Shell Triangular Nanoprisms by Heteroepitaxy: Facet

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Pt/Fe3O4 Core/Shell Triangular Nanoprisms by Heteroepitaxy: Facet Selectivity at the Pt
Fe3O4 Interface and the Fe3O4 Outer Surface Maowei Jiang,† Wei Liu,‡ Xiaoli Yang,† Zheng Jiang,§ Tao Yao,‡ Shiqiang Wei,*,‡ and Xiaogang Peng*,† †

Center for Chemistry of Novel & High-Performance Materials, Department of Chemistry, Zhejiang University, Hangzhou 310027, China, ‡National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, P. R. China, and §Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, P. R. China

ABSTRACT Pt/Fe3O4 core/shell triangular nanoprisms were synthesized using

seed-mediated heteroepitaxy. Their well-defined shape, facets, and orderedassembly allowed detailed analysis of mechanism of the heteroepitaxy. At the Pt
Fe3O4 interface, existence of both lattice and chemical mismatch resulted in facet-selective epitaxy along Æ111æ directions of two lattices. X-ray absorption fine structure measurements demonstrated that the Pt seed nanocrystals were composed of an iron-rich Pt
Fe metallic thin layer sandwiched between the Pt core and a Fe
O outer-surface. The Fe
O outer-surface of the seed nanocrystals presumably offered epitaxial sites for the following deposition of the Fe3O4 shell. Each tip and side of a triangular nanoprism respectively possessed a groove and a ridge, and a (111) plane parallel to the basal planes linked all grooves and ridges. This interesting (111) plane approximately bisected the triangle nanoprisms and located near the Pt-seed. The outer surface of the hybrid nanocrystals was also found to be facet-selective, that is, solely {111} facets of Fe3O4 lattice. These polar {111} facets allowed the surface to be only occupied with high-density iron ions, and thus offered best surface coordination for the electron donating ligands in the solution. KEYWORDS: nanoprism . heteroepitaxy . core/shell . facet-selectivity . interface . surface

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ybrid nanocrystals are colloidal crystalline nanostructures composed of multiple chemical components, which have attracted great attention in the fields of chemistry and materials over the past years.1
10 Hybrid nanocrystals with programmable compositions and defined geometrical arrangements may not only integrate different functionalities together, but also bring in unique properties that originate from the synergistic effects of different components.11
18 At present, the understanding of heteroepitaxy of two adjacent components within a colloidal hybrid nanocrystal is limited.19
25 Among those successful synthetic examples,1
10,26
34 some of them are particularly remarkable because there is significant mismatch on both lattice structure and chemical composition between different components. To fully exploit the great potential of hybrid nanocrystals, synthesis of hybrid nanocrystals with significant mismatch on both JIANG ET AL.

lattice structure and chemical composition is essential. Lattice mismatch is known to play a significant role in the epitaxial growth of heterogeneous structures.24,35 However, when multiple components with different chemical nature are grown into a single nanocrystal, lattice mismatch alone might not be sufficient to take into account all effects at the interface within a hybrid nanocrystal, such as charge balance, chemical bonding, and size variation of atoms/ ions and so on. These issues are evidently associated with the chemical nature of the interface between different components of a hybrid nanocrystal, which could thus be classified as chemical mismatch. Both lattice and chemical mismatch between two interfacial components determines the possibility of epitaxial growth. Specific selectivity at an epitaxial contact facet facilitates spatially selective growth of the second component and may further dictate the VOL. XXX

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* E-mail: [email protected]; [email protected]. Received for review July 6, 2015 and accepted October 2, 2015. Published online 10.1021/acsnano.5b04130 C XXXX American Chemical Society

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RESULTS AND DISCUSSIONS Synthesis and Characterization of Pt/Fe3O4 Core/Shell Nanoprisms. Pt/Fe3O4 hybrid nanocrystals have been studied by several research groups. The groups of Sun7 and Schaak34 systematically studied Fe/Fe3O4 dumbbell nanostructures. Briefly, both groups applied Fe(CO)5 JIANG ET AL.

as the Fe precursor and oleylamine/oleic acid as the ligands to grow the dumbbell-shaped hybrid nanocrystals with presynthesized Pt seeds. Because the solvent was ocadecene in both reports, the temperature range was limited to ∼300
310 C. With the same solvent and ligand system, Li and co-workers32 reported synthesis of a variety of M-Fe3O4 (M = Ag, Au, Pt, Pd) hybrid nanocrystals using iron oleate as the Fe precursor. In the latter cases, a small amount of AgNO3 was added and the resulting Pt
Fe3O4 hybrid nanocrystals were mostly irregular in shape. In the current work, Pt/Fe3O4 core/shell nanocrystals were synthesized using a two-step procedure. In the first step, platinum nanocrystal seeds were formed by decomposition of platinum acetylacetonate (Pt(acac)2) with eicosane as the solvent to afford relatively high reaction temperatures (up to 340 C) . In this seedformation step, a trace amount of iron stearate was added in the presence of fatty amine and fatty acid. Without addition of a small amount of iron stearate in the first step, the resulting Pt seeds were found to be unstable under high temperatures, which suggested that iron stearate likely bonded onto the Pt seeds as the stabilizers (see detail below). In the second step, the reaction mixture was allowed to cool down to 150 C, and then, iron palmitate (Fe(Pa)3) was added into the reaction system. The reaction mixture was again heated up to 330 C to complete the epitaxial growth. Details of the synthesis can be found in the Experimental Section. Two typical two-dimensional appearances of the resulting hybrid nanocrystals (Figure 1a) were identified under transmission electron microscope (TEM), triangular ones (I) and hexagonal ones (II). Both types of appearances possessed a high contrast area in the inner part of each nanocrystal, which should be the Pt seeds. The nanocrystals showed ordered arrangements in short-range, which should be resulted from their anisotropic shape. The entire view of Figure 1a was enlarged in Figure S1 (Supporting Information) to illustrate details. To our knowledge, nearly monodisperse Pt/Fe3O4 core/shell triangular nanoprisms have not been reported in the literature. Their wellcontrolled Pt
Fe3O4 interface and nanocrystal-organic (including ligands and solvent molecules) outersurface should offer a unique model system to study heteroepitaxy with both lattice and composition mismatch. The selective area electron diffraction (SAED) image revealed particularly strong diffraction intensity of iron oxide {220}, {440}, and {311} rings (Figure 1b). The related integrated pattern of the SAED is shown in Figure 1c (blue line). The peak positions in the integrated SAED pattern were consistent with those in the powder X-ray diffraction (XRD) pattern (red line, Figure 1c) of the same sample, which matched a superposition of the standard patterns of Pt and Fe3O4. VOL. XXX

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overall morphology of a hybrid nanocrystal.2,3,19
21 Both types of mismatch may also cause interface defects, which results in symmetry breaking at a given interface and further induces structural anisotropy of the resulting hybrid nanocrystal. Several groups have initiated mechanistic studies on shape control of hybrid nanocrystals with significant lattice and chemical mismatch. Fan et al. reported facet-selective epitaxial growth of ZnO with {0001} facets on the {111} facets of Ag nanocrystal seeds.20 The Shim's group introduced seed-size and epitaxialrate dependent growth of CdS with {111} facets onto the {111} facets of Fe3O4 nanocrystal, which results in structural diversification of the hybrid nanocrystals.21 For either Ag and ZnO or Fe3O4 and CdS, there exist substantial differences about their lattice parameters, contact facet symmetry, chemical bonding character, and charge balance at the interface. Besides the interface between two different components within a hybrid nanocrystal, the facets at the outer surface facing its surrounding environment, namely the solution for colloidal synthesis, should play a critical role in colloidal synthesis of hybrid nanocrystals. It is well-known that this factor also contributes to shape control of single-component nanocrystals by selective adsorption of solution species onto a specific facet.36
42 Similarly, selective adsorption differentiates the growth rates of different facets at the outer surface of a hybrid nanocrystal. While the facet-selective epitaxy at the interface within a hybrid nanocrystal discussed above initiates symmetry breaking for the epitaxial growth, the facet-selective growth at the outer surface of the hybrid nanocrystal shall dictate the survival facets at its outer surface. These two types of facet-selectivity together may determine the final morphology of hybrid nanocrystals. This report focuses on studying anisotropic heteroepitaxy of Pt/Fe3O4 core/shell triangular nanoprisms. This pair of heteroepitaxial components possesses substantial lattice and chemical mismatch, which offers an interesting model to explore the interface effects within a hybrid nanocrystal. In addition, interplay between the outer surface of hybrid nanocrystals and the solution environment is also evidenced in this system. Given the intrinsic difference on the surface polarity of Fe3O4 nanocrystals, facet-selective adsorption of the solution species seems to result in a single type of facets on the outer surface of Pt/Fe3O4 core/ shell triangular nanoprisms, that is, the {111} facets of Fe3O4 nanocrystals.

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ARTICLE Figure 1. (a) TEM picture of the hybrid nanocrystals with parts I and II enlarged to the right. (b) The corresponding SAED pattern of the nanocrystals. (c) Integrated SAED and XRD patterns with the standard XRD pattern for Fe3O4 bulk crystals (PDF No. 65-3107). The (111) and (200) peaks of fcc Pt are labeled by the green arrows. (d) and (e) HRTEM images with the corresponding FFT patterns of hybrid nanocrystal with different two-dimensional projections. (f) Area-scanning EDS mapping of a hybrid nanocrystal. (g) Line-scanning EDS mapping of one pair of hybrid nanocrystals.

However, in comparison with either the XRD pattern or the standard pattern of bulk Fe3O4, the integrated SAED pattern showed substantial intensity variations for some Fe3O4 peaks. Both (400) and (511) were significantly attenuated, and the (220), (422), and (440) peaks were greatly enhanced. Such variations indicated that the crystal lattice of the nanocrystals should be selectively oriented, indicating anisotropic morphology of the hybrid nanocrystals. Crystallographic structure of the hybrid nanocrystal was further investigated with high resolution TEM (HRTEM). Figure 1d illustrates the typical image of a nanocrystal with triangular appearance and the corresponding fast Fourier transform (FFT) pattern. In the light-contrast regime, cross fringes of both {220} and {422} Fe3O4 facets were identified, each of which was either perpendicular or parallel to the sides of the regular triangle. The high-contrast regime that should be the platinum part emerged with Moiré fringes. In the FFT pattern, both {220} and {422} diffraction spots showed 6-fold symmetry. The zone axis was identified as [111]. Moreover, platinum {220} spots were observed in the same diffraction directions of Fe3O4 {440}, which were outlined together within the green dashed ellipses. Figure 1e shows the HRTEM image of a typical nanocrystal with a hexagonal two-dimensional appearance JIANG ET AL.

and the corresponding FFT pattern. The zone axis of the light-contrast part was indexed to be [422]. With this specific view, one could see (022), (131), (113), and (111) fringes of Fe3O4. Different from the triangular case, platinum (111) fringes were clearly distinguishable and affirmed in the same direction with iron oxide (111) fringes, indicating epitaxial growth of two different crystal lattices along their [111] axis. The EDS (energy dispersion spectroscopy) mapping images with area and line scanning (Figure 1f,g) revealed the existence of platinum and iron elements and their relative locations. The elemental profile was found to be consistent with the TEM contrast images (Figure 1f). The light-contrast area of a hybrid nanocrystal in TEM image corresponded to the Fe location, and the high-contrast area and the Pt elemental location were overlapping. To further support this conclusion, the EDS line scanning was performed for a pair of hybrid nanocrystals (Figure 1g). Shape of the Hybrid Nanocrystals and Their Assembly. If the two-dimensional projection of a nanocrystal was a regular triangle, there could be two common models, either regular tetrahedron or triangular nanoprism with certain thickness. A regular tetrahedron would not form a two-dimensional hexagonal projection. Conversely, a corner-truncated triangular nanoprism could yield both VOL. XXX

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ARTICLE Figure 2. (a) Structural model of the hybrid nanocrystals with different viewing directions. (b) TEM images of an assembled pattern of the nanocrystals with three rotation angles. Scale bar: 50 nm. (c and d) TEM images and the models of the selected particle in panel b with high magnification. All blue arrows pointed to the {111} facets.

types of two-dimensional projections (Figure 2a). In addition, a corner-truncated triangular nanoprism should display uniform contrast under TEM when the thickness direction of a triangular nanoprism was perpendicular to the TEM substrate, which was found to be consistent with the experimental results (Figure 1). The corner-truncated triangular nanoprisms described above were found to be consistent with the HRTEM results as well. The HRTEM images and the corresponding FFT results (Figure 1d,e) revealed two different types of orientation of the crystal lattices, namely, [111] zone axis for the triangular ones and [422] zone axis for the hexagonal ones. All these results suggested that two appearances;triangular and hexagonal ones;were different two-dimensional projections of the same shape of the hybrid nanocrystals (Figure 2a). Simultaneously, the shape shown in Figure 2a well explained the interesting intensity deviation of the SAED patterns in Figures 1b and 1c. The regular and anisotropic shape of triangular nanoprisms with good size distribution induced interesting self-assembly patterns (see additional detail in Supporting Information). Briefly, there were two simplest assembly patterns in short-range (Figures 1a and S2). For type I, the nanocrystals were assembled via side-to-side contact of the regular triangles. For type II, nanocrystals were assembled into one string along their thickness direction via face-to-face contact. JIANG ET AL.

The interesting pattern in the middle TEM image in Figure 2b seemed to be different from either type I or type II by the first glance. After (30 rotation, the assembly of the nanocrystals emerged with the jagged and stagger patterns. Detailed analysis (Figures S2
S4) implied that the actual pattern should be the heterozygosis of type I and II, namely with both side-to-side and face-to-face contacts. Importantly, the rotation images in Figure 2b revealed detailed side-face geometry of the nanocrystals. One representative nanocrystal in Figure 2b was outlined in Figure 2c, and three types of two-dimensional projections of this nanocrystal could be distinguished in Figure 2d. It should be noted that the triangular nanoprisms possessed unambiguous groove and ridge as illustrated in Figure 2c and Supporting Information Figure S2c. The angles of interlaced sawteeth in the 30 rotation and
30 rotation images were found to be about 139 and 142, respectively. Theoretically, the intersection angle between two directions of the Æ111æ family is 70.5, approximately half of the average of 139 and 142. Within experimental errors, this observation and above structural analysis together suggested the nanoprisms were surrounded by {111} facets. Effects of Interface between Pt and Fe3O4 Components within a Hybrid Nanocrystal. When Pt and Fe3O4 form interfaces, the situation is quite different from epitaxial growth among II
VI semiconductors, such as CdSe/CdS core/ shell nanocrystals.9 Due to their stark difference on VOL. XXX

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ARTICLE Figure 3. (a) Unit cell of face-center-cubic lattice shared by platinum and oxygen atoms in Fe3O4. (b) Fe3O4 crystal structure with oxygen in red and iron in green. (c and d) Side view and top view of the (111) facet of Fe3O4. (e and f) Side view and top view of Fe3O4 (200) facet. (g and h) Side view and top view of Fe3O4 (220) facet.

Figure 4. (a) Top view and two side views of iron-on-platinum on Pt (111) facets. (b,c) HRTEM images of intermediate states of the heterogeneous epitaxy. (d) TEM image of a nanocrystal to outline a bright ellipse across the interface. Inset: the same nanocrystal but with a different contrast level. (e) TEM and SAED (inset) of pure Fe3O4 nanocrystals for control experiment.

chemical nature, the structural matching is quite complicated at an atomic level. The crystal structure of Fe3O4 is quite complex. For Fe3O4 (magnetite), the oxygen anions form regular fcc lattice, but the Fe(III) ions are randomly distributed in either octahedral or tetrahedral voids and Fe(II) ions only in the octahedral voids (Figure 3a,b). This complex atomic arrangement forms alternating packing of iron-only regions and oxygen-only monolayers along the Æ111æ direction (Figure 3c,d), which yields polar Fe3O4 {111} facets. Fe3O4 {200} facets form an alternating polar iron-only monolayer and nonpolar iron
oxygen mixed monolayer along the Æ200æ direction (Figure 3e,f). The {220} facets are always formed by a mixture of iron and oxygen ions (Figure 3g,h). In comparison, Pt atoms form regular face-center-cubic (fcc). Above description illustrates that Fe3O4 crystal possesses multiple lattices while fcc Pt crystal is formed with a single lattice. Specifically, the Pt single crystal is a single-element structure and does not have chemical JIANG ET AL.

distinction of polar and nonpolar facets. Without considering the chemical nature, a simple comparison of the lattice constants of two fcc lattices gives a lattice mismatch of 6.96%. Apart from the effects on lattice match described above, differences of chemical nature between Pt and Fe3O4 also cause other types of chemical mismatch, which should significantly impact the heterogeneous epitaxy. Given the common appearance of Pt
Fe alloys and uncommon Pt
O bonds, it would be reasonable to assume the first atomic layer of Fe3O4 grown onto the surface of Pt seeds as an iron layer.20 Among three low indexed facets, namely {111}, {200}, and {220} families, both {111} and {200} facets of Fe3O4 possess iron-only polar layers (Figure 3) to form Fe-on-Pt packing. The {111} facets offer a significantly larger density of iron atoms to overlap with Pt atoms than the {200} facets do (comparing Figures 3d and 3f with Figure 4a). Moreover, the atomic arrangement of Fe3O4 {111} facets shares the same symmetry as VOL. XXX

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JIANG ET AL.

spectroscopy (XAFS) at Fe K-edge and Pt L3-edge (Figure 5). Both Fe K-edge k3χ(k) oscillation curve (Figure 5a) and corresponding Fourier transform (FT) curve (Figure 5b) for Pt/Fe3O4 core/shell nanoprisms were found to be the same as those of pure Fe3O4 crystal, indicating that Fe mainly existed in the form of Fe3O4. This was considered to be reasonable because the Fe atoms within the Fe3O4 domain dominated the Fe atoms at either the Pt
Fe3O4 interface or the nanocrystal
organic outer-surface. However, the Fe K-edge k3χ(k) oscillation curve of Pt seed nanocrystals in Figure 5a exhibits remarkable differences from either of Fe foil, FeO, and Fe3O4. The corresponding FT curve of Pt seed nanocrystals shows an obvious peak at 1.62 Å (Figure 5b), which is similar to the Fe
O coordination of FeO (Figure 5b). Moreover, a peak at 2.61 Å attributed to a Fe
Pt coordination shell was confirmed by XAFS data analysis for the Pt-seed (Figure S6d). As for the Pt L3-edge XAFS results, both k3χ(k) oscillation and FT curves of Pt seed as well as Pt/Fe3O4 core/shell nanocrystals were found to be close to those of Pt foil (Figures 5c,d) and quite different with those of PtO2. These facts suggested that Pt atoms were mainly in the form of metallic Pt in the core without noticeable exposure to the oxygen atoms from either the surface ligands for the seeds or the Fe3O4 shell, in agreement with the HRTEM and XRD results above. The quantitative structural parameters of the samples obtained from Fe K-edge and Pt L3-edge XAFS spectra were listed in Tables S1 and S2. The fitting results clearly demonstrated that there were two coordination forms, that is, Fe
O coordination and Fe
Pt coordination, at the surface of Pt seed nanocrystals. Considering the fitted Fe
O coordination numbers of 6 and 1.7, respectively, for the reference FeO and Pt seed nanocrystals, the proportion of Fe existing in the Fe
O coordination form could be estimated as 30% among the total Fe in the Pt seed nanocrystals. Therefore, the Fe atoms mainly existed in the Pt
Fe coordination layer(s), indicating the interface between Pt and Fe3O4 was iron enriched. Pt seed nanocrystals were also synthesized without the addition of a small amount of Fe carboxylate under the same conditions otherwise. As mentioned in the first subsection, the resulting Pt nanocrystals would precipitate onto the bottom of the reaction flask at 325 C. This suggested that the trace amount of iron stearate added in the typical reaction was acting as stabilizers to the Pt nanocrystals, presumably in the form of surface ligands. To our knowledge, metal alkanoates were identified as surface ligands to semiconductor43 and oxide44 nanocrystals. Consistent with this surface picture, the oxygen coordination of the Fe atoms on the Pt seeds from the typical synthesis was found to be 1.7 as mentioned above, instead of 6 for the FeO crystal. Furthermore, we previously45 VOL. XXX

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Pt {111} facets, that is, hexagonal close packing (Figures 3d and 4a). It should be noticed that, for the {111} facets of Fe3O4, incomplete occupation of iron atoms in the oxygen voids results in atoms missing at some lattice sites (Figure 3d and 4a (top)). This type of vacancies might provide spatial locations for strain release of the iron layer at the interface, which possesses slightly larger atomic distances than the adjacent Pt layer does. In addition to the main causes described in the above paragraph, this possible type of interface reconstruction might further help to establish facet-specific heterogeneous epitaxy along the Æ111æ directions of both crystals at the interface. It is worth considering the different combinations of Fe3O4 polar facets (either {111} or {200}) and other types of low indexed facets;non-{111} facets;of Pt lattices, given the Pt seeds being three-dimensional objects in solution. Fe3O4 {111} facets and either {200} or {220} facets of Pt do not share the same symmetry, neither do Fe3O4 {200} facets nor {220} or {111} facets of Pt. For the combination of Fe3O4 {200} facets and Pt {200} facets, though they share the same symmetry, the atom density on Fe3O4 {200} facets (Figure 3f) are too low to match the close-packing of Pt {200} facets. The specific facet-specific epitaxy, namely heterogeneous epitaxy of Fe3O4 {111} facets onto Pt {111} facets, further implies that there would be a significant amount of strain at the interface between the two components. By adjusting the brightness and contrast during HRTEM experiments, one could observe a bright ellipse across the interface of platinum and iron oxide as shown in Figure 4d. Such a bright ellipse may reflect the specificity of the interfacial situation. For further understanding of the interface effects, control experiments were performed without Pt seeds (see Experimental Section). TEM measurements showed that truncated iron oxide nanocubes were produced in the control experiment (Figure 4e). The corresponding SAED pattern emerged with preponderant intensity of {400} and {440} diffraction rings as expected (Figure 4e, inset). HRTEM images of the hybrid nanocrystals with the Æ111æ direction as the zone axis in the early stage of the epitaxy (see Figure 4b as an example) were typically irregular in shape, instead of the two types of regular projections in Figure 1d,e. Specifically for the image in Figure 4b, only one ∼60 angle was nearly fully developed. Similarly, along the Æ422æ zone axis, the hybrid nanocrystals in the early stages were irregular in shape under HRTEM (see Figure 4c). These results implied that the epitaxy growth alone did not dictate the final shape of the hybrid nanocrystals, which shall be discussed in detail in the subsequent section. Atomic Structure of Pt Seed and Pt/Fe3O4 Core/Shell Nanocrystals. Local atomic structures of the nanocrystals were investigated by X-ray absorption fine structure

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ARTICLE Figure 5. Fe K-edge k3-weighted EXAFS oscillations [k3χ(k)] (a) and corresponding Fourier transforms (b) for Pt-seeds and Pt/Fe3O4 samples. The data for Fe foil, FeO, and Fe3O4 compounds are shown as references. Pt L3-edge k3-weighted EXAFS oscillations [k3χ(k)] (c) and corresponding Fourier transforms (d) for Pt-seeds and Pt/Fe3O4 samples. The data for Pt foil and PtO2 compounds are shown as references.

reported that FeIII stearate could be reduced to FeII and Fe0 through a pyrolysis reaction at high temperatures. Thus, formation of Pt
Fe metallic coordination layer(s) between the Pt core and Fe
O surface would be reasonable. Overall, the structure of the Pt seed nanocrystals could be described as an inner Pt crystal core and a Pt
Fe
O coordination outer layer(s) with a thin Pt
Fe metallic interlayer. Presumably, the interlayer was Fe rich. Existence of the Fe
O coordination layer(s), in the form of either FeO or Fe carboxylate, on the surface of the Pt seed nanocrystals would act as the epitaxial sites for the epitaxial growth of the Fe3O4 shell. This picture was evidently consistent with the epitaxial growth model described throughout this report. Effects of Outer Surface of the Hybrid Nanocrystal toward Solution. Without considering the environmental coordination, surface atoms on a single crystal would miss at least one coordination in comparison with the interior atoms of the given crystal. This is the main cause for high surface free energy of crystals, especially for ionic, metallic, and covalent crystals. When crystals are exposed in solution, interaction between the crystals and solution environment could make certain facets more stable than the others, which induces shape-selective growth of faceted nanocrystals. For faceted Fe3O4 nanocrystals without any seeds, cubic JIANG ET AL.

and octahedral shapes were common, which were proven to be surrounded by {200} and {111} facets.46
48 As shown in Figure 3, these two families of facets are polar (or partially polar) in nature, and it would be possible to have only iron ions exposed on the outer surface. These surface iron ions could be well passivated by the common ligands used in synthesis that have been mostly electron-donating ligands to the cations, such as carboxylate and amines. In addition to saturating coordination to the surface ions by ligands in solution, charge balance is another issue that may play a role in shape-selective growth. For instance, the negative charge of carboxylate ligands must be somehow balanced by the cations on the outer surface of a nanocrystal. This factor should also promote the outer surface of Fe3O4 nanocrystals to be terminated with {200} and {111} facets. Under some special conditions, Pt/Fe3O4 hybrid nanocrystals could grow into quasi-cubes that were with Pt seed partially exposed to the solution and the Fe3O4 portion terminated with {200} facets (Figure S5). However, such incomplete core/shell hybrid nanocrystals terminated with Fe3O4 {200} facets were not sufficiently stable under reaction conditions, which eventually evolved to irregular shapes. This phenomenon suggested that Fe3O4 {200} facets were less stable than the {111} family in the current system. VOL. XXX

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ARTICLE Figure 6. (a) XPS C 1s spectrum of the core/shell nanoprisms after magnetic separation (see Experimental). (b
f) TEM images of various hybrid nanocrystals synthesized by varying the mass ratio of octadecylamine and palmitic acid, 0:0.13 (b), 0.15:0.13 (c), 0.5:0.13 (d), 0.325:0.256 (e), 0.325:0.026 (f), with other parameters unaltered from the typical method, unit (g). Scale bar: 50 nm.

On another aspect, considering the facet-selective epitaxy of Fe3O4 {111} facet with Pt {111} facet at the interface, as discussed in above section, the exclusive exposure of the polar {111} facets would be explainable. Figure 6a shows an X-ray photoelectron spectrum (XPS) of the C 1s core levels of the core/shell nanoprisms after purification with magnetic separation (see Experimental Section). In addition to the common sp3 C peak, there is a peak at 288.8 eV, which could be assigned to the carboxylate carbon.49 This observation demonstrated the existence of carboxylate at the outer surface of nanocrystals. XPS measurements further revealed that, for both Pt seeds and Pt/Fe3O4 core/ shell nanocrystals, no signal of N was detected (data not shown). These results were consistent with the conclusion that carboxylates were the surface ligands for both types of nanocrystals. Control experiments were carried out to further confirm the conclusion mentioned above and illustrate the influence of fatty acids (or their iron salts) and fatty amines in the current system. The results are given in Figure 6b
f. As mentioned above, fatty amines were not appropriate ligands for the Pt seed nanocrystals at temperatures greater than ∼290 C, and the seeds would precipitate out of the reaction solution under high temperature. However, fatty amines with a certain concentration range were found to be necessary for formation of nearly monodisperse Pt/Fe3O4 core/shell nanoprisms. Without fatty amines, epitaxial growth of Fe3O4 was found to be negligible (Figure 6b), and with too much amine, the growth of hybrid nanocrystals was out of control to yield a significant portion of pure Fe3O4 nanocrystals (Figure 6d). This was found to be consistent with our previous observation, that fatty amines could activate decomposition of metal JIANG ET AL.

fatty acid salts through aminolysis.45,50 As surface ligands, the concentration of fatty acids and their salts was found to affect the size, shape, and size/shape distribution (Figure 6e,f). Formation of the Anisotropic Shape. Discussions above indicate that the anisotropic shape of the final hybrid nanocrystals was originated from the facet-selective growth at both epitaxial interface and nanocrystalligand outer surface. Similarly, during the growth of single-component nanocrystals, both inner defects of nanocrystals and outer surface were observed to collectively dictate the shape of the resulting nanocrystals. Formations of noble metal nanostructures with anisotropic shapes,40,41 such as Au nanoprisms, are well-known examples. The “twinning plane” defects in noble metal nanocrystals induced the initial symmetry breaking at the growth front of the nanostructures. Subsequently, selective adhesion of solution additives, such as halide ions, on the outer surface of the noble metal nanocrystals provided surface blocking to initiate formation of specific facets, such as Au {111} facets.39 An interesting crystal plane was illustrated with a dash line in Figure 7, which links the ridges and the tip points of the grooves in each Pt/Fe3O4 core/shell nanoprism. Appearance of such a plane for each hybrid nanocrystal should be associated with the inner defects at the Pt
Fe3O4 interface. In fact, this plane was a (111) plane and always near one side of the Pt
Fe3O4 interface. Furthermore, though the Pt seed was not at the center of a hybrid nanocrystal, this special (111) plane was approximately a geometric mirror plane of the hybrid nanocrystal and parallel to the basal planes of the nanoprisms. Existence of this special (111) plane discriminated the sides of a Pt/Fe3O4 core/shell nanoprism into VOL. XXX

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Figure 7. (a) Two different views of the atomic model of the hybrid nanocrystals with their polar surfaces. The green (top layer) and red (covered layer) balls represent iron and oxygen atoms, respectively. (b) Illustrations with grooves and ridges characters of the model.

different types, although all of them were Fe3O4 {111} facets. Similar to the noble metal nanoprisms,41 the side surfaces of the Pt/Fe3O4 core/shell nanoprisms were truncated by the grooves and ridges. As widely accepted in literature,41 a groove provided a selfperpetuating ledge to increase the nearest neighbors

EXPERIMENTAL SECTION Chemicals. Platinum acetylacetonate (Pt(acac)2, Pt 48%), stearic acid (HSt, 98%), palmitic acid (HPa, 95%), myristic acid (HMy, 98%), n-octadecylamine (NH2Oc, 98%), eicosane (99%), and octodecane (OTA, 99%) were purchased from Alfa-Aesar. Iron stearate (Fe(St)3) was obtained from Chemservice. Iron(III) chloride hexahydrate (FeCl3 3 6H2O, AR), sodium hydroxide (NaOH, AR), hexane (AR), ethanol (AR), and methanol (AR) were purchased from Sinopharm Reagents. Synthesis of Iron Palmitate (Fe(Pa)3) and Iron Myristate (Fe(My)3). HPa (2.73 g, 10.6 mmol) was dissolved in 40 mL of ethanol at 55 C. NaOH (0.4 g, 10 mmol) was dissolved in a mixture of 50 mL of water and 10 mL of ethanol. The NaOH solution was dropped into the aforesaid HPa solution to form sodium palmitate (NaPa) solution. FeCl3 3 6H2O (0.82 g, 3 mmol) was dissolved in a mixed solvent of 25 mL of ethanol and 25 mL of water, and this solution was added into the NaPa solution dropwise to form the yellowish-brown precipitate of Fe(Pa)3. After the precipitate was filtered and washed with methanol several times, it was collected and dried in a vacuum oven for 24 h at room temperature. The same procedure was applied for the synthesis of Fe(My)3, except HPa replaced HMy. Characterization. XRD patterns were acquired using a Rigaku Ultimate-IV X-ray diffractometer operating at 40 kV/30 mA using the Cu KR line (λ = 1.5418 Å). TEM and SAED images were taken on a Hitachi 7700 transmission electron microscope at 100 kV using copper grids coated with a pure carbon support film. HRTEM, EDS-mapping, and STEM images were taken on a JEM 2100F transmission electron microscope at 200 kV using a copper grid coated with an ultrathin carbon film as the substrate. X-ray photoelectron spectroscopy (XPS) was carried out with a Thermo Scientific ESCALAB 250Xi X-ray photoelectron spectrometer using a monochromatic aluminum KR X-ray source at 20 kV. Nanocrystal samples were purified by dispersion in hexanes/acetone (4/1 volume ratio) and collected using a laboratory magnet. The purified nanocrystals in hexane were drop-cast onto silicon substrates. The binding energy of all measurements was calibrated using the C 1s peak at 284.6 eV. XAFS Measurements and Data Analysis. XAFS spectra at the Fe K (7112 eV) and Pt L3 (11564 eV) edges were measured at the

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Hybrid Pt/Fe3O4 core/shell triangular nanoprisms were synthesized through seed-mediated epitaxy. Existence of both chemical and lattice mismatch induced a specific facet-selectivity for the heteroepitaxy, which was Fe3O4 {111} facets onto Pt {111} facets. This facetselectivity in epitaxy was due to the polar nature, symmetry matching, density matching of the iron-only monolayer of Fe3O4 {111} facets. Simultaneously, the polar nature of Fe3O4 {111} facets coupled with the facet-selectivity at the interface resulted in the outer surface of Fe3O4 being {111} facets. Overall, facet-selectivity at both the Pt
Fe3O4 interface and the nanocrystal-ligands outer surface likely played the key roles to dictate the anisotropic shape. Given the great potential of hybrid nanostructures, results presented in this work should be of interest for the fields of chemistry and materials.

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for an adatom to coordinate, and thus growth on the grooves should be faster than other types of surface structure, leading to the anisotropic shape of a triangular nanoprism.

beamline BL14W1 station of the Shanghai Synchrotron Radiation Facility, China. The storage ring was working at the energy of 3.5 GeV with an average electron current of 300 mA. The hard X-ray was monochromatized with Si(111) double-crystals. The acquired EXAFS data were processed according to the standard procedures using the ATHENA module implemented in the IFEFFIT software packages. The k3-weighted EXAFS spectra were obtained by subtracting the postedge background from the overall absorption and then normalizing with respect to the edge-jump step. Subsequently, k3-weighted χ(k) data in the k-space ranging from 2.6
13.4 Å
1 were Fourier transformed to real (R) space using hanning windows (dk = 1.0 Å
1) to separate the EXAFS contributions from different coordination shells. EXAFS Fitting Details. To obtain quantitative structural parameters around Fe and Pt atoms in the Pt-seed and Pt/Fe3O4 samples, least-squares curve parameter fitting was performed using the ARTEMIS module of IFEFFIT and USTCXAFS software packages.1 Effective scattering amplitudes and phase shifts of all the paths for fitting the EXAFS data were calculated by the ab initio code FEFF8.0. The amplitude reduction factor S02 was also treated as an adjustable variable, and the obtained value of 0.76 and 0.92 for Fe foil and Pt foil, respectively, was fixed in fitting the subsequent Fe and Pt edge data. The obtained results were all in accordance with the theoretical values, evidencing the high accuracy of EXAFS in determining structural parameters. The fits were done on the k3-weighted EXAFS function χ(k) data from 2.6 to 13.4 Å
1. The coordination numbers N, interatomic distances R, Debye
Waller factor σ2, and the edgeenergy shift ΔE0 were allowed to run freely. Following the above fitting strategy, we obtained satisfactory curve-fitting results as shown in Figures S6 and S7, and the resulting parameters were listed in Tables S1 and S2. Typical Synthesis. Typical synthesis was a two-step procedure. In Step I, a mixture of 4 mg of Pt(acac)2, 1.3 mg of Fe(St)3, 0.325 g of NH2Oc, 0.13 g HPa, and 2 g eicosane was heated to 120 C for 10 min with Ar flow. Subsequently, the temperature was increased to 325 C and remained at this temperature for 15 min to obtain a solution of the Pt seed nanocrystals. In Step II, the reaction mixture was allowed to cool down to below the flashing point of the solvent (∼120
150 C) by removing the

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Supporting Information Available: The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.5b04130. Rotary TEM images; synthetic procedures with different shapes of the iron oxide part; corresponding TEM images; XAFS analysis (PDF) Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grant 21233005) and Fundamental Research Fund for the Central Universities (Grant 2014FZA3006).

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heating mantle, and then, Fe(Pa)3 (0.12 g) was added into the Pt seed solution. The reaction temperature was again increased to 330 C. After 20 min of reaction at this temperature, the reaction was stopped by allowing cooling of the reaction mixture down to 50
100 C. The mixture was separated with a small laboratory magnet, and the collected products were dispersed into hexane. In the case of necessity, this magnetic separation procedure was repeated several times and the addition of a certain amount of acetone might help the magnetic purification. Synthesis of Fe3O4 Nanocrystals Shown in Figure 4e. Under Ar flow, a mixture of 0.12 g of Fe(Pa)3, 0.25 g of NH2Oc, 0.13 g of HPa, and 2 g of eicosane was heated up to 120 C and remained at this temperature for 10 min. The temperature was then increased to 330 C and remained for 20 min. The reaction was stopped by allowing cooling down to 50
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