WO2023166325A1

WO2023166325A1 - Laser-assisted method for high-quality, high conductivity graphene deposition for smart textile and flexible electronics

Info

Publication number: WO2023166325A1
Application number: PCT/GR2023/000004
Authority: WO
Inventors: Spyros N. YANNOPOULOS; Aspasia ANTONELOU; Antonios VAVOULIOTIS; Athanasios BALTOPOULOS; Despoina BATSOULI; Grigorios KOUTSOUKIS; Anastasios Iliopoulos
Original assignee: Foundation For Research And Technology Hellas - Institute Of Chemical Engineering Sciences (Forth/Ice-Ht); Adamant Composites
Priority date: 2022-03-01
Filing date: 2023-03-01
Publication date: 2023-09-07
Also published as: GR20220100190A

Abstract

The present invention relates to a method for the manufacturing of a wide range of conductive flexible substrates through the in-situ production and deposition of graphene on a flexible substrate surface, with emphasis on conventional and technical fabrics of various types. The method is suitable for industrial scale production, e.g. in a roll-to- roll process, with rapid deposition of graphene on the substrate, which takes place at ambient conditions. The graphene-coated substrates achieve very low ohmic resistance values. The method is applied for the manufacture of multifunctional or electronic textiles with applications in radiation shielding, harvesting and storage of energy in batteries and supercapacitors for charging portable devices, membranes and filters, sensors, and other applications where the use of flexible conductive surfaces is required.

Description

Title

LASER-ASSISTED METHOD FOR HIGH-QUALITY, HIGH CONDUCTIVITY GRAPHENE DEPOSITION FOR SMART TEXTILE AND FLEXIBLE ELECTRONICS

Description

Technical field of invention

[0001] The technical field of this invention relates to the production of lase-assisted deposition of high-quality graphene onto the surface of a substrate with emphasis on flexible surfaces, especially fabrics of various types, with applications in areas such as flexible electronics and multi-functional fabrics.

Background of invention and technical problem

[0002] The present invention discloses a method for producing conductive flexible surfaces (e.g., substrates that can be rolled, stretched and processed in Roll-to-Roll, R2R, production lines) by in-situ synthesis of high-quality graphene using laser radiation. The high quality is crucial since the method of the present invention achieves the growth of high purity graphene characterized by turbostratic stacking of the layers, with a negligible amount of other carbon forms or impurities, such as heteroatoms, e.g. oxygen. The growth takes place on a flexible surface resulting in a particularly low sheet resistance (high electrical conductivity), which is essential for the applications of the present invention.

[0003] The current invention finds applications in the development of multi-functional fabrics and other substrates (e.g., polymers, plastics, etc.) that are integral parts of devices in the area of flexible electronics. These multi-functional fabrics belong to the category of smart or electronic textiles and are related to a multitude of applications in the field of flexible electronics such as, for example, energy conversion and storage devices, sensors, and coatings for protection against external stimuli such as heat, humidity, radiation, pollutants, etc., among others.

[0004] The term “fabric/textile” in the present invention encompasses a broad category of flexible substrates such as: [i] conventional (or traditional) fabrics/textiles, i.e. those used in clothing manufacturing fashion products, furnishing products (e.g., sheets, curtains, towels, upholstery, etc.), and [ii] technical fabrics/textiles, such as those associated with certain specific applications, e.g., medical fabrics, synthetic reinforced fabrics, geotextiles, filters, etc., depending on the properties endowed on them, e.g. mechanical, chemical/physicochemical, waterproofing, etc. The term “fabrics/textiles” of the present invention also includes non-woven fabrics.

[0005] The properties of fabrics are determined not only by the nature of the raw materials but also by the surface treatments applied (finishing), which can not only improve the aesthetics and texture of the fabric but also endow the fabrics new properties. Most surface treatments are of chemical nature (chemical finishing) transforming a fabric into a technical product with “functional” properties. Examples of surface treatments aimed at protecting users and fabrics include, antimicrobial activity, hydrophobic or oleophobic behavior, resistance to combustion, radiation protection, anti-static properties, electrical conductivity, ballistic and knife (antistabbing) protection, and so on. A multitude of methods are used for surface treatment by depositing coatings on fabrics. For example, for liquid coating, knife, roller, dipping, and spraying techniques are used, while for dry coating, melt coating, calendering, and laminating are applied.

[0006] The need for the development of environmentally friendly methods for surface treatment of textiles has become increasingly urgent in view of the growing concern about protection of environment against pollution. The use of lasers for surface fabric/textile treatment offers several advantages over conventional chemical methods, since it radically reduces the use of water, eliminates waste production, while no further prost-treatment methods are required to clean the fabric/textile. In addition, laser treatment allows controlled surface modification in a short time, enabling the printing of predetermined shapes or patterns on the surface of the fabric/textile (laser patterning).

[0007] The use of lasers gradually replaces older processing technologies (e.g. fading) in various fabrics. Cutting and bonding of fabrics with laser technology has been used for several years. Lasers can also help in the controlled dyeing of fabrics since it is possible to achieve differential dyeing by irradiating specific areas. Another example of the use of laser radiation relates to the modification of the surface of fabrics in order to achieve a more efficient decoration with nanoparticles used e.g. as anti-microbial coating.

[0008] Endowing electrical properties (conductivity) to fabrics has emerged in recent years as a very important activity, because it is related to the development of fabrics that can be part of a flexible electronic device, which can perform a multitude of functions for the user of the smart textile, such as. e.g., temperature regulation, EM radiation shielding, energy conversion and storage, interfacing the user with the environment through the propagation of signals from embedded sensors, batteryless charging of portable devices, etc.

[0009] In the field of conductive fabrics, recent activity has been developed for the deposition of graphene-based thin layers, using various chemistry-based methods, both at the yarn and fabric level. These techniques are mainly based on the deposition of graphene oxide (GO) or reduced graphene oxide (rGO). The use of rGO in these applications is based on the affinity of rGO to the filaments due to the oxygen-containing chemical groups in rGO. The reduction of GO structures to the more conductive form of rGO in fabrics is achieved based on multiple time-consuming steps using either wet chemistry or heating methods. This bears significant negative environmental footprint but also results in low quality of the rGO due to undesirable chemical impurities.

[00010] A brief review of the prior art relevant to the subject matter of the present invention is provided below, highlighting the drawbacks of these approaches both in terms of complexity, cost, large scale scalability, and negative environmental and occupational health/safety impacts.

[0011] In an approach for yarn modification via proteins (bovine serum albumin) [1], it was found that GO can bind on the fibers’ surface of a fabric due to electrostatic forces arising from the presence of the protein. In that work, GO was reduced chemically at low temperatures. Immobilization of GO on cotton fabric [2] took place through the “dip and dry” process followed by chemical reduction. The sheet resistance was recorded to be always greater than 20 kQ sq'¹.

[0012] In an alternative approach [3], inkjet printing of rGO was employed on textile’s surface. A major problem relates to the homogeneous dispersion of carbon nanostructures in suspensions (inks). An additional drawback reported is that this method cannot provide large and homogeneous conductive paths on fabrics characterized by rough surfaces and pores. To address this problem, a film of organic nanoparticles was deposited onto the fabric using inkjet printing [4], followed by rGO printing. In the absence of this interlayer of nanoparticles, the sheet resistance was found in the range of MO sq'¹. With the use of the interlayer, this value of sheet resistance was reduced to ~2 kQ sq'¹. The same research team proposed a methodology for scaling up the process using an R2R production line, where the fabrics are immersed in an already chemically reduced suspension of GO. The sheet resistance for the impregnated fabric was ~361 kQ sq'¹, while after five immersion cycles the resistivity decreased to ~37 kQ sq'¹. These values are very high and are not considered to lead to viable applications.

[0013] In another study [5], a flexible capacitor was fabricated on fabrics by screen-printing using GO inks, followed by electrochemical reduction of GO. Depending on the number of iterations of screen-printing to create thicker films, the sheet resistance decreased, but the range varied in the range of 410-110 kQ sq'¹. More recently, the same research group [6] used a rapid technique for coating fabric fibers with rGO-based inks. The lowest sheet resistance achieved in this case, after three ink coating cycles, was ~43 kQ sq'¹. In another approach [7], coating of polyester fabric with GO by liquid spray took place. Also, in this case the reduction of GO was performed by thermal and chemical methods. The sheet resistance was measured in the range of hundreds kQ sq'¹.

[0014] In a recent patent [Pl], it has been reported an attempt to use laser radiation to reduce GO which was coated by dyeing on a nylon type fabric (Spandex Nylon Lycra Matte). The reduction took place with CO2-type laser (radiation at a wavelength of 10.6 pm, continuous wave) by direct irradiation on the fabric using 500 pulses per inch (ppi). In this study, only one type of fabric was used, the most thermally resistant one, because this radiation penetrates the GO film and destroys different types of fabrics. In addition, binder material was added in the GO suspension used to coat the film on the fabric. Remnants of binder remain after irradiation and contaminate the rGO with chemical groups that are deleterious to the rGO electrical properties. The deposited GO film thickness was 3 pm, while the rGO film thickness was found to be 10 pm. Based on the thickness and the conductivity value (20 S/m), the sheet resistance value is calculated to be ~5 kQ/sq which is considered as particularly high for applications, where electrical conductivity is the main property for this application, i.e. electrodes conductive textiles and so on. The reduction of GO took place using a laser source in the mid-infrared, i.e. CO2 laser (continuous wave) [Pl]. For this wavelength of radiation, there is today no possibility of waveguiding the laser light using commercially available optical fibers over practical distances, without significant losses. This drawback limits the outcome of [Pl] in relation to the size of the fabric that can be processed by the laser, the processing speed, and additionally raises safety issues due to the invisible nature of the radiation, as well as the high beam power of CO2 lasers.

[0015] Concerning the use of laser sources and beam propagation via an optical fiber, for the reduction of GO, a research group [8] used the 970 nm laser wavelength radiation for GO reduction. In that study, suspension of GO was spin-coated on a polymer substrate. After drying, the dry GO film was detached from the polymer substrate and was either placed on an aluminum substrate or was treated as self-standing film and was reduced by irradiation. The method disclosed by the current invention differs significantly from the method employed in Reference [8] in the following points: i. The spin coating technique applied in [8] is limited to very small area sizes where GO was deposited, e.g., 5-10 cm², while it is known that spin-coating cannot be applied to R2R process for large scale deposition. ii. The laser radiation pulses used in [8] range from 50 ms to continuous wave (CW) radiation. This range of the pulse width parameter lies in the very long pulse duration regime. Irradiation with such long pulses results in high energy power directed onto the GO film surface and can be destructive to underlying sensitive substrate, e.g., fabric or polymeric substrate. iii. The energy densities used in [8] for GO reduction, in the absence and in the presence of a glass cover on the GO surfaces, were 60 J/cm² and 180 J/cm², respectively. These energy densities are extremely high and can therefore be destructive to the underlying substrate if this substrate was a fabric or flexible polymeric material or other temperature-sensitive material. iv. Under the irradiation conditions of work [8], irradiation creates a crater at the focusing spot, i.e., a hole in the GO film under study. This is the effect of material ablation, which leads to the inhomogeneous removal of rGO around the irradiation point. To avoid a hole in the irradiated spot, the GO film was placed in reference [8] between two glass surfaces, i.e. the process took place with a glass cover over the irradiated GO surface. The glass cover led to less inhomogeneity in the crater area and was considered essential to avoid the ejection of a large fraction of the material at the focusing spot. In this context, the method of [8] is not suitable for the conformal coverage of fabric fibers or the homogeneous coverage of substrates by laser-assisted reduction of GO. v. As reported in [8], the use of a glass cover led to GO reduction at a lesser extent in relation to the case where a glass cover was not used. The rGO material was characterized by an atomic C/O ratio of 5.5:1 (i.e., -15% of O atoms remain) in the absence of a glass cover and 4.5:1 (i.e., -18% of O atoms remain) in the presence of a glass cover. These ratios indicate that a high concentration of O atoms exists in the reduced films, and imply low electrical conductivity of the reduced films.

[0016] The specifications regarding the electrical properties of the graphene (rGO) film deposited on a fabric substrate are determined by the sheet resistance (R_s) or alternatively the conductivity for a given thickness of the film. It has been unarguably arisen from the current state-of-the-art that the reduction of GO using by methods such as chemical, thermal and those using laser irradiation leads to graphitic structures with many defects in their structure (carbon sp³ hybridization and oxygen-containing groups). As a consequence, this results in low crystal quality and accordingly to very high sheet resistance (low conductivity) for a wide spectrum of practical applications in the field of smart textiles and flexible electronics.

[0017] The specifications regarding the structural characteristics of the carbon-based material on a textile’s surface - which ultimately endow the textile/fabric the desired properties stemming from graphene, and not the quality of microcrystalline graphite or graphite with lattice defects - can be determined following a detailed analysis of experimental data obtained by techniques such as Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and X- ray diffraction (XRD), among others. In this direction, a recent survey [9] on commercially available “graphene” materials, supplied by more than 60 companies worldwide, concluded that the quality of what is currently produced and sold as “graphene” by 90% of these companies is very low, and definitely not suitable for most practical applications. The reason is that the current commercially available “graphene” is actually a material composed of graphite microflakes or contains a very large fraction of defects and heteroatoms. These concerns about the quality of graphene also applies for the aforementioned state-of-the-art including the patent [Pl]. The prior state-of-the-art demonstrates that the values of the sheet resistance in rGO films prepared so far, by various methods, is very high, lying in the range of several kQ/sq, or even higher than that by one or two orders of magnitude.

[0018] Additional technical problems, in the field of conducive textiles/fabrics, are related to the methods followed up to now for the in-situ reduction of GO on a textiles/fabric surface. In chemical methods the reduction of GO necessitates the use of hazardous solvents and other chemical compounds, which entails high environmental risks, as well as residues on the fabric and on the graphene layer. In addition, the obtained reduced GO is characterized by very low electrical conductivity. In parallel, reduction by thermal methods is limited at low temperatures, typically in the range 150-200 °C, due to the sensitivity and degradation of the textiles/fabrics at higher temperatures. Thermal reduction also leads to electrical properties which are not suitable for applications.

[0019] Given the above description, it becomes evident that a reliable laser-assisted method for coating textiles/fabrics with high-quality, highly conductive graphene, which satisfies all the following characteristics: (a) high potential for industrial-scale scalability, (b) operation of the overall process under strictly ambient conditions, (c) avoiding hazardous chemicals/solvents and (d) being compatible with roll-to-roll (R2R) production lines of the textile/fabric technology and other flexible substrates, has not yet been established.

Summary of Invention

[0020] The present invention relates to a method for the coating of a surface with a film of a carbon source, e.g., graphene oxide, by spraying an aqueous suspension or other compatible process, the partial or total dehumidification of the film, and the irradiation of the film with radiation from an industrial-type laser directed to the film via an optical fiber. The whole process takes place in ambient conditions, in the absence of a protective chamber with inert gases and in the absence of a protective cover over the irradiated surface. The conversion of the material to graphene can be achieved with a single pulse with a pulse-width ranging between 50 psec to 30 msec and the beam focusing spot (trace) can be in the size of cm (successfully tested in the range of 0.05 to 3 cm), which leads to much higher processing speeds compared to the prior art.

[0021] The present invention is the only one that achieves a high-quality graphene coating and thus, low ohmic sheet resistance on the coated surface, in comparison to the prior state of the art. Although there have been efforts to develop methodologies aimed at preparing fabrics with a conductive coating based on graphene structures, the prior state of the art has not achieved the technology level required for viable, commercial applications. This is evident from the fact that in the prior art (for various fabrics of any type as mentioned above) the carbon nanostructure coating does not meet essential specifications In relation to atomic structure) to be characterized as graphene or graphene-like according to the existing “Good Practice Guide #145: Characterization of the Structure of Graphene, NPL, 2017” and “ISO ITS 21356-1:2021 Nanotechnologies - Structural characterization of graphene - Part 1: Graphene from powders and dispersions”.

[0022] The present invention overcomes the technical problems of the prior art, described in the preceding sections, by disclosing the development of a new reliable methodology for the direct synthesis of graphene onto any type of fabric by means of a high-degree reduction of graphene oxide using laser radiation, as described in the claims of the present invention. The methodology is characterized by a high degree of repeatability, demonstrating the potential of the method for industrial scale application. In addition to fabrics, the method has been successfully applied to other flexible substrates (e.g., polymeric materials) as well as other typical substrates such as glass, metal, ceramic, etc.

[0023] The proposed invention addresses important specific problems associated with the prior state of the art. In particular, the method of reducing graphene oxide by laser irradiation (with irradiation parameters as reported in the embodiments) leads to graphene structures of very high quality and electrical properties (low surface resistance) which are not possible to achieve with the techniques applied so far in the prior art. The process is a one-step method, namely the irradiation of the GO under ambient conditions, in the absence of post-treatment with chemicals and solvents after irradiation, as described in the claims of the present invention. [0024] The desired graphene quality is achieved with the present invention by focusing the beam over a much wider spatial area (the laser spot trace can reach 2 to 3 cm in diameter) compared to the prior state of the art. This innovation makes attractive and feasibly the appliance of this methodology on an industrial scale, e.g., in a roll-to-roll production line relevant for textiles. This is because the surface processing speed of a certain area can be increased by orders of magnitude in relation to the time scale needed for the same area using the methods of the prior art. The process requires only one pulse of radiation per unit surface area, which significantly reduces the time required to coat a certain area with graphene. The process also requires low radiation energy to protect the substrate and offers better control of the resulting coating quality in relation to the prior art. The graphene structures produced in this way consist of a very high percentage (>76%) of carbon atoms with sp² hybridization Raman spectra provided information confirming the high crystalline form of the graphene produced by irradiation. Raman spectra also provided evidence for a turbostratic-like structure, i.e. a nonBernal type stacking mode of the graphene layers.

[0025] The proposed method of reducing GO to generate graphene of very high quality and electrical properties (low surface resistivity) has not been reported so far using any other laser or other type of irradiation source.

[0026] The proposed method makes use of a laser source (Nd:YAG, 1064 nm) which is of industrial type with a pulse duration in the time scale of 50 ps to 30 msec, i.e. capable of operating in an industrial environment, as opposed to ultrashort pulse laser sources (e.g. ps or fs) which on one hand do not produce graphene with the desired structural and electrical properties, and in addition, typically operate in a laboratory or protected environment requiring frequent interventions by the user to maintain the quality and alignment of the light beam. Also, for laser-assisted GO reduction processes that use laser pulse durations longer that ~30 msec, the energy delivered can either be destructive to the underlying substrate supporting the GO film or can result in non-uniform coating of the surface with graphene.

[0027] A large body of attempts in the prior state for the laser-assisted GO reduction relates to the use of laser sources in the mid-infrared, i.e. CO2 laser (emission wavelength 10.6 pm) [Pl]. Currently, there is no feasibility for waveguiding this wavelength of laser radiation over long distances without significant losses, using commercially available optical fibers. This disadvantage limits the size of the fabric that can be processed by the laser, reduces the processing speed, and additionally raises safety concerns to the invisible nature of the radiation and due to the high beam power of CO2 lasers. The method proposed in the present disclosure is not limited by these weaknesses.

[0028] The method of the present disclosure comprises a process wherein the trace of the laser beam on the substrate/fabric can have a diameter between 0.05 and 3 cm without this altering the quality of the produced product (conductive graphene). This scale is 2 to 3 orders of magnitude larger than the corresponding sizes found in the prior art. In a previous patent [Pl] irradiation takes place at focused spots with a density of 500 dpi which corresponds to a step of 0.05 mm. Accordingly, the processing speed of the method disclosed by the present invention is much higher, up to 10² - 10³ times, in relation to the prior art.

[0029] In the prior art for GO reduction using laser sources [Pl], the laser reduction process on fabrics has been limited to only one type of fabric (Spandex Nylon Lycra Matte) due to the low thermal resistance of other types of fabrics. In the method disclosed by the present invention this limitation is overcome. The disclosed process has been applied to a number of different types of fabrics (see embodiments below) in such a way that the fabric fibers are not affected by the part of radiation that reaches and is absorbed by the textile/fabric, after passing through the reduced graphene layer. [0030] The advantages of the method for graphene production of present invention render the proposed technique suitable for adaptation and use in an R2R (Roll-to-Roll) type production line, which is the standard technology for processing conventional and technical fabrics.

[0031] Recently we have witnessed increased consumer demand for multi-functional fabrics and substrates, as new solutions are offered by advances in science and technology. One key reason is that conductive substrates are key components of flexible and wearable electronics. The increase in demand leads to the need for production units with a high increase in production capacity. The method disclosed in the current invention can satisfy the increased demand through its integration into an R2R production line and the development of smart processes that allow remote use of devices via computers and mobile phones (Internet of Things), storage, access and processing of information/data in centralized units (cloud computing), automation of production and important decision making based on existing information (Industry 4.0).

[0032] Compared to conventional methods, the R2R production line improves life cycle costs and increases scale of operation, making it a viable, cost-effective approach to manufacturing multi-functional fabrics or other types of flexible substrates that provide several applications for various industrial uses.

[0033] An essential aspect to enable the industrial application of the smart clothing sector, is related to the ability of the technique disclosed in the present invention, to process (i) larger amounts of substrate/textile areas in shorter time compared to techniques proposed in the prior art, (ii) larger areas at the same processing time, or (iii) need much shorter time for processing the same surface area. These benefits make it possible to use industrially the graphene production method of the current disclosure, in various sectors of applications.

[0034] The operation of the method disclosed in the present invention in standard industrial environments and its integration into production and material processing lines enables more efficient and automated control, and the characterization of the quality of the product.

[0035] The method disclosed in the present invention has been successfully applied to fabrics in an R2R pilot line, hence speeding up the production process. Adapting the process to an R2R line enables the automation of graphene-coated textile production. Some of the benefits of the automation of the method of the disclosed invention are: the improved product performance and uniformity, the optimization of product processing, the prevention of operational problems in production, increased productivity, flexibility and faster product changeovers, traceability, online monitoring and evaluation of the production process and the possibility of automatic and remote interventions, time savings, resource and energy savings, reduction of production and labor costs, and the positive environmental effects due to the speed of production, the reduction of energy consumption and the minimization of waste streams.

Description of Figures

[0036] A brief description of the schemes of the present invention follows.

[0037] Figure 1: Schematic of the R2R production line layout for in-situ production of graphene on the surface of fabrics and flexible substrates.

[0038] Figure 2: Flowchart of the deposition and irradiation process.

[0039] Figure 3 shows the optical image of the "polyester/white" substrate-fabric (A) before the GO spraying process, (B) after the spraying process, and (C) after laser irradiation at selected points. A typical Raman spectrum (D) from the irradiated region (rGO) is shown, and for comparison the Raman spectrum of GO is also depicted. Representative SEM images at low and high magnifications are shown in figures (E) and (F), respectively.

[0040] Figure 4: Corresponding to Figure 3 for the "polyester/blue" substrate-fabric.

[0041] Figure 5: Corresponding to Figure 3 for the "polypropyl ene/white" substrate-fabric.

[0042] Figure 6: Corresponding to Figure 3 for the "polypropylene/blue" substrate-fabric.

[0043] Figure 7: Corresponding to Figure 3 for the "polyamide" substrate-fabric.

[0044] Figure 8: Corresponding to Figure 3 for the "cotton" substrate-fabric.

[0045] Figure 9: Corresponding to Figure 3 for the woven glass-fiber textiles.

[0046] Figure 10: Corresponding to Figure 3 for the woven CF/Kevlar-fabric.

Description of Embodiments

[0047] Various embodiments and examples are referred below. Figure 1 shows a schematic of the R2R line which was used for the implementation of the examples of the current disclosure. Figure 2 shows schematically the steps of the process used for the deposition of GO on the various substrates.

[0048] In some embodiments, preparation of the GO film took place. In certain cases, the GO film was deposited on substrates/textiles using a commercially available aqueous GO suspension at a concentration in the range 1 - 4 mg/ml using a liquid spray method. Each substrate was subject to various successive steps of spray to increase the film thickness. Depending on the number of spray steps, and the dilution of the initial suspension, the final GO film thickness was found in the range from 10 nm to 1 pm. The film resulted by liquid spray process was subjected to controlled evaporation of the solvent (water). Evaporation took place following different ways in various experiments, such as physical evaporation at room temperature for a period between 1 and 24 hours to remove the solvent (water) or alternatively dehumidification took place by placing the substrate in a temperature controlled chamber or alternatively irradiation took place either immediately after spraying or at any time after spraying, i.e. at various stages of evaporation of the water, achieving simultaneous drying and reduction of GO to rGO by irradiation. Irradiation of a GO film which has not been fully dehydrated leads to a better adhesion of the rGO film on the surface of the fibers of fabrics and other substrates than the adhesion found in the case of irradiation of a completely dry GO film.

[0049] After the GO film deposition onto the fabric, the irradiation process takes place. In some embodiments, the irradiation process is described by the following steps: A laser beam of an appropriate wavelength (for example at 1064 nm) is focused onto the substrate at any angle with respect to the perpendicular direction in the plane of the substrate. In some embodiments, by using a galvo-mirror system, various irradiation schemes can be selected. For example, either continuous scanning to prepare a conductive graphene film on a continuous surface or by scribing specific conductive motifs on the fabric following a preselected pattern to create electrodes with desired geometric characteristics. The laser energy density takes values in the range of 1 to 12 J/cm² or alternatively in the range of 2 to 8 J/cm² or alternatively in the range of 4 to 6 J/cm². The values of the laser energy density depend on the thickness of the GO film that has been pre-deposited, the type of dispersion medium (e.g. water) and the fraction of evaporated water/solvent before the irradiation process. The above-mentioned ranges of the energy density values have been achieved using a diameter of the beam spot size in the range of 0,05 to 3 cm without affecting the quality of the product (e.g. graphene conductivity and structure). Preferably, the diameter of the beam spot size should be at the upper limit of the above range (> 1 cm in diameter) in order to increase the graphene scribing speed on the substrate/textile by a moving beam or through moving the substrate/textile. Experimental tests that have been performed have shown that higher values of energy density are either destructive to the underlying substrate e.g. by destroying the fibers of the textile/fabric or can cause a crater at the irradiation point leading to non-uniform coating; hence reducing conductivity.

[0050] In some embodiments of the present invention, irradiation occurs either directly, as the exits the laser device or by waveguiding the laser beam through an optical fiber, where at the exist of the fiber a lens, with suitable focal length and features, is attached. Based on the results of the present invention, irradiation through an optical fiber is preferable as the intensity profile of the laser beam attains a “flat-top” shape which endows to the beam profile a nearly constant intensity distribution over the entire irradiated spot area. In contrast, in the absence of the optical fiber, the beam profile has the conventional “Gaussian” type shape. In the case of using an optical fiber for waveguidng the laser radiation, s single pulse per unit area is able to convert the initial GO film into homogeneous graphene over the entire spot area. This is important because this condition relaxes the need for pulse overlap (for Gaussian beams) during the scanning of the substrate by the laser beam. The results of the present invention have also shown that the quality of the graphene does not deteriorate with additional re-irradiation of the same area with 2 or 3 pulses. Based on these findings, the irradiation takes place with one pulse, preferably with an overlap in the diameter of the imprint (laser footprint) of 60% to 10% with a preference for the overlap in the range of 30% to 10%.

[0051] The quality of the reduced graphene oxide in the different experiments/embodiments was examined by Raman spectroscopy and scanning electron microscopy (SEM). To enhance reliability in the evaluation of the results obtained by these two techniques, we have followed the protocols described in "Good Practice Guide #145: Characterization of the Structure of Graphene, NPL, 2017" and "ISO ITS 21356-1:2021 Nanotechnologies - Structural characterization of graphene - Part 1: Graphene from powders and dispersions". This is an important step for the reliable graphene characterization, because the vast majority of graphene- on-textile obtained by various methods of the prior art, the putative “graphenes” on textile, do not fulfill the criteria of the above Good Practice Guides. It is generally accepted that the reduction of GO through the removal of oxide groups can be achieved at various qualities, which depends on the percentage of oxygen (O) atoms remaining after reduction as well as the percentage of carbon (C) atoms with sp² hybridization. The reduced graphene oxide or alternatively the graphene, referred to in the claims of the present invention, refers to graphene having a structure and atomic arrangement that satisfies the specifications described in Good Practice Guide #145, cited above in this paragraph.

[0052] An embodiment includes the application of the present invention namely, graphene growth on a polyester/white fabric. Figure 3 shows a section of a commercially available polyester fabric in three snapshots, (A) prior to GO coating, (B) coated with GO after controlled water drying, and (C) after irradiation at selected points, with a trace diameter of ~10 mm. The irradiation spots are shown as darker footprints or spots in figure (C), which indicate that reduction of GO to the conducting graphene took place.

[0053] Several Raman spectra were recorded and analyzed to get a statistically significant picture of the graphene structure/quality. A representative Raman spectrum of laser-reduced rGO is shown in Figure 3(D), which is compared to the Raman spectrum of GO before irradiation (shown by dashed line). The spectrum of the irradiated material (rGO) reveals the spectral features of graphene, which according to the Good Practice Guide #145 discussed in [0051]) exhibits much higher quality in comparison to the corresponding attempts for GO reduction in the prior art. The Raman bands (in the rGO spectrum) denoted by the letters “D” and “G” have low half-widths, while the “D” band also exhibits much lower intensity than the “G” band. The spectral changes of the irradiated product with respect to the GO characteristics, demonstrate a very high quality of the graphene crystal after irradiation. The appearance and intensity enhancement of the band denoted as “2D” is a characteristic feature of the formation of graphene-like structures with few graphene layers. Analyzing a large number of spectyra, it was found that the intensity ratio of the G and 2D bands lies in the range of 0.6 < I^2D / I^G < 0.8 confirming that the generated structures have high-quality graphene characteristics, while no amorphous or / nanocrystalline graphite structures are observed, as is the typical case in the prior art.

[0054] Scanning electron microscope (SEM) images are shown in Figures 3(E) and 3(F). The first image shows a picture of a number of fibers revealing almost complete coverage of their surface by graphene layers. A detailed illustration of the texture of the graphene film covering the fabric fibers is provided in the higher magnification of Figure 1(F). These images demonstrate the conformal coverage and very good adhesion of the graphene film on the fiber, achieved by the method of the present invention.

[0055] An embodiment of the present invention relates to the growth of graphene on polyester/blue fabrics (of a different weaving motif than the previous example). Following the procedures for GO coating and irradiation of the present invention, the results of Raman spectroscopy and scanning electron microscopy demonstrate again, as in the previous embodiment, the growth of very high-quality graphene on the fabric surface. Figure 4 shows a section of commercially available polyester fabric in three snapshots, (A) before GO coating, (B) coated with GO after drying of the water and (C) after irradiation at selected spots. Figure 4D shows a typical Raman spectrum which also shows high quality graphene with low defect density and few-layer graphene characteristics. Figures 4E and 4F represent different electron microscopy magnifications, which reveal the complete coverage of the fibers with graphene, as well as the excellent adhesion of rGO on the fibers.

[0056] Another embodiment of the present invention relates to the growth of graphene on polypropylene/white fabrics. Following the procedures for GO coating and irradiation of the present invention, the results of Raman spectroscopy and scanning electron microscopy demonstrate again, as in the previous embodiment, the growth of very high-quality graphene on the fabric surface. Figure 5 shows a section of commercially available polypropylene fabric in three snapshots, (A) before GO coating, (B) coated with GO after water drying and (C) after irradiation at selected spots. Figure 5D shows a typical Raman spectrum which also shows high quality graphene with low defect density and few-layer graphene characteristics. Figures 5E and 5F represent different electron microscopy magnifications, which reveal the complete coverage of the fibers with graphene, as well as the excellent adhesion of rGO on the fibers.

[0057] Further proposed applications of the present invention relate to the growth of graphene on fabrics of different types, such as polypropylene, polyamide, cotton. Following the procedures for GO coating and irradiation of the present invention, the results of Raman spectroscopy and scanning electron microscopy demonstrate again, as in the previous embodiment, the growth of very high-quality graphene on these types of fabrics as well. The results are shown in Figure 6 (for polypropylene/blue fabric), Figure 7 (for polyamide) and Figure 8 (for cotton).

[0058] In some embodiments of the present invention, irradiation of the GO film (on polypropylene and polyester) was performed at a stage where the solvent (water) was partially evaporated. In this case, the irradiation plays dual role, on one hand part of the laser energy is used to evaporate the solvent (water) from the GO film, while simultaneously part of the energy is used to reduce the GO towards graphene with structural and electrical properties similar to those of the previous embodiments. The irradiation of the film in this embodiment resulted in a graphene film on the fabric fibers showing even better adhesion, in relation to the irradiation of the completely dry GO film, as observed by the reduced loss of material (graphene) from the fabric surface after the process.

[0059] Besides to the embodiments described above, using conventional fabrics, additional examples of embodiments of the present invention were implemented to develop graphene on the surface of technical textiles/fabrics. Typical examples are provided below.

[0060] An embodiment of the present invention relates to the growth of graphene on woven glass-fiber textiles with the procedure of the current invention. The results of Raman spectroscopy and scanning electron microscopy show the growth of high-quality graphene also on the surface of the glass fibers. The results are presented in Figure 9. The SEM images reveal complete and uniform coverage of the fibers with the graphene layer. Similarly, Figure 10 shows the results for GO reduction on fabrics produced by weaving carbon fiber and Kevlar fiber (woven CF/Kevlar-fabric) where they also exhibit high-quality graphene on the fiber surface.

[0061] For all examples of the embodiments where the process of the present invention is applied, an evaluation of their electrical properties took place, and in particular the surface resistance (R_s, sheet resistance) which is a quantity inversely proportional to the conductivity, was measured. In conventional fabrics, the values of the R_s parameter range in the interval 0.8 - 1 k£l sq'¹, i.e., lower than the R_s values reported in all previous studies in the prior art, including literature articles and relevant patents. The value of surface resistance in the technical fabrics of the current invention, was found to be much lower than that of the conventional fabrics, i.e. in the range 0.04 - 0.2 kQ sq'¹.

[0062] In some embodiments the method of the present invention has been implemented on an R2R pilot line, as illustrated in Figure 1. Fabrics with a wide of 30 cm belonging to the category of narrow fabrics were used. Rolls of fabrics were placed on the R2R line. The process includes sequentially the following: the GO dispersion spraying process, controlled drying to remove the solvent (water) and irradiation by the laser beam with the use of a galvo-mirror system. This process results in the preparation of conductive paths on the fabric surface in the areas where the laser spot was scanned. The automated process determines the speed of movement of the fabric roll by considering the synchronization of the spraying, drying and irradiation processes. Different implementations were tested where the fabric substrate coating and laser processing processes were performed either in a continuous or an intermittent production mode.

[00063] The process disclosed in the present invention provides the ability to produce flexible substrates with unique mechanical, electrical, optical and physical properties, to enable potential applications in various industrial fields, such as defense, space, naval, aeronautics, electronic devices and beyond. Potential applications of the present invention are related to multi-functional fibers, smart fabrics, e-textiles, electrodes for energy conversion and storage through flexible supercapacitors and batteries (energy storage), flexible electronics, flexible photovoltaics, wearable sensors, tactile sensors, artificial skin, membranes as filters for air pollution, face masks, coatings for corrosion protection, high temperature or flame protection, lightweight and conductive composites with enhanced mechanical properties, and combinations thereof. References

[Pl] WO 2020/237296 Al (ROYAL MELBOURNE INSTITUTE OF TECHNOLOGY) 03.12.2020.

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[2] Mohammad Shateri-Khalilabad, Mohammad E Yazdanshenas, Fabricating electroconductive cotton textiles using graphene, Carbohydr. Polym. 96 (2013) 190-195; 10.1016/j.carbpol.2013.03.052.

[3] Nazmul Karim, Shaila Afroj, Andromachi Malandraki, Sean Butterworth, Christopher Beach, Muriel Rigout, Kostya S. Novoselov, Alexander J. Casson and Stephen G. Yeates, AZZ inkjet-printed graphene-based conductive patterns for wearable e-textile applications, J. Mater. Chem. C. 5 (2017) 11640-11648; 10.1039/C7TC03669H.

[4] Nazmul Karim, Shaila Afroj, Sirui Tan, Pei He, Anura Fernando, Chris Carr, and Kostya S. Novoselov, Scalable Production of Graphene-Based Wearable E-Textiles, ACS Nano. 11 (2017) 12266-12275; 10.1021/acsnano.7b05921.

[5] Amr M Abdelkader, Nazmul Karim, Cristina Valles, Shaila Afroj, Kostya S. Novoselov and Stephen G. Yeates, Ultraflexible and robust graphene supercapacitors printed on textiles for wearable electronics applications, 2D Mater. 4 (2017) 035016; 10.1088/2053-1583/aa7d71.

[6] Shaila Afroj, Nazmul Karim, Zihao Wang, Sirui Tan, Pei He, Matthew Holwill, Davit Ghazaryan, Anura Fernando, and Kostya S. Novoselov, Engineering Graphene Flakes for Wearable Textile Sensors via Highly Scalable and Ultrafast Yarn Dyeing Technique, ACS Nano. 13 (2019) 3847-3857; 10.1021/acsnano.9b00319.

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Claims

1. A method of manufacturing graphene on a surface, comprising at least the following steps:

- depositing a layer of graphene oxide (GO) on a surface with a layer thickness between 10 nm and 1 pm or preferably in the range of 10 nm to 500 nm.

- irradiating the graphene oxide layer by application of a laser beam from a laser source focused on the substrate at any angle with respect to the perpendicular direction of the substrate, wherein the laser source has the following characteristics:

(i) a wavelength in the range 800 nm to 3 pm or alternatively in the range 1 pm to 3 pm, or preferably in the range 1 pm to 2 pm,

(ii) pulse duration on the time scale from 50 psec (50x10‘⁶ s) to 30 msec (30xl0'³ s),

(iii) radiation energy density in the range 1 to 12 J/cm² or alternatively in the range 2 to 8 J/cm² or alternatively in the range 4 to 6 J/cm².

2. A method of manufacturing graphene on a surface according to claim 1, wherein the surface refers to a surface of a flexible substrate and preferably a fabric/textile, woven or non-woven, technical or conventional, and more preferably fabric/textiles made of polypropylene, or polyester, or polyamide, or cotton, or woven glass fibers, or carbon fibers, or synthetic fibers, or Kevlar fibers, or prepregs of carbon fibers, or prepregs of glass fibers, or prepregs of natural fibers (such as flax).

3. A method of manufacturing graphene on a surface according to any one of the preceding claims, wherein the laser beam is applied using an optical fiber to transfer the laser radiation onto the graphene oxide film for the purpose of reducing the graphene oxide.

4. A method of manufacturing graphene on a surface according to any of the preceding claims, wherein the deposition of the graphene oxide layer onto the substrate is performed by liquid spraying an aqueous suspension of the graphene oxide, with one or more successive liquid spraying steps followed by gradual removal of the solvent.

5. A method of manufacturing graphene on a surface according to claim 4, wherein the removal of the solvent of the film prepared from depositing an aqueous suspension of graphene oxide, is partial and takes place either by leaving the film at room temperature for a period of 1 to 24 hours or alternatively by placing it in a temperature controlled chamber or alternatively total removal of the solvent of the film takes place upon irradiation of the wet film, either immediately after spraying, or at any time after spraying, at various stages of water evaporation, achieving simultaneous drying of the film and reduction of graphene oxide to reduced graphene by irradiation, and better adhesion of the graphene oxide to the surface of the fabric fibers.

6. A method of manufacturing graphene on a surface according to any of the preceding claims, wherein the diameter in the laser beam spot (trace) on the substrate emerging from the optical fiber is in the range from 500 pm to 3 cm, or more preferably in the range from 2 cm to 3 cm.

7. A method of manufacturing graphene on a surface according to any of the preceding claims, wherein the irradiation takes place with a single pulse.

8. A method of manufacturing graphene on a surface according to any one of the preceding claims, wherein during the reduction of the graphene oxide to reduced graphene, an overlap takes place on the traces of the laser beam imprint, with the percentage of the overlap being between 90% to 10% of the diameter of the imprint, preferably for the overlap in the range from 30% to 10%.

9. A method of manufacturing graphene on a surface according to any one of claims 2 to 8, wherein the flexible substrate is adapted to a roll-to-roll (R2R) type production line for continuous processing.

10. A graphene-coated flexible surface manufactured using the method of any of claims 1 to 9.

11. A graphene-coated flexible surface according to claim 10 where the graphene sheet resistance is less than 1 k sq^-1 for conventional textiles/fabrics and less than 0.2 kQ sq'¹ for technical textiles/fabrics.

12. Use of a textile/fabric flexible surface of claims 11 or 12 to manufacture: flexible electrodes for energy conversion and storage applications (batteries, supercapacitors, photovoltaics), flexible electronic devices, touch sensors for use as artificial skin in robotic applications, lightweight and conductive composites with enhanced mechanical properties, membranes and filters for air/water purification, electronic garments capable of harvesting mechanical energy from the environment to empower sensors and portable/wearable devices, as well as garments/clothing providing electromagnetic radiation shielding, temperature regulation, fire protection, and combinations of the above.