Solid-Phase Debundling of Single-Walled Carbon Nanotubes for the

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Solid-Phase Debundling of Single-Walled Carbon Nanotubes for the “Stock Solid” Delivery of Concentrated Nanotube Dispersions Gang Liu, Neng Liu, Ping Zhao, Xueying Zeng, Shengwei Shi, Can Qin, Shunjie Wang, and Wubin Dai ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00201 • Publication Date (Web): 14 Feb 2019 Downloaded from http://pubs.acs.org on February 16, 2019

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ACS Applied Nano Materials

Solid-Phase Debundling of Single-Walled Carbon Nanotubes for the “Stock Solid” Delivery of Concentrated Nanotube Dispersions Gang Liu,* Neng Liu, Ping Zhao, Xueying Zeng, Shengwei Shi,* Can Qin, Shunjie Wang and Wubin Dai* School of Materials Science and Engineering, Wuhan Institute of Technology, Guanggu 1st road, Wuhan 430205, China.

KEYWORDS：single-walled carbon nanotubes, debundling, stock solid, ball milling, dispersion

ABSTRACT: Debundling of single-walled carbon nanotubes (SWNTs) has been mostly limited to liquid-phase exfoliation. Since the SWNTs in dispersion are prone to aggregate and precipitate gradually during their storage, a solid-phase method which provides debundled SWNTs as “stock solid” is highly required. Herein, we report on a solution to this issue, which is based on solid-phase ball milling. The milled solid, composed of debundled SWNTs and surfactant, can be stored as “stock solid” and is readily dispersed in water just by shaking with hand, giving concentrated SWNTs dispersion (up to 1.16 mg/mL) with yield up to 60% after centrifugation.

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1. INTRODUCTION Single-walled carbon nanotubes (SWNTs) are one-dimensional cylinders constructed by sp2-carbon with high aspect ratio. Due to the specific structural characteristics, SWNTs show unique mechanical, thermal, and electrical properties, and thus have been used in various fields, including nanocomposite, nanoelectronics, and even nanomedicine.1 However, SWNTs are generally produced as insoluble bundles because of the van der Waals and hydrophobic interactions. For this reason, dispersion of SWNTs has emerged as a key issue for promoting their practical applications. Between the two approaches developed so far for preparing SWNTs dispersions, noncovalent functionalization is considered to be superior to the covalent one, in terms of preserving the conjugated structure and intrinsic properties of SWNTs. Therefore, a number of researchers have been concentrated on developing novel and efficient dispersants,2 including surfactants,3-8 biomolecules,9-11 aromatic compounds,12-17 and polymers,4,18,19 for noncovalent functionalization of SWNTs. On the other hand, the means employed to achieve noncovalent functionalization have been mostly limited to bath and probe type sonication.3,6,9-18 Although these liquid-phase sonication based techniques are sufficient for debundling and dispersing SWNTs in various solvents in the presence of dispersant, they suffer from either low concentration or low yield (< 60 μg/mL or < 15%, respectively).12 Furthermore, the presence of solvent may make the concentration less controllable and less precise. Therefore, the development of more effective and controllable technique for debundling and dispersing SWNTs is highly required.


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Ball milling, a technique first developed for breaking bulk materials, or cutting and functionalizing carbon nanotubes (CNTs),20-24 has recently been used to exfoliate and disperse two-dimensional (2D) layered materials including graphene.25-27 Since SWNTs possess similar π-conjugated sp2-carbon framework to graphene, ball milling has great potential in debundling and dispersing of SWNTs through noncovalent functionalization. In fact, Ikada et. al reported that pretreated SWNTs, which have been cut into short pieces and lightly oxidized, can be debundled and dispersed in aqueous solution by vibration milling.28 Masuda et. al demonstrated the dispersion of pulverized multiwalled carbon nanotubes in polymer matrix through melt mixing in a cup-and-rotor mixer.29 However, preparation of aqueous dispersion of pristine SWNTs with the assistance of dry ball milling has been rarely reported, although covalently

functionalized

SWNTs

dispersion

has

been

prepared

by

ball

milling.20,21,24,28 Herein, we report a simple, scalable, and high-yield production of debundled SWNTs from unpretreated SWNTs through dry ball milling process. Since no solvent was added, the milled powder can be stored as a “stock solid”, and readily dispersed in water simply by shaking with hand prior to use, avoiding the sedimentation from “stock dispersion”.2 These are considered to be advantages of dry ball milling over wet process including bath and probe type sonication, shearing, and wet ball milling, in terms of long term storage.3,6,7,9-18 Furthermore, through the ball milling debundling combined with shake-assisted dispersing process, about 60% SWNTs were well



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dispersed in H2O. This value is much higher than that through sonication (mostly < 30%),4,5,10,30 and vibration milling mentioned above (< 6%).28 2. EXPERIMENTAL DETAILS 2.1. Materials. HiPco and (7,6)-CoMoCAT SWNTs were purchased from Unidym Inc. (Product Number P2772, mean diameter ~1.1 nm) and Aldrich (Product Number MKBD4057, diameter: 0.7-1.1 nm), respectively. Multi-walled carbon nanotubes (MWNTs) with outer tube diameter < 8 nm was obtained from Nanjing XFNANO Materials Tech Co., Ltd (China). All the carbon nanotubes were used as received. Sodium deoxycholate (SDC) and Sodium dodecyl sulfate (SDS) were obtained from Wako Pure Chemical Industries, Ltd. Sodium cholate (SC) and sodium dodecyl benzene sulfonate (SDBS) were purchased from Aladdin Industrial Co., Ltd and Tokyo Chemical Industry Co., Ltd., respectively. All these surfactants were used as received. 2.2. Equipment. Ball milling was performed on a planetary ball milling apparatus (Changsha Mitr Instrument and Equipment Co., Ltd., YXQM-0.4L) by using a Zirconia grinding bowl (150 mL) with thirty Zirconia balls. The ratio of the balls was 6 (Φ=15 mm): 9 (Φ =10 mm):15(Φ =8mm). Bath sonication was carried out with Branson M5800-C. Centrifugation was performed on CenceH1850 (Hunan Xiangyi Laboratory Instrument Development Co., Ltd). UV-Vis-NIR absorption spectra were obtained

on

a

UV-3600

scanning

spectrophotometer

(Shimadzu

Co.).

Fourier-transform infrared spectroscopy (FTIR) was performed with a Shimadzu IRSpirit instrument (Shimadzu Co.). Raman spectra were recorded on LabRam


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HR800 (Horiba Ltd.) and take the average of more than seven different spots for the final curve. X-ray diffraction (XRD) was conducted on an X’Pert ProMPD, PANalytical diffractometer at 45 kV and 40 mA using Cu Kɑ radiation. 2.3. Preparation of SWNTs Dispersion with the Assistance of Ball Milling (Solid-Phase Debundling). Pristine SWNTs (7.5mg) and surfactant (52.5mg) were placed in a Zirconia container and ball milled at 150 rpm for 1 h under air. Since some of the powder was attached on the inside wall of the containers, it was scratched off after half hour milling to increase the efficiency of the ball milling. The black powder after ball milling was dispersed in Milli Q water (15 mL) by hand shaking for a few seconds. After centrifugation of the resulting suspension at 18500 rpm (23797g) for 1 h, the supernatant was subjected to UV-Vis-NIR and STEM measurement. 2.4. Preparation of SWNTs Dispersion with the Assistance of Sonication (Liquid-Phase Debundling). For comparison, liquid-phase debundling of SWNTs was carried out under the same conditions to that of solid-phase debundling, except for using bath sonication rather than ball milling. Pristine SWNTs (7.5mg) and surfactant (52.5mg) in Milli Q water (15 mL) were bath sonicated for 1 h at 20 ℃. After centrifugation of the suspension at 18500 rpm (23797g) for 1 h, the supernatant was subjected to UV-Vis-NIR measurement for the concentration determination. 2.5. XRD, FTIR, and Raman Characterization. To record the native spectral characteristics of the milled SWNTs, the dispersant was removed by repeating a filtration/dispersion process. Briefly, the aqueous SWNTs dispersion prepared by ball milling was filtered through a hydrophilic polytetrafluoroethylene (PTFE) filter



(Millipore omnipore membrane filter, 0.1 mm pore size). The resulted filter cake was then dispersed in Milli Q. After repeating the filtration/dispersion process for four times, the filter cake was collected and analyzed by XRD, FTIR, and Raman spectroscopy. 2.6. Stability. Stability of the dispersions was estimated by absorption spectrum. The 22 μg/mL sample was directly submitted for absorption spectra measurement after storage of different duration. In terms of higher concentration samples (300 and 345 μg/mL), after storage of the dispersion for different duration, a small part of the dispersion was diluted for the spectrum recording, as the spectrum was saturated at such high concentration. 3. RESULTS AND DISCUSSION 3.1. Preparation of SWNTs dispersion. Debundling of SWNTs was performed by ball milling of pristine HiPco SWNTs in the presence of dispersant at 150 rpm for 1 h (Figure 1a and b). The resulting powder exhibited very good dispersibility in water by shaking with hand for a few seconds. After centrifugation at 18500 rpm (23797g) for 1h, black supernatant (Figure 1c) was obtained and was picked up for further analysis. Control experiment without adding dispersant resulted in colorless supernatant as shown in Figure 1c. This phenomenon indicated that the dispersant plays an important role in debundling and dispersing the SWNTs.


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a)

b)

c)

Figure 1. (a) Schematic of the preparation of SWNTs dispersion. (b) Chemical structures of SC, SDC, SDS, and SDBS. (c) Photographs of aqueous dispersions of HiPco after ball milling with and without surfactant followed by dispersion in water (by shaking) and centrifugation at 18500 rpm (23797g) for 1h.

Among a large number of surfactants, we chose sodium cholate (SC), sodium deoxycholate (SDC), sodium dodecyl sulfate (SDS), and sodium dodecyl benzene sulfonate (SDBS), as they were reported to be effective in debundling and dispersing SWNTs by sonication in liquid phase.3-5 In terms of the feeding ratio of dispersant to SWNTs, 1:1, 3:1, 7:1, and 14:1 were performed and the resulting dispersing yields (defined by the dispersed SWNTs compare to the initial SWNTs) were summarized in Figure 2. At all feeding ratios we tried, SDS and SDBS exhibited less than 7% dispersing yields, in contrast to more than 40% for SC and SDC at feeding ratio larger than 3:1. The highest yield, 60% and 55%, were achieved at feeding of 7:1 by using SC and SDC, respectively.



Figure 2. Yield of dispersed SWNTs as a function of feeding ratio of dispersant to SWNTs.

Although SC, SDC, SDS, and SDBS have been extensively investigated as dispersant for SWNTs in liquid phase,3-5 the previous works were performed at different conditions (including debundling time, centrifugation speed and duration) to that of the solid-phase debunding mentioned above. For a precise comparison of the ability of dispersants in the solid and liquid phase, we performed the bath sonication assisted dispersion of HiPco under identical conditions to that of the solid method in Figure 1a, at a feeding ratio of 7:1. As summarized in Table 1, all these four dispersants can disperse more than 9% of the initial SWNTs by sonication for 1 h and centrifugation at 23797g. These dispersing yields are comparable to that of the published results.4,5 Note that higher yield can be achieved by increasing the sonication time or dispersant ratio. When applied to ball milling, only SC and SDC gave promising dispersing yield. As shown in Table 1, while more than 55% of initial SWNTs were dispersed in water in the presence of SC and SDC, only trace amount of


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SWNTs (< 2%, Table 1) was dispersed in the case of SDS and SDBS. We propose that the much better performance of SC and SDC than SDS and SDBS is correlated with their structural characteristics (Figure 1b). That is, the strong electron-donating properties of the hydroxy groups in SC and SDC facilitate electron transfer between dispersants and SWNTs, and thus accelerate the insertion of dispersant molecules into the SWNTs bundles.26 Furthermore, hydrogen-bonding networks between dispersant molecules may form to further improve the debundling of the SWNTs.26,31,32

Table 1 Concentration (μg/mL) and yield of dispersed SWNTs (%) after ball milling or bath sonication of HiPco with different dispersants (feeding ratio 7:1) for 1 h followed by centrifugation at 23797g for 1 h. The ball milled sample was dispersed in water by shaking before centrifugation.a

Concentration (μg/mL) and yield (%) Preparation method SC

SDC

SDS

SDBS

Ball milling

300 (60)

274 (55)

12 (2)

3 (

Solid-Phase Debundling of Single-Walled Carbon Nanotubes for the

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