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Synthesis and tribological properties of flower-like MoS2 microspheres
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CERAMICS INTERNATIONAL
Ceramics International ] (]]]]) ]]]–]]] www.elsevier.com/locate/ceramint
Synthesis and tribological properties of flower-like MoS2 microspheres Guogang Tanga,b, Jing Zhanga, Changchao Liua, Du Zhanga, Yuqi Wanga, Hua Tanga,n, Changsheng Lia a
School of Materials Science and Engineering, Jiangsu University, Jiangsu Province, Zhenjiang 212013, PR China b Department of Chemical Engineering, Zhenjiang College, Jiangsu, Zhenjiang 212003, PR China Received 10 December 2013; received in revised form 21 March 2014; accepted 21 March 2014
Abstract MoS2 flower-like microspheres with a mean diameter of about 1 μm, assembled by nanosheets, were successfully synthesized through a Pluronic F-127 (Ethylene Oxide/Propylene Oxide Block Copolymer) assisted hydrothermal method. The as-prepared products were characterized using powder X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM); UMT-2 multispecimen tribotester was used to assess their lubricating effect when used as additives in liquid paraffin dispersions. And the topography of worn scars was measured using a common scanning electron microscopy. Moreover, the relationship between the tribological properties and morphology of MoS2 were discussed. Tribological performance evidenced that the obtained flower-like MoS2 microspheres possessed superior anti-wear and friction-reducing properties as a lubrication additive compared with pure base oil and base oil containing commercial MoS2 plates, which will penetrate more easily into the interface with base oil, and form continuous film in concave of rubbing surface, enhancing the tribological properties. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Flower-like microspheres; Hydrothermal synthesis; MoS2; Tribological properties
1. Introduction Among various morphological structures of inorganic nanomaterials with different morphologies, three-dimensional (3D) nanostructures have been attracted much attention in many application areas of modern science and technology due to their inherent advantages, unique properties and potential applications [1–4]. Especially, they have enabled a number of key innovations in nanoscience in recent years. The simplest synthetic route to 3D nanostructures is probably self-assembly, in which ordered aggregates are formed through a spontaneous process [5,6]. To date, many graceful nano-architectures have been prepared by various techniques based on different mechanisms. For example, TiO2, ZnO, and Fe2O3 3D nanostructures [7–9] were successfully synthesized in the form of nanowire/-rod/-tube flower-like clusters via a wet-chemistry n
Corresponding author. Tel./fax: þ 86 511 8879 0268. E-mail address: [email protected] (H. Tang).
process. However, it still remains a challenge to synthesize high-quality 3D nanostructures at a high yield by effective and low cost methods which are highly demanded for new technological applications. In the past few years, layered transition metal dichalcogenides MS2 (M: Mo, W) nanomaterial have been receiving great attention because of their unique structure and superior properties, which exhibit extensive applications in catalysis, electrocatalysis, electrode materials for high-energy batteries, and lubricants [10–15]. As we know, transition metal dichalcogenides MS2 constitute a characteristic layered structure consisting of covalently bound S–M–S trilayers in analogy to graphite. Molybdenum disulfide (MoS2) with special layered closepacked hexagonal crystal structure is one of typical layered compounds, which is particularly important for solid lubrication or as an additive for lubricating oils and greases and has enjoyed the reputation of “the king of lubrication” for a long time. Previous studies have also shown that MoS2 exhibits superior tribological properties with numerous potential applications, such
http://dx.doi.org/10.1016/j.ceramint.2014.03.115 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: G. Tang, et al., Synthesis and tribological properties of flower-like MoS2 microspheres, Ceramics International (2014), http://dx.doi. org/10.1016/j.ceramint.2014.03.115
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as additives to lubricating fluids [16]; as self-lubricating coatings [17] and for improving the mechanical behavior of nanocomposites [18]. It has been well known that nanosized MoS2 usually has better tribological properties either in friction reduction or wear resistance than microsized and bulk MoS2. To date, various preparation methods have been developed to prepare MoS2 nanomaterials, including thermal reduction, hightemperature sulfurization, hydrothermal method, chemical vapor deposition (CVD), and even laser ablation [17–21]. However, MoS2 hierarchical self-assembled nanostructures have rarely been reported, and not to mention its application in the tribological field. In our previous study, we successfully
prepared MoS2 nanoflowers by using Cetyltrimethylammonium Bromide (CTAB) as a surfactant under hydrothermal conditions [22]. Previous investigations indicated that MoS2 nanoflowers were able to enhance the tribological properties of base oil as lubricant additives. Herein, we report the successful synthesis of flowerlike MoS2 microspheres via a surfactantassisted hydrothermal method. The products were characterized by X-ray powder diffraction (XRD), energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). In addition, we had also investigated the tribological properties of MoS2 microspheres as lubrication additives in paraffin base oil which
Fig. 1. XRD pattern and EDS of the as-prepared flower-like MoS2 microspheres.
Fig. 2. SEM (a, b), TEM (c) and HRTEM images (d) of the as-prepared MoS2 microspheres. Please cite this article as: G. Tang, et al., Synthesis and tribological properties of flower-like MoS2 microspheres, Ceramics International (2014), http://dx.doi. org/10.1016/j.ceramint.2014.03.115
G. Tang et al. / Ceramics International ] (]]]]) ]]]–]]]
indicate that as-synthesized MoS2 products are good oil additives with excellent anti-wear and friction-reducing properties. More importantly, this work not only gives some insight into the design of nanostructures for enhancing tribological performances, but also provides a simple and novel route for the hydrothermal preparation of other layered transition metal dichalcogenides nanomaterials.
2. Experimental 2.1. Materials All chemical reagents were of analytic purity and used directly without further purification. Commercial bulk MoS2 (microMoS2, 300 meshes, purity 498%) was purchased from Sinopharm Chemical Reagent Co. Ltd. (SCRC) (Shanghai, China). 2.2. Synthesis of MoS2 samples
3
(λ¼ 0.1546 nm). The morphologies and structures of the samples were characterized by scanning electron microscopy (SEM, JEOL JXA-840A) and transmission electron microscopy (TEM) with a Japan JEM-100CX II transmission electron microscopy.
3. Results and discussion Fig. 1 shows the XRD patterns of as-prepared MoS2 synthesized by the surfactant-assisted hydrothermal method. All labeled diffraction peaks in Fig. 1 can be indexed to those of the pure hexagonal phase of MoS2 with calculated lattice constants of a ¼ 3.161, c ¼ 12.84 Å, which is consistent with the values of standard card (JCPDS no. 37-1492). The EDS spectrum of the MoS2 microspheres (Fig. 1b) gives the signals of only element Mo and S; besides Cu, no other element was observed. Furthermore, the quantification of the peaks shows that the atom ratio of Mo:S is about 1.96:1, which is close to the stoichiometric ratio of MoS2.
MoS2 flower-like microspheres were prepared using a recently described protocol [24]. The typical experimental procedure was designed as follows: 0.44 g of (NH4)2MoO4, 0.62 g NH2OH HCl and 1.05 g of CH4N2S were dissolved in 80 ml deionized water, then 0.3 g of F127 was added into the solution under constant stirring, pH value of the mixture was adjusted to 6 by the addition of 2 mol/L HCl. The mixture was then transferred into a 100 ml Teflon-lined stainless steel autoclave and sealed, and the autoclave was placed in a preheated oven at 180 1C for 24 h and naturally cooled down to room temperature. Black precipitates were collected by centrifugation and washed with distilled water and absolute ethanol for several times, and finally dried in vacuum at 60 1C for 10 h. 2.3. Preparation and tribological properties of lubricating oil samples The liquid paraffin samples with 0.5–5 wt% MoS2 flowerlike microspheres and 2 wt% commercial MoS2 micro-plates were prepared by dispersing agent sorbitol monooleate (Span80) via 60 min ultrasonication. The tribological behaviors were investigated on a CETR Universal Micro-Tribometer (UMT-2) at a rotating speed of 100–500 rpm and a constant load of 5–50 N for 1 h at room temperature. The material of upper sample is 440C stainless steel ball with a diameter of 10 mm, hardness of 62 HRC, and the counterpart is 45 steel disc of Ø40 mm 3 mm in size. Each tribological test was repeated three times to ensure reproducibility. The obtained wear scars were characterized using a VEECO WYKO NT1100 noncontact optical profile testing instrument and an electronic beam-based microscope (SEM, HITACHI S-3400N). 2.4. Characterization of MoS2 samples The X-ray diffraction patterns were recorded using a D8 advance (Bruker-AXS) diffractometer with CuKα radiation
Fig. 3. Variations of friction coefficient of lubricant with increasing load (a) and under diverse speeds (b).
Please cite this article as: G. Tang, et al., Synthesis and tribological properties of flower-like MoS2 microspheres, Ceramics International (2014), http://dx.doi. org/10.1016/j.ceramint.2014.03.115
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The size, shape, and structure of the as-prepared MoS2 are characterized by SEM and TEM microscopy. SEM images (Fig. 2a and b) show that the obtained samples are uniform flower-like MoS2 microspheres with an average diameter of about 1 μm. Fig. 2c shows a typical TEM image of MoS2 microsphere, which reveals that the as-prepared MoS2 microspheres consist of many MoS2 nanosheets. The highly wrinkled surface and extruded lamella-like structure of the microspheres could be obviously observed, which indicates that the flower-like microspheres are composed of MoS2 nanosheets. More details for MoS2 structure are illustrated by HRTEM studies in Fig. 2d, which indicates that the nanosheets consist of about 10-layered structures. The distance between the lattice fringes is 0.63 nm, slightly larger than the reported data (0.62 nm) for the (002) planes of the hexagonal MoS2 structure. Recently, the formation mechanism of novel flower-like nanostructures has been widely reported and discussed [7–9,24]. Kamalianfar et al. [24] described the fabrication of ZnO flower-like multi-sheets via a vapor–liquid– solid process. They indicated that self-assembly growth and Ostwald ripening mechanism may account for the growth process of ZnO flowers. Very recently, our previous studies [22] reported that well-defined MoS2 nanoflowers are formed from several MoS2 nanosheets through a self-assembly process and Ostwald ripening process with help of CTAB based on the time-dependant shape evolution of MoS2 nanoflowers. In this study, we believe that the growing process of flower-like MoS2 microspheres is consistent with previous reports, the sheet-like MoS2 nanostructures gradually evolved to flowerlike MoS2 microspheres through the self-assembled process with help of F127.
In order to evaluate the friction and wear properties of flower-like MoS2 microspheres, fretting experiments were conducted with varying loads and rotation speeds at constant sliding distance and frequency. Fig. 3a shows the friction coefficient as a function of the concentration of as-prepared MoS2 samples and commercial MoS2 at different loads under a speed of 100 rpm for 1 h. It is observed that the friction coefficient of pure base oil without any additives is increased with increasing load. Moreover, the friction coefficient of the base oil containing as-prepared MoS2 samples and commercial MoS2 is always lower than that of pure base oil, and the base oil containing as-prepared MoS2 samples exhibit lower friction coefficient than that of commercial MoS2. And the friction coefficient of the base oil containing as-prepared MoS2 samples decreases with the mass percent of the additives increased. Especially, the base oil with 2% MoS2 flower-like microspheres has a lower and stable friction coefficient compared with other mass fraction of the additives at higher load. Fig. 3b shows the friction coefficient as a function of concentration of MoS2 flower-like microspheres and commercial MoS2 at 10 N loads under diverse speeds for 1 h. The friction coefficient of the basic oil containing MoS2 flower-like microspheres is lower than that of basic oil containing commercial MoS2 and pure basic oil at different rotating speeds. To further determine the wear resistance tribological properties of flower-like MoS2 microspheres, a VEECO WYKO NT1100 non-contact optical profile testing instrument is used for measuring the grinding crack and the 3-Dimensional Interactive Display images are at 100 rpm under 10 N loads for 1 h. It can be clearly seen that the grinding crack for base
Fig. 4. Non-contact optical profile testing instrument images of wear scar at 100 rpm under 10 N loads for 1 h: (a) paraffin base oil and, (b) paraffin base oil with 2.0 wt% MoS2 microspheres. Please cite this article as: G. Tang, et al., Synthesis and tribological properties of flower-like MoS2 microspheres, Ceramics International (2014), http://dx.doi. org/10.1016/j.ceramint.2014.03.115
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Fig. 5. Wear scar of plate: (a) paraffin base oil, (b) paraffin base oil with 2.0 wt% MoS2 microspheres, and (c) paraffin base oil with 2.0 wt% commercial MoS2.
oil is composed of wide grooves and irregular pits along the sliding direction (Fig. 4), and the grinding crack caused by the base oil with 2.0 wt% flower-like MoS2 microspheres (Fig. 4b) is shallower and smoother than that caused by the base oil (Fig. 4a). From these images we can see that the depth and width of the grinding crack for base oil with 2.0 wt% flowerlike MoS2 microspheres are about 0.59 μm and 73 μm respectively, while those for pure base oil are about 1 μm and 120 μm. This proves that the base oil with 2.0 wt% MoS2 flower-like microspheres shows better anti-wear capability than the pure paraffin base oil. In addition, SEM micrographs of wear scars of MoS2 microspheres samples are shown in Fig. 5. It could easily be found from SEM image that the rubbed surface lubricated by the base oil had lots of wide and deep furrows; the results are consistent with the above results of the non-contact optical profile test (Fig. 4). Compared with base oil and commercial MoS2, the surface lubricated with MoS2 flower-like microspheres only showed slender furrows. These experimental results confirm that the base oil containing flower-like MoS2 microspheres had better wear resistance than that of commercial MoS2. Therefore, we believe MoS2 microspheres consisting of many irregular nanosheets will penetrate more easily into the interface with base oil, and these nanosheets could strongly adhere to substrates and form continuous film in concave of rubbing surface, enhancing the tribological properties [23]. 4. Conclusions In summary, novel flower-like MoS2 microspheres with mean diameter about 1 μm were successfully synthesized via a
F-127 assisted hydrothermal method. The experimental results indicate that MoS2 flower-like microspheres can effectively improve the tribological properties of base oil especially in the field of friction reduction and wear resistance. In particular, the base oil with 2.0 wt% MoS2 flower-like microspheres shows better anti-wear capability than that with 2.0 wt% commercial MoS2 and pure base oil. Furthermore, the approach presented herein is simple, rapid, and reliable, and can be applied for surfactant-assisted hydrothermal preparation of other transition metal dichalcogenides nano/micromate. Acknowledgments This work was financially supported by Open Project of Key Laboratory of Tribology of Jiangsu Province (Kjsmcx2011002, kjsmcx1005), the Jiangsu National Nature Science Foundation (BK2011534), and the Senior Intellectuals Fund of Jiangsu University (09JDG009). References [1] C. Zhou, Y. Zhang, Y. Li, et al., Construction of high-capacitance 3D CoO@polypyrrole nanowire array electrode for aqueous asymmetric supercapacitor, Nano Lett. 13 (2013) 2078–2085. [2] D. Liu, Z. Yang, P. Wang, et al., Preparation of 3D nanoporous coppersupported cuprous oxide for high-performance lithium ion battery anodes, Nanoscale 5 (2013) 1917–1921. [3] S. Sattayasamitsathit, Y. Gu, K. Kaufmann, et al., Highly ordered multilayered 3D graphene decorated with metal nanoparticles, J. Mater. Chem. A 1 (2013) 1639–1645. [4] Y. Jia, X.Y. Yu, T. Luo, et al., PEG aggregation templated porous ZnO nanostructure: room temperature solution synthesis, pore formation
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Please cite this article as: G. Tang, et al., Synthesis and tribological properties of flower-like MoS2 microspheres, Ceramics International (2014), http://dx.doi. org/10.1016/j.ceramint.2014.03.115
CERAMICS INTERNATIONAL
Ceramics International ] (]]]]) ]]]–]]] www.elsevier.com/locate/ceramint
Synthesis and tribological properties of flower-like MoS2 microspheres Guogang Tanga,b, Jing Zhanga, Changchao Liua, Du Zhanga, Yuqi Wanga, Hua Tanga,n, Changsheng Lia a
School of Materials Science and Engineering, Jiangsu University, Jiangsu Province, Zhenjiang 212013, PR China b Department of Chemical Engineering, Zhenjiang College, Jiangsu, Zhenjiang 212003, PR China Received 10 December 2013; received in revised form 21 March 2014; accepted 21 March 2014
Abstract MoS2 flower-like microspheres with a mean diameter of about 1 μm, assembled by nanosheets, were successfully synthesized through a Pluronic F-127 (Ethylene Oxide/Propylene Oxide Block Copolymer) assisted hydrothermal method. The as-prepared products were characterized using powder X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM); UMT-2 multispecimen tribotester was used to assess their lubricating effect when used as additives in liquid paraffin dispersions. And the topography of worn scars was measured using a common scanning electron microscopy. Moreover, the relationship between the tribological properties and morphology of MoS2 were discussed. Tribological performance evidenced that the obtained flower-like MoS2 microspheres possessed superior anti-wear and friction-reducing properties as a lubrication additive compared with pure base oil and base oil containing commercial MoS2 plates, which will penetrate more easily into the interface with base oil, and form continuous film in concave of rubbing surface, enhancing the tribological properties. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Flower-like microspheres; Hydrothermal synthesis; MoS2; Tribological properties
1. Introduction Among various morphological structures of inorganic nanomaterials with different morphologies, three-dimensional (3D) nanostructures have been attracted much attention in many application areas of modern science and technology due to their inherent advantages, unique properties and potential applications [1–4]. Especially, they have enabled a number of key innovations in nanoscience in recent years. The simplest synthetic route to 3D nanostructures is probably self-assembly, in which ordered aggregates are formed through a spontaneous process [5,6]. To date, many graceful nano-architectures have been prepared by various techniques based on different mechanisms. For example, TiO2, ZnO, and Fe2O3 3D nanostructures [7–9] were successfully synthesized in the form of nanowire/-rod/-tube flower-like clusters via a wet-chemistry n
Corresponding author. Tel./fax: þ 86 511 8879 0268. E-mail address: [email protected] (H. Tang).
process. However, it still remains a challenge to synthesize high-quality 3D nanostructures at a high yield by effective and low cost methods which are highly demanded for new technological applications. In the past few years, layered transition metal dichalcogenides MS2 (M: Mo, W) nanomaterial have been receiving great attention because of their unique structure and superior properties, which exhibit extensive applications in catalysis, electrocatalysis, electrode materials for high-energy batteries, and lubricants [10–15]. As we know, transition metal dichalcogenides MS2 constitute a characteristic layered structure consisting of covalently bound S–M–S trilayers in analogy to graphite. Molybdenum disulfide (MoS2) with special layered closepacked hexagonal crystal structure is one of typical layered compounds, which is particularly important for solid lubrication or as an additive for lubricating oils and greases and has enjoyed the reputation of “the king of lubrication” for a long time. Previous studies have also shown that MoS2 exhibits superior tribological properties with numerous potential applications, such
http://dx.doi.org/10.1016/j.ceramint.2014.03.115 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: G. Tang, et al., Synthesis and tribological properties of flower-like MoS2 microspheres, Ceramics International (2014), http://dx.doi. org/10.1016/j.ceramint.2014.03.115
2
G. Tang et al. / Ceramics International ] (]]]]) ]]]–]]]
as additives to lubricating fluids [16]; as self-lubricating coatings [17] and for improving the mechanical behavior of nanocomposites [18]. It has been well known that nanosized MoS2 usually has better tribological properties either in friction reduction or wear resistance than microsized and bulk MoS2. To date, various preparation methods have been developed to prepare MoS2 nanomaterials, including thermal reduction, hightemperature sulfurization, hydrothermal method, chemical vapor deposition (CVD), and even laser ablation [17–21]. However, MoS2 hierarchical self-assembled nanostructures have rarely been reported, and not to mention its application in the tribological field. In our previous study, we successfully
prepared MoS2 nanoflowers by using Cetyltrimethylammonium Bromide (CTAB) as a surfactant under hydrothermal conditions [22]. Previous investigations indicated that MoS2 nanoflowers were able to enhance the tribological properties of base oil as lubricant additives. Herein, we report the successful synthesis of flowerlike MoS2 microspheres via a surfactantassisted hydrothermal method. The products were characterized by X-ray powder diffraction (XRD), energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). In addition, we had also investigated the tribological properties of MoS2 microspheres as lubrication additives in paraffin base oil which
Fig. 1. XRD pattern and EDS of the as-prepared flower-like MoS2 microspheres.
Fig. 2. SEM (a, b), TEM (c) and HRTEM images (d) of the as-prepared MoS2 microspheres. Please cite this article as: G. Tang, et al., Synthesis and tribological properties of flower-like MoS2 microspheres, Ceramics International (2014), http://dx.doi. org/10.1016/j.ceramint.2014.03.115
G. Tang et al. / Ceramics International ] (]]]]) ]]]–]]]
indicate that as-synthesized MoS2 products are good oil additives with excellent anti-wear and friction-reducing properties. More importantly, this work not only gives some insight into the design of nanostructures for enhancing tribological performances, but also provides a simple and novel route for the hydrothermal preparation of other layered transition metal dichalcogenides nanomaterials.
2. Experimental 2.1. Materials All chemical reagents were of analytic purity and used directly without further purification. Commercial bulk MoS2 (microMoS2, 300 meshes, purity 498%) was purchased from Sinopharm Chemical Reagent Co. Ltd. (SCRC) (Shanghai, China). 2.2. Synthesis of MoS2 samples
3
(λ¼ 0.1546 nm). The morphologies and structures of the samples were characterized by scanning electron microscopy (SEM, JEOL JXA-840A) and transmission electron microscopy (TEM) with a Japan JEM-100CX II transmission electron microscopy.
3. Results and discussion Fig. 1 shows the XRD patterns of as-prepared MoS2 synthesized by the surfactant-assisted hydrothermal method. All labeled diffraction peaks in Fig. 1 can be indexed to those of the pure hexagonal phase of MoS2 with calculated lattice constants of a ¼ 3.161, c ¼ 12.84 Å, which is consistent with the values of standard card (JCPDS no. 37-1492). The EDS spectrum of the MoS2 microspheres (Fig. 1b) gives the signals of only element Mo and S; besides Cu, no other element was observed. Furthermore, the quantification of the peaks shows that the atom ratio of Mo:S is about 1.96:1, which is close to the stoichiometric ratio of MoS2.
MoS2 flower-like microspheres were prepared using a recently described protocol [24]. The typical experimental procedure was designed as follows: 0.44 g of (NH4)2MoO4, 0.62 g NH2OH HCl and 1.05 g of CH4N2S were dissolved in 80 ml deionized water, then 0.3 g of F127 was added into the solution under constant stirring, pH value of the mixture was adjusted to 6 by the addition of 2 mol/L HCl. The mixture was then transferred into a 100 ml Teflon-lined stainless steel autoclave and sealed, and the autoclave was placed in a preheated oven at 180 1C for 24 h and naturally cooled down to room temperature. Black precipitates were collected by centrifugation and washed with distilled water and absolute ethanol for several times, and finally dried in vacuum at 60 1C for 10 h. 2.3. Preparation and tribological properties of lubricating oil samples The liquid paraffin samples with 0.5–5 wt% MoS2 flowerlike microspheres and 2 wt% commercial MoS2 micro-plates were prepared by dispersing agent sorbitol monooleate (Span80) via 60 min ultrasonication. The tribological behaviors were investigated on a CETR Universal Micro-Tribometer (UMT-2) at a rotating speed of 100–500 rpm and a constant load of 5–50 N for 1 h at room temperature. The material of upper sample is 440C stainless steel ball with a diameter of 10 mm, hardness of 62 HRC, and the counterpart is 45 steel disc of Ø40 mm 3 mm in size. Each tribological test was repeated three times to ensure reproducibility. The obtained wear scars were characterized using a VEECO WYKO NT1100 noncontact optical profile testing instrument and an electronic beam-based microscope (SEM, HITACHI S-3400N). 2.4. Characterization of MoS2 samples The X-ray diffraction patterns were recorded using a D8 advance (Bruker-AXS) diffractometer with CuKα radiation
Fig. 3. Variations of friction coefficient of lubricant with increasing load (a) and under diverse speeds (b).
Please cite this article as: G. Tang, et al., Synthesis and tribological properties of flower-like MoS2 microspheres, Ceramics International (2014), http://dx.doi. org/10.1016/j.ceramint.2014.03.115
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The size, shape, and structure of the as-prepared MoS2 are characterized by SEM and TEM microscopy. SEM images (Fig. 2a and b) show that the obtained samples are uniform flower-like MoS2 microspheres with an average diameter of about 1 μm. Fig. 2c shows a typical TEM image of MoS2 microsphere, which reveals that the as-prepared MoS2 microspheres consist of many MoS2 nanosheets. The highly wrinkled surface and extruded lamella-like structure of the microspheres could be obviously observed, which indicates that the flower-like microspheres are composed of MoS2 nanosheets. More details for MoS2 structure are illustrated by HRTEM studies in Fig. 2d, which indicates that the nanosheets consist of about 10-layered structures. The distance between the lattice fringes is 0.63 nm, slightly larger than the reported data (0.62 nm) for the (002) planes of the hexagonal MoS2 structure. Recently, the formation mechanism of novel flower-like nanostructures has been widely reported and discussed [7–9,24]. Kamalianfar et al. [24] described the fabrication of ZnO flower-like multi-sheets via a vapor–liquid– solid process. They indicated that self-assembly growth and Ostwald ripening mechanism may account for the growth process of ZnO flowers. Very recently, our previous studies [22] reported that well-defined MoS2 nanoflowers are formed from several MoS2 nanosheets through a self-assembly process and Ostwald ripening process with help of CTAB based on the time-dependant shape evolution of MoS2 nanoflowers. In this study, we believe that the growing process of flower-like MoS2 microspheres is consistent with previous reports, the sheet-like MoS2 nanostructures gradually evolved to flowerlike MoS2 microspheres through the self-assembled process with help of F127.
In order to evaluate the friction and wear properties of flower-like MoS2 microspheres, fretting experiments were conducted with varying loads and rotation speeds at constant sliding distance and frequency. Fig. 3a shows the friction coefficient as a function of the concentration of as-prepared MoS2 samples and commercial MoS2 at different loads under a speed of 100 rpm for 1 h. It is observed that the friction coefficient of pure base oil without any additives is increased with increasing load. Moreover, the friction coefficient of the base oil containing as-prepared MoS2 samples and commercial MoS2 is always lower than that of pure base oil, and the base oil containing as-prepared MoS2 samples exhibit lower friction coefficient than that of commercial MoS2. And the friction coefficient of the base oil containing as-prepared MoS2 samples decreases with the mass percent of the additives increased. Especially, the base oil with 2% MoS2 flower-like microspheres has a lower and stable friction coefficient compared with other mass fraction of the additives at higher load. Fig. 3b shows the friction coefficient as a function of concentration of MoS2 flower-like microspheres and commercial MoS2 at 10 N loads under diverse speeds for 1 h. The friction coefficient of the basic oil containing MoS2 flower-like microspheres is lower than that of basic oil containing commercial MoS2 and pure basic oil at different rotating speeds. To further determine the wear resistance tribological properties of flower-like MoS2 microspheres, a VEECO WYKO NT1100 non-contact optical profile testing instrument is used for measuring the grinding crack and the 3-Dimensional Interactive Display images are at 100 rpm under 10 N loads for 1 h. It can be clearly seen that the grinding crack for base
Fig. 4. Non-contact optical profile testing instrument images of wear scar at 100 rpm under 10 N loads for 1 h: (a) paraffin base oil and, (b) paraffin base oil with 2.0 wt% MoS2 microspheres. Please cite this article as: G. Tang, et al., Synthesis and tribological properties of flower-like MoS2 microspheres, Ceramics International (2014), http://dx.doi. org/10.1016/j.ceramint.2014.03.115
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Fig. 5. Wear scar of plate: (a) paraffin base oil, (b) paraffin base oil with 2.0 wt% MoS2 microspheres, and (c) paraffin base oil with 2.0 wt% commercial MoS2.
oil is composed of wide grooves and irregular pits along the sliding direction (Fig. 4), and the grinding crack caused by the base oil with 2.0 wt% flower-like MoS2 microspheres (Fig. 4b) is shallower and smoother than that caused by the base oil (Fig. 4a). From these images we can see that the depth and width of the grinding crack for base oil with 2.0 wt% flowerlike MoS2 microspheres are about 0.59 μm and 73 μm respectively, while those for pure base oil are about 1 μm and 120 μm. This proves that the base oil with 2.0 wt% MoS2 flower-like microspheres shows better anti-wear capability than the pure paraffin base oil. In addition, SEM micrographs of wear scars of MoS2 microspheres samples are shown in Fig. 5. It could easily be found from SEM image that the rubbed surface lubricated by the base oil had lots of wide and deep furrows; the results are consistent with the above results of the non-contact optical profile test (Fig. 4). Compared with base oil and commercial MoS2, the surface lubricated with MoS2 flower-like microspheres only showed slender furrows. These experimental results confirm that the base oil containing flower-like MoS2 microspheres had better wear resistance than that of commercial MoS2. Therefore, we believe MoS2 microspheres consisting of many irregular nanosheets will penetrate more easily into the interface with base oil, and these nanosheets could strongly adhere to substrates and form continuous film in concave of rubbing surface, enhancing the tribological properties [23]. 4. Conclusions In summary, novel flower-like MoS2 microspheres with mean diameter about 1 μm were successfully synthesized via a
F-127 assisted hydrothermal method. The experimental results indicate that MoS2 flower-like microspheres can effectively improve the tribological properties of base oil especially in the field of friction reduction and wear resistance. In particular, the base oil with 2.0 wt% MoS2 flower-like microspheres shows better anti-wear capability than that with 2.0 wt% commercial MoS2 and pure base oil. Furthermore, the approach presented herein is simple, rapid, and reliable, and can be applied for surfactant-assisted hydrothermal preparation of other transition metal dichalcogenides nano/micromate. Acknowledgments This work was financially supported by Open Project of Key Laboratory of Tribology of Jiangsu Province (Kjsmcx2011002, kjsmcx1005), the Jiangsu National Nature Science Foundation (BK2011534), and the Senior Intellectuals Fund of Jiangsu University (09JDG009). References [1] C. Zhou, Y. Zhang, Y. Li, et al., Construction of high-capacitance 3D CoO@polypyrrole nanowire array electrode for aqueous asymmetric supercapacitor, Nano Lett. 13 (2013) 2078–2085. [2] D. Liu, Z. Yang, P. Wang, et al., Preparation of 3D nanoporous coppersupported cuprous oxide for high-performance lithium ion battery anodes, Nanoscale 5 (2013) 1917–1921. [3] S. Sattayasamitsathit, Y. Gu, K. Kaufmann, et al., Highly ordered multilayered 3D graphene decorated with metal nanoparticles, J. Mater. Chem. A 1 (2013) 1639–1645. [4] Y. Jia, X.Y. Yu, T. Luo, et al., PEG aggregation templated porous ZnO nanostructure: room temperature solution synthesis, pore formation
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Please cite this article as: G. Tang, et al., Synthesis and tribological properties of flower-like MoS2 microspheres, Ceramics International (2014), http://dx.doi. org/10.1016/j.ceramint.2014.03.115