Technical Field

The present invention relates to a method of preparing a porous film. In particular, the invention relates to a method of preparing a porous graphene film. The porous graphene film may be used in electrical applications.

Background

It is known that lithium-ion battery electrodes store and release electrical energy by insertion and extraction of lithium ions and electrons through the electrode materials, where the surface area and the surface reaction are important for the lithium-ion battery.

Graphite is a commonly selected anode material in lithium-ion batteries because of its high Coulombic efficiency (the ratio of the extracted lithium to the inserted lithium) and cycle performance. However, the specific capacity of graphite is relatively low (theoretical value: 372 mAh/g) since every six carbon atoms can host only one lithium ion by forming an intercalation compound (LiC ₆ ). To enhance the battery performance, graphite has been replaced with other materials such as Sn, Si, Sn0 ₂ , and Co ₃ 0 ₄ . These materials have been reported to possess larger lithium storage capacities compared to graphite. For example, the theoretical capacity of Sn is reported to be about 993 mAh/g while that of Si is reported to be about 4200 mAh/g. A major drawback of these substances as anode materials is the huge volume variation during the charge/discharge process which causes the pulverization of the electrode, resulting in poor reversibility.

Alternatively, graphene, which exhibits exceptional electrical and mechanical properties, high surface-to-volume ratio, and chemical stability, has significant potential to work as electrode material for lithium ion batteries. The theoretical specific capacity of graphene exceeds that of graphite due to the fact that lithium adsorption can take place on two sides of the graphene sheet. For instance, it has been demonstrated that graphene-based anode materials have large initial discharge capacity (600-2042 mAh/g) and reversible capacity (540-1264 mAh/g), which are much better than the traditional graphite materials. Metal oxide nanoparticles encapsulated by graphene layers may have high specific capacity and excellent cycling performance as anode materials in recharge lithium battery compared to naked particles. The disadvantage of these materials is the complicated fabrication process which usually needs several steps to encapsulate the metal oxides into the hollow carbon spheres. Further, the adjustment of the graphene arrangement in the carbon is restricted. Another disadvantage is that the metal ions may leak into the electrolyte.

It is also known in the art that single layer graphene will aggregate when reduced from graphene oxide.

Graphene has many other applications. For example, graphene is impermeable to gases, which makes it useful for applications involving air-tight membranes.

There is therefore a need for an improved method for preparing a graphene film.

Summary

The present invention seeks to address these problems, and provides a method of preparing a porous graphene film.

According to a first aspect, there is provided a method of preparing a porous graphene film comprising:

mixing graphene oxide and a solvent to form a mixture;

applying the mixture to at least one surface of a substrate; and

cooling the at least one surface of the substrate to form water droplets on the surface, wherein the water droplets depress into the mixture on the substrate to form pores on the surface of the substrate.

The graphene oxide may be a complex of graphene oxide and a cationic surfactant. Any suitable cationic surfactant may be used for the purposes of the present invention. In particular, the cationic surfactant is an ammonium-containing surfactant. Even more in particular, the cationic surfactant is selected from the group consisting of: cetyl trimethylammonium bromide (CTAB), cetyl trimethylammonium chloride (CTAC), cetylpyridinium chloride (CPC), polyethoxylated tallow amine (POEA), benzalkonium chloride (BAC), benzethonium chloride (BZT), didodecyldimethylammonium bromide (DDAB), didodecyldimethylammonium chloride (DDAC), dimethyl ditetradecylammonium bromide (DTDA Br), dimethyl ditetradecylammonium chloride (DTDA CI), dimethyl dihexadecylammonium bromide (DHDA Br), dimethyl dihexadecylammonium chloride (DHDA CI), dimethyldioctadecylammonium bromide (DODA Br), dimethyldioctadecylammonium chloride (DODA CI), tetrabutyl ammonium bromide (TBAB), tetrabutyl ammonium chloride (TBAC), tetraheptyl ammonium bromide (THAB), tetraheptyl ammonium chloride (THAC), and a combination thereof. For example, the graphene oxide is a complex of graphene oxide and dimethyldioctadecylammonium bromide (DODA Br).

Any suitable solvent can be used for the mixing. For example, the solvent may be a hydrophobic solvent. In particular, the solvent may be a volatile hydrophobic solvent. The solvent may be selected from the group consisting of: toluene, chloroform, carbon disulphide, benzene, dichloromethane and a combination thereof. Even more in particular, the solvent is chloroform.

The cooling may be carried out under suitable conditions. The cooling may be carried out at a relative humidity of greater than or equal to about 60%. For example, the relative humidity may be about 80-85%. In particular, the cooling may comprise bringing the mixture applied on the substrate surface into contact with air having a relative humidity of about 60% or above to evaporate the solvent.

The method may further comprise transferring graphene oxide to an organic phase prior to the mixing. Any suitable method of transferring graphene oxide to the organic phase from an aqueous phase may be used for the purposes of the present application. For example, the graphene oxide may be transferred to the organic phase using any suitable organic solvent. The organic solvent may comprise at least one surfactant. In particular, the surfactant may be a cationic surfactant. For example, the surfactant may be as described above. The organic solvent may be selected from the group consisting of: chloroform, dichloromethane, benzene, toluene, and a combination thereof. In particular, the graphene oxide is transferred to the organic phase using chloroform. The method may further comprise reducing the porous graphene oxide film after the cooling to form a porous graphene film. Any suitable method of reducing the porous graphene oxide film to a porous graphene film may be used for the purposes of the present invention. For example, the step of reducing the porous graphene oxide film may comprise heating the porous graphene oxide film in the presence of a reducing agent. The heating may be carried out at any suitable temperature. Any suitable reducing agent may be used. For example, the reducing agent may be hydrogen gas, hydrazine (N ₂ H ₄ ), hydrazine hydrate, hydrazine monohydrate, dimethylhydrazine, hydroquinone, sodium borohydride (NaBH ), or a combination thereof. In particular, the reducing agent is hydrazine.

According to a second aspect, the present invention provides a porous graphene film prepared according to the method of the first aspect. The porous graphene film may be a monolayer porous graphene film.

According to a third aspect, there is provided a device comprising a porous graphene film prepared according to the first aspect of the present invention. The device may be an electronic device or a membrane. For example, the electronic device may be a field- effect transistor, an ultrasensitive sensor, an electrode, a capacitor or a battery. The electrode may be a transparent conducting electrode. The membrane may be an airtight membrane.

According to a fourth aspect, there is provided an electrode comprising at least one porous graphene film. The electrode may be a graphene electrode. The porous graphene film may be prepared according to any suitable method. In particular, the porous graphene film may be prepared according to the method as described above.

Brief Description of the Drawings

In order that the invention may be fully understood and readily put into practical effect there shall now be described by way of non-limitative example only exemplary embodiments, the description being with reference to the accompanying illustrative drawings. In the drawings: Figure 1 : (A) A schematic drawing of the electrostatic interaction between dimethyldioctadecylammonium bromide (DODA Br) and graphene oxide.

(B) Structural formula of DODA ion is shown.

Figure 2: A schematic illustration of the method of preparation of the porous graphene film according to an exemplary embodiment.

Figure 3: (A) An optical image of graphene oxide solution during the phase transfer process from the aqueous phase (water) to the organic phase

(comprising chloroform (CHCI ₃ )).

(B) A UV-visible spectra of graphene oxide solution in water, graphene oxide-DODA (GO/DODA) complex on quartz and reduced graphene oxide-DODA (rGO/DODA) complex on quartz.

Figure 4: (a) A photograph of the porous graphene film prepared according to an exemplary embodiment on glass.

(b) A photograph of the porous graphene film prepared according to an exemplary embodiment on copper foil.

(c) SEM image of porous graphene film prepared according to an exemplary embodiment prepared from 1 mg/mL of GO/DODA complex on silicon substrate.

(d) An enlarged SEM image of a section of (c).

(e) A section image of the graphene film of (b).

(f) The Raman spectral of GO/DODA complex and rGO/DODA complex under 488 nm laser excitation.

(g) Fourier-transform infrared (FT-IR) spectral of rGO/DODA and GO/DODA.

(h) A graph of current vs. voltage for the porous graphene film prepared according to an exemplary embodiment (inset is the microscopy image of the gold (Au) electrode).

Figure 5: (a) Charge and discharge curves of the porous graphene film prepared according to an exemplary embodiment, the film forming the anode in lithium cells at a rate of 50 mA/g.

(b) Charge and discharge capacity versus cycle number for the porous graphene film at 50 mA/g. The inset shows an SEM image of the porous graphene film. (c) Charge and discharge capacity versus cycle number for a non- porous graphene film at 50 mA/g. The inset shows an SEM image of the non-porous graphene film.

(d) Cyclic voltammetry (CV) curves of the porous graphene film vs Li/Li ⁺ at a rate of 0.1 mV/s.

(e) A schematic illustration of the intercalation between lithium ions within the pores of the porous graphene film prepared according to an exemplary embodiment.

Detailed Description of the Exemplary Embodiments

The exemplary embodiments aim to provide a simple, low-cost, green and scalable method of preparing graphene films. The method may have other advantages such as, for example, a shorter fabrication time and reduced fabrication cost. In particular, the method may be most suited for preparing porous graphene films.

According to a first aspect, there is a provided a method of preparing a porous graphene film comprising:

mixing graphene oxide and a solvent to form a mixture;

applying the mixture to at least one surface of a substrate; and

cooling the at least one surface of the substrate to form water droplets on the surface.

The water droplets formed on the surface of the substrate may depress into the mixture on the substrate to form pores on the surface of the substrate.

The porous graphene film prepared according to the method may be a single-layered film. In particular, the porous graphene film may be a monolayer porous graphene film.

A porous graphene film refers to a film of graphene in which pores or dimples oriented to the vertical direction to the film are provided along the plane direction of the film.

The pores may have any suitable shape. For example, the pores on the film may be like a honeycomb, spherical, oval, and the like. According to a particular aspect, the pores may be honeycomb shaped. The pores may have any suitable size. Pore size can be measured by (optical or electron) microscopy. Further, pore size of each pore refers to the average pore diameter. According to a particular aspect, the pores of the porous graphene film may have a substantially uniform pore size. For example, at least about 80% of the pores have a uniform pore size. In particular, at least about: 90%, 95%, 98% or 100% of the pores have a uniform pore size. According to another particular aspect, the pores of the porous graphene film may have a non-uniform pore size. Therefore, most of the pores have a different pore size. The average size of each pore may be less than about 5000 nm, 3500 nm, 3000 nm, 2500 nm, 2000 nm, 1000 nm, 800 nm, 600 nm, 500 nm, 350 nm, 300 nm, 250 nm, 200 nm, 100 nm, 75 nm, 50 nm, 20 nm, 10 nm, 5 nm. In particular, the average pore size is about 1500 nm.

According to a particular aspect, the porous graphene film may comprise an ordered array of pores. An ordered array of pores is an array of pores having a systematic arrangement. For example, the pore array may be such that there are a pre-determined number of rows and columns of pores, each row and column having a pre-determined number of pores. The pores in each row and/or column may be the same or different. An ordered array of pores may also be taken to comprise pores arranged in a non- random manner. For example, each pore may be spaced equidistant from one another.

According to a particular aspect, the porous graphene film prepared according to the method may comprise a non-ordered array of pores. A non-ordered array of pores is to be understood to refer to an array of pores having a random arrangement of pores. For example, the distance between adjacent pores may differ from pore to pore.

The graphene oxide used in the mixing may be in any suitable form. According to a particular aspect, the graphene oxide is a complex of graphene oxide and a surfactant. In particular, the graphene oxide is a complex of graphene oxide and a cationic surfactant. The cationic surfactant may be a surfactant which contains a net positive charge in its head portion. Any suitable cationic surfactant may be used. Any cationic surfactant which is capable of electrostatically adsorbing and self-assembling onto the surface of graphene oxide in alkaline solution to form a graphene oxide-surfactant complex in an organic solution may be used. The presence of the surfactant in the graphene oxide complex enables the formation of the porous single-layer porous graphene film. For example, the cationic surfactant may be selected from the group consisting of: alkyltrimethylammonium salts such as cetyl trimethylammonium bromide (CTAB) or cetyl trimethylammonium chloride (CTAC), cetylpyridinium chloride (CPC), polyethoxylated tallow amine (POEA), benzalkonium chloride (BAC), benzethonium chloride (BZT), , didodecyldimethylammonium bromide (DDAB), didodecyldimethylammonium chloride (DDAC), dimethyl ditetradecylammonium bromide (DTDA Br), dimethyl ditetradecylammonium chloride (DTDA CI), dimethyl dihexadecylammonium bromide (DHDA Br), dimethyl dihexadecylammonium chloride (DHDA CI), dimethyldioctadecylammonium bromide (DODA Br), dimethyldioctadecylammonium chloride (DODA CI), tetrabutyl ammonium bromide (TBAB), tetrabutyl ammonium chloride (TBAC), tetraheptyl ammonium bromide (THAB), tetraheptyl ammonium chloride (THAC), and a combination thereof. In particular, the graphene oxide is a complex of graphene oxide and dimethyldioctadecylammonium bromide (DODA Br). Figure 1(A) shows a schematic representation of the electrostatic interaction between DODA Br and graphene oxide and Figure 1(B) is a structural representation of a DODA ion.

Graphene oxide may be prepared according to any suitable method. The graphene oxide may be formed from a graphene or graphite. For example, the graphene oxide may be prepared by oxidising graphite via the Hummers method. The graphene oxide prepared is water soluble and therefore the graphene oxide may be transferred from the aqueous phase to the organic phase. Accordingly, the method may further comprise transferring graphene oxide to an organic phase prior to the mixing. In particular, the method may further comprise transferring a complex of graphene oxide and a surfactant as described above from the aqueous phase to the organic phase. Any suitable method may be used for transferring graphene oxide and/or a complex of graphene oxide and surfactant from the aqueous phase to the organic phase. For example, the graphene oxide may be transferred from the aqueous phase to the organic phase using a suitable surfactant. The surfactant may. be as described above. The graphene oxide may be transferred to the organic phase using any suitable organic solvent. The organic solvent may comprise at least one surfactant. In particular, the surfactant may be a cationic surfactant. For example, the surfactant may be as described above. The organic solvent may be selected from the group consisting of: chloroform, dichloromethane, benzene, toluene, and a combination thereof. In particular, the graphene oxide is transferred to the organic phase using chloroform.

The mixing comprises mixing graphene oxide of any embodiment described above and at least one solvent to form a mixture. Any suitable solvent may be used for the mixing. The solvent may be an organic solvent. The solvent may be a volatile solvent. In particular, the solvent is one which evaporates easily. The solvent may be a hydrophobic solvent. Even more in particular, the solvent is a hydrophobic evaporative solvent. The solvent may be toluene, chloroform, carbon disulphide, benzene, dichloromethane, or a mixture thereof. According to a particular aspect, the solvent is chloroform.

Any suitable substrate may be used. A suitable substrate may be selected depending on the application of the porous graphene film prepared according to the method described above. The substrate may be organic or inorganic. The substrate may be transparent or translucent. The portion of the surface of the substrate on which the porous film is in contact with may be flat and firm or semi-firm. The substrate may be composed of a metal, non-metal or a combination thereof. The substrate may comprise a material selected from a group consisting of silicon, quartz, glass, carbon, alumina, silicon nitride, mica, and a combination thereof. Many metals such as gold, platinum, aluminium, copper, titanium, and their alloys are also options for substrates. In addition, many ceramics and polymers may also be used as substrates. Polymers which may be used as substrates include, but are not limited to the following: polystyrene; poly(tetra)fluorethylene; (poly)vinylidenedifluoride; polycarbonate; polymethylmethacrylate; polyvinylethylene; polyethyleneimine; poly(etherether)ketone; polyoxymethylene (POM); polyvinylphenol; polylactides; polymethacrylimide (PMI); polyalkenesulfone (PAS); polyhydroxyethylmethacrylate; polydimethylsiloxane; polyacrylamide; polyvinyl chloride; poly(ethylene terephthalate); and/or polyimide. The substrate may also be the surface of water. In particular, the substrates may comprise silicon, glass, or copper.

The surface of the substrate on which the mixture is applied may be cooled to cause the formation of water droplets on the surface. Any suitable method of cooling the substrate surface may be used. In particular, the surface may be subjected to airflow with suitable humidity conditions. Even more in particular, the surface of the substrate is subjected to moist airflow. For example, the substrate surface may be brought in contact with ambient air with a suitable relative humidity. The relative humidity may be more than 60%. In particular, the relative humidity may be more than about 65%, 70%, 75%, 78%, 80%, 85%, 90%, 95%, 100%. Even more in particular, the relative humidity may be about 80-85%.

According to a particular aspect, the solvent in the mixture evaporates under ambient conditions. The evaporation of the solvent from the surface of the substrate on which the mixture is provided causes that surface to cool. As a result, breath figures are formed when the cool substrate surface, with the mixture film on the surface, is brought in contact with moist air having a suitable relative humidity. The breath figures result in the nucleation and growth of water droplets on the film of mixture on the surface of the substrate. The water droplets may form patterns in certain conditions.

When the substrate surface on which the mixture is provided is covered with water droplets, the temperature difference between the surface of the substrate and water droplets decreases and eventually dissipates. The water droplets may then depress into the mixture on the substrate as water is denser than the solvent in which the mixture is mixed. Pores are formed on the surface of the substrate upon further evaporation of the solvent and the condensed water droplets form a porous film. The use of a humidity chamber and external airflow may be required.

Controlling the conditions of cooling may result in the formation of pores of different shapes and sizes. For example, if pores of a hexagonal shape are desired, a higher relative humidity may be required. In particular, if pores of a hexagonal shape are desired, the relative humidity may be more than or equal to about 70%. The rate airflow in contact with the substrate surface and the rate of evaporation of the solvent may also be controlled to bring about different pore sizes.

The size of the pores formed on the porous graphene film may also be controlled by controlling the composition of the mixture applied on the surface of the substrate. For example, the ratio of the graphene oxide to the solvent in the mixture may be changed to form pores of different sizes. For example, a smaller graphene oxide to solvent ratio leads to the formation of bigger pore sizes on the film.

The method of preparing the porous graphene film may be by applying a solution of graphene oxide and dimethyldioctadecylammonium bromide (DODA Br) in chloroform onto a clean solid substrate to cover the surface of the substrate. The graphene oxide displaces the bromide ion and therefore the solution comprises a complex of graphene oxide and dimethyldioctadecylammonium ion (GO/DODA). The volatile chloroform evaporates rapidly, thereby inducing the surface of the substrate to cool rapidly. Breath figures form when the cold substrate surface with the graphene oxide/DODA film on the surface, is brought in contact with moist air. The cold surface of the substrate leads to the condensation, nucleation and growth of water droplets on the graphene oxide/DODA film on the substrate surface. The water droplets form patterns in ambient conditions, for example, with a relative humidity of about 70% and higher. When the substrate surface is completely covered with water droplets, the temperature difference between the surface and water droplets decreases and eventually dissipates, and the water droplets sink into the solution because they become heavier over time. Pores are generated on the film using the water droplets as a template upon the complete evaporation of the solvent and condensed water.

A schematic diagram of the formation of pores on a graphene film according to the method described above is shown in Figure 2. Figure 2 also shows a step of reducing the graphene oxide film with hydrazine subsequent to the formation of the porous film, which will be described below. In Figure 2, (1 ) when moist airflow passes across a volatile solution of graphene oxide/DODA complex in solvent, the rapid evaporation of the solvent induces a cooling of the solution surface followed by condensation of water vapour into droplets on the substrate surface. (2) The water droplets grow with time by molecular condensation until they reach a self-limiting, narrow size distribution. The water droplets begin to organise into a hexagonally-arranged, close-packed lattice with high surface coverage being driven by principle of the lowest free energy. Coalescence of the water droplets is inhibited by the complexes adsorbed at the interface between the water droplets and the solution. (3) Upon evaporation of the solvent, the organised film initially forms a template fashioned by the arrangement of water droplets. (4) Finally, the slow evaporation of the residual water droplets and the remaining solvent contribute to the further regulation and fixation of the film into the substrate. (5) Hydrazine (N ₂ H ₄ ) vapour is then used to reduce the porous graphene oxide/DODA film to form a reduced graphene/DODA film.

According to a particular aspect, the method further comprises a reducing the porous graphene oxide film after the cooling to form a porous graphene film. The porous graphene oxide film formed from the method may not be conductive. However, after reducing the porous graphene oxide film to the form porous graphene film, the film may be conductive.

Any suitable method of reducing the porous graphene oxide film to a porous graphene film may be used. For example, the porous graphene oxide film may be heated at a suitable temperature to reduce the film. The heating also thermally stabilises the film. The porous graphene oxide film may be heated at a temperature of greater than about 70°C, 80°C, 90°C, 100°C, 110°C, 120°C, or 150°C. In particular, the porous graphene oxide film may be heated at a temperature of about 80°C -120°C. Even more in particular, the porous graphene oxide film is heated at about 90°C. In particular, the porous graphene oxide film may be heated in the presence of a reducing agent. Any suitable reducing agent may be used. For example, the reducing agent may be selected from the group consisting of hydrogen gas, hydrazine (N ₂ H ₄ ), hydrazine hydrate, hydrazine monohydrate, dimethylhydrazine, hydroquinone, sodium borohydride (NaBH ₄ ) and a combination thereof. Even more in particular, the porous graphene oxide film is heated in the presence of N ₂ H vapour or hydrazine hydrate.

According to another aspect, there is provided a porous graphene film prepared according to the method described above. The porous graphene film may be a single- layered film. In particular, the porous graphene film may be a monolayer porous graphene film.

Graphene is known to be extremely tear-resistant and a thermal and electrical conductor. In fact, graphene is viewed as a possible substitute material for silicon in semiconductor technologies. Graphene is also impermeable to gases. Accordingly, the porous graphene film prepared according to any aspect of the method may be used in various applications. For example, the porous graphene film may be used in electrical applications or membrane applications. Therefore, according to another aspect, there is provided a graphene sheet comprising at least one porous graphene film prepared according to the method described above. The graphene sheet may be a monolayer graphene sheet.

There is also provided a device comprising at least one porous graphene film prepared according to the method described above. In particular, the device may comprise a plurality of porous graphene films. The plurality of porous graphene films may be in a disordered state.

The device may be a membrane or an electrical device. For example, the membrane may be an air-tight membrane. The electronic device may be a field-effect transistor, an ultrasensitive sensor, an electrode, a capacitor, or a battery. The electrode may be a transparent conducting electrode.

According to another aspect, there is provided an electrode comprising at least one porous graphene film. The electrode may be a graphene electrode. The pores of the at least one porous graphene film may have any suitable shape. For example, the pores on the film may be like a honeycomb, spherical, oval, and the like. According to a particular aspect, the pores may be honeycomb shaped.

The pores of the at least one porous graphene film may have any suitable size. Pore size can be measured by (optical or electron) microscopy. Further, pore size of each pore refers to the average pore diameter. According to a particular aspect, the pores of the porous graphene film may have a substantially uniform pore size. For example, at least about 80% of the pores have a uniform pore size. In particular, at least about: 90%, 95%, 98% or 100% of the pores have a uniform pore size. According to another particular aspect, the pores of the porous graphene film may have a non-uniform pore size. Therefore, most of the pores have a different pore size. The average size of each pore may be less than about 5000 nm, 3500 nm, 3000 nm, 2500 nm, 2000 nm, 1000 nm, 800 nm, 600 nm, 500 nm, 350 nm, 300 nm, 250 nm, 200 nm, 100 nm, 75 nm, 50 nm, 20 nm, 10 nm, 5 nm. In particular, the average pore size is about 1500 nm. According to a particular aspect, the at least one porous graphene film may comprise an ordered array of pores. An ordered array of pores is an array of pores having a systematic arrangement. For example, the pore array may be such that there are a predetermined number of rows and columns of pores, each row and column having a predetermined number of pores. The pores in each row and/or column may be the same or different. An ordered array of pores may also be taken to comprise pores arranged in a non-random manner. For example, each pore may be spaced equidistant from one another.

According to a particular aspect, the at least one porous graphene film may comprise a non-ordered array of pores. A non-ordered array of pores is to be understood to refer to an array of pores having a random arrangement of pores. For example, the distance between adjacent pores may differ from pore to pore.

The at least one porous graphene film comprised in the electrode may be prepared according to any suitable method. For example, the at least one porous graphene film may be prepared according to the method as described above.

The electrode may be used in any suitable application. For example, the electrode may be used as an electrode for a battery.

Having now generally described the invention, the same will be more readily understood through reference to the following examples which are provided by way of illustration, and are not intended to be limiting.

EXAMPLES

Materials, methods and results

(a) Preparation of graphene oxide (GO)

Graphene oxide (GO) was prepared from natural graphite powder (Sigma-Aldrich, 45 pm) via a modified Hummers method. 0.3 g of the graphite powder was added to a 80°C solution of concentrated H ₂ S0 ₄ (2.4 ml.) (VWR), K ₂ S ₂ 0 ₈ (0.5 g) (Sigma-Aldrich), and P ₂ 0 ₅ (0.5 g) (Scharlau). The mixture was kept at 80°C for 4 hours and then cooled to room temperature. The mixture was then diluted with 0.5 L deionized (Dl) water, and left to stand overnight. The mixture was filtered, washed with Dl water and dried. The pre-oxidized graphite was further oxidized in concentrated H ₂ S0 ₄ (12 mL) and KMn0 (1.5 g) (Sigma-Aldrich) for 2 hours under stirring and cooling conditions. After adding the KMn0 ₄ , the mixture was stirred at 35°C for 2 hours. The mixture was then diluted with Dl water (25 mL) in an ice bath to maintain the temperature below 50°C. After 2 hours and further dilution with Dl water (70 mL), 30% H ₂ 0 ₂ (2 mL) (VWR) was then added to the mixture. A brilliant yellow product was formed.

The product was filtered and washed with 1 :10 HCI (1 L) (Merck) aqueous solution and Dl water (1 L) to remove the metal ions and acid. The resulting graphite oxide sheets were dried at 80°C in a vacuum. Purified graphite oxide suspensions were then dispersed in water to form 0.1 wt% dispersion. Exfoliation of graphite oxide to graphene oxide was achieved by ultrasonication (Fisher Scientific, FB15051 ) for two hours. A brown dispersion was obtained.

This brown dispersion was then subjected to 30 minutes of centrifugation at 3000 r.p.m. to remove any unexfoliated graphite oxide (usually present in a very small amount), using a Thermo centrifuge with a rotor radius of 14 cm.

(b) Preparation of graphene oxide/dimethyldioctadecylammonium bromide (GO/DODA) complex

Since the GO prepared in (a) above is heavily oxygenated and water soluble, the GO was transferred to the organic phase from the aqueous phase. A cationic surfactant, dimethyldioctadecylammonium bromide (DODA-Br) was selected to electrostatically adsorb and self-assemble onto the surface of the highly negative charged GO in an alkaline solution to form GO/DODA complex in chloroform. The bromide ion in DODA-Br is displaced by the graphene in the solution, thus forming the complex GO/DODA. The method described in Liang Y et al. (Advanced Materials, 21 :1679- 1683, 2009) was followed in preparing the GO/DODA complex, except that the surfactant used for this method was dimethyldioctadecylammonium bromide (DODA-Br). The utilisation of a cationic surfactant effectively overcomes the incompatibility and aggregation problems associated with GO reducing to reduced graphene oxide. To ensure that a single layer of GO was obtained, the GO solution obtained in (a) was subjected to another 30 minutes of centrifugation at 14000 r.p.m. The top clean solution was obtained and the pH of the obtained solution was adjusted to about 9 using 1 mol/L NaOH (Sigma-Aldrich). In this way, the carboxyl and the phenolic hydroxyl groups became a salt structure.

Chloroform solution containing 1 mg/mL dimethyldioctadecylammonium bromide (DODA Br) was then titrated with the aqueous GO solution of (a). When the colour of the aqueous GO solution changed to light brown (as shown by the change in colour from a dark shade to a lighter shade of the top layer in Figure 3A), the addition of chloroform solution containing DODA Br was stopped. The aim was to make sure that enough, but not excess, DODA Br is added for the phase transfer of the GO from the aqueous phase to the organic phase. The colour change of the "water" layer in Figure 3A showed that the hydrophilic surfaces of GO had been successfully modified by the hydrophobic alkyl chains of surfactants and that GO had successfully transferred to the organic phase.

The organic phase was then separated and washed using Dl water. The graphene oxide-DODA (GO/DODA) complex sample was obtained by evaporating the chloroform to dryness. The sample was further dried under vacuum until the weight of the sample remained constant.

The GO/DODA complex was characterized by UV-visible spectrum to validate that the GO had indeed transferred to the organic phase. Referring to the UV-visible spectrum in Figure 3B, the characteristic peak was present at a wavelength of about 230 nm, corresponding to the π-π* transitions of carbon-carbon bonds, and a shoulder at about 298 nm, due to the η-π* transitions of carboxyl bonds.

For the absorbance of the chloroform at low wavelength, a film of GO/DODA casted from chloroform on quartz was tested. The film of GO/DODA complex exhibited a nearly identical absorption peak when compared to native GO in aqueous solution (Figure 3A). From the UV-visible spectrum, it can be deduced that GO has successfully been transferred to chloroform. The GO/DODA characteristic peak (230 nm) shifts to 264 nm when GO is chemically reduced into rGO/DODA (Figure 3B). The peak shift also implies that the GO sheets in the composition formed conjugated electronic graphene sheets during the reduction.

(c) Preparation of porous graphene oxide film

The breath figure method was used to prepare the porous graphene oxide films. The macroporous thin films were prepared by direct casting 40 pL of GO/DODA complex chloroform solution (1 mg/mL) onto the surface of a glass substrate under a moist airflow to provide humid conditions.

The humid conditions were achieved by bubbling nitrogen gas through a water-filled conical flask. As the nitrogen gas (temperature of about 25°C) percolated through the water, the nitrogen gas became saturated, with water vapour. The nitrogen gas which exited the flask through a glass nozzle was vertically applied above the surface of the substrate onto which the GO/DODA complex chloroform solution was casted. The internal diameter of the glass nozzle used to apply the moist airflow over the surface of the substrate was 0.6 cm, and the distance between the surface of the substrate and the nozzle was about. 1.2 cm. The relative humidity of the moist airflow over the substrate surface was about 85%, which was confirmed by a hygrometer. Following the same process, films on different substrates such as silicon wafer and copper foil were obtained.

A control experiment with an airflow having about 30% relative humidity was also performed in the desiccators. As a result, no macroporous film was obtained, leaving only a flat film without any pores. As no honeycomb morphology was observed, this confirms that the macroporous structure is templated by the water droplets condensed on the surface of the evaporating solution. It should be noted that the variation in humidity does not affect the macroporous structure of the film once assembled.

The films prepared covered an area of about 2 cm ² after the complete evaporation of the chloroform and water within 30-60 seconds. There is no limitation for the size of the porous honeycomb film, which only depends on the volume of the GO/DODA complex chloroform solution. For example, a film in the cm scale was obtained, as shown in Figure 4a, while Figure 4b shows a film on a copper foil substrate. The films obtained exhibit bright iridescent colours when viewed with reflected light, indicating a periodic refractive index of variation throughout the thickness of the film. The top view of the SEM image reveals that ordered honeycomb structures formed from the GO/DODA complex (Figure 4c). The structure of the walls can be clearly seen from the high-magnified SEM image (Figure 4d). According to the section image (Figure 4e), the thickness is about 2 pm (for 1 mg/mL of GO/DODA complex chloroform solution).

(d) Reduction of porous graphene oxide film

The porous graphene honeycomb films were reduced using hydrazine vapour (N ₂ H ₄ ). In order to keep the macroporous structure of the film, hydrazine vapour was used to reduce the film by placing the films in an autoclave and heated at 90°C for 16 hours together with 10 μΙ_ hydrazine monohydrate (Alfa Aesar). The hydrazine vapour reduced the GO/DODA to reduced GO/DODA (rGO/DODABr) to obtain porous graphene films from porous graphene oxide films.

The film was then cooled to room temperature. Subsequently, the film was heated to 120°C for 4 hours to remove the excess hydrazine.

After the reduction, the film could not dissolve in any organic solvent and water. The heating of the film also thermally stabilised the film. The film remained stable even after being heated at 400°C for one hour. The contact angle of the film did not change much before and after reduction (from 89° before reduction to 99° after reduction), showing that the surface property of the honeycomb film did not change as a result of the reduction.

Raman spectra with characteristic G and D bands sensitive to defects, disorder, and carbon grain size have extensively been used to characterize carbon materials. The G band arises from the zone centre E _2g mode, corresponding to ordered sp ² bonded carbon, whereas the D band is ascribed to edges, other defects, and disordered carbon. The /c / _G intensity ratio is a measure of disorder degree and average size of the sp ² domains. In the Raman spectrum (Figure 4f) of GO/DODA, the G band is 1600 cm ^"1 . In addition, the D band is at 1352 cm ^"1 , indicating the reduction in size of the in-plane sp ² domains, due to the extensive oxidation. The induced surfactant makes the graphite oxide insularly in the cubic material. The Raman spectrum of the rGO/DODA also contains both G and D bands (at 1597 and 1329 cm ^'1 , respectively). The D band becomes prominent, indicating that disordered carbon, or the edge defects, or other defects in the material may be present. However, the ID/I _G intensity ratio increased compared to that in GO/DODA film. The change indicates that the graphene oxide reduced to graphene in the porous graphene honeycomb film.

Figure 4g shows the FTIR spectra of GO/DODA and rGO/DODA. The FTIR spectra of GO/DODA shows a strong absorption band at 1721 cm ^"1 due to the C=0 stretching. It also shows aromatic C=C (1628 cm ^"1 ), carboxy C-0 (1418 cm ^"1 ), epoxy C-0 (1237 cm ^"1 ), and alkoxy C-0 (1070 cm ^"1 ) groups situated at the edges of the GO nanosheets. It also shows bands CH ₂ symmetry stretches (2918 cm ^"1 ) and CH ₂ antisymmetry stretches (2850 cm ^"1 ), CH ₂ -N ⁺ scissoring (1485 cm ^"1 ) and CH ₃ scissoring (1377cm ¹ ). These peaks also imply that the surfactant had successfully encapsulated the GO.

After reduction, peaks for oxygen functional groups were significantly reduced and perhaps entirely eliminated, and a broad peak at 1564 cm ^'1 was found, which is attributed to the skeletal vibration of the graphene sheets. The peak of alkyl chain still appears at 2918 and 2850 cm ^"1 , which implies that the organic molecule was still in the reduced composition to prevent the graphene from aggregating.

Figure 4h shows the l-V curve of the reduced honeycomb film. The current versus voltage curve was linear, with a 12 kO resistance (resistivity of 0.216 Qm). From the Raman spectra, it is deduced that the reduced graphene sheet in the honeycomb film is disordered, but it still gave a smaller resistance compared to GO/DODABr. The good conductivity of honeycomb film would ensure efficient electron transport for the charge and discharge curves.

Application of prepared porous graphene film The prepared porous graphene film was evaluated to test its abaility for use as an electrode for battery application.

The porous graphene film used for electrochemical measurements was formed directly on a copper foil substrate. Electrochemical evaluation was done using 2032 coin cells hardware. Half cells were then assembled in an Ar-filled dry box (<1 ppm H ₂ 0/0 ₂ ) using Li metal foils as counter electrodes and Celgard Septum separators saturated with 1 M LiPF ₆ in 1 :1 ethylene carbonate/dimethyl carbonate (EC/DMC, Ferro Corp). Several coin cells containing each sample were assembled and tested to ensure reproducibility. The charge/discharge tests were performed using a Battery Analyzer (MTI) at a current rate of 50 mA/g and with a voltage window of 0.01-3.0 V. Cyclic voltammetry (CV) curves were measured at 0.1 mV/s within the range of 0.01-3.0 V using an electrochemistry workstation (CHI, 660D). The capability of the obtained rGO/DODA honeycomb structures for lithium ion storage was also tested.

The high surface area, disordered graphene sheets, and numerous mesopores are favourable for the accessibility of the electrolyte, rapid diffusion of lithium ions, and host uptake. Furthermore, the graphene layers within the mesostructures facilitate the fast transport of electrons during the charge— discharge processes owing to the high electrical conductivity of graphene. In addition, the disordered dispersed graphene assists the diffusion of lithium ions, which is similar to the disordered carbons (in graphite) showing higher specific capacity than ordered graphitic carbons. The first 20 cycles galvanostatic charge/discharge curves of the rGO/DODA honeycomb films at a constant current of 50 mA/g with a potential window from 0.01 V to 3 V (versus Li ⁺ /Li) are shown in the Figure 5a. The first discharge process showed an enormous specific capacity (C1 ) of about 3025 mAh/g. However, the reversible capacity (C2) in the first process gave only about 1612 mAh/g. This large loss of capacity was about 1400 mAh/g and the ratio of C2/C1 was about 53%. The reversible capacity of the rGO/DODA honeycomb film was about 1313-1612 mAh/g in 25 cycles.

In the second cycle, the ratio of the C2/C1 increased to 93%, and in the subsequent fifth cycle the ratio was 96%. Although some initial specific energy capacity was lost over the first few cycles, it still reached a reversible high capacity plateau near 1149 mAh/g after 50 cycles (Figure 5b).

The cyclic voltammograms (CV) of the rGO/DODA electrode are illustrated in Figure 5d. Two cathodic (reduction) peaks in the potential range of 0.9-1.7 V and 0.3-0.6 V were observed in the first cycle, and these peaks disappeared during the second cycle. These peaks were associated with the solid electrolyte interface (SEI) film formation (0.9-1.7 V) and co-intercalation of solvated Li ⁺ (0.3-0.6 V), respectively. Evidently, the peak relating to co-intercalation of solvated Li ⁺ was smaller than the peak due to SEI film formation, which implies that SEI film formation contributed mostly to the irreversible capacity loss of rGO/DODA electrode. The two peaks disappeared/moved at the second cycles.

During the second cycles to the tenth cycles, there was no change of these cycles, implying the electrodes were stable during the charging/discharging cycles.

As a controlled experiment, a smooth rGO/DODA film without pores was formed to test the capability of lithium storage. The film exhibited a high capacity (1957 mAh/g) in the first discharge process (see Figure 5c). The reversible capacity of the smooth non- porous film is about 1076 mAh/g. The ratio of C2/C1 was about 55%, similar to that of the honeycomb porous film. After the first discharge, the charge-discharge cycle became stable. The capacity of the smooth film was 678 mAh/g after 50 cycles. The reversible retention capacity after 50 cycles was 71% and 63% for the honeycomb film and smooth film battery devices, respectively. The data shows that the fading capacity of the smooth film battery device is faster than that of the honeycomb film.

From the Raman spectrum, the reduced graphene sheets were disordered in the honeycomb structure, so that it was easy for Li ions to be electrochemically adsorbed on both sides of single-layer sheets that were arranged like "falling cards" (Figure 5e). The diffusion distance of the lithium ion into the host position will affect the capacity of the lithium battery.

In this example, a surfactant was used to encapsulate each graphene sheet. This helps increase the distance between two graphene sheets, and also helps to form the macroporous structure, which gives a bigger surface area. The longer distance between the graphene layers and the bigger surface area makes it easier for the lithium ions to reach the position. In addition, the honeycomb structures provide extra space for the electrolyte accommodation, and these properties facilitate electrolyte contact and electric and ionic diffusion/transfer and thereby result in a large capacity and favourable rate performance. Furthermore, the interfacial storage plays an important role to reach such high performance. The "house of cards" model suggests that the disordered carbon consists of very large number of single graphene sheets and the lithium may be adsorbed on both sides of the single-layer sheets. Finally, 3D cavities and nanopores in the films also contribute to the lithium storage, where lithium is intercalated in graphene layers and lithium stored in cavities (Figure 5e).

Honeycomb structures composed of graphene were prepared through controlling chemistries and assembly processes. The obtained rGO honeycomb structured films may show high conductivity, high porosity, and robust chemical and mechanical stability, which may lead to high performance energy-storage device.

The honeycomb graphene electrode may exhibit a large reversible capacity (1149 mAh g ^" after 50 cycles), excellent cyclic performance, and high Coulombic efficiency (above 97%), highlighting the advantages of mesostructures built from graphene sheets for the maximum utilization of electrochemically active carbon materials for energy storage applications in high-performance lithium ion batteries.

Porosity is important for improving the capacity and cycling performance of disordered carbon anode materials, since the porous framework may significantly decrease the diffusion distance of lithium ion into the disordered graphene layers, which may enhance the charge/discharge rate performance of rechargeable lithium batteries.

The fabrication process to fabricate such hierarchical structures is relatively simple, low-cost, green and scalable, which may make it advantageous to use these materials in electrode materials with enhanced performance and in the large-scale manufacturing of nanostructure materials and for their use in practical battery systems. Whilst the foregoing description has described exemplary embodiments, it will be understood by those skilled in the technology concerned that many variations in details of design, construction and/or operation may be made without departing from the present invention.