Dispersion of carbon nanotubes in low pH aqueous solutions by means of alumina-coated silica nanoparticles

Available online at www.sciencedirect.com

Carbon 45 (2007) 2823–2827 www.elsevier.com/locate/carbon

Dispersion of carbon nanotubes in low pH aqueous solutions by means of alumina-coated silica nanoparticles Yu-Chen Tsai

b

a,*

, Chian-Cheng Chiu a, Ming-Chieh Tsai a, Jeng-Yue Wu a, Tzu-Fan Tseng a, Tzong-Ming Wu b, Sung-Fu Hsu b

a Department of Chemical Engineering, National Chung Hsing University, Taichung 402, Taiwan, ROC Department of Materials Science and Engineering, National Chung Hsing University, Taichung 402, Taiwan, ROC

Received 22 March 2007; accepted 19 August 2007 Available online 1 September 2007

Abstract A novel noncovalent and inorganic method was used to disperse multiwalled carbon nanotubes (MWCNTs) in aqueous solutions through alumina-coated silica (ACS) nanoparticle halos. MWCNTs were directly dispersed into a highly charged ACS nanoparticle aqueous solution without functionalization of their surfaces. The dispersed MWCNTs were characterized by transmission electron microscopy and atomic force microscopy. Raman spectroscopy of MWCNTs prepared from dispersion in the ACS solution revealed reduced bundling compared to the corresponding untreated MWCNTs. The characteristic Raman peak at about 1570 cm 1, corresponding to the G band, shifted to a higher wavenumber with a narrower peak. It was possible to disperse up to 20 mg/mL of MWCNTs in a 1 wt% ACS nanoparticle aqueous solution at pH 2. This homogeneous MWCNT–ACS aqueous solution was stable for weeks after ultrasonication. Ó 2007 Elsevier Ltd. All rights reserved.

1. Introduction Carbon nanotubes (CNTs) have attracted significant scientific interest because of their unique structure and properties [1–6]. However, the major problem for the applications of CNTs is the insolubility of CNTs in most solvents because of the lack of side groups or other functionalities on CNTs that can interact with the surrounding solvent to overcome the large intertube van der Waals interactions. The methods to solubilize and separate discrete CNTs from the bundles of CNTs would broaden the applications of CNTs in nanotechnology, purification, and manipulation. Many attempts have been made toward the dispersion of CNTs in suitable solvents. These methods can be roughly classified as chemical and physical. Chemical modification involves the formation of covalent bonds between the CNTs and the reactants. Functionalization

*

Corresponding author. Fax: +886 4 22854734. E-mail address: [email protected] (Y.-C. Tsai).

0008-6223/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2007.08.034

of CNTs has been obtained by an oxidation process, which includes extensive ultrasonic treatment in a mixture of concentrated nitric and sulfuric acid [7]. Carboxyl groups can be introduced on CNTs by acid treatments. Physical modification of CNTs includes a solubilization procedure using different solubilizers such as surfactants, polymers, and p-aromatic compounds [8–13]. CNTs are placed in a solution with a solubilizer and then are sonicated for a period of time to de-bundle the CNTs. The homogeneity of the resulting solution depends on the interaction between the solubilizer with the sidewalls of CNTs and the solvation of the modified CNTs with the solvent. The strategy of solubilizing CNTs with noncovalent bonding can preserve the pristine CNT properties and maintain the as-prepared fulllength CNTs. In recent studies, a methodology for colloidal stabilization to the formation of nanoparticle haloing was introduced by Lewis and co-workers [14,15]. By adding charged nanoparticles to negligibly charged micrometersized spheres, halos occur because it is advantageous for the charged nanoparticles to be near the uncharged surface.

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Alumina had been found to be a good solid support for the immobilization of various biomolecules. Application of alumina for immobilization of biomolecules such as tyrosinase on biosensing application has been explored [16]. The alumina-coated silica (ACS) nanoparticles provide new opportunities for biosensing applications because their large specific surface area and high surface free energy. Therefore, the combination of CNTs and ACS is of great potential to chemical and biochemical areas in both fundamentals and applications. In this study, the feasibility to disperse multiwalled carbon nanotubes (MWCNTs) in ACS aqueous solution is described and discussed. 2. Experimental 2.1. Reagents Reagents used were of analytical grade or the highest commercially available purity and were used as received without further purification. The MWCNTs (TECO Nanotech Co., Ltd., Taiwan) used in this study was of 99% purity and synthesized by an electric arc discharge method. The MWCNTs are cylindrical with an outer diameter in the range 20– 40 nm, an inner diameter in the range 2–5 nm, and a length of up to several micrometers. Positively charged ACS spheres (Ludox CL) were the product from DuPont. The zeta potentials were measured by a NanoZS analyzer (Malvern, England) in 0.1 M HCl or NaOH. The average diameter of ACS is ca. 10–15 nm which was determined from transmission electron microscopy (TEM). The pH value of the ACS solution was adjusted with 0.1 M HCl. All solutions were prepared with demineralized and filtered water of resistivity of not less than 18 MX cm which was taken from a Milli-Q water purification system (Milli-Q, USA).

2.2. Instrumentation Atomic force microscopy (AFM) images were obtained using a SPA400 (Seiko, Japan) multiple function units together with SPI-3800N control station in tapping mode in air. TEM images were obtained using a JEM-2010 (JEOL, Japan). A 1 mL of MWCNT–ACS suspension (5 mg/ mL MWCNTs) was diluted with 50 mL of water and ultrasonicated, and then one drop of the diluted suspension was transferred to deposit on a copper grid and mica wafer for TEM and AFM study, respectively. The solvent was allowed to evaporate at room temperature in the air. Raman spectra were recorded with a micro-Raman spectrometer (JobinYvon Triax 550), equipped with a 25 mW He/Ne laser with an emission at 632.8 nm.

3. Results and discussion A 20 mg of MWCNTs was dispersed in 4 mL of 1 wt% ACS aqueous solutions at different pH values with the aid of ultrasonic agitation for 2 h in order to investigate the effect of pH value. A photograph of vials of 5 mg/mL MWCNTs dispersed in 1 wt% ACS aqueous solutions at different pH values after sonication is shown in Fig. 1. The solubility of MWCNTs in a 1 wt% ACS aqueous solution increases by decreasing the pH values of the ACS aqueous solution from 4 to 2. A homogeneous, well dispersed solution is observed after 12 h at pH 2; however, inhomogeneous ones are appeared when the pH values are 3 and 4. In pH 2, 1 wt% ACS aqueous solution, ACS colloidal particles are segregated to regions surrounding

Fig. 1. A photograph of vials taken 12 h after sonication of 1 wt% ACS aqueous solutions containing 5 mg/mL MWCNTs at pH values of (a) 2, (b) 3, and (c) 4.

MWCNTs. The colloidal particles are beneficial to dispersion of MWCNTs by strong electrostatic repulsion between nanoparticles and results in a homogeneous MWCNT– ACS aqueous solution. At pH 3 and pH 4, the halo formation around the MWCNTs is ineffective and results in the flocculation of MWCNTs. Therefore, a pH value of 2 was then employed to disperse the MWCNTs in a 1 wt% ACS aqueous solution. To investigate the maximum load of MWCNTs in a 1 wt% ACS aqueous solution (pH 2), different amounts of MWCNTs were added into 4 mL 1 wt% ACS aqueous solutions (pH 2). A photograph of vials taken 72 h after sonication of solutions of 5 mg/mL MWCNTs in an aqueous solution (pH 2) and different amounts of MWCNTs in a 1 wt% ACS aqueous solutions (pH 2) is shown in Fig. 2. In the aqueous solution (pH 2) without the ACS nanoparticles, MWCNTs were insoluble even after ultrasonication (Fig. 2a). This flocculation is a result of the significant van der Waals interaction between the sidewalls of the MWCNTs. This suggests that ACS nanoparticles act as modifier, resulting in the noncovalent functionalization of the MWCNTs and enhancing MWCNT solubility. An inhomogeneous solution of MWCNT–ACS in the case of 40 mg/mL was observed after 72 h. The inhomogeneous solution indicated that 40 mg/mL MWCNTs had exceeded the maximum load of MWCNTs in a 1 wt% ACS aqueous solution (pH 2). The dispersion of MWCNTs with ACS nanoparticles was stable for weeks after ultrasonication. With increasing ACS nanoparticle concentration up to 30 wt% in a 20 mg/mL MWCNTs aqueous solution (pH 2), a colloidal gel was observed 24 h after sonication. This is attributed to the ACS nanoparticle volume fraction above the upper critical value in the MWCNT–ACS aqueous solution [14]. To investigate the dispersion behavior of MWCNTs in ACS nanoparticles aqueous solutions, the zeta potentials

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Fig. 2. A photograph of vials taken 72 h after sonication of solutions of (a) 5 mg/mL MWCNTs in an aqueous solution (pH 2), (b) 5 mg/mL MWCNTs in a 1 wt% ACS aqueous solution (pH 2), (c) 10 mg/mL MWCNTs in a 1 wt% ACS aqueous solution (pH 2), (d) 20 mg/mL MWCNTs in a 1 wt% ACS aqueous solution (pH 2), and (e) 40 mg/mL MWCNTs in a 1 wt% ACS aqueous solution (pH 2).

of MWCNTs and ACS were examined. The zeta potential values of MWCNTs and ACS at different pH values are shown in Fig. 3. The isoelectric points are located at pH 4.5 and 7.8 for pristine MWCNTs and ACS, respectively. At pH 2, both MWCNTs and ACS have positive surface charge. The zeta potentials are around +30 and +35 mV for pristine MWCNTs and ACS at pH 2, respectively. The electrical surface charge for ACS nanoparticles is higher than MWCNTs. The nanoparticle halos mechanism can occur in this MWCNT–ACS aqueous solution. The ACS nanoparticles segregate to regions surrounding large MWCNTs and create stable bidispersed suspensions due to the repulsive potential. The similar dispersion behaviors of dynamic colloidal stabilization by nanoparticle halos

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have been reported for the dispersion of alumina powder with ceria nanoparticles [22] and silica nanoparticles [23]. The stabilization of the dispersed MWCNTs with the assistance of ACS nanoparticles was investigated by TEM and AFM. The TEM images of the MWCNTs dispersed without and with addition of ACS nanoparticles are shown in Fig. 4a and b, respectively. The aggregated MWCNTs ropes/bundles can be seen in Fig. 4a. On the contrary, well separated MWCNTs bundles are observed for the MWCNTs–ACS (Fig. 4b). It can be seen from the inset of Fig. 4b that the ACS nanoparticles with a diameter of ca. 10–20 nm were covered on the surface of MWCNTs. This result demonstrated that the dispersion and stabilization of MWCNTs are attributed to the ACS nanoparticle halos formation around the MWCNTs. This behavior is also supported by the AFM observation. The AFM images of the untreated MWCNTs and the dispersed MWCNTs are shown in Fig. 5a and b, respectively, which give further information about the nanoparticle halo mechanism in stabilization of MWCNTs. It can be seen from Fig. 5a that MWCNTs are cylindrical with a diameter of ca. 40 nm. The diameter of the MWCNTs directly measured from AFM image was slightly larger than the diameter obtained from TEM image, presumably due to the convolution of the AFM tip [17]. The shape of MWCNTs prepared by electric arc is uniform and rod-like, whereas materials produced when using chemical vapor deposition are curved and defect-rich. The typical AFM image of the dispersed MWCNTs with the aid of ACS nanoparticles is shown in Fig. 5b. It can be observed that the width was 100 nm after the covering of ACS nanoparticles on the sidewall of MWCNTs. This result indicated evidently that the MWCNTs were surrounded by ACS nanoparticles. The TEM and AFM observations strongly suggest that

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Fig. 4. TEM images of MWCNTs dispersed in (a) an aqueous solution (pH 2) and (b) a 1 wt% ACS aqueous solution (pH 2). The inset in Fig. 4(b) is the high magnification of MWCNT–ACS.

Fig. 5. AFM images of the (a) MWCNTs and (b) MWCNT–ACS.

Raman Intensity (counts)

the ACS nanoparticle halos formed near the external walls of the MWCNTs. Raman spectroscopy was used to ensure the debundling of MWCNTs with ACS colloidal. Raman spectroscopy is widely used for the surface analysis and an important tool for characterizing carbon materials because it provides information on the hybridization state, the size of the graphite crystalline, and the degree of ordering of the material. The ability of poly(4-vinylpyridine) for debundling of single-walled carbon nanotubes (SWCNTs) in alcohols investigated by Raman spectroscopy has been reported in the literature [18]. A blue shift with a narrower peak can be seen in the radial breathing mode and tangential mode region in the Raman spectroscopy after debundling of SWCNTs. The Raman spectroscopy of the MWCNTs and MWCNT–ACS composite in the range 150– 2650 cm 1 are shown in Fig. 6a and b, respectively. The band at 1325 cm 1 arises from the defects in the curved graphene sheet (D band), and the band at 2635 cm 1 (D* band) is a disorder-induced Raman band. The strong intensities of Raman-allowed phonon mode (G band) at 1572 cm 1 for MWCNTs and at 1578 cm 1 for

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MWCNT–ACS composite are identified as an E2g stretching mode [19–21]. The position of the G band peak of MWCNTs with and without addition of ACS nanoparticles showed a 6 cm 1 shift to higher frequency. The blue shift with a narrower peak of the Raman spectroscopy of the MWCNT–ACS composite can be attributed to the reduction in the amount of MWCNT–MWCNT contact with the aid of ACS nanoparticles. The low relative intensity of the G band for the MWCNT–ACS composite indicated that the fewer amount of MWCNTs was achieved after inclusion of ACS nanoparticles in MWCNT–ACS composite. 4. Conclusion In this paper, we utilized the ACS nanoparticles for the first time to disperse MWCNTs. This process constitutes a simple and versatile protocol for the noncovalent dispersion of MWCNTs in an aqueous solution. The MWCNTs can be suspended homogeneously in a 1 wt% ACS nanoparticle aqueous solution (pH 2). This novel methodology for the dispersion of MWCNTs in aqueous solutions can be applied in the fields of nanoelectronics, biological sensing, and nanostructured materials. This MWCNT–ACS composite might be used in biosensors after immobilizing biomaterials such as glucose oxidase and alcohol oxidase onto the surface of MWCNT–ACS composite to investigate the biological systems. The MWCNTs will catalyze the oxidation of hydrogen peroxide which is produced during the enzymatic reaction, and ACS nanoparticles are a suitable matrix for improving the stability of the immobilization of enzymes. The applications of this type of organic– inorganic composite for bioelectroanalysis are in progress. Acknowledgements The authors thank the National Science Council, Taiwan for financial support under the Contract No. NSC96-2221-E-005-050. This work is supported in part by the Ministry of Education, Taiwan under the ATU plan. References [1] Iijima S. Helical microtubules of graphitic carbon. Nature 1991;354:56–8. [2] Moore RR, Banks CE, Compton RG. Basal plane pyrolytic graphite modified electrodes: comparison of carbon nanotubes and graphite powder as electrocatalysts. Anal Chem 2004;76:2677–82. [3] Banks CE, Moore RR, Davies TJ, Compton RG. Investigation of modified basal plane pyrolytic graphite electrodes: definitive evidence

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