pubs.acs.org/Langmuir © 2010 American Chemical Society
Platinum-Centered Yolk-Shell Nanostructure Formation by Sacrificial Nickel Spacers† Ji Chan Park,‡ Jae Young Kim,‡ Eunjung Heo,§ Kang Hyun Park,*,§ and Hyunjoon Song*,‡ ‡
Department of Chemistry, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea, and § Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Busan, Korea Received March 29, 2010. Revised Manuscript Received May 4, 2010
We have synthesized Pt@silica/nickel phyllosilicate and Pt@silica yolk-shell nanostructures from NiPt@silica core-shell particles by simple chemical treatments. Silica coating of the NiPt alloy nanoparticles via the microemulsion method yielded spherical NiPt@silica core-shell nanoparticles with an average core diameter of 6.5 nm. Under a reflux condition in water, the core-shell structure transformed into Pt@silica yolk-shell nanoparticles with branched nickel phyllosilicate, which exhibited high surface area and large pore volume. The addition of hydrochloric acid selectively etched the nickel component from the NiPt cores and yielded Pt@silica yolk-shell nanoparticles with single-crystalline platinum cores. The average diameter of the metal cores was reduced to 4.5 nm. In both cases, the nickel components behaved as sacrificial spacers and successfully formed a vacancy between the metal cores and the silica hollow shells.
Introduction Hollow nanostructures bearing metal cores, so-called “nanorattles” or yolk-shell nanoparticles, have recently attracted much attention among complex hierarchical nanostructures.1,2 Although surroundings of the metal cores are confined by the hollow shells, the surface of the metal cores is fully accessible by diffusion through pores of the shell layers. Because of ready adjustability both in the metal cores and in the hollow shells, the yolk-shell nanoparticles have potential applications as to be nanoreactors and catalysts, drug delivery carriers, and surface-enhanced Raman scattering substrates.3-5 Alongside numerous synthetic methods of hollow nanostructures, the synthesis of yolk-shell nanoparticles has been developed in many ways.6 The methods can be categorized by two parts: manipulation of shell layers and metal cores. The former is based on either chemical conversion or etching of the shells without altering the core morphology.7,8 Slow oxidation of the outer metal layers could generate metal@metal oxide yolk-shell structures through the nanoscale Kirkendall effect.9 Galvanic replacement of the silver layers in metal@silver core-shell nanoparticles also yielded metal@ metal yolk-shell nanoparticles.10 In the case of silica shells, metal@ silica hollow shell structures were produced by a spontaneous dissolution-regrowth of silica in the presence of NaBH4.11 Silica could also act as a sacrificial template for providing hollow shells † Part of the Molecular Surface Chemistry and Its Applications special issue. *To whom correspondence should be addressed. E-mail: chemistry@ pusan.ac.kr (K.H.P.);
[email protected] (H.S.).
(1) Liu, J.; Liu, F.; Gao, K.; Wu, J.; Xue, D. J. Mater. Chem. 2009, 19, 6073. (2) Wu, X.-J.; Xu, D. J. Am. Chem. Soc. 2009, 131, 2774. (3) Lee, J.; Park, J. C.; Bang, J. U.; Song, H. Chem. Mater. 2008, 20, 5839. (4) Gao, J.; Liang, G.; Zhang, B.; Kuang, Y.; Zhang, X.; Xu, B. J. Am. Chem. Soc. 2007, 129, 1428. (5) Khalavka, Y.; Becker, J.; S€onnichsen, C. J. Am. Chem. Soc. 2009, 131, 1871. (6) Liu, S.; Han, M.-Y. Chem. Asian J. 2010, 5, 36. (7) Yin, Y.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorjai, G. A.; Alivisatos, A. P. Science 2004, 304, 711. (8) Zhang, Q.; Zhang, T.; Ge, J.; Yin, Y. Nano Lett. 2008, 8, 2867. (9) Tu, K. N.; G€osele, U. Appl. Phys. Lett. 2005, 86, 093111. (10) Sun, Y.; Wiley, B.; Li, Z.-Y.; Xia, Y. J. Am. Chem. Soc. 2004, 126, 9399. (11) Zhang, T.; Ge, J.; Hu, Y.; Zhang, Q.; Aloni, S.; Yin, Y. Angew. Chem., Int. Ed. 2008, 47, 5806.
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with other components. Removal of the interfacial silica layers in multiple core-shell structures provided metal cores within the hollow shells of dense silica, polymer, zirconia, and carbon.12 The latter is adjusting metal cores within the hollow shells. Metal salt reduction in the presence of silica hollow particles formed spherical metal cores inside. Selective etching of the metal cores in metal@ silica core-shell nanoparticles was established well in various metal and metal oxides. Gold cores were selectively dissolved by KCN, and metal oxides including NiO, CoO, and Fe2O3 were partially etched by hydrochloric acid without damaging silica shells.13,14 Subsequent reduction of the particles yielded corresponding metal@silica yolk-shell nanostructures. This etching process was applicable in a wide range of the metal components and was scaled up to more than grams. However, a significant loss of the metal cores was problematic particularly in expensive noble metals, and the metal core size could not be precisely controlled because of harsh etching conditions. In the present study, we report a new approach to synthesize welldefined metal-centered yolk-shell nanostructures by using nickel as a sacrificial spacer (Scheme 1). Bimetallic NiPt@silica core-shell nanoparticles were converted to two distinct yolk-shell structures, Pt@silica/nickel phyllosilicates, and Pt@silica, by simple chemical treatments. The nickel components involved in the original NiPt cores reacted with silica to form branched nickel phyllosilicates under a basic reaction condition. On the other hand, nickel was readily dissolved from the NiPt cores by acids to generate vacancy between platinum cores and silica shells. Because of its affordable price, nickel is potentially useful for providing various nanostructures with a combination of other elements including expensive noble metals.
Experimental Section Chemicals. Nickel(II) acetylacetonate (Ni(acac)2, 95%), platinum(II) acetylacetonate (Pt(acac)2, 97%), oleic acid (90%), oleylamine (70%), tetramethyl orthosilicate (TMOS, 98%), igepal (12) Arnal., P. M.; Comotti, M.; Sch€uth, F. Angew. Chem., Int. Ed. 2006, 45, 8224. (13) Lee, J.; Park, J. C.; Song, H. Adv. Mater. 2008, 20, 1523. (14) Park, J. C.; Bang, J. U.; Lee, J.; Ko, C. H.; Song, H. J. Mater. Chem. 2010, 20, 1239.
Published on Web 05/19/2010
DOI: 10.1021/la101248g
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Scheme 1. Synthesis of Pt@silica/Nickel Phyllosilicate and Pt@ silica Yolk-Shell Nanostructures from NiPt@silica Core-Shell Nanoparticles
CO-630, and benzyl ether (99%) were purchased from Aldrich. Ammonium hydroxide (NH4OH, 28% in water), hydrochloric acid (HCl, 35% in water), and cyclohexane (99.5%) were purchased from Junsei. The chemicals were used as received without further purification. Synthesis of NiPt Alloy Nanoparticles. Pt(acac)2 (0.20 g, 0.50 mmol) and Ni(acac)2 (0.64 g, 2.5 mmol) were mixed with oleylamine (2.0 mL), oleic acid (2.0 mL), and benzyl ether (20 mL). The mixture was slowly heated from room temperature to 270 °C for 20 min under an inert atmosphere and was allowed to stir at the same temperature for additional 40 min. After cooling the reaction mixture, the colloidal particles were separated by adding ethanol (40 mL) and centrifuging at 10 000 rpm for 20 min. Finally, the NiPt particles were dispersed in cyclohexane. Silica Coating of NiPt Alloy Nanoparticles. Cyclohexane (25 mL) was mixed with igepal CO-630 (8.0 mL) and NH4OH solution (1.0 mL), and the mixture was stirred for 10 min. A NiPt particle dispersion in cyclohexane (25 mL, 10 mM with respect to the precursor concentration) was added into the mixture. After 30 s, TMOS (1.0 mL) was injected, and the resulting mixture was stirred for 60 min at room temperature. The NiPt@SiO2 particles were precipitated by adding methanol (10 mL) and were purified by a repetitive dispersion/precipitation cycle in ethanol.
Synthesis of Pt@silica/Nickel Phyllosilicate Yolk-Shell Nanoparticles. A freshly prepared NiPt@SiO2 dispersion in deionized water (40 mL, 6.3 mM with respect to the precursor concentration) was refluxed in air for 2 h. After cooling the reaction mixture, the product was obtained by centrifuging at 10 000 rpm for 20 min and was thoroughly washed with ethanol. Synthesis of Pt@silica Yolk-Shell Nanoparticles. A mixture of the NiPt@SiO2 particle dispersion in ethanol (30 mL, 8.3 mM with respect to the precursor concentration) and hydrochloric acid (40 mL, 5.7 M) was refluxed in air for 12 h. After cooling the reaction mixture, the product was obtained by centrifuging at 10 000 rpm for 20 min and was thoroughly washed with ethanol. Characterization. The products were characterized by using Philips F20 Tecnai (200 kV) and Tecnai G2 F30 (300 kV) transmission electron microscopes at KAIST. Samples were prepared by putting a few drops of the corresponding colloidal solutions on carbon coated copper grids (Ted Pella, Inc.). X-ray powder diffraction (XRD) patterns were recorded on a Rigaku D/MAX-RB(12 kW) diffractometer. Nitrogen sorption isotherms were measured at 77 K in a BELSORP mini-II (BEL Japan Inc.) instrument. Before the measurements, the samples were degassed in a vacuum at 423 K for 6 h.
Results and Discussion The NiPt alloy nanoparticles were synthesized by thermal decomposition of the metal precursors in the presence of oleic acid and oleylamine as cosurfactants.15 The metal precursors, (15) Li, Y.; Zhang, X. L.; Qiu, R.; Qiao, R.; Kang, Y. S. J. Phys. Chem. C 2007, 111, 10747.
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nickel(II) acetylacetonate and platinum(II) acetylacetonate, were dissolved in benzyl ether in a mole fraction of 5:1 (Ni/Pt) with the surfactants. The transmission electron microscopy (TEM) image in Figure 1a shows monodisperse particles with an average diameter of 6.5 ( 0.6 nm. The high resolution TEM (HRTEM) image of a single nanoparticle represents the polycrystalline nature of the particles with a 5-fold symmetry, indicating a shape of truncated decahedral. The distance between adjacent lattice fringe images was measured to be 0.217 nm, which matches with that of (111) planes in face-centered cubic NiPt. The electron diffraction pattern has a characteristic set of spots with a 5-fold symmetry along the zone axis of [110] (Figure 1c), which is nearly identical to the pattern from a gold decahedron. The X-ray diffraction (XRD) spectrum in Figure 1d shows three broad signals that are comparable to (111), (200), and (220) peaks of the NixPt(1-x) nanoparticles.15 The average particle size was calculated to be 3.9 nm using the Debye-Scherrer equation based on the full-width at half-maximum of the (111) peak, and the small diameter indicated a single-crystalline domain size of the polycrystalline nanoparticles. The energy dispersive X-ray (EDX) spectrum reveals a relative atomic ratio of 2.7 Ni over Pt. The NiPt alloy nanoparticles were coated with silica using the microemulsion method.16 The particle dispersion in cyclohexane was mixed with igepal CO-630 and ammonia. It is known that ammonia catalyzes decomposition of the silica precursor and then accelerates silica polymerization. TMOS was added, and the mixture was stirred for 1 h. The NiPt@SiO2 core-shell nanoparticles were readily precipitated by adding methanol. Figure 2a,b shows that the entire particles are spherical with uniform silica layers. The average diameter of the NiPt@SiO2 spheres was measured to be 16.7 ( 1.3 nm with the shell thickness of 5.3 ( 0.5 nm (Figure S1 of the Supporting Information). The XRD spectrum includes a broad signal centered at θ = 23° for amorphous silica, as well as three characteristic peaks for face-centered cubic NiPt (Figure 2c). Deposition of Ni2þ onto the silica surface yields nickel silicate hydroxide or nickel phyllosilicate as a layered structure.17 Under a basic condition, Ni2þ generates nickel hydroxide species, which react with silicic acid and polymerize to form the layered nickel phyllosilicate phase.18 Chen et al. applied this reaction to bare silica spheres and produced nickel phyllosilicate hollow spheres by the dissolution of unreacted silica cores.19 We also reported that nickel nanoparticles supplied nickel sources to yield nickel phyllosilicate branches on the silica nanospheres.20 In order to check reactivity of the NiPt cores, the NiPt@SiO2 core-shell nanoparticles were dispersed in water and were refluxed for 2 h. It was interesting that the reaction condition was weakly basic at pH=9.8, presumably due to residual ammonia on the silica shells. The resulting particles had metal cores and silica hollow shells with thin branches, as seen in Figure 3a. The expanded image in Figure 3b shows that the small gaps are observed between the metal particles and the silica layers. The average core size was measured to be 4.5 ( 0.5 nm, which was smaller than that (6.5 nm) of the original NiPt cores. The total particle diameter was estimated to be 17.7 ( 1.2 nm (Figure S1b of the Supporting Information). The EDX analysis of the nanoparticles showed the atomic ratio of 2.7 Ni over Pt, which was identical to the original ratio of the NiPt alloy cores. It indicated (16) Chang, C.-L.; Fogler, H. S. Langmuir 1997, 13, 3295. (17) Burattin, P.; Che, M.; Louis, C. J. Phys. Chem. B 1998, 102, 2722. (18) Beverskog, B.; Puigdomenech, I. Corros. Sci. 1997, 39, 969. (19) Jin, P.; Chen, Q.; Hao, L.; Tian, R.; Zhang, L.; Wang, L. J. Phys. Chem. B 2004, 108, 6311. (20) Park, J. C.; Lee, H. J.; Bang, J. U.; Park, K. H.; Song, H. Chem. Commun. 2009, 7345.
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Figure 1. (a) TEM and (b) HRTEM images, and (c) electron diffraction and (d) XRD patterns of NiPt alloy nanoparticles. Bars represent (a) 20 nm and (b) 2 nm.
Figure 2. (a,b) TEM images and (c) XRD spectrum of NiPt@SiO2 core-shell nanoparticles. Bars represent (a) 50 nm and (b) 20 nm.
that there was no external release during the phase conversion and all components were still retained in each nanoparticle. Several peaks appear in the XRD spectrum (Figure 3c), where broad peaks at θ = 23° and 61° correspond to (004) and (060) planes of pecoraite of Ni3Si2O5(OH)4 nickel phyllosilicate (JCPDS no. 491859; space group, c2/m), respectively. Three characteristic peaks Langmuir 2010, 26(21), 16469–16473
at θ = 40°, 47°, and 68° were assigned to face-centerd cubic Pt (JCPDS no. 01-087-0647; space group, Fm3m), as well as an amorphous silica peak at θ=23°. These results indicated that the particles were converted to the Pt@silica/nickel phyllosilicate yolk-shell nanostructure. The N2 sorption experiment at 77 K shows a type IV isotherm with type H3 hysteresis according to the DOI: 10.1021/la101248g
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Figure 3. (a,b) TEM images, (c) XRD spectrum, and (d) N2 sorption isotherm of Pt@silica/nickel phyllosilicate yolk-shell nanoparticles. Bars represent (a) 50 nm and (b) 20 nm.
Figure 4. (a,b) TEM and (c) HRTEM images, and (d) XRD spectrum of Pt@SiO2 yolk-shell nanoparticles. Bars represent (a) 50 nm, (b) 20 nm, and (c) 2 nm.
IUPAC nomenclature (Figure 3d). The Brunauer-Emmett-Teller (BET) surface area and total pore volume were calculated to be 286 m2/g and 0.73 cm3/g, respectively, which largely increased from the values of 103 m2/g and 0.41 cm3/g in the NiPt@SiO2 16472 DOI: 10.1021/la101248g
core-shell nanoparticles. The high surface area and total pore volume of the Pt@silica/nickel phyllosilicate were basically attributed to the branches with the intrinsic layered structure of nickel phyllosilicate. Langmuir 2010, 26(21), 16469–16473
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By the addition of hydrochloric acid, the nickel components in the NiPt@SiO2 core-shell nanoparticles were completely etched under a prolonged reflux condition. Hydrochloric acid could penetrate grain boundaries or less polymerized sites on the amorphous silica layers to dissolve the metal cores.13 Dissolution of the nickel components was easily checked by the green color that appeared in the reaction mixture, which was due to the formation of hydrated nickel(II) chloride. Figure 4a shows that the spherical silica shells are preserved, whereas the metal cores are reduced to form a yolk-shell type structure. The expanded image in Figure 4b exhibits the formation of spherical voids around the spherical metal cores. The average size of the metal cores was estimated to 4.5 ( 0.3 nm with the average silica shell thickness of 5.6 ( 0.6 nm. It is remarkable that the inner cores are platinum single crystals, as shown in the HRTEM image of Figure 4c. The lattice fringe images of a single metal core are continuous over all nanoparticle projections with the distance between neighboring fringes of 0.194 nm, which matches with that of (200) in the face-centered cubic Pt phase. Note that the distance of the neighboring fringe images in the NiPt phase is 0.217 nm (Figure 1b), that confirms the complete metal core conversion from NiPt to pure Pt by the acid treatment. The XRD spectrum of the Pt@SiO2 yolk-shell nanoparticles in Figure 4d corresponds to the reflections of face-centered cubic Pt (JCPDS no. 01-087-0647; space group, Fm3m). The most intense peak at θ = 40.2° shifts to a lower angle than that at 41.7° of the original NiPt cores, indicating an increase of the lattice constant by removal of small Ni atoms (atomic radius: 124 pm) to form a pure Pt phase (atomic radius: 139 pm). After dissolution of the Ni components, the Pt atoms were recrystallized to become the most
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stable face-centered cubic phase with spherical morphology even at room temperature.
Conclusion We have fabricated two distinct yolk-shell nanostructures from the NiPt@SiO2 core-shell nanoparticles by simple chemical treatments. The labile nickel components behaved as sacrificial spacers that generated vacancy between Pt cores and silica hollow shells. The NiPt@SiO2 core-shell nanoparticles transformed to the Pt@silica yolk-shell nanostructure with branches of nickel phyllosilicate under a basic condition. Hydrochloric acid selectively dissolved the nickel components and formed the Pt@silica yolk-shell nanospheres. Because of its high reactivity and affordable price, nickel can act as a versatile sacrificial component for the formation of complex hierarchical structures with expensive noble metals, such as Rh, Ir, Pd, Pt, and Au. Acknowledgment. This work is dedicated to Prof. Gabor A. Somorjai on the occasion of his 75th birthday, and it was supported by the National Research Foundation (NRF) grants funded by the Ministry of Education, Science and Technology (MEST) through the Active Polymer Center for Pattern Integration (No. R11-2007-050-00000-0) and the Basic Science Research Program (2009-0070926). Supporting Information Available: Low-resolution TEM images of the NiPt@silica core-shell and Pt@silica/nickel phyllosilicate and Pt@silica yolk-shell nanoparticles. This material is available free of charge via the Internet at http:// pubs.acs.org.
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