Laser Reduction of Zn-Infiltrated Multilayered Graphene Oxide as

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Laser Reduction of Zn-Infiltrated Multilayered Graphene Oxide as Electrode Materials for Supercapacitors Seung-Mo Lee,*,†,‡ Yong-Jin Park,†,§ and Jae-Hyun Kim†,‡

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Department of Nanomechanics, Korea Institute of Machinery and Materials (KIMM), 156 Gajeongbuk-Ro, Yuseong-Gu, Daejeon 34103, South Korea ‡ Nano Mechatronics, Korea University of Science and Technology (UST), 217 Gajeong-Ro, Yuseong-Gu, Daejeon 34113, South Korea § Department of Energy Science and Technology, Graduate School of Energy Science and Technology (GEST), Chungnam National University, 99 Daehak-Ro, Yuseong-Gu, Daejeon 34134, South Korea S Supporting Information *

ABSTRACT: We demonstrate that under laser irradiation a tiny amount of Zn infiltrated into the graphene oxide enables massive production of the multilayered graphene with high electrochemical performance. The infiltrated Zn was found to lead to significant changes in microstructures, morphology, and surface chemistry of the resulting multilayer graphene. As compared to the graphene directly produced from raw graphene oxide, the graphene produced from the Zninfiltrated graphene oxide showed a nearly four times increase in energy density. Additional electrochemical properties promised that the resulting graphene could be widely used as electrode materials for high-performance supercapacitor. KEYWORDS: metal infiltration, multilayer grapheme, carbothermic reduction, laser reduction, supercapacitors

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patterned reduced graphene oxide (rGO)23 and the modulation of the laser beam can further improve the reduction efficiency.14,24 Among laser sources available commercially, continuous wave CO2 laser with mid-IR wavelength (∼10.6 μm) is one of the cheapest tools. It has been reported that the GO can directly absorb incident mid-IR25 and functional groups existing on the GO tune the optical absorption.26 The absorbed IR rapidly increases the temperature of the GO by exciting the CC bond, yielding phonons. It leads to a rapid reduction of GO (removing functional groups existing on the GO) accompanied by a drastic increase in volume as well as specific surface area (SSA). Although the optimization of laser reduction conditions and modulation of laser apparatus could increase the SSA and the electrochemical performance of the resulting rGO, this approach likely reaches the limit of improvement soon. Even until today, numerous researches have mainly focused on the light source for higher reduction efficiency. Here we demonstrate that simple preconditioning processes of the raw GO prior to the laser reduction could secure great improvement in the electrochemical performance of the resulting graphene. It was observed that a tiny amount of

INTRODUCTION The research on supercapacitor is likely never terminated until the energy density of the supercapacitor far surpasses that of the battery.1,2 Although the battery chemically stores energy through electrochemical reactions and can exhibit high energy density, the supercapacitor physically stores charge in electrochemical double layers and can only achieve low energy density. The challenge is to develop electrodes for the supercapacitor with the higher accessible surface area than that of electrochemical capacitor electrodes while maintaining high electrical conductivity.3 The graphene-based electrode could be a good candidate in this respect because the graphene has a large surface area (2630 m2/g), high conductivity (∼1 × 103 S/cm), and strong chemical stability.4,5 At present, the graphene-based supercapacitor electrode can be produced by direct laser irradiation on the graphene oxide (GO),6 which is found to be more advantageous than any other chemical way.7−13 This one-step approach has been continuously investigated with great promise for commercialization. Various laser sources characterized by a wide range of wavelengths (from UV to microwave) have been found to reduce successfully the GO without any use of reduction agents.14−20 It has been also reported that the laser yields higher reduction efficiency, as compared to other high-intensity light sources like a flash lamp.21,22 Recent studies have demonstrated that the programmed focused laser beam can easily produce a © 2019 American Chemical Society

Received: April 3, 2019 Accepted: May 17, 2019 Published: May 22, 2019 3711

DOI: 10.1021/acsanm.9b00618 ACS Appl. Nano Mater. 2019, 2, 3711−3717

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ACS Applied Nano Materials

Figure 1. Schematic showing the synthesis route of the multilayered graphene using laser reduction. By the low-temperature atomic layer deposition process of ZnO, Zn was infiltrated into the interlayer of the GO and ZnO was deposited on the outer surface of the GO. Under laser irradiation, Zn was rapidly evaporated by the heat generated by the laser, which removes many functional groups and changes microstructure significantly. In the case of ZnO, it was evaporated by carbothermic reduction (C(s) + ZnO(s) → C′(s) + Zn(g) + CO(g)), which led to mechanical and chemical activation of the GO29 (Figure S3). reduction (Figure S1). The LrGO, LrGO/Zn/100, LrGO/Zn/200, LrGO/Zn/500, and LrGO/Zn/1000 denote the reduced GO, GO/ Zn/100, GO/Zn/200, GO/Zn/500, and GO/Zn/1000, respectively (Figure S2). Among these samples, the LrGO/Zn/500 showed the best electrochemical performance. Therefore, in the following we discuss the details of the GO, LrGO, GO/Zn/500, and LrGO/Zn/ 500 in terms of physical and chemical properties. Physical Characterizations. The analysis of the surface morphology and the elemental composition of the samples was performed using field emission scanning electron microscopy (FESEM, JSM-7000F, JEOL) equipped with energy dispersive X-ray spectroscopy (EDX). FEI Talos F200X microscope equipped with Super-X EDX system was used for the transmission electron microscopy (TEM) investigation. In order to analyze the crystallinity of the samples, X-ray diffraction (XRD) analysis was performed with a Panalytical XPert pro X-ray diffractometer (Cu−Kα radiation, 40 kV, 30 mA, λ = 1.5418 Å). Raman spectroscopy analysis was performed with inVia Raman microscope (Renishaw) equipped with a 514 nm laser and a 50× objective lens. Using a Thermo Scientific Nicolet 6700 spectrometer, attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy analysis was conducted at room temperature in the range of 450−4000 cm−1. In order to measure the SSA of the samples, the N2 adsorption experiment at 77 K was performed with BELSORP-max (BEL Japan, Inc.). OriginLab 8.6 software was used for graphical tasks and data processing. Evaluation of Electrochemical Performance. Typical measurement procedures similar to our previous works14,29,30 were utilized for the electrochemical characterizations of the multilayer graphene films. In order to prepare the liquid-state device, first, the identical two electrodes (LrGO films with a size of ∼1 cm2) were immersed in a 0.5 M H2SO4 aqueous electrolyte. The two electrodes were then inserted into a test cell (HS-Flat Cell, Hohsen Corp.) after inserting the separator (Whatman filter papers, ashless, grade 40) between the electrodes. In order to allow the electrodes to uniformly wet, the constructed test cells were kept for 2 h before measurement. The electrochemical workstation (VSP, Biologic) with a two-electrode setup was used for characterizations, such as cyclic voltammetry (CV) measurement at various scan rates, galvanostatic charge−discharge (GCD) measurement between 0.0 to 1.0 V potential windows, and electrochemical impedance spectroscopy (EIS) measurement in the range of 100 MHz and 100 kHz. The cyclic stability test was performed by the cyclic voltammetry measurement at a scan rate of 200 mV/s for 10 000 cycles.

Zn infiltrated into GO leads to explosive reduction under laser irradiation. As compared to the LrGO (the laser-reduced GO produced from the raw GO), LrGO/Zn (the laser-reduced GO produced from the Zn-infiltrated GO) showed noticeable differences in microstructure, morphology, and SSA. More importantly, the LrGO/Zn exhibited superior electrochemical performances to the LrGO. It is believed that our approach could be widely used as one of novel means to produce the multilayer graphene assuring high electrochemical performance.

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EXPERIMENTAL SECTION

Preparation of Freestanding GO Films. Similar procedures to our previous works14 were performed. GO solution with a density of 6 g/L was purchased from Grapheneall. Using a bar coat system equipped with a film applicator, GO films were uniformly coated on a PET (polyethylene terephthalate) substrate. After drying, the dried GO films were gently peeled off from the substrate. Zn Infiltration into the GO Films. Similar to the previous experiment,28 the prepared GO films were transferred into ALD (atomic layer deposition) chamber (S200, Savannah, Cambridge NanoTech Inc.) and evacuated. For ZnO coating and Zn infiltration, diethylzinc (DEZ, ZnEt2) and water were used as precursors for zinc and oxygen source, respectively. The DEZ was purchased from SigmaAldrich. Each ALD cycle was composed of a pulse, exposure, and purge sequence for each precursor. First, the DEZ vapor was injected for 0.02 s into the chamber (pulse) and the GO was exposed to the DEZ vapor for 20 s (exposure). The excess DEZ was purged from the chamber for 30 s (purge). In the case of water vapor, similar steps were repeated (pulse for 0.1 s/exposure for 20 s/purge for 30 s). Nitrogen gas was used as carrier gas. The ALD processing of the GO films was conducted by 100, 200, 500, and 1000 cycles at 80 °C under ∼0.1 torrs, which were denoted as GO/Zn/100, GO/Zn/200, GO/ Zn/500, and GO/Zn/1000, respectively. Reduction Using CO 2 Laser. The information on the optimization of the used CO2 laser (Coryart, λ = 10.6 μm) via parametric study can be found in our previous work14 and similar experimental procedures were applied in this work. Square pieces of the raw GO and the Zn-infiltrated GO (GO/Zn/100, GO/Zn/200, GO/Zn/500, and GO/Zn/1000) films were transferred into the specially constructed chamber that prevents the films burning during 3712

DOI: 10.1021/acsanm.9b00618 ACS Appl. Nano Mater. 2019, 2, 3711−3717

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ACS Applied Nano Materials

Figure 2. Chemical and physical characterizations. (a) FTIR spectroscopy, (b) Raman spectroscopy, (c) XRD, and (d) nitrogen adsorption/ desorption isotherm measurement. The inset figure in (d) shows the pore size distributions of the LrGO and the LrGO/Zn/500, respectively. GO, GO/Zn/500, LrGO, and LrGO/Zn/500 denote raw graphene oxide, GO treated with ZnO ALD of 500 cycles, GO reduced by the laser irradiation, and GO/Zn/500 reduced by the laser irradiation, respectively (see the Experimental Section).

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activation (formation of wrinkles and cracks).29,30 Second, the infiltrated Zn triggered explosive reduction, thereby causing evident chemical and structural changes of the GO, as shown in Figure 2. The main functional groups on the GO are known to be epoxy (COC) and hydroxyl (COH) groups with some hydroxyl, carbonyl (CO), and carboxyl (COOH) groups at the edge.27 Fourier transformed infrared spectroscopy (FTIR) spectrum (Figure 2a) of the GO proved the existence of oxygen-containing groups. The absorption peaks at 3330, 1720, 1620, and 1044 cm−1 were assigned to the stretching vibration of the OH, COOH, CO, and CO functional groups, respectively. After laser irradiation, most of the absorption peaks were weakened or disappeared, suggesting that the reductions of the various GO samples have nearly completely occurred. It was noteworthy that the FTIR spectrum of GO/Zn/500 shows no significant absorption peaks, which is even similar as the spectrum of the LrGO and LrGO/Zn/500 (the laser-reduced GO/Zn/500). It implied that ZnO ALD treatment of the GO itself could lead to a direct reduction to some extent. In terms of quality of the produced multilayered graphene, some intriguing phenomena were also observed.

RESULTS AND DISCUSSION The structure of the GO depends on the synthesis methods and the degree of oxidation. The GO typically preserves the layered structure of the parent graphite, and the interlayer spacing of the layers is about two times larger (∼7 Å) than that of graphite.27 We have recently reported that Zn infiltration into the interlayer of the GO leads to a noticeable change in mechanical stability, gas permeation properties and microstructure.28 On the basis of our previous results, we presumed that the infiltrated Zn could also change the optical absorption properties of the GO. In view of the fact that the Zn has a relatively low melting and boiling point (∼419.5 °C and ∼907 °C, respectively), we thought that the instantaneous evaporation of Zn by the laser irradiation could give rise to a drastic change in SSA as well as microstructure of the resulting reduced GO. For the validation of our hypothesis, first, we artificially infiltrated the Zn into the interlayer of the GO via ALD process using diethylzinc and water as precursors (Figure 1). Subsequently, the reduction of the GO with the infiltrated Zn was performed by irradiation of the CO2 laser under N2 atmosphere at room temperature. First, it was found that the carbothermic reduction of ZnO (Figure S3) coated on the outer surface of the GO leads to chemical activation (C(s) + ZnO(s) → C′(s) + Zn(g) + CO(g)) as well as mechanical 3713

DOI: 10.1021/acsanm.9b00618 ACS Appl. Nano Mater. 2019, 2, 3711−3717

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Figure 3. Morphological and elemental analysis results. (a−d) Presumed microstructures, representative low/high magnification SEM images, and EDX point analysis spectra measured at the cross-section of GO, GO/Zn/500, LrGO, and LrGO/Zn/500, respectively (see text for details).

infiltration led to a significant reduction in the lattice spacing of the (001) plane together with the peak broadening (GO/Zn/ 500). It indicated that some of oxygen functional groups are likely removed by the random reactions with the ALD reactant (i.e., diethylzinc). Upon laser irradiation, the (001) peak eventually disappeared and the new (002) peak appeared at ∼26° (LrGO). The (002) peak of the LrGO/Zn/500 was shifted to a higher angle and became sharper as compared to the LrGO. The (002) peak broadening of the LrGO indicated that the resulting multilayered graphene has poorer ordering and stacking than the LrGO/Zn/500. The slight shift of the (002) peak signified that the layer spacing of the LrGO/Zn/ 500 is somehow narrower than the LrGO. The nitrogen adsorption/desorption measurement (Figure 2d) was conducted to examine the influence of the infiltrated Zn on the change in the surface area of the GO. Both the LrGO and the LrGO/Zn/500 showed a hysteresis loop from P/P0 = 0.5 to P/P0 = 1.0, which was due to the coexistence of micropores and mesopores. The BET (Brunauer, Emmett, and Teller) SSA and the pore volume of the LrGO/Zn/500 (378 m2/g and 1.72 cm3/g, respectively) were measured to be higher as compared to the LrGO (277 m2/g and 0.91 cm3/g, respectively). The plots of the corresponding pore size

Raman spectroscopy (Figure 2b, Table S1) is a useful tool for studying the quality of the graphene (defects and disorder in graphene). Quality is usually determined by the intensity ratio (ID/IG) between the D band (related to disorder) and the G band (related to sp2-hybridized carbon). All the samples showed typical spectra of the graphene-based materials, in particular, the multilayered graphene. The Zn-infiltrated GO (GO/Zn/500) exhibited a less significant shift of the G band (CC stretch). The ID/IG ratio, however, increased slightly as compared to the GO. After laser irradiation, the ID/IG ratio of both the LrGO and the LrGO/Zn/500 noticeably decreased as compared to the GO and the GO/Zn/500, respectively, which indicated the removal of most of the functional groups. Furthermore, the G bands slightly shifted to lower wavenumber after the laser irradiation, similar to the previous work.14 The reduction is known to start from edges of the GO and proceed into basal planes. Subsequently, the basal planes of the multilayered graphene snap together due to π−π interactions, which consequently reduces the interlayer spacing of the resulting reduced GO.31 X-ray diffraction (XRD) pattern of the GO (Figure 2c, Table S2) showed that the (001) peak appears at ∼10° (lattice spacing of ∼0.8 nm). Interestingly, the Zn 3714

DOI: 10.1021/acsanm.9b00618 ACS Appl. Nano Mater. 2019, 2, 3711−3717

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ACS Applied Nano Materials

Figure 4. Electrochemical characteristics of the multilayered graphenes. (a) CV profiles measured at the scan rate of 10 mV/s. (b) GCD curves collected at the current density of 1 A/g. (c) Specific capacitances at different current densities. (d) Ragone plot for the symmetric supercapacitors made of the multilayered graphene. In order to compare the performance with other energy storages, the plot in the literature was referenced2 on which our data were added.

in energy storage systems. Electrochemical properties of the produced multilayered graphenes as electrode materials for the supercapacitor were evaluated in aqueous 0.5 M H2SO4 electrolyte. Among the various samples, the LrGO/Zn/500 exhibited the best performance (Figure S6). Figure 4a presents CV profiles of the LrGO and the LrGO/Zn/500 electrodes, respectively. Each CV pattern showed a sharp vertical change in current density at the potential of the electrode’s polarity change. This suggested good internal cell conductivity, which was largely due to the high electrical conductivity of the LrGO and the LrGO/Zn/500 electrodes. It was further confirmed by the comparative GCD profiles of both electrodes at the current density of 1 A/g (Figure 4b). Both the LrGO and the LrGO/ Zn/500 showed linear and symmetrical GCD profiles, which signifies an ideal capacitance behavior of electrical double layer capacitor (EDLC) with the absence of any faradaic processes. In addition, the LrGO electrode exhibited much lower GCD time than that of the LrGO/Zn/500 electrode. GCD tests of both electrodes performed at different current densities also demonstrated good capacitance retention behavior. It indicated the less restricted flow of electrons at faster charge−discharge thanks to high electrical conductivity. The gravimetric specific capacitances of the LrGO and the LrGO/ Zn/500 were calculated to be 2132 and 83 F/g, respectively (Figure 4c). The Ragone plot for the LrGO/Zn/500 based supercapacitors displayed clear improvement in the energy density as compared to the LrGO (Figure 4d). ALD offers a wide spectrum of materials. It is also possible to infiltrate various metal elements. Also, the carbothermic reduction of diverse ALD materials coated on the graphene-based materials

distributions calculated by the Barrett−Joyner−Halenda method indicated the existence of well-developed mesoporosity with the pore distribution centered at about 4.5 nm. GO is formed by flat monolayers of carbon atoms arranged in a hexagonal lattice. Upon ZnO ALD processing, the overall macroscopic thickness of the GO film notably decreased by more than 50%, while the surface morphology remained nearly unchanged (Figure 3a,b). However, the thermal wave generated by the laser led to evident changes in the surface morphology of the GO as well as the GO/Zn/500 (Figure 3c,d). In particular, the volumetric expansions of the GO and the GO/Zn/500 film after laser reduction were remarkable. The LrGO and the LrGO/Zn/500 film expanded to be almost 15 and 40 times, as compared to the GO and the GO/Zn/500, respectively. The cross-section image of the LrGO exhibited numerous pores likely created by the removal of functional groups. The LrGO/Zn/500 also shows similar changes in morphology. The elemental analysis results measured at the cross-section of the LrGO indicated that oxygen atoms originally existing in the raw GO are mostly evaporated upon laser irradiation. The GO/Zn/500 was revealed to contain large amounts of Zn that were infiltrated by ZnO ALD (Figure S4). It was found that most of the infiltrated Zn existing in the GO/Zn/500 are removed by the laser reduction (Figure S4 and Figure S5), which eventually leads to significant chemical and microstructural changes. Obviously, the multilayered graphene with a larger SSA and abundance of mesopores can promote sufficient charge storage (high energy density) and fast charge transport kinetics (high power density), respectively, which are critical for applications 3715

DOI: 10.1021/acsanm.9b00618 ACS Appl. Nano Mater. 2019, 2, 3711−3717

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2014063700). We would like to thank Mr. Chang Hyun Kim for experimental support.

could lead to a drastic change in the topology of the resulting graphene via mechanical and chemical activation.29 Therefore, it is strongly believed that rigorous optimization could lead to a drastic increase in energy density, although the increase is less dramatic yet.

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CONCLUSION In this study, we showed that a tiny amount of Zn infiltrated into the GO plays an impressive role in an explosive reduction under laser irradiation. It was found that the infiltrated Zn leads to significant changes in microstructures and surface chemistry of the resulting multilayer graphene. More noticeably, the resulting multilayered graphene showed a nearly four times increase in the energy density likely thanks to the increase in specific surface area. It indicated that our method could be employed as one of the promising means to produce electrode materials for the graphene-based high-performance supercapacitor. In order to further improve the electrochemical performance of our multilayered graphene-based supercapacitor, we are planning to prepare and characterize a different configuration of the capacitor in which a negative and a positive electrode operate in electrolytes with various pHs (e.g., alkaline for the negative and neutral for the positive electrode, respectively).33 Our multilayer graphene films were observed to be mechanically less stable compared to the raw GO, hence the handling of the films still needs attention. As future work, we will develop a new method to produce multilayered graphene with high electrochemical performance as well as proper mechanical stability. Currently, the resulting multilayered graphene is expected to have other impressive performances, like filtration, which remains to be investigated.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.9b00618. Schematic of our laser equipment set up; schematic of Ellingham diagram (Gibb’s free energy vs temperature) of ZnO and carbon; electrochemical performance characterizations of the prepared multilayered graphenes; digital photos of GO, LrGO, GO/Zn, and LrGO/ Zn samples; table containing summarized data on Raman spectroscopy and XRD; EDX mapping data of various samples; TEM images of the multilayered graphene (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Seung-Mo Lee: 0000-0003-0732-4473 Yong-Jin Park: 0000-0003-4345-7492 Jae-Hyun Kim: 0000-0002-4327-2992 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to acknowledge the financial support from the internal research program of Korea Institute of Machinery and Materials (NK218C) and Global Frontier Project funded by the Ministry of Science and ICT (CAMM-N0. 2014063701, 3716

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DOI: 10.1021/acsanm.9b00618 ACS Appl. Nano Mater. 2019, 2, 3711−3717