Pulsed laser irradiation of colloidal nanoparticles: a new synthesis route for the production of non-equilibrium bimetallic alloy submicrometer spheres

Abstract

A new one-step approach was developed to synthesize bimetallic alloy submicrometer spheres, which are immiscible under equilibrium, using AuCo as a model system. Uniform, single-phase AuCo alloy submicrometer particles (∼230 nm) with a well-defined spherical morphology were successfully formed via pulsed laser irradiation of Au and Co-oxides nanoparticles dispersed in ethanol and characterized with a combination of electron microscopy, diffraction and magnetization measurements.

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Micron and submicron spheres have aroused great interest due to their useful functionalities and applications in many fields.^1,2 At the same time, alloys and intermetallic compound materials have attracted steadily growing attention because of their surprisingly diverse range of physical properties for potential applications in optics, magnetics, catalytic activity, corrosion resistance, electrochemistry and biotechnology.^3–8 Among the most important for this myriad of applications are the alloys and intermetallic compounds of noble metals with 3d transition metals. In particular, alloys of Au with the magnetic 3d elements Fe, Co, and Ni are fascinating materials because they are immiscible under equilibrium conditions but their non-equilibrium phases are of interest due to their potential multifunctional optical, catalytic, and magnetic properties.

Nowadays, the generation of bimetallic alloy particles has been investigated using chemical methods and the milling of powders.^9–15 However, the use of these methods may lead to contamination of the product as a result of chemical precursors or the abrasion of grinding elements.¹⁶

A laser ablation-in-liquid technique is considered a safe, simple and versatile way to produce nanocrystals and nano-sized oxides.^17–21 As an extension of this technology, our group proposed an innovative method of pulsed laser selective heating of colloidal nanoparticles in liquid in order to prepare submicrometer-sized spheres, such as metal or metal oxides.^22–25 In contrast to conventional chemical methods, this method avoids the use of toxic chemical agents to control the growth of the particles.²² Additionally, in this technique, when a pulsed laser is applied to the colloidal solution, only the nanoparticles are heated, not the solvent. This selective laser heating relies on the laser energy absorption of solid particles as well as the lack of thermal energy transfer to the solvent.²² In this communication, we demonstrate a facile but powerful strategy for the generation of AuCo alloy particles. An unfocused laser was used to irradiate a mixture of colloidal solutions of gold and cobalt oxides, resulting in submicrometer sphere formation. To the best of our knowledge, this is the first time that submicrometer AuCo alloy spheres have been synthesized.

Fig. 1 presents the typical morphologies of the starting materials and as-prepared particles. The initial average size of the Au and Co-oxide nanoparticles is 5 nm and 20 nm, respectively. Both initial particles were strongly agglomerated. After pulsed laser irradiation (Fig. 1c–d), spherical particles with an average diameter of 230 nm are produced. The spectrum resulting from energy dispersive X-ray spectroscopy (EDS) of a single submicrometer spherical particle indicates the formation of a multi-component particle, since the particle is composed of gold (Au), cobalt (Co) and oxygen (O) (Fig. S1, ESI†). The XRD profiles of the raw Au, raw Co-oxide nanoparticles and the as-synthesized AuCo particles are shown in Fig. 2.

Fig. 1 (a) SEM image of the raw Au nanoparticles, (b) SEM image of the raw Co-oxide nanoparticles, (c), (d) SEM, TEM images and particle size distribution histogram of the AuCo particles obtained by pulsed laser radiation.

Fig. 2 X-ray diffraction patterns of the raw Co-oxide, raw Au nanoparticles and AuCo alloy particles.

The initial XRD pattern of Au shows broad characteristic (111), (200), (220) and (311) reflections, consistent with the Fm3m face-centered cubic (fcc) crystal structure of pure gold. The Co-oxide nanoparticles are predominantly Co₃O₄ with a small fraction of CoO, indicated by peaks at 42.4°, 61.5°, and 77.5°. After laser irradiation, the diffraction peaks of the as-prepared particles revealed the presence of a (Fm3m) disordered Au–Co alloy crystal phase. The (111), (200), (220) and (311) reflections of the Au–Co alloy phase were shifted to higher 2θ angles with respect to pure Au, consistent with a contraction of the lattice and alloy formation. XRD analysis yields the lattice constant a = 3.961 Å, which is contracted relative to that of pure Au (a = 4.070 Å), indicating that Co is diffusing into the Au, forming the disordered solid solution Au_1−xCo_x. The XRD peaks of the alloy particles are relatively narrow compared to those of the Au and Co-oxides nanoparticles. This implies better crystallinity and an increase in the crystallite size of the obtained particles.

Fig. 3a depicts the phase evolution of the products obtained using pulsed laser irradiation with different laser irradiation times. After 5 min of irradiation, the crystalline product matches well with pure Au. A AuCo phase begins to appear after 10 min of irradiation and Au is also present. Increasing the irradiation time decreases the relative amount of the Au phase in the particles. After 60 min, only the AuCo phase exists. From the XRD data, we can calculate the lattice parameters of the Au–Co alloy particles obtained after 10, 15, 30 and 60 min as 3.996 Å, 3.997 Å, 3.989 Å and 3.961 Å. Based on these results, we estimated the concentration of cobalt in the particles.^26,27 The variation in the lattice parameters and the concentration of cobalt (determined via the lattice parameters) is shown as a function of irradiation time in Fig. 3b. In spite of the relatively large uncertainties in such calculated values of concentration, we have a clear tendency-concentration of Co which dramatically increases with irradiation time. The mechanism of the AuCo particles formation is quite intuitive (Fig. S2, ESI†). The starting materials include Au (5 nm) and Co-oxide (20 nm) nanoparticles. Because both particles are very small, we can expect strong agglomeration. There are the following possibilities: Co-oxide nanoparticles form agglomerates by themselves or/and agglomerate with Au. In this case, much smaller Au particles probably attach onto larger Co-oxide particles and agglomerate with a quasi core-shell structure. When the input laser fluence is high enough, the temperature of the nanoparticles dispersed in the solvent will rise dramatically up to the melting point due to the absorption of the laser-beam energy.

Fig. 3 (a) XRD patterns of the AuCo particles prepared by pulsed laser irradiation with different irradiation times from 5 to 60 min. For comparison, the XRD pattern for raw Au particles is presented. The broken lines in red are the peak positions from Au and those in green are from the AuCo alloy. (b) The lattice parameters and the Co concentration in the AuCo alloys obtained by pulsed laser irradiation with different irradiation times. (c) Au–Co binary alloy phase diagram (data from ref. 26).

After absorbing the laser pulse (during the time 10 ns) all the particles in agglomeration are melted and merged in one large particle. In one individual pulse such particles can not be spherical, not so homogeneous. But after many pulses the particles become spherical.

Through repeated agglomeration, laser energy absorption and melting–solidification of the particles, they become larger (Fig. S3 and S4, ESI†). Furthermore, Co₃O₄ decomposes thermally to CoO, and CoO can be decomposed to Co by ethanol pyrolysis products. Alloying requires a diffusion of the different chemical species. During each short period when the particle is in a melting condition, diffusion of Co into Au occurs in the melt pool. In the liquid state, diffusion is enhanced by convection movements in the melt pool. Three forces, gravity, viscosity and force due to surface tension gradients, enhance the diffusion and improve the homogenization of the alloy.^28,29 After the pulsed heating (within 10 ns), a quenching process occurs, which usually takes less than 10⁻⁴ s,³⁰ and this short cooling rate is helpful for retaining the structure formed during instantaneous heating. Rapid cooling and solidification overspreads the melt pool, producing the AuCo alloy. As a result of these processes, spherical submicrometer AuCo alloy particles can be formed.

To analyse the magnetic properties of the generated particles, we performed superconducting quantum interference device measurements (SQUID). The temperature dependence of the field cooled (FC) and zero-field-cooled (ZFC) branches of the DC susceptibility in a field of 100 Oe is shown in Fig. 4 a. There is a separation of the FC and ZFC branches up to room temperature and the ZFC curve shows a very broad hump. This could possibly be due to dipolar interactions among the nanoparticles,³¹ weak surface ferromagnetism or the presence of small quantities of elemental Co that are below the detection limit of XRD and TEM. Fig. 4b shows the magnetization as a function of magnetic field at 5 and 300 K. The coercivity of the AuCo particles is 102 Oe at 5 K and changes to 48 Oe at 300 K. This coercivity is similar to the coercivity obtained by Grass et al. for cobalt nanoparticles.³² The M(H) data at 300 K (showing weak ferromagnetism) is presumably due to small quantities of an impurity phase that is below the detection limit of XRD or TEM. Blocking behavior might occur at temperatures lower than our experimental limit but we could not see any evidence of superparamagnetism in our samples.

Fig. 4 (a) Zero-field-cooled and field cooled curves of the AuCo particles. The external magnetic field is 100 Oe, (b) magnetization vs. field at 5 and 300 K.

In this paper, we have demonstrated a new approach for the synthesis of non-equilibrium bimetallic alloy submicrometer spheres via pulsed laser irradiation of nanoparticle colloidal solutions. AuCo submicrometer spheres with 31 at% of Co in gold were successfully formed via pulsed laser irradiation of Au and Co-oxide nanoparticles dispersed in ethanol. To the best of the authors' knowledge, this is the first demonstration of a synthesis of AuCo alloy particles with a well-defined spherical morphology and uniform submicrometer-size. It should be noted that the AuCo binary system is a typical phase-separation system in a bulk phase diagram (Fig. 3c), and the synthesis of single-phase bimetallic alloy particles via laser irradiation is a significant result. Laser irradiation in a liquid environment is an alternative to commonly used chemical methods for the synthesis of colloidal particles. It has emerged as a versatile technique for the synthesis of a variety of nanomaterials, such as metal particles, semiconductor particles and metal oxide particles. The pulsed laser irradiation of colloidal nanoparticles is a chemically simple and clean synthesis process. In this technique, the conditions generated during the irradiation can be used for the synthesis of new phases. More importantly, this technology for preparing submicrometer spheres which are immiscible under equilibrium, was found to be applicable not only for a AuCo system but also for AuFe and AuNi (Fig. S5, ESI†). These advantages make pulsed laser irradiation of colloidal nanoparticles an effective and general route for the synthesis of a variety of bi- or multicomponent metallic submicrometer particles. Thus, we expect that our approach to the synthesis of alloy particles will be applicable to a wide range of inorganic solids, perhaps even yielding new metastable solids that are not stable in bulk systems. We believe that this facile laser irradiation approach represents a major step in the practical application of laser processing for technology.

Experimental section

Synthesis of AuCo particles: Raw nanoparticles of gold (Wako, colloidal solution, concentration 0.0069 wt.%) and Co-oxide nanoparticles (Nano Tek, powder form, average size 22 nm, purity 99.5% (atomic ratio of Au : Co = 60 : 40)) were dispersed in ethanol, mixed and transferred to a sealed cell with a quartz window to introduce laser light. The mixture was irradiated by unfocused laser light for 1 h using the second harmonic (532 nm) of an Nd:YAG laser operated at 30 Hz with a fluence of 100 mJ pulse^-1 cm^-2. During irradiation, ultrasonic stirring was used to prevent sedimentation and gravitational settling of the suspension.

Characterization: The morphology of the obtained gold/cobalt particles was observed by a field emission scanning electron microscope (SEM; Hitachi S4800) and a transmission electron microscope (TEM; JOEL JEM 2010). The average particle size was determined by measuring the diameters of 200 particles from the TEM images. The formed Au–Co phases and composition were determined by a powder X-ray diffractometer (XRD; RigakuUltima IV) with Cu–Kα radiation. A highly sensitive superconducting quantum interference device (SQUID; Quantum Design, MPMS) magnetometer was employed to measure the magnetic properties of the nanocomposite particles.

Acknowledgements

This work was supported by KAKENHI 2008734 and the magnetization measurements were conducted at the Nano-Processing Facility, supported by IBEC Innovation Platform, AIST.

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Footnote

† Electronic supplementary information (ESI) available: TEM images for the AuCo particles obtained by pulsed laser irradiation and EDS spectrum acquired from the center of a single particle. TEM image of the Au/Co-oxides particles after 5 min of irradiation and EDS spectra corresponding to different parts of the obtained particles. TEM images of the Au/Co-oxides after laser irradiation with various times (5 min, 10 min, 15 min, 30 min and 60 min). SEM image and XRD result of the Au/AuFe and Au/AuNi particles. See DOI: 0.1039/c2ra22119e

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