简易法制备竹节状非晶态碳纳米管

赵 率 郑明涛 谢 春 林 贺 文 启 易观贵 刘应亮

(暨南大学化学系，暨南大学纳米化学研究所，广州510632)

简易法制备竹节状非晶态碳纳米管

赵 率 郑明涛 谢 春 林 贺 文 启 易观贵 刘应亮＊

(暨南大学化学系，暨南大学纳米化学研究所，广州510632)

以正硅酸乙酯为碳源，采用简易热分解法，在无催化剂的条件下合成了竹节状非晶态碳纳米管。运用扫描电镜(SEM)和透射电镜(TEM)对样品进行表征，结果表明竹节状非晶态碳纳米管长几个微米，其外径和内径分别是30~40 nm和8~15 nm。该方法在无催化剂条件下制备出非晶态碳纳米管，对科学研究有着重要的意义和潜在的应用前景。

非晶态碳纳米管；热分解；正硅酸乙酯

0 Introduction

Among various types of nanomaterials,carbon nanotubes(CNTs)have attracted immense attention due to their mechanical,electrical,thermal,optical,and structural properties[1-3].Early works focused mainly on“ideal”carbon nanotubes with perfect concentric graphene shells,however,the defects in the carbon network can lead to interesting properties and new potential nanodevices[4-6].Thus,in recent years,much attention have been paid to a variety of special types of amorphous CNTs(ACNTs)such as bamboo-like[7-11], octopus[12],fish-bone[13],and so on.

Several methods such as electronic arc-discharge and chemical vapor deposition(CVD)are commonly used to prepare ACNTs[14-15].For instance,Liu and coworkers have carried out direct current arc discharge in a vacuum chamber to synthesis ACNTs.Zhao et al. have reported a deposition-precipitation method for the synthesis ACNTs.However,for both of the methods catalysts are essential as usual.Pyrolysis of organic precursors by using mesoporous AAO(anodic alumina oxide)template is a commonly used method[16-19]. However,the process method is tedious and thepurification is difficult.Ferrous or cobaltic salts were added as catalysts on the bottom of the meso-tubes of AAO in advance,which brings contamination into the end CNT product and is difficult to purify.A recent development in the field of organometallic chemistry has been the use of organometallic complexes in the high yield catalytic synthesis of ACNTs and other carbon nanomaterials with novel morphologies[20-27].For instance,Müllen and co-workers have carried out solidstate thermolysis of several organometallic precursors in asealed system[26,28].However,the choice of the precursors is critical,and the employed precursor is often expensive.Hence,there is a great demand for new strategies suitable for facile and cost-effective synthesis of ACNTs.

Herein,we reported a facile,catalyst-free,and cost-effective method to prepare bulk bamboo-like ACNTs in a stainless-steel autoclave system at 600℃. In this work,tetraethoxysilane(TEOS)was employed as carbon precursor and no catalyst was used,and ACNTs can be obtained only by the simple thermolysis of tetraethoxysilane(TEOS).Remarkably,the diameter and length of ACNTs can be controlled by the thermolysis procedure.

Fig.1XRD patterns of the samples obtained before(a) and after(b)HF etching

1 Experimental

All reag en ts used in our experiments were of analytical pure grade and used as the starting material without further purification.In a typical procedure,10 mL of tetraethoxysilane was put into a stainless-steel autoclave with 50mL of capacity.The autoclave was sealed and heated from room temperature to 600℃with the heating rate of 10℃·min-1in an electric furnace, and then maintained at this temperature for 12 h.After cooling to room temperature naturally,the black crude powderwascollectedandwashedwithdilute hydrofluoric(HF)acid,distilled water and absolute ethanol for several times.The product was dried in vacuum at 80℃for 10 h.

Structure characterization was performed by X-ray powder diffraction(XRD)pattern with a 2θ ranging from 10°to 80°with the scan speed of(2θ)2°·min-1on an MSAL-XD2 X-ray diffractometer using Cu Kα radiation (40 kV,20 mA,λ=0.154 06 nm).Scanning electron microscopy(SEM)image were recorded on a JSM-6700F field emission scanning electron microscope,in which the powder was directly revealed.Transmission electron microscopy(TEM),high-resolutionTEM (HRTEM),and fast Fourier transform(FFT)pattern were recorded on a JEOL JEM-2010HR microscope equipped with an Oxford Pentafet energy-dispersive X-ray fluorescence spectrometer(EDX)at an operating voltage of 200 kV.The sample was deposited from ethanol suspensions of the product onto amorphous carbon-coated copper grids.

2 Results and discussion

Fig.1 shows the XRD patterns of the as-prepared samples before and after HF etching.As shown in Fig. 1a,the broad and low intensity of the peak at range of 16°~30°indicates the disordered structures of the sample obtained before HF etching.It can be clearly seen from Fig.1b that there is broad peak at 25.6° which corresponds to the(002)plane of graphite.In addition,a small shoulder peak at 43.1°which corresponds to the(100)plane of graphite,can be observed.The broadening of the two peaks suggests the possible presence of an amorphous carbon phase within the CNTs.No impurity is observed in the XRD pattern.

The morphology of the samples obtained by thermolysis of TEOS at 600℃for 12 h was investigated by SEM and TEM observations.The results indicate that the samples are mainly composed of nanotubes.The yield of CNTs,based on statistical analyses of the SEM and TEM micrographs,is over 90%.Fig.2a shows the SEM image of the obtained sample before HF etching.It can be clearly seen that the sample exhibits one-dimensionalnanostructure.Somespherical particles can also be seen.After HF etching,no spherical particles can be observed,suggesting these spheres were resultant silica spheres(Fig.2b).The sample consists of a large number of nanotubes with diameter of about 30~40 nm and length of several micrometers.The inset shows the open-ended tips of the nanotubes(marked with black arrow),which indicates the hollow structure of the CNTs.

Fig.2SEM micrographs of the obtained samples before (a)and after(b)HF etching

Themorphologyandstructurewerefurther characterizedbylow-andhigh-resolutionTEM exzaminations,which reveals that all nanotubes exhibit a bamboo-like(compartmentalized)structure.It can be observed from Fig.3a that the as-synthesized nanotubes have an average length of several hundred nanometers and some other nanotubes up to several micrometers in length.The inset in Fig.3a displays the corresponding FFT pattern of the nanotubes.In Fig.3a,the conical nanoparticles of higher contrast visible within the nanotubes are identified as silica by the presented EDX investigation(Fig.3b).The silica particles mainly located at the tip of the tube and sometimes were also seen in the separated tube units of the bamboo-like structure.Fig.3c shows the TEM image of a crooked nanotube,clearlyrevealingthebamboo-shaped morphology.HRTEM images(Fig.3d)of the inner nanocompartments of the bamboo structure reveal the existence of amorphous material(marked with white arrows),which possibly caused by the formation of noncrystallinespeciesduringgrowth.Theenlarged HRTEM image(inset in Fig.3d)of the selected white square reveals the interior wall comprises well-ordered graphene layers(normally 10~20 layers in the inner wall).It is noteworthy that the exterior wall is composed of amorphous-like materials.Hence,the as-prepared bamboo-like CNTs is indeed a novel type of graphiticamorphouscore-shellstructure.Thehigher magnification image(Fig.3e and f)allows accurate measurement of the diameters of the obtained ACNTs with an outer diameter about 30~40 nm and an inner diameter of about 8~15 nm.

To determine the influence of different reaction conditions on the formation of ACNTs,we carried out the comparative experiments and found thatthe temperature and reaction time were important factors affecting the yields of ACNTs.When the temperature was lower than 500℃,such as 400℃,there were no ACNTs in the product.It is because that the carbon precursors can not be carbonized if the temperature is too low.When the temperature reached 600℃, predominant ACNTs were obtained.It was also found that the yield of ACNTs depends on the initial increase rate of reaction temperature.Quick initial increasing rate of reaction temperature usually gave higher yield of ACNTs.On the other hand,we found that the reaction time also play an important role on the morphology of ACNTs.The sample obtained by thermolysis at 600℃for 6 h showed short worm-like nanotube structures with average diameters of about 20~30 nm and lengths of less than 500 nm(Fig.4a).In contrast,a long period ofreaction time resulted in large-area of nanotubes with larger diameter and length than those of 12 h ones.For example,extending the thermolysis time to 48 h at 600℃dramatically improved the ACNT formation in terms ofdiameterandlength(Fig.4b).However,the uniformity of the product was reduced,and the bamboo-like sub-micrometer-sized tubes with an outer diameter of about 100~200 nm and length of about several micrometerswasobtained.Therefore,themost favorable temperature is about 600℃,and the optimal reaction time is about 12 h.

Fig.3TEM micrographs of the as-synthesized bamboo-like CNTs

Fig.4SEM images after thermolysis under different conditions

The results presented here are very different from those that were obtained from other systems where the thermolysis of organometallic compounds have been reported[28-32].In those cases,the organometallic complexes serve as both the catalyst precursors and as carbon sources and the resultant metal nanoparticles can catalyze the CNT growth.In the present case,the ACNTs are synthesized only by thermolysis of TEOS and no metal catalyst is introduced in the system. Establishingareasonfortheoutcomeofthese experiments is difficult,since results in the CNT syntheses can differ when slightly different analogs of the precursor complex are employed[33].We believe that the generated silica nanoparticles at the initial stage of the thermolysis process may be an explanation,and the excess of graphene sheets along the tube waist is caused by the conical shape of the silica nanoparticle, which is responsible of different rates of temperature precipitation that results in the formation of additional layers of carbon via a diffusion process originating at the particle tip.A possible mechanism was proposed addressing the formation of ACNTs and schematically shown in Scheme 1.Further work is needed for understanding the formation mechanism of the CNTs, and this work is in progress.

Scheme 1Schematic representation of the possible formation process of amorphous carbon nanotubes

3 Conclusion

In conclusion,we have presented a facile and costeffective synthesis of carbon nanotubes by simple thermolysis of tetraethoxysilane in the autoclave at 600℃for 12 h.The obtained carbon nanotubes exhibited a compartmentalizedbamboo-likestructurewithan average diameter of about 30~40 nm and length of several micrometers.Owing to its simplicity,low cost, and high yield of this synthesis technique,this method could be a promising way to the scale of industrial production of ACNTs.Moreover,the bamboo-like ACNTs have many application prospects in energy storage,drug delivery,and electrochemistry.

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A Facile Catalyst-Free Method for the Synthesis of Amorphous Carbon Nanotubes with Bamboo-Like Morphology

ZHAO ShuaiZHENG Ming-TaoXIE Chun-LinHE Wen-QiYI Guan-GuiLIU Ying-Liang＊
(Department of Chemistry,Institute of Nanochemistry,Jinan University,Guangzhou 510632,China)

A facile thermolysis route for the synthesis of amorphous carbon nanotubes(ACNTs)with bamboo-like morphology is presented,in which tetraethoxysilane is employed as carbon precursor.SEM and TEM results show that the obtained ACNTs exhibit a compartmentalized bamboo-like structure with an average outer diameter of about 30~40 nm and inner diameter of about 8~15 nm and several micrometers in length.The free of metal catalysts yields ACNTs,which is important for some fundamental scientific studies and also for promising applications.

amorphous carbon nanotubes;thermolysis;tetraethoxysilane

O613.71

A

1001-4861(2011)03-0397-06

2010-11-22。收修改稿日期：2010-12-23。

国家-广东联合基金(No.U0734005)，中央高校基本科研业务费专项资金(No.21610102)和国家自然科学青年基金(No.20906037)资助项目。

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