Nearly Monodispersion CoSm Alloy Nanoparticles Formed by an In-situ Rapid Cooling and Passivating Microfluidic Process
© to the authors 2009
Received: 25 March 2009
Accepted: 28 May 2009
Published: 14 June 2009
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.
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 [1–5]. 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, 6–13]. 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, 14–16]. 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.
- Watts P, Wiles C: Recent advances in synthetic micro reaction technology. Chem. Commun. (Camb) 2007, 5: 443–467. COI number [1:CAS:528:DC%2BD2sXotFKktQ%3D%3D] 10.1039/b609428gView ArticleGoogle Scholar
- Sounart TL, Safier PA, Voigt JA, Hoyt J, Tallant DR, Matzke CM, Michalske TA, et al.: Spatially-resolved analysis of nanoparticle nucleation and growth in a microfluidic reactor. Lab Chip 2007, 7: 908–915. COI number [1:CAS:528:DC%2BD2sXmvV2msbc%3D] 10.1039/b703810kView ArticleGoogle Scholar
- Pennemann H, Watts P, Haswell SJ, Hessel V, Lowe H: Benchmarking of microreactor applications. Org. Process. Res. Dev. 2004, 8: 422–439. COI number [1:CAS:528:DC%2BD2cXhs1Cmtbk%3D] 10.1021/op0341770View ArticleGoogle Scholar
- deMello AJ: Control and detection of chemical reactions in microfluidic systems. Nature 2006, 442: 394–402. COI number [1:CAS:528:DC%2BD28XnsVaju7o%3D]; Bibcode number [2006Natur.442..394D] 10.1038/nature05062View ArticleGoogle Scholar
- Sahoo HR, Kralj JG, Jensen KF: Multistep continuous-flow microchemical synthesis involving multiple reactions and separations. Angew. Chem. Int. Ed. 2007, 46: 5704–5708. COI number [1:CAS:528:DC%2BD2sXosFOktL4%3D] 10.1002/anie.200701434View ArticleGoogle Scholar
- Boleininger J, Kurz A, Reuss V, Sonnichsen C: Microfluidic continuous flow synthesis of rod-shaped gold and silver nanocrystals. Phys. Chem. Chem. Phys. 2006, 8: 3824–3827. COI number [1:CAS:528:DC%2BD28XotlOju7g%3D] 10.1039/b604666eView ArticleGoogle Scholar
- Song Y, Zhang T, Yang WT, Albin S, Henry LL: Fine crystal structure transition of cobalt nanoparticles formed in a microfluidic reactor. Cryst. Growth. Des. 2008, 8: 3766–3772. COI number [1:CAS:528:DC%2BD1cXhtVCrurjO] 10.1021/cg8003992View ArticleGoogle Scholar
- Edel JB, Fortt R, deMello JC, deMello AJ: Microfluidic routes to the controlled production of nanoparticles. Chem. Comm. 2002, 10: 1136–1137. COI number [1:CAS:528:DC%2BD38XjsFKgu7g%3D] 10.1039/b202998gView ArticleGoogle Scholar
- Krishnadasan S, Brown RJC, deMello AJ, deMello JC: Intelligent routes to the controlled synthesis of nanoparticles. Lab Chip 2007, 7: 1434–1441. COI number [1:CAS:528:DC%2BD2sXht1Snu7jJ] 10.1039/b711412eView ArticleGoogle Scholar
- Hung L-H, Lee AP: Microfluidic devices for the synthesis of nanoparticles and biomaterials. J. Med. Biol. Eng. 2007, 27: 1–6.Google Scholar
- Jahn A, et al.: Preparation of nanoparticles by continuous-flow microfluidics. J. Nanopart. Res. 2008, 10: 925–934. COI number [1:CAS:528:DC%2BD1cXnsFegtLk%3D] 10.1007/s11051-007-9340-5View ArticleGoogle Scholar
- Song Y, Kumar CSSR, Hormes J: Synthesis of palladium nanoparticles using a continuous flow polymeric micro reactor. J. Nanosci. Nanotechnol. 2004, 4: 788–793. COI number [1:CAS:528:DC%2BD2cXovVent7w%3D] 10.1166/jnn.2004.111View ArticleGoogle Scholar
- Gilbert B, et al.: Special phase transformation and crystal growth pathways observed in nanoparticles. Geochem. Trans. 2003, 4: 20–27. 10.1186/1467-4866-4-20View ArticleGoogle Scholar
- Song Y, Kumar CSSR, Hormes J: Microfluidic synthesis of nanomaterials. Small 2008, 4: 698–711. COI number [1:CAS:528:DC%2BD1cXosFynt7o%3D] 10.1002/smll.200701029View ArticleGoogle Scholar
- Ribeiro C, Lee EJH, Longo E, Leite ER: A kinetic model to describe nanocrystal growth by the oriented attachment mechanism. Chem. Phys. Chem. 2005, 6: 690–696. COI number [1:CAS:528:DC%2BD2MXjvVGrs7g%3D]Google Scholar
- Carslaw HS, Jaeger JC: Conduction of heat in solids. Clarendon Press, Oxford; 1956:232.Google Scholar
- Lienhard JHI: Lienhard JHV A heat transfer textbook. Phkigiston Press, Massachusetts; 2003:203–223.Google Scholar
- Inderhees SE, et al.: Manipulating the magnetic structure of Co core/CoO shell nanoparticles: implications for controlling the exchange bias. Phys. Rev. Lett. 2008, 101: 117202. COI number [1:CAS:528:DC%2BD1cXhtFWltr%2FK]; Bibcode number [2008PhRvL.101k7202I] 10.1103/PhysRevLett.101.117202View ArticleGoogle Scholar
- Kirkpatrick EM, Majetich SA, McHenry ME: Magnetic properties of single domain samarium cobalt nanoparticles. IEEE Trans. Magn. 1996, 32: 4502–4504. COI number [1:CAS:528:DyaK28XmsFWkurw%3D]; Bibcode number [1996ITM....32.4502K] 10.1109/20.538911View ArticleGoogle Scholar
- Y. Song, L.L. Henry, W.T. Yang, In situ rapid cooling microfluidic process for the formation of stable cobalt amorphous nanoparticles Langmuir (revised reversion under reviewing)Google Scholar