Packing Of Nanoparticles Research Articles

Fine-Tuning Nanoparticle Packing at Water-Oil Interfaces Using Ionic Strength.

Nanoparticle-surfactants (NPSs) assembled at water-oil interfaces can significantly lower the interfacial tension and can be used to stabilize liquids. Knowing the formation and assembly and actively tuning the packing of these NPSs is of significant fundamental interest for the interfacial behavior of nanoparticles and of interest for water purification, drug encapsulation, enhanced oil recovery, and innovative energy transduction applications. Here, we demonstrate by means of interfacial tension measurements the high ionic strength helps the adsorption of NPSs to the water-oil interface leading to a denser packing of NPSs at the interface. With the reduction of interfacial area, the phase transitions from a "gas"-like to "liquid" to "solid" states of NPSs in two dimensions are observed. Finally, we provide the first in situ real-space imaging of NPSs at the water-oil interface by atomic force microcopy.

Nanoporous Polymer-Infiltrated Nanoparticle Films with Uniform or Graded Porosity via Undersaturated Capillary Rise Infiltration.

In this work, we present the fabrication of nanoporous polymer-infiltrated nanoparticle films (PINFs) with either uniform or graded porosity based on undersaturated capillary rise infiltration (UCaRI) and study the processing-structure-property relationship of these nanoporous PINFs. The UCaRI process involves first generating a bilayer film of a randomly packed nanoparticle layer atop a polymer layer, such that the volume of the polymer is less than the void volume in the nanoparticle packing. Subsequently, the bilayer film is annealed above the glass transition temperature of the polymer to induce polymer infiltration into the voids of the nanoparticle packing. Using in situ spectroscopic ellipsometry and molecular dynamics simulations, we observe that the polymer transport occurs in two stages: capillarity-induced infiltration, followed by gradual spreading, likely via surface diffusion. By varying the annealing time, UCaRI enables the generation of graded or uniform nanoporous PINFs. We also show that these nanoporous PINFs have tunable optical and mechanical properties, which can be tailored simply by changing the nanoparticle to polymer layer thickness ratio in the initial bilayer. The UCaRI approach is versatile and widely applicable to various polymers, which allows generation of nanoporous PINFs for multiple applications.

Modeling nanofluid sessile drop evaporation

Modeling predictions for the evaporation of nanofluid pinned sessile drops are reported. Drops of fluids containing suspended nanoparticles have reduced evaporation rates relative to their pure fluid counterparts due to the agglomeration of nanoparticles at the surface resulting in a reduction in available liquid at the drop surface for evaporation. The present model implements a mechanism for the reduction in the surface concentration of the evaporating liquid based on the fractal geometry of nanoparticle agglomerates. Nanoparticle packing near the drop pinned contact line results in regions where a maximum nanoparticle volume fraction is attained, leading to significant reductions in the evaporative mass flux. Model predictions for the evaporation rate of pure ethanol and ethanol containing suspended aluminum nanoparticles are compared to experiments from the literature with excellent agreement for the reduction in evaporation rate due to nanoparticle loading and in reasonable quantitative agreement for the evaporation rate. The maximum allowable nanoparticle volume fraction is shown to be an important parameter in governing the evaporation rate of nanofluid sessile drops.

Emergence of hierarchical structural complexities in nanoparticles and their assembly.

We demonstrate that nanoparticle self-assembly can reach the same level of hierarchy, complexity, and accuracy as biomolecules. The precise assembly structures of gold nanoparticles (246 gold core atoms with 80 p-methylbenzenethiolate surface ligands) at the atomic, molecular, and nanoscale levels were determined from x-ray diffraction studies. We identified the driving forces and rules that guide the multiscale assembly behavior. The protecting ligands self-organize into rotational and parallel patterns on the nanoparticle surface via C-H⋅⋅⋅π interaction, and the symmetry and density of surface patterns dictate directional packing of nanoparticles into crystals with orientational, rotational, and translational orders. Through hierarchical interactions and symmetry matching, the simple building blocks evolve into complex structures, representing an emergent phenomenon in the nanoparticle system.

Progressive freezing of interacting spins in isolated finite magnetic ensembles

Self-organization of magnetic nanoparticles into secondary nanostructures provides an innovative way for designing functional nanomaterials with novel properties, different from the constituent primary nanoparticles as well as their bulk counterparts. Collective magnetic properties of such complex closed packing of magnetic nanoparticles makes them more appealing than the individual magnetic nanoparticles in many technological applications. This work reports the collective magnetic behaviour of magnetic ensembles comprising of single domain Fe3O4 nanoparticles. The present work reveals that the ensemble formation is based on the re-orientation and attachment of the nanoparticles in an iso-oriented fashion at the mesoscale regime. Comprehensive dc magnetic measurements show the prevalence of strong interparticle interactions in the ensembles. Due to the close range organization of primary Fe3O4 nanoparticles in the ensemble, the spins of the individual nanoparticles interact through dipolar interactions as realized from remnant magnetization measurements. Signature of super spin glass like behaviour in the ensembles is observed in the memory studies carried out in field cooled conditions. Progressive freezing of spins in the ensembles is corroborated from the Vogel–Fulcher fit of the susceptibility data. Dynamic scaling of relaxation reasserted slow spin dynamics substantiating cluster spin glass like behaviour in the ensembles.

Demagnetization effects in dense nanoparticle assemblies

We highlight the relevance of demagnetizing-field corrections in the characterization of dense magnetic nanoparticle assemblies. By an analysis that employs in-plane and out-of-plane magnetometry on cylindrical assemblies, we demonstrate the suitability of a simple analytical formula-based correction method. This allows us to identify artifacts of the demagnetizing field in temperature-dependent susceptibility curves (e.g., shoulder peaks in curves from a disordered assembly of essentially bare magnetic nanoparticles). The same analysis approach is shown to be a straightforward procedure for determining the magnetic nanoparticle packing fraction in dense, disordered assemblies.

In situ Electrochemical Impedance Spectroscopy of Electrostatically Driven Selective Gold Nanoparticle Adsorption on Block Copolymer Lamellae.

Electrostatic attraction between charged nanoparticles and oppositely charged nanopatterned polymeric films enables tailored structuring of functional nanoscopic surfaces. The bottom-up fabrication of organic/inorganic composites for example bears promising potential toward cheap fabrication of catalysts, optical sensors, and the manufacture of miniaturized electric circuitry. However, only little is known about the time-dependent adsorption behavior and the electronic or ionic charge transfer in the film bulk and at interfaces during nanoparticle assembly via electrostatic interactions. In situ electrochemical impedance spectroscopy (EIS) in combination with a microfluidic system for fast and reproducible liquid delivery was thus applied to monitor the selective deposition of negatively charged gold nanoparticles on top of positively charged poly(2-vinylpyridinium) (qP2VP) domains of phase separated lamellar poly(styrene)-block-poly(2-vinylpyridinium) (PS-b-qP2VP) diblock copolymer thin films. The acquired impedance data delivered information with respect to interfacial charge alteration, ionic diffusion, and the charge dependent nanoparticle adsorption kinetics, considering this yet unexplored system. We demonstrate that the selective adsorption of negatively charged gold nanoparticles (AuNPs) on positively charged qP2VP domains of lamellar PS-b-qP2VP thin films can indeed be tracked by EIS. Moreover, we show that the nanoparticle adsorption kinetics and the nanoparticle packing density are functions of the charge density in the qP2VP domains.

Strong surface enhanced Raman scattering from gold nanoarrays obtained by direct laser writing

We report for surface enhanced Raman scattering (SERS) from arrays of gold nanoparticles produced by 2-photons photo-reduction of the metallic precursor (HAuCl4) hosted in a Poly-Vinyl Alcohol (PVA) matrix, on glass substrates. Samples with the same pattern but featuring different nanoparticles size and density were obtained by varying the writing laser power and scanning speed. The Raman spectra were recorded from samples immersed in a solution of rhodamine-6G (R6G), as well as, after exposure of the samples in xylene. SERS enhancement factors of up to ∼104 were obtained for both analytes. The measurements show that the SERS enhancement is maximized on golden strips produced at higher writing laser power and lower scanning speed, where closer nanoparticles packing is obtained..

Precisely designing bimodal catalyst structure to trap cobalt nanoparticles inside mesopores and its application in Fischer-Tropsch synthesis

Bimodal catalyst support is a widely applied concept in solid catalysts but until now the supported metal on the bimodal support has no satisfied dispersion and the average metal particle size is generally larger than the pore size of small pores (mesopores) of the bimodal structure, lowering the power and significance of the bimodal structure. To overcome this problem, a novel ZrO2SiO2 bimodal pore support with enlarged depth of small pores, was successfully fabricated though self-assembly of nanosized ZrSiO4 inside pore of silica gel. Cobalt was finely dispersed on bimodal support by incipient-wetness impregnation. The bimodal support and catalyst were characterized by various techniques, such as BET, TEM, XRD, and TPR and H2-chemisorpion. The bimodal catalyst was applied in slurry-phase Fischer-Tropsch synthesis. Characterization results suggested that bimodal pore support processed larger BET surface than pristine silica gel, due to the newly formed small pores organized by packing of the ZrSiO4 nanoparticles. The enlarged BET surface area leaded to good dispersion of cobalt species. More importantly, it was observed that the deep small pores could serve as traps for Co nanoparticles, preventing sintering and agglomeration due to confinement effect. Co/30wt%ZrO2-Q-50 and Co/20wt%ZrO2-Q-100 bimodal catalyst with large BET surface and deep small pores exhibited excellent catalytic activity, and selectivity to heavy hydrocarbons due to high dispersion of cobalt and unique pore structure, while coexisting ZrO2 promoted the reduction of finely-dispersed cobalt oxide nanoparticles. It is interesting to find that different from the conventional theory where small metallic particle lowers chain-growth probability, the trapped smaller Co particles inside deep small pores achieved higher chain-growth probability due to the facilitated re-adsorption of olefinic intermediates happening inside deep small pores.

Modeling of nanoparticle agglomeration and powder bed formation in microscale selective laser sintering systems

Additive manufacturing (AM) has received a great deal of attention for the ability to produce three dimensional parts via laser heating. One recently proposed method of making microscale AM parts is through microscale selective laser sintering (μ-SLS) where nanoparticles replace the traditional powders used in standard SLS processes. However, there are many challenges to understanding the physics of the process at nanoscale as well as with conducting experiments at that scale; hence, modeling and computational simulations are vital to understand the sintering process physics. At the sub-micron (μm) level, the interaction between nanoparticles under high power laser heating raises additional near-field thermal issues such as thermal diffusivity, effective absorptivity, and extinction coefficients compared to larger scales. Thus, nanoparticle's distribution behavior and characteristic properties are very important to understanding the thermal analysis of nanoparticles in a μ-SLS process. This paper presents a discrete element modeling (DEM) study of how copper nanoparticles of given particle size distribution pack together in a μ-SLS powder bed. Initially, nanoparticles are distributed randomly into the bed domain with a random initial velocity vector and set boundary conditions. The particles are then allowed to move in discrete time steps until they reach a final steady state position, which creates the particle packing within the powder bed. The particles are subject to both gravitational and cohesive forces since cohesive forces become important at the nanoscale. A set of simulations was performed for different cases under both Gaussian and log-normal particle size distributions with different standard deviations. The results show that the cohesive interactions between nanoparticles has a great effect on both the size of the agglomerates and how densely the nanoparticles pack together within the agglomerates. In addition, this paper suggests a potential method to overcome the agglomeration effects in μ-SLS powder beds through the use of colloidal nanoparticle solutions that minimize the cohesive interactions between individual nanoparticles.

Chemical diffusion coefficient in dye sensitized solar cells as a function of porosity and surface roughness

In this work we introduce equations describing the morphological dependency of electron transport in nano-structured solar cells. Using the hopping model for electron transport and using the trap contained diffusion model we define an expression to explain the dependency of the electron diffusion coefficient and conductance versus porosity and surface roughness. Validity of the obtained theoretical formula is examined by a random walk simulation based on hopping model in a fully disordered medium with a random distribution of energy. A Monte-Carlo random walk simulation was used to investigate the effects of morphology on the electron transport in the porous nano-structures. For generation of the random packing of nanoparticles, a stochastic cluster model was utilized. The porosity and surface roughness of the generated porous media were tuned by changing the particle size and the overlap parameters.

Toward Ultrahigh‐Capacity V2O5 Lithium‐Ion Battery Cathodes via One‐Pot Synthetic Route from Precursors to Electrode Sheets

Acquisition of high‐energy density is the highest priority requirement and unending challenge in energy storage systems including lithium‐ion batteries (LIBs). One theoretically preferable way to reach this goal is the use of cathode active materials such as vanadium pentoxide (V2O5) that relies on multielectron insertion/extraction reactions. Application of V2O5 to LIB cathodes, however, has been mostly focused on V2O5 materials themselves with little emphasis on V2O5‐incorporated cathode sheets. Here, as an unusual electrode‐architecture approach to achieve ultrahigh‐capacity V2O5 cathode sheets, a new class of self‐standing V2O5 cathode sheets is demonstrated based on V2O5/multiwalled carbon tubes (MWNTs) mixtures spatially besieged by polyacrylonitrile nanofibers (referred to as “VMP cathode sheets”). Notably, the VMP cathode sheet is fabricated directly via one‐pot synthetic route starting from V2O5 precursor (i.e., through concurrent electrospraying/electrospinning followed by calcination), without metallic foil current collectors/carbon powders/polymeric binders. The one‐pot synthesis allows dense packing of V2O5 nanoparticles in close contact with MWNT electronic networks and also formation of well‐developed interstitial void channels (ensuring good electrolyte accessibility). This material/architecture uniqueness of the VMP cathode sheet eventually enables significant improvements in cell performance (particularly, gravimetric/volumetric capacity of cathode sheets) far beyond those accessible with conventional electrode technologies.

Antireflective Coatings for Glass and Transparent Polymers.

Antireflective coatings (ARCs) are applied to reduce surface reflections. We review coatings that reduce the reflection of the surface of the transparent substrates float glass, polyethylene terephthalate, poly(methyl methacrylate), and polycarbonate. Three main coating concepts exist to lower the reflection at the interface of a transparent substrate and air: multilayer interference coatings, graded index coatings, and quarter-wave coatings. We introduce and discuss these three concepts, and zoom in on porous quarter-wave coatings comprising colloidal particles. We extensively discuss the four routes for introducing porosity in quarter-wave coatings through the use of colloidal particles, which have the highest potential for application: (1) packing of dense nanospheres, (2) integration of voids through hollow nanospheres, (3) integration of voids through sacrificial particle templates, and (4) packing of nonspherical nanoparticles. Finally, we address the remaining challenges in the field of ARCs, and elaborate on potential strategies for future research in this area.

Fractal substructure of a nanopowder generated by repeated fragmentation and sedimentation: the rôle of the dust

Packings of cohesive nanoparticles, that is nanopowders, may be obtained as the result of repeated fragmentation–reagglomeration cycles (Schwager et al. in Phys Rev Lett 100:218002, 2008) such that the resulting sediment reveals a fractal structure. The size distribution of the fragments after a fragmentation step is a superposition of a narrow distribution of large particles (chunks) whose size is determined by the cutting length and a power-law distribution for small particles, representing scale invariant dust. It was shown that the exponent of the power-law, $$\tau $$ , is in non-trivial relation to the fractal dimension, $$d_f$$ , via $$d_f(2-\tau )=1$$ . This poses the question for the structure of the sediment created by repeated fragmentation–reagglomeration cycles when the dust particles are excluded from the reagglomeration step. We found that even in this case, repeated fragmentation–reagglomeration cycles yield a sediment of fractal structure with slightly reduced fractal dimension while the dust exponent, $$\tau $$ , remains unchanged.

Heterogeneity in the Small-Scale Deformation Behavior of Disordered Nanoparticle Packings.

Atomic force microscopy-based nanoindentation is used to image and probe the local mechanical properties of thin disordered nanoparticle packings. The probed region is limited to the size of a few particles, and an individual particle can be loaded and displaced to a fraction of a single particle radius. The results demonstrate heterogeneous mechanical response that is location-dependent. The weak locations may be analogous to the "soft spots" previously predicted in glasses and other disordered packings.

Growth of g-C3N4 on mesoporous TiO2 spheres with high photocatalytic activity under visible light irradiation

Growth of graphitic carbon nitride on mesoporous TiO2 spheres with well-controlled structures was achieved by melt-infiltrating dicyandiamide (DICY). The molten DICY diffused into the porous structure of TiO2, and g-C3N4 was produced inside the mesopores formed by packing of TiO2 nanoparticles at higher temperature, resulting in a better fused g-C3N4-TiO2 heterostructure. The formation of a strong interfacial connection between TiO2 and g-C3N4 by this preparation method greatly improved the separation efficiency of photo-generated electrons and holes. TiO2/g-C3N4 composites exhibited much higher photocatalytic activity than TiO2 and g-C3N4 towards the degradation of Rhodamine B under both UV light and UV–vis light irradiation. The heterostructured combination provided a synergistic photocatalytic activity through an efficient electron transfer process. Based on the experimental results, a possible mechanism for the improved photocatalytic performance was proposed. The factors affecting the photocatalytic activity were also discussed based on the result of structure analysis, optical and photoelectric characteristics, and photocatalytic reactions.

Influence of structural organization of silicas on interfacial phenomena

Initial and treated silicas of different origin, such as fumed and precipitated silicas and silica gel, were studied using infrared spectroscopy, DSC, NMR, and adsorption methods. Mechanochemical activation (MCA) of fumed silica A-300 in a ball-mill or a microbreaker affects the structure of aggregates of nanoparticles and agglomerates of aggregates becoming more compacted. Any treatment of ‘soft’ nanosilica affects the interfacial behavior of polar and nonpolar adsorbates because the rearrangement of secondary particles affects the freezing–melting point depression. Clusterization of adsorbates bound in pores causes diminution of heat effects during phase transition (freezing or fusion). The freezing point depression and increasing melting point lead to the hysteresis in freezing–melting of adsorbates bound in pores. The study shows that fumed and precipitated silicas can be more sensitive to external actions, such as wetting–drying and MCA, than silica gels because of less dense packing of nanoparticles in the secondary structures. These effects should be considered upon applications of similar materials in different media.

The effect of nanoparticle packing on capacitive electrode performance.

Nanoparticles pack together to form macro-scale electrodes in various types of devices, and thus, optimization of the nanoparticle packing is a prerequisite for the realization of a desirable device performance. In this work, we provide in-depth insight into the effect of nanoparticle packing on the performance of nanoparticle-based electrodes by combining experimental and computational findings. As a model system, polypyrrole nanospheres of three different diameters were used to construct pseudocapacitive electrodes, and the performance of the electrodes was examined at various nanosphere diameter ratios and mixed weight fractions. Two numerical algorithms are proposed to simulate the random packing of the nanospheres on the electrode. The binary nanospheres exhibited diverse, complicated packing behaviors compared with the monophasic packing of each nanosphere species. The packing of the two nanosphere species with lower diameter ratios at an optimized composition could lead to more dense packing of the nanospheres, which in turn could contribute to better device performance. The dense packing of the nanospheres would provide more efficient transport pathways for ions because of the reduced inter-nanosphere pore size and enlarged surface area for charge storage. Ultimately, it is anticipated that our approach can be widely used to define the concept of "the best nanoparticle packing" for desirable device performance.

Gold Nanoparticle Quantitation by Whole Cell Tomography.

Many proposed biomedical applications for engineered gold nanoparticles require their incorporation by mammalian cells in specific numbers and locations. Here, the number of gold nanoparticles inside of individual mammalian stem cells was characterized using fast focused ion beam-scanning electron microscopy based tomography. Enhanced optical microscopy was used to provide a multiscale map of the in vitro sample, which allows cells of interest to be identified within their local environment. Cells were then serially sectioned using a gallium ion beam and imaged using a scanning electron beam. To confirm the accuracy of single cross sections, nanoparticles in similar cross sections were imaged using transmission electron microscopy and scanning helium ion microscopy. Complete tomographic series were then used to count the nanoparticles inside of each cell and measure their spatial distribution. We investigated the influence of slice thickness on counting single particles and clusters as well as nanoparticle packing within clusters. For 60 nm citrate stabilized particles, the nanoparticle cluster packing volume is 2.15 ± 0.20 times the volume of the bare gold nanoparticles.

Metal ion-responsive photonic colloidal crystalline micro-beads with electrochemically tunable photonic diffraction colours

This work reports a metal ion-responsive photonic colloidal crystalline (PCC) microspheric material and the convenient, reversible switching of its photonic diffraction properties by the redox reaction of copper in a simple electrochemical cell. The PCC micro-beads were fabricated from the orderly three-dimensional packing of core-shell nanoparticles with hydrogel coatings laden with anionic phosphate functionalities. Electrostatic binding of metal cations by the hydrogel coating lowered its internal osmotic pressure and caused shrinkage of the core-shell nanoparticles. This brought about a blue-shift of the photonic diffraction λmax of the micro-beads. The maximum shift, from 650 to 590nm (colour change from magenta to green) was found to be produced by Cu2+ at a concentration of 1mM. A simple electrochemical photonic device was constructed by sandwiching a suspension of the photonic micro-beads in 1mM Cu2+ solution between two transparent ITO-glass electrodes. Photonic diffraction colour of the micro-beads was reversibly switched via the electrochemical reduction/oxidation of Cu2+ inside the device. The colour change can be observed by naked eyes under ambient light. This concept of rapid electrochemical regulation of photonic diffraction colour of PCC micro-beads may be useful in reflective displays applications.