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

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 𝜆=560nm (red shift) to 𝜆=501nm (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 Δ𝑓_Sch=−1.2(C_s𝜆)^1/2. 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.