Langmuir 2006, 22, 2371-2377
2371
Formation Mechanism of Pt Particles by Photoreduction of Pt Ions in Polymer Solutions Masafumi Harada*,† and Hisahiro Einaga‡ Department of Health Science and Clothing EnVironment, Faculty of Human Life and EnVironment, Nara Women’s UniVersity, Nara 630-8506, and National Institute of AdVanced Industrial Science and Technology (AIST), AIST Tsukuba West, 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan ReceiVed August 31, 2005. In Final Form: December 12, 2005 The formation mechanisms of metal particles (platinum (Pt) particles) in an aqueous ethanol solution of poly(Nvinyl-2-pyrrolidone) (PVP) by the photoreduction method have been studied by transmission electron microscopy (TEM) and in situ and ex situ X-ray absorption fine structure (XAFS) analysis. The average diameter of the dilute and concentrated Pt particles in the PVP solution is estimated from TEM to be 2.0 and 2.5 nm, respectively. XAFS analysis was performed for the reduction process of Pt4+ ions to metallic Pt particles for the Pt L3 edge of the colloidal dispersions of the concentrated Pt solutions. The photoreduction process proceeds by the following steps: (1) reduction of PtCl62- to PtCl42-, (2) dissociation of Cl from PtCl42-, followed by reduction of Pt2+ ionic species to Pt0, (3) formation of a Pt0-Pt0 bond and particle growth by the association of Pt0-Pt0. The reduction of PtCl42- to Pt0 is a slower process, compared with the reduction of PtCl62- to PtCl42-. There is a delay between the disappearance of PtCl42- and the formation of Pt0-Pt0 clusters.
1. Introduction Metal particles or metal clusters are of wide interest not only because of their large surface area, but also because of their specific functions, which are different from those of either bulk metal solids or metal atoms.1-3 Metal particles have been prepared by various methods, such as photochemical reduction, chemical reduction, chemical liquid deposition, and thermal decomposition. Among various methods, chemical reduction of metal ions using surfactants or polymers as protective reagents is one of the promising ways to prepare metal nanoparticles.4-7 The protective polymers in the chemical reduction tend to exhibit the properties of surfactants8-11 in controlling the size and shape of metal nanoparticles. Since the properties of particles strongly depend on their size and size distribution,12 control of particle size is of great importance in the practical synthesis of particles.13 The size and shape of the finally synthesized particles depends on the synthesis * To whom correspondence should be addressed. Phone/fax: +81-74220-3466. E-mail:
[email protected]. † Nara Women’s University. ‡ AIST. (1) Feldheim, D. L.; Foss, C. A., Jr. Metal Nanoparticles; Synthesis, Characterization, and Applications; Marcel Dekker: New York, 2002. (2) Fendler, J. H. Nanoparticles and Nanostructured Films; Wiley-VCH: Weinheim, Germany, 1998. (3) El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257. (4) Toshima, N.; Yonezawa, T. New J. Chem. 1998, 1179. (5) Duff, D. G.; Edwards, P. P.; Johnson, B. F. G. J. Phys. Chem. 1995, 99, 15934. (6) Harada, M.; Asakura, K.; Toshima, N. J. Phys. Chem. 1994, 98, 2653. (7) Harada, M.; Toshima, N.; Yoshida, K.; Isoda, S. J. Colloid Interface Sci. 2005, 283, 64. (8) (a) Lisiecki, I.; Pileni, M. P. J. Phys. Chem. 1995, 99, 5077. (b) Pileni, M. P. Langmuir 1997, 13, 3266. (c) Lisiecki, I.; Billoudet, F.; Pileni, M. P. J. Phys. Chem. 1996, 100, 4160. (9) (a) Esumi, K.; Matsuhisa, K.; Torigoe, K. Langmuir 1995, 11, 3285. (b) Itakura, T.; Torigoe, K.; Esumi, K. Langmuir 1995, 11, 4129. (10) Huang, H. H.; Ni, X. P.; Loy, G. L.; Chew, C. H.; Tan, K. L.; Loh, F. C.; Deng, J. F.; Xu, G. Q. Langmuir 1996, 12, 909. (11) (a) Nakao, Y.; Kaeriyama, K. J. Colloid Interface Sci. 1986, 110, 82. (b) Nakao, Y.; Kaeriyama, K. J. Colloid Interface Sci. 1989, 131, 186. (12) (a) Creighton, J. A.; Eadon, D. G. J. Chem. Soc., Faraday Trans. 1991, 87, 3881. (b) Cao, Y. W.; Jin, R.; Mirkin, C. A. J. Am. Chem. Soc. 2001, 123, 7961.
conditions under which clusters formed during the early stage of the reduction are very active intermediates in further particle growth. The final size and local structure of the particles is expected from the mechanism of the nucleation and growth process of the particles, for example, when generated by the chemical reduction of ionic precursors. For the preparation of metal nanoparticles, metal ions have often been reduced in various polymers and surfactants. As protective media for metal particles in solutions, water-soluble polymers such as poly(vinyl alcohol) (PVA), poly(N-vinyl-2pyrrolidone) (PVP), and poly(methyl vinyl ether) (PMVE) have been used. The reduction of metal ions can be achieved chemically (for example, by using sodium borohydride14,15) or radiolytically16 (by using γ-irradiation, UV irradiation, and so on). El-Sayed et al.17 have reported that the rate of the reduction process of Pt2+ ions is controlled very much in polymer solutions and determines the shape and distributions of Pt nanoparticles. They concluded a shape-controlled growth process in which there was a difference between the rates of the catalytic reduction of Pt2+ on the {111} and {100} faces, and competition between the Pt2+ reduction and the capping process of the polymer on the different nanoparticle surfaces took place. Furthermore, in recent years, considerable research efforts have been made in the preparation of one- or two-dimensional nanomaterials, such as nanowires,18 nanorods,19 and nanoplates,20 due to their size- and shape-sensitive surface plasmon resonance bands and their promising applications in optics. (13) (a) Petroski, J. M.; Wang, Z. L.; Green, T. C.; El-Sayed, M. A. J. Phys. Chem. B 1998, 102, 3316. (b) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 8410. (14) Shirtcliffe, N.; Nickel, U.; Schneider, S. J. Colloid Interface Sci. 1999, 211, 122. (15) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27. (16) (a) Kurihara, K.; Kizling, J.; Stenius, P.; Fendler, J. H. J. Am. Chem. Soc. 1983, 105, 2574. (b) Kurihara, K.; Fendler, J. H. J. Am. Chem. Soc. 1983, 105, 6152. (17) (a) Petroski, J. M.; Wang, Z. L.; Green, T. C.; El-Sayed, M. A. J. Phys. Chem. B 1998, 102, 3316. (b) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; El-Sayed, M. A. Science 1996, 272, 1924. (18) Sakamoto, Y.; Fukuoka, A.; Higuchi, T.; Shimomura, N.; Inagaki, S.; Ichikawa, M. J. Phys. Chem. B 2004, 108, 853.
10.1021/la052378m CCC: $33.50 © 2006 American Chemical Society Published on Web 01/26/2006
2372 Langmuir, Vol. 22, No. 5, 2006
Colloidal dispersions of Pt nanoparticles have been prepared by the photoreduction of Pt complexes in the presence of a protective polymer.21 Our recent research shows that this process is a promising method to prepare small Pt nanoparticles with a narrow particle size distribution, which can be easily deposited on TiO2. In this study, we investigated the mechanism for the photoreduction of the PtCl64- complex to give Pt nanoparticles by means of UV-vis, transmission electron microscopy (TEM), and X-ray absorption fine structure (XAFS) measurements. The concentrated colloidal dispersions of Pt particles with their diameters ranging from 1 to 4 nm could be prepared in a water/ ethanol (1/1) solution with PVP. We also investigated the role of PVP in the photoreduction process to clarify how the presence of a protecting polymer can affect the formation of Pt nanoparticles. 2. Experimental Section 2.1. Preparation of Colloidal Dispersions of Pt Particles. Colloidal dispersions of Pt particles were synthesized by the photoreduction of Pt4+ ions with the protective polymer PVP. The average molecular weight of PVP used here was 40000. Hexachloroplatinic(IV) acid (hydrated hydrogen hexachloroplatinate(IV), H2PtCl6‚6H2O; guaranteed reagent), PVP (K-30), ethanol (guaranteed reagent, 99.5%), and distilled water were purchased from Nacalai Tesque, and used without further purification. Dilute (0.66 mM) and concentrated (9.65 mM) colloidal dispersions of Pt particles were prepared from H2PtCl6‚6H2O by irradiation of a 500 W super-high-pressure mercury lamp in water/ethanol (1/1, v/v) solutions of PVP. The amount of PVP is 1.32 and 50.3 mmol of monomeric units in 50 mL of water/ethanol (1/1) in the case of dilute and concentrated colloidal solutions, respectively. N2 gas was bubbled into the solution in a Pyrex glass vessel with a quartz window, and vigorous stirring was carried out for 10 min to remove the dissolved O2. Then, the solution was photoirradiated through the quartz window with continuous stirring. The reduced samples prepared with a designated reduction time of up to 10 h were measured by a UV-vis spectrophotometer. 2.2. Characterization of the Pt Particles Prepared by Photoreduction. The UV-vis absorption spectra of the colloidal dispersions of Pt particles were measured by a Hitachi U-3010 spectrophotometer to pursue the reduction of Pt ions in the PVP solutions. Especially, in measuring the concentrated colloidal dispersions of PVP, 0.5 mL of the obtained samples was diluted in 10 mL of distilled water to adjust the concentration of metal for the UV absorption measurements. TEM micrographs of the colloidal dispersions were obtained using a JEM-2000FX instrument operated at 200 kV as the acceleration voltage. The high-resolution carbon-supported copper mesh was used to support the samples of colloidal dispersions. The diameter of each particle was determined from enlarged photographs. The histogram of the particle size distribution and the average diameter were obtained by measuring about 200 particles in arbitrarily chosen areas in the enlarged photograph. 2.3. Ex Situ and In Situ EXAFS Measurements and Data Analysis of the Pt Particles. EXAFS measurements of the Pt L3 edge were performed at room temperature (23 °C) in the transmission mode at the Photon Factory, High Energy Accelerator Research Organization (KEK-PF), in Japan, using the BL-10B and BL-7C stations.22 Monochromatic synchrotron radiation was obtained using a channel-cut Si(311) crystal in the BL-10B station or using a double Si(111) crystal in the BL-7C station. The storage ring was operated (19) (a) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 8410. (b) Nikoobakht, B.; El-Sayed, M. A. Chem. Mater. 2003, 15, 1957. (c) Jana, N. R.; Gearheart, L.; Murphy, C. J. J. Phys. Chem. B 2001, 105, 4065. (20) (a) Maillard, M.; Giorgio, S.; Pileni, M.-P. AdV. Mater. 2002, 14, 1084. (b) Chen, S.; Fan, Z.; Carroll, D. L. J. Phys. Chem. B 2002, 106, 10777. (c) Chen, S.; Carroll, D. L. Nano Lett. 2002, 2, 1003. (21) Einaga, H.; Harada, M. Langmuir 2005, 21, 2578. (22) (a) Nomura, M.; Koyama, A. KEK Rep. 1989, 89-16. (b) Nomura, M.; Koyama, A.; Sakurai, M. KEK Rep. 1991, 91-1.
Harada and Einaga
Figure 1. UV-vis absorption spectra of the Pt colloidal solutions produced from PVP aqueous solutions by the photoreduction method. The time dependence is examined during the photoreduction. The photoirradiation using the cut filter UV-30 was carried out in a quartz cell for up to 8 h. at 2.5 GeV with currents between 150 and 400 mV. Measurements were carried out using ionization chambers with optimized detecting gases to measure the radiation intensity (incident intensity I0 and transmitted intensity I). The data were collected by ionization chambers filled with Ar(15)N2(85) gas for I0 and with Ar for I. In the ex situ EXAFS measurements, the samples after the photoirradiation were kept for at most 2 h before the measurements. The samples were then poured into glass cells with an optical path length of 10 mm, which were sealed with polyimide film windows (Kapton200H, 50 µm thickness). In this experiment, the samples were directly photoirradiated with the 500 W super-high-pressure mercury lamp. In situ EXAFS measurements were performed under controlled UV light. Photoirradiation was carried out with a 500 W superhigh-pressure mercury lamp by using a cut filter, UV-30, and a reflecting mirror that can prevent heating of the samples. The sample solutions were prepared from H2PtCl6‚6H2O (9.65 mM) and water/ ethanol (1/1) solutions (2 mL) with or without PVP (223 mg, 2.02 mmol of monomeric units) by the same method mentioned above. The ionic solutions were poured into quartz cells (optical path length 10 mm) sealed with Kapton films, and the irradiation of the mercury lamp was started after N2 bubbling. The sample solutions were continuously stirred and photoirradiated during the in situ EXAFS measurements. EXAFS spectra were analyzed by the computer program REX2000 ver. 2.0.7 (supplied by Rigaku Corp.).23 EXAFS analysis was performed as described in detail elsewhere.24 The spectra were extracted using the cubic spline method and normalized to the edge height. The k3-weighted EXAFS function was Fourier transformed into r space, and the Fourier transformation range is between 3 and 16 Å-1. The inverse Fourier transform into k space was then performed. The Fourier-filtered EXAFS functions were fitted with empirically derived phase shift and amplitude functions obtained from the reference sample (Pt-Pt contribution from the Pt foil and Pt-Cl contribution from H2PtCl6‚6H2O).
3. Results 3.1. Formation of Pt Particles in Polymer Solutions and Their Ex Situ EXAFS Measurements. Figure 1 shows UVvis absorption spectra of the Pt solutions prepared from the mixed solution of water and ethanol (the volume ratio of water and ethanol is 1/1) containing PVP. No significant increase of the baseline in absorbance occurred in less than 15 min, while the absorption band around 300-400 nm gradually disappeared. With an increase of the irradiation time to 60 min, however, a reaction remarkably proceeded and a highly absorbent yellow(23) REX 2000 is a commercially available analytical program for EXAFS provided by Rigaku Corp. (24) Teo, B. K. EXAFS Basic Principles and Data Analysis, Inorganic Chemistry Concepts; Springer-Verlag: Berlin, 1986; Vol. 9.
Formation Mechanism of Pt Particles
Langmuir, Vol. 22, No. 5, 2006 2373 Table 1. Curve-Fitting Results Obtained from Ex Situ EXAFS Measurements for the Concentrated Colloidal Dispersions of Pt Particles Prepared in a PVP Solution during the Photoreduction without a UV Cut Filtera sample
bond
CN
r/Å
∆E
σ/Å
R/%
before irradiation irradiation 10 min 20 min 30 min 60 min
Pt-Cl
5.3
2.32
-0.98
0.041
2.03
Pt-Cl Pt-Cl Pt-Cl Pt-Cl Pt-Pt Pt-Cl Pt-Pt Pt-Cl Pt-Pt Pt-Cl Pt-Pt Pt-Cl Pt-Pt Pt-Cl Pt-Pt
4.2 3.5 3.1 2.4 3.8 2.4 3.8 2.0 4.7 1.3 5.8 1.1 6.5 0.8 7.7
2.31 2.30 2.31 2.31 2.70 2.31 2.73 2.31 2.73 2.31 2.74 2.31 2.74 2.32 2.74
-2.17 -3.03 -2.75 -1.57 -12.8 -1.13 -8.03 -0.95 -6.44 -1.50 -5.64 -0.85 -5.57 1.59 -6.17
0.047 0.053 0.051 0.053 0.099 0.066 0.089 0.068 0.089 0.063 0.082 0.070 0.084 0.073 0.084
1.45 1.04 1.34 0.88
90 min 120 min 180 min 240 min 360 min
0.48 0.58 1.66 1.38 0.67
a The R factor is defined as ∑[k3χ(k) 3 2 3 2 obsd - k χ(k)calcd] /∑[k χ(k)obsd] × 100. The estimated error bars in the CN and r values are (10% and (0.03 Å, respectively.
Figure 2. Fourier transforms of the Pt L3 edge ex situ EXAFS spectra for the (a) concentrated Pt colloidal solutions stabilized by PVP and (b) pure H2PtCl6‚6H2O aqueous solution. The time dependence is examined during the photoreduction for 6 h. Photoirradiation without the cut filter UV-30 was performed.
brown solution was obtained. The spectra of solutions irradiated for longer than 120 min are similar to those of solutions irradiated for 60 min. PVP is necessary for the stabilization of Pt colloidal dispersions, because the absence of a protecting polymer caused the deposition of Pt nanoparticles on the quartz cell. However, the UV-vis spectra of Pt-containing solutions without PVP do not show the significant increase of the baseline in absorbance (the spectra are not shown in this paper). Figure 2a shows the Pt L3 edge ex situ EXAFS Fourier transforms for the concentrated Pt colloidal solutions in the presence of PVP before and after photoreduction for several minutes. For a Pt ionic solution (before reduction), the peak observed at 0.20 nm (phase shift uncorrected) is assigned to a Pt-Cl bond with a bond distance of 0.232 nm, by comparison with the peak of a pure H2PtCl6‚6H2O aqueous solution as shown in Figure 2b. It is a general trend that an increase of the reduction time causes a decrease in the intensity of the peak around 0.20 nm, and a new peak is observed around 0.26 nm with the course of reduction. These results indicate the disappearance of the Pt-Cl bond in the PVP solution and the formation of Pt0 metal particles with a Pt-Pt bond of 0.276 nm. The structural parameters, such as the coordination number (CN), bond distance (r), energy shift (∆E), Debye-Waller factor (σ), and R factor, of the concentrated colloidal dispersions of Pt particles are listed in Table 1. The R factor (%) is defined as follows:
R)
∑{k3χ(k)obsd - k3χ(k)calcd}2 × 100 ∑{k3χ(k)obsd}2
The error bars of r and CN were estimated by varying the ∆E value ((10 eV) and the σ values ((0.01 Å), respectively. In the
curve-fitting analysis of the peaks at 0.12-0.30 nm, the peaks were attributed to Pt-Cl and Pt-Pt bonds without a contribution from the Pt-O bond, which coincides with the similar experiment on the formation of a Pt nanowire in mesoporous silica HMM1.18 The obtained result indicates that the CN (5.3) of the sample before reduction is reduced to 3.1 during a reduction of 30 min, and a gradual change from 3.1 via 2.4 to 2.0 occurred during the reduction between 30 and 120 min. The Pt-Pt bond is detected after a reduction time of 60 min, and finally the CN of the Pt-Pt bond after an irradiation time of 360 min is observed as 7.7 though the small contribution of the Pt-Cl bond still remains. The final particle size is expected from the CN to be approximately 1.4 nm in diameter. Parts a and b of Figure 3 show the TEM images of the dilute and concentrated colloidal dispersions of Pt particles, respectively, prepared in the PVP solutions. The size of each Pt particle is very small and has a narrow distribution. In the case of dilute colloidal dispersions, the image shown in Figure 3a indicates a particle size ranging from 1 to 3.5 nm, and the average particle size is 2.0 nm in diameter. On the other hand, as shown in Figure 3b, the particle size in the concentrated Pt colloidal dispersions ranges from 1 to 4 nm, and the average particle size is 2.5 nm in diameter. This indicates that the particle size distribution of concentrated colloidal dispersions of Pt particles is very similar to that of dilute colloidal dispersions of Pt particles. The particles are well dispersed, and the aggregation of each Pt particle does not remarkably occur. Thus, the isolation of the PVP-stabilized Pt particles is quite good in the colloidal solutions. 3.2. In Situ EXAFS Measurements of Colloidal Dispersions of Pt Particles in Photoirradiation. To investigate the precise mechanisms of reduction of Pt4+ ions and association of Pt atoms to create Pt particles, in situ EXAFS measurements have been carried out for the concentrated Pt colloidal solutions during the irradiation with a 500 W mercury lamp. Parts a and b of Figure 4 show the Pt L3 edge in situ EXAFS Fourier transforms for the concentrated Pt colloidal solutions with PVP and without PVP, respectively. As the same tendency is observed as in the case of ex situ EXAFS (as shown in Figure 2a), the intensity of the peak around 0.20 nm assigned to the Pt-Cl bond decreases with the course of reduction, and the intensity of the peak around 0.26 nm assigned to the Pt-Pt bond
2374 Langmuir, Vol. 22, No. 5, 2006
Harada and Einaga
Figure 3. TEM images and distributions of the diameter extracted from the corresponding image for the Pt colloidal solutions prepared by the photoreduction method: (a) dilute and (b) concentrated colloidal solutions containing PVP. The inset image was taken with high magnification. The photoreduction was carried out in a quartz cell for 5 h.
Figure 4. In situ EXAFS results of the Fourier transforms of the Pt L3 edge spectra for (a) the concentrated Pt colloidal solutions stabilized by PVP and (b) the concentrated Pt solutions without PVP. The time dependence is examined during the photoreduction for ca. 9 h. Photoirradiation with the cut filter UV-30 was performed.
appears after prolonged photoirradiation. In situ EXAFS measurements clearly show that a gradual decrease in the intensity of the peak attributed to the Pt-Cl bond is observed during the reduction from 30 to 120 min, although a peak attributed to a Pt-Pt metallic bond is still not detected. A new peak attributed to a Pt-Pt metallic bond appears around 0.26 nm after a reduction time of 240 min. Thus, there are significant time delays from the time when the contribution of the Pt-Cl bond disappears to the
time when the contribution of the Pt-Pt bond appears in the photoirradiation. It is also observed that the intensity of the peak at 0.26 nm becomes higher after the left in the dark condition without the irradiation of UV light. This means the growth of the particle has been continued without the photoirradiation and the particle size becomes larger than before the left in the dark condition. Figure 4b shows the effect of PVP in the solutions on the rate of the reduction of Pt4+ ions to Pt atoms and the formation of Pt particles, in comparison with Figure 4a. The measurements were carried out under the same conditions as those in Figure 4a, and the rate for Pt particle formation can be directly compared. It is remarkable from Figure 4b that the rate of particle growth to create Pt particles from Pt atoms is higher in the case without PVP than in the case with PVP. The Pt-Pt bond is detected after a reduction time of 60 min, which is a much shorter period compared with that of the reduction process with PVP. Especially, after a reduction time of more than 120 min, the peak height at 0.26 nm becomes about 2 times larger in the case of colloidal Pt solutions without PVP. Table 2 shows the structural parameters, such as the CN, r, ∆E, σ, and R factor, of the concentrated colloidal dispersions stabilized by PVP. Similar trends of the change of the CN are observed in comparison with those of Table 1. Here it is clear that the CN (5.8) of the sample before reduction is reduced to 3.6 during a reduction of 30 min, and a gradual change from 3.6 to 3.2 occurred during the reduction between 30 and 240 min. The Pt-Pt bond appears after a reduction time of 240 min, and the CN of the Pt-Pt bond after an irradiation time of 303 min is observed as 1.7 though the small contribution of the Pt-Cl bond still remains as 3.1. Consequently, the sample after a photoirradiation of 444 min keeps the left in the dark condition, resulting in the continuity of the particle growth so that the CN of the Pt-Pt bond increases to 5.9. The final particle after the left in the dark condition is expected to be approximately 0.9 nm in diameter. Here we note that the results of ex situ and in situ EXAFS measurements have basically the same trends, when comparing Table 1 with Table 2, although the reduction rates of
Formation Mechanism of Pt Particles
Langmuir, Vol. 22, No. 5, 2006 2375
Table 2. Curve-Fitting Results Obtained from In Situ EXAFS Measurements for the Concentrated Colloidal Dispersions of Pt Particles Prepared in a PVP Solution during the Photoreduction with the UV Cut Filter UV-30a sample
bond
CN
r/Å
σ/Å
R/%
before irradiation irradiation 3 min 30 min 60 min 120 min 240 min
Pt-Cl
5.8
2.32
1.16
0.043
1.31
Pt-Cal Pt-Cl Pt-Cl Pt-Cl Pt-Cl Pt-Pt Pt-Cl Pt-Pt Pt-Cl Pt-Pt
4.7 3.6 3.6 3.2 3.2 1.4 3.1 1.7 1.7 5.9
2.30 2.29 2.29 2.29 2.30 2.65 2.31 2.68 2.33 2.71
-0.83 -2.07 -2.22 -2.19 0.64 -21.4 1.12 -17.5 4.38 -12.3
0.056 0.056 0.060 0.058 0.067 0.070 0.072 0.071 0.079 0.087
0.58 0.93 1.31 1.67 0.61
303 min 1 day left
∆E
0.81 1.54
The R factor is defined as ∑[k3χ(k)obsd - k3χ(k)calcd]2/∑[k3χ(k)obsd]2 × 100. The estimated error bars in the CN and r values are (10% and (0.03 Å, respectively. a
Table 3. Curve-Fitting Results Obtained from In Situ EXAFS Measurements for the Concentrated Colloidal Dispersions of Pt Particles Prepared in an Aqueous Solution without PVP during the Photoreduction with the UV Cut Filter UV-30a sample
bond
CN
r/Å
σ/Å
R/%
before irradiation irradiation 30 min 60 min
Pt-Cl
5.7
2.31
0.98
0.045
1.89
Pt-Cl Pt-Cl Pt-Pt Pt-Cl Pt-Pt Pt-Cl Pt-Pt Pt-Cl Pt-Pt Pt-Cl Pt-Pt Pt-Pt
4.4 3.8 1.2 3.4 2.1 2.8 3.6 2.1 4.6 1.8 5.3 8.1
2.29 2.29 2.67 2.30 2.72 2.29 2.71 2.29 2.72 2.28 2.73 2.75
-2.45 -1.20 -17.7 -1.04 -7.29 -0.55 -8.75 -1.61 -6.15 -2.15 -3.43 -3.60
0.071 0.074 0.073 0.075 0.080 0.069 0.089 0.067 0.086 0.080 0.082 0.075
0.19 0.30
90 min 120 min 238 min 438 min 1 day left
∑[k3χ(k)
∆E
0.26 0.87 0.28 0.37 0.50
The R factor is defined as obsd × 100. The estimated error bars in the CN and r values are (10% and (0.03 Å, respectively. a
k3χ(k)
2 3 2 calcd] /∑[k χ(k)obsd]
Pt ions are different between these measurements. The reason for this difference is the photoirradiation condition (see the Experimental Section). In the former case, the photoirradiation was carried out to confirm the production of Pt nanoparticles. In the latter case, we investigated the precise mechanism of reduction of Pt4+ ions under controlled photoirradiation. On the other hand, Table 3 shows these structural parameters of concentrated colloidal dispersions of Pt particles without the stabilization of PVP. It is remarkable that the CN of the Pt-Pt bond of Pt particles prepared without PVP tends to increase much more than that prepared with PVP, compared at the same duration of irradiation. For example, the time (60 min) necessary for the appearance of the Pt-Pt bond is shorter than that (240 min) in the case with PVP as shown in Table 2. Therefore, the protective polymer PVP might suppress the rate of particle growth of Pt atoms in the solutions. Figure 5 shows the Pt L3 edge in situ XANES spectra for the concentrated Pt colloidal dispersions stabilized by PVP. Here we estimate qualitatively the electronic structure of the products during the photoreduction process by means of the position in energy and shape of the peak in the XANES regions. The maximum peak position in the energy region between 11560 and 11570 eV is gradually shifted to the direction of low energy. The peak position of Pt ionic solutions (before reduction) is the same as that (11566 eV) of the PtCl62- aqueous solution (reference
Figure 5. In situ XANES results of the Pt L3 edge spectra for the concentrated Pt colloidal solutions stabilized by PVP. The time dependence is examined during the photoreduction for ca. 8 h. Photoirradiation with the cut filter UV-30 was performed.
compound). In the reduction time from 3 to 30 min, the first peak position of the photoirradiated samples is shifted to that (11564 eV) of the PtCl42- aqueous solution (reference compound). Then in the reduction time between 30 and 240 min, the first peak position of the photoirradiated samples does not seem to shift from the peak position of the PtCl42- aqueous solution toward that of the Pt foil (11564 eV), because there is no effective difference among their positions of peaks. However, the second peak of the photoirradiated samples gradually approaches that (11578 eV) of the Pt foil in the course of reduction. Finally, the peak of the photoirradiated samples seems to be similar to that of the Pt foil when the reduction time reaches 444 min. These results suggest that the electronic structure of the photoirradiated samples is composed of three elemental states, Pt4+ (that is, a reactant, PtCl62-), Pt2+ (that is, an intermediate product, mainly PtCl42-), and Pt0 atom (or a Pt particle produced from a Pt0 atom). Additionally, the purity of the Pt particles produced by the prolonged photoirradiation is good enough to analyze XANES signals of the Pt0-Pt0 metallic bond, although several species such as Pt4+, Pt2+, and Pt0 coexist in the sample solutions during the early stage of the photoirradiation process.
4. Discussion 4.1. Formation Mechanism of Pt Particles in a Polymer Solution. Photoirradiation of UV light for PtCl62- ionic complexes in an ethanol-water solution leads to the reduction of Pt4+ to Pt0 metal particles via Pt2+ ions25 as described by eq 1. Bocarsly
PtCl62- f PtCl42- f Pt0
(1)
et al. have investigated the reduction process by using NMR techniques, and have reported that platinum metal (Pt0) formation is not found to occur until a ∼90% yield of PtCl42- has accumulated.25b Their findings showed that the increase of the PtCl62- concentration decreased the rate for metal particle formation. This suggests that PtCl62- acts as an inhibitor to platinum metal formation. In our previous research, we presented EXAFS results of the Pt L3 edge for the Pt solutions before and after the photoreduction21 and confirmed that the PtCl62- ionic complex completely disappeared and Pt0 metal clusters were formed by the photoirradiation in an ethanol-water solution. (25) (a) Bocarsly, A. B.; Cameron, R. E.; Zhou, M. In Proceedings of the SeVenth International Symposium on the Photochemistry and Photophysics of Coordination Compounds; Yersin, H., Vogler, A., Eds.; Springer: Berlin, 1987; pp 177-180. (b) Cameron, R. E.; Bocarsly, A. B. Inorg. Chem. 1986, 25, 2910. (c) Cameron, R. E.; Bocarsly, A. B. J. Am. Chem. Soc. 1985, 107, 6116.
2376 Langmuir, Vol. 22, No. 5, 2006
Harada and Einaga
Our UV-vis and EXAFS studies are consistent with the results previously reported.21 The reaction process for the transformation of PtCl62- to PtCl42has been well studied: Pt4+ is reduced to Pt3+ species along with the simultaneous oxidation of alcohols (eq 2),26-28 which are subsequently transformed to Pt2+ by further reduction (eq 4) and disproportionation (eq 5) via formation of PtCl52- complexes (eq 3).
PtCl62- + RCH2OH + hν f PtCl63- + •RCH2O + H+ (2) PtCl63- a PtCl52- + Cl-
(3)
PtCl52- + •RCH2O f PtCl42- + RCH2dO + HCl (4) 2PtCl52- a PtCl62- + PtCl42-
(5)
In the present study, we investigated the overall processes from the reduction of PtCl62- to the formation of Pt particles. The disappearance of the absorption band at 300-400 nm in UV-vis spectra shows the transformation of PtCl62- to PtCl42by only short-time photoirradiation (Figure 1). This process was also monitored by the color change of the solution from yellow to orange, which is characteristic of PtCl42- ionic species. The increase of the baseline by photoirradiation for 120 min is ascribed to the formation and growth of Pt metal particles, leading to a change to a black colloidal dispersion. The unchanged spectrum by further photoirradiation shows the completion of the growth of Pt particles. Ex situ EXAFS studies are consistent with the observations of UV-vis studies and further clarified these reduction processes. As shown in Table 1 and Figure 2a, the decrease of the Pt-Cl bond by short-time photoirradiation (10-30 min) is ascribed to the transformation of PtCl62- to PtCl42- ionic species. The continuous reduction of this bond indicates that the concentration of PtCl42- also decreased with reduction time. The long-time photoirradiation (180-360 min) resulted in the formation and growth of Pt metal particles, evidenced by the appearance of a Pt-Pt metallic bond. The lower height of the peak for the Pt-Pt bond compared with that of the Pt foil implies a lower coordination number of the Pt cluster in the resulting colloidal solutions. In situ EXAFS studies give us information on the reduction steps in detail. More precise observation, as shown in Table 2 and Figure 4a, can provide the following information: (1) PtCl62is transformed to PtCl42- by photoirradiation for 3-30 min. Thus, only short-time photoirradiation is sufficient for this reduction step. (2) During the reduction time from 30 to 120 min, the contribution of the Pt-Cl bond decreases with time, although a Pt-Pt metallic bond is not formed in this process. (3) After a reduction time of 240 min, a Pt-Pt metallic bond is formed and most of the Pt-Cl bond has disappeared. Additionally, in situ XANES studies give us qualitative information on the electronic structure of the products during the reduction processes. The energy shift of the maximum peak position to the direction of lower energy indicates the reduction of Pt4+ ionic species. The observation of the energy shift of the peak position in the XANES region (Figure 5) provides the following information. The Pt4+ ionic species is transformed to Pt2+ species by (26) Grivin, V. P.; Khmelinski, I. V.; Plyusnin, V. F.; Blinov, I. I.; Balashev, K. P. J. Photochem Photobiol., A 1990, 51, 167. (27) Grivin, V. P.; Khmelinski, I. V.; Plyusnin, V. F. J. Photochem. Photobiol., A 1990, 51, 379. (28) Grivin, V. P.; Khmelinski, I. V.; Plyusnin, V. F. J. Photochem. Photobiol., A 1991, 59, 153.
photoirradiation for 3-30 min, consistent with the observation from in situ EXAFS studies. The Pt2+ species mainly exists in the solution for 30-240 min, as is evidenced by the observation that the first peak position of the photoirradiated samples does not seem to shift. However, the move of the second peak to the value of the Pt foil’s peak implies that the Pt0 species is gradually formed during this time. This means that the main elements are Pt2+ ionic species and a small amount of Pt0 metallic element is created because of the difference in the rate between the dissociation of chloride atoms and the reduction of the Pt2+ ionic species to Pt0. Finally, the Pt2+ species is reduced to Pt0 after photoirradiation of 444 min. On the basis of the results obtained from in situ EXAFS and XANES studies, we can insist on the photoreduction process of PtCl62- ions as follows. It mainly consists of three steps, that is, (1) reduction of PtCl62- to PtCl42-, (2) dissociation of Cl from PtCl42-, followed by reduction of Pt2+ ionic species to Pt0 (that is, the coexistence of both Pt2+ and Pt0 species), and (3) formation of a Pt0-Pt0 bond and particle growth by the association of Pt0-Pt0, which can proceed even in the case of nonirradiation. The reduction from Pt2+ ionic species to Pt0 atoms is a slower process, compared with the reduction from Pt4+ to Pt2+ ionic species (or compared with the dissociation of chloride atoms from PtCl42-), and there is a delay between the formation of the Pt0-Pt0 metallic bond and the disappearance of PtCl42-. Once a Pt0-Pt0 bond is formed, the association of Pt0 atoms into Pt0Pt0 oligomeric clusters is activated to create a Pt particle. In recent work,29 an EXAFS study has been carried out to investigate the formation of platinum clusters produced using NaBH4 as a reducing agent in AOT reverse micelles. The authors have concluded a stepwise change in the coordination number NPt-Cl with an initial change from 6 to 4 and then to a final number of 0. This result is consistent with our present result. 4.2. Role of a Protective Polymer in Pt-Pt Formation. In situ EXAFS studies for the Pt-containing solution without PVP provide detailed information on the formation process of naked Pt particles without the stabilizer. The rate of particle growth to form the Pt nanoparticle is higher in the case without PVP than in the case with PVP. Therefore, the reduction steps for Pt complexes and Pt0-Pt0 formation are promoted when PVP is absent from the solution. These results suggest the protective PVP polymer can control the particle growth of Pt atoms in the solutions, especially in the case where PVP suppresses the growth of Pt particles during the photoirradiation process. El-Sayed and co-workers have investigated the effect of polyacrylate on the H2 reduction of PtCl42- to Pt colloidal particles. They reported that the rate of the reduction process of Pt2+ ions is controlled in polymer solutions and determines the shape and distributions of Pt nanoparticles. In the growth of Pt clusters, the H2 reduction of the Pt2+ complex occurs on the Pt particle and competes with polymer capping of the clusters. Thus, a high concentration of polymer prevents the rapid growth of Pt particles.17 From these results, we also consider the interaction between Pt species and PVP polymer chains. In the photoreduction process, dissociation of Cl from PtCl42- occurs and Pt2+ ionic species are created, and then the obtained Pt2+ ionic species are reduced to form Pt clusters. The presence of the PVP polymer inhibits the rapid formation of Pt particles by interacting with Pt2+ ionic species, Pt0 nuclei, and Pt clusters. This finding also implies the possibility that the protecting polymer can control the formation rate of Pt particles, leading to the control of their shapes and sizes in the photoreduction process. (29) Tsai, Y. W.; Tseng, Y. L.; Sarma, L. S.; Liu, D. G.; Lee, J. F.; Hwang, B. J. J. Phys. Chem. B 2004, 108, 8148.
Formation Mechanism of Pt Particles
5. Conclusion The colloidal dispersions of Pt particles stabilized by PVP were prepared by the photoreduction process. EXAFS analysis of the Pt L3 edge of the colloidal dispersions of concentrated Pt particles in PVP solutions suggests that the reduction process proceeds consequently from Pt4+ ions to Pt0 metal particles via Pt2+ ions. The reduction from PtCl62- to PtCl42- ions is quite rapidly completed by photoirradiation. The reduction from PtCl42to Pt0 atoms is a slower process, compared with the reduction from PtCl62- to PtCl42-. The reduction of PtCl42- to Pt0 proceeds by two steps: one is dissociation of Cl from PtCl42-, and the other is reduction of Pt2+ ionic species to Pt0 atoms. There is a delay between the disappearance of PtCl42- and the formation of the Pt0-Pt0 metallic bond. However, once a Pt0-Pt0 bond is
Langmuir, Vol. 22, No. 5, 2006 2377
formed, the association of Pt0 atoms into Pt0-Pt0 oligomeric clusters is easily activated to create a Pt particle. Acknowledgment. We are grateful to the Photon Factory Advisory Committee (PAC) (Proposal No. 2000G076) at the High Energy Accelerator Research Organization (KEK) for approval of the EXAFS measurements. We also thank Prof. S. Isoda at the Institute for Chemical Research, Kyoto University, for the TEM observations. A part of this work was supported by the “Nanotechnology Support Project” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. LA052378M
