WO2013040636A1

WO2013040636A1 - Reduced graphene oxide and method of producing same

Info

Publication number: WO2013040636A1
Application number: PCT/AU2012/001124
Authority: WO
Inventors: Jun Chen; Roderick Leonard SHEPHERD; Joselito Macabuhay RAZAL; Andrew Ian Minett
Original assignee: University Of Wollongong
Priority date: 2011-09-19
Filing date: 2012-09-19
Publication date: 2013-03-28
Also published as: CN104039695A; CN104039695B; KR20140093930A

Abstract

There is provided a method of producing reduced (rGO) or partially reduced (prGO) Graphene Oxide films that have improved conductivity. The method is capable of creating highly conducting (-2000 S/cm), flexible, printable, processable reduced graphene oxide materials without the need for harsh chemical treatment or high temperature annealing. In one embodiment, an electrical circuit consisting of reduced graphene oxide tracks may be patterned into a graphene oxide film by printing with a reducing agent, preferably ascorbic acid.

Description

REDUCED GRAPHENE OXIDE AND METHOD OF PRODUCING SAME Field

The present invention relates to a method for producing reduced and/or partially reduced graphene oxide.

Background

Graphene materials offer the highest known conductivity rates for single-layer systems. As such, Graphene and Graphene-like materials are expected to be the backbone of next generation electronic devices.

Graphene is typically produced by mechanical exfoliation or a chemical vapour deposition. (CVD) approach. Although these approaches are capable of producing highly crystalline graphene with high conductivity, they are typically not scalable or cost effective for mass production.

An alternative approach is the chemical reduction of graphene oxide (GO), which is easily produced from graphite. However, whilst it is possible to produce graphene oxide materials relatively easily, the subsequent reduction process to a graphene-like material has so far only resulted in poor conductivities (~10-100S/cm), far from that of pure Graphene (>3000 S/cm). The reduced product is generally referred to as Reduced Graphene Oxide (rGO or reGO). Reduced Graphene Oxide (rGO) and Partially Reduced Graphene Oxide (prGO) produced using existing methods resemble graphene in their lattice structure but contain residual oxygen and structural defects. These defects significantly lower reported conductivities compared to that of graphene. However conductivities are comparable to those of doped conductive polymers and silicon. Post-treating or high temperature annealing can result in conductivities as high as 500-700 S/cm, but such treatment conditions are not feasible in the semi-conductor industry and are not commercially viable on a large scale. To date, various chemical reduction methodologies have been reported in the literature to produce rGO from bulk GO material. For example, chemically derived graphenes have been produced by dipping GO flakes in hydrazine, exposure to hydrazine vapour, electrochemical reduction, thermal annealing, and combinations of these techniques. The most commonly utilised chemical route is via the use of hydrazine, a harsh toxic reductant, and subsequent high temperature annealing. However, the rGO films produced by these methods have been limited in their development and have to date resulted in rGO with low conductivity and high internal resistivity.

A further approach is the tuneable reduction of GO via resistive heating of an AFM cantilever. However, this method is not suitable for large-scale commercial production techniques.

Graphene Oxide (GO) based materials have potential for a wide range of applications, including memory devices, sensing platforms, cell culture, electrochemical energy devices, highly sensitive gas sensors and mechanical resonators with figures of merit surpassing those of graphene resonators. With a room-temperature band-gap larger than 0.5eV, GO possesses both localised semi-conductor and semi-metal characteristics as it is reduced towards a graphene-like material. The present invention seeks to provide a simple scalable and ecologically sustainable chemical process to create highly conducting (-2000 S/cm), flexible, printable, processable reduced graphene oxide materials without the need for harsh chemical treatment or high temperature annealing.

Summary of the Invention In one broad form, the present invention provides a method of producing reduced or partially reduced graphene oxide, the method including the steps of: (a) providing graphene oxide; and,

(b) applying a reducing agent to at least part of the graphene oxide.

In a further form, step (a) includes providing a substrate including graphene oxide on a surface thereof.

In a further form, the substrate in step (a) includes a layer of graphene oxide on the surface.

In a further form, a layer of reduced or partially reduced graphene oxide is produced on the surface.

In a further form, the reducing agent is selectively applied to the layer of graphene oxide such that the layer of reduced or partially reduced graphene oxide forms a pattern in the graphene oxide layer.

In one form, step (a) includes coating or depositing a graphene oxide dispersion onto the surface of the substrate.

In one form, the dispersion of graphene oxide comprises a dispersion of graphene oxide in water.

In another form, the reducing agent at step (b) is applied as part of a solution.

In a further form, step (b) includes immersing or dip coating the substrate surface including the graphene oxide in the solution including the reducing agent. In one form, step (b) includes printing the solution including the reducing agent on to the substrate including the graphene oxide.

In one form, printing includes gravure, roll-to-roll, reel-to-reel, inkjet or flexographic printing.

In a further form, the solution including the reducing agent is an aqueous solution. In one form, the applied reducing agent is ascorbic acid.

In another form, the conductivity of the reduced or partially reduced graphene oxide is between about 0 to about 5000 S/cm.

In a further form, the substrate in step (a) includes a layer of graphene oxide composite material on the surface thereof.

In a further form, the present invention provides a substrate including reduced or partially reduced graphene oxide on a surface thereof produced in accordance with any of the above forms.

In one form, the reduced or partially reduced graphene oxide is in the form of a circuit pattern.

In one form, the present invention provides a device including reduced or partially reduced graphene oxide produced in accordance with any one of the above forms.

In one form, the present invention provides, a substrate having a layer of reduced or partially reduced graphene oxide on a surface thereof, the layer having a thickness of less than 2um and a conductivity of greater than 1 OOOS/cm. In one form, the present invention provides use of reduced or partially reduced graphene oxide produced in accordance with the method of any one of above forms as a conducting or partially conducting coating, an anti-microbial coating, an anti-corrosion coating, or an anti-static coating.

In one form, the present invention provides use of reduced or partially reduced graphene oxide produced in accordance with the method of any one of the above forms as a conductor for the electrical conduction of electricity.

Brief Description of the Drawings

The present invention will become better understood from the following detailed description of various non-limiting embodiments thereof, described in connection with the accompanying figures, wherein:

Figure 1 shows a rGO tracks produced in a GO film using a conventional ink jet printer;

Figure 2 shows rGO layers of different thickness;

Figure 3a is X-Ray Diffraction spectra of the native GO, reGOl, 2, and 3;

Figure 3b is Raman spectra obtained at 633nm showing the D, G, 2D and G' regions of the spectra (inset - more detailed view of the 2D arid G' regions of the spectra);

Figure 3c is an X-Ray Photoelectron spectra of the C 1 s regions of GO ;

Figure 3d is an X-Ray Photoelectron spectra of the C 1 s regions of reGO 1 ;

Figure 3e is an X-Ray Photoelectron spectra of the Cls regions of reG03;

Figure 4a shows a flexible sandwich-type symmetric capacitor cell assembled using the prepared flexible rGO Polyvinyl diflouride (PVDF)/rGO membrane with same-size (mass weight 0.456 mg each) of rGO films on both sides;

Figure 4b shows the galvanostatic charge-discharge rate of the capacitor in figure 4a; Figure 4c is an SEM image of an rGO film including platinum nanoparticles;

Figure 4d is a linear sweep voltammogram of the rGO film including platinum nanoparticles; Figure 4e shows a partially reduced rGO film on ITO-coated PET sheet/glass;

Figure 4f is differential pulse voltammetry DPV of a partially reduced GO surface to detect deposited Dopamine;

Figure 5 is a Transmission Electron Microscope image of GO before reduction;

Figure 6 is a Scanning Electron Microscope image of GO before reduction;

Figure 7a shows inkjet patterned rGO circuits on GO on glass slide before lift-off;

Figure 7b shows the rGO circuits of figure 7a freestanding after tape lift-off.

Figure 8 shows examples of freestanding circuits and devices created by adhesive tape liftoff of GO/RGO from glass slides;

Figure 9a shows a RGO dipole RFID tag design patterned by inkjet printing on glass with tape applied and before lift-off;

Figure 9b shows RFID tag of figure 9a as a freestanding device after tape lift-off;

Figure 10a shows GO coated on ITO glass with a 10 x 10 mm converted RGO pattern; Figure 10b shows the RGO of figure 10a remaining after GO lifted off by adhesive tape; Figure 11a shows an inkjet patterned RGO circuit in GO on an ITO glass substrate; and Figure 1 lb shows inkjet pattered RGO of figure 1 la after GO removed with adhesive tape and with a surface mounted LED affixed with silver paint.

Detailed Description

The foregoing describes only some embodiments of the present invention, and modifications and/or changes can be made thereto without departing from the scope and spirit of the invention, the embodiments being illustrative and not restrictive.

In the context of this specification, the word "comprising" means "including principally but not necessarily solely" or "having" or "including", and not "consisting only of. Variations of the word "comprising", such as "comprise" and "comprises" have correspondingly varied meanings. According to certain embodiments, there is provided a method of producing reduced (rGO) or partially reduced (prGO) Graphene Oxide films that have improved conductivity when compared to rGO films prepared by other methods.

Furthermore, the present method permits selective reduction of Graphene Oxide (GO) films using an environmentally friendly and easily scalable process.

Rather than chemically reducing GO in solution and subsequently applying the rGO to a surface, the presently described method reduces GO deposited on a substrate. The modification of the processing sequence together with the novel use of specific reduction agents has been shown by the inventors to deliver significant advantages. Advantageously, with the present method, a GO film can be patterned or printed with reducing agent to produce highly conducting rGO tracks within a GO insulating matrix.

Advantages of the method described herein include:

- Measured conductivities approaching that of pure CVD grown Graphene (_^3000S/cm)

- No need for high temperatures or post-treatment procedures. Often the present method can be carried out in room temperature conditions. This would be an important factor in, for example, the electronics industry, as back-end chip technology requires procedure temperatures to be less than 300°C.

- Permits the use of environmentally and operator friendly reducing agents such as Ascorbic acid.

In accordance with certain embodiments, a substrate having graphene oxide deposited thereon is initially prepared. The substrate having graphene oxide may be prepared in a variety of ways. Typically, a slurry style dispersion of GO is first prepared. The dispersion of GO may comprise a dispersion of GO in water, ethanol, an ionic liquid, solvent or other liquid carrier. The liquid carrier may include a glycol ether, such as for example, diethylene glycol monobutyl ether, diethylene glycol n-butyl ether acetate, ethylene glycol n-butyl ether acetate or ethylene glycol monobutyl ether.

The wt% graphene oxide in the graphene oxide dispersion is variable and is typically selected to suit the ultimate application of the GO or subsequently produced rGO material. For example the graphene oxide dispersion may include low (less than 0.5 wt%), medium (between 0.5 wt% and 2.0 wt%) and high (more than 2 wt%) concentrations of graphene oxide. The GO dispersion may further include any one or a combination of the following: biological agents, enzymes, cells, polymers, fibres, ionic liquids, nanomaterials (e.g. nahoparticles, nanotubes,^{^} nanosheets, and nanorods) and/or other active precursors (e.g monomers, AgN0₃, AuCl₃ and tbPtCle, and other metal salts). Once prepared, the slurry style GO dispersion may then be deposited on to the surface of a substrate. A wide range of coating depositing techniques can be used to deposit the GO including knife blading, spraying (for example pressure spraying), inkjet printing, screen printing, spin coating, drop casting, dip coating, brush painting, gravure printing, flexographic printing, blade coating, meter bar coating, slot die coating etc. filtration, Langmuir-Blodgett, electro-spinning, or fibre spinning.

It will be appreciated that the substrate may be rigid, flexible or soft. For example, the substrate may include films, multilayered and 3D structures made from metals, polymers, membranes, glass, silicon, other coatings and even crystals. For example, the substrate may be a glass slide, a solid metal sheet, a metal or metal-based foil such as copper foil or aluminium foil, a plastic sheet such as PET or Au-Mylar, a membrane such as PVDF, paper, a polymer-based substrate, rubber, polymer gels, hydrogels, SPE membranes, crosslinked SPEs, textiles such as cloth, advanced textiles such as evlar or Gortex, or next generation fibres such as graphene oxide fibres. It will be appreciated that the substrate surface may include a GO composite material.

Furthermore the substrate surface may include, as part of a composite GO material or otherwise, any one or a combination of biological agents, enzymes, cells, polymers, fibres, ionic liquids, nanomaterials, nanoparticles, nanotubes, nanosheets, nanorods, active precursors, monomers, AgN0₃, AuCl₃ and H₂PtCl6 and other metal salts.

To obtain the rGO or prGO, a reducing agent is applied to the GO on the substrate as required. The reducing agent is typically part of a solution. The solution including the reducing agent may be an aqueous solution, organic solution, or an ionic liquid. The solution may include a glycol ether, such as for example, diethylene glycol monobutyl ether, diethylene glycol n-butyl ether acetate, ethylene glycol n-butyl ether acetate or ethylene glycol monobutyl ether. The reducing agent may be inorganic or organic. For example the reducing agent may include ascorbic acid, oxalic acid (C2H2O4), formic acid (HCOOH), Sodium borohydride (NaBHLj), lithium aluminium hydride (L1AIH₄), sulfite compounds, phosphites, phosphorous acid, or citric acid. The reducing agent may also be an electroactive polymer. Ascorbic acid is a naturally occurring compound and can be provided as an aqueous solution thus providing an environmentally friendly reducing agent.

The level of GO reduction and thus resultant conductivity of the produced rGO can be manipulated by altering the concentration of the reducing agent, the type of reducing agent, the temperature, and exposure time of GO to the reducing agent. The reducing agent may be selectively applied to provide conducting rGO patterns within an insulating GO matrix.

The reducing agent may be applied using a range of techniques including but not limited to dip coating or printing. Printing includes gravure, roll-to-roll, reel-to-reel, inkjet or flexographic printing. The reducing agent may also be applied using vapour phase patterning. Figure 1 shows the selected area reduction of the GO film using a conventional ink-jet printer loaded with the reducing agent. In this example, feature size is limited by the ink-jet nozzle, however a range of components can be one-step printed by this method. The simple circuit shown in Figure 1 shows rGO tracks with a surface resistivity of 200Ω/Ο patterned into the GO film by ink-jet printing. When using ink jet printing, conductivity of the rGO tracks produced in the GO film is further tuneable by adjusting the droplet size and drying time of the reducing agent.

The pattern of rectangles shown in Figure 2 demonstrate varying resistivity structures produced in accordance with the present method. Different resistivities may be achievable by varying the number of reducing agent printing passes or layers applied. Starting from the left, the rectangles represent resistivities of 120ΚΩ (1 pass), 13ΚΩ (2 passes), 2ΚΩ (5 passes), 600Ω (10 passes), 200 Ω/sq (15 passes) respectively.

The thickness of the layer of reduced or partially reduced graphene oxide is variable and depends on the desired application.

For 3D device applications a sequence of coating layers of GO "papers" could be deposited atop a substrate and thereafter the reducing agent applied. A structural fixing/bonding agent for the not-to-be-reduced GO may also be applied as each layer is deposited. This would permit production of highly conductive 3D rGO devices inside of highly insulating GO matrixes. These matrixes could potentially be machinable to create, for example, metal free solid state air or metal cored coils and toroids for inductive applications (e.g. speaker in all size ranges from earphone to large scale, electromagnets, motors, transformers etc) inside a structurally stable insulating platform. Alternatively rGO "papers" could themselves be deposited layer by layer, or alternatively pre-prepared "papers" of customisable thickness could be used for such a process. In addition, a facile selective lift-off process has been developed allowing for patterned rGO features to be removed from the substrate to form flexible freestanding rGO structures and devices. The process typically requires a piece of adhesive tape, which is placed adhesive side down onto the produced GO/rGO surface. The tape is then carefully lifted from the supporting substrate. The surface energy of the substrate determines the extent of rGO removal during lift-off. It is also possible to selectively remove the unconverted GO regions and leave the rGO pattern on the substrate surface.

Coated GO is relatively hydrophilic whereas rGO is more hydrophobic. Hence when rGO converted patterns are on a hydrophobic surface,, adhesion is sufficient to resist tape liftoff. On a less hydrophobic substrate, such as glass, rGO will be removed along with the surrounding GO regions. In this case a second piece of adhesive tape may be used to laminate the lifted piece protecting the rGO pattern and forming a flexible freestanding pattern.

Examples of lift-off from glass slides to produce freestanding devices are shown in figures 7 to 9. Selective removal of the unconverted GO material from more hydrophobic ITO coated glass is shown in figures 10 and 11.

For the example of figure 11, ITO coated glass with a rGO pattern produced thereon was scored to create two discrete contacts. An LED was affixed to the rGO of each discrete contact. Contact to the LED is made through the rGO contact pads from the underlying ITO conduction electrodes.

It should also be appreciated that the reducing agent may also impart other properties to the produced rGO layer. The reducing agent may act as both reducing agent and component of a composite rGO material post-reduction.

For example, an acidic electroactive conducting polymer may be used as the reducing agent. This polymer, being acidic, reduces the GO. In addition to reduction, the polymer provides the rGO with electroactive properties (e.g. may be reversibly switched between conducting and insulating states). The RFID tag of figure 9 was prepared by printing a water soluble electroactive conducting polymer.

The present method may therefore provide a 1-step method for fabricating rGO/composite devices by printing a functional reducing agent, rather than having to first form an rGO pattern and then overprint a functional material to complete the device.

Furthermore such a 1-step approach may give rise to a 3D network of the two materials whereas a multiple step approach may give rise to a film of polymer on the surface of an rGO pattern.

As described, the inventors have demonstrated a tuneable and scalable environmentally sustainable chemical method of preparing highly conducting (~2000S/cm) reduced graphene oxide architectures, without the need for harsh chemical post-treatment or annealing.

With the present methods it is also possible to produce patterns of varying resistivities on pre-formed graphene oxide architectures in ^' a manner that is scalable. By varying parameters such as the reductant concentration, the reduction agent or temperature, one can control the resulting resistivity of the rGO produced. The present method can obtain sheet resistivities as low as 2 Ohm/sq. This equates to conductivities approaching those of CVD grown graphene. Furthermore, the reducing agents applied in the method can be ink-jet printed, allowing one-step procedures for creating electronic circuit structures.

These results demonstrate unprecedented control over the properties of reduced graphene oxides that result in graphene-like conductivities. This control allows almost unlimited processing options that are applicable to a range of printing processes, such as ink-jet, roll- to-roll and reel-to-reel technologies. The method described herein provides opportunity to produce an array of next generation reduced graphene oxide applications and devices. For example, the present method may be . used in the production of electrodes, sensors, bionic devices, energy conversion devices, fuel cells, solar cells, water splitting cells, water cleaning cells, energy storage devices, rechargeable batteries, super capacitors, hybrid cells, electronic devices, display devices, electrochromic cells, communication/asset tracking devices, passive or active RFID tags, electrochemical devices and/or wearable devices.

Other applications may include:

- Sensors including variable conductivity/resistivity elements combined with

functionality.

- Biocompatible materials for biomimetic application.

- Electronics including for example printed circuitry, logic devices, components for anti-static devices and RFID tags.

- A variety of functional coatings and composites for example antifouling coatings for ships and marine structures or Anti-radar coating applications.

- Metal free inductive devices and other solid state 3D conductive devices.

Furthermore the rGO or prGO films may be used to provide: a conducting or partially conducting full coating or partial coating for metallic objects such as sheet steel and zinc alum;, an anti-microbial coating such as, for example, for freight liners and cruise ships; an anti-corrosion coatings such as, for example, for zinc alum fencing and roofing; or an antistatic film such as, for example, for computer casings, storage containers, and electrical isolation.

Examples

The present invention will become better understood from the following examples of preferred but non-limiting embodiments thereof. Example 1

Preparation of the Slurry Style Dispersion of Graphene Oxide The initial GO material was prepared by Modifed Hummers method, via the reaction of graphite with a mixture of potassium permanganate and concentrated sulphuric acid. See figures 5 and 6 for TEM and SEM images respectively of this starting GO material.

Dispersions of this GO material (0.5%) were then made by dispersing the GO flakes in pure Milli-Q water without any surfactant by low medium power sonication for 60 min. A slurry-style dispersion of GO (1 to 2%) was made by partially evaporating the Milli-Q water under vigorous stirring.

Example 2

GO (graphene oxide), reGOl and reG02 and reG03 ('fully' reduced GO) where prepared and characterised as follows:

Graphene oxide (GO) powder was prepared using modified Hummer's method, and dispersed in pure Milli-Q water (1 to 5 mg/mL, without any surfactant) in a 50mL glass bottle via ultrasonication (using flat head tip, 400W at 40% Amplitude) for 60 mins. The dispersion was then directly transferred onto a hot-plate (Crown Scientific, Ltd.) at 60°C and allowed continuous strong stirring until it changed to slurry-style dispersion (with volume change from 50 mL down close to 10 mL). The slurry-style dispersion was then used for preparing GO films. Thin GO films (brown colour) were coated onto different substrates (both rigid and flexible) using prepared slurry-style dispersion by knife-blading and/or vacuum spray methods, followed by immediately drying in a vacuum oven at 150°C in order to evaporate the water trapped in the wet GO films. As-prepared GO film coated substrates were immersed into ascorbic acid (reducing agent) solution with different concentrations (reGOl with 0.1M ascorbic acid, reG02 with 0.2M ascorbic acid and reG03 with 0.5M ascorbic acid) in order to obtain the reduced GO films with different electronic conductivities (related to different reduced levels). Then the reduced graphene oxide (reGO) films (black colour) were washed with both Milli-Q water and ethanol in order to remove the remaining salt (due to absorption) followed by a vacuum oven dry at 120°C for 30 mins.

This method can also be easily used to apply other reducing agents besides ascorbic acid by varying the reducing temperature. For example, reduced GO films with similar properties were also obtained by using NaBH4 as the reducing agent at lower temperature.

The graphene oxide reduction process was monitored and characterised by X-ray diffraction, Raman spectroscopy, and XPS. Figures 3(a) to (e) show the correlation among these reduced states in the Raman, the XRD and the XPS spectra. The XRD and the Raman spectra reflect the differing degrees of reduction of the GO films versus different concentration of Ascorbic acid solutions (0.1M to 0.4M) and scale appropriately (See Figure 3(a) and (b)). For the initial GO film, the main peak in the XRD appears at 2Θ angles of 11° corresponding to literature reports for the main peak in GO relating to both the presence of high-level oxygen-containing functional groups attached on both sides of the graphene sheet and the atomic scale roughness arising from structural defects generated on the original atomically flat graphene sheet. Upon reduction the XRD spectra changes considerably.

For the reGOl film, the 2Θ peak at 11° has shifted 1.5° upfield to 12.5°, whilst the 002 peak at 27° has sharpened with the increase in intensity, which is related to the pristine graphene sheet.

Further reduction to reG02 sees the 11° peak shift further upfield to 17° and the shoulder region in the 20-25° region has intensified. The reG03 film shows the disappearance of the main GO peak at 11° and the appearance of two peaks commensurate with reduced GO in the literature. That is a broad shoulder peak region centred at 22° and the intensity peak at 27.2°. The remaining broad shoulder- style peak (from 18° to 25°) indicates that the reGO film has the relatively disordered layered structure via retaining the corrugated structure, which is consistent with the results from HR SEM images. After reduction, it discerns a gradual change in the XRD patterns to finally achieved a randomly ordered layered reGO solid films.

These incremental changes are also reflected in the Raman spectra of the reGO films by the variation of the relative intensity of G and 2D band. Fig. 3(b) shows that GO and rGO sheets both contain three characteristic peaks at 1327, 1585 and 2628 cm-1, which are assigned to D band, G band and 2D band, respectively. Upon GO film, the 2D band is very weak small, while the Raman spectrum of reGO shows the peak intensity of 2D band (the insert in Fig.2b) increases clearly with the increase of reduction degree. Further changes in reduction also result in a further increase in sp2:sp3 ratio. It is noteworthy that reGO films present an obvious 2D band, indicating an increase in the average size of sp2 domains on reGO. These results are coincidence with the literature report for the thermal reduction of GO with hydrogen. The increases in the G' peak intensities further highlight a change to a more crystalline nature of the reGO and reduction in oxygenated species.

These significant structural changes and reduction in oxygenated species are confirmed in the XPS spectra of the GO and reGO films. Figure 3(c) to (e) show the Cls region of the XPS derived spectra for the GO, reGOl and reG03 samples. The GO Cls region of the spectrum clearly indicates a considerable degree of oxidation, with two distinct components and one minor component that can be associated with different functional groups on the C atom. These being non-oxygenated C from the hexagonal aromatic ring structure at 285eV, the C-0 bond at 287, carbonyl C=0 bonds at 288 and the carboxylate bound 0=C-0 groups at 289 and the π-π* satellite peak (290.6 eV) These assignments are in good agreement with the literature. In the corresponding reGOl and reG03 spectra, the spectral differences reflect the differing amounts of chemical reduction. In the reGOl spectra, there is a significant decrease in the Cls B peak C-0 assignment compared to the C-C peak. This decrease in intensity suggests considerable de- oxygenation via the acetic acid reduction process. Further reduction, as seen in the reG03 spectra show significant removal of the oxygenated species C-O, C=0 in the film. The minor peaks observable in graphene spectra in the literature become observable in this spectra (the CO(O) and C=0=0 peaks).

Example 3

rGO capacitor

Figure 4a shows a flexible sandwich-type symmetric capacitor cell assembled using a prepared flexible rGO Polyvinyl diflouride (PVDF)/rGO membrane with same-size (mass weight 0.456 mg each) of rGO films on both sides. The rGO/PVDF/rGO membrane was sandwiched with two Indium Tin Oxide (ITO) coated PET sheets (30Ω/ο) filled with degassed aqueous electrolyte (1.0M H2S04). This kind of flexible capacitor was preliminarily evaluated by a constant galvanostatic charge-discharge rate of 0.2A displayed in Figure 4b. It gives a stable specific capacitance of 160.2 F/g, with a high energy density of 26.1Wh kg.

Example 4

rGO composite including Platinum Nanoparticles To demonstrate the designed novel reducing protocol applicable for GO and/or rGO composites, platinum nanoparticles (PtNPs) were decorated onto the rGO film via reducing Pt⁴⁺ to Pt° by immersing the prepared Pt⁴⁺incorporated GO composite film coated glassy carbon (GC) into ascorbic acid. The SEM image (after ORE test, Figure 4c) and XRD pattern reveal that PtNPs are successfully loaded onto the reGO films during the reduction process. The as-prepared PtNPs/reGO coated GC was further preliminarily investigated for the electrocatalytic oxygen reduction reaction as shown in Figure 4(d). The linear sweep voltammograms of the PtNPs/reGO coated GC electrode indicated a substantial catalytic oxygen reduction with an onset potential of ca. +0.6 IV (vs. Ag/AgCl) in an oxygen saturated 0.5M H2S04/H20 solution. While there is no reduction response observed in a nitrogen atmosphere under identical testing conditions. This suggests that the developed rGO reduction protocol could be used to prepared reGO nanocomposites with the potentials for extending the applications into energy conversion and related areas.

Example 5

Investigation of bio-catalytic activity of rGO film

It is believed that the bio-catalytic activity of graphitic carbon electrodes mainly comes from functional groups - defect sites. Therefore, the partially reduced reGO film (on ITO- coated PET sheet/glass, Figure 4e) was investigated for the detection of dopamine (DA) by differential pulse voltammetry (DPV) in PBS buffer solution (pH=7.0). The DPV curves in Figure 4f show a clear oxidation peak of DA, increased with the increase of the concentration of DA (from 5μΜ to 100 μΜ). This suggests that the partially reduced reGO thin films could be promising electrode materials for DA detection via controlling of reduced-level of reGO film in order to not only manage/maintain the certain functional degree for bio-catalytic performance but also achieve a reasonable conductivity for charge transfer.

Claims

Claims:

1. A method of producing reduced or partially reduced graphene oxide, the method including the steps of:

(a) providing graphene oxide; and

(b) applying a reducing agent to at least part of the graphene oxide.

2. A method as claimed in claim 1 wherein step (a) includes providing a substrate including graphene oxide on a surface thereof.

3. A method as claimed in claim 2 wherein the substrate in step (a) includes a layer of graphene oxide on the surface.

4. A method as claimed in claim 3 wherein a layer of reduced or partially reduced graphene oxide is produced on the surface.

5. A method as claimed in claim 4 wherein the reducing agent is selectively applied to the layer of graphene oxide such that the layer of reduced or partially reduced graphene oxide forms a pattern in the graphene oxide layer.

6. A method as claimed in any one of claims 2 to 5 wherein step (a) includes coating or depositing a graphene oxide dispersion onto the surface of the substrate.

7. A method as claimed in claim 6 wherein the dispersion of graphene oxide comprises a dispersion of graphene oxide in water.

8. A method as claimed in any one of claims 2 to 7 wherein the reducing agent at step (b) is applied as part of a solution.

9. A method as claimed in claim 8 wherein step (b) includes immersing or dip coating the substrate surface including the graphene oxide in the solution including the reducing agent.

10. A method as claimed in claim 8 wherein step (b) includes printing the solution including the reducing agent on to the substrate including the graphene oxide.

11. A method as claimed in claim 10 wherein printing includes gravure, roll-to-roll, reel-to-reel, inkjet or flexographic printing.

12. A method as claimed in any one of claims 8 to 11 wherein the solution including the reducing agent is an aqueous solution.

13. A method as claimed in any one of the preceding claims wherein the applied reducing agent is ascorbic acid.

14. A method as claimed in any one of the preceding claims wherein the conductivity of the reduced or partially reduced graphene oxide is between about 0 to about 5000 S/cm.

15. A method as claimed in any one of claims 2 to 14 wherein the substrate in step (a) includes a layer of graphene oxide composite material on the surface thereof.

16. A substrate including reduced or partially reduced graphene oxide on a surface thereof produced in accordance with any one of the preceding claims.

17. A substrate as claimed in claim 16 wherein the reduced or partially reduced graphene oxide is in the form of a circuit pattern.

A .device including reduced or partially reduced graphene oxide produced in accordance with any one claims 1 to 15.

A substrate having a layer of reduced or partially reduced graphene oxide on a surface thereof, the layer having a thickness of less than 2um and a conductivity of greater than lOOOS/cm.

Use of reduced or partially reduced graphene oxide produced in accordance with the method of any one of claims 1 to 15 as a conducting or partially conducting coating, an anti-microbial coating, an anti-corrosion coating, or an anti-static coating.

21. Use of reduced or partially reduced graphene oxide produced in accordance with the method of any one of claims 1 to 15 as a conductor for the electrical conduction of electricity.