Laser-Induced Graphitization of Cellulose Nanofiber Substrates under

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Laser-induced Graphitization of Cellulose Nanofiber Substrates under Ambient Conditions Sanghee Lee, and Sangmin Jeon ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04955 • Publication Date (Web): 11 Dec 2018 Downloaded from http://pubs.acs.org on December 18, 2018

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ACS Sustainable Chemistry & Engineering

Laser-induced Graphitization of Cellulose Nanofiber Substrates under Ambient Conditions Sanghee Lee and Sangmin Jeon* Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), 77 Cheongam-Ro, Pohang, Gyeongbuk, Republic of Korea

Abstract A CO2 laser engraver was used to synthesize conductive graphitic carbon directly on cellulose nanofiber (CNF) substrates under ambient conditions. CNFs were prepared via a TEMPO (2,2,6,6-tetramethylpiperidin-1-oxyl radical)-mediated oxidation reaction of bleached pulp, and a porous paper or a transparent film was obtained based on the drying conditions employed. Laser irradiation on a porous CNF paper led to the formation of amorphous carbon owing to an increase in temperature. Subsequent lasing converted the amorphous carbon to conductive graphitic carbon. The conductivity of this carbon increased from 3 µS/cm to 60 mS/cm as the number of irradiations increased from one to four. Although the CNF paper was converted to graphitic carbon by means of multiple lasing, graphitic carbon was obtained for the CNF film by a single run of lasing owing to its very low oxygen permeability. The conversion of the CNF substrates to graphitic carbon under ambient conditions was attributed to the presence of sodium in CNFs. A control experiment using a CNF in which sodium was replaced with hydrogen demonstrated that only amorphous carbon was produced by laser exposure. Keywords: CO2 laser, Laser-induced graphitization, Ambient condition, Cellulose nanofiber, Conductive carbon pattern * Author to whom correspondence should be addressed. E-mail: [email protected] 1 ACS Paragon Plus Environment

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Introduction Graphene-based materials, which are characterized by excellent mechanical, thermal, and electrical properties, have received considerable attention due to their wide range of promising applications, such as electronic devices and energy storage devices.1,2 Graphene is generally produced by mechanical exfoliation or chemical vapor deposition,3,4 but these techniques require complex processes or equipment, which hinder mass production and reduce cost effectiveness. The laser-induced graphitization (LIG) technique is a good alternative to conventional methods and converts a substrate photothermally to graphitic carbon5 although it has more defects and higher electrical resistivity than graphene. Because various patterns can be fabricated easily over a large area by using LIG, the methods has been used to fabricate sensors and supercapacitors.6-9 The applications of LIG, however, are limited to materials such as high-temperature engineering plastics, including polyimide,10-13 and polyethersulfone.14,15 Recently, Tour et al. reported that LIG can be applied to any material that can be converted into amorphous carbon using the multiple lasing method.16,17 They demonstrated the application of the technique to a wide range of materials, including natural biodegradable substances, to convert them into graphitic carbon under ambient conditions. However, they could apply LIG to cellulose only in inert atmosphere or in the presence of fire retardants such as ammonium polyphosphate. Otherwise, cellulose would simply be burned to ashes because thermally weak cellulose was ablated by laser irradiation before it could be converted. Considering cellulose is the most abundant biopolymer on earth and possesses excellent mechanical properties,18 it is important to develop a method for applying LIG to cellulose under ambient conditions to expand its range of application. In the present study, we demonstrate the first experimental result pertaining to the conversion of cellulose nanofiber (CNF) to graphitic carbon by using LIG under ambient conditions. CNF was obtained by TEMPO-mediated oxidation of bleached pulp, and porous 2 ACS Paragon Plus Environment

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papers or transparent films were produced based on the drying conditions employed. Interestingly, the CNF film was converted to graphitic carbon by single lasing, whereas the CNF paper was converted to graphitic carbon only by multiple lasing. We found that the presence of inorganic sodium and the oxygen permeability played a key role in converting CNF to graphitic carbon.

Experimental Materials and chemicals Bleached Southern pine kraft pulp sample (Alabama River Cellulose) was obtained from Kuk-il Paper Co. (Seoul, Korea). TEMPO (2,2,6,6-tetramethylpiperidin-1-oxyl radical), sodium bromide, and sodium hydroxide solution were purchased from Sigma-Aldrich (St. Louis, MO, USA), and sodium hypochlorite solution (8 wt.%, NaClO) was purchased from Junsei (Japan). Deionized (DI) water (18.3 M ⋅ cm) was obtained using a reverse osmosis water system (Human Corporation, Korea).

Fabrication of cellulose nanofiber (CNF)-based papers and films 6.85 g of bleached pulp was dispersed in 400 mL of deionized water and stirred overnight. A solution containing 85.7 mg TEMPO in 12 mL of deionized water was added to the pulpdispersed solution. Subsequently, 0.857 g NaBr in 12 mL of deionized water was added to the solution. The solution was stirred vigorously for several minutes with the addition of 36.46 g NaClO. Solution pH was adjusted to 10 during the oxidation by adding 1M NaOH solution, and unreacted chemicals were removed by subjecting the solution to centrifugation several times at 12000 rpm for 15 min. TEMPO-mediated oxidation selectively produced sodium carboxylate groups (COONa) on the C6 carbon of each glucose unit in cellulose without 3 ACS Paragon Plus Environment


affecting the original crystallinity of the CNFs.19 The resulting TEMPO-treated cellulose fibers were disintegrated mechanically by using a high-speed blender (42000 rpm) and dispersed in DI water to a concentration of 1 wt.%. Depending on drying processes, cellulose substrates with different morphologies can be obtained. A porous paper was obtained by drying the CNF solution in a vacuum freeze drier and subsequent pressing at 10 MPa to flatten it. By contrast, a transparent film was obtained by drying the solution under ambient conditions without pressing. Scheme 1 shows a schematic illustration of CNF substrate preparation and direct fabrication of laser-induced graphitic carbon patterns.

Scheme 1. Schematic of preparation of CNF paper and CNF film. Various shapes were patterned on CNF substrates by using a computer-controlled CO2 laser engraver. The chemical structure shows selective replacement of the hydroxyl groups attached to the C6 carbon of cellulose with sodium carboxylate groups after TEMPO-mediated oxidation. Direct writing of conductive patterns on CNF substrates by laser-induced graphitization A continuous wave CO2 laser engraver (NCG-980) with a wavelength of 10.6 μm was purchased from NC GLOBAL (Daejeon, Korea). Various patterns generated by means of computer-controlled design were fabricated using the laser at a power of 5 W and a scanning speed of 40 mm/s at the focal point. The beam size of the laser at the spot was 40 µm and the power density at the LIG process was about 105 W/cm2. The intensity, moving speed, and number of irradiations were adjusted to investigate the produced carbonaceous material. 4 ACS Paragon Plus Environment

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Amorphous carbon was obtained after single laser exposure, and graphitic carbon was obtained after multiple laser exposure. Depending on the number of laser exposures (N), the resulting carbon layer was denoted LIG-N.

Characterization The morphologies and crystal properties of CNF and graphitic carbons were confirmed using scanning electron microscope (SEM, JSM-7401F, JEOL), transmission electron microscopy (TEM, JEM-2200FS, JEOL) and X-ray diffraction (XRD, D/MAX-2500, RIGAKU). Raman spectra of LIG-N were measured using confocal Raman microscopy (alpha300RA Plus, WITec). Elemental compositions of LIG-1 and LIG-4 were confirmed using X-ray photoelectron spectroscopy (VG ESCALAB250, Thermo Fisher Scientific). Thermogravimetric analysis (TGA) was carried out using SDT-Q600 (TA Instrument) under air atmosphere at a heating rate of 10°C min-1. Electrical properties of LIG-N samples were characterized using Keithley 2636B sourcemeter. Dynamic light scattering (DLS) measurements were conducted using a Zetasizer Nano (Malvern Instruments) to measure the average length of the CNF.

Results and Discussion Figure 1a shows an optical image of a centrifuge tube containing 0.65 wt.% CNF solution in water. A stable CNF suspension was obtained owing to strong electrostatic repulsion among CNFs, which originates from the formation of negatively charged carboxylate groups in the TEMPO-mediated oxidation reaction. The measured zeta potential of the CNF solution was about -70 mV (see Figure S1a in supporting information). The TEM image in Figure 1b shows that the diameters of the CNFs were of the order of several nanometers, and their average length 5 ACS Paragon Plus Environment


of 1.15 m was measured using DLS. Various computer-controlled designs were patterned on the surface of a CNF substrate using a CO2 laser engraver. Figures 1c and 1d show optical images of a porous CNF paper before and after character patterning, respectively. The surface of the transparent CNF film was converted into graphitic carbon with the shape of a graphene structure (Figure 1e and 1f). Both patterns demonstrated that only the desired region was photothermally converted into graphitic carbon, and the area not exposed to the laser did not change. UV-Vis measurements showed that the transmittance of the resulting film was ~80% over the entire visible light region (see Figure S1b in supporting information). Young’s modulus of the CNF film was measured to be 2.24 GPa, which is similar to that of polyethylene terephthalate.

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Figure 1. (a) Optical image of 0.65 wt.% CNF solution in water. (b) TEM image of CNF. Optical images of (c) CNF paper, (d) graphitic carbon pattern on a CNF paper, (e) transparent CNF film, and (f) graphitic carbon pattern on transparent CNF film. Figure 2 shows scanning electron microscopy (SEM) images of CNF taken after different numbers of laser exposure on CNF papers. The bottom panels show magnified versions of the images in the top panels. As the number of laser irradiations increased, the degree of photothermal conversion of CNF to carbonaceous materials increased. The magnified SEM images show that submicrometer-sized carbon particles disappear and porous carbonaceous materials are formed as the number of laser irradiations increases. In contrast to conventional thermal conversion, which heats the entire CNF, laser irradiation converts only the focused area and leaves the out-of-focus area unaffected. The width and thickness of the pattern formed on LIG-4 were ~300 m and ~70 m (inset of Figure 2d, Figure 5a), respectively.

Figure 2. Scanning electron microscopy (SEM) images of carbonaceous material patterned on CNF papers after different numbers of laser exposure: (a) LIG-1, (b) LIG-2, (c) LIG-3, and (d) LIG-4. The bottom panels show magnified versions of the images in the top panels. The inset of (d) shows a cross-sectional SEM image of the sample obtained using LIG-4.

The chemical composition and structure of the carbonaceous materials produced by laser exposure were identified using various characterization methods. Figure 3a shows a highresolution TEM (HR-TEM) image of LIG-4. The spacing between lattice fringes (0.33 nm) corresponds to the (002) plane of graphitic materials. Crystalline structures of the carbonaceous 7 ACS Paragon Plus Environment


materials were further investigated using X-ray diffraction. A new XRD peak at 26.76° (full width at half-maximum of 400 °C to produce a more condensed polycyclic aromatic structure. The oxygen-containing functional groups in the aromatic char were removed at higher temperatures, and aromatic char was converted to graphitic carbon.22 Almost no residues remained at temperatures higher than 500 °C in case of the protonated CNF and bleached pulp, whereas thermally stable graphitic materials remained in the pristine CNF even at 850 °C, which confirmed that the presence of sodium suppressed the thermal degradation of cellulose. Figure 4d shows optical microscopy images of the pristine and protonated CNF papers after a star shape was laser-engraved on them. While the star on the pristine CNF was black in color, it was tanned brown on the protonated CNF. The SEM images of the patterned regions showed that graphitic materials remained on the pristine CNF, whereas negligible carbon residue remained on the protonated CNF owing to laser ablation or burning, which is consistent with the TGA results. Interestingly, laser irradiation on the pristine CNF paper induced the emission of white light,27 whereas laser irradiation on the protonated CNF paper simply led to its burning with smoke, indicating that the laser irradiation did not convert the protonated CNF paper to thermally stable graphitic carbon.



Figure 5. Optical images of top and bottom of CNF substrates after a star shape was laserengraved onto them with different laser powers, scanning speeds, and numbers of laser irradiation: (a) LIG-4 (5 W, 40 mm/s), (b) LIG-1 (5 W, 10 mm/s) on CNF papers, and (c) LIG-1 (5 W, 10 mm/s) on CNF film. Cross-sectional SEM images were obtained from each LIG pattern. (d) Current-voltage (I-V) characteristics of fabricated LIG-1 on CNF film (black square) and LIG-4 on CNF-paper (red circle). Inset shows the top-view image of the fabricated LIG-1. Laser-induced graphitization is affected by laser power and scanning speed. Figure 5a shows the optical image of a star-shaped LIG-4 on CNF paper at the laser power of 5 W and scanning speed of 40 mm/s. Although laser irradiation was conducted four times, graphitic carbon was produced only on the top surface of the CNF paper without any damage on the bottom surface. The decrease in scanning speed to 10 mm/s, however, damaged the CNF paper even with one exposure, and laser irradiation could not be repeated (Figure 5b). As described earlier, single laser irradiation on a CNF paper produced amorphous carbon, and only multiple irradiation could produce conductive graphitic carbon (see Figure S2 in the supporting information). By contrast, single laser irradiation on the CNF film with a laser power of 5 W and scanning speed of 10 mm/s produced conductive graphitic carbon without damage to the 12 ACS Paragon Plus Environment

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bottom side of the film (Figure 5c and d). Considering that the CNF film (~70 m) was thinner than the CNF paper (~170 m), it was expected that the bottom side of the CNF film would be more damaged than that of the CNF paper. The conversion of the CNF film into graphitic carbon by single lasing was attributed to the very low oxygen contents and air permeability of the CNF film. Because the CNF paper had a porous structure and contained air inside the pores, laser heating increased the temperature of the paper efficiently in the presence of sufficient. However, the CNF film, which is used as a food packaging material owing to its very low oxygen permeability, had a dense structure and suppressed air supply during laser heating. Figure 5d shows that the electrical conductivity of LIG-1 on the CNF film is higher than that of LIG-4 on the CNF paper.

Conclusions It has been known that LIG cannot transform thermally weak cellulose into conductive graphitic carbon under ambient conditions because cellulose is ablated by laser irradiation prior to conversion. However, we have demonstrated that two types of the cellulose substrates, CNF paper and CNF film, could be converted into conductive graphitic carbon under ambient conditions by using a CO2 laser engraver for the first time to the best of our knowledge. In the case of the CNF paper, cellulose was converted into conductive graphitic carbon by multiple lasing, whereas in the case of the CNF film, one lasing was adequate to convert the cellulose in it to conductive carbon owing to the low oxygen permeability of the CNF film, which suppressed thermal degradation of the CNF. It was found that the presence of sodium played a key role in converting the CNF to conductive graphitic carbon. Because CNF films are transparent and mechanically robust, the conductive graphitic carbon-patterned CNF has great potential for use in a wide range of applications such as flexible electronic devices and sensors. 13 ACS Paragon Plus Environment


Supporting Information Zeta potential of CNF suspension, transmittance of pristine CNF film, electrical properties of LIG-N samples, optical images of incandescent light from LIG process, FT-IR spectra of CNF substrates, and elemental composition of LIG-1 and LIG-4.

Acknowledgments This work was supported by the Industrial Strategic Technology Development Program (10070241, Fabrication of High Bulk Porous Composite Sorbent with Pulp Support for Removal of Oil and Heavy Metals by Wet-laid Process) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea).

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Table of Contents

Cellulose nanofiber substrates were photothermally converted into conductive graphitic carbon under ambient conditions using CO2 laser engraver.



Scheme1 82x32mm (600 x 600 DPI)

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Figure 1



Figure 2


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Figure 3 86x105mm (600 x 600 DPI)



Figure 4 86x42mm (600 x 600 DPI)


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Figure 5 78x70mm (600 x 600 DPI)


Laser-Induced Graphitization of Cellulose Nanofiber Substrates under

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