Synthesis of silver nanoparticles with different shapes

1.1 Investigating the shape of synthesized silver nanoparticles

Incredible properties of nanomaterials strongly depend on size and, shape of NPs, their interactions with stabilizers and surrounding media and also on their preparation method. So, controlled synthesis of nanocrystals is a key challenge to reach their (nanoparticles) better applied characteristics (El-Kheshen and El-Rab, 2012). The optical, electronical, magnetic and catalytic properties of nanoparticles depend on their size, shape and chemical environment (Cao, 2004; Giri et al., 2011). In recent years, new methods have been proposed to synthesize non-spherical nanoparticles both planar (triangles, 5 or 6 diagonal, round surfaces, etc.) and three dimensional (cubic, pyramid, etc.). Spherical particles with the minimum surface for a given volume are thermodynamically more stable and if the reduction of one-capacity silver ions is performed under controlled thermodynamic conditions, the main product will be spherical nanoparticles (Krutyakov et al., 2008). The shapes of nanoparticles depend on their interaction with stabilizers and the inductors around them and also their preparation method (Haruta, 2004). It is also known that reaction rate is influenced by the shape of synthesized silver nanoparticles. Xu et al. studied the oxidation of styrene over three shapes (nano cube, semi round and triangular nano plate) of silver nanoparticles for this purpose. The results of this study showed that the reaction rate in cubic nanoparticles is 14 times more than triangular nanoplates and 4 times higher than the semi-spherical nanoparticles (Fig. 1) (Xu et al., 2006).

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Figure 1

(a). TEM images of truncated triangular Ag nanoplates (left), near-spherical (middle), and cubic (right) silver NPs supported on Cu-TEM grids and their structural models. The insets show scanning electron microscopy images (top left corner) and electron diffraction patterns from selected areas (top right corner).(b) Specific reaction rate of styreneconversion over Ag NPs with different shapes. Reaction time: 3 h (Xu et al., 2006).

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Abid et al. showed that by using various irradiation methods silver nanoparticles could be synthesized. Laser irradiation of aqueous solution of a silver salt and surfactant could synthesize silver nanoparticles with suitable shape and size (Abid et al., 2002).

2.1 Synthesis of cubic silver nanoparticles

Sun and Xia (2002a) could synthesize cubic silver nanoparticles by the reduction of silver nitrate using ethylene glycol in the presence of poly(vinyl pyrrolidone) (PVP). The results of this study have shown that the morphology of the product is strongly influenced by the reaction conditions such as temperature, AgNO₃ concentration and molar ratio of the units of PVP and AgNO₃. For example, when the temperature reduced from 161 °C to 120 °C or increased to 190 °C, the shapes of produced silver nanoparticles were irregular. Moreover, the input concentration of AgNO₃, as the next effecting factor should be higher than 0.1 M. Otherwise, silver nanowires will be the main product. If the molar ratio of the repeating unit of PVP and AgNO₃ increases from 1.5 to 3, the main product would be multiply twinned particles (MTPs) (Sun et al., 2002a). Im et al. could synthesize uniform silver nanocubes by reduction of silver nitrate using ethylene glycol at 140 °C in the presence of poly(vinyl pyrrolidone) (PVP) and HCl (Im et al., 2005).

In polyol process, alcohol containing hydroxyl groups such as ethylene glycol and pentanediol act as both solvent and reducing agent. A capping agent, poly(vinyl pyrrolidone) (PVP) was used to build the cubic shape. Finally, molar ratio of the repeating units of PVP and silver ions determines the morphology of the product (Tao et al., 2006). High molar ratio is used for nanocubes and low molar ratio is used for nanowires. In addition, very small amounts of chloride ions due to precipitation of the low-solubility of AgCl salt prevents the rapid reduction of metal ions, it eventually leads to the formation of nanocubes (Wiley et al., 2004). Tao et al. came to synthesize silver nanocubes using an experimental procedure (Tao et al., 2006), in which silver nitrate acted as precursor, PVP was used to control the shape, and pentanediol (PDO-1.5 H) was used as both solvent and reducing agent.

Wiley et al. (2006) synthesized three different shapes of silver nanoparticles using polyol chemical method, in which ethylene glycol (EG) acted as both solvent and reducing agent. The interesting point was that the reduction ratio could be controlled by changing the reaction temperature. In the experiment, PVP acted as a stabilizer to prevent the aggregation of nanoparticles, as a reducing agent and also as a substance to control the shape of nanoparticles. The group was able to synthesize silver nanocubes with controllable corner truncation using Cl⁻ (NaCl) (Wiley et al., 2006).

It was proved that in polyol synthesis, silver atoms are formed by reducing AgNO₃ with ethylene glycol through the following mechanism:

Young synthesized silver nanocubes with a diameter of 30–50 nm using the polyol process in which, ethylene glycol acted as both the reducing agent and the solvent. In their study silver nitrate was reduced by ethylene glycol in the presence of a capping agent, poly(vinyl pyrrolidone) (PVP) (Young et al., 2007). Skrabalak et al. (2007) could synthesize silver nanocubes with a rapid method (reaction time <15 min) by sulfide-mediated polyol method in which Ag(I) was reduced to Ag(0) by ethylene glycol in the presence of PVP and a trace amount of Na₂S. When the concentration of silver atoms reaches supersaturation, they agglomerate into seed shape and then grow to the nanostructures form. Presence of PVP and Na₂S facilitate the formation of silver nanocubes (Skrabalak et al., 2007). Rycenga et al. (2008) synthesized Ag nanocubes by reduction of Silver Nitrate (AgNO₃) with ethylene glycol (EG) in the presence of PVP and HCl. Following synthesis, the Ag nanocubes were isolated by centrifugation, washed with water to remove EG and excess PVP, and finally dispersed in deionized water for storage (Rycenga et al., 2008). Huang et al. synthesized silver nanocubes in an alkalic aqueous solution of AgNO₃/carboxymethylated chitosan (CMCTS) by ultraviolet (UV) light irradiation, in which (CMCTS) was used to reduce the silver cation and as the stabilizer of silver nanoparticles. The synthesized silver nanoparticles were in the range of 2–8 nm (Huang et al., 2008). Zhang et al. produced silver nanocubes with edge length of 30 to 70 nm using CF₃COOAg as a precursor. By adding trace amounts of sodium hydrosulfide (NaHS) and hydrochloric acid (HCl) into the polyol synthesis, Ag nanocubes were observed with good quality and high reproducibility (Zhang et al., 2010). Zeng et al., 2010 studied the effects of capping agents on shape control for Ag nanocrystals. They found that cubic shaped silver nanoparticles could be selectively obtained by introducing PVP, as the capping agent (Zeng et al., 2010). Poinern et al., synthesized silver nanocubes at room temperature using the leaf extracts from Eucalyptus macrocarpa. In the synthesis process, leaf extract acted as both reducing and stabilizing agent. TEM micrographs showed the presence of both spherical and cubic shaped silver nanoparticles. The spherical Ag NPs ranged in size from 10 to 100 nm, while the cubic shaped ranged in size from 10 to 50 nm. Three dimensional image of FESEM taken after a period of several hours revealed that the predominant shapes in the synthesis were cubic nanoparticles with 50 nm–1 μm size (Poinern et al., 2013). SEM image of nanocubes is shown in Fig. 2.

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Figure 2

SEM of sharp nanocubes (Wiley et al., 2006).

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2.2 Synthesis of silver nanorods

Generally nanorods can be made by thermal (Pérez-Juste et al., 2005), photochemical (Kim et al., 2002), and electrochemistry-based template (Martin, 1994) methods. Aslan et al. used a wet chemical method for deposition of silver nanorods on conventional glass substrates. First, silver seeds were prepared in solution. Silver nitrate is reduced to silver nanoparticles with a size of 2 ± 4 nm by sodium borohydride and in the presence of sodium citrate. Silver nanorods grow in the solution with the injection of silver seeds into the growth medium containing silver nitrate, ascorbic acid and cetyltrimethylammonium bromide (CTAB). The solution color was light first and then turned to green after completion of the reaction (Aslan et al., 2005).

Orendoff et al., could synthesize significant amounts of silver nanorods with transverse and longitudinal plasmon peaks. The synthesis process involved reduction of AgNO₃ with NaBH₄ in the presence of citrate and continued by growth of seeds into nanorods in the presence of ascorbic acid and cetyltrimethylammonium bromide (CTAB). Finally, a mixture of spherical and rod nanoparticles was synthesized that could be isolated and purified by centrifugation (Orendoff et al., 2006). Gu et al. synthesized silver nanorods. For the first time in their study, they used potassium tartaric for reduction of silver nitrate in the presence of poly(vinyl pyrrolidone) (PVP). By changing the AgNO₃/PVP ratio, diameter and length of silver nanorods can be controlled (Gu et al., 2006). Xu et al. were able to synthesize Ag nanorods using a simple chemical deposition method with the help of porous aluminum membrane (PAM). Ag⁺ ions in PAM nano channels were reduced by acetaldehyde, and leading in the formation of nanorod structures. The study found that the diameter and length of Ag nanorods are determined by PAM template and the length of the Ag nanorods depends on the reaction temperature. (Fig. 3) (Xu et al., 2010).

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Figure 3

A single Ag nanorod (Xu et al., 2006).

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Hu et al. (2012) could synthesize Ag nanorods using oxidation reduction growth (ORG) by sputtering without catalysts or chemical solutions (Hu et al., 2012). A schematic diagram for the oxidation reduction growth of silver nanorods in their study is shown in Fig. 4. In Fig. 4(a), at the beginning of sputtering, a thin silver film was predeposited on the Si surface with a constant flow of 15 sccm argon gas. Then, a trace amount of oxygen, as the source of plasma to form the silver oxide seed during sputtering, was added and mixed with argon Fig. 4(b). In the sputtering process, the sample temperature will increase to between 200 °C and 300 °C and because the melting point of silver oxide is normally around 300 °C, it allows silver oxide to dissolve and release O₂ (Fig. 4(c)). In the last part of Fig. 4(d), silver nanorods start to grow from the reduction metal silver site, without oxygen.

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Figure 4

Growth mechanism of Ag nanorods during sputtering process (Hu et al., 2012).

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2.3 Synthesis of silver nanowires

Liu et al., reported that formation of silver nanowires could not be completed without a template. These studies led to the belief that the formation of 1D silver nanostructures needed a physical template such as carbon nanotubes or zeolites in order to achieve nanowire shapes. So, they used AgBr crystals and a developer (such as AgNO₃) with gelatinous template and finally could synthesize silver nanowires with diameters of 80 nm and 9 μm length. This group discovered that formation of silver nanowires originated from silver nitrate (not from silver bromide crystals) (Liu et al., 2001). Jana et al., using rod-like micelle template in cetyltrimethylammonium bromide (CTAB) process, successfully synthesized silver nanowires (Jana et al., 2001). Sun et al. used soft, solution-phase method that did not require dissolution and disintegration of hard templates for the synthesis of silver nanowires. In their experiment, they found that reduction of silver nitrate is possible using ethylene glycol (EG) at the temperature of 160 °C and subsequently adding a solution of silver nitrate and polyvinylpyrrolidone (PVP) to the solution leading to the formation of silver nanowires with diameters of 30–40 nm and length of 50 μm (Sun et al., 2002a). Sun et al., were also able to synthesize uniform silver nanowires by taking advantage of the selectively adsorption on the {1 0 0} facets of PVP. They observed that, if PVP is absent or added in low amounts, the main products are mainly nanospheres. Meanwhile, they found that silver nanoparticles with irregular morphology were formed in the presence of lower molecular weight PVP (Sun et al., 2002b, 2003b; Sun and Xia, 2002b).

Hu et al. used a seedless surfactant process to produce high-quality silver nanowires in large quantities. Silver nitrate is reduced by Tri-sodium citrate (Na₃C₆H₅O₇) in the presence of sodium dodecylsulfonate (SDSN). Tri-sodium citrate was an important factor in this process and sodium dodecylsulfonate acted as a capping agent in the formation of Ag nanowires. It has been proved that by changing the concentration of tri-sodium citrate the diameters of produced silver nanowires can be controlled. It also became clear that in too high or too low concentrations of SDSN, silver nanowires cannot be produced (Hu et al., 2004). Fig. 5 shows the proposed schematic illustration for the growth of Ag nanorods and nanowires.

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Figure 5

Schematic illustration of the experimental mechanisms to generate spherical, rod and wire-like nanoparticles (Tang and Tsuji, 2010).

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Jiang et al., claimed that the use of physical templates limited the yield of silver nanowires production. So, synthesized silver nanowires using polyol reduction of silver with ethylene glycol in the presence of PVP. Finally, the diameter of produced silver nanoparticles was reported to be in the range of 150–200 nm and the length of 50–100 μm (Jiang et al., 2004). Wiley et al., synthesized silver pentagonal nanowires using polyol chemical method, where ethylene glycol (EG) acted as solvent and reducing agent. Silver nanowires can be grown from multiply twinned seeds in the presence of Cl⁻ if etching is prevented either by running the reaction under argon, or by simply adding Fe(II) or Fe(III) at a molar ratio of Ag:Fe = 11,000. The main material for the synthesis of silver nanowire was iron(II) acetylacetonate, which was added to PVP solution (Wiley et al., 2006). Zhang et al., using mild chemical reduction method in aqueous solutions of poly(methacrylic acid) produced silver nanowires at room temperature with 30–40 nm diameters (Zhang et al., 2004a). Cong et al., for the first time synthesized silver nanowires with different shapes: Y-shaped, K-shaped and multi-branched nanowires. The branched nano-structures were synthesized by reduction of silver nitrate (AgNO₃) in polyethylene glycol (PEG) with polyvinylpyrrolidone (PVP) as a capping agent. These nanostructures are typically synthesized by molar ratio of 3.3:1 between repeating units of PVP and AgNO₃. The synthesis of branched nanowires strongly depends on the molar ratio of PVP to AgNO₃, reaction temperature, polymerization degree of reducing agent and PVP (Cong et al., 2012). Hsieh et al., produced Ag NWs at 160 °C using a microwave-assisted (MA) approach from AgNO₃ and using PVP as the capping agent in the presence of ethylene glycol(EG) (Hsieh et al., 2012). Fu et al., synthesized Ag–Au bimetallic nanowires by a wet chemical method at room temperature. The nanowires were obtained by the reduction with vanadium oxide (V₂O₃) nanoparticles. Diameter of produced silver nanowires was ∼20 nm and their length reported to be up to 10 μm (Fu et al., 2013). Chen et al. could synthesize silver nanostructures including silver nanowires using microwave-assisted polyol method by adding sodium sulfide (Na(2)S) into the solution. The results showed that increasing the concentration of Na(2)S up to 750 microM in 400 W leads to the production of silver nanowires (Chen et al., 2010). Y. Chang et al. synthesized silver nanowires using ethylene glycol and the diameter of synthesized nanoparticles can be controlled by addition of chloride ions. Synthesis process consists of two distinct steps: nucleation and growth. The final diameter of nanowires which was in the range of 55–100 nm, became smaller by adding more chloride ions to the second phase of synthesis (Y. Chang et al., 2011). Zhang et al. (using a steel-assisted polyol SAP method could synthesize silver nanowires at high concentrations of silver nitrate (up to 0.5 M). The average diameter of products could be adjusted from ∼100 to 300 nm by increasing the synthetic concentration (input concentration of AgNO₃) (Zhang et al., 2008). Bhattacharyya et al. (2000) synthesized silver nanowires with a diameter of ∽40 nm and length of 0.3 mm. These nanowires were synthesized by electrodeposition in silica gel pores that were heated (in a temperature range between 523 and 823 K) and then soaked in a silver nitrate solution (Bhattacharyya et al., 2000). Zhu, et al. synthesized silver nanowires using polyol method at different molecular weights of poly vinyl pyrrolidone (PVP) as capping agent. The results of this study showed that the yields and aspect ratios of silver nanowires could be controlled by PVP chain length so that increasing the molecular weight of PVP, increased them. The proposed theory is that the chemical absorption of silver ions on PVP chain in the early stages leads the production of silver nanowires (Zhu et al., 2011).

Liu et al. (2006) found a simple aqueous route to synthesize silver nanowires in large-scale at room temperature through the reaction of AgNO₃ and sodium diphenylamine sulfonate. The advantage of this approach was that it did not require nanoporous membranes, surfactant, seed, and heating. It can be used for the synthesize of metallic nanoparticles with different shapes, due to its simplicity (Liu et al., 2006). Research results have shown that adding small amounts of salts such as NaCl, Fe(NO)_3, CuCl₂ and CuCl could affect on the shape of nanoparticle. Usually, salt-mediated synthesis is a simple and effective method for the synthesis of silver nanowires (Korte et al., 2008; Wiley et al., 2005a). For example, some researchers (Korte et al., 2008; Wiley et al., 2005a) synthesized silver nanowires by reduction of AgNO₃ with EG heated to 148 °C in the presence of PVP and a small amount of NaCl. In order to obtain silver nanowires, oxygen must be removed from the reaction mixture in the presence of Cl⁻ anions. In the synthesis of silver nanowires by Korte et al., the reduction reactions were performed using CuCl or mediated CuCl₂ in the polyol process, but it was done without introducing any inert gases (Korte et al., 2008). Wiley et al., used iron for polyol synthesis of silver nanoparticles. By adjusting the concentration of Fe(II) or Fe(III) in the polyol reduction of silver nitrate, by simply varying the concentration of iron ions, they could produce either silver nanocubes or nanowires. They showed that high concentrations of iron ions prevented selective oxidation of twinned seeds and led the formation of nanowires. Fig. 6(a) shows the role of Fe(II) in removing atomic oxygen from the surface of silver nanostructures (Wiley et al., 2005c). Fig. 6(b) shows the role of Cu⁻ containing salts in the polyol synthesis of Ag nanowires. Cu(I) can remove adsorbed atomic oxygen from the surface of silver seeds and leads the growth and formation of silver nanowires. A trace amount of Cl⁻ as an important role in the polyol synthesis of silver nanowires: (1) provides electrostatic stabilization for the initially formed silver seeds and (2) reduces the concentration of free Ag⁺ ions in the solution through the formation of AgCl nanocrystallites (Tang and Tsuji, 2010).

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Figure 6

(a) The proposed mechanism by which Fe(II) removes atomic oxygen from the surface of silver nanostructures. Reduction by ethylene glycol (EG) competes with oxidation by atomic oxygen to form an equilibrium between Fe(III) and Fe(II) (Wiley et al., 2005c). (b) A schematic illustration depicting the role of Cu-containing salts in the polyol synthesis of Ag nanowires. Molecular oxygen present during initial seed formation can absorb and dissociate on the (Oa) Ag seeds. Cu(I) rapidly scavenges this absorbed atomic oxygen (Tang and Tsuji, 2010).

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Synthesis of metal nanoparticles (such as Ag nanoparticles) using template method is cheap and easy to operate and can produce nanowires. Generally, various template methods for synthesizing multishaped nanostructures include (1) using hard templates such as anodic aluminum oxide (AAO), (2) using soft templates such as cetyltrimethylammonium bromide (CTAB) and (3) using sacrificial templates (Chen and Liu, 2011). Baoliang Sun et al. could synthesize silver nanowires by hard templates (AAO) method (Sun et al., 2009).

Nghia et al., synthesized silver nanowires by the polyol process in ethylene glycol as a reducing agent, (PVP) as a stabilizer, using a microwave technique. Presence of sodium chloride in the polyol reduction of silver nitrate facilitates silver nanowires production. The formation time was very short (about 3 min under microwave heating). Studies showed that the size and shape of silver nanostructures strongly depended on the reaction parameters such as concentration of PVP, NaCl, AgNO₃ and heating time. It was found that low concentrations of PVP (50 mM), 3 mM NaCl concentration in the culture and heating time of 3 min were favorable conditions for the production of silver nanowires. The advantage of this study was the use of NaCl which was much cheaper than H₂PtCl₆ (Nghia et al., 2012). Kou et al. could produce Ag nanowires with controlled diameters using Glycerol (the material is cheap, abundant and environmentally friendly), which was used as both solvent and reductant under non-stirred microwave irradiation. Nanowires in this test were reportedly formed at very short time (1 min) (Kou and Varma, 2013). SEM image of nanowires has been shown in Fig. 7 (Wiley et al., 2006).

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Figure 7

TEM image of silver nanowires, aspect ratio ∼100 (Murphy and Jana, 2002).

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Recently, a large number of capping agents such as PVP, CTAB, Sodium Dodecylsulfonate (SDS) and Vitamin B₂ have been used to control the anisotropic growth of silver seeds to obtain nanowires through soft solution reactions. Among these, PVP is the most popular one in the synthesis of silver nanowires. Also, it has been proved that NaCl, chloride ion, sodium chloride and controls in the polyol reduction of AgNO₃, have important function in the fabrication of silver nanowires (Yan et al., 2010).

2.4 Synthesis of silver nanobars

Wiley et al. (2007) in another article synthesized silver nanobars. The method used in this study was the polyol method, in which ethylene glycol (EG) acted as both solvent and reducing agent. At the beginning of the experiment, two solutions, one containing 48 mg AgNO₃ in 3 ml EG and the other containing 48 mg poly vinyl pyrrolidone (PVP) and 0.068 mg NaBr in 3 mL of EG using a two channel syringe pump added dropwise to 5 ml of EG, which reached 155 °C in the oil bath. The reaction was stopped after 1 h and Figures of silver nanobars were obtained using the scanning electron microscopy (SEM). The remarkable point here is the effect of NaBr concentration on the production of silver nanoparticles (Wiley et al., 2007). Wiley et al. in their earlier research used low concentrations of NaBr (Ag:Br = 850) in order to synthesize the pyramid-shaped silver nanoparticles. But they observed that the required concentration of NaBr for the synthesis of silver nanobars was double the amount required for the synthesis of bipyramids (Wiley et al., 2006). TEM image of the silver nanobars (Wiley et al., 2007) (Fig. 8).

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Figure 8

TEM image of the silver nanobars (Wiley et al., 2007).

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2.5 Synthesis of triangular (pyramid) silver nanoparticles

A photochemical route was the first reliable and high yielding method for making solution-phase triangular Ag nanoprisms in which, the edge length can be controlled with excitation wavelength (Jin et al., 2001). Yamamoto et al., synthesized triangular silver nanoplates using the microwave promoted (MW) reduction of silver nitrate in aqueous solutions containing PVP (Yamamoto et al., 2004). Wiley et al., synthesized right bipyramids shaped silver nanoparticles using chemical polyol method, in which ethylene glycol (EG) was the solvent and reducing agent applying (NaBr) Br⁻ (Wiley et al., 2006). Dong et al., 2010 could synthesize triangular silver nanoprisms using a stepwise reduction method in the absence of surfactant or special capping-agent. For this purpose, first small spherical silver nanoparticles were prepared by the rapid reduction of the precursor (silver nitrate) with NaBH₄ (the strong reductant which, acted at the low temperature) at ice-bath temperature. The remaining precursor was further reduced by citrate (the weak reductant which, acted at the high temperature) under 70 °C to result in the formation of additional small spherical nanoparticles and induce the transformation of the spherical nanoparticles into the triangular nanoprisms. In this study it was found that the formation of the triangular nanoprisms is dependent on the molar ratios of two reductants (NaBH₄ and trisodium citrate) used in the reactions. The residual silver ions after the formation of the spherical nanoparticles are necessary to promote their dissolution and transformation into the triangular nanoprisms. It can be said that, a balance between the precursor contributed to the formation of the small spherical particles and that to the transformation of the spherical nanoparticles is critical for the synthesis of the triangular nanoprisms (Dong et al., 2010). Kelly et al., synthesized the triangular-shaped silver nanoparticles. The used method was based on seed-mediated process involving reduction of silver ions by ascorbic acid in aqueous solution and also poly vinyl alcohol, citrate and polystyrenesulphonate as modifiers (Kelly et al., 2012). Millstone et al., reviewed a variety of solution-based methods like: Photochemical Syntheses and Thermal Syntheses (Chemical Reduction Methods), for synthesizing triangular Au and Ag triangular nanoprisms because of the mentioned reasons: (1) These structures have plasmonic features in the visible and IR regions, (2) they can be prepared in high yield and (3) can be readily functionalized with a variety of sulfur-containing adsorbates. They showed that in some cases, by adjusting experimental parameters, such as: metal ion and reducing agent ratios, surfactant concentrations, pH, irradiation wavelength, seed particle concentration and type, the dimensions of nanoprism can be controlled (Millstone et al., 2009).

Kai et al., could synthesize triangular silver nanoparticles by chemical reduction method. This was done via an appropriate method that was quick and easy. Finally, in order to observe and investigate silver nanoparticles, transmission electron microscopy and UV–visible spectrophotometer devices were used. Results showed that if silver nitrate, polyvinyl pyrrolidone and hydrazine hydrate were used as reagent, stabilizing factor and reducing agent, respectively, triangular silver nanoparticles with edge lengths in the size range of 50–200 nm were synthesized (Kai et al., 2012). Fig. 9 shows pyramid silver nanoparticles (Wiley et al., 2006).

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Figure 9

Pyramid silver nanoparticles (Wiley et al., 2006).

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2.6 Synthesis of silver nanoprisms

Generally, chemical reduction of metal salts (Métraux and Mirkin, 2005; Xue and Mirkin, 2007; Aherne et al., 2008) and photochemical growth (Bastys et al., 2006; Tsuji et al., 2012) are two methods which, have been used for the synthesis of silver Nanoprisms. It is proved that the former chemical reduction method is better than the photochemical method for mass production of silver prisms, which is necessary for industrial applications. Métraux and Mirkin used a unique simple chemical reduction method for the preparation of silver nanoprisms. They used a mixture of AgNO₃/NaBH₄/polyvinylpyrrolidone (PVP)/trisodium citrate (Na₃CA)/H₂O₂ in an aqueous solution as reagents to prepare silver nanoprisms at room temperature (Métraux and Mirkin, 2005).

Darmanin et al., could produce high concentrations of silver nanoprisms with controllable size ((height) of the pyramid) and size disparity. In this study, they showed that synthesis of silver nanoprisms can be performed by careful selection in the parameters of polyol method especially in the reducing agent (Darmanin et al., 2012). Fig. 10 shows TEM image of silver nanoprisms (Darmanin et al., 2012).

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Figure 10

TEM images of the solutions containing nanoprisms synthesized with ethylene glycol monoethyl ether (EEE) (Darmanin et al., 2012).

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2.7 Synthesis of flower-shaped silver nanoparticles

Cai and Zhai (2010) using a wet-chemical method synthesized different shaped silver microparticles, including flower-shaped silver microstructures. These particles were synthesized by reducing of silver nitrate (AgNO₃) using ascorbic acid and in the presence of PVP (Cai and Zhai, 2010). Pourjavadi and Soleyman used a wet-chemical method for producing flower-like microsized Ag crystals. Fig. 11(a) shows SEM image of produced particles (Pourjavadi and Soleyman, 2011).Zaheer and Rafiuddin (2011) could synthesize flower-like silver nanoparticles at room temperature using a simple chemical reduction method. The synthesis process occurred using silver nitrate, ascorbic acid (as a reducing agent) and cetyltrimethylammonium bromide (CTAB). Also, in this study the effect of reductant concentration, mixing ratio of the reactants and the concentration of cetyltrimethylammonium bromide as the important parameters for the formation of stable silver nanoparticles were studied. TEM image of produced silver nanoparticles has been shown in Fig. 11(B) (Zaheer and Rafiuddin, 2011).

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Figure 11

A: SEM images of flower-like silver NPs (Pourjavadi and Soleyman, 2011); B: TEM images of flower-like Ag-nanoparticles (Zaheer and Rafiuddin, 2011).

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2.8 Synthesis of Spherical silver nanoparticles

Kai et al., could synthesize spherical silver nanoparticles via convenient (quick and easy) method at room temperature and by chemical reduction method and finally used transmission electron microscopy and UV–visible spectrophotometer in order to observe and investigate the produced nanoparticles. If silver nitrate, polyvinyl pyrrolidone and sodium borohydride are used as the reagent, stabilizing factor and reducing agents respectively, spherical and monodisperse silver nanoparticles of 9.0 nm sizes are synthesized (Kai et al., 2012). Liang et al., could synthesize spherical silver nanoparticles using polyol method in which polyethylene glycol (PEG) acted as both solvent and reducing agent and polyvinyl pyrrolidone (PVP) as capping agent. Uniform nanospheres with an average diameter of 54 nm using molar ratio of 8 among repeating units of PVP and AgNO₃ at 260 °C were produced (Liang et al., 2010). Qin et al. synthesized spherical silver nanoparticles using ascorbic acid as a reducing agent and citrate as stabilizer in aqueous bath with 30 °C. It was observed that by increasing the pH from 6 to 11.5, the average size of produced silver nanoparticles declined from 73 to 31 nm. They also found that heating at the temperatures of 100 °C for 2 h changed the shape of synthesized silver nanoparticles to more spherical-like (Qin et al., 2010). Aguilar-Méndez et al. could synthesize spherical silver nanoparticles with a small size (less than 20 nm) by the reduction of silver nitrate solution using glucose in the presence of gelatin as a capping agent. The average size of produced nanoparticles was between 5 and 24 nm and the colloidal solution remained stable for more than 3 months at room temperature (Aguilare et al., 2011). Pyatenko et al. could synthesize spherical silver nanoparticles using a new method which was a combination of seed technique and laser treatment. The size of spherical silver nanoparticles ranged from 10 to 80 nm (Pyatenko et al., 2007).

Moghimi-Rad et al., could synthesize pure silver nanoparticles through wet chemical reaction in the presence of capping agent and surfactant under ultrasound radiation. In order to produce silver nanoparticles, ascorbic acid reduced aqueous solution of silver nitrate in the presence of dodecylbenzenesulfonic acid sodium, polyvinylpyrrolidone and a mixture of organic and aqueous solutions. In this study, the branched, cubic, spherical and porous structures were formed by self-arrangement of surfactant as a template under ultrasound radiation. Analysis of scanning electron microscopy and transmission electron microscopy demonstrated that the morphology and size of nanoparticles was influenced by: (1) capping agent, (2) presence of ultrasound radiation and (3) crystallization time (Moghimi-Rad et al., 2011).

AL-Thabaiti examined the effect of surfactants and polymers on metallic nanoparticle (silver). In this study, thiosulfate was used as a reducing agent, and cetyltrimethylammonium bromide (CTAB) and poly vinyl alcohol (PVA) were used as stabilizers. Their results showed that nano-materials having a range of optical properties can be prepared through careful control over the concentration of sodium thiosulfate in solution. As it can be seen in the figure, the presence or absence of stabilizers leads to the production of three different shapes (rod, spherical and hexagonal plates) of silver nanoparticles (AL-Thabaiti et al., 2013). Fig. 12 shows the sculpturing effects of CTAB and PVA in shape transformation of silver nanoparticles (AL-Thabaiti et al., 2013).

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Figure 12

Formation of rods, spherical and hexagonal plates of silver nano particles in the absence and presence of stabilizers (AL-Thabaiti et al., 2013).

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Tsuji et al., could synthesize silver nanoparticles with different shapes using microwave irradiation and glycol-H₂ [PtCl₆]-poly(vinylpyrrolidone) ethylene – silver nitrate solution. In this study a mixture of different shaped silver nanoparticles consist of: nanorods, nanowires, spherical, cubic, and triangular bipyramidal nanoparticles were prepared by reduction of Ag⁺ ions in ethylene glycol by the addition of H₂[PtCl₆] and poly-(vinylpyrrolidone) (PVP) within three minutes. Although, it was previously believed that 1-D Ag nanostructures can only be produced in the presence of a capping agent like PVP, Tsuji et al. in their study found that different shaped (Ag 1-D nanorods, nanowires, cubic and triangular-bipyramidal) silver nanoparticles can be produced without the addition of PVP in the presence of Cl⁻ (Tsuji et al., 2008).

2.9 Effective factors on the shape of produced silver NPs

As it has been shown in various studies, the shape and size of produced silver nanoparticles depend on the experimental conditions such as: temperature, the concentration of silver precursor, pH of the solution, the molar ratio between PVP (the capping agent) and AgNO₃ (silver precursor) (PVP:Ag⁺ ratio), the strength of chemical interaction between PVP and various crystallographic planes of silver (Wiley et al., 2005b), reducing agents (citric acid, l-ascorbic acid, and NaBH₄), the used method (chemical, physical or biological). Studies have shown that by using seed particles with a particular morphology (e.g., multifaceted or twinned), the final architecture of anisotropic nanoparticles can be controlled in some synthetic approaches (Liu and Guyot-Sionnest, 2005; Jiang et al., 2011). Also, by modulating the chemical and redox environment of the initial seed particle in situ, the shape of the produced nanoparticles can be changed. A change in seed concentration (Wiley et al., 2005a; Wu et al., 2008), surfactant (Sau and Murphy, 2004), pH (Xue and Mirkin, 2007), temperature (Sun et al., 2003a; Jiang et al., 2011), metal ion (Métraux and Mirkin, 2005), and even dopant concentrations (Millstone et al., 2008) can have drastic effects on the shape and size of the anisotropic product. Shervani et al. (2008) showed that by adjusting the capping and reducing agents Ag(0) colloids with various shapes can be prepared. PVP and PVE as capping agents also played an important role in the formation of different shaped Ag(0) nanomaterials like: pure cubes, stars, and mixed geometry. Spherical particles were prepared by reduction of aqueous starch solution of AgNO₃ precursor salt by d(+)-glucose and NaOH (Shervani et al., 2008). Taguchi et al. (2008) in their study found that the shape of silver nanoparticles during the polyol reduction of silver ions can be controlled by adjusting the flow rate of the oxygen gas. They proved that by adjusting this parameter different shapes of silver nanoparticles include: nanocubes, right bipyramids, nanowires, and spherical nanoparticles can be obtained.

Tang et al., in their experiment investigated the shape-controlled synthesis of silver NPs using a simple PVP-assisted sonoelectrochemical technique. It has been proved that the shape of the produced silver NPs depends on the current density (J) and the molar ratio (R) of PVP monomer to Ag⁺. Fig. 13 shows the effect of density on the morphology of silver NPs. In this experiment the current density varied from 0.75 mA cm⁻² to 1.25 mA cm⁻² and 2 mA cm⁻², while C, t (reaction time) and R (molar ratio) in all of the prepared samples were the same. As it can be seen by increasing the current density the shape of NPs has changed from irregular agglomerates (Fig. 13(a)) to flower-like (Fig. 13(c)) (Tang et al., 2009).

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Figure 13

TEM images of the produced silver NPs at various current densities of (a) 0.75 mA cm⁻², (b) 1.25 mA cm⁻², and (c) 2 mA cm⁻² when C, t and R were 3.0 × 10⁻³ mol L⁻¹, 10 min and 50, respectively (Tang et al., 2009).

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Effect of the PVP as another influential factor is shown in Fig. 14. Tang et al., for studying this effect, prepared samples with differing R while other parameters remained. They demonstrated that, PVP leads the formation of side branches and leaves and promotes the formation of finer, more hierarchical microstructures (Tang et al., 2009).

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Figure 14

TEM images of the products prepared at different PVP to Ag ratios R: (a) 10, (b) 80, and (c) 100 (Tang et al., 2009).

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Fig. 15 shows samples synthesized at a low J = 0.5 mA cm⁻² and various PVP concentrations. Fig. 15(a) shows spherical silver NPs with a narrow size distribution (7–10 nm) at R = 100. Fig. 15(b) shows spherical silver NPs with diameter of ∼15 nm which prepared at R = 50. These NPs have multiple domains, as shown in the HRTEM image of a typical particle (Fig. 15(c)). Fig. 15(d) shows TEM image of synthesized NPs with little PVP (R = 5). The size distribution of nanoparticles is shown in Fig. 15(e) (Tang et al., 2009).

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Figure 15

TEM image of Ag NPs at (a) R = 100, (b) R = 50 and (d) R = 5 when C, J, t remained at 3 × 10⁻³ mol L⁻¹, 0.5 mA cm⁻² and 10 min, respectively. The inset in (a) is a typical HRTEM image of an individual Ag particle. (c) HRTEM image of an Ag nanoparticle in (b). (e) A high-magnification TEM image of the Ag particle in (d) (Tang et al., 2009).

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Rivero et al., studied the effect of poly(acrylic acid, sodium salt) (PAA) and dimethylaminoborane (DMAB) as the protective and reducing agents respectively. Their results showed that by means of a fine control of the PAA and DMAB molar concentrations, silver nanoparticles of different colors, sizes and shapes (like: cubic, rod, triangle, hexagonal and spherical) can be synthesized (Rivero et al., 2013).

Various studies have shown that, the morphology of crystals depends on the distance of the formation conditions from thermodynamic equilibrium (Tang et al., 2009; Meng et al., 2010). It has been shown that increasing the driving force for crystallization leads the formation of polyhedrons NSs such as octahedrons, truncated octahedrons, cubes and pyramids (Tang et al., 2009; Meng et al., 2010) (see Fig. 16).

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Figure 16

Correlation between the distance of the formation conditions from the equilibrium and the shape of the produced Silver NPs (Tang et al., 2009).

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Dong et al., investigated the growth of silver nanoparticles by the citrate reduction of silver nitrate under the range of pH from 5.7 to 11.1. They showed that pH of the reaction solution affects the size/shape of the produced silver nanoparticles. It was observed that under high pH, the product was composed of both spherical and rod-like silver nanoparticles as a result of the fast reduction rate of the precursor while, under low pH, the main product was triangle or polygon silver nanoparticles due to the slow reduction rate of the precursor (Dong et al., 2009).