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Synthesis and characterization of DDP coated Ag nanoparticles
Materials Science and Engineering A 379 (2004) 378–383
Synthesis and characterization of DDP coated Ag nanoparticles L. Sun a,b , Z.J. Zhang b,∗ , Z.S. Wu b , H.X. Dang b,a a
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Science, Lanzhou 730000, PR China b Laboratory of Special Functional Materials, Henan University, Kaifeng 475001, PR China Received 2 December 2003; received in revised form 1 March 2004
Abstract Ag nanoparticles coated with di-n-octodecyldithiophosphate were chemically synthesized. The structure of the prepared Ag nanoparticles was investigated by means of X-ray diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy, thermogravimetry analysis and differential thermal analysis. The anti-wear properties of Ag nanoparticles as additives in liquid paraffin were investigated by a four-ball testing machine. The coated Ag nanoparticles with an average diameter of about 15 nm, are able to prevent water adsorption, and oxidation and are capable of being dispersed in organic solvents. Wear tests show that coated Ag nanoparticles as additives in liquid paraffin exhibit excellent anti-wear ability and can improve the load-carrying capacity of base oil effectively. © 2004 Elsevier B.V. All rights reserved. Keywords: Metals; Nanoparticles; Chemical synthesis; Anti-wear properties
1. Introduction In recent years, metal nanoparticles have been widely exploited for use in catalysis [1,2], magnetic recording media [3,4], biological labeling [5], and formulation of magnetic ferro fluids [6]. Many methods have been developed to produce metal nanoparticles. Examples include vapor deposition [7], electrochemical reduction [8], radiolytic reduction [9], chemical reduction [10–12], and thermal decomposition [13], etc. The chemical reduction in solution has been proven to be the most versatile and simple approach for preparation of metal nanoparticles. This method often involves the reduction of relevant metal salts in the presence of a suitable protecting agent, which is necessary in controlling the growth of metal colloids through agglomeration. The reduction agent used in this method may be hydrazine [14,15], sodium boron hydride [16,17], alcohol [18], and formaldehyde [19]. Ag nanoparticles have been applied in many scientific fields such as photonics [20], surface-enhanced Raman scattering [21]. But as a traditional solid lubricant, Ag has seldom been used as oil additive due to its poor oil solubility. This problem could be resolved by adding dis∗ Corresponding author. Tel.: +86-378-2852533; fax: +86-378-2867282. E-mail address: [email protected] (Z.J. Zhang).
0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.03.002
persing agent or using surface modification preparation technique. If the nanoparticles, thus prepared were coated with long chain hydrocarbons, they would have good compatibility with the lubrication base oil. Up to now, the di-n-octodecyldithiophosphate (DDP) coated nanoparticles such as MoS2 [22], Ni(OH)2 [23], Cu [24], and ZnS [25], used as oil additives have been investigated, results showed these coated nanoparticles exhibited good tribological properties. In this work, we describe the synthesis of DDP coated Ag nanoparticles, which have good compatibility with lubricating oil. Tannic acid was used as reduction agent. The characterization of the prepared product was performed with a variety of methods including X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), thermogravimetry analysis (TGA), and differential thermal analysis (DTA). The anti-wear properties of DDP coated Ag nanoparticles as oil additives in liquid paraffin (LP) were also studied.
2. Experimental 2.1. Reagents The silver nitrate (AgNO3 ), tannic acid, and ammonia solution are analytically pure reagents (AR). Pyridinium di-n-octodecyldithiophosphate (PyDDP) as modification
L. Sun et al. / Materials Science and Engineering A 379 (2004) 378–383
agent was synthesized according to the literature [26]; its structure is shown in Fig. 3(a). Deionized water and AR grade ethanol absolute were used as solvents. 2.2. Synthesis of nanoparticles AgNO3 , tannic acid, and ammonia solution were dissolved in deionized water individually to form stock solution. A series of samples were synthesized. Preparation methods of all nanoparticles are essentially the same, the difference being in the ratio of Ag+ to DDP− used. A typical procedure of [Ag+ ]:[PyDDP] = 4:1 is as follows: 0.36 g (0.5 mmol) of PyDDP dissolved in 60 ml of ethanol previously, the solution was then added into a 250 ml reaction flask with stirring. The transparent solution was pale yellow. Subsequently, 20 ml of 0.01 mol/l tannic acid solution was added to the flask, while PyDDP and tannic acid were stirred well together, 20 ml of 0.10 mol/l AgNO3 solution was added into the flask, and then 6 ml of 0.50 mol/l ammonia solution was added drop-wise to the flask, a black precipitate appeared immediately. The reaction was allowed to continue for 2 h at ambient temperature under vigorous stirring. At the end of the reaction, the precipitate was filtered, washed with ethanol and deionized water several times and dried in a degassed desiccator for 2 days at room temperature. Finally, the target product, a black powder of DDP coated Ag nanoparticles was obtained. Ag nanoparticles without the surface modification were prepared by the same procedures, except that no PyDDP was added. 2.3. Dispersion capacity The dispersion capacity of DDP coated Ag nanoparticles and bare (non-coated) Ag nanoparticles can be indicated
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by determination of their suspension time in solvents. The test method is as follows: DDP coated and non-coated Ag nanoparticles dispersed in chloroform, methylbenze, liquid paraffin, and distilled water, individually, at the same conditions (content: 0.5%, height of suspension 5 cm), and 15 min of ultrasonic was used to aid the dispersion. The suspension time was recorded to reflect the dispersion capacity of the nanoparticles in different solvents. 2.4. Instrumentation and characterization XRD patterns were recorded with a Philips X’per pro X-ray powder diffractometer using Cu K␣ radiation (λ = 1.5418 Å). The operation voltage and current were 40 kV and 40 mA, respectively. The TEM morphologies and high-resolution transmission electron microscope (HRTEM) image of Ag nanoparticles formed in ethanol–water mixture solvent were performed on a JEOL JEM-2010 transmission elcectron microscope (at an acceleration voltage of 200 KV). The prepared powders were dispersed in chloroform under ultrasonic bath agitation for 5 min, and then deposited on a copper grid covered with a perforated carbon film. The resulting specimen was then subjected to TEM analysis. FT-IR spectra were taken on an AVATAR 360 Fourier transform infrared spectrometer, which operated from 4000 to 400 cm−1 , to characterize the surface structure of the Ag nanoparticles. The prepared Ag nanoparticles were mixed with KBr powder and pressed into a pellet for measurement. Background correction was made using a reference blank KBr pellet. TGA and DTA were conducted in nitrogen on a Seiko EXSTAR 6000 thermal analyzer at a scanning rate of 10 ◦ C/min.
Fig. 1. XRD patterns of DDP coated Ag nanoparticles.
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Wear tests were carried out on a MRS-1J four-ball testing machine under a rotating speed of 1450 rpm at ambient conditions. A ball of 12.7 mm diameter made of GCr 15 steel with a HRc 61–64 was used. The base oil was chemically pure LP, which has a distillation range of 180–250 ◦ C, density of 0.835–0.855 g/cm3 . The Ag nanoparticles were dispersed in LP by ultrasonic agitation to form a transparent solution. Before each test, the balls and specimen holders were ultrasonically cleaned in a petroleum ether (normal alkaline with a boiling point of 60–90 ◦ C) bath and dried in hot air. At the end of each test, the wear scar diameters (WSD) on the three lower balls were measured using an optical digital-reading microscope to an accuracy of 0.01 mm. Then the arithmetic mean value of them was calculated as the average WSD.
3. Results and discussion Table 1 gives the dispersion capacity of the DDP coated and non-coated Ag nanoparticles in different solvents. It is indicated that the DDP coated Ag nanoparticles can be dispersed in organic solvents, including chloroform, methylbenze, and liquid paraffin for a long time, but it can not be dispersed in water. Whereas, non-coated Ag nanoparticles can hardly suspend stably in these organic solvents even after ultrasonic dispersion, so it is to conclude that after the surface modification with DDP, the dispersion capability in organic solvents of nano sized Ag is improved. The improvement in the dispersion capability enabled the DDP coated Ag nanoparticles to be used as additives in lubricating oils. Fig. 1 shows the XRD pattern of DDP coated Ag nanoparticles. It is seen that the peaks at 2θ = 38.1◦ , 44.2, 64.5, 77.3, and 81.4◦ are assigned to diffraction from the (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) lattice planes of silver, respectively, which are in agreement with cubic silver (Joint Committee on Powder Diffraction Standards File no. 87-0720). Thus, it is concluded that the Ag nanocores were successfully prepared in the DDP coated Ag nanoparticles. The crystalline size of DDP coated Ag nanoparticles was about 6 nm, which was calculated from the half-width of the diffraction peaks using the Scherrer formula. The peak at 2θ = 21.4◦ corresponds to the amorphous diffraction from organic modification layer. It also can be seen from Fig. 1 that there are no diffraction peaks of Ag2 O; this suggests that the existence of modification layer can prevent from the oxidation of Ag nanocores.
Fig. 2. TEM morphologies of (a) DDP coated Ag nanoparticles, (b) non-coated Ag nanoparticles and (c) HRTEM image of DDP coated Ag nanoparticles.
Table 1 Dispersion capacity of DDP coated and non-coated Ag nanoparticles
DDP coated Ag nanoparticles Non-coated Ag nanoparticles
Chloroform
Methylbenze
Liquid paraffin
Distilled water
15 days 30 min
7 days 1h
5 days 1 day
Cannot disperse 6h
The data in this table are suspend time of the nanoparticles.
L. Sun et al. / Materials Science and Engineering A 379 (2004) 378–383
Fig. 2 gives the TEM morphologies of DDP coated Ag nanoparticles (a), non-coated Ag nanoparticles (b), and the HRTEM image of DDP coated Ag nanoparticles (c). It is seen from Fig. 2(b) that non-coated Ag nanoparticles tend to agglomerate owing to the high surface energy, and the distribution of grain size is wide. On the contrary, the TEM image of DDP coated Ag nanoparticles show uniform distribution of fine particles and has an average size of about 15 nm. Comparing Fig. 2(a) and (b), it can be concluded that the existence of surface modification layer prevent the agglomeration of Ag nanoparticles. The mean size observed from Fig. 2(a) does not reconcile with the size estimated from XRD pattern. It is well known that XRD gives the crys-
381
tallite size. Therefore, we believe that some DDP coated Ag nanoparticles in the TEM photograph is an aggregate that consists of several Ag crystallites which are kept apart by the crystal boundary; this can be confirmed by the HRTEM of DDP coated Ag nanoparticles shown in Fig. 2(c). Fig. 3 shows the FT-IR spectra of (a) PyDDP and (b) DDP coated Ag nanoparticles. The structure of the modification agent PyDDP is also shown in Fig. 3(a). As is shown in Fig. 3(a), the band at 2900 cm−1 corresponds to C–H stretching vibration. The band of 1470 cm−1 is related to C–H bending vibrations. The vibration in 720 cm−1 region is characteristic of a minimum of four methyl groups, (CH2 )4 , in a row and assigned to the methylene rocking vibration. So
Fig. 3. FT-IR spectra of (a) PyDDP and (b) DDP coated Ag nanoparticles.
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Fig. 4. TGA and DTA curves of DDP coated Ag nanoparticles.
it is to conclude that the long alkyl chain does exist in PyDDP. The band at 801 cm−1 in Fig. 3(a) corresponds to P=S stretching band. This result indicates that the phosphorus atom is double-bond to the sulfur atom in PyDDP. In combination with the –OCH2 vibration band at 920–1000 cm−1 ; and pyridine bands, it can be concluded that the modification agent PyDDP was successfully prepared in this work. After the reaction, as shown in Fig. 3(b), the IR spectrum of DDP coated Ag nanoparticles displays marked changes from what is shown in Fig. 3(a). The most obvious change is the shifting of P=S from 801 to 832 cm−1 ; and the bands of pyridine disappear mostly. This indicates that a chemi-
cal reaction occurred between the polar radical in the PyDDP molecule and the Ag moiety during the synthesis process of DDP coated Ag nanoparticles. The bands of C–H vibrations at 2900, 1470, and 720 cm−1 and the bands of –OCH2 at 920–1000 cm−1 , which are observed in both PyDDP (Fig. 3(a)) and DDP coated Ag nanoparticles (Fig. 3(b)), indicate that the long alkyl chain in the backbone of DDP did not dissociate after the reaction of PyDDP with Ag nanoparticles. Since the surface of DDP coated Ag nanoparticles is hydrophobic, we infer that the relatively weak and broad band near 3400 cm−1 in Fig. 3(b) corresponds to the –OH on Ag nanocores.
Fig. 5. Wear scar diameter curves of liquid paraffin and liquid paraffin containing DDP coated Ag nanoparticles with applied load (four-ball testing machine, 0.50 wt.%, 1450 rpm, 30 mim).
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Fig. 4 shows the TGA and DTA curves of DDP coated Ag nanoparticles. It can be seen from the TGA curve that from 200 to 330 ◦ C, an obvious mass loss is observed, which corresponds to the decomposition of modification layer. The total mass loss of DDP coated Ag nanoparticles is about 54%. According to the stoichiometric relation of Ag+ and DDP− , nAg+ :nDDP− = 4:1, the amount of DDP coated on Ag nanoparticles should be 58%, which is approximately consistent with TGA result. This indicates that the reaction proceeded completely. From TGA we can also conclude that modification agent coated on Ag nanocores through chemical reaction but not simple adsorption, since the DDP can be rinsed off in the synthesis proceeding, if it does not react with Ag nanocores, there should not be so large amount of mass loss. This is consistent with the FT-IR analysis. It also can be seen from the TGA curve that there is no mass loss before 200 ◦ C, this result suggests that the DDP coated Ag nanoparticles have no surface-adsorbed water, which confirmed our assume in IR analysis that there are a little –OH on the Ag nanocores but no adsorption water on the surface of DDP coated Ag nanoparticles. It can be seen from Fig. 4 that there are two endothermic regions at 60.5 and 250 ◦ C, which corresponds to the melting and decomposition of modification layer, respectively. Fig. 5 gives the anti-wear properties of pure LP and LP with DDP coated Ag nanoparticles versus applied loads. The test conditions are as follows: four-ball testing machine, contents of additives 0.50% (weight ratio), rotating rate 1450 rpm, test duration 30 min. It can be seen that at the same applied load, the WSD of LP containing nanoparticles is smaller than that of pure LP, which indicates that the DDP coated Ag nanoparticles have good anti-wear ability. On the other hand, it also can be seen from Fig. 5 that the DDP coated Ag nanoparticles can improve the load-bearing capacity of LP remarkably (from 300 to 800 N). The detailed investigation of the tribological properties and its mechanism will be discussed in the future article.
4. Conclusions The synthesis process and characterization of DDP coated Ag nanoparticles were studied by XRD, TEM, FT-IR, TGA and DTA analysis. According to the results, we can conclude that the synthesized DDP coated Ag nanoparticles could be well dispersed in several organic solvents and LP, which enables it to be used as additives in lubricating oils. Because of the existence of DDP coating on the surface of
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the Ag nanoparticles, water adsorption and oxidation were prevented. The DDP coated Ag nanoparticles used as additives in LP exhibited good anti-wear ability and improved the latter’s load-carrying capacity remarkably. Acknowledgements The authors wish to acknowledge the financial support from Science Research Foundation of Henan University.
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Synthesis and characterization of DDP coated Ag nanoparticles L. Sun a,b , Z.J. Zhang b,∗ , Z.S. Wu b , H.X. Dang b,a a
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Science, Lanzhou 730000, PR China b Laboratory of Special Functional Materials, Henan University, Kaifeng 475001, PR China Received 2 December 2003; received in revised form 1 March 2004
Abstract Ag nanoparticles coated with di-n-octodecyldithiophosphate were chemically synthesized. The structure of the prepared Ag nanoparticles was investigated by means of X-ray diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy, thermogravimetry analysis and differential thermal analysis. The anti-wear properties of Ag nanoparticles as additives in liquid paraffin were investigated by a four-ball testing machine. The coated Ag nanoparticles with an average diameter of about 15 nm, are able to prevent water adsorption, and oxidation and are capable of being dispersed in organic solvents. Wear tests show that coated Ag nanoparticles as additives in liquid paraffin exhibit excellent anti-wear ability and can improve the load-carrying capacity of base oil effectively. © 2004 Elsevier B.V. All rights reserved. Keywords: Metals; Nanoparticles; Chemical synthesis; Anti-wear properties
1. Introduction In recent years, metal nanoparticles have been widely exploited for use in catalysis [1,2], magnetic recording media [3,4], biological labeling [5], and formulation of magnetic ferro fluids [6]. Many methods have been developed to produce metal nanoparticles. Examples include vapor deposition [7], electrochemical reduction [8], radiolytic reduction [9], chemical reduction [10–12], and thermal decomposition [13], etc. The chemical reduction in solution has been proven to be the most versatile and simple approach for preparation of metal nanoparticles. This method often involves the reduction of relevant metal salts in the presence of a suitable protecting agent, which is necessary in controlling the growth of metal colloids through agglomeration. The reduction agent used in this method may be hydrazine [14,15], sodium boron hydride [16,17], alcohol [18], and formaldehyde [19]. Ag nanoparticles have been applied in many scientific fields such as photonics [20], surface-enhanced Raman scattering [21]. But as a traditional solid lubricant, Ag has seldom been used as oil additive due to its poor oil solubility. This problem could be resolved by adding dis∗ Corresponding author. Tel.: +86-378-2852533; fax: +86-378-2867282. E-mail address: [email protected] (Z.J. Zhang).
0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.03.002
persing agent or using surface modification preparation technique. If the nanoparticles, thus prepared were coated with long chain hydrocarbons, they would have good compatibility with the lubrication base oil. Up to now, the di-n-octodecyldithiophosphate (DDP) coated nanoparticles such as MoS2 [22], Ni(OH)2 [23], Cu [24], and ZnS [25], used as oil additives have been investigated, results showed these coated nanoparticles exhibited good tribological properties. In this work, we describe the synthesis of DDP coated Ag nanoparticles, which have good compatibility with lubricating oil. Tannic acid was used as reduction agent. The characterization of the prepared product was performed with a variety of methods including X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), thermogravimetry analysis (TGA), and differential thermal analysis (DTA). The anti-wear properties of DDP coated Ag nanoparticles as oil additives in liquid paraffin (LP) were also studied.
2. Experimental 2.1. Reagents The silver nitrate (AgNO3 ), tannic acid, and ammonia solution are analytically pure reagents (AR). Pyridinium di-n-octodecyldithiophosphate (PyDDP) as modification
L. Sun et al. / Materials Science and Engineering A 379 (2004) 378–383
agent was synthesized according to the literature [26]; its structure is shown in Fig. 3(a). Deionized water and AR grade ethanol absolute were used as solvents. 2.2. Synthesis of nanoparticles AgNO3 , tannic acid, and ammonia solution were dissolved in deionized water individually to form stock solution. A series of samples were synthesized. Preparation methods of all nanoparticles are essentially the same, the difference being in the ratio of Ag+ to DDP− used. A typical procedure of [Ag+ ]:[PyDDP] = 4:1 is as follows: 0.36 g (0.5 mmol) of PyDDP dissolved in 60 ml of ethanol previously, the solution was then added into a 250 ml reaction flask with stirring. The transparent solution was pale yellow. Subsequently, 20 ml of 0.01 mol/l tannic acid solution was added to the flask, while PyDDP and tannic acid were stirred well together, 20 ml of 0.10 mol/l AgNO3 solution was added into the flask, and then 6 ml of 0.50 mol/l ammonia solution was added drop-wise to the flask, a black precipitate appeared immediately. The reaction was allowed to continue for 2 h at ambient temperature under vigorous stirring. At the end of the reaction, the precipitate was filtered, washed with ethanol and deionized water several times and dried in a degassed desiccator for 2 days at room temperature. Finally, the target product, a black powder of DDP coated Ag nanoparticles was obtained. Ag nanoparticles without the surface modification were prepared by the same procedures, except that no PyDDP was added. 2.3. Dispersion capacity The dispersion capacity of DDP coated Ag nanoparticles and bare (non-coated) Ag nanoparticles can be indicated
379
by determination of their suspension time in solvents. The test method is as follows: DDP coated and non-coated Ag nanoparticles dispersed in chloroform, methylbenze, liquid paraffin, and distilled water, individually, at the same conditions (content: 0.5%, height of suspension 5 cm), and 15 min of ultrasonic was used to aid the dispersion. The suspension time was recorded to reflect the dispersion capacity of the nanoparticles in different solvents. 2.4. Instrumentation and characterization XRD patterns were recorded with a Philips X’per pro X-ray powder diffractometer using Cu K␣ radiation (λ = 1.5418 Å). The operation voltage and current were 40 kV and 40 mA, respectively. The TEM morphologies and high-resolution transmission electron microscope (HRTEM) image of Ag nanoparticles formed in ethanol–water mixture solvent were performed on a JEOL JEM-2010 transmission elcectron microscope (at an acceleration voltage of 200 KV). The prepared powders were dispersed in chloroform under ultrasonic bath agitation for 5 min, and then deposited on a copper grid covered with a perforated carbon film. The resulting specimen was then subjected to TEM analysis. FT-IR spectra were taken on an AVATAR 360 Fourier transform infrared spectrometer, which operated from 4000 to 400 cm−1 , to characterize the surface structure of the Ag nanoparticles. The prepared Ag nanoparticles were mixed with KBr powder and pressed into a pellet for measurement. Background correction was made using a reference blank KBr pellet. TGA and DTA were conducted in nitrogen on a Seiko EXSTAR 6000 thermal analyzer at a scanning rate of 10 ◦ C/min.
Fig. 1. XRD patterns of DDP coated Ag nanoparticles.
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Wear tests were carried out on a MRS-1J four-ball testing machine under a rotating speed of 1450 rpm at ambient conditions. A ball of 12.7 mm diameter made of GCr 15 steel with a HRc 61–64 was used. The base oil was chemically pure LP, which has a distillation range of 180–250 ◦ C, density of 0.835–0.855 g/cm3 . The Ag nanoparticles were dispersed in LP by ultrasonic agitation to form a transparent solution. Before each test, the balls and specimen holders were ultrasonically cleaned in a petroleum ether (normal alkaline with a boiling point of 60–90 ◦ C) bath and dried in hot air. At the end of each test, the wear scar diameters (WSD) on the three lower balls were measured using an optical digital-reading microscope to an accuracy of 0.01 mm. Then the arithmetic mean value of them was calculated as the average WSD.
3. Results and discussion Table 1 gives the dispersion capacity of the DDP coated and non-coated Ag nanoparticles in different solvents. It is indicated that the DDP coated Ag nanoparticles can be dispersed in organic solvents, including chloroform, methylbenze, and liquid paraffin for a long time, but it can not be dispersed in water. Whereas, non-coated Ag nanoparticles can hardly suspend stably in these organic solvents even after ultrasonic dispersion, so it is to conclude that after the surface modification with DDP, the dispersion capability in organic solvents of nano sized Ag is improved. The improvement in the dispersion capability enabled the DDP coated Ag nanoparticles to be used as additives in lubricating oils. Fig. 1 shows the XRD pattern of DDP coated Ag nanoparticles. It is seen that the peaks at 2θ = 38.1◦ , 44.2, 64.5, 77.3, and 81.4◦ are assigned to diffraction from the (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) lattice planes of silver, respectively, which are in agreement with cubic silver (Joint Committee on Powder Diffraction Standards File no. 87-0720). Thus, it is concluded that the Ag nanocores were successfully prepared in the DDP coated Ag nanoparticles. The crystalline size of DDP coated Ag nanoparticles was about 6 nm, which was calculated from the half-width of the diffraction peaks using the Scherrer formula. The peak at 2θ = 21.4◦ corresponds to the amorphous diffraction from organic modification layer. It also can be seen from Fig. 1 that there are no diffraction peaks of Ag2 O; this suggests that the existence of modification layer can prevent from the oxidation of Ag nanocores.
Fig. 2. TEM morphologies of (a) DDP coated Ag nanoparticles, (b) non-coated Ag nanoparticles and (c) HRTEM image of DDP coated Ag nanoparticles.
Table 1 Dispersion capacity of DDP coated and non-coated Ag nanoparticles
DDP coated Ag nanoparticles Non-coated Ag nanoparticles
Chloroform
Methylbenze
Liquid paraffin
Distilled water
15 days 30 min
7 days 1h
5 days 1 day
Cannot disperse 6h
The data in this table are suspend time of the nanoparticles.
L. Sun et al. / Materials Science and Engineering A 379 (2004) 378–383
Fig. 2 gives the TEM morphologies of DDP coated Ag nanoparticles (a), non-coated Ag nanoparticles (b), and the HRTEM image of DDP coated Ag nanoparticles (c). It is seen from Fig. 2(b) that non-coated Ag nanoparticles tend to agglomerate owing to the high surface energy, and the distribution of grain size is wide. On the contrary, the TEM image of DDP coated Ag nanoparticles show uniform distribution of fine particles and has an average size of about 15 nm. Comparing Fig. 2(a) and (b), it can be concluded that the existence of surface modification layer prevent the agglomeration of Ag nanoparticles. The mean size observed from Fig. 2(a) does not reconcile with the size estimated from XRD pattern. It is well known that XRD gives the crys-
381
tallite size. Therefore, we believe that some DDP coated Ag nanoparticles in the TEM photograph is an aggregate that consists of several Ag crystallites which are kept apart by the crystal boundary; this can be confirmed by the HRTEM of DDP coated Ag nanoparticles shown in Fig. 2(c). Fig. 3 shows the FT-IR spectra of (a) PyDDP and (b) DDP coated Ag nanoparticles. The structure of the modification agent PyDDP is also shown in Fig. 3(a). As is shown in Fig. 3(a), the band at 2900 cm−1 corresponds to C–H stretching vibration. The band of 1470 cm−1 is related to C–H bending vibrations. The vibration in 720 cm−1 region is characteristic of a minimum of four methyl groups, (CH2 )4 , in a row and assigned to the methylene rocking vibration. So
Fig. 3. FT-IR spectra of (a) PyDDP and (b) DDP coated Ag nanoparticles.
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Fig. 4. TGA and DTA curves of DDP coated Ag nanoparticles.
it is to conclude that the long alkyl chain does exist in PyDDP. The band at 801 cm−1 in Fig. 3(a) corresponds to P=S stretching band. This result indicates that the phosphorus atom is double-bond to the sulfur atom in PyDDP. In combination with the –OCH2 vibration band at 920–1000 cm−1 ; and pyridine bands, it can be concluded that the modification agent PyDDP was successfully prepared in this work. After the reaction, as shown in Fig. 3(b), the IR spectrum of DDP coated Ag nanoparticles displays marked changes from what is shown in Fig. 3(a). The most obvious change is the shifting of P=S from 801 to 832 cm−1 ; and the bands of pyridine disappear mostly. This indicates that a chemi-
cal reaction occurred between the polar radical in the PyDDP molecule and the Ag moiety during the synthesis process of DDP coated Ag nanoparticles. The bands of C–H vibrations at 2900, 1470, and 720 cm−1 and the bands of –OCH2 at 920–1000 cm−1 , which are observed in both PyDDP (Fig. 3(a)) and DDP coated Ag nanoparticles (Fig. 3(b)), indicate that the long alkyl chain in the backbone of DDP did not dissociate after the reaction of PyDDP with Ag nanoparticles. Since the surface of DDP coated Ag nanoparticles is hydrophobic, we infer that the relatively weak and broad band near 3400 cm−1 in Fig. 3(b) corresponds to the –OH on Ag nanocores.
Fig. 5. Wear scar diameter curves of liquid paraffin and liquid paraffin containing DDP coated Ag nanoparticles with applied load (four-ball testing machine, 0.50 wt.%, 1450 rpm, 30 mim).
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Fig. 4 shows the TGA and DTA curves of DDP coated Ag nanoparticles. It can be seen from the TGA curve that from 200 to 330 ◦ C, an obvious mass loss is observed, which corresponds to the decomposition of modification layer. The total mass loss of DDP coated Ag nanoparticles is about 54%. According to the stoichiometric relation of Ag+ and DDP− , nAg+ :nDDP− = 4:1, the amount of DDP coated on Ag nanoparticles should be 58%, which is approximately consistent with TGA result. This indicates that the reaction proceeded completely. From TGA we can also conclude that modification agent coated on Ag nanocores through chemical reaction but not simple adsorption, since the DDP can be rinsed off in the synthesis proceeding, if it does not react with Ag nanocores, there should not be so large amount of mass loss. This is consistent with the FT-IR analysis. It also can be seen from the TGA curve that there is no mass loss before 200 ◦ C, this result suggests that the DDP coated Ag nanoparticles have no surface-adsorbed water, which confirmed our assume in IR analysis that there are a little –OH on the Ag nanocores but no adsorption water on the surface of DDP coated Ag nanoparticles. It can be seen from Fig. 4 that there are two endothermic regions at 60.5 and 250 ◦ C, which corresponds to the melting and decomposition of modification layer, respectively. Fig. 5 gives the anti-wear properties of pure LP and LP with DDP coated Ag nanoparticles versus applied loads. The test conditions are as follows: four-ball testing machine, contents of additives 0.50% (weight ratio), rotating rate 1450 rpm, test duration 30 min. It can be seen that at the same applied load, the WSD of LP containing nanoparticles is smaller than that of pure LP, which indicates that the DDP coated Ag nanoparticles have good anti-wear ability. On the other hand, it also can be seen from Fig. 5 that the DDP coated Ag nanoparticles can improve the load-bearing capacity of LP remarkably (from 300 to 800 N). The detailed investigation of the tribological properties and its mechanism will be discussed in the future article.
4. Conclusions The synthesis process and characterization of DDP coated Ag nanoparticles were studied by XRD, TEM, FT-IR, TGA and DTA analysis. According to the results, we can conclude that the synthesized DDP coated Ag nanoparticles could be well dispersed in several organic solvents and LP, which enables it to be used as additives in lubricating oils. Because of the existence of DDP coating on the surface of
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the Ag nanoparticles, water adsorption and oxidation were prevented. The DDP coated Ag nanoparticles used as additives in LP exhibited good anti-wear ability and improved the latter’s load-carrying capacity remarkably. Acknowledgements The authors wish to acknowledge the financial support from Science Research Foundation of Henan University.
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