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Tailored and highly efficient oxidation of various-sized graphite by kneading for high-quality graphene nanosheets
Carbon xxx (xxxx) xxx
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Tailored and highly efficient oxidation of various-sized graphite by kneading for high-quality graphene nanosheets Joon Young Cho a, b, Jung Hoon Kim a, Hye Jin Yang a, b, Jong Hwan Park a, Seung Yol Jeong a, b, Hee Jin Jeong a, Geon-Woong Lee a, Joong Tark Han a, b, * a b
Nano Hybrid Technology Research Center, Korea Electrotechnology Research Institute (KERI), Changwon, 51543, Republic of Korea Department of Electro-Functionality Material Engineering, University of Science and Technology (UST), Changwon, 51543, Republic of Korea
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Article history: Received 15 September 2019 Received in revised form 25 October 2019 Accepted 31 October 2019 Available online xxx
Graphene oxide (GO) exfoliated from graphite oxide is a precursor of graphene, which is a highly promising two-dimensional material for energy storage, printing electronics, composites, conducting fibers, etc. However, during the fabrication of GO by chemical oxidation, large amounts of oxidants and acid are used, which lead to many concerns, such as severe environmental pollution and high processing cost. In this study, we propose a highly efficient and rational oxidation of various-sized graphite powder by minimizing the amount of acid used (reducing 1/10 times) through kneading, inspired by the concept of kneading dough. First, NaClO3 and graphite were mixed together. Then, a small amount of fuming nitric acid was added, and the mixture was kneaded for several minutes. The ratio of oxidant to graphite and the reaction time, with and without kneading, were investigated based on the lateral size of graphite, because large-sized graphite requires more diffusion time for oxidizing the species through the graphitic layers. The chemically-reduced GO nanosheets exhibit highly crystalline structures based on the Raman spectroscopy and transmission electron microscopy studies, and their films exhibit a high electrical conductivity of over 45,000 S/m from large graphite, which is promising for electrical and electrochemical applications. © 2019 Elsevier Ltd. All rights reserved.
1. Introduction Graphene oxide (GO) is a fascinating material in terms of its solution processability and functional moieties introduced in its basal plane. Additionally, it is a precursor of the conducting graphene nanosheets for electrical and electrochemical applications [1e6]. The GO nanosheets are commonly synthesized by the exfoliation of graphite oxide (GrO), which can be produced by permanganate (MnO4)- or chlorate (ClO3)-based oxidation using a huge amount of a strong acid. However, this is recognized as a hurdle in the mass production of GO from graphite due to massive acid wastes and high processing costs. In 1859, Brodie oxidized graphite using KClO3 and fuming nitric acid [7]. After 100 years, Staudenmaier oxidized graphite using KClO3, sulfuric acid, and nitric acid [8]. Sixty years after Staudenmaier’s study, Hofmann and Hummers oxidized graphite using
* Corresponding author. Nano Hybrid Technology Research Center, Korea Electrotechnology Research Institute (KERI), Changwon, 51543, Republic of Korea. E-mail address: [email protected] (J.T. Han).
NaNO3, sulfuric acid, and KMnO4 [9]. The oxygen functional groups in GrO and degrees of oxidation were various by the oxidation methods [10]. However, the chemical structure of GO remains unproven. Recent studies insisted that GO contains sp2-hybridized carbon domains and oxidized domains with destroyed carbon domains and oxygen functional groups [11]. Their structures are critically dependent on the oxidizing agents and processing conditions of graphite powder [12]. Moreover, in terms of commercialization, minimizing the amount of acidic waste during the graphite oxidation remains a limitation. In case of H2SO4-based oxidation, some studies have been trying to recycle the used acid to reduce the processing cost of graphite and to overcome the environmental issues [13e17]. In chlorate-based oxidation, fuming nitric acid is typically used to fabricate high quality graphene, because fuming nitric acid and ClO3 can produce less defective graphene oxide nanosheets [18]. However, the fuming nitric acid cannot be reused as it releases toxic gases, which poses a threat to researchers’ health and the environment. Hence, there is a need to develop a method to oxidize graphite for high quality graphene via chlorate-based oxidation. Recently, the efficient oxidation of single-
https://doi.org/10.1016/j.carbon.2019.10.102 0008-6223/© 2019 Elsevier Ltd. All rights reserved.
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walled carbon nanotube (CNT) by kneading with oxidants and acid solution has been suggested [19]. Accordingly, this proposed method can be applied in the oxidation of graphite. However, the layered structure and lateral dimension of graphite needs to be considered for the optimal oxidation and efficient exfoliation of GrO. In this study, we demonstrated that graphite powder can be efficiently oxidized within a few hours through kneading, using a small amount of fuming nitric acid and oxidant molecules. To realize a rapid and highly efficient oxidation, oxidant powder was mixed with graphite powder, followed by the slow addition of fuming nitric acid onto the graphite/oxidant mixture while kneading. This process is similar to the kneading of dough for bread or noodles. Moreover, to optimize the oxidation condition of various-sized graphite, the reaction time and the graphite to oxidant ratio was controlled depending on the lateral size of the graphite. 2. Experimental details Materials. Small-, medium-, and large-sized graphite (99% purity) powders were obtained from Technokorea, Alfa Aesar, and Samjung, respectively. NaClO3 powder was purchased from Samchun Chemicals. Fuming nitric acid was obtained from TCI Chemicals. H2O2 and HCl were obtained from Daejung and were used without any further treatment. Oxidation of graphite and exfoliation into GO. GrO powder was prepared by the ClO3-based oxidation method through kneading. First, 1 g of graphite was evenly mixed with 7.5 g of sodium chlorate. Next, 10 mL of fuming nitric acid was added in a dropwise manner while kneading into the mixed graphite/NaClO3 powder, at room temperature, and then, the mixture was rested for 1 hat room temperature. In addition, for post-oxidation, 15 mL of water was poured onto the graphite dough and stirred for 24 h. Deionized (DI) water was used for the dilution of the acid, and H2O2 and HCl were subsequently added to the reactant mixture to terminate the reaction and remove metal ions, respectively. H2O2 can reduce sodium chlorate in acidic solutions [20]. Finally, GrO powder was purified by washing with DI water and freeze-dried. The synthesized GrO powder was immersed in aqueous KOH with a pH of 11 and was exfoliated into GO using a high-speed homogenizer at 10,000 rpm for 1 h. To obtain reduced graphene oxide (rGO), sodium carboxymethyl cellulose (SCMC) and hydriodic (HI) acid were mixed with the GO solution (1 g/L) using a highspeed homogenizer at 10,000 rpm for 3 h. Characterization. The crystalline structure of GrO powder was characterized by X-ray diffraction (XRD) using a Philips PW 3830 Xray diffractometer with Cu Ka radiation (l ¼ 1.5418 Å). The changes in the atomic ratio of carbon to oxygen on the oxidation level of the GO were identified by X-ray photoelectron spectroscopy (XPS) using a K-Alpha þ system (Thermo Fisher Scientific, U.K.) spectrometer with monochromated Al K a X-ray radiation as the X-ray excitation source. The thermal degradation behaviors of the GrO powder samples were confirmed by thermogravimetric analysis (TA Instruments, TGA Q500). 13C solid-state nuclear magnetic resonance (13C NMR) spectroscopy (AVANCE IIþ 400 MHz, Bruker, Germany) was used to analyze the functional groups on the GrO obtained with and without kneading. The surface morphologies of the GO sheets were imaged by field-emission scanning electron microscopy (FE-SEM, HITACHI S4800). Ultravioletevisible spectra were obtained using an ultravioletevisible (UVevis) spectrometer (Varian Cary 5000, Agilent). The structural characteristics of graphite, GrO, GO, and rGO were determined using Raman spectrometry (NTEGRA SPECTRA, NT-MDT) with excitation wavelengths of 532 nm. The electrical performance of the rGO films was
measured using a two-probe resistivity-measuring instrument with a uniform spacing of 50 mm. A high-resolution transmission electron microscope (HR-TEM, Titan G2 60e300, FEI) was used to image the surface of rGO at an acceleration voltage of 80 kV. 3. Results and discussion When the oxidant molecules are directly in contact with the graphite surface, the oxidation reaction rate can be accelerated, which depends on the ratio of oxidant/graphite and the reaction time. Hence, the concept of kneading, similar to kneading dough, is a good example for a highly efficient reaction of the powders. In this study, to maximize the reaction rate, the graphite and oxidant powder mixture was kneaded by adding a small amount of fuming nitric acid, as illustrated in Fig. 1. Moreover, to minimize the permanent defect formation on the basal plane of graphene, NaClO3 was used as an oxidant molecule as previously reported [18]. The graphite powders having different lateral sizes ranging from 4 to over 150 mm (Fig. S1), were used to optimize the oxidation condition through kneading because the oxidation of graphite is the diffusion-controlled reaction between the graphitic layers [21,22]. First, the oxidation of the small-sized graphite powder having a lateral size of 4e5 mm was controlled by varying the oxidation time of graphite from 1 to 3 h, and resting after mixing for several minutes at a constant graphite/NaClO3 ratio of 1/7.5. Fig. 2a depicts the XRD patterns of small GrO (s-GrO) powders by varying the oxidation time from 1 to 3 h. The Characteristic (002) plane peak of graphite at 26 , corresponding to an interlayer spacing of 3.35 Å, disappeared and the GrO peak generated at 16 , corresponding to 5.65 Å, with the increase in the oxidation time up to 3 h. However, even at an oxidation time of 2 h, the graphite peak at 26 remained, which implies that more resting time is required for the diffusion and reaction of the oxidizing agents through the graphitic layers. The TGA thermogram in Fig. 2b depicts the gradual increase in the oxidative functional groups with the increase in the oxidation time. As the oxidation time increased from 1 to 3 h, the peak of the sp2hybridized carbon atoms (284.4 eV) decreased and the peak of the sp3-hybridized carbon atoms (285.1 eV) increased. Furthermore, the peaks of the CeO bonds related to the epoxide group (286.5 eV) and the C]O bonds related to the carbonyl group (288.5 eV) increased (Fig. 2a). The XPS spectra in Fig. 2c depicts that the CeO bond related to the hydroxyl and epoxy groups was dominantly introduced by increasing the oxidation time, whereas only a small amount of carbonyl and carboxylic acid groups were detected. Then, the weight ratio of graphite to NaClO3 was varied from 1:5 to 1:10 (w:w) to investigate the effect of the abundance of oxidant molecules on the oxidation of graphite within a resting time of 1 h. As illustrated in Fig. 2c, the graphite peak almost disappeared at a NaClO3 to graphite ratio of 1:10, and more oxidative functional groups were introduced as indicated by the TGA data (Fig. 2d) and XPS spectra (Fig. 2e). The oxidation temperature of the graphite in TGA decreased with the addition of NaClO3, whereas as seen in Fig. 2b, the oxidation temperature did not change in a constant amount of NaClO3 even when the oxidation time was increased. The increase in the oxidative functional groups and decrease in the oxidation temperature of graphite indicate that more oxidant molecules were utilized to oxidize the inner basal plane of the graphite. However, in terms of commercialization, reducing the reaction time and the amount of used oxidant are profitable in minimizing the production cost. Thus, a more efficient oxidation process is needed to rapidly oxidize the graphite to obtain GrO powder without the existence of the graphite peak in the XRD spectra. To realize this, the kneading of the small-graphite dough (1:7.5 ¼ graphite: NaClO3) was continued through the oxidation
Please cite this article as: J.Y. Cho et al., Tailored and highly efficient oxidation of various-sized graphite by kneading for high-quality graphene nanosheets, Carbon, https://doi.org/10.1016/j.carbon.2019.10.102
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Fig. 1. Scheme of highly efficient oxidation of graphite inspired by the kneading of dough. A small amount of fuming nitric acid was slowly added to the graphite/NaClO3 mixture to form a dough, followed by kneading and resting for oxidation. (A colour version of this figure can be viewed online.)
Fig. 2. (a, d) XRD Plots, (b, e) TGA thermogram, and (c, f) XPS spectra of GrO by varying the oxidation time (aec) and by changing the weight ratio of graphite and NaClO3 (def). (A colour version of this figure can be viewed online.)
time. This resulted in the disappearance of the graphite peak within 1 h of the oxidation time, and the interlayer spacing of the GrO increased from 5.6 to 6.24 Å, as illustrated in Fig. 3a. This implies that kneading promoted the oxidation reaction of graphite by the light shear force, which was induced by the homogeneous mixing and rapid dissolution of NaClO3 powder in fuming nitric acid. An additional widening of the interlayer spacing of s-GrO powder up to 6.58 Å was observed in the XRD pattern after hydration and postoxidation by mixing some amount of water with the GrO dough, followed by stirring for 24 h. The XPS spectra (Fig. 3b) also indicates
the considerable increase in the CeO bonds and oxidative functional groups by kneading during oxidation. Based on the analysis of the XPS spectra, it was found that the C atomic ratio changed from 3.68 to 3.12 by kneading (Fig. 3c). However, the hydration and post oxidation by mixing water with the GrO dough only caused a slight oxidation, which corresponds to the TGA thermogram illustrated in Fig. 3d. Moreover, the 13C NMR in Fig. 3e and f indicates that the hydroxyl and epoxy groups in GrO powder quantitatively increased in comparison with the sp2 C]C bond by kneading during oxidation. These results clearly demonstrate that
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Fig. 3. Characterization of s-GrO samples prepared without kneading, with kneading, and additional post oxidation by adding water. (a) XRD plot; (b) XPS spectra; (c) C/O atomic ratio from XPS spectra; (d) TGA thermogram, 13C solid NMR spectra of s-GrO prepared without (e) and with (f) kneading. FESEM image of exfoliation of s-GrO prepared without (g) and with (h) kneading. (i) UVeVis absorption spectra of exfoliated s-GrO prepared without and with kneading. (j) Raman spectra of a small-sized graphite (s-graphite), s-GO, and srGO reduced by HI acid. (A colour version of this figure can be viewed online.)
continuous kneading during oxidation is beneficial for the rapid oxidation of graphite with a small amount of the acid solution, similar to the concept of kneading dough. The s-GrO powders were then exfoliated into graphene oxide using a high speed homogenizer at 10,000 rpm by adjusting the pH up to 11 [23]. Fig. 3g depicts the FESEM image of the exfoliated sGrO prepared without kneading. Most of the GrO particles were not fully exfoliated, whereas the s-GrO particles fabricated by kneading were fully exfoliated into GO (s-GO) nanosheets, as illustrated in Fig. 3h. In addition, the UVevis spectra of the GO solution (Fig. 3i) revealed the absorption peak near 250 nm, which indicates the pp* transitions of the CeC and C]C bonds. The other absorption peak near 300 nm indicates the n-p* transition of the CeO bonds. Typical polycyclic aromatic hydrocarbon peaks were observed in the fully exfoliated s-GO nanosheets, which only existed in the case of GO produced by ClO3-based oxidation [24]. To ensure that the GO
produced by kneading had a low defect, the GO and rGO were analyzed using Raman analysis. Raman spectroscopy is a useful analysis tool for characterizing. Structural defects. Fig. 3j depicts the Raman spectra of graphite, GO, and rGO, which features four prominent peaks, namely, D (1344 cm1), G (1581 cm1), D’ (1615 cm1), and 2D (2683 cm1). The intensity of the D peak is affected by the defective structures that appeared in the Raman spectrum of GO. However, after reduction with HI acid, a narrow and strong D peak indicates that rGO contains fewer number of permanent vacancies that are impossible to heal by chemical reduction [25]. In particular, the strong 2D peak also demonstrates the high crystalline structure of s-rGO fabricated through the kneading-assisted oxidation. The results described above were derived from 4 to 5 mm sized graphite. Small-sized graphite can be oxidized with an adequate amount of oxidant within 1 h through kneading. However, even by
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kneading, to fully oxidize the larger-sized graphite (<75 and >150 mm), 1 h of oxidation time was not enough, as indicated by the XRD plots in Fig. 4a. To optimize the oxidation of the large-sized graphite by kneading, the oxidation time and the graphite/NaClO3 weight ratio were controlled. First, the medium-sized graphite (<75 mm) was fully oxidized into GrO (m-GrO) by increasing the oxidation time, as indicated in the XRD plot (Fig. 4b), and could be exfoliated into single layer GO nanosheets. From the XPS spectrum, more CeO and C]O bonds were introduced in the GO after oxidation for 3 h. However, the large-sized graphite (>150 mm) required a more severe oxidation condition. With an oxidation time of 3 h and a graphite:NaClO3 weight ratio of 1:7.5, a graphite peak in the XRD plot still appeared, as illustrated in Fig. 4d. Based on the oxidation of small-sized graphite, the amount of oxidant was increased to 1:10 (w:w) with 3 h of oxidation time. A larger amount of NaClO3 resulted in fully oxidized large GrO (L-GrO) powders with 5.92 Å of d-spacing in the XRD plot (Fig. 4e). In the XPS spectrum of L-GrO, it was found that more number of C]O bonds were introduced, which was still a smaller amount than that in the GrO produced by KMnO4-based oxidation [18]. The exfoliated GO from m-GrO (m-GO) and L-GrO (L-GO) was reduced under the same conditions as that for s-GO and analyzed by Raman spectroscopy (Fig. 5a). The intensity of the 2D peaks of
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the m- and L-rGO was remarkably high, which implies that high quality graphene can be produced by kneading-assisted oxidation of large-sized graphite powders followed by exfoliation, oxidation, and chemical reduction. The high resolution TEM image in Fig. 5b also indicates a directly highly crystalline and large sp2 domain. These structural characteristics can affect the electrical conductivity of the rGO electrodes. The rGO pastes were fabricated by HI reduction and dispersion assisted by sodium carboxylmethyl cellulose. These pastes were coated on the substrate and their electrical performances were estimated based on their IeV plots determined using two-point probe tests. As depicted in Fig. 5c, the L-rGO film was more conductive than the m- and s-rGO films in accordance with the lateral size of GO. The electrical conductivity of the L-rGO film reached 46,000 S/m, whereas those of the m- and srGO films were 26,500 and 16,800, respectively. Furthermore, thin film heaters were fabricated using filtrated rGO films. Fig. 5d depicts the heating behavior of the s-, m-, and L-rGO films as a function of time at an applied voltage of 3 V. The saturation temperatures were dependent on the electrical performances of the rGO films in accordance with the measured current during Joule heating (Fig. 5e). The temperature of the L-rGO film reached over 70 C even at 3 V, which indicates the high electrical conductivity of the rGO film. Moreover, the Joule-heated rGO film retained a
Fig. 4. (a) XRD plots of GrO powders depending on the size of used graphite (4e5, <75, >150 mm) prepared by 1 h oxidation time under kneading and 1:7.5 wt ratio of graphite and NaClO3. (b) XRD plot and (c) deconvoluted XPS spectrum of m-GrO prepared by 3 h oxidation time under kneading and 1:7.5 wt ratio of graphite:NaClO3. Inset image in (b) depicts the FE-SEM image of exfoliated m-GrO. (d) XRD plot of L-GrO prepared by 3 h oxidation time under kneading and 1:7.5 wt ratio of graphite:NaClO3. (e) XRD plot and (f) deconvoluted XPS spectrum of L-GrO prepared by 3 h oxidation time under kneading and 1:10 wt ratio of graphite:NaClO3. Inset image in (e) depicts the FE-SEM image of exfoliated LGrO. (A colour version of this figure can be viewed online.)
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Fig. 5. (a) Raman spectra of s-, m-, and L-rGO samples. (b) High resolution TEM image of rGO. (c) IeV plots of rGO films prepared by s-, m-, and L-rGO pastes. (d) Temperature profiles and (e) current changes of rGO films by applying DC 3 V as a function of time. (f) IR temperature camera image of Joule-heated rGO film at 3 V. (A colour version of this figure can be viewed online.)
uniform heating behavior under bending, as illustrated in Fig. 5f.
Appendix A. Supplementary data
4. Conclusion
Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2019.10.102.
We introduced an effective oxidizing method for various-sized graphite, called kneading. When the graphite sheet size is small, the graphite can be effectively oxidized within an hour by using an appropriate amount of oxidant during kneading. However, with the increase in the graphite size, the conditions of oxidation need to be adjusted to fully oxidize the graphite by diffusion-controlled oxidation of the graphitic layers. Graphite can be oxidized within a few hours by kneading because the kneading promotes the intercalation of the oxidant through the graphite sheets. The disappearance of the graphite peak in the XRD analysis, and the XPS and Raman analysis revealed that the oxidation of graphite by kneading was an effective strategy. Additionally, the Raman spectrum of rGO demonstrated the recovery of the sp2 domain after chemical reduction. Therefore, kneading facilitated the efficient oxidation in a short time with a lesser amount of acid, and resulted in a highly crystalline structure of rGO after the chemical reduction of GO. Our proposed kneading method for GrO production can be applied to mass production of high quality graphene nanosheets. Declaration of competing interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Carbon journal homepage: www.elsevier.com/locate/carbon
Tailored and highly efficient oxidation of various-sized graphite by kneading for high-quality graphene nanosheets Joon Young Cho a, b, Jung Hoon Kim a, Hye Jin Yang a, b, Jong Hwan Park a, Seung Yol Jeong a, b, Hee Jin Jeong a, Geon-Woong Lee a, Joong Tark Han a, b, * a b
Nano Hybrid Technology Research Center, Korea Electrotechnology Research Institute (KERI), Changwon, 51543, Republic of Korea Department of Electro-Functionality Material Engineering, University of Science and Technology (UST), Changwon, 51543, Republic of Korea
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a b s t r a c t
Article history: Received 15 September 2019 Received in revised form 25 October 2019 Accepted 31 October 2019 Available online xxx
Graphene oxide (GO) exfoliated from graphite oxide is a precursor of graphene, which is a highly promising two-dimensional material for energy storage, printing electronics, composites, conducting fibers, etc. However, during the fabrication of GO by chemical oxidation, large amounts of oxidants and acid are used, which lead to many concerns, such as severe environmental pollution and high processing cost. In this study, we propose a highly efficient and rational oxidation of various-sized graphite powder by minimizing the amount of acid used (reducing 1/10 times) through kneading, inspired by the concept of kneading dough. First, NaClO3 and graphite were mixed together. Then, a small amount of fuming nitric acid was added, and the mixture was kneaded for several minutes. The ratio of oxidant to graphite and the reaction time, with and without kneading, were investigated based on the lateral size of graphite, because large-sized graphite requires more diffusion time for oxidizing the species through the graphitic layers. The chemically-reduced GO nanosheets exhibit highly crystalline structures based on the Raman spectroscopy and transmission electron microscopy studies, and their films exhibit a high electrical conductivity of over 45,000 S/m from large graphite, which is promising for electrical and electrochemical applications. © 2019 Elsevier Ltd. All rights reserved.
1. Introduction Graphene oxide (GO) is a fascinating material in terms of its solution processability and functional moieties introduced in its basal plane. Additionally, it is a precursor of the conducting graphene nanosheets for electrical and electrochemical applications [1e6]. The GO nanosheets are commonly synthesized by the exfoliation of graphite oxide (GrO), which can be produced by permanganate (MnO4)- or chlorate (ClO3)-based oxidation using a huge amount of a strong acid. However, this is recognized as a hurdle in the mass production of GO from graphite due to massive acid wastes and high processing costs. In 1859, Brodie oxidized graphite using KClO3 and fuming nitric acid [7]. After 100 years, Staudenmaier oxidized graphite using KClO3, sulfuric acid, and nitric acid [8]. Sixty years after Staudenmaier’s study, Hofmann and Hummers oxidized graphite using
* Corresponding author. Nano Hybrid Technology Research Center, Korea Electrotechnology Research Institute (KERI), Changwon, 51543, Republic of Korea. E-mail address: [email protected] (J.T. Han).
NaNO3, sulfuric acid, and KMnO4 [9]. The oxygen functional groups in GrO and degrees of oxidation were various by the oxidation methods [10]. However, the chemical structure of GO remains unproven. Recent studies insisted that GO contains sp2-hybridized carbon domains and oxidized domains with destroyed carbon domains and oxygen functional groups [11]. Their structures are critically dependent on the oxidizing agents and processing conditions of graphite powder [12]. Moreover, in terms of commercialization, minimizing the amount of acidic waste during the graphite oxidation remains a limitation. In case of H2SO4-based oxidation, some studies have been trying to recycle the used acid to reduce the processing cost of graphite and to overcome the environmental issues [13e17]. In chlorate-based oxidation, fuming nitric acid is typically used to fabricate high quality graphene, because fuming nitric acid and ClO3 can produce less defective graphene oxide nanosheets [18]. However, the fuming nitric acid cannot be reused as it releases toxic gases, which poses a threat to researchers’ health and the environment. Hence, there is a need to develop a method to oxidize graphite for high quality graphene via chlorate-based oxidation. Recently, the efficient oxidation of single-
https://doi.org/10.1016/j.carbon.2019.10.102 0008-6223/© 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: J.Y. Cho et al., Tailored and highly efficient oxidation of various-sized graphite by kneading for high-quality graphene nanosheets, Carbon, https://doi.org/10.1016/j.carbon.2019.10.102
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walled carbon nanotube (CNT) by kneading with oxidants and acid solution has been suggested [19]. Accordingly, this proposed method can be applied in the oxidation of graphite. However, the layered structure and lateral dimension of graphite needs to be considered for the optimal oxidation and efficient exfoliation of GrO. In this study, we demonstrated that graphite powder can be efficiently oxidized within a few hours through kneading, using a small amount of fuming nitric acid and oxidant molecules. To realize a rapid and highly efficient oxidation, oxidant powder was mixed with graphite powder, followed by the slow addition of fuming nitric acid onto the graphite/oxidant mixture while kneading. This process is similar to the kneading of dough for bread or noodles. Moreover, to optimize the oxidation condition of various-sized graphite, the reaction time and the graphite to oxidant ratio was controlled depending on the lateral size of the graphite. 2. Experimental details Materials. Small-, medium-, and large-sized graphite (99% purity) powders were obtained from Technokorea, Alfa Aesar, and Samjung, respectively. NaClO3 powder was purchased from Samchun Chemicals. Fuming nitric acid was obtained from TCI Chemicals. H2O2 and HCl were obtained from Daejung and were used without any further treatment. Oxidation of graphite and exfoliation into GO. GrO powder was prepared by the ClO3-based oxidation method through kneading. First, 1 g of graphite was evenly mixed with 7.5 g of sodium chlorate. Next, 10 mL of fuming nitric acid was added in a dropwise manner while kneading into the mixed graphite/NaClO3 powder, at room temperature, and then, the mixture was rested for 1 hat room temperature. In addition, for post-oxidation, 15 mL of water was poured onto the graphite dough and stirred for 24 h. Deionized (DI) water was used for the dilution of the acid, and H2O2 and HCl were subsequently added to the reactant mixture to terminate the reaction and remove metal ions, respectively. H2O2 can reduce sodium chlorate in acidic solutions [20]. Finally, GrO powder was purified by washing with DI water and freeze-dried. The synthesized GrO powder was immersed in aqueous KOH with a pH of 11 and was exfoliated into GO using a high-speed homogenizer at 10,000 rpm for 1 h. To obtain reduced graphene oxide (rGO), sodium carboxymethyl cellulose (SCMC) and hydriodic (HI) acid were mixed with the GO solution (1 g/L) using a highspeed homogenizer at 10,000 rpm for 3 h. Characterization. The crystalline structure of GrO powder was characterized by X-ray diffraction (XRD) using a Philips PW 3830 Xray diffractometer with Cu Ka radiation (l ¼ 1.5418 Å). The changes in the atomic ratio of carbon to oxygen on the oxidation level of the GO were identified by X-ray photoelectron spectroscopy (XPS) using a K-Alpha þ system (Thermo Fisher Scientific, U.K.) spectrometer with monochromated Al K a X-ray radiation as the X-ray excitation source. The thermal degradation behaviors of the GrO powder samples were confirmed by thermogravimetric analysis (TA Instruments, TGA Q500). 13C solid-state nuclear magnetic resonance (13C NMR) spectroscopy (AVANCE IIþ 400 MHz, Bruker, Germany) was used to analyze the functional groups on the GrO obtained with and without kneading. The surface morphologies of the GO sheets were imaged by field-emission scanning electron microscopy (FE-SEM, HITACHI S4800). Ultravioletevisible spectra were obtained using an ultravioletevisible (UVevis) spectrometer (Varian Cary 5000, Agilent). The structural characteristics of graphite, GrO, GO, and rGO were determined using Raman spectrometry (NTEGRA SPECTRA, NT-MDT) with excitation wavelengths of 532 nm. The electrical performance of the rGO films was
measured using a two-probe resistivity-measuring instrument with a uniform spacing of 50 mm. A high-resolution transmission electron microscope (HR-TEM, Titan G2 60e300, FEI) was used to image the surface of rGO at an acceleration voltage of 80 kV. 3. Results and discussion When the oxidant molecules are directly in contact with the graphite surface, the oxidation reaction rate can be accelerated, which depends on the ratio of oxidant/graphite and the reaction time. Hence, the concept of kneading, similar to kneading dough, is a good example for a highly efficient reaction of the powders. In this study, to maximize the reaction rate, the graphite and oxidant powder mixture was kneaded by adding a small amount of fuming nitric acid, as illustrated in Fig. 1. Moreover, to minimize the permanent defect formation on the basal plane of graphene, NaClO3 was used as an oxidant molecule as previously reported [18]. The graphite powders having different lateral sizes ranging from 4 to over 150 mm (Fig. S1), were used to optimize the oxidation condition through kneading because the oxidation of graphite is the diffusion-controlled reaction between the graphitic layers [21,22]. First, the oxidation of the small-sized graphite powder having a lateral size of 4e5 mm was controlled by varying the oxidation time of graphite from 1 to 3 h, and resting after mixing for several minutes at a constant graphite/NaClO3 ratio of 1/7.5. Fig. 2a depicts the XRD patterns of small GrO (s-GrO) powders by varying the oxidation time from 1 to 3 h. The Characteristic (002) plane peak of graphite at 26 , corresponding to an interlayer spacing of 3.35 Å, disappeared and the GrO peak generated at 16 , corresponding to 5.65 Å, with the increase in the oxidation time up to 3 h. However, even at an oxidation time of 2 h, the graphite peak at 26 remained, which implies that more resting time is required for the diffusion and reaction of the oxidizing agents through the graphitic layers. The TGA thermogram in Fig. 2b depicts the gradual increase in the oxidative functional groups with the increase in the oxidation time. As the oxidation time increased from 1 to 3 h, the peak of the sp2hybridized carbon atoms (284.4 eV) decreased and the peak of the sp3-hybridized carbon atoms (285.1 eV) increased. Furthermore, the peaks of the CeO bonds related to the epoxide group (286.5 eV) and the C]O bonds related to the carbonyl group (288.5 eV) increased (Fig. 2a). The XPS spectra in Fig. 2c depicts that the CeO bond related to the hydroxyl and epoxy groups was dominantly introduced by increasing the oxidation time, whereas only a small amount of carbonyl and carboxylic acid groups were detected. Then, the weight ratio of graphite to NaClO3 was varied from 1:5 to 1:10 (w:w) to investigate the effect of the abundance of oxidant molecules on the oxidation of graphite within a resting time of 1 h. As illustrated in Fig. 2c, the graphite peak almost disappeared at a NaClO3 to graphite ratio of 1:10, and more oxidative functional groups were introduced as indicated by the TGA data (Fig. 2d) and XPS spectra (Fig. 2e). The oxidation temperature of the graphite in TGA decreased with the addition of NaClO3, whereas as seen in Fig. 2b, the oxidation temperature did not change in a constant amount of NaClO3 even when the oxidation time was increased. The increase in the oxidative functional groups and decrease in the oxidation temperature of graphite indicate that more oxidant molecules were utilized to oxidize the inner basal plane of the graphite. However, in terms of commercialization, reducing the reaction time and the amount of used oxidant are profitable in minimizing the production cost. Thus, a more efficient oxidation process is needed to rapidly oxidize the graphite to obtain GrO powder without the existence of the graphite peak in the XRD spectra. To realize this, the kneading of the small-graphite dough (1:7.5 ¼ graphite: NaClO3) was continued through the oxidation
Please cite this article as: J.Y. Cho et al., Tailored and highly efficient oxidation of various-sized graphite by kneading for high-quality graphene nanosheets, Carbon, https://doi.org/10.1016/j.carbon.2019.10.102
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Fig. 1. Scheme of highly efficient oxidation of graphite inspired by the kneading of dough. A small amount of fuming nitric acid was slowly added to the graphite/NaClO3 mixture to form a dough, followed by kneading and resting for oxidation. (A colour version of this figure can be viewed online.)
Fig. 2. (a, d) XRD Plots, (b, e) TGA thermogram, and (c, f) XPS spectra of GrO by varying the oxidation time (aec) and by changing the weight ratio of graphite and NaClO3 (def). (A colour version of this figure can be viewed online.)
time. This resulted in the disappearance of the graphite peak within 1 h of the oxidation time, and the interlayer spacing of the GrO increased from 5.6 to 6.24 Å, as illustrated in Fig. 3a. This implies that kneading promoted the oxidation reaction of graphite by the light shear force, which was induced by the homogeneous mixing and rapid dissolution of NaClO3 powder in fuming nitric acid. An additional widening of the interlayer spacing of s-GrO powder up to 6.58 Å was observed in the XRD pattern after hydration and postoxidation by mixing some amount of water with the GrO dough, followed by stirring for 24 h. The XPS spectra (Fig. 3b) also indicates
the considerable increase in the CeO bonds and oxidative functional groups by kneading during oxidation. Based on the analysis of the XPS spectra, it was found that the C atomic ratio changed from 3.68 to 3.12 by kneading (Fig. 3c). However, the hydration and post oxidation by mixing water with the GrO dough only caused a slight oxidation, which corresponds to the TGA thermogram illustrated in Fig. 3d. Moreover, the 13C NMR in Fig. 3e and f indicates that the hydroxyl and epoxy groups in GrO powder quantitatively increased in comparison with the sp2 C]C bond by kneading during oxidation. These results clearly demonstrate that
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Fig. 3. Characterization of s-GrO samples prepared without kneading, with kneading, and additional post oxidation by adding water. (a) XRD plot; (b) XPS spectra; (c) C/O atomic ratio from XPS spectra; (d) TGA thermogram, 13C solid NMR spectra of s-GrO prepared without (e) and with (f) kneading. FESEM image of exfoliation of s-GrO prepared without (g) and with (h) kneading. (i) UVeVis absorption spectra of exfoliated s-GrO prepared without and with kneading. (j) Raman spectra of a small-sized graphite (s-graphite), s-GO, and srGO reduced by HI acid. (A colour version of this figure can be viewed online.)
continuous kneading during oxidation is beneficial for the rapid oxidation of graphite with a small amount of the acid solution, similar to the concept of kneading dough. The s-GrO powders were then exfoliated into graphene oxide using a high speed homogenizer at 10,000 rpm by adjusting the pH up to 11 [23]. Fig. 3g depicts the FESEM image of the exfoliated sGrO prepared without kneading. Most of the GrO particles were not fully exfoliated, whereas the s-GrO particles fabricated by kneading were fully exfoliated into GO (s-GO) nanosheets, as illustrated in Fig. 3h. In addition, the UVevis spectra of the GO solution (Fig. 3i) revealed the absorption peak near 250 nm, which indicates the pp* transitions of the CeC and C]C bonds. The other absorption peak near 300 nm indicates the n-p* transition of the CeO bonds. Typical polycyclic aromatic hydrocarbon peaks were observed in the fully exfoliated s-GO nanosheets, which only existed in the case of GO produced by ClO3-based oxidation [24]. To ensure that the GO
produced by kneading had a low defect, the GO and rGO were analyzed using Raman analysis. Raman spectroscopy is a useful analysis tool for characterizing. Structural defects. Fig. 3j depicts the Raman spectra of graphite, GO, and rGO, which features four prominent peaks, namely, D (1344 cm1), G (1581 cm1), D’ (1615 cm1), and 2D (2683 cm1). The intensity of the D peak is affected by the defective structures that appeared in the Raman spectrum of GO. However, after reduction with HI acid, a narrow and strong D peak indicates that rGO contains fewer number of permanent vacancies that are impossible to heal by chemical reduction [25]. In particular, the strong 2D peak also demonstrates the high crystalline structure of s-rGO fabricated through the kneading-assisted oxidation. The results described above were derived from 4 to 5 mm sized graphite. Small-sized graphite can be oxidized with an adequate amount of oxidant within 1 h through kneading. However, even by
Please cite this article as: J.Y. Cho et al., Tailored and highly efficient oxidation of various-sized graphite by kneading for high-quality graphene nanosheets, Carbon, https://doi.org/10.1016/j.carbon.2019.10.102
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kneading, to fully oxidize the larger-sized graphite (<75 and >150 mm), 1 h of oxidation time was not enough, as indicated by the XRD plots in Fig. 4a. To optimize the oxidation of the large-sized graphite by kneading, the oxidation time and the graphite/NaClO3 weight ratio were controlled. First, the medium-sized graphite (<75 mm) was fully oxidized into GrO (m-GrO) by increasing the oxidation time, as indicated in the XRD plot (Fig. 4b), and could be exfoliated into single layer GO nanosheets. From the XPS spectrum, more CeO and C]O bonds were introduced in the GO after oxidation for 3 h. However, the large-sized graphite (>150 mm) required a more severe oxidation condition. With an oxidation time of 3 h and a graphite:NaClO3 weight ratio of 1:7.5, a graphite peak in the XRD plot still appeared, as illustrated in Fig. 4d. Based on the oxidation of small-sized graphite, the amount of oxidant was increased to 1:10 (w:w) with 3 h of oxidation time. A larger amount of NaClO3 resulted in fully oxidized large GrO (L-GrO) powders with 5.92 Å of d-spacing in the XRD plot (Fig. 4e). In the XPS spectrum of L-GrO, it was found that more number of C]O bonds were introduced, which was still a smaller amount than that in the GrO produced by KMnO4-based oxidation [18]. The exfoliated GO from m-GrO (m-GO) and L-GrO (L-GO) was reduced under the same conditions as that for s-GO and analyzed by Raman spectroscopy (Fig. 5a). The intensity of the 2D peaks of
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the m- and L-rGO was remarkably high, which implies that high quality graphene can be produced by kneading-assisted oxidation of large-sized graphite powders followed by exfoliation, oxidation, and chemical reduction. The high resolution TEM image in Fig. 5b also indicates a directly highly crystalline and large sp2 domain. These structural characteristics can affect the electrical conductivity of the rGO electrodes. The rGO pastes were fabricated by HI reduction and dispersion assisted by sodium carboxylmethyl cellulose. These pastes were coated on the substrate and their electrical performances were estimated based on their IeV plots determined using two-point probe tests. As depicted in Fig. 5c, the L-rGO film was more conductive than the m- and s-rGO films in accordance with the lateral size of GO. The electrical conductivity of the L-rGO film reached 46,000 S/m, whereas those of the m- and srGO films were 26,500 and 16,800, respectively. Furthermore, thin film heaters were fabricated using filtrated rGO films. Fig. 5d depicts the heating behavior of the s-, m-, and L-rGO films as a function of time at an applied voltage of 3 V. The saturation temperatures were dependent on the electrical performances of the rGO films in accordance with the measured current during Joule heating (Fig. 5e). The temperature of the L-rGO film reached over 70 C even at 3 V, which indicates the high electrical conductivity of the rGO film. Moreover, the Joule-heated rGO film retained a
Fig. 4. (a) XRD plots of GrO powders depending on the size of used graphite (4e5, <75, >150 mm) prepared by 1 h oxidation time under kneading and 1:7.5 wt ratio of graphite and NaClO3. (b) XRD plot and (c) deconvoluted XPS spectrum of m-GrO prepared by 3 h oxidation time under kneading and 1:7.5 wt ratio of graphite:NaClO3. Inset image in (b) depicts the FE-SEM image of exfoliated m-GrO. (d) XRD plot of L-GrO prepared by 3 h oxidation time under kneading and 1:7.5 wt ratio of graphite:NaClO3. (e) XRD plot and (f) deconvoluted XPS spectrum of L-GrO prepared by 3 h oxidation time under kneading and 1:10 wt ratio of graphite:NaClO3. Inset image in (e) depicts the FE-SEM image of exfoliated LGrO. (A colour version of this figure can be viewed online.)
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Fig. 5. (a) Raman spectra of s-, m-, and L-rGO samples. (b) High resolution TEM image of rGO. (c) IeV plots of rGO films prepared by s-, m-, and L-rGO pastes. (d) Temperature profiles and (e) current changes of rGO films by applying DC 3 V as a function of time. (f) IR temperature camera image of Joule-heated rGO film at 3 V. (A colour version of this figure can be viewed online.)
uniform heating behavior under bending, as illustrated in Fig. 5f.
Appendix A. Supplementary data
4. Conclusion
Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2019.10.102.
We introduced an effective oxidizing method for various-sized graphite, called kneading. When the graphite sheet size is small, the graphite can be effectively oxidized within an hour by using an appropriate amount of oxidant during kneading. However, with the increase in the graphite size, the conditions of oxidation need to be adjusted to fully oxidize the graphite by diffusion-controlled oxidation of the graphitic layers. Graphite can be oxidized within a few hours by kneading because the kneading promotes the intercalation of the oxidant through the graphite sheets. The disappearance of the graphite peak in the XRD analysis, and the XPS and Raman analysis revealed that the oxidation of graphite by kneading was an effective strategy. Additionally, the Raman spectrum of rGO demonstrated the recovery of the sp2 domain after chemical reduction. Therefore, kneading facilitated the efficient oxidation in a short time with a lesser amount of acid, and resulted in a highly crystalline structure of rGO after the chemical reduction of GO. Our proposed kneading method for GrO production can be applied to mass production of high quality graphene nanosheets. Declaration of competing interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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