http://journals.cambridge.org Downloaded: 10 Jan 2016 IP address: 132.239.1.230

Large-scale synthesis of highly aligned nitrogen doped carbon

nanotubes by injection chemical vapor deposition methods

J. Liu, R. Czerw, and D.L. Carroll

The Center for Nanotechnology and Molecular Materials, Department of Physics,

Wake Forest University, Winston-Salem, North Carolina 27109

(Received 7 September 2004; accepted 24 November 2004)

In this study, we compare the effects of pyridine (C

N) and pyrimidine (C

)

precursors, using ferrocene as a metal source, in the production of nitrogen containing

multiwalled carbon nanotubes. Using standard chemical vapor deposition techniques,

highly aligned mats of carbon-nitrogen carbon nanotube were synthesized. The

maximum nitrogen concentration in these materials is between 1% and 2% when

pyridine is used as the precursor and can be increased to 3.2% when pyrimidine is

used as the precursor. However, the electronic structure of both materials, as

determined using scanning tunneling spectroscopy, suggests that the nitrogen is

incorporated into the nanotube lattice in the same way for both precursors.

I. INTRODUCTION

Since the first observation by Ijima,

there has been

much interest in preparing carbon nanotubes and related

materials. Modification of nanotube properties is of sig-

nificant interest to the materials science community. For

example, C

and CN nanotubes have been theoreti-

cally predicted to be superhard and metallic respec-

tively.

2–4

Efficient field emission has recently been

shown using nitrogen doped multi-walled nanotubes.

Thus, control over the electronic and structural properties

of nanotubes, through substitutional doping schemes,

provides an interesting route to broadening the potential

applications of these materials.

So far, many methods have been exploited to synthe-

size CN nanotubes, such as magnetron sputtering,

chemical deposition,

7–9

and pyrolysis.

10–17

In spite of

these efforts, control over N content and large-scale pro-

duction viability is still beyond reach. This comes from a

fundamental lack of understanding of growth mecha-

nisms for doped materials. For instance, it is not clear

how the bonding configuration of the substitutional do-

pant might change with the growth temperature and,

through this, how the different precursors might affect

the nitrogen concentration of carbon nanotubes.

In this study, we present a comparison of the large

scale synthesis of CN

nanotubes using injection-based

chemical vapor deposition (CVD) of ferrocene in

pyridine at different temperatures

13,18

with the use of

ferrocene in pyrimidine as a precursor to understand how

the doping level of carbon nanotubes might be better

controlled. Ferrocene is used as the catalyst throughout

the study.

15,16,19,20

X-ray photoelectron spectroscopy

(XPS) was used to check the overall nitrogen concentra-

tion of carbon nanotubes and how the different doping

changes with the growth temperature. The structure of

the CN

nanotubes is characterized by transmission elec-

tron microcopy (TEM) and the electronic structure by

scanning tunneling spectroscopy (STS).

II. EXPERIMENTAL PROCEDURE

The experimental set up used to synthesize CN

nano-

tubes consists of a two-stage tubular quartz furnace

(diameter ∼45mm, work length ∼450 mm). The first

stage is the preheater and the second is the growth oven,

similar to that reported by Andrews et al. and else-

where.

19,21

For all runs, hydrogen was used as the carrier

gas with a flow rate of 320 sccm. The preheater tempera-

ture was maintained at 160 °C, and the injection feed

rate was 5 ml/h using a syringe pump. Approximately

2.7 wt% of ferrocene was dissolved in pyridine or py-

rimidine and the solution was fed continuously through

the injection system. The synthesis temperature ranged

from 600 to 900 °C when using pyridine and pyrimidine

with the growth time of approximately 1 h. After the

reaction, the hydrogen gas was switched to argon with

the same flow rate so the preheater and the oven would

cool to room temperature.

Electron microscopy was carried out using a Hitachi

S-4700 (20 KV) scanning electron microscope (SEM)

and a Hitachi HD-2000 (200 KV) transmission electron

microscope (TEM). X-ray photoelectron spectroscopy

Address all correspondence to this author.

e-mail: [email protected]

DOI: 10.1557/JMR.2005.0069

http://journals.cambridge.org Downloaded: 10 Jan 2016 IP address: 132.239.1.230

XPS was acquired on a Kratos AXIS 165. Scanning tun-

neling microscopy and spectroscopy (STM/STS) was

performed under ultrahigh vacuum (<10

−9

Torr) on a

RHK UHV-300 microscope.

III. RESULTS AND DISCUSSION

When using pyridine as the precursor, a dense film of

nanotubes is observed to be deposited homogeneously

along the total length of the heating zone inside the

quartz tube for all temperatures studied, from 650 to

850 °C. At temperatures outside of this window, (600

and 900 °C for example) the film only forms at the ends

of the furnace heating zones. Figure 1 shows SEM mi-

crographs of the carbon nanotubes grown at 700 °C. It

should be noted that the films grown at every temperature

consist of highly aligned carbon nanotubes. The thick-

ness of the films (i.e., the length of the nanotubes) in-

creases with increasing the growth temperature until

750 °C and then decreases as the growth temperature is

increased further to 900 °C. The thickness of the film at

approximately 750 °C approaches about 100 ␮m after

one hour growth with a maximum yield of approximately

0.5 mg/cm

/h.

XPS spectra from the tubes grown at different tem-

peratures (ranging from 600 °C to 900 °C) shows that the

nanotubes have trace amounts of nitrogen associated

with them. In Fig. 2(a), the C 1s feature is at ∼285 eV

and the N 1s is around 400 eV. By taking the ratio of the

respective integrated peak areas and dividing them by the

respective photoionization cross-section ratios for the 1s

level,

the nitrogen concentration present in the dense

carbon nanotube mats is estimated to be between 1% and

2%. This is quite close to that reported by Sen.

How-

ever the nitrogen concentration changes little with the

growth temperature, which is different from that reported

FIG. 1. SEM micrographs of nanotubes synthesized at 700 °C with

320 sccm of H

, preheater at 160 °C, injection rate of 5 ml/h, 2.7 wt%

ferrocene in pyridine after 1 h growth.

FIG. 2. XPS analysis of CN nanotubes: (a) XPS spectrum of C and N

elements, (b) an enlargement of the nitrogen peak for the carbon nano-

tubes grown at 650 °C, and (c) ratio of the normalized intensities of the

peaks centered at 401 eV and 398.7 eV, which increases as the growth

temperature increases.

J. Liu et al.: Large-scale synthesis of highly aligned nitrogen doped carbon nanotubes by injection chemical vapor deposition methods

J. Mater. Res., Vol. 20, No. 2, Feb 2005 539

http://journals.cambridge.org Downloaded: 10 Jan 2016 IP address: 132.239.1.230

by other groups. Lee et al.

reported that the nitrogen

concentration increases with the growth temperature in-

creasing when NH

was used as nitrogen source, and

Tang et al.

reported that the nitrogen concentration

decreases with the growth temperature increasing when

dimethylformamide (DMF) was used as the precursor.

There are several ways in which N can be incorporated

in the carbon nanotube lattice.

3,4,24

However, there are

only two ways observed in our studies. One is pyridine-

like and the other is graphitelike as reported by other

groups.

12,20,25

TheN1s signal from the nanotubes grown

at a relatively low temperatures (below around 750

C),

for example, at 650 °C shows two distinct features at

398.7 and 401 eV as shown in Fig. 2(b), which have been

interpreted as corresponding to the pyridinelike and

graphitelike nitrogen placement in the lattice respec-

tively. However, this pyridinelike feature disappears

gradually when the growth temperature is increased.

Figure 2(c) shows that the ratio of the normalized inten-

sities of the peaks centered at 401 and 398.7 eV increases

as the growth temperature increases, and it is more pro-

nounced when the growth temperature is higher. This

suggests more nitrogen atoms present in the graphite

sheets rather than in pyridinelike structures at higher

temperature, a result similar to that reported previously.

However, the total nitrogen concentration of the carbon

nanotubes in our experiments did not change appreciably

when the growth temperature increased. If we consider

the pyridinelike structure to be more defective than the

graphitelike structure, XPS results might be interpreted

as indicating a decrease in defects in the nitrogen doped

carbon nanotubes with increasing growth temperature.

This has been confirmed by Raman studies on these ni-

trogen doped carbon nanotubes.

TEM observations show that at 600 °C, the diameter of

the nanotubes is not uniform and many more nanotubes

appear to be catalyst filled (in comparison to the nano-

tubes grown at higher temperatures). The nanotubes be-

come far more uniform with less amorphous carbon at-

tached when the growth temperature is raised to 700 °C

(Fig. 3) and higher. The average diameter of the nano-

tubes increases almost linearly from 20 to 45 nm when

the growth temperature increasing from 700 to 900 °C.

TEM also reveals a bamboo structure at all growth tem-

peratures, with it being most pronounced at higher

growth temperatures.

For comparison, the precursor pyrimidine (C

)

was also used to grow carbon nanotubes at temperatures

between 650 and 850 °C with other growth conditions

remaining the same (hydrogen flow rate was 320 sccm;

preheater temperature was 160 °C; the injection feed rate

was 5 ml/h; and approximately 2.7 wt% of ferrocene was

dissolved pyrimidine). SEM [Fig. 4(a)] observation

shows that highly aligned carbon nanotube films can also

be obtained by the pyrolysis of pyrimidine (here we show

FIG. 4. (a) SEM image and (b) TEM image of carbon nanotubes

grown at 750 °C with pyrimidine.

FIG. 3. TEM images of nanotubes synthesized at 700 °C with

320 sccm of H

, preheater at 160 °C, injection rate of 5 ml/h, 2.7 wt%

ferrocene in pyridine after 1 h growth.

J. Liu et al.: Large-scale synthesis of highly aligned nitrogen doped carbon nanotubes by injection chemical vapor deposition methods

J. Mater. Res., Vol. 20, No. 2, Feb 2005540

http://journals.cambridge.org Downloaded: 10 Jan 2016 IP address: 132.239.1.230

only the 750 °C growth temperature; other temperatures

are similar). The tubes have a very uniform diameter of

about 35 nm and exhibit a more pronounced bamboo

structure as compared to the carbon nanotubes by the

pyrolysis of pyridine at the same temperature [Fig. 4(b)].

XPS shows the maximum nitrogen concentration of the

tubes grown in this temperature range, from pyrimidine,

is about 3.2 at.%, higher than that of the carbon nano-

tubes grown from pyridine (1–2%). Pyridinelike struc-

ture was also found for these tubes, and the ratio of the

XPS intensities of the graphitelike/pyridinelike features

for these tubes is less than that of tubes grown from

pyridine at the same growth temperature. For instance,

the ratio at 750 °C is 1.5, which is less than the 2.7 ratio

obtained for the tubes grown from pyridine under the

same growth conditions. The lower ratio obtained from

the pyrimidine sample suggests that there is relatively

more nitrogen in pyridinelike structures for these carbon

nanotubes. We note here that the overall nitrogen con-

centration of the carbon nanotubes is much lower than

that in the precursors, no matter pyridine or pyrimidine

was used. Clearly, not all nitrogen is being consumed

in growth.

This result is contrast to the results from

Glerrup

and Tang et al.

They demonstrated that

nearly all of the nitrogen could be used in the reaction

and doped into the carbon lattice, resulting in much

higher concentration range from approximately 10% to

about 20% when acetonitrile or DMF was used as

precursors.

To understand the differences in electronic structure

that might arise from the use of the different precursors,

tunneling microscopy and spectroscopy was used. CN

nanotubes synthesized with pyridine and with pyrimidine

were compared for the different growth temperatures.

Typically, the materials were dispersed ultrasonically in

acetone, and the solution was then drop cast onto freshly

cleaved highly oriented pyrolitic graphite (HOPG). After

allowing for solvent evaporation under low vacuum in a

prechamber, the sample was transferred into ultrahigh

vacuum for evaluation. Fixed gap STS was carried out

using mechanically formed Pt–Ir tips. All spectra were

converted to the equivalent local density of states

(LDOS) using the accepted Feenstra algorithm of nu-

merical differentiation and normalization for the tip

height: (dI/dV)/(I/V).

Figures 5(a) and 5(b) shows a

comparison of typical spectra obtained from both pyridine-

and pyrimidine-grown nanotubes. The growth tempera-

ture compared for these materials was 750 °C; however,

similar results are seen for other temperatures. Notice

that the valence and conduction band features appear

symmetric about the Fermi level for both materials, while

an additional electronic feature occurs at approximately

0.18 eV as previously reported.

This donor feature

confirms XPS results since it is associated with pyridine-

like (sp

-like) structures within the lattice and suggests

FIG 5. LDOS of nanotubes synthesized at 750 °C: (a) LDOS of nano-

tubes grown from pyridine and (b) LDOS of nanotubes grown from

pyrimidine. Note the additional feature occurring at ∼0.18 eV (indi-

cated by the arrow) while other features are symmetric about the Fermi

energy. (c) LDOS similar to that of pure carbon nanotubes is also

observed.

J. Liu et al.: Large-scale synthesis of highly aligned nitrogen doped carbon nanotubes by injection chemical vapor deposition methods

J. Mater. Res., Vol. 20, No. 2, Feb 2005 541

http://journals.cambridge.org Downloaded: 10 Jan 2016 IP address: 132.239.1.230

that the pyridinelike structure is independent of the pre-

cursors used in our experiments. Also, this donor state is

in a fashion similar to that reported by Choi

and Nevi-

domskyy et al.

A small number of the tubes examined

show spectra as in which the valence band and conduc-

tion band features appear symmetric about the Fermi

level without the additional feature observed exactly like

the spectra from pure carbon nanotubes, as shown in

Fig. 5(c). This suggests that carbon nanotube growth

within the same growth zones may not be completely

heterogeneous. It should be noted that the raising of the

Fermi level due to the graphite like structure

is not

observed in our studies, maybe due to lower nitrogen

doping concentration. We do note that the predicted ap-

proximately 0.5 eV feature

in the conduction band is

observed in our spectra as a subtle splitting of the van

Hove singular point. However, further study is needed to

make a definite assignment of this feature with graphite-

like substitution N.

IV. CONCLUSIONS

The yield and quality of CN

nanotubes grown using

CVD techniques and pyridine or pyimidene, is com-

pletely comparable to that of pure carbon nanotubes by

similar methods and is evidence that such materials could

be produced in large quantities. The observed nitrogen

concentration can be increased through the substitution

of pyrimidine (3.2%) for pyridine (1–2%). However, we

do note that this may not correlate with the total nitrogen

concentration of the precursor since the kinetics and ther-

modynamics of the two precursors are expected to be

entirely different. In both cases the CN

nanotubes have

a uniform diameter and bamboo structure when grown at

700 °C and above. From XPS, the nitrogen is thought to

be incorporated into the nanotube lattice in two ways:

pyridinelike and graphitelike structure with the graphite-

like structures preferred at high growth temperatures

(fewer defects). STS further confirms the existence of the

pyridine-like structure. This work indicates that property

modification and engineered electronic structure in car-

bon nanotubes is possible through substitutional doping

schemes using CVD methods.

REFERENCES

1. S. Iijima: Helical microtubules of graphitic carbon. Nature 354, 56

(1991).

2. D.M. Teter and R.J. Hemley: Low-compressibility carbon ni-

trides. Science 271, 53 (1996).

3. Y. Miyamoto, M.L. Cohen, and S.G. Louie: Theoretical investi-

gation of graphitic carbon nitride and possible tubule forms. Solid

State Commun. 102, 605 (1997).

4. E. Hernandez, C. Goze, P. Bernier, and A. Rubio: Elastic prop-

erties of single-wall nanotubes. Appl. Phys. A: Mater. Sci. Proc.

68, 287 (1999).

5. M. Doytcheva, M. Kaiser, M.A. Verheijen, M. Reyes-Reyes,

M. Terrones, and N. de Jonge: Electron emission from individual

nitrogen-doped multi-walled carbon nanotubes. Chem. Phys. Lett.

396, 126 (2004).

6. K. Suenaga, M.P. Johansson, N. Hellgren, E. Broitman,

L.R. Wallenberg, C. Colliex, J-E. Sundgren, and L. Hultman:

Carbon nitride nanotubulite-densely-packed and well aligned tu-

bular nanostructures. Chem. Phys. Lett. 300, 695 (1999).

7. M. Yudasaka, R. Kikuchi, Y. Ohki, and S. Yoshimura: Nitrogen-

containing carbon nanotube growth from Ni phthalocyanine by

chemical vapor deposition. Carbon 35, 195 (1997).

8. S.L. Sung, S.H. Tsai, C.H. Tseng, F.K. Chiang, X.W. Liu, and

H.C. Shih: Well-aligned carbon nitride nanotubes synthesized in

anodic alumina by electron cyclotron resonance chemical vapor

deposition. Appl. Phys. Lett. 74, 197 (1999).

9. X. Ma and E.G. Wang: CN

/carobn nanotube junctions synthe-

sized by microwave chemical vapor deposition. Appl. Phys. Lett.

78, 978 (2000).

10. M. Reyes-Reyes, N. Grobert, R. Kamalakaran, T. Seeger,

D. Goldberg, M. Rühle, Y. Bando, H. Terrones, and M. Terrones:

Efficient encapsulation of gaseous nitrogen inside carbon nano-

tubes with bamboo-like structure using aerosol thermolysis.

Chem. Phys. Lett. 396, 167 (2004).

11. M. Terrones, N. Grobert, J.P. Zhang, H. Terrones, J. Olivares,

W.K. Hsu, J.P. Hare, A.K. Cheetham, H.W. Kroto, and

D.R.M. Walton: Preparation of aligned carbon nanotubes cata-

lysed by laser-etched cobalt thin films. Chem. Phys. Lett. 285, 299

(1998).

12. M. Terrones, P. Redlich, N. Grobert, S. Trasobares, W-K. Hsu,

H. Terrones, Y-Q. Zhu, J.P. Hare, C.L. Reeves, A.K. Cheetham,

M. Rühle, H.W. Kroto, and D.R.M. Walton: Carbon nitride

nanocomposites: Formation of aligned C

nanofibers. Adv.

Mater. 11, 655 (1999).

13. R. Sen, B.C. Satishkumar, A. Govindaraj, K.R. Harikumar,

G. Raina, J-P. Zhang, A.K. Cheetham, and C.N.R. Rao: B–C–N,

C–N, and B–N nanotubes produced by the pyrolysis of precursor

molecules over Co catalysts. Chem. Phys. Lett. 287, 671 (1998).

14. M. Nath, B.C. Satishkumar, A. Govindaraj, C.P. Vinod, and

C.N.R. Rao: Production of bundles of aligned carbon and carbon-

nitrogen nanotubes by the pyrolysis of precursors on silica-

supported iron and cobalt catalysts. Chem. Phys. Lett. 322, 333

(2000).

15. M. Terrones, H. Terrones, N. Grobert, S. Trasobares, W-K. Hsu,

Y-Q. Zhu, J.P. Hare, H.W. Kroto, D.R.M. Walton, P.K. Redlich,

M. Rühle, J.P. Zhang, and A.K. Cheetham: Efficient route to large

arrays of CN

nanofibers by pyrolysis of ferrocene/melamine mix-

tures. Appl. Phys. Lett. 75, 3932 (1999).

16. W-Q. Han, P.K. Redlich, T. Seeger, F. Emst, M. Rühle,

N. Grobert, W-K. Hsu, B-H. Chang, Y-Q. Zhu, H.W. Kroto,

D.R.M. Walton, M. Terrones, and H. Terrones: Aligned CN

nanotubes by pyrolysis of ferocene/C

under NH

atomosphere.

Appl. Phys. Lett. 77, 1807 (2000).

17. C.J. Lee, S.C. Lyu, H-W. Kim, J.H. Lee, and K.I. Cho: Syn-

thesis of bamboo-shaped carbon-nitrogen nanotubes using

–NH

–Fe(CO)

system. Chem. Phys. Lett. 359, 115 (2002).

18. R. Sen, B.C. Satishkumar, S. Govindaraj, K.R. Harikumar,

M.K. Renganathan, and C.N.R. Rao: Nitrogen-containing carbon

nanotubes. J. Mater. Chem. 12, 2335 (1997).

19. R. Andrews, D. Jacques, A.M. Rao, F. Derbyshire, D. Qian,

X. Fan, E.C. Dickey, and J. Chen: Continuous production of

aligned carbon nanotubes: A step closer to commercial realization.

J. Chen. Chem. Phys. Lett. 303, 467 (1999).

20. M. Terrones, P.M. Ajayan, F. Banhart, X. Blase, D.L. Carroll,

R. Czerw, B. Foley, N. Grobert, R. Kamalakaran, P. Kohler-

Redlich, M. Rühle, T. Seeger, and H. Terrones: N-doping and

J. Liu et al.: Large-scale synthesis of highly aligned nitrogen doped carbon nanotubes by injection chemical vapor deposition methods

J. Mater. Res., Vol. 20, No. 2, Feb 2005542

http://journals.cambridge.org Downloaded: 10 Jan 2016 IP address: 132.239.1.230

coalescence of carbon nanotubes: Synthesis and electronic prop-

erties. Appl. Phys. A: Mater. Sci. Proc. 74, 355 (2002).

21. J. Liu, R. Czerw, S. Webster, D.L. Carroll, J.H. Park, Y.W. Park,

and M. Terrones: Advances in CN

nanotube growth, in

Nanotube-Based Devices, edited by P. Bernier, D. Carroll,

G-T. Kim, and S. Roth (Mater. Res. Soc. Symp. Proc. 772,

Warrendale, PA, 2003), p. 105.

22. J.H. Scofiled: Hartree-slater subshell photoionization cross-

sections at 1254 and 1487 eV. J. Electron Spectrosc. 8, 129

(1976).

23. C. Tang, Y. Bando, D. Golberg, and F. Xu: Structure and nitrogen

incorporation of carbon nanotubes synthesized by catalytic py-

rolysis of dimethylformamide. Carbon 42, 2625 (2004).

24. I. Shimoyama, G.H. Wu, T. Sekiguchi, and Y. Baba: Evidence for

the existence of nitrogen-substituted graphic structure by polari-

zation dependence of near-edge x-ray-absorption fine structure.

Phys. Rev. B 62, R6053 (2000).

25. J. Casanovas, J.M. Ricart, J. Rubio, F. Illas, and J.M. Jimenez-

Mateos: Origin of large N 1s binding energy in x-ray photoelec-

tron spectra of calcined carbonaceous materials. J. Am. Chem.

Soc. 118, 8071 (1996).

26. S. Webster, J. Maultzsh, C. Thomsen, J. Liu, R. Czerw,

M. Terrones, F. Adar, C. John, A. Whitley, and D.L. Carroll:

Raman characterization of nitrogen doped multiwalled carbon

nanotubes, in Nanotube-Based Devices, edited by P. Bernier,

D. Carroll, G-T. Kim, and S. Roth (Mater. Res. Soc. Symp. Proc.

772, Warrendale, PA, 2003), p. 129.

27. M. Glerup, M. Castignolles, M. Holzinger, G. Hug, A. Loiseau,

and P. Bernier: Synthesis of highly nitrogen-doped multi-walled

carbon nanotubes. Chem. Comm. 20, 2542 (2003).

28. R.M. Feenstra: Tunneling spectroscopy of the (110) surface of

direct-gap III-V semiconductors. Phys. Rev. B 50, 4561 (1994).

29. R. Czerw, M. Terrones, J-C. Charlier, X. Blase, B. Foley,

R. Kamalakaran, N. Grobert, H. Terrones, D. Tekleab,

P.M. Ajayan, W. Blau, M. Rühle, and D.L. Carroll: Identification

of electron donor states in N-doped carbon nanotubes. Nano Lett.

1, 457 (2001).

30. H.J. Choi, J. Ihm, S.G. Louie, and M.L. Cohen: Defect, quasi-

bound states, and quantum conductance in metallic carbon nano-

tubes. Phys. Rev. Lett. 84, 2917 (2000).

31. A.H. Nevidomskyy, G. Csányi, and M.C. Payne: Chemically ac-

tive substitutional nitrogen impurity in carbon nanotubes. Phys.

Rev. Lett. 91, 105502 (2003).

J. Liu et al.: Large-scale synthesis of highly aligned nitrogen doped carbon nanotubes by injection chemical vapor deposition methods

J. Mater. Res., Vol. 20, No. 2, Feb 2005 543