Classifications

• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B32/00—Carbon; Compounds thereof
  • C01B32/15—Nano-sized carbon materials
  • C01B32/158—Carbon nanotubes
  • C01B32/16—Preparation
  • C01B32/162—Preparation characterised by catalysts
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B82—NANOTECHNOLOGY
  • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
  • B82Y30/00—Nanotechnology for materials or surface science, e.g. nanocomposites
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B82—NANOTECHNOLOGY
  • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
  • B82Y40/00—Manufacture or treatment of nanostructures
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B2202/00—Structure or properties of carbon nanotubes
  • C01B2202/02—Single-walled nanotubes
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B2202/00—Structure or properties of carbon nanotubes
  • C01B2202/06—Multi-walled nanotubes
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10S977/00—Nanotechnology
  • Y10S977/70—Nanostructure
  • Y10S977/734—Fullerenes, i.e. graphene-based structures, such as nanohorns, nanococoons, nanoscrolls or fullerene-like structures, e.g. WS2 or MoS2 chalcogenide nanotubes, planar C3N4, etc.
  • Y10S977/742—Carbon nanotubes, CNTs
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10S977/00—Nanotechnology
  • Y10S977/84—Manufacture, treatment, or detection of nanostructure
  • Y10S977/842—Manufacture, treatment, or detection of nanostructure for carbon nanotubes or fullerenes
  • Y10S977/843—Gas phase catalytic growth, i.e. chemical vapor deposition

Definitions

• the present invention relates to the synthesis of nanoscaled carbon materials, especially Fullerenic nano- structures such as single or multi-walled carbon nanotubes.
• Such materials have been produced previously using various approaches including the laser or arc-discharge ablation of a carbon/catalyst mixture target.
• CVD chemical vapour decomposition
• the catalyst may be in the form of a fragmented surface layer on a porous or non-porous macroscopic substrate (Ren et al, Bower et al, V I Merkulov, D H Lowndes, Y Y Wei et al,
• the catalyst may be a laser ablated nickel target exposed to a flow of reactant gas.
• the catalyst may be in finely divided form.
• the catalyst is constituted by nanometer sized metal particles supported on larger (10-20 nm) alumina particles. The particles are placed in the centre of a furnace and the carbon containing gas is passed over them.
• catalyst particles comprising two different metals supported on silica, alumina, magnesia, zirconia or zeolite are used, again placed in a tube within a furnace. It is also suggested that the metallic catalytic particles may be continuously fed to the furnace.
• catalyst nanoparticles are continuously produced within a furnace in the presence of reactant gas by decomposing a gaseous catalyst precursor (normally Fe(CO)s) in the presence of a ⁇ nucleation agency' .
• This may be a laser which provides some or all of the energy needed for photolysis of the catalyst precursor, or it may be a precursor moiety that stimulates clustering of catalyst atoms by decomposing more rapidly or binding to itself more strongly after dissociation.
• Ni(C0) 4 this is Ni(C0) 4 .
• the Ni may be acting as a co-catalyst, as the formation of carbon nanotubes using Ni as a catalyst is well known. Rather than being a well-defined substrate on which Fe atoms deposit in clusters, the Ni and Fe are essentially co- condensing. The formation of a solid solution of the metals would be expected.
• a continuing problem in this art is the control over the extent of the production of multi-walled nanotubes in preference to single walled nanotubes and the control of the diameter of the tubes.
• fine structures such as single walled nanotubes require very fine catalyst particles with diameters similar to that of the synthesised material (typically about 1 nm) .
• Maintaining the required catalyst particle size generally requires the use of a substrate to act as a carrier material to stabilise the catalyst itself.
• the production of very fine supported catalyst particles prior to use in the nanotube synthesis is generally complex and expensive involving for example aggressive reagents and supercritical drying.
• the process offers little control over the size of the nucleating ⁇ particle", or of the size of the final catalyst clusters. There is no controlled templating of the catalyst by the structure of the substrate.
• the carrier gas is preferably the gas comprising the carbon containing gas.
• a stream of reactant gas can have the catalyst precursor and the substrate particles injected into it at or upstream of a reaction zone, and the supported catalyst can be formed in situ in the reactant gas, which therefore acts as the carrier gas referred to.
• the carrier gas containing the dispersed supported-catalyst particles is mixed with a gas comprising said carbon containing gas immediately following the formation of the supported-catalyst particles.
• the supported catalyst particles are formed by decomposing the catalyst precursor material in the presence of the substrate particles in a carrier gas (suitably an inert gas under the conditions) and the supported catalyst particles so formed carried within the carrier gas are then mixed into a reactant gas .
• the zones in which the supported- catalyst particles are formed and in which the nanotube forming reaction takes place are then separate.
• Gases which may be mixed with the carbon containing gas include argon, hydrogen, nitrogen, ammonia, or helium.
• the gaseous effluent from the reaction zone may be recycled, with or without clean up.
• Fe, Ni, Co, Mo and mixtures thereof such as a 50/50 mixture (by weight) of Ni and Co, or a mixture of Fe and Ni, or a mixture of Fe and Mo.
• Any of these transition metals individually or in combination with any of the other transition metals listed may be used in clusters to serve as a catalyst for carbon nanotube growth.
• Particularly preferred catalysts are mixtures of two or more of the listed metals.
• the transition metal clusters may have a size from about 0.5 nr ⁇ to over 30 nm. Clusters in the range of 0.5 to 3 nm will produce singlewall nanotubes, while larger clusters tend to produce multiwall nanotubes with outer diameters greater than about 3 nm. Generally, using the process of this invention, catalytic production of nanotubes will be predominantly singlewall nanotubes.
• the catalyst precursor is gaseous prior to decomposition.
• catalyst precursors may be pre-heated to produce volatilisation prior to introduction into the presence of the substrate material.
• solid or liquid catalyst precursor may be directly fed to the reaction zone together with or separately from the substrate material and without pre-heating.
• Solid or liquid catalyst precursors and solid or liquid substrate materials may be entrained into gas flows for conveying to the reaction zone by known methods. These, include the use of a solution of each in each other or in a solvent (which may be the carbon source for the nanoparticle production, e.g. a hydrocarbon) or sublimation.
• formation of the nanoscale carbon materials takes place at a temperature of from 650°C to 1250°C, e.g. 850° to 1100°C.
• the substrate particles are conveyed from a supply of substrate material and are mixed with the catalyst precursor material either before or after the catalyst precursor material reaches the zone in which decomposition occurs.
• the substrate may be by way of example silica, alumina or a POSS (polyhedral oligomeric silsesquioxanes or polyhedral oligomeric silicates) . Some of these materials are liquid at room temperature. Generally a single POSS molecule will constitute a particle of substrate for nanotube growth. In the most straightforward case, the substrate particles are simply finely ground powders, such as silica or alumina. Finer material may be generated by a range of methods, known to those skilled in the art, such as colloidal processing, spray-drying, hydrothermal processing and so on.
• nanotubes may be derived by using structured substrate particles, particularly mesoporous silicas, anodised alumina membranes, or zeolites. These materials have channels of similar dimensions to nanotubes, and can further guide both the deposition of catalyst and synthesis of nanotubes.
• a particularly preferred approach is to use, so-called POSS (polyhedral oligomeric silsesquioxane) compounds, as the catalyst- substrate particles.
• POSS polyhedral oligomeric silsesquioxane
• a POSS molecule can act as a site for catalyst formation in situ or, as described below in connection with a second aspect of the invention, in a pre-reaction step.
• POSS functionalise POSS itself with metallic functionalities - creating a catalyst-substrate in a single molecule (strong substrate-catalyst interactions have been shown to favour single-wall nanotube production) .
• the advantages of using a POSS are numerous. They have a very high surface area. Their diameters are around 1 nm (the same size- as single wall nanotubes) but are tuneable as different POSS molecules have different sizes. They can be monodisperse (have specific molecular weights) and hence have the potential to generate well defined products. As they have molecular character, they may be liquid or may be dissolved in a suitable liquid carrier (and may potentially even be evaporated directly) for injection into the furnace. They have excellent thermal stability in themselves. They have the potential to form well-defined derivatives that potentially add catalytic metal particles (for example iron) .
• the finely divided substrate particles preferably have a size not smaller than 1 nm, e.g. not less than 5 nm. They may contain not less than 10 atoms, e.g. not less than 15 to 20 atoms, perhaps not less than 50 atoms or 75 atoms.
• the substrate is fed to the zone in which the catalyst precursor material is decomposed and preferably is essentially unchanged in the step of supported-catalyst particles, except for the deposition thereon of the catalyst material.
• some chemical modification of the substrate particles during the formation of the supported-catalyst particles is permissible, e.g. the removal of surface chemical groups or solvating chemical side chains.
• the size of the substrate particles remains substantially unchanged.
• an additional energy source (over and above the temperature of the decomposition zone) may be locally applied.
• a source is preferably a laser beam which may be directed into the catalyst precursor material in the presence of the dispersed substrate particles, but may be a plasma discharge or an arc discharge formed in the presence of the catalyst precursor material and the dispersed substrate particles.
• a pulsed or CW laser may be used, e.g. a KrF eximer laser or a NdrYAG laser.
• Preferred gas pressures are from 0.1 to 50 bar A, preferably from 0.5 to 5 bar A, more preferably 1 to 2 bar A.
• the ratio of catalyst metal to carbon fed to the reaction zone is preferably less than 1:100, e.g. 1:100 to 1:500.
• the present invention provides method for producing nanoscaled carbon materials comprising providing, dispersed in a reactant gas, finely divided supported-catalyst particles comprising catalyst atoms carried by substrate particles, wherein said reactant gas comprises a carbon containing gas at a temperature at which said carbon containing gas will react to form carbon when in the presence of said supported- catalyst particles and said substrate particles are POSS, forming nanoscaled carbon materials by said carbon forming reaction and recovering the nanoscaled carbon materials.
• the supported-catalyst particles may be produced in situ in accordance with the first aspect of the invention, they may also be pre-prepared by unrelated methods.
• the POSS used may be a metallo-organic-silica compound in which the catalyst metal is part of the POSS molecule, rather than being deposited thereon.
• a preferred option is to introduce the precursor from an inlet in the side wall of the main stream, so, ideally, the precursor nozzle is fashioned to turn the injection flow parallel to the main feedstock flow, either downstream or upstream.
• the latter option can be advantageous with regard to turbulent mixing, as discussed further below.
• the injected precursor may contain some reactant gas or other gas as carrier.
• Figure 2 shows the products obtained in Example 2, a) bright field image b) dark field image with position of the objective aperture on the 002 diffraction ring indicated;
• Figure 4 shows the apparatus used in Example 4.
• Figure 5 shows the products obtained in Example 4.
• a vertical furnace containing a silica tube (internal diameter 65 mm, length 90 cm) can be used to synthesis nanotubes.
• the tube is sealed at both ends by metal fittings (water-cooled where necessary) with suitable access ports for gas, and solid substrates and products.
• Two streams of dry, filtered, hydrogen are passed through the furnace from the top downwards, one is bubbled through a solution of toluene saturated with ferrocene.
• the flow rates in the two streams are around 300 cc/ in (bubbled) and 700 cc/min (pure) .
• the hopper may not be employed. Instead the POSS material (e.g. dodecaphenyl-POSS) is dissolved in the toluene carrier along with the ferrocene.
• POSS material e.g. dodecaphenyl-POSS
• Example 2 As shown in Figure 1, a vertical quartz reaction tube 10 (1.4 m long, 0.065 m internal diameter) having ends 12 sealed with plates 14 cooled by an electric fan (not shown) was placed inside a clam furnace 16 having a 0.9 m long hot zone 18 heated to 800 °C. A syringe 20 controlled by a syringe pump 22 was used to introduce a solution 24 of 4 wt% ferrocene and 1.12 wt% dodecaphenyl POSS in toluene into the upper end 12 of the quartz tube 10 via a steel needle 26. The fall of the solution 24 was impeded by a horizontal metal plate 28 suspended above the hot zone 18 such that it was at a temperature of 425 °C.
• the plate 28 was used to aid sublimation of the solution 24.
• Argon 30 was passed into the upper end 12 of the quartz tube 10 via an inlet pipe 32 at a flow rate controlled by a flowmeter (not shown) of 0.2 1/min.
• the reaction products (not shown) passed through the lower end 12 of the quartz tube 10 via a pipe 34 into an ice-cooled flask 36 then through a silicone oil bubbler 38.
• the reaction products (not shown) were collected from the exhaust 40.
• the reaction was run for 20 minutes.
• the product was found to consist of a mixture of aggregates and nanofibres, with the latter forming an estimated 5 % of the product.
• the nanofibres were solid, with no hollow and ranged in diameter from approximately 60 to 200 nm ( Figure 2) . Dark field imaging demonstrated that the nanofibres were graphitic and at the edges there was some preferential orientation of the graphite planes parallel to the long axis of the nanofibres ( Figure 2) . There were also small
• Example 3 The apparatus described in Example 2 was used with the metal plate 28 being replaced by a ceramic crucible (not shown) and a flow rate of 2.5 1/min of argon being used.
• the solution 24 was injected at a rate of 10 ml/hour.
• the solution 24 was sublimed from the crucible (not shown) and swept into the furnace 16. The reaction was run for 20 minutes and material was collected from a quartz substrate
• the product was found to consist of approximately 50 % multi-walled nanotubes and 50 % nanosized particles (Figure 3) .
• Example 4 The apparatus described in Example 4 was used, with the aerosol 45 replaced by a direct injection system (not shown) consisting of a syringe (not shown) driven by a syringe pump (not shown) .
• Argon 30 was passed through the quartz tube 10 at a rate of 0.4 1/min measured at 25 °C.
• a solution (not shown) of 1 wt% ferrocene and 2 wt% dodecaphenyl POSS in toluene was injected into the furnace 16.
• a size 30G needle (not shown) was used so that the solution (not shown) entered the furnace 16 as a steady stream (not shown) .
• the reaction was run for 4 minutes and the products (not shown) were collected from the back of the quartz tube 10. It was found that multi-walled nanotubes were formed with diameters of approximately 20 nm ( Figure 6) . These nanotubes had a carbon coating on their outsides.

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