WO2023047009A1 - Reduced graphene oxide with semiconductor containing photocatalysts - Google Patents

Reduced graphene oxide with semiconductor containing photocatalysts Download PDF

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WO2023047009A1
WO2023047009A1 PCT/ES2022/070609 ES2022070609W WO2023047009A1 WO 2023047009 A1 WO2023047009 A1 WO 2023047009A1 ES 2022070609 W ES2022070609 W ES 2022070609W WO 2023047009 A1 WO2023047009 A1 WO 2023047009A1
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rgo
nanoplatelets
photocatalyst
semiconductor
graphene oxide
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PCT/ES2022/070609
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Spanish (es)
French (fr)
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Radmila TOMOVSKA
Nikolaos POLITAKOS
Iranzu BARBARIN
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Universidad Del Pais Vasco / Euskal Herriko Universitatea
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    • B01J23/76—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of the iron group metals or copper combined with metals, oxides or hydroxides provided for in groups B01J23/02 - B01J23/36
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    • B01J27/00—Catalysts comprising the elements or compounds of halogens, sulfur, selenium, tellurium, phosphorus or nitrogen; Catalysts comprising carbon compounds
    • B01J27/02—Sulfur, selenium or tellurium; Compounds thereof
    • B01J27/04—Sulfides
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    • B01J35/50—Catalysts, in general, characterised by their form or physical properties characterised by their shape or configuration
    • B01J35/56—Foraminous structures having flow-through passages or channels, e.g. grids or three-dimensional monoliths
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    • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/02—Impregnation, coating or precipitation
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    • B—PERFORMING OPERATIONS; TRANSPORTING
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    • B01J37/16—Reducing
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    • C01B32/00—Carbon; Compounds thereof
    • C01B32/15—Nano-sized carbon materials
    • C01B32/182—Graphene
    • C01B32/198—Graphene oxide

Definitions

  • the present invention falls within the field of photocatalysts, in particular it refers to photocatalysts comprising a three-dimensional structure of reduced graphene oxide and platelets of at least one semiconductor. It also refers to their uses and methods for their preparation. BACKGROUND OF THE INVENTION
  • the invention relates to an integrated photocatalyst and in particular to a photocatalyst comprising a semiconductor and reduced graphene oxide as active support united in a monolithic and porous structure.
  • the photocatalytic process is based on the excitation of a solid (photocatalyst), usually a broadband semiconductor, by irradiation with light.
  • the photocatalytic process has multiple applications that include, for example, environmental decontamination treatments and the synthesis of high added value compounds, all in a sustainable way by using solar radiation.
  • One of the main challenges in this area is the development of new or modified photocatalytic materials that improve their efficiency by operating with a greater range of wavelengths.
  • Many of the semiconductors used as photocatalysts are metal oxides (for example, TiO2 and ZnO) or chalcogenides (for example, CdS, ZnS, CdSe, ZnSe, and CdTe). The most used is TiO2.
  • the invention relates to new photocatalysts and in particular to photocatalysts comprising a three-dimensional structure of reduced graphene oxide and nanoplatelets of at least one semiconductor. In such photocatalysts, the semiconductor nanoplatelets are embedded in the three-dimensional structure of reduced graphene oxide.
  • photocatalyst that comprises a three-dimensional structure of reduced graphene oxide and that incorporates semiconductor materials in the form of nanoplatelets (two-dimensional structures).
  • the photocatalysts described here have porous structures, and optionally comprise nanoparticles of at least one polymer.
  • these polymers are selected from a group formed by styrenes, acrylates, methacrylates, optionally functionalized, and their mixtures.
  • the semiconductor is selected from a group formed by GaN, CeO 2 , CdS, ZnO, MoS 2 , WS 2 , WO 3 and other semiconductor materials that allow the formation of nanoplatelets by exfoliation thereof, preferably being MoS 2 or WS 2 .
  • the photocatalyst comprises WS2 or MoS2 and optionally, functionalized polymethyl methacrylate nanoparticles. Also, according to the invention, a method of preparing said photocatalysts is provided. The synthesis method is simple, and can be carried out under mild reaction conditions.
  • the method comprises: a) preparing an aqueous dispersion of graphene oxide nanoplatelets, nanoplatelets of one or more semiconductors and a reducing agent; and heating the resulting dispersion, without stirring, to a temperature in the range of 45 to 90°C.
  • the reducing agent is preferably ascorbic acid or hydrazine.
  • the process described comprises a step c), between a) and b), in which the dispersion resulting in a) is stirred at room temperature for a period of time sufficient to homogenize the dispersion, typically for 15 minutes to 1 hour.
  • the resulting reaction product can be dried, for example using lyophilization or microwaves.
  • the photocatalysts described herein have been found to show catalytic properties superior to commercial photocatalysts and provide additional advantages.
  • the invention also relates to the use of said photocatalysts. For example, in environmental protection technologies, in the capture and/or chemical conversion of air, water or soil pollutants or as catalysts in chemical reactions. Also in applications that require the use of porous materials.
  • Figure 1g Results of MALDI-TOF mass spectrometry analysis of the aqueous MB solution after destaining done with 3D WS2/rGO and UV light (full line) and in the dark (dotted line) as indicated in Example 1. The characteristic MB peak is marked.
  • Figure 1h Results of MALDI-TOF mass spectrometry analysis of the ethanol solution obtained in the desorption process of 3D WS2/rGO material after the MB degradation reaction with UV light as indicated in Example 1
  • Figure 2b Kinetics of the bleaching process (Change of MB concentration with the time); 3D rGO pure with UV light; 2D WS2 pure with UV light; 3D WS2/rGO/Pol with UV light and 3D WS2/rGO/Pol D as indicated in Example 2
  • Figure 2c Results of MALDI-TOF mass spectrometry analysis of the aqueous MB solution
  • Figure 3b Kinetics of the bleaching process (Change of relative MB concentration with time); Pure 3D rGO under UV light, Pure 2D WS2 under UV light, 3D WS2/rGO under UV light and 3D WS 2 /rGO D in the dark as per Example 3
  • Figure 3c Results of MALDI-TOF mass spectrometry analysis of the aqueous solution of MB after destaining with 3D WS2/rGO as indicated in Example 3, with assignment of peaks
  • Figure 6c Results of MALDI-TOF mass spectrometry analysis of the aqueous MB solution after destaining with the 3D WS2/rGO system with light as indicated in Example 6, with indication of the specific chemical groups in different areas of the spectra.
  • Figure 7a Transmission Electron Microscopy (TEM) images of MoS2 nanoplatelets as indicated in Example 7
  • TEM Transmission Electron Microscopy
  • Figure 7c Kinetics of the decolorization of the MB solution (change of MB concentration over time) of 3D pure rGO under UV light, 2D pure MoS2 under UV light, 3D MoS2/rGO under UV light and 3D MoS2/rGO D in the dark as indicated in Example 7.
  • Figure 7d MALDI-TOF mass spectra of ethanol desorbed solution for the 3D MoS2/rGO system of Example 7.
  • Figure 8b Kinetics of decolorization of aqueous MB solution (change of MB concentration relative to time) by neat 3D rGO, neat 2D MoS 2 , 3D MoS 2 /rGO/Pol under light and 3D MoS 2 /rGO/ Pol D in the dark, as indicated in Example 8.
  • the materials of the present invention are integrated photocatalysts comprising a three-dimensional structure of reduced graphene oxide (rGO) into which two-dimensional semiconductor nanoplatelets are incorporated.
  • rGO reduced graphene oxide
  • This porous, three-dimensional, monolithic structure is a stable structure that functions as an integrated photocatalyst. This is advantageous because the material works as a photocatalyst without having to incorporate it into a support, as is the case with commercial catalysts.
  • the semiconductor absorbs the photon of UV light that carries more energy than the gap energy band
  • an electron-hole pair is generated in the semiconductor nanoplatelets that are capable of starting oxidation-reduction processes.
  • the electron-hole pair can undergo a recombination process and return to the state increases the effectiveness of the semiconductor as a photocatalyst.
  • the fact that the semiconductor is incorporated into the 3D rGO or rGO/polymer structures makes it possible to easily reuse the photocatalyst, after cleaning. Furthermore, since the semiconductor is immobilized, the loss of active material during cleaning is reduced.
  • the materials of the present invention have a three-dimensional macroscopic structure and are porous materials, with a sponge-like appearance.
  • the main advantage of the materials of the present invention is that it allows the union of the photocatalyst (semiconductor) and the support within the same structure.
  • the materials are also versatile, with many different applications.
  • the material of the invention has very advantageous properties in terms of porosity, mechanical and thermal resistance, shape and size, which can also be modulated.
  • the graphene oxide (GO) that is used to prepare the photocatalyst of the invention is graphene oxide in the form of nanoplatelets.
  • Graphene oxide in the form of nanoplatelets can be purchased commercially or prepared according to methods known in the state of the art. Nanoplatelets are usually purchased in the form of a dispersion, such as an aqueous dispersion.
  • the nanoplatelets are supplied in dispersion in water with concentrations that typically range from 0.1 to 5% by weight, preferably 0.4% - 1% by weight with respect to the total weight of the dispersion supplied with monolayer content of >90%, preferably >92%, or >95%, or >97%.
  • the starting graphene oxide has a high oxygen content of between 25% to 55% by weight, particularly from 30 to 50% by weight, or from 40% to 50% by weight, or from 45% to 50% by weight with with respect to the total weight of graphene oxide.
  • the nanoplatelets used in the present invention are also characterized by their lateral dimension.
  • the starting graphene oxide nanoplatelets have an oxygen content of 30 to 50 % by weight, 93 to 95% of them are nanoplatelets with a thickness equal to or less than 1 nm, and their lateral dimension is 10 microns.
  • semiconductor refers to a semiconductor material with photocatalytic characteristics.
  • Illustrative, but not limiting, examples of semiconductors that can be used in the preparation of the photocatalysts of the invention are GaN, CeO 2 , CdS, ZnO, MoS 2 , WS 2 , WO 3 and other semiconductors that have a layered structure and are therefore suitable for the formation of nanoplatelets by exfoliation.
  • Preferred semiconductors include CdS, MoS2, and WS2. Especially preferred is WS2.
  • Other semiconductors that can be used to prepare the photocatalysts of the invention include: (i) W: tungsten ditelluride (WTe 2 ) and tungsten diselenide (WSe 2 ), ii) Mo: molybdenum telluride (MoTe2) and molybdenum diselenide (MoSe2), iii) Cd: cadmium selenide (CsSe), iv) In: indium (III) sulfide In2S3) and indium (III) selenide (In2Se3), v) Ga: gallium (III) selenide (Ga2Se3 ), gallium(II) selenide (GaSe), gallium(II) sulfide (GaS) and gallium(II) telluride (GaTe ), vi) Bi: bismuth selenide (Bi2Se3), bismuth telluride (Bi2Te3) and bismuth
  • the semiconductor nanoplatelets that can be used to prepare the photocatalysts of the invention can have thicknesses varying between one or more atomic layers.
  • the thickness of an atomic layer varies depending on the particular semiconductor material. For example, it is 0.65 nm for MoS2 and 3.7 nm for WS2. As for their maximum thickness, it will depend on the number of atomic layers of the nanoplatelet. In the present invention, the thickness generally varies between that corresponding to one atomic layer and 40 nm according to the material, or between one atomic layer and 30 nm, or between one atomic layer and 20 nm, or between one atomic layer and 10 nm.
  • the lateral dimensions of the semiconductor nanoplatelets can also vary between a value greater than 40 nm and 3 microns on the condition that the lateral dimension particular embodiments the lateral dimensions are between 50 nm and 1 micron, or between 100 nm and 500 nm microns, or between 200 nm and 400 nm.
  • the nanoplatelets have thicknesses between 0.5 and 8 nm, and a lateral dimension between 25 and 300 nm, or thicknesses between 0.5 and 8 nm, and a lateral dimension between 50 and 250 nm, or 100 and 200 nm.
  • the thickness is between 0.5 and 4 nm and the lateral dimension between 25 and 300 nm, or 50 and 250 nm, or 100 and 200 nm.
  • the photocatalyst of the invention may comprise nanoplatelets of one or more semiconductor materials.
  • the nanoplatelets used for a particular embodiment may in turn have different structures and characteristics.
  • the composition of the photocatalyst of the invention can vary over a wide range. Elemental composition analysis determined from the EDX results demonstrate the incorporation of semiconductor nanoplatelets into the composite 3D structure. SEM images of photocatalysts show the three-dimensional and porous nanostructure.
  • the weight ratio of the starting graphene oxide to the weight of the starting semiconductor [GO/semiconductor] is typically in the range of 1:1 to 9:1. Some particular ratios are in the range of 2:1 to 8: 1, for example 3:1, 4:1, 5:1; 6:1, or 7:1. A more preferred ratio is 4:1.
  • polymer refers to any polymer composed of macromolecules (generally organic) formed by the union by covalent bonds of one or more simple units called monomers, such as: acrylates, methacrylates, styrenes, etc.
  • monomers such as: acrylates, methacrylates, styrenes, etc.
  • the polymer particles that are incorporated into the structure of the photocatalyst can be produced, for example, by polymerization techniques in a dispersed medium (water), such as in emulsion or the like.
  • the polymers can be functionalized, and for this, amounts of functional monomers are used, generally 1%-10% by weight. they are used in amounts between 2 and 8%, or between 4 and 6%.
  • Examples of functional monomers include, but are not limited to: 4-bromo styrene, 2-chloro ethyl acrylate, glycidyl methacrylate, 2-hydroxyethyl methacrylate, acrylamide, 2-amino ethyl methacrylate hydrochloride, acrylic acid, polystyrene sulfonate, and the like.
  • Illustrative, but not limiting, examples of polymers that can be used in the preparation of the photocatalysts of the invention are polystyrenes, polyacrylates, polymethacrylates, polyacrylamides, polyurethanes, etc., all of them optionally functionalized.
  • “Functionalized polymer” refers to a polymer that has functional groups incorporated. Examples of functional groups include epoxy, amide, amine, sulfonates, carboxyl, hydroxyl, ester, phenyl, ether, etc.
  • One of the preferred polymers in the present invention is functionalized polymethyl methacrylate, preferably functionalized with epoxy groups.
  • the photocatalyst comprises one or more polymers. The polymers can be selected based on the characteristics that the photocatalyst is desired to present.
  • the glass transition temperature (Tg) of the polymer used influences whether they tend to settle as nanoparticles on the rGO surface (when they have a high Tg) or whether they tend to form a layer covering the rGO (when they have a high Tg). have a low Tg) in the drying process of 3D materials.
  • Tg glass transition temperature
  • the photocatalyst comprises one or more polymers
  • the ratio between the weight of the "GO and semiconductor" starting material and the weight of the polymer, used in the preparation of the photocatalysts can vary.
  • they have been prepared with weight ratios of the starting materials (GO/semiconductor): polymer that vary from 1:1 to 10:1.
  • the synthesis method of the photocatalysts of the invention is a self-assembly process that occurs during the reduction reaction of the graphene oxide nanoplatelets in a colloidal dispersion that also contains the semiconductor nanoplatelets.
  • the three-dimensional porous structure is obtained spontaneously in the reduction reaction of graphene oxide with a reducing agent.
  • the reaction is carried to In particular, the temperature is between 25 and 45 oC.
  • the reaction time depends on the reaction temperature, the size of the photocatalyst to be obtained, etc.
  • the time is 30 minutes at 90 o C or several hours at 25 o C.
  • an aqueous dispersion of the starting materials is prepared: graphene oxide nanoplatelets, nanoplatelets semiconductor and optionally polymer nanoparticles.
  • the dispersion is stirred to homogenize the mixture. During this stirring period, it is verified that the semiconductor nanoparticles and the polymer nanoparticles adsorb on the surface of the graphene oxide nanoparticles. Then the temperature is set within the range mentioned above and the reducing agent (eg ascorbic acid (Asa) or hydrazine) is added when the required temperature is reached.
  • the reducing agent eg ascorbic acid (Asa) or hydrazine
  • the graphene oxide reduction process converts the graphene oxide material into a material that is much more hydrophobic and incompatible with the aqueous medium in which the reaction takes place.
  • the reduced graphene oxide nanoplatelets that are produced begin to spontaneously self-assemble, thus forming three-dimensional (3D) porous materials in the form of a monolithic structure.
  • “Monolithic structure” or “monolith” as used herein refers to a material composed of more than one type of material or component, in which the components form a single structure without joint points, which is made up of a single unit .
  • the material thus produced is a hydrogel swollen with a large amount of water.
  • the photocatalyst of the invention can be used in the form of a hydrogel or in a dry form, after a drying process. But before drying it has to be cleaned with distilled water by the dialysis process.
  • any of the known methods of drying hydrogels can be used. These include, for example, freeze drying, microwave or conventional oven drying.
  • the drying is carried out with lyophilization, in order to preserve the porous structure and the volume of the monolith.
  • reducing agent or "reductant” as used herein refers to any known reducing agent, that is to say an agent capable of donating electrons in an oxidation-reduction reaction according to the invention.
  • Borohydrides for example, NaBH4
  • Lithium aluminum hydride Lithium aluminum hydride
  • Hydrogen halides for example, HI
  • the reaction parameters can vary and the process can be controlled by changing the parameters, such as the type or amount of reducing agents or the temperature of the process.
  • parameters that can be changed and that influence the formation process of the structures and their properties include: the concentration and type of graphene oxide, reducing agent, polymer, and semiconductor; temperature and reduction time. As indicated before, it is important that the starting graphene oxide is highly oxidized. During the reduction process, oxygen-containing functional groups are rapidly lost, and this process provides the necessary conditions for the structure to form.
  • the amount of graphene oxide present in the reaction medium for the structures to form can vary.
  • the amount of GO is between 1 mg/mL and 20 mg/mL, or between 2 mg/mL and 15 mg/mL, or between 4 mg/mL and 10 mg/mL, preferably between 5 mg/mL and 6 mg/mL and more preferably is about 4 mg/mL.
  • the concentration of graphene oxide in the reaction medium determines the dimensions of the structure formed. a greater Concentration of dispersions in water results in the formation of larger volume structures.
  • the temperature at which graphene oxide is reduced (25o - 90oC) is a parameter that determines the speed of the reduction reaction and, as a consequence, the properties of the structures formed.
  • the reduction reaction is slower, and at higher temperatures, the structures are formed at a faster rate.
  • the amount and type of the agent are important for the reduction reaction. These parameters affect the rate of the reaction, which in turn influences the properties of the structure formed.
  • the concentration of the reducing agent affects above all the properties of the structures.
  • the weight ratios of GO:reducing agent can be in particular embodiments low (1:0.5), medium (1:1) or high (1:2). At lower proportions (for example when the ratio of GO:reducing agent is 1:0.5) the formation process is slower and the resulting structure is less compact and has less mechanical resistance, presents more macropores, that is, pores.
  • the structure formed is mechanically weaker, but richer in oxygen functional groups.
  • the temperature and the amount of reducing agent, in combination determine the properties of the structure, especially the mechanical properties, porosity and pore distribution, and also the surface chemistry of the structures, since it determines the level of reduction. and the amount of functional oxygen groups present. In order to save energy and produce materials with low energy demand, it is of interest to produce them at lower temperatures (25-45oC). Varying the amount of polymer can compensate for the reduction in mechanical properties.
  • the type of polymer and the type of semiconductor can influence the self-assembly process, since when adsorbed on graphene oxide platelets, they influence the mobility and assembly affinity. They can also affect the final properties. For example, by changing the semiconductor or by using a combination of two or more semiconductors, photocatalytic activity, light absorption and band gap energy can be optimized. Polymers are inert components that, although in general they do not influence photocatalytic activity, can improve physicochemical properties and contribute to the formation of a product with better consistency and mechanical resistance, longer duration and reusability. In the scaffold production process, the initial mixing of the starting materials is carried out with magnetic stirring (between 100 and 300 rpm, eg 150 rpm) to homogenize the mixture.
  • the preparation of the semiconductor nanochips is not restricted to a particular procedure. Rather, all known preparative methods can be used to prepare the nanoplatelets used in the process of the invention. These include chemical methods (Eda et al., 2011), ion exchange (Browning et al., 2015), powder evaporation (PV) (Najmaei et al., 2013), chemical vapor deposition (CVD) (Eichfeld et al., al., 2015), atomic layer deposition (ALD) (Song et al., 2013), molecular beam epitaxy (MBE) (Liu et al., 2015), thermal exfoliation, electrochemical and aqueous dispersion.
  • the semiconductor nanoplatelets are prepared by an exfoliation method, such as ultrasound.
  • exfoliation method such as ultrasound.
  • Other possible methods of exfoliation that can be used in the invention include high temperature or the use of solvents.
  • the exfoliation process is carried out starting from an aqueous dispersion of the semiconductor material.
  • three-dimensional or “3D” as used herein refers to a geometric structure of materials, in which each of the points that make it up is characterized by three dimensions, for example, width, height and depth.
  • the term “two-dimensional” or “2D” in the context of the invention refers to platelet-like and nanoplatelet-like materials that are characterized by their thickness and a lateral dimension which is always greater than the thickness.
  • polymeric nanoparticles in the context of the invention may have a mean diameter of between 0.5-1000 nm, for example between 1-500 nm, or between 1 and 250 nm.
  • integrated refers to the fact that the nanoplatelets of the semiconductor or semiconductors are incorporated into the three-dimensional structure of the reduced graphene oxide, distributed uniformly, homogeneously in the structure.
  • Ambient temperature refers to a temperature of 25°C.
  • the materials of the invention have many applications, especially in environmental protection technologies, in the capture and/or chemical conversion of air, water or soil pollutants or as catalysts in chemical reactions.
  • Possible uses include: adsorption and degradation of organic or inorganic contaminants from water, air or soil, eg heavy metals, petroleum, antibiotics, organic dyes or dyes and other contaminants, remediation of soils to remove pesticides and other contaminants; capture and photocatalytic conversion of CO2, photocatalytic dissociation of water to produce oxygen and hydrogen, capture and degradation of harmful tobacco products, etc.
  • Potential applications include but are not limited to: gas separation and storage, encapsulation agents in drug delivery systems, catalysts or catalyst supports, electrode and energy storage materials, supports for biomolecule immobilization (hybrid biocatalyst production) and scaffolds.
  • the starting graphene oxide (GO) was obtained from Graphenea, in the form of an aqueous dispersion of 4mg/mL and 10mg/mL, with a monolayer content of >95%, and pH between 2.2-2.5. Elemental analysis of the GO showed C: 49-56%, H: 0-1%, N: 0-1%, S: 2-4% and O: 41-50%. L-Ascorbic acid (AsA) was used as reducing agent ( ⁇ 99.0%, Sigma-Aldrich).
  • gallium(III) nitride - GaN (99.99%, Alfa Aesar) with a maximum of 0.01% metal impurities
  • tungsten(IV) sulfide -WS 2 (99%, Sigma-Aldrich) with a particle size of 2 ⁇ m
  • zinc oxide - ZnO 99.99%, Sigma-Aldrich
  • trace element analysis ⁇ 150.0 ppm iv) cadmium sulfide - CdS (Sigma-Aldrich);
  • cerium(IV) oxide - CeO 2 (99.9%, Sigma-Aldrich), with particle size ⁇ 5 ⁇ m, with rare earth trace analysis ⁇ 1500.0 ppm
  • molybdenum(IV) sulfide - MoS2 (99%, Sigma-Aldrich), with particle size ⁇ 2 ⁇ m.
  • KPS potassium persulfate
  • SDS sodium dodecyl sulfate
  • NaHCO3 sodium bicarbonate
  • Formulation for the preparation of MMA/GMA 90/10% by weight The reaction was carried out in a 250 ml reactor, equipped with a stainless steel rotor, a condenser and a nitrogen inlet.
  • the aqueous solution of the KPS initiator and NaHCO3 was prepared separately and allowed to homogenize at room temperature.
  • the emulsion was prepared by mixing the two phases, the organic phase of monomeric mixture and the aqueous phase of SDS solution in water. The emulsion was added to the reactor and stirred at room temperature for 15 minutes. Subsequently, the reactor was heated to 70 o C, the KPS/NaHCO 3 solution was added all at once and it was left to polymerize under a nitrogen atmosphere for 1.5 hours.
  • the resulting dispersion was used directly in the preparation of 3D materials. Analysis and structural characterization of the nanoplatelets was carried out by transmission electron microscopy (TEM). Synthesis of Three-Dimensional (3D) Structures of Reduced Graphene Oxide (rGO)/Two-Dimensional (2D) Platelets and of rGO/2D Platelets/Polymer A suitable amount of dispersion of graphene oxide GO was mixed with a dispersion of 2D semiconductor nanoplatelets. When the structure includes a polymer, ie, for the synthesis of the rGO/2D nanoplatelets/polymer material, a suitable proportion of polymeric particles was added to the GO:2D nanoplatelets dispersion.
  • the ratio between the weight of the GO/2D material and the weight of the polymer can vary and several ratios have been used, although in one particular case the ratio of 4 to 1.
  • the appropriate ratio of reducing agent was added, in this In the case of ascorbic acid, in three quantities with respect to the weight of graphene oxide (GO:AsA 1:0.5; 1:1; and 1:2). The mixture was stirred for 0.5 hour to homogenize.
  • the homogeneous mixture of GO/2D nanoplatelets/AsA or GO/2D nanoplatelets/polymer/AsA was heated in an oven (vacuum oven, Binder, Tuttlingen, Germany) and kept overnight at the chosen temperature ( 45 o C, 60 o C or 90 o C) for the formation of three-dimensional nanostructure materials. These materials are hydrogels and are highly swollen with water. Once out of the oven, the wetted material formed was washed by dialysis in Milli Q water for several days, changing the water daily until the conductivity of the water was constant (less than 10 ⁇ S/cm). The 3D framework was then dried by lyophilization to give the 3D monolithic material.
  • a blue colored aromatic molecule (organic methylene blue (MB)) was chosen.
  • MB organic methylene blue
  • This compound is used as a dye in the textile industry and wastewater generally contains this dye, causing serious problems for aquatic life and changes in the environment.
  • the adsorption and degradation of dissolved MB in aqueous solutions is therefore very important and of great environmental utility.
  • the 3D porous materials, rGO, rGO/2D nanoplatelets and rGO/polymer/2D nanoplatelets, and also 2D nanoplatelets in aqueous dispersion, were added to the aqueous MB solution with a concentration of 0.1 mg/ml.
  • Graphene oxide 3D materials are very good adsorbents of organic molecules, thus aqueous solutions quickly discolored for all 3D materials, even those without semiconductor nanoplatelets.
  • 3D materials containing semiconductor nanoplatelets were expected to be capable of degrading adsorbed MB molecules.
  • the photocatalytic experiments were carried out in a chamber equipped with 20 UV light lamps of 15 W each, (model BS 03, Dr. Gröbel UV-Elektronik GmbH). The irradiance of UV light is 7 mW/cm 2 .
  • the reactor containing the aqueous MB solution to which 1 mg of 3D material had been added for each 0.01 mg of MB was placed in the UV chamber.
  • Example 1 WS2/rGO, prepared from WS2/GO with a weight ratio of 1:4.
  • Example 2 WS2/rGO prepared from WS2/GO with a weight ratio of 1:4.
  • Example 3 WS2/rGO, prepared from WS2/GO with a weight ratio of 1:3.
  • Example 5 WS 2 /rGO/Pol prepared from WS 2 /GO with a weight ratio of 1:4, at high temperature.
  • Example 6 WS 2 /rGO prepared from WS 2 /GO with a weight ratio of 1:4, with less AsA.
  • Example 7 MoS 2 /rGO prepared from MoS 2 /GO with a weight ratio of 1:4.
  • Example 8 MoS 2 /rGO/Pol prepared from MoS 2 /GO with a weight ratio of 1:4.
  • Example 1 Synthesis and characterization of WS2/rGO, prepared from WS2/GO with a weight ratio of 1:4 Comparison of the photocatalytic activity of the WS2/rGO system with the pure WS2 nanoplatelets and with pure rGO
  • Figure 1a you can see the TEM images of the initial WS2 nanoparticles produced by sonication of the bulk crystal of WS2, in which you can see 2D nanoplatelets, with lateral dimension between 50-250nm.
  • Figure 1d shows the kinetics of the decolorization process of the aqueous methylene blue (MB) solution (followed by UV spectrophotometry). These results were obtained by placing the photocatalytic material in an aqueous MB solution and irradiating with UV light (368 nm). The MB concentration was 0.1 mg/mL and in all cases 1 mg of photocatalytic material was added for every 0.01 mg of MB in solution. The 3D material adsorbs the MB dye from the solution, and on the surface of the WS 2 it is degraded by photoinitiation produced by ultraviolet light.
  • UV light 368 nm
  • m/z 23 and 39 peaks are assigned to Na and K, and as the following peaks appear they are assigned to: hydrochloric acid, pentane, thioacetone, thiazoline, sulfate anion, 4 or 3-hydroxybutanoic acid, 2-aminophenol, benzene-Cl anion, 2-ethylthiazole, 4-nitrophenol, benzenesulfonic acid, potassium sulfite, sodium thiosulfate, 4-phenolsulfonic acid, sulfate potassium, sodium benzenesulfonate hydrate.
  • polymer nanoparticles were added to the same photocatalyst of Example 1 to improve handling, durability and increase ease of purification and reuse of photocatalysts.
  • the amount of polymer was 20% by weight based on the weight of WS2/rGO.
  • Table 2.1 Elemental composition of 3D WS 2 /rGO/Pol from Example 2, determined by EDX
  • Figure 2b presents the kinetics of the decolorization process of an aqueous solution of MB with a concentration of 0.1 mg/mL, by the 3D WS 2 /rGO/Pol nanostructure under UV light for 3 h. This result is compared with the same experiment performed in the dark and with the same experiments performed with pure 3D rGO and pure WS 2 .
  • Figure 2b shows that in the presence of the WS 2 /rGO/Pol compound under UV light, almost the entire amount of MB in the solution was adsorbed/degraded. This material exhibits improved efficiency compared to pure 2D WS 2 nanochips.
  • Table 3.1 presents the composition Table 3.1: Elemental composition of 3D WS 2 /rGO from Example 3, determined by EDX
  • Figure 3b shows the kinetics of the decolorization process of a 0.1 mg/mL aqueous solution of MB, with the 3D WS 2 /rGO photocatalyst of this example, in the presence of UV light.
  • a very rapid discoloration is observed in the presence of light (the MB is eliminated in less than 30 min).
  • the aqueous solution after destaining was examined by MALDI-TOF and the results are shown in Figure 3c.
  • the following degradation products are observed: nitrate, sulfur dioxide, hydrogen chloride from hydrochloric acid, pentane, benzene, thiazoline, sulfate anion, 4- or 3-acid hydroxybutanoic acid, benzene-Cl anion, 2-ethylthiazole, 4-nitrophenol, sodium benzenesulfonate hydrate.
  • C15H16N3S MB with one less -CH3
  • the kinetics of the decolorization process of the MB solution (0.1 mg/mL) is presented in Figure 4b.
  • Table 5.1 EDX results of the WS 2 /rGO/Pol system prepared at 90 o C Decolorization experiments with this material are shown in Figure 5b. The results obtained show that the compound photocatalysts are much more efficient than the 2D WS2 platelets alone. In the absence of light, discoloration occurs due to the adsorption of MB on the material. It is shown that the photocatalytic degradation of MB occurs in the presence of UV light, thanks to the integrated photocatalyst.
  • Example 6 Synthesis and characterization of WS2/rGO prepared from WS2/GO with a weight ratio of 1:4.
  • Table 6.1 shows the EDX results in which a large incorporation of WS 2 is observed in the 3D composite structure.
  • Figure 7a you can see the TEM images of the initial MoS 2 particles, which were obtained by sonication of the bulk MoS 2 crystal. 2D nanoplatelets with a lateral dimension in the range of 50 nm to 250 nm can be observed.
  • Figure 7b presents the SEM images of the 3D morphology of the MoS 2 /rGO structures.
  • Table 7.1 presents the elemental composition of the MoS 2 /rGO compounds, showing a large incorporation of the MoS2 photocatalyst.
  • Table 7.1 Elemental composition of 3D MoS2/rGO of Example 7, obtained by EDX
  • Figure 7c the kinetics of the decolorization of the aqueous MB solution is shown.
  • the MB removal efficiency is significantly improved when using 3D composites (77%, in the presence of light) compared to 2D MoS2 nanochips (11%).
  • the results show that the 3D MoS2/rGO structure presents better removal and degradation efficiency.
  • Figure 8a shows the SEM images and a porous and spongy material is observed). The pore size distribution at this scale is 1 to 10 ⁇ m.
  • Table 8.2 shows the elemental composition of MoS 2 /rGO/Pol.
  • Figure 8b shows the kinetics of the decolorization process of the aqueous MB solution using the 3D composite system compared to pure rGO and pure MoS 2 .
  • pure rGO has a high MB removal (adsorption) efficiency, rGO is only adsorbing MB, not degrading it.
  • the adsorbed MB is degraded very efficiently by the photocatalytic effect of the photocatalyst in the presence of light.
  • Other photocatalysts that have been prepared following the procedures as defined in the previous examples, except for the nanoplatelets, which were CdS: examples of CdS/rGO and CdS/rGO/Pol materials.
  • the processes carried out in the presence of UV light with the materials of the invention resulted in adsorption and photocatalytic degradation of the adsorbed agent.
  • the photocatalyst integrated into the three-dimensional structure of reduced graphene oxide demonstrated a much higher photocatalytic activity than that of the pure (non-integrated) semiconductor.
  • the integrated materials are monolithic materials, with a porous structure, easy to handle and can be reused without losing active components between cycles.

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Abstract

The present invention relates to a photocatalyst comprising a three-dimensional structure of reduced graphene oxide and nanoplatelets of at least one semiconductor, selected from a group consisting of GaN, Ce02, CdS, ZnO, MoS2, WS2, WO3, and other semiconductor materials which allow the formation of nanoplatelets by means of the exfoliation thereof, which nanoplatelets are integrated into the porous three-dimensional structure of reduced graphene oxide. Optionally, the photocatalyst comprises nanoparticles of a polymer. The preparation and uses thereof are also described.

Description

DESCRIPCION FOTOCATALIZADORES DE ÓXIDO DE GRAFENO Y SEMICONDUCTOR Campo de la invención La presente invención se encuadra dentro del campo de los fotocatalizadores, en particular se refiere a fotocatalizadores que comprenden una estructura tridimensional de óxido de grafeno reducido y plaquetas de por lo menos un semiconductor. También se refiere a los usos de los mismos y métodos para su preparación. Antecedentes de la invención La invención se refiere a un fotocatalizador integrado y en particular a un fotocatalizador que comprende un semiconductor y óxido de grafeno reducido como soporte activo unidos en una estructura monolítica y porosa. El proceso fotocatalítico está basado en la excitación de un sólido (fotocatalizador), normalmente un semiconductor de banda ancha, mediante la irradiación con luz. El proceso fotocatalítico tiene múltiples aplicaciones que incluyen, por ejemplo, tratamientos de descontaminación del medio ambiente y la síntesis de compuestos de alto valor añadido, todo ello de forma sostenible al utilizar la radiación solar. Uno de los principales retos en esta área es el desarrollo de materiales fotocatalíticos nuevos o modificados que mejoren su eficiencia operando con un mayor intervalo de longitudes de onda. Muchos de los semiconductores que se emplean como fotocatalizadores son óxidos metálicos (por ejemplo, TiO2 y ZnO) o calcogenuros (por ejemplo, CdS, ZnS, CdSe, ZnSe y CdTe). El más usado es TiO2. Los fotocatalizadores comerciales (generalmente TiO2) se venden normalmente en forma de nanopartículas en polvo. Para su uso es necesario incorporar estos fotocatalizadores a un soporte sólido. Ejemplos de nanopartículas de TiO2 comerciales son: CristalACTiVTM DT’20, Green Millenium and Microban. También se conocen algunos aparatos, por ejemplo, aparatos para la purificación del aire, que funcionan con nanopartículas de TiO2 integradas en distintos soportes. Ciertas estructuras tridimensionales de grafeno a las que se han incorporado fotocatalizadores como por ejemplo TiO2, AgX (X=Br, Cl), AgPO4, CuO, ZnO, CdS o C3N4, se conocen a través de algunas publicaciones (1) a (21). Estos materiales se limitan a estructuras tridimensionales de grafeno a las que se han incorporado fotocatalizadores en forma de nanopartículas (zerodimensional) o nanotubos (unidimensionales). El método de síntesis de estos materiales es hidrotermal, con frecuencia ejecutado a altas temperaturas (180 oC) durante periodos de tiempo prolongados (12-24 horas). La presencia de grafeno en estos compuestos proporciona, por un lado, un soporte adecuado para los fotocatalizadores y, por otro, debido a su buena aceptación de electrones y sus propiedades eléctricamente conductoras, resuelve el inconveniente principal de los materiales fotocatalíticos, la falta de una separación de cargas eficiente (Ref 22-24). Sin embargo, el rendimiento de los compuestos es insatisfactorio. Entre los objetivos de la presente invención está proporcionar un fotocatalizador mejorado, con una alta actividad fotocatalítica y que no sea necesario incorporarlo a un soporte sólido para su uso. Es también deseable proporcionar un fotocatalizador mejorado que se pueda preparar de forma sencilla y con un proceso que se pueda ejecutar en condiciones suaves, especialmente de temperatura. Resumen de la invención La invención se refiere a nuevos fotocatalizadores y en particular a fotocatalizadores que comprenden una estructura tridimensional de óxido de grafeno reducido y nanoplaquetas de por lo menos un semiconductor. En dichos fotocatalizadores, las nanoplaquetas de semiconductor están integradas en la estructura tridimensional de óxido de grafeno reducido. En el estado de la técnica no se conoce ningún fotocatalizador que comprenda una estructura tridimensional de óxido de grafeno reducido y que incorpore materiales semiconductores en forma de nanoplaquetas (estructuras bidimensionales). Los fotocatalizadores aquí descritos presentan estructuras porosas, y comprenden opcionalmente nanopartículas de por lo menos un polímero. En realizaciones particulares estos polímeros se seleccionan de un grupo formado por estirenos, acrilatos, metacrilatos, opcionalmente funcionalizados, y sus mezclas. En realizaciones particulares el semiconductor se selecciona de un grupo formado por GaN, CeO2, CdS, ZnO, MoS2, WS2, WO3 y otros materiales semiconductores que permiten la formación de nanoplaquetas por exfoliación de los mismos, preferiblemente siendo MoS2 o WS2. En realizaciones preferidas el fotocatalizador comprende WS2 o MoS2 y opcionalmente, nanopartículas de polimetil metacrilato funcionalizado. También, de acuerdo con la invención, se proporciona un método de preparación de dichos fotocatalizadores. El método de síntesis es sencillo, y se puede llevar a cabo en condiciones de reacción suaves. El método comprende: a) preparar una dispersión acuosa de nanoplaquetas de óxido de grafeno, nanoplaquetas de uno o más semiconductores y un agente reductor; y calentar la dispersión resultante, sin agitación, a una temperatura en el intervalo de 45 a 90 ºC. El agente reductor es preferentemente ácido ascórbico o hidracina. El procedimiento descrito comprende una etapa c), entre a) y b), en la que la dispersión resultante en a) se agita a temperatura ambiente durante un periodo de tiempo suficiente para homogeneizar la dispersión, típicamente durante 15 minutos a 1 hora. Después de la etapa b) el producto resultante de reacción puede secarse, por ejemplo, usando liofilización o microondas Se ha comprobado que los fotocatalizadores aquí descritos muestran propiedades catalíticas superiores a los fotocatalizadores comerciales y proporcionan ventajas adicionales. La invención también se refiere al uso de dichos fotocatalizadores. Por ejemplo, en tecnologías de protección ambiental, en la captura y/o conversión química de contaminantes del aire, agua o el suelo o como catalizadores en reacciones químicas. También en aplicaciones que requieren el uso de materiales porosos. Descripción de las Figuras Figura 1a: Imágenes de microscopia de trasmisión electrónica (TEM) de las nanoplaquetas de WS2 como indica el Ejemplo 1 Figura 1b: Imágenes de microscopía electrónica de barrido (SEM) de las estructuras tridimensionales (3D) de óxido de grafeno reducido (rGO) como indica el Ejemplo 1 Figura 1c: Imágenes SEM de nanoestructuras tridimensionales (3D) de WS2/rGO preparadas proporción de óxido de grafeno inicial (GO): ácido ascórbico (AsA) de 1: 1 en peso como indica el Ejemplo 1 Figura 1d: Cinética del proceso de decoloración (Cambio de la concentración de azul de metileno (MB) con el tiempo); 3D rGO puro con luz ultravioleta (UV); 2D WS2 puro con luz UV; 3D WS2/rGO con luz UV y 3D WS2/rGO D (D = en la oscuridad) (T = 60ºC, GO: AsA = 1: 1 en peso) como indica el Ejemplo 1 Figura 1e: Resultados del análisis de espectrometría de masas MALDI TOF [(Matrix-Assisted Laser Desorption/Ionization (desorción/ionización láser asistida por matriz) / Time-Of-Flight)] de una solución acuosa de MB, después de la reacción de decoloración de MB con 3D WS2/rGO y luz UV como indica el Ejemplo 1, con asignación de algunos de los picos Figura 1f: Resultados de análisis de espectrometría de masas MALDI-TOF de la solución de MB acuosa después de decoloración hecha con doble cantidad de fotocatalizador 3D WS2/rGO (2 mg de fotocatalizador por 0.1 mg MB) como indica el Ejemplo 1 con asignación de estructuras a los picos. Figura 1g: Resultados de análisis de espectrometría de masas MALDI-TOF de la solución de MB acuosa después de decoloración hecha con 3D WS2/rGO y luz UV (línea completa) y en la oscuridad (línea en puntos) como indica el Ejemplo 1. El pico característico de MB está marcado. Figura 1h: Resultados de análisis de espectrometría de masas MALDI-TOF de la solución de etanol obtenida en proceso de desorción de material 3D WS2/rGO después de la reacción de degradación de MB con luz UV como indica el Ejemplo 1 Figura 2a: Imágenes SEM de 3D WS2/rGO/Pol [Pol = polímero] preparado a partir de WS2/GO con una relación en peso de 1: 4, Temperatura (T) = 60ºC, con una proporción de óxido de grafeno inicial (GO): ácido ascórbico (AsA) de 1: 1 en peso y 20% en peso de polímero en relación con la cantidad en peso de WS2/GO, como indica el Ejemplo 2 Figura 2b: Cinética del proceso de decoloración (Cambio de la concentración de MB con el tiempo); 3D rGO puro con luz UV; 2D WS2 puro con luz UV; 3D WS2/rGO/Pol con luz UV y 3D WS2/rGO/Pol D como indica el Ejemplo 2 Figura 2c: Resultados de análisis de espectrometría de masas MALDI-TOF de la solución acuosa de MB después de la decoloración con 3D WS2/rGO/Pol como indica el Ejemplo 2, con asignación de estructuras a los picos. El pico característico de MB está marcado. Figura 2d: Resultados de análisis de espectrometría de masas MALDI-TOF de la solución acuosa de MB después de la decoloración con 3D WS2/rGO/Pol bajo luz UV (línea continua) y en la oscuridad (línea discontinua) como indica el Ejemplo 2. El pico característico de MB está marcado. Figura 3a: Imágenes SEM de 3D WS2/rGO preparado a partir de WS2/GO 1: 3 en peso, Temperatura (T) = 60ºC, con una proporción de óxido de grafeno inicial (GO): ácido ascórbico (AsA) de 1: 1 en peso, como indica el Ejemplo 3 Figura 3b: Cinética del proceso de decoloración (Cambio de la concentración de MB relativa con el tiempo); 3D rGO puro con luz UV, 2D WS2 puro con luz UV, 3D WS2/rGO con luz UV y 3D WS2/rGO D en la oscuridad como indica el Ejemplo 3 Figura 3c: Resultados de análisis de espectrometría de masas MALDI-TOF de la solución acuosa de MB después de decoloración con 3D WS2/rGO como indica el Ejemplo 3, con asignación de los picos Figura 4a: Imágenes SEM de 3D WS2/rGO preparado a partir de WS2/GO con una relación en peso de 1: 4, Temperatura (T) = 90ºC, con una proporción de óxido de grafeno inicial (GO): ácido ascórbico (AsA) de 1: 1 en peso, como indica el Ejemplo 4 Figura 4b: Cinética del proceso de decoloración (Cambio de la concentración de MB relativa con el tiempo); 3D rGO puro con luz UV, 2D WS2 puro con luz UV, 3D WS2/rGO con luz UV y 3D WS2/rGO D en la oscuridad como indica el Ejemplo 4 Figura 4c: Resultados de análisis de espectrometría de masas MALDI-TOF de la solución acuosa de MB después de decoloración con 3D WS2/rGO con luz como indica el Ejemplo 4, con asignación de los picos Figura 5a: Imágenes de SEM de WS2/rGO/Pol preparado a partir de WS2/GO con una relación en peso de 1: 4, Temperatura (T) = 90ºC, con una proporción de óxido de grafeno inicial (GO): ácido ascórbico (AsA) de 1: 1 en peso y 20% en peso de polímero en relación a la cantidad en Figura 5b: Cinética de la decoloración de la solución acuosa de MB (concentración relativa de MB frente al tiempo) de los sistemas: 3D rGO puro con luz; 2D WS2 puro con luz UV; y 3D WS2/rGO/Pol bajo luz UV y 3D WS2/rGO/Pol D (D: en la oscuridad), como indica el Ejemplo 5. Figura 6a: Imágenes de SEM de 3D WS2/rGO preparado a partir de WS2/GO con una relación en peso de 1: 4, Temperatura (T) = 60ºC, y con una proporción de óxido de grafeno inicial (GO): ácido ascórbico (AsA) de 1: 0.5 en peso, como indica el Ejemplo 6 Figura 6b: Cinética de la decoloración de la solución acuosa de MB (cambio de la concentración de MB relativa al tiempo) de los sistemas: 3D rGO puro bajo luz UV; 2D WS2 bajo luz UV; y 3D WS2/rGO bajo luz UV y 3D WS2/rGO D en la oscuridad, como indica el Ejemplo 6. Figura 6c: Resultados de análisis de espectrometría de masas MALDI-TOF de la solución acuosa de MB después de decoloración con el sistema 3D WS2/rGO con luz como indica el Ejemplo 6, con indicación de los grupos químicos específicos en diferentes áreas de los espectros. Figura 7a: Imágenes de microscopia de trasmisión electrónica (TEM) de las nanoplaquetas de MoS2 como indica el Ejemplo 7 Figura 7b: Imágenes de SEM de 3D MoS2/rGO preparado a partir de MoS2/GO con una relación en peso de 1: 4, Temperatura (T) = 60ºC, con una proporción de óxido de grafeno inicial (GO): ácido ascórbico (AsA) de 1: 1 en peso, como indica el Ejemplo 7. Figura 7c: Cinética de la decoloración de la solución MB (cambio de la concentración de MB en el tiempo) de los sistemas 3D rGO puro con luz UV, 2D MoS2 puro con luz UV, 3D MoS2/rGO bajo luz UV y 3D MoS2/rGO D en la oscuridad como indica el Ejemplo 7. Figura 7d: Espectros de masas MALDI-TOF de solución desorbida en etanol para el sistema 3D MoS2/rGO del Ejemplo 7. Figura 8a: Imágenes de SEM de 3D MoS2/rGO/Pol preparado a partir de MoS2/GO con una relación en peso de 1: 4, Temperatura (T) = 60ºC, con una proporción de óxido de grafeno inicial (GO): ácido ascórbico (AsA) de 1: 1 en peso y 20% en peso de polímero en relación a la cantidad en peso de WS2/GO, como indica el Ejemplo 8. Figura 8b: Cinética de la decoloración de la solución acuosa de MB (cambio de la concentración de MB relativa al tiempo) por 3D rGO puro, 2D MoS2 puro, 3D MoS2/rGO/Pol bajo luz y 3D MoS2/rGO/Pol D en la oscuridad, como indica el Ejemplo 8. ción detallada de la invención Los materiales de la presente invención son fotocatalizadores integrados que comprenden una estructura tridimensional de óxido de grafeno reducido (rGO) en el que están incorporadas las nanoplaquetas bidimensionales de un semiconductor. Esta estructura monolítica, tridimensional, porosa, es una estructura estable que funciona como un fotocatalizador integrado. Esto resulta ventajoso porque el material funciona como fotocatalizador sin necesidad de tener que incorporarlo a un soporte, como es el caso de los catalizadores comerciales. Se ha observado además que en comparación con ciertos aparatos fotocatalíticos integrados, en los que el material que se usa como soporte solamente actúa como tal para el semiconductor sin participar en la actividad catalítica, en el caso del fotocatalizador de la presente invención se observa sinergia entre la actividad fotocatalítica de las nanoplaquetas del semiconductor y del óxido de grafeno reducido que actúa como soporte activo. Dicha sinergia resulta en un desplazamiento de la absorbancia de luz hacia el rango de luz visible, la reducción del ancho de banda e incluso, en una mejor eficacia de separación de cargas. Sin ánimo de ceñirse a ninguna teoría en particular, se asume que estos efectos se acentúan debido a la estructura en forma de plaquetas del semiconductor, que resulta en una mayor interfase entre el óxido de grafeno reducido y el semiconductor, una mayor interacción entre el óxido de grafeno reducido y el semiconductor y un aumento del rendimiento catalítico. Otras ventajas del fotocatalizador de la invención incluyen mejor rendimiento, mayor durabilidad, la posibilidad de reutilizar el catalizador y la reducción de la pérdida de compuesto activo. Como resultado de la buena interacción entre el semiconductor y óxido de grafeno reducido, que facilita la separación de cargas, se ha notado la mejora en la eficacia del catalizador integrado. Cuando el semiconductor absorbe el fotón de la luz UV que lleva más energía que la banda de energía de la brecha, se genera un par electrón-hueco en las nanoplaquetas del semiconductor que son capaces de empezar procesos de óxido-reducción. Pero el par electrón-hueco puede tener un proceso de recombinación y volver al estado aumenta la eficacia del semiconductor como fotocatalizador. Que el semiconductor esté incorporado a las estructuras 3D de rGO o rGO/polímero hace que sea posible reutilizar el fotocatalizador fácilmente, después de la limpieza. Además, al estar el semiconductor inmovilizado, se reduce la pérdida del material activo durante la limpieza. Los materiales de la presente invención tienen una estructura macroscópica tridimensional y son materiales porosos, con un aspecto tipo esponja. Tienen también una gran capacidad de adsorción de moléculas y/o de partículas orgánicas o inorgánicas. La adsorción selectiva de ciertos materiales de interés se puede ajustar mediante el control de las propiedades físicoquímicas. Además, cuando se expone a la luz (UV o visible), el catalizador se activa y es capaz de degradar, por procesos de óxido-reducción, los materiales adsorbidos en su superficie. Los fotocatalizadores (semiconductores) comerciales suelen ser productos en polvo que tienen que ser integrados en el soporte adecuado antes de su uso. Tanto el proceso de inmovilización o integración del fotocatalizador en el soporte, como los soportes en sí, varían según las distintas aplicaciones. La principal ventaja de los materiales de la presente invención es que permite la unión del fotocatalizador (semiconductor) y el soporte dentro de una misma estructura. Los materiales son además versátiles, con muchas aplicaciones distintas. El material de la invención tiene unas propiedades muy ventajosas en cuanto a porosidad, resistencia mecánica y térmica, forma y tamaño, las cuales además pueden modularse. El óxido de grafeno (GO) que se utiliza para preparar el fotocatalizador de la invención es óxido de grafeno en forma de nanoplaquetas. El óxido de grafeno en forma de nanoplaquetas puede adquirirse de forma comercial o prepararse de acuerdo a métodos conocidos en el estado de la técnica. Habitualmente las nanoplaquetas se adquieren en forma de dispersión, tal como en dispersión acuosa. En una realización particular las nanoplaquetas se suministran en dispersión en agua con concentraciones que típicamente oscilan en el rango de 0.1 a 5% en peso, preferiblemente 0.4% - 1% en peso con respecto al peso total de la dispersión suministrada con contenido en monocapa de >90%, preferiblemente >92%, o >95%, o >97%. El óxido de grafeno de partida presenta alto contenido de oxígeno de entre 25% a 55% en peso, particularmente de 30 a 50% en peso, o de 40% a 50% en peso, o de 45% a 50% en peso con respecto al peso total del óxido de grafeno. En una realización particular de las nanoplaquetas de una capa de espesor igual o inferior a 1 nm. Las nanoplaquetas que se utilizan en la presente invención se caracterizan también por su dimensión lateral. Las dimensiones laterales de las mismas pueden oscilar entre 500 nm y 30 micras, por ejemplo, entre 1 micra y 20 micras, tal como 10 micras En una realización particular las nanoplaquetas de óxido de grafeno de partida presentan un contenido de oxígeno de 30 a 50% en peso, el 93 al 95% de las mismas son nanoplaquetas de una capa de espesor igual o inferior a 1 nm, y su dimensión lateral es de 10 micras. El término “semiconductor”, según se usa aquí se refiere a un material semiconductor con características fotocatalíticas. Ejemplos ilustrativos, pero no limitantes de semiconductores que se pueden usar en la preparación de los fotocatalizadores de la invención son GaN, CeO2, CdS, ZnO, MoS2, WS2, WO3 y otros semiconductores que tienen estructura en capas y que son por lo tanto aptos para la formación de nanoplaquetas por exfoliación. Los semiconductores preferidos incluyen CdS, MoS2 y WS2. Especialmente preferido es WS2. Otros semiconductores que se pueden usar para preparar los fotocatalizadores de la invención incluyen: (i) W: ditelururo de tungsteno (WTe2) y diselenuro de tungsteno (WSe2), ii) Mo: telururo de molibdeno (MoTe2) y diselenuro de molibdeno (MoSe2), iii) Cd: seleniuro de cadmio (CsSe), iv) In: sulfuro de indio (III) In2S3) y seleniuro de indio (III) (In2Se3), v) Ga: seleniuro de galio (III) (Ga2Se3), seleniuro de galio (II) (GaSe), sulfuro de galio (II) (GaS) y telururo de galio (II) (GaTe ), vi) Bi: seleniuro de bismuto (Bi2Se3), telururo de bismuto (Bi2Te3) y sulfuro de bismuto (Bi2S3), vii) Cu: sulfuro de cobre (Cu2S), viii) Sn: sulfuro de estaño (II) (SnS) y seleniuro de estaño (SnSe) y ix) Pb: yoduro de plomo (II) (PbI2). Las nanoplaquetas de semiconductor que se pueden usar para preparar los fotocatalizadores de la invención pueden tener espesores variables de entre una o más capas atómicas. El espesor de una capa atómica varía en función del material semiconductor concreto. Por ejemplo, es de 0.65 nm para el MoS2 y de 3.7 nm para el WS2. En cuanto al espesor máximo de las mismas, el mismo dependerá del número de capas atómicas de la nanoplaqueta. En la presente invención generalmente el espesor varía entre el correspondiente según el material a una capa atómica y 40 nm, o entre una capa atómica y 30 nm, o entre una capa atómica y 20 nm, o entre una capa atómica y 10 nm. Las dimensiones laterales de las nanoplaquetas de semiconductor también pueden variar entre un valor superior a 40 nm y 3 micras con la condición de que la dimensión lateral realizaciones particulares las dimensiones laterales están comprendidas entre 50 nm y 1 micra, o entre 100 nm y 500 nm micras, o entre 200 nm y 400 nm. En una realización particular las nanoplaquetas presentan espesores de entre 0.5 y 8 nm, y dimensión lateral de entre 25 y 300 nm, o espesores de entre 0.5 y 8 nm y de dimensión lateral de entre 50 y 250 nm, o 100 y 200 nm. En otras realizaciones el espesor está entre 0.5 y 4 nm y la dimensión lateral entre 25 y 300 nm, o 50 y 250 nm, o 100 y 200 nm. El fotocatalizador de la invención puede comprender nanoplaquetas de uno o más materiales semiconductores. Las nanoplaquetas utilizadas para una realización particular pueden a su vez tener estructuras y características diferentes. La composición del fotocatalizador de la invención puede variar en un amplio rango. Análisis de la composición elemental determinada a partir de los resultados de EDX demuestran la incorporación de nanoplaquetas de semiconductor en la estructura 3D compuesta. Imágenes SEM de fotocatalizadores muestran la nanoestructura tridimensional y porosa. La proporción en peso del óxido de grafeno de partida con respecto al peso del semiconductor de partida [GO/semiconductor] está típicamente en el rango de 1: 1 a 9: 1. Algunas proporciones particulares están dentro del rango 2:1 a 8:1, por ejemplo 3:1, 4:1, 5:1; 6:1, o 7:1. Una proporción más preferida es 4:1. De acuerdo con realizaciones de la invención, se ha observado que puede resultar ventajoso incorporar nanopartículas de un polímero al fotocatalizador. La incorporación de dichas nanopartículas de polímero ayuda a mejorar las propiedades físico-químicas del producto y el rendimiento mecánico, resulta en una estructura más flexible, fuerte y duradera y facilita su manipulación. Además, hace el fotocatalizador más efectivo en operaciones en ciclos. El término “polímero”, según se usa aquí se refiere a cualquier polímero compuesto de macromoléculas (generalmente orgánicas) formadas por la unión mediante enlaces covalentes de una o más unidades simples llamadas monómeros, tales como: acrilatos, metacrilatos, estirenos, etc.. Las partículas de polímero que se incorporan a la estructura del fotocatalizador se pueden producir por ejemplo por técnicas de polimerización en medio disperso (agua), como en emulsión o similar. En su caso los polímeros se pueden funcionalizar, y para ello se usan cantidades de monómeros funcionales generalmente de 1%-10 % en peso se utilizan en cantidades de entre 2 y 8%, o de entre 4 y 6%. Ejemplos de monómeros funcionales incluyen, sin estar limitados: 4-bromo estireno, acrilato de 2-cloro etilo, metacrilato de glicidilo, metacrilato de 2-hidroxietilo, acrilamida, hidrocloruro metacrilato de 2-amino etilo, ácido acrílico, sulfonato de poliestireno y similares. Ejemplos ilustrativos, pero no limitantes de polímeros que se pueden usar en la preparación de los fotocatalizadores de la invención son poliestirenos, poliacrilatos, polimetacrilatos, poliacrilamidas, poliuretanos, etc., todos ellos opcionalmente funcionalizados. “Polímero funcionalizado” se refiere a un polímero que tiene incorporados grupos funcionales. Ejemplos de grupos funcionales incluyen epoxi, amida, amina, sulfonatos, carboxílo, hidroxilo, éster, fenilo, éter, etc. Uno de los polímeros preferidos en la presente invención es polimetil metacrilato funcionalizado, preferiblemente funcionalizado con grupos epoxi. En realizaciones de la invención, el fotocatalizador comprende uno o más polímeros. Los polímeros pueden seleccionarse en función de las características que se desee presente el fotocatalizador. Los inventores han observado que la temperatura de transición vítrea (Tg) del polímero empleado influye en si éstos se tienden a colocar como nanopartículas en la superficie de rGO (cuando presentan una Tg alta) o si tienden a formar una capa recubriendo el rGO (cuando presentan una Tg baja) en el proceso de secado de los materiales 3D. Cuando el fotocatalizador comprende uno o más polímeros, la proporción entre el peso del material de partida “GO y semiconductor” y el peso del polímero, usados en la preparación de los fotocatalizadores, puede variar. En realizaciones particulares de los fotocatalizadores de la invención, éstos se han preparado con proporciones en peso de los materiales de partida (GO/semiconductor):polímero que varían desde 1:1 hasta 10:1. Algunas proporciones particulares están dentro del rango 2:1 a 9:1, por ejemplo 3:1, 4:1, 5:1; 6:1, 7:1 u 8:1. Las proporciones preferidas se encuentran en el rango 2:1 - 4:1, siendo 4:1 el rango más preferido. El método de síntesis de los fotocatalizadores de la invención es un proceso de autoensamblado que se produce durante la reacción de reducción de las nanoplaquetas de óxido de grafeno en una dispersión coloidal que contiene también las nanoplaquetas del semiconductor. La estructura porosa tridimensional se obtiene de forma espontánea en la reacción de reducción del óxido de grafeno con un agente reductor. La reacción se lleva a particular la temperatura es de entre 25 y 45 ºC. El tiempo de reacción depende de la temperatura de reacción, del tamaño del fotocatalizador a obtener, etc. En una realización particular el tiempo es de 30 minutos a 90 oC o de varias horas a 25 oC. Según el método de síntesis de la invención, se prepara una dispersión acuosa de los materiales de partida: nanoplaquetas de óxido de grafeno, nanoplaquetas de semiconductor y opcionalmente nanopartículas de polímero. La dispersión se agita para homogenizar la mezcla. Durante este periodo de agitación se comprueba que las nanoplaquetas de semiconductor y las nanopartículas de polímero se adsorben sobre la superficie de las nanoplaquetas de óxido de grafeno. Después se fija la temperatura dentro del rango mencionado anteriormente y se añade el agente reductor (por ejemplo, ácido ascórbico (Asa) o hidracina) cuando se alcanza la temperatura necesaria. Desde este momento se detiene la agitación para que se produzca el proceso de autoensamblado. El proceso de reducción del oxido de grafeno, gracias al agente reductor, convierte el material de óxido de grafeno en un material mucho más hidrofóbico e incompatible con el medio acuoso en el que se lleva a cabo la reacción. Como consecuencia, las nanoplaquetas de óxido de grafeno reducido que se producen, empiezan a autoensamblarse espontáneamente formando así los materiales porosos tridimensionales (3D) en forma de estructura monolítica. “Estructura monolítica” o “monolito” según se usa aquí, se refiere a un material compuesto de más de un tipo de material o componente, en el que los componentes forman una estructura única sin puntas de unión, que está constituido en una sola unidad. El material así producido es un hidrogel hinchado con gran cantidad del agua. El fotocatalizador de la invención se puede utilizar en forma de hidrogel o en forma seca, después de un proceso de secado. Pero antes que secar se tiene que limpiar con agua destilada por proceso de diálisis. Para el secado se puede utilizar cualquiera de los métodos conocidos de secado de hidrogeles. Estos incluyen, por ejemplo, liofilización, microondas o secado convencional en horno. En un aspecto preferido, el secado se lleva a cabo con liofilización, para preservar la estructura porosa y el volumen del monolito. Se puede secar también usando microondas, por ejemplo, con un programa de velocidad de calentamiento muy alto, en el que con el microondas a una potencia de 1400W, se aumenta la temperatura en el reactor hasta 200ºC por minuto y se deja actuar a esa temperatura durante unos de la invención. El fotocatalizador en forma de sólido liofilizado es otro aspecto preferido de la invención. El término “agente reductor” o “reductor”, según se usa aquí se refiere a cualquier agente reductor conocido, es decir a un agente capaz de ceder electrones en una reacción de óxido- reducción según la invención. Ejemplos ilustrativos, pero no limitantes de agentes reductores que se pueden usar en la preparación de los fotocatalizadores de la invención son: (i) Borohidruros (por ejemplo, NaBH4), (ii) Hidruro de litio y aluminio (LiAlH4), (iii) Halogenuros de hidrógeno (por ejemplo, HI), (iv) Agentes reductores que contienen azufre (por ejemplo, dióxido de tiourea/NaOH), (v) agente reductor que contiene nitrógeno (por ejemplo, hidracina), (vi) Agentes reductores que contienen oxígeno (por ejemplo, alcohol isopropílico), (vii) Metal-ácido (por ejemplo, Fe HCl), (viii) Metal-base (por ejemplo, Zn/NaOH), (ix) Aminoácido (por ejemplo, glicina ), etc. Preferidos son hidracina y ácido ascórbico (AsA). El más preferido es el ácido ascórbico ya que se disuelve fácilmente en agua, no es tóxico y la reducción se produce rápidamente. Los parámetros de reacción pueden variar y el proceso se puede controlar cambiando los parámetros, tales como el tipo o la cantidad de los agentes reductores o la temperatura del proceso. Hay parámetros que se pueden cambiar y que influyen en el proceso de formación de las estructuras y en las propiedades de las mismas. Estos incluyen: la concentración y el tipo de óxido de grafeno, de agente reductor, de polímero, y el semiconductor; la temperatura y el tiempo de la reducción. Como se ha indicado antes, es importante que el óxido de grafeno de partida esté muy oxidado. Durante el proceso de reducción, los grupos funcionales que contienen oxígeno se pierden rápido, y este proceso proporciona las condiciones necesarias para que se forme la estructura. La cantidad de óxido de grafeno presente en el medio de reacción para que se formen las estructuras puede variar. En realizaciones particulares la cantidad de GO está entre 1 mg/mL y 20 mg/mL, o entre 2 mg/mL y 15 mg/mL, o entre 4 mg/mL y 10 mg/mL, preferiblemente entre 5 mg/mL y 6 mg/mL y más preferiblemente es de aproximadamente 4 mg/mL. Con respecto a las propiedades de las estructuras, la concentración de óxido de grafeno en el medio de reacción determina las dimensiones de la estructura formada. Una mayor concentración de las dispersiones en agua resulta en la formación de estructuras de mayor volumen. La temperatura a la que se reduce el óxido de grafeno (25º - 90ºC) es un parámetro que determina la velocidad de la reacción de reducción y, como consecuencia, las propiedades de las estructuras formadas. A bajas temperaturas, la reacción de reducción, y por lo tanto la reacción de formación de las estructuras, es más lenta, y a temperaturas más altas, las estructuras se forman a mayor velocidad. En cuanto al agente reductor, la cantidad y el tipo de agente son importantes para la reacción de reducción. Estos parámetros afectan a la velocidad de la reacción, que a su vez influye en las propiedades de la estructura formada. La concentración del agente reductor afecta sobre todo a las propiedades de las estructuras. Las proporciones en peso de GO:agente reductor puede ser en realizaciones particulares baja (1:0.5), media (1:1) o alta (1:2). A proporciones más bajas (por ejemplo cuando la proporción de GO:agente reductor es 1:0.5) el proceso de formación es más lento y la estructura resultante es menos compacta y tiene menor resistencia mecánica, presenta más cantidad de macroporos, es decir, poros con un diámetro mayor que 50 nm, y es más rica en grupos funcionales de oxígeno, lo que puede ser importante para algunas aplicaciones. En cuanto al efecto de la temperatura en las propiedades de las estructuras, a temperaturas más bajas (25-45ºC) la estructura formada es más débil mecánicamente, pero más rica en grupos funcionales de oxígeno. La temperatura y la cantidad de agente reductor, en combinación, determinan las propiedades de la estructura, especialmente las propiedades mecánicas, la porosidad y la distribución de poros, y también la química de la superficie de las estructuras, ya que determina el nivel de reducción y la cantidad de grupos funcionales de oxígeno presentes. Con el objeto de ahorrar energía y producir materiales con baja demanda de energía es de interés producirlos a temperaturas más bajas (25-45ºC). Variando la cantidad de polímero se puede compensar la reducción en las propiedades mecánicas. El tipo de polímero y el tipo de semiconductor pueden influir en el proceso de autoensamblado, ya que al adsorberse en las plaquetas de óxido de grafeno, influyen en la movilidad y la afinidad de ensamblado. También pueden afectar las propiedades finales. Por ejemplo, cambiando el semiconductor o usando una combinación de dos o más semiconductores, se puede optimizar la actividad fotocatalítica, la absorción de la luz y la banda de la energía de brecha. Los polímeros son componentes inertes que, aunque en general no influyen en la actividad fotocatalítica, pueden mejorar las propiedades fisicoquímicas y contribuir a la formación de un producto con mejor consistencia y resistencia mecánica, mayor duración y capacidad de reutilización. En el proceso de producción de las estructuras, la mezcla inicial de los materiales de partida se lleva a cabo con agitación magnética (entre 100 y 300 rpm, por ejemplo, a 150 rpm) para homogenizar la mezcla. Sin embargo, a continuación, es esencial para la formación de las estructuras de la invención, detener la agitación. El tiempo que debe permanecer entonces la mezcla reaccionado depende de la temperatura. A temperaturas bajas (25-45ºC) el proceso puede necesitar entre 24h-48h mientras que a temperaturas altas (60-90ºC) el proceso puede completarse en 2-4 h. Otra ventaja del proceso de la invención es que no genera productos secundarios y por lo tanto genera una menor cantidad de residuos. Es además un proceso respetuoso con el medio ambiente ya que se puede llevar a cabo en dispersiones acuosas, a temperaturas suaves y que se puede llevar a cabo con reactivos inocuos como por ejemplo usando el ácido ascórbico como reductor. El método de la invención es fácil, versátil, respetuoso con el medio ambiente, de bajo coste y apto para producción a gran escala. La preparación de las nanoplaquetas de semiconductor no se restringe a un procedimiento particular. Por el contrario, todos los métodos preparativos conocidos pueden utilizarse para preparar las nanoplaquetas utilizadas en el proceso de la invención. Estos incluyen métodos químicos (Eda et al., 2011), intercambio iónico (Browning et al., 2015), evaporación en polvo (PV) (Najmaei et al., 2013), deposición química en fase vapor (CVD) (Eichfeld et al., 2015), deposición de capa atómica (ALD) (Song et al., 2013), epitaxia de haces moleculares (MBE) (Liu et al., 2015), exfoliación térmica, electroquímica y dispersión acuosa. Preferiblemente, las nanoplaquetas del semiconductor se preparan por un método de exfoliación, como por ejemplo con ultrasonido. Otros posibles métodos de exfoliación que se pueden usar en la invención incluyen alta temperatura o el uso de disolventes. Preferiblemente el proceso de exfoliación se lleva a cabo a partir de una dispersión acuosa del material semiconductor. El término “tridimensional” o “3D” como se usa aquí, se refiere a una estructura geométrica de los materiales, en la que cada uno de los puntos que la configuran está caracterizado por tres dimensiones, por ejemplo, ancho, altura y profundidad. El término “bidimensional” o “2D” en el contexto de la invención, se refiere a materiales tipo plaquetas y nanoplaquetas que se caracterizan mediante su espesor y una dimensión lateral la cual siempre es mayor que el espesor. El término “nanopartículas poliméricas” en el contexto de la invención pueden tener un diámetro medio de entre 0.5-1000 nm, por ejemplo, entre 1-500 nm, o entre 1 y 250 nm. El término “integrado” se refiere a que las nanoplaquetas del semiconductor o semiconductores están incorporadas a la estructura tridimensional del óxido de grafeno reducido, repartidas de forma uniforme, homogénea en la estructura. “Temperatura ambiente” se refiere a una temperatura de 25ºC. Los materiales de la invención tienen muchas aplicaciones, sobre todo en tecnologías de protección medioambiental, en la captura y/o conversión química de contaminantes del aire, agua o el suelo o como catalizadores en reacciones químicas. Posibles usos incluyen: adsorción y degradación de contaminantes orgánicos o inorgánicos del agua, aire o tierra, por ejemplo, metales pesados, petróleo, antibióticos, colorantes o tintes orgánicos y otros contaminantes, descontaminación de los suelos para eliminar pesticidas y otros contaminantes.; captura y conversión fotocatalítica del CO2, disociación fotocatalítica del agua para producir oxígeno e hidrógeno, captura y degradación de productos nocivos del tabaco, etc. Por su estructura porosa, los materiales de la invención tienen utilidad en aplicaciones que requieren el uso de materiales porosos. Posibles aplicaciones incluyen entre otras: separación y almacenamiento de gas, agentes de encapsulación en sistemas de administración de fármacos, catalizadores o soportes para catalizadores, materiales para electrodos y para almacenamiento de energía, soportes para inmovilización de biomoléculas (producción de biocatalizadores híbridos) y andamios celulares materiales para uso en cromatografía en Ejemplos Los ejemplos siguientes ilustran la invención. Materiales y métodos Materiales El óxido de grafeno de partida (GO) se obtuvo de Graphenea, en forma de dispersión acuosa de 4mg/mL y 10 mg/mL, con contenido en monocapa de >95%, y pH entre 2.2-2.5. El análisis elemental del GO mostraba C: 49-56%, H: 0-1%, N: 0-1%, S: 2-4% and O: 41-50%. Acido L-Ascórbico (AsA) se usó como agente reductor (≥99.0%, Sigma-Aldrich). Para la síntesis de las nanoplaquetas 2D, se usaron seis materiales semiconductores diferentes: i) nitruro de galio(III) - GaN, (99.99%, Alfa Aesar) con un máximo de 0.01% de impurezas metálicas; ii) sulfuro de tungsteno(IV) -WS2 (99%, Sigma-Aldrich), con tamaño de partícula 2µm; iii) óxido de zinc - ZnO (99.99%, Sigma-Aldrich) con análisis de elementos traza ≤ 150.0 ppm; iv) sulfuro de cadmio - CdS (Sigma-Aldrich); v) óxido de cerio(IV) - CeO2 (99.9%, Sigma-Aldrich), con tamaño de partícula <5µm, con análisis de trazas de tierras raras ≤ 1500.0 ppm; vi) sulfuro de molibdeno(IV) - MoS2 (99%, Sigma-Aldrich), con tamaño de partícula <2µm. Metacrilato de metilo, grado técnico (MMA, Quimidroga) y metacrilato de glicidilo (GMA) (≥97.0%, Sigma-Aldrich), se usaron sin purificación. Como iniciador radical soluble en agua se utilizó persulfato de potasio (KPS), como surfactante aniónico se utilizó ll dodecilsulfato sódico (SDS) y como tampón y para controlar la viscosidad de microemulsión por reducción de las interacciones electrostáticas se usó bicarbonato sódico (NaHCO3) todos obtenidos de Aldrich. Como medio de polimerización se utilizó agua desionizada. Para los experimentos de adsorción se usó azul de metileno (MB) (≥ 82% en peso contenido de colorante) de Sigma- Aldrich. Síntesis de nanopartículas poliméricas La preparación de partículas de PMMA (Polimetacrilato de metilo) funcionalizado con grupos epoxi, se llevó a cabo mediante polimerización en emulsión de mezcla monomérica de MMA/GMA en una relación en peso de 90/10 (Se pueden usar distintos monómeros funcionales en distintas cantidades y así obtener nanopartículas de polímeros funcionalizados con diversos grupos). La tabla 1 muestra un ejemplo de una formulación para sintetizar una dispersión acuosa de partículas de polímero con un contenido de sólidos del 20% en peso respecto al peso total de la dispersión resultante. Tabla 1. Formulación para la preparación de MMA/GMA 90/10 % en peso
Figure imgf000019_0001
La reacción se llevó a cabo en un reactor de 250 ml, equipado con un rotor de acero inoxidable, un condensador y una entrada de nitrógeno. La solución acuosa del iniciador KPS y NaHCO3 se preparó aparte y se dejó homogeneizar a temperatura ambiente. La emulsión se preparó mezclando las dos fases, la fase orgánica de mezcla monomérica y la fase acuosa de disolución de SDS en agua. La emulsión se añadió al reactor y se agitó a temperatura ambiente durante 15 minutos. Posteriormente se calentó el reactor a 70 oC, se añadió de una vez la disolución de KPS/ NaHCO3 y se dejó polimerizar bajo atmosfera de nitrógeno durante 1.5 horas. En la mezcla resultante de partículas de polímero en agua la conversión de monómeros era de 99% con diámetro medio de partícula de 70 nm. Preparación de nanoplaquetas de semiconductor Las nanoplaquetas bidimensionales (2D) se prepararon a partir de cristales de los respectivos materiales semiconductores (MoS2, WS2, CeO2, CdS, GaN, or ZnO). Los cristales se trituraron en un mortero hasta tener un tamaño de grano de menos de 2 ^m. Se preparó una dispersión del polvo resultante en agua/etanol y se sometió a sonicación con agitación durante 1 h a 360 W (1 seg encendido y 2 seg apagado) con un sonicador de punta Branson 450 (Danbury, CT). El matraz se mantuvo frío en un baño de agua-hielo durante la sonicación. La dispersión resultante se usó directamente en la preparación de materiales 3D. El análisis y la caracterización estructural de las nanoplaquetas se llevó a cabo mediante microscopía electrónica de trasmisión (TEM). Síntesis de estructuras tridimensionales (3D) de óxido de grafeno reducido (rGO)/plaquetas bidimensionales (2D) y de rGO /plaquetas 2D/polímero Una cantidad adecuada de dispersión de óxido de grafeno GO se mezcló con una dispersión de nanoplaquetas 2D de semiconductor. Cuando la estructura incluye un polímero, es decir, para la síntesis del material rGO /nanoplaquetas 2D/polímero se añadió a la dispersión de GO:nanoplaquetas 2D, una proporción adecuada de partículas poliméricas. La proporción entre el peso del material GO/2D y el peso del polímero puede variar y varias proporciones se han usado, aunque en un caso particular la proporción de 4 a 1. En este momento se añadió la proporción adecuada de agente reductor, en este caso el ácido ascórbico, en tres cantidades respecto al peso de óxido de grafeno (GO:AsA 1:0.5; 1:1; y 1:2). La mezcla se agitó durante 0.5 hora para homogenizar. Finalmente, la mezcla homogénea de GO/2D nanoplaquetas/AsA o de GO/2D nanoplaquetas/polímero/AsA se calentó en un horno (estufa de vacío, Binder, Tuttlingen, Germany) y se mantuvo durante toda la noche a la temperatura elegida (45 oC, 60 oC o 90 oC) para la formación de los materiales de nanoestructura tridimensional. Estos materiales son hidrogeles y están altamente hinchados de agua. Una vez fuera del horno, el material mojado formado se lavó mediante diálisis en agua Mili Q durante varios días, cambiando el agua diariamente hasta que la conductividad del agua era constante (menos de 10 µS/cm). A continuación, la estructura 3D se secó mediante liofilización para dar el material monolítico 3D. Caracterización La morfología de los materiales 3D secados obtenidos, se examinó mediante microscopia electrónica de barrido (SEM or EDX): modelo de mesa Hitachi TM3030, voltaje de aceleración de 15kV, después de cubrir las muestras con una capa fina de oro. En las tablas de los resultados, determinada por análisis EDX, se presenta la composición elemental de los materiales en % en peso Para determinar el tamaño y la forma de las nanoplaquetas 2D de seminconductor exfoliadas se utilizó microscopia de trasmisión electrónica (TEM). Una solución diluida (1 % en peso) en agua fue depositada en malla de cobre de 300 mesh. La caracterización se llevó a cabo con un microscopio Tecnai G220 Twin (FEI) a un voltaje de aceleración de 200 keV en modo de imagen de campo claro. Demostración de la actividad fotocatalítica Para examinar la actividad fotocatalítica de los materiales 3D, se eligió una molécula aromática de color azul (azul de metileno orgánico (MB)). Este compuesto se usa como tinte en la industria textil y las aguas residuales contienen generalmente este colorante, causando serios problemas en la vida acuática y cambios en el medio ambiente. La adsorción y degradación de MB disuelto en soluciones acuosas es por lo tanto muy importante y de gran utilidad medioambiental. Los materiales porosos tridimensionales 3D, rGO, rGO/nanoplaquetas 2D y de rGO/polímero/nanoplaquetas 2D, y también las nanoplaquetas 2D en dispersión acuosa, se añadieron a la disolución acuosa de MB con concentración de 0.1 mg/ml. Los materiales 3D de óxido de grafeno son muy buenos adsorbentes de moléculas orgánicas, por ello, las soluciones acuosas perdieron rápidamente el color en el caso de todos los materiales 3D, incluso con los que no contenían nanoplaquetas de semiconductor. Sin embargo, se esperaba que los materiales 3D que contenían nanoplaquetas de semiconductor fueran capaces de degradar las moléculas de MB adsorbidas. Los experimentos fotocatalíticos se llevaron a cabo en una cámara equipada con 20 lámparas de luz UV de 15 W cada una, (modelo BS 03, Dr. Gröbel UV-Elektronik GmbH). La irradiancia de la luz UV es de 7 mW/cm2. Para el experimento se colocó en la cámara UV el reactor que contenía la disolución acuosa de MB a la que se había añadido 1 mg del material 3D por cada 0,01 mg de MB. Para distinguir entre los mecanismos de adsorción y degradación del MB, se compararon los procesos en los que se habían usado los materiales 3D de rGO/nanoplaquetas 2D y formados de rGO. En este caso (solo rGO) se asumía que solo se produciría adsorción de MB en el material 3D. Para seguir la decoloración de la solución acuosa (desaparición de MB de ella), se utilizó espectroscopia UV. Para analizar la solución acuosa de MB y en el caso de degradación, para analizar los productos obtenidos, se utilizó espectrometría de masas MALDI-TOF. Las medidas de absorción UV-Vis se hicieron en un espectrómetro Shimadzu (modelo UV- 2550230 V). Las medidas en el rango 300–800 nm se llevaron a cabo a temperatura ambiente. Todas las soluciones se diluyeron a 1 ml de agua mili-Q, usando 50 µl de la disolución de azul de metileno (MB) inicial y/o para la solución de cada experimento de fotocatálisis. Las medidas de MALDI-TOF-MS se llevaron a cabo en un instrumento Bruker Autoflex Speed (Bruker, Germany) equipado con un láser de 355 nm Nd:YAG. Los espectros se obtuvieron en modo reflector de ion positivo y modo lineal (voltaje de aceleración de 20 kV, presión 5 x 10- 6 mbar). Muestras de aproximadamente 0.5 μL de solución resultante de los experimentos de decoloración fueron analizadas. Para cada espectro se acumularon 10000 disparos de láser. Resultados Caracterización de los sistemas fotocatalíticos 3D y su actividad fotocatalítica Ejemplo 1: WS2/rGO, preparado a partir de WS2/GO con una relación en peso de 1:4. Condiciones de síntesis: T = 60ºC, GO: AsA = 1:1 en peso Ejemplo 2: WS2/rGO preparado a partir de WS2/GO con una relación en peso de 1:4. Condiciones de síntesis: T = 60ºC, GO: AsA = 1:1 en peso y 20% en peso de polímero con relación a WS2/GO Ejemplo 3: WS2/rGO, preparado a partir de WS2/GO con una relación en peso de 1:3. Condiciones de síntesis: T = 60ºC, GO: AsA = 1:1 Ejemplo 4: WS2/rGO, preparado a partir de WS2/GO con una relación en peso de 1:4, a alta temperatura. Condiciones de síntesis: T = 90ºC, GO: AsA = 1:1 Ejemplo 5: WS2/rGO/Pol preparado a partir de WS2/GO con una relación en peso de 1:4, a alta temperatura. Condiciones de síntesis: T=90ºC, GO:AsA=1:1 y 20% en peso de polímero con relación a WS2/GO. Ejemplo 6: WS2/rGO preparado a partir de WS2/GO con una relación en peso de 1:4, con menor cantidad de AsA. Condiciones de síntesis: T=60 oC, GO:AsA=1:0.5 Ejemplo 7: MoS2/rGO preparado a partir de MoS2/GO con una relación en peso de 1:4. Condiciones de síntesis T = 60ºC, GO:AsA = 1: 1 Ejemplo 8: MoS2/rGO/Pol preparado a partir de MoS2/GO con una relación en peso de 1:4. Condiciones de síntesis: polímero: T = 60ºC, GO: AsA = 1:1 y 20% en peso de polímero con relación a MoS2/GO Ejemplo 1: Síntesis y caracterización de WS2/rGO, preparado a partir de WS2/GO con una relación en peso de 1:4 Comparación de la actividad fotocatalítica del sistema WS2/rGO con las nanoplaquetas de WS2 puro y con rGO puro Condiciones de síntesis: T = 60ºC, GO: AsA = 1:1 en peso; WS2:GO = 1: 4 en peso En la Figura 1a, se pueden ver las imágenes TEM de las nanopartículas WS2 iniciales producidas por sonicación del cristal a granel de WS2, en las que se pueden ver nanoplaquetas 2D, con dimensión lateral de entre 50 - 250 nm. Estos nanomateriales, combinados con GO, que posteriormente se redujo, produjeron 3D WS2/rGO. Las imágenes SEM de la estructura 3D hecha solo de rGO y de híbrido WS2/rGO se presentan en las Figuras 1b y 1c, respectivamente. Se obtuvieron materiales porosos en ambos casos. En la Tabla 1.1 se presentan los resultados obtenidos por el análisis EDX, mostrando la composición elemental de rGO y WS2/rGO en % en peso que demuestra la incorporación exitosa del fotocatalizador WS2 dentro de la estructura 3D rGO. Tabla 1.1: Composición elemental de las estructuras 3D rGO y WS2/rGO del Ejemplo 1, determinada por EDX.
Figure imgf000023_0001
Figure imgf000024_0001
En la Figura 1d se muestra la cinética del proceso de decoloración de la solución acuosa de azul de metileno (MB) (seguida de espectrofotometría UV). Estos resultados se obtuvieron colocando el material fotocatalítico en una solución acuosa de MB e irradiando con luz UV (368 nm). La concentración de MB era de 0.1 mg/mL y en todos los casos se añadió 1 mg de material fotocatalítico por cada 0.01 mg de MB en solución. El material 3D adsorbe el colorante MB de la solución, y en la superficie del WS2 se degrada por la fotoiniciación producida por la luz ultravioleta. El mismo experimento se realizó también sin luz ultravioleta (experimento en oscuro), en cuyo caso la decoloración de la solución acuosa de MB se produce simplemente por adsorción del MB por el material 3D. Las nanoplaquetas WS2 y la nanoestructura 3D hecha solo de rGO, se utilizaron también bajo luz ultravioleta como experimentos comparativos. Los resultados de UV de la Figura 1d muestran que, en 3 h, el rGO puro adsorbe el 82% del MB. En el caso del híbrido WS2/rGO, en la oscuridad se adsorbe el 80%. Bajo luz ultravioleta, la solución acuosa de MB se decolora por completo. La decoloración completa se produce debido a la eliminación del MB adsorbido gracias a la reacción fotocatalítica iniciada por la luz UV que activa los fotocatalizadores WS2. Los productos producidos por la degradación del MB son desorbidos desde la superficie del material 3D en la solución acuosa y analizados. También se analizó la solución acuosa. El WS2 puro (sin rGO) eliminó solo el 56% de MB. Esto demuestra que la presencia de rGO mejoró la actividad fotocatalítica del WS2 de manera importante ya que la decoloración fue más rápida (100% en 30 min) y completa. La degradación de MB se demostró mediante análisis MALDI-TOF-MS de la solución acuosa después de los experimentos de decoloración. La solución acuosa fue analizada. Los resultados en forma de espectros de masas, identificando la presencia de productos de degradación, se presentan en las Figuras 1e – 1h. En la Figura 1e, se muestran los espectros de masas de la solución acuosa después de la decoloración con 3D rGO/WS2 y con luz UV (1 mg de material 3D por 0.01 mg de MB). En la Figura 1e se presenta la ampliación de cada área (0-100 m/z; 100-200 m/z y 200-300 m/z), junto con la asignación de los picos. En el rango m/z de 0-100, la asignación de los picos es la siguiente: los picos de m/z 23 y 39 se asignan a Na y K, y a medida que aparecen los siguientes picos se asignan a: ácido clorhídrico, pentano, tioacetona, tiazolina, sulfato aniónico, ácido 4 o 3-hidroxibutanoico, 2-aminofenol, anión benceno-Cl, 2-etiltiazol, 4-nitrofenol, ácido bencenosulfónico, sulfito de potasio, tiosulfato de sodio, ácido 4-fenolsulfónico, sulfato de potasio, bencenosulfonato de sodio hidrato. En el rango de más de m/z 200, el pico m/z de 256 se asigna a C14H14N3S (MB con un -CH3 de menos) y el pico de 270 a C15H16N3S. Estos resultados indican que hay una serie de productos intermedios de la reacción de degradación de MB. Se observan solo trazas de MB. Para comprobar si existe la posibilidad de disminuir el número de intermedios creados, el experimento se realizó usando el doble de fotocatalizador 3D WS2/rGO (2 mg por 0.01 mg MB). En la Figura 1f se muestran los espectros de masas MALDI de la solución acuosa después de 3 h bajo irradiación UV. En comparación con los espectros de la Figura 1e, los mismos intermedios fueron identificados, pero en menor cantidad, lo que indica que al duplicar la cantidad de fotocatalizador, en proporción al MB, se produce más degradación. La aparición de picos de dióxido de azufre indica que una parte del MB se degradó por completo y la presencia de nuevos compuestos como benceno, dimetilanilina y el pico que aparece a m/z de 244, asignado a C12H10ON3S (MB con cuatro -CH3 menos), son resultado de la degradación de los primeros productos, observados previamente. Este resultado muestra que optimizando la cantidad de fotocatalizador WS2/rGO se puede lograr una degradación muy eficiente tanto del MB como de los primeros productos de la reacción. En la Figura 1g, se puede ver una comparación de la composición de la solución acuosa después de la degradación de MB con el 3D WS2/rGO realizada en presencia de luz UV y en la oscuridad. La solución acuosa del experimento en oscuro (curva superior en la Figura 1g) muestra la presencia de MB (el pico marcado), mientras que este pico está ausente en la solución acuosa después del tratamiento con luz UV. También se muestra la ampliación de las tres áreas características como comparación. Para comprobar si todo el MB estaba degradado después del experimento fotocatalítico con el doble de WS2/rGO (2 mg de material por 0.1 mg de MB), el material 3D se sometió a desorción con etanol para extraer los posibles componentes adsorbidos en su superficie (MB no degradado y productos de degradación). El análisis MALDI-TOF-MS de la solución obtenida de la desorción se muestra en la Figura 1h, e indica la ausencia de MB. Esto demuestra la degradación fotocatalítica completa de MB, a pesar de que todavía se observan algunos intermedios de reacción (Figura 1f). Ejemplo 2: Síntesis y caracterización de WS2/rGO, preparado a partir de WS2/GO con una relación 1:4 en peso Efecto de adición de polímero sobre la actividad fotocatalítica del sistema WS2/rGO Condición de síntesis: T = 60ºC, GO: AsA = 1:1; WS2:GO = 1: 4, cantidad de polímero 20% en peso en relación a WS2/GO En este ejemplo, se agregaron nanopartículas de polímero al mismo fotocatalizador del Ejemplo 1 para mejorar el manejo, la durabilidad y aumentar la facilidad de purificación y reutilización de los fotocatalizadores. La cantidad de polímero fue del 20% en peso en relación con el peso de WS2/rGO. En la Figura 2a se pueden ver las imágenes SEM del material compuesto 3D, mostrando una estructura esponjosa, con una morfología porosa muy bien desarrollada. Las nanoplaquetas WS2 se ven en la imagen como estructuras blancas sobre la estructura compuesta polímero/rGO en color gris. En la Tabla 2.1, se muestra la composición elemental determinada a partir de los resultados de EDX para este sistema, lo que demuestra la incorporación de nanoplaquetas 2D de WS2 en la estructura 3D compuesta. Tabla 2.1: Composición elemental de 3D WS2/rGO/Pol del Ejemplo 2, determinada por EDX
Figure imgf000026_0001
La Figura 2b presenta la cinética del proceso de decoloración de una solución acuosa de MB de concentración 0.1 mg/mL, por la nanoestructura 3D WS2/rGO/Pol bajo luz UV durante 3 h. Este resultado se compara con el mismo experimento realizado en la oscuridad y con los mismos experimentos realizados con 3D rGO puro y WS2 puro. La Figura 2b muestra que en presencia del compuesto WS2/rGO/Pol bajo luz UV, casi toda la cantidad de MB de la solución fue adsorbida/degradada. Este material presenta una eficiencia mejorada en comparación con las nanoplaquetas 2D WS2 puras. Los espectros de masas de la solución acuosa después de los experimentos de decoloración, obtenidos por MALDI-TOF, se presentan en la Figura 2c. Los espectros muestran que la degradación de MB tuvo lugar cuando se usó el fotocatalizador 3D compuesto en presencia de luz ultravioleta. Es de notar que, aunque todavía hay algo de MB presente en la solución, hay muy pocos intermedios de reacción (tales como: ácido bencenosulfónico, sulfito de potasio, tiosulfato de sodio, ácido 4-fenolsulfónico, sulfato de potasio y bencenosulfonato de sodio hidratado). En el rango de masas más altos, por encima de m/z 200, solo aparecen los picos característicos de las estructuras C14H14N3S y C15H16N3S y el MB residual. También se observan los nuevos picos de CO2, NO3-, benceno y 4-aminocatechol. Los productos de reacción indican una degradación eficaz del MB. En la Figura 2d, se puede ver una comparación entre los experimentos realizados con el material WS2/rGO/Pol en presencia de luz UV y en la oscuridad. Como es de esperar, de acuerdo con la invención, se observan muchos menos componentes en la solución acuosa después de la decoloración en presencia de luz UV. Ejemplo 3: Síntesis y caracterización de WS2/rGO, preparado a partir de WS2/GO con relación 1:3 en peso Efecto del aumento de cantidad de WS2 en la estructura 3D WS2/rGO Condición de síntesis: T = 60ºC, GO: AsA = 1:1; WS2:GO = 1: 3, En este ejemplo se muestran los resultados obtenidos con el fotocatalizador que contiene una mayor cantidad de WS2 en la estructura 3D WS2/rGO, preparado a partir de WS2:GO 1:3 en peso. Las imágenes SEM se muestran en la Figura 3a y la Tabla 3.1 presenta la composición Tabla 3.1: Composición elemental de 3D WS2/rGO del Ejemplo 3, determinada por EDX
Figure imgf000028_0001
En la Figura 3b, se muestra la cinética del proceso de decoloración de una solución acuosa de 0.1 mg/mL de MB, con el fotocatalizador 3D WS2 /rGO de este ejemplo, en presencia de luz UV. Al comparar con los resultados obtenidos para la misma reacción realizada en la oscuridad y con el rendimiento de los componentes puros, se observa una decoloración muy rápida en presencia de luz (el MB se elimina en menos de 30 min). La solución acuosa después de la decoloración fue examinada por MALDI-TOF y los resultados se muestran en la Figura 3c. En el primer rango de masas (m/z 100-200) se observan los siguientes productos de la degradación: nitrato, dióxido de azufre, cloruro de hidrógeno del ácido clorhídrico, pentano, benceno, tiazolina, anión sulfato, ácido 4 o 3-hidroxibutanoico, anión benceno-Cl, 2-etiltiazol, 4-nitrofenol, bencenosulfonato de sodio hidrato. Entre masas de m/z 200 y 270 se observa la presencia de C15H16N3S (MB con un -CH3 menos). Todo esto demuestra que la descomposición fotocatalítica de MB con este fotocatalizador es aún más avanzada que con el compuesto preparado a partir de GO y WS2 con una proporción 1:4, con muy pocos productos de degradación y en un área de masa baja. Esto demuestra una gran degradación de los productos de reacción especialmente la degradación de productos de masa molar muy alta que contienen anillos aromáticos conjugados y son los más tóxicos. En los siguientes dos ejemplos, se aumentó la temperatura de reacción de síntesis de los materiales 3D desde 60ºC a 90ºC y la síntesis se realizó con dos cantidades diferentes de agente reductor (ácido ascórbico, AsA). Ejemplo 4: Síntesis y caracterización de WS2/rGO, preparado a partir de WS2/GO con una relación en peso de 1:4, a alta temperatura Efecto del aumento de temperatura durante la síntesis de la estructura 3D WS2/rGO Condición de síntesis: T = 90ºC, GO: AsA = 1:1; WS2:GO = 1: 4, La morfología del 3D WS2/rGO obtenido a alta temperatura se presenta en la Figura 4a y la composición elemental del material se presenta en la Tabla 4.1. Table 4.1: EDX resultados del material 3D WS2/rGO del Ejemplo 4
Figure imgf000029_0001
La cinética del proceso de decoloración de la solución de MB (0.1 mg/mL) se presenta en la Figura 4b. Se puede observar que en presencia de luz UV, se logró la decoloración completa de la solución acuosa de MB en 60 minutos. El proceso de adsorción/degradación fotocatalítica en este caso es un poco más lento que para el mismo material obtenido a menor temperatura. La decoloración realizada en la oscuridad muestra eliminación del 68% del MB de la solución por adsorción. La solución acuosa después de la decoloración fue examinada por MALDI-TOF y los resultados se muestran en la Figura 4c. El espectro de masas completo muestra todavía la presencia del pico de MB (la técnica es muy sensible y aparece el pico en concentraciones mínimas). Los espectros ampliados muestran la presencia de los productos de degradación que se detallan a continuación. En el rango m/z de 100-200: dióxido de azufre, benceno, tiazolina, anión sulfato, ácido 4 o 3-hidroxibutanoico, 2-aminofenol, anión benceno-Cl, 4-nitrofenol. En el rango de m/z de 200 a 270: C15H16N3S (MB con un -CH3 menos). Los resultados demuestran que este material presenta una actividad fotocatalítica avanzada ya que se obtienen muy pocos productos de reacción, lo que indica la degradación y eliminación de MB y muchos de los intermedios tóxicos. Ejemplo 5. Síntesis y caracterización de WS2/rGO/Pol preparado a partir de WS2/GO con una relación en peso de 1:4, a alta temperatura Efecto de la adición de polímero sobre la estructura y el rendimiento fotocatalítico Condiciones de síntesis: T=90ºC, GO:AsA=1:1; WS2:GO=1:4; cantidad de polímero 20% en peso en relación a WS2/GO Al material producido a 90ºC, presentado en el Ejemplo 4, se le añadió polímero, 20% en peso respecto al peso de WS2/GO. La morfología de WS2/rGO/Pol, presentada en la Figura 5a muestra una estructura esponjosa y porosa. La Tabla 5.2 presenta la composición elemental de la estructura del compuesto 3D. Tabla 5.1: Resultados de EDX del sistema WS2/rGO/Pol preparado a 90 oC
Figure imgf000030_0001
Los experimentos de decoloración con este material se muestran en la Figura 5b. Los resultados obtenidos demuestran que los fotocatalizadores compuestos son mucho más eficientes que las plaquetas 2D WS2 en solitario. En ausencia de luz se produce decoloración debido a la adsorción de MB en el material. Se demuestra que la degradación fotocatalítica de MB se produce en presencia de luz UV, gracias al fotocatalizador integrado. Ejemplo 6: Síntesis y caracterización de WS2/rGO preparado a partir de WS2/GO con una relación en peso de 1:4. Efecto de reducir la cantidad de agente reductor AsA utilizado para la síntesis Condiciones de síntesis: WS2/GO =1:4, T=60 oC, GO:AsA=1:0.5 En este ejemplo, se estudió la influencia de la cantidad de AsA usado para la reducción de GO, en las características de la estructura 3D y en su rendimiento fotocatalítico. El AsA en este ejemplo se redujo a la mitad con respecto al usado en los Ejemplos 1-5, donde la relación de GO: AsA es 1:1 en peso. En la Figura 6a se pueden observar las imágenes de SEM de 3D WS2/rGO, y se observa que la muestra es menos porosa que el mismo material obtenido con mayor cantidad de AsA (Ejemplo 1, Figura 1c, GO: AsA = 1:1). La Tabla 6.1 muestra los resultados de EDX en los que se observa una gran incorporación de WS2 en la estructura compuesta 3D. Tabla 6.1: Composición elemental del 3D WS2/rGO del Ejemplo 6, determinada por EDX
Figure imgf000031_0001
Los experimentos de la decoloración se muestran en la Figura Figura 6b. Sorprendentemente, este material ha demostrado ser muy eficaz en la degradación fotocatalítica del MB, ya que el MB se eliminó completamente de la solución bajo luz UV en 30 min. La caracterización por MALDI-TOF demuestra la degradación fotocatalítica de MB, ya que se observa la formación de numerosos productos de degradación. Los espectros de masas de la solución acuosa se pueden ver en la Figura 6c en la que se observa la formación de los siguientes compuestos tras la degradación del MB. En el área de masas de m/z 100 -200: dióxido de azufre, cloruro de hidrógeno del ácido clorhídrico, pentano, tioacetona, benceno, tiazolina, anión de sulfato, ácido 4 o 3-hidroxibutanoico, 2-aminofenol, ácido bencenosulfónico, sulfito de potasio, tiosulfato de sodio, ácido 4-fenolsulfónico, sulfato de potasio, bencenosulfonato de sodio hidratado. En el área de masas m/z 200 -300 aparece el pico a m/z 256 asignado a C14H14N3S y m/z 270 asignado a C15H16N3S (MB con un -CH3 menos). El MB se degradó casi por completo en 30 minutos, como se muestra en la Figura Figura 6b determinado por espectroscopía UV. La presencia de MB en la solución (trazas) junto con una gran cantidad de productos de reacción intermedios se observa en la Figura 6c. Los resultados indican que para obtener una mayor eficiencia de degradación fotocatalítica en este sistema, puede ser beneficioso utilizar menos cantidad de agente reductor respecto a la cantidad de GO. Ejemplo 7: Síntesis y caracterización de MoS2/rGO preparado a partir de MoS2/GO con una relación en peso de 1:4 Actividad fotocatalítica del sistema MoS2/rGO Condiciones de síntesis MoS2/GO = 1:4, T = 60ºC, GO:AsA = 1: 1 En la Figura 7a, se pueden ver las imágenes del TEM de las partículas iniciales de MoS2, que se obtuvieron por sonicación del cristal a granel de MoS2. Se pueden observar nanoplaquetas 2D con una dimensión lateral en un rango de 50 nm a 250 nm. La Figura 7b presenta las imágenes SEM de la morfología 3D de las estructuras de MoS2/rGO. La Tabla 7.1 presenta la composición elemental de los compuestos MoS2/rGO, mostrando una gran incorporación del fotocatalizador MoS2. Tabla 7.1: Composición elemental del 3D MoS2/rGO del Ejemplo 7, obtenida por EDX
Figure imgf000032_0001
En la Figura 7c, se muestra la cinética de la decoloración de la solución acuosa de MB. La eficiencia de eliminación de MB mejora significativamente cuando se utilizan materiales compuestos 3D (77%, en presencia de luz) comparado con las nanoplaquetas 2D de MoS2 (11%). Los resultados demuestran que la estructura 3D MoS2/rGO presenta una mejor eficiencia de eliminación y degradación. Espectros de masas MALDI-TOF-MS de solución desorbida en etanol para el sistema 3D MoS2/rGO, presentado en la Figura 7d, demuestran que el MB se ha degradado por completo, como el pico característico de MB no aparece en el espectro. Ejemplo 8: Síntesis y caracterización de MoS2/rGO/Pol (1:4) preparado a partir de MoS2/GO con una relación en peso de 1:4 Efecto de la adición de polímero a MoS2/rGO Condiciones de síntesis MoS2:GO = 1:4, polímero: 20% en peso en relación a MoS2:GO, T = 60ºC, GO: AsA = 1:1 En este ejemplo se añadió 20% de polímero (% en peso en relación al peso de MoS2/GO). La Figura 8a muestra las imágenes SEM y se observa un material poroso y esponjoso). La distribución del tamaño de los poros a esta escala es de 1 a 10 µm. La Tabla 8.2 muestra la composición elemental de MoS2/rGO/Pol. Tabla 8.1: Composición elemental del MoS2/rGO/Pol del Ejemplo 8, determinada por EDX
Figure imgf000033_0001
La Figura 8b muestra la cinética del proceso de decoloración de la solución acuosa de MB mediante el sistema compuesto 3D en comparación con el rGO puro y el MoS2 puro. Aunque el rGO puro presenta una gran eficiencia de eliminación de MB (adsorción), el rGO solo está adsorbiendo el MB, no degradándolo. En la estructura 3D compuesta, el MB adsorbido se degrada de una forma muy eficaz por el efecto fotocatalítico del fotocatalizador en presencia de luz. Otros fotocatalizadores que se han preparado siguiendo los procedimientos tal como se han definido en los ejemplos anteriores, excepto por las nanoplaquetas, que fueron de CdS: ejemplos de materiales de CdS/rGO y CdS/rGO/Pol. Conclusiones generales de los experimentos UV y MALDI-TOF En la caracterización con MALDI-TOF se pueden observar algunos picos relacionados con la matriz empleada (por ejemplo Na o K) y otros son productos de degradación del MB, por ejemplo: i) Tiazolina, ii) SO₄²-, iii) Ácido 4 o 3- hidroxibutírico, iv ) 2-Aminofenol y v) MB sin un grupo metilo (-CH3). En la Tabla C1 se pueden ver los picos reconocidos y atribuidos a los productos de degradación. Los resultados obtenidos con las muestras del Ejemplo 1, muestran picos con una m/z baja. Esto indica que la degradación del MB ha avanzado mucho. Comparando los Ejemplos 1 y 2, los picos adicionales indican nuevos productos de degradación después de la adición del polímero (Ejemplo 2) a una m/z mucho más baja. Esto indica un progreso en la degradación tanto del MB como de los primeros productos de degradación, lo que indica que incluso una parte del MB estaba mineralizada. Estos picos son CO2, NO3-, SO2, C6H6 y 4-Catecolamina. Por el contrario, el sistema sin polímero muestra presencia de moléculas más grandes, con anillo aromático en su estructura o estructuras con Na/K con grupos -S- (158-174-198 m/z), (C6H6O3S, K2SO3, Na2S2O3, C6H6O4S, K2SO4 y C6H7NaO4S). De manera similar, en los Ejemplos 1 y 3 se puede ver el efecto de aumentar la cantidad de nanoplaquetas WS2. Al aumentar la cantidad de nanoplaquetas, se puede ver un mayor número de picos a valores de m/z más bajos, y compuestos aromáticos con una masa molar más baja, entre 158-197 m/z. Estos picos son de CO2, NO3-, SO2, C6H6 y 4-Catecolamina. Finalmente, comparando los Ejemplos 1, 5 y 7, donde se cambiaron los parámetros de preparación, se observa que las muestras preparadas a la misma temperatura (60 oC) muestran similar nivel de degradación, ya que se observan picos muy similares: ácido clorhídrico cloruro de hidrógeno, tioacetona, C6H6O3S, K2SO3, Na2S2O3, C6H6O4S, K2SO4, C6H7NaO4S y a 256 m/z el MB con 2 -CH3 menos. Tabla C1: Picos de MALDI reconocidos y atribuidos a los productos de degradación del azul de metileno para los Ejemplos 1, 2, 3, 4, 5, 7, que muestran la m/z y las estructuras químicas.
Figure imgf000034_0001
Figure imgf000035_0001
Figure imgf000036_0001
Figure imgf000037_0001
Figure imgf000038_0001
Los procesos llevados a cabo en presencia de luz UV con los materiales de la invención resultaron en la adsorción y degradación fotocatalítica del agente adsorbido. En todos los casos y de acuerdo con la invención, el fotocatalizador integrado en la estructura tridimensional de óxido de grafeno reducido demostró una actividad fotocatalítica muy superior a la del semiconductor puro (no integrado). Los materiales integrados son materiales monolíticos, de estructura porosa, fáciles de manipular y pueden ser reusados sin perder componentes activos entre ciclos. En todos los sistemas investigados, se ha demostrado que el semiconductor 2D se ha incorporado al rGO para obtener un fotocatalizador integrado y que la cinética de eliminación del agente (MB en los ejemplos) de la solución acuosa es mucho más rápida con el fotocatalizador integrado que con el fotocatalizador puro 2D. Los experimentos de los ejemplos también demuestran que los parámetros de la síntesis de los fotocatalizadores integrados (por ejemplo, temperatura, cantidad de agente reductor, proporción de los componentes GO y semiconductor) pueden afectar a la estructura y la eficacia de los fotocatalizadores integrados. Estas variaciones son ventajosas porque permiten adaptar el fotocatalizador dependiendo de la aplicación de interés para el mismo. Referencias [1] Nano Research 2017, 10(5), 1662–1672. [2] ACS Appl. Mater. Interfaces 2013, 5, 2227−2233. [3] J. Mater. Chem. A, 2014, 2, 3605 – 3612. [4] Applied Catalysis B: Environmental 2016, 199, 412–423. [5] Applied Catalysis B: Environmental 2018, 221, 36–46. [6] Separation and Purification Technology 2018, 194, 96–103. [7] ACS Sustainable Chem. Eng.2018, 6, 5718−5724. [8] Adv. Mater.2015, 27, 3767–3773 [9] Applied Catalysis B: Environmental 217 (2017) 65–80. [10] Journal of Colloid and Interface Science 536 (2019) 389–398. [11] New J. Chem., 2016, 40, 3208—3215. [12] Applied Catalysis B: Environmental 212 (2017) 41–49. [13] International Journal of Hydrogen Energy 2017, 42, 1554 - 15550. [14] Small 2015, 11, No.36, 4785–4792. [15] CrystEngComm, 2014, 16, 399–405. [16] Catal. Sci. Technol.,2017, 7, 1305–1314. [17] ACS Appl. Mater. Interfaces 2015, 7, 25693−25701. [18] Applied Catalysis B: Environmental 205 (2017) 228–237. [19] US 2016/0296909 A1 [20] US 2012/0149554 A1 [21]US 2015/0069295 A1 [22] Chem. Rev., 2015, 115, 10307–10377 23] Phys. Chem. Chem. Phys., 2013, 15, 19102–19118 [24] Chem. Soc. Rev., 2014, 43, 8240–8254 DESCRIPTION GRAPHENE OXIDE AND SEMICONDUCTOR PHOTOCATALYSTS Field of the invention The present invention falls within the field of photocatalysts, in particular it refers to photocatalysts comprising a three-dimensional structure of reduced graphene oxide and platelets of at least one semiconductor. It also refers to their uses and methods for their preparation. BACKGROUND OF THE INVENTION The invention relates to an integrated photocatalyst and in particular to a photocatalyst comprising a semiconductor and reduced graphene oxide as active support united in a monolithic and porous structure. The photocatalytic process is based on the excitation of a solid (photocatalyst), usually a broadband semiconductor, by irradiation with light. The photocatalytic process has multiple applications that include, for example, environmental decontamination treatments and the synthesis of high added value compounds, all in a sustainable way by using solar radiation. One of the main challenges in this area is the development of new or modified photocatalytic materials that improve their efficiency by operating with a greater range of wavelengths. Many of the semiconductors used as photocatalysts are metal oxides (for example, TiO2 and ZnO) or chalcogenides (for example, CdS, ZnS, CdSe, ZnSe, and CdTe). The most used is TiO2. Commercial photocatalysts (usually TiO2) are usually sold in the form of powdered nanoparticles. For their use it is necessary to incorporate these photocatalysts to a solid support. Examples of commercial TiO2 nanoparticles are: CristalACTiV TM DT'20, Green Millenium and Microban. Some devices are also known, for example devices for air purification, which work with TiO2 nanoparticles integrated into different supports. Certain three-dimensional structures of graphene to which photocatalysts have been incorporated, such as TiO 2 , AgX (X=Br, Cl), AgPO 4 , CuO, ZnO, CdS or C 3 N 4 , are known through some publications (1 ) to (21). These materials are limited to three-dimensional graphene structures to which photocatalysts have been incorporated in the form of nanoparticles (zerodimensional) or nanotubes (one-dimensional). The synthesis method for these materials is hydrothermal, often carried out at high temperatures (180 o C) for long periods of time (12-24 hours). The presence of graphene in these compounds provides, on the one hand, a suitable support for the photocatalysts and, on the other, due to its good acceptance of electrons and its electrically conductive properties, it solves the main drawback of photocatalytic materials, the lack of a efficient charge separation (Ref 22-24). However, the performance of the compounds is unsatisfactory. Among the objectives of the present invention is to provide an improved photocatalyst, with a high photocatalytic activity and which does not need to be incorporated into a solid support for its use. It is also desirable to provide an improved photocatalyst that can be prepared in a simple manner and with a process that can be performed under mild conditions, especially temperature. Summary of the invention The invention relates to new photocatalysts and in particular to photocatalysts comprising a three-dimensional structure of reduced graphene oxide and nanoplatelets of at least one semiconductor. In such photocatalysts, the semiconductor nanoplatelets are embedded in the three-dimensional structure of reduced graphene oxide. In the state of the art, no photocatalyst is known that comprises a three-dimensional structure of reduced graphene oxide and that incorporates semiconductor materials in the form of nanoplatelets (two-dimensional structures). The photocatalysts described here have porous structures, and optionally comprise nanoparticles of at least one polymer. In particular embodiments, these polymers are selected from a group formed by styrenes, acrylates, methacrylates, optionally functionalized, and their mixtures. In particular embodiments, the semiconductor is selected from a group formed by GaN, CeO 2 , CdS, ZnO, MoS 2 , WS 2 , WO 3 and other semiconductor materials that allow the formation of nanoplatelets by exfoliation thereof, preferably being MoS 2 or WS 2 . In preferred embodiments, the photocatalyst comprises WS2 or MoS2 and optionally, functionalized polymethyl methacrylate nanoparticles. Also, according to the invention, a method of preparing said photocatalysts is provided. The synthesis method is simple, and can be carried out under mild reaction conditions. The method comprises: a) preparing an aqueous dispersion of graphene oxide nanoplatelets, nanoplatelets of one or more semiconductors and a reducing agent; and heating the resulting dispersion, without stirring, to a temperature in the range of 45 to 90°C. The reducing agent is preferably ascorbic acid or hydrazine. The process described comprises a step c), between a) and b), in which the dispersion resulting in a) is stirred at room temperature for a period of time sufficient to homogenize the dispersion, typically for 15 minutes to 1 hour. After step b) the resulting reaction product can be dried, for example using lyophilization or microwaves. The photocatalysts described herein have been found to show catalytic properties superior to commercial photocatalysts and provide additional advantages. The invention also relates to the use of said photocatalysts. For example, in environmental protection technologies, in the capture and/or chemical conversion of air, water or soil pollutants or as catalysts in chemical reactions. Also in applications that require the use of porous materials. Description of the Figures Figure 1a: Transmission Electron Microscopy (TEM) images of the WS2 nanoplatelets as indicated in Example 1 Figure 1b: Scanning Electron Microscopy (SEM) images of the three-dimensional (3D) structures of reduced graphene oxide (rGO) as indicated in Example 1 Figure 1c: SEM images of prepared WS2/rGO three-dimensional (3D) nanostructures initial graphene oxide (GO) : ascorbic acid (AsA) ratio of 1:1 by weight as indicated in Example 1 Figure 1d: Kinetics of the bleaching process (Change of methylene blue (MB) concentration with time) ; 3D rGO pure with ultraviolet (UV) light; 2D WS 2 pure with UV light; 3D WS 2 /rGO with UV light and 3D WS 2 /rGO D (D = in the dark) (T = 60°C, GO:AsA = 1:1 by weight) as indicated in Example 1 Figure 1e: Results of spectrometric analysis Mass MALDI TOF [(Matrix-Assisted Laser Desorption/Ionization / Time-Of-Flight)] of an aqueous solution of MB, after destaining reaction of MB with 3D WS 2 / rGO and UV light as indicated in Example 1, with assignment of some of the peaks Figure 1f: Results of MALDI-TOF mass spectrometry analysis of the aqueous MB solution after destaining done with double amount of 3D WS 2 /rGO photocatalyst (2 mg photocatalyst per 0.1 mg MB) as indicated in Example 1 with structure assignment to the peaks. Figure 1g: Results of MALDI-TOF mass spectrometry analysis of the aqueous MB solution after destaining done with 3D WS2/rGO and UV light (full line) and in the dark (dotted line) as indicated in Example 1. The characteristic MB peak is marked. Figure 1h: Results of MALDI-TOF mass spectrometry analysis of the ethanol solution obtained in the desorption process of 3D WS2/rGO material after the MB degradation reaction with UV light as indicated in Example 1 Figure 2a: SEM images of 3D WS2/rGO/Pol [Pol = polymer] prepared from WS2/GO with a weight ratio of 1:4, Temperature (T) = 60ºC, with a ratio of initial graphene oxide (GO): ascorbic acid (AsA) of 1: 1 by weight and 20% by weight of polymer in relation to the amount by weight of WS2/GO, as indicated in Example 2 Figure 2b: Kinetics of the bleaching process (Change of MB concentration with the time); 3D rGO pure with UV light; 2D WS2 pure with UV light; 3D WS2/rGO/Pol with UV light and 3D WS2/rGO/Pol D as indicated in Example 2 Figure 2c: Results of MALDI-TOF mass spectrometry analysis of the aqueous MB solution after destaining with 3D WS 2 /rGO/Pol as indicated in Example 2, with structure assignment to the peaks. The characteristic MB peak is marked. Figure 2d: Results of MALDI-TOF mass spectrometry analysis of the aqueous MB solution after destaining with 3D WS 2 /rGO/Pol under UV light (solid line) and in the dark (dashed line) as indicated by Example 2. The characteristic MB peak is marked. Figure 3a: SEM images of 3D WS 2 /rGO prepared from WS 2 /GO 1:3 by weight, Temperature (T) = 60°C, with an initial graphene oxide (GO) : ascorbic acid (AsA) ratio of 1:1 by weight, as indicated in Example 3 Figure 3b: Kinetics of the bleaching process (Change of relative MB concentration with time); Pure 3D rGO under UV light, Pure 2D WS2 under UV light, 3D WS2/rGO under UV light and 3D WS 2 /rGO D in the dark as per Example 3 Figure 3c: Results of MALDI-TOF mass spectrometry analysis of the aqueous solution of MB after destaining with 3D WS2/rGO as indicated in Example 3, with assignment of peaks Figure 4a: SEM images of 3D WS2/rGO prepared from WS2/GO with a weight ratio of 1:4 , Temperature (T) = 90ºC, with a ratio of initial graphene oxide (GO): ascorbic acid (AsA) of 1: 1 by weight, as indicated in Example 4 Figure 4b: Kinetics of the bleaching process (Change of concentration relative MB over time); Pure 3D rGO in UV light, Pure 2D WS2 in UV light, 3D WS2/rGO in UV light, and 3D WS2/rGO D in the dark as indicated in Example 4 Figure 4c: Results of MALDI-TOF mass spectrometry analysis of the aqueous MB solution after 3D WS2/rGO light destaining as per Example 4, with peak assignment Figure 5a: SEM images of WS2/rGO/Pol prepared from WS2/GO with a weight ratio of 1: 4, Temperature (T) = 90ºC, with a proportion of initial graphene oxide (GO): ascorbic acid (AsA) of 1: 1 by weight and 20% by weight of polymer in relation to the amount in Figure 5b: Kinetics of decolorization of aqueous MB solution (relative MB concentration vs. time) of systems: 3D neat rGO with light; 2D WS 2 pure with UV light; and 3D WS 2 /rGO/Pol under UV light and 3D WS 2 /rGO/Pol D (D: in the dark), as indicated in Example 5. Figure 6a: SEM images of 3D WS 2 /rGO prepared from WS 2 /GO with a weight ratio of 1: 4, Temperature (T) = 60ºC, and with a ratio of initial graphene oxide (GO): ascorbic acid (AsA) of 1: 0.5 by weight, as indicated in the Example Figure 6b: Kinetics of decolorization of aqueous MB solution (change of MB concentration relative to time) of systems: pure 3D rGO under UV light; 2D WS 2 under UV light; and 3D WS 2 /rGO under UV light and 3D WS 2 /rGO D in the dark, as indicated in Example 6. Figure 6c: Results of MALDI-TOF mass spectrometry analysis of the aqueous MB solution after destaining with the 3D WS2/rGO system with light as indicated in Example 6, with indication of the specific chemical groups in different areas of the spectra. Figure 7a: Transmission Electron Microscopy (TEM) images of MoS2 nanoplatelets as indicated in Example 7 Figure 7b: SEM images of 3D MoS2/rGO prepared from MoS2/GO with a weight ratio of 1:4, Temperature (T) = 60ºC, with a ratio of initial graphene oxide (GO): ascorbic acid (AsA) of 1: 1 by weight, as indicated in Example 7. Figure 7c: Kinetics of the decolorization of the MB solution (change of MB concentration over time) of 3D pure rGO under UV light, 2D pure MoS2 under UV light, 3D MoS2/rGO under UV light and 3D MoS2/rGO D in the dark as indicated in Example 7. Figure 7d : MALDI-TOF mass spectra of ethanol desorbed solution for the 3D MoS2/rGO system of Example 7. Figure 8a: SEM images of 3D MoS2/rGO/Pol prepared from MoS2/GO with a weight ratio of 1 : 4, Temperature (T) = 60ºC, with a ratio of initial graphene oxide (GO): ascorbic acid (AsA) of 1: 1 by weight and 20% by weight of polymer in relation to the amount by weight of WS2/GO, as indicated in Example 8. Figure 8b: Kinetics of decolorization of aqueous MB solution (change of MB concentration relative to time) by neat 3D rGO, neat 2D MoS 2 , 3D MoS 2 /rGO/Pol under light and 3D MoS 2 /rGO/ Pol D in the dark, as indicated in Example 8. Detailed Information on the Invention The materials of the present invention are integrated photocatalysts comprising a three-dimensional structure of reduced graphene oxide (rGO) into which two-dimensional semiconductor nanoplatelets are incorporated. . This porous, three-dimensional, monolithic structure is a stable structure that functions as an integrated photocatalyst. This is advantageous because the material works as a photocatalyst without having to incorporate it into a support, as is the case with commercial catalysts. It has further been observed that in comparison with certain integrated photocatalytic apparatuses, in which the material used as a support only acts as such for the semiconductor without participating in the catalytic activity, in the case of the photocatalyst of the present invention synergy is observed between the photocatalytic activity of the semiconductor nanoplatelets and the reduced graphene oxide that acts as an active support. Such synergy results in a shift in light absorbance towards the visible light range, reduced bandwidth, and even better charge separation efficiency. Without intending to be bound by any particular theory, it is assumed that these effects are accentuated due to the platelet-like structure of the semiconductor, which results in a greater interface between the reduced graphene oxide and the semiconductor, a greater interaction between the oxide of reduced graphene and the semiconductor and an increase in catalytic performance. Other advantages of the photocatalyst of the invention include better performance, greater durability, the possibility of reusing the catalyst, and reduced loss of active compound. As a result of the good interaction between the semiconductor and reduced graphene oxide, which facilitates charge separation, the improvement in the efficiency of the integrated catalyst has been noted. When the semiconductor absorbs the photon of UV light that carries more energy than the gap energy band, an electron-hole pair is generated in the semiconductor nanoplatelets that are capable of starting oxidation-reduction processes. But the electron-hole pair can undergo a recombination process and return to the state increases the effectiveness of the semiconductor as a photocatalyst. The fact that the semiconductor is incorporated into the 3D rGO or rGO/polymer structures makes it possible to easily reuse the photocatalyst, after cleaning. Furthermore, since the semiconductor is immobilized, the loss of active material during cleaning is reduced. The materials of the present invention have a three-dimensional macroscopic structure and are porous materials, with a sponge-like appearance. They also have a great adsorption capacity for organic or inorganic molecules and/or particles. The selective adsorption of certain materials of interest can be tuned by controlling the physicochemical properties. Furthermore, when exposed to light (UV or visible), the catalyst is activated and is capable of degrading, through oxidation-reduction processes, the materials adsorbed on its surface. Commercial (semiconductor) photocatalysts are usually powdered products that have to be embedded in the appropriate support before use. Both the process of immobilization or integration of the photocatalyst in the support, as well as the supports themselves, vary according to the different applications. The main advantage of the materials of the present invention is that it allows the union of the photocatalyst (semiconductor) and the support within the same structure. The materials are also versatile, with many different applications. The material of the invention has very advantageous properties in terms of porosity, mechanical and thermal resistance, shape and size, which can also be modulated. The graphene oxide (GO) that is used to prepare the photocatalyst of the invention is graphene oxide in the form of nanoplatelets. Graphene oxide in the form of nanoplatelets can be purchased commercially or prepared according to methods known in the state of the art. Nanoplatelets are usually purchased in the form of a dispersion, such as an aqueous dispersion. In a particular embodiment, the nanoplatelets are supplied in dispersion in water with concentrations that typically range from 0.1 to 5% by weight, preferably 0.4% - 1% by weight with respect to the total weight of the dispersion supplied with monolayer content of >90%, preferably >92%, or >95%, or >97%. The starting graphene oxide has a high oxygen content of between 25% to 55% by weight, particularly from 30 to 50% by weight, or from 40% to 50% by weight, or from 45% to 50% by weight with with respect to the total weight of graphene oxide. In a particular embodiment of the nanoplatelets with a layer thickness equal to or less than 1 nm. The nanoplatelets used in the present invention are also characterized by their lateral dimension. Their lateral dimensions can range between 500 nm and 30 microns, for example, between 1 micron and 20 microns, such as 10 microns. In a particular embodiment, the starting graphene oxide nanoplatelets have an oxygen content of 30 to 50 % by weight, 93 to 95% of them are nanoplatelets with a thickness equal to or less than 1 nm, and their lateral dimension is 10 microns. The term "semiconductor" as used herein refers to a semiconductor material with photocatalytic characteristics. Illustrative, but not limiting, examples of semiconductors that can be used in the preparation of the photocatalysts of the invention are GaN, CeO 2 , CdS, ZnO, MoS 2 , WS 2 , WO 3 and other semiconductors that have a layered structure and are therefore suitable for the formation of nanoplatelets by exfoliation. Preferred semiconductors include CdS, MoS2, and WS2. Especially preferred is WS2. Other semiconductors that can be used to prepare the photocatalysts of the invention include: (i) W: tungsten ditelluride (WTe 2 ) and tungsten diselenide (WSe 2 ), ii) Mo: molybdenum telluride (MoTe2) and molybdenum diselenide (MoSe2), iii) Cd: cadmium selenide (CsSe), iv) In: indium (III) sulfide In2S3) and indium (III) selenide (In2Se3), v) Ga: gallium (III) selenide (Ga2Se3 ), gallium(II) selenide (GaSe), gallium(II) sulfide (GaS) and gallium(II) telluride (GaTe ), vi) Bi: bismuth selenide (Bi2Se3), bismuth telluride (Bi2Te3) and bismuth sulfide (Bi2S3), vii) Cu: copper sulfide (Cu2S), viii) Sn: tin(II) sulfide (SnS) and tin selenide (SnSe) and ix) Pb: lead(II) iodide ( PbI2). The semiconductor nanoplatelets that can be used to prepare the photocatalysts of the invention can have thicknesses varying between one or more atomic layers. The thickness of an atomic layer varies depending on the particular semiconductor material. For example, it is 0.65 nm for MoS2 and 3.7 nm for WS2. As for their maximum thickness, it will depend on the number of atomic layers of the nanoplatelet. In the present invention, the thickness generally varies between that corresponding to one atomic layer and 40 nm according to the material, or between one atomic layer and 30 nm, or between one atomic layer and 20 nm, or between one atomic layer and 10 nm. The lateral dimensions of the semiconductor nanoplatelets can also vary between a value greater than 40 nm and 3 microns on the condition that the lateral dimension particular embodiments the lateral dimensions are between 50 nm and 1 micron, or between 100 nm and 500 nm microns, or between 200 nm and 400 nm. In a particular embodiment, the nanoplatelets have thicknesses between 0.5 and 8 nm, and a lateral dimension between 25 and 300 nm, or thicknesses between 0.5 and 8 nm, and a lateral dimension between 50 and 250 nm, or 100 and 200 nm. In other embodiments the thickness is between 0.5 and 4 nm and the lateral dimension between 25 and 300 nm, or 50 and 250 nm, or 100 and 200 nm. The photocatalyst of the invention may comprise nanoplatelets of one or more semiconductor materials. The nanoplatelets used for a particular embodiment may in turn have different structures and characteristics. The composition of the photocatalyst of the invention can vary over a wide range. Elemental composition analysis determined from the EDX results demonstrate the incorporation of semiconductor nanoplatelets into the composite 3D structure. SEM images of photocatalysts show the three-dimensional and porous nanostructure. The weight ratio of the starting graphene oxide to the weight of the starting semiconductor [GO/semiconductor] is typically in the range of 1:1 to 9:1. Some particular ratios are in the range of 2:1 to 8: 1, for example 3:1, 4:1, 5:1; 6:1, or 7:1. A more preferred ratio is 4:1. In accordance with embodiments of the invention, it has been found that it may be advantageous to incorporate nanoparticles of a polymer into the photocatalyst. The incorporation of said polymer nanoparticles helps to improve the physical-chemical properties of the product and the mechanical performance, results in a more flexible, strong and durable structure and facilitates its handling. Furthermore, it makes the photocatalyst more effective in cycling operations. The term "polymer" as used here refers to any polymer composed of macromolecules (generally organic) formed by the union by covalent bonds of one or more simple units called monomers, such as: acrylates, methacrylates, styrenes, etc. The polymer particles that are incorporated into the structure of the photocatalyst can be produced, for example, by polymerization techniques in a dispersed medium (water), such as in emulsion or the like. Where appropriate, the polymers can be functionalized, and for this, amounts of functional monomers are used, generally 1%-10% by weight. they are used in amounts between 2 and 8%, or between 4 and 6%. Examples of functional monomers include, but are not limited to: 4-bromo styrene, 2-chloro ethyl acrylate, glycidyl methacrylate, 2-hydroxyethyl methacrylate, acrylamide, 2-amino ethyl methacrylate hydrochloride, acrylic acid, polystyrene sulfonate, and the like. . Illustrative, but not limiting, examples of polymers that can be used in the preparation of the photocatalysts of the invention are polystyrenes, polyacrylates, polymethacrylates, polyacrylamides, polyurethanes, etc., all of them optionally functionalized. "Functionalized polymer" refers to a polymer that has functional groups incorporated. Examples of functional groups include epoxy, amide, amine, sulfonates, carboxyl, hydroxyl, ester, phenyl, ether, etc. One of the preferred polymers in the present invention is functionalized polymethyl methacrylate, preferably functionalized with epoxy groups. In embodiments of the invention, the photocatalyst comprises one or more polymers. The polymers can be selected based on the characteristics that the photocatalyst is desired to present. The inventors have observed that the glass transition temperature (Tg) of the polymer used influences whether they tend to settle as nanoparticles on the rGO surface (when they have a high Tg) or whether they tend to form a layer covering the rGO (when they have a high Tg). have a low Tg) in the drying process of 3D materials. When the photocatalyst comprises one or more polymers, the ratio between the weight of the "GO and semiconductor" starting material and the weight of the polymer, used in the preparation of the photocatalysts, can vary. In particular embodiments of the photocatalysts of the invention, they have been prepared with weight ratios of the starting materials (GO/semiconductor): polymer that vary from 1:1 to 10:1. Some particular ratios are within the range 2:1 to 9:1, for example 3:1, 4:1, 5:1; 6:1, 7:1 or 8:1. Preferred ratios are in the range 2:1 - 4:1, with 4:1 being the most preferred range. The synthesis method of the photocatalysts of the invention is a self-assembly process that occurs during the reduction reaction of the graphene oxide nanoplatelets in a colloidal dispersion that also contains the semiconductor nanoplatelets. The three-dimensional porous structure is obtained spontaneously in the reduction reaction of graphene oxide with a reducing agent. The reaction is carried to In particular, the temperature is between 25 and 45 ºC. The reaction time depends on the reaction temperature, the size of the photocatalyst to be obtained, etc. In a particular embodiment, the time is 30 minutes at 90 o C or several hours at 25 o C. According to the synthesis method of the invention, an aqueous dispersion of the starting materials is prepared: graphene oxide nanoplatelets, nanoplatelets semiconductor and optionally polymer nanoparticles. The dispersion is stirred to homogenize the mixture. During this stirring period, it is verified that the semiconductor nanoparticles and the polymer nanoparticles adsorb on the surface of the graphene oxide nanoparticles. Then the temperature is set within the range mentioned above and the reducing agent (eg ascorbic acid (Asa) or hydrazine) is added when the required temperature is reached. From this moment the agitation stops for the self-assembly process to take place. The graphene oxide reduction process, thanks to the reducing agent, converts the graphene oxide material into a material that is much more hydrophobic and incompatible with the aqueous medium in which the reaction takes place. As a consequence, the reduced graphene oxide nanoplatelets that are produced begin to spontaneously self-assemble, thus forming three-dimensional (3D) porous materials in the form of a monolithic structure. "Monolithic structure" or "monolith" as used herein, refers to a material composed of more than one type of material or component, in which the components form a single structure without joint points, which is made up of a single unit . The material thus produced is a hydrogel swollen with a large amount of water. The photocatalyst of the invention can be used in the form of a hydrogel or in a dry form, after a drying process. But before drying it has to be cleaned with distilled water by the dialysis process. For drying, any of the known methods of drying hydrogels can be used. These include, for example, freeze drying, microwave or conventional oven drying. In a preferred aspect, the drying is carried out with lyophilization, in order to preserve the porous structure and the volume of the monolith. It can also be dried using microwaves, for example, with a very high heating speed program, in which with the microwave at a power of 1400W, the temperature in the reactor is increased up to 200ºC per minute and is allowed to act at that temperature. for a few of the invention. The photocatalyst in the form of a lyophilized solid is another preferred aspect of the invention. The term "reducing agent" or "reductant" as used herein refers to any known reducing agent, that is to say an agent capable of donating electrons in an oxidation-reduction reaction according to the invention. Illustrative, but not limiting, examples of reducing agents that can be used in the preparation of the photocatalysts of the invention are: (i) Borohydrides (for example, NaBH4), (ii) Lithium aluminum hydride (LiAlH4), (iii) Hydrogen halides (for example, HI), (iv) Sulfur-containing reducing agents (for example, thiourea dioxide/NaOH), (v) Nitrogen-containing reducing agents (for example, hydrazine), (vi) Reducing agents that contain oxygen (eg isopropyl alcohol), (vii) Metal-acid (eg Fe HCl), (viii) Metal-base (eg Zn/NaOH), (ix) Amino acid (eg glycine ), etc Preferred are hydrazine and ascorbic acid (AsA). Most preferred is ascorbic acid since it dissolves easily in water, is non-toxic, and reduction occurs rapidly. The reaction parameters can vary and the process can be controlled by changing the parameters, such as the type or amount of reducing agents or the temperature of the process. There are parameters that can be changed and that influence the formation process of the structures and their properties. These include: the concentration and type of graphene oxide, reducing agent, polymer, and semiconductor; temperature and reduction time. As indicated before, it is important that the starting graphene oxide is highly oxidized. During the reduction process, oxygen-containing functional groups are rapidly lost, and this process provides the necessary conditions for the structure to form. The amount of graphene oxide present in the reaction medium for the structures to form can vary. In particular embodiments, the amount of GO is between 1 mg/mL and 20 mg/mL, or between 2 mg/mL and 15 mg/mL, or between 4 mg/mL and 10 mg/mL, preferably between 5 mg/mL and 6 mg/mL and more preferably is about 4 mg/mL. Regarding the properties of the structures, the concentration of graphene oxide in the reaction medium determines the dimensions of the structure formed. a greater Concentration of dispersions in water results in the formation of larger volume structures. The temperature at which graphene oxide is reduced (25º - 90ºC) is a parameter that determines the speed of the reduction reaction and, as a consequence, the properties of the structures formed. At low temperatures, the reduction reaction, and therefore the formation reaction of the structures, is slower, and at higher temperatures, the structures are formed at a faster rate. As for the reducing agent, the amount and type of the agent are important for the reduction reaction. These parameters affect the rate of the reaction, which in turn influences the properties of the structure formed. The concentration of the reducing agent affects above all the properties of the structures. The weight ratios of GO:reducing agent can be in particular embodiments low (1:0.5), medium (1:1) or high (1:2). At lower proportions (for example when the ratio of GO:reducing agent is 1:0.5) the formation process is slower and the resulting structure is less compact and has less mechanical resistance, presents more macropores, that is, pores. with a diameter greater than 50 nm, and is richer in oxygen functional groups, which may be important for some applications. Regarding the effect of temperature on the properties of the structures, at lower temperatures (25-45ºC) the structure formed is mechanically weaker, but richer in oxygen functional groups. The temperature and the amount of reducing agent, in combination, determine the properties of the structure, especially the mechanical properties, porosity and pore distribution, and also the surface chemistry of the structures, since it determines the level of reduction. and the amount of functional oxygen groups present. In order to save energy and produce materials with low energy demand, it is of interest to produce them at lower temperatures (25-45ºC). Varying the amount of polymer can compensate for the reduction in mechanical properties. The type of polymer and the type of semiconductor can influence the self-assembly process, since when adsorbed on graphene oxide platelets, they influence the mobility and assembly affinity. They can also affect the final properties. For example, by changing the semiconductor or by using a combination of two or more semiconductors, photocatalytic activity, light absorption and band gap energy can be optimized. Polymers are inert components that, although in general they do not influence photocatalytic activity, can improve physicochemical properties and contribute to the formation of a product with better consistency and mechanical resistance, longer duration and reusability. In the scaffold production process, the initial mixing of the starting materials is carried out with magnetic stirring (between 100 and 300 rpm, eg 150 rpm) to homogenize the mixture. However, it is then essential for the formation of the structures of the invention to stop agitation. The time that the reacted mixture must then remain depends on the temperature. At low temperatures (25-45ºC) the process may need between 24-48h while at high temperatures (60-90ºC) the process can be completed in 2-4 h. Another advantage of the process of the invention is that it does not generate secondary products and therefore generates less waste. It is also an environmentally friendly process since it can be carried out in aqueous dispersions, at mild temperatures, and can be carried out with innocuous reagents, such as using ascorbic acid as a reducing agent. The method of the invention is easy, versatile, environmentally friendly, low cost and suitable for large-scale production. The preparation of the semiconductor nanochips is not restricted to a particular procedure. Rather, all known preparative methods can be used to prepare the nanoplatelets used in the process of the invention. These include chemical methods (Eda et al., 2011), ion exchange (Browning et al., 2015), powder evaporation (PV) (Najmaei et al., 2013), chemical vapor deposition (CVD) (Eichfeld et al., al., 2015), atomic layer deposition (ALD) (Song et al., 2013), molecular beam epitaxy (MBE) (Liu et al., 2015), thermal exfoliation, electrochemical and aqueous dispersion. Preferably, the semiconductor nanoplatelets are prepared by an exfoliation method, such as ultrasound. Other possible methods of exfoliation that can be used in the invention include high temperature or the use of solvents. Preferably the exfoliation process is carried out starting from an aqueous dispersion of the semiconductor material. The term "three-dimensional" or "3D" as used herein refers to a geometric structure of materials, in which each of the points that make it up is characterized by three dimensions, for example, width, height and depth. The term "two-dimensional" or "2D" in the context of the invention refers to platelet-like and nanoplatelet-like materials that are characterized by their thickness and a lateral dimension which is always greater than the thickness. The term "polymeric nanoparticles" in the context of the invention may have a mean diameter of between 0.5-1000 nm, for example between 1-500 nm, or between 1 and 250 nm. The term "integrated" refers to the fact that the nanoplatelets of the semiconductor or semiconductors are incorporated into the three-dimensional structure of the reduced graphene oxide, distributed uniformly, homogeneously in the structure. "Ambient temperature" refers to a temperature of 25°C. The materials of the invention have many applications, especially in environmental protection technologies, in the capture and/or chemical conversion of air, water or soil pollutants or as catalysts in chemical reactions. Possible uses include: adsorption and degradation of organic or inorganic contaminants from water, air or soil, eg heavy metals, petroleum, antibiotics, organic dyes or dyes and other contaminants, remediation of soils to remove pesticides and other contaminants; capture and photocatalytic conversion of CO2, photocatalytic dissociation of water to produce oxygen and hydrogen, capture and degradation of harmful tobacco products, etc. Because of their porous structure, the materials of the invention have utility in applications that require the use of porous materials. Potential applications include but are not limited to: gas separation and storage, encapsulation agents in drug delivery systems, catalysts or catalyst supports, electrode and energy storage materials, supports for biomolecule immobilization (hybrid biocatalyst production) and scaffolds. cell materials for use in chromatography in Examples The following examples illustrate the invention. Materials and methods Materials The starting graphene oxide (GO) was obtained from Graphenea, in the form of an aqueous dispersion of 4mg/mL and 10mg/mL, with a monolayer content of >95%, and pH between 2.2-2.5. Elemental analysis of the GO showed C: 49-56%, H: 0-1%, N: 0-1%, S: 2-4% and O: 41-50%. L-Ascorbic acid (AsA) was used as reducing agent (≥99.0%, Sigma-Aldrich). For the synthesis of the 2D nanoplatelets, six different semiconductor materials were used: i) gallium(III) nitride - GaN, (99.99%, Alfa Aesar) with a maximum of 0.01% metal impurities; ii) tungsten(IV) sulfide -WS 2 (99%, Sigma-Aldrich), with a particle size of 2µm; iii) zinc oxide - ZnO (99.99%, Sigma-Aldrich) with trace element analysis ≤ 150.0 ppm; iv) cadmium sulfide - CdS (Sigma-Aldrich); v) cerium(IV) oxide - CeO 2 (99.9%, Sigma-Aldrich), with particle size <5µm, with rare earth trace analysis ≤ 1500.0 ppm; vi) molybdenum(IV) sulfide - MoS2 (99%, Sigma-Aldrich), with particle size <2µm. Methyl methacrylate, technical grade (MMA, Quimidroga) and glycidyl methacrylate (GMA) (≥97.0%, Sigma-Aldrich), were used without purification. Potassium persulfate (KPS) was used as a water-soluble radical initiator, sodium dodecyl sulfate (SDS) was used as anionic surfactant, and sodium bicarbonate (NaHCO3) was used as a buffer and to control the viscosity of the microemulsion by reducing electrostatic interactions. Obtained from Aldrich. Deionized water was used as polymerization medium. For adsorption experiments methylene blue (MB) (≥ 82 wt% dye content) from Sigma-Aldrich was used. Synthesis of polymeric nanoparticles The preparation of PMMA (Polymethyl methacrylate) particles functionalized with epoxy groups was carried out by emulsion polymerization of a monomeric mixture of MMA/GMA in a weight ratio of 90/10 (Different functional monomers can be used in different amounts and thus obtain functionalized polymer nanoparticles with various groups). Table 1 shows an example of a formulation to synthesize an aqueous dispersion of polymer particles with a solids content of 20% by weight with respect to the total weight of the resulting dispersion. Table 1. Formulation for the preparation of MMA/GMA 90/10% by weight
Figure imgf000019_0001
The reaction was carried out in a 250 ml reactor, equipped with a stainless steel rotor, a condenser and a nitrogen inlet. The aqueous solution of the KPS initiator and NaHCO3 was prepared separately and allowed to homogenize at room temperature. The emulsion was prepared by mixing the two phases, the organic phase of monomeric mixture and the aqueous phase of SDS solution in water. The emulsion was added to the reactor and stirred at room temperature for 15 minutes. Subsequently, the reactor was heated to 70 o C, the KPS/NaHCO 3 solution was added all at once and it was left to polymerize under a nitrogen atmosphere for 1.5 hours. In the resulting mixture of polymer particles in water the monomer conversion was 99% with a mean particle diameter of 70 nm. Preparation of semiconductor nanoplatelets Two-dimensional (2D) nanoplatelets were prepared from crystals of the respective semiconductor materials (MoS 2 , WS 2 , CeO 2 , CdS, GaN, or ZnO). The crystals were crushed in a mortar until they had a grain size of less than 2 ^m. A dispersion of the resulting powder was made in water/ethanol and sonicated with shaking for 1 h at 360 W (1 sec on, 2 sec off) with a Branson 450 tip-top sonicator (Danbury, CT). The flask was kept cold in an ice-water bath during sonication. The resulting dispersion was used directly in the preparation of 3D materials. Analysis and structural characterization of the nanoplatelets was carried out by transmission electron microscopy (TEM). Synthesis of Three-Dimensional (3D) Structures of Reduced Graphene Oxide (rGO)/Two-Dimensional (2D) Platelets and of rGO/2D Platelets/Polymer A suitable amount of dispersion of graphene oxide GO was mixed with a dispersion of 2D semiconductor nanoplatelets. When the structure includes a polymer, ie, for the synthesis of the rGO/2D nanoplatelets/polymer material, a suitable proportion of polymeric particles was added to the GO:2D nanoplatelets dispersion. The ratio between the weight of the GO/2D material and the weight of the polymer can vary and several ratios have been used, although in one particular case the ratio of 4 to 1. At this time the appropriate ratio of reducing agent was added, in this In the case of ascorbic acid, in three quantities with respect to the weight of graphene oxide (GO:AsA 1:0.5; 1:1; and 1:2). The mixture was stirred for 0.5 hour to homogenize. Finally, the homogeneous mixture of GO/2D nanoplatelets/AsA or GO/2D nanoplatelets/polymer/AsA was heated in an oven (vacuum oven, Binder, Tuttlingen, Germany) and kept overnight at the chosen temperature ( 45 o C, 60 o C or 90 o C) for the formation of three-dimensional nanostructure materials. These materials are hydrogels and are highly swollen with water. Once out of the oven, the wetted material formed was washed by dialysis in Milli Q water for several days, changing the water daily until the conductivity of the water was constant (less than 10 µS/cm). The 3D framework was then dried by lyophilization to give the 3D monolithic material. Characterization The morphology of the obtained 3D dried materials was examined by scanning electron microscopy (SEM or EDX): Hitachi TM3030 tabletop model, 15kV accelerating voltage, after covering the samples with a thin layer of gold. In the results tables, determined by EDX analysis, the elemental composition of the materials is presented in % by weight Transmission electron microscopy (TEM) was used to determine the size and shape of the cleaved 2D semiconductor nanoplatelets. A diluted solution (1% by weight) in water was deposited on a 300 mesh copper mesh. Characterization was carried out with a Tecnai G220 Twin (FEI) microscope at an accelerating voltage of 200 keV in brightfield imaging mode. Demonstration of photocatalytic activity To examine the photocatalytic activity of the 3D materials, a blue colored aromatic molecule (organic methylene blue (MB)) was chosen. This compound is used as a dye in the textile industry and wastewater generally contains this dye, causing serious problems for aquatic life and changes in the environment. The adsorption and degradation of dissolved MB in aqueous solutions is therefore very important and of great environmental utility. The 3D porous materials, rGO, rGO/2D nanoplatelets and rGO/polymer/2D nanoplatelets, and also 2D nanoplatelets in aqueous dispersion, were added to the aqueous MB solution with a concentration of 0.1 mg/ml. Graphene oxide 3D materials are very good adsorbents of organic molecules, thus aqueous solutions quickly discolored for all 3D materials, even those without semiconductor nanoplatelets. However, 3D materials containing semiconductor nanoplatelets were expected to be capable of degrading adsorbed MB molecules. The photocatalytic experiments were carried out in a chamber equipped with 20 UV light lamps of 15 W each, (model BS 03, Dr. Gröbel UV-Elektronik GmbH). The irradiance of UV light is 7 mW/cm 2 . For the experiment, the reactor containing the aqueous MB solution to which 1 mg of 3D material had been added for each 0.01 mg of MB was placed in the UV chamber. To distinguish between the mechanisms of MB adsorption and degradation, we compared processes using 3D rGO/2D nanoplatelet materials and formed from rGO. In this case (rGO only) it was assumed that only MB adsorption would occur on the 3D material. To follow the discoloration of the aqueous solution (disappearance of MB from it), UV spectroscopy was used. To analyze the aqueous solution of MB and in the case of degradation, to analyze the products obtained, MALDI-TOF mass spectrometry was used. UV-Vis absorption measurements were made in a Shimadzu spectrometer (model UV-2550230 V). Measurements in the range 300–800 nm were carried out at room temperature. All solutions were diluted to 1 ml of milli-Q water, using 50 µl of the initial methylene blue (MB) solution and/or for the solution of each photocatalysis experiment. MALDI-TOF-MS measurements were carried out on a Bruker Autoflex Speed instrument (Bruker, Germany) equipped with a 355 nm Nd:YAG laser. Spectra were obtained in positive ion reflector mode and linear mode (20 kV accelerating voltage, 5 x 10-6 mbar pressure). Samples of approximately 0.5 μL of solution resulting from the decolorization experiments were analyzed. For each spectrum, 10,000 laser shots were accumulated. Results Characterization of the 3D photocatalytic systems and their photocatalytic activity Example 1: WS2/rGO, prepared from WS2/GO with a weight ratio of 1:4. Synthesis conditions: T = 60°C, GO: AsA = 1:1 by weight Example 2: WS2/rGO prepared from WS2/GO with a weight ratio of 1:4. Synthesis conditions: T = 60°C, GO: AsA = 1:1 by weight and 20% by weight of polymer with respect to WS2/GO Example 3: WS2/rGO, prepared from WS2/GO with a weight ratio of 1:3. Synthesis conditions: T = 60°C, GO: AsA = 1:1 Example 4: WS2/rGO, prepared from WS2/GO with a weight ratio of 1:4, at high temperature. Synthesis conditions: T = 90ºC, GO: AsA = 1:1 Example 5: WS 2 /rGO/Pol prepared from WS 2 /GO with a weight ratio of 1:4, at high temperature. Synthesis conditions: T=90ºC, GO:AsA=1:1 and 20% by weight of polymer with respect to WS 2 /GO. Example 6: WS 2 /rGO prepared from WS 2 /GO with a weight ratio of 1:4, with less AsA. Synthesis conditions: T=60 o C, GO:AsA=1:0.5 Example 7: MoS 2 /rGO prepared from MoS 2 /GO with a weight ratio of 1:4. Synthesis conditions T = 60°C, GO:AsA = 1:1 Example 8: MoS 2 /rGO/Pol prepared from MoS 2 /GO with a weight ratio of 1:4. Synthesis conditions: polymer: T = 60ºC, GO: AsA = 1:1 and 20% by weight of polymer with respect to MoS 2 /GO Example 1: Synthesis and characterization of WS2/rGO, prepared from WS2/GO with a weight ratio of 1:4 Comparison of the photocatalytic activity of the WS2/rGO system with the pure WS2 nanoplatelets and with pure rGO Synthesis conditions: T = 60°C, GO: AsA = 1:1 by weight; WS 2 :GO = 1: 4 by weight In Figure 1a, you can see the TEM images of the initial WS2 nanoparticles produced by sonication of the bulk crystal of WS2, in which you can see 2D nanoplatelets, with lateral dimension between 50-250nm. These nanomaterials, combined with GO, which was subsequently reduced, produced 3D WS2/rGO. SEM images of the 3D structure made only of rGO and WS2/rGO hybrid are presented in Figures 1b and 1c, respectively. Porous materials were obtained in both cases. Table 1.1 presents the results obtained by the EDX analysis, showing the elemental composition of rGO and WS2/rGO in wt%, demonstrating the successful incorporation of the WS2 photocatalyst into the 3D rGO structure. Table 1.1: Elemental composition of the 3D rGO and WS2/rGO structures of Example 1, determined by EDX.
Figure imgf000023_0001
Figure imgf000024_0001
Figure 1d shows the kinetics of the decolorization process of the aqueous methylene blue (MB) solution (followed by UV spectrophotometry). These results were obtained by placing the photocatalytic material in an aqueous MB solution and irradiating with UV light (368 nm). The MB concentration was 0.1 mg/mL and in all cases 1 mg of photocatalytic material was added for every 0.01 mg of MB in solution. The 3D material adsorbs the MB dye from the solution, and on the surface of the WS 2 it is degraded by photoinitiation produced by ultraviolet light. The same experiment was also carried out without ultraviolet light (experiment in the dark), in which case the discoloration of the aqueous MB solution occurs simply by adsorption of the MB by the 3D material. The WS2 nanoplatelets and the 3D nanostructure made only of rGO were also used under ultraviolet light as comparative experiments. The UV results in Figure 1d show that, within 3 h, pure rGO adsorbs 82% of the MB. In the case of the WS2/rGO hybrid, 80% is adsorbed in the dark. Under ultraviolet light, the aqueous MB solution is completely discolored. Complete decolorization occurs due to the removal of adsorbed MB thanks to the photocatalytic reaction initiated by UV light that activates the WS 2 photocatalysts. The products produced by the degradation of the MB are desorbed from the surface of the 3D material into the aqueous solution and analyzed. The aqueous solution was also analyzed. Pure WS2 (without rGO) removed only 56% of MB. This shows that the presence of rGO significantly improved the photocatalytic activity of WS2, since the decolorization was faster (100% in 30 min) and complete. MB degradation was demonstrated by MALDI-TOF-MS analysis of the aqueous solution after the decolorization experiments. The aqueous solution was analyzed. The results in the form of mass spectra, identifying the presence of degradation products, are presented in Figures 1e – 1h. In Figure 1e, the mass spectra of the aqueous solution after destaining with 3D rGO/WS 2 and UV light (1 mg 3D material per 0.01 mg MB) are shown. The magnification of each area (0-100 m/z; 100-200 m/z and 200-300 m/z) is presented in Figure 1e, together with the assignment of the peaks. In the m/z range 0-100, the peak assignment is as follows: m/z 23 and 39 peaks are assigned to Na and K, and as the following peaks appear they are assigned to: hydrochloric acid, pentane, thioacetone, thiazoline, sulfate anion, 4 or 3-hydroxybutanoic acid, 2-aminophenol, benzene-Cl anion, 2-ethylthiazole, 4-nitrophenol, benzenesulfonic acid, potassium sulfite, sodium thiosulfate, 4-phenolsulfonic acid, sulfate potassium, sodium benzenesulfonate hydrate. In the range over m/z 200, the m/z peak of 256 maps to C 14 H 14 N 3 S (MB with one less -CH 3 ) and the 270 peak to C 15 H 16 N 3 S. These results indicate that there are a number of intermediate products of the MB degradation reaction. Only traces of MB are observed. To check whether there is a possibility of decreasing the number of intermediates created, the experiment was performed using double the 3D WS2/rGO photocatalyst (2 mg per 0.01 mg MB). The MALDI mass spectra of the aqueous solution after 3 h under UV irradiation are shown in Figure 1f. Compared to the spectra in Figure 1e, the same intermediates were identified, but in smaller amounts, indicating that doubling the amount of photocatalyst, in proportion to MB, produces more degradation. The appearance of sulfur dioxide peaks indicates that a part of the MB was completely degraded and the presence of new compounds such as benzene, dimethylaniline and the peak that appears at m/z of 244, assigned to C12H10ON3S (MB with four less -CH3) , are the result of the degradation of the first products, previously observed. This result shows that by optimizing the amount of WS2/rGO photocatalyst, a very efficient degradation of both the MB and the first reaction products can be achieved. In Figure 1g, a comparison of the composition of the aqueous solution can be seen after MB degradation with 3D WS2/rGO performed in the presence of UV light and in the dark. The aqueous solution from the dark experiment (upper curve in Figure 1g) shows the presence of MB (the marked peak), while this peak is absent in the aqueous solution after UV light treatment. The magnification of the three characteristic areas is also shown for comparison. To check if all the MB was degraded after the photocatalytic experiment with double WS 2 /rGO (2 mg of material per 0.1 mg of MB), the 3D material was desorbed with ethanol to extract possible components adsorbed on its surface. (Undegraded MB and degradation products). The MALDI-TOF-MS analysis of the solution obtained from the desorption is shown in Figure 1h, and indicates the absence of MB. This demonstrates the complete photocatalytic degradation of MB, despite the fact that some reaction intermediates are still observed (Figure 1f). Example 2: Synthesis and characterization of WS 2 /rGO, prepared from WS 2 /GO with a 1:4 weight ratio Effect of polymer addition on the photocatalytic activity of the WS2/rGO system Synthesis condition: T = 60ºC, GO: AsA = 1:1; WS 2 :GO = 1: 4, amount of polymer 20% by weight in relation to WS2/GO In this example, polymer nanoparticles were added to the same photocatalyst of Example 1 to improve handling, durability and increase ease of purification and reuse of photocatalysts. The amount of polymer was 20% by weight based on the weight of WS2/rGO. In Figure 2a you can see the SEM images of the 3D composite material, showing a spongy structure, with a very well developed porous morphology. The WS2 nanoplatelets are seen in the image as white structures on top of the gray polymer/rGO composite structure. In Table 2.1, the elemental composition determined from the EDX results for this system is shown, demonstrating the incorporation of WS2 2D nanoplatelets into the composite 3D structure. Table 2.1: Elemental composition of 3D WS 2 /rGO/Pol from Example 2, determined by EDX
Figure imgf000026_0001
Figure 2b presents the kinetics of the decolorization process of an aqueous solution of MB with a concentration of 0.1 mg/mL, by the 3D WS 2 /rGO/Pol nanostructure under UV light for 3 h. This result is compared with the same experiment performed in the dark and with the same experiments performed with pure 3D rGO and pure WS 2 . Figure 2b shows that in the presence of the WS 2 /rGO/Pol compound under UV light, almost the entire amount of MB in the solution was adsorbed/degraded. This material exhibits improved efficiency compared to pure 2D WS 2 nanochips. The mass spectra of the aqueous solution after the decolorization experiments, obtained by MALDI-TOF, are presented in Figure 2c. The spectra show that MB degradation occurred when the composite 3D photocatalyst was used in the presence of ultraviolet light. It is noteworthy that although some MB is still present in the solution, there are very few reaction intermediates (such as: benzenesulfonic acid, potassium sulfite, sodium thiosulfate, 4-phenolsulfonic acid, potassium sulfate, and hydrated sodium benzenesulfonate). ). In the higher mass range, above m/z 200, only the characteristic peaks of the C 14 H 14 N 3 S and C 15 H 16 N 3 S structures and the residual MB appear. The new peaks of CO2, NO3-, benzene and 4-aminocatechol are also observed. The reaction products indicate efficient degradation of the MB. In Figure 2d, a comparison can be seen between the experiments performed with the WS2/rGO/Pol material in the presence of UV light and in the dark. As expected, according to the invention, much fewer components are observed in the aqueous solution after decolorization in the presence of UV light. Example 3: Synthesis and characterization of WS2/rGO, prepared from WS2/GO with a 1:3 weight ratio Effect of increasing the amount of WS2 on the 3D WS2/rGO structure Synthesis condition: T = 60ºC, GO: AsA =1:1; WS2:GO = 1: 3, This example shows the results obtained with the photocatalyst containing the highest amount of WS2 in the 3D WS2/rGO structure, prepared from WS2:GO 1:3 by weight. The SEM images are shown in Figure 3a and Table 3.1 presents the composition Table 3.1: Elemental composition of 3D WS 2 /rGO from Example 3, determined by EDX
Figure imgf000028_0001
Figure 3b shows the kinetics of the decolorization process of a 0.1 mg/mL aqueous solution of MB, with the 3D WS 2 /rGO photocatalyst of this example, in the presence of UV light. When compared with the results obtained for the same reaction carried out in the dark and with the yield of the pure components, a very rapid discoloration is observed in the presence of light (the MB is eliminated in less than 30 min). The aqueous solution after destaining was examined by MALDI-TOF and the results are shown in Figure 3c. In the first mass range (m/z 100-200) the following degradation products are observed: nitrate, sulfur dioxide, hydrogen chloride from hydrochloric acid, pentane, benzene, thiazoline, sulfate anion, 4- or 3-acid hydroxybutanoic acid, benzene-Cl anion, 2-ethylthiazole, 4-nitrophenol, sodium benzenesulfonate hydrate. Between masses of m/z 200 and 270 the presence of C15H16N3S (MB with one less -CH3) is observed. All this shows that the photocatalytic decomposition of MB with this photocatalyst is even more advanced than with the compound prepared from GO and WS2 with a 1:4 ratio, with very few degradation products and in a low mass area. This demonstrates a great degradation of the reaction products especially the degradation of very high molar mass products which contain conjugated aromatic rings and are the most toxic. In the following two examples, the synthesis reaction temperature of the 3D materials was increased from 60°C to 90°C and the synthesis was performed with two different amounts of reducing agent (ascorbic acid, AsA). Example 4: Synthesis and characterization of WS2/rGO, prepared from WS2/GO with a weight ratio of 1:4, at high temperature Effect of temperature increase during the synthesis of the 3D WS2/rGO structure Synthesis condition: T = 90°C, GO: AsA = 1:1; WS 2 :GO = 1: 4, The morphology of the 3D WS 2 /rGO obtained at high temperature is presented in Figure 4a and the elemental composition of the material is presented in Table 4.1. Table 4.1: EDX results of the 3D WS2/rGO material from Example 4
Figure imgf000029_0001
The kinetics of the decolorization process of the MB solution (0.1 mg/mL) is presented in Figure 4b. It can be seen that in the presence of UV light, complete decolorization of the aqueous MB solution was achieved within 60 minutes. The photocatalytic adsorption/degradation process in this case is slightly slower than for the same material obtained at a lower temperature. Decolorization performed in the dark shows removal of 68% of the MB from the solution by adsorption. The aqueous solution after destaining was examined by MALDI-TOF and the results are shown in Figure 4c. The complete mass spectrum still shows the presence of the MB peak (the technique is very sensitive and the peak appears at minimal concentrations). The extended spectra show the presence of the degradation products that are detailed below. In the m/z range of 100-200: sulfur dioxide, benzene, thiazoline, sulfate anion, 4- or 3-hydroxybutanoic acid, 2-aminophenol, benzene-Cl anion, 4-nitrophenol. In the m/z range of 200 to 270: C15H16N3S (MB with one less -CH3). The results show that this material presents an advanced photocatalytic activity since very few reaction products are obtained, indicating the degradation and elimination of MB and many of the toxic intermediates. Example 5. Synthesis and characterization of WS 2 /rGO/Pol prepared from WS 2 /GO with a weight ratio of 1:4, at high temperature Effect of polymer addition on structure and photocatalytic performance Synthesis conditions: T=90°C, GO:AsA=1:1; WS 2 :GO=1:4; amount of polymer 20% by weight relative to WS 2 /GO To the material produced at 90°C, presented in Example 4, polymer was added, 20% by weight relative to the weight of WS 2 /GO. The morphology of WS 2 /rGO/Pol, presented in Figure 5a shows a spongy and porous structure. Table 5.2 presents the elemental composition of the 3D compound structure. Table 5.1: EDX results of the WS 2 /rGO/Pol system prepared at 90 o C
Figure imgf000030_0001
Decolorization experiments with this material are shown in Figure 5b. The results obtained show that the compound photocatalysts are much more efficient than the 2D WS2 platelets alone. In the absence of light, discoloration occurs due to the adsorption of MB on the material. It is shown that the photocatalytic degradation of MB occurs in the presence of UV light, thanks to the integrated photocatalyst. Example 6: Synthesis and characterization of WS2/rGO prepared from WS2/GO with a weight ratio of 1:4. Effect of reducing the amount of AsA reducing agent used for the synthesis Synthesis conditions: WS2/GO =1:4, T=60 o C, GO:AsA=1:0.5 In this example, the influence of the amount of AsA used for the reduction of GO, in the characteristics of the 3D structure and in its photocatalytic performance. The AsA in this example was halved that used in Examples 1-5, where the ratio of GO:AsA is 1:1 by weight. Figure 6a shows the SEM images of 3D WS2/rGO, and it is observed that the sample is less porous than the same material obtained with higher amount of AsA (Example 1, Figure 1c, GO: AsA = 1:1). Table 6.1 shows the EDX results in which a large incorporation of WS 2 is observed in the 3D composite structure. Table 6.1: Elemental composition of the 3D WS 2 /rGO of Example 6, determined by EDX
Figure imgf000031_0001
The decolorization experiments are shown in Figure 6b. Surprisingly, this material has proven to be very effective in the photocatalytic degradation of MB, since MB was completely removed from solution under UV light within 30 min. The characterization by MALDI-TOF demonstrates the photocatalytic degradation of MB, since the formation of numerous degradation products is observed. The mass spectra of the aqueous solution can be seen in Figure 6c in which the formation of the following compounds after MB degradation is observed. In the mass area of m/z 100 -200: sulfur dioxide, hydrochloric acid hydrogen chloride, pentane, thioacetone, benzene, thiazoline, sulfate anion, 4- or 3-hydroxybutanoic acid, 2-aminophenol, benzenesulfonic acid, potassium sulfite, sodium thiosulfate, 4-phenolsulfonic acid, potassium sulfate, hydrated sodium benzenesulfonate. In the m/z 200 -300 mass area appears the peak am/z 256 assigned to C14H14N3S and m/z 270 assigned to C15H16N3S (MB with one less -CH3). The MB was almost completely degraded in 30 minutes, as shown in Figure 6b determined by UV spectroscopy. The presence of MB in the solution (traces) along with a large number of intermediate reaction products is seen in Figure 6c. The results indicate that to obtain a higher efficiency of photocatalytic degradation in this system, it may be beneficial to use less amount of reducing agent with respect to the amount of GO. Example 7: Synthesis and characterization of MoS 2 /rGO prepared from MoS 2 /GO with a weight ratio of 1:4 Photocatalytic activity of the MoS 2 /rGO system Synthesis conditions MoS 2 /GO = 1:4, T = 60°C, GO:AsA = 1:1 In Figure 7a, you can see the TEM images of the initial MoS 2 particles, which were obtained by sonication of the bulk MoS 2 crystal. 2D nanoplatelets with a lateral dimension in the range of 50 nm to 250 nm can be observed. Figure 7b presents the SEM images of the 3D morphology of the MoS 2 /rGO structures. Table 7.1 presents the elemental composition of the MoS 2 /rGO compounds, showing a large incorporation of the MoS2 photocatalyst. Table 7.1: Elemental composition of 3D MoS2/rGO of Example 7, obtained by EDX
Figure imgf000032_0001
In Figure 7c, the kinetics of the decolorization of the aqueous MB solution is shown. The MB removal efficiency is significantly improved when using 3D composites (77%, in the presence of light) compared to 2D MoS2 nanochips (11%). The results show that the 3D MoS2/rGO structure presents better removal and degradation efficiency. MALDI-TOF-MS mass spectra of ethanol-desorbed solution for the 3D MoS2/rGO system, presented in Figure 7d, demonstrate that MB has been completely degraded, as the characteristic MB peak does not appear in the spectrum. Example 8: Synthesis and characterization of MoS2/rGO/Pol (1:4) prepared from MoS2/GO with a weight ratio of 1:4 Effect of polymer addition to MoS2/rGO Synthesis conditions MoS 2 :GO = 1:4, polymer: 20% by weight relative to MoS 2 :GO, T = 60°C, GO: AsA = 1:1 In this example, 20% polymer (% by weight) was added in relation to the weight of MoS 2 /GO). Figure 8a shows the SEM images and a porous and spongy material is observed). The pore size distribution at this scale is 1 to 10 µm. Table 8.2 shows the elemental composition of MoS 2 /rGO/Pol. Table 8.1: Elemental composition of MoS 2 /rGO/Pol from Example 8, determined by EDX
Figure imgf000033_0001
Figure 8b shows the kinetics of the decolorization process of the aqueous MB solution using the 3D composite system compared to pure rGO and pure MoS 2 . Although pure rGO has a high MB removal (adsorption) efficiency, rGO is only adsorbing MB, not degrading it. In the composite 3D structure, the adsorbed MB is degraded very efficiently by the photocatalytic effect of the photocatalyst in the presence of light. Other photocatalysts that have been prepared following the procedures as defined in the previous examples, except for the nanoplatelets, which were CdS: examples of CdS/rGO and CdS/rGO/Pol materials. General conclusions of the UV and MALDI-TOF experiments In the characterization with MALDI-TOF some peaks related to the matrix used (for example Na or K) can be observed and others are MB degradation products, for example: i) Thiazoline, ii) SO₄²-, iii) 4 or 3-Hydroxybutyric acid, iv) 2-Aminophenol and v) MB without a methyl group (-CH3). Table C1 shows the peaks recognized and attributed to the degradation products. The results obtained with the samples of Example 1 show peaks with a low m/z. This indicates that the degradation of the MB has come a long way. Comparing Examples 1 and 2, the additional peaks indicate new degradation products after addition of the polymer (Example 2) at much lower m/z. This indicates a progress in the degradation of both the MB and the first degradation products, indicating that even a part of the MB was mineralized. These peaks are CO 2 , NO 3 -, SO 2 , C 6 H 6 and 4-Catecholamine. On the contrary, the system without polymer shows the presence of larger molecules, with an aromatic ring in its structure or structures with Na/K with -S- groups (158-174-198 m/z), (C 6 H 6 O 3 S, K 2 SO 3 , Na 2 S 2 O 3 , C 6 H 6 O 4 S, K 2 SO 4 and C6H7NaO4S). Similarly, in Examples 1 and 3, the effect of increasing the amount of WS2 nanoplatelets can be seen. By increasing the number of nanoplatelets, a greater number of peaks can be seen at lower m/z values, and aromatics with a lower molar mass, between 158-197 m/z. These peaks are for CO2, NO3-, SO2, C6H6 and 4-Catecholamine. Finally, comparing Examples 1, 5 and 7, where the preparation parameters were changed, it is observed that the samples prepared at the same temperature (60 o C) show a similar level of degradation, since very similar peaks are observed: hydrochloric acid hydrogen chloride, thioacetone, C6H6O3S, K2SO3, Na2S2O3, C6H6O4S, K2SO4, C6H7NaO4S and at 256 m/z the MB with 2 -CH3 less. Table C1: MALDI peaks recognized and attributed to methylene blue degradation products for Examples 1, 2, 3, 4, 5, 7, showing the m/z and chemical structures.
Figure imgf000034_0001
Figure imgf000035_0001
Figure imgf000036_0001
Figure imgf000037_0001
Figure imgf000038_0001
The processes carried out in the presence of UV light with the materials of the invention resulted in adsorption and photocatalytic degradation of the adsorbed agent. In all cases and according to the invention, the photocatalyst integrated into the three-dimensional structure of reduced graphene oxide demonstrated a much higher photocatalytic activity than that of the pure (non-integrated) semiconductor. The integrated materials are monolithic materials, with a porous structure, easy to handle and can be reused without losing active components between cycles. In all systems investigated, it has been shown that the 2D semiconductor has been incorporated into rGO to obtain an integrated photocatalyst and that the agent removal kinetics (MB in the examples) from the aqueous solution is much faster with the integrated photocatalyst than with the 2D pure photocatalyst. The example experiments also demonstrate that synthesis parameters of integrated photocatalysts (eg, temperature, amount of reducing agent, ratio of GO and semiconductor components) can affect the structure and efficiency of integrated photocatalysts. These variations are advantageous because they allow the photocatalyst to be adapted depending on the application of interest for it. References [1] Nano Research 2017, 10(5), 1662–1672. [2] DHW Appl. Mater. Interfaces 2013, 5, 2227−2233. [3] J. Mater. Chem. A, 2014, 2, 3605–3612. [4] Applied Catalysis B: Environmental 2016, 199, 412–423. [5] Applied Catalysis B: Environmental 2018, 221, 36–46. [6] Separation and Purification Technology 2018, 194, 96–103. [7] ACS Sustainable Chem. Eng. 2018, 6, 5718−5724. [8] Adv. Mater.2015, 27, 3767–3773 [9] Applied Catalysis B: Environmental 217 (2017) 65–80. [10] Journal of Colloid and Interface Science 536 (2019) 389–398. [11] New J. Chem., 2016, 40, 3208—3215. [12] Applied Catalysis B: Environmental 212 (2017) 41–49. [13] International Journal of Hydrogen Energy 2017, 42, 1554 - 15550. [14] Small 2015, 11, No.36, 4785–4792. [15] CrystEngComm, 2014, 16, 399–405. [16] Cat. Sci. Technol., 2017, 7, 1305–1314. [17] ACS Appl. Mater. Interfaces 2015, 7, 25693−25701. [18] Applied Catalysis B: Environmental 205 (2017) 228–237. [19] US 2016/0296909 A1 [20] US 2012/0149554 A1 [21] US 2015/0069295 A1 [22] Chem. Rev., 2015, 115, 10307–10377 23] Phys. Chem. Chem. Phys., 2013, 15, 19102–19118 [24] Chem. Soc. Rev., 2014, 43, 8240–8254

Claims

Reivindicaciones 1. Un fotocatalizador que comprende una estructura tridimensional de óxido de grafeno reducido y nanoplaquetas de por lo menos un semiconductor. 2. Un fotocatalizador integrado según la reivindicación 1, en el que las nanoplaquetas de semiconductor están integradas en la estructura tridimensional de óxido de grafeno reducido. 3. Un fotocatalizador según la reivindicación 1 o 2, que tiene una estructura porosa. 4. Un fotocatalizador según cualquiera de las reivindicaciones 1 a 3, que comprende nanopartículas de por lo menos un polímero. 5. Un fotocatalizador según cualquiera de las reivindicaciones anteriores, en el que dicho semiconductor se selecciona de un grupo formado por GaN, CeO2, CdS, ZnO, MoS2, WS2, WO3 y otros materiales semiconductores que permiten la formación de nanoplaquetas por exfoliación de los mismos. 6. Un fotocatalizador según la reivindicación 4 o 5, en el que dicho polímero se selecciona de un grupo formado por estirenos, acrilatos, metacrilatos, opcionalmente funcionalizados, y sus mezclas. 7. Un fotocatalizador según cualquiera de las reivindicaciones anteriores, en el que dicho semiconductor es WS2 o MoS2 y opcionalmente, el fotocatalizador comprende nanopartículas de polimetil metacrilato funcionalizado. 8. Procedimiento para la preparación de un fotocatalizador según cualquiera de las reivindicaciones 1 a 7, que comprende las siguientes etapas: a. Preparar una dispersión acuosa de nanoplaquetas de óxido de grafeno, nanoplaquetas de uno o más semiconductores y un agente reductor b. Calentar la dispersión resultante, sin agitación, a una temperatura en el intervalo de 45 a 90 oC 9. Procedimiento según la reivindicación 8, en el que el agente reductor es ácido ascórbico o hidracina. 10. Procedimiento según las reivindicaciones 8 o 9, en el que en la etapa b), la dispersión se calienta durante un periodo de tiempo de entre 3 y 24 horas 11. Procedimiento según cualquiera de las reivindicaciones 8 a 10, que comprende una etapa c), entre a) y b), en la que la dispersión resultante en a) se agita a temperatura ambiente durante un periodo de tiempo suficiente para homogeneizar la dispersión, opcionalmente de 15 minutos a 1 hora 12. Procedimiento según cualquiera de las reivindicaciones 8 a 11 que además comprende secar el producto de reacción usando liofilización o microondas 13. Uso del catalizador según cualquiera de las reivindicaciones 1 a 7, en tecnologías de protección ambiental, en la captura y/o conversión química de contaminantes del aire, agua o el suelo o como catalizadores en reacciones químicas. 14. Uso del material definido según cualquiera de las reivindicaciones 1 a 7, en aplicaciones que requieren el uso de materiales porosos. Claims 1. A photocatalyst comprising a three-dimensional structure of reduced graphene oxide and nanoplatelets of at least one semiconductor. An integrated photocatalyst according to claim 1, wherein the semiconductor nanoplatelets are integrated into the three-dimensional structure of reduced graphene oxide. A photocatalyst according to claim 1 or 2, having a porous structure. A photocatalyst according to any of claims 1 to 3, comprising nanoparticles of at least one polymer. 5. A photocatalyst according to any of the preceding claims, wherein said semiconductor is selected from a group consisting of GaN, CeO 2 , CdS, ZnO, MoS 2 , WS 2 , WO 3 and other semiconductor materials that allow the formation of nanoplatelets by their exfoliation. A photocatalyst according to claim 4 or 5, wherein said polymer is selected from the group consisting of optionally functionalized styrenes, acrylates, methacrylates, and mixtures thereof. A photocatalyst according to any of the preceding claims, wherein said semiconductor is WS2 or MoS2 and optionally, the photocatalyst comprises functionalized polymethyl methacrylate nanoparticles. 8. Process for the preparation of a photocatalyst according to any of claims 1 to 7, comprising the following steps: a. Prepare an aqueous dispersion of graphene oxide nanoplatelets, nanoplatelets of one or more semiconductors, and a reducing agent b. Heat the resulting dispersion, without stirring, to a temperature in the range from 45 to 90 o C 9. Process according to claim 8, in which the reducing agent is ascorbic acid or hydrazine. 10. Process according to claims 8 or 9, wherein in step b), the dispersion is heated for a period of between 3 and 24 hours. 11. Process according to any of claims 8 to 10, comprising a step c), between a) and b), in which the resulting dispersion in a) is stirred at room temperature for a period of time sufficient to homogenize the dispersion, optionally from 15 minutes to 1 hour 12. Process according to any of claims 8 to 11 which further comprises drying the reaction product using lyophilization or microwaves 13. Use of the catalyst according to any of the Claims 1 to 7, in environmental protection technologies, in the capture and/or chemical conversion of air, water or soil pollutants or as catalysts in chemical reactions. 14. Use of the material defined according to any of claims 1 to 7, in applications that require the use of porous materials.
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