METHOD OF FABRICATING A SELF-SUPPORTING EXPANDED 2D MATERIAL AND EXPANDED MATERIALS

This application claims priority from GB1517786.8 filed 08 October 2015, the contents of which are herein incorporated by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to expanded 2D material foams, methods of producing expanded 2D material foams, and uses of said 2D material foams. BACKGROUND

The most famous 2D material is graphene. Graphene is an atomically thick, two dimensional sheet composed of sp ² carbons in a honeycomb structure. It can be viewed as the building block for all the other graphitic carbon allotropes. Graphite (3-D) is made by stacking layers on top of each other, with an interlayer spacing of -3.4 A and carbon nanotubes (1 -D) are a graphene tube.

Single-layer graphene is one of the strongest materials ever measured, with a tensile strength of -130 GPa and possesses a modulus of -1 TPa. Graphene's theoretical surface area is - 2630 m2/g and the layers are gas impermeable. It has very high thermal

(5000+ W/mK) and electrical conductivities (up to 6000 S/cm). The observed superior properties of graphene introduced it as a potential candidate material for many applications including

but not limited to:

(a) additive for mechanical, electrical, thermal, barrier and fire resistant properties of a polymer;

(b) surface area component of an electrode for applications such as fuel cells, super- capacitors and lithium ion batteries;

(c) conductive, transparent coating for the replacement of indium tin oxide; and

(d) components in electronics.

Graphene was first reported in 2004, following its isolation by Professor Geim's group. Graphene research since then has increased rapidly. Much of the "graphene" literature is not on true monolayer graphene but rather two closely related structures:

(i) "few layer graphene", which is typically 2 to 10 graphene layers thick. The unique properties of graphene are lost as more layers are added to the monolayer and at 10 layers the material becomes effectively bulk graphite; and (ii) Graphene oxide (GO), which is a graphene layer which has been heavily oxidised in the exfoliation process used to make it and has typically 30at% oxygen content. This material has inferior mechanical properties, poor electrical conductivity and is hydrophilic (and hence a poor water barrier).

There are a variety of methods to produce graphene [Ruoff 2009]. Novoselov et al.

produced their first flakes by the mechanical exfoliation of graphite by using an adhesive tape to isolate individual layers [Novoselov 2004]. It has been shown subsequently that graphite can also be exfoliated by using ultrasonic energy to separate the layers when in an appropriate solvent, such as NMP (N-methyl pyrrolidone) [Coleman 2008 & 2009].

Graphite is an allotrope of carbon, the structure of which consists of graphene layers stacked along the c-axis in a staggered array usually denoted as ABAB. The layers are held together by weak van der Waals forces so that the separation between layers is 0.335 nm. Graphite is a cheap and abundant natural material, which makes it an excellent raw material for inexpensive production of graphene.

As noted above, graphite has been used to make graphene via exfoliation, wherein the stacked layers of graphite are separated to produce graphene. This has been achieved by using mechanical cleavage [Novoselov 2004], ultrasound (ultrasonic exfoliation, USE), using shearing forces exfoliation [Paton 2014 doi:10.1038/nmat3944, and US 20140044968 A1] and also by intercalating compounds into the graphite interlayer structure so as to weaken the interlayer bonding and promote layer separation. There are two routes that have been reported to intercalate compounds into graphitic structures: chemical and electrochemical. The chemical method is based on the direct reaction of solid graphite materials with the intercalation species (usually in liquid or vapour phase). This process is kinetically slow and usually assisted by sonication or heating. The second route, the electrochemical approach, involves generating the intercalated species through an electrochemical reaction on a graphite cathode or on a graphite anode.

Chemical intercalation of graphitic structures has been demonstrated using oxidizing or reducing agents. Reactions of graphite with oxidizing acids or molecular oxidants such as Br2, AsF ₅ or FeC result in intercalation compounds that contain both neutral and ionized guest species. [Schollhorn 1980, Zhao 201 1]. However, over-oxidation such as that by Bransted acids (phosphoric, sulfuric, dichloroacetic and alkylsulfonic acids) results in the formation of covalent bonds, as in the case of graphite oxide or fluorides, with loss of conductivity [Hofmann 1934, Worsley 2007]. By contrast, intercalation with reducing agent such as K, Li, Na, NH _n usually improves the conductivity of the graphite [Kwon 201 1 , Viculis 2003]. Subsequent exfoliation with these graphite intercalation compounds could

successfully produce a large quantity of graphene [Kwon 201 1 ].

Huang et al. [Huang 2012] have used molten LiOH at 600 °C to generate intercalated Li _x C _y species via an in situ reduction process. Huang reports that it is the reduced Li _x C _y species (and not the Li ions) that causes the desired expansion of the graphite. The expanded graphite is subsequently exfoliated in a distinct, separate aqueous sonication step.

The most famous example of the electrochemical approach is based on the lithium ion battery. For decades, graphite was used as negative electrode in lithium ion battery due to its high electrical conductivity and its ability to host lithium between the graphene layers. The lithium-graphite intercalation compounds decompose readily in water giving rise to lithium hydroxide and free standing graphene sheets. Loh et al. mimicked the lithium ion battery principle to intercalate Li into graphite and then applied a sonication step to exfoliate graphite [US 2013/0102084 A1 , and WO 201 1/162727]. This work is also discussed in a related paper [Wang 201 1 ]. However, due to the slow kinetic nature of the intercalation process, the lithium was limited to the areas close to the edges. Upon exfoliation in water, graphite with expanded edges was produced and further intercalation, water decomposition and sonication steeps were needed to achieve exfoliation.

The present inventors reported in WO2012/120264 A1 the exfoliation of graphite through the electrochemical ammonia-graphite intercalated compound. Without sonication or repeating the intercalation/decomposition steps, the product was few layer graphene with a particle size in the submicron level. Swager and Zhong [Zhong 2012] proposed a method to intercalate graphite with Li and then with ammonia in two separate steps. However, due to the expanding nature of the cathode, the electrodes distance was initially large and hence high voltage was applied to overcome the high internal resistance (IR) drop. As a result, the organic solvent used as electrolyte dissociated at later stages of the process and hindered the intercalation process. Therefore, an additional sonication step was necessary to achieve reasonable exfoliation.

Another electrochemical method has been introduced whereby double intercalation of graphite occurs with metal and organic ions. This method does not use any sonication, but it can suffer from decomposition of the organic solvent depending on the conditions used [WO 2013/132261 A1 ]. Intercalation compounds can also be produced by introducing a metal through the vapour phase and then reacting these ions. The layers of the intercalation compound can then be separated by stirring in an appropriate solvent, such as NMP [Valles 2008]. An intercalation approach has also been taken to separate graphene oxide aggregates by electrostatically attracting tetrabutylammonium cations in between the layers of the graphene oxide [Ang 2009]. This technique relies on the charges present in graphene oxide to attract the tetrabutylammonium cations. In summary, the intercalation of graphitic structures has been reported extensively in the art. These intercalated graphitic structures have most commonly been used in the production of graphene, exploiting the assistance of the intercalated species in the exfoliation of graphene layers. SUMMARY OF THE INVENTION

The invention is based on the inventors' insight that the intercalation of species in between graphitic layers may be exploited to produce expanded graphene foam "aerogels" having desirable graphene properties. This is thought to proceed via interlayer gas formation (assisted by the presence of the intercalation species) which leads to expansion and exfoliation with contemporaneous cross-linking of the layers.

The invention is further based on the inventors' insight that these methods are applicable to 2D materials other than graphene. The aerogels and methods described herein for the production of said aerogels represent a significant contribution to the art. A number of methods for the production of "graphene aerogels" have been described in the literature. However, these are not strictly graphene aerogels because their properties more resemble graphene oxide (GO) or graphite than graphene, limiting their usefulness for application relying on graphene's particular, and highly desirable, properties.

By contrast, the aerogels of the present invention show much better, indeed "true", graphene characteristics, as evidenced by a comparison of their Raman spectra, chemical purity, and high conductivity.

In addition, the processes of the invention provide a cheap, simple, and more eco-friendly process to prepare such a graphene aerogel. The process is not limited to the production of graphene aerogels, and it has been demonstrated to be suitable for the production of other foams based on atomically thick layered materials such as metal dichalcogenides.

The present invention therefore relates to an expanded 2D material; in other words, an expanded material comprising a 2D material.

The term 2D material is recognised in the art. It refers to super-thin mono- or few-layer sheets of material. It is understood that the term 2D material includes both atomically thick single layer covalent structures, of which graphene and boron nitride are perhaps the most well-known examples, and compounds exhibiting mono-layer and few layer structures, for example, transition metal dichalogenides, of which M0S2 is perhaps the best-known example.

Expanded 2D material, as used herein, describes a "foam / sponge"-like structure comprising interlocked and / or cross-linked 2D sheets and voids.

Accordingly, in a first aspect the invention may provide a method of fabricating an expanded 2D material, the method comprising:

- providing an intercalated bulk 2D material or intercalated 2D material, - heating so as to cause expansion of the intercalated bulk 2D material or intercalated 2D material to form the expanded 2D material.

Suitably, the method includes the step of locating the intercalated bulk 2D material or intercalated 2D material in an expansion vessel. Suitably, the heating step is a step of heating the expansion vessel so as to cause expansion of the intercalated bulk 2D material or intercalated 2D material to form the expanded 2D material.

Suitably the expansion vessel has a fixed volume. This can assist with producing an expanded material whose shape conforms to the internal shape / dimensions of the expansion vessel. Suitably the expansion vessel has at least one opening or aperture that is closable (e.g. to define a fixed volume), suitably with a closure means. The closure means can be permeable to gas, for example comprising a mesh or porous material. This permits exit of gas from the expansion vessel. Alternatively the closure means is not permeable to gas such that a gas tight / air tight arrangement can be achieved. That is, preferably the expansion vessel is sealable so as to provide a gas tight / air tight arrangement. More generally, the expansion vessel, when closed, can be gas tight / air tight. Suitably the expansion vessel is a crucible of fixed volume, which may be closed, for example with a mesh. The expansion vessel may also be a stainless steel cell, which may be airtight. Suitably, the expanded 2D material has a density of less than 1 g cm ^-3 , preferably less than 500 mg cm ^"3 . Preferably, the density is much lower, for example less than 100 mg cm ^"3 , suitably less than 75mg cm ^"3 , preferably less than 50mg cm ^"3 .

Suitably, the expanded 2D material is self-supporting. Suitably, the final shape and dimensions of the expanded 2D material are determined by the internal structure of the expansion vessel. Accordingly, the method may include the step of selecting an expansion vessel of suitable shape and size to produce an expanded 2D material having said shape and size. Furthermore, selection of a suitable vessel and selection of an appropriate amount of intercalated starting material may permit control of the density of the final product.

Suitably, the heating is at a temperature of greater than 200 °C, for example, greater than 300 °C, greater than 400 °C, greater than 500 °C, greater than 600 °C, greater than 700 °C, greater than 800 °C.

Suitably the heating is for a duration of greater than 30 minutes, preferably greater than 1 hour. For example, the duration may be 30 minutes to 10 hours, preferably 30 minutes to 5 hours, more preferably 30 minutes to 3 hours, more preferably 1 hour to 3 hours.

Without wishing to be bound by any particular theory, the inventors believe that the intercalated species may, during the expansion step, lead to a rapid gas formation that assists the formation of the desired foam structure. In preferred embodiments, the expanded 2D material produced is expanded graphene; that is, the bulk 2D material is graphitic material and/or the 2D material is graphene.

However, other 2D materials are also within the scope of the invention. For example, and as described herein, the inventors have demonstrated that the method is similarly applicable to the transition metal dichalcogenide M0S2. Accordingly, in some embodiments, the method is a method of fabricating an expanded 2D material, wherein the expanded 2D material is not graphene, for example wherein the expanded 2D material does not have a carbon-based structure.

2D materials other than graphene are known in the art. For example, they may include without limitation transition metal dichalcogenides, boron nitride and phosphorene. 2D transition metal dichalcogenides include M0S2 WS2, MoSe2, WSe2.

In some embodiments, the expanded 2D material produced is expanded M0S2; that is, the bulk 2D material is M0S2 and/or the 2D material is M0S2.

Without wishing to be bound to any particular theory, the inventors attribute the desirable foam properties, at least in part, to the formation of cross-linkers that form attachments between sheets during expansion. Suitably, cross-linkers include one or more of nitrogen, phosphorus and sulfur.

The cross-linker is derived from a cross-linking agent. In some embodiments, a cross-linking agent is also the intercalation species.

The intercalation species preferably includes nitrogen and / or phosphorus and / or sulfur. Suitable intercalation species are described herein and include tetraalkyl ammonium ions, a dialkyl ammonium ions, nitride-derived ions and phosphonium ions.

It may be desirable to include a cross-linking agent that is not an intercalation species.

These may be added during the intercalation step or after, prior to expansion. Suitable additional cross-linking agents are described herein.

The inventors have shown that the above method can use both electrochemically- and chemically-intercalated material. Accordingly, the step of providing an intercalated bulk 2D material or intercalated 2D material may comprise (a) intercalation in an electrochemical cell, wherein the cell comprises (i) a negative electrode and (ii) a positive electrode; at least one of which is a bulk 2D material; and (iii) an electrolyte which is ions in a solvent, wherein the ions include an intercalation species; and wherein the method comprises the step of passing a current through the cell. The step of providing an intercalated bulk 2D material or intercalated 2D material may comprise heating bulk 2D material or 2D material with an intercalation species. Preferably, electrochemical intercalation is used. This permits in situ intercalation and partial exfoliation/expansion. This may be advantageous as the intercalated material prior to expansion is already at least partially 2D material rather than bulk 2D material. Accordingly, in some embodiments, the expanded 2D material has a nitrogen and / or phosphorus and /or sulfur content of 5 to 30 atom %, preferably 15 to 20 atom %.

It will be appreciated that, preferably, the nitrogen and / or phosphorus and /or sulfur is bound to the structure within the cross-linkers. Accordingly, in some embodiments, the expanded 2D material has a bound nitrogen and / or phosphorus and /or sulfur content of 5 to 30 atom%; preferably a bound nitrogen and / or phosphorus content of 15 atom% to 20 atom%.

In a further aspect, the present invention provides an expanded 2D material produced by a method of the invention.

The present invention also relates to expanded 2D materials, which may also be referred to as foams. Advantageously, the present invention provides graphene foams having excellent graphenic properties. These represent a significant advance over the already known reduced graphene oxide aerogels that retain a high oxygen content.

Accordingly, in a further aspect, the present invention may provide an expanded graphene material. This expanded graphene material typically has a bound oxygen content of 10 atom% or less, for example 8 atom% or less, 6 atom% or less, 5 atom% or less. In this context, "bound" oxygen content refers to oxygen atoms linked covalently, either directly or indirectly, to the graphene sheets. For example, oxygen may be bound as hydroxyl groups or epoxy bridges, or within carboxylic acid groups (these functional groups are typically associated with graphene oxide structures). The low oxygen content results in expanded materials having desirable properties associated with graphene.

In some embodiments, the total oxygen content is 10 atom% or less, for example 8 atom% or less, 6 atom% or less, 5 atom% or less. In this context, "total" includes "bound" as described above, and additional oxygen atoms present in some other way, for example, adsorbed onto the graphene surface. Suitably, the expanded graphene foam has a density of less than 1 g cm ^-3 , preferably less than 0.5 g cm ^"3 , more preferably less than 0.3 g cm ^"3 , more preferably less than 0.1 g cm ^"3 , more preferably less than 50 mg cm ^"3 . Without wishing to be bound by any particular theory, the inventors speculate that properties of the expanded graphene material (foam) are in some embodiments enhanced by the presence of cross-linkers bound within the cavities between layers.

Accordingly, in some embodiments, the expanded graphene material may have a nitrogen content of 5 atom % or more, for example 10 atom % or more, or even 15 atom % or more.

In some embodiments, the nitrogen content of the expanded graphene material may be 5 to 30 atom %. In some embodiments, the nitrogen content of the expanded graphene material may be 10 to 25 atom %. In some embodiments, the nitrogen content of the expanded graphene material may be 15 to 20 atom %. Nitrogen content may refer to "bound" nitrogen content or, preferably, "total" nitrogen content, as defined above.

These same ranges may, in some embodiments, apply to phosphorus content and /or sulfur content.

In a further aspect, the present invention provides an expanded graphene material or expanded 2D transition metal dichalcogenide material having a nitrogen and / or phosphorus and / or sulfur content of 5 atom % or more, for example 10 atom % or more, or even 15 atom % or more. In some embodiments, the nitrogen and / or phosphorus and / or sulfur content of the expanded graphene material or expanded 2D transition metal dichalcogenide material may be 5 to 30 atom %. In some embodiments, the nitrogen and / or phosphorus and / or sulfur content of the expanded graphene material or expanded 2D transition metal dichalcogenide material may be 10 to 25 atom %. In some embodiments, the nitrogen and / or phosphorus and / or sulfur content of the expanded graphene material or expanded 2D transition metal dichalcogenide material may be 15 to 20 atom %. An exemplary transition metal dichalcogenide is M0S2.

The expanded graphene foams of the invention may have desirable capacitance properties, in other words, they may be useful for storing energy. In some embodiments, the present invention provides an expanded graphene material having a specific capacitance of 50 Fg ^"1 or greater, suitably 100 F g ¹ or greater. In some embodiments, the present invention provides an expanded graphene foam having a specific capacitance of 150 F g ^"1 or greater. In some embodiments, the present invention provides an expanded graphene foam having a specific capacitance of 200 F g ^"1 or greater.

Accordingly, the present invention further relates to a capacitor comprising an expanded 2D material of the invention, suitably expanded graphene material. Suitably, the capacitor is a supercapacitor. Therefore, in a further aspect, the present invention provides a

supercapacitor comprising an expanded 2D material of the invention, suitably expanded graphene material. Supercapacitors are also referred to as ultracapacitors (previously electric double layer capacitors) and are high capacity electrochemical capacitors. Methods for the construction of supercapacitors are known in the art, with a representative procedure described at example 1 . In brief, they typically consist of of two electrodes separated by an ion-permeable membrane (separator), and an electrolyte electrically connecting both electrodes One or more electrodes may comprise the expanded 2D material of the invention. The methods described herein for the production of expanded graphene foams have been demonstrated by the inventors to be applicable to the production of expanded foams of other 2D materials, for example M0S2.

The invention therefore further relates to expanded 2D materials other than graphene, for example wherein the expanded 2D material does not have a carbon-based structure.

Preferably, the expanded 2D material is a transition metal dichalcogenide. Accordingly, in a further aspect, the present invention relates to an expanded 2D M0S2 material having a density of less than 1 g cm ^"3 , for example, less than 500 mg cm ^"3 , less than 100 mg cm ^"3 , less than 50 mg cm ^"3 . A preferred transition metal dichalcogenide is M0S2.

Accordingly, in a further aspect, the present invention may provide an expanded 2D M0S2 material having a density of less than 1 g cm ^"3 , for example, less than 500 mg cm ^"3 , less than 100 mg cm ^"3 , less than 50 mg cm ^"3 . Preferably, the expanded 2D material has a nitrogen content of 5 to 30 atom %, for example a nitrogen and / or phosphorous and / or sulfur content of 15 to 20 atom %. As this nitrogen content is preferably retained within cross-linkers, the expanded 2D material may have a bound nitrogen and / or phosphorous and / or sulfur content of 5 to 30 atom %, for example a nitrogen and / or phosphorous and / or sulfur content of 15 to 20 atom %.

The expanded 2D materials of the invention other than expanded 2D graphene may also have desirable electronic properties. Accordingly, the invention further relates to a capacitor comprising an expanded 2D material of the invention other than expanded graphene.

Suitably, the capacitor is a supercapacitor.

For certain applications, it may be advantageous to dope the expanded material with an additional material. The term "dope" in this context means to include an additive within the structure. "Dopant" therefore will be understood to be an additive, for example a

nanoparticle supported on the expanded structure surface.

For clarity, it will be understood that the atom% ranges given above refer to the expanded material per se, and do not typically account for the presence of dopants. Dopant values are given in wt%.

It will be appreciated that suitable dopants are selected dependent on the intended purpose of the aerogels. Dopants may be selected to improve electrical properties, for example to produce doped expanded 2D material for use in capacitors, supercapacitors and dielectrics. Dopants may also be selected as catalysts such that the expanded 2D material forms a solid support for a catalyst.

Suitable dopants may include metals, for example, metal nanoparticles. For example, the dopant may be a transition metal, for example gold, platinum, chromium, palladium, rhodium, ruthenium, nickel, cobalt, rhenium, iridium, and / or tin. Suitable dopants may also be lanthanides or actinides. Suitable dopants may include semi-metals and non-metals, for example, silicon. Suitable dopants may include compounds, for example, metal oxides and hydroxides.

Suitable metal oxides may include transition metal oxides, lanthanide oxides and actinide oxides. A preferred dopant is iron oxide.

Suitable metal hydroxides may include transition metal hydroxides, lanthanide hydroxides and actinide hydroxides.

The dopant may be added such that it comprises 1 wt%, 5 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt%, 30 wt%, 35%, 40 wt%, 45 wt%, or even 50 wt% of the doped expanded product. It will be appreciated that, because the expanded materials of the invention are typically very low density, the dopant may for some applications comprise more than 50 wt% of the doped expanded product. It will be understood that options and preferences described herein with respect to materials are also applicable to methods and uses, while options and preferences for uses are applicable to material and methods, and options and preferences for the methods are applicable to the materials and uses, except where context dictates otherwise.

DETAILED DESCRIPTION

The invention will now be further described with reference to the following figures in which:

Figure 1 shows a Raman spectrum of a graphene aerogel according to the present invention.

Figure 2 shows a Raman spectrum of a reduced GO oxide product (representative of the prior art).

Figure 3 shows an XPS spectrum of a graphene aerogel of the present invention.

Figure 4 shows an N 1 s XPS spectrum of a graphene aerogel of the present invention. Figure 5 shows the 01 s XPS spectrum of a graphene material formed from graphite intercalated with H2SO4 and HNO3.

Figure 6 shows the 01 s XPS spectrum of a graphene aerogel formed from graphite intercalated with tetraethyl ammonium hydrogen chloride salt.

Figure 7 shows a scanning electron microscope (SEM) image of a graphene aerogel of the present invention.

Figure 8 shows a further scanning electron microscope (SEM) image of a graphene aerogel of the present invention.

In some embodiments, the present invention provides aerogels, especially graphene aerogels, that are self-supporting and exhibit desirable electronic properties.

Self-supporting, in the context of this application, refers to aerogels that, when removed from the expansion vessel, substantially maintain their shape. In other words, the produced material does not collapse when no longer confined. It will be appreciated that, importantly, the aerogels described herein are suitably self-supporting as a result of the relationship between 2D sheets and / or the cross-linkers between the sheets; as a result no external support is needed (for example, no resin, gelling agent, polymer, filler or similar is included, as described in WO2013/132259). "True" graphene aerogels

The graphene aerogels comprise graphenic material. In other words, the foam is formed predominately of mono- and few-layer graphene sheets, as evidenced by the Raman spectra, and in particular the 2D peak and similarly of the Raman spectra of the graphene aerogels of the present invention to Raman spectrum of exfoliated graphene (prior to expansion). The Raman spectra of the graphene aerogels of the present invention show comparatively few defects are present (Figure 1 ).

In these respect, the graphene aerogels of the present invention are distinguished from the graphene oxide "aerogels" previously described.

The graphene aerogels of the invention have very high conductivity (150 S nr ¹ ) as compared with the previously known "graphene" aerogels in the literature (the highest conductivity for the rGO aerogel is 64 S nr ¹ ) [Menzel 2015]. The building blocks of the graphene aerogels of the invention are substantially defective-free graphene sheets as can be seen from the Raman analysis. Other aerogels in the literature used heavily defective or oxidative graphene/ carbon to produce the aerogel, which have deleterious effect on the electric, mechanical and chemical properties of graphene [Menzel 2015, Qiu 2014]. The chemical purity of the graphene in the aerogel in the current invention is much higher than other aerogels. The current invention aerogel has less than 5% oxygen while Hu et al presented aerogel with oxygen content not less than 8.3% [Hu 2013]. By way of illustration, Figure 1 shows a Raman spectrum of a graphene aerogel of the present invention. Figure 2 shows a Raman spectrum of a thermally-reduced graphene oxide-based "aerogel", as described in US2010/0144904 [see also Becerril 2008, DOI: 10.1021/nn700375n]. This material was made as follows: concentrated H2SO4 (69 mL) was added to a mixture of graphite flakes (3.0 g) and NaNC (1 .5 g) and the mixture was cooled to 0 °C. KMn04 (9.0 g) was added slowly in portions, maintaining the reaction temperature below 20 °C. The reaction was warmed to 35 °C and stirred for 30 min, at which time water (138 mL) was added slowly, and then the mixture was heated on a hotplate to maintain the reaction temperature at 95 °C for 15 min. The reaction was then cooled down in a water bath for 10 min. The graphene oxide was filtered off and washed with HCI solution, water, and ethanol. The graphene oxide was the dried overnight at 30 °C under vacuum. In the second step, the dry graphene oxide was charged into sealed metallic crucible and rapidly heated to 800 °C under an argon atmosphere. The present inventors noted that, during expansion, no self- supporting foam was formed. Even increasing the load of the powder in an effort to obtain higher density (up to a theoretical density of 80 g cm ^-3 ) no foam formed. The Raman spectrum for the thermally-reduced GO-Based product produced above has two distinguished peaks (Figure 2). The two peaks are attributed to the G band at 1594 cm ^-1 and D band at 1348 cm ^-1 , respectively. The G band is related to the in-plane bond- stretching motion of pairs of sp ² -C atoms, while the D peak is associated with the disorder in the graphene layers, corresponding to the conversion of carbon sp ² to sp ³ . In the aerogel of the present invention (Figure 1 ), the intensity of the defects peak (D Band) is significantly decreased and a significant 2D was detected at -2600. This 2D peak is the most characteristic feature of graphene. Furthermore, the graphene aerogels of the present invention have comparatively low oxygen content (suitably, 10 atom% or less). The elemental content can be calculated using XPS (Figure 3). XPS data presented herein were collected using a Kratos Axis Ultra X-ray photoelectron spectrometer, equipped with an aluminium/magnesium dual anode and a monochromated aluminium X-ray sources.

The present inventors have found that retaining some nitrogen in the graphene aerogel is advantageous. Suitably, therefore, the graphene aerogels of the present invention may have a nitrogen content of 5 to 30 atom%, for example, a nitrogen content of 8 to 25 atom%, a nitrogen content of 10 to 20 atom%, a nitrogen content of 15 to 20 atom%.

Without wishing to be bound to any particular theory, the present inventors believe that the presence of some nitrogen may be advantageous in providing better mechanical properties and / or electrical properties. For example, the present inventors have found that graphene aerogels stripped of nitrogen (by controlling the baking / expansion process as described herein) are typically more brittle than graphene aerogels containing some nitrogen.

Once again, without wishing to be bound by any particular theory, the present inventors attribute this to cross-linking between the graphene platelets of the aerogel by bound nitrogen. The bound nature of the nitrogen is confirmed by XPS analysis.

As can be seen from Figure 4, there are four different components of nitrogen-containing groups in the produced materials, corresponding to pyridinic (398.3 eV), pyrrolic (400.1 eV), graphitic (401 .4 eV), and oxidized (403.7 eV) type N-functionalities, respectively. The appearance of these nitrogen components can be attributed to the complexity of the high temperature sintering, which facilitates the incorporation of elemental nitrogen into graphene with different oxidation states. Generally, the nitrogen functional groups are usually in the following molecular structures (chemical states): pyridinic-N refers to nitrogen atoms at the edge of graphene planes, each of which is bonded to two carbon atoms and donates one p- electron to the aromatic π-system; pyrrolic-N (refers to nitrogen atoms that are bonded to two carbon atoms and contribute to the π-system with two p-electrons; graphitic nitrogen is also called "quaternary nitrogen", in which nitrogen atoms are incorporated into the graphene layer and replace carbon atoms within a graphene plane; N-oxides are bonded to two carbon atoms and one oxygen atom.

The presence of oxygen-containing groups is confirmed in 01 s, C1 s XPS spectra and Raman spectra. The present inventors have found that oxygen content increases with the increase in the nitrogen content, and oxygen atoms are bonded with carbon in the form of C-O, C=0, and 0-C=0.

Representative results are present below:

Table 1

Furthermore, the present inventors have found that the bound nitrogen content affects the natural structure of function oxygen groups in the graphene / graphene oxide, as

demonstrated by a comparison of Figures 5 and 6.

Figure 5 shows the 01 s XPS spectrum of a graphene material formed from graphite intercalated with H2SO4 and HNO3. Figure 6 shows the 01 s XPS spectrum of a graphene aerogel formed from graphite intercalated with tetraethyl ammonium hydrogen chloride salt. The differing nature of the oxygen species can clearly be determined, indicating the role of nitrogen in forming graphene aerogels of the present invention.

Definitions

In the present application, the term "2D material" is used to describe materials consisting of ideally one to ten layers, preferably where the distribution of the number of layers in the product is controlled. For example, the term "graphene" is used to describe materials consisting of ideally one to ten graphene layers, preferably where the distribution of the number of layers in the product is controlled. The term nanoplatelet structures, for example, graphite nanoplatelet structures refers to structures under 100 nm in thickness, more preferably under 50nm in thickness, more preferably under 20 nm in thickness, and more preferably under 10 nm in thickness. The size of the graphene flakes described can vary from nanometres across to millimetres, depending on the morphology desired. In embodiments, the 2D material is material having up to ten layers. The 2D material may have one, two, three, four, five, six, seven, eight, nine or ten layers.

In embodiments, the graphene is graphene having up to ten layers. The graphene may have one, two, three, four, five, six, seven, eight, nine or ten layers.

In the present application, intercalated 2D material refers to 2D materials having up to ten layers and having ionic species located between at least some of the layers in the structure.

In the present application, intercalated graphene refers to graphene materials having up to ten layers and having ionic species located between at least some of the layers in the structure.

It will be appreciated that, after intercalation and before expansion, the intercalated material may not conform to the above definition regarding number of layers. In other words, the intercalated material may be intercalated "bulk" 2D material, for example, intercalated graphitic material. The layers of this intercalated bulk 2D material then separate during expansion such that the aerogel is a 2D material aerogel.

As used herein, the term "bulk 2D material" refers to the so-called bulk crystals; that is, those 3D layered materials that may be exfoliated to produce the corresponding 2D materials. The terms bulk material and bulk 2D material are used and understood in the art. For example, the bulk counterpart of graphene is graphite, while the bulk counterpart of 2D M0S2 may be referred to as bulk M0S2 (to distinguish it from single or few (up to 10) layer M0S2) or simply M0S2. The bulk material may comprise 10s, 100s or even more layers held together by Van der Waals forces. Where the material is less than 100 nm in thickness, it may be considered a nanoplatelet structure.

The graphene of the graphene aerogels may contain one or more functionalised regions. "Functionalised" and "functionalisation" in this context refers to the covalent bonding of an atom to the surface of graphene structures, such as the bonding of one or more hydrogen atoms (such as in graphane) or one or more oxygen atoms (such as in graphene oxide) or one or more oxygen-containing groups, etc. Typically, the aerogels described herein are substantially free of functionalisation, for instance, wherein less than 10% by weight, such as less than 5% by weight, preferably less than 2% by weight, more preferably less than 1 % by weight of the relevant product is functionalised. For instance, in the above aspect and embodiments it may be preferred that the material produced is substantially free of graphene oxide (i.e. wherein less than 10% by weight, such as less than 5% by weight, preferably less than 2%, more preferably less than 1 % by weight of the material produced is graphene oxide). Alternatively or additionally it may be preferred that the aerogel is substantially free of oxygen-containing groups such that the material contains less than 10atom%, more preferably less than 6atom%, and more preferably about the same, or less, at% as the graphitic starting material. In embodiments, the aerogel contains less than 5atom%, preferably less than 2atom%, preferably less than 1 atom% and most preferably less than 0.5atom% oxygen in the material. The functionalisation, where present, may occur on the material surface of the graphene sheets and / or near or at the edge of the graphene sheets. Typically, the functionalisation, where present, occurs at the grain boundary but not on the material surface. In preferred embodiments, the graphene aerogels are not functionalised. In other embodiments, it may be desirable to have higher levels of functionalisation. Thus, in embodiments, the aerogels may contain one or more functionalised regions such that more than 10% by weight, suitably more than 15% by weight, suitably more than 20% by weight, suitably more than 30% by weight, suitably more than 40% by weight, of the relevant product is functionalised. Additionally or alternatively the material produced by the present process contains more than 5 atom% total non-carbon elements (for example, oxygen and/or nitrogen and/or hydrogen) based on the total number of atoms in the material, suitably more than 10 atom%, preferably more than 15 atom%, preferably more than 20 atom%, and more preferably more than 30 atom%. The functionalised regions may for example comprise oxygen-containing groups covalently bonded to the carbon and/or nitrogen-containing groups bounded to carbon and/or hydrogen bonded to the carbon.

The definitions, ranges and explanations above in respect of graphene aerogels apply also to the non-graphene aerogels disclosed herein (e.g. transition metal dichalcongenide aerogels). Aerogel / Foam and Cross-linked

In the present specification, the terms expanded material, foam and aerogel are used interchangeably. While the term aerogel originally derives from materials produced from a "gel", aerogels are solid and rigid and do not resemble a gel in their physical properties. In the present application, the term is used to refer to such a structure not necessarily produced from a gel. Rather, the aerogels of the present application may be produced from intercalated flakes and / or 2D sheets as described herein.

As used herein, these terms refer to a structure composed of interlocked and / or cross- linked sheets defining voids. It will be understood cross-linked as used herein does not necessarily refer to any regular arrangement of cross-linkers. Rather, the aerogels of the invention have structures comprising sheets and interlocked clusters of sheets that are postulated to be cross-linked at the edges and / or at defects. This structure can be seen in the SEM images Figure 7 and Figure 8.

Intercalated material

In the present application, intercalated material refers to materials having ionic species located between at least some of the layers in the structure. The intercalated material may, for example, be an electrode that has undergone intercalation, partially exfoliated flakes that are intercalated, or bulk material that has undergone chemical intercalation.

There are two routes to intercalate compounds into structures: chemical and

electrochemical. The chemical method is based on the direct reaction of solid bulk material and / or 2D (exfoliated) materials with the intercalation species (usually in liquid or vapour phase). This process is kinetically slow and usually assisted by sonication or heating. The second route, the electrochemical approach, involves generating the intercalated species through an electrochemical reaction on a cathode or on an anode.

Suitably, the intercalation species contains nitrogen and/or phosphorus and / or sulfur, preferably nitrogen and / or phosphorous, preferably nitrogen. Without wishing to be bound by any particular theory, the present inventors believe that at least some

nitrogen/phosphorous / sulfur atoms within the intercalation species are retained in the final (graphene) aerogel and may contribute to the desirable mechanical properties and electrical properties of the aerogel. The inventors speculate that the retained intercalation species may provide some cross-linking between the layers. Suitable intercalation species may include, without limitation, ammonium ions obtained from ammonium salts. Ammonium salts may be alkylammonium salts, for example, dialkyl-, trialkyl- and tetraalkyl-ammonium salts. Suitable dialkylammonium salts include dibutyl ammonium diethyl ammonium ((C2H ₅ )2NH2 ⁺ ) and dimethyl ammonium ((CH3)2NH2 ⁺ ) salts. Suitable, trialkylammonium salts include tributyl ammonium ([(C4Hg]3NI-l ⁺ ), triethyl ammonium ((C2H ₅ )3NH ⁺ ), and trimethyl ammonium ((CH3)3NH ⁺ ) salts. Suitable tetraalkylammonium salts include tetrabutyl ammonium (TBA, [(C4Hg]4N ⁺ ), tetraethyl ammonium (TEA, (C2H ₅ )4N ⁺ ) and tetramethyl ammonium (TMA, (CH ₃ )4N ⁺ ) salts.

In such ammonium salts, the alkyl chains may contain up to 100 carbon atoms, more preferably up to 20 carbon atoms and most preferably up to 5 carbon atoms long. The alkyl chains may contain only a single carbon atom, but preferably contain at least two carbon atoms. The alkyl chains may all be the same, or may be different. Furthermore, a mixture of different ammonium ions may be used including a mixture of dialkylammonium cations, trialkylammonium cations and tetraalkyl ammonium cations. In such ammonium salts, and indeed for ionic solutions where the cation is other than an ammonium cation, e.g. an alkali metal cation, the counter-ions may be relatively lipophilic ions, e.g. tetrafluoroborate (BF4 ^" ), perchlorate (CICV) or hexafluorophosphate (PF6 ^~ ). Other soluble, inorganic ions may be used, such as tetraphenyl borate.

Other suitable intercalation species may include nitride (N ^3_ ) derived species, for example, protonated nitride species. Accordingly, metal nitride salts may be used. A suitable metal nitride salt is lithium nitride. Suitably, metal nitrides salts may be used to intercalate graphitic materials using chemical methods.

Other suitable intercalation species may include phosphate and phosphonium ions and sulphonium ions. Examples rinclude tetrabutylphosphonium hydroxide,

tetraphenylphosphonium bromide (TPPBr) and 1 1-metoxy-1 1 -oxo-undecyl- triphenylphosphonium bromide (MUTPBr). The intercalation may be either chemical or electrochemical.

It may be beneficial to include a cross-linking agent to improve the properties of the aerogel. Suitably, the cross-linking agent is a nitrogen-containing ionic species. Suitable cross-linking agents may include Λ/,Λ/'-dicyclohexylcarbodiimide (DCC), Λ/,Λ/'-diisopropylcarbodiimide (DIC), ethyl-(/V',/V'-dimethylamino)propylcarbodiimide hydrochloride (EDO), 4-(N,N- dimethylamino) pyridine (DMAP), (benzotriazol-1 -yloxy)tris(dimethylamino)phosphonium hexafluorophosphate (BOP), (benzotriazol-1 -yloxy)tripyrrolidinophosphonium

hexafluorophosphate, bromotripyrrolidinophosphonium hexafluorophosphate,

0-(benzotriazol-1 -yl)-/V,/V,/V',/V'-tetramethyluronium hexafluorophosphate (HBTU),

0-(benzotriazol-1 -yl)-/V,/V,/V',/V'-tetramethyluronium tetrafluoroborate (TBTU), 0-(7- azabenzotriazol-1 -yl)-/V,/V,/V',/V'-tetramethyluronium hexafluorophosphate (HATU), 0-(6- chlorobenzotriazol-1 -yl)-/V,/V,/V',/V'-tetramethyluronium hexafluorophosphate (HCTU), 0-(3,4- Dihydro-4-oxo-1 ,2,3-benzotriazine-3-yl)-/V,/V,/V',/V'tetramethyluronium tetrafluoroborate (TDBTU), 3-(diethylphosphoryloxy)-1 ,2,3 benzotriazin-4(3H)-one (DEPBT), and

carbonyldiimidazole (CDI).

A cross-linker may be used in addition to an intercalation species (as described above) or as the intercalation species itself. For example, cross-linkers as described herein may be used in place of, or in addition to, the ammonium cations, phosphonium cations, phosphate anions and nitride derived anions.

Accordingly, in some embodiments, a cross-linking agent is added after intercalation. In some embodiments, a cross-linking agent is added prior to intercalation. The cross-linking agent may itself act as an intercalating species, or may be added in addition an intercalating species.

Suitable nitrogen-containing intercalating and cross-linking agents (that is, the intercalating species also serves as the cross-linking agent) may include ammonium, imidazolium, piperidinium, pyridinium, or pyrrolidinium salts.

Suitable phosphorus-containing intercalating and cross-linking agents (that is, the intercalating species also serves as the cross-linking agent) may include phosphonium salts.

Suitable sulphur-containing intercalating and cross-linking agents (that is, the intercalating species also serves as the cross-linking agent) may include sulphonium salts.

The density of these aerogels, especially graphene aerogels is suitably less than 500 mg cm ^"3 , preferably less than 450 mg cm ^"3 , more preferably less than 400 mg cm ^"3 , more preferably less than 375 mg cm ^"3 , more preferably less than 350 mg cm ^"3 , more preferably less than 325 mg cm ^"3 , more preferably less than 300 mg cm ^"3 , more preferably less than 275 mg cm ^"3 , more preferably less than 250 mg cm ^"3 , more preferably less than

225 mg cm ^"3 , more preferably less than 200 mg cm ^"3 , more preferably less than 175 mg cm ^"3 , more preferably less than 150 mg cm ^"3 , more preferably less than

125 mg cm ^"3 , more preferably less than 100 mg cm ^"3 , more preferably less than 75 mg cm ^"3 , most preferably less than 50 mg cm ^"3 . It will be appreciated aerogels of the present invention may be very low density. For example, the density may be less than 40 mg cm ^"3 , more preferably less than 35 mg cm ^"3 , more preferably less than 30 mg cm ^"3 . It will be appreciated that preferred densities will vary with material. For example, a graphene aerogel may have a different preferred density to a M0S2 aerogel.

The density may be as low as 5 mg cm ^"3 , or even as low as 1 mg cm ^"3 . For example, the density may be 1 mg cm ^"3 to 50 mg cm ^"3 , for example 5 mg cm ^"3 to 50 mg cm ^"3 , for example 5 mg cm ^"3 to 30 mg cm ^"3 . Potential uses of the present invention

The aerogels of the present invention exhibit desirable electronic properties. Accordingly, they may have utility as electrodes or super-capacitors, for example, as described herein.

The aerogels of the present invention may be useful as solid supports for catalysts. For example, aerogels of the present invention may be doped with noble metals. These may be useful as photocatalysts for hydrogen evolution, as catalyst supports for proton exchange membrane (PEM) fuel cells, and for sensing applications.

The aerogels of the present invention may be doped with nanoparticles. For example, they may be doped with silicon and / tin for use in batteries.

As the aerogels of the present invention are porous, they may also have utility as capture devices, for example in water clean-up and purification applications. For example, an expanded 2D material may be able to absorb at least 50 times its weight of a solvent such as n-hexane, dichloromethane or /V-methylpyrrolidone (NMP). In some preferred embodiments, the expanded 2D material is able to absorb at least around 100 times its weight of a solvent.

The ratio increases with solvent density. Using NMP as an example, suitably, the aerogel is able to absorb at least 50 times its weight, preferably at least 100 times its weight, more preferably around 150 times its weight. For example, the aerogels of the present invention may be useful as heavy / toxic metal capturing devices for water purification.

Furthermore, the aerogels of the present invention may be useful in the cleaning-up of oil spills.

Other uses will be readily apparent to the skilled person. The invention encompasses these uses of aerogels of the invention. Example 1

Ammonium intercalated compounds of graphite were prepared by the electrochemical method described in the inventors' previous work [WO2013/132261 A1 , and Abdelkader, Kinloch and Dryfe, 2014, Doi: 10.1021/am404497n]. In summary, graphite was used as cathode in a 1 M LiCI 1 M TEAHCI (triethyl ammonium hydrogen chloride) DMSO solution. A potential difference of 15 V was applied between the graphite cathode and a Pt anode for 2 hours. The cathode material was then washed with water and dried under vacuum at 80°C. The powder was then loaded into Cu crucible (10 mm diameter by 30 mm height) and the top of the crucible sealed with a stainless steel mesh. The crucible was then loaded into one-end sealed quartz tube and flushed with argon for 2 hours before heating to 900°C. The sample was allowed to cool down under argon and then tested without further treatment.

A graphene aerogel formed inside the Cu crucible and upon removing from the crucible, the graphene aerogel was self-supporting (that is, it retained the shape of the internal part of the Cu crucible). The geometric density of the graphene aerogel was 9.3 mg cm ^-3 . The density was calculated by the weight of solid content without including the weight of entrapped air divided by the volume of the foam.

Scanning electron microscope (SEM) images (Figures 7 and 8) clearly indicate the formation of a porous 3D structure graphene aerogel with pore sizes in the range of sub- micrometer to several micrometers, as seen from a cross-sectional view. It is also evident from Figure 7 that the pore walls in the foams are cross-linked and not completely separated between different layers. In addition, the pore walls are solid or fully compact, but have some micro-size pores, increasing the overall porosity of the aerogel. Raman analysis confirmed the graphenic nature of the carbon materials. In general, the Raman spectrum of graphene has two characteristic peaks; the G peak at -1580 cm ^-1 and 2D peak at -2700 cm ^-1 . The exact positions of the Raman peaks depend on the excitation wavelength used and the level of doping in the sample. The 2D peak for monolayer graphene occurs at approximately 2637 cm ^"1 when measured using a 633 nm excitation laser. As seen in Figure 1 the Raman spectra show 2D peaks at 2662 and 2647 cm ^"1 indicating that the aerogel is composed of 1 - to 3-layer graphene.

Upon standing the graphene aerogel in /V-methyl-2-pyrrolidone (NMP), the weight of the aerogel increased from 24 mg to 3550 mg, indicating that the graphene aerogel is able to absorb around 150 times its weight of NMP. This ratio increased with the specific density of the organic solvent. For example, graphene aerogel is able to absorb 190 times its weight of dichloromethane and 98 times its weight of n-hexane.

The electric conductivity was measured to be 190 S nr ¹ for the sample with density

9.3 mg cm ^"3 and increased to 230 S nr ¹ and to 275 S nr ¹ when the density increased to 15 mg cm ^"3 and 21 mg cm ^"3 , respectively. Conductivity was measured using a standardised four-point probe to eliminate contact resistance.

This graphene aerogel was used to fabricate a supercapacitor device. The foam (as prepared above) was cut into pellets (3 mm thickness). Two of those pellets served as the capacitor electrode. Filter paper served as a separator and the electrolyte was 6 M KOH solution. The capacitance was measured in the two-electrode configuration, with a

Whatman filter paper as a separator, and 6 M aqueous KOH as the electrolyte. Cyclic voltammetry (CV) and galvanostatic charge/discharge between 0 to 1 V were carried out using an Iviumstat Electrochemical Interface.

The specific capacitance was calculated to be 265 F g ^"1 . Example 2

Phosphorus-containing cross-linking species have also been exemplified.

Using a similar procedure to example 1 , but with the electrochemical step conducted in glovebox and 0.5 M tetrabutylphosphonium hydroxide in DMSO as electrolyte. A potential of 3 V was applied for 5 hours. The cathode material was then washed with pure DMSO and then with ethanol and then dried under vacuum at 80 °C. The intercalated powder was then charged into an airtight stainless steel Swagelok cell. Then heated in a horizontal tube furnace at 400 °C for 2 hours. The Swagelok cell was then opened and the cork-like aerogel was removed. The density of the obtained aerogel was measured to be 26 mg/cm ³ . Example 3

Lithium nitride (1 g) was mixed with graphene (prepared by the electrochemical method described in WO2013/132261 A1 [and Abdelkader, Kinloch and Dryfe, 2014, Doi:

10.1021/am404497n] and heated at 900 °C in molten salts reactor for 4 hours under an argon atmosphere with manual stirring. The sample was allowed to cool down in the furnace. The mixture was then washed with water in an ice bath, then with dilute HCI solution and finally with water again with a filtering step after every washing step. The sample was dried at 40 °C under vacuum and then charged into a sealed metallic crucible. Foam was obtained following heating under an argon atmosphere following the same procedure described in example 1. The density of the obtained aerogel was measured to be 18.5 mg/cm ³ .

Example 4

Aerogel material other than graphene has also been produced. The following describes the production of a M0S2 aerogel.

The electrochemical intercalation was conducted in a glovebox under a flow of argon gas. M0S2 powder pellet was used as a cathode and Pt wire was used as an anode. The M0S2 pellet was wrapped in a cotton fabric. The electrolyte was 0.5 M triethylammonium chloride in DMSO. After 12 hours of applying 10 V, the powder was washed with pure DMSO and then filtered to collect the M0S2 intercalated compound powder. The intercalated powder was then charged into an airtight stainless steel Swagelok cell, removed from the glovebox and heated in a horizontal tube furnace at 400 °C for 2 hours. The Swagelok cell was opened and the cork like aerogel was removed. The density of the obtained aerogel was measured to be 43 mg/cm ³ .

Characterisation

Raman spectra were obtained using a Renishaw system 1000 spectrometer

coupled to a He-Ne laser (633 nm). The laser spot size was -1 -2 μηη, and the power was about 1 mW when the laser is focused on the sample using an Olympus BH-1

microscope. The SEM images were taken using a Zeiss Leo 1530 FEGSEM. TEM analysis was conducted using FEI Tecnai FZO 200kv FEGTEM. (XPS) were collected using a Kratos Axis Ultra X-ray photoelectron spectrometer, equipped with an aluminium/magnesium dual anode and a monochromated aluminium X-ray sources.

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