Colloidal Binary Supracrystals with Tunable Structural Lattices

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Colloidal Binary Supracrystals with Tunable Structural Lattices Peng-peng Wang, Qiao Qiao, Yimei Zhu, and Min Ouyang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b05643 • Publication Date (Web): 10 Jul 2018 Downloaded from http://pubs.acs.org on July 10, 2018

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Journal of the American Chemical Society

Colloidal Binary Supracrystals with Tunable Structural Lattices Peng-peng Wang†, Qiao Qiao‡§, Yimei Zhu‡, and Min Ouyang*† †Department of Physics and Center for Nanophysics and Advanced Materials, University of Maryland, College Park, MD 20742, USA. ‡Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, NY 11973, USA. §Department of Physics, Temple University, Philadelphia, PA 19122, USA.

Supporting Information Placeholder ABSTRACT: Colloidal binary supracrystals (SCs) possessing tunable and ordered assembly of two different types of functional nanoparticles (NPs) represent a unique class of artificial materials for both fundamental study and technological applications, but related study has been limited due to substantial challenges in materials growth. Here we report the controlled growth of colloidal binary SCs consisting of Au and Fe3O4 NPs via an oil-in-water emulsion process. The size, stoichiometry and lattice structure of the SCs can be broadly tuned by the growth parameters. Furthermore, our growth method is general and applicable to other NP building blocks to achieve various functional binary SCs. These as-grown free-standing binary SCs should therefore enable new test beds for exploring different nanoscale interactions ranging from the formation and stability of nanoscale binary phase to the emerging magneto-plasmonic coupling physics.

Self-assembled superstructures possessing long-range ordered lattice by a mixture of different types of functional nanoparticles (NPs) represent an important class of artificial materials in the range of nano- to micro-meter scale, which can not only help elucidate fundamental nanoscale phase diagram but also exhibit novel properties due to the collective interactions among functional NP building blocks.1,2 Although substantial advance in binary or even ternary superstructures has been achieved over the past decade, majority of work has been limited to thin film assembly with requirement of supporting substrates,3-6 and their important counterparts, i.e., colloidal supracrystals (SCs) have been lacking.7 Nevertheless, colloidal binary SCs are free-standing, and can offer unique test beds for understanding nucleation and growth of binary ordered state in bulk solution, without external constraints from a substrate. Furthermore, these colloidal binary SCs can serve as building blocks for hierarchical solids, whose properties and functionalities can be flexibly and precisely tailored by the compositions, shapes and sizes of the constituent NPs, with a prerequisite of formation of long-range ordered lattice.4 A few attempts have been made for colloidal binary SCs, but all efforts have only led to either amorphous or phase separated aggregates.8-13 The phase stability of free-standing binary SCs against phase separation with appearance of different structural domains remains an open question.14 Herein, we report for the first time the controlled

growth of colloidal binary SCs consisting of two distinct building blocks, Fe3O4 and Au NPs via an oil-in-water emulsion process. Importantly, we demonstrate that the precise stoichiometry and stable lattice structure of their binary SCs can be deliberately controlled by manipulating such as concentration ratio of two different types of NPs confined in the emulsion droplets, while the overall size of SCs can be independently tuned. The growth method is applicable to other types of constituent NPs to form various functional binary SCs. Our work represents the first achievement of colloidal binary SCs with tunable stoichiometry and lattice structure, thus paving the way to explore various underlying new physics.

Figure 1. Schematic of the formation of colloidal binary SCs consisting of two distinct NP building blocks. NPs are confined in an oil-in-water droplet, and then assembled to form a SC via gradual evaporation of oil phase within the droplet (blue and red spheres represent A and B NPs, respectively). By systematically varying growth condition, the SCs with a series of stable lattice structure and stoichiometry (AmBn, where m and n are integers) can be achieved. Our assembly of binary SCs starts from the formation of oil-in-water droplets, in which two distinct NPs with pre-determined concentration are confined and the interactions among NPs promote the assembly process as the organic solvent gradually evaporates from the droplets (see the Supporting

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Information, SI).15-17 In comparison with unary SCs composed of single species of NPs,16-21 the co-assembly process of two different types of NPs is much more challenging. For example, different type of NPs usually possess completely different surface chemistry and inter-particle interactions under the same growth condition, leading to dramatic modifications of their diffusion and bottom-up assembly kinetics as well as packing principles.2,4 As a result, it requires that the NPs need to be appropriately passivated by their surface ligands in order to ensure good dispersion inside the droplets and to allow effective diffusion of NPs to achieve equilibrium positions that is driven by evaporation of the organic solvent originating the emulsion droplets.1 In order to manipulate size of the SCs, high quality and uniform droplets should be necessary to act as the domains for the crystalline growth of the binary SCs (see the SI notes I). To demonstrate assembly and control of the binary SCs, we have chosen 10 nmsuperparamagnetic Fe3O4 and 5 nm- plasmonic Au NPs as exemplary building blocks (Figure S1). We particularly make the size of two building blocks different to highlight the generality of growth method because the ratio of their sizes is often a critical factor in the phase diagram of the binary solids.22-24 It is worth noting that the surface passivation of constituent NPs are crucial to the formation of the SCs.5,25,26 Therefore, before the co-assembly, all NPs are post-treated with long chain alkyl molecules (e.g., oleic acid or dodecanethiol molecules), and no excess ligands is needed thereafter. The co-assembly of the NPs without such surface treatment always leads to amorphous superstructures (Figure S2). Detailed discussion about effects of ligands and surface passivation is provided in the SI notes II.

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ratio of sizes of Fe3O4 and Au NPs is always kept the same as 0.5, but their concentration ratio is systematically varied to study the evolution of binary phases (Figure 1). The unary SCs of either Au or Fe3O4 NPs, as the two extreme scenarios of co-assembly, always manifest a face centered cubic (FCC) structure that is consistent with the prior studies (Figure S3 and S4).17,20 Figures 2 and 3 exemplify a few typical crystalline structures of as-grown SCs with different stoichiometry and lattice characteristics. Figure 2 presents one SC structure that is achieved under the condition of a high concentration ratio of Au to Fe3O4 NPs (~15:1). Both the low-magnification transmission electron microscopy (LM-TEM)

Figure 3. Other types of SCs. (a-c) Typical TEM image, HAADF-STEM image, and structural model of an individual binary SC with deficient Au NPs, respectively. (d) Typical HAADF-STEM image of an AlB2-type SC along [100] direction. (e) The unit cell of an AlB2-type lattice. (f) FFT pattern of (d). (g) [100] projection of an AlB2-type structure. (h) Typical HAADF-STEM image of an AuCu3-type SC along the [100] zone axis. (i) The unit cell of an AuCu3-type lattice. (j) FFT pattern of (h). (k) [100] projection of an AuCu3-type structure. Scale bar in all TEM images: 100 nm.

Figure 2. The ico-NaZn13-type SCs. (a) Typical LM-TEM image of SCs. Scale bar: 100 nm. (b) Typical HAADF-STEM image of an individual SC. Scale bar: 100 nm. (c, d) The unit cell and [100] projection of the ico-NaZn13-type lattice, respectively. The yellow and cyan spheres represent Au and Fe3O4 NPs, respectively. (e) FFT pattern of the image in (b). (f-h) EDX mapping of Au, Fe, and their overlay, respectively, which are acquired from the blue dashed square in (b). Scale bars: 10 nm. It has been predicted that co-crystallization of two different sets of particles (i.e., A and B) can lead to a rich binary crystalline phase diagram with versatile lattice symmetry and structure (AmBn), depending on a few key parameters of such as the ratios of sizes and concentrations of A and B.24,27,28 In our work, the

(Figures 2a and S5a) and high-angle annular dark field scanning TEM (HAADF-STEM) images (Figure 2b) show that the electron-dense Au NP clusters are separated by the less dense Fe3O4 NPs, which can be assigned to the ico-NaZn13-type lattice.3,29 The corresponding unit cell is shown in Figure 2c, consisting of eight cubic subsets. In each subset, eight large Fe3O4 NPs form a cubic framework, while thirteen small Au NPs are clustered with icosahedron configuration in the middle (i.e., twelve Au NPs are positioned at the vertexes of an icosahedron with one in the center).29 Both Figures 2a and 2b show images of [100] projection of the NaZn13 structure in which small Au NP clusters can be clearly resolved in the cubic lattice, agreeing well with the [100] plane of the model (Figure 2d). The fast Fourier transformation (FFT) of an individual SC in Figure 2b clearly reveals high degree of ordering in a single SC. Energy-dispersive X-ray (EDX) mapping identifies the arrangement of Au NP clusters surrounded by large Fe3O4 NPs, confirming the image contrast in the HAADF STEM image (Figure 2f-2h). In order to


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unambiguously confirm our assignment of lattice type, we have performed thorough angle-dependent TEM characterization by tilting the same SC over a wide range from [100] to [310] directions with a few selected images shown in Figure S5b, consistently agreeing with the ico-NaZn13 model. All these characterizations together have confirmed that the AB13 structure can be formed when the concentration of the Au NPs is much higher than that of the Fe3O4 NPs during the growth of SCs. Interestingly, a reverse growth condition with deficient Au NPs has led to a completely different lattice characteristic from Figure 2. Figures 3a-3c show one typical SC obtained when a ~1:10 concentration ratio of Au to Fe3O4 NPs is utilized. Under this growth condition, larger Fe3O4 NPs remain the ordered FCC lattice while Au NPs act as “dopants” by randomly residing in the interstitial sites.30 We have found that when the concentration of Au NPs is gradually increased during the growth, the ordered domains of Au NPs can appear, analogues to the nucleation process in the atomic or molecular systems, and the binary lattice can evolve to new and stable phases. For example, Figures 3d-3g and Figures 3h-3k highlight two other types of SCs, AlB2- and AuCu3structures, respectively, with comprehensive characterization and structural assignment (Figures S6-S8) in the SI notes III.

Figures 4a-4d, 4e-4h and 4i-4l, respectively. Our observation and control of distinct stable lattice structures of the binary SCs achievable under different growth condition corroborates the predicted lattice evolution for a binary nanosolid, and highlights the uniqueness of colloidal free-standing SCs as a useful platform for studying a few fundamental processes such as phase stability and the underlying assembly mechanism.24,31 In particular, the appearance of the AuCu3-type phase of SCs suggests that our assembly is not a direct consequence of hard sphere packing because only the AlB2- and AB13-type phases were predicted to exist for this particular size ratio of 0.5,32 whereas the AuCu3-type phase is allowed by treating NPs as soft spheres.33 Therefore, the short-range interactions involving surface ligands should be taken into account as the underlying driving forces for different lattice types of binary SCs, with more discussion in the SI notes I.14,31,34,35 The successful assembly and control of these free-standing colloidal binary SCs therefore provides a baseline to further guide theoretical efforts without constraints from the substrate that is unavoidable in the thin film counterparts. In conclusion, we have successfully achieved colloidal binary SCs with tunable crystal structure, stoichiometry and size via an oil-in-water emulsion process. While we have employed Au and Fe3O4 NPs as examples for demonstrating exceptional structural control of the binary SCs, our assembly approach can be readily extended to other functional NPs with facile control of surface chemistry (Figure S9-S10). Future work by more systematical control of the constituent NPs such as their size ratio should allow full determination of binary phase diagram of nanoscale solids. Furthermore, as compared with amorphous binary aggregates,9,12,36 the achievement of well-defined and tunable lattice order of SCs should pave the way to understand and control fundamental coupling physics between different functional NPs and to study their explicit structure-property relationship. For example, the Au-Fe3O4 SCs can integrate magnetism with plasmonics (Figure S11), and therefore the structures in Figures 2-4 should allow exploration of microscopic mechanism of the emerging magneto-plasmonic coupling with precise nanoscale spatial coordination that cannot be achieved otherwise.

ASSOCIATED CONTENT Supporting Information Synthesis and characterization details, additional (S)TEM images, and other characterization data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Figure 4. Size control of binary AlB2-type SCs. (a-d) Typical LM-TEM images, (e-h) Higher magnification TEM images, (i-l) Histogram plot of size distribution of the SCs with averaged size of 100±12, 137±26, 203±25 and 305±27 nm, respectively. The red curve in the plot represents a Gaussian fit. Scale bars in (a-d) and (e-h) are 200 nm and 100 nm, respectively. Our growth strategy of the binary SCs can also allow important size control by simply maneuvering growth conditions such as surfactant concentration in the water phase, NP concentration in the oil phase, and shear rate applied in the emulsification process (Scheme 1 in the SI). In general, smaller surfactant concentration and/or lower shear rate can lead to larger SCs. Figure 4 shows a series of the AlB2-type SCs with the averaged diameter ranging from ~100 to ~300 nm (see experimental details in the SI). For each size, typical TEM images of large-scale sample, individual SC and its corresponding histogram analysis are presented in

Corresponding Author [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT We thank Dr. Jing Tao for her assistance of TEM characterizations and stimulating discussion. Work in UMD was supported by the US Department of Energy, Office of Basic Sciences, Division of Materials Science and Engineering (DESC0010833) for materials synthesis, and the Office of Naval Research (N000141712885) for partial material characterization. We also thank TEM facility support from Maryland Nanocenter and its AIMLab. Q.Q was supported by the Center for the Computational Design of Functional Layered Materials, an



Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award No. DE-SC0012575, for her TEM work. Y.Z. was supported by the Materials Science and Engineering Divisions, Office of Basic Energy Sciences of the U.S. Department of Energy under Contract No. DE-SC0012704.

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(34) Tkachenko, A. V. Proc. Natl. Acad. Sci. U. S. A. 2016, 113, 10269. (35) Luo, D.; Yan, C.; Wang, T. Small 2015, 11, 5984. (36) Zhao, J.; Wu, J.; Xue, J.; Zhu, Q.; Ni, W. Isr. J. Chem. 2016, 56, 242.

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Figure 1. Schematic of the formation of colloidal binary SCs consisting of two distinct NP building blocks. NPs are confined in an oil-in-water droplet, and then assembled to form a SC via gradual evaporation of oil phase within the droplet (blue and red spheres represent A and B NPs, respectively). By systematically varying growth condition, the SCs with a series of stable lattice structure and stoichiometry (AmBn, where m and n are integers) can be achieved. 84x58mm (300 x 300 DPI)


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Figure 2. The ico-NaZn13-type SCs. (a) Typical LM-TEM image of SCs. Scale bar: 100 nm. (b) Typical HAADF-STEM image of an individual SC. Scale bar: 100 nm. (c, d) The unit cell and [100] projection of the ico-NaZn13-type lattice, respectively. The yellow and cyan spheres represent Au and Fe3O4 NPs, respectively. (e) FFT pattern of the image in (b). (f-h) EDX mapping of Au, Fe, and their overlay, respectively, which are acquired from the blue dashed square in (b). Scale bars: 10 nm. 84x83mm (300 x 300 DPI)



Figure 3. Other types of SCs. (a-c) Typical TEM image, HAADF-STEM image, and structural model of an individual binary SC with deficient Au NPs, respectively. (d) Typical HAADF-STEM image of an AlB2-type SC along [100] direction. (e) The unit cell of an AlB2-type lattice. (f) FFT pattern of (d). (g) [100] projection of an AlB2-type structure. (h) Typical HAADF-STEM image of an AuCu3-type SC along the [100] zone axis. (i) The unit cell of an AuCu3-type lattice. (j) FFT pattern of (h). (k) [100] projection of an AuCu3-type structure. Scale bar in all TEM images: 100 nm. 84x92mm (300 x 300 DPI)


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Figure 4. Size control of binary AlB2-type SCs. (a-d) Typical LM-TEM images, (e-h) Higher magnification TEM images, (i-l) Histogram plot of size distribution of the SCs with averaged size of 100±12, 137±26, 203±25 and 305±27 nm, respectively. The red curve in the plot represents a Gaussian fit. Scale bars in (a-d) and (e-h) are 200 nm and 100 nm, respectively. 84x92mm (300 x 300 DPI)



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Colloidal Binary Supracrystals with Tunable Structural Lattices

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