Y.H. Mo

Electrochemical hydrogen storage behaviors of CVD, AD and LA grown carbon nanotubes in KOH medium

Abstract Carbon nanotubes (CNTs) were synthesized catalytically by both chemical vapor deposition (CVD) method in C2H2/H2 atmosphere at 600°C and arc-discharge (AD) method under helium atmosphere. The structure and morphology of CNTs was determined employing scanning electron microscopy (SEM) and FT-Raman spectroscopy. Hydrogen storage capacities were electrochemically measured for undoped and alkali metal (Li, Na and K) doped CVD, AD and laser ablation (LA) grown CNTs. The amounts of hydrogen stored were calculated from the measured discharge capacities. LA grown CNTs produced the highest hydrogen storage capacity of 1.6 wt % . The alkali metal-doped CNTs showed higher hydrogen storage capacities than un-doped CNTs. Although the hydrogen storage capacities of Li-doped CVD and AD grown CNTs were 6 times higher than that of un-doped CNTs, it is only 0.6 wt % . For Li-doped LA grown CNTs, the hydrogen storage capacity reached 2.4 wt % . The high hydrogen storage capacity of alkali metal-doped CNTs seems to be originated from increasing the hydrogen adsorption sites of CNTs due to introducing the doped metals in nanotube bundles and separating the tubes but not from their chemical effects.

Synthesis of narrow-diameter carbon nanotubes from acetylene decomposition over an iron–nickel catalyst supported on alumina

Abstract Carbon nanotubes (CNTs) were synthesized by the catalytic decomposition of acetylene over 40Fe:60Al2O3, 40Ni:60Al2O3 and 20Fe:20Ni:60Al2O3 catalysts. High density CNTs of 20 nm diameter were grown over the 20Fe:20Ni:60Al2O3 catalyst, whereas low growth density CNTs of 40 and 50 nm diameter were found over 40Fe:60Al2O3 and 40Ni:60Al2O3 catalysts. Smaller catalyst particles enabled the synthesis of highly dense, long and narrow-diameter CNTs. It was found that a homogeneous dispersion of the catalyst was an essential factor in achieving high growth density. The carbon yield and the quality of CNTs increased with increasing temperature. For the 20Fe:20Ni:60Al2O3 catalyst, the carbon yield reached 121% after 90 min at 700 °C. The CNTs were grown according to the tip growth mode. Based on reports regarding hydrocarbon adsorption and decomposition over different faces of Ni and Fe, the growth mechanism of CNTs over the 20Fe:20Ni:60Al2O3 catalyst are discussed.

Synthesis of carbon nanotubes over nickel–iron catalysts supported on alumina under controlled conditions

Carbon nanotubes (CNTs) were synthesized by catalytic decomposition of acetylene over Fe, Ni and Fe–Ni catalysts supported on alumina. The growth of CNTs was carried out at various reaction conditions. The growth density and diameter of CNTs could be controlled by varying the catalyst composition and the growth parameters. The growth density of CNTs increased with increasing the activation time of catalysts in H2 atmosphere and/or decreasing acetylene concentration. At 600 °C, higher density of CNTs was observed at 60 min for higher Fe containing catalyst, whereas at 90 min for higher Ni containing catalyst. The growth density of CNTs highly increased with increasing reaction time from 30 to 60 min. For all the catalysts, the diameter of CNTs decreased with increasing growth time further mainly due to hydrogen etching. Bimetallic catalysts produced narrower diameter CNTs than single metal catalysts. The growth of CNTs followed the tip growth mode and the CNTs were multi-walled CNTs.

Growth Mechanism of Needle-shaped ZnO Nanostructures over NiO-coated Si Substrates

ZnO nanostructures were synthesized over NiO-coated Si substrate by a thermal evaporation of Zn powders in a vertical chemical vapor deposition reactor. The ZnO nanostructures had a needle-like morphology and the diameter of the structures decreased linearly from the bottom to the top. The bottom diameters of the ZnO nano-needles normally ranged from 20–100 nm and the lengths were in the range of 2–3 Μm. The clear lattice fringes in HRTEM image indicated the growth of good quality hexagonal single-crystal ZnO. Field emission characteristics of the ZnO nano-needles showed that the turn-on field was about 8.87 V/Μm with a field enhancement factor of about 1099. The growth mechanism of the ZnO nano-needles was proposed on the basis of experimental data.

Effects of Bimetallic Catalyst Composition and Growth Parameters on the Growth Density and Diameter of Carbon Nanotubes

Multi-wall carbon nanotubes (MWNTs) were synthesized by catalytic decomposition of acetylene over Fe, Ni and Fe-Ni bimetallic catalysts supported on alumina under various controlled conditions. The growth density and diameter of CNTs were markedly dependent on the activation time of catalysts in H2 atmosphere, reaction time, reaction temperature, flow rate of acetylene, and catalyst composition. Bimetallic catalysts were apt to produce narrower diameter of CNTs than single metal catalysts. For the growth of CNTs at 600 ‡C under 10/100 seem flow of C2H2/H2 mixture, the narrowest diameter about 20 nm was observed at the reaction time of 1 h for 20Fe : 20Ni : 60Al2O3 catalyst, but at that of 1.5 h for 10Fe : 30Ni : 60Al2O3 catalyst. It was considered that the diameter and density of CNTs decreased with the increase of the growth time mainly due to hydrogen etching. The growth of CNTs followed the tip growth mode.

Low cost growth route for single-walled carbon nanotubes from decomposition of acetylene over magnesia supported Fe-Mo catalyst

A large amount of single wall carbon nanotubes (SWNTs) was successfully produced by thermal decomposition of C2H, at 800 °C over magnesia supported Fe-Mo bimetallic catalysts in a tubular flow reactor under an atmosphere of hydrogen flow. The growth density of SWNTs increased with increasing the weight percent of the catalyst metals (wt% ratio of two metals: 50 : 50) supported on magnesia (MgO) from 5 to 30 wt%. The yield of SWNTs reached 144.3% over 30 wt% metal-loaded catalyst. Raman measurements showed the growth of bundle type SWNTs with diameters ranging from 0.81 to 1.96 nm. The growth of SWNTs was also identified by thermal gravimetric analysis (TGA) and Raman spectroscopy.

Growth and characterization of SiC micro-crystals on Si(100) substrate by the chemical vapor deposition method

Abstract Silicon carbide (SiC) micro-crystals were grown on an Si(100) substrate by the reaction of carbon nanotubes (CNTs) with tetramethysilane (TMS) in a chemical vapor deposition (CVD) reactor. CNTs were catalytically grown on an Si(100) substrate by the thermal cracking of acetylene. The grown CNTs were in situ reacted with TMS in the CVD reactor. Micro-sized SiC crystals were uniformly grown across the Si substrate. The growth of the SiC micro-crystals was characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), and Raman and photoluminescence (PL) spectroscopy. Higher temperature and longer time were favorable for the growth of good quality SiC micro-crystals. The blue shift of the PL spectra was observed from the SiC micro-crystals. The growth mechanism of micro-sized SiC crystals is discussed.

Growth of GaN nanowires on Si substrate using Ni catalyst in vertical chemical vapor deposition reactor

GaN nanowires were successfully grown on Ni-coated Si substrate by direct reaction of gallium with ammonia gas in a home-made vertical tubular chemical vapor deposition reactor. The growth of GaN nanowires was uniformly observed across the Si substrate surface, but the density and average diameter of the nanowires varied along the position of the substrate surface. At the position of 5 cm above Ga source surface, the growth of GaN crystal grains was observed with few nanowires. The length of the nanowires reaches several micrometers. The clear lattice fringes in HRTEM image revealed the growth of good quality hexagonal single-crystal GaN nanowires. Photoluminescence of the GaN nanowires showed a strong band edge emission at the energy position of /spl sim/3.4 eV with negligible deep level yellow emission. Field emission characteristics of the GaN nanowires showed that the turn-on field of GaN nanowires was /spl sim/7.4 V//spl mu/m with a field enhancing factor /spl beta/ of /spl sim/555. The catalytic growth mechanism of the GaN nanowires was discussed on the basis of experimental results in this work.

Effect of chemical vapor deposition energy sources on the structure of SiC prepared by carbon nanotubes-confined reaction

Silicon carbide (SiC) nanorods and microcrystals were synthesized with controlled structure through a two-step reaction scheme. Carbon nanotubes (CNTs) were first grown by the thermal decomposition of C2H2 over a Fe/Co metal catalyst loaded on a Si(100) wafer surface. The grown CNTs were then reacted with tetramethylsilane (TMS) to form β-SiC nanorods or microcrystals. SiC nanorods were observed when CNTs reacted with TMS at 1100 °C in an IR-induced heating chemical vapor deposition (CVD) reactor, whereas SiC microcrystals were formed at 1250 °C in an rf-induced-heating CVD reactor. Higher temperature and longer time were favorable for the growth of good quality SiC crystals. The blueshift of the PL spectra was observed from the SiC nanorods and SiC microcrystals. The structural change of CNTs in the growth was discussed in this work.

Growth and Field Emission of GaN Nanowires

GaN nanowires were successfully grown on a Ni-coated Si substrate with a direct reaction of gallium and ammonia in a home-made vertical tubular chemical vapor deposition (CVD) reactor. The grown GaN nanowires were uniformly distributed across the Si substrate surface, but the density and diameter of the nanowires varied along the length of the substrate surface. The clear lattice fringes in the high-resolution transmission electron microscopy (HRTEM) image indicated the growth of good quality hexagonal single-crystal GaN nanowires. The GaN nanowires produced a strong band edge emission at ~ 3.4 eV with a negligible deep level yellow emission. Field emission characteristics of the GaN nanowires showed that the turn-on field was ~ 7.4 V/μm with a field enhancing factor β of ~ 555.