Oriented Aggregation Mechanism Research Articles

Mesocrystal aggregation of biological apatite nanocrystals

AbstractA broader revelation of the mechanisms which contribute to the formation, growth, healing and remodelling of bone tissue is essential for advancing the design and development of biomaterials and devices which directly enhance bone health. Hydroxyapatite and associated calcium phosphate‐based minerals play an essential role in bone tissue formation. Further insights into how biomineral crystals form, grow and integrate within bone tissue will provide key information to direct efforts in more comprehensive bone tissue engineering products and therapies. While previous studies proposed the aggregation of amorphous calcium phosphate clusters as a precursor to biological hydroxyapatite, the exact formation mechanism of the plate‐like biological hydroxyapatite is still unclear. Here we report the analysis of high‐resolution electron microscopy images of bone biomineral precipitated in a biological environment. We propose that 3 nm primary biologically synthesized (biosynthesized) hydroxyapatite (BHAp) single crystal units assemble and coalesce via an oriented aggregation mechanism to form larger (approximately 46 nm × 25 nm) plate‐like biological hydroxyapatite mesocrystals. A better understanding of the biomineralization process can provide insights to improve the in vitro precipitation of bone biominerals with tailored properties and unique functionality. This will help to usher in the next generation of biobased biomaterials and devices to enhance the healing and remodelling of bone at the tissue, cell and subcellular level.

Synthesis and Adsorption Properties of Hierarchically Ordered Nanostructures Derived from Porous CaO Network

Using the porous framework of CaO as templates and reagents, we explored a surfactant-free and economical method for preparing calcium silicate hydrate (CSH) hierarchically ordered nanostructures. Incorporation of SiO2 nanoparticles into the CaO framework, followed by a reaction assisted by hydrothermal treatment, resulted in the formation of CSH with well-defined morphologies. The structural features of CSH were characterized by 3-D hierarchical networks, wherein nanofibers assembled to form nanosheets, and nanosheets assembled to form hierarchically ordered structures. Investigation of the crystal growth mechanism indicated that the key to forming the CSH ordered assembly structure was confining the Ca/Si ratio within a small range. Nonclassic oriented aggregation mechanism was used to describe the crystal growth of nanosheets, while the porous CaO framework served as template/reagents responsible for the formation of hierarchical structures. The resulting CSH adsorbent exhibited better performance in removing Pb(II) compared with other types of random CSH adsorbents. Additionally, the hierarchical structure of CSH provided more pores and active sites as support for other active functional materials such as zerovalent iron (Fe0). As-produced CSH@Fe nanocomposite with self-supported structures displayed high capacities for removal of Pb(II) after five adsorption-desorption cycles, and high capacities for other heavy metal ions (Cu2+, Cd2+, and Cr2O72-) and organic contaminants.

The pH-controlled {040} facets orientation of BiVO4 photocatalysts with different morphologies for enhanced visible light photocatalytic performance

Hierarchically structured m-BiVO4 samples with different morphologies have been synthesized via a template-free solvothermal route by adjusting the pH value of the precursor solution. The effects of pH value on structure, morphology and photocatalytic activity were investigated. The results showed that the pH value played a great influence not only on the formation of the hierarchical structures with different morphologies, but also on the preferential crystal growth of the BiVO4 oriented along {040} facets. The pinwheel-like sample with the highest relative intensities of the diffraction peaks I040/I–121 exhibited the best photocatalytic activity for the photodegradation of MB under visible light irradiation. According to the Ostwald ripening process, a possible oriented aggregation mechanism was proposed.

Facile synthesis of tetrahedral Ag3PO4 mesocrystals and its enhanced photocatalytic activity

A novel and rapid strategy has been developed to synthesize tetrahedral Ag3PO4 mesocrystals with the help of surfactant oleic acid. Homogeneous Ag3PO4 tetrahedron with average apex-to-apex length of 1.1μm could be synthesized under optimized conditions. Oleic acid is considered to be essential for the formation of this homogeneous tetrahedral morphology. The as-prepared tetrahedral Ag3PO4 mesocrystals could be formed through the close-packed assembly of Ag3PO4 nanoparticles, thus the formation of tetrahedral Ag3PO4 mesocrystals is explained by the oriented aggregation mechanism based on time-dependent experiments. Furthermore, the obtained tetrahedral Ag3PO4 mesocrystals exhibit enhanced visible-light photocatalytic degradation of methylene blue (MB) and methyl orange (MO) solutions compared to the reported micro-sized spherical Ag3PO4 particles with high photocatalytic performance.

Facile one-pot synthesis and photocatalytic properties of hierarchically structural BiVO4with different morphologies

Hierarchically structured m-BiVO4 crystals with twin-ball-like, cauliflower-like, pinwheel-like, columnar and spherical morphologies were synthesized by a facile one-pot method in the presence of soluble starch. The physicochemical properties of the material were characterized by means of XRD, SEM, HRTEM/SAED, UV-vis absorption and N2 adsorption techniques. The oriented aggregation mechanism may rationally explain the formation of the hierarchical structure. The assistance of the starch and the pH value of the precursor solution play pivotal roles in the formation of the hierarchical structures with different morphologies because the existing forms of bismuth and starch vary with the pH value of the precursor solution. The photocatalytic activities of the as-prepared BiVO4 samples were evaluated for the photodegradation of Rhodamine B (RhB) in aqueous solution under simulated sunlight irradiation. The highest photocatalytic activity of BiVO4 obtained at pH = 4 may result from several factors, such as the moderate BET surface area, the highest crystallinity, the lowest bandgap and the most exposed (040) face.

Monoclinic Ag2Mo2O7 Nanowire: A New Ag–Mo–O Nanophotocatalyst Material

We report a template-free facile technique that allows for the first ever synthesis of a monoclinic Ag2Mo2O7 nanowire (m-Ag2Mo2O7-NW), using a commercially available MoO3 particle. The nanowire possessed high crystallinity and structural homogeneity and strongly suggested that the nanowire was grown through an oriented aggregation mechanism in contrast to the case of a typical solution-phase method. The corresponding bulky counterpart showed no photoresponse; however, a complete structural transformation toward a nanowire triggered activity for O2 evolution in the presence of Ag(+) as an electron acceptor under visible-light irradiation.

Solid state synthesis of a new ternary nitride MgMoN2 nanosheets and micromeshes

In this study, a new ternary nitride MgMoN2 was synthesized by a solid-state reaction of Mo, Mg and NaN3 in a stainless steel autoclave at 700 °C. The crystal structure of MgMoN2 (a = 2.91081 A, c = 10.55029 A, Z = 2) was determined by Rietveld refinement based on the X-ray diffraction data (XRD), which comprises of alternating layers of MgN6 octahedral and MoN6 trigonal prisms. The field-emission scanning electron microscopy (FE-SEM) and the transmission electron microscopy (TEM) show that the obtained MgMoN2 was composed of nanosheets with a diameter of several micrometers and with a thickness of about 30 nm. As Mo was substituted by other molybdenum sources (such as MoO3, (NH4)6Mo7O24·4H2O or Na2MoO4·4H2O), the FE-SEM images, TEM images and the selected-area electron diffraction (SEAD) patterns show that the obtained MgMoN2 was composed of single-crystalline micromeshes with different pore sizes. An oriented aggregation mechanism was considered for the formation of MgMoN2 nanosheets and micromeshes. The as-obtained MgMoN2 micromesh is promising as a catalyst support in chemical filtration and in separations under severe operating conditions.

In Situ Crystallization and Growth Dynamics of Acentric Iron Iodate Nanocrystals in w/o Microemulsions Probed by Hyper-Rayleigh Scattering Measurements

A detailed experimental investigation of the formation mechanisms of acentric iron iodate [Fe(IO3)3] nanocrystals in AOT-based reverse micelles according to the water-in-oil (w/o) microemulsion (ME) composition and temperature was performed. A low chemical reaction rate was first demonstrated by means of UV−vis absorption spectroscopy. Attainment of iron iodate crystalline nanorods was then attributed to an oriented aggregation mechanism of 10−20-nm amorphous primary nanoparticles. As for the intermicellar exchange of reactants, the aggregation mechanism was also found to be determined by the ME composition because clear effects of the surfactant and reactant concentrations, the nature of the oil phase, and the water droplet size were observed. Moreover, a combination of dynamic light scattering, X-ray diffraction, and transmission electron microscopy experiments revealed that hyper-Rayleigh scattering is a fast, valuable, and nondestructive alternative to probe, in real time, the crystallization and form...

Specific Ion Effects on Nanoparticle Stability and Organocation−Particle Interactions in Tetraalkylammonium−Silica Mixtures

Tetramethylammonium (TMA)- and tetrapropylammonium (TPA)-silica mixtures containing monovalent salts were studied to determine how salt impacts nanoparticle stability and organocation-silica interactions. Dynamic light scattering (DLS) results show that salt concentrations as low as 5 mM can induce nanoparticle aggregation. The extent of aggregation increases with the ionic size of the alkali-metal cations, consistent with the Hoffmeister series. Thus specific ion effects are observed in these mixtures. Pulsed-field gradient (PFG) NMR shows a more obvious increase in the self-diffusion coefficient of TPA than TMA in the presence of salt, indicating TPA is more easily displaced from the nanoparticle surface due to the background electrolyte. A two-site model is used to describe the exchange between tetraalkylammonuim (TAA) adsorbed on the nanoparticles and TAA in solution, from which the binding isotherms of the organocations at low electrolyte concentration was obtained and analyzed using the Langmuir formalism. This analysis also shows specific-ion effects, with the amount of TPA adsorbed to be much smaller than TMA and also much more sensitive to the presence of salt. In the context of the oriented aggregation mechanism proposed previously in the literature, the current work suggests one route for tuning the organocation-particle interaction and thus a route to controlling the rates of some steps in the mechanism.

Fabrication and morphology control of BaWO 4 thin films by microwave assisted chemical bath deposition

Highly crystallized barium tungstate (BaWO 4) thin films with dumbbell-like, kernel-like, bowknot-like and cauliflower-like microstructure were synthesized from an aqueous solution containing barium nitrate, ethylenediamine tetraacetate acid disodium and sodium tungstate, via mild microwave assisted chemical bath deposition process. The resulting BaWO 4 films with different morphologies were characterized by X-ray diffraction spectrum, scanning electron microscope, Raman and photoluminescence spectra. The results indicate that the morphologies of final products significantly depend on the reaction conditions including the reaction time, the initial concentration of precursor reagent and the physicochemical characteristics of the substrates. Furthermore, the oriented aggregation mechanism is proposed as a possible formation mechanism of the films with specific morphologies.

One-Pot Assembly of Cu2O Chain-Like Hollow Structures

Novel Cu2O chain-like network was assembled via a facile one-pot solution process with the assistance of poly(vinylpyrrolidone) (PVP). TEM observations showed that the chain-like structures were aggregated by hollow spheres approximately 70 nm in diameter. To be worth mentioning, HRTEM image recorded from the coherent interfacial region demonstrated that the lattice fringes penetrate from one sphere into the adjacent one smoothly without apparent diffraction contrast, which indicated that the hollow spheres experienced lattice fusion and grew into each other. Based on the systematic studies, an oriented aggregation mechanism was proposed, i.e., Cu2O nanoparticles into hollow spheres and subsequently the hollow spheres into the chain-like structures. The Raman spectra of the Cu2O chain-like hollow nanostructures were also investigated. It was found that the Raman peak intensity is different from that in the previous reports, which might be originated from the structure defect resulted from the oriented attachment.

Nano to macroscale biomineral architecture of red coral (Corallium rubrum)

Different techniques have been used to characterize the physical and chemical structure of the red coral calcitic skeleton. A section normal to the axis of the skeleton shows a medullar zone surrounded by a circular domain composed of concentric rings. Growth rings are revealed by the cyclic variation of organic matter (OM) and Mg/Ca ratio. These growth rings are annual; thus, both OM and Mg/Ca ratio can be used to date red coral colonies. Growth rings display wavelets. The internal structure of each wavelet results from the stacking of layers with tortuous interfaces. Tortuosity is due to the presence of microprotuberances. Interfaces between layers may display sharp discontinuities indicative of interruption of the mineralizing process. SEM and TEM studies show that each layer is made of (1) fibers, organized or not in fan-shaped structures; and (2) submicrometer (apparently mono-) crystalline units. Fibers are superstructures made of submicrometer units possibly assembled by an oriented aggregation mechanism. HRTEM studies show that in spite of displaying single-crystal scattering behavior, the submicrometer crystalline units are made of 2-5 nm nanograins again possibly aggregated by a mechanism of oriented attachment. Thus, submicrometer crystalline units and polycrystalline fibers can be both defined as mesocrystals. The red coral skeleton is a hierarchically organized organic-inorganic composite that exhibits porosity and structural and compositional order on length scales from the nanoscale to the macroscale.

Growth and Morphology of Mesoporous SBA-15 Particles

The growth of single crystal particles of SBA-15 is explained. The morphology and size of the particles are highly dependent on synthesis temperature. The formation at certain temperatures proceeds via an oriented aggregation mechanism of primary particles into secondary (final) particles. Platelike SBA-15 particles with a uniform size are obtained.

Lotus-Root-Like In2O3 Nanostructures: Fabrication, Characterization, and Photoluminescence Properties

Novel lotus-root-like In2O3 nanostructures with a diameter of ca. 300 nm and a length of 1.5−4.0 μm have been prepared by annealing In(OH)3 nanostructures with the same morphology derived from a mild solution reaction. The hierarchical nanostructures are composed of several segments aggregated orderly from In2O3 nanorods with the length of 50−90 nm and diameter of 15−40 nm. The as-prepared products were characterized by X-ray powder diffractometry (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR), and X-ray photoelectron spectra (XPS) in detail. Furthermore, the effects of reaction parameters on the formation of nanostructures were also discussed, and the hydrolysis and oriented-aggregation mechanism was proposed. Room-temperature photoluminescence (PL) spectra of the In2O3 nanostructure showed the peculiar visible emission from the blue to orange region, with the strongest peak centered at 468 nm.

Synthesis of ZnWO4@MWO4 (M = Mn, Fe) Core−Shell Nanorods with Optical and Antiferromagnetic Property by Oriented Attachment Mechanism

Uniform core−shell heterostructured ZnWO4@MWO4 (M = Mn, Fe) nanorods with both optical and antiferromagnetic properties have been synthesized by a simple refluxing method under mild conditions in which the crystallization event of MWO4 happened on the backbone of ZnWO4 single crystalline nanorods in a ligand-free system. ZnWO4 nanorod-directed oriented aggregation mechanism has been clearly observed for the formation of heterostructured ZnWO4@MWO4 (M = Mn, Fe) nanorods. The shell thickness of MWO4 (M = Mn, Fe) could be tuned by changing the molar ratio of these raw materials. UV−visible absorption spectra and photoluminescence (PL) spectra of the as-prepared ZnWO4@MWO4 (M = Mn, Fe) core−shell nanorods show the similar “red-shift” trend, which can be ascribed to the influences of the out-layered shell. The ZnWO4@MWO4 (M = Mn, Fe) nanorods displayed both optical and antiferromagnetic properties. The result demonstrated that the multifunctional anisotropic nanostructures with a heteroshell could be synthesiz...

Morphology Transformation of Nanoscale Magnesium Hydroxide: from Nanosheets to Nanodisks

Nanoscale Mg(OH)2 with different morphologies, from nanosheets, nanosquares, and nanodisks, have been successfully synthesized via a simple hydrothermal reaction of Mg(CH3COO)2 and sodium hydroxide in the presence of citrate. Direct observation of transformation from nanosheets to nanodisks indicates clearly the growth of a Mg(OH)2 nanocrystal. The structure and morphology of the resultant Mg(OH)2 nanostructures were characterized by X‐ray powder diffraction (XRD), transmission electron microscopy (TEM), high‐resolution transmission electron microscopy (HRTEM), and selected area electron diffraction patterns (SAED). A possible formation mechanism of the Mg(OH)2 nanostructures is proposed based on the oriented aggregation mechanism.

Large-Scale Fabrication of Novel Hierarchical 3D CaMoO4 and SrMoO4 Mesocrystals via a Microemulsion-Mediated Route

Molybdates such as CaMoO4 and SrMoO4 have received much attention recently because of their interesting luminescence and structural properties. Herein, we report a microemulsion-mediated method as a novel technique to synthesize CaMoO4 and SrMoO4 mesocrystals with high hierarchy and with hollow structure. The “arms” of the flowerlike 3D mesocrystals are radial and are about 2−3 μm in length. They are constructed from nanoparticles with regular morphology and a size of about 150−300 nm. Study on the formation mechanism of the superstructures reveals that the oriented aggregation mechanism is responsible for the self-assembly. The mesocrystals of CaMoO4 and SrMoO4 are shown to be in a thermodynamically metastable state, which can be destroyed by further reaction time or temperature change.

Nanostructure and Irreversible Colloidal Behavior of Ca(OH)2: Implications in Cultural Heritage Conservation

Although Ca(OH)2 is one of the oldest art and building material used by mankind, little is known about its nanostructural and colloidal characteristics that play a crucial role in its ultimate performance as a binder in lime mortars and plasters. In particular, it is unknown why hydrated lime putty behaves as an irreversible colloid once dried. Such effect dramatically affects the reactivity and rheology of hydrated lime dispersions. Here we show that the irreversible colloidal behavior of Ca(OH)2 dispersions is the result of an oriented aggregation mechanism triggered by drying. Kinetic stability and particle size distribution analysis of oven-dried slaked lime or commercial dry hydrate dispersions exhibit a significant increase in settling speed and particle (cluster) size in comparison to slaked lime putty that has never been dried. Drying-related particle aggregation also leads to a significant reduction in surface area. Electron microscopy analyses show porous, randomly oriented, micron-sized clusters that are dominant in the dispersions both before and after drying. However, oriented aggregation of the primary Ca(OH)2 nanocrystals (approximately 60 nm in size) is also observed. Oriented aggregation occurs both before and during drying, and although limited before drying, it is extensive during drying. Nanocrystals self-assemble in a crystallographically oriented manner either along the 100 or equivalent 110 directions, or along the Ca(OH)2 basal planes, i.e., along [001]. While random aggregation appears to be reversible, oriented aggregation is not. The strong coherent bonding among oriented nanoparticles prevents disaggregation upon redispersion in water. The observed irreversible colloidal behavior associated with drying of Ca(OH)2 dispersions has important implications in heritage conservation, particularly considering that nowadays hydrated lime is often the preferred alternative to portland cement in architectural heritage conservation. Finally, our study demonstrates that, fortuitously, hydrated lime could be one of the first nanomaterials used by mankind.