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Synthesis of silver NPs
Physical methods Evaporation-condensation and laser ablation are the most important physical approaches. The absence of solvent contamination in the prepared thin films and the uniformity of NPs distribution are the advantages of physical synthesis methods in comparison with chemical processes. Physical synthesis of silver NPs using a tube furnace at atmospheric pressure has some disadvantages, for example, tube furnace occupies a large space, consumes a great amount of energy while raising the environmental temperature around the source material, and requires a lot of time to achieve thermal stability. Moreover, a typical tube furnace requires power consumption of more than several kilowatts and a preheating time of several tens of minutes to reach a stable operating temperature (12,13). It was demonstrated that silver NPs could be synthesized via a small ceramic heater with a local heating area (14). The small ceramic heater was used to evaporate source materials. The evaporated vapor can cool at a suitable rapid rate, because the temperature gradient in the vicinity of the heater surface is very steep in comparison with that of a tube furnace.
This makes possible the formation of small NPs in high concentration. The particle generation is very stable, because the temperature of the heater surface does not fluctuate with time. This physical method can be useful as a nanoparticle generator for long-term experiments for inhalation toxicity studies, and as a calibration device for nanoparticle measurement equipment (14). The results showed that the geometric mean diameter, the geometric standard deviation and the total number concentration of NPs increase with heater surface temperature. Spherical NPs without agglomeration were observed, even at high concentration with high heater surface temperature. The geometric mean diameter and the geometric standard deviation of silver NPs were in the range of 6.2-21.5 nm and 1.23-1.88 nm, respectively.
Silver NPs could be synthesized by laser ablation of metallic bulk materials in solution (15,16,17,18,19). The ablation efficiency and the characteristics of produced nano-silver particles depend upon many parameters, including the wavelength of the laser impinging the metallic target, the duration of the laser pulses (in the femto-, pico- and nanosecond regime), the laser fluence, the ablation time duration and the effective liquid medium, with or without the presence of surfactants (20,21,22,23).
One important advantage of laser ablation technique compared to other methods for production of metal colloids is the absence of chemical reagents in solutions. Therefore, pure and uncontaminated metal colloids for further applications can be prepared by this technique (24). Silver nanospheroids (20-50 nm) were prepared by laser ablation in water with femtosecond laser pulses at 800 nm (25). The formation efficiency and the size of colloidal particles were compared with those of colloidal particles prepared by nanosecond laser pulses. As a result, the formation efficiency for femtosecond pulses was significantly lower than that for nanosecond pulses. The size of colloids prepared by femtosecond pulses were less dispersed than that of colloids prepared by nanosecond pulses. Furthermore, it was found that the ablation efficiency for femtosecond ablation in water was lower than that in air, while in the case of nanosecond pulses, the ablation efficiency was similar in both water and air.
Tien and coworkers (26) used the arc discharge method to fabricate silver NPs suspension in deionized water with no added surfactants. In this synthesis, silver wires (Gredmann, 99.99%, 1 mm in diameter) were submerged in deionized water and used as electrodes. With a silver rod consumption rate of 100 mg/min, yielding metallic silver NPs of 10 nm in size, and ionic silver obtained at concentrations of approximately 11 ppm and 19 ppm, respectively. Siegel and colleagues (27) demonstrated the synthesis of silver NPs by direct metal sputtering into the liquid medium. The method, combining physical deposition of metal into propane-1,2,3-triol (glycerol), provides an interesting alternative to time-consuming, wet-based chemical synthesis techniques. Silver NPs possess round shape with average diameter of about 3.5 nm with standard deviation 2.4 nm. It was observed that the NPs size distribution and uniform particle dispersion remains unchanged for diluted aqueous solutions up to glycerol-to-water ratio 1:20.
Explanation:
EXTRA INFORMATION :-
Today the synthesis of silver nanoparticles is very common due to their numerous applications in various fields. Silver nanoparticles have unique properties such as: optical and catalytic properties, which, depend on the size and shape of the produced nanoparticles. So, today the production of silver nanoparticles with different shapes which have various uses in different fields such as medicine, are noted by many researchers. This article, is an attempt to present an overview of the shape-controlled synthesis of silver nanoparticles using various methods.
INTRODUCTION :-
Silver has too much of modern industrial uses and is considered a store of wealth. However, the story of this legendary precious metal begins with its use by ancient civilizations. Silver has many attributes that made it so valuable to early peoples. It is malleable, ductile, lustrous, resilient, conductive, antibacterial, and rare. Also, it was used as a precious commodity in currencies, ornaments, jewelry, electrical contacts and photography, among others. Although bulk silver is widely known for their brilliant surfaces and colors, there is a drastic color difference when the metal reduces in dimensions. Even though the craftsmen did not know nanoparticles in that period, the mixing of the metal chlorides with molten glass led to the formation of metallic nanoparticles of different shape and size, therefore the physical formats of the metal nanoparticles had interesting interactions with light and produced visibly beautiful colors.
Physical methods:-
The most important physical methods for the synthesis of the silver nanoparticles are evaporation-condensation, laser ablation, electrical irradiation, gamma irradiation, and lithography. Kimura and Bandow examined the measurement of the optical spectra of many metal colloid solutions and presented new preparation methods of metal colloids inorganic solvents without the chemicals such as redox reagents, polymers, electrolytes, glue or other kinds of colloid stabilizers. Three different preparation methods as the matrix isolation method, the gas flow-cold trap method, and the gas flow-solution trap method, were used to examine the synthesis of silver NPs . The laser ablation method, which has several types of different applications, is another method to study the synthesis of silver nanoparticles (Ag-N) .This method helps to control particle size of colloids by changing the number of laser pulses .Pyatenko et al. produced silver nanoparticles by irradiating an Ag target with a 532 nm laser beam in pure water. This technique is successfully applied to produce small nanoparticles with a narrow size distribution in pure water without using any chemical additives by using a high-power laser and small laser beam spot sizes . Sadrolhesseini et al. prepared a new method for the fabrication of silver nanoparticles which are dispersed in graphene oxide using the laser ablation and thermal effusivity of nanocomposite.
Chemical reduction of silver nanoparticles:-
The size, shape, and surface morphology play an important role in controlling the chemical, physical, optical, and electronic properties of nanomaterials. The chemical reduction is one of the most commonly used methods for the synthesis of silver nanoparticles by inorganic and organic reducing agents. In general, different reducing agents such as sodium citrate, ascorbate, sodium borohydride (NaBH4), elemental hydrogen, polyol process, Tollens reagent, N,N-dimethylformamide (DMF), and poly(ethylene glycol)-block copolymers, hydrazine, and ammonium formate are used for the reduction of the silver ions (Ag+) in the aqueous or nonaqueous solutions.
CONCLUSION :-
Silver nanoparticles can be obtained by physical, chemical, and biological synthesis methods. Hundreds of research articles reporting different synthesis methods for Ag-NP are published every year. In literature, all known applications for metallic silver may involve the use of nanosilver in place of silver to take advantage of nanosilver’s unique properties. Despite all beneficial uses for nanosilver, its impact on the environment is concerning. These synthesis methods may require the use of different raw materials and yield reaction by using toxic products or wastes. But in recent years, also known as “green chemistry”, an environmental-friendly approach has become a new option in chemistry, consisting of reduction and elimination of dangerous substantives for the design of products in the environment. However, as seen, there are numerous studies for the synthesis methods of the nanosilver in literature but the most commonly used methods in the industry are not yet known. For this reason, we suggest that researchers should be directed to work on the methods of synthesizing nanosilver used in the industry.