One-step hydrothermal synthesis of double core-shell oxygen-incorporated molybdenum disulﬁde and its tribological properties

Nowadays, with the rapid development of auto aftermarket, efficient lubrication technology plays a more and more important role in prolonging engine life and improving fuel economy [1]. However, traditional lubricant additives containing organophosphorus, sulphur and chlorine may cause subsequent pollution. Therefore, many studies have explored new lubricating additives to reduce environmental pollution. In recent years, with the development of nanotechnology, molybdenum disulfide (MoS2) nanomaterials have been widely used in various fields [2–8] such as enzyme-like applications [7] and photodegradation [9]. MoS2 has a typical sandwich-layered structure that makes it easy to slip and has good lubricating properties [10, 11]. Besides, there are tribological studies of MoS2 nanomaterials with various morphologies, such as flower and ball [12]. By introducing oxygen atoms into the MoS2, the interlayer spacing can be increased [13], and it is better to slide. There are few studies on oxygenincorporated MoS2. So we used a hydrothermal method to prepare double core-shell oxygen-incorporated MoS2 (DB-OMoS2) and studied its tribological properties by adding in engine oil (SN 5W-40 base oil) in this work.


INTRODUCTION
Nowadays, with the rapid development of auto aftermarket, efficient lubrication technology plays a more and more important role in prolonging engine life and improving fuel economy [1]. However, traditional lubricant additives containing organophosphorus, sulphur and chlorine may cause subsequent pollution. Therefore, many studies have explored new lubricating additives to reduce environmental pollution. In recent years, with the development of nanotechnology, molybdenum disulfide (MoS 2 ) nanomaterials have been widely used in various fields [2][3][4][5][6][7][8] such as enzyme-like applications [7] and photodegradation [9]. MoS 2 has a typical sandwich-layered structure that makes it easy to slip and has good lubricating properties [10,11]. Besides, there are tribological studies of MoS 2 nanomaterials with various morphologies, such as flower and ball [12]. By introducing oxygen atoms into the MoS 2 , the interlayer spacing can be increased [13], and it is better to slide. There are few studies on oxygenincorporated MoS 2 . So we used a hydrothermal method to prepare double core-shell oxygen-incorporated MoS 2 (DB-O-MoS 2 ) and studied its tribological properties by adding in engine oil (SN 5W-40 base oil) in this work.

Materials synthesis
In the first place (Figure 1 Preparation and tribological properties of lubricating oil samples: The samples prepared were distributed into the SN 5W-40 base oil utilising 60 min ultrasonication. The tribological properties of the oil with samples were examined on an MRS-10A four-ball testing machine. The testing of tribological properties was conducted at a rotating speed of 1200 rpm and a load of 392 N for 3600 s. The experimental steel balls' material conformed to the American ANSI standard E-52100 chromium alloy steel ball. Besides, the diameter of the steel ball was 12.7 mm, and the hardness of the ball was 61-66 HRC. The tribological behaviours of these samples were investigated under a rotational speed of 1200 rpm and a constant load of 392 ± 2 N at 75 ± 2 • C for 3600 s. The average wear scar diameter (AWSD; ± 0.01 mm) of the three bottom balls decided the wear rate. Each experiment was repeated three times under the same conditions.

Characterisation of the samples
The X-ray diffraction (XRD) patterns were operated by a D8 advance (Bruker-AXS) diffractometer with Cu Kα line (λ = 0.1546 nm). The microstructures of the sample were characterised by a scanning electron microscope (SEM; JEOL JXA-840A) at 20 kV equipped with an energy dispersive X-ray spectrometer (energy dispersive spectroscopy (EDS)) and transmission electron microscope (TEM; JEOL JEM-2100). Fourier transform infrared spectroscope (FT-IR; D/max2500, Rigaku Company, Japan) was used to determine the success of composite formation. N 2 adsorptiondesorption curve and pore size analysis (BET) were measured by an American NOVA2000e type-specific surface area and porosity analyser. Thermal stability was analysed by thermogravimetry (TG) and derivative thermogravimetry (DTG) (TG-DTG; NETZSCH STA 449F3). The tribological properties were tested by an MRS-10A four-ball testing machine (Yihua Company, China).

XRD analysis
The

Morphology analysis
According to the SEM images, Figures 3(a) and (b) show the pictures of DB-O-MoS 2 . The morphologies of products were primarily investigated by SEM measurement. Figure 3(a) indicates that the sample is of ultrathin nanosheet morphology with uniform lateral size in the range of about 100 nm. The sample self-assembled the binuclear shells from nanosheets. Figure 3(b) shows that the obtained samples had a double coreshell structure of about 900 nm in size and we can see the outer and the inner shells. EDS analysis was performed for the elemental composition of oxygen-incorporated MoS 2 . Figure 3(c) shows the EDS spectrum of oxygen-incorporated MoS 2 , which It could be clearly observed that DB-O-MoS 2 had a coreshell structure, and it was assembled from a large number of nanosheets.

FT-IR analysis
From the FT-IR spectra in Figure 4

TG-DTG analysis
The thermal gravimetric analysis of DB-O-MoS 2 is shown in Figure 5. In Figure 5, the TG-TDG curves of DB-O-MoS 2 underwent two stages of weight loss. The first stage was 50-200 • C with a weight loss. The weight loss in this stage was mainly water, which was attributed to water volatilisation. The second stage was 200-500 • C with a weight loss. The weight loss in the second stage was ascribed to the oxidation of DB-O-MoS 2 [14].

Friction and wear properties analysis
All the samples were used as lubricating additives for SN 5W-40 base oil; Figure 7 shows