Growth and morphology of carbon nanostructures on nickel oxide nanoparticles in catalytic chemical vapor deposition

The present study explores the conditions favorable for the growth of cylindrical carbon nanostructures such as multi-walled carbon nanotube (MWCNT) and carbon nanofiber by catalytic chemical vapor deposition (CCVD) method using nickel oxide-based catalyst nanoparticles of different average sizes as well as different levels of doping by copper oxide. The role of doping and the average size have been related to the observed melting behavior of nanoparticles of nickel oxide by thermal and diffraction analysis, and the importance of melting has been highlighted in the context of growth of cylindrical nanostructures. In the reducing environment prevailing in the CCVD chamber due to decomposition of flowing acetylene gas at elevated temperature, there is extensive reduction of oxide nanoparticles. Lack of melting and faster flow of carbon-bearing gases favor the formation of a carbon deposit cover over the catalyst nanoparticles giving rise to the formation of nanobeads. Melting allows rapid diffusion of carbon from the surface to inside catalyst particles, and reduced flow of gas lowers the rate of carbon deposit, both creating conditions favorable for the formation of cylindrical nanostructures, which grows around the catalyst particles. Smaller particle size and lower doping favor growth of MWCNT, while growth of fiber is commonly observed on larger particles having relatively higher level of doping.     

Flame synthesis of 1-D complex metal oxide nanomaterials

One dimensional (1-D) complex metal oxide nanomaterials, such as ternary oxides, doped oxides, and hierarchical structures containing several oxides, not only benefit from large aspect ratios, but also offer exciting opportunities to design materials with desired properties by tuning their chemical compositions and tailoring their sizes and morphologies at the nanometer scale. Flame synthesis is an attractive method to grow 1-D complex metal oxide nanostructures because of its high temperature, scalability, low-cost and rapid growth rate. Here, we present three new combined flame synthesis methods: (1) simultaneous vapor–vapor growth, (2) simultaneous solid–vapor growth, and (3) sequential solid–vapor growth, to grow 1-D complex metal oxide nanostructures with well-defined compositions and morphologies. These three methods combine the previously reported flame vapor deposition and solid diffusion growth methods that were separately used to grow 1-D simple binary metal oxide nanostructures, and significantly advance the capabilities of existing flame synthesis methods for the growth of 1-D nanomaterials. The first method, simultaneous vapor–vapor growth, combines the flame vapor deposition growth of two different metal oxides by oxidizing and evaporating two different metal sources. With this we have successfully grown W-doped MoO₃ nanoplates and nanoflowers. In the second method, simultaneous solid–vapor growth, one precursor is again provided by oxidizing and evaporating metal oxide from a metal, while the other precursor diffuses out from a different growth substrate. With this we have successfully grown ternary Cu₃Mo₂O₉ nanowires. The third method, sequential solid–vapor growth, essentially uses the 1-D nanostructures firstly grown by solid diffusion as the substrates for subsequent flame vapor deposition. With this we have successfully grown hierarchical CuO/MoO₃ core/shell nanowires and MoO₃-branched CuO nanowires. We believe that these three new combined flame synthesis methods will provide a general platform for the synthesis of 1-D complex metal oxide nanostructures with tailored properties.     

A facile strategy for covalent binding of nanoparticles onto carbon nanotubes

This paper describes a simple strategy for covalently attaching nanoparticles onto the carbon nanotubes (CNTs) to fabricate hybrid nanostructure. Densely distributed magnetite nanoparticles (MNPs) with a size of 8 nm have been deposited on the surface of carbon nanotubes by covalent interaction. Transmission electron microscopy (TEM), FT-IR spectroscopy, and X-ray diffraction (XRD) analysis have been used to study the formation of MNP/CNT nanostructure. The strategy employed herein is quite generic and applicable to a variety of nanoparticles, including metal, quantum dot and oxide. These composite nanostructures should open up new possibilities in areas such as nanoelectronics, chemical sensing, field-emission displays, nanotribology, and cell adhesion/biorecognition investigations.     

Plasma-chemical synthesis of carbon and composite nanostructure materials in an electric arc

The possibilities of plasma-chemical synthesis of carbon and composite nanomaterials in a dc electric arc are demonstrated in experiments. Synthesis of tungsten carbide nanoparticles is implemented, along with the known processes of synthesis of carbon nanostructures and metal nanoparticles.     

Flame synthesis of tungsten oxide nanostructures on diverse substrates

One-dimensional (1D) tungsten oxide nanostructures show great potential for applications in the areas of batteries, photoelectrochemical water-splitting, electrochromic devices, catalysts and gas sensors. 1D tungsten oxide nanostructures are currently synthesized by physical or chemical vapor deposition, which are limited by low temperatures, the need for vacuum conditions, frequently expensive catalysts, and difficulty in scaling up for mass-production. These limitations, however, can be overcome by flame synthesis. Here, using a co-flow multi-element diffusion burner, we demonstrate the atmospheric, catalyst-free, rapid, mild and scalable flame synthesis of diverse, quasi-aligned, large density, and crystalline tungsten oxide nanostructures on a variety of substrates. Specifically, under fuel-rich conditions, monoclinic 1D W₁₈O₄₉ nanowires and nanotubes were grown on tungsten, iron, steel and fluorinated tin oxide (FTO) substrates, with controlled diameters ranging from 10 to 400 nm and axial growth rates ranging from 2 to 60 μm/h. Monoclinic 1D WO₃ nanowires and nanotubes were grown, instead, on silicon and silicon dioxide substrates. Under fuel-lean conditions, diverse WO₃ nanostructures, including monoclinic 1D nanowires, cubic 2D nanobelts and monoclinic 3D nanocones were grown on tungsten and FTO substrates. The success of this versatile flame synthesis method is attributed to the large tunability of several synthesis parameters, including the flame stoichiometry, the tungsten source and growth substrate temperatures, the tungsten oxide vapor concentration, and the material of the growth substrate. This flame synthesis method can be extended to synthesize other 1D transition metal oxides as well, enabling many large-scale electronic and energy conversion applications.     

Preparation and characterization of carbon nanotubes encapsulated GaN nanowires

A novel two-step catalytic reaction is developed to synthesize gallium nitride nanowires encapsulated inside carbon nanotubes (GaN@CNT). The nanowires are prepared from the reaction of gallium metal and ammonium using metals or metal alloys as a catalyst. After the formation of the nanowires, carbon nanotubes are subsequently grown along the nanowires by chemical vapor deposition of methane. The structural and optical properties of pure GaN nanowires and GaN@CNT are characterized using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, energy dispersive X-ray spectroscopy and Raman spectroscopy. The results show that GaN nanowires are indeed encapsulated inside carbon nanotubes. The field emission studies show that the turn-on field of GaN@CNT is higher than that of carbon nanotubes, but substantially lower than that of pure GaN nanowires. This work provides a wide route toward the preparation and applications of new one-dimensional semiconductor nanostructures.     

Aerosol formation of Sea-Urchin-like nanostructures of carbon nanotubes on bimetallic nanocomposite particles

With the advantage of continuous production of pure carbon nanotubes (CNTs), a new simple aerosol process for the formation of CNTs was developed. A combination of conventional spray pyrolysis and thermal chemical vapor deposition enabled the formation unusual sea-urchin-like carbon nanostructures composed of multi-walled CNTs and metal composite nanoparticles. The CNTs formed were relatively untangled and uniform with a diameter of less than ~10 nm. The key to the formation of CNTs in this way was to create a substrate particle containing both a catalytic and non-catalytic component, which prevented coking. The density of the CNTs grown on the spherical metal nanoparticles could be controlled by perturbing the density of the metal catalysts (Fe) in the host non-catalytic metal particle matrix (Al). Mobility size measurement was identified as a useful technique to real-time characterization of either the catalytic formation of thin carbon layer or CNTs on the surface of the metal aerosol. These materials have shown unique properties in enhancing the thermal conductivity of fluids. Other potential advantages are that the as-produced material can be manipulated easily without the concern of high mobility of conventional nanowires, and then subsequently released at the desired time in an unagglomerated state.     

Study of the morphological evolution of ZnO nanostructures on various sapphire substrates

Zinc oxide (ZnO) nanostructures were grown on A-, C- and R-plane sapphires by metal organic chemical vapor deposition (MOCVD) technique. The shape of nanostructures was greatly influenced by the underlying sapphire substrate. Vertical aligned nanowires were observed on A- and C-plane sapphires, whereas the nanopencils were grown on R-plane sapphire. A correlation between the morphological and optical properties of the nanostructures has been established, where the morphological and structural characteristics are responsible for the evolution of optical properties. The nanowires, grown on C-plane sapphires, have shown superior optical properties. Comparatively higher photo-induced wettability transition has also been observed for ZnO nanostructures on R-plane sapphire.     

Synthesis,assembly and physical properties of magnetic nanoparticles

Compared with the top-down lithographic techniques, bottom-up chemical synthesis and self-assembly approaches offer much more flexibilities in creating magnetic nanostructures with controlled size, shape, composition and physical properties. This review summarizes some of the latest developments in this field, with emphasis mainly on transition metals, their alloys and metal oxide nanoparticles. The focus is directed towards the conditions of individual particles as well as large assemblies of particles through colloidal chemistry. Furthermore, some of the future directions in nanomagnetism from the perspective of physical chemists is also presented.     

Growth of carbon nanotubes by Fe-catalyzed chemical vapor processes on silicon-based substrates

In this paper, a site-selective catalytic chemical vapor deposition synthesis of carbon nanotubes on silicon-based substrates has been developed in order to get horizontally oriented nanotubes for field effect transistors and other electronic devices. Properly micro-fabricated silicon oxide and polysilicon structures have been used as substrates. Iron nanoparticles have been obtained both from a thin Fe film evaporated by e-gun and from iron nitrate solutions accurately dispersed on the substrates. Single-walled nanotubes with diameters as small as 1 nm, bridging polysilicon and silicon dioxide “pillars”, have been grown. The morphology and structure of CNTs have been characterized by SEM, AFM and Raman spectroscopy.     

Investigation of electrical transport in hydrogenated multiwalled carbon nanotubes

Highly disordered multiwalled carbon nanotubes of large outer diameter (∼60 nm) fabricated by means of chemical vapor deposition process inside porous alumina templates exhibit ferromagnetism when annealed in a H₂/Ar atmosphere. In the presence of an applied magnetic field, there is a transition from positive to negative magnetoresistance. The transition may be explained in terms of the Bright model for ordered and disordered carbon structures. Additionally, temperature dependent electrical transport experiments exhibit a zero-bias anomaly at low temperature.     

Iron Precursor Decomposition in the Magnesium Combustion Flame: A New Approach for the Synthesis of Particulate Metal Oxide Nanocomposites

Powders of Fe–Mg–O nanocomposite particles have been grown using a novel chemical vapor synthesis approach that employs the decomposition of a metalorganic precursor inside the metal combustion flame. After annealing in controlled gas atmospheres composition distribution functions, structure and phase stability of the obtained magnesiowüstite nanoparticles are measured with a combination of techniques such as inductively coupled plasma‐optical emission spectroscopy, energy dispersive X‐ray spectroscopy, X‐ray diffraction, and scanning and transmission electron microscopy. Complementary Mössbauer spectroscopy measurements reveal that depending on Fe loading and temperature of annealing either metastable and superparamagnetic solid solutions of Fe³⁺ ions in periclase (MgO) or phase separated mixtures of MgO and ferrimagnetic magnesioferrite (MgFe₂O₄) nanoparticles can be obtained. The described combustion technique represents a novel concept for the production of mixed metal oxide nanoparticles. Adressing the impact of selected annealing protocols, this study underlines the great potential of vapor phase grown non‐equilibrium solids, where thermal processing provides means to trigger phase separation and, concomitantly, the emergence of new magnetic properties.     

Carbon-coated SnO2 nanobelts and nanoparticles by single catalytic step

Several types of carbon nanostructures (amorphous and graphitic), for the coating of SnO₂ nanobelts and nanoparticles were obtained by a single catalytic process, during methane, natural gas, and methanol decomposition using the reactivity of surface-modified SnO₂ nanostructure as a nanotemplate. The nanostructured catalyst templates were based on transition metal nanoparticles supported on SnO₂ nanobelts previously prepared by a carbothermal reduction process. Carbon-coated SnO₂ nanopowders were also successfully synthesized for the fabrication of carbon spheres. The carbon coating process and yield, along with the nature of the nanostructured carbon, are strongly influenced by the chemically modified surface of the SnO₂ nanostructure template and the chemical reaction gas composition. The preliminary catalytic activity and gas-sensing properties of these novel materials based on metal nanoparticles and carbon-coated SnO₂ were determined.     

Morphology,chemical composition,and electrical characteristics of hybrid (Ni-C) nanocomposite structures grown on the van der Waals GaSe(0001) surface

The morphology and chemical composition of metal (Ni), carbon, and composite (Ni-C) nanostructures grown on oxidized and unoxidized (0001) surfaces of a layered GaSe crystal by electron beam vacuum evaporation of the material from a liquid ion source in an electric field have been investigated using atomic force microscopy and X-ray photoelectron spectroscopy. It has been demonstrated that this technology makes it possible to grow nanostructures with different morphologies depending on the growth mode and substrate surface state. Dense homogeneous arrays of nickel nanoparticles (Ni@C) (with geometrical sizes of ～1–15 nm and a lateral density of higher than 10¹⁰ cm^?2) encapsulated into carbon shells, as well as carbon layers (with a thickness of the order of several nanometers), are grown on the unoxidized van der Waals GaSe(0001) surface, whereas Ni-C composite nanostructures are grown on the oxidized surface. The formation of oxide nanostructures on the van der Waals surface and their chemical composition have been examined. Vertical hybrid Au/Ni/(Ni-C)/n-Ga₂O₃(Ni@C)/p-GaSe structures grown on the GaSe(0001) surface contain Ni@C nanoparticles embedded in the wide-band-gap n-Ga₂O₃ oxide. The current-voltage characteristics of these structures at temperatures close to T = 300 K exhibit specific features of the Coulomb blockade effect.     

Preparation and characterization of nickel nanoparticles in different carbon matrices

Using the solid-phase pyrolysis and chemical vapor deposition of nickel-phthalocyanine, we have fabricated ferromagnetic Ni nanoparticles in carbon matrices. The composition, structure, morphology, and magnetic properties of samples were investigated by means of scanning electron microscopy, energy dispersive X-ray microanalysis, X-ray diffraction technique, and ferromagnetic resonance. It is shown that the sizes of nanoparticles can be varied from ∼10 nm to ∼500 nm depending on the temperature and time of pyrolysis. The used method allows us to synthesize metal nanoparticles in different carbon matrices: in amorphous carbon plates, in graphitic capsules, and in carbon nanotubes.     

Synthesis of high nitrogen doping of carbon nanotubes and modeling the stabilization of filled DAATO@CNTs (10,10) for nanoenergetic materials

In this work, we have performed synthesis of nitrogen-doped carbon nanotubes using chemical vapor deposition method. Morphology, structure and composition of the carbon nanotubes (CNTs), as well as concentration and distribution of nitrogen inside CNTs are characterized by scanning electron microscopy, transmission electron microscopy, X-ray dispersive spectroscopy and X-ray photoelectron spectroscopy techniques. A bamboo-like structure of the nitrogen-doped CNTs has been observed. Temperature dependency on the synthesis of nitrogen-doped carbon nanotubes has been investigated and discussed. Diameter and growth rate of these hybrid materials are obviously temperature dependent. Nitrogen concentration inside the CNTs increases with declining synthesis temperature. Nitrogen-doped CNTs with nitrogen content up to 10.4 at% can be achieved at a low temperature of 800 ^oC. Synthesis of the high nitrogen CNTs proposes a feasible way to develop novel nanoenergetic materials. Besides the experimental study, we have carried out Density Functional Theory calculations on five energetic molecules named n-oxides of 3,3′-azo-bis(6-amino-1,2,4,5-tetrazine) (DAATO), where n=1-5 refer to oxygen atoms, encapsulated in CNTs (10,10), in order to investigate the chemical stabilization of filled DAATO_n inside CNTs (10,10). In fact, the predicted adsorption energy values confirmed the chemical stability of the hybrid systems DAATO_n@CNTs (10,10) under normal conditions.     

PVP Assisted Shape-Controlled Synthesis of Self-Assembled 1D ZnO and 3D CuO Nanostructures

Self-assembled one-dimensional (1D) zinc oxide (ZnO) rods and three-dimensional (3D) cupric oxide (CuO) cubes like nanostructures with a mean crystallite size of approximately 33 and 32 nm were synthesized through chemical route in the presence of polyvinylpyrrolidone (PVP) under mild synthesis conditions. The technique used for the synthesis of nanoparticles seems to be an efficient, inexpensive and easy method. X-Ray diffraction patterns confirmed well crystallinity and phase purity of the as prepared samples, followed by the compositional investigation using Fourier Transform Infrared (FT-IR) spectroscopy. The formation of ZnO nanorods and CuO nanocubes like structures were through Scanning Electron Microscopy (SEM) images. The mechanism and the formation factors of the self-assembly were discussed in detail. It was clearly observed from results that the concentration of precursors and PVP were important factors in the synthesis of self-assembly ZnO and CuO nanostructures. These self-assembly nanostructures maybe used as novel materials in various potential applications.     

Epitaxial growth of graphene on transition metal surfaces: chemical vapor deposition versus liquid phase deposition

The epitaxial growth of graphene on transition metal surfaces by ex situ deposition of liquid precursors (LPD, liquid phase deposition) is compared to the standard method of chemical vapor deposition (CVD). The performance of LPD strongly depends on the particular transition metal surface. For Pt(111), Ir(111) and Rh(111), the formation of a graphene monolayer is hardly affected by the way the precursor is provided. In the case of Ni(111), the growth of graphene strongly depends on the applied synthesis method. For CVD of propene on Ni(111), a 1 × 1 structure as expected from the vanishing lattice mismatch is observed. However, in spite of the nearly perfect lattice match, a multi-domain structure with 1 × 1 and two additional rotated domains is obtained when an oxygen-containing precursor (acetone) is provided ex situ.     

Preparation of nanosized metal (oxides) by gas phase hydrolysis using mesoporous materials as nanoreactors

Immobilized nanosized metal (oxides) on carbonaceous carriers were prepared by hydrolysis under mild conditions by using the carrier pores as a kind of nanoreactor. Metal alkoxide vapor was adsorbed on the carrier and then formed the product upon exposure to water vapor. With this facile method, Titania, Vanadia, Rhodium (oxide), and Platinum (oxide) nanostructures were prepared at high yields, high loadings, and good dispersion in the carrier material. High number concentrations of spheroidal nanoparticles of uniform size (diameter ca. 5 nm) were obtained from less reactive precursors, whereas with highly reactive precursors, such nanoparticles occurred only after subsequent calcination. Nanoparticles appeared to be the thermodynamically stable form of the metal (oxide) produced in the pores. Highly reactive precursors formed metastable seeds, which nucleated and restructured into nanoparticles upon subsequent exposure to heat. The presented method allows for preparation of metal (oxide) nanostructures and effective control of their size and shape.     

PECVD of Carbon Nanostructures in Hydrocarbon‐Based RF Plasmas

Different aspects of the plasma‐enhanced chemical vapor deposition of various carbon nanostructures in the ionized gas phase of high‐density, low‐temperature reactive plasmas of Ar+H₂+CH₄ gas mixtures are studied. The growth techniques, surface morphologies, densities and fluxes of major reactive species in the discharge, and effects of the transport of the plasma‐grown nanoparticles through the near‐substrate plasma sheath are examined. Possible growth precursors of the carbon nanostructures are also discussed. In particular, the experimental and numerical results indicate that it is likely that the aligned carbon nanotip structures are predominantly grown by the molecular and radical units, whereas the plasma‐grown nanoparticles are crucial components of polymorphous carbon films. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)