Structure and Magnetism Evolution from FeCo Nanoparticles to

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Structure and Magnetism Evolution from FeCo Nanoparticles to Hollow Nanostructure Conversion for Magnetic Applications Zhuolei Zhang, Jingming Zhang, Akila C. Thenuwara, Daniel R. Strongin, Yugang Sun, and Shenqiang Ren ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01488 • Publication Date (Web): 01 Oct 2018 Downloaded from http://pubs.acs.org on October 4, 2018

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ACS Applied Nano Materials

Structure and Magnetism Evolution from FeCo Nanoparticles to Hollow Nanostructure Conversion for Magnetic Applications Zhuolei Zhang†, Jingmin Zhang†, Akila C. Thenuwara‡, Daniel R. Strongin‡, Yugang Sun‡, Shenqiang Ren†* †

Department of Mechanical and Aerospace Engineering, and Research and Education in Energy,

Environment & Water (RENEW) Institute, University at Buffalo, The State University of New York, Buffalo, NY 14260 ‡

Department of Chemistry, Temple University, Philadelphia, PA 19122, USA.

KEYWORDS: Magnetic Nanoparticles • Kirkendall Effect • Hollow Structure • Exchange Coupling • Ion Exchange

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ABSTRACT

Hollow nanostructures have witnessed an increasing interest in emerging fields, including energy storage electrodes, nanocatalysis, photonics, and biomedical carriers, etc. The fundamental mechanisms for hollowing conversion chemistry of complex nanoalloys are indispensable to grow such nanostructures with controlled compositions and functionalities. Here, we study the Kirkendall effect on magnetic Fe-Co bimetallic alloys with the formation of hollow nanostructures for exchange interactions. The rich diffusion kinetics of binary Fe-Co alloys accompanied with its conversion from solid, core-shell to hollow nanostructures have been captured through X-ray diffraction pattern and electron microscopy. We further report the cation exchange improved magnetic performance of hollow nanostructures by increasing the crystallinity.


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Colloidal hollow nanoparticles (HNPs) have attracted tremendous attention due to their distinctive characteristics, such as high surface-to-volume ratio, low density, and large loading capacity, leading to a variety of emerging applications in catalysis, sensing, and energy storage.18

In this context, magnetic hollow nanoparticles of transition metals and their oxides provide an

extra degree of freedom to control their magnetic properties. For example, Wang et al. reported the preparation of hierarchical porous hollow nickel microspheres and studied their magnetic properties. Attributting to the hierarchical porous hollow structure, enhanced coercivity and remanent magnetization have been found as compared with bulk nickel, and free Ni nanoparticles. In addition, the structural evolution from the solid metal to hollow nanostructures enables a new pathway to control exchange interactions between the core and shell materials.9-10 Nemati et al. reported a interesting phenomenon that the low-temperature magnetic behavior of particles changes from a collective super-spin-glass system mediated by dipolar interactions for the Fe/γ-Fe₂O₂ core/shell nanoparticles to a frustrated cluster glass-like state for the shell nanograins in the γ-Fe₂O₂ hollow morphology.11 An enhanced surface area of the hollow nanoparticles leads to unique magnetic contributions from both inner and outer surface spins. The additional presence of an inner surface area results in an enhanced spin disorder and surface anisotropy.11-12 The magnetic hollow nanoparticles also shows great potential in biomedical application. The intrinsic properties of the hollow structures, such as large surface area, low density, abundant inner void space, and the magnetic property make it a good candidate to serve as carriers for potential drug delivery application. In addition, the hollow magnetic nanoparticles could not only be used for drug delivery but could also for hyperthermia therapy as dual therapy due to its magnetic property. Application of radio frequency field increases the temperature of



nanoparticles thereby increasing the temperature of nanoparticles containing cells as well, resulting in cell death by thermoablation.13 Different synthetic approaches have been applied to grow hollow nanoparticles, including the sacrificial template, selective etching, Ostwald ripening, galvanic erosion, and Kirkendall effect.2,14 The Kirkendall effect is known to control the core-shell morphology of monodispersed nanoparticles in the nanoscale precision with high crystallinity.15-16 Two consequential processes occurring in the Kirkendall effect include the core-shell heterostructure formation resulting from the surface oxidation using the oxidizing agent (sulfur, selenium, oxygen, phosphorus, etc), and the hollow structure formation by vacancy coalescence stemming from faster outward diffusion of the metal atoms than that of the inward diffusion of anions.16-18 Single component metal nanoparticles have been utilized to investigate the hollowing formation mechanism and Kirkendall kinetics.18-20 However, the Kirkendall kinetics and magnetic properties during the structural evolution of multi-component metal alloy nanoparticles remain unknown, such as binary and even ternary metal alloys. In this study, the magnetic binary FeCo particles are selected to serve as the prototypical system to investigate the kinetics, structure and magnetic property evolution during the hollowing process. Mainly, the FeCo metal alloy nanoparticles with sizes ranging from 7.7 nm to 16.3 nm are synthesized by the thermal decomposition method. The particles were then oxidized into hollow paticles by Kirkendall. The decreased saturation magnetization and the increased coercivity during the hollowing process is due to the formation of Co1-xFe (II)xFe2(III)O4 spinel thin layer with the high spin−orbit coupling. Moreover, applying the ion exchange approach by replacing Fe2+ by Co2+ and subsequent crystallinity increase through thermal treatment, the saturation magnetization and coercivity of hollow nanostructures can reach to 72 emu/g and 1240 Oe, respectively.


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By varying the reaction temperature from 453 K to 563 K, FeCo nanoparticles can be synthesized with different sizes ranging from 7.7 nm to 16.3 nm (Figure 1a-d and S1). A relatively lower injection temperature of 453 K leads to less nucleation but grows to a larger size by consuming all the remaining precursor monomers. On the contrary, a higher injection temperature of 563 K results in more substantial amount of nucleation and completes with a smaller size due to the limited remaining monomers. In addition, the subsequent not uniform growth process at 453 K results in a broader size distribution of ± 25% (Figure 1a), while the size distribution shows ±5% (Figure 1d) at a higher temperature due to the shorter growth period. The exact temperature dependent particle size is presented in Figure 1e. In addition, we further investigate the size effect on the magnetization of FeCo nanoparticles. As shown in Figure 1f, the magnetization increases with the increasing particle size of FeCo mainly due to the spin canting effect, where the smaller magnetic nanoparticles show the more substantial degree of spin-canting effect.21-22 Therefore, when the size of the FeCo nanoparticles increases from 7.7 to 16.3 nm, the corresponding saturation magnetization increases from 72 emu/g to 167 emu/g. To investigate the morphology and magnetic evolution during the oxidation process at 423 K, FeCo nanoparticle with 10.6 nm are selected for the following studies due to its nearly monodispersed spherical morphology and high saturation magnetization. Transmission electron microscope (TEM) is applied to visually reveal the structural transformation during the oxidation process. The controlled air flow through the colloidal solution makes it possible to oxidize the FeCo nanoparticles. At the early stage, a thin oxide layer is formed on the surface of the FeCo nanoparticles, which shows a subtle contrast change (Figure 2b). The air flow is bubbled at the central location in the solution and then diffused to the whole solution. The formation mechanism of oxygen atoms in liquid solution could be explained by the bubble template



mechanism.23 The nanocrystals tend to aggregate due to their high surface energy, while lots of bubbles produced during the reaction provide aggregation centers in the meantime. To decrease the interfacial energy to the minimum, small nanocrystals aggregated around the gas–liquid interface between bubble and solvent and finally hollow capsules formed. The outside oxygen atoms are adsorbed onto the surface oxide shell. Due to the tunneling electrons into the surface oxide layer, an equilibrium can be established between the metal core and the surface adsorbed oxygen, leading to the metal ion outward diffusion and oxygen atoms inward diffusion, accompanied by vacancy diffusion, which is resulted from the built-in electric field in the Co1xFe

(II)

xFe2

(III)

O4 thin oxide layer. The diffusion process of Fe and Co metal ions to the surface of

the nanoparticles is confirmed by the slight increase of average particle size (Figure 2g). The difference of the diffusion rate of inner FeCo metal atoms (faster) and outer oxygen atoms (slower) in the thin layer results in the formation of voids within the core. These separated voids start to occur at the interface between the FeCo core and the oxidized Co1-xFe (II)xFe2(III)O4 shell distinguished from the distinct contrast difference (Figure 2c). With further oxidation, the voids can become more prominent and then coalesce into single/double voids in the shape of a halfmoon (Figure 2d), and eventually, the core disappears and the voids become a full moon-like shape, due to the fully outward diffusion of the metal atoms. In the hollowing process, the evolution of the cores does not have uniform geometric shape, which makes it challenging to evaluate the remaining core volume accurately. However, the oxidized shell thickness increases gradually in the way of circular rings. Therefore, the hollowing evolution process can be evaluated by the average particle size evolution and oxidized Co1-xFe

(II) (III) O4 xFe2

layer thickness evolution. The real time core and shell dimension

monitorings are shown in Figure 2g-i. Not only the oxidation time influences the hollowing


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process, but several other factors also have effect on the formation of the hollow nanostructures, such as the oxidation reaction temperature (Figure 2g), the size of the inner FeCo cores (Figure 2h), and the oxidative species (Figure 2i and S2), etc. To get insight into the void evolution, we measured the metal ion diffusion rate by calculating the radius/volume of the shell as a function of time. The Fick’s law could be used for estimating the diffusion coefficient in the oxide shell. The diffusion flux in the oxide shell is approximately

, where ε is the diffusion coefficient, ρ is the density, and d is the

average oxide shell thickness. The diffusion cross section as the inner surface area of the shell is calculated as S = 4πr2, where r is the inner radius of the shell. The time t1/2 for FeCo in the core decreases to half of its initial volume can be expressed as:

Therefore, the average diffusion coefficient ε of the 10.7 nm nanoparticles oxidization at 423K is 9.46×10-3 nm2/min, and it increased to 20.16×10-3 nm2/min when the oxidation temperature is at 463 K. In addition, This is because the reaction temperature under the ambient environment can drastically affect the Kirkendall reaction by modifying the diffusion rate, activation energy. At a temperature below 353 K, the FeCo particles can be oxidized with a thin Co1-xFe (II)xFe2(III)O4 surface layer. However, the thickness of thin oxide layer barely changes even with a prolonged time due to the low diffusion rate and activation energy of inner metal atoms. The diffusion rate of the inward oxygen and outward metal atoms are balanced, and therefore, no apparent voids can be found in these particles. While the formation of the hollow structure becomes much faster when the temperature is increased to 493 K, and the FeCo nanoparticles can be fully oxidized in



3 hours under the same environment. In addition, when the average size of the nanoparticles decreased from 10.6 nm to 7.7 nm, the average diffusion coefficient is 33.92×10-3 nm2/min at the temperature of 463 K. This is because the smaller particles have larger surface-volume ratio, leading to relatively more oxygen atoms adsorbing at the particles surface (more reaction action sites), thus facilitating the hollowing process. By mixing small and large FeCo nanoparticles together with an average size of 7.7 nm and 10.6 nm, they are oxidized under the same condition which show the smaller particles with a fully hollow structure and the larger particles with the core-shell structure (Figure S3). Furthermore, the oxidative species play an essential role in the hollowing process of FeCo binary alloys. By replacing the oxygen with the sulfur solution, the hollowing process can be completed in ~ 60 s. The average diffusion coefficient increased significantly to 4.85 nm2/min when the oxidizing species changed from air gas flow to S-ODE solution. The fast hollowing process of sulfurization could be explained as that the sulfur precursor is more active when reacting with metal particles due to its larger electron affinity of zero-valent element (1.461 eV for O and 2.077 eV for S),24 which increases the built-in electric field in the thin transition layer and facilitates the electrons tunneling. To reveal the magnetic property and structure information, the magnetization hysteresis loops together with X-ray diffraction (XRD) measurements are investigated on the FeCo nanoparticles at different oxidation stages from solid, core-shell and hollow nanostructures. The FeCo particles with 9.6 nm shows high saturated magnetization of 124 emu/g and the coercivity of 200 Oe, which is consistent with previous report at the size of ~10 nm without thermal annealing (Figure 3a and b).25-28 The corresponding XRD pattern is shown in Figure 3c where the sharp diffraction peaks can be assigned to the (110) and (200) reflections of the bcc-FeCo alloy nanoparticles,


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indicating the crystalline nature of the as-prepared nanoparticles.29 When the particles are gradually oxidized at the surface, the saturated magnetization of the core-shell structure at ~1h oxidation time is decreased to ~100 emu/g while the coercive field is slightly increased to 310 Oe (Figure 3b). The intensity of the FeCo peak is decreased in the XRD pattern. We didn’t see clear typical diffraction peaks from Co3O4 structure and CoO structure in the core/shell structure,30-31 but we can see typical diffraction peak at about 2theta= 42 from (400) of CoFe2O4 or Fe3O4 structure. This is because Fe has a stronger chemical affinity with oxygen 116.619 Kcal, the than that of Co (

=-

standard magnitudes refer to chemical affinities of metals for oxygen.) = -108.222 Kcal).32 The iron oxide is formed first and then the cobalt is

oxidized and diffused into iron oxide to form CoFe2O4 structure. The increased coercive field can be assigned to the formation of the CoFe2O4 structure in the shell with the higher spin−orbit coupling. The similar phenomenon is also found in previous literature that during the oxidation of FeCo nanoparticles, the Fe atoms are oxidized first, and subsequently the formation of CoFe2O4 structure.33-34 The saturated magnetization is further decreased to ~90 emu/g at 3h oxidation time and ~82 emu/g at 5h oxidation time together with the coercive field is slightly increased to 420 Oe and 530 Oe, respectively. The increased coercive field can be assigned to the formation of the CoFe2O4 structure in the shell with the higher spin−orbit coupling, which can be distinguished from the XRD pattern by the (311), (222), and (400) peaks. The peaks of FeCo core in the XRD pattern completely disappear with the saturated magnetization of 78 emu/g and coercive field of 610 Oe in the final hollow crystals. Due to the solution oxidation process, it is challenging to fully oxidize all zero-valent Fe to Fe3+ ions.15 The existance of Fe3O4 (or FeFe2O4, it is difficult to distinguish between Fe2+ and Fe3+ in the XRD measurement due to the similar spinel structure of FeFe2O4 and CoFe2O4) would



significantly decrease the phase purifity and subsequently the magnetic property of the oxidized material. A simple post-synthetic treatment, named cation exchange, is introduced to replace Fe2+ by new Co2+ cations with the preservation of the original sublattice (Figure 4a), leading to an increased phase purity and enhanced magnetic property of the hollow structures. This procedure greatly preserves the structures, sizes, and shapes of nanocrystals (Figure 4b). In addition, as a strong coordination ligand, hydrophobic trioctylphosphine oxide (TOPO) helps to decrease the ripening effect, while the nanocrystals undergo fast Ostwald ripening without TOPO ligands, results in irregular nanoparticles. The advantages of using TOPO as a solvent, surfactant, and capping agent include the high boiling point, allowing reactions to proceed even higher than 613 K, facilitating high-temperature synthesis and further annealing. Furthermore, oleylamine in the original system has high affinity with Co2+ and Fe2+, which means it is readily to dissolve some insoluble oxide by etching at high temperature. While the phosphine ligand has much lower affinity with Fe2+ and Co2+, the addition of stronger TOPO ligands would significantly decrease the size and shape change by influencing the equilibrium between solvated Fe2+ or Co2+ ions and cobalt/iron in the lattice.35-36 After the introduction of the cobalt precursor, the saturated magnetization of the CoFe2O4 hollow particles gradually decreased to 61 emu/g while the coercivity increased to 1050 Oe (Figure 4c-d). To further improve the coercivity and saturated magnetization, the particles are thermally treated with the coercivity of 1240 Oe and saturated magnetization of 71 emu/g, due to the enhancement of particle crystallinity (Figure 4e). In conclusion, the kinetics, structure and magnetic property evolution of FeCo metal alloy nanoparticles during the oxidation hollowing process are investigated. The conversion from solid, core-shell to hollow nanostructures is captured through X-ray spectroscopy and electron microscopy. The decreased saturation magnetization and the increased coercivity during the


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hollowing process is due to the formation of Co1-xFe

(II)

xFe2

(III)

O4 thin layer with the high

spin−orbit coupling. Finally, applying the ion exchange approach by replacing Fe2+ by Co2+ and subsequent crystallinity increase through thermal treatment, the saturation magnetization and coercivity of hollow nanostructures can reach to 72 emu/g and 1240 Oe, respectively. The synthetic hollowing scheme presented here could be expanded to other magnetic material systems for improved exchange interactions.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. TEM images of FeCo nanoparticles with different sizes. Hollowing process with oxidizing species of S-ODE. TEM image of mixed ~7.7 nm and ~10.6 nm particles oxidized under the same condition at 150 oC for 400 mins. Corresponding Author Email: [email protected] Author Contributions All authors discussed the results and commented on the manuscript. Z.Z. carried out experiments and drafted the paper. J.Z. participated into the discussion. A.T., D.S. performed, analyzed the XPS experient and write the corresponding part. Y.S. and S.R. designed, guided and directed the project. Notes The authors declare no conflict of interest.



ACKNOWLEDGMENT S.R. thanks the financial support from the U.S. National Science Foundation (NSF) under the CAREER Award No: NSF-DMR-1830749. Work (D.S.) was supported as part of the Center for the Computational Design of Functional Layered Materials, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award #DE-SC0012575. Y.S. acknowledges the startup support from Temple University.

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Figure Captions Figure 1. The FeCo nanoparticles with different size synthesized by thermal decomposition method. a. FeCo nanoparticles synthesized at 453 K with the size distribution diagram in the inset. b. FeCo nanoparticles synthesized at 493 K with the size distribution diagram in the inset. c. FeCo nanoparticles synthesized at 523 K with the size distribution diagram in the inset. d. FeCo nanoparticles synthesized at 563 K with the size distribution diagram in the inset. e. Temperature dependent average size synthesized by thermal decomposition method. f. M-H loops of FeCo nanoparticles with various average dimension ranging from 7.7 nm to 16.3 nm.

Figure 2. The morphology evolution of FeCo nanoparticles during the oxidation process and the kinetics investigation on the impact of different factors. a-e. The morphology evolution of FeCo nanoparticles from 0h (a), 1h (b), 3h (c), 5h (d),7h (e). f. The scheme of Kirkendall effect during the hollowing process. g. Average particle dimension evolution during the hollowing process with different oxidizing temperature of 423 K and 463 K. The inset is the average thickness of the oxidation layer. h. Average particle dimension evolution during the hollowing process with different core size of 7.7 nm and 10.6 nm. The inset is the average thickness of the oxidation layer. i. Average particle dimension evolution during the hollowing process with different oxidizing species of S-ODE and oxygen (air flow). The inset is the average thickness of the oxidation layer.

Figure 3. Magnetic property and structure information during the oxidizing hollowing process. The M-H loop of nanoparticles at different oxidation stage from pure metal particles, core/shell


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particles and hollow particles (a) and corresponding saturated magnetization and coactivity field (b). (c) The structure information at different oxidation stage recorded by XRD measurement.

Figure 4. Cation exchange and thermal annealing as the post synthetic treatment for the magnetic property enhancement. a. Scheme illustration of the cation exchange by Co2+ treatment in the hollow nanoparticles. b. TEM images of hollow nanoparticles before and after cobalt treatment. c. The M-H loop of nanoparticles during the Co2+ treatment. d. The corresponding saturated magnetization and coactivity field during the Co2+ treatment. e. The M-H loop of hollow nanoparticles before and after thermal annealing process.



Figure 1


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Figure 2



Figure 3


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Figure 4



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Structure and Magnetism Evolution from FeCo Nanoparticles to

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