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Ionic liquids gels with in situ modified multiwall carbon nanotubes towards high-performance lubricants
Tribology International 88 (2015) 179–188
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Ionic liquids gels with in situ modified multiwall carbon nanotubes towards high-performance lubricants Xiaoqiang Fan a,b, Liping Wang b,n a b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China University of Chinese Academy of Sciences, Beijing 100049, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 20 November 2014 Received in revised form 6 March 2015 Accepted 18 March 2015
Two ionic liquids (ILs) gels were prepared by grinding multiwall carbon nanotubes (MWCNTs) in two kinds of ILs, and their physical and tribological properties were investigated in detail. Results demonstrate that ILs gels possess high conductivity and excellent tribological performance which mainly depends on the synergy of ILs and MWCNTs with their respective outstanding characteristics. ILs modified MWCNTs through van der Waals and π–π stacting interactions significantly improve the dispersibility and compatibility with lubricants, which greatly enhances the conductivity and tribological properties of the lubricants. The friction mechanism for the ILs gels is attributed to the synergetic lubrication of ILs and MWCNTs. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Carbon nanotubes Ionic liquids Modification Tribology
1. Introduction Since carbon nanotubes (CNTs) were found in 1991, they have aroused intensive interest in both scientific and industrial communities due to their extraordinary mechanical strength, electrical and thermal conductivity [1,2]. Intense research activities have made remarkable scientific and technological progress in the development of such nano-structure materials [3–5]. This state-of-the-art carbon nano-material, considered as rolled-up graphene sheets with hexagonally arranged sp2-hybridized carbon atoms holds great promise for a wide range of applications in fabrication of composite, gas storage, optical and electronic devices [6,7]. However, practical applications of CNTs have to face one major challenge about their poor dispersibility and compatibility with other materials because CNTs are heavily entangled with each other to form agglomerates by the high intermolecular cohesive forces. Modification of CNTs is of particular interest to weakening the van der Waals interactions between CNTs and improving their dispersion and solubility in organic/inorganic solvents, which would open up new routes to fabricate soluble and dispersible CNTs for widespread application [8–10]. The common approaches for the modification of CNTs can be divided into two main categories including both the covalent combination of functional groups through interaction with the conjugated skeleton of CNTs and the noncovalent adsorption or wrapping of various functional molecules onto CNTs by means of van der Waals forces and/or
n
Corresponding author. Tel.: þ 86 931 4968080. E-mail address: [email protected] (L. Wang).
http://dx.doi.org/10.1016/j.triboint.2015.03.026 0301-679X/& 2015 Elsevier Ltd. All rights reserved.
π-stacking interactions [11–13]. These two types of bonding modes in CNTs are similar to that of graphene, while the curvature of CNTs' sidewalls renders more favorable reactions to the cylindrical nano-structure than that on a flat graphene sheet. Early surface modification techniques used chemical oxidation methods to enhance the dispersion and solubility of CNTs, these approaches require harsh solvents such as hot nitric acid or sulfuric/nitric acid mixtures to introduce some oxygen-containing groups on the sidewalls of CNTs, and expensive equipment with prolonged reaction times to obtain more amounts of unbundled product. The interest of ionic liquids (ILs) as green solvents have dramatically grown in the past decade due to their unique properties such as negligible vapor pressure, excellent thermal stability, wide temperature range, high electrical conductivity, broad electrochemical potential window, good tribological properties and miscibility with a range of organic/inorganic compounds [14–17]. ILs as a versatile fluid composed of a relatively large organic cation and a weakly coordinating inorganic anion, have attracted tremendous attention on recyclable candidates to substitute traditional volatile organic solvents for wet processes including chemical syntheses, catalyzes, liquid/liquid extractions, and so forth [18,19]. Therefore, the dispersion and compatibility of CNTs are expected to be significantly improved through modification using ILs for expanding their potential applications in electrochemical devices (sensors, capacitors and actuators) and mechanical systems [20]. Recently, it has been reported that ILs as novel dispersant can exfoliate CNTs bundles into individuals. Aida’s group and other groups have prepared ILs gels through grinding CNTs in ILs and investigated several unique aspects of the gels from ILs and CNTs, such as rheological, thermal and conductive properties [21–23]. In addition, the tribological properties of ILs gel as a novel lubricant
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have also been studied and found that ILs gel showed good frictionreduction and anti-wear properties which are attributed to the loadcarrying ability of the lubricant and the more effective separation of the sliding surfaces [24–26]. In this paper, two ILs gels were prepared by grinding multiwall carbon nanotubes (MWCNTs) with two kinds of imidazolium ILs in an agate mortar with a pestle at room temperature for minutes, and an easily and in situ modified MWCNTs were also obtained through van der Waals and π–π stacting interactions during the grinding process. This moderate and green modification process without any solvents can exfoliate MWCNTs bundles in ILs to form relaxed bundles and is not accompanied by the disruption of the π-conjugated nanotube structure, which provides an opportunity for high-yield production and large-scale applications of modified MWCNTs. Here, the physicochemical and tribological properties of ILs gels with MWCNTs were investigated in detail. 2. Experimental section 2.1. Preparation and characterization of ionic liquids gel with CNTs Multiwall carbon nanotubes (MWCNTs) were commercially obtained from Beijing DK nano technology Co. LTD., (Beijing, China). As literature reported [27], alkyl imidazolium ionic liquids (1-butyl-3methylimidazolium tetrafluoroborate (LB104) and 1-butyl-3-methylimidazolium hexafluorophosphate (LP104) were synthesized. The ILs gel was prepared via mixing 0.75 wt% MWCNTs with ILs to form suspension by sonication or stirring, then grinding this suspension in an agate mortar with a pestle for minutes. In this work, two kinds of imidazolium ILs (LB104 and LP104) were used to prepare two ILs gels (were abbreviated as LB104–CNT and LP104–CNT) in the manner as just described. To confirm that MWCNTs have been modified by ILs in the process of gel formation, a small amount of two ILs gels were washed with absolute ethanol and ethyl acetate several times, then were centrifuged with the speed of 3000 10 r/min for 10 min to obtain modified MWCNTs from the gels. Finally the modified MWCNTs by ILs (CNT–LB104 and CNT–LP104) were obtained after drying in the vacuum oven for 24 h at 60 1C under 0.08 MPa, these modified CNT-ILs from ILs gels were tested using the following instruments. The morphology and structure of the original and modified MWCNTs were observed with FEI Tecnai F300 high-resolution transmission electron microscope (HRTEM). Their Raman spectra were obtained by Renishaw inVia Raman microscope with 532 nm laser excitation. Fourier transform infrared (FTIR) spectra were recorded in the wavenumber range of 4000–500 cm–1 with Bruker IFS 66v/s Fourier transform infrared analysis (FTIR) using a KBr wafer. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were carried out on a ZRY2P TGA at a heating rate of 10 1C/min in flowing air. Their functional groups were analyzed by a PHI5702 multifunctional Xray photoelectron spectroscope (XPS) made by American Institute of Physics Electronics Company using KAlpha irradiation as the excitation source. The binding energies of the target elements were determined at a pass energy of 29.3 eV, with a resolution of about 70.3 eV, using the binding energy of contaminated carbon (C1s: 284.8 eV) as the reference. The elemental surface distribution of functional groups was examined by scanning electron microscopy–Energy dispersive Xray spectroscopy (SEMEDS) (JEOL JEM5600LV SEM, Oxford IE250 Energy Dispersive Spectrometer, EDS) under 20 kV accelerating voltage with 10 nA beam current. 2.2. Physical and tribological properties of ILs gels The contact angle, zeta potential and conductivity of the ILs gels were measured on a Krüss DSA100 optical CA meter, a Zetasizer Nano 3600
dynamic laser scattering instrument and a DDSJ-308A conductivity meter, respectively. For the measurement of static contact angle, about 5 μl ILs gel droplet was gently deposited on the stainless steel disk using a micropipette at room temperature. The extreme pressure of ILs gels were performed on an Optimal-SRV-IV reciprocation friction tester with a ball-on-block configuration. The upper balls (diameter of 10 mm, AISI 52100 steel, hardness 710 HV) slide reciprocally at an amplitude of 1 mm against the stationary lower steel disks (AISI 52100 steel, Ф24 7.9 mm with hardness of about 645 HV). All the tests were conducted at a ramp test from 100 N up to 500 N in 100 N intervals with a 3 min test duration for each applied load with the frequency of 25 Hz at high temperature (200 1C). Prior to the friction tests, 0.05 ml ILs gel was introduced to the ball–disk contact area. The friction coefficient was recorded automatically by a computer connected to the Optimal-SRV-IV tester. The vacuum friction behaviors of the ILs gels were performed on a self-made rotational ball-on-disk vacuum tribometer. The stationary upper specimens were an AISI 52100 steel ball with standard 6 mm diameter, while the lower specimens were the stainless steel disk. The tests were determined under rotational radius of 6 mm, the sliding speed of 300 r/min, applied load of 5 N and duration of 60 min at room temperature and under vacuum conditions (8.9 10–5 Pa). The tribological properties of ILs gels were investigated by the reciprocating ball-ondisk UMT-2MT sliding tester at ambient temperature. Commercially available AISI steel balls with a diameter of 6 mm were used as the stationary upper counterparts, and the lower specimens (the stainless steel disks) were mounted onto a reciprocating table with a traveling distance of 5 mm. The friction tests were conducted at a sliding frequency of 5 Hz, applied load of 10 N and 20 N, test duration of 60 min. Friction experiments were repeated three times in order to ensure the repeatability and reliability. After the friction test, the residual ILs gels on the friction pairs were collected to investigate the structure of MWCNTs after friction, then the stainless steel disks were cleaned ultrasonically several times in baths of petroleum ether and dried with pure nitrogen. The wear losses of the lower disks were measured as following: firstly, the 2D morphology of the wear scar was measured using a MicroXAM 3D surface mapping microscope profilometer; secondly, cross sectional area of the wear scar was calculated by scanning probe image processing software SPIP which was installed on the computer connected to the 3D surface mapping microscope profilometer; finally, the wear loss was obtained through cross sectional area multiplied by the length of wear scar. The morphologies of wear surfaces and the chemical states of elements on the wear scar were analyzed by SEM and XPS. The structure of MWCNTs after the friction test was also analyzed by HRTEM.
3. Results and discussion 3.1. Analysis of modified MWCNTs CNTs have poor dispersibility in organic/inorganic solvents due to strong van der Waals interactions that lead to be tightly bound together, which has impeded the widespread application. Chemical modification of CNTs could enhance dispersion in solvents which mainly depends on the oxidative treatments using strong oxidizing reagents [28]. Therefore, in situ modification of CNTs with ILs during the grinding process is superior to these chemical methods using harsh reaction conditions. To obtain an intuitional insight into the morphologies of MWCNTs, HRTEM as a powerful tool was used to characterize the change of MWCNTs structure before and after modification. Fig. 1a and b shows the HRTEM images of original MWCNTs, Fig. 1c and d shows the HRTEM images of the modified MWCNTs with the corresponding selected-area electron diffraction pattern (SAED). Compared with their HRTEM images, the modified MWCNTs exhibit a hollow structure and keep almost similar morphology,
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Fig. 1. HRTEM micrographs of the MWCNTs before and after modification with high magnification image and the selected-area electron diffraction pattern (SAED).
Fig. 2. Raman spectra (a), Fourier transform infrared analysis spectra (b) of MWCNTs before and after modification.
whereas their high magnification images and the corresponding selected-area electron diffraction pattern are slightly different because the ILs could append to their sidewalls. Raman spectroscopy has historically played an important role in probing structural and electronic characteristics of carbon-based materials through the most intense features of the G band (1580 cm–1, inplane vibration of sp2 carbon atoms) and 2D band ( 2670 cm–1, two phonon double resonance Raman scattering process) as well as D band (introduction of defects) [29,30]. For sp2 hybridized carbon such as graphene and CNTs, Raman spectroscopy can provide a great deal of information including number of graphene layers, nanotube diameter,
clustering of the sp2 phase, the presence of sp2–sp3 hybridization, the introduction of chemical impurities, defects and crystal disorder, and so forth [31]. Fig. 2a shows the Raman spectroscopy of MWCNTs before and after modification. Compared with their Raman spectroscopy, the intensity of the D band at 1350 cm–1 always keeps the minimal value before and after modification which illustrates that the number of sp3carbons did not change because modification did not introduce new defects, whereas the intensity of the G band at 1590 cm–1 obviously weakened because the in-plane vibration of sp2 carbon atoms was suppressed by chemical impurities, hence it can be confirmed that the ILs have appended to their sidewalls.
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Fig. 3. X-ray photoelectron spectra of C, N, F, B and P elements of the modified MWCNTs using ionic liquids functional groups.
FTIR was used to confirm the presence of functional groups of modified MWCNTs. Fig. 2b shows the FTIR spectra of the samples before and after modification. As compared with the FTIR spectra, the characteristic peaks of the HC ¼ CH scissoring vibration on the imidazolium ring were observed at 1560 cm–1, the peaks of B–F and P–F stretching vibration of the anions from LB104 to LP104 are assigned at 1082 and 839 cm–1 [32,33], whereas the peaks of the imidazolium framework vibration and NC(H)N antisymmetric stretching on the imidazolium ring of ILs cannot be determined because the MWCNTs with oxygen-containing groups cause a certain influence on detecting the functional groups of ILs. XPS spectra of modified MWCNTs provide a deeper insight into the functional groups of ILs. Fig. 3 presents the XPS spectra of typical elements on ILs. The strong and weak C1s peaks appear at the binding energy of 283.8 and 285.9 eV, combining with N1s peaks at 400.8 eV, which is assigned to NC(H)N of the ILs
imidazolium ring and C–N bond due to the interaction between N atoms of imidazolium ring and C atoms of MWCNTs' sidewalls. The symmetrical F1s binding energies at 687.2 and 687.6 eV combined with the B1s at 194.8 eV and P 2p at 137.5 eV are attributed to BF4 of LB104 and PF6 of LP104 [34]. The results suggest that the ILs as a whole were attached to the surface of MWCNTs by covalent/noncovalent binding. Hence the spectroscopic analyses are consistent with the microscopy. Fig. 4 shows the TGA curves of ILs and DSC curves of modified MWCNTs. The decomposition temperature of LB104 and endothermic peak of CNT–LB104 are both at 410 1C, and that of LP104 and CNT–LP104 are both at 355 1C. Compared with DSC curve of MWCNTs, the endothermic peak of ILs modified MWCNTs is consistent with the decomposition temperature of corresponding ILs, which confirms that MWCNTs were effectively modified by ILs which as a whole were appended to the surface of MWCNTs.
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Fig. 4. TGA and DSC curves of the modified MWCNTs compared with that of the original MWCNTs.
To obtain a direct evidence of the modification, SEM–EDS provides a significant discrimination for chemical elements composition and distribution of CNT-ILs. Fig. 5 provides the surface distribution of the typical elements of CNT–LB104 and CNT–LP104. It is visible that the functional groups have reached high-density coverage which proves that modification of MWCNTs during the grinding process is high-efficiency. Combined with these characterization and analysis, the modification of MWCNTs using alkyl imidazolium ILs in the formation process of gels can be readily occurred and can achieve high-density coverage due to the specific interactions between the imidazolium groups of ILs and the π-electronic MWCNT surfaces. This process did not require any solvents or reagents for dispersion or oxidation of MWCNTs, so it could be allowed large-scale preparation of high-density modified CNTs. In addition, the processing is not accompanied by the structural disruption of CNTs and is environmentally friendly because ILs as a class of green solvent are nonvolatile and nonflammable [35]. With the outstanding advantage, ILs gels with effective modification of CNTs are opening a variety of possibilities for development and application of CNTs-based composite materials. 3.2. Characterization and analysis of the ILs gels Great efforts have been made to introduce the green and simple method to modify carbon nano-materials for improving the dispersibility and compatibility with other materials. Here the MWCNTs separated from ILs gels have been demonstrated the efficient modification by ILs. With known the exceptional properties of ILs and CNTs, we were motivated to explore the physical and tribological properties of ILs gels because they might display some synergistic performance of ILs and CNTs. In order to confirm the potential of ILs gels for wide applications, we investigate their physical properties like conductivity, stability and dispersion. Fig. 6 shows the contact angle data on the
smooth steel surface to examine the wettability of ILs gels for the stainless steel disks. The contact angle of two kinds of ILs gels is smaller than that of the neat ILs because the ILs gels with highdensity modified MWCNTs have formed the homogeneous systems and could provide better wettability. Table 1 shows the zeta potential and conductivity of the ILs gels. The zeta potential is a key parameter indicating the dispersion of charged particle, which depends on the potential difference between the dispersion medium and the stationary layer of fluid attached to the dispersed particle [36]. The magnitude of the zeta potential illustrates the degree of electrostatic repulsion between adjacent charged particles in a dispersion system. Very small molecules and particles have a high zeta potential which can provide good stability for resisting aggregation [37]. The LB104–CNT and LP104–CNT gels have the high negative zeta potential ( 59 and 48 mV), which indicates that modified MWCNTs have excellent stability and dispersion in the ILs gels. In contrast with the LB104 and LP104, the conductivity of LB104–CNT and LP104–CNT was enhanced by 61.5% from 3.5 mS of LB104 to 9.1 mS and by 50% from 1.8 mS of LP104 to 3.6 mS, respectively. The significant improvement of conductivity may be attributed to the synergy of electronic conductivity of CNTs and ion conductivity of ILs. In previous study, Hao’s and other groups have investigated in detail the rheological behaviors of ILs-CNTs fluids including the steady shear and oscillatory measurements. The results exhibited a shear thinning behavior at rather low concentrations (r0.1 wt% CNTs) which indicated that a transient network was formed through the nanotube–nanotube and nanotube–matrix interactions, whereas the rheological behaviors of the ILs-MWCNTs (Z0.1 wt%) fluid were similar to those of the worm-like micelles systems. The shear viscosity of the fluids was lower than the neat ILs especially under high shear rates. This phenomenon could be attributed to the selflubrication of CNTs [22,23].
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Fig. 5. EDS elemental surface distribution images of the modified MWCNTs.
Table 1 Physical properties of the as-prepared lubricants. Sample
Zeta potential (mV)
Conductivity (mS cm 1)
Kinematic viscosity (mm2 s 1)
Viscosity index
40 1C 100 1C LB104 LB104–CNT LP104 LP104–CNT
Fig. 6. Contact angle of 5 μl ILs gels in contact with a surface-smooth stainless steel disk.
ILs as versatile lubricants have been intensively investigated for different counterpart materials under various conditions and environment and have shown good friction reducing and anti-wear behaviors [38–40]. So far, it is not clear whether the ILs gels have better tribological performance, hence we investigated in detail
– 59 — 48
3.5 9.1 1.8 3.6
27.9 23.8 72.7 56.3
5.7 4.9 10.2 7.9
151 133 124 106
their tribological properties with the neat ILs as a comparison. Fig. 7 displays the extreme pressure properties of ILs gels at 200 1C and their friction curves under high vacuum (8.9 10 5 Pa). The ILs gels keep more stable and lower fricton coefficient ( 0.1) than the neat ILs that show extremely instable and higher friction coefficient ( 0.15) before applied load up to 500 N. Under high vacuum, the ILs gels provide very stable and lower friction coefficient (0.065)
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Fig. 7. High temperature extreme pressure property (200 1C) of the ILs and ILs gels and their friction curves under vacuum.
Fig. 8. Friction coefficient values and wear volume of the ILs and ILs gels at the applied loads of 10 N and 20 N.
than that of the ILs (0.08). The results indicate that the ILs gels have better extreme properties at high temperature and frictionreducing ability under vacuum condition, which is attributed to the high thermal stability, nonvolatility, the unique molecular composition and structure, moreover the MWCNTs improve the thermal conductivity and heat capacity as well as tribological performance of ILs. Fig. 8 shows the average friction coefficient values and wear volume of the lower disk lubricated by the ILs and ILs gels at applied loads of 10 and 20 N. Compared with the ILs, the ILs gels provide relatively low friction coefficient values and wear volume at low applied loads (10 N) because one-dimensional MWCNTs easily roll between the friction pairs and can form a physical protective layer for preventing the rubbing surfaces from coming in direct contact [41]. At a high applied load of 20 N, the friction coefficient values of the ILs gels slightly reduce, whereas their wear volume significantly reduces because the MWCNTs were exfoliated into the graphene-like fragments during the friction process and the unwrapping MWCNTs could weaken their rolling ability, but these graphene-like fragments can readily adsorbed on
the sliding surfaces which as an anti-wear protective film provides higher load-carrying ability and more effective separation of the sliding surfaces for drastically reducing the wear volume. Fig. 9 shows the morphologies of the wear surface at applied load of 20 N. All of the SEM images were obtained at the same conditions. It can be observed that the wear surface lubricated by the ILs is rough with wide and deep wear tracts, whereas the wear surface lubricated by the ILs gels is relatively smooth and appears small and narrow tracts, which is attributed to the excellent wear resistance performance of the ILs gels with MWCNTs because MWCNTs greatly enhance their tribological properties. Observing XPS spectra of the typical elements on the wear surfaces (as shown in Fig. 10), the Fe2p and strong F1s peaks appear at about 711.2 and 685.1 eV, respectively, which are assigned to fluoride because the tetrafluoroborate and hexafluorophosphate anions would be readily decomposed to form antiscratch component (FeF2) on the friction pairs under the severe friction conditions [25,42]. Fig. 11 presents the TEM micrographs of MWCNTs after friction tests, it is observed that the structure of
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Fig. 9. SEM and 3D images of the worn surfaces lubricated by the ILs and ILs gels at applied load of 20 N.
MWCNTs have been changed because the high friction has unwrapped and tore MWCNTs into graphene-like sheets and resulted in the change of the regularity and length of MWCNTs, which reflects the friction reducing and wear resistance mechanism of MWCNTs [43]. The excellent tribological properties of the ILs gels depend on the synergies of ILs and MWCNTs. The related literatures on the tribological properties of ILs have demonstrated that the hexafluorophosphate and tetrafluoroborate anions under the friction conditions would be readily decomposed and react with metal atoms on the
friction pairs to form a boundary lubricating protective film composed of anti-scratch components like fluoride, phosphate and boride, which improves the friction reducing and anti-wear abilities [38–40,42]. MWCNTs on the sliding surfaces can enhance the load carrying capacity and can form a physical protective film which prevents the sliding surfaces from coming direct contact for improving the wear resistance ability [43,44]. Therefore, the synergistic effects of ILs and MWCNTs with their respective characteristics significantly improve the physicochemical and tribological properties of ILs gels and make them hold great promise for widespread applications.
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Fig. 10. XPS spectra of Fe 2p and F 1s elements on the wear surface lubricated the neat ILs and the ILs gels at the applied load of 20 N.
References
Fig. 11. HRTEM images of the MWCNTs after friction tests.
4. Conclusions The modified carbon nanomaterials with several outstanding characteristics have attracted intensive interest in different fields. The ILs gels with in situ and high-density modified MWCNTs have been developed, which does not require any solvent and is readily prepared for minutes at room temperature. The ILs gels possess high conductivity, good stability and excellent tribological performance which are attributed to the synergy of ILs and MWCNTs with their respective characteristics (ILs with high thermal stability, high ion conductivity and good boundary lubrication behavior; MWCNTs with unique electrical conductivity, high thermal capacity and high mechanical strength). Hence, the ILs gels as a kind of green lubricant hold great promise for lubrication at different environmental conditions. In addition, the ILs gels with the efficiently modified MWCNTs offer a great deal of flexibility in preparation of novel CNTs-reinforced and soft modified composites for real-world and large-scale applications in both conventional technological fields and emerging areas.
Acknowledgments This work was supported by National Natural Science Foundation of China (Grants. 11172300 and 21373249) and Nature Science Foundation of Gansu Province of China (Grant. 145RJDA329).
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Contents lists available at ScienceDirect
Tribology International journal homepage: www.elsevier.com/locate/triboint
Ionic liquids gels with in situ modified multiwall carbon nanotubes towards high-performance lubricants Xiaoqiang Fan a,b, Liping Wang b,n a b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China University of Chinese Academy of Sciences, Beijing 100049, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 20 November 2014 Received in revised form 6 March 2015 Accepted 18 March 2015
Two ionic liquids (ILs) gels were prepared by grinding multiwall carbon nanotubes (MWCNTs) in two kinds of ILs, and their physical and tribological properties were investigated in detail. Results demonstrate that ILs gels possess high conductivity and excellent tribological performance which mainly depends on the synergy of ILs and MWCNTs with their respective outstanding characteristics. ILs modified MWCNTs through van der Waals and π–π stacting interactions significantly improve the dispersibility and compatibility with lubricants, which greatly enhances the conductivity and tribological properties of the lubricants. The friction mechanism for the ILs gels is attributed to the synergetic lubrication of ILs and MWCNTs. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Carbon nanotubes Ionic liquids Modification Tribology
1. Introduction Since carbon nanotubes (CNTs) were found in 1991, they have aroused intensive interest in both scientific and industrial communities due to their extraordinary mechanical strength, electrical and thermal conductivity [1,2]. Intense research activities have made remarkable scientific and technological progress in the development of such nano-structure materials [3–5]. This state-of-the-art carbon nano-material, considered as rolled-up graphene sheets with hexagonally arranged sp2-hybridized carbon atoms holds great promise for a wide range of applications in fabrication of composite, gas storage, optical and electronic devices [6,7]. However, practical applications of CNTs have to face one major challenge about their poor dispersibility and compatibility with other materials because CNTs are heavily entangled with each other to form agglomerates by the high intermolecular cohesive forces. Modification of CNTs is of particular interest to weakening the van der Waals interactions between CNTs and improving their dispersion and solubility in organic/inorganic solvents, which would open up new routes to fabricate soluble and dispersible CNTs for widespread application [8–10]. The common approaches for the modification of CNTs can be divided into two main categories including both the covalent combination of functional groups through interaction with the conjugated skeleton of CNTs and the noncovalent adsorption or wrapping of various functional molecules onto CNTs by means of van der Waals forces and/or
n
Corresponding author. Tel.: þ 86 931 4968080. E-mail address: [email protected] (L. Wang).
http://dx.doi.org/10.1016/j.triboint.2015.03.026 0301-679X/& 2015 Elsevier Ltd. All rights reserved.
π-stacking interactions [11–13]. These two types of bonding modes in CNTs are similar to that of graphene, while the curvature of CNTs' sidewalls renders more favorable reactions to the cylindrical nano-structure than that on a flat graphene sheet. Early surface modification techniques used chemical oxidation methods to enhance the dispersion and solubility of CNTs, these approaches require harsh solvents such as hot nitric acid or sulfuric/nitric acid mixtures to introduce some oxygen-containing groups on the sidewalls of CNTs, and expensive equipment with prolonged reaction times to obtain more amounts of unbundled product. The interest of ionic liquids (ILs) as green solvents have dramatically grown in the past decade due to their unique properties such as negligible vapor pressure, excellent thermal stability, wide temperature range, high electrical conductivity, broad electrochemical potential window, good tribological properties and miscibility with a range of organic/inorganic compounds [14–17]. ILs as a versatile fluid composed of a relatively large organic cation and a weakly coordinating inorganic anion, have attracted tremendous attention on recyclable candidates to substitute traditional volatile organic solvents for wet processes including chemical syntheses, catalyzes, liquid/liquid extractions, and so forth [18,19]. Therefore, the dispersion and compatibility of CNTs are expected to be significantly improved through modification using ILs for expanding their potential applications in electrochemical devices (sensors, capacitors and actuators) and mechanical systems [20]. Recently, it has been reported that ILs as novel dispersant can exfoliate CNTs bundles into individuals. Aida’s group and other groups have prepared ILs gels through grinding CNTs in ILs and investigated several unique aspects of the gels from ILs and CNTs, such as rheological, thermal and conductive properties [21–23]. In addition, the tribological properties of ILs gel as a novel lubricant
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have also been studied and found that ILs gel showed good frictionreduction and anti-wear properties which are attributed to the loadcarrying ability of the lubricant and the more effective separation of the sliding surfaces [24–26]. In this paper, two ILs gels were prepared by grinding multiwall carbon nanotubes (MWCNTs) with two kinds of imidazolium ILs in an agate mortar with a pestle at room temperature for minutes, and an easily and in situ modified MWCNTs were also obtained through van der Waals and π–π stacting interactions during the grinding process. This moderate and green modification process without any solvents can exfoliate MWCNTs bundles in ILs to form relaxed bundles and is not accompanied by the disruption of the π-conjugated nanotube structure, which provides an opportunity for high-yield production and large-scale applications of modified MWCNTs. Here, the physicochemical and tribological properties of ILs gels with MWCNTs were investigated in detail. 2. Experimental section 2.1. Preparation and characterization of ionic liquids gel with CNTs Multiwall carbon nanotubes (MWCNTs) were commercially obtained from Beijing DK nano technology Co. LTD., (Beijing, China). As literature reported [27], alkyl imidazolium ionic liquids (1-butyl-3methylimidazolium tetrafluoroborate (LB104) and 1-butyl-3-methylimidazolium hexafluorophosphate (LP104) were synthesized. The ILs gel was prepared via mixing 0.75 wt% MWCNTs with ILs to form suspension by sonication or stirring, then grinding this suspension in an agate mortar with a pestle for minutes. In this work, two kinds of imidazolium ILs (LB104 and LP104) were used to prepare two ILs gels (were abbreviated as LB104–CNT and LP104–CNT) in the manner as just described. To confirm that MWCNTs have been modified by ILs in the process of gel formation, a small amount of two ILs gels were washed with absolute ethanol and ethyl acetate several times, then were centrifuged with the speed of 3000 10 r/min for 10 min to obtain modified MWCNTs from the gels. Finally the modified MWCNTs by ILs (CNT–LB104 and CNT–LP104) were obtained after drying in the vacuum oven for 24 h at 60 1C under 0.08 MPa, these modified CNT-ILs from ILs gels were tested using the following instruments. The morphology and structure of the original and modified MWCNTs were observed with FEI Tecnai F300 high-resolution transmission electron microscope (HRTEM). Their Raman spectra were obtained by Renishaw inVia Raman microscope with 532 nm laser excitation. Fourier transform infrared (FTIR) spectra were recorded in the wavenumber range of 4000–500 cm–1 with Bruker IFS 66v/s Fourier transform infrared analysis (FTIR) using a KBr wafer. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were carried out on a ZRY2P TGA at a heating rate of 10 1C/min in flowing air. Their functional groups were analyzed by a PHI5702 multifunctional Xray photoelectron spectroscope (XPS) made by American Institute of Physics Electronics Company using KAlpha irradiation as the excitation source. The binding energies of the target elements were determined at a pass energy of 29.3 eV, with a resolution of about 70.3 eV, using the binding energy of contaminated carbon (C1s: 284.8 eV) as the reference. The elemental surface distribution of functional groups was examined by scanning electron microscopy–Energy dispersive Xray spectroscopy (SEMEDS) (JEOL JEM5600LV SEM, Oxford IE250 Energy Dispersive Spectrometer, EDS) under 20 kV accelerating voltage with 10 nA beam current. 2.2. Physical and tribological properties of ILs gels The contact angle, zeta potential and conductivity of the ILs gels were measured on a Krüss DSA100 optical CA meter, a Zetasizer Nano 3600
dynamic laser scattering instrument and a DDSJ-308A conductivity meter, respectively. For the measurement of static contact angle, about 5 μl ILs gel droplet was gently deposited on the stainless steel disk using a micropipette at room temperature. The extreme pressure of ILs gels were performed on an Optimal-SRV-IV reciprocation friction tester with a ball-on-block configuration. The upper balls (diameter of 10 mm, AISI 52100 steel, hardness 710 HV) slide reciprocally at an amplitude of 1 mm against the stationary lower steel disks (AISI 52100 steel, Ф24 7.9 mm with hardness of about 645 HV). All the tests were conducted at a ramp test from 100 N up to 500 N in 100 N intervals with a 3 min test duration for each applied load with the frequency of 25 Hz at high temperature (200 1C). Prior to the friction tests, 0.05 ml ILs gel was introduced to the ball–disk contact area. The friction coefficient was recorded automatically by a computer connected to the Optimal-SRV-IV tester. The vacuum friction behaviors of the ILs gels were performed on a self-made rotational ball-on-disk vacuum tribometer. The stationary upper specimens were an AISI 52100 steel ball with standard 6 mm diameter, while the lower specimens were the stainless steel disk. The tests were determined under rotational radius of 6 mm, the sliding speed of 300 r/min, applied load of 5 N and duration of 60 min at room temperature and under vacuum conditions (8.9 10–5 Pa). The tribological properties of ILs gels were investigated by the reciprocating ball-ondisk UMT-2MT sliding tester at ambient temperature. Commercially available AISI steel balls with a diameter of 6 mm were used as the stationary upper counterparts, and the lower specimens (the stainless steel disks) were mounted onto a reciprocating table with a traveling distance of 5 mm. The friction tests were conducted at a sliding frequency of 5 Hz, applied load of 10 N and 20 N, test duration of 60 min. Friction experiments were repeated three times in order to ensure the repeatability and reliability. After the friction test, the residual ILs gels on the friction pairs were collected to investigate the structure of MWCNTs after friction, then the stainless steel disks were cleaned ultrasonically several times in baths of petroleum ether and dried with pure nitrogen. The wear losses of the lower disks were measured as following: firstly, the 2D morphology of the wear scar was measured using a MicroXAM 3D surface mapping microscope profilometer; secondly, cross sectional area of the wear scar was calculated by scanning probe image processing software SPIP which was installed on the computer connected to the 3D surface mapping microscope profilometer; finally, the wear loss was obtained through cross sectional area multiplied by the length of wear scar. The morphologies of wear surfaces and the chemical states of elements on the wear scar were analyzed by SEM and XPS. The structure of MWCNTs after the friction test was also analyzed by HRTEM.
3. Results and discussion 3.1. Analysis of modified MWCNTs CNTs have poor dispersibility in organic/inorganic solvents due to strong van der Waals interactions that lead to be tightly bound together, which has impeded the widespread application. Chemical modification of CNTs could enhance dispersion in solvents which mainly depends on the oxidative treatments using strong oxidizing reagents [28]. Therefore, in situ modification of CNTs with ILs during the grinding process is superior to these chemical methods using harsh reaction conditions. To obtain an intuitional insight into the morphologies of MWCNTs, HRTEM as a powerful tool was used to characterize the change of MWCNTs structure before and after modification. Fig. 1a and b shows the HRTEM images of original MWCNTs, Fig. 1c and d shows the HRTEM images of the modified MWCNTs with the corresponding selected-area electron diffraction pattern (SAED). Compared with their HRTEM images, the modified MWCNTs exhibit a hollow structure and keep almost similar morphology,
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Fig. 1. HRTEM micrographs of the MWCNTs before and after modification with high magnification image and the selected-area electron diffraction pattern (SAED).
Fig. 2. Raman spectra (a), Fourier transform infrared analysis spectra (b) of MWCNTs before and after modification.
whereas their high magnification images and the corresponding selected-area electron diffraction pattern are slightly different because the ILs could append to their sidewalls. Raman spectroscopy has historically played an important role in probing structural and electronic characteristics of carbon-based materials through the most intense features of the G band (1580 cm–1, inplane vibration of sp2 carbon atoms) and 2D band ( 2670 cm–1, two phonon double resonance Raman scattering process) as well as D band (introduction of defects) [29,30]. For sp2 hybridized carbon such as graphene and CNTs, Raman spectroscopy can provide a great deal of information including number of graphene layers, nanotube diameter,
clustering of the sp2 phase, the presence of sp2–sp3 hybridization, the introduction of chemical impurities, defects and crystal disorder, and so forth [31]. Fig. 2a shows the Raman spectroscopy of MWCNTs before and after modification. Compared with their Raman spectroscopy, the intensity of the D band at 1350 cm–1 always keeps the minimal value before and after modification which illustrates that the number of sp3carbons did not change because modification did not introduce new defects, whereas the intensity of the G band at 1590 cm–1 obviously weakened because the in-plane vibration of sp2 carbon atoms was suppressed by chemical impurities, hence it can be confirmed that the ILs have appended to their sidewalls.
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Fig. 3. X-ray photoelectron spectra of C, N, F, B and P elements of the modified MWCNTs using ionic liquids functional groups.
FTIR was used to confirm the presence of functional groups of modified MWCNTs. Fig. 2b shows the FTIR spectra of the samples before and after modification. As compared with the FTIR spectra, the characteristic peaks of the HC ¼ CH scissoring vibration on the imidazolium ring were observed at 1560 cm–1, the peaks of B–F and P–F stretching vibration of the anions from LB104 to LP104 are assigned at 1082 and 839 cm–1 [32,33], whereas the peaks of the imidazolium framework vibration and NC(H)N antisymmetric stretching on the imidazolium ring of ILs cannot be determined because the MWCNTs with oxygen-containing groups cause a certain influence on detecting the functional groups of ILs. XPS spectra of modified MWCNTs provide a deeper insight into the functional groups of ILs. Fig. 3 presents the XPS spectra of typical elements on ILs. The strong and weak C1s peaks appear at the binding energy of 283.8 and 285.9 eV, combining with N1s peaks at 400.8 eV, which is assigned to NC(H)N of the ILs
imidazolium ring and C–N bond due to the interaction between N atoms of imidazolium ring and C atoms of MWCNTs' sidewalls. The symmetrical F1s binding energies at 687.2 and 687.6 eV combined with the B1s at 194.8 eV and P 2p at 137.5 eV are attributed to BF4 of LB104 and PF6 of LP104 [34]. The results suggest that the ILs as a whole were attached to the surface of MWCNTs by covalent/noncovalent binding. Hence the spectroscopic analyses are consistent with the microscopy. Fig. 4 shows the TGA curves of ILs and DSC curves of modified MWCNTs. The decomposition temperature of LB104 and endothermic peak of CNT–LB104 are both at 410 1C, and that of LP104 and CNT–LP104 are both at 355 1C. Compared with DSC curve of MWCNTs, the endothermic peak of ILs modified MWCNTs is consistent with the decomposition temperature of corresponding ILs, which confirms that MWCNTs were effectively modified by ILs which as a whole were appended to the surface of MWCNTs.
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Fig. 4. TGA and DSC curves of the modified MWCNTs compared with that of the original MWCNTs.
To obtain a direct evidence of the modification, SEM–EDS provides a significant discrimination for chemical elements composition and distribution of CNT-ILs. Fig. 5 provides the surface distribution of the typical elements of CNT–LB104 and CNT–LP104. It is visible that the functional groups have reached high-density coverage which proves that modification of MWCNTs during the grinding process is high-efficiency. Combined with these characterization and analysis, the modification of MWCNTs using alkyl imidazolium ILs in the formation process of gels can be readily occurred and can achieve high-density coverage due to the specific interactions between the imidazolium groups of ILs and the π-electronic MWCNT surfaces. This process did not require any solvents or reagents for dispersion or oxidation of MWCNTs, so it could be allowed large-scale preparation of high-density modified CNTs. In addition, the processing is not accompanied by the structural disruption of CNTs and is environmentally friendly because ILs as a class of green solvent are nonvolatile and nonflammable [35]. With the outstanding advantage, ILs gels with effective modification of CNTs are opening a variety of possibilities for development and application of CNTs-based composite materials. 3.2. Characterization and analysis of the ILs gels Great efforts have been made to introduce the green and simple method to modify carbon nano-materials for improving the dispersibility and compatibility with other materials. Here the MWCNTs separated from ILs gels have been demonstrated the efficient modification by ILs. With known the exceptional properties of ILs and CNTs, we were motivated to explore the physical and tribological properties of ILs gels because they might display some synergistic performance of ILs and CNTs. In order to confirm the potential of ILs gels for wide applications, we investigate their physical properties like conductivity, stability and dispersion. Fig. 6 shows the contact angle data on the
smooth steel surface to examine the wettability of ILs gels for the stainless steel disks. The contact angle of two kinds of ILs gels is smaller than that of the neat ILs because the ILs gels with highdensity modified MWCNTs have formed the homogeneous systems and could provide better wettability. Table 1 shows the zeta potential and conductivity of the ILs gels. The zeta potential is a key parameter indicating the dispersion of charged particle, which depends on the potential difference between the dispersion medium and the stationary layer of fluid attached to the dispersed particle [36]. The magnitude of the zeta potential illustrates the degree of electrostatic repulsion between adjacent charged particles in a dispersion system. Very small molecules and particles have a high zeta potential which can provide good stability for resisting aggregation [37]. The LB104–CNT and LP104–CNT gels have the high negative zeta potential ( 59 and 48 mV), which indicates that modified MWCNTs have excellent stability and dispersion in the ILs gels. In contrast with the LB104 and LP104, the conductivity of LB104–CNT and LP104–CNT was enhanced by 61.5% from 3.5 mS of LB104 to 9.1 mS and by 50% from 1.8 mS of LP104 to 3.6 mS, respectively. The significant improvement of conductivity may be attributed to the synergy of electronic conductivity of CNTs and ion conductivity of ILs. In previous study, Hao’s and other groups have investigated in detail the rheological behaviors of ILs-CNTs fluids including the steady shear and oscillatory measurements. The results exhibited a shear thinning behavior at rather low concentrations (r0.1 wt% CNTs) which indicated that a transient network was formed through the nanotube–nanotube and nanotube–matrix interactions, whereas the rheological behaviors of the ILs-MWCNTs (Z0.1 wt%) fluid were similar to those of the worm-like micelles systems. The shear viscosity of the fluids was lower than the neat ILs especially under high shear rates. This phenomenon could be attributed to the selflubrication of CNTs [22,23].
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Fig. 5. EDS elemental surface distribution images of the modified MWCNTs.
Table 1 Physical properties of the as-prepared lubricants. Sample
Zeta potential (mV)
Conductivity (mS cm 1)
Kinematic viscosity (mm2 s 1)
Viscosity index
40 1C 100 1C LB104 LB104–CNT LP104 LP104–CNT
Fig. 6. Contact angle of 5 μl ILs gels in contact with a surface-smooth stainless steel disk.
ILs as versatile lubricants have been intensively investigated for different counterpart materials under various conditions and environment and have shown good friction reducing and anti-wear behaviors [38–40]. So far, it is not clear whether the ILs gels have better tribological performance, hence we investigated in detail
– 59 — 48
3.5 9.1 1.8 3.6
27.9 23.8 72.7 56.3
5.7 4.9 10.2 7.9
151 133 124 106
their tribological properties with the neat ILs as a comparison. Fig. 7 displays the extreme pressure properties of ILs gels at 200 1C and their friction curves under high vacuum (8.9 10 5 Pa). The ILs gels keep more stable and lower fricton coefficient ( 0.1) than the neat ILs that show extremely instable and higher friction coefficient ( 0.15) before applied load up to 500 N. Under high vacuum, the ILs gels provide very stable and lower friction coefficient (0.065)
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Fig. 7. High temperature extreme pressure property (200 1C) of the ILs and ILs gels and their friction curves under vacuum.
Fig. 8. Friction coefficient values and wear volume of the ILs and ILs gels at the applied loads of 10 N and 20 N.
than that of the ILs (0.08). The results indicate that the ILs gels have better extreme properties at high temperature and frictionreducing ability under vacuum condition, which is attributed to the high thermal stability, nonvolatility, the unique molecular composition and structure, moreover the MWCNTs improve the thermal conductivity and heat capacity as well as tribological performance of ILs. Fig. 8 shows the average friction coefficient values and wear volume of the lower disk lubricated by the ILs and ILs gels at applied loads of 10 and 20 N. Compared with the ILs, the ILs gels provide relatively low friction coefficient values and wear volume at low applied loads (10 N) because one-dimensional MWCNTs easily roll between the friction pairs and can form a physical protective layer for preventing the rubbing surfaces from coming in direct contact [41]. At a high applied load of 20 N, the friction coefficient values of the ILs gels slightly reduce, whereas their wear volume significantly reduces because the MWCNTs were exfoliated into the graphene-like fragments during the friction process and the unwrapping MWCNTs could weaken their rolling ability, but these graphene-like fragments can readily adsorbed on
the sliding surfaces which as an anti-wear protective film provides higher load-carrying ability and more effective separation of the sliding surfaces for drastically reducing the wear volume. Fig. 9 shows the morphologies of the wear surface at applied load of 20 N. All of the SEM images were obtained at the same conditions. It can be observed that the wear surface lubricated by the ILs is rough with wide and deep wear tracts, whereas the wear surface lubricated by the ILs gels is relatively smooth and appears small and narrow tracts, which is attributed to the excellent wear resistance performance of the ILs gels with MWCNTs because MWCNTs greatly enhance their tribological properties. Observing XPS spectra of the typical elements on the wear surfaces (as shown in Fig. 10), the Fe2p and strong F1s peaks appear at about 711.2 and 685.1 eV, respectively, which are assigned to fluoride because the tetrafluoroborate and hexafluorophosphate anions would be readily decomposed to form antiscratch component (FeF2) on the friction pairs under the severe friction conditions [25,42]. Fig. 11 presents the TEM micrographs of MWCNTs after friction tests, it is observed that the structure of
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Fig. 9. SEM and 3D images of the worn surfaces lubricated by the ILs and ILs gels at applied load of 20 N.
MWCNTs have been changed because the high friction has unwrapped and tore MWCNTs into graphene-like sheets and resulted in the change of the regularity and length of MWCNTs, which reflects the friction reducing and wear resistance mechanism of MWCNTs [43]. The excellent tribological properties of the ILs gels depend on the synergies of ILs and MWCNTs. The related literatures on the tribological properties of ILs have demonstrated that the hexafluorophosphate and tetrafluoroborate anions under the friction conditions would be readily decomposed and react with metal atoms on the
friction pairs to form a boundary lubricating protective film composed of anti-scratch components like fluoride, phosphate and boride, which improves the friction reducing and anti-wear abilities [38–40,42]. MWCNTs on the sliding surfaces can enhance the load carrying capacity and can form a physical protective film which prevents the sliding surfaces from coming direct contact for improving the wear resistance ability [43,44]. Therefore, the synergistic effects of ILs and MWCNTs with their respective characteristics significantly improve the physicochemical and tribological properties of ILs gels and make them hold great promise for widespread applications.
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Fig. 10. XPS spectra of Fe 2p and F 1s elements on the wear surface lubricated the neat ILs and the ILs gels at the applied load of 20 N.
References
Fig. 11. HRTEM images of the MWCNTs after friction tests.
4. Conclusions The modified carbon nanomaterials with several outstanding characteristics have attracted intensive interest in different fields. The ILs gels with in situ and high-density modified MWCNTs have been developed, which does not require any solvent and is readily prepared for minutes at room temperature. The ILs gels possess high conductivity, good stability and excellent tribological performance which are attributed to the synergy of ILs and MWCNTs with their respective characteristics (ILs with high thermal stability, high ion conductivity and good boundary lubrication behavior; MWCNTs with unique electrical conductivity, high thermal capacity and high mechanical strength). Hence, the ILs gels as a kind of green lubricant hold great promise for lubrication at different environmental conditions. In addition, the ILs gels with the efficiently modified MWCNTs offer a great deal of flexibility in preparation of novel CNTs-reinforced and soft modified composites for real-world and large-scale applications in both conventional technological fields and emerging areas.
Acknowledgments This work was supported by National Natural Science Foundation of China (Grants. 11172300 and 21373249) and Nature Science Foundation of Gansu Province of China (Grant. 145RJDA329).
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