Investigation of Co nanoparticles with EXAFS and XANES

Article abstract of DOI:10.1016/j.cplett.2004.10.095

3, 5, and 12-nm Co nanoparticles were synthesized via thermo-decomposition. Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) were used to probe the structures of these nanoparticles. Simulations were carrie

Full text of DOI:10.1016/j.cplett.2004.10.095

Chemical Physics Letters 400 (2004) 122–127
www.elsevier.com/locate/cplett
Investigation of Co nanoparticles with EXAFS and XANES
1
Guangjun Cheng , Joshua D. Carter, Ting Guo
*
Chemistry Department, University of California, One Shields Avenue, Davis, CA 95616, USA
Received 5 August 2004; in final form 15 October 2004
Available online 11 November 2004
Abstract
3, 5, and 12-nm Co nanoparticles were synthesized via thermo-decomposition. Extended X-ray absorption fine structure
(EXAFS) and X-ray absorption near-edge structure (XANES) were used to probe the structures of these nanoparticles. Simulations
were carried out to compare the EXAFS profiles of three different crystalline phases of Co. The results of EXAFS and XANES
suggested that the reactivity of these nanoparticles towards their environment is inversely proportional to their size. Sharp decreases
in the amplitude of EXAFS of these nanoparticles were observed, which could be attributed to the existence of the disorder in those
crystalline nanostructures.
Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction
it is well suited for determining the local structures of
both non-crystalline and crystalline materials. A number
of colloidal nanocrystals have been characterized by
EXAFS and XANES, including CdS [12], CdSe [13],
SnO2 [14], Manganese oxides [15,16], MoS2 [17], and
Au [18,19].
The aim of this study is to characterize the structure
and stability of as-prepared Co nanoparticles, and the
outcome of which will be useful to in situ EXAFS meas-
urements of these Co nanoparticle catalysts. We will
describe the results of the synthesis and characterization
of three different size Co nanoparticles using XAS,
transmission electron microscopy (TEM), and atomic
force microscopy (AFM). The X-ray absorption meas-
urements were performed under both ambient and
anaerobic conditions. Theoretical simulations were car-
ried out to illustrate the effect of crystalline structures on
EXAFS.
Nanoscale materials are of great importance because
of their potential applications in electronics, optics, and
catalysis [1,2]. The properties of these nanoscale materi-
als can be expected to be between those of the bulk and
isolated atoms. To exploit their full application poten-
tials, it is important to thoroughly investigate the struc-
tural, electronic, magnetic and optical properties of
these nanomaterials. Recently X-ray absorption spectr-
oscopy (XAS), in particular extended X-ray absorption
fine structure (EXAFS) and X-ray absorption near-edge
structure (XANES), has been used as powerful tools for
studying the structures and dynamics of the nanoscale
materials [3–9].
EXAFS have been used extensively in the investiga-
tion of local atomic structures such as the number and
type of neighboring atoms, inter-atomic distances, and
disorder. Since the application of EXAFS does not re-
quire the materials to have a long-range order [10,11],
2. Experimental
*
Corresponding author.
E-mail address: tguo@ucdavis.edu (T. Guo).
Dicobalt octacarbonyl Co2(CO)8 containing 1–5%
hexane stabilizer, oleic acid (OA, 99%), anhydrous
o-dichlorobenzene (DCB, 99%), tri-octylamine (TOA,
1
Current address: Physics Laboratory, National Institute of
Standards and Technology, Gaithersburg, MD 20899, USA.
0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2004.10.095

G. Cheng et al. / Chemical Physics Letters 400 (2004) 122–127
123
98%), cobalt(II) oxide and boron nitride were purchased
from Aldrich. Tri-octylphosphine oxide (TOPO, 90%)
was purchased from Alfa Aesar. All chemicals were used
without further treatment.
EXAFS data were collected using the XAS program at
SSRL.
EXAFS data analysis was performed using EXAF-
SPAK [22]. The subtraction of polynomial fit pre-edge
and spline-fit atomic absorption curves, and k3 factor
multiplication were all built-in in the EXAFSPAK pack-
age. The processed EXAFS data in k reciprocal space
was directly obtained from that in the energy space.
The radial distribution function (RDF) in real space
was then obtained via Fourier transformation of the
Co nanoparticles were synthesized using standard
procedures [20,21]. In brief, TOPO was degassed in Ar
in a three-neck flask for 20 min. A 15 mL OA in DCB
solution was introduced into the flask under Ar. The
solution was then heated to the reflux temperature of
DCB (ꢀ182 °C). 0.54 g of Co2(CO)8 diluted in 3 mL
of DCB was quickly injected into the refluxing solution.
In order to obtain different sizes of Co nanoparticles,
different combinations of surfactants were used. Typi-
cally, the combination of 0.1 mL OA, 0.9 mL TOA
and 0.05 g TOPO was used to produce 3 nm nanoparti-
cles; the combination of 0.2 mL OA and 0.1 g TOPO
yielded 5 nm nanoparticles; and the combination of
0.1 mL OA and 0.2 g TOPO was used to make 12 nm
nanoparticles.
Nanoparticle size, morphology, structure, and se-
lected area diffraction (SAD) were probed using a
Philips CM-12 TEM (100 kV). TEM samples were pre-
pared by dropping the colloids solution onto carbon-
coated TEM grids. AFM (Nanoscope IIIA, Digital
Instruments) measurements operated in the tapping
mode were performed to determine the size of 3-nm
nanoparticles. AFM samples were prepared by immers-
ing a Si wafer cleaned with Piranha solution (4:1 conc.
H2SO4:H2O2 (30%)) into the 3-nm nanoparticle colloid
solution for several minutes, followed by rinsing with
toluene and drying with Ar gas.
EXAFS measurements were performed on the K edge
of Co in Co nanoparticles and a 3-lm thick Co thin foil
standard. Co nanoparticle samples were prepared under
the ambient condition by directly dropping the colloid
Co nanoparticle solutions onto Kapton tapes to form
thin layers in air. Another set of Co nanoparticle samples
were prepared using the same method of dropping and
drying, but all the procedures were performed in a dry-
box (<1 ppm oxygen and water). After dried in the dry-
box, the samples were sealed with Kapton tapes, and then
taken out of the dry-box. CoO sample was prepared by
grinding with boron nitride and pressing into a pellet.
The experiments were performed at the beamline 4–1
at the Stanford Synchrotron Radiation Laboratory
(SSRL) with a double silicon crystal (220) monochroma-
tor [5]. Harmonics were suppressed by detuning the crys-
tal spectrometer. All the samples were measured at room
temperature. EXAFS data were obtained by detecting
the fluorescence of K lines from Co in Co nanoparticles
using a Lytle detector. Simultaneously, transmission
mode detected with ion chambers was employed to
acquire the EXAFS data on the standard Co foil. In flu-
orescence mode, thin Mn foil filter placed in front of the
Lytle detector was used to reduce the scattering of inci-
dent X-rays by the sample and sample holder. The
˚
EXAFS data in k space. A 1–5 A range was used for
displaying the RDFs.
3. EXAFS simulations
The EXAFS patterns for three different phases of Co
bulk, face-centered cubic (fcc), hexagonal close packed
(hcp) and epsilon-Co were simulated. For hcp Co, the
program ÔATOMSÕ was used to generate the Ôfeff.inpÕ files,
which were used for EXAFS simulation using FEFF8
[23,24]. The Ôfeff.inpÕ file provided a list of atomic coor-
dinates in the crystal. For other two phases of Co, fcc
and epsilon phase, the Ôfeff.inpÕ files were generated
based on their X-ray crystallography data. For all the
simulations, the Debye temperature and the measuring
temperature were set to be 450 and 300 K, respectively.
We assumed that Co nanoparticles had the same Debye
temperature as the bulk.
The generated Ôfeff.inpÕ files were used in FEFF8 pro-
gram, which calculated extended X-ray absorption fine
structure (EXAFS) using an ab-initio self-consistent real
space multiple scattering (RSMS) approach [23,24].
F
EFF8 program generated a large number of scattering
˚
paths. All scattering paths shorter than 5.0 A, including
multiple scatterings, were used to simulate the EXAFS
pattern. The OPT program in EXAFSPAK were used to
treat all these scattering paths, which produced the sim-
ulated EXAFS results and their corresponding FFT
patterns.
4. Results and discussions
We simulated the EXAFS patterns for three different
crystalline phases of Co bulk: fcc, hcp and epsilon. For
fcc and hcp Co bulk, there is only one type of Co atom
in a unit cell, in which all the Co atoms have the same
surrounding environments. However, there are two
types of Co atoms in epsilon, which have different sur-
rounding environments in a unit cell [25]. Thus, two
Ôfeff.inpÕ files were generated for two types of Co atoms
appearing in one unit cell.
Fig. 1a shows the EXAFS simulation results for two
types of Co atoms. The total EXAFS for epsilon-Co and
their corresponding FFT patterns are shown in Fig. 1b.

124
G. Cheng et al. / Chemical Physics Letters 400 (2004) 122–127
Fig. 2. Fourier transforms of three different phases of Co, fcc, hcp and
epsilon.
lower due to the fact that the atoms in epsilon-Co are
not closely packed. In addition, the peak positions for
higher shells are different from those in fcc or hcp-Co.
˚
There are two peaks at 3.5 and 4.5 A, which do not ap-
pear in fcc-Co or hcp-Co either. The two peaks around
˚
3.2 and 3.8 A in fcc-Co and hcp-Co do not show up in
epsilon-Co. Therefore, EXAFS can be used to differen-
tiate these three different phases of Co bulk, provided
that the Co material under investigation has a well-
defined crystalline structure.
Fig. 3a shows the TEM micrograph of 12-nm Co
nanoparticles, and Fig. 3b shows the SAD pattern on
this sample. It indicates that 12-nm Co nanoparticles
adopt epsilon-Co. Fig. 3c shows the TEM micrograph
of 5-nm Co nanoparticles. Although 12-nm Co nano-
particles give a clear electron diffraction pattern, which
indicates that there is a long range ordering in it, 5-
and 3-nm Co nanoparticles do not. This could be caused
by the limited amount of samples in selected area or
amorphous/crystalline mixture or oxidations of these
small nanoparticles.
Fig. 1. Co K EXAFS simulations of two types of Co atoms in epsilon-
Co and the epsilon-Co bulk (a), and the corresponding Fourier
transforms (b).
Because of the limitation of the resolution of the
TEM used here, AFM was used to characterize 3-nm
Co nanoparticles (Fig. 3d). The section analysis shown
in Fig. 3e clearly shows that the average size of nano-
particle is ꢀ3 nm.
The simulated FFT patterns from three different
phases of Co bulk are shown in Fig. 2. From these simu-
lations, it can be seen that the FFT patterns of fcc and
˚
hcp are similar in the first shell (2.0 A) and the second
˚
˚
shell (3.3 A). The third peak (3.8 A) in the hcp structure
has a lower magnitude, and no obvious peak on the
fourth shell for hcp-Co. However, the fourth peak (4.6
Fig. 4a show normalized XANES spectra of as-pre-
pared Co nanoparticles in an open-air environment at
room temperature. XANES from a standard Co foil
and CoO are also shown as the references. As shown
in Fig. 4a, the XANES pattern of the 12-nm sample
˚
A) is intense for fcc due to the multiple scattering. The
FFT of epsilon-Co is much different from that of fcc-
Co and hcp-Co. The magnitudes for all the peaks are

G. Cheng et al. / Chemical Physics Letters 400 (2004) 122–127
125
Fig. 3. TEM micrographs of 12-nm Co nanoparticles (a), 5-nm Co
nanoparticles (c), and selected area diffraction pattern for 12-nm Co
nanoparticles (b). AFM images for 3-nm Co nanoparticles (d) and its
section analysis (e).
resembles closely to that of the Co foil, which has a sig-
nificant shoulder around 7712 eV. The XANES of 5-nm
nanoparticles has a much smaller shoulder, and that of
3-nm nanoparticle shows almost no shoulder around
7712 eV. In the meantime, the patterns between 20
and 40 eV above the absorption edges of these three
samples are all somewhat between that of Co bulk and
CoO, with 12 nm nanoparticles closest to the bulk,
and 3 nm particles closest to CoO. These results can
be explained by the reactions of these nanoparticles with
oxygen in the air to form oxides.
The XANES results for the Co nanoparticles pre-
pared in the dry-box are shown in Fig. 4b. The 12-nm
nanoparticles show the same result as the sample pre-
pared in the open-air environment. Under protection,
the XANES of 3- and 5-nm nanoparticles become
slightly closer to that Co bulk in the XANES region
20–40 eV above the edge. Therefore, the preparation
in dry-box protects both nanoparticles, to some extent.
From Fig. 4, it is obvious that the XANES patterns of
3- and 5-nm are still far from being identical to that of
CoO. Although the main component in those nano-
Fig. 4. Normalized XANES patterns for Co foil and 3, 5, 12-nm Co
nanoparticles prepared under ambient conditions (a), and under
anaerobic conditions (b).
particles is oxygen around Co, as can be seen below in
the EXAFS section, apparently these nanoparticles
adopt different structures than CoO bulk, possibly due
to the large portion of the atoms in the surface layer that
are bonded to surfactants.
Fig. 5 shows the FFT of the experimental EXAFS
data for three different sizes of Co nanoparticles sealed
in the dry-box. As can be seen from Fig. 5, the FFT pat-
tern for 12-nm nanoparticles is similar to the theoretical
epsilon-Co. The ripples in the low R range for the 12-nm
nanoparticles are caused by the noise of the EXAFS
data in the high k region. The main features in FFT

126
G. Cheng et al. / Chemical Physics Letters 400 (2004) 122–127
reported that a 31% and 39% reduction in the Fourier
transform peaks of the first and third shell for nanocrys-
talline fcc Cu compared to polycrystalline Cu, respec-
tively. It was suggested that these nanocrystalline Cu
structurally consists of two components with compara-
ble volume fractions: a crystalline component and a
grain boundary component, which exhibits a new solid
state structure with random atomic arrange. The reduc-
tion of the signal in nanocrystalline Cu was interpreted
by progressive decreasing of the effective coordination
number with increasing shell number and by a wide dis-
tribution of bond length in the grain boundaries. A 52%
reduction for the first shell was reported by the same
group for the EXAFS studies on nanocrystalline body-
centered-cubic (bcc) tungsten [28]. Sobczak and Dorozh-
kin [29] calculated EXAFS spectra for an Fe nanocrystal
in the form of a cube of size ranging from 3 to 10 nm.
However, the theoretical model calculations of grains
with an ideal surface cannot explain the significant
reduction of the experimental EXAFS magnitude for
nanocrystalline Fe. Therefore, they have postulated a
model of nanograins with a statistically disordered
boundary for nanocrystalline Fe. A more severe reduc-
tion to the peak of the first shell in FFT of nanocrystal-
line Co was observed by Babanov et al. [30], and they
suggested that the nanocrystalline Co consisted of an or-
dered crystalline phase and a disordered phase, and the
latter was responsible for the reduction of the magnitude
of the first peak in FT.
Fig. 5. Experimental EXAFS for 3, 5 and 12-nm Co nanoparticles
prepared under anaerobic conditions and the corresponding Fourier
transforms.
remained the same when the high k noise was removed.
˚
Two peaks at 2.1 and 3.5 A (phase uncorrected) are
seen. Therefore, 12-nm nanoparticles are considered to
adopt the epsilon structure, which matches the result
from SAD. For 5-nm nanoparticles, two peaks at 2.2
Therefore, we attribute the strong reductions of the
EXAFS oscillations and Fourier transformations for
our Co nanoparticles to the existence of the disordered
phase (or boundary) in the nanocrystalline structure.
A high-temperature annealing process may help reduce
the disorder, and improve the EXAFS and Fourier
transformation signal strength.
˚
˚
A (Co–Co) and 1.6 A (Co–O) can be seen, with the
Co–Co peak higher than the Co–O peak. Three-nano-
meter-nanoparticles also show two adjacent peaks at
˚
2.2 and 1.6 A, although the Co–Co peak is weaker than
the Co–O peak. The oxidation could happen due to
imperfect sealing of the sample holders. These results
show that the smaller the nanoparticles, the more reac-
tive they are.
However, there is a noticeable reduction to the mag-
nitude of EXAFS patterns even for 12-nm nanoparticles
in comparison with the theoretical EXAFS pattern for
epsilon-Co. The first peak is filtered and fitted with
Co–Co, and the best fit results are 3.36 for number of
5. Conclusions
XAS, TEM, and AFM have been used to characterize
three different sizes of Co nanoparticles synthesized by
the thermo-decomposition method. TEM and AFM
show that Co nanoparticles have average diameters of
roughly 3, 5, and 12 nm, respectively. Electron diffrac-
tion data indicate that 12-nm nanoparticles adopt
long-ranged ordering in the form of epsilon-Co struc-
ture. EXAFS has shown to be able to differentiate the
epsilon from fcc and hcp. The experimental measure-
ments of XAS spectra for three different sizes of Co
nanoparticles have shown that the smaller the nanopar-
ticles are, the more reactive they are. The existence of the
disorder phase (or boundary) in the nanocrystalline
structure may cause sharp decreases in the EXAFS pat-
terns and Fourier transformations for Co nanoparticles.
˚
the atoms in the first shell, 2.547 A for the radial dis-
tance, and 0.00772 for the square of Debye–Waller fac-
tor. For nanoparticles with the size of 12 nm, the surface
and size effects do not have a large impact on the EX-
AFS patterns [26]. On the basis of TEM and EXAFS
results, 12-nm nanoparticles adopt epsilon structure,
which means they are crystalline and possess a long
range ordering. However, the experimental EXAFS
and FFT results clearly show reduced magnitudes. For
instance, the first peak in FFT is reduced by 70%.
Several studies have been published concerning
EXAFS investigations of nanocrystalline materials
prepared by physical methods. Haubold et al. [27]

G. Cheng et al. / Chemical Physics Letters 400 (2004) 122–127
127
[11] E.E. Koch (Ed.), Handbook on Synchrotron Radiation, vol. 1,
North-Holland, New York, 1983.
Acknowledgements
[12] J. Rockenberger, L. Tro¨ger, A. Kornowski, T. Vossmeyer, A.
Eychmu¨ller, J. Feldhaus, H. Weller, J. Phys. Chem. B 101 (1997)
2691.
We thank the National Science Foundation (CHE
0135132) the Camille and Dreyfus Foundation for their
financial supports. We thank the excellent staff at Stan-
ford Synchrotron Radiation Laboratory (SSRL) for
experimental support. We are grateful to DOE for sup-
port of the facility.
[13] M.A. Marcus, L.E. Brus, C. Murray, M.G. Bawendi, A. Prasad,
A.P. Alivisatos, Nanostruct. Mater. 1 (1992) 323.
[14] S. Davis, A. Chadwick, J. Wright, J. Phys.Chem. B 101 (1997)
9901.
[15] T. Ressler, S.L. Brock, J. Wong, S.L. Suib, J. Phys. Chem. B 103
(1999) 6407.
[16] S. Hwang, J. Choy, J. Phys. Chem. B 107 (2003) 5791.
[17] T. Shido, R.J. Prins, Phys. Chem. B 102 (1998) 8426.
[18] R.E. Benfield, D. Grandjean, M. Kro¨ll, R. Pugin, T. Sawitowski,
G. Schmid, J. Phys. Chem. B 105 (2001) 1961.
[19] D. Zanchet, H. Tolentino, M.C. Alves, O.L. Alves, D. Ugarte,
Chem. Phys. Lett. 323 (2000) 167.
References
[1] Z.L. Wang (Ed.), Characterization of Nanophase Materials,
Wiley–VCH, Weinheim, 1999.
[20] V.F. Puntes, K.M. Krishnan, A.P. Alivisatos, Science 291 (2001)
2115.
[2] A.S. Edelstein, R.C. Cammarata (Eds.), Nanomaterials: Synthe-
sis, Properties and Applications, Institute of Physics Publishing,
Bristol, 1998.
[21] V.F. Puntes, D. Zanchet, C. Erdonmenz, A.P. Alivisatos, J. Am.
Chem. Soc. 124 (2002) 12874.
[3] J.H. Sinfelt, Bimetallic Catalysts: Discoveries, Concepts, and
Applications, Wiley, New York, 1983.
[22] Available from: <http://www-ssrl.slac.stanford.edu/exafspak.
html>.
[4] A.I. Frenkel, C.W. Hills, D. Johnson, R.G. Nuzzo, J. Phys.
Chem. B 105 (2001) 12689.
[23] Available from: <http://leonardo.phys.washington.edu/feff/>.
[24] Available from: <http://feff.phys.washington.edu/~ravel/software/
atoms/aboutatoms.html>.
[5] G. Cheng, T. Guo, J. Phys. Chem. B 106 (2001) 5833.
[6] M.S. Nashner, A.I. Frenkel, D. Somerville, C.W. Hills, J.R.
Shapley, R.G. Nuzzo, J. Am. Chem. Soc. 120 (1998) 8093.
[7] C.W. Hills, M.S. Nashner, A.I. Frenkel, J.R. Shapley, R.G.
Nuzzo, Langmuir 15 (1999) 690.
[25] D.P. Dinega, M.G. Bawendi, Angew. Chem., Int. Ed. Engl. 38
(1999) 1788.
[26] A. Jentys, Phys. Chem. Chem. Phys. 1 (1999) 4059.
[27] T. Haubold, R. Birringer, B. Lengeler, H. Gleiter, Phys. Lett. A
135 (1989) 461.
[8] P.N. Floriano, C.O. Noble, J.M. Schoonmaker, E.D. Poliakoff,
R.L. McCarley, J. Am. Chem. Soc. 123 (2001) 10545.
[9] G. Apai, J.F. Hamilton, J. Sto¨hr, A. Thompson, Phys. Rev. Lett.
43 (1979) 165.
[28] T. Haubold, W. Krauss, H. Gleiter, Philos. Mag. Lett. 63 (1991)
245.
[29] E. Sobczak, D.D. Dorozhkin, J. Alloys Comp. 286 (1999) 108.
[30] Y. Babanov, I.V. Golovshchinkova, F. Boscherini, T. Haubold,
S. Mobilio, Physica B 208 and 209 (1995) 140.
[10] D.C. Koningsberger, R. Prins (Eds.), X-ray Absorption: Princi-
ples, Applications, Techniques of EXAFS, SEXAFS, XANES,
Wiley, New York, 1988.