Scalable Assembly of Crystalline Binary Nanocrystal Superparticles

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Scalable Assembly of Crystalline Binary Nanocrystal Superparticles and Their Enhanced Magnetic and Electrochemical Properties Yuchi Yang, Biwei Wang, Xiudi Shen, Luyin Yao, Lei Wang, Xiao Chen, Songhai Xie, Tongtao Li, Jianhua Hu, Dong Yang, and Angang Dong J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b09779 • Publication Date (Web): 16 Oct 2018 Downloaded from http://pubs.acs.org on October 17, 2018

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Journal of the American Chemical Society

Yuchi Yang,†, ‡ Biwei Wang,‡ Xiudi Shen,† Luyin Yao,† Lei Wang,† Xiao Chen,† Songhai Xie,‡ Tongtao Li,‡ Jianhua Hu,† Dong Yang,† and Angang Dong*,‡ †

State Key Laboratory of Molecular Engineering of Polymers and Department of Macromolecular Science, Fudan University, Shanghai 200433, China. ‡

iChem, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, and Department of Chemistry, Fudan University, Shanghai 200433, China. ABSTRACT: Self-assembled binary nanocrystal superlattices (BNSLs) represent an important class of solid-state materials with potentially designed properties. In pursuit of widening the range of applications for binary superlattice materials, it is desirable to develop scalable assembly methods that enable high-quality BNSLs with tailored compositions, structures, and morphologies. Here we report the gram-scale assembly of crystalline binary nanocrystal superparticles with high phase purity through an emulsion-based process. The structure of the resulting BNSL colloids can be tuned in a wide range (AB13, AlB2, MgZn2, NaCl, and CaCu5) by varying the size and/or number ratios of the two nanocrystal components. Access to large-scale, phase-pure BNSL colloids offers vast opportunities for investigating their physiochemical properties, as exemplified by AB13-type CoFe2O4-Fe3O4 binary superparticles. Our results show that CoFe2O4-Fe3O4 binary superparticles not only display enhanced magnetic coupling but also exhibit superior lithium-storage properties. The non-closed-packed NC packing arrangements of AB13-type binary superparticles are found to play a key role in facilitating lithiation/delithiation kinetics and maintaining structural integrity during repeated cycling. Our work establishes the scalable assembly of high-quality BNSL colloids, which is beneficial for accelerating the exploration of multicomponent nanocrystal superlattices toward various applications.

INTRODUCTION Co-assembly of multicomponent nanocrystals (NCs) into ordered superstructures opens an unprecedented route for creating designer materials with diverse structures and properties.1-13 Among them, binary NC superlattices (BNSLs) are of particular interest,6-9 as they integrate prescribed nanoscale components with different functionalities in a single material in a precisely controlled manner.5 Self-assembly of BNSLs with tailored structures and geometries,6,14-17 which will introduce many new opportunities for enhancing the functionality, represent an important step toward the realization of BNSL-based devices. Usually, solvent-evaporation-induced assembly of colloidal NCs is considered the method of choice in preparing BNSLs.1,5 Early attempts rely on the controlled drying of a mixed NC solution on a solid substrate such as silicon wafers and carbon-coated copper grids.6,18,19 Although binary superlattices are obtainable by this method, the resulting BNSLs typically exist as randomly scattered islands,19 which can hardly be manipulated for further applications. The recently developed liquid-air interfacial assembly technique allows the growth of BNSL as two-dimensional (2D) membranes.7 Of particular significance is that the asassembled membranes can be readily transferred onto arbitrary substrates, enabling extensive exploration of BNSLs as thin-film devices in the past few years.20-23 Despite the tremendous progress, we note that the 2D

geometry is unsuitable for many applications beyond thin-film devices. In addition, integration of BNSLs with existing technologies requires the production of phase-pure superlattice materials in large quantities, which is difficult, if not impossible, to be achieved using the current assembly techniques. Emulsion-based assembly, in which the selfassembly of NCs is confined within oil droplets stabilized by surfactants, has been widely adopted to construct crystalline superparticles.24-27 Besides being potentially scalable, another attractive aspect of this methodology is that the as-assembled NC superparticles, existing as freestanding, micrometersized colloids, can be readily isolated for further processing.26,28,29 This, combined with the well-defined spherical morphology, renders NC superparticles particularly suitable for magnetic,24 bioimaging,30 light-emitting,31,32 and energy storage applications.26,33 Fundamentally, recent studies on single-component superparticles have shown that the geometrical confinement of emulsion droplets can alter the NC assembly behaviors upon reducing the size, resulting in lower-symmetry geometries such as icosahedrons which have been not observed in extended superlattices.34,35 This offers new possibilities for designing NC superstructures with unconventional structures and morphologies. In addition to single-component superparticles, many efforts have been devoted to the emulsion-based

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assembly of binary NC systems,36-44 with a primary goal of achieving colloidal spheres of BNSLs. However, extending this concept to multicomponent NCs is not trivial, and most binary superparticles reported previously are composed of randomly packed NCs.36-41 The assembly bottlenecks might be caused by the stringent NC size and concentration ratios, complex surface chemistry, or geometrical confinement. To the best of our knowledge, there are only a few reports on the growth of crystalline binary superparticles.43,44 The first such report was by Kraus and co-workers,43 in which AB13-type BNSL colloids composed of two different-sized Au NCs were obtained by carefully regulating the assembly conditions. More recently, Ouyang and co-workers reported the growth of AuFe3O4 binary superparticles with tunable lattice structures.44 While the feasibility has been demonstrated experimentally, it remains for future work to investigate the experimental conditions that govern the successful growth of BNSL colloids and to further explore their potential applications.

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Chemical Reagent Co., Ltd. All chemicals were used as received without further purification. Synthesis of Fe3O4 NCs. Monodisperse Fe3O4 NCs were synthesized by the thermal decomposition of iron oleate,45,46 which was pre-synthesized by the reaction of iron chloride with sodium oleate. In a typical synthesis of 17.0 nm Fe3O4 NCs, 9.0 g of ion oleate, and 2.0 g of OA were dissolved in 50 g of ODE in a 250 mL three-neck flask. This mixture was degassed at 120 oC for 30 min before heating to 320 oC under N2. The reaction was allowed to proceed for 60 min before cooling to room temperature. The as-synthesized Fe3O4 NCs were precipitated by the addition of ethanol and isopropanol followed by centrifugation. The precipitated NCs were redispersed in hexane to form a stable colloidal solution with a concentration of ~75 mg mL-1. Fe3O4 NCs with other sizes were synthesized through a similar procedure. The corresponding reaction conditions were summarized in Supporting Information (Table S1). Synthesis of CoFe2O4 NCs. Monodisperse CoFe2O4 NCs with a size of 4.5 and 8.0 nm, respectively, were synthesized according to the literature methods with slight modifications.47-49 In a typical procedure to synthesize 4.5 nm CoFe2O4 NCs, 5.6 g of Fe(acac)3, 2.0 g of Co(acac)2, 4.5 g of OA, and 21.0 g of OAm were dissolved in 50 mL of benzyl ether in a 250 mL threeneck flask. The resulting mixture was degassed at 120 o C for 30 min before heating to 200 oC for 90 min and further heating to 295 oC for 60 min under N2. For the synthesis of 8.0 nm CoFe2O4 NCs, 5.6 g of Fe(acac)3, 2.0 g of Co(acac)2, 4.5 g of OA, and 21.0 g of OAm were dissolved in 80 mL of benzyl ether. The solution was degassed at 120 oC for 30 min. After that, 672 mg of 4.5 nm CoFe2O4 NC seeds dispersed in 10 mL of hexane was added. After reaction at 120 °C for 30 min, the solution was heated at 200 °C for 60 min and was further heated to 295 °C for 40 min. The isolation and purification of CoFe2O4 NCs were conducted similarly to Fe3O4 NCs. The as-synthesized CoFe2O4 NCs were redispersed in hexane with a concentration of ~75 mg mL-1.

Herein, we report the gram-scale assembly of crystalline binary NC superparticles through an emulsion-based assembly approach. The resulting BNSL colloids feature a high degree of phase purity, with the superlattice structure tunable by adjusting the size and/or number ratios of the two NC components. We show that unlike the case of singlecomponent superparticles, downsizing the size of emulsion droplets does not change the NC packing arrangements of binary NC systems. Our AB13-type binary superparticles composed of CoFe2O4 and Fe3O4 NCs exhibit enhanced magnetic properties relative to their disordered counterparts, resulting from strong dipolar coupling between Fe3O4 and CoFe2O4 NCs. We further explore the applications of binary superparticles by demonstrating their use as anode for lithium-ion batteries (LIBs). It is found that AB13-type CoFe2O4-Fe3O4 binary superparticles exhibit superior electrochemical performance relative to their singlecomponent counterparts and stoichiometric mixtures thereof. Electron microscopies reveal that the enhanced electrochemical performance of AB13-type binary superparticles is due in large part to their structural robustness during cycling, as a result of the non-closed-packed superlattice structure.

Synthesis of Pd NCs. Monodisperse Pd NCs were synthesized according to the literature method with slight modifications.50 In a typical synthesis of 6.0 nm Pd NCs, 122 mg of Pd(acac)3, 1.6 mL of TOP, and 2.6 mL of OAm were mixed in 15 mL of benzyl ether. The reaction mixture was degassed at 90 °C for 30 min and was then heated to 290 °C for 20 min under N2. After precipitation with the addition of isopropanol, Pd NCs were redispersed in hexane with a concentration of ~20 mg mL-1.

EXPERIMENTAL SECTION Materials. Oleic acid (OA, 90%), oleylamine (OAm, 70%), trioctylphosphine (TOP, 97%), 1-octadecene (ODE, 90%), benzyl ether (98%), dodecyltrimethylammonium bromide (DTAB, 98%), Triton X-100, sodium dodecyl sulfate (SDS), Pluronic F127, and TBAB (tertbutylamine-borane complex) were purchased from Sigma-Aldrich. Sodium oleate, 1hexadecene (HDE, 90%), and 1-tetradecene (TDE, 90%) were purchased from TCI. Iron chloride tetrahydrate (FeCl3·4H2O) was purchased from Aladdin. Iron acetylacetonate (Fe(acac)3), palladium acetylacetonate (Pd(acac)3), and cobalt acetylacetonate (Co(acac)2) were purchased from Alfa Aesar. HAuCl4·4H2O (Au content: ≥47.8%) was purchased from Sinopharm

Synthesis of Au NCs. Monodisperse Au NCs were synthesized according to the literature method with slight modifications.51 In a typical synthesis of 4.5 nm Au NCs, 0.25 mmol of HAuCl4·4H2O and 10 mL of OAm were dissolved in 10 mL of hexane under stirring. To the above solution, 0.25 mmol of TBAB dissolved in a mixture of 1 mL of hexane and 1 mL of OAm was injected, resulting in a purple solution after ~10 s. The reaction was allowed to proceed at 20 °C for 60 min. After precipitation with the addition of ethanol, the as-

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synthesized Au NCs were redispersed in hexane to form a stable colloidal solution with a concentration of ~20 mg mL-1. Monodisperse 7 nm Au NCs were synthesized by a seeded growth process. Typically, 0.25 mmol of HAuCl4·4H2O and 2.0 mL of OAm were mixed in 20 mL of ODE at 60 °C under N2. After that, 4 mg of 4.5 nm Au NC seeds was injected and the reaction was allowed to proceed at 60 °C for 3 h. After precipitation with the addition of ethanol, the assynthesized Au NCs were redispersed in hexane with a concentration of ~20 mg mL-1.

emission spectroscopy (ICP-OES, iCAP 7400, Thermo Fisher Scientific). Electrochemical Measurements. Prior to electrochemical measurements, the as-assembled binary or single-component superparticles were heated at 400 oC in Ar for 2 h, converting the surface-coating ligands into thin and conformal carbon-coating layers.26,52 The working electrode was prepared by mixing carbon-coated superparticles (active materials), carbon black (Super P), and polyvinylidene fluoride (PVDF) binder with a mass ratio of 7:2:1 in N-methyl2-pyrrolidone (NMP). The slurry was cast onto the Cu foil followed by drying at 90 oC under vacuum for 12 h. 2016R type coin cells were assembled in a dry argonfilled glovebox, with lithium metal foil as the counter electrode and polypropylene membrane (Celgard-2300) as the separator. The electrolyte contained 1 M LiPF6 in a mixed solvent of ethylene carbonate, ethyl methyl carbonate, and dimethylcarbonate (1:1:1 by volume). The cells were tested in galvanostatic mode on a Neware cell test instrument in the voltage range of 0.01-3.00 V (vs. Li+/Li). Cyclic voltammetry (CV) measurements were recorded at a scan rate of 0.1 mV s-1 on an Autolab N 204 electrochemical workstation. To study the structural evolution of active materials after cycling, the battery cell was disassembled in the argon-filled glovebox. The cycled electrodes were soaked in dimethylcarbonate for 6 h and then in acetonitrile for 12 h to remove residual electrolytes. Then, the electrodes were taken out of the glovebox, washed with ethanol, and were finally soaked in deionized water for 24 h.

Self-Assembly of BNSL Colloids. Colloidal spheres of BNSLs were obtained by an emulsion-based assembly process. In general, 20 mL of hexane solution containing two types of NCs with desired particle number ratios and 0.2 mL of OA were mixed with 200 mL of deionized water containing 4.0 g of DTAB (or Triton X-100, SDS, F127). The biphase mixture obtained was vigorously stirred using a homogenizer (8000 rpm) to produce a hexane-in-water emulsion. The as-formed emulsion was then heated at 40 oC under N2 flow for 2 h to evaporate hexane. The resulting BNSL colloids suspended in water were collected by centrifugation. The precipitated binary superparticles were washed twice with water to remove residual surfactants. The superlattice structure of BNSL colloids could be tuned in a wide range by varying the size and/or number ratios of the two NC components, as summarized in Table S2. Self-Assembly of Single-Component NC Superparticles. Single-component superparticles featuring face-centered cubic (fcc) packing symmetry were obtained by a similar emulsion-based assembly method. Typically, 20 mL of hexane solution containing 8.0 nm CoFe2O4 NCs (or 17.0 nm Fe3O4 NCs) and 0.2 mL of OA was mixed with 200 mL of aqueous solution containing 4.0 g of DTAB. The resulting biphase mixture was vigorously stirred using a homogenizer. The subsequent evaporation of hexane at 40 oC under N2 flow induced the self-assembly of fcc superparticles, which were collected by centrifugation followed by washing before further use.

RESULTS AND DISCUSSION Self-Assembly and Structural Characterization of BNSL Colloids. Our method to construct BNSL colloids is based on an emulsification process followed by controlled solvent evaporation, as illustrated in Figure 1a. In a typical procedure for forming CoFe2O4-Fe3O4 BNSL colloids, a hexane solution containing 8.0 nm CoFe2O4 and 17.0 nm Fe3O4 NCs (Figure S1) is combined with an aqueous solution of surfactant stabilizers such as DTAB. The resulting mixture is stirred to afford a hexane-in-water emulsion. To obtain BNSL colloids, the resulting emulsion is simply heated at 40 oC under N2 flow for 2 h, during which the gradual evaporation of hexane from emulsion droplets induces the confined co-crystallization of two NC components into ordered arrays.

Characterization. Transmission electron microscopy (TEM) images, high-angle annular darkfield scanning TEM (HAADF-STEM) images, and energy dispersive X-ray spectroscopy (EDS) were conducted on a Tecnai G2 F20 S-Twin microscope operating at 200 kV. Scanning electron microscopy (SEM), highresolution SEM (HRSEM), and EDS were carried out on a Zeiss Ultra-55 microscope operating at 5 and 15 kV, respectively. Small-angle X-ray scattering (SAXS) was performed on the beamline (BL16B1) at the Shanghai Synchrotron Radiation facility with the incident X-ray photon energy at 10 keV. Dynamic light scattering (DLS) was performed on a Malven Zetasizer Nano ZS instrument. Cross-sectioned samples of binary NC superparticles were prepared on a Leica triple-ionbeam cutter (EM TIC 3X). Magnetic property measurements were performed on a superconducting quantum interference device (SQUID, Quantum Design MPMS). The Fe and Co contents in the electrolyte were examined using inductively coupled plasma-optical

Figure 1b shows a photograph of an aqueous suspension containing the as-formed colloids selfassembled from CoFe2O4 and Fe3O4 NCs with a particle number ratio of ~13:1. DLS measurements indicate that the as-assembled colloids are relatively uniform with an average hydrodynamic diameter of ~420 nm, which is smaller than that (~840 nm) of emulsion droplets prior to hexane evaporation (Figure 1d). This result suggests that the self-assembly of NCs is accompanied by the shrinkage of oil droplets, which is expected considering the progressive evaporation of hexane. The resulting superparticles show enhanced magnetic response resulting from NC assembly, such that they

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STEM and the corresponding elemental mapping corroborate the ordered assembly of two NC components (Figure 2e). It should be noted that although structural defects such as grain boundaries were occasionally observed (Figure S3), most binary superparticles are single crystalline. Moreover, unlike previous extended BNSLs where superlattice domains of different structures (e.g., AB13 and AlB2) commonly co-exist,6,7,19 TEM survey over 200 superparticles shows that all the examined superparticles exhibit an AB13-type structure without the presence of other binary phases, demonstrating the high phase purity of BNSL colloids.

Figure 1. (a) Schematic illustration of the preparation of BNSL colloids. (b) Photograph of an aqueous suspension containing the as-assembled CoFe2O4-Fe3O4 BNSL colloids. (c) Isolation of BNSL colloids using a magnet. (d) DLS measurements of emulsion droplets and BNSL colloids. (e) Photograph of binary superparticle powders from one-pot assembly. (f) Low-magnification SEM image and (g) size distribution histogram of CoFe2O4-Fe3O4 binary superparticles. (h) HRSEM image of a sectioned binary superparticle, showing the interior NC packing arrangements.

can be readily collected using a magnet (Figure 1c). The subsequent removal of redundant surfactants by washing followed by drying leads to black powders of binary superparticles. Notably, gram-scale quantities of binary superparticles are routinely achievable by one-pot assembly (Figure 1e). SEM reveals that most binary superparticles exist as discrete spheres or quasi-spheres (Figure 1f). Statistical analysis gave rise to an average diameter of 380 ± 50 nm (Figure 1g), which is slightly smaller than the value determined from DLS measurements. To reveal the interior NC packing arrangements, the binary superparticles were sectioned using a triple-ion-beam cutter and were then imaged by SEM. Cross-sectional HRSEM reveals that the two NC components are organized in a square pattern spanning the entire superparticle (Figure 1h), suggesting the successful growth of crystalline binary superparticles.

Figure 2. (a) Low- and (b) high-magnification TEM images of the as-assembled AB13-type CoFe2O4-Fe3O4 binary superparticles. (c) FFT of the TEM image shown in (b). (d) Crystallographic model of AB13-type BNSLs onto the (001) lattice plane. (e) HAADF-STEM image and the corresponding elemental mapping of a single binary superparticle. (f) SAXS pattern of AB13-type CoFe2O4-Fe3O4 binary superparticles. Inset shows the 2D diffusion rings.

SAXS, which is a powerful tool to evaluate the NC packing arrangements over large areas,53,54 was conducted to further verify the phase purity of our binary superparticles. Figure 2f shows a typical synchrotron-based SAXS pattern, in which at least 20 well-resolved peaks characteristic of NaZn13 are discernible, thus corroborating their long-range NC ordering and high phase purity. The lattice constant of the as-assembled CoFe2O4-Fe3O4 binary superparticles was determined to be ~48 nm. The emergence of diffuse rings in the 2D SAXS pattern (Figure 2f, inset) is due to the random orientation of individual superparticles.53 Simultaneous formation of different types of structures in previous BNSLs is believed to be caused by the variations of local NC concentrations during solvent evaporation.6,7,18 The formation of phase-pure BNSLs in this work is presumably attributed to the confined assembly process, in which each emulsion droplet acts as a microreactor with identical NC concentrations, thus ensuring the evolution of a single binary phase.

TEM was employed to examine the precise structure, revealing that individual superparticles predominantly adopt a lattice isostructural with NaZn13 (Figure 2a). Figure 2b presents a high-magnification TEM image a single superparticle onto the (001) lattice plane, the high crystallinity of which was also verified by fast Fourier transform (FFT, Figure 2c). Additional TEM images viewed from other directions can be found in the Supporting Information (Figure S2). In this cubic superlattice structure, each Fe3O4 NC is surrounded by eight icosahedra, while each icosahedron consists of 13 small CoFe2O4 NCs, as illustrated in Figure 2d. HAADF-

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Journal of the American Chemical Society the cluster size shrinks below ~200 nm (Figure S6), as a result of geometrical confinement.34 Our results thus strongly suggest the high tolerance of binary NC assembly to geometrical confinement, which appears to be associated with interparticle interactions in addition to entropic contribution.19,55 Future theoretical work may provide insights into the underlying mechanism.

Figure 4. (a, b) TEM images of disordered superparticles self-assembled from 8.5 nm and 17.0 nm Fe3O4 NCs with a number ratio of 7:1 and 2:1, respectively. (c, d) TEM images of core-shell superparticles featuring AB13-type cores (as indicated by the dashed circles) formed with a number ratio of 26:1 and 78:1, respectively. (e) SAXS patterns of various binary superparticles. The SAXS pattern of AB13-type binary superparticles was also

Figure 3. (a) TEM and (b) the corresponding HAADF-STEM images of a single AB13-type Fe3O4-Fe3O4 binary superparticle onto the (001) lattice plane. Inset in (a) shows the FFT. (c) SAXS pattern of AB 13-type Fe3O4Fe3O4 binary superparticles. (d-h) TEM images of a series of AB13-type binary superparticles with sizes gradually decreasing from 600 to 50 nm. (i) Schematic of a unit cell of AB13-type BNSLs.

provided for comparison. In addition to the CoFe2O4-Fe3O4 combination, AB13type BNSL colloids can also be obtained by coassembling 8.5 nm and 17.0 nm Fe3O4 NCs with a number ratio of ~13:1 (Figure S4). The TEM and the corresponding HAADF-STEM images of a single Fe3O4Fe3O4 binary superparticle are shown in Figure 3a and 3b, respectively, revealing its single crystallinity onto the (001) lattice plane. The high phase purity of Fe3O4Fe3O4 binary superparticles was further confirmed by SAXS (Figure 3c), as evidenced by the prominent diffraction peaks exclusively ascribed to NaZn13. The diameter of Fe3O4-Fe3O4 BNSL colloids ranges from several tens to a few hundreds of nanometers (Figure 3d-h), with an average diameter of 400 nm (Figure S5). Notably, access to binary superparticles with varying sizes from one-pot assembly allows us to evaluate the influence of geometrical confinement on the selfassembly of binary NC systems. Similar to CoFe2O4Fe3O4 BNSL colloids, TEM reveals that all the Fe3O4Fe3O4 binary superparticles, despite different diameters, exhibit an AB13-type superlattice structure (Figure 3d-h), consistent with their high phase purity. In particular, Figure 3h shows the TEM image of an ultrasmall binary superparticle having a diameter of ~50 nm, roughly corresponding to a unit cell composed of 8 large and 104 small Fe3O4 NCs (Figure 3i). To our knowledge, this is arguably the smallest BNSLs reported to date. The formation of stabilized singleunit-cell BNSLs indicates that decreasing the size of emulsion droplets does not alter the NC packing arrangements. By contrast, single-component superparticles composed of 17.0 nm Fe3O4 NCs were found to experience fcc-to-icosahedron transition as

It is also noteworthy that in addition to the size ratio, the particle number ratio is another critical parameter dictating the successful growth of BNSL colloids. In our experiments, phase-pure AB13-type BNSL colloids are obtainable only when the number ratio of small to large NCs is close to 13:1, whereas disordered superparticles would be formed when the particle number ratio is lower than 13:1 (i.e., a deficient amount of small NCs). Figure 4a and 4b show TEM images of disordered superparticles selfassembled from 8.5 nm and 17.0 nm Fe3O4 NCs with a number ratio of 7:1 (A-B7) and 2:1 (A-B2), respectively. The randomly packed structure of disordered superparticles was further confirmed by SAXS. As shown in Figure 4e, both A-B7 and A-B2 superparticles exhibited two peaks in the SAXS pattern, which were ascribed to 8.5 nm and 17.0 nm Fe3O4 NCs, respectively, indicative of the occurrence of phase segregation. On the other hand, core-shell superparticles featuring AB13-type BNSL cores and small NC shells were formed when the particle number ratio is higher than 13:1. Figure 4c shows the TEM image of such core-shell Fe3O4-Fe3O4 superparticles formed with a particle number ratio of 26:1 (A-B26). Notably, AB13-type BNSL cores are still observable even with a particle number ratio of 78:1 (A-B78, Figure 4d), suggesting that the nucleation of BNSLs is more tolerant to excessive small NCs. SAXS provides further evidence of the formation of core-shell superparticles (Figure 4e). As the characteristic peak of 8.5 nm Fe3O4 NCs nearly overlaps with the (600) peak of AB13-type BNSL cores, increasing the amount of 8.5 nm NCs enhances the

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intensity of the (600) peak while diminishing the first three prominent peaks ascribed to BNSL cores. The formation of core-shell superparticles also suggests that the nucleation of binary superlattices initiates in the emulsion core and proceeds outwards; however, further work such as in situ SAXS and simulation is needed to better reveal the growth pathway of BNSL colloids.

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conjunction with SAXS establishes the high phase purity for each type of superparticles. BNSL colloids composed of other NC combinations, such as Pd-Fe3O4 and Au-Fe3O4, can also be obtained by optimizing assembly conditions (Figure S9). Moreover, BNSL colloids can also be obtained by replacing DTAB with other types of ionic or non-ionic surfactants including Triton X-100, SDS, and F127 (Figure S10). Taken together, our results demonstrate the effectiveness of emulsion-based assembly in growing high-quality BNSL colloids with tunable compositions and lattices, provided that the size and number ratios of the two NC components are in the optimal range.

Of particular significance is that the structure and stoichiometry of BNSL colloids can be tuned in a wide range by varying NC combinations. For example, emulsion-based assembly of 4.5 nm CoFe2O4 NCs and 11.0 nm Fe3O4 NCs with a particle number ratio of ~2:1 leads to AlB2-type BNSL colloids (Figure 5). Figure 5b shows the TEM image of a single AlB2-type superparticle onto the (001) lattice plane, the single crystallinity of which was also confirmed by HAADFSTEM (Figure 5c) and FFT (Figure 5d). TEM images shown in Figure 5f and 5g are AlB2-type BNSL colloids viewed from (111) and (100) lattice planes, respectively. The high phase purity of AlB2-type BNSL colloids was verified by SAXS (Figure 5h), as evidenced by the multiple well-resolved scattering peaks exclusively ascribed to an AlB2 lattice. The lattice constants were determined to be a = b = 14.14 nm and c = 14.02 nm. Similar to the case of AB13-type BNSL colloids, decreasing the size of AlB2-type binary superparticles even to 50 nm does not change the superlattice structure (Figure S7), further demonstrating the high tolerance of binary NC assembly to geometrical confinement. Likewise, increasing the number ratio of small to large NCs far beyond 2:1 does not inhibit the nucleation of BNSLs, resulting in core-shell superparticles with AlB2-type cores as revealed by TEM and SAXS (Figure S8).

Figure 6. TEM images and the corresponding SAXS patterns of various binary superparticles: (a, d) MgZn2-type superparticles self-assembled from 6.5 nm and 9.0 nm Fe3O4 NCs; (b, e) CaCu5-type superparticles composed of 4.5 nm CoFe2O4 NCs and 7.0 nm Fe3O4 NCs; (c, f) NaCltype superparticles composed of 4.5 nm CoFe2O4 NCs and 15.0 nm Fe3O4 NCs. Insets in (d-f) show the respective crystallographic models.

Magnetic Properties of Binary NC Superparticles. Growing BNSLs as freestanding colloidal spheres facilitates their isolation and purification for further processing and property investigation. We choose bimagnetic CoFe2O4-Fe3O4 binary superparticles as an example to illustrate magnetic property studies, mainly because of their potential to achieve exchange coupling between magnetically hard CoFe2O4 and magnetically soft Fe3O4 NCs.56-58 Figure 7a and b show the magnetic hysteresis loops, measured at 100 K, of single-component fcc superparticles self-assembled from 8.0 nm CoFe2O4 NCs and 17.0 nm Fe3O4 NCs, respectively. Structural characterization of fcc superparticles was given in Figure S11. As expected, superparticles of CoFe2O4 NCs are ferromagnetic with a high coercivity of ~7.0 KOe (Figure 7a), consistent with their high magnetocrystalline anisotropy feature.56 In contrast, Fe3O4 NC superparticles display magnetically soft characteristics with a small coercivity of ~0.5 KOe (Figure 7b).

Figure 5. (a) Low-magnification TEM image of AlB2-type CoFe2O4Fe3O4 binary superparticles. (b) TEM and (c) the corresponding HAADFSTEM images of a single AlB2-type binary superparticle onto the (001) lattice plane. (d) FFT and (e) crystallographic model of AlB2-type BNSLs onto the (001) lattice plane. (f, g) TEM images of AlB2-type binary superparticles onto the (111) and (100) lattice planes, respectively. Insets show the corresponding crystallographic models. (h) SAXS pattern of AlB2-type binary superparticles.

Upon co-assembly of CoFe2O4 and Fe3O4 NCs, the resulting AB13-type binary superparticles show a hysteresis loop characteristic of nearly single-phase magnetic switching, with a large coercivity of ~4.0 KOe (Figure 7c). Despite the slight kinks in the hysteresis loop (indicated by arrows in Figure 7c), the single-phase-like magnetic switching and the large

In addition to AB13 and AlB2, BNSL colloids isostructural with MgZn2, NaCl, and CaCu5, respectively, have also been realized through a similar emulsification procedure (Figure 6). TEM in

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Journal of the American Chemical Society indeed caused by the enhanced interparticle magnetic coupling interactions. It is worth mentioning that further reducing the interparticle spacing by removing the organic ligands at the NC surface could lead to fully exchange-coupled binary superparticles.56 Electrochemical Lithiation of Binary NC Superparticles. Apart from interesting magnetic properties, our BNSL colloids are also of scientific and technological interests for a variety of energy-related applications.37,61,62 A notable example is LIBs, in which secondary superparticles are known to be the ideal architecture for high-performance electrode materials.26,33 Additionally, from a fundamental perspective, the employment of structurally ordered NC superlattices enables better visualization of structural evolution of electrode materials,26 which may provide valuable insights into the structureperformance relationship. As such, self-assembled NC superparticles have been widely explored as electrode materials in the past few years.26,33 In contrast, electrochemical lithiation of BNSLs has been rarely reported, mainly because of the forbidden challenge in scalable fabrication of binary superlattices with desired structures and morphologies. Access to gramscale, phase-pure binary superparticles in this work allows us to investigate the electrochemical properties of BNSLs for the first time.

Figure 7. Magnetization hysteresis loops of various superparticles measured at 100 K: (a) fcc superparticles of 8.0 nm CoFe2O4 NCs; (b) fcc superparticles of 17.0 nm Fe3O4 NCs; (c) AB13-type CoFe2O4-Fe3O4 binary superparticles; (d) disordered CoFe2O4-Fe3O4 superparticles.

coercivity are indicative of strong magnetic coupling between two NC components.56 Considering the nonphysical contact of NC constituents due to the presence of long-chain OA ligands, we suspect that the origin of the interaction effect observed in Figure 7c should be primarily contributed by interparticle dipolar coupling rather than exchange coupling.59 The slight kinks in the hysteresis loop in Figure 7c also support the negligible contribution from exchange coupling. Nonetheless, this result is somewhat surprising, as Murray and co-workers have demonstrated that the asassembled AB13-type CoFe2O4-Fe3O4 BNSLs display obvious two-phase magnetic switching with a rather small coercivity.56 Compared with the previous Fe3O4 and CoFe2O4 NCs used for constructing AB13-type BNSLs,56 we note that the two NC components employed in this work have much larger diameters (17.0 nm vs. 12.8 nm for Fe3O4 NCs; 8.0 nm vs. 6.8 nm for CoFe2O4 NCs). We surmise that the use of larger NCs for self-assembly should lead to a superlattice with shorter interparticle spacing due to stronger van der Waals interactions between inorganic cores,60 which could enhance dipolar coupling between neighboring NCs in the as-assembled superlattices.

Again, AB13-type CoFe2O4-Fe3O4 binary superparticles were chosen as a model system to illustrate the electrochemical lithiation of BNSLs, as both CoFe2O4 and Fe3O4 are promising transition metal oxide (TMO) anode materials with high and similar theoretical specific capacities of ~1000 mAh g-1.63-65 For comparison, single-component fcc superparticles composed of CoFe2O4 and Fe3O4 NCs as well as their stoichiometric physical mixture were also tested under identical conditions. The electrochemical performance of various superparticles as LIB anodes was investigated using the standard half-cell configuration. Prior to electrode preparation, the native OA ligands tethered to NCs were thermally converted into conformal carbon-coating shells in order to improve the electrical conductivity of NC superparticles.26,29,52 For all superparticle samples, TEM and SAXS establish that both the spherical morphology and local NC ordering were well retained during ligand carbonization despite the reduced interparticle spacing (Figure S12).

To verify this, we intentionally prepared disordered superparticles composed of the same CoFe2O4 and Fe3O4 NCs with a particle number ratio of ~13:1 by rapidly evaporating hexane. As shown in Figure 7d, this control sample displayed a distinct necking hysteresis loop with a much smaller coercivity at ~2.8 KOe, a characteristic of apparent two-phase switching.56,57 Compared with the case of BNSLs which enable maximized interactions between distinct NCs, the interparticle dipolar coupling in disordered superparticles should be effectively weaker, which accounts for the necking characteristic and small coercivity. This result also confirms that the singlephase-like switching exhibited by BNSL colloids is

Figure 8a shows the CV curves of AB13-type binary superparticles for the initial four cycles, in which the redox features are in general consistent with those of TMO-based anodes reported previously.26,33,66,67 Specifically, the pronounced peak at ~0.52 V during the first cathodic sweep is ascribed to the reduction of CoFe2O4 and Fe3O4 NCs by Li (CoFe2O4 + 8Li+ + 8e- → Co + 2Fe + 4Li2O; Fe3O4 + 8Li+ +8e- → 3Fe + 4Li2O)33,67 as well as the formation of solid electrolyte interface (SEI),33,67 while the anodic peaks at ~1.70 V correspond to the oxidation of Fe and Co. The CV curves nearly overlap in the following cycles, indicative of good electrochemical reversibility. Notably, the CV curves of

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maintaining a high capacity of ~800 mAh g-1 even after 500 cycles. In comparison, the two types of singlecomponent superparticles and their physical mixture delivered a comparable capacity of only ~630 mAh g-1 after 500 cycles.

CoFe2O4-Fe3O4 binary superparticles are similar to those of their single-component counterparts (Figure S13), which is expected considering the similar electrochemical pathways of CoFe2O4 and Fe3O4.

It is well known that the electrochemical performance is closely related to the structural evolution of electrode materials during cycling.26,33 To gain deeper insights into the superior performance exhibited by CoFe2O4-Fe3O4 binary superparticles, we disassembled the battery cells to examine the structural changes of electrodes after cycling. Electron microscopies revealed that for both types of singlecomponent superparticles, a large portion of spheres cracked after 50 cycles at 2 A g-1 (Figure 9a-c), presumably caused by the mechanical strain accumulated from the drastic volumetric variations of TMO NCs during cycling.26 In stark contrast, CoFe2O4Fe3O4 binary superparticles largely preserved their spherical morphology without rupture under identical cycling conditions (Figure 9d). Besides, the local superlattice structure of binary superparticles was also well retained without particle agglomeration, as evidenced by STEM and elemental mapping (Figure 9e). These results indicate that CoFe2O4-Fe3O4 binary superparticles are structurally more stable than their single-component counterparts during repeated lithiation and delithiation. The structural changes of superparticles may also affect the dissolution of active materials. To verify this, we examined the metal (i.e., Fe and Co) concentration in the electrolyte after 50 cycles at 2 A g-1 by ICP-OES. Our results show that compared with the two types of single-component superparticles, the metal level leached from binary superparticles is significantly lower (Table S3). We suspect that the well-retained spherical morphology of CoFe2O4-Fe3O4 binary superparticles effectively reduces the contact between NC constituents and the electrolyte, thereby decreasing the leaching probability of active materials during cycling.

Figure 8. (a) Representative CV curves, (b) rate capabilities, and (c) cycling performance of AB13-type CoFe2O4-Fe3O4 binary superparticles. The rate and cycling performances of two types of single-component superparticles and their physical mixture were also provided in (b) and (c) for comparison.

Despite the similar lithiation/delithiation pathways, these NC superparticles behave quite differently in capacity retention. Figure 8b shows the rate capabilities of various superparticle anodes, which were charged and discharged at current densities varying from 0.1 to 5 A g-1. Clearly, the average discharge capacity of CoFe2O4-Fe3O4 binary superparticles is much higher than that of singlecomponent superparticles at each current rate, whereas the physical mixture exhibited a performance only comparable to that of the comprising singlecomponent superparticles. As the major difference between various superparticles lies in the superlattice structure, these results demonstrate the local packing arrangements of NCs have a tremendous impact on electrochemical kinetics of superparticles. To further elucidate the structural merits of binary NC assembly in enhancing performance, the superparticle electrodes were subjected to long-term cycling at a high current density of 2 A g-1. As shown in Figure 8c, the initial discharge and charge capacities of CoFe2O4-Fe3O4 binary superparticles are 1255 and 782 mAh g-1, respectively, corresponding to a Coulombic efficiency of 62.3%. This first-cycle capacity loss should be mainly caused by the irreversible formation of SEI.33,65 The Coulombic efficiency continuously increased from the second cycle and stabilized over 98% from the fifth cycle (Figure 8c). It is worth mentioning that after the initial capacity decay, all superparticle anodes experienced continuous capacity increase before reaching maximum after ~200 cycles. This capacity recovery, presumably originating from electrode material reactivation,33,68 has been commonly observed in TMO-based anodes. Despite the similar trend in capacity retention, CoFe2O4-Fe3O4 binary superparticles exhibited the best performance,

Figure 9. (a) Low- and (b) high-magnification SEM images of delithiated fcc superparticles of Fe3O4 NCs. (c) TEM image of delithiated fcc superparticles of CoFe2O4 NCs. (d) Low-magnification SEM image and (e) HAADF-STEM image and the corresponding elemental mapping of delithiated AB13-type CoFe2O4-Fe3O4 binary superparticles.

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The distinct structural evolution behaviors of various NC superparticles are presumably attributable to their different NC packing arrangements. Singlecomponent superparticles are known to adopt closepacked fcc symmetry with a packing fraction of ~74%,5 whereas AB13-type BNSLs are non-closed packed arrays with lower packing density. Based on SAXS results, the packing fraction of carbon-coated CoFe2O4-Fe3O4 binary superparticles was calculated to be ~69% (see Supporting Information for calculation details), much lower than that of fcc superparticles. This result indicates that there is more open space in AB13-type binary superparticles compared with fcc superparticles. We surmise that such percolating open space can facilitate mass transport while alleviating volumetric changes of individual NCs during repeated lithiation and delithiation, which explains the excellent rate performance and structural durability exhibited by CoFe2O4-Fe3O4 binary superparticles. A schematic illustration depicting the structural evolution difference between binary and fcc superparticles was given in Figure S14.

[email protected] The authors declare no competing financial interest.

A.D. acknowledges the financial support from NSFC (21872038, 21373052), MOST (2017YFA0207303), and Key Basic Research Program of Science and Technology Commission of Shanghai Municipality (17JC1400100). D.Y. thanks the financial support from NSFC (51573030, 51573028, and 51773042).

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CONCLUSIONS In summary, we have demonstrated the gram-scale synthesis of crystalline binary NC superparticles based on an emulsion-based assembly process. The asassembled BNSL colloids exhibit unprecedented phase purity, with compositions and structures tunable in a wide range by adjusting NC combinations. Unlike the case of single-component NCs, the geometrical confinement of emulsion droplets is found to have negligible influence on the self-assembly behaviors of binary NC systems. Access to large-scale, phase-pure BNSL colloids allows us to investigate their physicochemical properties. We show that the asassembled AB13-type CoFe2O4-Fe3O4 binary superparticles display single-phase-like magnetic switching resulting from enhanced dipolar coupling between CoFe2O4 and Fe3O4 NCs. Furthermore, AB13type CoFe2O4-Fe3O4 binary superparticles exhibit superior lithium-storage properties relative to their single-component counterparts when evaluated as anode materials for LIBs, retaining a stabilized capacity of ~800 mAh g-1 after 500 cycles at 2 A g-1. Our results demonstrate that the NC local packing arrangements have profound influence on lithiation/delithiation kinetics and structural durability of NC superparticles during cycling. Given the flexibility in tuning compositions and structures, we anticipate that such freestanding BNSL colloids may find wide applications ranging from catalysis and bioimaging to energy storage and conversion.

Supporting Information. Supplementary results. This material is available free of charge via the Internet at http://pubs.acs.org.

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Scalable Assembly of Crystalline Binary Nanocrystal Superparticles

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