Magnetic properties of dense carbon nanospheres prepared by chemical vapor deposition

Article abstract of DOI:10.1016/j.cplett.2007.09.036

Carbon nanospheres (? ～ 50 nm) prepared by chemical vapor deposition exhibit Curie-type paramagnetism, with a localized spin density N_S = 4.01 × 10¹⁹ spins/g. The spheres are solid, as shown by their cross-sections. Their low surface area excludes the presence of exposed graphene layers at the surface being the origin of paramagnetic centers. Inside the carbon spheres, HRTEM reveals a concentric wavy pattern of graphitic (sp²-bonded) clusters (size < 2 nm) embedded in disordered carbon. The high density of carbon atoms with sp²-sp³ hybrid orbitals acting as paramagnetic centers, at interfaces between graphitic (sp²-bonded carbon) clusters and amorphous carbon, can explain the magnetic measurements.

Full text of DOI:10.1016/j.cplett.2007.09.036

Available online at www.sciencedirect.com
Chemical Physics Letters 447 (2007) 295–299
www.elsevier.com/locate/cplett
Magnetic properties of dense carbon nanospheres prepared
by chemical vapor deposition
a,b,
c
c
d
*
´
´
´
, Amılcar Labarta , Xavier Batlle , Trinitat Pradell ,
Jose M. Calderon-Moreno
a
e
Daniel Crespo , Vu Thien Binh
a
`
Applied Physics Dept., Univ. Politecnica de Catalunya, EPSC, Castelldefels 08860, Spain
b
`
Center for Research in Nanoengineering, CRNE, Univ. Politecnica de Catalunya, Spain
Fundamental Physics Dept., Univ. Barcelona, Av. Diagonal 647, 08028, Barcelona, Spain
c
d
`
Physics and Nuclear Engineering Dept.,Univ. Politecnica de Catalunya, ESAB, Castelldefels, 08860, Barcelona, Spain
e
Equipe Emission Electronique, LPMCN-CNRS, Universite´ Lyon 1, Villeurbanne 69622, France
Received 15 January 2007; in final form 12 September 2007
Available online 15 September 2007
Abstract
Carbon nanospheres (B ꢀ 50 nm) prepared by chemical vapor deposition exhibit Curie-type paramagnetism, with a localized spin
density NS = 4.01 · 1019 spins/g. The spheres are solid, as shown by their cross-sections. Their low surface area excludes the presence
of exposed graphene layers at the surface being the origin of paramagnetic centers. Inside the carbon spheres, HRTEM reveals a con-
centric wavy pattern of graphitic (sp2-bonded) clusters (size < 2 nm) embedded in disordered carbon. The high density of carbon atoms
with sp2–sp3 hybrid orbitals acting as paramagnetic centers, at interfaces between graphitic (sp2-bonded carbon) clusters and amorphous
carbon, can explain the magnetic measurements.
ꢀ 2007 Elsevier B.V. All rights reserved.
1. Introduction
ablation [4,5], pitch-based activated carbon filaments with
specific surfaces up to 3000 m2/g [6] or heat treated nano-
powders produced from explosion-induced techniques
and disordered at the atomic level with mixed sp2:sp3 bond-
ing [7]. The presence of non-bonding p-electrons at the
edges of nanometer size graphene clusters have been pro-
posed to explain Curie-like paramagnetic contributions
[7]. A tight-binding analytical model [8] has predicted a
weak paramagnetic contribution from edge states localized
at zig-zag edge-carbon atoms in nanosized graphene rib-
bons, but the presence of localized edge states of graphene
layers in the absence of graphitic clusters is yet an open
question in non-graphitic disordered carbon.
Paramagnetic centers in annealed a-C:H films have been
associated with intraband electronic transitions between
localized states near the Fermi energy [9]. Druz et al con-
sider that paramagnetic centers (PC) are associated to
stress concentrations in diamond-like carbon films, either
at boundaries between sp2 and sp3 clusters or between
sp3 clusters with different orientations [10]. They have
Crystalline carbon allotropes are all diamagnetic: dia-
mond, graphite, fullerenes and carbon nanotubes, either
single wall or multiwall, exhibit significant diamagnetism
[1]. Disordered non-crystalline forms of solid carbon, such
as diamond-like carbon or tetrahedral amorphous carbon
(ta-C) and amorphous carbon (a-C) generally exhibit also
diamagnetic behavior. Increasing the disorder brings about
the reduction of diamagnetism [2]. Different groups [3–6]
have obtained carbon materials disordered at the atomic
scale, with surface areas similar to the surface area of a sin-
gle graphene sheet, exhibiting paramagnetic behavior: car-
bon thin films obtained by pulsed laser deposition [3],
nanofoams obtained by high-repetition-rate pulsed laser
*
Corresponding author. Address: Applied Physics Dept., Univ. Polit-
`
ecnica de Catalunya, EPSC, Castelldefels 08860, Spain. Fax: +34 93
4137007.
´
E-mail address: jose.calderon@upc.edu (J.M. Calderon-Moreno).
0009-2614/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2007.09.036

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J.M. Caldero´n-Moreno et al. / Chemical Physics Letters 447 (2007) 295–299
questioned the association of PC with sp2 clusters for ta-C
films since the maximum concentration of paramagnetic
centers was observed in ta-C films with the minimum con-
tent of the graphite phase [11,12]. Shibayama et al also
observed that the spin density decreases with the increase
in the size of sp2 clusters [6]. Those results indicate that
the density of paramagnetic centers must correlate rather
with the surface area of graphitic domains and not with
their volume fraction. A simultaneous effect is the increase
in anti-ferromagnetic interaction between the localized
spins. A higher interaction is caused by a closer vicinity
of the interacting spins. Those observations agree well with
the location of spin centers at the boundaries of graphitic
domains, thus explaining the strong paramagnetism
observed in sp2-bonded carbon with high specific surfaces
and the reduction in the number of paramagnetic centers
with increasing sizes of graphitic domains, due to the
reduction of surface area for higher domain sizes.
2.2. Synchrotron
Measurements were taken at station 9.6 of the Synchro-
tron Radiation Source (SRS) at Daresbury Laboratory.
SR-XRD diffraction data were collected with an intense
beam with high energy photons (E = 14.25 KeV,
˚
k = 0.87 A), and a small (100 lm) beam footprint defined
by the aperture of a cylindrical collimator. The XRD pat-
tern was collected in transmission geometry by a QUAN-
TUM-4 CCD area detector. Powders were supported in a
0.5 mm quartz capillary which could be oscillated by a
given angle about the phi-axis during exposure. In all cases
the background scatter due to the support and air was sub-
tracted before to data analysis using the ESRF package
FIT2D [21].
2.3. Magnetic characterisation
To our knowledge, paramagnetism has been previously
reported in pure carbon only in porous bodies and films
with high surface area (>300 m2/g) [4–7], except for a
recent report [13] of the magnetic properties of micro-sized
carbon filaments grown under strong electric and magnetic
fields [14,15], obtained via the thermal dissociation of
mesitylene, method used peviously to produce spherical
carbon bodies made of nanosized sp2-bonded graphitic
clusters [16,17].
Magnetic characterisation using a superconductor quan-
tum interference device (SQUID) magnetometer (quantum
design) in the temperature range of 1.7–300 K under
magnetic fields up to 5.0 T revealed a Curie-type para-
magnetic behavior, with a magnetic susceptibility v =
3.7 · 10À6 emu/g at 2 K. The sample was rolled up with
plastic para-film and was suspended from a cotton thread
in order to minimize non-intrinsic magnetic contributions
during SQUID measurements. The diamagnetic magneti-
zation coming from the para-film was determined previ-
ously and subtracted from the total signal.
2. Experimental
3. Results and discussion
2.1. CVD growth process of carbon nanopearls
The current study reports the magnetic properties of
nanosized carbon spheres, called hereupon ‘nanospheres’
with ꢀ85% monodispersity and a solid non-porous struc-
ture composed of both amorphous and disconnected nano-
crystalline (ꢀ2 nm) regions. They were synthesized by
CVD using a mixture of acetylene, as the source of carbon,
and nitrogen (20% C2H2, 80% N2). Surface area
determined by BET was 15 m2/g [18–20], much reduced
compared with that of carbon nanofoam made of nanocl-
usters of pure carbon (from 300 to >1000 m2/g) [4,5].
TEM measurements of the as-obtained product showed
spherical particles organized into a 3D foam-like configu-
ration, as shown in Fig. 1. Elemental analysis by energy
dispersive X-ray spectroscopy (EDX) detected only carbon
element.
Fig. 2 shows the typical X-ray diffraction pattern
obtained using synchrotron radiation (SR-XRD). The pat-
terns in Fig. 2 clearly show the 002 and 100/101 reflec-
tions of graphitic carbon. The width of the peaks results
from the small size of the graphitic domains. The interlayer
spacing of graphitic layers calculated from the 002 diffrac-
tion line is roughly 0.39 nm, compared to about 0.34 nm
for macrocrystalline graphite. The plane coherent length
or crystalline domain size of the sp2-bonded clusters in
the basal plane, a, and in the graphene layers stacking
A catalytic chemical vapor deposition (CVD) process at
ambient pressure was used to produce the carbon nano-
spheres [18–20]. The catalysts were Ni nanoparticles
100 nm in diameter, synthesized by a sol–gel combustion
process. For a typical procedure, an aqueous solution con-
taining nickel acetate tetrahydrate and methyl hydrazine
was mixed in propan-2-ol solution at room temperature.
With constant stirring, during a day, the dissolution of
metal acetate was obtained. This solution was dried at
ꢀ50 ꢁC in air atmosphere until complete evaporation of
the solvent. The resulting complex powder was heat treated
at 400 ꢁC for 1 h under nitrogen atmosphere. Finally, by a
mechanical filtration, the powder obtained was calibrated
in order to have Ni nanoparticles with a diameter in the
range of 100 nm. Metallic W tips having apex radius in
the range of 5–10 lm were covered with these Ni catalysts,
before putting them in the CVD reactor. The conditions for
the CVD growth process were the following: the feed gas
was C2H2 with a ratio C2H2:N2 kept constant at
80:400 sccm; the temperature was 700 ꢁC; and the deposi-
tion time was in the range of 10 s. The nanopearls grew
directly on the metallic tips covered with Ni catalyst having
apex radius in the range of 5–10 lm, as a uniform layer
with a thickness of about 20 lm.

J.M. Caldero´n-Moreno et al. / Chemical Physics Letters 447 (2007) 295–299
297
Fig. 1. TEM micrograph of the carbon nanospheres.
direction, c, were 1.42 and 2.22 nm, respectively, deter-
mined using Scherrer’s method on the 100 and 002 peaks.
The electron diffraction pattern in Fig. 2 displays diffuse
rings at the position of the 002 and 100/101 diffraction
features of graphite, indicating the presence of small clus-
ters of graphitic carbon. The electron diffraction results
are consistent with nanospheres made of graphite nano-
crystallites, sized about 1–2 nm, embedded in disordered
amorphous carbon, as it was confirmed by Raman mea-
surements [18–20]. The Raman spectra showed the two
characteristic peaks of disordered graphitic carbon, a G-
peak at 1590 cmÀ1 and a D-peak around 1350 cmÀ1
.
High resolution TEM measurements, shown in Fig. 3,
confirmed that the nanospheres were solid spheres showing
an internal architecture of curved layers made of crystalline
patches, ꢀ2 nm long, embedded in an amorphous matrix
and arranged concentrically in the spheres. The crystalline
nuclei were oriented preferentially, in a concentric arrange-
ment, perpendicularly to the radius and in parallel with the
surface of the nanosphere. This arrangement is typical of
carbon nanoparticles condensed from the gas phase [22].
Fig. 4 shows the temperature dependence of the mag-
netic susceptibility measured under 3 T. The carbon nano-
spheres exhibit paramagnetic behavior superimposed to the
constant contribution v0 arising from core and orbital dia-
magnetic terms, and Pauli paramagnetic susceptibility. The
total susceptibility at 2 K was v = 3.7 · 10À6 emu/g,
decreasing up to v = À4.1 · 10À7 emu/g at room tempera-
ture, compared to v values in the range 5–20 · 10À6 emu/
g at 2 K in carbon nanofoam [23]. The experimental sus-
ceptibility could be fitted by the sum of a temperature-
dependent Curie–Weiss term, C/(T À H) – the solid line
in Fig. 4a – originating from localized spins and a constant
diamagnetic susceptibility v0. From the obtained value of
the Curie constant, C = 2.5 · 10À5 emu K/g, and assuming
S = 1/2 the localized spin density was estimated to be
Fig. 2. Synchrotron X-ray diffraction and electron diffraction patterns of
the carbon nanospheres.
NS = 4 · 1019 spins/g. This spin density corresponds to an
average separation between spins of 2.45 nm. A small
negative value of the Curie temperature H = À4 K was
estimated by this fitting procedure, indicating the exis-
tence of weak anti-ferromagnetic interactions among the
localized spins. The obtained value of v0 was À0.47 ·
10À6 emu/g, which is about 6% of the value corresponding
to bulk graphite. The Curie–Weiss behavior related to the
localized spins could be also made evident by plotting the
reciprocal of the susceptibility obtained after subtracting
v0 from the total susceptibility v, as it is shown in the inset
in Fig. 4a. A further insight in the origin of this paramag-
netic signal can be gained by analyzing the magnetization
curves M(H,T) measured at low-temperatures (Fig. 4b).
The magnetization curve at 10 K and up 5T should be a
straight line for localized spins of S = 1/2. However, a
downward curvature is clearly observed below about
5000 Oe arising from the abrupt magnetization process of

298
J.M. Caldero´n-Moreno et al. / Chemical Physics Letters 447 (2007) 295–299
Fig. 4. (a) Temperature dependence of the susceptibility. Inset: reciprocal
of the susceptibility as a function of the temperature. (b) Magnetization
curves at low-temperature (symbols) fitted to the superimposition of two
Brillouin functions corresponding to S = 1/2 and SFM = 75 (solid lines).
Fig. 3. TEM and HRTEM micrographs of individual carbon nanospheres.
The small radius of curvature of the nanospheres, and
the corrugation at the atomic scale of the surface resulting
from the unclosed graphitic patches arranged in concentric
wavy architecture results in the observed high density of
localized spins. Our explanation agrees with that of Rode
et al. [5] for carbon foam made of nanoclusters. Our results
seem to support spin density calculations of Park et al pos-
tulating the presence of unpaired spins in graphitic nano-
structures with negative Gaussian curvature [24]. Our
results indicate that carbon bodies made of corrugated gra-
phitic carbon nanostructures (i.e., nanocells [25], curved
graphitic shells in core–shell nanoparticles [26,27], but also
carbon microbeads [28]) harbour unpaired spins by virtue
of their structure.
residual ferromagnetic (FM) impurities. The contribution
of these FM impurities to the total magnetic signal
can be estimated by fitting M(H,T = 10 K) to the superim-
position of two Brillouin functions corresponding to an
arbitrary spin SFM and S = 1/2, M(H,T = 10 K) =
AFMBSFM(H/T) + A1/2B1/2(H/T) + v0H = MFM(H/T) +
M1/2(H/T) + v0H, where AFM and A1/2 are constants which
define the relative importance of the two Brillouin func-
tions and v0 is fixed to the value obtained from the suscep-
tibility curve in Fig. 4a. The result of the fitting for
SFM = 75 is shown in Fig. 4b, in which both contributions
are also shown separately. Assuming that the FM contribu-
tion (MFM) is temperature independent below 10 K the rest
of the M(H,T) curves can be well fitted to M(H,T) =
MFM(H) + M1/2(H/T) + v0H (solid lines in Fig. 4b). It is
worth noting that good fittings can only be achieved with
S = 1/2, fact which corroborates the validity of the
assumption made for the S value of the localized spins.
Carbon nanospheres exhibit also excellent field emission
properties due to its geometry and specific surface structure
of densely packed unclosed graphitic domains, arranged in
concentric waves, with currents up to 50 lA, i.e., current
densities in the range of 1 A/cm2, readily obtainable [19].

J.M. Caldero´n-Moreno et al. / Chemical Physics Letters 447 (2007) 295–299
299
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Spherical carbon nanoparticles of uniform diameter
around 50 nm obtained by CVD using acetylene as the
source of carbon exhibit significant low-temperature para-
magnetic behavior. The non diamagnetic behavior and the
high density of localized spins acting as PC is related to the
characteristic architecture of the carbon nanospheres,
made of open graphitic domains arranged concentrically
and densely packed in an amorphous matrix, and to the
homogeneity of the nanospheres, nearly monodisperse.
Similar magnetic properties have been previosuly observed
in a nanofoam constituted from nanoclusters with similarly
curved internal layers, despite the fact that the nanofoam
was produced by a different synthesis method under very
different conditions. Nanospheres clearly harbour unpaired
spins by virtue of their structure rather than formation con-
ditions. It is the first time that significant paramagnetism
have been measured in dendritic aggregates of solid gra-
phitic carbon bodies with low specific surface area
(15 m2/g), rather than in mesoporous carbon, excluding
that exposed graphene layers at the surface of nanoclusters
are the only origin of paramagnetic centers.
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Finantial support from grants MAT2004-01214,
´
MAT2006-03999 (Ministerio de Educacion y Ciencia)
SGR2005-00535 and SGR2005-00969 (DURSI, Generali-
tat de Catalunya) and CGR 16.01.663 in the beamline
BM-16 (ESRF) are gratefully acknowledged.
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