Synthesis and characterisation of carbon nanotubes grown ... M... · Synthesis and characterisation of carbon nanotubes grown on silica ... Imperial College London, London SW7 2AZ,

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Synthesis and characterisation of carbon

nanotubes grown on silica fibres by injection

CVD

Hui Qian1,2

Alexander Bismarck2

Emile S. Greenhalgh3

Milo S.P. Shaffer1,*

1 Department of Chemistry, Imperial College London, London SW7 2AZ, UK

2 Polymer and Composite Engineering (PaCE) Group, Department of Chemical

Engineering, Imperial College London, London SW7 2AZ, UK

3 The Composites Centre, Imperial College London, London SW7 2AZ, UK

* Corresponding author. Tel.: +44 (0)20 7594 5825; Fax.: +44 (0)20 7594 5801; E-mail

address: [email protected] (M. S. P. Shaffer).

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Abstract

Carbon nanotube grafted primary reinforcing fibres are under development for a new

generation of hierarchical composites. Pure and nitrogen-doped multi-walled carbon

nanotubes (MWCNTs) were grown on silica fibres using the injection chemical vapour

deposition method. The morphology and size of the nanotubes were controlled by

varying the growth time. The surface structure of the silica fibres after the grafting

process was studied by electron microscopy following focused ion beam sectioning; the

images confirmed a base growth mechanism and a shallow iron-rich layer.

Thermogravimetric analysis indicated a shorter induction period and a faster growth rate

for pure rather than nitrogen-doped MWCNTs. Raman characterisation of the grafted

MWCNTs showed a decreasing intensity ratio of the D to G modes, moving from the

tip to base in both cases; a detailed comparison of different characteristic Raman ratios

is provided, using both the peak intensity and the area for the D, G, and G′ signals.

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1. Introduction

Carbon nanotubes (CNTs) continue to receive huge interest due to their extraordinary

mechanical, thermal and electronic properties [1], [2] and potential applications in a

wide range of fields, such as composites [3] and electronic devices [4]. Amongst a

variety of methods to produce CNTs; catalytic chemical vapour deposition (CVD) is

widely accepted as the most efficient for large scale synthesis of CNTs [5, 6], tending to

produce relatively pure, but defective and entangled materials. Alternatively, aligned

CNT arrays can be grown by CVD, often using an injection method (ICVD) which

involves the simultaneous pyrolysis of solutions containing both the catalyst precursor

and the hydrocarbon source [7-10], without the time-consuming preparation of catalyst

pre-coated substrates. This method has been used to grow well-aligned nitrogen-doped

carbon nanotubes (CNx-CNTs) by injecting nitrogen-containing organic precursors [11-

16], such as pyridine, diazine, acetonitrile and so on. The addition of diazine or triazine

precursors has been shown to improve the internal crystalline order, straightness and

packing of ICVD-grown nanotubes [11, 17]. CNx-CNTs are either metals or narrow

energy gap semiconductors, thus possibly offering better electrical conductivity than

pure CNTs [16, 18]. In addition, the nitrogen sites in the graphene network can also

improve the nanotube-matrix interactions in composites [19].

Over the past decade, many researchers have explored the use of CNTs in composites,

as a replacement for conventional microscale reinforcing fillers, such as carbon or glass

fibres [20]. However, the direct use of CNTs remains limited by the quality of the

materials available in bulk and the difficulties associated with dispersion, alignment,

and the CNT/matrix interface. Meanwhile, interest is growing in the opportunity to

combine CNTs with conventional fibres to create hierarchical composites. Of the

available processing strategies, one of the most appealing is to grow CNTs directly onto

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fibre surfaces [21-24]; this approach has the potential to provide high loadings of well-

spaced CNTs, with a potentially optimal, radial orientation around the fibres. It is

reported that the grafting of CNTs on fibres improves the interfacial properties between

fibres and matrix [21, 23]. In addition, the presence of CNTs in matrix-rich regions

could alleviate some of the drawbacks of conventional fibre reinforced polymer

composites, especially longitudinal compression performance, interlaminar properties,

and potentially damage tolerance [22, 24].

Ci et al. [25] first reported the synthesis of CNTs on the surface of ceramic fibres by

using the ICVD method and deduced that the base growth mechanism applies. Since

then, CNTs have been grown on various substrates, including carbon [26], SiC [22, 27],

aluminia [24], quartz and aluminium silicate [28] fibres or woven fabrics. The present

study systematically investigated the growth of pure and N-doped MWCNTs on silica

fibres using the ICVD method. The morphology, length and diameter of nanotubes

grown at different growth times were characterised using scanning electron microscopy

(SEM). The consistency of the ICVD-grown MWCNTs along the nanotube array and

fibres was also investigated. The interface structure between nanotubes and fibres is of

particular relevance to composite performance; the local structure of the interface was

explored for the first time by using the focused ion beam (FIB) technique to prepare

cross-sections, from which the growth mechanism of nanotubes on fibres can be

deduced. Moreover, the effects of the nitrogen-incorporation on the growth rate,

oxidation resistance and crystallinity of MWCNTs grown on fibres were studied using

thermogravimetric analysis (TGA) and Raman characterisation.

2. Experimental

2.1. Materials

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Commercially available Silfa silica fibres (~9 μm in diameter, Albert Hellhake GmbH)

were used for this work. A thermal desizing treatment of the fibres was carried out prior

to CNT grafting at 600 ºC for 1 h. All chemicals were used as received.

2.2. Growth of carbon nanotubes on fibres

Pure and N-doped MWCNTs were synthesised on silica fibres using the injection

chemical vapour deposition method [8, 11, 13]. The reactions were carried out in a

tubular quartz reactor (50 mm in diameter) equipped with an electrical furnace (PTF

12/50/610, Lenton). The silica fibres were held by an alumina frame and placed in the

middle of the furnace to expose them to the hydrocarbon source. For pure MWCNTs, a

feed solution of 3 wt% ferrocene (98%, Sigma-Aldrich) in toluene (BDH AnalaR grade,

VWR International Ltd.) was injected continuously into the reaction tube at a rate of 5

ml/h using a syringe pump (KDS100, Linton). For CNx-MWCNTs, a feed solution

consisting mixture of 3 wt% ferrocene, 58.5 wt% toluene and 38.5 wt% pyrazine

(Kosher grade, Sigma-Aldrich) was used. The liquid feed was preheated to 200 ºC,

immediately volatilised, and swept into the furnace by a flow of argon (Ar) as carrier

gas. The reaction temperature was 760 ºC. The growth times for pure MWCNTs were

15, 30 and 120 min. Longer growth times, of 60, 120 and 240 min, were used for the

synthesis of CNx-MWCNTs, due to their slower growth rate.

2.3. Electron microscopy characterisation

The morphology, alignment and size of the grafted nanotubes on the silica fibres were

characterised using a field emission gun scanning electron microscope (Gemini LEO

1525 FEG-SEM, Carl Zeiss NTS GmbH). The surface of modified fibres was studied

using a transmission electron microscope (TEM, JEM-2000FXII Electron Microscope,

JEOL, Inc.) operating at 200 kV. Energy dispersive X-ray (EDX) analysis was

performed using the INCA X-ray microanalysis system (4.08 suite version, Oxford

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Instruments). FIB sectioning [29] was carried out using FIB 200 TEM focused ion beam

instrument (FEI, Inc.). After deliberate removal of the CNT layer, a fibre was glued

onto a copper TEM grid, which was partly cut for better handling. The specimen was

pre-coated with gold using a sputter coater (K-550X, Emitech Ltd.). During the FIB

sectioning process, a gallium liquid metal ion source (Ga+) was used as a primary ion

beam with an energy of 30 keV. The area of interest on the fibre surface was protected

by coating with a thin layer of platinum prior to etching. The ultimate thickness of the

defined area on the specimen was approximately 100 nm (Supporting information,

Figure S1).

2.4. Thermogravimetric analysis

TGA measurements were carried out using a PerkinElmer Pyris 1 thermogravimetric

analyser (PerkinElmer, Inc.). Samples were analysed in a platinum pan at a heating rate

of 2 ºC /min up to 900 ºC, in an atmosphere of air flowing at 20 mL/min. Sample

masses ranged from 1-5 mg.

2.5. Raman spectroscopy

The Raman spectra presented in this report were collected in the backscattered geometry

using a LabRam infinity analytical Raman spectrometer (HORIBA Jobin Yvon Ltd.), at

room temperature, using a linearly polarised Helium-Neon laser (λ = 632.8 nm). The

diameter of the laser spot was a few micrometres and the resolution of the spectrometer

was about 1 cm−1

. All the spectra were fitted with a mixed Gaussian-Lorentzian

function using GRAMS software.

3. Results and discussion

3.1. Pure MWCNT-grafted silica fibres

3.1.1 Growth of MWCNTs on silica fibres

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SEM images (Figure 1) showed that well-aligned MWCNTs were successfully grown

onto the silica fibres at all growth times between 15 and 120 min; however, different

morphologies were obtained. Radial growth in the early stages resulted in a ‘brush-like

structure’ (Figure 1b), which later ‘parted’ asymmetrically. A random orientation

observed during nucleation [9], rapidly gives way to radial growth, due to steric

interactions between MWCNTs, once the density of catalytic sites is sufficiently high

[8], producing the symmetric ‘brush-like’ structure. As the MWCNTs continue to

elongate via the base growth mechanism (see detailed discussion below), the

circumference described by the tips expands. Eventually, the circumferential tension

exceeds the inherent lateral strength of the growing nanotube layer, which has been

mostly attributed to the inter-tube entanglements [30]. The layer then splits, or ‘parts’,

breaking the cylindrical symmetry, and causing growth to continue in one direction

(Figure 1c, d). Similar behaviour has been reported for MWCNTs grown on other fibres

[27]. In addition, gravity, steric hindrance within the fibre tow, and gas flow effects may

also contribute to this splitting phenomenon [31]. In Figure 1d, regular bands of high

contrast are visible, most likely due to variable iron content, as the iron-containing

precursor was drip-fed into the vapourisation furnace [32].

A statistical analysis of the gross length and the outer diameter of the MWCNTs was

carried out using high resolution SEM images taken from different areas (tip, middle

and base of the nanotube array). More than 600 tube diameters per sample were

measured using the open-source Java software ImageJ [33]. The average lengths and the

outer diameters at different growth times are summarised in Table 1 and the outer

diameter distributions are shown in Figure 2a. As expected, the MWCNT lengths

increased with growth time from a few tens of micrometers to around half a millimetre.

It was also found that both the average diameter of the MWCNTs, and the breadth of

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the diameter distributions increased with growth time. The measurements along the

length of the MWCNTs clearly show the cause; namely that the diameter gradually

thickened at the base of the MWCNTs. This trend could be due to either continued

pyrolytic carbon deposition during tip growth or catalyst ripening during base growth.

However, the literature suggests that injection CVD usually proceeds by base growth [8,

9]; this conclusion is supported by the presence of iron particles at the MWCNT roots

observed in this study (see detailed discussion below). Thus, the increasing MWCNT

outer diameters at the base can be mainly attributed to catalyst ripening.

In order to explore the nature of the MWCNT-fibre interaction further, the surfaces of

the silica fibres after the reaction were characterised after deliberately removing the

grafted MWCNTs using a razor blade. As shown in Figure 3a, b, many particles and

holes with diameters around 115 ± 13 nm were observed, which were a similar size to

the grafted MWCNTs (refer to Dbase in Table 1), again suggesting a base growth

mechanism (and catalyst ripening). The pitting also showed that the MWCNTs were

attached to the fibre surface via etching or chemical reactions between the catalyst and

the substrate. In some cases, the catalyst particles remained visible, embedded in the

surface, in other cases, they had been pulled out with the MWCNTs (Figure 3c); either

way, the outline of the carbon walls was visible as a ring. Note that the MWCNTs

shown in Figure 3c are at the larger end of the size distributions shown in Figure 2a, as

they are located at the base of the longest growth experiment, and have been subjected

to the greatest ripening; the majority of MWCNTs in the sample are significantly

smaller. At high temperature, iron may diffuse into silica, forming iron oxide and an

iron-silicon alloy [34, 35]. Qu et al. [36] has reported similar surface damage caused by

the high-temperature pyrolysis of iron phthalocyanine (FePc) on silica-coated carbon

fibres. Cross-sections of the damaged surface were prepared by FIB sectioning

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(Supplementary Information, Figure S1) and studied using TEM (Figure 3d, e). The

holes caused by the catalyst etching were clearly observed, with depth ranged between

20 and 55 nm (Figure 3d). However, few catalyst particles remained attached to the

surface after the FIB sectioning, which may be due to the relatively small thickness of

the specimen, i.e., around 100 nm. Close-up images of the holes (Figure 3e) displayed a

number of dark features (around 15 nm), which are likely to be iron residue, as

suggested by the EDX analysis (Figure 3f). Other signals shown in the EDX,

specifically C, Au, Si and Cu, came from the carbon residue, metal coating, fibre and

TEM grid, respectively. The oxidation state of the iron is unclear, but the catalyst

remaining after growth could either be iron carbide (Fe3C), α-iron, or γ-iron [8, 37].

3.1.2 TGA characterisation

TGA was applied to assess the oxidation temperature and weight fraction of the

MWCNT-grafted silica fibres (Figure 4a). The onset oxidation temperatures for weight

loss, corresponding to the decomposition of MWCNTs, were 582 ºC, 620 ºC and 654 ºC

for the MWCNTs grown during 15, 30 and 120 min respectively, suggesting that the

degree of crystalline perfection of the MWCNTs was improved as the reaction

continued. Assuming that the mass of iron was negligible (< 3 wt.%), the mass ratios of

grafted MWCNTs to silica fibres, fCNTm

mmR / , were calculated from the mass of

residue, i.e. the mass of silica fibres, determined by TGA. Thus, the mass yield of the

MWCNTs per unit area, CNT

, can be evaluated from the following equations:

ff

CNT

f

CNT

CNTlrn

m

S

m

2 (1)

ff

f

f

flrn

mV

2

(2)

By substituting equation 2 into 1, the yield can be obtained using the equation:

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ff

m

ff

f

CNT

CNT

rR

r

m

m (3)

where CNT

m and f

m are the masses of the grafted MWCNTs and silica fibres,

respectively, andm

R is their ratio,.f

S , n , f

r , f

l , f

, and f

V are the surface area,

number, radius, length, density and volume of the silica fibres, respectively. f

and f

r

are specified as 2.15 g/cm3 and 9 μm respectively by the manufacturer.

The average yields of MWCNTs grown in the 15, 30 and 120 min reactions were 0.09,

0.74 and 4.93 mg/cm2, respectively. The MWCNT yield is plotted as a function of the

growth time in Figure 4b. The growth rate of MWCNTs on silica fibres under the

experimental conditions used in this work, was linear at around 0.044 mg/(cm2min).

The linear fit crossed the x axis, indicating that the induction period for the growth of

MWCNTs was around 13 min. This result is consistent with the previous data shown in

the literature [8], suggesting that the initial deposition of ferrocene took about 13.75 min,

during which time the nucleation sites for MWCNT growth developed. Singh et al. [8]

reported that the growth rate of MWCNTs on quartz materials is initially linear, but

eventually reduces, probably due to the diffusion limitation of the hydrocarbon gas

through the increasing thickness of the growing films. For the cylindrical geometry of

grafting MWCNTs on fibres, diffusion limitations are less likely to be significant.

3.1.3 Raman characterisation

Raman spectra of the silica fibres before and after MWCNT grafting are shown in

Figure 5a and b. Before the growth, only the silica signal at around 440 cm-1

was

detected [38]. The spectra taken from the silica fibres after 15 and 30 min reactions

showed three main bands, which are the disorder-induced D mode (~ 1321 cm-1

), the

tangential G mode (~ 1570 cm-1

) and second order G' mode (~ 2642 cm

-1), consistent

with the synthesis of MWCNTs during the reactions; no signal was observed for the

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radial breathing mode (RBM), range between 120 and 400 cm-1

[39]. It is widely

accepted that the D mode acts as a diagnostic for disruptions in the hexagonal

framework of MWCNTs and the intensity ratio of the D mode to G mode can be used

qualitatively to compare the crystallinity of CNTs [40]. Figure 5b shows that the

intensity of the D mode was approximately half that of the G mode. The value is typical

for ICVD-grown MWCNTs [8, 25], but higher than that for most commercial CVD

MWCNTs, suggesting a relatively high degree of crystallinity.

A more detailed Raman study was carried out on the MWCNTs grafted in the 120 min

reaction (Figure 1d & 5c). The Raman spectra (Figure 5d), taken at four different

positions through the thickness of the layer, confirmed that only MWCNTs were

synthesised during the reaction. The laser power and integration times used to record

these spectra were identical. At least three spectra were taken from each position within

the MWCNT array (Figure 5c). The D:G intensity ratio, ID/IG, was about 0.62 at the

MWCNT tips and fell to approximately 0.23 at the base (see Figure 5e), close to the

silica fibre, indicating a very high degree of crystallinity, particularly for CVD-grown

materials. Assuming a base growth mechanism, the data once again show that the

crystallinity increased during the growth, as the nanotubes thickened, in agreement with

the TGA data.

Several studies [41-43] have reported that normalising the D mode intensity with

respect to the intensity of the G' mode is a more reliable method for estimating the

defect concentration in nanotube samples [42]. A comparison of the various possible

ratios of the Raman signals as a function of position, calculated based on both peak

intensity (Figure 5e) and area (Figure 5f) showed similar trends in all cases. Of the

three ratios tested, D:G' showed the greatest relative variation from tip to base. This

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effect is more apparent while plotting using the inverse ratios (Supporting information,

Figure S2).

3.2. N-doped MWCNT-grafted silica fibres

3.2.1 Growth of CNx-MWCNTs on silica fibres

Aligned CNx-MWCNTs were grown on silica fibres for all reactions, with growth times

ranging from 60 to 240 min (Figure 6a-c), showing the radial ‘brush-like’ morphology,

that was observed in the growth of pure CNTs at short reaction times (Figure 1b).

Figure 6d shows a close-up image of the fibre surface after the growth reaction, with a

few CNx-MWCNTs having been pulled out from the fibres; it is clear that the catalyst

sites were very densely distributed on the surface. However, relatively few embedded

catalyst particles remained on the fibre surface, suggesting a different balance of fibre-

catalyst-CNT interactions than for the pure MWCNTs (refer to Figure 3b). A change in

catalyst behaviour was also evidenced by the elongated iron particles (indicated by

arrows) found at the root of the nanotubes. The growth of CNx-MWCNTs also appears

to have followed a base growth mechanism, in accordance with previous work [14, 44];

the elongated catalyst shape is also characteristic of CNx-MWCNTs [11, 14, 17].

The statistical analysis of the absolute length and the outer diameter of the CNx-

MWCNTs (Table 1) showed the same trends as that observed for pure MWCNTs; the

average length and outer diameter, as well as the outer diameter distribution (see Figure

2b) increased with increasing growth time. Again, the effect can be attributed to catalyst

ripening during the reaction, particularly with the continued supply of iron. After the

same reaction time (120 min), the length of CNx-MWCNTs was 91 ± 11 μm, only 14%

of that of pure MWCNTs, 654 ± 23 μm. The reduced growth rate is thought to be

related to the altered nature of the catalyst-nanotube interaction [15, 45].

3.2.2 TGA characterisation

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The onset temperatures for the TGA of CNx-MWCNTs (Figure 4c) were 475 ºC, 484 ºC

and 487 ºC after the 60, 120 and 240 min reactions, respectively. The trend is similar to

the pure CNTs, indicating that the degree of crystalline perfection of the nanotubes was

slightly improved as the reaction continued. The lower combustion temperature of CNx-

MWCNTs, compared to that of pure MWCNTs, can be attributed to the increasing

defect concentration and reactivity on incorporation of nitrogen [15]. Based on the

analysis of TGA data, the average yields of CNx-MWCNTs grown in the 60, 120 and

240 min reactions were 0.04, 0.80 and 2.33 mg/cm2, respectively. By plotting the yield

as a function of growth time (Figure 4d), a linear growth rate of 0.013 mg/(cm2min) was

identified, with an induction period of around 55 min. The lower growth rate and

longer induction period of CNx-MWCNTs suggested that the incorporation of nitrogen

inhibited the growth of nanotubes. Theoretical calculations [45] have indicated that

nitrogen prefers to segregate to the nanotube edge, inhibiting growth.

3.2.3 Raman characterisation

The Raman spectra taken from three different regions along the CNx-MWCNT arrays

are displayed in Figure 7a, b. The D:G ratio, was used to describe the degree of disorder

in this case, since the G' mode was relatively weak. For CNx-MWCNTs, the disorder

introduced by the nitrogen, both in local bonding and overall microstructure [12],

tended to increase the D:G ratio. In addition, both intensity (ID/IG) and area (AD/AG)

ratios decreased from the tip to the base of the arrays, with the effect more evident in the

case of the area ratio (see Figure 7c), again indicating increasing crystalline quality as

growth proceeded, consistent with the TGA data. As an alternative approach, Figure 7d

shows the changes of the full width at half maximum (FWHM) of the D and G modes as

a function of position. The values of the FWHM for both D and G modes dropped from

tip to base, with a stronger effect on the D mode. The narrowing Raman signals

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indicated a larger graphitic crystal domain size and consequently a lower defect

concentration [12, 46].

4. Conclusions

Aligned, pure and N-doped carbon nanotubes were grown on silica fibres simply by

using the ICVD method; the thickness of the grafted nanotube layer can be controlled

from a few tens of micrometres to over half a millimetre by varying the growth time.

SEM and TEM observation on fibre surfaces after the growth reaction indicated that the

nanotubes were bonded to fibres through the catalyst etching into the fibre surfaces; the

location of the catalyst implied a base growth mechanism. The average diameter of the

nanotubes increased with prolonged growth time in both cases due to the catalyst

ripening during the reaction; at the same time, the crystalline quality increased, as

shown by both TGA and spatially-resolved Raman. The increase in crystallinity could

be a size effect and/or due to the increasing packing density of the MWCNTs. The

CNx-MWCNTs have a higher intrinsic packing density, associated with a slower growth

rate and longer induction time. However, they appear more defective in both TGA and

Raman, due to the disorder introduced by the nitrogen. This paper demonstrates an

effective method for producing (both pure and N-doped) nanotube grafted silica fibres

and provides a further understanding of the growth mechanism. CNx-MWCNTs may

offer advantages in the context of hierarchical composites due to the greater packing

density and potential for chemical interaction with a matrix. Improved electrical

conductivity associated with nitrogen doping may also be an advantage, for example,

for lightening strike protection. Further investigations of mechanical and electrical

properties of these nanotube-grafted fibres and the CNT-fibre interaction are required to

establish their potential in a new generation of composite materials.

Acknowledgements

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The authors would like to thank DSTL and QinetiQ for the financial support of this

work. The authors also thank Mahmoud G Ardakani and Richard J Chater (Imperial

College London) for their assistance on TEM and FIB.

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