Synthesis of C–N nanotube blocks and Y-junctions in bamboo-like C–N nanotubes

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  We report the observations made on the synthesis and characterization of C–N nanotube blocks and Y-junctions in bamboo-like C–N nanotubes. The C–N nanotube Blocks have been synthesized by pyrolyzing the mixture of silver nitrate acetonitrile solution
  BRIEF COMMUNICATION Synthesis of C–N nanotube blocks and Y-junctionsin bamboo-like C–N nanotubes Ram Manohar Yadav   Dinesh Pratap Singh   T. Shripathi   O. N. Srivastava Received: 18 June 2007/Accepted: 23 April 2008/Published online: 29 May 2008   Springer Science+Business Media B.V. 2008 Abstract  We report the observations made on thesynthesis and characterization of C–N nanotubeblocks and Y-junctions in bamboo-like C–N nano-tubes. The C–N nanotube Blocks have beensynthesized by pyrolyzing the mixture of silvernitrate acetonitrile solution and ferrocene benzenesolution. The structural/microstructural characteriza-tion of the as-synthesized material has been doneusing scanning electron microscopy (SEM) andtransmission electron microscopy (TEM). X-rayphotoelectron spectroscopic (XPS) analysis has beencarried out to confirm the presence of nitrogen innanotubes. These investigations reveal the formationof blocks of bamboo-like nanotubes having thedimension 300  9  200  9  30  l m and the diameter is20–50 nm. We also observe the formation of Y-junctions in bamboo-like nanotubes as we spraythe acetonitrile ferrocene and AgNO 3  mixture. Thelength of the synthesized Y-junction nanotube bun-dles is  * 2  l m. Some more complex  W -shaped junctions are also found to be present. The diametersof the Y-junction nanotubes is  * 80 nm at the junction and 25–50 nm at the branches. Keywords  Nanotubes    Bamboo-like   Y-junction    Electron microscopy   Nanostructure Introduction Device miniaturization in semiconductor technologyis expected to reach its limits due to the inherentquantum effects as one goes toward smaller size. Insuch a scenario, an alternative would be nano-electronics based on a molecule. The opportunity of creating and tailoring beautiful symmetric 3D struc-tures has propelled the science of carbon nanotubes(CNTs) to become one of the most promising areas inthe hot filed of nanotechnology (Dai et al. 1996; Kouwenhoven 1997; Mc Euen et al. 1998). Recent breakthrough (Frank et al. 1998; Tans et al. 1998; Collins et al. 2001) in CNT-based nano-electronicdevices have marked a new milestone for furtherminiaturization of circuit elements in the integratedcircuit industry. WhileResearch is going on to explorethe application of individual nanotubes, bulk orbundled nanotube have been explored for mechanicalactuator (Banghman et al. 1999) and filed emission display (Choi et al. 1999). Experimental findings show that bulk CNTs are promising for use as thermal R. M. Yadav    D. P. Singh    O. N. SrivastavaDepartment of Physics, Banaras Hindu University,Varanasi, India 221005R. M. Yadav ( & )Department of Physics, VSSD (PG) College Kanpur,Kanpur, India 208002e-mail: ShripathiUGC-DAE Consortium for Scientific Research, IndoreCentre, Khandwa Road, Indore, India 452 017  1 3 J Nanopart Res (2008) 10:1349–1354DOI 10.1007/s11051-008-9408-x  and anemometrical sensors (Lee et al. 2000). Two terminal heterojunctions of nanotubes have beenproposed as a model of nano-scale rectifying switchesand their behavior has been verified in experiments aswell (Collins and Avouris 2000; Andriotis and Menon 2006; Yao et al. 1999; Papadopoulos et al. 2004). Multi-terminal junctions (MTJs), such as T-, Y-, or X- junctions, have generated intense interest as buildingblocks for extended nanoelectronic devices, since thethird or fourth terminal can be used to control thepower gain and switching mechanism (Meng et al.2006). For any current based architecture of nanotube-based electronics, three-terminal nanotube junctionsare essentially required. The three-terminal multi-walled junctions (Y-junction) have been synthesizedin recent experiments and electronic transport usingY-junction shows asymmetric and rectifying charac-teristics (Papadopoulos et al. 2000; Satish Kumar et al. 2000). To the best of our knowledge, thesynthesis of Y-junction in the bamboo-like structureof C–N nanotube has not been reported so far.Keeping these aspects in view, we attempted thegrowth of C–N nanotube blocks and Y-junctions inbamboo-like carbon nanotubes employing the simple,one step, and economically viable spray pyrolysistechnique using a mixture of acetonitrile, ferrocene,benzene, and silver nitrate. Experimental details The spray pyrolysis setup consists of a nozzle (innerdiameter of0.5 mm)attachedatoneendofa containerusedforstoringandreleasingtheprecursors.Theotherendofthenozzleisfixedtoasilicatube(innerdiameterof11 mm),whichisfittedintoa330-mm-longfurnace.The nozzle is contained in a tube with an exit diameterof 2 mm, which directs the carrier gas flow around thenozzle. Argon was used as the carrier gas in theexperiment. The concentrations of AgNO 3  in CH 3 CNand (C 5 H 5 ) 2 Fe in C 6 H 6  were 10 and 25 mg/mL,respectively. The silica tube was preheated to atemperature of 900   C. The precursor was sprayed atanoptimumflowrateof2 mL/min.Thesilicatubewasmaintained at 900   C for an additional 15 min toanneal the products. The as-grown materials weretakenoutfromthe silicatube andfurthercharacterizedbyascanningelectronmicroscope(SEM)(PhilipsXL-20) and transmission electron microscope (TEM)(Philips EM CM-12). SEM was further used to studythe surface morphology of the nanotube blocks. TEMwas used to investigate the structure of as-grownnanotubes. X-ray photoelectron spectroscopic (XPS)analysiswasalsocarriedouttoconfirmthepresenceof nitrogen in nanotubes. Results and discussions Homogeneous dense film-like deposition takes placealong the total heating zone inside the silica tube byspray pyrolysis of mixture (silver nitrate solution inacetonitrile and ferrocene in benzene) at 900   Cunder argon atmosphere. SEM images show theformation of blocks of aligned C–N nanotubes, asshown in Fig. 1a. Figure 1b, c are the magnified micrographs of a block of Fig. 1a. Figure 1b shows that the nanotube are aligned in the blocks. Inspite of the formation of nanotubes blocks, some othermaterials are also found to be present in the SEMimages, as shown, in Fig. 1a, b. These are fewcarbonaceous materials (e.g. vitreous and amorphouscarbon) and silver glue, which are necessary in excessto hold these nanotubes blocks on the SEM sampleholder. The magnified image (Fig. 1c) shows that thenanotubes in the block are highly compact in nature.The average length of these nanotubes blocks comesout to be 30  l m.In order to investigate the structure of nanotubes,we analyze the TEM images of CN nanotubes shownin Fig. 1d, e. The image in Fig. 1d shows the nanotubes with bamboo-like and nested cone-shapedmorphology. Figure 1e is the magnified picture of Fig. 1d showing the nested cone-shaped morphology,which may be the initial stage of the formation of theY-junction. The diameter of the bamboo-like andnested cone-shaped carbon nanotubes is 20–50 nm.The bamboo-like structures of the nanotubes are dueto nitrogen incorporation (Nath et al. 2000; Yadav et al. 2004, 2005). The nitrogen comes from the decomposition of acetonitrile and silver nitrate.We also analyze the SEM (Fig. 2) and TEMimages of the C–N nanotubes (Fig. 3a–c), where wetake only AgNO 3 , (C 5 H 5 ) 2 Fe, and CH 3 CN as theprecursor. Figure 2 is the SEM picture of C–Nnanotube bundles containing Y-junction nanotubes.The length of the synthesized Y-junction nanotubebundles was found to be * 2  l m. 1350 J Nanopart Res (2008) 10:1349–1354  1 3  The scanning electron microscopic (SEM) investi-gations reveal that the length of the nanotubesdecreasesdrasticallyintheformationoftheY-junctionnanotubebundles(showninFig. 2)ascomparedtotheformation of nanotubes blocks (shown in Fig. 1).Probably, there are two points which are responsiblefor the differences in the length of nanotubes. Firstpoint is the higher concentration of ferrocene in theprecursor solution used in the Y-junction formation ascompared to the ferrocene concentration in theprecursor used for the synthesis of nanotube blocks.This is because we have not taken benzene in theprecursorsolutionusedforthesynthesisofY-junctionsand the relative concentration of ferrocene in thesolution increased. We have already reported inour previous paper (Yadav et al. 2004) that the Fig. 1  ( a ) SEM images of the as-synthesized C–Nnanotube blocks. ( b, c )Magnified images of a C–Nnanotube block. ( d ) TEMmicrograph of the C–Nnanotubes shows bamboo-like and nested cone-shapedmorphology. ( e ) Magnifiedversion of   dFig. 2  SEM image of the C–N nanotube bundles containingY-junction bamboo-like nanotubeJ Nanopart Res (2008) 10:1349–1354 1351  1 3  length of the nanotubes decreases rapidly when theconcentration of ferrocene increases in the precursor.When the solution is sprayed in the silica tube, growthof nanotubes takes place on the cluster of Fenanoparticles, which act as a catalyst for the growthof the carbon nanotubes. As the concentration of ferrocene increases, more iron nanoparticles areavailable for the catalytic growth of nanotubes withthe formation of a new cluster of iron nanoparticles,growth of fresh nanotubes, and their agglomeration inbundlesstarts.ThesupplyofcarbonandnitrogentoC–Nnanotubeswhicharegettingformedearlierwillstop.Thus, the length of the C–N nanotubes will have aspecificextent.ThesecondpointistheformationoftheY-branch disturbs the growth of neighboring nano-tubes in the bundles and therefore the bundles of Y- junction nanotubes have a smaller linear extent ascompared to the C–N nanotube blocks. Figure 3a, bshows the presence of the Y-junction in the bamboo-shaped carbon-nitrogen nanotubes. In both the cases,the outer diameter of the C–N nanotube are nearly thesame and is found to be around 50 nm. Theseinvestigations reveal that the synthesis of a three point junction (Y-shaped) and a more complex junction( W -shaped), as shown in Fig. 3c, is possible by spraypyrolysis of AgNO 3 , (C 5 H 5 ) 2 Fe, and CH 3 CN mixtureat around 900   C. The diameter of the nanotubes is * 80 nm, whereas the diameter of the branches is25–50 nm.The result of XPS of as-grown Y-junction nano-tubes is shown in Fig. 4a, b. This analysis shows thatthe nanotubes consist of carbon accompanied bytraces of nitrogen. Figure 4a shows that the C1ssignal is at 284.7 eV. The nitrogen 1s peak at400.7 eV [as shown in Fig. 4b] is likely due to thenitrogen present in the graphene sheets of carbonnanotubes (Yadav et al. 2005). This corresponds to pyrrolic-type trivalent nitrogen replacing the carbonin hexagonal structures. The nanotubes are C–Nnanotubes as reported by us (Yadav et al. 2004, 2005).In both the cases, the nanotubes are hollow insidehaving bamboo-like structures with compartmentlayers. We presume that nitrogen doping favors theformation of pentagons in addition to hexagons in thegraphene networks, which is responsible for bamboo-like structures. It seems that the base growth mech-anism is the dominating growth mechanism in thepresent investigations. As for the growth mechanismfor Y-junction formations, it is not yet clear, but afeasible mechanism may be given as follows.We presume that the silver nanoparticle comesfrom the decomposition of silver nitrate interacting Fig. 3  ( a ,  b ) TEM images of Y-junction bamboo-like C–Nnanotube. ( c )  W -shaped junctions in bamboo-like C–Nnanotubes1352 J Nanopart Res (2008) 10:1349–1354  1 3  with catalysts (iron nanoparticle) and changes theshape of iron nanoparticles. Therefore, the changednanoparticles shape is responsible for different typesof nanotubes junctions as suggested by Heyning et al.2005. The growth of branched CNTs with bamboo-shaped structures was a catalytic growth process,which could be influenced by many factors. Anyfluctuation in temperature, gas flow, or carbon sourcesupply could change the distribution of carbon atomson the catalyst particles and alter the growth process.Fluctuations could change the orientations of thecatalysts, and new branches would grow in differentorientations. We expect that if the catalyst particlesremained at the location of a junction and changedthe orientation two times before being pushed away, a W -junction would form (Qiang et al. 2007). Conclusions On the basis of the present investigation followingconclusions can be drawn. The C–N nanotube blocksof dimension * 300  9  200  9  30  l m having a bam-boo-like structure have been synthesized by thepyrolysis of the mixture of AgNO 3 , CH 3 CN ferro-cene, and benzene. The new precursor silver nitrateferrocene and acetonitrile mixture leads to theformation of a three point junction (Y-shaped) usingthe spray pyrolysis technique. We presume that thegrowth of multiple junction nanotubes is due to thedifferent shapes of catalyst particles (iron nanoparti-cles) altered by silver–iron nanoparticle interactions. Acknowledgements  The authors are extremely grateful toProf. CNR Rao, Late Dr. A. K. Singh, Prof. R. S. Tiwari, &Prof. S. Ram for their encouragement. The authorsacknowledge with gratitude the financial supports fromDST:UNANST, Council of Scientific and Industrial Research(CSIR) and University Grant Commission (UGC) New Delhi,India. References Andriotis AN, Menon M (2006) Are electrical switching andrectification inherent properties of carbon nanotube Y junctions? Appl Phys Lett 89:132116–132119Banghman RH, Cui C, Zakhidov AA, Iqbal Z, Barisci JN,Spinks GM, Wallace GG, Mazzoldi A, Rossi DD, RinzlerAG, Jaschinski O, Roth S, Kertesz M (1999) Carbonnanotube actuators. Science 284:1340–1344Choi WB, Chung DS, Kang JH, Kim HY, Jin YW, Han IT, LeeYH, Jung JE, Lee NS, Park GS, Kim JM (1999) Fullysealed, high-brightness carbon-nanotube field-emissiondisplay. Appl Phys Lett 75:3129–3131Collins P, Avouris P (2000) Nanotubes for electronics. Sci Am62:283–291Collins PG, Arnold MS, Avouris P (2001) Engineering carbonnanotubes and nanotube circuits using electrical break-down. Science 292:706–709Dai H, Wong EW, Lieber CM (1996) Probing electricaltransport in nanomaterials: conductivity of individualcarbon nanotubes. Science 272:523–526Frank S, Poncharal P, Wang ZL, de Heer WA (1998) Carbonnanotube quantum resistors. Science 280:1744–1746 Fig. 4  ( a ) XPS spectra of as-grown Y-junction nanotubes. Thepeak shows that the C1s signal is at 284.7 eV. ( b ) XPS spectraof as-grown Y-junction nanotubes. The nitrogen 1s peak hasbeen observed at 400.7 eVJ Nanopart Res (2008) 10:1349–1354 1353  1 3
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