J. Phys. Chem. C 2008, 112, 19257–19262
19257
Ultrasonic Activation of Platinum Catalysts Darya Radziuk,* Helmuth Mo¨hwald, and Dmitry Shchukin Max-Planck Institute of Colloids and Interfaces, D14424 Potsdam, Germany ReceiVed: July 23, 2008; ReVised Manuscript ReceiVed: August 15, 2008
The effect of ultrasonic treatment on the crystallinity and activity of platinum nanoparticles is demonstrated. Preformed platinum nanoparticles stabilized with citrate ions were ultrasonically modified in water, poly(vinylpyrrolidone) aqueous solution, and ethylene glycol solution. The rapid heating/cooling cycles of the cavitation microbubbles resulted in melting and formation of amorphous or crystalline platinum nanoparticles with controllable catalytic activity. After 1 h of ultrasonic treatment in all solutions platinum nanoparticles were found to be more crystalline and ordered. Amorphous platinum nanostructures were formed after 20 min of sonication in water, whereas in poly(vinylpyrrolidone) or ethylene glycol solutions they became just less crystalline and more disordered. The catalytic activity of the ultrasonically modified platinum nanoparticles was examined by means of the reaction of the hexacyanoferrate(III) reduction by thiosulfate ions. The fastest catalysis was enabled by platinum nanoparticles after sonication in poly(vinyl pyrrolidone) solution for 1 h, while the lowest activity was found for particles after the ultrasonic treatment for 20 min in ethylene glycol solution. Platinum nanoparticles before and after ultrasonic treatment in water have similar catalytic efficiencies. Introduction For acceleration of technological processes various physical treatments can be applied, in particular acoustic techniques. Acoustic waves with frequencies of more than 20 kHz interacting with a species can cause structural changes and accelerate chemical reactions.1 The initialization of the majority of sonochemical reactions in aqueous solution applying acoustic vibrations is caused by cavitation. Acoustic cavitation is a disturbance of the continuousness of a liquid, which is connected to the creation, growth, oscillation, and collapse of steam-togas bubbles inside the liquid. The evolution of cavitation bubbles follows the sound field in a liquid during the cycles of compression and expansion and is stimulated by time-varying pressure. Bubbles either oscillate around their equilibrium position over several expansion/compression cycles or grow over one (sometimes two or three) acoustic cycles to double their initial size and finally collapse violently. It was experimentally proven that this short-time cavitational collapse produces extreme conditions inside the cavitation medium: temperatures of 5000 K and pressures of 1800 atm can be observed inside the microvolume of a collapsing bubble.2,3 If a medium is heterogeneous and has metallic particles in it, tremendous heating/cooling rates of bubbles and transient high temperature inside hot spots cause the dispersion of particles, their decrease in size, activation, and violent changes of the crystalline structure, up to the complete destruction of the last one. Suslick and Bellissent after sonication of solutions containing the volatile transition-metal carbonyls Fe(CO)5 and Co(CO)3NO produced highly porous aggregates of nanometer-sized clusters of amorphous metals.4,5 In 1999 amorphous silver nanoparticles were prepared by the sonochemical reduction of an aqueous silver nitrate solution in an atmosphere of argon-hydrogen.6 Amorphous silver sulfide nanowires have been prepared via thioglycolic acid assisted sonochemistry.7 Recently, much attention has been directed toward the sonochemical synthesis * To whom correspondence should be addressed. Phone: +49 (0)331-5679447. Fax: +49 (0)331-567-9202. E-mail:
[email protected].
of amorphous alloy catalysts such as Co-B,8 Ni-B,9 and crystalline Pt-Ru alloys.10,11 Plenty of results have been obtained in the synthesis of nanocrystals,12 semiconductor nanoparticles,13 core-shell gold-silver nanoparticles,14 and small crystalline colloidal Pt.15,16 Owing to corrosion stability and strong acceleration capability, platinum is commonly employed as a catalyst. Nowadays applications of platinum catalysts span a wide range from catalytic converters for the treatment of automotive exhaust17 to excellent catalysts for the total oxidation of hydrocarbons and carbon monoxide.18,19 Since platinum is rare, the smallest amounts of platinum with the highest reactivity are demanded. Therefore, much effort is spent on the synthesis and activation of platinum nanoparticles (Pt NPs). It was revealed that the surface and phase morphology as well as the average size distribution of the platinum catalysts on the nanometer scale strongly influence the activation energy in catalysis.20-24 To control the size of Pt nanoparticles in shortchain alcohols, poly(N-vinyl-2-pyrrolidone) (PVP) has been used as a capping material.25 The same reductant has been widely applied as a regulating agent for the selective growth of nanocrystals with well-defined shapes such as rods,26,27 prisms,28 and cubes.29 In contrast to the effect of PVP on the reduction of Pt NPs, ethylene glycol (EG) substantially slows the reduction rate of Pt(II) and Pt(IV), resulting in accurate control of the anisotropic growth of the nanoparticles.30 Taking into account these two contradictory effects, Kim et al. synthesized metal nanoparticles with various shapes by refluxing metal precursor in ethylene glycol in the presence of PVP.31 In sonochemistry the accumulation of surface-active materials at the gas/liquid interface of cavitation microbubbles is known to play an important role in a number of ultrasonically induced processes. This is mainly because the surface energy of the bubble in sonochemical reactions is transformed into heat during the bubble collapse, thus bringing the species together and making them react. Surface-active substances with various hydrophilicities are normally used for enrichment at the interface.11 For example, poly(vinylpyrrolidone) is a hydrophilic
10.1021/jp806508t CCC: $40.75 2008 American Chemical Society Published on Web 11/12/2008
19258 J. Phys. Chem. C, Vol. 112, No. 49, 2008 protecting polymer that strongly affects the shape of the platinum nanoparticles.32 Ethylene glycol is considered to influence strongly the reduction of the metals. In view of these facts, our interest arose to sonicate preformed platinum nanoparticles in PVP or EG solution, taking a pure water solution for comparison. In most cases ultrasonic treatment is applied to the synthesis of metallic catalysts with their expected crystal structure. Up to now there have been no papers in which ultrasonic cavitation is used not as one of the methods to synthesize colloidal particles stabilized with surfactants, but rather as a means for activation of already preformed nanoparticles in the same surfactants. The duration of sonication was varied to produce well-dispersed platinum catalysts on the nanometer scale with different crystallinities and, therefore, activities. The main aim of the present work is to illuminate the effect of ultrasonic treatment on the crystal structure of preformed platinum nanoparticles in poly(vinylpyrrolidone) or ethylene glycol solutions. Concurrently, these preformed platinum nanoparticles were sonicated in water under the same conditions to elucidate and compare the influence of the surfactants on the structure of the nanoparticles. The peculiarities of the crystal structure were examined by applying electron diffraction (ED) and X-ray scattering (WAXS). The shape and size of platinum nanoparticles were assessed by transmission electron microscopy (TEM). The catalytic properties of platinum nanoparticles were studied by means of the electron-transfer reaction between hexacyanoferrate(III) and thiosulfate ions resulting in the formation of hexacyanoferrate(II) ions and tetrathionate ions33 and monitored via UV-vis spectroscopy. Experimental Section Chemicals. Chloroplatinic acid hydrate (H2PtCl6 · H2O, 99.9%), sodium citrate (Na3C6H5O7), sodium borohydride (NaBH4, 98%), ethylene glycol (EG, 99%), poly(vinylpyrrolidone) K-30 (PVP K-30, 55 kDa, 99%), potassium hexacyanoferrate(III) (K3Fe(CN)6, 99.99%), and sodium thiosulfate pentahydrate (Na2S2O3 · 5H2O, 99.5%) were purchased from Sigma-Aldrich (Germany). These chemicals were used without any further purification. In all experiments the water was obtained from a three-stage Millipore “Milli-Q” purification system with a conductivity of less than 10-6 S · cm-1 and a surface tension of 72.3 mN · m-1 at 25 °C. Synthesis of Citrate-Protected Platinum Nanoparticles. All glassware used in the following procedures was cleaned in a bath of freshly prepared 3:1 v/v HCl:HNO3 (aqueous) and rinsed thoroughly with Milli-Q grade water. For the preparation of platinum nanoparticles, as the first step 5 mL of 10 mM H2PtCl6 was added to 30 mL of H2O and stirred vigorously for 1 min. Then this solution was diluted by 0.5 mL of 38.8 mM sodium citrate and stirred vigorously for 1 min. For further reduction 800 µL of 75 mM NaBH4 was freshly prepared in 90 mL of 38.8 mM sodium citrate and dropwise added to the mixture. Additional stirring followed for 5 min. The final colloidal solution resulted in a dark brown color, which indicated the formation of nanoparticles, and was stored in a dark bottle at 4 °C.35 The concentration of the platinum nanoparticles formed was 1.6 µmol/L. Sonication. An UIP100-230 ultrasonic processor (Hielscher GmbH, Germany, 20 kHz) was employed for continuous sonication. For ultrasonic treatment two mixtures were prepared: (1) 6 mL of Pt NPs was mixed with 4 mL of ethylene glycol solution and (2) 6 mL of Pt NPs was mixed with 4 mL of 6 mg/mL poly(vinylpyrrolidone) solution. The horn tip (9/13 in.) was immersed into each of the 10 mL mixtures to the same
Radziuk et al. depth (5 mm) and provided ultrasonic irradiation for 20 min (27 W/cm2) and for 60 min (22 W/cm2). For comparison the same ultrasonic procedure was carried out with 6 mL of Pt NPs diluted by 4 mL of H2O. Characterization. The catalytic activity of 600 µL of platinum nanoparticles was examined via the electron-transfer reaction between 200 µL of 0.01 M Fe(CN)36- and 200 µL of 0.1 M S2O32-. The kinetics of this catalytic reaction was monitored by employing a Varian CARY 50 UV-vis spectrophotometer. The morphological analysis of the Pt NPs was performed by a Zeiss EM 912 Omega transmission electron microscope, and their crystalline properties were analyzed by electron diffraction and X-ray scattering performed by a D8 Bruker Diffractometer. The diameter of the selected area in electron diffraction corresponded to 580 nm, determined by the camera length. The size of the platinum nanoparticles was estimated by applying the Debye-Scherer formula. Results and Discussion To examine the influence of ultrasonic treatment on the activity and crystalline structure, Pt nanoparticles were sonicated in water. They were stabilized with citrate ions in water, resulting in assemblies of spherical shape (Figure 1A). The size distribution of preformed Pt nanoparticles is available in the Supporting Information. After 20 min of ultrasonic treatment the decomposition of Pt assemblies was observed (Figure 1B). Highly monodisperse platinum nanoparticles of smaller size were obtained after 1 h of sonication (Figure 1C). In this case very interesting opposite effects on the crystalline structure, depending on sonication time, were found for platinum nanoparticles. Both thin sharp and diffuse rings with several spots were observed in the diffraction pattern of the Pt NPs before sonication (Figure 1D). Diffuse rings are due to small platinum grains, whereas the sharp ones are mainly ascribed to their assemblies. The presence of bright sharp dots points to the specific orientation of small platinum seeds. Twenty minutes of ultrasonic treatment reduced the nanoparticle size, and the crystallinity was lost. This can be deduced from the very diffuse rings in Figure 1E. Narrow diffraction rings were in turn found for platinum nanoparticles after ultrasonic treatment for 1 h (Figure 1F). The change from diffuse to sharp rings is characteristic of recrystallization of the platinum nanoparticles. For comparison, preformed platinum nanoparticles were sonicated in PVP aqueous solution and ethylene glycol. Monodisperse platinum nanoparticles with discrete orientation before ultrasonic treatment in PVP are shown in Figure 2A. After 20 min of sonication a drastic change in the stability was observed (Figure 2B). New monodisperse crystalline Pt NPs were found after 60 min of ultrasonic treatment (Figure 2C). The electron diffraction pattern of platinum nanoparticles in the PVP aqueous solution reveals broken rings and streaks that indicate disordered polycrystalline platinum nanoparticles (Figure 2D). After 20 min of sonication the numbers of streaks decreased and weak diffuse rings were observed (Figure 2E). This indicates a higher disorder and a loss of crystallinity of the nanoparticles. As soon as 1 h of ultrasonic treatment was completed, the electron diffraction of Pt nanoparticles revealed a few elongated bright streaks (Figure 2F). These correspond to crystals that change their round shape to an elongated one with order along only one axis. The absence of diffuse rings confirms the higher crystallinity of the formed nanoparticles. In comparison with the TEM image, one deduces a slight increase of the size of final Pt NPs. Unlike the diffraction patterns of Pt nanoparticles before
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Figure 1. TEM images and electron diffraction patterns of platinum nanoparticles before (A, D), after 20 min (B, E), and after 60 min (C, F) sonication in water.
Figure 2. TEM images and electron diffraction patterns of platinum nanoparticles before (A, D), after 20 min (B, E) and after 60 min (C, F) sonication in poly(vinylpyrrolidone) aqueous solution.
and after 20 min sonication, a large bright diffuse stain appeared in the middle of the pattern in the case of 1 h sonication. This is due to inelastic scattering of the electrons on the surface of the final Pt nanoparticles.33 Preformed Pt nanoparticles in ethylene glycol solution resulted in small monodisperse crystals (Figure 3A). Twenty minutes of sonication was enough to form large Pt aggregates (Figure 3B). Their size did not change much, in contrast to the structure of Pt NPs obtained after 60 min of ultrasonic treatment (Figure 3C). A thin diffuse diffraction ring was found for platinum nanoparticles in ethylene glycol solution
before ultrasonic treatment (Figure 3D). Comparing with the TEM image, one concludes that there were well-ordered small monodisperse Pt nanoparticles. The fact that only one ED ring is observed is mainly due to the camera length and intensity contrast of the electron microscope. After 20 min of sonication the diffraction ring becomes more diffuse (Figure 3E). Compared with the TEM data and according to ED analysis, the diffuse pattern characteristic of a particular area of the specimen indicates small nanoparticles. Moreover, the number of such nanoparticles is relatively large. The bright thin diffraction ring reveals polycrystalline small Pt
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Figure 3. TEM images and electron diffraction patterns of platinum nanoparticles before (A, D), after 20 min (B, E) and after 60 min (C, F) sonication in ethylene glycol solution.
Figure 4. Decay of the concentration of Fe (CN)36- during its reduction by S2O32- accelerated by platinum nanoparticles ultrasonically modified in water (A), poly(vinylpyrrolidone) aqueous solution (B), or ethylene glycol solution (C).
TABLE 1: Interplanar Spacing of Ultrasonically Modified Platinum Nanoparticles Calculated from Electron Diffraction Patterns (nm)a hkl sampleb Pt Pt Pt Pt Pt Pt
control H2O 20 min H2O 60 min PVP bf PVP 20 min PVP 60 min
111
200
220
311
0.23 amorphous 0.22 0.20 0.20 0.23
0.19 0.19 0.17 0.16 0.21
0.14 0.13 0.13 0.13 -
0.12 0.11 0.11 -
a The error amounts to 0.01 nm. b Pt H2O 20 and 60 min, Pt nanoparticles after sonication for 20 and 60 min correspondingly; Pt PVP bf, 20, and 60 min, Pt nanoparticles before sonication and after 20 and 60 min of sonication.
nanoparticles after sonication for 60 min (Figure 3F). This suggestion was confirmed by TEM analysis. The interplanar spacing between crystalline planes of ultrasonically modified platinum nanoparticles in water or PVP solution was calculated from the ED patterns (Table 1). Strong changes of the interplanar spacing were observed for the planes (111) and (200). Platinum nanoparticles in PVP solution before sonication had smaller interplanar spacing in comparison with the preformed Pt NPs. This effect is most pronounced for the (111) plane. It is known that the adsorption of PVP prefers (111)
to the other facets.34 According to the Laplace-Young equation, the decreased interplanar spacing of the (111) plane for small Pt NPs may be ascribed as due to the increase of the pressure at constant surface tension. The largest distance between (111) planes was found for Pt nanoparticles in water before sonication and Pt NPs ultrasonically modified for 60 min with PVP. The interplanar spacing of Pt nanoparticles in PVP aqueous solution before and after 20 min of ultrasonic treatment did not change. Hence 20 min of sonication is not enough for altering the crystalline structure of nanoparticles in contrast to 60 min of sonication. The same observations were found for the (200) and (220) planes. A small decrease of the interplanar spacing for the planes (111), (200), (220), and (311) was obtained for Pt NPs after sonication in water for 60 min. The d value for the (220) plane remained the same for Pt NPs after sonication for 60 min in water and 20 min in PVP solution as well as for Pt NPs in PVP before ultrasonic treatment. The diffraction rings for the (220) and (311) planes of Pt NPs disappeared after 1 h of sonication in PVP. Thus significant changes of the crystallinity of Pt nanoparticles occurred in PVP solution after 1 h of ultrasonic treatment. The strongest effect of sonication on the crystallinity was demonstrated for Pt NPs in water. That is, after 20 min of ultrasonic treatment, Pt NPs completely lost their crystalline structure and became amorphous, while 60 min of sonication resulted in recrystallization of Pt NPs.
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TABLE 2: X-ray Powder Scattering Data for Platinum Nanoparticles before and after Their Sonication in Ethylene Glycol Solutiona hkl
2θ
d, nm
I/I0, au
fwhm
size, nm
111 200 220 311
39.90 46.55 67.86 81.42
Pt Control 0.22 226 0.19 99 0.14 72 0.12 78
3.0 3.4 3.1 4.6
4.9 4.4 5.3 3.9
111 200 220 311
39.90 46.55 67.86 81.42
EG bf 0.23 0.19 0.14 0.12
165 75 56 61
1.2 1.7 2.0 2.4
12.1 8.8 8.3 7.5
111 200 220 311
39.90 46.55 67.86 81.42
EG 20 min 0.23 163 0.19 70 0.14 64 0.12 83
1.0 1.1 1.4 1.6
14.6 14.9 11.8 11.3
111 200 220 311
39.90 46.55 67.86 81.42
EG 60 min 0.22 230 0.19 93 0.14 91 0.12 95
1.5 1.3 1.4 1.2
9.7 11.5 11.8 15.0
a
The error in lengths amounts to 0.01 nm, that in intensities to
5%.
X-ray powder scattering data were obtained for Pt nanoparticles ultrasonically modified in ethylene glycol solution (Table 2). Slight changes of the interplanar spacing for the (111), (200), (220), and (311) planes were found for Pt NPs before and after ultrasonic treatment. In contrast, the intensities of the diffraction peaks were different for all Pt specimens (see the Supporting Information). This indicates shape changes and different anisotropies along different lattice vectors. The strongest peaks for all four planes appeared for Pt NPs after 60 min of sonication. Compared to the electron diffraction patterns, 1 h of ultrasonic treatment strongly increased the crystallinity of Pt NPs in either PVP or EG solutions. The ultrasonic treatment of platinum nanoparticles in water resulted in the decrease of the size of final Pt NPs after 1 h of sonication. On the contrary, PVP protected platinum nanoparticles kept their size after either 20 or 60 min of ultrasonic treatment. In the presence of ethylene glycol a greater amount of Pt nanoparticles increased their size by a factor of 2.4, whereas the rest of them roughly doubled it before the ultrasonic treatment. This is due to the fact that ethylene glycol is well adsorbed on the surface of Pt NPs. This effect is stronger with longer sonication, when the degradation of polymer occurred. More than half of the platinum nanoparticles resulted in an average size of 13 nm after 20 min and of 10 nm after 1 h of sonication in EG solution; that fits well with the TEM and ED data. Thus 20 min of ultrasonic treatment in EG solution leads to the formation of platinum nanoparticles of larger size, but 60 min of sonication results in smaller Pt NPs in water or ethylene glycol. The electron-transfer reaction between the hexacyanoferrate(III) and thiosulfate ions was chosen to monitor the activity of already preformed Pt nanoparticles before and after ultrasonic treatment (see the Supporting Information). The same catalytic reaction was carried out with Pt NPs ultrasonically modified in water (Figure 4A), PVP aqueous solution (Figure 4B), and EG solution (Figure 4C). The fastest catalysis was demonstrated by Pt nanoparticles ultrasonically modified in poly(vinyl pyrrolidone) solution for 1 h, while the lowest one was found for
Pt nanoparticles obtained after sonication for 20 min in ethylene glycol solution. This weak catalytic effect of Pt NPs in EG is due to many poorly ordered nanoparticles of various sizes, which is opposite for Pt NPs modified in PVP solution. Platinum nanoparticles formed in PVP or EG accelerate the catalytic reaction to a greater extent after 60 min of sonication than those after 20 min of ultrasonic treatment. However, amorphous and crystalline platinum nanoparticles prepared in water drove the catalysis with a slight difference. Crystalline nanoparticles accelerated the catalytic reaction during the first 20 min of the reaction, whereas the amorphous Pt NPs did the same for a further 20 min of the reaction. This is due to the more pronounced poisoning by the more effective amorphous Pt NPs of the S2O32- oxidation compared to crystalline Pt. In conclusion, ultrasonic treatment of preformed Pt NPs has the following influence: (1) In all solutions 20 min of sonication provides fewer crystalline or amorphous Pt nanoparticles, whereas 60 min of ultrasonic treatment suffices to form new Pt NPs with higher crystallinity. (2) The crystallinity of Pt NPs in pure aqueous solution is reduced by sonication strongly up to its complete loss, while PVP or ethylene glycol prevent the destruction of Pt crystals, but enable changes in the shape, orientation, and size. (3) Spherical amorphous and crystalline platinum nanoparticles ultrasonically modified in water are catalysts of similar activities. The best catalysis is performed by Pt catalysts with higher crystallinity formed after 60 min of ultrasonic treatment in PVP, while Pt nanoparticles with low crystallinity formed after 20 min of sonication in ethylene glycol were found to be the least efficient catalysts. Acknowledgment. We thank Rona Pitschke for electron microscopy analysis and Ju¨rgen Hartmann for electron diffraction discussions. This work was supported by the MATSILC, EU FP6 project, and by the Gay-Lussac/Humboldt award to H.M. Supporting Information Available: Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Mason, T.; Lorimer, J. P. Sonochemistry. Theory, applications and uses of ultrasound in chemistry; Ellis Horwood: Chinchester, 1988; Chapter 1.4, pp 13-15. (2) Suslick, K. S. Science 1990, 247, 1439. (3) Flint, E. B.; Suslick, K. S. Science 1991, 253, 1397. (4) Suslick, K. S.; et al. Annu. ReV. Mater. Sci. 1999, 29, 295–326. (5) Bellissent, R.; Gali, G.; Grinstaff, M. W.; Migliardo, P.; Suslick, K. S. Phys. ReV. B 1993, 48, 15797–15800. (6) Salker, R. A.; Jeevanandam, P.; Aruna, S. T.; Kolyptin, Y.; Gedanken, A. J. Mater. Chem. 1999, 9, 1333–1337. (7) Du, N.; Zhang, H.; Sun, H.; Yang, D. Mater. Lett. 2007, 61, 235– 238. (8) Li, H.; Zhang, J.; Dai, W.; Qiao, M. J. Catal. 2007, 246, 301–307. (9) Li, H.; Zhang, J.; Li, H. Catal. Commun. 2007, 8, 2212–2216. (10) Angelucci, C.; Silva, M.; Nart, F. Electrochim. Acta 2007, 52, 7293– 7299. (11) Shchukin, D. G.; Mo¨hwald, H. Phys. Chem. Chem. Phys. 2006, 8, 3496–3506. (12) Dhas, N. A.; Suslick, K. S. J. Am. Chem. Soc. 2005, 127, 2368– 2369. (13) Didenko, Y. T.; Suslick, K. S. J. Am. Chem. Soc. 2005, 127, 12196– 12197. (14) Radziuk, D.; Shchukin, D.; Mo¨hwald, H. J. Phys. Chem. C 2008, 112, 2462–2468. (15) Caruso, R. A.; Ashokumar, M.; Grieser, F. Colloids Surf., A: Physicochem. Eng. Apsects 2000, 169, 219–225.
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