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Sn composite film
Journal of Alloys and Compounds 800 (2019) 107e115
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
High humidity and high vacuum environment performance of MoS2/Sn composite film Yizhou Bai a, b, Jibin Pu a, *, Haixin Wang a, Liping Wang a, Qunji Xue a, Shuan Liu a, ** a Key Laboratory of Marine Materials and Related Technologies, Zhejiang Key Laboratory of Marine Materials and Protective Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbom, Zhejiang, 315201, PR China b Hohai University, Nanjing, 210098, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 January 2019 Received in revised form 27 May 2019 Accepted 4 June 2019 Available online 5 June 2019
In order to improve the service performance of key mechanical parts in high humidity and high vacuum environments, the MoS2/Sn composite films with different Sn contents were prepared by magnetron sputtering technique. The morphology, composition and structure of the as-deposited films were investigated by SPM, SEM, XRD and XPS. The mechanical and tribological properties of MoS2/Sn composite films in different humidity and vacuum environments were investigated. The results show that Sn-doping changes the crystal structure of MoS2, increases the hardness and elastic modulus, and reduces the oxidation activity of the MoS2 films. Especially, the MoS2/Sn composite film with optimized Sn content of 7.2 at.% has better tribological properties compared with pure MoS2 films in a high humidity of 75% RH and high vacuum environments. © 2019 Elsevier B.V. All rights reserved.
Keywords: MoS2/Sn composite film High humidity High vacuum Mechanical properties Tribological properties
1. Introduction With the in-depth development of the aerospace industry, the harsh conditions such as high temperature, high humidity and high vacuum exceed the limits of conventional liquid lubricants. The solid lubricant can ensure the normal operation of the machine under special circumstances, improve the safety factor and prolong the service life of the machine. Molybdenum disulfide (MoS2) has broad application prospects in photoelectrocatalysis and photovoltaic field due to its unique physical and chemical properties [1e3]. Its typical layered structure also makes it an excellent solid lubricating material. In the process of sliding friction, MoS2 is easy to slip along (0002) crystal plane due to weak van der Waals forces between unit layers [4,5]. However, the tribological properties of MoS2 is particularly sensitive to humidity. The friction coefficient of MoS2 film is significantly increased in the humidity environment, and after storage for a period of time in a humid environment, the life of the film is reduced to 1/10 [6]. This is mainly because the dangling bonds and interlayer edges on the surface of the MoS2 film easily react with water and oxygen in the air, the generated MoO3 hinders the interlayer slip of the MoS2. Thus, it exhibits a high
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Pu), [email protected] (S. Liu). https://doi.org/10.1016/j.jallcom.2019.06.038 0925-8388/© 2019 Elsevier B.V. All rights reserved.
coefficient of friction (0.15e0.3), and the wear resistance drops sharply in the humidity environment. It has been reported that MoS2 film often loses lubrication after more than a thousand cycles [7]. In order to improve the oxidation resistance of MoS2 in a humid environment, metal-doping and structural adjustment are the main methods. In general, the addition of metal to the MoS2 film results in a denser structure and higher hardness [8]. Cai Z B et al. found that lead-doping can effectively improve the impact wear resistance of MoS2 film. Under the same impact force, the MoS2/Pb nanocomposite had better wear resistance than the 304 stainless steel matrix [9]. In addition, the doped metal can protect MoS2 as an oxygen getter. Wang P et al. reported that titanium-doping not only improved the mechanical properties of MoS2 films, but also improved its resistance to the atomic oxygen [10]. As a metal lubricating material, tin has strong ductility, plastic fluidity and low melting point. In order to improve the service life of MoS2 both in high humidity and high vacuum environments, a new kind of MoS2/Sn composite film were prepared for the first time by the magnetron sputtering technique. The effects of Sn-doping on the tribological properties of the composite film were investigated under different humidity conditions. A surprising finding was that the sensitivity of MoS2 to humidity was significantly reduced, and the MoS2/Sn composite film maintained good tribological performance in vacuum at the same time.
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2. Experimental details
Table 1 The chemical components of the as-deposited films.
2.1. Sample preparation All samples were prepared by a Teer CF-800 closed field unbalanced magnetron sputtering apparatus. A titanium target, two tin targets and two molybdenum disulfide targets were selected with a purity of 99.99%. The tin target was placed symmetrically with the molybdenum disulfide target. The substrate materials include polished 304 stainless steel sheet and silicon wafer. All test pieces were sonicated in acetone and alcohol respectively for 15 min to remove surface contamination, and were fixed on a twoaxis rotary stand. When the vacuum in the chamber reached 5 104 Pa, the deposition procedure of the film stared. A transition layer of 200 nm was first deposited to improve the bonding force between the film and the substrate. Then different Sn-doped composite films were prepared by adjusting the tin target current, and six films of different Sn content were prepared by gradient. 2.2. Thin film characterization Surface morphologies and film thicknesses were investigated by thermal field emission scanning electron microscopy Quanta 250 (SEM). The surface roughness of as-deposited composite films was measured by scanning probe microscopy (SPM). The chemical composition of the deposited composite films was determined by energy dispersion spectrum (EDS). The crystal structures of asdeposited composite films were characterized by glancing angle X-ray diffraction (XRD), adopting the Cu K ray, and scanning from 5 to 80 with the scanning speed of 7 /min. The structures were further investigated by the confocal micro Raman spectrometer from 100 to 1200 cm1 with the laser wavelength of 532 nm. The nanometer-hardness (H) and Young's modulus (E) of the asdeposited film were determined by using the MTS NanoIndenter G200 system. Six repeated indentations were conducted for each film in the mode of continuous stiffness to obtain the average value. Tribological tests were conducted by CSM tribo-tester against 52100 steel ball with a diameter of 6 mm as the counterpart under a normal load of 5 N, corresponding to a Hertzian contact pressure of 1.0 GPa. A reciprocating mode was applied for 18,000 cycles at sliding speed of 5 Hz and track length of 5 mm. The atmospheric tests were carried out at 20 Ce25 C with the ambient humidity around 15 ± 3% RH, 45 ± 3% RH and 75 ± 3% RH, respectively. The vacuum tests were performed under 3.6 103 Pa at room temperature. All sliding experiments were repeated at least three times. After the tests, the wear rate (K) of the films was calculated by the formula (1):
K ¼ ð1 SÞ=
Run No.
S (at.%)
Mo (at.%)
Sn (at.%)
O (at.%)
NS/NMo
1 2 3 4 5 6 7
56.4 55.37 56.44 54.15 54.20 52.81 52.23
36.3 38.67 36.74 37.48 33.03 31.16 29.77
0 1.8 2.5 4.0 7.2 10.8 14.5
7.3 4.16 4.35 4.37 5.65 5.24 3.5
1.55 1.43 1.54 1.44 1.64 1.69 1.75
100 at.%. Oxygen element in the films originated from the residual gas in the chamber. It can be seen that Sn content of the MoS2/Sn composite film increases from 0 to 14.5 at.%, and O content decreases from 7.3 at.% to 3.5 at.%. It is reported that the content of O is related to the tightness and density of MoS2 film [11]. Furthermore, the S/Mo ratio of the as-deposited composite films is less than 2, attributing to the preferential re-sputtering of S atoms and the ion bombardment during deposition [12]. Fig. 1 shows the surface roughness of the pure MoS2 and MoS2Sn composite films. The surface of the undoped MoS2 film is very rough, showing a distinct granular structure with high surface roughness Ra of 8.93. With the incorporation of Sn element, the surface of the composite film becomes smooth, and the roughness Ra decreases gradually from 7.07 to 0.74 nm with increase of Sn content.
(1)
where V is the wear volume loss (mm3 ), Fn and L represented the normal load (N) acting on the sliding surface and total sliding distance (m), respectively, l is track length (mm) of reciprocating friction, S is the cross-section area measured by a profilometer (AlphaStep D-100). In addition, wear tracks of the films were observed by thermal field emission scanning electron microscopy Quanta 250 (SEM) and Raman spectroscopy, and wear scars on counterpart steel ball surface were analyzed by EDS and Raman spectroscopy. 3. Results and discussion 3.1. Film composition and structure The chemical components of all the films were analyzed by EDS (Table 1), and the elemental composition was normalized to
Fig. 1. The surface roughness of (a) the pure MoS2 film and the MoS2/Sn composite films with different Sn content of (b) 1.8 at.%, (c) 2.5 at.%, (d) 4.0 at.%, (e) 7.2 at.%, (f) 10.8 at.% and (g) 14.5 at.%.
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Fig. 2. XRD spectra and Raman patterns of the pure MoS2 film and MoS2/Sn composite films with different Sn content of 2.5 at.%, 4.0 at.%, 7.2 at.%, 10.8 at.% and 14.5 at.%.
The XRD spectra of the pure MoS2 film and the MoS2/Sn composite films are shown in the Fig. 2a. The diffraction peaks of the pure MoS2 film appearing at 13 , 33 and 59 correspond to the (002), (100) and (110) crystal plane of MoS2 [13,14]. The intensity of (002) diffraction peak of the pure MoS2 film is significantly higher than that of the (100) and (110) crystal planes, which indicates that the MoS2 film tends to grow parallel to the (002) plane. This orientation gives the film excellent friction properties [15,16]. The (002) diffraction peak intensity of the MoS2/Sn composite film decreases gradually with the increase of Sn content, and forms a very broad peak. Meanwhile, the (100) and (110) diffraction peaks disappear. This phenomenon indicates that the incorporation of Sn limits the growth of MoS2 grain. The relationship between the grain size and the half-height width of the diffraction peak are formulated by the Scheller formula (2):
D ¼ ðK lÞ=ðb cosqÞ
(2)
where l is the X-ray wavelength of 0.154056 nm, K is grain shape factor between 0.62 and 2.08, b is half-height width of the diffraction peak, q is Bragg diffraction angle in degrees. As the b increases, the grain size decreases [17]. So for the MoS2/Sn composite film, the decrease of (002) diffraction peak intensity suggests deterioration of crystallinity, and that the structure gradually change from crystal to amorphous. Raman was applied to further determine the superficial structure of the as-deposited films (Fig. 2b). The characteristic peaks of the undoped MoS2 at 380 cm1 and 413 cm1 correspond to E2g in the MoS2 layered unit layer and interlayer A1g vibration [18,19]. It can be seen that the characteristic peak intensity of MoS2 gradually decreases with the incorporation of Sn. When Sn content reaches 7.2 at.% or higher, the characteristic peaks substantially disappear, and no distinct peaks appear. This suggests that the structure of MoS2 changes to a quasi-amorphous structure, which is in agreement with the result in XRD.
Fig. 3 shows the cross-sectional SEM images of the pure MoS2 film and the MoS2/Sn composite films. The thickness of all films is about 4 mm, and the thickness of transition layer is about 200e250 nm. It can be seen that the pure MoS2 film exhibits a distinct columnar structure, while the composite films become finer with the incorporation of Sn element. When the Sn content is higher, the MoS2/Sn composite films grow from a columnar structure to a non-type structure, and the films are flatter and denser. According to the XRD and Raman analysis, the incorporation of Sn reduces the crystallinity and crystal size of MoS2, and disturbs the directional growth of MoS2. The microstructure of the composite films was further analyzed by HRTEM. Fig. 4(a, b) show the SAED pattern and HTEM image of the MoS2/Sn composite film with Sn content of 4.0 at.%. The Sn element exists in the form of nanocrystals in the film, and thus there is no corresponding diffraction ring. By measuring MoS2 nanocrystals in HTEM images, it can be found that these lattice fringes belong to the (002) plane of MoS2, which indicates that the (002) plane is parallel to the substrate. It is noteworthy that the incorporation of Sn causes the bending of MoS2 lattice fringes, hinders the growth of MoS2 crystals, and induces the densification, fine crystallization and amorphization of film structure. As shown in Fig. 4c, d, when Sn content is more, the lattice fringes of MoS2 are greatly curved and the structure is non-uniform, the composite films exhibit a quasi-amorphous structure, which is consistent with the XRD and Raman results above. 3.2. Film mechanical properties The hardness, elastic modulus and H/E value of the pure MoS2 and the MoS2/Sn composite films are showed in Fig. 5. It can be seen that with the increase of Sn content, the hardness of the composite film increases firstly from 4.51 GPa of the pure MoS2 film to 9.15 GPa, and then decreases to 6.25 GPa. This is because that the columnar structure of the MoS2 films are changed and the crystal
Fig. 3. Cross-sectional SEM micrographs of (a) the pure MoS2 film and MoS2/Sn composite films with different Sn content of (b) 7.2 at.% and (c) 14.5 at.%.
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Fig. 4. The SAED pattern and HRTEM images of (a, b) MoS2/Sn (4.0 at.%) and (c, d) MoS2/Sn (7.2 at.%) films.
grains are finer with the addition of Sn element, which makes the film denser to get high hardness. Meanwhile, the more grain boundaries increase yield strength of the films, so increases the hardness of the film. However, when the Sn content reaches a certain value, on the one hand, the deformation of soft metal tin causes the hardness of the composite film to decrease. On the other hand, when the grain size is small to some extent, the anti-HallPetch effect will occur, which will reduce the hardness of the film through the grain boundary sliding [20e22]. Elastic modulus of the composite films shows a consistent trend as hardness. It increased firstly from 48.00 GPa of the pure MoS2 film to 128.74 GPa, then decreased to 104.13 GPa. The plasticity factor H/E shown in the right figure can effectively predict the wear resistance of the films. When Sn content increases to 7.2 at.%, the H/E ratio reaches the maximum of 0.079, and then decreases with the further increase of Sn content. 3.3. Friction and wear performance 3.3.1. Atmospheric friction test The limited use of MoS2 in the atmosphere is mainly due to
oxidation in the humid atmosphere. In order to explore the humidity resistance of the doped composite films, the friction tests were carried out in dry environment (15 ± 3% RH), general environment (45 ± 3% RH) and humid environment (75 ± 3% RH). As shown in Fig. 6, all samples have a low coefficient of friction in a dry environment (15 ± 3% RH), and the pure MoS2 film exhibits excellent friction properties of about 0.065. Then, as the humidity increase to 45 ± 3% RH, the friction coefficient of the pure MoS2 film increases obviously, and reaches about 0.21 with the increase of the cycle. At same time, that of the Sn-doped composite films increase slightly. When the humidity further increases to 75 ± 3% RH, the friction coefficient of the pure MoS2 film is higher than 0.26 and the fluctuation is large, while that of the Sn-doped composite films is less than 0.15 and is more stable. Especially, the composite film with Sn-doping amount of 7.2 at.% has the lowest friction coefficient of 0.08. The results show that Sn effectively improves the moisture resistance of MoS2 film. The wear rates are summarized in Fig. 6. All films have an optimum anti-wear property in dry environment (15 ± 3% RH). However, the wear rate of the pure MoS2 film under high humidity increased to 18.2 107 mm3/Nm. Meanwhile, the wear rate of the Sn-doped composite films is significantly lower than that of the pure MoS2. Similar to the trend of friction coefficient, the composite film with Sn-doping amount of 7.2 at.% has a lowest wear rate of 4.5 107 mm3/Nm, which is mainly attributed to the improvement of the oxidation resistance. When Sn content is more than 7.2 at.%, the wear rate increased accordingly up to about 8.5 107 mm3/Nm. It is mainly because of decrease in the mechanical properties of the composite films. The typic wear tracks of the films formed under 15 ± 3% and 75 ± 3% RH are showed in Fig. 7. It can be seen that the wear scar width of all the samples is narrow in the dry environment. When the humidity reaches 75 ± 3% RH, the pure MoS2 film has a widest wear mark, and there are a lot of wear debris around it. It is also noted that the internal transverse groove of the wear marks is deep and the stripes are more, which is consistent with poor friction and wear properties under high humidity. The MoS2/Sn composite film with Sn content of 7.2 at.% exhibits the best friction performance in terms of the width of the wear scar and the amount of wear debris. However, the width of the wear scar increases as the Sn content further increases, and the friction performance decreases. In order to explore the friction and wear mechanism of the MoS2/Sn composite films in humid environment. Fig. 8 shows an XPS spectrum of the wear scar of the composite film with Sn content of 7.2 at.% and the pure MoS2 in humid environment (75 ± 3% RH). It can be seen that the undoped and doped composite film has a peak at a binding energy of 226.15 eV corresponding to the S2s peak of MoS2 [23], and the peaks at 228.35 eV and 231.6 eV
Fig. 5. The hardness, elastic modulus and H/E value of (a) the pure MoS2 and (b) MoS2/Sn composite films.
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Fig. 6. Friction coefficients and specific wear rate of the pure MoS2 film and the MoS2/Sn composite films under different humidity.
correspond to Mo 3d5/2 and Mo 3d3/2 of MoS2-x, the peaks at 229.15 eV and 232.2 eV correspond to the Mo 3d5/2 and Mo 3d3/2 of MoS2 (Mo4þ), respectively. The peak of the binding energy at 235.0 eV corresponds to the Mo6þ (Mo-O bond) in MoO3 [24]. It is noted that the MoO3 characteristic absorption peak of the pure MoS2 is obvious, while the MoO3 peak of the doped composite film is almost invisible. It means that the pure MoS2 film is severely oxidized under high humidity environment, resulting in a high friction coefficient and wear rate. Fig. 9 shows the SEM and EDS images of the wear scars in environment of 15 ± 3% and 75 ± 3% RH. It can be seen that a transfer layer composed mainly of Mo and S elements has been formed in wear scars of the pure MoS2 film in the dry environment. The O signal in EDS is blurred, which indicates that there is less oxidation in a dry environment. As the humidity increases to 75 ± 3% RH, it is apparent that the wear spots increase and the wear
debris increases. As for the MoS2/Sn composite film with Sn content of 7.2 at.%, the Mo, S and Sn elements are obvious while Fe signal is weak in the wear scars. This indicates that a compact and uniform transfer layer with high adhesion formed on the counterface steel ball surface during the rubbing process, which effectively improves the friction performance. However, for the MoS2/Sn composite film with Sn content of 14.5 at.%, the signal of Mo, S and Sn elements in transfer film is weakened and the Fe signal is strengthened. This indicates that excessive Sn addition not only cause the decline of mechanical properties of the composite film, but also is unfavorable to formation of transfer layer, resulting in the decrease of friction performance and the increase of wear spot area. Raman spectra were used to further analyze the composition of the transfer layer formed at 75 ± 3% RH. As shown in Fig. 10, both the pure MoS2 film and the Sn-doped composite film have two characteristic peaks at 375 cm1 and 413 cm1, corresponding to
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Fig. 7. Wear tracks of (a) the pure MoS2 film and the MoS2/Sn composite films with Sn content of (b) 7.2 at.% and (c) 14.5 at.% under 15 ± 3% and 75 ± 3% RH.
Fig. 8. XPS spectrum of the wear scar of (a) the pure MoS2 film and (b) the composite film with Sn content of 7.2 at.% under a humidity environment of 75 ± 3% RH.
E2g in the MoS2 layered unit layer and interlayer A1g vibration [25]. The shoulders appearing around 526 cm1 and 650 cm1 correspond to the secondary vibration of MoS2 [26e28]. The Raman peak of the pure MoS2 around 926 cm1 corresponds to b-FeMoO4 [29,30]. For the MoS2/Sn composite film with Sn content of 7.2 at.%, the intensity of the b-FeMoO4 peak weakens. However, that of the MoS2/Sn composite film with Sn content of 14.5 at.% strengthens again, which is responsible for the high friction and wear of the film. Thus, appropriate Sn content can effectively inhibit the oxidation of MoS2. Above all, the appropriate Sn-doping of 7.2 at.% can promote
formation of a more compact MoS2 transfer film on the counterface steel ball surface. At this time, Sn acts as an oxygen absorber to protect MoS2 from H2O and O2 [31], thus maintaining a low coefficient of friction in high humidity environment. Meanwhile, the low wear rate of the MoS2/Sn composite film also attributed to the incorporation of Sn element, which changes the film from crystal to amorphous structure, and obtains a higher hardness/elastic modulus ratio. However, as the content of Sn further increases to 10.8 at.% and 14.5 at.%, the mechanical properties of the films degrade, and the transfer film are easier wear through, so that the friction coefficient and wear rate increased significantly. Therefore,
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the appropriate Sn-doped composite film has a better tribological performance than the pure MoS2 film in high humidity environment.
Fig. 9. SEM and EDS images of the wear scars on the counterface ball surface in atmospheric friction test at 15 ± 3% and 75 ± 3% RH.
Fig. 10. Raman spectra of the wear scars on the counterface ball surface after sliding 20,000 cycles at 75 ± 3% RH.
3.3.2. Vacuum friction test In order to investigate the performance of the Sn-doped composite film under high vacuum (less than 3.6 105pa), vacuum friction tests were carried out. As shown in Fig. 11, the pure MoS2 film have highest friction coefficient of 0.05 and wear rate of 8.7 107 mm3/Nm among all the films. As for the Sn-doped composite films, both friction coefficient and wear rate first decrease and then increase with the increase of Sn content. When Sn-doping amount is 7.2 at.%, the composite film has a best tribological performance in vacuum, the friction coefficient is as low as 0.015, the wear rate is about 4.8 107 mm3/Nm. The wear tracks of the films after vacuum friction test were observed by SEM. As shown in Fig. 12, the wear track of the pure MoS2 film is significantly wider than that of the doped composite films, and there are more wear debris around the wear marks. Meanwhile, many grooves are observed in the wear track of the pure MoS2 film, suggesting a relatively poor wear resistance. When Sn-doping amount reaches 7.2 at.%, the wear scar is obviously narrower and smoother, and the surrounding wear debris is significantly reduced. This is consistent with its minimal friction coefficient and wear rate. However, with the further increase of Sn content, the wear scar becomes slightly wider, and some lateral grooves appear again. Fig. 13 is SEM and EDS images of transfer layers on wear scars after vacuum friction tests. It can be seen that the area of the wear spot in the high vacuum environment is much smaller than the humid atmospheric environment. After high vacuum friction tests, the pure MoS2 film has the largest wear spot and more scattered debris around it. With the increase of Sn content, the wear spot area and the amount of wear debris of the composite film first increase and then decrease. Among them, the MoS2/Sn composite film with Sn content of 7.2 at.% has the smallest area of the wear spot and less wear debris around it, which is consistent with its corresponding wear track pattern on surface of the composite film. EDS is used to analyze the composition of the wear scars, it can be seen that Mo, S and other element signals mainly appear around the wear spots and wear debris. Thus, as for the MoS2/Sn composite film with Sn content of 7.2 at.%, the small wear scar and the less wear debris around it makes the Mo, S and Sn elements signal weak. The relatively ambiguous O signal indicates that the oxidation is not significant under high vacuum. In addition, the friction performance in the dry environment is better than that in high vacuum environment, which is consistent with the weaker EDS signal of Mo, S and Sn in the wear spot after high vacuum friction test. This
Fig. 11. Friction coefficients and specific wear rate of the pure MoS2 film and the MoS2/Sn films in vacuum environment.
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Fig. 12. Wear tracks formed in vacuum tests for (a) the pure MoS2 film and the MoS2/Sn composite films with different Sn content of (b) 7.2 at.% (c) 14.5 at.%.
Fig. 13. SEM and EDS images of the wear scars on the counterface ball surface after vacuum tests.
means that the transfer films are formed more easily in the dry environment to reduce friction and wear, which is consistent with the weaker signal of Mo, S and Sn. 4. Conclusions A new kind of MoS2/Sn composite film were prepared for the first time by the magnetron sputtering technique. The incorporation of Sn led to a structural transformation of MoS2 film from crystal to amorphous structure, which makes the composite film structure more compact to get a higher hardness and elastic modulus. Compared with the pure MoS2 film, the composite films with appropriate Sn amount of 7.2 at.% have superior tribological performance in the humidity of 45% RH, 75% RH and vacuum environments, and the friction coefficients are as low as about 0.075, 0.08 and 0.015, respectively. Besides its excellent mechanical properties, this was mainly due to the formation of denser transfer film and the lubricating properties of the Sn element itself. Acknowledgement The work was supported by National Natural Science Foundation of China (Grant No. U1737214, 51775539) and Industrial major science and technology projects of Ningbo City (Grant No. 2017B10004). References [1] K.P. Kamal, N. Sreekanth, K.B. Ravi, et al., Solar light driven photoelectrocatalytic hydrogen evolution and dye degradation by metal-free fewlayer MoS2 nanoflower/TiO2(B) nanobelts heterostructure, Sol. Energy Mater. Sol. Cells 185 (2018) 364e374. [2] H. Xiaolin, Z. Hang, et al., Synthesis of few-layer MoS2 nanosheets-coated TiO2 nanosheets on graphite fibers for enhanced photocatalytic properties, Sol.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
High humidity and high vacuum environment performance of MoS2/Sn composite film Yizhou Bai a, b, Jibin Pu a, *, Haixin Wang a, Liping Wang a, Qunji Xue a, Shuan Liu a, ** a Key Laboratory of Marine Materials and Related Technologies, Zhejiang Key Laboratory of Marine Materials and Protective Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbom, Zhejiang, 315201, PR China b Hohai University, Nanjing, 210098, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 January 2019 Received in revised form 27 May 2019 Accepted 4 June 2019 Available online 5 June 2019
In order to improve the service performance of key mechanical parts in high humidity and high vacuum environments, the MoS2/Sn composite films with different Sn contents were prepared by magnetron sputtering technique. The morphology, composition and structure of the as-deposited films were investigated by SPM, SEM, XRD and XPS. The mechanical and tribological properties of MoS2/Sn composite films in different humidity and vacuum environments were investigated. The results show that Sn-doping changes the crystal structure of MoS2, increases the hardness and elastic modulus, and reduces the oxidation activity of the MoS2 films. Especially, the MoS2/Sn composite film with optimized Sn content of 7.2 at.% has better tribological properties compared with pure MoS2 films in a high humidity of 75% RH and high vacuum environments. © 2019 Elsevier B.V. All rights reserved.
Keywords: MoS2/Sn composite film High humidity High vacuum Mechanical properties Tribological properties
1. Introduction With the in-depth development of the aerospace industry, the harsh conditions such as high temperature, high humidity and high vacuum exceed the limits of conventional liquid lubricants. The solid lubricant can ensure the normal operation of the machine under special circumstances, improve the safety factor and prolong the service life of the machine. Molybdenum disulfide (MoS2) has broad application prospects in photoelectrocatalysis and photovoltaic field due to its unique physical and chemical properties [1e3]. Its typical layered structure also makes it an excellent solid lubricating material. In the process of sliding friction, MoS2 is easy to slip along (0002) crystal plane due to weak van der Waals forces between unit layers [4,5]. However, the tribological properties of MoS2 is particularly sensitive to humidity. The friction coefficient of MoS2 film is significantly increased in the humidity environment, and after storage for a period of time in a humid environment, the life of the film is reduced to 1/10 [6]. This is mainly because the dangling bonds and interlayer edges on the surface of the MoS2 film easily react with water and oxygen in the air, the generated MoO3 hinders the interlayer slip of the MoS2. Thus, it exhibits a high
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Pu), [email protected] (S. Liu). https://doi.org/10.1016/j.jallcom.2019.06.038 0925-8388/© 2019 Elsevier B.V. All rights reserved.
coefficient of friction (0.15e0.3), and the wear resistance drops sharply in the humidity environment. It has been reported that MoS2 film often loses lubrication after more than a thousand cycles [7]. In order to improve the oxidation resistance of MoS2 in a humid environment, metal-doping and structural adjustment are the main methods. In general, the addition of metal to the MoS2 film results in a denser structure and higher hardness [8]. Cai Z B et al. found that lead-doping can effectively improve the impact wear resistance of MoS2 film. Under the same impact force, the MoS2/Pb nanocomposite had better wear resistance than the 304 stainless steel matrix [9]. In addition, the doped metal can protect MoS2 as an oxygen getter. Wang P et al. reported that titanium-doping not only improved the mechanical properties of MoS2 films, but also improved its resistance to the atomic oxygen [10]. As a metal lubricating material, tin has strong ductility, plastic fluidity and low melting point. In order to improve the service life of MoS2 both in high humidity and high vacuum environments, a new kind of MoS2/Sn composite film were prepared for the first time by the magnetron sputtering technique. The effects of Sn-doping on the tribological properties of the composite film were investigated under different humidity conditions. A surprising finding was that the sensitivity of MoS2 to humidity was significantly reduced, and the MoS2/Sn composite film maintained good tribological performance in vacuum at the same time.
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2. Experimental details
Table 1 The chemical components of the as-deposited films.
2.1. Sample preparation All samples were prepared by a Teer CF-800 closed field unbalanced magnetron sputtering apparatus. A titanium target, two tin targets and two molybdenum disulfide targets were selected with a purity of 99.99%. The tin target was placed symmetrically with the molybdenum disulfide target. The substrate materials include polished 304 stainless steel sheet and silicon wafer. All test pieces were sonicated in acetone and alcohol respectively for 15 min to remove surface contamination, and were fixed on a twoaxis rotary stand. When the vacuum in the chamber reached 5 104 Pa, the deposition procedure of the film stared. A transition layer of 200 nm was first deposited to improve the bonding force between the film and the substrate. Then different Sn-doped composite films were prepared by adjusting the tin target current, and six films of different Sn content were prepared by gradient. 2.2. Thin film characterization Surface morphologies and film thicknesses were investigated by thermal field emission scanning electron microscopy Quanta 250 (SEM). The surface roughness of as-deposited composite films was measured by scanning probe microscopy (SPM). The chemical composition of the deposited composite films was determined by energy dispersion spectrum (EDS). The crystal structures of asdeposited composite films were characterized by glancing angle X-ray diffraction (XRD), adopting the Cu K ray, and scanning from 5 to 80 with the scanning speed of 7 /min. The structures were further investigated by the confocal micro Raman spectrometer from 100 to 1200 cm1 with the laser wavelength of 532 nm. The nanometer-hardness (H) and Young's modulus (E) of the asdeposited film were determined by using the MTS NanoIndenter G200 system. Six repeated indentations were conducted for each film in the mode of continuous stiffness to obtain the average value. Tribological tests were conducted by CSM tribo-tester against 52100 steel ball with a diameter of 6 mm as the counterpart under a normal load of 5 N, corresponding to a Hertzian contact pressure of 1.0 GPa. A reciprocating mode was applied for 18,000 cycles at sliding speed of 5 Hz and track length of 5 mm. The atmospheric tests were carried out at 20 Ce25 C with the ambient humidity around 15 ± 3% RH, 45 ± 3% RH and 75 ± 3% RH, respectively. The vacuum tests were performed under 3.6 103 Pa at room temperature. All sliding experiments were repeated at least three times. After the tests, the wear rate (K) of the films was calculated by the formula (1):
K ¼ ð1 SÞ=
Run No.
S (at.%)
Mo (at.%)
Sn (at.%)
O (at.%)
NS/NMo
1 2 3 4 5 6 7
56.4 55.37 56.44 54.15 54.20 52.81 52.23
36.3 38.67 36.74 37.48 33.03 31.16 29.77
0 1.8 2.5 4.0 7.2 10.8 14.5
7.3 4.16 4.35 4.37 5.65 5.24 3.5
1.55 1.43 1.54 1.44 1.64 1.69 1.75
100 at.%. Oxygen element in the films originated from the residual gas in the chamber. It can be seen that Sn content of the MoS2/Sn composite film increases from 0 to 14.5 at.%, and O content decreases from 7.3 at.% to 3.5 at.%. It is reported that the content of O is related to the tightness and density of MoS2 film [11]. Furthermore, the S/Mo ratio of the as-deposited composite films is less than 2, attributing to the preferential re-sputtering of S atoms and the ion bombardment during deposition [12]. Fig. 1 shows the surface roughness of the pure MoS2 and MoS2Sn composite films. The surface of the undoped MoS2 film is very rough, showing a distinct granular structure with high surface roughness Ra of 8.93. With the incorporation of Sn element, the surface of the composite film becomes smooth, and the roughness Ra decreases gradually from 7.07 to 0.74 nm with increase of Sn content.
(1)
where V is the wear volume loss (mm3 ), Fn and L represented the normal load (N) acting on the sliding surface and total sliding distance (m), respectively, l is track length (mm) of reciprocating friction, S is the cross-section area measured by a profilometer (AlphaStep D-100). In addition, wear tracks of the films were observed by thermal field emission scanning electron microscopy Quanta 250 (SEM) and Raman spectroscopy, and wear scars on counterpart steel ball surface were analyzed by EDS and Raman spectroscopy. 3. Results and discussion 3.1. Film composition and structure The chemical components of all the films were analyzed by EDS (Table 1), and the elemental composition was normalized to
Fig. 1. The surface roughness of (a) the pure MoS2 film and the MoS2/Sn composite films with different Sn content of (b) 1.8 at.%, (c) 2.5 at.%, (d) 4.0 at.%, (e) 7.2 at.%, (f) 10.8 at.% and (g) 14.5 at.%.
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Fig. 2. XRD spectra and Raman patterns of the pure MoS2 film and MoS2/Sn composite films with different Sn content of 2.5 at.%, 4.0 at.%, 7.2 at.%, 10.8 at.% and 14.5 at.%.
The XRD spectra of the pure MoS2 film and the MoS2/Sn composite films are shown in the Fig. 2a. The diffraction peaks of the pure MoS2 film appearing at 13 , 33 and 59 correspond to the (002), (100) and (110) crystal plane of MoS2 [13,14]. The intensity of (002) diffraction peak of the pure MoS2 film is significantly higher than that of the (100) and (110) crystal planes, which indicates that the MoS2 film tends to grow parallel to the (002) plane. This orientation gives the film excellent friction properties [15,16]. The (002) diffraction peak intensity of the MoS2/Sn composite film decreases gradually with the increase of Sn content, and forms a very broad peak. Meanwhile, the (100) and (110) diffraction peaks disappear. This phenomenon indicates that the incorporation of Sn limits the growth of MoS2 grain. The relationship between the grain size and the half-height width of the diffraction peak are formulated by the Scheller formula (2):
D ¼ ðK lÞ=ðb cosqÞ
(2)
where l is the X-ray wavelength of 0.154056 nm, K is grain shape factor between 0.62 and 2.08, b is half-height width of the diffraction peak, q is Bragg diffraction angle in degrees. As the b increases, the grain size decreases [17]. So for the MoS2/Sn composite film, the decrease of (002) diffraction peak intensity suggests deterioration of crystallinity, and that the structure gradually change from crystal to amorphous. Raman was applied to further determine the superficial structure of the as-deposited films (Fig. 2b). The characteristic peaks of the undoped MoS2 at 380 cm1 and 413 cm1 correspond to E2g in the MoS2 layered unit layer and interlayer A1g vibration [18,19]. It can be seen that the characteristic peak intensity of MoS2 gradually decreases with the incorporation of Sn. When Sn content reaches 7.2 at.% or higher, the characteristic peaks substantially disappear, and no distinct peaks appear. This suggests that the structure of MoS2 changes to a quasi-amorphous structure, which is in agreement with the result in XRD.
Fig. 3 shows the cross-sectional SEM images of the pure MoS2 film and the MoS2/Sn composite films. The thickness of all films is about 4 mm, and the thickness of transition layer is about 200e250 nm. It can be seen that the pure MoS2 film exhibits a distinct columnar structure, while the composite films become finer with the incorporation of Sn element. When the Sn content is higher, the MoS2/Sn composite films grow from a columnar structure to a non-type structure, and the films are flatter and denser. According to the XRD and Raman analysis, the incorporation of Sn reduces the crystallinity and crystal size of MoS2, and disturbs the directional growth of MoS2. The microstructure of the composite films was further analyzed by HRTEM. Fig. 4(a, b) show the SAED pattern and HTEM image of the MoS2/Sn composite film with Sn content of 4.0 at.%. The Sn element exists in the form of nanocrystals in the film, and thus there is no corresponding diffraction ring. By measuring MoS2 nanocrystals in HTEM images, it can be found that these lattice fringes belong to the (002) plane of MoS2, which indicates that the (002) plane is parallel to the substrate. It is noteworthy that the incorporation of Sn causes the bending of MoS2 lattice fringes, hinders the growth of MoS2 crystals, and induces the densification, fine crystallization and amorphization of film structure. As shown in Fig. 4c, d, when Sn content is more, the lattice fringes of MoS2 are greatly curved and the structure is non-uniform, the composite films exhibit a quasi-amorphous structure, which is consistent with the XRD and Raman results above. 3.2. Film mechanical properties The hardness, elastic modulus and H/E value of the pure MoS2 and the MoS2/Sn composite films are showed in Fig. 5. It can be seen that with the increase of Sn content, the hardness of the composite film increases firstly from 4.51 GPa of the pure MoS2 film to 9.15 GPa, and then decreases to 6.25 GPa. This is because that the columnar structure of the MoS2 films are changed and the crystal
Fig. 3. Cross-sectional SEM micrographs of (a) the pure MoS2 film and MoS2/Sn composite films with different Sn content of (b) 7.2 at.% and (c) 14.5 at.%.
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Fig. 4. The SAED pattern and HRTEM images of (a, b) MoS2/Sn (4.0 at.%) and (c, d) MoS2/Sn (7.2 at.%) films.
grains are finer with the addition of Sn element, which makes the film denser to get high hardness. Meanwhile, the more grain boundaries increase yield strength of the films, so increases the hardness of the film. However, when the Sn content reaches a certain value, on the one hand, the deformation of soft metal tin causes the hardness of the composite film to decrease. On the other hand, when the grain size is small to some extent, the anti-HallPetch effect will occur, which will reduce the hardness of the film through the grain boundary sliding [20e22]. Elastic modulus of the composite films shows a consistent trend as hardness. It increased firstly from 48.00 GPa of the pure MoS2 film to 128.74 GPa, then decreased to 104.13 GPa. The plasticity factor H/E shown in the right figure can effectively predict the wear resistance of the films. When Sn content increases to 7.2 at.%, the H/E ratio reaches the maximum of 0.079, and then decreases with the further increase of Sn content. 3.3. Friction and wear performance 3.3.1. Atmospheric friction test The limited use of MoS2 in the atmosphere is mainly due to
oxidation in the humid atmosphere. In order to explore the humidity resistance of the doped composite films, the friction tests were carried out in dry environment (15 ± 3% RH), general environment (45 ± 3% RH) and humid environment (75 ± 3% RH). As shown in Fig. 6, all samples have a low coefficient of friction in a dry environment (15 ± 3% RH), and the pure MoS2 film exhibits excellent friction properties of about 0.065. Then, as the humidity increase to 45 ± 3% RH, the friction coefficient of the pure MoS2 film increases obviously, and reaches about 0.21 with the increase of the cycle. At same time, that of the Sn-doped composite films increase slightly. When the humidity further increases to 75 ± 3% RH, the friction coefficient of the pure MoS2 film is higher than 0.26 and the fluctuation is large, while that of the Sn-doped composite films is less than 0.15 and is more stable. Especially, the composite film with Sn-doping amount of 7.2 at.% has the lowest friction coefficient of 0.08. The results show that Sn effectively improves the moisture resistance of MoS2 film. The wear rates are summarized in Fig. 6. All films have an optimum anti-wear property in dry environment (15 ± 3% RH). However, the wear rate of the pure MoS2 film under high humidity increased to 18.2 107 mm3/Nm. Meanwhile, the wear rate of the Sn-doped composite films is significantly lower than that of the pure MoS2. Similar to the trend of friction coefficient, the composite film with Sn-doping amount of 7.2 at.% has a lowest wear rate of 4.5 107 mm3/Nm, which is mainly attributed to the improvement of the oxidation resistance. When Sn content is more than 7.2 at.%, the wear rate increased accordingly up to about 8.5 107 mm3/Nm. It is mainly because of decrease in the mechanical properties of the composite films. The typic wear tracks of the films formed under 15 ± 3% and 75 ± 3% RH are showed in Fig. 7. It can be seen that the wear scar width of all the samples is narrow in the dry environment. When the humidity reaches 75 ± 3% RH, the pure MoS2 film has a widest wear mark, and there are a lot of wear debris around it. It is also noted that the internal transverse groove of the wear marks is deep and the stripes are more, which is consistent with poor friction and wear properties under high humidity. The MoS2/Sn composite film with Sn content of 7.2 at.% exhibits the best friction performance in terms of the width of the wear scar and the amount of wear debris. However, the width of the wear scar increases as the Sn content further increases, and the friction performance decreases. In order to explore the friction and wear mechanism of the MoS2/Sn composite films in humid environment. Fig. 8 shows an XPS spectrum of the wear scar of the composite film with Sn content of 7.2 at.% and the pure MoS2 in humid environment (75 ± 3% RH). It can be seen that the undoped and doped composite film has a peak at a binding energy of 226.15 eV corresponding to the S2s peak of MoS2 [23], and the peaks at 228.35 eV and 231.6 eV
Fig. 5. The hardness, elastic modulus and H/E value of (a) the pure MoS2 and (b) MoS2/Sn composite films.
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Fig. 6. Friction coefficients and specific wear rate of the pure MoS2 film and the MoS2/Sn composite films under different humidity.
correspond to Mo 3d5/2 and Mo 3d3/2 of MoS2-x, the peaks at 229.15 eV and 232.2 eV correspond to the Mo 3d5/2 and Mo 3d3/2 of MoS2 (Mo4þ), respectively. The peak of the binding energy at 235.0 eV corresponds to the Mo6þ (Mo-O bond) in MoO3 [24]. It is noted that the MoO3 characteristic absorption peak of the pure MoS2 is obvious, while the MoO3 peak of the doped composite film is almost invisible. It means that the pure MoS2 film is severely oxidized under high humidity environment, resulting in a high friction coefficient and wear rate. Fig. 9 shows the SEM and EDS images of the wear scars in environment of 15 ± 3% and 75 ± 3% RH. It can be seen that a transfer layer composed mainly of Mo and S elements has been formed in wear scars of the pure MoS2 film in the dry environment. The O signal in EDS is blurred, which indicates that there is less oxidation in a dry environment. As the humidity increases to 75 ± 3% RH, it is apparent that the wear spots increase and the wear
debris increases. As for the MoS2/Sn composite film with Sn content of 7.2 at.%, the Mo, S and Sn elements are obvious while Fe signal is weak in the wear scars. This indicates that a compact and uniform transfer layer with high adhesion formed on the counterface steel ball surface during the rubbing process, which effectively improves the friction performance. However, for the MoS2/Sn composite film with Sn content of 14.5 at.%, the signal of Mo, S and Sn elements in transfer film is weakened and the Fe signal is strengthened. This indicates that excessive Sn addition not only cause the decline of mechanical properties of the composite film, but also is unfavorable to formation of transfer layer, resulting in the decrease of friction performance and the increase of wear spot area. Raman spectra were used to further analyze the composition of the transfer layer formed at 75 ± 3% RH. As shown in Fig. 10, both the pure MoS2 film and the Sn-doped composite film have two characteristic peaks at 375 cm1 and 413 cm1, corresponding to
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Fig. 7. Wear tracks of (a) the pure MoS2 film and the MoS2/Sn composite films with Sn content of (b) 7.2 at.% and (c) 14.5 at.% under 15 ± 3% and 75 ± 3% RH.
Fig. 8. XPS spectrum of the wear scar of (a) the pure MoS2 film and (b) the composite film with Sn content of 7.2 at.% under a humidity environment of 75 ± 3% RH.
E2g in the MoS2 layered unit layer and interlayer A1g vibration [25]. The shoulders appearing around 526 cm1 and 650 cm1 correspond to the secondary vibration of MoS2 [26e28]. The Raman peak of the pure MoS2 around 926 cm1 corresponds to b-FeMoO4 [29,30]. For the MoS2/Sn composite film with Sn content of 7.2 at.%, the intensity of the b-FeMoO4 peak weakens. However, that of the MoS2/Sn composite film with Sn content of 14.5 at.% strengthens again, which is responsible for the high friction and wear of the film. Thus, appropriate Sn content can effectively inhibit the oxidation of MoS2. Above all, the appropriate Sn-doping of 7.2 at.% can promote
formation of a more compact MoS2 transfer film on the counterface steel ball surface. At this time, Sn acts as an oxygen absorber to protect MoS2 from H2O and O2 [31], thus maintaining a low coefficient of friction in high humidity environment. Meanwhile, the low wear rate of the MoS2/Sn composite film also attributed to the incorporation of Sn element, which changes the film from crystal to amorphous structure, and obtains a higher hardness/elastic modulus ratio. However, as the content of Sn further increases to 10.8 at.% and 14.5 at.%, the mechanical properties of the films degrade, and the transfer film are easier wear through, so that the friction coefficient and wear rate increased significantly. Therefore,
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the appropriate Sn-doped composite film has a better tribological performance than the pure MoS2 film in high humidity environment.
Fig. 9. SEM and EDS images of the wear scars on the counterface ball surface in atmospheric friction test at 15 ± 3% and 75 ± 3% RH.
Fig. 10. Raman spectra of the wear scars on the counterface ball surface after sliding 20,000 cycles at 75 ± 3% RH.
3.3.2. Vacuum friction test In order to investigate the performance of the Sn-doped composite film under high vacuum (less than 3.6 105pa), vacuum friction tests were carried out. As shown in Fig. 11, the pure MoS2 film have highest friction coefficient of 0.05 and wear rate of 8.7 107 mm3/Nm among all the films. As for the Sn-doped composite films, both friction coefficient and wear rate first decrease and then increase with the increase of Sn content. When Sn-doping amount is 7.2 at.%, the composite film has a best tribological performance in vacuum, the friction coefficient is as low as 0.015, the wear rate is about 4.8 107 mm3/Nm. The wear tracks of the films after vacuum friction test were observed by SEM. As shown in Fig. 12, the wear track of the pure MoS2 film is significantly wider than that of the doped composite films, and there are more wear debris around the wear marks. Meanwhile, many grooves are observed in the wear track of the pure MoS2 film, suggesting a relatively poor wear resistance. When Sn-doping amount reaches 7.2 at.%, the wear scar is obviously narrower and smoother, and the surrounding wear debris is significantly reduced. This is consistent with its minimal friction coefficient and wear rate. However, with the further increase of Sn content, the wear scar becomes slightly wider, and some lateral grooves appear again. Fig. 13 is SEM and EDS images of transfer layers on wear scars after vacuum friction tests. It can be seen that the area of the wear spot in the high vacuum environment is much smaller than the humid atmospheric environment. After high vacuum friction tests, the pure MoS2 film has the largest wear spot and more scattered debris around it. With the increase of Sn content, the wear spot area and the amount of wear debris of the composite film first increase and then decrease. Among them, the MoS2/Sn composite film with Sn content of 7.2 at.% has the smallest area of the wear spot and less wear debris around it, which is consistent with its corresponding wear track pattern on surface of the composite film. EDS is used to analyze the composition of the wear scars, it can be seen that Mo, S and other element signals mainly appear around the wear spots and wear debris. Thus, as for the MoS2/Sn composite film with Sn content of 7.2 at.%, the small wear scar and the less wear debris around it makes the Mo, S and Sn elements signal weak. The relatively ambiguous O signal indicates that the oxidation is not significant under high vacuum. In addition, the friction performance in the dry environment is better than that in high vacuum environment, which is consistent with the weaker EDS signal of Mo, S and Sn in the wear spot after high vacuum friction test. This
Fig. 11. Friction coefficients and specific wear rate of the pure MoS2 film and the MoS2/Sn films in vacuum environment.
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Fig. 12. Wear tracks formed in vacuum tests for (a) the pure MoS2 film and the MoS2/Sn composite films with different Sn content of (b) 7.2 at.% (c) 14.5 at.%.
Fig. 13. SEM and EDS images of the wear scars on the counterface ball surface after vacuum tests.
means that the transfer films are formed more easily in the dry environment to reduce friction and wear, which is consistent with the weaker signal of Mo, S and Sn. 4. Conclusions A new kind of MoS2/Sn composite film were prepared for the first time by the magnetron sputtering technique. The incorporation of Sn led to a structural transformation of MoS2 film from crystal to amorphous structure, which makes the composite film structure more compact to get a higher hardness and elastic modulus. Compared with the pure MoS2 film, the composite films with appropriate Sn amount of 7.2 at.% have superior tribological performance in the humidity of 45% RH, 75% RH and vacuum environments, and the friction coefficients are as low as about 0.075, 0.08 and 0.015, respectively. Besides its excellent mechanical properties, this was mainly due to the formation of denser transfer film and the lubricating properties of the Sn element itself. Acknowledgement The work was supported by National Natural Science Foundation of China (Grant No. U1737214, 51775539) and Industrial major science and technology projects of Ningbo City (Grant No. 2017B10004). References [1] K.P. Kamal, N. Sreekanth, K.B. Ravi, et al., Solar light driven photoelectrocatalytic hydrogen evolution and dye degradation by metal-free fewlayer MoS2 nanoflower/TiO2(B) nanobelts heterostructure, Sol. Energy Mater. Sol. Cells 185 (2018) 364e374. [2] H. Xiaolin, Z. Hang, et al., Synthesis of few-layer MoS2 nanosheets-coated TiO2 nanosheets on graphite fibers for enhanced photocatalytic properties, Sol.
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