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Chemical Communications www.rsc.org/chemcomm
Volume 46 | Number 42 | 14 November 2010 | Pages 7845–8072
ISSN 1359-7345
COMMUNICATION Ekaterina V. Skorb et al. Ultrasound-Assisted Design of Metal Nanocomposites
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COMMUNICATION
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Ultrasound-assisted design of metal nanocompositesw Ekaterina V. Skorb,*a Helmuth Mo¨hwald,a Torsten Irrgang,b Andreas Feryc and Daria V. Andreevac
Downloaded by UNIVERSITAT BAYREUTH on 14 February 2011 Published on 30 July 2010 on http://pubs.rsc.org | doi:10.1039/C0CC00965B
Received 15th April 2010, Accepted 2nd July 2010 DOI: 10.1039/c0cc00965b A one-step method was developed to produce metal nanocomposites from metal alloys under ultrasound irradiation. Systematic investigation of ultrasound effects on various metal particles reveals cavitation-induced recrystallization and oxidation of metals as main factors in the process. The fact that different metals react in dramatically different fashion towards ultrasound irradiation was exploited for the formation of nanoscale composites. Results from the application of ultrasound to formation of nanocatalysts are reported. Metal particles with tuneable structures combining the beneficial properties of metal nature and a developed nanostructure have gained interest due to their potential application in nanocatalysis,1,2 as building blocks in photovoltaic and fuel cells2,3 as well as ‘‘smart’’ delivery systems for biomedicine.4 Approaches to the fabrication of metal particles with controlled nanostructure are based on electroplating into the self-assembled liquid crystal surfactants,5 colloidal crystals,6 porous block copolymers7 or anodic porous alumina.8 Nanoporous metals have been synthesized by chemical reduction,9 plasma spraying10 and electrodeposition11 followed by template decomposition, etc.12 Some of these methods have an advantage of precise control over the structure of a final metallic system. However, they are multistage, expensive and time-consuming to implement. Thus, it is very important to develop novel, general, energy effective and environmentally friendly approaches for manipulation of metal structure which can be applied to a broad range of metals and alloys. We explored a novel approach to manipulate metallic structures in aqueous media using a low frequency highintensity ultrasound as a tool. The generation, growth, and collapse of bubbles stimulated by ultrasound exposure of liquids initiates intense shock waves that propagate through the liquid. Sonochemical effect induced by the shock waves are high velocity collisions among solid particles suspended in liquids. The collisions result in extreme heating at the point of impact, which can lead to local melting and increases in the rates of many solid–liquid reactions.13,14 Here, we focused on the investigation of the structural and morphological changes of a variety of metal particles under extreme conditions a
Max Planck Institute of Colloids and Interfaces, Wissenschaftspark Golm, Am Mu¨hlenberg 1, Golm 14476, Germany. E-mail:
[email protected] b Inorganic Chemistry II, University of Bayreuth, Universita¨tstr. 30, Bayreuth 95440, Germany c Physical Chemistry II, University of Bayreuth, Universita¨tstr. 30, Bayreuth 95440, Germany. E-mail:
[email protected] w Electronic supplementary information (ESI) available: Additional details concerning experimental conditions, results concerning noble metals/magnesium and zinc modification, BET, ED data and catalytic runs. See DOI: 10.1039/c0cc00965b
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generated by cavitation as well as understanding of the mechanism of ultrasound-stimulated manipulation of metal structures. We propose that the mechanism of the modification of metal structure under ultrasound exposure involves physical and chemical aspects. Cavitation-generated intensive heat provides an efficient way to manipulate the structure and the properties of the metals by changing grain size and composition. Fast local heating with high temperature gradient, phase and structural changes is a physical effect of cavitation.14 Oxidation of metal surfaces by free radicals15 also generated by cavitation is a chemical aspect of ultrasound modification of metals. We propose that surface oxidation should be an important aspect in structure stabilization and formation of ‘‘frozen’’ nanostructures. In order to clarify the underlying ultrasound driven physical and chemical processes in liquid–metal system, we investigated several groups of metals. For convenience, we divided the investigated metals in accordance with melting points and tendency to oxidation. Depending on the metal nature we observed three possible modification processes in 40 min of applying 20 kHz 57 W cm 2 ultrasound irradiation: complete conversion in oxide particles for Zn, surface modification for Ni and Ti and decomposition of particles and formation of mesoporous sponge structure for Al and Mg. The noble metals demonstrate maximum resistance to ultrasound treatment. Au, Ag and Pt (see Fig. S1, ESIw) form neither porous structure nor developed surface after sonication, even in cases when their melting point is actually lower than the one of metals that can be modified slightly (like Ni or Ti). High resistance to oxidation (stability to chemical aspect of ultrasound) of noble metals might explain their stability to sonication. SEM images (Fig. 1) show that ultrasound-stimulated interparticle collisions have results common for all used metal particles except noble metals: (I) formation of a developed outer surface (the rough surface can be observed even in 5 min of sonication); (II) involving of inner structure of the particles; (III) particle breakage (approximately 100-mm initial particles are broken into 1–5-mm pieces after 60 min of sonication); (IV) oxidation of metal particles. Commonly, ultrasound modification of metal particles is accompanied by oxidation of the surface of a metal and a formation of a rough metal oxide (hydroxide) layer (Al, Mg particles, Fig. 2a, Fig. S2 (ESIw)). The surface oxidation was observed besides aluminium and magnesium for Zn, Ti and Ni particles. Development of a metal oxide layer was proved by XRD and solid state NMR. The 27Al MAS NMR spectra (Fig. 2e) of the samples treated by ultrasound exhibit two signals at 1200 ppm and 67 ppm that can be assigned to Al and Al3+, respectively.16 Thus, Chem. Commun., 2010, 46, 7897–7899
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Fig. 1 SEM images of the surfaces of initial metal particles: aluminium (a), magnesium (b), nickel (c) and titanium (d) and after 60 min sonication at 57 W cm 2: aluminium (e), magnesium (f), nickel (g) and titanium (h).
Fig. 3 One-step formation of Al/Ni mesoporous system: TEM microphotograph of nickel particles distributed in the porous aluminium support (a); effect of sonication time on efficiency of Al/Ni catalyst in hydrogenation of acetophenone (b). TEM of palladium particles distributed in the porous aluminium support formed after high-intensity ultrasound (57 W cm 2) treatment of Pd/Al alloy (the inset shows the sketch image of the Pd/Al nano-composite) (c); TEM composite formed by high-intensity ultrasound (57 W cm 2) treatment of Cu/Pd/Al alloy (the insert shows the higher magnification) (d).
Fig. 2 SEM images of Al particles sonicated at 57 W cm 2 for 1 min (a), 3 min (b) and 5 min (c). Influence of ultrasound time on structural changes of the aluminium particles. TEM images, XRD (black squares show the peaks assigned to bayerite) and 27Al MAS NMR spectra of the initial aluminium particles (d) and after 60 min sonication (e). Influence of the sonication time on the specific surface area (BET analysis) and surface morphology of the aluminium particles (SEM images). The initial 100 mm Al particles were sonicated at 57 W cm 2 for 5 min (f), 60 min (g) and 120 min (h).
ultrasound irradiation stimulates formation of a nanostructure consistent with metallic skeleton stabilized by an oxide layer. The XRD patterns given in Fig. 3 indicate that an admixture of bayerite Al(OH)3 (black squares on XRD patterns) with 7898
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boehmite AlO(OH) are formed during sonication.17 The quantity of bayerite increases during formation of the porous structure which corresponds to the formation of a more developed surface of the samples. Peak at around 401 which is observed after sonication indicates a possible break of symmetry. Nature of metal strongly affects the structure of metal particles and ultrasound-induced modification process. Al and Mg demonstrate decomposition of particles and formation of channel-like structures (Fig. 2b, Fig. S2 (ESIw)). The TEM image (Fig. 2e, the insert) demonstrates 200 nm thick lamellas forming the channels ca. 50 nm in width. The electron microdiffraction measurements (Fig. S3, ESIw) provide evidence that lamellas are built from highly ordered aluminium crystallites. XRD diffraction patterns agree with observed preferred ordering in the Al samples. After 5 min of sonication formation of a porous inner structure became noticeable on the electron micrographs (Fig. 2, Fig. S4 (ESIw)). Development of the inner structure is clearly demonstrated by the dependence of surface area vs. sonication time (Fig. 2, Fig. S4 (ESIw)). The surface area for 60 min-sonicated Al particles reaches 80 m2 g 1 (Fig. 2g, Fig. S4 (ESIw)). Further increase of This journal is
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irradiation time leads to regression of structure development due to complete transformation of metal into metal oxides (after 2 h treatment (Fig. 2h)). The average pore diameter for Al particles after 60 min of sonication is ca. 4 nm (Fig. S5, ESIw). Additionally, the XRD patterns of the aluminium particles after sonication (Fig. 2e) reveals some intensity changes of the signals. Intensity of the signals is decreased and slightly shifted due to grain destruction under ultrasound irradiation. The most stable grain corresponds to (111) planes, the maximum density plane for a cubic face centered structure.18 The observed changes are probably caused by the effect of ultrasound on grain formation and orientation. Simultaneously zinc particles have the lowest melting point among the used metals. Ultrasound treatment of Zn particles results in complete conversion of Zn in Zn oxide—zincite. The ultrasound-stimulated formation of zinc oxide on the surface of zinc particles occurs already after short-term sonication yielding 2 mm zinc oxide crystals of tubular morphology (Fig. S6, ESIw). Probably due to their brittleness they are decomposed by further sonication. Long-term treatment (>30 min) stimulates formation of stable nanorods with length E 100 nm and diameter approximately 20 nm (Fig. S3f, ESIw). This structure is stable and is able to resist the impact of ultrasound cavitation. XRD patterns reveal presence of zinc oxide (zincite) after 90 min of sonication and, therefore, complete conversion of Zn to ZnO. The particles of Ni and Ti (the highest melting points) demonstrate increasing of surface roughness after 60 min of sonication (Fig. 1g and h). XRD and SEM detected formation of the developed oxide layer on the particle surface. The BET and TEM data have no evidence of formation of developed inner structure for both the metals. The fact that different metals react in dramatically different fashion towards ultrasound irradiation can be exploited for the formation of nanoscale composites. Thus, if alloy particles consisting of a resistant-to-ultrasound metal and a sensitive one are treated, one would consequently expect that novel nanocomposite structures are developed. We tested this idea for the Al/Ni (Fig. 3(a)), Pd/Al (Fig. 3(c)) and Cu/Pd/Al (Fig. 3(d)) systems. Indeed the treatment of the aqueous alloy dispersions with highly-intense ultrasound leads not only to the formation of porous structure but also results in phase segregation yielding ultrasound-resistant nickel, palladium and copper nanoparticles homogeneously distributed in the porous matrix as evidenced by the TEM micrographs given in Fig. 3. Simultaneously BET analysis even shows the more porous structure formation in the case of nanocomposites (the surface area in this case achieved 280 m2 g 1) for the Pd/Al particles and 210 m2 g 1 for those of Cu/Pd/Al after 60 min of modification. Thus, phase segregation process in multi-metal systems stimulates formation of mesoporous structures by sonication. In the long run this principle is universal and opens perspective for a whole new class of composite-mesoporous
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materials that can be prepared in a one-step process. We performed a standard hydrogenation of acetophenone catalyzed by Ni supported by mesoporous Al in order to prove this principle and the efficiency of ultrasound in the formation of heterogenic catalysts. It is known that the preparation method and structures of support exert an important influence on the catalytic activity of Ni.19 The system prepared by 60 min of sonication (the highest porosity) exhibits >99% conversion in aqueous solution (Fig. 3(b), Fig. S7 (ESIw)) and a very high selectivity in hydrogenation of ketones due to a very narrow pore distribution in mesoporous metal system formed by ultrasound. Furthermore, the catalyst prepared by sonication exhibits stability unusual for RANEYs catalyst and does not require an oxygen-free storage conditions. The proposed ‘‘green chemistry’’ method of ultrasoundinduced generation of metal nanocomposites is applicable on a range of materials and provides the basis for applications in nanocatalysis, nanosensors, and multifunctional delivery systems. We thank Mrs Brenda Ip (Free University Berlin) for help with the solid state NMR experiments and Prof. Rhett Kempe (University of Bayreuth) for the discussion on catalytic activity. We also thank Jana Scha¨ferhans, Dr Dmitry Shchukin and Prof. Dmitry Sviridov for helpful suggestions. E. S. thanks Alexander von Humboldt Foundation. The present research was supported by SFB840.
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