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Journal Pre-proofs Full Length Article A General Route to Free-standing Films of Nanocrystalline Molybdenum Chalcogenides at a Liquid/Liquid Interface under Hydrothermal Conditions Ramya Prabhu B., Kaushalendra K. Singh, Chandraraj Alex, Neena S. John PII: DOI: Reference:
S0169-4332(20)30335-4 https://doi.org/10.1016/j.apsusc.2020.145579 APSUSC 145579
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Applied Surface Science
Received Date: Revised Date: Accepted Date:
10 October 2019 28 January 2020 29 January 2020
Please cite this article as: R. Prabhu B., K.K. Singh, C. Alex, N.S. John, A General Route to Free-standing Films of Nanocrystalline Molybdenum Chalcogenides at a Liquid/Liquid Interface under Hydrothermal Conditions, Applied Surface Science (2020), doi: https://doi.org/10.1016/j.apsusc.2020.145579
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A General Route to Free-standing Films of Nanocrystalline Molybdenum Chalcogenides at a Liquid/Liquid Interface under Hydrothermal Conditions Ramya Prabhu B,a,b Kaushalendra K Singh,a Chandraraj Alexa and Neena S John *a a
Centre for Nano and Soft Matter Sciences, Jalahhali, Bengaluru-560013,India
b
Manipal Academy of Higher Education, Manipal-576104, India
*Corresponding author Email: [email protected]
Abstract: The utility of hydrothermal method applied to liquid/liquid interface for obtaining freestanding films of molybdenum chalcogenides is demonstrated. A general route for obtaining films of MoO3, MoS2 and MoSe2 at a water/toluene interface is illustrated employing a universal Mo precursor and the desired chalcogenide reactant in the two different phases. The hydrothermal conditions promote the formation of emulsions of water and toluene. A mechanism is proposed wherein the in-situ chalcogenation happens at the interfaces of tiny droplets of toluene in water and the nanosheets of the chalcogenide self-assemble at the regenerated interface during quenching to form films. The free-standing films are transferred on to various substrates for characterization. MoO3 films consist of nanobelts while MoS2 and MoSe2 films consist of a dense assembly of nanosheets. The advantage of transferring the films on arbitrary substrates is exemplified for electrochemical applications. Keywords: Molybdenum chalcogenide, free–standing films, Liquid/liquid interface, hydrothermal, electrocatalysis
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1. Introduction
Molybdenum chalcogenides (MoS2, MoO3, MoSe2 and MoTe2) form an important class of 2D materials and are often referred to as inorganic graphene analogues [1]. For applications as 2D materials in electronic devices, monolayers and few layers of Mo-chalcogenides with high crystalline quality are obtained by mechanical or liquid phase exfoliation of their respective bulk crystals and in-situ growth on substrates by chemical vapour deposition [2,3]. Thin films of the above chalcogenides with controlled thicknesses can be obtained by physical vapour deposition processes such as sputtering or evaporation of Mo metal or Mo-oxide and their further conversion to sulfide or selenide via high temperature treatment in S or Se atmosphere [4,5]. Chemical syntheses of Mo-chalcogenides are generally achieved by hydrothermal method or thermal decomposition of molybdates [6,7]. Large area thin films of MoS2 on glass and Si substrates have been achieved by coating the substrate with precursors such as ammonium molybdate, ammonium thiomolybdate, etc followed by thermal decomposition at higher temperatures in the presence of sulfur or hydrogen [8,9]. Recently, there is a report on obtaining large area MoS2 film with extreme control on the thickness assisted by the use of polyenimine along with molybdate precursor and their application as photodetector [10]. These methods are demonstrated only for MoS2 synthesis on specific substrates and require post-annealing treatment at a high temperature in the gas atmosphere. Liquid/liquid interface has emerged as an attractive route to generate large area thin films and has been demonstrated for self-assembly of diverse materials such as metal and semiconductor nanoparticles, 2D materials, metal-organic frameworks and even polymers. [11-16] In our previous work, we have been employing liquid/liquid (L/L) interface as a tool to
obtain free-standing thin films of reduced graphene oxide (rGO) and their hybrids [17-20]. The interface formed by two immiscible liquids provides a constrained environment for the spontaneous assembly of nanosheets during in situ synthesis. Extensive studies have shown that the assembly is directed by the lowering of interfacial tension by the adsorbed particles and the film formation at the interface is controlled by capillary forces and surface wettability of the flakes or particles [21-23]. Thin films obtained by this method have been demonstrated for applications in surface enhanced Raman scattering, energy storage and electrocatalysis [18-20]. Among recent reports on interfacial assembly of 2D materials, Russell and co2
workers have achieved assembly of MXene–surfactant at toluene/water interface towards 2D and 3D functionalized Mxene [24]. L/L interface has been exploited for orthogonal functionalization of graphene to impart Janus characteristics to graphene [25]. The interfacial assembly of transition metal chalcogenides, MoS2 and WS2 and their hetero structures have been achieved by self-assembly of their respective dispersions of exfoliated flakes. [26-29] Usually, in situ L/L reaction is performed at temperatures below 100 °C and ambient pressures with least disturbance to the interface. It has been shown earlier and also by us that stirring and heating can accelerate the reactions occurring at the interface [17,30]. In this work, we have employed the L/L interface method for the in-situ synthesis of various layered Mo-chalcogenides under hydrothermal conditions. The higher pressures and temperatures achieved inside the sealed Teflon vessels are conducive for the formation of crystalline inorganic nanomaterials without any post treatment. Hydrothermal assisted organic–water interface has been employed for the preparation of TiO2 nanorod superstructures and ferrite nanoparticles [31,32]. For the same reason, supercritical water is used to synthesize nanocrystals with controlled size and shape [33]. In the present report, we show that the advantages of L/L interface and hydrothermal method can be combined to form substratefree, thin Mo-chalcogenide films extending over a large area. We illustrate a general route involving only a change in the aqueous phase precursor to obtain films of α-MoO3, MoS2 and MoSe2. The films transferred from the interface on to the electrodes are demonstrated for electrocatalytic hydrogen evolution from water. 2. Experimental Section 2.1 Materials and methods Molybdenyl acetylacetonate (>99%), sodium selenide (95%) and hydrazine hydrate (50-60%) were purchased from sigma Aldrich. Nitric acid (69%) and sodium sulphide (LR) were purchased from Merck and SD – Fine, respectively. 2.2 Synthesis of Mo-chalcogenide thin films The syntheses of MoO3, MoS2 and MoSe2 thin films were achieved employing liquid-liquid interface method. For MoO3, 300 μmoles of molybdenylacetylacetonate (MoO2(acac)2) was dissolved in 40 mL of toluene and 15 mL of aqueous 0.2 M HNO3 was added to it. The solution was transferred to a Teflon autoclave and kept in an oven at 130 °C for 20 h. In the case of MoS2 and MoSe2 films, 200 μmoles of MoO2(acac)2 was dissolved in 40 mL toluene and 3
poured over 400 µmoles of Na2S or Na2Se dissolved in 10 mL water taken in a Teflon autoclave. The pH of aqueous phase was prior adjusted to 4 using dil. H2SO4. 100 µL of hydrazine hydrate was then injected to the aqueous phase using a micropipette and the autoclave was sealed and kept in an oven at 200 °C for 40 h to obtain MoS 2 and MoSe2 films. To study the effect of precursor concentration, the synthesis was also carried out with 100 and 300 µmoles of MoO2(acac)2 at 200 °C. After 40 h, the autoclave was quenched in an ice bath instantly. After cooling, the autoclave was opened and a film was observed floating at the interface. Excess toluene was slowly removed without disturbing the film. After the removal of the toluene, the films were fished out on the desired substrates with the help of tweezers. The films were then washed, dried and used for subsequent characterization. 2.3 Characterization X-ray diffraction (XRD) of the thin films was carried out using a Rigaku Smart Lab diffractometer equipped with parallel beam optics and Cu Kα radiation (40 kV, 30 mA) was incident at a grazing angle of 2°. Raman spectroscopy studies were carried out using Horiba XploRA PLUS spectrometer and an excitation wavelength of 532 nm was focused on to the thin films collected on the Si substrate. Surface morphology of as-synthesized films was examined by a field emission scanning electron microscope (FESEM), TESCAN MIRA3 LM (Czech Republic). For FESEM imaging, the thin films were lifted on Si substrates, washed with ethanol and dried under nitrogen gas. Transmission electron microscopy (TEM) images were acquired using FEI Technai (T20 S-TWIN TEM, 200 kV). The samples for TEM were prepared by dispersing synthesized films in ethanol and drop casting on the Cu grids supporting a holey carbon film and were dried under vacuum. X-ray photoelectron spectroscopy (XPS) study of synthesized thin films was carried out in the ultra-high vacuum chamber of Kratos Axis DLD XPS. For this, the synthesized film was collected on a Si substrate. The thickness of the thin films was measured using an atomic force microscope (AFM), Agilent 5500, operating in contact mode equipped with a tip having a resonance frequency 13 kHz and a force constant 0.18 Nm-1 (Mikro Masch, USA). All electrochemical experiments were conducted using potentiostat (CH Instruments, CHI 660E), with a three electrode configuration consisting of Pt wire as the counter electrode and saturated Hg/Hg2SO4 as reference electrode in 0.5 M H2SO4 (pH ≈1) electrolyte. The synthesized thin films were directly lifted on to fluorine doped tin oxide (FTO) substrates and used as working electrodes to study the HER activity. All voltammetric measurements were obtained at a scan rate of 5 mVs-1. 4
3. Results and discussion In the conventional L/L interface reaction at a planar interface, the thin layer of interface formed between the two immiscible liquids extends to nanometer dimensions and has the liquid molecules from two phases interpenetrating with each other. Accordingly, the enrichment of each phase varies at various length scales from the actual interface [34,35]. The reactants dissolved in each phase diffuse to the interface and nucleates are formed, which further grow to form nanocrystals of metal, metal oxides etc [36,37]. By supplying heat or mechanical energy, the diffusion of the reactants can be made faster expediting the reaction [17,30]. In the present work, we have facilitated the reaction MoO 2(acac)2 in the toluene phase and nitric acid, sodium sulfide or selenide in the aqueous phase under sealed conditions in the hydrothermal autoclave. At higher temperatures and pressures achieved under hydrothermal conditions, dielectric constant of the water and the polarity of the water will decrease [38]. Deguchi et al. have reported the formation of dodecane in water nanoemulsions employing a bottom-up approach in which homogenous solutions of water and hydrocarbon with surfactants, obtained at a higher temperature are cooled to obtain (B)
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Figure 1. (A) Schematic of the reaction mechanism in hydrothermal-assisted L/L interface for obtaining free-standing films of Mo-chalcogenides (B) Photographs of top view of greyish MoS2 film floating on water after the removal of toluene over layer and the films lifted on to glass and Si substrates.
stable nanometer sized droplets of water-dodecane mixtures [38]. They have shown that in the absence of surfactants, the water-dodecane interface will separate upon reducing the temperature. Similarly, under sealed conditions, the miscibility of water and toluene will increase resulting in the emulsion of toluene and water. The reaction between MoO2(acac)2
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Figure 2. (A) XRD pattern and (B) Raman Spectra of (a) MoO3 (b) MoS2 and (c) MoSe2 thin films obtained at a L/L interface and chalcogenide precursor in the two phases is assisted by the increased miscibility and the reaction probably takes place at the tiny interfaces of toluene in water droplets. Finally, when the reaction is quenched, the droplets coalesce and the organic and aqueous phase separates out as there is no surfactant. Consequently, the Mo-chalcogenide flakes will assemble at the
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interface and can be obtained as a thin film floating at the interface of the two liquids. Some aggregated flakes or particles are also observed to settle down. The reaction process is schematically depicted in Figure 1A. For MoO3 synthesis, nitric acid acts as the disproportionation agent in aqueous phase while for MoS2 and MoSe2, sulfidation and selenization of Mo precursor are achieved in the presence of hydrazine hydrate as the reducing agent. Figure 1B shows the photograph of MoS2 film floating on aqueous phase after removal of toluene along with pictures of MoO3 and MoS2 films transferred on to 1 × 1 cm2 glass substrates. X-ray diffraction (XRD) and Raman Spectroscopy (Figures 2A and 2B) confirmed the formation of MoO3, MoS2 and MoSe2 films formed after the interface reaction. For MoO3, the peaks at 12.8°, 25.8°, 39.1°, in the XRD pattern could be indexed to (020), (040), (060) reflections of αMoO3 (JCPDS Card No. 89-5108). The two prominent peaks in Raman spectra of MoO3 located at 814 cm-1 and 991 cm-1 corresponding to symmetric stretching (Ag B1g) and asymmetric stretching (Ag B1g) modes, respectively, (Fig.2B(a)) clearly distinguishes α-MoO3 from other two polymorphs of MoO3. The peak at 285 cm-1 is due to the wagging mode of terminal oxygen and 657 cm-1 is due to asymmetric stretching of Mo-O-Mo bridge along the c-axis [39]. In Figure 2A(b) XRD pattern of MoS2 film shows a broad peak around 33° corresponding to the (100) plane indicating that the films are nanocrystalline (JCPDS 87-2416). The absence of (002) reflection peak and a broad (100) peak indicates that the synthesized films are a few layer graphene-like MoS2 [40]. Raman spectra of MoS2 films (Fig.2B(b)) exhibit the characteristic in-plane (E12g) and out-of-plane (A1g) vibrational modes of MoS2 at 379 cm-1 and 404 cm-1, respectively, of 2H MoS2 [41]. The separation between the two peaks; 25 cm-1 lies close to that for bulk MoS2 (26 cm-1) indicating that the MoS2 films consists of a few layers [41,42]. The XRD pattern of MoSe2 is shown in the figure 2A(c) and the diffraction peak observed at 13.6° corresponds to (002) plane of hexagonal MoSe2 (JCPDS87-2419).Other peaks corresponding to MoSe2 are seen at 32.2°(101), 41.5°(006), 47.6°(105) as denoted by the symbol ‘•’. The peaks due to monoclinic selenium formed also during the reaction are seen at 23.5°(022), 29.7°(500), 45.5°(224), indicated by the symbol ‘•’. Raman spectra of MoSe2 (Fig. 2B (c)) shows only one resonance peak at 237 cm-1, which can be indexed to inplane A1g mode of 2H MoSe2[43].
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FESEM images of the various synthesized films collected on Si substrates are shown in Figure 3. Figure 3a shows that the MoO3 film consists of crystallites with belt like morphology (Fig. 3d). A large area view under FESEM shows that the nanobelts overlap thickly to form an interwoven mat morphology. Figure 3b shows continuous MoS2 films extending over micron scale are with a few pinholes. High resolution FESEM (Fig.3e) reveals that MoS 2 film consists of nanosheets curled at the edges. The curly nanosheets are typically observed for MoS 2 preparations employing solvothermal routes, however, the sheets are often found aggregated to form flower-like or spherical particles when obtained from solution [40,44,45]. In the present case, the nanosheets assemble in a two-dimensional fashion at the L/L interface generating a continuous film. In the case of MoSe 2, the film morphology appears rough with largely aggregated features (Fig. 3c). High resolution SEM shows that the above aggregates consist of sheet morphology (Fig. 3f).
Figure 3. FESEM images of (a, d) MoO3 and (b, e ) MoS2 and (c,f) MoSe2 films obtained at a L/L interface.
In order to glean further structural information of the synthesized films, transmission electron microscopy (TEM) was carried out. TEM images of α-MoO3 film are shown in figures 4a and 4b, comprising of nanobelt features. The width of the belts ranges from 140 to 180 nm. The 8
selected area diffraction pattern (SAED) given in the inset confirms that prepared MoO 3 is highly crystalline with (021) and (020) planes of orthorhombic α-phase. The high-resolution TEM (HRTEM) of the edge of a nanobelt clearly indicates the layered morphology of the nanostructure with lattice planes corresponding to a d spacing of 0.32 nm of the (021) plane of α-MoO3 (Fig. 4b). The inset images give SAED for the respective films.
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Figure 4. TEM images of (a) MoO3 (b) HRTEM of MoO3 (c) MoS2 (d) HRTEM of MoS2 (e) MoSe2; insets show SAED pattern 1 0 0
n m
The MoS2 film (Fig. 4c) shows well-defined flakes formed by the assembly of curly nanosheets in accordance with the observation from FESEM. The dark regions denote the dense assembly of MoS2 curls (figure 4c). HRTEM images show the (100) lattice fringes of MoS2 and also layers with 0.62 nm spacing of (002) plane, which can be associated with the interlayer spacing of layered MoS2 (Fig. 4d) [5]. The MoS2 nanosheets are observed to exhibit short-range order as revealed by the diffusive nature of SAED pattern. The TEM image of MoSe 2 in Figure 4e confirms the sheet like morphology of the aggregates observed in FESEM. The SAED pattern in the inset shows the crystalline nature of the nanosheets and the diffraction pattern can be indexed to the (100) plane of 2H-MoSe2.
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Figure 5. AFM images (5µm×5µm) of the synthesized thin films of MoS 2 at various molybdenum precursor concentrations (a) 100 (b) 200 (c) 300 μmoles. The thickness of the film formed at the L/L interface can be controlled to a certain extent by varying the concentration of the molybdenum precursor, particularly, in the case of MoS 2 film synthesis. Figures 5a-c display the AFM images of the synthesized films with the addition of 100, 200 and 300 μmoles of the Mo precursor, respectively. At a lower concentration of the precursor i.e. 100 micromoles, particles in the thin film are relatively uniform and agglomeration is avoided compared to the other concentrations. From the height profile of the image w r t the underlying substrate, the thickness of the film is estimated to be 20 nm. The roughness (rms value) of the film is found to be 12.8 nm. As the concentration of precursor is increased to 200 μmoles, we can see inhomogeneous distribution of the particles leading to the increase in the roughness to 18.3 nm and thickness to ~40 nm. The thin film synthesized at 300 μmoles is found to be highly non-uniform with irregular agglomerated
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particles causing high surface roughness of 76 nm. The thickness of the film also increased to 125 nm. We can notice that the thickness of the film increases proportionally with concentration of the Mo precursor. Obviously, as the precursor concentration is increased, more MoS2 sheets are formed forming aggregates and thicker layer at the interface.
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Binding energy (eV) Figure 6. XPS spectra of MoS2 films (a) Mo 3d and (b) S 2p. The chemical composition of MoS2 films prepared at the L/L interface was further examined by X-ray photoelectron spectroscopy (XPS) and is given in Figure 6. The Mo 3d core level spectra of MoS2 films can be fitted with two sets of doublet peaks. The two prominent peaks in the lower binding energy region, 228.5 eV and 232 eV correspond to the 3d 5/2 and 3d3/2 of Mo4+ in MoS2 [46-48] .The weak peaks at 232.8 and 235.9 eV correspond to 3d 3/2 and 3d5/2 of Mo6+. The Mo6+ peak indicates the presence of oxidized form of MoS 2 [46-48]. A very weak peak at 225.6 eV is assigned to S 2S. The peaks of S 2p spectra at 161.3 and 163 eV are attributed to S 2p3/2 and S 2p1/2 corresponding to S2− (Fig. 6b) [46, 47]. A very weak peak at 166.3 is likely due to the surface plasmon of MoS2 [46, 47]. A broad peak observed near 168.52 eV corresponds to the binding energy of S6+ and may be due to the presence of any oxidized form of MoS2 formed during the synthesis [48].
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Figure 7. (a) LSV of MoS2 film on FTO at a scan rate of 5 mV/sec (b) Tafel plot As an illustration, we demonstrate the use of such free-standing films by transferring MoS2 film directly on to a conducting substrate (fluorine doped tin oxide, FTO) for electrocatalytic water splitting. Figure 7 shows the linear sweep voltammetry (LSV) of a 40 nm MoS 2 film coated with Nafion. A three electrode configuration consisting of Pt wire as the counter electrode and saturated Hg/Hg2SO4 as the reference electrode was employed in 0.5 M H2SO4 (pH=1) electrolyte. MoS2 film exhibits an onset potential (280 mV) to attain 1 mA/cm2 current density towards hydrogen evolution reaction (HER). After 1000 cycles, the film exhibits only a 10 mV increase in overpotential (290 mV) to retain 1mA/cm 2 as shown in Fig.7a. This indicates that the films are reasonably stable for electrochemical HER. To understand the kinetics of hydrogen evolution, Tafel plots were calculated by plotting the overpotential (η) vs. current density (j) in logarithmic scale (Fig. 7b). The estimated Tafel slope for MoS2 film is 136 mV/dec indicating Volmer reaction as the rate determining step [49]. The HER performance parameters such as onset potential and Tafel slope values [50,51] have the scope for
further improvement by combination with carbon nanomaterials towards
applications in electrochemical energy conversion and storage [40,44,45,52,53]. The growth of MoS2 on graphene derivatives employing L/L interface strategy is currently in progress. 4. Conclusions In conclusion, the preparation of large area, free-standing, thin films of layered molybdenum chalcogenides (MoO3, MoS2, MoSe2) employing a liquid/liquid interface route under hydrothermal conditions is demonstrated. Molybdenum acetylacetonate is taken in toluene phase along with ntiric acid in aqueous phase for obtaining MoO3 and sodium sulphide or
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selenide for obtaining MoS2 or MoSe2. It is proposed that the higher pressures and temperatures achieved under hydrothermal conditions promote mixing of water and toluene phases and the reaction happens at the interface of water-toluene emulsion droplets. The films are formed by the self-assembly of nanosheets of the layered chalcogenides at the regenerated interface during quenching as evidenced by electron microscope images. The thickness of the films can be tuned by changing the precursor concentration and is demonstrated in the case of MoS2 films. The synthesized films can be lifted on to any substrate for promising applications in electrochemical energy storage and generation. The prepared MoS2 films transferred on to FTO substrates show an onset potential of 280 mV and Tafel slope of 136 mV/dec for HER. The method holds immense scope towards the synthesis of other diverse transition metal chalcogenide thin films. Declarations of interest: none Author Contributions RPB and KKS contributed to synthesis and characterization. CA conducted electrochemical measurements. NSJ and RPB contributed to analysis and manuscript preparation. Acknowledgements The authors acknowledge Tata Steel Advanced Material Research Centre (TSAMRC) at Centre for Nano and Soft Matter Sciences (CeNS) for financial support. One of the authors, RPB is grateful to DST, India for INSPIRE fellowship (IF160653). The authors acknowledge AFMM and CeNSE at IISc, Bengaluru, for TEM and XPS facilities. References [1] M. K. Jana, C.N.R.Rao, Two-dimensional inorganic analogues of graphene: transition metal dichalcogenides, Phil. Trans. R. Soc. A 374 (2016) 20150318. [2] J. Shen, Y. He, J. Wu, C. Gao, K. Keyshar, X. Zhang, Y.Yang, M. Ye, R. Vajtai, J. Lou, P. M. Ajayan, Liquid phase exfoliation of two-dimensional materials by directly probing and matching surface tension components, Nano Lett. 15 (2015) 5449–5454. [3] Y. Shi, H. Li , L-J. Li, Recent advances in controlled synthesis of two-dimensional transition metal dichalcogenides via vapour deposition techniques, Chem. Soc. Rev. 44 (2015) 27442756.
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Graphical Abstract
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A general method is developed to synthesise large area, free-standing films of Molybdenum chalcogenides. Liquid /liquid interface formed between organic and aqueous phase when subjected to hydrothermal conditions is conducive for formation of nanocrystalline films. Synthesized MoS2 film is transferred to FTO substrate and tested for electrocatalytic water splitting.
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CRediT author statement RPB: Investigation, Writing-Original Draft KKS: Methodology, Investigation, Validation CA: Investigation, NSJ: Conceptualization, Writing - Review & Editing, Supervision.
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S0169-4332(20)30335-4 https://doi.org/10.1016/j.apsusc.2020.145579 APSUSC 145579
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
10 October 2019 28 January 2020 29 January 2020
Please cite this article as: R. Prabhu B., K.K. Singh, C. Alex, N.S. John, A General Route to Free-standing Films of Nanocrystalline Molybdenum Chalcogenides at a Liquid/Liquid Interface under Hydrothermal Conditions, Applied Surface Science (2020), doi: https://doi.org/10.1016/j.apsusc.2020.145579
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© 2020 Published by Elsevier B.V.
A General Route to Free-standing Films of Nanocrystalline Molybdenum Chalcogenides at a Liquid/Liquid Interface under Hydrothermal Conditions Ramya Prabhu B,a,b Kaushalendra K Singh,a Chandraraj Alexa and Neena S John *a a
Centre for Nano and Soft Matter Sciences, Jalahhali, Bengaluru-560013,India
b
Manipal Academy of Higher Education, Manipal-576104, India
*Corresponding author Email: [email protected]
Abstract: The utility of hydrothermal method applied to liquid/liquid interface for obtaining freestanding films of molybdenum chalcogenides is demonstrated. A general route for obtaining films of MoO3, MoS2 and MoSe2 at a water/toluene interface is illustrated employing a universal Mo precursor and the desired chalcogenide reactant in the two different phases. The hydrothermal conditions promote the formation of emulsions of water and toluene. A mechanism is proposed wherein the in-situ chalcogenation happens at the interfaces of tiny droplets of toluene in water and the nanosheets of the chalcogenide self-assemble at the regenerated interface during quenching to form films. The free-standing films are transferred on to various substrates for characterization. MoO3 films consist of nanobelts while MoS2 and MoSe2 films consist of a dense assembly of nanosheets. The advantage of transferring the films on arbitrary substrates is exemplified for electrochemical applications. Keywords: Molybdenum chalcogenide, free–standing films, Liquid/liquid interface, hydrothermal, electrocatalysis
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1. Introduction
Molybdenum chalcogenides (MoS2, MoO3, MoSe2 and MoTe2) form an important class of 2D materials and are often referred to as inorganic graphene analogues [1]. For applications as 2D materials in electronic devices, monolayers and few layers of Mo-chalcogenides with high crystalline quality are obtained by mechanical or liquid phase exfoliation of their respective bulk crystals and in-situ growth on substrates by chemical vapour deposition [2,3]. Thin films of the above chalcogenides with controlled thicknesses can be obtained by physical vapour deposition processes such as sputtering or evaporation of Mo metal or Mo-oxide and their further conversion to sulfide or selenide via high temperature treatment in S or Se atmosphere [4,5]. Chemical syntheses of Mo-chalcogenides are generally achieved by hydrothermal method or thermal decomposition of molybdates [6,7]. Large area thin films of MoS2 on glass and Si substrates have been achieved by coating the substrate with precursors such as ammonium molybdate, ammonium thiomolybdate, etc followed by thermal decomposition at higher temperatures in the presence of sulfur or hydrogen [8,9]. Recently, there is a report on obtaining large area MoS2 film with extreme control on the thickness assisted by the use of polyenimine along with molybdate precursor and their application as photodetector [10]. These methods are demonstrated only for MoS2 synthesis on specific substrates and require post-annealing treatment at a high temperature in the gas atmosphere. Liquid/liquid interface has emerged as an attractive route to generate large area thin films and has been demonstrated for self-assembly of diverse materials such as metal and semiconductor nanoparticles, 2D materials, metal-organic frameworks and even polymers. [11-16] In our previous work, we have been employing liquid/liquid (L/L) interface as a tool to
obtain free-standing thin films of reduced graphene oxide (rGO) and their hybrids [17-20]. The interface formed by two immiscible liquids provides a constrained environment for the spontaneous assembly of nanosheets during in situ synthesis. Extensive studies have shown that the assembly is directed by the lowering of interfacial tension by the adsorbed particles and the film formation at the interface is controlled by capillary forces and surface wettability of the flakes or particles [21-23]. Thin films obtained by this method have been demonstrated for applications in surface enhanced Raman scattering, energy storage and electrocatalysis [18-20]. Among recent reports on interfacial assembly of 2D materials, Russell and co2
workers have achieved assembly of MXene–surfactant at toluene/water interface towards 2D and 3D functionalized Mxene [24]. L/L interface has been exploited for orthogonal functionalization of graphene to impart Janus characteristics to graphene [25]. The interfacial assembly of transition metal chalcogenides, MoS2 and WS2 and their hetero structures have been achieved by self-assembly of their respective dispersions of exfoliated flakes. [26-29] Usually, in situ L/L reaction is performed at temperatures below 100 °C and ambient pressures with least disturbance to the interface. It has been shown earlier and also by us that stirring and heating can accelerate the reactions occurring at the interface [17,30]. In this work, we have employed the L/L interface method for the in-situ synthesis of various layered Mo-chalcogenides under hydrothermal conditions. The higher pressures and temperatures achieved inside the sealed Teflon vessels are conducive for the formation of crystalline inorganic nanomaterials without any post treatment. Hydrothermal assisted organic–water interface has been employed for the preparation of TiO2 nanorod superstructures and ferrite nanoparticles [31,32]. For the same reason, supercritical water is used to synthesize nanocrystals with controlled size and shape [33]. In the present report, we show that the advantages of L/L interface and hydrothermal method can be combined to form substratefree, thin Mo-chalcogenide films extending over a large area. We illustrate a general route involving only a change in the aqueous phase precursor to obtain films of α-MoO3, MoS2 and MoSe2. The films transferred from the interface on to the electrodes are demonstrated for electrocatalytic hydrogen evolution from water. 2. Experimental Section 2.1 Materials and methods Molybdenyl acetylacetonate (>99%), sodium selenide (95%) and hydrazine hydrate (50-60%) were purchased from sigma Aldrich. Nitric acid (69%) and sodium sulphide (LR) were purchased from Merck and SD – Fine, respectively. 2.2 Synthesis of Mo-chalcogenide thin films The syntheses of MoO3, MoS2 and MoSe2 thin films were achieved employing liquid-liquid interface method. For MoO3, 300 μmoles of molybdenylacetylacetonate (MoO2(acac)2) was dissolved in 40 mL of toluene and 15 mL of aqueous 0.2 M HNO3 was added to it. The solution was transferred to a Teflon autoclave and kept in an oven at 130 °C for 20 h. In the case of MoS2 and MoSe2 films, 200 μmoles of MoO2(acac)2 was dissolved in 40 mL toluene and 3
poured over 400 µmoles of Na2S or Na2Se dissolved in 10 mL water taken in a Teflon autoclave. The pH of aqueous phase was prior adjusted to 4 using dil. H2SO4. 100 µL of hydrazine hydrate was then injected to the aqueous phase using a micropipette and the autoclave was sealed and kept in an oven at 200 °C for 40 h to obtain MoS 2 and MoSe2 films. To study the effect of precursor concentration, the synthesis was also carried out with 100 and 300 µmoles of MoO2(acac)2 at 200 °C. After 40 h, the autoclave was quenched in an ice bath instantly. After cooling, the autoclave was opened and a film was observed floating at the interface. Excess toluene was slowly removed without disturbing the film. After the removal of the toluene, the films were fished out on the desired substrates with the help of tweezers. The films were then washed, dried and used for subsequent characterization. 2.3 Characterization X-ray diffraction (XRD) of the thin films was carried out using a Rigaku Smart Lab diffractometer equipped with parallel beam optics and Cu Kα radiation (40 kV, 30 mA) was incident at a grazing angle of 2°. Raman spectroscopy studies were carried out using Horiba XploRA PLUS spectrometer and an excitation wavelength of 532 nm was focused on to the thin films collected on the Si substrate. Surface morphology of as-synthesized films was examined by a field emission scanning electron microscope (FESEM), TESCAN MIRA3 LM (Czech Republic). For FESEM imaging, the thin films were lifted on Si substrates, washed with ethanol and dried under nitrogen gas. Transmission electron microscopy (TEM) images were acquired using FEI Technai (T20 S-TWIN TEM, 200 kV). The samples for TEM were prepared by dispersing synthesized films in ethanol and drop casting on the Cu grids supporting a holey carbon film and were dried under vacuum. X-ray photoelectron spectroscopy (XPS) study of synthesized thin films was carried out in the ultra-high vacuum chamber of Kratos Axis DLD XPS. For this, the synthesized film was collected on a Si substrate. The thickness of the thin films was measured using an atomic force microscope (AFM), Agilent 5500, operating in contact mode equipped with a tip having a resonance frequency 13 kHz and a force constant 0.18 Nm-1 (Mikro Masch, USA). All electrochemical experiments were conducted using potentiostat (CH Instruments, CHI 660E), with a three electrode configuration consisting of Pt wire as the counter electrode and saturated Hg/Hg2SO4 as reference electrode in 0.5 M H2SO4 (pH ≈1) electrolyte. The synthesized thin films were directly lifted on to fluorine doped tin oxide (FTO) substrates and used as working electrodes to study the HER activity. All voltammetric measurements were obtained at a scan rate of 5 mVs-1. 4
3. Results and discussion In the conventional L/L interface reaction at a planar interface, the thin layer of interface formed between the two immiscible liquids extends to nanometer dimensions and has the liquid molecules from two phases interpenetrating with each other. Accordingly, the enrichment of each phase varies at various length scales from the actual interface [34,35]. The reactants dissolved in each phase diffuse to the interface and nucleates are formed, which further grow to form nanocrystals of metal, metal oxides etc [36,37]. By supplying heat or mechanical energy, the diffusion of the reactants can be made faster expediting the reaction [17,30]. In the present work, we have facilitated the reaction MoO 2(acac)2 in the toluene phase and nitric acid, sodium sulfide or selenide in the aqueous phase under sealed conditions in the hydrothermal autoclave. At higher temperatures and pressures achieved under hydrothermal conditions, dielectric constant of the water and the polarity of the water will decrease [38]. Deguchi et al. have reported the formation of dodecane in water nanoemulsions employing a bottom-up approach in which homogenous solutions of water and hydrocarbon with surfactants, obtained at a higher temperature are cooled to obtain (B)
(A)
Mo precursor in organic Chalcogen precursor NH2-NH2 in aqueous
Before the reaction
MoS2
Mo-Chalcogenide film at interface
During the reaction
Reaction quenched
Free- standing MoS2 film
MoO3
Figure 1. (A) Schematic of the reaction mechanism in hydrothermal-assisted L/L interface for obtaining free-standing films of Mo-chalcogenides (B) Photographs of top view of greyish MoS2 film floating on water after the removal of toluene over layer and the films lifted on to glass and Si substrates.
stable nanometer sized droplets of water-dodecane mixtures [38]. They have shown that in the absence of surfactants, the water-dodecane interface will separate upon reducing the temperature. Similarly, under sealed conditions, the miscibility of water and toluene will increase resulting in the emulsion of toluene and water. The reaction between MoO2(acac)2
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Figure 2. (A) XRD pattern and (B) Raman Spectra of (a) MoO3 (b) MoS2 and (c) MoSe2 thin films obtained at a L/L interface and chalcogenide precursor in the two phases is assisted by the increased miscibility and the reaction probably takes place at the tiny interfaces of toluene in water droplets. Finally, when the reaction is quenched, the droplets coalesce and the organic and aqueous phase separates out as there is no surfactant. Consequently, the Mo-chalcogenide flakes will assemble at the
6
interface and can be obtained as a thin film floating at the interface of the two liquids. Some aggregated flakes or particles are also observed to settle down. The reaction process is schematically depicted in Figure 1A. For MoO3 synthesis, nitric acid acts as the disproportionation agent in aqueous phase while for MoS2 and MoSe2, sulfidation and selenization of Mo precursor are achieved in the presence of hydrazine hydrate as the reducing agent. Figure 1B shows the photograph of MoS2 film floating on aqueous phase after removal of toluene along with pictures of MoO3 and MoS2 films transferred on to 1 × 1 cm2 glass substrates. X-ray diffraction (XRD) and Raman Spectroscopy (Figures 2A and 2B) confirmed the formation of MoO3, MoS2 and MoSe2 films formed after the interface reaction. For MoO3, the peaks at 12.8°, 25.8°, 39.1°, in the XRD pattern could be indexed to (020), (040), (060) reflections of αMoO3 (JCPDS Card No. 89-5108). The two prominent peaks in Raman spectra of MoO3 located at 814 cm-1 and 991 cm-1 corresponding to symmetric stretching (Ag B1g) and asymmetric stretching (Ag B1g) modes, respectively, (Fig.2B(a)) clearly distinguishes α-MoO3 from other two polymorphs of MoO3. The peak at 285 cm-1 is due to the wagging mode of terminal oxygen and 657 cm-1 is due to asymmetric stretching of Mo-O-Mo bridge along the c-axis [39]. In Figure 2A(b) XRD pattern of MoS2 film shows a broad peak around 33° corresponding to the (100) plane indicating that the films are nanocrystalline (JCPDS 87-2416). The absence of (002) reflection peak and a broad (100) peak indicates that the synthesized films are a few layer graphene-like MoS2 [40]. Raman spectra of MoS2 films (Fig.2B(b)) exhibit the characteristic in-plane (E12g) and out-of-plane (A1g) vibrational modes of MoS2 at 379 cm-1 and 404 cm-1, respectively, of 2H MoS2 [41]. The separation between the two peaks; 25 cm-1 lies close to that for bulk MoS2 (26 cm-1) indicating that the MoS2 films consists of a few layers [41,42]. The XRD pattern of MoSe2 is shown in the figure 2A(c) and the diffraction peak observed at 13.6° corresponds to (002) plane of hexagonal MoSe2 (JCPDS87-2419).Other peaks corresponding to MoSe2 are seen at 32.2°(101), 41.5°(006), 47.6°(105) as denoted by the symbol ‘•’. The peaks due to monoclinic selenium formed also during the reaction are seen at 23.5°(022), 29.7°(500), 45.5°(224), indicated by the symbol ‘•’. Raman spectra of MoSe2 (Fig. 2B (c)) shows only one resonance peak at 237 cm-1, which can be indexed to inplane A1g mode of 2H MoSe2[43].
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FESEM images of the various synthesized films collected on Si substrates are shown in Figure 3. Figure 3a shows that the MoO3 film consists of crystallites with belt like morphology (Fig. 3d). A large area view under FESEM shows that the nanobelts overlap thickly to form an interwoven mat morphology. Figure 3b shows continuous MoS2 films extending over micron scale are with a few pinholes. High resolution FESEM (Fig.3e) reveals that MoS 2 film consists of nanosheets curled at the edges. The curly nanosheets are typically observed for MoS 2 preparations employing solvothermal routes, however, the sheets are often found aggregated to form flower-like or spherical particles when obtained from solution [40,44,45]. In the present case, the nanosheets assemble in a two-dimensional fashion at the L/L interface generating a continuous film. In the case of MoSe 2, the film morphology appears rough with largely aggregated features (Fig. 3c). High resolution SEM shows that the above aggregates consist of sheet morphology (Fig. 3f).
Figure 3. FESEM images of (a, d) MoO3 and (b, e ) MoS2 and (c,f) MoSe2 films obtained at a L/L interface.
In order to glean further structural information of the synthesized films, transmission electron microscopy (TEM) was carried out. TEM images of α-MoO3 film are shown in figures 4a and 4b, comprising of nanobelt features. The width of the belts ranges from 140 to 180 nm. The 8
selected area diffraction pattern (SAED) given in the inset confirms that prepared MoO 3 is highly crystalline with (021) and (020) planes of orthorhombic α-phase. The high-resolution TEM (HRTEM) of the edge of a nanobelt clearly indicates the layered morphology of the nanostructure with lattice planes corresponding to a d spacing of 0.32 nm of the (021) plane of α-MoO3 (Fig. 4b). The inset images give SAED for the respective films.
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Figure 4. TEM images of (a) MoO3 (b) HRTEM of MoO3 (c) MoS2 (d) HRTEM of MoS2 (e) MoSe2; insets show SAED pattern 1 0 0
n m
The MoS2 film (Fig. 4c) shows well-defined flakes formed by the assembly of curly nanosheets in accordance with the observation from FESEM. The dark regions denote the dense assembly of MoS2 curls (figure 4c). HRTEM images show the (100) lattice fringes of MoS2 and also layers with 0.62 nm spacing of (002) plane, which can be associated with the interlayer spacing of layered MoS2 (Fig. 4d) [5]. The MoS2 nanosheets are observed to exhibit short-range order as revealed by the diffusive nature of SAED pattern. The TEM image of MoSe 2 in Figure 4e confirms the sheet like morphology of the aggregates observed in FESEM. The SAED pattern in the inset shows the crystalline nature of the nanosheets and the diffraction pattern can be indexed to the (100) plane of 2H-MoSe2.
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Figure 5. AFM images (5µm×5µm) of the synthesized thin films of MoS 2 at various molybdenum precursor concentrations (a) 100 (b) 200 (c) 300 μmoles. The thickness of the film formed at the L/L interface can be controlled to a certain extent by varying the concentration of the molybdenum precursor, particularly, in the case of MoS 2 film synthesis. Figures 5a-c display the AFM images of the synthesized films with the addition of 100, 200 and 300 μmoles of the Mo precursor, respectively. At a lower concentration of the precursor i.e. 100 micromoles, particles in the thin film are relatively uniform and agglomeration is avoided compared to the other concentrations. From the height profile of the image w r t the underlying substrate, the thickness of the film is estimated to be 20 nm. The roughness (rms value) of the film is found to be 12.8 nm. As the concentration of precursor is increased to 200 μmoles, we can see inhomogeneous distribution of the particles leading to the increase in the roughness to 18.3 nm and thickness to ~40 nm. The thin film synthesized at 300 μmoles is found to be highly non-uniform with irregular agglomerated
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particles causing high surface roughness of 76 nm. The thickness of the film also increased to 125 nm. We can notice that the thickness of the film increases proportionally with concentration of the Mo precursor. Obviously, as the precursor concentration is increased, more MoS2 sheets are formed forming aggregates and thicker layer at the interface.
(a)
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Binding energy (eV) Figure 6. XPS spectra of MoS2 films (a) Mo 3d and (b) S 2p. The chemical composition of MoS2 films prepared at the L/L interface was further examined by X-ray photoelectron spectroscopy (XPS) and is given in Figure 6. The Mo 3d core level spectra of MoS2 films can be fitted with two sets of doublet peaks. The two prominent peaks in the lower binding energy region, 228.5 eV and 232 eV correspond to the 3d 5/2 and 3d3/2 of Mo4+ in MoS2 [46-48] .The weak peaks at 232.8 and 235.9 eV correspond to 3d 3/2 and 3d5/2 of Mo6+. The Mo6+ peak indicates the presence of oxidized form of MoS 2 [46-48]. A very weak peak at 225.6 eV is assigned to S 2S. The peaks of S 2p spectra at 161.3 and 163 eV are attributed to S 2p3/2 and S 2p1/2 corresponding to S2− (Fig. 6b) [46, 47]. A very weak peak at 166.3 is likely due to the surface plasmon of MoS2 [46, 47]. A broad peak observed near 168.52 eV corresponds to the binding energy of S6+ and may be due to the presence of any oxidized form of MoS2 formed during the synthesis [48].
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MoS2 1st LSV MoS2 after 1000 cycles
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Figure 7. (a) LSV of MoS2 film on FTO at a scan rate of 5 mV/sec (b) Tafel plot As an illustration, we demonstrate the use of such free-standing films by transferring MoS2 film directly on to a conducting substrate (fluorine doped tin oxide, FTO) for electrocatalytic water splitting. Figure 7 shows the linear sweep voltammetry (LSV) of a 40 nm MoS 2 film coated with Nafion. A three electrode configuration consisting of Pt wire as the counter electrode and saturated Hg/Hg2SO4 as the reference electrode was employed in 0.5 M H2SO4 (pH=1) electrolyte. MoS2 film exhibits an onset potential (280 mV) to attain 1 mA/cm2 current density towards hydrogen evolution reaction (HER). After 1000 cycles, the film exhibits only a 10 mV increase in overpotential (290 mV) to retain 1mA/cm 2 as shown in Fig.7a. This indicates that the films are reasonably stable for electrochemical HER. To understand the kinetics of hydrogen evolution, Tafel plots were calculated by plotting the overpotential (η) vs. current density (j) in logarithmic scale (Fig. 7b). The estimated Tafel slope for MoS2 film is 136 mV/dec indicating Volmer reaction as the rate determining step [49]. The HER performance parameters such as onset potential and Tafel slope values [50,51] have the scope for
further improvement by combination with carbon nanomaterials towards
applications in electrochemical energy conversion and storage [40,44,45,52,53]. The growth of MoS2 on graphene derivatives employing L/L interface strategy is currently in progress. 4. Conclusions In conclusion, the preparation of large area, free-standing, thin films of layered molybdenum chalcogenides (MoO3, MoS2, MoSe2) employing a liquid/liquid interface route under hydrothermal conditions is demonstrated. Molybdenum acetylacetonate is taken in toluene phase along with ntiric acid in aqueous phase for obtaining MoO3 and sodium sulphide or
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selenide for obtaining MoS2 or MoSe2. It is proposed that the higher pressures and temperatures achieved under hydrothermal conditions promote mixing of water and toluene phases and the reaction happens at the interface of water-toluene emulsion droplets. The films are formed by the self-assembly of nanosheets of the layered chalcogenides at the regenerated interface during quenching as evidenced by electron microscope images. The thickness of the films can be tuned by changing the precursor concentration and is demonstrated in the case of MoS2 films. The synthesized films can be lifted on to any substrate for promising applications in electrochemical energy storage and generation. The prepared MoS2 films transferred on to FTO substrates show an onset potential of 280 mV and Tafel slope of 136 mV/dec for HER. The method holds immense scope towards the synthesis of other diverse transition metal chalcogenide thin films. Declarations of interest: none Author Contributions RPB and KKS contributed to synthesis and characterization. CA conducted electrochemical measurements. NSJ and RPB contributed to analysis and manuscript preparation. Acknowledgements The authors acknowledge Tata Steel Advanced Material Research Centre (TSAMRC) at Centre for Nano and Soft Matter Sciences (CeNS) for financial support. One of the authors, RPB is grateful to DST, India for INSPIRE fellowship (IF160653). The authors acknowledge AFMM and CeNSE at IISc, Bengaluru, for TEM and XPS facilities. References [1] M. K. Jana, C.N.R.Rao, Two-dimensional inorganic analogues of graphene: transition metal dichalcogenides, Phil. Trans. R. Soc. A 374 (2016) 20150318. [2] J. Shen, Y. He, J. Wu, C. Gao, K. Keyshar, X. Zhang, Y.Yang, M. Ye, R. Vajtai, J. Lou, P. M. Ajayan, Liquid phase exfoliation of two-dimensional materials by directly probing and matching surface tension components, Nano Lett. 15 (2015) 5449–5454. [3] Y. Shi, H. Li , L-J. Li, Recent advances in controlled synthesis of two-dimensional transition metal dichalcogenides via vapour deposition techniques, Chem. Soc. Rev. 44 (2015) 27442756.
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Graphical Abstract
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A general method is developed to synthesise large area, free-standing films of Molybdenum chalcogenides. Liquid /liquid interface formed between organic and aqueous phase when subjected to hydrothermal conditions is conducive for formation of nanocrystalline films. Synthesized MoS2 film is transferred to FTO substrate and tested for electrocatalytic water splitting.
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CRediT author statement RPB: Investigation, Writing-Original Draft KKS: Methodology, Investigation, Validation CA: Investigation, NSJ: Conceptualization, Writing - Review & Editing, Supervision.
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