3.1 Composition Structure and Microstructure of MoS2@SiO2
The morphology, structure and dimensions of the synthesized samples are shown in Fig. 3. From Fig. 3a, it shows that the MoS2 formed nanoflower-shaped particles with an average diameter of 250 nm and dispersed well. The morphology of the particles can be directly observed in the corresponding TEM image (Fig. 3c). From Fig. 3b, it shows that the surface of the sample became smooth after the silica shell was deposited. At the same time, it can be clearly observed from Fig. 3d that the bright outer layer contains dark cores, indicating that a core-shell structure of silica-coated MoS2 is formed. Two main findings are obtained in the figure: (1) The previous MoS2 nanoflowers structure was completely preserved; (2) The edges of the "petals" of MoS2 nanoflowers are covered by SiO2 to form a spherical shape with good dispersibility. From these results it can be demonstrated that SiO2 was successfully deposited on the surface of MoS2 nanoflowers during the reaction. Apparently, a clear core-shell structure can be seen for all these MoS2@SiO2 nanospheres, and the average thickness of the SiO2 shell is about 30 nm.
The XRD patterns of the synthesized MoS2 and MoS2@SiO2 are shown in Fig. 3e. The diffraction peaks of MoS2 at 2θ = 13.94°, 33.14°, 38.7°, 46.06° and 69.88° correspond to the (002), (004), (110), (103) and (110) planes of the hexagonal MoS2, respectively (JCPDS No.37-1492)[19, 20]. From the diffraction pattern of MoS2@SiO2, the characteristic diffraction peaks of MoS2@SiO2 at 38.7° and 69.88° respectively correspond to the (100) and (110) planes of MoS2. In addition, the broad diffraction peak at 20-30° is amorphous SiO2, respectively (JCPDS No.29-0085)[21, 22]. The results confirmed that the MoS2 nanoflowers were coated with amorphous SiO2.
As shown in Fig. 3f, Raman spectroscopy is used to further characterize the samples. From the fitted results, MoS2 nanoflowers had characteristic peaks at 380.3 cm−1 and 402.7 cm−1, which correspond to the E1 2g and A1g modes of MoS2. E1 2g mode is the in-plane vibration of Mo-S bond and A1g is the out-of-plane vibration of S atom[23]. Since the layer number of a MoS2 flake can be identified based on the frequency difference between E1 2g peak and A1g peak[24], it indicates that each petal of the MoS2 nanoflowers prepared in this work was composed of 2 layers of MoS2. For the MoS2@SiO2 nanospheres, due to the influence of Si and O atoms on the vibration of MoS2, the E1 2g peak and A1g peak were slightly shifted to 378.7 cm−1 and 402.8 cm−1, respectively. Thus, the Raman results show that the SiO2 shell coating did not affect the structure of MoS2 nanoflowers.
To further confirm the characterization results, the composition and content of MoS2@SiO2 nanospheres were investigated. EDS map scanning spectra reflecting the spatial elemental distribution of Silicon, oxygen, sulfur and molybdenum. Fig. 4b and Fig. 4c show that the Si and O elements were distributed throughout the outer layer of the particle; while in Fig. 4d and Fig. 4e, the S and Mo elements were only distributed in the core part. This result proves the existence of the core-shell structure, which means that the outer layer and the inner core are composed of different materials. Fig. 4f illustrates that the atomic ratios of Si to O and Mo to S were both approximately 1:2, which proves the presence of SiO2 and MoS2. All the results indicate that MoS2@SiO2 core-shell structure was prepared by depositing amorphous SiO2 shells on the crystalline MoS2 nanoflowers.
3.2 Effect of confined space on the formation of α-MoO3
The thermal stability and oxidation behavior of MoS2 and MoS2@SiO2 nanoflowers were investigated by thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) to reveal their high-temperature reaction processes.TG and DSC curves are presented in Fig. 5. It can be seen that there are four weightless stages on the TG curve of MoS2. The 10% weight loss before 100 ℃ is due to the volatilization of water molecules in the sample. The organic molecules in the sample are oxidized to CO/CO2, and an exothermic peak appears at about 350 ℃. MoS2 was oxidized to α-MoO3 at 440 ℃ to 550 ℃. The DSC curve had a clear exothermic peak at about 525°C, at which the oxidation rate was the fastest. The stage after 650 ℃ is the sublimation of MoO3 and the sample begins to lose weight rapidly.
The DSC curve of MoS2@SiO2 shows that the oxidation temperature of organics is delayed from 300 ℃ to 400 ℃, while the violent oxidation process of MoS2 originally at 500 ℃ disappeared. This shows that the shell composed of SiO2 can slow down the chemical reaction at high temperature, so that MoS2 can be converted into α-MoO3 while keeping its structure unchanged. From the TG curve, it can be seen that, the produced α-MoO3 is still confined in the SiO2 shell, so that the existence of core-shell structure prevents α-MoO3 from sublimation at high temperature.
Based on the above analysis, the α-MoO3@SiO2 material required for lubrication was obtained after baking the MoS2@SiO2 nanospheres at 680 ℃ for 10 min. XRD patterns and Raman spectroscopy were used to characterize the chemical composition of the fired samples, as shown in Fig. 6. MoS2 nanoflowers were also baked at 680 ℃ under the same conditions. However, without the protection of the silica shell, α-MoO3 has sublimated at 680 ℃. So the powder left after MoS2 baking is the intermediate oxidation product, MoO2.
Figure 6a shows the XRD patterns of MoO2 and α-MoO3@SiO2. The diffraction peaks at 2θ = 26.05°, 37.04°, 53.57° and 66.71° correspond to the (011), (-211), (-311) and (-402) planes of MoO2 (JCPDS No. 73-1249). It can be seen from the results of thermal analysis that MoS2 is oxidized to MoO3 and volatilized after being baked at 680°C. Only the intermediate oxide MoO2 remains in the sample, which is consistent with the XRD results. From the diffraction pattern of α-MoO3@SiO2, the diffraction peaks at 12.85°, 23.38°, 27.38°, 33.81° and 49.33° correspond to the (020), (110), (021), (111) and (002) planes of MoO3 (JCPDS No. 76-1003). The broad diffraction peak corresponding to amorphous SiO2 (JCPDS No.29-0085) still exists. It is proved that MoS2 is completely oxidized to α-MoO3 during high temperature baking, and α-MoO3 is not sublimated under the protection of SiO2 shell. In addition, the inset of Figure 6a shows the TEM image of the α-MoO3@SiO2 nanospheres. It can be seen that the core-shell structure is obvious, and the morphology and size are consistent with MoS2@SiO2.The core-shell α-MoO3@SiO2 nanospheres were successfully prepared.
Figure 6b shows the Raman spectroscopy of MoO2 and α-MoO3@SiO2. For MoO2, there is no characteristic peak of MoS2 in the spectrum, indicating that MoS2 in the sample is completely sublimated. From the result, it can be found that the spectrum of α-MoO3@SiO2 is consistent with that of layered orthorhombic ɑ-MoO3[25, 26]. The peak at 285 cm−1 is attributed to the peaks of the B2g and B3g vibrational modes of the Mo-O bond. The peak at 819 cm−1 is attributed to the peaks of the Ag and B1g vibrational modes of the Mo-O-Mo bond, and the peak at 994 cm−1 is attributed to the peaks of the Ag and B1g vibrational modes of the Mo=O bond[27, 28]. The results show that the core in the core-shell MoO3@SiO2 is layered α-MoO3.
3.3 Tribological Properties of α-MoO3@SiO2
The tribological properties of the α-MoO3@SiO2 and other reference samples were tested using a ball-to-disk tester. As shown in Fig. 7a, the friction coefficient of α-MoO3@SiO2 is low, about 0.2. The friction coefficient of MoO2 is 0.7. It shows that α-MoO3@SiO2 generated at high temperature still has lubricating properties, while MoO2 does not have lubricating properties. The α-MoO3 composites formed in the confined space of the core-shell structure can be used in high-temperature lubrication. The friction coefficient of SiO2 is 0.58 on average, which is significantly higher than that of α-MoO3@SiO2. Compared with silica alone, it shows that the lubricating properties of α-MoO3@SiO2 come from MoO3. In addition, the friction coefficients of MoS2 and MoS2@SiO2 can be kept around 0.05, showing excellent lubricating properties. It shows that the prepared SiO2 shell will not reduce the lubricating performance.
To explore the surface morphology, SEM images show the wear scar morphologies of different lubricated surfaces. As shown in Fig. 7b, when α-MoO3@SiO2 is used as lubricant, the wear scar has a rough morphology and fine wear debris on the surface, and slight abrasive wear occurs during the friction process. It can be seen from Fig. 7c that the wear scars when lubricated by MoS2 are very shallow and smooth. Combined with the friction coefficient in Fig. 7a, it can be demonstrated that the prepared MoS2 nanoflowers have excellent lubricating properties. For MoS2@SiO2, the wear scar area in Fig. 7d is relatively flat with some small wear scars. The wear scar in Fig. 7e has a rough surface and abrasive grains. MoO2 has no lubricating effect in friction. In Fig. 7f, the wear scar is severe under pure SiO2 lubrication, and the surface has obvious grooves. This is abrasive wear caused by flaking debris.
The wear scar width can also show the anti-wear advantage of the core-shell structure. When MoS2@SiO2 is used as a lubricant, although the friction coefficient is not the lowest, it has the smallest wear scar width of 256 µm. It shows that the prepared composite material has excellent wear resistance and wear resistance. The wear scar widths of MoS2 and MoS2@SiO2 are 325 µm and 256 µm, respectively. Although their friction coefficients are similar, the wear scar widths are quite different. The core-shell structure was found to be responsible for the decrease in wear. MoO2 and pure SiO2 wear a lot due to their lack of lubricating ability, and their wear scar widths are 409 µm and 352 µm, respectively. This also shows that SiO2 alone cannot reduce the wear, while the 2D material and SiO2 will have a synergistic effect when they work together, so that the wear rate is significantly reduced.
3.4 Tribological Mechanism
As the morphology and structure of the sample changed during the friction process, in order to further explore the friction mechanism, the surface element distribution changes of the sample after friction were further studied. The SEM image of the wear scar and the EDS analysis (the substrate is SiO2) are shown in Fig. 8. For MoS2, a dense lubricating film is formed on the friction surface, and a small amount of agglomeration of MoS2 occurs. No ball flower was observed, indicating that the ball flower structure assembled by the nanosheets is easy to peel off under the action of friction. Due to the unique two-dimensional structure of the MoS2 nanosheet, interlayer sliding occurs and spreads on the friction surface to achieve a good lubricating effect. For MoS2@SiO2, MoS2 agglomeration also exists on the friction surface. No spherical structure was observed, combined with EDS analysis and low friction coefficient, it is speculated that the SiO2 shell ruptured during the friction process to release MoS2 to achieve a good lubricating effect.
As shown in Fig. 8, there is a small amount of lubricant on the friction surface of MoO2. EDS results show that the friction surface is free of elements and MoS2 is completely oxidized. Combined with the XRD pattern, MoO3 sublimates at high temperature, and there is only a small amount of MoO2 on the friction surface, resulting in insufficient lubrication. For MoO3@SiO2, no spherical structure was observed on the friction surface. Combining the Raman spectroscopy of Fig. 6b with the EDS results, during the baking process, MoS2 is oxidized to layered MoO3, but the SiO2 shell protects α-MoO3 from sublimation and still maintains a layered structure. During the friction process, the SiO2 shell ruptures and releases layered α-MoO3, which continues to work as a lubricant.
Based on the above results, the mechanism of the core-shell structure samples during the lubrication process is summarized. As shown in Fig. 9a, for MoS2@SiO2, the SiO2 shell cracks during friction, releasing MoS2. MoS2 lamellar structure has excellent lubricating properties and synergizes with nano-sized SiO2, which has good anti-wear and anti-friction properties. As shown in Fig. 9b, for α-MoO3@SiO2, the existence of the SiO2 shell has two functions: (1) It protects MoO3 from sublimation at high temperature and greatly reduces the weight loss rate. (2) The broken SiO2 shell and layered α-MoO3 work together to still provide a good lubricating effect. The presence of the core-shell structure plays a crucial role in improving the tribological properties of the lubricant.