Direct Synthesis of CarbonâMolybdenum Carbide Nanosheet

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Direct Synthesis of Carbon-Molybdenum Carbide Nanosheet composites via a Pseudo-Topotactic Solid-State Reaction Katsutoshi Fukuda, Masahito Morita, Satoshi Toyoda, Akiyoshi Nakata, Koji Tanaka, Yoshiharu Uchimoto, and Eiichiro Matsubara Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b03095 • Publication Date (Web): 28 Nov 2016 Downloaded from http://pubs.acs.org on November 28, 2016

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Chemistry of Materials

Direct Synthesis of Carbon-Molybdenum Carbide Nanosheet composites via a Pseudo-Topotactic SolidState Reaction Katsutoshi Fukuda, † ,* Masahito Morita, † Satoshi Toyoda, ‡ Akiyoshi Nakata, † Koji Tanaka, § Yoshiharu Uchimoto,⊥ Eiichiro Matsubara†, ‡ †

Office of Society-Academia Collaboration for Innovation, Kyoto University, Gokasho, Uji, Kyoto 611-

0011, Japan ‡

Department of Materials Science and Engineering, Kyoto University, Yoshida-honmachi, Kyoto 606-

8501, Japan §

Materials Science Research Group, National Institute of Advanced Industrial Science and Technology,

1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan ⊥

Graduate School of Human and Environmental Studies, Kyoto University, Yoshida-nihonmatsu-cho,

Sakyo-ku, Kyoto 606-8501, Japan

RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to)

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ABSTRACT We report the solid-state reaction of MoO2 nanosheets, obtained from the soft-chemical delamination of Na0.9Mo2O4, into metallic Mo2C single layers that constitute a new family of versatile carbide nanosheets. This so-called pseudo-topotactic reaction, i.e. conversion from nanosheet to nanosheet, is aided by the use of cationic polymers as binders for the film growth based on electrostatic self-assembly. Compared to Mo2C in the bulk form, 2D anisotropic Mo2C sheets having a larger surface-area-tovolume ratio are of significant use in potential electrochemical applications and it is also worth noting that the thickness of Mo2C sheets can be controlled in the nanometer range by altering the stacking number of the precursor nanosheets.

INTRODUCTION Physical or chemical delamination of various layered compounds such as clays,1,2 sulfides,3-5 oxides,69

hydroxides,10,12 graphite13,14 and carbides,15,16 historically centers around the chemistry of two-

dimensional (2D) nanomaterials. Technologies used for the exfoliation of such compounds are based on the use of the anisotropy of chemical bonding, since the targeted stimulation of the weaker bonds in the starting three-dimensional (3D) materials isolates the more tightly bonded 2D slabs, which are known as “exfoliated nanosheets”. The resultant exfoliated nanosheets, which inherit their structure and composition from a part of the parent phase, often possess unique physicochemical properties that cannot be attained in other nanomaterials due to their nanometer-scale thickness and a lateral dimension in the bulk. One of the most famous examples of a useful exfoliated nanosheet is graphene, which is physically peeled from graphite using adhesive tapes;13 graphene has led to a boom in the production of 2D nanomaterials that are fundamental to science and industry. In contrast to the physical peeling, two chemical exfoliation methods have so far been proposed according to the nature of contact with the weak bonds mentioned above. One of the methods is based on acid/base exfoliation, in which alkali groups in layered compounds are substituted with bulky foreign species,6,7 and the other involves selective extraction based on the dissolution and corrosion of a vulnerable part of the 3D ACS Paragon Plus Environment

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compound.8,15,16 Irrespective of the chosen chemical exfoliation method, its success is governed by the choice of thermodynamically stable crystalline phase as a precursor, which has been a long-standing issue for 2D nanomaterials. In the last decade, the primitive structural conversion of exfoliated nanosheets has demonstrated significant potential in the design of 2D materials; for example, the exfoliated nanosheets of the TiO2 system in the monolayer state transformed into platy TiO2 anatase nano-crystallites upon heating to temperatures above 800˚C.17,18 Moreover, it is well known that the structural conversion of chalcogenide nanosheets such as MoS2 produces several polymorphs with different properties.19-22 In contrast to these structural conversions in binary systems, reduction of graphite oxide nanosheets into graphene without disruption of the sheet-structure can be depicted as a simplification of the material composition from a C-O binary system to a C mono-component system.14 More recently, we have succeeded in producing unusual metal nanosheets via a similar topotactic reduction of oxide nanosheets,23,24 indicating that a binary transition metal oxide system can be simplified in only transition metal elements. Thus, structural conversions using the exfoliated nanosheets are known to be mainly attained in the same composition system or a simpler composition system. Should the exfoliated nanosheets topotactically react with foreign species in a solid-state reaction, various derivative nanosheets would be created depending on the ingredient used. Also, Leonard’s group has recently reported that metal carbides can be readily synthesized using a diverse range of amines as a source of carbon via solid-state reactions with conventional metal oxides rather than solely carbon,25 and this pioneering work on organic/inorganic hybrid precursors has prompted us to further examine the phenomenon in atomically-thin film consisting of exfoliated molybdenum oxide nanosheets9 and amine-related organic polymers. Fortunately, electrostatic selfassembly using anionic nanosheets and cationic polymers yields a layer-by-layer structure in which the constituent atoms are well adjoined,26 and limiting the stacking number of the nanosheets/polymers pair on the substrate is expected to show diffusion-limitations in phase transformation, which is reported to


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inhibit undesirable aggregation.18 For these reasons it is useful to investigate whether or not a topotactic solid-state reaction can be induced in the nanosheet reactant system. In this study, we propose a direct synthetic route for Mo2C nanosheets based on a pseudo-topotactic reaction of electrostatic self-assembled monolayer pairs consisting of MoO2 nanosheets obtained from the delamination of Na0.9Mo2O49 and organic block polymers. Furthermore, we have focused on the metallic nature of the obtained Mo2C nanosheets as well as their potential electrochemical application.

EXPERIMENTAL SECTION Materials synthesis. A detailed procedure for preparation of the MoO2 nanosheets has been reported in the literature.9 Concisely, a layered molybdenum oxide (Na0.9Mo2O4) was synthesized by heating a mixture of Na2MoO4, MoO2, and Mo (2.55 : 1 : 0.85 in molar ratio) in a vacuum-encapsulated quarts glass tube at 750˚C for more than 36 hours. The produced black powder was then washed with ultrapure water and treated with 1 mol dm-3 HCl three times to exchange Na+ for H+. The protonated form was added to tetrabutylammonium hydroxide (TBAOH) aqueous solution (1.46 × 10-3 mol dm-3), yielding a dark-green suspension of the monodispersed MoO2 nanosheets. Film fabrication. The substrate (e.g., Si wafer, SiO2 glass) was cleaned by dipping in a mixture of 12 mol dm-3 HCl and 24 mol dm-3 CH3OH (1 : 1 ratio by volume) solution followed by 18 mol dm-3 H2SO4 solution to obtain hydrophilic surfaces. This was followed by immersion in an aqueous solution, of 1 wt% cationic diblock copolymer, composed of ~ 14% polyvinylamine and ~ 86% polyvinylalcohol (PVA), for 10 min to pre-coat the surface. The substrate was then dipped in a colloidal suspension of negatively charged MoO2 nanosheets (0.3 g dm-3) for 10 min to assemble a monolayer film, in which the MoO2 nanosheets were adsorbed in random azimuth via electrostatic self-assembly onto the substrate surface. Multilayer films of the nanosheets/polymers pairs were then fabricated by repeating the aforementioned process up to ten times, and heat-treatments of these films were performed by increasing the temperature from that of the surrounding environment at a rate of 10˚C min-1 to set temperatures of 200, 300, 400, 500, 600 and 700˚C under gas-flow-controlled conditions (5% H2 and 95% Ar, 250 ml ACS Paragon Plus Environment

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min-1). After heating at the selected temperature for 1 hour, the samples were left to cool in a quartz tube furnace. Measurements and analysis. An amplitude-modulation atomic force microscope (AFM; Bruker Innova AFM) with a 10 nm diameter Si tip cantilever (42 N m-1) was used to study the morphological features of the MoO2 nanosheets on the Si substrate before and after heating. The in-plane X-ray diffraction (XRD) data for the as-grown film and its heated derivatives were acquired using a four-axis diffractometer equipped with a NaI scintillation counter in BL6C of the Photon Factory at the High Energy Accelerator Research Organization in Tsukuba, Japan. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) was employed to observe the nanosheet sample on a Si grid with a Si3N4 porous membrane, and was conducted using a “Titan Cube” microscope (FEI co.) with the correction of spherical aberration (Cs-Corrector) and Si drift detector for energy-dispersive Xray (EDX) analysis. The accelerating voltage applied for both the observation and EDX analysis was 300 kV. An Ulvac-Phi Quantera SXMTM X-ray photoelectron analyzer using a monochromatic Al Kα (1486.6 eV) radiation source was used for X-ray Photoelectron Spectroscopy (XPS) analysis, with a beam spot of ~ 100 µm being used for the measurements and yielding the total energy resolutions of 0.60.7 eV. The binding energies obtained via this process were calibrated using the Fermi edge of Au, and optical absorption spectra for the nanosheet films on the SiO2 substrate were recorded on a Hitachi UH5300 spectrophotometer. Electrochemical test. In order to investigate the potential of two types of Mo2C-related electrode, cyclic-voltammetry (CV) measurements were conducted using a potentiostat (Bio-Logic Science Instruments SP-300). Firstly, the 10-layered MoO2 nanosheet film, which was coated across the entire surface of a Cu current collector, was heated at 600˚C under 5% H2 to yield a metallic-colored film-type electrode. Assuming a chemical composition of “Mo2C + 3C” and 100% coverage of the Mo2C nanosheets following carbonization, the weight of the active materials at both sides of the rectangular electrode of 5 mm × 30 mm in dimensions was calculated as 15.3 µg. A composite reference electrode was subsequently prepared by pasting a slurry composed of standard Mo2C powder (10.02 mg), ACS Paragon Plus Environment

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acetylene black, and polyvinylidene fluoride (8 : 1 : 1, wt%) in N-methylpyrrolidone onto the current collector. Each electrode as a working electrode was then incorporated into an aluminum pouch-type cell with two metallic lithium foils as the counter and reference electrodes in addition to 1 M LiPF6 electrolyte in a mixed solvent composed of ethylene carbonate and ethyl methyl carbonate (3 : 7 in volume) under an inert atmosphere. Both cells were tightly sealed and then subjected to discharge/charge cycles over a potential range of 3.0 V to 0. 1 V vs. Li/Li+ at a scan rate of 1 mV sec-1.

RESULT AND DISCCUSION Topographical and structural changes occurring within MoO2 nanosheets deposited on a Si substrate before and after heat-treatment under a gas atmosphere (composed of 5% H2 and 95% Ar) were examined by in-plane XRD analyses and tapping-mode AFM (see Figure 1 and Supporting Information S1, respectively). As shown in Figure S1a, the as-deposited nanosheets are visualized as lamellar objects below 2 nm in thickness and sub-micrometer in lateral size in this study although it must be noted that this value lies close to the height resolution limit of the AFM instrument used. Most of the MoO2 nanosheets were adsorbed onto the substrate as monolayers despite the inevitability of some overlap and gaps between the nanosheets under the present fabrication process. The in-plane XRD pattern of MoO2 nanosheets (Figure 1a) is indexable to a 2D rectangular unit cell of b = 0.290(1) and c = 0.502(5) nm, which is fairly consistent with those reported in the literature.9 The XRD patterns and morphology remained fairly unchanged until the MoO2 nanosheets were heated to 300˚C, indicating the structural robustness of the MoO2 nanosheets. However, heating at 400˚C triggered peak-broadening and a change in the intensity ratios of reflections 11 and 20, and at 500˚C the 2D rectangular unit cell seemed to transform into a 2D hexagonal one (d = 0.2887(2) nm) due to the loss of reflections 12 and 03. This symmetry improvement may originate from the reduction of the MoO2 nanosheets induced by heating under the reduction atmosphere including H2 gas, a theory that is supported by the XPS spectra of Mo 3d (see Supporting Information S2). After heating above 600˚C, these XRD peaks distinctively shift to smaller d-1 values, yielding a larger 2D hexagonal unit cell of d = 0.3000(3) nm. This shift cannot simply ACS Paragon Plus Environment

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be attributed to deoxygenation as in the case of several exfoliated nanosheets, e.g. graphite oxides14 and ruthenium oxides23 due to the magnitude of the shift, for example the shift observed in this case is a stark contrast to the d-value decrease from 0.294 to 0.27 nm in the topotactic reduction of RuO2 nanosheets. Considering the d-value of ~ 0.3 nm in the heated film, the most likely phase is β-Mo2C (a = 0.2997 nm, c = 0.47270 nm in P63/mmc, JCPDS No. 075-6678), which will be discussed in further analyses. Surprisingly, lamellar objects are still observed in the sample heated to 600˚C (see Figure S1f). Coverage analysis based on the height histogram of Figures S1a and S1f indicates a significant decrease in coverage from ~ 95% to ~ 80% despite the expansion of the 2D lattice, which leads us to consider a change in the lateral density of Mo atoms. A simple estimation of the structural conversion from the MoO2 nanosheet (~ 0.073 nm2 per Mo atom) to the single Mo2C sheet (~ 0.039 nm2 per Mo atom) predicts shrinkage of approximately 50% in terms of the sheet area, and the contrast between the experimental and predicted values can be attributed to the decrease in the regions of overlap and undetectable cracks that are not accounted for in the coverage analysis of the sheets. The average thickness of the generated Mo2C sheets (~ 2 nm), acquired from the AFM image, seems to be much larger than c-axis of β-Mo2C, which indicates a formation of unexpected phase or a few layers of the Mo2C slab. Judging from the little change in the sheet area, the latter is hardly considered. This will be discussed later. Either way, these results imply that a pseudo-topotactic conversion from nanosheet to nanosheet occurs at 600˚C, and further heating to 700˚C makes the 10, 11, 20 reflections more sharp as well as generating a very weak peak associated with bcc-Mo (Figure 1g). The AFM image revealed that the overlapping region started to transform into larger nanocrystals that can therefore be identified as bcc-Mo. A monolayer film of the MoO2 nanosheets, directly self-assembled with cationic polymers on a highly stable Si3N4 membrane, was heated at 600˚C under a H2 atmosphere and the sample then studied using HAADF-STEM. This technique enabled us to focus on the element-weighted structural changes occurring on a single nanosheet. Figure 2a displays a HAADF-STEM image, where ~ 20 nm sized black spots represent holes in the membrane whilst the distinct white islands correspond to the newly formed ACS Paragon Plus Environment

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nanosheets. The EDX spectrum (Figure 2c) for the single sheet shows characteristic X-ray peaks of Mo L-edges and C K-edges in addition to the peaks representing Si-, N- and O K-edges, which are assignable to the used membrane having a partial oxide layer, or adsorption of water during exposure to air prior to the measurement. Although such an atomic-scale sheet presents a significant hurdle for quantitative EDX analysis, the faint but detectable X-ray that is characteristic of C K-edges indicates Mo2C formation. The heavy element-weighted atomic-scale STEM image shown in Figure 2b indicates a lateral order of Mo in the nanosheets. With the help of atomic modelling and based on the Mo symmetry inferred from the image, it was estimated that the nanosheets have a 2D hexagonal unit cell having a cell edge of ~ 0.3 nm, which is fairly consistent with that obtained from the in-plane XRD analysis. Fundamentally, this carbonization appears to have taken place over the whole area and the carbon sources for the reaction should be distributed on the substrate in the same level as the MoO2 nanosheets, particularly the polymer used as a binder for the film fabrication. We can therefore conclude that the reaction of the MoO2 nanosheets with the polymer yields highly anisotropic 2D Mo2C nanosheets. In similarity with the carbonization, the production of bulk β-Mo2C can be observed in the restacked materials in which the MoO2 nanosheets are stacked with TBA ions as gest species (see Supporting Information S3). The resultant bulk Mo2C shows undoubtedly metallic color, which prompted us to research into the optical and conductive properties of the Mo2C nanosheets. The as-grown film of the MoO2 nanosheets exhibits three peaks at 226, 411, and 780 nm in the ultra-violet and visible (UV-vis) ranges (see Supporting Information S4). Heating to 600˚C significantly modified the optical properties, i.e., absorption at wavelengths longer than those across the visible range intensifies as absorption in the UV range weakens, suggesting that the Mo2C nanosheet formed demonstrates metallic properties even in single layers. The valence-band spectrum (see Supporting Information S2) indicates distinct electron density at the Fermi level, which is also the case with the bulk Mo metal. Unlike the case with the insulating Ru metal nanosheets in single layer environments, 23,27 the Mo2C nanosheet having the wellestablished conductive properties promises to work as an elementary unit of conductive nanosheets. ACS Paragon Plus Environment

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More recently, Gotosi and coworkers shed light upon revisiting the selective extraction and corrosion of layered materials and succeeded in the synthesis of novel transition metal carbide materials called MXenes,15,16 where M refers to an early transition metal and X is C or N. Unlike in this case, the present bottom-up approach based on the structural conversion of the precursor nanosheets is advantageous in terms of the availability of atomically-thin carbides, which further strengthens this field, as well as the recently reported morphology-controlled syntheses of transition metal carbides28-30. Let us consider how the polymer works as a carbon source for the formation of the Mo2C nanosheets on the basis of XPS analysis for the light elements, C and O, as shown in Figure 3. The as-grown film shows at least three kinds of O 1s peaks; 532.8, 532.0 and 530.5 eV peaks, which can be assigned to the SiO2 substrate, the water molecules31 adsorbed on the nanosheets and polymers and the MoO2 nanosheets, respectively. Note that the peak at 532.8 eV may include a contribution from the PVArelated polymer.32 On the other hand, a broad peak centered at 285.8 eV for C 1s (Figure 3b-i) originates from the polymers used in this study. After heating to 500˚C, the C 1s peak significantly shifts to a lower energy due to the formation of sp2-like carbons (284.6 eV), which corresponds to the elimination of the water-derived O 1s at this temperature. At the carbonization temperature (more than 600˚C), the O 1s peak for the MoO2 nanosheets almost disappears and a new C 1s peak at 283.3 eV appears. Similar C 1s peak attributable to Mo-C bonding, more reduced carbon, can be seen in the recent report on softchemically-induced MXenes.33 At the same temperature, the Mo atoms in the sample were clearly reduced (see Figure S2). These observations suggest that the deoxygenation of the MoO2 nanosheets and the formation of Mo-C bonds occur almost concurrently, and this carbonization phenomenon can therefore be considered as a solid-state reaction between the MoO2 nanosheets and the sp2-like (or graphite-like) carbon layers. In other words, the formation of the sp2-like carbons at the temperature less than 500˚C is a key to triggering the carbonization, which emphasizes the importance of using the reducing atmosphere in this experiment. The remaining sp2-like carbons after the carbonization indicate a carbon-excess system (see Figure 3b-iv). Considering the conductive nature of the Mo2C product, the shapes of the peaks representing the C 1s for the sp2-like carbons and the Mo2C can be reproduced by ACS Paragon Plus Environment

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symmetric and asymmetric Voigt functions, respectively. Comparison of the integrated intensities of the two peaks gives an approximation of the C atomic ratio in the combination of ingredients, which is 1 : 3 in Mo2C : the sp2-like carbons (C). The excess carbon species may work as a passivation layer to make the produced C-Mo2C nanosheet composite stable to exposure to air, being compatible with the height of the C-Mo2C sheets, ~2 nm, in the AFM analysis. As a result of preventing the Mo2C sheets from undesired oxidation, the amount of impurity phase associated with Mo, about 3-4% (see Figure S2), can be negligible in this study. One of the most lucrative applications of Mo2C-C system may be a Li-ion battery (LIB) based on the reversible Li storage-release reaction according to the innovative research undertaken by Gao and coworkers,34 which demonstrated the huge potential application of the porous hybrid structure of Mo2C nanoparticles and carbons in LIB anodes as a result of smooth charge transfer and Li+ diffusion in the 3D architecture. To assess the electrochemical potential of the Mo2C nanosheets obtained in this study, we fabricated the 10-layered multilayers and found that heating of these yielded a metallic colored film consisting of 2D anisotropic Mo2C nanomaterials (Supporting Information S5 and S6). The Mo2C product remains similar in terms of morphology and has an exquisite c-axis orientation, indicating the successful conversion into a “thick nanosheet” composed of several layers of the Mo2C slab unit. Assuming that the remaining sp2-like carbons are almost graphite (C), the ideal chemical composition of Mo2C + 3C can be used to estimate the specific capacity. Figure 4 displays cyclic-voltammograms (CVs) for the obtained Mo2C/C film electrode and conventional Mo2C composite electrode. Note that the constant-current charge-discharge voltage profiles are displayed in Supporting Information S7. The Mo2C/C electrode exhibits a redox couple around 1.35/1.45 V and a specific capacity of more than 800 mAh g-1 in this potential range at a scan rate of 1 mV sec-1 after 5 discharge/charge cycles despite the distinctive capacity fading presumably because of unexpected separation of the composite from the current collector, and it must be noted that this specific capacity more than 10 times greater than the ~ 70 mAh g-1 capacity of the bulk Mo2C may stem from the condition peculiar to the ultrathin electrode system, i.e. little diffusion limitation. These CV features including peak, shape and capacity of the ACS Paragon Plus Environment

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nanosheet-derived electrode widely resemble those of the Mo2C/C porous material,34 implying that similar Li conversion or alloying (Mo2C + xLi → 2Mo + LixC) may occur. Furthermore, the remaining sp2-like carbons should also contribute to the huge capacity. Although the mechanism is still under debate, this attempt demonstrates significant potential for the use of C-Mo2C nanosheet composite even in nano-scaled thickness in Li storage media.

CONCLUSION In conclusion, we have demonstrated the reaction of exfoliated MoO2 nanosheets with organic polymers as binders in a self-assembled reaction, and hence the pseudo-topotactical conversion into Mo2C nanosheets. This is because the uniformly-adsorbed polymer on the Si substrate begins to graphitize in advance and consequently works as a homogeneous carbon source for the carbonization of the MoO2 nanosheet. This nano-scaled solid-state reaction of the exfoliated nanosheet reactants is potentially a bold and direct approach for designing various carbide nanomaterials. The number of possible combinations of various polymers and other nanosheets is in the myriad, and presents an opportunity to encounter unexplored nanosheet system in the future.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS publications website at DOI: [DOI number]: Experimental methods, cell design for CV measurements, XPS spectra, XRD patterns, UV-vis absorption spectra, layer-by-layer self-assembled film growth and its monitoring and Sam-ple picture. This material is available free of charge online at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: (K. F.) [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was partially supported by the Research and Development Initiative for Scientific Innovation of New Generation Batteries (RISING) and RISING II under the auspices of the New Energy and Industrial Technology Development Organization (NEDO), Japan. Synchrotron radiation analyses were performed at the Institute of Materials Structure Science in KEK (2014G701). Figure 2 was drawn using a 3D Visualization System for Crystal. Structures and Electron/Nuclear Densities, VENUS, (National Institute for Materials Science).35


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FIGURE CAPTIONS

Figure 1. In-plane XRD patterns for (a) as-grown film of the MoO2 nanosheet, and its heated films at: (b) 200, (c) 300, (d) 400, (e) 500, (f) 600, and (g) 700˚C for 1 hour under H2 flow. The asterisk refers to 110 of bcc-Mo.

Figure 2. HAADF-STEM images of the MoO2 nanosheet film following carbonization: (a) wide-rangeview and (b) atom level image including a top view of Mo2C slab unit. (c) EDX spectra acquired from the single nanosheet. The broken lines refer to a 2D hexagonal cell.

Figure 3. Core level spectra of (a) O 1s and (b) C 1s for the MoO2 nanosheets (i) before heating, and after heating at: (ii) 500, (iii) 600, and (iv) 700˚C under H2 flow. The broken lines represent the best fit curves.

Figure 4. Cyclic-voltammograms acquired at a scan rate of 1 mV sec-1 for: (a) the Mo2C nano-film electrode obtained from the 10-layered MoO2 nanosheet film, and (b) the conventional Mo2C composite electrode.


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Figure 1


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Figure 2


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Figure 3


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Figure 4


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Table of Contents artwork (TOC)


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Direct Synthesis of CarbonâMolybdenum Carbide Nanosheet

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