NANOPARTICLES *
CHAPTER 5 CHAPTER 5
5.2 SYNTHESIS AND CHARACTERISTICS
5.2.1 MAGNETIC NANOPARTICLES
5.2.1.1 Characteristics of magnetic nanoparticles
Among all kinds of nanomaterials, magnetic nanoparticles earn their position by their special features and wide-used applications. Compared to regular magnetic materials, nanoparticles differ from the domain structure to the classic quantum theory. Thus, they have a more advanced technology and more applications due to different physical and chemical properties.
The physical properties of magnetic nanoparticles can be determined by the chemical compositions, the type and the degree of defectiveness of the domain structure including the particle size and shape, and the interaction of the atoms among the molecular structure. However, due to the limitations of pre- sent technology, the above factors cannot be controlled all the time during the synthesis. Furthermore, the relationship between the properties and the structure is unknown, so the same type of material with different concentrations could have big differences.
Even though magnetic nanoparticles are basically metals with magnetism, living creatures also have magnetic nanoparticles within their bodies. For examples, migratory birds and fish have magnetic nanoparticles inside their sense system to guide them during their migration between south and north every year. Even human beings have magnetic nanoparticles inside their brains. It is estimated that the human brain includes about 100 million magnetic nanoparticles per gram of tissue. So magnetic nanoparticles are not just a science, but are related to our daily life and affect it in many different ways.
There are many kinds of magnetic nanoparticles due to different chemical properties: metals, rare earth metals, oxidation of metallic nanoparticles, and magnetic alloys. Since the metal nanoparticles include most of the metal magnetic materials and oxidation of metallic nanoparticles, the following section will be focusing on the different magnetic alloys (Gubin, 2009).
Fe–Co alloys: Since Co and Fe are body-centered cubic structures in a nanoparticle, the allied order of both metals is very soft and very suitable to be raw material for nanoparticles. The maximum con- centration of Co in the alloy is 35%. This is the saturation concentration of Co. The related magnetic properties also increase with the mixing level (Gubin, 2009).
Fe–Ni alloys: Samples of Fe and Ni in experiments can have nonmagnetic or magnetic soft ferro- magnets. For the alloy compound of iron and nickel, they have a much lower saturation magnetization compared to pure samples of each metal. For example, when we have 37% of Ni, it has a low curie point and an FCC structure. Theoretical calculations estimate a more complicated magnetic structure for these types of alloys due to the different combinations.
Fe–Pt alloys: Because of the wide application on the information recording density of materials, these types of alloys are being studied a lot recently. They have the face-centered tetragonal structure and thus obtain a unique property of recording advantage of large coercively when in room tempera- ture, no matter how small the particles are. Prepared by joint thermolysis in the presence of oleic acid and oleyl amine, Fe–Pt nanoparticles have a narrow size distribution. After further heating, a protective film is formed on the surface of alloys, which remain about the same size.
Co–Pt alloys: In high-density information storage field, nanoparticles of Co–Pt have a lot of advan- tages due to their form, size, and crystal structure, which makes them chemically stable and mag- netically crystalline. One of the many methods is the polyol method that does not use organ metallic
95 5.2 SyNThESIS ANd ChARACTERISTICS
precursors. Even though the same solution is processed, sometimes two different concentrations of alloys are formed with different structures, in this case, the different concentration can be studied.
Since most of magnetic materials are metal or metal oxides, the raw material is straight forward and can be easily found. However, the technology difficulties remain in the purity section. As men- tioned above, the different chemical properties are based on the structure and the concentration of the alloys, so the synthesis process has to be controlled very carefully. Furthermore, when it comes to the nanoscale, many of the classic physics laws are not applicable and quantum physics is required.
The synthesis of ferrofluid consists of synthetic procedures. Basically, it is the thermal decom- position of metal organic compounds or the thermal decomposition of monometallic metal organic compounds. Fig. 5.1 shows examples of magnetic nanoparticles for one simple structure and for metal alloy compounds.
From Fig. 5.1, we can see the Co particles prepared by thermal decomposition; the raw materials are octacarbonyldicobalt and dichlorobenzene.
Twenty years ago, Massart developed a method to synthesize magnetic ferrofluids based on the co-precipitation of Fe salts in aqueous solutions using repulsive electrostatic forces. The image of Fe oxidation product is slightly different from the Co product, as it is shown in Fig. 5.2.
As we can see from Fig. 5.2, the left side is the iron oxide product prepared in aqueous solution and the right side is the product formed from the toluene solution, which have oleic acid on the surface.
5.2.1.2 Synthesis method of magnetic nanoparticles
The synthesis of magnetic nanoparticles has been developed through over 30 years. The raw materials have been used from metal to nonmetal and from gas to liquid phases. The most commonly used metal FIGURE 5.1
Transmission electron microscopy (TEM) images at different magnifications of monodisperse Co particles on a carbon coated Cu grid.
From Giersig, M., & Hilgendorff, M. (2005). Magnetic nanoparticle superstructures. Germany: Wiley-VCH Verlag.
96 CHAPTER 5 NANOPARTICLES
oxides are the Fe, Co, Mg, and Mn with their alloy compounds. In the recent years, many experiments have been done on the control of shape, crystalline, and stable surface of magnetic nanoparticles. As mentioned earlier, the shape and orientation affect the chemical properties greatly. The most common way is co-precipitation, thermal synthesis, and microemulsion. The following section will focus on each of the techniques.
5.2.1.2.1 Coprecipitation
For most of the iron oxides, this is a very convenient way to process aqueous Fe salt solutions. By add- ing a base on normal temperature and pressure, the process could be controlled to get ideal shape and size of the magnetic nanoparticles. However, there are also other factors to consider with this kind of method: the Fe ratio, the reaction rate and temperature, and the pH value of the solution. These will also affect the smoothness of the reaction. After the preparations are done, the experiment will proceed to a point where the solution reaches magnetic saturation (Lu, Salabas, & Schth, 2007).
5.2.1.2.2 Thermal decomposition
In order to control shape and size more precisely, the method of thermal decomposition is developed.
This is a method similar to the synthesis of semiconductors with high-quality nanocrystals. The smaller size magnetic nanocrystals can be formed from organometallic compounds in organic solvents. By adding precursor in zerovalent, thermal decomposition will have metal formed in the end. When the decomposition happens, cationic metal will lead the electrons to the oxides and the reaction solution will have metal acid salts in non-aqueous solution. With the metal fatty acid compound in the solution, the metal will reach to saturation and the metal magnetic nanoparticles will precipitate out from the solutions.
FIGURE 5.2
Typical TEM images of iron oxide particles.
From Giersig, M., & Hilgendorff, M. (2005). Magnetic nanoparticle superstructures. Germany: Wiley-VCH Verlag.
97 5.2 SyNThESIS ANd ChARACTERISTICS
Fig. 5.3 demonstrates the process of thermal decompositions.
Fig. 5.3 shows the TEM images of synthesized nanocrystals at different reaction times. At the beginning, the crystals are in random arranged orders and with random size arrangements. After further heating, the order is clear at the end with regular size and shape.
5.2.1.2.3 Microemulsion
A microemulsion process is the process that two immiscible liquids will go through a dispersion stage where both liquids are stabilized by the surfactant molecules. For example, oil and water are two immiscible liquids. The disperse phase of oil will be surrounded by surfactant molecules. With the desired reactants in the solution, the micro droplets will form and then break to precipitate in the micelles. Acetone and ethanol can be used as solvent for micro emulsions to extract the precipitate by changing the structure. Fig. 5.4 shows the nanorod synthesis process (Lu et al., 2007).
From Fig. 5.4, we can see the different compounds and structures are formed by the different raw materials. The steroid acid products are straight structures, while the octanoic acid products are short and thick.