Formation of gold nanoparticles catalyzed by platinum nanoparticles: Assessment of the catalytic mechanism

Article abstract of DOI:10.1021/jp0642342

The understanding of how the formation of metal nanoparticles in aqueous solutions is influenced by the presence of presynthesized nanoparticles is important for precise control over size, shape, and composition of nanoparticles. New insights into the catalytic mechanism of Pt nanoparticles are gained by studying the formation of gold nanoparticles from the reduction of AuCL₄^- in aqueous solution in the presence of presynthesized Pt nanoparticles as a model system. The measurement of changes of the surface plasmon resonance band of gold nanoparticles, along with TEM analysis of particle size and morphology, provided an important means for assessing the reaction kinetics. The reductive mediation of Pt-H species on the Pt nanocrystal surface is believed to play an important role in the Pt-catalyzed formation of gold nanoparticles. This important physical insight is evidenced by comparison of the rates of the Pt-catalyzed formation of gold nanoparticles in the presence and in the absence of hydrogen (H₂), which adsorb dissociatively on a Pt nanocrystal surface forming Pt-H species. Pt-H effectively mediates the reduction of AuCl₄^- toward the formation of gold nanoparticles. Implications of the findings to the controllability over size, composition, and morphology of metal nanoparticles in the aqueous synthesis environment are also discussed.

Full text of DOI:10.1021/jp0642342

J. Phys. Chem. B 2006, 110, 22503-22509
22503
Formation of Gold Nanoparticles Catalyzed by Platinum Nanoparticles: Assessment of the
Catalytic Mechanism
Peter N. Njoki, Aisley Jacob, Bilal Khan, Jin Luo, and Chuan-Jian Zhong*
Department of Chemistry, State UniVersity of New York at Binghamton, Binghamton, New York 13902
ReceiVed: July 5, 2006; In Final Form: September 5, 2006
The understanding of how the formation of metal nanoparticles in aqueous solutions is influenced by the
presence of presynthesized nanoparticles is important for precise control over size, shape, and composition
of nanoparticles. New insights into the catalytic mechanism of Pt nanoparticles are gained by studying the
-
formation of gold nanoparticles from the reduction of AuCl4 in aqueous solution in the presence of
presynthesized Pt nanoparticles as a model system. The measurement of changes of the surface plasmon
resonance band of gold nanoparticles, along with TEM analysis of particle size and morphology, provided an
important means for assessing the reaction kinetics. The reductive mediation of Pt-H species on the Pt
nanocrystal surface is believed to play an important role in the Pt-catalyzed formation of gold nanoparticles.
This important physical insight is evidenced by comparison of the rates of the Pt-catalyzed formation of gold
nanoparticles in the presence and in the absence of hydrogen (H2), which adsorb dissociatively on a Pt
-
nanocrystal surface forming Pt-H species. Pt-H effectively mediates the reduction of AuCl4 toward the
formation of gold nanoparticles. Implications of the findings to the controllability over size, composition,
and morphology of metal nanoparticles in the aqueous synthesis environment are also discussed.
Introduction
Au nanoparticles were reacted with H2PtCl6 and ascorbic acid
to form Pt-coated Au nanoparticles.14
The use of metal and metal oxide nanoparticles to catalyze
various homogeneous and heterogeneous chemical reactions has
rejuvenated research interests as a result of the rapidly emerging
nanotechnology.1 In comparison with the vast majority of recent
studies of nanoparticle-catalyzed reactions in solution phase,1a
little has been reported for nanoparticle-catalyzed formation of
nanoparticles. We have recently shown that preformed Pt
nanoparticles in aqueous solutions exhibit catalytic activity
toward the synthesis of AuPt alloy nanoparticles.2 An under-
standing of how the reaction rate for the formation of metal
nanoparticles is influenced by the presence of metal or alloy
nanoparticles is important for control over size, morphology,
and composition. Such control is challenging because of the
propensity of aggregation of aqueous-soluble metal nanopar-
ticles3,4 in comparison with the high stability of their organic-
soluble nanoparticles,5-9 e.g., alkanethiolate-protected nano-
particles derived from two-phase synthesis. There are a number
of synthetic methods reported for the preparation of water-
soluble Pt and Au nanoparticles in the size range of a few to a
hundred nanometers.3-4,10-15 For example, Pt nanoparticles were
synthesized by reducing tetrachloroplatinate in an aqueous
solution of acrylate or polyacrylate with hydrogen3. This
preparation method produced Pt nanoparticles of different shapes
(cubic, tetrahedra, and truncated octahedra). Gold and platinum
nanoparticles were also synthesized by the polyol method and
stabilized with poly(vinylpyrrolidone) (PVP) under controlled
temperature.11 Au nanoparticles were also synthesized by
reducing an aqueous solution of HAuCl4 in the presence of
polymers such as N,N-dimethylacetoacetamide12 and poly(diallyl
dimethylammonium) chloride.13 Core-shell Au-Pt nanopar-
ticles were synthesized by a seed growth in which citrate capped
Despite the extensive reports on the synthetic aspects, there
is a limited understanding of the factors controlling the size,
morphology, and composition. While our recent observation that
the reaction rate for the formation of gold nanoparticles was
dramatically increased in the presence of Pt nanoparticles2
suggests catalytic activity of Pt nanoparticles for the reduction
of AuCl4-, questions concerning the catalytic mechanism remain
quite elusive. Two possible scenarios can be considered in
developing a mechanistic assessment of the Pt-catalyzed syn-
thesis of gold nanoparticles. One involves direct redox reaction
on the surface Pt atoms in reducing AuCl4-, and the other
involves chemical mediation by species adsorbed on the surface
of Pt in reducing AuCl4-. Both scenarios seem to be intuitively
possible based on the difference of the redox potentials of Pt
and Au16 and some earlier experimental observations on
chemical adsorption on Pt surfaces.17 However, experimental
data are not available for the mechanistic understanding of how
the detailed surface chemistry of Pt nanocrystals is operative.
Such understanding has important implications to the precise
control of the nanoscale interfacial reactivity in the synthesis
of nanoparticles with well-defined sizes, shapes, and composi-
tion.
In this paper, we report new experimental findings from an
investigation of Pt nanoparticle catalyzed formation of gold
nanoparticles by reduction of AuCl4- in aqueous solution. The
measurement of changes in the surface plasmon resonance
optical properties of gold nanoparticles, along with TEM
analysis of the particle sizes and morphology, allowed us to
assess the reaction kinetics and to gain insights into the catalytic
mechanism of Pt nanoparticles in the formation of gold
nanoparticles.
* To whom correspondence should be addressed. E-mail: cjzhong@
binghamton.edu.
10.1021/jp0642342 CCC: $33.50 © 2006 American Chemical Society
Published on Web 10/24/2006

22504 J. Phys. Chem. B, Vol. 110, No. 45, 2006
Njoki et al.
Experimental Section
Chemicals. Potassium tetrachloroplatinate (K2PtCl4, 99.99%),
hydrogen tetrachloroaurate (HAuCl4, 99%), and sodium acrylate
(CH2CHCO2Na, (A), 97%) were purchased from Aldrich and
used as received. Water was purified with a Millipore Milli-Q
water system. Hydrogen (99.95%) was purchased from Air Gas
and used as received.
Synthesis. Pt nanoparticles encapsulated with acrylate were
synthesized based on a reported protocol,3 where aged K2PtCl4
was dissolved in 50 mL of deionized water. The pH was adjusted
to 7.0 using a dilute NaOH solution. Ar gas was then purged
through the solution for 5 min. Sodium acrylate solution was
then added to the metal-precursor solution. The solution
displayed a light orange color. H2 gas was then bubbled into
the solution for 30 min. The reaction flask was sealed and stirred
overnight in the dark at room temperature. After 12 h the
solution turned brown as a result of the formation of Pt
nanoparticles capped with acrylate. The Pt nanoparticles can
be isolated by a salting out method or high-speed centrifugation
(>16 000 rpm). The particles were redispersable in water and
have been stored in an aqueous solution for at least 1 year. As
for the yield, these isolation and cleaning processes were found
to lead to a significant loss of particles.
Figure 1. UV-vis spectra monitoring the formation of Au nanopar-
The synthesis of Au nanoparticles was carried out either in
a solution of Au precursors without presynthesized Pt nano-
particles or in the presence of presynthesized Pt nanoparticles;
both were under the same conditions except for use of Pt
nanoparticles. For the synthesis of Au nanoparticles in the
presence of Pt nanoparticles, an aged solution of HAuCl4 of
known concentration was mixed with a solution of Pt nanopar-
ticles of known concentration. The solution was diluted with
deionized water, and the pH was adjusted to 7 using a dilute
NaOH solution. Pt nanoparticles and aged HAuCl4 were used
at different concentration ratios. A known volume of sodium
acrylate was added into the mixed solution, typically in the range
of 1.0 × 10-3 - 1.0 × 10-2 M. The reactions were also
followed by spectrophotometric monitoring of the optical
absorption band. In the experiment to examine the stability of
the catalytic activity of the Pt nanoparticles, the Pt nanoparticles
used from a previous run of the reaction were separated by the
centrifugation method. The obtained Pt nanoparticles were
cleaned and reused for the reduction of HAuCl4 following the
same protocol as that described above. In the experiment to
examine the involvement of adsorbed H-species in the reaction,
H2 gas was bubbled through the reaction solution containing Pt
nanoparticles and AuCl4- under conditions similar to those from
the above experiments.
-
ticles upon reduction of AuCl4 by acrylate in the presence of Pt
nanoparticles of different concentrations. Inserts: time dependence of
λmax and Amax of the SP band. The concentrations of Pt nanoparticles
and AuCl4-, ([Pt-NPs], [AuCl4-]: (1.37 × 10-8, 2.4 × 10-4 M) (A)
and (1.37 × 10-8, 4.8 × 10-4 M)(B), which used the same amount of
reducing agent.
Results and Discussion
Gold nanoparticles (NPs) of larger than 2 nm exhibit a surface
plasmon (SP) resonance band in the visible region (∼520 nm),
whereas this band is largely damped for Pt nanoparticles (its
SP band being shifted largely into the UV region). A change in
absorbance or wavelength of the SP band18-20 provides a
measure of particle size, shape, concentration, and dielectric
medium properties. In the cases when particle sizes, shapes,
and solvent properties are comparable, the absorbance of the
SP band is largely related to the concentration of the particles.
As described below, the measurement of the SP band during
the synthesis of Au nanoparticles provide useful information
for assessing the reaction kinetics.
Figure 1 shows two typical sets of UV-vis spectra monitoring
the formation of gold nanoparticles in the presence of Pt
nanoparticles. Set B is obtained under the same concentration
of Pt nanoparticles as that for set A, but with a 2× concentration
of AuCl4-. A gradual increase of absorbance for the SP band
in the ∼520 nm region was observed, which is a characteristic
of the formation of gold nanoparticles. Note that the rising
background is due to the spectrum of Pt nanoparticles (i.e., the
spectrum at t ) 0). As shown in the insert, while the absorbance
(Amax) of the SP band increased with reaction time, there seemed
Instrumentation and Measurements. UV-visible (UV-
vis) spectra were acquired with an HP 8453 or HP 8452
spectrophotometer. The spectra were collected over the range
200-1100 nm or 190-820 nm (HP 8452). The UV-vis spectra
were collected as a function of reaction time either by directly
monitoring the solution in a reaction cuvette (with a stirring
bar in the bottom to stir the solution when a spectrum was not
taken) or by taking solution samples from a separate 200-mL
reaction vessel under constant stirring. The reactions were
carried out under ambient conditions.
to be a subtle decrease in the wavelength of the SP band (λmax
)
in the initial ∼10 h, which remained largely constant in the
rest of the reaction. This initial decrease of λmax may be due to
difficulty in peak maximum identification for the initially weak
and broad SP band feature. The change in the Pt nanoparticle
concentration did not seem to show a significant shift for the
SP band. However, there appeared to be a significant difference
in the rate for the absorbance change; i.e., the rate increases
with the concentration of Pt nanoparticles. Under the same
concentration of Pt nanoparticles but different AuCl4- concen-
Transmission electron microscopy (TEM) was performed on
a Hitachi H-7000 microscope (100 kV). The aqueous nanopar-
ticle samples were drop cast onto a carbon-coated copper grid
sample holder followed by natural evaporation at room tem-
perature.

Au Nanoparticle Formation by Pt Nanoparticles
J. Phys. Chem. B, Vol. 110, No. 45, 2006 22505
Figure 2. TEM micrographs and size distributions of Pt, Au in the presence of Pt nanoparticles and Au in the absence of Pt nanoparticles. The
concentrations of K2PtCl4 and A are 4.8 × 10-4 and 9.6 × 10-3 M) (A); the concentrations of Pt nanoparticles and AuCl4- ([Pt-NPs], [AuCl4-]):
(2.74 × 10-8, 2.5 × 10-4) (B), and (0, 2.5 × 10-4) (C), which used the same amount of reducing agent.
tration, the rate for the absorbance increase of the SP band (538
nm) was found to increase with AuCl4 concentration (B). As
will be shown later, the spectral evolution rate for the same
reaction in the absence of Pt nanoparticles is much slower than
those in the presence of Pt nanoparticles.
Before a detailed analysis of the reaction kinetics, the
morphology and size of the nanoparticles were examined using
TEM. Figure 2 shows a representative set of TEM micrographs
for the Au nanoparticles formed in the presence of presynthe-
sized Pt nanoparticles (B). The inclusion of the morphological
features of the presynthesized nanoparticles (A) and gold
nanoparticles formed in the absence of Pt nanoparticles (C)
serves the purpose of comparison. Note that the basic feature
of the Pt nanoparticles is largely dominated by the morphology
of cubes.
nanoparticles (C), the data for the gold nanoparticles formed in
the presence of Pt nanoparticles reveal particles with distribu-
tions of two different average sizes, 7.0 ( 0.8 nm and 18.2 (
1.7 nm. The former was almost the same as that for the
presynthesized Pt nanoparticles before being used in the reaction
(8.1 ( 0.7 nm). The latter are Au nanoparticles formed in the
presence of Pt nanoparticles, which are apparently much smaller
than those formed in the absence of Pt nanoparticles (51.7 (
4.2 nm).
In Figure 3, two typical TEM micrographs are compared for
Au nanoparticles synthesized in the presence of two different
concentrations of Pt nanoparticles. Micrograph A in Figure 3
is for a sample corresponding to that for spectra set A in Figure
1. Nanoparticles with bimodal or trimodal distributions are
clearly observed. The average sizes of the particles determined
from the TEM data are 24.3 ( 2.0 and 7.6 ( 0.6 nm for A,
and 34.1 ( 2.9, 26.5 ( 1.9, and 8.2 ( 1.3 nm for B. Under
-
In contrast to the morphological features for the Pt nanopar-
ticles (A) and the Au nanoparticles formed in the absence of Pt

22506 J. Phys. Chem. B, Vol. 110, No. 45, 2006
Njoki et al.
Figure 3. TEM micrographs and size distributions for Au nanoparticles
formed in the presence of presynthesized Pt nanoparticles of different
concentrations. (A) The concentrations correspond to those for spectral
set A in Figure 1. (B) The concentration of Pt nanoparticles is twice
that of A, whereas other concentrations are the same as in those in A.
Figure 4. Kinetic plots for the formation of Au nanoparticles in the
absence of (a) and in the presence of (b) Pt nanoparticles at room
temperature. The concentrations of Pt nanoparticles and AuCl4- ([Pt-
NPs], [AuCl4-], are (2.74 × 10-8, 2.5 × 10-4 M) (a) and (0, 2.5 ×
10-4 M) (b) which use the same amount of reducing agent.
each of these conditions, the size of the presynthesized Pt
nanoparticles before and after the reaction remains almost
unchanged (i.e., 7.6-8.2 nm), whereas the newly formed Au
nanoparticles showed sizes which fall in the range 21-34 nm.
For example, nanoparticles of the larger sizes were observed
when the concentration of Pt nanoparticles was relatively large.
The difference in the average size of the gold nanoparticles
formed is rather small, and there is an insignificant change in
the wavelength of the SP band. The change in the absorbance
is a function of the volume fraction of Au nanoparticles in the
solution. The increase in particle size is usually accompanied
by a red shift of the SP band for particle sizes larger than 30
nm. Our experimental data showed however the lack of such a
red shift of the SP band as a function of time (see Supporting
Information). It is also supported by a simulation of the SP band
based on Mie theory (see Supporting Information). The results
from the simulation of the SP bands in Figure 1 reveal that the
volume fraction of the Au nanoparticles (φ) shows a gradual
increase as a function of reaction time in different concentra-
tions. This finding, together with the TEM data, substantiates
that the SP band evolution reflects largely the dependence of
the concentration of the formed Au nanoparticles on the reaction
time. In separate experiments (see Supporting Information), the
dependence of λmax and Amax on the size and concentration of
Au nanoparticles showed a clear trend of increasing λmax with
size for particles larger than ∼30 nm, which is in fact quite
comparable with the theoretical simulation. The experimentally
observed λmax values remained largely constant for most of the
kinetic region where the absorbance rising is observed. The final
size of the Au particles was found to be 30-50 nm. The
experimental data thus suggest that the growth is fast, whereas
the nucleation is slow. These findings constituted the basis for
using the change in absorbance, which is a function of the
change in volume fraction of Au nanoparticles in the system,
to assess the reaction kinetics. The reaction rate, i.e., the change
TABLE 1: Fitting Results Based on Eq 1a
reaction condition
k
n
(a) in absence of Pt nanoparticles
(b) in the presence of Pt nanoparticles
1.05 × 10-6
3.6
2.5
1.17 × 10-3
a Note: The fittings were for the data in Figure 4. The data were
background subtracted.
on Avrami’s theoretical model of crystallization and growth,21
which was shown to be useful for assessing the crystallization
kinetics of macromolecules or polymers.22-23 This model was
also recently found to be viable for describing the formation of
thin film assemblies of gold nanoparticles mediated by thiol-
based linker molecules.24 Using this model, the absorbance can
be related to the volume fractional crystallinity of the nanopar-
ticles formed.
n
A ) C(1 - e-kt
)
(1)
where k is the apparent rate constant, n is the critical growth
exponent, and C is the proportionality constant. The fitting
results are summarized in Table 1.
The overall apparent rate constant is evidently increased by
about 3 orders of magnitude in the presence of Pt nanoparticles
in comparison with that in the absence of Pt nanoparticles. Under
the condition of diffusion-controlled mass flow to the crystal
nucleus, a critical exponent of 2.5 or less has been observed
for thermal nucleation.22 Smaller values of n were considered
as indications of lower growth dimensionality. The fitted n
values in each case are greater than 2.5, suggestive of a
mechanism involving a thermal nucleation and three-dimen-
sional crystallization as often found for macromolecule
systems.22-23
in the concentration of AuCl4-, -(dC(AuCl )/dt), should be
In Figure 5, the kinetic plots based on the spectral evolution
of the SP band data are compared, which involved different
concentrations of Pt nanoparticles but the same concentration
-
4
proportional to the amount of Au in the nanoparticles or the
change in volume fraction of Au nanoparticles (φ),
-
-
-
-(dC(AuCl )/dt)
φ. Since the particle size seemed to be
of AuCl4 (a, b, and c), or different concentrations of AuCl4
4
relatively constant in most of the rising region in the Amax-t
curve, the change in absorbance (dAmax/dt) can be used to assess
the reaction rate.
but the same concentration of Pt nanoparticles (b and d). Note
that the absorbance data were all corrected against the absor-
bance at t ) 0.
Figure 4 shows a representative set of data monitoring the
formation of Au nanoparticles in the absence of presynthesized
Pt nanoparticles (a), which is also compared with that obtained
in the absence of the Pt nanoparticles (b). In comparison with
the data obtained in the absence of Pt nanoparticles (b), the
reaction rate for the formation of gold nanoparticles in the
presence of Pt nanoparticles (a) was clearly much faster (by a
factor of 30-50). The fitting to the experimental data was based
The above data yield several important pieces of information.
First, the overall reaction rate seems to display the exponential
type of kinetics in which the absorbance values approach the
same plateau value for the three different concentrations (a, b,
and c). Second, the increase in concentration of AuCl4- (d) with
the same concentration of Pt nanoparticles led to an increase of
both the rate and the final plateau. Such dependence is expected
since the formation of Au nanoparticles involves the aggregation

Au Nanoparticle Formation by Pt Nanoparticles
J. Phys. Chem. B, Vol. 110, No. 45, 2006 22507
which is followed by an electron-transfer pathway for the
reduction of Au(III) to Au(0). Such a scenario is possible based
on a comparison of redox potentials between the two metals,16
i.e.,
PtCl4(aq)2- + 2e f Pt(s) + 4Cl-
E° ) 0.758 V
(aq)
and
AuCl4(aq)- + 3e f Au(s) + 4Cl-
E° ) 1.002 V
(aq)
-
This mechanism requires direct adsorption of AuCl4 on the
surface of Pt and facile oxidation of the surface Pt. However,
the presence of acrylate capping agents on the surface of Pt
creates a steric barrier to the surface adsorption and oxidation.
2-
To assess this possibility, PtCl4 was examined as a catalyst
to substitute Pt nanoparticles. The results (see Supporting
Information) seemed to show some indication of activity for
the formation of Au nanoparticles, but the SP characteristics
and the resulting particles determined were very different from
those using Pt nanoparticles as catalysts. It showed a weak and
broad SP band in the 500-700 nm region; the TEM showed
only small particles (<5 nm) (smaller than Pt particles). This
is in contrast to the strong SP band characteristic and the large
particle size feature (30-50 nm) obtained for the case of Pt
catalyst. Importantly, when H2 was introduced into the above
reaction solution, no SP band was detected in the 500-700 nm
region, consistent with the formation of AuPt alloy nanopar-
ticles.2 While the observed activity appeared to be consistent
with the above redox reaction, the clear difference from the
activity observed in the presence of Pt nanoparticles and H2
indicated that Pt(II) was not a major contributor. The fact that
H2 reduces Pt(II) to Pt nanoparticles in the presence of acrylate
(which was how the Pt nanoparticles were prepared) further rules
out the possibility of Pt(II) as the catalytic origin when both H2
and Pt nanoparticles were present in the reaction solution.
In a second scenario, the catalytic activity originates from
chemical mediation of species preadsorbed on the Pt nano-
crystals surface. Under the experimental conditions, hydrogen
adsorbed on the Pt surface (i.e., Pt-H) is one of the most likely
chemical species. The source of hydrogen could be H+ or H2O.
The possibility of H+ seemed to be ruled out because the
reaction rate was found to be reduced when the pH was lowered
to less than 7. The dissociative adsorption of H2O on the Pt
surface is another likely source for Pt-H and Pt-OH species.
If this possibility is operative, the enrichment of Pt-H species
on the catalyst surface is expected to accelerate the reaction.
An experiment was thus performed to assess the viability of
the surface Pt-H species in the catalytic reaction based on the
fact that H2 can dissociatively be adsorbed on the Pt surface to
form Pt-H species.17
Figure 5. Kinetic plots monitoring the formation of Au nanoparticles
in the presence of presynthesized Pt nanoparticles of different concen-
trations. The concentrations of Pt nanoparticles and AuCl4- ([Pt-NPs],
[AuCl4-] are (2.74 × 10-8, 2.4 × 10-4 M) (a), (1.37 × 10-8, 2.4 ×
10-4 M) (b), (6.85 × 10-9, 2.4 × 10-4 M) (c), and (1.37 × 10-8, 4.8
× 10-4 M) (d), which used the same amount of reducing agent. The
inset is a plot of the apparent rate constant (k) vs concentration of Pt
nanoparticles (CPt).
of Au atoms as a result of the reduction of AuCl4-. Finally, a
close examination of the initial reaction kinetics reveals that
the initial formation rate is linearly dependent on the concentra-
tion of the Pt nanoparticles, even though the overall reaction
rate seems to be described by the exponential type of kinetics
(eq 1). The fact that the rate increases with concentration of Pt
nanoparticles (a > b > c) under the relatively low concentration
-
of AuCl4 (<∼0.4 mM) further substantiates the catalytic
activity of Pt nanoparticles.
In view of the catalyst role of Pt nanoparticles, one question
concerns whether Pt nanoparticles were coated by Au nano-
particles formed in the catalytic reaction. In the above experi-
ments, this question seemed partially addressed by an unde-
tectable size increase of Pt nanoparticles in the TEM images
shown earlier. In addition, the catalytic activity of the re-used
Pt nanoparticles was examined in a control experiment, in which
the Au nanoparticles were removed as precipitate by centrifuga-
tion since their sizes are larger than Pt nanoparticles. The
supernatant containing Pt nanoparticles was then used again to
catalyze the formation of new Au nanoparticles. The results
revealed catalytic activity similar to the case using fresh Pt
nanoparticles. This result seemed to suggest that Pt nanoparticles
catalyzed the formation of Au nanoparticles without being
coated by Au. However, as it will be described later by more
careful examinations, this is not the case.
In the experiment, H2 gas was bubbled through the reaction
solution containing Pt nanoparticles and AuCl4-. Figure 6 shows
a representative set of results. In contrast to the previous data
in the absence of H2, the SP band (Figure 6A) exhibits a
remarkably rapid increase in the presence of H2. As shown in
Figure 6B, the Pt-catalyzed nanoparticle formation in the
presence of H2 seemed to be completed in less than 10 min (a),
in sharp contrast to the corresponding time length of ∼20 h for
the same reaction in the absence of H2 (b). The time length for
this reaction in the absence of Pt and H2 was about 50 h (c).
Again, while Amax increased with reaction time, λmax remained
largely constant (540 nm) in the rest of the reaction except for
the subtle decrease in the initial ∼5 min which may be due to
Having established the apparent catalytic activity of Pt
-
nanoparticles for the reduction of AuCl4 in the formation
process of Au nanoparticles, a fundamental question is how the
-
reduction of AuCl4 is mechanistically catalyzed by Pt nano-
particles. In the absence of prior literature directly related to
this system, two possible scenarios have been considered. The
-
first scenario involves the adsorption of AuCl4- (or AuCl2(OH)2
in the slightly basic condition4a) on a Pt nanocrystal surface

22508 J. Phys. Chem. B, Vol. 110, No. 45, 2006
Njoki et al.
Figure 7. (A) UV-vis spectra monitoring the formation of Au
nanoparticles in the presence of Pt nanoparticles, H2, and acrylate.
Concentrations of Pt nanoparticles and AuCl4-: 4.38 × 10-10 and 2.4
× 10-4 M. (B) Kinetic plots for three different conditions (same
[AuCl4-] and [acrylate]): in the presence of H2 and Pt nanoparticles
with concentration of Pt nanoparticles at 8.77 × 10-10 M (a, 1) and
4.38 × 10-10 M (b, b); reused b after removing gold nanoparticles (c,
O).
Figure 6. (A) UV-vis spectra monitoring the formation of Au
nanoparticles in the presence of Pt nanoparticles, H2, and acrylate.
Insert: time dependence of λmax and Amax of the SP band. (B) Kinetic
plots for three different conditions (the same [AuCl4-]): in the presence
of Pt nanoparticles and H2 (a, squares), of Pt nanoparticles but without
H2 (b, circles), and without Pt nanoparticles and H2 (c, diamonds).
Concentrations of Pt nanoparticles and AuCl4-: 1.37 × 10-8 and 2.4
× 10-4 M, which used the same amount of reducing agent.
this condition, the spectral feature (Figure 7A) is similar to what
was observed in the previous cases except subtle differences in
peak position and width. The absorbance increased with time.
The average size of Au nanoparticles thus formed was deter-
mined to be 30-50 nm. Under such low concentrations of Pt
nanoparticles, it is still evident that the Pt nanoparticles catalyze
the formation of Au nanoparticles, as supported by the increase
of apparent rate constant almost 1 order of magnitude when
the catalyst concentration was doubled (3.0 × 10-1 h-1 (a) and
3.2 × 10-2 h-1 (b)) (Figure 7B). When the supernatant of
solution b after centrifugation was reused as catalyst for the
formation of Au nanoparticles, the reaction rate was found to
be much lower with an apparent rate constant almost 2 orders
of magnitude lower (6.0 × 10-4 h-1 (c)) than that of the fresh
catalyst (3.2 × 10-2 h-1(b)) (Figure 7B). This finding suggests
that Pt particles were likely occluded in the Au nanoparticles
formed, in sharp contrast to the conclusion from the previous
observation of unchanged catalytic activity when the Pt nano-
particle concentration was much higher (>102×) where the loss
of Pt particles was insignificant. It is therefore likely that at
least part of the Pt nanoparticles were occluded forming core-
(Pt)-shell(Au) type nanoparticles, which explains the sharp
decrease of the reaction rate in the reused Pt nanoparticle catalyst
solution.
difficulty in peak maximum identification for the initially weak
and broad band feature.
Based on the approximate fitting results using eq 1, the
apparent rate constant for the Pt-catalyzed Au nanoparticles
formation in the presence of H2, 1.13 × 102 h-1 (n ) 2.4) (a),
was found to be ∼5 orders of magnitude greater than the rate
constant for the same reaction in the absence of H2, 1.17 ×
10-3 h-1 (n ) 2.5) (b). The latter was already shown to be ∼3
orders of magnitude greater that the apparent rate constant for
the reaction in the absence of Pt and H2 (1.05 × 10-6 h-1 (n )
3.6)) (c). These findings clearly demonstrate the enhanced
catalytic activity of Pt nanoparticles in the presence of H2.
In view of the concentration difference between the Pt
nanoparticles used as catalysts and the Au nanoparticles formed
in the above measurements in which Pt nanoparticle concentra-
tion is about 10-100 times larger than that for Au particles
formed, the assessments on whether the Pt nanoparticles were
encapsulated by a Au shell and whether the reused solution of
Pt nanoparticles was still active were not well substantiated.
Reaction systems with more than 1 order of magnitude lower
concentrations of Pt nanoparticles were further examined, where
Pt nanoparticles used were about the same as the Au nanopar-
ticles formed. Figure 7 shows a representative set of data for
such an experiment. The λmax values determined from this set
of spectra remained constant at 546-448 nm for the first ∼100
min and then were gradually decreased to 333 nm in the end.
The experimental data, in comparison with calibration and
simulation results (see Supporting Information), suggest that the
initial particle sizes were large (∼60 nm) and the final sizes
were ∼50 nm. The latter was in fact confirmed by TEM.
When the concentration of Pt nanoparticles was about half
of that of the Au nanoparticles expected to be 30-nm sizes under
As another important piece of information for assessing the
reaction mechanism, a decrease in pH was detected in the Pt-
catalyzed reaction solution. For example, the experiment showed
a pH change from pH ) 7.5 at t ) 0 to 7.3 at t ) 15 min in
case a of Figure 6. This change is consistent with the release of
H+ from the oxidation of Pt-H species in mediating the
reduction of AuCl4-. Control experiments showed that the
reaction solution in the presence of H2 but absence of Pt
nanoparticles displayed observable reactivity only after ∼70 h,

Au Nanoparticle Formation by Pt Nanoparticles
J. Phys. Chem. B, Vol. 110, No. 45, 2006 22509
SCHEME 1: Schematic Illustration of the Catalytic
Mediation of the Adsorbed H Species on Pt Nanocrystal
Surface for the Reduction of AuCl4-, Forming Pt@Au
Nanoparticles via Pt Nucleation Site to Grow Au Shell or
Au Nanoparticles via Au Nucleation and Growth
currently working on using Pt nanoparticles from a different
preparation method and using a dynamic light scattering
technique to monitor the size changes, the results of which will
be reported soon. A similar catalytic mechanism may also be
operative for the formation of other nanoparticles in the presence
of Pt nanoparticles. A more extensive investigation of the nano-
particle formation in such systems is part of our ongoing work.
Acknowledgment. Financial support of this work from
National Science Foundation (CHE 0349040) is gratefully
acknowledged.
Supporting Information Available: UV-vis spectra in
Figure 1 after background correction; simulation of SP band
based on Mie theory; UV-vis spectra monitoring the formation
of Au nanoparticles in presence of Pt(II) and acrylate; Mie
theory simulation results for Au nanoparticles of different sizes
and comparison of λmax vs particle size. This material is available
free of charge via the Internet at http://pubs.acs.org.
References and Notes
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mediated by the surface Pt-H species. There are two pathways
for resulting Au atoms: one uses the Pt nanoparticle as a
nucleation site to grow a Au shell forming core@shell type
Pt@Au particles, and the other undergoes nucleation and growth
forming Au nanoparticles. Preliminary DCP analysis of the
composition of the large-sized nanoparticle product indicated
the presence of Pt in addition to the predominated Au, but a
more precise analysis of the composition is needed to make a
quantitative assessment of the two products.
Conclusions
In conclusion, we have shown that the Pt nanoparticle
catalyzed formation of gold nanoparticles in aqueous solution
involves the mediation of H species preadsorbed on a Pt
nanocrystal surface (Pt-H). This insight is supported by
comparison of the Au nanoparticle formation rates in the
presence of Pt nanoparticles in solutions with and without H2.
The favorable dissociative adsorption of H2 on the Pt nanocrystal
surface produces Pt-H species, which has a higher reducing
power than H2 or acrylate in the solution for the reduction of
AuCl4-. The reductive mediation by the surface Pt-H species
is believed to be responsible for the formation of Au and core-
(Pt)-shell(Au) nanoparticles. These findings have important
implications to the development of abilities in enhancing the
controllability over size, composition, and morphology of metal
nanoparticles in the aqueous synthesis environment. We are
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