Metal@SiO2 CoreâShells with Self-Arrested Migrating Core - Nano

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Metal@SiO2 Core-Shells with Self-Arrested Migrating Core Dong Kyoung Yoon, Amit Kumar, Dong-Gyu Lee, Jihwan Lee, Taewan Kwon, Jungkyu Choi, Taewon Jin, Ji-Hoon Shim, and In Su Lee Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.9b00653 • Publication Date (Web): 09 May 2019 Downloaded from http://pubs.acs.org on May 10, 2019

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Metal@SiO2 Core-Shells with Self-Arrested Migrating Core Dong Kyoung Yoon,†‡§ Amit Kumar,†‡§ Dong-Gyu Lee,†‡ Jihwan Lee,†‡ Taewan Kwon,†‡ Jungkyu Choi,†‡ Taewon Jin,‡ Ji Hoon Shim,‡ In Su Lee*†‡ †

National Creative Research Initiative Center for Nanospace-confined Chemical Reactions (NCCR),

Pohang University of Science and Technology (POSTECH), Pohang 37673 (Korea) ‡

Department of Chemistry, Pohang University of Science and Technology (POSTECH), Pohang 37673

(Korea) §

These authors contributed equally to this work.

*

To whom correspondence should be addressed

E-mail: [email protected] (I. S. L.) .

1 ACS Paragon Plus Environment

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ABSTRACT: Developing easy and customizable strategies for the directional structure modulation of multi-component nanosystems to influence and optimize their properties, are paramount but challenging task in nanoscience. Here, we demonstrate highly controlled eccentric off-center positioning of metalcore in metal@silica core-shells by utilizing an in situ generated biphasic silica-based intraparticle solidsolid interface. In the synthetic strategy, by including Ca2+-ions in silica-shell and successive oxidative and reductive annealing at high temperature, a unique hairline-biphasic-interface is evolved via the heatinduced concentric radial segregation of calcium silicate phase at the interior and normal silica phase at the exterior of core-shell, which can effectively arrest the outwardly migrating metal-core within rubbery calcium silicate phase, affording various eccentric core-shells, where core-positions are flexibly controlled by the annealing time and amounts of initially added Ca2+-ions. In the structure-property correlation study, the strategy allows fine-tuning of dipolar interaction-based blocking temperatures and magnetic anisotropies of different eccentric core-shells as the function of variable off-center distance of magnetic core, without changing overall size of nanoparticles. This work demonstrates the discovery and potential application of biphasic solid-solid media interface in controlling the heat-induced migration of metal nanocrystals and opens the avenues for exploiting the rarely studied high-temperature solid-state nanocrystal conversion chemistry and migratory behavior for directional nanostructure engineering.

KEYWORDS: core-shells, eccentric nanoparticles, solid-state migration, magnetic nanoparticles, silica nanoparticles


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Major challenges of nanoscience underlie not only to synthesize intricate structures with well-defined dimensions and compositions, but also to perpetuate their structural controllability, integrity and functionality under extreme conditions. Hybrid nanoparticles (NPs) consisting of metal cores and silica (SiO2) shells encompass diverse research lines including biomedicine, catalysis, and sustainable energy storage/conversions;1-2 while, introducing asymmetry and directionality in such NPs by generating wellengineered eccentric structures, have direct implications in optical, magnetic, adsorption and catalytic properties3-4 and well-ordered self-assemblies.5-6 For example, to direct sophisticated self-assembly of magnetic NCs with fine-tuning their coupled magnetic moments, it is important to synthesize magneticNCs protected in non-magnetic shells with controllable asymmetric core-positions.7-9 However, unlike plethora of synthetic methods for concentric core@shells synthesized by symmetrical silica-condensation around spherical core10-11 and also, Janus structures synthesized by kinetically biased asymmetric silicacondensation directed by using either a mixture of differently reactive silica-precursors12-13 or by partially blocking the core-surface with polymers/surfactants,14-17 or by using immiscible biphasic liquid media1819

― so far, there is no report for the synthesis of eccentric metal@SiO2 NPs with thermally stable and

tunable core-positions. This paper reports a solid-state confined NC-migration strategy towards various eccentric metal@silica core-shells with highly controlled off-center core positions, which utilizes an in situ generated biphasic silica-based solid-solid interface (Scheme 1). Analogous to the immiscible biphasic liquid-liquid system assembling NPs within interfacial region, we discovered a unique solidsolid biphasic nanomedia comprising radially segregated SiO2-based hairline biphasic interfacial trap, which can effectively control and arrest the heat-induced outward migration of metal NC,20-23 generating eccentric core-shells. In the present work, silica nanospheres, encapsulated with a Fe3O4/PdO NC and homogeneously dispersed Ca2+-ions as phase-segregators, were subjected to the annealing, which resulted the radial segregation of CaSiO3 phase in the core-region surrounded by immiscible normal SiO2 phase; concomitantly, the synthesized magnetic FePd NC migrates outwardly, and is eventually trapped at in situ 3 ACS Paragon Plus Environment

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formed CaSiO3-SiO2 interface. This strategy provides precise control over positioning of magnetic core in the produced eccentric metal@SiO2 NPs because of the synthetic privilege of finely tuning the radial volume of CaSiO3 phase based on the initially added Ca2+ and in turn assigning the position of biphasic interfacial trap for arresting the movement of NC. Finally, investigation on the magnetic properties of different synthesized eccentric core-shells revealed that the magnetic blocking temperature of FePd@SiO2 NP is highly correlated and tunable with the off-center positions of FePd core; hence, validating the utility of such engineered structures as model system for studying and controlling their ensemble magnetic properties. Before the synthesis of eccentric core-shells, the formation of well-segregated SiO2 and CaSiO3 phases and evolution of biphasic solid-solid interface was observed in Ca2+-ions containing SiO2 NPs (SiO2/Ca2+) [114 ± 10 nm diameter, 18 wt% Ca] under air annealing at different temperatures (Tann = 500 C – 600 C – 800 C); and characterized by the increase in the radial dark-contrast in transmission electron microscopy (TEM) images and gradual increase in the Ca/Si ratio and central localization of Caelement in energy-dispersive X-ray spectroscopy (EDS)-elemental mapping and line profiling, X-ray diffraction (XRD) and differential scanning calorimetry (DSC) (Figure 1a & Figure S1-3). In the SiO2CaSiO3 segregation process: as the annealing temperature increases, randomly distributed Ca2+-ions in amorphous silica voids tend to migrate and occupy anionic silicate sites, eventually, synthesizing thermodynamically stable CaSiO3 phase, which finally becomes well-segregated in the central volume of nanosphere and generates a hairline biphasic solid-solid interface.24-25 In an attempted to use other metal ions to form metal (Mn+) silicate-SiO2 biphase (Mn+ = Zn2+, Mn2+, Ni2+) similar to CaSiO3-SiO2: SiO2/Mn2+, SiO2/Mn2+ and SiO2/Ni2+ NPs were synthesized and annealed which resulted the formation of particulate-like island-domains composed of corresponding metal oxides (ZnO, Mn3O4, NiO) inside SiO2 nanosphere without the formation of any biphasic interface (Figure S4). Next, to study the metal NC migration behaviour in CaSiO3-SiO2 media, silica nanospheres pre-encapsulated with single Fe3O4 NC, 4 ACS Paragon Plus Environment

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Pd2+-ions and Ca2+-ions [designated as Red-Fe3O4@(SiO2/Ca2+/Pd2+)], were synthesized using a modified reverse-microemulsion method [Supporting Information (SI)]. In a typical procedure, presynthesized oleic acid-stabilized Fe3O4 NCs, 100 µL Na2PdCl4 (16 mg/mL) and a 40 µL Ca(NO3)2·4H2O (1000 mg/mL) were added during TEOS condensation reaction tempalted on Triton-X and 1-hexanolderived reverse micelles in cyclohexane medium, resulting Red-Fe3O4@(SiO2/Ca2+/Pd2+) core-shells (87 ± 7 nm overall diameter) containing Fe3O4 NC (7 ± 1 nm diameter) at the center and Pd2+ (0.4 wt% Pd from ICP) and Ca2+ (7.9 wt% from EDS) distributed homogeneously throughout the silica shell, as characterized by TEM, EDS and XRD (Figure 1b & 1h). Next, to remove any oxidizable organic contents and

to

partially

segregate

CaSiO3-SiO2

phases,

freeze-dried

powder

form

of

Red-

Fe3O4@(SiO2/Ca2+/Pd2+) NPs were subjected to air annealing at 500 C for 12 h in a box-furnace, which also led to the growth of PdO grain on the oxidized Fe3O4 NC (Ox-Fe3O4), generating Ox-Fe3O4/PdOheterogeneous

phase

at

the

center

of

CaSiO3-SiO2

nanosphere

[designated

as

(Ox-

Fe3O4/PdO)@(CaSiO3-SiO2)], characterized by changes in lattice spacings in high resolution TEM (HRTEM) images, EDS-elemental maps and the presence of characteristic XRD peaks corresponding to iron oxide and palladium oxide phases (Figure 1c & 1h).26 Subsequently, to inflict the heat induced phase transition and migration of core metal NC, (Ox-Fe3O4/PdO)@(CaSiO3-SiO2) NPs were reductively annealed under a flow of Ar and 4% H2 at 800 °C for 48 h in a quartz tube furnace, which resulted selfcatalytic reduction of Ox-Fe3O4/PdO into a single FePd NC of bimetallic random alloy phase (7 ± 1 nm diameter, Fe : Pd = 5 : 1 from EDS)26 which was characterized by HRTEM-based lattice spacing and emmergence of characteristic XRD peaks corresponding to reduced alloy of Fe and Pd (Figure 1d-h). With increasing tann, an obvious and gradual increase in the contrast between inner CaSiO3 and outer SiO2 phases (26 ± 2 nm and 87 ± 7 nm diameters, repectively) and the emergence of hairline biphasic CaSiO3SiO2 interface was found; and simultaneously, an outward migration of the FePd NC was also noticed, producing an eccentric core-shell [designated as ece-FePd@(CaSiO3-SiO2)] (Figure 1d-g). X-ray 5 ACS Paragon Plus Environment

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photoelectron spectroscopy (XPS)-based depth profiling analysis on ece-FePd@(CaSiO3-SiO2) showed increase in Ca 2p peak intensity from the surface to the interior of the nanosphere due to the concentration of CaSiO3 phase in the core region, which was also supported by the EDS- elemental mapping results (Figure S5 & Figure 1b-d). In order to understand NC-migration behaviour in details, the time-course TEM analysis on the samples isolated at different annealing times was conducted which showed a gradual increase in average off-center distances of FePd NC [Doff, estimated from the distribution of their offcenter displacement in plane-projected TEM images (dproj),27 (details in SI)] to be Doff = 5 (± 2), 8 (± 3), 11 (± 1) and 17 (± 2) nm at tann = 0, 12, 24, 48 h, respectively, untill approaching to the CaSiO3-SiO2 interface within 48 h; further increase in tann (up to 96 h) neither changed the volume of CaSiO3 phase nor altered the position of FePd NC (Figure 1d-g) — notably, unlike the initial facile migration of FePd alloy NC within the internal CaSiO3 medium, it could not pursue any such movement into outer SiO2 medium, finally, arresting the NC inside CaSiO3-SiO2 interfacial trap. As compared to our previous studies on unrestrained off-center migration of alloy NCs in pure silica medium which upon extended annealing (tann > 48 h) led to escaping of NC from silica-surface,21 presently discovered CaSiO3-SiO2 biphasic media self-limits and guides the NC-migration via arresting the core-position at biphasic intrefacial trap leading to thermally stable and uniformly high yields of eccentric core-shells. At sufficiently high temperatures [Tann > Tg (~770 °C)], amorphous CaSiO3-SiO2 medium undergoes glass-liquid transition to a movable rubbery phase and in situ evolves in to a biphasic immiscible system where thermal fluctuations and the consequent outward migration of FePd NC compete with the change in interfacial energy which in turn determines the position of FePd NC at the CaSiO3-SiO2 interface. Taking the analogy with the scenario of NC in immiscible liquid-liquid system in to account,28-29 the change in the interfacial energy (∆E) of FePd NC at the CaSiO3-SiO2 interface can be represented by:


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∆𝐸 = −𝜋𝑅 𝜸𝐶𝑎𝑆𝑖𝑂3 /𝑆𝑖𝑂2 [1 −

(𝜸𝐹𝑒𝑃𝑑/𝐶𝑎𝑆𝑖𝑂3 −𝜸𝐹𝑒𝑃𝑑/𝑆𝑖𝑂2 ) 𝜸𝐶𝑎𝑆𝑖𝑂3 /𝑆𝑖𝑂2

2

] ; where, R is the radius of FePd NC and

𝜸𝐶𝑎𝑆𝑖𝑂3/𝑆𝑖𝑂2 , 𝜸𝐹𝑒𝑃𝑑/𝐶𝑎𝑆𝑖𝑂3 and 𝜸𝐹𝑒𝑃𝑑/𝑆𝑖𝑂2 are the interfacial tensions of CaSiO3/SiO2, FePd/CaSiO3 and FePd/SiO2 interfaces, respectively. For the stabilization of FePd NC at CaSiO3-SiO2 interface, 𝜸𝐶𝑎𝑆𝑖𝑂3/𝑆𝑖𝑂2 > │𝜸𝐹𝑒𝑃𝑑/𝐶𝑎𝑆𝑖𝑂3 –𝜸𝐹𝑒𝑃𝑑/𝑆𝑖𝑂2 │; otherwise, no such arrest of FePd NC is possible. Also, the ∆E for the NC at interface by critically balanced interfacial tensions should be much larger than the thermal energy, kBT (kB is Boltzmann constant and T is temperature), so that the NC is strongly held to the interface and doesn’t migrate afterwards. For theoretical understanding, simplified interfaces: Fe/CaO, Fe/SiO2 and CaO/SiO2 were constructed based on fcc Fe(111), fcc CaO(111) and hcp SiO2 (001) and their formation energies (Ef) were calculated using density functional theory (DFT)-based Vienna ab-initio simulation package (VASP)30-31 which resulted Ef(SiO2/CaO), Ef(Fe/SiO2) and Ef(Fe/CaO) to be 4.99, 4.34 and 3.47 J/m2 respectively, validating the fact that the thermodynamically favored situation emerges when Fe is sandwiched between CaO and SiO 2 media, avoiding the relatively unfavorable direct contact of SiO2/CaO (details in SI). In contrast to the FePd NC having reduced metallic surface, Fe3O4/PdO with CaSiO3-compatible oxygenated surface, can be stabilized in CaSiO3 medium; therefore, upon switching the chemical state of metal NC it should be shifted to the position of minimized sum of surface tensions. To test this hypothesis, we subjected eceFePd@(CaSiO3-SiO2) NPs under air annealing condition which resulted a slight inward migration of oxidized core-NC, changing Doff = 29 (± 1) nm to Doff = 23 (± 2) nm which upon successive reductive annealing reverted back the position of reduced FePd NC to the original interfacial region with Doff = 28 (± 1) nm (Figure 2) and such redox-based reversible core-positioning can be repeated multiple times by switching the annealing conditions. This reversible core-positioning depicts the crucial role of chemical state of core NC in defining its migration behavior.


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Next, with the intention of varying the volume of CaSiO3 phase and in turn controlling the off-center position of biphasic interface, Red-Fe3O4@(SiO2/Ca2+/Pd2+) NPs were synthesized by adding different amounts of Ca2+ and subjected to the annealing, producing various ece-FePd@(SiO2-CaSiO3)-1-3 NPs possessing finely controlled off-center positions [Doff = 15 (± 2), 19 (± 1), 29 (± 1) nm] of FePd NCs arrested at CaSiO3-SiO2 interfacial traps, sophisticatedly emerged by the different volumes of CaSiO3 phases in each NP, as characterized by TEM and EDS-elemental mapping (Figure 3) — sparse Fe and Pd signals in the EDS-elemental maps were found at random regions, however, the majority of Fe/Pd EDS bright signals emerged from the location reasonably well-overlapped with the STEM image of FePd nanocrystal in each case; such unexpected minor EDS signals at random locations could be due to the noise and instrumental error. First, different Red-Fe3O4@(SiO2/Ca2+/Pd2+) NPs possessing varied amounts of Ca2+ were synthesized by adding 40 µL aliquots of Ca(NO3)2 aqueous solution (1.0 g/mL) 1, 2 and 5 times at certain intervals (at 24, 36, 48, 60 and 72 h reaction times) during the synthesis; on the other hand, by adding higher amounts of Ca(NO3)2 in a single portion, resulted the formation of aggregated nanospheres with poor yields (Figure S6), therefore it was necessary to use small portions (1000 mg/mL, 40 μL) of Ca(NO3)2 multiple times during the silica-condensation process. Subsequent oxidative and reductive annealing of different Red-Fe3O4@(SiO2/Ca2+/Pd2+) NPs resulted the formation of ece-FePd@(SiO2-CaSiO3)-1-3 NPs having 8, 11 and 17 wt% Ca (from EDS) within different CaSiO3 segregated volumes having radii 13 (± 3), 20 (± 3) and 30 (± 3) nm (Table S1). As revealed by TEMbased time-course studies (tann = 0, 12, 24, 48 h), in all the eccentric NPs, in situ generated FePd NCs underwent the similar thermal migation in CaSiO3 phase and got captured at CaSiO3-SiO2 interface located at variable radial distances (Figure S7). At high temperature, randomly distributed Ca2+ within amorphous SiO2, form new segregated internal CaSiO3-volume via homogeneous and self-limited saturation of anionic silicate sites by Ca2+-cations, and the simultaneous movement of FePd NC, which is driven by the thermal vibrations and movable rubbery phase of CaSiO3, precisely stops at hairline CaSiO38 ACS Paragon Plus Environment

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SiO2 interface and eccentricity of the resulting structure is overall determined by the initially added Ca2+ amount. Such synthetic flexibility by simply adding the different amounts of Ca2+ as phase-segregator, and by tuning the radial distances of in situ generated biphasic interface, provided us the opportunity to tune the eccentricity of core-shell nanostructure. In the ecentric NPs having variably off-centred FePd-cores obtained from annealing of RedFe3O4@(SiO2/Ca2+/Pd2+) NPs for different tann (0, 12, 24, 48 h) (Figure 4a), magnetic dipoles are progressively shifted from the centre of mass of the NP, such internal asymmetry may affect their magnetic dipolar interacations. To investigate such corellation, a DC zero field cooling-field cooling (ZFC-FC) experiment was conducted, measuring the temperature (T) dependence of magnetic moment (M) which showed a gradual increase in the blocking temperature maxima [TB(dc) = 42, 54, 61 and 68 K] with the increasing Doff [15 (± 2), 19 (± 2), 26 (± 2), and 33 (± 1) nm, respectively] (Figure 4b). Such magnetization trend reflects stronger interparticle magnetic interactions resulting from the successive directional reduction of dipole-dipole distances in the increasingly eccentric core-shells.32-33. Further, the effective magnetic anisotropy resulting from different orientations of surface spins and dipolar interactions of the variably eccentric core-shells, was characterized by the temperature dependence of AC susceptibility (Figure 4c-e) ― the measured AC blocking temeprature [TB(AC)] shifted positively upon increasing the field frequencies (10, 102, 103, 104 Hz) in susceptibility curves for each NP (Figure S8). In AC field, the characteristic relaxation time (τ0) is governed by the energy barrier which is the product of effective anisotropy constant (Keff) and particle volume. Following the Neel-Arrhenius and Vogel-Fulcher equations, τ0 and Keff for each eccentric core-shell were calculated (details in SI): as shown in Figure 4e, upon gradual increase in Doff, Keff followed an aproximately linear progressive trend, reflecting the gradual increase in magnetic anisotropies originated from the increasing dipolar interactions.34-35


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In conclusion, a strategy towards eccentric metal@silica core-shells is developed, which renders precise directional control of core positions. By the inclusion of variable amounts of Ca2+ into SiO2 and redox annealing at high temperatures triggers: solid-solid CaSiO3-SiO2 phase segregation, a hairline biphasicinterface evolution and off-center core-migration, where heat-induced movement of metal core NC is selfarrested at controllable radial distances. The magnetic property-measurements revealed that the dipolar interactions and effective magnetic surface anisotropies of different ece-FePd@(CaSiO3-SiO2) NPs could be finely tuned according to the eccentricity of core-shell. The in-depth mechanistic understanding of guided nanoscale migration behavior of NC in solid-solid biphasic media opens avenues for possible design and implementation of plethora of mixed phase systems with engineered interfaces for solid-state synthesis and engineering of nanostructures having variety of unique and tunable properties and applications in the field of magnetically guided and tunable self-assemblies, augmented plasmonics and molecular sensing (in case of plasmonic-hybrid nanoparticles), selective catalysis and biomedical science.


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ASSOCIATED CONTENT Supporting Information. Experimental Details, Additional TEM images, XRD patterns, XPS spectra, magnetization curves. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected]. Author Contributions §

contributed equally to this work.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (MSIP) (Grant NRF-2016R1A3B1907559) (I.S.L.)..


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17. Chen, T.; Chen, G.; Xing, S.; Wu, T.; Chen, H. Scalable Routes to Janus Au-SiO2 and Ternary Ag-Au-SiO2 Nanoparticles. Chem. Mater. 2010, 22, 3826–3828. 18. Wu, B.; Tang, S.; Chen, M.; Zheng, N. Amphiphilic Modification and Asymmetric Silica Encapsulation of Hydrophobic Au–Fe3O4 Dumbbell Nanoparticles. Chem. Commun. 2013, 50, 174−176. 19. Crane, C. C.; Tao, J.; Wang, F.; Zhu, Y.; Chen, J. Mask-Assisted Seeded Growth of Segmented Metallic Heteronanostructures. J. Phys. Chem. C 2014, 118, 28134−28142. 20. Wang, D.; Wang, X.; Li, Z.; Chi, M.; Li, Y.; Liu, Y.; Yin, Y. Migration of Iron Oxide Nanoparticle through a Silica Shell by the Redox-Buffering Effect. ACS Nano 2018, 12, 10949−10956. 21. Kim, Y. J.; Choi, J. K.; Lee, D. G.; Baek, K.; Oh, S. H.; Lee, I. S. Solid-State Conversion Chemistry of Multicomponent Nanocrystals Cast in a Hollow Silica Nanosphere: MorphologyControlled Syntheses of Hybrid Nanocrystals. ACS Nano 2015, 9, 10719−10728. 22. Bubenhofer, S. B.; Krumeich, F.; Fuhrer, R.; Athanassiou, E. K.; Stark, W. J.; Grass, R. N. From Embedded to Supported Metal/Oxide Nanomaterials: Thermal Behavior and Structural Evolution at Elevated Temperatures. J. Phys. Chem. C 2010, 115, 1269−1276. 23. George, C.; Dorfs, D.; Bertoni, G.; Falqui, A.; Genovese, A.; Pellegrino, T.; Cingolani, R. A Castmold Approach to Iron Oxide and Pt/Iron Oxide Nanocontainers and Nanoparticles with a Reactive Concave Surface. J. Am. Chem. Soc. 2011, 133, 2205−2217. 24. Martel, L.; Allix, M.; Millot, F.; Sarou-Kanian, V.; Véron, E.; Ory, S.; Deschamps, M. Controlling the Size of Nanodomains in Calcium Aluminosilicate Glasses. J. Phys. Chem. C 2011, 115, 18935−18945. 14 ACS Paragon Plus Environment

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25. Hudon, P.; Baker, D. R. The Nature of Phase Separation in Binary Oxide Melts and Glasses. I. Silicate Systems. J. Non-Cryst. Solids 2002, 303, 299−345. 26. Shin, J.; Kim, H.; Lee, I. S. Synthesis of Fe3O4/PdO Heterodimer Nanocrystals in Silica Nanospheres and Their Controllable Transformation into Fe3O4/Pd Heterodimers and FePd Nanocrystals. Chem. Commun. 2008, 43, 5553−5555. 27. Jeon, K. W.; Lee, D. G.; Kim, Y. K.; Baek, K.; Kim, K.; Jin, T.; Lee, I. S. Mechanistic Insight into the Conversion Chemistry between Au-CuO Heterostructured Nanocrystals Confined inside SiO2 Nanospheres. Chem. Mater. 2017, 29, 1788−1795. 28. Lin, Y.; Skaff, H.; Emrick, T.; Dinsmore, A. D.; Russell, T. P. Nanoparticle Assembly and Transport at Liquid-liquid Interfaces. Science 2003, 299, 226−229. 29. Pieranski, P. Two-dimensional Interfacial Colloidal Crystals. Phys. Rev. Lett. 1980, 45, 569-572. 30. Kresse, G.; Furthmuller, J. Efficiency of ab-initio Total Energy Calculations for Metals and Semiconductors Using a Plane-wave Basis Set. J. Comp. Mater. Sci. 1996, 6, 15−50. 31. Kresse, G.; Furthmuller, J. Efficient Iterative Schemes for ab-initio Total-energy Calculations Using a Plane-wave Basis Set. Phys. Rev. B 1996, 54, 11169−11186. 32. Mamiya, H.; Kanatani, I.; Furubayashi, T. Blocking and Freezing of Magnetic Moments for Iron Nitride Fine Particle Systems. Phys. Rev. Lett. 1998, 80, 177−180. 33. Tronc, E.; Prene, P.; Jolivet, J. P.; d'Orazio, F.; Lucari, F.; Fiorani, D.; Godinho, M.; Cherkaoui, R.; Nogues, M.; Dormann, J. L. Magnetic Behaviour of γ-Fe2O3 Nanoparticles by Mössbauer Spectroscopy and Magnetic Measurements. Hyperfine Interact. 1995, 95, 129−148.


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34. Coşkun, M.; Can, M. M.; Coşkun, Ö. D.; Korkmaz, M.; Firat, T. Surface Anisotropy Change of CoFe2O4 Nanoparticles Depending on Thickness of Coated SiO2 Shell. J. Nanopart. Res. 2012, 14, 1197−1205. 35. Ma, D.; Veres, T.; Clime, L.; Normandin, F.; Guan, J.; Kinston, D.; Simard, B. Superparamagnetic FexOy@SiO2 Core-shell Nanostructures: Controlled Synthesis and Magnetic Characterization. J. Phys. Chem. C 2007, 111, 1999−2007.

Scheme 1. Schematic for the synthesis of eccentric core-shells with arrested metal core at CaSiO3SiO2 interface.


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Figure 1. (a) TEM images, EDS elemental mapping and line profiles [Ca (red), Si (yellow)] of SiO2/Ca2+ NPs before and after annealing at 500 C, 600 C and 800 C in air. (b-c) TEM images (up, inset: lattice fringe of core), HRTEM (down, inset: EDS mapping [Ca (red)]) of Fe3O4@(SiO2/Ca2+/Pd2+) before (b) and after air annealing at 500 C (c). (d-g) Time course TEM images for reductive thermal conversion of air annealed Fe3O4@(SiO2/Ca2+/Pd2+) to ece-FePd@(CaSiO3-SiO2) at 800 C for 0 h, 12 h, 24 h, and 48 h. Corresponding histograms (below) show the off-centered distance of the FePd core. (h) XRD patterns of Fe3O4@(SiO2/Ca2+/Pd2+) before (black), after air annealing at 500 °C (blue) and reductive annealing at 800 °C (red).


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Figure 2. TEM images, histogram and HRTEM (inset) for showing reversible shuttling of FePd coreposition at CaSiO3-SiO2 interface and in CaSiO3 phase in ece-FePd@(CaSiO3-SiO2) during the successive annealing with repeatedly switching the flowing gas between air and hydrogen.


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Figure 3. Different ece-FePd@(CaSiO3-SiO2) with various volume of CaSiO3, synthesized using different amounts of Ca2+ for showing the controllable off-center distances (Doff) with their TEM images, and EDS mapping [Pd (blue), Fe (green), Ca (red), Si (yellow)].


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Figure 4. (a) TEM images of ece-FePd@(CaSiO3-SiO2) NPs obtained after different reductive annelaing times for magnetism studies. (b) DC magnetization versus temperature curves for various eceFePd@(CaSiO3-SiO2) having increasing core Doff and correlation between core Doff and TB(DC) (inset). (c) Neel-Arrhenius equation plotting for ece-FePd@(CaSiO3-SiO2) with Doff = 15, 21, 26 and 29 nm. (d) Vogel-Fulcher equation plotting for ece-FePd@(CaSiO3-SiO2) with Doff = 15, 21, 26 and 29 nm. (e) AC magnetization based trends of calculated τ0 and Keff values for various ece-FePd@(CaSiO3-SiO2) having increasing Doff.


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Metal@SiO2 CoreâShells with Self-Arrested Migrating Core - Nano

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Metal@SiO2 CoreâShells with Self-Arrested Migrating Core - Nano