Influence of the nanostructure of silica supports on the growth and morphology of MWCNTs synthesized by CCVD method

Due to the influence of its superficial physical parameters on the interaction with catalyst, mesoporous silica is commonly referred as suitable support to grow carbon nanotubes (CNTs) by catalytic chemical vapor deposition (CCVD) method. Faced with the various possibilities of applying nanostructured SiO₂/CNTs composites, this work aims to evaluate and clarify the influence of the silica mesoporosity and morphology on the quality and amount of CNTs produced by CCVD process. Five different nanostructured silicas (n-SiO₂) were produced by sol-gel method. Basically, four silica samples were synthesized with the addition of an acidic catalyst and one with a basic catalyst. Thermogravimetric analysis, Raman spectroscopy, transmission electron microscopy and scanning electron microscopy were used to characterize the silica supports and the as-grown CNTs produced in this work. The obtained results show differences in the morphology of the synthesized CNTs according to the physical properties of each n-SiO₂. The mesoporous silica structure, due to different pore size distribution and volume, affected the interaction between the support and the catalyst, and, consequently, the quality and amount of the synthesized multi-walled carbon nanotubes (MWCNTs). Silica supports with either high mesopore volume, or high mesopore size, provide the highest quantities of as-grown CNTs materials. However, in terms of quality of as-grown CNTs, the supports with lower mesopores volume were more adequate to the MWCNTs synthesis. Nevertheless, the presence of pores with compatible size may have allowed an improved anchorage of catalyst particles inside these pores favoring the growth of CNTs with good quality.     

Production of Carbon Nanotube by Ethylene Decomposition over Silica-Coated Metal Catalysts

Formation of carbon nanofibers (CNFs) and carbon nanotubes (CNTs) through the decomposition of ethylene at 973 K was achieved using various metal catalysts covered with silica layers. CNFs of various diameters were formed by ethylene decomposition over a Co metal catalyst supported on the outer surface of the silica. In contrast, silica-coated Co catalysts formed CNTs with uniform diameters by ethylene decomposition. Silica-coated Ni/SiO₂ and Pt/carbon black also formed CNTs with uniform diameters, while CNFs and CNTs with various diameters were formed over Ni/SiO₂ and Pt/carbon black without a silica coating. These results indicate that silica layers that envelop metal particles prevent sintering of the metal particles during ethylene decomposition. This results in the preferential formation of CNTs with a uniform diameter.     

Preparation of hollow platinum nanospheres/carbon nanotubes nanohybrids and their improved stability for electro-oxidation of methanol

We report a facile method for the synthesis of hollow platinum nanospheres/carbon nanotubes nanohybrids (CNTs-G-PtHNs). Silver nanoparticles were used as sacrificial templates and uniformly deposited on the functionalized carbon nanotubes (CNTs). By galvanic replacement reaction between CNTs-supported silver and PtCl₆²⁻, well-dispersed hollow platinum nanospheres (PtHNs) were “grown” on CNTs. The morphology and electrochemical properties of the CNTs-G-PtHNs nanohybrids have been investigated by transmission electron microscopy and cyclic voltammetry, respectively. PtHNs in the CNTs-G-PtHNs nanohybrids have an average diameter of about 8 nm and the CNTs-G-PtHNs nanohybrids have higher electrochemical surface area and better electrocatalytic performance towards methanol oxidation than CNTs-A-PtHNs nanohybrids which were obtained by adsorbing the pre-synthesized PtHNs onto CNTs. Most importantly, the long-term stability of CNTs-G-PtHNs nanohybrids for methanol electro-oxidation has obviously improved compared with that of the CNTs-A-PtHNs nanohybrids.     

Controllable‐Nitrogen Doped Carbon Layer Surrounding Carbon Nanotubes as Novel Carbon Support for Oxygen Reduction Reaction

Novel nitrogen‐doped carbon layer surrounding carbon nanotubes composite (NC‐CNT) (N/C ratio 3.3–14.3 wt.%) as catalyst support has been prepared using aniline as a dispersant to carbon nanotubes (CNTs) and as a source for both carbon and nitrogen coated on the surface of the CNTs, where the amount of doped nitrogen is controllable. The NC‐CNT so obtained were characterized with scanning electron microscopy (SEM), Raman spectroscopy, X‐ray photoelectron spectroscopy (XPS), and nitrogen adsorption and desorption isotherms. A uniform dispersion of Pt nanoparticles (ca. 1.5–2.0 nm) was then anchored on the surface of NC‐CNT by using aromatic amine as a stabilizer. For these Pt/NC‐CNTs, cyclic voltammogram measurements show a high electrochemical activity surface area (up to 103.7 m² g^–1) compared to the commercial E‐TEK catalyst (55.3 m² g^–1). In single cell test, Pt/NC‐CNT catalyst has greatly enhanced catalytic activity toward the oxygen reduction reaction, resulting in an enhancement of ca. 37% in mass activity compared with that of E‐TEK.     

Preparation of gold nanoparticles on poly(methyl methacrylate) nanospheres with surface-grafted poly(allylamine)

A facile method for in situ anchoring of gold nanoparticles onto the surface of polymer nanospheres was successfully developed in this study. As polymer nanospheres, amphiphilic poly(methyl methacrylate) (PMMA)/poly(allylamine) (PAA) nanospheres were prepared by graft copolymerization of methyl methacrylate from PAA. The gold nanoparticles anchored were spherically symmetric and the average sizes were ∼12 nm for all samples. It was found that surface-grafted PAA effectively anchored and stabilized gold nanoparticles for a long period of time.     

Enrichment of metallic carbon nanotubes by electric field-assisted chemical vapor deposition

We report herein a rational approach to increase the proportion of metallic carbon nanotubes (CNTs) in horizontally aligned ultralong CNT arrays by electric field-assisted chemical vapor deposition. In a gas flow-directed growth mode, the buoyancy caused by temperature differences near the substrate can lift catalyst particles or CNTs from the substrate into the laminar flow so that ultralong CNT arrays with mixed metallic (m-) and semiconducting (s-) CNTs can be obtained. It was verified that the percentage of m-CNTs was about 47% for pristine CNTs. When an electric field was introduced during CNT growth, the grown CNTs were polarized and the generated electric field force assisted them into the laminar flow. The greater polarizability of m-CNTs compared to s-CNTs resulted in more m-CNTs lifted and an increased m- to s-CNT ratio in the array. Measurements of CNT electrical properties showed that the percentage of m-CNTs could reach 80% when the electric field intensity was set at 200 V/cm.     

The density control of carbon nanotubes using spin-coated nanoparticle and its application to the electron emitter with triode structure

The density-controlled carbon nanotubes (CNTs) were grown on the iron nanoparticles by using the freeze–dry method. The iron-acetate [Fe(II)(CH₃COO)₂] solution was used for the preparation of the catalytic iron nanoparticles. The density of CNTs was controlled in order to achieve the enhancement in the field emission process. Furthermore, the patterning of the iron nanoparticle catalyst layer for the fabrication of electronic devices was simply achieved by using an alkaline solution, TMAH (tetramethylammonium hydroxide). We applied this patterning process of catalyst layer to the formation of the electron emitter with under-gate type triode structure.     

氟烷基改性的二氧化硅纳米球的制备与应用研究

以浓氨水为催化剂、正硅酸乙酯(TEOS)为原料,通过种子生长法制得二氧化硅纳米球;进一步以十三氟辛基三乙氧基硅烷(F-8261)对二氧化硅纳米球的表面进行改性,得到氟烷基改性二氧化硅纳米球.利用IR、UV、TEM等手段对氟烷基改性纳米球进行了表征.结果表明,改性单体F-8261通过化学键结合在二氧化硅纳米球表面;通过改变催化剂的用量,可制得平均粒径分别为35 nm和86 nm的纳米球.用含氟烷基改性二氧化硅纳米球的乙醇溶液处理玻璃、基准混凝土,其表面与水的的静态接触角分别从32°和0°提高到104°和141°,表明处理过的基材表面具有良好的防水和防污性能.     

Facile preparation of highly dispersed Pt nanoparticles supported on heteroatom-containing porous carbon nanospheres and their catalytic properties for the reduction of 4-nitrophenol

Heteroatom-containing porous carbon nanospheres with a high surface area were firstly fabricated by pyrolysis of poly(cyclotriphosphazene-co-4,4′-sulfonyldiphenol) (PZS) nanospheres which were fabricated by a facile polycondensation between hexachlorocyclotriphosphazene and 4,4′-sulfonydiphenol. Then the porous carbon nanosphere-supported Pt nanoparticles (Pt NPs@C-PZS) were synthesized by a simple microwave reduction method, during which Pt NPs were highly dispersed on the surface of carbon supports. The surface morphologies and chemical composition of the as-obtained C-PZS and Pt NPs@C-PZS nanocomposites were characterized by SEM, TEM, XRD, XPS, and Raman spectroscopy. Characterization results showed that the Pt NPs with an average diameter of 2 nm was well anchored onto the surface of C-PZS nanospheres. In addition, the as-prepared Pt NPs@C-PZS nanocomposites exhibited an excellent catalytic capability towards the reduction of 4-nitrophenol to 4-aminophenol by excessive sodium borohydride (NaBH₄) at room temperature.     

Electrochemical deposition of Pt nanoparticles on CNTs for fuel cell electrode

In order to increase the performance of fuel cell electrode, carbon nanotubes (CNTs) were used as support instead of conventional carbon black, and the Pt catalyst was synthesized by using electrochemical deposition (ECD) method which has recently been adopted as a synthetic tool of metal nanoparticles. CNTs used in this paper were grown directly on carbon paper by chemical vapor deposition (CVD) of acetylene. Highly dispersed and nano-sized Pt particles were electrochemically deposited on CNTs surface, which would simplify the manufacturing process of membrane-electrode-assembly (MEA). Pt particles on CNTs were investigated by SEM and TEM. The particle size of Pt is less than 2 nm, which is relatively small compared to that of conventional wet impregnated catalyst (2–8 nm). CO chemisorption results show that the amounts of catalytic sites are about three times larger in Pt/CNT prepared by ECD than those in conventional wet-impregnated one. The mass activity of the former catalyst for oxygen reduction is more than three times higher compared to that of the latter one. This paper was presented at the 11th Korea-Japan Symposium on Catatysis held at Seoul, Korea, May 21–24, 2007.     

Bimetallic Ru-Fe Nanoparticles Supported on Carbon Nanotubes for Ammonia Decomposition and Synthesis

Alloy catalysts can achieve superior performance to single metal while reducing the cost by fine-tuning the composition and morphology. Bimetallic Ru-Fe nanoparticles were synthesized via liquid-phase reduction method followed by impregnation with multiwall carbon nanotubes (CNTs) to prepare Ru-Fe/CNTs catalysts. The Ru₃Fe/CNTs catalyst yields a superior catalytic stability for ammonia decomposition compared to the Ru/CNTs catalyst. Hence, the ammonia synthesis rate of the Ru₃Fe/CNTs catalyst was significantly higher than that of Ru/CNTs catalyst. The potential of bimetallic catalysts with reasonable composition and proportion will expand the research of efficient catalysts for ammonia decomposition and synthesis.     

Improved tensile strength of carbon fibers undergoing catalytic growth of carbon nanotubes on their surface

We demonstrate that the tensile strength of carbon fibers (CFs) can be increased by more than 14% by the catalytic growth of carbon nanotubes (CNTs) onto their surface. Repair to some of the damage incurred during the formation of catalyst nanoparticles, an increase in the carbon crystal size, and the formation of crosslinks of neighboring crystals by CNTs all occur during the chemical vapor deposition process, and are the main reasons for the improvement. The interfacial shear strength of the CFs is also shown to be significantly improved due to the CNTs grown on the CF surface.     

Controlled assembly of graphene shells encapsulated gold nanoparticles and their integration with carbon nanotubes

Controlled growth and uniform patterning of graphene/carbon shells encapsulated gold nanoparticles (GNPs) on silicon wafer or on high curvature carbon nanotubes (CNTs) is reported here. This was achieved by utilizing patterned gold nanoparticles with controlled sizes (∼30–600 nm) via gold film dewetting process. Surface-oxidized and patterned nanoparticles were used as sacrificial catalysts for the chemical vapor deposition (CVD) growth of graphene/carbon shells. The shell morphological evolution and thickness as well as surface migration of nanoparticles during the CVD process were studied as a function of the gold nanoparticles size. Reduced surface migration and coalescence was observed for gold nanoparticles after the CVD growth and this was attributed to the initial formation of graphene/carbon shells as well as stable dispersion of the dewetted gold nanoparticles. It is proposed that graphene/carbon shell growth was controlled by Ostwald’s ripening, surface gold oxide, and reducing CVD growth environment. Furthermore, complex heterostructures based on CNTs coated with GNPs were fabricated by dewetting Au films on CNTs and followed by surface oxidation and CVD growth steps. CNTs successfully survived multiple processing steps and selective growth of graphene shells around Au nanoparticles was achieved and studied using microscopic and spectroscopic methods.     

Femtosecond laser modification of an array of vertically aligned carbon nanotubes intercalated with Fe phase nanoparticles

Femtosecond lasers (FSL) are playing an increasingly important role in materials research, characterization, and modification. Due to an extremely short pulse width, interactions of FSL irradiation with solid surfaces attract special interest, and a number of unusual phenomena resulted in the formation of new materials are expected. Here, we report on a new nanostructure observed after the interaction of FSL irradiation with arrays of vertically aligned carbon nanotubes (CNTs) intercalated with iron phase catalyst nanoparticles. It was revealed that the FSL laser ablation transforms the topmost layer of CNT array into iron phase nanospheres (40 to 680 nm in diameter) located at the tip of the CNT bundles of conical shape. Besides, the smaller nanospheres (10 to 30 nm in diameter) are found to be beaded at the sides of these bundles. Some of the larger nanospheres are encapsulated into carbon shells, which sometime are found to contain CNTs. The mechanism of creation of such nanostructures is proposed.     

Pd/CNTs金属纳米催化剂的制备及其催化对氯硝基苯加氢性能

通过硝酸浸渍法对碳纳米管(CNTs)进行预处理改性,并采用乙二醇液相还原法制备Pd/CNTs纳米催化剂,通过XRD、TEM、BET、拉曼光谱、ICP等手段对其进行表征。结果表明,Pd/CNTs催化剂中Pd的负载量为0.9 wt%,Pd纳米颗粒均匀地分散在CNTs表面,粒径平均大小为5 nm。该催化剂用于对氯硝基苯的选择加氢反应,在常压、60℃下反应2 h,转化率为94.7%,选择性为89.2%。     

Pt-supported Spherical Mesoporous Silica as a Nanosized Catalyst for Efficient Liquid-Phase Hydrogenation

Mesoporous silica nanospheres with an average diameter of approximately 240 and 600 nm have been used as catalyst supports of active Pt nanoparticles, which were found to be highly efficient for liquid-phase hydrogenation under atmospheric H₂ compared to the micron-sized conventional mesoporous silica because of the suppression of mass-transport limitation toward active sites.     

Bridging mesoporous carbon particles with carbon nanotubes

The enhancement of the electrical conductivity (EC) of a porous carbon is highly desirable in many applications, especially in those associated with storage and conversion of electrochemical energy. In this work, we demonstrated an approach to largely increasing the EC of ordered mesoporous carbon (OMC) by bridging the OMC particles with carbon nanotubes (CNTs). Infiltration of the pores of ordered mesoporous SBA-15 silica with a carbon precursor yielded a carbon/mesoporous silica composite, which was further used as a support for Ni catalyst. Subsequently, catalytic growth of CNTs on the Ni-supported composite surface was carried out using the chemical vapor deposition (CVD) method with benzene as the carbon precursor. Removal of the silica framework and the metal catalyst left behind OMC particles bridged with CNTs. The EC of the OMC was increased from 138 S/m (before bridging) to 645 S/m (after bridging). Because of the significant enhancement of EC and the availability of mesopores, the cyclability of the hybrid carbon materials as a negative electrode used in rechargeable lithium-ion batteries was significantly improved.     

Synthesis and characterisation of silica–carbon nanotube hybrid microparticles and their effect on the electrical properties of poly(vinyl alcohol) composites

Hybrid silica–carbon nanotube (CNT) particles with a radial symmetry were produced by the growth of nanotubes onto spherical, mesoporous silica gel particles using the floating catalyst chemical vapour deposition (FC-CVD) method. Characterisation of the hybrid particles, using electron microscopy, Raman spectroscopy and thermogravimetry showed the geometry and porosity of the silica particles to influence the alignment and density of the CNTs produced. CNT growth initiated in the pores of the gel particles and three hours of CVD growth were required to get extensive surface coverage. In the early stages of growth, the reactants diffused inside the mesoporous silica and consequently the CNTs grew mainly within the silica gel rather than on the surface. Some indication of catalyst templating was observed within the smaller (<10 nm) pores, but this templating did not result in aligned CNTs. Composite films of hybrid silica–CNT particles in poly(vinyl alcohol) were cast and their impedance measured. An electrical percolation threshold of 0.62 wt.% was found for the hybrid particles, of which 0.20 wt.% were CNTs.     

Preparation of Catalyst Ni–Cu/CNTs by Chemical Reduction with Formaldehyde for Steam Reforming of Methanol

The novel catalyst Ni–Cu alloys supported on carbon nanotubes (CNTs) was prepared by reduction with formaldehyde and applied in steam reforming of methanol. With nitric acid and sulfuric acid to create defects on the surface of CNTs and using ethanol to improve the hydrophilicity of CNTs, the Ni–Cu alloys were anchored on the surface of CNTs by co-reduction of Ni- and Cu-precursors under the use of tetra-n-methylammonium hydroxide to reduce the aggregation of Ni–Cu particles. In contrast, Ni–Cu catalyst supported on activated carbon (Ni–Cu/C) was prepared as well, and the bimetal of Ni and Cu supported on CNTs (Ni/Cu/CNTs) was attained by successive reduction of first Cu- and then Ni-precursors. The catalysts were characterized with XRD, ED, FESEM, transmission electron microscopy, and Thermogravimetric analysis. The hydrogen yield in steam reforming of methanol was near 100% at 360 °C over 20 wt.% Ni₂₀–Cu₈₀/CNTs. The catalytic activity of Ni₂₀–Cu₈₀/CNTs is much higher than that of Ni₂₀–Cu₈₀/C and Ni₂₀/Cu₈₀/CNTs.     

Shortened Carbon Nanotubes as Enhanced Support for High‐performance Platinum Catalysts

Carbon nanotubes (CNTs) were shortened from 5 to 15 μm to ca. 200 nm using ball milling with ethanol as the milling aid agent, and a platinum catalyst with these shortened carbon nanotubes (SCNTs) as the support was prepared by a high‐pressure colloidal method. It was found that this catalyst with SCNTs showed much higher activity than a platinum catalyst with normal CNTs as support; for methanol anodic oxidation, the activity of the Pt/SCNTs was 50% higher than that of the Pt/CNTs, and the Pt/SCNTs also showed higher activity for the cathodic reduction of oxygen. The Pt/SCNTs were characterised by X‐ray diffraction scanning and transmission electron microscropy. It is suggested that the significant performance enhancement when SCNTs are used as support might result from the generation of new surfaces and defects, the opening of closed nanotubes in the process of milling, higher platinum dispersion on the shortened nanotubes and the interaction of platinum nanoparticles with the SCNTs.