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Quality and high yield synthesis of Ag nanowires by microwave-assisted hydrothermal method.

Meléndrez MF, Medina C, Solis-Pomar F, Flores P, Paulraj M, Pérez-Tijerina E - Nanoscale Res Lett (2015)

Bottom Line: One of the drawbacks presented so far in the synthesis of nanostructures by polyol path is the high temperature used in the process, which is superior than the boiling point of solvent (ethylene glycol), and also its excessive reaction time.It was found that the reaction time needs to be decreased because of the NWs which start to deform and break up due to significant increase in the pressure's system.Energy-dispersive X-ray spectroscopy and electron diffraction analysis (SAED) did not show corresponding phases of AgO.

View Article: PubMed Central - PubMed

Affiliation: Advanced Nanocomposites Research Group (GINA), Faculty of Engineering, University of Concepcion, 270 Edmundo Larenas, Box 160-C, Concepcion, 4070409 Chile ; Hybrid Materials Laboratory (HML), Faculty of Engineering, University of Concepcion, 270 Edmundo Larenas, Box 160-C, Concepcion, 4070409 Chile ; Department of Materials Engineering (DIMAT), Faculty of Engineering, University of Concepcion, 270 Edmundo Larenas, Box 160-C, Concepcion, 4070409 Chile.

ABSTRACT
Silver nanowires (Ag-NWs) were obtained using microwave-assisted hydrothermal method (MAH). The main advantage of the method is its high NWs production which is greater than 90%. It is also easy, fast, and highly reproducible process. One of the drawbacks presented so far in the synthesis of nanostructures by polyol path is the high temperature used in the process, which is superior than the boiling point of solvent (ethylene glycol), and also its excessive reaction time. Here, Ag-NWs with diameters of 70 to 110 nm were synthesized in 5 min in large quantities. Results showed that dimensions and shape of nanowires were very susceptible to changes with reaction parameters. The reactor power and reactor fill capacity were important for the synthesis. It was found that the reaction time needs to be decreased because of the NWs which start to deform and break up due to significant increase in the pressure's system. Energy-dispersive X-ray spectroscopy and electron diffraction analysis (SAED) did not show corresponding phases of AgO. Some aspects about synthesis parameters which are related to the percent yield and size of nanowires are also discussed.

No MeSH data available.


Ag-NWs images obtained with a 20% of reactor filling capacity at 800 W. (a, b) Ag nanowires before washing process. (c, d) After washing process.
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Fig4: Ag-NWs images obtained with a 20% of reactor filling capacity at 800 W. (a, b) Ag nanowires before washing process. (c, d) After washing process.

Mentions: MAH was a quick method compared to the conventional hydrothermal method, since the conventional methods require a reaction time exceeding 2 h and temperatures above 160°C. MAH is a fast method and economical due to the reaction time of the process which is not greater than 10 min. The main disadvantage in the conventional hydrothermal method was to take care of a number of synthesis parameters in order to improve the yield, which was avoided in this method. Ag-NWs synthesized using MAH method are shown in Figures 1, 2, 3, 4, and 5; all reactions were performed by using the following molar relation (1:6) AgNO3 (0.1 mol/L) and PVP (0.6 mol/L), respectively. The above molar relation was established in preliminary tests with different reactant ratios and concentration by using various microwave powers. The 1:6 reagent relation was the most efficient to obtain Ag-NWs of high quality. PVP must always be in excess because it acts as a precursor molecule for preferential growth of NWs. If the molar ratios (1:6) were maintained and the reactant concentration increases during a typical synthesis, the reaction product rich in Ag nanoparticles will be obtained due to the strong interaction of PVP with the silver seed surface, avoiding the preferential growth to obtain 1D nanostructures. This situation is favored by the large amount of PVP molecules that are present in the solution and also preventing the diffusion of Ag2+ species to the active growth centers. Figure 1 shows micrographs of Ag-NWs obtained for 5 min of reaction time and 20% of reactor fill capacity, respectively. Different reactor powers 200 W (Figure 1a,b), 400 W (Figure 1c,d), and 600 W (Figure 1e,f) were used in these experiments. When the reaction was carried out using small reactor power, homogeneous silver nanoparticles were obtained with sizes ranging between 50 and 70 nm. Instead, if the reactor power is raised to 400 W, then the mixture of the nanoparticles and nanowires of lesser proportion is produced. Particle size in this treatment ranged between 60 and 90 nm. Ag-NPs were slightly larger compared to those obtained with 200 W. NWs had lengths above 5 μm with diameters about 90 nm. When the reaction was performed at 600 W, the NWs proportion compared to the NPs was found to be greater. These had the same dimensions than those synthesized at 400 W. In conventional experiments, it was not possible to obtain high production of Ag-NWs. NPs and NWs mixture were always obtained except for treatment at 200 W, which only produces NPs. This may be due to inadequate pressure inside the chamber that would not have been enough for Ag2+ ion migration towards the active growth seeds, which also interacts with PVP that induces preferential growth for nanostructures. Unlike conventional heating, microwave directly heats the volume of the liquid while leaving the surroundings (i.e., the containers) untouched. Moreover, the dipolar molecules, e.g., ethylene glycol and PVP oscillates due to microwave fluctuating field. This oscillation generates a molecular movement resulting in friction and therefore generates heat. The reactor was placed in a fixed position in the microwave oven; for this reason, molecules having an ionic structure (e.g., AgNO3) get aligned along the electromagnetic field, according to the above, with the rapid heating of the solution, nucleation and growth process for 1D nanostructures is facilitated. MAH provides a combination of rapid and efficient heating which overheats the solvent above its boiling point, and this leads to obtain products much faster. To achieve overheating, a considerable reactor power is needed to increase the molecular friction above the boiling point of solvent. Therefore, temperature and pressure of the synthesis process is not just enough to induce nanowire growth. Figure 1a,b,c,d,e,f shows that on increasing reactor power, the Ag-NWs production improves substantially. With these reaction conditions, reaction time was not sufficient for full conversion of silver seed into nanowires.Figure 1


Quality and high yield synthesis of Ag nanowires by microwave-assisted hydrothermal method.

Meléndrez MF, Medina C, Solis-Pomar F, Flores P, Paulraj M, Pérez-Tijerina E - Nanoscale Res Lett (2015)

Ag-NWs images obtained with a 20% of reactor filling capacity at 800 W. (a, b) Ag nanowires before washing process. (c, d) After washing process.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4384996&req=5

Fig4: Ag-NWs images obtained with a 20% of reactor filling capacity at 800 W. (a, b) Ag nanowires before washing process. (c, d) After washing process.
Mentions: MAH was a quick method compared to the conventional hydrothermal method, since the conventional methods require a reaction time exceeding 2 h and temperatures above 160°C. MAH is a fast method and economical due to the reaction time of the process which is not greater than 10 min. The main disadvantage in the conventional hydrothermal method was to take care of a number of synthesis parameters in order to improve the yield, which was avoided in this method. Ag-NWs synthesized using MAH method are shown in Figures 1, 2, 3, 4, and 5; all reactions were performed by using the following molar relation (1:6) AgNO3 (0.1 mol/L) and PVP (0.6 mol/L), respectively. The above molar relation was established in preliminary tests with different reactant ratios and concentration by using various microwave powers. The 1:6 reagent relation was the most efficient to obtain Ag-NWs of high quality. PVP must always be in excess because it acts as a precursor molecule for preferential growth of NWs. If the molar ratios (1:6) were maintained and the reactant concentration increases during a typical synthesis, the reaction product rich in Ag nanoparticles will be obtained due to the strong interaction of PVP with the silver seed surface, avoiding the preferential growth to obtain 1D nanostructures. This situation is favored by the large amount of PVP molecules that are present in the solution and also preventing the diffusion of Ag2+ species to the active growth centers. Figure 1 shows micrographs of Ag-NWs obtained for 5 min of reaction time and 20% of reactor fill capacity, respectively. Different reactor powers 200 W (Figure 1a,b), 400 W (Figure 1c,d), and 600 W (Figure 1e,f) were used in these experiments. When the reaction was carried out using small reactor power, homogeneous silver nanoparticles were obtained with sizes ranging between 50 and 70 nm. Instead, if the reactor power is raised to 400 W, then the mixture of the nanoparticles and nanowires of lesser proportion is produced. Particle size in this treatment ranged between 60 and 90 nm. Ag-NPs were slightly larger compared to those obtained with 200 W. NWs had lengths above 5 μm with diameters about 90 nm. When the reaction was performed at 600 W, the NWs proportion compared to the NPs was found to be greater. These had the same dimensions than those synthesized at 400 W. In conventional experiments, it was not possible to obtain high production of Ag-NWs. NPs and NWs mixture were always obtained except for treatment at 200 W, which only produces NPs. This may be due to inadequate pressure inside the chamber that would not have been enough for Ag2+ ion migration towards the active growth seeds, which also interacts with PVP that induces preferential growth for nanostructures. Unlike conventional heating, microwave directly heats the volume of the liquid while leaving the surroundings (i.e., the containers) untouched. Moreover, the dipolar molecules, e.g., ethylene glycol and PVP oscillates due to microwave fluctuating field. This oscillation generates a molecular movement resulting in friction and therefore generates heat. The reactor was placed in a fixed position in the microwave oven; for this reason, molecules having an ionic structure (e.g., AgNO3) get aligned along the electromagnetic field, according to the above, with the rapid heating of the solution, nucleation and growth process for 1D nanostructures is facilitated. MAH provides a combination of rapid and efficient heating which overheats the solvent above its boiling point, and this leads to obtain products much faster. To achieve overheating, a considerable reactor power is needed to increase the molecular friction above the boiling point of solvent. Therefore, temperature and pressure of the synthesis process is not just enough to induce nanowire growth. Figure 1a,b,c,d,e,f shows that on increasing reactor power, the Ag-NWs production improves substantially. With these reaction conditions, reaction time was not sufficient for full conversion of silver seed into nanowires.Figure 1

Bottom Line: One of the drawbacks presented so far in the synthesis of nanostructures by polyol path is the high temperature used in the process, which is superior than the boiling point of solvent (ethylene glycol), and also its excessive reaction time.It was found that the reaction time needs to be decreased because of the NWs which start to deform and break up due to significant increase in the pressure's system.Energy-dispersive X-ray spectroscopy and electron diffraction analysis (SAED) did not show corresponding phases of AgO.

View Article: PubMed Central - PubMed

Affiliation: Advanced Nanocomposites Research Group (GINA), Faculty of Engineering, University of Concepcion, 270 Edmundo Larenas, Box 160-C, Concepcion, 4070409 Chile ; Hybrid Materials Laboratory (HML), Faculty of Engineering, University of Concepcion, 270 Edmundo Larenas, Box 160-C, Concepcion, 4070409 Chile ; Department of Materials Engineering (DIMAT), Faculty of Engineering, University of Concepcion, 270 Edmundo Larenas, Box 160-C, Concepcion, 4070409 Chile.

ABSTRACT
Silver nanowires (Ag-NWs) were obtained using microwave-assisted hydrothermal method (MAH). The main advantage of the method is its high NWs production which is greater than 90%. It is also easy, fast, and highly reproducible process. One of the drawbacks presented so far in the synthesis of nanostructures by polyol path is the high temperature used in the process, which is superior than the boiling point of solvent (ethylene glycol), and also its excessive reaction time. Here, Ag-NWs with diameters of 70 to 110 nm were synthesized in 5 min in large quantities. Results showed that dimensions and shape of nanowires were very susceptible to changes with reaction parameters. The reactor power and reactor fill capacity were important for the synthesis. It was found that the reaction time needs to be decreased because of the NWs which start to deform and break up due to significant increase in the pressure's system. Energy-dispersive X-ray spectroscopy and electron diffraction analysis (SAED) did not show corresponding phases of AgO. Some aspects about synthesis parameters which are related to the percent yield and size of nanowires are also discussed.

No MeSH data available.