8100
Langmuir 1999, 15, 8100-8104
Preparation and Characterization of Surface-Coated ZnS Nanoparticles Shuang Chen* and Weimin Liu Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Science, Lanzhou 730000, People’s Republic of China Received June 2, 1999. In Final Form: July 22, 1999 ZnS nanoparticles coated with di-n-hexadecyldithiophosphate (DDP) were chemically synthesized. The structure of the prepared ZnS nanoparticles was investigated by means of transmission electron microscopy, electron diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The thermal stability of DDP coated on nanoparticles was compared with that of pyridinium di-nhexadecyldithiophosphate (PyDDP) using a thermogravimetric analyzer. The tribological properties of ZnS nanoparticles as an additive in tetradecane were investigated by a SRV tester in a ball-on-disk configuration. DDP-coated ZnS nanoparticles, with an average diameter of about 4 nm, are able to prevent water adsorption, and oxidation and are capable of being dispersed stably in organic solvents or mineral oil. Thermal stability of DDP coating on ZnS nanoparticles was superior to that of PyDDP. Wear tests show that DDP-coated ZnS nanoparticles as additive in tetradecane are capable of reducing friction and wear of steel.
Introduction The properties of nanocrystalline materials are quite different from those of bulk materials or individual molecules. Therefore, research on nanocrystalline materials is gaining popularity in scientific and industrial communities.1-6 Intense investigations have been stimulated by several envisaged application areas for this new class of materials. Advanced materials built up from nanoscale particles give promise of a wide range of innovative applications in catalysis, sensors, nonlinear optics, and molecular electronics.7-9 However, relatively limited attention has been paid to their tribological properties, especially as additives in lubricating oils. The insolubility and dispersion difficulties of inorganic powders limit their applications in lubricating oils. Hence, research in this area is urgently needed to resolve these difficulties. The above problems could be resolved by using some special preparation technique, for example, preparation of the nanoparticles with surface modifications.10-13 If the surface modification agents are high-weight hydrocarbons, * To whom correspondence should be sent. Phone: 86-9318860209. Fax: 86-931-8417088. E-mail:
[email protected]. (1) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (2) Chakravorty, D.; Giri, A. K. Chemistry for the 21st Century. In Chemistry of Advanced Materials; Rao, C. N. R., Ed.; Blackwell: Oxford, 1993; p 217. (3) Henglein, A. Chem. Rev. 1989, 89, 1861. (4) Yu, W. Y.; Tu, W. X.; Liu, H. F. Langmuir 1999, 15, 6. (5) Kim, D. W.; Oh, S. G.; Lee, J. D. Langmuir. 1999, 15, 1599. (6) Løver, T.; Henderson, W.; Bowmaker, G. A.; Seakins, J. M.; Cooney, R. P. Chem. Mater. 1997, 9, 1878. (7) Wen, J.; Wilkes, G. L. Chem. Mater. 1996, 8, 1667. (8) Schubert, U.; Tewinkel, S.; Lamber, R. Chem. Mater. 1996, 8, 2047. (9) Miyake, M.; Torimoto, T.; Nishizawa, M.; Sakata, T.; Mori, H.; Yoneyama, H. Langmuir 1999, 15, 2714. (10) Herron, N.; Wang, Y.; Eckert, H. J. Am. Chem. Soc. 1990, 112, 1322. (11) Steigerwald, M. L.; Alivisatos, A. P.; Gibson, J. M.; Harris, T. D.; Kortan, R.; Muller, A. J.; Thayer, A. M.; Duncan, T. M.; Douglass, D. C.; Brus, L. E. J. Am. Chem. Soc. 1988, 110, 3046. (12) Johnson, S. R.; Evans, S. D.; Mahon, S. W.; Ulman, A. Langmuir 1997, 13, 51. (13) Zhang, Z. J.; Zhang, J.; Xue, Q. J. J. Phys. Chem. 1994, 98, 12973.
the nanoparticles thus prepared will have good dispersion capacity in lubrication oils. The roughness of friction surfaces is often several micrometers thick, so micrometer particles of certain hardness can be used as a grinding material. An example of this was investigated by Knapp and Nitta,14 in which zirconium oxide of less than 3 µm diameter was used as an abrasive. Nanoparticles, however, can deposit on the rubbing surfaces. Xue et al. 15 investigated the tribological properties of surface-coated TiO2 nanoparticles as an additive in liquid paraffin, and found that TiO2 deposited on rubbing surfaces, exhibited good antiwear and frictionreduction behavior. Other nanoparticles, such as surfacecoated PbS nanoparticles, exhibit good tribological properties because of tribochemical reactions occurring during the rubbing process, with the formation of a complex boundary film.16,17 In this work, we describe the synthesis process of din-hexadecyldithiophosphate (DDP)-coated ZnS nanoparticles, and the characterization of the prepared product with a variety of methods including transmission electron microscopy (TEM), electron diffraction (ED), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA). The tribological properties of ZnS nanoparticles as an additive in tetradecane were also studied. Experimental Section Materials. Pyridinium di-n-hexadecyldithiophosphate (PyDDP) as modification agent was synthesized and characterized according to the literature.18 Its structure is shown in Figure 3a. Analytically pure zinc acetate [Zn(Ac)2‚2H2O] and sodium sulfide (Na2S‚9H2O) were used as the raw materials for synthesis. Analytically pure ethanol and deionized water were used as solvents. Tetradecane (Fluka Chemie AG, >99%) was used as base oil. Preparation of ZnS Nanoparticles. Preparation methods of all nanoparticles are essentially the samesthe difference being (14) Knapp, J. K.; Nitta, H. Tribol. Int. 1997, 30, 225. (15) Xue, Q. J.; Liu, W. M.; Zhang, Z. J. Wear 1997, 213, 29. (16) Chen, S.; Liu, W. M.; Yu, L. G. Wear 1998, 218, 153. (17) Chen, S.; Liu, W. M.; Yu, L. G. J. Mater. Res. 1999, 14, 2147. (18) Waters, D. N.; Paddy, J. L. Spectrochim. Acta 1988, 44A, 393.
10.1021/la9906875 CCC: $18.00 © 1999 American Chemical Society Published on Web 09/18/1999
Surface-Coated ZnS Nanoparticles in the ratio of sulfide to DDP used. A typical procedure [S2-]: [PyDDP] ) 1:2 is as follows: Two stock solutions were prepared: (A) 0.12 g (0.5 mmol) of Na2S‚9H2O dissolved in 5 mL of deionized water; (B) 0.22 g (1 mmol) of Zn(Ac)2‚2H2O dissolved in 5 mL of deionized water. The preparation procedure was as follows: First, put 100 mL of ethanol-water mixed solvent into a 250-mL reaction flask, and heat to 55 °C with stirring. Then, 0.66 g (1 mmol) of modification agent PyDDP was added into the mixture. After PyDDP was completely dissolved, solution A was added to the flask while stirring. While PyDDP and solution A were stirred well together, solution B was added dropwise with vigorous stirring, and a white precipitate appeared immediately. After a few hours of reaction time, the white precipitate was filtered, washed with ethanol-deionized water, and ethanol, respectively, and dried in a degassed desiccator for 48 h. Finally, the target product, a white powder of DDP-coated ZnS nanoparticles, was obtained. It was found that the particle size was sensitive to temperature and to the ratio of S2-/DDP-. As the temperature increased, the size of the coated ZnS nanoparticles increased slightly, and when the ratio of S2-/DDP- was higher, the nanoparticles produced were a bit larger. The same conclusion was obtained by Herron et al.10 in the synthesis of thiophenolate-capped CdS nanoparticles, and by Zhang et al.13 in the preparation of DDP-capped MoS2 nanoparticles. ZnS nanoparticles without the surface modification were prepared by the same procedures, except that no PyDDP was added. Dispersion Capacity. The DDP-coated ZnS nanoparticles can be dispersed in several organic solvents, including chloroform, benzene, methylbenzene, and liquid paraffin, whereas bare ZnS nanoparticles cannot be dispersed in these solvents. So it is concluded that after the surface modification with DDP, the dispersion capability of nano-sized ZnS is improved. The improvement in the dispersion capability enabled the DDP-coated ZnS nanoparticles to be used as an additive in lubricating oils. Instrumentation and Characterization. The morphology and crystalline structure of the ZnS nanoparticles formed in ethanol-water mixture solvent was studied by JEM-1200 EX/S TEM. The prepared powders were dispersed in benzene under ultrasonic bath agitation for 5 min, then deposited on a copper grid covered with a perforated carbon film. The resulting specimen was then subjected to TEM and ED analyses. FTIR spectra were taken on a Bio-Rad FTS-165 IR spectrometer, which operated from 4000 to 500 cm-1, to characterize the structure of the ZnS nanoparticle surface. The prepared ZnS nanoparticles were mixed with KBr powder and pressed into a pellet for measurement. Background correction was made using a reference blank KBr pellet. XPS analysis was performed on a PHI-5702 photoelectron spectrometer using a pass energy of 29.35 eV and a MgKR 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. TGA was conducted in nitrogen on a Perkin-Elmer TGA-7 at a scanning rate of 10 °C/min. Wear tests were carried out on an Optimol Schwingung Reibung Verschlei β (SRV) wear tester, which provided a ballon-disk configuration. The steel disk and ball used in the wear tests were both GCr15-bearing steel (SAE 52100 steel; composition: 0.95-1.05% C, 0.15-0.35% Si, 0.20-0.40% Mn,
