DE102011000662A1 - Process for the preparation of graphene nanolayers - Google Patents

Abstract

The present invention relates to a method for producing nano-layers from graphene comprising the following steps: a) producing a mixture of at least one graphite compound and at least one polymerizable medium to form at least one graphite compound intercalated with the polymerizable medium, b) polymerizing the in step a) dispersion produced with exfoliation of the at least one intercalated graphite compound, and c) isolation of the graphene nano-layers formed in step b).

Description

The present invention relates to a process for the preparation of nanolayers from graphene according to claim 1.

Carbon usually has four distinct crystalline structures. These structures are commonly known by the terms diamond, graphite, fullerenes and carbon nanotubes.

Carbon nanotubes have a tubular structure of single-walled or multi-walled tubes. Conceptually, obtainable by rolling up a graphite sheet such as a graphene sheet or a plurality of graphite sheets to form a concentric hollow structure. Carbon nanotubes can be used either as conductors or as semiconductors depending on the coiled shape and diameter of the helical tubes. The elongated, hollow structure of carbon nanotubes gives this material unique mechanical, electrical, thermal, and chemical properties.

Carbon nanotubes can be fabricated by plasma deposition at a density of 10 ¹¹ cm ^-3 or more on suitable substrates, preferably using metal-coated silicon or polysilicon substrates. For growth of a layer of carbon nanotubes, hydrocarbon gases may be used as the plasma gas, and the temperature of the substrate suitable for deposition may be in the range of 600 to 900 ° C, and the pressure used in the method may be in the range of 10 to 1000 mtorr. Disadvantages are thus the rather expensive process for the production of carbon nanotubes and the rather low yield. Because of this, far-reaching application of carbon nanotubes has not been possible hitherto.

A possible alternative to the use of carbon nanotubes are nanolages of graphene.

Graphene is another modification of carbon with a two-dimensional, aromatic structure in which each carbon atom is surrounded by three other carbon atoms, forming a honeycomb pattern. Such two-dimensional carbon structures, however, have until recently not been thought possible because they were considered thermodynamically unstable. It was therefore all the more astonishing that the scientists Konstantin Novoselov, André Geim and their staff were able to prepare free, one-layer graphene structures in 2004. The previously unexpected stability could be explained by the existence of metastable states or wrinkling of the graphene.

Individual graphene nanolayers and clusters of multiple nanoscale graphene sheets, collectively referred to as Graphene Nanolagen (GNS), provide a unique opportunity for solid phase scientists to study the structure and properties of nanocarbon materials. These nanomaterials can also be a cost-effective substitute for carbon nanotubes or other types of nanorods for various scientific and process applications.

Initially, graphene was isolated by mechanical exfoliation of graphite by stripping surface layers of pyrolytic graphite ( Ponomarenko et al., Science 2008, 320, 356 ). However, this method is not suitable for large-scale applications.

A recently developed process involves the reduction of graphene oxide to graphene. Here graphite oxide is first deposited on a silicon substrate and then chemically reduced to individual layers of graphene ( Gummes-Navarro et al., Nanoletters, 2007, Vol. 7, 3499 ). But even this method is only suitable for a limited application.

The production of larger quantities of functionalized graphene oxide is based on the exfoliation of strong acid oxidized graphite by rapid thermal expansion ( Schniepp et al., Journal of Physical Chemistry B, 2006, Vol. 110, 8535 ) or ultrasonic dispersion ( Niyogi et al., Journal of American Chemical Society, 2006, Vol 128, 7720 ). The oxidation chemistry is similar to that used to functionalize single-walled carbon nanotubes (SWNT) and allows for the production of a variety of oxygen functionalities, such as -OH or -CO ₂ H, primarily at so-called defect sites the ends of the nanotubes.

Using highly oxidizing agents, it was also possible to produce functionalized defects on the wall surfaces of the carbon nanotubes. For this reason included independently From the mechanism of exfoliation, the graphene oxides prepared from oxidized graphite occupy a substantial proportion of oxygen functionalities and defects, so that the structural and electronic perturbances caused by the oxidation must be repaired to preserve the unusual properties of graphene.

These pertubations or perturbations can be improved on the surface by means of the so-called passivation chemistry, for example by the reaction of graphene oxides with amines. However, due to residual defects, the resulting materials do not have the electronic properties of graphene.

Ideally, after exfoliation, the graphene oxide must be reduced under rigorous conditions to obtain the desired graphene properties. Thus, polymer coated graphite nanoplatelets were obtained by reduction of exfoliated graphene oxide in the presence of polysodium 4-styrenesulfonate. However, the presence of a polymeric dispersant in the graphene composition is undesirable in some applications. The reduction of exfoliated graphite oxide in the presence of ammonia can lead to the formation of graphene nanolayers, which, however, have a limited water solubility of less than 0.5 mg / ml ( Lij et al. Nat. Nano. 2008, Vol. 3, 101 ).

The disadvantages of the previously developed processes for the production of graphene via the approach of graphite extrusion are manifold. Thus, on an industrial scale, it is currently only possible to use graphite in a very low concentration of 0.2 to 0.3% by weight in the reaction batch. Another disadvantage of these methods is the long exfoliation time, which can take up to 400 hours. Also, the methods used to date include a variety of reaction steps and complex reaction conditions that prevent cost-effective production of graphene and thus their large-scale use.

The present invention is therefore based on the problem to provide a process for the preparation of graphene nanolayers, which does not have the disadvantages mentioned above, is easy to apply and allows the production of graphene nanolayers in a simple and cost-effective manner.

This object is achieved by a method having the features of claim 1.

Accordingly, a method for producing nanolayers from graphene is provided by the steps described below.

In a first step a), the preparation of a mixture of at least one graphite compound and at least one polymerizable medium, wherein it comes to the formation of at least one intercalated with the polymerizable medium graphite compound. In the polymerization step b) following step a), the polymerization of the dispersion prepared in step a) is carried out with simultaneous exfoliation or delamination of the at least one intercalated graphite compound. Following exfoliation, the formed graph is isolated in step c).

The inventive method thus enables the production of graphene nanolayers (GNS), which consists essentially of a graphite layer or a plurality of graphite layers. Each of these graphite layers, also referred to as graphite nanolay, comprises a two-dimensional hexagonal structure of carbon atoms. Each ply has a length, and a width parallel to the graphite ply, and a thickness orthogonal to the graphite ply, wherein at least one of these values is equal to or less than 100 nm. The thickness of the layer is preferably less than 100 nm.

The first step of the process according to the invention for producing the graphene nanolayers comprises the dispersion of graphite in a suitable polymerisable medium. Accordingly, the polymerizable medium should consist of at least one compound having at least one double bond suitable for the polymerization, in particular unsaturated fatty acids.

The graphite dispersion prepared in step a) in the at least one polymerisable medium preferably contains graphite crystals having a size in the micron and / or nanometer range, each graphite crystal consisting of one or more graphite layers. Preferably, the carbon used is chopped or ground to improve the dispersion and to allow for better penetration or penetration of the polymerizable medium between the graphite sheets. At this time, the graphite used preferably has a thickness of less than 1 mm, in particular less than 0.1 mm.

The second step b) comprises the step of exfoliating the graphite crystal by in situ polymerization, in particular thermal in situ polymerization, of the at least one polymerizable medium used.

Exfoliation typically involves penetration or absorption of the molecules of the polymerizable medium between the individual graphite layers of the graphite compound used. After absorption or penetration of the molecules of the polymerizable medium into the graphite compound, the intercalated graphite compound thus prepared is heated together with the polymerizable medium, resulting in polymerization of the molecules of the polymerizable medium and growth of the polymerization takes place. The purpose of this Exfoliationbehandlung is the delamination or at least in the production of a gap between the individual graphene layers of the graphite compound or the partial or complete separation of the individual graphene layers in the graphite compound used, so as individual Graphenlagen (single-layered GNS) or blocks of bonded graphene layers (multilayer GNS) produce.

After the intercalated graphite compound has been polymerized, the nanolayers formed are isolated in step c). Here, the graphene nanolayers are separated from the polymerized medium. The separation or isolation typically involves repeated washing with a suitable solvent, followed by filtration.

In the direction of the thickness or C-axial direction with respect to the graphene sheets, there may exist a small number of graphene sheets which are still tied together by van der Waals forces, which usually hold together the individual graphene sheets in the naturally occurring graphite.

Preferably, the graphene produced by the present method has less than 100 layers, preferably less than 20 layers, more preferably less than 5 layers of graphene layers, each of these graphene layers having a length and width less than 100 nm, preferably less than 50 nm.

The simplicity of the process and the wide range of properties of the graphene produced by the process of the invention allow multiple applications. The electronic, thermal and mechanical properties of graphene are comparable to those of carbon nanotubes, wherein when using the method according to the invention graphene is much cheaper and can be produced in larger quantities.

As graphite compound from which the graphene or the graphene layers can be produced, preference is given to using at least one graphite compound selected from the group consisting of natural graphite, synthetic graphite, expanded graphite and graphite fibers. As already described, the medium suitable for the polymerization in step b) comprises at least one compound having at least one double bond suitable for the polymerization, in particular unsaturated fatty acids. Vegetable and / or animal oils have proven to be particularly suitable and cost-effective. Thus, the at least one vegetable oil selected from the group comprising linseed oil, tung oil, soybean oil, canola oil, sunflower oil, olive oil, castor oil, peanut oil, corn oil, thistle oil or mixtures thereof. If animal oils are to be used as the polymerizable medium, preferably at least one animal oil is selected from the group comprising fish oil, in particular anchovy oil (anchovy oil), salmon oil, Pronova fish oil, shark liver oil, sturgeon oil, eel oil, anchovy oil, herring oil (shark oil, river herring oil) or Used skate oil.

In one embodiment of the method according to the invention are between 1 to 50 wt .-%, preferably 2 to 20 wt .-% of the at least one graphite compound and between 50 to 99 wt .-%, preferably 80 to 98 wt .-% of at least one polymerizable Medium used.

The polymerization in step b) can be carried out up to a viscosity of the polymerizable medium between 40 mPas and 10000 mPas, preferably 100 to 8000 mPas, particularly preferably 500 to 5000 mPas. The viscosity to be achieved depends on the type of polymerizable medium used and can therefore vary considerably. In particular, the viscosity is dependent on the prevailing ambient or reaction temperature and therefore vary greatly.

Polymerization and exfoliation in step b) are preferably carried out at a temperature of between 100 and 500 ° C., preferably between 200 and 350 ° C., more preferably between 250 and 300 ° C.

The duration of the polymerization and exfoliation in step b) preferably ranges from 0.5 to 50 hours, preferably from 5 to 40 hours, more preferably from 10 to 30 hours. Ideally, the polymerization time can be 16 to 24 hours. The longer the polymerization time, the thinner graphene sheets can be produced by the present method. For shorter reaction times of z. B. 30 min only layers of graphene with a layer thickness of up to 800 nm can be produced. If the reaction time is increased successively, the layer thickness of the prepared graphene layers is also reduced. Over a period of up to 5 hours, layers with a layer thickness of 500 nm and at 16 hours graphene layers with a layer thickness of 200 nm are obtained. Increasing the reaction time to 24 hours or more reduces the film thickness to 100 nm and below.

Likewise, the polymerization or reaction time has an influence on the distribution of the prepared graphene layers. As the reaction time increases, the proportion of graphene layers with layer thicknesses of 3 μm decreases successively and shifts towards graphene layers with smaller layer thicknesses of less than 100 nm. The decrease in layer thicknesses is therefore to be regarded as a kinetic process that depends on the external reaction conditions such as time, temperature , the amount of graphite used and the type of polymerization medium is dependent.

In step c) of the present process, the nanolayers formed from graphene can be isolated by means of addition of at least one solvent to the polymerized dispersion obtained in step b) and subsequent filtration. The preferred solvent is selected from a group containing chloroform, toluene or petroleum. Essential in the choice of a suitable solvent is that this can dissolve the polymerizable medium used.

The graphene sheets produced by means of the method according to the invention can be used for heat dissipation or else in the form of coatings or printing inks, due to their excellent conductive properties in polymer composites, lubricants and greases.

Due to the particularly high electrical conductivity of graphene nanolayers, a particularly preferred field of use is the use of graphene nanolayers as semiconductor material, for. B. in transistors. It is even conceivable that graphene can replace what is currently the most commonly used silicon. Clock rates of up to 1000 GHz are possible with garphene-based transistors, while with silicon-based transistors only clock rates of up to 5 GHz are currently possible.

The invention will be explained in more detail with reference to several figures in conjunction with exemplary embodiments.

Show it:

1 a TEM image of Graphenlagen produced by the method according to the invention; and

2 a Raman spectrum of graphite used as the starting component and graphene nanolayers prepared by the method according to the invention.

1 shows a recording of graphene layers by means of transmission electron microscopy (TEM). The picture shows graphene layers comprising one, two or more layers. The dimensions of the graphene layers are in the range of 10 μm × 4.5 μm. With reference to the particles which are arranged perpendicular to the graphene layers, the thickness of the graphene layers can be estimated to a thickness between 10 to 40 nm.

Because of the oil or polyol layer deposited on the graphene layers, it is also possible with reference to the TEM image to determine the layer number of the graphene layers at 1 to 5.

2 shows a Raman spectrum of the graphite used for the preparation of graphene layers (lower line) and produced by the process according to the invention Graphennanolagen (upper line). The graphene was prepared using soybean oil and a polymerization time of 24 hours. At a frequency of 963 cm ^-1 , a peak can be seen which indicates the presence of a polymer layer on the particle or graphene surface.

It can also be seen that the intensity of the D peak at a frequency of 1364 cm ^-1 in the graphene layer increases in comparison to the graphite used as starting material. This increase is due to an increase in the number or concentration of carbon atoms which are not sp ² -carbon atoms, which are typical for the graphite. In other words, the increase in the number of non-sp ² carbon atoms clearly indicates the change in valence and thus the formation of graphene sheets.

The broadening of the 2D peak at a frequency of 2711 cm ^-1 underpins the structural change of the graphite particles to form individual graphene layers.

The peak at a frequency of 1582 cm ^-1 is the so-called G peak, which is caused by the sp ² hybridization of the carbon atom in the graphite used as the starting material.

Exemplary Embodiment 1 Dispersion of Graphite in Soybean Oil

Suitable graphite compounds are u. a. natural graphite (RMC Remacon GmbH), synthetic graphite (Thielmann Graphit GmbH & Co. KG) and expanded graphite (Timcal Ltd).

1 g of synthetic graphite is added to 99 g of soybean oil in three wide-necked flasks equipped with thermometer and reflux condenser and stirred for 2 hours.

Embodiment 2: Exfoliation

The dispersion of synthetic graphite and soybean oil of Example 1 is heated to 290 ° C in a heating mantle with temperature control with stirring. The reaction mixture is held at this temperature for 24 hours. Subsequently, the reaction mixture is cooled to room temperature, diluted with 300 ml of chloroform and filtered to remove the solid from the oil. The black solid is collected on the filter and washed repeatedly with chloroform until a colorless supernatant is obtained.

Embodiment 3

Analogous conditions are selected as in working examples 1 and 2, wherein the polymerization time varies between 0.35 hours and 28 hours.

Embodiment 4

Analogous conditions are selected as in working examples 1 and 2, the amount of graphite used varying between 2 g and 20 g.

Embodiment 5:

Analogous conditions are selected as in working examples 1 and 2, whereby instead of the synthetic graphite natural and expanded graphite is used.

Embodiment 6:

Analogous conditions are selected as in working examples 1 and 2, fish oil being used as the polymerisable medium.

Embodiment 7:

Analogous conditions are selected as in working examples 1 and 2, the polymerization temperature varying between 200 and 350.degree.

Embodiment 8: Characterization of the formed graphene

1 mg of the black solid collected on the filter after in situ polymerization was dispersed in 10 ml of chloroform. The slightly grayish dispersion is spin-coated on a silicon substrate and examined by atomic force microscopy (AFM). The results of the examples tested are summarized in Table 1. Ex. graphite oil Polymerization time [h] Amount of graphite [g] Size d10 [μm] Size d50 [μm] Size d90 [μm] 2 Synth. Soybean 24 1 0003 1.1 3.0 3a Synth. Soybean 12:35 1 0.8 1.2 3.0 3b Synth. Soybean 5 1 0.5 1.2 3.0 3c Synth. Soybean 16 1 0.2 1.1 3.0 4a Synth. Soybean 24 11 12:01 1.0 2.0 4b Synth. Soybean 24 20 0021 0.8 1.1 5 Expand. Soybean 24 10 0025 5 10 6 Synth. fish oil 24 1 0045 1.2 3.2 Table 1: AFM results

Table 1 summarizes the experimental results for the preparation of graphene nanoparticles or graphene nanolayers by in situ polymerization of natural oils. The polymerization of a mixture of graphite and soybean oil or fish oil leads to the formation of graphene nanolayers by exfoliation of the graphite during the polymerization. As can be seen from Table 1, the polymerization time or exfoliation time has a decisive influence on the thickness of the graphan olanols produced. Thus, the proportion of graphene layers with a thickness below 100 nm increases with increasing polymerization time, with the best results being achieved at a polymerization time of 24 h. The amount or concentration of the used graphite has a smaller influence on the thickness of the synthesized graphene layers.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited non-patent literature

• Ponomarenko et al., Science 2008, 320, 356 [0008]
• Gummes-Navarro et al., Nanoletters, 2007, Vol. 7, 3499 [0009]
• Schniepp et al., Journal of Physical Chemistry B, 2006, Vol. 110, 8535 [0010]
• Niyogi et al., Journal of American Chemical Society, 2006, Vol 128, 7720 [0010]
• Lij et al. Nat. Nano. 2008, Vol. 3, 101 [0013]

Claims (12)

Process for the preparation of nanolayers from graphene comprising the following steps: a) preparing a mixture of at least one graphite compound and at least one polymerisable medium to form at least one graphite compound intercalated with the polymerisable medium, b) polymerization of the dispersion prepared in step a) with exfoliation of the at least one intercalated graphite compound, and c) isolation of the nanolayers formed in step b) from graphene. A method according to claim 1, characterized in that the at least one graphite compound is selected from a group containing natural graphite, synthetic graphite, expanded graphite and graphite fibers. A method according to claim 1 or 2, characterized in that the polymerizable medium comprises at least one compound having at least one suitable for the polymerization double bond, in particular unsaturated fatty acids. Method according to one of the preceding claims, characterized in that at least one vegetable and / or animal oil is used as a polymerizable medium. Method according to one of the preceding claims, characterized in that at least one vegetable oil selected from the group consisting of linseed oil, tung oil, soybean oil, canola oil, sunflower oil, olive oil, castor oil, peanut oil, corn oil, thistle oil or mixtures thereof is used as the polymerizable medium. Method according to one of the preceding claims, characterized in that as polymerizable medium at least one animal oil selected from the group containing fish oil, in particular anchovy oil (anchovy oil), salmon oil, Pronova fish oil, shark liver oil, sturgeon oil, eel oil, anchovy oil, herring oil (Maifischöl, Flussheringsöl ) or ray oil is used. Method according to one of the preceding claims, characterized in that between 1 to 50 wt .-%, preferably 2 to 20 wt .-% of the at least one graphite compound and between 50 to 99 wt .-%, preferably 80 to 98 wt .-% of the at least one polymerizable medium are used. Method according to one of the preceding claims, characterized in that the polymerization in step b) to a viscosity of the polymerizable medium between 40 mPas and 10,000 mPas, preferably 100 to 8000 mPas, particularly preferably 500 to 5000 mPas is performed. Method according to one of the preceding claims, characterized in that the polymerization and exfoliation in step b) at a temperature between 100 to 500 ° C, preferably between 200 to 350 ° C, more preferably between 250 to 300 ° C is performed. Method according to one of the preceding claims, characterized in that the polymerization and exfoliation in step b) for a period of between 0.5 to 50 hours, preferably between 5 to 40 hours, more preferably carried out for 10 to 30 hours. Method according to one of the preceding claims, characterized in that the nanolayers formed from graphene in step c) are isolated by adding at least one solvent to the polymerized dispersion obtained in step b) and subsequent filtration. A method according to claim 11, characterized in that the at least one solvent is selected from a group containing chloroform, toluene and petroleum.

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