- Email: [email protected]
Thermal properties of NO2–graphite intercalation compound prepared by gas-phase processing
Materials Letters 189 (2017) 279–281
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Thermal properties of NO2–graphite intercalation compound prepared by gas-phase processing Lv Xiaomeng, Wang Xuanjun n, Li Xia Research Institute of High Technology, Xi'an, Shaanxi 710025, China
art ic l e i nf o
a b s t r a c t
Article history: Received 20 September 2016 Received in revised form 1 December 2016 Accepted 7 December 2016 Available online 18 December 2016
Natural flake graphite and nitrogen dioxide (NO2) gas were used as host material and intercalating agent respectively, to prepare NO2–graphite intercalation compound (NO2–GIC) by direct gas-phase processing. Thermal properties of NO2–GIC were investigated by storage experimental and thermal gravity analysis, Fourier transform infrared spectrometry, and elemental analysis. Temperatures higher than 50 °C were disadvantageous to the maintenance of the chemical stability of NO2–GIC, easily resulting in the thermal decomposition of NO2–GIC. The intensity of the infrared absorption peaks at 147.29 °C and was stronger than that at other temperatures. This phenomenon suggests the large release of heat and gaseous molecules during decomposition processing of NO2–GIC. The mole ratio of NO2 and H2O gas molecules released was approximately 5.02:1, as analyzed by each infrared absorption peak area and elemental analysis results. & 2016 Elsevier B.V. All rights reserved.
Keywords: Thermal properties Carbon materials NO2–GIC Thermal gravity analysis FTIR
1. Introduction
2. Experimental
Exfoliated graphite (EG) and graphene are very important carbon materials that have attracted considerable attention owing to their remarkable unique properties for manufacturing flexible graphite [1], thermal energy storage materials [2], and thermal insulators [3–6], and so on. In recent years, a more increasing interest has been observed in graphite intercalation compound (GIC) prepared by gas-phase processing [7,8] without treatment of strong corrosive acid wastes or mixtures than that prepared by liquid-phase processing [9,10]. Nitrogen dioxide–graphite intercalation compound (NO2–GIC), as a new precursor for exfoliated graphite and graphene [11,12], could be prepared by natural flake graphite (NFG) and NO2 gas by direct gas-phase processing [13]. Thermal properties of NO2–GIC at high temperature environment should be studied for its influence on exfoliation volume (EV) of EG and quality of graphene. In this study, a comparative investigation on the chemical stability of NO2–GIC is carried out by storage experimental. Thermal decomposition processing of NO2– GIC is analyzed by thermal gravity (TG) analysis, Fourier transform infrared spectrometry (FTIR), and elemental analysis (EA).
2.1. Materials and methods
n
Corresponding author. E-mail address: [email protected] (W. Xuanjun).
http://dx.doi.org/10.1016/j.matlet.2016.12.023 0167-577X/& 2016 Elsevier B.V. All rights reserved.
NO2–GIC was prepared by direct reaction of nitrogen dioxide gas with natural flake graphite. This preparation involved a onestep method under optimum conditions of 50 °C and 0.45 MPa for 48 h in a dry, sealed steel container [13]. The twelve 1.000g NO2–GIC samples were placed in a quartz beaker and then left in the air for 60 days. The mass and EV of sample were measured and recorded every five days. Another eight 1.000g NO2–GIC samples were placed into a furnace at 50 °C, 75 °C, 100 °C, 150 °C, 200 °C, 250 °C, 300 °C, and 400 °C heating for 30 min, respectively. The mass and EV of samples were also measured. A comparative study of elemental distinctions between NO2–GIC and EG sample was carried out. Thermal decomposition processing of NO2–GIC was studied by thermal gravity–derivative thermal gravity (TG–DTG) analysis and FTIR. 2.2. Characterization The EV of EG sample was measured according to the Chinese National Standard GB 10698-1989. Elemental analysis was performed in a VARIO EL III elemental analyzer system (German, ELEMENTAR, CHNS and O modes, and high temperature combustion method, 1150 °C). Thermal gravity analysis apparatus (German, NETZSCH STA 409PC, 30–800 °C, 10 K/min) was used for the TG–DTG analysis to show the mass loss changing during thermal decomposition of NO2–GIC samples. Fourier transform infrared
280
L. Xiaomeng et al. / Materials Letters 189 (2017) 279–281
spectrometry (German, BRUKER, VERTEX 70, 700–4000 cm 1) was used to analyze infrared (IR) spectra details changing during thermal decomposition of NO2–GIC samples.
3. Results and discussion Fig. 1 shows the chemical stability of NO2–GIC samples left in the air for 60 days (Fig. 1A) and placed into the furnace at different temperatures for 30 min (Fig. 1B). The mass of NO2–GIC and the EV of EG remain almost the same in the air as shown in Fig. 1A. This condition indicates that NO2–GIC exhibits good chemical stability and can easily be stored at room temperature. Fig. 1B shows that the increase in heating from 50 °C to 400 °C results in the slow decrease of NO2–GIC mass from 1.000g to 0.891 g and EV of EG from 240 mL/g to 0 mL/g dramatically, especially in the range from 100 °C to 150 °C. This result indicates that temperature higher than 50 °C is disadvantageous to the chemical stability of NO2–GIC, especially to EV of EG. Thermal investigation of NO2–GIC was studied by TG–DTG analysis (heating rate: 10 K min 1) as shown in Fig. 2. The result shows a complicated thermal decomposition character of NO2–GIC comprising two stages of mass loss. The first endothermic peak of NO2–GIC from 47.28°C to 88.74 °C with a mass loss of 3.56% should be evidence for the desorption and volatilization of adsorbed small gas molecules with low stability. The second endothermic peak shows that the thermal decomposition of NO2–GIC in the range of 88.74–169.2 °C with a mass loss of 7.32% is responsible for
Fig. 1. Chemical stability of NO2–GIC samples (A) left in the air at room temperature for 60 days and (B) placed into the furnace at different temperatures for 30 min.
Fig. 2. TG–DTG curves of NO2–GIC (10 K min 1).
exfoliation. The two temperature points 71.3 °C and 114.3 °C had the most dramatic mass losses, respectively. The total mass loss ratio was 10.88% during the whole thermal decomposition process at the temperature range of 47.28–169.2 °C. A few mass increases were observed at the beginning of the endothermic curve because of the liberation of small gaseous molecules, thereby resulting in microbalance buoyancy. The amounts of small gaseous molecules released evolving in three stages at different temperatures were measured as shown in Fig. 3. More amounts of gaseous molecules were released at stage 3 (111.84–200.5 °C) than that at stage 1 (30–70.39 °C) and stage 2 (70.39–111.84 °C). Gaseous molecules released at stages 1 and 2 at below 111.84 °C are mainly responsible for the desorption and volatilization of small molecules. In addition, the increased amounts of gaseous molecules released at stage 3 upon decomposition reaction of NO2–GIC were advantageous in generating high gas pressure for high expansion of exfoliation. As shown in Fig. 4, the FTIR spectra of gaseous molecules released at different temperatures were determined by Fourier transform infrared spectrometry. Weak infrared absorption peaks were obtained from the curve at 70.39 °C. Less strong infrared absorption peaks at 892, 1316, 1338, and 3253 cm 1 were obtained from curves at 102.27 °C and 111.84 °C. These peaks were accompanied by the desorption of adsorbed NO2 and a few H2O molecules at the edge of adjacent carbon layers. These infrared absorption peaks obtained at stages 1 and 2 were the secondary reasons for exfoliation only, thereby corresponding with the measurement of EV of EG. At stage 3, the strongest peaks were obtained from the curve at 147.29 °C, wherein the intensity was approximately 10 times stronger than that at 70.39 °C, 102.27 °C,
Fig. 3. Amounts of gaseous molecules released at the curve of NO2–GIC at different temperatures.
L. Xiaomeng et al. / Materials Letters 189 (2017) 279–281
281
released was approximately 5.02:1 during the decomposition processing. This result corresponded with the data of chemical stability (Fig. 1) and the infrared absorption peak areas (Fig. 4) at 112.13 °C, 147.29 °C, and 200.5 °C.
4. Conclusions
Fig. 4. FTIR spectra of gaseous molecules released at different temperatures. Table 1 Elemental analysis of NFG, NO2–GIC, and EG. Mass content [%]
NFG NO2–GIC EG (EV ¼240 mL/g)
C
H
N
S
O
99.85 85.15 99.66
0.015 0.121 0.026
0.009 4.053 0.102
0.005 0.012 0.003
0.122 10.67 0.211
and 111.84 °C. The sharp peaks at 1596 and 1630 cm 1 were assigned to release large amounts of NO2 molecules because of thermal decomposition reaction of NO2–GIC occurrence. This condition is mainly responsible for the large exfoliation. The weak peaks at 893, 1316, 3255, and 3567 cm 1 belonged to H2O, –OH, and from the curve at 147.29 °C. This phenomenon is due to the water molecules adsorbed on the graphite surface. The water molecules are involved in the intercalation processes with NO2 forming nitric acid and corresponded with the result of elemental analysis (Table 1). As shown in Table 1, the mass content of C in NO2–GIC decreased to 85.15%. Meanwhile, the mass content of N and O in NO2–GIC increased to 4.053% and 10.67%, respectively, compared with that of NFG. This result suggests an intercalation reaction between graphite and NO2 gas forming NO2–GIC. The appearance of 0.121% mass content of H in NO2–GIC could be related to the intercalation reaction between graphite and nitric acid forming HNO3–GIC because of the adsorbed water. After exfoliation, the mass content of C in EG increased to 99.66%. Meanwhile, the mass content of H, N, and O in EG decreased to 0.026%, 0.102%, and 0.211%, respectively. This result suggests that gaseous molecules obtained during thermal decomposition of NO2–GIC were released. In NO2–GIC, the atomic ratio of H, N, and O was 1:2.39:5.52, and the corresponding mole ratio of NO2 and H2O gaseous molecules
In the present study, a series of comparative investigation on the chemical stability of NO2–GIC is carried out by storage experimental analysis in the air for 60 days and in the furnace at different temperatures. Thermal decomposition processing of NO2–GIC was investigated by TG–DTG analysis, FTIR, and EA. The intensity of the infrared absorption peaks at 147.29 °C was stronger than at other temperatures, which suggests the large release of heat and gaseous molecules during thermal decomposition processing. The corresponding mole ratio of NO2 and H2O gaseous molecules released was approximately 5.02:1. NO2–GIC as a new precursor for manufacturing important carbon materials, should be stored at a proper temperature environment lower than 50 °C to maintain good chemical stability and to establish an effective route for safe treatment of waste nitro oxidizer propellant.
References [1] D.D.L. Chung, Exfoliation of graphite, J. Mater. Sci. 22 (12) (1987) 4910–4918. [2] G.Y. Fang, H. Li, Z. Chen, X. Liu, Preparation and characterization of stearic acid/ expanded graphite composites as thermal energy storage materials, Energy 35 (12) (2010) 4622–4626. [3] M.S. Dresselhaus, G. Dresselhaus, Intercalation compounds of graphite, Adv. Phys. 51 (1) (2010) 5–35. [4] A.K. Geim, K.S. Novoselov, The rise of graphene, Nat. Mater. 6 (3) (2007) 183–191. [5] H.K. Shin, K.Y. Rhee, S.J. Park, Effects of exfoliated graphite on the thermal properties of erythritol-based composites used as phase-change materials, Compos. Part B 96 (2016) 350–353. [6] P.H. Chen, D.D.L. Chung, Dynamic mechanical behavior of flexible graphite made from exfoliated graphite, Carbon 50 (1) (2012) 283–289. [7] V.V. Avdeev, I.U. Martynov, I.V. Nikol`skaya, L.A. Monyakina, N.E. Sorokina, Investigation of the graphite-H2SO4-gaseous oxidizer (Cl2, O3, SO3) system, J. Phys. Chem. Solids 57 (1996) 837–840. [8] B.J. Lee, Characteristics of exfoliated graphite prepared by intercalation of gaseous SO3 into graphite, Bull. Korean Chem. Soc. 23 (12) (2002) 1801–1805. [9] N.E. Sorokina, N.V. Maksimova, V.V. Avdeev, Anodic oxidation of graphite in 10% to 98% HNO3, Inorg. Mater. 37 (4) (2001) 360–365. [10] A.V. Dunaev, I.V. Arkhangelsky, Y.V. Zubavichus, V.V. Avdeev, Preparation, structure and reduction of graphite intercaltion compounds with hexachloroplatinic acid, Carbon 46 (5) (2008) 788–795. [11] R. Park, R.S. Ruoff, Chemical methods for the production of graphenes, Nat. Nanotechnol. 4 (4) (2009) 217–224. [12] O.C. Compton, S.T. Nguyen, Graphene oxide, highly reduced grapheme oxide and graphene: versatile building blocks for carbon based materials, Small 6 (6) (2010) 711–723. [13] X.M. Lv, X.J. Wang, Z.Y. Huang, X.X. Liu, et al., Preparation of exfoliated graphite intercalated with nitrogen dioxide by direct gas-phase processing, Mater. Lett. 136 (2014) 48–51.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Thermal properties of NO2–graphite intercalation compound prepared by gas-phase processing Lv Xiaomeng, Wang Xuanjun n, Li Xia Research Institute of High Technology, Xi'an, Shaanxi 710025, China
art ic l e i nf o
a b s t r a c t
Article history: Received 20 September 2016 Received in revised form 1 December 2016 Accepted 7 December 2016 Available online 18 December 2016
Natural flake graphite and nitrogen dioxide (NO2) gas were used as host material and intercalating agent respectively, to prepare NO2–graphite intercalation compound (NO2–GIC) by direct gas-phase processing. Thermal properties of NO2–GIC were investigated by storage experimental and thermal gravity analysis, Fourier transform infrared spectrometry, and elemental analysis. Temperatures higher than 50 °C were disadvantageous to the maintenance of the chemical stability of NO2–GIC, easily resulting in the thermal decomposition of NO2–GIC. The intensity of the infrared absorption peaks at 147.29 °C and was stronger than that at other temperatures. This phenomenon suggests the large release of heat and gaseous molecules during decomposition processing of NO2–GIC. The mole ratio of NO2 and H2O gas molecules released was approximately 5.02:1, as analyzed by each infrared absorption peak area and elemental analysis results. & 2016 Elsevier B.V. All rights reserved.
Keywords: Thermal properties Carbon materials NO2–GIC Thermal gravity analysis FTIR
1. Introduction
2. Experimental
Exfoliated graphite (EG) and graphene are very important carbon materials that have attracted considerable attention owing to their remarkable unique properties for manufacturing flexible graphite [1], thermal energy storage materials [2], and thermal insulators [3–6], and so on. In recent years, a more increasing interest has been observed in graphite intercalation compound (GIC) prepared by gas-phase processing [7,8] without treatment of strong corrosive acid wastes or mixtures than that prepared by liquid-phase processing [9,10]. Nitrogen dioxide–graphite intercalation compound (NO2–GIC), as a new precursor for exfoliated graphite and graphene [11,12], could be prepared by natural flake graphite (NFG) and NO2 gas by direct gas-phase processing [13]. Thermal properties of NO2–GIC at high temperature environment should be studied for its influence on exfoliation volume (EV) of EG and quality of graphene. In this study, a comparative investigation on the chemical stability of NO2–GIC is carried out by storage experimental. Thermal decomposition processing of NO2– GIC is analyzed by thermal gravity (TG) analysis, Fourier transform infrared spectrometry (FTIR), and elemental analysis (EA).
2.1. Materials and methods
n
Corresponding author. E-mail address: [email protected] (W. Xuanjun).
http://dx.doi.org/10.1016/j.matlet.2016.12.023 0167-577X/& 2016 Elsevier B.V. All rights reserved.
NO2–GIC was prepared by direct reaction of nitrogen dioxide gas with natural flake graphite. This preparation involved a onestep method under optimum conditions of 50 °C and 0.45 MPa for 48 h in a dry, sealed steel container [13]. The twelve 1.000g NO2–GIC samples were placed in a quartz beaker and then left in the air for 60 days. The mass and EV of sample were measured and recorded every five days. Another eight 1.000g NO2–GIC samples were placed into a furnace at 50 °C, 75 °C, 100 °C, 150 °C, 200 °C, 250 °C, 300 °C, and 400 °C heating for 30 min, respectively. The mass and EV of samples were also measured. A comparative study of elemental distinctions between NO2–GIC and EG sample was carried out. Thermal decomposition processing of NO2–GIC was studied by thermal gravity–derivative thermal gravity (TG–DTG) analysis and FTIR. 2.2. Characterization The EV of EG sample was measured according to the Chinese National Standard GB 10698-1989. Elemental analysis was performed in a VARIO EL III elemental analyzer system (German, ELEMENTAR, CHNS and O modes, and high temperature combustion method, 1150 °C). Thermal gravity analysis apparatus (German, NETZSCH STA 409PC, 30–800 °C, 10 K/min) was used for the TG–DTG analysis to show the mass loss changing during thermal decomposition of NO2–GIC samples. Fourier transform infrared
280
L. Xiaomeng et al. / Materials Letters 189 (2017) 279–281
spectrometry (German, BRUKER, VERTEX 70, 700–4000 cm 1) was used to analyze infrared (IR) spectra details changing during thermal decomposition of NO2–GIC samples.
3. Results and discussion Fig. 1 shows the chemical stability of NO2–GIC samples left in the air for 60 days (Fig. 1A) and placed into the furnace at different temperatures for 30 min (Fig. 1B). The mass of NO2–GIC and the EV of EG remain almost the same in the air as shown in Fig. 1A. This condition indicates that NO2–GIC exhibits good chemical stability and can easily be stored at room temperature. Fig. 1B shows that the increase in heating from 50 °C to 400 °C results in the slow decrease of NO2–GIC mass from 1.000g to 0.891 g and EV of EG from 240 mL/g to 0 mL/g dramatically, especially in the range from 100 °C to 150 °C. This result indicates that temperature higher than 50 °C is disadvantageous to the chemical stability of NO2–GIC, especially to EV of EG. Thermal investigation of NO2–GIC was studied by TG–DTG analysis (heating rate: 10 K min 1) as shown in Fig. 2. The result shows a complicated thermal decomposition character of NO2–GIC comprising two stages of mass loss. The first endothermic peak of NO2–GIC from 47.28°C to 88.74 °C with a mass loss of 3.56% should be evidence for the desorption and volatilization of adsorbed small gas molecules with low stability. The second endothermic peak shows that the thermal decomposition of NO2–GIC in the range of 88.74–169.2 °C with a mass loss of 7.32% is responsible for
Fig. 1. Chemical stability of NO2–GIC samples (A) left in the air at room temperature for 60 days and (B) placed into the furnace at different temperatures for 30 min.
Fig. 2. TG–DTG curves of NO2–GIC (10 K min 1).
exfoliation. The two temperature points 71.3 °C and 114.3 °C had the most dramatic mass losses, respectively. The total mass loss ratio was 10.88% during the whole thermal decomposition process at the temperature range of 47.28–169.2 °C. A few mass increases were observed at the beginning of the endothermic curve because of the liberation of small gaseous molecules, thereby resulting in microbalance buoyancy. The amounts of small gaseous molecules released evolving in three stages at different temperatures were measured as shown in Fig. 3. More amounts of gaseous molecules were released at stage 3 (111.84–200.5 °C) than that at stage 1 (30–70.39 °C) and stage 2 (70.39–111.84 °C). Gaseous molecules released at stages 1 and 2 at below 111.84 °C are mainly responsible for the desorption and volatilization of small molecules. In addition, the increased amounts of gaseous molecules released at stage 3 upon decomposition reaction of NO2–GIC were advantageous in generating high gas pressure for high expansion of exfoliation. As shown in Fig. 4, the FTIR spectra of gaseous molecules released at different temperatures were determined by Fourier transform infrared spectrometry. Weak infrared absorption peaks were obtained from the curve at 70.39 °C. Less strong infrared absorption peaks at 892, 1316, 1338, and 3253 cm 1 were obtained from curves at 102.27 °C and 111.84 °C. These peaks were accompanied by the desorption of adsorbed NO2 and a few H2O molecules at the edge of adjacent carbon layers. These infrared absorption peaks obtained at stages 1 and 2 were the secondary reasons for exfoliation only, thereby corresponding with the measurement of EV of EG. At stage 3, the strongest peaks were obtained from the curve at 147.29 °C, wherein the intensity was approximately 10 times stronger than that at 70.39 °C, 102.27 °C,
Fig. 3. Amounts of gaseous molecules released at the curve of NO2–GIC at different temperatures.
L. Xiaomeng et al. / Materials Letters 189 (2017) 279–281
281
released was approximately 5.02:1 during the decomposition processing. This result corresponded with the data of chemical stability (Fig. 1) and the infrared absorption peak areas (Fig. 4) at 112.13 °C, 147.29 °C, and 200.5 °C.
4. Conclusions
Fig. 4. FTIR spectra of gaseous molecules released at different temperatures. Table 1 Elemental analysis of NFG, NO2–GIC, and EG. Mass content [%]
NFG NO2–GIC EG (EV ¼240 mL/g)
C
H
N
S
O
99.85 85.15 99.66
0.015 0.121 0.026
0.009 4.053 0.102
0.005 0.012 0.003
0.122 10.67 0.211
and 111.84 °C. The sharp peaks at 1596 and 1630 cm 1 were assigned to release large amounts of NO2 molecules because of thermal decomposition reaction of NO2–GIC occurrence. This condition is mainly responsible for the large exfoliation. The weak peaks at 893, 1316, 3255, and 3567 cm 1 belonged to H2O, –OH, and from the curve at 147.29 °C. This phenomenon is due to the water molecules adsorbed on the graphite surface. The water molecules are involved in the intercalation processes with NO2 forming nitric acid and corresponded with the result of elemental analysis (Table 1). As shown in Table 1, the mass content of C in NO2–GIC decreased to 85.15%. Meanwhile, the mass content of N and O in NO2–GIC increased to 4.053% and 10.67%, respectively, compared with that of NFG. This result suggests an intercalation reaction between graphite and NO2 gas forming NO2–GIC. The appearance of 0.121% mass content of H in NO2–GIC could be related to the intercalation reaction between graphite and nitric acid forming HNO3–GIC because of the adsorbed water. After exfoliation, the mass content of C in EG increased to 99.66%. Meanwhile, the mass content of H, N, and O in EG decreased to 0.026%, 0.102%, and 0.211%, respectively. This result suggests that gaseous molecules obtained during thermal decomposition of NO2–GIC were released. In NO2–GIC, the atomic ratio of H, N, and O was 1:2.39:5.52, and the corresponding mole ratio of NO2 and H2O gaseous molecules
In the present study, a series of comparative investigation on the chemical stability of NO2–GIC is carried out by storage experimental analysis in the air for 60 days and in the furnace at different temperatures. Thermal decomposition processing of NO2–GIC was investigated by TG–DTG analysis, FTIR, and EA. The intensity of the infrared absorption peaks at 147.29 °C was stronger than at other temperatures, which suggests the large release of heat and gaseous molecules during thermal decomposition processing. The corresponding mole ratio of NO2 and H2O gaseous molecules released was approximately 5.02:1. NO2–GIC as a new precursor for manufacturing important carbon materials, should be stored at a proper temperature environment lower than 50 °C to maintain good chemical stability and to establish an effective route for safe treatment of waste nitro oxidizer propellant.
References [1] D.D.L. Chung, Exfoliation of graphite, J. Mater. Sci. 22 (12) (1987) 4910–4918. [2] G.Y. Fang, H. Li, Z. Chen, X. Liu, Preparation and characterization of stearic acid/ expanded graphite composites as thermal energy storage materials, Energy 35 (12) (2010) 4622–4626. [3] M.S. Dresselhaus, G. Dresselhaus, Intercalation compounds of graphite, Adv. Phys. 51 (1) (2010) 5–35. [4] A.K. Geim, K.S. Novoselov, The rise of graphene, Nat. Mater. 6 (3) (2007) 183–191. [5] H.K. Shin, K.Y. Rhee, S.J. Park, Effects of exfoliated graphite on the thermal properties of erythritol-based composites used as phase-change materials, Compos. Part B 96 (2016) 350–353. [6] P.H. Chen, D.D.L. Chung, Dynamic mechanical behavior of flexible graphite made from exfoliated graphite, Carbon 50 (1) (2012) 283–289. [7] V.V. Avdeev, I.U. Martynov, I.V. Nikol`skaya, L.A. Monyakina, N.E. Sorokina, Investigation of the graphite-H2SO4-gaseous oxidizer (Cl2, O3, SO3) system, J. Phys. Chem. Solids 57 (1996) 837–840. [8] B.J. Lee, Characteristics of exfoliated graphite prepared by intercalation of gaseous SO3 into graphite, Bull. Korean Chem. Soc. 23 (12) (2002) 1801–1805. [9] N.E. Sorokina, N.V. Maksimova, V.V. Avdeev, Anodic oxidation of graphite in 10% to 98% HNO3, Inorg. Mater. 37 (4) (2001) 360–365. [10] A.V. Dunaev, I.V. Arkhangelsky, Y.V. Zubavichus, V.V. Avdeev, Preparation, structure and reduction of graphite intercaltion compounds with hexachloroplatinic acid, Carbon 46 (5) (2008) 788–795. [11] R. Park, R.S. Ruoff, Chemical methods for the production of graphenes, Nat. Nanotechnol. 4 (4) (2009) 217–224. [12] O.C. Compton, S.T. Nguyen, Graphene oxide, highly reduced grapheme oxide and graphene: versatile building blocks for carbon based materials, Small 6 (6) (2010) 711–723. [13] X.M. Lv, X.J. Wang, Z.Y. Huang, X.X. Liu, et al., Preparation of exfoliated graphite intercalated with nitrogen dioxide by direct gas-phase processing, Mater. Lett. 136 (2014) 48–51.