Recently, the nanomaterials (the metal nanoparticles or polymer/metal nanocomposites) are the subject of increased attention of researchers in different areas of science and technology, due to its applications in many fields in our life such as solar cells, novel photodiodes and sensors. So, the synthesis routes of these nanomaterials have a much interest from the scientist and the researchers. From our lab experience, the γ irradiation method is one of the most interested methods for the preparation the nanomaterials. This chapter is devoted to introduce an introduction in the importance of the gamma irradiation method in the nanotechnology. Then, explain simply and in details and with examples the preparation of some metal nanoparticles and metal/polymer nanocomposites by using the γ method in both the colloidal phase and in the solid phase. This method presents some benefits such as a fully reduced particles and extremely pure nanoparticles free from any impurities or residual reducing agents, and is capable of getting a specific particle size and structure. In addition to its provide a good dispersion of nanoparticles in the polymer matrix in the polymer/metal nanocomposites. The nucleation mechanism of the metallic nanoparticles and also their growth process under the irradiation by gamma rays are also explained. The nanoparticles size is depend on the gamma irradiation doses and also on the competition and the race between nucleation and growth process. Also, open the window of the sun shine on the explanation with comparison of the selected electrical properties of these nanocomposites, which irradiated with and without γ method.
Keywords: gamma rays; polymer nanocomposites; nucleation mechanism; growth process; metal nanoparticles;
1. Introduction
In the past two decades, science and engineering have a nemours revolution developments towards materials synthesis in the nanosize scale in order to achieve specific and special properties that are significantly and completely different from the materials of the individual atoms and the materials in bulk state [1-2]. The particle exhibits many exciting and excellent properties when its size decreases below 100 nm, these properties mainly comes from two physical effects. First, the quantization or quantum confinement of electronic states which making the physical, optical, magnetic, properties are very sensitive to the size and depend on it [3]. Second, the high amount of surface area per unit volume (surface-to-volume ratio) and high surface area alters the material properties such as mechanical, thermal, and chemical properties [4]. Generally, from the top viewpoint it can be classified the techniques that used in the preparation of the nanoparticle and nanocomposite materials into two categories like top-down and bottom-up approaches [5]. In the previous approaches, the nanoparticles can be prepared by many different traditional techniques such as milling or lithography, etc., which produces a particles with very small size from the identical bulk materials [6]. However, nanoparticles can be formed atom-by-atom, in the latter approach, in the gas, solid, or liquid phases [7]. Also, the preparation of the nanoparticles can be done by the chemical way in the liquid state that containing precursors salt, a solvent, and a capping and reducing agents [8,9]. Although the good structure such as crystallinity, morphology, composition of the colloidal synthesis and its potential to produce the nanoparticles with large quantity such as surface chemistry, good control of size and shape at reasonably low cost. But these methods facing a huge problem, where the nanoparticles obtained need to modified their surfaces. Where, its can conclude the Goals and Problems in the Synthesis of Metallic Nanoparticles as a following. Ideally, the conditions that must be existent in the method that can be used in the preparation of metallic nanoparticles are:
1- Easy processability and its ability to reproduce again.
2- Its ability to control the size, shape and the structure of the produced particles.
3- Achieving the metallic nanoparticles with mono-dispersion.
4- Must be simple, safe and economically.
5- Must be environmental friendly and less harmful precursors.
6- Use the small amount of reagents.
7- The reaction should be done in ambient temperature, near the room temperature.
8- One‐pot reaction as possible, with as few preparation steps as it can (time faster).
9- Free from impurities, waste and by‐products as possible.
So, researchers have resorted to a way to beat this problem. The gamma irradiation method can be done in room temperature and ambient pressure and this can be considered as one of the great advantages when compared with the other methods.
The gamma irradiation method has the great advantages when compared with the other methods and are listed as a following:
1- The experiment method can be performed in the mild conditions ”room temperature and ambient pressure”[10].
2- Its high reproducibility
3- the hydrated electron is the prime reducing agent, without oxygenation, with redox potential negative.
4- Without additional or external chemical reducing agents, the zero valent metal atoms obtained from reducing the metal ions.
5- Thus, and as an separately stages, the primary atoms created and the atoms are separately isolated, at the origin, and regularly distributed as were the ionic metal ions[11,12].
In other words, there are two main factors to obtain highly stable nanoparticles with homogenously dispersed and without unwanted by-products of the reductant. These two factors are the abstraction of the extravagant chemical reducing agents and homogeneous formation of nuclei.
The most important, wonderful and crucial thing in this method is controlling the particle size and crystal structure by the choice and selecting the absorbed dose. This is done and in special case for multi-metallic clusters by the exact tuning and adjusting the tone between the nucleation and growth steps [13]. Therefore, the gamma radiation technique is low-cost and an environmentally friendly method for preparation the metal nanoparticles with increasing dominance of the size and structure and a large quantity[14-15]. This chapter is devoted to introduce an introduction in the importance of the gamma irradiation method in the nanotechnology. Then, explain simply and in details and with examples the preparation of some metal nanoparticles and polymer/metal nanocomposites by using the γ method and shows the advantage of this method. Also, open the window of the sun shine on the explanation with comparison of the selected electrical properties of these nanocomposites, which synthesized with and without γ method.
2. Synthesis of metallic nanoparticles in Colloidal phase.
Colloids in chemistry are consists of suspensions of one substance in a liquid substance [16]. Because of their high surface-to-volume ratio or amount of surface area per unit volume, they are very attractive and also their huge specific surface area or the total area that occupy by the surface of the object. This assures to a large part of the particle atoms to contact with the liquid surrounding it, which leads to increase the interactions or larger and faster reactions [14]. In this chapter, we are concerned with the colloids which are metallic elements particles with respect to their phase surrounding it. The techniques used in the synthesis of the metal nanoparticles in the colloidal phase are depend on the reduction of metal ions in the liquid, which containing a stabilizing agent. The chemical reduction, thermolysis, sonochemical route [17-20] are the techniques usually used in the preparation the nanoparticles in addition to the irradiation methods [21,22].
Gamma irradiation of aqueous solutions use to produce a large number of hydrated electrons ( ) and H• atoms (Equation 1). The ( ) and H• are strong reducing agents with E0 (H2O/ ) and E0 (H+/H•) equal -2.87 and −2.3 VNHE, respectively, where E0 is the redox potential [30]. Therefore, metal ions can be reduced into zero-valent metal particles by these hydrated electrons ( ) and H• atoms (Equations 2 and 3).
Eq.(1) Eq.(2)
Eq.(3)
The additional reducing agents and the following side reactions have been avoided by this mechanism. Moreover, the amount of nuclei with zero valent can be controlled by varying the gamma irradiation doses. On the other hand, another strong reducing agents hydroxyl radicals (OH•) with redox potential E0 = (OH•/H2O) = +2.8 VNHE, which induced in the gamma irradiation of water, which the ions oxidized or the atoms into a higher oxidation state. Before gamma irradiation the primary or secondary alcohols, which called an hydroxyl OH• radical scavenger, is added into the precursor solutions. For example, the reducing agent isopropanol used to reduce metal ions (M+) into zero-valent atoms (M0), which can used scavenge OH• and H• radicals and at the same time changes into the secondary radicals, as shown in the below reactions [24]:
Eq.(4)
Eq.(5)
Eq.(6)
In chemistry, any material inserted to a mixture to react with the impurities and remove the by-product and also undesired reaction products from the reaction, called a scavenger. Also, a schematic diagram in Figure 1, illustrated the processes of reduction the multivalent ions. This reduction process produced by multi-step processes including inequality and variance in lower valence states where the multivalent ions are reduced to the atoms.
2.1 Nucleation and growth mechanisms under gamma irradiation.
Before we explained the nucleation and growth of the metal nanoparticles under the gamma irradiation, we need to define some expression such as nucleation, growth and secondary growth. Nucleation occur when the solution reach to the supersaturation, at this stage the solution is thermodynamically unstable. The solution must be reach to supersaturation state for occurring the process of nucleation, to produce an extremely small size particle.
Growth process take place after the formation the nuclei from the solution, they grow when the soluble species deposited onto the solid surface (molecular addition). Important notice, the small size particles has relative rate of growth completely different from the larger size one due to the growth of the particle, when the reactants are consumed in the reaction.
Also, when the particles growth by the aggregation is faster than that by molecular addition the secondary growth occur and it takes place by the combination of stable particles with smaller unstable nuclei.
In the liquid-phase system the term “nucleation” in the metallic nanoparticles synthesis can be defined as the small cluster formation from aggregation of newly atoms formed in the solution [23]. The newly neutral M0 atoms formed at first dimerize, an addition reaction in which two molecules of the same compound react with each other to give the adduct, when connecting with the excess M+ ions due to the binding energy between the atoms of two metals or atoms with unreduced ions is high. (Eqs. 3 and 4).
Eq.(7)
Eq.(8)
The (M+2) charged clusters formed from the dimerization process reduced to form a cluster nucleation which considered to a center of the growth process of the clusters. The fasten association processes was produced by the interaction of the clusters with unreduced ions or by the strong bonding between two charged clusters (Eq. 5 and 6).
Eq.(9)
Eq.(10)
where the nuclearities represented by the subscripts m, n, and p and the number of associated ions represents by x, y, and z. These nuclei grow and setup a seeds with various shapes and critical and definite size. The most important key role played in controlling the shape of final products is the seeds crystallinity which evaluated by the minimization of surface energy [24].
The schematic diagram in Figure 1 shows the different shapes of the noble metal nanoparticles formation which can be grown from multiply twinned particle or a single crystal with respect to the different steps of the reaction. [25-27]. According to the Wulff’s theory, which describe the facets bounded of the crystals. The facets of the crystal bounded to reach the thermally equilibrium state by minimization the total surface energy [28, 29]. The growth in the noble metals which characterized by the face centered cubic (fcc) increase in the order of the plane axes a(111) < a(100) < a(110), due to the surface energies [30, 31]. In the next section, we introduce an example for the reduction and growth of the noble nanoparticles by using the gamma irradiation method.
2.2 Examples for the preparation the metal nanoparticles in liquid phase.
Goals and problems regarding the synthesis of Ag NPs
• The huge problem that facing the conventional methods, is a limited the particles size that can be created by these methods and such methods are usually sold on their ability to make < 10 nm
• For the non‐ conventional methods, the large problems are often a vast distribution of the particle size, the reduction in the crystallinity of the particle, and the cost and the ability to the produce
• The difficulty the facing the preparation of silver nanoparticles, is obtaining the Ag nanoparticles with uniform, stable and control size.
• The perfect synthetic method should manipulate all of the previous problems and also create particles with no bizarre chemicals that can be change the optical behaviour of the particle’s and their surface chemistry
Krstić et al, [32] reported the radiolytically in situ synthesized of the Ag nanoparticles capped by poly(vinyl alcohol)/chitosan blends. In this study the 2-propanol was used as a scavenger, to remove or de-activate and unwanted impurities and reaction products. By directing the aqueous solutions which containing the metal ions to gamma rays, the noble metals can be created. The radiolysis of water lead to formation the primary radicals ( ), H•, OH•, H2 and H2O2 due to the absorption of the gamma energy occurred by the solvent, Eq. (1).
The scavenger 2-propanol was used to convert OH• radicals to 2-propanol radicals ((CH3)2C*OH)
Eq. (11)
when a solid chitosan exposed to the gamma irradiation. the chitosan degredate through the bond breaking of the glycosidic which accompanied by the carbonyl groups formation, the amine group elimination and the release hydrogen and ammonia. On the other hand, a large part of the gamma radiation energy is absorbed by a diluted aqueous solution of chitosan and PVA, and then formed the water radiolysis products (Eq. (1)). As mentioned above, 2-propanol react with the OH* radicals and also the OH* radicals abstract the hydrogen atoms from the both polymers (Cs and PVA) and macroradicals formation of CS* and PVA*.
The reaction rate of interaction the radicals OH* with the polymers CS and PVA is 109 dm3/mol s and has the similar the reduction rate of the bimolecular . The reactions (scission of chains in the chitosan and the recombination of the intra and intermolecular reaction and also the transfer of hydrogen) are the subsequent reactions of macroradicals and also finally disproportionation of macroradicals. When macroradicals are interacted with the different six carbon atoms in the glucosamine unit (C1–C6), and double bond C=O formed when only the radicals at C1 and C4 atoms formed and then rearrangement involving the bonds of 1–4 glycosidic breakage. However, due to the presence of, OH* radical scavenger, 2-propanol in reaction solution the impossible considerable extent for induced degradation of CS by the gamma irradiation. One can divided the process of the Ag nanoparticles creation, under the gamma radiolytic method into two main steps. First, the nucleation processes where the atoms formation and then followed aggregation processes, where the nanoparticles formation. The below equations described the nucleation process of the Ag nanoparticles.
The silver Ag ions (Ag+) can be reduced by a strong reducing species such as, 2-propanol ((CH3)2C*OH),the solvated electrons ( ) and PVA radicals into zero-valent Ag atoms (Ag0) (Eqs. (5–7)). Moreover, the Ag+ ions forms Complexes with high number of hydroxyl groups in the CS giving the ions (Eq. (8)). Also, according to the (Eq. (9)), the ions can be reduced by , and formed the species. Because the reducing species in the solution are randomly distributed like the silver Ag+ ions , the nanoparticles Ag0 atoms in the solution are formed with a homogenous distribution. Because the binding energy between two metal atoms is stronger than the energy between the solvent atoms or atom-ligand bond energy. Therefore, the formed Ag0 atoms dimerize when they combine with a similar molecule to form a dimer or encounter with each other (Eqs. (10 and 11)) and/or connect with an spare ions (Eqs. (12–15)), and then the metal clusters with higher nuclearities formation by a continuous of coalescence processes the progressively growth obtained.
The very important parameter in the mechanism of the growth of the clusters is the fast collision reactions of ions with atoms or clusters. The reduction are competitive processes between the reduction of the free ions and the Ag ions and this reduction is controlled by formation rate of reduction radicals. Accordingly to that, when obtained the nanoparticles with smaller dimension, the clusters formation with collision, or by direct reduction is dominant at higher dose rate. The adsorbed ions on the Clusters, at any coalescence stage, may be reduced by , PVA*, (CH3)2C*OH and CS* radicals (Eqs. (16–20)).
The polymer CS and PVA molecules with functional groups were added to prevent nanoparticles from the growth to bigger nanoparticles and inhabit the clusters collision and form the larger nanoparticles. Where the PVA has a hydroxyl (-OH) groups and also CS has amino (-NH2) that interact with the atoms lies on the nanoparticles surface and stabilizing them.
Another important example for synthesis nanoparticles by using gamma irradiation method is the formation of CdS nanoparticles in the PVA solution [34](Alireza Kharazmi). This can be illustrated in the combination of cadmium and sulfur ions. Indeed, according to Equation (1), the radicals hydroxyl radical(OH*) and hydrogen radical (H*) and the hydrated electrons ( ) was produced by the interaction of gamma rays with water. In aqueous solution, the sulfur (S 2-) ions was formed by the radiation decomposes of Na2S2O3. Formerly, the dissolving of the Cd(NO3)2 in the solution produced the cadmium (Cd2+) ions. Therefore, the CdS nanoparticles formed by coupling the cadmium ions with sulfur ions. The reaction described as in the following reactions:
In the final term, the polyvinyl alcohol (PVA) polymer used as stabilizer to prevent the CdS nanoparticles agglomeration into larger particles. Recently, El-Shamy et al. [35], reported a promising method for preparation the (PVA/Ag nanorods) nanocomposites by using the gamma rays in solid phase. They prepared the (PVA/Ag nanorods) nanocomposites at a gamma doses (125 KGy), also they showed the Ag nanorods on the back surface (the surface not face the gamma source), as shown in the figure 2. In this article, the author observed two behavior. First, the embedding of the Ag nanoparticles in the polyvinyl alcohol matrix from the surface of the PVA into the PVA body itself and then segregate from the other surface of the PVA films with increasing the gamma irradiation doses from 0 to 125 KGy, as shown in the figure 2. Second observation is the formation of the Ag nanorods specially at the gamma irradiation doses 125 KGy.
Here, we can illustrate the reason for the formation of the two previous observation as a following: we started with the second observation. Two real mechanisms happen at the same time in the reaction. the Ag nanoparticles firstly created via homogenous nucleation and then the Ag nanoparticles grow into the direction of the plane with the lowest energy {111}. At first, the creation of Ag nanoparticles takes place by the reduction of the silver nitrate by using the gamma irradiation. Concurrently the homogenous nucleation process occur from the silver seeds which served as nuclei for the creation of the silver atoms. It was found that the PVA under the gamma irradiation can take a shape like rods micelles structure, and this behavior was strongly depend on the mole ratio of (PVA:AgNO3). According to, the polyvinyl skeleton of PVA containing the polar groups (OH) groups. This facilitates the interaction between the oxygen atoms in the (OH) group and Ag formed a complex by the covalent bond. Second stage contains merging, inclusion and fusing of the Ag nanoparticles to procedure the Ag nanorods in the PVA matrix by photo-thermal effect produced from the gamma irradiation.
Here, Two functions was observed for the PVA in this reaction (1) make a complex with Ag+ as intermediate state through the reaction (PVA/Ag+) complexs. (2) coated and capsulated the planes of the Ag nanoparticles. It’s known that the PVA contains (–C=O group) in its chemical structure. So, the possibility of the binding capacity of the PVA to the planes/surface of Ag particle is higher due to the presence of this group in PVA structure. Also, its known that the plane {100} is lower in energy than the plane {111}, so the possibility to the growth of the Ag nanoparticles in the direction of {100} is higher than in {111}. According to this illustration, there is a large difference in the reactivity of the planes between planes {100} and the planes {111} of Ag rods. But the presence of (–C=O group) in the PVA structure make the PVA is more strongly interaction with Ag crystals. Consequently, the PVA interacts with the plane {100} stronger than the plane {111} of Ag. So, the PVA coated and adsorb the side planes {100} and make this plane completely blocked from the growth. On the other hand, PVA coated the ends planes {111} and make this plane partially blocked with PVA. Finally, the growth of Ag crystal along the direction of the plane {111} increases as compared with plane {100}. There is a strong preferred orientation along the plane {111} of the Ag crystals as shown in figure 3. These investigations show that, why the Ag nanoparticles grow in the direction of the plane {111} rather than the plane {100}.
Now, we return to the first observation. First observation can be illustrated in the basis of the thermodynamic and the factors affecting the embedding of noble metal NPs: Driving forces, surface and interface energies.
Thermodynamically, the Ag nanoparticles located at the surface of the PVA are not much stable on the surfaces and undergo or tend to reactions to be thermodynamically stable either by wetting or embedding mechanisms. When the PVA matrix has surface energies higher than that of the surface energy of the Ag nanoparticle, wetting phenomenon occurs. On the other hand, when the surface energy of PVA film is lower enough than that of the Ag nanoparticle, Ag nanoparticles tend to embed into the PVA surface. The study of embedding Ag nanoparticles in PVA surface and the technological interest lead to explore the fundamental principles of the thermodynamic i.e. thermodynamic driving force that controls the embedding of nanoparticles into the polymeric surface [33]. From the fact that when the Ag nanoparticles embedded into the PVA surface, it’s surface Gibbs free energy becomes lower as compared to when it is on the surface, according to this fact the driving force for the embedding metal nanoparticles takes place or originates. In the case of Ag nanoparticles on the surface of the PVA, the surface Gibbs free energy of the PVA is lower than that of Ag nanoparticles because the cohesive energy of PVA is lower than the cohesive energy of the Ag nanoparticles. The thermodynamic driving force produces when the system has a tendency to decrease this surface energy. The thermodynamic behavior of Ag nanoparticles on the PVA surfaces has been predicted and formulated in equation for embedding of nanoparticles. The surface Gibbs free energy of the Ag nanoparticle, with surface area (σ), on the PVA surface can be given as:
Eq. (37)
where the surface energy of the Ag nanoparticle is denoted by γAg. In the above equation, the interfacial energy of the Ag nanoparticles and the surrounding PVA replace the surface energy, that is only when the Ag nanoparticles is embedded into the PVA surface. Therefore, the net change in the Gibbs free energy (ΔGs) of the system in this consideration, would be equal the difference of the surface energies of the Ag particle when it is on and inside the PVA and can be given as:
Eq. (38)
where the interfacial energy between Ag nanoparticles and PVA is denoted by γPVA/Ag. Also, (σ) is the surface area of the Ag particle, (γPVA) is the surface energy of the PVA. On the other hand, by measuring the work (W) which the Ag nanoparticles taken when moving from the PVA surface to the final rest position inside the PVA, the change in the Gibbs free energy (ΔGs) can be calculated. A void is created in the PVA, when the Ag particle is embedded in a PVA, and this void required free energy of (σγPVA). The energy (σW) is regained when the Ag nanoparticle filled this void, where W is the work of adhesion between the Ag nanoparticle and the PVA. The net change in the free energy of the PVA/Ag nanocomposite would be:
Eq. (39)
From Equations ( and )
Eq. (40)
Embedding is favorable, when ΔGs < 0, then:
˃ Eq. (41)
Eventually, they determined that the complete embedding could be possible when
˃ Eq. (42)
while interfacial energy can be estimated using expression
Eq. (43)
where the average contact angle is denoted by θcontact (Figure 4). Therefore, using this equations (6 and 7) and with the Ag particles satisfies all the conditions in this two equations, the complete embedding of the Ag nanoparticles into the PVA surface is done. One of the two mechanisms, either wetting or dewetting of the Ag nanoparticles can occur when the Ag metal on the surface of the PVA under some conditions such as gamma irradiation or annealing and also depend on its ability to reaction (reactive or unreactive) with the PVA surfaces. Also, weak interaction of Ag nanoparticles to the PVA polymer and the tendency of the Ag nanoparticles to minimize their surface free energy tend to dewet from the PVA surface leading to the creation of the Ag nanoparticles. We have shown that, under gamma irradiation and the Ag metals have a strong tendency to aggregate on the PVA surface because of the Ag nanoparticles have high cohesive energy leads to dewetting of the Ag from the surface and formed the Ag nanoparticles on the surface. Thus, the surface energy of the formed Ag nanoparticles is greater than the estimated interfacial energy of Ag nanoparticles and PVA, and get embedded Ag nanoparticles into PVA surface with further gamma irradiation due to the thermodynamic driving forces (capillary) and photon induced viscous flow in PVA polymer substrate. However, according to the Fick’s law for the atoms diffusion flux and the systemic geometry [ ],
J = the diffusion flux
D = the diffusion coefficient
c = the concentration
R = universal gas constant
T = the absolute temperature
μ = the chemical potential .
the growth rate of particles which is depend on the temperature is principally given by the coefficient of diffusion D which is depend on temperature. From the relation of Stokes–Einstein [ ],
D= the diffusion coefficient
, a= radius of a spherical particle
= a fluid of dynamic viscosity
T =absolute temperature
with increasing temperature the diffusion coefficient increase, accelerating the growth rate of the Ag particles resulting in considerably larger particles.
The introduction of metallic Ag atoms by gamma irradiation into a dielectric PVA matrix leads to the creation of metallic Ag clusters in the nanoscale (as shown schematically in Fig. 9), and this effect provides interesting optical properties with possible applications in optoelectronics. In this work, gamma irradiation has been used to make metal Ag nanoparticles embedded in PVA. Also, PVA/Ag nanocomposites were synthesized by gamma irradiation, carried out at gamma doses of 25–125 KGy, where Ag nanoparticles were found to be dispersed in the PVA matrix and isolated from the PVA surface (figure 5).
The gamma irradiation by heavy Ag+ ions at relatively low doses and low energy and by interaction of the PVA atoms with the accelerated photons, the nuclear collisions was prevailing in the reaction i.e. atoms displacement in the PVA matrix and some chemical bonds breaking by the gamma irradiation degradation or by the displacement of the atoms. Target Ag atoms lose electrons due to this chemical alteration in the PVA matrix, and then the neutral Ag atoms (Ag0) formed by the deionize of the Ag+ ions. Principally, Ag atoms may react with PVA radicals or share in the oxidation reaction. However, the bonding (Ag–Ag) is energetically more predominant, because of a great difference in Gibbs free energy (ΔGs) between the atoms of the PVA and Ag. Therefore, the creation of Ag nanoparticles clusters in the PVA is caused by the high cohesive energy of the Ag and low interaction energy between the PVA and Ag. This nucleation process consists of silver (Ag) accumulation up to supersaturation state, formation of few atoms nuclei and their growth [125]. The gamma irradiation doses, energy and composition, structure of the PVA results from the gamma irradiation are the parameters that affected on the growth of Ag nanoparticles. The physical properties of the PVA polymer also plays an important role in formation of the Ag nanoparticles and their growth in the PVA.
Figure 2 shows the SEM micrographs of the PVA/Ag system irradiated with gamma rays at different doses. It can be clearly seen that starting from the Ag on the surface of the PVA film, the nanostructuring induced by gamma photon and embedding of Ag nanoparticles in PVA matrix take place by gamma irradiation. Local melting induced by the gamma rays due to photo-thermal and formation of the crater which is the primary mechanism responsible for the dewetting of Ag metal from the PVA surface whereas, with subsequent gamma irradiation interface mixing and PVA decomposition leads to the Ag nanoparticles formation at the surface and embedded nanostructures. Thermal spike induced by gamma photons is produced in the Ag metal leads to melting of Ag metal due to collision cascades, and formation a craters and holes in the PVA as a result of outflow of the Ag atoms from the hot molten zones of the cascade. The induced molten zone size leading to dewetting was depended on the melting temperature Tm of the PVA polymer. The Ag nanoparticles has a higher surface energy with respect to the PVA polymer substrate promotes the out flow of Ag atoms from the molten zones and promotes dewetting of the Ag from the PVA polymer substrate because the higher surface energy of Ag than that of the PVA film, responsible for dewetting during gamma irradiation.
To understand the dewetting of the Ag metal in the PVA and then the Ag nanoparticles formation also the embedding of Ag induced by gamma irradiation. The parameter is responsible for the embedding of Ag NPs into the PVA surface is the thermodynamic driving forces. Which produced by minimization the free energy of the surface and interfacial energies.
The scenario for the synthesis of Ag nanoparticles and embedding phenomena of the Ag in the PVA matrix and the companying thermodynamic behaviour were showed in (figure 6).
Using the gamma irradiation with high energy, PVA/Ag nanocomposite has been synthesized by gamma induced the Ag nanoparticles on PVA surfaces which results in embedded Ag nanostructures in the PVA matrix rich with carbon. The high energy photons cause modification in the structure of the materials by inducing a high degree of localized electronic excitation through the photon track. On the other hand, the material will be completely amorphous and rich with amorphous carbon, due to the processes that occurred in the material such as non-thermodynamic relaxation processes and densification due to dehydrogenation, outgassing of hydrocarbon fragments, that arising from the nuclear energy deposition through cascade collision of the photons within the PVA polymer.
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