On the Structure of Surface-Modified Triboactive

Petroleum Chemistry, Vol. 45, No. 1, 2005, pp. 17–20. Translated from Neftekhimiya, Vol. 45, No. 1, 2005, pp. 21–24. Original Russian Text Copyright © 2005 by Suslov, Bondarenko, Bakunin, Kuz’mina, Parenago. English Translation Copyright © 2005 by MAIK “Nauka /Interperiodica” (Russia).

On the Structure of Surface-Modified Triboactive Nanoparticles of Molybdenum Trisulfide A. Yu. Suslov, G. N. Bondarenko, V. N. Bakunin, G. N. Kuz’mina, and O. P. Parenago Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Leninskii pr. 29, Moscow, 119992 Russia Received July 5, 2004

Abstract—Surface-modified nanoparticles of molybdenum trisulfide were obtained by the hydrogen sulfide treatment of inverse microemulsions of molybdic acid salts. The structure of the nanoparticles was studied by IR and NMR spectroscopy. The molybdenum nanoparticles were proposed for use as antioxidant and frictionreducing additives to lubricating oils. It was shown that an anion-exchange reaction occurred between cetyltrimethylammonium bromide used to stabilize a inverse microemulsion and thiomolybdate, which had been formed in the system as a result of the replacement of oxygen atoms in the starting molybdate with sulfur by the action of hydrogen sulfide. Isolated molybdenum trisulfide nanoparticles in organic solvents, including hydrocarbons, can be dissolved by treating the nanoparticle surface with nitrogen-containing modifiers, which are attached to the sulfur-containing core of nanoparticles, probably via a Mo–N bond. A plausible model for the structure of modified MoS3 nanoparticles is proposed.

containing ammonium or sodium molybdates and stabilized by the cationic surfactant cetyltrimethylammonium bromide (CTAB) in a medium of organic solvents (chloroform–hexane, 1 : 1 vol/vol) according to the procedure described earlier [15]. Butylamine, succinimide, and alkenylsuccinimide (ACI) were used as modifiers. Bis(hexadecyltrimethylammonium) tetrathiomo2– lybdate [(ë16ç33)(ëç3)3N+]2Mo S 4 was synthesized by the exchange reaction of CTAB with ammonium thiomolybdate in an aqueous medium. IR spectra were recorded on a Specord M-82 (Carl Zeiss) for samples as KBr pellets in the region of 400−4000 cm–1 and as CsI pellets in the region of 200−2000 cm–1. Proton magnetic resonance (1H NMR) spectra were recorded on an MSL-300 instrument (Bruker) at a frequency of 300 MHz in a CDCl3 solution.1

It is known from the literature that nanoparticles of some inorganic metal compounds obtained by different methods exhibit new and unusual properties and, therefore, can be promising for use in many areas of science and technology. First, they can be used in different catalytic processes [1, 2] as semiconductors and sensors [3, 4], in photochemistry and nanophotonics [5, 6], in the production and application of nanotubes [7], and finally, in biology and medicine [8]. The main problem of using nanoparticles in organic dispersion media is their low aggregation stability. Since they tend to minimize their surface energy, nanoparticles coagulate to form large aggregates. Therefore, for the practical use of nanoparticles, their surface needs to be protected by either encapsulating into a matrix, for example, into polymers [9–11], or modifying the surface [12, 13]. The latter seems more promising for the use of nanoparticles as a component of lubricating oils. The synthesis and application of molybdenum trisulfide nanoparticles as additives to lubricating oils have previously been reported [14, 15]. After treatment with special modifying agents to impart the necessary solubility in hydrocarbon media, these compounds manifested antifriction and antioxidant properties, allowing for their use as additives to lubricating oils [14–16]. The purpose of this work was to study the structure of modified molybdenum trisulfide nanoparticles by the IR and NMR spectroscopy techniques.

RESULTS AND DISCUSSION According to published data [17, 18] on the IR spectral characteristics of molybdenum sulfides, the longwavelength region of the spectrum of MoS3 usually exhibits bands of stretching vibrations of Mo=S bonds at 460–480 cm–1 and bridging Mo–S bonds (240–400 cm–1). In addition, absorption bands at 510–544 cm–1 characterizing different S–S bond types in polynuclear molybdenum sulfides were reported in the cited works.

EXPERIMENTAL Nanoparticles of MoS3 were synthesized by the hydrogen sulfide treatment of inverse microemulsions

1 The

authors thank Cand. Sci. (Chem.) M.P. Filatova for the NMR analysis.

17

18

SUSLOV et al.

Characteristic stretching frequencies in the long-wavelength spectral region for samples of MoS3 nanoparticles obtained under different conditions Bond stretching frequency ν, cm–1

Reactants molybdenum compound Na2MoO4 (NH4)6Mo7O24 –

modifier

Mo=S

Mo–Sbridg

S–S

Mo–N

481 483 479

334, 363 341, 385 342, 384

545 539 537

451 451 459

sec-C4H9NH2 – alkenylsuccinimide

Indeed, the long-wavelength region of the spectrum of molybdenum trisulfide nanoparticles contains all the indicated characteristic bands (table) for the nanoparticles synthesized by different methods. In all cases, the 480-cm–1 band was the most intense, which is quite consistent with its assignment to the Mo=S bond. The bands at 360–320 and 510–550 cm–1 due to the bridging Mo–S and S–S bonds, respectively, have different relative intensities and insignificantly vary in the position of their maximums depending on the synthesis conditions. A comparison of the spectra of the synthesized nanoparticles with those of the modifiers and surfactant (CTAB) used in the work shows that the spectra of the isolated products contain all main bands inherent in these reagents. In particular, the spectra of all nanoparticles contain the bands (characteristic of the hexadecyl ligand) of stretching vibrations at 720 cm–1 attributed to methylene groups (ëç2)n, where n > 4. The bands at 900–1000 cm–1 assigned to skeletal vibrations in the N(CH3)3 unit are also present. These results indicate that CTAB molecules are not completely removed dur-

Skeletal vibrations of R

+

N

CH3 CH3 CH3

1 2

1400

1200

1000

3 800 cm–1

Fig. 1. IR spectra of (1) nanoparticles modified by alkenylsuccinimide, (2) dicetyltrimethylammonium tetrathiomolybdate salt, and (3) cetyltrimethylammonium bromide (CTAB).

ing the isolation of nanoparticles and are retained in the composition of the final products. Moreover, the spectra of all nanoparticles exhibit significant changes in band intensities of the N(CH3)3 group in CTAB, indicating that CTAB molecules interact with molybdenum sulfide. For example, the spectrum of the starting CTAB contains three distinct bands at 997, 964, and 913 cm–1 (Fig. 1), of which the 964-cm–1 band has the lowest intensity. These bands are also present in the spectra of the nanoparticles; however, the intensity of the 964-cm–1 band sharply increases in this case, and the band becomes predominant. Since a similar ratio of intensities of these bands is observed for specially synthesized bis(hexadecyltrimethylammonium) tetrathiomolybdate as well, the data obtained can be explained by a change in the geometry of the tetraalkylammonium cation upon anion replacement, i.e., when the bromide anion is replaced by the tetrathiomolybdate ion: +

2R'–NR+3 Br–

+

MoS4–2

R' NR3 S– R'

+ NR3 S–

S Mo

+ 2Br– .

S

It was of interest to perform an IR spectroscopic study of the role of nitrogen-containing modifiers added, along with CTAB, to MoS3 nanoparticles to impact the necessary solubility in organic media. In the case of the modification of the nanoparticles with butylamine, bending vibrations of the NH2 group (δ = 1605 cm–1), which are observed in the spectrum of secbutylamine, also appear in the spectrum of the nanoparticles, but as a split band at δ = 1600–1630 cm–1. The band of the stretching mode νN–C in the spectrum of secbutylamine lies at 1156 cm–1, and in the spectrum of the nanoparticles this band is observed at 1140 cm–1. The shift and splitting of the main bands of sec-butylamine observed in the spectrum of the nanoparticles can be explained by the interaction of butylamine with molybdenum sulfide. When alkenylsuccinimide is used as a modifier of the nanoparticles, the spectrum of the resulting compound is poorly informative because of a complex structure of ASI itself. Therefore, to simulate the interaction of the succinimide group with the nanoparticles, their synthesis involved succinimide as a modifier. A comparison of the spectra of the starting succinimide and succinimide-modified nanoparticles shows PETROLEUM CHEMISTRY

Vol. 45

No. 1

2005

ON THE STRUCTURE OF SURFACE-MODIFIED TRIBOACTIVE NANOPARTICLES

R

+

N

19

CH3 CH3 CH3 O NH

1

1

2

2

3

3 9

8

7

6

5

4

3

2

1

0 ppm

Fig. 2. NMR spectra of (1) dicetyltrimethylammonium tetrathiomolybdate, (2) cetyltrimethylammonium bromide (CTAB), and (3) succinimide-modified MoS3 nanoparticles.

that two bands (characteristic of succinimide) at 1700– 1800 cm–1 appear, in addition to the bands of CTAB. These bands are attributed to stretching vibrations of the carbonyl group. One band has a slight shift toward a similar band in the individual succinimide (1700 and 1770 cm–1 in the starting succinimide and 1700 and 1750 cm–1 in the nanoparticles). Most likely, this indicates that the succinimide ring interacts with molybdenum trisulfide. In addition to the presented indirect evidence for the interaction of modifiers (sec-butylamine, succinimide) with MoS3, an obvious spectral corroboration of their bonding would be the direct observation of spectral bands corresponding to a bond between molybdenum and nitrogen. To find this proof, we used published data on the assignment of the Co–N bonds in the cobalt ammoniate complexes. The bands due to Co–N stretching in these complexes are known to lie near 470 cm–1 [19]. Using the classical expression relating the frequency of vibrational motion of two nuclei to the mass of the nuclei and bond rigidity, and taking into account that the force constants for the Co–N and Mo–N bonds should be close in value, we write 2

ν vibr = (1/2π)2Kvibr /å, where K is the force constant of a bond and M is the reduced mass: M = å1å2/å1 + å2; we calculated the frequency of stretching vibrations of the Mo–N bond, which turned out to be equal to the frequency at 450 cm–1. An analysis of the spectra of the synthesized nanoparticles (see table) showed the presence of a rather intense band at 451–459 cm–1, which may confirm the interaction of the modifiers with MoS3 via the molybdenum–nitrogen bond. PETROLEUM CHEMISTRY

Vol. 45

No. 1

2005

O

9

8

7

6

5

4

3

2

1

0 ppm

Fig. 3. NMR spectra of (1) succinimide, (2) dicetyltrimethylammonium tetrathiomolybdate with a succinimide additive, and (3) succinimide-modified molybdenum trisulfide nanoparticles.

It should be mentioned that the spectrum of the nanoparticles contains no intense bands at 800– 900 cm–1 characteristic of the Mo=O bonds in the starting molybdenum salt. This confirms that the oxygen atoms were completely replaced by sulfur during the hydrogen sulfide treatment of ammonium or sodium molybdates. The analysis of the NMR spectra showed, by analogy with the IR data, the presence of the cetyltrimethylammonium group in the synthesized nanoparticles. The chemical shift of signals of methylene groups (ë3−ë15) in the cetyl radical for individual CTAB (Fig. 2) and for the isolated nanoparticles (Fig. 3) is the same (1.24 ppm). At the same time, the signals of protons of the groups in the α-position to the nitrogen atom in the CTAB molecule are displaced for the nanoparticles and have different chemical shifts depending on the type of modifier used. The maximum upfield chemical shift is observed for signals due to methyl groups on the nitrogen atom in CTAB. For example, it is 3.46 ppm for individual CTAB, 3.35 ppm for the nanoparticles modified by alkenylsuccinimide, and 3.30 ppm for the succinimidemodified nanoparticles (Figs. 2 and 3). Since the upfield shift of the signal of methyl groups at the nitrogen atom is also observed for dihexadecyltrimethylammonium tetrathiomolybdate (3.175 ppm), this change can be explained by a distortion of the tetrahedral structure of the tetraalkylammonium cation—the stronger the interaction of this cation with the anion, the stronger the distortion in the tetraalkylammonium group. Most likely, this distortion is caused by the exchange reaction of the bromide and molybdate anions in the CTAB molecule, as has already been shown above by IR spectroscopy.

20

SUSLOV et al.

Evidently, the structure proposed for the modified nanoparticles is hypothetical and needs further experimental investigation. MoS3

+ +N O N O N +N – S H S– S– Mo Mo Mo Mo S S S S S S S S Mo S Mo S

Fig. 4. Model of the structure of surface-modified nanoparticles of molybdenum trisulfide.

To reveal whether molybdenum trisulfide nanoparticles are bound to the modifier or not, we compared the proton NMR spectra of the starting succinimide with those of the products obtained and with the spectrum of dihexadecyltrimethylammonium tetrathiomolybdate with a succinimide additive. Relative to the spectrum of individual succinimide, the signal of the NH group in the product (nanoparticles + succinimide) is somewhat shifted upfield (in succinimide, 8.707 ppm; in a nanoparticle, 8.651 ppm). In addition, the intensity of the signal from this group decreased (for the starting succinimide, the integral ratio of signals is NH–/CH2– = 1 : 4.16; for the product, this ratio is equal to 1 : 4.73). The signal of the NH group is completely absent from the spectrum of dicetyltrimethylammonium tetrathiomolybdate with a succinimide additive (Fig. 3). These changes also indicate that the nitrogen atom of the modifiers is involved in the interaction with molybdenum trisulfide. In summary, based on the IR and 1H NMR data, we can assume that the core of the modified nanoparticles consists of amorphous molybdenum trisulfide, the surface layer is close in structure to the tetrathiomolybdate anions, and the modifying layer of the nanoparticles includes both the hexadecylammonium cation and modifier molecules, in particular, alkenylsuccinimide, which reacts with molybdenum trisulfide, most probably, at the N–H bond (Fig. 4).

REFERENCES 1. U. Schubert, New J. Chem. 18, 1049 (1994). 2. G. B. Sergeev, Usp. Khim. 70, 915 (2001). 3. Zh. I. Alferov, Fiz. Tekh. Poluprovodn. (St. Petersburg) 32, 3 (1998). 4. M. Albrecht, M. Lutz, A. L. Spek, et al., Nature (London) 406, 970 (2000). 5. C. S. Ah, H. S. Han, K. Kim, and D. J. Jang, J. Phys. Chem., B 104, 8153 (2000). 6. Y. Shen, C. S. Friend, Y. Jiang, et al., J. Phys. Chem., B 104, 7577 (2000). 7. B. P. Tarasov, N. F. Gol’dshleger, and A. P. Moravskii, Usp. Khim. 70, 149 (2001). 8. C. F. Mirkin, J. Nanoparticle Res. 2, 121 (2000). 9. L. Quaroni and G. Chumanov, J. Am. Chem. Soc. 121, 10642 (1999). 10. S. P. Gubin, Colloids Surf., A 202, 155 (2002). 11. M. Moffitt, L. McMahon, V. Pessel, et al., Chem. Mater., No. 7, 1185 (1995). 12. E. Caponetti, L. Pedone, M. D. Chillura, et al., Mater. Sci. Eng. 4, 531 (2003). 13. M. L. Steigerwald, A. P. Alivisatos, J. M. Gibson, et al., J. Am. Chem. Soc. 110, 3046 (1998). 14. O. P. Parenago, V. N. Bakunin, A. Yu. Suslov, et al., Dokl. Akad. Nauk 383, 84 (2002). 15. A. Yu. Suslov, V. N. Bakunin, G. N. Kuz’mina, et al., Neftekhimiya 43, 200 (2003) [Pet. Chem. 43, 192 (2003)]. 16. V. N. Bakunin, A. Yu. Suslov, G. N. Kuz’mina, et al., in Proceedings of 14 International Colloquium “Tribology and Lubrication Engineering,” Ostfildern, Germany, 2004, Vol. 1, p. 99. 17. Th. Weber, J. C. Muijsers, and J. W. Niemantsverdriet, J. Phys. Chem. 99, 9194 (1995). 18. J. W. McDonald, G. D. Friesen, et al., Inorg. Chim. Acta 72, 205 (1983). 19. K. Nakamoto, Infrared Spectra of Inorganic and Coordination Compounds (Wiley, New York, 1963; Mir, Moscow, 1966).

PETROLEUM CHEMISTRY

Vol. 45

No. 1

2005