Electronic state of nitrogen incorporated into CNx nanotubes

Article abstract of DOI:10.1140/epjd/e2005-00156-y

CN_x nanotubes have been prepared by acetonitrile decomposition over Ni, Co and Ni/Co catalysts. X-ray photoelectron spectroscopy study on the samples revealed a change of nitrogen concentration and shape of N 1s line with variation of the catalyst used. Quantum-chemical calculations on tube fragments showed the energy of N 1s level depends on the atomic structure of carbon tube and kind of incorporated nitrogen. The largest binding energies were found to be characteristic of three-coordinated nitrogen atoms doping the zigzag and chiral carbon nanotubes. EDP Sciences, Societa? Italiana di Fisica, Springer-Verlag 2005.

Full text of DOI:10.1140/epjd/e2005-00156-y

Eur. Phys. J. D 34, 271–274 (2005)
DOI: 10.1140/epjd/e2005-00156-y
THE EUROPEAN
PHYSICAL JOURNAL D
Electronic state of nitrogen incorporated into CN nanotubes
x
1
,a
1
1
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3
L.G. Bulusheva , A.V. Okotrub , A.G. Kudashov , I.P. Asanov , and O.G. Abrosimov
1
Nikolaev Institute of Inorganic Chemistry, SB RAS, pr. Ak. Lavrentieva 3, Novosibirsk 630090, Russia
Samsung Advanced Institute of Technology, PO Box 111, Suwon 440-600, Korea
Boreskov Institute of Catalysis, SB RAS, pr. Ak. Lavrentieva 5, Novosibirsk 630090, Russia
2
3
Received 6 September 2004
Published online 13 July 2005 – ꢀc EDP Sciences, Societ `a Italiana di Fisica, Springer-Verlag 2005
x
Abstract. CN nanotubes have been prepared by acetonitrile decomposition over Ni, Co and Ni/Co cata-
lysts. X-ray photoelectron spectroscopy study on the samples revealed a change of nitrogen concentration
and shape of N 1s line with variation of the catalyst used. Quantum-chemical calculations on tube fragments
showed the energy of N 1s level depends on the atomic structure of carbon tube and kind of incorporated
nitrogen. The largest binding energies were found to be characteristic of three-coordinated nitrogen atoms
doping the zigzag and chiral carbon nanotubes.
PACS. 61.46.+w Nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals – 71.23.An
Theories and models; localized states – 79.60.-i Photoemission and photoelectron spectra
1
Introduction
the atomic structure of pristine tube [6]. The impurity
state is totally delocalized in the case of the armchair tube
and spatially localized in the semiconducting zigzag tube.
Electron localization makes the impurity site chemically
and electronically active.
Electronic properties of carbon nanotubes are strongly
sensitive to the atomic arrangement of tube, namely,
diameter, chirality, defects and impurities [1]. Nitrogen
atom, having a size close to that of carbon atom, is an
ideal substitution impurity, which can be easy incorpo-
rated into the tube walls through various synthetic tech-
niques [2]. Nitrogen doping was demonstrated to improve
electric conductivity, field electron emission properties of
carbon nanotubes and their chemical activity towards the
gaseous molecules [3]. Electron energy loss spectroscopy
Recently we have demonstrated that composition of
the catalyst, used in chemical vapor deposition (CVD)
process, can influence on the proportion of different kinds
of nitrogen in multiwall CNx nanotubes [7]. The greatest
proportion of the pyridine-like nitrogen was found for the
sample obtained with the Ni/Co 1:1 catalyst. The equal
quantity of Ni and Co in the catalyst is likely to provide
the higher solubility of nitrogen in metal particle and more
high kinetics of growth of the CNx nanotube that result in
formation of many vacancies with nitrogen atoms on the
boundaries. The purpose of the present work is to study
a change of binding energy of N 1s electrons for doped
tubes differed by the atomic structure. We invoke ab initio
calculations on armchair, zigzag, and chiral carbon tubes
doped with three-coordinated and pyridinic nitrogen for
interpretation of XPS measured for CNx nanotubes syn-
thesized using different catalysts.
(
EELS) and X-ray photoelectron spectroscopy (XPS) have
revealed two types of bonding of the nitrogen and carbon
within the hexagonal network [4]. The higher binding en-
ergy in the N 1s spectra corresponds to three-coordinated
nitrogen atoms replacing carbon ones, the lower bind-
ing energy is attributed to pyridinic nitrogen incorpo-
rated into tube walls with a vacancy defect formation.
The holes in the CNx nanotube network have been ob-
served using scanning tunneling microscopy (STM) [4].
Density functional theory (DFT) calculations on metallic
armchair carbon nanotubes doped with different kinds of
nitrogen showed that three-coordinated atom produces a
donor energy level, while pyridinic atom acts as an ac- 2 Experimental
ceptor impurity [5]. This suggests a way to tune the elec-
tronic property of carbon nanotubes by adjusting methods CNx nanotubes were obtained using a CVD method de-
for incorporation of certain form of nitrogen. Moreover, scribed in details elsewhere [8]. The catalysts were synthe-
the electronic state of three-coordinated substitutional ni- sized by the thermal decomposition of bimaleates of Ni
trogen was found using DFT computations to depend on and Co and their mutual solid solutions with the ratios
of Ni/Co equal to 1:1. CNx nanotubes grew via pyrolysis
a
◦
e-mail: bul@che.nsk.su
of acetonitrile in an argon flow (3 l/min) at 850 C and

272
The European Physical Journal D
Fig. 1. Transmission electron micrographs of the samples produced by acetonitrile pyrolysis over Ni, Ni/Co, and Co catalyst.
Fig. 2. N 1s spectra measured for the samples produced using Ni, Ni/Co, and Co catalyst. The spectra were fitted by
three Gaussian components.
atmospheric pressure. The synthesized samples were stud- 4 Results and discussion
ied with a transmission electron microscope JEOL-100C
and an XPS spectrometer Quantum 2000 Scanning Transmission electron microscopy (TEM) images of the
ESCA Microprobe. The concentration of nitrogen in the samples obtained are presented in Figure 1. The synthe-
samples was estimated from the ratio of the areas of the sized materials contain multiwall carbon nanotubes, cata-
N 1s and C 1s lines with taking into account the pho- lyst particles capsulated into graphite shells, and amor-
toionization cross-sections. The samples obtained over Ni phous carbon. The maximal amount of nanotubes was
and Ni/Co catalysts involve ∼1.2% of nitrogen, using of found in the sample obtained over Ni/Co catalyst. Using
the Co catalyst reduces the nitrogen concentration up to of Ni and Co catalysts yields carbon nanotubes with diam-
∼
0.7%.
eter varying within 20–40 nm. The sample obtained over
Ni/Co catalyst contains tubular structures of diameter
6
0–90 nm and thinner nanotubes 15–30 nm in diameter.
The XPS of N 1s line measured for the samples pro-
duced are compared in Figure 2. The spectra exhibit
two peaks located around 399.0 and 401.4 eV. The relative
3
Calculations
Geometry of fragments of nitrogen-doped carbon tubes intensity of the peaks is changed with catalyst variation.
was relaxed in Hartree-Fock self-consistent field using The spectra were fitted by three Gaussian components of
3
-21G basis set within the quantum chemical package the width 2.1 eV. The binding energy corresponding to
Jaguar [9]. This approach calculates ground state of a the components and their energy separation are collected
system while the XPS lines arise in the result of sample in Table 1.
ionization. Thus, for the XPS spectra interpretation, the
To interpret the experimental data we calculated
calculated N 1s level energy (E) should be corrected to three tubes, which are close in diameter and drastically
take into account the relaxation for the ionized state. The differed in the atomic arrangement (Fig. 3). Two bonds of
correction factor was determined from the correlation de- carbon hexagons are oriented perpendicularly and along
pendence between the experimental and theoretical values a tube axis in armchair (7,7) and zigzag (12,0) carbon
for ten nitrogen-containing molecules [7]. The theoretical tube respectively. The (10,4) carbon tube has chiral struc-
BE
binding energies E
linear dependence E
were computed using the derived ture. Two different nitrogen impurities were introduced in
BE
= 75.69 + 0.78271E.
a carbon tube: (1) a three-coordinated atom and (2) three

L.G. Bulusheva et al.: Electronic state of nitrogen incorporated into CN
x
nanotubes
273
Table 1. Parameters of the N 1s spectra of samples synthesized using different catalysts.
Catalyst composition Binding energy for feature (eV) Separation of features (eV)
A
B
C
A and B
2.4
B and C
3.6
Ni
Ni/Co
Co
399.1
399.1
398.8
401.5
401.3
401.3
402.7
402.7
402.7
2.2
3.6
2.5
3.9
Fig. 3. Optimized geometry of fragments of nitrogen-doped carbon nanotubes. Nitrogen atoms are indicated by dark circles,
hydrogen atoms (white circles) saturate dangling bonds at the fragment boundaries.
Table 2. Energy of N 1s level, calculated for pyridinic (Epyr
)
Comparison between the experimental data and calcu-
and three-coordinated (Eth) nitrogen incorporated into frag- lation results (Tabs. 1 and 2) shows the feature A of the
ments of carbon tubes and graphite, and difference between N 1s spectra of CN nanotubes corresponds to the pyri-
x
these values (∆E).
dinic nitrogen. The features B and C can be attributed
to the three-coordinated nitrogen atoms incorporated re-
Fragment
graphite
E
pyr(eV)
399.1
399.2
399.5
398.9
E
th(eV)
∆E (eV)
2.6
spectively into armchair tubes and zigzag or chiral tubes.
401.7
401.9
403.2
403.3
The N 1s spectrum of the CN nanotubes synthesized over
x
Co catalyst exhibits the greatest relative intensity of the
(
7,7) tube
2.7
feature C and the lowest energy of the feature A. Both
these facts suggest that Co particles promote growth of
chiral nanotubes. The Ni and Ni/Co catalysts produce the
carbon nanotubes with similar arrangement, being arm-
chair or zigzag type mainly. However, the Ni/Co solid so-
lution provides large inserting of pyridinic nitrogen into
(
(
12,0) tube
10,4) tube
3.7
4.4
pyridinic atoms located at the edges of one-atomic va- tube walls. The N 1s spectrum of CNx nanotubes synthe-
cancy. The boundary dangling bonds of fragments were sized using Ni/Co catalyst shows the smallest separation
saturated by hydrogen atoms and the nitrogen impurities of the features A and B. The calculation of N 1s elec-
were maximally spaced from fragment boundary and from tron energies for doped graphite fragments [7] has shown
each other. The energy of N 1s levels was corrected in ac- the decrease of A and B features distance can be expected
cordance with the correlation dependence, described in the when nitrogen atoms occupy one or two sites at the bound-
Calculations part, and the work function of the spectrome- aries of one-atomic vacancy.
ter (6 eV). The values obtained for the pyridinic and three-
coordinated nitrogen incorporated into the carbon tubes
In summary, the N 1s spectra of CNx nanotubes were
are listed in Table 2 together with those for nitrogen atoms shown to be decomposed on three components which rela-
embedded into the graphite fragments [7]. The 1s electron tive intensity and separation depend on the catalyst, used
energies for the pyridinic nitrogen atoms have the similar in the CVD process. The ab initio Hartree-Fock calcu-
values for all considered carbon structures, while those lations on the fragments of nitrogen-doped carbon nan-
for the three-coordinated nitrogen atoms are strongly de- otubes indicated the low-energy spectral component cor-
pendent of a tube arrangement. The obtained variation in responds to the pyridinic nitrogen. The electronic state of
the 1s energies for the latter kind of nitrogen could be due three-coordinated nitrogen atoms was found to be largely
to a difference in local distribution of density associated sensitive to the atomic arrangement of carbon nanotube
with the unpaired electron. The difference has been found that leads to appearance of two high-energy components
to be caused by breaking of the left-right mirror symme- in N 1s spectrum of CNx nanotubes. The relative intensity
try in zigzag carbon nanotube with the nitrogen impurity of these components varies with change of a catalyst and
inserting [6].
could be a mark for indication of chiral tubes formation.

274
The European Physical Journal D
The work was financially supported by the Russian Basic Re-
search Foundation (grants 03-03-32286, 03-03-32336) and Eu-
ropean Office of Aerospace Research and Development (grant
CRDF RUP1-1501-NO-04).
4. M. Terrones, P.M. Ajayan, F. Banhart, X. Blas ´e , D.L.
Carroll, J.C. Charlier, R. Czerw, B. Foley, N. Grobert, R.
Kamalakaran, P. Kohler-Redlich, M. Ruhle, T. Seeger, H.
Terrones, Appl. Phys. A 74, 355 (2002)
5
6
7
. M. Zhao, Y. Xia, J.P. Lewis, R. Zhang, J. Appl. Phys. 94,
2398 (2003)
. H. Nevidomskyy, G. Cs ´a nyi, M.C. Payne, Phys. Rev.
Lett. 91, 105502 (2003)
. A.G. Kudashov, A.V. Okotrub, L.G. Bulusheva, I.P.
Asanov, Yu.V. Shubin, N.F. Yudanov, L.I. Yudanova, V.S.
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