In situ TEM observation of the nucleation and growth of silver oxide nanoparticles

The nucleation, growth, and coalescence of silver oxide nanoparticles have been investigated dynamically and at high spatial resolution by using the electron beam of a transmission electron microscope to stimulate and to observe the processes. Under the assumption the particles are hemispherical, the growth rate was found to be proportional to the square root of the electron irradiation time. This result suggests that the rate-limiting step is the attachment of atoms to the nanoparticles. Growth of the nanoparticles occurred by the addition of columns of atoms on {111} planes. Particle impingement resulted in interpenetration of the particles and, ultimately, the formation of a grain boundary.     

Coalescence of oxide nanoparticles: In situ HRTEM observation

The coalescence process of copper oxide particles of 3∼ ∼8 nm was studied by in-situ HRTEM observation with no external heating. We found that merging of two particles is much faster as compared to the reshaping process. The coalescence of pre-aligned nanoparticles leads to the formation of a single nanocrystal. The lattice mismatch between two merged misaligned particles can be released by structural relaxation in small nanoparticles and by interface movement in large nanoparticles. Surface atom migration is the leading mechanism during the whole coalescence process.     

Study of Au-Pd core-shell nanoparticles by using slow positron beam

Binary Au-Pd nanoparticles were synthesized by ultrasonic irradiation of solutions containing Au³⁺ and Pd²⁺ ions (the ion ratio from 0.3:0.7 to 0.9:0.1 mM) and cationic surfactant (SDS: sodium dodecyl sulfate). In each case the core-shell structure (Au core, Pd shell) was confirmed by scanning transmission electron microscopy (STEM). The mean diameters of them were all about 9 nm, and the thickness of the Pd shell depends on the ratio of Pd²⁺ and Au³⁺ ions in solution. In order to study the electronic states of core-shell nanoparticles and their dependence on shell thickness, Doppler broadening measurements were performed for Au-Pd core-shell nanoparticles by using slow positron beam technique. The ratio curves of Au-Pd particles did not match with those of pure Pd and pure Au, but a small difference in the low electron momentum region was observed among nanoparticles depending on Pd shell thickness.     

Assembly and interaction of Au/C core-shell nanoparticles

The formation of Au/C core-shell structures from C-supported Au nanoparticles, and their thermally and electron beam induced interactions are studied by real-time TEM. At temperatures below 400 °C no C-shell is assembled, and closely spaced Au nanoparticles interact by coalescence. At high temperatures (400-800 °C) the Au particles are transformed into Au/C core-shell structures via encapsulation into curved, fullerene-like C shells. The shells initially passivate the Au cores and inhibit their coalescence. But under electron irradiation, the Au cores can break free from their shells, and hence can coalesce. Surprisingly, at this stage the assembled C-sheets may actually enhance the coalescence process by driving the directed motion of Au/C particles and causing the efficient contraction of widely spaced particle ensembles.     

On the high temperature coalescence of metallic nanocrystals

The high temperature properties of metallic particles with a size of a few nanometers are very important in modern materials science. In the present work we report in situ transmission electron microscope studies of coalescence behavior of nanoparticles at high temperatures. At T > 700 K a new mechanism for coalescence was found. This phenomenon occurs on a time scale 3 to 20 times faster than the classical liquid-like coalescence reported by Pashley [Adv. Phys. 14 (1965) 327]. Before the coalescence the particles undergo shape convulsions of the type described by Iijima and Ichihashi [Phys. Rev. Lett. 56 (1986) 616] which has been termed “quasimelting”, then fast coalescence occurs. The newly formed particles also undergo convulsions until stabilized by the substrate. It is also shown that the electron beam plays a significant role on this process.     

A novel approach for preparing silver nanoparticles under electron beam irradiation

Silver (Ag) nanoparticles were obtained when Ag microparticles were exposed to an electron beam in a transmission electron microscope (TEM). Results from TEM characterization indicated that the morphologies of the prepared Ag nanoparticles were quasi-circular, and the sizes were mainly in the range of 5–60 nm. The effect of irradiation time (t) on size and distribution of Ag nanoparticles was investigated. It was found that the sizes of Ag nanoparticles increased with the increase of t. The bigger Ag nanoparticles were near the Ag microparticle and the smaller ones were far from it. In addition, these Ag nanoparticles were monodisperse. This approach offered a new route for preparing Ag nanoparticles under electron beam irradiation, and the forming process of Ag nanoparticles was explained by the nucleation-growth mechanism.     

Radiation ordering in quenched alloy sobserved “in situ” in the high voltage microscope

Different alloys with a face centered cubic disordered structure have been electron irradiated in the quenched or short range order state under direct observation in a high voltage electron microscope. Ordering due to 1 MeV irradiation has been observed in Au₄Mn, Ni₄Mo and Cu₃Pd. Care has been taken to avoid ordering due to the thermal effect of the electron beam. It has been demonstated that although similar states of order can be achieved by thermal and irradiation ordering, the path followed can be different however.     

Transmission electron microscopy of fine aromatic hydrocarbon particles

Fine perylene and pyrene particles were produced by evaporation in helium gas. The particles were sensitive to electron beam radiation. The pyrene particles sublimed under observation. These difficulties were cleared up by reducing electron beam exposure and sandwiching a specimen between two Formvar films. The fine perylene particles were rectangular or hexagonal plates about 50nm thick. The size was about 200-2000nm. The fine pyrene particles were polymorphic with a size of about 300-2000nm. The crystal forms of the perylene and pyrene particles were determined from the electron diffraction patterns to be alpha-perylene and pyrene I, respectively.     

Targeting EGFR-overexpressed A431 cells with EGF-labeled silica-coated magnetic nanoparticles

Electrode catalysts composed of carbon-supported PtRu nanoparticles (PtRu/C) for use as a direct methanol fuel cell anode were synthesized by the reduction of precursor ions in an aqueous solution via irradiation with a high-energy electron beam. The effect of pH control in the precursor solution on the PtRu mixing state and the methanol oxidation activity was studied in order to enhance the catalytic activity for methanol oxidation. The PtRu/C structures were characterized by transmission electron microscopy, inductively coupled plasma atomic emission spectrometry, X-ray fluorescence spectrometry, and X-ray diffraction and X-ray absorption fine structure techniques. The methanol oxidation activity was evaluated by linear sweep voltammetry. The initial pH of the precursor solution has little influence on the average grain size for the metal particles (approximately 3.5 nm) on the carbon particle supports, but the dispersibility of the metal particles, PtRu mixing state, and methanol oxidation activity differed. The maintenance of a low pH in the precursor solution gave the best dispersibility of the PtRu nanoparticles supported on the surface of the carbon particles, whereas, a high pH gave the best PtRu mixing state and the highest oxidation current although a low dispersibility of the PtRu nanoparticles supported on the surface of the carbon particles was obtained. The PtRu mixing state strongly correlated with the methanol oxidation current. In addition, a high pH was more effective for PtRu mixing when using an electron beam irradiation reduction method, because the complexation reaction of the chelating agents was improved, which resulted in an enhancement of the catalytic activity for methanol oxidation.     

Observation of Undamped 3D Brownian Motion of Nanoparticles Using Liquid-Cell Scanning Transmission Electron Microscopy

In theory, liquid-cell (scanning) transmission electron microscopy (LC(S)TEM) is the ideal method to measure 3D diffusion of nanoparticles (NPs) on a single particle level, beyond the capabilities of optical methods. However, particle diffusion experiments have been especially hard to explain in LC(S)TEM as the observed motion thus far has been slower than theoretical predictions by 3–8 orders of magnitude due to electron beam effects. Here, direct experimental evidence of undamped diffusion for two systems is shown; charge-neutral 77 nm gold nanoparticles in glycerol and negatively charged 350 nm titania particles in glycerol carbonate. The high viscosities of the used media and a low electron dose rate allow observation of Brownian motion that is not significantly altered by the electron beam. The resulting diffusion coefficient agrees excellently with a theoretical value assuming free diffusion. It is confirmed that the particles are also moving in the direction parallel to the electron beam by simulating STEM images using Monte Carlo simulations. Simulations and experiments show blurring of the particles when these move out of focus. These results make clear that direct observation of 3D diffusion of NPs is possible, which is of critical importance for the study of interparticle interactions or in situ colloidal self-assembly using LC(S)TEM.     

Engineering embedded metal nanoparticles with ion beam technology

In this paper, we summarize our recent results of study on how to engineer the embedded metal nanoparticles in silica by ion implantation and ion irradiation technologies, including controlling the size, distribution and morphology of nanoparticles. The optical properties of the tailored nanoparticle composites are studied. Thermal annealing, electron beam irradiation, and chemical erosion are used to study the stability of these embedded nanoparticles by ex situ or in situ transmission electron microscopy observation.     

In situ electron beam irradiated rapid growth of bismuth nanoparticles in bismuth-based glass dielectrics at room temperature

In this study, in situ control growth of bismuth nanoparticles (Bi⁰ NPs) was demonstrated in bismuth-based glass dielectrics under an electron beam (EB) irradiation at room temperature. The effects of EB irradiation were investigated in situ using transmission electron microscopy (TEM), selected-area electron diffraction and high-resolution transmission electron microscopy. The EB irradiation for 2–8 min enhanced the construction of bismuth nanoparticles with a rhombohedral structure and diameter of 4–9 nm. The average particle size was found to increase with the irradiation time. Bismuth metal has a melting point of 271 °C and this low melting temperature makes easy the progress of energy induced structural changes during in situ TEM observations. This is a very useful technique in nano-patterning for integrated optics and other applications.     

Non-aggregated Pd nanoparticles deposited onto catalytic supports

Nanostructured powders have shown great promise for a variety of applications including chemical gas sensors, high surface area supports for catalysis, tribology, chemical mechanical polishing, and optoelectronics. In this report, highly dispersed Pd nanoparticles with a narrow size distribution, and mean diameter of 2±0.2 nm, were deposited at room temperature onto amorphous carbon and oxide supports (TiO₂, Al₂O₃) by pulsed-laser ablation of a Pd sputtering target. Depositions were performed in Ar at a back-fill pressure of 3 mTorr after reaching a base pressure of 10^-7 Torr. Populations of uniformly dispersed particles with an interparticle spacing of 3 to 10 nm were observed by high-resolution transmission electron microscopy with little evidence of nanoparticle aggregation. The chemical compositions of individual nanoparticles were confirmed by high spatial resolution energy-dispersive X-ray spectroscopy.     

Photo-chemical synthesis and deposition of noble metal nanoparticles

Highly dispersed nanoparticles of transition and noble metals are utilized for hydrocarbon reactions and rearrangements important to the chemical industry. The need to obtain 1 to 3 nm particles with narrow size distributions has prompted the development of alternative processing methods. In this paper, a novel, dry method to synthesize nanoparticles from a frozen salt solution is reported. Pd nanoparticles were synthesized by photo-chemical decomposition of palladium acetate (PdAc) within a host matrix of chloroform using an excimer laser operating at 248 nm. Frozen composite targets were ablated at fluences ranging from 0.25 J/cm² to 0.75 J/cm² at a processing pressure of 10 mTorr. The ejected nanoparticles were deposited on continuous carbon coated and lacey carbon transmission electron microscopy (TEM) grids at ambient temperature. Characterization was performed by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDXS). High-resolution TEM analysis showed definitive evidence that the size distributions of the nanoparticles were narrow, exhibiting mean diameters ranging from 2.15 nm to 2.62 nm. PACS 81.15.Fg; 68.37.Lp; 81.07.Wx; 81.07.Bc; 81.10.Dn     

Direct and sensitized liquid phase photodeposition for the preparation of alumina supported Pd nanoparticles for applications to heterogeneous catalysis

Different photochemical approaches have been investigated to prepare alumina supported Pd nanoparticles to be used as heterogeneous catalysts. The employed techniques were: (i) direct photodeposition, (ii) sensitized photodeposition, and (iii) photodeposition in the presence of a protecting agent (polyvinylpirrolidone). The sensitized photodeposition with acetone resulted to be the most proper technique, allowing to obtain, in very short irradiation time, Pd particles homogeneously distributed over the support surface, with a very narrow and symmetrical monomodal distribution centered at 4–5 nm, found to be appropriate for the deep oxidation of methanol.     

Ion beam shaping of embedded metal nanoparticles by Si+ ion irradiation

Fine Co and Pt nanoparticles are nucleated when a silica sample is implanted with 400 keV Co⁺ and 1370 keV Pt⁺ ions. At the implanted range, Co and Pt react to form small Co _x Pt_(1?x) nanoparticles during Si⁺ ion irradiation at 300 °C. Thermal annealing of the pre-implanted silica substrate at 1000 °C results in the formation of spherical nanoparticles of various sizes. When irradiated with Si⁺ ions at 300 °C, particles in the size range of 5–17 nm undergo rod-like shape transformation with an elongation in the direction of the incident ion beam, while those particles in the size range of 17–26 nm turn into elliptical shape. Moreover, it is suspected that very big nanoparticles (size >26 nm) decrease in size, while small nanoparticles (size <5 nm) do not undergo any transformation. During Si⁺ ion irradiation, the crystalline nature of the nanoparticles is preserved. The results are discussed in the light of the thermal spike model.     

Study of size dependent features of swift heavy ion irradiation in nanosized zinc ferrite

In the present work zinc ferrite nanoparticles of different crystallite size were irradiated with 200 MeV Ag¹⁵⁺ ion beam. The structural and magnetic characterization performed for these samples indicate the presence of size dependent irradiation induced changes in the nanoparticles. The superparamgnetic nanoparticles do not alter their behavior after irradiation; however paramagnetic samples exhibit weak ferrimagnetism in the irradiated specimen. Results obtained from these measurements are in agreement with results obtained from the electron paramagnetic resonance spectroscopy.     

EXAFS study of size dependence of atomic structure in palladium nanoparticles

Dependence of atomic structure of Palladium nanoparticles on supports Al₂O₃ and SiO₂ upon their size, changed from 1.3 to 10.5 nm, was studied by Pd K-edge EXAFS. To determine the structure of the interior (core) and the near surface regions of nanoparticle, the fitting technique of the Fourier-transforms F(R) of spectra was used, which enabled to overcome instabilities of the obtained structural parameters values. The processing of experimental data was performed using results of the study of features formation in │F(R)│ of Pd K-EXAFS in Pd foil. By this approach it was revealed that the local structure of Pd atoms in the core is similar to fcc structure of bulk Pd, irrespective of size. The percentage of Pd atoms, which can be attributed to the core, upon the particles size was determined and the obtained dependence was described by the “cluster size equation”. In the near surface region of nanoparticles, nearest-neighbors Pd–Pd distances show a large Debye–Waller parameters and the mean bond length slightly contracted for nanoparticles of sizes less than ~2 nm. The effect of small structural distortions in the vicinity of absorbing Pd atom in the near surface region was studied using the cluster model of nanoparticle.     

Controlled synthesis of pompon-like self-assemblies of Pd nanoparticles under microwave irradiation

Pd nanoparticles with uniform, self-assembled pompon-like nanostructure were synthesized by thermal decomposition of palladium acetate under microwave irradiation with methyl isobutyl ketone (MIBK) as a solvent in the presence of a little amount of ethylene glycol (EG) and KOH without using any special stabilizers. The as-synthesized Pd nano-pompons were characterized by transmission electron microscopy, X-ray photoelectron spectroscopy and X-ray powder diffraction. The results show that the as-prepared Pd nano-pompons with the average diameters in the range of 28–81 nm were self-assemblies organized by hundreds of smaller primary nanoparticles with an average dimension of about 2.4 nm. The sizes of Pd nano-pompons can be well controlled by adjusting the concentration of palladium acetate. A little amount of EG and KOH also plays an important role in controlling the size, uniformity and dispersion of Pd nano-pompons. The Pd nano-pompons can be easily supported on γ-Al₂O₃ and their catalytic activity was examined preliminarily.