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Nanotechnology is expected to be the basis of many of the main technological innovations of the 21st century. The research and development in the field of nanotechnology is growing rapidly in most of the countries. The major output of this activity is the development of the new materials in the nanometer scale, which are nanoparticles. In nanotechnology, a particle is defined as a small object that behaves as a whole unit in terms of its transport and properties . The size of the particles is classified according to the diameter. Fine particles cover the range from 100 to 2500 nanometers and for the size between 1 and 100 nanometers is called ultrafine particles, also known as nanoparticles. Nanoparticles may or may not exhibit size-related properties that differ significantly from those observed in fine particles or bulk materials. Although the size of most molecules would fit into the above outline, individual molecules are usually not referred to as nanoparticles.
The transition from microparticles to nanoparticles can lead to a number of changes in physical properties. Two of the major factors in this are the increase in the ratio of surface area to volume, and the size of the particle moving into the realm where quantum effects predominate . The increase in the surface-area-to-volume ratio, which is due to the particle getting smaller, lead to the increase dominance of the behavior of atoms on the surface of a particle over that of those in the interior of the particle. This affects both the properties of the particle in isolation and its interaction with other materials. High surface area is a critical factor in the performance of catalysis and structures such s electrodes, allowing improvement in performcance of such technologies as fuel cells and batteries . Besides that, the large surface area of nanoparticles also leads to the interactions between the intermixed materials in nanocomposites, leading to special properties such as increased strength and/or increased chemical/heat resistance.
Nanoparticles are zero-dimensional nanostructures and are generally classified according to their composition: pure metal, metal oxides, noble metals, transition metals, magnetic metals and others . Like all nanostructures, the properties of nanoparticles are dependent on their size and shape. The variation of their properties at the nanoscale is not a result of a scaling factor, but stems from different causes in different materials. In semiconductor, it is due to the further confinement of the electronic motion to a length scale that is comparable to or less than the length scale of the electronic motion in bulk semiconductors. In noble metals, it results from the strong absorption of radiation within the visible region leading to the collective oscillation of the electrons in the conduction band, called surface plasmon resonance, from the surface of one particle to another. In transition metals, it arises from the large surface to volume ratio resulting to high chemical activities. And in magnetic metals, it is due to finite-size and surface effects, which become increasingly important as the particle size of the magnetic material is reduced.
Nanoparticles are of great scientific interest as they are effectively a bridge between bulk materials and atomic structures . For a bulk material, it has constant physical properties regardless of its size, but size-dependent properties are able to observe in nano-scale. Therefore, as the size of the particles approaching nanoscale and as the percentage of atoms at the surface of a material become more significant, the properties of materials change. The percentage of atoms at the surface is insignificant in relation to the number of atoms in the bulk of the material for bulk materials larger than one micrometer. The interesting and sometimes unexpected properties of nanoparticles are therefore largely due to the large surface area of the material, which dominates the contributions made by the small bulk of the material .
Magnetic nanoparticles are a class of nanoparticle which can be manipulating using magnetic field . Such particles commonly consist of magnetic elements such as iron, nickel, cobalt and their chemical compounds. Magnetic nanoparticles are of great interest for researchers from a wide range of disciplines, including magnetic fluids, catalysis, biotechnology/biomedicine, magnetic resonance imaging, data storage, and environmental remediation . There are a number of suitable methods that have been developed for the synthesis of magnetic nanoparticles with various different compositions; successful application of such magnectic nanoparticles in the areas listed above is highly dependent on the stability of the particles under a range of different condition. In most of the envisioned applicatios, particles will perform best when the size of the nanoparticles is below a critical value, which is depending on the types of materials but is typically around 10-20nm. Then each nanoparticle becomes a single magnetic domain and shows superparamagnetic behavior when the temperature is above the so-called blocking temperature . These individual nanoparticles have a large constant magnetic moment and it behave like a giant paramagnetic atom with a fast response to applied magnetic fields with a fast response to applied magnetic fields with negligible remanence (residual magnetism) and coercivity (the field required to bring the magnetization to zero). Superparamagnetic nanoparticles with these features are very attractive for a broad range of biomedical applications because the risk of forming agglomerates can be negligible at room temperature.
In spite of that, there is an unavoidable problem associated with particles in this size range is their intrinsic instability over longer periods of time. This nano-scale of particles tends to form agglomerates to reduce the energy associated with the high surface area to volume ratio of the nanosized particles. In addition, naked metallic nanoparticles are chemically highly active, and the particles are easily oxidized in the air, this results in the loss of magnetism and dispersibility of the nanoparticles. It is thus crucial to develop protection strategies to chemically stabilize the naked magnetic nanoparticles against degradation during or after the synthesis for many applications. These strategies comprise grafting of or coating with organic species, including surfactants or polymers, or coating with an inorganic layer, such as silica or carbon . It is remarkable that in many cases, the protecting shells not only stabilize the nanoparticles, but it can also be used for further functionalization with other nanoparticles or various ligand, depending on the desired application.
Functionalized nanoparticles are very promising for applications in catalysis, biolabeling, and bioseparation. Especially in liquid-phase catalytic reactions, such small and magnetically separable particles may be useful as quasihomogeneous systems that combine the advantages of high dispersion, high reactivity, and easy separation. In the following, after briefly addressing the magnetic phenomena specific for nanoparticles, we focus mainly on recent developments in the synthesis of magnetic nanoparticles, and various strategies for the protection of the particles against oxidation and acid erosion. Further functionalization and application of such magnetic nanoparticles in catalysis and bioseparation will be discussed in brief. Readers who are interested in a more detailed treatment of the physical properties and behavior of these magnetic nanoparticles, or biomedical and biotechnology applications, are referred to specific reviews.
Application using Magnetic Nanoparticles
In these recent years, the magnetic nanoparticles are widely used to manufacture high density storage. Currently, multiple grains are used to store each bit of information, and it is estimated a ten-fold increase in capacity could be achieved if this could be reduced to one grain per bit. One way to achieve this would be using nanoparticles. Magnetic nanoparticles with long relaxation times (thermally blocked nanoparticles) with a stable remanent magnetization can be used as information carriers in magnetic identification and data storage systems where it is crucial to have small regions of magnetic material. The two directions of the magnetic moments (the remament magnetization) of the magnetic nanoparticles gives the zeros (0) and ones (1) that make it possible to store information on a hard disk in a computer or in other types of media. The directions of the magnetic moment of the nanoparticles must be stable with time, otherwise information can be lost. Today, research into using magnetic nanoparticles for information storage is developing rapidly.
The biological application of nanoparticles is a rapidly developing area of nanotechnology that raises new possibilities in the diagnosis and treatment of human cancers. In cancer diagnostics, fluorescent nanoparticles can be used for multiplex simultaneous profiling of tumor biomarkers and for detection of multiple genes and matrix RNA with fluorescent in-situ hybridization. In breast cancer, three crucial biomarkers can be detected and accurately quantified in single tumor sections by use of nanoparticles conjugated to antibodies. In the near future, the use of conjugated nanoparticles will allow at least ten cancer-related proteins to be detected on tiny tumor sections, providing a new method of analyzing the proteome of an individual tumor. Supermagnetic nanoparticles have exciting possibilities as contrast agents for cancer detection in vivo, and for monitoring the response to treatment. Several chemotherapy agents are available as nanoparticle formulations, and have at least equivalent efficacy and fewer toxic effects compared with conventional formulations.
Ultimately, the use of nanoparticles will allow simultaneous tumor targeting and drug delivery in a unique manner. In this review, we give an overview of the use of clinically applicable nanoparticles in oncology, with particular focus on the diagnosis and treatment of breast cancer.
Special Features of Magnetic Nanoparticles
Two key issues dominate the magnetic properties of nanoparticles: finite-size effects and surface effects which give rise to various special features, as summarized in
The different magnetic effects occurring in magnetic nanoparticles. The spin arrangement in a) a ferromagnet (FM) and b) an antiferromagnet (AFM); D=diameter, Dc=critical diameter. c) A combination of two different ferromagnetic phases (magenta arrows and black arrows in (a)) may be used for the creation of novel functional nanomaterials, for example, permanent magnets, which are materials with high remanence magnetization (Mr) and high coercivity (HC), as shown schematically in the magnetization curve (c), d) An illustration of the magnetic moments in a superparamagnet (SPM). A superparamagnet is defined as an assembly of giant magnetic moments which are not interacting, and which can fluctuate when the thermal energy, kBT, is larger than the anisotropy energy. Superparamagnetic particles exhibit no remanence or coercivity, that is, there is no hysteresis in the magnetization curve (d). e) The interaction (exchange coupling; linked red dots) at the interface between a ferromagnet and an antiferromagnet produces the exchange bias effect. In an exchange-biased system, the hysteresis is shifted along the field axis (exchange bias field (Heb)) and the coercivity increases substantially. f) Pure antiferromagnetic nanoparticles could exhibit superparamagnetic relaxation as well as a net magnetization arising from uncompensated surface spins (blue arrows in (b)). This Figure, is a rather simplistic view of some phenomena present in small magnetic particles. In reality, a competition between the various effects will establish the overall magnetic behavior.
The two most studied finite-size effects in nanoparticles are the singledomain limit and the superparamagnetic limit. These two limits will be briefly discussed herein. In large magnetic particles, it is well known that there is a multidomain structure, where regions of uniform magnetization are separated by domain walls. The formation of the domain walls is a process driven by the balance between the magnetostatic energy (DEMS), which increases proportionally to the volume of the materials and the domain-wall energy (Edw), which increases proportionally to the interfacial area between domains. If the sample size is reduced, there is a critical volume below which it costs more energy to create a domain wall than to support the external magnetostatic energy (stray field) of the single-domain state. This critical diameter typically lies in the range of a few tens of nanometers and depends on the material. It is influenced by the contribution from various anisotropy energy terms. The critical diameter of a spherical particle, Dc, below which it exists in a single-domain state is reached when DEMS=Edw, which implies Dc_18 where A is the exchange constant, Keff is anisotropy constant, m0 is the vacuum permeability, and M is the saturation magnetization. Typical values of Dc for some important magnetic materials are listed in Table 1
A single-domain particle is uniformly magnetized with all the spins aligned in the same direction. The magnetization will be reversed by spin rotation since there are no domain walls to move. This is the reason for the very high coercivity observed in small nanoparticles. Another source for the high coercivity in a system of small particles is the shape anisotropy. The departure from sphericity for single-domain particles is significant and has an influence on the coercivity as is shown, for instance, in Table 2 for Fe nanoparticles.[
It must be remembered that the estimation of the critical diameter holds only for spherical and non-interacting particles. Particles with large shape anisotropy lead to larger critical diameters. The second important phenomenon which takes place in nanoscale magnetic particles is the superparamagnetic limit. The superparamagnetism can be understood by considering the behavior of a well-isolated single-domain particle. The magnetic anisotropy energy per particle which is responsible for holding the magnetic moments along a certain direction can be expressed as follows: E(q)=KeffVsin2q, where V is the particle volume, Keff anisotropy constant and q is the angle between the magnetization and the easy axis. The energy barrier KeffV separates the two energetically equivalent easy directions of magnetization. With decreasing particle size, the thermal energy, kBT, exceeds the energy barrier KeffV and the magnetization is easily flipped. For kBT>KeffV the system behaves like a paramagnet, instead of atomic magnetic moments, there is now a giant (super) moment inside each particle (Figure 1d). This system is named a superparamagnet. Such a system has no hysteresis and the data of different temperatures superimpose onto a universal curve of M versus H/T. The relaxation time of the moment of a particle, t, is given by the N;el-Brown expression [Eq. (1)rsqb; where kB is the Boltzmann=s constant, and t0_10_9 s. If the particle magnetic moment reverses at times shorter than the experimental time scales, the system is in a superparamagnetic state, if not, it is in the so-called blocked state. The temperature, which separates these two regimes, the so called blocking temperature, TB, can be calculated by considering the time window of the measurement. For example, the experimental measuring time with a magnetometer (roughly 100 s) gives: TB ¼ Keff V 30 kB
The blocking temperature depends on the effective anisotropy constant, the size of the particles, the applied magnetic field, and the experimental measuring time. For example, if the blocking temperature is determined using a technique with a shorter time window, such as ferromagnetic resonance which has a t_10_9 s, a larger value of TB is obtained than the value obtained from dc magnetization measurements. Moreover, a factor of two in particle diameter can change the reversal time from 100 years to 100 nanoseconds. While in the first case the magnetism of the particles is stable, in the latter case the assembly of the particles has no remanence and is superparamagnetic. Many techniques are available to measure the magnetic properties of an assembly of magnetic nanoparticles. In the following, only some of the more important techniques are briefly discussed, and for more detailed information, the reader is referred to the cited references. SQUID magnetometry[ 22] and vibrating sample magnetometry (VSM) are powerful tools to measure the sample=s net magnetization. Like most conventional magnetization probes, both techniques are not element specific but rather measure the whole magnetization. Ferromagnetic resonance (FMR) probes the magnetic properties in the ground state and provides information about magnetic anisotropy, magnetic moment, relaxation mechanism of magnetization, and g-factor. Xray absorption magnetic circular dichroism (XMCD) is the method of choice to determine the orbital and spin magnetic moments. It is based on the changes in the absorption cross section of a magnetic material and uses circularly polarized photons.[25, 26] The magneto-optical Kerr effect (MOKE) is also used as a magnetization-measuring tool. The basic principle behind MOKE is that as polarized light interacts with a magnetic material the polarization of the light can change. In principle, this method is very useful for qualitative magnetic characterization, for imaging domain patterns, and for measuring the magnetic hysteresis. Qualitative information on magnetization, exchange and anisotropy constants from magnon spectra are provided by Brillouin light scattering (BLS). This technique is an optical method capable of detecting and determining the frequency of magnetic excitations (surface spin waves) that can interact with visible photons in magnetic systems. A simple and rapid way to estimate the blocking temperature is provided by dc magnetometry measurements, in which a zero-field-cooled/field-cooled procedure is employed. Briefly, the sample is cooled from room temperature in zero magnetic field (ZFC) and in a magnetic field (FC). Then a small magnetic field is applied (about 100 Oe) and the magnetization is recorded on warming. As temperature increases, the thermal energy disturbs the system and more moments acquire the energy to be aligned with the external field direction. The number of unblocked, aligned moments reaches a maximum at TB. Above the blocking temperature the thermal energy is strong enough to randomize the magnetic moments leading to a decrease in magnetization.
A distribution of the particle sizes results in a distribution of the blocking temperatures. As pointed out already, the above discussion about the time evolution of the magnetization only holds for particles with one single-domain. Taking into account the magnetic interactions between nanoparticles which have a strong influence on the superparamagnetic relaxation, the behavior of the system becomes more complicated. The main types of magnetic interactions which can be present in a system of small particles are: a) dipole-dipole interactions, b) direct exchange interactions for touching particles, c) superexchange interactions for metal particles in an insulating matrix, d) RKKY (Ruderman-Kittel-Kasuya- Yosdida) interactions for metallic particles embedded in a metallic matrix. Dipolar interactions are almost always present in a magnetic particle system and are typically the most relevant interactions. They are of long-range character and are anisotropic. From an experimental point of view, the problem of interparticle interactions is very complex. First, it is very complicated to separate the effects of interactions from the effects caused by the random distributions of sizes, shapes, and anisotropy axes. Second, several interactions can be present simultaneously in one sample. This situation makes it even more complicated to assign the observed properties to specific interactions.
As the particles size decreases, a large percentage of all the atoms in a nanoparticle are surface atoms, which implies that surface and interface effects become more important. For example, for face-centered cubic (fcc) cobalt with a diameter of around 1.6nm, about 60%of the total number of spins are surface spins. Owing to this large surface atoms/bulk atoms ratio, the surface spins make an important contribution to the magnetization. This local breaking of the symmetry might lead to changes in the band structure, lattice constant or/and atom coordination. Under these conditions, some surface and/ or interface related effects occur, such as surface anisotropy and, under certain conditions, core-surface exchange anisotropy can occur.
2.2.1. No or Magnetically Inert Surface Coatings
Surface effects can lead to a decrease of the magnetizationof small particles, for instance oxide nanoparticles, with respect to the bulk value. This reduction has been associated with different mechanisms, such as the existence of a magnetically dead layer on the particle=s surface, the existence of canted spins, or the existence of a spin-glass-like behavior of the surface spins. On the other hand, for small metallic nanoparticles, for example cobalt, an enhancement of the magnetic moment with decreasing size was reported as well. Respaud et al. associated this result with a high surface-to-volume ratio, however, without more detailed explanation. Another surface-driven effect is the enhancement of the magnetic anisotropy, Keff, with decreasing particle size.[29, 30] This anisotropy value can exceed the value obtained from the crystalline and shape anisotropy and is assumed to originate from the surface anisotropy. In a very simple approximation, the anisotropy energy of a spherical particle with diameter D, surface area S, and volume V, may be described by one contribution from the bulk and another from the surface:
Keff ¼ KV þ 6 DKS, where KV and KS are the bulk and surface anisotropy energy constants, respectively. Bøder et al. have shown that Keff changes when the surfaces are modified or adsorb different molecules, which explains very well the contribution of the surface anisotropy to Keff. For uncoated antiferromagnetic nanoparticles, weak ferromagnetismcan occur at low temperatures (Figure 1 f),which has been attributed to the existence of uncompensated surface spins of the antiferromagnet.[31-34] Since this situation effectively corresponds to the presence of a ferromagnet in close proximity to an antiferromagnet, additional effects, such as exchange bias, can result (see Section 2.2.2). However, only in some cases can a clear correlation between the surface coating and the magnetic properties be established. For example, a silica coating is used to tune the magnetic properties of nanoparticles, since the extent of dipolar coupling is related to the distance between particles and this in turn depends on the thickness of the inert silica shell. A thin silica layer will separate the particles, thereby preventing a cooperative switching which is desirable in magnetic storage data. In other cases, the effect of the coating is less clear. A precious-metal layer around the magnetic nanoparticles will have an influence on the magnetic properties. For example, it was shown that gold-coated cobalt nanoparticles have a lower magnetic anisotropy than uncoated particles, whereas gold coating of iron particles enhances the anisotropy, an effect which was attributed to alloy formation with the gold. Hormes et al. also discussed the influence of various coatings (e.g., Cu, Au) on the magnetic properties of cobalt nanoparticles, and came to the conclusion that a complex interplay between particle core and coating determines the properties, and tuning may therefore be difficult. Organic ligands, used to stabilize the magnetic nanoparticles, also have an influence on their magnetic properties, that is, ligands can modify the anisotropy and magnetic moment of the metal atoms located at the surface of the particles. As Paulus and co-workers reported, cobalt colloidal particles stabilized with organic ligands show a reduction of the magnetic moment and a large anisotropy. Leeuwen et al. proposed that surface-bonded ligands lead to the quenching of the surface magnetic moments, resulting in the reduction of magnetization. In the case of nickel nanoparticles, Cordente et al. have demonstrated that donor ligands, such as amines, do not alter the surface magnetism but promote the formation of rods, whereas the use of
trioctylphosphine oxide leads to a reduction in the magnetization of the particles. Overall, it must be concluded that the magnetic response of a system to an inert coating is rather complex and system specific, so that no firm correlations can be established at present.
2.2.2. Magnetic Coatings for Magnetic Nanoparticles
A magnetic coating on a magnetic nanoparticle usually has a dramatic influence on the magnetic properties. The combination of two different magnetic phases will lead to new magnetic nanocomposites, with many possible applications. The most striking feature which takes place when two magnetic phases are in close contact is the exchange bias effect. A recent review of exchange bias in nanostructured systems is given by Nogu;s et al. The exchange coupling across the interface between a ferromagnetic core and an antiferromagnetic shell or vice versa, causes this effect. Exchange bias is the shift of the hysteresis loop along the field axis in systems with ferromagnetic (FM)-antiferromagnetic (AFM) interfaces (Figure 1e). This shift is induced by a unidirectional exchange anisotropy created when the system is cooled below the N;el temperature of the antiferromagnet. This exchange coupling can provide an extra source of anisotropy leading to magnetization stabilization. The exchange bias effect was measured for the first time in cobalt nanoparticles surrounded by an antiferromagnetic CoO layer. There are numerous systems where the exchange bias has been observed, and some of the most investigated systems are: ferromagnetic nanoparticles coated with their antiferromagnetic oxides (e. g., Co/CoO, Ni/ NiO), nitrides (Fe-Fe2N), and sulfides (Fe-FeS), ferrimagnetic- antiferromagnetic (Fe3O4-CoO), or ferrimagnetic-ferromagnetic (TbCo-Fe20Ni80) nanoparticles. Recently, single-domain pure antiferromagnetic nanoparticles have shown an exchange-bias effect arising from uncompensated spins on the surface. This reveals a complicated surface spin structure which is responsible for the occurrence of a weak ferromagnetism (Figure 1 f), the exchange bias effect, and the so-called training effect. The training effect represents a reduction of the exchange bias field upon subsequent field cycling.
Metallic particles embedded in a matrix are interesting systems of magnetic-coated particles. Skumryev et al. have demonstrated the role of the matrix in establishing the magnetic response of small particles. The magnetic behavior of the isolated 4-nm Co particles with a CoO shell changes dramatically when, instead of being embedded in a paramagnetic matrix, they are embedded in an antiferromagnetic matrix. The blocking temperature of Co particles embedded in an Al2O3 or C matrix was around 10 K, but by putting them in a CoO matrix, they remain ferromagnetic up to 290 K. Thus, the coupling of the ferromagnetic particles with an
antiferromagnetic matrix is a source of a large additional anisotropy. Exchange biased nanostructures have found applications in many fields, such as permanent magnets (Figure 1c), recording media, and spintronics. A new approach to produce high-performance permanent magnets is the combination of a soft magnetic phase (easily magnetized), such as Fe3Pt, and a hard magnetic phase (difficult to magnetize and thus having high coercivity), such as Fe3O4 which interact through magnetic exchange coupling. The right choice of ferromagnetic and antiferromagnetic components can provide a structure suitable for use as a recording medium. The exchange coupling can supply the extra anisotropy which is needed for magnetization stabilization, thus generating magnetically stable particles. Another interesting aspect related to a magnetic coating is given by the bimagnetic core-shell structure, where both the core and the shell, are strongly magnetic (e. g., FePt/ CoFe2O4). These bimagnetic core-shell nanoparticles will allow a precise tailoring of the magnetic properties through tuning the dimensions of the core and shell, which selectively controls the anisotropy and the magnetization. Some important aspects should be emphasized. The magnetic behavior of an assembly of nanoparticles is a result of both the intrinsic properties of the particles and the interactions among them. The distribution of the sizes, shapes, surface defects, and phase purity are only a few of the parameters influencing the magnetic properties, which makes the investigation of the magnetism in small particles very complicated. One of the great challenges remains the manufacturing of an assembly of monodisperse particles, with well-defined shape, a controlled composition, ideal chemical stability, tunable interparticle separations, and a functionalizable surface. Such particles will tremendously facilitate the discrimination between finite-size effects, interparticle interactions, and surface effects. Thus, the synthesis of magnetic nanoparticles with well-controlled characteristics is a very important task, which will be described in more detail in the next sections.
Method to measure Co nanoparticles size
It is important to make sure the nanoparticles to be monodispersed in the array. There are few methods can be used to measure the size of the cobalt nanoparticles, which are:
2) Transmission Electron Microscope (TEM)
3) X-Ray Diffraction (XRD)
Zetasizer is a device that uses light scattering techniques to measure hydrodynamic size, zeta potential and molecular weight of proteins and nanoparticles. Zetasizer Nano-ZS can be used for a measurement of hydrodynamic size, zeta potential and molecular weight of particles in solution. It has a size range for particles of 0.6 nm to 6 microns, for zeta potential of 5 nm to 10 microns and for molecular weight of 1-20,000kDa. The concentration range for the Zetasizer Nano-ZS is 0.1 ppm to 40 weight %. This instrument can be used for determining particle size in solution for nanoparticles, colloids and biomolecules. The zeta potential is measured as an indication of the overall charge and dispersion stability of the particles in solution. The zeta potential provides information about how a particle will interact electrostatically. The instrument features a cell chamber and offers measurement in the size range of 0.6nm to 6um and a concentration range of 0.1mg/ml lysozyme to 40 percent w/v.
Figure 2.3 Image of zetasizer
Transmission Electron Microscope (TEM)
The size of the nanoparticles can be measured by using transmission electron microscope. Transmission electron microscopy (TEM) is a microscopy technique whereby a beam of electrons is transmitted through an ultra thin specimen, interacting with the specimen as it passes through. An image is formed from the interaction of the electrons transmitted through the specimen; the image is magnified and focused onto an imaging device, such as a fluorescent screen, on a layer of photographic film, or to be detected by a sensor such as a CCD camera. TEMs are capable of imaging at a significantly higher resolution than light microscopes, owing to the small de Broglie wavelength of electrons. This enables the instrument's user to examine fine detail even as small as a single column of atoms, which is tens of thousands times smaller than the smallest resolvable object in a light microscope. TEM forms a major analysis method in a range of scientific fields, in both physical and biological sciences. TEMs find application in cancer research, virology, materials science as well as pollution and semiconductor research. By using the image obtained from TEM, measures the size of 50 particles and obtained the average size of it. This method is widely used after TEM is introduced, but this method is not as accurate as the measurement is differ depends on different individual.
Figure 2.4 Image of the Transmission electron microscopy (TEM)
X-Ray Diffraction (XRD)
XRD method is a measurement of X-ray powder diffraction (XPD) line integrated intensity fluctuations caused by fluctuations of grains number in a scattering volume. The
XRD method is a nondestructive technique which allows measuring the grain size on sufficiently large area of surface. A shape factor is used in XRD and crystallography to correlate the size of sub-micrometer particles or crystallites, in a solid to the broadening of a peak in a diffraction pattern. In the Scherrer equation,
The Scherrer equation is limited to nano-scale particles. It is not applicable to grain larger than about 0.1 μ m, which precludes those observed in most metallographic and
Image 2.5 Image of the X-ray Diffractometer
Method to control Co nanoparticles size
The size of the cobalt nanoparticles can be varied by few methods, which are:
1) Adjusting the reaction temperature
2) Tailoring the ratio of the concentration of the reagents to that of surfactants
3) Adding different combination of surfactants
Adjusting the reaction temperature
From the research of the influence of the temperature towards the particle size, we know that the size of particle increased as the reaction temperature increased, vice versa. In cobalt nanoparticles, hexagonal-centered cubic (HCP) phase will formed when the reaction temperature is below 700°C and when the reaction temperature reached 700°C, the phase of the cobalt will change to face-centered cubic (FCC).
TEM micrographs of hcp Co nanoparticles. (c) TEM micrographs of fcc Co
The images above are the TEM images of the cobalt nanoparticles produced by different reaction temperature. Image (a) is produced with the reaction temperature at 500°C; the average size of the particles is 2-4nm. Image (c) produced by increasing the reaction temperature to 700°C, the average size of the particles increased to 5-10nm. Therefore, this clearly showed that the increase of reaction temperature influence on the size of the particles.
Adding different combination of surfactants
In this method, the size of the particles is influenced by the types of surfactants used in the experiment. In this experiment, the different combinations of trioctylphosphine (TOP), oleylamine and oleic acid were added as surfactants to control the particles size. The presence of TOP led the particles to be well dispersed with no agglomeration, while much larger particles flocculating together were synthesized in the absence of TOP. TOP is a high-boiling point surfactant with a patulous long chain structure providing greater steric hindrance. So, this agent might slow the addition rate of materials to the nanoparticles during the growth, resulting in much smaller nanoparticles. Futhermore, the surfactants in the solution adsorbed onto the surface of the nanoparticles, providing a dynamic organic structure that stabilizes the nanoparticles in solution. The addition of oleic acid into the mixture of TOP and oleylamine as an additional surfactant reduced the particle size much further and resulted in very uniform size distribution. Oleic acid is known as a surfactant that binds tightly to the metal nanoparticles surface. The combined effects of TOP and oleic acid were much more profound than those of individual contributions.
TEM images of cobalt nanoparticles coated with various surfactants: (a) oleylamine, (b)
TOP and oleylamine, and (c) TOP,oleylamine and oleic acid
The figures above showed the TEM images of cobalt nanoparticles when coated with various surfactants. As additional surfactants were added, the average particle size decreased from about 200nm to 8nm. The particles in (c) were added with TOP, oleylamine and oleic acid, showed the particles are well dispersed and having a smaller particle size.
Tailoring the ratio of the concentration of the reagents to that of surfactants
The synthesis of cobalt nanoparticles in the presence of different ratio of triphenylphosphine (TPP) and oleic acid can vary the size of the particles. The TPP and oleic acid are employed as stabilizers to control particle growth, stabilize the particles and prevent the particles from oxidation. Through judicious adjustment of the ratio of the TPP and oleic acid stabilizers, the size of the particles can be controlled. The oleic acid binds tightly to the particle surface during synthesis that hinders the particle from growing. While for the TPP, it reversibly coordinates neutral metal surface sites that favor rapid growth. Judicious adjustment of the ratio of TPP to oleic acid stabilizers can control the size of particles. If the amount of the TPP added is more, the particles will tend to grow larger, vice versa. The addition of oleic acid will reduce the size of the particles.
TEM image of two-dimensional superlattice of Co NCs at different concentration ratios
of TPP to OA: a 6.5 nm at TPP/OA=3 : 1, b 8 nm at TPP/OA = 5 : 1, c 9.5 nm at
TPP/OA = 7 : 1;
Surfactants effect on the synthesis process
The surfactants play an important role in controlling the shapes and sizes of cobalt nanoparticles. The shapes of the cobalt nanoparticles include: spherical, triangular, rod-like, and hexagonal. The shapes of cobalt nanoparticles depend on the type of the surfactant used in the synthesis and the temperature at which the cobalt precursor was added to the reaction mixture. Surfactant that can be used to synthesis the cobalt nanoparticles are:
Trioctylphosphine oxide (TOPO)
TOP led the particles to be well dispersed with no agglomeration while much larger particles flocculating together were synthesized in the absence of TOP. Top is a high-boiling point surfactant with a patulous long chain structure providing greater steric hindrance. So, this agent might slow the addition rate of materials to the nanoparticles during their growth, resulting in much smaller nanoparticles. Furthermore, the surfactants in the solution adsorbed onto the surface of the nanoparticles, providing a dynamic organic structure that stabilizes the nanoparticles in solution.
Oleylamine acts as a stabilizer in the synthesis of cobalt nanoparticles. The addition of oleylamine to the synthesis of cobalt nanoparticles helps protecting ligand for the synthesis of a variety of metal nanoparticles. Besides, it helps to reduce the particle size and resulted in very uniform size distribution.
The addition of oleic acid as a surfactant reduced the particle size much further and resulted in very uniform size distribution. Oleic acid is known as a surfactant that binds tightly to the metal nanoparticles surface. Oleic acid is employed as stabilizers to control particle growth, stabilize the particles and prevent particles from oxidation.
TPP exists as relatively air stable, colorless crystals at room temperature. It dissolves in non-polar organic solvents such as benzene and diethyl ether. The steric hindrance of the phenyl group in TPP in the traverse direction is larger than that of the alkyl chain, so that it can be used to control cobalt nanoparticles with small size. The TPP reversibly coordinates neutral metal surface sites that favor rapid growth. The sizes of the Co NCs increase with the increasing of TPP molarities.
TDDA consisted of one nitrogen atom and three alkyl group chains each with twelve carbon atoms. It can cause the particles to change from spheres to hexagonal platelets but resulted in loss of monodispersity.
Trioctylphosphine oxide (TOPO)
The presence of TOPO narrowed the size distribution but played no role in the formation of the disk shaped cobalt nanoparticles. Besides, the presence of TOPO induces monodispersity to the nanoparticles so that the size of the particles is almost the same.