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used a combination of CVD and template synthesis methods to synthesize highly aligned, uniform, hollow and open ended (suitable for filling with other materials) CNTs with diameters ranging from 20 to 200 nm. Among all the methods of preparation of CNTs, the template synthesis method has the advantage of controlling composition, morphology and the size of the nanotubes [25]. Nitrogen containing carbon nanotubes prepared by template synthesis using precursors such as polyacrylonitrile [26] and polypyrrole [27], have been reported. In this letter, use of polyvinylpyrrolidone as a new precursor for the synthesis of nitrogen containing carbon nanotubes by template synthesis is discussed. The well-aligned nitrogen containing CNTs have been formed. The morphology and structural features of the nanotubes have been investigated using electron microscopy and Raman spectroscopy. The composition and bonding of the nanotubes were determined using IR and X-ray photoelectron spectroscopy (XPS).
2. Experimental 2.1. Chemicals All the chemicals used were of analytical grade. Polyvinyl pyrrolidone (Sisco Research Laboratories, India), dichloromethane and concentrated HF (both from Merck) were used. Alumina template membranes were obtained from Whatman Anopore Filters. 2.2. Synthesis of nitrogen containing carbon nanotubes Polyvinylpyrrolidone (PVP, 5 g) was dissolved in dichloromethane (20 ml) and impregnated directly into the pores of the alumina template by wetting method [28]. After complete solvent evaporation, the membrane was placed in a quartz tube (30 cm length, 3.0 cm diameter) kept in a tubular furnace and carbonized at 1173 K under Ar gas flow. After 3 h of carbonization, the quartz tube was naturally cooled to room temperature. The resulting carbon–nitrogen composite was immersed in 48% HF at room temperature for 24 h to remove the alumina template and the nitrogen containing CNTs were obtained as an insoluble fraction. The nanotube were then washed with distilled water to remove the residual HF and was dried at 393 K. 2.3. Characterization The total chemical composition of the nanotubes was determined by elemental analysis using Hereaus CHN analyzer after the removal of alumina template. The scanning electron micrographs were obtained using JEOL JSM-840 model working at 15 keV. For transmission electron microscopic studies, the nanotubes dispersed in ethanol were drop cast on to a carbon-coated copper grid followed by solvent evaporation in air at room temperature and the images were
obtained using Phillips 420 model, operating at 120 keV. The nanotubes were sonicated in acetone for 20 min and then were dropped on cleaned Si substrates and AFM imaging was performed in air using a Nanoscope IIIA atomic force microscope (Digital Instruments, St. Barbara, CA) operated in contact mode. The CNTs placed in glass capillary tubes were used for recording Raman spectrum at the room temperature with 514.5 nm excitation in backscattered mode using Bruker FRA106 FT-Raman instrument. The IR spectrum was recorded with Perkin-Elmer (L-710) spectrophotometer. The X-ray photoelectron spectroscopic analysis was carried out using VG ESCALAB 220 iXL instrument at Al K␣ (1486.5 eV) radiation.
3. Results and discussion Elemental analysis was first conducted to examine whether nitrogen has really entered the carbon nanotube framework. It has been found that the samples prepared contained about 87.2% carbon and 6.6% nitrogen (w/w). Scanning electron microscopic images given in Fig. 1 shows the morphology of the nitrogen containing CNTs after the removal of alumina template. The nanotubes are aligned and grouped into high-density arrays, appearing like the bristles of a brush (Fig. 1a) and this reflects the highly dense straight pores of the alumina membrane (∼109 pores cm−2 ). Images b and c of Fig. 1 show the lateral view of the wellaligned nanotubes of the length 50–60 m with open ends (indicated by arrows in Fig. 1c) similar to the thickness of the alumina membranes. Moreover, we performed transmission electron microscopy (TEM) analysis to determine the morphology and the diameter of the nanotube. Transmission electron microscopy images of the nitrogen containing carbon nanotubes prepared from polyvinylpyrrolidone are shown in Fig. 2. The carbonized specimens are found to have a hollow structure with open ends. Because the channels of the membranes being cylindrical with uniform diameter and the tube diameter closely matching the pore size of the template used, monodispersed nanotubes of the desired material were obtained. Since no catalyst has been used for synthesis of nitrogen containing carbon nanotubes, it is worth pointing out that the nanotubes produced by template synthesis under normal experimental conditions are almost free from impurities. AFM images of the nitrogen containing carbon nanotubes deposited on a silicon substrate are shown in Fig. 3. The AFM tip was carefully scanned across the tube surface in a direction perpendicular to the tube axis. From the AFM images, a part of the long nanotube is appeared to be cylindrical in shape and to be terminated by a symmetric hemispherical cap. Raman spectrum is a unique tool to characterize the nanotubes under study, since the amount of ordering and degree of sp2 and sp3 bonding leaves a characteristic Raman “finger print”. Thus, the graphitic nature of the nitrogen