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

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.


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][8][9][10][11]. Optical properties can be tuned by controlling the coating thickness; previous studies indicate the possibility to tune surface plasmonic properties of Fe 3 O 4 /Au/Ag from λ = 560 nm (red shift) to λ = 501 nm (blue shift) with the addition of nonmagnetic layers (Au or Ag); however, it will reduce magnetic strength of (Fe 3 O 4 ) nanoparticles [12]. Other authors achieved spindle-shaped hematite (Fe 2 O 3 ) 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 Fe 3 O 4 /Au nanoparticles along with atom-resolved scanning transmission electron microscopy and energy disperse X-ray spectroscopy profiles.

Synthesis of AuFe 3 O 4 Cubic 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/HNO 3 = 3 : 1. The synthesis consisted of two main steps. (1)   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 Fe 3 O 4 and potassium tetrachloroaurate (III) in distilled water were dissolved to create gold shell onto Fe 3 O 4 nanoparticles. This second reaction will reduce gold from Au +3 to Au 0 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: (

Scanning Transmission Electron and Energy Disperse Xray.
Morphology of products (AuFe 3 O 4 ) 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 AuFe 3 O 4 /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 Δ f Sch = −1.2(C s λ) 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.

Conclusions
A successful synthesis of cubic-shaped core-shell Fe 3 O 4 /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.