Nanoparticle Chain Aggregates Research Articles

Charge transfer effect: a new assignment of the abnormal optical absorption band of gold nanoparticles

As a significant accompanying phenomenon of surface-enhanced Raman scattering (SERS), the addition of foreign molecules to colloidal gold or silver nanoparticles results in a new abnormal optical absorption (AOA) band, which usually appears in the long-wavelength region. The assignment of this AOA band has long been debated as an important issue that is desired to be addressed in the SERS field, which is crucial for a clear understanding of the SERS enhancement mechanism and beneficial to surface plasmonics. In this study, both the calculated and measured optical absorptions of gold nanoparticle monomers and dimers as well as their interactions with adsorbed molecules, showed that the AOA band in the long-wavelength region which was assigned to the characteristic longitudinal localized surface plasmon resonance (LSPR) effect of gold nanoparticle chain aggregates in conventional SERS electromagnetic theory, should be attributed to the charge-transfer resonance absorption from gold nanoparticles to adsorbed molecules. This was further confirmed by the corresponding SERS effects. As the excitation wavelength at 785 nm was resonant with the broad AOA band centered at 750 nm, the SERS peaks of the adsorbed pyridine molecules could be dramatically enhanced due to the charge-transfer resonance effect. In contrast, under an excitation wavelength of 532 nm, the SERS peaks appeared very weak, although the excitation wavelength was resonant with the LSPR absorption band of the individual gold nanoparticles.

A general method for precise chain assembly of noble metal nanoparticles

We report a general method for obtaining precise chain aggregates of noble metal nanoparticles, which could be generally applied to AuNPs, AgNPs and PtNPs of various sizes, giving single long chains.

Synthesis and structures of Al–Ti nanoparticles by hydrogen plasma-metal reaction

Three kinds of Al–Ti nanoparticles (7.7, 27.8, and 42.6 at.% Ti) have been prepared from Al–65, Al–85, and Al–88 at.% Ti master alloys by hydrogen plasma-metal reaction, with average particle sizes of 30, 25, and 80 nm, respectively. The higher evaporation rate of Al than Ti resulted in the low Ti contents in the nanoparticles than those in the master alloys. Microscopy observation revealed that the primary nanoparticles are spherical in shape, and occur as chain aggregates of several individual nanoparticles due to the faster collision rate than the coalescence rate. All the Al–Ti nanoparticles contain amorphous alumina layers of about 2–3 nm in thickness surrounding the crystalline core. AlTi intermetallic nanoparticles were successfully produced for Al–27.8 at.% Ti, with a single crystal of AlTi in one chain aggregate. The composite nanoparticles of Al together with some Al3Ti phases are prepared for Al–7.7 at.% Ti, with each phase in the individual particle of one chain aggregate. The composite nanoparticles of AlTi with some AlTi3 were produced for Al–42.6 at.% Ti, with each phase in the individual particle of one chain aggregate. The formation mechanism of Al–Ti nanoparticles was interpreted in terms of phase transition and the effect of hydrogen.

A rheological study on temperature dependent microstructural changes of fumed silica gels in dodecane

The effect of temperature on colloid systems of hydrophobic and hydrophilic fumed silica in dodecane was investigated mainly by means of dynamic rheology. Dynamic frequency sweeps showed frequency-independent elastic modulus (G′), indicating the presence of gel networks in both dispersions. Dynamic temperature sweeps consisting of a heating and cooling cycle were also conducted. For the hydrophobic fumed silica gel, G′ slowly decreased with rising temperature owing to the increased particle mobility and decreased strength of hydrogen bonds due to the higher thermal energy. However, the hydrophilic fumed silica gel showed a rather unusual temperature dependent behavior involving a transition temperature (Tc) and an irreversible increase in G′ during the cooling cycle. This unusual behavior was believed to be related to the restructuring of nanoparticle chain aggregates (NCA) of fumed silica in gels as also revealed by FTIR results. Further analysis on this behavior showed that elevated temperature tended to increase the fractal dimension (Df) of aggregates, resulting in more compact NCAs and thereby a denser gel network with a higher G′. These results suggest that temperature is an important external variable that can be regarded as an optional simple way to tailor the viscoelastic properties and to control the microstructure of colloidal systems such as fumed silica dispersions in dodecane.

Structure of fumed silica gels in dodecane: enhanced network by oscillatory shear

The structure and viscoelastic properties of fumed silica gels in dodecane were studied by means of dynamic rheology. With increasing the specific surface area of fumed silica nanoparticles, the plateau elastic modulus (G′), which is frequency-independent and shows the characteristic of a network of the fumed silica gels, decreases. Such networks of fumed silica gels show a significant temperature-dependent behavior and a transition temperature (T c) related with the restructuring of nanoparticle chain aggregates of fumed silica in gels. Under oscillatory shear, the fumed silica gels experience disorganization and reorganization and present strong structural recovery ability after adjusting oscillatory shear (AOS) at small strain amplitudes (1–10%), and a more perfect network structure than that in origin gels can be induced. Elevated temperature (above T c) improves the network structure to be more compact and stronger than that at a lower temperature, as a result, the deformation resistance during the AOS period and the structural recovery after AOS are enhanced. These results indicate that the network structure and viscoelastic properties of fumed silica gels can be tailored and optimized by performing small-amplitude oscillatory shear at a properly selected temperature.

Reversible Tuning of Plasmon Coupling in Gold Nanoparticle Chains Using Ultrathin Responsive Polymer Film

We demonstrate large and reversible tuning of plasmonic properties of gold nanoparticles mediated by the reversible breaking and making of linear and branched chains of gold nanoparticles adsorbed on an ultrathin (1 nm) responsive polymer film. Atomic force microscopy revealed that at pH below the isoelectric point of the polybase (extended state of the polymer chains), gold nanoparticles adsorbed on the polymer layer existed primarily as individual nanoparticles. On the other hand, at higher pH, the polymer chains transition from coil to globule (collapsed) state, resulting in the formation of linear and branched chains with strong interparticle plasmon coupling. Reversible aggregation of the nanoparticles resulted in large and reversible change in the optical properties of the metal nanostructure assemblies. In particular, we observed a large redistribution of the intensity between the individual and coupled plasmon bands and a large shift (nearly 95 nm) in the coupled plasmon band with change in pH. Large tunability of plasmonic properties of the metal nanostructure chains reported here is believed to be caused by the chain aggregates of nanoparticles and un-cross-linked state of the adsorbed polymer enabling large changes in polymer chain conformation.

Estimation of contact area of nanoparticles in chains using continuum elastic contact mechanics

The key challenge to understand the nanomechanical behavior of nanoparticle chain aggregates (NCA) is strongly dependent on accurate determination of the contact area between individual particles. Experimental determination of the contacting radius is extremely difficult and often not reliable. We present calculations based on elastic contact mechanics models, that predict the contact radii of nanoparticles. The model considers important parameters, such as the particle size, the elastic property of the materials and the effect of humidity that intervenes between the contacts. Calculations carried out for four selected materials show that there is a large difference between the radii of contacts formed in vacuum and in ambient conditions. In vacuum, the surface energy of the material is higher compared to that in ambient condition and the contact formation is dominated by the strong interactions between the surfaces. In ambient condition, the contact is dominated by the Laplace pressure due to the condensed water meniscus between the contacts. Calculations presented in this article show that the contact radius between nanoparticles depends on materials properties and the environment.

Electron microscopic study of chain aggregates of MoO3 nanoparticles

Chain aggregates of MoO3 nanoparticles have been obtained and their polymer-like properties revealed. Chain aggregates of MoO3 nanoparticles, like polymers, form parallel packings, coils and exhibit coil-helix transition, Brownian motion, and high flexibility.

Determination of surface area and volume of nanoparticle aggregates deposited in the human respiratory tract using DMA data

A method is proposed for using size distribution data obtained with a differential mobility analyzer (DMA) to calculate the total surface area and volume of electrically conducting nanoparticle chain aggregates deposited in the human respiratory tract. The method consists of two steps: (1) the electrical mobility diameter of conducting aggregates determined with a DMA is used to calculate the diffusion-equivalent diameter and (2) the theory of idealized aggregates developed by Lall and Friedlander [(2006). On-line measurement of ultrafine aggregate surface area and volume distributions by electrical mobility analysis: I. Theoretical analysis. Journal of Aerosol Science 37, 260] is applied in calculations for the aggregate surface area and volume. The results of calculations using the volume distribution of a sample of diesel exhaust particles indicate that deposited aggregates have a larger total surface area than do electrical-mobility-equivalent spheres, even though the deposited aggregates have a much smaller total volume. The proposed method for calculating diffusion-equivalent diameter from electrical mobility diameter is also employed to show that the deposition data reported in literature for diesel exhaust particles and petrol particles agree well when the deposition fractions are plotted as functions of diffusion-equivalent diameter.

Development of a Simple Sintering Law for Fractal Aggregates Composed of Unequal Sized Primary Particles

ABSTRACTSintering of silicon nanoparticle chain aggregates composed of unequal sized primary particles are investigated using molecular dynamics (MD) simulations at 1500 K. We consider straight chain aggregates consisting of up to 40 2.5 and 5.4 nm primary particles. The sintering time increases with increase in the total volume of the chain aggregate or with increase in the exposed initial surface area of the chain. A mathematical model was developed to describe the dynamics of sintering of such chain aggregates. The model is a power law modification of the Frenkel sintering equation with the Koch-Friedlander model to include primary particle size dependence. We found that the particle size effect is a local process, and important only at the initial stage of the sintering. Thus, the effect is not significant when the aggregate becomes large. The model is amenable for use in aerosol models that might include sintering effects.

Mechanical Properties of Nanoparticle Chain Aggregates by Combined AFM and SEM: Isolated Aggregates and Networks

Mechanical properties of nanoparticle chain aggregates (NCA) including tensile strength and Young's modulus were measured using an instrument incorporating an AFM tip under SEM imaging. The NCA were studied individually and as network films. Carbon NCA were made by laser ablation of graphite, and SnO2 NCA were made by oxidation of a tin compound. The films were deformable and showed elastic behavior. NCA serve as reinforcing fillers in rubber and films of SnO2 NCA for trace gas detection.

Inertial deposition of nanoparticle chain aggregates: Theory and comparison with impactor data for ultrafine atmospheric aerosols

Nanoparticle chain aggregates (NCAs) are often sized and collected using instruments that rely on inertial transport mechanisms. The instruments size segregate aggregates according to the diameter of a sphere with the same aerodynamic behavior in a mechanical force field. A new method of interpreting the aerodynamic diameter of NCAs is described. The method can be used to calculate aggregate surface area or volume. This is useful since inertial instruments are normally calibrated for spheres, and the calibrations cannot be directly used to calculate aggregate properties. A linear relationship between aggregate aerodynamic diameter and primary particle diameter based on published Monte-Carlo drag calculations is derived. The relationship shows that the aggregate aerodynamic diameter is independent of the number of primary particles that compose an aggregate, hence the aggregate mass. The analysis applies to aggregates with low fractal dimension and uniform primary particle diameter. This is often a reasonable approximation for the morphology of nanoparticles generated in high temperature gases. An analogy is the use of the sphere as an approximation for compact particles. The analysis is applied to the collection of NCAs by a low-pressure impactor. Our results indicate the low-pressure impactor collects aggregates with a known surface area per unit volume on each stage. Combustion processes often produce particles with aggregate structure. For diesel exhaust aggregates, the surface area per unit volume calculated by our method was about twice that of spheres with diameter equal to the aerodynamic diameter. Measurements of aggregates collected near a major freeway and at Los Angeles International Airport (LAX) were made for two aerodynamic cutoff diameter diameters (d a,50), 50 and 75 nm. (Aerodynamic cutoff diameter refers to the diameter of particles collected with 50% efficiency on a low-pressure impactor stage.) Near-freeway aggregates were probably primarily a mixture of diesel and internal combustion engine emissions. Aggregates collected at LAX were most likely present as a result of aircraft emissions. In both measurements, the aggregate aerodynamic diameters calculated from the primary particle diameter were fairly close to the stage cutoff diameter. The number of primary particles per aggregate varied one order of magnitude for particles depositing on the same stage. The average aggregate surface area per unit volume was 2.41 × 106 cm−1 and 2.59 × 106 cm−1 (50 nm d a,50) and 1.81 × 106 cm−1 and 1.68 × 106 cm−1 (75 nm d a,50) for near-freeway and LAX measurements, respectively. These preliminary measurements are consistent with values calculated from theory.

Synthesis of ultra fine particles by plasma transferred arc: Influence of anode material on particle properties

There is a great deal of interest today in the special properties of nanoparticles and their potential applications. Gas-phase process, although having some drawbacks, has the largest control possibilities and is therefore the method chosen here. The size of the primary particles depends on the temperature/time history and material properties. Further growth of the particles strongly depends on the properties of the flow into which they are imbedded. In the current work, a transferred arc is used to produce nanometric particles from the condensation of metallic vapours obtained by controlled evaporation of the anode material, which becomes the solid precursor of the synthesis. As all types of anode materials would be used, depending of the nature of the desired particles, this technique requires a good control of the heat transfer and its application time to a given anode location, as well as the separation of evaporation/ nucleation-growth steps. That is why a new and original experimental set-up was built in order to control vapour production and its thermal history. Experiments showed that the heat transfer at the anode precursor strongly depends on the cold boundary layer (CBL) properties close to the anode. For adequate parameters, it becomes possible to generate either diffuse or constricted stable arc root, and so to control vapour production. Orientation and so dissociation of evaporation and nucleation events is also achieved by tilting the angle between the jet issued from the cathode and the anode. The vapours produced are then naturally entrained towards a temperature controlled zone, where they are collected onto a water cooled substrate. It thus becomes possible to control the residence time and the thermal vapour history of the particles. Whereas aluminium oxide particles synthesized are clearly nanoparticle chain aggregates, iron oxides particles are spherical, in the micrometric range and no aggregates or agglomerates are visible. These experiments show that particle morphology, size and shape, for given working parameters, strongly depend on the properties of the material to be vaporized.

Molecular Dynamics Simulation of Nanoparticle Chain Aggregate Sintering

AbstractSintering of silicon nanoparticle chain aggregates are investigated using molecular dynamics (MD) simulations at 1500 K, which is about melting temperature at the size range we tested. The straight chain aggregates consist of upto 40 particles and the primary particles of 2.5 to 5 nm sizes are considered. The sintering time increases with increase the total volume of the chain aggregate or with increase the exposed initial surface area of the chain. A mathematical model was developed to describe the dynamics of sintering of chain aggregates. The model was able to predict the sintering time with excellent agreement with the results obtained from MD simulations. We also studied the chain aggregate that has a secondary branch coming out from the edge of the primary branch (L-shape) and from the middle of the primary branch (T-shape). In general, sintering time changes as much as 30% of that of a straight chain which contains the same volume of particles.

On-line measurement of ultrafine aggregate surface area and volume distributions by electrical mobility analysis: I. Theoretical analysis

Electrical mobility analyzers are usually calibrated for spherical particles, and provide number, area and volume distributions for spherical particles. However, these instruments cannot be directly used to obtain the surface area and volume distributions for aggregates. Aggregates are important in technological applications, such as the manufacture of fine powdered materials, and in air pollution and atmospheric sciences. Thus, nanoparticle chain aggregates of low fractal dimension are another important limiting case, in addition to spheres; a method is described which makes it possible to relate aggregate surface area and volume distributions to the electrical mobility diameter. This is accomplished by equating the migration velocity of an aggregate to that of a sphere. Particles of equal migration velocities will trace similar paths in the mobility analyzer and have the same mobility diameter (neglecting the Brownian diffusive spread). By equating the migration velocities of a sphere and aggregate, the number and size of the primary particles composing the aggregate can be related to the diameter of a sphere with the same migration velocity. The calculation of aggregate surface areas and volumes requires two theoretical “modules”, one for the drag on the aggregates and the other for aggregate charging efficiency. Two modules selected from the literature were used. The results indicate that the surface area distributions of aggregates with random orientation are somewhat over-predicted when calculated directly from the mobility diameter. However, the volume distributions are greatly over-predicted, up to a factor of ten compared with values based on the mobility diameter. The affect of aggregate orientation on surface area estimates was also examined.

Molecular dynamics simulations of the straining of nanoparticle chain aggregates: the caseof copper

Previous studies in our laboratory have shown that individual nanoparticle chainaggregates (NCAs) exhibit unusual mechanical behaviour when under strain inside thetransmission electron microscope. NCAs made of various materials (e.g. carbon, metaloxides and metals) were strained by as much as 100% under tension. The nanoparticlesthat compose the chains were 5–10 nm in diameter and the chains of the order of1 µm in length. Such aggregates are of technological importance in the manufacture ofnanocomposite materials (e.g. rubber), aggregate break-up (e.g. sampling diesel emissions)and chemical–mechanical planarization.The goal of this study was to simulate the mechanical behaviour of chain aggregates withmorphological properties similar to those of technological interest. Molecular dynamics(MD) and energy minimization computer simulations are employed to investigate, at theatomic scale, the behaviour of short nanoparticle aggregates under strain and toobtain quantitative information on the forces involved in aggregate straining andfracturing. The interaction potential used is that of copper obtained with theembedded atom method (EAM). Two seven-nanoparticle aggregates are studied, onelinear and the other kinked. The seven nanoparticles in both aggregates are singlecrystals and about 2.5 nm in diameter each. The aggregates are strained alongtheir longest dimension, to the breaking point, at strain rates spanning from2.5 × 107 to8.0 × 108 s−1 (MD simulations). The linear aggregate yield strain is about 0.1. The kinked aggregateelastic limit is also about 0.1, but only one-third of the stress develops along the strainingdirection compared to the linear aggregate. The kinked aggregate breaks at a strain ofabout 0.5, five times higher than the breaking strain of the linear aggregate. The ability ofthe kinked aggregate to straighten through combined nanoparticle interface sliding androtation accounts for the extra strain accommodation. Simulation strain ratesare orders of magnitude higher than the experimental ones. However, aggregatebehaviour is independent of strain rates over the range studied here. The MD andenergy minimization straining gave very similar results. In the elastic regime, the1/S11 modulus for the seven-nanoparticle kinked aggregate is about one-fifth of the bulkvalue. This is due to a combined effect of the small primary particle diameterand the aggregate kinked structure. If this softening behaviour also occurs fornanoparticle aggregates of other materials (e.g. carbon, silica), nanoparticle aggregates,in some cases, may be strained along with the nanocomposite they reinforce.

Complementary TEM and AFM Force Spectroscopy to Characterize the Nanomechanical Properties of Nanoparticle Chain Aggregates

Nanoparticle chain aggregates are used as reinforcing fillers in elastomers (rubber) and significantly enhance the mechanical properties (tensile strength, Young's modulus) of elastomers. Insight into mechanisms of the performance of nanoparticle chain aggregates can be obtained from force vs displacement (force spectroscopy) measurements made with an AFM tip and an aggregate-coated substrate. Distinctive sawtooth patterns were observed which were interpreted as aggregate stretching and breaking events. Based on the measurements, estimates of the aggregate nanomechanical properties were made.

A lattice chain model for the thermal restructuring of nanoparticle chain aggregates

Chain aggregates composed of nanoparticles of carbon black, silica, titania and other metal oxides are produced commercially using aerosol reactors. Properties of nanoparticle chain aggregates (NCA), such as restructuring when heated in gaseous suspension, may play an important role during synthesis. It is hypothesized that in some cases a separate step in NCA creation is the restructuring of the chains initially formed by rigid body addition processes, for example cluster–cluster aggregation. A model of a freely rotating bead chain (zero activation energy for particle rotation) is adapted here from the polymer literature to describe nanoparticle chain restructuring resulting from interaction with the surrounding gas. The link between the physical properties of the surrounding gas and the dynamics of the nanoparticle chain is provided through the diffusion coefficient of the chain center of mass. We have studied the restructuring dynamics for primary particles of various diameters, different chain lengths and at different temperatures, and the model predicts a fast transition from a non-equilibrium initial chain configuration to a relaxed state. For a stretched chain of 64 particles, each 35 nm in diameter, in air at 1800 K and atmospheric pressure, the characteristic time for relaxation was 0.34 ms. This time is consistent with a 12 ms upper limit in restructuring time for soot aggregates reported in the literature. It is possible that in practice the approach to equilibrium may be delayed by constraints on particle rotation.

Dynamics of Chain Aggregates of Carbon Nanoparticles in Isolation and in Polymer Films: Implications for Nanocomposite Materials

Composite materials composed of polymers blended with reinforcing fillers play a major role in various technologies. An important example, commercial rubber, incorporates carbon black fillers, which significantly enhance mechanical properties such as Young's modulus and ultimate strength. The carbon black fillers consist of nanoparticle chain aggregates (NCAs). Because the mechanisms behind the filler effects are poorly understood, we have studied the dynamics of NCAs composed of carbon particles that we synthesize by laser ablation. In situ stretching of the NCA was carried out in the transmission electron microscope (TEM), first in the absence of a supporting polymer matrix and then embedded in a polymer film. The samples studied were first deposited across a micron-sized slit mounted on a commercial specimen holder in the TEM. For an isolated NCA, after initial reorientation and unfolding of the chain, a large strain led to its elongation, plastic deformation, and breakage, followed by fast recoil of t...

A Molecular Dynamics Study of Short Nanoparticle Chains under Mechanical Strain: Linear and Kinked Configurations

AbstractPrevious studies in our laboratory have shown that individual nanoparticle chain aggregates (NCA) exhibit remarkable mechanical behavior when under strain inside the transmission electron microscope. NCA made of various materials (e.g. carbon, metal oxides, metals, etc.) were strained by as much as 100% when tension was applied to them. After breaking, the NCA rapidly contracted to form more compact structures. In this study, molecular dynamics (MD) computer simulations are employed to investigate, at the atomic scale, the behavior of short nanoparticle chains under strain and to obtain quantitative information of the forces involved in chain straining and fracturing. The interaction potential used is that of copper obtained with the embedded atom method (EAM). Although the methodology is generally applicable, copper was selected as a test material because reliable interatomic potentials are available. Seven single- crystal nanoparticles, each 2.452 nm in diameter, are placed in contact in two chain configurations, linear and kinked. The structures are initially relaxed adiabatically with MD steps for 225 ps, at a starting temperature of 300 K. The bonding energy between any two nanoparticles in contact ranges from about 20 eV to 30 eV at 0 K. The two relaxed chain configurations are strained along their longest dimension, to the breaking point, at strain rates spanning from 0.3 m/s to 10 m/s. We identify mechanisms of stress accommodation that lead to plastic deformation and eventually fracture for both chain configurations, linear and kinked, and we construct the corresponding stress-strain curves. The two chain configurations exhibit different mechanical behavior. Applications of our experimental and simulation studies on NCA are to the behavior of nanocomposite materials, including carbon black reinforced rubber, sampling of aggregates by high speed impactors and the formation of flexible coatings of nanoparticles.