Scalable Synthesis of Few-Layered 2D Tungsten Diselenide (2H-WSe2) Nanosheets Directly Grown on Tungsten (W) Foil Using Ambient-Pressure Chemical Vapor Deposition for Reversible Li-Ion Storage

We report a facile two-furnace APCVD synthesis of 2H-WSe2. A systematic study of the process parameters is performed to show the formation of the phase-pure material. Extensive characterization of the bulk and exfoliated material confirm that 2H-WSe2 is layered (i.e., 2D). X-ray diffraction (XRD) confirms the phase, while high-resolution scanning electron microscopy (HRSEM), high-resolution transmission electron microscopy (HRTEM), and atomic force microscopy (AFM) clarify the morphology of the material. Focused ion beam scanning electron microscopy (FIB-SEM) estimates the depth of the 2H-WSe2 formed on W foil to be around 5–8 μm, and Raman/UV–vis measurements prove the quality of the exfoliated 2H-WSe2. Studies on the redox processes of lithium-ion batteries (LiBs) show an increase in capacity up to 500 cycles. On prolonged cycling, the discharge capacity up to the 50th cycle at 250 mA/g of the material shows a stable value of 550 mAh/g. These observations indicate that exfoliated 2H-WSe2 has promising applications as an LiB electrode material.


■ INTRODUCTION
As the demand for sustainable energy storage devices increases, we require new materials for energy storage systems that will alleviate the cost and increase their longevity. A worthwhile and prominent energy storage technology for modern applications is the development of lithium-ion batteries. These batteries are currently used in electric vehicles, portable electronics, etc. and have become a part of our daily life. 1−3 In this regard, two-dimensional (2D) materials have also attracted substantial and considerable attention due to their unique properties and abundant potential in various applications, including batteries. 4−6 A controllable synthesis of 2D materials with high quality and high efficiency is essential for their largescale applications, for instance, their functions as electrode material in batteries. 7 There are many methods for the synthesis of two-dimensional materials, including mechanical exfoliation, 8 liquid exfoliation, 9 liquid-phase routes, 1,10 and chemical vapor deposition (CVD). 11,12 Among these processes, CVD offers better quality, efficiency, consistency, and control over the others. 13 The efficiency of CVD-assisted synthesis is quite higher compared to mechanical exfoliation, and the quality control is better than liquid-phase routes, due to which CVD has been recognized as a reliable route for preparing high-quality two-dimensional (2D) materials. 11 In this perspective, the family of 2D transition-metal dichalcogenides (TMDCs) gained significant attention due to their ability to store metal ions such as Li, Na, and Mg. 14,15 The behaviors of 2D TMDCs are varied, ranging from insulating, semiconducting, and metallic to superconducting. 16 TMDCs of group 6 transition metals (MX 2 where M = Mo, W, and X = S, Se, Te) have a lamellar structure (space group P63/mmc) similar to graphite but with larger interlayer spacing. 12 Here, in this 2D family, tungsten dichalcogenides are significant because of the larger size of W, which provides a further alteration of the 2D structure. 17 Although WO 3 and WS 2 are still the popular members of this TMDC subfamily in various applications, tungsten diselenide (WSe 2 ) is among the emerging candidates in the family of TMDC and is successfully utilized for various applications such as in photodetectors, 18 field-effect transistors (FETs), 19 etc. It has a typical group 6 TMDC lattice structure, where W atoms are confined in a trigonal prismatic coordination sphere near Se atoms. 12 WSe 2 has a layered crystal structure similar to WS 2 . The latter (i.e., WS 2 ) is also extensively studied as a possible anode material for LiBs, 20 and the reaction mechanism of WSe 2 in LiB application is also found to be similar to that of WS 2 . 19 WSe 2 possesses unique properties such small band gap (1.6 eV) and large adjacent layer spacing (0.648 nm, 2 times larger than that of graphite), which can effectively facilitate reversible ion intercalation and extraction. Because of its very high density of 9.32 g cm −3 , WSe 2 also has a high volumetric capacity, and these properties help manifest this material as a prospective electrode material. 21−24 Usually, the synthesis methods for TMDCs using CVD is either a top-down or a bottom-up approach. 25 CVD-assisted processes include precursors that are reacted and/or decomposed and deposited as a monolayer or few-layered thin films on appropriate substrates such as sapphire, Si/SiO 2 , etc. The four common routes of synthesis can be summarized based on (1) thermal decomposition of precursors, (2) selenization of predeposited metal or metal oxide films on suitable substrates, (3) physical vapor transport between presynthesized TMDC and an insulating substrate, or (4) vapor-phase reaction between two precursors (i.e., the transition-metal oxide/halide and chalcogen precursor). 26 The vapor-phase reaction of the CVD process is classified into atmospheric pressure CVD (APCVD), 27,28 modified metal−organic CVD (m-MOCVD), 29,30 and low-pressure CVD (LPCVD). 31,32 Our method of synthesis combines the synthetic routes mentioned in (3) and (4), which helps us to report a facile and scalable two-furnace bulk synthesis of tungsten diselenide (WSe 2 ) on tungsten (W) foil using only elemental selenium (Se) powder as a precursor, using an atmospheric pressure chemical vapor deposition system (APCVD). Several reports of WSe 2 thin-film growth by CVD on different substrates are reported, but using different precursors, such as WF 6 , H 2 Se, W(CO) 6 , SeCl 4 , etc., 33 as the starting materials, which are often tricky to handle involving burdensome processes. For example, using H 2 S or H 2 Se as the chalcogen precursor or involving an intermediate process of annealing the substrate with Ar/H 2 or using chalcogen precursors such as thiourea in solvothermal methods gives rise to problems in handling these gases as a byproduct of the reaction because they are hazardous to human health. 34,35 In this article, we report a facile synthesis of 2H-WSe 2 on W foil in an ambient-pressure chemical vapor deposition (APCVD) system using only argon as a carrier gas without the formation of any hazardous waste products during the reaction. The idea behind the preparation of this material using APCVD is (1) to show that synthesis of transition-metal dichalcogenides with a higher melting point of the metal is possible using a two-furnace system in the absence of high pressure and continuous heating at higher temperatures for days, thereby simplifying our procedure for this synthesis, (2) to study the possible growth route of 2H-WSe 2 formation on W foil for exploring how this material can be easily exfoliated and integrated into a possible electrode material for lithium-ion batteries, (3) to estimate the maximum yield obtained from liquid-phase exfoliations of the 2H-WSe 2 grown on the W foil, and (4) to observe if a single-step reaction was feasible in producing phase-pure material on the exposed surface of W foil. Since the bulk material was exfoliated to prepare the electrodes, we also investigated if the yield from the reactions was increased as a function of reaction time. Conventional LiB electrodes are usually composed of materials often synthesized using hydrothermal or solvothermal procedures with specific morphologies (such as spheres) in favor of the performance of the material in LiB. 36−38 Since, among the TMDCs, 2H-WSe 2 synthesized in APCVD has a platelike morphology, we decided to inspect it as a possible electrode material for battery application because there are very few reports on the ability of exfoliated 2H-WSe 2 to function as a lithium-ion battery electrode.

■ RESULTS AND DISCUSSION
To the best of our knowledge, a phase diagram for W and Se is not available in the literature. Therefore, to understand the possible reactions between Se and W precursors, as a function of temperature and pressure, we relied on previous reports of this material. It is reported that 2H-WSe 2 can be synthesized using traditional solid-state mixing of WO 3 powder with elemental Se, sealing them in quartz ampoules, and heating at ca. 700−800°C for a few days in an inert atmosphere. The final product is usually crushed or ground to obtain flakes of layered WSe 2 . 39 Other methods of preparation include surface selenization of tungsten (W) or tungsten oxide (WO 3 ) thin films, 40 using magnetron sputtering, 41 molecular beam epitaxy, 40,42 etc., but, in our experiments, we found out that the reaction between the tungsten foil and the elemental selenium takes place (in the absence of pressure) at 900°C (indicating the reaction did not occur between 600 and 850°C ) in F2 while heating the elemental Se precursor at 650°C in F1. (The schematic of the furnace system is shown in Figure  1). This temperature gradient of 250°C between the two furnaces is vital to maintain throughout the reaction time because it allows the vapor from the elemental Se to react and deposit on the surface of the W metal foil. The Se boat was immediately pulled out once the reaction was over, and the WSe 2 sample was allowed to cool gradually to room temperature. For optimizing the mentioned reaction, we tested the effect of several reaction parameters, which we summarize as follows: Figure 1. Photo of the APCVD system used with a schematic of the growth of 2H-WSe 2 in a two-furnace CVD system and a summary of optimization of reaction parameters. (Photograph was taken by Rajashree Konar, Copyright 2020).
(1) Optimization of carrier gas flow: To study the dependency of the WSe 2 -film growth on the flow rates of the carrier gas (argon in this case), the Ar flow rate was varied from 50 to 300 sccm during the reaction. It was observed that at lower Ar flow rates (i.e., from 50 to 100 sccm), no growth was observed at 900°C. Higher flow rates, starting from 150 sccm onward, resulted in a substantial growth of WSe 2 . The reaction conditions were finally optimized at 200 sccm because the growth of WSe 2 was found to be the most uniform with this Ar flow. Further increase in the flow rate to 250 and 300 sccm did not lead to any significant reaction between the W foil and the elemental Se precursor because a significant amount of selenium vapor was found to be condensed at the end of the quartz tube outside the furnace F2. (2) Optimization of reaction time: Different reaction times from 30 s to 3 h were investigated to achieve efficient synthesis protocol. Initial results of X-ray diffraction (XRD) and high-resolution scanning electron microscopy (HRSEM) indicated that the reaction did not occur for reaction times shorter than 15 min, from which we can assume that the minimum time to drive this reaction is 15 min in configuration. Reactive annealing did not result in uniform coverage of the W foil with WSe 2 , so we preannealed the W foil in the inert atmosphere before introducing the elemental Se vapor from furnace F1. We observed that a preannealing step of 5 min was the best for 2H-WSe 2 growth on W foil. Longer reaction times between the acid-etched W foil and elemental Se from 2.5 h up to 3 h led to bending of the foil itself and failed to provide us with any critical data. The optimized reaction time in our case is 1 h between W foil and Se. We also tried to understand the physical changes associated with the synthesis of this material. Since the reaction occurred mainly between the W foil and the elemental selenium, we varied the reaction times between them, ranging from 1 min up to 2 h. We observed for the reaction times of 1, 5, and 10 min that there was no growth of WSe 2 on the surface. For reaction times of 15 min and above, we observed a thin grayish coating on the top of the W foil. At longer reaction times of 30 min, 45 min, and 1 h, we observed a significant growth of WSe 2 , as indicated by the subtle changes in the color (which was becoming relatively darker) of the exposed surface of the W foil. But at further reaction times of 2 and 3 h, there were no significant visual changes, and these were observed according to the high-resolution scanning electron microscopy (HRSEM) images (although we could not image or analyze the material formed at 3 h because it was stuck to the boat and got damaged). (3) Optimization of Se precursor quantity: The dependency of WSe 2 -film growth on the amount of elemental selenium was also investigated by varying the precursor quantity in the range of 0.1−2 g. No significant reaction was observed for loadings of less than 1 g. However, when the Se quantity was 0.5−0.9 g, deposits of unreacted Se were found adsorbed on the W foil. For Se loadings greater than 1 g (1.5 and 2 g), a black deposit of excess Se vapor was observed along with the 2H-WSe 2 at the end of the quartz tube. Consequently, 1 g of elemental Se powder was found to be the optimized loading and was used throughout for the synthesis of 2H-WSe 2 . (4) Optimization of inert carrier gas: The role of the inert atmosphere during the synthesis was also carefully investigated. The study was carried out using three different inert carrier gases: nitrogen (N 2 ), helium (He), and argon (Ar). The results showed that there was no reaction in a nitrogen atmosphere. Helium atmosphere showed some growth (only at longer reaction times of 1.5 and 2 h between W foil and elemental Se (Supporting Figure S1)), but the yield and the coverage of the foil with WSe 2 were very low. Only argon showed significant growth and yield compared to a helium atmosphere at specified reaction times. (5) Effect of acid etching: All of the aforementioned reaction conditions were implemented for two sets of W foils. One was cleaned with 0.01 (M) HCl and isopropanol before the reaction, while the other was cleaned only with isopropanol and used in the as-purchased condition. We observed that the acid-etched W foil exhibited much better growth in comparison to the unetched W foil. The growth observed on the unetched W foil was nonconformal, similar to the growth patterns observed in the acid-etched W foil (reacting with Se at 15 min reaction time) but only at longer reaction times (1.5 and 2 h). This indicated that the nucleation sites occurring due to acid etching, as well as the removal of native oxide from W foil surface, helped in the formation of 2H-WSe 2 sheets on the W foil.
We also conducted a postannealing of the as-grown WSe 2 at 900°C in argon after the selenization reaction was completed. This resulted in (i) more substantial grain growth of the asgrown WSe 2 as well as (ii) significant damage to the W foil itself (on prolonged heating), which was stuck to the quartz boat, being ineffective and too much degraded for further analyses.
Several researchers have found that hydrogen plays a vital role in the growth of TMDCs. It acts as an additional reducing agent with Se during the growth of nanosheets and thin films when elemental Se and metal oxides are used as precursors. 7 But in our case, the absence of H 2 as a reducing agent simplified our synthetic procedure and dismissed the possibility of having H 2 Se (very toxic gas) as a byproduct. It also indicated that the synthesis could take place only with a simply diluted acid etching (taking 0.01 (M) of HCl) of the W foil before selenization. Keeping in mind the above reaction parameters, we have summarized the optimization process, as shown in Figure 1. The optimized reaction was also employed to a larger set of W foils (having cross section 4 cm × 4 cm with the same thickness of 0.127 mm), and we observed that this process could be scaled up easily.
The X-ray diffractogram analysis of tungsten diselenide in an argon atmosphere synthesized at 1 h reaction time between W foil and Se in Figure 2 indicated that a phase-pure material of high crystallinity was synthesized with a hexagonal crystal system of space group P63/mmc and space group no. 194, where all of the diffraction peaks could be indexed to the ICDD no: 00-038-1388 of 2H-WSe 2 , according to the literature. The peaks of both bulk and exfoliated 2H-WSe 2 were compared, and it showed that there were changes in the intensities of the (002) peak at 13.6°and the (105)  2°respectively. The analysis of the XRD data also showed an interesting detail regarding our material. The area of each peak is different, signifying that there is anisotropic broadening in the XRD pattern of the material. Such an anomaly can be expected because WSe 2 has a crystal structure in which the atoms are connected by covalent bonds along the in-plane direction. In contrast, the atoms interacted via van der Waals forces along the through-plane direction. 41,43 Therefore, the different interactions and bond strengths, as well as the particle size, can lead to different distortions along different planes of growth, which is observed in the X-ray diffractogram in our case. Also, it must be noted that, in general, peak broadening in X-ray diffraction measurements also indicates the formation of nanometer-sized sheets. This signifies that the material also contains both micron-and nanosized sheets. It can happen due to several reasons, the prominent one being the agglomeration of the nanosized sheets into a larger mass, which ultimately breaks up into individual sheets following the ultrasonic liquidassisted exfoliation. From the comparison of the XRD pattern between the bulk and the exfoliated 2H-WSe 2 , the intensity of the (002) peak seems to decrease, while the intensities of the (100), (103), (105), and (0010) peaks seem to show an increasing trend. So, we can conclude from the XRD measurements that 2H-WSe 2 shows a particular orientation of growth along the aforementioned planes. In addition to analyzing both the bulk and exfoliated materials, we also checked for free peaks of metallic tungsten and elemental selenium in both cases and observed that there was none. This indicated that the samples we synthesized were of high purity, and no unreacted W or Se was present. The investigation of the nanosheet size was established further using highresolution scanning electron microscopy (HRSEM) and transmission electron microscopy (TEM) characterizations. The high-magnification HRSEM image of the nanosheets is shown in Figure 2c.
From the HRSEM and HRTEM images, we observed that the material synthesized at 1 h reaction time (i.e., optimized reaction time) exhibited both bigger and smaller flakes/sheets. The smaller sheets were around 50 nm, and the larger sheets were around 80 nm in length (Supporting Figure S2). From previous reports, it is understood that CVD-assisted syntheses tend to lead to the formation of the thermodynamically stable phase, i.e., 2H-WSe 2 . In our case, we obtained the same results and reproducibility from all XRD and HRSEM imaging data at different reaction times. Based on this, we hypothesize the following: the bare unetched as well as etched W foils were unreactive at short reaction times of 1, 5, and 10 min. This indicates that the surface roughness of the W foil was a significant indicator of growth. But since the reaction occurred from 15 min onward, we can assume that both thermal annealing and a longer exposure time of the etched W foil to Se led to a reaction on the surface of the W foil. This was also according to the HRSEM images in Figure 3a−f, indicating that the bulk material had layered growth on it, and it was consistent throughout the surface of the W foil. A closer inspection at higher magnifications of the exfoliated material sheds light on the shape of the nanosheets, which were stacked on top of each other.
The observations above prompted us to present a possible reaction mechanism for the formation of 2H-WSe 2 on W foil, with the reaction scheme shown in Figure 4. The bare W foil  ACS Omega http://pubs.acs.org/journal/acsodf Article shows striations, which are even more accentuated after acid etching. Since after growth, we also observe striations, we infer that these striations are the initial nucleation sites for the reaction between W and Se to form 2H-WSe 2 . The reaction yield increases with time, with a burst after 15 min reaction time, which saturates beyond 1 h ( Figure S2 in the Supporting Information). The yield curve appears as an S-curve, which is very typical of coarsening. We also observe that with increasing reaction time, the morphology shows more layers, with the best result after 1 h. However, for too long reaction times (above 1 h), we start losing the layered morphology with the formation of agglomerated areas. After 2 h, we observe the formation of a fluffy powderlike material (as observed while physically handling the bulk material), indicating that the reaction was already complete and that further heat treatment led to weaker adhesion among the individual sheets. This characteristic is also observed while preparing dispersions from the bulk material formed after the 2 h reaction, which led to easier exfoliation of the individual layers. Additionally, after 15 min, we observe many small spherical nanosized structures, which appear to increase in size until 45 min reaction time, which could indicate a secondary growth. The presence of both W and Se in the bulk 2H-WSe 2 was further qualitatively established from elemental maps obtained using energy-dispersive spectroscopy (EDS). The mapping data were obtained for a small area in a bulk sample of WSe 2, and it exhibited a W/Se ratio very close to 1:2. The overlapped maps for W and Se also indicated a uniform and complete coverage of the W foil with 2H-WSe 2 formation after reacting with elemental selenium. We estimated the amount/yield of 2H-WSe 2 obtained from the liquid exfoliation of the bulk samples. From a 2 cm × 2 cm W foil covered with 2H-WSe 2 in 10 mL of ethanol solution, the yields after drying the ethanol dispersions for each reaction time were: (a) @15 min ∼ 2 mg, (b) @ 30 min ∼ 2.5 mg, (c) @45 min ∼ 4 mg, (d) @1 h ∼ 20 mg, (e) @ 1.5 h ∼ 20.3 mg, and (f) @ 2 h ∼ 20.5 mg (Supporting Figure S2). We, therefore, observe that an increase in reaction time leads to an increase in the yield of the exfoliated material from the bulk 2H-WSe 2 , with a significant increase in the yield at 1 h reaction time. Since there was no pronounced increase in the yield after 1 h, we can safely assume that the reaction was already complete.
The depth of the 2H-WSe 2 formed on W foil is estimated using FIB-SEM to be around 5−8 μm (synthesized at 1 h reaction time between W foil and elemental Se ( Figure  7a,b8)). The solid-state indirect band gap measurement of the bulk 2H-WSe 2 from the Kubelka−Mulk function is calculated to be around 1.37 eV, which is in agreement with the value of 1.3 eV reported for bulk 2H-WSe 2 in the literature. 41,44,45 Since the evolution in morphology was most pronounced in the reaction at 1 h (after which the reaction probably saturates, as we mentioned before), we decided to proceed further with this particular reaction set for further analysis using atomic force microscopy (AFM). The bulk tungsten diselenide (2H-WSe 2 ) samples were prepared for atomic force microscopy (AFM) measurements to see if the material had layers, using a liquid exfoliation technique, where a tiny amount from the bulk 2H-WSe 2 was taken into an appropriate solvent and sonicated in a bath for 60 min until the individual flakes were completely detached from each other and formed a homogeneous dispersion (using 80 kHz and 100% power on the sonicator). A single drop of this ethanol dispersion on a Si/ SiO 2 wafer was left to dry for 24 h, and then the portion of the 2H-WSe 2 that was left on the Si/SiO 2 wafer was examined by AFM. For our case, we used three different solvents (ethanol, isopropanol, and hexane) to find which was most appropriate. Only ethanol and isopropanol proved to be the ideal solvents in our liquid exfoliation experiments because hexane degraded our as-synthesized material to some extent. Theoretically, a monolayer should be obtained if the solvent is suitable for exfoliation. Still, we found that we could only exfoliate the bulk material to two layers in both ethanol and isopropanol. A comparison of both the bulk 2H-WSe 2 and exfoliated 2H-WSe 2 (keeping bare W foil as a reference) was made, which helped to identify the difference in the surface microstructure and exfoliated microstructure of the as-synthesized material. The terraces or the steps in the AFM image of bulk WSe 2 were found to be tens of nanometers thick, and the as-grown material is still too thick to point out the monolayers/bilayers correctly. The step-size profile in Figure 5b indicates that the material we prepared was layered, and it was composed of both bigger and smaller flakes/sheets. The stacking of each of the layers is visible. Since the HRSEM measurements also specify that the individual sheets of 2H-WSe 2 are arranged on top of each other, the AFM measurements help to identify the individual heights and also the distribution of the layers in the exfoliated sample. The deposited sheets of WSe 2 on the Si/ SiO 2 substrate have different thicknesses and heights, mainly being bilayers and few layers. The bilayer heights or step sizes range around 1.7 ± 0.3 nm. The few-layer step sizes, as observed from Figure 5c, are almost around 4 ± 0.3 nm, From the AFM and XRD images of the bulk and the exfoliated 2H-WSe 2 sample, we get an idea that the sample might be layered, two-dimensional, or contain some degree of anisotropy. To endorse our claims, we prepared similar dispersions of 2H-WSe 2 by sonicating at 80 kHz and 100% power for 60 min until we obtained a homogeneous dispersion of the material in the solvent. The solution was then centrifuged, and we took a small amount of this dispersion from the supernatant of the resulting solution and placed it on a copper grid (mesh 300). This solution was dried and observed under a transmission electron microscope (TEM) to identify its characteristic properties. The low-magnification TEM image in Figure 5f gives an idea as to how the exfoliated material looks like when deposited on the copper grid and/or if they are disintegrated on sonication. A closer inspection of the nanosheets revealed that there are distinct layers in the material we synthesized and that each of the layers has almost identical platelike shapes. The portions of the agglomerates, as observed under the TEM, also showed how the individual flakes in the exfoliated 2H-WSe 2 are stacked on top of each other, helping to confirm the findings of the HRSEM imaging and the AFM microscopy results. The length of the nanosheets was also estimated to be around 50−80 nm. The thinnest portions of the material, as observed in the HRTEM brightfield (BF) imaging, could be speculated as a single layer. However, in our case, the presence of bilayers and multilayers (5L) is more prevalent. The high-resolution transmission electron microscopy (HRTEM) measurements help us to identify the SAED pattern ( Figure 5i) and calculate the lattice fringes. The (100), (101), (103), and (006) planes were recognized and calculated from the fringes in this particular case. The HRTEM images also help to identify the characteristic honeycomb-like lattice, which is very consistent with the formation of the thermodynamically stable 2H phase of WSe 2 . Due to the absence of any other phase of WSe 2 in the HRTEM measurements, it confirms that the material we prepared is indeed phase-pure, whereby all of the peaks obtained in the XRD measurements could be easily correlated. Besides, the high-magnification images (@1 nm) of the HRTEM measurements at different reaction times (15 min (beginning), 1 h (optimized), and 2 h (end)) showed the typical honeycomb arrangement of W and Se throughout the exfoliated samples of 2H-WSe 2 . The high-magnification HRTEM images are shown in Figure 6i−iii. In our case, it seems that mostly Se vacancies are present, but this is qualitative information based on the EDS measurements obtained from the HRTEM (where W/Se ∼ 37:62), which can be explained from a previous report. 46 The d-values calculated from the lattice fringes and FFT patterns of the HRTEM images are slightly deviated compared to the original values in the ICDD reference. Further, HRTEM measurements  (i−iii) high-magnification HRTEM images of (a−c) at three different reaction times (beginning @15 min, optimized @1 h, and prolonged reaction @ 2 h, respectively).

ACS Omega
http://pubs.acs.org/journal/acsodf Article of the 2H-WSe 2 samples synthesized in the helium atmosphere at 1.5 and 2 h reaction times (because in He only the more prolonged reaction times were responsive) were also analyzed. They showed similar fast Fourier transforms (FFTs) as the samples synthesized in the Ar atmosphere ( Figure S3). We performed a Raman analysis of the exfoliated tungsten diselenide (formed at a 1 h reaction between W foil and elemental Se in Ar). The principle bands were observed at 180 cm −1 , and a double peak was observed around 248−256 cm −1 ; these bands correspond to the frequencies of the Raman-active A g 1 , E 2g 1 , and, E 1g modes of the 2H-WSe 2 crystal structure, which is in correlation with some previous works. 47,48 The absence of the B 2g 1 peak at 304 cm −1 is an indicator that the flakes of 2H-WSe 2 obtained are 3L−4L. The UV−visible absorption spectra of the WSe 2 show the expected two typical characteristic absorption peaks in the region around 475−800 nm, which correspond to the A1 and B1 direct excitonic transitions of the 2H-WSe 2 originated from the energy split of valence−band and spin−orbital coupling. 49 These two peaks also additionally indicate that WSe 2 was dispersed in ethanol only as of the 2H variant because the excitonic transitions are characteristic of the 2H phase of WSe 2 (Figures 7 and 8). 44 So, all of the aforementioned analyses are found to be consistent with the formation of the thermodynamically favorable phase-pure 2H crystal structure of WSe 2 on the W foil, and all of the characterizations of the material enabled us to check it further for application as possible electrode material in lithium-ion battery.
Electrochemical Characterization. The theoretical capacity of WSe 2 is about 313.7 mAh/g based on the following conversion reaction 15 When WSe 2 is utilized as a LiB electrode, a voltage window of 0.01−3.0 V is generally used for test. 42 To understand if our material had prospect as electrode material in LiB, the redox processes of the as-synthesized 2H-WSe 2 at 1 h reaction between W foil and elemental Se as precursor and cyclic voltammetry (CV) measurements are tested. The cyclic voltammograms corresponding to the first two cycles at a scan rate of 0.1 mV/s over a potential window of 0.01 V−3.0 V (vs Li/Li + ) are shown in Figure 9a. The voltammogram for the first cycle mainly exhibits a sharp reduction peak at around 0.77 V followed by an oxidation peak at 2.2 V. The reduction peak observed around 0.77 V during the first scan is attributed to the initial conversion of WSe 2 to metallic tungsten and lithium selenide according to eq 1 mentioned above besides the formation of a solid electrolyte interphase on the electrode surface. A careful analysis of the first reduction scan shows multiple reduction peaks at 1.7, 1.54, and 1.37 V (Figure 9a, inset). The appearance of these peaks is associated with the formation of the Li X WSe 2 phase according to the following equations Li WSe (2)  Here, we propose a stepwise formation of the Li X WSe 2 phase that occurs within the potential window of 1.8−1.3 V during the first discharge. The occurrence of these multiple peaks, which corroborated to the formation of the Li X WSe 2 phase, maybe because of the variable electrochemical environment present within the material. To understand the conversion mechanism, we performed an ex situ XRD study of the working electrode at two different discharge states, 0.5 and 0.01 V (vs Li/Li + ). The results are presented in Figure S6. Two as-assembled cells consisting of WSe 2 as the working electrode were discharged to different potentials of 0.5 and 0.01 V (vs Li/Li + ), and X-ray diffractograms were taken from the respective electrodes after dismantling the cells. Figure S6 shows that the intensities of all of the 2H-WSe 2 diffraction peaks diminish gradually, while a new peak at ∼25.8°emerged in the diffraction patterns of the discharged electrode. This new peak (2θ ∼ 25.8°) is corroborated to a newly formed Li 2 Se phase (JCPDS card no. 65−2985). Since no phases related to Li x WSe 2 can be seen in our samples, we propose that it exists as an intermediate state, which undergoes final conversion to Li 2 Se following eq 3. Furthermore, diffraction peaks of metallic tungsten (W 0 ) were not found in the sample discharged to 0.01 V, which might be attributed to the poor crystallinity of the W resulted after the conversion. 15 The CV profile for the second cycle shows two broad cathodic peaks at 1.88 and 2.0 V coupled with a sharp anodic peak at 2.24 V. This redox couple corresponds to the reversible lithiation of WSe 2 to form Li x WSe 2 /Li 2 Se and its conversion back to the WSe 2 at 2.24 V during the anodic scan. Furthermore, the weak cathodic and anodic peaks at 0.01 and 0.20 V, respectively, are associated with the intercalation and deintercalation of a minor quantity of Li ions into the electrode material. 50,51 From both the voltammograms, the reversible redox processes of 2H-WSe 2 and its ability to reversibly host Li + can be inferred, and we carried out detailed electrochemical investigations. The galvanostatic charge− discharge profile for the first two cycles at a current density of 50 mA/g is presented in Figure 9b. During the first cycle, clear plateaus for the lithiation and delithiation are observed at ∼0.86 and 2.1 V, respectively. WSe 2 exhibits an initial charge/ discharge capacity of 403/550 mAh/g, demonstrating a significant first irreversible capacity for solid electrolyte interface formation involving the irreversible consumption of electrolytes on the electrode surface. Here, we observe a specific charge capacity value that is beyond the theoretical capacity [according to eq 1]. This phenomenon is reasonably common for the conversion-type electrodes because of the two crucial facts: (i) interfacial lithium storage through the kinetic activation of the electrode materials 52 and (ii) a partial reversible conversion of the electrode material, which can be coupled with the conductive carbonaceous matrix via a synergistic effect that can perform as the local source for additional Li storage. 53 After the first cycle, lithiation and delithiation are seen at 1.95 and 2.1 V, respectively, which are in good agreement with the peaks observed during the cyclic voltammetry. The proximity of the lithiation and delithiation potentials of WSe 2 further confirms the involvement of the reversible redox process in the charge storage mechanism. Also, a decrease of 100 mAh/g in the specific capacity is observed from the first to the second cycle, which then stabilizes to ∼450 mAh/g. The electrochemical performance as a function of the increasing current densities is then investigated to understand the rate performance of the synthesized 2H-WSe 2 . Figure 10 represents the rate performance of the 2H-WSe 2 . As can be seen from the figure, the specific discharge capacity decreases from 446 (2nd cycle) to 438 (5th cycle), 440 (10th cycle), 409 (15th cycle), and 347 mAh/g (20th cycle) when the current density was increased from 50 to 100, 150, 250, and 500 mA/g, respectively. Additionally, discharge capacities of ∼340 and ∼280 mAh/g were obtained at high current densities of 1000 and 1500 mA/ g, respectively ( Figure S7). A reversible discharge capacity of 434 mAh/g (25th cycle) is obtained when the current density is decreased to 100 mA/g. Comparing the discharge capacity of 438 mAh/g at the same current density in the fifth cycle, 2H-WSe 2 exhibits a capacity loss of only 0.91%. It can be assumed that the superior electrochemical performance of our material is due to its 2D layered structure, which facilitated the diffusion and storage of Li + in the WSe 2 matrix. Next, the cycling stability of the material was investigated. In the initial cycles, a gradual decrease in the discharge capacity was witnessed, which can be attributed to the structural

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http://pubs.acs.org/journal/acsodf Article reorganization, volume expansion, as well as sluggish electrolyte penetration into the porous material, resulting in poor wettability during the initial cycles. 54,55 Interestingly, after 50 cycles, a gradual increase in the capacity is seen till 400 cycles, after which capacity was found to be stabilized at ∼555 mAh/g at a current density of 250 mA/g till 700 cycles ( Figure 11). The increase in capacity after 50 cycles can be linked to additional contribution from the interfacial/surface Li + storage mechanism through the kinetic activation of electrode material and accumulation of nanosized conversion residues like W 0 dispersed in layered structures of 2D materials. 56−58 These dispersed redox-active particles in the matrix with a binder and conductive carbon better accommodate the volume changes in addition to shortening the Li + diffusion path leading to enhanced capacity. 59,60 A gradual decrease in R CT values observed from the electrochemical impedance spectroscopy (EIS) measurements further supports the aforementioned explanations confirming better charge transfer kinetics of the material after 100 cycles ( Figure S8c). Furthermore, 2H-WSe 2 exhibited a capacity of ∼300 mAh/g when the current density was increased to 500 mA/g after ∼750 cycles at 250 mA/g. Impressively, even after an additional 500 cycles (total of 1200+ cycles) at 500 mA/g, a capacity of 271 mAh/g with a retention of ∼91% was recorded. The observed results for 2H-WSe 2 are quite commendable with the reported results. 22,61−63 The superior rate and stable cycling performance of this electrode material in its pristine form are directly related to the unique morphology. Most importantly, WSe 2 can be a possible conversion electrode material that can substitute the largely studied WS 2 -based electrodes because of the significantly reduced overpotential in comparison to WS 2 , showing the possibilities for high-energyefficient devices. 21 The detailed characterization of the phenomenon and further optimization of the material is underway and beyond the scope of this paper, which mainly focuses on the facile and scalable synthesis of 2D layered nanosheets of 2H-WSe 2 .

■ CONCLUSIONS
We reported a facile atmospheric pressure CVD-based synthesis route for synthesizing 2D, layered 2H-WSe 2 in this paper. By carefully studying the process parameters, i.e., the effect of temperature and inert atmospheres, we observed that the material formed at a temperature of 900°C (at the metal foil side in F2) only in the Ar atmosphere at 1 h reaction between W foil and elemental Se. The phase-pure 2H-WSe 2 we obtained contains a certain degree of anisotropy with some possible defects that need further detailed investigations. We also show a probable reaction mechanism for the growth of 2H-WSe 2 on the W foil, which indicated how surface roughness due to acid etching might have contributed to the formation of 2H-WSe 2 . The yield of the exfoliated material we obtained after liquid-based exfoliation of the bulk material (2 cm × 2 cm of W foil on which 2H-WSe 2 has grown) was checked, and it shows an increasing trend (displaying a characteristic S-curve) to increasing time. A significant change in morphology is observed at a 1 h reaction between W foil and Se, leading to the formation of nanoplates/nanosheets. The reaction could also be upscaled easily when we used a W foil of 4 cm x 4 cm cross section using the same optimized process parameters. The bulk and exfoliated material after thorough characterization revealed its structural and physical properties. XRD measurements confirmed the formation of a single phase, while HRSEM, HRTEM, Raman, and band gap measurements helped to identify the 2D layered nature of the material. We carried out electrochemical investigations of the material (optimized at 1 h reaction time) building on its layered structure with a tuned interlayer distance and the ability of 2H-WSe 2 to host the Li + to act as a possible electrode material for LiB. Our exfoliated material shows a reversible capacity of 446 mAh/g with appreciable stability and rate performance, signifying its possible application as an electrode material for LiB. Overall, 2H-WSe 2 alone can work as a possible LiB electrode. Further modifications are required related to its structure as well as in-depth study of intermediates formed during prolonged cycling, which are beyond the scope of this paper as it investigates this layered 2D material more from a synthetic point of view.

■ EXPERIMENTAL SECTION
Synthesis of 2H-WSe 2 . The reagents used in our synthesis are of analytical grade and are used in all experiments without further purification. We synthesized tungsten diselenide (WSe 2 ) using an atmospheric pressure chemical vapor deposition (APCVD), composed of two furnaces (Lindberg Blue) located under a fume hood. The furnaces were equipped with a fused silica (quartz) tube, while the temperatures of the furnaces were monitored using built-in furnace thermocouples. During the growth of WSe 2 , the temperatures were set at 650°C for the first furnace (F1) for elemental Se and 900°C for the W foil at the second furnace (F2). Argon gas flow (99.9999%, from Gas Technologies) was maintained using electronic mass flows MKS GM50A and controller MKS 946. The synthetic procedure is as follows: We placed a quartz boat Figure 11. Prolonged electrochemical cycling performance of 2H-WSe 2 electrode in half coin cell configuration (vs Li/Li + ). After the initial 27 cycles of rate performance, the cells were subjected to long-term cycling for 700 cycles at 250 mAh/g and another 500 cycles at 500 mAh/g.

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http://pubs.acs.org/journal/acsodf Article with 1 g of selenium powder (Alfa Aesar, 99.999%) outside of the furnace F1, upstream of the gas flow, and a second quartz boat with two 2 cm × 2 cm pieces of cleaned tungsten (W) foil (Alfa Aesar, 99.999%) downstream, also outside the heated portion of the furnace F2. One of the W foils was cleaned before selenization with 0.01 (M) hydrochloric acid and isopropanol, while the other was cleaned with isopropanol only. After purging the quartz tube with argon for 30 min keeping both boats at room temperature until the furnaces reached equilibrium at the selected temperatures in them, we proceeded to start our experiment. For the growth of tungsten diselenide, we pushed the quartz tube inside the furnace to position the W foil in the middle of the furnace F2 (900°C). We then introduced the selenium boat at the edge of the first furnace F1 and then slowly pushed it inside the heated area of furnace F1 (set at 650°C) using an external magnet. At the end of the reaction, we first pulled out the selenium boat from F1. We allowed the as-synthesized material at F2 to stay inside the furnace until it reached room temperature (we turned off the heat of both furnaces), under a constant flow of argon, before removing it from the furnace and exposing it to air. Note that we used the "fast heat technique" according to the thermal annealing reported in our previous papers, 64 but performed a slow cooling of our sample to room temperature while it was still inside the furnace. We also synthesized our material in a two-furnace system rather than a three-zone furnace, which we described during our earlier reports. 65 When we took out the final product from the furnace, we observed a dull gray coating covering the upper exposed surface of the W foil. We refined the process parameters further by varying the growth times of tungsten diselenide (WSe 2 ) at 1 min, 5 min, 15 min, 30 min, 45 min, 1 h, 1.5 h, and 2 h. We also tested lower growth temperatures of 600, 650, 700, 750, 800, and 850°C , but none of them showed any observable growth on the surface of the tungsten (W) foil (the details being explained further in the Results and Discussion section). Similar experiments were carried out for another two sets of 4 cm × 4 cm pieces of W foil to check the scalability to the optimized reaction parameters.
Exfoliation of Bulk Tungsten Diselenide (2H-WSe 2 ). The as-prepared bulk tungsten diselenide (2H-WSe 2 ) material was exfoliated into few-layer nanosheets using the liquid-phase exfoliation (LPE) method. Typically, the bulk samples were dispersed in ethanol (10 mL, Romical). The dispersions were sonicated continuously for 60 min (optimized for obtaining maximum yield from the bulk material) at room temperature using an ultrasonic bath (Elmasonic P) at a frequency of 80 kHz and power output of 100 W. The dispersions were utilized in subsequent characterizations and dried separately for further usage as a powder material for LiB electrode.
Characterization. Physical Characterizations. The X-ray diffraction measurements of the bulk tungsten diselenide (WSe 2 ) on tungsten (W) foil were performed using a Rigaku SmartLab X-ray diffractometer. We analyzed a 2 cm × 2 cm square sample of bulk tungsten diselenide (WSe 2 ). The data were collected in the 2θ range of 10−80°, with a step size of 0.004°and a scanning rate of 0.4°/min. The X-ray generator was operated at 40 kV and 30 mA with Cu Kα radiation (λ = 1.542 Å). The crystallinity and purity of the as-synthesized tungsten diselenide (WSe 2 ) on tungsten (W) foil was further analyzed by X'Pert HighScore Plus software, and the corresponding peaks in the XRD were identified. Highresolution scanning electron microscopy (HRSEM) of the bulk material was carried out using the instrument Magellan 400 FEI for further examination. We performed additional examinations with atomic force microscopy (AFM) measurements. This was done using a Bio Fast Scan scanning probe microscope (Bruker AXS). All images were obtained using tapping mode with a FastScan-A (Bruker) silicon probe (spring constant of 18 N/m). The resonance frequency of the cantilever was approximately 1400 kHz (in air). The measurements were performed under environmental conditions. The images were captured in the retrace direction with a scan rate of 2.0 Hz. The resolution of the images was 512 samples/line. For image processing, Nanoscope Analysis software was used. Before the analysis of the images, the "flatting" and "planefit" functions were applied to each image. We carried out HRTEM analysis of the sample in a JEOL JEM 2100 (operating at 200 keV). We used focused ion beam (FIB) using a Helios 600 FEI to estimate the depth of the 2H-WSe 2 formed on W foil. UV−vis measurements were done in the range of 280−900 nm for 10 mL of the sample in a quartz cell using a Cary 100 spectrophotometer (Varian), operated by LabSphere software. Raman measurements were done using a Thermo Scientific iXR Raman spectrometer. The bulk band gap measurements were done using a Bruker Vertex 70 FT-IR spectrophotometer in DRIFT mode.
Electrochemical Characterizations. Testing of the electrochemical performance of our exfoliated material (2H-WSe 2 ) is done by fabricating two-electrode coin-type cells. We used a slurry casting method and a standard doctor blade to prepare the working electrodes on a Cu-foil substrate (15 μm thick). A 70:10:10:10 wt % slurry composition mixture containing the electroactive material (2H-WSe 2 ) (70 wt %), Super C65 carbon (10 wt %), Graphite KS6 66 (10 wt %), and poly(vinylidene fluoride) (10 wt %) in N-methyl-2 pyrrolidone was prepared. The cast film (green thickness, 100 mm) kept at 110°C for 6 h in a vacuum oven vaporized the solvent, and after calendaring at 80°C in a roller press, it was cut into circular disks of 14 mm diameter. The typical active material loading in these circular electrodes was 1.2−1.4 mg. For coin cell assembly, Li metal was the counter electrode as well as the reference electrode, Celgard 2500 was the separator, and LiPF 6 in EC/EMC (3:7 vol%; LP57) was the electrolyte. Cell fabrication was performed inside an argon-filled glovebox keeping the H 2 O and O 2 levels at <0.5 ppm. Electrochemical properties were evaluated via cyclic voltammetry (CV) and galvanostatic charge−discharge (GCD) measurements. CV was performed using a VSP potentiostat (BioLogic Science Instruments) in the potential window of 0.01−3.0 V at a scan rate of 0.1 mV/s.
HRSEM images of 2H-WSe 2 grown in the helium atmosphere ( Figure S1); estimated particle size and the optimization of yield from bulk as well as the optimization of the exfoliation time ( Figure S2); TEM/HRTEM imaging to determine the d-values of 2H-WSe 2 synthesized in helium atmosphere ( Figure  S3); elemental mapping, EDS spectra, and HRTEM (FFT image) of postannealed 2H-WSe 2 in Ar ( Figure  S4); HRSEM imaging of the electrode after 70 cycles ACS Omega http://pubs.acs.org/journal/acsodf Article and 127 cycles ( Figure S5); X-ray diffraction patterns of 2H-WSe 2 electrodes (performed ex situ) at different states of discharge 0.5 and 0.01 V (vs Li/Li + ) with respect to pristine 2H-WSe 2 electrode shown for comparative understanding ( Figure S6); variation of the discharge capacity as a function of applied current density measured in a Li half-cell ( Figure S7); and impedance spectra of 2H-WSe 2 as-assembled, after the first formation cycle and after 127 cycles ( Figure S8) (PDF)

■ AUTHOR INFORMATION
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