- Email: [email protected]
Synthesis of dodecanethiol monolayer-stabilized nickel nanoparticles
Materials Science and Engineering A 452–453 (2007) 262–266
Synthesis of dodecanethiol monolayer-stabilized nickel nanoparticles Lei Chen a,b , Jianmin Chen a,∗ , Huidi Zhou a , Dingjun Zhang a,b , Hongqi Wan a,b a
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China b Graduate School, Chinese Academy of Sciences, Beijing 100039, China
Received 1 December 2005; received in revised form 17 October 2006; accepted 18 October 2006
Abstract Nickel nanoparticles surface-capped with self-assembled monolayer of dodecanethiol were prepared by the controlled reduction of nickel chloride (NiCl2 ·6H2 O) in the presence of dodecanethiol of different concentrations as the ligand and hydrazinium hydroxide as the reductant. The morphology and structure of the resulting surface-capped nickel nanoparticles were analyzed by means of transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, Fourier transformation infrared spectroscopy and thermogravimetric analysis. It was found that dodecanethiol played a critical role in controlling the radius and dispersibility of the surface-capped Ni nanoparticles. Namely, the nickel nanoparticles prepared in the absence of the surface-capping agent were liable to aggregate, while those prepared in the presence of decanethiol with a large enough concentration had a very small grain size. It was supposed that the surface-capped nickel nanoparticles with significantly increased stability and dispersibility as compared to the non-surface-capped nickle nanoparticles could be used as a kind of novel nanoscale lubricating additives. © 2006 Elsevier B.V. All rights reserved. Keywords: Nickel; Nanoparticles; Dodecanethiol; Surface capping
1. Introduction Increased attention has been recently paid to the study of metal nanoparticles exhibiting novel optical, electronic, magnetic, and chemical properties that are ascribed to the extremely small dimensions and special surface nature [1–5]. The significance of the metallic nanoparticles as a kind of special functional materials would be more promising on condition that they can be stabilized by self-assembled monolayers (SAMs). This may be due to the fact that the nanoparticles protected by SAMs have great potentials in the development of multifunctional catalysts, chemical sensors, and circuit components such as single electron transistors [6–9]. On one hand, gold and silver have been extensively focused on among the metal nanoparticles protected by SAMs, due to their relatively high stability [10–12]. On the other hand, various alkanethiols have been found to be able to form SAMs on many kinds of metallic nanoparticles [12–15]. However, little has so far been reported on the surface modification of nickel nanoparticles by SAMs, which
could be related to the oxidation susceptibility and relatively strong intermolecular forces as well. Indeed, the effect of the intermolecular forces such as van der Waals attraction and – interaction would be more significant in the aggregation of the nickel nanoparticles with magnetic dipole–dipole interaction [16–18]. This could also partly reflect the difficulty in the surface capping of the nickel nanoparticles using SAMs. With that perspective in mind, the present work deals with the synthesis and characterization of surface-capped Ni nanoparticles, using dodecanethiol as the capping agent, nickel chloride (NiCl2 ·6H2 O) as the nickel source, and hydrazinium hydroxide as the reductant. Dodecanethiol was selected in the present research, since it has been found to be critical in controlling the stability and radius of the target surface-capped metal nanopariticles [12–15]. 2. Materials and experiment 2.1. Materials
∗
Corresponding author. Tel.: +86 931 4968018; fax: +86 931 8277088. E-mail address: [email protected] (J. Chen).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.10.140
Nickel chloride (NiCl2 ·6H2 O, AR grade) was commercially obtained from Tianjin Chemicals Factory of China.
L. Chen et al. / Materials Science and Engineering A 452–453 (2007) 262–266
Hydrazine hydrate (AR grade, 80 wt.%), sodium hydroxide (AR grade, purity 99%), and dodecanethiol (C12 SH, CP grade, purity 96%) were produced by Shanghai Chemical Company of China. Acetone (AR grade) and ethanol (AR grade 95 wt.%) without further purification were used as the organic solvents.
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2.2. Preparation and characterization of Ni nanoparticles Typically, an appropriate amount of nickel chloride (8 mmol) and dodecanethiol (8 mmol) was directly dissolved in 60 ml ethanol (95%) contained in a round-bottomed flask by vigorous stirring on a magnetic-stirrer equipped with a heating unit.
Fig. 1. TEM images of nickel nanoparticles on prepared at different concentrations of dodecanethiol.
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L. Chen et al. / Materials Science and Engineering A 452–453 (2007) 262–266
The nickel chloride and decanethiol mixture was continuously stirred and heated to 60 ◦ C, then 10 ml hydrazinium hydroxide solution (purity 80%) was added into the reaction mixture of nickel chloride and dodecanethiol and the pH value of the mixed solution was adjusted to 12 using sodium hydroxide. The mixture of the three kinds of reactants was kept at 60 ◦ C and continuously vigorously stirred for 2 h, to allow the completion of the following reduction reaction: 2Ni2+ + N2 H4 + 4OH− → 2Ni + N2 + 4H2 O
(1)
At the end of the reduction reaction, the mixed solution in the round-bottomed flask was cooled in ambient condition to room temperature. The product was collected by filtering and the residue unreacted decanethiol was removed by fully rinsing with ethanol and acetone. The other synthesis with respect to different concentrations of dodecanethiol were carried out in the same manner as above-mentioned. The collected products were carefully dried below 80 ◦ C in vacuum to obtain black (Ni-S1) or dark brown (Ni-S2, Ni-S3) powders as the target products, where S1 refers to the sample prepared that no dodecanethiol introduced into the system during the synthesis process, while S2 and S3 refer to the samples prepared that 4 and 8 mmol of dodecanethiol were introduced during the synthesis process, respectively. 2.3. Characterization The average size of the nickel nanoparticles was determined using a Hitachi-H 600 transmission electron microscope (TEM), and the particle sizes were determined from the maximum length of the nanoparticles. The Fourier transformation infrared spectra (FTIR) of the target products (KBr pellet) were recorded on a Bio-Rad FTS-165 IR spectrometer. A Philips X’Pert-pro X-ray diffractometer (XRD) operating with graphite monochromatized Cu K␣ radiation (λ = 0.1540598 nm) was performed to record the XRD patterns of the target products. A PHI-5702 multifunctional X-ray photoelectron spectroscope (XPS) operating with monochromated Al K␣ irradiation at a pass energy of 29.4 eV was used to examine the chemical states of some typical elements in the target products. The binding energy of the core levels was calibrated against the binding energy of contaminated carbon (C1s: 284.8 eV) and recorded to an accuracy of ±0.2 eV. The desorption temperature of the dodecanethiol adsorbing on the Ni nanocores and the weight loss percentage of the nanoclusters in nitrogen gas were determined using a TA Instruments PE-7 thermal analysis device, by performing thermogravimetric analysis (TGA) at a heating rate of 10◦ /min. 3. Results and discussion Fig. 1 shows the TEM micrographs of the nickel nanoparticles with respect to different concentrations of dodecanethiol. It can be seen that the radius and dispersibility of the surfacecapped Ni nanoparticles change remarkably when increased the concentration of dodecanethiol. Namely, the target sample NiS1 was liable to aggregate, while the target sample Ni-S3 had a very small grain size and showed almost no signs of aggre-
Fig. 2. XRD patterns of nickel nanoparticles prepared at different concentrations of dodecanethiol.
gation. This could imply that the dodecanethiol molecules are densely packed on the Ni nanocores and thereby capable of preventing the Ni nanocores from aggregation and improving their dispersibility. It was indeed found in our experiment that the surface-capped nickel nanoparticles prepared at a large enough dodecanethiol concentration were highly hydrophobic and readily dissolved in a variety of organic solvents such as hexane, benzene, toluene, dichloromethane, and chloroform, but undissolved in various polar solvents such as alcohols, acetone, and water. The above observation agrees well with the report on other alkanethiolate-protected transition-metal nanoparticles such as gold and silver [10–12]. Thus it was concluded that the dodecanethiol was effective in controlling the size and improving the dispersibility of nickel nanoparticles. Fig. 2 shows the XRD patterns of the surface-capped nickel nanoparticles prepared at different concentrations of dodecanethiol. It is seen that all the surface-capped Ni nanoparticles prepared at different concentrations of dodecanethiol show the characteristic peaks of nickel at 2θ = 44.5, 51.8 and 76.4 corresponding to Miller indices (1 1 1), (2 0 0) and (2 2 2), respectively, which indicates that the resultant Ni nanocores are composed of pure cubic nickel. Interestingly, all the XRD patterns show no signals assigned to nickel oxides or hydroxides such as NiO, Ni2 O3 , and Ni(OH)2 , indicating that it could be effective in preventing the Ni nanocores from oxidation by the surface-capping with dodecanethiol in Ni-S2 and Ni-S3. However, though there is no conclusive evidence for oxidation presence in the Ni-S1 nanoparticle. It is difficult to say exactly when the oxidation took place. But it must have happened either during the reduction or later when the particles were exposed to the air [19]. This supposition could be further confirmed by the XPS analysis of the Ni-S1 and Ni-S3 nanoparticles. As shown in Fig. 3a, the Ni2p3/2 peak at 853.1 eV corresponds to elemental Ni [20]. The feature extending from 855.0 to 865.0 eV is attributed to nickel oxide species [21], and the XPS peak intensity of nickel oxide in this region decreases for the nickel nanoparticles surface-capped with dodecanethiol. The broad O1s peak at 530.0 eV is attributed to the oxygen atom bonded to divalent nickel (see Fig. 3b), and similar to what has
L. Chen et al. / Materials Science and Engineering A 452–453 (2007) 262–266
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Fig. 3. Typical XPS spectra of dodecanethiol surface-capped nickel nanoparticles: (a) Ni2p spectra of Ni-S1 and Ni-S3, (b) O1s spectra Ni-S1 and Ni-S3, and (c) S2p spectra of Ni-S3.
been observed for the Ni2p spectrum, the intensity of O1s also assumes a significant decrease for the Ni nanoparticles surfacecapped with the dodecanethiol. Moreover, the S2p spectrum of Ni-S3 after mixed Gaussian–Lorentzian curve-fitting shows the S2p peaks of S2p3/2 and S2p1/2 spinning at 162.5 and 163.6 eV, respectively, which correspond to the S in thiolate (metal S ) and thiol (H S ) [22]. Interestingly, the S2p1/2 peak intensity ascribed to free thiols at 163.6 eV is much smaller than the S2p3/2 peak intensity attributed to the sulfur atoms linked to metallic Ni at 162.5 eV, which is in line with a nearly ideal ratio of 2 between the intensities of the S2p3/2 and S2p1/2 components. Therefore, it was concluded that the oxidation of the nickel nanoparticles could be indeed effectively retarded by the chemically adsorbed dodecanethiol.
Fig. 4 shows the transmission FTIR spectra of the Ni nanoparticles prepared at different concentrations of dodecanethiol (KBr pellet), where the C H stretching (Fig. 4a) and low frequency (Fig. 4b) regions are separately presented, while the spectrum of neat dodecanethiol is also given for a comparison. It is seen that the neat dodecanethiol shows the symmetric and asymmetric IR absorbance of C H stretching at 2852 and 2925 cm−1 , respectively. Different from the neat dodecanethiol, the Ni nanoparticles surface-capped by dodecanethiol records the symmetric and asymmetric IR absorbance C H stretching at 2848 and 2917 cm−1 , respectively. Such a shift of the C H vibration to lower wavenumbers as compared with that of the neat thiols implies that the chemisorbed alkanethiol species were well ordered on the surface of the surface-capped nickel nanocrystals
Fig. 4. The symmetrical and asymmetrical methylene C H stretching vibrations (a) and the low frequency peaks detected for Ni-S3 (b). The IR spectrum of neat dodecanethiol is included for comparison.
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the radius and increasing the dispersibility in apolar solvents and resistance to oxidation of the nickel nanoparticles. Therefore the synthetic surface-capped nickel nanoparticles had significantly increased dispersibility and thermal-oxidation stability than the non-surface-capped counterparts. This could make it feasible to use the surface-capped nickel nanoparticles as a kind of novel nanoscale lubricating additives. Acknowledgements The authors are grateful to the National Natural Science Foundation of China (Grant No. 50575217), the Innovative Group Foundation from NSFC (Grant No. 50421502), and “Top Hundred Talents Program” of Chinese Academy of Sciences for financial support. Fig. 5. TGA curve of Ni-S3.
References [23,24]. Besides, as shown in Fig. 4b, the absorbance band at 722 cm−1 is ascribed to the C S stretching, and the absorbance band at 1463 cm−1 is assigned to the methylene scissoring. All the above FTIR features of the target products indicate that the chemical integrity of the dodecanethiol was maintained during the formation of the surface-capped nickel nanoparticles. Fig. 5 shows the TGA curve of nickel nanoparticles Ni-S3 as an example. It is seen that Ni-S3 nanoparticles begins to lose weight at 263.4 ◦ C and the total weight loss is about 70.9%, which corresponds to the decomposition of the dodecanethiol coating around the Ni nanocores. Assuming that the residue at the end of heating in nitrogen gas was pure nickel and the total weight loss of the surface-capped Ni nanoparticles was solely attributed to the desorption of dodecanethiol, the molar ratio of the Ni nanocores and dodecanethiol in the target product Ni-S3 was calculated to be 1.43:1 (The mole fraction of thiol was equal to 70.9/202 = 0.35, the mole fraction of nickel to 29.1/58.69 = 0.50, thus the molar ratio of Ni nanocores to dodecanethiol was equal to 0.50:0.35 = 1.43:1.). 4. Conclusions The nickel nanoparticles protected by self-assembled monolayers of dodecanethiol were synthesized via a facile and inexpensive chemical reduction route, using dodecanethiol of different concentrations as the ligand. The resulting surfacecapped nickel nanoparticles were characterized by means of transmission electron spectroscopy, X-ray diffraction, Fourier transformation infrared spectroscopy and TGA. It was found that the target products were composed of cubic nickel nanoparticles. Dodecanethiol as the ligand was effective in controlling
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Synthesis of dodecanethiol monolayer-stabilized nickel nanoparticles Lei Chen a,b , Jianmin Chen a,∗ , Huidi Zhou a , Dingjun Zhang a,b , Hongqi Wan a,b a
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China b Graduate School, Chinese Academy of Sciences, Beijing 100039, China
Received 1 December 2005; received in revised form 17 October 2006; accepted 18 October 2006
Abstract Nickel nanoparticles surface-capped with self-assembled monolayer of dodecanethiol were prepared by the controlled reduction of nickel chloride (NiCl2 ·6H2 O) in the presence of dodecanethiol of different concentrations as the ligand and hydrazinium hydroxide as the reductant. The morphology and structure of the resulting surface-capped nickel nanoparticles were analyzed by means of transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, Fourier transformation infrared spectroscopy and thermogravimetric analysis. It was found that dodecanethiol played a critical role in controlling the radius and dispersibility of the surface-capped Ni nanoparticles. Namely, the nickel nanoparticles prepared in the absence of the surface-capping agent were liable to aggregate, while those prepared in the presence of decanethiol with a large enough concentration had a very small grain size. It was supposed that the surface-capped nickel nanoparticles with significantly increased stability and dispersibility as compared to the non-surface-capped nickle nanoparticles could be used as a kind of novel nanoscale lubricating additives. © 2006 Elsevier B.V. All rights reserved. Keywords: Nickel; Nanoparticles; Dodecanethiol; Surface capping
1. Introduction Increased attention has been recently paid to the study of metal nanoparticles exhibiting novel optical, electronic, magnetic, and chemical properties that are ascribed to the extremely small dimensions and special surface nature [1–5]. The significance of the metallic nanoparticles as a kind of special functional materials would be more promising on condition that they can be stabilized by self-assembled monolayers (SAMs). This may be due to the fact that the nanoparticles protected by SAMs have great potentials in the development of multifunctional catalysts, chemical sensors, and circuit components such as single electron transistors [6–9]. On one hand, gold and silver have been extensively focused on among the metal nanoparticles protected by SAMs, due to their relatively high stability [10–12]. On the other hand, various alkanethiols have been found to be able to form SAMs on many kinds of metallic nanoparticles [12–15]. However, little has so far been reported on the surface modification of nickel nanoparticles by SAMs, which
could be related to the oxidation susceptibility and relatively strong intermolecular forces as well. Indeed, the effect of the intermolecular forces such as van der Waals attraction and – interaction would be more significant in the aggregation of the nickel nanoparticles with magnetic dipole–dipole interaction [16–18]. This could also partly reflect the difficulty in the surface capping of the nickel nanoparticles using SAMs. With that perspective in mind, the present work deals with the synthesis and characterization of surface-capped Ni nanoparticles, using dodecanethiol as the capping agent, nickel chloride (NiCl2 ·6H2 O) as the nickel source, and hydrazinium hydroxide as the reductant. Dodecanethiol was selected in the present research, since it has been found to be critical in controlling the stability and radius of the target surface-capped metal nanopariticles [12–15]. 2. Materials and experiment 2.1. Materials
∗
Corresponding author. Tel.: +86 931 4968018; fax: +86 931 8277088. E-mail address: [email protected] (J. Chen).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.10.140
Nickel chloride (NiCl2 ·6H2 O, AR grade) was commercially obtained from Tianjin Chemicals Factory of China.
L. Chen et al. / Materials Science and Engineering A 452–453 (2007) 262–266
Hydrazine hydrate (AR grade, 80 wt.%), sodium hydroxide (AR grade, purity 99%), and dodecanethiol (C12 SH, CP grade, purity 96%) were produced by Shanghai Chemical Company of China. Acetone (AR grade) and ethanol (AR grade 95 wt.%) without further purification were used as the organic solvents.
263
2.2. Preparation and characterization of Ni nanoparticles Typically, an appropriate amount of nickel chloride (8 mmol) and dodecanethiol (8 mmol) was directly dissolved in 60 ml ethanol (95%) contained in a round-bottomed flask by vigorous stirring on a magnetic-stirrer equipped with a heating unit.
Fig. 1. TEM images of nickel nanoparticles on prepared at different concentrations of dodecanethiol.
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L. Chen et al. / Materials Science and Engineering A 452–453 (2007) 262–266
The nickel chloride and decanethiol mixture was continuously stirred and heated to 60 ◦ C, then 10 ml hydrazinium hydroxide solution (purity 80%) was added into the reaction mixture of nickel chloride and dodecanethiol and the pH value of the mixed solution was adjusted to 12 using sodium hydroxide. The mixture of the three kinds of reactants was kept at 60 ◦ C and continuously vigorously stirred for 2 h, to allow the completion of the following reduction reaction: 2Ni2+ + N2 H4 + 4OH− → 2Ni + N2 + 4H2 O
(1)
At the end of the reduction reaction, the mixed solution in the round-bottomed flask was cooled in ambient condition to room temperature. The product was collected by filtering and the residue unreacted decanethiol was removed by fully rinsing with ethanol and acetone. The other synthesis with respect to different concentrations of dodecanethiol were carried out in the same manner as above-mentioned. The collected products were carefully dried below 80 ◦ C in vacuum to obtain black (Ni-S1) or dark brown (Ni-S2, Ni-S3) powders as the target products, where S1 refers to the sample prepared that no dodecanethiol introduced into the system during the synthesis process, while S2 and S3 refer to the samples prepared that 4 and 8 mmol of dodecanethiol were introduced during the synthesis process, respectively. 2.3. Characterization The average size of the nickel nanoparticles was determined using a Hitachi-H 600 transmission electron microscope (TEM), and the particle sizes were determined from the maximum length of the nanoparticles. The Fourier transformation infrared spectra (FTIR) of the target products (KBr pellet) were recorded on a Bio-Rad FTS-165 IR spectrometer. A Philips X’Pert-pro X-ray diffractometer (XRD) operating with graphite monochromatized Cu K␣ radiation (λ = 0.1540598 nm) was performed to record the XRD patterns of the target products. A PHI-5702 multifunctional X-ray photoelectron spectroscope (XPS) operating with monochromated Al K␣ irradiation at a pass energy of 29.4 eV was used to examine the chemical states of some typical elements in the target products. The binding energy of the core levels was calibrated against the binding energy of contaminated carbon (C1s: 284.8 eV) and recorded to an accuracy of ±0.2 eV. The desorption temperature of the dodecanethiol adsorbing on the Ni nanocores and the weight loss percentage of the nanoclusters in nitrogen gas were determined using a TA Instruments PE-7 thermal analysis device, by performing thermogravimetric analysis (TGA) at a heating rate of 10◦ /min. 3. Results and discussion Fig. 1 shows the TEM micrographs of the nickel nanoparticles with respect to different concentrations of dodecanethiol. It can be seen that the radius and dispersibility of the surfacecapped Ni nanoparticles change remarkably when increased the concentration of dodecanethiol. Namely, the target sample NiS1 was liable to aggregate, while the target sample Ni-S3 had a very small grain size and showed almost no signs of aggre-
Fig. 2. XRD patterns of nickel nanoparticles prepared at different concentrations of dodecanethiol.
gation. This could imply that the dodecanethiol molecules are densely packed on the Ni nanocores and thereby capable of preventing the Ni nanocores from aggregation and improving their dispersibility. It was indeed found in our experiment that the surface-capped nickel nanoparticles prepared at a large enough dodecanethiol concentration were highly hydrophobic and readily dissolved in a variety of organic solvents such as hexane, benzene, toluene, dichloromethane, and chloroform, but undissolved in various polar solvents such as alcohols, acetone, and water. The above observation agrees well with the report on other alkanethiolate-protected transition-metal nanoparticles such as gold and silver [10–12]. Thus it was concluded that the dodecanethiol was effective in controlling the size and improving the dispersibility of nickel nanoparticles. Fig. 2 shows the XRD patterns of the surface-capped nickel nanoparticles prepared at different concentrations of dodecanethiol. It is seen that all the surface-capped Ni nanoparticles prepared at different concentrations of dodecanethiol show the characteristic peaks of nickel at 2θ = 44.5, 51.8 and 76.4 corresponding to Miller indices (1 1 1), (2 0 0) and (2 2 2), respectively, which indicates that the resultant Ni nanocores are composed of pure cubic nickel. Interestingly, all the XRD patterns show no signals assigned to nickel oxides or hydroxides such as NiO, Ni2 O3 , and Ni(OH)2 , indicating that it could be effective in preventing the Ni nanocores from oxidation by the surface-capping with dodecanethiol in Ni-S2 and Ni-S3. However, though there is no conclusive evidence for oxidation presence in the Ni-S1 nanoparticle. It is difficult to say exactly when the oxidation took place. But it must have happened either during the reduction or later when the particles were exposed to the air [19]. This supposition could be further confirmed by the XPS analysis of the Ni-S1 and Ni-S3 nanoparticles. As shown in Fig. 3a, the Ni2p3/2 peak at 853.1 eV corresponds to elemental Ni [20]. The feature extending from 855.0 to 865.0 eV is attributed to nickel oxide species [21], and the XPS peak intensity of nickel oxide in this region decreases for the nickel nanoparticles surface-capped with dodecanethiol. The broad O1s peak at 530.0 eV is attributed to the oxygen atom bonded to divalent nickel (see Fig. 3b), and similar to what has
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265
Fig. 3. Typical XPS spectra of dodecanethiol surface-capped nickel nanoparticles: (a) Ni2p spectra of Ni-S1 and Ni-S3, (b) O1s spectra Ni-S1 and Ni-S3, and (c) S2p spectra of Ni-S3.
been observed for the Ni2p spectrum, the intensity of O1s also assumes a significant decrease for the Ni nanoparticles surfacecapped with the dodecanethiol. Moreover, the S2p spectrum of Ni-S3 after mixed Gaussian–Lorentzian curve-fitting shows the S2p peaks of S2p3/2 and S2p1/2 spinning at 162.5 and 163.6 eV, respectively, which correspond to the S in thiolate (metal S ) and thiol (H S ) [22]. Interestingly, the S2p1/2 peak intensity ascribed to free thiols at 163.6 eV is much smaller than the S2p3/2 peak intensity attributed to the sulfur atoms linked to metallic Ni at 162.5 eV, which is in line with a nearly ideal ratio of 2 between the intensities of the S2p3/2 and S2p1/2 components. Therefore, it was concluded that the oxidation of the nickel nanoparticles could be indeed effectively retarded by the chemically adsorbed dodecanethiol.
Fig. 4 shows the transmission FTIR spectra of the Ni nanoparticles prepared at different concentrations of dodecanethiol (KBr pellet), where the C H stretching (Fig. 4a) and low frequency (Fig. 4b) regions are separately presented, while the spectrum of neat dodecanethiol is also given for a comparison. It is seen that the neat dodecanethiol shows the symmetric and asymmetric IR absorbance of C H stretching at 2852 and 2925 cm−1 , respectively. Different from the neat dodecanethiol, the Ni nanoparticles surface-capped by dodecanethiol records the symmetric and asymmetric IR absorbance C H stretching at 2848 and 2917 cm−1 , respectively. Such a shift of the C H vibration to lower wavenumbers as compared with that of the neat thiols implies that the chemisorbed alkanethiol species were well ordered on the surface of the surface-capped nickel nanocrystals
Fig. 4. The symmetrical and asymmetrical methylene C H stretching vibrations (a) and the low frequency peaks detected for Ni-S3 (b). The IR spectrum of neat dodecanethiol is included for comparison.
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the radius and increasing the dispersibility in apolar solvents and resistance to oxidation of the nickel nanoparticles. Therefore the synthetic surface-capped nickel nanoparticles had significantly increased dispersibility and thermal-oxidation stability than the non-surface-capped counterparts. This could make it feasible to use the surface-capped nickel nanoparticles as a kind of novel nanoscale lubricating additives. Acknowledgements The authors are grateful to the National Natural Science Foundation of China (Grant No. 50575217), the Innovative Group Foundation from NSFC (Grant No. 50421502), and “Top Hundred Talents Program” of Chinese Academy of Sciences for financial support. Fig. 5. TGA curve of Ni-S3.
References [23,24]. Besides, as shown in Fig. 4b, the absorbance band at 722 cm−1 is ascribed to the C S stretching, and the absorbance band at 1463 cm−1 is assigned to the methylene scissoring. All the above FTIR features of the target products indicate that the chemical integrity of the dodecanethiol was maintained during the formation of the surface-capped nickel nanoparticles. Fig. 5 shows the TGA curve of nickel nanoparticles Ni-S3 as an example. It is seen that Ni-S3 nanoparticles begins to lose weight at 263.4 ◦ C and the total weight loss is about 70.9%, which corresponds to the decomposition of the dodecanethiol coating around the Ni nanocores. Assuming that the residue at the end of heating in nitrogen gas was pure nickel and the total weight loss of the surface-capped Ni nanoparticles was solely attributed to the desorption of dodecanethiol, the molar ratio of the Ni nanocores and dodecanethiol in the target product Ni-S3 was calculated to be 1.43:1 (The mole fraction of thiol was equal to 70.9/202 = 0.35, the mole fraction of nickel to 29.1/58.69 = 0.50, thus the molar ratio of Ni nanocores to dodecanethiol was equal to 0.50:0.35 = 1.43:1.). 4. Conclusions The nickel nanoparticles protected by self-assembled monolayers of dodecanethiol were synthesized via a facile and inexpensive chemical reduction route, using dodecanethiol of different concentrations as the ligand. The resulting surfacecapped nickel nanoparticles were characterized by means of transmission electron spectroscopy, X-ray diffraction, Fourier transformation infrared spectroscopy and TGA. It was found that the target products were composed of cubic nickel nanoparticles. Dodecanethiol as the ligand was effective in controlling
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