Chemical Engineering Journal 228 (2013) 1050–1056

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Chemical Engineering Journal

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Controlling the crystalline quality of carbon nanotubes with processingparameters from chemical vapor deposition synthesis

1385-8947/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.cej.2013.05.088

⇑ Corresponding authors. Tel.: +1 607 220 7844; fax: +1 607 255 9166 (D. Cho),tel.: +82 2 820 0667; fax: +82 2 817 8346 (Y. Jeong).

E-mail addresses: dc575@cornell.edu (D. Cho), yjeong@ssu.ac.kr (Y. Jeong).

Yeonsu Jung a, Junyoung Song a, Wansoo Huh b, Daehwan Cho c,⇑, Youngjin Jeong a,⇑a Department of Organic Materials and Fiber Engineering, Soongsil University, Seoul 156-743, South Koreab Department of Chemical Engineering, Soongsil University, Seoul 156-743, South Koreac School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, NY 14853, USA

h i g h l i g h t s

� CNT fibers could be selectively synthesized by the processing conditions.� Type and crystalline quality of CNTs were characterized by RBM and IG/ID ratio.� Crystalline quality of CNTs could be improved by the processing parameters.

a r t i c l e i n f o

Article history:Received 26 February 2013Received in revised form 9 May 2013Accepted 22 May 2013Available online 2 June 2013

Keywords:Carbon nanotube fiberDirect spinningCrystalline qualityChemical vapor deposition

a b s t r a c t

Carbon nanotubes (CNTs) were synthesized by direct spinning from chemical vapor deposition (CVD)reactions and their assembly for continuous fibers was fabricated through post-processing devices. Thecomplicated processing parameters that include a precursor solution composition, reaction temperature,flow rate of a carrier gas, and injection rate of the precursor solution were precisely controlled to synthe-size CNTs during the reaction process. The effects of the processing factors were analyzed with respect tothe formation of CNTs and the crystallinity in the CNT structure. The CNT fibers were characterized by ascanning electron microscope, high-resolution transmission electron microscopy, and Raman spectros-copy. The type and crystalline quality of the CNT fibers were characterized by a Raman radial breathingmode and its relative intensities of the G and D bands. A high reaction temperature, high H2 flow rate, andlow injection rate of solution precursor were important factors to improve the crystalline quality of theCNT fibers. Low concentrations of iron catalysts in the reaction were favorable for the synthesis of single-walled carbon nanotubes (SWCNTs) that have high electric conductivity, superior specific strength, andhigh crystallinity. The CNT fibers could be tunable to meet a particular application by varying the reactionconditions.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Fiber morphology has a great potential for developing materialsfor two- or three-dimensional structures, such as textile fabrics ornozzles for launch vehicles. Fiber morphology can be also used tobridge objects to allow the transfer of electrical signals or energy.The unique mechanical and physical properties of the nano-scaleof CNT fibers have attracted a lot of attentions of researchers fordecades [1–8]. However, the nano-scale of CNTs has severely re-stricted the handling and/or precise control of their use in particu-lar application. For example, an array of CNTs vertically aligned tothe gold surface of an electrode would require a strong covalent

bond to construct molecular wires acting as biosensors [1]. How-ever, the complicated self-assembly process could be avoided bya judicious selection of macro-scale processing such as the prepa-ration of a velvet-like carpet of CNT fibers followed by textile pro-cess of brushing to create fuzz on the CNT fabric surface. Likewise,macro-scale fibers formed from nano-scale elements provide animportant material for use in a wide range of applications, includ-ing multifunctional fabrics [2], power-transmission cables [3],electric devices [4], heating wires, films, fabrics [5,6], artificialmuscles [7], and dye-sensitized solar cells [8]. Thus, the macro-sizeCNT fibers would have great potential if their properties and theirprocessing flexibility could meet the requirements needed forthese applications. The unique properties of CNTs, such as theirelectrical and thermal conductivity and high strength are relatedto chirality, number of walls, and crystalline perfection. It wouldbe valuable to know the factors that influence structure ofmacro-size CNT fibers.

Fig. 1. (a) Schematic illustration of the synthesis of CNTs and their assembly intocontinuous fibers by chemical vapor deposition, (b) SEM image of CNT assembly,and (c) photograph of winding process using a roller (the width of roller is 30 mm).

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Methods for manufacturing macro-size CNT fibers have beendescribed previously [9–15], including the coagulation method[9,10], liquid crystal spinning [11], spinning from a nanotube mat[12–14], and direct growth of CNT fibers [15]. Of these methods, di-rect growth appears to be the most viable commercially, becauseCNT fibers can be produced continuously without a substrate.The direct growth method was introduced by Li et al. who synthe-sized CNTs and formed their assembly using a vertical furnace anda post-processing technique [15]. The assemblies could be formedinto films or fibers, depending on the type of winder needed. Sincethe method was firstly reported, researchers have focused onunderstanding how the assembly mechanism of CNTs fiber im-proves their mechanical properties [15–21]. Li et al. showed thatthe different types of CNTs were synthesized under different pro-cessing conditions; however, it was not clear how the processingconditions affected the internal structure of CNTs. Later, Mottaet al. carried out a parametric study of the direct spinning processfocused on how the processing conditions influenced the diame-ters of CNTs [16,18]. In their study, CNT diameter was presentedas a representative index of the CNTs’ quality. The proportions ofSWCNTs increased as the iron catalyst concentration decreased,and this effect was attributed to the small size of the iron catalyst[18]. A theoretical modeling study was conducted to understandhow the concentration of ferrocene and the hydrogen flow rate af-fected the iron catalyst size in the reactor, which, in turn, affectedthe diameter of the CNTs synthesized [17]. Their focus on catalystdiameter was motivated by the link to the unique properties ofCNTs, such as band gap and number of CNT walls [22].

A parametric study on the qualities of the CNTs producedthrough direct spinning methods has not been carried out previ-ously, to the best of our knowledge. Realizing the unique advanta-ges offered by CNT fibers, a parametric study of this nature shouldprovide insights that could greatly enhance the quality of CNT fi-bers and facilitate control over their properties. In this study, thecrystalline qualities of CNT fibers have been mainly analyzed, be-cause this property influences the physical and mechanical proper-ties of CNT fibers, such as thermal stability, conductivity, andspecific strength [23]. For these experiments, various CNT fiberswere produced from a set of precursor solutions by varying solu-tion composition, temperature, H2 flow rate, and solution injectionrate. These studies analyzed the processing conditions that yieldthe highest quality of CNTs and control the type of CNTs whichconstitute the fibers.

Table 1Processing parameters and their ranges for the synthesis of CNTs.

Parameter Range

Ethanol (wt%) 95.5–99.8Ferrocene (wt%) 0.1–2.5Thiophene (wt%) 0.05–5.0Temperature (�C) 1130–1250H2 flow rate (sccm) 800–1600Solution injection rate (ml/h) 5–25

2. Experimental

Ethanol was obtained from Samchun Chemical Co., Ltd. (Korea).Ferrocene and thiophene were purchased from Sigma Aldrich,which were used as received. The synthesis was carried out byinjecting a mixture solution with a hydrogen gas flow into a heatedreactor where the flow rates of the mixture solution and the hydro-gen gas could be separately controlled [15,19]. CNT sock descendedinto a water tank at one end of the reactor and was wound at a rateof 5 m/min (in Fig. 1). After the CNT assembly passed through thewater tank, they formed a yarn from the sock. Standard synthesisconditions for CNTs were taken to be 98.0 wt% ethanol, 1.0 wt% fer-rocene, and 1.0 wt% thiophene injected at a 10 ml/h rate into thecarrier hydrogen gas (1000 sccm) in a furnace at 1170 �C. The com-position of the precursor solutions and the other parameters werevaried until reaching a range in which the CNT fibers could be spuncontinuously (Fig. 1).

The Raman spectra of the CNT fibers were obtained using aHORIBA LabRAM HR polarized Raman instrument with a radiationwavelength of 514 nm and a 50� objective lens. The exposure timeof the laser was set to 20 s. A JEM-3010 HRTEM (JEOL) was used to

investigate the structures of the CNTs synthesized under each con-dition. Energy dispersive spectroscopy (EDS, EX-250 model ofHORIBA) measurements were collected to analyze the residual ironcontent in the CNT fibers. The mechanical properties of the CNT fi-bers were tested using a universal tensile machine (Instron, model4464). The sample gauge length and rate of extension were 10 mmand 3 mm/min, respectively. The electrical conductivity of the fi-bers was measured using a mX HiTESTER model from HIOKI usingthe 4-point probes method.

3. Results and discussion

The recipes for precursor solution, temperature, flow rate ofhydrogen gas, and injection rate of precursor solution are impor-tant factors that influence the physical properties and nanostruc-tures of the as-synthesized CNTs using CVD techniques [24]. Inthis study, CNTs were synthesized and their assembly for continu-ous fibers was fabricated to analyze the effects of these fourparameters on the formation of CNTs, their crystalline qualities,and mechanical properties by controlling the ranges in the param-eters (in Table 1).

3.1. Thiophene effect

Sulfur affects the formation of CNTs during CVD processes.Although the mechanisms of sulfur’s influence in this process havebeen explained, these mechanisms are not completely understood[25–32]. These mechanisms include deactivation of catalysts byblocking active sites, a decrease in the melting point of catalysts,and interaction with growing CNTs. Sulfur is generally regardedas a promoter by causing the carbon atoms produced by the

(a) (b)

Fig. 2. (a) Raman spectra of the CNT fibers synthesized with different amounts of thiophene. (b) RBM peaks identified in (a), which indicate the presence of SWCNTs.

Fig. 3. HRTEM images of CNTs synthesized with different thiophene concentrations; (a) 0.1 wt%, (b) 0.3 wt%, and (c) 1.5 wt%, respectively.

(a) (b) (c)

Fig. 4. (a) Raman spectra of CNT fibers synthesized with (a) 0.1 wt% and (b) 1.0 wt% thiophene at various ferrocene concentrations and (c) enlarged RBM peaks of the CNTfibers marked with dashed squares on (a) and (b).

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decomposition of carbon sources to rapidly diffuse into an iron cat-alyst due to the iron sulfide formed on the catalyst. However, sulfurmay act as a poison to catalytic particles above a certain sulfurconcentration.

The effects of sulfur concentrations on the synthesis of CNTswere evaluated using thiophene as a sulfur source. Thiophenewas added to the carbon source solution to make 0.05–2.0 wt%concentration of thiophene, while keeping all other conditions con-stant. As shown in Fig. 2a and b, Raman spectroscopy was per-formed on the CNT fibers prepared with different amounts of

thiophene. Raman radial breathing mode (RBM) frequency (x)was closely related to the nanotube diameter (d) according to therelation of d (nm) = 248/x (cm�1), because the RBM peaks in thelower wavenumber region (100–500 cm�1) provided direct evi-dence of the presence of SWCNTs [34–36].

The G-band (near 1580 cm�1) indicated features of graphite, butthe D-band (near 1350 cm�1) suggested that there were disorderedfeatures of graphitic sheets [37,38]. As shown in Fig. 2a and b, theRBM peaks became weaker and the D-band peaks gradually in-creased in intensity with an increase in the thiophene concentration.

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The RBM peaks disappeared in the CNT fibers when the fibers syn-thesized at a higher thiophene concentration than 0.3 wt%. The CNTsthat exhibited the RBM peaks were 1–2 nm in diameter as shown inthe HRTEM images of Fig. 3. Although RBM peaks can originate fromthe thin innermost tubes in multi-walled carbon nanotubes(MWCNTs) prepared by direct current (DC) arc discharge [39], it isinteresting to note that the RBM peaks in CNT fibers synthesizedby direct spinning appeared to be related to single- or double-walledstructures. The presence of sulfur as an additive may increase thediameters of CNT fibers by promoting carbon diffusion on the sur-face of the sulfur-rich layer [33,37,38,40–44].

Similar results (Fig. 3) were obtained when SWCNTs andMWCNTs were synthesized at low and high thiophene concentra-tions, respectively. High thiophene concentrations in Fig. S1 mightresult in a low iron content in CNT fibers. As a result, the elementalratio of iron/sulfur was greatly changed with thiophene concentra-tion. Otherwise, the change of iron/carbon ratio was not significantas a function of thiophene concentration (Fig. S1), which showedthat the carbon delivery was not influenced by the thiopheneconcentration.

3.2. Ferrocene effect

Ferrocene is a good precursor for the iron catalyst particles thatseed nanotube growth [45]. CNT fibers were synthesized at variousconcentrations of ferrocene (0.1–2.5 wt%) using two thiophene

Fig. 5. Raman spectra of CNT fibers synthesized with precursor solution a under various Crate.

concentrations (0.1 and 1.0 wt%). All other conditions were heldconstant. The Raman results in Fig. 4a and b suggested that thesynthesis of CNTs was influenced by the ferrocene and the thio-phene concentrations. At the 0.1 wt% thiophene concentration,the RBM peaks began to appear at a ferrocene concentration of0.2 wt% (Fig. 4c).

At higher concentrations of thiophene, more iron sulfides wereformed on the catalyst [3], which promoted the diffusion of carboninto the iron catalyst and led to the production of multi-walledstructures. Although the concentration of thiophene was 0.1 wt%,the low concentration of ferrocene (0.1 wt%) yielded multi-walledCNT, while D-band appeared very small but there was no RBMpeak. A high ratio of thiophene to iron particles increased the dif-fusion layer on the catalyst surface. High concentrations of ironcatalysts did not produce SWCNT fibers effectively, because theyagglomerated and then formed a large-sized iron catalyst at highconcentrations of ferrocene. A combination of large-sized iron cat-alysts and a high concentration of thiophene favored the produc-tion of MWCNT fibers, because the large-sized iron catalystsreduced overall surface area in the reaction mixture and increasedthe thickness of the iron sulfide on catalysts.

3.3. Temperature effect

CNTs were synthesized by using two precursor solutions (solu-tion a: ferrocene 1.0 wt% and thiophene 0.1 wt%, solution b: ferro-

VD synthesis conditions: (a) temperature, (b) H2 flow rate, and (c) solution injection

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cene 1.0 wt% and thiophene 1.0 wt%) at 1130–1250 �C. The compo-sition of the solution was chosen to synthesize SWCNTs andMWCNTs; that is, SWCNTs were synthesized from precursor aand MWCNTs from precursor b. Raman spectra (Fig. 5) illustratethe changes of IG/ID ratios in the SWCNT fibers prepared under var-ious conditions. A higher value of IG/ID ratio indicated that the CNTfibers featured fewer defects and a higher crystallinity [46,47]. TheIG/ID ratios of the SWCNT fibers increased steadily from 5.4 to 27.0as the temperature increased from 1130 to 1250 �C (Fig. 5a), con-sistent with previous studies of nanotubes synthesized at hightemperature. The high temperature led to high crystallinity,straightness of the nanotubes, and rapid growth of carbon nano-tubes, which achieved a clean amorphous carbon-free deposition[48–50]. Otherwise, the IG/ID ratios (2.1 ± 0.3) of the MWCNT fiberssynthesized with solution b showed little change (Fig. S3a).

As a result, the CNT type was not changed by temperature,which was contrary to the results that were previously reported[51]. Conroy et al. [17] reported that the synthesis temperatureinfluenced the reaction rate of ferrocene and the collision rate ofiron particles. At high temperature, the dissociation rate of ferro-cene increased and it caused the iron particle to become largerprior to the reaction zone of the carbon sources to form CNTs.The large iron particles might not produce the SWCNTs, but theyfacilitated the synthesis of thicker CNTs. This study suggests that

Fig. 6. Overall trends of IG/ID ratios in CNT fibers according to the processing paramerespectively.

Fig. 7. Raman spectra of (a) SWCNT fibers (IG/ID = 62.5) and (b) MW

SWCNTs could be synthesized from the solution a at high temper-ature. Therefore, the effect of temperature on the CNT type wasnegligible.

3.4. Effect of hydrogen flow rate

CNT fibers were synthesized at various hydrogen gas flow ratesfrom 800 to 1600 sccm. Fig. 5b shows the IG/ID ratios of theSWCNTs synthesized with the precursor solution a. These valueswere enhanced according to the flow rate of hydrogen gas as a car-rier. It has been known that the CNT synthesis might be influencedby the total flow rate and the degree of catalyst poisoning with H2

[52–56], but in this study, the solution injection rate was four or-ders lower than the H2 flow rate (see Fig. S2 in the Supplementaryinformation). By comparing Fig. S2a with b, it is apparent that thechange in the IG/ID ratio was mainly due to the change in the ele-mental ratio of hydrogen to carbon, which influenced the rate ofcarbon production on dehydrogenation.

The chemistry of CNT growth by CVD is closely related to theamount of H2, which leads to three regimes of pyrolysis, growth,and inactiveness [56]. In the pyrolysis regime, the hydrocarbondecomposition is vigorous due to the negligible inhibition by H2.The hydrocarbon decomposition leads to a low CNT yield and apoisoned catalyst from the amorphous carbons generated during

ters. SWCNT and MWCNT fibers were synthesized from the precursors a and b,

CNT fibers (IG/ID = 6.1) synthesized at each optimized condition.

Table 2Properties of the SWCNT and MWCNT fibers synthesized under the variousconditions.

Properties SWCNT fiber MWCNT fiber

Linear density (tex)a 0.06 0.16Average diameter (lm) 10.6 20.7Specific strength (GPa/SGb, N/tex) 0.53 0.31Specific stiffness (GPa/SG, N/tex) 19.2 7.9Breaking strain (%) 6.0 12.5Electrical conductivity (S/m) 3.2 � 105 1.1 � 105

a Linear density presents a measure of mass per unit of length and ‘‘tex’’ isdefined as the mass in grams per 1000 m.

b SG is specific gravity.

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the pyrolysis regime. As shown in Fig. 5b, the low concentration ofH2 to carbon resulted in low IG/ID ratios. In the growth regime, theconcentration of H2 retained a good balance between the produc-tion of undesired amorphous carbons and the hydrocarbon decom-position, which resulted in catalytic CNT growth. In the inactiveregime, the concentration of H2 reduced the hydrocarbon decom-position that resulted in a lack of carbon feedstock to the catalyst.Anyone of the three regimes could be determined by the hydrogenflow rate during the CVD process, which was largely influenced bythe processing temperature and the carbon source [30].

The IG/ID ratio rapidly increased as the hydrogen gas flow rate wasincreased from 800 to 1200 sccm. Considering the high value of theIG/ID ratio, the range of H2 flow rates corresponded to the SWCNTgrowth regime. Hydrogen flow rates exceeding 1200 sccm graduallyincreased the IG/ID ratios of SWCNTs. During the synthesis process inthe inactive regime, the SWCNTs did not grow well because thehydrocarbon decomposition rate was slower than the carbon diffu-sion rate on the catalyst. On the other hand, it was observed that theIG/ID ratios of MWCNTs were not changed with the H2 flow rate(Fig. S3b). The thick layer of iron sulfide facilitated the carbon diffu-sion, which was less sensitive to the H2 flow rate.

3.5. Effect of solution injection rate

Fig. 5c shows the changes of the IG/ID ratios in the CNT fiberssynthesized with different solution injection rates varying from 5to 15 ml/h. The IG/ID ratios of the SWCNTs decreased drasticallywith the injection rate of the precursor solution a. A higher injec-tion rate had the same effect as a decrease in the hydrogen flowrate, because the ratio of hydrogen to hydrocarbon was reducedin both cases. This synthesis process might be considered as thepyrolysis regime, in which the catalyst poisoning and the lowSWCNT yields predominate. Therefore, the IG/ID ratio for theSWCNT fibers decreased from 34.5 to 6.3 at high injection rates.On the other hand, the IG/ID ratio for the MWCNT fibers changedslightly from 2.3 to 1.6, as observed by the effect of the hydrogenflow rate (Fig. S3c). Fig. 6 illustrates the overall trends of crystallineperfection of SWCNT and MWCNT fibers that were synthesizedfrom two precursor solutions (a and b) according to the processingparameters, respectively. As a result, the MWCNT fibers had amuch lower sensitivity over the three parameters than the SWCNTfibers. Parameter space presents the values of the parameters inTable 1, which were normalized using their minimum and maxi-mum values of 0 and 1, respectively.

3.6. Properties of CNT fibers

The differences in the number of walls and crystalline perfec-tion of the CNT fibers should yield different mechanical and phys-ical properties. In this study, two types of CNT fibers were preparedto compare their mechanical and physical properties. The SWCNTfibers were synthesized by using a precursor solution of ferrocene

1.5 wt% and thiophene 0.1 wt% under a hydrogen flow rate of1600 sccm, an injection rate of 5 ml/h, and a furnace temperatureof 1250 �C. MWCNT fibers were synthesized using a precursorsolution of ferrocene 0.5 wt% and thiophene 1.0 wt% under ahydrogen flow rate of 1600 sccm, an injection rate of 10 ml/h,and a furnace temperature of 1250 �C. These synthesis conditionswere selected to obtain a high IG/ID ratio and good spinnability ofthe CNT fibers.The SWCNT fibers showed a very high IG/ID ratioof 62.5 with a very small D-band peak, indicating a high crystallinequality (Fig. 7). Otherwise, the IG/ID ratio for the MWCNT fibers wasabout 6.1, which is regarded as a high crystalline quality [57]. Also,Breit–Wigner–Fanoline shaped lines (Fig. S4) in the Raman spec-trum of SWNCT fibers suggest that the fibers consist primarily ofmetallic SWCNTs. It is noted that a certain type of CNT fiber mightbe synthesized and its properties could also be adjusted by control-ling the processing parameters. As shown in Fig. 7 and Table 2, thetwo types of CNT fibers have significantly different properties. Thehigher crystallinity of SWCNT fiber shows higher mechanical andelectrical conductivity due to stronger Van Der Waals bondingcompare to MWCNT fiber. In addition, changes in resistance wereas a function of temperature (Fig. S5).

4. Conclusion

The controlled synthesis of CNT fibers by CVD with ethanol, fer-rocene, and thiophene under a flowing H2 was studied. Tempera-ture, reactant gas flow rate, precursor solution flow rate, amountof catalyst, and the promoter were comprehensively investigatedto analyze their effects on CNT formation. The types of CNTs andtheir crystalline properties were characterized by HRTEM and Ra-man spectroscopy. The CNT type was very sensitive to the amountsof thiophene and ferrocene. The synthesis of SWCNTs was efficientat low concentrations of the thiophene and ferrocene, which pro-vided a favorable environment for the formation of small iron cat-alysts. SWCNTs could not be obtained by controlling temperaturealone. In addition, the hydrogen flow rate and precursor solutioninjection rate might not change the types of CNTs. Otherwise, theprocessing parameters influenced the crystalline quality of CNT fi-bers, which could be verified by the magnitude of the IG/ID ratio inthe Raman spectra. This parametric study found several importantinsights on the processing conditions by which high-qualitySWCNT and MWCNTs could be selectively synthesized. Themechanical and physical properties of the produced CNT fiberswere significantly different according to the processing conditions.This thorough parametric study demonstrates that the synthesis ofCNT fibers can be precisely controlled to customize their propertiesfor use in various applications.

Acknowledgements

This work was supported by the Human Resources Develop-ment Program (No. 20124030200070) of the Korea Institute of En-ergy Technology Evaluation and Planning (KETEP) Grant funded bythe Korea government Ministry of Trade, Industry and Energy.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.cej.2013.05.088.

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