Formation of Platinum and Palladium Bimetallic Superfine Particles by Ultrasonic Irradiation
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MW 40000) added as a protecting group. The concentration of PVP was ca. 0. 1 mmol-dm-3 .
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line The metal salt solution (24 ml) and 1 ml of ar- O-ringGasNacuum gon-saturated ethanol were put into the cylindrical sonication reactor (glass made, 25 mm diameter) which was placed in a water bath. WE , Because the local heating produced by the cavisolution Sample temperasolution the on depends tation strongly ture [10), the temperature of the water bath was Water bath held at 278 ± 0.5 K during sonications. Ultrasonic irradiations were made with a collimated 20 kHz beam from a ceramic transducer with titanium amplifying horn (Sonics and Materials, VCX-600) directly immersed in the solu- Figure 1 Schematics of the Sonication Reactor tion. The power of the ultrasound was 100 W. The sonication reactor and titanium horn were connected by an O-ring. A schematic of the reactor is represented in Figure 1. Since the reactor has a teflon valve, the solutions can be treated without exposing to air by standard Schlenk techniques. The irradiation was stopped every minute and UV-VIS spectra were observed using a UV-VIS2000 (Shimazu) instrument. Some drops of the solutions were placed onto a carbon support film stuck on a copper microgrid, and vacuum dried. Transmission electron microscopy was performed using a JEOL2000FXII. The sizes of 500 particles were measured on micrographs in order to obtain averaged diameters and size distributions. All manipulations were performed under an argon atmosphere except when introducing the microgrids to the electron microscope. RESULTS Metal ions can be reduced by radicals produced by acoustic cavitation. The expected sequence of reactions that would lead to the formation of novel metal nanoparticles are shown in equations (1) to (6) [7,11 ] H + HO* sonication )))))) H2 0 H + HO + R + RO + 0 sonication )))))) ROH R° + H2 0 -> RH + HO° where RH denotes PVP or ethanol -> M + nH+ nH° + Mn+ M + R' + nH+ -> nR° + Mn+ where R' + H = R (M)n -> n(M)
(1) (2) (3) (4) (5) (6)
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0 Particle size (nm)
Figure 2 Electron micrograph and particle size distribution of Pt nanoparticles prepared from S(1:0)
The clear yellow-colored solution of S(1:0) changed to very pale grayish-yellow and then turned to dark brown upon ultrasonic irradiation. In the UV-VIS spectrum, absorption bands around 375 nm and below 300 nm firstly decayed (0-50 minutes) and after that an increase of the base line was observed during the sonication. Only a few Pt particles larger than 2 nm were observed in electron micrographs of the sample prepared from the solution after the first stage of sonication. This result indicates that the consumption of PtCI6 2 - predominantly occurred at an early stage of the sonication, and the reduced Pt atoms and/or small clusters grow to collo
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