ISRN NanotechnologyVolume 2011 (2011), Article ID 893879, 3 pagesdoi:10.5402/2011/893879

Research Article

Microwave-Assisted Synthesis Core-Fe₃O₄ Shell-Au Cubic Nanoparticles

Manuel Ramos,1,2 Karina Castillo,¹ Domingo A. Ferrer,³ Rurik J. Farias,² Sergio Flores,² and Russell R. Chianelli¹

¹Materials Research and Technology Institute, UT-El Paso, El Paso, TX 79968, USA²Departamento de Física y Matemáticas, Universidad Autónoma de Ciudad Juárez, Ciudad Juárez, MX 32300, USA³Microelectronics Research Center, UT-Austin, Austin, TX 78751, USA

Received 18 July 2011; Accepted 17 August 2011

Academic Editors: I.-C. Chen and J. Das

Copyright © 2011 Manuel Ramos et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Core-Shell (Fe₃O₄/Au) nanoparticles were synthesized using iron II chloride tetrahydrate (FeCl₂H₂O) and potassium tetrachloroaurate III (AuCl₄K) precursors under microwave-assisted conditions. Products were analyzed using field emission gun electron microscope in transmission and scanning modes; energy disperse X-ray spectroscopy performed during STEM measurements indicated a signal for gold K and M signals at 9 keV and 13 keV, respectively, confirming Au atoms at nanoparticle's perimeter and Fe-L signal at 8 keV to be at the center.

1. Introduction

Chemical synthesis, fabrication, and applications of nanoparticles have been an evolving topic in the material science of advanced materials; this is attributed mainly to their specific electronic properties, which in many cases differ from as when they are present in bulk, making them strong chemical entities as antibacterial [1], solid-state electronics [2], catalytic reactions [3], optical physics, and petroleum research [4]. In particular, magnetic nanoparticles have attracted a special interest for two main reasons: (1) implementation as contrast agents for magnetic resonance imaging (MRI) [5], (2) magnetic material for data storage in solid-state electronics (SSE) [6]. The achievement of standardized shape and high-quality nanoparticles properties will depend solely on synthesis-fabrication method which is dependable on appropriate precursor solutions ratios and in some occasions a reductant agent [7–11]. Optical properties can be tuned by controlling the coating thickness; previous studies indicate the possibility to tune surface plasmonic properties of Fe₃O₄/Au/Ag from (red shift) to (blue shift) with the addition of nonmagnetic layers (Au or Ag); however, it will reduce magnetic strength of (Fe₃O₄) nanoparticles [12]. Other authors achieved spindle-shaped hematite (Fe₂O₃) using a hydrothermal method of synthesis, and particle shapes depend only on 3-aminopropyl trimethoxysilane (APTMS) which acted as a reduction agent in generating amine moiety-coated surface [13, 14]. This paper presents a microwave-assisted synthesis of cubic core-shell Fe₃O₄/Au nanoparticles along with atom-resolved scanning transmission electron microscopy and energy disperse X-ray spectroscopy profiles.

2. Synthesis of AuFe₃O₄Cubic Nanoparticles

To avoid any contaminant variations on the results, before any chemical reaction, all glassware was cleaned using aqua regia in a concentration ratio of HCl/HNO₃ = 3 : 1. The synthesis consisted of two main steps. (1) Synthesis of Fe₃O₄ by using iron II chloride tetrahydrate (FeCl₂·H2O Alfa Aesar) by dissolving 70 mg in distilled water and slowly titrated for 4 h with 40 mL of 5 M NaOH solution to form iron II hydroxide (Fe(OH)₂). The iron II hydroxide solution was oxidized to form Fe₃O₄ using microwave-assisted synthesis (Multiwave 2000) at a constant temperature of 120°C for 30 minutes; products were washed and centrifuged to remove any sodium chloride (NaCl) residue and then set to dry in an open-flow furnace at 100°C for 10 min.

(2) There is a second solution, where Fe₃O₄ and potassium tetrachloroaurate (III) in distilled water were dissolved to create gold shell onto Fe₃O₄ nanoparticles. This second reaction will reduce gold from Au⁺³ to Au⁰ sodium citrate tribasic dehydrate. To quench the reaction, large amounts of distilled water were applied, followed by filtration of products and drying in open-flow furnace at 80°C for 30 min.

The stoichiometry of both reactions is as follows: (1)FeCl2 · H₂O + 2NaOH → Fe(OH)₂ + 2NaCl + H₂O(2)Fe₃O₄ + KAuCl₄ + Na₃C₆H₅O₇  → AuFe₃O₄ + NaC₆H₅O₇ + KCl + 2NaCl + Cl₂

3. Results and Discussion

3.1. Scanning Transmission Electron and Energy Disperse X-ray

Morphology of products (AuFe₃O₄) was studied by high-resolution transmission electron microscopy using an FEI Tecnai TF20 equipped with an STEM unit, high-angle annular dark-field (HAADF) detector, and X-Twin lenses. Just one drop of AuFe₃O₄/isopropanol solution was placed into a lacey/carbon (EMS LC225-Cu) grid. The operational voltage was kept constant at 200 kV in both dark field (DF) and bright field (BF) mode images. Scherzer defocus condition was set at . Energy disperse X-ray analysis (EDX) was performed using a solid angle of 0.13 sr in the detector. Cubic structure is observed by HRTEM as presented in Figure 1; in order to locate the gold on nanoparticles surface, a profile was created using EDX while performing STEM as presented in Figures 2 and 3; clearly K and M signals at 9 to 13 keV indicate the presence of gold at perimeter, while Fe-L signal at 8 keV appears when probing nanoparticle center.

(a)

(b)

Figure 1: (a) HRTEM image of Fe₃O₄/Au nanoparticles at scale of 100 nm confirming cubic shape and (b) STEM image indicate that the majority of Fe₃O₄/Au nanoparticles have cubic shape.

Figure 2: Energy disperse X-ray profile and STEM image indicating L and M signals at perimeter (red cross) corresponding to gold.

Figure 3: Energy disperse X-ray showing K and L signals corresponding to Fe when probe is at center and STEM image.

4. Conclusions

A successful synthesis of cubic-shaped core-shell Fe₃O₄/Au nanoparticles was achieved using microwave-assisted synthesis. STEM and HRTEM confirm cubic shape. Energy disperse X-ray analysis profiles indicate peak intensities from 9 to 13 keV for gold at the perimeter and 8 keV for iron at the center also confirming a core-shell array.

Acknowledgment

The authors thank The National Nanotechnology Infrastructure Network (NNIN) Research Program of the Microelectronic Research Center of UT-Austin, the Consejo Nacional de Ciencia y Tecnología, México for their economic support, and the Materials Research and Technology Institute of UT-El Paso.

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