Single-Crystal Metal Nanoplatelets: Cobalt, Nickel, Copper, and Silver

Jul 27, 2007 - Single-Crystal Metal Nanoplatelets: Cobalt, Nickel, Copper, and Silver. Run Xu,Ting Xie,Yonggang Zhao, andYadong Li*. Department of Che...
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Single-Crystal Metal Nanoplatelets: Cobalt, Nickel, Copper, and Silver Run

Xu,†

Ting

Xie,†

Yonggang

Zhao,‡

and Yadong

Li*,†

Department of Chemistry and Department of Physics, Tsinghua UniVersity, Beijing, 100084, P. R. China

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 9 1904-1911

ReceiVed September 8, 2006; ReVised Manuscript ReceiVed June 7, 2007

ABSTRACT: In this paper, single-crystal metallic nanoplatelets, such as cobalt, nickel, copper, and silver, have been successfully synthesized based on a facile hydrothermal/solvothermal synthetic method. The as-prepared cobalt nanoplatelets had the {001} crystal facets as the basal plane, while the exposed nanoplatelet planes were assigned to be {111} crystal facets of the face-centered cubic nickel, copper, and silver crystals. Owing to the interesting combination of novel nanostructures, remarkable magnetic anisotropy was found on the cobalt and nickel nanoplatelets. The anisotropic copper and silver nanoplatelets exhibited a distinct surface plasmon resonance effect. These metallic nanoplatelets could be expected to bring new opportunities in the vast research areas of and application in magnetic storage devices, catalysts, and surface-enhanced Raman scattering. 1. Introduction Controlling the shape of metallic nanostructures has been a subject of intensive research in recent years because it provides an effective strategy for tuning the electronic, magnetic, optical, and catalytic properties of a metal.1-3 For example, the sensitivity of surface-enhanced Raman scattering has been shown to depend on the exact morphology of a silver nanoparticle.4-6 El-Sayed and co-workers demonstrated a beautiful example of controlling the shape and the size of platinum nanoparticles and also reported the strong relationship between their shape and their catalytic properties.7,8 It is wellknown that the crystal plane of catalyst nanoparticles plays an essential role in determining their catalytic properties and that the synthesis of nanoparticles with well-controlled shape and a well-defined reactive crystal plane could be critical for their applications.9 For instance, hydrogen tends to be absorbed on cubic platinum nanoparticles, the surface of which are bounded by {100} facets, while buckyball-shaped platinum nanoparticles bounded by {210} faces prefer to be covered by carbon monoxide.10,11 Therefore, it could be critical to develop an effective preparation method for particles with well-controlled shapes and sizes. The primary target of this work is to fabricate metal nanoplatelets with well-defined crystal planes via a lowtemperature chemical process. The two-dimensional (2D) structures such as nanoplates, nanoprisms, nanodisks, and nanorings are believed to have a marvelous ability to control physical and chemical properties because of their high aspect ratio (the edge length over thickness).12 The replacement of equiaxed-shaped metal particles with metal platelets offers the opportunity to form thinner layers due to the efficiency of the surface coverage/unit volume of the anisotropically shaped platelets relative to spherical particles. Moreover, the ability to control nanocrystal shape and produce anisotropic structures such as platelets provides an opportunity to test the coupling of shape anisotropy with quantum confinement effects.13 Cobalt and nickel nanoparticles have attracted special attention because of their high saturation magnetization and high * To whom tsinghua.edu.cn. † Department ‡ Department

correspondence should be addressed. E-mail: ydli@ Phone: 86-10-62772350. Fax: 86-10-62788765. of Chemistry. of Physics.

chemical activity.14,15 Many cobalt and nickel nanocrystals with different morphologies have been synthesized.16-22 In most cases, the magnetic nanocrystals are superparamagnetic at room temperature because of their small dimensions. Thus, their applications for magnetic recording are limited. Controlling the shape of nanoparticles is an effective method to increase the magnetic anisotropy. Attempts have been made to improve the magnetic properties of nanocrystals by preparing highly anisotropic 1D nanostructured materials, such as rods23,24 and wires.25 Enhanced magnetic properties are also expected in 2D nanostructures. However, in this area it is poorly reported with the exception of some work on cobalt nanodisks26,27 and nickel nanosheets.28,29 The thermal decomposition of organometallic precursors in solvent with surfactant is widely used to fabricate cobalt and nickel nanodisks and nanosheets. Shape-controlled synthesis of copper and silver nanoparticles has stimulated intensive research in recent years because of their interesting electrical, catalytic, and optical properties.30,31 Especially, the shape variety of silver leads to different surface plasma resonance (SPR) effects and has great effect on surfaceenhanced Raman scattering (SERS).32 Since Jin et al.33 developed the photoinduced method for converting large quantities of silver nanospheres into nanoprisms, many methods have been reported for the synthesis of anisotropic silver nanoparticles. However, to the best of our knowledge, there have been only a few reported on the synthesis of copper nanoplatelets.34 In the present study, we report a facile hydrothermal synthetic method for one-step preparation of single-crystal cobalt nanoplatelets. This synthetic strategy can be applied in fabricating other metal nanoplatelets, such as nickel and copper. Furthermore, single-crystal silver nanoplatelets can be successfully obtained via a solvothermal method using N, N-dimethylformamide as reducing agent and solvent. The magnetic and optical properties of these metallic nanoplatelets are also investigated. 2. Experimental Section In a typical synthesis of metal nanoplatelets, an aqueous solution of 30 mL was first prepared by dissolving metal salt (50 mM CoCl2, NiCl2, or CuCl2), sodium dodecylsulfate (SDS, 15 mM), and NaOH (3 M) in distilled water. After addition of NaH2PO2‚H2O (0.4 M), the solution was vigorously stirred and then transferred into a Teflon cup in a stainless steel-lined autoclave. The autoclave was maintained at 110 °C for 24 h for Co nanoplatelets, 110 °C 14 h for Ni nanoplatelets,

10.1021/cg060593a CCC: $37.00 © 2007 American Chemical Society Published on Web 07/27/2007

Single-Crystal Metal Nanoplatelets and 100 °C 12 h for Cu nanoplatelets and then was allowed to cool to room temperature. A black fluffy solid (red solid for copper) product was deposited on the bottom of the Teflon cup, indicating the formation of metallic products. The final product was centrifuged, rinsed with distilled water and ethanol several times to remove any alkaline salt and surfactants that remained in the final products, and then dried in a vacuum oven at 60 °C for 4 h. In typical synthesis of silver nanoplates, 20 mL of N,N-dimethyl formamide (DMF) solution of 50 mM AgNO3 was added dropwise into 20 mL of DMF solution of 50 mM poly(vinyl pyrrolidone) (PVP), then transferred into a 50-mL autoclave and heated at 140 °C under autogenetic pressure for 8 h. The final sample was obtained by centrifugation and washed with acetone and water. X-ray diffraction (XRD) testing was performed with a Bruker D8 Advance X-ray diffractometer with monochromatized Cu KR radiation (λ ) 1.5418 Å). For electron microscopy, a few drops of the solution were diluted into 1 mL of ethanol, and the resulting ethanol solution was placed onto a carbon-coated copper grid and allowed to evaporate. Micrographs were recorded using a HITACHI H-1200 transmission electron microscopy (TEM), and high-resolution TEM (HRTEM) images were obtained with a HITACHI H-2010F electron microscope. The UV-vis spectra were obtained using UV-3101 by Hitachi U-3010 spectrophotometer. Magnetic studies were carried out using MPMS XL-7 Quantum Design superconducting quantum interference device magnetometer (SQUID). A small amount of Co nanoplatelets was dispersed in ethanol by ultrasonic treatment for about 10 min. And then one drop of the colloidal solutions was placed onto a Si wafer and dried in oven at 60 °C for 10 min. In order to characterize the anisotropic magnetic properties of the cobalt nanoplatelets, the zero-field cooled/field cooled magnetization curve were recorded with magnetic fields applied parallel or perpendicular to the cobalt nanoplatelets. The applied magnetic field strength is 50 Oe.

3. Results and Discussions 3.1. Controlled Synthesis and Formation Mechanism of Cobalt Nanoplatelets. The morphology of the products was characterized by transmission electron microscopy (TEM), selected-area electron diffraction (SAED), and high-resolution transmission electron microscopy (HRTEM). Figure 1a shows a typical TEM pattern of nanometer-sized Co hexagonal platelets obtained by heating at 110 °C for 24 h using NaH2PO2 as a reducing agent. The edge length of the cobalt nanoplatelets is in the range of 100-120 nm. Because of the strong magnetic interaction between different nanoplatelets and their very thin nature, the nanoplatelets tend to overlap. The corresponding SAED pattern is shown in Figure 1b. The SAED pattern indicates that the Co nanoplatelets are single crystalline and can be indexed as the hexagonal structure with the incident electron beam parallel to the [001] orientation. The HRTEM image in Figure 1c indicates a highly crystalline character with a lattice spacing of 2.2 Å, which can be indexed to the (100) plane of hexagonal close-packed (hcp) Co. Therefore, the predominantly exposed planes of the Co nanoplatelets are the (001). The side-view image (Figure 1d) indicates that the thickness of the plates is about 8 nm. Electron dispersive spectrum (EDS) of the product (Figure 1e) shows that the sample is essentially pure cobalt. Only a small amount of oxygen is present, suggesting that the surface of the Co nanoplatelets are partly oxygenized. The crystal structure of the product was further confirmed by X-ray diffraction (XRD). As shown in Figure 1f, the recorded diffraction peaks are well-assigned to the structure of Co with hexagonal phase, indicating the formation of hcp Co metals (space group P63/mmc; a ) 2.5031 Å, c ) 4.0605 Å; JCPDS No. 5-727). Broadening of the peaks exhibits the nanocrystalline nature of the sample. No CoO or Co3O4 peak is observed. To investigate the influence of the surfactant, sodium dodecylsulfate (SDS) was substituted with sodium oleate (SO),

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cetyltrimethylammonium bromide (CTAB) and poly(vinyl pyrrolidone) (PVP). When SO was used as surfactant, Co hexagonal platelets also could be obtained, and the size of products was about 200 nm. However, the other substitutions resulted in the formation of Co nanoparticles with different shapes. The product was dominated by nanoparticles with irregular shapes when CTAB was used as surfactant, while semi-plates with irregular morphologies were synthesized when PVP was used as surfactant. The result indicates that the type of surfactant is a key to achieve nanoplatelets. The anionic surfactants, such as SDS and SO, favor the formation of platelets, probably because of interaction between the anion and the cobalt ion or because of the anion-selective adsorption on the particle surface. The cationic and nonionic surfactants, such as CTAB and PVP, cannot produce regular shapes because of their weak interaction with cobalt ion. In the present work, the concentration of NaOH played an important role in the formation of Co nanoplatelets. A possible reduction reaction during the hydrothermal process can be formulated as

Co2+ + H2PO2- + 3OH- f Co + HPO32- + 2H2O (1) In the absence of NaOH, metallic Co cannot be produced. When the concentration of NaOH was increased in the range of 1-5 M, the size of product increased (Figure 2a). Furthermore, when the concentration of NaOH was increased to 7 M, mesoscale sheets can be obtained (Figure 2b). This result is reasonable because, when the amount of NaOH is increased appropriately, the reaction velocity of eq 1 is increased and the formation of cobalt nanoplatelets is enhanced. In order to investigate the formation mechanism of the Co nanoplatelets, the synthesis was stopped at different stages during the synthesis process. When the synthesis had proceeded for 6 h, a red-pink precipitate product was obtained on the bottom of the Teflon cup. As shown in Figure 3a, the products are hexagonal nanoplatelets with a size of about 100-150 nm. The XRD pattern of the product exhibits a predominant wellcrystalline b-cobalt hydroxide phase (JCPDS No. 30-443) (Figure 3b). Co(OH)2 prefers to grow into platelets due to its intrinsic lamellar structures.35 When the synthesis had proceeded for 12 h, the product was composed of Co(OH)2 and metallic Co. When the reaction time exceeds 18 h, pure metallic Co was obtained. This result indicates that the Co(OH)2 nanoplatelets are the predecessors of the Co nanoplatelets. By keeping the concentration of NaOH at 7 M and the other reaction conditions constant, one obtains a product composed of irregularly shaped particles and platelets with various sizes in the absence of SDS. This demonstrates that SDS and the concentration of alkali are important parameters to promote the formation of Co nanoplatelets. As a result, one possible formation mechanism of Co nanoplatelets is presented in Scheme 1. First, the cobalt hydroxide colloid precipitate was produced when the NaOH was added into the CoCl2 solution. Under the hydrothermal conditions, Co(OH)2 hexagonal nanoplatelets were produced at initial stages of the synthesis process. And then the Co(OH)2 reacted with H2PO2- to form metallic Co in the presence of SDS. The product sustains platelet structure during the reducing process. The present mechanism is different from some reported formation mechanisms of metallic nanocrystals, which generally conclude the fabrication of seeds, surfactant selective adsorption on various crystal planes, and the anisotropic growth of seeds crystals. In our process, the morphology of product is inherited from the hydroxide precursor. This implies that the formation

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Figure 1. (a) TEM image, (b) SAED pattern, (c) HRTEM image, (d) the side view, (e) EDS patterns, and (f) the XRD patterns of the cobalt nanoplatelets.

mechanism could be employed to synthesize other kinds of metal nanoplatelets, which could form the platelet structure hydroxide precursors. 3.2. Characterization of Nickel and Copper Nanoplatelets. In our previous study, the Ni(OH)2 and CuO nanoplatelets could be synthesized by a hydrothermal method.36,37 In terms of the formation mechanism of Co nanoplatelets, we tried to fabricate Ni and Cu nanoplatelets via the hydrothermal redox reaction. Figure 4a shows a typical TEM pattern of nanometer-sized Ni hexagonal platelets obtained by heating at 110 °C for 14 h using NaH2PO2 as a reducing agent. The SAED of one nanoplatelet revealed that they are single crystal with a {111} crystal plane as the basal plane (the inset of Figure 4a). The XRD pattern

depicted in Figure 4c shows that the Ni nanoplatelets possess a face-centered cubic (fcc) structure, in accordance with reported data (JCPDS No. 4-485). A typical TEM image of the Cu nanoplatelets shows that the as-obtained products are more like belts (Figure 4b). They are about 100 nm in width and a few hundreds of nanometers in length. The inset shows a SAED pattern from the single Cu nanoplatelet. The set of spots with the strongest intensity could be indexed to (220) reflections, which indicates that the asprepared nanoplates are single crystals with a {111} lattice plane as the basal plane. The XRD measurement has proven the successful synthesis of fcc structure Cu nanoplatelets (Figure 4d). The products are very active. Slow oxidation of Cu

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Figure 2. TEM images of the cobalt nanoplatelets: (a) [NaOH] ) 5 M, (b) [NaOH] ) 7 M.

nanoplatelets occurred when they were separated from solution. After 24 h, the red product becomes black indicating that the Cu was oxidized to CuO. The formation mechanisms of the Ni and Cu nanoplatelets are similar to that of Co nanoplatelets. When the synthesis of Ni nanoplatelets had proceeded for 3 h, a green precipitate product was obtained. The XRD pattern of the product exhibits a predominant well-crystalline nickel hydroxide phase (Figure 4c). The products are hexagonal nanoplatelets with a size of about 100-150 nm. In the same way, when the synthesis process of Cu was stopped at 3 h, a black suspension was obtained. The product is a single phase of well-crystallized CuO with the monoclinic structure (Figure 4d). TEM image reveals that they are belt-like. These results indicate that the hydroxide and oxide nanoplatelets are the precursors of the Ni and Cu nanoplatelets. That greatly supports our conclusion about the formation mechanism of metallic nanoplatelets. 3.3. Characterization of Silver Nanoplatelets. We attempted to utilize this method to prepare Ag nanoplatelets. But Ag nanoplatelets cannot be synthesized via a similar hydrothermal redox reaction. When the NaH2PO2 is introduced, the reducing reaction occurs immediately. This indicates that the reduction ability of NaH2PO2 is too strong for Ag ion. As a result, N,Ndimethylformamide (DMF) was used as reducing agent and solvent because of its weak reducing property and easy operation advantages. Figure 5a is the TEM image of as-obtained Ag nanoplatelets, and it demonstrated products with an average edge length of 100-120 nm. The inset shows a SAED pattern from the single nanoplatelet. The set of spots with the strongest intensity could be indexed to (220) reflections, which indicates that the Ag nanoplatelets are single crystals with a {111} lattice

Figure 3. (a) TEM images of the cobalt hydroxide nanoplatelets, and (b) the XRD patterns for the as-obtained cobalt hydroxide nanoplatelets.

Scheme 1. An Illustration of the Possible Formation Mechanism of the Cobalt Nanoplatelets

plane as the basal plane. The X-ray diffraction patterns of the Ag nanoplatelets are shown in Figure 5b. The overwhelmingly intensive peak located at 2θ ) 38.02° corresponds to the diffraction of (111) lattice plane of fcc structure, while peaks belonging to other lattice planes were quite weak (JCPDS No. 04-0783). This indicates that (111) planes of Ag nanoplates were highly oriented parallel to the supporting substrate. The formation mechanism is different from that of Co, Ni, and Cu because the Ag ion is easier to reduce. Analogous to some reports of Ag synthesis,33,38,39 the preparation of Ag nanoplatelets here was a seed growth process through Ostwald ripening. 3.4. Magnetic Properties of Cobalt and Nickel Nanoplatelets. Magnetic Property Measurement System (MPMS) was used

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Figure 4. TEM images and the corresponding SAED patterns of (a) nickel nanoplatelets and (b) copper nanoplatelets and the XRD patterns of (c) the nickel hydroxides and the nickel nanoplatelets and (d) the copper oxides and copper nanoplatelets.

to characterize the magnetic properties of cobalt nanoplatelets. As shown in Figure 6a, the zero-field cooling (ZFC) and field cooling (FC) temperature-dependent magnetization (M(T)) measurements indicate that the Co nanoplatelets are ferromagnetic. There is no obvious blocking temperature peak even when the temperature rises to 300 K. The ZFC/FC magnetization curve is different from that for Co nanocrystals, in which the blocking temperature is below 165 K.19 However, a similar ZFC/FC curve was found for the Co chains formed by nanoparticles, and the blocking temperature increased as the strength of interaction between the particles increased.40 This suggests that probably there is an interaction occurring between the Co nanoplatelets. Cobalt in hcp crystallographic form has the c-axis as the easy axis of magnetization. When the c-axis is perpendicular to the film plane, the film will demonstrate a large perpendicular magnetic anisotropy.41 Figure 6b represents the hysteresis loops of the ferromagnetic Co nanoplatelets with magnetic fields applied parallel (H|) and perpendicular (H⊥) to the nanoplatelets at 3 K. It can be seen that the loop measured in H| has a larger coercivities (772 Oe) and remanences (29%) in comparison to that measured in H⊥, indicating a remarkable magnetic anisotropy. Such phenomenon suggests that most Co nanoplatelets are arrayed in the same direction. So, the preparative method of Co nanoplatelets on Si wafer is very important. Both coercivities and remanences in parallel or perpendicular direction could be improved if all nanoplatelets can be arrayed in same direction. At 300 K, the coercivity for Co nanoplatelets is 218 and 176 Oe for H| and H⊥, respectively (Figure 6c). Compared with the coercivity value of the bulk one (a few tens of oersteds at room temperature)42 and that of the Co spheres (ca. 590 Oe

at 5 K),43 the Co nanoplatelets exhibit a distinct enhanced coercive force, which is attributed to their 2D structure. The magnetic properties of the as-obtained Ni nanoplatelets were assessed by recording the temperature-dependent magnetization curves at an applied field of 50 Oe and the hysteresis loops. Both the temperature-dependent magnetization M(T) (Figure 7a) and hysteresis loops M(H) (Figure 7b) curves of Ni nanoplatelets demonstrate typical ferromagnetic behaviors. The blocking temperature clearly exceeds 300 K, suggesting ferromagnetic characteristics of as-prepared nanoplatelets at room temperature. It is different from the superparamagnetism at room temperature of nickel nanoparticles44 but similar to the bulk one. This may be ascribed to the large edge lengths as well as good crystallinity of Ni nanoplatelets. The FC curve deviates from the ZFC curve from the highest measurement temperature, indicating high anisotropic energy of the as-prepared nanoplatelets. The increase in shape anisotropy results in the high blocking temperature as a reflection of the improved anisotropic energy. The coercive forces at 3 and 300 K for Ni nanoplatelets are 240 and 120 Oe, respectively. Compared with the coercive forces value of the bulk one (ca. 0.7 Oe) and that of the Ni hollow nanometer-sized spheres (ca. 102 Oe)45 at room temperature, the Ni nanoplatelets also exhibit a distinct enhanced coercive force. These results indicate that by increasing the shape anisotropy of Co and Ni, its blocking temperature and the coervice forces are greatly enhanced as a reflection of the magnetic shape anisotropy. 3.5. Optical Properties of Copper and Silver Nanoplatelets. According to Mie’s theory,46 the optical properties of particles depend on their shapes and size because anisotropic metal

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Figure 5. (a) TEM image of the silver nanoplatelets and the corresponding SAED pattern and (b) the XRD patterns of the silver nanoplatelets.

particles should exhibit distinct surface plasmon resonance (SPR) effects. The study of the optical properties of Cu and Ag nanoplatelets is very interesting because very few reports have been published on the formation of metal nanoplatelets, especially for Cu nanoplatelets. The successful synthesis of Cu and Ag nanoplatelets allows the experimental confirmation of theoretical calculations. Using the discrete dipole approximation, Salzemann et al.47,48 calculated the UV-visible spectrum of various Cu clusters with different shapes. Their results indicate that for Cu nanoparticles one main peak should be observed at 560 nm, corresponding to the plasmon band of the Cu particle surface, and that it strongly depends on the copper nanocrystal size. The other peak should be observed at 640 nm, corresponding to the in-plane dipole resonance. Figure 8 shows the UVvis absorption spectra of Cu and Ag nanoplatelets. The prepared Cu nanoplatelets display two absorption peaks at 540 and 610 nm. The absorption spectrum of Cu nanoplatelets is a little different from the reported results probably because of the prepared Cu nanoplatelets with high aspect ratio.49,50 As shown in Figure 8, for Ag nanoplatelets, there are three parts of the peak, 350 nm, 380 nm, and a wide absorption in the visible region 500-800 nm. The 350 and 380 nm peaks should be attributed to the weak out-of-plane dipole and quadrupole resonance, respectively. As calculated before,51 the wide absorption in the visible region is due to the red shift of

Figure 6. (a) Temperature dependence of magnetization measured with 50 Oe for cobalt nanoplatelets after field cooling (FC) and zero-field cooling (ZFC) and (b, c) the hysteresis loops of the cobalt nanoplatelets with the magnetic field applied parallel and perpendicular to the nanoplatelets at 3 and 300 K, respectively. M/Ms indicates the magnetization (M) normalized by the saturated magnetization (Ms).

relatively intense in-plane dipole resonance because of the size distribution and the truncation degree of the angles. We expect that these particles should have promising usage in biomolecule detection by surface-enhanced resonance Raman scattering.

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could be obtained via a solvothermal method using N,Ndimethylformamide as reducing agent and solvent. The asprepared Co nanoplatelets had the {001} crystal facets as the basal plane with a hexagonal close-packed (hcp) structure. The Ni, Cu, and Ag nanoplatelets possess a face-centered cubic (fcc) structure, and the exposed nanoplatelet planes were assigned to be {111} crystal facets. Our research demonstrated that the Co, Ni, and Cu nanoplatelets resulted from the reducing of hydroxide and oxide nanoplatelets, which were generated at initial stage of synthesis process. By increase of the shape anisotropy of Co and Ni, their blocking temperature and the coervice forces were greatly enhanced as a reflection of the magnetic shape anisotropy. The anisotropic Cu and Ag nanoplatelets exhibited distinct surface plasmon resonance effect. We expect that these nanoplatelets should have promising applications for magnetic materials, catalysts, biomolecule detection by surface-enhanced resonance Raman scattering, and other related nanodevices. Acknowledgment. This work was supported by NSFC (Grant 90406003), the Foundation for the Author of National Excellent Doctoral Dissertation of the P. R. China and the state key project of fundamental research for nanomaterials and nanostructures (Grant 2003CB716901). References (1) (2) (3) (4) (5) (6)

Figure 7. (a) Temperature dependence of magnetization measured with 50 Oe for nickel nanoplatelets after field cooling (FC) and zero-field cooling (ZFC) and (b) the hysteresis loops of the nickel nanoplatelets at 3 and 300 K, respectively. The inset shows the details of the hysteresis loops at low fields.

(7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23)

Figure 8. UV-vis spectra of the copper and silver nanoplatelets.

(24)

4. Conclusions

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In summary, Co hexagonal nanoplatelets with well-defined planes were successfully synthesized by a facile hydrothermal synthetic method. This synthetic strategy could be applied in fabricating other metal nanoplatelets with anisotropic structure, such as Ni and Cu. Moreover, single-crystal Ag nanoplatelets

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Lewis, L. N. Chem. ReV. 1993, 93, 2693. Kamat, P. V. J. Phys. Chem. B 2002, 106, 7729. Wang, Z. L. AdV. Mater. 1998, 10, 13. Kottmann, J. P.; Martin, O. J. F.; Smith, D. R.; Schultz, S. Phys. ReV. B 2002, 64, 235402. Dick, L. D.; Mcfarland, A. D.; Haynes, C. L.; Van Duyne, R. P. J. Phys. Chem. B 2002, 106, 853. Zhang, J. T.; Li, X. L.; Sun, X. M.; Li, Y. D. J. Phys. Chem. B 2005, 109, 12544. Narayanan, R.; El-Sayed, M. A. Nano Lett. 2004, 4, 1343-1348. Narayanan, R.; El-Sayed, M. A. J. Phys. Chem. B 2005, 109, 12663. Zhou, K. B.; Wang, X.; Sun, X. M.; Peng, Q.; Li, Y. D. J. Catal. 2005, 229, 206. Shi, A.-C.; Masel, R. I. J. Catal. 1989, 120, 421. Falicov, L. M.; Somorjai, G. A. Proc. Natl. Acad. Sci. U.S.A. 1985, 82, 2207. Jiang, L. P.; Zhu, J. M.; Zhang, J. R.; Zhu, J. J.; Chen, H. Y. Inorg. Chem. 2004, 43, 5877. Ghezelbash, A.; Sigman, M. B.; Korgel, B. A. Nano Lett. 2004, 4, 537. Raabe, J.; Pulwey, R.; Sattler, R.; Schweinbock, T.; Zweck, J.; Weiss, D. J. Appl. Phys. 2000, 88, 4437. Park, J.; Kang, E.; Song, S. U.; Park, H. M.; Lee, M. K.; Kim, J.; Kim, K. W.; Noh, H. J.; Park, J. H.; Bae, C. J.; Park, J. G.; Hyeon, T. AdV. Mater. 2005, 17, 429. Li, Y. D.; Li, C. W.; Wang, H. R.; Li, L. Q.; Qian, Y. T. Mater. Chem. Phys. 1999, 59, 88. Li, Y. D.; Li, L. Q.; Liao, H. W.; Wang, H. R. J. Mater. Chem. 1999, 9, 2675. Petit, C.; Taleb, A.; Pileni, M. P. AdV. Mater. 1998, 10, 259. Sun, S. H.; Murray, C. B. J. Appl. Phys. 1999, 85, 4325. Dumestre, F.; Chaudret, B.; Amiens, C.; Respaud, M.; Fejes, P.; Renaud, P.; Zurcher, P. Angew. Chem., Int. Ed. 2003, 42, 5213. Lee, G. H.; Huh, S. H.; Park, J. W.; Ri, H.-C.; Jeong, J. W. J. Phys. Chem. B 2002, 106, 2123. Gibson, C. P.; Putzer, K. J. Science 1995, 267, 1338. Cordente, N.; Respaud, M.; Senocq, F.; Casanove, M. J.; Amiens, C.; Chaudret, B. Nano Lett. 2001, 1, 565. Dumestre, F.; Chaudret, B.; Amiens, C.; Respaud, M.; Fejes, P.; Renaud, P.; Zurcher, P. Angew. Chem., Int. Ed. 2003, 42, 5213. Beecher, P.; Shevchenko, E. V.; Weller, H.; Quinn, A. J.; Redmond, G. AdV. Mater. 2005, 17, 1080. Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115. Puntes, V. F.; Zanchet, D.; Erdonmez, C. K.; Alivisatos, A. P. J. Am. Chem. Soc. 2002, 124, 12874. Bradley, J. S.; Tesche, B.; Busser, W.; Maase, M.; Reetz, M. T. J. Am. Chem. Soc. 2000, 122, 4631.

Single-Crystal Metal Nanoplatelets (29) Leng, Y. H.; Zhang, Y. H.; Liu, T.; Suzuki, M.; Li, X. G. Nanotechnology 2006, 17, 1797. (30) Salzemann, C.; Urban, J.; Lisiecki, I.; Pileni, M.-P. AdV. Funct. Mater. 2005, 15, 1277. (31) Panigrahi, S.; Kundu, S.; Ghosh, S. K.; Nath, S.; Praharaj, S.; Basu, S.; Pal, T. Polyhedron 2006, 25, 1263. (32) Nie, S. M.; Emery, S. R. Science 1997, 275, 1102. (33) Jin, R. C.; Cao, Y. W.; Mirkin, C. A.; Kelly, K. L.; Schatz, G. C.; Zheng, J. G. Science, 2001, 294, 1901. (34) Sun, Y.; Xia, Y. Science 2002, 298, 2176. (35) Hou, Y. L.; Kondoh, H.; Shimojo, M.; Kogure, T.; Ohta, T. J. Phys. Chem. B 2005, 109, 19094. (36) Wang, D. S.; Xu, R.; Wang, X.; Li, Y. D. Nanotechnology 2006, 17, 979. (37) Li, X. L.; Liu, J. F.; Li, Y. D. Mater. Chem. Phys. 2003, 80, 222. (38) Sun, Y.; Mayers, B.; Xia, Y. Nano Lett. 2003, 3, 675. (39) Sun, Y.; Xia, Y. AdV. Mater. 2003, 15, 695. (40) Gross, A. F.; Diehl, M. R.; Beverly, K. C.; Richman, E. K.; Tolbert, S. H. J. Phys. Chem. B 2003, 107, 5475. (41) Ge, S. H.; Li, C.; Ma, X.; Li, W.; Xi, L.; Li, C. X. J. Appl. Phys. 2001, 90, 509.

Crystal Growth & Design, Vol. 7, No. 9, 2007 1911 (42) Cao, H. Q.; Xu, Z.; Sang, H.; Sheng, D.; Tie, C. Y. AdV. Mater. 2001, 13, 121. (43) Hou, Y. L.; Kondoh, H.; Toshia, T. Chem. Mater. 2005, 17, 3994. (44) Roy, A.; Srinivas, V.; Ram, S.; De Toro, J. A.; Mizutani, U. Phys. ReV. B 2005, 71, 184443. (45) Liu, Q.; Liu, H.; Han, M.; Zhu, J.; Liang, Y.; Xu, Z.; Song, Y. AdV. Mater. 2005, 17, 1995. (46) Mie, G. Ann. Phys. 1908, 25, 377. (47) Salzemann, C.; Lisiecki, I.; Brioude, A.; Urban, J.; Pileni, M.-P. J. Phys. Chem. B 2004, 108, 13242. (48) Salzemann, C.; Brioude, A.; Pileni, M-P. J. Phys. Chem. B 2006, 110, 7208. (49) Dujardin, E.; Hsin, L.-B.; Wang, C. R. C.; Mann, S. Chem. Commun. 2001, 14, 1264. (50) Zhong, Z.; Luo, J.; Ang, T. P.; Highfield, J.; Lin, J.; Gedanken, A. J. Phys. Chem. B 2004, 108, 18119. (51) Mock, J. J.; Barbic, M.; Smith, D. R.; Schultz, D. A.; Schultz, S. J. Chem. Phys. 2002, 116, 6775.

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