Platinum nanoparticles prepared by a plasma ...

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www.rsc.org/materials | Journal of Materials Chemistry

Platinum nanoparticles prepared by a plasma-chemical reduction method{ Published on 10 August 2005. Downloaded by Inha University Jungseok Memorial Library on 27/08/2015 13:56:28.

Il Gyo Koo,a Myoung Seok Lee,a Jae Hee Shim,a Jae Hwan Ahnb and Woong Moo Lee*a Received 14th June 2005, Accepted 13th July 2005 First published as an Advance Article on the web 10th August 2005 DOI: 10.1039/b508420b Platinum nanoparticles were prepared by reducing H2PtCl6 dissolved in water with the help of hydrogen plasma generated right above the aqueous solution, thus the small surface area of the solution contacting the plasma becomes the active reaction zone. This simple method to fabricate the nanoparticles did not necessitate use of reducing agents and polymer protective materials. The average particle size was around 2 nm with the solution temperature set at 10 uC.

Introduction Noble metal nanoparticles are used as electrochemical catalysts for numerous electrochemical reactions such as hydrogen oxidation, methanol oxidation, and oxygen reduction. The use of Pt as a catalyst requires that the active area remains stable with time of operation and that the expensive Pt metal is efficiently used. Thus, numerous methods to produce and utilize high surface area forms of Pt nanoparticles have been developed.1–18 Recently it was reported that the catalytic activity of the metal depends on the particle shape.19 Conventional methods for preparing Pt nanoparticles are mostly based on colloidal techniques.1–14 There are roughly two different approaches available in preparation of the nanosize particles using colloidal systems; one approach prefers direct formation of the metal particles by reducing the metal salt dissolved in a solvent9–14 and the other one takes a hydrolysis/condensation route of forming the particles and their aggregates, namely the sol–gel method.15,16 In the former approach the dispersion of the resulting particles on a substrate surface can be achieved by impregnating the salt solution into the substrate followed by reduction. Either approach necessitates the use of a stabilizer or stabilizing agents during the phase of particle formation. The nanoparticles produced by any approach can be dispersed on a substrate surface by their self-assembly or deposition. Some drawbacks associated with the conventional approaches include long aging and reduction times, involvement of organic solvents in some processes, necessity of removing the stabilizers at the final stage, and preparation of complex molecular precursors. A variety of reducing agents are used in conventional reduction methods; they can be hydrogen gas,4–9 LiBH4,10 NaBH4,11,12 ethylene glycol,2,6–8,13 and various alcohols.3,14 In rare cases, radicals produced by acoustic cavitation can function as reducing agents.5 It was also reported that microwave irradiation of the colloidal solution can shorten the reduction time.6–8,18

Our work was motivated by a search for a method that enables simple and fast preparation of Pt nanoparticles without using stabilizers and reducing agents, which can be toxic, so that the whole process would involve the simplest possible chemical system. The critical step in the preparation of its colloidal solution is the reduction of the metal by various reducing agents under the protection of stabilizers. Our group recently showed that PdCl2 can be reduced to metallic Pd when the chloride reacts with hydrogen plasma. The plasmachemical conversion of a 3 mm thick PdCl2 film coated on an alumina substrate into a Pd film took only 30 seconds without heating the reacting system;20 atmospheric pressure hydrogen plasma was generated by RF discharge through H2–He working gas in contact with the film. We extended this technique to plasma reactions at the liquid–vapor interface so that hydrogen plasma in contact with an aqueous Pt salt solution can possibly reduce the metal complex ions to produce Pt colloids. We used the ionic compound, hexachloroplatinic acid (H2PtCl6), as the precursor. This approach is the first example of noble metal nanoparticles production from the solution phase using low power hydrogen plasma. The novelty of the approach was manifested in fast kinetics, low temperature reaction, and no use of stabilizing agents. In this paper, we present the outline of the approach and some evidence of the nanoparticles produced under the circumstances described. Although it is for very different applications, the hydrogen plasma technique can be used for archaeological metal reduction.21 Application of electrical discharge techniques to nanoparticles preparation is nothing new. But such applications have resorted to brute force of dissipated electrical energy in decomposing related compounds. For example, it was reported that carbon nanoparticles or onions could be synthesized by an electrical arc formed between two carbon electrodes.22 What we report here does not bear any resemblance to this type of work.

Experimental a

Department of Molecular Science and Technology, Ajou University, Suwon, Korea. E-mail: [email protected]; Fax: 00 82-31-219-2969; Tel: 00 82-31-219-2607 b Division of Chemical and Materials Engineering, Ajou University, Suwon, Korea { Electronic supplementary information (ESI) available: EDX spectrum of Pt nanoparticles. See http://dx.doi.org/10.1039/b508420b

This journal is ß The Royal Society of Chemistry 2005

Fig. 1 shows the experimental setup used in our work. The principle of the plasma generation in the setup is basically the same as the one for the air plasma discharge.23 A 60 Hz AC discharge was sustained through flowing He–H2 gas between the stainless steel upper electrode and the surface of 2.44 mM J. Mater. Chem., 2005, 15, 4125–4128 | 4125

Published on 10 August 2005. Downloaded by Inha University Jungseok Memorial Library on 27/08/2015 13:56:28.

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Fig. 1 Experimental setup for plasma generation.

H2PtCl6 solution contained in a quartz container. The AC discharge consumed around 10 watts power to sustain the plasma, with the voltage and current being approximately 1 kV and 10 mA, respectively. The gap between the solution surface and the tip of the upper electrode was adjusted to be 5 mm. The inside of the tubular upper electrode (2 mm OD, 1.5 mm ID) shielded by alumina provided the path through which the mixture of He and H2 flew at the rate of 500 and 100 ml min21, respectively with their total pressure approaching the atmospheric value. The surface of the metal salt solution acted as the water electrode while a stainless steel disk placed at the bottom of the solution completed the electrical connection to the power source. The temperature of the solution was set at a value between 10 and 40 ¡ 0.1 uC using a water bath (JEIO TECH, RW0525G). The voltage and current were measured using a Tektronix 400 Mhz digital oscilloscope with current probe. The reduction of the Pt salt solution and the gas phase plasma reaction were monitored using a UV-Vis spectrometer (Scinco, model UV-2100) and an emission spectrometer (ARC, model Spectra pro 275 and PSI, model Darsa pro 5100), respectively. The Pt nanoparticles were characterized by transmission electron microscopy (TEM) using a TECNAI G2 microscope operated at 200 kV. TEM samples were prepared by placing a drop of the Pt solution onto a carboncoated copper grid, followed by drying under ambient conditions. Over 500 particles were measured on the enlarged TEM micrograph in order to obtain the average diameter and size distribution. The compositional information about the nanoparticles was obtained using an energy dispersive X-ray spectrometer (Oxford, INCA Energy) equipped in the scanning electron microscope (Leica Cambridge, Stereoscan440).

Fig. 2 UV-Vis absorption spectra of H2PtCl6 solution before and after the onset of discharge.

column. Fig. 2 shows that the characteristic absorption peak of PtCl622 at around 260 nm6,7,18 disappeared 5 minutes after the onset of the discharge. The reduction rate of the metal salt was estimated to be about 1.2 6 1025 mol min21 from the change of UV-Vis absorbency. The black particles were formed at the interface where the plasma touched the liquid solution and the spread of the particles to other regions of the solution was clearly observable. The plasma formed with He gas alone without H2 did not result in any reaction with the metal salt solution. Fig. 3 shows the emission spectra taken on the plasma generated right above the liquid surface during the discharge. For comparison purposes, aqueous NaCl and HCl solutions were used in some experiments instead of the H2PtCl6 solution. The spectra show that the characteristic HCl+ (2S+ A X2Pi) peak24 appears at 300–350 nm, except for the NaCl solution. The plasma-generated active hydrogens, either atoms of excited electronic states or ion, could trigger fast reduction of PtCl622 ions by reacting with Cl atoms of the complex ion to generate HCl while the left-out and reduced Pt atoms form nanoparticles. The strongly sorbed Cl2 ions of NaCl solution have a much lower chance of reacting with the active hydrogen

Results and discussion When the discharge circuit shown in Fig. 1 was closed by switching on the AC power source and the power consumption reached about 10 watts, a stable glow discharge (named just plasma from now on) column of about 2 mm diameter was generated in the region between the upper electrode tip and the solution surface. The salt solution changed colour from pale yellow to brownish as black particles were being formed at the surface region of the liquid solution near the gas phase plasma 4126 | J. Mater. Chem., 2005, 15, 4125–4128

Fig. 3 Emission spectra taken on the discharge region near the surface of solutions having different compositions. An arrow indicates the characteristic HCl peak.



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species in the plasma region, thus they are less likely to produce HCl. The HCl solution exhibits the most intense HCl+ line since the solution can drive HCl out by evaporation. The characteristic peak of Cl at 388.3 nm appeared in each system. The spectra profile was cut at 320 nm due to the appearance of strong He peaks right below that value. The emission spectra also confirmed the presence of species such as H and OH. The formation of pure Pt particles was confirmed by an EDX spectrum taken on the Pt sample deposited on carbon conductive tape, which showed only the peaks of Pt (see ESI{). Typical TEM images of the Pt particles at different magnifications are shown in Fig. 4. At higher magnification (Fig. 4b), the lattice planes can be seen, which demonstrates the nanocrystalline nature of the particles. The selected area diffraction (SAD) pattern (Fig. 5) shows four Debye–Scherrer rings corresponding to the {111},{200},{220},and {311} planes of a FCC lattice25 indicating that the Pt nanoparticles prepared in the present work have such an FCC structure as bulk Pt. The electron microscopy investigation revealed that most of the particles have irregular shapes and uniform size. It remains to be seen that the plasma method favors any particular shapes without using any capping agents26 or foreign ions like Ag+ ion.2 The TEM image discloses that some particles are not completely separated from one another, but they are not coalesced into tight agglomerates either, allowing size measurements of each particulate to be still possible. Agglomerations of these nanoparticles may have

Fig. 4

TEM images of the Pt nanoparticles at different magnifications.


Fig. 5 Selected area diffraction (SAD) pattern of the Pt nanoparticles.

taken place in the drying procedure during TEM sample preparation. The average size of the Pt nanoparticles was around 2 nm with diameters ranging from approximately 0.98 to 4.2 nm when the solution temperature was set at 10 uC. The histogram of the particle size distribution is shown in Fig. 6. As the solution temperature was raised to 40 uC, a substantial increase in the average size was observed, i.e. around 5 nm at 40 uC. Some conjectures are inevitable as to plausible explanations for no materialization of coalescence of the particles and for the average size of the particles falling in the range of 2–5 nm despite the absence of stabilizers in this work. We believe this is attributed to some conditions unique to the plasma method we employed. First, the surface of the solution in our experiment plays the role of a water electrode so that the liquid layer encompassing the region from the solution surface down to the surface of the immersed electrode in the container acts basically as a capacitor. Thus during the AC discharge some net charge of a given polarity should accumulate on the solution surface in alternate manner. Pt particles being formed at the surface region could bear some net charge, perhaps

Fig. 6 The size distribution histogram of the Pt nanoparticles.

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through association with charged species other than Pt, thus expelling any other particles of the same polarity surrounding them. Second, the reduction of the PtCl622 ions and thus the formation of the particles during the AC discharge is an extremely local process. Owing to the narrowness of the discharge column, the diameter of which is less than 2 mm, the active hydrogen species generated in the column should contact the metal salt ions residing only at the very top layer at the plasma/solution interface with the diameter less than 2 nm. The Pt particles formed in this very localized region from the plasma-chemical reduction should diffuse out rapidly into other regions and fresh ions diffuse into the local area, thus the chance for particle agglomeration is low. Another important factor for creating smaller particles can be associated with the fast nucleation rate27 inherent to the plasma process. Active hydrogen generated continuously in fresh state can enhance the reduction rate and thus the formation rate of the nanoparticles.

Conclusions Platinum nanoparticles were prepared by reducing PtCl622 dissolved in water with the help of active hydrogen generated in the plasma region in contact with the surface of the aqueous salt solution. Active hydrogen species were produced in the region when AC powered discharge was sustained in H2–He gas flowing toward the surface. Thus the small surface region of the solution contacting the plasma actually became the active reaction zone. The reduction was complete in 5 minutes when 2.44 mM H2PtCl6 solution was exposed to the discharge consuming 10 W power. This simple method to fabricate the nanoparticles did not necessitate the use of reducing agents and polymer protective materials. The average particle size was around 2 nm with the solution temperature set at 10 uC and the size increased to 5 nm as the temperature was raised to 40 uC.

Acknowledgements This study was supported by the National Nuclear Technology Program administered by Korea Science and Engineering Foundation (KOSEF). We appreciate the help by Mr Kim Soo Chul of KIST for taking TEM images of our samples.

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