Synthesis of NiAu alloy and core–shell nanoparticles in water-in-oil microemulsions
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- Chiu, H., Chiang, I. & Chen, D. J Nanopart Res (2009) 11: 1137. doi:10.1007/s11051-008-9506-9
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NiAu alloy nanoparticles with various Ni/Au molar ratios were synthesized by the hydrazine reduction of nickel chloride and hydrogen tetrachloroaurate in the microemulsion system. They had a face-centered cubic structure and a mean diameter of 6–13 nm, decreasing with increasing Au content. As Au nanoparticles did, they showed a characteristic absorption peak at about 520 nm but the intensity decreased with increasing Ni content. Also, they were nearly superparamagnetic, although the magnetization decreased significantly with increasing Au content. Under an external magnetic field, they could be self-organized into the parallel lines. In addition, the core–shell nanoparticles, Ni3Au1@Au, were prepared by the Au coating on the surface of Ni3Au1 alloy nanoparticles. By increasing the hydrogen tetrachloroaurate concentration for Au coating, the thickness of Au shells could be raised and led to an enhanced and red-shifted surface plasmon absorption.
Composite nanoparticles combine two or more components in each individual particle. Their properties not only depend on the size and structure but also are markedly influenced by the composition and composition distribution. So, the characteristics of bi- and multi-metallic nanoparticles in the alloy or core–shell structures are quite different from those of single-component nanoparticles. They exhibit novel or multiple properties and hence have much broader applications than their single-component counterparts. It is of great interest and a challenge to prepare bimetallic nanoparticles with a controlled composition distribution. Their applications in the fields of catalysts, biotechnology, biomedicine, and optical, electronic, magnetic, thermal, and mechanic materials also have been extensively investigated (Caruso 2001; Niemeyer 2001; Liao and Chen 2002; Hofman-Caris 1994; Wang et al. 2003; Wooley 2000; Zhong and Maye 2001).
A lot of processes have been developed for the synthesis of bimetallic nanoparticles, including alcohol reduction (Wang and Toshima 1997; Yonezawa and Toshima 1995), citrate reduction (Link et al. 1999), the polyol process (Lee and Chen 2006), solvent extraction reduction (Han et al. 1998; Esumi et al. 1991), the sonochemical method (Mizukoshi et al. 1997), photoreduction (Remita et al. 1996), decomposition of organometallic precursors (Chiang and Chen 2007), and electrolysis of bulk metals (Reetz et al. 1995). Water-in-oil (w/o) microemulsions are thermodynamically stable systems and can be formed spontaneously. It is clear, transparent, and isotropic complex fluids media with mixtures of oil, water, and surfactant which increases the oil–water contact by a monolayer. The nano-sized water droplets with salts dispersed in a continuous oil phase and stabilized by surfactant molecules at the water/oil interface. The surfactant-stabilized water pools provide a microenvironment for the preparation of a nanoparticle by exchanging their contents via the fusion–redispersion process and preventing the excess aggregation of particles. As a result, the particles obtained in such a medium are generally very fine and monodispersed (Wu et al. 2001a, b).
Magnetic nanoparticles can be widely used in magnetic recording devices, bioseparation, medical diagnoses, magnetically targeted therapy, magneto-optical systems, and electromagnetic wave absorption (Bergemann et al. 1999; Knauth et al. 2001; Mykhaylyk et al. 2001; Liao and Chen 2001; Chen and Liao 2002; Lee and Chen 2007. Ni and Au nanoparticles are important magnetic and optical materials, respectively, and both are useful in catalytic field. Their bimetallic nanoparticles are interesting and may exhibit combined or novel properties. Until now, only few works on the preparation of NiAu colloid dispersion have been reported, and they usually were formed in the films or on the substrates (Lu et al. 2002; Tsaur and Maenpaa 1981; Lahr and Ceyer 2006). In this work, synthesis of NiAu alloy nanoparticles in w/o microemulsions of water/CTAB/1-butanol/isooctane by the co-reduction of nickel chloride and hydrogen tetrachloroaurate with hydrazine at 65 °C is reported. In addition, we also used NiAu alloy nanoparticles as the cores to further prepare the core–shell nanoparticles, NiAu@Au. They are expected to possess enhanced surface plasmon resonance owing to the increase in the thickness of Au shells. Also, because no Ni atoms were exposed on the surface, they should be less toxic as compared to NiAu alloy nanoparticles and may find potential application in biomedicine. Another reason why NiAu alloy nanoparticles were used as the cores rather than Ni nanoparticles is the surface Au atoms of NiAu alloy nanoparticles may facilitate the coating of Au shells.
Nickel(II) chloride was the product of Showa (Tokyo). Hydrogen tetrachloroaurate was obtained from Alfa Aesar (Ward Hill). Cetyltrimethylammonium was purchased from Across Organics (Belgium). 1-Butanol and isooctane were supplied by J. T. Baker (Phillipsburg). Sodium hydroxide was a product of Hayashi (Osaka). Ammonium hydroxide was obtained from TEDIA (Fairfield). Hydrazinium hydroxide was guaranteed reagent of E. Merck (Darmstadt). Polyethyleneimine was supplied by Fluka (Buchs). Ethanol was purchased from Seoul Chem. Ind. Co. (Kyungki-do).
Synthesis of NiAu alloy nanoparticles
Synthesis of NiAu@Au nanoparticles
Two equal volumes of microemulsion solutions, one containing an aqueous solution of hydrogen tetrachloroaurate and the other containing an aqueous solution of hydrazine, were prepared at first. The concentration of hydrogen tetrachloroaurate was 1 wt%. The molar ratio of water/CTAB/1-butanol/isooctane, ω0 value, and CTAB and hydrazine concentrations were all the same as those for the synthesis of NiAu alloy nanoparticles. Secondly, these two microemulsion solutions were mixed with an equal volume of microemulsion solution of Ni3Au1 alloy nanoparticles as synthesized according to the above. At 65 °C for 10–15 min, NiAu@Au nanoparticles were formed. By increasing the concentration of hydrogen tetrachloroaurate to 3 wt%, the thickness of Au shell could be raised.
Particle size was determined by transmission electron microscopy (TEM) using a Hitachi Model HF-2000 field emission transmission electron microscope at an accelerating voltage of 80 kV. The sample for TEM analysis was obtained by placing a drop of the colloidal solution onto a Formvar-covered copper grid and evaporating it in air at room temperature. The electron-diffraction patterns were obtained by a JEOL Model JEM-2100F electron microscope at 200 kV. X-ray diffraction (XRD) measurement was carried out on a Shimadzu Model RX-III X-ray diffractometer at 40 kV and 30 mA with Cu-Kα radiation (λ = 0.1542 nm). The UV/Vis absorption spectra of NiAu colloid dispersions were analyzed by a Hitachi U-3000 spectrophotometer. Magnetic measurement was done using a superconducting quantum interference device (SQUID) magnetometer (MPMS7, Quantum Design). The real compositions of NiAu alloy nanoparticles were determined by dissolving the sample in a concentrated HCl/HNO3 (3:1 v/v) mixture solution and analyzing the solution composition using a GBC Model SDS-270 atomic absorption spectrometer (AAS).
Results and discussion
Synthesis and characterization of NiAu alloy nanoparticles
A list of the Ms, Mr, and Hc values for Ni and NiAu alloy nanoparticles with various molar ratios
Ni/Au molar ratio
Ms (emu g−1)
Mr (emu g−1)
Synthesis and characterization of Ni3Au1@Au nanoparticles
The synthesis of NiAu alloy (NiAuNi3Au1, Ni1Au1, and Ni1Au3) and core–shell (Ni3Au1@Au) nanoparticles have been achieved by the hydrazine reduction of nickel chloride and hydrogen tetrachloroaurate in the water-in-oil microemulsion system of water/CTAB/1-butanol/isooctane at 65 °C. The resultant alloy nanoparticles had a mean diameter of 6–13 nm, decreasing with increasing Au content due to the presence of more nuclei at the beginning of reaction. They had a fcc structure with Au and Ni atoms homogeneously distributed throughout the bulk phase of nanoparticles. Their real compositions were confirmed to be consistent with those of the feed solutions by AAS analysis. They showed the surface plasmon absorption at about 520 nm and were nearly superparamagnetic. Also, by varying the composition in feed solution, the optical and magnetic properties could be tuned. In addition, under an external magnetic field, they could be self-assembled into parallel stripes in the direction of magnetic field. Furthermore, Ni3Au1@Au nanoparticles were prepared by coating Au shells on the surface of Ni3Au1 alloy nanoparticles. By increasing the hydrogen tetrachloroaurate concentration for Au coating, the thickness of Au shells was raised and the surface plasmon absorption was enhanced and red-shifted. The NiAu alloy and core–shell nanoparticles obtained in this work may be useful in optical, catalytic, and biomedicine fields.
We are grateful to the National Science Council of the Republic of China for the support of this research (Contract No. NSC 94-2214-E006-006).