Journal of Crystal Growth 237–239 (2002) 1942–1945
Formation of ultrafine platinum particles in an aqueous solution with a surfactant S. Hahakura, S. Isoda, T. Ogawa*, S. Moriguchi, T. Kobayashi Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan
Abstract The formation process of ultrafine metal particles in water was examined with a cryogenic transmission electron microscope by using a rapid-freezing specimen-preparation technique. The particles were formed by hydrogen reduction in an aqueous solution of chloroplatinic acid with or without a surfactant. The ultrafine platinum particle was obtained with the surfactant, and they were concluded as formed not in a micelle as a nucleation site, but in arbitrary places in solution. During the formation process, the particles are coated and stabilized by surfactant molecules so as to not aggregate, when using the surfactant. r 2002 Elsevier Science B.V. All rights reserved. PACS: 81.10.A; 81.30.F; 61.46; 64.60.Q Keywords: A1. Nucleation; A1. Solidification; B1. Metals; B1. Nanomaterials
1. Introduction Interest in the physicochemical properties of colloidal metal particles has been growing since the time of Faraday [1]. Ultrafine metal particles have drawn attention not only because of their catalytic properties due to the large surface area, but also because of their specific functions which are different from those of bulk metal solids [2]. There have been a number of researches on the preparation methods and functions of metal clusters [3]. One of the promising methods is the chemical synthesis in a solution with some dispersion agent to fabricate fine metal particles. For example, Toshima et al. have reported on the preparation of *Corresponding author. Tel.: +81-774-38-3051; fax: +81774-38-3055. E-mail address:
[email protected] (T. Ogawa).
colloidal dispersions of platinum clusters of wellcontrolled nano-size diameter by hydrogen reduction in an aqueous solution with surfactants [4]. However, it is still unclear whether the formation of metal particle occurs at a micelle of the surfactant as a nucleation site or is initiated independent of the micelles but stabilized after the fine metal formation. In the present study, we examined the relation between ultrafine particles and a surfactant by using small-angle X-ray scattering (SAXS) and cryogenic transmission electron microscopy (cryo-TEM). The main advantage of the cryo-TEM is the fact that we can observe the structure directly just as it is in water [5,6].
2. Experimental procedure An aqueous solution of 0.2 mmol/l chloroplatinic acid (CPA) and a chosen concentration of
0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 2 2 5 5 - 2
S. Hahakura et al. / Journal of Crystal Growth 237–239 (2002) 1942–1945
Fig. 1. A cryo-TEM image of micelles formed in the aqueous solution of 100 mmol/l SDS. The micelles with dark contrast are dispersed homogeneously in the solution with an average size of 2 nm, some of which are indicated with the arrows.
Scattering intensity (a.u.)
sodium dodecyl sulfate (SDS) were reduced with hydrogen gas. At room temperature, platinum sol was formed in o10 min. With SDS, the color of the solution changed from light yellow to light brown by the full reduction, and without SDS to black which indicates the occurrence of large-scale aggregation. For the cryo-TEM observation, a thin layer of sample solution was rapidly frozen to achieve cooling sufficiently fast, so as to not rearrange the water molecules into a crystalline form [5–7]. After a little solution was placed on an electron microscopy microgrid, the excess solution on the grid was drained off with a filter paper, and the grid was immediately plunged into liquid propane maintained at E100 K in an immersion cryofixation apparatus. Then it was placed in a compartment of a specially designed cryotransfer system attached to a cryo-TEM (JEM-4000SFX). This instrument has a helium stage to keep specimens around 4.2 K [5], so that structures formed in the solution can be observed in vitreous water as they are in solution. SAXS was measured for the solutions with a SAXS camera at the High Intensity X-ray Laboratory of Kyoto University. The X-ray used was CuKa from a fine-focus X-ray generator (RU1000C3 of Rigaku Corporation, Japan).
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3. Results and discussion As well known, SDS surfactant forms micelle in an appropriate concentration in water [8]. Fig. 1 shows a typical cryo-TEM image of SDS micelles in vitreous water with a SDS concentration of 100 mmol/l. The dark spherical contrast of micelle has a diameter of E2 nm, which corresponds well with a result from the SAXS. The SAXS curve in Fig. 2 shows a scattering curve taken from a water solution of 100 mmol/l SDS, which has a maximum at a scattering vector of q ¼ 1:6 nm1; that is, a diameter of 2.2 nm can be expected by assuming a homogeneous spherical scattering object. When CPA is added in the aqueous solution of SDS and reduced with hydrogen gas, ultra-fine platinum particles were formed. The 0.2 mmol/l of
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Scattering vector q (nm ) Fig. 2. An SAXS pattern taken from the aqueous solution of SDS. The peak around q ¼ 1:6 nm1, that is, a diameter of 2.2 nm, is observed clearly, which corresponds well in size to the micelle in the cryo-TEM image.
CPA was reduced to form platinum sols with an SDS concentration of 100 mmol/l as shown in Fig. 3, where the reducing solution, after 6 min, was quenched into liquid propane and platinum particles are observed as individually dispersed ones with a diameter of E2 nm. Due to such regularity of the particle size, a surface plasmon peak has been reported in this case [7]. The reduction was completed almost in 6 min at room temperature, which can be confirmed by ultraviolet absorption spectroscopy for the reducing solution. In non-reduced solution, owing to the
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Fig. 3. A cryo-TEM image of platinum ultrafine particles formed through the hydrogen-gas reduction of CPA in the aqueous solution of SDS. The particle size seems to be very regular around 2 nm, which corresponds well to the diameter of the micelle. If the reduction proceeds without SDS, the metal particles are found as a larger aggregate with the size of several tens of nanometers in diameter as shown in the inset where larger particles of several tens of nanometers in diameter are observed. 2 existence of ions of PtCl2 4 and PtCl6 , absorption peaks appear at 220 and 260 nm, respectively [9]. In the due course of hydrogen reduction, these peaks disappear almost in 6 min and completely in 10 min. When the CPA in water is reduced without SDS, the produced platinum particles are aggregated to form large particles 20–70 nm in diameter as shown in the inset of Fig. 3 which indicates that the SDS should work as an agent to disperse the nano-particles. In addition to the difference in the states of aggregation with or without SDS, considering the correspondence in the diameters of SDS micelle and the produced metal particle, one might conclude that the particle formation occurs in or at the micelle. However, it is still not verified from the direct experimental evidence. To understand the mechanism, we carried out the following experiment. An aqueous solution of CPA was reduced without SDS by hydrogen gas, and as soon as the solution turned brownish (after 6 min), we added quickly the SDS with a concentration of 100 mmol/l into the brownish solution as schematically illustrated in scheme (iii) in Fig. 4. In this case, the color of the solution did not change to black as in the case of reduction
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Fig. 4. Schematic illustration on the time process: (i) reduction of CPA and SDS aqueous solution, (ii) reduction without SDS, and (iii) to put the SDS in the reducing aqueous solution of CPA. In the first 6 min, the reduction proceeds without SDS, in which the CPA has almost reduced as indicated by ultra-violet spectroscopy and the platinum particles are already formed as shown in Fig. 2.
without SDS in scheme (ii) in Fig. 4. A cryo-TEM image in Fig. 5 exhibits particles formed in scheme (iii), where the isolated fine particles are observed with only a slightly larger diameter than that in Fig. 3. Consequently, the micelle is not the nucleation site of metal particle formation, but is responsible to protect the particles from the further growth or aggregation. Actually, in the completely reduced solution without SDS, the large particle of several tens of nanometers diameter was found to be composed of nano-size particles of E2 nm in diameter by deep inspection of the large particles as shown in Fig. 6. This indicates that the platinum atoms are crystallized at arbitrary places in the solution by the reduction, and the size may be controlled by the concentration of CPA and/or temperature. With increase in the concentration of CPA, the particle size becomes larger in the same SDS concentration of 100 mmol/l and the produced metal particles were found to be precipitated even at a CPA concentration of 0.5 mmol/l. Consequently, by the hydrogen reduction, firstly platinum metals start to form as nanoparticles
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Fig. 5. A cryo-TEM image of the platinum particles formed through the process illustrated in scheme (iii) of Fig. 4. Although, in the first 6 min, the reduction was carried out without SDS, the platinum particles are nearly isolated from each other, not larger particles as in the inset of Fig. 3.
Fig. 7. A schematic drawing of the process of ultrafine particles formation in aqueous solution of CPA with SDS by hydrogen reduction. When the CPA is reduced in solution, platinum atoms come together as ultrafine particle with a size of 2 nm at a CPA concentration around 0.2 mmol/l, and then the particle is stabilized by being covered quickly with the SDS.
platinum particles are, therefore, protected by the surfactant molecules, so that the size of the particles is controlled to not grow larger. Often, the aggregation of particles occurs artificially in the preparation process for conventional TEM observation, because it is necessary to dry the sample. In contrast to this, the cryo-TEM method provides more reliable evidence on the morphology in solutions.
References
Fig. 6. A cryo-TEM image of platinum particle formed without SDS, where the large particle is observed to be clearly composed of ultrafine particles with 2 nm diameter.
sporadically in the solution, and secondly the surfactant molecules come to cover the particle so that the metal particles do not aggregate. The process is drawn schematically in Fig. 7. The
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