Fabrication method of parallel mesoporous carbon nanotubes
- 格式：pdf
- 大小：1.24 MB
- 文档页数：6
Colloids and Surfaces A:Physicochem.Eng.Aspects 377 (2011) 150–155Contents lists available at ScienceDirectColloids and Surfaces A:Physicochemical andEngineeringAspectsj o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /c o l s u r faFabrication method of parallel mesoporous carbon nanotubesXuecheng Chen a ,∗,Krzysztof Cendrowski a ,Joanna Srenscek-Nazzal a ,Mark Rümmeli b ,Ryszard J.Kalenczuk a ,Hongmin Chen c ,Paul K.Chu c ,Ewa Borowiak-Palen a ,∗aNanotechnology Center,West Pomeranian University of Technology,Szczecin,Poland bTechnische Universität Dresden,01062Dresden,Germany cDepartment of Physics and Materials Science,City University of Hong Kong,Hong Kong,Chinaa r t i c l e i n f o Article history:Received 15October 2010Received in revised form 1December 2010Accepted 21December 2010Available online 13 January 2011Keywords:Carbon nanotube Mesoporous Surfactant AAO templatea b s t r a c tA facile approach to prepare parallel mesoporous carbon nanotubes fabricated from an anodised alu-minum oxide (AAO)template loaded with mesoporous silica by combining sol–gel and chemical vapor deposition (CVD)techniques is presented.The carbon nanotube walls that grow during CVD process copy the mesoporous structure of silica without removing the surfactant.The mechanism of formation of mesoporous carbon nanotubes was also studied.The technique results in a simple and efficient manner to produce parallel mesoporous carbon nanotubes exhibiting high specific surface area.© 2011 Elsevier B.V. All rights reserved.1.IntroductionAmong all investigated materials,ordered mesoporous mate-rials (OMM)and carbon nanotubes (CNTs)gained a tremendous attention due to their fantastic properties since their first discov-ery in 1992,and 1991,respectively.A combination of large specific surface area and high size-and shape-selectivity makes OMM a great candidate for applications such as catalysis,adsorption,and separation [1].CNTs exhibit extraordinary mechanical,electrical and ther-mal properties that make them promising for many applications,such as in structural materials,electronic and optical devices,supercapacitor electrodes and hydrogen storage [2].Several direct methods have been developed for the synthesis of CNTs,including arc-discharge,laser ablation,and chemical vapor deposition [3].Recently,templating methods have been developed to synthesize CNTs by carbonization of either polymer or pre-organized disclike molecules in porous anodic aluminum oxide (AAO)membranes [4].Being carbon based,such structures offer significant benefits rang-ing from electrical conductivity to enhanced chemical functionality and stability as compared to their silica mesophase analogues.Among all the applications,the use of CNTs as a catalyst support seems to be one of the more promising fields with large economi-cal implications.The results published to date [5]seem to indicate∗Corresponding author.Tel.:+48914494742;fax:+48914494686.E-mail addresses:xchen@.pl (X.Chen),eborowiak@.pl (E.Borowiak-Palen).that nanostructured carbon can represent a new class of advanced materials for catalytic applications allowing the modification of the reaction rate and of the final product selectivity rendering the process more environmentally benign because of the interaction between metal nanoparticles and graphic Ts used as catalyst supports induce peculiar activity and selectivity in several catalytic reactions such as in the hydrogenation of olefins [6]and nitrobenzene into aniline [7],selective hydrogenation of the C C bond in an-unsaturated aldehyde [8].It was also found that the tubular morphology and the high aspect ratio of CNTs could induce a confinement effect on the gas or liquids trapped inside the tube leading to completely different physical behavior when compared to conventional bulk material.Several reports recently dealt with the behavior of fluid trapped inside these high aspect ratio tubular materials [9].In addition,many experimental results have shown that the small size of the carbon nanostructured materials significantly contributes to the final catalytic performance of the system since catalytic reactions are governed by the phenomena of mass and heat transfer between the catalyst particles and the reactants.It is expected that reducing the catalyst size can make the reaction faster during the catalytic process.However,multi-walled carbon nanotubes (MWNTs)have a low specific surface area,what constitutes a limitation for applications such as those regarding gas,energy storage and catalyst support.A combination of the advantages of mesoporous materials and tubular CNTs to form CNTs with fine mesochannels with a size of about a few nanometers is of much importance and would provide wider applications in nanodevice fabrication,biomacromolecule0927-7757/$–see front matter © 2011 Elsevier B.V. All rights reserved.doi:10.1016/j.colsurfa.2010.12.044X.Chen et al./Colloids and Surfaces A:Physicochem.Eng.Aspects377 (2011) 150–155151Scheme1.Schematic of the synthesis steps for mesoporous CNTs.separations and more extensive applications in many otherfields. In this sense,nowadays there is great interest to obtain high specific surface area MWNTs to improve the performance of these mate-rials.Up to now,only several research groups have reported the synthesis of mesoporous CNTs[10].In this article,we report the synthesis of MWNTs with a meso-porous wall from an AAO template through a simple sol–gel and CVD method.The diameter of formed MWNTs is in the range of 200–270nm,in agreement with the pore sizes of the used AAO template.The walls forming the MWNT have mesochannels with a channel size of about1.15nm.The purified CNT have high spe-cific surface area(108m2/g),which may serve as a new and efficient mold andfind wider applications in nanodevice fabrication,catalyst support and bio-macromolecule separation.2.Experimental2.1.MaterialsAAO template was purchased from Waitman Company (200nm).Tetraethyl orthosilicate(TEOS)and cetyltrimethyl ammonium bromide(CTAB)were bought from Aldrich.These reagents were used as received.2.2.Synthesis of mesoporous carbon nanotubeInitially,a silica sol–gel precursor was prepared by adding 50:5:1ratio of absolute ethanol:tetraethoxysilane(TEOS):1M HCl, respectively.The mixture was allowed to hydrolyze for1h at room temperature.An AAO template(average pore diameter200nm, Fig.1.SEM images of parallel CNT composed of mesoporous walls.152X.Chen et al./Colloids and Surfaces A:Physicochem.Eng.Aspects377 (2011) 150–155Fig.2.TEM images of purified meso-CNT.60m thick)was sonicated inside the mixture for 10min while maintaining a constant water bath temperature.The loaded alu-mina template was then dried at 100◦C for 24h.In the next step,the dried loaded template was introduced to a cetyltrimethyl ammonium bromide (CTAB)solution and TEOS.In this way a surfactant-templating approach is applied to create mesoporous silica in the following manner:a solution containing CTAB (30mg),deionized water (8mL),concentrated ammonia aqueous solution (110l,28wt %)and ethanol (6mL)were prepared.The mixed solution was homogenized for 0.5h to form a uniform dispersion.50010001500200025000200400I n t e n s i t yWavenumber (cm -1)Fig.3.(a)Raman spectra of purified CNT with mesoporous walls.21.5l of TEOS was added dropwise to the dispersion with contin-uous stirring.Then dried SiO 2@AAO was re-dispersed in the above solution.During this process,vacuum was applied to ensure the solution was incorporated into the SiO 2@AAO.After this process,a meso-structured CTAB/silica composite was deposited inside the SiO 2@AAO channels.In the third step,without removing the CTAB,mesoporous silica containing CTAB in AAO template was put into the ceramic boat,Ar was used as carrier gas,when the tempera-ture reached 800◦C,C 2H 4was introduced,flow rate of C 2H 4was set 20sccm.After 2h reaction,graphite carbon was formed in the pores and partially on the surface of mesoporous SiO 2(C-m-SiO 2@SiO 2@AAO).In the last step,AAO templates and SiO 2were removed via HF treatment to form a mesoporous carbon wall,resulting in CNTs with mesoporous walls.2.3.CharacterizationTransmission electron microscopy (TEM)measurements were carried out on a transmission electron microscope (HR-TEM,Tec-nai F30from FEI)operated at an accelerated voltage of 200kV.The specimens for the microscopic studies were prepared on standard TEM copper grids.The prepared samples were also observed by scanning electron microscopy (SEM)on a Hitachi S-4300field emis-sion scanning electron microscope operated at 10kV.N 2adsorption and desorption measurements were carried out at −196◦C using a Quantachrome Instrument.The sample was degassed at 300◦C in high vacuum before measurements.The pore surface area distribu-tion against pore size was also obtained from nitrogen desorption data by the BJH method.X.Chen et al./Colloids and Surfaces A:Physicochem.Eng.Aspects 377 (2011) 150–155153Fig.4.TEM images of solid wall CNT liberated from CNT@AAO .3.Results and discussionThe preparation route for the mesoporous CNT is depicted in Scheme 1.Initially,A silica sol–gel precursor was prepared from ethanol,TEOS and HCl,respectively.An AAO template was dipped into the mixture.The loaded alumina template was then dried1.00.80.60.40.20.0101520253035V o l u m e a d s o r b e d [c m 3/g ]Relative press (p/p o )V o l u m e a d s o r b e d [c m 3/g ]Relative press [p/p o ]Fig.5.N 2adsorption–desorption isotherms and the corresponding pore size distri-butions (insets)of a)solid CNT from CNT@AAO and mesoporous CNT.(SiO 2@AAO).In the next step,the dried loaded template was intro-duced to a cetyltrimethyl ammonium bromide (CTAB)solution and TEOS.In this way a surfactant-templating approach is applied to create a thin layer of mesoporous silica in the AAO template.After this process,a meso-structured CTAB/silica composite was deposited inside the SiO 2@AAO channels (m-SiO 2@SiO 2@AAO).In the third step,without removing the surfactant (CTAB),CVD process was directly performed with C 2H 4as a carbon source.Graphite carbon was formed in the pores of mesoporous SiO 2(C-m-SiO 2@SiO 2@AAO).In the last step,AAO templates and SiO 2were removed via HF treatment to form a mesoporous carbon wall,resulting in CNTs with mesoporous walls.Fig.1a displays an SEM image of purified mesoporous CNT.This image shows that the resulting one-dimensional nanostructures are continuous and tightly arranged parallel to one another.Fig.1b presents a typical SEM image of the as-prepared CNTs from an end-on view,confirming that the as-prepared CNT array comprises open-ended CNTs.The presence of so many bundles implies that most of the tubes are connected at both ends of each tube,because the carbon deposition also took place on the outer surface of the anodic film.However,some of the tubes were separated from the others while stirring in HF solution,as can be seen in Fig.1a and d.The lengths of the arrays are up to 60m,which closely matches the thickness of the AAO template used.Fig.1b and c shows SEM images of an array of aligned CNTs at a higher magnification,suggesting the formation of an ordered array of nanotubes with uniform diameter.Fig.2shows representative transmission electron microscopy (TEM)images of the resulting mesoporous CNTs after removal of the AAO template.The nanotubes are straight and monodisperse with ca.200nm diameter corresponding closely to the pore size of the used AAO template.The high-resolution transmission elec-tron microscopy images (HRTEM)(Fig.2c and d and Fig.S1–S3)revealed that the nanotube wall was nanoporous in the tube wall.The tube wall is about 20nm thick.The typical graphitic interlattice (∼0.35nm)structure was observed in the HRTEM,which suggests that the pore wall is composed of graphite.In Fig.3,the purified mesoporous CNTs were also studied by Raman spectroscopy.As shown in Fig.1b,two strong peaks appear at 1597and 1362cm −1,respectively,in the Raman spectrum.The former (so-called G-band)is attributed to an E 2g mode of hexago-nal graphite and is associated with the vibration of sp 2-hybridized carbon atoms in a graphite layer.This means the mesoporous wall are composed of graphitic carbon.The latter (so-called D-band)is ascribed to the vibration of carbon atoms with dangling bonds in the plane terminations of disordered graphite.Thus,the Raman spec-troscopic results agree with the HRTEM observations.Two weak154X.Chen et al./Colloids and Surfaces A:Physicochem.Eng.Aspects377 (2011) 150–155Fig.6.(A)(a)MCM-41after CVD reaction,(b)MCM-41CTAB after CVD reaction and (c)MCM-41CTAB heated at 800◦C for 3h in Ar.(B)TEM image of MCM-41CTAB heated at 800◦C for 3h in Ar.peaks also appear in a higher frequency range (not shown here),which are attributed,respectively,to the double frequencies of the D-and G-bands.In order to compare the structural properties of mesoporous CNTs and typical CNTs grown directly in AAO (this sample will be refered to solid wall CNTs),the synthesis of solid wall CNT was per-formed.Fig.4displays a TEM image of CNTs synthesized directly in an AAO template with a pore size of 200nm.11The diameter and shell thickness of the CNTs are 200nm and 30–40nm,respec-tively.Fig.4b provides an HRTEM image of a CNT prepared in this way.Well-ordered graphitic layers are clearly seen.No mesoporous structure appeared.Typical nitrogen adsorption–desorption isotherms at 77K for solid wall CNTs and mesoporous CNTs and their corresponding pore size distributions are shown in Fig.5,which showed type IV curves indicating that the pore sizes are in the mesopore range.In Fig.5b,the pore size distribution data calculated from the des-orption of the nitrogen isotherms by the Barrett–Joyner–Halenda (BJH)method showed that wide distributions and only a weak peak at 16nm and two strong weak at 3.7and 6.9nm,which matches very well with the TEM results.The mesoporous CNT exhibited a Brunauer–Emmett–Teller (BET)surface area of 108m 2/g.While the BET area of solid wall CNTs from CNT@AAO is only 14m 2/g,which is far lower than mesoporous CNT.This is also consistent with the TEM data.From BET data of meso-CNT and solid wall CNT one can see an increase by a factor of eight in the mesoporous structure.This means that the mesopores are formed in the wall of meso-CNT.This provides additional free space in meso-CNT in respect to the solid wall CNT.This should also provide wider applications in H 2storage,biomacromolecule separations and in many other fields [11].The mechanism of formation of mesoporous carbon nanotubes was also studied.Through the mechanism study,we found that CTAB is crucial for the synthesis of mesoporous carbon nanotubes during the CVD.It was observed that before the C 2H 4was intro-duced,CTAB was converted into graphitic carbon seeds in the silica channels and partially on the silica surface during the heat treat-ment in Ar.After the introduction of C 2H 4,this formed graphitic carbon seeds played a role of the catalyst seeds for the nanotubes formation.Those catalyst seeds accelerated the carbon deposition process.After the removal of silica,mesoporous carbon nanotubes were obtained.In order to prove our claims,mesoporous silica sphere (MCM-41)was used as a model.Two kinds of MCM-41silica spheres were used for this experiment,MCM-41with or without CTAB trapped into the channels.The experimental conditions were kept the same as in CVD described above.In the first experiment,the same CVD was applied using two kinds of MCM-41silica spheres,namely (a)–MCM-41and (b)–MCM-41CTAB.The heat treatment at 800◦C for 2h in Ar without C 2H 4introduction was also performed for MCM-41CTAB (c).In the same reaction temperature and time,three kinds of products after CVD were obtained.As one can see in Fig.6A,only in sam-ple (b)the product exhibited black color indicating aboundance of carbon deposition.In both remaining samples (a and c)the color of the material was greyish.This observation was also confirmed by TEM analysis of the products.Fig.6B shows the presence of the graphitic carbon seeds deposited on the sphere surface as indicated by arrows after heating in Ar of sample (c).Therefore,it can be con-cluded that the existence of CTAB can accelerate the CVD deposition process what is consistent with our previous assumption.4.ConclusionsIn conclusion,we have demonstrated the synthesis of paral-lel CNTs with mesoporous wall.The mesoporous CNTs possess a very high surface area,large pore volume,and uniform accessi-ble mesopores as compared to the solid CNTs from AAO template.Because of the useful unique mesoporous carbon structure,the nan-otubes may provide great promise for many applications such as catalysts,adsorbents,sensors,electrode materials,and advanced storage materials and beyond.The presence of a macro-cavity in the carbon tubes facilitates the bidirectional diffusion of reactants to both interior and exterior surfaces of the tubes.AcknowledgementThe work was supported by Foundation for Polish Science within Focus with contract F4/2010and City University of Hong Kong Strategic Research Grant (SRG)7008009.Appendix A.Supplementary dataSupplementary data associated with this article can be found,in the online version,at doi:10.1016/j.colsurfa.2010.12.044.References[1](a)J.S.Beck,J.C.Vartuli,W.J.Roth,M.E.Leonowicz,C.T.Kresge,K.D.Schmitt,C.T.W.Chu,D.H.Olson,E.W.Sheppard,A new family of mesoporous molecu-lar sieves prepared with liquid crystal templates,J.Am.Chem.Soc.114(1992)10834–10843;(b)C.T.Kresege,M.E.Leonowicz,W.J.Roth,J.C.Vartuli,J.S.Beck,Ordered meso-porous molecular sieves synthesized by a liquid-crystal template mechanism,Nature 359(1992)710–712.X.Chen et al./Colloids and Surfaces A:Physicochem.Eng.Aspects377 (2011) 150–155155[2](a)M.S.Dresselhaus,G.Dresselhaus,P.Avouris(Eds.),Carbon Nanotubes:Advanced Topics in the Synthesis,Structure,Properties and Applications, Springer,New York,2001,pp.1–692;(b)E.Frackowiak,F.Beguin,Carbon39(2001)937;(c)X.Li,H.Zhu,L.Ci,C.Xu,Z.Mao,B.Wei,Hydrogen uptake by graphitizedmulti-walled carbon nanotubes under moderate pressure and at room temper-ature,Carbon39(2001)2077–2079;(d)C.Niu,E.K.Sichel,R.Hoch,D.Moy,H.Tennet,High power electrochemicalcapacitors based on carbon nanotube electrodes,Appl.Phys.Lett.70(1997) 1480–1482.[3]H.J.Dai,Carbon nanotubes:synthesis,integration,and properties,Acc.Chem.Res.35(2002)1035–1044.[4](a)V.S.Iyer,K.P.C.Vollhardt,R.Wilhelm,Near-quantitative solid-state synthe-sis of carbon nanotubes from homogeneous diphenylethynecobalt and nickel complexes,Angew.Chem.Int.Ed.42(2003)4379–4383;(b)S.Rahman,H.Yang,Nanopillar arrays of glassy carbon by anodic aluminumoxide nanoporous templates,Nano Lett.3(2003)439–442.[5](a)J.M.Planeix,N.Coustel,B.Coq,V.Brotons,P.S.Kumbhar,R.Dutartre,P.Gen-este,P.Bernier,P.M.Ajayan,Application of carbon nanotubes as supports in heterogeneous catalysis,J.Am.Chem.Soc.116(1994)7935–7936;(b)G.Mestl,N.I.Maksimova,N.Keller,V.V.Roddatis,R.Schlögl,Carbon nanofil-aments in heterogeneous catalysis:an industrial application for new carbon materials?Angew.Chem.Int.Ed.40(2001)2066–2068;(c)C.Pham-Huu,N.Keller,M.J.Ledoux,L.Charbonnière,R.Ziessel,Carbonnanofiber supported palladium catalyst for liquid-phase reactions.An active and selective catalyst for hydrogenation of C C bonds,mun.19 (2000)1871–1872;(d)M.J.Ledoux,R.Vieira,C.Pham-Huu,N.Keller,New catalytic phenomena onnanostructured(fibers and tubes)catalysts,J.Catal.216(2003)333–342. [6](a)A.Chambers,T.Nemes,N.M.Rodriguez,R.T.K.Baker,Catalytic behavior ofgraphite nanofiber supported nickel parison with other sup-port media,J.Phys.Chem.B102(1998)2251–2258;(b)C.Park,R.T.K.Baker,Catalytic behavior of graphite nanofiber supportednickel particles.2.The influence of the nanofiber structure,J.Phys.Chem.B 102(1998)5168–5177;(c) F.Salman, C.Park,R.T.K.Baker,Hydrogenation of crotonaldehydeover graphite nanofiber supported nickel,Catal.Today53(1999)385–394.[7]H.P.Boehm,Some aspects of the surface chemistry of carbon blacks and othercarbons,Carbon32(1994)759–769.[8](a)S.S.Barton,M.J.B.Evans,E.Halliop,J.A.F.MacDonald,Acidic and basic siteson the surface of porous carbon,Carbon35(1997)1361–1366;(b)C.N.R.Rao,indaraj,B.C.Satishkumar,Functionalised carbon nan-otubes from solutions,mun.(1996)1525–1526.[9](a)K.Koga,G.T.Gao,H.Tanaka,X.C.Zeng,Formation of ordered ice nanotubesinside carbon nanotubes,Nature412(2001)802–805;(b)M.S.P.Sansom,P.C.Biggin,Biophysics:water at the nanoscale,Nature414(2001)156–159;(c)M.C.Gordillo,J.Marti,Hydrogen bonding in supercritical water confined incarbon nanotubes,Chem.Phys.Lett.341(2001)250–254;(d)C.Pham-Huu,N.Keller,G.Ehret,M.J.Ledoux,Thefirst preparation of siliconcarbide nanotubes by shape memory synthesis and their catalytic potential,J.Catal.200(2001)400–410;(e)X.L.Pan,Z.L.Fan,W.Chen,Y.J.Ding,H.Y.Luo,X.H.Bao,Enhanced ethanolproduction inside carbon-nanotube reactors containing catalytic particles,Nat.Mater.7(2007)507–511.[10](a)A.T.Rodriguez,M.Chen,Z.Chen,C.J.Brinker,H.Y.Fan,Nanoporous carbonnanotubes synthesized through confined hydrogen-bonding self-assembly,J.Am.Chem.Soc.128(2006)9276–9277;(b)M.Kim,K.Sohn,J.Kim,T.Hyeon,Synthesis of carbon tubes with mesoporouswall structure using designed silica tubes as templates,mun.(2003) 652–653.[11]H.Gao,X.B.Wu,J.T.Li,G.T.Wu,J.Y.Lina,Hydrogen adsorption of open-tippedinsufficiently graphitized multiwalled carbon nanotubes,Appl.Phys.Lett.83 (2003)3389–3391.。
