Nanoscale Mosaic Works: Tetragonal Lattices of Iso-Oriented

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Nanoscale Mosaic Works: Tetragonal Lattices of Iso-Oriented Heterogeneous Nanocubes Riho Matsumoto, Hiroya Yamazaki, Mihiro Takasaki, Yuya Oaki, Tetsuya Sato, and Hiroaki Imai* Center of Material Design Science, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan ABSTRACT: Tetragonal 2D lattices are spontaneously formed by the self-assembly of homogeneous nanocubes. However, ordered arrays consisting of differently sized rectangular nanoblocks have not been achieved because the regulation of the assembly is weakened by the combination of binary units. In the present work, ordered arrays comprising binary nanocubes were investigated using the combination of 10 nm Pt nanocubes and 20 nm BaTiO3 nanocubes. Heterogeneous but ordered 2D tetragonal lattices were successfully produced using differently sized rectangular nanoblocks. The highly ordered self-assembly in the heterogeneous system requires the matching of the size ratio of binary nanocubes with the buffer effect of oleic acid covering the building units.

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INTRODUCTION The self-assembly of nanometric building blocks has been studied for the fabrication of a wide variety of novel functional materials having ordered architectures.1−6 Various arrangements depending on the size and shape of the building units have been produced via self-assembly. Hexagonal lattices are usually produced by monodispersed homogeneous spherical blocks.1,7,8 Other arrangements, such as tetragonal lattices, via the combination of binary and ternary nanoblocks have been reported.9−12 However, the crystallographic direction has not been controlled in the ordered arrays of spherical building blocks. When rectangular nanoblocks, such as cubed and cuboid nanocrystals, are used as a building unit, single-crystallike superlattices or mesocrystals are formed via oriented assembly by facing well-defined facets of {100} or (001).13−17 Excellently ordered 2D arrays were reported by the selfassembly of magnetite nanocubes.18,19 Nanocubes and nanocuboids are generally synthesized with amphiphilic molecules from cubic and tetragonal crystals including metals and metal oxides.20−25 Regulation of the direction of the oriented selfassembly has been achieved by using anisotropic rectangular nanoblocks without any external fields.14 The 1D-, 2D-, and 3D-ordered arrays were fabricated via controlled assembly of the nanoblocks by changing the preparation conditions. Recently, spatial switching of the crystallographic direction of nanoblocks in 2D arrays was achieved using micron-scale trenches on a substrate.26 A wide variety of functions have been created by the combination of heterogeneous materials through the conjugation of various properties.27−36 Recently, the nanometric conjunction of heterogeneous crystals has attracted much attention with regard to the emergence of novel functions.28−36 In a previous work,36 a monolayer of Pt nanocubes was stacked © XXXX American Chemical Society

on a monolayer of CeO2 nanocubes to enhance the catalytic activity. However, the crystallographic orientations of binary nanocubes were not strictly controlled by this method. In another work,15 ordered arrays were produced by randomly mixing the same-size nanocubes (∼20 nm) of BaTiO3 (BT) and SrTiO3. However, ordered arrays consisting of differently sized rectangular nanoblocks have not been achieved because the regulation of the assembly is weakened by the combination of building units having multimodal size distribution. In the present study, we report ordered 2D lattices composed of heterogeneous and differently sized binary nanocubes. Here, ordered 2D tetragonal lattices were produced by basal 10 nm Pt nanocubes with 20 nm BT nanocubes. A small amount of the larger nanocubes was incorporated into the tetragonal lattices of the smaller nanocubes without deformation. The matching of the size ratio of binary nanocubes with the moderate homogeneity of their shape is required for the formation of ordered self-assembly in the heterogeneous system. Moreover, the buffer effect of oleic acid covering the building units is essential for ordering the binary system.

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EXPERIMENTAL SECTION

Preparation of Pt Nanocubes. Cubic Pt (a = 0.392 nm) nanocubes were synthesized via a solvothermal method reported in previous works.20 We loaded 0.020 g of platinum(II) acetylacetonate, 8.0 cm3 of oleylamine, and 2.0 cm3 of oleic acid into a three-necked flask equipped with a condenser and attached to a Schlenk line. The mixture was heated to 130 °C with vigorous stirring under an argon stream. Then, 0.05 g of tungsten hexacarbonyl was added into the solution, and the temperature was subsequently raised to 240 °C and Received: February 2, 2018 Revised: March 7, 2018

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DOI: 10.1021/acs.langmuir.8b00364 Langmuir XXXX, XXX, XXX−XXX

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Langmuir maintained for 30−60 min with vigorous agitation. The resultant products were isolated by centrifugation and washed with anhydrous hexane for several cycles. The products were redispersed in hexane. Preparation of BT Nanocubes. Tetragonal BT nanocubes (a = 0.399 nm and c = 0.404 nm) were synthesized via a hydrothermal method reported in previous works.21 In a typical synthesis process, we introduced titanium bis(ammonium lactato) dihydroxide (0.05 mol dm−3, Ba/Ti = 1:1) into 24 cm3 of Ba(OH)2 aqueous solution, and then 5 mol dm−3 NaOH aqueous solution was added under mechanical stirring. The initial concentration of NaOH aq was 1 mol dm−3. The aqueous solution was transferred to a 50 cm3 autoclave, and t-butylamine and oleic acid (Ba/oleic acid/butylamine = 1:8:8 molar ratio) were then added into the solution. The sealed autoclave was heated at 200 °C for 72 h and then cooled at room temperature. After the synthesis, the precipitate was centrifugally separated and washed twice with ethanol and then dispersed into toluene. Formation and Observation of Ordered Arrays on Microgrids. A mixed dispersion of Pt nanocubes and BT nanocubes was arranged at several concentrations in a toluene/hexane mixture (1:1 (v/v)). A drop of the mixed dispersion was loaded on a copper microgrid covered with a collodion film that was placed on a piece of filter paper. After the excess dispersion was absorbed by another piece of filter paper, the nanoblocks were deposited on the grid with the evaporation of a liquid medium. The morphologies and crystallographic orientation of nanocrystals were characterized using transmission electron microscopy (TEM, FEI Tecnai F20 operated at 200 kV), selected area electron diffraction (SAED), and energy-dispersive X-ray (EDX).

100 and 001 in the SAED pattern were obtained from the crystals (Figure 1f), tetragonal BT nanocubes were deduced to exhibit four {100} and two (001) faces. We found that 2D tetragonal arrays with a periodicity of ∼12 nm were spontaneously formed with the Pt nanocubes on the collodion film of the microgrid on a copper mesh (Figure 1c,d). The nanocubes were separated with bilayers of oleic acid ∼3 nm thick in the arrays. According to the SAED pattern of the ordered arrays (Figure 1b), the crystal lattices of the nanocubes were aligned in the same orientation because of the attachment of their (100) faces. The largest domain size of the region consisting of iso-oriented nanocubes was about 500 nm, whereas the voids were included in the ordered area (Figure 1i). The tetragonal arrays were slightly distorted because of the distribution of the nanocube size and shape. As shown in Figure 2, vacancies and edge dislocations were occasionally observed

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RESULTS AND DISCUSSION The self-assembly of the nanoblocks was performed using 10 nm Pt nanocubes and 20 nm BT nanocubes. We obtained homogeneous Pt nanocubes exhibiting six {100} faces that were covered with oleic acid (Figure 1a,b). The average size and standard deviation of the Pt nanocubes were 11.7 and 0.9 nm, respectively (Figure 1j). Homogeneous BT nanocubes covered with oleic acid were obtained, as shown in Figure 1e. From the TEM images, the average size and standard deviation of the BT nanocubes were estimated to be 23.0 and 3.6 nm, respectively (Figure 1j). Because two sets of diffraction spots assigned to

Figure 2. Scanning TEM (STEM) images (a,b) and schematic illustrations (c,d) of vacancies and edge dislocations observed in 2D arrays of Pt nanocubes. Yellow arrows indicate vacancies and a dislocation.

in the tetragonal lattice with the distribution of size and shape (Figure 2). The presence of oleic acid bilayers between the nanocubes buffered the strains in the ordered arrays because of the distribution of their size and shape. As shown in Figure 1g,h, 2D tetragonal arrays with a periodicity of ∼23 nm was spontaneously formed by 20 nm BT nanocubes on the collodion film of a copper microgrid. The crystal lattices of the nanocubes were aligned in the same orientation because of the attachment of their {100} faces, according to the single-crystal-like SAED pattern (Figure 1f). However, the distortion of nanocube lattices was relatively large, and the domain size consisting of iso-oriented nanocubes (200−300 nm) was smaller than that of the Pt nanocube arrays. The disorder of the nanocube lattices is ascribed to the wide distribution of their size (Figure 1j). Moreover, the buffer effect of the oleic acid bilayers ∼3 nm thick was relatively low for the assembly of ∼20 nm nanocubes because of the larger size difference between the buffer layer and building units. We produced the ordered arrays on a collodion film from a mixture of 20 nm BT nanocubes and 10 nm Pt nanocubes. When the number of BT nanocubes was much smaller than that of Pt nanocubes (90%) in the dispersion system, ordered lattices were not formed with the binary system (Figure 5d). The disorder of the nanocube lattices is ascribed to the wide size distribution of BT

Figure 5. TEM images of disordered regions in the binary arrays with a differently sized nanocube (a), deformed BT nanocubes (b,c), and a large number of BT nanocubes (d). Yellow arrows indicate vacancies and a dislocation.

nanocubes. The highly ordered self-assembly requires the matching of the size ratio of binary nanocubes with the moderate homogeneity of their shapes. Here, the buffer effect of the oleic acid bilayers between the binary nanocubes was insufficient for the ordering. The ordered arrays were small, and the TEM images present snapshots of the most ordered arrays. However, our results C

DOI: 10.1021/acs.langmuir.8b00364 Langmuir XXXX, XXX, XXX−XXX

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Langmuir

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become clear that a geometrical ordering of differently sized nanoparticles with the same shape is possible. This is a promising first step toward a “nanoparticle Lego” where different nanoparticles can be coassembled in the same superstructure to build multifunctional superstructures. The present work is of particularly interest to the self-organization of nanoparticle structures with potential applications in plasmonics and metasurfaces.37−39

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CONCLUSIONS We performed a nanometer-scale mosaic work using tetragonal lattices of binary nanocubes. Iso-oriented ordered arrays comprising heterogeneous and differently sized nanocubes were produced using 10 nm Pt nanocubes and 20 nm BT nanocubes. Heterogeneous but ordered tetragonal lattices are produced by small cubes with a small amount of the large cubes. The matching of the size ratio with moderate homogeneity of their shapes is essential for highly ordered self-assembly of heterogeneous nanoblocks. The covering organic molecules function as a buffer for the oriented assembly. Because the chemical composition is not important for the oriented assembly, the fabrication routes for ordered arrays consisting of heterogeneous materials would be applicable to a wide variety of novel nanometer-scale structures providing novel functions.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Mihiro Takasaki: 0000-0001-9183-7698 Yuya Oaki: 0000-0001-7387-9237 Hiroaki Imai: 0000-0001-6332-9514 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by Grant-in-Aid for Challenging Exploratory Research (15K14129) and Grant-inAid for Scientific Research (A) (16H02398) from the Japan Society for the Promotion of Science.

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DOI: 10.1021/acs.langmuir.8b00364 Langmuir XXXX, XXX, XXX−XXX