Surface-Stress-Induced Phase Transformation of Ultrathin FeCo

Dec 28, 2015 - Large surface-stress-induced phase transformation in Au nanowires triggers and stabilizes metastable tetragonal FeCo nanostructure to e...
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Surface-Stress-Induced Phase Transformation of Ultrathin FeCo Nanowires Jian Shen, Maogang Gong, Qilin Dai, and Shenqiang Ren* Department of Mechanical Engineering, Temple University, Philadelphia, Pennsylvania 19122, United States S Supporting Information *

ABSTRACT: Ultrathin metal nanowires have attracted wide attention becau se oftheir unique anisotropy and surface-to-volume effects. In this study, we use ultrathin Au nanowires as the templating core to epitaxially grow magnetic iron−cobalt (FeCo) shell through metal-redox with the control on their thickness and stoichiometry. Large surface-stress-induced phase transformation in Au nanowires triggers and stabilizes metastable tetragonal FeCo nanostructure to enhance its magnetic anisotropy and coercivity. Meanwhile, under illumination, plasmon-induced hotspot in ultrathin Au nanowires enables the light-control on magnetic characteristics of FeCo shell. This study demonstrates the feasibility of surface-stress-induced phase transformation to stabilize and control metastable nanostructures for enhanced magnetic anisotropy, which is one of the key properties of functional magnetic materials. KEYWORDS: FeCo, magnetism, core/shell nanowires, phase transformation

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Therefore, one of the most challenging focuses is to control surface reconstruction of Au nanowires, and to guide the phase transformation in its nearest neighbors of surface layers,16 particularly exciting potential for Au nanowire-based core/shell nanostructures. In such nanostructures, phase transformation of surface shell could exhibit dramatically different physical and chemical characteristics from their bulk counterpart. Iron cobalt (FeCo) alloys constitute an interesting class of soft magnetic materials, exhibiting high saturation magnetization, large permeability, and high Curie temperatures.17 However, as a “soft” magnetic material, FeCo has a bodycentered cubic (bcc) nanostructure with small magnetocrystalline anisotropy energy (MAE), which limits their application in energy-critical technologies requiring high energy product.18 Though computational efforts have predicted a giant uniaxial MAE in tetragonal FeCo nanostructures,17,19 it is still a challenging task to grow metastable tetragonal FeCo nanostructures. In this study, we report a facile epitaxial metal-redox growth method to deposit the FeCo shell onto

ne-dimensional nanostructures, such as metal nanowires, have attracted a great deal of interest because of their unique physical and chemical properties, which are associated with their highly anisotropic geometry and strong finite size effect, as well as increasingly large surface-to-volume ratio at nanoscale length.1,2 The remarkable surface and anisotropic properties of metal nanowires have motivated the development of functional metal nanostructures, such as high magnetocrystalline anisotropy materials and transparent conductive interconnects, as well as tandem nanocatalysts.3−6 There are numerous studies dedicated to the structural change of ultrathin nanowires via theoretical calculation, which is predominantly driven by surface-stress-induced phase transformation.7−10 The large tensile surface-stress component within {100} face-centered cubic (fcc) 5d metal surfaces tends to reconstruct into a {110} body-centered tetragonal (bct) structure.11,12 Especially, ultrathin gold (Au) nanowires with the diameter smaller than 2 nm have been the subject of intense theoretical and experimental studies to understand surface energy effect on their phase transformation in the [110] crystallographic orientation,13,14 where the wringling structure of ultrathin Au nanowires is resulted from the atomic displacment in the (110) facet under surface stress. 15 © XXXX American Chemical Society

Received: November 13, 2015 Accepted: December 28, 2015

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DOI: 10.1021/acsami.5b10991 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces

Figure 1. Schematic structural evolution from (a) ultrathin gold nanowire core, (b) (220) fcc-Au/(200) bcc-FeCo (core/shell) nanowires, (c) surface stress induced phase transformation of (220) bct-Au/ (200) bct-FeCo (core/shell) nanowires. From top to bottom, the diagram is threedimension crystal structure, cross-section of nanowires, and three-dimension crystal structure observed from the x and y directions, respectively.

Figure 2. (a) Transmission electron microscopy (TEM) image of ultrathin gold nanowires (the inset shows their diameter distribution). (b) UV− vis−NIR spectrum of ultrathin gold nanowires. (c) TEM image of ultrathin Au/FeCo (core/shell) nanowires with the FeCo shell thickness of 0.83 nm. (d) The scanning TEM (STEM) image of HRTEM of ultrathin Au/FeCo (core/shell) nanowires. (e−g) The elemental mapping images of the squared area in c, exhibiting the Au, Fe/Co elements and Au/FeCo (core/shell).

ultrathin Au nanowire core (Figure 1a, b) to enable the core/ shell nanostructures, where we select the lattice-matched Au and FeCo phases as a prototypical example to illustrate the effect of surface-stress-induced phase transformation on the

magnetic performance of FeCo shell. Because of the role of surface stress, the structure of ultrathin Au nanowires can be transformed from fcc to bct structure, especially, in the crystallographic [110] orientation which may induce the B

DOI: 10.1021/acsami.5b10991 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces tetragonal FeCo nanostructures. Therefore, we study the surface-stress-induced phase transformation of ultrathin Au nanowires to control and achieve tetragonal FeCo nanostructures. Figure 1b, c shows the structural change of Au core and FeCo shell before and after phase transformation. In addition, the thickness and stoichiometry of the FeCo shell could be tuned by controlling the reaction conditions, which have been confirmed from both structural and chemical analysis. Because of the plasmonic nature of the Au nanowire core, we also demonstrate the plasmon-induced hotspot effect on the tunability of magnetic characteristics of the FeCo shell. Our interest in the epitaxial templating effect of ultrathin Au nanowires has mainly risen due to the experimental observation and theoretical prediction on surface reconstruction and atomic displacement in the (110) plane of ultrathin Au nanowires under surface stress. In this study, single crystalline ultrathin Au nanowires with a large aspect ratio are synthesized at room temperature.20 Figure 2a shows transmission electron microscopy (TEM) image of uniform Au nanowires with an average diameter of 1.72 nm (the diameter distribution is shown in the inset of Figure 2a, where high resolution TEM (HRTEM) image indicates the axial growth direction [111] of ultrathin fccAu nanowires with {110} surfaces for the sequential FeCo shell coating (Figure S1). In addition, the broad photoabsorption is shown in its UV−vis-NIR spectrum, which is resulted from plasmonic Au nanowires with high aspect ratios (Figure 2b).21 Furthermore, it should be noted that these broad-spectral Au nanowires exhibit extraordinary photothermal effect, which could efficiently convert plasmon induced hotspots to thermal heat for surface structural control (Figure S2).22 By calculating the lattice mismatch between Au fcc (110) and FeCo bcc (100), it is found that the mismatch value is 0.7% (the interplanar distance of Au fcc (110) and FeCo bcc (100) is 0.288 and 0.286 nm, respectively). Therefore, it is inferred that FeCo could be epitaxially coated on the surface of Au (110) facet. The magnetic FeCo shell is coated onto ultrathin Au nanowire core by simultaneously decomposing the iron and cobalt chemical precursors in the solution through the metalredox method,23 where this spontaneous reaction allows the nanoalloying for the FeCo shell formation. The TEM images (Figure 2c, d) show that the FeCo shell uniformly deposits on Au nanowire core, and magnetic FeCo shell thickness could be varied from 0. 83 to 1.59 nm by controlling the reaction time from 30 to 120 min, respectively (The details are shown in Figure S3). Figure 2e−g show the scanning TEM (STEM) and elemental mapping images of ultrathin Au/FeCo (core/shell) nanowires, which further confirm the chemical composition of the Au core and FeCo shell. The Au core is fully encapsulated by the FeCo shell with a well-defined interface. Meanwhile, as shown in HRTEM image of Figure S1, the interplanar distances of the core and the shell regions show 0.235 and 0.143 nm, respectively, confirming the growth of (200) bcc-FeCo shell along the axis direction of the (111) fcc-Au core. The magnetic characteristics of Au/FeCo (core/shell) nanowires exhibit a soft magnet behavior, which show the saturation magnetization (Ms) of 59.5 emu/g and the coercivity (Hc) of 298 Oe (Figure 3a). Furthermore, magnetic performance of nanowires could be tuned by the thickness and stoichiometry of the FeCo shell, which are controlled by the reaction time and tuning iron and cobalt precursor concentrations. As a noble metal, Au exhibits plasmon-resonanceinduced hotspot under photoexcitation by absorbing incident photons,24 which enables surface thermal expansion through

Figure 3. (a) Magnetic hysteresis (M−H) loops of ultrathin Au/FeCo (core/shell) nanowires. (b) Schematic measurement of plasmoninduced hotspots of Au nanowire core on magnetic characteristics of FeCo shell. (C) Light intensity-dependent magnetization plot of ultrathin Au/FeCo (core/shell) nanowire films under a bias magnetic field (298 Oe).

local heating at the surface of Au nanowires,25,26leading to the structural change in ultrathin FeCo shell (Figure 3b). As shown in Figure S2, the structural change of ultrathin Au nanowires is indeed observed under light illumination (550 nm wavelength and light intensity of 20 mW/cm2). Because of the structuredependent uniaxial magnetocrystalline anisotropy of FeCo,27 the magnetic properties of the ultrathin Au/FeCo (core/shell) nanowires could be tuned by using the plasmon effect of ultrathin Au core under light illumination. Figure 3c shows that light intensity-dependent magnetization plot of ultrathin Au/ FeCo (core/shell) nanowires under a bias magnetic field (298 Oe), where the thermal expansion by plasmon hotspot indeed induces the magnetization change of FeCo surface shells. This confirms the plasmonic Au core effect on structure-controlled magnetic characteristics of FeCo shell. To further investigate the thermal effect on phase transformation, we annealed the core/shell nanowires at a selected temperature. Magnetic characteristics of the core/shell nanowires could be tuned by the thickness and stoichiometry of FeCo shell, which are controlled by the reaction time and the injection mole ratios of cobalt and iron precursors. Figure 4a shows the magnetic hysteresis (M−H) loops of ultrathin Au/ FeCo (core/shell) nanowires at different shell thickness after annealing at 633 K. As shown in Figure 4a, the optimum magnetic coercivity could be reached up to 982 Oe with the

Figure 4. (a) The M−H loops of ultrathin Au/FeCo (core/shell) nanowires at different shell thickness after annealing at 633 K. (b) Shell thickness and stoichiometry of FeCo on the coercivity of ultrathin Au/FeCo nanowires. C

DOI: 10.1021/acsami.5b10991 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 5. (a) M−H loops of ultrathin Au/FeCo nanowires with an average shell thickness of 0.83 nm and the cobalt mole percentage of 66.7%, annealed at different temperatures. (b) Annealing temperature-dependent coercivity of ultrathin Au/FeCo nanowires. (c) X-ray diffraction (XRD) patterns of ultrathin Au/FeCo nanowires before and after thermal annealing. The blue and red color labeled peaks represent the diffraction from FeCo and Au phases. (d) HRTEM images of ultrathin Au/FeCo nanowires after annealing.

temperature of 633 K. It has been predicted that the structural change of surface atoms within ultrathin Au nanowires is triggered by internal stress along [100] and [110] orientations due to thermal expansion as elevating the temperature below the melting point (650 K).7,26 Therefore, in our study, the optimum coercivity of ultrathin Au/FeCo nanowires annealed at 633 K could be attributed to the tetragonal distortion of thin FeCo shell under surface stress induced phase transformation of Au nanowire core. However, further increasing the annealing temperature beyond the melting point of ultrathin Au nanowire core,28,29 ultrathin Au/FeCo core/shell nanowires are converted into the separate spherical nanocrystals between Au and FeCo, where surface stress induced phase transformation does not apply in this case, resulting in a reduction of coercivity. The TEM images of ultrathin Au/FeCo core/shell structures after annealing at 773 K is presented in Figure S4. To obtain the detailed information on phase transformation within ultrathin Au nanowire core and tetragonal distortion of FeCo shell, X-ray diffraction (XRD) study is carried out before and after annealing at 633 K (Figure 5c). Before annealing, the (220) peak represents (220)-fcc Au nanowires.30 During thermal annealing, surface tensile stress within the Au core is built up from the thermal expansion, which could cause the surface atom displacement of Au (220) plane, ultimately, resulting in reconstructure from a fcc (220) structure to a bct (220) structure.Therefore, we infer that the (220) peak of ultrathin Au nanowires could be transformed into the (220) Au bct structure after annealing at 633 K, which further triggers the tetragonal distortion of FeCo shell. As shown in the XRD spectra, the peaks of Au (220) and FeCo (200) overlap before and after annealing, which indicate that the reconstructure of Au (220) facet may results in the tertragonal structure of FeCo

FeCo shell thickness of 0.83 nm. It should be noted that the coercivity of FeCo shell decreases as increasing its thickness (Figure 4b). We have attributed this behavior due to the critical thickness hc of FeCo shell (0.585 nm), above which the strain energy induced by phase transformation of the Au nanowire core does not effectively reconstruct the FeCo surface shell (the detailed calculation is shown in Supporting Information). Therefore, when the FeCo shell thickness exceeds extensively above the critical length, the outer atomic layers of FeCo shell relaxes and shows a weak tetragonal distortion, leading to a reduced coercivity. In addition, by varying the injection concentration of cobalt and iron precursors, the optimum coercivity is achieved at the stoichiometry of Fe34Co66 with a constant shell thickness of 0.83 nm (Figure 4b). Therefore, it is important to obtain the optimum shell thickness and stoichiometry of FeCo shell to enable tetragonal distortion for enhanced magnetic anisotropy and coercivity, which is trigged by surface-stress-induced phase transformation of ultrathin Au nanowire core under thermal annealing. The annealing temperature is the key parameter to control phase transformation of Au fcc (110) nanowires, which dictates the tetragonal distortion degree within the FeCo shell. Figure 5a shows the M-H loops of ultrathin Au/FeCo nanowires with an average shell thickness of 0.83 nm and the stoichiometry of Fe34Co66, annealed at different temperatures. As shown in Figure 5b, the coercivity of ultrathin Au/FeCo nanowires increases as increasing the annealing temperature from 473 to 633 K. Further increasing the annealing temperature up to 773 K, the coercivity begins to reduce drastically. Compared with the coercivity of the pristine Au/FeCo nanowires (298 Oe, Figure 3a), the coercivity of ultrathin Au/FeCo nanowires achieves the highest value of 982 Oe at the annealing D

DOI: 10.1021/acsami.5b10991 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

ACS Applied Materials & Interfaces (200) after annealing. In order to verify this hypothesis, we further calculated Bain strain mismatch of the (220)-(200) structure relationship between Au nanowires core and FeCo shell. This distortion between the two phases can be quantified via using Bain strain mismatch m = (d2 − d1)/d14; d1 and d2 are the lattice parameters of FeCo shell (200) and ultrathin Au nanowires core bct (220) plane, respectively. The calculated mismatches between bct-Au/bct-FeCo and bct-Au/bcc-FeCo are 4.9 and 18.7%, respectively, implying that the final structure prefers more stable bct-AuCu/bctFeCo (the detailed calculation is shown in Supporting Information). The interface between ultrathin Au nanowire core and FeCo shell is also presented after thermal annealing at 633 K (Figure 5d), and the lattice distance of 0.121 and 0.127 nm represents the interplanar distance of (220) bct Au and (200) bctFeCo nanostructures, respectively. Therefore, under the surfacestress-induced phase transformation, ultrathin (220) fcc Au nanowires transform into (220) bct Au nanostructures, accompanied by tetragonal distortion within FeCo shell phase. As the above disscussion from structural and chemical analysis, it indeed proves our rational design that the metastable tetragonal FeCo nanostructures could be triggered by the surface reconstruction of ultrathin Au nanowire core from (220)-fcc to (220)-bct structures because of phase transformation (Figure 1). Ultimately, ultrathin Au/FeCo (core/ shell) nanowires with high coercivity and magnetic saturation could be obtained, which opens up a new route to manufacture high MAE rare-earth-free magnetic nanomaterials. In summary, ultrathin Au nanowires are selected as the templating core to grow rare-earth-free FeCo magnetic shell, which works as a prototypical example to illustrate the surfacestress-induced phase transformation for the formation of metastable tetragonal FeCo nanostructures with increased magnetocrystalline anisotropy. The unique feature of ultrathin core/shell nanowires is that phase transformation of the Au core from fcc to bct nanostructures triggers the tetragonal distortion within surface FeCo shell. The magnetic properties could be tuned by controlling the Fe/Co stochiometry and FeCo shell thickness. The optimum coercivity and magnetic saturation (982 Oe and 162 emu/g) are achieved in the ultrathin Au/FeCo (core/shell) nanowires, which open up a new strategy to utilize the surface stress induced phase transformation to control the structural change, as one of unique characteristics of ultrathin metal nanowire systems.

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ACKNOWLEDGMENTS

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REFERENCES

S.R. thanks the financial support from the U.S. National Science Foundation (NSF) under the CAREER Award No: NSF-DMR1551948.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b10991. Materials and Methods, Figures S1−S6, lattice mismatch calculations, and critical FeCo shell thickness (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

J.S. carried out the experiments and wrote the manuscript. All authors have contributed to the manuscript revision. S.R. designed and guided the project. Notes

The authors declare no competing financial interest. E

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ACS Applied Materials & Interfaces (21) Guidez, E. B.; Aikens, C. M. Theoretical Analysis of The Optical Excitation Spectra of Silver and Gold Nanowires. Nanoscale 2012, 4, 4190−4198. (22) Cui, H.; Hong, C.; Ying, A.; Yang, X.; Ren, S. Ultrathin Gold Nanowire-Functionalized Carbon Nanotubes for Hybrid Molecular Sensing. ACS Nano 2013, 7, 7805−7811. (23) Kirkeminde, A.; Spurlin, S.; Draxler Sixta, L.; Cooper, J.; Ren, S. Metal Redox Processes for the Controlled Synthesis of Metal Alloy Nanoparticles. Angew. Chem. 2015, 127, 4277−4281. (24) Verhagen, E.; Spasenović, M.; Polman, A.; Kuipers, L. Nanowire Plasmon Excitation by Adiabatic Mode Transformation. Phys. Rev. Lett. 2009, 102, 203904. (25) Govorov, A. O.; Richardson, H. H. Generating Heat with Metal Nanoparticles. Nano Today 2007, 2, 30−38. (26) Pissuwan, D.; Valenzuela, S. M.; Cortie, M. B. Therapeutic Possibilities of Plasmonically Heated Gold Nanoparticles. Trends Biotechnol. 2006, 24, 62−67. (27) Ohtsuki, T.; Kojima, T.; Kotsugi, M.; Ohkochi, T.; Mizuguchi, M.; Takanashi, K. Magnetic Domain Observation of FeCo Thin Films Fabricated by Alternate Monoatomic Layer Deposition. J. Appl. Phys. 2014, 115, 043908. (28) Hui, L.; Pederiva, F.; Wang, B. L.; Wang, J. L.; Wang, G. H. How Does The Nickel Nanowire Melt? Appl. Phys. Lett. 2005, 86, 011913. (29) Hu, W.; Xiao, S.; Yang, J.; Zhang, Z. Melting Evolution and Diffusion Behavior of Vanadium Nanoparticles. Eur. Phys. J. B 2005, 45, 547−554. (30) Feng, H.; Yang, Y.; You, Y.; Li, G.; Guo, J.; Yu, T.; Shen, Z.; Wu, T.; Xing, B. Simple and Rapid Synthesis of Ultrathin Gold Nanowires, Their Self-assembly and Application in Surface-enhanced Raman Scattering. Chem. Commun. 2009, 15, 1984−1986.

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DOI: 10.1021/acsami.5b10991 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX