Formation Mechanism of Boron-Based Nanosheet through the

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Article

Formation Mechanism of Boron-Based Nanosheet Through the Reaction of MgB with Water 2

Hiroaki Nishino, Takeshi Fujita, Akiyasu Yamamoto, Tomohiro Fujimori, Asahi Fujino, Shin-ichi Ito, Junji Nakamura, Hideo Hosono, and Takahiro Kondo J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 02 May 2017 Downloaded from http://pubs.acs.org on May 7, 2017

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The Journal of Physical Chemistry

Formation Mechanism of Boron-Based Nanosheet through the Reaction of MgB2 with Water

Hiroaki Nishino1, Takeshi Fujita2, Akiyasu Yamamoto3,4, Tomohiro Fujimori5, Asahi Fujino5, Shin-ichi Ito6, Junji Nakamura7,8, Hideo Hosono4,9, and Takahiro Kondo*4,7,8

1

Institute of Materials Science, Graduate school of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8573, Japan

2

Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

3

Institute of Engineering, Tokyo University of Agriculture and Technology, Tokyo, 183-8538, Japan

4

Materials Research Center for Element Strategy, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan

5

College of engineering sciences, University of Tsukuba, Tsukuba 305-8573, Japan

6

Technical Service Office for Pure and Applied Sciences, Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8573, Japan

7

Division of Materials Science, Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8573, Japan

8

Tsukuba Research Center for Interdisciplinary Materials Science, and Center for Integrated Research in Fundamental Science and Engineering, University of Tsukuba, Tsukuba 305-8571, Japan

9

Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan

*Correspondence to: [email protected]

1

ACS Paragon Plus Environment


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Abstract A recent experiment demonstrated that ultrasonication of MgB2 in water yields Mg-deficient hydroxyl-functionalized boron nanosheets at room temperature. Herein, we examined the mechanism of nanosheet formation. Analysis of the reaction products and temporal variation in pH and H2 production shows that the reaction between MgB2 and water comprises two steps: i) an ion-exchange process between protons and a part of Mg cations of MgB2 with its exfoliation and ii) the hydrolysis reaction between Mg-deficient boron hydride and water to produce H2 and Mg-deficient hydroxyl functionalized boron sheets. The sheets with a stacking periodicity of 0.70 nm, were obtained as the supernatant of the reaction product of water with MgB2. The stacking sheets can be further exfoliated if the reaction is conducted under ultrasonication. The derived nanosheets are composed of sp2-bonded boron framework and possess a disordered structure containing hydroxyl species and oxidized magnesium.

2


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1. Introduction A two-dimensional (2D) boron sheet (borophene) was recently synthesized by the deposition of boron atoms on a Ag(111) surface following a bottom-up synthesis method. 1, 2 However, a top-down method for the mass production of borophene, such as a liquid exfoliation method,3 has not been established. Considering the application of borophene or borophene-related 2D sheets in a wide variety of fields, such as electronic devices, batteries, catalysts, and hydrogen storage materials as predicted by the theoretical calculations,4, 5 it is essential to develop both top-down and bottom-up methods for its production. We have focused on magnesium diboride (MgB2), a binary compound composed of hexagonal boron sheets alternating with Mg cations, as the parent material in the top-down approach for borophene or borophene-related 2D sheets. Since MgB2 inherently contains 2D boron sheets in the material, it is of interest to determine whether borophene could be formed simply by exfoliation and deintercalation of Mg in a top-down approach. According to the recent report, however, ultrasonication of water with MgB2 at room temperature produces not pure boron sheets but Mg-deficient hydroxyl-functionalized boron nanosheets.6

The

presence of Mg and hydroxyl species in nanosheets is probably caused by the instability of charged boron sheets in water derived from MgB2 by the exfoliation. We thus consider that the designed ion-exchange method between Mg cations and other cations is the way to produce borophene-related stable new type of 2D materials in a top-down approach. From this point of view, the Mg-deficient hydroxyl-functionalized boron nanosheets can be classified as one of the stabilized borophene-related 2D sheets as a reaction product of MgB2 and water. It can be obtained simply by the mixture of MgB2 with water at room temperature under the ultrasonication and shows an intriguing optical property.6 However, the reaction mechanism driving the formation of the nanosheets including the role of the ultrasonication is yet to be analyzed. Also, the presence of hydroxyl species in the sheets 3



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cannot be simply explained by the ion-exchange process. For the further application of the ion-exchange and liquid exfoliation methods to a metal diboride as the production method of several types of borophene-related 2D sheets, a deeper understanding of the formation mechanism of nanosheets on reaction with water is fundamental and essential. Therefore, we examined the mechanism of formation of nanosheets during a reaction between water and MgB2.

2. Methods MgB2 powder (99%, 100 mesh size, rare metallic,) was mixed with distilled water at room temperature, with and without ultrasonication. The volume of gas generated during the reaction was measured by collecting it over water. The collected gas was periodically sampled and analyzed using a gas chromatograph (GC-8A, Shimadzu Corporation, Ltd, Kyoto, Japan), equipped with Molecular Sieves 5A (60-80 mesh, GL Sciences, Inc., USA) and Porapak Q (50-80 mesh, Waters Chromatography Ireland Ltd, Ireland) column. Independently, the variation in pH of the aqueous solution was measured using a pH meter (HI 2002-01, HANNA instruments Japan Ltd., Japan). After standing for more than 5 days, the supernatant was dried on a hot plate (RSH-1DN, As One Corporation. Ltd., Japan) at 95 °C. The precipitate and dried supernatant were analyzed by transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), electron energy loss spectroscopy (EELS), energy dispersive X-ray spectroscopy (EDS), electron diffraction, scanning electron microscopy (SEM), Fourier transform infrared absorption spectroscopy (FTIR), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). TEM, STEM, EELS, EDS, and electron diffraction measurements were performed at room temperature using a JEM-2100F TEM/STEM (JEOL, Ltd., Japan) with double spherical aberration (Cs) correctors (CEOS GmbH, Heidelberg, Germany). High-contrast images with 4


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a point-to-point resolution of 1.4 Å were obtained. The lens aberrations were optimized by evaluating the Zemlin tableau of amorphous carbon as a control. The residual spherical aberration was almost zero (Cs = −0.8±1.2 µm with 95% certainty). The acceleration voltage was set to 120 kV, which is the lowest voltage effective with the Cs correctors in this system. SEM measurements were performed on a JSM-521 scanning electron microscope (JEOL, Ltd., Japan) operating at 10 kV. Samples were mounted on Cu or Mo TEM sample holders. XRD measurements were performed at room temperature using a two circle diffractometer (PW 3050 Philips X’pert Pro, PANalytical, Almelo, the Netherlands) with Cu Kα radiation (1.5418 Å). X-rays were generated using the line focus principle. A reflection-free Si plate was used as the sample stage. Cu Kα radiation was obtained by reflection from a singly bent highly oriented pyrolytic graphite crystal. The diffraction pattern was recorded using a solid-state detector (X’Celerator, PANalytical) at a scan speed of 0.05° 2θ/s up to 90°. The FTIR spectrum was measured at room temperature using a FT/IR-300 spectrophotometer (JASCO Analytical Instruments, Japan). The sample was analyzed as a KBr pellet. The background signal was subtracted using Spectra Manager Software (JASCO Analytical Instruments, Japan). XPS measurements were performed at room temperature using a JPS 9010 TR (JEOL, Ltd., Japan) with an ultra-high vacuum chamber and an Al Kα X-ray source (1486.6 eV). The pass energy was 10 eV, the energy resolution (estimated from the Ag 3d5/2 peak width of a clean Ag sample) was 0.635 eV, and the uncertainty in the binding energy was ± 0.05 eV. The sample was mounted on the sample holder using a graphite tape with a metal contact and introduced into the ultrahigh vacuum chamber for measurement. The Shirley background was subtracted from the spectrum using SpecSurf software (version 1.8.3.7, JEOL, Ltd., Japan). Charge-up in the sample causes higher binding energy shifts for these spectra. Thus, we 5



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calibrated the charge up amount assuming the B1s binding energy for MgB2 to be 188.2 eV .7,8

3. Results and discussion Reaction process Mixing MgB2 with distilled water at room temperature results in gas production and a black turbid suspension of water (Fig. 1a). The produced gas was identified as H2, which was independently analyzed during the reaction by gas chromatography. As shown in Fig. 1b, H2 production saturated at about 6,000 min with a total H2 production of about 150 ml, when 255 mg of MgB2 in 255 ml water was used in a reaction without ultrasonication. The saturation amount of H2 (6.25 mmol) corresponds to about 1.1 times excess compared with Mg atoms in the starting material (5.55 mmol). These results indicate that H2 was produced as the major reaction product between MgB2 and water. After attaining saturation with respect to H2 production, the water in the reaction medium containing MgB2 becomes relatively transparent with the formation of a precipitate (Fig. 1a), suggesting that the reaction reaches an equilibrium and the reaction products are separated as hydrogen gas, precipitate, and a colloid suspended in the solution. As an evidence indicating the presence of colloid in the solution, a distinct Tyndall effect, confirming the formation of the colloid, was observed in the aqueous solution with dried supernatant, as highlighted by the arrows in Fig. 1c. The incident green laser was scattered in the solution, and its intensity increases with an increase in the density of the dried supernatant in water. The time required for the saturation of H2 production becomes much shorter when the reaction was carried out under ultrasonication, as shown in Fig. 1b. The total amount of H2 produced is not significantly different (though slightly larger) compared with the reaction without ultrasonication. The total weights of precipitate and dried supernatant are comparable 6


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between the reaction conditions (with and without ultrasonication) as shown in Table 1. It can be concluded that ultrasonication promotes the reaction probably by increasing the frequency factor for the reaction, while the fundamental chemical reaction between MgB2 and water is essentially the same with and without ultrasonication. Figure 1d shows the temporal variation in the pH of water after the mixing of MgB2 without ultrasonication. A significant increase in pH from 6.0 to 10.5 was observed within about 1.5 min. This is in sharp contrast to temporal variation in H2 production, that is, the production of H2 is almost zero at 1.5 min even with ultrasonication as shown in Fig. 1d. This difference indicates that the chemical reaction between MgB2 and water, including H2 production, is not a single process but comprises multiple steps. The pH variation can be explained by the chemical reaction Mg(OH)2 → Mg2+ +2OH− in water, taking into account the solubility of Mg(OH)2 (9 mg/L).9 That is, the pH of water containing a saturation amount of soluble Mg(OH)2 is estimated to be 10.49, which is almost identical to the pH at the saturation condition in Fig. 1d (pH = 10.5). The pH variation indicates that Mg(OH)2 is formed

by

the

reaction

between

MgB2

and

water.

To

verify

this

reaction,

ethylenediaminetetraacetic acid (EDTA 2Na) chelator was added to the solution at about 8,400 min. As shown in Fig. 1b, an increase in H2 production and a decrease in pH to 5.55 were observed, indicating that Mg cations are captured by EDTA and protons are released so that Mg(OH)2 completely dissolves in the solution. The amount of OH− released at the moment when pH reaches 10.5 in Fig. 1d (1.5 min) can be estimated to be 0.077 mmol in 255 ml of water. The corresponding amount (0.077 mmol) of H2 (1.85 ml) was not produced at 1.5 min in Fig. 1d. These results quantitatively indicate that the chemical reaction between MgB2 and water, including H2 production, is composed of multiple steps. Based on these observations, the essential chemical reactions between MgB2 and water can be described as follows: 7



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MgB2 + 2H2O → 2BH + Mg2+ + 2OH−

(i)

2BH + 2H2O → 2B(OH) + 2H2 ↑

(ii)

Here, both BH and B(OH) are assumed to form nanosheets. The reaction (i) is the ion-exchange between proton and Mg cation forming boron hydride nanosheet. Subsequently, boron hydride nanosheet may react with water, as shown in reaction (ii), to produce H2, as in the case of diborane hydrolysis.10 Hydroxyl functionalized boron is then expected to be formed, which forms a colloid in aqueous solution exhibiting a Tyndall effect (Fig. 1c). The weights of dried supernatant and precipitate obtained from 1.0 g of MgB2 and 500 ml of distilled water at different conditions are shown in Table 1. We used three different types of MgB2 (m-MgB2: commercial MgB2, c-MgB2: synthesized using crystalline boron, and a-MgB2: synthesized using amorphous boron, details in Supporting information and Fig. S1) with or without ultrasonication. In every case, the total weights are slightly larger, but close to the calculated weight of 2.48 g assuming the reaction products as Mg(OH)2 + 2B(OH). The major reactions for MgB2 and H2O can thus be concluded as reactions (i) and (ii). This conclusion is consistent with the results from the SEM, TEM, FTIR and XRD analyses of precipitate and dried supernatant, which will be described later. The slight difference in weight is probably caused by the different process including in the reaction between MgB2 and H2O. Indeed, the amount of detected H2 produced by the reaction (Fig. 1b) is about 56-60% compared to the expected H2 amount by the reactions (i) and (ii).

Formation of nanosheets Sheets with 100 µm size were observed in SEM and TEM images of the dried supernatant (Figs. 2a and 2b). Observed sheets contain clear bent sections, indicating the flexible nature of the sheets. The sheet thickness can be roughly estimated to be less than 10 nm based on the cross-section of the sheet indicated by arrows in Fig. 2b. According to the 8


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electron diffraction measurement, no ordered structure was observed (Fig. 2b). Conversely, two distinct boron peaks were detected in EELS spectrum as shown in Fig. 2c together with a small oxygen peak. Two boron peaks at 188.4 and 195.3 eV can be assigned as EELS peaks originating from the K-shell excitation from B1s to π* and B1s to σ*.11, 12 The sheets are thus suggested to be composed of a sp2–bonded boron framework with a disordered structure. The sp2–bonded boron framework is consistent with the hexagonal structure of boron in the starting material, MgB2. EDS shows the presence of magnesium and oxygen (Fig. 2d). Presence of Mg and O is probably the cause for the disordered structure of the sheet by bonding with boron randomly (possibly by forming sp3–like bonding). Indeed, the observed species (B, Mg, and O) are found to be uniformly distributed in the sheet by STEM, as seen from the elemental mapping shown in Fig. 2e. The elemental mapping is constructed by plotting the EELS intensities for B and O (B peak area at 189–214 eV and O: peak area at 534–560 eV) and EDS intensity for Mg (peak intensity at 1.25 keV). Clear diffraction peaks were observed in the XRD pattern of the dried supernatant without ultrasonication as shown in Fig. 3a, which is in contrast to the electron diffraction measurement (Fig. 2b). The observed distinct XRD peak at 12.6° (0.702 nm) and broad, weak peaks at 36.7° (0.245 nm), 59.3° (0.156 nm), 60.8° (0.152 nm), and 61.2° (0.151 nm) are different from peaks observed in case of MgB2. These peaks were reproducibly observed even for samples derived from different types of MgB2 (Fig. S2). Among them, the distinct peak at 12.6° probably originates from the stacking periodicity of the sheets, since such an intense single peak is known to appear in two-dimensional layered materials such as graphite-intercalated compounds. However, the origin of other peaks is not clear, and it is possible that they could arise either from sheet or from nanoparticles on the sheet. The possible nanoparticles on the sheet are Mg(OH)2 particles because the peaks at 36.7° and 59.3° are not different from the (101) and (2-10) peaks of Mg(OH)2 (see standard spectrum of 9



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Mg(OH)2 shown in Fig. S3), and we occasionally observed nanoparticles on the sheet in TEM images (Fig. S4). Conversely, no XRD diffraction peaks were observed for the dried supernatant when the reaction was carried out under ultrasonication as shown in Fig. 3a, suggesting that ultrasonication induces perfect exfoliation of the stacking sheets, so that the flexible monolayer structure causes the absence of the diffraction peaks, as in the case of XRD for the flexible single layer graphene.13 Significant exfoliation of the sheet due to ultrasonication is consistent with a previous report on Mg-deficient hydroxyl-functionalized boron nanosheets manufactured using ultrasonication, where formation of a two-dimensional nanosheet with a thickness of a few layers was reported.6 In our current work, we observed an intermediate state between MgB2 and such hydroxyl-functionalized boron nanosheets as layered sheets with a stacking periodicity of 0.70 nm, as shown in Fig. 3a. The intermediate layered sheets identified in this work contain hydroxyl species as observed by FTIR (Fig. 3b), which is consistent with the above our proposed reaction (ii). In the FTIR spectrum, clear BH stretching vibrational modes were also observed as absorptions at 2488 and 1640 cm-1,14 which is probably due to the unreacted BH in the above our proposed reaction (ii). The latter IR peak corresponds to BH stretching vibrational mode originating from the three-center two-electron bonds of boron hydrides. We note here that while there are no indications of the formation of B(OH)3 in the current experimental condition, B(OH)3 formation was clearly identified in XRD patterns and FTIR spectra when there is a complete exchange of Mg ions with protons in water using acid.15 This is because of the repetitive hydrolysis of borane as in the case of diborane.10 In other words, the presence of uniformly distributed Mg in the sheet suppresses the hydrolysis of boron hydrides to form B(OH)3 but leads to the formation of the Mg-deficient, hydroxyl- functionalized boron nanosheets. Sample color and XRD patterns of the precipitate significantly depend on the differences 10


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in the starting material of MgB2 (m-MgB2, c-MgB2, and a-MgB2) in contrast to the dried supernatant (Figs. S2 and S3). In every case, however, the precipitate contained Mg(OH)2 as observed in the XRD patterns (Fig. S3). This is consistent with our proposed reaction (i). The origin of the difference in sample color and XRD patterns is due to the difference of the amount of impurities present in the starting material, such as amorphous boron and hexagonal boron nitride, as they precipitates rather than form colloids in the solution. In other words, impurities in the starting material do not affect the purity of the dried supernatant. Indeed, sample color and XRD of the dried supernatant has no dependence on the starting material (Fig. S2). XPS spectra for the dried supernatant prepared without ultrasonication and those for the starting material MgB2 are shown in Fig. 4. In both cases, boron, oxygen and magnesium signals are detected. This is consistent with EELS and EDS results (Figs. 2c and 2d). Here, we focus on B and Mg signals in XPS, since oxygen signals originate not only from the sample but also from the graphite tape supporting the sample. In case of the starting material (MgB2), the peak at the lower binding energy (188.2 eV) corresponds to the negatively charged boron species in MgB2, while the peak at the higher binding energy (193.2 eV) corresponds to the positively charged boron found in boron oxides such as B2O3 on the MgB2 surface as reported previously.8 Mg2p peak from MgB2 at 51.3 eV is in accordance with the value reported in literature, which is known to reflect Mg positive charge greater than 0 but less than +2.16,17 For the dried supernatant, B1s intensity becomes larger and Mg2p intensity becomes smaller compared to that of MgB2. Quantitatively, the atomic ratio of Mg and B can be estimated as 68.3 and 31.7 at% (Mg:B = 6.8:3.2) for MgB2 and 41.1 and 58.9 at% (Mg:B = 4.1:5.9) for the dried supernatant, based on the peak area taking into consideration the sensitivity factors (B1s: 2.1017 and Mg2p: 1.4025). The significant deviation of B/Mg ratio from actual stoichiometric ratio of 2 for MgB2 is probably due to the difference of the escape 11



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depth of photo electron between for B1s and that for Mg2p, which is not considered here. That is, the elemental ratio of Mg and B for the dried supernatant estimated by XPS may also different from actual ratio (i.e. the amount of B is likely significantly underestimated by XPS). In any case, the nanosheets obtained as dried supernatant of water with MgB2 are composed of boron framework as indicated by the presence of planar sp2 bonded boron (Fig. 2c) and possess non-negligible amount of Mg as forms of oxidized magnesium, Mg(OH)2 nanoparticles (Fig. S4), and/or unreacted MgB2 (as shown by soled inverse triangles in Fig. 3a) as well as OH species. Here we note that unreacted MgB2 were mostly found in the precipitate as indicated by XRD pattern (Fig. S3) while small amount of MgB2 signals were detected from the dried supernatant by XRD (Figs. 3a and S2) which is probably due to the presence of unreacted MgB2 left in the supernatant. We consider that the exfoliation process of MgB2 occurs accompanying with the above our proposed reaction (i). Indeed, the complete ion-exchange between Mg cations and protons, leading to the formation of boron hydride (borophane) exfoliated-nanosheets, can be achieved only by ion-exchange method without using water (i.e. the exfoliation can be realized only by conducting reaction (i) with suppressing the reaction (ii)). The complete ion-exchange between protons and Mg cations of MgB2 will be reported elsewhere with detailed structure characterizations about the product of boron hydride sheets.15

Summary We have investigated the mechanism of nanosheet formation by the reaction between MgB2 and water. Based on the characterization of the reaction products and variations in pH and H2 production with time after the mixing of MgB2 and water, the essential reaction is considered to be an ion exchange between protons and a part of Mg cations followed by the hydrolysis reaction between the Mg-deficient boron hydride sheets and water to produce H2 12


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and Mg-deficient hydroxyl functionalized boron sheets. The nanosheets with a stacking periodicity of 0.70 nm can be obtained by drying the supernatant from the reaction of the water with MgB2, wherein the stacking sheets can be further exfoliated if the reaction was performed under ultrasonication. The nanosheets were found to be composed of sp2-bonded boron framework and possess a disordered structure, while the hydroxyl-species and oxidized magnesium were uniformly distributed in the sheets.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc Materials, XRD patterns and TEM images of the sheets or precipitate derived from m-MgB2, c-MgB2, or a-MgB2.

Acknowledgments This work was supported by the PRESTO program of the Japan Science and Technology Agency (JST) and JSPS KAKENHI Grant Numbers JP26107504, JP16H00895, and JP16H03823. We thank Dr. M. Takeguchi and T. Taniguchi at NIMS, Mr. Y. Akasu, Mr. W. Ooki, Mr. S. Morohoshi and Prof. Y. Yamamoto at the University of Tsukuba and Dr. M. Yuki in Asahi Glass Co., Ltd. for their help and suggestions with these experiments. HH, AY, and TK were supported by MEXT Element Strategy Initiative to Form Core Research Center.

Author information: The authors declare no competing financial interests.

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New York, 1969.

(15) Nishino, H.; Fujita, T.; Cuong, N.T.; Tominaka, S.; Miyauchi, M.; Iimura, S.; Hirata, A.; Umezawa, N.; Okada, S.; Nishibori, E. et al. Boron Hydride Sheets Derived from MgB2 by Cation Exchange, submitted.

(16) A.Talapatra; Bandyopadhyay, S. K.; Sen, P.; Barat, P.; Mukherjee, S.; Mukherjee, M. X-Ray Photoelectron Spectroscopy Studies of MgB2 for Valence State of Mg. Phys. C Supercond. Its Appl. 2005, 419, 141–147.

(17) Kurmaev, E. Z.; Lyakhovskaya, I. I.; Kortus, J.; Moewes, A.; Miyata, N.; Demeter, M.; Neumann, M.; Yanagihara, M.; Watanabe, M.; Muranaka, T.; et al. Electronic Structure of MgB2 : X-Ray Emission and Absorption Studies. Phys. Rev. B 2002, 65.

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Figure 1 (a) Photographs of distilled water after the mixing of MgB2 at room temperature. Photographs of the dried supernatant and precipitate are also shown. (b) Variations in the amount of the H2 produced from water after the mixing of MgB2 with (red circles) or without (blue triangles) ultrasonication. In case of ultrasonication, ethylenediaminetetraacetic acid chelator was added to the solution at about 8,400 min. The plots were generated based on the amount of 255 mg of MgB2 and 255 ml of water (in the case of the plot with ultrasonication, 510 mg MgB2 and 510 ml of water were used, and thus the produced H2 amount was divided by two). (c) Tyndall effect in a solution containing different amounts (0, 1.6, 5.7, and 11 mg from the left) of the dried supernatant in 5.0 ml of water, when a green laser was introduced from the left. Due to the scattering at the ampule wall, only the region shown by arrows can be used to evaluate the presence of Tyndall effect. (d) Variation of pH of water with time after the mixing of MgB2 without ultrasonication (green open triangle). For the comparison, the results in panel b (H2 amount) are also plotted together. 16


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Figure 2 (a) SEM image, (b), TEM images and electron diffraction, (c) EELS spectrum, (d) EDS spectrum, and (e) element mapping (B: peak at 189-214 eV in EELS, Mg, peak at 1.25 keV in EDS, O: peak at 534-560 eV in EELS) of dried supernatant of MgB2 + H2O.

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Figure 3 (a) XRD patterns for MgB2, dried supernatant of MgB2 + H2O (without and with ultrasonication). (b) FTIR spectrum of dried supernatant of MgB2 + H2O without ultrasonication.

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Figure 4 XPS (a) survey spectrum, O1s, B1s, and Mg2p spectra for (b) MgB2 and (c) dried supernatant of MgB2 + H2O.

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Table 1 The weights of dried supernatant, precipitate and its summation (total) for different experimental lots with different conditions (starting material and with or without ultrasonication) about the reaction between MgB2 (1 g) and water (500 ml) at room temperature. The calculated weight for the total amount of assumed reaction Mg(OH)2 + 2B(OH) is also shown. Sample

Dried

Precipitate (g)

Total (g)

supernatant (g) m-MgB2 : 1 g

Lot 1

1.66

0.94

2.60

H2O : 500 ml

Lot 2

1.72

1.17

2.89

with ultrasonication

1.65

1.30

2.95

c-MgB2 : 1 g

Lot 1

1.50

0.94

2.44

H2O : 500 ml

Lot 2

1.53

1.01

2.54


1.55

0.85

2.40

a-MgB2 : 1 g

Lot 1

1.87

0.97

2.84

H2O : 500 ml

Lot 2

1.91

1.09

3.00


1.65

0.78

2.43

——

——

2.48

Calculated weight if the reaction products were Mg(OH)2 + 2B(OH)

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Formation Mechanism of Boron-Based Nanosheet through the

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