Evolution of Co3O4 Nanocubes through Stepwise Oriented Attachment

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Evolution of Co3O4 Nanocubes through Stepwise Oriented Attachment Keishi Tsukiyama, Mihiro Takasaki, 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

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S Supporting Information *

ABSTRACT: Uniformly sized building units are generally required to construct highly elaborate architectures over a wide range. Defined nanocubes of Co3O4 evolved from deformed precursor nanograins 2−5 nm in diameter through direct oriented attachment in a nonpolar medium. Uniformly sized primary nanocubes ∼8 nm on a side with {100} faces were formed by adjusting the coverage of the oxide nanograins with oleic acid. Larger nanocubes 20−40 nm on a side were produced with further direct oriented attachment of the primary nanocubes. Ordered arrays, such as superlattices, were found to be constructed by the indirect oriented attachment of the primary and larger nanocubes covered with organic molecules.

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with the lattice fusion in the nonpolar medium.29,30 The direct and indirect oriented attachments are regarded as a nonclassical route in contrast to the conventional ion-by-ion crystal-growth route. The fabrication method via oriented attachment in nonaqueous media is applicable to various nanoscale particles of other metals and metal oxides. In the present work, we applied the nonclassical growth route to fabricate uniformly sized nanocubes of Co3O4 that are worthy of attention because of their potential applications, such as active catalysts,45−48 antiferromagnets,49,50 and electrode materials.51−53 A wide variety of shapes, such as nanosheets,54,55 nanocubes,56,57 nanorods,47,58,59 and nanodisks60,61 have been synthesized to improve the specific properties of Co3O4 nanocrystals. The size of the Co3O4 nanocrystal also influences its property.62,63 However, the controllability of the size and shape has been insufficient for the formation of superlattices consisting of nanoscale building units through self-assembly. Here, we studied the simple formation process of certain-sized primary nanocubes of Co3O4 using a nonclassical method, such as the direct oriented attachment of tiny metastable grains, in a water−toluene twophase system. The growth mechanism of uniformly sized nanocubes is discussed on the basis of the influence of growth conditions on size and morphology. We observed the formation of larger nanocubes via further direct oriented attachment of the certain-sized primary nanocubes. Moreover, ordered arrays, such as superlattices, were found to be

INTRODUCTION Syntheses of inorganic nanoparticles with controlled morphologies, such as nanospheres, nanooctahedra, nanorods, and nanoplates, have attracted much attention1−14 because morphology control is important for the emergence of optical,15,16 magnetic,17,18 electrochemical,19,20 and catalytic functions.21−26 Recently, shaped nanocubes and nanocuboids have been studied as building units to fabricate ordered structures through self-assembly for the improvement of their functions.27−29 Uniformly sized building units are generally required to construct highly elaborate architectures over a wide range. In the present study, we report on a simple method for the fabrication of certain-sized metal oxide nanocubes and discuss the oriented attachment as a nonclassical growth mechanism for the nanocube formation. Definite nanocubes with six exposed {100} faces are excellent building units for the production of two- and threedimensional single-crystal-like superlattices or supercrystals through self-assembly in which nanocubes are ordered in the same crystallographic orientation. Here, the self-assembly of nanocubes covered with organic molecules is regarded as “indirect” oriented attachment of nanocubes through the organic layers. Uniformly sized nanocubes and nanocuboids have been synthesized with noble metals (such as Au,30 Ag,31,32 Pt,33 and Pd34,35), metal oxides, and halides (such as BaTiO3,36 CeO2,37−39 Mn3O4,40,41 Fe3O4,42,43 and CsPbBr344). Rectangular nanocubes with high dispersibility in nonaqueous media are easily obtained using water−toluene two-phase solvothermal methods.37−40 The formation of CeO2 nanocubes and Mn3O4 nanocuboids was reported to be achieved through “direct” oriented attachment of precursor particles © XXXX American Chemical Society

Received: February 14, 2019 Revised: April 16, 2019

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DOI: 10.1021/acs.langmuir.9b00342 Langmuir XXXX, XXX, XXX−XXX

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Langmuir constructed by the indirect oriented attachment of the primary and larger nanocubes covered with organic molecules. Our findings are important for the fabrication of highly designed nanomaterials.

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EXPERIMENTAL SECTION

Synthesis. Nanocubes of Co3O4 were synthesized using the water−toluene two-phase solvothermal method reported in previous studies.38,40 In a standard process, 0.51 mmol of cobalt(II) nitrate and 35 wt % hydrogen peroxide (1.00 cm3) were dissolved in 15 cm3 of water in a 50 cm3 Teflon container. Oleic acid (2.00 mmol) and tertbutylamine (5.76 mmol) were added into 14.2 cm3 of toluene. The organic mixture was gently added to the Teflon container. Oxygen gas was then generated through decomposition of hydrogen peroxide. When the generation of oxygen gas stopped, the Teflon container was put into a stainless-steel autoclave and then heated at 180 °C for a certain period. After the reaction, the resultant dark black liquid (upper phase) was transferred into a glass vial. We varied the concentrations of cobalt nitrate, tert-butylamine, and hydrogen peroxide to study the growth mechanism of metal oxide nanocubes. A thin mesocrystalline film was fabricated by evaporation-induced self-assembly on a silicon substrate. Fabrication methods are described in the Supporting Information (Figure S1). Characterization. A copper grid covered with a collodion film was placed on a piece of filter paper. A drop of the resultant dispersion was placed on the grid. After the excess medium of the dispersion was absorbed by the filter paper, the nanocubes were deposited on the grid. The morphology of nanocrystals was characterized using transmission electron microscopy (TEM, FEI Tecnai F20 operated at 200 kV), selected area electron diffraction (SAED), and fast Fourier transform (FFT) profiles. Scanning electron microscopy (SEM, JEOL JSM-7100F) was used for morphological observation. The resultant dispersion and the equal amount of ethanol were mixed and then centrifuged at 13500 rpm for 5 min. Precipitates were calcinated at 300 °C for 6 h to remove organic molecular materials. Obtained powders were used for the powder X-ray diffraction (XRD, Bruker D8 Advance). The sizes of nanocubes in the dispersion were measured using a dynamic light-scattering photometer (DLS, Otsuka Electronics ELSZ-2000). Characterization of adsorbed molecules was performed via Fourier transform infrared spectroscopy (FTIR, JASCO FT/IR-4200).

Figure 1. A typical XRD profile (a), size distributions (b, c), and TEM images (d, e, g, h) with schematic illustrations (f, i) of Co3O4 nanocubes. The reaction times were 15 min (d−f) and 720 min (a, c, g−i). The size distributions (b) and (c) were obtained via TEM observation and DLS, respectively.

was confirmed by FTIR spectra (Figure S2). Thus, certainsized primary Co3O4 nanocubes were stabilized by oleic acid coverage. Moreover, Ostwald ripening through dissolution/ reprecipitation would hardly occur due to the extremely low solubility of Co3O4 to the organic phase. These results indicate that the primary nanocubes are formed through direct oriented attachment of the deformed metastable nanograins. This nonclassical growth process of defined nanocubes is similar to that of Mn3O4 rectangular nanocuboids.40 Figure 2 shows the influence of the concentration of tertbutylamine on the shape, size, and arrangement of Co3O4 nanoparticles produced in the two-phase system. When the concentration of tert-butylamine (1.44 mmol) was smaller than the standard condition (5.76 mmol), the deformed grains 3−5 nm in diameter were stabilized even after the reaction for 720 min (Figure 2a, c). Disordered arrangements were observed due to the heterogeneity of the shape and size of the building blocks (Figure 2b). The number of nanocubes increased with the increasing concentration of tert-butylamine (2.88 mmol). Tetragonal superlattices in which the crystallographic orientation was ordered were formed by the indirect oriented attachment of the nanocubes covered with oleic acid (Figure 2d−f). The gap between the nanocubes was about 3 nm, which corresponds to twice the chain length of oleic acid. Uniformly sized primary nanocubes were produced with the formation of ordered tetragonal superlattices over a wide range at a sufficient concentration of tert-butylamine (4.32 and 5.76 mmol) (Figure 2g−l). This suggests that tert-butylamine, which is an initiator of the crystal growth, must be present in more than the critical amount in order to form certain-sized nanocubes. When the amount of tert-butylamine was smaller than 2.88 mmol, the production rate of Co3O4 was relatively small by comparing with the stabilizing agent. Thus, a large amount of oleic acid prohibited the formation of facets of cubic grains. As the grain size increased and the facets became

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RESULTS AND DISCUSSION According to a typical XRD pattern (Figure 1a) of the resultant precipitates, cubic Co3O4 (a = 0.8083 nm) was produced by the two-phase solvothermal method. The crystallite size was calculated to be ∼9 nm from the broadened width of the 311 diffraction signals by applying the Scherrer equation. TEM images and size distribution (Figure 1b, d, e, g, h) show the shape evolution of Co3O4 nanocrystals with increasing reaction time. Deformed tiny grains 2−5 nm in diameter were observed in the early stage of the reaction (15 min) (Figure 1b, d). Most of the precursor grains were found to attach to each other (Figure 1d, e). According to the high-resolution TEM images, the attached grains were aligned in the same crystallographic orientations (Figure 1e, f). The precursor grains were isolated due to coverage of oleic acid in a nonpolar liquid medium. Thus, dispersion evaporation did not cause direct attachment of precursor grains. These results suggest that the crystal growth occurred through direct oriented attachment of the metastable grains in a nonpolar liquid medium. Certain-sized primary nanocubes ∼8 nm in size on a side with {100} faces were formed by prolongation of the reaction time (720 min) (Figure 1b, g−i). The grain sizes of the primary nanocubes were confirmed via DLS measurement (Figure 1c). The adsorption of oleic acid on the surface of Co3O4 nanocubes B

DOI: 10.1021/acs.langmuir.9b00342 Langmuir XXXX, XXX, XXX−XXX

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Figure 3. (a, b) SEM images, (c) schematic illustration, (d, e) TEM images, and (f) SAED pattern of Co3O4 mesocrystals on a silicon substrate.

Basically, hydrogen peroxide acts as an oxidant for the oxidation of Co2+ to Co3+. However, we obtained the samesized primary Co3O4 nanocubes without hydrogen peroxide (Figure 4a). This suggests that dissolved oxygen in the aqueous

Figure 2. TEM images of Co3O4 nanocubes (a, d, g, j), SAED images of the corresponding area (b, e, h, k), and grain-size distribution (c, f, i, l). Inset images are high-magnification images. The amount of tertbutylamine loaded in the toluene phase was 1.44 (a−c), 2.88 (d−f), 4.32 (g−i), and 5.76 (j−l) mmol.

Figure 4. TEM images of Co3O4 nanocubes synthesized without hydrogen peroxide (tert-butylamine: 1.44 mmol).

phase oxidizes the cobalt ions. In this case, interestingly, we found that the interparticle spaces were thinner than twice the chain length of oleic acid molecules. Moreover, partial direct attachment of the primary nanocubes was observed in the ordered arrangements (Figure 4b). The promotion of the direct attachment is ascribed to the thinner or weaker coverage of oleic acid. The adsorption of negatively charged oleic acid molecules would be weakened with the increasing amount of Co2+ on the surface due to insufficient oxidation without hydrogen peroxide. Larger nanocubes were then formed with ∼8 nm primary nanocubes by addition of twice amount of the cobalt source (1.02 mmol) without hydrogen peroxide (Figure 5a, b). The size distribution is as wide as 20−40 nm (Figure 5c). According to TEM observation (Figure 5d−h), the larger nanocubes are deduced to be produced through the direct oriented attachment of the primary nanocubes. We successfully separated larger nanocubes from the mixture by centrifugation. Tetragonal superlattices were also obtained with the larger nanocubes by indirect oriented attachment through organic layers on a silicon substrate by evaporation-induced selfassembly (Figure 6). Under the synthesis condition, threedimensional superlattices were occasionally observed with attachment of the primary nanocubes (Figure S4). Figure 7 shows a schematic illustration of the evolution mechanism of Co3O4 nanocubes through stepwise oriented attachment. The decomposition of tert-butylamine raises the pH of the water phase and induces the nucleation of Co3O4 near the two-phase interface. The precursor nanograins are

clearer, the indirect oriented attachment was promoted, and the crystal orientation was aligned (Figure 2b, e, h, k). The size of the primary nanocubes was not changed by the addition of twice the amount of the cobalt source (1.02 mmol) (Figure S3). Since excess cobalt ions remain in the aqueous phase, certain-sized primary nanocubes are formed by the coverage of oleic acid in the nonpolar medium in the two-phase system. Unfortunately, the number of nanocubes produced here cannot be estimated because of the large amount of Co3O4 precipitates in the aqueous phase. The size of the nanocubes is determined by the adsorption of the organic molecule. Therefore, smaller nanocubes would be obtain by introducing organic molecule which adsorbed to the crystal surface more strongly. Tetragonal superlattices were fabricated on a substrate with uniformly sized primary Co3O4 nanocubes that are covered with oleic acid through indirect oriented attachment due to their distinct facets and narrow size distribution. Figure 3a−c shows SEM images with schematic illustration of the tetragonal superlattice of Co3O4 nanocubes on a silicon substrate by evaporation-induced self-assembly. After the removal of oleic acid with calcination, the fusion of oriented nanocubes was confirmed by SAED patterns from TEM images of a thin film that was peeled from the substrate (Figure 3d−f). We succeeded in fabricating a thin macroscale film with a uniform crystal orientation over several micrometers due to the high dispersibility and narrow size distribution of the primary nanocubes covered with oleic acid as building blocks. C

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surfaces. Tetragonal superlattices are formed through indirect oriented attachment of the primary nanocubes covered with organic molecules. A large amount of oleic acid stabilizes the nanograins and induces the formation of disordered arrangements. When the coverage of oleic acid molecules is insufficient, larger nanocubes 20−40 nm on a side were formed by further direct oriented attachment of the metastable primary nanocubes.

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CONCLUSION We synthesized uniformly sized Co3O4 nanocubes ∼8 nm on a side by using the two-phase solvothermal method and adjusting the synthesis conditions. The primary nanocubes covered with oleic acid are produced through the direct oriented attachment of deformed metastable nanograins. Tetragonal superlattices with a uniform crystal orientation are constructed through indirect oriented attachment of the uniformly sized building units. When the coverage with oleic acid was relatively weak, larger nanocubes are formed by further direct attachment of the primary nanocubes.

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Figure 5. TEM images (a, b, d, g, h), size distribution (c), FFT images (e, f), and schematic illustration (i) of Co3O4 nanocubes synthesized with twice the amount of the cobalt sources (1.02 mmol) without hydrogen peroxide (tert-butylamine: 1.44 mmol).

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.9b00342. Experimental procedure of the fabrication of mesocrystal films; (Figure S1) schematic illustration of the fabrication system for thin films consisting of nanocubes thorough evaporation-induced self-assembly using dispersion; (Figure S2) FTIR spectra of Co3O4 nanocubes, commercial Co3O4 powder, and oleic acid; (Figure S3) TEM images of Co3O4 nanocubes; (Figure S4) TEM, SAED, and SEM images of three-dimensional cubic superlattice of Co3O4 synthesized without hydrogen peroxide (PDF)

Figure 6. SEM images of Co3O4 mesocrystals produced with larger nanocubes through indirect oriented attachment by evaporationdriven self-assembly on a silicon substrate.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Keishi Tsukiyama: 0000-0002-0996-8187 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 a Grant-in-Aid for Challenging Exploratory Research (15K14129) and Scientific Research (A) (16H02398) from the Japan Society for the Promotion of Science.

Figure 7. Schematic illustration of the evolution mechanism of Co3O4 nanocubes through stepwise oriented attachment in the two-phase system.

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covered with oleic acid and transported to the toluene phase. The deformed metastable nanograins 3−5 nm in diameter grow through direct oriented attachment to each other in toluene. Certain-sized primary cubes ∼8 nm on a side are formed with the suitable coverage of oleic acid. The adsorption of the organic molecules suppresses the further growth of Co3O4 nanocubes and stabilizes the certain-sized cubic

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DOI: 10.1021/acs.langmuir.9b00342 Langmuir XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.langmuir.9b00342 Langmuir XXXX, XXX, XXX−XXX