Metallic nanoparticles and its composites have emerged as valuable asset in all phases of material science and engineering including electronic, optics and electromagnetic domains. Silver nanoparticles (Ag NPs) are one of the most vital and fascinating nanomaterials among several metallic nanoparticles due to its large surface ratio and outstanding properties with diverse field of potential applications. We demonstrated various synthesis techniques of nanocomposites, silver nanoparticles and composite based on these particles have shown great importance because of the remarkable properties (high electrical and thermal conductivity, good chemical stability and catalytic properties) of silver nanoparticles. This chapter provides various synthesis techniques for preparation of silver nanoparticles and their composites with dielectric and electrical properties in a lucid manner. The detail discussions of silver-polymer nanocomposites, emphasizing on each individual synthesis routes and properties have been carried out.
- Ag nanoparticles
- polymer manocomposites
In recent decade nanotechnology (nano signify very small that denotes to one billionth or 10−9 m in size) is a recognized as one of the most emerging fields of contemporary research deals with synthesis, manufacturing, strategy and tailoring of particle size approximately varying from 1 to 100 nm. The nanoparticles have unique magnetic, electronic and optical properties because of their high surface area to volume ratio and wide variety of applications including environmental health, optics, electronics; optoelectronics, catalysis and energy storage devices [1, 2, 3, 4, 5, 6]. Nanoparticles possess small size; composition and shape have differences in their physical and chemical behaviors from their parent materials. Moreover, the smaller size of the nanomaterials also helps them to penetrate exact cellular locations and additional surface area facilitates increased absorption and targeted delivery of the substances [6, 7, 8]. A large number of nanomaterials have multitude of technological applicability in the field of engineering including electronics, biomedical, drug-gene delivery, environment, catalysis, light emitters, single electron transistors, non-linear optical or photo-electrochemical devices [9, 10]. The synthesis of nanomaterials by using chemical and physical methods is relatively expensive and potentially hazardous to the environment due to the effect of toxic chemicals and it is responsible for various biological risks . Nanomaterials have rised as appropriate alternatives to overcome the shortcomings of micro-composites and monolithics, while challenges exist related to preparation for controlling elemental composition and stoichiometry in the nanocluster phase. Engineered nanomaterials produced with nanoscale dimension are commonly grouped into four kinds: carbon, metal oxides, dendrimers and composites . However, composites are another powerful tool for the progress of specific materials according to our requirements. Nanocomposites are composites in which at least one of the phases exhibit dimensions in the nanometer range (1 nm = 10−9 m) and also shows high performance material with unusual performance and unique design possibilities . The nanoparticles based composite by using various polymeric matrices and fillers such as metal powders (conductive filler) have made immense interest from academia and industry [14, 15]. These nanocomposites show various performances like high strength, stiffness at elevated temperature, corrosion resistance, low weight, low maintenance cost and attractive thermal coefficient . Besides, composites generally divided into two stages such as continuous phase which is commonly uses as matrix and the other phases such as reinforcement which is embedded into the matrix. Moreover, the unique combination of matrix materials including polymer, carbon, metal, ceramics and different types of the reinforcements such as fibers, particles and layered materials have been widely utilized for the fabrication of composites . It has been reported that, there is change in particle properties when size of the particle is less than a particular level called critical size . In addition, as dimensions reach the nanometer level, interactions at phase interfaces become largely improved and this is important to enhance material properties. In view of this, the surface area-volume ratio of reinforced materials employed in the preparation of nanocomposites is the crucial role to the understanding of their structure–property co-relations. Furthermore, the discovery (1991) of carbon material especially carbon nanotube (CNTs)  and their subsequent use to fabricate composites with improved mechanical, thermal and electrical properties [13, 19] added a new and attractive dimension in the field of nanotechnology. Currently, nanocomposites offer new technology and business opportunities for all areas of industry making environmental friendly .
The synthesis and characterization of composites using organic and inorganic nanomaterials have been extensively investigated area of research with improved functional properties and wide range of potential applications such as coating, packaging materials, sensors, energy storage, etc. [21, 22]. The measurement of various properties of the prepared composite materials mostly depends on the characteristics of the original materials. There is a variety of properties of the matrix materials, for instance it combines the dispersed phase mutually. It protects the dispersed phase from chemical action and maintain in suitable position and its direction . It is revealed that the properties of the polymer composites associated with chemistry extend of polymer chain and thermoset cure can change from the interface between reinforcement and matrix. The silver nanoparticle based polymer composites can be used as biocompatible materials with improved antimicrobial activity as well as good electrical conductivity and catalytic properties [24, 25, 26]. The polymer based nanocomposites using nanoparticles have been successfully synthesized by various researchers via free radical thermal polymerization  and photo polymerization . There are several attempts to synthesize Ag based polymer composites: For example; Nikfarjam
Urged by scientific interest and potential application for the various green synthetic techniques of silver nanoparticle related polymer based composites research has increased to a surprising scale, opening new challenges and opportunity for the further modulation of properties. This typical chapter is primarily divided into two major sections. The first section covers various synthetic routes of silver (Ag) nanoparticles. The second section covers the synthesis, dielectric and electrical properties of silver-polymer nanocomposites in the field of energy storage devices. We end this chapter with a few words on this new and exciting research area of the Ag nanoparticles followed by summary and perspective.
2. Synthesis of Ag nanoparticles
The new synthesis strategy for the material fabrications are of essential importance of nanostructure material in the field of nanotechnology. The applications of nanomaterials are possible only when nano-structured materials are made available with required size, morphology, crystal, chemical composition and their unique properties in the field of various technological applications [29, 30, 31]. Silver nanoparticles (Ag NPs) have outstanding microbial resistant ability. These nanoparticles have variety of applications in our daily life including clothes, household and personal care products and mostly owing to their antimicrobial properties. Moreover, the silver based nanostructured materials with specific physical, chemical and optical properties, especially altering sizes and shapes have been widely used in the field of electronic devices, paints, coatings, soaps, detergents, etc. . In the above discussion, the following details of Ag based nanomaterials are significant to consider in their synthesis methods such as surface property, particle composition, size distribution, morphology and different types of reducing and capping agents used. Generally, the methods used for the preparation of metallic Ag Nps are classified into two categories, namely top-down or bottom-up approach . The top down approach involves bulk materials and decrease them into nano-sized particles by using physical/chemical and mechanical processes . The top-down approach is also used for the fabrication of many materials including semiconductor industry , in this approach metal oxide semiconductor field effect transistor (MOSFET) are imprinted onto a silica wafer by lithography based technique . On the other hand, the preparation in bottom-up method requires single atoms and molecules into larger nanostructures to achieve nano-size particles . Currently, the synthetic methods are divided into physical, chemical and biological green syntheses. In this respect, the physical and chemical synthesis method tend to more serious and hazardous as compared to the biological synthesis of Ag nanoparticles. This shows outstanding properties including high yield, solubility and stability . The following segment discuss different synthesis methods in detail of Ag nanoparticle and their mechanisms, explaining how shape and size controlled Ag Nps can be achieved by proper selection of precursor chemicals, reducing and capping agent as well as concentration and molar ratio of chemicals.
2.1 Physical methods
The physical synthesis of Ag nanoparticles involves various processes such as evaporation-condensation process and the laser ablation technique [38, 39]. This technique is to synthesize large amount of Ag nanoparticles with high purity without use of chemicals that release toxic substances and expose human health and environment. However, there is a great challenge of agglomeration of nanoparticles because it is not used in the capping agents. Besides, both methods consume more power and require relatively longer duration of synthesis and difficult equipment, all of which increase their operating cost. The evaporation-condensation method commonly uses a gas phase technique that utilizes a tube furnace to synthesize nanospheres at atmospheric pressure. A variety of nanospheres using several materials including Au, Ag and PbS have been prepared by this method . The centre of the tube furnace comprises a vessel carrying a base metal source which is evaporated into the carrier gas, permitting the final of nanoparticles. The shape, size and yield of the nanoparticles can be controlled by changing the plan of reaction facilities. Further, the synthesis of Ag nanoparticles by evaporation-condensation technique through tube furnace has various drawbacks. In this technique, the tube furnace occupies a huge space, consumes high energy elevating the surrounding temperature of the metal source and gives a longer period to maintain its thermal stability. To overcome these difficulties, Jung
Furthermore, another crucial approach for physical synthesis of Ag nanoparticle using laser ablation technique. The preparation of Ag nanoparticles through laser ablation of a bulk metal source placed in a liquid environment. Once irradiating with a pulse laser, the liquid environment may contain Ag nanoparticle of the base metal source, cleared from other compounds, ions or reducing agents . However, there are various factors like laser power, duration of irradiation, type of base metal source and property of liquid media manipulate the features of the metal nanoparticles produced. Consequently, different chemical synthesis, the preparation of nanoparticles via laser ablation technique is pure and uncontaminated and in this method it uses mild surfactants in the solvent without using any other chemical reagents .
2.2 Chemical method
The synthesis of silver (Ag) nanoparticles by chemical method is the most commonly used technique and it is stable, colloidal dispersions in water or other appropriate organic solvents. The chemical process for synthesis of Ag nanoparticles in solution comprises the three major behaviors: (i) metal precursor, (ii) reducing agent and (iii) stabilizing or capping agent. The Ag nanoparticles are mainly chemically synthesized through Brust-Schiffrin synthesis (BSS) or the Turkevich method [43, 44, 45, 46]. In order to achieve specific shape, size and various optical properties of the metal nanoparticles, it is essential to control the reducing agents and stabilizers are also taken into consideration. The uses of stabilizing agent during the preparation of metal nanoparticles are typically for avoiding aggregation . So therefore, the following factors are needed to be considered for the safety and effectiveness of this technique, which includes the choice of appropriate solvent, use of environment friendly reducing agent and selection of non-toxic substances. There are various reducing agents used for the preparation of nanoparticles such as NaBH4, N2H4, tri-sodium citrate (TSC), sodium citrate and N,N-dimethylformamide (DMF). Besides, in order to avoid aggregation between Ag nanoparticles, surfactant can be used such as sodium dodecyl sulphate (SDS), oleylamine and some polymeric materials like polyvinylpyrrolidone (PVP), polymethacrylic acid, polyethylene glycol (PEG) and polymethylmethacrylate have been reported to be the efficient protecting agents to stabilize as capping agents for stabilization [48, 49, 50]. This stabilization or capping agents can be accomplishing either through electrostatic or steric repulsion. For example, electrostatic stabilization is generally reached through anionic species like citrate, halide, carboxylates that adsorb or interact with Ag nanoparticles to impart a negative charge on the Ag nanoparticle surfaces. Thus, the surface charge of Ag nanoparticles can be controlled by coating with citrate ions to give a strong negative charge. Further, the polyethylene glycol (PEG) coated nanoparticles show good stability in highly concentrated salt solution, whereas lipoic acid coated particles with carboxylic group can also be used for bio conjugation. Meanwhile, the morphology of nanoparticles is strongly manipulated by the temperature variant were implemented during the preparation. The Ag nanoparticles exhibit much deviation in shape ranging from spherical to trigonal/hexagonal. It is also reported that ascorbic acid used as reducing agent for the formation of room temperature flower like silver nano architecture with average particle size is about 20 nm. Consequently, Ag nanoparticles have been synthesized by the polyol process with the support of supercritical carbon dioxide from nitrate salt of silver as the base material, polyvinylpyrrolidone (PVP) as the stabilizer for the silver clusters and ethylene glycol act as the reducing agent and solvent. However, polyvinylpyrrolidone (PVP) not only protected the nanosized silver particles from aggregation but also help nucleation phenomenon [51, 52].
2.3 Photochemical method
The Ag nanoparticle was successfully synthesized by using photo irradiation technique. In this technique, photo assisted synthesis of Ag used for the preparation of stable Ag nanoparticles by irradiation of a reaction mixture with a light source including laser or lamp in presence of photo reducing agents without introducing stabilizers or surfactants [53, 54, 55]. For instance, the laser irradiation of an aqueous solution of Ag salt and surfactant can made stable homogeneous dispersion of Ag nanoparticles with good distribution of shape and size of the particles. However, the syntheses of silver nanoparticles with narrow size distribution through ethylene glycol-water solvent system without use of a stabilizer. Further, it is also observed that Ag nanoparticles are synthesized by using per chlorate salt via pulse radiography technique. In this technique, there is a reduction of Ag+ to Ag0 was accomplished successfully using UV light as a substitute of chemical materials in a rubber matrix using photo reduction of film cast from natural rubber latex (NRL) comprising silver salt with an average size about 10 nm. It is also reported that other synthesis route such as microwave irradiation have also been utilized and this route is of much faster rate than that of the conventional heating via conduction and convection. Their size and preparation time of the nanoparticles is directly proportional to the irradiation power of the source of light . In addition, photochemical processes also suggest a reasonable potential synthesis of controlled shape and size of Ag nanoparticles while multiple preparation steps might be required.
3. Synthesis of polymer-silver nanocomposites
3.1 In-situ polymerization
In-situ polymerization is a very efficient technique for the carbon based conductive fillers to be dispersed homogeneously in the polymer matrix, so it gives a strong interaction between the matrix and the filler particles. However, in-situ polymerization technique normally involves the addition of nanoparticle in a pristine monomer or a solution of monomer through polymerization in the presence of nanomaterials . Many studies have been made to synthesize nanocomposites using in-situ polymerization techniques and also showed that the covalent linkages between the matrix and nanomaterials. Besides, the fabrication of silver nanoparticle is relatively simple, efficient process by using in-situ technique. It is a one step process for manufacture of nanoparticles which uses the corresponding precursors for synthesis and these nanoparticles can be directly grown using this technique. The most significant benefit of this technique is that it avoids particle agglomeration and maintains homogeneous distribution of the particles in the polymer matrix at the same time but, the main shortcoming of this technique is the slight possibility of left un-reacted educts in path of the reaction. These results may be control on the properties of the final product. For instance: Li
3.2 Ex-situ polymerization
Another important technique for preparation of nanoparticles based polymer composites by using ex-situ polymerization. This technique is more appropriate for large scale industrial applications. The main challenges of this method are preparation of nanoparticles with good homogeneity into the polymer matrix and have good thermal stability against aggregation . In order to solve these difficulties sonication techniques were used to disperse the nanoparticles in the polymer matrix. For instance: Dhibar
4. Different properties of polymer-silver nanocomposites
4.1 Dielectric properties
The dielectric properties of poly (vinylidene fluoride) (PVDF) based composites filled with nanosilver (nAg) deposited nickelate (Mg doped La1.9Sr0.1NiO4) particles, which was reported by Thongbai
The enhancement of the thermal conductivity for Ag deposited alumina sphere through interfacial thermal resistance has been reported by Sun
The enhancement of both dielectric constant and breakdown strength of the modified Ag-OMMT nanoparticles incorporated in to the P(VDF-HFP) matrix to form P(VDF-HFP)-Ag-OMMT composites. However, the dielectric constant and breakdown strength of the P(VDF-HFP)-Ag-OMMT composites significantly increased with 4 vol% of Ag-OMMT contents and showed an energy density of 10.51 J cm−3 at 400 MV m−1, which is ~2.25 times higher than that of pristine P(VDF-HFP) film. Thus, it is a simple and facile method to increase the energy density of polymer dielectric films by the addition of very low loading hybrid fillers . Zou
The new strategy of core–shell graphene@polydopamine-Ag nanoplatelets and corresponding thermoplastic polyurethane (TPU)/Gns@PDA-Ag nanocomposites was reported by Zhu
4.2 Electrical properties
The simple method for the synthesis of electrically conductive composites comprising silver nanoparticles decorated on the carbon nanotubes (CNT) (Ag@CNT) incorporated into the epoxy resins by using wet chemistry reaction. The obtained result of the composites showed that higher electrical conductivity 0.10 wt% of Ag@CNTs, which was four times higher than the pure and functionalized CNTs. Thus, this enhancement of electrical conductivity of the Ag@CNTs-epoxy based composites was used as a potential application in the field of electronic package industries . Sahu
In summary, the syntheses of Ag nanoparticles have significant aspects of nanotechnology and it is used as nanofillers for fabricating nanocomposites. The Ag nanoparticle is a highly efficient, reliable high yielding and low cost technique. These nanoparticles have gained immense interest due to their unique physical and chemical properties as well as confirmed applicability in diverse fields such as electronics, catalysis, biotechnology and medicine. However, the shape and size distribution of silver nanoparticles can be controlled by adjusting reaction conditions such as reducing agent, stabilizing agent or using various synthesis techniques. The use of silver in the polymer based nanocomposites shows the enhancement in various properties such as dielectric and electrical in the field of energy storage devices. The Ag based nanocomposites have an enormous research interest in recent few times and potentially applicable in various fields especially in embedded high energy storage devices.
The authors gratefully acknowledge DST-SERB for financial support obtained through project grant of (CRG/2018/004101), New Delhi, Government of India.
Conflicts of interest
The authors declare no conflict of interest.