WO2024253578A1 - Method for photochemical functionalization of 2d materials using a flow setup - Google Patents

Abstract

Disclosed is a method of oxidizing and/or functionalizing a two-dimensional (2D) material, comprising: a. exfoliating the 2D material in an aqueous dispersion using sonication or shear mixing; b. exposing the exfoliated 2D material to an oxidizing gas comprising ·O· and ·OH radicals in a batch or flow reaction setup.

Description

METHOD FOR PHOTOCHEMICAL FUNCTIONALIZATION OF 2D MATERIALS

USING A FLOW SETUP

FIELD OF THE INVENTION

[0001] The present invention generally relates to a method of functionalization of 2D materials. In particular, the present invention relates to a method of photochemical functionalization of 2D materials.

BACKGROUND

[0002] The oxidation of graphite⁴¹ or graphene^[2] into graphenea oxide (GO) is an appealing solution for the compatibility and stability issues preventing the use of graphene in specific applications¹³¹. Due to the hydrophilic properties of GO, it is often used to form stable aqueous dispersions and incorporated into various macrostructures such as conductive films, membranes, and aerogels¹⁴¹. Therefore, besides many technical, economical, and environmental issues generally associated with GO production, it has proven to be effective enough to reach the market as a viable product¹⁵¹. The three main large-scale methods currently being used rely on oxidizing graphite with excess amounts of concentrated sulfuric acid H2SO4 and strong oxidizing acids (nitric acid for Staudenmaier and Hofmann methods) or strong oxidizing agents (NaNO₃ and KMnO4 for Hummers method)¹⁶¹. These aggressive and highly explosive oxidative processes, led by Mn₂O₇ intermediates, radical formation, and a chaotic modification process, are very effective in modifying the graphitic structure but also bring together technical or quality-related issues, as they permanently “scar” the graphitic basal plane (and consequently the final product’s properties)^11,61. The latter affects especially GO application in electronic and thermal management purposes since, differently from graphene, GO is an insulating material and demands another synthetic step related to the thermal or chemical reduction into reduced graphene oxide (rGO)^[6]. Furthermore, the electric and thermal conductivities arc very sensitive to the structural defects left by the functional groups, and even after the reduction process, rGO does not yield good conductivity values, demanding further anncaling/graphitizalion steps¹⁷¹.

[0003] As the environmental impact of the manufacturing industry is a growing concern, a green, scalable, and cost-effective process for the production of graphene derivatives is in high demand. Thus, there is a need to overcome the above concerns associated with existing methods and to develop a novel method to functionalize 2D materials such as graphene. Accordingly, this invention presents a photocatalytic setup, both for continuous flow or batch, using only mild solvents (e.g. water, isopropanol), high 2D material concentration in the reaction (up to 20 mg/mL), and solely ozone as the reactant for the modification of 2D material such as graphene. The higher the concentration of the 2D material in the reaction, the higher is the reaction rate/yield. Highly concentrated reaction setups allow for easier scale-up, enabling more product per liter of reactor. This invention not only provides a clean, cheap, and environmentally friendly method for 2D material such as graphene modification, but it also offers an industrial-oriented approach that favours the mass production of GO, functionalized graphene, other 2D materials, their oxides and their functionalized version. Although the method completely avoids the use of acids and inorganic oxidants, leaving only water (or another non-hazardous solvent of choice) as waste, it can oxidize even extremely stable 2D materials such as hexagonal boron nitride (hBN). Moreover, by controlling the reaction parameters such as temperature, this process allows selective oxidation by exploiting the higher reactivity of graphene in comparison to graphite¹²¹, and also its regio selectivity (edges vs. basal plane)¹⁸¹.

SUMMARY

[0004] In one aspect, the present disclosure refers to a method of oxidizing and/or functionalizing a two-dimensional (2D) material, comprising: a. exfoliating the 2D material in an aqueous dispersion using sonication or shear mixing; b. exposing the exfoliated 2D material to an oxidizing gas comprising • O• and • OH radicals in a batch or flow reaction setup.

[0005] Advantageously, the method as disclosed herein is a non-explosive and acid-free method for oxidizing 2D materials. The method as disclosed herein avoids the usage of hazardous chemicals such as concentrated sulfuric acid and potassium permanganate. Therefore, the method is safe and easy to handle.

[0006] Advantageously, the method as disclosed herein is a waste-free 2D materials oxidation process. This process docs not generate by-products that require a tedious and expensive purification process to remove the unwanted residue. Thus, it increases the purity of the product and decreases the production cost. [0007] Advantageously, the method as disclosed herein can be an oxidation process with self -harvesting chemical reactants. The oxidizing gas, which is the only chemical component used in the oxidation process, can be directly harvested from atmospheric air by a sequential connection between an oxygen concentrator and an ozone generator, which is then converted using a UV source. This dramatically reduces the production costs.

[0008] Advantageously, the method as disclosed herein enables scalable and cost-effective production of exfoliated and functionalised 2D materials. The reaction setup can utilize both batch and flow synthesis, and can be easily scaled up. In addition, by mainly utilizing light sources (UV) and gases, the reaction requires a minimum amount of chemicals for synthesis, which also reduces the production cost significantly. Further, the solvent in the flow system is recyclable.

[0009] Advantageously, the method as disclosed herein is a versatile approach for modification of 2D materials. The photochemical flow setup can be used for the modification of different 2D materials with different functional groups, depending on the type of solvent or reactant used in the system.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The invention will be better understood with reference to the detailed description when considered in conjunction with the non-limiting examples and the accompanying drawings, in which:

[0011] Figure 1 are schematic representations of photochemical reaction configurations coupled with different oxidation approaches and exfoliation techniques: (a) bath sonication; (b) probe sonication; (c) high shear mixing; and (d) high shear mixing with direct oxidizing air injection to a batch or a flow reactor.

[0012] Figure 2 show (a) a photo of modified graphene (water) dispersion in water, (b) Raman spectra of pristine graphene and modified graphene (water), (c) UV-vis spectrum of modified graphene (water), (d) a photo of modified graphene (amine) dispersion in waler, (c) Raman spectra of pristine graphene and modified graphene (amine), (f) UV-vis spectrum of modified graphene (amine), and (g) SEM and EDX elemental mapping analysis of the modified graphene (amine). [0013] Figure 3 show (a) a photo of 4 mg/mL modified graphene dispersion in an isopropanol/water mixture, (b) a photo of pressed pellet of glycol/water- modified graphene, and (c) SEM image of modified graphene flakes on silicon substrate.

[0014] Figure 4 show (a) a photo of 0.5 mg/mL modified graphite in an isopropanol/water mixture dispersion, (b) a photo of film casted on a weighing boat, and (c) SEM image with its corresponding (d) EDX elemental mapping of the modified graphite flake.

[0015] Figure 5 show (a) a photo of modified natural graphite dispersion (10 mg/mL) in isopropanol/water mixture, (b) Raman intensity maps of D, G, and 2D bands for the modified natural graphite flake, (c) EDX elemental mapping of the pristine natural graphite flake and modified natural graphite flake, (d) thermogravimetric analysis (TGA) and derivative thermogravimetric (DTG) (inset) curves of pristine natural graphite and modified natural graphite flake.

[0016] Figure 6 show (a) a schematic diagram showing the photochemical reaction setup with irradiation of gaseous ozone, (b) EDX elemental mapping, and (c) Raman intensity maps of D, G, and 2D bands of the modified graphene (with the gaseous radicals).

[0017] Figure 7 show (a) a photo of water dispersion of modified hBN at 2 mg/mL, (b) EDX elemental mapping of the pristine hBN (top row) and modified hBN (bottom row), (c) SEM image and EDX analysis of modified hBN from 60-min and 120-min photochemical reaction.

DETAILED DESCRIPTION

[0018] The present disclosure describes a method of using a reaction platform containing a series of batch and flow photochemical reaction setups for acid-free production of water-stable 2D materials and selective oxidation/functionalization thereof.

[0019] In one aspect, the present disclosure refers to a method of oxidizing and/or functionalizing a two-dimensional (2D) material, comprising: a. exfoliating the 2D material in an aqueous dispersion using sonication or shear mixing; b. exposing the exfoliated 2D material to an oxidizing gas comprising • O• and • OH radicals in a batch or flow reaction setup.

[0020] The process of the present invention is inspired by the intermediates formed in the water-enhanced chemical oxidation processes used for graphene oxidation into graphene oxide (GO)^[9], however, completely eliminating the use of oxidizing acids and inorganic oxidizers. The conventional chemical processes require strong acids and oxidizers to form highly oxidizing intermediates (e.g. Mn₂O₇), which form O₃ in the presence of water, subsequently degrading into O• and • OH (the latter is favoured in the presence of water)^19-111. Differently, the present invention avoids suing strong acids and oxidizers, making it safe and easy to handle. [0021] As used herein, "exfoliate", "exfoliation", "exfoliating" or grammatical variants thereof means a series of operations to reduce the number of layers of a bulk material, to produce a 2D material. For example, exfoliation of graphene can occur in liquid phase, in a media such as ionic liquids, solutions of water/polymer or surfactant, green solutions and organic solvents, to separate the individual graphene layers forming the graphite structure. The advantage of exfoliation is to increase the surface area available for reacting, since • O• and •OH do not permeate effectively the graphite interlayer galleries.

[0022] In one example, liquid-phase exfoliation of graphene is performed using shear exfoliation. In another example, liquid-phase exfoliation of graphene is performed using high- shear mixing, based on shear forces in high-shear mixer. The high-shear exfoliation resembles the sonication technique since it also relies on the cavitation process to promote exfoliation, and aqueous solutions can be employed to facilitate the graphite exfoliation and create unstable dispersion environment as well as eliminate the use of hazardous organic solvents. Once a local shear with proper magnitude is applied, graphite exfoliation occurs in turbulent and laminar regions. Compared to sonication, high-shear mixing has higher efficiencies and is more suitable for large volume (in liters) processing.

[0023] In another example, liquid-phase exfoliation of graphene is performed using cavitation in sonication. In another example, liquid-phase exfoliation of graphene is performed using probe sonication. In another example, liquid-phase exfoliation of graphene is performed using bath sonication. Sonication is employed to trigger physical chemical perturbances in liquid systems through generating cavitation bubbles. As the ultrasonic waves propagate throughout the medium, rarefactions and compressions exert low and high pressures. In the course of the rarefaction, bubbles in micron size begin to form and grow bigger with each cycle until an unstable state (cavitation) is achieved and powerful shockwaves are imploded. Specifically, there arc two main techniques of sonication including probe sonication and bath sonication that are employed simultaneously or separately in order to fabricate graphene, from multilayer to monolayer, through exfoliating graphite. Compared to shear exfoliation, sonication is more suitable for processing smaller volumes of sample. [0024] The exfoliated 2D material is then exposed to an oxidizing gas comprising • O• and •OH radicals. It is shown in this invention the development of a synthesis platform with a batch or flow reaction setup (Figure 1). In one example, O₃ can be directly applied to the aqueous solution and decomposed, forming the O and OH radicals in situ. In other words, O₃ is directly injected into a batch or a flow reactor that is been irradiated with UV irradiation. The good solubility of O₃ in water, especially at relatively low temperatures (0.640 L ozone/L water at 0°C and insoluble at 60°C) ^[13], allows a direct bubbling and re-feeding of O₃ until saturation. In one example, the temperature of the reaction setup is about 0 °C. In one example, the temperature of the reaction setup is 0 — 50 °C. In another example, the temperature of the reaction setup is 5-10 °C. In one example, the temperature of the reaction setup is about 5 °C. In one example, the temperature of the reaction setup is about 10 °C. In one example, the temperature of the reaction setup is about 15 °C. In one example, the temperature of the reaction setup is about 20 °C. In one example, the temperature of the reaction setup is about 25 °C. In one example, the temperature of the reaction setup is about 30 °C. In one example, the temperature of the reaction setup is about 35 °C. In one example, the temperature of the reaction setup is about 40 °C. In one example, the temperature of the reaction setup is about 45 °C. In one example, the temperature of the reaction setup is about 50 °C. The temperature of the reaction setup is well regulated, to ensure solubility of ozone, and to prevent the random damaging of the 2D material by heat generated from the exfoliation process, such as sonication. [0025] Moreover, the O₃ decomposition into • O• and OH radicals is photocatalyzed by a UV lamp with a specific wavelength (254 nm)^[14]. In one example, the O₃ decomposition into .O. and OH radicals is photocatalyzed by a UV lamp with wavelengths within the short wave

UV region (UVC) of 200-280 nm.

[0026] It is worth noting that for the processes involving in situ formation of radicals, by first dissolving O₃ to the dispersion followed by UV irradiation, two crucial issues need to be addressed to ensure effective oxidation. First, for this method involving in situ formation of radicals, a dilution effect usually lakes place, wherein there is an overall lower concentration of the oxidizing air within the dispersion. Second, many 2D materials are high-absorbance and low -reflectance materials, thus, UV irradiation directly onto the liquid phase leads to a decrease of the area where the irradiation reaches. For mitigating these issues, the following actions are necessary. The first action is to increase the O₃ concentration and residence time in the liquid dispersion. This may be achieved by formation of microbubbles (using gas spargers), decreasing the temperature to closer to freeze point (increasing Oi solubility), and/or using pure O₃ instead of an O₃-concentrated air mixture. To increase residence time, it is important to avoid O₃ evaporation prior to reacting with the 2D materials. In one example, the amount of ozone that is directly applied to the aqueous solution and decomposed to the O• and OH radicals in. sila is 1 to 5 wt.%. It is important to increase the O₃ concentration and residence time in the liquid dispersion. The second action is to increase the area with direct UV irradiation in the reactor. This may be achieved by using long and thin reactors (along the lamp region), or by using hehcoidal flow tubes (such as the ones in condensation columns) made of UV- transparent materials (e.g. quartz) along the UV lamp. In both cases, the intention is to increase the surface area directly exposed to irradiation and reduce the liquid volume shadowed by the dispersion of the 2D materials.

[0027] In another example, an oxidizing gas (containing • O• and • OH) can be directly applied to an aqueous dispersion of 2D materials. The • O • and • OH radicals are formed directly from ozone in a gas phase. The O₃ decomposition into • O• and OH radicals is photocatalyzed by a UV lamp with a specific wavelength (254 nm). In one example, the O₃ decomposition into O• and • OH radicals is photocatalyzed by a UV lamp with wavelengths within the short wave UV region (UVC) of 200-280 nm.

[0028] When using the process as disclosed herein, with direct oxidizing gas formation (gas phase), the issues associated with the processes involving in situ formation of radicals, by first dissolving O₃ to the dispersion followed by UV irradiation, as described above, are partially or totally avoided due to the following reasons. First, for direct oxidizing gas formation (gas phase), there is no UV blockage related to dispersion absorbance, and conversion from O₃ to oxidizing gas occurs in the whole chamber volume. Second, for direct oxidizing gas formation (gas phase), there is no side reaction or dilution effect taking place during the oxidizing gas formation, as it happens in a separated chamber. Third, for direct oxidizing gas formation (gas phase), when the oxidizing gas is added to the liquid dispersion, a large amount of gas is added forming a concentration gradient around the nozzle, increasing the reaction rate (although partially decreasing selectivity). In one example, for direct oxidizing gas formation (gas phase), the amount of O₃ added, forming the oxidizing gas comprising the O and • OH radicals, is from 0.35 to 5 wt.%.

[0029] Advantageously, the method as disclosed herein is a non-explosive and acid-free method for oxidizing 2D materials. The method as disclosed herein avoids the usage of hazardous chemicals such as concentrated sulfuric acid and potassium permanganate. Therefore, the method is safe and easy to handle.

[0030] Advantageously, the method as disclosed herein can be an oxidation process with self -harvesting chemical reactants. The oxidizing gas, which is the only chemical component used in the oxidation process, can be directly harvested from atmospheric air by a sequential connection between an oxygen concentrator and an ozone generator, which is then converted using a UV source. This dramatically reduces the production costs. In the photochemical setup, the usage of chemicals is minimal (utilizing water, ozone, and UV lamp), leaving much lesser chemical residue in the product and liquid waste.

[0031] In more details about the reaction mechanism of the oxidation or functionalization of a 2D material by the oxidizing gas as disclosed herein. O₃ undergoes photolysis into the ()• and • OH radicals (also forming hydrogen peroxide by further reacting with water in the solvent). The excess water leads to the predominant formation of OH, and the radical species formed can react to the 2D materials (2DM) and functionalize them (2Df), as described in Equations 1-4.

[0032] Advantageously, the method as disclosed herein is a waste-free 2D materials oxidation process. This process does not generate by-products that require a tedious and expensive purification process to remove the unwanted residue. Thus, it increases the purity of the product and decreases the production cost.

[0033] In another example, the metastable radical formation process also allows adding other molecules that can receive this radical via radical transfer and use it as a channel to be inserted in the 2D materials’ structure. Although those two phenomena are described separately, both reactions using water or other multifunctional heteroatomic molecules as functional group sources follow the same rationale, fitting to the general molecular formula RXH (Equations 5-7).

[0034] According to that, the multifunctional heteroatomic molecule may be any multifunctional molecule with terminal functional groups based on oxygen, nitrogen, sulphur, phosphorus, boron and/or silicon (X). Examples of multifunctional heteroatomic molecules are, but are not limited to, multifunctional alcohols, multifunctional amines, amino alcohols. amino acids, thiols, or a mixture thereof. Multifunctional refers to the molecule having more than one functional group containing heteroatoms. Heteroatomic refers to the molecule having

[0035] Advantageously, the method as disclosed herein is a versatile approach for modification of 2D materials. The photochemical flow setup can be used for the modification of different 2D materials with different functional groups, depending on the type of solvent or reactant used in the system.

[0036] In this invention, the photochemical process can be designed to oxidize/functionalise different 2D materials with different functional groups, depending on the types of solvents used. In addition, the degree of modification is tuneable by regulating the reaction parameters such as the type of protocol (liquid or gas phase O₃ irradiation), reaction temperature (within the range as disclosed herein), OVoxidizing gas concentration and type of solvent used (both change the amount of • O• or HO• radicals formed).

[0037] In the method as disclosed herein, ozone can be generated in a few ways. In one example, an O₃ generator is fed by air (from ambient air, oxygen-enriched air, or pure oxygen gas) and converts the O₂ into O₃ using a photocatalytic process with UV irradiation under a specific wavelength (X = 185 nm) ^[12]. O₃ is formed (photocatalytically) from O₂ within the vacuum UV range (100-200 nm) and the formation rate peaks at 185 nm. In another example, ozone can be supplied by corona discharge ozone generator. In a corona discharge ozone generator, the electrical discharge will lake place in an air gap within the corona cell designed specifically to split the oxygen molecule and produce ozone. In this air gap a dielectric is used to distribute the electron flow evenly across this gap to spread the electron flow to as great a volume of oxygen as possible.

[0038] In one example, the 2D material which is oxidized or functionalized by the method as disclosed herein is graphene. In another example, the 2D material which is oxidized or functionalized by the method as disclosed herein is hexagonal boron nitride (hBN). In another example, the 2D material which is oxidized or functionalized by the method as disclosed herein is graphite. Other examples of 2D material which are oxidized or functionalized by the method as disclosed herein include, but are not limited to, molybdenum disulphide (M0S2), boron/borophene, graphyne, graphane, germanene, silicene, stanine, plumbene, antimonene, bismuthinc, phosporcnc, transition metal dichalcogcnidcs, MXcncs such as transition metal carbides, nitrides, carbonitrides, or a combination thereof. After the photochemical process, the modified 2D material can be easily dispersed, showing excellent solubility in water.

[0039] The oxidization/functionalization process can be performed using different solvents or a solvent mixture system. The broad compatibility with many different solvents and mixtures thereof allow for tuning the viscosity of the system for optimum exfoliation and stability of the different 2D materials during oxidation. In one example, the solvent system is water. In another example, the solvent system is a mixed solvent system, for example, a glycol/water solvent system. In another example, the solvent system is 90% ethylene glycol/10% water. In another example, the solvent system is 10% triethylene glycol/90% water. In another example, the solvent system is 25% triethylene glycol/75% water, hi another example, the solvent system is an alcohol/water solvent system. In another example, the solvent system is 10% isopropyl alcohol/90% water. In another example, the solvent system is 25% isopropyl alcohol/75% waler. In another example, the solvent system is 50% isopropyl alcohol/50% waler. In another example, the solvent system is 10% ethyl alcohol/90% water. In another example, the solvent system is 25% ethyl alcohol/75% water. In another example, the solvent system is 50% ethyl alcohol/50% water.

[0040] The oxidizing air ( O• and OH) or O₃-saturated 2D material/water dispersion is circulated between a sheer/cavitation chamber (leading to and maintaining the exfoliated species) and a photocatalytic immersion well, to ensure uniform distribution. Therefore, different exfoliation techniques can be applied (Figure 1), depending on the starting materials, desired working volume, and degree of exfoliation. Typically, large volumes (in litres) arc processed by shear exfoliation and small volumes are processed by sonication.

[0041] The four different process setups depicted in Figure 1 are only examples of the different combinations possible with this approach. However, the different setups affect oxidation and selectivity, i.e., how many functional groups are added to the 2D materials and where they are located (basal plane or edge). Setups with in situ formation of radicals (Figure l a-c) are milder and allow better control of the edge vs. basal plane selectivity. On the other hand, gas phase formation of radicals (oxidizing gas) and its insertion into the cavitation process allows much more powerful oxidation and accessing the oxidation of very chemically stable materials such as hBN.

[0042] Advantageously, the method as disclosed herein enables scalable and cost-effective production of exfoliated and functionalized 2D materials. The reaction setup can utilize both batch and flow synthesis, and can be easily scaled up. In addition, by mainly utilizing light sources (UV) and gases, the reaction requires a minimum amount of chemicals for synthesis, which also reduces the production cost significantly. Further, the solvent in the flow system is recyclable. Due to the simplicity of the reaction medium/solvent, after filtering out the product the same liquid medium can be reused.

[0043] As used in this application, the singular form “a,” “an,” and “the” include plural references unless the context clearly dictates otherwise. For example, the term “a metal” includes a plurality of metals, including mixtures and combinations thereof.

[0044] As used herein, the term “comprising” means “including.” Variations of the word "comprising", such as “comprise” and “comprises,” have correspondingly varied meanings. Thus, for example, a composition “comprising” X may consist exclusively of X or may include one or more additional unrecited components.

[0045] As used herein, the term “about” in the context of concentration of a substance, size of a substance, length of time, or other stated values means +/- 5% of the stated value, or +/- 4% of the staled value, or +/- 3% of the stated value, or +/- 2% of the staled value, or +/- 1% of the stated value, or +/• 0.5% of the stated value.

[0046] Throughout this disclosure, certain embodiments may be disclosed in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the disclosed ranges. Accordingly, the description of a range should be considered to have specifically disclosed all the possible sub-ranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numbers within that range, for example, 1, 2, 3, 4, 5, and 6. This applies regardless of the breadth of the range.

[0047] The disclosure illustratively described herein may suitably be practiced in the absence of any element or elements, limitation or limitations, not specifically disclosed herein. Thus, for example, the terms "comprising", "including", "containing", etc. shall be read expansively and without limitation. Additionally, the terms and expressions employed herein have been used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications arc possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by preferred embodiments and optional features, modification and variation of the inventions embodied herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention.

[0048] The invention has been described broadly and generically herein. Each of the narrower species and subgeneric groupings falling within the generic disclosure also form part of the invention. This includes the generic description of the invention with a proviso or negative limitation removing any subject matter from the genus, regardless of whether or not the excised material is specifically recited herein.

[0049] Unless otherwise indicated, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which the invention belongs.

[0050] Other embodiments are within the following claims and non-limiting examples. EXAMPLES

[0051] Non-limiting examples of the disclosure will be further described in greater detail by reference to specific Examples, which should not be construed as in any way limiting the scope of the disclosure.

[0052] Materials

[0053] The generality of the process was evaluated by using different graphene (2DM, Tianyuan Empire and KNano), graphite (Sigma Aldrich and Ceylon), hexagonal boron nitride (Sigma Aldrich and 3M) sources for functionalization.

[0054] Characterization methods

[0055] Samples were deposited onto substrates (Si, Si/SiO2 or Si/Au), pre-washed by immersion in acetone and isopropanol alcohol under sonication (5 min each). The morphology of the products was investigated by electron microscopy techniques. For scanning electron microscopy (SEM), samples were drop casted directly onto Si and Au-coated Si substrates and the analyses were carried out in a FEZ Vcrios 460L field-emission scanning electron microscope (FESEM) operating at 2 kV. SEM-EDX mappings were performed using a Zeiss Evo 10, operating, operated al 6.3 kV. For scanning/transmission electron microscopy (STEM), samples were drop casted on Lacey carbon gold TEM grids (TedPella). Optical images were obtained on Si/SiO2 substrates using an Olympus optical microscope. Atomic force microscopy (AFM) images were acquired in a Bruker Dimension Icon Microscope operated in tapping mode and scan lines of 512 and the height profile images were obtained using the open• source AFM image processing tool Gwyddion. Confocal Raman spectroscopy is earned out in a Witec Alpha 300R, with excitation wavelength of 532 nm and a 100x objective with a numeric aperture of 0.9. The spectra were normalized with respect to the G band intensity. Thermal diffusivity measurements: Free standing samples were pre-cut into a circular shape with diameter - 24.6 mm before loading into the standard through-plane and customized in-plane sample holders for Netzsch LFA 467. A simultaneous TGA/DSC analyzer SDT 650 (TA Instruments) calibrated with sapphire and zinc standards was used to study the thermal behavior of the materials. Film pieces (~ 15 mg) were placed in a ceramic pan (90 μl) with the punctured lid and heated at a constant rate (0.5°C/min) under an air atmosphere. An empty ceramic pan was used as a reference. As the temperature changes, the instrument simultaneously measured the sample's weight change and heat flow'. Sheet resistance measurements: Same batch of samples used for thermal diffusivity measurements, without further preparation, was used for sheet resistance measurements using a dasolcng 4-probc measurement setup. Through-plane electrical test: A device was fabricated on a 4-inches SiO2 (285 A)/Si wafer. First, the bottom layer of metal contacts which consists of Ti (50 A)/Au (1000 A) was deposited via electron beam evaporator system (AJA ATC-E) with the aid of a thermal tape mask. Next, the sample dispersion was drop-casted onto a pre-defined square with an area of 20 mm x 20 mm. The sample was left to dry in fume -hood for few hours. Finally, the top layer of metal contacts which consists of Au (1000 A) was deposited using the same method as described above.

[0056] Mild oxidation/functionalization and selectivity

[0057] Using variations of the processes presented in Figure la-c, e.g., graphene or graphite could be processed resulting in functionalized graphene dispersions that were homogenously well-dispersed in water and stable for at least days (Figure 2a). Raman spectroscopy was employed to characterize the defects of graphene, which was an indication of the degree of functionalization. The D band (1350 cm ¹ 1 represented the disordered sp' carbon atoms, and the G band at 1580 cm"¹ was assigned to the sp²-hybridised carbon atoms in the graphitic layers¹¹³¹. As shown in Figure 2b, the intensity ratio of the D band to the G band (I_D//_G ) of modified graphene was higher than that of pristine graphene, indicating the modification of graphene after the photochemical reaction. Moreover, pressing the modified graphene with a pellet press resulted in a silver-colored, sturdy pellet with a mirror-like appearance. The pellet presented a high electrical conductivity of 9740 S/m, whereas the pristine graphene did not form an intact specimen for the measurement. Additionally, UV-vis spectroscopy showed that although stable in polar solvents, the modified graphene presented a characteristic absorption peak at -276 nm, which was in agreement with the value reported for high-quality graphene dispersions by liquid -phase exfoliation¹¹⁶¹ (Figure 2c).

[0058] Depending on the desired types of functionalization, different solvents can be used in the reaction. For example, amines such as Jeffamine® D230 can be used to functionalize graphene with amine groups. The I_D//_G ratio of modified graphene (amine) was higher than that of pristine graphene (Figure 2e). As shown in Figure 2g, the graphene modified using D230 had a nitrogen content of 9.45% by weight, indicating the insertion of nitrogcn-conlaining functional groups into the graphene sheets. Similarly, UV-vis spectroscopy showed that the modified graphene (amine) presented a similar characteristic absorption peak as graphene (Figure 2f). [0059] The modification process can also be achieved by using a mixed solvent system, for example, a glycol/water solvent system. The modified graphene (90% ethylene glycol/10% water) showed an excellent stability in isopropanol/watcr mixture at 4 mg/mL (Figure 3a). The modified graphene (25% triethylene glycol/75% water) formed pellet with a smooth and shiny surface (Figure 3b), and exhibits electrical conductivity of 46400 S/m. In terms of thermal property, the in-plane and through-plane thermal conductivity of pressed pellet were 31.42 W/mK and 7.38 W/mK, respectively, resulting in an anisotropy of -5. As the modification approach was able to preserve the intrinsic electrical and thermal properties of graphene, it was indicated that the degree of modification of the graphene sheets can be mild and different from the conventional graphene oxidation process that formed GO with low electrical conductivity. As evidenced in the SEM image in Figure 3c, the modified graphene flakes appeared to be less defective.

[0060] The modification approach aiso applies to the bulk material graphite. The modified graphite prepared from synthetic graphite powder could disperse well in an isopropanol/watcr mixture (Figure 4a). Quantitative analysis showed that modified graphite has an oxygen content of 11.84% by weight (Figure 4d). Also, the electrical conductivity of graphite had improved after the modification process. The pellet of pristine synthetic graphite could not be measured for electrical conductivity due to its crumbly property, whereas the modified graphite formed a pellet and exhibited electrical conductivity of 7890 S/m. Moreover, the electrical conductivity of the modified graphite film produced via solvent casting (Figure 4b) nearly doubled (657 S/m) when compared to that of the pristine graphite (367 S/m).

[0061 ] Besides synthetic graphite, the present method can also be employed to modify other forms of graphite such as natural graphite flakes . Taking the natural graphite with a large lateral size as an example, after mechanical grinding, the natural graphite flakes in the designated solvent (pure solve nt/solvent mixture) were subjected to the photochemical reaction. For the setup using ethylene glycol as a solvent, the pressed pellet of modified natural graphite showed electrical conductivity of 96100 S/m, and the film from solvent casting had an electrical conductivity of 715 S/m. In the reaction using diluted isopropanol (25%) as a solvent, the resulting modified graphite was able to disperse well in isopropanol/watcr dispersion but stays partially stable (Figure 5 a). This could be explained by the mild level of modification of the graphite flake, as evidenced by the Raman mapping results, where the intensity of D bands was higher along the edge of the graphite flakes as compared to the basal plane (Figure 5b). Based on the energy dispersive X-ray (EDX) elemental map of oxygen, the graphite after the modification process showed a higher concentration of oxygen at the edges of the flake as compared to the basal plane, indicating the edge functionalization of oxygen functional groups (Figure 5c). Both Raman spectroscopy and EDX results indicated that the graphitic properties at the basal plane of graphite flake had been preserved. In terms of thermal stability, the temperature of maximum weight loss (T_max) of modified graphite (-851 °C) was slightly lower than that of the natural graphite (~870°C) (Figure 5d), probably due to the reduction of the number of graphene stacked layers after the modification process^[17].

[0062] Up to now, the large-scale exfoliation of graphite into graphene sheets without causing many defects is still challenging. Prolonged sonication is also not ideal as it results in a small size of graphene flakes and limits the applications^[18]. Therefore, graphene that is free of basal-plane defects from this production method could serve the demand in the industries ^[19]. Besides, the resulting graphene from the current process is also a potential precursor material for graphene oxide synthesis via the modified Hummers method. As the graphene flakes arc pre-exfoliated before the Hummers reaction, intercalating agents and excessive sonication steps can be excluded, allowing the process to be more energy-efficient with less environmental impact ^[20] [0063] High shear and high oxidation processes [0064] As previously mentioned, besides producing radicals in the reaction solvent mixture (Figure 1 a-c), ozone could be exposed to U V light to allow the formation of radicals at the gas phase, which increased the oxidizing power of the process. In more detail, the system shown in Figure 6a is an adaptation of the one in Figure I d, illustrating a high shear and high oxidation setup, where ozone- saturated air was irradiated with UV light in the dry chamber and it formed a continuous supply of gaseous oxygen radicals. Then, the formed radicals entered the reaction solvent through the bubbling and react with the 2D materials (in the wet chamber). Moreover, since the UV irradiation for radical formation occurred in an isolated chamber in the gas phase (Figure 6a, dry chamber), no irradiation blockage was incurred by liquid dispersion and the reaction concentration can be dramatically increased (up to ~20mg/mL). Additionally, in an all-in-one setup as demonstrated in Figure 6a, the residual UV irradiation could also interact with the liquid phase and further react with eventual unreacted ozone. This method was applied forthe oxidation of graphene and graphite, and an increase in oxidation is observed (C/O -1.5), where oxygen was uniformly distributed on the graphene flakes (Figure 6b). Also, Raman mapping shows that the defects appeared both at the basal plane and edges of the graphene flakes (Figure 6c).

[0065] Other than graphene-based materials, the reports on the large-scale synthesis of other newly discovered 2D materials and their derivatives are limited. The existing synthesis techniques^{121 22} are mostly bottom-up approaches that are generally costly with low yield. Although the results of graphene -based materials are mostly highlighted here, this photochemical reaction is a versatile approach that can be applied for the modification of other layered materials such as hexagonal boron nitride (hBN) into their respective 2D materials and oxides thereof. After the photochemical process, the modified hBN powder could be easily dispersed, showing excellent solubility in water (Figure 7a). In Figure 7b, the EDX analysis showed that hBN was successfully oxidized after the photochemical reaction. To regulate the degree of modification of 2D materials, the reaction parameters such as concentration of oxygen input for ozone generation, intensity of UV irradiation, concentration of 2D materials, and reaction time could be adjusted. As demonstrated in Figure 7c, a longer reaction time allowed a higher degree of oxidation of hBN, as indicated by the higher percentage of oxygen content measured by EDX analysis. Therefore, the present method is a potential sustainable and cost-effective solution to address the scalable issue in the production of a wide range of 2D materials.

[0066] Altogether, herein it is demonstrated a reaction platform containing a series of new batch and flow photochemical reaction setups for the oxidation/functionalization of 2D materials. Two main reaction types could be exploited: (1) in situ conversion reaction from ozone to radical, forming mostly • OH radical and only mildly functionalizing/oxidizing the 2D materials mainly at the edges; (2) gas phase conversion of ozone to radicals, forming both • OH and • O' radicals that react both to the edges and basal plane of 2D materials. Overall, the modified 2D materials exhibited improved solubility in the aqueous solvents, due to the functionalization during the photochemical process, while being simultaneously exfoliated. Since this functionalization method could be performed at mild conditions, high concentrations and without the use of hazardous chemicals, it has the potential to be applied for industry-scale production of graphene oxide, functionalized graphene and other water-stable 2D materials and oxides thereof.

[0067] Industrial Applicability [0068] This invention serves as an alternative method for the production of water• stable graphene/graphene oxide or other oxidized/functionalised 2D materials, avoiding the hazards of using a strong acid. Without the acid waste management issue, the production cost can be greatly reduced and the application of graphene-based materials in commercial products can be easily expanded. Moreover, since the degree of oxidation can be broadly expanded, the products of this process can reach almost the totality of both graphene and graphene oxide markets. In addition, the method is applicable to other 2D materials, including very chemically stable ones such as hexagonal boron nitride, thus expanding their applications range in industrial processes.

[0069] Various other modifications and adaptations of the invention will be apparent to the person skilled in the art after reading the foregoing disclosure without departing from the spirit and scope of the invention and it is intended that all such modifications and adaptations come within the scope of the appended claims.

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Claims

Claims

1. A method of oxidizing and/or functionalizing a two-dimensional (2D) material, comprising: a. exfoliating the 2D material in an aqueous dispersion using sonication or shear mixing; b. exposing the exfoliated 2D material to an oxidizing gas comprising • O- and • OH radicals in a batch or flow reaction setup.

2. The method of claim 1, wherein the . O. and . OH radicals are produced from ozone in situ dissolved in a reaction solvent mixture using ultraviolet (UV) irradiation of 200-280 nm.

3. The method of claim 1, wherein the oxidizing gas comprising O- and -OH radicals are produced from ozone in a gas phase using UV irradiation of 200-280 nm.

4. The method of claim 2 or 3, wherein the UV irradiation has a wavelength of about 254 nm.

5. The method of claim 2, wherein the amount of ozone is 1 to 5 wt.%.

6. The method of claim 3, wherein the reaction setup comprises helicoidal flow tubes made of UV-transparent material.

7. The method of claim 6, wherein the UV-transparent material is quartz.

8. The method of claim 3, wherein the amount of O₃ forming the oxidizing gas comprising the O- and - OH radicals is 0.35 to 5 wt.%.

9. The method of claim 2 or 3, wherein the ozone is converted from oxygen directly from atmospheric air, oxygen-enriched air or pure oxygen gas using UV irradiation of 100-200 nm.

10. The method of claim 9, wherein the UV irradiation has a wavelength of about 185 nm.

11. The method of claim 2 or 3, wherein the ozone is supplied by corona discharge ozone generator.

12. The method of claim 2 or 3, wherein the temperature of the reaction setup is close to 0 °C.

13. The method of any one of claims 1-12, wherein the 2D material is selected from the group consisting of graphene, graphite, and hexagonal boron nitride (hBN).

14. The method of claim 1, wherein a mixture of solvent is used in the method, wherein the mixture of solvent is a glycol/water solvent or an alcohol/water solvent.

15. The method of claim 14, wherein the mixture of solvent is selected from the group consisting of 90% ethylene glycol/10% water, 10% triethylene glycol/90% water, 25% triethylcnc glycol/75% water, 10% isopropyl alcohol/90% water, 25% isopropyl alcohol/75% water, 50% isopropyl alcohol/50% water, 10% ethyl alcohol/90% water, 25% ethyl alcohol/75% water, and 50% ethyl alcohol/50% water.

16. The method of claim 14, wherein the solvent in the flow reaction setup is recyclable.

17. The method of claim 1, wherein in step b a multifunctional heteroatomic molecule was added to form a RX radical, along with O• and • OH, which react to the 2D material forming functional groups.

18. The method of claim 17, wherein the multifunctional heteroatomic molecule is selected from the group consisting of a multifunctional alcohol, a multifunctional amine, an amino alcohol, an amino acid, and a thiol; and wherein X is selected from the group consisting of oxygen, nitrogen, sulphur, phosphorus, boron and silicon.

19. The method of claim 17, wherein the multifunctional heteroatomic molecule is triethylene glycol, R = Cr.OrHir and X = O.

20. The method of claim 17, wherein the multifunctional heteroatomic molecule is tricthylcnctctraminc, R — C₆N₃H₁₇ and X — N.

21. The method of claim 1, wherein exfoliation is done by a method selected from the group consisting of bath sonication, probe sonication, and high shear mixing.