Bimetallic AgPt and AuPt aggregates synthesized by radiolysis

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Radiat. Phys. Chem. Vol. 47, No. 2, pp. 275-279, 1996 Copyright © 1996 ElsevierScienceLtd 0969-806X(94)00172-3 Printed in Great Britain. All rights reserved 0969-806X/96 $15.00+ 0.00

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BIMETALLIC Ag-Pt AND Au-Pt AGGREGATES SYNTHESIZED BY RADIOLYSIS S. R E M I T A , M. M O S T A F A V I and M. O. D E L C O U R T Laboratoire de Physico-Chimie des Rayonnements, CNRS URA 75, Brit. 350, Universit6 Paris-Sud, 91405 Orsay Cedex, France

(Received 14 July 1994,"accepted 9 November 1994) Abstract--Irradiating aqueous solutions containing both Ag2SO4 and K2PtCI 4 leads to intermetallic aggregates of various sizes according to the stabilizing agent: polyvinylalcohol, polyacrylic acid or polyacrylate. In the last case, the particle diameter is 1.5 nm. The bimetallic character is evidenced in all cases by the spectral changes of such sols compared to pure silver sols which display a characteristic surface plasmon absorption band. This plasmon band disappears when 10 to 20 at.% or more Pt is present. Observation by TEM gives an estimation of the particle sizes. Comparable results have been obtained for Au-Pt particles.

INTRODUCTION Radiolysis of aqueous solutions of metal ions is well known to lead to reduced metal sols. A wide variety of sizes can be obtained according to the stabilizing agent used. With PVA (polyvinyalcohol), long-lived sols are obtained for Ag as well as for Pt. While PVA can limit the sizes of Pt aggregates in the nanometer range (Delcourt et al., 1983), it stabilizes only much bigger sizes in the case of Ag (Mostafavi, 1989). Recently, we have shown that P A A (polyacrylic acid) was more efficient towards Ag; its deprotonated form PA (polyacrylate) revealed to be even much more efficient since oligomer aggregates have been stabilized for long periods (Mostafavi et al., 1990). The different sizes have different spectra in the case of silver. In the present work, we study irradiated solutions containing both Ag and Pt in order to evidence their intermetallic character. Indeed several bimetallic aggregates have been synthesized at r o o m temperature by using the radiolytic reduction process (Marignier et al., 1985; Belloni et al., 1988; Georgopoulos and Delcourt, 1989). We also look for mixed oligomer aggregates. PVA, PAA and PA have been used in order to limit the particle sizes. Spectral information has been complemented by T E M (Transmission Electron Microscope) observation. The study has been extended to Au-Pt particles under comparable conditions.

CLAL, 2-propanol from Prolabo and the polymers PVA and P A A from Aldrich. The irradiations were carried out with a 6°Co gamma source. In aqueous solutions containing 0.2 m o l . l- ~2-propanol, the radiolytic reduction yield for noble metals is the sum of all the radical yields, i.e. 6.0 elementary reductions per 100eV. On this basis, typically 20 krad, i.e. 200 Gy have been applied to 10 4m01"1-1 Ag +, 40krad (400Gy) to 10 -4 mol" I J Pt II (a double dose is necessary for Pt because of its two reduction steps), and 60 krad (600 Gy) to 10 4 m o l . 1 i AuIH (three reduction steps) for complete reduction. Ag2SO 4 and K2PtCI 4 mixtures have been found not to precipitate AgCI since no scattered light could be observed on their UV-visible spectra. We have also checked that Pt~lCl4 cannot be reduced by silver particles: adding ptlICl4 to a silver sol does not change its spectrum significantly. Transmission Electron Microscopy has been conducted on a J E O L 2000 EX apparatus equipped with a T R A C O R N O R T H E R N (series 2) energy dispersive X-ray analyzer (X-EDS). RESULTS AND DISCUSSION

(a) Ag-Pt in the presence o f PVA (natural p H ~ 3) Solutions containing a total metal quantity 10 -4 mol • l ~ at various Ag-Pt ratios in the presence of 10 4 m o l ' 1-~ PVA have been irradiated: the dose was in any case sufficient to quantitatively reduce both metals. Figure 1 is a typical micrograph of the particles obtained: the sizes are in the 1-10 nm range, which is much smaller than in the case of pure silver, rather

EXPERIMENTAL The chemicals used are pure grade reagents: Ag2SO 4 from Aldrich, K2PtCI 4 and HAuCI 4 from RPc47/2-H

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S. Remita et al. spectrum (60-40), the silver plasmon band has almost completely disappeared. The other spectra show mainly scattering as for pure platinum. Let us note that the observed Pt spectra are not similar to those calculated by the Mie theory (Creighton and Eadon, 1991) likely due to the presence of clumps as shown by TEM (Fig. 1). The intensity of the spectrum is minimum for the 50-50 mixture, then a progressive increase in the band with the Pt content is observed. It is clear that the presence of Pt, even in minor quantities, strongly affects the silver spectrum. In order to have a better knowledge of such mixed Ag-Pt sols, we prepared separately a pure silver sol and a pure platinum sol which were then mixed in various proportions so as to be compared to the previous spectra. We found a good additivity of their spectra, thus indicating that the possible interaction between both particles does not result in substantial changes of the spectrum. In particular, the silver plasmon band was always observed when platinum was present. This is illustrated in Fig. 3 where the signal at 410nm (empty circles) follows the linear additivity law. On the opposite, the signal obtained when irradiating mixed solutions is far from this straight line (Fig. 3. Solid squares: values taken from Fig. 2). From these observations, it must be concluded that irradiating mixed salts leads to bimetallic Ag-Pt particles. Such a spectral behaviour has been also observed in other studies showing that small quantities of a second metal at the surface of silver aggregates strongly affect the plasmon band (Henglein et al., 1992; Mulvaney et al., 1993). The bimetallic character is also confirmed by the changes in the corrodability of the particles. Various

Fig. 1. TEM micrograph of particles obtained after reduction of a 75 25 at.% Ag-Pt mixture in the presence of PVA. Scale bar: 20 nm. comparable to pure Pt/PVA samples (Delcourt et al., 1983). X-EDS indicates the presence of both metals Pt and Ag in any place where particles are observed; no signal of these elements is obtained on the naked carbon support. Figure 2 shows the absorption spectra of radiolytically reduced solutions. The first spectrum (100-0) corresponds to pure silver: the surface plasmon absorption band is observed with a maximum at 410 nm. In the second spectrum (80-20, i.e. 80 at.% Ag-20% Pt), the plasmon band appears drastically lowered while some scattering is observed. In the next

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L (rim) Fig. 2. UV-visible spectra obtained for irradiated solutions containing AgzSO4 and K2PtCI4 in various ratios. Total metal concentration: 10-4 mol. 1-~, [PVA] = 10-4 mol. 1-t. Doses for total reduction. The numbers are referred to the atomic percentages Ag-Pt. Optical length: 1 cm.

Bimetallic aggregates synthesized by radiolysis

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sols have been prepared in the absence of a stabilizing polymer: pure silver, Ag-Pt 90-10 and 50-50. Immediately after irradiation, they have been brought to acidic medium (by HCIO4 addition) from natural pH ~ 5 to 2 in an air open ceil. At pH = 2 the corrosion of the pure silver sol is fast as observed by the fading of the spectrum. The Ag-Pt sols are more resistant to corrosion, all the more when the Pt content is high (50-50). This indicates that platinum

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(b) Ag-Pt in the presence of PAA (natural pH ~ 3) In the presence of another stabilizing agent PAA, the features of the spectra are very similar (Fig. 4). The main difference is that for pure Ag, the spectrum consists in an overlap between the silver aggregate band peaking at 380 nm and a broad polymer

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~. (nm) Fig. 5. Spectra of irradiated solutions containing 5.10-5mol • 1 ~ Ag_~SO 4 and increasing quantities of K2PtCI 4 in the presence of 10-2 mol. 1-~ polyacrylate. Dose: 30 krad (300 Gy). Optical length: 1 cm. interaction band with a maximum at ca. 460 nm (Mostafavi et al., 1990; 1993). In the presence of some platinum, this last band disappears. The additivity of the spectra of pure colloids is observed as above, and the signal for the irradiated mixture is different from the sum of the signals of the pure sols similarly to the PVA stabilized sols. Consequently, the particles must contain both metals. T E M analysis shows that the particle sizes are in the 1-6 nm range.

agent for Ag-Pt particles as already known for pure silver.

(d) Au-Pt colloids A similar study has been conducted on Au-Pt mixtures. Figure 7 shows the UV-visible absorption spectra obtained after half-dose irradiation of sol-

(c) Ag-Pt in the presence of PA (pH = 10) In basic medium, polyacrylate ions have been shown to be extremely powerful in stabilizing silver oligomer aggregates (Mostafavi et al., 1990; 1993). In Fig. 5 the full line shows two oligomer bands at 295 and 340 nm in addition to the plasmon band. When adding some platinum salt before irradiation, the oligomer bands are kept when Pt is no more than 10% of the silver quantity. The silver plasmon band has already almost completely disappeared. With 5 ' 10 5mol.1 - I P t or more, only the m o n o t o n o u s spectrum is observed, which increases with the Pt content. We conclude as above that the particles are bimetallic. No new oligomer band is observed as might be expected for bimetallic oligomers. (However a blue shift of the U V oligomer bands for the sample [Ag] = 10 -4 and [Pt]= 10-Smol '1 1 might be an indication of such intermetallic oligomers). When studied by T E M (Fig. 6), the particles appear gathered in a polymer mantle so that it is not possible to get them individually. X - E D S analysis reveals that both metals are present in any place where particles are observed. The population is remarkably homodisperse with a particle size ca. 1.5 nm: polyacrylate is an excellent size limiting

Fig. 6. TEM micrograph of a sample containing 5.10 Smol.1 ' Ag2SOn, 10 5mol.1 i, K2PtC14 and 10-2mol.l -f polyacrylate irradiated by 5krad (50Gy). The big spots are polyacrylate. The small spots are the metal particles (0 ~ 1.5 nm) containing both Ag and Pt. Scale bar: 20 rim.

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)~ (nm) Fig. 7. UV-visible spectra obtained for half-reduced solutions containing HAuC14 and KzPtCI4 in various ratios. Total metal concentration: 5.10 4mol. 1 1. [PAA] = 5.10-4 tool -1-~. Optical length: 1 cm. utions containing HAuC14 and K2PtCI 4. The 540 nm band is the plasmon band of pure Au and the 300 nm band corresponds to the remaining AuC14 ions. If 2 0 a t . % K2PtCI 4 is added, the AuC14 band totally disappears while the 540 nm plasmon band subsists: this indicates that Au has been preferentially reduced. At 40% Pt, the plasmon band becomes minor although still detectable. At 60% Pt, the shape of the spectrum suddenly changes. At higher Pt %, the effects are similar to those obtained in the case of Ag-Pt: a flat spectrum without any more Ag plasmon band. The same results are obtained when PVA is replaced by PAA or in the absence of polymer. The preferential reduction of Au can be explained by the redox potentials (Handbook of Chemistry and Physics, 1992): AulHCl4/Au°metal

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which make the reduction of oxidized forms of Au by Pt H possible. The slow reaction: 2AuUlC14 + 3PtHC12~ ~ 2Au ° + 3PtlVCl~ can also be observed without irradiation by thermal contact between both salts, in the scale of a few hours. Preferential reduction of Au is obtained in the same way from the mixtures AutHC14 + ptJvCl~ (for which the reduction steps include Pt").

CONCLUSION The radiolytic process allows the synthesis of Ag-Pt and Au-Pt bimetallic aggregates. The UV-visible absorption spectra of such particles in aqueous solution are noticeably different from those obtained with pure metals. In any case the plasmon band of Ag or Au fades in the presence of some platinum. An important size effect has also been shown: the mixed Ag-Pt particles are much smaller than the pure silver ones under comparable conditions. However no evidence has been found for oligomer mixed aggregates.

REFERENCES Belloni J., Marignier J. L., Delcourt M. O. and Minana M. (1988) U.S. Patent 4,745,094. Creighton J.A. and Eadon D. G. (1991) J. Chem. Soc. Faraday Trans. 87, 3881. Delcourt M. O., Keghouche N. and Belloni J. (1983) New. J. Chem. 7, 131. Georgopoulos M. and Delcourt M. O. (1989) New. J. Chem. 13, 519. Handbook of Chemistry and Physics (1992) 72nd edn. CRC Press, Boca Raton, Fla. Henglein A., Mulvaney P., Linnert T. and Holzwarth A. (1992) J. Phys. Chem. 96, 2411 Marignier J. L., Belloni J., Delcourt M. O. and Chevalier J. P. (1985). Nature 317, 344. Mostafavi M. (1989) Ph.D thesis pp. 93-95. Orsay, France. Mostafavi M., Keghouche N. and Delcourt M. O. (1990) Chem. Phys. Lett. 169, 81. Mostafavi M., Delcourt M. O. and Picq G. (1993) Radiat. Phys. Chem. 41, 453. Mulvaney P., Giersig M. and Henglein A. (1993) J. Phys. Chem. 97, 7061.