Control of catalyst particle crystallographic orientation in vertically aligned carbon nanofiber synthesis

Vertically aligned carbon nanofibers (VACNF) have been synthesized where the crystallographic orientation of the initial catalyst film was preserved in the nanoparticle that remained at the nanofiber tip after growth. A substantial percentage of catalyst particles (75%), amounting to approximately 200 million nanofibers over a 100 mm Si wafer substrate, exhibited a sixfold symmetry attributed to a cubic Ni(1 1 1)∥Si(0 0 1) orientation relationship which was verified by X-ray diffraction studies. The Ni catalyst films were prepared by rf-magnetron sputtering under substrate bias conditions to yield a single (1 1 1) film texture. The total energy of the Ni thin film was estimated by calculating the sum of the surface free energy and strain energy. The total film energy was minimized by the evolution of the plane of lowest surface free energy, the (1 1 1) texture. This result was in agreement with X-ray diffraction measurements. The preferred orientation present in the Ni catalyst film prior to nanofiber growth was preserved in the Ni catalyst particles throughout the VACNF growth process. The Ni catalyst particles at the nanofiber tips were not pure single crystals but rather consisted of a mosaic structure of Ni nanocrystallites embedded within Ni catalyst nanoparticles (200-400 nm). The tip-located nanoparticles exhibited a faceted, crystal morphology with the faceting transferred to the underlying carbon nanofiber during the growth process. The possibility of precisely and accurately controlling VACNF growth velocity over macroscopic wafer dimensions with uniformly aligned catalyst particles is discussed.     

Direct growth of MWCNTs on 316 stainless steel by chemical vapor deposition: Effect of surface nano-features on CNT growth and structure

Multi-walled carbon nanotubes were directly grown by chemical vapor deposition on as-received or pretreated 316 SS without application of an external catalyst. A detailed study of the size distribution of surface features formed by different steps of the synthesis process showed that the heating cycle and any complementary pretreatment may produce significant changes of the surface topography, thus suggesting that the influence of any primary characteristics of the original surface, as well as those caused by a pretreatment, should be assessed in conjunction with the effects of heating. Average lateral size of nano-features less than 60 nm (after heating) were shown to favor mainly the carbon nanotube growth while a larger features size was associated predominantly to the carbon nanofiber synthesis. Scanning and transmission electron microscopy observations suggest two different mechanisms for nanotube/nanofiber growth: (1) base growth mode caused by nanosized hills on the surface catalyzing the nanotube/nanofiber synthesis, (2) tip growth mode requiring substrate surface break-up as a preliminary step to form catalytic particles, with similarities to the “metal dusting” mechanisms. While untreated steel showed the best results concerning carbon nanotube coverage and homogeneity, oxidized-reduced samples showed an almost exclusive growth of carbon nanofibers with a full coverage.     

High performance of carbon nanowall supported Pt catalyst for methanol electro-oxidation

The performance of Pt catalyst supported on carbon nanowall (CNW) and vertically aligned carbon nanofiber (VACNF) for methanol electro-oxidation has been compared. Pt/CNW and Pt/VACNF electrodes were fabricated by growing CNW and VACNF on carbon papers with inductively coupled plasma enhanced chemical vapor deposition, followed by sputter deposition of Pt nanoparticles using a radio-frequency magnetron sputtering system. Scanning electron microscopy and transmission electron microscopy results show that the Pt nanoparticles are homogeneously dispersed on the surface of CNW and VACNF. The histograms of Pt nanoparticle diameter for both electrodes reveal that the Pt/CNW electrode shows a broader particle size distribution. Cyclic voltammetric measurements show that the Pt/CNW electrode has a better electrochemical activity and methanol oxidation property than Pt/VACNF electrode. The unique structure of CNW ensures that Pt/CNW electrode has a faster electron transport rate and shorter electron transport path, which lead to an obvious improvement of electro-catalysis activity compared to the Pt/VACNF electrode and show a further potential application in direct alcohol fuel cells.     

Structure of flattened carbon nanotubes

A large number of flattened multi-walled carbon nanotubes have been synthesized using chemical vapor deposition. They are often capped with elongated crystalline cobalt catalyst nanoparticles. Sample rotation studies inside the transmission electron microscope show that the catalyst nanoparticles have a cylindrical rather than a rectangular shape, indicating that the flattened structure of the nanotubes is not templated by the shape of the catalyst particles. Observations reveal the existence of nanotube flattening parallel to the long axis and a spiraling of fully flattened nanotube around the long axis of the nanotube. The flattening of the nanotubes is attributed to the pressure difference between the inside and outside of the nanotubes which are sealed by the catalyst nanoparticles during their growth. The flattening and the spiraling are ascribed to axial compression and torsion, respectively.     

Critical role of gas phase diffusion and high efficiency in vertically aligned carbon nanotube forest growth

Here the growth kinetics of vertically aligned carbon nanotube forests depend on the size of the patterned catalyst films from which they grow. Forests are grown using chemical vapor deposition on thin film catalyst islands patterned at the 100 μm scale on silicon wafers. The smaller the pattern, the faster the forest grows and the earlier it stops growing. Furthermore, the shape and structure of the forests, in particular the concavity of their top surface, also depend on the size of the catalyst islands. This result can be understood as a consequence of the high efficiency by which the acetylene source gas is converted into carbon nanotubes (here ∼30%) and a varying local amount of acetylene source gas available for growth. A diffusion model can explain the observed shape and structure of the forests and their growth kinetics by using experimentally measured parameters. This model also gives insight into the density and growth rate of carbon nanotube forests and suggests a mechanism that coordinates growth rates across the sample and, under certain conditions, can limit the fraction of catalyst nanoparticles that produce nanotubes.     

Effects of Crystalline Anisotropy and Indenter Size on Nanoindentation by Multiscale Simulation

Nanoindentation processes in single crystal Ag thin film under different crystallographic orientations and various indenter widths are simulated by the quasicontinuum method. The nanoindentation deformation processes under influences of crystalline anisotropy and indenter size are investigated about hardness, load distribution, critical load for first dislocation emission and strain energy under the indenter. The simulation results are compared with previous experimental results and Rice-Thomson (R-T) dislocation model solution. It is shown that entirely different dislocation activities are presented under the effect of crystalline anisotropy during nanoindentation. The sharp load drops in the load–displacement curves are caused by the different dislocation activities. Both crystalline anisotropy and indenter size are found to have distinct effect on hardness, contact stress distribution, critical load for first dislocation emission and strain energy under the indenter. The above quantities are decreased at the indenter into Ag thin film along the crystal orientation with more favorable slip directions that easy trigger slip systems; whereas those will increase at the indenter into Ag thin film along the crystal orientation with less or without favorable slip directions that hard trigger slip systems. The results are shown to be in good agreement with experimental results and R-T dislocation model solution.     

Investigating the mechanism of collective bidirectional growth of carbon nanofiber carpets on metallic substrates

Bidirectional-growth of carbon nanofibers is a rare phenomenon found on free-standing catalyst particles, in contrast to the most commonly studied tip- and base-growth mechanisms for carbon nanostructures synthesized through thermal chemical vapor deposition. We reveal the underlying mechanisms of collective bidirectional growth in Ni_xPd_1−x-catalyzed carbon nanofiber carpets grown on a palladium substrate with varying nickel film thickness by monitoring the fiber growth evolution. The results show that the collective bidirectional growth is promoted and controlled by the chemical and physical restructuring of the sub-surface portion of the metal stack which undergoes micro-fragmentation as a result of the incorporation, diffusion, and precipitation of carbon. Carbon nanofiber growth can be controlled by engineering the catalyst-underlayer materials properties such as grain size, chemical composition and alloying. Since the determining factor whether carbon nanofibers or nanotubes are obtained is a strong function of catalyst size, the understanding of this growth mechanism can be transferred to the field of carbon nanotube synthesis. By keeping the grain size small enough to ensure carbon nanotube instead of carbon nanofiber growth, achieving dense, vertically aligned carbon nanotube carpets on metallic substrates might be possible, which is a prerequisite for carbon nanotube integration in integrated circuits.     

ZnO Nanofiber and Nanoparticle Synthesized Through Electrospinning and Their Photocatalytic Activity Under Visible Light

In this paper, we fabricate ZnO nanofibers and nanoparticles through electrospinning precursor solution zinc acetate(ZnAc)/cellulose acetate(CA) in mixed-solvent N , N -dimethylformamide/acetone. Depending on the posttreatment of precursor ZnAc/CA composite nanofibers, both ZnO nanofibers and nanoparticles were synthesized after calcination of precursor nanofibers. The morphology and crystal structure of the ZnO nanofiber and nanoparticle were characterized by scanning electron microscopy, transmission electron microscopy, atomic force microscopy, and X-ray diffraction. It was found that the mean diameter of the ZnO nanofiber and nanoparticle was ca. 78 and 30 nm, respectively. The photo-degradation of dye molecules such as Rhodamine B and acid fuchsin catalyzed by the ZnO nanofiber and nanoparticle was evaluated under the irradiation of visible light. Both morphological ZnO species showed strong photocatalytic activity. However, the ZnO nanofiber in the form of nanofibrous mats showed much higher efficiency than the nanoparticle although the latter has a smaller size than the former. The porous structure of ZnO nanofibrous mats is believed to improve the contacting surface areas between the catalyst and the dye molecules, while the aggregation of ZnO nanoparticle in the solution lowers the photocatalytic efficiency.     

PREPARATION OF ZINC OXIDE NANOPARTICLE VIA UNIFORM PRECIPITATION METHOD AND ITS SURFACE MODIFICATION BY METHACRYLOXYPROPYLTRIMETHOXYSILANE

Zinc oxide nanoparticles were prepared by uniform precipitation using urea hydrolysis. The ZnO precursor was slowly deposited from aqueous solution. Anionic surfactant was added into solution to block ZnO crystal growth and its agglomeration. Then ZnO nanoparticles were synthesized by the calcination of the precursor at high temperature. Transmission electron microscope (TEM) observation and particle size analyzer demonstrated that the ZnO nanoparticle exhibited nearly spheric shape with 10-40 nm particle size. The surface of the ZnO nanoparticle was modified by methacryloxypropyltrimethoxysilane (MPS). FT-IR (Fourier transform-infrared spectrophotometry) and XPS (X-ray photoelectron spectrophotometry) revealed that MPS was grafted onto the zinc oxide nanoparticle. XRD (X-ray diffraction) showed that the ZnO nanoparticle was a hexagonal crystal with a perfect crystalline structure, and its crystalline morphology was not altered through surface modification. The activation index (AI) of the modified ZnO nanoparticle was measured. It was found that the surface of the ZnO nanoparticle was changed from hydrophilicity into hydrophobicity via surface modification, implying the enhancement of its compatibility with organic polymers. FE-SEM (field scanning electron microscopy) showed that the modified ZnO nanoparticles were homogeneously dispersed in PVC matrices. Consequently, ZnO nanoparticles were integrated with PVC matrices by the grafting organic molecule.     

3-Orders-of-magnitude density control of single-walled carbon nanotube networks by maximizing catalyst activation and dosing carbon supply

Tailoring the density of random single-walled carbon nanotube (SWCNT) networks is of paramount importance for various applications, yet it remains a major challenge due to the insufficient catalyst activation in most growth processes. Here we report on a simple and effective method to maximise the number of active catalyst nanoparticles using catalytic chemical vapor deposition (CCVD). By modulating short pulses of acetylene into a methane-based CCVD growth process, the density of SWCNTs is dramatically increased by up to three orders of magnitude without increasing the catalyst density and degrading the nanotube quality. In the framework of a vapor-liquid-solid model, we attribute the enhanced growth to the high dissociation rate of acetylene at high temperatures at the nucleation stage, which can be effective in both supersaturating the larger catalyst nanoparticles and overcoming the nanotube nucleation energy barrier of the smaller catalyst nanoparticles. These results are highly relevant to numerous applications of random SWCNT networks in next-generation energy, sensing and biomedical devices.     

PREPARATION OF ZINC OXIDE NANOPARTICLE VIA UNIFORM PRECIPITATION METHOD AND ITS SURFACE MODIFICATION BY METHACRYLOXYPROPYLTRIMETHOXYSILANE

Zinc oxide nanoparticles were prepared by uniform precipitation using urea hydrolysis. The ZnO precursor was slowly deposited from aqueous solution. Anionic surfactant was added into solution to block ZnO crystal growth and its agglomeration. Then ZnO nanoparticles were synthesized by the calcination of the precursor at high temperature. Transmission electron microscope (TEM) observation and particle size analyzer demonstrated that the ZnO nanoparticle exhibited nearly spheric shape with 10–40 nm particle size. The surface of the ZnO nanoparticle was modified by methacryloxypropyltrimethoxysilane (MPS). FT-IR (Fourier transform-infrared spectrophotometry) and XPS (X-ray photoelectron spectrophotometry) revealed that MPS was grafted onto the zinc oxide nanoparticle. XRD (X-ray diffraction) showed that the ZnO nanoparticle was a hexagonal crystal with a perfect crystalline structure, and its crystalline morphology was not altered through surface modification. The activation index (AI) of the modified ZnO nanoparticle was measured. It was found that the surface of the ZnO nanoparticle was changed from hydrophilicity into hydrophobicity via surface modification, implying the enhancement of its compatibility with organic polymers. FE-SEM (field scanning electron microscopy) showed that the modified ZnO nanoparticles were homogeneously dispersed in PVC matrices. Consequently, ZnO nanoparticles were integrated with PVC matrices by the grafting organic molecule.     

Growth and morphology manipulation of carbon nanostructures on porous supports

Structural polymorphism of carbon grown on porous supports by chemical vapor deposition (CVD) is demonstrated. Combining three different supports (activated carbon, macroporous polymeric beads, and microporous aluminophosphate molecular sieves), with two catalysts (Ni and Fe), yielded a variety of carbon nanostructures ranging from coils and belts to fibers and tubes, without changing CVD temperature, time, and precursor composition and flow rate. Ion exchange between ammonium-impregnated supports and the metals was necessary in order to achieve a fine dispersion of the catalyst over the support surface. Metal-support interactions and the balance between ammonium and metal concentrations were investigated and found to considerably affect catalyst dispersion, shape, and crystallographic orientation, which in turn determined the morphological and structural characteristics, and yield of the carbon product. The catalyst-loaded supports and the resulting carbon materials were characterized by scanning and transmission electron microscopy, thermogravimetic analysis, X-ray diffraction, and Raman spectroscopy, while nitrogen and mercury porosimetry were used to characterize the supports and evaluate the degree of support pore blocking by the carbon deposits.     

Microstructure and microscopic deposition mechanism of twist-shaped carbon nanocoils based on the observation of helical nanoparticles on the growth tips

Carbon nanocoils possessing a twisting form were prepared from an Fe-based alloy (SUS410) catalyst, the crystallographic properties of the catalyst surface, which was present on the growth tip, were examined, and a novel growth mechanism of the nanocoils is presented. Single-helix twisted carbon nanocoils with a coil diameter of 300-400 nm were obtained at 700-800 °C with a yield of 60% and a purity of nearly 100%. The catalyst structure observed on the growth tip was a Fe₅C₂ (monoclinic) or Fe₇C₃ (orthorhombic) single crystal, in which some of the Fe atoms were substituted by Cr. Carbon nanoparticles with a helical structure were observed on the surface of catalyst particle. It is considered that the carbon nanoparticles are a helical carbon supply source. That is, the catalyst particle supplies microscopic carbon nanoparticles by helical deposition patterns on the catalyst surface to form macroscopic helical patterns of carbon nanocoils.     

Surface Energy Anisotropy of SrTiO3 at 1400°C in Air

Geometric and crystallographic measurements of grain-boundary thermal grooves and surface faceting behavior as a function of orientation have been used to determine the surface energy anisotropy of SrTiO₃ at 1400°C in air. Under these conditions, thermal grooves are formed by surface diffusion. The surface energy anisotropy was determined using the capillarity vector reconstruction method under the assumption that Herring's local equilibrium condition holds at the groove root. The results indicate that the (100) surface has the minimum energy. For surfaces inclined between 0° and 30° from (100), the energy increases with the inclination angle. Orientations inclined by more than 30° from (100) are all about 10% higher in energy and, within experimental uncertainty, energetically equivalent. A procedure for estimating the uncertainties in the reconstructed energies is also introduced. Taken together, the orientation dependence of the surface-facet formation and the measured energy anisotropy lead to the conclusion that the equilibrium crystal shape is dominated by {100}, but also includes {110} and {111} facets. Complex planes within about 15° of {100} and 5° of {110} are also part of the equilibrium shape.     

Antibacterial activity of photocatalytic electrospun titania nanofiber mats and solution-blown soy protein nanofiber mats decorated with silver nanoparticles

Highly porous photocatalytic titania nanoparticle decorated nanofibers were fabricated by electrospinning nylon 6 nanofibers onto flexible substrates and electrospraying TiO₂ nanoparticles onto them. Film morphology and crystalline phase were measured by SEM and XRD. The titania films showed excellent photokilling capabilities against E. coli colonies and photodegradation of methylene blue under moderately weak UV exposure (≤ 0.6 mW/cm² on a 15-cm illumination distance). In addition, solution blowing was used to form soy protein-containing nanofibers which were decorated with silver nanoparticles. These nanofibers demonstrated significant antibacterial activity against E. coli colonies without exposure to UV light. The nano-textured materials developed in this work can find economically viable applications in water purification technology and in biotechnology. The two methods of nanofiber production employed in this work differ in their rate with electrospinning being much slower than the solution blowing. The electrospun nanofiber mats are denser than the solution-blown ones due to a smaller inter-fiber pore size. The antibacterial activity of the two materials produced (electrospun titania nanoparticle decorated nanofibers and silver-nanoparticle-decorated solution-blown nanofibers) are complimentary, as the materials can be effective with and without UV light, respectively.     

Catalytic soot oxidation in microscale experiments

The oxidation of soot agglomerates over catalytically active surfaces is of interest for the development of catalytic reactors for the control of soot emissions. The process involves the transport and deposition of nanoparticle aggregates to a surface on which catalyst particles are deposited. To simulate this process, graphitized carbon nanoparticles and platinum nanoparticles were separately deposited on an oxidized silicon wafer by laser ablation and electro hydro dynamic atomization. Changes in particle morphology produced by the reaction were visualized ex situ by scanning electron microscopy. In this way chemical reaction data could be correlated with the local surface coverage and particle size of the catalytically active material and the morphology of the reacting particles, resulting in detailed local information on their interaction, which is not available in studies on bulk samples. The contact between catalyst and soot particles was loose, simulating the behavior of catalyst systems used in practice. The activation energy of the oxidation in air was found to be 40 kJ/mol whereas the activation energy in air/NO was found to be 160 kJ/mol, both in presence of Pt deposited on a SiO₂ support. Notwithstanding the higher activation energy, the reaction rate of soot oxidation in air/NO is about two to three orders of magnitude higher than in air. A linear relationship between the relative Pt surface and reaction rate was found for the oxidation in an air/NO atmosphere. In air, the relationship has a minimum which indicates that there are different simultaneous mechanisms of reaction. Although activation energies are different from other studies, the oxidation temperatures are comparable. The EHDA and laser ablation produced platinum catalysts behave similarly and show potential to be used as model catalyst.     

CO Poisoning of Ethylene Hydrogenation over Pt Catalysts: A Comparison of Pt(111) Single Crystal and Pt Nanoparticle Activities

The ethylene hydrogenation reaction was studied on two platinum model catalyst systems in the presence of carbon monoxide to examine poisoning effects. The catalysts were a Pt(111) single crystal and lithographically fabricated platinum nanoparticles deposited on alumina. Gas chromatographic results for Pt(111) show that CO adsorption reduces the turnover rate from 10¹ to 10^-2 molecules/Pt site/s at 413 K, and the activation energy for hydrogenation on the poisoned surface becomes 20.2 ± 0.1 kcal/mol. The activation energy for ethylene hydrogenation over Pt(111) in the absence of CO is 10.8 kcal/mol. The Pt nanoparticle system shows the same rate for the reaction as over Pt(111) in the absence of CO. When CO is adsorbed on the Pt nanoparticle array, the rate of the reaction is reduced from 10² to 10⁰ nmol/s at 413 K. However, the activation energy remains largely unchanged. The Pt nanoparticles show an apparent activation energy for ethylene hydrogenation of 10.2 ± 0.2 kcal/mol in the absence of CO and 11.4 ± 0.6 kcal/mol on the CO-poisoned nanoparticle array. This is the first observation of a significant difference in catalytic behavior between Pt(111) and the Pt nanoparticle arrays. It is proposed that the active sites at the oxide--metal interface are responsible for the difference in activation energies for the hydrogenation reaction over the two model platinum catalysts.     

First-principles based kinetic modeling of effect of hydrogen on growth of carbon nanotubes

We investigate the influence of hydrogen on carbon nanotube (CNT) growth in thermal catalytic chemical vapor deposition. Kinetic calculations of gas-phase transformations of hydrocarbons show that hydrogen interacts with gaseous carbon precursors, resulting in modification of the carbon supply rate to the catalyst particle. A surface-kinetic model of CNT growth is developed to study adsorption and decomposition kinetics of precursors on Ni catalyst particles. The detailed surface kinetics of carbon precursors and transport of carbon atoms through the catalyst particle are described in the framework of the surface/bulk site formalism, with the parameters of the reactions determined on the basis of first-principles calculations for Ni (1 1 1) and (1 1 3) surfaces. Using this model, different regimes of CNT growth, with and without hydrogen in the system, are analyzed. Hydrogen is shown to enhance desorption of hydrocarbons, leading to a decrease of the surface coverage and effective carbon supply rate.     

Surface and interface properties of alumina via model studies of microdesigned interfaces

The ability to produce controlled-geometry, controlled-crystallography internal voids in ceramics has made possible several new model experiments for studying the high-temperature properties of surfaces and interfaces in ceramics. Recent advances have enabled the production of more complex microdesigned internal defect structures, and have exploited new means of examining them, thus, broadening the range of problems that can be addressed. A particular topic of concern is the effect of surface energy anisotropy on both the driving force for and the mechanism of shape changes. This paper reviews and previews recent research focussing on improving our understanding of surface diffusion in ceramics. Rayleigh instabilities provide one means of examining morphological evolution. The modelling of Rayleigh instabilities in materials with surface energy anisotropy is reviewed, and the results of experiments utilizing microdesigned pore arrays in sapphire are summarized. In a material with anisotropic surface energy and a facetted Wulff shape, the driving force for shape changes hinges on both the absolute and relative surface energies. Microdesigned pore structures have been used to determine the stable surfaces in both undoped and doped sapphire and to provide the relative values of the energies of these stable surfaces. Nonequilibrium shape, controlled-crystallography cavities have been introduced into undoped sapphire, and the effect of crystallographic orientation on their morphological evolution has been studied. Comparisons of the results with predictions of models of surface-diffusion-controlled evolution indicate that surface-attachment-limited kinetics (SALK) play an important role.     

The 13th International Symposium on Relations Between Homogeneous and Heterogeneous Catalysis—An Introduction

Over 40 years, there have been major efforts to aim at understanding the properties of surfaces, structure, composition, dynamics on the molecular level and at developing the surface science of heterogeneous and homogeneous catalysis. Since most catalysts (heterogeneous, enzyme and homogeneous) are nanoparticles, colloid synthesis methods were developed to produce monodispersed metal nanoparticles in the 1–10 nm range and controlled shapes to use them as new model catalyst systems in two-dimensional thin film form or deposited in mezoporous three-dimensional oxides. Studies of reaction selectivity in multipath reactions (hydrogenation of benzene, cyclohexene and crotonaldehyde) showed that reaction selectivity depends on both nanoparticle size and shape. The oxide-metal nanoparticle interface was found to be an important catalytic site because of the hot electron flow induced by exothermic reactions like carbon monoxide oxidation.