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J. Phys. Chem. C 2009, 113, 3426–3429
Preparation of Nearly Monodisperse Nickel Nanoparticles by a Facile Solution Based Methodology and Their Ordered Assemblies Deepti S. Sidhaye,† Tanushree Bala,† S. Srinath,‡,§ H. Srikanth,‡ Pankaj Poddar,† Murali Sastry,*,†,⊥ and B. L. V. Prasad*,† Materials Chemistry DiVision, National Chemical Laboratory, Pune-411 008, India, and Department of Physics, UniVersity of South Florida, Tampa, Florida 33620 ReceiVed: August 23, 2008; ReVised Manuscript ReceiVed: December 29, 2008
Nickel nanoparticles have been synthesized by a simple wet chemical reduction technique in the presence of a surfactant sodium dodecyl sulfate (SDS) and a capping agent oleic acid. Varying the concentration of the surfactant and the capping agent has been found to critically govern the nature of the nanoparticles prepared. It is observed that at an optimum concentration of SDS and oleic acid, nearly monodisperse Ni nanoparticles are obtained that form highly ordered hexagonally close-packed structures on electron microscopy grids by a simple drop coating procedure. It is also shown that the presence of oleic acid is necessary for the stability of the Ni nanoparticles synthesized. Structural and magnetic characterizations of the nanoparticles are also presented. Introduction There is plenty of action in the research world that is aimed at developing new methods to prepare different kinds of magnetic nanoparticles. These include methods for preparing metal nanoparticles,1 metal oxides like iron oxide,2 alloys containing magnetic components,3 core-shell systems,4 and ferrites.5 Among these, nickel and cobalt nanoparticles are important magnetic materials and are expected to find applications in diverse areas6 including interesting magnetocaloric properties for spot cooling of microelectronic devices.6d Different groups have already reported the formation of Co and Ni nanoparticles, but most of these methods use organometallic precursors which undergo reactions like decomposition at high temperatures.7 Other successful routes involve the utilization of microemulsions/reverse micelles8 and soft templates.9 Obtaining ordered arrangements of nanoparticles is a prerequisite for many of the envisaged applications of these systems. However, such arrangements are not commonly observed from nanoparticles synthesized by the aqueous medium based methods. Further, if the nanoparticles are monodisperse in nature, it would be an added advantage from the application point of view.10 Therefore, development of a simple wet chemical process for synthesizing nearly monodisperse Ni and Co nanoparticles that self-assemble to form ordered structures has excellent potential for applications. Herein, we report a protocol based on metal reduction to synthesize nearly monodisperse nickel as well as cobalt nanoparticles in aqueous medium, where the fidelity of the reaction is seen to depend on the selection of surfactant and cosurfactant concentration. We found that sodium dodecyl sulfate and oleic acid serve the required purpose, and when they are taken at * Authors to whom correspondence should be addressed. E-mail:
[email protected] (M.S.);
[email protected] (B.L.V.P.). Phone: 91-20-25902013. Fax: 91-20-25902636. † National Chemical Laboratory. ‡ University of South Florida. § Current address: School of Physics, University of Hyderabad, Hyderabad, India. ⊥ Current address: Tata Chemicals Limited, Pune 411 045, India.
appropriate concentration ratios, nearly monodisperse nanoparticles readily form and further lead to the assembly of highly ordered structures as demonstrated through the transmission electron microscopy images. While as-prepared systems did not show any discernible X-ray diffraction pattern, after heating the powders display clear XRD patterns corresponding to the fcc phase of these metals. We found that the presence of oleic acid is very important for the stability of nanoparticles. As the results obtained for nickel and cobalt nanoparticles follow exactly the same trends, we report herein results for the nickel system for brevity and focused discussion of the results. Presented below are the details of the investigation. Materials and Methods Materials. Nickel nitrate hexahydrate (Ni(NO3)2 · 6H2O), sodium dodecyl sulfate (SDS), oleic acid (9-octadecenoic acid), and sodium borohydride (NaBH4) were purchased from Sigma Aldrich and used as-received. Preparation of Nickel Nanoparticles. In a typical experiment, an aqueous mixture of 1 × 10-3 M Ni(NO3)2 · 6H2O was taken with the following combination of sodium dodecyl sulfate (SDS) and oleic acid (cis-9-octadecenoic acid): (A) 1 × 10-2 M SDS and 1 × 10-4 M oleic acid, (B) 5 × 10-4 M SDS and 5 × 10-6 M oleic acid, (C) 5 × 10-2 M SDS, without oleic acid, and (D) 1 × 10-4 M oleic acid without SDS. Total volume of the aqueous mixture was maintained to be 100 mL. Please note that we use methanolic solution of oleic acid as it is not soluble in water by itself. In all above combinations of Ni2+ ions, SDS, and oleic acid, addition of sodium borohydride (∼0.025 g) led to the reduction of metal ions. The color of the solution turned black almost immediately after the addition of the reducing agent. The solutions were kept at ambient conditions for 1 h to ensure the completion of the reaction. The solutions were then subjected to repeated centrifugation at 9000 rpm for 20 min followed by the separation of supernatant and pellet. The pellet thus obtained was washed by a copious amount of deionized water and recentrifuged under the same conditions. The pellet received after the second centrifugation was used for characterization such as TEM, XRD, and magnetic measure-
10.1021/jp807542w CCC: $40.75 2009 American Chemical Society Published on Web 02/06/2009
Nearly Monodisperse Ni Nanoparticles
J. Phys. Chem. C, Vol. 113, No. 9, 2009 3427
Figure 1. Photograph displaying the solutions A, B, C, and D (the sequence is same as mentioned in Materials and Methods) of Ni nanoparticles.
ments (solution pH ∼8) Also, the dried pellet from solution A was heated at 300 °C for 30 min and then characterized by the above-mentioned techniques. Instrumental Details. TEM and Electron Diffraction Measurements. Samples for transmission electron microscopy (TEM) and selected area electron diffraction (SAED) analyses were prepared by putting a drop of the nickel nanoparticle solution on a carbon-coated copper grid, allowing the solution to dry up. TEM analysis was performed on a JEOL model 1200EX instrument operated at an accelerating voltage of 120 kV. HRTEM Analysis. For HRTEM analysis, the samples were prepared by drop casting the solution on carbon-coated copper grids, and measurements were performed on a Tecnai G2 F30 S-Twin (FEI; Super Twin lens with Cs ) 1.2 mm) instrument operated at an accelerating voltage at 300 kV having a point resolution of 0.2 nm and lattice resolution of 0.14 nm. XRD. XRD measurement of drop-coated film of the sample on glass substrate was carried out on an X’Pert Pro model from Panalytical Instruments operated at a voltage of 40 kV and a current of 30 mA with Cu KR radiation. FTIR. Fourier transform infrared (FTIR) spectroscopic measurements were carried out on the samples after mixing them with potassium bromide. The measurements were carried out on a Perkin-Elmer-Spectrum One FTIR spectrometer operated at a resolution of 4 cm-1. The FTIR spectrum of pure oleic acid and SDS were also measured for comparison. ThermograWimetric Analysis. Thermogravimetric analysis (TGA) of the dried powdered samples was carried out on a TGA-7 Perkin-Elmer instrument in the temperature range 30-800 °C at a scan rate of 10 °C/min. Magnetic Measurements. The magnetic measurements were performed using a commercial Physical Property Measurement System (PPMS) from Quantum Design. The magnetic field was maintained at 1000 Oe for both field-cooled (FC) and zerofield-cooled (ZFC) measurements of the samples. Results and Discussion After addition of NaBH4, all the solutions turned black irrespective of the chosen concentration of SDS and oleic acid as the Ni2+ ions are reduced by borohydride. However, there are vast variations in the stability of the Ni0 systems formed. The change in color of these solutions with time is quite interesting and shown in the photograph of Figure 1. For the sake of clarity in discussion, we label the solutions as A, B, C, and D. Solution A, containing both SDS (concentration just above its CMC; CMC of SDS ) 8.1 mM) and oleic acid, is found to be highly stable with no color change with time and so is solution D, where only oleic acid is present. On the other
Figure 2. (A1, A2) Representative TEM images recorded from the solution A showing spontaneous ordered arrangement of nickel nanoparticles. (B) Agglomerated Ni particles precipitated out from solution B where SDS concentration is below the CMC. (D) TEM picture showing polydisperse Ni nanoparticles synthesized in the presence of only oleic acid (solution D).
Figure 3. Representative TEM images obtained from redispersion of the heated sample. Inset of (B) shows HRTEM of the same sample depicting the crystalline nature of the sample.
hand, solution B (having SDS concentration less than CMC) shows appreciable amount of precipitate within 30-40 min of the reaction, and in solution C (containing only SDS), fast decolorization occurs. The stable black color in the cases of solutions A and D clearly suggests the formation of stable dispersions, while in the cases of solutions B and C, the stability is an issue. Representative TEM images of Ni nanoparticles prepared by varying the concentration of SDS and oleic acid are shown in Figure 2. In this figure, A1 and A2 are the TEM micrographs from solution A and show highly monodisperse particles arranged in a nice hexagonal order. The average particle size is found to be ∼30 nm. The particles in Figure 2B are from solution B that show a high degree of coagulation as indicated by their precipitation in solution, while those in Figure 2D are from solution D. As can be seen from the image, these are highly polydisperse in nature. We could not get any image from the samples where only SDS was present (solution C) suggesting the complete degradation of the particles in these cases. We would like to mention here that further increase in the oleic acid concentration from 10-4 to 10-2 M rendered the solution turbid before the reduction itself and on addition of reducing agent led to unstable colloid formation and precipitation of the particles. Figure 3 depicts the TEM images of the heated sample clarifying that the particles remain independent (do not aggregate) and that there is no significant change in their size. The nanoparticles show marked improvement in the crystallinity as can be seen in the HRTEM image (inset of Figure 3B)
3428 J. Phys. Chem. C, Vol. 113, No. 9, 2009
Figure 4. (A) X-ray diffractogram of Ni nanoparticles before (curve 1) and after (curve 2) heating at 300 °C. (B) Thermogravimetric analysis of Ni nanoparticles prepared in the presence of oleic acid only (curve 1) and both SDS and oleic acid (curve 2). (C) FTIR spectra of pure SDS (curve 1), pure oleic acid (curve 2), and Ni nanoparticles capped by both SDS and oleic acid (curve 3).
showing d spacings corresponding to (111) planes for the fcc phase of nickel (JCPDS card no. 1-1258). The improved crystallinity of the heated sample is also clear from the XRD measurements that are presented in Figure 4. Figure 4A shows XRD spectra of Ni nanoparticles (solution IB) before (curve 1) and after heating (curve 2). The sample does not show any distinguishable peaks before heating. But the crystallinity is seen to improve dramatically after heating it at 300 °C. The peaks marked have d values matching the d spacings for (111), (200), and (220) planes of fcc nickel nanoparticles (JCPDS card no. 1-1258). TGA curves are shown in Figure 4B. Curve 1 in Figure 4B corresponds to Ni nanoparticles capped only with oleic acid (solution D). It shows that about 8% weight is lost at 250 °C with total weight loss of 20% including a small loss at ∼100 °C. Curve 2 is for Ni nanoparticles synthesized in the presence of both SDS and oleic acid (case IB). Here, the weight decreases by 30%. Figure 4C depicts the FTIR spectra of pure SDS (curve 1) and pure oleic acid (curve 2), along with Ni nanoparticles (sample IB) in curve 3. The carboxylic stretching mode of pure oleic acid molecule is observed at 1709 cm-1. In the spectrum of SDS, a sharp peak is observed at 1660 cm-1. These two peaks are absent in the spectrum of Ni nanoparticles capped by oleic acid and SDS. Instead, a single peak is observed at 1629 cm-1. The formation of nanoparticles can plausibly proceed via two different pathways. The first hypothesis is the formation of vesicles when SDS is taken above its CMC along with the oleic acid.11 The fact that oleic acid can form vesicular structure with anionic surfactants has been well documented in the literature.11 However, as the typical vesicle sizes are big (greater than 100
Sidhaye et al. nm), the metal particles, if formed inside these vesicular structures, are also expected to be in the range of 100 nm. Since the individual particles formed with SDS and oleic acid, revealed by the TEM images in our experiments, are much smaller than that, this mechanism may be ruled out. The second hypothesis is that SDS and oleic acid can form micellar structures. When the metal ions are added to these micellar structures, monodisperse aggregates of micelles form, and upon reduction of the metal ions the formation of nearly monodisperse nanoparticles is realized. The oleic acid present in the solution can then stabilize them immediately. However, at this present jucnture, it is difficult to conclusively speak in favor of one mechanism, and in situ studies like dynamic light scattering are needed to really discern these details; experiments toward that effect are underway. TGA studies and FTIR studies were undertaken to see whether in the final system of monodisperse particles only oleic acid is present or both SDS and oleic acid are present. Our results clearly suggest that stable, nearly monodisperse particles form only when the SDS is present above its CMC (solution A). Taking the concentration of SDS above its CMC is sufficient because even with further increase in its concentration, the stability and size of the nanoparticles remain the same. But when the SDS is below CMC (solution B), the particle stability, as well as monodispersity, is compromised. Here, as the concentration of SDS is below CMC, micelle formation may not be well alleviated, and hence they lead to coagulation of the particles. When only oleic acid is present, the particles are stable but are highly polydisperse. The nanoparticles obtained from solution A are stabilized/capped by both SDS and oleic acid as proved by the thermogravimetric analysis (TGA) of the samples, as these nanoparticles show more weight loss as compared to the only oleic acid capped/stabilized ones. The excess weight loss in curve 1 can be attributed to the presence of SDS molecules. The possibility of weight loss due to unbound SDS can be ruled out as the samples are washed thoroughly several times with water. The main role of oleic acid seems to be that of a capping agent12 of these magnetic nanoparticles. The shift in carboxylic stretch of oleic acid and disappearance of olefinic C-H band (observed at 3009 cm-1 for oleic acid) indicate an interaction of these groups with the nanoparticles. The importance of oleic acid in stabilizing the nanoparticles is exemplified by the observation that when no oleic acid is present in the experimental recipe, a complete degradation of the nanoparticles occurs within 30-40 min of their preparation. The presence of only oleic acid is sufficient to stabilize the nanoparticles, but the monodispersity and ordered arrangements of the nanopar-
Figure 5. (A) Field dependent magnetization curves for Ni nanoparticles (before heating) above (300 K) and below (10 K) the blocking temperature (TB). Temperature dependent magnetization of the Ni nanoparticles in zero-field-cooled and field-cooled mode are given in the inset. (B) Field dependent magnetic measurements of the heated sample with the inset showing its temperature dependent magnetic measurements.
Nearly Monodisperse Ni Nanoparticles ticles can only be achieved in the presence of an adequate amount of SDS in the reaction mixture. Preparation of Ni nanoparticles by borohydride reduction can be susceptible to contamination with their respective borides.13 But it has been shown that when the reduction is carried out in ambient atmosphere, there is a greater chance of pure Ni particle formation,14 and our experiments also were carried out under similar conditions. It should be worth noting that simple heating for few minutes leads to the development of XRD peaks corresponding to pure metal phases. Such results have earlier been observed by Pileni et al.9a and Bawendi et al.7f In our case, small particles could be forming in the initial conditions which develop the crystallinity upon heating. The heating could also result in elimination of metal boride if present.13 The field dependent magnetization (M-H) measurements for Ni (sample A) before and after heat treatment and are shown in Figure 5A and 5B, respectively, with the temperature dependent magnetization curves as the insets. The zero-fieldcooled (ZFC) and field-cooled (FC) curves for Ni are typical of superparamagnetic particles.15 The peak observed at ∼14 K in the ZFC curve could be attributed to the blocking temperature (TB). Superparamagnetic nanoparticles should exhibit hysteresis below this temperature but not at room temperature, which is clearly supported by the field dependent magnetization studies. The field dependent magnetization curves display no hysteresis at 300 K, and a distinct loop is seen at 10 K. Also, the saturation magnetization is seen to increase in the case of the heated samples. At 10 K, the saturation magnetization values for the unheated and heated samples were ∼10 and ∼44 emu/g, respectively. Hence, heating of the nanoparticles is seen to lead to improved crystallinity and enhanced magnetic characteristics while the monodispersity is not compromised. Conclusion In conclusion, the formation of nearly monodisperse Ni nanoparticles by the soft templating action of SDS and oleic acid is presented. Simple drop coating of the nanoparticle solutions on a TEM grid leads to nice ordered assemblies. The choice of appropriate concentration combination of SDS and oleic acid plays a crucial role in this preparation procedure. Control experiments reveal that while SDS is necessary for the formation of monodisperse nanoparticles, oleic acid plays the role of stabilizing agent. Simple heating of the sample at 300 °C in air results in improved crystallinity and better magnetic properties. Acknowledgment. This work was funded by DST to set up a unit on Nano Science and Technology (DST-UNANST) at NCL. D.S.S. and T.B. thank the Council for Scientific and Industrial Research (CSIR), government of India, for a research fellowship. Work at USF is supported by NSF through grant no. CTS-0408933.
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