Cork derived laser-induced graphene for sustainable green electronics

Agglomerated cLIG was firstly produced using a 50 W 10.6 μm CO₂ infrared laser (VLS3.5 Laser Platform by Universal Laser Systems, Inc.) and a 50 W 1.06 μm Nd:YAG fiber laser (PLS6MW Multi-wavelength Laser Platform by Universal Laser Systems, Inc.), both equipped with 2.0'' lens. A preliminary study to compare both laser systems were performed by varying laser parameters such as (a) power (adjustable up to 50 W in both lasers), (b) speed (adjustable up to 1.27 m s^−1 on CO₂ laser and up to 1.78 m s^−1 on fiber laser), and (c) focal point defocus (from 0.76 mm above focal point to 1.52 mm below). A fixed 1000 pulses per inch was established on the CO₂ laser, and a fixed frequency of 100 kHz was established for the fiber laser. Finally, after all laser parameters optimization, electrodes were produced with fiber laser using 5.5 W, 17.8 mm s^−1 and 1.52 mm below focal point.

LIG was characterized by SEM in a Hitachi TM 3030Plus tabletop workstation to examine the structure, morphology, and cross-section of the produced LIG, while carbon and oxygen content was evaluated by EDS. XPS was performed using a Kratos Axis Supra with monochromated Al Kα and Al Lα sources. The Al Kα source was running at 225 W and detailed scans were recorded with 5 eV pass energy. The Ag Lα source was running at 300 W and the respective detailed scans were recorded with 40 eV pass energy. CasaXPS was used for data analysis. Raman microscopy was carried out with a Renishaw® inViaTM Qontor® confocal Raman microscope equipped with a Renishaw Centrus 2957T3 detector and a 532 nm laser (green) operating at 10 mW. A 50× Olympus objective lens was used to focus the laser beam. All measurements were made with three scans of 10 s laser exposure. For electrical characterization, cLIG was engraved in 5 × 5 mm² squares and then silver ink contacts were placed on the ends of the squares. A multimeter was used for preliminary assessments, followed by resistivity measurements using BioRad HL5500 at room temperature to determine cLIG sheet resistances. cLIG surface wettability was evaluated through contact angle (CA) measurements with Dataphysics OCA15 plus using the sessile drop method. Briefly, 3 μl of electrolyte (H₂SO₄) droplets were placed on the substrate surface and CAs were measured for 15 min, to assess the spreadability of the droplet. The CA analysis was performed with embedded software (SCA 20) using Laplace–Young approximation model with at least three measurements per sample.

For the MSCs development an extra process was added, a pre-treatment of wax to the cork, followed by LIG synthesis and subsequent MSC fabrication/characterization, described below.

2.3.1. Pre-treatment of the agglomerated cork substrate

2.3.2. MSCs production and electrochemical characterization

Agglomerated cork sheets were used as the substrate for LIG production. This composite material, made from the agglomeration of natural cork granules with binding agents, results in different size grains being compressed into each other while adhering together. Using agglomerated cork instead of pristine cork brings scalability for industrial use, being currently one of the most marketable materials in the world, allowing the production of multiple substrate shapes, from millimeter sheets to bulky pieces. Also, being a composite material with sophisticated properties can be beneficial for the desired type of application, especially for flexible wearables. Figure 1 shows the substrate morphology, and as expected, coming from a natural source to a processed sheet, different dimensions of granules are observed.

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The substrate presents different natural cork granules randomly oriented, and its cellular structure is composed of hexagonal and pentagonal structures as confirmed in the SEM micrographs (figure 1) [32, 40]. It can also be observed that the alveoli walls of the agglomerates are ∼1.1 µm thick. The characteristic morphology of the cork is of utter importance, as it enables the production of 3D porous LIG exhibiting high surface area (inside the cork cells and even on the walls). These properties are highly beneficial for electrochemical applications, as they may allow an enhanced charge transfer rate in the LIG electrodes, large available surface area and, consequently, a good energy storage performance [26]. Therefore, the study of cLIG synthesis on this substrate started with a systematic optimization of the laser system, by varying parameters such as power, speed, and focal plane defocus on cork substrate. Regarding the laser wavelength influence, as already reported, infrared lasers mainly result in photothermal conversion, which means that covalent bond breaking occurs by local high temperature induction. Thus, the laser spot size and laser pulse duration influence the LIG formation [41, 42]. These outcomes vary using lasers with different wavelengths, in which a higher resolution laser, such as the fiber laser, reflects in a smaller spot size and a shorter pulse duration than the CO₂ laser [43].

In this work, a fiber laser, with a 1.06 μm wavelength, is reported for the first time, to the best of the author's knowledge, for LIG production on cork. To identify the conditions of power and speed that result in a homogeneous LIG formation, the laser was focused on the substrate's surface at its focal point. As reported in the literature, higher laser powers increase LIG thickness, and lower laser speeds result in better quality LIG exhibiting an improved electrical conductivity [24, 31]. It is clear that cork is very sensitive to the variation of the laser conditions since 1% variation in power and speed is enough to promote carbonization or nothing at all, which can be visually inspected in figure S1. The results suggest that laser power cannot be increased too much at lower scanning speed since it results in undesirable outcomes such as laser ablation, thermal damage of the surrounding zones of established pattern, and formation of brittle powder LIG (figure S2(a)). For lower power and higher scanning speed, no changes were visually identified on the substrate or LIG formation did not occur on the entire surface of the defined square. This can be explained by the low energy density delivered to the substrate unable to induce a photothermal conversion [17] (figure S2(b)). After establishing the proper power and speed conditions for fiber laser (5.5 W @ 17.8 mm s^−1), a study on the influence of focal point defocus on LIG synthesis was conducted. As reported by Tour et al [29], when the laser beam is in a defocused position, the spot size increases, subsequently laser spots overlap, and a multiple lasing effect can be achieved [30]. The multiple lasing method suggests that in some cases LIG formation occurs by two steps: conversion of a carbon precursor into amorphous carbon followed by a conversion to graphene upon subsequent lasing [25, 31]. Using the defocusing method, the process is simplified by only one laser passage, resulting in multiple exposures in the overlapping area [20]. Thus, the effects of defocusing on the characteristics of LIG were explored in this work. The structures obtained by defocusing the laser, from 0.76 mm above and 1.52 mm below the focal plane, were characterized through SEM, as shown in figure S3. Variations in defocusing distances and laser wavelengths result in clearly different LIG morphologies.

These results suggest that cork photothermal conversion into cLIG is in good agreement with the above explanation about the defocusing method. Using the fiber laser at the focal plane (Z = 0.00 mm) cork receives a larger amount of energy per unit area, resulting in depth powerful engravings onto the substrate instead of a uniform and continuous carbonization.

By moving the substrate far from the focal plane, in both positive and negative directions of the Z plane, the laser spot size expands over a larger area. This leads to an overlapping of the laser spots with a multiple lasing effect, resulting in better quality LIG [16, 44]. In addition, by defocusing 1.52 mm below the focal point, it produces less depth of focus, and the initial porous honeycomb flake structure of cork (figure 1) prevails even after irradiation (figure S3). Thus, conferring the porous cLIG produced by fiber laser a hexagonal and pentagonal structure with an extensive accessible surface area for posteriorly electrochemical reactions.

Raman spectra of cLIG obtained from fiber laser, show the three prominent peaks of graphitic materials, ranging from 1000 to 3000 cm^−1. Two distinctive first-order peaks are presented at ∼1350 cm^−1 (D peak) and ∼1580 cm^−1 (G peak), and a second-order 2D peak is presented at ∼2700 cm^−1, showing the presence of graphitic carbon [45]. The D band can be related to the formation of defects, vacancies and bent sp² bonds, while the G band results from the first-order inelastic scattering process being highly sensitive to the number of layers present in the LIG [46]. The 2D band corresponds to the second-order Raman peak, which results from two photon lattice vibrational process [47]. When relating the SEM images with Raman spectra and sheet resistances, there is a correlation between Raman peaks intensity, the conductivity of the cLIG and structural morphology obtained (figure S4). By defocusing substrate from the focal plane, cLIG samples show a decrease in D peak relative intensity, meaning that fewer defects are induced during LIG conversion, which is in accordance with previously reported works [29, 32].

In terms of sheet resistance, for LIG induced by IR (10.6 μm), visible (450 nm), and UV lasers (355 nm) on cork, there are reported values in the range 46–115 ohm sq^−1 [32–34]. In the current study using a fiber laser (1.06 μm) with defocusing the distance from the focal plane, cLIG conductivity improves significantly, showing an incredible enhancement in LIG quality with a sheet resistance almost ten times lower (R_s ∼ 9.86 ± 0.12 ohm sq^−1). This significant improvement, as already mentioned, is essentially due to the combination and interaction of the type of laser with the substrate used and the laser conditions applied.

XPS was used to characterize cork prior laser irradiation and cLIG conversion, supporting the results above (figure S5). Regarding high-resolution spectrum of the C 1s, cLIG produced revealed the presence of graphene structures. Regarding the variability of the XPS spectra and partial oxidation observed, a factor to be considered is the influence of the place where the measurement is performed. In figure S6 it is shown that in fact the most superficial zones of the sample are more oxidized, while an inner zone results in a LIG with lower oxygen content.

In this work, fiber laser shows its promising application for converting carbon-based materials into LIG, taking advantage of its interaction with the substrate. As CO₂ lasers (with a 10.6 μm wavelength) are the most commonly used systems for LIG conversion on cork substrates [29, 33], a comparison in terms of structural morphology and quality obtained with this system and the fiber laser is described in the supporting information. It was observed that LIG morphology obtained with 10.6 μm CO₂ and 1.06 μm fiber lasers are quite different. In samples produced in the fiber laser a less ablative effect is observed, and better uniformity is achieved, contrarily to the fragile and powdery cLIG microstructure obtained from the CO₂ laser.

A comparison of the optimized laser conditions and resultant LIG properties for both lasers is briefly shown in figure 2. In figures 2(a) and (b) it is visible a significant difference in the LIG morphology produced with different lasers' wavelengths. LIG induced on cork present structures similar to porous and powdery honeycomb flakes, as previously described.

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It seems that cLIG produced in the fiber laser is thicker (150 μm), exhibiting a continuous and uniform layer, with a less defective appearance compared to the LIG produced by the CO₂ laser. This can be supported and confirmed with the Raman spectra and ratios from figures 2(c) and (d). As already mentioned, the D peak corresponding essentially to the formation of defects, decreases when switching from the CO₂ to the fiber laser.

It was demonstrated that cLIG quality and the low sheet resistance makes this material attractive for circuit conductive tracks, electrodes, sensors, or even full devices. In the following, and as a proof of concept, w-cLIG electrodes prepared at optimized conditions were tested as a flexible and sustainable MSC.

After the optimization process, cLIG was used to design interdigitated electrodes for MSC applications. However, during MSC preparation it was verified a random spreading when the electrolyte was drop-casted onto the MSCs' active area. Hence, there was no control in defining the active area where the electrolyte should be deposited nor was it guaranteed that the electrolyte dried on the substrate surface where the electrodes were engraved. Consequently, a proper assessment of the device electrochemical properties was not possible. To circumvent this issue, a wax-based pre-treatment on cork was performed. The wax-based ink can be absorbed throughout the entire cork bulk volume when heated, occupying the free spaces between the granules. In the supporting information is described a brief characterization of the cLIG induced into waxed-cork substrate (figure S7).

CAs of untreated and post-treated substrate surfaces were measured before laser irradiation, to assess wettability of different surfaces over time (table S1). The wax pre-treatment on cork resulted in a hydrophobic interaction between electrolyte and surface of the LIG/substrate, solving the uncontrolled spread of the electrolyte. Furthermore, compared with untreated substrate, only a minimal reduction in sheet resistance was observed with an R_s of approximately 7.5 ohm sq^−1 (figure S8), proving that this step is not relevant for conductivity improvement but an important approach for MSC assembly. The possibility of maintaining good properties of LIG and improving other aspects such as the surface wettability and adhesion, makes this treatment very innovative and promising.

After this process, MSC electrodes based on w-cLIG were patterned and integrated in flexible MSCs. A schematic representation of the MSC fabrication process is shown in figure 3.

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For the electrochemical performance assessment, w-cLIG flexible MSCs were fabricated and tested using different electrolytes. Comparison of the correspondent areal and volumetric capacitances in function of scan rate (from 5 to 500 mV s^−1), obtained from CV curves are shown in figure S9. Although using the same electrode dimensions for all the MSCs produced, different CV curves were obtained suggesting different electrolyte-LIG electrodes interaction. By increasing the scan rates, 'quasi-rectangular' to a 'fish' shape curves are identified for all devices produced, which is more prominent on w-cLIG MSCs with PVA-H₂SO₄. This electrolyte is well-known to remain a viscous gel with an apparent higher water content upon drying, which enhances the electrochemical performance for graphene based MSCs [48, 49]. Since the PVA-H₂SO₄ electrolyte rendered w-cLIG MSC the best electrochemical performance among the tested electrolytes, further characterization is shown in figure 4. The GCD curves of the MSC at different current densities of 0.005, 0.01, 0.02, 0.05 and 0.1 mA cm^−2 were measured to determine the electrochemical performance of the device over time (figure 4(b)).

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Pseudosymmetrical response curves were observed, which mean that its potential changes linearly with time due to its stable and reversible properties.

In figures 4(c), a gradual decrease in MSCs capacitance is observed with the scan rate, where the maximum areal capacitance (C_A) reached was 1.35 mF cm^−2 (with a corresponding C_V of 103.63 mF cm^−3 for w-cLIG with 130 μm of thickness) at a scan rate of 5 mV s^−1, respectively.

This phenomenon can be explained by the ion diffusion mechanisms in the LIG-MSCs. At lower scan rates, electrolyte ions have more time to move and to penetrate deeply into the available pores in the LIG structure, which results in a higher areal capacitance [50]. At higher scan rates, a loss of efficiency of ions to infiltrate the LIG occurs, leading to a reduction on capacitance [51–53]. Therefore, figure 4(d) shows the capacitance retention of the MSCs using PVA-H₂SO₄, which confirmed device excellent stability after 10 000 cycles at a current density of 0.05 mA cm^−2. During the cyclability test, it was observed a slight increase in capacitance retention with a tendency to stabilize over operation time like already reported [54]. A loss of MSC capacity is observed mostly due to parasitic irreversible electrochemical reactions that occur in the first phase of the cyclability test. Over a number of cycles, the electrodes adapt, and the pores open within its structure. This enhances the wettability of the electrolyte within the LIG structure, promoting greater activation of the ionic diffusion paths and electrochemical reactions. Also, unwanted species of the electrolyte are consumed over a larger number of cycles, tending to increase the supercapacitor capacity until it becomes stable. Therefore, throughout the charge and discharge process of the device, a gradual increase of the capacitance retention is observed. This is not a common electrode behavior, however it was already observed in other nanomaterials based systems [55]. For this reason, w-cLIG electrodes exhibit long-term cyclical stability, which proves to be an excellent material with promising functionality in MSC applications.

To study the flexibility of the device and corresponding electrochemical performance, bending tests were carried out as presented in figure 5. Comparing the flat and bent MSC (with 35° angle), an increase in charge–discharge time in the GCD curve is observed. This behavior can be related with the deformation submitted to its structure and an opening of the spongy pores, which suggests an enhancing of the charge storage performance with expansion of the active area [56]. Also, when the MSC is bent, it benefits from the infiltration of the electrolyte through longer paths leading to a better charge carrier distribution.

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In figures 5(c) and (d), the areal and volumetric capacitances were plotted at different current densities, calculated from the GCD curves using equations (3) and (4) from supporting information.

Flat conformation yields a C_A of 1.43 mF cm^−2 at 0.1 mA cm^−2 (C_V corresponding to 109.62 mF cm^−3) and bent MSC resulted on a C_A of 2.20 mF cm^−2 at 0.1 mA cm^−2 (C_V corresponding to 169.23 mF cm^−3). These values confirm the suggestions above, where it was mentioned that a bent conformation increases LIG paths and facilitate ionic diffusion, consequently yielding a better capacitance.

In addition, areal and volumetric energy, and power densities of the flat and bent MSCs were evaluated, and the Ragone plots are shown in figure 6. Greater areal and volumetric capacitances obtained with the bent MSC reflects in better energy-power characteristics, which is in accordance with results demonstrated above. Flat w-cLIG MSC exhibits an E_A of 0.13 μWh cm^−2 and an E_V of 9.74 μWh cm^−3 at the current density of 0.1 mA cm^−2. For the bent conformation, the MSC was able to deliver the highest E_A of 0.20 μWh cm^−2 and an E_V of 15.04 μWh cm^−3 at the same current value. The areal power densities resulting from flat and bent conformation reached 80 μW cm^−2 and a corresponding P_V of 6154 μW cm^−3 at the same current value.

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By focusing this work on the sustainability and simplicity of the processes used, it is shown that exploring the use of renewable materials to develop electronics can achieve equally good or even better performances than what is currently found in the literature (table 1). Similar works have already reported the conversion of natural carbon substrates into LIG electrodes, using laser sources ranging from UV to IR wavelengths. Thus, the properties of LIG can be manipulated according to each target application, achieving tailored electrical, physical, chemical resistance, and structural properties. However, different substrates can have limitations in terms of compatibility radiation absorption for specific wavelength lasers. This work introduces the importance of exploring the application of an alternative laser source (fiber) for LIG synthesis in an agglomerated cork, without the need to use doping strategies or pre-treatments on the substrate. In contrast with that, the properties of the electrodes vary and, a porous w-cLIG with excellent properties was achieved with greater conductivity and fewer defects. Also, the optimized laser conditions, the electrodes design/geometry can also influence LIG properties. Overall, LIG electrodes induced on cork substrates with a fiber laser have the potential for low-cost MSCs electrodes fabrication compared to previous LIG reports produced on natural substrates by DLW, with similar electrochemical performances. It is important to highlight the fact that different lasers used on similar substrates do not always achieve the same results and therefore, the obtain metrics and consequent comparisons are not always straightforward.

MSC development on cork substrates is just one example of one possible application for LIG. It holds a great potential for future technology not only due to the simplicity of optimizing the electrodes but also due to the intrinsic properties of the substrate itself. Additionally, these metrics can be translated to other systems, such as transistors, sensors and general circuitry or conductive paths. Additionaly, it is still possible to explore multiple lasing and doping strategies and other pre-treatments to the cork substrate to obtain better quality LIG, with enhanced properties adapted to the desired application. With this work, it is shown that LIG is to be a good candidate for further technological advances in electronics, due to its low sheet resistance, adaptability, low cost, and simplicity.