10747
2007, 111, 10747-10750 Published on Web 07/03/2007
Chemical Synthesis and Silica Encapsulation of NiPt Nanoparticles Yan Li, Xiao Li Zhang, Ri Qiu, Ru Qiao, and Young Soo Kang* Department of Chemistry, Pukyong National UniVersity, 599-1 Daeyon-3-Dong, Namgu, Busan 608-737, Korea ReceiVed: April 3, 2007
NiPt nanoparticles were one-step synthesized by the reduction of nickel acetylacetonate and platinum acetylacetonate in the presence of oleic acid and oleylamine as stabilizers. The as-synthesized NiPt nanoparticles have a face-centered-cubic (fcc) structure. Silica shells were then coated with tunable thickness from 8 to 16 nm that could be controlled by varying the ratio of the NiPt nanoparticles to the silica precursor. This is the first study of NiPt@SiO2 (core/shell) nanoparticles. The size of the nanoparticles was confirmed by transmission electron microscopy (TEM). The crystal structure was characterized by X-ray diffraction pattern (XRD). The chemical structure and morphology were studied with X-ray photoelectron spectroscopy (XPS) and energydispersive X-ray microanalysis (SEM-EDX). The magnetic properties were characterized by a superconducting quantum interference device (SQUID).
Introduction Nanotechnology is covering a broad range of topics in the field of applied science and technology.1 In the area from flat panel displays to medical implants, nanoscale materials play an important role because of their remarkable structural, electrical, magnetic, and optical properties. Nanocrystalline alloys and magnetic nanomaterials present a comprehensive overview of this fast-moving field such as magnetic resonace imaging for medical diagnosis, high-density magnetic recording, controlled drug delivery, biological targeting or separation, and catalysis.2-14 In recent years, there has been an increased interest in coating surfaces of magnetic nanoparticles with a thin-shell material for various electronic and biomedical applications.15,16 NixPt(1-x) exhibits favorable catalytic activities such as hydrolysis and the thermolysis of ammonia borane.17 Compared with the pure Pt, the alloying of Pt with transition-metal Ni not only exhibits the catalytic activities but also reduces the cost of the catalyst. This alloy system is also interesting from the fundamental properties, which must be related to the electronic structures. Among the proposed approaches like thermolysis or reduction of organometallic precursors to produce magnetic nanoparticles, Sun et al.18 have developed a chemical process to synthesize monodispersed FePt nanoparticles. Other groups reported on the synthesis of FePt nanoparticles and then coated with different shells, such as SiO2,19 MnO,20 and Fe3O4.21 Silica shell could provide a chemically inert surface for magnetic nanoparticles and protect them from insulating the acidic environment.17 Inspired by their synthetic work on magnetic nanoparticles, we endeavored to make NiPt magnetic nanoparticles via chemical means. Furthermore, the NiPt nanoparticles were coated with silica shells by the colloidal-coating deposition method. In this paper, we report a chemical synthetic means for making NiPt nanoparticles. Considered the toxicity and evapo* Corresponding author. E-mail:
[email protected].
10.1021/jp072610s CCC: $37.00
TABLE 1: Concentrations of the Precursors for Preparing NixPt(1-x) sample
A
B
C
D
Ni(acac)2 (mmol) Pt(acac)2 (mmol) benzyl ether (mL) oleic acid (mL) oleylamine (mL)
0.5 0.5 10 1.2 1.2
0.6 0.2 10 1.2 1.2
0.5 0.1 10 1.2 1.2
0.7 0.1 10 1.2 1.2
TABLE 2: Elemental Composition of NixPt(1-x) sample
A
B
C
D
Ni(acac)2:Pt(acac)2 elemental composition
1:1 Ni48Pt52
3:1 Ni68Pt32
5:1 Ni72Pt28
7:1 Ni81Pt19
ration of the carbonyl compound, we chose Ni(acac)2 and Pt(acac)2 to be the precursors and demonstrate the benefits. They have the similar reaction rate and not much vapor during the reaction. Furthermore, NiPt nanoparticles were encapsulated with a silica shell. By controlling the molar ratio between the NiPt particles and the silica precursor, we get the shell with a tunable thickness from 8 to 16 nm. Experiment Materials. Platinum acetylacetonate (Pt(acac)2), nickel (II) acetylacetonate (Ni(acac)2), benzyl ether, oleic acid, igepal CO520 (4-(C9H19)C6H4(OCH2CH2)nCH, n ∼ 5; igepal CO-520 is a nonionic surfactant that is composed of a nonylphenol tail coupled to a head group of five ethylene oxide units.), NH4OH aqueous solution (28%), tetraethylorthosilicate (TEOS), absolute ethanol, hexane, and cyclohexane were purchased from Aldrich Chemical Co. Oleylamine was obtained from Fluka. All chemicals were used without further purification. Preparation of NiPt Nanoparticles. Under a flow of nitrogen, Pt(acac)2 (0.5 mmol) and Ni(acac)2 (0.5 mmol) were mixed with 10 mL benzyl ether with magnetic stirring at 100 °C for 10 min; 4 mmol of oleic acid and 4 mmol of oleylamine were then injected to the mixture. The compositions of materials © 2007 American Chemical Society
10748 J. Phys. Chem. C, Vol. 111, No. 29, 2007
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Figure 1. XRD patterns of all of the NixPt(1-x) samples.
Figure 3. TEM images of NiPt@SiO2 (Ni48Pt52 coating with SiO2) with the different thicknesses of the silica shells: (a) 8 nm, (b) 10 nm, and (c) 16 nm. (d) Sample prepared by using 20 mg of NiPt and 0.25 mL of TEOS.
Figure 4. XPS pattern of NiPt and NiPt@SiO2 (Ni48Pt52 and Ni48Pt52 coating with SiO2).
Figure 2. TEM images of NiPt nanoparticles: (a) Ni48Pt52, (b) Ni68Pt32, (c) Ni72Pt28, and (d) Ni81Pt19.
are listed in Table 1. The reaction mixture was heated to 275 °C at a rate of ∼20 °C/min for 1 h and then allowed to be refluxed at 298 °C for 2 h. The reaction mixture was cooled to room temperature and gave a black dispersion. The product was precipitated by adding 20 mL ethanol and isolated by centrifuging more then three times. The nanoparticles were stored in hexane under nitrogen. Silica Coating of NiPt Nanoparticles. The colloid coating deposition method19,20,23 was used here for preparing NiPt@SiO2 nanoparitcles. Freshly prepared NiPt particles were dispersed in cyclohexane to get the NiPt solution (1 mg/mL). In the other flask, 150 mL of cyclohexane was mixed with 8 mL of igepal
CO-520 and stirred. And then the NiPt solution was added to it in different amounts (10, 15, 20, and 20 mL) to get the different thicknesses of the shell. One milliliter of 28% NH4OH solution was dropped into the above dispersion. Depending on the desired silica shell thickness, TEOS was added at the amount ranged from 0.25 to 1 mL. The mixture was stirred for more than 72 h before washing by hexane and ethanol. The final product was stored in organic solvents with low polarity, such as ethanol and toluene, as well as in ethanol/toluene mixture. Characterization. The size and morphology of NiPt and NiPt@SiO2 nanoparticles were obtained on a JEOL JEM-2010 (high-resolution) transmission electron microscope (TEM). TEM samples were prepared by dropping the dilute colloidal solution of the nanoparticles onto a carbon-coated TEM grid (Formvar/ Carbon Cu grids, purchased from Ted Pella, Inc. Redding, CA). In this case, the solvent was allowed to evaporate in air. The nanocrystallite structure was analyzed by X-ray diffraction
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J. Phys. Chem. C, Vol. 111, No. 29, 2007 10749
Figure 5. (A) SQUID patterns of NiPt (a) and NiPt@SiO2 (Ni48Pt52 and Ni48Pt52 coating with SiO2) with the different thicknesses of the silica shells: (b) 8 nm, (c) 10 nm, and (d) 16 nm. (B) An expanded plot for field strengths between -1000 and 1000 Oe.
(XRD) using a Philips X’Pert-MPD System with a Cu KR radiation source (λ ) 0.154056 nm). The components of the product were measured by energy-dispersive X-ray microanalysis (SEM-EDX, HITACHI S-2400). The chemical structure and morphology were studied with X-ray photoelectron spectroscopy (XPS, MUTILLAB 2000). The samples were compressed into a pellet of 2-mm-diameter and fixed to the holder. The sample holder was then placed into a fast entry air load-look chamber without exposure to air and evacuated under vacuum (
