碳纳米管改性

Journal of Colloid and Interface Science322(2008)321–326/locate/jcisFabrication of silica nanotubes using silica coated multi-walled carbonnanotubes as the templateMyunghun Kim a,Jinho Hong a,Jeongwoo Lee a,Chang Kook Hong b,Sang Eun Shim a,∗a Department of Chemical Engineering,Inha University,253Yonghyundong,Namgu,Incheon402-751,South Koreab School of Applied Chemical Engineering,Chonnam National University,300Yongbong-dong,Puk-gu,Gwangju500-757,South KoreaReceived15October2007;accepted28March2008Available online3April2008AbstractSilica nanotubes were synthesized using the multi-walled carbon nanotubes(MWCNTs)as the template material.First,we prepared silica coated MWCNT composites by surface oxidation of MWCNTs using KMnO4in the presence of a phase transfer catalyst and followed by grafting of2-aminoethyl3-aminopropyl trimethoxy silane,AEAPS.The amine groups in grafted AEAPS on MWCNTs could activate the silica shell formation by acid–base interaction.The synthesized silica was formed a uniform layer on MWCNTs with a controllable thickness and possessed sturdy3-dimensional stability.After calcinations at800◦C,the inner MWCNTs of the composite were completely decomposed and the outer silica shell layer maintained without distortion of its original shape.Finally,we could obtain the silica nanotubes having13.0nm of average layer thickness.©2008Elsevier Inc.All rights reserved.Keywords:Carbon nanotube;Silica;Surface modification;Nanotube;Encapsulation1.IntroductionCurrently,carbon nanotubes(CNTs)play an essential role in variousfields of nano based science and technology[1–4]. Due to their extraordinary mechanical,thermal,electrical,and optical properties,industry and academic communities have fo-cused on CNTs.Moreover,CNTs are expected to substitute a variety of classical materials because of their great possibil-ity in near future[5].Recently,silica coated CNT composites are of interest because they are useful for the advancement of nanoscale sensors and electric devices as well as optical,mag-netic,and catalytic applications[6].Especially,they can pro-mote compatibility with existing silicon based technology.Sev-eral methods have been explored to fabricate the silica coated CNT composites such as(1)synthesis of silica on noncova-lently(usingπ–πinteraction)or covalently(by formingσbond)functionalized CNTs[7–10],and(2)direct coupling re-action with previously modified silica particles and CNTs[11]. *Corresponding author.Fax:+82328720959.E-mail address:seshim@inha.ac.kr(S.E.Shim).In addition,(3)surfactants were employed to give negative or positive charges on CNTs and followed by the synthesis of sil-ica onto surface of CNTs[12–14].In most cases,a uniform silica layer on CNTs was not achieved,whereas dangling silica particles were adsorbed on CNTs,resulting in the incomplete coverage of the surface of CNTs by the silica layer.The insuf-ficient coverage originates from the lack of chemical affinity between the surface of CNTs and silica.The major important considerations in the synthesis of silica coated CNTs are(1)the less-destructive modification of CNTs in a primary functional-izing step,(2)the enhancement of interfacial adhesion between silica and CNTs,and(3)the formation of a uniform silica layer on CNTs with a controllable thickness in nanometer scale.In the chemical oxidation process of CNTs,various oxidants have been conventionally employed,which include concen-trated HNO3,concentrated H2SO4,aqua regia,KMnO4solu-tion(acid/alkali),an aqueous OsO4/NaIO4and so forth[15].It has been known that the genuine CNTs structure experiences severe damage causing the deterioration of their superior prop-erties during the chemical oxidation processing using strong acids at high temperature above100◦C.Therefore,a novel chemical oxidation of CNTs in the presence of a phase transfer0021-9797/$–see front matter©2008Elsevier Inc.All rights reserved. doi:10.1016/j.jcis.2008.03.045322M.Kim et al./Journal of Colloid and Interface Science 322(2008)321–326catalyst (PTC)has been emerged recently in order to func-tionalize CNTs.There exist a few publications regarding the utilization of PTCs for the surface modification of fullerene or CNTs consisting of a planar wall and a hemispherical cap [16–18].It has been found that PTC methodology possesses several advantages over conventional oxidation methods using strong acids:(1)efficiency of oxidation drastically increases,(2)product selectivity between carboxylic acid and hydroxyl group is improved,(3)reaction condition is very mild,and (4)damage of CNTs is minimized [16].In this study,hydroxyl groups were first introduced on the MWCNTs using tetrapropyl ammonium bromide (TPABr)as a PTC in conjunction with potassium permanganate.In order to maximize the silica encapsulation efficiency,hydroxyl groups of MWCNTs reacted with a silane coupling agent containing amine groups,AEAPS.This silane-modified MWCNTs were used to prepare silica coated MWCNTs by the following poly-condensation of tetraethyl orthosilicate (TEOS).Due to strong interaction between basically modified CNTs and acidic TEOS,sturdy silica shell with a controllable uniform thickness was achieved.Such silica coated CNTs were further utilized as the template for the synthesis of hollow silica nanotubes after cal-cinations.Finally,the stable hollow silica nanotubes were fab-ricated using this method.2.Materials and methods 2.1.MaterialsThe MWCNTs synthesized by a thermal chemical vapor deposition (CVD)method using an aluminum nitrate cata-lyst were used (approximately 97%pure,Iljin Nanotech,Ko-rea).The MWCNTs have a diameter of 10–20nm and length of 10–50µm.Hydrochloric acid (36%),potassium perman-ganate (99.3%),methylene chloride (99%),ethanol,methanol (99.9%),ammonium hydroxide (27%),and TEOS (98%)were all purchased from Samchun Chemical Co.,Korea.TPABr (99%)was purchased from Aldrich.Acetic acid (99.8%)was purchased from Duksan Chemical Co.,Korea.AEAPS was pur-chased from Shin-Etsu Chemical Co.,Japan.2.2.Grafting hydroxyl groups on the surface of MWCNTs The impurities such as amorphous carbon and metallic cat-alyst in the MWCNTs were removed by 37%HCl at 60◦C for 4h prior to experiment.0.2g raw MWCNTs and 100ml methylene chloride were added into a 250ml flask and the mix-ture was dispersed by ultrasonication (Bandelin Sonoplus)for 10min in the flask.1g TPABr dissolved in 10ml H 2O,10ml acetic acid,and KMnO 4solution (0.25g of KMnO 4dissolved in 5ml H 2O)were added to the flask.The mixture was stirred vigorously at 25◦C for 24h.The mixture was washed with HCl and filtered with methanol and distilled water through 0.2µm Teflon filter (Adventec MFS).The washing step was repeated at least 4cycles.After vacuum drying the filtrate,0.18g hy-droxyl groups functionalized MWCNTs (MWCNT-OH)wereobtained.Fig.1.Scheme of the reaction between MWCNT-OH and AEAPS for the fol-lowing synthesis of silica coated MWCNTs.2.3.Modification of MWCNT-OH with AEAPS0.05g as-prepared hydroxyl functionalized MWCNTs were dispersed in 40ml toluene by ultrasonication for 10min.After adding an excess of AEAPS,the suspension was refluxed under N 2at 100◦C for 6h.After reaction,the resultant was washed repeatedly with methanol to remove residual AEAPS.2.4.Synthesis of silica coated MWCNTs0.005g of AEAPS-functionalized MWCNTs and 50ml ethanol were charged into the 100ml flask and mixed by ultra-sonication for 10min.After dispersion,ammonium hydroxide (diluted with 5ml ethanol)was dissolved in the mixture.TEOS (diluted with 5ml ethanol)was injected into the flask and the reaction vessel was stirred vigorously at room temperature un-der N 2atmosphere for 24h.After synthesis,the composite was washed with ethanol twice for the removal of secondary silica particles.Finally,silica coated MWCNTs were prepared and characterized.2.5.CharacterizationA scanning electron microscopy (SEM)(S-4300,Hitachi)and transmission electron microscopy (TEM)equipped with selected area electron diffraction (SAED)(CM200,Philips)were used for observation of the structure variation of sil-ica coated MWCNT.Thermogravimetry analysis (TGA)was performed at a heating rate of 20◦C /min under oxygen at-mosphere on TGA/SDTA 851,Mettler Toledo Co.X-ray dif-fraction (XRD)was measured using Ni-filtered Cu K radiation (Miniflex,Rigaku).The average diameter of silica coated MW-CNTs was obtained using Scion image analyzer ®by measuring at least 100points from TEM microphotographs.3.Results and discussionFig.1shows the procedure starting from the hydroxyl groups functionalized MWCNTs treated by KMnO 4with the aid of a PTC for the synthesis of silica coated MWCNTs.Chemical ox-idation reaction with permanganate in the presence of phaseM.Kim et al./Journal of Colloid and Interface Science322(2008)321–326323Fig.2.TEM microphotographs of silica coated MWCNTs related to amount of reactants,TEOS(dissolved in5ml EtOH)and ammonia solution(dissolved in5ml EtOH);(a–c,scale bar=50nm;d–f,scale bar=100nm).Refer to Table1for recipes.transfer catalysts can be carried out in very mild conditions to modify CNTs or fullerene and the reaction takes place at room temperature.Furthermore,it offers a high concentration of surface acid groups and high yield of hydroxyl group[17]. It is noted that the modification reaction is carried out in mild conditions so that the damage of MWCNTs can be minimized. During the surface modification reaction in the presence of a PTC,the color of KMnO4changed from dark purple to dark brown.This fact implies the transformation of Mn7+to Mn4+. The surface hydroxyl groups developed by permanganate to-gether with a phase transfer catalyst,TPABr,are subsequently reacted with AEAPS to give amine groups terminated MWC-NTs.The opulent basic amine groups on modified MWCNTs can activate acidic precursor TEOS by acid–base interaction with an assist from a basic catalyst ammonium hydroxide.Due to the strong chemical attraction in the silica synthesis process, MWCNTs are evenly coated with silica and the thickness of sil-ica shell can be readily controlled by adjusting the amounts of reagents.Fig.2shows TEM microphotographs of silica coated MW-CNTs where the amount of reactants is varied.The weight ratio of NH4OH to TEOS isfixed at3.75whereas the weight ratio of TEOS to modified MWCNTs is increased from0.48 to9.60.In Fig.2a,silica is hardly coated on the surface of MWCNTs,and uniform coating by the silica layer is obtained above4.80weight ratio of[TEOS]/[MWCNTs].For the inter-mediate weight ratios of[TEOS]/[MWCNTs]between1.20and 4.80,incomplete coating is resulted,remaining the bare MW-CNT surface.It is noted that dangling silica particles are not produced at any experimental conditions,indicating that sil-ica instantly coats MWCNTs upon the generation due to strong interaction between basic amine groups on MWCNTs.The av-erage diameter of silica coated MWCNTs increases with the amount of reactants,TEOS.The increase in the average di-ameter of MWCNTs is summarized in Table1.The average Table1The variation of silica coated MWCNT’average diameter prepared with an increasing amount of TEOS with afixed weight ratio of[NH4OH]/[TEOS]= 3.75Exp.No.MWCNT(g)TEOS(g)TEOS/MWCNTNH4OH(g)Average diameter(nm)(a)0.050.0240.480.09019.5(b)0.050.06 1.200.22517.8(c)0.050.12 2.400.45026.1(d)0.050.24 4.800.90039.5(e)0.050.367.20 1.35048.9(f)0.050.489.60 1.80062.4diameter of pristine MWCNTs with11.8nm increased up to 62.4nm by uniform coating of silica shell.The variation of average diameter of silica coated MWC-NTs with respect to the amount of TEOS is plotted in Fig.3. From TEM images in Fig.2,the silica layer thickness can-not be measured because the boundary between MWCNTs and silica is clear.Therefore the layer thickness of the coated sil-ica was calculated after calcination of silica coated MWCNT hybrid composites.The diameter of silica coated MWCNTs lin-early increases with the amount of TEOS,suggesting that one can control the thickness of silica layer by changing the amount of TEOS.Figs.4and5respectively show TEM and SEM micropho-tographs of silica coated MWCNTs synthesized with an in-creasing amount of NH4OH at afixed concentration of TEOS, [TEOS]/[MWCNTs]=4.80.The variation of average diame-ter of silica coated MWCNTs related to amount of NH4OH is listed in Table2and the result is plotted in Fig.6.TEM mi-crographs of silica coated MWCNTs in Figs.4a and4b show that some parts of MWCNTs’surface are not sufficiently coated with silica.Above the weight ratio of[NH4OH]/[TEOS]=3.75 whole surfaces of MWCNTs are encapsulated by uniform silica324M.Kim et al./Journal of Colloid and Interface Science 322(2008)321–326layer shown in Figs.4c and 4d .In addition,the SEM micro-graphs of silica coated MWCNTs are represented in Fig.5.The difference in the increase of the diameter of the composites is evident when silica is sufficiently synthesized on the surface of modified MWCNTs.As the amount of NH 4OH increases,the average diameter of the hybrid nanotubes linearly increased up to [NH 4OH]/[TEOS]=3.75,then the diameter is not further increased with the amount of NH 4OH since TEOS is depleted at this concentration of NH 4OH (Fig.6).Thermogravimetry analysis (TGA)was performed for the quantitative analysis of functionalized MWCNTs andsilicaFig. 3.The variation of silica coated MWCNT’average diameter pre-pared with an increasing amount of TEOS with a fixed weight ratio of [NH 4OH]/[TEOS]=3.75.coated MWCNT composites.The weight loss curves of raw MWCNTs,hydroxylated MWCNTs (MWCNT-OH),silane-functionalized MWCNTs (MWCNT-AEAPS),and silica coated MWCNTs are presented in Fig.7.Because MWCNTs did not decompose under nitrogen atmosphere up to 1000◦C,we per-formed analyses under oxygen atmosphere.For raw MWCNTs,a constant weight is maintained up to 400◦C and slowly de-creases at 550◦plete decomposition occurs at around 650◦C whilst slight lower complete decomposition tempera-ture is observed for surface treated MWCNTs due to apossibleFig.4.TEM microphotographs of silica coated MWCNTs related to amount of NH 4OH.The silica layer was synthesized using 0.24g TEOS and various amounts of NH 4OH:(a)0.45,(b)0.675,(c)0.9,and (d)1.8g.(a,scale bar =50nm;b,scale bar =50nm;c,scale bar =200nm;d,scale bar =100nm.)Fig.5.SEM microphotographs of silica coated MWCNTs related to amount of NH 4OH (scale bar =500nm).The silica layer was synthesized using 0.24g TEOS and various amounts of NH 4OH:(a)0.45,(b)0.675,(c)0.9,and (d)1.8g.M.Kim et al./Journal of Colloid and Interface Science 322(2008)321–326325Table 2The variation of silica coated MWCNT’average diameter related to amount of NH 4OH at a fixed amount of TEOS Exp.No.TEOS (g)NH 4OH (g)Average diameter (nm)(a)0.240.4520.7(b)0.240.67530.7(c)0.240.940.5(d)0.241.843.6Fig.6.The variation of silica coated MWCNT’average diameter related to amount of NH 4OH at a fixed amount of TEOS.destruction of a perfect MWCNT structure during the chemical oxidation process.However,the degree of lowering decomposi-tion temperature is much lesser than the conventional oxidation processes using strong acids as reported elsewhere [19].For MWCNT-OH,the weight loss decreases for the decomposition of hydroxyl groups in 200–500◦C and finally MWCNTs de-composed at around 650◦C.For AEAPS grafted MWCNTs,the weight loss of silane coupling agent is about 10wt%and activation site unit in AEAPS on MWCNTs is about 1.5wt%.These are observed for the decomposition of grafted silane units in 280–420◦C.For silica coated MWCNTs,the main weight loss is obtained for the decomposition of MWCNTs in the silica coated MWCNT composite in the temperature range of 400–650◦C.The decreased weight loss is decomposi-tion of functionalized MWCNTs and the remaining material is silica from the composite.For the silica coated MWNTs prepared using 0.24g TEOS and 0.9g NH 4OH,the TGA thermogram indicates that silica is synthesized about 80wt%on MWCNTs.X-ray diffraction patterns are characterized to compare the structures of the synthesized hybrid composite in Fig.8.For the raw MWCNTs in Fig.8a,the peaks are observed at the angles 2theta =26.4◦,37.9◦,and 44.2◦.The angle 2theta at 26.4◦corresponds to the (002)reflection for the feature peak ofrawFig.7.TGA weight loss curves of (a)raw MWCNTs,(b)MWCNT-OH,(c)MWCNT-AEAPS,and (d)silica coated MWCNTs.Silica coated MWCNTs were prepared using 0.24g TEOS and 0.9g NH 4OH.Fig.8.XRD patterns of (a)raw MWCNTs and (b)silica coated MWCNTs.Silica coated MWCNTs were prepared using 0.24g TEOS and 0.9g NH 4OH.MWCNTs.The two sharp peaks at 37.9◦and 44.2◦are char-acteristic peaks of the used cell so that they are observed for all of the characterized samples.For silica coated MWCNT in Fig.8b,the feature peak of MWCNTs at 26.4◦disappeared be-cause the raw MWCNTs were perfectly coated with amorphous silica.This amorphous silica caused the hindrance of MWCNT’characteristic peak due to the interference of the X-ray.In addi-tion,the inset to Fig.4c is an electron diffraction image of silica coated MWCNT which indicates no crystallinity of the formed silica.Throughout the synthesis process and characterizations,we successfully obtained MWCNTs uniformly coated with con-326M.Kim et al./Journal of Colloid and Interface Science 322(2008)321–326Fig.9.TEM microphotographs of silica coated MWCNTs after calcination.(a,scale bar =100nm;b,scale bar =200nm.)trollable silica shell layer.Since silica is synthesized from TEOS,the silica layer possesses a sturdy 3-dimensional intra-structure.Fig.9shows the TEM morphology of calcinated hybrid composite nanotubes.The silica coated MWCNTs were calcinated at 800◦C for two hours.The inner MWC-NTs of the composite were completely decomposed and the outer silica shell layer of the composite maintained with-out distortion of its original shape.Finally,we could obtain the silica nanotubes having 13.0nm of average layer thick-ness.4.SummarySilica nanotubes are synthesized using MWCNTs as the template material.In order to minimize the destruction of MW-CNTs during a chemical oxidation process,MWCNTs are oxi-dized using KMnO 4in the presence of a phase transfer catalyst,TPABr at room temperature under a nearly neutral condition.The hydroxyl functionalized MWCNTs further reacted with a silane coupling agent bearing amine groups,AEAPS.Ap-proximately 10wt%of AEAPS is grafted on MWCNTs and the basic characteristics of the AEAPS aggrandizes the for-mation of acidic silica on the surface of MWCNTs due to strong acid–base interaction.The uniformly silica coated MW-CNTs are synthesized and the 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