Diffusion-limited Aggregation Mechanism Research Articles

Aqueous dispersions of highly conductive carbon nanomaterials: Unusual aggregation mechanism

Determination of the optimal conductive composition and operational flow conditions are of profound significance for aqueous–based semisolid flow battery and supercapacitor applications. We systematically investigated the development of equilibrium microstructure and flow–induced structural transitions in aqueous dispersions of carbon black (CB) in 1 M LiNO3 at 25 °C. Simultaneous measurements of the impedance spectra and optical texture distinguished two unique percolated dispersions with distinct microstructure and aggregation mechanisms around a critical CB content: CCB = 4 wt%. A tenuous network of weakly–linked aggregates with rapid diffusion–limited aggregation mechanism (DLA) is formed at CCB < 4 wt%. A compact network of strongly fused aggregates with slow reaction–limited aggregation mechanism (RLA) dominates the dispersion microstructure above the critical CCB. The percolated dispersions commonly exhibited a three–regime (two thinning separated by thickening regime) flow curve consistent with structural breaking up (conductivity loss) and alignment of flowing units (conductivity recovery) at a critical shear rate that pertain to CCB.

In situ surface evolution dynamics of external-electric-field-triggered structural oscillation on Au(111)

Surface nanostructures serve as an essential role in determining intrinsic physical features and device performance in solid materials. Here, this work systematically investigates the surface dynamical evolution at the nanoscale on Au(111) induced by a “tip-to-surface” external electric field through a scanning tunneling microscope (STM). The Au(111) steps exhibit a “compact to fractal” reconstruction under a relatively high electric field, in which the transition is strengthened with increasing the applied electric field. Inversely, a “fractal to compact” morphological transition enables to be self-triggered at Au(111) surface steps upon a normal STM imaging electric field (very low). These two reversible structural changes are attributed to the diffusion-limited aggregation mechanism where the diffusion barriers were quantified as 0.64–0.75 eV varied with the regularity in step edges. In addition, we further simulate a “terrace-edge-kink” model to trace the effect of atomic coordination on structural transition, determining the surface step edge stability. This study presents insight into morphological and structural transformation at surface steps induced by variant external electric fields, establishing a deep understanding in the nature of surface evolution dynamics.

Solvent effects on the rheology of fuel cell catalyst ink and the adsorption of ionomers on the particles

The solvent affects the morphology of ionomers, agglomeration structure, adsorption of ionomers on carbon or catalyst particles, and rheology in the proton exchange membrane fuel cell (PEMFC) ink, which are all related to the microstructure of catalyst inks. We investigated the link between the morphology of the ionomers or the fractal dimension (df) of graphitized carbon (GC) dispersion and the microstructure of the catalyst ink by solvent to change the morphology of the ionomers or df of GC dispersion. The nuclear magnetic resonance results showed relatively low mobility of the ionomer in ethylene glycol (EG), resulting in a weak interaction between the ionomer and EG. GC in water, EG, and n-propanol (nPA) form transition gels with a network structure by a diffusion-limited aggregation mechanism, with df of 1.76 ± 0.03, 1.95 ± 0.04, and 2.01 ± 0.06, respectively. Moreover, the addition of ionomers reduces the viscosity of inks due to the introduction of steric hindrance and an increase in electrostatic repulsion, transforming the ink from shear-thinning to Newtonian fluids. Further, the interaction between ionomer and particles in GC and graphitized-carbon-supported platinum (Pt/GC) inks is solvent-dependent, and nPA is more favorable than water and EG for ionomer adsorption on GC. These findings provide valuable insight into the internal interactions and microstructure of PEMFC catalyst inks.

Two-Dimensional to Three-Dimensional Growth of Transition Metal Diselenides by Chemical Vapor Deposition: Interplay between Fractal, Dendritic, and Compact Morphologies

We investigate the role of growth temperature and metal/chalcogen flux in atmospheric pressure chemical vapor deposition growth of MoSe2 and WSe2 on Si/SiO2 substrates. Using scanning electron microscopy and atomic force microscopy, we observe that the growth temperature and transition metal flux strongly influence the domain morphology, and the compact triangular or hexagonal domains ramify into branched structures as the growth temperature (metal flux) is decreased (increased). The competition between adatom attachment to the domain edges and diffusion of adatoms along the domain boundary determines the evolution of the observed growth morphology. Depending on the growth temperature and flux, two different branched structures-fractals and dendrites-grow. The fractals (with a dimension of ∼1.67) obey a diffusion-limited aggregation mechanism, whereas the dendrites with a higher fractal dimension of ∼1.80 exhibit preferential growth along the symmetry-governed directions. The effect of chalcogen environment is studied, where a Se-rich condition helps restrict Mo-rich nucleus formation, promoting lateral growth. For a Se-deficient environment, several multilayer islands cluster on two-dimensional domains, suggesting a transition from lateral to vertical growth because of insufficient Se passivation. X-ray photoelectron spectroscopy analysis shows a near perfect stoichiometry (Mo/Se = 1:1.98) of MoSe2 grown in a Se-rich environment, whereas in the Se-deficient condition, a ratio of Mo/Se = 1:1.68 is observed. This also supports the formation of metal-rich nuclei (Mo1+xSe2-x) under Se-deficient conditions, leading to three-dimensional clustering. Tuning the growth temperature and metal/chalcogen flux, we propose an optimized CVD growth window for synthesizing large-area Mo(W) selenide.

Asymmetric Growth of Tetragon-Shaped Single-Crystalline Graphene Flakes on Copper Foil by Annealing Treatment under Oxygen-Free Conditions

Engineering graphene into a particular shape is vital for potential industrial applications. To this end, better understanding of the growth mechanism is needed to control the growth behavior of graphene on a substrate surface with a specific shape. In this work, a tetragon-shaped graphene single crystal (TS_GSC) with millimeter-scale grain size was achieved on copper foil, which was annealed at oxygen-free conditions (AOF) prior to graphene growth. The TS_GSC grains are featured by two dendritic Frontiers at the shorter-edge (SE) sides and two sharp Frontiers at the longer-edge (LE) sides of graphene grain. By combining scanning electron microscopy, optical microscopy, and Raman mapping with carbon isotope labeling, we revealed for the first time an asymmetric growth behavior of TS_GSC grains on an AOF-treated copper substrate. It was supposed that the growth of graphene was determined by the diffusion-limited aggregation mechanism at the SE side, whereas it was governed by the edge-determined atom-attac...

A completely green approach to the synthesis of dendritic silver nanostructures starting from white grape pomace as a potential nanofactory

A simple, eco-friendly, cost-effective and rapid microwave-assisted method has been developed to synthetize dendritic silver nanostructures, composed of silver nanoparticles (AgNPs), using white grape pomace aqueous extract (WGPE) as both reducing and capping agent. With this aim, WGPE and AgNO3 (1 mM) were mixed at different ratio, and microwave irradiated at 700 W, for 40 s. To understand the role of bioactive compounds involved in the green synthesis of AgNPs, preliminary chemical characterization, FT-IR analysis and 1H NMR metabolite profiling of WGPE were carried out. The effects of bioactive extract concentration and stability over time on AgNPs formation were also evaluated. WGPE-mediated silver nanostructures were then characterized by UV–vis, FTIR analyses, and scanning electron microscopy. Interestingly, the formation of dendritic nanostructures, originated from the self-assembly of Ag rounded nanoparticles (average diameter of 33 ± 6 nm), was observed and ascribed to the use of microwave power and the presence of organic components within the used WGPE, inducing an anisotropic crystal growth and promoting a diffusion-limited aggregation mechanism. The bio-dendritic synthetized nanostructures were also evaluated for potential applications in bio-sensing and agricultural fields. Cyclic voltammetry measurements in 0.5 M phosphate + 0.1 M KCl buffer, pH 7.4 showed that green AgNPs possess the electroactive properties typical of AgNPs produced using chemical protocol. The biological activity of synthetized AgNPs was evaluated by in-vitro antifungal activity against F. graminearum. Additionally, a phytotoxicity evaluation of synthetized green nanostructures was carried out on wheat seed germination. Results highlighted the potential of WGPE as green agent for bio-inspired nanomaterial synthesis, and of green Ag nanostructures, which can be used as antifungal agent and in biosensing applications.

Mechanisms of precipitate formation during the purification of an Fc-fusion protein.

Protein precipitates that arise during bioprocessing can cause manufacturing challenges, but they can also aid in clearance of host-cell protein (HCP) and DNA impurities. Such precipitates differ from many protein precipitates that have been studied previously in their heterogeneous composition, particularly in the presence of high concentrations of the product protein. Here, we characterize the precipitates that form after neutralization of protein A purified and viral-inactivated material of an Fc-fusion protein produced in Chinese hamster ovary cells. The physical growth of precipitate particles was observed by optical microscopy, transmission electron microscopy, dynamic light scattering, and small-angle and ultra-small-angle X-ray scattering to characterize the precipitate microstructure and growth mechanism. The precipitate microstructure is well-described as a mass fractal with fractal dimension approximately 2. The growth is governed by a diffusion-limited aggregation mechanism as indicated by a power-law dependence on time of the size of the principal precipitate particles. Optical microscopy shows that these primary particles can further aggregate into larger particles in a manner that appears to be promoted by mixing. Absorbance experiments at varying pH and salt concentrations reveal that the growth is largely driven by attractive electrostatic interactions, as growth is hindered by an increase in ionic strength. The solution conditions that resulted in the most significant particle growth are also correlated with the greatest removal of soluble impurities (DNA and HCPs). Proteomic analysis of the precipitates allows identification of unique HCP impurities, depending on the buffer species (acetate or citrate) used for the viral inactivation. Most of these proteins have pI values near the precipitation pH, supporting the likely importance of electrostatic interactions in driving precipitate formation.

Abnormal growth kinetics of h-BN epitaxial monolayer on Ru(0001) enhanced by subsurface Ar species

Growth kinetics of epitaxial films often follows the diffusion-limited aggregation mechanism, which shows a “fractal-to-compact” morphological transition with increasing growth temperature or decreasing deposition flux. Here, we observe an abnormal “compact-to-fractal” morphological transition with increasing growth temperature for hexagonal boron nitride growth on the Ru(0001) surface. The unusual growth process can be explained by a reaction-limited aggregation (RLA) mechanism. Moreover, introduction of the subsurface Ar atoms has enhanced this RLA growth behavior by decreasing both reaction and diffusion barriers. Our work may shed light on the epitaxial growth of two-dimensional atomic crystals and help to control their morphology.

Solubility of C60 and PCBM in Organic Solvents.

The ability to correlate fullerene solubility with experimentally or computationally accessible parameters can significantly facilitate nanotechnology nowadays for a wide range of applications, while providing crucial insight into optimum design of future fullerene species. To date, there has been no single relationship that satisfactorily describes the existing data clearly manifesting the effects of solvent species, system temperature, and isomer. Using atomistic molecular dynamics simulations on two standard fullerene species, C60 and PCBM ([6,6]-phenyl-C61-butyric acid methyl ester), in a representative series of organic solvent media (i.e., chloroform, toluene, chlorobenzene, 1,3-dichlorobenzene, and 1,2-dichlorobenzene), we show that a single time constant characterizing the dynamic stability of a tiny (angstrom-sized) solvation shell encompassing the fullerene particle can be utilized to effectively capture the known trends of fullerene solubility as reported in the literature. The underlying physics differs substantially between the two fullerene species, however. Although C60 was previously shown to be dictated by a diffusion-limited aggregation mechanism, the side-chain-substituted PCBM is demonstrated herein to proceed with an analogous reaction-limited aggregation with the "reaction rate" set by the fullerene rotational diffusivity in the medium. The present results suggest that dynamic quantities-in contrast to the more often employed, static ones-may provide an excellent means to characterize the complex (entropic and enthalpic) interplay between fullerene species and the solvent medium, shed light on the factors determining the solvent quality of a nanoparticle solution, and, in particular, offer a practical pathway to foreseeing optimum fullerene design and fullerene-solvent interactions.

Tunable morphology from 2D to 3D in the formation of hierarchical architectures from a self-assembling dipeptide: thermal-induced morphological transition to 1D nanostructures

Construction of complex three-dimensional (3D) architectures through hierarchical self-assembly of peptide molecules has become an attractive approach of fabricating multifunctional advanced materials due to their various potential applications in bionanotechnology. This paper describes the tunable formation of flower-like 3D hierarchical architectures of intricate morphology from a simple self-assembling dipeptide phenylalanine–tyrosine with a facile preparative method by applying a range of voltages through a drop of peptide solution. The fine-tuning of voltages and their application time enable to produce morphological changes of the microstructures from 2D to 3D and also control their formation. The morphology has been characterized by the gradual change in the height-to-diameter ratio of the microstructures with change in the applied voltages. Moreover, these microstructures show significant thermal stability over a wide range of temperatures, whereas adequately high temperature promotes the morphological transformation of the microstructures into different types of ultrathin 1D nanostructures such as nanowires, nanofibrils, etc. Furthermore, we have suggested a possible growth model for the fabrication of unique hierarchical architectures through diffusion-limited aggregation mechanism.

Second-order metropolitan urban phase transitions

The morphology evolution of Metropolitan Urban Areas constituted by different Central Business Districts is studied in this paper. For this matter, we propose a stochastic model which combines an initial percolation setting followed by a diffusion-limited aggregation mechanism. Our model mimics better than either case (percolation or diffusion-limited aggregation) the Metropolitan Urban Areas formation progress. We argue that the Metropolitan Urban Areas case introduced in this paper, grows in such a way that undergoes a non-equilibrium second-order phase transition during this process. This conclusion is supported by a fractal dimension and configurational entropy analysis, as well as by studying an empirical case.

Self-organization of polyaniline during oxidative polymerization: formation of granular structure

AbstractThe paper is focused on oxidative polymerization of aniline proceeding in an acid medium with a strong oxidant; formation of polyaniline (PANI) granular structures in different steps of the synthesis was studied. The relationship between the processes of self-organization of the growing polymer into supramolecular structures and the steps of molecular synthesis has been revealed. It was shown that during the induction period (the initial synthesis step), insoluble non-conducting products are formed. They are characterized by the absorption band at 430 nm corresponding to the wavelength of the phenazinium cation radical peak. In the second step, the polymer chain growth, conducting PANI granules with the diameter of 50 nm were obtained. These granules consist of spherical particles with the diameter as small as several nanometers. Then, the granule dimensions increased to 200 nm due to the growth of the spheres; the sphere diameter reached 20 nm. The number of spheres in a granule remained constant. Both precipitate and PANI film consist of common structural elements, polymer spheres, organized into granules and larger structures. Suppression of the polymer chain growth leads to the formation of non-conducting aniline oligomers which are self-organized into large particles with fractal structure. To describe the self-organization processes of a growing polymer chain, the diffusion-limited aggregation mechanism was used.

Formation of fractals by the self-assembly of interpolymer adducts of polymethacrylic acid with complementary polymers in aqueous solution

Interpolymer adducts of poly(methacrylic acid), (PMAA), with poly(vinylpyrrolidone) in presence of sodium chloride or potassium chloride form highly ordered fractal patterns in films on glass surface on drying at ambient temperature. The structure, morphology and the conditions under which the formation of fractal patterns occurs were investigated by SEM, EDX and confocal microscopic techniques. Self-organization of PMAA with complementary polymers such as poly(vinylpyrrolidone) is well-known and in the presence of sodium chloride formation of the fractals in films of the adducts is a novel observation. Fractal formation occurs due to the aggregation of interpolymer adducts. The composition of the fractals in the film is studied by EDX and confocal microscopic images of the fluorophores covalently bound to PMAA. In presence of salts, sodium chloride or potassium chloride, micellar like entities of 80 nm size were formed which further aggregate to form fractal patterns. It is suggested that the fractals result from the interpolymer adduct by Diffusion Limited Aggregation mechanism.

Electrochemical Impregnation of Silver Nanostructures in Titanium Dioxide Nanotubes

Highly-ordered titanium dioxide nanotubes array was prepared in fluoride based electrolyte and was subsequently deposited with silver nanoparticles and decorated by silver dendritic structures in silver ions-rich ethanolic electrolyte through conventional two-electrode configuration electrochemical method. The nanoparticles conglomerated at the surface of titanium dioxide nanotubes to form a hierarchy dendritic nanostucture, analogous to a structure consists of a trunk with branches and leaves, through the diffusion-limited aggregation mechanism and oscillatory character in nucleation kinetics. Structural investigation by X-ray diffraction measurements revealed that titanium dioxide nanotubes are comprised of multiple anatase and rutile phases. The finding is aligned with the observation of active anatase and rutile Raman modes through Raman measurements. In comparison of silver loaded titanium dioxide nanotubes to unloaded nanotubes in Raman measurements, surface-enhanced Raman scattering effect was demonstrated by silver incorporated samples whereby the Raman signal is intensified to more than sixfold in certain sample.

Insight into Fractal Self-Assembly of Poly(diallyldimethylammonium chloride)/Sodium Carboxymethyl Cellulose Polyelectrolyte Complex Nanoparticles

Poly(diallyldimethylammonium chloride)-sodium carboxymethyl cellulose polyelectrolyte complexes (PDDA-CMCNa PECs) solids were prepared and dispersed in NaOH aqueous solution. Self-assembly of PECs nanoparticles during the solvent evaporation was examined by field emission electron microscopy (FESEM), atomic force microscopy (AFM), and fractal dimension analysis. It was found that tree-shaped fractal patterns formed after the solvent (water) was dried at ambient temperatures, and the fractal pattern is composed of needle-shaped PEC aggregate (PECA) nanoparticles. Time-dependent FESEM observation revealed that the fractal pattern started with the formation of initial nucleon and it is growing, during which the diffusion limited aggregation (DLA) mechanism revealed and made the pattern branched. Physical insight into the DLA mechanism was discussed in detail. Effects of PEC concentrations, PEC compositions, solvent evaporation temperatures, pH of PEC dispersion, and chemical structures of PECs on the formation of self-assembled fractal pattern were studied. Generally, it was found that the morphologies, charge characters of PEC particles, and the solvent evaporation conditions play important roles during the fractal self-assembly process.

A simple pyrolysis route to synthesize leaf-like carbon sheets

Leaf-like carbon sheets have been obtained by the pyrolysis of dichloromethane and ferrocene in an autoclave in 300–600 °C on Cu ribbons for 90 min. The thickness of the sheets could be controlled from 50 to 200 nm by adjusting the experimental parameters. The formation process was studied by observing product-evolution utilizing the real-time imaging capabilities of emission scanning electron microscope, and it is considered that Diffusion-Limited Aggregation mechanism could be responsible for their growth. Thermo-gravimetric analysis curves show that the initial thermal decomposition temperature of the carbon sheets increases from 300 to 420 °C with the increasing preparation temperature. The Brunauer–Emmett–Teller specific surface area was 398 m 2/g and with a narrow pore distribution in the range of 2–5 nm. By changing the carbon sources, solid carbon spheres as well as chain-like solid carbon spheres (3–5 μm) could be selectively prepared using one-pot reactions.

Mechanisms of Asphaltene Aggregation in Toluene and Heptane Mixtures

The nature of aggregation kinetics of colloidal asphaltene particles in mixture of toluene and heptane was investigated by utilizing a polarizing microscope with appropriate magnification. The concentration of asphaltene in toluene-heptane mixture varied from 1 g/l to 8 g/l. There are two main mechanisms associated with the aggregation process. These mechanisms are diffusion limited aggregation (DLA) and reaction limited aggregation (RLA). Each mechanism has its own characteristics and acts on limited values of asphaltene concentration. At the asphaltene concentrations below the critical micelle concentration (CMC), the DLA mechanism is dominant, while at concentrations above the CMC, at the initial stage of aggregation process, the RLA mechanism is observed and then the mechanism tends toward DLA (crossover behavior). It should be noted that the CMC for asphaltene in the solution is around 3 g/l.

Topological Self-Similar Networks Introduced by Diffusion-Limited Aggregation Mechanism

We propose a model for growing fractal networks based on the mechanisms learned from the diffusion-limited aggregation (DLA) model in fractal geometries in the viewpoint of network. By studying the DLA network, our model introduces multiplicative growth, aging and geographical preferential attachment mechanisms, whereby featuring topological self-similar property and hierarchical modularity. According to the results of theoretical analysis and simulation, the degree distribution of the proposed model shows a mixed degree distribution (i.e., exponential and algebraic degree distribution) and the fractal dimension and clustering coefficient can be tuned by changing the values of parameters.

Morphology of semicrystalline oxyethylene/oxybutylene block copolymer thin films on mica

The morphology of as-cast and annealed thin films of four symmetric semicrystalline block copolymers on mica was investigated by tapping mode atomic force microscopy (AFM) and grazing incidence X-ray diffraction (XRD). It is found that the morphology of the thin films is dependent on chain length of oxyethylene/oxybutylene block copolymers. The as-cast thin films of the shorter EmBn block copolymers on mica exhibit a multi-layered lamellar structure parallel to the surface, in which the stems of the E crystals in the first half polymer layer contacting mica are parallel to the mica surface and perpendicular to the mica surface in the upper polymer layers. In contrast, the as-cast thin film of longer E224B114 exhibits a structure with mixed orientations of lamellar microdomains on a half polymer layer parallel to the surface. After annealing, the multi-layered structure on mica is transformed into a half-layered, densely branched structure, which is formed following a diffusion-limited aggregation mechanism, opposed to the featureless half-layered structure on silicon. Upon annealing, the upper polymer layers gradually retreat and the remaining area becomes thicker, but in contrast the first half polymer layer contacting mica becomes thinner due to wetting and the parallel orientation of the E crystal stems. The densely branched structure and the different chain orientations of the E crystal stems in the first half polymer layer contacting mica are attributed to the strong interaction between the E block and mica, as revealed by our previous work. The width of branches was employed to analyze the kinetics of secondary crystallization. It is also found that the width of the branches and the velocity of crystal front decrease as the chain length increases.

Mechanistic Study of the Electrodeposition of Nanoporous Self-Assembled ZnO/Eosin Y Hybrid Thin Films: Effect of Eosin Concentration

ZnO films prepared by one-step electrodeposition in the presence of dissolved eosin molecules present an internal nanoporous hybrid structure resulting from self-assembling processes occurring in solution between ZnO and eosin components. This study aims to better understand the underlying growth mechanism, which is still unexplained. The films were deposited by cathodic electrodeposition from an oxygen-saturated aqueous zinc chloride solution. The effects of the addition of 10 to 100 micromol.L(-1) eosin Y, as a sodium salt, on the growth rate and film properties, were systematically studied while all other parameters remained constant (concentrations of zinc salt and supporting electrolyte, applied potential of -1.4 V versus the mercurous sulfate electrode (MSE), temperature of 70 degrees C, rotating disk electrode at 300 rotations per min, and a glass-coated tin oxide electrode). It is shown that the addition of eosin provokes the formation of a nanoporous "cauliflower" structure whose nodule size and composition depend on the eosin concentration in the bath. The growth rate of the hybrid films increases markedly with the eosin concentration. The ZnO and eosin contents of the films are determined for each concentration by chemical analysis. Comparing with thickness determinations, it is shown that the total porosity increases up to 60-65% in volume fraction toward an eosin concentration of 100 micromol.L(-1). The empty pore volume fraction increases up to about 30% at an eosin concentration of about 20 micromol.L(-1) and then decreases. These correlations have been precisely established for the first time. It is shown that the global composition is fixed by the relative rate of deposition for zinc oxide, which is constant, and for the relative rate of eosin inclusion, which is proportional to the concentration in solution. This is explained on the basis of different steps in the growth mechanism, in particular, a diffusion effect limitation for both oxygen and eosin. This variation explains part of the increase in the growth rate. Another contribution is related by the structural effect on the nanoscale leading to the formation of the interpenetrated porous network. Competition between empty and eosin-filled parts of the pore network is evidenced. The formation of the porous network structure could be governed by a diffusion-limited aggregation mechanism. The system may represent a reference case of competing reactions in the electrochemical self-assembly of hybrid nanostructures.