Halide Ion-mediated Synthesis of L10-FePt Nanoparticles with

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Halide Ion-mediated Synthesis of L10-FePt Nanoparticles with Tunable Magnetic Properties Wenjuan Lei, Junjie Xu, Weiwei Yang, Yongsheng Yu, Yanglong Hou, and Dafa Chen Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b03603 • Publication Date (Web): 15 Nov 2018 Downloaded from http://pubs.acs.org on November 15, 2018

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Halide Ion-mediated Synthesis of L10-FePt

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Nanoparticles with Tunable Magnetic Properties

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Wenjuan Lei,† Junjie Xu,‡ Yongsheng Yu,*,† Weiwei Yang,*,† Yanglong Hou*,‡ and

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Dafa Chen†

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†

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Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology,

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Harbin, Heilongjiang 150001, China

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‡

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Beijing Innovation Center for Engineering Science and Advanced Technology (BIC-

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ESAT), Department of Materials Science and Engineering, College of Engineering,

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Peking University, Beijing 100871, China

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ABSTRACT: L10-FePt nanoparticles (NPs) have great potential in areas of advanced

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magnetic and catalytic applications. Here, we present a facile control route for synthesis

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of hard magnetic L10-FePt NPs, in which halide ions (Cl-, Br- or I-) were added to the

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synthetic process to promote the phase transfromaion. It is confirmed that the strong ionic

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binding force between halide ions and Fe3+ or Pt2+ ions could facilitate the formation of

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L10-FePt phase due to favoring growth of FePt NPs in a more thermodynamically stable

MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and

Beijing Key Laboratory for Magnetoeletric Materials and Devices (BKL-MEMD),

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ACS Paragon Plus Environment

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way, which enables the formation of an ordered structure. L10-FePt NPs with the highest

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coercivity of 8.64 kOe and saturation magnetization of 64.21 emu/g at room temperature

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can be directly obtained by controlling the amount of the halide ions. Comparing with

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conventional solution phase reduction methods, the halide ions-assisted method shows

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enhanced capability to tune the growth of hard magnetic bimetallic NPs, particularly Pt-

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based bimetallic NPs.

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KEYWORDS: L10-FePt, nanoparticle, hard magnetic, coercivity, halide ions

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L10-ordered FePt nanoparticles (NPs) with face-centered-tetragonal (fct) phase

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structure have aroused extensive attention for potential applications in ultra-high-

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density magnetic recording media, magnetic energy storage and electrocatalysis

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because of their large magneto-crystalline anisotropy constant (Ku > 107 erg/cm3),

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high coercivity, outstanding corrosion resistance and eminent catalytic activity.1-5

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Solution phase chemical synthesis of monodispersed FePt NPs with controllable

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size, composition and regular shapes have been reported over the last few decades.6-

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centered cubic structure (fcc), which is superparamagnetic at room temperature and

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cannot be applied in areas of data storage and permanent magnetic materials.10-13

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For obtaining hard magnetic L10-phase, high temperature (>550 oC) annealing of

In general, the as-synthesized FePt NPs from solution phase reactions have a face-

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the as-made NPs under a reducing or inert atmosphere is required.14-17 However,

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the protective organic layer surrounding the NPs will decompose at high

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temperature condition, and resulting in particle agglomeration that will lead to the

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growth up of particles with irregular shapes and sizes.18-21 Among various

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preparation processes, a direct synthetic process that can produce fct phase FePt

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NPs without post-synthesis annealing has attracted special interest.

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In previous studies, a series of strategies have been reported for preparing fct-

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FePt NPs. One strategy is to prepare the fcc-FePt NPs in a high temperature

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solution route first. Next the fcc-FePt NPs are coated with high melting point

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oxides such as SiO2, MgO or MnO to inhibit NPs growth and aggregation under

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the high temperature annealing process. Then the coated oxide on the surface of

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the NPs can be removed by acid pickling.

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contribute to obtaining great dispersible fct-FePt NPs, the post annealing step is

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still necessary. Another strategy is to decrease the ordered temperature of fct-FePt

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phase by addition of other noble metals or transition metal atoms, Ag, Au, Cu, Zn

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or Sb, for example.27-31 Wang et al. found that Ag additive can decrease the L10-

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FePt phase ordered temperature in hexadecylamine.28 The ordering degree of L10-

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FePt phase is sensitive to the amount of Ag. By increasing the Ag mole ratio to

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29%, the coercivity of the NPs can reach 7.6 kOe. However, further increase of

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Although the coated oxides can

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the Ag mole ratio leads to a decrease in the coercivity. Recent research shows that

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moderate amount of Cu additives can also be effective in facilitating the ordering

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degree of L10-FePt NPs.29 A coercivity of 4.8 kOe can be achieved for FePtCu

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NPs due to the fact that Cu doping promotes grain growth and increases the driving

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force for ordering of FePt NPs. In our previous studies, we prepared monodisperse

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L10-FePtAu NPs by addition of Au precursors in the high temperature solution

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phase process.30 When the Au mole ratio increases to 32%, the coercivity can

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reach up to 12.15 kOe, which is far higher than other literature values concerning

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the direct liquid phase synthesis of L10-FePt NPs. The results showed that Au or

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Ag atoms alloy with FePt to form fcc-FePtAu at a lower temperature. With the

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temperature increasing, Au or Ag atoms segregate from fcc-FePtM (M=Ag or Au)

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lattice, leaving lots of vacancies for the Fe and Pt atoms to rearrange in an orderly

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manner. Despite the recent progress in the synthesis of these materials (or NPs),

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the use of noble metal atoms will increase the economic cost in addition to needing

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an acid treatment process. Thus, it is still desirable, albeit challenging to directly

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prepare L10-FePt NPs by a liquid phase synthesis process that does not require

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further annealing or the addition of noble metal atoms additives.

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Here, we present a new route to directly prepare L10-FePt NPs by liquid phase

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synthesis in the presence of halide ions (Cl-, Br- and I-). Heating the mixture of 4


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Fe(acac)3, Pt(acac)2 and oleylamine (OAm) to 350 oC in the presence of halide ions,

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L10-FePt NPs with high room temperature coercivity can be directly obtained

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without further annealing. The phase structure and magnetic property of the FePt

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NPs can be tuned by the addition of different halide ions. The results demonstrated

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that the halide ions played a key part in the formation of L10-FePt NPs. The strong

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binding forces of halide ions to Fe3+ or Pt2+ ions retarded the FePt growth kinetics

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and facilitated the Fe and Pt atoms to alloy in a more thermodynamically stable way

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and form the L10-FePt ordered structure. These halide ions effects may be

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generalized to tune the bimetallic NPs growth, especially Pt-based bimetallic NPs,

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into desired crystal structures.

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We took NH4Cl as an example to synthesize the FePt NPs. In a typical synthesis,

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Fe(acac)3, Pt(acac)2, OAm and NH4Cl were mixed and stirred under nitrogen. The

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mixture was heated at 100 oC for 0.5 h to remove air and moisture from the reaction

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system. Then the mixture was heated to 350 °C for a different amount of time, and

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cooled to room temperature. The resulting product was collected and washed with

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a mixture of ethanol and hexane, finally redispersed in hexane. The typical low-

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magnification TEM image of the NPs (Figure 1a) showed that the 3:1 mole ratio

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of NH4Cl to Pt(acac)2 (Cl- to Pt2+), when heated at 350 oC for 3 h, yielded FePt NPs

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with a diameter of ~70 nm. After ultrasonic dispersion for 3 h, the smaller FePt NPs 5


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could be seen (Figure S1), suggesting that each large FePt NP was composed of

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small particles. High-resolution TEM (HRTEM) image revealed the NPs possessed

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a poly-crystalline structure (Figure 1b). The fringe distances were 0.37 nm and

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0.22 nm, which were in accord with the (001) and (111) planes of the fct FePt alloy

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structure, respectively. And the electron diffraction rings corresponded to the (001)

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and (110) planes (Figure 1c), further confirming the formation of the L10-FePt

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phase in the as-synthesized NPs. The elemental distribution of L10-FePt NPs

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showed that both Fe and Pt atoms distributed uniformly in the L10-FePt alloy

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structure (Figure 1d-f).

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Figure 1. (a) The low- and (b) high-magnification TEM images of the fct-FePt NPs

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synthesized at 350 oC for 3 h with a Cl-/Pt2+ mole ratio of 3:1. (c) SAED pattern of

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the fct-FePt NPs. (d-f) Elemental mappings of Fe (green) (d), Pt (red) (e) and

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combined signals (f) of the fct-FePt NPs.

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To tune the crystal structure and magnetic properties of the FePt NPs, we studied the

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effects of the reaction time on their formation. We have synthesized the FePt NPs with a

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Cl-/Pt2+ mole ratio of 3:1 at 350 oC from 0.5 h to 9 h, respectively. The TEM images of

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the NPs synthesized for a different amount of time showed that the NPs were nearly in the

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same shape (Figure S2). The composition of the FePt NPs, however, varied with the

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reaction time (Fiugre 2a). When the mixture was heated at 350 oC for 0.5 h, Fe43Pt57 NPs

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were formed, suggesting that the reductive rate of the Fe atoms was slower than Pt atoms.

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Because of the different reductive rates, Pt-riched NPs were firstly formed. By extending

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the reaction time, the Fe precursors were further reduced to Fe atoms and diffused into the

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Pt-riched NPs, which led to an increase in the amount of Fe in the NPs while the relative

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the amount of Pt descreased. Finnally, Fe49.2Pt50.8 NPs were obtained after heating at 350

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oC

for 9 h.

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The phase structure of the reaction time-dependent NPs are shown in Figure 2b. The

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appearance of (001) and (110) diffraction peaks for 1 h NPs confirms the formation of L10

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ordered phase. The intensity of (001) and (110) peaks increased and the (200) and (220) 7


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peaks split with the extension of reaction time, indicating an improvement in the ordering

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degree of L10 phase. These results also show that the ordering degree of L10 phase

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increases with the reaction time.

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Figure 2. (a) Relationship between the reaction time and the Fe and Pt compositions in

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the NPs for the Cl-/Pt2+ mole ratio of 3:1. The XRD patterns (b) and the room-temperature

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hysteresis loops (c) of FePt NPs synthesized with a Cl-/Pt2+ mole ratio of 3:1 at 350 oC for

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a different reaction time. (d) The low-temperature hysteresis loops of FePt NPs

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synthesized with a Cl-/Pt2+ mole ratio of 3:1 at 350 oC for 9 h.

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Figure 2c shows the room-temperature magnetic properties of the NPs

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synthesized with a Cl-/Pt2+ mole ratio of 3:1 at 350 oC for various reaction time.

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When the reaction mixture was heated at 350 oC for 0.5 h, the NPs showed weak

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hard magnetism with only 0.40 kOe coercivity and 19.5 emu/g saturation

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magnetization. By increasing the reaction time, the hard magnetic property of FePt

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NPs was enhanced. When the reaction time was 3 h, the coercivity of the NPs was

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6.89 kOe and the saturation magnetization was 23.37 emu/g. Further extension of

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the reaction time led to higher coercivity and saturation magnetization. By

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increasing the reaction time to 6 h, the room temperature coercivity and saturation

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magnetization of the NPs increased to 7.22 kOe and 44.86 emu/g, respectively. The

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NPs synthesized at 9 h showed the highest coercivity of 8.64 kOe and saturation

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magnetization of 64.21 emu/g at room temperature. Both coercivity and saturation

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magnetization increased with the reaction time up to 9 h because of the increased

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L10-FePt ordering and Fe composition. These results suggested that the magnetic

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property of the fct-FePt NPs can be controlled by simply altering the reaction time.

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The magnetic properties of the FePt NPs synthesized at 350 oC for 9 h with a Cl-

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/Pt2+ mole ratio of 3:1 were also measured at lower temperatures such as 5 K, 100

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K and 200 K (Figure 2d). The low-temperature hysteresis loops showed that the

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FePt NPs were magnetically hard, with coercivity reaching 8.93 kOe (200 K), 9.84

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kOe (100 K) and 11.01 kOe (5 K). 9


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Figure 3. (a) Low- and (b) high-magnification HAADF-STEM images of

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representative fct-FePt NPs synthesized at 350 oC for 9 h with a Cl-/Pt2+ mole ratio

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of 3:1. (c) Corresponding HAADF line profiles across the line scan position in (b)

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normalized with Z contrast of metal atoms.

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To verify the FePt NPs ordering, we characterized the fct-FePt NPs synthesized at 350

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oC

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field scanning transmission electron microscopy (HAADF-STEM). The typical HAADF-

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STEM image of the L10-FePt NPs also showed that each FePt NPs was composed of small

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NPs (Figure 3a). The intermetallic structure, represented by the periodic contrast change,

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was clearly visible in the high-magnification HAADF-STEM image (Figure 3b). The

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layered Fe/Pt structure in the fct-FePt NPs was obviously revealed by the high (Pt) and

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low (Fe) Z contrasts, which were consistent with line profiles (Figure 3c) across the line

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scans No 1 and 2 along the direction drawn in Figure 3b. The alternating intensity

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profile of the line scan No 3 along the direction (Figure 3c) confirmed that the

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L10-FePt NPs consisted of the layered Fe/Pt structure and the Fe/Pt atoms arranging

for 9 h with a Cl-/Pt2+ mole ratio of 3:1 by aberration-corrected high-angle annular dark

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alternatively in the order of FePt along the direction. These results fully confirm

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the formation of ordered bimetallic fct-FePt NPs in the current synthesis.

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In addition, the effects of a Cl-/Pt2+ mole ratio on the composition, morphology,

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crystal structure and magnetic properties of the FePt NPs synthesized at 350 oC for

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3 h were investigated. The compositions of the FePt NPs synthesized with a

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different amount of Cl- were characterized by ICP-AES. Figure 4a shows that

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without Cl-, the Fe/Pt atoms mole ratio of the as-prepared FePt NPs was 49:51. By

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increasing the Cl-/Pt2+ mole ratio from 1:1 to 3:1, the Fe/Pt atoms ratio in the FePt

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NPs changed from 47.4:52.6 to 47.1:52.9, suggesting that the Cl-/Pt2+ mole ratios

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of lower than 3:1 had little influence on the FePt NPs composition. Further

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increasing the Cl-/Pt2+ mole ratio to higher than 3:1, the composition of the as-

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prepared FePt NPs was greatly affected. And the amount of Fe in the FePt NPs

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started to decrease sharply while relative Pt amount increased. The as-prepared

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FePt NPs compositions approached to be Fe40Pt60, Fe30Pt70 and Fe25Pt75 when the

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Cl-/Pt2+ mole ratios increased to 4:1, 5:1 and 6:1, respectively. Therefore, it can be

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concluded that excess Cl- could slow down the Fe3+ and Pt2+ reduction rate and

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inhibit the formation of the FePt alloy with Fe and Pt atoms ratio of 1:1 in the OAm.

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The crystal structure of the NPs synthesized at 350 oC for 3 h in the presence of

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different amount of Cl- was characterized by XRD, as shown in Figure 4b. A strong 11


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peak (37.2o ～44.4o) and a weak broad peak (44.4o ～50.5o) were observed in the

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FePt NPs synthesized without Cl-, corresponding to (111) and (200) diffraction

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peaks of fcc-FePt phase structure, respectively. When the Cl-/Pt2+ mole ratio was

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1:1, weak (001) and (110) peaks certified the formation of L10-FePt NPs. The (001)

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and (110) peaks relative intensity enhanced with the Cl-/Pt2+ mole ratio increasing

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from 2:1 to 3:1. And more splitting of (200) and (002) peaks meant higher ordering

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degree of L10-FePt NPs. The peaks of (001) and (110) relative intensity started to

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decrease and the (111) peak split to two peaks, indicating the formation of L12-

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FePt3 in the sample with a Cl-/Pt2+ mole ratio of 4:1. When the Cl-/Pt2+ mole ratio

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increased to 6:1, no other diffraction peaks from L10-FePt phase could be observed

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in the XRD pattern, finally only L12-FePt3 NPs were separated. These results

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suggest that L10-FePt structure could be directly obtained by co-reduction of

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Fe(acac)3 and Pt(acac)2 with OAm by simply controlling the amount of Cl-.

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Figure 4. (a) Relationship between the Cl-/Pt2+ mole ratio and the Fe and Pt

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compositions in the NPs synthesized at 350 oC for 3 h. (b) The XRD patterns of

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FePt NPs synthesized with a different Cl-/Pt2+ mole ratio synthesized at 350 oC for

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3 h.

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In order to further characterize the size and morphology of the Cl- amount-dependent

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NPs, we also performed TEM measurements, as shown in Figure S3. It can be seen that

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the amount of NH4Cl added in the synthesis controlled not only the crystal structure, but

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also the size and morphology of the as-synthesized NPs. For the synthesis without NH4Cl,

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polyhedral NPs with their size being at around 5 nm could be obtained, as shown in Figure

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S3a. When the Cl-/Pt2+ mole ratio was lower than 3:1, the NPs with irregular shape could

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be obtained (Figure S3b). The NPs with dendritic nanostructures were achieved by further

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increasing the Cl-/Pt2+ mole ratio from 3:1 to 6:1, which were composed of a number of

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densely packed branches (Figure S3c-f). Increasing the amount of Cl- in the synthesis

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caused particle size enlargement and size distribution widening. The diameter of the L10-

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FePt NPs increased from ∼39 to ∼90 nm with the Cl-/Pt2+ mole ratio changing from 2:1

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to 6:1. The color of the reaction mixture slowly changed from brown to black at about 195

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oC,

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was tuned from 1:1 to 6:1, suggesting that the NH4Cl additive resulted in slower metal

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ions reduction rate. In comparison, in the absence of NH4Cl, the color of the reaction

225 oC , 239 oC, 248 oC, 252 oC and 259 oC , respectively, when the Cl-/Pt2+ mole ratio

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mixture changed from yellow to black at about 184 °C. These results indicated that the Cl-

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retarded the nucleation rate of the NPs and enabled the NPs to grow to a larger size.

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Room temperature magnetic properties of the NPs syntesized with a different Cl-/Pt2+

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mole ratio are shown in Figure S4. Without the addition of NH4Cl in the synthesis, the

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as-synthesized NPs showed superparamagnetic property at room temperature. When the

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Cl-/Pt2+ mole ratio varied from 1:1 to 2:1, the coercivity of the NPs increased from 2.31

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kOe to 2.84 kOe, suggesting that the FePt NPs contained hard magnetic fct phase. A room

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temperature coercivity of 6.89 kOe was achieved when the Cl-/Pt2+ mole ratio reached 3:1.

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Further increasing the Cl-/Pt2+ mole ratio to 4:1 resulted in a decrease in the coercivity.

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When the Cl-/Pt2+ mole ratio was increased to 6:1, the NPs were paramagnetic at room

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temperature, suggesting that only L12-FePt3 phase was obtained in the production. From

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this series of magnetic measurement, we can also confirm that excess Cl- inhibit the fct-

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FePt phase formation.

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To investigate the determining factor for the fct-FePt phase formation, we

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synthesized the NPs under the same condition other than replacing NH4Cl with

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(NH4)2SO4 or the precursor of Fe(acac)3 with FeCl2·4H2O in the absence of NH4Cl.

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When FeCl2·4H2O was used as precursors, 24.9 nm fct-FePt NPs with the

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coercivity of 1.74 kOe were formed (Figure S5-S7). In the presence of (NH4)2SO4,

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only fcc-FePt NPs were obtained (Figure S8 and S9). These studies indicate that

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Cl- has a strong effect on the fct-FePt growth.

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We further tested the effects of other halide ions of Br- and I- on the growth of

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FePt NPs. Thermally unstable compounds such as KBr and KI were used as halide

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addition agent to replace the NH4Cl in the synthesis. When Br- and I- were

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introduced into the reaction mixture, the product showed the (001) and (110) peaks

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of fct-FePt phase in the corresponding XRD patterns (Figure S11). The as-

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synthesized NPs also showed the ferromagnetic properties at room temperature

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(Figure S12). With the addition of KBr and KI, the coercivity of the as-prepared

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NPs can reach 1.68 kOe and 5.72 kOe, respectively. Our studies indicate that halide

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ions (Cl-, Br- or I-) are the key controlling factor of the fct-FePt phase formation.

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Figure 5. The schematic illustration of fct-FePt NPs formation in the presence of

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Cl-.

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On the basis of the results described above, it can be concluded that only fcc-

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FePt NPs were formed due to the fast reduction reaction rate in the absence of halide

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ions. An appropriate amount of halide ions (Cl-, Br- or I-) can efficiently retard the

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fcc-FePt growth kinetics, facilitating the formation of ordered fct-FePt NPs.32-34

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Increasing the amount of halide ions in the synthesis will slow down metal

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precursors reduction rate, further decreasing the nucleation rate of the NPs and

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resulting in the larger size NPs.35,36 As illustrated in Figure 5, we hypothesize that

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strong binding forces of Fe-halide ions and Pt-halide ions slow down the growth of

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Fe and Pt atoms on the surface of seeding NPs, then facilitate the NPs growth by a

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thermodynamic more stable way to form fct phase.

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In summary, a halide-ions (Cl-, Br- or I-) mediated strategy for synthesis of fct-

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FePt NPs was developed. The FePt NPs structure can be controlled to be either fcc

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(without halide ions) or poly-crystalline fct (with different amounts of halide ions)

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through co-reduction of Fe(acac)3 and Pt(acac)2 in OAm. The systematic

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investigation on the synthetic mechanism confirm that the key control factor to

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form fct-FePt NPs is the strong binding force between halide ions and Fe or Pt ions,

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which are crucial for favoring FePt NPs growing in a more thermodynamic stable 16


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way and forming more stable fct phase structure. L10-FePt NPs with a room-

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temperature coercivity as high as 8.64 kOe and saturation magnetization of 64.21

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emu/g can be directly obtained by controlling the amount of the halide ions. This

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halide ion-assisted strategy may demonstrate a general route to tune the phase

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structure, magnetic properties and morphology of the FePt NPs by addition of

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different kinds of halide ions or in various amounts. The strategy also provides

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opportunities to prepare other magnetically hard Pt-based bimetallic NPs in a more

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controllable manner.

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ASSOCIATED CONTENT

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Supporting Information. The Supporting Information is available free of charge

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on the ACS Publications website, and includes experiment section, TEM images,

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XRD patterns and hysteresis loops (Figures S1-S12).

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AUTHOR INFORMATION

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Corresponding Author

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* E-mail: [email protected]

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* E-mail: [email protected]

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* E-mail: [email protected] 17


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ORCID

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Yongsheng Yu:

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Weiwei Yang:

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Yanglong Hou:

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Author Contributions

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The manuscript was written through contributions of all authors. All authors have

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given approval to the final version of the manuscript.

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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENT

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This work was supported by the National Natural Science Foundation of China under

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Grant (No. 51571072, 51590882, 51631001), the National Key R&D Program of China

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(2017YFA0206301), Fundamental Research Funds for the Central Universities (No.

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AUGA5710012715), Heilongjiang Postdoctoral Scientific Research Development Fund

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(No. LBH-Q14058), China Postdoctoral Science Foundation (No. 2015M81436) and

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Heilongjiang Postdoctoral Science Foundation (No. LBH-Z15065).

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REFERENCES

18

(1)

Yue, M.; Zhang, X.; Liu, J. P. Nanoscale. 2017, 9 (11), 3674-3697. 18


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Page 19 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

1 2

(2)

3090-3094.

3

(3)

4

190.

5

(4)

6 7 8

Yu, Y.; Mendozagarcia, A.; Ning, B.; Sun, S. H. Adv. Mater. 2013, 25 (22),

Zeng, H.; Li, J.; Wang, Z.; Liu J. P.; Sun, S. H. Nano. Lett. 2015, 4 (1), 187-

Yu, Y. S.; Sun, K.; Tian, Y.; Li, X. Z.; Kramer, M. J.; Sellmyer, D. J.; Shield

J. E.; Sun, S. H. Nano. Lett. 2013, 13 (10), 4975-4979. (5)

Zeng, H.; Li, J.; Liu, J. P.; Wang, Z. L.; Sun, S. H. Nature. 2002, 420 (6914),

395-398.

9

(6)

Min, C.; Liu, J. P.; Sun, S. H. J. Am. Chem. Soc. 2004, 126 (27), 8394-8395.

10

(7)

Yu, Y. S.; Yang, W.; Sun, X. L.; Zhu, W. L.; Li, X. Z.; Sellmyer, D. J.; Sun,

11 12 13

S. H. Nano. Lett. 2014, 14 (5), 2778-2782. (8)

(39), 15354-15357.

14

(9)

15

2882.

16

(10)

17

Guo, S. J.; Zhang, S.; Sun, X. L.; Sun, S. H. J. Am. Chem. Soc. 2011, 133

Yang, W.; Yu, Y.; Wang, L.; Yang, C.; Li, H. Nanoscale. 2015, 7 (7), 2877-

Bu, L.; Ding, J.; Guo, S.; Zhang, X.; Su, D.; Zhu, X.; Yao, J.; Guo, J.; Lu, G.;

Huang, X. Adv. Mater. 2015, 27 (44), 7204-7212.

18

(11)

Wang, C.; Hou, Y.; Kim, J.; Sun, S. Angew. Chem. 2007, 46 (33), 6333-6335.

19

(12)

Bu, L.; Guo, S.; Zhang, X.; Shen, X.; Su, D.; Lu, G.; Zhu, X.; Yao, J.; Guo,

20

J.; Huang, X. Nat. Commun. 2016, 7, 11850-11860. 19


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1 2 3 4 5 6 7 8

(13)

Bian, B.; Xia, W.; Du, J.; Zhang, J.; Liu, J. P.; Guo, Z.; Yan, A. Nanoscale.

2013, 5 (6), 2454-2459. (14)

Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science. 2000, 287

(5460), 1989-1992. (15)

Jeyadevan, B.; Shinoda, K.; Justin, R. J.; Matsumoto, T. IEEE. T. Magn.

2006, 42 (10), 3030-3035. (16)

Chuanbing, R.; Daren, L.; Vikas, N.; Narayan, P.; Yong, D.; Zhonglin, W.;

Hao, Z.; JiuPing, L. Cheminform. 2007, 38 (6), 2984-2988.

9

(17)

Sun, S. H. Adv. Mater. 2006, 18 (4), 393-403.

10

(18)

Zeynali, H.; Akbari, H.; Arumugam, S. J, Ind. Eng. Chem. 2015, 23, 235-

11

237.

12

(19)

Lai, J.; Luque R.; Xu, G. ChemCatChem, 2015, 7 (20), 482-488.

13

(20)

Hu, X.; Agostinelli, E.; Ni, C.; Hadjipanayis, G.; Capobianchi, A. Green.

14 15 16 17 18 19

Chem. 2014, 16 (4), 2292-2297. (21)

Yamamoto, S.; Morimoto, Y.; Ono, T.; Takano, M. Appl. Phys. Lett. 2005,

87 (032503), 1-3. (22)

Tamada, Y.; Yamamoto, S.; Takano, M.; Nasu, S.; Ono, T. Appl. Phys. Lett.

2007, 90 (162509), 1-3. (23)

Kim, J.; Rong, C.; Liu, J. P.; Sun, S. H. Adv. Mater. 2010, 21 (8), 906-909.

20


Page 20 of 22

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Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

(24)

Kim, J.; Rong, C.; Lee, Y.; Liu, J. P.; Sun, S. H. Chem. Mater. 2012, 20 (23),

7242-7245. (25)

Kang, S.; Miao, G.; Shi, S.; Jia, Z.; Nikles, D. E.; Harrell, J. J. Am. Chem.

Soc. 2006, 128 (4), 1042-1043. (26)

Li, Q.; Wu, L.; Wu, G.; Su, D.; Lv, H.; Zhang, S.; Zhu, W.; Casimir, A.; Zhu,

H.; Mendozagarcia, A. Nano. Lett. 2014, 15 (4), 2468-2473. (27)

Kang, S.; Jia, Z.; Nikles, D. E.; Harrell, J. W. IEEE. T. Magn. 2003, 39 (5),

2753-2757. (28)

Wang, H.; Shang, P.; Zhang, J.; Guo, M.; Mu, Y.; Li, Q.; Wang, H. Chem.

Mater. 2013, 25 (12), 2450-2454. (29)

Wang, H.; Li, Y.; Chen, X.; Shu, D.; Liu, X.; Wang, X.; Zhang, J.; Wang, H.;

Wang, Y.; Ruterana, P. J. Magn. Magn. Mater. 2017, 422, 470-474. (30)

Yu, Y. S.; Mukherjee, P.; Tian, Y.; Li, X. Z.; Shield, J. E.; Sellmyer, D. J.

Nanoscale. 2014, 6 (20), 12050-12055. (31)

Maeda, T.; Kai, T.; Kikitsu, A.; Nagase, T. Appl. Phys. Lett. 2002, 80 (12),

2147-2149. (32)

Zhang, S.; Jiang, G.; Filsinger, G. T.; Wu, L.; Zhu, H.; Lee, J.; Wu, Z.; Sun,

S. H. Nanoscale. 2014, 6 (9), 4852-4856. (33)

Yang, Z.; Zhao, T.; Huang, X.; Chu, X.; Tang, T.; Ju, Y.; Wang, Q.; Hou, Y.;

Gao, S. Chem. Sc. 2017, 8 (1), 473-481. 21


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1 2 3 4 5 6

(34)

Yang, C.; Zhao, H.; Hou, Y.; Ma, Ding. J. Am. Chem. Soc. 2012, 134 (38),

15814-15821. (35)

Yu, Y.; Goodfellow, B. W.; Rasch, M.; Bosoy, C.; Smilgies, D.; Korgel, B.

A. Langmuir. 2015, 31 (24), 6924-6932. (36)

Xie, S.; Peng, H. C.; Lu, N.; Wang, J.; Kim, M. J.; Xie, Z.; Xia, Y. J. Am.

Chem. Soc. 2013, 135 (44), 16658-16667.

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Halide Ion-mediated Synthesis of L10-FePt Nanoparticles with

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