Solution Phase Mass Synthesis of 2D Atomic Layer with Hexagonal

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Solution phase mass synthesis of 2D atomic layer with hexagonal boron network Tetsuya Kambe, Reina Hosono, Shotaro Imaoka, and Kimihisa Yamamoto J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b06110 • Publication Date (Web): 02 Aug 2019 Downloaded from pubs.acs.org on August 2, 2019

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Journal of the American Chemical Society

Solution phase mass synthesis of 2D atomic layer with hexagonal boron network Tetsuya Kambe1,2, Reina Hosono1, Shotaro Imaoka1, Akiyoshi Kuzume2, Kimihisa Yamamoto1,2* 1) Laboratory for Chemistry and Life Science, Tokyo Institute of Technology, Yokohama 226-8503, Japan 2) JST-ERATO, Tokyo Institute of Technology, Yokohama 226-8503, Japan

Supporting Information Placeholder ABSTRACT: Borophene and the analogs are attractive 2D-materials showing unique mechanical and electronic properties. In this study, the bottom-up synthesis of an atomic boron network possessing a completely planar skeleton was achieved from KBH4. The borophene-analog was stabilized by oxygen atoms positioned on the same plane, providing holes and the anionic state of the layer. Potassium cations between the layers enabled crystalline stacking of the layers, as well as dissolution in solvents as atomically thin layers. The conductivity measurements revealed the electronic feature. The activation energy of the in-plane conductivity suggested a metal-like behavior whereas that of the inter-plane showed a semiconducting nature.

The chemical synthesis of atomically thin two-dimensional materials from molecular compounds is attracting much interest.1-3 The archetypical two-dimensional material is graphene, where changing the shape and structures is interesting for providing different functionalities.4 The chemical synthesis of such two-dimensional structures enables expanding the variations of the materials and the components to introduce functionalities.5,6 Two-dimensional networks constructed via coordinating bonds or dynamic covalent bonds were reported. For example, atomically-thin coordination networks were investigated by the interfacial synthesis using metal and ligand solutions.7-9 This method enables introduction of functionalities derived from metal complexes into two-dimensional sheets. Covalent organic frameworks (COFs) also provide two-dimensional networks,6, 10-12 although the fabrication of networks with a single atom thickness like graphene are still difficult. It prevents the materials from exhibiting properties derived from delocalized electrons on the two-dimensional plane. This study is focused on borophene,13-15 which was recently reported as a two-dimensional boron network. While various two-dimensional atomic layered materials have been developed over the past decade,13-19 borophene is expected to have unique properties including an extraordinary mechanical property and metallic behavior.13,20 However, despite the attractive properties of borophene, it is difficult to identify applications of this material because it is not stable without a substrate.15 As a boron-rich layered material, MgB2 is famous. The MgB2 inherently possesses borophene-like hexagonal boron structure, however, it cannot be exfoliated by simple physical treatments. Recently, hydroxyl-functionalized boron nanosheets with borophanelike structure were reported through reaction with water.21,22 The

produced boron nanosheet is not atomically flat because of the addition of hydroxyl groups. According to borates, various layered materials were also reported,23-29 while they had 3D structure due to the coordination fashion of the boron units. On the other hand, atomically planer structure is available in COFs.30,31 They were generally synthesized by formation of the boroxine rings with various linkers of carbon aromatic rings. As above, atomically thin boron network was still difficult. In this work, we synthesized an inorganic atomic layer of oxidized borophene analog from solution system with B-O bonds in the atomic planes (Figure 1). The hexagonal network contains boron and bridging oxygen atoms. Such stabilization of borophene with incorporated oxygen had been theoretically expected.32 The resulting structure is quite different from that of a three-dimensional B2O3 network. Consequent borophene oxide layers demonstrated an anisotropic conductivity based on the two-dimensional structures.

Figure 1. 2D structure of borophene-oxide layers obtained from a single crystal X-ray structure analysis (green: boron, red: oxygen, purple: potassium). Occupancy of oxygen is 0.5. Boron atoms form hexagonal 2D network. The edge part is reflected in the occupancy of boron atoms. Detailed data is summarized in the crystallographic information file (CIF), methods and Table S1.

The borophene-oxide layers (BoL) were fabricated in a facile manner via the oxidation of KBH4 in air dissolved in organic solvents (method, Figure 2A). In short, a MeOH solution of KBH4 was

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added to an acetonitrile or 1:1 mixed solution of chloroform and acetonitrile in an Ar-filled glove box. The solution was heated for 1 hour at 40 °C in air, then it was allowed to stand for several days. During this process, crystals of the BoL grew with partial oxidation, and colorless crystals of BoL having a prismatic column shape were observed (Figures 2B and C). The BoL crystal was also obtained from acetonitrile solution of KBH4, while it was not formed in other solutions including toluene, THF, chloroform, dioxane and ethanol. In addition, the aspect ratio of the BoL crystal was varied by the cooling rate. Short crystals were obtained by rapid cooling from 40 °C of the heated solution (Fig. S1). The size of the cations also affected the formation of the crystal. LiBH4 or NaBH4 with smaller cations were not applicable for this synthesis. A single crystal structure analysis revealed a structure consisting of stacked monoatomic boron layers with the composition of B4.27KO3 (Figure 2D, Table S1 and supporting crystal file). Hydrogen atoms can be involved in the crystal, while strict position of the hydrogen atoms are not determined in this crystal structure analysis. Each boron layer had a hexagonal network structure (Figure S2A) in which the average distance between the boron atoms (1.78 Å, Figure S2B) was comparable to that in borophene.33 These layers contained pores the edge of which contains bridged oxygen atoms, where potassium cations could be sandwiched between the layers. As a result, layers of boron networks and potassium cations were positioned in an alternating manner throughout the crystal (Figure 2D). Direction-controlled X-ray diffraction (XRD) using a plane index demonstrated that the c-axis direction of the crystal structure matched the long axis of the columnar crystals (Figures S2C-F). XRD measurement by irradiation in a vertical fashion to the columnar crystal gave diffraction peaks without those containing the caxis factor (Figure S2F). Such layered crystal suggested the weak interaction between layers compared to the in-plane covalent network, which was confirmed by the coefficient value of thermal expansion along each axis. The diffraction peak of (001) clearly shifted to the lower angle whereas that of (100) was almost same (Figure 2E). The temperature dependency of XRD peaks revealed each coefficient values (c-axis: 1.2 × 10−4 K−1, a or b-axis: 3.5 × 10−5 K−1) (Figure 2F, method), suggesting the significant anisotropic properties of this crystal. In addition, the coefficient value along the c-axis was much higher than that of other layered materials such as graphite or MgB2.34

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Figure 2. Synthesis and structure of BoL crystal. (A) Synthesis of the BoL crystals in solution. The blue arrows show the as-synthesized BoL crystals. The photographs of (A) were samples synthesized only from an acetonitrile solution (see method). (B,C) Optical microscopy (B) and SEM (C) images of the BoL crystal after cutting. (D) Side view of the crystal structure of BoL (green: boron, red: oxygen, purple: potassium). (E) Temperature dependency of XRD measurement. Red lines show (100) (left) and (001) (right), respectively. Peaks of * were derived from the sample stage. (F) Temperature dependency of the thermal expansion along c-axis of the BoL crystal and graphite.

Fourier transform infrared (FT-IR) and Raman spectra of this material were found to contain B-O in-phase stretching at 800-900 cm-1, out-of-phase stretching at 1350-1450 cm-1, along with a deformation at 510-520 cm-1, indicating the presence of the B-O bond (Figure S3A).35 In addition, Raman spectrum showed several peaks at lower frequency range 100-800 cm−1 indicating the involvement of B-B bond in borophene-like structure. 20d,36 X-ray photoelectron spectroscopy (XPS) data acquired from the BoL exhibited three peaks corresponding to two boron types in the network and one at the edges. Based on the binding energies of KBH4 and B2O3, the boron atoms in the BoL were evidently partially oxidized compared to those in KBH4 (Figures S3B and C). The layered crystal of BoL was easily split by the application of physical pressure to the thin layers (Figure 3A). Scanning electron microscopy (SEM) was used to elucidate the layered structure of BoL crystal. The ordered lines were observed at the side surface of the crystal (Figure 3B). The physical pressure to the crystal provided thin sheets exfoliated along the layers. The thin-sheet form was clearly observed by SEM measurement (Figure 3C). TEM images of the mechanically exfoliated sample confirmed the thin film morphology (Figure 3D) forming sheets with sizes greater than several hundred nanometers. Small flakes were also observed on the thin film. They could be generated by the exfoliation process, or derived from small domains existing in the crystals as defects. Selected area diffractions of the thin-layer suggested the hexagonal structure (inset of Figure 3D).


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Figure 3. Physical exfoliation of BoL. (A) Schematic illustration of procedure. SEM images demonstrating layered crystal (B) and exfoliated thin (C) structures. (D) A TEM image of the physically exfoliated thin sheets. The inset shows selected area diffraction pattern.

Atomically-thin sheets of BoL were obtained by dissolving crystals in dimethyl formamide (DMF) (Figure 4A). Solubility of BoL was summarized in the Table S2. The crystal of BoL was dissolved in DMF within several minutes (Figure 4B). The dissolution without decomposition was confirmed by UV-vis absorption spectroscopy. The peaks at 1400-1500 nm observed in the crystal sample kept the positions in the absorption spectrum of the DMF solution (Figure 4C). The dissolution process was also confirmed by recrystallization from the solution. The crystalline phase of the solid obtained by drying of the DMF solution was revealed by polarized optical microscopy (Figure S4A). The SEM images showed the crystalline and layered morphology (Figures S4B and C). In addition, the powder XRD revealed the same crystalline patterns to the initial crystal although some other peaks derived from different crystalline phase were also observed (Figure S4D). The DMF solution of BoL was drop-casted on substrates to prepare the atomically thin sheets (Figure 4D). Retention of the structure was confirmed by IR spectroscopy. The IR spectrum of the drop-cast sample on KBr pellet kept the similar vibration peaks with that of the BoL crystal (Figure 4E). TEM observation of the thin sample revealed the sheet form with a hexagonal pattern from the selected area electron diffraction (Figure 4F and the inset). The drop-cast sample on HOPG was found to consist of a flat and thin sheet with a height about 2 nm (Figure 4G). This value is much larger than the considerable value as a single layer, therefore double layer or attachment of solvent on the layer or potassium cations were conceivable. Single layered sheet was observed by spin-coat sample. The thickness was found to be about 0.6 nm (Figure S5). This value is reasonable considering the distance between layers of the BoL crystal.37 The observed domain sizes prepared by DMF dissolution looked smaller than that by physical pressure (Fig. 3C). It suggested that splitting was initiated from the defect of the boron sheet.

Figure 4. Atomic layers of BoL. (A,B) Illustration and photographs showing dissolution of BoL to DMF. (C) Diffuse reflectance absorption spectra of BoL crystal (up) and absorption spectra of DMF solution (down). (D) Preparation scheme of atomically-thin sheets on a substrate. (E) IR spectra of BoL crystal (blue) and drop-casted sheets on KBr substrate (orange). The latter drop-casted sample was measured by ATR method. (F) An STEM image of the thin sheets casted on TEM-grid. Inset shows a hexagonal pattern from selected area electron diffraction (SAED). (G) An AFM image of the thin sheets on an HOPG prepared from a DMF solution.

The synthesized BoL showed an anisotropic electronic property, as clearly observed in the conductance measurement (Figure 5). It is important to note that there are few conductivity values reported for other boron based crystals. The measurements were carried out by an impedance method using comb electrodes both along the inplane and inter-plane directions (Figure S6). The resistances were obtained from curve-fittings of the observed Nyquist-plots considering the equivalent circuits using a constant-phase element (Figures S7-S9).38 The observed Nyquist-plots shows almost perfect circle suggests that the carrier is electron. The activation energies were obtained from the temperature dependency of the conductance which was calculated from the resistance. Those activation energies were totally different from each other (Figures 5B-E). Whereas the inter-plane conductivity had an activation energy of 0.2 eV, the in-plane conductivity had almost zero activation energy like metals. The former is considered to be derived from the electron hopping mechanism through the separated layers by potassium cations. In contrast, the metal-like conductivity behavior suggested the formation of hexagonal boron networks like borophene.



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manuscript. All authors have given approval to the final version of the manuscript. K.Y. supervised the project through discussing with T.K.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT The authors wish to thank A. Watanabe and M. Morita for their assistance during the experimental work and H. Sato at Rigaku for performing XRD analyses. This study was supported in part by JST ERATO grant (no. JEMJER1503), JSPS KAKENHI grants (nos. 17K05804 and 15H05757), and the Cooperative Research Program of “Network Joint Research Center for Materials and Devices.” The authors are also grateful to the Suzukakedai Materials Analysis Division, Technical Department, at the Tokyo Institute of Technology. XPS analyses were conducted at the Advanced Characterization Nanotechnology Platform of the University of Tokyo, supported by the Nanotechnology Platform of MEXT, Japan.

REFERENCES

Figure 5. Conductivity of stacked BoL crystal. (A) Measurement directions of the BoL structure. (B,C) Nyquist-plots of the impedance measurements at several temperatures along (B) inter-plane and (C) in-plane directions. (D,E) Temperature dependency of the observed conductivity. The observed activation energies were about 0.2 (D, inter-plane) and 0 (E, in-plane) eV, respectively.

In conclusion, we demonstrated the bottom-up synthesis of crystalline-stacked boron atomic layers from solution process. The structure was totally revealed by various measurements including a single crystal analysis. The weak interaction between layers enabled exfoliation into thin or atomically-thin layers by physical pressure or dissolution in the solvent without decomposition. The anisotropic physical and electronic properties of the BoL crystal were revealed by the thermal expansion and conductivity. A semiconducting property was observed in the inter-plane direction, while a metal-like behavior was suggested in the in-plane boron network.

ASSOCIATED CONTENT Supporting Information Materials and methods, synthetic conditions, detailed crystal data, IR, Raman, XPS, details about re-crystallization from solution, AFM, set-up and analysis with curve-fitting of conductivity measurements are included in the supporting information. This material is available free of charge via the internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected]

Author Contributions T.K., R.H., A.K. and S.I. carried out experiments. T.K., R.H., S.I. and K.Y. discussed to design the research. T.K. and K.Y. wrote the

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Solution Phase Mass Synthesis of 2D Atomic Layer with Hexagonal

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