Formation of Monocrystalline 1D and 2D Architectures via Epitaxial

Article pubs.acs.org/Langmuir

Formation of Monocrystalline 1D and 2D Architectures via Epitaxial Attachment: Bottom-Up Routes through Surfactant-Mediated Arrays of Oriented Nanocrystals Yoshitaka Nakagawa, Hiroyuki Kageyama, Yuya Oaki, and Hiroaki Imai* Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan ABSTRACT: Monocrystalline architectures with well-defined shapes were achieved by bottom-up routes through epitaxial attachment of Mn3O4 nanocrystals. The crystallographically continuous 1D chains elongated in the a axis and 2D panels having large a or c faces were obtained by removal of the organic mediator from surfactant-mediated 1D and 2D arrays of Mn3O4 nanocrystals, respectively. Our basal approach indicates that the epitaxial attachment through the surfactantmediated arrays is utilized for fabrication of a wide variety of micrometric architectures from nanometric crystalline units.

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INTRODUCTION The size and shape control of crystals on a nanoscale provides characteristic properties for optical,1 magnetic,2 and electrochemical applications.3 In general, microscale architectures of functional crystalline materials have been fabricated by topdown approaches including photolithography. However, bottom-up routes are also important for fabrication of mesoscopically designed architectures. A wide variety of nanoscale crystals, such as nanocubes,4−9 nanorods,10,11 and nanosheets,12 have been produced by conventional crystal growth techniques. Our next challenge is manipulation of the arrangement and connection of the nanoscale crystals as a building unit instead of the conventional ion-by-ion routes. Whereas the crystallographic continuity of the microarchitectures is essential for their functions, the crystallinity of the assembly of the nanoscale units has not been sufficiently investigated. The functions of the isolated nanograins are limited even if the grains are specifically arranged in the same crystallographic direction. The 1D and 2D continuous lattices would provide particular electrical, magnetic, optical, or mechanical properties. In the current study, we exhibited alternative routes for formation of monocrystalline 1D and 2D architectures via epitaxial attachment of oriented nanocrystals. Our results indicate that the epitaxial attachment through the surfactant-mediated arrays of nanometric crystalline units has a potential for bottom-up fabrication of specific micrometric architectures. Recently, hierarchically organized structures consisting of oriented nanocrystals have been revealed in the microstructures of biominerals.13 Mesostructured crystals similar to the biominerals have been fabricated artificially from a variety of crystals in solution systems.14−19 One of the proposed © 2015 American Chemical Society

formation mechanisms of the mesostructured crystals is the ordered assembly of nanocrystals covered with organic molecular layers (molecular-mediated arrangement).17 Nanometric crystals synthesized using organic molecules as capping agents are dispersed in a liquid medium. The molecularmediated arrays of the monodisperse nanocrystals were formed through a self-assembly process.20,21 Notably, cube-shaped nanocrystals, so-called nanocubes, which have six {100} facets, tend to assemble in the same crystallographic direction.6−8,22−24 Recently, moreover, a wide variety of ordered assemblies including 1D, 2D, and 3D microarrays have been achieved by using rectangular nanoblocks.25−31 However, monocrystalline architectures with continuous lattices have not been achieved form the shape-controlled microarrays of nanounits covered with the organic molecules. Experimental evidence for the epitaxial attachment between the nanocrystals by removal of the organic mediator is insufficient although the crystallographic continuity in the entire body of the oriented arrays is essential for the property of the architectures. Monocrystalline architectures would be formed by epitaxial attachment of the nanocrystals oriented in the same direction.17,32−35 The ordered arrays of oriented nanocrystals can be changed into crystallographically continuous structures with the removal of the organic molecules. In previous works, the epitaxial attachment of oriented nanocrystals was partially observed in molecular-mediated arrays.24,36,37 In these cases, unfortunately, the defined morphologies have been hardly Received: February 10, 2015 Revised: May 14, 2015 Published: May 14, 2015 6197

DOI: 10.1021/acs.langmuir.5b00502 Langmuir 2015, 31, 6197−6201

Article

Langmuir

grid covered with a collodion film was placed on a piece of filter paper. A drop of the dispersion of rectangular nanoblocks was placed on the grid. After the dispersion medium was absorbed by the filter paper, arrays of the nanoblocks were deposited on the grid. Morphologies of the arrays and crystallographic orientation of nanocrystals were characterized by the transmission electron microscopy (TEM), highresolution TEM (HRTEM), and fast Fourier transform (FFT) profiles using an FEI Tecnai-F20. Fabrication and Characterization of Continuous Nanochains and Nanopanels. Absorbed organic molecules onto the surfaces of the nanoblocks deposited on the substrate were removed by heat treatment at 500 °C for 18 h. The morphologies of the products after the heat treatment were observed via SEM. The substrate after the heat treatment or air plasma treatment for 10 min (Meiwafosis SEDEGE) was immersed in NaOHaq (0.1 mol/dm3, 0.5 cm3) for 3 weeks or more to detach the products from the substrate. After that, a drop of the liquid was placed on a copper grid covered with a collodion film. After the liquid was evaporated, morphologies of the products and epitaxial attachment of nanoblocks were characterized by TEM, HRTEM, and FFT profiles.

achieved by direction- and dimension-control of the epitaxial attachment. In the current work, monocrystalline 1D and 2D architectures were achieved through epitaxial attachment of truncated cuboid-shaped Mn3O4 nanocrystals (Scheme 1). We Scheme 1. Bottom-up Fabrication Routes of Specifically Designed Chains and Panels through Epitaxial Attachment from Surfactant-Mediated 1D and 2D Arrays of Anisotropic Rectangular Nanoblocks

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RESULTS AND DISCUSSION We prepared truncated nanocuboids of tetragonal Mn3O4 through a liquid−liquid two-phase (water and toluene) solvothermal method as reported in our previous article.25 The cuboids were covered with four {100} faces and two (001) faces. The typical width and length of the Mn3O4 nanocuboids synthesized under the standard condition were ∼20 nm and ∼30 nm, respectively. The ordered arrays were fabricated from a dispersion of the cuboids by evaporation of a mixture of toluene and hexane (1:1 in volume). As shown in Figure 1a, 1D chains consisting of the rectangular nanoblocks were observed demonstrated bottom-up routes for fabrication of 1D chains elongated in the a direction and two types of 2D panels having large a and c faces. It is difficult to make these nanostructures by using conventional crystal growth methods. This is a novel approach to microscale architectures with specifically designed shapes and crystallographic directions from the nanoscale blocks.

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EXPERIMENTAL SECTION

Synthesis of Mn3O4 Rectangular Nanoblocks. 1.20 mmol manganese(II) chloride and 35 wt % hydrogen peroxide (4 cm3) were dissolved in 31 cm3 of water in a 100 cm3 Teflon container. Oleic acid (7.94 mmol) and tert-butylamine (4.62 mmol) were added into 30 cm3 toluene. The organic mixture was added to the Teflon container without stirring. At this time, oxygen gas was generated through decomposition of hydrogen peroxide. When the generation of oxygen gas roughly stopped, the Teflon container was put into a stainless steel autoclave. The autoclave was heated at 115 °C for 12 h. After the reaction, the resultant dark brown liquid (upper phase) was transferred into a glass vial. Fabrication and Characterization of Surfactant-Mediated Arrays Consisting of Mn3O4 Rectangular Nanoblocks. The resultant dispersion was centrifuged at 13 500 rpm for 5 min. The precipitates were redispersed into a hexane−toluene mixture (1:1 in volume) or toluene in a 6 cm3 vial by sonication for 30 min. The volume of the dispersions was 0.5 cm3. The particle concentration was adjusted to 2.2 × 10−3 g/dm3 (1D arrays) or 2.8 × 10−1 g/dm3 (2D arrays). A silicon substrate treated by acetone with sonication for 30 min was put in the dispersion in the vial. The dispersion spread on the substrate by its surface tension. The dispersion medium was evaporated with heating on a hot plate (1D arrays) or at room temperature (2D arrays). When the drying was completed, nanoblocks were deposited on the surface of the substrate. The morphologies of superstructures consisting of the nanoblocks were observed via the scanning electron microscopy (SEM) using an FEI-Sirion. A copper

Figure 1. TEM image and schematic illustration of surfactantmediated 1D arrays consisting of Mn3O4 rectangular nanoblocks (a, b). HRTEM image (c, d) of two adjacent Mn3O4 nanocuboids and their FFT pattern (e). We observed the interparticle space and the misalignment of the lattice fringes that were separated by the organic mediator (d). 6198

DOI: 10.1021/acs.langmuir.5b00502 Langmuir 2015, 31, 6197−6201

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Langmuir on a collodion film after evaporation of the dispersion medium. The interparticle distance (ca. 3 nm) between adjacent building blocks is close to twice the molecular length of oleic acid (1.7 nm). Thus, the thickness suggests the presence of bilayer of oleic acid between the nanocrystals. However, the Mn3O4 nanocuboids were found to be aligned in the ⟨100⟩ direction (the a direction) with a high degree of crystalline uniformity from HRTEM images and FFT profiles (Figure 1b−e). Therefore, the rectangular nanoblocks formed the molecularmediated 1D arrays. The relatively large faces of the nanocuboids easily attached to each other at the air−liquid interface, and the arrays were elongated in the a direction. However, we observed misalignment of the lattice fringes that were separated by the organic mediator (Figure 1d). This means that the lattices were not completely matched each other although their directions are the same. We applied plasma treatment in air to remove oleic acid at room temperature from the molecular-mediated arrays prepared on a silicon substrate. The epitaxial attachment of the nanocrystals occurred through the removal of oleic acid because continuous lattice fringes were observed between adjacent nanocuboids after the plasma treatment (Figure 2a,c).

room temperature. Thus, we obtained monocrystalline architectures that have specific shapes from the nanoscale blocks. We also removed oleic acid from the ordered arrays by heat treatment in air at 500 °C on a silicon substrate. Here, we utilized the calcination to remove the organic mediator instead of the plasma treatment at room temperature. The boundaries between adjacent nanocuboids of the 1D arrays in scanning electron microscope (SEM) and TEM images were obscured after the removal of the organic molecules (Figure 3a−d). The

Figure 3. Schematic illustration of the formation of monocrystalline Mn3O4 chains from 1D arrays consisting rectangular nanoblocks by the removal of the organic mediator (a). SEM image of 1D arrays before the treatment (b). SEM and TEM images of the continuous chains after the treatment (c, d). HRTEM images of the connecting regions of nanoblocks (scale bars are 2 nm) and corresponding FFT patterns (e). (Numbers (1−5) described in each HRTEM image correspond to numbers described in (d)).

Figure 2. TEM image and schematic illustration of four epitaxially attached Mn3O4 rectangular nanoblocks obtained after plasma treatment (a, b). HRTEM images of the joint part of epitaxially attached nanoblocks and the FFT patterns (c, d). (Numbers (1, 2) described in each HRTEM and FFT image correspond to the numbers described in (a)).

epitaxial attachment of the rectangular nanoblocks was clarified by the presence of continuous lattice fringes between the building blocks. The FFT images indicate that the nanoblocks attached to each other have the same crystalline orientation with a deviation of ∼5° (Figure 3e). By increasing the concentration of the nanoblocks in the toluene−hexane mixture, we obtained 2D arrays with a faces of Mn3O4 nanoblocks parallel to the substrate (Figure 4a,b). The formation mechanism of the specifically ordered arrays was discussed in our previous article.25 The continuous lattice fringes were observed at the boundaries between adjacent nanocuboids in the 2D arrays through removal of the organic

The corresponding FFT pattern (Figure 2d) indicates that the nanocuboids were crystallographically oriented as illustrated in Figure 2b. We observed the direct connection with the continuous lattice fringes by removal of the organic mediator in the HRTEM images. Thus, the removal of the organic molecules covering the nanoblocks promoted epitaxial attachment of the nanoblocks by facing bare {100} facets even at 6199

DOI: 10.1021/acs.langmuir.5b00502 Langmuir 2015, 31, 6197−6201

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Figure 4. Schematic illustration of the formation of monocrystalline 2D panels with a broad a face parallel to a substrate from 2D arrays of Mn3O4 rectangular nanoblocks by the removal of the organic mediator (a). SEM image of 2D arrays of Mn3O4 rectangular nanoblocks before the treatment (b). SEM and TEM images of 2D panels after the treatment (c, d). Schematic illustration of panels (e), HRTEM images of the connecting regions of nanoblocks within the nanopanels (f, g), and corresponding FFT pattern (h).

Figure 5. Schematic illustration of the formation of Mn3O4 panels with c faces parallel to a substrate from 2D arrays of rectangular nanoblocks by the removal of the organic mediator (a). SEM image of 2D arrays of Mn3O4 rectangular nanoblocks before the treatment (b). SEM and TEM images of nanopanels after the treatment (c, d). Schematic illustration (e) and TEM image (f) of the nanopanels and corresponding FFT pattern (g). HRTEM images of the connecting regions of nanoblocks within the panels (h). (Numbers (1−4) described in each HRTEM image correspond to the numbers described in (f). Scale bars are 2 nm in (h)).

molecules by heat treatment at 500 °C (Figure 4a and c). Monocrystalline 2D panels were formed through epitaxial attachment of the nanocuboids in the same orientation with a deviation of ∼10° (Figure 4d−g). When toluene was used as a dispersion medium to increase the polarity, another type of 2D array with c faces of Mn3O4 parallel to the substrate was obtained (Figure 5a,b).25 The 2D panels were produced from the 2D arrays through removal of the organic mediator by heat treatment (Figure 5a, c, and d). The epitaxial attachment between the nanoblocks was confirmed from the continuous lattice fringes over the adjacent units observed in TEM images (Figure 5e, f, and h). The FFT pattern indicates that nanounits were attached in the same crystalline orientation (Figure 5g). As discussed in the previous section, separate nanocrystals covered with organic molecules form the 1D and 2D molecular-mediated arrangements with a high degree of crystallographic orientation. Because the plasma treatment induced the formation of the continuous lattices at room temperature, the removal of the organic mediator is essential for the epitaxial attachment rather than the sintering effect at a high temperature. The thermal diffusion may strengthen the necking between the nanoblocks. Finally, monocrystalline architectures with the specific 1D and 2D shapes were fabricated through the surfactant-mediated ordered arrays of the nanocrystalline units. The epitaxial attachment that occurs in the well-ordered structures consisting of oriented

nanocrystals is clearly different from the sintering phenomena, because the crystalline lattices are not aligned in the same direction through the fusion of the adjacent particles by the conventional calcination technique.

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CONCLUSIONS We demonstrated bottom-up routes for fabrication of specifically designed architectures though epitaxial attachment of nanocrystals. The monocrystalline 1D chains elongated in the a axis and 2D panels having large a or c faces were obtained by removal of the organic mediator from surfactant-mediated ordered arrays of Mn3O4 rectangular nanocrystals. Our results provide a novel approach to microscale architectures with specifically designed shapes and crystallographic directions from the nanoscale blocks.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 6200

DOI: 10.1021/acs.langmuir.5b00502 Langmuir 2015, 31, 6197−6201

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ACKNOWLEDGMENTS This work was partially supported by the Advanced Low Carbon Technology Research and Development Program (ALCA) from Japan Science and Technology Agency (JST) and by Grant-in-Aid for Scientific Research (No. 22107010) on Innovative Areas of “Fusion Materials: Creative Development of Materials and Exploration of Their Function through Molecular Control” (No. 2206) from the Ministry of Education, Culture, Sports, Science and Technology.

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DOI: 10.1021/acs.langmuir.5b00502 Langmuir 2015, 31, 6197−6201