Surface Modification of Multiwall Carbon …

1 Surface Modification of Multiwall Carbon Nanotubes by Sulfuric acid and nitric acid Muhammad Ali Turgunov1, Jin Oh Oh1, and Sung Ho Yoon1,* 1 Department of Mechanical Engineering, Kumoh National Institute of Technology, 61 Daehak-ro, Gumi, 730-701, Gyeongbuk, Korea * Corresponding Author: +82-54-478-7299, Fax: +82-54-478-7319, e-mail: Abstract. Surface Modification of Multiwall Carbon nanotubes (MWCNTs) was performed by a mixture of sulfuric acid and nitric acid . Pristine MWCNTs, vacuum heating dry MWCNTs, and vacuum freezing dry MWCNTs were prepared. According to the results, acid treatments give more carboxyl groups on the Surface of the MWCNTs. In addition, vacuum freezing dry treatment provides less damaged MWCNTs than vacuum heating dry treatment.

2 Keywords: Surface Modification ; acid mixture; MWCNTs; vacuum heating dry treatment; vacuum freezing dry treatment. 1 Introduction Scientists have been opened a new era of remarkable applications to apply Multiwall Carbon nanotubes (MWCNTs) in many industries based on their excellent physical, mechanical, and electrical properties [1]. However, the MWCNTs have poorly dispersed in polymer matrix because of their bundled and entangled structures. acid treatment for purification and oxidation is gaining particular interest for the MWCNTs to be well dispersed in polymer matrix [2, 3]. Strong acids such as sulfuric acid and nitric acid would provide carboxyl groups at open ends and at defect sites of the MWCNTs [4, 5].

3 Significant damage to the molecular framework of the MWCNTs may be inevitable. The aim of this study is to investigate Surface Modification for purification and oxidation of the MWCNTs using acid treatment. The acid mixture of sulfuric acid and nitric acid was prepared for acid treatment of the MWCNTs. Three types of the MWCNTs were prepared: pristine MWCNTs, vacuum heating dry MWCNTs, and vacuum freezing dry MWCNTs. The characterization of the MWCNTs were conducted by Fourier transform infrared spectroscopy, Raman spectroscopy, and scanning electron microscopy with energy dispersive X-ray. Advanced Science and Technology Letters (Materials 2014), : 2287-1233 ASTL Copyright 2014 SERSC 2 Experimental Pristine MWCNTs were purchased and used as received.

4 Vacuum heating dry MWCNTs and vacuum freezing dry MWCNTs were obtained by acid treatment of pristine MWCNTs using a mixture of sulfuric acid and nitric acid . acid treatments procedures are as follows: Pristine MWCNTs were added into a flask containing the acid mixture. After the sonication was treated on the flask containing the MWCNTs at room temperature for 10 minutes, the mixture was refluxed at 130 C for 6 hours. After cooling to room temperature, it was diluted with deionized water and was vacuum filtered. The MWCNTs were washed thoroughly with distilled water until it becomes neutral. For vacuum heating dry treatment, acid treated MWCNTs were left to a vacuum dry oven (OV-12, JEIO Tech, Korea) at 120 C for 24 hours.

5 The MWCNTs were transferred into an alcohol filled glass cup. After the sonication for 1 hour, the MWCNTs were dried overnight at 50 C. For vacuum freezing dry treatment, acid treated MWCNTs collected after the filtering and drying process were transferred into capped glass bottles. They were connected to a vacuum centrifugal concentrator (SVQ-70, OPERON, Korea) and dried at -80 C for 24 hours. shows acid treatment procedures and Surface structures of MWCNTs. Fig. 1. acid treatment procedures and Surface structures of MWCNTs. 3 Experimental Results and Discussion Fourier Transform Infrared Spectroscopy Fourier transform infrared spectroscopy (VERTEX-80v, BR KER Optics, Germany) was performed to verify the existence of carboxyl groups.

6 Fourier transform infrared spectra were recorded from 900 cm-1 to 4000 cm-1. Fourier transform infrared spectra of pristine MWCNTs, vacuum heating dry MWNCTs, and vacuum freezing dry MWNCTs are shown in Fig. 2. In (a), C-O stretching at 1160 cm 1, C=C at 1645 cm 1, C-H stretching at 2923 cm 1, but no carboxyl groups. In (b), C=O stretching at 1737 cm 1 due to the presence of carboxyl groups, O-H stretching of hydroxyl groups at 3315 cm 1, C O stretching at 1155 cm 1, C=C stretching at 1560 cm 1. In (c), O-H Advanced Science and Technology Letters (Materials 2014)Copyright 2014 SERSC23 stretching at 3315 cm 1 due to hydroxyl groups, C=O at 1737 cm 1 due to carboxyl groups. Fig. 2.

7 Fourier transform infrared spectroscopic spectra: (a) pristine MWCNTs, (b) vacuum heating dry MWCNTs, (c) vacuum freezing dry MWCNTs. Raman Spectroscopy Raman spectroscopy was performed to analyze defect sites of the MWCNTs. Raman spectrometer (RENISHAW M820, RENISHAW, UK) was also used to confirm defect sites. Raman shift from 1000 cm-1 to 2000 cm-1 was collected. Each of the MWCNTs consists of two characteristic bands: D band and G band. The peak of the D band is related to defect derived mode and the peak of the G band to graphite structure derived mode. After acid treatment, the peak intensity of the D band at 1345 cm-1 increases due to defects of the MWCNTs. The G band at 1568 cm-1 is used as a measure of quality of the nanotubes.

8 The ratio of the D to G peak intensity increases as the number of defects increases. The peak of acid treated MWCNTs was significantly increased compared to that of pristine MWCNTs. It is obvious that the peak of the G band was strongly affected by acid treatment. The peak intensity explains the presence of defects but no disruption of graphitic structures on the Surface . Scanning Electron Microscopy Surface morphology was observed for three types of the MWCNTs through a scanning electron microscope as shown in Fig. 3. Scanning electron microscopy (JSM-6380, JEOL, Japan) was conducted to examine the Surface morphology. Pristine MWCNTs have smooth Surface with more agglomeration in (a).

9 It is difficult to align the MWCNTs without physical intervention due to agglomeration. acid treatment of pristine MWCNTs using the acid mixture makes the Surface roughened in (b) and (c). No existence of impurities was detected in acid treated MWCNTs due to the Advanced Science and Technology Letters (Material 2014)24 Copyright 2014 SERSC purification of the MWCNTs. The roughness of the side walls is attributed by defect sites with carboxyl groups on the Surface . Fig. 3. Surface morphology examined by scanning electron microscope: (a) pristine MWCNTs, (b) vacuum heating dry MWCNTs, (c) vacuum freezing dry MWCNTs. 4 Conclusions A series of experiments were conducted on three types of the MWCNTs.

10 Vacuum freezing dry treatment was led to less damage and more interaction with the MWCNTs. The existence of carboxyl groups was verified from Fourier transform infrared spectroscopy. The peaks of the D and G band were found on Raman spectra. The presence of defects but no disruption of graphitic structures was observed on the Surface after acid treatment. Scanning electron microscopy showed that acid treated MWCNTs had less agglomeration than pristine MWCNTs. acid treatment would be effective candidate for Surface Modification of the MWCNTs. Acknowledgments. This work was supported by the research fund, Kumoh National Institute of Technology. References 1. Grobert, N.: Carbon Nanotubes - Becoming Clean.