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The synthesis and characterization of carbon nanotubesgrown by chemical vapor deposition using a stainless steelcatalyst

Luca Camilli a, Manuela Scarselli a,*, Silvano Del Gobbo a, Paola Castrucci a,Francesca Nanni b, Eric Gautron c, Serge Lefrant c, Maurizio De Crescenzi a

a Dipartimento di Fisica, Universita di Roma Tor Vergata, 00133 Roma, Italyb INSTM – Unita di ricerca di Roma Tor Vergata – Dipartimento di Scienze e Tecnologie Chimiche, Universita di Roma Tor Vergata,

00133 Roma, Italyc Institut des Materiaux Jean Rouxel, CNRS-UMR 6502, 44322 Nantes Cedex 3, France

A R T I C L E I N F O

Article history:

Received 1 March 2011

Accepted 7 April 2011

Available online 13 April 2011

0008-6223/$ - see front matter � 2011 Elsevidoi:10.1016/j.carbon.2011.04.014

* Corresponding author: Fax: +39 06 2023506.E-mail address: manuela.scarselli@roma2

A B S T R A C T

Multi wall carbon nanotubes (MWCNTs) were grown on a stainless steel (SS) sheet by

chemical vapor deposition without the addition of external metal catalyst. We found that

the key for highly efficient growth includes the nanoscale roughness of the SS surface, as

shown by scanning tunneling microscopy, that acts as catalyst/template in the nanotube

formation. Raman spectroscopy and electron microscopy were used to check the nature

and quality of the synthesized nanotubes. We conclude that stainless steel favors a base-

growth mechanism. Transmission electron energy loss spectroscopy performed on single

metallic particles found inside the nanotubes clarified the atomic nature of the catalytic

particles supplied by the steel. Only unoxidized iron was found and no traces of nickel

and chromium were detected. In addition, the SS substrate has been used for a second

growth process after carefully removing the synthesized CNTs, proving that a continuous

production of CNTs from the same substrate is achievable.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Since Iijima’s landmark paper in 1991 [1], carbon nanotubes

(CNTs) have received wide-spread attention in terms of both

fundamentals and practical applications [2]. Indeed, numer-

ous nanoscale electronic devices based on CNTs such as

transistors [3–6], gas sensors [7,8], bio-sensors [9,10], super-

capacitors [11] and solar cells [12–15] have been successfully

achieved.

As production of CNTs moves from laboratory-scale to

industrial-scale, their worldwide market is expected to grow

considerably [16]. Continuous and massive production is only

possible using a material that can play at the same time the

er Ltd. All rights reserved.infn.it (M. Scarselli).

role of support and catalyst, so as to be rapidly reused for sub-

sequent growth processes. In this context, we have elucidated

the basic mechanism governing the carbon nanotube growth

on stainless steel without the addition of external metal cat-

alysts. Since 2003, Vander Wal et al. [17] have successfully

synthesized CNTs directly on stainless steel meshes in a

CVD chamber with benzene/C2H2–CO mixtures as carbon pre-

cursor. According to these authors, some redox pre-treat-

ments were needed to prevent the clustering of the metallic

particles and make them active as catalysts.

Other examples of CNT synthesis on pre-treated steel of

various shapes and composition were presented in the fol-

lowing years [18–24]. Other research groups have used steel

.

Fig. 1 – XPS spectrum of the surface of pristine AISI 316

stainless steel sheet. As expected, the three principal

metals of the alloy are present: Cr, Fe and Ni. In minor

quantity we can also reveal Manganese (Mn). Calcium (Ca)

features are due to the washing treatment.

3308 C A R B O N 4 9 ( 2 0 1 1 ) 3 3 0 7 – 3 3 1 5

only as support, adding external metal catalysts to improve

the quality of CNT produced [25–28]. Despite the great num-

ber of publications on this issue, the mechanism of nucle-

ation and growth of carbon deposits is still controversial

and more investigations are needed to elucidate the funda-

mental reasons behind the grow of high quality carbon nano-

structures on SS substrate without addition of metal

catalysts.

Fig. 2 – 150 nm · 150 nm constant current STM images of the SS

lower panels show the relative cross section, corresponding to

VSample = 700 mV.

Here, we illustrate a simplified recipe to synthesize multi

wall carbon nanotubes on a sheet of AISI 316 stainless steel

by chemical vapor deposition (CVD), without any pretreat-

ment stages of the substrate. More important it is evidenced

that acid treatment and oxidation–reduction stages on this

type of surface are not necessary because of the native nano-

scale roughness of the substrate, as clearly shown by scan-

ning tunneling microscopy (STM) images. Furthermore, after

the first growth, the SS substrate has been used for a second

time just carefully removing the synthesized CNTs in an

ultrasonic bath. Thus the present method results to be partic-

ularly promising and it paves the way to a continuous produc-

tion of CNTs by using the same substrate.

The focus of this work is to assess that, among the main

components of the steel substrate (i.e. iron, chromium,

nickel), only nanoparticles of iron are involved in the growth

mechanism, as evidenced by near edge local atomic investi-

gation by electron energy loss spectroscopy (EELS).

Transmission electron microscopy performed on several

carbon nanotubes suggests that a base-growth mechanism

is mainly involved.

2. Experimental

2.1. Synthesis of MWCNTs on stainless steel sheet

A 30 · 40 mm2 piece of as-received AISI 316-stainless steel

(SS) sheet (from Goodfellow Cambridge, Ltd.) was carefully

sonicated in deionized water and degreased in isopropyl

alcohol for 10 min. Then the steel substrate was placed on a

surface before (a) and after (b) use for the CNT growth. The

the white arrows in the STM images. I = 0.3 nA,

Fig. 3 – Histograms of the diameter of the nano-hills

obtained from STM images. Dotted line is the lognormal fit ;

the average size found is 6.5 ± 1 nm for the SS substrate

before (a) and 8.5 ± 1.3 nm after the growth (b).

C A R B O N 4 9 ( 2 0 1 1 ) 3 3 0 7 – 3 3 1 5 3309

molybdenum sample holder acting also as resistive heater

and inserted into an ultra high vacuum (UHV) chamber.

The pressure was brought up to 10�2 Torr by a rotary pump.

At this stage, argon gas (500 sccm) was inserted at 12 Torr and

then the heater was increased to about 730 �C. The sample

temperature was checked with an optical pyrometer. When

the substrate reached the desired temperature, acetylene

(C2H2) was introduced (200 sccm) in the chamber and CNTs

grew in dynamic condition, since the rotary pump was kept

in operation during the process. After 10 min of growth argon

gas (500 sccm) was inserted in chamber for 5 min.

2.2. Characterization of stainless steel substrate andMWCNTs

A STM/AFM (Omicron, Germany) system was used in UHV

(7 · 10�11 Torr) to analyze the SS sheet both before and after

the CNT growth, to investigate down to the nanometric scale

the substrate surface roughness. The measurements were

performed at room temperature in constant current mode

with I = 0.3 nA, VSample = 700 mV.

X-ray photoelectron spectroscopy (XPS) data were ac-

quired with a semi-imaging analyzer MAC2 (Riber Instru-

ments) operating in the constant pass energy mode (with

the total energy resolution of 1.1 eV) using a non-monochro-

matized Al Ka radiation source. The distance between the

sample and the anode was about 40 mm, the illumination

area was about 5 · 5 mm2 and the takeoff angle between the

sample surface and the photoelectron energy analyzer was

kept fixed at 45�. The energy scale with reference to the bind-

ing energy of the C1s was calibrated at 284.7 eV with respect

to the Fermi level.

Raman spectroscopy was also employed to characterize

the synthesized sample. The Raman spectra were obtained

by a micro-Raman setup equipped with Rayleigh scattering

micro imaging. The setup is made up of a Coherent Sabre

Ar+ laser, a Dilor triple monochromator equipped with LN2

cooled CCD (Horiba 3000C), confocal optics and a nanometric

piezo actuated positioning/scanning table. The confocal op-

tics allows a spatial resolution of �350 nm in the xy plane

and of �1 lm along the z-axis. The spectra was acquired

using an exciting laser wavelength of 514 nm in the backscat-

tering configuration z(x,x)-z.

The morphology and structure of the CNTs were examined

using a field emission gun scanning electron microscope

(FEG-SEM, Leo Supra 35) equipped with energy dispersive X-

ray (EDX) and a cold FEG Hitachi HF2000 transmission elec-

tron microscope (TEM) operated at 200 kV. In order to collect

images from the self-supported nanostructures, TEM samples

were prepared scratching with a razor blade the synthesis

product from the SS surface directly on a gold TEM grid (mesh

1000). Transmission EELS measurements performed on single

particles found inside the tubes were carried out with the

same TEM but with an accelerating voltage of 100 kV,

equipped with Gatan 666 parallel spectrometer. The energy

resolution of the spectra was estimated to be about 1.5 eV

measuring the full width at half-maximum of the zero loss

peak.

In order to demonstrate the possibility to recover and to re-

use the SS sheet after the growth stage, CNTs were removed

from the surface by sonication in isopropyl alcohol. Half of

the SS piece was treated with hydrochloric acid 32% in weight

(HCl) for 1 h, prior to be used for a second growth process.

3. Results and discussion

3.1. Surface atomic composition and morphology ofstainless steel sheet

XPS data (shown in Fig. 1) have been used to assess the

presence, on the stainless steel surface, of all the three

main metals (Fe, Cr and Ni) in the alloy. This analysis

showed that any segregation phenomena were not observed.

We also noticed the presence of Mn, as reported in the

nominal composition of the AISI 316 steel (1.5% atomic

composition). Traces of Ca have been found probably due

to the washing treatment.

We stress that the reported surface morphology has been

obtained without any surface pretreatment (any acid or other

physical–chemical removal of the first outer layers). The

height profile of the nanostructures is reported in the lower

part of Fig. 2a, showing a height distribution ranging from

3.0 nm to 4.5 nm. The presence of these hills reveals the na-

tive nanoscale substrate structure, constituted by small

Fig. 4 – (a and b) Low and high magnification SEM images of the MWCNTs synthesized directly on SS sheet. (c) Blow up of (b)

showing some capped CNTs with catalyst particle at the tip end (red arrows) and some uncapped CNTs (white arrows). (For

interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 5 – Raman spectrum of MWCNTs grown on SS sheet.

The D-band intensity is almost one third of G-band feature,

proving high structural quality of the sample. The arrow

shows the feature located at higher energy with respect to

the G band and it is characteristic of the formation of multi

walled structure.

3310 C A R B O N 4 9 ( 2 0 1 1 ) 3 3 0 7 – 3 3 1 5

islands which are essential (coupled to suitable thermody-

namic conditions) for carbon nanotube synthesis by chemical

vapor deposition [29,30]. This observation clearly shows that

any physical–chemical pretreatment of SS surface is not

necessary to create the morphology requested for the CNT

synthesis, contrary to a number of earlier studies [17,19,

21,22,24,27].

Once grown, the carbon nanotubes have been removed by

sonication and the SS surface was investigated with STM

again. Fig. 2b shows that the nano-hills are still present and

this observation explains why repetitive growths are possible

on stainless steel, as shown in the following and by others

[23]. We also performed a detailed statistical analysis of sev-

eral STM images collected on both substrates. The results ob-

tained are reported in Fig. 3. The average lateral size of the

nanostructures found is 6.5 ± 1 nm in the case of the SS sub-

strate before use and 8.5 ± 1.3 nm after the growth.

We conclude that the metallic composition and the sur-

face morphology of SS substrate favor in a straightforward

way the CNT synthesis as a natural and almost infinite source

of catalyst.

3.2. Carbon nanotube characterization

Fig. 4a and b reports SEM images of our synthesized nano-

structures showing that random-oriented CNTs are uniformly

grown on the SS sheet with very-high density. After only

10 min of acetylene exposure we estimated that the length

of the carbon nanotubes reaches 50 lm. Over the large SS area

Fig. 6 – TEM images showing the multi wall nature of CNTs grown on SS sheet. (a) Red arrows indicate uncapped CNTs. (b)

High resolution of a typical MWCNT with 23 walls. (For interpretation of the references to color in this figure legend, the

reader is referred to the web version of this article.)

Fig. 7 – HRTEM images illustrating the different observed morphologies: (a and b) two examples of a shoot-shaped catalytic

particle inside a tube; (c) the formation of the fullerene cap around the catalytic particle on the tip of a CNT; (d) a capped and (e)

an uncapped tube.

C A R B O N 4 9 ( 2 0 1 1 ) 3 3 0 7 – 3 3 1 5 3311

used, only few traces of carbonaceous nanostructures have

been detected. Fig. 4c is a blow up of Fig. 4b, showing a wide

distribution of tube diameters. Red arrows indicate small

bright catalyst particles located at the tip of the CNTs, while

white arrows the CNTs without caps. This finding will be

extensively explained below.

Raman spectrum of the as-grown CNTs is reported in

Fig. 5. The arrow shows the feature located at higher energy

Fig. 8 – Schematic depiction of growth process of the

MWCNTs on stainless steel. At the working temperature

(730 �C), the nano-hills on the stainless steel surface are in

liquid state, so the stretching force applied on them during

the formation of the CNT make them elongated and finally

broken into two parts. According to this model the tube

grows upward, leaving the nano-hill (or, at least, a part of it)

on the SS surface.

Fig. 9 – Typical EELS analysis performed on single metal

nanoparticle located inside a carbon nanotube. The

presence of Fe L2,3 features at around 710 eV energy loss

region revealed that it is made of pure iron. No chromium

and nickel atoms have been measured (due to the absence

of ionization edges at around 570 eV and 850 eV,

respectively) within the minimum detectable

concentrations. Also the oxygen K ionization edge resulted

to be absent, thus showing that iron nanoparticles are not

oxidized. This finding is confirmed by the intensity ratio

between Fe L3 and L2 ionization edges, as reported in the

inset, which amounts to about three [32].

Fig. 10 – EDX spectra of the stainless steel surface before

(blue curve) and after (red curve) the CNT growth. We

observe that among the main metallic elements (Fe, Cr and

Ni) of the steel alloy only iron can be detected within the

carbon nanotube network. (For interpretation of the

references to color in this figure legend, the reader is

referred to the web version of this article.)

3312 C A R B O N 4 9 ( 2 0 1 1 ) 3 3 0 7 – 3 3 1 5

with respect to the G band and it is characteristic of the for-

mation of multi walled structure. The intensity of the D-band,

generally attributed to the amount of structural defects, re-

sulted to be about one third of the G-band feature. This

strongly suggests that our carbon nanotubes are character-

ized by very few structural imperfections. The reported ratio

is similar to that obtained by Kim et al. [25] who have synthe-

sized CNTs on SS by water-assisted chemical vapor deposi-

tion with the addition of external metallic catalyst. It is

worth noting that this ratio is well lower than that we mea-

sured in case of MWCNTs synthesized with the same CVD

apparatus by using Fe catalyst [14] on a silicon substrate.

TEM images prove the multi walled nature of carbon nano-

tubes, as reported in Fig. 6. The average inner diameter (ID) of

the nanotubes resulted to be 9 ± 3 nm, while the outer diam-

eter (OD) was 19 ± 6 nm. Interestingly, we note that the ID va-

lue closely corresponds to the average size of nano-hills on

the SS surface evaluated from the statistical analysis

(8.5 ± 1.3 nm) reported in Fig. 3b.This is a direct evidence that

the native nanostructures of the SS act as catalyst/template

in the nanotube formation.

Fig. 6a shows also the presence of nanotubes without cap,

as indicated by white arrows, in agreement with SEM images

(see Fig. 4). In Fig. 6b is reported a high resolution TEM

(HRTEM) image of a typical MWCNT with 23 walls, pointing

up the high quality and good graphitization degree of the ob-

tained nanostructures.

Fig. 7a–e are HRTEM images of different morphologies ob-

served in our sample. In some cases we detected shoot-

shaped nanoparticles inside tubes (see Fig. 7a and b). The

explanation of this phenomenon was given by Chen et al.

[31] who suggested that the catalytic nanoparticles are in a

Fig. 11 – SEM images collected on the SS substrate after the second growth process; (a) as used and (b) chemically etched SS.

C A R B O N 4 9 ( 2 0 1 1 ) 3 3 0 7 – 3 3 1 5 3313

liquid state at working temperature (730 �C) and the stretch-

ing force causes them to elongate and finally break. In this

way the metallic nanoparticles are dragged by the tube during

growth, as schematically illustrated in Fig. 8. This observation

is a clear-cut proof of a base-growth mechanism for multi

wall carbon nanotube on SS substrate. In Fig. 7c we report

an example of the encapsulation of catalyst particles at the

top end of the CNT. It is evident the formation of the fullerene

shell around the particle, which will form the cap of the tube.

In our case we find no particles attached to the base of carbon

nanotubes in HRTEM images, but only inside them (Fig. 7a and

b) or at the tip (Fig 7c). We suggest that this is due to our pe-

culiar growth method that employs the whole SS sheet as cat-

alyst. The nano-hills that act as template for the CNT

formation are well adhering to the SS bulk underneath and

therefore are not stripped during the detachment of nano-

tubes, before the TEM analysis. In Fig. 7d and e we illustrate

the two cases that occur in the terminal part of tube: an un-

capped and a capped one. Probably, these morphologies are

possible because the nano-hills on the surface of SS sheet

have different average size and we suggest that the smallest

ones should favor the formation of the fullerene shell, while

the larger ones should lead to the open end. This finding dem-

onstrates that a wide number of situations can be achieved

from the described growth process.

Many EELS analyses (shown in Fig. 9), performed on single

nanoparticles inside the tubes with an electron probe smaller

than the particle, revealed that they are made of pure iron. No

nickel and chromium atoms have been detected within the

limits of the minimum detectable concentrations (about

0.5 wt.%).

The result is confirmed by EDX analysis (reported in

Fig. 10) performed on the pristine stainless steel (blue curve)

and on the as-grown carbon nanotube carpet (red curve). It

is worth noting the difference in the intensity of C-relative

peak in the two cases, proving the presence of a great num-

ber of produced MWCNTs. We observe that only iron feature

is detected on the whole network of the carbon nanostruc-

tures.

This finding has been already reported by Mordkovich

et al. [20] in the case of CNTs grown with Fe–Ni catalytic film

deposited on SS-304 substrate; thus, also in pristine SS-316,

the competition between these two metals in the CNT growth

process leads to total suppression of the catalytic activity of

nickel in favor of iron. This is one of the main results of our

investigation, because we know very well that both iron and

nickel (as well as cobalt) are the most used catalytic materials

for the synthesis of CNTs via chemical vapor deposition. At

present, we do not have a clear model to explain this impor-

tant issue in which the formation of carbon nanotubes occurs

only by extracting iron from the SS surface, leaving the other

metals unreacted on the surface (as schematically depicted in

Fig. 8). We suspect that our observation could be attributed to

the different diffusion kinetics of C atoms in iron or in nickel

lattice at the temperature utilized for the synthesis (730 �C).

Thus a theoretical explanation is needed to clarify this exper-

imental observation with the aim to understand the selective

catalyst action and extraction of a metal over all others of the

SS alloy.

Surprisingly, we observed that iron is not oxidized, as we

can clearly see from a careful analysis of the Fe L2,3 edge line

shapes [32] and from the total absence of any signal coming

from the oxygen K ionization edge (Fig. 9). From the near edge

features shown in the inset of Fig. 9 we estimated that the ra-

tio between L3 and L2 lines (white-line ratio) is very close to

the value reported in Ref. [32] for pure iron (ratio of about 3).

This is a clear hint of its metallic character, in fact any other

form of oxidized iron reports a statistical value of the ratio

much higher, ranging from 4.3 for FeO up to 5.1 for Fe2O3

[33]. Thus, the intercalation in the graphitic planes hinders

any oxidation process of these metal nanoparticles. This

observation could be of some interest for practical applica-

tions, such as magnetic recording [34] and bio-application

[35,36], where the use of pure iron is preferred to that of its

oxidized form.

SEM images were also collected on the CNT grown on a re-

used SS sheet as shown in Fig. 11.

At this point it is worth reminding that, after the first CNT

synthesis and complete removing cycle, only half of SS sheet

has been chemically etched before the second CNT growth.

No significant difference was revealed in the density and

quality of the carbon nanotube film on the two surface sides.

This proves that no further treatment is necessary to the steel

surface for reuse in the growth, as already discussed by

Baddour et al. [21].

4. Conclusions

We have reported a careful method to grow high quality

MWCNTs directly on stainless steel without any additional

3314 C A R B O N 4 9 ( 2 0 1 1 ) 3 3 0 7 – 3 3 1 5

metal catalyst and any surface pretreatment. The method

allows to produce large quantities of CNTs with a very sim-

ple procedure and cheap materials so that we believe a

mass production could be implemented in a near future.

In fact, pieces of steels heated in hydrocarbon atmosphere

are just necessary to obtain long and almost defect free

nanotubes, as supported by Raman spectroscopy. The meth-

od allows an easy and quick reuse of the same SS substrate

without any additional process, apart a careful removal of

synthesized carbon nanotubes. The key role of the nano-

structured surface character of the stainless steel has been

assessed by STM investigation, revealing the presence of

small islands of the same order of magnitude of the internal

channel of the produced CNTs. The native metallic surface

morphology is the principal ingredient that excludes the

addition of external metallic catalyst. A surprising discover

of our EELS investigation performed above the L2 and L3 ion-

ization edges of Fe, Ni and Cr is that inside the nanotube we

only detected unoxidized iron nanoparticles. This finding is

extremely important to understand the thermodynamic of

the interaction of carbon atoms, coming from gas feedstock

(i.e. acetylene), with stainless steel substrate surface. We re-

ported evidence that the catalytic activity of nickel is com-

pletely suppressed even though it is well known that

nickel is an efficient catalyst for CNTs growth through

CVD method. Therefore, further experiences are still needed

to investigate the full role of single metallic components

present into the SS alloy. On this last point, a theoretical

investigation of the growth mechanism is highly expected

to explain the selectivity of carbon nanotubes to grow on

iron instead on nickel.

Acknowledgements

This work was supported by the French-Italian Galileo 2009–

2010 project. The Italian authors acknowledge the financial

support of the Queensland Government smart futures fund

National and International Research Alliances Program (NIR-

AP): Solar powered nano-sensors for data acquisition and sur-

veying in remote areas‘, ABN 83 791 724 622. We are grateful to

Dr. M. Salvato for thickness measurements of the samples.

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