Synthesis, stabilization, and applications of nanoscopic siloxane-metal particle conjugates

Article abstract of DOI:10.1016/S0022-328X(03)00612-0

This account reports the new directions in the synthesis and characterization of polysiloxane-encapsulated metal nanoparticles and their applications to catalysis. These materials are prepared by sequestering metal ions with hydropolysiloxanes followed by

Full text of DOI:10.1016/S0022-328X(03)00612-0

Journal of Organometallic Chemistry 686 (2003) 24ꢀ31
/
www.elsevier.com/locate/jorganchem
Synthesis, stabilization, and applications of nanoscopic siloxaneꢀ
/
metal particle conjugatesꢀ
Bhanu P.S. Chauhan *, Jitendra Rathore, Rajesh Sardar, Pankaj Tewari, Umar Latif
Polymers and Engineered Nanomaterials Laboratory, Department of Chemistry and Graduate Center, City University of New York at The College of
Staten Island, 2800 Victory Boulevard, Staten Island, New York, NY 10314, USA
Received 1 April 2003; accepted 28 May 2003
Abstract
This account reports the new directions in the synthesis and characterization of polysiloxane-encapsulated metal nanoparticles
and their applications to catalysis. These materials are prepared by sequestering metal ions with hydropolysiloxanes followed by
chemical reduction to yield the corresponding zerovalent metal nanoparticles. The size and processability of such particles depends
on the metal to polysiloxane ratios. Our method enables routine formation of stable nanometallic reservoirs in organic solvents
avoiding particle aggregation during the storage as well as nucleation and growth process. Catalytic activity of nanoparticles vs
metal complexes compares favorably for silaesterification reactions. We also demonstrate the utility of such reservoirs in grafting the
surface properties of nanosized silver particles by ligand exchange reactions with thiols and phosphine oxide surfactants.
#
2003 Elsevier B.V. All rights reserved.
Keywords: Polymer; Metallic nanoparticles; Silaesterifications; Polysiloxanes; Silver organosols
1
. Introduction
technological applications [4ꢀ6], including photonic
/
devices, catalysis, corrosion protection, solar energy
conversion, and chemical or biochemical sensors.
Interest in condensed matter at size scales larger than
atoms but much smaller than bulk solids has grown
rapidly over the last few years. Matter containing from
tens to thousands of atoms can have structures and
properties significantly different from those of conven-
tional materials [1]; consequently, the current research
on nanostructured materials is principally devoted to the
synthesis, characterization, and understanding of
changes in the fundamental properties.
Particularly interesting is the study of metal properties
behavior on a nanometric scale [2,3]. Size-dependent
changes in band-gap energy, excited-state electronic
behavior, and optical spectra are generated that differ
drastically from those known for the bulk limit. In
addition, the new characteristics of this class of materi-
als make them really attractive for a number of
Preparation and characterization of nanocrystallites is
a very critical area of research for fundamental under-
standing and tailoring of materials properties of prac-
tical use. Nanocomposite materials require nanometric
particles with uniform size, controlled dimensions, and
regular shape. Such particles can be obtained by
solution chemistry (chemical precipitation and solꢀgel
/
technique) and by vapor deposition (gas evaporation,
laser ablation, and sputtering) [7]. However, solution
chemistry is the only technique that provides a cost-
effective method for the production of large quantities
of nanoparticles and allows one to manipulate matter at
the molecular level. Solution chemistry is the most
practical route for the synthesis of nanoscale particles,
but the control of size distribution, particle morphology,
and crystallinity still need further investigation.
The synthesis of soluble transition-metal nanoclusters
has been accomplished using five general methods [8,9]:
ꢀ
Invited contribution for ‘‘What’s new in Silicon Chemistry in
2
003’’.
(
i) the chemical reduction of transition-metal salts; (ii)
*
910.
Corresponding author. Tel.: ꢁ1-718-982-3902; fax: 1-718-982-
/
the electrochemical reduction of transition-metal salts;
(iii) thermal or photochemical decomposition of transi-
3
E-mail address: chauhan@postbox.csi.cuny.edu (B.P.S. Chauhan).
0022-328X/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0022-328X(03)00612-0

B.P.S. Chauhan et al. / Journal of Organometallic Chemistry 686 (2003) 24ꢀ
/
31
25
tion-metal precursors; (iv) ligand reduction and displa-
cement from organometallic compounds; and (v) metal
vapor synthesis. Some synthetic techniques use a
combination of these five methods; e.g., sonochemical
While our group is involved in devising versatile and
practical routes to semiconductor and metallic nano-
crystals and their property profiling, due to space
limitations, we will limit our discussion to novel metals.
In this account, we will summarize, the synthesis and
characterization of polysiloxane-encapsulated metal na-
noparticles their thermal, catalytic, optical, and photo-
thermal properties.
[
10] preparations of nanoclusters involve either (i) or (iii)
or a combination of (i) and (iii) [10ꢀ15]. Of the five
/
methods, the chemical reduction of transition-metal
salts is by far the most common.
Transition-metal nanoclusters are only kinetically
stable because the formation of bulk metal is the
thermodynamic minimum [16,17]. Therefore, nanoclus-
ters that are freely dissolved in solution must be
stabilized in a way that prevents the nanoclusters from
2. Motivations and rationale for approach
Siloxane polymers are a very versatile class of
polymers, which have been used as additives to produce
or enhance a number of physical properties ranging
from water repellency to thermal stability. Monomeric
hydrosilanes are known reducing agents and have been
successfully used for the generation of Pt, Pd, and Rh
nanosized particles in the context of metal-catalyzed
hydrosilylation of alkenes [26]. On the other hand,
investigations of polyhydrosilanes as reducing agents
for the generation of nanosized silver particles have not
been explored. Though, their property profile may
provide the means of directing metallic particles into
specific physicochemical environments in addition to
their utility as reducing agents. In our approach, well-
defined silicon polymers (polysiloxanes) are envisioned
to act as reducing agents, as well as templates to control
the size, stability, and solubility of nanoparticles ranging
in diameter from less than 1 up to 10 nm.
diffusing together and coalescing*any such agglomera-
/
tion would eventually lead to the formation of the
thermodynamically favored bulk metal [16]. Nanoclus-
ter stabilization is usually discussed in terms of two
general categories of stabilization, electrostatic and
steric [18,19]. Electrostatic stabilization is achieved by
the coordination of anionic species, such as halides,
carboxylates or polyoxoanions, to the coordinatively
unsaturated surface metal atoms of the metal particles
[
18]. This results in the formation of an electrical double-
layer (really a diffuse electrical multilayer) [20], which
causes coulombic repulsion between the nanoclusters.
Steric stabilization is achieved by the presence of bulky,
typically organic materials that, due to their bulk,
impede the nanoclusters from diffusing together [18].
Polymers, dendrimers, and large alkylammonium ca-
tions are examples of organic steric stabilizers. The
choice of stabilizer also allows one to tune the solubility
of the nanoclusters [21,22].
Monolithic ceramic and polymeric templates have
also been used for preparing nanomaterials. For exam-
ple, the well-defined pores in alumina or polymeric
filtration membranes can be used to define the geome-
trical and chemical properties of metal, semiconductor,
and polymeric nanomaterials [23]. In many cases, the
template can be removed chemically or thermally,
leaving behind the naked nanomaterial. The obvious
advantage of this technique is that highly monodisperse
particles with a variety of shapes, sizes, and chemical
compositions can be prepared [24].
The polymer template usually serves to both control
particle size and passivate the surface of the nanopar-
ticles against agglomeration. A number of polymers can
be used as protective agents (e.g. poly(vinylalcohol),
poly(methylvinylether), sodium polyacrylate, poly(N-
vinylpyrrolidone) (PVP)) [25]; such a component allows
one to recover the fine particles as a polymer-based
composite. Moreover, diblock copolymers with metal-
ion affinities can be used to sequester metal ions into
localized domains that can subsequently be converted
into metal nanostructures. A wide range of metal
particles has been formed within such polymer templates
including Cu, Ag, Au, Pt, Pd, and Rh.
Polysiloxanes are, particularly, well suited for hosting
metal nanoparticles for the following reasons: (1) the
selected polysiloxane templates themselves are of fairly
uniform composition and structure, and, therefore, they
may yield well-defined nanoparticle replicas, (2) unusual
freedom of rotation around the SiÃ
polymers a high degree of flexibility while maintaining
their integrity, (3) polysiloxanes with SiÃH functional-
/O bond allows these
/
ities can act as reducing agents and thus can eliminate
the need for extra reducing agents, (4) the encapsulated
nanoparticles are confined primarily by steric effects,
and, therefore, a substantial fraction of their surface is
unpassivated and available to participate in surfactant
exchange and catalytic reactions, (5) the substituent
branches can be used as selective gates to control access
of small molecules (substrates) to the encapsulated
(catalytic) nanoparticles, and (6) the terminal groups
can be tailored to control solubility of the hybrid
nanocomposite and used as handles for facilitating
linking to surfaces and other polymers. As will be
discussed later, these attributes take full advantage of
the unique structural and chemical properties of poly-
siloxanes. Indeed, polysiloxanes/nanoparticle compo-
sites can represent an unusual case of the template and
replica working in concert to exhibit functions that
exceeds those of the individual components. That is, in

26
B.P.S. Chauhan et al. / Journal of Organometallic Chemistry 686 (2003) 24ꢀ31
/
the studies reported here the polysiloxane templates play
a role well beyond that of a simple casting mold.
3. Pd-nanoweb: synthesis and applications of Pd
polysiloxane conjugates
/
ꢀ
Recently, we have described a novel route to biode-
gradable polysilylesters, via esterification of polyhydro-
siloxanes in presence of Pd(OAc) (Scheme 1) [27].
2
Fig. 1. UVꢀVis spectra of the reaction mixture.
/
During these studies, following observations led us to
investigate this catalysis in detail [28]: (i) when hydro-
silanes were added to the reaction mixture containing
indicates fully condensed network. FT-IR spectra also
displayed characteristic signals associated with SiH, and
catalytic amounts of Pd(OAc) , color of the solution
2
SiÃ
also undertaken. Particles were found to be in nan-
ometer size regime (40ꢀ50 nm) and stabilized by
polymeric network.
After product isolation, remaining sticky solid was
washed with benzene and same catalysis was repeated
again. A black solution was obtained after addition of
the reactants. Catalysis was continued until total con-
version to silylester, indicating that presence of silicon
network stabilization facilitates the redispersion of
catalytically active ‘Pd’ particles.
/
OÃ
/
Si bonds. SEM analysis (Fig. 3) of this solid was
turned black accompanied by gas formation presumably
H , (ii) after the transformation was complete, a black
2
/
precipitate was formed and the solution became color-
less, and (iii) the black precipitate can be redispersed in
the solution and was found catalytically active for
silylesterification reactions thus allowing recyclability.
Based on these elucidations, we decided to investigate
the possibility if ‘‘Pd’’ colloids were formed during this
catalysis. Thus, in a Schlenk tube, Pd(OAc) (0.004 gm,
2
0
.02 mmol) and acetic acid (0.06 ml, 1.00 mmol) were
dissolved in 2.5 ml of benzene, and the mixture was
examined by UVꢀVis spectroscopy. A peak at 400 nm
To further probe the identity of the ‘‘real catalyst’’,
poisoning experiments were also performed [29]. When
the silaesterification was carried out in presence of PPh3,
/
indicative of Pd(OAc) was observed (Fig. 1a). Poly-
methylhydrosiloxane (1) (PMHS; 0.06 ml, 1 mmol,
2
(3:1, PPh :Pd(OAc) ), no reaction took place under
standard conditions even after 18 h. If PPh was added
3 2
M Â
/
2000, 33ꢀ
/
35 SiÃ
/
H units) was added, which was
w
3
accompanied by generation of a gas presumably H2.
after the generation of the Pd-colloids, only 25%
conversion of reactants to product was obtained after
After 5 min of addition of 1, Pd(OAc) peak (400 nm)
2
disappeared completely (Fig. 1) indicating formation of
new metallic species.
2
4 h of reaction. Under identical reaction conditions,
quantitative conversion to corresponding silylester takes
place after 3.5ꢀ4 h.
One drop of the reaction mixture was deposited on
formvar/carbon-coated grid and analyzed by transmis-
sion electron microscopy (TEM; Fig. 2). As is evident,
from Fig. 2, ‘‘Pd’’ colloids were formed after the
addition of PMHS and were stabilized by siloxane
polymer network. Particle size analysis showed the
average particle size of 6 nm with a standard deviation
of 1 nm. High resolution TEM indicated that Pd-
particles form web structures in the nanosize regime.
After completion of the catalysis, precipitated black
sticky solid was analyzed by scanning electron micro-
scopy (SEM), IR, and multinuclear NMR techniques.
Silicon moieties were present in the solid as evidenced by
/
Catalyst poisoning tests along with UV, TEM, and
catalyst isolation (evacuation to dryness) and then reuse
experiments firmly establish that Pd-colloids are the real
catalysts in the present system. Based on these evidences,
it seems plausible that the silicones play the role of
intermediate host stabilizing agents.
We also investigated independent generation of ‘‘Pd’’
siloxane-networked particles by reacting 1 (0.06 ml, 1
mmol) with Pd(OAc) (0.09 gm, 0.4 mmol) in 20 ml
2
benzene. Preliminary evidence for this transformation
was obtained from UVꢀ
spectra of Pd(OAc) in benzene show a peak at 400
nm (Fig. 1). When PMHS was added to this solution,
the color of the mixture changed from yellow to dark
brown and was accompanied by generation of a gas
/
Vis spectroscopy. UVꢀVis
/
2
2
9
Si-NMR. The peaks at d ꢂ
5.74 ppm correspond to a polymeric network contain-
ing SiÃH (assigned after gated as well as off-resonance
decoupling experiments), SiOCOR, and SiÃOH moi-
eties, respectively. The signal at d ꢂ110.94 ppm
/
36.15, ꢂ
/
64.74, and ꢂ
/
7
/
/
presumably H . After 5 min of addition of PMHS,
2
/
Pd(OAc) peak (400 nm) disappeared completely (Fig.
2
1), and a featureless transition indicating formation of
new metallic species was observed.
After 2 h of stirring at room temperature, obtained
black precipitate was filtered and washed with excess of
Scheme 1. Pd-catalyzed silylester synthesis.

B.P.S. Chauhan et al. / Journal of Organometallic Chemistry 686 (2003) 24ꢀ
/
31
27
Fig. 2. TEM image and the particle size analysis of the ‘‘Pd’’ colloids obtained after the addition of starting materials (no staining was required).
Table 1
Comparison of the catalytic activity
Acid
Silane Pd-colloids
2% Pd(OAc)
2
Yield
%)
(
CH
3
3
COOH
COOH
2
1
Room tempera-
ture, 5 h
Room tempera-
ture, 8 h
95
CH
70 8C, 12 h
70 8C, 6 h
70 8C, 24 h
70 8C, 3.5 h
70 8C, 6 h
70 8C, 24 h
95
95
85
C
C
6
6
H
H
5
COOH 2
COOH 1
5
steps that particleꢀparticle adhesion and sintering must
/
be avoided. Prevention of particle sintering is achieved
by adding a critical dosage of polysiloxane protective
agent whose function is to cover the particles, thus
Fig. 3. SEM image of the gummy solid obtained after the catalysis.
effectively eliminating any possibility of palladiumꢀ
palladium particle bond formation. The presence of
this agent at the solidꢀliquid interface does not interfere
with the palladium diffusionꢀsurface deposition pro-
cess, and the particles can grow to a definite size.
/
benzene. SEM showed similar particle size distribution
2
9
and, Si displayed peaks at same positions as in case of
the solid obtained from catalytic reactions. This powder
was used as catalyst for silaesterification reactions of
/
/
polymer
1
1
,1,1,3,5,5,5-heptamethyl trisiloxane (2). Under the
and its monomeric model siloxane
identical reaction conditions and molar ratios, indepen-
dently generated ‘‘Pd’’ colloids showed similar catalytic
efficiency as in the case of previous experiments (see
Table 1).
Based on initial evidences, the reaction scheme for
producing nanoweb of palladium particles involves the
reduction of the soluble Pd complex species, nucleation
of metallic particles, and growth of the individual nuclei
in the presence of a suitable reducing and protective
agent. The presence of polysiloxane is essential for
preventing the coalescence of the nuclei during the
growth step. It is during the nucleation and growth
4
. Silver nanoparticles: synthesis and applications
Silver nanoparticles are of great interest due to their
role in photographic processes [30], their utility as
substrates for surface-enhanced Raman spectroscopy
(SERS) [31], and also due to their application in
catalysis [32]. Silver nanocrystallites, mostly hydrosols,
have been widely studied because of the ease of their
preparation. Since most of the catalytic reactions are
performed in organic solvents, it is desirable to design
synthetic methods, which lead to the stabilization of
metal nanoparticles in such solvents. Colloidal disper-

28
B.P.S. Chauhan et al. / Journal of Organometallic Chemistry 686 (2003) 24ꢀ31
/
sions of silver in non-aqueous liquids (organosols) are
rare and more difficult to prepare and stabilize.
this case, well-separated nanoscopic silver particles were
obtained (Scheme 2).
It has been observed that the stability, particle size,
and properties of metal colloids strongly depend on the
specific method of preparation and the experimental
conditions applied. In most cases, nanoparticles are
stabilized with strong coordinating surfactants to pre-
vent the agglomeration and to provide specific surface
properties [33]. On the other hand, utility and activity of
such particles is compromised due to the difficulty in
ligand exchange reactions in catalytic processes. In this
context, it is desirable to develop synthetic strategies to
fabricate nanoparticles, which are stabilized by non-
coordinating environments. Such preparations will pro-
vide flexibility to functionalize nanoparticles according
to the need, hence tailoring of nanoparticle surfaces.
Moreover, for catalytic applications, such conjugates
may have superior activity and selectivity over the
nanoparticles passivated by strong coordinating ligands.
In this part, we summarize a versatile method and
first example of polyhydrosiloxanes-induced generation
of functionalizable monodisperse silver sols [34]. This
method enables routine formation of stable nanosilver
reservoirs in organic solvents avoiding particle aggrega-
tion during the storage as well as nucleation and growth
process. We also demonstrate the utility of such
reservoirs in grafting the surface properties of nanosized
silver particles by ligand exchange reactions with thiols
and phosphine oxide surfactants [34].
This diversity led us to examine the metal salt
reduction under various reaction conditions for tailoring
the morphological features of the nanoparticles. In
exploratory experiments, when PMHS (0.024 ml, 0.4
mmol, M Â
5
/
2000, 33ꢀ
0 ml toluene suspension of silver acetate (0.032 g, 0.2
mmol), mixture turned faint yellow and showed a peak
at 435 nm in UVꢀVis spectra indicating formation of
/
35 SiÃ
/
H units) was added to the
w
/
silver nanoparticles. But, it was observed that reduction
process was slow (24 h) and was accompanied by
particles precipitation.
In order to accelerate the reduction process and to
develop the synergy between nucleation and growth
process, an amine catalyst was added to the reaction
mixture. This modification is based on the fact that
dynamic coordination of amine to silicon hydrides leads
to hypercoordinated silicon centers and thus very active
siliconhydrides [35]. Thus, in an optimized procedure,
AgOAc (0.032 g, 0.2 mmol) was suspended in 50 ml of
toluene, and PMHS (0.072 ml, 1.2 mmol) was added
while stirring gently at room temperature under nitro-
gen. The solution was stirred for 1 h, and trioctylamine
(0.005 ml, 0.01 mmol) was added as a catalyst. Solution
was stirred intermittently for around 5 min after every
half an hour and the formation of silver nanoparticles
was probed by monitoring the absorption changes at
different time intervals. A peak at 430ꢀ435 nm was
/
observed after 30 min of reaction, indicating the
formation of Ag nanoparticles. After 5 h of reaction,
peak intensity became constant and mixture turned light
yellow. At this juncture, yellow solution was centrifuged
and analyzed.
As in the case of Pd(OAc) reduction process, when
2
PMHS was added to a mixture of AgOAc and benzoic
acid, reduction of silver salt to silver nanoparticles was
observed. Surprisingly, in contrast to Pd-nanoweb, in
One drop of the solution was deposited on formvar-
coated carbon grid and visualized by transmission
electron microscope (TEM), without any additional
staining. TEM analysis demonstrated that indeed silver
particles were formed. The average particle size was
found to be 2 nm with a standard deviation of 0.6 nm.
Careful visualization at higher resolution indicated that
all the particles were wrapped by lighter matrix most
probably polysiloxane. Though, it was difficult to
achieve enough contrast (PMHS is not visible at such
resolutions) to conclude this observation. Due to the
stability of these particles, we infer that polysiloxane
wraps around the particles during the formation stage
and prevents them from further agglomeration.
There was, however, an effect against this colloidal
stability, namely adsorption on the walls of silica
containers, especially for the higher concentrations.
This effect is mostly observed for quartz, while it is
hardly noticeable if the dispersions are stored in glass
vials. This tendency to attach onto silica surfaces
represented a serious problem for the spectroscopic
study of the kinetics of particle formation. Hence,
Scheme 2. Generation and stabilization of silver nanosols.

B.P.S. Chauhan et al. / Journal of Organometallic Chemistry 686 (2003) 24ꢀ
/
31
29
Fig. 4. TEM and UVꢀVis spectrum of as-prepared polysiloxane-stabilized silver nanoparticles. No additional staining was required.
/
advantage of this property can be taken for the
application of the particles to the preparation of thin
films.
0.8 mmol) was added and solution was kept at room
temperature for 24 h and analyzed by TEM, NMR, IR,
and UVꢀ
/
Vis spectroscopy. The color of the sol changed
Reduction process was examined with various
amounts of polysiloxane reducing agent. In the cases
of 1:2 and 1:6 (AgOAc:PMHS) ratios, particles were
to golden yellow and UVꢀ
/
Vis analysis indicated forma-
Table 2
Comparison of absorption maxima and average particle size
found to be stable and almost monodisperse (Â2 nm).
/
a
On the other hand, in the presence of 1:4 AgOAc:PMHS
ratio, reduction process was also accompanied by
Entry Silver sols UV-absorbance
Color
Particle size
2.00 (0.60)
particles precipitation (Fig. 4, UVꢀ/Vis spectra).
1
2
3
Agꢀ
Agꢀ
Agꢀ
/
PMHS 435ꢀ
/
440 nm
Faint yellow
Golden yellow 3.63 (0.96)
DDT Very weak 422 nm Dark yellow 4.40 (1.19)
In order to investigate the utility of polysiloxaneꢀ
/
/TOPO 421ꢀ
/
425 nm
silver conjugates, surface grafting studies were per-
formed. In as-prepared samples of polysiloxane-conju-
gated silver particles, trioctylphosphineoxide (TOPO;
/
a
In all the cases, ca. 150 particles were counted and then combined
into histograms to obtain the size distributions.
Fig. 5. TEM of (a) TOPO, and (b) DDT-functionalized silver particles.

30
B.P.S. Chauhan et al. / Journal of Organometallic Chemistry 686 (2003) 24ꢀ31
/
tion of new type of silver sols (421 nm, lmax). TEM
analysis (Fig. 5a) showed that TOPO-capped silver
nanoparticles spontaneously self assemble, when depos-
ited on a copper grid. The average particle size was
found to be 3.6 nm. Spontaneous self-assembly has been
observed before for highly monodispersed TOPO-
capped semiconductor nanoparticles [10]. In a similar
fashion and under identical molar ratios, addition of 1-
dodecanethiol (DDT) led to the formation of thiol
functionalized silver sols (Fig. 5b).
References
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1] (a) V. Kojarnovitch (Ed.), New and Advanced Materials, Emer-
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(
1
(
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/
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displayed an intense absorption peak around Â
A comparative study of UVꢀVis absorption spectra and
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/
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[
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