Scalable microwave assisted continuous flow process for preparation of silver nanowires and nanorods

Abstract

The present invention relates to a continuous flow process for the synthesis of silver nanowire in the microwave. Specifically, the present invention relates to a process that uses a tubular reactor under microwave assisted conditions for synthesizing silver nanowires. Further, the process of the present invention is carried out in shorter reaction time with high yields and throughput.

Description

[0001] SCALABLE MICROWAVE ASSISTED CONTINUOUS FLOW PROCESS FOR PREPARATION OF SILVER NANO WIRES AND NANORODS

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a scalable microwave assisted continuous flow process for the synthesis of silver nanowires. Particularly, the present invention relates to a process that uses a tubular reactor under microwave assisted conditions for synthesizing silver nanowires without any clogging and wall deposition.

[0004] BACKGROUND OF THE INVENTION

[0005] Metal nanowires (NWs) such as silver (Ag), copper (Cu) and gold(Au) NWs have unique dimension-dependent electrical, catalytic, optical and antimicrobial properties that have potential benefits in applications such as transparent electronics, flexible / stretchable conductors, photonics, solar cells, wearable devices, sensor, electro catalyst, Surface- enhanced Raman spectroscopy (SERS) technology and lithium-ion batteries. Among all the metals, silver nanowires have huge commercial potential because it has low cost, high stability, high transparency, high electrical and thermal conductivities. Silver nanowires are an alternative to high-cost indium tin oxide (ITO) for use as a transparent conductor in touch screens. It can be synthesized by physical and chemical synthesis route. Some of the well- known synthesis methods are template assisted, UV irradiation induced photo -reduction technique, hydrothermal synthesis, wet chemical synthesis, and microwave assisted method and polyol synthesis. Among these, microwave assisted polyol method is time efficient, eco- friendly, cost effective and a sustainable method. This method has the potential to synthesize thin silver nanowire in a few minutes that has application in the touch screen panels.

[0006] Microwave (MW) heating reduces reaction time significantly due to the influence of ‘microwave dielectric heating’ effect driven by the mechanism of ionic conduction and dipole polarization. Using microwaves, it is possible to expedite the inorganic / organic material synthesis to larger volumes, which avoids inducing specific thermal gradient and hot injection that was seen in the convective heating process. The morphology and disparity of the nanomaterial is generally affected by the microwave power, temperature and reaction time. Conventional processes provide poor yields, are time consuming and being batch processes show batch to batch variation. However, the microwave method has potential to synthesize silver nanowires in short reaction time with higher yield. The major drawback of the batch microwave assisted synthesis of silver nanowire is scale up issues due to limited penetration depth of microwave irradiation and sparking due to deposition of metal on the reactor wall. The limited penetration depth is a constraint of the energy efficient microwave process, which may cause the wide particle size distribution and variation in the product yield due to the large temperature gradient in the reaction volume. The microwave's limited penetration depth prevents the batch reactor from being scaled up.

[0007] The large quantity of silver nanowire synthesis by batch reactor is insufficient for the current requirements in the application. This can be addressed by using a continuous flow reactor, which has the advantages of enhancing mass and heat transfer. The continuous flow reactor has potential to overcome the limitation of the microwave batch reactor. Continuous flow systems enable real time analysis and control of reaction conditions, enhancing the repeatability and precision of the synthesis procedure. The continuous flow reactor offers high throughput and yield synthesis of organic / inorganic materials in the microwave. The congestion of the channel caused by the continual deposition of nanomaterial on the reactor's wall can be overcome by ensuring a superficial velocity higher than the particle sedimentation velocity.

[0008] Reference may be made to the patent application “CN103878387A”,which discloses the rapid synthesis of silver nanowire having pentagonal cross section in the microwave by using sodium sulphide as etching agent. In this patent application, they synthesized silver nanowire with diameter 60-480 nm and length 10-30 pm. The microwave power used for the synthesis of silver nanowire is 280-500 W. Moreover, it is batch synthesis and requires an additional 10 min stirring of the reaction mixture before putting it in the microwave oven, which increases the overall reaction time.

[0009] Reference may be made to the thesis “Microwave assisted synthesis of silver nanorods” by Srichandana Nandikonda [Nanotechnol. 2011, 104086 (2011)] which discloses using a 2 cm diameter glass tube as a continuous flow reactor to synthesize silver nanoparticles. Silver nanorods were synthesized in a batch method, which exhibits a very low aspect ratio.

[0010] Reference may be made to a journal “Appl Nanosci (2015) 5:881-890”, which discloses synthesis of silver nanowires by the polyol process with ethylene glycol as a reducing agent and polyvinylpyrrolidone as a stabilizer, using microwave technique. However, a reaction time is significantly higher (upto 1 hr. or more) and used a high molecular weight solvent (polyethylene glycol) for the synthesis of silver nanowires. Moreover, very low aspect ratio nanorods were formed using this method with a large population of nanoparticles.

[0011] Moreover, the aforesaid reported reactors comprise surface area to volume ratio at higher side; hence, Ag nanostructures formed therein are more likely to be deposited on the reactor wall. This can cause tube clogging after several hours of continuous operation, resulting in sparking in the reactor. The higher dielectric constant of Ag metal absorbs more electromagnetic radiation; leading to excess energy is emitted in the form of sparks. The silver nanomaterial was more likely to adhere to the wall because of the rough surface of borosilicate glass.

[0012] Therefore, there is an unmet need in the art to solve the aforementioned problems, wherein all prior arts are batch synthesis of silver nanowires under microwave with no precise control on diameter or aspect ratio. Also, there is a need for a large-scale continuous process of synthesizing silver nanowires using tubular reactor with a reactor surface having a lower contact angle (which minimizes the risk of deposition and addresses abovementioned problems), and also, it provides a high yield of the silver nanowires.

[0013] OBJECTS OF THE INVENTION

[0014] Main object of the present invention is to provide a scalable microwave assisted continuous flow process for the synthesis of silver nanowire.

[0015] Another object of the present invention is to providea large-scale continuous flow process for the synthesis of silver nanowires in the microwave.

[0016] Yet another object of the present invention is to provide a process that uses a tubular reactor under microwave assisted conditions for synthesizing silver nanowires.

[0017] Yet another object of the present invention is to provide a process that is carried out in shorter reaction time with high yields and throughput.

[0018] Yet another object of the present invention is to provide a process that is carried out without any wall deposition.

[0019] SUMMARY OF THE INVENTION Accordingly, present invention provides a microwave assisted continuous flow process for the synthesis of metal nanowire, comprising the steps of: a) adding metal salt in a solvent at a temperature in the range of 25-35°C to obtain a solution A; b) dissolving polyvinylpyrrolidone (PVP) of molecular weight MW=3,60,000 and 55000 in 2: 1 ratio in a solvent at a temperature in the range of 25-35°C to obtain a solution B; c) adding metal halide in a solvent at a temperature in the range of 25-35 °C to obtain a solution C; d) mixing the solution of step b) and c) to obtain a solution mixture D; e) feeding continuously the solution A of step a) and solution mixture D of step d) in the helical tubular reactor placed in the microwave oven; f) obtaining metal nanowire after complete conversion at the outlet of the reactor.

[0020] In an embodiment of the present invention, the process is carried out for a time period in the range of 2-4 minutes.

[0021] In another embodiment of the present invention, the metal salt is silver nitrate and solvent is ethylene glycol.

[0022] In yet another embodiment of the present invention, the metal halide is selected from the group consisting of sodium bromide, sodium chloride, copper chloride and ferric chloride hexahydrate.

[0023] In yet another embodiment of the present invention, at least two molecular weights of polyvinylpyrrolidone (PVP) is based on the range of about 55000 to 360000.

[0024] In yet another embodiment of the present invention, the microwave power is in the range of 300-1500 W and microwave temperature is in the range of 150 - 210°C.

[0025] In yet another embodiment of the present invention, the flow rate of solution of step a) and solution of step d) are 3 ml / min and 24 ml / min, respectively.

[0026] In yet another embodiment of the present invention, the helical tubular reactor is made of a material selected from PTFE (Polytetrafluoroethylene) and PFA (Perfluoroalkoxy alkanes).

[0027] In yet another embodiment of the present invention, the PTFE tubular reactor having an inner diameter of 1.5 mm -5 mm and volume of 12 - 500 ml. In yet another embodiment of the present invention, the continuous flow is in upward direction in the tubular reactor.

[0028] In yet another embodiment of the present invention, the aspect ratio of obtained metal nano wire is in the range of 120-1200, diameter in the range of 10-100 nm, and length in the range of 5 to 30 microns.

[0029] In yet another embodiment of the present invention, the obtained metal nanorods have diameter in the range of 100 nm - 200 nm, length in the range of 1 - 20 microns, and aspect ratio in the range of 10 to 100.

[0030] In yet another embodiment of the present invention, the polyvinylpyrrolidone (PVP) is a capping agent that allows silver nanowires to grow unidirectionally and act as stabilizer.

[0031] BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Fig. 1 illustrates the temperature Vs time for reaction at different microwave power.

[0033] Fig. 2 illustrates the comparison of UV-Vis analysis of reaction at different microwave (MW) wattages.

[0034] Fig. 3 illustratestheUV-Vis analysis of silver nano wire at different residence time.

[0035] Fig. 4A to 4C illustrates the FESEM image (Field Emission Scanning Electron Microscope) of silver nanowires synthesis at varied reaction conditions.

[0036] Fig. 5 illustrates the schematic of the process of the present invention.

[0037] DETAILED DESCRIPTION OF THE INVENTION

[0038] The present invention relates to a continuous flow process for the synthesis of silver nanowire in the microwave. Specifically, the present invention relates to a process that uses a tubular reactor under microwave assisted conditions for synthesizing silver nanowires. Further, the process of the present invention is carried out in shorter reaction time with high yields and throughput.

[0039] The present invention relates to a microwave assisted continuous flow process for the synthesis of metal nanowire, comprising the steps of: a) preparing metal salt solution in a solvent to obtain a solution A; b) dissolving polyvinylpyrrolidone (PVP) of two different molecular weight (MW=360000 and 55000 in 2:1 ratio) in a solvent at room temperature in the range of 25 to 35°C to obtain a solution B ; c) preparing metal halide solution in a solvent to obtain a solution C; d) mixing the solution of step b) and c) to obtain a mixture D; e) feeding continuously the solution A of step a) and mixture D of step d) in the helical tubular reactor placed in the microwave oven; and f) obtaining metal nanowire after complete conversion at the outlet of the reactor.

[0040] In the present invention, the metal salt used is silver nitrate.

[0041] The solvent used is ethylene glycol, which also acts as a reducing agent. Ethylene glycol acts as a reducing agent at higher temperature (>120 °C), converting into acetaldehyde to reduce the metal precursor (silver nitrate).

[0042] The metal halide is selected from the group consisting of sodium bromide, sodium chloride, copper chloride and ferric chloride hexahydrate. Preferably, metal halide is ferric chloride hexahydrate, which is used as an etching agent to control silver nanowires yield and diameter. The polyvinylpyrrolidone (PVP) is a capping agent that allows silver nanowires to grow unidirectionally and act as stabilizer.

[0043] All solutions of the process of the present invention are prepared at room temperature. Further, changing the flow rate of solutions affects the dimension and yield of nanowire.

[0044] The microwave power is in the range of 300-1500 W and microwave temperature is in the range of 150 - 210 °C.

[0045] The process is carried out for a time period in the range of 2-4 minutes. Also, changing of residence time will affect the dimension of the nanowire, yield of nanowire and conversion of metal salt.

[0046] In an embodiment, nanorods can be prepared from the nanowires on the sonication application.

[0047] The present invention relates to a continuous flow process for synthesizing silver nanowires by using the microwave assisted polyol method. Since the process is continuous, it can easily tune diameter and aspect ratio. The process involves the reduction of silver nitrate by using ethylene glycol solvent preferably. The silver nanowire synthesis is done by the polyol method. This method has the advantage of synthesis of the silver nanowire at a larger scale. This polyol method can achieve controlled growth of nanowires and can easily tune the dimension of the nanomaterial by changing the reaction parameter. All the reactants are mixed and kept for heating for a few minutes. The nucleation process starts, once the supersaturation Ag+concentration is achieved in the reaction mixture. The formation of the silver nanowire method is three steps and follows the autocatalytic route. The first step is nucleation, followed by the growth of nuclei to form thermodynamically stable multiple-twinned particles (MTPs). Once the MTPs are developed, Ag+ion gets reduced on the active site of MTPs allowing it to grow unidirectionally.

[0048] The present invention provides a tubular reactor under microwave assisted conditions for synthesizing silver nanowires. Figure 5 shows the schematic of the process of the present invention, wherein silver nitrate solution (120) is prepared by mixing silver nitrate salt in ethylene glycol, and followed by preparing a mixture (110) by mixing ferric chloride hexahydrate and PVP in ethylene glycol. Feeding continuously the silver nitrate solution (120) and mixture (110) in the helical tubular reactor (130) placed in the microwave oven (140) and obtaining silver nanowire after complete conversion at the outlet (150) of the reactor.

[0049] The material of the tubular reactor is selected from PTFE (Polytetrafluoroethylene) and PFA (Perfluoroalkoxy alkanes). Preferably, PTFE tubular reactor is used. Thus, provide a reactor surface with a lower contact angle that minimizes the risk of deposition on the wall of the reactor.

[0050] The continuous flow is in upward direction in the tubular reactor. Further, there is no wall deposition on the tubular reactor and no moving parts in direct contact with the reaction mass. Also, no need for any other heating fluid for taking the reaction mass to reaction temperature. The PTFE tubular reactor having an inner diameter of 1.5 mm -5 mm and volume of 12 - 500 ml is used.

[0051] The parameters of metal nanowires like length, diameter and aspect ratio are controlled and tuned based on tubular reactor and temperature.

[0052] The process of the present invention enables the continuous flow synthesis of silver nanowires by using a PTFE tubular reactor. This setup allows for precise control of nucleation and growth of nanowires, resulting in the production of silver nanowires with controllable dimensions at a production rate of 4 grams per hour. The process successfully produced silver nanowires with a diameter of 10-100 nm and 5-30 pm length without deposition inside the reactor.

[0053] EXAMPLES

[0054] Following examples are given by way of illustration and therefore should not be construed to limit the scope of the invention.

[0055] Materials:

[0056] All the chemicals were used without further purification. Metal precursor silver nitrate (99.8 %) was purchased from Merck Chemicals, India. Polyvinylpyrrolidone (PVP, K90:360000 MW, and K30 = 58000 MW), iron chloride hexahydrate (FeCh. 6H3O), and ethylene glycol (anhydrous 99.9%) used as stabilizing agent, etching agent and reducing agent respectively were purchased from the Loba chemicals, India.

[0057] EXAMPLE 1: Batch synthesis of silver nanowires

[0058] A 50 ml round bottom flask containing reactant of composition 0.350 M AgNCh, 600 pM FeCh, 0.077 M PVP (mol. wt. 3,60,000 + 55,000 in 2: 1 ratio) and 27.5 ml ethylene glycol. All the reactants mixed in a flask and subjected into the microwave cavity. Experiments have been done at ten different microwave power (300 -1500 W) and varying the reaction temperature (160 -190 °C). As the microwave power increases from 300 to 1500 W, the rate of nucleation and growth increases. This resulted in a significant reduction in reaction time from 50 to 3 minutes, with almost complete conversion of metal precursor. It is clear that, heating rate increases from 11.60 to 122.29 °C / min for 300 to 1500 W respectively (Figure 1). The rate of heating of reaction mixture increases as microwave power increases. At higher micro wave power, solution temperature reached to 120 °C within a min, which is sufficient to activate the reductant (ethylene glycol) to reduce the Ag+to Ag° in very short reaction time. At higher microwave powers (1200 W), nanowires with a diameter of 35 nm and a yield of 94% were obtained within 4 minutes of reaction time. However, the limited penetration depth of the microwave irradiation constrains the scalability of the process. EXAMPLE 2: Effect of etching agent

[0059] Etching agents such as sodium halide, copper chloride, potassium halide and ferric chloride hexahydrate scavenge the oxygen from the surface of multiple twinned particles and reduce the reduction rate Ag+ into Ag° in the solution. The different molar ratio of etching halide to metal precursor, affects the aspect ratio of nanowire. The dual etching agent, such as sodium bromide and sodium chloride, reduced the diameter of nanowire to 22 nm with 90 % conversion and 56 % of nano wire yield.

[0060] EXAMPLE 3: Synthesis of silver nano wires in helical coil reactor

[0061] Experiments were carried out using 1 / 8” PTFE helically coiled tubular reactor (VICI make) having volume 32 ml. The temperature was monitored by inserting the temperature sensor inside the round bottom flask filled with 22 ml ethylene glycol. The temperature was set to 190-210 °C. Same stoichiometry of reactants maintained as mentioned in the Example 1. The micro wave power is set-to 1200 W. The atomic absorption spectrophotometer confirms the 76 % conversion of silver precursor with 74 % of nano wires yield. The aspect ratio of silver nanowire at 4 min of residence time is 220, (diameter 54 nm and length 12 pm).

[0062] EXAMPLE 4: Effect of thickness of tubular reactor

[0063] Using the different thickness of the tubular reactor for the nucleation and growth of nanowire in the microwave oven controlled the growth kinetics and gave a narrower distribution of nanowires diameter and length. The higher thickness of tubing for the growth of nanowire reduced growth rate, resulting in the long length nanowire due the preferentially deposition of metal ions on the facet of particles. The AAS analysis confirms that the 72 % conversion of metal precursor with higher yield (81 %) of nano wires as compared to the earlier case.

[0064] EXAMPLE 5: Effect of Microwave heating followed by conventional heating

[0065] Set up is arranged in such a way that nucleation happens in the microwave oven followed by the conventional heating for the growth of nano wire. A 10 ml of reactor placed in the microwave oven followed by 22 ml of reactor for the further growth of nanowires by conventional heating. Here, due to the rapid / selective heating of microwaves, they produce the small size thermodynamically stable MTPs which act as a seed for the nanowire growth. The conventional heating for the growth step of nanowire over seeds allows the preferentially and kinetically controlled deposition of silver ions on the surface of MTPs. This experiment reduces the formation of nanoparticles significantly. The AAS analysis of supernatant confirms the 66 % conversion of metal precursor with 91 % yield of nanowire.

[0066] EXAMPLE 6: Effect of micro wave power

[0067] The microwave oven was operated at a different power level to produce the different sizes of silver nanowires. As the microwave decreases from 1200 to 750 W, reaction time increases from 4 to 16 min to achieve the same conversion of metal precursor. Figure 2shows the comparison of UV-Vis analysis of reaction at different microwave (MW) wattages. The results for all the experiments are shown in Table 1.

[0068] Table 1. Effect of microwave power on the dimension of Silver nanowires.

[0069] EXAMPLE 7: Large scale synthesis of silver nano wires

[0070] A helical coil reactor made of 1 / 4” PTFE (polytetrafluoroethylene) with an inner diameter of 0.475 cm was used. The reactor had a volume of 108 ml. The microwave power was set to 1200 W and temperature ranged from 190 to 210 °C. Solution 1 consisted of 0.35 M silver nitrate dissolved in ethylene glycol, while solution 2 was a mixture of 500 - 700 pM FeCF. 6H2O, 0.065- 0.08 M PVP (polyvinylpyrrolidone) in ethylene glycol. Solution 1 and 2 were introduced into the PTFE tubular reactor via Tee connector using a peristaltic pump. The flow rate of solution 1 and solution 2 were 3 and 24 ml / min respectively. The total residence time in the reactor was maintained between 4 - 8 minutes and outflow from the reactor was analyzed by the AAS. The experimental results demonstrated a 71 % conversion of metal precursor along with a nanowire yield of 56 %. The FESEM image of silver nanowires showed that the average diameter of nano wire is 63 ± 14 nm and a length of 14 pm. Figure 4(A) shows the FESEM image of silver nanowires synthesized at reaction conditions of microwave power 900 W and reaction time of 6 min. Figure 4(B) shows the FESEM image of silver nanowires synthesized at reaction conditions of microwave power 1050 W and reaction time of 5 min. Figure 4(C) shows the FESEM image of silver nanowires synthesized at reaction conditions of microwave power 1200 W and reaction time of 4 min.

[0071] EXAMPLE 8: Effect of convectional - micro wave - conventional heating

[0072] In this experiment, the process of growing nanowires involved a sequence of different heating methods. Firstly, conventional heating at a temperature of 150-170 °C was applied in the nucleation section, which was then followed by microwave heating at 160-190 °C for the growth of nanowires. To further increase the length of the nanowires, conventional heating at a temperature of 150-170 °C was provided. Two solutions were used: Solution 1 containing ethylene glycol, ferric chloride hexahydrate, PVP K90 and PVP K30 and Solution 2 containing silver nitrate and ethylene glycol. Solution 1 was pumped into the 1 / 8” PTFE tubular reactor using a peristaltic pump, while Solution 2 was pumped using a syringe pump. The total volume of the reactor was 70 ml. The reactor with a volume of 18 ml was first treated with convectional heating, then microwave heating for 32 ml and finally conventional heating for another 20 ml. This experimental setup resulted in a higher conversion of the silver precursor (96 %) compared to the previous cases. The nanowires have a diameter of 63 nm and a length of 21 pm.

[0073] EXAMPLE 9: Synthesis of silver nanowires in CSTR in micro wave (Comparative example)

[0074] Three neck 100 ml continuous stirred tank reactor (CSTR) with reflux condenser was used. The inlet stoichiometry of reactant kept the same as Example 1 and residence time 4 - 8 minutes. Figure 3 shows theUV-Vis analysis of silver nanowire at different residence time. The resulting silver nanowires aspect ratio is 108 (diameter 112 nm and length 12 pm) and the overall conversion of silver precursor was 67 % were analyzed by Field emission scanning electron microscopy and atomic absorption spectrophotometer respectively. The 49 % yield of nanowires achieved. The dimension of nanowire was changing with residence time as inlet reactantstended to form secondary seeds. A formed silver nanostructure found deposited on the surface of temperature surface and wall of the reactor, which could generate sparking due to the excessive heat generation.

[0075] ADVANTAGES OF THE INVENTION

[0076] • The present invention provides a large-scale continuous flow process for the synthesis of silver nanowires.

[0077] • The present invention provides a continuous process with tubular reactor which covers: (i) no wall deposition, (ii) it is scalable with an unidirectional flow of solutions, (iii) it has controlled nucleation and growth rates, (iv) it has no moving parts in direct contact with the reaction mass, (v) it requires lower footprint, (v) it has high throughput, and (vi) there is no need of any other heating fluid for taking the reaction mass to reaction temperature.

[0078] • The present invention provides a continuous flow process with shorter reaction time.

[0079] • The present invention provides a continuous flow process with high yield.

[0080] • The present invention provides a tubular reactor that is easy to operate, since there are no moving parts.

[0081] • The present invention provides silver nanowires with tunable aspect ratio.

Claims

We Claim1. A microwave assisted continuous flow process for the synthesis of metal nanowire, comprising the steps of: a) adding a metal salt in a solvent at a temperature in the range of 25-35°C to obtain a solution A; b) dissolving polyvinylpyrrolidone (PVP) of molecular weight MW=3,60,000 and 55000 in 2: 1 ratio in a solvent at a temperature in the range of 25-35°C to obtain a solution B; c) adding a metal halide in a solvent at a temperature in the range of 25-35 °C to obtain a solution C; d) mixing the solution B of step b) and solution C of step c) to obtain a solution mixture D; e) feeding continuously the solution A of step a) and solution mixture D of step d) in a helical tubular reactor placed in microwave oven; f) obtaining the metal nanowire after complete conversion at outlet of the reactor.

2. The process as claimed in claim 1, wherein the process is carried out for a time period in the range of 2-4 minutes.

3. The process as claimed in claim 1, wherein the metal salt is silver nitrate and the solvent is ethylene glycol.

4. The process as claimed in claim 1, wherein the metal halide is selected from the group consisting of sodium bromide, sodium chloride, copper chloride and ferric chloride hexahydrate.

5. The process as claimed in claim 1, wherein at least two molecular weights of PVP is based on the range of 55000 to 360000.

6. The process as claimed in claim 1, wherein the microwave power is in the range of 300- 1500 W and microwave temperature is in the range of 150 - 210°C.

7. The process as claimed in claim 1, wherein the flow rate of solution of step a) and solution of step d) are 3 ml / min and 24 ml / min, respectively.

8. The process as claimed in claim 1 , wherein the helical tubular reactor is made of a material selected from polytetrafluoroethylene ( PTFE) and perfluoroalkoxy alkanes ( PFA).

9. The process as claimed in claim 8, wherein the PTFE tubular reactor is having an inner diameter of 1.5 mm -5 mm and volume of 12 - 500 ml.

10. The process as claimed in claim 1, wherein the continuous flow is in upward direction in the tubular reactor.

11. The process as claimed in claim 1 , wherein the aspect ratio of obtained metal nanowire is in the range of 120-1200, diameter in the range of 10-100 nm, and length in the range of 5 to 30 microns.

12. The process as claimed in claim 1, wherein the obtained metal nanorods have diameter in the range of 100 nm - 200 nm, length in the range of 1 - 20 microns, and aspect ratio in the range of 10 to 100.

13. The process as claimed in claim 1, wherein the PVP is a capping agent that allows silver nanowires to grow unidirectionally and act as stabilizer.

Images