**2. Preparation methods**

#### **2.1. Ex situ technique**

problems. For practical applications of the nanoparticles, they are embedded in the polymers to produce the nanocomposite polymers since these nanocomposite polymers may have optical, electrical and thermal insulators or conductor, mechanical and a variety of properties. The nanocomposites may have mechanically plastic behavior or elastic behavior and may have a water-loving or a water-hating nature. Finally, polymer doping with the metal nanoparticles is the easiest and widely convenient way for stabilization and handling the nanostructured metals [1]. The most interesting metals that were used in the nanocomposite materials are the noble metals. Noble metals lie in group 11 in the periodic tables and called (d-blocks). The most important thing that characterizes the noble metals in nanoscale is the surface plasmon resonance (SPR) and is formed due to the collective oscillations of the electrons that are located on the nanoparticles surface. This electron pulsation is proportionally related to the light electromagnetic field which fall on the electrons, that is, the conduction electrons symmetrically vibrate at its location when exposed to the light, as

The simplest and normal shape of the produced composites is the films or powders, and also these are good for exploiting the desired properties. The combination between the metal nanoparticles and the polymer is very wonderful because its composite has good and promising physical and chemical properties. One of the most important things that make this combination to be excellent is the method that combines/connects them together (preparation

There are two general and principle approaches for preparation of the polymer/metal nanocomposite: the ex situ and in situ methods [1]. In the ex situ route, at first, the metal nanoparticles are synthesized, and the surface of the created particles is encapsulated and passivated with organic polymer materials. Then, the metal nanoparticle derivatives are dispersed into the liquid monomer of the polymer solution that is then polymerized. Contrarily, the metal ions are located on site with the monomer and the monomer occurred, where the metal ion reduced chemically, thermally, or by UV irradiation during the polymerization process to obtain the nanoparticles; this method is called the in situ methods. Illustration with more information and details of some of these key

**Figure 1.** Cartoon showing the happy noble metal possessing the surface plasmon resonance.

shown in **Figure 1**.

46 Nanocomposites - Recent Evolutions

method).

methods follows.

The metal nanoparticles are created in the ex situ method by any traditional method (chemical reduction, precipitation, laser ablation, etc.) and then the surface of the created particles is stabilized, capsulated and passivated by using stabilizing agent. This can be achieved by the reduction of the metal precursors which was dissolved in the appropriate selected solvent such as water or ethanol, which often contain a polymer as a stabilizing agent [2, 3]. Otherwise, it can be stabilized and be ready by controlling micelle, reverse micelle or micro-emulsion reactions [4–6]. Often, the surface of the obtained particles by the ex situ method is manipulated by covalent bonds, metal-thiol or ligand with other ions to inhibit the agglomeration and aggregation processes [7] or by encapsulating a polymer [8]. Then, the produced metal nanoparticles are inserted into the polymer matrix. This is achieved by combining the obtained nanoparticles with the polymer solution, or by interacting with the monomer, and then followed by the casting techniques, etc. to obtain the nanocomposite films [9, 10], as shown in **Figure 2**.

The ex situ technique was successfully used to prepare many metals in nanoscale such as Ag, Cu, Pt and Au with specific size and shape by using an external reducing agent (sodium borohydride NaBH4 , tannic acid and sodium carbonate Na2 CO3 , hydrazine hydrate), then added to the polymer matrix such as (polyvinyl alcohol (PVA), polyvinyl pyrrolidone (PVP), polymethyl methacrylate (PMMA)) to produce the polymer/metal nanocomposites [11–13].

Sharma et al. [14], used the ex situ method to synthesize the polyaniline/copper nanocomposites by using the NaBH4 as a reducing agent to reduce the copper salt to copper nanoparticles. Also, Yao et al. [15] used the trisodium citrate to reduce the gold salt to gold nanoparticles and then embedded in the PVA to produce the PVA/Au nanocomposites. Feng et al. [16] obtained the PVA/Ag nanocomposite films via the ex -situ method. They used the tannic acid and

**Figure 2.** Scenario that depicts the ex situ technique (left side) and the in situ technique (right side).

Na2 CO3 as a reducing and stabilizing agent to synthesize the silver nanoparticles. Then, the PVA solution was added to produce the nanocomposites. Another work by Campos et al. [17] was reported by the ex situ synthesis of the PVA/Ag nanocomposite films. The Ag nanoparticles powder with an average particle size of 25 nm was obtained and then mixed it with the PVA solution to produce the nanocomposite films.

However, this method has a shortcoming such as the aggregation, agglomeration and dispersion problems. It is necessary to manipulate and passivate the surface of the metal nanoparticles after the reduction process, in order to disperse them in the matrix. This surface manipulation led to changes in the properties of the nanoparticles. However, and after the surface manipulation process, it is very difficult to get well dispersion of nanoparticles in the composite. And also still a certain degree of agglomeration and aggregation is found in the composite. Also, the compatibility is another problem that facing the ex situ method, because of the difficulty of selecting the solvent which facilitates the compatibility between the particle-polymer- solvent systems. So, it was necessary to find a method (the in situ method) to overcome these problems.

#### **2.2. In situ polymerization**

The polymeric materials doped with metal nanofillers have been created by the in situ polymerization methods, which are composed of various techniques. The in situ methods have more benefits as compared to the ex situ methods such as its more simple, easy and straight-forward, and producing the class of nanocomposite materials with a higher feature and a higher quality, and more precise controlling. Firstly, the dispersion of metal nanofiller in a polymer monomer is utilized in this in situ method. Also, a technique similar to bulk polymerization is used to polymerize the resulting mixture. **Figure 2** shows a schematic diagram for the in situ technique.

distribution over the surface from the nonbonding hybrid electrons 2p2

as (4) an arbitrary spiral of three layers. [18]. Copyright 2007. Reused with permission from Elsevier Ltd.

reaction of the Ag + → Ag, which can be expressed as follows [18]:

Ag+ + e<sup>−</sup> (from a PVA molecule) Active

via an intermediate product of a polymer PVA/Ag+

complex of an intermediate state Agq+

reactions. Where and simply, the PVA<sup>−</sup>

[−R − OH−]

drives the Ag+ → Ag reduction,

rupts the PVA/Ag+

(the head groups) to interact with the metal ions to create a complex. Thermodynamically, the fugacity of the Ag surface is enhanced by Van der Waals interaction and facilitates a surface

**Figure 3.** (1) A model coplanar structure of PVA molecules with interchain bridging via H-bonding between (2) the monomers forming (3) a molecular layer of extended surface. Such layers recombine further in different structures such

The Ag metal, which passes in steps with temporary intermediate Ag oxidation states, creates

rapid transform in the mixture color. Where the Ag metal creates through a polymer PVA/Ag+

to Ag atoms followed by coalescence, and then the Ag cluster forms and grows to achieve Ag particles. The Ag nanoparticles capsulated by the PVA molecules are stabilized from oxidative

that is, one hydroxyl group "OH" is substituted by oxygen 'O' group, with the number of monomers (n = Ω1n') in the native polymer molecule. Where the produced hydrogen atoms

<sup>n</sup> → Ω1 [−R − OH]

→

*PVA* [Ag+ –PVA]

complex structure. The Ag clusters formed from the Agq+ species convert

     Ag + PVA<sup>−</sup>

−

↓

(q < 1), the PVA oxidation during this reaction inter-

represents a partially oxidized state of PVA as follows:

(complex)

complex in this model reaction, reflecting a

<sup>n</sup>" − CH2 − CH − O + 12 H2 (2)

(O) in such OH groups

Polymer/Noble Metal Nanocomposites http://dx.doi.org/10.5772/intechopen.79016 49

(1)

In general, the in situ reduction mechanism in the PVA/Ag nanocomposite films is described as reported as example for the PVA/Ag nanocomposite as the following: the polymer PVA has a linear structure with the principle carbon backbone chains. Polyvinyl alcohol (PVA) molecules are hardly aggregate in dilute solution (water). Nevertheless, **Figure 3** shows the linear chains in a PVA planar structure, which interbridged by the H bonding coming from (or through) the OH groups. **Figure 3(a**, **b)** shows the PVA polymer in model configuration, where the PVA monomer is denoted by the symbol (R) and the PVA in part with an OH group. However, **Figure 3(c)** shows a cross section of the PVA structure. As apparent from the classical structure, especially when the PVA molecules dispersed in a dilute solution (water), the lateral or side growth to the backbone occurs. PVA structure contains weak hydrogen bonds, which extended to a little interbridging chain. In the coplanar PVA molecules structure, the hydrogen H-bond in a warm liquid at 60–70°C showed as floating and handling individually atoms and separately isolated with no much interaction with one another. The dispersion was facilitated by the thermo-mechanical stirring. It is possible that PVA molecular layers transform to a fibril structure or a favored structure, spiral in shape in linear polymer molecules (**Figure 3(d)**). Where the molecular layers of H-bonded monomers were interbridged from the OH groups, this is a compact structure, which takes place preferably in small island and dispersed molecules. At the surfaces, any type of the PVA molecular configurations has plenty of OH groups free from the H-bonding. There are electrons arranged in a definite localized

Na2 CO3

48 Nanocomposites - Recent Evolutions

as a reducing and stabilizing agent to synthesize the silver nanoparticles. Then, the

PVA solution was added to produce the nanocomposites. Another work by Campos et al. [17] was reported by the ex situ synthesis of the PVA/Ag nanocomposite films. The Ag nanoparticles powder with an average particle size of 25 nm was obtained and then mixed it with the

However, this method has a shortcoming such as the aggregation, agglomeration and dispersion problems. It is necessary to manipulate and passivate the surface of the metal nanoparticles after the reduction process, in order to disperse them in the matrix. This surface manipulation led to changes in the properties of the nanoparticles. However, and after the surface manipulation process, it is very difficult to get well dispersion of nanoparticles in the composite. And also still a certain degree of agglomeration and aggregation is found in the composite. Also, the compatibility is another problem that facing the ex situ method, because of the difficulty of selecting the solvent which facilitates the compatibility between the particle-polymer- solvent systems. So, it was necessary to find a method (the in situ method)

The polymeric materials doped with metal nanofillers have been created by the in situ polymerization methods, which are composed of various techniques. The in situ methods have more benefits as compared to the ex situ methods such as its more simple, easy and straight-forward, and producing the class of nanocomposite materials with a higher feature and a higher quality, and more precise controlling. Firstly, the dispersion of metal nanofiller in a polymer monomer is utilized in this in situ method. Also, a technique similar to bulk polymerization is used to polymerize the resulting mixture. **Figure 2** shows a schematic diagram for the in situ technique.

In general, the in situ reduction mechanism in the PVA/Ag nanocomposite films is described as reported as example for the PVA/Ag nanocomposite as the following: the polymer PVA has a linear structure with the principle carbon backbone chains. Polyvinyl alcohol (PVA) molecules are hardly aggregate in dilute solution (water). Nevertheless, **Figure 3** shows the linear chains in a PVA planar structure, which interbridged by the H bonding coming from (or through) the OH groups. **Figure 3(a**, **b)** shows the PVA polymer in model configuration, where the PVA monomer is denoted by the symbol (R) and the PVA in part with an OH group. However, **Figure 3(c)** shows a cross section of the PVA structure. As apparent from the classical structure, especially when the PVA molecules dispersed in a dilute solution (water), the lateral or side growth to the backbone occurs. PVA structure contains weak hydrogen bonds, which extended to a little interbridging chain. In the coplanar PVA molecules structure, the hydrogen H-bond in a warm liquid at 60–70°C showed as floating and handling individually atoms and separately isolated with no much interaction with one another. The dispersion was facilitated by the thermo-mechanical stirring. It is possible that PVA molecular layers transform to a fibril structure or a favored structure, spiral in shape in linear polymer molecules (**Figure 3(d)**). Where the molecular layers of H-bonded monomers were interbridged from the OH groups, this is a compact structure, which takes place preferably in small island and dispersed molecules. At the surfaces, any type of the PVA molecular configurations has plenty of OH groups free from the H-bonding. There are electrons arranged in a definite localized

PVA solution to produce the nanocomposite films.

to overcome these problems.

**2.2. In situ polymerization**

**Figure 3.** (1) A model coplanar structure of PVA molecules with interchain bridging via H-bonding between (2) the monomers forming (3) a molecular layer of extended surface. Such layers recombine further in different structures such as (4) an arbitrary spiral of three layers. [18]. Copyright 2007. Reused with permission from Elsevier Ltd.

distribution over the surface from the nonbonding hybrid electrons 2p2 (O) in such OH groups (the head groups) to interact with the metal ions to create a complex. Thermodynamically, the fugacity of the Ag surface is enhanced by Van der Waals interaction and facilitates a surface reaction of the Ag + → Ag, which can be expressed as follows [18]:

$$\begin{array}{l} \text{\(\textbf{Ag}^\*\text{'} \rightarrow \textbf{Ag}\text{, \(\textbf{m}\)}\text{\(\textbf{m}\)}\text{ can be expressed as follows [10]:}\\\\ \text{Ag}^\* + \text{e}^- \text{ (from a PVA molecule)} \stackrel{\text{\(\textbf{A}\)}}{\rightarrow} \begin{array}{l} \text{\(\textbf{Ag}^\*\text{-} \text{PVA}\)}^- \text{\(\textbf{complex}\)}\\ \downarrow\\ \text{\(\textbf{A}\)}\\ \text{\(\textbf{Ag} + \text{PVA}^-\)} \end{array} \tag{1}$$

The Ag metal, which passes in steps with temporary intermediate Ag oxidation states, creates via an intermediate product of a polymer PVA/Ag+ complex in this model reaction, reflecting a rapid transform in the mixture color. Where the Ag metal creates through a polymer PVA/Ag+ complex of an intermediate state Agq+ (q < 1), the PVA oxidation during this reaction interrupts the PVA/Ag+ complex structure. The Ag clusters formed from the Agq+ species convert to Ag atoms followed by coalescence, and then the Ag cluster forms and grows to achieve Ag particles. The Ag nanoparticles capsulated by the PVA molecules are stabilized from oxidative reactions. Where and simply, the PVA<sup>−</sup> represents a partially oxidized state of PVA as follows:

$$\text{[}\text{-R}-\text{OH}-\text{]}\_{\text{n}} \rightarrow \text{ }\Omega1\text{[}-\text{R}-\text{OH]}\_{\text{n}}\text{---CH}\_{2}\text{--CH}-\text{O}+12\,\text{H}\_{2}\tag{2}$$

that is, one hydroxyl group "OH" is substituted by oxygen 'O' group, with the number of monomers (n = Ω1n') in the native polymer molecule. Where the produced hydrogen atoms drives the Ag+ → Ag reduction,

$$\text{Ag}^+ + 12\,\text{H}\_2 \rightarrow \text{Ag} + \text{H}^+ \text{(here, nitric acid)}\tag{3}$$

Ag nanoparticles and obtained the nanocomposites. Ananth et al. [11] prepared the PVA/Ag nanocomposites by using the in situ method for the SPR-based protein sensors. Karthikeyan et al. [24] synthesized free-standing PVA/Pt nanocomposite films via in situ method for ultra-

The radiolytic process means that the molecules separate to smaller atoms, radicals or ions by ionizing radiation like γ rays, UV irradiation. In fact, the gamma irradiation method is one of the most interested methods for preparing the polymer/metal nanocomposites. The desired highly reducing radicals generated free from any by-product are the main feature of gamma irradiation method for the creation of noble metallic nanoparticles. The primary and first

radicals and upon gamma irradiation, the molecules produced in water are [25].

is converted from colorless to pale yellow color as a result of the Ag+

**<sup>−</sup>** (2.7), OH∗(2.7) H∗(0.6), H2

The numbers in parentheses represents the G values. The G value for a given irradiated system is the absolute chemical yield expressed as the number of individual chemical events occurring per 100 eV of absorbed energy. Thus the G (eaq-), G (OH•), etc. are the number of solvated electrons, hydroxyl radicals, etc., formed per 100 eV of absorbed energy. The alcohol radical is produced when the radicals (OH\* and H\*) are able to capture hydrogen from the alcohol group. This way, oxidizing OH\* radicals are transformed into reducing alcohol radicals. The scavenger material was used to make the reaction free from the OH\* radicals. The

and OH\* radicals possess the electron transfer reaction between the PVA and Ag+, and

It is well known that the OH\* radicals induce the cross-linking of PVA molecules in aqueous medium by/through the gamma radiation (with the G value of the intermolecular cross-

2PVA(H) + 2OH<sup>∗</sup> → PVA―PVA(cross − linked polymer) + 2H2 O (6)

hydrated electrons, reducing agent and the PVA radicals created by the H atom, which is

The hydroxyl radicals formed by the hydrated electrons during the gamma irradiation of

ions are reduced, under the given conditions of the experiment, with highly reducing

(0.45), H2 O2

ions to Ag particles takes place. The color of the composite solution

**<sup>−</sup>** /PVA<sup>∗</sup> → (Ag)n (7)

**<sup>−</sup>** → N2 + 2OH<sup>∗</sup> (8)

(0.7) (5)

Polymer/Noble Metal Nanocomposites http://dx.doi.org/10.5772/intechopen.79016 51

ions reduction in PVA

fast optical power-limiting applications.

**2.3. Radiolytic method**

H2 O → eaq

then the reduction of Ag+

solution by the radiolytic method.

linking induced via the gamma irradiation is 0.48).

abstracted from the (OH\*) radicals in the PVA chains.

nAg + +n eaq

N2 O + 2 eaq

O-saturated aqueous solution are as follows:

eaq **−**

The Ag+

N2

The byproduct nitric acid (HNO3 ) evaporates during the heating process, and the PVA polymer molecules capping the Ag metal results in small size particles (nanoparticles). To obtain the Ag metal of neat dispersed particles in the PVA matrix, the external heating is very important and a crucial process. Otherwise, the residual of (HNO3 ) acid interacts with Ag metal back to AgNO3 . The PVA has two functions in the reaction, first encapsulating the Ag particles and separating the Ag+ → Ag reactions in small isolated groups. Second, the structure of the isolated Ag particles can be controlled by the PVA. Under a hot condition of 60–70°C and mechanical stirring, the PVA molecules dissolved in the water are difficult to agglomerate and aggregate. From the models shown in **Figure 3**, the surface energy of the planar structure is high because of the molecular stretching of the bonds. Also, the PVA molecules interacted with OH groups via the hydrogen bonds, which work as follows:


The nucleation and growth processes of the Ag particles start and depend on their morphology, the Ag size, and also on its template stability. A spiral structure stimulates a fibril of the PVA, whereas the different shapes (platelet or spheroids) grow by a PVA crystalline lamellar regions. The small spherical templates participate to evolve the spherical or near special shapes of the nanoparticles according to the reaction species interactions through the interface layer.

The reported mechanism of this reaction based on 'polyol route,' the PVA monomer, involves the secondary alcohol groups, R2 CHOH, on the polymer.

$$\text{R}\_2\text{CHOH} + \text{AgNO}\_3 \rightarrow \text{R}\_2\text{CO} + \text{H}\_2\text{O} + \text{NO}\_2 + \text{Ag} \tag{4}$$

Here, R denotes a PVA monomer.

Recently, Deb and Sarkar [19] used the in situ method to prepare the PVA/Ag nanocomposite films and then followed the films by thermal annealing to obtain the PVA/Ag nanorods. In the same context, Llorens et al. [20] synthesized the cellulose/copper nanocomposite fibers by using in situ thermal treatment. Bogdanovic et al. [21] prepared the polyaniline/copper nanocomposites via the in situ method, wherein the Cu nanoparticles and polyaniline are created at the same instant. In this case, the reaction takes place at room temperature to obtain the nanocomposite. This route can be counted as a simple and inexpensive method of preparation. Also, Becerra et al. [22] prepared the poly(vinyl chloride)/copper nanocomposite films by the in situ method for antibacterial applications. Also, El-Shamy et al. [23] recently used the in situ method to produce the PVA/Ag nanocomposites, where they used the PVA polymer as a reducing agent by activation of the OH group in the PVA at 60°C to reduce the AgNO3 to Ag nanoparticles and obtained the nanocomposites. Ananth et al. [11] prepared the PVA/Ag nanocomposites by using the in situ method for the SPR-based protein sensors. Karthikeyan et al. [24] synthesized free-standing PVA/Pt nanocomposite films via in situ method for ultrafast optical power-limiting applications.

#### **2.3. Radiolytic method**

Ag+ + 12 H2 → Ag + H+ (here, nitric acid) (3)

mer molecules capping the Ag metal results in small size particles (nanoparticles). To obtain the Ag metal of neat dispersed particles in the PVA matrix, the external heating is very impor-

and separating the Ag+ → Ag reactions in small isolated groups. Second, the structure of the isolated Ag particles can be controlled by the PVA. Under a hot condition of 60–70°C and mechanical stirring, the PVA molecules dissolved in the water are difficult to agglomerate and aggregate. From the models shown in **Figure 3**, the surface energy of the planar structure is high because of the molecular stretching of the bonds. Also, the PVA molecules interacted

**4.** A protecting surface coating to inhibit the growth of the Ag nanoparticles and obtain a

The nucleation and growth processes of the Ag particles start and depend on their morphology, the Ag size, and also on its template stability. A spiral structure stimulates a fibril of the PVA, whereas the different shapes (platelet or spheroids) grow by a PVA crystalline lamellar regions. The small spherical templates participate to evolve the spherical or near special shapes of the nanoparticles according to the reaction species interactions through the interface layer. The reported mechanism of this reaction based on 'polyol route,' the PVA monomer, involves

CHOH, on the polymer.

R2 CHOH + AgNO3 → R2 CO + H2 O + NO2 + Ag (4)

Recently, Deb and Sarkar [19] used the in situ method to prepare the PVA/Ag nanocomposite films and then followed the films by thermal annealing to obtain the PVA/Ag nanorods. In the same context, Llorens et al. [20] synthesized the cellulose/copper nanocomposite fibers by using in situ thermal treatment. Bogdanovic et al. [21] prepared the polyaniline/copper nanocomposites via the in situ method, wherein the Cu nanoparticles and polyaniline are created at the same instant. In this case, the reaction takes place at room temperature to obtain the nanocomposite. This route can be counted as a simple and inexpensive method of preparation. Also, Becerra et al. [22] prepared the poly(vinyl chloride)/copper nanocomposite films by the in situ method for antibacterial applications. Also, El-Shamy et al. [23] recently used the in situ method to produce the PVA/Ag nanocomposites, where they used the PVA polymer as a reducing agent by activation of the OH group in the PVA at 60°C to reduce the AgNO3

. The PVA has two functions in the reaction, first encapsulating the Ag particles

) evaporates during the heating process, and the PVA poly-

) acid interacts with Ag metal

to

The byproduct nitric acid (HNO3

50 Nanocomposites - Recent Evolutions

stable PVA/Ag surface-interface.

the secondary alcohol groups, R2

Here, R denotes a PVA monomer.

back to AgNO3

tant and a crucial process. Otherwise, the residual of (HNO3

with OH groups via the hydrogen bonds, which work as follows:

**2.** A weak reducing agent, at moderate rate, to give Ag+ → Ag reaction; **3.** A surface stabilizer to maintain the Ag nanoparticles in the sample;

**1.** A matrix to Ag+ → Ag reaction occurs over such surfaces;

The radiolytic process means that the molecules separate to smaller atoms, radicals or ions by ionizing radiation like γ rays, UV irradiation. In fact, the gamma irradiation method is one of the most interested methods for preparing the polymer/metal nanocomposites. The desired highly reducing radicals generated free from any by-product are the main feature of gamma irradiation method for the creation of noble metallic nanoparticles. The primary and first radicals and upon gamma irradiation, the molecules produced in water are [25].

$$\mathrm{H}\_{\mathrm{2}}\mathrm{O} \rightarrow \mathrm{e}\_{\mathrm{aq}}\mathrm{-}(2.7), \mathrm{OH}\mathrm{+}(2.7)\,\mathrm{H}\mathrm{+}(0.6), \mathrm{H}\_{\mathrm{2}}\mathrm{(0.45)}, \mathrm{H}\_{\mathrm{2}}\mathrm{O}\_{\mathrm{2}}\mathrm{(0.7)}\tag{5}$$

The numbers in parentheses represents the G values. The G value for a given irradiated system is the absolute chemical yield expressed as the number of individual chemical events occurring per 100 eV of absorbed energy. Thus the G (eaq-), G (OH•), etc. are the number of solvated electrons, hydroxyl radicals, etc., formed per 100 eV of absorbed energy. The alcohol radical is produced when the radicals (OH\* and H\*) are able to capture hydrogen from the alcohol group. This way, oxidizing OH\* radicals are transformed into reducing alcohol radicals. The scavenger material was used to make the reaction free from the OH\* radicals. The eaq **−** and OH\* radicals possess the electron transfer reaction between the PVA and Ag+, and then the reduction of Ag+ ions to Ag particles takes place. The color of the composite solution is converted from colorless to pale yellow color as a result of the Ag+ ions reduction in PVA solution by the radiolytic method.

It is well known that the OH\* radicals induce the cross-linking of PVA molecules in aqueous medium by/through the gamma radiation (with the G value of the intermolecular crosslinking induced via the gamma irradiation is 0.48).

$$2\text{PVA(H)} + 2\text{OH}^{\*} \rightarrow \text{PVA} \xrightarrow{\text{PVA}} \text{PVA(cross-linked polymer)} + 2\text{H}\_{2}\text{O} \tag{6}$$

The Ag+ ions are reduced, under the given conditions of the experiment, with highly reducing hydrated electrons, reducing agent and the PVA radicals created by the H atom, which is abstracted from the (OH\*) radicals in the PVA chains.

$$\mathsf{nAg} \star \mathsf{\taun} \mathrm{e}\_{\mathsf{aq}}^{-} / \mathsf{PVA}^{\*} \to \left( \mathsf{Ag} \right)\_{\mathsf{n}} \tag{7}$$

The hydroxyl radicals formed by the hydrated electrons during the gamma irradiation of N2 O-saturated aqueous solution are as follows:

$$\mathrm{N}\_{2}\mathrm{O} + 2\,\mathrm{e}\_{\mathrm{aq}}\mathrm{"\rightarrow N}\_{\mathrm{2}} + 2\mathrm{OH}^{\*}\tag{8}$$

According to the reaction in Eq. (6), the PVA interacts with the hydroxyl (OH\*) radicals and is lost in the reaction to obtain the polymeric PVA\* radicals. After the gamma irradiation of the (PVA/Ag+ ions) hydrogel, the color of the hydrogel is changed to pale yellow color due to the electron transfer interaction between the PVA and Ag+. Reduction of the silver ions in PVA/Ag+ hydrogel induced the creation of Ag nanoparticles with a characteristic fingerprint surface plasmon resonance (SPR) band.

The advantage of the gamma irradiation method comes from the gamma rays and is as follows: (1) hydrated electron resulted from the gamma radiolysis can reduce metal ions to metal nanoparticles. (2) Escaping from the use of external reducing agent and the resultant side reactions like oxidation reactions in UV irradiations and other byproducts produced in the reducing agent methods. (3) The gamma rays used to control the reduction reaction by controlling the doses of the irradiation and also the amount of the reduced nanoparticles nuclei by controlling the amount of radicals. (4) The gamma radiation used to reduce the AgNO3 to Ag seeds point which serves as nuclei or nucleation sites for Ag atoms formation, to start the Ag nanoparticles growing directly on the PVA backbone. Also, this method has the ability to produce the metal nanoparticles in different shape and size.

Recently, El-Shamy and his group [26] reported a promising route for the creation of the PVA/Ag and the (PVA/Ag nanorods) nanocomposites by using the gamma rays. After producing the PVA/Ag nanocomposite films via the chemical reduction in situ route, the films were directed to gamma rays with different irradiation doses from 25 to 100 KGy with steps 25 KGy, and at special case, the Ag nano-rods were produced at 125 KGy, and the Ag nanoparticles appeared on the back surface (the surface not facing the gamma source) as nanorods, as shown in **Figure 4**.

In the real reaction, two processes may occur simultaneously. At first, Ag nanoparticles are created through homogeneous nucleation process and then grow along the direction of the lowest energy {111} plane. The Ag nanoparticles were firstly created by the homogeneous nucleation process, through the silver nitrate reduction by gamma rays. The nucleation process was done by the Ag seeds which was a source of the formation of Ag atoms. The rod-like PVA micelles were created through the gamma irradiation, and this is strongly related to the PVA: Ag + molar ratio. The backbone chain of PVA contains oxygen atoms from the (-OH) groups in the PVA. This oxygen atoms coordinate with the Ag to form complexes (PVA/Ag+) as an intermediate state in the reaction via the covalent bonds. The second step includes merging and fusing of the Ag nanoparticles to create Ag nanorods in the matrix via the photothermal effect of gamma irradiation.

Here, the PVA polymer has two functions in the reaction: (1) the first function is the PVA forming a complex (PVA/Ag+) with Ag + through the coordination reaction and (2) the second function is the PVA used to adsorb on the Ag nanoparticles facets. From the PVA structure, the binding capacity of PVA to the Ag surface increases, because of the PVA containing the ▬C=O groups. So, the adsorption of the PVA on the Ag nanoparticles surface increases. From this fact, the Ag crystals interact with the PVA groups (▬C=O), leading to a decrease in the crystal growth of the {100} plane as compared to the {111} plane. The plane {100} has energy lower than the plane {111}, so there is a high energy difference between the two surfaces. According

**Figure 5.** Schematic illustration of the formation of the Ag nanorods in the PVA matrix.

**Figure 4.** SEM images of front and back surfaces for samples 0 (b1), 100 (b4) and 125 KGy (b5) [26]. Copyright 2018.

Polymer/Noble Metal Nanocomposites http://dx.doi.org/10.5772/intechopen.79016 53

Reused with permission from Elsevier Ltd.

According to the reaction in Eq. (6), the PVA interacts with the hydroxyl (OH\*) radicals and is lost in the reaction to obtain the polymeric PVA\* radicals. After the gamma irradiation of

to the electron transfer interaction between the PVA and Ag+. Reduction of the silver ions in

The advantage of the gamma irradiation method comes from the gamma rays and is as follows: (1) hydrated electron resulted from the gamma radiolysis can reduce metal ions to metal nanoparticles. (2) Escaping from the use of external reducing agent and the resultant side reactions like oxidation reactions in UV irradiations and other byproducts produced in the reducing agent methods. (3) The gamma rays used to control the reduction reaction by controlling the doses of the irradiation and also the amount of the reduced nanoparticles nuclei by controlling the amount of radicals. (4) The gamma radiation used to reduce the AgNO3

Ag seeds point which serves as nuclei or nucleation sites for Ag atoms formation, to start the Ag nanoparticles growing directly on the PVA backbone. Also, this method has the ability to

Recently, El-Shamy and his group [26] reported a promising route for the creation of the PVA/Ag and the (PVA/Ag nanorods) nanocomposites by using the gamma rays. After producing the PVA/Ag nanocomposite films via the chemical reduction in situ route, the films were directed to gamma rays with different irradiation doses from 25 to 100 KGy with steps 25 KGy, and at special case, the Ag nano-rods were produced at 125 KGy, and the Ag nanoparticles appeared on the back surface (the surface not facing the gamma source) as nanorods, as shown in

In the real reaction, two processes may occur simultaneously. At first, Ag nanoparticles are created through homogeneous nucleation process and then grow along the direction of the lowest energy {111} plane. The Ag nanoparticles were firstly created by the homogeneous nucleation process, through the silver nitrate reduction by gamma rays. The nucleation process was done by the Ag seeds which was a source of the formation of Ag atoms. The rod-like PVA micelles were created through the gamma irradiation, and this is strongly related to the PVA: Ag + molar ratio. The backbone chain of PVA contains oxygen atoms from the (-OH) groups in the PVA. This oxygen atoms coordinate with the Ag to form complexes (PVA/Ag+) as an intermediate state in the reaction via the covalent bonds. The second step includes merging and fusing of the Ag nanoparticles to create Ag nanorods in the matrix via the photo-

Here, the PVA polymer has two functions in the reaction: (1) the first function is the PVA forming a complex (PVA/Ag+) with Ag + through the coordination reaction and (2) the second function is the PVA used to adsorb on the Ag nanoparticles facets. From the PVA structure, the binding capacity of PVA to the Ag surface increases, because of the PVA containing the ▬C=O groups. So, the adsorption of the PVA on the Ag nanoparticles surface increases. From this fact, the Ag crystals interact with the PVA groups (▬C=O), leading to a decrease in the crystal growth of the {100} plane as compared to the {111} plane. The plane {100} has energy lower than the plane {111}, so there is a high energy difference between the two surfaces. According

ions) hydrogel, the color of the hydrogel is changed to pale yellow color due

to

hydrogel induced the creation of Ag nanoparticles with a characteristic fingerprint

the (PVA/Ag+

52 Nanocomposites - Recent Evolutions

surface plasmon resonance (SPR) band.

thermal effect of gamma irradiation.

produce the metal nanoparticles in different shape and size.

PVA/Ag+

**Figure 4**.

**Figure 4.** SEM images of front and back surfaces for samples 0 (b1), 100 (b4) and 125 KGy (b5) [26]. Copyright 2018. Reused with permission from Elsevier Ltd.

**Figure 5.** Schematic illustration of the formation of the Ag nanorods in the PVA matrix.

to this fact, the reactivity of the plane {100} to interact with PVA is larger than the reactivity of the plane {111} with PVA. So, there is a large difference in reactivity between the two surfaces of Ag nanorods. Consequently, the PVA coated the plane {100} of the Ag nanorods and completely blocked Ag nanorods from growing. On the other hand, the PVA partially coated the plane {111} of the Ag and also PVA partially blocked the Ag nanorods from growing along this direction as shown in **Figure 5**. This investigation confirms that the PVA macromolecule interacts with the Ag plane {100}, which is stronger than the Ag plane {111}.

Yonghong et al. [27] successfully prepared the polyacrylamide/gold (PAM/Au) nanocomposites by γ (gamma)-irradiation in an ethanol system. In a similar way, Krkljes et al. [28] prepared the PVA/gold nanocomposites via the gamma irradiation, in situ method. Also, Ali et al. [29] recently used the gamma irradiation to prepare the PVA/copper nanocomposites, with the obtained copper size ranging from 13.9 to around 19 nm. Graeser et al. [30] used gamma irradiation to reduce Ag + ions in the presence of polyvinylpyrrolidone (PVP) to produce the PVP/Ag nanocomposites.
