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 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 understanding 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 nanoparticles3,4 in comparison with the high stability of their organicsoluble nanoparticles,5-9 e.g., alkanethiolate-protected nanoparticles derived from two-phase synthesis. There are a number of synthetic methods reported for the preparation of watersoluble 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 nanoparticles were synthesized by a seed growth in which citrate capped * To whom correspondence should be addressed. E-mail: cjzhong@ binghamton.edu.
Au nanoparticles were reacted with H2PtCl6 and ascorbic acid to form Pt-coated Au nanoparticles.14 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 synthesis 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 composition. 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.
10.1021/jp0642342 CCC: $33.50 © 2006 American Chemical Society Published on Web 10/24/2006
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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. The synthesis of Au nanoparticles was carried out either in a solution of Au precursors without presynthesized Pt nanoparticles 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 nanoparticles 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. Instrumentation and Measurements. UV-visible (UVvis) 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. Transmission electron microscopy (TEM) was performed on a Hitachi H-7000 microscope (100 kV). The aqueous nanoparticle samples were drop cast onto a carbon-coated copper grid sample holder followed by natural evaporation at room temperature.
Figure 1. UV-vis spectra monitoring the formation of Au nanoparticles 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 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-
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 presynthesized 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. In contrast to the morphological features for the Pt nanoparticles (A) and the Au nanoparticles formed in the absence of Pt
nanoparticles (C), the data for the gold nanoparticles formed in the presence of Pt nanoparticles reveal particles with distributions 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
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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.
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 concentrations. 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 in the concentration of AuCl4-, -(dC(AuCl4-)/dt), should be proportional to the amount of Au in the nanoparticles or the change in volume fraction of Au nanoparticles (φ), -(dC(AuCl4-)/dt) ∝ φ. Since the particle size seemed to be 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. 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
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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- ([PtNPs], [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.
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 1.17 × 10-3
3.6 2.5
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 thiolbased linker molecules.24 Using this model, the absorbance can be related to the volume fractional crystallinity of the nanoparticles formed.
A ) C(1 - e-kt ) n
(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-dimensional crystallization as often found for macromolecule systems.22-23 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 of AuCl4- (a, b, and c), or different concentrations of AuCl4but the same concentration of Pt nanoparticles (b and d). Note that the absorbance data were all corrected against the absorbance at t ) 0. 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-(aq)
E° ) 0.758 V
AuCl4(aq)- + 3e f Au(s) + 4Cl-(aq)
E° ) 1.002 V
and
Figure 5. Kinetic plots monitoring the formation of Au nanoparticles in the presence of presynthesized Pt nanoparticles of different concentrations. 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 concentration 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- (
