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Mechanism of Silver Particle Formation during Photoreduction Using In Situ Time-Resolved SAXS Analysis Masafumi Harada* and Etsuko Katagiri Department of Health Science and Clothing Environment, Faculty of Human Life and Environment, Nara Women’s University, Nara 630-8506, Japan
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Received July 6, 2010. Revised Manuscript Received October 4, 2010 Formation mechanisms of silver (Ag) particles in an aqueous ethanol solution of poly(N-vinyl-2-pyrrolidone) (PVP) by the photoreduction of AgClO4 were investigated by means of in situ small-angle X-ray scattering (SAXS) measurements. The kinetics of association process (nucleation, growth, and coalescence) of Ag0 atoms to produce Ag particles was successfully revealed by the quantitative SAXS analysis for the number-average of radius (R0), number of particles (nAg), reduced standard deviation (σR/R0), and volume fraction (φAg) of Ag particles produced by the photoreduction. The rate of nucleation and growth process during Ag particle formation strongly depend on the initial metal concentration. The time evolution of radius and number of Ag particles indicates that a mechanism of Ag particle formation is composed of different three processes, that is, reduction-nucleation, Ostwald ripening, and particle coalescence. In a rapid reduction-nucleation process, small nuclei or particles (average radius ∼2.5 nm) are produced by an autocatalytic reduction. After the formation of small nuclei or particles proceeds, Ostwald ripening and particle coalescence, predicted by the Lifshitz-Slyozov-Wagner theory (LSW theory), subsequently occur, resulting in the particle growth (average radius ∼11.5 nm).
1. Introduction Metal particles in nanoscales have received great attention in recent years because they exhibit unique optical, electronic, magnetic, and catalytic properties that differ from those of bulk metals.1-3 It is well known that these properties are highly dependent on their size and shape.4,5 To establish the control of size and shape of metal particles, a detailed understanding of nucleation and growth mechanism during chemical synthesis is required for a variety of organic and inorganic crystallization. The reduction of gold or silver salts to synthesize size-defined Au or Ag particles in solution can be achieved by several classes of synthesis routes. For example, one of the most common chemical routes is the citrate reduction method (Turkevich method),6 where sodium citrate plays a significant role as a weak reducing agent and a capping agent that stabilizes the particles. Kumar et al.7 reported that the ratio of initial concentrations of citrate to gold influences the particle size, and there is a large dependence of particle size on the low ratio of citrate to gold. In their kinetic model, the particle size is determined by the balance between the rates of degradation of dicarboxy acetone and nucleation in the citrate method. In *To whom correspondence should be addressed. Tel: þ81-742-20-3466. Fax: þ81-742-20-3466. E-mail:
[email protected].
(1) (a) Kamat, P. V. J. Phys. Chem. B 2002, 106, 7729. (b) Sudeep, P. K.; Kamat, P. V. Chem. Mater. 2005, 17, 5404. (2) (a) Xia, Y.; Xiong, Y.; Lim, B.; Skrabalak, S. E. Angew. Chem., Int. Ed. 2009, 48, 60. (b) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Annu. Rev. Mater. Sci. 2000, 30, 545. (3) (a) Feldheim, D. L., Foss, C. A., Jr. Metal Nanoparticles; Synthesis, Characterization, and Applications; Marcel Dekker: New York, 2002. (b) Corain, B.; Schmid, G.; Toshima, N. Metal Nanoclusters in Catalysis and Materials Science: The Issue of Size Control; Elsevier: Amsterdam, 2008. (4) (a) Narayanan, R.; El-Sayed, M. A. J. Phys. Chem. B 2005, 109, 12663. (b) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. Rev. 2005, 105, 1025. (5) (a) Murphy, C. J.; Sau, T. K.; Gole, A. M.; Orendorff, C. J.; Gao, J.; Gou, L.; Hunyadi, S. E.; Li, T. J. Phys. Chem. B 2005, 109, 13857. (b) Hu, M.; Petrova, H.; Wang, X.; Hartland, G. V. J. Phys. Chem. B 2005, 109, 14426. (c) Rocha, T. C. R.; Winnischofer, H.; Westphal, E.; Zanchet, D. J. Phys. Chem. C 2007, 111, 2885. (6) (a) Turkevich, J.; Stevenson, P. C.; Hillier, J. Discuss. Faraday Soc. 1951, 11, 55. (b) En€ust€un, B. V.; Turkevich, J. J. Am. Chem. Soc. 1963, 85, 3317. (c) Turkevich, J. Gold Bull. 1985, 18, 86. (d) Turkevich, J. Gold Bull. 1985, 18, 125. (7) Kumar, S.; Gandhi, K. S.; Kumar, R. Ind. Eng. Chem. Res. 2007, 46, 3128.
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contrast with the chemical synthesis route, the usage of UVinitiated growth8-10 is a promising method to prepare size-defined metal particles and to monitor the formation mechanism of metal particles because of a slow process of the neutral atom formation or an autocatalytic reaction on the particle surfaces. In general, nucleation represents the very first stage of any crystallization process. Despite the scientific and technological importance of this phenomenon and the tremendous efforts that have been devoted to studying this subject, only a few reliable accounts of the mechanism of nucleation and growth of metal particles have been reported.11,12 This is due to the fact that the lack of experimental “in situ” tools capable of capturing, identifying, and monitoring the nuclei, that is, the metal clusters consisting of very few atoms, ions, or both, formed in the earliest stage of nanoparticle synthesis. The in situ technique that is most powerful to determine the size and size distribution of metal particles during the growth process is small-angle X-ray scattering (SAXS) measurements13-19 and grazing incidence small-angle X-ray scattering (GISAXS),20,21 (8) (a) Eustis, S.; Hsu, H.-Y.; El-Sayed, M. A. J. Phys. Chem. B 2005, 109, 4811. (b) Eustis, S.; El-Sayed, M. A. J. Phys. Chem. B 2006, 110, 14014. (c) Eustis, S.; Krylova, G.; Eremenko, A.; Smirnova, N.; Schill, A. W.; El-Sayed, M. A. Photochem. Photobiol. Sci. 2005, 4, 154. (9) (a) Scaiano, J. C.; Aliaga, C.; Maguire, S.; Wang, D. J. Phys. Chem. B 2006, 110, 12856. (b) Gaddy, G. A.; McLain, J. L.; Korchev, A. S.; Slaten, B. L.; Mills, G. J. Phys. Chem. B 2004, 108, 14858. (c) Malone, K.; Weaver, S.; Taylor, D.; Cheng, H.; Sarathy, K. P.; Mills, G. J. Phys. Chem. B 2002, 106, 7422. (d) Zhu, J.; Shen, Y.; Xie, A.; Qiu, L.; Zhang, Q.; Zhang, S. J. Phys. Chem. C 2007, 111, 7629. (10) (a) Harada, M.; Takahashi, S. J. Colloid Interface Sci. 2008, 325, 1. (b) Harada, M.; Inada, Y. Langmuir 2009, 25, 6049. (11) (a) Watzky, M. A.; Finke, R. G. J. Am. Chem. Soc. 1997, 119, 10382. (b) Finney, E. E.; Finke, R. G. J. Colloid Interface Sci. 2008, 317, 351. (12) (a) Viswanatha, R.; Santra, P. K.; Dasgupta, C.; Sarma, D. D. Phys. Rev. Lett. 2007, 98, 255501. (b) Viswanatha, R.; Amenitsch, H.; Sarma, D. D. J. Am. Chem. Soc. 2007, 129, 4470. (c) Bilecka, I.; Elser, P.; Niederberger, M. ACS Nano 2009, 3, 467. (13) Plech, A.; Kotaidis, V.; Siems, A.; Sztucki, M. Phys. Chem. Chem. Phys. 2008, 10, 3888. (14) Abecassis, B.; Testard, F.; Spalla, O.; Barboux, P. Nano Lett. 2007, 7, 1723. (15) Bera, M. K.; Sanyal, M. K.; Yang, L.; Biswas, K.; Gibaud, A.; Rao, C. N. R. Phy. Rev. B 2010, 81, 115415.
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provided that a strong X-ray source (synchrotron radiation) makes sufficient time resolution possible. This offers a direct probe because the scattering cross section depends directly on the shape and size of the metal particles. However, there are a few reports in the literature where the growth kinetics of metal particles is described on the basis of the SAXS analysis. For example, Plech et al.13 investigated the nucleation and growth of spherical gold nanoparticles prepared by the Turkevich method and proposed that smaller and larger particles coexist in the solution during growth, which contradicts the simple burst nucleation model. Abecassis et al.14 presented an in situ SAXS study on Au nanoparticle growth in an organic solvent by the use of a fast-mixing stopped-flow device. They concluded that a sort of the capping agent influences the seed formation. Recently, Bera et al.15 have demonstrated that the initial nucleation and growth of Au particles and their aggregation formed at the toluene-water interface could be observed by in situ grazing-incidence SAXS. They revealed that by combining both the results of fractal analysis of low-q scattering data and multiple cluster analysis, Au particles with a size of 1.3 nm aggregate and then reorganize to form compact planner structure with a gradual increase in fractal dimension from 1.82 to 2.05. To our best knowledge, unfortunately, the particle nucleation process in the initial stage of the synthesis has remained unclear because of the lack of time resolution of in situ SAXS. To overcome this difficulty of monitoring the growth process of metal particles in real time, we have performed the in situ time-resolved SAXS measurements in the nucleation, growth, and coalescence of Ag particles in polymer solutions. We have chosen the synthetic procedure of metal particles by the photoreduction of metallic precursor in the presence of poly(N-vinyl2-pyrrolidone) (PVP).22,23 In this article, we present the nucleation and growth mechanism of Ag particle formation during the photoreduction of different concentrations ([Ag] = 5, 10, 20, and 40 mM) of silver perchlorate (AgClO4) by means of the in situ time-resolved SAXS in conjunction with the conventional techniques of UV-vis and TEM.
2. Experimental Section 2.1. Materials. Silver perchlorate (AgClO4 3 H2O, Nacalai
Tesque, guaranteed reagent), PVP (K-30, average MW =40 000, Tokyo Kasei Kogyo), benzoin (C6H5CH(OH)COC6H5, guaranteed reagent), ethanol (Nacalai Tesque, guaranteed reagent, 99.5%), and distilled water were used without further purification.
2.2. Preparation and Characterization of Colloidal Dispersions of Ag Particles. Colloidal dispersions of Ag particles (16) (a) Polte, J.; Erler, R.; Th€unemann, A. F.; Sokolov, S.; Ahner, T. T.; Rademann, K.; Emmerling, F.; Kraehnert, R. ACS Nano 2008, 2, 1305. (b) Polte, J.; Emmerling, F.; Radtke, M.; Reinholz, U.; Riesemeier, H.; Th€unemann, A. F. Langmuir 2010, 26, 5889. (17) (a) Biswas, K.; Das, B.; Rao, C. N. R. J. Phys. Chem. C 2008, 112, 2404. (b) Biswas, K.; Varghese, N.; Rao, C. N. R. Small 2008, 4, 649. (18) Kammler, H. K.; Beaucage, G.; Kohls, D. J.; Agashe, N.; Ilavsky, J. J. Appl. Phys. 2005, 97, 054309. (19) (a) Harada, M.; Saijo, K.; Sakamoto, N.; Einaga, H. Colloids Surf., A 2009, 345, 41. (b) Harada, M.; Saijo, K.; Sakamoto, N. Colloids Surf., A 2009, 349, 176. (c) Sakamoto, N.; Harada, M.; Hashimoto, T. Macromolecules 2006, 39, 1116. (20) (a) Renaud, G.; Lazzari, R.; Leroy, F. Surf. Sci. Rep. 2009, 64, 255. (b) Revenant, C.; Leroy, F.; Lazzari, R.; Renaud, G.; Henry, C. R. Phys. Rev. B 2004, 69, 035411. (c) Lazzari, R.; Leroy, F.; Renaud, G. Phys. Rev. B 2007, 76, 125411. (21) (a) Winans, R. E.; Vajda, S.; Ballentine, G. E.; Elam, J. W.; Lee, B.; Pellin, M. J.; Seifert, S.; Tikhonov, G. Y.; Tomczyk, N. A. Top. Catal. 2006, 39, 145. (b) Vajda, S.; Winans, R. E.; Elam, J. W.; Lee, B.; Pellin, M. J.; Seifert, S.; Tikhonov, G. Y.; Tomczyk, N. A. Top. Catal. 2006, 39, 161. (22) (a) Sun, Y.; Gates, B.; Mayers, B.; Xia, Y. Nano Lett. 2002, 2, 165. (b) Sun, Y.; Yin, Y.; Mayers, B.; Herricks, T.; Xia, Y. Chem. Mater. 2002, 14, 4736. (c) Sun, Y.; Mayers, B.; Herricks, T.; Xia, Y. Nano Lett. 2003, 3, 955. (23) (a) Harada, M.; Einaga, H. Langmuir 2006, 22, 2371. (b) Harada, M.; Einaga, H. Langmuir 2007, 23, 6536.
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Article ([Ag] = 5, 10, 20, and 40 mM) were prepared from the ionic precursor of AgClO4 3 H2O in the presence of benzoin as a photoactivator10,24,25 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 50.2 mmol of monomeric unit in 50 mL of water/ethanol(1/1) solutions. The ionic solution was poured in a cell made of stainless steel (SUS316) with four optical windows. Two of them were sapphire windows (15 mm in diameter and 6 mm in thickness) for the direct observation inside the cell, and the other two were quartz windows (15 mm in diameter and 8 mm in thickness) for the irradiation of UV light. The inner volume of the cell was ca. 9.5 mL. The ionic solution was then photoirradiated with continuous stirring using a magnetic stirrer. The reduced samples prepared with a designated reduction time of up to 180 min were measured by a UV-vis spectrophotometer. UV-vis absorption spectra were measured by a Hitachi U-3010 spectrophotometer to confirm the formation of Ag particles in PVP solutions. To measure the obtained colloidal dispersions ([Ag] = 10 mM), we diluted 0.1 mL of the obtained samples in 3 mL of water/ethanol(1/1) mixture to adjust the metal concentration for the UV-vis measurements. TEM micrographs of the colloidal dispersions were obtained using a JEM-2000FX operated at acceleration voltage of 200 kV. The samples after the photoirradiation of 90 min (that is, after the in situ SAXS measurement of 90 min) were diluted in an appropriate amount of water/ethanol(1/1) mixture for the TEM observations. A drop of the dispersions was deposited on a highresolution carbon-supported copper mesh and then dried completely in ambient air. 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 the enlarged photograph.
2.3. In Situ SAXS Measurements of the Colloidal Dispersions of Ag Particles. In situ time-resolved SAXS measurements for the colloidal dispersions of Ag particles ([Ag] = 5, 10, 20, and 40 mM) in the PVP solutions were performed at the BL45XU beamline26 at SPring-8 in Japan. The beamline was set to a sample-to-detector distance of 2500 mm and an X-ray wavelength of 0.9 A˚. A CCD camera with 6 in. image intensifier was used as the 2-D detector, and the measured q range was between 0.06 and 1.6 nm-1. Here q is the magnitude of the scattering wave vector defined as q ¼ ð4π=λÞ sinðθ=2Þ
ð1Þ
where θ is the scattering angle and λ is the wavelength of X-ray. Silver behenate (CH3(CH2)20-COOAg), with a d-spacing of 58.38 A˚,27 was used as a standard to calibrate the angular scale of the 2-D detector. The X-ray path was evacuated, except at the position where the sample cell was set. All measurements were carried out at room temperature. To compensate for the relatively small dynamic range of the detector, an absorber mask of lead was placed in front of the detector. The typical measuring time was 1800 ms. The obtained data were corrected by air scattering and electrical background. Then, we obtained the 1D SAXS profiles by circularly averaging the 2D data. For in situ SAXS measurements, the colloidal samples were synthesized in a stainless steel cell (inner volume of 13.5 mL) equipped with four optical windows; two windows were single crystal diamond (0.7 mm thickness, optical path length of 1 mm) for the X-ray path, and the other two were quartz for the irradiation by UV-light. The direction of the photoirradiation was perpendicular to that of the direct X-ray beam. The sample solution was continuously stirred (24) Shrestha, N. K.; Yagi, E. J.; Takatori, Y.; Kawai, A.; Kajii, Y.; Shibuya, K.; Obi, K. J. Photochem. Photobiol., A 1998, 116, 179. (25) Kometani, N.; Kohara, Y.; Yonezawa, Y. Colloids Surf., A 2008, 313314, 43. (26) Fujisawa, T.; Inoue, K.; Oka, T.; Iwamoto, H.; Uruga, T.; Kumasaka, T.; Inoko, Y.; Yagi, N.; Yamamoto, M.; Ueki, T. J. Appl. Phys. 2000, 33, 797. (27) Huang, T. C.; Toraya, H.; Blanton, T. N.; Wu, Y. J. Appl. Crystallogr. 1993, 26, 180.
DOI: 10.1021/la102705h
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Figure 1. UV-vis absorption spectra of the colloidal dispersions of Ag particles ([Ag]=10 mM) produced from aqueous PVP solutions by the photoreduction in the presence of benzoin ([benzoin]= 10.4 mM). To adjust the concentration of metal for the UV absorption measurements, 0.1 mL of the obtained colloidal dispersions was diluted in 3 mL of water/ethanol(1/1) mixture. The time evolution was examined during the photoreduction up to 180 min.
Figure 3. Particle size distributions extracted from the corresponding TEM image shown in Figure 2. Metal concentration [Ag] in the colloidal dispersions is (a) 5, (b) 10, (c) 20, and (d) 40 mM. Figure 2. Transmission electron micrographs for the colloidal dispersions of Ag particles produced from aqueous PVP solutions by the photoreduction. Metal concentration [Ag] in the colloidal dispersions is (a) 5, (b) 10, (c) 20, and (d) 40 mM. The photoreduction was carried out over 90 min. and photoirradiated during the acquisition of SAXS spectra. The spectra were typically acquired at an interval of 30 s during the photoirradiation for up to 90 min.
3. Results and Discussion 3.1. Formation of the Colloidal Dispersions of Ag Particles during Photoirradiation. Figure 1 shows the UV-vis absorption spectra of the colloidal dispersions of Ag particles prepared by photoreduction in the presence of benzoin. With an increase in the irradiation time of UV light, a narrow plasmon absorption peak around 410 nm increases in time because of the Ag particle formation. No significant changes are observed for the samples after the irradiation for 120 min, indicating that the consumption of AgClO4 precursor to produce Ag particles is completed around this time. Furthermore, a small amount of precipitates was 17898 DOI: 10.1021/la102705h
spontaneously formed after a couple of weeks without photoirradiation. Even in the [Ag] concentration higher than 10 mM, the photoreduction of Agþ ions to Ag0 atoms proceeds easily using ketyl radicals as reducing reagents such as benzoin. Figure 2 shows the TEM images of the colloidal dispersions of Ag particles prepared by the photoreduction of the different concentration of AgClO4, and the corresponding particle size distribution histograms (Figure 3) are obtained from the images. The TEM images (Figure 2a,b) of the Ag particles show the presence of irregularly shaped larger particles (>40 nm in diameter) but only a small percentage. The Ag particles have a broader size distribution ranging from 5 to 60 nm, and the average particle diameter is 23.8 (for [Ag]=5 mM) and 14.4 nm (for [Ag]=10 mM), respectively. The smaller particle less than ca. 20 nm in diameter has a spherical shape, and the larger particles (>40 nm in diameter) are probably multicrystalline. With increasing Ag concentrations, as shown in Figure 2c,d, we observe a decrease in the particle size and polydispersity. The particles formed in the case of [Ag] = 20 and 40 mM are spherical in shape with 2-20 nm in diameter. The increase in [Ag] concentration is expected to generate a greater number Langmuir 2010, 26(23), 17896–17905
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of Ag nuclei, thus producing stable Ag particles with a diameter
