CuS Nanoparticle Additives for Enhanced Ester Lubricant

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CuS Nanoparticle Additives for Enhanced Ester Lubricant Performance Chenxu Liu, Ofir H. Friedman, Yonggang Meng, Yu Tian, and Yuval Golan ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01632 • Publication Date (Web): 04 Dec 2018 Downloaded from http://pubs.acs.org on December 10, 2018

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ACS Applied Nano Materials

CuS Nanoparticle Additives for Enhanced Ester Lubricant Performance Chenxu Liu1, Ofir Friedman2, Yonggang Meng1*, Yu Tian1, Yuval Golan2 1State

Key Laboratory of Tribology, Tsinghua Uinversity, Beijing 100084, China

2Department

of Materials Engineering and Ilse Katz Institute for Nanoscale Science and

Technology, Ben-Gurion University of the Negev, Be’er-Sheva 84105, Israel KEYWORDS: CuS nanoparticle, lubricant, electric potential, adsorption behavior, friction reduction

ABSTRACT: Lubrication performance of CuS nanoparticle additive in ester lubricant for the friction pair of ZrO2 ball and copper plate were investigated under different electric potential conditions. When the potential of the copper plate is lower than -14 V, the friction coefficient decreases from 0.18 to 0.05 after an induction period of running-in, the time of which is shorter if the voltage is lower. The significant friction reduction is attributed to the excess of positively charged nanoparticles in the vicinity of the negatively charged surface.

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1. INTRODUCTION Nanomaterials have great potential as advanced lubricant additives for green lubrication. It has been found that various nanoparticles such as oxides,[1, 2] sulfides,[3] hydroxides[4] and metals[5] as oil additives are capable of considerably reducing friction and wear of materials. Non-coated inorganic fullerene-like nanoparticles of MoS2 and WS2 have been applied as additives to lubricating oils[6-9] or by directly using them as solid-state lubricants.[10-14] Research results have shown that WS2 nanoparticles can reduce the coefficient of friction of paraffin oil by ~30%.[7] Surfactant-coated nanoparticles offer certain advantages over bare nanoparticles: they disperse more steadily in different organic solvents due to the screening effect of the van der Waals force between particles by the surfactant layer, and they do not come into intimate contact with the friction surfaces, reducing the chance for their degradation due to material transfer or delamination.[10,

15]

It has been shown that surfactant-coated nanorods can provide both low

friction and good wear protection when used as additives in fluids that are not good lubricants on their own.[16,

17]

At present, many studies have been focused on novel methodology for the

preparation of nanomaterials and on the development of novel multi-functional and environmental friendly nanoparticles as additives of lubricating oils and greases. Meanwhile, boundary lubrication widely exists in most cases of frictional interfaces. Improving the performance of boundary lubrication could not only increase the wear life of friction pairs but also decrease the frictional loss. Furthermore, controlling the friction in boundary lubrication, either by increasing or by reducing, is crucial according to different working conditions. Hence, active control of friction has been a goal pursued by scientists for many years.[18]


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Bearing great promise for enhancing boundary lubrication control, potential-controlled friction has been increasingly studied by researchers who have interest in developing smart surfaces and interfaces. Systematical research[19] on active control over friction in aqueous solutions via electrical field has been reported in the past two decades. Recently, some tribological studies[20, 21]

have been carried out under electrical fields for several non-aqueous lubricants with ionic

surfactants or ionic liquids as additives, and friction modulation via electrical potential has been demonstrated. Potential dependent friction for non-aqueous lubricants can be explained by electrostatic interaction, similar to the case of aqueous solutions. As mentioned above, previous studies on potential-controlled friction in aqueous or nonaqueous lubricants are mostly based on the regulation at molecular scale[19-21]. However, there are few reports on the effect of electrical potential on the lubrication behavior of nanoparticle additives. Nanoparticles with surfactant layer can also show significant electrical response, which may affect the boundary lubrication behavior under different applied electric potentials. Most importantly, the corrosion issue generally caused by the electrochemical reactions[18, 20] will be weakened in suspensions of surfactant-coated nanoparticles. In the present work, we report for the first time on the response of surface modified nanoparticles in lubricant systems under electric potentials, and describe the mechanism of friction regulation. 2. EXPERIMENTAL METHODS Nanoparticles were synthesized and then dispersed in ester lubricant by ultrasonic treatment. Subsequently, tribological experiments, quartz crystal micro-balance and zeta potential were carried out to study the response of nanoparticle additive in ester lubricant under applied electric potentials.


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2.1 Materials Octadecylamine (C18H37NH2, 99%, ODA) and copper perchlorate hexahydrate (CuCl2·6H2O) were purchased from Sigma-Aldrich. Potassium-ethyl-xanthogenate (CH3CH2OCS2K, 98%) was purchased from Fluka. Methanol (absolute) and chloroform (AR-b) were purchased from BioLab. Deionized water (DI water, with resistivity of 18.2 M cm) was obtained from a Millipore filter system. The materials were used without further purifications. Diethyl succinate (C8H14O4, 99.0%, DES, manufactured by Tokyo Chemical Industry Co., Ltd) was selected as the base lubricant, owning to its stable physical and chemical properties. Zirconium dioxide (ZrO2) ball was used as the upper friction pair with a diameter of 6.35 mm. Copper (Cu) plate with a dimension of 20 mm × 20 mm × 2 mm was used as the lower friction pair. Acetone (analytically pure), ethanol (analytically pure) and DI water were used for washing and cleaning of the friction pairs. 2.2 Synthesis of CuS nanoparticles and their dispersion in lubricant The synthesis of CuS nanoparticles was based on a previously reported study.[22] Illustration of the system configuration and reagent used for CuS nanoparticles synthesis is shown in Figure 1. 3 g of potassium-ethyl-xanthogenate were mixed in 400 mL of DI water. 3.54 g of copperperchlorate hexahydrate were dissolved in 100 ml of DI water. The two solutions were mixed to form copper-ethylxanthate (C2H5OCS2)2Cu). 81 mg of copper-ethylxanthate and 1.53 g octadecylamine were used to synthesize CuS nanoparticles. The synthesis was conducted under N2 flow and magnetic stirring at 90˚C for 60 min. After the nanoparticles were formed, methanol and chloroform were used in order to wash and remove the excess ODA. Thus, the alkylamine-coated CuS nanoplatelet array samples were produced.


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Figure 1. Illustration of the system configuration and reagent used for nanoparticles synthesis. After the preparation, the nanoparticles were dispersed in DES by ultrasonic rinse for about 3 hours at room temperature, with a concentration of 1 mg/mL. In addition, 10 mg/mL CuS nanoparticles were also uniformly dispersed in DES in the same way for a verification experiment. 2.3 Characterization of CuS nanoparticles and their suspensions Transmission electron microscopy (TEM) analyses were performed using a JEOL JEM-2100F with an accelerating voltage of 200 kV. X-ray data were collected on a Panalytical Empyrean powder diffractometer equipped with a position-sensitive X'Celerator detector using Cu Kα radiation (λ = 1.5405 Å) operated at 40 kV and 30 mA. θ/2θ scans were ran for 20 min in a 2θ range of 10-60°, in steps of ~0.033°. A quartz crystal microbalance (QCM, Q-sense E4 system, Biolin Scientific, Sweden) was used to simultaneously measure the changes of both resonance frequency (△f) and dissipation (△D) for the adsorption of nanoparticles on the copper coated quartz crystal sensors. DES base oil or the suspensions of the CuS nanoparticles in DES was injected into the cell in the flow rate of


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0.120 mL/min, and the decrease in frequency and increase in dissipation caused by adsorption were recorded for the third, fifth, seventh and ninth overtones at 23.7 ℃ . When dissipation is small enough (△D＜10△f), which means that the adsorbed film is rigid with low viscoelasticity, the Sauerbrey equation

[23-25]

can be used to convert the frequency changes into adsorbed mass

changes (△m), according to the following Equation (1):

m  C0

f N

(1)

where, C0 is a constant of 17.7 ng Hz-1 cm-2 which is related to the properties of the quartz crystal, and N is the overtone of the oscillations. Zeta potential of the suspension of surfactant-coated CuS nanoparticles in DES lubricant and particle size distribution were measured with Zetasizer (Malvern, UK). 2.4 Potential-controlled friction test UMT-3 Micro-Tribometer (Bruker, US) was used for ball-on-flat friction experiments at 25℃. Before the friction tests, the copper plate (used as the lower friction pair) was polished using silicon carbide paper and diamond paste, until its roughness Ra was about 10 nm. Then, both the upper and lower friction pairs were cleaned in an ultrasonic bath successively with acetone, ethanol and deionized water, and dried with compressed air. The force applied to the ball-on-flat surface was slowly increased to the set value of 3 N and allowed to equilibrate at that load for at least 5 s before the lower friction pair was set into reciprocating motion. Tests were performed at the sliding speed of 10 mm/s over 5-mm track. Reciprocating frequency was controlled at 1 Hz, and time about 30 min.


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Direct-current (DC) power supply was used in the experiment to provide electric potential (from -30 V to +30 V, minus value means the copper plate is connected to the negative pole, and vise verse). Pure DES base oil or DES oil added with CuS nanoparticles were dropped on the lower friction pair. Wires were used to connect the lower friction pair and a stainless steel holder (acting as the counter electrode) of the ZrO2 ball. To control potential changes, a switch was designed to control the variation of voltages. Figure 2 showed the schematic of the potential-controlled friction tests and the control switch. The connecting wires were named as “A” to “F”, respectively. When C and A, D and B were connected respectively, the lower friction pair was the cathode; when C and E, D and F were connected, the lower friction pair was the anode. The magnitude of the potential could be set up in the DC power. Tribological experiments were carried out in the range from -30 V to +30 V. After the friction test, the post-experiment CuS nanoparticles in DES suspension were centrifuged at 5000 rpm for 10 minutes, washed by methanol and dried in the air for 24 hours (temperature about 15~25˚C and humidity about 30%). Then, the obtained solid samples were studied again by using XRD and TEM characterization, to analyze the changes in composition and structure of the surfactant coated nanoparticles after being dispersed in the DES and the tribological testing.


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Figure 2. Schematic of the potential-controlled friction test and the control switch. 3. RESULTS AND DISCUSSIONS Transmission electron microscopy (TEM) and X-ray diffraction (XRD) analyses, as shown in Figure 3, were conducted for characterization of the nanoparticle morphology and of the ordering of the octadecylamine (ODA) surfactant following the synthesis and different tribology procedures. Figure 3a presents the XRD patterns of as prepared CuS nanoparticles and post tribology test with external voltage of 0 V or -30 V. Registration of the peaks in the XRD diffractogram is based on previous reports.[22] The results indicate that no structural change was made to the nanoparticles due to the tribology tests. Notably, the peak around 5°, which represents the (003) Bragg reflection of the ODA decreased after the tribology test and when external voltage was applied. The decrease in the basal plane peak intensity suggests breaking of the stacked superstructure into single platelets. Furthermore, each nanoparticle maintained its shape with very little evidence for degradation as observed in the bright-field (BF) TEM images of as prepared (Figure 3b), post tribology experiment without applied potential (Figure 3c) and with -30 V (Figure 3d). All three BF-TEM images shows nanoparticles with two-dimensional structure with 20~100nm in size.


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Figure 3. (a) XRD diffractogram of ODA surfactant coated CuS nanoparticles at different stages of experiment. Bright-field (BF) TEM image of CuS nanoparticles: (b) as prepared, (c) post tribology experiment without applied potential and (d) post tribology experiment conducted with -30V. After the preparation, the nanoparticles were dispersed in diethyl succinate (DES) lubricant by ultrasonic rinse for about 3 hours at room temperature, with a concentration of 1 mg/mL. The average particle size of the CuS nanoparticles in DES lubricant is about 277.5 nm (see Figure S1). Figure 4a shows the average values of friction coefficient of copper (Cu) versus potentials


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obtained for DES based oil and for a 1 mg/mL suspension of the CuS nanoparticles in DES oil, under different potentials in the range from -30 V to +30 V. Every point in Figure 4a means an individual friction test lasting about 30 min for different locations of the same lower friction pair after washing. In the pure DES, friction coefficient is almost the same and remains at about 0.24, regardless of the applied voltage. In the DES with CuS nanoparticles, the plotted points are the average values of friction coefficient after they decreased significantly and kept stable (see Figure 4b). When the potentials are higher (more positive) than -12 V, friction coefficient remains at the value of ca. 0.18. When the potentials are ≤ -14 V (more negative than -14V), friction coefficient significantly decreases to 0.05. Figure 4b shows the friction coefficient versus test time in DES lubricant with 1 mg/mL of CuS nanoparticles under different applied voltages (from -10 V to -30 V). When the voltages are higher (more positive) than -12 V, the friction coefficient keeps at the constant value of ca. 0.18 during the entire test period. When the voltages are ≤ -14 V, friction coefficient decreases with rubbing time down to 0.05; shorter induction periods were obtained for increasingly negative potentials, indicating faster response of the nanoparticles.


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Figure 4. Potential-controlled friction results for CuS nanoparticle suspensions in DES lubricant under different conditions. (a) Average friction coefficients obtained for pure DES lubricant and for the suspension of CuS nanoparticles in DES lubricant. The coefficients are average values after they keep stable; (b) Friction coefficients of Cu versus time in DES lubricant with CuS nanoparticles under different applied potentials from -10 V to -30 V. In order to analyze the mechanism of the electric field effect on the lubrication performance of the CuS nanoparticle additive, adsorption isotherms were measured by using the quartz crystal microbalance (QCM) for the mass changes on Cu surface in DES without or with CuS nanoparticles under different potentials, as shown in Figure 5a. The adsorbed mass was


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calculated according to the Sauerbrey equation, [23-25] owing to small enough dissipation (△D ＜ 10△f, summarized in Figure S2). In order to describe the relationship between adsorption mass and time quantitatively, an adsorption-settlement model of nanoparticles in ester lubricant has been proposed in this study, by combining the adsorption kinetic model for molecules or ions

[26-28]

with the Stokes’ law for

the settlement of nanoparticle dispersions [29, 30]. The isothermal adsorption kinetics of molecules or ions is usually expressed by using either the pseudo-first-order or pseudo-second-order model, while the Stoke’s law for the particle settlement is a linear equation. Thereby, the adsorptionsettlement kinetics of naoparticles can be described by Equation (2) or Equation (3), named as Model-I and Model-II respectively.

q  qe  10

q

(log qe 

k1 t ) 2.303

 v t

(2)

qe 2  t  v t k2  qe  t

(3)

Where k1 and k2 are the adsorption rate constants for the Equations (2) and (3) respectively, v is the settlement rate constant, qe represents the amount of substance adsorbed at equilibrium on a solid surface, q is the amount of substance adsorbed and settled on the surface at time t. Table S1 and Figure S3 in the Supporting Information show that both models can fit the experimental data (including the DES with and without CuS nanoparticles) well (the correlation coefficient R2>0.98). For the DES with CuS nanoparticles, both the adsorption rate constant (k1 or k2 in the two different models) and the settlement rate constant (v) are much greater than the pure DES lubricant, which implies that both the adsorption and settlement of CuS nanoparticles


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play an important role in the mass changes on the solid surface in the suspension of the nanoparticles. Considering that the reciprocating frequency was 1 Hz in the friction test, the adsorption film on the surface of the friction track would be destroyed every rubbing cycle. Thus, the incipient stage of each change in the QCM adsorption isotherms is focused. In the case of pure DES (the red points in Figure 5a), a relative slow mass increase can be observed, which is considered as the adsorption of the DES molecules on the copper surface. When a potential is applied, the amount of mass changes increases, while the instantaneous rate of adsorbed mass shows no significant change, which means that the applied potential has little influence on the adsorption of the DES molecules. For the DES with CuS nanoparticles, the rate of the adsorbed mass is about 1.0 ng cm-2 s-1 in the absence of an electric potential (the blue points and the inset picture in Figure 5a). When the negative potential is applied, the instantaneous rate significantly increases to about 6.5 ng cm-2 s-1 in the first second (1 s). When the positive potential is applied, the rate decreases to about 0.3 ng cm-2 s-1. These results illustrate that the adsorption behavior of the nanoparticles on the copper surface under different potentials has strong correlation with the change of friction coefficient. Zeta potential analysis indicated that the surfactant coated CuS nanoparticles in DES oil are positively charged (about +11.7 mV), and are due to migrate toward the negatively charged copper surface. Indeed, the data in Figure 4b and in Figure 5a show a significant friction reduction under negative electric potentials. Figures 5b-d shows the schematic illustrations of the CuS nanoparticles in lubricant under no potential (0 V), negative potential (-30 V) and positive potential (+30 V), respectively. When a potential is applied on the lubricant film, the nanoparticles within the lubricant can move under


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electrostatic forces, owing that the surfactant coated CuS nanoparticles are positively charged. Thus, CuS nanoparticles gather and align near the surface of the lower friction pair when the copper surface is suitably negatively charged, resulting in a significant drop of friction coefficient. When the applied potential is less negative than -14 V, the electrostatic energy is not high enough to drive the nanoparticles to the rubbing surface. The more negative the applied potential, the larger the electrostatic energy - resulting in faster movement of the nanoparticles. On the other hand, when a positive potential is applied, the nanoparticles are driven away from the copper surface, while some nanoparticles can still be present at the ball and plate interface owing to fluid flow and uniform distribution of the pressure. Hence, friction coefficient is slightly lower than that of the pure DES lubricant. To further validate the above considerations, friction tests were also carried out at high concentration of CuS nanoparticles. Figure 5e shows the friction coefficient of Cu versus time in DES lubricant with 10 mg/mL CuS nanoparticles at voltages of 0 V, -30 V and +30 V. In the absence of applied potential, the friction coefficient decreases with sliding time to a steady value of 0.05, almost the same as the value obtained for 1 mg/mL CuS nanoparticles under the potentials below -14 V. When -30 V is applied, friction coefficient further reduces slightly to the value of 0.045. When +30 V is applied, friction coefficient gradually goes up from 0.05 to 0.13. These results also demonstrate the effect of electric field on the lubrication performance of the nanoparticles. Mostly, high concentration of CuS nanoparticles can reduce the friction coefficient, supporting the significant friction reduction of CuS nanoparticles under the negative potentials: the excess of the positively charged CuS nanoparticles in the vicinity of the negatively charged copper surface cause the low friction coefficient (see also Figure S4). The state of the surfactant on the surface of the CuS nanoparticles has been shown to strongly affect their optical


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properties, [22] and is likely to play a key role on their response under electric potentials, and their subsequent tribological behavior.

Figure 5. (a) adsorption isotherms measured by QCM for the adsorption on copper surface in DES without or with CuS nanoparticles (1 mg/mL) under different applied potentials, the inset picture shows the relationship of the instantaneous adsorption rate (slope of the tangent line in the blue data) and time in DES with nanoparticles; (b-d) schematic illustrations of the CuS nanoparticles (1 mg/mL) in lubricant with no potential, negative potential (-30 V) and positive potential (+30V), respectively; (e) friction coefficients in DES lubricant with 10 mg/mL CuS nanoparticles under different applied potentials. 4. CONCLUSIONS


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We have studied the effect of electric field on the lubrication performance of CuS nanoparticle arrays dispersed in diethyl succinate lubricant. The suspensions showed good friction reduction effect and notably showed significant and promising effects of potential control. Friction tests indicate that the nanoparticles can reduce the coefficient of friction of DES lubricant by ~25%, from 0.24 to 0.18. When a negative potential lower than -14 V was applied, the friction coefficient markedly decreased to 0.05. The surfactant coated CuS nanoparticles maintained their structures and morphologies of after the dispersion and testing processes. According to this research, shape controlled nanoscale lubricant additives are likely to have profound technological importance in reducing and controlling friction in engineering systems. Moreover, applying electric field is considered to be of great interest and importance for controlling friction in systems with nanoparticle additives, ultimately providing the scientific and technological basis for electrical field controlled devices of the future such as automotive transmissions. ASSOCIATED CONTENT Supporting Information Supplementary experimental results are presented in the supporting information. AUTHOR INFORMATION Corresponding Author * Email: [email protected]. ORCID Chenxu Liu: 0000-0002-7147-7063


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Ofir Friedman: 0000-0001-5903-7797 Yonggang Meng: 0000-0002-8274-5131 Yu Tian: 0000-0001-7742-5611 Yuval Golan: 0000-0003-1369-0285 Author Contributions Nanoparticles were designed, synthesized and characterized in Ben-Gurion University of the Negev, while tribological characterization was carried out in Tsinghua University. C.L. performed the tribology tests, Zeta potential and QCM measurements and drafted the manuscript. O.F. synthesized the CuS nanoparticles, conducted XRD and TEM characterization and edited the manuscript. Y.M., Y.T. and Y.G. proposed the research hypotheses, planned the experiments and edited the final manuscript. All authors have given approval to the final version of the manuscript. Notes The authors declare no conflict of interest. ACKNOWLEDGMENT This work was financially supported by the Chinese National Key R&D Plan (Grant No. 2016YFE0130300) in the framework of the China-Israel bilateral research program in nanotechnology of the Ministry of Science and Technology of the People’s Republic of China and the Ministry of Science and Technology of the State of Israel, and by the National Natural Science Foundation of China (No.51635009). The authors thank Mr. Hui Cao, Dr. Xiaoxi Qiao and Dr. Jun Zhang for their help with the QCM and friction tests.


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Modified CuS Nanorods as Lubricating Oil Additives. Ceram. Int. 2017, 43, 4246-4251. (18) Meng, Y.; Jiang, H.; Wong, P. L. An Experimental Study on Voltage-controlled Friction of Alumina/Brass Couples in Zinc Stearate/Water Suspension. Tribol. T. 2001, 44, 567-574. (19) Jiang, H.; Meng, Y.; Wen, S.; Ji, H. Effects of External Electric Fields on Frictional Behaviors of Three Kinds of Ceramic/Metal Rubbing Couples. Tribol. Int. 1999, 32, 161-166. (20) Yang, X.; Meng, Y.; Tian, Y. Potential-controlled Boundary Lubrication of Stainless Steels in Non-aqueous Sodium Dodecyl Sulfate Solutions. Tribol. Lett. 2014, 53, 17-26. (21) Yang, X.; Meng, Y.; Tian, Y. Effect of Imidazolium Ionic Liquid Additives on Lubrication Performance of Propylene Carbonate under Different Electrical Potentials. Tribol. Lett. 2014, 56, 161-169. (22) Rabkin, A.; Friedman, O.; Golan, Y. Surface Plasmon Resonance in Surfactant Coated Copper Sulfide Nanoparticles: Role of the Structure of the Capping Agent. J. Colloid. Interf. Sci. 2015, 457, 43-51. (23) Sauerbrey, G. Verwendung Von Schwingquarzen Zur Wägung Dünner Schichten und Zur Mikrowägung. Zeitschrift Für Physik 1959, 155, 206-222. (24) Thavorn, J.; Hamon, J. J.; Kitiyanan, B.; Striolo, A.; Grady, B. P. Competitive Surfactant Adsorption of AOT and TWEEN 20 on Gold Measured using a Quartz Crystal Microbalance with Dissipation. Langmuir 2014, 30, 11031-11039. (25) Zhang, J.; Meng, Y.; Tian, Y.; Zhang, X. Effect of Concentration and Addition of Ions on the Adsorption of Sodium Dodecyl Sulfate on Stainless Steel Surface in Aqueous Solutions. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2015, 484, 408-415.


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(26) Dai, M.; Xia, L.; Song, S.; Peng, C.; Rangel-Mendez, J. R.; Cruz-Gaona, R. Electrosorption of As(III) in Aqueous Solutions with Activated Carbon as the Electrode. Appl. Surf. Sci. 2018, 434, 816-821. (27) Chen, Z.; Song, C.; Sun, X.; Guo, H.; Zhu, G. Kinetic and Isotherm Studies on the Electrosorption of NaCl from Aqueous Solutions by Activated Carbon Electrodes. Desalination 2011, 267, 239-243. (28) Ho, Y. S. Citation Review of Lagergreen Kinetic Rate Equation on Adsorption Reaction. Scientometrics 2004, 59, 171-177. (29) Millikan, R. A. The Isolation of an Ion, a Precision Measurement of its Charge, and the Correction of Stokes’s Law. Phys. Rev. 1911, 32, 350-396. (30) Cheng, P. Y.; Schachman, H. K. Studies on the Validity of the Einstein Viscosity Law and Stokes' Law of Sedimentation. J. Polym. Sci. 1955, 16, 19.

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Illustration of the system configuration and reagent used for nanoparticles synthesis. 119x70mm (300 x 300 DPI)



Schematic of the potential-controlled friction test and the control switch.


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(a) XRD diffractogram of ODA surfactant coated CuS nanoparticles at different stages of experiment. Brightfield (BF) TEM image of CuS nanoparticles: (b) as prepared, (c) post tribology experiment without applied potential and (d) post tribology experiment conducted with -30V. 140x139mm (300 x 300 DPI)



Potential-controlled friction results for CuS nanoparticle suspensions in DES lubricant under different conditions. (a) Average friction coefficients obtained for pure DES lubricant and for the suspension of CuS nanoparticles in DES lubricant. The coefficients are average values after they keep stable; (b) Friction coefficients of Cu versus time in DES lubricant with CuS nanoparticles under different applied potentials from -10 V to -30 V.


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(a) adsorption isotherms measured by QCM for the adsorption on copper surface in DES without or with CuS nanoparticles (1 mg/mL) under different applied potentials, the inset picture shows the relationship of the instantaneous adsorption rate (slope of the tangent line in the blue data) and time in DES with nanoparticles; (b-d) schematic illustrations of the CuS nanoparticles (1 mg/mL) in lubricant with no potential, negative potential (-30 V) and positive potential (+30V), respectively; (e) friction coefficients in DES lubricant with 10 mg/mL CuS nanoparticles under different applied potentials.



Particle size distribution of 1 mg/mL CuS nanoparticles in DES lubricant.


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Change in frequency (△F,△F = △f/N) and dissipation (△D) for the adsorption of nanoparticles on the copper coated quartz crystal sensors in (a) air to DES with CuS nanoparticles and (b) air to pure DES, illustrated as the third overtone normalized by overtone number (N = 3). The black line indicates the variation of frequency versus time and the blue line represents the variation of dissipation versus time. Blue background denotes negative applied potential (-30 V), while yellow background denotes positive applied potential (+30 V).



Fitting curves calculated by the adsorption-settlement models of naoparticles in (a) DES with CuS nanoparticles (1 mg/mL) and (b) pure DES lubricant under different applied potentials: the black points are the original data measured by QCM, the green line is the fitting curve of Mode-I and the red line is the fitting curve of Mode-II


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Friction coefficient of Cu versus time from dry sliding to pure lubricant, and then to lubricant with untreated CuS nanoparticles when the applied potentials changes continuously. In the pure DES, the coefficient generally does not change with the applied potentials. However, in the DES suspension with CuS nanoparticles, the coefficient changes with the potentials: When negative potential (-30V) is applied, the coefficient decreases to about 0.1. When no potential (0V) or positive potential (+30V) is applied, the coefficient increases to about 0.2. The reversible responses indicate that the adsorption/desorption of the nanoparticles during actuation of different potential sequences is most likely the physical interaction responsible for the effect of applied external potential on the tribological behavior of this system.


CuS Nanoparticle Additives for Enhanced Ester Lubricant

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