Water-Controlled Crystallization of CaCO₃, SrCO₃, and MnCO₃ from Amorphous Precursors

Calcium carbonate is the most abundant biomineral, whose amorphous form is stabilized in nature by a variety of organic additives and water. It is used to manipulate the morphology of new materials and to make strong inorganic/organic hybrid materials. However, the crystallization pathways (e.g., nucleation and growth, two-step nucleation pathways involving disordered, amorphous, or dense liquid states preceding the appearance of crystalline phases) remain often unclear. We have synthesized three amorphous carbonates, CaCO₃ (ACC), SrCO₃ (ASC), and MnCO₃ (AMnC), that do not require any stabilization by additives to study their crystallization kinetics and mechanisms in the presence of water. The evolution of the carbonate concentration during crystallization was monitored potentiometrically with a pH electrode. The crystallization of ASC proceeds extremely fast, whereas the transformation of AMnC is relatively slow. ACC is an intermediate case between these extremes. The kinetic data were interpreted by a mathematical model based on a dissolution–recrystallization reaction. For high water concentrations, the dissolution rate (and for lower concentrations, the crystallization rate) determines the reaction kinetically. For all three carbonates, the crystallization rate increases with increasing water content. A comparison with the Pearson hardness of the cations indicates that the hydration energy and the binding strength of the hydration shell pose the main kinetic barrier for recrystallization

Monitoring Thiol–Ligand Exchange on Au Nanoparticle Surfaces

Surface functionalization of nanoparticles (NPs) plays a crucial role in particle solubility and reactivity. It is vital for particle nucleation and growth as well as for catalysis. This raises the quest for functionalization efficiency and new approaches to probe the degree of surface coverage. We present an (in situ) proton nuclear magnetic resonance (¹H NMR) study on the ligand exchange of oleylamine by 1-octadecanethiol as a function of the particle size and repeated functionalization on Au NPs. Ligand exchange is an equilibrium reaction associated with Nernst distribution, which often leads to incomplete surface functionalization following “standard” literature protocols. Here, we show that the surface coverage with the ligand depends on the (i) repeated exchange reactions with large ligand excess, (ii) size of NPs, that is, the surface curvature and reactivity, and (iii) molecular size of the ligand. As resonance shifts and extensive line broadening during and after the ligand exchange impede the evaluation of ¹H NMR spectra, one- and two-dimensional ¹⁹F NMR techniques (correlation spectroscopy and diffusion ordered spectroscopy) with 1<i>H</i>,1<i>H</i>,2<i>H</i>,2<i>H</i>-perfluorodecanthiol as the fluorinated thiol ligand were employed to study the reactions. The enhanced resolution associated with the spectral range of the ¹⁹F nucleus allowed carrying out a site-specific study of thiol chemisorption. The widths and shifts of the resonance signals of the different fluorinated carbon moieties were correlated with the distance to the thiol anchor group. In addition, the diffusion analysis revealed that moieties closer to the NP surface are characterized by a broader diffusion coefficient distribution as well as slower diffusion

Effect of Isovalent Substitution on the Thermoelectric Properties of the Cu₂ZnGeSe_4–<i>x</i>S_<i>x</i> Series of Solid Solutions

Knowledge of structure–property relationships is a key feature of materials design. The control of thermal transport has proven to be crucial for the optimization of thermoelectric materials. We report the synthesis, chemical characterization, thermoelectric transport properties, and thermal transport calculations of the complete solid solution series Cu₂ZnGeSe_4–<i>x</i>S_<i>x</i> (<i>x</i> = 0–4). Throughout the substitution series a continuous Vegard-like behavior of the lattice parameters, bond distances, optical band gap energies, and sound velocities are found, which enables the tuning of these properties adjusting the initial composition. Refinements of the special chalcogen positions revealed a change in bonding angles, resulting in crystallographic strain possibly affecting transport properties. Thermal transport measurements showed a reduction in the room-temperature thermal conductivity of 42% triggered by the introduced disorder. Thermal transport calculations of mass and strain contrast revealed that 34% of the reduction in thermal conductivity is due to the mass contrast only and 8% is due to crystallographic strain

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

Optimizing the Binding Energy of the Surfactant to Iron Oxide Yields Truly Monodisperse Nanoparticles

Despite the great progress in the synthesis of iron oxide nanoparticles (NPs) using a thermal decomposition method, the production of NPs with low polydispersity index is still challenging. In a thermal decomposition synthesis, oleic acid (OAC) and oleylamine (OAM) are used as surfactants. The surfactants bind to the growth species, thereby controlling the reaction kinetics and hence playing a critical role in the final size and size distribution of the NPs. Finding an optimum molar ratio between the surfactants oleic OAC/OAM is therefore crucial. A systematic experimental and theoretical study, however, on the role of the surfactant ratio is still missing. Here, we present a detailed experimental study on the role of the surfactant ratio in size distribution. We found an optimum OAC/OAM ratio of 3 at which the synthesis yielded truly monodisperse (polydispersity less than 7%) iron oxide NPs without employing any post synthesis size-selective procedures. We performed molecular dynamics simulations and showed that the binding energy of oleate to the NP is maximized at an OAC/OAM ratio of 3. The optimum OAC/OAM ratio of 3 is allowed for the control of the NP size with nanometer precision by simply changing the reaction heating rate. The optimum OAC/OAM ratio has no influence on the crystallinity and the superparamagnetic behavior of the Fe₃O₄ NPs and therefore can be adopted for the scaled-up production of size-controlled monodisperse Fe₃O₄ NPs

Joining Two Natural Motifs: Catechol-Containing Poly(phosphoester)s

Numerous catechol-containing polymers, including biodegradable polymers, are currently heavily discussed for modern biomaterials. However, there is no report combining poly­(phosphoester)­s (PPEs) with catechols. Adhesive PPEs have been prepared via acyclic diene metathesis polymerization. A novel acetal-protected catechol phosphate monomer was homo- and copolymerized with phosphoester comonomers with molecular weights up to 42000 g/mol. Quantitative release of the catechols was achieved by careful hydrolysis of the acetal groups without backbone degradation. Degradation of the PPEs under basic conditions revealed complete and statistical degradation of the phosphotri- to phosphodiesters. In addition, a phosphodiester monomer with an adhesive P–OH group and no protective group chemistry was used to compare the binding to metal oxides with the multicatechol-PPEs. All PPEs can stabilize magnetite particles (NPs) in polar solvents, for example, methanol, due to the binding of the phosphoester groups in the backbone to the particles. ITC measurements reveal that multicatechol PPEs exhibit a higher binding affinity to magnetite NPs compared to PPEs bearing phosphodi- or phosphotriesters as repeating units. In addition, the catechol-containing PPEs were used to generate organo- and hydrogels by oxidative cross-linking, due to cohesive properties of catechol groups. This unique combination of two natural adhesive motives, catechols and phosphates, will allow the design of novel future gels for tissue engineering applications or novel degradable adhesives

The “Needle in the Haystack” Makes the Difference: Linear and Hyperbranched Polyglycerols with a Single Catechol Moiety for Metal Oxide Nanoparticle Coating

Multifunctional linear (CA-<i>lin</i>PG) and hyperbranched polyglycerols (CA-<i>hb</i>PG) bearing a single catechol unit were synthesized by use of an acetonide-protected catechol initiator for the anionic polymerization of ethoxyethyl glycidyl ether (EEGE) and glycidol, respectively. A key feature for the synthesis of the hyperbranched structures was a selective, partial acetal deprotection step. The single catechol unit among a large number of aliphatic 1,2- and 1,3-diol moieties (i.e., the “needle in the haystack”) in both linear and hyperbranched polyglycerols permits dispersion of transition metal oxide nanoparticles in brine, as demonstrated for manganese oxide (MnO) nanoparticles. Molecular weights of the single catechol bearing PGs ranged from 950 to 2350 g·mol^–1 for CA-<i>lin</i>PG and from 3750 to 5750 g·mol^–1 for CA-<i>hb</i>PG with narrow and monomodal molecular weight distributions (<i>M</i>_w/<i>M</i>_n < 1.23 for <i>lin</i>PG and <i>M</i>_w/<i>M</i>_n = 1.22–1.48 for <i>hb</i>PG). Both C-<i>lin</i>PGs and C-<i>hb</i>PGs are suitable hydrophilic capping agents to generate highly hydroxyl-functional nanoparticles with hydrophilic PG shell. The PG content of the polymer-coated MnO nanoparticles (diameter 17 nm) was in the range 21–54 wt %, as determined via TGA. The MnO nanoparticles with a hydrophilic, multifunctional polyglycerol shell may represent a promising alternative to iron oxide or gadolinium contrast agents for MRI

Enzymatic Synthesis and Surface Deposition of Tin Dioxide using Silicatein-α

Nanostructured tin dioxide was synthesized by making use of the catalytic activity of silicatein-α. TEM, HRTEM, and XRD revealed the formation of cassiterite SnO₂. Surface bound silicatein retains its biocatalytic activity. This was demonstrated by immobilizing silicatein on glass surfaces using a histidine-tag chelating anchor. The subsequent deposition of SnO₂ on glass was monitored by quartz crystal microbalance (QCM) measurements and scanning electron microscopy (SEM). This new aspect of silicatein activity toward the formation of metal oxides other than SiO₂, TiO₂, and BaTiO₃ opens up new vistas in composite material synthesis

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

Phonon Scattering through a Local Anisotropic Structural Disorder in the Thermoelectric Solid Solution Cu₂Zn_1–<i>x</i>Fe_<i>x</i>GeSe₄

Inspired by the promising thermoelectric properties of chalcopyrite-like quaternary chalcogenides, here we describe the synthesis and characterization of the solid solution Cu₂Zn_1–<i>x</i>Fe_<i>x</i>GeSe₄. Upon substitution of Zn with the isoelectronic Fe, no charge carriers are introduced in these intrinsic semiconductors. However, a change in lattice parameters, expressed in an elongation of the <i>c</i>/<i>a</i> lattice parameter ratio with minimal change in unit cell volume, reveals the existence of a three-stage cation restructuring process of Cu, Zn, and Fe. The resulting local anisotropic structural disorder leads to phonon scattering not normally observed, resulting in an effective approach to reduce the lattice thermal conductivity in this class of materials