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Formation of Gold Nanoparticles Catalyzed by Platinum Nanoparticles: Assessment of theCatalytic Mechanism

Peter N. Njoki, Aisley Jacob, Bilal Khan, Jin Luo, and Chuan-Jian Zhong* Department of Chemistry, State Uni V ersity of New York at Binghamton, Binghamton, New York 13902

Recei V ed: July 5, 2006; In Final Form: September 5, 2006

The understanding of how the formation of metal nanoparticles in aqueous solutions is influenced by thepresence of presynthesized nanoparticles is important for precise control over size, shape, and compositionof nanoparticles. New insights into the catalytic mechanism of Pt nanoparticles are gained by studying theformation of gold nanoparticles from the reduction of AuCl 4- in aqueous solution in the presence of presynthesized Pt nanoparticles as a model system. The measurement of changes of the surface plasmonresonance band of gold nanoparticles, along with TEM analysis of particle size and morphology, provided animportant means for assessing the reaction kinetics. The reductive mediation of Pt - H species on the Ptnanocrystal 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 goldnanoparticles in the presence and in the absence of hydrogen (H 2), which adsorb dissociatively on a Ptnanocrystal surface forming Pt - H species. Pt - H effectively mediates the reduction of AuCl 4- 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

The use of metal and metal oxide nanoparticles to catalyzevarious homogeneous and heterogeneous chemical reactions hasrejuvenated research interests as a result of the rapidly emergingnanotechnology. 1 In comparison with the vast majority of recentstudies of nanoparticle-catalyzed reactions in solution phase, 1alittle has been reported for nanoparticle-catalyzed formation of nanoparticles. We have recently shown that preformed Ptnanoparticles 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 metalnanoparticles is influenced by the presence of metal or alloynanoparticles is important for control over size, morphology,and composition. Such control is challenging because of thepropensity 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 numberof synthetic methods reported for the preparation of water-soluble Pt and Au nanoparticles in the size range of a few to ahundred nanometers. 3- 4,10- 15 For example, Pt nanoparticles weresynthesized by reducing tetrachloroplatinate in an aqueoussolution of acrylate or polyacrylate with hydrogen 3. This

preparation method produced Pt nanoparticles of different shapes(cubic, tetrahedra, and truncated octahedra). Gold and platinumnanoparticles were also synthesized by the polyol method andstabilized with poly(vinylpyrrolidone) (PVP) under controlledtemperature. 11 Au nanoparticles were also synthesized byreducing an aqueous solution of HAuCl 4 in the presence of polymers such as N , N -dimethylacetoacetamide 12 and poly(diallyldimethylammonium) chloride. 13 Core - shell Au - Pt nanopar-ticles were synthesized by a seed growth in which citrate capped

Au nanoparticles were reacted with H 2PtCl6 and ascorbic acidto form Pt-coated Au nanoparticles. 14

Despite the extensive reports on the synthetic aspects, thereis a limited understanding of the factors controlling the size,morphology, and composition. While our recent observation thatthe reaction rate for the formation of gold nanoparticles wasdramatically increased in the presence of Pt nanoparticles 2

suggests catalytic activity of Pt nanoparticles for the reductionof AuCl4- , questions concerning the catalytic mechanism remain

quite elusive. Two possible scenarios can be considered indeveloping a mechanistic assessment of the Pt-catalyzed syn-thesis of gold nanoparticles. One involves direct redox reactionon the surface Pt atoms in reducing AuCl 4- , and the otherinvolves chemical mediation by species adsorbed on the surfaceof Pt in reducing AuCl 4- . Both scenarios seem to be intuitivelypossible based on the difference of the redox potentials of Ptand Au16 and some earlier experimental observations onchemical adsorption on Pt surfaces. 17 However, experimentaldata are not available for the mechanistic understanding of howthe detailed surface chemistry of Pt nanocrystals is operative.Such understanding has important implications to the precisecontrol 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 goldnanoparticles by reduction of AuCl 4- in aqueous solution. Themeasurement of changes in the surface plasmon resonanceoptical properties of gold nanoparticles, along with TEManalysis of the particle sizes and morphology, allowed us toassess the reaction kinetics and to gain insights into the catalyticmechanism of Pt nanoparticles in the formation of goldnanoparticles.

* To whom correspondence should be addressed. E-mail: [email protected].

22503 J. Phys. Chem. B 2006, 110, 22503- 22509

10.1021/jp0642342 CCC: $33.50 2006 American Chemical SocietyPublished on Web 10/24/2006


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Experimental Section

Chemicals. Potassium tetrachloroplatinate (K 2PtCl4, 99.99%),hydrogen tetrachloroaurate (HAuCl 4, 99%), and sodium acrylate(CH2CHCO 2Na, (A), 97%) were purchased from Aldrich andused as received. Water was purified with a Millipore Milli-Qwater system. Hydrogen (99.95%) was purchased from Air Gasand used as received.

Synthesis. Pt nanoparticles encapsulated with acrylate were

synthesized based on a reported protocol,3

where aged K2PtCl4was dissolved in 50 mL of deionized water. The pH was adjustedto 7.0 using a dilute NaOH solution. Ar gas was then purgedthrough the solution for 5 min. Sodium acrylate solution wasthen added to the metal-precursor solution. The solutiondisplayed a light orange color. H 2 gas was then bubbled intothe solution for 30 min. The reaction flask was sealed and stirredovernight in the dark at room temperature. After 12 h thesolution turned brown as a result of the formation of Ptnanoparticles capped with acrylate. The Pt nanoparticles canbe isolated by a salting out method or high-speed centrifugation(> 16 000 rpm). The particles were redispersable in water andhave been stored in an aqueous solution for at least 1 year. Asfor the yield, these isolation and cleaning processes were foundto lead to a significant loss of particles.

The synthesis of Au nanoparticles was carried out either ina 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 Ptnanoparticles. For the synthesis of Au nanoparticles in thepresence of Pt nanoparticles, an aged solution of HAuCl 4 of known concentration was mixed with a solution of Pt nanopar-ticles of known concentration. The solution was diluted withdeionized water, and the pH was adjusted to 7 using a diluteNaOH solution. Pt nanoparticles and aged HAuCl 4 were usedat different concentration ratios. A known volume of sodiumacrylate was added into the mixed solution, typically in the range

of 1.0 10- 3

- 1.0 10- 2

M. The reactions were alsofollowed by spectrophotometric monitoring of the opticalabsorption band. In the experiment to examine the stability of the catalytic activity of the Pt nanoparticles, the Pt nanoparticlesused from a previous run of the reaction were separated by thecentrifugation method. The obtained Pt nanoparticles werecleaned and reused for the reduction of HAuCl 4 following thesame protocol as that described above. In the experiment toexamine the involvement of adsorbed H-species in the reaction,H2 gas was bubbled through the reaction solution containing Ptnanoparticles and AuCl 4- under conditions similar to those fromthe above experiments.

Instrumentation and Measurements. UV- visible (UV -vis) spectra were acquired with an HP 8453 or HP 8452spectrophotometer. The spectra were collected over the range200- 1100 nm or 190 - 820 nm (HP 8452). The UV - vis spectrawere collected as a function of reaction time either by directlymonitoring the solution in a reaction cuvette (with a stirringbar in the bottom to stir the solution when a spectrum was nottaken) or by taking solution samples from a separate 200-mLreaction vessel under constant stirring. The reactions werecarried out under ambient conditions.

Transmission electron microscopy (TEM) was performed ona Hitachi H-7000 microscope (100 kV). The aqueous nanopar-ticle samples were drop cast onto a carbon-coated copper gridsample holder followed by natural evaporation at room tem-perature.

Results and Discussion

Gold nanoparticles (NPs) of larger than 2 nm exhibit a surfaceplasmon (SP) resonance band in the visible region ( 520 nm),whereas this band is largely damped for Pt nanoparticles (itsSP band being shifted largely into the UV region). A change in

absorbance or wavelength of the SP band18- 20

provides ameasure of particle size, shape, concentration, and dielectricmedium properties. In the cases when particle sizes, shapes,and solvent properties are comparable, the absorbance of theSP band is largely related to the concentration of the particles.As described below, the measurement of the SP band duringthe synthesis of Au nanoparticles provide useful informationfor assessing the reaction kinetics.

Figure 1 shows two typical sets of UV - vis spectra monitoringthe formation of gold nanoparticles in the presence of Ptnanoparticles. Set B is obtained under the same concentrationof Pt nanoparticles as that for set A, but with a 2 concentrationof AuCl4- . A gradual increase of absorbance for the SP bandin the 520 nm region was observed, which is a characteristic

of the formation of gold nanoparticles. Note that the risingbackground is due to the spectrum of Pt nanoparticles (i.e., thespectrum at t ) 0). As shown in the insert, while the absorbance( Amax) of the SP band increased with reaction time, there seemedto be a subtle decrease in the wavelength of the SP band ( max)in the initial 10 h, which remained largely constant in therest of the reaction. This initial decrease of max may be due todifficulty in peak maximum identification for the initially weak and broad SP band feature. The change in the Pt nanoparticleconcentration did not seem to show a significant shift for theSP band. However, there appeared to be a significant differencein the rate for the absorbance change; i.e., the rate increaseswith the concentration of Pt nanoparticles. Under the sameconcentration of Pt nanoparticles but different AuCl 4- concen-

Figure 1. UV- vis spectra monitoring the formation of Au nanopar-ticles upon reduction of AuCl 4- by acrylate in the presence of Ptnanoparticles of different concentrations. Inserts: time dependence of max and Amax of the SP band. The concentrations of Pt nanoparticlesand AuCl 4- , ([Pt- NPs], [AuCl 4- ]: (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.

22504 J. Phys. Chem. B, Vol. 110, No. 45, 2006 Njoki et al.


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tration, the rate for the absorbance increase of the SP band (538nm) was found to increase with AuCl 4- concentration (B). Aswill be shown later, the spectral evolution rate for the same

reaction in the absence of Pt nanoparticles is much slower thanthose in the presence of Pt nanoparticles.Before a detailed analysis of the reaction kinetics, the

morphology and size of the nanoparticles were examined usingTEM. Figure 2 shows a representative set of TEM micrographsfor the Au nanoparticles formed in the presence of presynthe-sized Pt nanoparticles (B). The inclusion of the morphologicalfeatures of the presynthesized nanoparticles (A) and goldnanoparticles formed in the absence of Pt nanoparticles (C)serves the purpose of comparison. Note that the basic featureof the Pt nanoparticles is largely dominated by the morphologyof cubes.

In contrast to the morphological features for the Pt nanopar-ticles (A) and the Au nanoparticles formed in the absence of Pt

nanoparticles (C), the data for the gold nanoparticles formed inthe 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 thepresynthesized Pt nanoparticles before being used in the reaction(8.1 ( 0.7 nm). The latter are Au nanoparticles formed in thepresence of Pt nanoparticles, which are apparently much smallerthan those formed in the absence of Pt nanoparticles (51.7 (4.2 nm).

In Figure 3, two typical TEM micrographs are compared forAu nanoparticles synthesized in the presence of two differentconcentrations of Pt nanoparticles. Micrograph A in Figure 3is for a sample corresponding to that for spectra set A in Figure1. Nanoparticles with bimodal or trimodal distributions areclearly observed. The average sizes of the particles determinedfrom 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

Figure 2. TEM micrographs and size distributions of Pt, Au in the presence of Pt nanoparticles and Au in the absence of Pt nanoparticles. Theconcentrations of K 2PtCl4 and A are 4.8 10 - 4 and 9.6 10 - 3 M) (A); the concentrations of Pt nanoparticles and AuCl 4- ([Pt - NPs], [AuCl 4- ]):(2.74 10 - 8, 2.5 10 - 4) (B), and (0, 2.5 10 - 4) (C), which used the same amount of reducing agent.

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each of these conditions, the size of the presynthesized Ptnanoparticles before and after the reaction remains almostunchanged (i.e., 7.6 - 8.2 nm), whereas the newly formed Aunanoparticles showed sizes which fall in the range 21 - 34 nm.For example, nanoparticles of the larger sizes were observedwhen the concentration of Pt nanoparticles was relatively large.

The difference in the average size of the gold nanoparticlesformed is rather small, and there is an insignificant change in

the wavelength of the SP band. The change in the absorbanceis a function of the volume fraction of Au nanoparticles in thesolution. The increase in particle size is usually accompaniedby a red shift of the SP band for particle sizes larger than 30nm. Our experimental data showed however the lack of such ared shift of the SP band as a function of time (see SupportingInformation). It is also supported by a simulation of the SP bandbased on Mie theory (see Supporting Information). The resultsfrom the simulation of the SP bands in Figure 1 reveal that thevolume fraction of the Au nanoparticles ( ) shows a gradualincrease as a function of reaction time in different concentra-tions. This finding, together with the TEM data, substantiatesthat 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), thedependence of max and Amax on the size and concentration of Au nanoparticles showed a clear trend of increasing max withsize for particles larger than 30 nm, which is in fact quitecomparable with the theoretical simulation. The experimentallyobserved max values remained largely constant for most of thekinetic region where the absorbance rising is observed. The finalsize of the Au particles was found to be 30 - 50 nm. Theexperimental data thus suggest that the growth is fast, whereasthe nucleation is slow. These findings constituted the basis forusing the change in absorbance, which is a function of thechange in volume fraction of Au nanoparticles in the system,to assess the reaction kinetics. The reaction rate, i.e., the changein the concentration of AuCl 4- , - (dC (AuCl 4- ) /dt ), should be

proportional to the amount of Au in the nanoparticles or thechange in volume fraction of Au nanoparticles ( ),- (dC (AuCl4- ) /dt ) . Since the particle size seemed to berelatively constant in most of the rising region in the Amax- t curve, the change in absorbance (d Amax /dt ) can be used to assessthe reaction rate.

Figure 4 shows a representative set of data monitoring theformation of Au nanoparticles in the absence of presynthesizedPt nanoparticles (a), which is also compared with that obtainedin the absence of the Pt nanoparticles (b). In comparison withthe data obtained in the absence of Pt nanoparticles (b), thereaction rate for the formation of gold nanoparticles in thepresence of Pt nanoparticles (a) was clearly much faster (by afactor of 30- 50). The fitting to the experimental data was based

on Avramis theoretical model of crystallization and growth, 21which was shown to be useful for assessing the crystallizationkinetics of macromolecules or polymers. 22- 23 This model wasalso 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 canbe related to the volume fractional crystallinity of the nanopar-ticles formed.

where k is the apparent rate constant, n is the critical growth

exponent, and C is the proportionality constant. The fittingresults are summarized in Table 1.The overall apparent rate constant is evidently increased by

about 3 orders of magnitude in the presence of Pt nanoparticlesin comparison with that in the absence of Pt nanoparticles. Underthe condition of diffusion-controlled mass flow to the crystalnucleus, a critical exponent of 2.5 or less has been observedfor thermal nucleation. 22 Smaller values of n were consideredas indications of lower growth dimensionality. The fitted nvalues in each case are greater than 2.5, suggestive of amechanism involving a thermal nucleation and three-dimen-sional crystallization as often found for macromoleculesystems. 22- 23

In Figure 5, the kinetic plots based on the spectral evolution

of the SP band data are compared, which involved differentconcentrations of Pt nanoparticles but the same concentrationof AuCl4- (a, b, and c), or different concentrations of AuCl 4-

but the same concentration of Pt nanoparticles (b and d). Notethat the absorbance data were all corrected against the absor-bance at t ) 0.

The above data yield several important pieces of information.First, the overall reaction rate seems to display the exponentialtype of kinetics in which the absorbance values approach thesame plateau value for the three different concentrations (a, b,and c). Second, the increase in concentration of AuCl 4- (d) withthe same concentration of Pt nanoparticles led to an increase of both the rate and the final plateau. Such dependence is expectedsince the formation of Au nanoparticles involves the aggregation

Figure 3. TEM micrographs and size distributions for Au nanoparticlesformed in the presence of presynthesized Pt nanoparticles of differentconcentrations. (A) The concentrations correspond to those for spectralset A in Figure 1. (B) The concentration of Pt nanoparticles is twicethat of A, whereas other concentrations are the same as in those in A.

Figure 4. Kinetic plots for the formation of Au nanoparticles in theabsence of (a) and in the presence of (b) Pt nanoparticles at roomtemperature. The concentrations of Pt nanoparticles and AuCl 4- ([Pt -NPs], [AuCl 4- ], 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.

TABLE 1: Fitting Results Based on Eq 1 a

reaction condition k n

(a) in absence of Pt nanoparticles 1.05 10- 6 3.6(b) in the presence of Pt nanoparticles 1.17 10- 3 2.5a Note: The fittings were for the data in Figure 4. The data were

background subtracted.

A ) C (1 - e- kt n) (1)



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of Au atoms as a result of the reduction of AuCl 4- . Finally, aclose examination of the initial reaction kinetics reveals thatthe initial formation rate is linearly dependent on the concentra-tion of the Pt nanoparticles, even though the overall reactionrate seems to be described by the exponential type of kinetics

(eq 1). The fact that the rate increases with concentration of Ptnanoparticles (a > b > c) under the relatively low concentrationof AuCl4- (< 0.4 mM) further substantiates the catalyticactivity of Pt nanoparticles.

In view of the catalyst role of Pt nanoparticles, one questionconcerns 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 imagesshown earlier. In addition, the catalytic activity of the re-usedPt nanoparticles was examined in a control experiment, in whichthe Au nanoparticles were removed as precipitate by centrifuga-tion since their sizes are larger than Pt nanoparticles. Thesupernatant containing Pt nanoparticles was then used again to

catalyze the formation of new Au nanoparticles. The resultsrevealed catalytic activity similar to the case using fresh Ptnanoparticles. This result seemed to suggest that Pt nanoparticlescatalyzed the formation of Au nanoparticles without beingcoated by Au. However, as it will be described later by morecareful examinations, this is not the case.

Having established the apparent catalytic activity of Ptnanoparticles for the reduction of AuCl 4- in the formationprocess of Au nanoparticles, a fundamental question is how thereduction of AuCl 4- is mechanistically catalyzed by Pt nano-particles. In the absence of prior literature directly related tothis system, two possible scenarios have been considered. Thefirst scenario involves the adsorption of AuCl 4- (or AuCl2(OH) 2-

in the slightly basic condition 4a) on a Pt nanocrystal surface

which is followed by an electron-transfer pathway for thereduction of Au(III) to Au(0). Such a scenario is possible basedon a comparison of redox potentials between the two metals, 16i.e.,

and

This mechanism requires direct adsorption of AuCl 4- on thesurface of Pt and facile oxidation of the surface Pt. However,the presence of acrylate capping agents on the surface of Ptcreates a steric barrier to the surface adsorption and oxidation.To assess this possibility, PtCl 42- was examined as a catalystto substitute Pt nanoparticles. The results (see SupportingInformation) seemed to show some indication of activity forthe formation of Au nanoparticles, but the SP characteristicsand the resulting particles determined were very different fromthose using Pt nanoparticles as catalysts. It showed a weak andbroad SP band in the 500- 700 nm region; the TEM showed

only small particles ( 102 ) where the loss

of Pt particles was insignificant. It is therefore likely that atleast part of the Pt nanoparticles were occluded forming core-(Pt) - shell(Au) type nanoparticles, which explains the sharpdecrease of the reaction rate in the reused Pt nanoparticle catalystsolution.

As another important piece of information for assessing thereaction mechanism, a decrease in pH was detected in the Pt-catalyzed reaction solution. For example, the experiment showeda pH change from pH ) 7.5 at t ) 0 to 7.3 at t ) 15 min incase a of Figure 6. This change is consistent with the release of H+ from the oxidation of Pt - H species in mediating thereduction of AuCl 4- . Control experiments showed that thereaction solution in the presence of H 2 but absence of Ptnanoparticles displayed observable reactivity only after 70 h,

Figure 6. (A) UV - vis spectra monitoring the formation of Aunanoparticles in the presence of Pt nanoparticles, H 2, and acrylate.Insert: time dependence of max and Amax of the SP band. (B) Kineticplots for three different conditions (the same [AuCl 4- ]): in the presenceof Pt nanoparticles and H 2 (a, squares), of Pt nanoparticles but withoutH2 (b, circles), and without Pt nanoparticles and H 2 (c, diamonds).Concentrations of Pt nanoparticles and AuCl 4- : 1.37 10 - 8 and 2.4 10 - 4 M, which used the same amount of reducing agent.

Figure 7. (A) UV - vis spectra monitoring the formation of Aunanoparticles in the presence of Pt nanoparticles, H 2, and acrylate.Concentrations of Pt nanoparticles and AuCl 4- : 4.38 10 - 10 and 2.4 10- 4 M. (B) Kinetic plots for three different conditions (same[AuCl 4- ] and [acrylate]): in the presence of H 2 and Pt nanoparticleswith concentration of Pt nanoparticles at 8.77 10 - 10 M (a, 1 ) and4.38 10- 10 M (b, b ); reused b after removing gold nanoparticles (c,O ).



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indicating that H 2 alone is not reactive. These findings clearlydemonstrate that the catalytic activity is dramatically enhancedin the presence of both Pt nanoparticles and H 2.

On the basis of the above findings, we believe that thecatalytic mechanism of Pt nanoparticles for the formation of Au nanoparticles in the presence of H 2 involves the adsorbedH species on Pt nanocrystals surface, i.e., Pt - H (Scheme 1).In the absence of H2, it is believed that the formation of thesurface Pt - H species via dissociation of water on Pt nanopar-ticles must have played an important role in the Pt nanoparticlecatalyzed reduction of AuCl 4- toward the formation of Aunanoparticles.

The adsorbed Pt - H species has a higher reducing power thanH2 or acrylate for the reduction of AuCl 4- in the solution, e.g.,

This reaction pathway involves the reduction of AuCl 4-mediated by the surface Pt - H species. There are two pathwaysfor resulting Au atoms: one uses the Pt nanoparticle as anucleation site to grow a Au shell forming core@shell typePt@Au particles, and the other undergoes nucleation and growthforming Au nanoparticles. Preliminary DCP analysis of thecomposition of the large-sized nanoparticle product indicatedthe presence of Pt in addition to the predominated Au, but amore precise analysis of the composition is needed to make aquantitative assessment of the two products.

ConclusionsIn conclusion, we have shown that the Pt nanoparticle

catalyzed formation of gold nanoparticles in aqueous solutioninvolves the mediation of H species preadsorbed on a Ptnanocrystal surface (Pt - H). This insight is supported bycomparison of the Au nanoparticle formation rates in thepresence of Pt nanoparticles in solutions with and without H 2.The favorable dissociative adsorption of H 2 on the Pt nanocrystalsurface produces Pt - H species, which has a higher reducingpower than H2 or acrylate in the solution for the reduction of AuCl4- . The reductive mediation by the surface Pt - H speciesis believed to be responsible for the formation of Au and core-(Pt)- shell(Au) nanoparticles. These findings have importantimplications to the development of abilities in enhancing thecontrollability over size, composition, and morphology of metalnanoparticles in the aqueous synthesis environment. We are

currently working on using Pt nanoparticles from a differentpreparation method and using a dynamic light scatteringtechnique to monitor the size changes, the results of which willbe reported soon. A similar catalytic mechanism may also beoperative for the formation of other nanoparticles in the presenceof 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 fromNational Science Foundation (CHE 0349040) is gratefullyacknowledged.

Supporting Information Available: UV- vis spectra inFigure 1 after background correction; simulation of SP bandbased on Mie theory; UV - vis spectra monitoring the formationof Au nanoparticles in presence of Pt(II) and acrylate; Mietheory simulation results for Au nanoparticles of different sizesand comparison of max vs particle size. This material is availablefree of charge via the Internet at http://pubs.acs.org.

References and Notes(1) (a) Burda, C.; Chen, X. B.; Narayanan R.; El-Sayed, M. A. Chem.

ReV . 2005 , 105 , 1025. (b) Zhong, C. J.; Luo, J.; Maye, M. M.; Han, L.;Kariuki, N. N. In Nanotechnology in Catalysis ; Zhou, B., Hermans, S.,Somorjai, G. A., Eds.; Kluwer Academic/Plenum Publishers: 2004; Vol.

1, Chapter 11, pp 222-

248.(2) Njoki, P. N.; Luo, J.; Wang, L.; Maye, M. M.; Quaizar, H.; Zhong,C. J. Langmuir 2005 , 21 , 1623.

(3) Petroski, J. M.; Green, T. C.; El-Sayed, M. A. J. Phys. Chem. A2001 , 105 , 5542.

(4) (a) Goia, D. V.; Matijevic, E . Colloids Surf., A 1999 , 146 , 139. (b)Sau, T. K.; Pal, A.; Jana, N. R.; Wang, Z. L.; Pal, T. J. Nanopart. Res.2001 , 3 , 257.

(5) (a) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman,R. J. Chem. Soc., Chem. Comm . 1994 , 801. (b) Brust, M.; Kiely, C. J .Colloids Surf., A 2002 , 202 , 175.

(6) (a) Hostetler, M. J.; Wingate, J. E.; Zhong, C. J.; Harris, J. E.;Vachet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.;Wignall, G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998 , 14 , 17. (b) Hostetler, M. J.; Zhong, C. J.; Yen, B. K. H.;Anderegg, J.; Gross, S. M.; Evans, N. D.; Porter, M. D.; Murray, R. W. J. Am. Chem. Soc. 1998 , 120 , 9396.

(7) (a) Maye, M. M.; Zheng, W. X.; Leibowitz, F. L.; Ly, N. K.; Zhong,

C. J. Langmuir 2000 , 16 , 490. (b) Maye, M. M.; Zhong C. J. J. Mater.Chem. 2000 , 10 , 1895.(8) (a) Kariuki, N. N.; Luo, J.; Maye, M. M.; Hassan, A.; Menard, T.;

Naslund, H. R.; Lin, Y.; Wang, C.; Engelhard, M. H.; Zhong, C. J. Langmuir 2004 , 20 , 11240. (b) Sakai, T.; Alexandridis, P. Chem. Mater. 2006 , 18 ,2577

(9) Luo, J.; Njoki, P. N.; Lin, Y.; Mott, D.; Wang, L.; Zhong, C. J. Langmuir 2006 , 22 , 2892.

(10) Daniel, M. C.; Astruc, D. Chem. Re V . 2004 , 104 , 293.(11) Garcia-Gutierrez, D. I.; Gutierrez-Wing, C. E.; Giovanetti L.;

Ramallo-Lopez, J. M.; Requejo, F. G.; Jose-Yacaman, M. J. Phys. Chem. B 2005 , 109 , 3813.

(12) Song, J. H.; Kim, Y.-J.; Kim, J.-S. Curr. Appl. Phys. 2006 , 6 , 216.(13) Chen, H.; Wang, Y.; Wang, Y.; Dong, S.; Wang, E. Polymer 2006 ,

47 , 763.(14) Zhang, B.; Li, J.-F.; Zhong Q.-L.; Ren, B.; Tian, Z.-Q. Zou, S.-H

Langmuir 2005 , 21 , 7449.(15) (a) Kim, K. S.; Demberelnyamba, D.; Lee, H. Langmuir 2004 , 20 ,

556. (b) Hussain, I.; Brust, M.; Papworth, A. J.; Cooper, A. I. Langmuir 2003 , 19 , 4831.(16) Bard, A. J.; Parsons, R.; Jordan, J. Standard Potentials in Aqueous

Solutions ; Marcel Dekker Inc.: New York, Basel, 1985; pp 318 and 353.(17) Angel, P. D.; Dominguez, J. M.; Angel, G. D.; Montoya, J. A.;

Pitara, E. L.; Labruquere, S.; Barbier, J. Langmuir 2000 , 16 , 7210.(18) Luo, J.; Maye, M. M.; Han, L.; Kariuki, N. N.; Jones, V. W.; Lin,

Y.; Engelhard, M. H.; Zhong, C. J. Langmuir 2004 , 20 , 4254.(19) (a) Link, S.; El-Sayed, M. A. Int. Re V . Phys. Chem. 2000 , 19 , 409.

(b) Papavassiliou, G. C. Prog. Solid State Chem. 1979 , 12 , 185.(20) Ung, T.; Liz-Marzan, L.; Mulvaney, P. J. Phys. Chem. B 2001 ,

105 , 3441.(21) Avrami, M. J. Chem. Phys. 1940 , 8 , 212.(22) Exarhos, G. J.; Aloi M. Thin Solid Films 1990 , 193 , 42.(23) Cheng, S. Z. D.; Wunderlich, B. Macromolecules 1988 , 21 , 3327.(24) Han, L.; Maye, M. M.; Leibowitz, F. L.; Ly, N. K.; Zhong, C. J.

J. Mater. Chem. 2001 , 11 , 1258.

SCHEME 1: Schematic Illustration of the CatalyticMediation of the Adsorbed H Species on Pt NanocrystalSurface for the Reduction of AuCl 4- , Forming Pt@AuNanoparticles via Pt Nucleation Site to Grow Au Shell orAu Nanoparticles via Au Nucleation and Growth

AuCl4- + Pt- H f Pt@Au (andor Au) + Pt + H+ + Cl-

Au Nanoparticle Formation by Pt Nanoparticles J. Phys. Chem. B, Vol. 110, No. 45, 2006 22509

formation of gold nanoparticles catalyzed by platinum nanoparticles assessment of the catalytic...

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