Formation of well-packed TiO<Subscript>2</Subscript> nanoparticles on multiwall carbon nanotubes using CVD method to fabricate high sensitive gas sensors

Titanium oxide nanoparticles were coated on multiwall carbon nanotubes (MWCNTs) using an atmospheric pressure chemical vapor deposition (CVD) to achieve highly compact nanoparticles of about 5 nm on CNT structure. The CNTs with a diameter of about 50 nm were grown by plasma enhanced CVD. Gas sensitivity of the fabricated structure was investigated and compared with TiO₂/CNT composite-based gas sensors. The effect of the structural interaction between the nanoparticles and the CNT wall on sensing mechanism of the as-prepared gas sensors was investigated. Ultrasensitive gas sensors were obtained by TiO₂/CNT nanostructures with strong interaction between the MWCNT and the TiO₂ nanoparticles. The measurements show high chemical activity and exceptional electrical response of the as-prepared structure being exposed to gases. Scanning and transmission electron microscopy and X-ray diffraction analysis were used to obtain structural information.     

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.     

Modelling the effects of nitrogen doping on the carbon nanofiber growth via catalytic plasma‐enhanced chemical vapour deposition process

An analytical model is developed to describe the effects of nitrogen doping on the growth of the carbon nanofibers (CNFs) and to elucidate the growth mechanism of nitrogen‐contained carbon nanofibers (N‐CNFs) on the catalyst substrate surface through the plasma‐enhanced chemical vapour deposition (PECVD) process. The analytical model accounts for the charging of CNFs, kinetics of all plasma species (electrons, ions, and neutrals) in the reactive plasma, generation of carbon species on the catalyst nanoparticle surface due to dissociation of hydrocarbons, CNF growth due to diffusion and precipitation of carbon species, and various other processes. First‐order differential equations have been solved for glow discharge plasma parameters for undoped CNFs (CNF growth in C₂H₂/H₂ plasma) and nitrogen‐doped CNFs (N‐CNF growth in C₂H₂/NH₃ plasma). Our investigation found that nitrogen‐doped CNFs exhibit lower tip diameters and smaller heights compared to the undoped CNFs. In addition, we have estimated that nitrogen‐doped CNFs have more enhanced field emission characteristics than the undoped CNFs. Moreover, we have also observed that N‐CNFs' growth rate increases and tip diameter decreases as the C₂H₂/NH₃ gas ratio decreases. The theoretical results of the present investigation are consistent with the existing experimental observations.     

Gas phase kinetic and optical emission spectroscopy studies in plasma-enhanced hot filament catalytic CVD production of carbon nanotubes

Optical emission spectroscopy (OES) is used as the main experimental tool for comparison with simulations of the plasma and gas phase composition during plasma-enhanced hot filament catalytic chemical vapor deposition (PE HF CCVD) growth of carbon nanotubes (CNTs). Calculated concentration of more than 45 species in model of the CVD reactor is acquired by Chemkin™ software. Study of different conditions is performed and a close relationship can be found between the nature and the growth rate of carbon nanostructures and the concentration of the active gas phase species. Moreover it is shown that significant changes in the density and morphology of the CNTs grown in the presence of NH₃ could be mainly explained by the gas phase formation of CN and HCN.     

Tailoring optical properties of TiO2 nanowires coated with Ag nanoparticles by plasmon coupling of Ag nanoparticles

TiO₂ nanowires were grown on titanium foil by an alkali hydrothermal growth method. The as-synthesized nanowires are structurally uniform with diameters of 50-100 nm and lengths of up to a few micrometers. The as-prepared TiO₂ nanowires were coated with Ag nanoparticles by reducing AgNO₃ in solution. The experimental results indicate that the Ag nanoparticles can aggregate together on the surfaces of TiO₂ nanowires by interconnection between nanoparticles. The degree of aggregation of Ag nanostructures can be controlled by changing the concentrations of Ag nanoparticles. The as-prepared nanostructures exhibit a wide optical absorption from 387 to 580 nm that can be easily tuned by controlling the degree of aggregation of Ag nanostructures. The results reveal that optical properties of the Ag-coated TiO₂ nanowires can be enhanced by plasmon coupling of Ag nanoparticles. The as-prepared nanostructures may find potential applications in the field of solar cells.     

ZnO–SnO2 branch–stem nanowires based on a two-step process: Synthesis and sensing capability

ZnO–SnO₂ branch–stem nanostructures were realized on a basis of a two-step process. In step 1, SnO₂-stem nanowires were synthesized. In step 2, ZnO-branch nanowires were successfully grown on the SnO₂-stem nanowires through a simple evaporation technique. We have pre-deposited thin Au layers on the surface of SnO₂ nanowire stems and subsequently evaporated Zn powders on the nanowires. The ZnO branches, which sprouted from the SnO₂ stems, had diameters in a range of 30–35 nm. As-synthesized branches were of single crystalline hexagonal ZnO structures. Since the branch tips were comprised of Au-containing nanoparticles, the Au-catalyzed vapor–liquid–solid growth mechanism was more likely to control the growth process of the ZnO branches. To test a potential use of ZnO–SnO₂ branch–stem nanostructures in chemical gas sensors, their sensing performances with respect to NO₂ gas were investigated, showing the promising potential in chemical gas sensors.     

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.     

Investigations on the effect of process parameters on the growth of vertically oriented graphene sheet in plasma-enhanced chemical vapour deposition system

A multistage numerical model comprising the plasma kinetics and surface deposition sub-models is developed to study the influence of process parameters, namely, total gas pressure and input plasma power on the plasma chemistry and growth characteristics of vertically oriented graphene sheets (VOGS) grown in the plasma-enhanced chemical vapour deposition system containing the Ar + H₂ + C₂H₂ reactive gas mixture. The spectral and spatial distributions of temperature and number densities, respectively, of plasma species, that is, charged and neutral species in the plasma reactor, are examined using inductively coupled plasma module of COMSOL Multiphysics 5.2 modelling suite. The numerical data from the computational plasma model are fed as the input parameters for the surface deposition model, and from the simulation results, it is found that there is a significant drop in the densities of various plasma species as one goes from the bulk plasma region to the substrate surface. The significant loss of the energetic electrons is observed in the plasma region at high pressure (for constant input power) and low input power (for constant gas pressure). At low pressure, the carbon species generate at higher rates on the catalyst nanoislands surface, thus enhancing the growth and surface density of VOGS. However, it is found that VOGS growth rate increases when input plasma power is raised from 100 to 300 W and decreases with further increase in the plasma power. A good comparison of the model outcomes with the available experimental results confirms the adequacy of the present model.     

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.     

XAFS analysis of indium oxynitride thin films grown on silicon substrates

Local structure of indium oxynitride thin films grown on silicon substrates was investigated by X‐ray absorption fine structure technique incorporated with first principle calculations. The thin films were grown by using reactive gas timing radio frequency (RF) magnetron sputtering technique with nitrogen (N₂) and oxygen (O₂) as reactive gasses. The reactive gasses were interchangeably fed into sputtering system at five different time intervals. The gas feeding time intervals of N₂:O₂ are 30 : 0, 30 : 5, 30 : 10, 30 : 20 and 10 : 30 s, respectively. The analysis results can be divided into three main categories. Firstly, the films grown with 30 : 0 and 30 : 5 s gas feeding time intervals are wurtzite structure indium nitride with 25 and 43% oxygen contaminations, respectively. Secondary, the film grown with 10 : 30 s gas feeding time intervals is bixbyite structure indium oxide. Finally, the films are alloying between indium nitride and indium oxide for other growth condition. The fitted radial distribution spectra, the structural parameters and the combination ratios of the alloys are discussed. Copyright © 2012 John Wiley & Sons, Ltd.     

Carbon nanotubes as nanoparticles collector

This communication reports on a new method for the collection of nanoparticles using carbon nanotubes (CNT) as collecting surfaces, by which the problem of agglomeration of nanoparticles can be circumvented. CNT (10–50 nm in diameter, 1–10 μm in length) were grown by thermal CVD at 923 K in a 7 v/v% C₂H₂ in N₂ mixture on electroless nickel-plated copper transmission electron microscopy (TEM) grids and Monel coupons. These samples were then placed downstream of an arc plasma reactor to collect individual copper nanoparticles (5–30 nm in diameter). It was observed that the Cu nanoparticles preferentially adhere onto CNT and that the macro-particles (diameter >1 μm), a usual co-product obtained with metal nanoparticles in the arc plasma synthesis, are not collected. Cu–Ni nanoparticles, a catalyst for CNT growth, were deposited on CNT to grow multibranched CNT. CNT-embedded thin films were produced by re-melting the deposited nanoparticles.     

Tuning Carbon Content and Morphology of FeCo/Graphitic Carbon Core–Shell Nanoparticles using a Salt‐Matrix‐Assisted CVD Process

Large‐scale and tunable synthesis of FeCo/graphitic carbon (FeCo/GC) core–shell nanoparticles as a promising material for multipurpose biomedical applications is reported. The high‐quality graphitic structure of the carbon shells is demonstrated through high‐resolution transmission electron microscopy (HRTEM), X‐ray diffraction (XRD), and Raman spectroscopy. A saturation magnetization of 80.2 emu g^?1 is reached for the pure FeCo/GC core–shell nanoparticles. A decrease in the saturation magnetization of the samples is observed with an increase in their carbon content with different carbon morphologies evolved in the process. It is also shown how hybrid nanostructures, including mixtures of the FeCo/GC nanoparticles and multi‐walled carbon nanotubes (MWNTs) or carbon nanorods (CNRs), can be obtained only by manipulation of the carbon‐bearing gas flow rate.     

The effects of hydrogen plasma pretreatment on the formation of vertically aligned carbon nanotubes

The effects of H₂ plasma pretreatment on the growth of vertically aligned carbon nanotubes (CNTs) by varying the flow rate of the precursor gas mixture during microwave plasma chemical vapor deposition (MPCVD) have been investigated in this study. Gas mixture of H₂ and CH₄ with a ratio of 9:1 was used as the precursor for synthesizing CNTs on Ni-coated TiN/Si(1 0 0) substrates. The structure and composition of Ni catalyst nanoparticles were investigated by using scanning electron microscopy (SEM) and cross-sectional transmission electron microscopy (XTEM). Results indicated that, by manipulating the morphology and density of the Ni catalyst nanoparticles via changing the flow rate of the precursor gas mixture, the vertically aligned CNTs could be effectively controlled. The Raman results also indicated that the intensity ratio of the G and D bands (I_D/I_G) is decreased with increasing gas flow rate. TEM results suggest H₂ plasma pretreatment can effectively reduce the amorphous carbon and carbonaceous particles and, thus, is playing a crucial role in modifying the obtained CNTs structures.     

Growth mechanism and photoluminescence of the SnO2 nanotwists on thin film and the SnO2 short nanowires on nanorods

SnO2 nanotwists on thin film and SnO2 short nanowires on nanorods have been grown on single silicon substrates by using Au-Ag alloying catalyst assisted carbothermal evaporation of SnO2 and active carbon powders.The morphology and the structure of the prepared nanostructures are determined on the basis of field-emission scanning electron microscopy(FESEM),transmission electron microscopy(TEM),selected area electronic diffraction(SAED),high-resolution transmission electron microscopy(HRTEM),x-ray diffraction(XRD),Raman and photoluminescence(PL) spectra analysis.The new peaks at 356,450,and 489 nm in the measured PL spectra of two kinds of SnO2 nanostructures are observed,implying that more luminescence centres exist in these SnO2 nanostructures due to nanocrystals and defects.The growth mechanism of these nanostructures belongs to the vapour-liquid-solid(VLS) mechanism.     

Formation of Sn@C Yolk–Shell Nanospheres and Core–Sheath Nanowires for Highly Reversible Lithium Storage

As one promising anode material with high theoretical capacity, metallic tin has attracted much research interest in the field of lithium‐ion batteries. Here, two types of tin/carbon (Sn@C) core–shell nanostructures with inner buffering voids are fabricated from SnO₂ hollow nanospheres via a facile chemical vapor deposition (CVD) method. The crystallinity and surface topography of SnO₂ hollow nanospheres are found to affect the morphology of resultant Sn@C materials. Sn@C yolk–shell nanospheres and core–sheath nanowires are obtained from the as‐prepared SnO₂ and high‐temperature annealed SnO₂ nanospheres, respectively. The unique Sn@C nanostructures can mitigate the agglomeration/pulverization of Sn nanoparticles and electrical disconnection from the current collector caused by the large volume change during the lithium alloying/dealloying process. Both Sn@C yolk–shell and core–sheath nanostructures show stable cycling performance up to 500 cycles with specific capacities of ca. 430 and 520 mA h g^?1, respectively.     

微量水对碳纳米管形貌的影响及其机理研究

利用介质阻挡放电等离子体化学气相沉积技术,在蒸镀有25nm Ni催化剂层的Si基片上,以CH₄和H₂作为反应气体,在973K下制备了碳纳米管,并研究了微量水的引入对碳纳米管形貌的影响.场发射扫描电子显微镜结果表明,不加水时,制备出的碳纳米管直径不均匀,分布在40—90nm之间,呈链节状的结构;加入少量水时,制备出的碳纳米管直径比较均匀,集中在70nm左右,表面为瘤状结构;当水的流量进一步增加时,得到的碳纳米管表面光滑,出现了枝状结构.原位测量了加水前后等离子体区的发射光谱,分析了微量水的引入对于碳纳米管形貌变化的影响机理. 关键词： 碳纳米管 介质阻挡放电 水 发射光谱     

Defect-free growth of epitaxial silicon at low temperatures (500–800 °C) by atmospheric pressure plasma chemical vapor deposition

Epitaxial Si growth at low temperatures (500–800 °C) by atmospheric pressure plasma chemical vapor deposition has been investigated. Silicon films are deposited on (001) Si wafers using gas mixtures containing He, H₂, and SiH₄. The effects of deposition parameters (composition of reactive gases, very high frequency (VHF) power, and substrate temperature) on film properties are investigated by reflection high-energy electron diffraction, atomic force microscopy, cross-sectional transmission electron microscopy, and plasma emission spectroscopy. It is found that epitaxial temperature can be reduced by increasing VHF power, and that an optimum range of VHF power exists for Si epitaxy, depending on the substrate temperature and the composition of the reactive gases. The result of the H₂ concentration dependence of H_α emission intensity, shows that hydrogen atoms generated in the atmospheric pressure plasma play an important role in Si epitaxial growth. Under the optimized growth conditions, defect-free epitaxial Si films (as observed by transmission electron microscopy) with excellent surface flatness are grown at 500 °C with an average growth rate of approximately 0.25 μm/min. PACS 81.05.Cy; 81.15.Gh; 68.55.Jk     

Growth of undoped and Fe doped TiO2 nanostructures and their optical and photocatalytic properties

Undoped and Fe-doped TiO₂ nanostructures have been successfully grown on Pt-coated quartz and Si (100) substrates using vapor-liquid-solid (VLS) growth method. The scanning electron microscopy (SEM) image showed that TiO₂ grew in nanowires (NWs) with diameters of 200–400 nm and lengths greater than 12 μm. However, the morphology of Fe-doped TiO₂ consists of chunk shaped nanoparticles (NPs). The X-ray diffraction analysis for undoped TiO₂ NWs clearly showed the formation of tetragonal rutile TiO₂, whereas for the Fe-doped TiO₂ NPs it showed orthorhombic TiO₂ phase and there are no crystalline peaks for iron or iron oxide. The refractive index and extinction coefficient values of undoped and Fe-doped TiO₂ nanostructures were derived from the ellipsometric measurements. Enhanced photocatalytic activities were obtained for undoped and Fe-doped TiO₂ nanostructures. The obtained results may find potential applications in optical devices and degradation of organic wastes.     

Structural and catalytic properties of ZnO and Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanostructures loaded with metal nanoparticles

Nanostructured zinc oxide (ZnO) nanobelts and aluminum oxide (Al₂O₃) nanoribbons have been grown successfully from the vapor phase. XRD results confirmed the purity and the high quality of the formed crystalline materials. TEM images showed that ZnO nanostructures grew in the commonly known tetrapod structure with nanobelts separated from the tetrapods with an average width range of 10–30 nm and a length of about 500 nm. Al₂O₃ nanostructures grew in the form of nanoribbons with an average width range of 20–30 nm and a length of up to 1 μm. The catalytic oxidation of CO gas into CO₂ gas over the synthesized nanostructures is also reported. Higher catalytic activity was observed for Pd nanoparticles loaded on the ZnO nanobelts (100% conversion at 270 °C) and Al₂O₃ nanoribbons (100% conversion at 250 °C). The catalytic activity increased in the order Cu < Co < Au < Pd for the metal-loaded nanostructures. The preparation methods could be applied for the synthesis of novel nanostructures of various materials with novel properties resulting from the different shapes and morphologies.     

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.