Article pubs.acs.org/JPCC
Ultrashort Single-Walled Carbon Nanotubes: Density Gradient Separation, Optical Property, and Mathematical Modeling Study Yun Kuang, Junfeng Liu, and Xiaoming Sun* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China
ABSTRACT: The density gradient ultracentrifuge separation (DGUS) method for obtaining ultrashort single-walled carbon nanotubes (SWNTs) was systematically investigated. A twice separation was used for further narrowing the length distribution. Investigations on Raman and absorbance spectra evidenced the concomitant chirality separation of the NTs. The lengthdependent blue shift recorded on the absorbance spectra confirmed band gap widening on finite semiconductive SWNTs, which showed a linear relationship to the inverse of the length of the NTs. Laser ablation NTs were used to demonstrate DGUS as a general method for separation of different type of SWNTs. A possible vertical sedimentation separation mechanism is proposed, and a mathematical model was set up to give a quantitive description on separation results, which was further demonstrated by time-dependence experiments.
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up vectors.22,23 Dai’s group used fluorecein−polyethylene glycol (PEG) to wrap and efficiently cut NTs.24 Consequent length separation using DGUS method yielded short enough (7−60 nm) SWNT fractions with narrow enough length distribution and eventually evidenced the predicted “quantum size effect” on finite lengthed SWNTs.25 Although considerable efforts have been devoted to develop the methods of sorting SWNTs and their corresponding properties, further optimization of the DGUS method to narrow the length distribution of the SWNTs, demonstrate the versatility of the separation method, provide more quantitive analysis on optical properties of USNTs, and develop a greater understanding of the DGUS mechanism is still insufficient. Herein, DGUS for separating SWNTs to get USNTs was further investigated. Optimizing the separation parameters and applying a secondary separation was found to further focus the length distribution. Investigations on Raman and absorbance spectra revealed that the optical properties of separated NTs are highly dependent on their length, and also linked to their diameter and chirality. Blue shifts observed in the absorbance spectra of NTs with different lengths further evidenced the predicted “quantum size effects” on finite length NTs.
INTRODUCTION Single-walled carbon nanotubes (SWNTs) have been intensively studied owing to their unique electronic, optical, and mechanical properties and potential applications.1−3 They are usually considered as typical one-dimensional structures with an intact graphene cylinder.4,5 Ultrashort SWNTs (USNTs) with finite length are theoretically predicted to show “quantum size effects” due to the reduction in dimensionality (1D→0D).6−9 However, SWNTs longer than 100 nm were achieved by common approaches. Though a series of cutting methods have been developed,4,10−12 accurate control to get nanotubes (NTs) with suitable length needs further exploration. For instance, harsh chemical reactions (such as fuming sulfonic acid exfoliation/oxidization4), commonly used to get short SWNTs, lead to the destruction of side wall and loss of intrinsic properties of SWNTs. On the other hand, the length distribution of the NTs was not narrow enough, though separation of as-prepared materials has helped focus the length distribution to a certain degree.13−17 How to get such USNTs with narrow enough length distribution and evidence the “quantum size effects” prediction on SWNTs experimentally has remained a challenge for chemists and materials scientists for a long time. Recently, density gradient ultracentrifugation separation (DGUS) has emerged as an efficient way to sorting carbon nanotubes according to their diameter,18 length,19−21 and roll© 2012 American Chemical Society
Received: March 25, 2012 Revised: November 5, 2012 Published: November 5, 2012 24770
dx.doi.org/10.1021/jp3028337 | J. Phys. Chem. C 2012, 116, 24770−24776
The Journal of Physical Chemistry C
Article
Quantitative analysis indicated that the band gap widening was proportion to the inverse of the length of the NTs. This DGUS method was also shown as a general method for separation of different type of SWNTs (such as laser ablation NTs). Finally, a possible vertical sedimentation separation mechanism was proposed, which was strongly supported by the spatial length distribution of NTs.
This was used to correct for the length of very short SWNTs measured: true length = measured length − 2(tip size) = measured length + height − measured width
According to our measurement, a typical tip had size 5−9 nm. Raman and UV−vis−NIR Spectroscopy. Raman spectra were recorded from 800 to 3600 cm−1 on a Renishaw 1000 Confocal Raman Microprobe (Renishaw Instruments) using a 785 nm argon ion laser. UV−vis−NIR measurement was carried out using a Cary 6000i spectrophotometer. Fractions after separation were used directly for measurement. The path length used for the measurements was 1 mm.
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EXPERIMENTAL SECTION Preparation of Functionalized SWNTs. A 0.25 mg portion of HiPco SWNTs (Carbon Nanotechnologies, Inc.) was bath sonicated with 5 mg/mL FITC−PEG (fluorescein− PEG5000−NHS, Nektar Therapeutics) for 4 h. The resulting dark suspension was centrifuged at 25 000 g for 6 h and the aggregate-containing pellet at the bottom of the centrifuge tube was discarded. Separation of SWCNTs in Step Density Gradient. The length separation of SWNTs was performed in three-layer density gradients made from 5%, 10%, and 15% iodixanol solution (0.6 mL each layer). In a typical procedure, OptiPrep (60% (w/v) iodixanol, 1.32 g/cm3, Sigma-Aldrich Inc.) was diluted with water (HPLC grade) to form gradient solutions of different concentrations. A step gradient was created directly in Beckman centrifuge tubes (polycarbonate, inner diameter 13 mm, length 51 mm) by adding layers to the tube with increasing density. For instance, to make a 5%+10%+15% gradient, 0.6 mL of 5% iodixanol was added to the centrifuge tube first, and then 0.6 mL of 10% iodixanol was added below the 5% layer by dipping the pipet tip deeply to the very bottom of the centrifuge tube. This process resulted in a density gradient that increased stepwise in density from the top to the bottom of the centrifuge tube. Finally, 0.4 mL of 60% iodixanol was added to the bottom of the centrifuge tube to raise the height of the gradient in the centrifuge tube, and to ensure that, following ultracentrifugation, the sorted SWNTs were above the hemispherical bottom cap of the centrifuge tube, facilitating their fractionation and recovery. For ultrashort laser ablation nanotube separation, 3 layers of 10%, 12.5%, and 15% iodixanol solutions were used. A 0.2 mL aliquot of SWNT/PL-PEG solution with ∼1 mg/ mL SWNT concentration was layered on top of the gradient before centrifugation. Typical centrifugation conditions were 2 h at 50k rpm. Calibrated micropipet was used to manually extract 100 μL fractions at various positions along the centrifuge-tube for characterization. Characterizations on Separated SWNTs. Atomic Force Microscopy (AFM). APTES (amine-propanol triethyl silicate, 0.6%) was used to treat the SiO2 surface to make the surface positively charged, which can make the substrate more sticky to the NTs. Separated fractions were deposited by soaking a piece of SiO2 in the fractionated solution for ∼15 min. The substrate was rinsed briefly with water, blow dried with air, and imaged by tapping mode AFM; ∼100 SWNTs were measured to get a lengths histogram and the average length. AFM Image Tip Correction for SWNT Length Measurements. This correction was especially important when the SWNTs were in
