“On demand” triggered crystallization of CaCO3 from solute precursor species stabilized by the water-in-oil microemulsion

Can we control the crystallization of solid CaCO3 from supersaturated aqueous solutions and thus mimic a natural process predicted to occur in living organisms that produce biominerals? Here we show how we achieved this by confining the reaction between Ca2+ and CO32- ions to the environment of nanosized water cores of water-in-oil microemulsions, in which the reaction between the ions is controlled by the intermicellar exchange processes. Using a combination of in situ small-angle X-ray scattering, high-energy X-ray diffraction, and low-dose liquid-cell scanning transmission electron microscopy, we elucidate how the presence of micellar interfaces leads to the formation of a solute CaCO3 phase/species that can be stabilized for extended periods of time inside micellar water nano-droplets. The nucleation and growth of any solid CaCO3 polymorph, including the amorphous phase, from such nano-droplets is prevented despite the fact that the water cores in the used microemulsion are highly supersaturated with respect to all known calcium carbonate solid phases. On the other hand the presence of the solute CaCO3 phase inside of the water cores decreases the rigidity of the micellar surfactant/water interface, which promotes the aggregation of micelles and the formation of large (>2 μm in diameter) globules. The actual precipitation and crystallization of solid CaCO3 could be triggered “on-demand” through the targeted removal of the organic-inorganic interface and hence the destabilization of globules carrying the CaCO3 solute

Crystallization of citrate-stabilized amorphous calcium phosphate to nanocrystalline apatite : a surface-mediated transformation

This work explores the mechanisms underlying the crystallization of citrate-functionalized amorphous calcium phosphate (cit-ACP) in two relevant media, combining in situand ex situ characterization techniques. Results demonstrate that citrate desorption from cit-ACP triggers the surface-mediated transformation to nanocrystalline apatite (Ap). Our findings shed light on the key role of citrate, an important component of bone organic matrix, and the medium composition in controlling the rate of transformation and the morphology of the resulting Ap phase

Semiconductor-Metal Nano-Floret Hybrid Structures by Self-Processing Synthesis

We present a synthetic strategy that takes advantage of the inherent asymmetry exhibited by semiconductor nanowires prepared by Au-catalyzed chemical vapor deposition (CVD). The metal–semiconductor junction is used for activating etch, deposition, and modification steps localized to the tip area using a wet-chemistry approach. The hybrid nanostructures obtained for the coinage metals Cu, Ag, and Au resemble the morphology of grass flowers, termed here Nanofloret hybrid nanostructures consisting of a high aspect ratio SiGe nanowire (NW) with a metallic nanoshell cap. The synthetic method is used to prepare hybrid nanostructures in one step by triggering a programmable cascade of events that is autonomously executed, termed self-processing synthesis. The synthesis progression was monitored by ex situ transmission electron microscopy (TEM), in situ scanning transmission electron microscopy (STEM) and inductively coupled plasma mass spectrometry (ICP-MS) analyses to study the mechanistic reaction details of the various processes taking place during the synthesis. Our results indicate that the synthesis involves distinct processing steps including localized oxide etch, metal deposition, and process termination. Control over the deposition and etching processes is demonstrated by several parameters: (i) etchant concentration (water), (ii) SiGe alloy composition, (iii) reducing agent, (iv) metal redox potential, and (v) addition of surfactants for controlling the deposited metal grain size. The NF structures exhibit broad plasmonic absorption that is utilized for demonstrating surface-enhanced Raman scattering (SERS) of thiophenol monolayer. The new type of nanostructures feature a metallic nanoshell directly coupled to the crystalline semiconductor NW showing broad plasmonic absorption

Processus contrôlant les concentrations des phosphates dans les eaux naturelles

Le phosphore est un élément indispensable à la vie, provoquant notamment une forte croissance des végétaux quand il est en forte concentration dans l'eau. Cette étude est centrée sur le comportement des phosphates dans les eaux naturelles, afin d'éviter son apport excessif au milieu aquatique et d'améliorer son utilisation en tant que fertilisant. Nous avons mesuré les vitesses de dissolution et de précipitation des principaux minéraux phosphatés. Les phosphates étudiés sont la struvite (MgNH4PO4,6H20), la fluorapatite Ca5(PO4)3F, la variscite (AlPO4,2H2O), la strengite (FePO4,2H2O) et certains phosphates de terres rares, rhabdophane (LaPO4,2H2O et NdPO4,2H2O). Les vitesses de dissolution ont été mesurées à température ambiante (25 C) et à différents pH. La vitesse de dissolution, normalisée à une surface spécifique constante (BET), évolue dans l'ordre suivant: sturvite > fluorapatite > variscite > rhabdophane. Cette vitesse dépend de la force des liaisons cation-oxygène assurant le maintien des tétraèdres isolés de phosphate dans la structure du minéral. Les taux de précipitation ont été mesurés à différentes températures en condition acide. Les phosphates d'aluminium et de fer précipitent en tant que phases amorphes. Ils évoluent ensuite en une phase cristalline en fonction du temps et de la température. Le rhabdophane précipite rapidement, directement d'une phase aqueuse à une phase cristalline. La concentration des terres rares dans les systèmes naturels est influencée par le rhabdophane et par vitesse de précipitation. Les minéraux phosphatés tamponnent la concentration en phosphates des eaux naturelles du fait de leur faible solubilité et de leur grande réactivité. Pour une même composition des eaux, la phase solide contrôlant la teneur en phosphate dissous dépend du pH. La variscite régule la teneur en phosphate dissous à pH acide, l'apatite à pH neutre et la struvite à pH basique si la solution est chargée en ammonium.Phosphorus is an essential element for life. This study focuses on the behaviours of phosphate minerals in natural waters. We have experimentally determined the dissolution and precipitation kinetic laws of the main phosphate minerals, at several temperatures and 1 atm. The studied phases were struvite (MgNH4PO4,6H2O), fluorapatite Ca5(PO4)3F, variscite (AlPO4,2H2O), strengite (FePO4,2H2O) and REE-phosphates as rhabdophane (LaPO4,2H2O and NdPO4,2H2O). These dissolution rates, normalized to a constant BET surface area, follow the approximate order struvite > fluorroapatite > variscite > rhabdophane. Since the phosphate group passes directly into the aqueous solution, it is reasonable to assume that the rate controlling process is related to the progressive disruption of metal oxygen bonds holding the PO4 tetrahedra together in the mineral structure. The precipitation rates were measured at different temperatures and acid conditions. During the experimental stages, aluminium and iron phosphates precipitated as amorphous phases, becoming crystalline as temperature rises and reaction time progresses. Rhabdophane, however, quickly precipitated directly from the solution as a crystalline phase. The low solubility and the big reactivity of phosphate minerals limit phosphate availability in natural waters. At constant solution composition, the precipitated phosphate-bearing solid phase depends on the induced pH conditions. At acid conditions, variscite is the phosphate dominant phase, while at moderately to high alkaline conditions apatite formation occurs. Further basic pH conditions combined with high ammonium concentrations results in struvite precipitation., at medium pHs is apatite and at basic pH in presence of ammonium is struvite.TOULOUSE3-BU Sciences (315552104) / SudocSudocFranceF

The direct precipitation of rhabdophane (REEPO<SUB>4</SUB>· nH<SUB>2</SUB>O) nano-rods from acidic aqueous solutions at 5-100 °C

International audienceThe precipitation of lanthanum and neodymium phosphate phases from supersaturated aqueous solutions at pH 1.9 was studied at 5, 25, 50, and 100 °C in batch reactors for up to 168 h. Crystalline La and Nd-rhabdophane phases precipitated immediately upon mixing of the initial aqueous La or Nd and PO4 solutions. Changes in aqueous PO4 and Rare Earth Element (REE) concentrations during the experiments were determined by ICP-MS and UV-Vis spectrophotometry, while the resulting solids were characterized via powder XRD, SEM, TEM, and FTIR. All precipitated crystals exhibited a nano-rod morphology and their initial size depended on temperature and REE identity. At 5 °C and immediately after mixing the La and Nd-rhabdophane crystals averaged 44 and 40 nm in length, respectively, while at 100 °C lengths were 105 and 94 nm. After 168 h of reaction, the average length of the La and Nd rhabdophanes increased by 23 and 53% at 5 °C and 11 and 59% at 100 °C, respectively. The initial reactive solutions in all experiments had activity quotients for rhabdophane precipitation: {{REE}}^{ 3+ } + {{PO}}4^{3 - } + n{{H}}2 {{O}} = {{REEPO}}4 \cdotn{{H}}2 {{O}} of 10-20.5. This activity quotient decreased with time, consistent with rhabdophane precipitation. The rapid equilibration of rhabdophane supersaturated solutions and the progressive rhabdophane crystal growth observed suggests that the REE concentrations of many natural waters may be buffered by rhabdophane precipitation. In addition, this data can be used to guide crystallization reactions in industrial processes where monodisperse and crystalline La or Nd rhabdophane materials are the target

Crecimiento Epitaxial de Calcita (CaCO3) sobre las Superficies de Exfoliación de Anhidrita (CaSO4)

Trabajo presentado en la XXXVI Reunión de la Sociedad Española de Mineralogía, celebrada en Oviedo (España) del 4 al 7 de julio de 2017Las reacciones de disoluciónprecipitación conducen frecuentemente a la nucleación y posterior crecimiento de la fase que precipita (fase secundaria) sobre el mineral que se está disolviendo (substrato). Cuando esto ocurre, pueden desarrollarse procesos de remplazamiento mineral, cuyo avance vendrá favorecido por el desarrollo de una red de poros en la fase secundaria que garantice una comunicación continua entre la solución acuosa y el substrato (Putnis, 2002).Este trabajo ha sido financiado por los proyectos CGL2013-47988-C2-1-P y CGL2016-77138-C2-1-P. Iris Cuesta agradece el disfrute de una beca FPI.Peer reviewe