WO2014202740A1

WO2014202740A1 - Systems and methods for synthesis of carbon nanotubes

Info

Publication number: WO2014202740A1
Application number: PCT/EP2014/062978
Authority: WO
Inventors: Jin Won Seo; Ignaas Verpoest; Luis GONZALEZ-URBINA; Roel LOCUS; Filip de Clippel; Bert Sels
Original assignee: Katholieke Universiteit Leuven
Priority date: 2013-06-19
Filing date: 2014-06-19
Publication date: 2014-12-24

Abstract

The present invention provides systems and methods and porous substrates (hierarchical or not) for synthesis of carbon nanotubes. A fibrous material comprising such nanowires, use of such nanowires and a holder for such a substrate are also disclosed.

Description

SYSTEMS AND METHODS FOR SYNTHESIS OF CARBON NANOTUBES

Field of the invention

The present invention relates to the synthesis of nanostructures, and more particularly, to the a method and system for synthesis of nanostructures, to the use of a membrane, (optionally a hierarchical membrane), for the synthesis of such nanostructures and to methods and systems for preparing a porous body.

Background of the invention

Carbon nanotubes (CNTs) show extremely high stiffness (up to 1060 GPa) and high strength (up to 60 GPa has been reported) owing to their perfect and oriented structure of a tubular graphene sheet. Combined with the low density (1 .8 kg/lit) they have high potential to be used as reinforcing element.

The major problem of CNTs is currently that their handling is very difficult due to the fact that their diameter is only a few nanometer. Moreover, as-produced CNTs are mostly entangled, and randomly oriented, unless they are grown as aligned CNT forest. In the latter case, however, the density (number of CNTs/cm2) is very difficult to control. Typically a CNT forest is grown from a metal catalyst deposited as a thin film on a flat substrate. The film breaks up into small nanoparticles upon heating to CVD process temperatures (400-1000 °C). Hence, the location and the CNTs characteristics are rather randomly determined by the nanoparticles formed. For instance in WO2005098084 a CVD based process is described for the growth of CNTs.

The length of CNTs is also rather limited. In particular for CNTs in a carpet, diffusion of feedstock gas to catalyst particles becomes more and more difficult with the dense CNT carpet thickness, and the growth finally stops because of the obstruction of the gas flow. Before it really happens, the growth strongly comes at the expense of increase in amorphous carbon for high thickness carpets, which limits the interaction between individual CNTs in a carpet.

This possibility of growing CNTs with tunable density and length directly finds an application in the spinning of CNTs into fibers. Although individual CNTs seem to be the stiffest and strongest materials so far, currently they are no real alternative for steel wires or carbon fibers as direct reinforcement for polymers for structural applications. Several methods described in the prior art try to provide a method that assembles CNTs into macroscopic fibers, mainly following two main methods, namely liquid-state (wet) spinning and solid-state (dry) spinning. However, the wet spinning involves CNTs dispersions stabilized in a surfactant solution or super acids and the subsequent spinning of the solution. In contrast, during the dry spinning process fibers are drawn directly from a vertically aligned CNT forest or from a gas phase suspension formed by an aerogel in the CVD reactor. Both spinning methods have various advantages as well as disadvantages. Especially the liquid-state spinning provides easily km-long fibers but their mechanical property is rather poor because of the surfactant (or solvent) present between the CNTs. The dry-spinning method is more difficult in terms of fibre compaction and process scalability. In any case, both processes currently use rather short (up to mm-sized) CNTs because long (a few cm) and ultra-long (>10cm) CNTs have been difficult to grow as well as to process.

The fibers spun from a gas phase suspension has shown so far the best mechanical properties with stiffness of up to 350GPa and strength up to 8GPa. These values are beyond any other strongest commercially available fibers. The main drawback of that approach is however that small catalyst particles are present weakening the fibers, such as in WO2005098084. Moreover, the CNT length is rather limited to about 1 mm.

Therefore, there is a need of a novel approach for synthesis of nanostructures.

Summary of the invention

A need still exists for an improved system and method for synthesis of nanostructures. It is an object of the present invention to provide an alternative system and method for synthesis of nanostructures.

It is another object of the present invention to provide an alternative system and method for synthesis of extended length nanostructures.

It is another object of the present invention to provide an alternative porous body and a method for producing such body.

These objects are met by the method and apparatus according to the independent claims of the present invention. The dependent claims relate to preferred embodiments.

It is an advantage of embodiments of the present invention that the pores may extend through the full thickness of the substrate. The synthesis can then be based on a flow through mechanism. It is an advantage of embodiments of the present invention that an efficient method for synthesis is obtained.

It is an advantage of embodiments of the present invention that the growth is performed directly in the pores, followed by a graphitizing process afterwards. It is an advantage of embodiments of the present invention that by growing directly in the pores the catalytic particles are not enclosed in the nanostructures. Avoiding enclosure of the catalytic particles in the nanostructures is advantageous as catalytic particles could destroy the walls of the nanostructures and may result in inferior mechanical properties.

If on the other hands nanostructures are to be decorated with metal particles, the latter can be performed in a simple process, by for instance coating CNTs with a solution containing metal particles and subsequent drying or using a metal particles containing aerosol. Such structures can lead to hybrid structures able for hydrogen storage, chemical detection, catalysis etc.

It is an advantage of at least some embodiments of the present invention that a higher density of nanostructures can be obtained, due to the direct exposure of catalyst particles to the reaction gasses and due to the separation of the up-stream and down-stream directions. It thereby is to be noticed that the exhaust gas could influence the growth of nanostructures.

It is an advantage of embodiments of the present invention that the growth of nanostructures is not limited by gas obstruction caused by the densely growing nanostructures or by a strongly limited diffusion process of gas through the substrate.

The present invention provides substrates for the synthesis of nanostructures. In preferred embodiments a substrate is provided which preferably comprises a porous body having an upstream surface and a downstream surface. The substrate further may include a plurality of catalyst particles deposited inside the pores of the porous body, and from which nanostructures may be synthesized. Advantageously, the porosity of the body provides pathways through which a reaction gas can travel across the upstream surface and out the downstream surface to initiate growth of nanostructures from the catalyst particles. The substrate, in one embodiment, may be provided with pore size ranging from about 0.5 nm to about 500 microns, and a void fraction of from about 10 percent to about 95 percent. The catalyst particles in the pores or pore channels of the substrate may range from about 0.5 nm to about 50 nm.

It is an advantage of embodiments of the present invention that positioning of the catalyst particles in the pores of the porous body results in a good and efficient positioning of the catalyst particles. It is furthermore an advantage of embodiments of the present invention that a single pore provides the possibility for accommodating one or more than one catalyst particle.

It is an advantage of embodiments of the present invention that the area for accommodating catalyst particles can be drastically increased when accommodating the catalyst particles in the pores.

In preferred embodiments the catalyst particles are deposited in the pores of the substrate in such a way that they advantageously enable a pathway through which a reaction gas can travel across the upstream surface and out the downstream surface, and thus the catalyst particles do not block the pathway of the reaction gas. The catalyst particles in the pores or pore channels of the substrate may have a size from about 0.5 nm to about 50 nm but not exceeding 50 percent of the pore size.

In some preferred embodiments, the average number (i.e. averaged over all pathways) of catalyst particles positioned in the pathways is higher than 1 .0. It was found that advantageously, the number of particles in the pores can be larger than one, resulting in a higher growth efficiency, because the chance that a carbon precursor molecule flowing through a specific channel is "caught" is at least twice as high as compared to the case where there was only a single catalyst particle per pathway. In this way, a high efficiency for growing nanostructures can be obtained leading to high number of nanostructures per pore as well as highly efficient use of the process gases.

It is an advantage that the catalyst particles are primarily located inside the pathways through which the reaction gas flows, rather than on the downstream surface of the substrate, in that a higher density of nanostructures, e.g. carbon nanotubes CNTs, can be obtained because of the direct exposure of catalyst particles to the reaction gases, and because the carbon concentration of the gas is higher inside the pathways than at the downstream surface location. In preferred embodiments each of the pathways has a substantially gas-impermeable wall (unlike e.g. an array of needles). This offers the advantage that the growth will be more efficient. It also offers a higher implementation of the carbon atoms into the nanostructures because the movement of the precursor gas is confined by the pathways walls.

In preferred embodiments for instance an Anopore™ inorganic membrane (Anodisc™) of Whatman, may be used. A porous membrane, according to embodiments of the invention, advantageously has a precise, nondeformable honeycomb pore structure with no lateral crossovers between individual pores, that preferably may filter at precisely the stated cut-off, allowing no larger sized particles to pass through the membrane. Embodiments of the invention provide the use of inorganic porous membranes, whereby said membranes may be composed of a high purity alumina matrix that is manufactured electrochemically. This porous substrate, known in the art, has been used for capture of micro-organisms and particulate material on the surface of the membrane for subsequent analysis by light or electron microscopy, however embodiments of the present invention provide the novel use of such a porous membrane as a substrate for the synthesis of nanostructures, whereby catalyst particles are deposited inside the pores. The term "in" or "inside" used herein may refer to that a catalyst particle is positioned at an inner or interior part of the pore or pore channel.

In preferred embodiments a substrate for the synthesis of nanostructures is a membrane which preferably is hydrophilic and is compatible with most solvents and aqueous material. In other preferred embodiment such membrane, preferably used as a porous substrate, is preferably peripherally bonded to e.g. an annular polypropylene ring for ease of handling and is suitable for both vacuum and pressure filtration.

In preferred embodiments catalyst particles are provided in or inside the pores or pore channels of a porous substrate by modifying the surface of the substrate by e.g. chemical etching and bringing in the catalyst inside the pores by impregnation, e.g. dipping into a catalyst solution, e.g. a metal-salt catalyst solution, and preferably drying said substrate comprising the dipped catalyst, e.g. metal-salt, in order to obtain a substrate comprising the catalyst inside the pore channels. Advantageously, by dipping, instead of e.g. ion beam sputtering or molecular beam deposition, the catalyst particles are distributed evenly along the pore channels driven by the wetting of the channel walls. If one would use a physical deposition technique such as ion beam sputtering or molecular beam epitaxy, the catalyst deposition distribution will be inhomogeneous and difficult to control, especially the pores on the left- and righthand side will be deposited differently (unless the sample is rotated) because of the different angle of the directed beam of sputtered material or molecular beam as well as because of the shallow opening of the pores. In preferred embodiments, by dipping a substrate in order to enable the introduction of catalyst particles inside the pore channel of the substrate, one can use this method for substrates with different pore diameters. For e.g. sputtering, a substrate comprising pores with large diameters may be used, because shallow pores will disadvantageously lead to a high shadowing effect.

In preferred embodiments the chemical etching may be performed by for instance NaOH/HCI etching, the latter advantageously provides a modification of the surface of a porous substrate, for instance by providing a cleaner surface. Moreover, said modification advantageously improves the adhesion qualities of the substrate surface. In addition, said modification, e.g. etching, increases the roughness of the walls of the pore channels of the membrane or substrate providing nucleation sizes for catalyst particles after dip-coating.

In preferred embodiments a membrane, which in embodiments of the invention is used as a substrate, may be hardened or coated e.g. with a sol-gel of alumina in order to increase its mechanical stability.

In preferred embodiment the catalyst particles are preferably evenly distributed along the pore channel of the porous body, e.g. a substrate used for the synthesis of nanostructures.

The porous substrate may be a porous hierarchical substrate, said porous hierarchical substrate comprising a porous parent body, said parent body comprising a plurality of pores and whereby at least one pore of said parent body comprises at least one porous child body, whereby said porous child body comprises a plurality of pores, and wherein said plurality of catalyst particles are comprised in said child body. The child body may comprise a thickness of at least 0.3% and up to 1 00% of the parent body thickness. The pore diameter of the child body may be smaller than the pore diameter of the parent body. A pore of the parent body may be defined by a channel and whereby said channel extends through the whole parent body. A pore of the child body may be defined by a channel and whereby said channel extends through the child body. The channel of the parent and/or child body may comprise a hexagonal or circular or cubic or lameral cross-section. The porous parent body may comprise a first and second outer body surface. The parent or child pores channels may be oriented perpendicularly with respect to the first and second outer body surface. The channel may be straight or is inclined with respect to the surface with a degree of 0 to 90 °. The channel may be straight or may be inclined with respect to the surface with a degree of 5 to 90 °. The parent body may have a thickness ranging from 20 μηι up to 1 0 mm, for example from 60 μηι up to 1 mm or from 60 μηι to 200 μηι. The channel length of the parent pore channel may range from 20 μηι up to 1 0 mm, for example from 60 μηι up to 1 mm or from 60 μηι to 200 μηι. The child body may have a thickness ranging from 1 00 nm up to 1 0 mm, for example from 150 nm up to 1 mm or from 200 nm up to 1 μηι. The parent body of the substrate may be made from a material provided with pore size ranging from about 20 nm to about 2 mm. The child body of the substrate may be made from a material provided with pore diameter ranging from about 2nm to about 50 nm. The parent and/or child porous body may comprise a void fraction of from about 1 0 percent to about 90%. The catalyst particles may be substantially evenly distributed across the child body pores.

In another aspect the invention provides holders for substrates used for the synthesis of nanostructures. A holder, according to embodiments of the invention, advantageously enables that reactive gas may go through the membrane pores of a substrate according to embodiments of the invention in order to separate an inlet, which e.g. supplies reactive gasses, from an outlet, whereby the exhaust gasses might influence the growth process. Thus advantageously a holder according to embodiments of the invention enables that the gas flow fully or mainly goes through the pores of the substrates, whereby said pore channels of the substrate comprise catalyst particles, and thus generates CNTs inside of the pores of the substrate and limited on the surface of the substrate. The holder is a passageway across the thickness of the holder, for instance a cylindrical tube, with a partially closed ending on one side and an open ending at the opposite side (see drawing). The diameter of the holder preferentially corresponds to the inner diameter of the reactor tube and can be scaled from 1 mm to 1 m. The closed ending is directed towards the inlet of the gases, preferably at the upstream side, and the open side towards the outlet of the reactor, preferably at the downstream side. The partially closed side contains one or several apertures depending on the size of the reactor and the substrates. The apertures preferably have the design of a truncated-cone forming pockets where the substrates are placed. The smaller base of the truncated cone of the pockets and the substrates must be of the same size and shape. In further embodiments, an accessory in the shape of smaller cylinder with a conical ground join may be provided which ensures the fixation of the substrate in the pocket.

In preferred embodiments a holder may be provided which allows hosting of one or more membranes (e.g. 7 membranes such as in the drawing). The basic principle of the holder can be adapted to include 1 membrane as well as numerous membranes. The number of the pockets is limited by the holder diameter dh as well as by the membrane diameter dm. Preferentially, the pockets are arranged in hexagonal packing. The central pocket is surrounded by 6 other pockets. With increasing diameter of the holder multiples of 6 pockets come into addition, hence the number of the pockets goes from 1 to 6-( with n being an integer number (e.g. 7, 19, 37 etc.).

The holder diameter is given by dh > (2n+1 )d_m , hence for n=0, only one membrane can be placed with dh > dm. For n=1 ,2 and 3, 7,19 and 37 membranes can be mounted, respectively.

The holder material is chosen such that it withstands the high temperature of 400 °C up to 1400 °C and the exposure to the reactive gases. The holder is therefore preferentially made of quartz.

Preferred embodiments of the invention provide the use of inorganic membranes, such as for instance an anodised aluminium oxide, as a growing substrate in a system for synthesis of nanostructures.

In an aspect the invention provides methods of preparing a porous body, said method comprising:

-providing a porous parent body;

-partial filling of at least one pore of the parent body along a pore channel, with a filling substance, resulting in a filled and in an empty part of the parent pore;

-providing a child body in the empty part of the parent pore;

-removing the filling substance in the filled part of the parent pore;

-creating porosity in the child body, resulting in a porous child body;

In preferred embodiments the child body comprises a metal oxide or metal oxide gel. Preferably providing the metal oxide gel may be performed by synthesis of the metal oxide in said empty part with or without precursors. In preferred embodiments providing the metal oxide gel is performed by mechanical force, sonication, vibration, evaporation-intrusion or liquid filtration.

In preferred embodiments the porous child body comprises a thickness of at least 0.3% and up to 99.7% of the parent body thickness.

In other embodiments of the invention, the method of preparing a porous body further comprises providing metal particles in said pores of the porous child body. Preferably providing the metal particles may be performed by selective deposition of the metal particles in the porous child body.

In preferred embodiments the porous child body comprises channels extending throughout the child body, e.g. extends over its entire thickness.

In some embodiments the porous child body may fill the pores of the parent body over substantially their full width, i.e. over substantially the full cross-section of the channels. The latter may be for example, over at least 95% of the cross-section of the pore the parent body, e.g. over at least 99% of the cross-section of the pore of the parent body

In preferred embodiments, the average number (i.e. averaged over all pore channels) of catalyst particles positioned in the pore channels of the porous child body is 1 .0 or higher. This offers the advantage of a higher growth efficiency, because the chance that a carbon precursor molecule flowing through a specific channel is "caught" is higher (if more than one catalyst particle is used) compared to the case where there was only a single catalyst particle per channel.

It is an advantage that the catalyst particles are primarily located inside the pore channels through which the reaction gas flows, rather than on the downstream surface of the substrate, in that a higher density of CNTs can be obtained because of the direct exposure of catalyst particles to the reaction gases, and because the carbon concentration of the gas is higher inside the pore channels than at the downstream surface location.

In preferred embodiments each of the pore channels has a substantially gas-impermeable wall (unlike e.g. an array of needles). This offers the advantage that the process gases will be more efficiently exploited. It also offers a higher growth efficiency because the diffusion of carbon through the pore channel walls is limited.

In other preferred embodiments the body is a layer or a particle. Preferably the particle is a silica particle. More preferably the body may be a porous membrane.

Embodiments of the method for providing a porous body according to the invention provide a porous body having a hierarchical structure, whereby the porous body is manufactured using a method according to embodiments of the invention.

In another aspect the present invention provides porous bodies comprising a porous parent body, said porous parent body comprising a plurality of pores and whereby at least one pore of said parent body comprises at least one porous child body, whereby said porous child body comprises a plurality of pores, whereby said porous child body comprises a thickness of at least 0.3%. In other preferred embodiments said porous child body comprises a thickness of at least 0.3% and up to 99.7% of the parent body thickness. Preferably the pore diameter of the porous child body is smaller than the pore diameter of the porous parent body.

In preferred embodiments a pore of the porous parent body is defined by a channel and whereby said channel extends through the whole porous parent body.

In other preferred embodiments the pore of the child body is defined by a channel and whereby said channel extends through the porous child body.

In preferred embodiments the body is a layer or particle.

In preferred embodiments of the porous body according to the invention, the channel of the parent and/or child body comprises a hexagonal or circular or cubic or lameral cross-section. In preferred embodiments the child body pores may comprise metal particles.

In preferred embodiments of the porous body the metal particles are positioned along and in the porous child body channel.

In other preferred embodiments of the porous body the porous parent body comprises a first and second outer body surface. Preferably the parent or child pores channels may be oriented perpendicularly with respect to the first and second outer body surface.

In preferred embodiments of the porous body the channel is straight or is inclined with respect to the surface with a degree of 0 to 90 °. In other preferred embodiments the channel is straight or is inclined with respect to the surface with a degree of 5 to 90 °.

In preferred embodiments of the porous body the child porous body is positioned at the first outer body surface or at the second outer body surface or in-between the first and second outer body surface.

In preferred embodiments of the porous body the parent body has a thickness ranging from 20 μηι up to 10 mm. In preferred embodiments of the porous body the parent body has a thickness ranging from 60 μηι up to 1 mm. In other preferred embodiments the parent body has a thickness ranging from 60 μηι to 200 μηι. In preferred embodiments the channel length of the parent pore channel ranges from 20 μηι up to 10 mm. In other preferred embodiments the channel length of the parent pore channel ranges from 60 μηι up to 1 mm. More preferably the channel length of the parent pore channel may range from 60 μηι to 200 μηι.

In preferred embodiments the child body has a thickness ranging from 1 00 nm up to 1 0 mm. Preferably the child body may have a thickness ranging from 150 nm up to 1 mm. More preferably the child body may have a thickness ranging from 200 nm to 1 μηι.

In preferred embodiments of the porous body according to the invention the porous body is a membrane.

In preferred embodiments the invention provides a porous body, whereby said porous parent body comprises a pore diameter between 20 nm and up to 2mm. Preferably the parent body may comprise a void fraction between 1 0 and 90%.

In preferred embodiments the porous child body comprises a pore diameter between 2nm and 50nm and a void fraction between 1 0 and 90%.

In other preferred embodiments the porous child body is a catalytic active thin metal oxide layer.

Preferred embodiments of the invention provide a porous body, whereby the porous body is a hierarchical porous body. Preferably a hierarchical porous body may link the parent and/or at least one child body either directly or indirectly, and either vertically or horizontally.

In preferred embodiments the porous parent body is made from a material including carbon foams, glassy carbon, silica, alumina, alumina coated with silica, zirconia, zeolites, sintered titanium, titania, magnesia, yttria, copper, iron, iron nickel, iron cobalt, cobalt, steel, iron carbide, nickel, or a combination thereof.

In yet another aspect the invention provides the use of the porous membrane according to embodiments of the invention for synthesis of nanostructures.

In a further aspect the invention provides systems for producing a porous membrane body having a hierarchical structure, the system comprising at least means for providing a porous parent body, means for filling of at least one pore of the parent body along a pore channel, with a filling substance, resulting in a filled and in an empty part of the parent pore, means for providing a child body in the empty part of the parent pore and means for removing the filling substance in the filled part of the parent pore, for performing a method according to embodiments of the invention for provide a porous body.

Preferably the pore channel of a child porous body is parallel to the pore channel of a parent porous body.

A parent body, preferably a layer or a membrane, used in embodiments of the present invention may be commercial layers or membranes known in the art. Preferably after the deposition of e.g. a layer or child particle such as metal oxides or a hexagonal COK-12 silica particle respectively, advantageously provide a child body, e.g. a membrane, in at least one pore of the parent body e.g. layer or membrane, moreover embodiments of the present invention enable a hierarchical body e.g. a hierarchical membrane which can be used as a substrate for e.g. CNT growth. In examples of the present invention, a hierarchical membrane may be a parent crystalline mesoporous parent silica material, being obtained by an assembly of nanometer size building units having a zeolite framework, said crystalline mesoporous silica material having two or more children levels of porosity and structural order. The crystalline mesoporous hierarchical materials, e.g. silica materials, according to embodiments of the invention are useful in a number of industrial applications, such as, but not limited to, the fixation of biologically active species, as well as an electro-optical or dielectric material or for catalysis, molecular separation or adsorption of metal ions. Moreover the hierarchical bodies according to embodiments of the invention can be used for biosensing and for photo conversion or photo devices (e.g. fluorescent carbon dots or CNTs).

In the substantially crystalline mesoporous oxide based material according to the invention, said oxide based material may comprise one or more oxides selected from the group consisting of silica, germanium oxide and metallic oxides. A preferred non-metallic oxide is silica. The metallic oxides may derive from any metal selected from groups 4 to 12 of the periodic table. Preferred metals are aluminum and transition metals. Exemplary metallic oxides are preferably selected from the group consisting of alumina, titania, zirconia, ceria, manganese oxide, niobium oxide, tantalum oxide, tungsten oxide, tin oxide, gallium oxide, iron oxide, and hafnium oxide. In other embodiments ternary or quaternary oxides can be used, which can be a combination of two or three of the named oxides. The parent, e.g. mesoporous oxide, based material according to the invention may comprise silica in combination with one or more such metallic oxides, which will be selected according to the intended end use of the material. For instance silica, alumina, titania, zirconia and their mixtures in a wide range of proportions may be considered for use as acidic catalysts. A mesoporous tungsten oxide material may be used as a carrier for a number of ruthenium based catalysts for diverse chemical reactions including polymerization, metathesis, epoxidation, hydro-amination, aziridination and the like. A mesoporous semiconducting oxide material may be used for industrial applications such as the construction of fuel cells.

Embodiments of the present invention advantageously provide a novel hierarchical body or e.g. membrane as support or substrate for e.g. vertical CVD growth, the membrane according to embodiments of the invention comprises a parent layer e.g. membrane, whereby pores of the parent membrane comprise a porous metal oxide as a child membrane or e.g. a hexagonal COK-12 silica particle as a child porous particle, preferably within the channels of the parent layer e.g. membrane. The orientation of the pores and/or pore channels in the layer, e.g. parent or child membrane, is preferably perpendicular with respect to the surface of the layers.

The hierarchical nature of a porous body or membrane according to embodiments of the invention can be preferably obtained by e.g. filling a parent layer or membrane channel with silica for a limited part along its axial length or by partial filling along the full length of the membrane channel or by deposition of individual metal oxide particles along the membrane channel.

In preferred embodiments a method for providing a porous layer or particle e.g. membrane, preferably a catalytic active thin metal oxide layer in the parent membrane can comprise at least the steps of:

- Partial filling of the meso/macroporous pores of the parent membrane with a filling substance via a top-down or bottom up approach

- Synthesis of the metal oxide, with or without metal precursors, in the residual empty channel space of the parent membrane

-removing of the filling substance in the parent membrane

-creating of porosity in the metal oxide, e.g. by removal of surfactant

-deposition of the metal species

In other preferred embodiments the metal oxide synthesis gel may be introduced in the open membrane channel of the parent membrane by mechanical force i.e. sonication, vibration, evaporation-intrusion and liquid filtration. Preferred embodiments of the invention provide a method to control metal deposition at a thin top layer of a membrane by selective deposition of metal particles on the newly introduced metal oxide phase, preferably in the child membrane structure.

In preferred embodiments a continuous gas flow through the porous membrane is enabled by limiting the amount of growth nuclei in each pore of the child membrane and providing gas large transport channels in the hierarchical structure.

The membrane method according to embodiments of the invention can be used for growing fibrous carbon nanostructures using CVD synthesis conditions, like for example the one proposed in WO'084. The carbon nanostructures are preferably grown after providing contact between the metal comprising hierarchical membranes with at least a carbon-containing gas and optionally a support or carrier gas at elevated temperature between 200 °C and 1400 °C, preferably between 400 °C and 1200 °C and more preferably between 600 °C and 1000 °C. The metal catalyst particles can be provided on or preferably within the pores of the child body or membrane.

Embodiments of the present invention provide a method to align the produced fibrous carbon nanostructures under influence of gravimetric forces, gas flow and an increased density of growth nuclei and optionally further supported by a magnetic field.

Hierarchical membranes according to embodiments of the present invention can be used for the synthesis for any kinds of nanostructures by means of a CVD process, whenever catalyst particles, gas flow, formation of nanostructures are involved. This can be for instance the case for the growth of nanowires based on the catalytic VLS-growth. In embodiments of the present invention hierarchical membranes can be used for carbon nanostructures as well and more specifically for CNTs.

In an aspect the invention provides systems for synthesis of nanostructures, the system comprising:

-a housing having a first end, an opposite second end, and a passageway extending between the first and second ends;

-a porous hierarchical substrate, said hierarchical porous substrate comprising a porous parent body, said parent body comprising a plurality of pores and whereby at least one pore of said parent body comprises at least one porous child body, whereby said porous child body comprises a plurality of pores and whereby said porous substrate is situated within the passageway of the housing;

-said child body comprising a plurality of catalyst particles, and from which nanostructures can be synthesized upon interaction with a reaction gas;

- a heating mechanism circumferentially placed about the substrate for generating energy sufficient to maintain an environment within which nanostructures are synthesized within a specified temperature range;

-a pair of flanges, each capping one end of the housing; and an inlet positioned across the flange capping the first end of the housing and through which reaction gas can be directed to/through the porous substrate.

In preferred embodiments of the system for synthesis of nanostructures, the body is a layer or particle, e.g. a silica particle. In other preferred embodiments the child body comprises a thickness of at least 0.3% and up to 100% of the parent body thickness.

In other preferred embodiments of the system for synthesis of nanostructures the pore diameter of the child body is smaller than the pore diameter of the parent body. In preferred embodiments the pore of the parent body is defined by a channel and whereby said channel extends through the whole parent body. In other preferred embodiments the pore of the child body is defined by a channel and whereby said channel extends through the child body.

In other preferred embodiments of the system for synthesis of nanostructures the channel of the parent and/or child body comprises a hexagonal or circular or cubic or lameral cross-section. Preferably the porous parent body may comprise a first and second outer body surface. Preferably the parent or child pores channels may be oriented perpendicularly to the first and second outer body surface. In alternative embodiments the channel may be straight or is inclined with respect to the surface with a degree of 0 to 90 °. More preferably the channel may be straight or is inclined with respect to the surface with a degree of 5 to 90 °.

In preferred embodiments the parent body has a thickness ranging from 20 μηι up to 10 mm. In other preferred embodiments the parent body may have a thickness ranging from 60 μηι up to 1 mm. In preferred embodiments the parent body has a thickness ranging from 60 μηι to 200 μηι. In preferred embodiments the channel length of the parent pore channel may range from 20 μηι up to 10 mm. In other preferred embodiments the channel length of the parent pore channel may range from 60 μηι up to 1 mm. In preferred embodiments the channel length of the parent pore channel ranges from 60 μηι to 200 μηι. In other preferred embodiments the child body may have a thickness ranging from 100 nm up to 10 mm.

In preferred embodiments of a system comprising a hierarchical substrate, the hierarchical substrate according to embodiments of the invention, whereby the child body has a thickness ranging from 150 nm up to 1 mm. Preferably the child body may have a thickness ranging from 200 nm to 1 μηι.

Preferred embodiments of a system for synthesis of nanostructures according to embodiments of the invention further may include an exhaust port through which reaction waste product flowing from the substrate may be removed. Preferably the housing may be made from a strong, substantially gas-impermeable material. In other preferred embodiments the material is substantially resistant to corrosion and/or high temperature. In alternative preferred embodiments the material may be quartz.

In preferred embodiments of a system for synthesis of nanostructures the substrate is sufficiently porous so that a pressure difference between both surfaces of the substrate can be substantially low, so as to permit the substrate to maintain its structural integrity. In other preferred embodiments of a system synthesis of nanostructures the parent body of the substrate is made from a material provided with a pore size ranging from about 20 nm to about 2 mm. In further preferred embodiments the child body of the substrate is made from a material provided with pore diameter ranging from about 2nm to about 50 nm. Preferably the parent and/or child porous body may comprise a void fraction of from about 1 0 percent to about 90%.

In preferred embodiments of a system for synthesis of nanostructures according to the invention the substrate is made from a material including carbon foams, glassy carbon, silica, alumina, alumina coated with silica, zirconia, zeolites, sintered titanium, titania, magnesia, yttria, copper, iron, iron nickel, iron cobalt, cobalt, steel, iron carbide, nickel, or a combination thereof. In other preferred embodiments the substrate is one of foam, channel plate, felt, wool, fibers, cloth, or array of needles.

In preferred embodiments of a system for synthesis of nanostructures according to the invention the catalyst particles may be substantially evenly distributed across the child body pores. In other preferred embodiments the catalyst particles are made from a material including iron, nickel, cobalt, iron oxides, nickel oxides, cobalt oxides, metal salts with sulfate, metal salts with sulfamates, acetate, citrate, oxalates, nitrites, nitrates, or a combination thereof. In other preferred embodiments the catalyst particles range from about 0.5 nm to about 50 nm in size. In preferred embodiments of a system for synthesis of nanostructures according to the invention the heating mechanism can maintain the temperature of the growth environment within a range of from about 200 °C to about 1400 °C. Preferably the heating mechanism can maintain the temperature of the growth environment within a range of from 400 °C up to 1200 °C. More preferably the heating mechanism can maintain the temperature of the growth environment within a range of from 600 °C to 1 000 °C.

Preferred embodiments of a system for synthesis of nanostructures according to the invention provide that the energy generated from the heating mechanism may include thermal energy, frictional energy, visible light photons or other types of electromagnetic radiation, chemical, electrical, or electrochemical energy, microwave radiation, eddy currents, or ultrasound shock waves or compression.

In preferred embodiments of a system for synthesis of nanostructures according to the invention the flanges are substantially gas- impermeable.

Preferred embodiments of a system for synthesis of nanostructures according to the invention, further include a tube for accommodating the porous substrate, the tube being situated within the passageway of the housing for providing a pathway for the reaction gas from the inlet to the substrate. Preferably the substrate may be situated within the tube. In preferred embodiments the tube is made from a strong, substantially gas-impermeable material. Preferably the material may be quartz.

Preferred embodiments of a system according to invention further may include: a first electrode situated within the pathway of the tube upstream of the porous substrate; and a second electrode situated within the pathway of the tube downstream of the porous substrate. In preferred embodiments the upstream electrode and the downstream electrode may be designed to generate an electric field there between to provide physical support to the nanostructures growing from the substrate. In other preferred embodiments the upstream and downstream electrodes may be designed to generate an electric field there between to control and maintain direction of growth to the nanostructures growing from the substrate. Preferably the electrodes permit flow of reaction gas within the tube there through. In alternative preferred embodiments the electrodes are made from an electrically conductive material that is non-reactive and resistant to nanostructure growth. In preferred embodiments the material from which the electrodes are made includes graphite, copper, titanium, vitreous carbon, a combination thereof, or other conductive materials not catalytic to carbon nanostructure formation.

In preferred embodiments of a system according to invention provide that a porous substrate, according to embodiments of the invention, is positioned circumferentially about an exterior surface of the tube at one end of the tube, and the system further includes: a tubular electrode having an open end and sufficiently sized so as to concentrically accommodate the porous substrate through the open end; and a plurality of guides, positioned between the tubular electrode and the substrate, and over which growing nanostructures can be directed. In preferred embodiments the porous substrate may be designed to be an electrode, such that along with the tubular electrode, an electric field can be generated there between to provide physical support to the nanostructures growing from the substrate. In other preferred embodiments the porous substrate and the tubular electrode designed to generate an electric field there between to control and maintain direction of growth to the nanostructures growing from the substrate. In yet other preferred embodiments the substrate may be made from an electrically conductive material including, glassy carbon, porous titania, porous zirconia, or sintered titanium powder. In other preferred embodiments the substrate is made from an inert material coated with an electrically conductive material including, copper, tin oxide, titania, titanium, tungsten or platinum.

In preferred embodiments the tubular electrode is rotatable and retractable from its position over the substrate so as to pull growing nanostructures there along.

In other preferred embodiments the guides may include a series of rings circumferentially placed about the substrate. Preferably a system according to embodiments of the invention further may include a second inlet situated across the flange capping the first end of the housing and through which an evacuation gas can be directed into the passageway of the housing to displace and remove reaction waste product within the housing. In another aspect the invention provides substrates for synthesis of nanostructures, the substrate comprising: a porous parent body having an upstream surface and a downstream surface; the porous body further comprising at least a porous child body in said parent pore and a plurality of catalyst particles, deposited in said child body, whereby said child body is positioned or on the upstream or on the downstream surface or in-between the upstream and downstream surface of the substrate, and from which nanostructures can be synthesized; wherein the porous body provides pathways through which a reaction gas can travel across the upstream surface and out the downstream surface to initiate growth of nanostructures from the catalyst particles. Preferably said porous substrate is hierarchical or ordered.

In preferred embodiments the porous substrate is sufficiently porous so that a pressure difference between the upstream surface and the downstream surface can be substantially low, so as to permit the body to maintain its structural integrity.

In a further aspect the invention provides methods for synthesizing nanostructures, the method comprising: -providing a porous hierarchical substrate;

-directing a flow of a reaction gas across the upstream surface and through the downstream surface of the substrate;

-decomposing the reaction gas about the catalyst particles to generate constituent atoms;

-allowing for diffusion of the constituent atoms on or in the catalyst particles and thereon for the synthesis of nanostructures therefrom. In preferred embodiments the step of directing includes allowing the gas, after exiting the downstream surface of the substrate, to flow past growing nanostructures, so as to provide support to the growing nanostructures. In preferred embodiments the step of directing includes adding a carbon-containing source to the reaction gas prior to directing the reaction gas to the substrate. Preferably in the step of adding, the carbon- containing source may include ethanol, methane, methanol, ethylene, acetylene, xylene, carbon monoxide, or toluene.

In preferred embodiments the method further may include introducing an evacuation gas to displace and remove reaction waste product.

In preferred embodiments the step of decomposing may include generating energy to temperatures ranging from about 200 °C to about 1400 °C.

In other preferred embodiments the step of decomposing may include generating energy to temperatures ranging from about 400 °C to about 1400 °C.

In yet other preferred embodiments, in the step of generating, the energy may include thermal energy, frictional energy, visible light photons or other types of electromagnetic radiation, chemical, electrical, or electrochemical energy, microwave radiation, eddy currents, or ultrasound shock waves or compression.

In preferred embodiments the step of allowing includes growing the nanostructures in a direction substantially parallel to the flow of reaction gas.

In other preferred embodiments the step of allowing may include growing the nanostructures in a direction substantially radially to the flow of reaction gas. Preferably the step of allowing may include growing the nanostructures to a specified length. In preferred embodiments, in the step of growing, the specified length may be at least 1 cm. In other preferred embodiments, in the step of growing, the specified length may be at least 1 m.

In yet other preferred embodiments the method further may include generating an electrostatic field having a strength between about 5 kV/m and about 1000 kV/m to provide support the nanostructures as they grow from the substrate, so as to contribute to the straightness of the nanostructures.

In preferred embodiments the method further may include generating a magnetic field having a magnitude between about 0.1 T and about 50T to provide support the nanostructures as they grow from the substrate, so as to contribute to the straightness and/or alignment of the nanostructures. More preferably a magnetic field having a magnitude between about 0.1 T and about 30T may be generated.

The values of the magnetic field according to embodiments of the invention are chosen such to enable in situ orientation but also to enable a base growth mechanism. One preferably uses a high magnetic field in order to orient CNTs after the growth (up to 50T, preferably up to 30T), for instance CNTs dispersed in a solution or CNTs powder etc. During the growth (in-situ), the magnetic field applied is much lower because of the magnetic catalyst particles involved. As a result the magnetic field is preferably chosen such that the magnetic metal particle (e.g. all CNT growing particles such as Co, Ni, Fe, etc.) stay attached to the membrane surface according to embodiments of the invention.

In yet other preferred embodiments the method further may include generating an electric field having a magnitude between about 5 kV/m and about 1000 kV/m to provide support to the nanostructures as they grow from the substrate, so as to contribute to the straightness and alignment of the nanostructures.

In preferred embodiments the method further may including collecting the nanostructures once the nanostructures have grown to a desired length.

In another aspect the invention provides a fibrous material comprising a plurality of nanostructures according to embodiments of the invention. Preferably said fibrous material may be used in thermal management applications.

In yet another aspect the invention provides heat conductors comprising the fibrous material set obtained by using method according to the present invention.

Preferred embodiments of the invention provide a low eddy current, low resistance winding for an electric motor comprising the fibrous material according to embodiments of the invention.

Preferred embodiments of the invention provide a low eddy current, low resistance winding for a high frequency solenoid comprising fibrous material according to embodiments of the invention.

Preferred embodiments of the invention provide a winding for a high frequency transformer comprising the fibrous material according to embodiments of the invention.

Preferred embodiments of the invention provide a fabric comprising the fibrous material according to embodiments of the invention.

Preferred embodiments of the invention provide a protective armour or clothing comprising the fibrous material according to embodiments of the invention.

Preferred embodiments of the invention provide a rope or cable comprising the fibrous material according to embodiments of the invention.

Preferred embodiments of the invention provide a yarn comprising the fibrous material according to embodiments of the invention.

Preferred embodiments of the invention provide a sheet comprising the fibrous material according to embodiments of the invention.

Preferred embodiments of the invention provide a heat sink comprising the fibrous material according to embodiments of the invention.

Preferred embodiments of the invention provide a high strength, low eddy current, low resistance electric power transmission line comprising the fibrous material according to embodiments of the invention.

Preferred embodiments of the invention provide a process for packing of carbon nanotubes comprising: generating the nanotubes according to embodiments of the invention; coating the nanotubes with one of furfuryl alcohol, epoxy resin, or Resol; and arranging the coated nanotubes in bundles for packing.

It is an advantage of embodiments of the present invention that multi-walled nanoparticles, e.g. carbon nanotubes, as well as single-walled nanoparticles can be obtained using embodiments of the present invention. It is an advantage of embodiments of the present invention that large as well as small catalyst particles can be used.

In some embodiments, the smallest pore diameter determines the lowest limit of the nanoparticles diameter and the upper limit of the nanoparticles diameter, e.g. carbon nanotube diameter, is determined by the metal catalyst particles formed on the pore side walls.

It is an advantage of embodiments of the present invention that these allow growing of individual nanoparticles as well as growing of multiple nanoparticles inside one pore.

It is an advantage of embodiments according to the present invention that high density nanoparticle arrays, e.g. high density carbon nanotube arrays, can be obtained inside the pores, which may advantageously be used in chemical sensor applications. Such arrays can be densely packed but are still permeable for gasses. The obtained arrays typically are highly efficient and highly sensitive.

It is an advantage of embodiments of the present invention that a fast and efficient reaction can be obtained as the reaction gas can already interact with the catalyst particles in the pore. It is an advantage of embodiments of the present invention that the highest density of nanoparticles will be formed in the pores. The latter is especially advantageous when used in chemical sensors as this may allow interaction with the entire surface, including the side walls, of the nanoparticle, e.g. carbon nanotube. For chemical sensing, this significantly increases the number of active sites.

It is an advantage of embodiments of the present invention that catalyst particles are not in the nanoparticles grown and therefore do not weaken the materials. It is an advantage of embodiments of the present invention that the nanoparticles can be properly oriented, resulting in a good close packaging.

It is an advantage of embodiments of the present invention that the reaction gas flows through the pores, resulting in advantageous growth of nanoparticles as described.

Particular and preferred aspects of the invention are set out in the accompanying independent and dependent claims. Features from the dependent claims may be combined with features of the independent claims and with features of other dependent claims as appropriate and not merely as explicitly set out in the claims. For example, the different methods and systems comprising or using a porous substrate may make use of a hierarchical structured substrate, i.e. a substrate comprising a porous parent body and comprising a child body in the pores of the porous parent body.

Brief description of the drawings

Further features of the present invention will become apparent from the examples and figures, wherein:

Fig. 1 illustrates, in correspondence with one embodiment of the present invention, an example (vertical) set-up of the chemical vapour deposition setup as used for the growth of fibrous carbon nanostructures.

Fig. 2 shows an exemplary strategy in order for the preparation of a hierarchical membrane applied as a substrate in the CVD set-up.

Fig. 3a shows the targeted localization of the catalytic active metal particles within the pores of the membrane substrate according to embodiments of the invention.

Fig. 3b shows the targeted localization of the catalytic active metal particles within the pores of the membrane substrate according to alternative embodiments of the invention

Fig. 4 summarizes the different steps involved in the synthesis of the fibrous carbon nanostructures according to embodiments of the invention.

Fig. 5 is a SEM image and illustrates a partial pore filling as intermediate step toward hierarchical membranes, according to embodiments of the invention.

Fig. 6a is a SEM image and shows the partial filling of the membrane channels with a mesoporous solid, according to embodiments of the invention.

Fig. 6b is a SEM image and shows the complete filling of the membrane channels with a mesoporous solid according to embodiments of the invention.

Fig. 7 illustrates the deposition of mesoporous particles inside the membrane channels according to embodiments of the invention.

Fig. 8 illustrates schematically a porous ordered membrane.

Fig. 9a schematically illustrates a porous anodised aluminim oxide (AAO) membrane known in the art, where said AAO membrane is a relatively inexpensive and commercially available membrane, such a membrane is for instance described by Sonny S. Mark et al., in Colloids and Surfaces B: Biointerfaces (2008) vol 65, pp. 230-238. Fig 9b illustrates images showing an AAO membrane comprising catalyst particles inside the pore channels or columns of the AAO membrane, according to embodiments of the invention.

Figs. 10a and 10b illustrate a holder, according to embodiments of the invention, which may comprise a porous membrane or an porous ordered membrane, whereby the membranes both can be used as a substrate in systems for synthesis of nanostructures according to embodiments of the invention. Such a holder preferably is used for CVD growth systems of nanostructures. Fig. 10 illustrates an image of the membrane holder according to embodiments of the invention, whereas Fig. 10b schematically illustrates a holder, according to embodiments of the invention, when used for a large CVD oven that allows inserting 7 membranes into the holder or a small CVD oven that allows mounting only 1 membrane. Fig. 1 1 illustrates a holder according to embodiments of the invention, whereby said holder is used for the synthesis of extended length carbon nanotubes with one or more pockets, whereby he amount of pockets in the holder depends on the size of of the reactor and the diameter of the membrane, which can be up to 100 or even more), more specifically in this figure 7 pockets are provided an example. Moreover Figure 1 1 depicts the plane view (on the left) and the lateral view (on the right).

Fig. 12 illustrates how a holder, according to embodiments of the invention, and in addition illustrates how the diameter of such a holder (dh) depends on the size, e.g. diameter (dm), of the membrane.

Definitions

The term "membrane" used in this text relates to a thin, film-like structure that separates two phases, for instance in fluids, like for instance liquid and/or gas environments It acts as a selective barrier, allowing some particles or chemicals to pass through, but not others. Membranes can be of various thicknesses, with homogeneous or heterogeneous structure. Membrane can also be classified according to their pore diameter (dp). According to lUPAC, there are three different types of pore size classifications: microporous (dp < 2 nm), mesoporous (2 nm < dp < 50 nm) and macroporous (dp > 50 nm). Membranes can be neutral or charged, and particles transport can be active or passive. The latter can be facilitated by pressure, concentration, chemical or electrical gradients of the membrane process. Membranes can be generally classified into synthetic membranes and biological membranes.

The term "hierarchical" used in this text relates to a structure arranged in a graded order which as a result form a hierarchy. A hierarchy is then preferably an arrangement of items, in this text of porous bodies or structures or substrates, in which the items can represented as being "above", such as a parent, "below" such as a child, or "at the same level as" one another. Abstractly, a hierarchy can be modeled mathematically as a rooted tree: the root of the tree forms the top level, and the children of a given vertex are at the same level, below their common parent. A hierarchy can link entities either directly or indirectly, and either vertically or horizontally. Indirect hierarchical links can extend "vertically" upwards or downwards via multiple links in the same direction, following a path. All parts of the hierarchy which are not linked vertically to one another nevertheless can be "horizontally" linked through a path by traveling up the hierarchy to find a common direct or indirect superior, and then down again. A substrate having a hierarchical structure according to embodiments of the present invention thus may be a substrate comprising at least one porous parent body having pores and comprising at least one child body positioned in a pore of the porous parent body.

Detailed description of preferred embodiments

The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. Where the term "comprising" is used in the present description and claims, it does not exclude other elements or steps. Where an indefinite or definite article is used when referring to a singular noun e.g. "a" or "an", "the", this includes a plural of that noun unless something else is specifically stated.

The term "comprising", used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. Thus, the scope of the expression "a device comprising means A and B" should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B.

Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

Moreover, the terms top, bottom, over, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other orientations than described or illustrated herein.

In the drawings, like reference numerals indicate like features; and, a reference numeral appearing in more than one figure refers to the same element. The drawings and the following detailed descriptions show specific embodiments of devices and methods for synthesis of nanostructures.

Embodiments of the present invention relate to a method for the synthesis of a hierarchical porous body, e.g. a layer or membrane. The invention further relates to a method to form fibrous carbon nanostructures in high yields via a chemical vapour deposition approach from inside the body pores, e.g. membrane pores, which functions as the support.

Embodiments of the present invention relate to a novel method to deposit and control the amount of (transition) metal nanoparticles in a porous body, e.g. a membrane, the metal being crucial to initiate the growth of the carbon nanostructures. Preferably said metal nanoparticles are provided in a child body of the porous parent body.

More specifically the invention relates the maximization of the metal dispersion and density in order to increase the amount of carbon nanostructure nuclei exiting the membrane surface. Related due to confinement of the metal particles in the selected e.g. membrane pores the final properties of the carbon nanostructure can be partially controlled.

More specifically hierarchical membranes according to embodiments of the invention guarantee a continuous growth by offering straight transport channels, connecting both surface ends of the porous body, e.g. membrane, and in the size range preferably of 10 nm to 0.9 mm, for the reaction gas and by preventing pore blocking during carbon growth due to selective metal deposition. The invention implies a sustained gas flow without pressure built-up and continuous growth of the fibrous carbon nanostructures in order to increase their overall length. The present invention is further related to the field of reinforcing materials and the use of fibrous carbon nanostructures in composite materials. The invention is further related to the alignment and consequent bundling of the individual fibrous carbon nanostructures into larger scale threads.

Advantageously embodiments of the present invention provide a method to grow CNTs with a tunable density, whereby density refers to the number of CNTs per cm², as well as with infinite or ultra-long (preferably greater than 10 cm) length by using a membrane, more specifically a hierarchical membrane, according to embodiments of the invention, as support for the growth of CNTs. This membrane advantageously enables tuning of the CNT density by tuning the pore density of the membrane. It also enables that the gas obstruction does not occur because the catalyst preferably will remain accessible via CNTs growing along the downstream direction according to embodiments of the invention. According to embodiments of the present invention, fibrous carbon nanostructures can be formed in the presence of metals, preferably transition metals (such as nickel, cobalt, iron) or combinations thereof in multimetallic clusters (bi- or trimetallic) in said porous child body. Metal precursors are deposited into a hierarchical membrane, according to embodiments of the invention, as a metal salt or as present in a substance with ion exchange properties (e.g. hyperbranched polymer) using a standard procedure preferably ion exchange, wet/incipient wetness impregnation or electrodeposition. The formation of metal nanoparticles can be preferably obtained after an optional oxidation step, performed e.g. via gaseous or chemical pretreatment, and preferably a reduction step, e.g. performed via a gaseous or chemical pretreatment or after decomposition of the carbon reaction gas and simultaneous formation of hydrogen gas at the growth conditions. In preferred embodiments the temperature is chosen between 200 and 1200 °C, a heating rate preferably between 0.01 °C min ¹ to 50 °C min ¹and a duration time preferably from 0 to 1200 min of the pretreatment/reaction control the final size of the metal particles, next to the confinement effect of the porous hierarchical membrane described below.

Embodiments of the present invention advantageously enable the synthesis and the use of CNTs with significantly longer, preferably greater than 1 cm or at least 1 mm of length. It is an object of the present invention to provide a method to incorporate ordered porous metal oxides in a meso/macroporous membrane. The porous parent body, e.g. membrane, comprises pores with a pore diameter between 20 nm and 2 mm and provides a void fraction between 1 0 and 90 %. In accordance with an aspect of the present invention the child porous body, e.g. metal oxide, has pores with an orientation which is defined by orientation of the membrane channels and results in pores perpendicular to the surface of the porous body, e.g. membrane. The present invention enables a large increase, up to a factor of 1 x 1 01 1 , of the pore density with a maximum of 1 pore per 7.79 nm², while controlling pore orientation and allows a dense and aligned growth of fibrous carbon nanostructures out from e.g. the child membrane pores. Control of the porous child body, whereby said porous child body comprises a pore diameter in the mesopore size between 2 and 50 nm, allows confinement of the deposition of the metal particles advantageously yielding better-controlled size-distribution and spatial distribution of catalyst particles (limits agglomeration). Moreover, the confinement provides that catalyst particles are well separated pore-wise and the residual gas originating from individual CNT growth will significantly less impact the neighboring growth. Hence, this configuration enables a more homogenous product and further increases the product density. The invention further provides an improved method for growing carbon nanostructures within the (hierarchical) membrane by improving the gas supply to the metal particle via large macro/meso-porous transport channels toward the mesoporous grown channels of the metal oxide. Alternatively, selective deposition of metal particles on the metal oxide phase allows control of the amount of nuclei sites in each pore channel thus reducing interdependent pore blocking. The improved growth method gives a significantly higher carbon density (yield) compared to state of the art techniques.

Embodiments of the invention solve the problem of state of the art techniques to combine high metal particle density (and dispersion) and carbon nanostructure yield and a continuous growth of aligned using a membranes support.

The problem is solved by the use of hierarchical porous bodies, such as for instance membranes, or the selective deposition of metal particles in the child body, e.g. a mesoporous oxide layer, with a thickness ranging from 200 nm up to 3 mm, preferably covering only a top part, e.g. stretching from the parent body outer surface, e.g. membrane surface, to a well-defined depth, of the full porous body, e.g. membrane cross section which can vary between 60 μηι and 3 mm. As such approximation to the one-to-one relation between metal particle and growth channel may be obtained. In alternative embodiments, the porous child body can not only be positioned in the top part of the parent body, but also in the middle part of the parent body pore, in the bottom part of the parent body, or a combination thereof. The porous child body can be a layer or a particle, according to embodiments of the invention.

In an embodiment the catalyst particles are preferably positioned inside the pores and/or pore channels of the membrane, rather than on the upstream and downstream surface of the membrane. In an embodiment the number of catalyst particles positioned inside the membrane is higher than the number of catalyst particles positioned on the upstream surface resp. the downstream surface of the membrane. The substrate may comprise at least one pore where more than one catalyst particle is present. The substrate may comprise on average more than 1 .0 nanostructures per pore. In some embodiments, the substrate may comprise on average one or more than 1 nanostructures per pore. This offers the opportunity to provide more than one particle, and hence more than one nanostructure per pore or pore channel.

Furthermore embodiments of the present invention provide the advantage that the predetermined size, between 2 and 50 nm of the child porous body, and cross-section geometry (e.g. either lamellar, cubic, hexagonal or disordered) of the mesopores within the pores of the parent porous body, e.g. meso/macroporous membrane, can be used to determine the diameter and hence properties of the synthesized carbon nanostructures (e.g. chirality and amount of wall in CNTs). In prior art this is disadvantageously done by providing an additional design pattern as to form a designed pattern to synthesize specific CNT shapes, e.g. prismatic structures.

Embodiments of the method of the invention to provide carbon nanostructures within hierarchical (i.e. bi- or trimodal pore size distribution) membranes with pore channels perpendicular to the membrane surface have the advantage to grow easily carbon nanostructures in away from the substrate thus reducing pore blocking at the downstream side of the porous body or parent body, e.g. membrane. The pore orientation is crucial in order to allow good contact of the carbon precursor gas to the metal particles, to sustain gas flow over the metal particles and to reduce pressure built-up endangering the integrity of the support membrane.

The vertical set-up and perpendicular growth allow for the aligned growth of bundles of fibrous carbon nanostructures.

The invention also provides a method to integrate membrane supported CVD growth of aligned bundles of carbon fibrous nanostructures to its further processing in weaving or incorporation in polymer matrices as reinforcing material.

According to embodiments of the invention, a method is disclosed for the synthesis of a porous child body, e.g. mesoporous metal oxides with controlled pore size and orientation, within the channels of a porous parent body, e.g. membrane. The method for providing mesoporous metal oxide into the porous membrane may comprise at least the steps of:

Providing a porous parent body, e.g. membrane, comprising an open channel system

Preparation of a mesoporous solid synthesis solution

· Applying mechanical forces to incorporate (part of) the synthesis solution into the parent body, e.g. membrane, channels

Applying (hydro)thermal conditions to create the appropriate hydrolysis and condensation conditions in the soft-templating synthesis

Rinsing and drying the parent body, e.g. membrane, to remove non-attached mesoporous solid or building blocks thereof

Removal of the organic surfactant molecules to create the open pore structure in said channels of the parent body.

Embodiments of the invention include a method to use the novel membrane support to grow fibrous carbon nanostructures from within the hierarchical body pores, e.g. membrane pores. Its embodiment may comprise at least the steps of:

Positioning a hierarchical porous body, e.g. membrane, according to embodiments of the invention in the CVD set-up

Preheating the gas supply consisting of a carrier, support and carbon precursor gas, preferably at a temperature between 350 °C and 950 °C. Heating the membrane and reaction gas at the metal particles between 400 and 1200 °C in order to initiate the growth of fibrous carbon nanostructures

Alignment of the fibrous nanostructures under influence of gravimetry, magnetic fields, dense packing or gas flow.

According to embodiments of the present invention, an improved method is provided to obtain a continuous growth of fibrous carbon nanostructures (e.g. CNT) from within the pores of a hierarchical membrane. The improved method gives rise to a significantly improved density as well as tuning of the density of the carbon product in comparison to non-modified and non-hierarchical meso/macroporous membranes known in the art. In comparison with mesoporous membranes as known in the art which are provided in Fig. 8, these known membranes are for instance used for biosensing means, however there are not used in systems which are adapted for synthesis of nanostructures. Moreover, advantageously embodiments of the present invention provide reducing of pore blocking by providing a hierarchical membrane and reducing the thickness of the mesoporous layer or offering the availability of additional transport channels in the mesoporous layer. Moreover, in prior art membranes pressure which built up is an additional problem. In other preferred embodiments of the invention a hierarchical membrane is provided, comprising a parent and child porous body, whereby the child porous body has a thickness of at least 0.3% of the parent body thickness.

Embodiments of the method of the present invention advantageously achieves metal deposition either preferably directly via the synthesis gel or via a post synthesis introduction (e.g. impregnation or ion-exchange) of the required metals.

In embodiments of the invention control of the metal deposition can be either controlled via controlling the degree of ion exchange site via isomorphous substitution either by modifying the concentration of the metal salt during impregnation.

In preferred embodiments, sintering of the metals can be reduced via the optimization of the thermal treatment or by the use of polymers with a fixed amount of exchange sites (e.g. hyper branched polymers or dendrimers). Embodiments of the present invention provide methods to grow a porous child body, e.g. a mesoporous porous metal oxide, in at least one pore of a parent body, in order to create new hierarchical membranes as described above which are preferably compatible with an improved method to grow aligned fibrous carbon nanostructures (e.g. CNT) from within the pores of the membrane. According to embodiments of the invention, the improved method may comprise at least the steps of:

· Providing a porous parent body, e.g. membrane, comprising an open channel system

Preparation of a mesoporous solid synthesis solution used for the making of the child body

Applying mechanical forces to incorporate (part of) the synthesis solution into the porous parent body e.g. membrane channels and as a result providing a child body.

Removal of the organic surfactant molecules to create the open pore structure in the child body and thus creating a porous child body

· Positioning the hierarchical membrane, comprising at least one parent and one child body, in the CVD set-up

Preheating the gas supply consisting of a carrier, support and carbon precursor gas.

Heating the membrane and reaction gas at the metal particles between 400 and 1200 °C in order to initiate the growth of fibrous carbon nanostructures Alignment of the fibrous nanostructures under influence of gravimetry, magnetic or electrical fields, dense packing or gas flow.

According to embodiments of the invention, the macro pore membrane, e.g. the parent layer, may be any suitable membrane which is chosen in function of further applications. For fibrous carbon nanostructure growth applications the membrane is preferably highly thermally stable as well as chemically stable against the reactive gasses used, therefor the membrane is preferably made from an inorganic material e.g. aluminum oxide, silica or titania.

According to embodiments of the invention, the meso/macroporous parent membrane has preferentially one- dimensional pores perpendicular to the surface of the membrane and a diameter in the range of 20 nm up to 1 μηι. In order to ensure maximal pore and carbon product density porosity should be maximal while sustaining a high thermal stability. Porosity should be preferably in the range of 1 0 to 90%, and more preferably in the range between 20 and 70%.

According to preferred embodiments, a porous parent body, e.g. a meso/macroporous membrane, is preferably present comprising a limited thickness from 1 0 μηι to 3 mm in order to combine the retention of structure integrity under the applied synthesis conditions (e.g. gas flow and temperature) and pressure built-up. Advantageous, a hierarchical porous body, e.g. membrane, according to embodiments of the invention can reduce the pressure built up, caused by hampered gas flow, by limiting the thickness of the layer of the porous child body, with small mesopores, which is deposited inside the meso/macroporous membrane according to embodiments of the invention. Reduction of the thickness of the layer wherein the metal catalyst particles are located, preferably in the porous child body, can ensure a reduction of the amount of fibrous structures in one pore thus providing continuous gas flow and reduction of pore blocking inside the porous oxide channels.

According to embodiments of the invention metal oxide synthesis solution, which is used to create a child body, may comprise a metal oxide precursor, a surfactant, and a source of acidity/basicity and optionally hetero- elements such as Al, P, etc. for the execution of an isomorphous substitution in order to create ion exchange sites in the final porous metal oxide solid.

The metal oxide used in embodiments of the invention is preferably produced via the soft templating methodology using a bi-constituent synthesis mixture. Si is preferred but others metals (group I I, IV and V) and transition metal are included in the invention.

According to embodiments of the invention regarding the in situ synthesis of metal oxide particles. An ordered mesoporous silica with 1 D-pores and a high width-to-thickness ratio such as COK-12 is preferred. Alternatively, members of the following families of ordered mesoporous silica materials can be used: TUD, SBA, COK, MCM, FSM, KIT, MSU, HMS and HMM.

According to embodiments of the invention to force the synthesis solution into the meso/macroporous membrane the synthesis gel can be introduced into the pores via mechanical forces out from the following selection: sonication, vibration and liquid filtration.

The typical bi-constituent mixture can be mixed inside the membrane pores by a two-step intrusion method. First the surfactant and (co)solvent are introduced into the membrane pores via aforementioned method after which the membrane is put into a solution of the metal oxide precursor and the proper pH controlling agent.

Excess of synthesis gel from the membrane is preferably removed via spin-coating condition, washing for limited time, or mechanically e.g. by whipping.

According to embodiments of the invention the hydrothermal treatment is used modify the silica pore size and hydrothermal stability. Suitable hydrothermal conditions comprise the heating of the substrate under autogeneous pressure for 2-48h at 40 to 1 00 °C.

According to embodiments of the invention washing and drying step to remove excess synthesis solution is performed using doubly-distilled water.

Partial filling of the parent body ,e.g. membrane, channels may be achieved by the use of a temporarily filling substance. In preferred embodiments the filling substance can be introduced into the porous parent body, e.g. meso/macroporous membrane, by for instance a top-down or bottom up approach and is removed chemically or by thermal treatment after the deposition and growth of the metal oxide in the remainder parent body, e.g. membrane, channel voids.

In embodiments of the invention, the top-down approach for partial filling of the membrane channels is preferably achieved by an in situ polymerization or intrusion of a polymer (e.g. melting) into the parent body, e.g. membrane, channels. After polymerization selective etching of the polymer is achieved by chemical treatment, laser, etc.

In other preferred embodiments, a bottom-up approach for partial filling of the parent body, e.g. membrane, channels may be achieved by the use of thermo hardening chemicals and its subsequent local heating under influence of a laser equipment.

According to embodiments of the invention the step of growing the carbon nanostructures may be achieved using high temperature CVD conditions. The reaction gas is a carbon containing gas, e.g. acetylene, ethylene and methane (possibly in high pressure conditions), or liquid precursor (ethanol, methanol etc.). Optionally an additional carrier (inert gas e.g. He, N2, Ar) or support gas may be used (e.g. slightly oxidative to remove amorphous carbon i.e. CO2, H2O or reducing H2, NH3). In other embodiments nitrogen containing carbon sources can be used in order to prepare N-doped CNTs, as these functionalized CNTs can interesting for catalysis, gas sensors, batteries, nanoelectronics etc....

According to embodiments of the invention the growth of the fibrous carbon nanostructures may be performed with a, preferentially vertical, CVD setup. The gas reactor preferably is capable of optionally preheating the gas flow and heating the membrane support up to 1200 °C. Gas flow is then preferably controlled with a combined flow rate between 10 ml/min^"1 to 10 l/min^-1 , and more preferably between 10 ml/min^-1 to 5 ml/min^-1 , as the membrane and tube diameter may vary between 1 to 500 cm, preferably between 1 to 100 cm, and more preferably between 1 to 10 cm. Its relation is of upmost importance in order to carefully control the contact time between carbon precursor gas and the metal catalyst particle.

Figure 1 illustrates a set-up, as used in embodiments of the invention, for the growth of the fibrous carbon nanostructures 1 . The orientation of the set-up, here illustrated, is vertical although other positions are also possible. The substrate 2, provided using methods of the present invention, is hierarchical and preferably is hold into position by a substrate e.g. membrane support 3 and as such positioned along the full cross section of reaction tube 8. The gas inlet which is connected to the reaction tube provides a gas mixture according to prior art consisting of either a pure carbon source gas 5 or its mixture with a support gas 6 such as e.g. CO2, CO, H2, O2 and H2O or a carrier gas 7 such as He, Ar and N2 or combination thereof. The reaction tube 9 is preferably made from quartz or metal (if non-atmospheric pressures are applied). A pre-heating element 10 preferably ensures the attainment of the required temperature of the reaction gas when contacted with the metal catalyst particles and can be installed in the first part of the reaction tube 6 or at the tubing of the gas inlet 4. The preheating element 10 commonly operates at slightly lower temperature (100 to 1200 °C) then the chosen reaction temperature according to embodiments of the invention e.g. 400-1200 °C. The heating element 1 1 is preferably positioned at the location of the substrate, e.g. membrane, and extending along both sides of the membrane for preferably 0,1 cm to 1 m. Insulation 12 is preferably provided in the reactor embodiment around 6 and 1 1 , to control the temperature and out of safety/economic motives. Optionally a magnetic field and/or an electric field 13 can be created to govern the orientation and alignment of the fibrous carbon nanostructures which are then governed toward a collector. The waste gas is preferably collected at the end of the reaction tube 9 and governed toward the exhaust.

Figure 2 schematically illustrates the steps involved in the methodology to prepare a hierarchical porous body 20, e.g. membrane, according to embodiments of the invention. The meso- or macropores 15, preferably ranging between 20 nm to 2 mm, of the parent body e.g. membrane 14, can be prepared using techniques known by the skilled person, are preferably filled with a filling substance 1 7. This substance can be a polymer or monomer solution. Hardening of the polymer is then performed preferably by cross-linking or polymerisation in situ. A partial pore filling can be obtained via a top down or bottom up approach. The bottom up approach preferably comprises the local hardening of the filling substance under influence of chemicals, e.g. cross-linker, initiator, etc. or under radiation e.g. laser. The top down approach preferably embodies a first complete filling of the membrane pore followed by the partial removal by locally heating or one side chemical treatment (e.g. salt, acid or base solution). Consequently a metal oxide synthesis solution is loaded into the free pore voids 1 6 of the modified parent body, e.g. membrane. Preferably a mesoporous metal oxide 1 8, with pores between 2 and 50 nm, is formed by a hydrothermal gel synthesis method or via e.g. EISA synthesis methods known in the art, which provides at least a child body in said parent pore channel. The remainder of the filling substance 1 7 is preferably finally removed to ensure the accessibility of the pore system of the final hierarchical membrane 20 by e.g. heat treatment or chemical etching.

Figures 3a show the selective deposition of the metal particles 21 , which preferably can function as a catalyst in the synthesis of fibrous carbon nanostructures. Selective positioning in the porous child body, e.g. a thin mesoporous metal oxide top layer of the hierarchical porous body e.g. membrane, is preferably obtained by introducing metal particles prior to removal of the filling substance 1 7 or by isomorphous substitution in the metal oxide layer 1 8 thus creating exchange sites for the catalyst metal precursor solution. Figure 3b shows different embodiments of the hierarchical porous body used as a substrate to synthesize fibrous carbon nanostructures. The left figure schematically illustrates a porous body according to embodiments of the invention, whereby the porous body comprises a particle as porous child body. The particle can be a hexagonal prism and be made of COK-12 silica particle which is preferably placed in a AAO parent pore. Preferably the silica child particle is positioned in on top or bottom of the parent pore. In alternative embodiments the silica child particle can be positioned in the middle of the parent pore. The middle figure illustrates a porous body according to embodiments of the invention, whereby the channels defining the pores of the porous child body extend through the thickness of the parent porous body. The right figure of Fig. 3b schematically illustrates a hierarchical porous body according to embodiments of the invention, whereby the channel of the child body partially extends over the channel of the parent porous body.

Figure 4 shows a flowchart 23 showing a preferred embodiment of the processing steps to synthesize fibrous carbon nanostructures using a hierarchical porous body, e.g. membrane, according to preferred embodiments of the invention. The first step 24 comprises a preheating of the reaction gas. Secondly, in step 25 the reaction gas is brought to the hierarchical membrane 22, which is positioned in a tubular flow reactor which is positioned at a fixed angle, e.g. vertically as illustrated in Fig. 1 or horizontally (not shown), and covering the complete cross section of the reactor tube, and decorated with active metal particles 21 . In a next step 26 the metal particles deposited 21 inside the pores of the hierarchical membrane 22 gas are brought in contact with the gas flow. Next, according to commonly known mechanisms, in step 27 the gas is decomposed by action of the metal particles 21 . Consequently, carbon is dissolved into the metal particles 28 and after oversatu ration growth of fibrous carbon nanostructures occurs. The specific design of the hierarchical membrane (controlled metal deposition to locate and limit the formation and agglomeration of metal catalyst particles joint with the presence of transport channels and reduction of pore blocking) ensures a continuous flow through the porous membrane in step 29, in order to have a controlled and non-variable contact time (during carbon growth) between gas and metal particle to sustain the prolonged growth of the carbon nanostructures. Alignment of the fibrous carbon nanostructures is obtained in the final step 30 as a direct result of the high density of the carbon product or by the usage of either one of the following techniques or a combination thereof: gravimetric (via positioning of the membrane, e.g. vertical), magnetic fields, electric fields and gas flow. As it is known that alignment can provided by high density growth procedures, the creation of highly porous membrane according to embodiments of the invention may advantageously contribute to the alignment.

Figure 5 is a SEM photograph of a parent porous body, e.g. a meso/macroporous membrane, filled with a polymeric filling substance 17 which was later partially removed by a top down approach using chemical etching according to embodiments of the invention. Consequently a partially filled membrane is obtained. Such membrane can be used for further processing toward the deposition of a thin mesoporous metal oxide layer, which can act as a child body, inside the remaining open voids 16 of the parent porous body. The picture in Fig. 5 thus illustrates step 2 in the formation of a hierarchical porous body, e.g. membrane, according to a methodology depicted in Figure 2.

Figure 6 illustrates the growth of mesoporous metal oxide inside the parent meso-macroporous membrane 14 according to embodiments of the invention. The variation in morphology of the final metal oxide layer is shown in Figure 6 and Figures 7 (a) and (b). Hollow 31 and full 32 cylinders of a metal oxide can be formed inside the parent membrane channels according to embodiments of the invention. Figure 7 shows the deposition of individual metal oxide particles 33, size controlled by their confinement, along the membrane channel.

As indicated, embodiments of the invention advantageously provide continuous fabrication of carbon nanotubes using chemical vapour deposition (CVD). Embodiments of the invention furthermore employ a carbon source in the form of a gas and high temperatures to dissociate its molecules into atoms. The gas is preferably brought in contact with catalyst nanoparticles that are distributed inside of a porous substrate and helps to direct the continuously growing carbon nanotubes towards the exhaust of the CVD system. In Figure 1 1 a holder 1 1 according to embodiments of the invention, to support the porous substrate is provided. The holder 1 1 is preferably a cylindrical tube with a partially closed ending 1 1 1 on one side and an open ending 1 12 in the opposite side. The closed ending 1 1 1 is preferably directed towards the inlet of the gases and the open side 1 12 preferably towards the outlet of the reactor. The partially closed side 1 1 1 contains one or several apertures 12, for instance from 1 to 15 depending on the size of the reactor and the substrates. The apertures preferably have the design of a truncated-cone forming pockets 12, where one places a substrate 14 according to embodiments of the invention. The smaller base of the truncated cone of the pockets 12 and the substrates 14 are preferably of the same size and shape. In further embodiments an accessory 13 in the shape of smaller cylinder with a conical ground glass join preferably is provided which can ensure the fixation of the substrate 14 in the pocket 12. The gas is then forced to go through the pores of the substrate where the catalyst particles are located in. The material of the holder 1 1 and the accessories 13 might be the same or different, and able to stand high temperatures, from 400 to 1000^eC, without reacting with the gases (eg. Quartz).

It is to be understood that this invention is not limited to the particular features of the means and/or the process steps of the methods described as such means and methods may vary. It is also to be understood that the terminology used herein is for purposes of describing particular embodiments only, and is not intended to be limiting. It must be noted that, as used in the specification and the appended claims, the singular forms "a" "an" and "the" include singular and/or plural referents unless the context clearly dictates otherwise. It is also to be understood that plural forms include singular and/or plural referents unless the context clearly dictates otherwise. It is moreover to be understood that, in case parameter ranges are given which are delimited by numeric values, the ranges are deemed to include these limitation values.

Claims

1 . A system for synthesis of nanostructures, the system comprising:

-a porous substrate, said porous substrate is situated within the passageway of the housing;

-a plurality of catalyst particles deposited in the pores and/or in the pore channels of said porous substrate, and from which nanostructures can be synthesized upon interaction with a reaction gas;

- a heating mechanism circumferentially placed about the substrate for generating energy sufficient to maintain an environment within which nanostructures are synthesized within a specified temperature range; -a pair of flanges, each capping one end of the housing; and an inlet positioned across the flange capping the first end of the housing and through which reaction gas can be directed to/through the porous substrate.

2. The system according to claim 1 , wherein the porous substrate is an inorganic membrane.

3. The system according to any of the previous claims, whereby the porous substrate is an anodic aluminum oxide channel membrane.

4. A system according to any of the previous claims, wherein the porous substrate is made from a material provided with pore size ranging from about 0.5 nm to about 500 microns.

5. A system according to any of the previous claims, wherein the porous substrate is made from a material having a void fraction of from about 10 percent to about 95 percent.

6. A system according to any of the previous claims, wherein the catalyst particles are substantially evenly distributed along the pores and/or along the pore channels of the porous substrate.

7. A system according to any of the previous claims, wherein the porous substrate is hardened or coated.

8. A system according to any of the previous claims, further comprising a holder with a thickness, whereby said holder comprises at least one passageway across the thickness, whereby said at least one passageway has a truncated-cone shape.

9. A system according to any of the previous claims, whereby said at least one passageway comprises a partially closed ending and an open ending on the opposite side, whereby said partially closed ending is positioned towards the inlet and the open ending positioned towards the outlet of the system.

10. A system according to any one of the previous claims, further including an exhaust port through which reaction waste product flowing from the porous substrate may be removed.

1 1 . A system according to any one of the previous claims, wherein the housing is made from a strong substantially gas-impermeable material.

12. The system according to any of the previous claims, wherein the housing is made from a material being substantially resistant to corrosion and/or high temperature.

13. The system according to any of the previous claims, wherein the housing is made from a material being quartz.

14. The system according to any one of the previous claims, wherein the porous substrate is sufficiently porous so that a pressure difference between both surfaces of the porous substrate can be substantially low, so as to permit the porous substrate to maintain its structural integrity.

15. A system according to any one of the previous claims, wherein the porous substrate is made from a material including carbon foams, glassy carbon, silica, alumina, alumina coated with silica, zirconia, zeolites, sintered titanium, titania, magnesia, yttria, copper, iron, iron nickel, iron cobalt, cobalt, steel, iron carbide, nickel, or a combination thereof.

16. A system according to any one of the previous claims, wherein the porous substrate is one of foam, channel plate, felt, wool, fibers, cloth, or array of needles.

1 7. A system according to any one of the previous claims, wherein the catalyst particles are positioned inside the porous substrate and whereby the catalyst particles are substantially evenly distributed across the pore channels of the substrate.

1 8. A system according to any of the previous claims, wherein the average number of catalyst particles positioned in the pore channels of the porous substrate is higher than 1 .0, advantageously at least 2.0 .

1 9. A system according to any of the previous claims, wherein the pore channels of the porous substrate have substantially gas-impermeable walls.

20. A system according to any one of the previous claims, wherein the catalyst particles are made from a material including iron, nickel, cobalt, iron oxides, nickel oxides, cobalt oxides, metal salts with sulfate, metal salts with sulfamates, acetate, citrate, oxalates, nitrites, nitrates, or a combination thereof.

21 . A system according to any one of the previous claims, wherein the catalyst particles range from about 0.5 nm to about 50 nm in size, but not exceeding 50 percent of the pore size.

22. A system according to any one of the previous claims, wherein the heating mechanism can maintain the temperature of the growth environment within a range of from about 200 °C to about 1400 °C.

23. A system according to any one of the previous claims, wherein the heating mechanism can maintain the temperature of the growth environment within a range of from 400 °C up to 1200 °C.

24. A system according to any one of the previous claims, wherein the heating mechanism can maintain the temperature of the growth environment within a range of from 600 °C to 1 000 °C.

25. A system according to any one of the previous claims, wherein energy generated from the heating mechanism includes thermal energy, frictional energy, visible light photons or other types of electromagnetic radiation, chemical, electrical, or electrochemical energy, microwave radiation, eddy currents, or ultrasound shock waves or compression.

26. A system according to any one of the previous claims, wherein the flanges are substantially gas- impermeable.

27. A system according to any one of the previous claims, further including the holder for accommodating the porous substrate, the holder being situated within the passageway of the housing for providing a pathway for the reaction gas from the inlet to the porous substrate.

28. A system according to any of the previous claims, wherein the holder is a tube.

29. A system according to any of claims 27 to 28, wherein the porous substrate is situated within the holder.

30. A system according to any of claims 27 to 29, wherein the holder is made from a strong, substantially gas- impermeable material.

31 . A system according to any of the claims 27 to 30, wherein the holder is made from quartz.

32. A system according to any of the previous claims, further including: a first electrode situated within the pathway of a tube upstream of the porous substrate; and a second electrode situated within the pathway of a tube downstream of the porous substrate.

33. A system as set forth in claim 32, wherein the upstream electrode and the downstream electrode are designed to generate an electric field there between to provide physical support to the nanostructures growing from the porous substrate.

34. A system as set forth in claim 32 or 33, wherein the upstream and downstream electrodes are designed to generate an electric field there between to control and maintain direction of growth to the nanostructures growing from the porous substrate.

35. A system as set forth in any of the claims 32 to 34, wherein the electrodes permit flow of reaction gas within the tube there through.

36. A system as set forth in any of the claims 32 to 35, wherein the electrodes are made from an electrically conductive material that is non-reactive and resistant to nanostructure growth.

37. A system as set forth in any of the claims 32 to 36, wherein the material from which the electrodes are made includes graphite, copper, titanium, vitreous carbon, a combination thereof, or other conductive materials not catalytic to carbon nanostructure formation.

38. A system as set forth in any of the previous claims, wherein the porous substrate is positioned circumferentially about an exterior surface of the tube at one end of the tube, and the system further includes: a tubular electrode having an open end and sufficiently sized so as to concentrically accommodate the porous substrate through the open end; and a plurality of guides, positioned between the tubular electrode and the porous substrate, and over which growing nanostructures can be directed.

39. A system as set forth in claim 38, wherein the porous substrate is designed to be an electrode, such that along with the tubular electrode, an electric field can be generated there between to provide physical support to the nanostructures growing from the porous substrate.

40. A system as set forth in claim 38 or 39, wherein the porous substrate and the tubular electrode designed to generate an electric field there between to control and maintain direction of growth to the nanostructures growing from the porous substrate.

41 . A system as set forth in any of the claims 38 to 40, wherein the tubular electrode is rotatable and retractable from its position over the porous substrate so as to pull growing nanostructures there along.

42. A system as set forth in any of the claims 37 to 41 , wherein the guides include a series of rings circumferentially placed about the porous substrate.

43. A system as set forth in any of the previous claims, wherein the porous substrate is made from an electrically conductive material including, glassy carbon, porous titania, porous zirconia, or sintered titanium powder.

44. A system as set forth in any of the previous claims, wherein the porous substrate is made from an inert material coated with an electrically conductive material including, copper, tin oxide, titania, titanium, tungsten or platinum.

45. A system as set forth in any of the previous claims, further including a second inlet situated across the flange capping the first end of the housing and through which an evacuation gas can be directed into the passageway of the housing to displace and remove reaction waste product within the housing.

46. The system according to any of the previous claims, wherein said body is a layer or article.

47. The system according to any of the previous claims , wherein said porous substrate is a porous hierarchical substrate, said porous hierarchical substrate comprising a porous parent body, said parent body comprising a plurality of pores and whereby at least one pore of said parent body comprises at least one porous child body, whereby said porous child body comprises a plurality of pores, and wherein said plurality of catalyst particles are comprised in said child body.

48. The system according to claim 47, whereby said child body comprises a thickness of at least 0.3% and up to 100% of the parent body thickness.

49. The system according to any of claims 47 to 48, whereby the pore diameter of the child body is smaller than the pore diameter of the parent body.

50. The system according to any of claims 47 to 49, whereby a pore of the parent body is defined by a channel and whereby said channel extends through the whole parent body.

51 . The system according to any of claims 47 to 50, whereby a pore of the child body is defined by a channel and whereby said channel extends through the child body.

52. The system according to any of claims 47 to 51 , whereby said channel of the parent and/or child body comprises a hexagonal or circular or cubic or lameral cross-section.

53. The system according to any of claims 47 to 52, whereby said porous parent body comprises a first and second outer body surface.

54. The system according to claim 53, whereby the parent or child pores channels are oriented perpendicularly with respect to the first and second outer body surface.

55. The system according to claim 54, whereby the channel is straight or is inclined with respect to the surface with a degree of 0 to 90 °.

56. The system according to claim 54, whereby the channel is straight or is inclined with respect to the surface with a degree of 5 to 90 °.

57. The system according to any one of claims 47 to 56 whereby said parent body has a thickness ranging from 20 μηι up to 1 0 mm, for example from 60 μηι up to 1 mm or from 60 μηι to 200 μηι.

58. The system according to any of claims 47 to 57, whereby the channel length of the parent pore channel ranges from 20 μηι up to 1 0 mm, for example from 60 μηι up to 1 mm or from 60 μηι to 200 μηι.

59. The system according to any of claims 47 to 58, whereby said child body has a thickness ranging from 1 00 nm up to 1 0 mm, for example from 150 nm up to 1 mm or from 200 nm up to 1 μηι.

60. The system according to any of claims 47 to 59, wherein the parent body of the substrate is made from a material provided with pore size ranging from about 20 nm to about 2 mm.

61 . The system according to any of claims 47 to 60, wherein the child body of the substrate is made from a material provided with pore diameter ranging from about 2nm to about 50 nm.

62. A system according to any one of claims 47 to 61 , wherein the parent and/or child porous body comprise a void fraction of from about 1 0 percent to about 90%.

63. A system according to any one of previous claims, wherein the catalyst particles are substantially evenly distributed across the child body pores.

64. A substrate for synthesis of nanostructures, the substrate comprising a porous substrate having an upstream surface and a downstream surface; and a plurality of catalyst particles deposited in said pores and/or pore channels of the porous substrate, and from which nanostructures can be synthesized; wherein the porous substrate provides pathways through which a reaction gas can travel across the upstream surface and out the downstream surface to initiate growth of nanostructures from the catalyst particles.

65. A substrate as set forth in claims 64, wherein the porous substrate is sufficiently porous so that a pressure difference between the upstream surface and the downstream surface can be substantially low, so as to permit the body to maintain its structural integrity.

66. A substrate according to any of claims 64 to 65, wherein the porous substrate is a porous parent body having an upstream surface and a downstream surface, the porous parent body comprising at least a porous child body in a parent pore wherein said child body is positioned or on the upstream or on the downstream surface or in between the upstream and the downstream surface of the porous parent body, the plurality of catalyst particles being deposited in pores of said child body.

67. A substrate according to any of the claims 64 to 66, wherein the average number of catalyst particles positioned in pore channels of the porous substrate is one or higher than 1 .0 .

68. A substrate according to any of the claims 64 to 67, wherein the pore channels of the porous substrate have substantially gas-impermeable walls.

69. A method for synthesizing nanostructures, the method comprising:

-providing a porous substrate;

-providing catalyst particles in the pores and/or in the pore channels of the porous substrate;

-directing a flow of a reaction gas through the pores of the substrate, across the upstream surface and through the downstream surface of the substrate; -decomposing the reaction gas about the catalyst particles to generate constituent atoms;

-allowing for diffusion of the constituent atoms on or in the catalyst particles and thereon for the synthesis of nanostructures therefrom.

70. A method according to claim 69, wherein providing a porous substrate comprises providing a porous hierarchical substrate.

71 . A method according to any of claims 69 to 70 wherein the step of directing includes allowing the gas, after exiting the downstream surface of the substrate, to flow past growing nanostructures, so as to provide support to the growing nanostructures.

72. A method according to any of claims 69 to 71 , wherein providing catalyst particles in the pores and/or pore channels includes impregnating or dipping said porous substrate in a catalyst solution comprising said catalyst particles.

73. A method as set forth in any of the claims 69 to 72, wherein said porous substrate is chemically etched before providing said catalyst particles.

74. A method as set forth in any of the claims 69 to 73, wherein the step of directing includes allowing the gas, after exiting the downstream surface of the substrate, to flow past growing nanostructures, so as to provide support to the growing nanostructures.

75. A method as set forth in any of the claims 69 to 74, wherein the step of directing includes adding a carbon- containing source to the reaction gas prior to directing the reaction gas to the substrate.

76. A method as set forth in claim 75, wherein, in the step of adding, the carbon- containing source includes ethanol, methane, methanol, ethylene, acetylene, xylene, carbon monoxide, or toluene.

77. A method as set forth in any of the claims 69 to 76, further including introducing an evacuation gas to displace and remove reaction waste product.

78. A method as set forth in any of the claims 69 to 77, wherein the step of decomposing includes generating energy to temperatures ranging from about 200 °C to about 1400 °C.

79. A method as set forth in any of the claims 69 to 78, wherein the step of decomposing includes generating energy to temperatures ranging from about 400 °C to about 1400 °C.

80. A method as set forth in claim 79, wherein, in the step of generating, the energy includes thermal energy, frictional energy, visible light photons or other types of electromagnetic radiation, chemical, electrical, or electrochemical energy, microwave radiation, eddy currents, or ultrasound shock waves or compression.

81 . A method as set forth in any of the claims 69 to 80, wherein the step of allowing includes growing the nanostructures in a direction substantially parallel to the flow of reaction gas.

82. A method as set forth in any of the claims 69 to 80, wherein the step of allowing includes growing the nanostructures in a direction substantially radially to the flow of reaction gas.

83. A method as set forth in any of the claims 69 to 82, wherein the step of allowing includes growing the nanostructures to a specified length.

84. A method as set forth in claim 83, wherein, in the step of growing, the specified length is at least 1 cm.

85. A method as set forth in claim 83, wherein, in the step of growing, the specified length is at least 1 m.

86. A method as set forth in any of the claims 69 to 85, further including generating an electrostatic field having a strength between about 5 kV/m and about 1000 kV/m to provide support the nanostructures as they grow from the substrate, so as to contribute to the straightness of the nanostructures.

87. A method as set forth in any of the claims 69 to 85, further including generating an electric field having a magnitude between about 5 kV/m and about 1000 kV/m to provide support to the nanostructures as they grow from the substrate, so as to contribute to the straightness and/or alignment of the nanostructures.

88. A method as set forth in any of the claims 69 to 87, further including generating a magnetic field having a magnitude between about 0.1 T and about 50T to provide support the nanostructures as they grow from the substrate, so as to contribute to the straightness and/or alignment of the nanostructures.

89. A method as set forth in any of the claims 69 to 87, further including generating a magnetic field having a magnitude between about 0.1 T and about 30T to provide support to the nanostructures as they grow from the substrate, so as to contribute to the straightness and/or alignment of the nanostructures.

90. A method as set forth in any of the claims 69 to 89, further including collecting the nanostructures once the nanostructures have grown to a desired length.

91 . A fibrous material comprising a plurality of nanostructures as produced by a method according to any of the claims 49 to 68 or produced by a method according to any of the claims 69 to 90.

92. A fibrous material as set forth in claim 91 for use in thermal management applications.

93. A heat conductor comprising the fibrous material set forth in claim 91 or 92.

94. A low eddy current, low resistance winding for an electric motor comprising the fibrous material as set forth in claim 91 or 92.

95. A low eddy current, low resistance winding for a high frequency solenoid comprising fibrous material as set forth in claim 91 or 92.

96. A winding for a high frequency transformer comprising the fibrous material as set forth in claim 91 or 92.

97. A fabric comprising the fibrous material as set forth in claim 91 or 92.

98. A protective armour or clothing comprising the fibrous material as set forth in claim 91 or 92.

99. A rope or cable comprising the fibrous material as set forth in claim 91 or 92.

1 00. A yarn comprising the fibrous material as set forth in claim 91 or 92.

1 01 . A sheet comprising the fibrous material as set forth in claim 91 or 92.

1 02. A heat sink comprising the fibrous material as set forth in claim 91 or 92.

1 02. A high strength, low eddy current, low resistance electric power transmission line comprising the fibrous material as set forth in claim 91 or 92.

1 03. A process for packing of carbon nanotubes comprising: generating the nanotubes as set forth in any of claims 69 to 90; coating the nanotubes with one of furfuryl alcohol, epoxy resin, or Resol; and arranging the coated nanotubes in bundles for packing.

1 04. A method of preparing a porous body, said method comprising:

- providing a porous parent body;

- partial filling of at least one pore of the parent body along a pore channel, with a filling substance, resulting in a filled and in an empty part of the parent pore;

- providing a child body in the empty part of the parent pore;

- removing the filling substance in the filled part of the parent pore;

- creating porosity in the child body, resulting in a porous child body;

1 05. The method of claim 1 04, whereby said child body comprises a metal oxide or metal oxide gel.

1 06. The method according to claim 1 05, whereby providing said metal oxide gel is performed by synthesis of the metal oxide in said empty part with or without precursors.

1 07. The method of any one of the claims 1 05 to 1 06, whereby said providing metal oxide gel is performed by mechanical force, sonication, vibration, evaporation-intrusion or liquid filtration.

1 08. The method according to any one of the claims 1 04 to 1 07, whereby said porous child body comprises a thickness of at least 0.3% and up to 99.7% of the parent body thickness.

1 09. The method according to any one of the claims 104 to 1 08, further comprising providing metal particles in said pores of the child body.

1 1 0. The method according to claim 1 09, whereby said providing metal particles is performed by selective deposition of the metal particles in the child body.

1 1 1 . The method according to any one of the claims 104 to 1 10, whereby said porous child body comprises channels extending throughout the child body.

1 12. The method according to any of the claims 104 to 1 1 1 , wherein the average number of catalyst particles positioned in the channels of the child body is 1 .0 or higher than 1 .0 .

1 13. The method according to any of the claims 104 to 1 12, wherein the channels of the child body have substantially gas-impermeable walls.

1 14. The method according to any of the claims 104 to 1 13, whereby said body is a layer or a particle.

1 15. The method according to claim 1 14, whereby said particle is a silica particle.

1 16. The method according to any of claims 104 to 1 14, whereby said body is a porous membrane.

1 17. A porous body having a hierarchical structure, the porous body being manufactured using a method according to any of the claims 104 to 1 16.

1 18. A porous body comprising a porous parent body, said porous parent body comprising a plurality of pores and whereby at least one pore of said parent body comprises at least one porous child body, whereby said porous child body comprises a plurality of pores, whereby said porous child body comprising a thickness of at least 0.3% of the parent body thickness.

1 19. The porous body according to claim 1 18, whereby said porous child body of at least 0.3 % and up to 99.7% of the parent body thickness.

120. The porous body according to any of the claims 1 18 to 1 19, whereby the pore diameter of the child body is smaller than the pore diameter of the parent body.

121 . The porous body according to any one of the claims 1 18 to 120, whereby a pore of the parent body is defined by a channel and whereby said channel extends through the whole parent body.

122. The porous body according to any one of the claims 1 18 to 121 , whereby a pore of the child body is defined by a channel and whereby said channel extends through the child body.

123. The porous body according to any one of the claims 1 18 to 122, whereby said body is a layer or particle, e.g. a silica particle.

124. The porous body according to any one of the claims 1 18 to 123, whereby said channel of the parent and/or child body comprises a hexagonal or circular or cubic or lameral cross-section.

125. The porous body according to any one of the claims 1 18 to 124, whereby the child body pores comprise metal particles.

126. The porous body according to claim 125, whereby the metal particles are positioned along and in the porous child body channel.

127. The porous body according to any one of the claims 1 18 to 126, whereby said porous parent body comprises a first and second outer body surface.

128. The porous body according to any of the claims 1 18 to 127, whereby the parent or child pores channels are oriented perpendicularly with respect to the first and second outer body.

129. The porous body according to any of the claims 1 18 to 127, whereby the channels are straight or are inclined with respect to the surface with a degree of 0 to 90 °.

130. The porous body according to any of the claims 1 18 to 127, whereby the channels are straight or are inclined with respect to the surface with a degree of 5 to 90 °.

131 . The porous body according to any of the claims 1 18 to 130, whereby said child porous body is positioned at the first outer body surface or at the second outer body surface or inbetween the first and second outer body surface.

132. The porous body according to any one of the claims 1 18 to 131 , whereby said parent body has a thickness ranging from 20 μηι up to 1 0 mm.

133. The porous body according to any one of the claims 1 1 8 to 131 , whereby said parent body has a thickness ranging from 60 μηι up to 1 mm.

134. The porous body according to any one of the claims 1 1 8 to 131 , whereby said parent body has a thickness ranging from 60 μηι to 200 μηι.

135. The porous body according to any one of the claims 1 1 8 to 131 , whereby the channel length of the parent pore channel ranges from 20 μηι up to 1 0 mm, e.g. from 60 μηι up to 1 mm..

136. The porous body according to any one of the claims 1 1 8 to 135, whereby the child body is filling the channels of the parent body substantially over their full cross-section. .

137. The porous body according to any one of the claims 1 1 8 to 135, whereby the channel length of the parent pore channel ranges from 60 μηι to 200 μηι.

138. The porous body according to any one of the claims 1 1 8 to 137, whereby said child body has a thickness ranging from 1 00 nm up to 1 0 mm.

139. The porous body according to any one of the claims 1 1 8 to 137, whereby said child body has a thickness ranging from 150 nm up to 1 mm.

140. The porous body according to any one of the claims 1 1 8 to 137, whereby said child body has a thickness ranging from 200 nm to 1 μηι.

141 . The porous body according to any one of the claims 1 18 to 140, whereby said porous body is a membrane.

142. The porous body according to any one of the claims 1 1 8 to 141 , whereby said porous parent body comprises a pore diameter between 20 nm and up to 2mm.

143. The porous body according to any one of the claims 1 1 8 to 142, whereby said parent body comprises a void fraction between 1 0 and 90%.

144. The porous body according to any one of the claims 1 1 8 to 143, whereby said porous child body comprises a pore diameter between 2 nm and 50 nm and a void fraction between 1 0 and 90%.

145. The porous body according to any one of the claims 1 18 to 144, whereby the porous child body is a catalytic active thin metal oxide layer.

146. The porous body according to any one of the claims 1 1 8 to 145, whereby the porous body is a hierarchical porous body.

147. The porous body according to claim 146, whereby the hierarchical porous body links the parent and/or at least one child body either directly or indirectly, and either vertically or horizontally.

148. The porous body according to any one of the claims 1 1 8 to 147, whereby the porous parent body is made from a material including carbon foams, glassy carbon, silica, alumina, alumina coated with silica, zirconia, zeolites, sintered titanium, titania, magnesia, yttria, copper, iron, iron nickel, iron cobalt, cobalt, steel, iron carbide, nickel, or a combination thereof.

149. Use of the porous body according to any one of the claims 1 1 8 to 148 for synthesis of nanostructures.

150. Use of a porous body according to any one of the claims 1 1 8 to 148 for biosensing.

151 . Use of a porous body according to any one of the claims 1 1 8 to 148 for photo conversion or photo devices.

152. Use of a substrate according to any of the claims 64 to 68, wherein the substrate is a membrane made of an inorganic material, as a growing substrate in a system for synthesis of nanostructures according to any of the claims 1 to 63.

153. A system for producing a porous membrane body having a hierarchical structure, the system comprising at least means for providing a porous parent body, means for filling of at least one pore of the parent body along a pore channel, with a filling substance, resulting in a filled and in an empty part of the parent pore, means for providing a child body in the empty part of the parent pore and means for removing the filling substance in the filled part of the parent pore, for performing a method according to any of claims 104 to 1 16.

154. A holder for a substrate according to any of claims 64 to 68, whereby said holder having a thickness, whereby the holder comprises a least one passageway across the thickness of the holder whereby said at least one passageway is a passageway having a truncated-cone shape.

155. A holder as set forth in claim 154, whereby said at least one passageway comprises a partially closed ending and an open ending on the opposite side, whereby said closed ending is positioned towards the inlet of the reactive gas and whereby the open ending is positioned towards an outlet of a system according to any of the claims 1 to 63.