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Materials Chemistry and Physics 98 (2006) 183–189

Oleic acid capped PbS nanoparticles: Synthesis, characterization and tribological properties Shuang Chen a,∗ , Weimin Liu b a

Department of Chemical and Environmental Engineering, Wuyi University, Jiangmen, Guangdong 529020, China b State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Science, Lanzhou 730000, China Received 19 August 2004; received in revised form 19 August 2005; accepted 5 September 2005

Abstract Oleic acid (OA) capped PbS nanoparticles were chemically synthesized and characterized by means of Fourier transform-infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), X-ray electron diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The triboligical properties of the capped PbS nanoparticles as additive in liquid paraffin was investigated using a four-ball machine. The lubricating mechanisms were discussed along with the analyses results of XPS and scanning electron microscope (SEM). Results show that OA-capped PbS nanoparticles, with an average diameter of about 8 nm, are able to prevent water adsorption, oxidation and are capable of being dispersed stably in organic solvents or mineral oil. OA-capped PbS nanoparticles as an additive in liquid paraffin perform good antiwear and friction-reduction properties owing to the formation of a boundary film. © 2005 Elsevier B.V. All rights reserved. Keywords: Nanoparticles; Oil additive; Capping; Tribology

1. Introduction Nanoparticles have been intensively investigated for the past decade because of their distinctive physical and chemical properties [1–5]. In particular, organic compound monolayer capped nanoparticles have attracted extensive attention owing to their special properties and wide application areas [6–8], especially as additives in lubricating oils [9,10]. The dispersion capacity of inorganic nanoparticles in lubricating oils and their stability in air can be improved effectively by means of capped with a layer of a long hydrocarbon chains. It is well known that zinc dialkyldithiophosphate (ZDDP) is a popular used oil additive, so in the previous work, we chose DDP as the capping agent, and synthesized DDP capped PbS, ZnS and PbO nanoparticles, respectively [11–13]. The widely used four-ball wear tester was chosen to evaluate the tribological behaviour of the capped nanoparticles mentioned above

∗

Corresponding author. Fax: +86 750 3299390. E-mail address: [email protected] (S. Chen).

0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.09.043

as oil additives, liquid paraffin (LP) was used as the base oil. Results show that they both can effectively improve the antiwear ability of LP at an extremely low additive concentration. Unfortunately, the friction coefficient is even higher than use LP alone, which means they cannot reduce the friction of the system. Both the antiwear and friction-reduction ability is important to evaluate the capacity of an oil additive. Since all the DDP capped PbS, ZnS and PbO nanoparticles cannot lubricating the system effectively, the surface-capping agent was changed in this work. We chose oleic acid (OA) as the new surfacecapping agent, and synthesized OA-capped PbS nanoparticles. The prepared product was characterized with a variety of methods, including Fourier transform-infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), X-ray powder diffractometer (XRD), and X-ray photoelectron spectroscopy (XPS). The tribological properties of OA-capped PbS nanoparticles as an oil additive in LP were also studied using the four-ball wear tester. It is anticipated that the OA-capped PbS nanoparticles will perform better tribological properties than DDP capped PbS, ZnS and PbO nanoparticles does, especially for the frictionreduction ability.

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2. Experimental Analytically pure lead acetate, sodium sulfide and oleic acid were used as the raw materials for synthesis. Analytically pure ethanol and distilled water were used as solvents. Capped PbS nanoparticles were prepared by a chemical co-deposition method using OA as a capping agent. Lead acetate and sodium sulfide were dissolved in distilled water, respectively, to prepare the stock solution. The preparation technique was as follows. Ethanol–water mixture was used as the solvent to suffice the reaction system to be homogeneous. Capping agent OA and sodium sulfide stock solution was added to the mixture solution mentioned above in turn, and heated to 333 K with vigorous stirring, adjusting the ratio of ethanol and distilled water to ensure the solution to be homogeneous. After they were well mixed together, lead acetate stock solution was added drop by drop under agitation, and black precipitate appeared immediately. After further stirring for 3 h, the black precipitate was filtered, washed with ethanol-distilled water, and ethanol, respectively, and dried in a degassed desiccators for 48 h. Finally, the target product, a black powder of OA-capped PbS nanoparticles, was obtained. Non-capped PbS nanoparticles were prepared for comparison by the same procedures as the capped ones, except that no capping agent was added during the preparation process. The prepared OA-capped PbS nanoparticles can be well dispersed in several organic solvents, including chloroform, benzene, toluene, and liquid paraffin, whereas non-capped PbS nanoparticles cannot be dispersed in the above solvents. So it is concluded that after the surface-capping with OA, the dispersion capability of nano-sized PbS is improved. The improvement in the dispersion capability enables the OA-capped PbS nanoparticles to be used as an additive in lubricating oils. FT-IR spectra were taken on a Bio-Red FTS-165 IR spectrometer, to investigate the chemical structures of the capping layer on the PbS nanoparticles’ surface. The morphology of the PbS nanoparticles was studied by JEM-1200 EX/S TEM. The crystal phase of the prepared powders was identified by a D/max-RB ˚ XPS X-ray powder diffractometer (XRD) using Cu K␣ radiation (λ = 1.5418 A). analysis was conducted on a PHI-5702 photoelectron spectrometer using a pass energy of 29.35 eV and a Mg K␣ line excitation source with the reference of C1s at 284.6 eV. Because a monochromator was used, the resolution for binding energy reached 0.3 eV. The tribological properties of the OA-capped PbS nanoparticles as an additive in liquid paraffin were evaluated on a four-ball machine under a rotating speed of 1450 r min−1 at ambient conditions. The test duration was 30 min and the load applied was 100, 200, 300 and 400 N, respectively. The 12.7 mm diameter balls used in the test were made of GCr15 bearing steel (SAE52100 steel) with a HRc of 59–61. Before each test, the balls and the specimen holders were all cleaned in petroleum ether (normal alkane with a boiling point of 60–90 ◦ C). For each sample, three identical tests were performed so as to minimize data scattering. At the end of each test, the wear scar diameters on the three stationary balls were measured on a digital-reading optical microscope to an accuracy of 0.01 mm in the direction parallel and perpendicular to the sliding motion, and the average wear scar diameter of the three identical tests was calculated as the wear scar diameter in this work. The steel ball, after wear tests, was washed with ultrasonic waves in petroleum ether twice, totaling 20 min before surface analysis. A JSM-5600 LV scanning electron microscope (SEM) equipped with KEVEX energy dispersion X-ray analyzer (EDS) was employed to examine the morphologies and the element distributions in the worn surfaces.

3. Results and discussion Fig. 1 shows the FT-IR transmission spectra of OA, OAcapped PbS nanoparticles, and non-capped PbS nanoparticles, respectively. Characteristics of OA can be clearly seen in Fig. 1a, it relates to the long alkyl chain, the double bond between carbon atoms and the carboxylic acid group. Fig. 1a shows the antisymmetric and symmetric C H stretching vibration of the CH2 group around 2920 and 2850 cm−1 , respectively, the CH2 deformation vibration at 1465 cm−1 , and the rocking vibration

Fig. 1. FT-IR spectra of: (a) OA, (b) OA-capped PbS nanoparticles and (c) noncapped PbS nanoparticles.

at 722 cm−1 . This band is typical of (CH2 )n chains with n > 3. The band at 3006 cm−1 is assigned to the stretching vibration of C H, the strong band at 1710 cm−1 is the characteristic stretching vibration of C O in carboxylic acid. Compare Fig. 1a and b, it was found that the peaks around 3006, 2900, 2850, 1465, and 722 cm−1 also exist in the IR spectra of OA-capped PbS nanoparticles (Fig. 1b), indicate the existence of C H group and long alkyl chain in the OA-capped PbS nanoparticles. This reveals that OA was successfully capped on the surface of PbS nanoparticles. The characteristic band of carbonyl in carboxyl acid at 1710 cm−1 was not found in Fig. 1b, but a new peak at 1544 cm−1 appeared. The band at 1544 cm−1 demonstrates the existence of carboxylic acid salt in OA-capped PbS nanoparticles. The disappearance of the peak at 1710 cm−1 for carboxylic acid and the appearance of the peak at 1544 cm−1 for carboxylic acid salt reveals that the capping agent OA does react with PbS nanoparticles, and form carboxylic acid salt. Namely, the surface-capping agent capped on the surface of PbS nanoparticles not by physically adsorption, but undergoes chemical reaction process. The broad band near 3400 cm−1 in Fig. 1c indicates that there are large amounts of adsorbed water in non-capped PbS nanoparticles due to the high surface energy of nanoparticles. In combination with the results in Fig. 1b, it is subsequently concluded that the surface-capping layer can effectively prevent the adsorption of water on the surface of PbS nanoparticles. Fig. 2 shows the TEM image of non-capped PbS nanoparticles and OA-capped PbS nanoparticles, respectively. It was

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Fig. 2. TEM image of: (a) non-capped PbS nanoparticles and (b) OA-capped PbS nanoparticles.

found that non-capped PbS nanoparticles tended to agglomerate and did not permit identification of a single particle (Fig. 2a) owing to the high surface energy of nanoparticles. Unlike the non-capped ones, the OA-capped PbS nanoparticles had a narrow size distribution and the average diameter was about 8 nm (Fig. 2b). Comparing Fig. 2a and b, it can be concluded that the existence of the surface-capping layer can effectively prevent the agglomeration and growth of PbS particles and led to the formation of fine nanoparticles. Besides, the blurry spots in Fig. 2a indicate the existence of some surface adsorbed water around the non-capped PbS nanoparticles, which is also confirmed by the above-mentioned FT-IR observation. No similar observation is availed in Fig. 2b. This reveals that the existence of the surface-capping layer cannot only effectively prevent agglomeration between PbS nanoparticles, but also effectively prevent the surface adsorbed water around PbS nanoparticles. Fig. 3 shows the XRD pattern of the OA-capped PbS nanoparticles. It revealed several broad diffractions, which were in good agreement with face-centered cubic (fcc) PbS phase (reference:

JCPDS 5-0592). The widened X-ray diffraction peaks indicate that the synthesized product consists of very small PbS crystallites. It is thus concluded that the PbS nanocore in the capped PbS nanoparticles has fcc structure and the surface-capping agent restrains the particle size. Figs. 4 and 5 shows the XPS spectra of non-capped and OAcapped PbS nanoparticles, respectively. It is seen from Fig. 4 that the PbS nanoparticles without capping show a Pb4f7/2 peak at 137.4 eV and a S2p peak at 160.5 eV, which are in agreement with the reported values of PbS (Pb4f7/2 : 137.6 eV, S2p : 160.8 eV) [14]. Besides, the spectra of Pb4f7/2 in Fig. 4 show a shoulder peak at 138.5 eV which corresponds to PbO (Pb4f7/2 : 138.8 eV) [14], that of S2p show a peak at 168.6 eV which indicates the existence of S(VI) [14]. Thus it can be concluded that the PbS nanoparticles without capping exhibits poor stability in air. In other words, non-capped PbS nanoparticles are liable to oxidation as exposed in atmosphere because of the high surface energy. OA-capped PbS nanoparticles show a Pb4f7/2 peak at 137.8 eV, which is the signals for PbS and a single S2p peak at

Fig. 3. XRD spectra of OA-capped PbS nanoparticles.

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Fig. 4. XPS spectra of non-capped PbS nanoparticles.

Fig. 5. XPS spectra of OA-capped PbS nanoparticles.

161.2 eV (Fig. 5), a little higher than what was reported for the signals of S2p in PbS. It was supposed that the above-mentioned changes in the binding energies could be attributed to the influence of the coordination action of Pb moiety and the capping agent OA. No peaks of PbO and S(VI) were found in Fig. 5, indicating that surface-capping layer could prevent the oxidation of PbS nanoparticles effectively. According to the results of FT-IR, TEM, XRD and XPS, the model of the monolayer OA-capped PbS nanoparticles was

Fig. 6. Model of OA-capped PbS nanoparticles.

Fig. 7. Tribological properties as a function of additive concentration (four-ball, 1450 r min−1 , 300 N, 30 min).

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Fig. 8. Wear scar diameter as a function of applied load with the lubrication of LP alone or that containing 0.2 wt.% OA-capped PbS nanoparticles (four-ball, 1450 r min−1 , 30 min).

Fig. 9. Friction coefficient as a function of applied load with the lubrication of LP alone or that containing 0.2 wt.% OA-capped PbS nanoparticles (four-ball, 1450 r min−1 , 30 min).

established and shown in Fig. 6. Since the PbS nanocore was embedded in the monolayer of OA, the dispersion capacity in the organic solvent and stability in the air was both improved, accordingly. The repulsion between the surface-capping layers enables the PbS nanocore away from agglomeration. Namely,

the capping layer of OA stabilized the PbS nanoparticles, and made the OA-capped PbS nanoparticles to be a potential oil additive. Fig. 7 shows the tribological behaviour as a function of the additive concentration of OA-capped PbS nanoparticles in LP

Fig. 10. SEM morphologies and element distribution of the worn surface.

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under a load of 300 N. Results show that with the addition of OA-capped PbS nanoparticles to the LP, the wear scar diameter and friction coefficient both reduced remarkably. The effective additive concentration of OA-capped PbS nanoparticles as an additive is above 0.05 wt.%, which is much lower than the commercial oil additives (usually larger than 0.5 wt.%). As the additive concentration lower than 0.05 wt.%, the wear scar diameter and friction coefficient become larger, which means the boundary lubrication film could not be completely formed with such a low additive concentration. Further addition of OA-capped PbS nanoparticles to LP can reduce wear but cannot reduce friction coefficient any more. The lowest wear scar diameter was obtained at the additive concentration reach to 0.2 wt.%, under this condition, the wear scar diameter can be reduced by 31%, and the friction coefficient can be reduced by 30% as compared to LP. Since the optimal additive concentration was 0.2 wt.% according to both antiwear and friction-reduction ability, this concentration was chosen to investigate the influence of the load. Figs. 8 and 9 show the variation of the wear scar diameter and friction coefficient with the applied load for LP alone and for LP containing 0.2 wt.% OA-capped PbS nanoparticles. With LP alone, the wear scar diameter and friction coefficient is relatively large, and the friction system scuffs at a load higher than 300 N. However, with LP containing 0.2 wt.% OA-capped PbS nanopar-

ticles, a smaller wear scar diameter and friction coefficient was generated, and the friction system could be effectively lubricated even at a load of 400 N. It is concluded that OA-capped PbS nanoparticles as an additive can effectively improved tribological properties of LP. Fig. 10 gives the SEM morphologies and elemental distributions of Pb, S and O of the worn surfaces (four-ball machine, load: 300 N, test duration: 30 min, lubricant: LP containing 0.2 wt.% OA–PbS nanoparticles). The distributions of Pb, S and O thereon give strong evidence that a protective film was indeed formed during the friction process. Since XPS is very informative for analyzing the composition and chemical characteristics of the elements in a material, XPS analysis of the boundary film formed on the worn surfaces was performed. Fig. 11 shows the binding energies of some elements on the worn surfaces. From the results in Fig. 11, it is concluded that the boundary film on the rubbed surfaces is composed of PbS (Pb4f7/2 : 137.6 eV, S2P : 160.8 eV), PbO (Pb4f7/2 : 138.8 eV, O1S : 529.4 eV), FeS (Fe2P : 710.3 eV, S2P : 161.6 eV), Fe2 O3 (Fe2P : 710.8 eV, O1S : 530.2 eV) and SO4 2− (S2P : 168.8 eV, O1S : 532.4 eV) [14]. Compare the XPS results of Figs. 5 and 11, it is indicated that during the friction process, the chemical bond between Pb and capping agent OA fractured. Namely, the long alkyl chain in OA molecule dissociated,

Fig. 11. XPS spectra of Pb4f , S2P , P2P , Fe2P and O1S in the boundary film.

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PbS nanocore became exposure and be oxidized to PbO and S(VI). Accordingly, we supposed that the excellent antiwear and friction-reducing capacity of OA-capped PbS nanoparticles could be attributed to the formation of the boundary film on the steel surface. 4. Conclusions OA-capped PbS nanoparticles were successfully prepared by a chemical method. The synthesized nanoparticles with an average diameter of 8 nm could be well dispersed in several organic solvents and liquid paraffin, which enables the synthesized particles to be used as additives in lubricating oils. Surface-capping agent capped on the PbS nanoparticles’ surface by chemical bond, not by physically adsorption. The existence of OA capping layer on the surface of the PbS nanoparticles restrained water adsorption and the growth of the nanoparticles. OA-capped PbS nanoparticles as an additive in LP can effectively reduce friction and wear at a low additive concentration owing to the formation of a complex boundary film. Acknowledgments The authors wish to acknowledge the financial support of the National Natural Science Foundation of China (50272068,

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