- Email: [email protected]
The preparation of nickel nanorods in water-in-oil microemulsion
Journal of Crystal Growth 252 (2003) 612–617
The preparation of nickel nanorods in water-in-oil microemulsion Xiao-Min Ni*, Xiao-Bo Su, Zhi-Ping Yang, Hua-Gui Zheng Department of Chemistry, University of Science and Technology of China, Heifei, Anhui 230026, People’s Republic of China Received 27 January 2003; accepted 2 February 2003 Communicated by M. Schieber
Abstract Nickel nanorods with a diameter of 8–10 nm and a length of 100–200 nm have been prepared by reduction of nickel chloride with hydrazine hydrate in water/butanol/potassium oleate/kerosene microemulsion. Transmission electron microscope (TEM) and X-ray diffraction characterizations indicated that these nickel nanocrystallites were rod-shaped with a face-center-cubic phase. The possible growth process of the particles was investigated. TEM images of the samples taken out from the reaction system at different periods revealed that as-prepared particles grew from spherical to rod-like shape. The size of the nanorods was influenced by the length of micelles and the molar ratio of water to surfactant. The effect of the weight ratio of cosurfactant to surfactant on the formation of the particles was also discussed. The coercivity of as-prepared nanorods reached as high as 332 Oe at room temperature, superior to that of bulk nickel and spherical nanoparticles with a mean size of 10 nm. r 2003 Elsevier Science B.V. All rights reserved. PACS: 75.50.Kj; 81.05.Ys; 61.46.+w Keywords: A1. Nanostructures; A1. Crystal morphology; B1. Metals; B1. Nanomaterials; B2. Magnetic materials
1. Introduction Transition metal nanoparticles such as Fe, Co and Ni have been given much attention due to their wide applications in the magnetization, catalysis, electronic and conduction fields [1–5]. Therefore, their synthesis has become an active area. Many methods including hydrogen arc plasma [6], borohydride reduction of metal salts [7], rapid expansion of supercritical fluid solutions *Corresponding author. Tel.: +865513606144; +865513606144. E-mail address: [email protected] (X.-M. Ni).
fax:
[8,9], g-ray irradiation [10], sonochemical and thermal decomposition of organic metal complexes [11–15] and electrodeposition on alumina template [16] have been proved effective for the synthesis of these transition metals. Among them, the morphology control was more challenging and mainly performed in nonaqueous solvents with organometallic compounds as precursors. For example, nanorods and trigonal nanoparticles of nickel were achieved in THF by decomposition of Ni (COD)2 [14,15]. Recently, reduction of the nickel salt in microemulsion and aqueous solution in the presence of cationic surfactant has been reported [17,18], but the particles obtained were all spherical.
0022-0248/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-0248(03)00954-0
X.-M. Ni et al. / Journal of Crystal Growth 252 (2003) 612–617
Microemulsion is a simple and convenient method widely used in controlling the size and shape of nanoparticles [19–23]. Some metal nanocrystallites with different morphologies have been successfully prepared through this soft technique [24–26]. However, to the best of our knowledge, articles on the synthesis of nickel nanorods in microemulsion have seldom been seen up to date. In this paper, we reported the preparation of nickel nanorods in a microemulsion system using potassium oleate as surfactant. The reactants were the facile nickel chloride and hydrazine hydrate. Contrast experiments were carried out to study the factors that worked on the formation and size of the nanorods.
613
NiCl2 microemulsion was added in 30 min. Samples were taken out from the mixtures with a straw at different reaction periods. The samples were washed with absolute ethanol and distilled water to remove the impurities. The X-ray diffraction (XRD) patterns were collected on a Japan Rigaku Damax gA diffractometer with graphite monochromized CuKa radiation (l ¼ 0:15418 nm). Transmission electron microscope (TEM) was performed by a Hitachi Model H-800 TEM with an accelerating voltage of 200 KV. The M–H hysteresis loops were measured by a model BHV-55 vibrating sample magnetometer.
3. Results and discussion 2. Experimental procedure The anion surfactant C17H35COOK, nC4H9OH, N2H4 H2O (85%) and NaOH were all analytical grade. Kerosene was purchased from the market and used without further purification. Two types of microemulsions were prepared in advance, respectively. In a typical experiment, 6 ml NiCl2 aqueous solution (0.5 M) containing tartrate sodium (0.001 M) was added dropwise into the mixture which was made up of 4 g potassium oleate, 6 ml n-butanol and 25 ml kerosene under magnet stirring. After continuous stirring at 400 rpm for an hour, a homogenous and transparent solution could be obtained. The other microemulsion was made in the same manner except that the N2H4 H2O (4 M) solution dissolved a certain amount of NaOH replaced the NiCl2 aqueous solution. Nickel nanorods were synthesized as follows: The microemulsion containing N2H4 H2O was refluxed (B90 C) in a three-necked round bottom flask equipped with a condenser, a stirring bar and a thermometer. After the temperature was invariable, 3 ml NiCl2 microemulsion was added into the flask through a syringe. Five minutes later, the reaction mixture turned dark which indicated the production of the nickel nanoparticles. Then quenched the reaction system with ice water to decrease the temperature to 65 C to slow the reaction. All the rest of the
TEM images of the three as-prepared samples taken out at t ¼ 6; 40; and 150 min were shown in Fig. 1, respectively. Fig. 1a was the image of the particles produced in the initial reaction process. They were all spherical particles with a diameter of about 6–10 nm. Fig. 1b gave the image of the sample obtained at t ¼ 40 min. Spherical nanoparticles and quite a few nickel nanorods coexisted. After the reaction had proceeded for 150 min, a large amount of nanorods with aspect ratio of 10–20 (the ratio of length to diameter) appeared as shown in Fig. 1c. From studies of these TEM images, we tried to propose the formation process of these nanorods. At the beginning of the reaction, the added nickel ions were all reduced by hydrazine at 90 C. These initially reduced nickel atoms collided with each other and nucleated in the micelles. During this stage, the particles are mainly spherical. After cooling the system, these particles acted as seeds for the later born particles to grow on. Due to the fact that all these particles were constrained in the microreactors offered by the microemulsion, shape of the particles formed therein was tuned according to the template [27]. With more NiCl2 microemulsion added, more nickel atoms were reduced and absorbed on the nuclei. Then they aggregated into larger particles along a certain orientation under the direction of micelles. Further growth of the initially produced particles in the
X.-M. Ni et al. / Journal of Crystal Growth 252 (2003) 612–617
614
Fig. 1. TEM images of samples taken at different reaction time.
Table 1 Changes of particle size with the reaction time
Intensity
(111)
(200)
Reaction time (min)
Full width at medium Particle size height of peak (1 1 1) (nm)
6 40 150
0.960 0.551 0.436
8.9 15.6 19.7
(220)
30
40
50
60
70
80
2θ (degree) Fig. 2. XRD pattern of the sample taken out at t ¼ 40 min.
constrained environment offered by micelles resulted in rod-like shapes. With the time prolonged, these nanorods transformed into longer ones. Fig. 2 was a typical XRD pattern of the sample obtained at t ¼ 40 min. Three peaks for nickel (2y ¼ 44:6 ; 51.8 , 76.2 ) were observed. Average particle sizes of the three samples calculated from the full width at medium height of peaks in their XRD patterns using Scherrer formula were summarized in Table 1. The increasing size confirmed the fact that the size of particles was
increasing with reaction time, which was consistent with TEM results. It is well known that the basic function of microemulsion droplets is to provide a limited volume for the formation of nanoparticles. The micelles of the potassium oleate in aqueous solution are thread-like [28] and probably this thread-like shape can still be maintained in the microemulsion. Contrast experiment was carried out in aqueous solution with potassium oleate just as surfactant, keeping the concentrations of reactants and temperatures constant. TEM image indicated that just a few short nanorods were obtained (Fig. 3). This difference suggested that the micelles in the microemulsion had better control on the shape of particles than those in the aqueous solution. In the microemulsion, both
X.-M. Ni et al. / Journal of Crystal Growth 252 (2003) 612–617
the quantity and the stability of micelles were greatly raised because of the high concentration of surfactants and the existence of cosurfactant. Accordingly, much more nanorods could be achieved. We attempted to add trace tartrate sodium into the NiCl2 aqueous solution, but no trigonal particles were obtained as expected like
Fig. 3. TEM image of the sample prepared in aqueous solution with potassium oleate just as surfactant.
615
that in the literature [14]. It was observed that rodlike particles were also achieved without tartrate sodium. And this suggested that the key factor that affected the morphology of particles was the shape control from micelles. Since the role of micelles is to induce the paricles to aggregate along a certain direction, the length of micelles should have some influence on the length of the nanorods. In the article of Zechen Lin et al. they deduced that the entangled thread-like micelles of potassium oleate were at least 100 nm long [28]. We found that the nanorods obtained in our experiment are also centralized around 100 nm. It was likely that once the rods reached the length of micelles, further aggregation of the particles cannot occur owing to the absorption of micelles on the particles. Meanwhile, it was found that more NiCl2 microemulsion could not make the as-formed rods grow longer after the reaction had proceeded to a certain degree. This result further proved the hypothesis proposed above. In the microemulsion, the size of water pools is mainly dependent on the molar ratio (o) of water to surfactant [24]. Increasing o can raise the size of the particles formed therein. Two samples were prepared in microemulsions with different o values. The size of nickel nanorods prepared in
Fig. 4. TEM images of the samples prepared in two microemulsions with different o values.
X.-M. Ni et al. / Journal of Crystal Growth 252 (2003) 612–617
616 8000 6000
M (emu)
4000 2000 0 -2000 -4000 -6000 -8000 -10000
-5000
0
5000
10000
H (Oe)
be explained from the contrast experimental results. The shape control of the micelles is a key factor that influences the formation of the nanorods. The length of micelles affects the size of the nanorods. Increasing the molar ratio of water to surfactant and the weight ratio of cosurfactant to surfactant is beneficial to create larger rods. Thus-prepared nickel nanorods showed superior magnetization to that of bulk nickel. The present microemulsion system is expected to be used to fabricate other transition metal nanocrystals and the relative studies are in progress.
Fig. 5. M–H loop of the as-prepared nanorods.
the microemulsion with a higher o was larger than that prepared in the other one with a lower o (seen in Fig. 4). The weight ratio of cosurfactant to surfactant is another factor influencing the formation of nanorods. Cosurfactant can help decrease the fraction of the micellar head groups, thus a more stable and rigid interface can be got [29]. Experimental results indicated that when the weight ratio of n-butanol to potassium oleate was only 0.8, only a few short nanorods could be obtained; but if the value was above 1.0, nearly all the particles were rod-shaped. However, further increase of the ratio showed little influence on the morphology of the particles. As the presence of shape anisotropy can significantly enhance the magnetic properties [30], a higher aspect ratio can favor the increase of the coercivity. We measured the M–H hysteresis loop of thus-prepared nanorods at room temperature. A typical pattern was shown in Fig. 5. Its coercivity (332 Oe) was superior to that of the bulk nickel (100 Oe) [31] and spherical nanoparticles with a diameter of about 10 nm, which was measured as 173 Oe. Differences in their microstucture result in the rise of coecivity.
4. Conclusion Nickel nanorods had been successfully prepared in the water-in-oil microemulsion under mild conditions. The possible formation process can
Acknowledgements We are grateful to the electron microscope and X-ray diffraction facilities of University of Science and Technology of China for assistance in XRD and TEM measurement.
References [1] T. Kato, Ind. Mater. 31 (1983) 18. [2] M. Koda, Applications collection of super fine particles, Argil Technology Center, Tokyo, Japan, 1985, p. 117. [3] M. Ozaki, Mater. Res. Soc. Bull. 14 (1989) 35. [4] M.L. Wagner, L.D. Schmidt, J. Phys. Chem. 99 (1999) 805. [5] G. Schmid, L.F. Chi, Adv. Mater. 10 (1998) 515. [6] Z.L. Cui, L.F. Dong, C.C. Hao, Mater. Sci. Eng. A 286 (2000) 205. [7] J.P. Chen, C.M. Sorensen, K.J. Klabunde, Phys. Rev. B 17 (1995) 11527. [8] G.N. Glavee, K.J. Klabunde, C.M. Sorensen, G.C. Hadjipannayis, Inorg. Chem. 32 (1993) 474. [9] Y.P. Sun, H.W. Rollins, R. Guduru, Chem. Mater. 11 (1999) 7. [10] F. Wang, Z. Zhang, Z. Chang, Mater. Lett. 55 (2002) 27. [11] K.S. Suslick, M. Fang, T. Hyeon, J. Am. Chem. Soc. 118 (1996) 11960. [12] S.J. ParK, S. Kim, S. Lee, Z.G. Khim, K. Char, T. Hyeon, J. Am. Chem. Soc. 122 (2000) 8581. [13] V.F. Puntes, K.M. Krishnan, A.P. Alivisatos, Science 291 (2001) 2115. [14] J.S. Bradley, B. Tesche, W. Busser, M. Maase, M.T. Reetz, J. Am. Chem. Soc. 122 (2000) 4631. [15] N. Coydente, M. Respaud, F. Secocq, M.J. Casanove, C. Amiens, B. Chaudret, Nano Lett. 1 (2001) 565. [16] J. Bao, C. Tie, Z. Xu, Q. Zhou, D. Shen, Q. Ma, Adv. Mater. 13 (2001) 1631.
X.-M. Ni et al. / Journal of Crystal Growth 252 (2003) 612–617 [17] D.H. Chen, S.H. Wu, Chem. Mater. 12 (2000) 1354. [18] D.H. Chen, C.H. Hsieh, J. Mater. Chem. 12 (2002) 2412. [19] S. Qiu, J. Dong, G. Chen, J. Colloid Interface Sci. 216 (1999) 230. [20] M. Li, S. Mann, Langmuir 16 (2000) 7088. [21] M.L. Wu, D.H. Chen, T.C. Huang, Chem. Mater. 13 (2001) 599. [22] Z.H. Zhou, J. Wang, X. Liu, H.S.O. Chan, J. Mater. Chem. 11 (2001) 1704. [23] Z.X. Lu, S.M. Kanan, C.P. Tripp, J. Mater. Chem. 12 (2002) 983. [24] I. Lisiecki, M.P. Pileni, J. Am. Chem. Soc. 115 (1993) 3887.
617
[25] M.P. Pileni, Langmuir 13 (1997) 3226. [26] M. Maillard, S. Giorgio, M.P. Pileni, Adv. Mater. 14 (2002) 1084. [27] N.R. Jana, L. Gearheart, C.J. Murphy, Adv. Mater. 13 (2001) 1389. [28] Z. Lin, C.D. Eads, Langmuir 13 (1997) 2647. [29] M. Valiente, E.J. Rodenas, J. Am. Chem. Soc. 95 (1991) 3368. [30] D.L.L. Pelecky, R.D. Rieke, Chem. Mater. 38 (1996) 1770. [31] J.H. Hwang, V.P. Dravie, M.H. Teng, J.J. Host, B.R. Euiott, D.L. Johnson, T.O. Mason, J. Mater. Res. 12 (1997) 1676.
The preparation of nickel nanorods in water-in-oil microemulsion Xiao-Min Ni*, Xiao-Bo Su, Zhi-Ping Yang, Hua-Gui Zheng Department of Chemistry, University of Science and Technology of China, Heifei, Anhui 230026, People’s Republic of China Received 27 January 2003; accepted 2 February 2003 Communicated by M. Schieber
Abstract Nickel nanorods with a diameter of 8–10 nm and a length of 100–200 nm have been prepared by reduction of nickel chloride with hydrazine hydrate in water/butanol/potassium oleate/kerosene microemulsion. Transmission electron microscope (TEM) and X-ray diffraction characterizations indicated that these nickel nanocrystallites were rod-shaped with a face-center-cubic phase. The possible growth process of the particles was investigated. TEM images of the samples taken out from the reaction system at different periods revealed that as-prepared particles grew from spherical to rod-like shape. The size of the nanorods was influenced by the length of micelles and the molar ratio of water to surfactant. The effect of the weight ratio of cosurfactant to surfactant on the formation of the particles was also discussed. The coercivity of as-prepared nanorods reached as high as 332 Oe at room temperature, superior to that of bulk nickel and spherical nanoparticles with a mean size of 10 nm. r 2003 Elsevier Science B.V. All rights reserved. PACS: 75.50.Kj; 81.05.Ys; 61.46.+w Keywords: A1. Nanostructures; A1. Crystal morphology; B1. Metals; B1. Nanomaterials; B2. Magnetic materials
1. Introduction Transition metal nanoparticles such as Fe, Co and Ni have been given much attention due to their wide applications in the magnetization, catalysis, electronic and conduction fields [1–5]. Therefore, their synthesis has become an active area. Many methods including hydrogen arc plasma [6], borohydride reduction of metal salts [7], rapid expansion of supercritical fluid solutions *Corresponding author. Tel.: +865513606144; +865513606144. E-mail address: [email protected] (X.-M. Ni).
fax:
[8,9], g-ray irradiation [10], sonochemical and thermal decomposition of organic metal complexes [11–15] and electrodeposition on alumina template [16] have been proved effective for the synthesis of these transition metals. Among them, the morphology control was more challenging and mainly performed in nonaqueous solvents with organometallic compounds as precursors. For example, nanorods and trigonal nanoparticles of nickel were achieved in THF by decomposition of Ni (COD)2 [14,15]. Recently, reduction of the nickel salt in microemulsion and aqueous solution in the presence of cationic surfactant has been reported [17,18], but the particles obtained were all spherical.
0022-0248/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-0248(03)00954-0
X.-M. Ni et al. / Journal of Crystal Growth 252 (2003) 612–617
Microemulsion is a simple and convenient method widely used in controlling the size and shape of nanoparticles [19–23]. Some metal nanocrystallites with different morphologies have been successfully prepared through this soft technique [24–26]. However, to the best of our knowledge, articles on the synthesis of nickel nanorods in microemulsion have seldom been seen up to date. In this paper, we reported the preparation of nickel nanorods in a microemulsion system using potassium oleate as surfactant. The reactants were the facile nickel chloride and hydrazine hydrate. Contrast experiments were carried out to study the factors that worked on the formation and size of the nanorods.
613
NiCl2 microemulsion was added in 30 min. Samples were taken out from the mixtures with a straw at different reaction periods. The samples were washed with absolute ethanol and distilled water to remove the impurities. The X-ray diffraction (XRD) patterns were collected on a Japan Rigaku Damax gA diffractometer with graphite monochromized CuKa radiation (l ¼ 0:15418 nm). Transmission electron microscope (TEM) was performed by a Hitachi Model H-800 TEM with an accelerating voltage of 200 KV. The M–H hysteresis loops were measured by a model BHV-55 vibrating sample magnetometer.
3. Results and discussion 2. Experimental procedure The anion surfactant C17H35COOK, nC4H9OH, N2H4 H2O (85%) and NaOH were all analytical grade. Kerosene was purchased from the market and used without further purification. Two types of microemulsions were prepared in advance, respectively. In a typical experiment, 6 ml NiCl2 aqueous solution (0.5 M) containing tartrate sodium (0.001 M) was added dropwise into the mixture which was made up of 4 g potassium oleate, 6 ml n-butanol and 25 ml kerosene under magnet stirring. After continuous stirring at 400 rpm for an hour, a homogenous and transparent solution could be obtained. The other microemulsion was made in the same manner except that the N2H4 H2O (4 M) solution dissolved a certain amount of NaOH replaced the NiCl2 aqueous solution. Nickel nanorods were synthesized as follows: The microemulsion containing N2H4 H2O was refluxed (B90 C) in a three-necked round bottom flask equipped with a condenser, a stirring bar and a thermometer. After the temperature was invariable, 3 ml NiCl2 microemulsion was added into the flask through a syringe. Five minutes later, the reaction mixture turned dark which indicated the production of the nickel nanoparticles. Then quenched the reaction system with ice water to decrease the temperature to 65 C to slow the reaction. All the rest of the
TEM images of the three as-prepared samples taken out at t ¼ 6; 40; and 150 min were shown in Fig. 1, respectively. Fig. 1a was the image of the particles produced in the initial reaction process. They were all spherical particles with a diameter of about 6–10 nm. Fig. 1b gave the image of the sample obtained at t ¼ 40 min. Spherical nanoparticles and quite a few nickel nanorods coexisted. After the reaction had proceeded for 150 min, a large amount of nanorods with aspect ratio of 10–20 (the ratio of length to diameter) appeared as shown in Fig. 1c. From studies of these TEM images, we tried to propose the formation process of these nanorods. At the beginning of the reaction, the added nickel ions were all reduced by hydrazine at 90 C. These initially reduced nickel atoms collided with each other and nucleated in the micelles. During this stage, the particles are mainly spherical. After cooling the system, these particles acted as seeds for the later born particles to grow on. Due to the fact that all these particles were constrained in the microreactors offered by the microemulsion, shape of the particles formed therein was tuned according to the template [27]. With more NiCl2 microemulsion added, more nickel atoms were reduced and absorbed on the nuclei. Then they aggregated into larger particles along a certain orientation under the direction of micelles. Further growth of the initially produced particles in the
X.-M. Ni et al. / Journal of Crystal Growth 252 (2003) 612–617
614
Fig. 1. TEM images of samples taken at different reaction time.
Table 1 Changes of particle size with the reaction time
Intensity
(111)
(200)
Reaction time (min)
Full width at medium Particle size height of peak (1 1 1) (nm)
6 40 150
0.960 0.551 0.436
8.9 15.6 19.7
(220)
30
40
50
60
70
80
2θ (degree) Fig. 2. XRD pattern of the sample taken out at t ¼ 40 min.
constrained environment offered by micelles resulted in rod-like shapes. With the time prolonged, these nanorods transformed into longer ones. Fig. 2 was a typical XRD pattern of the sample obtained at t ¼ 40 min. Three peaks for nickel (2y ¼ 44:6 ; 51.8 , 76.2 ) were observed. Average particle sizes of the three samples calculated from the full width at medium height of peaks in their XRD patterns using Scherrer formula were summarized in Table 1. The increasing size confirmed the fact that the size of particles was
increasing with reaction time, which was consistent with TEM results. It is well known that the basic function of microemulsion droplets is to provide a limited volume for the formation of nanoparticles. The micelles of the potassium oleate in aqueous solution are thread-like [28] and probably this thread-like shape can still be maintained in the microemulsion. Contrast experiment was carried out in aqueous solution with potassium oleate just as surfactant, keeping the concentrations of reactants and temperatures constant. TEM image indicated that just a few short nanorods were obtained (Fig. 3). This difference suggested that the micelles in the microemulsion had better control on the shape of particles than those in the aqueous solution. In the microemulsion, both
X.-M. Ni et al. / Journal of Crystal Growth 252 (2003) 612–617
the quantity and the stability of micelles were greatly raised because of the high concentration of surfactants and the existence of cosurfactant. Accordingly, much more nanorods could be achieved. We attempted to add trace tartrate sodium into the NiCl2 aqueous solution, but no trigonal particles were obtained as expected like
Fig. 3. TEM image of the sample prepared in aqueous solution with potassium oleate just as surfactant.
615
that in the literature [14]. It was observed that rodlike particles were also achieved without tartrate sodium. And this suggested that the key factor that affected the morphology of particles was the shape control from micelles. Since the role of micelles is to induce the paricles to aggregate along a certain direction, the length of micelles should have some influence on the length of the nanorods. In the article of Zechen Lin et al. they deduced that the entangled thread-like micelles of potassium oleate were at least 100 nm long [28]. We found that the nanorods obtained in our experiment are also centralized around 100 nm. It was likely that once the rods reached the length of micelles, further aggregation of the particles cannot occur owing to the absorption of micelles on the particles. Meanwhile, it was found that more NiCl2 microemulsion could not make the as-formed rods grow longer after the reaction had proceeded to a certain degree. This result further proved the hypothesis proposed above. In the microemulsion, the size of water pools is mainly dependent on the molar ratio (o) of water to surfactant [24]. Increasing o can raise the size of the particles formed therein. Two samples were prepared in microemulsions with different o values. The size of nickel nanorods prepared in
Fig. 4. TEM images of the samples prepared in two microemulsions with different o values.
X.-M. Ni et al. / Journal of Crystal Growth 252 (2003) 612–617
616 8000 6000
M (emu)
4000 2000 0 -2000 -4000 -6000 -8000 -10000
-5000
0
5000
10000
H (Oe)
be explained from the contrast experimental results. The shape control of the micelles is a key factor that influences the formation of the nanorods. The length of micelles affects the size of the nanorods. Increasing the molar ratio of water to surfactant and the weight ratio of cosurfactant to surfactant is beneficial to create larger rods. Thus-prepared nickel nanorods showed superior magnetization to that of bulk nickel. The present microemulsion system is expected to be used to fabricate other transition metal nanocrystals and the relative studies are in progress.
Fig. 5. M–H loop of the as-prepared nanorods.
the microemulsion with a higher o was larger than that prepared in the other one with a lower o (seen in Fig. 4). The weight ratio of cosurfactant to surfactant is another factor influencing the formation of nanorods. Cosurfactant can help decrease the fraction of the micellar head groups, thus a more stable and rigid interface can be got [29]. Experimental results indicated that when the weight ratio of n-butanol to potassium oleate was only 0.8, only a few short nanorods could be obtained; but if the value was above 1.0, nearly all the particles were rod-shaped. However, further increase of the ratio showed little influence on the morphology of the particles. As the presence of shape anisotropy can significantly enhance the magnetic properties [30], a higher aspect ratio can favor the increase of the coercivity. We measured the M–H hysteresis loop of thus-prepared nanorods at room temperature. A typical pattern was shown in Fig. 5. Its coercivity (332 Oe) was superior to that of the bulk nickel (100 Oe) [31] and spherical nanoparticles with a diameter of about 10 nm, which was measured as 173 Oe. Differences in their microstucture result in the rise of coecivity.
4. Conclusion Nickel nanorods had been successfully prepared in the water-in-oil microemulsion under mild conditions. The possible formation process can
Acknowledgements We are grateful to the electron microscope and X-ray diffraction facilities of University of Science and Technology of China for assistance in XRD and TEM measurement.
References [1] T. Kato, Ind. Mater. 31 (1983) 18. [2] M. Koda, Applications collection of super fine particles, Argil Technology Center, Tokyo, Japan, 1985, p. 117. [3] M. Ozaki, Mater. Res. Soc. Bull. 14 (1989) 35. [4] M.L. Wagner, L.D. Schmidt, J. Phys. Chem. 99 (1999) 805. [5] G. Schmid, L.F. Chi, Adv. Mater. 10 (1998) 515. [6] Z.L. Cui, L.F. Dong, C.C. Hao, Mater. Sci. Eng. A 286 (2000) 205. [7] J.P. Chen, C.M. Sorensen, K.J. Klabunde, Phys. Rev. B 17 (1995) 11527. [8] G.N. Glavee, K.J. Klabunde, C.M. Sorensen, G.C. Hadjipannayis, Inorg. Chem. 32 (1993) 474. [9] Y.P. Sun, H.W. Rollins, R. Guduru, Chem. Mater. 11 (1999) 7. [10] F. Wang, Z. Zhang, Z. Chang, Mater. Lett. 55 (2002) 27. [11] K.S. Suslick, M. Fang, T. Hyeon, J. Am. Chem. Soc. 118 (1996) 11960. [12] S.J. ParK, S. Kim, S. Lee, Z.G. Khim, K. Char, T. Hyeon, J. Am. Chem. Soc. 122 (2000) 8581. [13] V.F. Puntes, K.M. Krishnan, A.P. Alivisatos, Science 291 (2001) 2115. [14] J.S. Bradley, B. Tesche, W. Busser, M. Maase, M.T. Reetz, J. Am. Chem. Soc. 122 (2000) 4631. [15] N. Coydente, M. Respaud, F. Secocq, M.J. Casanove, C. Amiens, B. Chaudret, Nano Lett. 1 (2001) 565. [16] J. Bao, C. Tie, Z. Xu, Q. Zhou, D. Shen, Q. Ma, Adv. Mater. 13 (2001) 1631.
X.-M. Ni et al. / Journal of Crystal Growth 252 (2003) 612–617 [17] D.H. Chen, S.H. Wu, Chem. Mater. 12 (2000) 1354. [18] D.H. Chen, C.H. Hsieh, J. Mater. Chem. 12 (2002) 2412. [19] S. Qiu, J. Dong, G. Chen, J. Colloid Interface Sci. 216 (1999) 230. [20] M. Li, S. Mann, Langmuir 16 (2000) 7088. [21] M.L. Wu, D.H. Chen, T.C. Huang, Chem. Mater. 13 (2001) 599. [22] Z.H. Zhou, J. Wang, X. Liu, H.S.O. Chan, J. Mater. Chem. 11 (2001) 1704. [23] Z.X. Lu, S.M. Kanan, C.P. Tripp, J. Mater. Chem. 12 (2002) 983. [24] I. Lisiecki, M.P. Pileni, J. Am. Chem. Soc. 115 (1993) 3887.
617
[25] M.P. Pileni, Langmuir 13 (1997) 3226. [26] M. Maillard, S. Giorgio, M.P. Pileni, Adv. Mater. 14 (2002) 1084. [27] N.R. Jana, L. Gearheart, C.J. Murphy, Adv. Mater. 13 (2001) 1389. [28] Z. Lin, C.D. Eads, Langmuir 13 (1997) 2647. [29] M. Valiente, E.J. Rodenas, J. Am. Chem. Soc. 95 (1991) 3368. [30] D.L.L. Pelecky, R.D. Rieke, Chem. Mater. 38 (1996) 1770. [31] J.H. Hwang, V.P. Dravie, M.H. Teng, J.J. Host, B.R. Euiott, D.L. Johnson, T.O. Mason, J. Mater. Res. 12 (1997) 1676.