Ni Nanoparticle Catalysts Research Articles

Novel Fe–Ni nanoparticle catalyst for the production of CO- and CO2-free H2 and carbon nanotubes by dehydrogenation of methane

Abstract A novel nanoparticle impregnation method was used to prepare an Fe–Ni nanoparticle (np) catalyst supported on Mg(Al)O for the production of CO- and CO 2 -free H 2 and carbon nanotubes (CNT) by non-oxidative dehydrogenation of methane. This novel catalyst and a catalyst of similar composition prepared by incipient wetness (IW) were evaluated for their catalytic performance and their structures were determined by several microscopic and spectroscopic techniques. Monosized Fe 0.65 –Ni 0.35 oxide nanoparticles with an average particle size of 9 nm were prepared by thermal decomposition of an Fe–Ni oleate–surfactant complex in octadecene under reflux; these nanoparticles were dispersed onto a Mg(Al)O support to form a supported Fe–Ni np/Mg(Al)O catalyst. Compared with the Fe–Ni IW/Mg(Al)O catalyst, the nanoparticle catalyst was more easily reduced at a lower temperature (600 °C in H 2 ) and exhibited enhanced methane dehydrogenation and longer life-times at both 600 and 650 °C. Each reduced Fe–Ni nanoparticle functioned as an active site for the growth of CNT. The CNT were in the form of multi-walled nanotubes (MWNT) of relatively uniform diameter. An invar-like Fe–Ni–C alloy phase is believed to be the active phase for methane dehydrogenation. The deactivation of the nanoparticle catalyst is principally due to encapsulation of catalyst particles by the CNT.

Carbon saturation of arrays of Ni catalyst nanoparticles of different size and patternuniformity on a silicon substrate

The kinetics of saturation of Ni catalyst nanoparticle patterns of the three different degreesof order, used as a model for the growth of carbon nanotips on Si, is investigatednumerically using a complex model that involves surface diffusion and ion motionequations. It is revealed that Ni catalyst patterns of different degrees of order, with Ninanoparticle sizes up to 12.5 nm, exhibit different kinetics of saturation with carbon on theSi surface. It is shown that in the cases examined (surface coverage in the rangeof 1–50%, highly disordered Ni patterns) the relative pattern saturation factorcalculated as the ratio of average incubation times for the processes conducted in theneutral and ionized gas environments reaches 14 and 3.4 for Ni nanoparticles of 2.5and 12.5 nm, respectively. In the highly ordered Ni patterns, the relative patternsaturation factor reaches 3 for nanoparticles of 2.5 nm and 2.1 for nanoparticles of12.5 nm. Thus, more simultaneous saturation of Ni catalyst nanoparticles of sizes inthe range up to 12.5 nm, deposited on the Si substrate, can be achieved in thelow-temperature plasma environment than with the neutral gas-based process.

The Influences of H2Plasma Pretreatment on the Growth of Vertically Aligned Carbon Nanotubes by Microwave Plasma Chemical Vapor Deposition

The effects of H2flow rate during plasma pretreatment on synthesizing the multiwalled carbon nanotubes (MWCNTs) by using the microwave plasma chemical vapor deposition are investigated in this study. A H2and CH4gas mixture with a 9:1 ratio was used as a precursor for the synthesis of MWCNT on Ni-coated TaN/Si(100) substrates. The structure and composition of Ni catalyst nanoparticles were investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The present findings showed that denser Ni catalyst nanoparticles and more vertically aligned MWCNTs could be effectively achieved at higher flow rates. From Raman results, we found that the intensity ratio of G and D bands (ID/IG) decreases with an increasing flow rate. In addition, TEM results suggest that H2plasma pretreatment can effectively reduce the amorphous carbon and carbonaceous particles. As a result, the pretreatment plays a crucial role in modifying the obtained MWCNTs structures.

Non-lithographic organization of nickel catalyst for carbon nanofiber synthesis onlaser-induced periodic surface structures

We present a non-lithographic technique that produces organized nanoscale nickel catalystfor carbon nanofiber growth on a silicon substrate. This technique involves threeconsecutive steps: first, the substrate is laser-irradiated to produce a periodicnanorippled structure; second, a thin film of nickel is deposited using glancing-angleion-beam sputter deposition, followed by heat treatment, and third, a catalytic dcplasma-enhanced chemical vapor deposition (PECVD) process is conducted to produce thevertically aligned carbon nanofibers (VACNF). The nickel catalyst is distributedalong the laser-induced periodic surface structure (LIPSS) and the Ni particledimension varies as a function of the location on the LIPSS and is correlated to thenanoripple dimensions. The glancing angle, the distance between the ion beamcollimators and the total deposition time all play important roles in determiningthe final catalyst size and subsequent carbon nanofiber properties. Due to thegradual aspect ratio change of the nanoripples across the sample, Ni catalystnanoparticles of different dimensions were obtained. After the prescribed threeminute PECVD growth, it was observed that, in order for the carbon nanofibers tosurvive, the nickel catalyst dimension should be larger than a critical value of∼19 nm, below which the Ni is insufficient to sustain carbon nanofiber growth.

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

The effects of H 2 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 H 2 and CH 4 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 H 2 plasma pretreatment can effectively reduce the amorphous carbon and carbonaceous particles and, thus, is playing a crucial role in modifying the obtained CNTs structures.

Carbon Nanotubes Grown by Catalytic CVD on Silicon Based Substrates for Electronics Applications

To exploit the impressive electronic, mechanical and thermal properties of Carbon Nanotubes (CNTs) in the nanoelectronics technology, the development of deposition methods enabling the synthesis of well ordered, properly located and reproducible CNTs structures, is strongly recommended. We have been developing catalytic CVD synthesis of CNTs in order to get aligned nanotubes for applications ranging from nuclear particle-position detectors and cold cathode emitters for storage devices, to interconnects, vias and CNT-FETs. In this paper, the significant achievements gained on CVD growth processes for the CNTs deposition are presented. Ni and Fe catalyst nanoparticles have been obtained starting from thin films evaporated on silicon based substrates. The growth of vertically aligned carpets of MWNTs and horizontally aligned SWNTs, having a diameter of about 1 nm and bridging between patterned catalyst islands, has been accomplished. The SEM, TEM, Raman spectroscopy and AFM characterizations are discussed.

Synthesis of multi-walled carbon nanotubes for NH 3 gas detection

It has been recently demonstrated that carbon nanotubes (CNTs) represent a new type of chemical sensor capable of detecting a small concentration of molecules such as CO, NO 2, NH 3. In this work, CNTs were synthesized by chemical vapor deposition (CVD) on the SiO 2/Si substrate by decomposition of acetylene (C 2H 2) on sputtered Ni catalyst nanoparticles. Their structural properties are studied by atomic force microscopy, high-resolution scanning electron microscopy (HRSEM) and Raman spectroscopy. The CNTs grown at 700 °C exhibit a low dispersion in size, are about 1 μm long and their average diameter varies in the range 25–60 nm as a function of the deposition time. We have shown that their diameter can be reduced either by annealing in oxygen environment or by growing at lower temperature (less than 600 °C). We developed a test device with interdigital Pt electrodes on an Al 2O 3 substrate in order to evaluate the CNTs-based gas sensor capabilities. We performed room temperature current–voltage measurements for various gas concentrations. The CNT films are found to exhibit a fast response and a high sensitivity to NH 3 gas.

Supported Ni catalysts from nominal monolayer grow single-walled carbon nanotubes

Fe, Co, and Ni are catalytically effective for growing single-walled carbon nanotubes (SWNTs). On substrates, however, Ni tends to yield only multi-walled carbon nanotubes. Because enhanced surface diffusion at the elevated growth temperature required for deposition might cause coarsening of Ni catalyst nanoparticles, adjusting the nominal Ni thickness should be crucial for controlling the particle size. Using our previously developed combinatorial method, we prepared a thickness profile of Ni on a quartz glass (SiO 2) substrate and found that Ni nanoparticles catalyzed the growth of SWNTs by chemical vapor deposition only when nominal Ni thickness was in the monolayer range.

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.

5483806 Refrigeration system

Carbon nanofibres (CNFs) obtained by plasma-enhanced chemical vapour deposition are made of cone-shaped graphene layers, the opening angle of which has a significant influence on their properties: the smaller the angle, the closer the properties to those of carbon nanotubes. That angle is determined by the shape of the metal nanoparticle used to catalyse the growth. We show in this paper that the shape of Ni nanoparticle catalysts, and in turn the CNF properties, can be tuned during plasma-enhanced chemical vapour deposition, by the choice of the etchant gas. We show in particular that a water-containing etchant (H2O or H2O + H2) increases the growth rate by an order of magnitude at 600 °C compared to an ammonia-containing etchant (NH3 or NH3 + H2), and leaves more elongated Ni particles with a cone angle three times smaller. We conclude that the cone angle and the growth rate are directly related, and propose a mechanism to explain that large difference between the two etchants