Ma Zhansheng; Liu Zan; Cheng Zhilin

(School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002)

Abstract: In this work, the 2D nanostructured h-BN/WS2 heterojunction was successfully prepared by the hydrothermal method-assisted exfoliation of bulk h-BN and WS2. The morphology and structure of the as-prepared heterojunction were determined by a series of characterization techniques. The mechanism for the formation of the as-prepared heterojunction was proposed. The friction coefficient and wear rate of the as-prepared h-BN/WS2 heterojunction-grease was decreased by 33.9% and 45.9%, respectively, as compared to those of the neat grease.

Key words: heterojunction; h-BN; WS2; tribological property

1 Introduction

In recent years, two-dimensional heterostructures have gained much attention due to their uniquely tailored physical properties[1-3]. Particularly, the heterojunction consisting of tungsten disulfide (WS2) and hexagonal boron nitride (h-BN) has been discovered, which exhibits the robust superlubricity property[4]. For example, the graphene/h-BN heterojunction can significantly reduce the friction and wear in nano mechanical systems.

As far as the preparation of heterojunction was concerned,there was rarely a low-cost method mentioned in the past. Sediri, et al.[5]reported a controlled preparation of large-area h-BN/graphene heterojunction via a simple chemical deposition of h-BN layers on epitaxial graphene/SiC. Liu, et al.[6]developed the direct CVD growth of h-BN on the mechanically exfoliated graphene,thereby preparing the heterojunction consisting of fewlayer graphene and h-BN. Ionescu, et al.[7]reported the preparation of the atomically-thin vertically-stacked MoS2/WS2heterojunction through the two-step chemical vapor deposition (CVD) growth process. Definitely, it is noted that the CVD method can ensure the integrality of heterostructure, but the reaction condition is extremely harsh (which usually requires a high-temperature vacuum environment). Simultaneously, the CVD preparation of heterojunction had the disadvantages of higher cost,more complicated process, and lower output. Hence, it was urgent to probe the desirable method for prospective applications. To our best knowledge, the liquid phase exfoliation method has been rarely carried out to prepare h-BN/graphene heterostructure. Nevertheless,some investigators have attempted to use this chemical method to create heterostructures, like the metal oxide heterojunction[8]and the LDH/rGO heterojunction[9-10].

In this paper, we developed a simple method to prepare the 2D nanostructured h-BN/WS2heterojunction from bulk h-BN and WS2. The typical hydrothermal exfoliation method was adopted in the mixed NaOH/KOH solution.Through the self-assembly of h-BN and WS2nanosheets in exfoliation, the heterojunction was successfully formed depending on the interaction of the as-exfoliated h-BN nanosheets into the interlayer of WS2nanosheets. Finally,the tribological properties of the 2D nanostructured h-BN/WS2heterojunction were examined after it was added to a specified amount of grease.

2 Experimental

2.1 Preparation of 2D nanostructured h-BN/WS2 heterojunction

Firstly, 4.26 g of NaOH and 1.62 g of KOH were added into 75 mL of deionized water to form a transparent solution, followed by adding 0.6 g of h-BN powder and 0.9 g of WS2into the above solution. Subsequently,the mixture solution was transferred into a Teflonlined stainless steel autoclave, which was then put into a rotation oven operating at a rotary speed of 10 r/min at 220 °C for 6 h. After the termination of reaction,the resulting solution was placed in an ultrasonic bath for 30 minutes. Finally, the resulting suspension was centrifuged at 3 000 r/min for 10 min so that the largesized BN and WS2particles were removed thoroughly.

The supernatant was operated by the centrifugation process and separated to form a solid precipitate, which was repeatedly washed with deionized water until the pH value was equal to 7, and the final product was denoted as h-BN/WS2. The yield of h-BN/WS2heterostructure could reach 10.53%. The h-BN/WS2-grease was prepared by adding a proper amount of h-BN/WS2into the grease prior to being subject to grinding until a homogeneous mixture was obtained. The mechanism for the formation of 2D nanostructured h-BN/WS2heterojunction is proposed in Scheme 1.

Scheme 1 The mechanism for formation of 2Dnanostructured h-BN/WS2 heterojunction via self-assembly of h-BN and WS2 nanosheets during the exfoliation process

2.2 Preparation of h-BN-grease, WS2- grease and h-BN/WS2-grease

In order to examine the tribological properties of h-BN/WS2heterojunction in grease (calcium-based grease No. 2, featuring a dropping point of 187 °C, a cone penetration of 274 (0.1 mm), a highest working temperature of 100 °C, and a lowest working temperature of -20 °C), which was purchased from the Sinopec Lubricants Co., Ltd. The h-BN/WS2-grease was prepared by the following procedure. According to the required dosage added to the grease, 0.024 g of the as-prepared h-BN/WS2was added into 20 g of grease via pre-grinding of the mixture for 20 min in a mortar, and then the sample was heated at 105 °C under mechanical stirring at a rate of 180 r/min for 10 min, resulting in a flowing state grease with high viscosity. Subsequently, the resulting grease was processed for 10 min in a household wall-breaking machine with a heating capacity of 750—1 000 W.The operation mentioned above ensured the uniform dispersion of h-BN/WS2in grease. In other cases, the h-BN-grease and the WS2-grease with the same dosage of chemical feedstock added to the grease also were prepared by the above procedures.

2.3 Tribological properties testing

The tribological properties of the h-BN/WS2-grease were performed by a reciprocating sliding frictional pair testing system (MFT-5000). The testing conditions covered: a frequency of 2 Hz, a load of 10 N, an amplitude of 5 mm,and a test time of 60 min. Each testing was repeated at least three times and the average value was adopted.

2.4 Characterization

The X-ray diffraction (XRD) analysis was carried out by a powder X-ray diffractometer using CuKa radiation source at a tube voltage of 40 kV and a tube current of 100 mA(Bruker D8 Advance, Germany). The Fourier transform infrared spectroscopy (FTIR) analysis was performed on a Cary 610/670 infrared micro-spectrometer (Varian, USA).The Raman spectroscopic analysis was conducted on a Raman spectrometer (Renishaw, Britain). The energydispersive X-ray spectroscopy (EDS) analysis was carried out by a Philips XL-30 environmental scanning electron microscope. The ultraviolet-visible (UV-vis) spectra were collected on a Cary 5000 spectrophotometer. The surface morphology was detected by a S-4800II field (HITACHI,Japan) emission scanning electron microscope (SEM) and a Tecnai 12 (Philips, Netherlands) transmission electron microscope (TEM). The high resolution transmission electron microscopy (HRTEM) analysis was performed on a Tecnai G2 F30 S-TWIN field emission transmission electron microscope (FEI, USA).

3 Results and Discussion

The XRD pattern of the as-exfoliated h-BN/WS2is shown in Figure 1a. The peak at 2θ = 14.26° corresponds to the reflection of (002) plane of WS2and the peak at 2θ = 27°matches with the reflection of (002) plane of h-BN. The result suggests that the as-exfoliated h-BN nanosheets are probably embedded into the interlayer of WS2nanosheets[11]. Additionally, it is found that there is a low angle shift of about 0.64° associated to the (002) plane of h-BN. This should be ascribed to the insertion of the asexfoliated BN nanosheets in the interlayers of WS2[11]. The intensity of (002) peak of h-BN is dramatically reduced,the reason of which is ascribed to the bigger size of WS2than h-BN. Figure 1b exhibits the Raman spectra of the asexfoliated h-BN/WS2. Apparently, the characteristic peak of the as-exfoliated h-BN is located at 1 362 cm-1and those of the as-exfoliated WS2lie at 349 cm-1and 420 cm-1. The as-exfoliated h-BN/WS2has these features, suggesting the formation of the 2D nano-structured heterojunction[12].In the UV spectra (Figure 1c), the characteristic peaks of as-exfoliated h-BN/WS2are composed of h-BN and WS2nanosheets. Definitely, the feature peak of h-BN at 203 cm-1moves to 206 cm-1by the red shift due to the nanostructured heterojunction[13]. Figure 1d shows the FTIR spectra of the nanostructured heterojunction. Interestingly,the spectrum of the as-exfoliated h-BN/WS2can display the characteristic peak of WS2, whereas the peak of h-BN almost disappears. These results demonstrate that the asexfoliated h-BN/WS2arises from the h-BN nanosheets intercalated into the interlayers of WS2to form the socalled h-BN/WS2heterojunction.

Figure 1 XRD patterns (a), Raman spectra (b), UV-vis spectra(c), FT-IR spectra (d) of h-BN, WS2 and h-BN/WS2 nanosheets

Figure 2 shows the SEM, TEM images and EDS spectrum of the as-exfoliated h-BN/WS2. Distinguishingly, the h-BN/WS2has the morphology of nanosheets and the foldover of h-BN and WS2nanosheets. In Figure 2a, the h-BN nanosheets with a diameter of less than 100 nm are distinct and dispersed between the layers of WS2to form the BN/WS2heterostructure. Figure 2b depicts the BN/WS2heterostructure regions, in which the smaller h-BN nanosheets are covered by the larger WS2sheets[14]. This structure size is in accordance with the previous SEM results of h-BN and WS2. As shown in Figure 2c and 2d, the composition of the as-exfoliated h-BN/WS2is compatible with h-BN and WS2[15].

Figure 2 SEM (a, c), TEM (b), EDS images (c) and spectrum (d) of the as-exfoliated h-BN/WS2

Figure 3 HRTEM images (a, b, d, e) and SAED (c, f) patterns of the as-exfoliated h-BN/WS2

Figure 3 gives the HRTEM images and SAED patterns of the as-exfoliated h-BN/WS2in different areas. Obviously,the h-BN/WS2has the morphology of the nanoflakes(Figure 3a, 3d). Furthermore, the space lattice property clearly demonstrates the stacked feature of h-BN nanosheets intercalated into the interlayers of the few-layer WS2(Figure 3b, 3e). The measured interlayer distance in the h-BN/WS2confirms the (002) plane of h-BN with an interplanar spacing of 0.33 nm and the (002) plane of WS2with an interplanar spacing of 0.68 nm. The calculated plane spacing of the two points was in good agreement with XRD test results. Figure 2c and 2f can display the electron diffraction (SAED) patterns of the h-BN/WS2in the selected area. The stacked diffraction rings of different structures are an obvious indication of the formation of heterojunction[16]. Hence, these results can demonstrate the formation of 2D nanostructured h-BN/WS2heterojunction.The tribological properties of the h-BN/WS2-grease are performed by a reciprocating sliding frictional pair testing system. Figure 4a presents the variation of the friction coefficients of grease, h-BN-grease, WS2-grease,and h-BN/WS2-grease as a function of friction time.Throughout the running time, the friction coefficients of h-BN/WS2-grease is lower than other samples, showing a better antifriction performance. Additionally, the stable curve of h-BN/WS2-grease demonstrates the preferable tribological stability. The friction coefficient (COF)of h-BN/WS2-grease is decreased by about 33.9% as compared to that of the neat grease. Encouragingly, we found that the friction coefficient is close to 0.01 at some intervals of the running stages (in inset). This suggests that the 2D nanostructured h-BN/WS2heterojunction in grease is promising to exhibit its super-lubricating property even though there is the presence of a partial physical mixture of h-BN and WS2to some extent. As shown in Figure 4b, the wear track depths of the scar cross section indicate that the h-BN/WS2-grease has the preferable antiwear performance, which is decreased by 45.9% compared with that of grease.

Figure 4 The COF curves of the grease, h-BN-grease, WS2-grease and h-BN/WS2-grease (a), the wear track depths of cross sections (b)

4 Conclusions

In summary, the 2D nanostructured h-BN/WS2heterojunction was successfully fabricated by the self-assembly of the as-exfoliated nanosheets. The characterization results demonstrated that the structure of the heterojunction was constructed by the stacked h-BN nanosheets intercalated into the interlayers of the fewlayer WS2. Moreover, the friction performance proved that the as-exfoliated h-BN/WS2heterojunction in grease exhibited a dramatic friction reducing performance.

Acknowledgment: This work was funded by the Zhenjiang High Technology Research Institute of Yangzhou University(2017), the Yangzhou Social Development Project (YZ2016072),the Jiangsu Province Six Talent Peaks Project (2014-XCL-013),and the Jiangsu Industrial-Academic-Research Prospective Joint Project (BY2016069-02).