pubs.acs.org/Langmuir © 2010 American Chemical Society
Hydrothermal Microemulsion Synthesis of Oxidatively Stable Cobalt Nanocrystals Encapsulated in Surfactant/Polymer Complex Shells Xian-Hua Zhang,† Kin Man Ho,† Ai-Hua Wu,† Kin Hung Wong,‡ and Pei Li*,† †
Department of Applied Biology and Chemical Technology and ‡Department of Applied Physics, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, P. R. China Received December 5, 2009. Revised Manuscript Received March 14, 2010
Air-stable magnetic cobalt nanocrystals have been conveniently prepared via a reverse micellar synthesis, followed by a hydrothermal treatment. The synthesis was carried out by first mixing an aqueous solution containing cobalt chloride and poly(sodium 4-styrenesulfonate) (PSS) with an organic mixture containing cetyltrimethylammonium bromide (CTAB) to form reverse micelles, followed by reducing cobalt ions with sodium borohydride. The resultant nanoparticles were then undergone a hydrothermal treatment at 165 °C for 8 h to generate well-dispersed CTAB/ PSS-encapsulated cobalt nanocrystals with an average diameter of 3.5 ( 0.5 nm. The nanoparticles were highly crystalline with a hexagonal close-packed crystal phase. The presence of CTAB/PSS complex coatings was identified by FT-IR and UV-vis spectroscopies as well as thermogravimetry analyses. The nanocrystals exhibited superparamagnetic property at room temperature with a saturation magnetization (Ms) of 95 emu/g. The magnetization could be largely preserved after storage at room temperature for 4 months as the Ms value only slightly decreased to 88 emu/g (measured at 300 K). Thus, the polymer encapsulation could not only improve thermal stability of the micelles for the growth and nucleation of Co atoms but also protect the resulting cobalt nanocrystals from oxidation through forming an oxygen impermeable sheath.
Introduction Magnetic nanoparticles with diameters less than 20 nm and high crystallinity are of great interest for many potential applications such as magnetic resonance imaging (MRI),1 magnetic separation,2 magnetic guided drug delivery,3,4 magnetic-mediated hyperthermia,2,5 electromagnetic interference shielding,6 electronic device,7,8 and information storage.9 Among various types of magnetic nanoparticles, cobalt (Co) nanoparticle is one of the most promising nanomaterials for electronic and information storage devices because it has one of the largest magnetic susceptibilities as compared to other metal nanoparticles. However, the use of Co nanoparticles in these potential applications is hindered by two intrinsic problems: first, the Co nanoparticles are easily oxidized upon exposure to air. This leads to the formation of a layer of antiferromagnetic cobalt oxide on the nanoparticle surface, thus significantly reducing their magnetic susceptibility.10,11 In fact, this problem will become more and more serious when the *Corresponding author: e-mail
[email protected].
(1) Na, H. B.; Song, I. C.; Hyeon, T. Adv. Mater. 2009, 21, 2133–2148. (2) Pankhurst, Q. A.; Connolly, J.; Jones, S. K.; Dobson, J. J. Phys. D: Appl. Phys. 2003, 36, R167–R181. (3) Arrueboa, M.; Fernandez-Pachecoa, R.; Ibarraa, M. R.; Santamarı´ a, J. Nano Today 2007, 2, 22–32. (4) Veiseh, O.; Gunn, J.; Zhang, M. Adv. Drug Delivery Rev. 2010, 62, 284–304. (5) Berry, C. C.; Curtis, A. S. G. J. Phys. D: Appl. Phys. 2003, 36, R198–R206. (6) Han, Z.; Li, D.; Wang, H.; Liu, X. G.; Li, J.; Geng, D. Y.; Zhang, Z. D. Appl. Phys. Lett. 2009, 95, 023114. (7) Black, C. T.; Murray, C. B.; Sandstrom, R. L.; Sun, S. Science 2000, 290, 1131–1134. (8) Yakushiji, K.; Ernult, F.; Imamura, H.; Yamane, K.; Mitani, S.; Takanashi, K.; Takahashi, S.; Maekawa, S.; Fujimori, H. Nat. Mater. 2005, 4, 57–61. (9) Gambardella, P.; Rusponi, S.; Veronese, M.; Dhesi, S. S.; Grazioli, C.; Dallmeyer, A.; Cabria, I.; Zeller, R.; Dederichs, P. H.; Kern, K.; Carbone, C.; Brune, H. Science 2003, 300, 1130–1133. (10) Rutnakornpituk, M.; Thompson, M. S.; Harris, L. A.; Farmer, K. E.; Esker, A. R.; Riffle, J. S.; Connolly, J.; St. Pierre, T. G. Polymer 2002, 43, 2337– 2348. (11) Zalich, M. A.; Baranauskas, V. V.; Riffle, J. S.; Saunders, M.; St. Pierre, T. G. Chem. Mater. 2006, 18, 2648–2655.
Langmuir 2010, 26(8), 6009–6014
size of the Co particle is reduced to a few nanometers. This phenomenon is attributed to the fact that the surface cobalt atoms become more and more susceptible to oxidation with the increase of surface to volume ratio.12 Second, the nanoparticles have a strong tendency to form agglomerates because of their strong magnetic and van der Waals attractive forces acting on them.12,13 Therefore, it is difficult to produce stable cobalt nanocrystals with sizes of a few nanometers. To address the aforementioned problems, various protective coating strategies to stabilize the naked cobalt nanoparticles against oxidation and agglomeration have been developed using diverse synthetic methods.12 These coatings include carbon,11,14 silica,15-17 aluminum alkyls,18 surfactants,19-22 and polymers.23,24 Among various synthetic strategies to prepare well-dispersed and oxidatively stable cobalt nanoparticles, microemulsion-mediated (12) Lu, A.; Salabas, E. L.; Schuth, F. Angew. Chem., Int. Ed. 2007, 46, 1222– 1244. (13) Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115– 2117. (14) El-Gendy, A. A.; Ibrahim, E. M. M.; Khavrus, V. O.; Krupskaya, Y.; Hampel, S.; Leonhardt, A.; B€uchner, B.; Klingeler, R. Carbon 2009, 47, 2821–2828. (15) Horie, M.; Konno, M.; Rodriguez-Gonzalez, B.; Liz-Marzan, L. M. J. Phys. Chem. B 2003, 107, 7420–7425. (16) Salgueirino-Maceira, V.; Correa-Duarte, M. A. J. Mater. Chem. 2006, 16, 3593–3597. (17) Agustı´ n, M.; Gonzalo, P. Catal. Commun. 2007, 8, 1479–1486. (18) Behrens, S.; B€onnemann, H.; Matoussevitch, N.; Dinjus, E.; Modrow, H.; Palina, N.; Frerichs, M.; Kempter, V.; Maus-Friedrichs, W.; Heinemann, A.; Kammel, M.; Wiedenmann, A. Z. Phys. Chem. (Munich) 2006, 220, 3–40. (19) Puntes, V. F.; Zanchet, D.; Erdonmez, C. K.; Alivisato, A. D. J. Am. Chem. Soc. 2002, 124, 12874–12880. (20) Song, Y.; Modrow, H.; Henry, L. L.; Saw, C. K.; Doomes, E. E.; Palshin, V.; Hormes, J.; Kumar, C. S. S. R. Chem. Mater. 2006, 18, 2817–2827. (21) Cheng, G.; Dennis, C. L.; Shull, R. D.; Walker, A. R. H. Langmuir 2007, 23, 11740–11746. (22) Wu, N.; Fu, L.; Su, M.; Aslam, M.; Wong, K. C.; Dravid, V. P. Nano Lett. 2004, 4, 383–386. (23) Baranauskas, V. V.; Zalich, M. A.; Saunders, M.; St. Pierre, T. G.; Riffle, J. S. Chem. Mater. 2005, 17, 5246–5254. (24) Zalich, M. A.; Vadala, M. L.; Riffle, J. S.; Saunders, M.; St. Pierre, T. G. Chem. Mater. 2007, 19, 6597–6604.
Published on Web 03/25/2010
DOI: 10.1021/la9045918
6009
Article
synthesis of cobalt nanoparticles through reduction of cobalt ions in a reverse micellar system is a very attractive route to obtain bulk quantity of nanoparticles because of the use of inexpensive reagents, mild reaction conditions, and the capability to control size of the magnetic nanoparticles in a few nanometers through varying the water to surfactant ratio.25-28 For example, Lin et al. have synthesized cobalt nanoparticles using sodium borohydride reduction of cobalt chloride in a didodecyldimethylammonium bromide (DDAB)/toluene inverse micelle solution and studied morphology, size, and magnetic properties of the nanoparticles formed at different temperatures.29,30 Pileni et al. have also reported the synthesis of cobalt nanocrystals using cobalt(II) bis(2-ethylhexyl)sulfosuccinate [Co(AOT)2] reverse micelles.31,32 Although microemulsion-mediated synthesis of cobalt nanoparticles have been studied,25 the problem of long-term oxidative stability of the nanoparticles was rarely addressed. In fact, metallic magnetic nanoparticles stabilized by single or double layer(s) of surfactant molecules are not good enough to prevent oxidation of the highly reactive metal nanoparticles. Thus, the development of novel coating strategy based on the reversemicellar system for protecting cobalt nanoparticles against oxidative deterioration and improving particle crystallinity is of great importance for their potential applications. Herein we report a simple strategy to synthesize surfactant/ polymer-encapsulated Co nanocrystals with an average diameter of ∼3.5 nm based on the hydrothermal microemulsion synthesis. Our approach involves the formation of thermally stable reverse micelle shells through electrostatic complexation between poly(sodium 4-styrenesulfonate) (PSS) and cetyltrimethylammonium bromide (CTAB) at the micellar interface. The aqueous core of the stabilized reverse micelle acts as a nanoreactor where the reduction of cobalt ions, growth of cobalt nanoparticle, and coating of the polymer onto the newly formed nanoparticle all take place. The nanoparticles formed are subsequently subjected to a hydrothermal treatment at 165 °C to produce cobalt nanocrystals. Although hydrothermal microemulsion technique has been used to produce crystalline nanomaterials such as ZnS,33,34 ZnO,35 SnO2,36 and NiS,37 this technique has rarely been used in the preparation of highly reactive cobalt nanoparticles.38 The success of our approach contributes to the creation of stable micellar shell of polymer/surfactant complexes. Thus, the shell can not only prevent Co nanocrystals from agglomeration during the hydrothermal process but also tightly and completely encapsulate the newly formed cobalt nanocrystals, resulting in the formation of an oxygen impermeable sheath. (25) Capek, I. Adv. Colloid Interface Sci. 2004, 110, 49–74. (26) Kumbhar, A.; Spinu, L.; Agnoli, F.; Wang, K. Y.; Zhou, W.; O’Connor, C. J. IEEE Trans. Magn. 2001, 37, 2216–2218. (27) Wilcoxon, J. P.; Venturini, E. L.; Provencio, P. Phys. Rev. B 2004, 69, 172402. (28) Ahmed, J.; Sharma, S.; Ramanujachary, K. V.; Lofland, S. E.; Ganguli, A. K. J. Colloid Interface Sci. 2009, 336, 814–819. (29) Lin, X. M.; Sorensen, C. M.; Klabunde, K. J.; Hadjipanayis, G. C. Langmuir 1998, 14, 7140–7146. (30) Lin, X. M.; Sorensen, C. M.; Klabunde, K. J.; Hajipanayis, G. C. J. Mater. Res. 1999, 14, 1542–1547. (31) Lisiecki, I.; Pileni, M. P. Langmuir 2003, 19, 9486–9489. (32) Pileni, M. P. Langmuir 1997, 13, 3266–3276. (33) Xu, S. J.; Chua, S. J.; Liu, B.; Gan, L. M.; Chew, C. H.; Xu, G. Q. Appl. Phys. Lett. 1998, 73, 478–480. (34) Liu, J.; Ma, J.; Liu, Y.; Song, Z.; Sun, Y.; Fang, J.; Liu, Z. J. Alloys Compd. 2009, 486, L40–L43. (35) Wang, J.; Shi, N.; Qi, Y.; Liu, M. J. Sol-Gel Sci. Technol. 2010, 53, 101–106. (36) Chen, D.; Gao, L. J. Colloid Interface Sci. 2004, 279, 137–142. (37) Chen, D.; Gao, L.; Zhang, P. Chem. Lett. 2003, 32, 996. (38) Liu, W.; Zhong, W.; Wu, X.; Tang, N.; Du, Y. J. Cryst. Growth 2005, 284, 446–452.
6010 DOI: 10.1021/la9045918
Zhang et al.
Experimental Section Materials. Cobalt(II) chloride hexahydrate (Panreac), cetyltrimethylammonium bromide (CTAB, BDH), sodium borohydride (NaBH4, Aldrich), hexane (Fisher), and 1-hexanol (Aldrich) were used as received. Poly(sodium 4-styrenesulfonate) (PSS, Aldrich) with a weight-average molecular weight of 70 000 g mol-1 was purified through recrystallization in anhydrous ethanol. A homemade cylindrical stainless steel autoclave (height: 80 mm; diameter: 30 mm) with a Teflon inner wall (thickness: 2 mm) was used in all experiments. Synthesis of CTAB-Coated Cobalt Nanocrystals. A 0.45 mL stock solution of CoCl2 3 6H2O (0.5 mol/L) was mixed with an organic solution containing CTAB (1.64 g), 1-hexanol (1.5 mL), and hexane (4.5 mL) under sonication (International Laboratory USA, AS10200A ultrasonic cleaner, medium power) for about 30 min to form a clear reverse micellar solution. An excess amount of NaBH4 powders (0.8 mmol, Co2þ/BH4- molar ratio = 2:7) was added into the solution under sonication for about 5 min at room temperature, giving a black dispersion. The colloidal dispersion was then transferred to an autoclave and heated at 165 °C for 8 h. To remove excess amounts of CTAB, the nanoparticle dispersion was washed three times with ethanol via magnetic separation of the nanoparticles and decantation of the supernatant. Synthesis of CTAB/PSS-Coated Cobalt Nanocrystals. The cobalt ion/polymer solution was first prepared by dissolving CoCl2 3 6H2O (53.5 mg) with a PSS aqueous solution (0.45 mL, PSS concentration = 0.5, 1, or 2 mg/mL). The aqueous solution was then mixed with an organic solution containing CTAB (1.64 g), 1-hexanol (2 mL), and hexane (7 mL) under sonication for 30 min to form a clear reverse micelle solution. An excess amount of NaBH4 (30 mg, 0.79 mmol) was added directly to the reverse micelle solution under sonication for about 5 min, giving a black dispersion. The colloidal dispersion was then transferred to an autoclave and heated at 165 °C for 8 h. In order to remove free surfactants and PSS, the nanoparticle dispersion was collected by a permanent magnet (∼0.4 T), followed by three repeated cycles with ethanol washing, magnetic separation, decantation, and redispersion. Measurements and Characterization. The crystallographic structure of Co nanocrystals was studied with a Rigaku DMAX/ RC X-ray diffractometer (XRD) using a Cu KR radiation (λ = 0.154 178 nm). The infrared spectrum was recorded on a Nicolet Avatar 360 FT-IR spectrophotometer using a KBr disk. UV-vis spectra of PSS and PSS-coated Co nanoparticles were obtained using a Shimadzu UV-2101PC spectrophotometer. The samples, either dissolved or dispersed in deionized water, were scanned at the wavelength of 200-400 nm. The zeta-potential of the nanocrystals was measured with a Malvern Zetasizer 3000HSA (Malvern, UK) using a 1.0 mM NaCl solution as a suspension fluid. The thermogram was obtained with a PerkinElmer thermogravimetry analyzer TGA7 (TGA) at a heating rate of 20 °C/min under a N2 atmosphere. Magnetic properties were characterized using a SQUID XL-7 magnetic property measurement system (Quantum Design Inc.) with applied fields from (50 kOe at 300 and 2 K. Transmission electron microscope (TEM) and highresolution transmission electron microscope (HRTEM) images were obtained from a JEOL JSM 2010 operating at an accelerating voltage of 200 kV. The sample was prepared by adding a drop of dilute nanoparticle dispersion (∼100 ppm) onto a carboncoated copper grid, followed by drying at room temperature in a dust-free chamber. The average size of the nanocrystals was determined by statistical analysis of at least 100 nanoparticles in the TEM images.
Results and Discussion The cobalt nanocrystals encapsulated with surfactant/polymer complexes were synthesized via a two-step process as illustrated in Scheme 1. Stable reverse micelles are first generated which contain Langmuir 2010, 26(8), 6009–6014
Zhang et al.
Article Scheme 1. Synthesis of Cobalt Nanocrystals with Surfactant/Polymer Complex Coatings
Scheme 2. Chemical Reactions of the Cobalt Ion Reduction with Sodium Borohydride
cobalt ions and PSS in the aqueous cores. The cobalt ions are then reduced with NaBH4 to generate amorphous cobalt nanoparticle precursors. Crystallinity of the precursor is subsequently enhanced through a hydrothermal treatment. In this synthetic approach, the creation of a thermally stable reverse micelle is a key step. The rationale of using PSS with a weight-average molecular weight of 70 000 g mol-1 is as follows: (1) The PSS is a water-soluble polymer that contains negatively charged sulfonate groups; thus, it is able to interact with Co2þ ions through charge interaction. Hence, the cobalt nanoparticles subsequently formed can anchor onto the polymer. (2) The negatively charged PSS molecules are able to interact strongly with the cationic CTAB to form a stable polyelectrolyte complex layer via the electrostatic interaction. As a result, the thermal stabilities of the CTAB molecules and the reverse micelles could be considerably enhanced. (3) The CTAB/PSS complexed layer can act as an oxygen impermeable shell to protect cobalt nanocrystals from oxidation. After forming the stable reverse micelles, a NaBH4 was added to reduce the Co2þ ions inside the micellar aqueous core to form Co nanoparticles. Scheme 2 shows the chemical reactions between cobalt ion and sodium borohydride.39 In our case, the sulfonate group of the PSS acts as a ligand (L) to form a complex with Co(II) ion. Thus, the resulting Co nanoparticles can anchor to the PSS. In fact, a similar approach to generate metallic nanoparticles onto polyelectrolyte molecules through NaBH4 reduction of the affixed metal complex has been recently reported by Ballauff and co-workers.40-42 The cobalt nanoparticles prepared via the above reduction process usually have low crystallinity. In order to generate highly crystalline Co nanoparticles, the as-formed nanoparticles were subjected to a hydrothermal treatment at 165 °C for 8 h. The hydrothermal treatment is a process in which both high temperature and high vapor pressure are employed to facilitate the dissolution and recrystallization of substances that are relatively insoluble under ordinary conditions. This method is widely used for crystal growth. However, the use of hydrothermal process for the growth of cobalt nanoparticles prepared in a reverse micelle system is quite difficult because the surfactant-assembled reverse (39) Petit, C.; Taleb, A.; Pileni, M. P. J. Phys. Chem. B 1999, 103, 1805–1810. (40) Sharma, G.; Mei, Y.; Lu, Y.; Ballauff, M.; Irrgang, T.; Proch, S.; Kempe, R. J. Catal. 2007, 246, 10–14. (41) Schrinner, M.; Ballauff, M.; Talmon, Y.; Kauffmann, Y.; Thun, J.; Moller, M.; Breu, J. Science 2009, 323, 617–620. (42) Proch, S.; Mei, Y.; Villanueva, J. R.; Lu, Y.; Karpov, A.; Ballauff, M.; Kempe, R. Adv. Synth. Catal. 2008, 350, 493–500.
Langmuir 2010, 26(8), 6009–6014
micelles are often unstable under such high temperature and pressure. Hence, our approach to create CTAB/PSS complex layer at the micellar interface of the reverse micelle could considerably improve the micellar stability (melting point of PSS is around 450 °C). As a result, the nanostructure of the CTAB/PSS-encapsulated nanoparticles can remain intact at high temperature and pressure during the hydrothermal treatment, thus allowing the cobalt nanoparticles to nucleate and recrystallize without agglomeration. It is known that PSS has a strong affinity toward metal ions through complexation between metal ion and sulfonate group.43 Thus, PSS concentration may affect the nanocrystal formation and morphology. When the Co nanocrystals were synthesized in the absence of PSS polymer, the nanoparticles produced had an average diameter of around 10 nm with a serious particle aggregation (Figure 1A). In contrast, when a small amount of PSS, equivalent to the molar ratio of Co2þ to -SO3- of 225:1, was added and subjected to the synthesis, well-dispersed Co nanocrystals with an average diameter of 3.7 ( 0.7 nm were generated (Figure 1B). Increasing the amount of PSS to a molar ratio of Co2þ to -SO3- of PSS equal to 103:1 gave a comparable size of the nanoparticles. Figure 1C shows TEM micrograph of the cobalt nanoparticles which are well dispersed on the substrate without agglomeration. Statistical analysis of the nanocrystals in the TEM image indicates that they have an average size of 3.5 ( 0.5 nm (Supporting Information, Figure S1). Since Co nanocrystals are known to exist in three polymorphs, the face-centered cubic (fcc), hexagonally close packed (hcp), and epsilon (ε) phases,11 high-resolution TEM (HRTEM) and X-ray diffraction (XRD) were employed to elucidate the crystallography of the CTAB/PSS-encapsulated Co nanocrystals. Figure 1C (inset) reveals the atomic lattice fringes of the nanocrystal, indicating the formation of highly crystalline cobalt nanocrystals. The calculated planar spacing of the fringes is 2.02 A˚, which is in a good agreement with the (002) or (101) crystal plane of hcp Co.11 The XRD spectrum shown in Figure 2 also confirms the hcp structure since the XRD pattern of the CTAB/PSS Co nanocrystals is consistent with the standard Co crystal structure data (ref code: 00-005-0727) reported in the literature.44 Furthermore, calculations of lattice spacing based on the XRD pattern of the nanocrystals match well with the hcp Co nanocrystals (Supporting Information, Table S1). The above results suggest that pure hcp Co nanocrystals have been produced. Further increasing the amount of PSS up to a molar ratio of Co2þ to -SO3- of 52:1 could also produce nanocrystals with an average diameter of 3.5 ( 0.6 nm (Figure 1D). However, some faint fiberlike materials were observed, which may be due to the presence of CTAB/PSS complexes in solution. These complexes (43) Sui, Z. M.; Chen, X.; Wang, L. Y.; Xu, L. M.; Zhuang, W. C.; Chai, Y. C.; Yang, C. J. Physica E 2006, 33, 308–314. (44) Lisiecki, I.; Walls, M.; Parker, D.; Pileni, M. P. Langmuir 2008, 24, 4295– 4299.
DOI: 10.1021/la9045918
6011
Article
Zhang et al.
Figure 1. TEM images of Co nanocrystals synthesized in the presence of different amounts of PSS: (A) in the absence of PSS, the average diameter is 10.0 ( 2.5 nm; (B) Co2þ to -SO3- of PSS (molar ratio) = 225:1, the average diameter is 3.7 ( 0.7 nm (CTAB/PSS-1); (C) Co2þ to -SO3- of PSS (molar ratio) = 103:1, the average diameter is 3.5 ( 0.5 nm (CTAB/PSS-2) (inset with a scale bar of 5 nm is the HR-TEM image of a single Co nanocrystal); (D) Co2þ to -SO3- of PSS (molar ratio) = 52:1 (CTAB/PSS-3), the average diameter is 3.5 ( 0.6 nm (some fiberlike materials are observed).
Figure 2. XRD patterns of CTAB/PSS-coated Co nanocrystals.
may appear like tubular materials during drying. Results from this study suggest that variation of Co2þ to PSS ratio has little effect on the particle size and morphology. In fact, the particle size should be controlled by the water to surfactant ratio, which determines the micellar size. The chemical structures of the Co nanocrystals were identified with FTIR spectroscopy. Figure 3 displays the FTIR spectra of CTAB, PSS, and CTAB/PSS-encapsulated Co nanocrystals. Comparing the spectrum of the Co nanocomposites with pure CTAB and PSS, characteristic peaks of the PSS polymer at 1186 and 1128 cm-1 (stretching vibrations of SO3-) and CTAB 6012 DOI: 10.1021/la9045918
Figure 3. FTIR spectra of (A) CTAB, (B) CTAB/PSS-encapsulated Co nanocrystal (CTAB/PSS-2), and (C) PSS.
molecule at 2855 and 2925 cm-1 (symmetric and asymmetric stretching of -CH2- vibrations of alkyl chains) were found.43 These results suggest that the Co nanocrystals contain both PSS matrix and CTAB molecules. The presence of the PSS was also confirmed with the UV-vis spectroscopy since the PSS molecule has a strong absorption peak at 225 nm in water. Figure 4 illustrates that the CTAB/PSS-encapsulated Co nanocrystals display Langmuir 2010, 26(8), 6009–6014
Zhang et al.
Figure 4. UV-vis spectra of CTAB/PSS-encapsulated Co nanocrystals and poly(sodium 4-styrenesulfonate) in water.
Figure 5. (A) TGA thermograms of CTAB (dotted line), PSS (solid line), and CTAB/PSS complex (dashed line). The complexes were obtained through physically mixing a CTAB and a PSS solution with a CTAB to PSS weight ratio of 2:1). (B) CTABcoated Co nanocrystals (dashed line) and CTAB/PSS-encapsulated Co nanocrystals (solid line).
an absorption peak, which is similar to the PSS molecule, thus indicating the existence of the PSS on the Co nanocrystals. The presence of CTAB molecules on the Co nanocrystals was confirmed with zeta-potential analysis. When the Co nanocrystals were dispersed in a 1 mM NaCl solution, a positive zeta-potential magnitude (22.5 ( 0.7 mV) was obtained. The positive surface charge must be attributed to the quaternary ammonium ions of the CTAB molecules, which may assemble to a bilayer structure on the nanocrystal surface in water. The thermal stabilities of CTAB, PSS, and Co nanocomposites (CTAB/PSS-2) were examined with a thermogravimetry analyzer (TGA). Figure 5A shows that CTAB molecules started to decompose at 300 °C and completely decomposed at 351 °C (dotted line). But the PSS molecules only started to decompose at 520 °C (solid line) (initial 20% weight loss at 163 °C was due to the presence of water molecules). These results indicate that thermal stability of the PSS is much higher than that of the CTAB molecules. To examine the thermal stability of the CTAB/PSS Langmuir 2010, 26(8), 6009–6014
Article
complexes, the CTAB and PSS solutions were physically mixed in a 2:1 weight ratio. The thermogram of the CTAB/PSS complexes shows that the weight loss up to 350 °C is attributed to the loss of water and free CTAB molecules, while the subsequent weight loss between 350 and 580 °C represents the loss of the complexed CTAB. The weight loss after 580 °C is due to the degradation of the PSS molecules. These results clearly suggest that the CTAB/ PSS reverse micelles have much higher thermal stability than the CTAB reverse micelles. Figure 5B compares the TGA thermograms of the CTAB and CTAB/PSS-encapsulated Co nanocrystals. For CTAB-coated Co nanocrystals, there was a 25% weight loss up to 500 °C. Such weight loss was probably due to the loss of adsorbed water molecules and decomposition of CTAB coatings. In contrast, the CTAB/PSS-encapsulated Co nanocrystals only had a 10% weight loss up to 900 °C. The inset diagram indicated that the first 1% weight loss at up to 181 °C was due to the presence of moisture in the sample. The second step of the weight loss between 181 and 422 °C, which accounted for 5% of the weight might be due to the uncomplexed CTAB molecules. The continuous weight loss up to 580 °C contributed to the loss of complexed CTAB molecules. These results confirm that the CTAB/PSS-encapsulated Co nanocrystals possess a higher thermal stability than the CTABcoated Co nanocrystals, thus preventing the Co nanocrystals from aggregation during the hydrothermal treatment. For bulk magnetic materials, it is well-known that there is a multidomain structure where regions of uniform magnetization are separated by domain walls. When the particle size is reduced to a critical diameter (Dc), typically in the range of a few tens of nanometers, a single domain state becomes favorable. The value of Dc for hcp Co is reported to be 15 nm.45 Therefore, the nanocrystals synthesized via our route can be regarded as a single-domain particle. The magnetic anisotropy energy of the single-domain particle, which is responsible for holding the magnetic moments along a certain direction, can be expressed as E(θ) = KeffV sin2 θ, where V is the particle volume, Keff is the anisotropy constant, and θ is the angle between the magnetization and the easy axis. The energy barrier KeffV separates the two energetically easy directions of magnetization. When the particle volume (V ) becomes very small, thermal energy (kBT ) is sufficient to overcome the anisotropy barrier (KeffV ) of orientation of a well-isolated single domain particle. Thus, the magnetization is easily flipped. When kBT > KeffV, a single nanoparticle behaves like a giant paramagnet and shows superparamagnetism.12 In contrast, when the temperature is decreased to a value at which the thermal energy (kBT ) is lower than the anisotropy energy (KeffV ), the nanoparticles exhibit ferromagnetism. To characterize the superparamagnetic property of the CTAB/ PSS-encapsulated nanocrystals, the field-dependent magnetization and magnetic hysteresis of the nanocrystals were examined at 300 and 2 K by applying external magnetic fields from (50 kOe using a superconducting quantum interference device. Results in Figure 6A show that the saturation magnetization (Ms) value of the CTAB/PSS-coated nanocrystals is 95 emu/g at 300 K, which is lower than that of the bulk cobalt materials (Mbulk = 166 emu/g). A lower Ms value may be due to the presence of more structural distortion on the particle surface in nanoscale materials.12,45 Figure 6A also shows that there is no hysteresis loop at 300 K, suggesting that the nanocrystals exhibit a superparamagnetic behavior.45 At low temperature (2 K), the nanocrystals exhibit ferromagnetism (i.e., hysteresis loop), and they have a magnetic remanence of 29 emu/g and a coercivity of 659 Oe. (45) Batlle, X.; Labarta, A. J. Phys. D: Appl. Phys. 2002, 35, R15–R42.
DOI: 10.1021/la9045918
6013
Article
Zhang et al. Table 1. Comparison of Oxidative Stability of the CTAB-Coated and CTAB/PSS-Encapsulated Co Nanocrystals in Terms of Their Magnetic Properties CTAB-coated Co nanocrystalsa measurement
CTAB/PSS-coated Co nanocrystals
freshly after freshly prepared 4 months prepared
after 4 months
saturation magnetization 103 13 95 88 at 300 K (emu/g) saturation magnetization 112 22 103 94 at 2 K (emu/g) magnetic remanence 31 2.8 29 22 at 2 K (emu/g) coercivity (Oe) at 2 K 883 564 659 526 a Room temperature and low temperature magnetization curves of CTAB-coated Co nanocrystals are provided in the Supporting Information S2 and S3.
unlike other surface coating methods which may have many defects due to lose coatings, our polymer encapsulation within confined micellar core enables the coating of entire cobalt nanoparticles once they are formed inside the micellar core. The resulting compact and oxygen impermeable polymer encapsulation provide effective sheath to reduce oxygen penetration to the surface of cobalt nanoparticles. Figure 6. Room and low temperature magnetization measurements: (A) freshly prepared CTAB/PSS-encapsulated Co nanocrystals and (B) CTAB/PSS-encapsulated Co nanocrystals after storage at room temperature in air for 4 months.
Metallic magnetic nanoparticles coated with single or double layer(s) of surfactant or polymer are not good enough barrier to prevent oxygen diffusion to the surface of the cobalt nanoparticles, thus forming antiferromagnetic cobalt oxide. To evaluate the effectiveness of our coating strategy against oxidation during storage, the magnetization values at 300 and 2 K of the CTABcoated and the CTAB/PSS-encapsulated Co nanocrystals before and after storage for 4 months under air at room temperature are compared (Table 1). The Ms values of the freshly prepared CTAB-coated Co nanocrystals are 103 and 112 emu/g at 300 and 2 K, respectively, while the Ms values for freshly prepared CTAB/PSS-encapsulated Co nanocrystals are 95 and 103 emu/g at 300 and 2K, respectively. The slightly lower Ms values of the CTAB/PSS nanocrystals may be attributed to the more pronounced surface distortion effect due to their smaller sizes (average 3.5 nm) than the CTAB-coated nanocrystals (∼10 nm). Upon exposure of both types of the nanoparticles in air at room temperature for 4 months, the Ms value of CTAB-coated Co nanoparticles decreased significantly from 103 to 13 emu/g (measured at 300 K), indicating that the CTAB-coated Co nanoparticles are not air-stable, forming nonmagnetic cobalt oxide. On the other hand, the Ms value of the Co nanoparticles with CTAB/PSS shells (measured at 300 K) is only slightly reduced from 95 to 88 emu/g, as illustrated in Figure 6B. This impressive protection ability may be attributed to the fact that,
6014 DOI: 10.1021/la9045918
Conclusion In summary, a novel approach to the synthesis of highly crystalline, well-dispersed, and oxidatively stable Co nanocrystals has been developed based on the hydrothermal microemulsion technique. Well-dispersed hcp Co nanocrystal coated with a surfactant/polymer complex layer has been produced with average diameters in the range of 3.5-3.7 nm. These nanocrystals exhibit superparamagnetism at room temperature with a saturation magnetization (Ms) of 95 emu/g, and they are air-stable. The novel aspect of this synthetic approach is the creation of a stable micellar complex; thus, the micellar nanostructure can remain intact under high temperature and pressure during the hydrothermal process. As a result, highly crystalline Co nanocrystals with a narrow particle size distribution can be produced. In addition, the presence of PSS polymer in the micellar core can fully encapsulate the newly generated cobalt nanoparticles, thus forming an oxygen-impermeable sheath. Acknowledgment. We gratefully acknowledge The Hong Kong Polytechnic University Postdoctoral Fellowship Scheme (Projects G-YX56 and G-YX0E) for their financial support on this research. Supporting Information Available: Particle size distribution and lattice spacings of CTAB/PSS-encapsulated Co nanocrystals as well as magnetic properties of CTAB-coated Co nanocrystals. This material is available free of charge via the Internet at http://pubs.acs.org.
Langmuir 2010, 26(8), 6009–6014
