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Letter
The Smallest (1.6 nm) Bimetallic CoPt3 Superparamagnetic Nanoparticles Mohamed Karmaoui, Joao S Amaral, Luc Lajaunie, Harinarayanan Puliyalil, David Maria Tobaldi, Robert C. Pullar, João António Labrincha, Raul Arenal, and Uros Cvelbar J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b01768 • Publication Date (Web): 27 Sep 2016 Downloaded from http://pubs.acs.org on September 28, 2016
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The Smallest Bimetallic CoPt3 Superparamagnetic Nanoparticles Mohamed Karmaoui,a* J. S. Amaral,b Luc Lajaunie,c Harinarayanan Puliyalil,d,e David M. Tobaldi,f Robert C. Pullar, f,g João A. Labrincha,f Raul Arenal,c,h and Uroš Cvelbar d,e
a
School of Chemistry-College of Engineering and Physical Sciences-University of Birmingham Edgbaston, Birmingham B15 2TT UK b
c
Department of Physics / CICECO – Aveiro Institute of Materials, University of Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal
Laboratorio de Microscopías Avanzadas, Instituto de Nanociencia de Aragón, Universidad de Zaragoza, 50018 Zaragoza, Spain d
e
f
Department F4, Jozef Stefan Institute, Jamova cesta 39, Ljubljana-1000, Slovenia
Jozef Stefan international Postgraduate School, Jamova cesta 39, Ljubljana-1000, Slovenia
Department of Materials and Ceramic Engineering / CICECO – Aveiro Institute of Materials, University of Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal g
Department of Materials, Imperial College London, London, SW7 2AZ, UK h
ARAID Foundation, 50018 Zaragoza, Spain
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Abstract We present for the first time a non-aqueous sol–gel route to produce ultra-small (< 2 nm) magnetic bimetallic CoPt3 nanoparticles (NPs). The one-pot procedure is carried out at low-temperature (180 ºC) using benzyl alcohol, acting as both reducing agent and solvent. The highly monodisperse CoPt3 NPs were investigated with innovative advanced x-ray methods (whole powder pattern modelling, WPPM), HR-TEM, XPS and SQUID magnetometry. XPS showed Co was mostly in metallic form, but with a very small amount of CoO on the NP surface. The spherical NPs had an ultra-small diameter of 1.6 nm, and could self-assemble in aligned linear chains, or nanobelts, of single NPs. They are superparamagnetic, with blocking temperature ~20 K, and coercivity at 10 K of 27.9 kA m-1 (~350 Oe). However, there is evidence of a second magnetic phase (probably CoO) in the ZFC magnetisation curve, which enhances their magnetisation values, without significantly affecting their superparamagnetism.
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Keywords: Sol-Gel Process, Solvothermal Synthesis, Bimetallic Nanocrystals, HR-STEM/HAADF, XPS, Coercivity
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General interest in magnetic nanoparticles (MNPs) has grown due to the need generated by novel biomedical, diagnostic and therapeutic applications, which require combinations of electrochemical and other multifunctional properties. As a part of this research flow, bimetallic nanoparticles (NPs) have emerged for applications in the fields of heterogeneous catalysis, electrochemistry, magnetic storage, spintronics, biomaterials and biomedical systems.1-5 As an example, high-coercivity magnetic NPs, especially FePt and CoPt alloys, have in particular attracted much attention, due to applications in magnetic recording6 or as components for rare-earth-free permanent magnet composites (this is depending on size, composition or shape of the bimetallic NPs).7 It is also well known that all these factors exhibit a strong influence on the catalytic and ferromagnetic properties of bimetallic CoPt3 NPs.8-13 It is essential to understand the relationship between NP synthesis and the resultant properties. Moreover, the tendency is to achieve the goal of a single domain within one NP, which will allow the tailoring of resulting magnetic properties. Therefore, making ultra-small NPs (below the single magnetic domain size limit), with a narrow size distribution (monodisperse), is crucial in order to increase the maximum information density in magnetic recording, to optimise the magnetic coupling, and increase the volume of soft magnetic phase in permanent magnet nanocomposites. To this end, we selected cobalt-platinum (CoPt3) NPs as an object of study. Reviewing potential synthesis methods, many chemical and physical processes have been developed over the years to accomplish a controlled synthesis of such NPs. These methods can be classified into several different synthetic approaches, including impregnation methods,14 solution phase synthesis,15 wet chemical routes with organometallic approaches,16 simultaneous reduction/thermal decomposition,17-18 pulsed laser deposition (PLD),19 synthesis in ionic liquids,20 transmetalation reactions synthesis,21 polyol processing,9 plasma processing22-24 or hybrid processing with liquids and plasma.25-26 Recently, the most interesting route to date was reported, where monodisperse MPt (M = Fe, Co, Ni, Cu, Zn) NPs were prepared from the oleylamine reduction of metal salts. The particles were an egg-like shape, and 9.5 nm in diameter.27 4 ACS Paragon Plus Environment
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However, none of the existing methods yielded satisfactorily small bimetallic NPs. Therefore, we have to explore new potential routes of controlled synthesis. In general, going back to solid state ionics, alcohols and amines can all serve as the reactant source to ensure fast reduction of various metal salts. Among these options, benzyl alcohol offers a unique example, since it can serve as both the reducing agent and stabilizer at low temperatures, to produce different metal/bimetal NPs with small sizes and variable shape.28 In the frame of a non-aqueous approach, the preparation of a large diversity of NPs by the benzyl alcohol route has been demonstrated over the last few years, including the preparation of hybrid materials,29-30 titanium metal oxides31 and alkalineearth/transition metal aluminate NPs.32-33 Here we present a completely new route for producing metal and bimetal NPs by means of a non-hydrolytic sol-gel method. With this we answer the challenge of low-temperature synthesis for the production of stable, consistently sized and shaped, and highly crystalline ultra-small magnetic bimetallic CoPt3 NPs. This process in general relates to an advanced method for the production of magnetic metal and bimetallic NPs.34 The “benzyl alcohol route” presented here is able to produce magnetic bimetallic CoPt3 NPs with ultra-small crystal size as well as spherical shape. In comparison with other existing methods, it can yield high quality particles of a very small spherical size (
