Solid-State Phase Transformation and Self-Assembly of Amorphous Nanoparticles into Higher-Order Mineral Structures

Digging into nonclassical pathways to crystallization to unearth design principles for fabricating advanced functionalized materials shapes the future of materials science. Nature has long since been exploiting such nonclassical pathways to crystallization to build inorganic-organic hybrid materials that fulfill support, mastication, defense, attack, or optical functions. Especially, various biomineralizing taxa such as stony corals deposit metastable, magnesium-rich, amorphous calcium carbonate nanoparticles that further transform into higher-order mineral structures. Here we examine whether a similar process can be duplicate in abiogenic conditions using synthetic, amorphous calcium magnesium carbonate nanoparticles. Applying a combination of ultrahigh-resolution imaging, and, in situ, solidstate nuclear magnetic resonance (NMR) spectroscopy, we reveal the underlying mechanism of the phase transformation of these synthetic amorphous nanoparticles into crystals. When soaked in water, these synthetic amorphous nanoparticles are coated by a rigid hydration layer of bound water molecules. In addition, fast chemical exchanges occur between hydrogens from the nanoparticles and those from the free water molecules of the surrounding aqueous medium. At some stage, crystallization spontaneously occurs, and we provide spectroscopic evidence for a solid-state phase transformation of the starting amorphous nanoparticles into crystals. Depending on their initial chemical composition, and especially on the amount of magnesium, the starting amorphous nanoparticles can aggregate and form ordered mineral structures through crystal growth by particle attachment, or rather dissolve and reprecipitate into another crystalline phase. The former scenario offers promising prospects for exerting some control over such non-classical pathway to crystallization to design mineral structures that could not be achieved through a classical layer-by-layer growth.<br /

Solid-State Phase Transformation and Self-Assembly of Amorphous Nanoparticles into Higher-Order Mineral Structures

International audienceMaterials science has been informed by nonclassical pathways to crystallization, based on biological processes, about the fabrication of damage-tolerant composite materials. Various biomineralizing taxa, such as stony corals, deposit metastable, magnesium-rich, amorphous calcium carbonate nanoparticles that further assemble and transform into higher-order mineral structures. Here, we examine a similar process in abiogenic conditions using synthetic, amorphous calcium magnesium carbonate nanoparticles. Applying a combination of high-resolution imaging and in situ solid-state nuclear magnetic resonance spectroscopy, we reveal the underlying mechanism of the solid-state phase transformation of these amorphous nanoparticles into crystals under aqueous conditions. These amorphous nanoparticles are covered by a hydration shell of bound water molecules. Fast chemical exchanges occur: the hydrogens present within the nanoparticles exchange with the hydrogens from the surface-bound H2O molecules which, in turn, exchange with the hydrogens of the free H2O molecule of the surrounding aqueous medium. This cascade of chemical exchanges is associated with an enhanced mobility of the ions/molecules that compose the nanoparticles which, in turn, allow for their rearrangement into crystalline domains via solid-state transformation. Concurrently, the starting amorphous nanoparticles aggregate and form ordered mineral structures through crystal growth by particle attachment. Sphere-like aggregates and spindle-shaped structures were, respectively, formed from relatively high or low weights per volume of the same starting amorphous nanoparticles. These results offer promising prospects for exerting control over such a nonclassical pathway to crystallization to design mineral structures that could not be achieved through classical ion-by-ion growth

Self-Organization of Zr(IV) Porphyrinoids on Graphene Oxide Surfaces by Axial Metal Coordination

The protruding oxophilic central metal ion of Zr^IV porphyrinoids facilitates axial coordination to the oxygen bearing functional groups on graphene oxide (GO) surfaces to result in new supramolecular photonic materials with high dye loading especially on edges and large defects. The reaction proceeds at room temperature with GO dispersed in tetrahydrofuran and GO films on glass. Since the Zr^IV serves as a conduit, the photophysical properties of the dye sensitized GO derive from both the axially bound chromophores and the GO substrate. Self-organization of metalloporphyrinoids on GO mediated by axial coordination of group (IV) metal ions allows for direct sensitization of graphene and graphenic materials without requiring covalent chemistries with poorly conducting linkers

Single Atomic Vacancy Catalysis

International audienceSingle atom catalysts provide exceptional activity. However, measuring the intrinsic catalytic activity of a single atom in real electrochemical environments is challenging. Here, we report the activity of a single vacancy for electrocatalytically evolving hydrogen in two-dimensional (2D) MoS2. Surprisingly, we find that the catalytic activity per vacancy is not constant but increases with its concentration, reaching a sudden peak in activity at 5.7 x 10(14) cm(-2) where the intrinsic turn over frequency and Tafel slope of a single atomic vacancy was found to be similar to 5 s(-1) and 44 mV/dec, respectively. At this vacancy concentration, we also find a local strain of similar to 3% and a semiconductor to metal transition in 2D MoS2. Our results suggest that, along with increasing the number of active sites, engineering the local strain and electrical conductivity of catalysts is essential in increasing their activity