Green wet chemical route for the synthesis of silver and palladium dendrites

Article abstract of DOI:10.1002/ejic.201001259

A novel and very effective wet chemical route for the preparation of metal dendritic nanostructures was developed by starting from VOSO₄ and Ag₂SO₄ (or K₂PdCl₄) in aqueous solution at room temperature. No template or surfactant was used. VOSO ₄ serves as the reducing agent for the silver salt, and the resulting silver nanoparticles and nanocrystallites are then arranged into dendritic structures. The morphologies of the dendrites are found to be heavily influenced both by the molar ratio and the concentrations of the reactants. Observation of symmetric and asymmetric dendrites with variations of these parameters supports the proposal that diffusion and oriented attachment of silver nanoparticles or crystallites might compete, to various extents, and dictate the final morphologies of dendritic nanostructures. To the best of our knowledge, this is the first example of a surfactant-free, template-free, fully inorganic wet chemical route to metal dendritic structures. The method was also successful in the preparation of palladium nanodendrites. The simple mixing of Ag⁺ and VO²⁺ in water, at room temperature, results in the very efficient synthesis of silver nanodendrites in the absence of any template or surfactant. The same procedure proved successful for the synthesis of Pd nanodendrites. Copyright

Full text of DOI:10.1002/ejic.201001259

SHORT COMMUNICATION
DOI: 10.1002/ejic.201001259
Green Wet Chemical Route for the Synthesis of Silver and Palladium
Dendrites
[
a]
[a,b]
[a]
[a]
Bineta Keita,* L. R. Brudna Holzle,
Rosa Ngo Biboum, Louis Nadjo,
[c]
[d]
[d]
[e]
Israel M. Mbomekalle, Sylvain Franger, Patrick Berthet, François Brisset,
[f]
[a]
Frederic Miserque, and Georges A. Ekedi
Keywords: Nanoparticles / Nanostructures / Dendrites / Silver / Palladium
A novel and very effective wet chemical route for the prepa-
ration of metal dendritic nanostructures was developed by
tants. Observation of symmetric and asymmetric dendrites
with variations of these parameters supports the proposal
that diffusion and oriented attachment of silver nanoparticles
or crystallites might compete, to various extents, and dictate
the final morphologies of dendritic nanostructures. To the
best of our knowledge, this is the first example of a surfac-
tant-free, template-free, fully inorganic wet chemical route
to metal dendritic structures. The method was also successful
in the preparation of palladium nanodendrites.
starting from VOSO
solution at room temperature. No template or surfactant was
used. VOSO serves as the reducing agent for the silver salt,
4 2 4 2 4
and Ag SO (or K PdCl ) in aqueous
4
and the resulting silver nanoparticles and nanocrystallites
are then arranged into dendritic structures. The morpho-
logies of the dendrites are found to be heavily influenced
both by the molar ratio and the concentrations of the reac-
Introduction
free synthesis of Ag nanodendrites by Yang et al. was per-
formed with use of a suspension of zinc microparticles as a
heterogeneous reducing agent. As a truly wet chemical
The more attractive electrocatalysts currently in use for
redox processes are based on metal nanostructures. The
major challenge remains the development of efficient,
stable, cheap, and environmentally friendly materials. Den-
dritic architectures have been recognized in several reports
to fulfil most of the requirements for exhibiting enhanced
properties relative to both discrete nanoparticles and bulk
powders. Applications of such structures in the catalysis
and technology fields are gaining widespread impor-
[4]
route, a novel preparation of snowflake-like dendritic nano-
structures of Ag or Au at room temperature was proposed
[5]
with p-phenylenediamine or dendritic Ag with ascorbic
[6]
acid, provided the organic reducing agent was in large ex-
cess. Finally, dendritic silver synthesis by a surfactant-free,
mixed-solvent water–acetone route by using l-ascorbic acid
[7]
was recently published. Examples of wet chemical routes
for the synthesis of dendritic Pd nanostructures are even
[
1–3]
tance.
In most cases, the electrochemical method is used
[8,9]
more scarce.
for the synthesis of metal dendritic structures. Concentrat-
ing specifically on the case of Ag, template- and surfactant-
We now report on silver and palladium dendrites gener-
ated by a simple wet chemical route using only VOSO in
4
water at room temperature, without any template or surfac-
tant. One of us (B. K.) described this method recently in a
[
a] Groupe d’Electrochimie et de Photoélectrochimie, Laboratoire
de Chimie Physique, UMR 8000, CNRS, Université Paris-Sud
XI,
[
10]
patent.
In the global search for platinum-free catalysts,
Bâtiment 350, 91405 Orsay Cedex, France
E-mail: bineta.keita@u-psud.fr
silver and palladium could be good candidates, provided
one can elaborate judicious nanostructures in order to im-
[
[
[
b] Universidade Federal do Pampa, Campus de Bagé,
Rua Carlos Barbosa, s/n°, Getúlio Vargas Bagé – RS, CEP prove their efficiencies. Despite extensive previous use of
9
6412-420, Brazil
oxides and polyoxometalates (POMs) for the generation of
c] Institut Lavoisier, UMR 8180, CNRS, Université de Versailles
Saint-Quentin,
[
11]
metal nano-objects,
the present report constitutes the
4
5, avenue des Etats-Unis, 78035 Versailles Cedex, France
first example of surfactant-free, template-free, fully inor-
ganic wet chemical route to dendritic structures.
d] Laboratoire de Physico-chimie de l’Etat Solide, ICMMO,
UMR 8182, CNRS, Université Paris-Sud XI,
Bâtiment 410, 91405 Orsay Cedex, France
[
[
e] ICMMO, UMR 8182 CNRS, Université Paris-Sud XI,
9
1405 Orsay Cedex, France
f] Commissariat à l’Energie Atomique (Saclay)/Département de Results and Discussion
Physico-Chimie/Laboratoire Réactivité des Surfaces et
Interfaces,
The precursor salts were Ag SO and VOSO . In a typi-
2
4
4
Bâtiment 391, 91191 Gif-sur-Yvette cedex, France
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201001259.
cal experiment for Ag dendrite synthesis, a mixture contain-
+
ing 2 mm Ag SO (4 mm Ag ) and 4 mm VOSO was as-
2
4
4
Eur. J. Inorg. Chem. 2011, 1201–1204
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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B. Keita et al.
SHORT COMMUNICATION
Figure 1. (A) Low-magnification TEM image of a typical symmetric silver dendrite obtained for γ = 1 and C0Ag+ = 4.0 mm; (B) XRD
pattern of as-synthesized silver nanodendrites; (C) deconvolution of the silver core 3d level XPS spectrum; (D) EDS microanalysis of the
dendrites.
sembled in Millipore water and kept at room temperature. 374.2Ϯ0.3 eV, respectively, indicating that metallic silver
+
The excess parameter is defined as γ = [Ag ]/[VOSO ]. The was deposited (Figure 1C). X-ray energy dispersive spec-
4
color of the solution changed rapidly, and a large amount troscopy (EDS) microanalysis (Figure 1D) shows only the
of precipitate gradually formed. The solid was collected by Ag peak at about 3 eV. The other peaks in this spectrum
centrifugation, thoroughly washed several times with water, can be assigned to Cu from the Cu grid, C from holey car-
and finally re-dispersed in water for characterization. A rep- bon on the grid, in particular, Al and Si coming from the
resentative low-magnification TEM image of a typical den- sample holder (STEM detector). It is worth noting that the
dritic nanostructure is shown in Figure 1A for C0
4
=
amount of vanadium is negligibly small. In summary, pure
mm and γ = 1. The dendrite appears as a remarkably hier- Ag metal nanodendrites are obtained. Detection of vana-
Ag+
archical structure constituted by several generations with dium in XPS spectra in the as-prepared mixtures is dis-
apparent self-similarity. An overall high symmetry is ob- cussed and rationalized in the Supporting Information in
served, with angles between the stem and branches uni- conjunction with XRD and EDS analyses (Figure S2).
formly in the range 50°–60°. Branches at the tip of the stem
Depending on the initial concentrations of reactants, dif-
are shorter than those at its base, thus creating a tree-like ferent silver nanostructures are observed (see Figure 2 and
structure. The selected area electron diffraction (SAED) the accompanying Figure S3). In Figure 2, the value γ = 1
spectra recorded at several positions, including stem and was kept throughout. With C0Ag+ = 0.25 mm, a representa-
branches, exhibit a typical hexagonal symmetry (that in the tive TEM image in Figure 2A shows long nanowires. An-
inset of Figure 1A indicates that the dendrite is a single other domain shows that these nanowires are actually sur-
crystal; see also Figure S1, obtained from a whole branch). rounded by an abundance of nanoparticles (Figure S3A).
The XRD diffraction peaks collected in the range 10° Ͻ2θ Also, Figure 2B includes some nanowires with very short
Ͻ80° at 2θ = 38.14, 44.35, 64.47, and 77.47 (Figure 1B) can branches prefiguring dendritic structures. Finally, starting
be indexed to the (111), (200), (220), and (311) reflections from values of C0Ag+ = 1.0 mm and higher, dendrites be-
of the cubic Ag, respectively. The intensity ratio between come by far the dominant morphological structures with
the (111) and the (220) XRD diffraction peaks is higher two simultaneously observed main variants: symmetrical
than that of the standard file (JCPDS 04–0783) (5.7 vs. 2.5), dendrites (Figures 1A and 2C) and asymmetrical ones (Fig-
which confirms that the dendrites grow with a preferred ure 2D) observed for all concentrations C0Ag+ Ն1.0 mm.
orientation [large fraction of (111) facets]. No vanadium Asymmetrical dendritic structures appear dense, with fewer
oxide was detected in this diffraction spectrum. The XPS straight stems and branches. The angles between stems and
spectrum in the Ag-3d5/2 and Ag-3d3/2 binding energy branches are rather random, giving the appearance of a dis-
region shows peaks centered at 368.2Ϯ0.3 and ordered structure (see Figures S3C and S3D). In addition,
1202
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Eur. J. Inorg. Chem. 2011, 1201–1204

Green Wet Chemical Route to Silver and Palladium Dendrites
Figure S3D highlights the presence of an environment of
organized nanostructures, which seem to consist of vana-
dium oxide, containing Ag nanoparticles (see also Fig-
ure S2). Future work will try to elucidate the very nature of
this environment. No significant new feature was observed
0
for other combinations of γ and C Ag+.
0
Figure 3. TEM images of typical Pd dendritic structures obtained
0
for γ = 1 and C Ag+ = 0.25 mm: (A) very low magnification; (B)
high magnification.
scarce.[4–9] Second, whatever the method, the observed final
morphology of the nanodendrites depends on several pa-
rameters, including the extent of adsorption or complex-
ation among reactants, the working electrode potential in
electrochemistry, and the injection sequence of reactants
and eventual surfactants in wet chemical routes. In the pres-
ent example of palladium nanodendrites, it is likely that the
observed morphology, as also described in the literature,[
must be ascribed to the interplay of such parameters, re-
sulting in the modulation of the reduction kinetics at the
early stages of the reaction.
9]
Conclusions
Figure 2. (A) Low-magnification TEM image of the centrifuged
The present observations support the assumption that
the simple use of VOSO as reducing agent, in water, at
nanowires (γ = 1 and C0Ag+ = 0.25 mm); (C) new representative room temperature, is a general method for the preparation
0
0
and washed Ag nanowires (γ = 1 and C Ag+ = 0.25 mm); (B) high-
0
magnification TEM image of the centrifuged and washed Ag
4
low-magnification TEM image of a typical symmetric silver den-
of dendritic Ag and Pd nanostructures from Ag SO and
2
4
drite obtained for γ = 1 and C0Ag+ = 4.0 mm; (D) TEM image of
K PdCl , respectively. To the best of our knowledge, this is
0
2
4
a typical asymmetric silver dendrite obtained for γ = 1 and C Ag+
the first use of VOSO as reducing agent for the synthesis
of dendritic metal nanostructures. The method might con-
4
=
10.0 mm.
Various models are useful to analyze fractal phenom- stitute a general wet chemical route for the fabrication of
ena,[
12]
including the deposition–diffusion aggregation noble metal nanodendrites, including Ag and Pd, and will
model, the diffusion-limited aggregation (DLA) model, and be extended to other metals. Future work will explore the
the cluster–cluster aggregation model. Possible mechanistic properties of these structures, including electrocatalytic ap-
pathways for the formation of symmetrical and asymmetri- plications.
cal dendrites are developed in the Supporting Information.
Supporting Information (see footnote on the first page of this arti-
cle): Experimental conditions, mechanisms of dendrite formation,
2
The same approach was applied to a mixture of K PdCl4
and VOSO with γ = 1. Figure 3 shows representative den- additional TEM images with X-ray diffraction, XPS, XRD and
4
dritic features observed at a low palladium concentration. EDS analyses.
Further details are presented in the Supporting Infor-
mation. Figure 3B shows a representative magnified image
Acknowledgments
of the blackberry-like dendritic structure obtained for
0
C
= 0.25 mm with a diameter around 185 nm. The
Pd2+
Financial support from the Centre National de la Recherche Sci-
entifique (UMR 8000, UMR8180 and 8182), Université Paris-Sud
same dendritic structure was observed with higher concen-
trations (Figure S4). However, its size and dispersity depend 11 and Université de Versailles are gratefully acknowledged.
on the initial concentration. XRD spectra for several initial
concentrations of reactants (Figure S4C) indicate the good
crystallinity of the Pd deposits.
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[
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position
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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