Method for preparing a carbon nanomaterial

Abstract

Comprising the steps of (i) providing a mass of cellulosic plant fibres; (ii) treating the mass with a base at a temperature less than 100 °C to form a treated mass; (iii) recovering the treated mass from the base; and (iv) heating the recovered mass to form carbon nanomaterial. Prior to heating the mass of fibres may be comminuted. The heating temperature may be from 10-50°C, preferrable 15-40°C and the mass may be heated for at leas 24 hours, preferably at least 5 days. The fibres may be obtained from hemp, bamboo, flax, coconut shell, coconut coir, sisal, cotton, jute, ramie, kapok, milkweed, pina, abaca, sedges, rushes and / or reeds. The base may comprise a calcium compound such as hydroxide, oxide, carbonate, hydrogen carbonate or combinations thereof. The recovered treated mass may be treated with an activating agent. Forming the nano material may comprising heating to at least 300 °C for at least 10 minutes; Also include is a claim to carbon nanomaterial obtained by such a method.

Description

The present invention relates to a method for preparing a carbon nanomaterial. In particular, the present invention relates to a method for preparing a graphene-like carbon nanomaterial that is environmentally friendly, economical and straightforward to perform, while providing good material properties. Due to its observed physical properties and theoretical physical properties (including its high surface area, fracture strength, electrical conductivity and thermal conductivity), graphene has been a significant focus of research interest in recent years. These properties make graphene and graphene-like materials excellent candidates for use in a broad range of applications including drug delivery, carbon capture, filtration devices, nanoelectronics, batteries, sensors, computer chips, composites, functional coatings and paints, thermal management material, supercapacitors and solar radiation absorbers amongst many others. Existing methods for synthesising graphene and graphene-like materials typically require bulk graphite crystals, complicated methods or expensive substrates. Such processes are often difficult to scale up, expensive and environmentally unfriendly. The biggest tonnage of graphene or graphene-like material is currently from non-renewable sources. US 2014 / 0328006 discloses a combined hydrothermal and activation process that uses hemp bast fibre as the precursor to produce carbon nanosheets. The document teaches the importance of relatively aggressive hydrothermal conditions (e.g., at 180 °C using a sulphuric acid, iron oxide or iron chloride catalyst) to exfoliate and partially carbonise the crystalline cellulose hemp fibrils present in the hemp bast fibre, prior to a KOH activation and carbonisation step at 700-800 °C to generate micropores and mesopores within the structure. While this process has the advantage of using plant matter as a starting material, the hydrothermal treatment step is energy intensive, and the process is still relatively complicated to perform. Moreover, the carbon nanosheets are at least partially graphitised, limiting their similarity to graphene. There is a need for further methods for preparing graphene or graphene-like materials from plant matter, which are more sustainable, more environmentally friendly, more economical and / or easier to perform than the methods of the prior art. 10 05 22 Accordingly, it is one aspect of the present invention to provide a method for preparing a carbon nanomaterial, such as a graphene or graphene-like material, that is more sustainable than the methods of the prior art while providing excellent material properties. It is an alternative and / or additional aspect of the present invention to provide a method for preparing a carbon nanomaterial, such as a graphene or graphene-like material, that is less complex and / or more economical than the methods of the prior art while providing excellent material properties. GB 1284415 A discloses a process for carbonising cellulose fibres or products thereof. According to a first aspect, the present invention provides a method for preparing a carbon nanomaterial in accordance with claim 1. Advantageously, the present inventors have found that a carbon nanomaterial having graphene-like properties can be prepared from plant fibres without requiring a high-temperature hydrothermal treatment step. Instead, it has been found that treating the plant fibres with a base (at much lower temperatures), recovering the treated mass of fibres and heating the recovered mass of fibres will provide a material with graphene-like properties. The process of the present invention has the additional advantages of being less energy intensive, more sustainable, less complex and / or more economical than the processes of the prior art. The present invention will now be described further. In the following passages different aspects / embodiments of the invention are defined in more detail. Each aspect / embodiment so defined may be combined with any other aspect / embodiment or aspects / embodiments unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous. The present invention provides a method for preparing a carbon nanomaterial. By carbon nanomaterial it is meant a carbon-containing or carbon-based material possessing at least one external dimension measuring at most 100 nm. The carbon nanomaterial is preferably 23 09 24 a graphene or graphene-like material. These terms are known in the art. A single-layer graphene sheet is composed of carbon atoms occupying a two-dimensional hexagonal lattice. It is accepted by those skilled in the art that the hexagonal lattice of carbon atoms may be perfect or may include defects that do not detract significantly from the physical 5 properties. The term “graphene” is typically used to refer to materials with one or several stacked layers of such two-dimensional sheets, but with far fewer stacked layers than graphite. In particular, the term “graphene” encompasses monolayer graphene, bilayer graphene and few-layer graphene (FLG). Few-layer graphene may comprise up to 30 stacked layers of the aforementioned two-dimensional sheets, typically from 3 to 20 or from 10    3 to 10. As such, graphene materials have a substantially two-dimensional morphology. Beyond 30 layers, the properties of the material start to more closely resemble those of graphite and the material cannot be considered to be graphene or graphene-like. The term “graphene-like material”, as used herein, refers to a carbon-based nanomaterial having a similar sheet or layered sheet morphology to graphene, with the number of layers 15 preferably as set out above^ Preferably, the carbon nanomaterial has a carbon content of at least 90 wt% by weight of the carbon nanomaterial, more preferably at least 95 wt% or from 95 to 99 wt%. Preferably, the carbon nanomaterial has an oxygen content of at most 10 wt% by weight of the carbon 20 nanomaterial, more preferably from 1 to 5 wt%. Preferably, the carbon and oxygen contents are measured by Transmission Electron Microscopy Energy Dispersive Spectroscopy (TEM-EDS). The method of the present invention comprises a number of steps. As will be appreciated, 25 while these steps are intended to be sequential, there may be some overlap between the steps when the process is carried out in a continuous or scaled up manner. The first step comprises providing a mass of cellulosic plant fibres. By cellulosic plant fibres it is meant fibres containing cellulose. The mass of cellulosic plant fibres may be obtained 30 from a variety of plant-based materials. In particular, the cellulosic plant fibres are obtained from hemp, bamboo, flax, coconut shells, coconut coir, sisal, cotton, jute, ramie, kapok, milkweed, piha, abaca, sedges, rushes and / or reeds. Preferably, the cellulosic plant fibres are obtained from hemp. Because the method of the present invention uses plant-based renewable source starting materials, it is more environmentally friendly than many non-35 renewable methods used in the prior art to prepare graphene-like materials. Preferably, the cellulosic plant fibres comprise cellulose, hemicellulose and lignin. Preferably, the cellulosic plant fibres comprise bast fibres, more preferably hemp bast fibres. The next step comprises treating the mass of cellulosic plant fibres with a base at a temperature of less than 100 °C to form a treated mass of fibres. Preferably, the base comprises or consists of a calcium compound. Preferably, the calcium compound is selected from the group consisting of calcium hydroxide (Ca(OH)2), calcium oxide (CaO), calcium carbonate (CaCOs), calcium hydrogen carbonate (Ca(HCOs)2) and combinations of two or more thereof. Most preferably, the calcium compound is or comprises calcium hydroxide (Ca(OH)2). Preferably, the temperature is from 10 to 50 °C, more preferably from 15 to 40 °C, and most preferably from 20 to 35 °C. Advantageously, the temperature at which this step is carried out is significantly less than that of the hydrothermal treatment step used in US 2014 / 0328006, and can occur both at natural ambient temperatures (e.g., outdoors) and at controlled temperatures. The energy requirement of the base treatment step of the present invention is therefore lower than the aggressive hydrothermal treatments required by the prior art. Without wishing to be bound by theory, it is believed that one function of the base treatment step is to at least partially separate the cellulose present in the plant fibres from other components such as hemicellulose and lignin. While the reaction mixture of the mass of cellulosic plant fibres and the base may be heated (to a temperature of less than 100 °C) in order to speed up the rate at which the plant fibre structure breaks down, this is not necessary, and the desired separation of the cellulose will occur at ambient temperature. The temperature referred to in this paragraph is the temperature of the reaction mixture of the mass of cellulosic plant fibres and the base at the start of the reaction. It is possible that the temperature of the reaction mixture will increase over the course of the reaction due to the exothermicity of the reaction. However, the temperature of the reaction mixture preferably never exceeds 100 °C and may optionally be controlled such that it lies within the above ranges. Preferably, the step of treating the mass of cellulosic plant fibres with a base at a temperature of less than 100 °C to form a treated mass of fibres does not involve a heat source other than ambient conditions provide. Preferably, the step of treating the mass of cellulosic plant fibres with a base to form a treated mass of fibres is carried out for a period of at least 24 hours, preferably for at least 5 days, still more preferably from 6 to 12 days and most preferably from 7 to 10 days. Preferably, the step is carried out for a period sufficient to remove at least a portion of the hemicellulose and lignin from the mass of cellulosic fibres. Preferably, in the step of treating the mass of cellulosic plant fibres with a base to form a treated mass of fibres, the mass of cellulosic plant fibres is combined with the base in a weight ratio of the mass of cellulosic plant fibres to the base of at most 2:1, preferably at most 1:1, more preferably from 1:2 to 1:10, and most preferably from 1:4 to 1:6. Preferably, the base is provided as an aqueous solution. Advantageously, the aqueous solution helps to dissolve soluble organic compounds formed from the treatment of the fibres with the base, aiding their removal from the cellulosic mass of fibres and the recovery of the cellulose. For the avoidance of doubt, in this embodiment, the ratios in the preceding paragraph apply to the base itself (e.g., the calcium compound), rather than the aqueous solution of the base. The next step comprises recovering the treated mass of fibres from the base. Preferably, there are no intervening steps between the step of treating the mass of cellulosic fibres at a temperature of less than 100 °C to form a treated mass of fibres and the step of recovering the treated mass of fibres from the base. Preferably, this step comprises draining a first aqueous portion from the treated mass of fibres. Optionally, this step further comprises: (i) washing the recovered mass of fibres with water and draining a second aqueous portion from the treated mass of fibres, and / or (ii) drying the recovered mass of fibres. For example, the treated mass of fibres, after the first aqueous portion has been drained, may be washed with water and spun dried. The first aqueous portion and / or second aqueous portion may contain solids of inorganic and / or organic nature, and / or may contain solutes of inorganic and / or organic nature. Preferably, the first aqueous portion and / or the second aqueous portion is recovered and used in agriculture. For instance, the first aqueous portion and / or the second aqueous portion may be recovered and used to treat soil. The recovered first aqueous portion and the recovered second aqueous portion may be combined to form a mixture prior to use. Optionally, the recovered first aqueous portion, the recovered second aqueous portion and / or the mixture may be further treated prior to use. For instance, the recovered first aqueous portion, the recovered second aqueous portion and / or the mixture may be 10 05 22 concentrated, diluted, acidified, basified, fermented, composted, partitioned into solid and liquid portions, ground and / or stirred. The recovery and use of the first and / or second aqueous portion in agriculture (e.g., in 5 treating soil) is especially preferred when the base comprises or consists of a calcium compound. Those skilled in the art of gardening or agriculture recognise the historic use of calcium compounds in soil improvement. An amateur gardener is taught the use of lime, which is a word commonly used to refer not inclusively to any or all of the following: calcium hydroxide, calcium carbonate, slaked lime, hydrated lime, milk of lime, lime wash, 10 whitewash, chalk, limestone, ground limestone, dolomite limestone, coral, calcified seaweed. Those skilled in the art of agriculture or ecology understand from the commonly described “lime cycle” that calcium hydroxide and calcium oxide absorb a proportion of carbon dioxide from the atmosphere. Calcium oxide is commonly referred to, not exclusively as burnt lime, quicklime, and lumplime. Those skilled in the art of agriculture 15 and gardening understand the addition of solid pieces of organic material to soil is necessary for the soil structure. This practice is commonly referred to as, not exclusively, mulching, digging through dead plant material or humus making material, fibrous materials. Those skilled in the art of agriculture and gardening understand that the addition of plant derived organic and inorganic solutes to the soil is necessary for soil fertility, such solutes 20 are commonly referred to (not exclusively) as humus, fertilizers, nutrients. Accordingly, the first and / or second aqueous portion are useful in agriculture, especially when the base is or comprises a calcium compound. Prior to the step of treating the mass of cellulosic plant fibres with a base at a temperature 25 of less than 100 °C to form a treated mass of fibres, the method comprises comminuting the mass of cellulosic plant fibres to form a comminuted mass of cellulosic plant fibres. The base treatment step is then performed on the comminuted mass of cellulosic plant fibres. The term “comminuting” is known in the art and refers to the mechanically assisted breakdown of solid materials into smaller pieces. Preferably, the comminuting comprises 30 crushing, grinding or cutting the mass of cellulosic plant fibres to form the comminuted mass of cellulosic plant fibres. In some embodiments, the comminuting is conducted using a hammer mill or other comminuting equipment. The comminuted mass of cellulosic plant fibres is made up of smaller pieces of plant fibres than those provided in the first step and may, for instance, be in the form of a dust, a fibrous dust or a powder. In other 35 embodiments described herein but not claimed, prior to the step of treating the mass of cellulosic fibres with a base at a temperature of less than 100 °C to form a treated mass of fibres, the mass of cellulosic plant fibres is subjected to a bioextrusion step, preferably a thermomechanical bioextrusion step. After the step of recovering the treated mass of fibres from the base, the method may comprise a step of treating the recovered mass of fibres with an activating agent. In such embodiments, there are preferably no intervening steps between the step of recovering the treated mass of fibres from the base and the step of treating the recovered mass of fibres with an activating agent. Preferably, the activating agent is an inorganic acid or an inorganic base. Preferably, the activating agent is selected from the group consisting of KOH, Ca(OH)2, NaOH, ZnCI2, H3PO4 and combinations of two or more thereof. Other carbon activation agents are known in the art. Most preferably, the activating agent is or comprises KOH or Ca(OH)2. Preferably, the activating agent is provided as an aqueous solution, preferably having a concentration of the activating agent of from 25 to 75 wt%, more preferably from 40 to 60 wt% by weight of the solution. Preferably, the recovered mass of fibres and the activating agent are combined in a weight ratio of the recovered mass of fibres to the activating agent of from 2:1 to 1:5, preferably from 1:1 to 1:3, still more preferably from 1:1 to 1:2 and most preferably about 1:1.5. For the avoidance of doubt, the ratios in this paragraph apply to the activating agent itself (e.g. the KOH or Ca(OH)2), rather than an aqueous solution of the activating agent. Preferably, the step of treating the recovered mass of fibres with an activating agent comprises a step of heating to a temperature of from 50 to 100 °C, more preferably from 70 to 90 °C. That is, the recovered mass of fibres and the activating agent are preferably heated to these temperatures. Preferably, the step is carried out for a period of from 1 to 40 hours, more preferably from 10 to 40 hours, more preferably from 20 to 30 hours. The step may, for instance, be carried out in an oven or other heating device. After the step of recovering the treated mass of fibres from the base, the method comprises a step of heating the recovered mass of fibres to form the carbon nanomaterial. Preferably, the step of heating the recovered mass of fibres to form the carbon nanomaterial comprises carbonising the recovered mass of fibres. The term “carbonising” is known in the art and refers to the heating of organic matter, in this instance the recovered mass of fibres, to convert it substantially into carbon. Many different reactions take place in the carbonisation process concurrently, such as dehydrogenation, condensation, hydrogen transfer and isomerisation. In some embodiments, this step is performed under an inert atmosphere, such as nitrogen or argon gas. In such embodiments, the heating step is a step of pyrolysing the recovered mass of fibres to form the carbon nanomaterial. In other embodiments, the step is performed in a restricted air flow, for example in an oven or a furnace that is not air-tight, but has controlled restricted air flow (e.g., such as an historical charcoal oven or coking oven). Preferably, the step of heating the recovered mass of fibres to form the carbon nanomaterial comprises heating the recovered mass of fibres to a temperature of at least 300 °C, more preferably at least 500 °C, and most preferably from 600 to 900 °C. Preferably, the step of heating the recovered mass of fibres to form the carbon nanomaterial comprises heating the recovered mass of fibres for at least 10 minutes, more preferably from 30 minutes to 2.5 hours. The heating step may be conducted, for instance, in a furnace. The furnace may, for instance, be a batch furnace or a continuous furnace. Those skilled in the art know that the furnace type may alter the required heating time due to the different rates at which the material is heated or cooled. In some embodiments, the furnace is a rotary furnace. In embodiments in which the recovered mass of fibres is treated with an activating agent, the step of heating the recovered mass of fibres to form the carbon nanomaterial is carried out after the step of treating the recovered mass of fibres with the activating agent. In other words, the step of treating the recovered mass of fibres with the activating agent (which, as noted above, may itself be conducted at an elevated temperature) is between the step of recovering the mass of fibres from the base and the step of heating the recovered mass of fibres to form the carbon nanomaterial. In such embodiments, the recovered mass of fibres, having been treated with the activating agent, may be dried prior to the heating (e.g. carbonising) step. The activating agent is preferably not removed or recovered from the mass of fibres prior to the step of heating the recovered mass of fibres to form the carbon nanomaterial. That is, the step of heating the recovered mass of fibres to form the carbon nanomaterial is preferably performed on the (optionally dried) combination of the recovered mass of fibres and the activating agent. In such embodiments, there are preferably no intervening steps between the step of treating the recovered mass of fibres with an activating agent (or the optional subsequent drying step) and the step of heating the recovered mass of fibres to form the carbon nanomaterial. In embodiments in which the activating agent is not removed or recovered from the mass of fibres prior to the step of heating the recovered mass of fibres to form the carbon nanomaterial, the heating step is preferably an activation and carbonisation step. In embodiments in which the recovered mass of fibres is not treated with an activating agent, there are preferably no intervening steps between the step of recovering the treated mass of fibres from the base and heating the recovered mass of fibres to form the carbon nanomaterial. Preferably, prior to the step of heating the recovered mass of fibres to form the carbon nanomaterial, the mass of cellulosic plant fibres, the treated mass of fibres and the recovered mass of fibres are not subjected to hydrothermal treatment at a temperature of more than 150 °C, and / or are not subjected to hydrothermal treatment using sulphuric acid, iron oxide or iron chloride. Preferably, prior to the step of heating the recovered mass of fibres to form the carbon nanomaterial, the mass of cellulosic plant fibres, the treated mass of fibres and the recovered mass of fibres are not subjected to sonication. Preferably, the method of the present invention does not comprise a step of applying electrical pulse discharges to the fibres at any stage of the method. Preferably, the fibres are not at any stage of the method subjected to hydrothermal treatment at a temperature of more than 150 °C, hydrothermal treatment using sulphuric acid, iron oxide or iron chloride, and / or sonication. After the step of heating the recovered mass of fibres to form the carbon nanomaterial, the method may further comprise washing the carbon nanomaterial, preferably with water or an acidic or alkaline solution. The washing step may remove the activating agent from the carbon nanomaterial and / or remove non-carbon impurities. The washing solution may be selected depending on the nature of the activating agent. For example, when an acidic activating agent is used, an alkaline washing solution is preferred. Conversely, when an alkaline activating agent is used, an acidic washing solution is preferred. Preferably, the carbon nanomaterial is washed with a washing solution (e.g. an acidic or alkaline solution) followed by water. The washed carbon nanomaterial may then be dried, for example at a temperature of from 60 to 100 °C for a period of from 1 hour to 48 hours. Preferably, the carbon nanomaterial is dried until the mass of the carbon nanomaterial remains stable despite further drying time. The drying may be performed in an oven or a dryer, and with or without a desiccant. After the step of heating the recovered mass of fibres to form the carbon nanomaterial and any subsequent washing and / or drying steps, the carbon nanomaterial may optionally be subjected to an oxidation step. After the oxidation step, the carbon nanomaterial may have a carbon content of from 80 to 90 wt% and an oxygen content of from 10 to 20 wt% by weight of the carbon nanomaterial. In some embodiments, the carbon nanomaterial after the oxidation step has a carbon content of from 84 to 88 wt % and an oxygen content of 12 to 16 wt%. Preferably, the carbon and oxygen contents are measured by Transmission Electron Microscopy Energy Dispersive Spectroscopy (TEM-EDS). After the oxidation step, the carbon nanomaterial may be a graphene oxide material. In an especially preferred embodiment, there is provided a method for preparing a carbon nanomaterial, the method comprising: (i) providing a mass of cellulosic plant fibres; (ii) comminuting the mass of cellulosic plant fibres to form a comminuted mass of cellulosic plant fibres; (iii) treating the comminuted mass of cellulosic plant fibres with a base at a temperature of less than 100 °C to form a treated mass of fibres; (iv) recovering the treated mass of fibres from the base; (v) optionally treating the recovered mass of fibres with an activating agent; and (vi) carbonising the recovered mass of fibres to form the carbon nanomaterial. wherein the step of recovering the treated mass of fibres from the base comprises draining an aqueous portion from the treated mass of fibres and using the recovered aqueous portion to treat soil. In a further especially preferred embodiment, there is provided a method for preparing a carbon nanomaterial, the method comprising: (i) providing a mass of cellulosic plant fibres; (ii) comminuting the mass of cellulosic plant fibres to form a comminuted mass of cellulosic plant fibres; (iii) treating the comminuted mass of cellulosic plant fibres with a base at a temperature of from 10 to 50 °C, preferably from 15 to 35 °C, to form a treated mass of fibres, wherein the base is provided as an aqueous solution; (iv) recovering the treated mass of fibres from the base by draining an aqueous portion from the treated mass of fibres, and optionally using the recovered aqueous portion to treat soil; (v) optionally treating the recovered mass of fibres with an activating agent at a temperature of from 50 to 100 °C; and (vi) carbonising the recovered mass of fibres by heating the recovered mass of fibres to a temperature of at least 500 °C to form the carbon nanomaterial. In a further especially preferred embodiment, there is provided a method for preparing a carbon nanomaterial, the method comprising: (i) providing a mass of cellulosic plant fibres; (ii) comminuting the mass of cellulosic plant fibres to form a comminuted mass of cellulosic plant fibres in the form of a dust; (iii) treating the comminuted mass of cellulosic plant fibres with a base at a temperature of from 10 to 50 °C, preferably from 15 to 30 °C, to form a treated mass of fibres, wherein the base is provided as an aqueous solution of Ca(OH)2 and the comminuted mass of cellulosic plant fibres is combined with the Ca(OH)2 at a weight ratio of the comminuted mass of cellulosic plant fibres to the Ca(OH)2 of from 1:2 to 1:10; (iv) recovering the treated mass of fibres from the base by draining an aqueous portion from the treated mass of fibres, and optionally using the recovered aqueous portion to treat soil; (v) treating the recovered mass of fibres with an activating agent at a temperature of from 50 to 100 °C, wherein the activating agent is selected from the group consisting of KOH, Ca(OH)2, NaOH, ZnCI2, H3PO4 and combinations of two or more thereof and the recovered mass of fibres and the activating agent are combined in a weight ratio of the recovered mass of fibres to the activating agent of from 1:1 to 1:2; and then (vi) carbonising the recovered mass of fibres by heating the recovered mass of fibres to a temperature of at least 500 °C to form the carbon nanomaterial. In a further especially preferred embodiment, there is provided a method for preparing a carbon nanomaterial, the method comprising: (i) providing a mass of cellulosic plant fibres; (ii) comminuting the mass of cellulosic plant fibres to form a comminuted mass of cellulosic plant fibres in the form of a dust; (iii) treating the comminuted mass of cellulosic plant fibres with a base at a temperature of from 10 to 50 °C, preferably from 15 to 35 °C, to form a treated mass of fibres, wherein the base is provided as an aqueous solution of Ca(OH)2 and the comminuted mass of cellulosic plant fibres is combined with the Ca(OH)2 at a weight ratio of the comminuted mass of cellulosic plant fibres to the Ca(OH)2 of from 1:2 to 1:10; (iv) recovering the treated mass of fibres from the base by draining an aqueous portion from the treated mass of fibres, washing the recovered mass of fibres and drying the recovered mass of fibres; (v) treating the recovered mass of fibres with an activating agent at a temperature of from 50 to 100 °C, wherein the activating agent is selected from the group consisting of KOH, NaOH, ZnCI2, H3PO4 and combinations of two or more thereof and the recovered mass of fibres and the activating agent are combined in a weight ratio of the recovered mass of fibres to the activating agent of from 1:1 to 1:2; and then (vi) carbonising the recovered mass of fibres by heating the recovered mass of fibres to a temperature of at least 500 °C to form the carbon nanomaterial; (vii) washing the carbon nanomaterial to remove the activating agent and any noncarbon impurities from the carbon nanomaterial; and (viii) drying the carbon nanomaterial. In these especially preferred embodiments, the carbon nanomaterial is preferably a graphene or graphene-like material. Preferably, the method of the present invention consists of the steps specified in these especially preferred embodiments, optionally in combination with a step of oxidising the carbon nanomaterial as discussed above. The present invention will now be described in relation to the following non-limiting figures: Figure 1 is a Transmission Electron Microscopy (TEM) image (scale bar 200 nm) of a portion of a carbon nanomaterial produced in accordance with the method of Example 1. Figure 2 is a TEM image (scale bar 1 pm) of a portion of a carbon nanomaterial produced in accordance with the method of Example 1. Figure 3 is a TEM image (scale bar 200 nm) of a portion of a carbon nanomaterial produced in accordance with the method of Example 1. Figure 4 is a Power X-Ray Diffraction (PXRD) pattern of a carbon nanomaterial produced in accordance with the method of Example 1 (A) and then after the oxidising method (B) disclosed in Example 2. Figure 5 is an Attenuated Total Reflection Infrared (ATR-IR) spectrum of a carbon nanomaterial produced in accordance with the method of Example 1 (A) and then after the oxidising method disclosed in Example 2 (B). Figure 6 is a TEM image (scale bar 200 nm) of a portion of an oxidised carbon nanomaterial produced in accordance with the method of Example 2. Figure 7 is a TEM image (scale bar 20 nm) of a portion of an oxidised carbon nanomaterial produced in accordance with the method of Example 2. The present invention will now be described in relation to the following non-limiting examples. Example 1 Method for preparing graphene-like material A graphene-like material was prepared from hemp fibre using the following method. A 300g mass of hemp fibres was comminuted to a dust-like material in a Hammer mill and combined with calcium hydroxide (1 part of hemp to 5 parts of calcium hydroxide by weight) and 20 litres of water and mixed together. The mixture was left for 7 to 10 days at room temperature in an unheated room in winter for the hemp fibre structure to break down. The liquid and bigger lumps of non-organic solid were then drained away and the recovered hemp fibre remains were washed with water and spun dried. A mixture of potassium hydroxide and water (in a 1:1 ratio by weight recovered fibre remains to KOH) was prepared and applied to the recovered hemp fibre remains and dried in an oven for 24 hours at a temperature of around 80 °C. The reaction mixture was then transferred to a furnace and heated for 2 hours at 800 °C to carbonise the material. The carbonised material was then washed in a weak acidic solution followed by a few clean water washes. It was then placed back in the oven for 24 hours at around 80 °C to dry. The drained away liquid and bigger lumps of non-organic solid mentioned above were retained and could be used in agriculture e.g., to treat soil. The wash water from washing the hemp fibre remains could also have been saved and added to the drained liquid and bigger lumps of non-organic solid. Characterisation of the carbon nanomaterial Figure 1 Transmission Electron Microscope (TEM) image taken using a 120kV Tungsten filament. The carbon nanomaterial is supported by a Lacy Carbon Grid. The Lacy Carbon Grid is the stripe with darker edges going from the bottom of the image to the bottom right of the image. The scale bar is the length 200 nanometers. The image shows a sheet of material that is only a few nanometers thick. Figure 2 Transmission Electron Microscope (TEM) image taken using a 120kV Tungsten filament. The carbon nanomaterial is supported by a Lacy Carbon Grid. The scale bar is the length 1 micrometer. The image shows sheets of material that is only a few nanometers thick and some microstructure that is thicker in parts. Figure 3 Transmission Electron Microscope (TEM) image taken using a 120kV Tungsten filament. The carbon nanomaterial is supported by a Lacy Carbon Grid. The Lacy Carbon Grid is the stripe with darker edges going from the bottom of the image to the bottom right of the image. The scale bar is the length 200 nanometers. The image shows the area A. Transmission Electron Microscopy Energy Dispersive Spectroscopy (TEM-EDS) was used on area A. Using TEM-EDS area had 95.32 ± 0.47 weight % (or 97.23% atomic%) carbon and 2.95 ± 0.39 weight% ( 2.26% atomic%) oxygen atoms. Example 2 The graphene-like material made in accordance with the method of Example 1 was oxidised by exposure to H2SO4, KMnO4, heat, H2O2 and then the suspension was then filtered and washed with 1:10 HCL Characterisation of the oxidised carbon nanomaterial Figure 4 Powder X-Ray Diffraction (PXRD) of the carbon nanomaterial as made by the method described in examplel before (A) and after the oxidising method (B) as described in example 2. The graphene oxide (B) is the oxidised carbon nanomaterial made in example 1. Stars highlight the PXRD peaks. The carbon nanomaterial (A) exhibits two peaks (*) that are commensurate with those of a graphene standard material. The graphene oxide (B) created by oxidising the starting carbon nanomaterial one peak (#) that is commensurate with that of graphene oxide standard material. Figure 5 Attenuated Total Reflection Infrared (ATR-IR) spectroscopy of the carbon nanomaterial (A) (as made by the method described in examplel) before and after the oxidising method (B) as described in example 2. The graphene oxide (B) is the oxidised carbon nanomaterial made in example 1. The oxidised material exhibits the C=O stretching 1759cm-1, 0-H bending 1364cm-1 and C-0 stretching 1216cm-1 spectra indicative of graphene oxide. The carbon nanomaterial (A) does not show these, indicating that these bonds are not present in the carbon nanomaterial. Figure 6 Transmission Electron Microscope (TEM) image taken using a 120kV Tungsten filament. The oxidised carbon nanomaterial (as described in method 2) is supported by a Lacy Carbon Grid. The scale bar is the length 200 nanometers. The image shows a sheet of material that is only a few nanometers thick. Figure 7 Transmission Electron Microscope (TEM) image taken using a 120kV Tungsten filament. The oxidised carbon nanomaterial is supported by a Lacy Carbon Grid. The scale bar is the CM CM length 20 nanometers. The image shows an eancircled area. Transmission Electron Microscopy Energy Dispersive Spectroscopy (TEM-EDS) was used on the area. Using TEM-EDS the area had 86.4 ± 0.6 weight % (or 89.64% atomic%) carbon and 13.1 ± 0.6 weight% (10.2.% atomic%) oxygen atoms.

Claims

1. A method for preparing a carbon nanomaterial, the method comprising:(i)     providing a mass of cellulosic plant fibres;5 (ii)     treating the mass of cellulosic plant fibres with a base at a temperature of less than100 °C to form a treated mass of fibres;(iii)    recovering the treated mass of fibres from the base; and(iv)    heating the recovered mass of fibres to form the carbon nanomaterial,wherein prior to the step of treating the mass of cellulosic plant fibres with a base at10 a temperature of less than 100 °C to form a treated mass of fibres, the method comprises comminuting the mass of cellulosic plant fibres to form a comminuted mass of cellulosic plant fibres, andwherein the step of providing a mass of cellulosic plant fibres comprises obtaining a mass of cellulosic plant fibres from hemp, bamboo, flax, coconut shells, coconut coir, sisal, 15 cotton, jute, ramie, kapok, milkweed, piha, abaca, sedges, rushes and / or reeds.

2. A method according to claim 1, wherein the step of treating the mass of cellulosic plant fibres with a base to form a treated mass of fibres is carried out at a temperature of from 10 to 50 °C, preferably from 15 to 40 °C and / or for a period of at least 24 hours,20 preferably for at least 5 days.

3. A method according to claim 1 or claim 2, wherein the base comprises a calcium compound, preferably wherein the calcium compound is selected from the group consisting of calcium hydroxide (Ca(OH)2), calcium oxide (CaO), calcium carbonate (CaCOs), calcium25 hydrogen carbonate (Ca(HCO3)2) and combinations of two or more thereof.

4. A method according to any of the preceding claims, wherein the mass of cellulosic plant fibres is combined with the base in a weight ratio of the mass of cellulosic plant fibres to the base of at most 2:1, preferably at most 1:1, more preferably from 1:2 to 1:10.

305. A method according to any of the preceding claims, wherein the base is provided as an aqueous solution.

6. A method according to any of the preceding claims, wherein the step of recovering 35 the treated mass of fibres from the base comprises draining a first aqueous portion from23 09 24the treated mass of fibres and, optionally: (i) washing the recovered mass of fibres with water and draining a second aqueous portion from the treated mass of fibres, and / or (ii) drying the recovered mass of fibres.5    7. A method according to claim 6, wherein the method further comprises recoveringthe first aqueous portion and / or the second aqueous portion and using the recovered first aqueous portion and / or the recovered second aqueous portion to treat soil.

8. A method according to any of the preceding claims, wherein between the step of 10 recovering the treated mass of fibres from the base and heating the recovered mass of fibres to form the carbon nanomaterial, the method comprises a step of treating the recovered mass of fibres with an activating agent.

9. A method according to claim 8, wherein the activating agent is selected from the 15 group consisting of KOH, Ca(OH)2, NaOH, ZnCl2, H3PO4 and combinations of two or more thereof.

10. A method according to claim 9, wherein the activating agent is provided as an aqueous solution, preferably having a concentration of the activating agent of from 25 to 75 20 wt% by weight of the solution.

11. A method according to claim 9 or claim 10, wherein the recovered mass of fibres and the activating agent are combined in a weight ratio of the recovered mass of fibres to the activating agent of from 2:1 to 1:5.2512. A method according to any of claims 8 to 11, wherein the step of treating the recovered mass of fibres with an activating agent comprises heating to a temperature of from 50 to 100 °C and / or for a period of from 1 to 40 hours.30    13. A method according to any of the preceding claims, wherein the step of heating therecovered mass of fibres to form the carbon nanomaterial comprises carbonising the recovered mass of fibres.

14. A method according to any of the preceding claims, wherein the step of heating the recovered mass of fibres to form the carbon nanomaterial comprises heating the recovered mass of fibres to a temperature of at least 300 °C and / or for a period of at least 10 minutes.5    15. A method according to any of the preceding claims, wherein the method furthercomprises washing the carbon nanomaterial, optionally with an acidic solution, and, optionally, drying the carbon nanomaterial.

16. A method according to any of the preceding claims, wherein the method further10 comprises oxidising the carbon nanomaterial.

17. A method according to any of the preceding claims, wherein the carbon nanomaterial is a graphene or graphene-like material.23 09 24

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