Ultrathin Interface Regime of Core shell Magnetic Nanoparticles for Effective Magnetism Tailoring

Transcription

1 Supporting Information Ultrathin Interface Regime of Core shell Magnetic Nanoparticles for Effective Magnetism Tailoring Seung Ho Moon,,, Seung-hyun Noh,,, Jae-Hyun Lee,,, Tae-Hyun Shin,,, Yongjun Lim,,, and Jinwoo Cheon *,,, Center for Nanomedicine, Institute for Basic Science (IBS), Seoul 03722, Republic of Korea. Yonsei-IBS Institute, Yonsei University, Seoul 03722, Republic of Korea. Department of Chemistry, Yonsei University, Seoul 03722, Republic of Korea. *Correspondence to: J.C. This file contains: Supporting sections on methods, materials, results and analyses Figures S1 S7 Tables S1 S3 Supporting References

2 Supporting Information 1: Synthesis of magnetic nanoparticles All chemicals are purchased from Sigma-Aldrich. Iron(III) acetylacetonate, cobalt(ii) acetylacetonate, manganese(ii) acetylacetonate, iron(ii) acetylacetonate, nickel(ii) acetylacetonate, cobalt(ii) chloride, oleic acid (technical grade, 90%), oleylamine (70%), benzyl ether (98%) and octyl ether (99%) are used as received. a. Synthesis of 30 nm cubic CoFe 2 O 4 (CF) nanoparticles Iron(III) acetylacetonate (3.60 mmol) and cobalt(ii) acetylacetonate (3.20 mmol) are placed in a 100 ml three-neck round-bottom flask in the presence of oleic acid (31.54 mmol) and benzyl ether ( mmol). The mixture solution is degassed at room temperature for 1 h, heated to 290 C under an argon atmosphere and then kept at that temperature for 30 min. After cooling down to room temperature, ethanol is added to the mixture under ambient conditions, resulting in black precipitate. The obtained precipitate after centrifugation and isolation is then dispersed in toluene. b. Synthesis of CF@MnFe 2 O 4 (CF@MF) core shell nanoparticles with a various shell thickness CF@MF core shell nanoparticles are synthesized by the typical seed-mediated growth method. The 30 nm CF are mixed with iron(iii) acetylacetonate, manganese(ii) acetylacetonate, oleic acid, and benzyl ether. The mixture solution is degassed at room temperature for 1 h, heated to 290 C under an argon atmosphere and then kept at that temperature for 30 min. After cooling down to room temperature, ethanol is added to the mixture under ambient conditions, resulting in black precipitate. The obtained precipitate after centrifugation and isolation is then dispersed in toluene. For the control of shell thickness, the portion of metal precursors is increased. The use of 0.2, 0.25, 0.3, 0.4, and 0.6 mmol of iron(iii) acetylacetonate and 0.16, 0.2, 0.24, 0.32, and 0.48 mmol of manganese(ii) acetylacetonate yields CF@MF nanoparticles with varying shell thicknesses of 0.5 nm (CF@MF 0.09 ), 1 nm (CF@MF 0.18 ), 1.5 nm (CF@MF 0.25 ), 2.5 nm (CF@MF 0.37 ), and 5 nm (CF@MF 0.58 ), respectively. c. Synthesis of CF@XFe 2 O 4 (X=Fe, Co and Ni) core shell nanoparticles CF@XFe 2 O 4 (X=Fe, Co, and Ni, shell volume fraction of 0.09) core shell nanoparticles are synthesized by the seed-mediated growth method, which is utilized for CF@MF. To introduce divalent metals, a stoichiometric amount of divalent metal acetylacetonates (X(acac) 2, X= Fe, Co, and Ni) are utilized while other synthetic conditions are unchanged. d. Synthesis of CF@MF core shell nanoparticles with different shape and size Cuboctahedron-shaped CF core nanoparticles with an identical magnetic volume of 30 nm cubic nanoparticles are prepared by changing a heating rate from 18 C/min to 4.5 C/min as compared to the synthetic condition described in SI 1a. As for synthesis of 20 nm cubic CF core nanoparticles, 3 mmol of iron(iii) acetylacetonate and 2.67 mmol of cobalt(ii) acetylacetonate are used, while the other conditions are followed by the synthetic protocol for the 30 nm cubic CF core. For 12 nm CF core, the mixture solution of cobalt(ii) chloride (1.95 mmol), iron(iii) acetylacetonate (3 mmol), oleic acid (9.5 mmol), oleyamine (36.5 mmol) and octyl ether (30 mmol) is heated to 300 C under an

3 argon atmosphere with a heating rate of 2.3 C/min and then kept for 1 hour. After cooling to a room temperature, the solution is mixed with ethanol and centrifuged to obtain black precipitate of nanoparticles. The isolated product is redispersed in toluene. Core shell nanoparticles are prepared by the typical seed-mediated growth method presented in SI 1b. To control the shell thickness, the amount of metal precursors of iron(iii) acetylacetonate and manganese(ii) acetylacetonate is increased from 0.2, 0.25, and 0.3 mmol and 0.16, 0.2, and 0.24 mmol, respectively, yielding an increase in a shell thickness of 0.5 nm, 1 nm, and 1.5 nm.

4 Supporting Information 2: Characterization of magnetic nanoparticles Transmission electron microscopy (TEM), high-resolution TEM and scanning transmission electron microscopy (STEM) analyses are carried out using JEOL-2100, JEM-ARM 1300S, and JEM-ARM 200F instruments, respectively. Elemental analysis of nanoparticles is conducted using energy dispersive X-ray spectroscopy (EDS) (INCA, Oxford Instruments). The quantitative analysis of Fe and Mn atoms in as-prepared nanoparticles is performed with inductively coupled plasma atomic emission spectroscopy (ICP-AES) (OPTIMA 4300DV, PerkinElmer). Herein, as a sample pretreatment, the nanoparticles are dissolved in a piranha solution. Elemental distribution and chemical state of as-prepared nanoparticles are analyzed using angle-resolved X-ray photoelectron spectroscopy (AR-XPS) with an argon-ion beam etching method (K-Alpha, Thermo Scientific). The magnetic properties of as-prepared nanoparticles are examined by measuring the field-dependent magnetization in the range of field strength from 2 to +2 T at 5 K, 150 K, and 300 K using a superconducting quantum interference device (SQUID) (MPMS-7, Quantum Design).

5 Figure S1. TEM images and size distribution of the 30 nm CF core and the core shell nanoparticles with a various shell thickness. The number average size distribution of nanoparticles (total counts= 100) is determined by measuring the edge lengths of nanoparticles. (i) CF core nanoparticles show an average edge length of 30±0.47 nm. (ii vi) CF@MF nanoparticles have an average edge length of 31±0.49 nm, 32±0.49 nm, 33±0.48 nm, 35±0.47 nm, and 40±0.52 nm, which are denoted as CF@MF 0.09, CF@MF 0.18, CF@MF 0.25, CF@MF 0.37, and CF@MF 0.58, respectively.

6 Figure S2. ICP-AES analysis of the 30 nm CF core and the various core shell nanoparticles. The results show that the amounts of metal atoms in the shell (i.e., Fe and Mn) increase accordingly with an increasing shell volume fraction. Table S1. The quantitative analysis for the metal atoms in the as-prepared nanoparticles using ICP-AES. concentration sample element SD RSD (%) (mg/l) CF core Co Fe Co Mn Fe Co Mn Fe Co Mn Fe Co Mn Fe Mn Co Fe

7 Figure S3. The XPS analysis for as-prepared nanoparticles. Depth profile of elemental distribution and chemical states for the (i) CF core and the (ii,iii) core shell nanoparticles with a various shell thickness of (ii) 5 nm and (iii) 1.5 nm using angle-resolved XPS (AR-XPS). The kinetic energy of the electrons in the Co and Mn atoms are measured depending on a profile depth of 2.7, 4, 5, 6, and 8 nm with varying emission angle of 0, 40, 50, 60, and 70, at which the electrons are collected. In case of the (i) CF core, Mn peaks are not detected at all angles. As for the (ii) CF@MF 0.58 (i.e., 5 nm-thick shell), both peaks of Co and Mn are detected at low angle ranging from 0 to 50, which corresponds to a profile depth of 8 nm and 5 nm, respectively. Meanwhile, the Co peaks completely disappear at higher angles than 50 (i.e., shell thickness less than 5 nm), indicating that the intra-particle diffusion rarely happen at the core shell interface. As for the CF@MF 0.25 (i.e., 1.5 nm-thick shell), (iv) argon-ion beam etching method with a sputtering rate of 0.5 angstrom/sec is additionally used to clearly identify the thin-shelled structure in detail. After applying the ion beam for 30 sec (i.e., an etching depth of ca. 1.5 nm), the negligible change in Mn 2P 3/2 peak is observed, verifying that the thin-shell is well-preserved without changing in structure and dimension.

8 Figure S4. Magnetic properties of core shell nanoparticles with various shell volume fractions in the EC regime. Field-dependent magnetization curves of CF core and nanoparticles with different shell volume fractions of 0.37, 0.58, and 0.78 at 5 K.

9 Figure S5. Magnetic properties of core shell nanoparticles with various shell volume fractions in the ESC regime. Field-dependent magnetization curves of 30 nm CF core and nanoparticles with different shell volume fractions at (i) 150 K and (ii) 300 K.

10 Figure S6. Magnetic properties of CF core and core shell nanoparticles with cuboctahedron shape. (i) TEM image of 0.17 with 31.9 nm cuboctahedral core, which has the identical volume of 30 nm cubic core. (ii-iv) Field dependent magnetization curves of nanoparticles at (ii) 5 K, (iii) 150 K and (iv) 300 K.

11 Figure S7. Magnetic properties of CF core and core shell nanoparticles with various size. (i) TEM image of 0.25 with 20 nm cubic core and (ii) field dependent magnetization curves of nanoparticles at 5 K. (iii) TEM image of CF@MF 0.37 with 12 nm spherical core and (iv) field dependent magnetization curves of nanoparticles at 5 K.

12 Supporting Information 3: Measurement of blocking temperature and calculation of the surface anisotropy The temperature-dependent magnetic properties of core and core shell nanoparticles are measured with a superconducting quantum interference device (SQUID) (MPMS-7, Quantum Design). The effective magnetic anisotropy and surface magnetic anisotropy are calculated using the following equations and parameters (ref #1,2): T B = K eff V k B ln ( τ m τ0 ) (1) T B : blocking temperature K eff : effective anisotropy V: volume of magnetic nanoparticles k B : Boltzmann constant ( J/K) τ m : measurement time τ 0 : attempt time ln τ m τ 0 = 25 (typical laboratory measurement) K eff = K V + 6 D K S (2) K S : surface anisotropy K V : volume anisotropy ( J/m 3 ) (ref #2) D: diameter of nanoparticles Table S2. Effective and surface anisotropy constants calculated using equations (1) and (2) T B (K) K eff (J/m 3 ) K S (J/m 2 ) CF core CF@MF CF@MF CF@MF CF@MF CF@MF

13 Supporting Information 4: The magnetic spin state simulation of core shell nanoparticles using the Object-Oriented Micro-Magnetic Framework (OOMMF) The magnetic spin state of CF core, 0.18 core shell, and 0.37 core shell nanoparticles are simulated using the object-oriented micro-magnetic framework (OOMMF) program based on the Landau Lifshitz Gilbert (LLG) equation: dm dt = γ G[M H eff ] + α G M S [M dm dt ] (3) M: magnetization γ G : electron gyromagnetic ratio H eff : effective magnetic field α G : Landau Lifshitz phenomenological damping parameter Table S3. The parameters for the LLG equation in the OOMMF simulation exchange coefficient (J/m) particle size (nm) anisotropy constant (J/m 3 ) saturation magnetization (A/m) simulation parameter anisotropy type parameter value core core (ref #3) shell shell (ref #4) core shell (ref #4) Cubic Close Packing anisotropy direction (x, y, z) (1 0 0) cell size (nm) 1.0 core (edge length) 30 shell (thickness) 1 core (ref #5) shell (ref #6) core (ref #7) shell (ref #8)

14 Supporting Information 5: Calculation of the maximum energy product ((BH) max ) of various nanoparticles M H curves were obtained from physical property measurement system (PPMS, Quantum Design PPMS-14). M H curves were transformed into B H curves by using the equation (4) and (BH) max was maximum value of B H in the second quadrant. B = H + 4πM (4) B = Magnetic flux density (G) H = Magnetic field (Oe) M = Magnetization (emu/cm 3 ) To calculate (BH) max of CF@MF theor.limit, M r = 0.5M S is obtained from the Stoner Wohlfarth model (ref #9) and H C,b = H C,j from ref #1. H C,b = Coercivity of B H curves H C,j = Coercivity of M H curves Substitute H = 0, M r = 0.5M S, we have B r = 2430 G. B r = 0 G, we have H C,b = 293 Oe. theoretical (BH) max = GOe = J/m 3 (5)

15 Supporting Information 6: Measurement of specific loss power (SLP) The SLP values of CF core and various nanoparticles are measured using high-frequency heating (HF 10K, Taeyang System Co., Korea), which generates an alternating magnetic field (AMF) at a frequency (f) of 500 khz and a strength (H 0 ) of 10, 20 and 30 ka/m. The nanoparticles (4 mg) are dispersed in toluene (2 ml), and then, the solution is placed in the center of a water-cooled magnetic induction coil with a diameter of 5 cm insulated by Styrofoam. The time-dependent temperature curves are measured using a fiber optic thermometer (M602, Lumasense technologies, Inc. USA) under AMF magnetic heating. The SLP values (W/g) of samples are derived from equation (6). SLP = CV s dt m dt (6) C: volumetric specific heat capacity of the sample solution (J/L. K) Vs: sample volume (L) m: mass of magnetic material in the sample (g) dt/dt: initial slope of the change in temperature versus time curve (K/s)

16 References (1) Lee, J.-H.; Jang, J.-t.; Choi, J.-s.; Moon, S. H.; Noh, S.-h.; Kim, J.-w.; Kim, J.-G.; Kim, I.-S.; Park, K. I.; Cheon, J. Nat. Nanotechnol. 2011, 6, (2) Hiratsuka, N.; Suzuki, S.; Kakizaki, K.; Sugimoto, M. J. Jpn. Soc. Powder. Powder Metall. 1995, 42, (3) Torres, T. E.; Lima, E., Jr.; Mayoral, A.; Ibarra, A.; Marquina, C.; Ibarra, M. R.; Goya, G. F. J. Appl. Phys. 2015, 118, (4) Rafiquea, M. Y.; Li-Qing, P.; Javed, Q.; Iqbal, M. Z.; Hong-Mei, Q.; Farooq, M. H.; Zhen-Gang, G.; Tanveer, M. Chin. Phys. B 2013, 22, (5) Ichiyanagi, Y.; Moro, Y.; Katayanagi, H.; Kimura, S.; Shigeoka, D.; Hiroki, T.; Mashino, T. J. Therm. Anal. Calorim. 2010, 99, (6) Rechenberg, H. R. and Tourinho, F. A. Hyperfine Interactions 1991, 67, (7) Dantas, C. C.; Gama, A. M. J. Magn. Magn. Mater. 2010, 322, (8) Coey, J. M. D. Magnetism and magnetic materials; Cambridge University Press, New York, 2010; p.423. (9) Hadjipanayis, G. C. Magnetic Hysteresis in Novel Magnetic Materials; Springer, Netherlands, 1997; p.359.