Enhanced Superhydrophobic Performance of BN-MoS2

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Enhanced superhydrophobic performance of BN-MoS2 heterostructure prepared via a rapid, one-pot supercritical fluid processing Pitchai Thangasamy, Thamodaran Partheeban, Subramanian Sudanthiramoorthy, and Marappan Sathish Langmuir, Just Accepted Manuscript • Publication Date (Web): 30 May 2017 Downloaded from http://pubs.acs.org on May 30, 2017

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Enhanced superhydrophobic performance of BN-MoS2 heterostructure prepared via a rapid, one-pot supercritical fluid processing Pitchai Thangasamy, Thamodaran Partheeban, Subramanian Sudanthiramoorthy and Marappan Sathish* Functional Materials Division, CSIR-Central Electrochemical Research Institute, Karaikudi-630 003, Tamil Nadu, India. E-mail address: [email protected]; [email protected]

KEYWORDS: BN-MoS2 heterostructure; supercritical fluid processing; exfoliation; nanoscale roughness; superhydrophobic performance ABSTRACT: Fabrication of highly crystalline BN-MoS2 heterostructure with >95% yield was demonstrated using one-pot supercritical fluid processing within 30 min. The existence of 20-50 layers of BN-MoS2 in the prepared heterostructure was confirmed by AFM analysis. The HR-TEM imaging and mapping analysis revealed the well melded BN and MoS2 nanosheets in the heterostructure. The drastic reduction in XRD line intensities corresponding to (002) plane and broadening of the peaks for BN system over MoS 2 indicated the effective exfoliation and lateral size reduction in BN nanosheets during SCF processing. Also, the exfoliated MoS 2 nanosheets are preferentially exposed rather than BN nanosheets consequently the MoS 2 nanosheets sturdily covered on BN nanosheets in the heterostructure. The exfoliated BN and MoS2 nanosheets with nanoscale roughness makes the surface highly hydrophobic in nature. As a results the BN-MoS2 heterostructure showed superior superhydrophobic performance with high water contact angle of 165.9°, which is much higher than the value reported in literature. semiconductor (1.8 eV), consists of tri-sublayer (S-Mo-S) where Mo atomic plane is sandwiched between two planes of Introduction sulfur atoms in a trigonal prismatic arrangement. It can be used for diverse applications such as anode in lithium ion batteries, Superhydrophobic coatings have gathered significant attention lubricant, photovoltaic, electrode coating in dye sensitized solar in recent days due to their unique properties such as water recells, biomolecule and gas sensors, memory devices and catapelling, self-cleaning, anti-icing and antifouling properties.1,2 lyst for the hydrogen evolution reaction.13,16–21 The combination Any materials with highly hydrophobic surface exhibit a water of two different thin 2D nanosheets, called as heterostructure contact angle greater than 150° and a small sliding angle of 5can be formed by vertical or horizontal stacking of atomically 10° results in the superhydrophobicity. 3,4 Thus, materials with thin 2D layered structures. By combining two or three different high hydrophobic in nature with nano/micro roughness is highly van der Waals heterostructures such as graphene/BN or warranted for superhydrophobic applications. Recently, graBN/MoS2 or MoS2/WS2, the chemical and mechanical properphene and its analogous have great impact in various fields for ties of individuals could be significantly tuned desirably. The realistic/potential applications owing to their fascinating suractivity of such a heterostructure could be enhanced by miniface properties. Interestingly, the physico-chemical and surface mizing the restacking behaviour of individual exfoliated layers. properties of 2D nanosheets vary with thickness and size. Thus, So far, limited attempts have been made to synthesis the differthere is a growing interest to synthesize the ultra-thin 2D ent van der Waals heterostructures and their potential applicananosheets based on either by top-down or bottom-up approach. tions in different fields have been demonstrated.22–26 Here, the To date, many attempts have been made to isolate the single/few preparation of BN-MoS2 heterostructure using a facile superlayer 2D nanosheets using various methods such as mechanical critical fluid (SCF) processing method was demonstrated. Reexfoliation, chemical exfoliation, and electrochemical exfoliacently, SCF processing has been considered as the promising tion.5–14 However, each methods has its own disadvantages like and effective route for the synthesis of energy storage/converpoor graphene quality, low yield, scalability, expensive, large sion materials with high yield in a short reaction time. The SCF defect surface etc. A simple, facile and efficient methodology behaves like liquid and diffuse like vapour when the temperais yet to be reported for controlled layer preparation of 2D ture and pressure of the solvent attains its critical state. Attempts nanosheets from their bulk materials with high yield. Notably, being made to exfoliate/disintegrate/functionalize the 2D inorthe extensive research activities have been executed on the graganic layered materials and synthesis of metal oxides/sulphides phene nanosheets and now the attention is being put forward in a short reaction time.27–31 Recently, we reported a simple, towards other layered materials such as boron nitride (BN) and rapid, one-pot route for generating surfactant free h-BN molybdenum disulphide (MoS2) due to their similar structural nanosheets from bulk h-BN using isopropanol-water mixture as integrity. Analogous to carbon, BN also exists in several forms, SCF in a short reaction time of 15 min.32 Also, we demonstrated among them hexagonal (h-BN) and cubic (c-BN) BN are quite the synthesis of MoS2 nanoscrolls instead of MoS2 nanosheets common. The h-BN nanosheets with graphene like structure is in DMF medium with a reaction time of 30 min.33 It is important called ‘white graphene’, an electrical insulator (> 6 eV). 15 The to note here that unlike BN nanosheets, the exfoliated MoS 2 structure contains covalently bonded boron and nitrogen atoms, nanosheets are found to be undergo scrolling to minimize their arranged in an alternating manner with sp2 bonding and it dissurface energy. Indeed, the produced BN nanosheets and MoS 2 plays some intriguing properties like high chemical inertness, ACS Paragon Plus Environment nanoscrolls are well suspended in the corresponding supernathermal stability and excellent resistance to oxidation or corrotant SCF solution. In this work, the heterostructure composed sion. On the other hand, single layer MoS2, a direct band gap

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of BN and MoS2 with high yield (> 95%) have been prepared in a short reaction time of 30 min. Remarkably, the obtained heterostructure exhibited an excellent water repelling tendency with high water contact angle of 165.9°, which is significantly higher than individually exfoliated BN (125°) and MoS 2 (124.8°) nanosheets. Owing to the disintegration of BN and MoS2 nanosheets and nano/micro-roughness causes BN-MoS2 heterostructure to engender superhydrophobicity. Contrarily, the bulk BN and MoS2 do not exhibit contact angle hysteresis when the droplets of water falls on it due to their smooth surface nature. It is worthy to mention here that there are no reports are available for superhydrophobicity applications for BN-MoS2 heterostructure. Very recently, Venkatesan et al., attempted to study the wettability of van der Waals heterostructures and they reported a very low water contact angle of 61.6° for BN/MoS2/SiO2/Si heterostructure synthesized by wet transfer method.34 To the best of our knowledge, the contact angle value obtained for BN-MoS2 heterostructure in the present investigation is much higher than the value reported in previous studies.

Page 2 of 9 Reflectance Spectra (DRS), FT-IR and Raman spectroscopic analysis.

Synthesis of BN-MoS2 heterostructure: In a typical BN-MoS2 heterostructure synthesis, the bulk BN (0.15 g) and bulk MoS2 (0.15 g) powders were dispersed in 25 mL DMF solvent and sonicated for 30 min. Then, the homogeneous solution was filled in the stainless steel reactor and subjected to SCF treatment at 400°C for 30 min in a pre-heated furnace. At the end of the reaction, the stainless steel reactor was removed and quenched in an ice cold water bath. The resulting SCF treated supernatant solution was then removed and the residue was washed with ethanol-water mixture for several times to completely remove the DMF molecules by centrifugation. Finally, the obtained product is dried at 60 °C in a vacuum oven. For the comparison study, bare exfoliated BN (E-BN) and MoS2 (E- MoS2) have also been prepared separately using the similar experimental conditions. Results and discussions: In Figure 1A, the XRD patterns of bulk BN and bulk MoS2 showed a strong diffraction lines at 2θ = 26.7 and 14.4° respectively due to (002) plane of hexagonal phase, which is in good agreement with ICDD card No: 01-073-2095 and 00-037-1492, respectively. The observed strong line indicates the existence of many layers that are stacked/oriented predominantly along the z-axis direction in bulk BN and MoS2. After the SCF treatment, the intensity of (002) plane corresponding to BN-MoS2 heterostructure is considerably reduced with slight peak broadening compared to bulk BN and MoS2. In addition, the drastic reduction in the line intensities and broadening of the peaks observed for BN system is significantly higher than MoS 2 in the heterostructure. It is due to the effective exfoliation and lateral size reduction in BN under SCF condition. It may also due to the possibility of exposing more exfoliated MoS2 nanosheets rather than BN nanosheets i.e., MoS2 nanosheets may be sturdily covered on BN nanosheets in the heterostructure. It is worthy to note that high crystalline nature of heterostructure have been achieved with higher yield (> 95%) in a short reaction time using single step SCF exfoliation method. The existence of BN and MoS2 in the heterostructure is further confirmed by Diffuse

Figure 1. A) XRD patterns and B) FT-IR spectra of bulk BN, bulk MoS2 powder and BN-MoS2 heterostructure. Figure 1B shows representative FT-IR spectra of bulk BN, bulk MoS2 and BN-MoS2 heterostructure. For BN, two characteristic vibration bands are observed at 1375 and 810 cm-1 which can be assigned to in plane B-N stretching vibration (E1u mode) and out of plane B-N-B bending vibration mode (A2u mode), respectively.35,36 The existence of BN in the heterostructure is evident from the presence of these two modes in BN-MoS2 heterostructure. Similarly, the vibration peaks observed for the bulk MoS2 are also observed for the heterostructure as well. In addition, FT-IR spectra clearly indicate that the surface of BN and MoS2 nanosheets in the heterostructure were not oxidized under the SCF condition. The UV-Vis DRS spectra (Figure S1) of bulk BN showed a sharp absorption band edge at 215 nm (5.76 eV),37 ascribed to h-BN and the bulk MoS2 showed a broad absorption peaks between 600 and 700 nm. 38,39 Whereas the observed absorption peaks in BN-MoS2 confirms the heterostructure nature. In order to examine the exfoliation of 2D nanosheets during the SCF processing, Raman spectroscopic analysis were carried out (Figure 2). Raman spectra of bulk BN shows a high intense peak corresponding to E2g phonon mode at 1367.4 cm-1.40 While, the bulk MoS2 powder shows two discrete peaks at 383.4 and 409.1 cm-1 attributed to in-plane mode (1E2g) and out-of-plane mode (A1g), respectively.41 The extent of exfoliation in the BN-MoS2 heterostructure was confirmed by observing the peak shift of BN i.e., either blue or red shift and

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peak spacing values between 1E2g and A1g modes of MoS2 with respect to corresponding bulk materials.41 It is important to note that the monolayered BN nanosheets show blue shift while red shift was observed for the few layered BN nanosheets.42 On the other hand, when the MoS2 layers increases the frequency of 1 E2g mode decreases and A1g mode increases with respect to bulk MoS2. The observed red shift in BN42 and broadening of 1 E2g and A1g modes of MoS2 in the heterostructure with respect to their corresponding bulk materials confirm the existence of few layers in BN-MoS2 heterostructure. However, the red shift observed for both 1E2g and A1g modes of MoS2 in the heterostructure without any significant reduction in peak spacing values. This may be attributed to the strain introduced into these bonds and also the heterostructure consists of more than ten MoS2 layers. Consequently, the difference in peak spacing values between 1E2g and A1g modes as similar to bulk values.43–45 In addition, the few layers BN and MoS2 in the heterostructure was further confirmed by the substantial reduction in the above peak intensities.42

Figure 3. SEM images of (a) bulk BN, (b) bulk MoS2 powder and (c-d) BN-MoS2 heterostructure.

Figure 2. Raman spectra of (a) bulk BN, (b) bulk MoS2 powder and (c) BN-MoS2 heterostructure. To investigate the distribution of BN and MoS2 nanosheets in the heterostructure, electron microscopic analysis were carried out. From the SEM images of bulk BN, bulk MoS2 and BNMoS2 heterostructure, it is apparent that bulk BN and bulk MoS2 powder have the disk-like and sheet-like morphology with lateral dimensions ranging from 0.1-2 µm and 0.2-20 µm, respectively (Figure 3 & S2). The SEM images of nanosheets with reduced lateral size and thickness in the BN-MoS2 heterostructure clearly confirms the simultaneous exfoliation of BN and MoS2 from their bulk. The apparent distribution of BN and MoS2 in the heterostructure is indicated by blue and red circles, respectively. It is worthy to mention here that after SCF processing, both BN and MoS2 lateral size and thickness is reduced significantly compared to their respective starting materials.

TEM images of bulk BN, bulk MoS2 and BN-MoS2 heterostructure were shown in Figure 4 & S3A. It also affirms the uniform distribution of BN and MoS2 nanosheets with reduced lateral size and thickness in the heterostructure. The observed lateral size of bulk BN, bulk MoS2 and BN-MoS2 heterostructure are in good agreement with the SEM analysis. Moreover, TEM analysis reveal that the surface of bulk BN and MoS2 powder was smooth in nature that leads to wetting of surfaces when the water droplet was added on it. But, after the SCF processing the rough surface could be observed in both BN and MoS2 nanosheets in the heterostructure that are mainly responsible for non-wetting characteristics. It is worthy to note here that in our earlier observation, the SCF processing of MoS2 alone will results the MoS2 nanosheets with scrolled morphology due to high surface energy.46 However, such scrolling behavior was not observed in BN-MoS2 heterostructure, this may be attributed to the presence of BN nanosheets that hindered the scrolling or reduced the MoS2 surface energy. This clearly indicates that the BN and MoS2 in the heterostructure have some chemical interaction. In the BN-MoS2 heterostructure, the BN nanosheets shows good transparency compared to MoS2, this may be ascribed to persuasive exfoliation of BN than MoS2 under SCF condition which is well consistent with the XRD results. The acquired SAED patterns in different places of the heterostructure confirms the high crystalline nature that is not damaged by the SCF processing (Figure S3Ae-f). However, it is very difficult to identify the individual layered structures of the corresponding planes in the obtained diffraction patterns. It reflects the both BN and MoS2 nanosheets in the heterostructure are the source of diffraction that also confirms the heterostructure nature of BN-MoS2. The TEM-EDX (Figure S3B) and STEM elemental mapping (Figure S4) analysis further confirms the existence of BN and MoS2 nanosheets that are uniformly distributed in the heterostructure.

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Figure 4. TEM images of (a) bulk BN, (b) bulk MoS2 powder and (c-d) BN-MoS2 heterostructure. The thickness and roughness of as prepared BN-MoS2 heterostructure are measured using AFM analysis. From the height profile analysis (Figure 5A-B& S5A), it was deduced that the heterostructure has thickness of 20 to 50 nm corresponding to 10-50 monolayers of BN and MoS2 nanosheets. It clearly indicates that our one-step, rapid and environmentally benign SCF exfoliation process is a promising route for the fabrication of high yield of ultrathin exfoliated nanosheets/heterostructures with reduced lateral size compared to bulk layered materials. The strong water repelling tendency or non-wetting nature of heterostructure leads to high water contact angle that can be attributed to its nanoscale roughness and it is effectively determined from 3D AFM topography image (Figure 5C). The additional 3D AFM topography images with the corresponding roughness parameter tables have been placed in the supporting information (Figure S5B-C). It suggests that the BN-MoS2 heterostructure has nanoscale roughness that are comprised of few layer BN and MoS2 nanosheets with reduced thickness and lateral size. The surface roughness of BN-MoS2 heterostructure coated films was also characterized using confocal microscopic analysis (Figure S6). It can be clearly seen that the coated heterostructure films has few microns rough surface.

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Figure 5. A-B) AFM images and height profile analysis of BNMoS2 heterostructure and C) 3D image of BN-MoS2 heterostructure on silica substrate. Water Contact Angle In general, the wetting property of solid materials mainly depends on their surface free energy and morphology with nano/micro roughness.47–50 The contact angle (CA) have been measured in static contact angle mode for BN-MoS2 heterostructure, E-BN and E-MoS2 when the water droplets on their surface. It is worthy to mention here that we have recorded the CA measurement for at least 10 times in order to get the average value of WCA and the perceived values are 165.9o, 124.8o and 125.0° for BN-MoS2 heterostructure, E-MoS2 and E-BN, respectively (Figure 6). The high water contact angle of BN-MoS2 heterostructure compared to bare exfoliated materials indicate that the synthesized heterostructure can be utilized for large scale superhydrophobicity applications. According to reported literatures, the high water contact angle have been found in the entirely and partially vertically aligned BNNSs since these nanosheets are well separated and they have very high surface area with nanoscale rough surface.36,51 An attempt have been made to synthesize the porous BNNSs for the removal of organic solvents and oils from the water surface.52 Similarly, porous BN foams could be also used effectively for the removal of used engine oil from the surface of water. 53 In general, the BNNSs could exhibit such a high contact angle values compared to vertically aligned or porous BNNSs, when the surface roughness is high or if it is hybridized with other materials. In contrast to BNNSs, MoS2 nanosheets exhibit a very low water contact angle (less than 100°), it may be due to the dual nature of MoS2 nanosheets i.e., both hydrophobic and hydrophilic. 54,55

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The exposed surface (or top surface) for CA measurement are hydrophilic due to the reactive nature of edges and also hydrophobic due to sulfur surface termination of MoS2 nanosheets. Thus, the sulfur surface termination of MoS2 nanosheets can increase the CA of pure MoS2. It can significantly affect the CA of BN-MoS2 heterostructure because of the exfoliation of MoS2 along the (002) plane during the SCF processing that leads to the possibility of exposing more sulfur surface termination of MoS2 in the heterostructure which is in good agreement with the XRD results. In the present work, the obtained CA value of BN-MoS2 heterostructure is better than previous reported literature on BNNSs.36,51 Thus, the SCF processing is an effective and simple method for synthesis of heterostructure in a short reaction time with high yield. It is worth noting that in case of heterostructure, the disintegrated exfoliated nanosheets with nano/micro roughness are obtained after the SCF processing. This is foremost responsible to provoke superhydrophobicity feature as predicted. In addition to this, partially rolled nanosheets in the heterostructure also accountable since liquid spreading over this surface differs from plain surface. Thus the fabricated heterostructure exhibit the high water contact angle of 165.9° with a sliding angle of 6°.

Figure 6. The static water contact angle of (a) E-BN, (b) EMoS2 and (c) BN-MoS2 heterostructure. The non-wetting (superhydrophoibic) nature of BN-MoS2 heterostructure has been further confirmed by surface area fraction value of liquid water. The superhydrophoibic behaviour of BN-MoS2 heterostructure can be explained using Cassie - Baxter model.56 This can be done by measuring the CA (θ) of a spherical water drop on rough surface of the material using the following equation. Cos θr = f (Cos θs + 1) – 1 Where f, θr and θs are the surface area fraction, CA on rough and smooth surface of solid, respectively. Air trapped in the valleys and troughs of BN-MoS2 heterostructure surface results in a large water–air interface. Thus, the water droplets are not able to penetrate into the valleys and troughs of BN-MoS2 heterostructure surface which gives rise to superhydrophobicity. Miwa et al.,57 derived the relationship between water contact angle and surface area fraction on a rough surface of the solid. It states that the material with superhydrophobic properties has very low surface area fraction value. Interestingly, our attained surface area fraction value is in good agreement with the above speculation and it is calculated to be 0.0991. In addition, we calculated the work of adhesion in order to understand the easiness of movement of water drops on the surface of BN-MoS2 heterostructure based on the Young-Dupre equation.58 Cos θ = (γSA – γSL / γLA) = (W / γLA ) – 1

Where γLA is the liquid–air interfacial surface tension and W is the work of adhesion of water droplets on heterostructure. In the BN-MoS2 heterostructure, water droplets might have partially sitting on the solid surface and hence W depends on the surface fraction (f). Thus the equation becomes W = γLA f (1 + Cos θ) The calculated work of adhesion of water droplets on BN-MoS2 heterostructure is 1.26 mN/m. This value indicates that the water droplets are almost suspended on a layer of air with negligible interaction between the solid and liquid phases. 59 From the values of surface tension and radius of water droplet (R), the surface free energy of BN-MoS2 heterostructure can be calculated using the equation (Esurf = 4πR2 γLV) and it is found to be around 1.39 × 10-4 J. It further confirms that BN-MoS2 heterostructure has the ability to generate the superhydrophobic feature since the heterostructure has very low surface energy as the above equation stated.60 In general, two common approaches such as creating micro/nanoscale roughness on the surface and chemical functionalization to lower the surface energy have been employed to obtain non-wetting surfaces with high water contact angles.61,62 The BN coatings with and without nanostructure displays superb hydrophobicity and hydrophilic feature, respectively.63 Hence the nanostructured BN with micro/nanoscale roughness is highly desired for superhydrophobic applications. The plausible mechanism for superhydrophobic nature of BN-MoS2 heterostructure can be surmised to the addition of anisotropic vdW bonded MoS2 to the nanostructured BN i.e., BN-MoS2 heterostructure. The nanoscale rough surface with low surface energy and lateral size/thickness reduced BNMoS2 heterostructure was synthesized using a simple and onepot SCF processing for superhydrophobic applications. XRD pattern results suggesting that BN and MoS2 nanosheets could be exfoliated from their bulk powders along the Z-axis direction i.e., (002) plane and therefore more thickness reduction with non-polar surface. Further, AFM analysis of BN-MoS2 heterostructure reveal that the heterostructure has nanoscale roughness on its nano surfaces which are more important to obtain non-wetting surfaces. In addition, the calculated surface free energy (1.39 x 10-4 J) and work of adhesion of water (1.26 mN/m) values for the BN-MoS2 heterostructure further confirms the superhydrophobic features. Air trapped in the nanoscale rough surface of BN-MoS2 heterostructure based on Cassie-Baxter model that causing the water droplets to roll by forming large water-air interface. Effect of temperature on water contact angle The effect of temperature on water contact angle of BN-MoS2 heterostructure has been studied by heating the sample at different temperatures for 5 h (Figure 7). The water contact angles of the sample measured at 100, 200 and 300 °C are 161.3, 158.3 and 153.1°, respectively. BN-MoS2 heterostructure maintains the superhydrophobic character even after treating at higher temperature. The decrease in contact angle at higher temperature may be due to the reduction of roughness by the ordered structure of the BN-MoS2 crystal at higher temperature. Generally, air was trapped in the nanoscale rough surface materials and it couldn’t able to escape when the water droplet is in contact with the rough surface. Thus causing the large water-air interface results in a high water contact angle. Whereas,

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reducing the surface roughness minimize the air trapping and facilitate the release of air from the surface when water droplet added, which significantly reduces the water contact angle. 64–66

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AUTHOR INFORMATION Corresponding Author E. mail: [email protected]; [email protected]

ACKNOWLEDGMENT We thank CSIR, India for financial support through MULTIFUN project, (CSC 0101). PT thanks UGC-CSIR, India for SRF fellowship.

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Figure 7. Effect of temperature on water contact angle of BNMoS2 heterostructure.

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Conclusions: In summary, a simple, rapid and one-pot SCF processing technique was demonstrated for the preparation of BN-MoS2 heterostructure. DMF is used as SCF and the reaction time and temperature are 30 min and 400 °C, respectively. The exfoliation of nanosheets, size reduction and preferential combination of BN and MoS2 nanosheets in the heterostructure was confirmed from the XRD analysis. The existence of BN and MoS 2 nanosheets in the heterostructure was confirmed using the IR and Raman spectroscopic analysis. The uniform distribution of the exfoliated BN and MoS2 nanosheets with reduced thickness in the heterostructure was confirmed using SEM, HR-TEM and HRTEM mapping analysis. The existence of nanoscale roughness in the heterostructure was identified from the TEM and AFM analysis. The AFM images revealed the formation of heterostructure by combination of 20-50 layers of BN and MoS2 nanosheets. Interestingly, the heterostructure with nanoscale roughness demonstrated excellent superhydrophoibic behaviour with a very high water contact angle of 165.9°. To the best of our knowledge, the observed high water contact angle for BN-MoS2 heterostructure is much higher than their individual counter parts and not yet reported for this heterostructure. Thus, the BN-MoS2 nanostructures are potential and promising material for superhydrophobicity applications. And, the in situ preparation of BN-MoS2 heterostructure using the SCF method is very simple, convenient, scalable and economic for viable applications. The same preparation strategy could be extended to the preparation of other 2D layered heterostructure.

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ASSOCIATED CONTENT Supporting Information. Structural characterization, additional

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SEM and TEM images of BN-MoS2 heterostructure are available in the Electronic Supplementary Information (ESI): This material is available free of charge via the Internet at http://pubs.acs.org.

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Graphical abstract Enhanced superhydrophobic performance of BN-MoS2 heterostructure prepared via a rapid, one-pot supercritical fluid processing Pitchai Thangasamy, Thamodaran Partheeban, Subramanian Sudanthiramoorthy and Marappan Sathish*

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