J. Phys. Chem. 1994,98, 12973-12977
12973
Synthesis and Characterization of a Molybdenum Disulfide Nanoclustert Zhi Jun Zhang,*y$Jun Zhang, and Qun Ji Xue Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, People’s Republic of China Received: December 6, 1993; In Final Form: September 13, 1994@
The synthesis of a molybdenum disulfide (MoSZ) nanocluster prepared by dialkyldithiophosphate ion modification is reported. The nanocluster was characterized by XPS, IR, UV-Vis, TEM, and elementary analysis. Results show that the MoSz nanocluster is capped with a layer of dialkyldithiophosphate molecules. The lubrication properties of the capped nanocluster were also primarily investigated. The results show that such a nanocluster has a low kinetic friction coefficient @k 0.1) and a good load-canying capacity.
1. Introduction The properties of nanometer size materials are quite different from that of the bulk material or individual molecule, so that the research has been conducted extensively in many Recently, new methods of nanocluster synthesis have been developed, and meanwhile the structure and physicochemical properties of all kinds of nanometer materials have been Dichalcogenides are not only a class of useful lubricants but also semiconductor material^.^ Peterson et al.1° and Henglien et al.” had used the formation of intercalates and intense ultrasonic treatment to obtain MoS2 and WS2 nanoclusters, which represented clear quantum confinement effects. In the present work, we synthesized a novel MoSz nanocluster which was “capped” by dialkyldithiophosphate (DDP) molecules in order to combine the different functions of MoS2 particles and hydrocarbon chains. This capped nanocluster was prepared by an ion modification method, and the nanocluster obtained had good solubility in organic solvents such as acetone, DMF, THF, and others. Its structure was characterized by U S ,IR, UVVis, TEM, and elementary analysis. The results indicated that the MoSz nanocluster was capped by a single layer of DDP molecules and well protected from oxidation. Granick12 had reviewed the results obtained using the surface force apparatus and molecular tribometers comprehensively and advanced a new concept called “nanotribology”. Ovemey and co-workers13 investigated the friction behavior of mixed Langmuir-Blodgett (LB) films down to the nanometer scale by use of the friction force microscope (FFM). But up to now, there has been no report about nanometer materials used in tribological investigation. Hence we studied the lubrication properties of the DDPcapped MoSz nanocluster by depositing the particles on a glass plate. It was found that the kinetic friction coefficient of the sample was very low &. < O.l), and the sample had good loadcarrying capacity compared with the capped agent DDP and bulk MoS2. Since Mo is a multivalent element and Mo(1V) is acidic in solution, the formation of the DDP-capped MoS2 nanocluster is greatly influenced by reaction time, nature of the reductant, acidity of the reaction solution, and reaction temperature. In this paper we report a typical synthesis process, the results of +This project was supported by NSFC and the Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences. Surface Science Laboratory, Henan University, Kaifeng 475001, China. Abstract published in Advance ACS Abstracts, October 15, 1994.
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0022-365419412098- 12973$04.50/0
primary characterization, and the lubrication properties of the DDP-capped MoSz nanocluster. 2. Experimental Section 2.1 Chemicals and Equipment. The sodium molybdate (Na2Mo04*2HzO), sodium sulfide ( N ~ z S ~ H Z O and ) , hydroxylamine hydrochloride (NH20WHCl) are analytical pure regents (AR) purchased and used without further treatments. The pyridinium di-n-octadecyldithiophosphate(PyDDP) synthesis and characterization were based on references. l4 Deionized water and AR grade acetone were used as solvents. The XPS data were taken from a PHI-550 multitechnique spectrometer with Mg K a radiation as source (power, 320 W; energy, 1253.6 eV). A EM-1200 EWS transmission electron microscope was used to examine the morphology of the nanocluster. IR and UV-vis spectral measurements were carried out on a Nicolet 10 DX-FTIR spectrometer and a Beckman Du-50 spectrophotometer, respectively. Elementary analysis data were acquired from a Carlo Extra-1106 elemental analyzer. The friction experiments were performed on a DPPM type reciprocating friction tester. 2.2 Synthesis of MoS2 Nanoclusters. The synthesis was performed in an inert atmosphere of purified nitrogen as reported.I5 We found that the structure and properties of the obtained MoSz nanoclusters are very sensitive to the ratio of S2- ion and PyDDP added and are also closely related to the reaction conditions such as the acidity and temperature of the reaction solution. Therefore, we take a typical synthesis process with S2-:PyDDP = 20: 1 (mole ratio) as an example. First, we filled a 250-mL reaction flask with 120 mL of deionized water, heated it in a thermostat-equipped heating unit with stirring devices up to 70 OC, and maintained it at this temperature for 10 min. We then added 10 mmol(2.4 g) of sodium molybdate. After the sodium molybdate was completely dissolved, 10 mmol (0.7 g) of NH20WHCl dissolved in 10 mL of deionized water was injected into the flask with stirring. After the hydroxylamine hydrochloride was added, the color of the reaction solution became darker and darker as the reaction proceeded, and finally the color looked deep blue-green. The process should take an hour. Then 30 mL of distilled acetone was injected into the flask with stimng, and no precipitates could be observed at this time. After 10 min, 1.0 mmol (0.7 g) of PyDDP was added; the reaction solution became sticky, and the color tumed blue-violet. Finally 20 mmol (4.3 g) of NazS (predissolved in 10 mL of deionized water) was injected, and the reaction continued for 3 h. Then the temperature fell to 65
0 1994 American Chemical Society
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12974 J. Phys. Chem., Vol. 98, No. 49, 1994
0
Wavelength (nm)
Wavelength (run)
Figure 1. Absorption spectra of (a) PyDDP, (b) MoDDP, and (e) DDP -capped MoS2 nanoclusters in ethanol solution.
"C, and a certain amount of HC1 was added to the flask to control the acidity of the reaction solution under vigorous stimng. The reaction was allowed to proceed with stirring for 2 h further. During this period, a large amount of brown precipitate was produced. The reaction completed, we removed the flask from the heating unit and cooled the solution to room temperature. The solution, which had a blue-green color, was filtered, and the precipitate was rinsed thoroughly with deionized water. Then the precipitate was extracted with acetone and recrystallized after a minor insoluble residue was removed. The final product was kept in a vacuum desiccator for 48 h. A reddish brown powder was obtained, which is the expected nanocluster of MoS2. The nanocluster can be dissolved in THF, DMF, and acetone, and it can be stably dispersed in liquid paraffin also. The filtrate was evaporated by decompression, and the dark green residue obtained could not be dissolved in organic solvent. The kinetic friction coefficient, Pk, was measured on a reciprocating friction tester where the plate sample was sliding against a GCrl5 steel ball at a sliding velocity of 35 mm min-' under a load of 1 N. The antiwear ability was estimated by the sliding time elapsed or the amount of sliding that had proceeded before the Pk value increased abruptly.
3. Results and Discussion Figure 1 shows the UV-vis absorption spectra of DDPcapped MoSz nanocluster, small molecular or cluster MoDDP, and PyDDP. The structures of MoDDP, which was received from the Fujian Institute of Material Structure, and PyDDP are as follows:
MoDDP
PyDDP
From Figure 1, the absorption spectrum of nanocluster MoS2 is apparently different from those of MoDDP and PyDDP. This means that PyDDP had reacted with molybdenum and produced a novel material different from MoDDP. At the same time, the UV-vis spectrum of nanocluster MoS2 is identical with that of the bulk semiconductor M o S ~ ~ Oexcept J ~ for its onset of absorption at the shorter wavelength. This blue shift of the onset results from the quantum size effects of the nanocluster.' Therefore, we can primarily conclude that the product is nanocluster MoS2 capped or modified by PyDDP rather than MoDDP.
Figure 2 shows the XPS patterns of the Mo, S, P, 0, and C in the sample. As there are a large number of CH2 groups in the nanocluster, the binding energy of C1, in polyethylene (284.6 eV) is used as the reference. The binding energies of the elements are listed in Table 1. In the X P S pattern, the SzP, OI,, and PzP orbitals all show a single peak, which indicates that the valency states of these atoms and their surroundings are similar at least near the surface of the nanoclusters. The binding energy of M03d5/2 is 230.1 eV, which is close to that of Mo3,35n in MoS2 (229.6 eV)" and equal to that of Mo(1V) (230.1).18 But MOgfi/2has an absorption shoulder at 226.7 eV, which is the binding energy of the S2$ electron. The spectrum peak of SzPis at 162.6 eV, which is the signal of S2- in MoSz, and the 01, peak is at 532.3 eV, which is different from that in Moo2 (529.9 eV)18 or 0 2 - (533.4 eV).I9 Therefore, it is considered that the main compound of Mo is MoS2 in the reddish brown powder and that the MoS2 nanocluster capped by DDP molecules was well protected from oxidation since no higher valance sulfur was distinguished. At the same time, the possibility that polysulfides are present in the MoS2 nanocluster was eliminated on the basis of the binding energy of S2p in polysulfide, which is 164.1 eV25rather than 162.6 eV. The S:P:Mo atom ratio from XPS analysis is about 9.6:1.3: 3.3. If the S atoms in DDP are eliminated, the S:Mo atom ratio is 7.0:3.3, which approximates 2 1 . This result also shows that the main component of the reddish brown powder is MoS2. The P:Mo atom ratio approximates 1:3. Because the probe depth of X-rays is about 40-100 A for organic compounds, XPS analysis only represents the element composition near the surface, while the P:Mo atom ratio in the whole should be less than 113. Figure 3 illustrates the IR absorption spectra of PyDDP and DDP-capped MoS2 nanoclusters. In Figure 3, the strongest absorption is the long alkyl chain absorption in the 2900-cm-' region which is typical of compounds with long aliphatic chains. The strong absorption of P=O at 1250 cm-' disappears, and at the same time a sharp peak at 804 cm-l appears that is known to be the P-S stretching band." This indicates that the phosphorus atom is double bonded to the sulfur atom in PyDDP. Moreover, the OCH2 vibration at 920-980 cm-l and the displaced pyridine absorption all support the PyDDP structure. After the reaction, the IR spectrum of the DDP-capped MoS2 nanocluster apparently changes; the most obvious change is the disappearance of the P=S vibration at 804 cm-' and the appearance of two new peaks at 335 and 1645 cm-'. The disappearance of the peak at 804 cm-' indicates the reaction occurred between the polar region of the PyDDP molecule and
Molybdenum Disulfide Nanoclusters
J. Phys. Chem., Vol. 98, No. 49, 1994 12975
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element binding energy (eV)
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the Mo moiety. The sharp peak at 2900 cm-l and the broad OCH2 absorption also c o n f i this conclusion and at the same time suggest that the long alkyl chain does not dissociate in the reaction. The peak at 335 cm-' was assigned to the MwS-Mo vibration.16,20The strong peak at 1645 cm-I could be related to the conjugate structure between DDP and Mo atoms on the surface. Besides the broad peak of OH (free or bound) at 3400 cm-' is appreciable after reaction, which illustrates that the capped MoS2 nanoclusters still contain a small amount of water or OH, which can be either inserted in the MoS2 cluster or coordinated with it even after evacuation in a vacuum desiccator. This result is compatible with the XPS data where we have noted that the oxygen can be in the form of OH or the like. Figure 4 shows a TEM photograph of the DDP-capped MoS2 nanoclusters. The average diameter of the MoS2 nanocluster is ca. 40-50 nm. Suppose the MoS2 nanocluster were capped with a single layer of DDP molecule; then the diameter of the MoS2 nanocluster should be a ca. 35-45 nm. Obviously, this size does not reconcile with the quantum size effects revealed in the absorption spectrum. The blue shift of the absorption onset shows that the size of MoS2 nanoclusters should be smaller than 10 nm,lo while in fact the size of DDP-capped MoS2 nanoclusters in Figure 4 is much larger than that. It is well known that the blue shift of the absorption onset is related to the crystallite size.24 Therefore, we believe that each MoSz nanocluster in the TEM photograph is an aggregate which consists of several MoS2 crystallites which are kept apart by the crystal boundary. The elementary analysis results of the C, H, and N percentage content in DDP-capped MoS2 nanoclusters were 24.24%, 4.3%, and 2.10%, respectively. Element N may be introduced from the oxidation products of NH2OH.HCl or a compound containing N inserted between the laminar layers. If the deviation coming from the compounds containing nitrogen was not considered,
and assuming that the DDP molecule does not dissociate during reaction, then the mass of a quasi molecule containing one DDP molecule can be estimated from the percentage content in elementary analysis. It is 1782 or 1721 g mol-' according to the percentage content of C and H, respectively. The average mass of such a quasi molecule is 1752 g mol-'. Hence, the mass number of MoS2 in the quasi molecule is 1157 g mol-'. This means there are about seven MoS2 molecules in each quasi molecule. Suppose that each Mo atom on the surface of the nanocluster is bonded to a DDP molecule so that the MoSz nanocluster is capped with DDP molecules; then about 14% of the Mo atom is on the surface. Of course, if the compound containing N is considered, the number of MoS2 molecules in each quasi molecule is fewer. In addition, not all Mo atoms on the surface connect with DDP one to one. Therefore, the elementary analysis data is an approximate value. In spite of this, the elementary analysis data can still confirm that the MoS2 nanocluster is capped with DDP molecules, which caused the capped nanocluster to have good solubility in nonaqueous solution. It is worthwhile to note that if the product obtained in our experiment is the compound MoDDP, the percentage contents of C and H should be 54.41% and 9.33%, respectively. This result further shows that the product is the nanocluster of MoS2 rather than the small molecule compound. At the same time, the ratio DDPMo = 1:7 is higher than the 1:10 ratio before reaction; it is in agreement with the experiment: not all of the molybdate formed capped nanocluster MoS2. On the other hand, the solubility of the sample obtained also shows that it is capped nanocluster MoS2 rather than a mixture of MoS2 particles and MoDDP. The formation mechanism of DDP-capped MoSz nanoclusters is complex because of the multivalent state of Mo and the acidity of Mo(1V). According to the experimental results, we proposed the following mechanism: At first NazMoO4 reacted with N H 2 OH-HCl to produce Mo(IV) at a certain temperature. The addition of Na2S and PyDDP made MoDDP, M o S ~ ~and - , other molybdenum species form respectively. When HC1 was injected into the reaction system slowly, M o S ~ ~was - decomposed to MoS2 step by step. It is well known that the initial crystallite
Zhang et al.
12976 J. Phys. Chem., Vol. 98, No. 49, 1994
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Figure 5. Plot of friction coefficient versus reciprocating times: (a) is extremely active;21there are a large number of defect sites and dangling bonds on its surface. So the initial MoS2 will react with MoDDP or DDP in the solution to form a surfacemodification layer which defers the growth and condensation of MoS2. Those bare MoS2 and other molybdenum compounds are removed during the processes of filtration and recrystallization. The structure of the MoS2 nanocluster is perhaps similar to that proposed by Steigerwald.22 Our experiments prove that the reproducibility is greatly influenced by the acidity of the reaction solution and the reaction temperature. Even under identical experimental condition, the size of the obtained nanocluster is difficult to maintain identically. Figure 5 shows the pk value of the DDP-capped MoS2 nanocluster and hydrogen di-n-octadecyldithiophosphate(HDDP), which was prepared under the same experimental conditions
HDDP, (b) bulk MoS2, and (c) DDP-capped MoS2 nanoclusters.
as nanocluster MoS2 without addition of Na2MoO4 and deposited on a glass plate and a bulk MoS2 brushed glass plate, versus reciprocating times under a 1.0 N load. This primary result indicates that both antiwear and friction-reducing properties of HDDP and bulk MoS2 are poor under our experimental conditions, while DDP-capped MoS2 nanoclusters not only reduce the friction coefficient but also increase the antiwear ability apparently. This may result from the structure of nanocluster MoS2 because the spheroid structure of nanocluster MoS2 particles changes sliding friction into rolling friction partially, thereby reducing the friction coefficient, and the organic arrangement of hydrocarbon chain on the surface of nanocluster can also made friction coefficient reduced, owing
Molybdenum Disulfide Nanoclusters to “the brush m e ~ h a n i s m ” . At ~ ~ the same time, the cohesive effect between long hydrocarbon chains increases the antiwear ability of nanoclusters. Although the measured ,&value is much larger than those of long-chain fatty acid LB films transferred onto the glass plate @k 0.02), the load-carrying capacity is rather higher than that of LB films.23 MoS2 is a well-known lubrication material; its laminar structure and good load carrying capacity make it widely used in lubrication. Capped MoS2 nanoclusters are much more stable than MoS2 particles in air, and they have good dispersity in oil. Further, tribology study down to the nanocluster scale is just under investigation; the introduction of nanoclusters into the tribology field, especially in boundary lubrication, may have potential importance both in theory and in practice.
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4. Conclusion A new kind of surface-capped MoS2 nanocluster has been synthesized by an ion modification method. The MoS2 nanocluster has good chemical stability and solubility in organic solvents because its surface was modified by addition of a longchain organic compound. The tribology study of the nanometer scale deposited layer reveals its potential use in lubrication. Acknowledgment. We are grateful to professors Ziqiang Zhu, Zhongyi Zhang, Hanqing Wan, and Zhensheng Jin for fruitful discussions. References and Notes (1) Henglein, A. Chem. Rev. 1989, 89, 1861. (2) Wang, Y.; Herron, H. J. Phys. Chem. 1991, 95, 525.
J. Phys. Chem., Vol. 98, No. 49, 1994 12977 (3) Henglein, A. Top. Curr. Chem. 1988, 143, 113. (4) Ficher, C. H.; Giersig, M. Langmuir 1992, 8, 475. ( 5 ) Mo&, L.; Petit, C.; Boulanger, L; Lixon, P.; Pileni, M. P. Langmuir 1992, 8, 1049. (6) Zhou, H. S.; Honma, I.; Komiyama, H.; Haus, J. W.; J. Phys. Chem. 1993, 97, 895. (7) Kormann, C.; Bahnemann, D. W.; Hoffmann, M. R. J. Phys. Chem. 1988, 92, 5196. (8) Vukovic, V., Nedeljkovic, J. M. Langmuir 1993, 9, 980. (9) Tributsch, H. 2.Naturforsch. 1977, 32a, 972. (10) Peterson, M. W.; Nenadovic, M. T.; Rajh, T.; Herak, R.; Micic, 0. I.; Goral, J. P.; Nozik, A. J. J . Phys. Chem. 1988, 92, 1400. (11) Gutierrez, M.; Henglein, A. Ultrasonics 1989, 27, 259. (12) Granick, S. MRS Bull. 1991, 16 (lo), 30. (13) Ovemey, R. M.; Meyer, E.; Frommer, T.; Brodbeck, D.; Luthi, R.; Howald, L.; Guntherodt, H.-J.; Fujihira, M.; Takano, H.; Gotoh, Y. Nature 1992, 359, 133. (14) Elliot, J. S.; Jayne, G. J. J.; Barber, R. I. J . Inst. Pet. 1969, 55, 219. Waters, D. N.; Paddy, J. L. Spectrochim. Acta 1988, 44A, 393. (15) Zhang, Z.; Jin, Z.; Zhu, Z. Ninth International Conference on Photochemical Conversion and Storage of Solar Energy, Beijing, China, Book of Abstracts 1992, p 97. (16) Chianelli, R. R.; Dines, M. B. Inorg. Chem. 1987, 17, 2758. (17) Suzuki, K.; Soma, M.; Omishi, T.; Tamam, K. J . Electron Spectrosc. Relat. Phenom. 1981, 24, 283. (18) Spevack, P. A.; Molntyre, N. S. J . Phys. Chem. 1992, 96, 9029. (19) Du, Z.; Zhang, Z.; Zhao, W.; Zhu, Z. Thin Solid Films 1992, 210/ 211, 404. (20) Mitchell, P. C. H. Wear 1984, 100, 287. (21) Hoffmann, A. J.; Mills, G.; Yee, H.; Hoffmann, M. R. J . Phys. Chem. 1992, 96, 5546. (22) Steigenvald, M. L.; Alivisatos, A. P.; Gibson, J. M.; et al. J . Am. Chem. SOC.1988, 110, 3046. (23) Du, Z. L.; Zhu, Z. Q.;Zhang, J.; Xue, Q, J. Proceedings of China International Symposium for Youth Tribologists, Lanzhou, China, 1992; p 184. (24) Steigerwald, M. L.; Brus, L. E. Acc. Chem. Res. 1990, 23, 183. (25) Riga, R.; Verbist, J. J. J . Chem. Soc., Perkin Trans. 2 1983, 1545.
