In-situ transmission electron microscopy observation of the helium bubble evolution in pre-irradiated fluorapatite during annealing

The polycrystalline fluorapatite Ca₁₀(PO₄)₆F₂ ceramic synthesized by a standard solid-state sintering method was pre-irradiated with 80 keV He⁺ ions to a fluence of 5 × 10¹⁶ ions/cm² at room temperature. After that, an in-situ annealing experiment was performed inside a transmission electron microscope to monitor the evolution of helium bubbles during heating to 723 and 823 K. Initially, no helium bubble formation was observed in the damage layers of the pre-irradiated samples. However, as the temperature increased, helium bubbles first became visible and then began coarsening, ultimately reaching an asymptotic radius during annealing. The migration and coalescence of helium bubbles in the fluorapatite matrix was complete at a temperature of 823 K, and its likely mechanism involved the existence of two different types of coalescing bubbles.     

Platinum catalysts supported on graphitized carbon black—II: Characterization of the platinum by Small Angle X-ray scattering and transmission electron microscopy

Particle sizes of platinum dispersed on a graphitized carbon black (Vulcan 3G) and samples resulting from its prior oxidation in air at 798 K have been measured by Small Angle X-ray Scattering (SAXS) and Transmission Electron Microscopy (TEM). Mean diameters of platinum particles decrease with increasing burn-off of the carbonaceous supports. The values of mean diameters as determined by these physical methods agree well with those estimated from gas titration procedures. Size distributions of the platinum particles are also described in terms of log-normal functions.     

The conversion of methane with silica-supported platinum catalysts: the effect of catalyst preparation method and platinum particle size

The particle size distribution of platinum in silica prepared by the complexing agent-assisted sol–gel method and impregnation method and also in MCM-41 has been compared and its influence on the product distribution in the non-oxidative dehydrogenation of methane has been investigated. The sol–gel method gives a narrow range of platinum particle size distribution compared to the impregnation method. It was found that as the particle size increases, the selectivity for the higher hydrocarbons increases though the yield decreases.     

The effect of particle size on nitric oxide decomposition and reaction with carbon monoxide on palladium catalysts

The effect of palladium particle size on its catalytic activity was investigated by the decomposition of chemisorbed nitric oxide and the reaction of nitric oxide with carbon monoxide in flow conditions. Palladium particles (30–500 Å) were prepared on silica thin films (100 Å) which were supported on a Mo(110) surface. The reactivity of the supported palladium varied with the metal particle size. On large palladium particles, nitric oxide (NO) reacts to form nitrous oxide (N₂O), dinitrogen (N₂) and atomic oxygen during temperature-programmed reaction, whereas on small particles (< 50 Å), nitrous oxide is not formed. Similarly, reactions of NO with CO on large particles, in flow conditions produce N₂O, N₂ and CO₂, whereas N₂O is not produced on small particles. In addition, more extensive NO decomposition is observed on the smaller particles.     

The use of transmission electron microscopy tomography to correlate copper catalyst particle morphology with carbon fiber morphology

In this study carbon fibers of various morphologies have been synthesized by the decomposition of acetylene over copper supported catalysts (Cu(NO₃)₂/TiO₂, at 250 °C). In order to investigate the relationship between the morphologies of the copper catalyst particle and that of the carbon fiber extruded, tilting procedures (±60 A axis and ±30 B axis) under TEM (120 kV) were performed. In this way a 3-D (rather than a 2-D) image of the Cu crystallites were obtained which permitted for the 3-D shape of the Cu catalyst particle to be linked to the morphology of the extruded carbon fiber. Whereby copper particles that give distorted decahedra are apt to form helical fibers, trigonal bi-pyramidal particles give linear fibers. Various plate-like particles were also investigated, and it was observed that as the number of sides varied (3, 4, 5 or 6) there was a corresponding change in the carbon fiber helicity. Herein we report on our findings that reveal the geometric structures involved in controlling the shape of morphologically diverse carbon coils, illustrating the influential role catalyst particle shape plays in carbon fiber synthesis.     

Impact of sonication pretreatment on carbon nanotubes: A transmission electron microscopy study

Sonication treatments are commonly used for debundling and dispersing carbon nanotubes (CNTs) in liquid media prior to chemical functionalization. However, this step may lead to the stripping of the outer graphitic layers and the scission of the CNTs, and can therefore have a deleterious effect on the achievable properties of the functionalized CNTs. Thus, knowledge on the structural integrity of the modified CNTs is required to understand its influence on the device performance of hybrid nanocarbon-based composites. Here we report on the impact of a sonication pretreatment on the structure of multiwalled CNTs, and on the role of the induced modifications on the subsequent attachment of ferrimagnetic Fe₃O₄ nanoparticles. Decoration of the CNTs with Fe₃O₄ nanoparticles is achieved by a microwave-assisted synthesis route involving the reaction of iron acetylacetonate with 2-pyrrolidinone. Employing a combination of atomic resolution transmission electron microscopy, electron energy-loss spectroscopy, energy-filtered transmission electron microscopy and electron tomography, we provide evidence that significant degradation of the CNT structure takes place during the dispersion process. Moreover we find that the sp² system is more heavily disrupted at the interface between the CNTs and the surface-deposited nanoparticles suggesting that nucleation of Fe₃O₄ preferentially occurs at the nanotube defect sites.     

Temperature effect on the formation of catalysts for growth of carbon nanofibers

Turbostratic carbon nanofibers (CNFs), platelet graphite nanofibers (PGNFs) and tubular graphite nanofibers (TGNFs, also called multi-walled carbon nanotubes) were synthesized using thermal decomposition from a mixture of poly(ethylene glycol) and NiCl₂. A detailed study found that the synthesis temperature dramatically affected the morphology and topography of the catalysts, which play an important role in the synthesis of the various CNFs. At the temperature of 600 °C, irregular shape nanocatalysts with very rough surfaces were formed for the synthesis of turbostratic CNFs. Cubic-like nanocatalysts were formed at 750 °C for PGNFs and truncated cone-like nanocatalysts were formed at 850 °C for TGNFs. The surface roughness and the shape of the catalysts determined the stacking order of the graphene layers so that different types of CNF were formed. The growth direction of the graphene layers was from the Ni(1 1 1) plane for PGNFs and from the Ni(1 1 0) plane for TGNFs. Characterizations and field emission properties of these materials were also studied and compared.     

Direct observation of ferroelectric domain formation by environmental scanning electron microscopy

The evolution of the ferroelectric domain pattern in donor-doped barium calcium titanate during heating and cooling across the Curie transition temperature was imaged by orientation contrast imaging using an environmental scanning electron microscope (ESEM) equipped with a heating stage. The vanishing of the ferroelectric domains above the transition temperature as well as their recurrence during cooling was observed. The domain pattern of the sample largely remained the same, confirming that domain orientation is determined by elastic energy conditions (clamping), which scarcely change within the investigated temperature range. In a few cases a change in the domain pattern of single grains was observed during heating and cooling.     

Influence of Pt particle size on catalytic combustion of xylenes on carbon aerogel-supported Pt catalysts

Pt catalysts supported on a carbon aerogel with different Pt particle sizes were studied in the combustion of o-xylene and m-xylene. Results found show that the activity of the catalysts increased with larger Pt particle size. In addition, the catalysts were activated during consecutive combustion runs and during time on stream. This activation depends on the Pt particle size and type of xylene isomer. Activation was due to the increase in Pt particle size during reaction. The lower activity of catalysts with smaller Pt particle size was due to the stronger PtO bonds formed during xylene combustion by the smaller Pt particles.     

Precise estimation of HPHT nanodiamond size distribution based on transmission electron microscopy image analysis

Fluorescent nanodiamonds (NDs) are extremely promising fluorescent probes and sensors in experimental biology. For these applications, NDs obtained from high-pressure, high-temperature (HPHT) synthesis are used. Commercial HPHT NDs are polydisperse and of irregular shape. In this work, we used image analysis of transmission electron microscopy (TEM) micrographs to determine the actual size distribution of an HPHT ND sample with high precision. We developed novel TEM sample preparation procedure, which includes hydrophilization of a carbon-coated TEM grid by UV/ozone treatment and subsequent charge reversal and charge-controlled adsorption of nanoparticles. This method results in NDs spread evenly on the grid and radically improves the quality of TEM samples. Image analysis of TEM micrographs of commercial NDs revealed an unexpectedly high abundance of sub-10 nm NDs in the sample. This method can also be used for preparing TEM samples of other negatively charged nanoparticles, such as citrate-stabilized gold.     

Clustering of platinum atoms into nanoscale particle and network on NaY zeolite

Platinum has been supported on NaY zeolite by ion exchange of Pt(NH₃)₄ ²⁺. Clustering of Pt atoms into a 1 nm cluster, and three-dimensional quantum-size wire (3 nm thick) network (greater than 100 nm) from reduction of the Pt species has been investigated by extended X-ray absorption fine structure transmission electron microscopy and xenon adsorption measurements.     

The effect of SiO_2 particle size on iron based F–T synthesis catalysts

The effect of particle size of silica, as catalyst binder, on the chemical and mechanical properties of iron based FT catalyst was studied in this work. The samples were characterized using XRD, BET, TEM, FT-IR, and H2-TPR, respectively. The attrition resistance and the FT activity were tested. Si-8–Si-15 catalysts prepared with 8–15 nm silica sol show good attrition resistance(attrition loss b 4%), especially Si-13 with an attrition loss of 1.89%. Hematite appeared in XRD patterns when silica sol above 15 nm is used. TEM micrographs show that no obvious Si O_2 particles appear when silica sol particle with size less than 8 nm was used, but Si O_2 particles coated with small ferrihydrite particles appear when silica sol above 8 nm was used. Si–O–Si vibration peak in FT-IR spectra increases with increasing silica sol size. Samples prepared with silica sol show good stability of FT reactions, and the average molecular weight of FT products increases with the increase of Si O_2 particle.     

Distribution of carbon black in semicrystalline polypropylene studied by transmission electron microscopy

Polypropylene (PP) samples filled with different carbon blacks (CB) were prepared by the conventional melt‐mixing method. The distribution of the CB in the PP matrix was investigated by using the RuO₄ vapor‐staining technique and transmission electron microscopy (TEM). Our results indicate that amorphous regions of PP are sandwiched between the crystalline lamellae. The thickness of individual amorphous layers is a few nanometers. The amorphous region was measured to be about 45 wt % using a differential scanning calorimeter (DSC). The thickness of the lamellae is much smaller than the dimensions of the CB particles, and the CB particles or aggregates are dispersed inside the spherulites. There is a thin layer of amorphous PP encapsulating the CB particles and aggregates. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 358–365, 2002     

High resolution transmission electron microscopy study on polyacrylonitrile/carbon nanotube based carbon fibers and the effect of structure development on the thermal and electrical conductivities

Polyacrylonitrile (PAN) and PAN/carbon nanotube (CNT) based carbon fibers at various CNT content have been processed and their structural development was investigated using high resolution transmission electron microscope (HR-TEM). In CNT containing carbon fibers, the CNTs act as templating agents for the graphitic carbon structure development in their vicinity at the carbonization temperature of 1450 °C, which is far below the graphitization temperature of PAN based carbon fiber (>2200 °C). The addition of 1 wt% CNT in the gel spun precursor fiber results in carbon fibers with a 68% higher thermal conductivity when compared to the control gel spun PAN based carbon fiber, and a 103% and 146% increase over commercially available IM7 and T300 carbon fibers, respectively. The electrical conductivity of the gel spun PAN/CNT based carbon fibers also showed improvement over the investigated commercially available carbon fibers. Increases in thermal and electrical conductivities are attributed to the formation of the highly ordered graphitic structure observed in the HR-TEM images. Direct observation of the graphitic structure, along with improved transport properties in the PAN/CNT based carbon fiber suggest new applications for these materials.     

The preparation of silica supported Pd catalysts: the effect of pretreatment variables on particle size

The effect of pretreatment on the dispersion of Pd catalysts supported on silica has been studied. The catalysts were prepared from [Pd(NH₃)₄] (NO₃)₂ as the metal precursor at a pH = 9. The resulting catalysts were characterized using both CO and H₂ chemisorption, transmission electron microscopy, and in-situ UV reflectance spectroscopy. Pretreatment in H₂ resulted in poor Pd dispersions while pretreatment in He or Ar resulted in very high dispersions. Pretreatment in O₂ resulted in moderate dispersions. The results are explained by considering the chemical structure of the adsorbed surface complex under different pretreatment conditions. The chemistry of the decomposition process is considered in detail.     

High resolution,in-situ controlled atmosphere transmission electron microscopy (CATEM) of heterogeneous catalysts

A catalyst has been imaged with lattice resolution whilst surrounded by flowing gas, using a specially constructed in-situ cell for an electron microscope. This advancement demonstrates the feasibility of directly studying atomic rearrangements in catalysts and catalyst supports under reaction conditions by electron microscopy. Such studies show promise of providing a detailed understanding of the mechanisms involved in certain types of heterogeneous catalyst.     

Effect of platinum particle size and catalyst support on the platinum catalyzed selective oxidation of carbohydrates

The effect of the platinum particle diameter and the catalyst support was investigated for the aqueous phase selective oxidation of methyl α-D-glucopyranoside, octyl α-D-glucopyranoside and α-cyclodextrin with molecular oxygen. No platinum particle size effect was observed in the platinum particle diameter range of 1.4 to 3.0 nm. The structure-insensitivity is attributed to the high degree of surface coverage by oxygen. The rate of deactivation due to over-oxidation of the catalyst increases to a small extent with decreasing platinum particle diameter. The effect of the catalyst support is much larger than the platinum particle size effect. The turnover frequency for platinum on activated carbon and carbon fibrils supported catalyst is 3-7 times higher than for graphite as the catalyst support. The support effect is attributed to the hydrophilic character of the support. This revised version was published online in July 2006 with corrections to the Cover Date.