Study On Shape-Controlled Synthesis And Their Applications Of Silver Nanostructures

Silver nanostructures have garnered a significant amount of attention because of their unique physical and chemical properties, as well as their wide applications in catalytic materials, cathode materials, optical materials, antibacterial materials, coatings, and so on. The intrinsic properties of silver nanostructures are mainly determined by their shapes and sizes. Apparently, shape-controlled synthesis of silver nanostructures has been and continues to be an area of active research. Firstly, the preparation and properties of silver nanostructures with different morphologies are introduced in brief. In particular, we discuss the polyol synthesis of silver nanostructures with different shapes in detail. In the end, the shortages in the relevant research fields are pointed out. Therefore, we research on shape-controlled synthesis and growth mechanism of silver nanostructures. With different reducing agents and solvents, such as ethanol, ethylene glycol and N,N-dimethylformamide (DMF), the effects of different reaction parameters on the final morphologies and sizes of silver nanostructures have been discussed. In addition, the conductivity of electrically conductive adhesives (ECAs) and LiFePO4 cathodes can be improved by the addition of silver nanostructures with different shapes. This work mainly includes the following aspects:Uniform silver nanoparticles have been synthesized by using ethanol as the reducing agent and solvent in the presence of poly(vinylpyrrolidone) (PVP). PVP can combine Ag+ to form a complex compound. This effectively decreases chemical potential and further enables Ag+to be reduced by ethanol more easily. An increased reaction time to 1.5h leads to the synthesis of silver nanoparticles with large sizes. When the concentration of AgNO3 is as high as 0.15 M, the nucleation rate is greater than the growth rate, resulting in the synthesis of the particles with small sizes. Otherwise, PVP can easily adsorb and cover the surfaces of these particles because of their high surface energies, preventing the agglomeration between these particles. It facilitates the preparation of silver nanoparticles with small sizes. In addition, with the high reaction temperature (60℃), the rate of the nucleation is more than that of the growth. Silver nanoparticles with small sizes can be obtained in the presence of PVP.Silver nanowires (< 3μm) can be obtained via a solvothermal-assisted polyol method by using FeCl3 as controlling agents. When the concentration of FeCl3 is low, ranging from 5μM to 50μM, the main product of the reaction is a mixture of silver nanoparticles and nanowires. When the FeCl3 concentration is more than 100μM, large amounts of AgCl colloids form in the early stage, reducing the concentration of free Ag+ions in the solution. This results in decreasing the nucleation rate. Multiply twinned seeds of decahedral shape form preferentially. These seeds can grow into silver nanowires because selective adsorption of PVP on the{100} facets leads to preferential addition of silver atoms to the{111} facets. Otherwise, silver nanowires with large sizes can be obtained by increasing the FeCl3 concentration. Under the same concentration of Cl-, compared to Na+ Fe3+can react with O2, preventing the silver seeds from being etched with O2. As a result, pure silver nanowires can be obtained. Different species of the anions play important roles on the final morphologies of the products. When the concentration of anions is the same as Cl- (300μM), a few silver nanowires (> 4μm) can be synthesized with Br-, and the presence of OH-facilitates the synthesis of uniform silver nanoparticles. In addition, reaction times and the molar ratio of the repeat unit of PVP to AgNO3 also have significant effects on morphologically controlled silver nanostructures.Silver nanowires and nanocubes are synthesized via a solvothermal-polyol method by adding different concentrations of Na2S into the solution. A low-concentration Ag2S (< 50 μM) which is an n-type semiconductor acts as the catalysis, resulting in increasing the rate of the reduction. Single-crystalline seeds are grown preferentially. Silver nanocubes can be obtained in the presence of PVP. Furthermore, by adjusting the reaction temperature (135-155℃) and the amounts of Ag2S colloids (12.5-50μM), silver nanocubes with adjustable sizes can be acquired. A high-concentration Ag2S (>100μM) acts as the controlling agent, decreasing the rate of the reduction. Multiply twinned seeds can form preferentially in the solution. It facilitates the preparation of silver nanowires (>10μm) in the existence of PVP. When the concentration of Ag2S ranges from 100 to 300μM, silver nanowires with large diameters can be obtained.In nothing flat (3.5 min), pure silver nanocubes and nanowires can be produced via a microwave-assisted polyol method by adding different concentrations of Na2S into the solution. It is found that the ideal concentration of Ag2S is 31.25-500μM for producing monodispersed silver nanocubes at 300 W. When the heating power is increased to 400 W, 62.5-250μM is the ideal concentration of Ag2S for the synthesis of silver nanocubes. And that silver nanocubes with controllable sizes can be obtained by adjusting the Ag2S concentration. In addition, an increase in the heating power from 250 to 450 W leads to the synthesis of silver nanocubes with adjustable sizes. With a high-concentration Ag2S (750μM), pure silver nanowires can be obtained at 400 W. Compared to the existence of S2-, there are only a few silver nanowires synthesized in the presence of Cl-.We describe a solvothermal procedure for the synthesis of silver nanoplates, based on the use of N,N-dimethylformamide as the solvent and reducing agent, in the presence of FeCl3. The final morphologies of the products can be controlled by adjusting the concentration of free Ag+ions. A low-concentration FeCl3 (50μM) leads to the formation of thin silver nanoplates, but a high-concentration FeCl3 (100μM) facilitates the synthesis of thick silver nanoplates. When the concentration of FeCl3 is as high as 300μM, silver nanostructures with different shapes can be obtained. In addition, hexagonal silver nanoplates can transform to triangular silver nanoplates in the presence of 50μM FeCl3 when the reaction temperature ranges from 150 to 160℃.The conductivity of electrically conductive adhesives (ECAs) can be improved by adding silver nanostructures with different shapes to ECAs. Silver nanoparticles can fill the gaps between micrometer-size silver flakes, increasing the conductive paths. This results in decreasing the resistivity of ECAs. In addition, a mixture of silver nanoparticles and nanowires is added to ECAs, besides the effects of silver nanoparticles, silver nanowires can contact silver flakes together while silver flakes supply junctions for wire-build network. The number of conductive pathways can be increased. As a result, this two factors greatly decrease the resistivity of ECAs.Silver nanostructures with different shapes can improve the conductive properties of LiFePO4 cathodes. In the presence of silver nanowires, they can disperse on the surface of electrodes and inside the electrodes, resulting in decreasing the contact resistance and forming wire-build network respectively. Compared to LiFePO4 cathodes, the discharge capacity of LiFePO4/Ag nanowires cathodes can be all improved at various rates. Especially, the discharge capacity of these cathodes reaches-150 mAhg-1 at the 0.2 C rate. With the existence of silver nanoplates, they can decrease the contact resistance when they are on the surface of the electrodes. Inside the electrodes, these plates act as conducting routes between LiFePO4 particles, improving the conductivity of the cathodes. In consequence, the discharge capacity of LiFePO4/Ag nanoplates cathodes is 148 mAhg-1 at the 0.2 C rate. In particular, discharge capacities are improved from 60 mAhg-1 of LiFePO4/Ag nanowires cathodes to 80 m mAhg-1 of these cathodes at the 5 C rate.