Structural study of bimetallic CoxRh1-x nanoparticles: Size and composition effects

Article abstract of DOI:10.1103/PhysRevB.69.235416

The structure of ultrafine bimetallic Co_xRh_1-x nanoparticles synthesized in mild conditions by codecomposition of organometallic precursors in the presence of a polymer or a ligand has been studied using high-resolution electron microscopy and wide-angle x-ray scattering techniques. While pure rhodium particles exhibit the main structural features of a face centered cubic (fcc), alloying with cobalt induces a progressive loss of periodicities, leading in high-cobalt-content particles to a polytetrahedral structure close to the one already encountered in pure-cobalt particles. When increasing the synthesis temperature, the polytetrahedral structure remains remarkably stable, while particles with higher rhodium content clearly evolve towards perfect fcc. Increasing the size of the particles up to 5-6 nm stabilizes the structural phases encountered in the phase diagram of the bulk alloy. Different element-sensitive techniques, x-ray absorption spectroscopy (XANES and EXAFS) and energy-filtering transmission electron microscopy, have also been implemented in order to get chemical information. Evidence is given for a cobalt surface segregation in these bimetallic particles, highly favorable for magnetic-moment enhancement.

Full text of DOI:10.1103/PhysRevB.69.235416

PHYSICAL REVIEW B 69, 235416 (2004)
Structural study of bimetallic Co Rh nanoparticles: Size and composition effects
x
1−x
M. C. Fromen, P. Lecante, and M. J. Casanove*
Centre d’Élaboration de Matériaux et d’Études Structurales, C.N.R.S., 29, Rue Jeanne Marvig, 31055 Toulouse Cedex 4, France
P. Bayle Guillemaud
CEA, Département de Recherche Fondamentale sur la Matière Condensée, SP2M 17, Rue des Martyrs, 38054 Grenoble, France
D. Zitoun, C. Amiens, and B. Chaudret
Laboratoire de Chimie de Coordination, C.N.R.S., 205, route de Narbonne, 31077, Toulousse Cedex 4, France
M. Respaud
Laboratoire de Physique de la Matière Condensée, I.N.S.A., 135 Avenue de Rangueil, 31077 Toulouse Cedex 4, France
R. E. Benfield
Functional Materials Group, School of Physical Sciences, University of Kent, Canterbury, CT2 7NR, United Kingdom
(
Received 15 December 2003; published 28 June 2004)
The structure of ultrafine bimetallic Co Rh1−x nanoparticles synthesized in mild conditions by codecompo-
x
sition of organometallic precursors in the presence of a polymer or a ligand has been studied using high-
resolution electron microscopy and wide-angle x-ray scattering techniques. While pure rhodium particles
exhibit the main structural features of a face centered cubic (fcc), alloying with cobalt induces a progressive
loss of periodicities, leading in high-cobalt-content particles to a polytetrahedral structure close to the one
already encountered in pure-cobalt particles. When increasing the synthesis temperature, the polytetrahedral
structure remains remarkably stable, while particles with higher rhodium content clearly evolve towards perfect
fcc. Increasing the size of the particles up to 5–6 nm stabilizes the structural phases encountered in the phase
diagram of the bulk alloy. Different element-sensitive techniques, x-ray absorption spectroscopy (XANES and
EXAFS) and energy-filtering transmission electron microscopy, have also been implemented in order to get
chemical information. Evidence is given for a cobalt surface segregation in these bimetallic particles, highly
favorable for magnetic-moment enhancement.
DOI: 10.1103/PhysRevB.69.235416
PACS number(s): 61.46.ϩw, 61.10.Eq, 68.37.Lp, 61.10.Ht
I. INTRODUCTION
magnetization in particles with 1.65 nm of mean diameter
compared to the predicted value for the bulk alloy. In par-
6
Investigations of size effects in nanometer-sized particles
have been intensively carried out in the last few years, both
in theoretical and in experimental approaches. In particular,
evidence has been given of strong size-related magnetic ef-
fects in metallic particles. For instance, in a 4d metal as
rhodium, the occurrence of ferromagnetism in small clusters
including less than 50 atoms was both calculated and experi-
mentally demonstrated.1 Combining size effects and alloy-
ing with a 3d metal, already known as an effective way to
induce spin polarization in 4d metals, was recently under-
allel, size reduction is also known to influence the structural
organization and to exalt the role played by surface segrega-
tion in metallic alloys. The first structural investigations, per-
formed by high-resolution transmission electron microscopy
(
HRTEM) and wide angle x-ray scattering (WAXS), on
Co Rh particles with 0ഛxഛ1 synthesized in PVP gave
x
1−x
7,8
evidence for two major features. First, the mean metal-
metal bond-length ͑dmm͒ in the alloyed particles remains
close to the pure Rh mean metal-metal distance, whatever
the composition, provided some rhodium is introduced. Its
value is then systematically higher than the one reported in
the bulk alloy of same composition. Secondly, the crystallo-
graphic order in the bimetallic CoRh particles tends to im-
prove with increasing Rh content. The structure of pure co-
balt particles with same size and synthesized by the same
chemical route has already been proposed as a well-defined
,2
3
taken in our group. In this previous paper, we reported the
synthesis and magnetic study of bimetallic Co Rh par-
0
.5
0.5
ticles with diameters lying in the 1–3 nm range. The synthe-
sis process, already successfully followed for cobalt
4
,5
clusters, was based on the decomposition of organometal-
lic precursors in mild conditions of pressure and temperature
in the presence of a polymer, the polyvinylpyrrolidone
5
polytetrahedral arrangement. Alloying with rhodium, as al-
(PVP). Both the magnetic and the structural results obtained
loying with platinum that we studied in a previous work,9
seems to introduce structural disorder in the Co-rich par-
ticles, at least at the surface, which impedes their unambigu-
ous structural determination.
on cobalt particles demonstrated that they behave as free
particles, with very weak or none interaction with the PVP
polymer. The results obtained on CoRh particles stabilized in
PVP also agreed with a good surface state of the particles.
The cooperative role of alloying and size reduction was dem-
onstrated through the strong enhancement of the saturation
In relation with these first results, we focus in this paper
on the structural evolution of Co Rh particles with 0ഛx
x
1−x
ഛ1 both with composition and synthesis conditions, likely to
0163-1829/2004/69(23)/235416(9)/$22.50
69 235416-1 ©2004 The American Physical Society

M. C. FROMEN et al.
PHYSICAL REVIEW B 69, 235416 (2004)
influence the size and the structure of the particles. Particular
attention was also paid to the chemical distribution inside the
particles. Indeed, the presence of weakly ordered structures
copper mesh. In order to limit oxidation effects, the deposits
were done in an argon atmosphere and the microgrids were
kept in this atmosphere until they were placed in the TEM
specimen holder. However, during this last step, the grids are
still exposed to air for a short time. The HRTEM experi-
ments were performed on a Philips CM30/ST operated at
300 kV, with a point resolution of 0.19 nm. For each sample,
a size distribution histogram of at least 200 particles was
plotted from the numerical analysis of several micrographs.
HRTEM images of isolated particles were recorded at a reso-
lution of 0.03 nm/pix using a Gatan slow-scan charge
coupled device (CCD) camera. When present, the lattice pe-
riodicities in the particles were analyzed using numerical dif-
fractograms of the image (Fourier transforms). EFTEM ex-
periments were performed on some samples using a JEOL
3010 microscope operated at 300 kV and fitted with a post-
column filter (GATAN 200 model). The filtered images were
recorded on a CCD camera. A three-window procedure was
used to extract the characteristic signal of the cobalt and
rhodium edges.11 In this procedure three filtered images were
recorded, two before the edge and one after, using the same
width for the energy window. The images recorded before
the edge allowed us to extrapolate the background to be ex-
tracted from the third filtered image. This last one contains
the information on the spatial distribution of the analyzed
element. Analyses were performed on the Co-L23 and the
Rh-M45 edges with energy window widths of 15 and 10 eV,
respectively.
in PVP stabilized Co Rh1−x particles can result from the for-
x
mation of metastable states at room temperature. Syntheses
at higher temperature, 80°C, have then been carried out and
we follow the evolution of the structural features for the
different compositions. Increasing the size of the particles is
also likely to bring elements for solving the Co Rh par-
x
1−x
ticles structure. This size is, however, largely influenced by
the nature of the stabilizer. The synthesis of larger particles
has then been performed in an organic ligand, the hexadecy-
lamine (HDA). The structure of the different samples is in-
vestigated by a combination of HRTEM and WAXS experi-
ments. HRTEM provides direct size estimation, morphology,
and spatial distribution of individual particles, as well as
structural evidences, mainly through the analysis of lattice
periodicities. WAXS provides an accurate set of interatomic
distances. This technique, dedicated to amorphous or weakly
crystallized materials, is particularly suited for solving non-
periodic structures. The chemical distribution inside the par-
ticles is analyzed using element-sensitive techniques as ex-
tended x-ray absorption fine structure (EXAFS) and energy
filtering transmission electron microscopy (EFTEM). The
latter technique enables local analyzes on a single particle, at
least the larger ones, and thus provides a chemical map of the
different species. The former probes the short-range order
around a specific element, whatever the particle size. The
chemical information is, however, indirectly retrieved from
these structural features through numerical refinement of
models.
C. WAXS and EXAFS experiments
WAXS experiments were mainly performed at CEMES
using a dedicated two-axis diffractometer. Measurements of
the intensity scattered by the samples were performed at the
II. SYNTHESIS, STRUCTURAL AND EXPERIMENTAL
DETAILS
molybdenum K ͑0.071069 nm͒ wavelength. Since only the
␣
A. Synthesis
primary beam was graphite monochromatized, an aluminium
filter was inserted in the diffracted beam in order to suppress
fluorescence from cobalt. Data typically include 457 mea-
surements in the range 0° ഛ␪ഛ65° for equidistant s values
͓s=4␲͑sin ␪/␭͔͒. Additional measurements were also per-
formed at the European Synchrotron Radiation Facility
The different Co Rh1−x samples were all synthesized us-
x
ing a chemical route based on the codecomposition under
dihydrogen (3 bars) of two organometallic precursors,
3
4
4
namely Co͑␩ -C H ͒͑␩ -C H ͒ and Rh͑acetylacetate͒͑␩
8
13
8
12
−
C H ͒, in a solvent containing either a polymer or a
8 12
(
ESRF) in Grenoble (France) on the high-energy diffraction
beamline ID15B. The much higher energy available
90 keV͒ combined to the very high intensity and parallel
ligand. The polymer/solvent couple is PVP/tetrahydrofuran
THF). The toluene was used for the samples in HDA. The
(
͑
final composition of the particles was adjusted by varying the
relative initial proportion of the two precursors. Except for
the pure Co or Rh particles, three typical compositions were
studied in the polymer PVP: Co Rh , Co0.25Rh0.75, and
measurement of all angles using an image plate as detector
led to faster measurements (typically 20 min instead of 20 h
in laboratory), better statistics and a larger cutoff for s
0
.5
0.5
−
1
−1
(
26 Å vs 17 Å for our diffractometer). The same samples
Co0.75Rh0.25, respectively, named in the following Co1Rh1,
Co1Rh3, and Co3Rh1. The final compositions, determined
by chemical microanalysis, are no more than 2.7% far from
the nominal composition. Four different compositions have
been studied in HDA (see Table III). A more complete de-
scription of the synthesis can be found in Refs. 8 and 10.
were used for both measurements: small amounts of dry col-
loidal powder sealed in thin-walled Lindemann glass capil-
laries of 1.5 mm diameter. The same treatments were applied
to all data sets: physical corrections for polarization and self-
absorption as well as subtraction of the corrected scattering
from glass capillary and PVP. Scattering from the relatively
very small amount of ligands surrounding bigger particles
could actually be neglected. An additional benefice of the
high energy used on ID15B was to greatly reduce absorption
effects. Data were normalized to one metal atom, taking into
account the mean scattering power for the binary systems.
B. HRTEM and EFTEM experiments
Specimens for TEM investigations were prepared by slow
evaporation of droplets of a solution of the colloid diluted in
methanol deposited on high-resolution carbon supported
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STRUCTURAL STUDY OF BIMETALLIC Co Rh1−x…
PHYSICAL REVIEW B 69, 235416 (2004)
x
The so-called reduced intensity function was extracted then
Fourier transformed in order to obtain the radial distribution
function (RDF). A full description of this procedure can be
found in Ref. 12. Analysis of the experimental RDF provides
good estimates of the mean metal-metal bond length and the
order extent inside the particles, hereafter called the coher-
ence length. In the more favorable situation, the position and
relative intensity of the different peaks provide a signature of
a single three-dimensional structure. However, in the general
case, comparison with computed theoretical RDF, also nor-
malized to one atom, using different models are required.
The size of the theoretical model is chosen to reproduce the
experimental coherence length, and the cell parameter is con-
tracted or dilated in order to match the experimental first
metal-metal bond length. The computed RDF are obtained
using the Debye formula
N−1
N
sin͑srij͒
2
iD͑s͒ = 2
f ͑s͒f ͑s͒
exp͑− b s ͒,
͑1͒
͚ ͚
i
j
ij
srij
i=1 j=i+1
FIG. 1. TEM image of the RT-PVP-Co1Rh3 colloid with size
histogram inset.
where N is the total number of atoms in the model, f is the
i
atomic scattering factor for atom i, r is the distance between
ij
atoms i and j, and b is a dispersion factor affecting the i-j
interaction.
ij
total phase shift. MacKale functions16 were also used for Fj
and ⌽ . Final values of the refinement are given in Table IV.
X-ray absorption (XANES and EXAFS) experiments
were performed for some samples on beamline X1 at Hasy-
lab in Hamburg. The samples were prepared by compacting a
pellet of the powder in a polyethylene matrix, in order to
have a higher metal density, and to give protection from
oxidation (at least for a short time). The measurements were
done in transmission mode at 30 K in a vapor flow cryostat
using two different silicon monochromators, set for diffrac-
tion from (111) planes at Co K edge ͑7709 eV͒ and from the
ij
III. RESULTS
A. A PVP-CoRh: Structural evolution with temperature
1. Room temperature
Whatever the composition, the particles synthesized in
PVP at room temperature (RT-PVP-colloids) look spherical
and well dispersed in the polymer matrix when observed by
TEM. As for pure cobalt and rhodium particles, the size dis-
tribution of the bimetallic particles is narrow. Size histo-
grams, drawn for Co3Rh1, Co1Rh1, and Co1Rh3 colloids,
showed a mean size of 2.1, 1.7, and 2.5 nm, respectively,
with respective standard deviations of 0.6, 0.4, and 0.9 nm.
A TEM image of Co1Rh3 particles with size histogram inset
is displayed in Fig. 1. The experimental RDFs of the differ-
(311) planes for Rh K edge ͑23220 eV͒. It is worth noticing
that the sample is exposed to air for roughly 30 min when
positioned on the sample holder and introduced in the cry-
ostat. Both edges were measured with the samples main-
tained in situ at low temperature using the quick monochro-
mator change capability of beamline X1 (less than 15 min to
move between the two metal absorption edge energies, in-
cluding change of detection gas in the ion chambers measur-
ing x-ray intensity). Metal foils were measured simulta-
neously in the monitor position for accurate calibration of
x-ray absorption energies. The EXAFS signal was extracted
using the ATHENA program.13 Further treatments were per-
formed using classical procedures. For the best data sets, the
ent RT-PVP colloids are reproduced in Fig. 2 together with
the RDFs of pure Co 5 and pure Rh
17
particles. For each
colloid, the coherence length measured from the reduced
RDF is shorter than the size measured on TEM images:
around 1.4 nm for Co3Rh1, 1.5 nm for Co1Rh1, and 2.0 nm
for Co1Rh3. The higher statistics of synchrotron data give
more accurate evaluation of the distance for complete damp-
ing of oscillations, which is specially difficult for poorly or-
dered particles. On the measurements performed on the same
samples and both instruments, as for Co1Rh1 reported in
Fig. 2, very close RDFs were obtained and differences in
coherence length were never larger than one period of the
last observable oscillation. This leads us to evaluate the un-
certainty to 0.2 nm. Apart from the coherence length, an im-
portant value taken from the RDF is the position of the first
peak which gives the mean metal-metal bond length ͑dmm͒.
In the absence of fitting with an accurate model, this value
remains approximate: here we took the center of the peak at
1
4
MINUIT program from CERN was used to refine parameters
of the one-shell EXAFS function [see Eq. (2)]:
−
2r
j
Nj
kr2j
2 2
−2k ␴
␹fit͑k͒ = Si
F ͑k͒ · exp
j
· exp ␭j · sin͓2krj
͚
j
+
⌽ ͑k͔͒,
ij
͑2͒
where S is an amplitude scaling factor, F is the backscatter-
i
j
ing amplitude from N neighboring atoms of the jth type at a
j
distance r from the absorbing atom i, ␭ is the electron mean
j
j
1
5
4
free path approximated by ͓͑3.913/k͒ +k͔/0.95, ␴ is the
standard deviation of the interatomic distance, and ⌽ is the
j
ij
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M. C. FROMEN et al.
PHYSICAL REVIEW B 69, 235416 (2004)
FIG. 3. Comparison between the evolution of dmm in the bulk
alloy (solid line) and in the RT-PVP particles (crosses). The discon-
tinuity of the solid line corresponds to the phase transition hcp to
fcc in the bulk.
that the dmm evolution in the particles is completely different
from the one observed in the bulk alloy.
According to the phase diagram in the bulk alloy, Co3Rh1
crystallizes in the hcp structure. However, the comparison
between the experimental RDF of RT-PVP- Co3Rh1 par-
ticles and the simulated RDF of a hcp particle of same size
and metal-metal distance, clearly shows strong differences,
see Fig. 4. In fact, as can be observed in Fig. 2, this experi-
mental RDF displays the same main features as the one of
pure RT-PVP-cobalt particles, except for a larger dmm. This
FIG. 2. Experimental WAXS RDF of CoRh particles with dif-
ferent compositions synthesized in PVP at room temperature to-
gether with the experimental RDF of pure-cobalt and pure-rhodium
particles synthesized in identical conditions. Dashed line: RDF
from high energy measurements on ESRF beamline ID15B, for the
Co1Rh1 sample.
particular pattern with broad peaks, a short coherence length
half maximum and found dmm=0.263, 0.269, and 0.269 nm
for Co3Rh1, Co1Rh1, and Co1Rh3, respectively. The error
on this measurement can be estimated to less than ±0.5 pm.
The different parameters measured from the TEM and
WAXS experiments are summarized in Table I. Clearly, dmm
is not proportional to the composition. It increases strongly
between pure cobalt and Co3Rh1, then it remains nearly
steady from Co1Rh1 to pure rhodium. A comparison of the
evolution of the dmm with the composition in the particles
and in the bulk, as deduced from the lattice parameters of the
ͱ
and the lack of a specific peak located at the distance dmm
2
was identified as the signature of a polytetrahedral atomic
5
organization. It was, however, impossible to decide in favor
of a particular polytetrahedral model, as they all display the
1
8
bulk phases, is plotted in Fig. 3. In this figure, the plot
corresponding to the bulk alloy consists in two different seg-
ments according to the structural phases stable at this com-
position: hexagonal close packed ͑hcp͒ below 50.5 at% Rh
and face centered cubic (fcc) above. In spite of this structural
transition, the first interatomic distance in the alloy ͑dmm͒ at
room temperature nearly follows a Vegard’s law between
pure Co ͑0.250 nm͒ and pure Rh ͑0.269 nm͒. Figure 3 shows
TABLE I. Data extracted from the TEM images and the RDF of
the PVP-CoRh colloids synthesized at room temperature.
Co3Rh1
Co1Rh1
Co1Rh3
Mean diameter (nm)
Standard deviation (nm)
Coherence length (nm)
dmm ͑nm±0.5 pm͒
2.1
0.6
1.7
0.4
2.5
0.9
FIG. 4. Comparison between the experimental RDF of RT-
PVP-Co3Rh1 and top: a computed RDF of a 129-atom hcp model,
bottom: a simulated RDF corresponding to a 105-atom polytetrahe-
1.4±0.2
0.263
1.5±0.2
0.269
2±0.2
0.269
dral model with same dmm
.
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STRUCTURAL STUDY OF BIMETALLIC Co Rh1−x…
PHYSICAL REVIEW B 69, 235416 (2004)
x
fcc model with a cuboctahedral fcc model containing an in-
trinsic stacking fault considerably improves the fit, particu-
larly the amplitudes of the different peaks (see the best fit in
Fig. 6). The proportion of the two models in the RDF was
adjusted using weighting coefficients K . These stacking
w
faults are likely to increase the compacity of the structure
and can therefore result from a pure size effect. Besides, two
different sizes of particles were required to reproduce more
suitably the amplitudes of all the peaks: a 561-atom and a
110-atom models. This can be understood in different ways.
First, it can result from a size dispersion. Indeed, the size of
the different models, respectively, 2.5 and 1.6 nm, would
agree with the TEM results owing to the standard deviation
measured in this sample (Table I). However, the respective
weight of the models is not consistent with the TEM size
histogram. More probably, the need for models with two dif-
ferent sizes results from a progressive loss of coherency with
increasing distance due to some structural disorder in the
particles.
FIG. 5. HRTEM micrograph and corresponding Fourier trans-
form of a fcc RT-PVP-Co1Rh3 particle observed along a [011]
zone axis.
same structural features. One of these models, including 105
atoms and having the suitable dmm, was built according to the
cluster of clusters approach proposed by Van de Waal.19
A
comparison between the simulated RDF of this model and
the experimental RDF of Co3Rh1 is displayed in Fig. 4.
Although some differences, particularly in the larger dis-
tances, remain, the main features of the experimental RDF
are correctly reproduced by the model like the absence of the
As observed in Fig. 2, the different peaks get sharper and
more numerous with increasing rhodium content. For in-
stance, the peak at dmmͱ
2 just appears in the equiatomic
sample Co1Rh1, while the coherence length of this sample
remains quite short. This colloid so presents features from
both surrounding compositions, i.e., from two completely
different models, a nonperiodic polytetrahedral and a peri-
odic fcc models. It is therefore impossible to fit its RDF with
a single model. HRTEM observations reveal very few crys-
tallized particles in this sample.
ͱ
peak at dmm 2 (near 0.35 nm) observed in the hcp model and
more generally in all compact structures. HRTEM investiga-
tions of single Co3Rh1 particles do not provide additional
structural information as no lattice periodicities were ob-
served.
On the contrary, HRTEM observations of the Co1Rh3
sample clearly reveal lattice fringes in the particles. The
analysis of these periodicities agrees with the fcc structure,
see Fig. 5. The experimental RDF of this sample has indeed
many common features with a fcc structure, particularly in
the shorter distances, as can be observed in Fig. 6. The main
differences with the 147-atom fcc model, which presents a
comparable coherence length, are the sharpness of the peaks
and their relative amplitude. Mixing a perfect cuboctahedral
2. Synthesis at 80°C
These samples have been synthesized in conditions simi-
lar to the previous ones except for a temperature of 80°C
(80°C-PVP colloids). The microanalysis gives Co:Rh com-
positions close to the initial Co:Rh precursors ratio and TEM
investigations show well-dispersed particles with diameters
around 2 nm. Therefore, these particles are comparable to
the ones synthesized at room temperature. The RDF corre-
sponding to the different samples are reproduced in Fig. 7
together with the RDF of the same samples synthesized at
room temperature.
As can be observed, the synthesis temperature only affects
the atomic organization in samples with a high rhodium con-
tent. Indeed the RDF of 80°C-PVP-Co3Rh1 seems steady. It
should be mentioned that this RDF still remains unchanged
after annealing of the sample, kept sealed in the capillary, at
1
20°C during one week. Clearly, the 80°C-PVP-Co1Rh1
sample keeps its coherence length but its pattern is now
sharper and closer to a fcc one. Besides its dmm is lower
͑
0.265 nm͒ than in the RT sample although still higher than
expected from the bulk alloy. Enhanced crystallization is also
supported by the relative intensities of the main peaks. While
the envelope of the RDF for Co3Rh1 and RT-PVP-Co1Rh1
share the same monotonously decreasing shape, the three
first peaks in the 80°C-PVP-Co1Rh1 sample are much
closer in amplitude which is consistent with an increase of
order inside the particles. The RDF of this last sample is in
fact very similar to the one of the RT-PVP-Co1Rh3 sample,
which means that Co1Rh1 now presents a partially faulted
FIG. 6. Comparison between the experimental RDF of RT-
PVP-Co1Rh3 particles and top: the computed RDF of a 147-atom
perfect fcc cluster, bottom: the computed RDF of a model corre-
sponding to the mixture {Kw1* 561-atom perfect fcc model
+
Kw2*110-atom faulted fcc model} with Kw1=0.2 and Kw2=3.
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M. C. FROMEN et al.
PHYSICAL REVIEW B 69, 235416 (2004)
TABLE II. Data extracted from the RDF of the 80°C-PVP-
CoRh colloids.
Co3Rh1
Co1Rh1
Co1Rh3
Coherence length (nm)
dmm ͑nm±0.5 pm͒
1.4±0.2
0.263
1.4±0.2
0.265
1.8±0.2
0.266
B. HDA-CoRh: Structural evolution with size
The particles synthesized in HDA are much larger, which
was the expected effect. They do not adopt a spherical shape,
indicating that most of them have coalesced during the
growth process. Their size varies between 4.3 and 6.8 nm,
with important standard deviations ͑1.2–2.5 nm͒.
The particles are formed by several grains as expected
from their morphology and they are all fully crystallized. For
all the compositions studied, both the fcc and the hcp phases
are present, although in different proportions. The results
from TEM investigations are gathered in Table III. The
Co-rich sample (Co0.75Rh0.25, i.e., Co3Rh1) presents a large
majority of hcp grains, while the Rh-rich sample
͑
Co0.39Rh0.61͒ is mainly fcc, although its composition is
closer to Co2Rh3 than to Co1Rh3. Finally, the two samples
with compositions close to Co1Rh1 (Co0.53Rh0.47 and
Co0.45Rh0.55) are mainly fcc. However, many regions of these
samples exhibit structural distorsions, probably due to the
presence of numerous faults, which prevents us from unam-
biguous determination of the structure in as much as one
investigated area over four. Figure 9 shows HRTEM images
of two particles having different structures, together with
their Fourier transforms. The RDF patterns (not shown) are
consistent with the structural characteristics determined by
HRTEM. For all the compositions, the fit requires a mixture
of fcc and hcp models. In the Rh-rich samples (Co0.45Rh0.55
and Co0.39Rh0.61) the main structure is clearly fcc, while the
RDF of HDA-Co0.75Rh0.25 is closer to a hcp structure. No
evidence for a major structure was obtained in the
Co0.53Rh0.47 sample which presents a broader pattern. The
coherence length of these HDA samples are larger than in the
PVP samples, but not as large as the diameters measured on
TEM images. The dmm bond lengths are shorter than in the
PVP samples, but still higher than in the bulk (see Table III).
FIG. 7. Evolution of the experimental RDF of the different
samples with the synthesis temperature (RT stands for room
temperature).
fcc structure. The organization in Co1Rh3 also improves
with the temperature and the RDF of the 80°C sample shows
sharper peaks, the position and intensity of which agree with
a perfect fcc structure. A very good fit, reproduced in Fig. 8,
is obtained with a model including two different sizes of fcc
cuboctahedral particles: a 561-atom and a 147-atom models,
with respective diameters of 2.5 and 1.8 nm. The distance
dmm is here equal to 0.266 nm. The data extracted from the
WAXS experiments in the high-temperature samples are
summarized in Table II.
C. Chemical distribution
The bimetallic character of these particles has been estab-
3
lished by different results, in particular the magnetism for
TABLE III. HDA-CoRh particles: chemical composition (ob-
tained by microanalysis), particle mean size ͑Ø͒ with standard de-
viations ͑␴͒ and percentage of identified fcc grains deduced from
HRTEM investigations. Coherence length ͑Lcohe
͒
and dmm
͑
±0.5 pm͒ obtained on the RDFs.
Size ͑TEM͒
%fcc
Lcohe
dmm
Composition
Ø͑nm͒
␴͑nm͒
(nm)
(nm)
Co0.75Rh0.25
Co0.53Rh0.47
Co0.45Rh0.55
Co0.39Rh0.61
6.8
4.3
5.6
5.7
2.5
1.2
1.2
1.5
41
63
58
59
2.4±0.2
2.1±0.2
3.1±0.2
2.1±0.2
0.260
0.266
0.265
0.266
FIG. 8. Comparison between the experimental RDF of 80°C-
PVP-Co1Rh3 particles with the computed RDF of a model corre-
sponding to the mixture {Kw1* 561-atom fcc cuboctahedral model
+
Kw2*147-atom fcc cuboctahedral model} with Kw1=0.1, Kw2=2.
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STRUCTURAL STUDY OF BIMETALLIC Co Rh1−x
…
PHYSICAL REVIEW B 69, 235416 (2004)
x
FIG. 10. Uncorrected exafs Fourier transforms modulus of PVP-
Co1Rh3 and PVP-Co3Rh1 at the Co and Rh edges, compared with
a Rh foil or with simulations.
FIG. 9. High-resolution micrographs and Fourier transforms in
inset (a) Co0.39Rh0.61 in HDA, fcc zone axis [011], (b) Co0.75Rh0.25
in HDA, polycrystalline particle with a hcp structure (bottom left
and FT) and a fcc one (top right).
sample. Indeed, the experimental cobalt K-edge FTs are
close to the one calculated with a CoO structure.
The Co-rich particles present a different behavior. At the
Rh edge, the uncorrected FT presents a single peak, which is
located at a distance too short for a Rh-Rh bond, but too long
for a Rh oxide. The same peak is present at the cobalt edge
together with an oxide peak. A direct comparison of these
FTs with simple models is not sufficient for accurate struc-
ture determination. However, fitting attempts at the Rh edge
give some indications of the presence of Co in the first
neighbors shell.
These different elements, particularly the results obtained
on the Rh-rich sample (Rh neighbors around Rh atoms and
partial oxidation of cobalt), are consistent with a surface seg-
regation of cobalt atoms. EFTEM experiments are likely to
provide a direct proof of such a chemical distribution. How-
ever, the PVP particles are very small and the signal of the
electron energy loss is too weak to be processed. Only the
particles stabilized in HDA can be successfully analyzed. A
typical EFTEM analysis of the HDA-Co0.75Rh0.25 sample is
presented in Fig. 11. In the zero-loss image (a), formed by
the elastically transmitted electron beam, some contrast al-
ready appears in the particle: small dark nuclei are sur-
rounded by light gray areas. Such contrast is not observed in
the high-resolution mode which mainly displays a phase con-
the PVP samples. However, the way the different species
organize inside a particle requires the use of element-
sensitive techniques. EXAFS experiments have been carried
out on the RT-PVP samples, which present the highest size
effects. We first worked on the Fourier transforms of the ␹
function without any phase correction. The modules of these
uncorrected Fourier transforms (FT) have been compared ei-
ther to experimental reference data (Rh foil for instance) or
to simulations (see Fig. 10). At the Rh edge, the FT of the
Co1Rh3 is very similar to the one obtained on a rhodium foil
but with an amplitude twice lower. Both plots show a main
peak close to 2.5 nm followed by three much smaller peaks.
Let us mention that the same main peak is obtained by mod-
eling the FT corresponding to the first neighbors shell in the
rhodium fcc structure. The Co1Rh1 sample (not shown) pre-
sents the same main peak but it is associated to a shorter
distance, probably a Rh-O bonding. The overall amplitude is
lower. In these two samples, the environment of the rhodium
atoms looks very close to the one they have in bulk rhodium,
in spite of a partial oxidation in the case of Co1Rh1. For the
Co1Rh3 sample, it has been possible to fit the inverse Fou-
rier transform of the main peak using the parameters listed in
Table IV. The number of first neighbors has been fixed to
TABLE IV. Fitting parameters at the Rh-edge for RT-PVP-
Co1Rh3 and for a rhodium foil. Values with à are fixed during the
9
.9, which is the mean value in particles of about 2 nm.20
2
fit (NRh: number or Rh neighbors, RRh-Rh: distance, ␴ : Debye-
The best fit is obtained for rhodium neighbors at a distance
close to the one in pure fcc Rh, thus confirming the resem-
blance with bulk rhodium. XANES analysis of the Co edge
reveals partial oxidation of cobalt in this sample. The pre-
edge inflexion characteristic of metallic cobalt is suppressed,
and the intensity of the white line at the absorption edge is
increased, showing a higher mean oxidation state of the co-
balt atoms. This is also the case for cobalt in the Co1Rh1
Waller factor, ⌬E : edge shift).
0
␴2
Scaling RRh-Rh
⌬E0
͑10−3 Å2͒ (eV) Residual
NRh
factor
(Å)
Co1Rh3 9.9Ã
0.8Ã
2.692
2.705
7.57
4.62
−1.39
−1.03
0.117
0.013
Rh foil
12Ã
0.83
235416-7

M. C. FROMEN et al.
PHYSICAL REVIEW B 69, 235416 (2004)
other compositions, the bigger particles present both fcc and
hcp areas. However, none of these structures is particularly
favored, which agrees with the occurrence of the hcp to fcc
phase transition at this composition in the bulk alloy. It is
worth recalling that the study of the structural evolution with
size has required a change in stabilizer, i.e., the use of the
HDA ligand instead of the PVP polymer. We are thus not
able to distinguish between a pure size effect and a combined
size and environment effect.
The particular evolution of the dmm in the RT-PVP-CoRh
samples has also been pointed out. Synthesis at higher tem-
peratures as synthesis in other media do not change drasti-
cally this particular evolution. In other words, the first metal-
metal bond length remains in all the studied samples, higher
than in the bulk alloy with same composition. In particular,
for samples with at least 50 at%Rh, the dmm comes very
close to the one in pure Rh particles (same as in bulk Rh). As
the core-atoms are involved in more bondings then the sur-
face atoms, this feature could be attributed to a Rh-rich core.
Surface segregation in binary alloys is driven by different
forces: the difference between the surface energies of the
involved elements, their size difference, and their ability (in-
ability) to mix in the bulk (tendency to phase separation or to
order). Cobalt has a slightly lower surface energy than
rhodium21 so that, at any composition, the system will lower
its surface energy if cobalt comes to the surface. Rhodium
having a larger atomic radius than cobalt, the elastic strain
energy will be lower when Rh lies at the surface in Co-rich
alloys. Finally, the CoRh alloy forms a solid solution all over
the phase diagram, so that the heteroatomic bond driving
force will not have an important effect on the segregation of
this alloy. Ellison et al.22 carried out an experimental study
of the segregation in macroscopic specimens of CoRh and
showed some surface segregation of the solute element. Ac-
cording to their results, the driving force of the segregation
would depend on the composition: surface energy in Co-rich
samples and atomic size in Rh-rich ones. In conclusion,
some cobalt surface segregation can be expected in the
Rh-rich particles while the chemical distribution in the
Co-rich particles remains difficult to foresee because of the
competition between the surface energy and the atomic size
driving forces. Our investigations by EFTEM clearly showed
a surface segregation of cobalt in Co-rich HDA particles.
However, contrary to the PVP polymer which has nearly no
interactions with the particles, the HDA ligand can have
more affinities with the cobalt atoms and thus play a role in
the observed segregation. Therefore, the direct use of this
result to conclude on the chemical distribution in the PVP-
CoRh particles remains questionable. For these small par-
ticles, EXAFS experiments have given different clues for the
presence of a Rh-rich core. Indeed, in the Rh-rich particles,
our results show that the rhodium atoms have mainly the
same environment as in bulk rhodium and reveal the pres-
ence of oxide at the cobalt edge. The chemical distribution in
the Co-rich sample is not so clear although evidence for oxi-
dation, only at the cobalt edge, would also agree with cobalt
surface segregation. The overall difficulty to prevent the
samples from any oxidation and though to extract reliable
information from these experiments prevents us from further
analysis in most of the samples. Finally, the link between the
FIG. 11. EFTEM images of HDA-Co0.75Rh0.25 particles (a)
zero-loss image, (b) Rh and Co maps.
trast. The elemental maps obtained by energy filtering on the
Rh-M4,5 and the Co-L2,3 edges show that rhodium atoms
concentrate in the nuclei while cobalt atoms spread over
much larger zones corresponding to the particles size. Be-
cause of the two-dimensional projection in the TEM analy-
ses, we cannot say if cobalt atoms are only located around
the rhodium cores or also inside them. In any cases, this
result shows without ambiguity the occurrence of a cobalt
surface segregation in these particles.
IV. DISCUSSION
We have previously shown that the structure of PVP-
CoRh nanoparticles vary from a polytetrahedral arrangement
to a structure close to fcc with increasing Rh content. Using
°
a higher temperature during the synthesis ͑80 C͒ confirms
this behavior. The same polytetrahedral structure is observed
in the cobalt-rich particles, at least at this temperature,
whereas the structure of the rhodium-rich particles gets
°
closer to perfect fcc. The RDF of the 80 C-PVP-Co1Rh1
samples, which presents the more ambiguous pattern for the
room temperature synthesis, evolves towards a pattern char-
acteristic of a partially faulted fcc, i.e., very similar to the
one in the RT-PVP-Co1Rh3 sample. This is an indication of
the low stability of the polytetrahedral structure at this com-
position. The structural evolution of the Co1Rh1 and
Co1Rh3 particles with the temperature is accompanied by a
slight decrease of the first metal-metal bond length ͑dmm͒.
Increasing the size of the particles also affects their structural
behavior. The big cobalt-rich HDA particles mainly display a
hcp structure, like in the bulk alloy, even if some fcc par-
ticles are observed. For the Co3Rh1 composition, the critical
size, above which the bulk structure ͑hcp͒ becomes more
stable than the polytetrahedral one, thus lies between 2 and
6
nm. Whatever the size, the Rh-rich particles always crys-
tallize in the fcc structure of the bulk alloy. However, the
bigger ones also present some hcp regions. The case of the
equiatomic composition is again more complex. As for the
235416-8

STRUCTURAL STUDY OF BIMETALLIC Co Rh1−x…
PHYSICAL REVIEW B 69, 235416 (2004)
x
value of the metal-metal bond length and the chemical dis-
tribution in the particles is probably not as straightforward as
mentioned previously. Indeed, the slight decrease of the dmm
in the Rh-rich 80°C samples could be interpreted as the
result of some intermixing of the chemical species instead of
a core-shell distribution. However, this decrease is accompa-
nied by an increase of the crystallographic order in these
samples. For comparison, room temperature Rh-rich
HDAsamples which are fully crystallized in compact struc-
tures (mainly fcc) also present a lower dmm. Besides, the
Co-rich HDA samples, which also crystallize in compact
structures, present both characteristics: a lower dmm and a
rhodium core, as evidenced by EFTEM. The difference be-
tween the respective weights of Rh-Rh, Rh-Co, and Co-Co
contributions to scattering probably also contribute to the
overall increase of the dmm measured by WAXS compared to
the one expected from the bulk.
The experimental techniques implemented in this study
reach their limits in these ultrafine particles. In order to bring
more information on these samples and more specifically on
the cobalt-rich ones, molecular dynamics and Monte-Carlo
calculations using a n-body semiempirical interaction model
for the particles are now in progress. In any case, if not
definitely proved for the different compositions, all our ex-
perimental results agree with a cobalt segregation at the sur-
face, which should play an important part in the magnetic
behavior of these particles.
have shown that the structure of the small particles ͑2 nm͒
evolves from a polytetrahedral arrangement in Co-rich par-
ticles to a faulted fcc structure in Rh-rich ones. Using a
higher temperature during the synthesis does not affect the
polytetrahedral arrangement in the Co-rich particles but in-
creases the crystallographic order in the Rh-rich ones. In-
creasing the size of the particles allowed us to stabilize the
bulk phases, with a majority of hcp in the cobalt rich side,
and fcc in the Rh-rich one, in particles 4–6 nm large. In all
the studied particles, the first metal-metal bond length proves
larger than in the bulk alloy of same composition. Moreover,
introduction of a small amount of rhodium in the particles
induces a larger shift towards the mean bond length in pure
rhodium than expected from a Vegard’s law. This particular
behavior can be explained by the occurrence of both some
surface disorder and cobalt segregation. Evidence for this
surface segregation has indeed been given by element-
sensitive techniques on some of the samples, if not all. In
particular, some ambiguity remains concerning the smaller
cobalt-rich samples. Complementary information should
soon be brought by calculations using a n-body semiempir-
ical interaction model, now in progress.
ACKNOWLEDGMENTS
The authors acknowledge the European Synchrotron Ra-
diation Facility for provision of synchrotron radiation facili-
ties and we would like to thank Dr. V. Honkimaki for assis-
tance in using beamline ID15B. They also acknowledge
financial support through the IHP-Contract No. HPRI-CT-
V. CONCLUSION
1
999-00040/2001-00140 of the European Commission for
In this paper, we have completed the structural and chemi-
cal study of CoRh nanoparticles synthesized in mild condi-
tions using a wide range of experimental techniques. We
the measurements on beamline X1 at Hasylab, and the assis-
tance of Dr. P. Kappen and Dr. J. Wienold in using this
beamline.
*
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