Preparation of nanocomposite materials from mercaptoacetate-modified platinum nanoparticles and a layered nickel hydroxyacetate salt

Article abstract of DOI:10.1016/j.materresbull.2009.04.004

Nanocomposites were prepared by a very simple route from preformed platinum particles and a nickel layered hydroxide salt (LHS), these compounds being first synthesized separately by the polyol process. The nanoparticles were treated with mercaptoacetate

Full text of DOI:10.1016/j.materresbull.2009.04.004

Materials Research Bulletin 44 (2009) 1692–1699
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Preparation of nanocomposite materials from mercaptoacetate-modified
platinum nanoparticles and a layered nickel hydroxyacetate salt
a,
a
a
a
a
b
Lorette Sicard *, Souad Ammar , Fr e´ d e´ ric Herbst , Claire Mangeney , Fernand Fi e´ vet , Guillaume Viau
a
ITODYS (UMR 7086) Universit e´ Paris-Diderot, B aˆ timent Lavoisier, 15 rue Jean-Antoine de Ba ı¨ f, 75205 Paris Cedex 13, France
b
LPCNO (UMR 5215), INSA Toulouse, 135 avenue de Rangueil, 31077 Toulouse, France
A R T I C L E I N F O
A B S T R A C T
Article history:
Nanocomposites were prepared by a very simple route from preformed platinum particles and a nickel
layered hydroxide salt (LHS), these compounds being first synthesized separately by the polyol process.
The nanoparticles were treated with mercaptoacetate before being brought into contact with the
lamellar compound. XPS and IR spectroscopies give clear evidence for interactions between the platinum
nanoparticles and the mercaptoacetate species. XRD, TEM and magnetic characterizations show that the
structure of the nickel hydroxide layers is retained and that some of the LHS sheets contain incorporated
nanoparticles.
Received 27 January 2009
Received in revised form 31 March 2009
Accepted 13 April 2009
Available online 19 April 2009
Keywords:
A. Layered compounds
A. Composites
ß 2009 Elsevier Ltd. All rights reserved.
B. Intercalation reactions
1
. Introduction
precipitation of a multicationic lamellar hydroxide, [7] or by
impregnation with a metallic complex or salt, followed by in situ
Layered double hydroxides (LDH) and layered hydroxide salts
LHS) have attracted increasing interest during the last decades.
reduction [8–9]. However, these routes do not allow the
incorporation of much metal, particularly in the case of Pt, [7]
and they lead to the precipitation of polydisperse particles.
Nanoparticles have also been incorporated by exfoliation and
restacking [10]. An alternative method consists in incorporating
preformed monodisperse metal nanoparticles. To the best of our
knowledge, this method is seldom reported. Recently, G e´ rardin
(
Their properties arise from their foliated structure: they consist of
positively charged layers, generally of the brucite type. Anionic
species are intercalated in the interlamellar space together with
water molecules. This is due, in the case of LDH compounds, to the
partial substitution of divalent cations by trivalent cations in
octahedral sites, leading to a positive charge on the layers which
need to be compensated by intercalated anionic species. In LHS
compounds, only divalent cations are present. The insertion of
anions has been explained by: (i) the lack of some hydroxyl groups
in the sheets, (ii) the partial replacement of hydroxyl groups, or (iii)
the protonation of some hydroxyl groups. The large range of
compositions possible for the host matrix and the possibility of
exchanging the anionic species offer potential for tuning the
properties of such materials. Consequently, layered hydroxides are
of interest in numerous fields, such as catalysis, electrochemistry,
magnetism, environment, medicine, etc. [1–6].
6 7 3 x y
et al. [11] incorporated preformed Ni(C H O ) (OH) colloids by
anionic exchange into the lamellar space of Mg–Al LDH. However,
the reduction of nickel necessitates a further calcination step,
followed by reduction at high temperature in dihydrogen, and
leads to particles coated by a carbon-rich shell. The present work
aims at incorporating directly a large quantity of preformed
monodisperse metal nanoparticles into lamellar compounds
without any post-synthetic treatment.
Since it is a promising chimie douce route, the polyol process
was chosen to synthesize the starting compounds. The polyol acts
as an amphiprotic solvent, as well as complexant, reductant and
Surprisingly, while several papers discuss of the intercalation of
nanoparticles in clays, there are few reports of the introduction of
metal nanoparticles in LDH or LHS. However, this would greatly
improve the properties of the layered material, or confer additional
properties. Noble metals have been introduced either by co-
surfactant. In the absence of water and at a reasonable
temperature, monodisperse nanoparticles of noble metals can
be produced [12]. In the presence of water, this route allows the
preparation of transition metal layered hydroxyacetate salts [13].
The LHS chosen for this study was a nickel hydroxyacetate
(
Ni(OH)1.6(CH
3
CO
2
)
0.4Án(H
2
O)) whose synthesis in polyol is now
well controlled [13]. The choice of platinum particles was directed
by two considerations. The first was the importance of platinum in
catalysis applications, the second the ability of our group to obtain
*
Corresponding author. Tel.: +33 1 57 27 87 59; fax: +33 1 57 27 72 63.
E-mail address: sicard@itodys.univ-paris-diderot.fr (L. Sicard).
0025-5408/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2009.04.004

L. Sicard et al. / Materials Research Bulletin 44 (2009) 1692–1699
1693
very small (diameter below 2 nm), monodisperse platinum
particles [14].
dissolved in DEG (250 ml) at 100 8C. MilliQ water (110 ml) was
added to the cooled solution (hydrolysis ratio, n(H O)/n(Ni) = 250),
2
Our strategy was therefore to first synthesize separately both
components by the polyol process, then to make the two systems
compatible by grafting a small molecule with a double function-
ality on the nanoparticle surface, and finally to bring the two
components together. The nanocomposites obtained after one and
two loading steps are characterized from chemical, morphological,
structural and magnetic points of view. The particle concentration
in the final composite was chosen on purpose to be higher than
required for many applications (for example, catalysis) in order to
facilitate characterization. For specific applications the system will
need to be optimized.
which was then heated at 90 8C for one night. A green solid was
recovered by centrifugation and washed three times with ethanol.
Platinum nanoparticles were obtained by a method derived
from that previously described [14]. It was based on the reduction
of a solution containing K
(4.0 mmol) in DEG (200 ml) at 90 8C. To functionalize the
nanoparticles, a solution of HSCH CO Na (1.2 mmol) in DEG
2 4
PtCl (1.2 mmol) and sodium acetate
2
2
(100 ml) was then added. Thus, the molar ratio of mercaptoacetate
to platinum in the colloidal solution is equal to 1.0. The solution,
noted PtMA, was used without centrifugation to make the
composites. However, for further characterizations, a volume of
the PtMA solution was also centrifuged after addition of ethanol to
obtain a powder, noted PtMAp.
2
. Experimental
A first composite material was obtained by adding the previous
colloidal solution, PtMA (150 ml, containing 0.6 mmol of both Pt
and MA) to the LHS-Ac dispersion (300 mg, 2.5 mmol) in fresh
MilliQ water (300 ml) (molar ratio Pt/Ni = 0.15) and filtering a
product, noted LHS-PtMA-1, after 20 h. A second material was
made by redispersing this product in MilliQ water (300 ml), adding
a second batch of grafted platinum (150 ml) and stirring for
another 20 h. This material is noted LHS-PtMA-2.
2
.1. Chemicals
Nickel acetate tetrahydrate (Ni(CH
3
COO)
2
Á4H
2
O 99%), sodium
acetate trihydrate (CH
tate (HSCH COONa 98%, MA) and diethylene glycol (DEG 99%)
were purchased from Acros. Potassium tetrachloroplatinate
PtCl 99.9%) was from Alfa Aesar.
3
COONaÁ3H
2
O 99%), sodium mercaptoace-
2
(K
2
4
Finally, LHS-Ac (400 mg, 3.4 mmol) was left with sodium
mercaptoacetate (342 mg, 3.0 mmol) in fresh MilliQ water
2.2. Synthesis procedures
(
200 ml) with stirring under nitrogen for 20 h before filtering
A reaction scheme describing the different steps of the
and washing with water to obtain a solid denoted LHS-MA.
synthesis of the composites and summarizing the products is
given in Fig. 1.
2.3. Characterizations
The first step consists in synthesizing
a layered nickel
hydroxyacetate salt (solid noted LHS-Ac) according to a previously
described procedure [13]. Nickel acetate (6.222 g, 25 mmol) was
The chemical content of Ni, S and Pt of each sample was checked
by using an energy dispersive spectrometer (EDX) EDAX Genesis
(
SUTW) mounted on a Jeol-JSM 6100 scanning electron microscope
operating at 25 kV. The oxygen content was omitted, as it cannot
be measured correctly.
X-ray photoelectron spectrometry (XPS) measurements were
performed using a Thermo VG Escalab 250 instrument equipped
with a monochromatic Al K (h
n
= 1486.6 eV) 200 W X-ray source.
a
The X-ray spot size was 650
mm. The pass energy was set at 150
and 40 eV for the survey and the narrow scans, respectively. An
À8
electron flood gun was used, under 2 Â 10 mbar partial pressure
of argon, for static charge compensation. These conditions
resulted in a negative but uniform static charge. Measurements
were carried out on powdered samples mounted on sample
holders. Data acquisition and processing were performed with the
Avantage software, version 2.20. The surface composition was
determined using the integrated peak areas and the correspond-
ing Scofield sensitivity factors corrected for the analyser
transmission function. For the majority of the compounds under
study, spectra were calibrated by setting the main C(1s)
component, assigned to aliphatic carbons, to 285 eV [15].
However, such a peak is not expected in the XPS spectrum of
the free mercaptoacetate salt. Consequently, in this case the
highest energy component, assigned to carboxylate-type carbons,
was set to 289.5 eV.
Fourier transform infra-red (FTIR) spectra were collected on KBr
pellets with a PerkinElmer 1750 spectrophotometer in the
À1
transmittance mode between 4000 and 600 cm with a resolution
À1
of 4 cm , 64 scans being accumulated.
Transmission electron microscopy (TEM) images were obtained
on a Jeol-100-CX II microscope operating at 100 kV. The samples
were dispersed in a solvent (ethanol or toluene); one drop of this
solution was deposited on the carbon membrane of the microscope
grid and the solvent was left to evaporate at room temperature.
Microtomy was used to locate more precisely the nanoparticles in
the composite materials. The samples were first embedded in an
Fig. 1. Reaction scheme for the synthesis of composites.

1
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L. Sicard et al. / Materials Research Bulletin 44 (2009) 1692–1699
epoxy resin (from Fluka). They were then sliced with a LEICA ultra-
microtome (EM-UC6) to prepare thin specimens (70 nm thick).
Powder X-ray diffraction (XRD) experiments were carried out
on a X’Pert Pro Panalytical diffractometer equipped with a Co
anode (l = 0.17889 nm) and a multichannel detector (X’Celerator).
The magnetic susceptibility and hysteresis loop measurements
were performed on a Quantum Design MPMS-5S SQUID magnet-
ometer. The DC magnetic susceptibility
field cooling (FC) mode between 2.5 and 320 K in a magnetic field
H) of 200 Oe. The magnetization curves M(H) were obtained at
x(T) was measured in the
(
H = À50 to 50 kOe, after zero-field cooling down to 5 K. All
measurements were performed on the as-prepared powders
slightly compacted in a plastic sampling tube, in order to prevent
their physical movement during the experiments.
3
. Results and discussion
3.1. Formation and chemical analysis of the composite materials
Fig. 2. XPS survey scans of (a) sodium mercaptoacetate, and (b) PtMAp
The powder (LHS-PtMA-1) obtained after a first addition of the
nanoparticles. Insets show the S(2p) high-resolution spectra.
dark colloidal solution of preformed PtMA (platinum nanoparticles
with mercaptoacetate) in DEG to a dispersion of preformed nickel
layered hydroxyacetate salt (LHS-Ac) in water was very dark,
whereas the supernatant was quasi-colorless. Moreover, it is
noteworthy that, in the absence of the layered hydroxyacetate salt,
the particles cannot be centrifuged without the addition of a co-
solvent. This indicates that all the PtMA particles remain trapped in
the powder. EDX analysis was carried out in order to quantify the
Pt and S incorporated in the composite (Table 1). The ratio of Pt/Ni
found in LHS-PtMA-1 is equal to 0.25 and the sodium counter-ion
of the mercaptoacetate salt was absent from the composite
material. Finally, the S/Pt ratio was estimated to be approximately
PtMAp nanoparticles, the S(2p3/2) signal is shifted down to
d
À
163.9 eV, indicating charge transfer and the presence of S
species [16]. An additional contribution is detected at ca. 169.1 eV,
originating from very oxidized sulfur species such as sulfonates. By
peak fitting they are estimated to represent 57% of the total sulfur
species. They correspond to mercaptoacetate molecules not bound
by their mercapto extremity, oxidized after centrifugation in
contact with air. This phenomenon of the oxidation of thiol groups
on exposure to air is classically observed after a few days [16–18].
The Pt(4f) signal observed for PtMAp particles is also split into two
components, Pt(4f7/2) and Pt(4f5/2), at 72.2 and 76.0 eV, respec-
tively. These figures are significantly higher than those obtained
for a Pt(0) compound (70.9 and 74.2 eV). Moreover, the contribu-
tions of two Pt species can be evidenced. Their proportion has been
estimated by fitting the whole Pt(4f) signal, considering two
1
.0, the same ratio as initially introduced. For comparison, the
solution of PtMA alone was also centrifuged after addition of
ethanol. The S/Pt ratio obtained for this PtMAp powder is only 0.5.
This means that only half of the mercaptoacetate ions are grafted
onto the platinum particles, the remaining half staying in solution
in the case of PtMAp, while both the mercaptoacetate grafted to
platinum and the free mercaptoacetate ions interact with the
composite material in LHS-PtMA-1.
In LHS-PtMA-2, obtained after treating LHS-PtMA-1 with a
solution of PtMA, the S/Pt ratio is unchanged and no sodium was
found either. On the other hand, a great increase in the platinum
content is observed: it is twice that obtained after one loading (Pt/
Ni = 0.5 in LHS-PtMA-2). These figures prove the possibility of
obtaining a powder containing a large amount of platinum
nanoparticles.
oxidation states for Pt and, for each species, two peaks (Pt(4f7/2
)
and Pt(4f5/2)). In order to account for the multiplet splitting,
asymmetric peaks were used. On the basis of this analysis the main
component can be assigned to neutral Pt (Pt(4f7/2) peak at 71.8 eV),
d
+
while the other corresponds to Pt species (72.7 eV). In conclusion,
the XPS experiments carried out on PtMAp show clearly an
interaction between the surface Pt of the nanoparticles and the
d
+
thiol extremity of mercaptoacetate, with the formation of a Pt
S
–
d
À
bond.
XPS was then carried out on LHS-Ac and LHS-PtMA-2 (Fig. 3a
and b, respectively). Clear modifications are observed after
addition of the colloidal PtMA solution: the nickel and oxygen
signals are attenuated compared to bare LHS, while new peaks
appear at ca. 72 and 320 eV due to Pt(4f) and Pt(4d), respectively.
Although almost indistinguishable on the survey spectrum, a
signal originating from sulfur core electrons also appears, at ca.
3
.2. Characterization of the chemical interactions between the
elements of the composite material
X-ray photoelectron spectroscopy was used to accurately
analyze the chemical interactions in the composite materials.
First, PtMAp was compared to the free mercaptoacetate salt (see
Fig. 2). The PtMAp spectrum presents additional peaks due to Pt
163 eV. The S(2p) high-resolution spectrum (inset of Fig. 3) reveals
the presence of two types of sulfur species, as previously observed
for PtMAp nanoparticles, with a sulfonate-type contribution at
(Pt(4f) at ca. 72 eV and Pt(4d) at ca. 320 eV) whereas the sodium
1
68.3 eV and a sulfide-type one at 163.4 eV. The proportion of
oxidized species is between 45% and 60% depending on the sample
LHSPtMA-1 or 2). The Pt(4f) signal also resembles that of PtMAp
samples with a broad peak with binding energy of ca. 72 eV for
signal disappears. The S(2p) signal for the mercaptoacetate, shown
in the inset of Fig. 2, displays an asymmetric peak resulting from
the doublets S(2p3/2) (at 164.6 eV) and S(2p1/2). In the case of the
(
0
d
+
Table 1
Pt(4f7/2). It contains the two components Pt and Pt . These
observations confirm that mercaptoacetate ions are still in
interaction with the Pt particles. Finally, the nickel signal is
Atom percentages of platinum, sulfur and nickel in the composite materials.
Sample
Ni
Pt
S
unchanged after PtMA loading, with a Ni(2p) spectrum (Ni(2p3/2
at ca. 857.2 eV) characteristic of Ni species, indicating that the
sheets of the LHS are preserved.
)
LHS-PtMA-1
LHS-PtMA-2
67.1
50.5
16.3
25.0
16.6
24.5
2+

L. Sicard et al. / Materials Research Bulletin 44 (2009) 1692–1699
1695
Fig. 5. FTIR spectra of (a) LHS-Ac, (b) LHS-PtMA-1, (c) LHS-PtMA-2, (d) LHS-MA.
Fig. 3. XPS survey scans of (a) LHS-Ac, and (b) LHS-PtMA-2. Insets show the S(2p)
and Ni(2p) high-resolution spectra.
The spectra of LHS-Ac, LHS-MA, LHS-PtMA-1 and LHS-PtMA-2
are displayed in Fig. 5. The most striking feature is the presence in
all the spectra of the stretching vibrations of acetate groups. The
The system was further characterized by FTIR analysis. The
bands were mainly attributed using reference [19]. As previously,
the sodium mercaptoacetate and the centrifuged PtMAp particles
were first analyzed (Fig. 4a and b, respectively).
The spectrum of mercaptoacetate (Fig. 4a) presents the typical
antisymmetric and symmetric stretching vibrations of (CO
sodium acetate salt at 1588 and 1400 cm , respectively. The
spectrum of PtMAp (Fig. 4b) is quite different. The first striking
observation is the appearance of a band at 1734 cm , which can
À À1
first one,
MA and 1600 cm
observed at 1374 cm
nas(COO ), is observed at 1610 cm for LHS-Ac and LHS-
À1
À
for LHS-PtMA-2. The second,
n
s
(COO ), is
À1
for LHS-MA,
À1
for LHS-Ac, 1350 cm
À1
1355 cm
for LHS-PtMA-2. In each case, the difference
À
À1
2
) of a
Dn
=
n
as
À
n
s
is between 220 and 260 cm , suggesting that the
À1
acetate ions are coordinated to nickel as unidentate species [21]. In
no sample is a band characteristic of C O stretching vibration near
À1
À1
1740–1700 cm detected. This means that all the acetate and
be assigned to the C O stretching vibration of a carboxylic acid.
The second difference is the appearance of two intense bands at
mercaptoacetate terminal groups are in the ionic form rather than
the acid form, –COOH, contrary to the case of the PtMAp powder.
The second point is the presence of intense bands at 1125 and
À1
À
1
139 and 1052 cm . These can be assigned to SO
3
antisymmetric
À
+
À1
and symmetric stretching vibrations of RSO
a
3
H
3
O
type
1028 cm for the three samples LHS-MA, LHS-PtMA-1 and 2. They
À
compound [20]. Consequently, it can be concluded that the
mercaptoacetate ions are present in the PtMAp sample but have
undergone several changes: (i) some of the thiol extremities have
been oxidized to hydrated sulfonic acid after centrifugation in
contact with air, in agreement with XPS conclusions and other
studies [16–18]; (ii) some acetate extremities have been proto-
nated, explaining the absence of sodium, established by XPS. The
protons gained by the acetate groups probably come from the thiol
extremities. This coexistence of both the carboxylic acid and the
carboxylate form has already been observed when sodium
mercaptoacetate was used to coat gold nanoparticles [20].
were already present in PtMAp and assigned to SO
metric and symmetric stretching vibrations of a RSO
3
antisym-
3À
3
+
H O type
compound. These observations lead to the conclusion that the
sulfur-containing extremity of mercaptoacetate is not much
modified during the adsorption on LHS, whereas the other
extremity, –COOH in the PtMAp sample, is deprotonated during
the adsorption process even though the experiment is carried out
in water. This point, together with the absence of sodium, suggests
that some mercaptoacetate ions interact with the LHS sheets
through their carboxylate extremities. This can be related to the
chemical analysis previously reported: the amount of mercaptoa-
cetate in LHS-PtMA-1 and 2 (S/Pt = 1) is twice that in PtMAp (S/
Pt = 0.5). This means that 0.5 mercaptoacetate per Pt, and thus
0
.125 and 0.25 mercaptoacetate per Ni (=S/Pt  Pt/Ni, see Table 1),
respectively, have replaced acetate ions in LHS-PtMA-1 and 2 to
assure the neutrality of the LHS sheets.
3.3. Morphological characterization
A TEM image of the platinum particles is presented in Fig. 6.
Very small, almost isotropic, monodisperse and non-agglomerated
objects with a mean diameter of 1.7 Æ 0.3 nm are observed. These
features are the result of a very rapid precipitation (high heating rate
and quenching just after the appearance of the particles) and of the
interaction with acetate ions which limit particle growth and prevent
aggregation.
TEM images of LHS-Ac, LHS-MA, LHS-PtMA-1 and LHS-PtMA-2
materials are reported in Fig. 7a–f. The particles of LHS-Ac appear
as thin sheets forming aggregates (Fig. 7a). No significant evolution
of the morphology is noticed after 20 h in a mercaptoacetate
solution (Fig. 7b).
Fig. 4. FTIR spectra of (a) sodium mercaptoacetate, and (b) PtMAp nanoparticles.

1
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L. Sicard et al. / Materials Research Bulletin 44 (2009) 1692–1699
non-aggregated thin platelets interacting with densely packed
platinum particles are observed all over the grid (Fig. 7e and f). In
the two samples, LHS-PtMA-1 and 2, quasi-parallel lines of
particles are observed at the edges of the LHS platelets, which
are probably bent up (Fig. 7d–f). In order to be sure that these lines
cannot be attributed to artefacts and to determine whether the
platinum particles are incorporated within the interlamellar space
of the LHS, powders were embedded in a resin and TEM was
performed after microtomy. The aspects of the particles of LHS-Ac
(
Fig. 8a) and LHS-PtMA-2 (Fig. 8b and c) are very different. For the
composite, alignments of particles are seen all over the grid. The
distance between two adjacent particles in one interslab is
estimated to be ca. 3.0 nm and that between two lines ca.
4.0 nm. The fact that the lines are parallel and that the distance
between two lines is more or less constant argues in favor of an
intercalation of the particles.
Fig. 6. TEM images of as-synthesized platinum particles.
3.4. Structural characterization
The X-ray powder pattern of the LHS-Ac phase (Fig. 9a) can be
The composite obtained after one contact with the solution of
indexed in the hexagonal system and is characteristic of a stacking
of brucite-like sheets, as previously described by Poul et al. [13]
and Taibi et al. [22] The first three peaks correspond to (0 0 l)
reflections with an interlayer spacing c = 0.96 nm. The fourth peak
corresponds to the (1 0 0) reflection with the in-plane parameter
a = 0.31 nm. The asymmetry of the in-plane reflections is due to the
turbostratic character. The structural parameter a is the same as
that previously reported by Poul and Taibi, but c is slightly lower
(reported interlayer spacing of 1.06 nm). This variation can be
explained on the basis of different water contents and/or a change
PtMA presents nearly the same sheet morphology without well
defined shape and arrangement (Fig. 7c). However, the LHS
particles are less aggregated and they seem very thin, as the
contrast between the layers and the carbon grid is very low
(
Fig. 7d). The Pt particles adsorb preferentially on some sheets,
while other sheets remain free from particles, resulting in an
inhomogeneous material. Finally, very few particles are detected
beside the LHS sheets, confirming the strong interaction with the
LHS sheets. After a second contact with the PtMA solution, mainly
Fig. 7. TEM images of (a) LHS-Ac, (b) LHS-MA, (c–d) LHS-PtMA-1, (e–f) LHS-PtMA-2.

L. Sicard et al. / Materials Research Bulletin 44 (2009) 1692–1699
1697
Fig. 8. TEM images after microtomy of (a) LHS-Ac, and (b) and (c) LHS-PtMA-2.
in their arrangement in the interlayer spacing. For comparison, if
the LHS is left for 20 h in a mercaptoacetate solution to produce the
LHS-MA phase, the structure is maintained, with the same in-plane
parameter (XRD pattern not given). However, the distance
between layers shrinks (c = 0.89 nm) owing to hydrogen bonding
between the salt and the layers.
formula with a shape factor for spheres (K = 0.89). The particle size
evaluated in this way is about 1.5 nm, which is in good agreement
with the TEM measurements.
Finally, a fast increase in intensity is visible as the angle
becomes very small. Consequently, a new diagram was recorded at
2u from 28 to 158 with narrower slits in order to detect the peaks
After the first contact of LHS-Ac with the PtMA particles, the
characteristic peaks of the LHS structure are still visible, even if less
intense (Fig. 9b). The structural parameter a remains constant as
compared to LHS-Ac and c is increased to 1.06 nm, the value found
for the LHS-Ac phase by Poul et al. [13]. The relative intensities of
the peaks are modified: the ratio between the intensities of the
which could be present in this interval (see inset of Fig. 9). Besides
the (0 0 1) peak of the LHS structure near 10.68, a broad peak which
does not exist in the pristine LHS sample (XRD pattern not given at
low angles) appears at d = 2.2 nm. It cannot be attributed to the
distance between two particles adsorbed on a same LHS sheet, as
this distance was estimated by TEM to be ca. 3.0 nm.
(
0 0 1) and (1 0 0) peaks (I(0 0 1)/I(1 0 0)) decreases when the LHS
The decrease in the intensity of the (0 0 1) peak of the LHS-Ac
structure together with the appearance of a new broad peak at
2.2 nm could be more likely attributed to the co-existence of two
phases: the initial one (LHS-Ac) and a new one (poorly organized, as
only one peak is observed). The new phase which is formed cannot
is loaded with nanoparticles. This observation is also true for the
ratio I(0 0 1)/I(1 1 0). This suggests that the order within the sheets
is retained whereas the correlation length between two sheets is
not always the same in the whole sample.
Besides the LHS structure peaks, a very broad peak appears on
be a lamellar phase in which the acetate is replaced by
the XRD pattern near 2
reflection of platinum, the (1 1 1) peak. The width of the peak can
be related to the size of the platinum particles using the Scherrer
u
= 468 corresponding to the most intense
mercaptoacetate (and/or sulfonate) anions. Indeed, the (0 0 1) peak
should appear at 0.89 nm in this case. TEM images show parallel
lines of particles spaced out by 4.0 nm. This distance corresponds
approximately to the diameter of one particle and the length of two
mercaptoacetateionsand twoLHSwallthickness. Consequently, the
new phase could correspond to the LHS in which one out of two
(
most of the time) interslabs of LHS is filled with nanoparticles. Such
compounds are known as second-stage intercalation compounds,
whoseformationhasbeendescribedbyDaumasandH e´ rold[23]and
which have been observed in several LDH compounds [24–26]. For
these materials containing alternate layers with two different basal
1 2
spacings, d and d , then the basal reflections expected in XRD are
(d
1
+ d )/n, with n = 2 usually the most intense. In the present case,
2
thisreflectioncanbeexpectedataspacingofca. 2 nm.Consequently,
the peak at low angles can be due to this phase. The other reflection
peaks should be less intense and could contribute to the width of the
peaks. However, the structure is clearly not so well organized, and
has filling defects and medium-range disorder.
Finally, LHS-PtMA-2 was also characterized. For this sample, the
peaks characteristic of the LHS structure continue to decrease in
intensity compared to the broad peak due to platinum nanopar-
ticles at 0.23 nm and that at 2.2 nm (Fig. 9c), in agreement with the
EDX results. The intensity ratios (I(0 0 1)/I(1 0 0) and I(0 0 1)/
I(1 1 0)) continue to decrease compared to their values in LHS-
Fig. 9. XRD patterns of (a) the LHS-Ac, (b) the LHS-PtMA-1, and (c) LHS-PtMA-2. Low
angles are presented in the insets.

1
698
L. Sicard et al. / Materials Research Bulletin 44 (2009) 1692–1699
Table 2
c c
Curie temperature (T ), coercivity (H ), saturation magnetization per gram of
sample (Msat) and remanence to saturation ratio (M
r
/Msat) of LHS salts before and
after loading with platinum particles.
À1
Sample
T
c
(K)
H
c
(Oe)
M
sat (emu g
)
r
M /Msat (%)
LHS-Ac
14
14
10
10
750
745
285
285
97.8
96.0
50.0
39.0
39
38
26
19
LHS-MA
LHS-PtMA-1
LHS-PtMA-2
PtMA-1 and LHS-Ac. This correlates well with the TEM images,
which show that more LHS particles contain PtMA particles.
3.5. Magnetic measurements
Fig. 11. Hysteresis loops (per gram of product) measured at T = 5 K for: (a) LHS-Ac,
b) LHS-PtMA-1, (c) LHS-PtMA-2, (d) PtMAp, (e) LHS-MA.
(
The temperature dependence of the mass magnetic suscept-
ibility
m m
x (per gram of product) and the x T product are plotted in
Fig. 10. The variation of the mass magnetization (M), when the
magnetic field (H) is cycled at 5 K, is plotted in Fig. 11 for the
reference LHS-Ac, LHS-MA and PtMAp compounds and for the two
composites, LHS-PtMA-1 and LHS-PtMA-2.
Apart from PtMAp, which exhibits a Pauli-like paramagnetic
behavior characterized by a magnetic susceptibility temperature-
independent, all samples exhibit ferromagnetic behavior: (i) at
2
+
3
À1
3
À1
with respect to Ni ) and 0.412 cm g K (89 cm K mol ) for
LHS-PtMA-1 and 2 respectively. Similarly, the 5 K saturation
magnetization (Msat) of the two reference compounds, LHS-Ac and
LHS-MA, obtained by extrapolating the curve of M vs. 1/H to zero, is
À1
À1
2+
found to be ca. 97 emu g , corresponding to 1.9
This value is very close to that expected (2 /Ni) for an oxygen-
coordinated Ni ion. The values obtained for LHS-PtMA-1 and 2
(Table 2) are significantly lower due to the very weak magnetic
contribution of Pt. When expressed per mol of Ni , Msat is equal to
1.6 and 1.5 for LHS-PtMA-1 and 2, respectively. The weakening
of the magnetization when Pt is intercalated in the interslabs
contributes also to a decrease in the coercitive field (H ) from LHS-
Ac and LHS-MA to LHS-PtMA-1 and 2. These net modifications of
the characteristic magnetic values after loading indicate modifica-
tions of the interlayer interactions and agree well with the
intercalation of platinum nanoparticles within the interlamellar
space.
m mol of Ni .
B
m
B
2
+
high temperature, the
x
m
T product increases when the tempera-
ture decreases, suggesting the occurrence of short-range ferro-
magnetic interactions, namely ferromagnetic in-layer nickel
2
+
hydroxide plane interactions, (ii) at lower temperature
x
m
m
B
increases to reach a kind of plateau when the temperature
decreases, a phenomenon which can be related to the occurrence
of 3D ferromagnetic correlations and (iii) the variation of M versus
c
H presents an hysteresis loop. The Curie temperature, T
determined at the maximum (not shown) of the curve of T as a
function of T. The values obtained are summarized in Table 2. T is
c
, is
x
m
c
equal to ca. 14 K for LHS-Ac and LHS-MA, and 10 K for LHS-PtMA-1
and 2. This variation is probably due to modifications of the
interactions between the layers with the introduction of Pt
4. Conclusions
particles. The maximum value of
m
x T as a function of T decreases
3
À1
3
À1
2+
from 1.016 cm K g
of product (121 cm K mol
of Ni ) to
In this paper we report the formation of nanocomposites made
from platinum nanoparticles and a lamellar nickel hydroxyacetate
salt by the mediation of a short organic molecule: a mercaptoa-
cetate salt. The two starting blocks of the nanocomposite were
made separately by the polyol process, a simple and rapid method
which assures very good control of the particle morphology. The
platinum particles are very small, monodisperse objects (1.7 nm in
diameter). The acetate ions adsorbed at their surface were
exchanged for mercaptoacetate species which are compatible
with both the metal particles and the LHS compound. This
functionalization of the platinum particles by mercaptoacetate
species was followed by XPS and IR spectroscopies. The shift of the
surface platinum binding energy in the XPS spectra indicates that
strong bonds are formed with the salt. Infrared spectra indicate
that the grafted molecules are in the acid rather than the acetate
form. This probably results from proton release by the thiol group
when it bonds to platinum. This is confirmed by the absence of a Na
signal in the XPS spectra. Moreover, an oxidation of some thiol
extremities is observed. The contact between a colloidal solution of
grafted platinum particles and a layered hydroxyacetate nickel salt
leads to the formation of a composite with a high particle content.
Structural study and magnetic characterizations show that the
structure of the nickel hydroxide layers is retained in the
composite materials. Mercaptoacetate ions are in this case in
their basic form, as attested by IR spectroscopy. TEM images show
the incorporation of the platinum nanoparticles within the layers
of some lamellar sheets. XRD and magnetic measurements are in
3
3
À1
0
.773 cm K (101 cm K mol ) as the basal spacing decreases
from 0.96 to 0.89 nm on going from LHS-Ac to LHS-MA. This
interesting behavior is related to the influence of the weakening of
the demagnetization field when the interlayer distance increases
3
À1
[
m
22]. The maximum value of x T falls to 0.428 (79 cm K mol
Fig. 10. Variation of the susceptibility (per gram of product) vs. temperature
measured at H = 200 Oe for: (a) LHS-Ac, (b) LHS-PtMA-1, (c) LHS-PtMA-2, (d)
PtMAp, (e) LHS-MA. The variations of
inset.
m
x T as a function of T are presented in the

L. Sicard et al. / Materials Research Bulletin 44 (2009) 1692–1699
1699
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