Nearly Monodispersion CoSm Alloy Nanoparticles Formed by an In-situ Rapid Cooling and Passivating Microfluidic Process
An in siturapid cooling and passivating microfluidic process has been developed for the synthesis of nearly monodispersed cobalt samarium nanoparticles (NPs) with tunable crystal structures and surface properties. This process involves promoting the nucleation and growth of NPs at an elevated temperature and rapidly quenching the NP colloids in a solution containing a passivating reagent at a reduced temperature. We have shown that Cobalt samarium NPs having amorphous crystal structures and a thin passivating layer can be synthesized with uniform nonspherical shapes and size of about 4.8 nm. The amorphous CoSm NPs in our study have blocking temperature near 40 K and average coercivity of 225 Oe at 10 K. The NPs also exhibit high anisotropic magnetic properties with a wasp-waist hysteresis loop and a bias shift of coercivity due to the shape anisotropy and the exchange coupling between the core and the thin oxidized surface layer.
KeywordsNanoparticles Microfluidic reactor Synthesis Monodispersion Alloy Cobalt Samarium
Over the years, microfluidic reactor (MR) processes have gained much attention in the preparation of specific materials due to its in situ spatial and temporal control of reaction kinetics, in addition to efficient mass and heat transfer [1, 2, 3, 4, 5]. Recently, application of microfluidic reactors has been expanded from the improvement of chemical reaction efficiency to the controlled synthesis of micro and nanoscale materials [4, 6, 7, 8, 9, 10, 11, 12, 13]. Although significant progress has been achieved in size and shape control of NPs using microfluidic reactors, it is still challenging to obtain monodispersed NPs with controlled crystal structures . One reason is possibly the difficulty in preventing aggregation and coarsening [caused by Ostwald Ripening (OR) and Oriented Attachment (OA) process and the concurrent phase transformation] of the NPs [8, 14]. These problems, aggregation and coarsening, often occurs in the bottled batch process and in MR processes if the growth of NPs is not carefully controlled. It is therefore important that process optimization be performed to suppress these processes, even in the MR process [8, 14, 15, 16]. According to the stability principle of NPs, elimination of defects in the crystal structure, passivation of the nanoparticle growth, and the deactivation of nanoparticle surfaces can be considered to suppress the OR and OA processes, and the in-time termination of nanoparticle aggregation .
A typical reaction process is as follows: 25 mL of a mixture of CoCl2and SmCl3(28.5 mM CoCl2, 5.7 mM SmCl3in tetrahydrofuran, THF) is delivered into a heater (H1) by a pump (P1), the mixture entering into the inlet 1 after it is heated to 50 °C. A volume of 25 mL of the reducing agent, which is a mixture of 90 mM LiBEt3H and 0.24 mM PVP in TH; PVP: Mw = 29,000, is delivered into a heater (H2) by a pump (P2), and heated to 52 °C before it is pumped into inlet 2. At the Y mixer 1, the salt mixture from inlet 1 mixes with the reducing agent, and the metal salts are rapidly reduced to metal atoms. The resulting metal atoms will nucleate and grow in the nucleation and growth area to form NPs at a constant temperature of 50 °C. When the formed nanoparticle solution meets the cold quenching solution (2 °C, 10% acetone in THF) at the Y mixer 2, both the nanoparticle growth and the soon coming OR and OA processes can be suppressed, and the surfaces of NPs will be rapidly deactivated by acetone through a process of suddenly forming an ultra-thin oxidation layer. When the nanoparticle solution is collected in the chiller-cooled receiver, both the nanoparticle growth and the OR and OA processes continue to be suppressed by the cold environment and the inert surfaces, until the particle synthesis is completed.
In summary, nearly monodispersed amorphous Co5Sm alloy NPs were fabricated by an IRCPM process. The resulting NPs retain their primary amorphous crystal structures and nonspherical shapes that are formed at elevated temperature without further Ostwald ripening and oriented attachment processes. The shape anisotropy and exchange coupling between the ferromagnetic core and the antiferromagnetic oxidized surface cause the NPs magnetic hysteresis loop at 10 K to show a wasp-waist character with a significant coercivity bias shift. To conclude, we have developed a method for producing nearly monodispersed magnetic CoSm NPs with desired structure and surface properties by using a rapid quenching technique.
Author Y. Song is grateful for the financial support received from New Teacher Funds (2008-00061025) and SRF for ROCS and SEM by the Chinese Education Ministry, and Innovative Research Team of Chinese Education Ministry in University (IRT0512) at Beihang University. Y. Song also appreciates the kind suggestions from reviewers.
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