Elsevier

Surface Science

Volume 601, Issue 18, 15 September 2007, Pages 4352-4357
Surface Science

Structure, morphology and magnetic properties of Fe–Au core-shell nanoparticles

https://doi.org/10.1016/j.susc.2007.06.024Get rights and content

Abstract

The aim of the present work is the investigation of the core-shell Fe@Au nanoparticles obtained by the inverse micelles method. Also hybrid structures between Fe@Au nanoparticles and the well-known conducting polymer polypyrrole (PPY) were obtained and investigated. The properties of the composites were investigated by TEM, HRTEM, X-ray diffraction (XRD), X-ray Photoelectron spectroscopy (XPS) and magnetization measurements. The TEM, HRTEM measurements show that two categories of core-shell nanoparticles with mean diameters of 5 nm and 25 nm, respectively, are formed. The XPS investigations of the core-shell Fe@Au nanoparticles indicate that besides Fe0 inside the cores, small amounts of FeII,III, located onto the gold surface ,were also formed during the samples preparation. The magnetization of the investigated nanocomposites measured versus increasing and decreasing magnetic field shows no hysteresis loop, this behavior being consistent with a superparamagnetic behavior. The behavior of the SQUID magnetization vs. temperature under field cooled (FC) and zero field cooled (ZFC) at 0.1 T magnetic field for Fe@Au nanoparticles confirmed the existence of superparamagnetism. The superparamagnetic model was used to fit the experimental magnetization vs. applied magnetic field as it was measured by SQUID up to 5 T.

Introduction

Studies of magnetic nanoparticle systems have attracted much interest in the last few years from both fundamental and applicative point of view [1], [2], [3]. The key is that when decreasing the size of magnetic particles, a transition occurs from polydomain to monodomain magnetic systems [4], [5]. Over the last decade, additional control and design of magnetic nanoparticles was achieved by developing core-shell structured nanosystems. This type of particles have drawn the attention of physicist, chemists and material scientists since they allow the tailoring of the combined surface and core properties providing an increased number of applications like catalysis, and coatings to hyperthermia and magneto-resistant materials [6], [7], [8]. The key for the applications of these systems is the control of the magnetic properties of one particle. One possible way results in the “dressing” the magnetic core with a suitable shell for the adjustment of the properties of the system. Recently, gold-coated magnetic core-shell nanoparticles are reported to enhance chemical stability by protecting the core from oxidation and corrosion, and to exhibit good biocompatibility and affinity via amine/thiol terminal groups. Various core-shell systems with gold or silver shell onto iron oxide (Fe2O3 or partially oxidized Fe3O4) core nanoparticles (9 nm) have also been synthesized [9], [10]. Also, core-shell Fe@Au nanoparticles synthesis was recently reported [11], [12], [13], [14]. As it concerns core-shell magnetic composites with polymers the preparation of superparamagnetic polymeric nanoparticles consisting of a magnetite core and polymeric shell represents a novel recently reported approach [15].

In this paper we present the structural magnetic properties of the core-shell iron–gold (Fe@Au) nanoparticles and their combination with conducting polypyrrole (PPY). Our main goal is to gain insight into the effects of the core-shell structuralizing process onto the magnetic properties of these systems in order to obtain the required characteristics for specific applications. For instance, various systems obtained from the combinations of Fe@Au nanoparticles and PPY with attached biofunctional entities (oligonucleotides, amino acids) for instance, or by thiolation of the Au surface would result in potential applications as hyperthermia; catalytic uptake of organic pollutants from water, targeted magnetic separations in biological systems etc.

Section snippets

Samples preparation

Gold-coated iron nanoparticles generally were obtained by reverse micelle method [11]. We used FeSO4 · 7H2O and HAuCl4 as precursors and NaBH4 as reducing agent; 4.8 ml FeSO4 0.5 M (aq.), 6 g cetyltrimethylammonium bromide (CTAB), 12.48 ml 1-butanol, 42.8 ml octane were mixed up with 4.8 ml NaBH4 1 M, 6 g CTAB, 12.48 ml 1-butanol and 42.8 ml octane. This mixture was stirred 1 h, at the room temperature, under inert atmosphere. Another mixture containing: 3.6 ml NaBH4 1.6 M + 3 g CTAB + 6.2 ml 1-butanol + 28.6 ml

Characterization methods

The morphology of the Fe@Au and Fe@Au@PPy hybrid structures was determined by TEM and HRTEM using 1010 JEOL and Hitachi H9000NAR transmission electron microscopes. Structural characterization of the samples was performed by X-ray diffraction (XRD). The standard sample for the instrumental function was a silicon powder.

X-ray photoelectron spectroscopy (XPS) was carried out on a VG Scientific ESCA-3 Mk-II spectrometer having as X-ray source the Al Kα radiation (1486.6 eV, non-monochromatic) of an

Results and discussions

The analysis of TEM image (Fig. 1) shows the formation of nanoparticles having various sizes. It should be noticed that the CTAB nanocrystals are very difficult to remove completely by sample washing. These organic crystals appearing in many TEM and HRTEM images obscure the Au@Fe systems. The complete CTAB removing was obtained by pyrrole polymerization around to Fe@Au nanoparticles or by dispersion in DBSA of Fe@Au nanoparticles (Fig. 2).

By analyzing the HRTEM image from Fig. 2a one can see

Conclusions

Core-shell Fe@Au nanoparticles were obtained by the inverse micelles method. We established that two distributions exists concerning the diameters of nanoparticles: one centered around 5 nm and another one centered at 25 nm, respectively. The majority number of the particles are in the low diameters size distribution. As a consequence only large particles with diameters above 15 nm could be evidenced from the analysis of the XRD line profiles, because the majority of Au atoms (and probably of the

Acknowledgment

This work was supported by the Romanian Ministry of Education and Research under the research programs, CEEX-MATNANTECH Project No. 12/2005, CEEX-CNMP Project No. 208/2006. We acknowledge the participation in the EU NoE Nanofun-Poly.

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