Synthesis and Characterization of Colloidal Cobalt Nano-particles

1. Organophosphine Superhydride Oleic Acid Organic Solvent Metal NP Metal (aq.) Metal Salt ~200°C Nitrogen Purge ~100°C Nitrogen Purge Organic Cap Layer Synthesis and Characterization of Colloidal Cobalt Nano-particles Abhishek Singh, Dr. Gregory L. Young, Dr. Kiumars Parvin, Dr. David Bruck San Jose State University, San Jose, CA Introduction Nano-technology is defined as a field of science in which the goal is to control individual atoms and molecules to create materials and products which are thousands of times smaller than current technologies permit [1]. In physical dimensions, nano-technology encompasses dimensions of 100 nanometers and smaller. A subset of nano-technology is magnetic nanotechnology. This field focuses on magnetic nano-materials and their applications. Magnetic materials at the nano-scale have been studied longer than any other type of material [2]. The applications of magnetic nano-technology are diverse and immense including: data storage, DNA sequencing, drug delivery, biomedical sensors, magnetic resonance imaging, radiation therapy, nano-electronics, military weapons and defense. An example is the utilization of gold-coated iron, nickel, and cobalt ferromagnetic nano-particles for detoxification of soldiers exposed to toxins. The magnetic nano-particles are injected into the human body and bind with the toxins [3]. Then, a magnetic field is used to extract the toxins bonded to the nano-particles [3]. Another example is the utilization of magnetic nano-particles in cancer treatment. Hysteresis is the result of magnetized remnants that persist after an applied magnetic field is removed. Each cycle of applying a magnetic field generates a hysteresis loop, which has an associated energy loss proportional to the area of the hysteresis loop [4]. Magnetic nano-particles with an effective coercivity can be utilized at a cancerous site in the body. Application of an alternating magnetic field will result in selective warming at the cancerous site due to hysteretic energy loss [2]. This increase in temperature can be used to increase the effectiveness of chemotherapy and radiation based therapies [2]. Characterization Methods Characterization is critical to ensure that the synthesis procedure is yielding nano-particles that are of targeted measures. Physical characterization of magnetic nano-particles includes x-ray diffraction (XRD) and transmission electron microscopy (TEM). XRD allows the determination of elemental identification, which verifies that nano-particles are existent in the resulting product of synthesis [7]. A transmission electron microscope is utilized to capture images of the magnetic nano-particles, which are subsequently utilized for sizing and distribution analysis. TEM analysis is critically dependent on the quality of sample preparation. Improper sample preparation can lead to futile TEM analysis. Magnetic analysis is critical for complete characterization of magnetic nano-particles. In this study, magnetic analysis will be conducted using a vibrating sample magnetometer (VSM). A VSM consists of an electromagnet, which consists of a soft magnetic core surrounded by wires, pick-up coils mounted at the pole faces, and a rod where the sample is mounted on the end [8]. The rod is vibrated at a frequency using the cone of a loudspeaker [8]. Another set of coils is coupled to a fixed magnetic specimen, which serves as a reference to the lock-in detector [8]. The VSM generates voltage readings which are proportional to the magnetization and volume of the sample [8]. Varying the applied field allows one to generate a hysteresis loop. Capturing hysteresis behavior of the magnetic nano-particles serves to provide characteristic parameters such as: saturation magnetization, remnant magnetization, coercivity, hysteretic square-ness, coercivity square-ness, and energy loss of a material. Experimental Setup Figures 2, 3, 4 display the synthesis and anti-chamber, transmission electron microscope, and vibrating sample magnetometer, respectively, used for characterization in this research. XRD characterization was performed courtesy of Alza Corporation. Figure 2. Synthesis and Anti-Chamber Figure 3. TEM Figure 4. VSM Results from Analysis Figures 5 is a TEM micrograph of Cobalt nanoparticles. The particles where synthesized from a cobalt chloride salt. Figure 6 is an XRD spectra that confirms the particles synthesized are cobalt. Significance The applications and utilization of magnetic nano-particles and nano-technology is clearly a vast field of opportunity. However, in order to make use of magnetic nano-particles into a viable product it is essential that synthesis and characterization can be accomplished at a high level of precision and accuracy. For all nano-particle applications, magnetic or otherwise, it is critical to be able to control physical properties such as size, structure, shape, and self-assembly. For magnetic nano-particle applications, it is further necessary to characterize magnetic properties including: saturation magnetization, coercivity, and hysteretic behavior. Only when these properties are characterized and demonstrated repeatedly can magnetic nano-technology become viable on an industrial scale. Research Objectives The overall goal of this research is to synthesize magnetic cobalt nano-particles and characterize their structural and magnetic properties. The primary objective is to characterize average nano-particle size as a function of surfactant. The secondary objective is to characterize size distribution as a function of molar ratio of surfactant-to-reagent. The tertiary objective is to analyze magnetic behavior as a function of average nano-particle size and temperature. Figure 5. Co nanoparticles 8 nm to 15 nm in size Figure 6. XRD spectra of synthesized Co nanoparticles And Co standard Research Justification The work of several researchers has suggested that variation in choice of surfactant used in synthesis can yield a resultant variation in size, with bulkier surfactants yielding smaller nano-particles. Literature has also suggested that increasing the ratio of surfactant-to-reagent can affect sizing distribution in nano-particles. Thus far, however, there has been no formal experiment conducted in which the two parameters, type of surfactant and surfactant-to-reagent molar ratio, have been varied and the subsequent results presented. Magnetic analysis work thus far has shown that a compensation temperature or blocking temperature is existent, above which synthesized nano-particles exhibit paramagnetic behavior and below which they exhibit hysteretic ferromagnetic behavior. Literature has also shown increases in coercivity with size. Experimental work in which a formal characterization of magnetic behavior as a function of average nano-particle size and temperature has yet to be conducted. Figure 7 shows the size distribution of the cobalt nanoparticles. The range of particles is between 5 nm to 10 nm Synthesis Method Current research has shown that solution-based high-temperature synthesis based on oxidation-reduction of a metallic salt is the pre-dominant method for synthesizing mono-disperse cobalt nano-particles [4, 5]. This technique involves the addition of reagents into a homogeneous solvent at elevated temperatures, which serves to provide discrete nucleation sites and allow size control [4]. The resultant nano-particles are composed of an inorganic crystalline core surrounded by an organic monolayer, which prevents oxidation and conglomeration of the nano-particles [4]. Figure 1 displays the typical reaction. Recent experiments have shown results of cobalt nano-particles ranging in size from 5-30nm with standard deviations of 5-15% [4,5,6]. Figure 7: Co particle size distribution References 1. [Online]. Available at http://www.webopedia.com/TERM/N/nanotechnology.html (accessed 16 February 2006). 2. K. O’Grady, “Biomedical Applications of Magnetic Nanoparticles,” Journal of Physics D: Applied Physics, Vol. 36, Issue 13 (July 2003). 3. Strem Chemicals, Inc., Nanomaterials for Defense and Security, [Online]. Available at www.strem.com (accessed 21 February 2006). 4. Y.K. Su, C.M. Shen, T.Z. Yang, H.T. Yang, H.L. Li and H.J. Gao, “Synthesis and Characterization of Monodisperse Cobalt Nanocrystals and Nanocrystalline Superlattices,” Advanced Nanomaterials and Nanodevices IUMRS-ICEM, pp. 486-500 (June 2002). 5. V.F. Puntes, K.M. Krishnan and A.P. Alivisatos, “Colloidal Nanocrystal Shape and Size Control: The Case of Cobalt,” Science, Vol. 291, pp. 2115-2117 (March 2001). 6. C.B. Murray, S. Sun, W. Gaschler, H. Doyle, T.A. Betley and C.R. Kagan, “Colloidal Synthesis of Nanocrystals and Nanocrystal Superlattices,” IBM Journal of Research and Development, Vol. 45, No. 1, pp. 47-56 (January 2001). 7. B.E. Warren, X-Ray Diffraction, (Dover Publications, Inc., New York, 1990), pp. 7-12, 27-35. 8. R.L. Comstock, Introduction to Magnetism and Magnetic Recording, (John Wiley & Sons, Inc., New York, 1999), pp. 52-56. Experimental Program The experiments in this research will be conducted based on a 2 x 2 full-factorial design. The design variables of this experiment are type of surfactant and surfactant-to-reagent molar ratio. Two surfactants will be evaluated at two different surfactant-to-reagent molar ratios, yielding a total of four unique conditions. Duplicate trials will be performed for each unique condition to prove repeatability, yielding a total of 8 synthesis runs. For these 8 runs XRD, TEM, and VSM characterization will be performed. Acknowledgements DARPA Grant #: HR0011-05-0046 NSF-RUI #: DMR-0514068 Alza Corporation Figure 1. Reduction of a metal-salt to metal nano-particles.