Few-layer thick WS₂ nanosheets produced by intercalation/exfoliation route

Abstract

Thin two-dimensional (2D) nanostructures of layered materials are of importance for both science and technology due to their unique properties and promising applications. Few-layer tungsten disulfide (WS₂) presents some amazing optoelectronic behaviors that have attracted great interest. In this work, WS₂ nanosheets with few-layer (<10 layers) thickness have been successfully produced by Li ions intercalation/exfoliation of bulk WS₂ via a solvothermal technique. X-ray diffraction indicates that hexagonal crystal structure of bulk WS₂ remains the same after the intercalation/exfoliation treatment. Scanning electron microscopy and transmission electron microscopy characterizations reveal that the exfoliated products are single-crystalline thin nanosheets with high crystallinity. Atomic force microscopy reveals that 8-layer thick WS₂ nanosheets can be fabricated by the present approach. Raman phonon modes of the prepared WS₂ nanosheets present the redshifts due to their decreased thickness. The work provides a facile and affective route to high-quality WS₂ thin 2D nanostructures that would have great potentials for optoelectronics.

Introduction

Two-dimensional (2D) nanostructure of carbon, i.e., graphene, has inspired great interesting in the last decade because of its unique structure and physical properties [1–4]. Meanwhile, as the analogues of graphene, some atomically thick 2D nanostructures of layered inorganic compounds, such as transition metal dichalcogenides (TMDs, including MoS₂, WS₂, WSe₂, TiS₂, etc.), transition metal oxides (TMOs, including MoO₃, WO₃, etc.), and hexagonal boron nitride (h-BN), have drawn considerable attention from both fields of sciences and technologies [1, 5–8]. These 2D nanostructures present unique physical and chemical properties, and some applications in optoelectronics and photonics have already been demonstrated [9, 10].

Among those materials, it is known that hexagonal 2H-WS₂ has typical layered structure in which each layer consists of a plane of tungsten atoms between two close-packed planes of sulfur atoms [11]. Bulk WS₂ is a narrow band semiconductor with an indirect band gap of 1.35 eV that limits its applications in optoelectronics, while a direct band gap can be achieved in 2D monolayer WS₂ [12–14], which makes great change in its physical properties and enables it to possess a series of potential applications [15, 16]. Nowadays, some reports have shown that 2D WS₂ can be obtained by different methods including direct pyrolysis [17], atmospheric pressure chemical vapor deposition [18], thermal reduction sulfurization [19], chemical exfoliation [20], modified l-cysteine-assisted solution-phase method [21], and so on.

For layer-structured compounds, it is reasonable to expect that the exfoliation approaches are simple and effective to realize their 2D nanostructures. In particular, Li-ion-assisted wet chemical intercalation/exfoliation method has been successfully used to fabricate a series of layered materials. Zhong et al. developed a hydrothermal Li intercalation and exfoliation route using ethylene glycol as both reductant and solvent to obtain ultrathin Bi₂Te₃ nanosheets from Bi₂Te₃ bulk [22]. Similarly, the ultrathin MoS₂ nanosheets with good quality and high yield were produced [15]. Xie et al. reported that freestanding five-atom-thick Bi₂Se₃ single layers were first synthesized via a scalable intercalation/exfoliation strategy [23]. Using n-butyllithium as intercalation precursor, Rao’s group synthesized WS₂ layers by a 72-h intercalation treatment in hexane solvent under N ₂ atmosphere and ultrasonication [6].

In this work, we report the fabrication of two-dimensional WS₂ thin nanosheets by Li-ion intercalation/exfoliation route [23]. Li ions were intercalated in bulk WS₂ through a solvothermal technique, and the 2D WS₂ nanosheets were finally obtained by the exfoliation process using sonication treatment. The phase, morphology, microstructure, and Raman scattering of the obtained products were characterized. The results show that the as-prepared WS₂ nanosheets are well-crystallized single crystals and their thickness can be down to 8-layer WS₂. Compared with the previous reports on liquid or chemical exfoliation of WS₂ [6, 20], the present method is low cost and facile because no dangerous n-butyllithium or supercritical CO₂ was involved. The work provides an effective route to high-quality WS₂ thin layers with promising applications in optoelectronics.

Experimental procedure

Materials preparation

The starting materials, commercial available WS₂ (99 wt%) and Li₂CO₃ (99 wt%) powders, were directly used without further purification. The synthesis of the WS₂ nanosheets involves two steps, i.e., Li intercalation and exfoliation procedures, which is similar with the previous report [23]. In a typical run, 352 mg WS₂ powders and 142 mg Li₂CO₃ were mixed and put into a Teflon-lined stainless-steel autoclave with the capacity of 50 mL. Then 40 mL benzyl alcohol was added into the autoclave and vigorously stirred for 60 min. After that, the autoclave was sealed and maintained at 200 °C for 48 h. Cooled to room temperature naturally, the precipitates, i.e., Li-intercalated WS₂, were collected at the bottom of the autoclave and repeatedly rinsed by deionized (DI) water, acetone, and alcohol to remove soluble inorganic and organic impurities. Then the Li-intercalated WS₂ were dried at 50 °C for 6 h in vacuum. 30 mg Li-intercalated WS₂, 30 mL distilled water, and 30 mL dimethyl formamide (DMF) were added into a jar with a capacity of 125 mL, which was sealed and sonicated at for 12 h. The resultant dispersions were centrifuged at 600 rpm for 30 min and then the supernatant was collected. After centrifuging the supernatant at 6000 rpm for 10 min, the collected products were orderly washed with 3 % HCl and DI water to remove the residual Li₂CO₃. Finally, the as-prepared products were collected by repeatedly washing with ethanol and drying at 50 °C for 6 h in vacuum.

Characterization of the materials

The final products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM), and Raman scattering.

Results and discussion

Figure 1 schematically depicts the formation processes of two-dimensional tungsten disulfide (WS₂) nanosheets produced by Li intercalation/exfoliation of layered bulk WS₂ via the solvothermal and sonication treatment, which is similar with the cases of Bi₂Te₃ [21], Bi₂Se₃ [23], and MoS₂ [15]. Briefly, because the bulk WS₂ layers are connected by weak van der Waals interaction, lithium atoms of Li₂CO₃ dissolved in benzyl alcohol can be easily intercalated into the interlayers of bulk WS₂ under appropriate solvothermal condition. When the Li-intercalated WS₂ were sonicated in the mixed solution of distilled water and DMF for 12 h, the lithium atoms between layers of bulk WS₂ were extracted out, leading to the production of the exfoliated WS₂ layers. Finally, two-dimensional WS₂ nanosheets powders were collected from the suspension by centrifuging.

Figure 1

Schematic illustration of the formation of WS₂ nanosheets

As shown in Fig. 2a, b, the morphologies of bulk WS₂ and the exfoliated WS₂ layers are characterized by SEM. It can be obviously observed that the bulk WS₂ precursor consists of many thick plates with native layered structure. On the contrary, the exfoliated products are thin two-dimensional WS₂ nanosheets with flat and smooth surfaces. There is no distinct difference in the lateral sizes of the synthesized WS₂ nanosheets and the starting bulk WS₂ plates.

Figure 2

SEM images of WS₂ bulk (a) and the exfoliated WS₂ nanosheets (b)

Figure 3 shows the XRD patterns of the exfoliated WS₂ products and commercial WS₂ powders. The main reflections of the WS₂ nanosheets can be well indexed to hexagonal 2H-WS₂ with lattice constants of a = 3.1532 Å and c = 12.323 Å (ICDD PDF Card No. 08-0237) except for an unknown peak marked by square. The sharp profiles of the reflections suggest good crystallization of the sample. Compared with the XRD pattern of the commercial WS₂ powders, the exfoliated WS₂ products show the same crystal structure except for the size-related broadening of the reflections.

Figure 3

The XRD patterns of the exfoliated WS₂ nanosheets and commercial WS₂ powders

Figure 4a, b shows the low-magnification TEM images of exfoliated WS₂ nanosheets. The individual WS₂ nanosheets are randomly stacked and look transparent under the irradiation of electron beam, which implies that the obtained WS₂ nanosheets are very thin. Microstructures of the WS₂ nanosheets are further checked by HRTEM. Representative HRTEM crystal lattice images taken from the as-prepared samples are shown in Fig. 4c. Clear lattice fringes with a spacing of 0.270 nm shown in the HRTEM image can be indexed to the {100} plane of 2H-WS₂, revealing the single-crystal nature of the prepared nanosheets. The well-arranged lattice with hexagonal symmetry indicates that the top surface of the nanosheet is {001} plane. The SAED pattern shown in Fig. 4d can be indexed to 2H-WS₂, further demonstrating the nanosheet is well-crystallized single crystal. The zone axis of the SAED pattern is along [001], which reveals the [001] preferential orientation of the nanosheet, too.

Figure 4

a, b Low-magnification TEM images of the exfoliated WS₂ nanosheets. c High-resolution TEM (HRTEM) image and SAED pattern (d) of the as-prepared WS₂ nanosheets

As shown in Fig. 5a, a typical AFM topography image is shown to confirm the thickness of the as-prepared nanosheets. The nanosheet has smooth and flat surface with a uniform thickness across the lateral dimension. Figure 5b shows the height profile of the nanosheet. It can be seen that the WS₂ nanosheet has a thicknesses of about 6 nm, which corresponds to about 8 times the thickness of WS₂ monolayer [24].

Figure 5

a Typical AFM image of the exfoliated WS₂ nanosheet. b Height profile corresponding to the dashed line in the image of the exfoliated WS₂ nanosheet

Room-temperature Raman spectrum of the prepared WS₂ nanosheets (Fig. 6) shows two peaks at 349.5 and 416.5 cm⁻¹, which can be assigned to E ¹_2g and A_1g phonon modes of 2H-WS₂ [11], respectively. Compared with the corresponding phonon modes of bulk WS₂ shown in Fig. 6, there is slightly redshifts in those of nanosheets. It is known that A_1g and E ¹_2g modes are due to the vibrations perpendicular and parallel to the S–W–S layer, respectively [11, 25]. A_1g mode is sensitive to the thickness of WS₂ nanosheets, and the observed redshifts are consistent with the previous reports on the few-layer WS₂ [11, 24, 25] which indicates the weakened interaction between WS₂ layers with decreasing the thickness. On the other hand, E ¹_2g mode has less dependence on the thickness and is thought to be influenced by the long-range Coulomb interactions [25]. The frequency difference of the two modes of the prepared WS₂ nanosheets is smaller than that of bulk WS₂, which also demonstrates their decreased thicknesses [11].

Figure 6

Raman spectra for WS₂ nanosheets (a) and WS₂ bulk (b)

Conclusions

In summary, thin two-dimensional tungsten disulfide (WS₂) nanosheets have been successfully produced by Li intercalation/exfoliation of bulk WS₂ via solvothermal technique. The as-prepared WS₂ nanosheets are high-quality single crystals with hexagonal structure and have thin thicknesses of few-layer WS₂. The work provides a facile and efficient route to large-scale preparation of two-dimensional WS₂ nanosheets with potential applications in optoelectronic nanodevices.