains approximately five molecules of structural water per formula unit as determined by thermal analysis (TGA) and quantitative IR spectroscopy. The Co²⁺ coordination in ACP is tetrahedral, as shown by XANES/EXAFS spectroscopy, but octahedral in crystalline CPO. ACP is stable in the absence of water even at 500 °C. In the wet state, the transformation of ACP to CPO is triggered by the diffusion and incorporation of water into the structure. Quantitative in situ IR analysis allowed monitoring the crystallization kinetics of ACP in the presence of water

Effect of Charge Transfer in Magnetic-Plasmonic Au@MO_<i>x</i> (M = Mn, Fe) Heterodimers on the Kinetics of Nanocrystal Formation

Heteronanoparticles represent a new class of nanomaterials exhibiting multifunctional and collective properties, which could find applications in medical imaging and therapy, catalysis, photovoltaics, and electronics. This present work demonstrates the intrinsic heteroepitaxial linkage in heterodimer nanoparticles to enable interaction of the individual components across their interface. It revealed distinct differences between Au@MnO and Au@Fe₃O₄ regarding the synthetic procedure and growth kinetics, as well as the properties to be altered by the variation of the electronic structure of the metal oxides. The chemically related metal oxides differ concerning their band gap; while MnO is a Mott-Hubbard insulator with a large band gap, Fe₃O₄ is a semimetal with thermally activated conductivity. The fluorescence dynamics indicate a prolonged relaxation time (>2 ns) for electrons of the conduction band of the Au nanoparticles after interfacing to Fe₃O₄. Here, the semiconductor is not depleted and forms an ohmic contact to the Au domain. In contrast, the fluorescence dynamics and ESCA of Au@MnO affirmed the weak interaction with the electrons of the Au domain, where the junction behaves as a Schottky barrier

Thermally Highly Stable Amorphous Zinc Phosphate Intermediates during the Formation of Zinc Phosphate Hydrate

The mechanisms by which amorphous intermediates transform into crystalline materials are still poorly understood. Here we attempt to illuminate the formation of an amorphous precursor by investigating the crystallization process of zinc phosphate hydrate. This work shows that amorphous zinc phosphate (AZP) nanoparticles precipitate from aqueous solutions prior to the crystalline hopeite phase at low concentrations and in the absence of additives at room temperature. AZP nanoparticles are thermally stable against crystallization even at 400 °C (resulting in a high temperature AZP), but they crystallize rapidly in the presence of water if the reaction is not interrupted. X-ray powder diffraction with high-energy synchrotron radiation, scanning and transmission electron microscopy, selected area electron diffraction, and small-angle X-ray scattering showed the particle size (≈20 nm) and confirmed the noncrystallinity of the nanoparticle intermediates. Energy dispersive X-ray, infrared, and Raman spectroscopy, inductively coupled plasma mass spectrometry, and optical emission spectrometry as well as thermal analysis were used for further compositional characterization of the as synthesized nanomaterial. ¹H solid-state NMR allowed the quantification of the hydrogen content, while an analysis of ³¹P­{¹H} C rotational echo double resonance spectra permitted a dynamic and structural analysis of the crystallization pathway to hopeite

Wet Chemical Synthesis and a Combined X-ray and Mössbauer Study of the Formation of FeSb₂ Nanoparticles

Understanding how solids form is a challenging task, and few strategies allow for elucidation of reaction pathways that are useful for designing the synthesis of solids. Here, we report a powerful solution-mediated approach for formation of nanocrystals of the thermoelectrically promising FeSb₂ that uses activated metal nanoparticles as precursors. The small particle size of the reactants ensures minimum diffusion paths, low activation barriers, and low reaction temperatures, thereby eliminating solid–solid diffusion as the rate-limiting step in conventional bulk-scale solid-state synthesis. A time- and temperature-dependent study of formation of nanoparticular FeSb₂ by X-ray powder diffraction and iron-57 Mössbauer spectroscopy showed the incipient formation of the binary phase in the temperature range of 200–250 °C

From Single Molecules to Nanostructured Functional Materials: Formation of a Magnetic Foam Catalyzed by Pd@Fe_<i>x</i>O Heterodimers

Multicomponent nanostructures containing purely organic or inorganic as well as hybrid organic–inorganic components connected through a solid interface are, unlike conventional spherical particles, able to combine different or even incompatible properties within a single entity. They are multifunctional and resemble molecular amphiphiles, like surfactants or block copolymers, which makes them attractive for the self-assembly of complex structures, drug delivery, bioimaging, or catalysis. We have synthesized Pd@Fe_<i>x</i>O heterodimer nanoparticles (NPs) to fabricate a macroporous, hydrophobic, magnetically active, three-dimensional (3D), and template-free hybrid foam capable of repeatedly separating oil contaminants from water. The Pd domains in the Pd@Fe_<i>x</i>O heterodimers act as nanocatalysts for the hydrosilylation of polyhydrosiloxane and tetravinylsilane, while the Fe_<i>x</i>O component confers magnetic properties to the final functional material. Pd@Fe_<i>x</i>O heterodimers were synthesized by heterogeneous nucleation and growth of the iron oxide domain onto presynthesized Pd NPs at high temperatures in solution. The morphology, structure, and magnetic properties of the as-synthesized heterodimers were characterized by transmission electron microscopy (TEM), X-ray diffraction, Mössbauer spectroscopy, and a superconducting quantum interference device. The epitaxial growth of the Fe_<i>x</i>O domain onto Pd was confirmed by high-resolution TEM. A potential application of the 3D hydrophobic magnetic foam was exploited by demonstrating its ability to soak oil beneath a water layer, envisioning its use in oil sampling during oil prospection drilling, or to remove oil films after oil spills

Influence of Compensating Defect Formation on the Doping Efficiency and Thermoelectric Properties of Cu_2‑ySe_1–<i>x</i>Br_<i>x</i>

The superionic conductor Cu_2−δSe has been shown to be a promising thermoelectric at higher temperatures because of very low lattice thermal conductivities, attributed to the liquid-like mobility of copper ions in the superionic phase. In this work, we present the potential of copper selenide to achieve a high figure of merit at room temperature, if the intrinsically high hole carrier concentration can be reduced. Using bromine as a dopant, we show that reducing the charge carrier concentration in Cu_2−δSe is in fact possible. Furthermore, we provide profound insight into the complex defect chemistry of bromine doped Cu_2−δSe via various analytical methods and investigate the consequential influences on the thermoelectric transport properties. Here, we show, for the first time, the effect of copper vacancy formation as compensating defects when moving the Fermi level closer to the valence band edge. These compensating defects provide an explanation for the often seen doping inefficiencies in thermoelectrics via defect chemistry and guide further progress in the development of new thermoelectric materials

Pd@Fe₂O₃ Superparticles with Enhanced Peroxidase Activity by Solution Phase Epitaxial Growth

Compared to conventional deposition techniques for the epitaxial growth of metal oxide structures on a bulk metal substrate, wet-chemical synthesis based on a dispersible template offers advantages such as low cost, high throughput, and the capability to prepare metal/metal oxide nanostructures with controllable size and morphology. However, the synthesis of such organized multicomponent architectures is difficult because the size and morphology of the components are dictated by the interplay of interfacial strain and facet-specific reactivity. Here we show that solution-processable two-dimensional Pd nanotetrahedra and nanoplates can be used to direct the epitaxial growth of γ-Fe₂O₃ nanorods. The interfacial strain at the Pd−γ-Fe₂O₃ interface is minimized by the formation of an Fe_<i>x</i>Pd “buffer phase” facilitating the growth of the nanorods. The γ-Fe₂O₃ nanorods show a (111) orientation on the Pd(111) surface. Importantly, the Pd@γ-Fe₂O₃ hybrid nanomaterials exhibit enhanced peroxidase activity compared to that of isolated Fe₂O₃ nanorods with comparable surface area because of a synergistic effect for the charge separation and electron transport. The metal-templated epitaxial growth of nanostructures via wet-chemical reactions appears to be a promising strategy for the facile and high-yield synthesis of novel functional materials