Solid-State Conversion Chemistry of Multicomponent Nanocrystals

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Solid-State Conversion Chemistry of Multicomponent Nanocrystals Cast in a Hollow Silica Nanosphere: Morphology-Controlled Syntheses of Hybrid Nanocrystals Yeon Jun Kim,†,§ Jung Kyu Choi,†,§ Dong-Gyu Lee,† Kyungjoon Baek,‡ Sang Ho Oh,‡ and In Su Lee*,† †

Department of Chemistry, and ‡Department of Materials Science & Engineering, Pohang University of Science and Technology (POSTECH), Pohang, Gyeongbuk 790-784, Korea. §Y.J.K. and J.K.C. contributed equally to this work.

ABSTRACT During thermal transformation of multicomponent

nanocrystals in a silica nanosphere, FeAuPd alloy nanocrystals migrate outward and thereby leave a cavity in the silica matrix. Oxidation then converts these nanocrystals back into phasesegregated hybrid nanocrystals, AuPd@Fe3O4, with various morphologies. The FeAuPd-to-AuPd@Fe3O4 transformation was cast by the in situ generated hollow silica mold. Therefore, the morphological parameters of the transformed AuPd@Fe3O4 are defined by the degree of migration of the FeAuPd in the hollow silica nanoshell. This hollow silica-cast nanocrystal conversion was studied to develop a solid state protocol that can be used to produce a range of hybrid nanocrystals and that allows for systematic and sophisticated control of the resulting morphologies. KEYWORDS: nanostructures . nanoparticles . solid-state reactions . hollow nanoparticle . nanoreactor

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eterostructured hybrid nanocrystals (HNCs) consist of two or more inorganic domains of chemically dissimilar materials that have fused at a nanoscale interfacial area. Synthesis of HNCs is an important and challenging subject in nanochemistry.1
4 HNCs composed of metal oxides and noble metals, including M/Fe3O4 (M = Au, Ag, Ni, Pd, Pt, PdPt), M/MnO (M = Au, Pd, Pt), and Au/In2O3, have synergistically enhanced catalytic activity and plasmonic resonance, which are influenced by the shape and the heterojunction structure of the component domains.5
12 Moreover, optical coupling and photocatalytic activity of a novel metal
semiconductor (Au/CdX, X = Se, Te, S) HNC can be tuned by a symmetry evolution of morphology of the metal
semiconductor HNC.13
15 More recently, the nanocrystal conversion chemistry concept has been adopted to produce KIM ET AL.

several Au-containing HNCs, such as Au/Cu2O, Au/Cu2S, Au/CuInSe2, Au/In2O3, Au/Ag2S, and Au/MnO.7
12 This method exploits the phase segregation of metallic alloys during solution-mediated oxidation or chalcogenization, and represents an alternate method to produce diverse and complex materials in a well-controlled and predefined manner.5,6 This paper reports a solid-state conversion-based strategy to synthesize HNCs by exploiting the transformation behavior of multicomponent nanocrystals in a hollow nanostructured reaction medium.16
21 In previous work, Fe3O4/PdO HNC generated from silica-encapsulated Fe3O4 nanocrystal and Pd2þ complexes can be transformed at high temperature T into a FePd alloy nanocrystal while maintaining nanometer size in a thermally stable silica medium.21 In higher multicomponent systems that contain VOL. XXX

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* Address correspondence to [email protected]. Received for review April 8, 2015 and accepted October 27, 2015. Published online 10.1021/acsnano.5b05860 C XXXX American Chemical Society

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ARTICLE Scheme 1. Schematic illustration of the solid-state protocol for synthesis of morphology-controlled AuPd@Fe3O4 HNCs depended on the positions of the FeAuPd nanocrystal in the silica shell.

Fe3O4, Au, and Pd species, the in situ reduced metal alloy nanocrystal moves outward, and thereby leaves a space at the core; the resulting hollow structure traps the migrating metallic nanocrystal in a silica nanoshell. The migration of the in situ reduced metal alloy nanocrystal in a silica matrix is related to the thermal properties of the silica matrix that is used as the reactor. At above glass transition temperature Tg, amorphous silica changes to glass transition state; as a result its kinematic viscosity η decreases but its elasticity is maintained.22
24 During annealing at near the glass transition temperature Tg of silica under a reductive atmosphere, multicomponent alloy nanocrystals are generated, but are also mobile due to the low viscosity of the glassy silica matrix. Noble metals (e.g., Au, Pt, Pd) migrate and sinter with adjacent metals in a silica matrix during calcination.25
27 Especially, Pd in SiO2 transforms to Pd on SiO2 nanocomposite at 500 e T e 900 °C due to diffusion of whole metallic Pd clusters within that silica matrix; the diffusity of the Pd clusters in the silica shell is related to T, η, and the size of the Pd cluster.25 Here we quantified how the migration of metal alloy nanocrystals in a silica matrix depends on the reductive Tann, η of the silica shell, and the size of the metal nanocrystals. Moreover, while guided by the in situ developed hollow silica mold, the subsequent oxidative process could convert the alloy nanocrystal back to phase-segregated HNCs with diverse morphologies. The morphological parameters of the resultant HNCs, including the number and shape of component domains, were defined by the degree of migration of the alloy nanocrystal in the hollow silica nanoshell. We report successful syntheses of a range of Fe3O4/ metal HNCs by using a novel solid-state protocol, which entails a series of nanocrystal conversion processes, including alloying, migration, and phase segregation, all within the confines of a silica nanosphere (Scheme 1). Exploitation of the understanding of the alloy-to-HNC transformation cast in the hollow silica mold enabled systematic and sophisticated control of the diverse morphologies of the resulting HCNs. KIM ET AL.

RESULTS AND DISCUSSION HNC-to-Alloy Transformation of the (Fe3O4/Au)@(SiO2/Pd2þ) during the Reductive Annealing. As a starting template for the conversion chemistry, the (Fe3O4/Au)@(SiO2/Pd2þ) nanosphere, was synthesized using reverse microemulsion (Figure 1a).28 This nanosphere may contain an embedded satellite-type hybrid nanocrystal composed of Fe3O4 and many surrounding tiny Au nanocrystals with Pd2þ complexes dispersed over a silica sphere. The (Fe3O4/Au)@(SiO2/Pd2þ) was calcined in air at 500 °C to burn off any interfering organic molecules. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) analyses revealed that the tiny Au nanocrystals coalesced into larger ones and that Pd2þ were concentrated on the Fe3O4 surface into a PdO grain, thereby generating a ternary hybrid nanocrystal, Fe3O4/Au/PdO. The Fe3O4/Au/PdO consists of small individual Au and PdO nanocrystals with similar sizes of 2.7 ((0.6) nm, which dangle around the 12.6 ((0.9)nm Fe3O4 nanocrystal (Figures 1b and S1). EDS elementary mapping analysis showed Au elements concentrated at one dot and Pd elements at another dot as comparing with HRTEM image (Figure S1a). When the resulting (Fe3O4/Au/PdO)@SiO2 was treated at 700 °C under a flow of Ar and 4% H2, the Fe3O4 was reduced to metallic Fe, most likely with the catalytic aid of the adjacent AuPd grain that was observed during the reductive annealing at 600 °C (Figure S2), and then converted into a 9.8 ((0.7) nm-sized spherical nanocrystal of the ternary-alloyed FeAuPd phase (Figure 1c).21,29,30 The decrease in the size of the metal core in Fe3O4, Au and PdO grains by reductive alloying to FeAuPd led to generation of a crack (∼3 nm) between the FeAuPd and the silica surface (Figure 2a inset). Other evidence for this alloy transformation includes the absence of peaks that correspond to Fe3O4, and the generation of a major peak at 44°, corresponding to the bcc crystalline phase in the XRD pattern (Figure 1d). The peak was found to be just slightly shift to the lower angle from the standard XRD peak of Fe bcc phase, indicating a slight increase of VOL. XXX

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ARTICLE Figure 1. TEM and HRTEM images of (a) (Fe3O4/Au)@(SiO2/Pd2þ) as starting template in this study, (b) (Fe3O4/Au/PdO)@SiO2, air-annealed (a) at 500 °C, generated one larger Au nanocrystal and an oxidized PdO nanocrystal on the Fe3O4 surface. (c) FeAuPd@h-SiO2(12h), reductive annealed (b) at 700 °C for 12 h (d) XRD patterns of (i) (Fe3O4/Au)@(SiO2/Pd2þ), (ii) (Fe3O4/ Au/PdO)@SiO2, (iii) FeAuPd@h-SiO2(12h), and (iv) (AuPd@dum-Fe3O4)@SiO2. (e) EELS elementary mapping image of the FeAuPd@h-SiO2(12h) for each metals [Fe (blue), Au (green), and Pd (red)].

Figure 2. TEM and HRTEM images of FeAuPd@h-SiO2. A series of migration of the FeAuPd nanocrystals in the rubbery silica shell as a function of time. TEM (upper), HRTEM (right lower), and histogram off-center distances of the FeAuPd nanocrystals in TEM images (left lower). (a) 0 h, 3 nm off-centered, (b) 12 h, 7 nm off-centered, (c) 24 h, 13 nm off-centered, and (d) 48 h, 15 nm off-centered (extrude out of silica shell). (e) Series of the captured pictures from the recorded video through in situ heating TEM. (Real-time movie is presented in the Supporting Information.)

the lattice parameter by the random incorporation of Au and Pd atoms into the lattice of Fe bcc phase. Moreover, high resolution-TEM (HRTEM) and elementary mapping analyses revealed that the Fe, Au, and KIM ET AL.

Pd elements are all homogeneously distributed over the entire region of the single crystalline sphere, which ensures the development of a random-alloy phase of trimetallic FeAuPd (Figures 1e and S1b). VOL. XXX

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the glassy silica matrix at 700 °C. Migration of the FeAuPd nanocrystal in the silica shell was definitely observed using in situ heating TEM. Figure 2e is a series of the captured pictures from the recorded video during the in situ heating TEM observation, showed the whole metallic migration outward the silica shell without any decomposition or transformation of the FeAuPd nanocrystal (this process was recorded in a real-time movie through in situ heating TEM and is presented in the Supporting Information). Grass et al. had reported the structural evolution of Pd clusters within rubbery silica matrix by migration and aggregation at elevated temperatures.25 They described the structural transformation of the Pd in SiO2 to Pd on SiO2 nanocomposite was induced by migration of whole metallic sphere rather than by Pd atomic diffusion.25 They also described in the paper the diffusivity D [m2 3 s
1] of the metal particle can be described by the Stokes
Einstein equation: D ¼

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Migration of the Alloyed Nanocrystals in the Silica Shell during the Reductive Annealing. The FeAuPd nanocrystal was displaced from the center of the sphere, thereby leaving a void in its trajectory and creating a unique hollow nanostructure, FeAuPd@h-SiO2, which traps the migrating FeAuPd nanocrystal in the middle of the silica nanoshell (Figure 2). To elucidate the displacement of the nanocrystal, the effect of annealing time tann on the transformation was examined by analyzing the nanospheres annealed for tann = 0, 12, 24, or 48 h. This investigation revealed that the movement of the nanocrystal was triggered by the reduction of the Fe3O4 at 700 °C and continued outward to the external surface of the silica sphere (Figure 2). The migration distances of the FeAuPd nanocrystals, which were each estimated by the most populated off-center displacement in plane-projected TEM images, increased gradually and continuously with increasing tann up to 48 h (Figure 2d). The positions of most of the FeAuPd nanocrystals in the silica sphere were away from the center of sphere by an amount that increased with tann: 3 nm at 0 h, 7 nm at 12 h, 13 nm at 24 h, and 15 nm at 48 h (Figures 2 and S3). Meanwhile, the voids generated by the migration of FeAuPd nanocrystals in the silica shell expanded from 34.6 nm2 at 0 h to 64.3 nm2 at 12 h, 89.5 nm2 at 24 h, and 86.7 nm2 at 48 h (Figure S4). As the reductive tann was increased, the size of the FeAuPd nanocrystals did not change, but the size of the FeAuPd@h-SiO2 decreased slightly due to densification of the silica matrix after phase transformation to the glass transition phase (Figure S4). This evolution can be interpreted as phenomenon by which the metallic nanocrystal relieves the high interfacial strain between it and the unfavorably interacting silica.31,32 A TEM image of nanospheres that had been treated for 48 h revealed that some of FeAuPd nanocrystals fully escaped the protective silica sphere; these were readily oxidized to AuPd@Fe3O4 HNCs, presumably upon exposure to air after annealing (Figures 2d and S3d). This displacement of the FeAuPd nanocrystals in the spherical silica shell is plausibly induced by migration of the whole alloyed metal. The alternative possibility is permeation into the silica matrix with atomic diffusion of melted metals; this process seems improbable because it would lead to change in metal size, and to omnidirectional diffusion, but the measurement results showed no change in the size of the FeAuPd nanocrystals (Figure S4) and a directionality for the displacements of the FeAuPd nanocrystals in the spherical silica shell (Figures 2 and S4). The EELS mapping image of the FeAuPd@h-SiO2 that had been annealed for 12 h showed no metal ions or metal grains except the FeAuPd nanocrystal in the silica shell (Figure 1e), and differential scanning calorimetry (DSC) did not show any melting (Figure S5a). Based on these results, the displacements of the FeAuPd nanocrystals probably occurred by migration of the whole alloyed metal through

kB T 6πηa

where kB is the Boltzmann constant, T is the absolute temperature, η is the dynamic viscosity of the matrix, and a is the diameter of the particle.25 According to the equation, D of the FeAuPd nanocrystals in the silica shell is proportional for Tann.25 To test this prediction, isolated FeAuPd@h-SiO2 nanocrystals were annealed under a flow of Ar and 4% H2 for 12 h at Tann = 700, 730, 750, or 800 °C and their positions were estimated as the modal off-center displacement in plane-projected TEM images (Figure S6). As Tann increased, the migration distances of the FeAuPd nanocrystals increased. The positions of the modal concentration of FeAuPd nanocrystals in the silica sphere departed from the center of sphere by 7 nm at 700 °C, 9 nm at 730 °C, 14 nm at 750 °C, and 16 nm at 800 °C (Figure S6). Not only that, but also the cavity generated by migration of the FeAuPd nanocrystals disappeared at Tann > 700 °C. To investigate this temperature relation in detail, the thermal properties of the FeAuPd@h-SiO2 were characterized using DSC and thermogravimetric analysis (TGA). Based on the DSC curve, FeAuPd@h-SiO2 has Tg = 695 °C, and TGA detected no change in mass up to 1100 °C (Figure S5a). During annealing at 700 °C, the phase of the amorphous silica shell changed to glass
liquid transition phase, which was getting decreased viscosity, so the FeAuPd nanocrystals can move relatively easily outward silica shell through the glassy silica matrix to relieve the high interfacial strain with the unfavorably interacting silica. The Stokes
Einstein equation indicates that D should also increase as η decreases, with all other factors constant.25 It is well-known that increasing the porosity of amorphous silica or polymer network causes reduction in Tg and the viscosity of both.33 VOL. XXX

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displacement of ∼7 nm, so the Fe3O4 segregation refilled the neighboring cavity and, as a result, developed a dumbbell-like structure, AuPd@dum-Fe3O4 that consists of two fused Fe3O4 grains of similar size of 10.0 ((0.9) nm, one of which embraces a 3.6 ((0.7) nm sized AuPd nanocrystal (Figure 3b). A time-course TEM study of the samples during annealing at 200 °C samples revealed that the oxidized Fe3O4 sprouted from the cavity-side surface of the FeAuPd sphere and then grew by filling the cavity; the result was a 7.8 ((0.7) nm sphere at 12 h. During this process the FeAuPd grain shrank to 6.9 ((0.6) nm (Figure 4). Subsequently, segregation of the Fe3O4 phase proceeded within the remaining FeAuPd sphere, thereby completing the structure of the AuPd@dum-Fe3O4. Moreover, when the (AuPd@dum-Fe3O4)@SiO2 was retreated under Ar and 4% H2 at 700 °C, the grains of the dumbbell nanocrystal merged back into a spherical FeAuPd nanocrystal with a cavity remaining at the center of the silica sphere; this hollow led in turn to regeneration of the initial hollow structure of the FeAuPd@h-SiO2(12h). In addition, the reversible conversion between AuPd@Fe3O4 HCN and the alloyed FeAuPd nanocrystal could be cycled repeatedly by switching the flowing gas environment during the annealing (Figure S8).20,37,38 Air-annealing of the FeAuPd@h-SiO2(0h), which contains an FeAuPd nanocrystal that is displace by ∼ 3 nm from a minute interior void space, converted the FeAuPd nanocrystal to AuPd@sp-Fe3O4 HNCs that consist of an AuPd core nanocrystal in a nearly spherical Fe3O4 grain with a somewhat bumpy surface (Figure 3a). For FeAuPd@ h-SiO2 that had been annealed for 24 h (FeAuPd@ h-SiO2(24h)), in which the FeAuPd nanocrystal with ∼13 nm displacement is caught at the halfway point of the silica nanoshell, the growth of the segregated Fe3O4 phase proceeded simultaneously in both directions toward the internal and external spaces. Consequently, the FeAuPd nanocrystal was transformed to an AuPd@tri-Fe3O4 HNC with a three-ball-snowman structure, in which two spherical Fe3O4 grains with similar size of 9. ((0.9) nm are linked to a central Fe3O4 grain that embraces an AuPd nanocrystal (Figure 3c). Air-annealing of the FeAuPd@h-SiO2(48h) yielded a mixture of silica nanospheres, that is, with an AuPd@ tri-Fe3O4 HNC embedded through the shell and an AuPd@mush-Fe3O4 HNC patched at the surface (Figure S9). When the (Fe3O4/Au/PdO)@SiO2 was treated under Ar and 4% H2 at Tann = 780 °C, with the purpose of inducing further migration of the FeAuPd nanocrystals, nearly all of them leached out of the silica sphere during annealing, were converted to AuPd@ Fe3O4 HNC by the oxidation in air, and became attached to the external silica surface. During oxidative conversion, the segregated Fe3O4 grew outward to form a somewhat flat grain patched onto the outside of the silica sphere, while still holding the metallic AuPd VOL. XXX

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Here, we used an organosilane agent (C18TMS, n-octadecyltrimethoxysilane) as a porogen to increase the porosity of the silica shell.33 After reductive annealing at 700 °C, FeAuPd alloyed nanocrystals embedded in C18TMS treated silica shell were further away from the center than the C18TMS untreated particles (Figure S7). Also, the Tg values for each particle were compared by measuring the DSC, which showed a lower Tg (667 °C) for the C18TMS-treated FeAuPd@h-SiO2 than for the untreated ones (Figure S5). The Stokes
Einstein equation indicates that D should also increase as metal size (a) decreases, with all other factors constant. When the iron oxides were encapsulated with the Au and Pd precursors, smaller iron oxides (9.8 nm average core size) were used instead of the 12.6 nm ones. The size of the alloyed FeAuPd nanocrystals was decreased to 6.7 nm by the reductive annealing process at 700 °C, and the metals migrated further to the exterior of the silica shell than did the large crystals (Figure S7). A void in the migration trajectory of the FeAuPd nanocrystals in the silica shell was generated during reductive annealing at 700 °C and widened with increasing tann due to outward migration of the FeAuPd nanocrystals. The amorphous silica matrix was a viscous and elastic fluid at T ≈ Tg.22
24 Despite the fact that a glass, like a liquid, has a topologically disordered structure, at the same time, it has elastic properties of an isotropic solid at the glass transition state.22
24 As temperature increased, the silica matrix became increasingly viscous behavior, and lost its elastic character; these trends explain why the cavity disappeared after annealing Tann > 700 °C. Moreover, the failure of TGA to detect mass loss indicates that the generation of the cavity is not a result of decomposition of silica. Therefore, the cavity may record the trajectory of the migration of the metallic FeAuPd nanocrystal through the silica matrix, which had viscoelastic properties at T ≈ Tg. Reversion of the Alloyed Nanocrystal to the HNC through the Oxidative Phase Segregation Process. To investigate the possible reversal of the alloy transformation, the FeAuPd@h-SiO2 was treated at 700 °C under air atmosphere, which may induce the separation of phases and elements based on their respective susceptibilities to oxidation. XRD and TEM analyses after air-annealing confirmed the preferential oxidation of Fe and segregation of the oxidized Fe3O4 phase, as well as retention of the metallic AuPd grain, which represents the reversion of the alloyed nanocrystal to the HNC, AuPd@Fe3O4, with metal
metal oxide heterojuction (Figure 3).34
36 Interestingly, the segregation of the Fe3O4 occurred in the hollow silica nanoshell, which acted as a reaction medium, to the resultant structures took the form of the nanoshell, rather than a spherical form. FeAuPd@h-SiO2 that had been annealed for 12 h (i.e., FeAuPd@h-SiO2(12h)) has an off-center

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ARTICLE Figure 3. Oxidative phase segregations of the FeAuPd nanocrystal with different morphologies depending on the positions of the FeAuPd nanocrystal. TEM (upper) and HRTEM (lower) images of (a) (AuPd@sp-Fe3O4)@SiO2 (3 nm off-centered; spherical), (b) (AuPd@dum-Fe3O4)@SiO2 (7 nm off-centered; dumbell), (c) (AuPd@tri-Fe3O4)@SiO2 (13 nm off-centered; three ball snowman), and (d) (AuPd@mush-Fe3O4)@SiO2 (15 nm off-centered; mushroom). The morphology-controlled cast multicomponent nanocrystals after an etching-treatment of silica shell. TEM (upper) and HRTEM (lower) images of (e) AuPd@sp-Fe3O4 (spherical), (f) AuPd@dum-Fe3O4 (dumbell), (g) AuPd@tri-Fe3O4 (three ball snowman), and (h) AuPd@ mush-Fe3O4 (mushroom) HNCs. Insets of (e)
(h): EDS element maps [Fe (blue), Au (green), and Pd (red)].

nanocrystals inside. This phase segregation created a mushroom-like structure of the AuPd@mush-Fe3O4, composed of a cap of Fe3O4 grain and a spherical stipe of AuPd coated with a thin Fe3O4 shell, which straddles the external surface of the silica nanosphere (Figure 3d). Consideration of Hollow Silica-Cast Phase Segregation Process: Morphology-Controlled Synthesis of HNCs. In all observations, phase segregation during oxidation of FeAuPd proceeded by preferential growth of a separate Fe3O4 grain on the silica
air interface at either the internal or external surface, followed by conversion of the remaining metallic grain into a core@shell-type AuPd@Fe3O4 structure. Accordingly, the distances of the converted FeAuPd nanocrystal from the internal and external KIM ET AL.

surfaces of the hollow silica nanoshell could be critical in defining the transformed structure. Therefore, the number of Fe3O4 grains and their integrated array structure in the resulting HCN must vary according to the radial locations of the FeAuPd nanocrystal in the starting FeAuPd@h-SiO2. These locations represent the migration degree of the nanocrystal during the preceding reductive annealing. Therefore, the final morphology of the AuPd@Fe3O4 HNC is produced through a series of sequential transformations, and can be determined by controlling Tann or tann during conversion from (Fe3O4/Au/PdO)@SiO2 to FeAuPd@h-SiO2. HRTEM images of the surfactant-free AuPd@Fe3O4 HNCs isolated from the silica nanospheres, that had been annealed reductively for different durations or at VOL. XXX

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ARTICLE Figure 4. Air-annealing process of the FeAuPd@h-SiO2(12h) at 200 °C as a function of time. The oxidized Fe3O4 was phase segregated from the FeAuPd nanocrystal with a gradually filling the center cavity. TEM (upper) and HRTEM (lower) images of samples isolated after the air-annealing of the FeAuPd@h-SiO2(12h) for (a) 0 h, (b) 5 h, (c) 12 h, and (d) 24 h. Insets: EDS element maps [Fe (blue), Au (green), and Pd (red)].

Figure 5. Applying for the morphology controlled syntheses of varying hybrid nanocrystals. TEM (upper) and HRTEM (lower) images of M@dum-Fe3O4 HNCs. M = (a) AuPt, (b) Au, (c) Pt, and (d) Pd.

different temperatures with the (Fe3O4/Au/PdO)@SiO2, clearly revealed bumpy spherical and dumbbell-, three-ball-snowman-, and mushroom-like morphologies after tann = 0, 12, or 24 h at Tann = 700 °C, or tann = 12 h at Tann = 780 °C. (Figure 3). These results confirm the sophisticated controllability of the resulting HNC morphology. To evaluate the wide-scale applicability of this approach, further investigation was performed by starting with (Fe3O4/AuPt2þ)@SiO2 that had been prepared by incorporating Pt2þ instead of Pd2þ.29,30 The (Fe3O4/AuPt2þ)@SiO2 was subjected to the current KIM ET AL.

transformation protocol, which entails consecutive annealing processes with switching between oxidative and reductive gas environments. The encapsulated nanocrystals underwent a series of transformations, including reductive alloying, outward migration, and hollow-silica-cast phase segregation, which together yielded AuPt@dum-Fe3O4 HNCs with a structure analogous to that of AuPd@dum-Fe3O4 (Figures 5a and S10). The current solid-state conversion strategy was also verified to be applicable to production of various bimetallic HCNs, such as Au@dum-Fe3O4, Pt@dumFe3O4, and Pd@dum-Fe3O4, by the transformation of VOL. XXX

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CONCLUSION In summary, by exploiting the conversion chemistry of multicomponent nanocrystals in a nanosized solid media, we devised a novel solid-state protocol for synthesizing a range of noble-metal/Fe3O4 HNCs with diverse heterostructures. This protocol, composed of a series of successive thermal conversion processes, was demonstrated through an in-depth investigation of the alloy-to-HNC transformation that is cast in the hollow nanostructured silica mold. Therefore, we could

METHODS General Consideration. All reagents, including FeCl3 3 6H2O (Acros), sodium oleate (TCI), oleic acid (Aldrich), 1-octadecene (Aldrich), Igepal CO-520 (Aldrich), tetraethyl orthosilicate (Acros), HAuCl4 3 xH2O (Strem), Na2PtCl4 3 xH2O (Strem), Na2PdCl4 3 3H2O (Strem), NH 4OH (Samchun), and NaOH (Samchun), were used as purchased. Transmission electron microscopy (TEM) analyses were conducted with JEOL JEM2100 and JEM-ARM200F instruments. All samples for the TEM analyses were prepared by immersing powders in water and then dispersed by repeating the vortexing and the sonication. The resulted suspension was then dropped (20 μL) on a carboncoated copper grid and dried under air. Powder X-ray diffraction patterns were obtained by using an X-ray diffractometer (18 kW, Rigaku, Japan). Differential scanning calorimetry/thermogravimetry analysis (DSC/TGA) was measured on SDT Q600 (TA Instruments). The DSC/TGA curve was obtained from the first heating run at a rate of 10 °C/min under Ar atmosphere. Preparation of the (Fe3O4/Au)@(SiO2/Pd2þ) and (Fe3O4/AuPt2þ)@SiO2. The (Fe3O4/Au)@(SiO2/Pd2þ) and (Fe3O4/AuPt2þ)@SiO2 nanospheres, which were used as starting template in this study, was performed through the previously reported reverse microemulsion procedure.28 Oleic acid stabilized Fe3O4 nanocrystals of 13.0 ((0.6) nm of average core size were prepared through the previously reported procedure.39 Igepal CO-520 (0.4 mL) was dispersed by sonication in a round-bottom flask containing cyclohexane (10 mL). Fe3O4 nanoparticles (3 mg) dispersed in cyclohexane were added to the reaction solution. An aqueous solution of HAuCl4 (16 mg/mL, 0.05 mL) was added dropwise to the reaction mixture to form a transparent suspension. Then aqueous solutions of Na2PdCl4 or Na2PtCl4 (16 mg/mL, 0.05 mL) were added to the suspension, followed by addition of ammonium hydroxide solution (28
30%, 0.13 mL) to the reaction mixture with vigorous stirring. Lastly, tetraethyl orthosilicate (TEOS, 0.4 mL) was added and the solution was stirred for 21 h. Methanol (MeOH) was added to the reaction suspension, and (Fe3O4/Au)@(SiO2/Pd2þ) or (Fe3O4/ AuPt2þ)@SiO2 precipitated as a dark brown solid, which was collected by centrifugation of the MeOH layer at the bottom. The resulted nanospheres were purified by repeated redispersion in EtOH and centrifugation. The (Fe3O4/Au)@SiO2, Fe3O4@(SiO2/Pt2þ), and Fe3O4@(SiO2/Pd2þ) nanospheres, which were used to produce Au@dum-Fe3O4, Pt@dum-Fe3O4, and Pd@dum-Fe3O4 respectively, had been prepared using a similar procedure that entails addition of aqueous solutions of HAuCl4, Na2PtCl4, and Na2PdCl4, respectively. Thermal Annealing Process. For the transformation to (Fe3O4/ Au/PdO)@SiO2, the powder of (Fe3O4/Au)@(SiO2/Pd2þ) nanospheres was placed in a box-type furnace, heated at 5 °C 3 min
1, then annealed in air at 500 °C for 5 h. Reductive annealing, which led to the transformation of the (Fe3O4/Au/PdO)@SiO2 to the FeAuPd@h-SiO2, was conducted in a tube furnace under a flow of Arþ 4% H2 at 700 °C for different tann. To convert FeAuPd@h-SiO2 to (AuPd@Fe3O4)@SiO2, the powder of

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demonstrate that the morphological parameters of the resulting HNC, including the number of grains and their integrated array structure, could be manipulated in a sophisticated, systematic, and well-controlled manner. We believe that the nanocrystal migration phenomenon, discovered in this study, can be developed into a model system that can be used to find explanations for the mobility and reactivity of nanoparticles in a solid-state medium. Use of this system could be very helpful to interpret complicated macroscopic phenomena such as the decline in the effectiveness of supported catalysts during operation at high temperature.

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(Fe3O4/Au)@SiO2, Fe3O4@(SiO2/Pt2þ) and Fe3O4@ (SiO2/Pd2þ), respectively (Figures 5b
c and S11).

FeAuPd@h-SiO2 nanospheres was air-annealed in a box-type furnace at 700 °C for 12 h. Each sample for the time-course TEM study during evolution of AuPd@dum-Fe3O4 was prepared by treating the FeAuPd@h-SiO2(12h) powder at 200 °C in air for 0, 5, 12, or 24 h. (AuPt@dum-Fe3O4)@SiO2, (Au@dum-Fe3O4)@SiO2, (Pt@dum-Fe3O4)@SiO2, and (Pd@dum-Fe3O4)@SiO2 nanospheres were synthesized from (Fe3O4/AuPt2þ)@SiO2, (Fe3O4/Au)@SiO2, Fe3O4@(SiO2/Pt2þ), and Fe3O4@(SiO2/Pd2þ) nanospheres, respectively, using a similar annealing protocol that entails calcinations in air, reductive annealing, and air-annealing. Preparation of the C18TMS Treated (Fe3O4/Au)@(SiO2/Pd2þ). Synthesis of C18TMS-treated (Fe3O4/Au)@(SiO2/Pd2þ) was similar to the procedures (above) that were used to synthesize untreated (Fe3O4/Au)@(SiO2/Pd2þ). C18TMS (0.08 mL) was added at the same time as the TEOS (0.32 mL). The resulting nanospheres were purified by repeated redispersion in EtOH and centrifugation. The resulting nanosphere powder was placed in a box-type furnace, heated at 5 °C 3 min
1, then annealed in air at 500 °C for 5 h. Reductive annealing, which led to the transformation of the (Fe3O4/Au/PdO)@SiO2 to the FeAuPd@h-SiO2, was conducted a tube furnace under a flow of Ar þ 4% H2 at 700 °C. Control Experiment for Relation between Diffusivity and Size of FeAuPd Nanocrystal. For the control experiment for the relation between the diffusivity and the size of the FeAuPd nanocrystal, the (Fe3O4/Au)@(SiO2/Pd2þ) was prepared using oleic-acid-stabilized Fe3O4 nanocrystals of 9.7 ((0.7) nm of average core size instead of those of 13.0 ((0.6) nm. Igepal CO-520 (0.4 mL) was dispersed in a round-bottom flask containing cyclohexane (10 mL) by sonication. Fe3O4 nanoparticles (3 mg) dispersed in cyclohexane were added to the reaction solution. An aqueous solution of HAuCl4 (16 mg/mL, 0.05 mL) was added dropwise to the reaction mixture; a transparent suspension resulted. Then aqueous solutions of Na2PdCl4 (16 mg/mL, 0.05 mL) was added to the suspension, was followed by the addition of ammonium hydroxide solution (28
30%, 0.13 mL) to the reaction mixture with vigorous stirring. Lastly, tetraethyl orthosilicate (TEOS, 0.4 mL) was added, and the solution was stirred for 21 h. Then MeOH was added to the reaction suspension; a dark brown solid (Fe3O4/Au)@(SiO2/Pd2þ) precipitate formed and was collected by centrifugation of the MeOH layer at the bottom. The resulted nanospheres were purified by repeated redispersion in EtOH and centrifugation. For the transformation to (Fe3O4/Au/PdO)@ SiO2, the powder of (Fe3O4/Au)@(SiO2/Pd2þ) nanospheres was placed in a box-type furnace, heated at 5 °C/min, then annealed in air at 500 °C for 5 h. Reductive annealing, which led to the transformation of the (Fe3O4/Au/PdO)@SiO2 to the FeAuPd@ h-SiO2, was conducted in a tube furnace under a flow of Ar þ 4% H2 at 700 °C for different tann. Isolation of Surfactant-free HNCs. HNCs were isolated from the annealed silica nanospheres by immersing (AuPd@Fe3O4)@SiO2 powder (15 mg) in 3 M NaOH solution (3 mL) and stirring for 24 h at room temperature. The solid of the resulting surfactant-free AuPd@Fe3O4 HNC was collected by centrifugation and purified by three repetitions of dispersion in water and centrifugation.

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Acknowledgment. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (2011-0017377). Supporting Information Available: The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.5b05860. Additional experimental data including DSC and TGA data, STEM-HADDF images, and TEM images (PDF) Video of in situ heating TEM (AVI)

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In Situ Heating TEM. In situ heating experiment was carried out in a transmission electron microscope (JEOL 2100F, JEOL) operated at 200 kV and under a column base pressure of ∼1  10
10 bar. The sample was first heated from 298 to 1098 K at constant heating rate (50 K min
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2 throughout the in situ heating experiments. Conflict of Interest: The authors declare no competing financial interest.

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