Journal of Nanoparticle Research

, Volume 11, Issue 5, pp 1137–1144

Synthesis of NiAu alloy and core–shell nanoparticles in water-in-oil microemulsions

Authors

  • Hsin-Kai Chiu
    • Department of Chemical EngineeringNational Cheng Kung University
  • I-Chen Chiang
    • Department of Chemical EngineeringNational Cheng Kung University
    • Department of Chemical EngineeringNational Cheng Kung University
Research Paper

DOI: 10.1007/s11051-008-9506-9

Cite this article as:
Chiu, H., Chiang, I. & Chen, D. J Nanopart Res (2009) 11: 1137. doi:10.1007/s11051-008-9506-9

Abstract

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.

Keywords

NiAuAlloyCore–shellNanoparticlesMicroemulsionOpticalNanocomposite

Introduction

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.

Experimental

Materials

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

The synthesis of NiAu alloy nanoparticles were achieved by mixing equal volumes of two w/o microemulsion solutions at the same composition, one containing an aqueous solution of the metal salts and the other containing an aqueous solution of hydrazine. The molar ratio of water/CTAB/1-butanol/isooctane was 1/0.123/0.503/0.98. The CTAB concentration was 0.45 M. The overall concentration of nickel chloride and hydrogen tetrachloroaurate in water phase was fixed at 1 wt%. The pH value of hydrazine solution (0.3 M) was adjusted to 10–11 with ammonium hydroxide. The reduction of HAuCl4 and NiCl2 were
$$ 4 {\text{HAuCl}}_{ 4} + 3 {\text{N}}_{ 2} {\text{H}}_{ 5} {\text{OH}} \to 4 {\text{Au}} + 1 6 {\text{HCl}} + 3 {\text{N}}_{ 2} + 3 {\text{H}}_{ 2} {\text{O}} $$
(1)
$$ 2 {\text{NiCl}}_{ 2} + {\text{N}}_{ 2} {\text{H}}_{ 5} {\text{OH}} \to 2 {\text{ Ni}} + 4 {\text{ HCl}} + {\text{N}}_{ 2} + {\text{H}}_{ 2} {\text{O}} $$
(2)
At 65 °C, NiAu alloy nanoparticles were formed after about 10–15 min in a capped bottle. The product was magnetically recovered and washed several times with ethanol for further characterization. By varying the Ni/Au molar ratio, the composition of NiAu alloy nanoparticles could be adjusted. In the absence of nickel chloride or hydrogen tetrachloroaurate, pure Ni and Au nanocrystals could be obtained, respectively.

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.

Characterization

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

The UV/Vis absorption spectra of Au and NiAu alloy nanoparticles at various molar ratios are shown in Fig. 1. Ni3Au1, Ni1Au1, and Ni1Au3 denote the NiAu alloy nanoparticles obtained at the Ni/Au molar ratios of 3/1, 1/1, and 1/3, respectively. Obviously, NiAu alloy nanoparticles exhibited significant surface plasmon absorption at about 520 nm as Au nanoparticles did. Also, the characteristic absorption band essentially remained unchanged, but the absorbance decreased with increasing Ni content. This implied the formation of NiAu alloy nanoparticles and revealed the electron cloud oscillation of surface Au atoms might be perturbed by Ni atoms, because Ni nanoparticles did not show any characteristic absorption band in the examined wavelength range owing to the absence of surface plasmon resonance. Similar phenomenon was also observed in other Au-containing alloy nanoparticles (Wu et al. 2001a, b). By AAS, the real mole fractions of Au in the Ni3Au1, Ni1Au1, and Ni1Au3 alloy nanoparticles were determined to be 0.26, 0.51, and 0.78, respectively, revealing that the real Au contents were similar that those in the feed solutions (i.e., 0.25, 0.50, and 0.75). Because the alloy nanoparticles were recovered magnetically, they must contain Ni element in each particle. This provided an evidence for the formation of NiAu alloy nanoparticles.
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Fig. 1

UV/Vis absorption spectra of Au and NiAu alloy nanoparticles at various molar ratios

The typical TEM images of Ni, Au, and NiAu alloy nanoparticles are shown in Fig. 2. Obviously, they all were very fine and discrete with mean diameters of 19.6, 5.0, 12.9, 8.9, and 6.2 nm for Ni, Au, Ni3Au1, Ni1Au1, and Ni1Au3 nanoparticles, respectively. It was found that the mean diameters of the alloy nanoparticles varied with the Ni/Au molar ratios as shown in Fig. 2f. The descending tendency of particle size with increasing Au content could be explained from the influence of reduction rate on the nucleation and growth process, which are determined mainly by the probabilities of the collisions between several atoms, between one atom and a nucleus, and between two or more nuclei. It is known that more nuclei may lead to the formation of smaller particles at a constant concentration. The increase in Au content might increase the number of Au nuclei and lead to the formation of smaller NiAu alloy nanoparticles. In addition, as shown in Fig. 3, the electron-diffraction pattern of NiAu nanoparticles indicated four main fringe patterns with their radii in the ratio of 31/2:2:81/2:111/2. They related to the (111), (200), (220), (311) planes of face-centered cubic (fcc) NiAu.
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Fig. 2

Transmission electron micrographs of the individual metallic and alloy nanoparticles: Ni (a), Au (b), Ni3Au1 (c), Ni1Au1 (d), Ni1Au3 (e), and composition dependence of particle size (f)

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Fig. 3

The electron diffraction pattern of alloy nanoparticles: Ni3Au1 (a), Ni1Au1 (b), and Ni1Au3 (c)

Figure 4a presents the XRD patterns of Au, Ni, and NiAu alloy nanoparticles. This revealed that the resultant Au and Ni nanoparticles were in the fcc structure. For Au nanoparticles, four characteristic peaks corresponding to the (111), (200), (220), and (311) planes were observed at 2θ = 38.1, 44.4, 64.6, and 77.6°, respectively. For NiAu alloy nanoparticles, four characteristic peaks also occurred at similar diffraction angles. Although the characteristic peaks for the (111) and (220) planes of fcc Ni at 2θ = 44.6 and 76.6° were almost overlapped with those for the (200) and (311) planes of Au at 2θ = 44.4 and 77.6°, the formation of NiAu alloy nanoparticles could be confirmed from the fact that the (200) plane of fcc Ni at 2θ = 52.0° was not found, which revealed no pure Ni nanoparticles were formed individually. Furthermore, the characteristic peaks of NiAu alloy nanocrystals were slightly broader than those of Au nanoparticles, implying their poorer crystallinity, resulting from less ordered structures as usually observed for nanoparticles. In addition, it was noted that the diffraction angles of NiAu alloy nanoparticles at (200) plane decreased with increasing Au content. Their dependence on the real composition as measured above was shown in Fig. 4b. According to the Vegard’s law, the diffraction peak of the metal alloy with homogeneous composition should lie between the two set peaks of pure metals and change linearly (Tsaur and Maenpaa 1981). Figure 4b indicated that the diffraction peaks of NiAu alloy nanocrystals indeed lay between those of pure Ni and Au, revealing the formation of metal alloy. Also, the composition dependence was almost linear, implying Au and Ni atoms were homogeneously distributed throughout the bulk phase of nanocrystals.
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Fig. 4

XRD patterns of Ni, Au, and NiAu alloy nanoparticles with various initial molar ratios (a) and the composition dependence of the diffraction angle at (200) plane (b)

To investigate the magnetic properties of NiAu alloy nanoparticles, the field dependences of magnetization for Ni and NiAu alloy nanoparticles with various molar ratios were measured at 298 K as illustrated in Fig. 5. The hysteresis phenomenon was weak for each case. This could be attributed to their small particle size and revealed that they were nearly superparamagnetic. From Fig. 5, the corresponding saturation magnetization (Ms), remanent magnetization (Mr), and coercivity (Hc) could be determined as listed in Table 1. It was noted that Ni1Au1 and Ni1Au3 nanoparticles had significantly lower Ms, Mr, and Hc values as compared to Ni3Au1 nanoparticles and pure Ni nanoparticles. This could be attributed to the fact that most of Ni atoms were dispersed in the Au matrix at lower Ni content.
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Fig. 5

Isothermal hysteresis loops of Ni and NiAu alloy nanoparticles at 298 K

Table 1

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)

Hc (Oe)

1/0

35.02

10.31

168

3/1

29.27

4.05

80

1/1

7.56

0.07

12

1/3

7.14

0.09

20

As demonstrated above, NiAu alloy nanoparticles possessed the optical property of Au nanoparticles and the magnetic property of Ni nanoparticles. In spite of the decrease in magnetization, they still could be manipulated by an external magnetic field. By dropping the alcohol solution of Ni3Au1 alloy nanoparticles (0.3 mg mL−1) containing 1 wt% polyethyleneimine (MW = 70,000) on the transparency film (0.07 mL cm−2) on a permanent magnet, it was found that these nanoparticles were quickly aligned into stripes in the direction of magnetic field as shown in Fig. 6. Such a one-dimensional pattern might be useful in the fields of anisotropic optical, magnetic, or conducting materials.
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Fig. 6

SEM micrographs of Ni3Au1 alloy nanoparticles on a planar substrate at an external magnetic field

Synthesis and characterization of Ni3Au1@Au nanoparticles

The UV/Vis absorption spectra of Ni3Au1@Au nanoparticles obtained at different concentrations of hydrogen tetrachloroaurate are shown in Fig. 7, in which the UV/Vis absorption spectrum of Ni3Au1 alloy nanoparticles was also given for comparison. Obviously, with increasing concentration of hydrogen tetrachloroaurate for Au coating, the characteristic absorption bands were slightly red-shifted and their intensity increased. This revealed the formation of Au shells on the surface of Ni3Au1 alloy nanoparticles.
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Fig. 7

UV/Vis absorption spectra of Ni3Au1@Au nanoparticles obtained at 3 (a) and 1 (b) wt% of hydrogen tetrachloroaurate. The absorption spectrum of Ni3Au1 alloy nanoparticles (c) was given for comparison

Figure 8 shows the TEM images of Ni3Au1@Au nanoparticles obtained at different concentrations of hydrogen tetrachloroaurate. When the concentrations of hydrogen tetrachloroaurate for Au coating were 1 and 3 wt%, the mean diameters of the resultant core–shell nanoparticles were found to be 16.9 and 18.3 nm, respectively. Compared to Ni3Au1 alloy nanoparticles, the larger mean diameter confirmed the formation of Au shell on the surface of Ni3Au1 alloy nanoparticles. Also, the thickness of Au shells could be calculated to be 2.0 and 2.7 nm, respectively, increasing with the increase in the hydrogen tetrachloroaurate concentration for Au coating. This revealed that the thickness of Au shells could be tuned easily.
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Fig. 8

TEM images of Ni3Au1@Au nanoparticles obtained at 1 (a) and 3 (b) wt% of hydrogen tetrachloroaurate

By further comparing the XRD patterns as indicated in Fig. 9, it was noted that Ni3Au1@Au nanoparticles showed higher crystallinity than Ni3Au1 alloy nanoparticles. Also, the intensity ratio of the peak at 2θ = 38.1° to that at 2θ = 44.4° increased with increasing hydrogen tetrachloroaurate concentration for Au coating. Because the peak at 2θ = 38.1° was contributed only by Au (111) plane while that at 2θ = 44.4° was resulted from Au (200) and Ni (111) planes, this revealed that Ni3Au1@Au nanoparticles had higher Au content than Ni3Au1 nanoparticles. This was consistent with the above optical, composition, and TEM analyses.
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Fig. 9

XRD patterns of Ni3Au1@Au nanoparticles obtained at 3 (a) and 1 (b) wt% of hydrogen tetrachloroaurate. The XRD pattern of Ni3Au1 alloy nanoparticles (c) was given for comparison

Conclusions

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.

Acknowledgment

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).

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© Springer Science+Business Media B.V. 2008