WO2022023600A1 - Method for obtaining single-layer carboxylated graphene oxide and single-layer carboxylated graphene oxide obtained - Google Patents

Method for obtaining single-layer carboxylated graphene oxide and single-layer carboxylated graphene oxide obtained Download PDF

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WO2022023600A1
WO2022023600A1 PCT/ES2021/070521 ES2021070521W WO2022023600A1 WO 2022023600 A1 WO2022023600 A1 WO 2022023600A1 ES 2021070521 W ES2021070521 W ES 2021070521W WO 2022023600 A1 WO2022023600 A1 WO 2022023600A1
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graphene oxide
graphite
sheets
monolayer
carboxylated graphene
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Spanish (es)
French (fr)
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Iluminada Rodríguez Pastor
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Applynano Solutions, S.L.
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    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B32/00—Carbon; Compounds thereof
    • C01B32/15—Nano-sized carbon materials
    • C01B32/182—Graphene
    • C01B32/198—Graphene oxide
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y30/00—Nanotechnology for materials or surface science, e.g. nanocomposites
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y40/00—Manufacture or treatment of nanostructures
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B32/00—Carbon; Compounds thereof
    • C01B32/15—Nano-sized carbon materials
    • C01B32/182—Graphene
    • C01B32/184—Preparation
    • C01B32/19—Preparation by exfoliation
    • C01B32/192—Preparation by exfoliation starting from graphitic oxides
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B32/00—Carbon; Compounds thereof
    • C01B32/20—Graphite
    • C01B32/21—After-treatment
    • C01B32/23—Oxidation

Definitions

  • the present invention relates to a process for the synthesis of graphene oxide with specific properties for its use, among other uses, as a reinforcing material in polymer matrix composite materials and for the formation of membranes.
  • the graphene oxide obtained is monolayer, so that each of the layers is functionalized with selective oxygenated groups by oxidation process carried out on previously exfoliated monolayers, and not on the stacked layers, unlike the usual processes of synthesis of graphene. graphene oxide.
  • the object of the invention is to make it possible to obtain an exfoliated graphene oxide with a size and degree of functionalization that can be adjusted to the needs of the application and the matrix where it is to be incorporated, the membrane to be formed, or simply joined. through functionalization with any other entity.
  • the graphene oxide obtained by the process of the present invention has suitable surface properties to achieve adequate dispersion in polymeric matrices, together with greater control of the dimensions and number of graphene oxide layers.
  • Graphene oxide is obtained by exfoliation of an intermediate product called graphite oxide, which comes from an intercalation compound of graphitic structure materials, mainly graphite with acids and oxysalts, which is unstable and decomposes into graphite oxide.
  • an intermediate product called graphite oxide which comes from an intercalation compound of graphitic structure materials, mainly graphite with acids and oxysalts, which is unstable and decomposes into graphite oxide.
  • a large number of functional groups are formed between the stacked graphitic planes, adding water intercalated between the oxygenated layers to produce the separation between the layers that in graphite have a separation of 0.34 nm and in graphite oxide have a separation of 0.9 nm.
  • JEONG, HK et al Evidence of graphitic AB stacking order of graphite oxides, J. Am. Chem. Soc. 130 (2008) 1362-1366. ISSN 0002-7863.
  • This separation in layers of graphite oxide allow
  • the Hummers and Offeman method is the chemical exfoliation method that allows obtaining a high degree of intercalation and oxidation between the layers of the prepared graphite oxide, obtaining a graphene oxide with a higher proportion. of monolayers, as disclosed in the publication of BOTAS, C. et al. Graphene materials with different structures prepared from the same graphite by the Hummers and Brodie methods. Carbon 65 (2013) 156-164. ISSN 0008-6223.
  • PCT international patent application no. WO2011016889 discloses a method for producing highly oxidized graphene oxide with high structural quality, with a higher proportion of rings and aromatic domains than that of graphene oxide, where the presence of at least one protective agent is added. They are based on the Hummers-Offeman method using potassium permanganate as oxidant, sulfuric acid reaction medium, and different non-aqueous weak acids as protective agent (eg phosphoric acid and its related, trifluoroacetic acid, boric acid), to avoid the formation of defects in graphene sheets that occur during the oxidation process.
  • protective agent eg phosphoric acid and its related, trifluoroacetic acid, boric acid
  • patent no. US10336619B2 discloses a modification of the Hummers and Offeman method by which graphene oxide with a single layer or with few layers is obtained, where after the reaction of KMn0 and graphite in a sulfuric acid medium, quenching is carried out by pouring the mixture into a solution aqueous hydrogen peroxide.
  • the invention does not use NaN0 3 or H 3 P0 4 as protective agent for graphene oxide.
  • the patent discloses that the intercalation process in the graphite sheets occurs completely only with the presence of H 2 S0 and KMn0 4 .
  • the graphene oxide obtained in this way has high solubility both in aqueous medium and in polar organic solvents.
  • the incorporation of graphene oxide into polymeric matrices is one of the current challenges in the field of composite materials in order to obtain materials with improved properties, such as glass or carbon fiber, which use thermosetting resins as a matrix.
  • materials with improved properties such as glass or carbon fiber, which use thermosetting resins as a matrix.
  • the graphene oxide in order to achieve a high interaction between the polymeric matrix, the graphene oxide and the fiber, it is necessary for the graphene oxide to have a controlled size and number of planes, as well as the selective type of oxygenated groups.
  • the incorporation of nanomaterials is essential to obtain materials with improved properties (such as mechanical properties, thermal and electrical conductivity, anti-corrosion, etc.) but whose limitation lies in the difficulty of obtaining homogeneous dispersions of nanomaterials in said matrices.
  • the use of graphene oxide as a second reinforcing filler - in addition to the fiber - improves the mechanical properties of the material.
  • the selection of the filler and resin is essential to avoid problems in processing, such as premature curing of the resin or failure to fill the mold during the infusion process.
  • graphene oxide to carbon fibers has also been studied, verifying the influence of the functional groups of graphene oxide on the reinforcing capacity of epoxy resin composite materials processed by vacuum infusion, concluding that graphene oxide increases the toughness of epoxy resin laminates with carbon fiber.
  • graphene oxide mainly has hydroxyl, ether-epoxy, quinone and lactol groups, as detailed in the disclosure by MARTIN GULLON, I. et al. New insights into oxygen surface coverage and the resulting two-component structure of graphene oxide.
  • carboxyl and carboxylate groups are necessary, so it is necessary to previously anchor the carboxyl groups to the graphene surface.
  • the applicant of the present patent application detects the need to develop a procedure for obtaining graphene oxide with a low number of planes or stacked layers and with the majority or selective presence of carboxylic groups, so that the oxide
  • the graphene obtained has characteristics that favor its effective dispersion in a polymeric matrix, improving its compatibility and better fiber-resin interfacial interaction.
  • the present invention recommends a procedure for obtaining monolayer carboxylated graphene oxide whose majority presence of selective functional groups such as carboxyl, carboxylate, lactone and anhydride allow an exhaustive control of the resin-graphene oxide interface through an interlaminar functionalization with oxygenated groups that enables the best dispersion of graphene oxide sheets in the resin.
  • the process for obtaining monolayer carboxylated graphene oxide is made up of at least the following stages:
  • the material with the crystalline graphitic structure is natural graphite, synthetic graphite, carbon nanotubes, helical tape carbon nanofibers, carbon fiber and/or carbon black.
  • the monolayer carboxylated graphene oxide sheets formed are made up of a sheet where the functional groups formed are mostly selective of the carboxyl, carboxylate, lactone and anhydride type, which are located at the edges of the carboxylated graphene oxide sheets. monolayer and in the structural defects of its interior, that is, located in defects such as the holes located in the sheets.
  • the precursor material is oxidized - the material with a crystalline graphitic structure - to obtain the graphite oxide and in it, oxo acids such as sulfuric acid or nitric acid are preferably used, while the oxysalts used are, preferably potassium permanganate or potassium chlorate.
  • the oxidation of the precursor material is carried out by the Brodie method
  • the oxidation is carried out using potassium chlorate in a fuming nitric acid medium.
  • the oxidation of the precursor material is carried out by the Staudenmeier method, the oxidation of the precursor material is carried out using potassium chlorate in a nitric and sulfuric acid medium.
  • potassium permanganate is used in a sulfuric acid medium, with or without the presence of sodium nitrate.
  • the oxidation of the material with a crystalline graphitic structure is carried out using the Hummers and Offeman method, being formed by, at least, the following stages:
  • the material with a crystalline graphitic structure is suspended in a sulfuric acid medium or in a sulfuric acid-sodium nitrate medium with a ratio (precursor mass:volume) of between 1:20 and 1:150 (g graphite:ml of H 2 S0 ), carrying out mechanical stirring for at least 15 minutes at room temperature.
  • a graphite:sulfuric acid intercalation compound is generated, separating the graphite layers.
  • a preliminary graphite oxide reaction mixture is generated, which presents functional groups on the structural defects of the graphitic structure (basal planes and edges) by decomposing the intercalation compound formed.
  • the material with a crystalline graphitic structure is suspended in a sulfuric acid medium or in a sulfuric acid-sodium nitrate medium.
  • a suspension of sodium nitrate in sulfuric acid in a mass ratio preferably between 1:1 and 1:2 (NaN0:precursor).
  • the second stage of the procedure for obtaining monolayer carboxylated graphene oxide refers to the thermal exfoliation of the graphite oxide obtained in the first oxidation, the thermal exfoliation being formed by, at least, the following stages:
  • hydrophobic sheets of reduced graphene oxide also called chemically exfoliated graphene
  • hydrophobic sheets of reduced graphene oxide also called chemically exfoliated graphene
  • the functional groups dragged and eliminated in the thermal exfoliation stage of the graphite oxide are those that were formed in the defects generated, and subsequent propagation, during the oxidation of the crystalline graphitic structure.
  • the temperature increase in the thermal exfoliation stage is generated by microwave irradiation, since it is a much more efficient heating process on materials rich in carbon and water than convective heating by temperature gradient.
  • the next and last stage is a second oxidation of the hydrophobic sheets of chemically exfoliated graphene, which is formed by the following stages:
  • hydrophobic sheets of chemically exfoliated graphene are suspended in a sulfuric acid medium or in a sulfuric acid-sodium nitrate medium with a mass ratio of hydrophobic sheets:volume between 1:20 and 1:150 (g of hydrophobic sheet: ml of H 2 S0 ), carrying out mechanical stirring for at least 15 minutes at room temperature. In this case, no intercalation compound is formed since the sheets do not have crystalline stacking, so their solvation by the acid is less.
  • the pre-oxidation is weaker (forming hydroxyl groups) and localized in the larger defects, mainly edges.
  • the hydrophobic sheets of chemically exfoliated graphene are suspended in a sulfuric acid medium or in a sulfuric acid-sodium nitrate medium.
  • a suspension of sodium nitrate in sulfuric acid in a mass ratio preferably between 1:1 and 1:2.
  • the graphene oxide obtained has a controlled plane size, standardized and smaller than that obtained with the conventional process of Hummers and Offeman.
  • the exfoliation and subsequent oxidation steps make it possible to obtain a graphene oxide with a high rate of monolayers without ordered stacking.
  • the graphene oxide obtained has a functionalized lamellar structure with selectively oxygenated groups (carboxylates, carboxylic, alkoxy, etc.), which allows greater chemical interaction and compatibility with the polar groups of a polymeric matrix and the fiber.
  • oxygenated groups present in the sheets of synthesized graphene oxide also have the possibility of surface functionalization, offering a high application in the field of nanocomposites by allowing compatibility with a greater number of polymeric matrices and even providing special properties. .
  • the characteristics of the monolayer carboxylated graphene oxide obtained allow its use as a reinforcing material in materials composed of thermosetting polymeric matrices (epoxy, polyester, vinyl ester resins) reinforced with fiber (for example, fiberglass, . carbon, aramid) in order to obtain materials with improved mechanical properties, where the matrix phase is dominant.
  • thermosetting polymeric matrices epoxy, polyester, vinyl ester resins
  • fiber for example, fiberglass, . carbon, aramid
  • the addition of monolayer carboxylated graphene oxide sheets of the present invention allow an exhaustive control of the resin- oxide.
  • the monolayer carboxylated graphene oxide presents a selectivity of oxygenated functional groups and, consequently, its presence in the mixture with fibers and/or resins favors the dispersion of the graphene oxide sheets in the resin and/or fiber. and an increase in the mechanical properties of the composite materials obtained.
  • monolayer carboxylated graphene oxide of the present invention allows obtaining mechanically reinforced plastics and polymers with antibacterial properties, a decrease in permeability due to a barrier effect, etc., with innovative applications in the packaging sector or 3D printing.
  • monolayer carboxylated graphene oxide allows subsequent functionalization processes for use in other matrices (for example, amine groups, silanes, etc.) and applications in the formation of graphene-based membranes for water treatment, in microfiltration, ultrafiltration , nanofiltration and desalination.
  • Figure 1. Shows a representation of an X-ray photoelectronic spectrometry of the material with a crystalline graphitic structure to be oxidized.
  • Figure 2. Shows a representation of an X-ray photoelectronic spectrometry of the graphite oxide obtained after the first oxidation.
  • Figure 3. Shows a representation of an X-ray photoelectronic spectrometry of exfoliated graphene oxide.
  • Figure 4. Shows a representation of an X-ray photoelectronic spectrometry of the monolayer carboxylated graphene oxide obtained after the second oxidation.
  • Figure 5. Shows a representation of the X-ray diffractogram of graphite oxide, exfoliated graphene oxide and monolayer carboxylated graphene oxide.
  • Figure 6 Shows a transmission electron microscopy image of different sheets of monolayer carboxylated graphene oxide, where it is observed that they have a very similar size.
  • the oxidation step of the crystalline graphitic structure material is carried out using the Flummers and Offeman method, as it is the most effective in the intercalation of anhydrous sulfuric acid. between each and every one of the layers, and subsequently oxidize the interior by the action of the permanganate.
  • the presence of sodium nitrate during the first and second oxidation helps the sulfuric acid and potassium permanganate to penetrate into the galleries or structural defects both in the material with a crystalline graphitic structure and in the galleries or structural defects of the graphene oxide.
  • the material with a crystalline structure used as a precursor is natural graphite.
  • the oxidation of the precursor material is carried out using the Flummers and Offeman method as it is the most efficient in the intercalation of anhydrous sulfuric acid between each and every one of the layers, and subsequently oxidizing the interior of the layers by the action of permanganate.
  • the presence of sodium nitrate encourages the penetration of sulfuric acid and sodium permanganate in the galleries or defects present in the graphite layers.
  • natural graphite and sodium nitrate are suspended, in a 1:1 mass ratio, in sulfuric acid (preferably 98%) with a precursor mass ratio: sulfuric acid volume of 1:50 (g graphite: ml of H 2 S0 ), in such a way that 70 ml of sulfuric acid and mixed with mechanical stirring for several hours at room temperature.
  • a graphite:sulfuric acid intercalation compound is generated, separating the graphite layers.
  • the corresponding quantity of potassium permanganate (mass ratio KMn0 4 :precursor of 4:1) is slowly added , controlling the temperature below 25°C. Stirring is then maintained at a temperature of 35-55°C , for one or two hours.
  • the preliminary graphite oxide is generated, when the potassium permanganate acts in the galleries, with the sulfuric acid, generating functional groups on the structural defects of the graphitic structure.
  • the reaction mixture is poured onto water (quenching) in a relative volumetric ratio of sulfuric acid:water of 1:1.8. This process can produce up to 100 e C, so it is advisable to extract heat and keep the mixture at 60 e C.
  • the liquid phase is separated from the solid phase by centrifugation or filtration processes, discarding the clear liquid phase.
  • the solid is washed with water at pH 2 and separated again, and finally washed with water.
  • the wet solid phase is then dried by convection at temperatures between 60 and 70 C.
  • the product thus obtained corresponds to graphite oxide, which is a powdery and dry material.
  • the graphite oxide obtained is subjected to a thermal exfoliation treatment. This process takes place at a temperature between 140 e and 200 e C. The increase in temperature of the particle produces the violent exit of the large amount of water intercalated between the layers, both physisorbed and chemisorbed, exfoliating the material.
  • the reduced graphene oxide sheets are separated and do not show graphitic stacking.
  • the graphene oxide sheets are smaller than the starting graphite, since the planes of the original graphite were cut to a large extent with the generation of the functional groups and subsequent breakage of these bonds in the thermal exfoliation. Therefore, the result of the thermal exfoliation of graphite oxide by thermal shock is a plurality of chemically exfoliated graphene sheets, without ordered stacking and hydrophobic, as part of the oxygen is eliminated by the thermal decomposition of the oxygenated groups. Hydrophobic sheets of a similar size and smaller than that of the starting graphite oxide are generated.
  • a second oxidation is carried out by the same modified Hummers-Offeman chemical method described above.
  • 1 g of the exfoliated graphene oxide is suspended in 70 ml of H 2 S0 together with 1 g of NaN0 and stirred on a magnetic hot plate for three hours at room temperature.
  • Potassium permanganate is added in a 3:1 ratio and stirred for two hours at 35°C.
  • the mixture is then heated to 55°C and allowed to stabilize. Allow to cool and pour over about 160 ml of water and ice, stir and add 8 ml of hydrogen peroxide.
  • the solution is filtered and the solid is washed with water for 30 min, filtered from new and the product is dried in the oven at 65 e C.
  • the product thus obtained is monolayer carboxylated graphene oxide that presents a selective functionalization with carboxylic, carboxylate groups, with hardly any change in the size of the sheets in this last stage, but with a hydrophilic character, resulting in a very fine and dry black powder.
  • Figures 1, 2, 3 and 4 are attached, which correspond, respectively, to the representations of the X-Ray Photoelectronic Spectrometry for the material with a crystalline graphitic structure to be oxidized, graphite oxide, chemically exfoliated graphene and graphene oxide.
  • carboxylated monolayer obtained, representing the binding energy (eV) on the abscissa axis and the intensity on the ordinate axis.
  • eV binding energy
  • the graphite oxide obtained in the first stage has a higher percentage of oxygen than the graphene oxide obtained after the second stage (figure 4).
  • the atomic C/O ratio is higher in the monolayer carboxylated graphene oxide (figure 4) obtained after the second oxidation than in the conventional graphite oxide (figure 2) obtained in the first oxidation.
  • the following table shows the evolution of the carbon-oxygen ratio (C/O) for each of the compounds involved in the steps of the process object of the present invention, measured by X-ray Photoelectron Spectroscopy.
  • C/O carbon-oxygen ratio
  • the exfoliation process - second stage of the procedure of the invention removes oxygenated groups, obtaining a reduced graphene oxide with a surface composition similar to that of the initial crystalline graphitic structure material.
  • exfoliation allows the separation of the graphite layers, which in the following oxidation stage enables the introduction of new oxygenated groups, in this case with a higher degree of oxidation, in the form of anhydrides, lactones and/or carboxylates (15% , 289.0 eV), exceeding the peak of 286.7 eV, different from the oxygenated groups obtained in the conventional graphite oxide obtained with the first oxidation stage.
  • the mentioned effect is consistent with the measurements made by zeta potential to aqueous suspensions of 0.1 mg/ml_ of conventional graphite oxide and monolayer carboxylated graphene oxide.
  • the first has a value of -33.3 mV, while the second - 48.1 mV, indicating that the monolayer carboxylated graphene oxide has more polarity, and more stability in water, because its functional groups are preferentially carboxyl and carboxylate, even when the oxygen content is lower than conventional graphite oxide.
  • Figure 5 shows the X-Ray Diffraction (XRD) diffractograms of graphite oxide (corresponds to line I), chemically exfoliated graphene (or reduced graphene oxide) (corresponds to line J) and graphene oxide.
  • carboxylated graphene (corresponds to line K), inventive product of this patent.
  • the abscissa axis represents the angle 2Q and the ordinate axis the intensity. It is observed that the graphite oxide presents stacking, given the prominence of the 002 peak around 10 e of the diffraction angle. Thermal exfoliation eliminates this stacking, since no peak is present in the interval, as well as in the carboxylated graphene oxide. It is confirmed that the product of the third stage, the oxidation of the hydrophobic sheets of chemically exfoliated graphene, gives a product very different from that of stage 1, consisting of monolayers.
  • Figure 6 shows an image obtained by Transmission Electron Microscopy (TEM) on a grid support in which a ruler (2) corresponding to 2 micrometers is represented.
  • TEM Transmission Electron Microscopy
  • the homogeneity between sheets (1) is due to the fact that the second oxidation, as it does not take place between the layers, is not the result of a chain reaction that leads to the sheets being cut, but rather acts punctually, only on the defects and previously formed edges, resulting in planes of similar sizes.
  • the monolayer carboxylated graphene oxide obtained from the second oxidation has three key characteristics:
  • the monolayer carboxylated graphene oxide obtained is a nanomaterial suitable for incorporation into materials composed of thermosetting polymeric matrices to obtain materials with improved mechanical properties.

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Abstract

The present invention relates to a method for obtaining single-layer carboxylated graphene oxide, without ordered stacking, which selectively functionalises oxygenated groups by means of a consecutive oxidation-exfoliation-oxidation process. The obtained graphene oxide has a laminar structure functionalised with selectively oxygenated groups, which allows greater chemical interaction and compatibility with polar groups of a polymer matrix and of a fibre. The invention also relates to the single-layer carboxylated graphene oxide obtained using the described method, which can be widely applied in the field of nanocomposites, as it allows compatiblisation with a greater number of polymer matrices.

Description

PROCEDIMIENTO DE OBTENCIÓN DE ÓXIDO DE GRAFENO CARBOXILADO MONOCAPA Y OXIDO DE GRAFENO CARBOXILADO MONOCAPA OBTENIDO
Figure imgf000002_0001
PROCEDURE FOR OBTAINING SINGLE-LAYER CARBOXYLATED GRAPHENE OXIDE AND SINGLE-LAYER CARBOXYLATED GRAPHENE OXIDE OBTAINED
Figure imgf000002_0001
SECTOR DE LA TÉCNICA TECHNICAL SECTOR
La presente invención se refiere a un procedimiento de síntesis de óxido de grafeno con propiedades específicas para su utilización, entre otros usos, como material de refuerzo en materiales compuestos de matrices poliméricas y para la formación de membranas. Concretamente, el óxido de grafeno obtenido es monocapa, de forma que cada una de las capas es funcionalizada con grupos oxigenados selectivos por proceso de oxidación realizado sobre monocapas previamente exfoliadas, y no sobre las capas apiladas, a diferencia de los procesos habituales de síntesis de óxido de grafeno. The present invention relates to a process for the synthesis of graphene oxide with specific properties for its use, among other uses, as a reinforcing material in polymer matrix composite materials and for the formation of membranes. Specifically, the graphene oxide obtained is monolayer, so that each of the layers is functionalized with selective oxygenated groups by oxidation process carried out on previously exfoliated monolayers, and not on the stacked layers, unlike the usual processes of synthesis of graphene. graphene oxide.
El objeto de la invención es posibilitar la obtención de un óxido de grafeno exfoliado con un tamaño y grado de funcionalización que puede ajustarse a las necesidades de la aplicación y la matriz donde se vaya a incorporar, membrana que vaya a formar, o simplemente se una a través de la funcionalización con cualquier otro ente. The object of the invention is to make it possible to obtain an exfoliated graphene oxide with a size and degree of functionalization that can be adjusted to the needs of the application and the matrix where it is to be incorporated, the membrane to be formed, or simply joined. through functionalization with any other entity.
Ventajosamente, el óxido de grafeno obtenido por el procedimiento de la presente invención presenta propiedades superficiales idóneas para conseguir una adecuada dispersión en matrices poliméricas, conjuntamente con un mayor control de las dimensiones y número de capas del óxido de grafeno. Advantageously, the graphene oxide obtained by the process of the present invention has suitable surface properties to achieve adequate dispersion in polymeric matrices, together with greater control of the dimensions and number of graphene oxide layers.
ANTECEDENTES DE LA INVENCIÓN BACKGROUND OF THE INVENTION
El óxido de grafeno se obtiene por exfoliación de un producto intermedio denominado óxido de grafito, el cual procede a partir de un compuesto de intercalación de materiales de estructura grafitica, principalmente grafito con ácidos y oxisales, que es inestable y descompone en óxido de grafito. Así, se forman gran cantidad de grupos funcionales entre los planos apilados grafiticos, añadiéndose agua intercalada entre las capas oxigenadas para producir la separación entre las capas que en el grafito presentan una separación de 0.34 nm y en óxido de grafito presentan una separación de 0.9 nm tal como queda divulgado en la publicación de JEONG, H.K. et al. Evidence of graphitic AB stacking order of graphite oxides, J. Am. Chem. Soc. 130 (2008) 1362-1366. ISSN 0002-7863. Esta separación en las capas del óxido de grafito permite su exfoliación más fácilmente mediante procesos tanto en solución como térmicos, con un alto contenido en monocapas. Graphene oxide is obtained by exfoliation of an intermediate product called graphite oxide, which comes from an intercalation compound of graphitic structure materials, mainly graphite with acids and oxysalts, which is unstable and decomposes into graphite oxide. Thus, a large number of functional groups are formed between the stacked graphitic planes, adding water intercalated between the oxygenated layers to produce the separation between the layers that in graphite have a separation of 0.34 nm and in graphite oxide have a separation of 0.9 nm. as disclosed in the publication by JEONG, HK et al. Evidence of graphitic AB stacking order of graphite oxides, J. Am. Chem. Soc. 130 (2008) 1362-1366. ISSN 0002-7863. This separation in layers of graphite oxide allow its exfoliation more easily through both solution and thermal processes, with a high content of monolayers.
Entre los métodos de síntesis del óxido de grafeno conocidos, el método de Hummers y Offeman es el método de exfoliación química que permite obtener un elevado grado de intercalación y oxidación entre las capas del óxido de grafito preparado, obteniendo un óxido de grafeno con mayor proporción de monocapas, tal como se divulga en la publicación de BOTAS, C. et al. Graphene materials with different structures prepared from the same graphite by the Hummers and Brodie methods. Carbón 65 (2013) 156-164. ISSN 0008-6223. Among the known graphene oxide synthesis methods, the Hummers and Offeman method is the chemical exfoliation method that allows obtaining a high degree of intercalation and oxidation between the layers of the prepared graphite oxide, obtaining a graphene oxide with a higher proportion. of monolayers, as disclosed in the publication of BOTAS, C. et al. Graphene materials with different structures prepared from the same graphite by the Hummers and Brodie methods. Carbon 65 (2013) 156-164. ISSN 0008-6223.
En este sentido, son conocidas modificaciones del método de Hummers y Offeman. Cabe citar la solicitud internacional de patente PCT núm. WO2010042912 que divulga un método para preparar óxido de grafito en la que se utiliza una sal de permanganato potásico en una disolución de nitrato sódico y ácido sulfúrico en agua a temperaturas hasta 100eC. Mediante la citada invención se obtienen láminas de óxido de grafeno mediante exfoliación. Adicionalmente, el óxido de grafito puede reducirse químicamente mediante hidrazina, hidroquinona, plasma, etc. Sin embargo, la citada solicitud de patente no divulga datos sobre el tipo y propiedades del óxido de grafeno sintetizado. In this sense, modifications of the Hummers and Offeman method are known. It is worth citing the PCT international patent application no. WO2010042912 disclosing a method for preparing graphite oxide in which a potassium permanganate salt is used in a solution of sodium nitrate and sulfuric acid in water at temperatures up to 100 e C. By means of the aforementioned invention, sheets of graphene oxide are obtained by exfoliation. Additionally, graphite oxide can be chemically reduced by hydrazine, hydroquinone, plasma, etc. However, the cited patent application does not disclose data on the type and properties of the synthesized graphene oxide.
Adicionalmente, la solicitud internacional de patente PCT núm. WO2011016889 divulga un método de producción de óxido de grafeno altamente oxidado y de gran calidad estructural, con una proporción de anillos y dominios aromáticos superior a la que presenta el óxido de grafeno, donde se añade la presencia de al menos un agente protector. Se basan en el método de Hummers-Offeman empleando como oxidante permanganato potásico, medio de reacción ácido sulfúrico, y como agente protector diferentes ácidos débiles no acuosos (p. ej. ácido fosfórico y sus relacionados, ácido trifluoroacético, ácido bórico), para evitar la formación de defectos en las láminas de grafeno que se producen durante el proceso de oxidación. Additionally, PCT international patent application no. WO2011016889 discloses a method for producing highly oxidized graphene oxide with high structural quality, with a higher proportion of rings and aromatic domains than that of graphene oxide, where the presence of at least one protective agent is added. They are based on the Hummers-Offeman method using potassium permanganate as oxidant, sulfuric acid reaction medium, and different non-aqueous weak acids as protective agent (eg phosphoric acid and its related, trifluoroacetic acid, boric acid), to avoid the formation of defects in graphene sheets that occur during the oxidation process.
También la patente núm. US10336619B2 divulga una modificación del método de Hummers y Offeman por la cual se obtiene óxido de grafeno de una sola capa o con pocas capas, donde tras la reacción de KMn0 y grafito en medio ácido sulfúrico, se realiza el quenching volcando la mezcla en una solución acuosa de peróxido de hidrógeno. La invención no utiliza NaN03 ni H3P04 como agente protector del óxido de grafeno. La patente divulga que el proceso de intercalación en las láminas de grafito se produce completamente únicamente con la presencia de H2S0 y KMn04. El óxido de grafeno obtenido de esta forma presenta alta solubilidad tanto en medio acuoso como en disolventes orgánicos polares. Also patent no. US10336619B2 discloses a modification of the Hummers and Offeman method by which graphene oxide with a single layer or with few layers is obtained, where after the reaction of KMn0 and graphite in a sulfuric acid medium, quenching is carried out by pouring the mixture into a solution aqueous hydrogen peroxide. The invention does not use NaN0 3 or H 3 P0 4 as protective agent for graphene oxide. The patent discloses that the intercalation process in the graphite sheets occurs completely only with the presence of H 2 S0 and KMn0 4 . The graphene oxide obtained in this way has high solubility both in aqueous medium and in polar organic solvents.
Por otro lado, la incorporación del óxido de grafeno a matrices poliméricas es uno de los retos actuales en el campo de los materiales compuestos con el fin de obtener materiales de propiedades mejoradas, como es el caso de la fibra de vidrio o de carbono, las cuales emplean resinas termoestables como matriz. En particular, se debe destacar, que para alcanzar una elevada interacción entre la matriz polimérica, el óxido de grafeno y la fibra es necesario que el óxido de grafeno presente un tamaño y número de planos controlado, así como el selectivo tipo de grupos oxigenados. On the other hand, the incorporation of graphene oxide into polymeric matrices is one of the current challenges in the field of composite materials in order to obtain materials with improved properties, such as glass or carbon fiber, which use thermosetting resins as a matrix. In particular, it should be noted that in order to achieve a high interaction between the polymeric matrix, the graphene oxide and the fiber, it is necessary for the graphene oxide to have a controlled size and number of planes, as well as the selective type of oxygenated groups.
De esta forma, la incorporación de nanomateriales es esencial para obtener materiales con propiedades mejoradas (tales como propiedades mecánicas, conductividad térmica y eléctrica, anticorrosión, etc.) pero cuya limitación reside en la dificultad de obtener dispersiones homogéneas de los nanomateriales en dichas matrices. In this way, the incorporation of nanomaterials is essential to obtain materials with improved properties (such as mechanical properties, thermal and electrical conductivity, anti-corrosion, etc.) but whose limitation lies in the difficulty of obtaining homogeneous dispersions of nanomaterials in said matrices.
Por ejemplo, en la síntesis de resinas, la utilización del óxido de grafeno como segunda carga de refuerzo - además de la fibra - mejora las propiedades mecánicas del material. Sin embargo, la selección de la carga y resina es fundamental para evitar problemas en el procesado, como un curado prematuro de la resina o fallos en el llenado del molde durante el proceso de infusión. For example, in the synthesis of resins, the use of graphene oxide as a second reinforcing filler - in addition to the fiber - improves the mechanical properties of the material. However, the selection of the filler and resin is essential to avoid problems in processing, such as premature curing of the resin or failure to fill the mold during the infusion process.
También se ha estudiado la adición del óxido de grafeno en fibras de carbono, verificando la influencia de los grupos funcionales del óxido de grafeno en la capacidad de refuerzo de materiales compuestos de resina epoxi procesadas mediante infusión a vacío, concluyendo que el óxido de grafeno incrementa la tenacidad de los laminados de resina epoxi con fibra de carbono. The addition of graphene oxide to carbon fibers has also been studied, verifying the influence of the functional groups of graphene oxide on the reinforcing capacity of epoxy resin composite materials processed by vacuum infusion, concluding that graphene oxide increases the toughness of epoxy resin laminates with carbon fiber.
Por otro lado, en los últimos años se han desarrollado membranas basadas en grafeno para el tratamiento de aguas, donde las membranas son producidas por apilamiento capa a capa de láminas de óxido de grafeno funcionalizado, donde la distancia entre los distintos planos o capas del óxido de grafeno es pequeña y dificulta su dispersión efectiva. On the other hand, in recent years graphene-based membranes have been developed for water treatment, where the membranes are produced by stacking layers of functionalized graphene oxide layer by layer, where the distance between the different planes or layers of the oxide of graphene is small and makes it difficult to disperse effectively.
Cabe resaltar que el óxido de grafeno presenta mayoritariamente grupos hidroxilo, éter- epoxi, quinona y lactoles tal como se detalla en la divulgación de MARTIN GULLON, I. et al. New insights into oxygen surface coverage and the resulting two-component structure of graphene oxide. Carbón 158 (2020) 406-417. ISSN 0008-6223. Sin embargo, para muchas de las reacciones de funcionalización es necesaria la presencia de grupos carboxilo y carboxilato, por lo que es necesario anclar previamente los grupos carboxilo a la superficie grafénica. It should be noted that graphene oxide mainly has hydroxyl, ether-epoxy, quinone and lactol groups, as detailed in the disclosure by MARTIN GULLON, I. et al. New insights into oxygen surface coverage and the resulting two-component structure of graphene oxide. Carbon 158 (2020) 406-417. ISSN 0008-6223. However, for many of the functionalization reactions, the presence of carboxyl and carboxylate groups is necessary, so it is necessary to previously anchor the carboxyl groups to the graphene surface.
Es por ello que el solicitante de la presente solicitud de patente detecta la necesidad de desarrollar un procedimiento para la obtención de óxido de grafeno con un bajo número de planos o capas apiladas y con presencia mayoritaria o selectiva de grupos carboxílicos, de forma que el óxido de grafeno obtenido presente unas características que favorezcan su dispersión efectiva en una matriz polimérica, mejorando su compatibilidad y mejor interacción interfacial fibra-resina. That is why the applicant of the present patent application detects the need to develop a procedure for obtaining graphene oxide with a low number of planes or stacked layers and with the majority or selective presence of carboxylic groups, so that the oxide The graphene obtained has characteristics that favor its effective dispersion in a polymeric matrix, improving its compatibility and better fiber-resin interfacial interaction.
DESCRIPCIÓN DE LA INVENCIÓN DESCRIPTION OF THE INVENTION
La presente invención preconiza un procedimiento de obtención de óxido de grafeno carboxilado monocapa cuya presencia mayoritaria de grupos funcionales selectivos tipo carboxilo, carboxilato, lactona y anhídrido permiten un control exhaustivo de la interfase resina-óxido de grafeno mediante una funcionalización interlaminar con grupos oxigenados que posibilita la mejor dispersión de las láminas de óxido de grafeno en la resina. The present invention recommends a procedure for obtaining monolayer carboxylated graphene oxide whose majority presence of selective functional groups such as carboxyl, carboxylate, lactone and anhydride allow an exhaustive control of the resin-graphene oxide interface through an interlaminar functionalization with oxygenated groups that enables the best dispersion of graphene oxide sheets in the resin.
Así, se propone la obtención de óxido de grafeno carboxilado monocapa que presenta una funcionalización selectiva de grupos oxigenados mediante proceso consecutivo de oxidación-exfoliación-oxidación, tal como se detalla seguidamente. Thus, it is proposed to obtain monolayer carboxylated graphene oxide that presents a selective functionalization of oxygenated groups through a consecutive process of oxidation-exfoliation-oxidation, as detailed below.
Concretamente, el procedimiento de obtención del óxido de grafeno carboxilado monocapa está integrado por, al menos, las siguientes etapas: Specifically, the process for obtaining monolayer carboxylated graphene oxide is made up of at least the following stages:
- Oxidación de un material de estructura grafitica cristalina mediante oxoácidos y oxisales que se intercalan entre las capas del material de estructura grafitica a temperaturas de entre 5eC y 20eC, formándose el compuesto de intercalación de grafito, el cual se descompone a temperaturas de entre 25 y 100eC, oxidando las capas grafiticas para la obtención de óxido de grafito, - Exfoliación térmica del óxido de grafito por choque térmico, eliminando parte del oxígeno de los grupos oxigenados y generando la separación en láminas hidrófobas de grafeno exfoliado químicamente en una sola capa, sin apilamiento ordenado, y- Oxidation of a material with a crystalline graphitic structure by means of oxoacids and oxysalts that are intercalated between the layers of the material with a graphitic structure at temperatures between 5 and 20 eC , forming the graphite intercalation compound, which decomposes at temperatures between 25 and 100 e C, oxidizing the graphitic layers to obtain graphite oxide, - Thermal exfoliation of the graphite oxide by thermal shock, eliminating part of the oxygen from the oxygenated groups and generating the separation in hydrophobic sheets of chemically exfoliated graphene in a single layer, without ordered stacking, and
- Oxidación de las láminas hidrófobas de grafeno exfoliado químicamente para la obtención de láminas de óxido de grafeno carboxilado monocapa. - Oxidation of hydrophobic sheets of chemically exfoliated graphene to obtain monolayer carboxylated graphene oxide sheets.
Preferentemente, el material de estructura grafitica cristalina es grafito natural, grafito sintético, nanotubos de carbono, nanofibras de carbono de cinta helicoidal, fibra de carbono y/o negro de carbono. Preferably, the material with the crystalline graphitic structure is natural graphite, synthetic graphite, carbon nanotubes, helical tape carbon nanofibers, carbon fiber and/or carbon black.
Ventajosamente, las láminas de óxido de grafeno carboxilado monocapa formadas están constituidas por una lámina donde los grupos funcionales formados son mayoritariamente selectivos de tipo carboxilo, carboxilato, lactona y anhídrido, los cuales se encuentran localizados en los bordes de las láminas del óxido de grafeno carboxilado monocapa y en los defectos estructurales de su interior, es decir, localizados en defectos como son los agujeros localizados en las láminas. Advantageously, the monolayer carboxylated graphene oxide sheets formed are made up of a sheet where the functional groups formed are mostly selective of the carboxyl, carboxylate, lactone and anhydride type, which are located at the edges of the carboxylated graphene oxide sheets. monolayer and in the structural defects of its interior, that is, located in defects such as the holes located in the sheets.
En la primera etapa se oxida el material precursor - el material de estructura grafitica cristalina - para la obtención del óxido de grafito y en ella se emplea, preferentemente, oxoácidos tales como el ácido sulfúrico o el ácido nítrico, mientras que las oxisales empleadas son, preferentemente, permanganato potásico o clorato potásico. In the first stage, the precursor material is oxidized - the material with a crystalline graphitic structure - to obtain the graphite oxide and in it, oxo acids such as sulfuric acid or nitric acid are preferably used, while the oxysalts used are, preferably potassium permanganate or potassium chlorate.
De esta forma, cuando la oxidación del material precursor se realiza mediante el método de Brodie, la oxidación se realiza usando clorato potásico en medio ácido nítrico fumante. Thus, when the oxidation of the precursor material is carried out by the Brodie method, the oxidation is carried out using potassium chlorate in a fuming nitric acid medium.
Si la oxidación del material precursor se realiza mediante el método de Staudenmeier, la oxidación del material precursor se realiza usando clorato potásico en medio ácido nítrico y sulfúrico. If the oxidation of the precursor material is carried out by the Staudenmeier method, the oxidation of the precursor material is carried out using potassium chlorate in a nitric and sulfuric acid medium.
Mientras que, si la oxidación del material precursor se realiza mediante el método de Hummers y Offeman, se utiliza permanganato potásico en medio ácido sulfúrico, con presencia o no de nitrato sódico. Whereas, if the oxidation of the precursor material is carried out using the Hummers and Offeman method, potassium permanganate is used in a sulfuric acid medium, with or without the presence of sodium nitrate.
Preferentemente, la oxidación del material de estructura grafitica cristalina se realiza empleando el método de Hummers y Offeman, estando formado por, al menos, las siguientes etapas: Preferably, the oxidation of the material with a crystalline graphitic structure is carried out using the Hummers and Offeman method, being formed by, at least, the following stages:
- Se suspende el material de estructura grafitica cristalina (precursor) en un medio ácido sulfúrico o en un medio ácido sulfúrico-nitrato sódico con proporción (masa del precursor:volumen) de entre 1 :20 y 1 :150 (g grafito:ml de H2S0 ), realizando una agitación mecánica durante, al menos, 15 minutos, a temperatura ambiente. - The material with a crystalline graphitic structure (precursor) is suspended in a sulfuric acid medium or in a sulfuric acid-sodium nitrate medium with a ratio (precursor mass:volume) of between 1:20 and 1:150 (g graphite:ml of H 2 S0 ), carrying out mechanical stirring for at least 15 minutes at room temperature.
- Se genera un compuesto de intercalación grafito:ácido sulfúrico, separando las capas de grafito. - A graphite:sulfuric acid intercalation compound is generated, separating the graphite layers.
- Adición de permanganato potásico en una proporción másica KMn04:precursor de entre 7:1 y 1:1 a una temperatura inferior a 25eC. - Addition of potassium permanganate in a KMn0 4 :precursor mass ratio of between 7:1 and 1:1 at a temperature below 25°C.
- Agitación manteniendo una temperatura de entre 25eC y 70eC durante, al menos, 15 minutos, donde se favorece la penetración del permanganato entre las capas. - Agitation maintaining a temperature of between 25 and 70 eC for at least 15 minutes, where the penetration of the permanganate between the layers is favored.
- Se genera una mezcla de reacción de óxido de grafito preliminar, que presenta grupos funcionales sobre los defectos estructurales de la estructura grafitica (planos básales y bordes) al descomponer el compuesto de intercalación formado. - A preliminary graphite oxide reaction mixture is generated, which presents functional groups on the structural defects of the graphitic structure (basal planes and edges) by decomposing the intercalation compound formed.
- Vertido de la mezcla de reacción sobre agua en proporción volumétrica ácido sulfúrico:agua de entre 1 :1 y 1 :10, generando calor y una reacción en cadena de oxidación-hidrolización provocada por la presencia del permanganato potásico, generando Mn02 (insoluble), multiplicando la presencia de grupos funcionales oxigenados por esa reacción en cadena que llega a cortar las capas grafiticas, reduciendo su tamaño. Estos grupos funcionales formados son, mayoritariamente, hidroxilo, lactol, etér-epoxi, quinonas, y en menor medida, carboxilos. La velocidad e intensidad de la reacción en esta etapa es controlada actuando sobre la temperatura, mediante la extracción de parte del calor generado, con el fin de mantenerse preferentemente por debajo de 60eC. - Pouring the reaction mixture over water in a sulfuric acid:water volumetric ratio of between 1:1 and 1:10, generating heat and an oxidation-hydrolyzation chain reaction caused by the presence of potassium permanganate, generating Mn0 2 (insoluble ), multiplying the presence of oxygenated functional groups by this chain reaction that cuts the graphitic layers, reducing their size. These functional groups formed are mainly hydroxyl, lactol, ether-epoxy, quinones, and to a lesser extent, carboxyl. The speed and intensity of the reaction in this stage is controlled by acting on the temperature, by extracting part of the heat generated, in order to keep it preferably below 60 ° C.
- Adición, aplicando agitación, de una solución acuosa de peróxido de hidrógeno, con proporción volumétrica de peróxido de hidrógeno y la mezcla anterior de entre 1 :20 y 1 :50, convirtiendo todas las especies de manganeso presentes, el Mn02 formado (insoluble) y el permanganato remanente a manganeso (II) (soluble). - Addition, applying stirring, of an aqueous solution of hydrogen peroxide, with a volumetric proportion of hydrogen peroxide and the previous mixture between 1:20 and 1:50, converting all the manganese species present, the Mn0 2 formed (insoluble ) and the remaining permanganate to manganese (II) (soluble).
- Separación de la fase líquida y la fase sólida por procesos de centrifugación o filtración, descartando la fase líquida clara. - Separation of the liquid phase and the solid phase by centrifugation or filtration processes, discarding the clear liquid phase.
- Lavado del sólido con agua, preferentemente a pH superior a 2. - Washing of the solid with water, preferably at a pH greater than 2.
- Separación. - Separation.
- Lavado con agua. - Wash with water.
- Secado de la fase sólida húmeda por convección a temperaturas entre 50eC y 95eC para la obtención del óxido de grafito en estado polvo y seco. - Drying of the wet solid phase by convection at temperatures between 50 e C and 95 e C to obtain graphite oxide in powder and dry state.
Tal como se mencionaba anteriormente, en la primera etapa de la oxidación del material de estructura grafitica cristalina, éste se suspende en un medio ácido sulfúrico o en un medio ácido sulfúrico-nitrato sódico. En este sentido, cuando la suspensión del material precursor se realice sobre el medio ácido sulfúrico - nitrato sódico será necesario preparar una suspensión de nitrato sódico en ácido sulfúrico en una proporción másica preferentemente entre 1 :1 y 1 :2 (NaN0 :precursor). As mentioned above, in the first stage of the oxidation of the material with a crystalline graphitic structure, it is suspended in a sulfuric acid medium or in a sulfuric acid-sodium nitrate medium. In this sense, when the suspension of the precursor material is carried out on the sulfuric acid - sodium nitrate medium, it will be necessary to prepare a suspension of sodium nitrate in sulfuric acid in a mass ratio preferably between 1:1 and 1:2 (NaN0:precursor).
Por otro lado, la segunda etapa del procedimiento de obtención de óxido de grafeno carboxilado monocapa se refiere a la exfoliación térmica del óxido de grafito obtenida en la primera oxidación, estando la exfoliación térmica formada por, al menos, las siguientes etapas: On the other hand, the second stage of the procedure for obtaining monolayer carboxylated graphene oxide refers to the thermal exfoliation of the graphite oxide obtained in the first oxidation, the thermal exfoliation being formed by, at least, the following stages:
- Aumento de temperatura entre 90eC y 250eC, produciendo la salida violenta del agua intercalada entre las capas, tanto fisisorbida como quimisorbida, exfoliando el material. - Temperature increase between 90 e C and 250 e C, producing the violent exit of the water intercalated between the layers, both physisorbed and chemisorbed, exfoliating the material.
- Arrastre de grupos funcionales existentes en el óxido de grafito, junto con la salida de agua quimisorbida alcanzándose presiones que exceden las fuerzas de Van der Waals que unen las láminas del grafito. Los grupos funcionales arrastrados/eliminados son hidroxilo, éter, epoxi, lactol, anhídrido, quinonas y carboxilos. - Drag of existing functional groups in the graphite oxide, together with the exit of chemisorbed water, reaching pressures that exceed the Van der Waals forces that join the graphite sheets. The functional groups removed/removed are hydroxyl, ether, epoxy, lactol, anhydride, quinones and carboxyls.
- Obtención de láminas hidrófobas de óxido de grafeno reducido (también denominado grafeno exfoliado químicamente) que están separadas y no presentan apilamiento grafitico ordenado. - Obtaining hydrophobic sheets of reduced graphene oxide (also called chemically exfoliated graphene) that are separated and do not present ordered graphitic stacking.
De esta forma, los grupos funcionales arrastrados y eliminados en la etapa de exfoliación térmica del óxido de grafito son los que se formaron en los defectos generados, y posterior propagación, durante la oxidación de la estructura grafitica cristalina. In this way, the functional groups dragged and eliminated in the thermal exfoliation stage of the graphite oxide are those that were formed in the defects generated, and subsequent propagation, during the oxidation of the crystalline graphitic structure.
Opcionalmente, y de forma preferente, el aumento de temperatura en la etapa de exfoliación térmica se genera por irradiación por microondas, dado que es un proceso de calentamiento mucho más eficiente sobre los materiales ricos en carbono y agua que el calentamiento convectivo por gradiente de temperatura. Finalmente, la siguiente y última etapa es una segunda oxidación de las láminas hidrófobas de grafeno exfoliado químicamente, la cual está formada por las siguientes etapas: Optionally, and preferably, the temperature increase in the thermal exfoliation stage is generated by microwave irradiation, since it is a much more efficient heating process on materials rich in carbon and water than convective heating by temperature gradient. . Finally, the next and last stage is a second oxidation of the hydrophobic sheets of chemically exfoliated graphene, which is formed by the following stages:
- Se suspenden las láminas hidrófobas de grafeno exfoliado químicamente en un medio ácido sulfúrico o en un medio ácido sulfúrico-nitrato sódico con proporción masa de láminas hidrófobas:volumen de entre 1 :20 y 1 :150 (g de lámina hidrófoba: mi de H2S0 ), realizando una agitación mecánica durante, al menos, 15 minutos, a temperatura ambiente. En este caso no se forma compuesto de intercalación ya que las láminas no tienen apilamiento cristalino, por lo que la solvatación de éstas por el ácido es menor. - The hydrophobic sheets of chemically exfoliated graphene are suspended in a sulfuric acid medium or in a sulfuric acid-sodium nitrate medium with a mass ratio of hydrophobic sheets:volume between 1:20 and 1:150 (g of hydrophobic sheet: ml of H 2 S0 ), carrying out mechanical stirring for at least 15 minutes at room temperature. In this case, no intercalation compound is formed since the sheets do not have crystalline stacking, so their solvation by the acid is less.
- Adición de permanganato potásico en una proporción másica KMn0 :láminas hidrófobas de entre 7:1 y 1 :1 a una temperatura inferior a 25eC. - Addition of potassium permanganate in a mass ratio of KMn0:hydrophobic sheets between 7:1 and 1:1 at a temperature below 25°C.
- Agitación manteniendo una temperatura de entre 25eC y 70eC durante, al menos, 15 minutos. - Stirring maintaining a temperature between 25 and 70 eC for at least 15 minutes.
- Preoxidación de los defectos presentes en las láminas hidrófobas por la generación de una estructura intermedia entre las láminas hidrófobas. En este caso, al no haberse formado compuesto de intercalación, la preoxidación es más débil (formando grupos hidroxilos) y localizada en los defectos mayores, principalmente bordes. - Pre-oxidation of the defects present in the hydrophobic sheets by the generation of an intermediate structure between the hydrophobic sheets. In this case, since no intercalation compound has been formed, the pre-oxidation is weaker (forming hydroxyl groups) and localized in the larger defects, mainly edges.
- Vertido de la mezcla de reacción sobre agua en proporción volumétrica ácido sulfúrico:agua de entre 1 :1 y 1 :10, generando un calor de dilución que controla que la temperatura sea menor de 100eC. Opcionalmente, el calor de dilución producido al verter la mezcla de reacción sobre agua es extraída para mantener la mezcla por debajo de 60eC. - Pouring of the reaction mixture over water in a sulfuric acid:water volume ratio of between 1:1 and 1:10, generating a heat of dilution that controls the temperature to be less than 100 e C. Optionally, the heat of dilution produced by pouring the reaction mixture over water it is extracted to keep the mixture below 60 e C.
- Reacción de oxidación-hidrolización generada por la presencia del permanganato potásico combinada con el calor dilución, donde el permanganato potásico (transformándose a Mn02 insoluble) completa la oxidación de los defectos de las láminas hidrófobas de grafeno exfoliado químicamente a preferentemente grupos mayoritariamente carboxílicos, carboxilato y anhídrido, en los bordes de éstas, sin que en este caso haya reacción de propagación en cadena que corte las láminas, por no haber apilamiento de éstas y haber menor solvatación del ácido. - Oxidation-hydrolyzation reaction generated by the presence of potassium permanganate combined with heat dilution, where potassium permanganate (transforming into insoluble Mn0 2 ) completes the oxidation of defects in the hydrophobic sheets of chemically exfoliated graphene to preferably mostly carboxylic groups, carboxylate and anhydride, at the edges of these, without in this case there being a chain propagation reaction that cuts the sheets, because there is no stacking of these and there is less solvation of the acid.
- Adición, aplicando agitación, de una solución acuosa de peróxido de hidrógeno, con proporción volumétrica de peróxido de hidrógeno:mezcla de entre 1 :20 y 1 :50, produciendo la reducción de las especies de manganeso presentes (manganeso (IV) del Mn02 (insoluble) y manganeso (Vil) del permanganato) a manganeso (II) (soluble), pasando a la disolución acuosa. - Addition, applying agitation, of an aqueous solution of hydrogen peroxide, with a volumetric ratio of hydrogen peroxide: mixture between 1:20 and 1:50, producing the reduction of the manganese species present (manganese (IV) of Mn0 2 (insoluble) and manganese (Vil) permanganate) to manganese (II) (soluble), passing into aqueous solution.
- Separación de la fase líquida y la fase sólida por procesos de centrifugación o filtración, descartando la fase líquida clara. - Separation of the liquid phase and the solid phase by centrifugation or filtration processes, discarding the clear liquid phase.
- Lavado del sólido con agua, preferentemente a pH superior a 2. - Washing of the solid with water, preferably at a pH greater than 2.
- Separación. - Separation.
- Lavado con agua. - Wash with water.
- Secado de la fase sólida húmeda por convección a temperaturas entre 50 y 95eC para la obtención del óxido de grafito en estado polvo y seco funcionalizado con grupos carboxílicos, carboxilatos y anhídridos. - Drying of the wet solid phase by convection at temperatures between 50 and 95 C to obtain graphite oxide in powder and dry state, functionalized with carboxylic, carboxylate and anhydride groups.
Tal como se mencionaba anteriormente, en la primera etapa de la oxidación de las láminas hidrófobas de grafeno exfoliado químicamente, éstas se suspenden en un medio ácido sulfúrico o en un medio ácido sulfúrico-nitrato sódico. En este sentido, cuando la suspensión de las láminas hidrófobas de grafeno exfoliado químicamente se realice sobre el medio ácido sulfúrico - nitrato sódico será necesario preparar una suspensión de nitrato sódico en ácido sulfúrico en una proporción másica preferentemente entre 1 :1 y 1 :2. As mentioned above, in the first stage of the oxidation of the hydrophobic sheets of chemically exfoliated graphene, they are suspended in a sulfuric acid medium or in a sulfuric acid-sodium nitrate medium. In this sense, when the suspension of chemically exfoliated graphene hydrophobic sheets is carried out on the sulfuric acid - sodium nitrate medium, it will be necessary to prepare a suspension of sodium nitrate in sulfuric acid in a mass ratio preferably between 1:1 and 1:2.
El procedimiento que se preconiza posibilita la obtención de un óxido de grafeno carboxilado monocapa que ofrece las siguientes ventajas: The procedure that is recommended makes it possible to obtain a monolayer carboxylated graphene oxide that offers the following advantages:
- El óxido de grafeno obtenido tiene un tamaño de plano controlado, uniformizado e inferior al obtenido con el proceso convencional de Hummers y Offeman. Ventajosamente, las etapas de exfoliación y posterior oxidación permiten obtener un óxido de grafeno con una elevada tasa de monocapas sin apilamiento ordenado. - The graphene oxide obtained has a controlled plane size, standardized and smaller than that obtained with the conventional process of Hummers and Offeman. Advantageously, the exfoliation and subsequent oxidation steps make it possible to obtain a graphene oxide with a high rate of monolayers without ordered stacking.
- El óxido de grafeno obtenido presenta una estructura laminar funcionalizada con grupos oxigenados de forma selectiva (carboxilatos, carboxílicos, alcoxi, etc.), que permite una mayor interacción química y compatibilidad con los grupos polares de una matriz polimérica y la fibra. Ventajosamente, los grupos oxigenados presentes en las láminas del óxido de grafeno sintetizado presentan a su vez la posibilidad de funcionalización superficial, ofreciendo una elevada aplicación en el campo de los nanocompuestos al permitir una compatibilización con un mayor número de matrices poliméricas e incluso aportar propiedades especiales. Por todo lo anterior, las características del óxido de grafeno carboxilado monocapa obtenido permiten su utilización como material de refuerzo en materiales compuestos de matrices poliméricas termoestables (resinas epoxi, poliéster, vinil éster) reforzadas con fibra (por ejemplo, fibra de vidrio, .de carbono, de aramida) con objeto de obtener materiales con propiedades mecánicas mejoradas, donde la fase matriz es dominante. - The graphene oxide obtained has a functionalized lamellar structure with selectively oxygenated groups (carboxylates, carboxylic, alkoxy, etc.), which allows greater chemical interaction and compatibility with the polar groups of a polymeric matrix and the fiber. Advantageously, the oxygenated groups present in the sheets of synthesized graphene oxide also have the possibility of surface functionalization, offering a high application in the field of nanocomposites by allowing compatibility with a greater number of polymeric matrices and even providing special properties. . For all of the above, the characteristics of the monolayer carboxylated graphene oxide obtained allow its use as a reinforcing material in materials composed of thermosetting polymeric matrices (epoxy, polyester, vinyl ester resins) reinforced with fiber (for example, fiberglass, . carbon, aramid) in order to obtain materials with improved mechanical properties, where the matrix phase is dominant.
Concretamente, en la fabricación por infusión a vacío de materiales compuestos con resinas - donde la interfase entre la fibra y la matriz es débil - la adición de láminas de óxido de grafeno carboxilado monocapa de la presente invención permiten un control exhaustivo de la interfase resina-óxido. De esta manera, el óxido de grafeno carboxilado monocapa presenta una selectividad de grupos funcionales oxigenados y, en consecuencia, su presencia en la mezcla con fibras y/o resinas favorece la dispersión de las láminas de óxido de grafeno en la resina y/o fibra y un incremento de las propiedades mecánicas de los materiales compuestos obtenidos. Specifically, in the manufacture by vacuum infusion of composite materials with resins - where the interface between the fiber and the matrix is weak - the addition of monolayer carboxylated graphene oxide sheets of the present invention allow an exhaustive control of the resin- oxide. In this way, the monolayer carboxylated graphene oxide presents a selectivity of oxygenated functional groups and, consequently, its presence in the mixture with fibers and/or resins favors the dispersion of the graphene oxide sheets in the resin and/or fiber. and an increase in the mechanical properties of the composite materials obtained.
Igualmente, la incorporación del óxido de grafeno carboxilado monocapa de la presente invención en el sector de los termoplásticos permite la obtención de plásticos y polímeros reforzados mecánicamente y con propiedades antibacterianas, una disminución de la permeabilidad por efecto barrera, etc, con aplicaciones innovadoras en el sector del embalaje o la impresión 3D. Así, el óxido de grafeno carboxilado monocapa permite procesos de funcionalización posterior para su utilización en otras matrices (por ejemplo, grupos amina, silanos, etc) y aplicaciones en la formación de membranas basadas en grafeno para el tratamiento de aguas, en microfiltración, ultrafiltración, nanofiltración y desalinización. Likewise, the incorporation of the monolayer carboxylated graphene oxide of the present invention in the thermoplastics sector allows obtaining mechanically reinforced plastics and polymers with antibacterial properties, a decrease in permeability due to a barrier effect, etc., with innovative applications in the packaging sector or 3D printing. Thus, monolayer carboxylated graphene oxide allows subsequent functionalization processes for use in other matrices (for example, amine groups, silanes, etc.) and applications in the formation of graphene-based membranes for water treatment, in microfiltration, ultrafiltration , nanofiltration and desalination.
BREVE DESCRIPCIÓN DE LOS DIBUJOS BRIEF DESCRIPTION OF THE DRAWINGS
Para complementar la descripción que seguidamente se va a realizar y con objeto de ayudar a una mejor comprensión de las características del invento, de acuerdo con un ejemplo preferente de realización práctica del mismo, se acompaña como parte integrante de dicha descripción, un juego de figuras en donde con carácter ilustrativo y no limitativo, se ha representado lo siguiente: To complement the description that will be made below and in order to help a better understanding of the characteristics of the invention, according to a preferred example of its practical embodiment, a set of figures is attached as an integral part of said description. where, by way of illustration and not limitation, the following has been represented:
Figura 1.- Muestra una representación de una espectrometría fotoelectrónica de Rayos X del material de estructura grafitica cristalina a oxidar. Figura 2.- Muestra una representación de una espectrometría fotoelectrónica de Rayos X del óxido de grafito obtenido tras la primera oxidación. Figure 1.- Shows a representation of an X-ray photoelectronic spectrometry of the material with a crystalline graphitic structure to be oxidized. Figure 2.- Shows a representation of an X-ray photoelectronic spectrometry of the graphite oxide obtained after the first oxidation.
Figura 3.- Muestra una representación de una espectrometría fotoelectrónica de Rayos X del óxido de grafeno exfoliado. Figure 3.- Shows a representation of an X-ray photoelectronic spectrometry of exfoliated graphene oxide.
Figura 4.- Muestra una representación de una espectrometría fotoelectrónica de Rayos X del óxido de grafeno carboxilado monocapa obtenido tras la segunda oxidación. Figure 4.- Shows a representation of an X-ray photoelectronic spectrometry of the monolayer carboxylated graphene oxide obtained after the second oxidation.
Figura 5.- Muestra una representación del difractograma de Rayos X del óxido de grafito, el óxido de grafeno exfoliado y el óxido de grafeno carboxilado monocapa. Figure 5.- Shows a representation of the X-ray diffractogram of graphite oxide, exfoliated graphene oxide and monolayer carboxylated graphene oxide.
Figura 6. Muestra una imagen de microscopía electrónica de transmisión de diferentes láminas del óxido de grafeno carboxilado monocapa, donde se observa que tienen un tamaño muy similar. Figure 6. Shows a transmission electron microscopy image of different sheets of monolayer carboxylated graphene oxide, where it is observed that they have a very similar size.
REALIZACIÓN PREFERENTE DE LA INVENCIÓN PREFERRED EMBODIMENT OF THE INVENTION
En la realización preferente del procedimiento de obtención del óxido de grafeno carboxilado monocapa de la presente invención, la etapa de oxidación del material de estructura grafitica cristalina se realiza mediante el método de Flummers y Offeman por ser el más eficaz en la intercalación de ácido sulfúrico anhidro entre todas y cada una de las capas, y posteriormente oxidar el interior por la acción del permanganato. In the preferred embodiment of the procedure for obtaining the monolayer carboxylated graphene oxide of the present invention, the oxidation step of the crystalline graphitic structure material is carried out using the Flummers and Offeman method, as it is the most effective in the intercalation of anhydrous sulfuric acid. between each and every one of the layers, and subsequently oxidize the interior by the action of the permanganate.
Ventajosamente, la presencia de nitrato sódico durante la primera y segunda oxidación ayuda a que penetre el ácido sulfúrico y el permanganato potásico en las galerías o defectos estructurales tanto del material de estructura grafitica cristalina como en las galerías o defectos estructurales del óxido de grafeno. Advantageously, the presence of sodium nitrate during the first and second oxidation helps the sulfuric acid and potassium permanganate to penetrate into the galleries or structural defects both in the material with a crystalline graphitic structure and in the galleries or structural defects of the graphene oxide.
Preferentemente, el material de estructura cristalina empleado como precursor es el grafito natural. Preferably, the material with a crystalline structure used as a precursor is natural graphite.
De esta forma, en una realización preferente de la invención, la oxidación del material precursor se realiza mediante el método de Flummers y Offeman por ser el más eficaz en la intercalación de ácido sulfúrico anhidro entre todas y cada una de las capas, y posteriormente oxidar el interior de las capas por la acción del permanganato. La presencia del nitrato sódico fomenta la penetración del ácido sulfúrico y el permanganato sódico en las galerías o defectos presentes en las capas de grafiticas. In this way, in a preferred embodiment of the invention, the oxidation of the precursor material is carried out using the Flummers and Offeman method as it is the most efficient in the intercalation of anhydrous sulfuric acid between each and every one of the layers, and subsequently oxidizing the interior of the layers by the action of permanganate. The presence of sodium nitrate encourages the penetration of sulfuric acid and sodium permanganate in the galleries or defects present in the graphite layers.
Preferentemente, se suspende grafito natural y nitrato sódico, en proporción másica 1 :1 , en ácido sulfúrico (preferentemente al 98%) con proporción masa precursor:volumen ácido sulfúrico de 1 :50 (g grafito:ml de H2S0 ), de 70 mi de ácido sulfúrico y se mezcla con agitación mecánica durante varias horas, a temperatura ambiente. En este proceso se genera un compuesto de intercalación grafito:ácido sulfúrico, separando las capas de grafito. Preferably, natural graphite and sodium nitrate are suspended, in a 1:1 mass ratio, in sulfuric acid (preferably 98%) with a precursor mass ratio: sulfuric acid volume of 1:50 (g graphite: ml of H 2 S0 ), in such a way that 70 ml of sulfuric acid and mixed with mechanical stirring for several hours at room temperature. In this process, a graphite:sulfuric acid intercalation compound is generated, separating the graphite layers.
Se añade lentamente la cantidad correspondiente de permanganato potásico (proporción másica KMn04:precursor de 4:1) controlando la temperatura por debajo de 25eC. A continuación, se mantiene la agitación a temperatura de 35-55eC, durante una o dos horas. The corresponding quantity of potassium permanganate (mass ratio KMn0 4 :precursor of 4:1) is slowly added , controlling the temperature below 25°C. Stirring is then maintained at a temperature of 35-55°C , for one or two hours.
En esta etapa se genera el óxido de grafito preliminar, al actuar el permanganato potásico en las galerías, con el ácido sulfúrico, generando grupos funcionales sobre los defectos estructurales de la estructura grafitica. Transcurrido el tiempo de reacción, se vierte la mezcla de reacción sobre agua ( quenching ) en proporción volumétrica relativa ácido sulfúrico:agua de 1 :1.8. Este proceso puede producir hasta 100eC, por lo que es recomendable extraer calor y mantener la mezcla a 60eC. In this stage, the preliminary graphite oxide is generated, when the potassium permanganate acts in the galleries, with the sulfuric acid, generating functional groups on the structural defects of the graphitic structure. After the reaction time, the reaction mixture is poured onto water (quenching) in a relative volumetric ratio of sulfuric acid:water of 1:1.8. This process can produce up to 100 e C, so it is advisable to extract heat and keep the mixture at 60 e C.
En este proceso, la presencia del agua junto con el calor de dilución, desencadenan la acción del permanganato potásico en las galerías grafiticas mediante la reacción en cadena de oxidación-hidrolización generando también Mn02 (insoluble), siendo una reacción muy rápida y cortando las láminas grafiticas. In this process, the presence of water together with the heat of dilution, trigger the action of potassium permanganate in the graphitic galleries through the chain reaction of oxidation-hydrolyzation, also generating Mn0 2 (insoluble), being a very fast reaction and cutting the graphite sheets.
A continuación, se añade con agitación una solución acuosa de peróxido de hidrógeno (preferentemente 30% v/v), con proporción volumétrica de peróxido de hidrógeno:mezcla de quenching de 1 :30. De esta forma, se reduce el manganeso a manganeso (II), se detiene la acción del manganeso (Vil) sin reaccionar y se solubiliza el manganeso (IV) formado. Next, an aqueous solution of hydrogen peroxide (preferably 30% v/v) is added with stirring, with a volumetric proportion of hydrogen peroxide: quenching mixture of 1:30. In this way, manganese is reduced to manganese (II), the action of unreacted manganese (Vil) is stopped and the manganese (IV) formed is dissolved.
Finalmente, se separa la fase líquida de la fase sólida por procesos de centrifugación o filtración, descartando la fase líquida clara. Se lava el sólido con agua a pH 2 y se vuelve a separar, y finalmente se lava con agua. A continuación, la fase sólida húmeda se seca por convección a temperaturas entre 60 y 70eC. El producto así obtenido se corresponde con el óxido de grafito, que es un material en polvo y seco. Finally, the liquid phase is separated from the solid phase by centrifugation or filtration processes, discarding the clear liquid phase. The solid is washed with water at pH 2 and separated again, and finally washed with water. The wet solid phase is then dried by convection at temperatures between 60 and 70 C. The product thus obtained corresponds to graphite oxide, which is a powdery and dry material.
El óxido de grafito obtenido se somete a un tratamiento de exfoliación térmica. Este proceso tiene lugar a una temperatura entre 140e y 200eC. El incremento de temperatura de la partícula produce la salida violenta de la gran cantidad de agua intercalada entre las capas, tanto fisisorbida como quimisorbida, exfoliando el material. The graphite oxide obtained is subjected to a thermal exfoliation treatment. This process takes place at a temperature between 140 e and 200 e C. The increase in temperature of the particle produces the violent exit of the large amount of water intercalated between the layers, both physisorbed and chemisorbed, exfoliating the material.
Simultáneamente, esta salida de agua quimisorbida arrastra grupos funcionales existentes en el óxido de grafito, alcanzándose presiones que exceden las fuerzas de Van der Waals que unen las láminas del grafito. Los grupos funcionales arrastrados son hidroxilo, éter, lactol, y quinonas y, en menor medida, carboxilos, los cuales están preferentemente situados en la gran cantidad de defectos producidos durante la generación del óxido de grafito saliendo como CO, C02 y H20 en el espacio entre las láminas. Esto da lugar a la exfoliación del óxido de grafeno y a su reducción, ya que pierde la mayoría del contenido en oxígeno. Simultaneously, this exit of chemisorbed water drags functional groups existing in the graphite oxide, reaching pressures that exceed the Van der Waals forces that join the graphite sheets. The dragged functional groups are hydroxyl, ether, lactol, and quinones and, to a lesser extent, carboxyls, which are preferentially located in the large number of defects produced during the generation of graphite oxide, leaving as CO, C0 2 and H 2 0 in the space between the sheets. This results in exfoliation of the graphene oxide and its reduction as it loses most of the oxygen content.
Las láminas de óxido de grafeno reducido están separadas y no presentan apilamiento grafitico. Igualmente, las láminas de óxido de grafeno presentan un tamaño inferior al del grafito de partida, dado que se cortaron en gran medida los planos del grafito original con la generación de los grupos funcionales y posterior rotura de esos enlaces en la exfoliación térmica. Por tanto, el resultado de la exfoliación térmica del óxido de grafito por choque térmico es una pluralidad de láminas de grafeno exfoliado químicamente, sin apilamiento ordenado e hidrófobas, al ser eliminada una parte del oxígeno por la descomposición térmica de los grupos oxigenados. Se generan láminas hidrófobas de un tamaño similar y más reducido que el del óxido de grafito de partida. The reduced graphene oxide sheets are separated and do not show graphitic stacking. Similarly, the graphene oxide sheets are smaller than the starting graphite, since the planes of the original graphite were cut to a large extent with the generation of the functional groups and subsequent breakage of these bonds in the thermal exfoliation. Therefore, the result of the thermal exfoliation of graphite oxide by thermal shock is a plurality of chemically exfoliated graphene sheets, without ordered stacking and hydrophobic, as part of the oxygen is eliminated by the thermal decomposition of the oxygenated groups. Hydrophobic sheets of a similar size and smaller than that of the starting graphite oxide are generated.
A partir de las láminas de óxido de grafeno exfoliado químicamente se realiza una segunda oxidación por el mismo método químico de Hummers-Offeman modificado descrito anteriormente. Se suspende 1g del óxido de grafeno exfoliado en 70 mi de H2S0 junto con 1 g de NaN0 y se agita en una placa calefactora magnética durante tres horas a temperatura ambiente. Se añade el permanganato potásico en proporción 3:1 y se agita dos horas a 35eC. A continuación, se calienta la mezcla a 55eC y se deja estabilizar. Se deja enfriar y se vierte sobre unos 160 mi de agua y hielo, se agita y se añaden 8 mi de peróxido de hidrógeno. La disolución se filtra y el sólido se lava con agua durante 30 min, se filtra de nuevo y el producto se seca en la estufa a 65eC. Finalmente, el producto así obtenido es óxido de grafeno carboxilado monocapa que presenta una funcionalización selectiva con grupos carboxílico, carboxilato, sin apenas cambiar el tamaño de las láminas en esta última etapa, pero con carácter hidrofílico, resultando ser un polvo negro muy fino y seco. From the chemically exfoliated graphene oxide sheets, a second oxidation is carried out by the same modified Hummers-Offeman chemical method described above. 1 g of the exfoliated graphene oxide is suspended in 70 ml of H 2 S0 together with 1 g of NaN0 and stirred on a magnetic hot plate for three hours at room temperature. Potassium permanganate is added in a 3:1 ratio and stirred for two hours at 35°C. The mixture is then heated to 55°C and allowed to stabilize. Allow to cool and pour over about 160 ml of water and ice, stir and add 8 ml of hydrogen peroxide. The solution is filtered and the solid is washed with water for 30 min, filtered from new and the product is dried in the oven at 65 e C. Finally, the product thus obtained is monolayer carboxylated graphene oxide that presents a selective functionalization with carboxylic, carboxylate groups, with hardly any change in the size of the sheets in this last stage, but with a hydrophilic character, resulting in a very fine and dry black powder.
Se adjuntan las figuras 1 , 2, 3 y 4 que corresponden, respectivamente, a las representaciones de las Espectrometrías Fotoelectrónicas de Rayos X para el material de estructura grafitica cristalina a oxidar, el óxido de grafito, el grafeno exfoliado químicamente y el óxido de grafeno carboxilado monocapa obtenido, representándose en el eje de abscisas la energía de enlace (eV) y en el eje de ordenadas la intensidad. Se indica a continuación cada letra de las representadas en las figuras 1 , 2, 3 y 4 a qué corresponden: Figures 1, 2, 3 and 4 are attached, which correspond, respectively, to the representations of the X-Ray Photoelectronic Spectrometry for the material with a crystalline graphitic structure to be oxidized, graphite oxide, chemically exfoliated graphene and graphene oxide. carboxylated monolayer obtained, representing the binding energy (eV) on the abscissa axis and the intensity on the ordinate axis. Each letter of those represented in figures 1, 2, 3 and 4 is indicated below to which they correspond:
A corresponde a los datos B corresponde al ajuste global C corresponde a deconvolución por sp2C = sp2C D corresponde a deconvolución sp3C - OH/ sp3C - sp3C E corresponde a sp3C - O - sp3C F corresponde a O - sp2C = O A corresponds to the data B corresponds to the global fit C corresponds to deconvolution by sp 2 C = sp 2 CD corresponds to deconvolution sp 3 C - OH/ sp 3 C - sp 3 CE corresponds to sp 3 C - O - sp 3 CF corresponds to aO - sp 2 C = O
El óxido de grafito obtenido en la primera etapa (figura 2) presenta un mayor porcentaje de oxígeno que el óxido de grafeno obtenido después de la segunda etapa (figura 4). En este sentido, la relación atómica C/O es más alta en el óxido de grafeno carboxilado monocapa (figura 4) obtenido tras la segunda oxidación que en el óxido de grafito convencional (figura 2) obtenido en la primera oxidación. The graphite oxide obtained in the first stage (figure 2) has a higher percentage of oxygen than the graphene oxide obtained after the second stage (figure 4). In this sense, the atomic C/O ratio is higher in the monolayer carboxylated graphene oxide (figure 4) obtained after the second oxidation than in the conventional graphite oxide (figure 2) obtained in the first oxidation.
En la siguiente tabla se observa la evolución de la relación carbono-oxígeno (C/O) para cada uno de los compuestos que intervienen en las etapas del procedimiento objeto de la presente invención, medido mediante Espectroscopia Fotoelectrónica de Rayos X.
Figure imgf000015_0001
Sin embargo, a pesar de que el óxido de grafeno carboxilado monocapa tiene menos oxígeno que el óxido de grafito convencional, presenta mayor funcionalización de grupos carboxílicos, anhídridos, carboxilatos y/o lactonas, tal como se observa en la comparativa de las figuras 2 y 4. The following table shows the evolution of the carbon-oxygen ratio (C/O) for each of the compounds involved in the steps of the process object of the present invention, measured by X-ray Photoelectron Spectroscopy.
Figure imgf000015_0001
However, despite the fact that monolayer carboxylated graphene oxide has less oxygen than conventional graphite oxide, it presents greater functionalization of carboxylic groups, anhydrides, carboxylates and/or lactones, as observed in the comparison of figures 2 and Four.
De esta forma, se confirma que con la primera etapa de oxidación, el material de estructura grafitica cristalina introduce un elevado porcentaje de grupos éter, epoxi, lactol y quinonas (51% del total de los enlaces de C, 286.7 eV, incluso más que enlaces sp2C=sp2C), así como un 10% de grupos COOH (289.0 eV). El proceso de exfoliación - segunda etapa del procedimiento de la invención - elimina grupos oxigenados, obteniendo un óxido de grafeno reducido con una composición superficial similar a la del material de estructura grafitica cristalina inicial. Adicionalmente, la exfoliación permite la separación de las capas del grafito, que en la siguiente etapa de oxidación posibilita la introducción de nuevos grupos oxigenados, en este caso con mayor grado de oxidación, en forma de anhídridos, lactonas y/o carboxilatos (15%, 289.0 eV), superando al pico de 286.7 eV, diferentes a los grupos oxigenados obtenidos en el óxido de grafito convencional obtenido con la primera etapa de oxidación. In this way, it is confirmed that with the first oxidation stage, the material with a crystalline graphitic structure introduces a high percentage of ether, epoxy, lactol and quinone groups (51% of the total C bonds, 286.7 eV, even more than sp2C=sp2C bonds), as well as 10% COOH groups (289.0 eV). The exfoliation process - second stage of the procedure of the invention - removes oxygenated groups, obtaining a reduced graphene oxide with a surface composition similar to that of the initial crystalline graphitic structure material. Additionally, exfoliation allows the separation of the graphite layers, which in the following oxidation stage enables the introduction of new oxygenated groups, in this case with a higher degree of oxidation, in the form of anhydrides, lactones and/or carboxylates (15% , 289.0 eV), exceeding the peak of 286.7 eV, different from the oxygenated groups obtained in the conventional graphite oxide obtained with the first oxidation stage.
El efecto mencionado es coincidente con las medidas realizadas por potencial zeta a suspensiones acuosas de 0.1 mg/ml_ de óxido de grafito convencional y óxido de grafeno carboxilado monocapa. El primero tiene un valor de -33.3 mV, mientras que el segundo - 48.1 mV, indicativo de que el óxido de grafeno carboxilado monocapa tiene más polaridad, y más estabilidad en agua, debido a que sus grupos funcionales son preferencialmente carboxilo y carboxilato, aun cuando el contenido en oxígeno es inferior al óxido de grafito convencional. The mentioned effect is consistent with the measurements made by zeta potential to aqueous suspensions of 0.1 mg/ml_ of conventional graphite oxide and monolayer carboxylated graphene oxide. The first has a value of -33.3 mV, while the second - 48.1 mV, indicating that the monolayer carboxylated graphene oxide has more polarity, and more stability in water, because its functional groups are preferentially carboxyl and carboxylate, even when the oxygen content is lower than conventional graphite oxide.
Se adjunta la figura 5, que muestra los difractogramas de Difracción de Rayos X (DRX) del óxido de grafito (corresponde a línea I), el grafeno exfoliado químicamente (u óxido de grafeno reducido) (corresponde a línea J) y el óxido de grafeno carboxilado (corresponde a línea K), producto inventivo de esta patente. El eje de abscisas representa el ángulo 2Q y el eje de ordenadas la intensidad. Se observa que el óxido de grafito presenta apilamiento, dada la prominencia del pico 002 alrededor de 10e del ángulo de difracción. La exfoliación térmica elimina dicho apilamiento, dado que no se presenta ningún pico en el intervalo, así como en el óxido de grafeno carboxilado. Queda comprobada que el producto de la tercera etapa, la oxidación de las láminas hidrófobas de grafeno exfoliado químicamente, da un producto muy diferente al de la etapa 1 , constituido por monocapas. Figure 5 is attached, which shows the X-Ray Diffraction (XRD) diffractograms of graphite oxide (corresponds to line I), chemically exfoliated graphene (or reduced graphene oxide) (corresponds to line J) and graphene oxide. carboxylated graphene (corresponds to line K), inventive product of this patent. The abscissa axis represents the angle 2Q and the ordinate axis the intensity. It is observed that the graphite oxide presents stacking, given the prominence of the 002 peak around 10 e of the diffraction angle. Thermal exfoliation eliminates this stacking, since no peak is present in the interval, as well as in the carboxylated graphene oxide. It is confirmed that the product of the third stage, the oxidation of the hydrophobic sheets of chemically exfoliated graphene, gives a product very different from that of stage 1, consisting of monolayers.
La figura 6 muestra una imagen obtenida por Microscopía Electrónica de Transmisión (TEM) sobre soporte de rejilla en la que aparece representada una regleta (2) que corresponde a 2 micrómetros. De esta forma, se observa que las láminas (1) de óxido de grafeno carboxilado obtenidas conforme al procedimiento de la invención presentan un tamaño más homogéneo entre ellas. Figure 6 shows an image obtained by Transmission Electron Microscopy (TEM) on a grid support in which a ruler (2) corresponding to 2 micrometers is represented. In this way, it is observed that the sheets (1) of carboxylated graphene oxide obtained according to the method of the invention have a more homogeneous size between them.
La homogeneidad entre láminas (1) se debe a que la segunda oxidación, al no tener lugar entre las capas, no es el resultado de una reacción en cadena que desemboca en que las láminas se corten, sino que actúa puntualmente, únicamente sobre los defectos y bordes previamente formados, resultando planos de tamaños similares. The homogeneity between sheets (1) is due to the fact that the second oxidation, as it does not take place between the layers, is not the result of a chain reaction that leads to the sheets being cut, but rather acts punctually, only on the defects and previously formed edges, resulting in planes of similar sizes.
Con el proceso de exfoliación térmica, el tamaño de plano disminuye y se hacen similares, y tras la segunda oxidación, del mismo tamaño anterior, pero planos por el carácter hidrofílico adquirido. With the thermal exfoliation process, the size of the plane decreases and they become similar, and after the second oxidation, the same size as before, but planes due to the acquired hydrophilic character.
Por tanto, el óxido de grafeno carboxilado monocapa obtenido de la segunda oxidación presenta tres características clave: Therefore, the monolayer carboxylated graphene oxide obtained from the second oxidation has three key characteristics:
Está funcionalizado específicamente con grupos oxigenados carboxilato, lactonas y/o anhídridos; It is specifically functionalized with oxygenated carboxylate groups, lactones and/or anhydrides;
La funcionalización tiene lugar en las monocapas separadas durante el proceso de exfoliación. Functionalization takes place in the separated monolayers during the exfoliation process.
Menor tamaño de plano y más homogéneos. Smaller plane size and more homogeneous.
Siendo el óxido de grafeno carboxilado monocapa obtenido un nanomaterial adecuado para su incorporación en materiales compuestos de matrices poliméricas termoestables para obtener materiales de propiedades mecánicas mejoradas. The monolayer carboxylated graphene oxide obtained is a nanomaterial suitable for incorporation into materials composed of thermosetting polymeric matrices to obtain materials with improved mechanical properties.

Claims

R E I V I N D I C A C I O N E S R E I V I N D I C A T I O N S
1a.- Procedimiento de obtención de óxido de grafeno carboxilado monocapa caracterizado por que comprende las siguientes etapas: 1 a.- Procedure for obtaining monolayer carboxylated graphene oxide characterized in that it comprises the following stages:
- Oxidación de un material de estructura grafitica cristalina - precursor - mediante oxoácidos y oxisales que se intercalan entre las capas del material de estructura grafitica a temperaturas de entre 5eC y 20eC, formándose el compuesto de intercalación de grafito, el cual se descompone a temperaturas de entre 25 y 100eC, oxidando las capas grafiticas para la obtención de óxido de grafito, - Oxidation of a material with a crystalline graphitic structure - precursor - by means of oxoacids and oxysalts that are intercalated between the layers of the material with a graphitic structure at temperatures between 5 and 20 eC , forming the graphite intercalation compound, which is decomposes at temperatures between 25 and 100 C, oxidizing the graphitic layers to obtain graphite oxide,
- Exfoliación térmica del óxido de grafito por choque térmico, eliminando parte del oxígeno de los grupos oxigenados y generando la separación en láminas hidrófobas de grafeno exfoliado químicamente en una sola capa, sin apilamiento ordenado, y- Thermal exfoliation of the graphite oxide by thermal shock, eliminating part of the oxygen from the oxygenated groups and generating the separation in hydrophobic sheets of chemically exfoliated graphene in a single layer, without ordered stacking, and
- Oxidación de las láminas hidrófobas de grafeno exfoliado químicamente para la obtención de láminas de óxido de grafeno carboxilado monocapa, caracterizado porque la etapa de oxidación de las láminas hidrófobas de grafeno exfoliado químicamente para la obtención de láminas de óxido de grafeno carboxilado monocapa comprende las siguientes etapas: - Oxidation of the hydrophobic sheets of chemically exfoliated graphene to obtain monolayer carboxylated graphene oxide sheets, characterized in that the oxidation step of the hydrophobic sheets of chemically exfoliated graphene to obtain monolayer carboxylated graphene oxide sheets comprises the following stages:
- Se suspenden las láminas hidrófobas de grafeno exfoliado químicamente en un medio ácido sulfúrico o en un medio ácido sulfúrico-nitrato sódico con proporción g de lámina hidrófoba:ml de H2S0 de entre 1 :20 y 1 :150, realizando una agitación mecánica durante, al menos, 15 minutos, a temperatura ambiente,- Chemically exfoliated graphene hydrophobic sheets are suspended in a sulfuric acid medium or in a sulfuric acid-sodium nitrate medium with a ratio g of hydrophobic sheet: ml of H 2 S0 between 1:20 and 1:150, performing mechanical agitation for at least 15 minutes at room temperature
- Adición de permanganato potásico en una proporción másica KMn04:láminas hidrófobas de entre 7:1 y 1 :1 a una temperatura inferior a 25eC, - Addition of potassium permanganate in a mass ratio of KMn0 4 : hydrophobic sheets between 7:1 and 1:1 at a temperature below 25°C ,
- Agitación manteniendo una temperatura de entre 25eC y 70eC durante, al menos, 15 minutos, - Agitation maintaining a temperature between 25 and 70 eC for at least 15 minutes,
- Preoxidación de los defectos presentes en las láminas hidrófobas de grafeno exfoliado químicamente por la generación de una estructura intermedia entre las láminas hidrófobas de grafeno exfoliado químicamente. - Preoxidation of the defects present in the hydrophobic sheets of chemically exfoliated graphene by the generation of an intermediate structure between the hydrophobic sheets of chemically exfoliated graphene.
- Vertido de la mezcla de reacción sobre agua en proporción volumétrica ácido sulfúrico:agua de entre 1 :1 y 1 :10, generando un calor de dilución que controla que la temperatura sea menor de 100eC. - Reacción de oxidación-hidrolización generada por la presencia del permanganato potásico combinada con el calor de dilución, generando Mn02 insoluble, de forma que el permanganato potásico oxida y forma preferentemente grupos carboxílicos en los defectos de las láminas hidrófobas de grafeno exfoliado químicamente.- Pouring of the reaction mixture over water in a volumetric ratio of sulfuric acid:water between 1:1 and 1:10, generating a heat of dilution that controls the temperature to be less than 100 ° C. - Oxidation-hydrolyzation reaction generated by the presence of potassium permanganate combined with the heat of dilution, generating insoluble Mn0 2 , so that potassium permanganate oxidizes and preferentially forms carboxylic groups in the defects of the hydrophobic sheets of chemically exfoliated graphene.
- Adición, aplicando agitación, de una solución acuosa de peróxido de hidrógeno, con proporción volumétrica de peróxido de hidrógeno:mezcla de entre 1 :20 y 1 :50, produciendo la reducción de las especies de manganeso (IV), del Mn02 insoluble y manganeso (Vil) del permanganato a manganeso (II) soluble, pasando a la disolución acuosa. - Addition, applying stirring, of an aqueous solution of hydrogen peroxide, with a volumetric proportion of hydrogen peroxide: mixture between 1:20 and 1:50, producing the reduction of the manganese species (IV), of the insoluble Mn0 2 and manganese (VIl) from permanganate to soluble manganese (II), passing to the aqueous solution.
- Separación de la fase líquida y la fase sólida por procesos de centrifugación o filtración, descartando la fase líquida clara. - Separation of the liquid phase and the solid phase by centrifugation or filtration processes, discarding the clear liquid phase.
- Lavado del sólido con agua a pH superior a 2. - Washing of the solid with water at a pH higher than 2.
- Separación. - Separation.
- Lavado con agua. - Wash with water.
- Secado de la fase sólida húmeda por convección a temperaturas entre 50 y 95eC para la obtención del óxido de grafeno en estado polvo y seco funcionalizado con grupos carboxílicos, carboxilatos y anhídridos. - Drying of the wet solid phase by convection at temperatures between 50 and 95 C to obtain graphene oxide in powder and dry state, functionalized with carboxylic, carboxylate and anhydride groups.
2a.- Procedimiento de obtención de óxido de grafeno carboxilado monocapa, según reivindicación 1a, caracterizado porque en la oxidación del material de estructura grafitica cristalina para la obtención de óxido de grafito se emplea ácido sulfúrico como oxoácido, mientras que se emplea permanganato potásico como oxisal. 2 .- Procedure for obtaining monolayer carboxylated graphene oxide, according to claim 1, characterized in that sulfuric acid is used as oxo acid in the oxidation of the crystalline graphitic structure material to obtain graphite oxide, while potassium permanganate is used as oxysalt.
3a.- Procedimiento de obtención de óxido de grafeno carboxilado monocapa, según reivindicación 1a o 2a, caracterizado porque la oxidación del material de estructura grafitica cristalina comprende las siguientes etapas: 3.- Process for obtaining monolayer carboxylated graphene oxide, according to claim 1 or 2, characterized in that the oxidation of the material with a crystalline graphitic structure comprises the following stages:
- Se suspende el material de estructura grafitica cristalina - precursor - en un medio ácido sulfúrico o en un medio ácido sulfúrico-nitrato sódico con proporción g grafito:ml de H2S0 de entre 1 :20 y 1 :150, realizando una agitación mecánica durante, al menos, 15 minutos, a temperatura ambiente, - The material with a crystalline graphitic structure - precursor - is suspended in a sulfuric acid medium or in a sulfuric acid-sodium nitrate medium with a ratio g graphite: ml of H 2 S0 between 1:20 and 1:150, carrying out mechanical stirring for at least 15 minutes at room temperature
- Se genera un compuesto de intercalación grafito:ácido sulfúrico, separando las capas de grafito, - A graphite:sulfuric acid intercalation compound is generated, separating the graphite layers,
- Adición de permanganato potásico en una proporción másica KMn04:precursor de entre 7:1 y 1:1 a una temperatura inferior a 25eC, - Agitación manteniendo una temperatura de entre 25eC y 70eC durante, al menos, 15 minutos, - Addition of potassium permanganate in a KMn0 4 :precursor mass ratio of between 7:1 and 1:1 at a temperature below 25°C , - Agitation maintaining a temperature between 25 and 70 eC for at least 15 minutes,
- Se genera una mezcla de reacción de óxido de grafito preliminar que presenta grupos funcionales sobre los defectos estructurales de la estructura grafitica. - A preliminary graphite oxide reaction mixture is generated that has functional groups on the structural defects of the graphitic structure.
- Vertido de la mezcla de reacción sobre agua en proporción volumétrica ácido sulfúrico:agua de entre 1 :1 y 1 :10, generando calor y una reacción en cadena de oxidación-hidrolización provocada por la presencia del permanganato potásico, generando Mn02 y originando el corte de las capas grafiticas, con multitud de grupos funcionales oxigenados. - Pouring of the reaction mixture on water in a sulfuric acid:water volumetric ratio of between 1:1 and 1:10, generating heat and an oxidation-hydrolyzation chain reaction caused by the presence of potassium permanganate, generating Mn0 2 and causing the cut of the graphitic layers, with a multitude of oxygenated functional groups.
- Adición, aplicando agitación, de una solución acuosa de peróxido de hidrógeno, con proporción volumétrica de peróxido de hidrógeno y la mezcla anterior de entre 1 :20 y 1 :50, convirtiendo las especies insolubles de Mn02 y el permanganato remanente a manganeso (II) el cual es soluble. - Addition, applying stirring, of an aqueous solution of hydrogen peroxide, with a volumetric ratio of hydrogen peroxide and the previous mixture between 1:20 and 1:50, converting the insoluble species of Mn0 2 and the remaining permanganate to manganese ( II) which is soluble.
- Separación de la fase líquida y la fase sólida por procesos de centrifugación o filtración, descartando la fase líquida clara. - Separation of the liquid phase and the solid phase by centrifugation or filtration processes, discarding the clear liquid phase.
- Lavado del sólido con agua a pH superior a 2. - Washing of the solid with water at a pH higher than 2.
- Separación. - Separation.
- Lavado con agua. - Wash with water.
- Secado de la fase sólida húmeda por convección a temperaturas entre 50eC y 95eC para la obtención del óxido de grafito en estado polvo y seco. - Drying of the wet solid phase by convection at temperatures between 50 e C and 95 e C to obtain graphite oxide in powder and dry state.
4a.- Procedimiento de obtención de óxido de grafeno carboxilado monocapa, según reivindicación 1a o 3a, caracterizado porque el medio ácido sulfúrico - nitrato sódico se prepara al suspender el nitrato sódico en el ácido sulfúrico, en una proporción másica entre 1 :1 y 1 :2 de nitrato sódico:precursor. 4 .- Procedure for obtaining monolayer carboxylated graphene oxide, according to claim 1 or 3, characterized in that the sulfuric acid - sodium nitrate medium is prepared by suspending sodium nitrate in sulfuric acid, in a mass ratio between 1: 1 and 1:2 sodium nitrate:precursor.
5a.- Procedimiento de obtención de óxido de grafeno carboxilado monocapa, según reivindicación 1a, caracterizado porque la exfoliación térmica del óxido de grafito comprende las siguientes etapas: 5.- Process for obtaining monolayer carboxylated graphene oxide, according to claim 1, characterized in that the thermal exfoliation of the graphite oxide comprises the following stages:
- Aumento de temperatura entre 90eC y 250eC, produciendo la salida del agua intercalada entre las capas, tanto fisisorbida como quimisorbida, exfoliando el material. - Increase in temperature between 90 e C and 250 e C, producing the exit of the water intercalated between the layers, both physisorbed and chemisorbed, exfoliating the material.
- Arrastre de grupos funcionales existentes en el óxido de grafito, junto con la salida de agua quimisorbida alcanzándose presiones que exceden las fuerzas de Van der Waals que unen las láminas del grafito, de forma que los grupos funcionales arrastrados y eliminados son hidroxilo, éter, epoxi, lactol, anhídrido, quinonas y carboxilos, - Drag of existing functional groups in the graphite oxide, together with the exit of chemisorbed water, reaching pressures that exceed the Van der forces Waals that join the graphite sheets, so that the functional groups dragged and eliminated are hydroxyl, ether, epoxy, lactol, anhydride, quinones and carboxyls,
- Obtención de láminas de óxido de grafeno reducido que están separadas sin apilamiento grafitico. - Obtaining reduced graphene oxide sheets that are separated without graphitic stacking.
6a.- Procedimiento de obtención de óxido de grafeno carboxilado monocapa, según reivindicación 5a, caracterizado porque el aumento de temperatura se genera por irradiación por microondas. 6 .- Process for obtaining monolayer carboxylated graphene oxide, according to claim 5, characterized in that the temperature increase is generated by microwave irradiation.
7a.- Procedimiento de obtención de óxido de grafeno carboxilado monocapa, según la reivindicación 1 a, caracterizado porque el calor de dilución producido al verter la mezcla de reacción sobre agua es extraído para mantener la mezcla por debajo de 60eC. 7 .- Process for obtaining monolayer carboxylated graphene oxide, according to claim 1, characterized in that the heat of dilution produced by pouring the reaction mixture on water is extracted to keep the mixture below 60 e C.
8a.- Procedimiento de obtención de óxido de grafeno carboxilado monocapa, según cualquiera de las reivindicaciones anteriores, caracterizado porque el material de estructura grafitica cristalina es grafito natural, grafito sintético, nanotubos de carbono, nanofibras de carbono de cinta helicoidal, fibra de carbono y/o negro de carbono, 8 .- Procedure for obtaining monolayer carboxylated graphene oxide, according to any of the preceding claims, characterized in that the material with crystalline graphitic structure is natural graphite, synthetic graphite, carbon nanotubes, carbon nanofibers of helical tape, carbon fiber and/or carbon black,
9a.- Óxido de grafeno carboxilado monocapa obtenido según el procedimiento detallado en cualquiera de las reivindicaciones anteriores caracterizado por que las láminas de óxido de grafeno carboxilado monocapa obtenidas están constituidas por una lámina donde los grupos funcionales formados son mayoritariamente selectivos de tipo carboxilo, carboxilato, lactona y anhídrido, los cuales se encuentran localizados en los bordes de las láminas del óxido de grafeno carboxilado monocapa y en los defectos estructurales de su interior, y donde las láminas de óxido de grafeno carboxilado monocapa no presentan un pico alrededor de 10e del ángulo de difracción en el difractograma de Rayos X al no estar apiladas. 9 .- Monolayer carboxylated graphene oxide obtained according to the procedure detailed in any of the preceding claims, characterized in that the monolayer carboxylated graphene oxide sheets obtained consist of a sheet where the functional groups formed are mostly selective of the carboxyl, carboxylate type , lactone and anhydride, which are located at the edges of the monolayer carboxylated graphene oxide sheets and in the structural defects inside them, and where the monolayer carboxylated graphene oxide sheets do not present a peak around 10 e of the diffraction angle in the X-ray diffractogram when not stacked.
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