8148
J. Phys. Chem. B 2004, 108, 8148-8152
Genesis of Pt Clusters in Reverse Micelles Investigated by in Situ X-ray Absorption Spectroscopy Yin W. Tsai,† Yu L. Tseng,† Loka S. Sarma,† Din G. Liu,‡ Jyh F. Lee,‡ and Bing J. Hwang*,† Department of Chemical Engineering, National Taiwan UniVersity of Science and Technology, Taipei 106, Taiwan, R.O.C. ReceiVed: January 31, 2004; In Final Form: April 14, 2004
In situ X-ray absorption spectroscopy (XAS) has been successfully applied to explore the formation mechanism of Pt clusters at the early stages within AOT reverse micelles. Transmission electron microscopy was also utilized to observe the size of the platinum clusters. Upon successive addition of the reducing agent, hydrazine (N2H4), six distinguishable steps were observed for the formation of Pt clusters at the early stage. Both in situ X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analysis for the Pt LIII-edge confirms the Pt4+ f Pt2+ f Pt0 reduction sequence. A corresponding structural model was proposed for each step so that this work provides a detailed insight into the mechanism of nucleation and growth of platinum clusters.
1. Introduction The metallic nanoparticle has drawn much attention due to its unique catalytic performance, electrochemical properties, and other related physicochemical properties.1-3 The materials synthesized from micellar solutions of surfactants, which combine the advantages of high specific surface area and uniform particle size distribution, have particular potential for applications in catalysis and fuel cells. Since the properties of the nanoparticle strongly depend on its particle size and particle size distribution (PSD), control of them is of great importance in the practice of the synthesis of nanoparticles. Since the particle size distribution of nanoparticles is associated with the formation of nanoparticles at the early stage, it is essential to investigate the genesis of nanoparticles during the course of formation. However, it is difficult to get the exact structural information of nanoparticles at early stages using ex situ techniques such as transmission electron microscopy (TEM) and X-ray diffraction (XRD) methods because the nanoparticle structure would change during the preparation or transportation of the samples or by the lack of long range ordering. In situ X-ray absorption spectroscopy has been successfully used to study the structural transformation of metallic nanoparticles.4,5 It can provide information on short range ordering around a specific type of atom, which is especially important while nanoparticles form at the early stage. Although a large number of studies have focused on the synthesis and characterization of Pt nanoparticles, to our knowledge no work deals with the formation mechanism of Pt nanoparticles at the early stages. Meanwhile, a detailed understanding of the formation mechanism of nanoparticles is necessary for both a successful particle design and a scale-up process. Although several methods have been developed for the synthesis of nanoparticles,6-11 it has been well demonstrated that a fairly uniform size distribution of nanoparticles can be synthesized in reverse micelles. By studying XAS spectra at the Pt LIII-edge, here we propose a mechanism * Corresponding author. E-mail:
[email protected]. † Department of Chemical Engineering, National Taiwan University of Science & Technology, Taipei 106, Taiwan. ‡ National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan.
based on the derived parameters such as coordination number, bond distance, and structural and vibrational disorder in bond distances during the course of the formation of Pt clusters in AOT reverse micelles. A structural model was proposed for each step in the reduction process based on the XAS results. 2. Experimental Section Preparation of Pt Clusters in AOT Reverse Micelles. The Pt nanoclusters were prepared in a microemulsion system containing n-heptane, water, and surfactant. The surfactant used was sodium bis(2-ethyl hexyl) sulfosuccinate (AOT) (99%). The n-heptane and the surfactant were thoroughly mixed, and this was followed by addition of an aqueous solution of the H2PtCl6 to form a microemulsion with the Pt complex in the water pool. The volume ratio of aqueous phase and organic phase has been kept at 1:10 (0.25 mL of aqueous phase is mixed with 5.0 mL of organic phase). The water to surfactant molar ratio was equal to 5.55, and the aqueous component contained 0.25 M H2PtCl6. This microemulsion solution was placed in the liquid cell for in situ XAS study. This liquid cell was carefully designed to avoid excessive attenuation of the X-ray beam. A microemulsion, of the same composition of n-heptane, water, and the surfactant that contained the reducing agent, hydrazine (N2H4), was also prepared. An appropriate amount of N2H4-containing microemulsion was then added to the H2PtCl6-containing microemulsion using a microsyringe, whereby the Pt complex was reduced. All reactions were carried out at 298 K unless otherwise specified. Nitrogen gas was purged into the microemulsion system throughout the experiment to prevent the nanoparticles from being oxidized by air. Transmission electron microscopy (TEM) measurements were carried out using a Hitachi model H-7100 electron micrograph. The sample for TEM analysis was prepared by placing a drop of the dispersed solution onto a carbon-coated copper grid and allowing the solvent to be evaporated in air at room temperature. XAS Measurements. XAS measurements were made at the Wigger Beam Line 17C at the National Synchrotron Radiation Research Center, Hsinchu, Taiwan. The beam size was 4 × 2 mm2. The channel-cut Si(111) monochromator was used. The
10.1021/jp0495592 CCC: $27.50 © 2004 American Chemical Society Published on Web 05/25/2004
Genesis of Pt Clusters in Reverse Micelles
J. Phys. Chem. B, Vol. 108, No. 24, 2004 8149 TABLE 1: Edge Energies and White Line Areas of the Pt Microemulsion System as a Function of N2H4 Dosage
Figure 1. In situ XANES of Pt microemulsion system.
storage ring was operated at 1.5 GeV, and the ring current was in the range of 100-200 mA. The monochromator was detuned by 10% to reject higher harmonics. The experiments were done in the transmission mode with three detectors. The third detector was used in conjunction with the reference sample, Pt foil. The lengths of the incident (I0), transmission (It), and reference (Ir) ionization chambers were 30, 30, and 30 cm, respectively. The I0 and It ion chambers were filled with a mixture of He + N2 and Ar + N2, respectively, for the Pt LIII measurement. Standard procedures were followed to analyze the EXAFS data. First, the raw absorption spectrum in the preedge region was fit to a straight line and the background above the edge was fit with a cubic spline. The EXAFS function, χ, was obtained by subtracting the postedge background from the overall absorption and then normalizing with respect to the edge jump step. The normalized χ(E) was transformed from energy space to k-space, where k is the photoelectron wave vector. The χ(k) data were multiplied by k3 to enhance the damping of EXAFS oscillations in the high-k region. Subsequently, k3-weighted χ(k) data in the k-space ranging from 3.06 to 11.41 Å-1 for Pt were Fourier transformed (FT) to r-space in order to separate the EXAFS contributions from different coordination shells. A nonlinear least-squares algorithm was applied for curve fitting of EXAFS in r-space between 0.98 and 2.98 Å. All the computer programs were implemented in the UWXAFS 3.0 package12 with the backscattering amplitude and the phase shift for the specific atom pair theoretically calculated using the FEFF6.01 code.13 3. Results and Discussion In Situ XANES at the Pt LIII-Edge of Pt Clusters. Figure 1shows the Pt LIII-edge XANES spectra recorded during the formation of Pt clusters in the microemulsion system as a function of dosage of hydrazine. The edge energies and white line areas of the Pt microemulsion system are shown in Table 1. The white line at the Pt LIII-edge is an absorption threshold resonance, attributed to electronic transitions from the 2p3/2 to unoccupied states above the Fermi level and is sensitive to changes in electron occupancy in the valence orbitals of the absorber.14 Hence, changes in the white line intensity have been
amount of N2H4 (moles)
energy of absorption edge (eV)
Pt LIII-edge energy of main peak (eV)
area of white line
none 4.39 × 10-5 4.39 × 10-5, 1.5 h 8.78 × 10-5 1.32 × 10-4 2.20 × 10-4 3.51 × 10-4, 41 min 5.71 × 10-4, 16 min 7.90 × 10-4, 1 h 40 min 7.90 × 10-4, 4 h 9.22 × 10-4, 1.5 h 1.05 × 10-3, 2.5 h 1.10 × 10-3, 6 h Pt foil
11564.5 11564 11564 11564 11564 11564 11564 11564 11564 11564 11564 11564 11564 11564
11567 11566.6 11566.5 11566.5 11566.5 11566.5 11566.5 11566.5 11566.5 11566.5 11566.5 11566.5 11566.5 11566.5
5.865 2.895 2.667 2.663 2.272 2.141 2.390 3.269 3.507 3.527 3.233 2.863 2.669 1.513
directly related to the density of unoccupied d states and indicate the changes in the oxidation state of the Pt absorber. In general, if the white line intensity decreases, the density of unoccupied d states is lower and the oxidation state of Pt is lower. As can be seen in Figure 1, the white line (represented as WL) intensity decreases gradually as the dosage of hydrazine increases from 4.39 × 10-5 to 2.20 × 10-4 moles. The area of the white line (the intensity of the absorption peak) was found to be 5.865 in the absence of the reducing agent and decreased sharply to 2.895 upon addition of 4.39 × 10-5 M of hydrazine. The white line area decreased further to 2.141 due to the decrease in the density of unoccupied d states up to the addition of 2.20 × 10-4 M of hydrazine. This indicates the gradual change in Pt oxidation state from +4 to +2. Further increase of the dosage of reducing agent up to 9.22 × 10-4 M of hydrazine resulted in the formation of hydroxide species on Pt, and there is a corresponding small increase in the area of the white line to 3.233. This variation could be explained by the ligand effect of OHand Cl-. The electron-withdrawing power of OH- is comparatively higher than that of Cl-, and hence formation of OH- on Pt increases the density of unoccupied d states and hence increases the area of the white line. The XANES pattern at this stage is similar to that of the reference H2Pt(OH)6. However, an increase in reducing agent dosage to 1.10 × 10-3 M decreases the density of unoccupied d states and the area of white line to 2.669, and this indicates the change in Pt oxidation state from +2 to 0. The XANES pattern at this stage is similar to that of the reference Pt foil. Another peak, postedge, at 11579 eV (represented as HP) could be assigned as a “hybridization peak”. This peak arises due to hybridization of the Pt d photoelectron state with the unoccupied atomic Cl 3d states, mediated by multiple scattering. Appearance of the HP peak is consistence with the ab initio self-consistent XANES calculations for the Pt LIII-edge of PtCl compounds.15 The intensity of the HP peak also decreases with an increase in reducing agent from 4.39 × 10-5 to 2.20 × 10-4 M of hydrazine, which indicates the extent of decrease in Pt-Cl coordination and corresponding decrease in Pt oxidation state. The disappearance of the HP peak starts after the addition of 3.51 × 10-4 M of hydrazine and completely disappeares at 9.22 × 10-4 M of N2H4. This is the indication of progressive omission of Cl d states, and at this stage the coordination to Pt is entirely due to hydroxide species. The hybridization peak also observed in the reference H2Pt(OH)6 is located at higher energy compared to that for the PtCl6 compound and is consistent with the ab initio XANES calculations for the Pt LIII-edge of Pt-O compounds.16 Observations from both the preedge white line
8150 J. Phys. Chem. B, Vol. 108, No. 24, 2004
Tsai et al.
TABLE 2: Structural Parameters of the Pt-Cl, Pt-O, and Pt-Pt Shells at the Pt LIII-Edgea reducing agent dosage (moles) none 4.39 × 10-5 4.39 × 10-5, 1.5 h 8.78 × 10-5 1.32 × 10-4 2.20 × 10-4 3.51 × 10-4, 41 min 5.71 × 10-4, 16 min 7.90 × 10-4, 1 h 40 min 7.90 × 10-4, 4 h 9.22 × 10-4, 1.5 h 1.05 × 10-3, 2.5 h 1.05 × 10-3, 6 h a
bond
N
R (Å)
σ2 × 10-3 (Å2)
E0 (eV)
Pt-Cl Pt-Cl Pt-Cl Pt-Cl Pt-Cl Pt-Cl Pt-Cl Pt-O Pt-Cl Pt-O Pt-O Pt-O Pt-O Pt-Pt Pt-O Pt-Pt
5.7(0.4) 4.0(0.3) 4.1(0.3) 3.8(0.4) 3.7(0.7) 4.2(0.8) 3.9(0.7) 1.8(0.4) 1.4(0.2) 3.5(0.5) 3.4(0.4) 4.0(0.6) 3.1(0.9) 1.1(0.9) 2.7(0.7) 3.2(1.2)
2.328(0.004) 2.319(0.003) 2.319(0.003) 2.317(0.005) 2.313(0.011) 2.283(0.013) 2.273(0.012) 1.988(0.001) 2.258(0.001) 2.045(0.009) 2.043(0.009) 2.042(0.011) 2.049(0.017) 2.756(0.047) 2.023(0.018) 2.837(0.031)
2.4(0.5) 2.6(0.4) 2.6(0.4) 4.0(0.7) 4.5(1.4) 7.2(1.6) 7.7(1.5) 3.7(0.9) 4.1(0.7) 5.5(1.2) 4.8(1.2) 7.1(1.5) 3.3(2.3) 0.4(4.9) 4.9(2.2) 7.9(4.0)
12.8(0.9) 12.8(0.8) 12.7(0.8) 13.1(1.1) 13.1(2.2) 8.8(2.3) 8.1(2.1) 4.8(0.2) 2.7(1.1) 14.7(1.6) 15.3(1.6) 14.8(1.7) 16.5(3.4) 23.7(6.6) 12.1(3.5) 30.1(6.0)
R factor 0.003 0.003 0.0025 0.006 0.012 0.013 0.022 0.034 0.023 0.015 0.009 0.057 0.049
R, bond distance; N, coordination number; σ2, Debye-Waller factor; E0, inner potential shift; S02, amplitude reduction factor (0.985).
Figure 2. FT EXAFS spectra at Pt LIII-edge of Pt microemulsion system.
(WL) as well as the postedge hybridization peak (HP) reasonably support the oxidation state change of Pt from the initial +4 state to the final 0 state through the +2 state, and these results will be confirmed later with EXAFS results. In Situ EXAFS at the Pt LIII-Edge of Pt Clusters. Figure 2 displays the Fourier transforms of Pt LIII-edge EXAFS at various dosages of hydrazine in the microemulsion system. The main peak appearing in the range of 1.85-1.92 Å without phase correction is assigned to the Pt-Cl bond. The peak located in the range of 1.65-1.75 Å is associated with the Pt-O bond. The peak observed around 2.7 Å is attributed to the Pt-Pt bond. The magnitude of the Pt-Cl bond decreases gradually with an increase in the dosage of hydrazine. In contrast, the magnitude of the Pt-O bond increases up to the addition of 7.90 × 10-4 moles of hydrazine. When the dosage of the reducing agent increases, a peak corresponding to the Pt-Pt bond begins to appear. These observations indicate the reduction of Pt ions and formation of Pt clusters. The structural parameters (coordination number N, bond distance R, Debye-Waller factor σ2, inner potential shift E0) were derived from the Pt LIII-edge EXAFS data analysis and are shown in Table 2. The coordination number NPt-Cl was found to be around 6 in the absence of the reducing agent. However, this coordination
number was reduced to around 4 once N2H4 was added. The value of NPt-Cl does not change further until the dosage of N2H4 exceeds 3.51 × 10-4 moles. In this stage, XAS results indicate the reduction of Pt4+ to Pt2+ species. The appearance of the Pt-O bond was found as the extent of reduction increased.The value of NPt-O became close to 4 when the dosage of N2H4 added was 7.90 × 10-4 moles after having been kept for 100 min. In the case of addition of N2H4 with 1.05 × 10-3 moles to this microemulsion system, NPt-O was determined as 3.1 and 2.7 and NPt-Pt was 1.1 and 3.2 after having been kept for 2.5 and 6.0 h, respectively. This indicated the formation of Pt clusters by the further reduction of Pt(OH)42- to Pt0. The formation of Pt clusters was clearly observed step by step in this work. From all the observations, a model was proposed for the formation mechanism of Pt clusters at early stages, as shown in Scheme 1. Discussion of the Model Proposed for the Formation Mechanism of Pt Clusters at Early Stages. Six steps were proposed in this model. In the first step, the microemulsion system containing the PtCl62- complex/Pt ion is surrounded by six chloride ions. After the addition of N2H4, in the second step, the PtCl62- ions were reduced due to intermicellar exchange processes.17 Reverse micelles in solution will exchange the contents of their cores via both fusion and redispersion processes.9 The exchange process occurs when micelles collide because of the Brownian motion and the attractive forces between the micelles. These collisions result in fusion of the reverse micelles, an exchange of the contents within the cores, and a redispersion of the micelles.17-19 As a result, the reduction of a metal salt within the cores of the reverse micelles can result in the growth of Pt nanoparticles within the core of the micelle. It was found that NPt-Cl was reduced from 6 to around 4, revealing the formation of PtCl42- ions in this step:
N2H4 + 2PtCl62- + 4OH- f N2 + 2PtCl42- + 4Cl- + 4H2O (1) In the third step, after the addition of 5.71 × 10-4 moles of N2H4, the NPt-O and NPt-Cl became 1.8 and 1.4, respectively. It is presumed that the oxygen contribution comes from OH- ions. This means that Pt2+ ions were complexed with Cl- ions in some of the micelles and with OH- ions in some other micelles (note that two micelles are shown in this step to show the coordination of Pt2+ ions with Cl- ions and OH- ions). Since Pt species were completely complexed with OH- ions, the
Genesis of Pt Clusters in Reverse Micelles
J. Phys. Chem. B, Vol. 108, No. 24, 2004 8151
Figure 3. Transmission electron micrograph of platinum clusters.
SCHEME 1: Model Proposed for the Formation Mechanism of Pt Clusters at Early Stages
Pt-Cl coordination was not observed when the dosages of N2H4 were between 7.90 × 10-4 and 9.22 × 10-4 moles. The Pt-Pt coordination was also not observed, indicating no formation of Pt atoms (the fourth step). However, Pt-O coordination was observed in this step. The Pt-O coordination distance and the coordination numbers were observed as 2 Å and around 4, respectively. These observations are consistent with the formation of [Pt(OH)4]2- species.20 It established the formation of [Pt(OH)4]2through ligand exchange in aqueous solution. The Pt-Pt coordination starts to appear only from the fifth step after having kept the microemulsion system containing 1.05 × 10-3 moles of reducing agent for more than 2.5 h. This indicated the formation of Pt clusters by the further reduction of Pt(OH)42-
to Pt0. The corresponding chemical reaction can be written as follows:
N2H4 + 2Pt(OH)42- f N2 + 2Pt + 4OH- + 4H2O (2) At the final step, it was clear that Pt atoms from the individual micelles aggregate together and form clusters due to intermicellar exchange processes when the microemulsion system was kept for a longer time, 6 h. A typical transmission electron micrograph (TEM) for the Pt nanoclusters obtained in a microemulsion system of nheptane/water/AOT at the final step of Scheme 1 is shown in Figure 3. This image shows that the size distribution of the clusters is monodisperse. This implies that the microemulsion system containing micelles are homogeneous.
8152 J. Phys. Chem. B, Vol. 108, No. 24, 2004 However, the size of the clusters (ca. 8 nm) obtained from TEM is very much larger than that obtained from XAS analysis (
