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Preparation and characterization of sulfur-free exfoliated graphite with large exfoliated volume
Materials Letters 73 (2012) 11–13
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Preparation and characterization of sulfur-free exfoliated graphite with large exfoliated volume Xiu-Juan Yu a,⁎, Juan Wu a, b, 1, Qi Zhao a, 1, Xiu-Wen Cheng a, 1 a b
Department of Environmental Science and Engineering, Heilongjiang University, Harbin 150080, PR China Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People's Republic of China, Heilongjiang University, Harbin 150080, PR China
a r t i c l e
i n f o
Article history: Received 30 December 2010 Accepted 19 November 2011 Available online 6 December 2011 Keywords: Carbon materials Porous materials Exfoliated graphite Sulfur-free Exfoliated volume
a b s t r a c t Sulfur-free exfoliated graphite with large exfoliated volume was prepared via chemical oxidation combined with microwave irradiation by using nature flake graphite, perchloric acid and potassium permanganate as raw materials. The influences of some experimental parameters on the exfoliated volume were discussed in detail. It was found that the optimum preparing conditions went as follows: m(nature flake graphite): m(perchloric acid):m(potassium permanganate) = 1:4:0.5, the reaction temperature and time were 35 °C and 40 minutes, respectively. The exfoliated volume of exfoliated graphite which was obtained by microwave irradiation could reach up to 760 ml g− 1. Many network pores existed in the worm-like particles of exfoliated graphite according to scanning electron microscope images. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.
1. Introduction
2.2. Sample preparation
Exfoliated graphite (EG) is a new kind of functional nano-carbon material which is made from natural flake graphite, through the process of intercalating, washing, drying and heat expansion [1]. Exfoliated graphite has been widely used in gasket, electromagnetic interference shielding, electrochemical applications, stress sensing and thermal insulator due to its lightweight, soft, porosity, high-surface area and high-surface activity [2–4]. Sulfuric acid often served as intercalating agent to prepare exfoliated graphite in chemistry oxidation, but the residue acids will limit its application, such as flexible graphite in seal materials [5]. In this work, EG was prepared by chemical oxidation combined with microwave irradiation and perchloric acid was used to replace the concentrated sulfuric acid that served as oxidizing and intercalating agent . The influences of some experimental parameters on the exfoliated volume (EV) were discussed in detail. The porosity of EG was characterized by scanning electron microscope (SEM).
The preparation process of EG went as follows: under stirring, the mixture consisted of 2 g nature flake graphite and potassium permanganate was added into perchloric acid in a water bath at a certain temperature. After continuously stirring at a scheduled time, the GICs were synthesized and then the mixture was washed close to neutrality with distilled water by a vacuum filter, which is aimed to remove the remaining acid on the surface of GICs. Then the GICs were dried for 12 h in an oven at 353 K. Finally, exfoliation occurred in a domestic microwave oven at 750 W for 20 s, and the worm-like EG was obtained.
2. Experimental procedure 2.1. Raw materials Nature flake graphite (carbon content, 95 wt.%; 50 mesh, produced in the city of Jixi, China). Perchloric acid (72 wt.%; A.R.); potassium permanganate (99.8 wt.%; A.R.) ⁎ Corresponding author. Tel.: + 86 451 86608549; fax: + 86 451 86603259. E-mail address: [email protected] (X.-J. Yu). 1 Tel.: + 86 451 86608549; fax: + 86 451 86603259.
2.3. The measure of exfoliated volume The exfoliated volume (EV) was measured using a 50 ml graduated cylinder and the operation went as follows: EG was transferred into graduated cylinder lightly, then the volume (V) and quantity (M) of EG were recorded and the EV was calculated by the following equation: EV = V/M (ml·g − 1).
2.4. Materials characterization The microstructure of sample was characterized by SEM. SEM images were obtained with an instrument (SEM: model XL-30-ESEMFEG, Philips Co., Holland). The acceleration voltage was 20.00 kV and EG was dealt with gold jetting before scanning.
0167-577X/$ – see front matter. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.11.078
12
X.-J. Yu et al. / Materials Letters 73 (2012) 11–13
is A2B3C2D4. That is, the ratio of weight is m(nature flake graphite): m(perchloric acid):m(potassium permanganate) = 1:4:0.5, and the reacting temperature and time were 35 °C and 40 min, respectively. The EV of EG could reach up to 760 ml g − 1 under optimum condition.
Table 1 Orthogonal experiment and results of EG. Sample
A
B
C
D
EV
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 k1 k2 k3 k4 R
6.0 6.0 6.0 6.0 8.0 8.0 8.0 8.0 10.0 10.0 10.0 10.0 12.0 12.0 12.0 12.0 485.00 498.75 487.50 469.25 29.50
0.4 0.8 1.0 1.2 0.4 0.8 1.0 1.2 0.4 0.8 1.0 1.2 0.4 0.8 1.0 1.2 363.00 467.75 559.00 550.75 196.00
25 35 45 60 35 25 60 45 45 60 25 35 60 45 35 25 480.75 537.25 474.25 448.25 89.00
10 20 30 40 30 40 10 20 40 30 20 10 20 10 40 30 453.50 485.50 492.75 508.75 55.25
294 542 568 536 435 513 526 521 400 408 556 586 323 408 586 560 – – – – –
3.2. Influence of the dosage of raw materials on expanded volume
A—perchloric acid (g); B—potassium permanganate (g); C—reaction temperature (°C); D—reaction time (minutes).
3. Results and discussions 3.1. The optimum preparing conditions Table 1 shows the orthogonal experiment and results of EG at different reaction conditions caused different EVs, and the mass of potassium permanganate is the most important factor for exfoliated volume according to R value. The most suitable factor combination
Fig. 1(A) shows that when the dosage of potassium permanganate is too large or too small, EV is not the maximum. Firstly, the more the mass of potassium permanganate, the larger the EV; then when the dosage of potassium permanganate exceeds 1.0 g, EV decreases sharply. The reason is that potassium permanganate served as assistant oxidant could open the layer of nature flake graphite and ensures intercalating agent gets into the interlayer smoothly. If there are excessive potassium permanganate in the reacting system, the structure of EG will be destroyed due to the excessive oxidization or the inserting reagent will outflow from the overlarge width of layer. If the dosage of potassium permanganate is not enough, it cannot open the layer of nature flake graphite due to lack of oxidizing. From Fig. 1(B), we can see the influence of the dosage of perchloric acid on EV clearly. The variation trend of perchloric acid is similar to potassium permanganate as discussed above. Perchloric acid served as oxidizing reagent and intercalating agent in the preparation of EG. If the dosage of perchloric acid exceeds, it will cause the over oxidant of nature flake graphite and the waste of raw material. If the dosage of perchloric acid is not enough to satisfy the requirement of inserting, EV will decrease sharply. 3.3. Influence of the reaction time and temperature on expanded volume From Fig. 1(C), 35 °C is the optimum temperature for the preparation of EG; as the temperature increases, the reaction speed of 560
600
B
540
A
Exfoliated graphite/mL/g
Exfoliated graphite/mL/g
550 500 450 400 350 300 250
520 500 480 460 440 420
200 150
0.4
0.6
0.8
1.0
1.2
400
1.4
4
5
510
Exfoliated graphite/mL/g
Exfoliated graphite/ml/g
C
520 500 480 460 440
8
9
10
11
12
13
14
D
500 490 480 470 460 450
420
400 20
7
520
560 540
6
the dosage of perchloric acid/g
the dosage of potassium permanganate/g
440 25
30
35
40
45
50
reaction temperature/0C
55
60
65
0
10
20
30
40
50
60
reaction time/min
Fig. 1. The effect of reaction parameters on the exfoliated volume: A. potassium permanganate; B. perchloric acid; C. reaction temperature; D. reaction time.
X.-J. Yu et al. / Materials Letters 73 (2012) 11–13
a
b
200um
d
13
c
200um
e
5um
500um
f
20um
50um
Fig. 2. SEM images of carbon material: (a) nature flake graphite; (b)GICs;(c) exfoliated graphite(d) the layer structure of nature flake graphite; (e)the layer structure of GICs;(f) the inner structure of exfoliated graphite.
oxidizing and intercalating will accelerate, but higher temperature will accelerate the volatilization of perchloric acid and over oxidation. From Fig. 1(D), the EV increases as the reaction time increase, and the longer the time, the larger the EV. In order to investigate the influence of extending reaction time on EV, the experiment was operated as follows: the ratio of weight is m(nature flake graphite):m(perchloric acid):m(potassium permanganate) = 1:4:0.5, the reaction temperature is 35 °C and the reaction time is 60 min, respectively. The EV of EG could reach to 780 ml g − 1. When the inserting agent inserted into the interlayer reach to saturation, the differences in EV are not significant. Forty minutes was chosen as the optimum reaction time in this experiment.
4. Conclusions Using perchloric acid and potassium permanganate as oxidant and intercalating agent is an effective method to prepare sulfur-free exfoliated graphite with large expanded volume. The optimum condition for the preparation of exfoliated graphite went as follows: m(nature flake graphite):m(perchloric acid):m(potassium permanganate) = 1:4:0.5, the reaction temperature and time was 35 °C and 40 minutes, respectively. The EV of EG which obtained at the optimum conditions by microwave irradiation could reach up to 760 ml g − 1. The SEM images showed that many network pores existed inside the worm-like particles.
3.4. Analysis of porosity References Fig. 2(a–e) showed the SEM images of the nature flake graphite, GICs and exfoliated graphite. The relationships among six images were analyzed by comparing the obviously alteration of appearance, the distance of layer and pore structure, respectively. By comparing (a) and (b) images, the surface of GICs is cracked and the edge of GICs is curly. Similarly, the SEM image of (d) and (e) showed that oxidation enlarged the distance of layer and the layered structure of flake graphite was not destroyed. After microwave exfoliation, worm-like particles was observed from Fig. 2(c) and many irregular network pores existed in the worm-like particles of EG from Fig. 2(f). According to Fig. 2(c) and Fig. 2(f), microwave irradiation could distort the graphite and change its appearance and structure.
[1] Zheng Y-P, Wang H-N, Kang F-Y, Wang L-N, Inagaki M. Sorption capacity of exfoliated graphite for oils sorption in and among wormlike particles. Carbon 2004;42: 2603–7. [2] Chung DDL. Exfoliation of Graphite. J Mater Sci 1987;22(12):4190–8. [3] Kang F, Zheng YP, Wang HN, Nishi Y, Inagaki M. Effect of preparation conditions on the characteristics of exfoliated graphite. Carbon 2002;40:1575–81. [4] Celzard A, Mareche JF, Furdin G. Modelling of exfoliated graphite. Prog Mater Sci 2005;50:93–179. [5] Inagaki M. Application of graphite intercalation compounds. J Mater Res 1989;4(8):1560–6.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Preparation and characterization of sulfur-free exfoliated graphite with large exfoliated volume Xiu-Juan Yu a,⁎, Juan Wu a, b, 1, Qi Zhao a, 1, Xiu-Wen Cheng a, 1 a b
Department of Environmental Science and Engineering, Heilongjiang University, Harbin 150080, PR China Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People's Republic of China, Heilongjiang University, Harbin 150080, PR China
a r t i c l e
i n f o
Article history: Received 30 December 2010 Accepted 19 November 2011 Available online 6 December 2011 Keywords: Carbon materials Porous materials Exfoliated graphite Sulfur-free Exfoliated volume
a b s t r a c t Sulfur-free exfoliated graphite with large exfoliated volume was prepared via chemical oxidation combined with microwave irradiation by using nature flake graphite, perchloric acid and potassium permanganate as raw materials. The influences of some experimental parameters on the exfoliated volume were discussed in detail. It was found that the optimum preparing conditions went as follows: m(nature flake graphite): m(perchloric acid):m(potassium permanganate) = 1:4:0.5, the reaction temperature and time were 35 °C and 40 minutes, respectively. The exfoliated volume of exfoliated graphite which was obtained by microwave irradiation could reach up to 760 ml g− 1. Many network pores existed in the worm-like particles of exfoliated graphite according to scanning electron microscope images. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.
1. Introduction
2.2. Sample preparation
Exfoliated graphite (EG) is a new kind of functional nano-carbon material which is made from natural flake graphite, through the process of intercalating, washing, drying and heat expansion [1]. Exfoliated graphite has been widely used in gasket, electromagnetic interference shielding, electrochemical applications, stress sensing and thermal insulator due to its lightweight, soft, porosity, high-surface area and high-surface activity [2–4]. Sulfuric acid often served as intercalating agent to prepare exfoliated graphite in chemistry oxidation, but the residue acids will limit its application, such as flexible graphite in seal materials [5]. In this work, EG was prepared by chemical oxidation combined with microwave irradiation and perchloric acid was used to replace the concentrated sulfuric acid that served as oxidizing and intercalating agent . The influences of some experimental parameters on the exfoliated volume (EV) were discussed in detail. The porosity of EG was characterized by scanning electron microscope (SEM).
The preparation process of EG went as follows: under stirring, the mixture consisted of 2 g nature flake graphite and potassium permanganate was added into perchloric acid in a water bath at a certain temperature. After continuously stirring at a scheduled time, the GICs were synthesized and then the mixture was washed close to neutrality with distilled water by a vacuum filter, which is aimed to remove the remaining acid on the surface of GICs. Then the GICs were dried for 12 h in an oven at 353 K. Finally, exfoliation occurred in a domestic microwave oven at 750 W for 20 s, and the worm-like EG was obtained.
2. Experimental procedure 2.1. Raw materials Nature flake graphite (carbon content, 95 wt.%; 50 mesh, produced in the city of Jixi, China). Perchloric acid (72 wt.%; A.R.); potassium permanganate (99.8 wt.%; A.R.) ⁎ Corresponding author. Tel.: + 86 451 86608549; fax: + 86 451 86603259. E-mail address: [email protected] (X.-J. Yu). 1 Tel.: + 86 451 86608549; fax: + 86 451 86603259.
2.3. The measure of exfoliated volume The exfoliated volume (EV) was measured using a 50 ml graduated cylinder and the operation went as follows: EG was transferred into graduated cylinder lightly, then the volume (V) and quantity (M) of EG were recorded and the EV was calculated by the following equation: EV = V/M (ml·g − 1).
2.4. Materials characterization The microstructure of sample was characterized by SEM. SEM images were obtained with an instrument (SEM: model XL-30-ESEMFEG, Philips Co., Holland). The acceleration voltage was 20.00 kV and EG was dealt with gold jetting before scanning.
0167-577X/$ – see front matter. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.11.078
12
X.-J. Yu et al. / Materials Letters 73 (2012) 11–13
is A2B3C2D4. That is, the ratio of weight is m(nature flake graphite): m(perchloric acid):m(potassium permanganate) = 1:4:0.5, and the reacting temperature and time were 35 °C and 40 min, respectively. The EV of EG could reach up to 760 ml g − 1 under optimum condition.
Table 1 Orthogonal experiment and results of EG. Sample
A
B
C
D
EV
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 k1 k2 k3 k4 R
6.0 6.0 6.0 6.0 8.0 8.0 8.0 8.0 10.0 10.0 10.0 10.0 12.0 12.0 12.0 12.0 485.00 498.75 487.50 469.25 29.50
0.4 0.8 1.0 1.2 0.4 0.8 1.0 1.2 0.4 0.8 1.0 1.2 0.4 0.8 1.0 1.2 363.00 467.75 559.00 550.75 196.00
25 35 45 60 35 25 60 45 45 60 25 35 60 45 35 25 480.75 537.25 474.25 448.25 89.00
10 20 30 40 30 40 10 20 40 30 20 10 20 10 40 30 453.50 485.50 492.75 508.75 55.25
294 542 568 536 435 513 526 521 400 408 556 586 323 408 586 560 – – – – –
3.2. Influence of the dosage of raw materials on expanded volume
A—perchloric acid (g); B—potassium permanganate (g); C—reaction temperature (°C); D—reaction time (minutes).
3. Results and discussions 3.1. The optimum preparing conditions Table 1 shows the orthogonal experiment and results of EG at different reaction conditions caused different EVs, and the mass of potassium permanganate is the most important factor for exfoliated volume according to R value. The most suitable factor combination
Fig. 1(A) shows that when the dosage of potassium permanganate is too large or too small, EV is not the maximum. Firstly, the more the mass of potassium permanganate, the larger the EV; then when the dosage of potassium permanganate exceeds 1.0 g, EV decreases sharply. The reason is that potassium permanganate served as assistant oxidant could open the layer of nature flake graphite and ensures intercalating agent gets into the interlayer smoothly. If there are excessive potassium permanganate in the reacting system, the structure of EG will be destroyed due to the excessive oxidization or the inserting reagent will outflow from the overlarge width of layer. If the dosage of potassium permanganate is not enough, it cannot open the layer of nature flake graphite due to lack of oxidizing. From Fig. 1(B), we can see the influence of the dosage of perchloric acid on EV clearly. The variation trend of perchloric acid is similar to potassium permanganate as discussed above. Perchloric acid served as oxidizing reagent and intercalating agent in the preparation of EG. If the dosage of perchloric acid exceeds, it will cause the over oxidant of nature flake graphite and the waste of raw material. If the dosage of perchloric acid is not enough to satisfy the requirement of inserting, EV will decrease sharply. 3.3. Influence of the reaction time and temperature on expanded volume From Fig. 1(C), 35 °C is the optimum temperature for the preparation of EG; as the temperature increases, the reaction speed of 560
600
B
540
A
Exfoliated graphite/mL/g
Exfoliated graphite/mL/g
550 500 450 400 350 300 250
520 500 480 460 440 420
200 150
0.4
0.6
0.8
1.0
1.2
400
1.4
4
5
510
Exfoliated graphite/mL/g
Exfoliated graphite/ml/g
C
520 500 480 460 440
8
9
10
11
12
13
14
D
500 490 480 470 460 450
420
400 20
7
520
560 540
6
the dosage of perchloric acid/g
the dosage of potassium permanganate/g
440 25
30
35
40
45
50
reaction temperature/0C
55
60
65
0
10
20
30
40
50
60
reaction time/min
Fig. 1. The effect of reaction parameters on the exfoliated volume: A. potassium permanganate; B. perchloric acid; C. reaction temperature; D. reaction time.
X.-J. Yu et al. / Materials Letters 73 (2012) 11–13
a
b
200um
d
13
c
200um
e
5um
500um
f
20um
50um
Fig. 2. SEM images of carbon material: (a) nature flake graphite; (b)GICs;(c) exfoliated graphite(d) the layer structure of nature flake graphite; (e)the layer structure of GICs;(f) the inner structure of exfoliated graphite.
oxidizing and intercalating will accelerate, but higher temperature will accelerate the volatilization of perchloric acid and over oxidation. From Fig. 1(D), the EV increases as the reaction time increase, and the longer the time, the larger the EV. In order to investigate the influence of extending reaction time on EV, the experiment was operated as follows: the ratio of weight is m(nature flake graphite):m(perchloric acid):m(potassium permanganate) = 1:4:0.5, the reaction temperature is 35 °C and the reaction time is 60 min, respectively. The EV of EG could reach to 780 ml g − 1. When the inserting agent inserted into the interlayer reach to saturation, the differences in EV are not significant. Forty minutes was chosen as the optimum reaction time in this experiment.
4. Conclusions Using perchloric acid and potassium permanganate as oxidant and intercalating agent is an effective method to prepare sulfur-free exfoliated graphite with large expanded volume. The optimum condition for the preparation of exfoliated graphite went as follows: m(nature flake graphite):m(perchloric acid):m(potassium permanganate) = 1:4:0.5, the reaction temperature and time was 35 °C and 40 minutes, respectively. The EV of EG which obtained at the optimum conditions by microwave irradiation could reach up to 760 ml g − 1. The SEM images showed that many network pores existed inside the worm-like particles.
3.4. Analysis of porosity References Fig. 2(a–e) showed the SEM images of the nature flake graphite, GICs and exfoliated graphite. The relationships among six images were analyzed by comparing the obviously alteration of appearance, the distance of layer and pore structure, respectively. By comparing (a) and (b) images, the surface of GICs is cracked and the edge of GICs is curly. Similarly, the SEM image of (d) and (e) showed that oxidation enlarged the distance of layer and the layered structure of flake graphite was not destroyed. After microwave exfoliation, worm-like particles was observed from Fig. 2(c) and many irregular network pores existed in the worm-like particles of EG from Fig. 2(f). According to Fig. 2(c) and Fig. 2(f), microwave irradiation could distort the graphite and change its appearance and structure.
[1] Zheng Y-P, Wang H-N, Kang F-Y, Wang L-N, Inagaki M. Sorption capacity of exfoliated graphite for oils sorption in and among wormlike particles. Carbon 2004;42: 2603–7. [2] Chung DDL. Exfoliation of Graphite. J Mater Sci 1987;22(12):4190–8. [3] Kang F, Zheng YP, Wang HN, Nishi Y, Inagaki M. Effect of preparation conditions on the characteristics of exfoliated graphite. Carbon 2002;40:1575–81. [4] Celzard A, Mareche JF, Furdin G. Modelling of exfoliated graphite. Prog Mater Sci 2005;50:93–179. [5] Inagaki M. Application of graphite intercalation compounds. J Mater Res 1989;4(8):1560–6.