Use of long chain amine as a reducing agent for the synthesis of high quality monodisperse iron(0) nanoparticles

Abstract

This article reports the synthesis of iron(0) nanoparticles at moderate temperature-from 120 C to 150 C-using the reduction of the organometallic iron(II) precursor {Fe[N(SiMe 3 ) 2 ] 2 } 2 by hexadecylamine (HDA) in the absence of dihydrogen (H 2 ). The nanoparticles are monodisperse in size and self-assemble into 2D super-lattices suitable for transport measurements. The nanoparticles are stabilized in mesitylene by a mixture of HDA and hexadecylammonium chloride (HDA$HCl). The resulting truncated single-crystalline nanocubes have a narrow size distribution and a high magnetization close to the bulk value. The products are characterized by transmission electronic microscopy (TEM and HRTEM), SQUID measurements, M€ ossbauer and Infra-Red spectroscopies. Fe(II) reduction is accompanied by oxidation of amines into imines which was detected as a by-product. This reduction occurs at 120 C and above. The temperature, in conjunction with the reaction time, allows for a fine control of the nano-objects final size. The latter can also be tuned with the HDA$HCl concentration. Finally, this one-pot synthesis produces high-quality magnetic nanoparticles with mean sizes in the range 6 to 10 nm depending on the conditions. Due to their potential interest in many application fields, iron(0) nanoparticles (NPs) well-dispersed in a common solvent and with mean sizes that cover the range from 1 to 20 nm are highly desirable. Indeed, metallic iron has a high magnetization at room temperature, a superparamagnetic behaviour for sizes ranging from 1 to 12 nm and a ferromagnetic behaviour above those. These properties, from a physical point of view, make it a material of choice for biomedical applications such as magnetic fluid hyperthermia (MFH) and magnetic resonance imaging (MRI). 1 Furthermore, their relatively high reactivity and low cost make iron(0) NPs ideal candidates for catalysis in a non-oxidizing environment (e.g. formation or cleavage of stable bonds). 1 Iron NPs can also be interesting building blocks for applications in electronic and spin-dependent tunnelling devices too. In the field of spintronics, only two groups have measured a magnetoresistive response in assemblies of ferromagnetic metallic NPs. 2 The main difficulty is related to the unclean surface state, where the oxidation leads to an insulating surface with a high degree of spin disordering. Well controlled elaboration routes under preserved atmosphere are thus necessary, the stabilizing ligands should preserve the surface spin polarisation. Moreover, the control of the particle growth to get real monodisperse size distributions is a key parameter to get long-range well ordered two dimensional and three dimensional assemblies. However, partly due to its high sensitivity to oxidation and despite a number of efforts recently reported in the literature, synthesis of such full metallic Fe nano-objects remains a challenge.The two well-known synthesis routes to iron(0) NPs are the thermal decomposition and the chemical reduction of an iron molecular complex. In t...

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“…Interestingly, amines can be used both as solvent, surfactant and reducing agent, thus simplifying the synthetic processes [128], [129]. As well, carboxylic acids or mixed systems such as acid/amine, acid/phosphine, acid/phosphine oxide, amine/phosphine oxide, phosphine/thiol or N/O combinations are reported to provide excellent stabilization to many different metal NPs [119], [13], [130], [131], [132], [133], [134]. Alkylsilanes fragments are another class of molecules binding to the surface of NPs such as Pt [135] or Ru [136] ones through the silicon atom after Si-H bond activation.…”

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“…Synthesis of iron NPs (I-NPs): Iron NPs were prepared according to a published procedure [13]. The powder obtained contains 92% of iron (as deduced from TG analysis); isolated yield: 76% (80 mg).…”

“…These NPs have been prepared in organic solvents [13] using an organometallic approach and were initially hydrophobic. We have thus increased the hydrophilicity of the NPs surface to transfer them into water and afford good relaxometric properties.…”

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“…The first step involvest he synthesis of Fe 0 NPs by the reduction of an organometallicc omplex [Fe{N(SiMe 3 ) 2 } 2 ] 2 under H 2 in the presence of am ixture of hexadecylamine (HDA) and HDA·HCl as ligands. [15][16] In the second step, the resultingN Ps were reacted with Fe(CO) 5 and H 2 to produce iron carbiden anocrystals.…”