Electric Response of CuS Nanoparticle Lubricant Additives: The Effect

Article Cite This: Langmuir XXXX, XXX, XXX−XXX

pubs.acs.org/Langmuir

Electric Response of CuS Nanoparticle Lubricant Additives: The Effect of Crystalline and Amorphous Octadecylamine Surfactant Capping Layers Chenxu Liu,†,‡ Ofir Friedman,‡,§ Yuanzhe Li,† Shaowei Li,† Yu Tian,† Yuval Golan,*,§ and Yonggang Meng*,† †

State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China Department of Materials Engineering and Ilse Katz Institute for Nanoscale Science and Technology, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel

Downloaded via GUILFORD COLG on August 1, 2019 at 13:13:28 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

§

S Supporting Information *

ABSTRACT: Octadecylamine-coated CuS nanoparticles were designed and confirmed to play an important role in their electric response and boundary lubrication in the ester lubricant. For the case of CuS nanoparticles coated with crystalline surfactant, the surface potential is 18.47 ± 0.99 mV higher than with amorphous surfactant, owing to the random chain conformations of the octadecylamine molecules. When used as a lubricant additive, CuS nanoparticles (in the form of nanoplates or nanoarrays) with a crystalline surfactant were positively charged due to the presence of the amino headgroup in octadecylamine. The observed friction coefficient decreased from 0.18 to 0.09 and 0.05, respectively, when negative potential (for the copper lower pair) was applied across untreated CuS nanoparticles. However, thermally treated CuS nanoparticles showed good lubricating effect, but almost no effect of potential control since the amino groups were obscured by the disordered carbon chains, hindering electron transfer and weakening the response to externally applied electric field. being modified with stearic acid.6 Their results indicated that the low friction can be ascribed to self-lubricious amorphous tribofilm formation, which was composed of ZnO and a lowsurface-energy surfactant. To compare the effect of different surfactant structures, Yoshizawa and Israelachvili studied the adhesion and boundary friction of two surfactant-coated mica surfaces in the surface forces apparatus.7 They found that the friction force, as well as the adhesion energy hysteresis increased and then decreased when the monolayers went from being solid-like to amorphous and liquid-like. Interdigitation of the chains and rearrangement of the molecules on the surface were considered to be key factors in this effect. While the state of the surfactant coating (crystalline vs amorphous) affects the tribological behavior, shear forces can, at the same time, affect the structure of the surfactant and particularly induce its crystallization. Khatri and Biswas studied the load-induced microstructure evolution and friction in an organic monolayer self-assembled on a silicon substrate.8 They

1. INTRODUCTION Nanoparticles possess unique chemical and physical properties due to their large fractions of surface atoms and quantum-scale dimensions as compared to their bulk counterparts. However, owing to the high surface energy of nanoparticles, aggregation is generally inevitable during the storage and operation processes, causing deterioration of their originally high performance.1 Hence, studies concerning the stability of nanoparticles in liquid systems are essential for their future potential application. Surfactants are often used to lower the interfacial tension between two or more components in a materials system, thereby increasing stability and dispersion of nanoparticles.2,3 According to the different chemical constitutions of the surfactant, various properties of nanoparticles can be tailordesigned.4,5 However, there are few studies focusing on the structural nature of the surfactant and its effect on the performance of the nanoparticles. Generally, the chemical and structural nature of the surfactant can play an important role on the tribological properties of nanoparticles. Zhang et al. studied that the friction coefficient of ZnO nanorods (synthesized by a lowtemperature hydrothermal route on Si substrates) against the Si3N4 ball in dry sliding decreased from 0.33 to 0.15, after © XXXX American Chemical Society

Special Issue: Intermolecular Forces and Interfacial Science Received: June 6, 2019 Revised: July 19, 2019

A

DOI: 10.1021/acs.langmuir.9b01714 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

Figure 1. Bright-field TEM images of ODA-coated CuS nanoplates (a) as synthesized (untreated), and (b) thermally treated; (c) X-ray diffractograms of untreated and thermally treated nanoplates. Bright-field TEM images of ODA-coated CuS nanoarrays (d) as-synthesized (untreated) and (e) treated; (f) X-ray diffractograms of untreated and thermally treated nanoarrays.

weak or without polarity and its relevance to potential controlled boundary lubrication performance of nanoparticle suspensions.

observed that the area averaged friction force was dominated by the low-friction crystalline tiles at low loads, while high friction was dominated by amorphous tiles at high loads. Zhang et al. found that polyetheretherketone crystallization occurred at the beginning of the frictional process, owing to the shear force as well as the friction-induced heat.9 Also, Jiang et al. reported shear-induced crystallization of bimodal polyethylene at low shear rate and high temperatures.10 In summary, it is clear that the state of the surfactant and shear forces are interrelated. Nanoparticles have great potential as lubricating materials and for the development of advanced lubrication technology.11 It was found that various inorganic nanoparticles such as oil additives are capable of considerably lubricating and enhancing the load-carrying capability.12 For example, MoS2 nanoparticles with a laminar structure were applied as additives to fluid lubricants.13 Results showed that friction reduction of 0.25 wt % MoS2 nanoparticles in reciprocating mode reaches 53.1% compared with that of base oil. Most importantly, surfactant-coated inorganic nanoparticles applied as additives to fluid lubricants were widely studied.14−16 It was shown that surfactant-coated nanoparticles offer certain advantages over bare ones, such as dispersion stability, friction reduction, and antiwear effects. In a recent study, we found a significant response to externally applied electric potentials when surfactant-coated CuS nanoparticle additives were dispersed in diethyl succinate (a common ester lubricant).17 Although the mechanism of friction regulation was considered to be mainly due to enhanced adsorption of the nanoparticles, there were key issues that required further exploration. For example, how are the nanoparticles charged in ester lubricant and what is the effect of the nature of the surfactant (amorphous versus crystalline) on the electric response of the nanoparticles, which are addressed in detail in the present article. Subsequently, we discuss the nature of the electric double layer in solvents with

2. EXPERIMENTAL DETAILS 2.1. Materials. Copper perchlorate hexahydrate (CuCl2·6H2O) and octadecylamine (C18H37NH2, 99%, ODA) were obtained from Sigma-Aldrich. Potassium-ethyl-xanthogenate (CH3CH2OCS2K, 98%) was purchased from Fluka. Methanol (absolute) and chloroform (AR-b) were purchased from Bio-Lab. Deionized water (DI water) with a resistivity of 18.2 MΩ·cm was obtained from a Millipore filter system. Diethyl succinate (C8H14O4, 99%, DES) was used as the base fluid lubricant. Acetone (analytically pure), ethanol (analytically pure), and DI water were used for rinsing and cleaning of the friction pairs. 2.2. Synthesis of Surfactant-Coated CuS Nanoparticles and Thermally Treatment. The synthesis of the surfactant-coated CuS nanoparticles was carried out according to a previously reported study.18 A total of 3 g of potassium-ethyl-xanthogenate was mixed in 400 mL of DI water. A total of 3.54 g of copper-perchlorate hexahydrate was dissolved in 100 mL of DI water. The two solutions were mixed to form copper-ethylxanthate, (C2H5OCS2)2Cu. A total of 81 mg of copper-ethylxanthate and 1.53 g octadecylamine were used to synthesize CuS nanoparticles. The synthesis was conducted under a N2 flow and a magnetic stirring for 60 min at temperatures of 65 and 90 °C for nanoplates and nanoarrays, respectively. After the nanoparticles were formed, methanol and chloroform were used in order to wash and remove the excess ODA. Finally, the CuS nanoparticles were thermally treated in an environmental furnace at 130 °C for 4 h under ambient conditions. Thus, four varieties of alkylamine-coated CuS nanoparticles samples were synthesized: untreated CuS nanoplates (CuS-UPs), treated CuS nanoplates (CuS-TPs), untreated CuS nanoarrays (CuS-UAs), and treated CuS nanoarrays (CuS-TAs). 2.3. Characterization of the Surfactant-Coated CuS Nanoparticles and Their Suspensions. The nanoparticle morphologies were characterized by transmission electron microscopy (TEM, JEOL JEM-2100F, 200 kV). The phases present were characterized by X-ray diffraction (XRD) using Cu Kα radiation (λ = 1.5405 Å) operated at B


Article

Langmuir

Figure 2. (a) AFM topography and (b) surface potential image of CuS sample coated with crystalline ODA surfactant. (c) AFM topography and (d) surface potential image of CuS sample coated with amorphous ODA surfactant. 40 kV and 30 mA. θ/2θ scans were taken for 20 min in a 2θ range of 5−60°, in steps of 0.033°. Atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM) were used for characterization of topography and surface potential distribution of the nanoparticles. First, the nanoplates were chosen, owing their relatively flat surface. Powder samples before and after heat treatment were dispersed in methanol in an ultrasonic bath, deposited onto ultraflat Au substrates (roughness Ra about 0.7 nm), and dried in air. The AFM/KPFM instrument (Bruker Dimension ICON) was operated in the tapping mode using a Cr/Pt-coated silicon cantilever (Multi75E-G) with a force constant and a resonant frequency of 3 N/m and 75 kHz, respectively. A 0.5 V AC voltage at the resonant frequency of the cantilever was applied between the probe tip and the sample. The lift scan height during the interleave scan is 116 nm. Dispersions of 1 mg/mL CuS nanoparticles in DES were prepared by ultrasonic treatment for 1 h at room temperature. Zeta potential and particle size distributions of the suspensions of CuS nanoparticles were measured with a zetasizer (Malvern, U.K.). Moreover, a quartz crystal microbalance (QCM, Q-sense E4 system, Biolin Scientific, Sweden) was used to measure mass variation per unit area by measuring the change in frequency of a quartz crystal resonator. The

changes of both resonance frequency (Δf) and dissipation (ΔD) for the adsorption of nanoparticles on the quartz crystal sensors (QSX 301 Gold) were measured. Liquid suspensions were injected into the cell in a flow rate of 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 298 K. 2.4. Friction Test. Tribology tests were conducted on a MicroTribometer (UMT-3, Bruker) at room temperature. The normal load was set as 3 N, corresponding to a maximum Hertz contact pressure of 690.84 MPa. Tests were performed at the sliding speed of 10 mm/s over 5 mm track. Reciprocating frequency was controlled at 1 Hz and time was about 30 min. Before the friction tests, the Cu plate with dimensions 20 mm × 20 mm × 2 mm (used as the lower friction pair) was polished using silicon carbide paper and diamond paste, until its roughness Ra was about 10 nm. A zirconium dioxide (ZrO2, Ra ≈ 10 nm) ball with a diameter of 6.35 mm was used as the upper friction pair. The upper and lower friction pairs were cleaned in an ultrasonic bath using acetone, ethanol, and deionized water and were finally dried with compressed air. Direct-current power supply was used in the experiment to apply electric potential (minus value means the copper plate is connected to the negative pole and vice versa). Lubricant was dropped on the lower C


Article

Langmuir

Figure 3. Schematic illustrations and EDL models of the ODA surfactant-coated CuS nanoparticles in DES (a) before and (b) after thermal treatment. (c) Zeta potential distributions (d) and particle size distribution of the untreated and treated CuS nanoplates dispersed in DES lubricant. 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. After the friction test, the postexperiment DES suspensions of CuS nanoparticles were collected and centrifuged at 5000 rpm for 10 min, washed with methanol, and dried in the air for 24 h (temperature about 15−25 °C and humidity about 30%) and examined again by using XRD and TEM.

before and after thermal treatment, where in the untreated sample, the ODA surfactant capping layer shows sharp Bragg peaks typical for crystalline materials, and in the thermally treated sample, the ODA capping layer turns amorphous. KPFM and AFM were employed to probe the surface potential and topography of ODA surfactant-coated CuS nanoplates on Au substrate. Figures 2a and 2b respectively show representative topographic and surface potential images of the untreated CuS nanoplates with crystalline ODA surfactant. The thickness of the nanoplates appears in multiples of 6.7 nm (6.7 ± 0.73 nm and 13.3 ± 0.63 nm), suggesting occasional stacking of the CuS plates. As shown in Figure 2b, the surface potential of the corresponding nanoparticles in Figure 2a is higher than that of the surrounding Au substrate. The average potential difference is 18.47 ± 0.99 mV. For the thermally treated CuS nanoplates in Figure 2c, the thickness is about 51.9 ± 2.2 nm, indicating aggregation of the nanoplates after the heat treatment. Notably, almost no potential difference is measured between the surrounding substrate and the treated nanoplate, as shown in Figure 2d. Combining the surface potential images with the XRD results in Figure 1, the surfactant structures of ODA on the surface of the CuS before and after thermal treatment are further discussed. As previous studies reported, surface potential arises from, and provides information on, the dipole moments of the molecules forming the monolayer.19 Most surface potential measurements can be interpreted using the Helmholtz equation.20 For an array of polarizable molecules,

3. RESULTS AND ANALYSIS 3.1. Surfactant Structures on the Surface of CuS Nanoparticles. TEM and XRD analyses were carried out in order to study the effect of thermal treatment at 130 °C for 4 h under ambient conditions. Figure 1a,b shows TEM micrographs of the nanoplates before and after thermal treatment, respectively. The result indicates that both of the samples retain the platelet structure with typical in-plane dimensions of 20−70 nm. Figure 1c presents the XRD patterns obtained from these samples. The Bragg peaks in 2θ range from 18 to 25 degrees arise from the crystalline form of the ODA surfactant, as described previously.18 In further agreement with our previous work, these peaks disappear and merge into one broad peak after heat treatment, indicating the liquid-like form of the ODA surfactant. Figure 1d,e shows TEM micrographs of the nanoarrays before and after thermal treatment. The CuS particles appear to maintain their platelet array morphology for both untreated and thermally treated samples. As in the case of the individual platelets, XRD analyses in Figure 1f showed marked difference D


Article

Langmuir

structure of the EDL is considered as the same, regardless of solvent polarity. The largest differences are found between the protic solvents, where there are proton exchanges between the solvent molecules, while for aprotic solvents there is exchange of an electron pair. Namely, there exist ions or dipoles in any pure nonpolar solvent, though the concentration is very low for most of pure lubricating liquids. In order to define the shortest distance that can separate cation and anion pairs, the Bjerrum length has been defined as follows:24

such as the ODA surfactants in this research, the Helmholtz equation is described as follows: ΔV =

μ cos θ Aεε0

(1)

where μ is the moment of the isolated molecule, θ is the angle between the molecule, the subphase, A, is the area occupied by each molecule, ε0 is the permittivity of free space, and ε is the apparent relative permittivity that accounts for dipole−dipole interactions in the monolayer. For the crystalline surfactants, it is reasonable to assume that the angle θ between the ODA molecules and the subphase is almost the same. However, after the thermal treatment, the surfactant state was changed from crystalline to amorphous and isotropic, and θ can assume a large range of angles. Hence, regardless of the other parameters in the equation, the surface potential variation between the amorphous phase and the Au substrate is found to be much smaller than with the crystalline phase. A related conclusion has been previously reported by Bush et al. who used KPFM to investigate the adsorption and morphology of octadecyltrichlorosilane monolayers on silicon and found that high surface potentials correspond to a highly ordered phase of the monolayer.21 According to the above discussion, it can be obtained that thermal treatment results in random chain conformations of the ODA surfactants with respect to the surface of the nanoparticles and, therefore, shows smaller surface potential values according to the Helmholtz model. In a previously reported study,18 ODA molecules were considered to be physically adsorbed on the surface of the nanoparticle substrate. Thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) of the surfactantcoated CuS nanoparticles prepared by the same conditions were carried out, and the data are used for further analysis in this research. For the nanoplates (CuS-UPs) heated to 500 °C at a scan rate of 5 °C/min, the residue was about 26 wt %. For the nanoarrays (CuS-TAs), the residue was around 13 wt %. The final residue is likely to be Cu2S or Cu1.8S, owing to the decomposition reaction of the CuS.22,23 Thus, there is 1 mol CuS with 0.78 mol ODA in CuS-UPs, and 1 mol CuS with 1.91 mol ODA in CuS-UAs. Most importantly, the TGA curves presented that the surfactant molecules began to desorb at about 150 °C. Combing the result with the X-ray diffractograms in Figure 1, the ODA molecules were considered to melt and mix with each other, instead of desorbing from the nanoparticle surface after the thermal treatment at 130 °C. 3.2. Effect of the Surfactant States on Electrical Properties of the Nanoparticles. Combining with the XRD, KPFM, and TGA analysis, we have studied the electrical properties of nanoparticles with different surfactant capping layers. A total of 1 mg/mL CuS nanoparticles before and after heat treatment were dispersed in DES lubricant by ultrasonic treatment. Then, zeta potential distributions of CuS nanoparticle suspensions were tested and shown in Figure 3. The corresponding mean zeta potentials were +9.75 and −0.29 mV (near 0 mV) for the untreated and thermally treated surfactantcoated CuS nanoplate suspensions, respectively. It is well-known that the electric potential difference exists between any contacted phases, owing to the different Fermi levels of the materials. Electrical double layer (EDL) exists in all interfaces between two phases. In the liquid phase, the basic

λB =

e2 4πε0εrkBT

(2)

where e is the elementary charge, εr is the relative dielectric constant of the solvent, ε0 is the dielectric constant of vacuum, kB is the Boltzmann constant, and T is the absolute temperature in kelvin degrees. The λB is the separation at which the electrostatic interaction between two elementary charges is comparable in magnitude to the thermal energy, kBT. If we assume that the temperature is constant, the charge number of the ions is 1, and the ions are spherical in DES lubricant, some conclusions can be obtained according to the above analysis. There exist ions in almost any pure solvents (polar or nonpolar). The larger the εr, as well as dion, the easier is the formation and more stable existence of the ions. For DES used in this research, e = 1.602 × 10−19 C, ε0 = 8.854 × 10−12 F/m, kB = 1.38 × 10−23 J/K, T = 298 K, and εr = 6.6. Then λB can be calculated and equal to 8.5 nm. This value is obviously much larger than the diameter of the DES molecule. Ions are far more likely to be solvated than to exist alone. Hence, solvated ions are considered as the basic unit of double layer, as shown in the Figure 3a,b. After the discussion of the EDL structure in the liquid phase, the effect of the surfactant molecules on the nanoparticles was analyzed as follows. In this research, ODA was used as the surfactant coating on the surface of the nanoparticles. Before heat treatment, the ODA surfactant on the untreated CuS nanoplatelets shows in-plane crystalline order according to XRD. ODA molecules can adsorb on the sample surface by two types of adsorption, adsorption of the electron rich amine head-groups onto the positively charged Cu2+ cationic sites on the surface of the CuS substrate, and van der Waals adsorption between ODA molecules. The schematic diagram is shown in Figure 3a. Owing that the amino (−NH2) in the physically adsorbed ODA molecule is an electron donor, the composite nanoparticles tend to be positively charged in the lubricant. When CuS nanoplates are subject to mild thermal treatment, the state of their surfactant capping layer is changed and the surfactant becomes amorphous, as evident from the broadening of the surfactant Bragg peaks in Figure 1d. Previous research reported the influence of pentacene molecule structures on the carrier mobility and found that amorphous phase had extremely lower conductivity comparing with the stable single-crystal phase.25,26 Similar to the above research, the −NH2 groups in this experiment are surrounded by the irregular carbon chains, thus, inhibiting electron transfer. Hence, the EDL structures are different from the untreated samples, as shown in Figure 3b, and the zeta potential is about 0 mV for the amorphous surfactant coated nanoparticles. In fact, zeta potential could not only show the electrical charge on the particle surface, but also provides indication on the stability of colloidal dispersions.27,28 Though the particle sizes of the CuS nanoparticles shown in TEM observation E


Article

Langmuir

Figure 4. Friction coefficient vs time in DES lubricant with (a) CuS-UPs, (b) CuS-TPs, (c) CuS-UAs, and (d) CuS-TAs under different applied potentials.

applied, the COF drops to about 0.09, thus, negative applied potential gives rise to a prominent friction reduction in the case of CuS-UPs in DES lubricant. Figure 4b shows that the COFs versus time for CuS-TPs in DES lubricant are near 0.15 under different applied potentials. Hence, the electric response of the CuS-TPs is weakened compared with the response of CuS-UPs, shown in Figure 4a. Nevertheless, the lowest COF values are obtained for an applied potential of −30 V as in the case of CuS-UPs suspensions, and loss of crystallinity of the surfactant capping layer due to thermal treatment clearly gives rise to higher friction forces. Similar behavior is observed in the case of CuS nanoarrays (CuS-UAs) in Figure 4c,d, showing superior friction reduction properties compared to the individual CuS platelet samples. The COF decreases from 0.23 to 0.18 when adding the CuSUAs to the pure DES lubricant. Moreover, the CuS-UAs show better potential control effect than the CuS-UPs for boundary lubrication. When −30 V potential is applied, the COF sharply drops to about 0.05. When +30 V potential is applied, the COF more or less remains at the same value as compared to the case without externally applied potential. In the case of the DES with CuS-TAs, the COF decreases sharply compared to pure DES, yet as opposed to the CuS-UAs, it is hardly affected by the potential. Notably, also in the case of CuS arrays, the lowest value of COF, 0.05, is obtained in the case of untreated nanoparticles, while thermal treatment results in higher COF values of 0.07−0.10, regardless of applied potential. For studying the mechanism of the electric field effect on the tribological performance of the CuS nanoparticle lubricant additives, their adsorption behavior was characterized by QCM on gold surfaces for pure DES and for CuS nanoparticle suspensions in DES under different potentials. The data shows the change in frequency of the crystal in different environments at the fifth overtone normalized by the overtone number, as

(Figure 1a,b) are quite uniform, there are obvious differences when nanoparticles before and after heat treatment are dispersed in DES liquid, as shown in Figure 3d, measured by Zetasizer, owing to the agglomeration of particles in DES lubricant. For the treated CuS nanoplates, the average particle size is about 1280 nm, larger than that of the untreated (about 825 nm), which indicates the more agglomeration of amorphous ODA-coated CuS. The result of the particle size distributions is agreement with that of the zeta potential measurement. For the untreated CuS nanoarrays in DES liquid, the particle size is about 300 nm and zeta potential is about 11.7 mV, as reported in our previous study.17 While the average particle size of the treated CuS nanoarrays is about 943 nm, the zeta potential is about 0.05 mV measured by the zetasizer. The data show the similar regularity with the untreated and treated CuS nanoplates. 3.3. Electric Response of CuS Nanoparticles in Ester Lubricant. A recent area of great interest is the research of nanoparticles dispersed in lubricants as friction modifier additives.11,12 In order to explore the influence of the surfactant structures on the electric response and tribological behavior of the nanoparticles, the potential controlled boundary lubrication performances of the CuS nanoparticles with same shape but different surfactant states were analyzed. Figure 4 shows the friction coefficient (COF) versus time in DES lubricant and for DES suspensions of CuS nanoparticles (including CuS-UPs, CuS-TPs, CuS-UAs, and CuS-TAs) under different applied potentials. Figure 4a shows that, for 1 mg/mL CuS-UPs in pure DES lubricant, the COF decreases from 0.23 to 0.16. Due to the unique platelet structure of the nanoparticles, reasonable to assume that their main effect is to increase the contact area or directly reduce the shear force. When 30 V is applied, the average value of COF remains nearly unchanged, 0.16−0.17. However, when −30 V is F


Article

Langmuir

To evaluate the durability of the nanoparticle additives and to further characterize the influence of the solvent and friction shear force on the state of nanoparticles and their surfactants, the postexperiment CuS nanoparticles in DES suspension were collected, centrifuged, and characterized. Figure 6 shows the

shown in Figure 5. First, the frequency was measured in the pure DES or the suspensions in their adsorption equilibrium.

Figure 5. Change in frequency for adsorption of nanoparticles on the quartz crystal sensors in pure DES, in DES suspensions with CuSUPs, CuS-TPs, CuS-UPs, or CuS-TAs under different applied potentials (0 and ±20 V), illustrated as the fifth overtone normalized by overtone number (N = 5).

After 180 s, positive potential (+20 V) was applied in the yellow background. Then, negative potential (−20 V) was applied, as shown in the blue background. For pure DES, slow frequency reduction is obtained when positive potential is applied, which is considered as the adsorption of the DES molecules onto the metal surface. When negative potential is applied, the variation in frequency tends to decrease and finally remained unchanged again. For the DES with CuS-UPs, the change of frequency is slight when positive potential is applied. While, the frequency decreases significantly from about 0 to 30 Hz when negative potential is applied, indicating rapid adsorption on the surface of the quartz crystal sensor. The case in DES with CuS-UAs is similar to that of the CuS-UPs. Mentionable, the response of the nanoparticles (CuS-UAs) is not fast. When the negative potential is turned off, there is still some continued adsorption. Comparing with the untreated nanoparticles, the adsorption behavior of the thermally treated samples (CuS-TPs and CuSTAs) has no marked and regular effect, regardless of the applied potentials. This can be attributed to electrical properties of the nanoparticles with different surfactant states. Both the zeta potential and QCM data show that the untreated nanoparticles are positively charged, while the treated ones have almost no charge. When negative potentials are applied, the positively charged nanoparticles are adsorbed onto the surface of the lower pair and reduce the force of friction. However, the thermally treated samples show almost no electric response, owing to the amorphous surfactants. Another parameter obtained from the QCM measurement is dissipation. The frequency is related to the mass of the film, as mentioned above, and the dissipation is related to the viscoelastic properties of the absorbed film. According to the change in dissipation on the quartz crystal sensors in Figure S1 in the Supporting Information, the adsorption of the DES molecules can be considered as a rigid film, owing to the negligible change in dissipation (near 0). However, the capping layer formed by the adsorbed particles can be regarded as a soft film, as the change of the dissipation is considerably greater than 1 × 10−6.

Figure 6. Bright-field (BF) TEM images of CuS-UPs post tribology experiment: (a) at −30 V, (b) without applied potential, and (c) at +30 V; and CuS-TPs post tribology experiment: (d) at −30 V, (e) without applied potential, and (f) at +30 V. Scale bars represent 50 nm.

TEM images of CuS-UPs and CuS-TPs after the friction tests with different applied potentials, which show that the tribology tests apparently did not damage the CuS nanoplates. Figure 7 presents XRD patterns of CuS-UPs and CuS-TPs after the friction tests with an external voltage of −30, 0, or +30 V. The Bragg peaks in the 2θ range that are 18−25° (marked in light orange in Figure 7) indicate the state of the ODA surfactant. No significant change was observed in the ODA coating of the untreated samples, as shown in Figure 7a− c, while Figure 7d−f shows that the ODA coating of the thermally treated samples tends to lose its liquid-like structure following the tribology tests and transforms to crystalline ODA. This effect is most significant when an external field of −30 V is applied. It is considered that shear force during the friction test can cause the amorphous surfactant to shear align and to crystallize. Examples for shear force induced crystallization of polymers, such as polypropylene, are wellknown in the literature.10 G


Article

Langmuir

Figure 7. XRD diffractograms of CuS-UPs post tribology experiment: (a) at −30 V, (b) without applied potential, and (c) at +30 V; and CuS-TPs post tribology experiment: (d) at −30 V, (e) without applied potential, and (f) at +30 V.

XRD and TEM analyses were conducted also for the CuS nanoarrays after different tribology procedures. In general, the results in Figures S2 and S3 indicate that tribology tests did not damage the CuS nanoarrays structure. Nevertheless, samples tested with external potential of +30 V (Figure S2) showed visual evidence of morphological degradation. No significant changes were observed in the CuS nanoparticles and ODA coating for both treated and untreated samples. XRD did not confirm less crystalline material in the sample and did not provide any additional information on this matter. In the case of treated nanoplates, the ODA is completely exposed to the shearing surfaces. As a result, recrystallization of the ODA in CuS nanoplates was observed. On the other hand, treated nanoarrays contain a large quantity of masked ODA between the stacked plates. It is possible that some ODA recrystallizes on the outer faces of the arrays yet masked in the XRD signal due to large amounts of amorphous ODA material remaining between the plates.

implying that amorphous surfactants weaken the response to electric field. (3) For the case of nanoparticles coated with crystalline ODA, the surfactants maintain their structure after dispersion in DES and tribological testing. In the case of thermally treated samples, the amorphous surfactants tend to partially crystallize due to shear alignment during the friction test.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.9b01714. Supplementary experimental results (PDF)

■

4. CONCLUSIONS We have studied the surface charge and boundary lubrication behavior of CuS nanoparticles coated with crystalline and amorphous surfactants.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

(1) The ODA surfactant state of the CuS nanoparticles prepared by colloidal chemical method is crystalline. Following thermal treatment at 130 °C for 4 h under ambient conditions, the surfactant becomes amorphous and isotropic. The surface potential of the CuS nanoplates with crystalline surfactant is 18.47 ± 0.99 mV, which is higher than that obtained with amorphous surfactant. Both the zeta potential and QCM data show that the untreated nanoparticles are positively charged, while the treated ones show almost no charge.

ORCID

(2) Electric field controlled lubrication performance of the untreated CuS nanoparticles, including nanoplates and nanoarrays, has been reported to be affected by the electromigration of the nanoparticles.11 When negative potentials are applied, the positively charged nanoparticles are adsorbed on the surface of the lower pair, reducing the friction forces. However, the thermally treated samples show almost no potential control effect,

This work was financially supported by the Chinese National Key R&D Plan (Grant No. 2016YFE0130300) and by the China−Israel bilateral research program in nanotechnology of the Ministry of Science and Technology of the People’s Republic of China and the Israeli Ministry of Science and Technology.

Chenxu Liu: 0000-0002-7147-7063 Ofir Friedman: 0000-0001-5903-7797 Yu Tian: 0000-0001-7742-5611 Yuval Golan: 0000-0003-1369-0285 Yonggang Meng: 0000-0002-8274-5131 Author Contributions ‡

C.L. and O.F. contributed equally.

Funding

Notes

The authors declare no competing financial interest. H


Article

Langmuir

■

layer on Silicon Surfaces†. J. Phys. Chem. A 2007, 111 (49), 12339− 12343. (22) Ikram, M.; Nafees, M.; Ali, S. Thermal behavior and decomposition of copper sulfide nanomaterial synthesized by aqueous sol method. Digest Journal of Nanomaterials and Biostructures 2015, 2 (10), 635−641. (23) Westrum, E. F., Jr.; Stolen, S.; Gronvold, F. Thermodynamics of copper sulfides II. Heat capacity and thermodynamic properties of synthetic covellite, CuS, from 5 to 780.5 K. Enthalpy of decomposition. J. Chem. Thermodyn. 1987, 19 (11), 1199−1208. (24) Russel, W. B., Saville, D. A., Schowalter, W. R. Colloidal Dispersions; Cambridge University Press, 1989. (25) Dimitrakopoulos, C. D.; Brown, A. R.; Pomp, A. Molecular beam deposited thin films of pentacene for organic field effect transistor applications. J. Appl. Phys. 1996, 80 (4), 2501−2508. (26) Dimitrakopoulos, C. D.; Mascaro, D. J. Organic thin-film transistors: A review of recent advances. IBM J. Res. Dev. 2001, 45 (1), 11−27. (27) Masoudipour, E.; Kashanian, S.; Azandaryani, A. H.; Omidfar, K.; Bazyar, E. Surfactant effects on the particle size, zeta potential, and stability of starch nanoparticles and their use in a pH-responsive manner. Cellulose 2017, 24 (10), 4217−4234. (28) Soukupová, J.; Kvítek, L.; Panácě k, A.; Nevěcň á, T.; Zbořil, R. Comprehensive study on surfactant role on silver nanoparticles (NPs) prepared via modified Tollens process. Mater. Chem. Phys. 2008, 111 (1), 77−81.

REFERENCES

(1) Mulvihill, M. J.; Habas, S. E.; Jen-La Plante, I.; Wan, J.; Mokari, T. Influence of Size, Shape, and Surface Coating on the Stability of Aqueous Suspensions of CdSe Nanoparticles. Chem. Mater. 2010, 22 (18), 5251−5257. (2) Heinz, H.; Pramanik, C.; Heinz, O.; Ding, Y.; Mishra, R. K.; Marchon, D.; Flatt, R. J.; Estrela-Lopis, I.; Llop, J.; Moya, S.; Ziolo, R. F. Nanoparticle decoration with surfactants: Molecular interactions, assembly, and applications. Surf. Sci. Rep. 2017, 72 (1), 1−58. (3) Israelachvili, J. N. Intermolecular and Surface Forces, 3rd ed.; Academic Press, 2011. (4) Kango, S.; Kalia, S.; Celli, A.; Njuguna, J.; Habibi, Y.; Kumar, R. Surface modification of inorganic nanoparticles for development of organic−inorganic nanocompositesA review. Prog. Polym. Sci. 2013, 38 (8), 1232−1261. (5) Zhu, K.; Ju, Y.; Xu, J.; Yang, Z.; Gao, S.; Hou, Y. Magnetic Nanomaterials: Chemical Design, Synthesis, and Potential Applications. Acc. Chem. Res. 2018, 51 (2), 404−413. (6) Zhang, J.; Zhang, J. Surfactant Inducing Phase Change of ZnO Nanorods to Low Friction. Tribol. Lett. 2013, 49 (1), 77−83. (7) Yoshizawa, H.; Chen, Y.; Israelachvili, J. Fundamental Mechanisms of Interfacial Friction. 1. Relation between Adhesion and Friction. J. Phys. Chem. 1993, 97 (16), 4128−4140. (8) Khatri, O. P.; Biswas, S. K. Load Induced Microstructure Evolution and Friction in an Organic Monolayer Self-assembled on a Silicon Substrate. Tribol. Lett. 2008, 32 (3), 179−188. (9) Zhang, G.; Yu, H.; Zhang, C.; Liao, H.; Coddet, C. Temperature dependence of the tribological mechanisms of amorphous PEEK (polyetheretherketone) under dry sliding conditions. Acta Mater. 2008, 56 (10), 2182−2190. (10) Jiang, L.; An, M.; Wu, F.; Miao, W.; Wang, Z.; Zhang, Y.; Hsiao, B. S. The influence of short chain branch on formation of shear induced crystals in bimodal polyethylene at high shear temperatures. Eur. Polym. J. 2018, 105, 359−369. (11) Spikes, H. Friction Modifier Additives. Tribol. Lett. 2015, 60 (1), na. (12) Dai, W.; Kheireddin, B.; Gao, H.; Liang, H. Roles of nanoparticles in oil lubrication. Tribol. Int. 2016, 102, 88−98. (13) Wu, H.; Li, X.; He, X.; Lu, J.; Wang, L.; Zhou, B.; Dong, G. An investigation on the lubrication mechanism of MoS 2 nanoparticles in unidirectional and reciprocating sliding point contact: The flow pattern effect around contact area. Tribol. Int. 2018, 122, 38−45. (14) Akbulut, M.; Belman, N.; Golan, Y.; Israelachvili, J. Frictional Properties of Confined Nanorods. Adv. Mater. 2006, 18 (19), 2589− 2592. (15) Min, Y.; Akbulut, M.; Prud'homme, R. K.; Golan, Y.; Israelachvili, J. Frictional Properties of Surfactant-Coated Rod-Shaped Nanoparticles in Dry and Humid Dodecane†. J. Phys. Chem. B 2008, 112 (46), 14395−14401. (16) Aralihalli, S.; Biswas, S. K. Grafting of Dispersants on MoS2 Nanoparticles in Base Oil Lubrication of Steel. Tribol. Lett. 2013, 49 (1), 61−76. (17) Liu, C.; Friedman, O.; Meng, Y.; Tian, Y.; Golan, Y. CuS Nanoparticle Additives for Enhanced Ester Lubricant Performance. ACS Applied Nano Materials 2018, 1 (12), 7060−7065. (18) 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 Interface Sci. 2015, 457, 43−51. (19) Ellison, D. J.; Lee, B.; Podzorov, V.; Frisbie, C. D. Surface Potential Mapping of SAM-Functionalized Organic Semiconductors by Kelvin Probe Force Microscopy. Adv. Mater. 2011, 23 (4), 502− 507. (20) Taylor, D. M. Developments in the theoretical modelling and experimental measurement of the surface potential of condensed monolayers. Adv. Colloid Interface Sci. 2000, 87 (2), 183−203. (21) Bush, B. G.; DelRio, F. W.; Opatkiewicz, J.; Maboudian, R.; Carraro, C. Effect of Formation Temperature and Roughness on Surface Potential of Octadecyltrichlorosilane Self-Assembled MonoI


Electric Response of CuS Nanoparticle Lubricant Additives: The Effect

Recommend Documents