ES2937334B2 - GRAPHENE OXIDE PHOTOCATALYSTS AND SEMICONDUCTOR - Google Patents

Description

DESCRIPTION

GRAPHENE OXIDE PHOTOCATALYSTS AND SEMICONDUCTOR

field of invention

The present invention falls within the field of photocatalysts, in particular it refers to photocatalysts that comprise 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 an 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 compounds with high added value, 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 (e.g., TiO ² and ZnO) or chalcogenides (e.g., CdS, ZnS, CdSe, ZnSe, and CdTe). The most used is TiO ₂ .

Commercial photocatalysts (usually TiO ² ) are typically sold in the form of powdered nanoparticles. For their use, it is necessary to incorporate these photocatalysts into a solid support. Examples of commercial TiO ² nanoparticles are: CristalACT N ™ DT'20, Green Millenium and Microban. Some devices are also known, for example, devices for air purification, which work with TiO ² nanoparticles integrated into different supports.

Certain three-dimensional graphene structures 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 (zero-dimensional) or nanotubes (one-dimensional). The method of (180 oC) for extended periods of time (12-24 hours).

The presence of graphene in these compounds provides, on the one hand, a suitable support for photocatalysts and, on the other hand, 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 composites is unsatisfactory.

Among the objectives of the present invention is to provide an improved photocatalyst, with 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 simply and with a process that can be carried out 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 these photocatalysts, semiconductor nanoplatelets are integrated into the three-dimensional structure of reduced graphene oxide. In the state of the art, there is no known 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. In particular embodiments, these polymers are selected from a group consisting of 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 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 range from 45 to 90 °C. The reducing agent is preferably ascorbic acid or hydrazine.

The described procedure 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 freeze-drying or microwaves.

The photocatalysts described herein have been found to exhibit superior catalytic properties over 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 WS ² 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 three-dimensional (3D) WS ² /rGO nanostructures prepared from WS ² /GO with a weight ratio of 1: 4, Temperature (T) = 60 °C, and with an oxide ratio of Initial graphene (GO): ascorbic acid (AsA) 1:1 by weight as indicated in Example 1

Figure 1d: Kinetics of the decolorization process (Change of methylene blue (MB) concentration with time); Pure 3D rGO with ultraviolet (UV) light; 2D WS ² pure with UV light; 3D WS ² /rGO with UV light and 3D WS ² /-GO D (D = in the dark) (T = 60°C, GO: AsA = 1: 1 by weight) as indicated in Example 1

Figure 1e : Results of MALDI TOF [(Matrix-Assisted Laser Desorption/Ionization)/Time-Of-Flight] mass spectrometry analysis of an aqueous solution of MB, after reaction of discoloration of MB with 3D WS ² /rGO and UV light as indicated in Example 1, with assignment of some of the peaks

Figure 1f: MALDI-TOF mass spectrometry analysis results of aqueous MB solution after decolorization done with double amount of 3D photocatalyst of structures to the peaks.

Figure 1g: MALDI-TOF mass spectrometry analysis results of aqueous MB solution after decolorization done with 3D WS ² /rGO and UV light (full line) and in the dark (dotted line) as indicated in Example 1 .The characteristic peak of MB is marked.

Figure 1h : MALDI-TOF mass spectrometry analysis results of the ethanol solution obtained in the desorption process of 3D WS ² /rGO material after the degradation reaction of MB with UV light as indicated in Example 1

Figure 2a : SEM images of 3D WS ² /rGO/Pol [Pol = polymer] prepared from WS ² /GO with a weight ratio of 1: 4, Temperature (T) = 60 °C, with an oxide ratio of initial graphene (GO): ascorbic acid (AsA) of 1: 1 by weight and 20% by weight of polymer in relation to the amount by weight of WS ² /GO, as indicated in Example 2

Figure 2b : Kinetics of the decolorization process (Change of MB concentration with time); Pure 3D rGO with UV light; 2D WS ² pure with UV light; 3D WS ² /rGO/Pol with UV light and 3D WS ² /rGO/Pol D as indicated in Example 2

Figure 2c : Results of MALDI-TOF mass spectrometry analysis of the MB aqueous solution after decolorization with 3D WS ² /rGO/Pol as indicated in Example 2, with assignment of structures to the peaks. The characteristic peak of MB is marked.

Figure 2d : MALDI-TOF mass spectrometry analysis results of MB aqueous solution after decolorization with 3D WS ² /rGO/Pol under UV light (solid line) and in the dark (dashed line) as indicated in the Example 2. The characteristic peak of MB 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 a ratio of initial graphene oxide (GO): ascorbic acid (AsA ) of 1: 1 by weight, as indicated in Example 3

Figure 3b : Kinetics of the decolorization process (Change of relative MB concentration with time); 3D rGO pure with UV light, 2D WS ² pure with UV light, 3D WS2/rGO with UV light and 3D WS ² /rGO D in the dark as indicated in Example 3

Figure 3c: MALDI-TOF mass spectrometry analysis results of MB aqueous solution after decolorization with 3D WS ² /rGO as indicated in Example 3, with peak assignment

by weight of 1: 4, Temperature (T) = 90 ° C, 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 decolorization process (Change of relative MB concentration with time); 3D rGO pure with UV light, 2D WS ² pure with UV light, 3D WS2/rGO with UV light and 3D WS ² /rGO D in the dark as indicated in Example 4

Figure 4c : Results of MALDI-TOF mass spectrometry analysis of the MB aqueous solution after decolorization with 3D WS ² /rGO with light as indicated in Example 4, with assignment of the peaks

Figure 5a : SEM images of WS ² /rGO/Pol prepared from WS ² /GO with a weight ratio of 1: 4, Temperature (T) = 90 °C, 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 by weight of WS ² /GO, as indicated in Example 5

Figure 5b : Kinetics of discoloration of MB aqueous solution (relative concentration of MB vs. time) of the systems: 3D rGO pure 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) = 60 °C, and with a proportion of initial graphene oxide (GO ): ascorbic acid (AsA) of 1: 0.5 by weight, as indicated in Example 6

Figure 6b : Kinetics of discoloration of MB aqueous solution (change of MB concentration relative to time) of the 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 : MALDI-TOF mass spectrometry analysis results of the MB aqueous solution after decolorization with the 3D WS ² /rGO system with light as indicated in Example 6, with indication of the specific chemical groups in different areas of the spectral.

Figure 7a: Transmission electron microscopy (TEM) images of the MoS ² nanoplatelets as indicated in Example 7

Figure 7b : SEM images of 3D MoS ² /rGO prepared from MoS ² /GO with a weight ratio of 1: 4, Temperature (T) = 60 °C, with a proportion of initial graphene oxide (GO) : ascorbic acid (AsA) of 1:1 by weight, as indicated in Example 7.

in time) of the systems 3D pure rGO with UV light, 2D pure MoS ² with UV light, 3D MoS ² /rGO under UV light and 3D MoS ² /rGO D in the dark as indicated in Example 7.

Figure 7d: MALDI-TOF mass spectra of solution desorbed in ethanol for the 3D MoS ² /rGO system of Example 7.

Figure 8a : SEM images of 3D MoS ² /rGO/Pol prepared from MoS ² /GO with a weight ratio of 1: 4, Temperature (T) = 60 °C, with an initial graphene oxide ratio ( GO): ascorbic acid (AsA) of 1: 1 by weight and 20% by weight of polymer in relation to the amount by weight of WS ² /GO, as indicated in Example 8.

Figure 8b : Kinetics of discoloration of MB aqueous solution (change of MB concentration relative to time) by pure 3D rGO, pure 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 description of the invention

The materials of the present invention are integrated photocatalysts that comprise a three-dimensional structure of reduced graphene oxide (rGO) in which two-dimensional nanoplatelets of a semiconductor are incorporated. This monolithic, three-dimensional, porous 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 also been observed that in comparison with certain integrated photocatalytic devices, 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. This synergy results in a shift of light absorbance towards the visible light range, a reduction in bandwidth and even better charge separation efficiency. Without wishing to adhere to any particular theory, it is assumed that these effects are accentuated due to the platelet-shaped 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 the reduction of the loss of active compound. As a result of the good interaction between the semiconductor and graphene oxide integrated catalyst. When the semiconductor absorbs the UV light photon that carries more energy than the band gap energy, an electron-hole pair is generated in the semiconductor nanoplatelets that are capable of starting oxidation-reduction processes. But the electron-hole pair can have a recombination process and return to the main state. The interaction between graphene and the semiconductor decreases this probability and increases the effectiveness of the semiconductor as a photocatalyst. 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 molecules and/or organic or inorganic 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 photocatalysts (semiconductors) are usually powder products that have to be integrated into 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) 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 dispersion form, such as 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 30 to 50% by weight, or 40% to 50% by weight, or 45% to 50% by weight. starting nanoplatelets, 90% to 97%, for example, 93 to 95% of them are 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 layer 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 layer structure and that are therefore suitable for the formation of nanoplatelets by exfoliation. Preferred semiconductors include CdS, MoS ² and WS ² . Especially preferred is WS ² .

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 (MoTe ² ) and diselenide molybdenum (MoSe ² ), iii) Cd: cadmium selenide (C ₈ Se), iv) In: indium (III) sulfide In ² S ³ ) and indium (III) selenide (In ² Se ³ ), v) Ga: gallium(III) selenide (Ga ² Se ³ ), gallium(II) selenide (GaSe), gallium(II) sulfide (GaS) and gallium(II) telluride (GaTe), vi) Bi: selenide of bismuth (Bi ² Se ³ ), bismuth telluride (Bi ² Te ³ ) and bismuth sulfide (Bi ² S ³ ), vii) Cu: copper sulfide (Cu ² S), viii) Sn: tin sulfide ( II) (SnS) and tin selenide (SnSe) and ix) Pb: lead (II) iodide (Pbh).

The semiconductor nanoplatelets that can be used to prepare the photocatalysts of the invention can have variable thicknesses of one or more atomic layers. The thickness of an atomic layer varies depending on the specific 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, depending on the material, to an atomic layer and 40 nm, or between an atomic layer and 30 nm, or between an atomic layer and 20 nm, or between an 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, with the condition that the lateral dimension must always be greater than the thickness to meet the condition of being nanoplatelets. In 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.

lateral dimension of between 25 and 300 nm, or thicknesses of between 0.5 and 8 nm and lateral dimension of 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 in a wide range. Elemental composition analysis determined from EDX results demonstrates 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 within the range 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.

According to embodiments of the invention, it has been observed 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, stronger and durable structure and facilitates its handling. In addition, it makes the photocatalyst more effective in cycle operations.

The term "polymer", as used herein, refers to any polymer composed of macromolecules (generally organic) formed by the union through 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 similar. In this case, the polymers can be functionalized, and for this, amounts of functional monomers generally of 1%-10% by weight with respect to the total weight of monomers. In particular embodiments of the invention these are used in amounts of between 2 and 8%, or between 4 and 6%. Examples of functional monomers include , without being limited: 4-bromo styrene, 2-chloroethyl acrylate, glycidyl methacrylate, 2-hydroxyethyl methacrylate, acrylamide, 2-aminoethyl 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. "Polymer 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 depending on the desired characteristics of the photocatalyst. The inventors have observed that the glass transition temperature (Tg) of the polymer used influences whether they tend to be placed as nanoparticles on the surface of rGO (when they have a high Tg) or if they tend to form a layer covering the rGO (when present 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 starting material "GO and semiconductor" and the weight of the polymer, used in the preparation of the photocatalysts, may vary. In particular embodiments of the photocatalysts of the invention, These have been prepared with weight ratios of the starting materials (GO/semiconductor):polymer varying 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 out under mild conditions, at a temperature between 25 and 90 oC. In a particular embodiment the temperature is between 25 and 45 °C. 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 oC or several hours at 25 oC.

According to the synthesis method of the invention, an aqueous dispersion of the starting materials is prepared: graphene oxide nanoplatelets, semiconductor nanoplatelets and optionally polymer nanoparticles. The dispersion is stirred to homogenize the mixture. During this stirring period, it is found that the semiconductor nanoplatelets and polymer nanoparticles are adsorbed on the surface of the graphene oxide nanoplatelets. The temperature is then set within the range mentioned above and the reducing agent (e.g., ascorbic acid (Asa) or hydrazine) is added when the required temperature is reached. From this moment the stirring is stopped so that the self-assembly process can occur. The process of reducing much more hydrophobic material 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 joining tips, which is constituted as 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 dry form, after a drying process. But before drying it has to be cleaned with distilled water through the dialysis process. Any of the known methods for drying hydrogels can be used for drying. These include, for example, freeze-drying, microwave or conventional oven drying. In a preferred aspect, drying is carried out with freeze-drying, to preserve the pore structure and 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 200°C per minute and left to act at that temperature for a few minutes, for example 2 min. The photocatalyst in hydrogel form is a preferred aspect of the invention. The photocatalyst in the form of a lyophilized solid is another preferred aspect of the invention.

The term "reducing agent" or "reducing agent" as used herein refers to any known reducing agent, that is, to an agent capable of giving up electrons in a redox 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 (e.g., NaBH4), (ii) Lithium aluminum hydride (LiAlH4), (iii) Hydrogen halides (e.g. HI), (iv) Sulfur-containing reducing agents (e.g. thiourea dioxide/NaOH), (v) Nitrogen-containing reducing agent (e.g. hydrazine), (vi) Reducing agents containing contain oxygen (e.g. isopropyl alcohol), (vii) Metal-acid (e.g. Fe HCl), (viii) Metal-base (e.g. Zn/NaOH), (ix) Amino acid (e.g. glycine), etc Preferred are hydrazine and ascorbic acid (AsA). The most preferred is ascorbic acid since it dissolves easily in water, is non-toxic and reduction occurs quickly.

Reaction parameters can vary and the process can be controlled by changing parameters, such as the type or amount of reducing agents or the process temperature. There are parameters that can be changed and that influence the training process type of graphene oxide, reducing agent, polymer, and semiconductor; the temperature and time of reduction.

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 it is about 4 mg/mL. With respect to the properties of the structures, the concentration of graphene oxide in the reaction medium determines the dimensions of the structure formed. A higher concentration of the dispersions in water results in the formation of larger volume structures.

The temperature at which graphene oxide is reduced (25° - 90°C) 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 structure formation reaction, is slower, and at higher temperatures, structures form at a higher rate.

As for the reducing agent, the amount and type of 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 mainly affects the properties of the structures. The weight ratios of GO:reducing agent may in particular embodiments be low (1:0.5), medium (1:1) or high (1:2). At lower proportions (for example when the proportion of GO:reducing agent is 1:0.5) the formation process is slower and the resulting structure is less compact and has lower mechanical resistance, presenting 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 (25-45°C) 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, as it determines the level of reduction and the amount of oxygen functional groups present. In order to save energy lower (25-45°C). By varying the amount of polymer the reduction in mechanical properties can be compensated.

The type of polymer and the type of semiconductor can influence the self-assembly process, since when adsorbed on the graphene oxide platelets, they influence the mobility and assembly affinity. They can also affect the final properties. For example, by changing the semiconductor or using a combination of two or more semiconductors, the 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, greater durability and reusability.

In the production process of the structures, the initial mixing of the starting materials is carried out with magnetic stirring (between 100 and 300 rpm, for example, at 150 rpm) to homogenize the mixture. However, it is then essential for the formation of the structures of the invention to stop stirring. The time that the reacted mixture must then remain depends on the temperature. At low temperatures (25-45°C) the process may take between 24h-48h while at high temperatures (60-90°C) the process can be completed in 2-4h.

Another advantage of the process of the invention is that it does not generate secondary products and therefore generates a smaller amount of waste. It is also an environmentally friendly process since it can be carried out in aqueous dispersions, at mild temperatures and it can be carried out with harmless reagents such as using ascorbic acid as a reducer.

The method of the invention is easy, versatile, environmentally friendly, low cost and suitable for large scale production.

The preparation of semiconductor nanoplatelets is not restricted to a particular procedure. On the contrary, 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., 2015), atomic layer deposition (ALD) (Song et al., 2013), molecular beam epitaxy (MBE) (Liu et al., 2015), thermal exfoliation, electrochemistry, and aqueous dispersion. Preferably, the semiconductor nanoplatelets are prepared by an exfoliation method, such as invention include high temperature or the use of solvents. Preferably the exfoliation process is carried out from an aqueous dispersion of the semiconductor material.

The term “three-dimensional” or “3D” as used here, 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- and nanoplatelet-type 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 an average 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 throughout 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 pollutants from air, water or soil or as catalysts in chemical reactions. Possible uses include: adsorption and degradation of organic or inorganic contaminants from water, air or land, for example, heavy metals, petroleum, antibiotics, organic dyes or dyes and other contaminants, decontamination of soils to remove pesticides and other contaminants; capture and photocatalytic conversion of CO ² , photocatalytic dissociation of water to produce oxygen and hydrogen, capture and degradation of harmful tobacco products, etc.

Due to their porous structure, the materials of the invention are useful in applications that require the use of porous materials. Possible applications include among others: gas separation and storage, encapsulation agents in drug delivery systems, catalysts or supports for catalysts, materials for electrodes and for energy storage, supports for immobilization of biomolecules (production of hybrid biocatalysts) and scaffolds cells, materials for use in chromatography, photonic materials, separation membranes and anti-reflective coatings.

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 4 mg/mL and 10 mg/mL, with a monolayer content of >95%, and a pH between 2.2-2.5. The elemental analysis of 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 a 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 2 μm; 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 trace analysis of rare earths ≤ 1500.0 ppm; vi) molybdenum(IV) sulfide - MoS ² (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 an anionic surfactant, and sodium bicarbonate (NaHCO ₃ ) was used as a buffer and to control the microemulsion viscosity by reducing electrostatic interactions. all obtained from Aldrich. Deionized water was used as the polymerization medium. Methylene blue (MB) (>82 wt% dye content) from Sigma-Aldrich was used for adsorption experiments.

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 MM N GMA in a weight ratio of 90/10 (Different functional monomers can be used in different amounts. and thus obtain nanoparticles of functionalized polymers 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.

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 dissolving SDS in water. The emulsion was added to the reactor and stirred at room temperature for 15 minutes. Subsequently, the reactor was heated to 70 oC, the KPS/NaHCO3 solution was added all at once and allowed to polymerize under a nitrogen atmosphere for 1.5 hours. In the resulting mixture of polymer particles in water, the conversion of monomers was 99% with an average particle diameter of 70 nm.

Preparation of semiconductor nanoplatelets

The two-dimensional (2D) nanoplatelets were prepared from crystals of the respective semiconductor materials (MoS2, WS2, CeO2, CdS, GaN, or ZnO). The crystals were crushed in a mortar until they had a grain size of less than 2 Dm. A dispersion of the resulting powder was prepared in water/ethanol and sonicated with shaking for 1 h at 360 W (1 sec on and 2 sec off) with a Branson 450 tip sonicator (Danbury, CT). The flask was kept cold in an ice-water bath during sonication.

The resulting dispersion was directly used in the preparation of 3D materials.

The 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

An appropriate amount of GO graphene oxide dispersion was mixed with a 2D semiconductor nanoplatelet dispersion.

/2D nanoplatelets/polymer, an appropriate proportion of polymer 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 various ratios have been used, although in one particular case the ratio was 4 to 1. At this time the appropriate proportion of reducing agent was added, in this case 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 oC, 60 oC or 90 oC) for the formation of three-dimensional nanostructure materials. These materials are hydrogels and are highly swollen with water. Once out of the oven, the wet material formed was washed by dialysis in Mili Q water for several days, changing the water daily until the water conductivity was constant (less than 10 pS/cm). The 3D structure was then dried by freeze-drying to give the 3D monolithic material.

Characterization

The morphology of the dried 3D materials obtained was examined by scanning electron microscopy (SEM or EDX): Hitachi TM3030 table model, accelerating voltage of 15kV, after covering the samples with a thin layer of gold. In the tables of the results, 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 exfoliated 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 acceleration voltage of 200 keV in bright field 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 in aquatic life and changes in the environment. The adsorption and degradation of MB dissolved in aqueous solutions is therefore very important and of great environmental utility.

rGO/polymer/2D nanoplatelets, and also the 2D nanoplatelets in aqueous dispersion, were added to the MB aqueous solution with a concentration of 0.1 mg/ml. Graphene oxide 3D materials are very good adsorbents of organic molecules, therefore, the aqueous solutions quickly lost color in the case of all 3D materials, even those that did not contain semiconductor nanoplatelets. However, it was expected that 3D materials containing semiconductor nanoplatelets would be able to degrade the 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. Grobel UV-Elektronik GmbH). The irradiance of UV light is 7 mW/cm2. For the experiment, the reactor containing the aqueous MB solution to which 1 mg of the 3D material had been added for every 0.01 mg of MB was placed in the UV chamber.

To distinguish between the adsorption and degradation mechanisms of MB, the processes in which the 3D materials of rGO/2D nanoplatelets and rGO/2D nanoplatelets/polymer had been used were compared with the processes in which the 3D materials were only 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 on a Shimadzu spectrometer (model UV-2550 230 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 (acceleration voltage 20 kV, pressure 5 x 10-6 mbar). 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 3D photocatalytic systems and their photocatalytic activity Example 1: WS ² /rGO, prepared from WS ² /GO with a weight ratio of 1:4. Synthesis conditions: T = 60°C, GO: AsA = 1:1 by weight

Example 2: WS ² /rGO prepared from WS ² /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 in relation to WS ² /GO

Example 3: WS ² /rGO, prepared from WS ² /GO with a weight ratio of 1:3. Synthesis conditions: T = 60°C, GO: AsA = 1:1

Example 4: WS ² /rGO, prepared from WS ² /GO with a weight ratio of 1:4, at high temperature. Synthesis conditions: T = 90°C, 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=90°C, GO:AsA=1:1 and 20% by weight of polymer in relation to WS ² /GO.

Example 6: WS ² /rGO prepared from WS ² /GO with a weight ratio of 1:4, with a lower amount of AsA. Synthesis conditions: T=60 oC, 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 = 60°C, GO: AsA = 1:1 and 20% by weight of polymer relative to MoS ² /GO

Example 1: WS synthesis and characterization ² /rGO, prepared from WS ² /GO with a weight ratio of 1:4

Comparison of the photocatalytic activity of the WS ² /rGO system with pure WS ² nanoplatelets and with pure rGO

Synthesis conditions: T = 60°C, GO: AsA = 1:1 by weight; WS2:GO = 1:4 by weight

In Figure 1a , the TEM images of the initial WS ² nanoparticles produced by sonication of the bulk WS ² crystal can be seen, in which 2D nanoplatelets can be seen, with lateral dimension between 50 - 250 nm.

These nanomaterials, combined with GO, which was subsequently reduced, produced 3D WS ² /rGO. The SEM images of the 3D structure made of rGO alone and WS ² /rGO hybrid are presented in Figures 1b and 1c , respectively. Porous materials were obtained in showing the elemental composition of rGO and WS ² /rGO in wt% demonstrating the successful incorporation of the WS ² photocatalyst within the 3D rGO structure.

Table 1.1 : Elemental composition of the 3D rGO and WS ² /rGO structures of Example 1, determined by EDX.

Figure 1d shows the kinetics of the decoration process of the aqueous methylene blue (MB) solution (followed by UV spectrophotometry). These results were obtained by placing the photocatalytic material in an aqueous solution of MB and irradiating with UV light (368 nm). The concentration of MB 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 performed without UV light (dark experiment), in which case the discoloration of the MB aqueous solution occurs simply by adsorption of the MB by the 3D material. The WS ² nanoplatelets and the 3D nanostructure made only of rGO were also used under UV 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 WS ² /rGO hybrid, 80% is adsorbed in the dark.

Under UV light, the MB aqueous solution is completely decolorized. Complete discoloration 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 MB are desorbed from the surface of the 3D material into the aqueous solution and analyzed. The aqueous solution was also analyzed.

Pure WS ² (without rGO) removed only 56% of MB. This demonstrates that the presence of rGO improved the photocatalytic activity of WS ² significantly since the decolorization was faster (100% in 30 min) and complete.

The degradation of MB was demonstrated by MALDI-TOF-MS analysis of the aqueous solution after the decolorization experiments. The aqueous solution was analyzed. The degradation, are presented in Figures 1e - 1h.

In Figure 1e , the mass spectra of the aqueous solution after decolorization with 3D rGO/WS ² and with UV light (1 mg of 3D material per 0.01 mg of 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 , along with the assignment of the peaks. In the m/z range of 0-100, the peak assignment is as follows: peaks at m/z 23 and 39 are assigned to Na and K, and as the following peaks appear they are assigned to: hydrochloric acid, pentane, thioacetone, thiazoline, anionic sulfate, 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 above m/z 200, the m/z peak of 256 is assigned to C ¹⁴ H ¹⁴ N ³ S (MB with a -CH ³ minus) and the peak of 270 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 if there is a possibility of reducing the number of intermediates created, the experiment was carried out using twice the amount of 3D WS ² /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 lower quantities, indicating that doubling the amount of photocatalyst, in proportion to the MB, more degradation occurs. 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 C ¹² H ¹⁰ ON ³ S (MB with four -CH ³ less), are the result of the degradation of the first products, previously observed. This result shows that by optimizing the amount of WS ² /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 after degradation of MB with the 3D WS ² /rGO performed in the presence of UV light and in the dark can be seen. 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. Magnification of the three characteristic areas is also shown for comparison.

twice WS ² /rGO (2 mg of material per 0.1 mg of MB), the 3D material was subjected to ethanol desorption to extract possible components adsorbed on its surface (non-degraded MB and degradation products). MALDI-TOF-MS analysis of the solution obtained from the desorption is shown in Figure 1h , and indicates the absence of MB. This demonstrates complete photocatalytic degradation of MB, even though some reaction intermediates are still observed ( Figure 1f ).

Example 2: WS synthesis and characterization ² /rGO, prepared from WS ² /GO with a 1:4 weight ratio

Effect of polymer addition on the photocatalytic activity of the WS ² /rGO system Synthesis condition: T = 60°C, GO: AsA = 1:1; WS2:GO = 1: 4, polymer amount 20% by weight in relation to WS ² /GO

In this example, polymer nanoparticles were added to the same photocatalyst from Example 1 to improve handling, durability, and increase the ease of purification and reuse of the photocatalysts. The amount of polymer was 20% by weight relative to the weight of WS ² /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 WS ² nanoplatelets are seen in the image as white structures on the gray polymer/rGO composite structure. The elemental composition determined from EDX results for this system is shown in Table 2.1 , demonstrating the incorporation of WS ² 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 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 composite under UV light, almost the entire amount of MB in the solution with pure 2D WS ² nanoplatelets.

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.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 took place when the composite 3D photocatalyst was used in the presence of UV light. It is noteworthy that although there is still some MB present in the solution, there are very few reaction intermediates (such as: benzenesulfonic acid, potassium sulfite, sodium thiosulfate, 4-phenolsulfonic acid, potassium sulfate, and sodium benzenesulfonate hydrate ). In the higher mass range, above m/z 200, only the characteristic peaks of the structures appear.

and the residual MB. The new peaks of CO ² , NO ³ -, benzene and 4-aminocatechol are also observed. The reaction products indicate efficient degradation of MB. In Figure 2d , a comparison can be seen between the experiments carried out with the WS ² /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: WS synthesis and characterization ² /rGO, prepared from WS ² /GO with 1:3 weight ratio

Effect of increasing amount of WS ² in 3D WS structure ² /rGO

This example shows the results obtained with the photocatalyst containing a greater amount of WS ² in the 3D WS ² /rGO structure, prepared from WS ² :GO 1:3 by weight. The SEM images are shown in Figure 3a and Table 3.1 presents the elemental composition (wt %) of pure rGO and WS ² /rGO composite nanostructure.

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 an aqueous solution of 0.1 mg/mL of MB, with the 3D WS ² /rGO photocatalyst of this example, in the presence of darkness and with the yield of pure components, a very rapid discoloration is observed in the presence of light (MB is eliminated in less than 30 min).

The aqueous solution after decolorization 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 of 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 C ¹⁵ H ¹⁶ N ³ S (MB with one -CH ³ less) is observed. All this demonstrates that the photocatalytic decomposition of MB with this photocatalyst is even more advanced than with the composite prepared from GO and WS ² with a ratio of 1:4, with very few degradation products and in a low mass area. This demonstrates a large degradation of the reaction products especially the degradation of very high molar mass products that 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: WS synthesis and characterization ² /rGO, prepared from WS ² /GO with a weight ratio of 1:4, at high temperature

Effect of temperature increase during the synthesis of 3D WS structure ² /rGO Synthesis condition: T = 90°C, GO: AsA = 1:1; WS2: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 WS ² /rGO material from Example 4

Figure 4b. It can be seen that in the presence of UV light, complete decolorization of the MB aqueous solution was achieved within 60 min. The photocatalytic adsorption/degradation process in this case is a little 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 decolorization 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 minimum concentrations). The magnified spectra show the presence of the degradation products 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 -CH3 less). The results demonstrate that this material has 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; WS2:GO=1:4; polymer amount 20% by weight in relation to WS2/GO

To the material produced at 90°C, presented in Example 4 , polymer was added, 20% by weight with respect to the weight of WS2/GO. The morphology of WS2/rGO/Pol, presented in Figure 5a shows a spongy and porous structure. Table 5.2 presents the elemental composition of the 3D composite structure.

Table 5.1 : EDX results of the WS2/rGO/Pol system prepared at 90 oC

The results obtained demonstrate that the composite photocatalysts are much more efficient than the 2D WS ² platelets alone. In the absence of light, discoloration occurs due to the adsorption of MB on the material. It is demonstrated that the photocatalytic degradation of MB occurs in the presence of UV light, thanks to the integrated photocatalyst.

Example 6: Synthesis and characterization of WS ² /rGO prepared from WS ² /GO with a weight ratio of 1:4. Effect of reducing the amount of reducing agent AsA used for synthesis

Synthesis conditions: WS2/GO =1:4, T=60 oC, GO:AsA=1:0.5

In this example, the influence of the amount of AsA used for the reduction of GO on the characteristics of the 3D structure and its photocatalytic performance was studied. The AsA in this example was reduced by half with respect to that used in Examples 1-5 , where the ratio of GO: AsA is 1:1 by weight. In Figure 6a, the SEM images of 3D WS ² /rGO can be seen, and it is observed that the sample is less porous than the same material obtained with a greater 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

The bleaching experiments are shown in Figure 6b . Surprisingly, this material has been shown to be very effective in photocatalytic degradation of MB, as MB was completely removed from the solution under UV light within 30 min.

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 is observed after the degradation of MB. In the mass range of m/z 100 -200: sulfur dioxide, hydrogen chloride of hydrochloric acid, pentane, thioacetone, benzene, benzenesulfonic acid, potassium sulfite, sodium thiosulfate, 4-phenolsulfonic acid, potassium sulfate, hydrated sodium benzenesulfonate. In the mass area m/z 200 -300 the peak appears at m/z 256 assigned to C ¹⁴ H ¹⁴ N ³ S and m/z 270 assigned to C ¹⁵ H ¹⁶ N ³ S (MB with one -CH ³ less). The MB was almost completely degraded within 30 min, as shown in Figure 6b determined by UV spectroscopy. The presence of MB in the solution (trace) along with a large amount of intermediate reaction products is observed in Figure 6c . The results indicate that to obtain a greater 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 M0S ² /GO with a weight ratio of 1:4

Photocatalytic activity of MoS system ² /rGO

Synthesis conditions M0 S2/GO = 1:4, T = 60°C, GO:AsA = 1:1

In Figure 7a , one 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 composites, showing a large incorporation of the MoS ² photocatalyst.

Table 7.1: Elemental composition of the 3D MoS ² /rGO of Example 7, obtained by EDX

The kinetics of decolorization of the MB aqueous solution are shown in Figure 7c . The MB removal efficiency is significantly improved when using 3D composites (77%, in the presence of light) compared to 2D MoS ² nanoplatelets (11%). The results demonstrate that the 3D MoS ² /rGO structure presents better removal and degradation efficiency.

MoS2/rGO, presented in Figure 7d, demonstrate that MB has completely degraded, as the characteristic peak of MB does not appear in the spectrum.

Example 8: Synthesis and characterization of MoS ² /rGO/Pol (1:4) prepared from M0S ² /GO with a weight ratio of 1:4

Effect of polymer addition to MoS ² /rGO

Synthesis conditions MoS ² :GO = 1:4, polymer: 20% by weight relative to MoS ² :GO, T =

In this example, 20% polymer was added (% by weight 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 1–10 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 8b shows the kinetics of the decolorization process of MB aqueous solution by the 3D composite system compared with pure rGO and pure MoS ² . Although pure rGO has 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 made of CdS: examples of CdS/rGO and CdS/rGO/Pol materials.

General conclusions from 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 degradation products of the MB, for example. methyl group (-CH ³ ).

In Table C1 you can see 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 MB degradation has progressed a lot.

Catecolamina. Por el contrario, el sistema sin polímero muestra presencia de moléculas más grandes, con anillo aromático en su estructura o estructuras conComparing Examples 1 and 2, additional peaks indicate new degradation products after addition of the polymer (Example 2) at a much lower m/z. This indicates 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

Catecholamine. On the contrary, the system without polymer shows the presence of larger molecules, with an aromatic ring in its structure or structures with

-Catecolamina.Similarly, in Examples 1 and 3 the effect of increasing the amount of WS ² nanoplatelets can be seen. By increasing the amount of nanoplatelets, a greater number of peaks can be seen at lower m/z values, and aromatic compounds with a lower molar mass, between 158-197 m/z. These peaks are

-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 oC) show a similar level of degradation, since very similar peaks are observed: acid

Table C1: MALDI peaks recognized and attributed to methylene blue degradation products for Examples 1,2, 3, 4, 5, 7, showing m/z and chemical structures.

The processes carried out in the presence of UV light with the materials of the invention resulted in the adsorption and photocatalytic degradation of the adsorbed agent. In all cases and in accordance with the invention, the photocatalyst integrated into the three-dimensional structure of reduced graphene oxide demonstrated a photocatalytic activity much higher than that of the pure (non-integrated) semiconductor. Integrated materials are monolithic materials, with a porous structure, easy to manipulate and can be reused without losing active components between cycles.

In all the systems investigated, it has been shown that the 2D semiconductor has been incorporated into the rGO to obtain an integrated photocatalyst and that the removal kinetics of the agent (MB in the examples) from the aqueous solution is much faster with the integrated photocatalyst than with the pure 2D photocatalyst.

The experiments in the examples also demonstrate that the synthesis parameters of integrated photocatalysts (e.g., 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.

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Claims (13)

1. A photocatalyst comprising a three-dimensional structure of reduced graphene oxide and nanoplatelets of at least one semiconductor characterized in that the nanoplatelets of at least said semiconductor integrated into the three-dimensional structure of reduced graphene oxide. 2. A photocatalyst according to claim 1, having a porous structure. 3. A photocatalyst according to any of claims 1 to 2, comprising nanoparticles of at least one polymer. 4. A photocatalyst according to any of the preceding claims, wherein said semiconductor is selected from a group consisting of GaN, CeO ² , CdS, ZnO, MoS2, WS2, WO ₃ and other semiconductor materials that allow the formation of nanoplatelets by exfoliation thereof. 5. A photocatalyst according to claim 3 or 4, wherein said polymer is selected from a group consisting of styrenes, acrylates, methacrylates, optionally functionalized, and mixtures thereof. 6. A photocatalyst according to any of the preceding claims, wherein said semiconductor is WS ² or MoS ² and optionally, the photocatalyst comprises functionalized polymethyl methacrylate nanoparticles. 7. Procedure for the preparation of a photocatalyst according to any of claims 1 to 6, comprising the following steps: to. 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 of 45 to 90 oC. 8. Method according to claim 7, wherein the reducing agent is ascorbic acid or hydrazine. 9. Method according to claims 7 or 8, wherein in step b), the dispersion is heated for a period of time between 3 and 24 hours. 10. Procedure according to any of claims 7 to 9, which comprises a step c), between a) and b), in which the dispersion resulting in a) is stirred at temperature ambient for a period of time sufficient to homogenize the dispersion, optionally 15 minutes to 1 hour 11. Method according to any of claims 7 to 10 which further comprises drying the reaction product using lyophilization or microwaves 12. Use of the catalyst according to any of claims 1 to 6, in environmental protection technologies, in the capture and/or chemical conversion of pollutants from the air, water or soil or as catalysts in chemical reactions. 13. Use of the material defined according to any of claims 1 to 6, in applications that require the use of porous materials.