WO2020260710A1

WO2020260710A1 - Method for producing porous graphene or porous graphtic carbon

Info

Publication number: WO2020260710A1
Application number: PCT/EP2020/068292
Authority: WO
Inventors: Michael Burke; Aidan Quinn; Daniela Iacopino; Mary Manning
Original assignee: University College Cork, National University Of Ireland, Cork
Priority date: 2019-06-27
Filing date: 2020-06-29
Publication date: 2020-12-30
Also published as: GB201909271D0

Abstract

A method is provided for producing a porous graphene or porous graphitic carbon material. The method comprised creating a biopolymer-based feedstock material from a biopolymer formulation (8). The bio polymer is derived from a synthetic or naturally occurring mucopolysaccharide or other heteropolysaccharide. The method comprises irradiating the biopolymer-based feedstock material with coherent light (12) to produce porous graphene or porous graphitic carbon.

Description

METHOD FOR PRODUCING POROUS GRAPHENE OR POROUS GRAPHTIC CARBON

Field of Invention

The current invention relates to a method for producing porous graphene or porous graphitic carbon from a biopolymer derived from a mucopolysaccharide (also known as a glycosaminoglycan) or another heteropolysaccharide. In particular, but not exclusively, graphene is produced by laser irradiation of the biopolymers.

Background of the Invention

Graphene is a two-dimensional (2D) array of densely packed sp²-hydridised carbon atoms in a honeycomb pattern. Graphene has attracted immense attention resulting from its unique chemical and physical properties. Since 2004, when single layers of graphene were first isolated by mechanical exfoliation of graphite, various other methods have been developed such as chemical vapour deposition (CVD) and epitaxial growth on various substrates.

More recently, 3-dimensional (3D), porous graphene structures, comprising non-planar graphene structures with pentagonal-heptagonal defects allowing formation of high surface area, micro- and nano- porous graphene (some with nanometre-scale pore dimensions), have attracted strong attention.¹¹¹

Such structures exhibit unique properties which are distinct from pristine single layer graphene, potentially making them attractive in a wide variety of applications such as energy storage applications, sensors, filters and gas purification.

Synthesis of porous graphene structures traditionally either involved multi-step chemical synthesis routes, which can be time consuming and/or expensive, or high temperature processing, limiting their commercial viability.

In this regard, patterning of graphene-based nanomaterials on substrates seems to offer a promising route towards commercialisation, as it is proven to be scalable and cost-effective. For example, the production of laser-induced graphene has been demonstrated using commercially available polyimide and poly-etherimide irradiated by a C0₂ laser (at a wavelength of 10.6 pm).¹¹¹ However, it has been shown from molecular dynamics simulations that graphene formation from polyimide without metal catalysts requires both high temperature and high pressure, as high temperature alone (e.g. through furnace pyrolysis of polyimide) is not sufficient to produce graphene.¹²¹

Nevertheless, it has been subsequently shown that laser-induced graphene can be produced from wood under a controlled atmosphere,¹³¹ and from lignin-containing, cellulose-based biomaterials (wood, potato skins, coconut shells) by multiple lasing using a C0₂ laser under ambient conditions.¹⁴¹ The proposed mechanism involves the conversion of a carbon precursor to amorphous carbon (first lasing step) followed by conversion to graphene. By contrast, Tour et al. focused extensively on natural materials derived from cellulose (a homopolysaccharide biopolymer).¹⁴¹ Further reductions in cost and environmental footprint can be achieved by using other naturally abundant, environmentally friendly materials such as, for example, mucopolysaccharides, ^[51other heteropolysaccharide biopolymer materials, alginates,¹⁵³¹ and carrigeenans. ^[5t>1 The present invention aims to address the aforementioned issues by providing an environmentally friendly, cost-effective and scalable solution to produce porous graphene structures using biopolymers, including naturally derived biopolymers.

Summary of the invention

According to a first aspect of the present invention, there is provided a method for producing graphene or porous graphitic carbon. The method comprises creating a biopolymer-based feedstock material from a biopolymer formulation, where the biopolymer may be derived from a synthetic or naturally occurring heteropolysaccharide such as a mucopolysaccharide. The method may further comprise irradiating the biopolymer feedstock with coherent light to produce porous graphene or porous graphitic carbon.

In one embodiment, the coherent light is laser light. Producing graphene using a laser-based technique means graphene can be easily be patterned onto or integrated into various electronic components in a scalable, cost effective way. This approach is also compatible with roll-to-roll processing of electronic devices. The use of biopolymer materials derived from naturally derived biopolymers means further reductions in the costs are possible, whilst also making the process more environmentally friendly.

According to an embodiment of the present disclosure, the biopolymer formulation is deposited onto a host substrate, and then cured to create the biopolymer feedstock.

The biopolymer formulation may be deposited onto the substrate by either casting, spin coating, doctor blading or by thin film deposition, and cured to create a biopolymer-based feedstock. The various methods for casting, spin coating, doctor blading or thin film deposition of the biopolymer formulation may be adapted to be suitable for depositing the feedstock onto different substrates. These substrates include, but are not limited to food, paper, polymers, metals, inorganic or organic semiconductors, inorganic or organic insulators, ceramics, making the method highly versatile. Curing processes include, but are not limited to, drying under ambient conditions for periods up to several days and also annealing to temperatures ~ 40 °C in different gas environments (including air). In some embodiments, the host substrate is or comprises an electrically conductive material or a non-conductive material, making it suitable for their incorporation into various devices.

In some embodiments, the laser is operated in pulsed mode or continuous mode. The biopolymer-based feedstock may be irradiated with one or more lasers. The one or more lasers may have or be characterised by one or more laser parameters which are adjustable to optimise the biopolymer- laser interaction. These parameters may include, but are not limited to, laser wavelength, pulse duration, laser power, laser power density and laser scan rate. Typical process parameter ranges include, for example, laser powers substantially in the range 1.5-4 W and scan speeds substantially in the range 10- 400 mm/second for a 10.6 pm C0₂ laser. Process parameter example ranges for irradiation with visible lasers include laser powers substantially in the range 0.5-4 W and scan speeds substantially in the range 10-500 mm/second for a 405 nm or 450 nm laser. The laser beam diameter of the/each C0₂ laser may be in the range substantially 15-300 pm. The laser beam diameter of the/each visible laser may be in the range substantially 5-300 pm. The laser power density may be calculated from the beam diameter and the laser power. In other embodiments, irradiating the biopolymer-based feedstock is with multiple lasers in sequence or simultaneously, and, optionally or preferably, involves pre-conditioning the biopolymer-based feedstock by irradiating the biopolymer-based feedstock with an infra-red laser. The infrared laser can have a wavelength between substantially 0.7-20 pm. In an embodiment, microblister formation may comprise irradiation with a 10.6 pm C0₂ laser for site-specific dehydration and/or microblister or porous network formation using a laser power substantially in the range 1 .5-4 W and a scan speed substantially in the range 10-400 mm/second. The laser beam diameter may be in the range substantially 15-300 microns. The laser power density may be calculated from the beam diameter and the laser power. The pre-condition step allows the formation of micro-blisters within the biopolymer-based feedstock material. It is speculated that the infra-red laser illumination improves local dehydration of the cured feedstock by releasing solvents or vaporising solvent molecules such as aqueous acetic acid or water trapped in the cured feedstock, resulting in the formation of micro-blisters.

The micro-blisters comprise micron-scale or sub-micron pores and optionally contain connected pore networks.¹⁶¹ Thus, the pre-conditioning step increases the porosity of the feedstock material. The micropores possess a cross-section in one plane with a diameter less than 10 pm; and mesopores possess a cross-section in one plane with a diameter less than 1 pm. The areal density of pores in a plane parallel to the host substrate exceeds 1 micropore or 1 mesopore in an area of 500 (pm)².

In another embodiment, the biopolymer-based feedstock is further irradiated with a visible laser, with a wavelength between substantially 400-700 nm. This results in the formation of a high surface area porous graphitic carbon structure with a 3D hierarchical structure. This activation step enables the formation of non-planar graphitic carbon or graphene structures, allowing the graphene to conform to the 3D surface.

In another embodiment, the biopolymer-based feedstock is irradiated with an infra-red laser in a further post-processing step, with a wavelength between substantially 0.7-20 pm. The post-processing step can also be used to manipulate the surface topography, such as enhancing the surface area of the structure further.

In an embodiment, the biopolymer-based feedstock is irradiated with a 10.6 pm C0₂ laser for site-specific dehydration and/or microblister or porous network formation. The biopolymer-based feedstock may then be further irradiated with a 405 nm or 450 nm laser. This provides for graphitisation. The biopolymer-based feedstock may then be further irradiated with a 10.6 pm C0₂ laser to enable formation of graphene-like carbon.

The C0₂ laser(s) may have a power substantially in the range 1 .5-4 W and a scan speed substantially in the range 10-400 mm/second may be used. The laser beam diameter of the C0₂ laser may be in the range substantially 15-300 pm.

The visible lasers (405 nm or 450 nm) may have a laser power substantially in the range 0.5-4 W and a scan speed substantially in the range 10-500 mm/second. The laser beam diameter of the visible wavelength laser may be in the range substantially 5-300 pm.

Porous graphene exhibits properties that are distinct from monolayer graphene, leading to a wide variety potential applications that require non-planar, 3D, high surface area graphene (e.g. various electrodes). In addition, the graphene may also be highly electrically conductive, such that it can be patterned into electrodes for supercapacitors. Other applications can include energy storage, sensors (electrochemical, gas/humidity, pressure etc...), flexible heaters and gas purification. In other embodiments, the biopolymer may be irradiated with the one or more lasers under ambient and/or atmospheric conditions, alleviating the complexity of the process further.

In some embodiments, the heteropolysaccharide or mucopolysaccharide may comprise a nitrogen group. In other embodiments, the heteropolysaccharide or mucopolysaccharide may comprise an amino or an amide group. The mucopolysaccharide may be chitosan, which can be formed by the deacetylation of chitin.¹⁵¹ Chitin may be found naturally in abundance. The heteropolysaccharide may be an alginate such as seaweed or carrigeenan.

In some embodiments, the biopolymer formulation may comprise chitosan and a naturally occurring ion source, such as acetic acid, and/or a naturally occurring plasticizer, such as glycerol or sorbitol. The ratios of chitosan, ion source and plasticizer may be varied to generate different types of formulations, such as paper-like material, plastic or rubber. For example, there may be substantially 5g of chitosan, substantially 96 ml of a 0.5% aqueous acetic acid solution and substantially 4 ml of glycerol or sorbitol to produce a paper-like substrate; or substantially 5g of chitosan, substantially 98 ml of a 5% aqueous acetic acid solution and substantially 2 ml of glycerol or sorbitol to produce a rubber-like substrate; or substantially 5g of chitosan and substantially 100 ml of a 10% aqueous acetic acid solution produce a hard plastic-like substrate.

In other embodiments, the biopolymer formulation further comprises a naturally occurring additive, such as calcium lactate. This additive modifies the mechanical properties of the cured biopolymer formulation and also acts as a sensitizer to improve the efficiency of the laser-induced graphene formation process using C0₂ laser irradiation, allowing formation of laser-induced graphene with a single lasing step (C0₂ laser irradiation), thus obviating the need for a pre-conditioning step (typically using C0₂ laser irradiation) or a laser activation step (405 nm or 450 nm laser irradiation).

In some embodiments, the biopolymer formulation may comprise substantially 100 ml of a 10% aqueous acetic acid solution, substantially 5 g of chitosan, substantially 2.5 g of calcium lactate and substantially 2 ml of glycerol or sorbitol.

In addition, the biopolymer may further comprise quinine or quinine-derived compounds. Quinine improves the biopolymer absorption near 405 nm. Quinine also improves the stiffness of the materials, reduces the steps required to produce the material by eliminating the need for a laser pre-conditioning step and enhances the fluorescence of the biopolymer for various applications.

The above method may be used to pattern porous graphene structures directly onto electrodes for use in supercapacitors. Graphene is widely considered a prime candidate this purpose instead of activated carbon, because of its high electrical conductivity and surface area. A higher surface area means better electrostatic charge storage, potentially offering higher capacitances.

Features which are described in the context of separate aspects and embodiments of the invention may be used together and/or be interchangeable. Similarly, where features are, for brevity, described in the context of a single embodiment, these may also be provided separately or in any suitable sub-combination. Features described in connection with the device may have corresponding features definable with respect to the method(s), and vice versa, and these embodiments are specifically envisaged. Brief description of drawings

Embodiments will be described, by way of example only, with reference to the drawings, in which:

Figure 1 shows a schematic of a process for producing graphene in accordance with an embodiment; Figure 2 shows a schematic of the process flow for LIG formation from a chitosan-based biopolymer; Figure 3 shows low-magnification SEM images showing surface topography for cured feedstock film and cured film exposed to C0₂ laser;

Figure 4 shows high-magnification SEM images of micron-scale surface topography for cured feedstock film and cured film exposed to C0₂ laser;

Figure 5 shows high-magnification SEM images showing micron-scale surface topography for feedstock film after C0₂ laser exposure and 405 nm laser exposure (patterning step);

Figure 6 shows schematic of the patterning step and post-processing step, and SEM images of the resulting surface topography;

Figure 7 shows Fourier Transform Infrared Spectroscopy data;

Figure 8 shows a typical Raman spectrum of a porous graphitic carbon film;

Figure 9 shows single Lorentzian fits to each of the D and G peaks for the data shown in figure 8;

Figure 10 shows a typical Raman spectrum of the same film following the post-processing step; and Figure 11 show fits to the D, G, D’, D+D”, 2D and D+D’ peaks.

Detailed Description

Figure 1 shows a schematic of the set-up of an apparatus or device for use in an embodiment of the present disclosure. A chitosan/biopolymer formulation 8 (shown in figure 2) is deposited onto a substrate 16 and cured to create a biopolymer feedstock 14. The cured feedstock film 14 can range in thickness from microns to millimetres. Curing processes include, but are not limited to, drying under ambient conditions for periods up to several days, drying under vacuum conditions and also annealing to temperatures approximately 40 °C under vacuum or different gas environments (including air).

The method of deposition may be casting the biopolymer formulation 8 onto the substrate, doctorblading or spin-coating. The biopolymer formulation 8 may also be deposited onto the substrate 16 by using thin film deposition, such as by chemical vapour deposition (CVD) or physical vapour deposition (PVD). The substrate 16 may be an electrically conductive material such as a metal; or non-conductive, such as glass, paper, plastic or rubber.

In some embodiments, the biopolymer feedstock 14 is derived from a naturally occurring mucopolysaccharide such chitosan. In an embodiment, the mucopolysaccharide is or contains a nitrogen group and an amino or an amide group. In other embodiments, the biopolymer feedstock 14 is derived from the formulation 8 containing chitosan and a naturally occurring ion source, such as acetic acid and a naturally occurring plasticizer such as glycerol. The ratios of chitosan, ion source and plasticizer may be altered to produce feedstocks 14 with different mechanical properties, such as paper-like, rubber-like or plastic-like feedstock 14.

In some embodiments, the biopolymer may also contain a naturally occurring additive such as calcium lactate, or blends of additives. The additives modify the mechanical properties of the cured biopolymer, and can also act as a sensitizer to improve the efficiency of the laser-induced graphene formation process or reduce the number of process steps needed. In addition, the biopolymer may also contain quinine. The addition of quinine improves the stiffness of the materials, and/or also reduces the number of process steps required to produce the material by eliminating the need for a laser pre-conditioning step, and enhances the fluorescence of the bio polymer.

Figure 2 shows a schematic of the process flow for laser-induced graphene (LIG) formation from a chitosan-based biopolymer. In figure 2a the formulation 8 containing chitosan is deposited onto the substrate 16 to form the feedstock 14. In figure 2b a specific region of the feedstock 14 is pre-conditioned using a C0₂ laser at a wavelength of 10.6 pm. The pre-conditioned region of the feedstock 14 is then illuminated with a laser 12 at a wavelength of 405 nm to form a porous graphitic carbon pattern (figure 2c), as evidenced by Raman data (see figures 8 and 9). It can be appreciated, however, that patterning of porous graphitic carbon may be achieved by illuminating the feedstock 14 using a laser with a wavelength ranging anywhere between 400-700 nm. The porous graphitic carbon structure is subsequently processed again using a C0₂ laser (figure 2d) to create porous graphene, as evidenced by Raman data (figures 10 & 11). Figure 2e shows a photograph of a typical biopolymer based LIG pattern. It can be appreciated, however, that the pre-conditioning and the post-processing steps may be achieved by illuminating the feedstock 14 using a laser with a wavelength ranging anywhere between 0.7-20 pm.

Various stages of the process are shown in figure 2f. Corresponding low-magnification SEM images of figure 2f are shown in figures 2g-h: figure 2g shows an image of the cured chitosan feedstock film 14, and figure 2h shows an image of the feedstock film 14 after being exposed to a 10.6 pm C0₂ laser to create micro-blisters. The micro-blisters increase the porosity of the feedstock material. It is speculated that the illumination using a 10.6 pm laser improves local dehydration of the cured feedstock 14 by releasing or vaporizing solvents such as aqueous acetic acid trapped in the cured feedstock 14, resulting in the formation of micro-blisters. The resulting material is subsequently exposed to a 405 nm laser. The sequential illumination of the cured feedstock 14 with 10.6 pm and the 405 nm lasers enable the formation of a porous graphitic carbon structure with a 3D hierarchical structure. This is shown in figure 2i, where the surface topography resulting from exposure of the micro-blister feedstock film 14 to a 405 nm laser is clearly visible. Finally, figure 2j shows the surface topography of the porous biopolymer after the post-processing using a 10.6 pm C0₂ laser. The latter step enhances the surface area of the porous morphology and enables the formation of non-planar graphene structures, allowing the formation of high surface area, micro-, meso and nano-porous graphene.

In an embodiment, a 10.6 pm C0₂ laser is used with a power substantially in the range 1 .5-4 W, with a scan speed substantially in the range 10-400 mm/second. The laser beam diameter of the C0₂ laser may be in the range substantially 15-300 pm. For the visible lasers, e.g. the 405nm or 450nm lasers, process parameter example ranges for irradiation include laser powers substantially in the range 0.5-4 W and scan speeds substantially in the range 10-500 mm/second. The laser beam diameter of the visible laser may be in the range substantially 15-300 microns.

Tuning the biopolymer formulation process, feedstock deposition process, pre-conditioning of the feedstock, laser activation of the feedstock and laser post-processing of the feedstock allows control of the porosity of the graphene or graphitic carbon and to produce a hierarchical porous network of graphene or graphitic carbon, with pore dimensions ranging from < 10 nm to curved surfaces with of radii of curvature in excess of 10 microns. In particular, the laser pre-conditioning step can be employed to create micron-scale micropores and sub-micron-scale mesopores in the feedstock material (figure 4c). Furthermore, the post-processing step can be used to manipulate the surface topography (see figure 6g and figure 6h).

Figure 3a further shows low-magnification SEM images of the surface topography for a cured chitosan feedstock film exposed to a 10.6 pm C0₂ laser to create micro-blisters. The boundary between the as-cured region (right) and a pre-conditioned region (left) is shown in figure 3b. Figure 3c is a higher magnification SEM image showing the same boundary between the as-cured region and a preconditioned region. Further high magnification images are shown in figure 4. Here, micron-scale surface topography for cured (figure 4a) and pre-conditioned chitosan feedstock film 14 (figure 4b and figure 4c) is shown. Figure 4b and Figure 4c clearly demonstrate the formation of micro-pores (with diameters below 10 microns) and meso-pores (with diameters below 1 micron).

A comparison of the change in the surface topography after the pre-conditioning step and subsequent exposure of the 405 nm laser is illustrated in figures 5a and b. In particular, figure 5a shows the boundary between the region of as-cured chitosan feedstock film 14 (right side) that is exposed to the C0₂ laser (pre-conditioning) to create micro-/meso-pores, and when the same region is subsequently exposed to a 405 nm laser (left side) to create a porous graphitic carbon. Figure 5b shows a higher magnification SEM image of the boundary in figure 5a.

Figure 6a shows a schematic of the laser activation/patterning step (405 nm laser irradiation) following the laser pre-conditioning step. Figures 6b-d show successive higher magnification SEM images showing the surface topography of the biopolymer based porous graphitic carbon following 405 nm laser patterning. Figure 6e shows a schematic of the laser post-processing step to convert porous graphitic carbon to porous graphene (C0₂ laser irradiation at 10.6 pm) following the 405 nm laser activation/patterning step. Figures 6f-h show successive higher magnification SEM images showing the surface topography of the biopolymer-based porous graphene following 405 nm laser patterning and subsequent C0₂ laser irradiation at 10.6 pm. The SEM images clearly demonstrate the surface area enhancement arising from the post-processing step.

In some embodiments, a laser light 12 is directed or focused onto the biopolymer feedstock 14 using a lens 10. In other embodiments, laser light 12 may be directed onto the biopolymer without using a lens 10. The wavelength of the laser may be in the infra-red, visible or UV spectrum. In other embodiments the laser 12 may be operated in pulsed or continuous mode. One or more laser parameters may also be tuned. These include, but are not limited to, laser power, laser power density, laser pulse duration and laser scanning rate. Using multiple laser sources of different wavelengths in sequence may be useful in controlling the degrees of graphene formation on different regions of the feedstock 14. In some embodiments, the one or more laser parameters may also be varied according to the one or more characteristics of the feedstock. These characteristics may include, for example, thickness, composition, morphology and absorption characteristics. In other embodiments, one or more laser parameters may also be varied to control the interaction of the laser with the feedstock. The C0₂ laser(s) may have a power substantially in the range 1.5-4 W and a scan speed substantially in the range 10-400 mm/second may be used. The laser beam diameter of the C0₂ laser may be in the range substantially 15-300 pm. The visible lasers (405 nm or 450 nm) may have a laser power substantially in the range 0.5-4 W and a scan speed substantially in the range 10-500 mm/second. The laser beam diameter of the visible wavelength laser may be in the range substantially 5-300 pm.

In some embodiments, patterning of the biopolymer feedstock 14 with graphene may be achieved by scanning the laser across the surface of the biopolymer feedstock 14. Various techniques can be employed to expose the feedstock 14 to a laser source. For example, in some embodiments patterning of biopolymer feedstock may be achieved using a computer-controller mechanism.

In some embodiments, the laser 12 wavelength may be selected to match the absorbance of the biopolymer feedstock 14. This results in a higher proportion laser light being absorbed by the biopolymer feedstock 14, enabling graphene formation at lower laser power. In order to maximise the absorption of laser light, the biopolymer may be chemically modified to optimise absorption at the desired wavelength (such as UV, visible or infra-red). In some embodiments, the biopolymer is irradiated with laser under ambient conditions. It may be appreciated that graphene formation can also result from laser irradiation of biopolymers under a controlled atmosphere (such as under nitrogen gas) or an inert atmosphere (such as under argon or helium gas) or in vacuum.

Without reference to theory, the above processes may be influenced by photo-thermal and/or photo-chemical degradation processes. The absorption of infra-red laser energy by the biopolymer feedstock 14 can result in strong lattice vibrations when the laser energy lies within an absorption band of the feedstock, which in turn can induce extremely high temperatures and pressures in the region of the biopolymer irritated with the laser 12. Consequently, the sp³-carbon bonding in the biopolymer feedstock 14 converts to sp²-carbon bonding; and C-O, C=0 and N-C bonds break in the biopolymer, leading to the formation of porous graphene or porous graphitic carbon.

In some instances, graphene formation can result from a combination of both photo-thermal and photo-chemical degradation processes, especially when multiple lasers with different wavelengths are used. Photo-chemical processes may dominate at shorter laser wavelengths (such as UV/visible), whereas photo-thermal effects tend to dominate at longer wavelengths (such as infra-red).

Figure 7 shows a Fourier Transform Infrared Spectroscopy (FTIR) data for cured chitosan feedstock film 14 (solid curve) and cured feedstock film 14 exposed to a 10.6 pm C0₂ laser preconditioning step (dashed curve). These data indicate that the pre-conditioning step does not substantially alter the chemical composition of the cured feedstock 14. Without reference to theory, the increase in transmittance over the 2600-3500 cm ¹ range may reflect a reduction in the water and/or acetic acid content of the feedstock following the pre-conditioning step. The FTIR spectrum of water shows a broad transmittance minimum (with transmittance values ~0-0.1) over the spectral range ~3100-3600 cm ¹ ,^[11] while the FTIR spectrum of acetic acid shows a broad transmittance minimum (with transmittance values ~0.4-0.6) over the spectral range ~2600-3200 cm ¹. ¹¹²¹

Figure 8 shows a typical Raman spectrum (acquired using 514 nm laser excitation) of a biopolymer-based porous graphitic carbon film following curing, pre-conditioning (10.6 pm C0₂ laser) and direct-write patterning (405 nm diode laser 12) of the feedstock 14. Only two dominant peaks are clearly visible: a D peak centred near 1350 cm ¹ and a G peak centred at 1580 cm ¹. The G peak corresponds to high-frequency E_2g phonon modes for pairs of sp² carbon atoms and the defect-activated D peak is due to radial breathing modes of six-atom aromatic rings . Figure 9 shows the fits to the D and G peaks. The D peak fit is centred at Pos(D) = 1354 cm ¹ with full-width at half maximum FWHM(D) = 181 cm ¹ , while Pos(G) = 1579 cm ¹ and FWHM(G) = 108 cm ¹. The intensity ratio of the peaks is /(D)//(G) ~ 0.75. These Raman data are consistent with other reports of disordered nanocrystalline graphitic carbon^[8l[9l[101. Ferrari and Robertson proposed an “amorphization trajectory” through a phase diagram from pure graphite to disordered nanocrystalline graphite to amorphous carbon and discussed the associated changes in Raman spectra¹⁸¹. Lespade and co-workers demonstrated the evolution of the Raman spectrum of single crystal graphite through introduction of disorder by ion implantation¹⁹¹. In particular, they showed that the FWHM of both the D and G peaks increase with increasing disorder, which corresponds to smaller crystallite size.

Figure 10a shows a typical Raman spectrum of the porous graphitic carbon film with Raman data shown in figure 8 following the post-processing step (10.6 pm C0₂ laser irradiation). Three significant peaks are now evident: D, G and a 2D peak centred close to 2700 cm ¹. After the post-processing step the FWHM(D) = 61 cm ¹ and FWHM(G) = 53 cm ¹. The significant narrowing of the D and G peaks with respect to the peaks for the laser activated graphitic carbon shown in figure 9 is indicative of increased ordering.

Figure 10b shows the Raman data between 1500 cm ¹ and 1700 cm ¹ , which shows the shoulder on the G peak above 1600 cm ¹. Figure 1 1 a and figure 1 1 b show Lorentzian fits to the D and G and 2D regions, respectively for the Raman data shown in figure 10. Three Lorentzian lineshapes were used to fit the data in Figure 1 1 a, to take account of the clear asymmetry in the G peak. This asymmetric results from the D’ peak and can be fitted with a single Lorenztian with Pos(D) = 1620 cm ¹ and a FWHM(D) = 21 cm ¹ , as shown in figure 1 1 a. The latter peak has been assigned to a double-resonance intravalley process, that is, connecting two points belonging to the same cone around K (or K^') in the Brillouin Zone.¹⁷¹ The D’ peak makes a small contribution to the overall Raman spectrum in this region. The data are dominated by the D and G peaks.

Figure 1 1 b further shows Lorentzian fits to the D+D”, 2D, D+D’ peaks for the Raman data shown in figure 10a, FWHM(D+D’) = 287 cm ¹ and FWHM(D+D) = 184 cm ¹. The D+D” and D+D’ peaks result from two-phonon defect-assisted processes. The 2D peak, centred at 2696 cm ¹ dominates the total signal in this region. This sharp peak, which can be fitted with a single Lorentzian with FWHM(2D) = 81 cm ¹ indicates formation of graphene. A FWHM(2D) < 100 cm ¹ is indicative of graphene formation from a biopolymer, however, it can be appreciated that there may exist other mucopolysaccharides or heteropolysaccharides or biopolymers derived from such mucopolysaccharides or heteropolysaccharides where graphene formation may be indicated by a FWHM(2D) exceeding 100 cm ¹. In addition, it may be appreciated that porous graphene produces a Raman 2D peak with Pos(2D) values in the range 2650- 2750 cm ¹ and a peak intensity ratio 1_2D/I _G of greater than 0.2.

It can be appreciated that other that various other techniques may also be employed in the characterisation of the resultant graphene structures; including, but not limited to, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), atomic force microscopy (AFM) and electrical conductivity measurements.

The porous graphene structures produced using the method of the present disclosure may be incorporated into various electronics devices, resulting from its unique structural and electrical properties. For example, the graphene produced using this method may form part of a cathode or an anode (or both) of an electronic device or may be utilized as conductive fillers or wires, as a current collector, and/or as additives in an electronic device. Graphene or graphene structures of the present disclosure may also be incorporated into various electrochemical sensors, strain sensors, gas/humidity bimorph actuators, flexible heaters and pressure sensors. These devices can find applications across numerous industries, including medical device development, wearables consumer electronics, industrial and home automation, food and beverage, packaging and automotive and as catalysts. Additional uses can include DNA sequencing, and incorporation into energy storage or generation devices such as super- and microcapacitors, batteries, sodium-ion batters, magnesium-ion barriers, micro-batteries and for hydrogen storage.

From reading the present disclosure, other variations and modifications will be apparent to the skilled person. Such variations and modifications may involve equivalent and other features which are already known in the art, and which may be used instead of, or in addition to, features already described herein.

Although the appended claims are directed to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention.

Features which are described in the context of separate embodiments may also be provided in combination in a single embodiment. Conversely, various features which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination.

For the sake of completeness it is also stated that the term "comprising" does not exclude other elements or steps, the term "a" or "an" does not exclude a plurality, and any reference signs in the claims shall not be construed as limiting the scope of the claims.

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Claims

1 . A method for producing a porous graphene or porous graphitic carbon material, said method comprising the steps of: creating a biopolymer-based feedstock material from a biopolymer formulation, wherein the said biopolymer is derived from a synthetic or naturally occurring mucopolysaccharide or other heteropolysaccharide; and irradiating the biopolymer-based feedstock material with coherent light to produce porous graphene or porous graphitic carbon.

2. The method of claim 1 , wherein irradiating the biopolymer with the coherent light comprises using a laser, wherein the laser is operated in a pulsed mode or a continuous mode.

3. The method of claim 2, further comprising depositing a biopolymer formulation onto a host substrate, and curing the biopolymer formulation to create the biopolymer-based feedstock before irradiating.

4. The method of claim 3, wherein depositing the biopolymer formulation onto the host substrate comprises casting, doctor blading, spin coating, and/or thin film deposition of the biopolymer formulation.

5. The method of claim 3 or claim 4, wherein the host substrate is made from an electrically conductive

material or a non-conductive material.

6. The method of any preceding claim, wherein irradiating the biopolymer-based feedstock is with one or more lasers, wherein the one or more lasers have one or more laser parameters, such as laser wavelength, pulse duration, laser power, laser power density and laser scan rate, that are adjustable to optimise biopolymer-laser interaction.

7. The method of claim 6, wherein irradiating the biopolymer-based feedstock is with multiple lasers in

sequence or simultaneously, and, optionally or preferably, involves pre-conditioning the biopolymer-based feedstock by irradiating the biopolymer-based feedstock with an infrared laser to form micro-blisters.

8. The method of claim 7, comprising irradiating the biopolymer-based feedstock with multiple lasers in

sequence or simultaneously to form micro-blisters comprising micropores and mesopores which are of micron-scale and sub-micron scale, respectively, and optionally contain connected pore networks.

9. The method of claim 7 or 8, comprising irradiating the biopolymer-based feedstock with multiple lasers in sequence or simultaneously to form micropores having a cross-section in one plane with a diameter less than 10 pm, and mesopores having a cross-section in one plane with a diameter less than 1 pm.

10. The method of claim 9, comprising irradiating the biopolymer-based feedstock with multiple lasers in

sequence or simultaneously to form pores with an areal density of pores in a plane parallel to the host substrate greater than 1 micropore or 1 mesopore in an area of substantially 500 (pm)².

1 1 . The method claim 10, wherein irradiating the biopolymer-based feedstock with one or more lasers in

sequence or simultaneously further involves irradiating the biopolymer-based feedstock with a visible laser.

12. The method of claim 1 1 , wherein the visible laser has:

a) a wavelength between substantially 400-700 nm and, optionally or preferably, that is 405nm or 450nm; and/or

b) a laser power substantially in the range 0.5-4 W; and/or

c) a laser scan speed substantially in the range 10-500 mm/second; and/or

d) a laser beam diameter substantially in the range 5-300 microns.

13. The method of any of claim 12, wherein irradiating the biopolymer-based feedstock with the visible laser produces porous graphitic carbon with a porous morphology.

14. The method claim 13, wherein irradiating the biopolymer-based feedstock with one or lasers in sequence or simultaneously further involves post-processing the biopolymer-based feedstock by irradiating the biopolymer feedstock with an infrared laser.

15. The method of any of claims 7 to 14, wherein pre-conditioning the biopolymer-based feedstock is with an infrared laser having:

a) a wavelength between substantially 0.7-20 pm and optionally or preferably 10.6 pm, and/or the method of claim 14, wherein post-processing the biopolymer-based feedstock is with an infrared laser having a wavelength between substantially 0.7-20 pm and optionally or preferably 10.6 pm; and/or

b) a laser power substantially in the range 1 .5-4 W; and/or

c) a laser scan speed substantially in the range 10-400 mm/second; and/or

d) a laser beam diameter substantially in the range 15-300 microns.

16. The method of any of claim 15, wherein post-processing the biopolymer-based feedstock with an infrared laser produces porous graphene with porous morphology.

17. The method of claim 16, wherein the steps of pre-conditioning and irradiating the biopolymer-based

feedstock with a visible laser produces porous graphitic carbon having a high surface area and/or is highly conductive; and wherein the further step of post-processing produces porous graphene having a high surface area and/or is highly conductive.

18. The method of any of claims 6 to 17, where irradiating the biopolymer-based feedstock with the one or more lasers occurs under ambient atmospheric conditions.

19. The method of any preceding claim, wherein creating the biopolymer-based feedstock material from a biopolymer formulation is from a heteropolysaccharide or a mucopolysaccharide which is or comprises one or more of a nitrogen group, an amino or amide group or chitosan or an alginate or a carrigeenan.

20. The method of any preceding claim, wherein creating the biopolymer-based feedstock material is from a biopolymer formulation comprising chitosan and a naturally occurring ion source, such as acetic acid, and/or a naturally occurring plasticizer, such as glycerol or sorbitol.

21. The method of claim 20, wherein creating the biopolymer-based feedstock material is from a biopolymer formulation comprising substantially 5g of chitosan, substantially 96 ml of a 0.5% aqueous acetic acid solution and substantially 4 ml of glycerol or sorbitol.

22. The method of claim 20, wherein creating the biopolymer-based feedstock material is from a biopolymer formulation comprising substantially 5g of chitosan, substantially 98 ml of a 5% aqueous acetic acid solution and substantially 2 ml of glycerol or sorbitol.

23. The method of claim 20, wherein creating the biopolymer-based feedstock material is from a biopolymer formulation comprising substantially 5g of chitosan, and substantially 100 ml of a 10% aqueous acetic acid solution.

24. The method of any of claims 1 to 19, wherein creating the biopolymer-based feedstock material is from a biopolymer formulation further comprising a naturally occurring additive, such as calcium lactate and, optionally, wherein the biopolymer formulation further comprises quinine or quinine-derived compounds.

25. The method of claim 24, wherein creating the biopolymer-based feedstock material is from a biopolymer formulation comprising substantially 100 ml of a 10% acetic acid solution, substantially 5 g of chitosan, substantially 2.5 g of calcium lactate and substantially 2 ml of glycerol or sorbitol.