WO2019034623A1

WO2019034623A1 - Process for the manufacture of metal nanowires

Info

Publication number: WO2019034623A1
Application number: PCT/EP2018/071955
Authority: WO
Inventors: Lauriane D'ALENCON; Thierry Le Mercier; Marie PLISSONNEAU; Mona Treguer-Delapierre
Original assignee: Rhodia Operations; Le Centre National De La Recherche Scientifique; Universite de Bordeaux
Priority date: 2017-08-16
Filing date: 2018-08-13
Publication date: 2019-02-21

Abstract

Process for the preparation of metal nanowires in which metal salts, capping agents and ionic additives are reacted in specific reducing solvents under specific reaction conditions.

Description

Process for the manufacture of metal nanowires

[0001] This application claims priority to European application

No. 17306072.4 filed on August 16, 2017, the whole content of this application being incorporated herein by reference for all purposes.

[0002] The present invention relates a process for the manufacture of metal nanowires.

[0003] Transparent conductive electrodes are commonly used in organic electronic devices such as organic light emitting diodes, displays and photovoltaic cells.

[0004] At present mainly Indium Tin Oxide (ITO) is used for the

manufacture of such electrodes, but ITO has some drawbacks like its brittleness and the need of high processing temperatures to achieve the desired properties. Furthermore, ITO has high manufacturing costs as it is made by vapour phase deposition. The use of ITO in new and flexible organic electronic devices is thus difficult and has severe limitations.

[0005] In the recent past nanoparticulate products with a high aspect ratio such as nanowires and nanotubes have been investigated as a replacement material for ITO. Such high aspect ratio materials have the benefit compared to spherical or close to spherical particles (i.e. particles with low aspect ratio) that the amount needed to achieve a percolation network (which is necessary to achieve sufficient conductivity) is significantly lower.

This is an economic as well as a technical advantage as the transparency of an electrode deteriorates with increasing content of nanoparticulate material. Thus, because of the need of higher amounts of low-aspect nanoparticles the transparency of respective electrodes made therefrom is worse than respective products obtained from nanowires and nanotubes. [0006] Among the nanowires and nanotubes silver nanowires offer a good potential as silver is the metal with the highest metal conductivity. Several other metallic nanowires have also been investigated and some of them show interesting properties. In particular copper, gold and cupronickel nanowires may be mentioned in this regard.

[0007] However, in many synthesis processes for nanowires (which hereinafter are generally to be understood as nanoparticulate products with aspect ratios of at least 10, cf. later), nanoparticles with aspect ratios of less than 10 are produced as by-products in significant amounts. These nanoparticles have to be removed to restore transparency and they don't provide a beneficial effect in terms of conductivity (they do not improve the electrical percolation).

[0008] Several routes have been described in the literature for the

manufacture of in particular silver nanowires. Respective methods include the hydrothermal method, microwave assisted processes, electrochemical techniques, photochemical techniques and other template techniques.

[0009] From the known methods the so called polyol-process appears to be a promising technology in terms of ease of mass production, cost and simplicity. In this process metallic salts are reduced by a polyol, mostly ethylene glycol. Crystallization of noble metals usually leads to highly symmetric cubic unit cells, i.e. rather to the formation of products with a low aspect ratio (an aspect ratio of 1 representing the ideal sphere with highest symmetry). To promote the growth of nanowires, i.e. products with a high aspect ratio, some form of anisotropic confinement has to be applied in order to achieve one-dimensional growth.

[0010] The exact mechanism of nanowire growth in the polyol process is complex and has not yet been entirely clarified. In a first step, the glycol reduces the metal cations in the salt to the neutral metal which induces metal crystal formation and the subsequent growth of nanostructures. The synthesis of nanowires includes three distinct steps of nucleation, evolution of nuclei into seeds and growth of seeds into nanocrystals. By modifying the

thermodynamics and kinetics of each of these steps, it is possible to control the form of the synthesized nanostructures to a certain degree. The addition of particles playing the role of seeds can change the process of nucleation and various nucleating agents have been studied in combination with ethylene glycol. Polyvinyl pyrrolidone has been found to be capable to control the growth rates of different faces of metal nanospecies and thus contributes to the introduction of anisotropic confinement.

[001 1 ] Johan et al. J. of Nanomaterials Volume 2014, Article ID 105454 (http://dx.doi.org/10- 1 155/2014/105454) discloses the synthesis of silver nanowires through different mediated agents (CuCh and NaCI) in a polyol process. In the process described propane diol is added to a flask preheated at 170°C, then a solution of silver nitrate (0.0005M) is added under vigorous stirring in a first step. Subsequently a 0.1 M solution of AgNO3 and a propane diol solution of polyvinyl pyrrolidone (PVP) are added dropwise. The reaction is continued for one hour and heated at 170°C for 30 minutes. The suspension obtained was allowed to cool down to room temperature and the solution was diluted with acetone, centrifuged and washed with deionized water.

[0012] Wen et al., Chinese Physics, Vol. 14, No. 1 1 (November 2005), 2269-2275 discloses the controlled growth of silver nanowires via reducing silver nitrate with 1 ,2-propane diol in the presence of polyvinyl pyrrolidone. It is said that the diameter of the nanowires could be controlled in the range of from 100 to 400 nm by varying the experimental conditions. The interaction between PVP and silver nanowires is ascribed to the oxygen atom in the carbonyl group of the PVP.

[0013] US 2008/0210052 discloses a process of forming monodispersed metal nanowires comprising forming a reaction mixture including a metal salt, a capping agent and a quaternary ammonium chloride in a reducing solvent (which in some of the working examples is propylene glycol) and forming metal nanowires by reducing the metal salt in the reaction mixture.

[0014] US 201 1/0174190 discloses a method comprising growing metal nanowires from a reaction solution including a metal salt and a reducing agent wherein the reaction is carried out in two stages with sequential addition of the metal salt. Propylene glycol is used as reducing agent and solvent in some of the working examples.

[0015] WO 2015/157741 discloses a process for the manufacture of

silver nanowires wherein the reaction is carried out in two stages, allowing a first stage reaction mixture comprising a polyol solvent, a first portion of a silver salt, a capping agent, a chloride source and a co-additive to react for a first period of time and thereafter gradually adding a second portion of the silver salt while maintaining a concentration of less than 0.1 w/w of the silver salt. Propylene glycol is used as the glycol solvent in some of the examples.

[0016] After the synthesis, the product is usually subjected to certain purification steps.

[0017] The processes described in the prior art are not fully satisfactory in terms of ease of mass production, product quality and simplicity of the process.

[0018] It was therefore an object of the present invention to provide an improved process for the manufacture of metal nanowires which offers advantages over the prior art processes. [0019] This object is achieved with the process in accordance with clainn 1.

[0020] Preferred embodiments are set forth in the dependent claims and in the detailed specification hereinafter.

[0021] The process in accordance with the present invention comprises the following steps

a) forming a first solution comprising a capping agent dissolved in a reducing solvent selected from glycols having at least three carbon atoms,

b) forming a second solution comprising a metal in a reducing solvent, identical or different to the reducing solvent used in step a), and selected from glycols having at least three carbon atoms,

c) preparing a solution of an ionic additive in a reducing

solvent, identical or different to the reducing solvent used in step a), and selected from glycols having at least three carbon atoms,

d) combining the solutions obtained in steps a) and b) under stirring at a temperature in the range of from 0 to 60 °C, in amounts that the molar ratio of capping agent to metal salt in the combined solutions is in the range of from 2: 1 to 7: 1 e) adding the solution obtained in step c) to the mixture

obtained in step d) under stirring,

f) heating the reaction mixture obtained in step e) to a temperature in the range of from 1 10 to 190 °C for a period of 20 to 300 min, and thereafter cooling the system to room temperature, and , optionally,

g) subjecting the slurry obtained in step f) to a purification treatment.

[0022] The term metal nanowires, as used herein, is intended to denote elongated nanoparticulate materials having an aspect ratio of at least 10, preferably in the range of from 10 to 10000, more preferably in the range of from 10 to 5000 and even more preferably in the range of from 20 to 2000 and most preferred in the range of form 250 to 1200..

[0023] Suitable metal nanowires include, without being limited to, silver, gold, copper, nickel, palladium, platinum and gold-plated silver nanowires.

[0024] The term "aspect ratio of a geometric shape", as used herein, is the ratio of the extensions of a material in different dimensions, i.e. the ratio of the largest diameter of a particle to the smallest diameter orthogonal to the largest diameter (or, generally speaking the ratio of length to width). Thus, the aspect ratio is a shape factor numerically describing the shape of a particle independent of its absolute size. The aspect ratio of an ideal sphere is 1 (the size in all dimensions is equal). Fibers and wires have a high aspect ratio, i.e. their size in one dimension exceeds the size in other dimensions significantly. An aspect ratio of at least 10 means a size of the particle in one axis being at least 10 times the size in another axis.

[0025] In the first step of the process in accordance with the present invention, a first solution comprising a capping agent dissolved in a reducing solvent selected from glycols having at least three carbon atoms is formed.

[0026] Examples of suitable reducing solvents derived from polyols with at least three carbon atoms are 1 ,2- and 1.3-propane diol, glycerol and glucose, to mention only the most common ones. 1 ,2- and 1 ,3-propane diol and in particular 1 ,2-propane diol are preferred.

[0027] The term capping agent, when used herein, refers to a chemical compound that preferentially interacts and adheres to a lateral surface of a growing nanowire such that the said capping agent prevents or inhibits said lateral surface from growing. As a result, the growth takes place preferably in one direction leading to particles with a high aspect ratio (the aspect ratio of a perfect spherical particle would be 1 ). The capping agent interacts with one surface of the growing nanocrystal more strongly than with a second surface. As a result the first surface is passivated for growth and only the second surface is available for further crystallization thereby yielding elongated structures in the form of wires.

[0028] Suitable capping agents which have proven to be effective

include, without limitation, polyvinyl pyrrolidone), polyarylamides, polyacrylic acid, polyvinyl alcohol, poly(ethylene imine), poly(2- ethyloxazoline), polycaprolactam, polypropylene carbonate), hydroxyl propyl cellulose, hydroxyl propyl methyl cellulose, gelatin, and bovine serum albumin and copolymers thereof, with polyvinyl pyrrolidone) - hereinafter referred to as PVP - being particularly preferred.

[0029] The molecular weight of the capping agent is not subject to

particular limitations. Products with molecular weight (weight average molecular weight) in the range from 10 000 to 5 000 000, preferably from 25 000 to

2 000 000 and even more preferably in the range of from 30 000 to 500 000 may be used. In some cases (in particular with PVP as capping agent) it has been found that increasing the molecular weight of the capping agent increases the length and the diameter of the nanowires obtained, with the growth in length being more pronounced than the growth in diameter. As a result, using a capping agent with higher molecular weight usually increases the aspect ratio of the nanowires obtained but also leads to an increased thickness. [0030] The weight average molecular weight of the capping agent may be determined by standard methods known to the skilled person which have been described in the prior art. Size Exclusion Chromatography is a generally accepted method which may also be applied to determine the weight average molecular weight of the capping agent. Other suitable methods are viscosimetry and ultracentrifugation.

[0031] The concentration of the capping agent in the reducing solvent obtained in step a) is not subject to particular limitations and is generally in the range of from 0.05 M to 2M, preferably from 0.08 to 1 M. Solubility of the capping agent in the reducing solvent may limit the maximum concentration of the capping agent in the solution.

[0032] In step b), a solution of a metal salt in a reducing solvent (said solvent being identical or different to the solvent used in step a)) selected from glycols having at least three carbon atoms is prepared. In some cases it has found to be advantageous if the solvent used in step b) is identical to the solvent used in step a).

[0033] The term metal salt, when used herein, refers to a neutral

compound having a positively charged metal ion and a negatively charged counterion, which counterion may be organic or inorganic. Suitable metals include but are not limited to silver, gold, copper, nickel, gold-plated silver, platinum and palladium, with silver and copper being preferred. Particularly preferred is silver. Preferred counterions include, without limitation, nitrate, chloride, perchlorate, and acetate. Just by way of example silver nitrate, silver acetate, silver perchlorate, gold perchlorate, palladium chloride, platinum chloride and the like may be mentioned. Silver nitrate is a particularly preferred metal salt frequently used for the synthesis of silver nanowires.

[0034] The concentration of the metal salt in the reducing solvent is not subject to particular limitations. In some cases concentrations in the range of from 0.1 to 20 wt% , preferably of from 0.5 to 16 wt%, based on the combined weight of salt and solvent, have shown certain advantages. The solubility of the salt in the reducing solvent is basically the upper limit as the salt should be completely dissolved in the reducing solvent.

[0035] The reduction of the metal salt by the reducing solvent produces corresponding elemental metal. The elemental metal crystallizes or grows into a one-dimensional nanostructure under the conditions of the reaction.

[0036] For the manufacture of solutions a) and b) in the first two steps of the invention it is beneficial to remove any oxygen dissolved in the glycols which are used as solvents in step a) and b) (as described above, the solvent in step b) can be identical to or different from the solvent used in step a)). This can be achieved by any conventional method for degassing, e.g. by bubbling with inert gas for a certain period of time (in most cases for a time period of between 2 and 10 hours).

[0037] Furthermore, it is also beneficial to prepare solution b) fresh

shortly before use to avoid any undesired reduction of the silver during storage of the solution.

[0038] In step c) of the present invention, a solution of an ionic additive in a reducing solvent (being identical or different to the solvent used in step a)) selected from glycols having at least three carbon atoms is prepared. In some cases it has found to be advantageous if the solvent used in step c) is identical to the solvent used in step a) and/or the solvent used in step b).

[0039] In certain cases it has been found advantageous if the solvents used in steps a) , b) and c) are identical.

[0040] Ionic additives which may be used are generally salts having an anion selected from the group consisting of chloride, perchlorate, tetrafluoroborate, hexafluorophosphate, triflate, phosphate, thiosulphate and salicylate or their mixtures. Chlorides are preferred ionic additives. The chloride source may be metal chlorides, including chlorides of alkali, alkaline earth and transition metals. Examples of metal chlorides include, without being limited to, sodium chloride, potassium chloride, lithium chloride, cesium chloride, magnesium chloride, calcium chloride, iron (II) chloride, iron(lll)chloride, copper(l)chloride,

copper(ll)chloride, nickel chloride, indium chloride, zinc chloride and the like. In yet further embodiments, the chloride source may be a quaternary ammonium chloride NR₄CI wherein individually and independently may be hydrogen, alkyl, alkenyl, alkynyl, aryl or aralkyl. Examples of quaternary ammonium chlorides include, without being limited to, ammonium chloride, tetramethyl ammonium chloride, tetrabutylammonium chloride, cetyl trimethylammonium chloride, alkyl dimethyl benzyl ammonium chlorides with Ce-C-ie alkyl groups, methyl trioctylammonium chloride. Another suitable chloride is tetraphenylphosphonium chloride. Sodium chloride and CuCh are preferred chloride ionic additives which may be advantageously used.

The ionic additives preferably have a solubility in the reducing solvent of at least 0.01 , preferably at least 0.1 wt%.

In accordance with another embodiment of the present invention, a co-additive is used in combination with the ionic additive. Such co-additive is preferably selected from nitrogen containing bases, optionally substituted phenols or hydroquinones, optionally substituted anilines and mixtures thereof. Further details on suitable co-additives in this regard are disclosed in WO

2015/0157741 to which reference is made for further details. The co-additive, if present is preferably used in an amount of 5 to120, preferably of 10 to 80 wt%, relative to the amount of ionic additive.

[0043] In step d) the solutions obtained in steps a) and b) are combined under stirring , eventually accompanied by diluting with additional solvent. The amounts of solutions a) and b) are chosen in accordance with the specific application case so that the molar amounts of capping agent (calculated based on the monomeric unit of the polymeric capping agent) and metal salt are in the range of from 2: 1 to 7:1 , preferably of from 2.5: 1 to 4: 1

Depending on the concentration of the two solutions, same are combined in the respective amounts. The mixing or combination of the two solutions is effected at a temperature in the range of from 0 to 60, preferably of from 15 to 50°C. Generally speaking, the mixing is preferably carried out without external heat being applied during the mixing.

[0044] In case silver salts and in particular silver nitrate is the metal salt, it is advantageous to protect the mixture from light to avoid decomposition of the silver salt. The same applies to other metals respectively metal salts should they be light sensitive.

[0045] In some cases it has been found that certain benefits are given if capping agent and metal salt are combined prior to heating.

[0046] The mixing of the two solutions obtained in steps a) and b) may be effected by simply pouring one solution into the other solution or by dropping one of the solutions into the other. For economic reasons it is preferred to carry out the mixing in a relatively short time interval of preferably less than thirty minutes, i.e. the full amounts of metal salt and capping agent are combined in less than thirty, preferably less than twenty minutes.

[0047] In step e) of the process in accordance with the present

invention, the solution of the ionic additive obtained in step c) is added under stirring to the mixture obtained in step d). This can be achieved preferably by slowly pouring the solution obtained in step c) into the nnixture obtained after step d). This can be effected by pouring the total amount of the solution obtained in step c) or the addition can be made in several steps or portions.

[0048] The relative molar ratio between the ionic additive (preferably chloride) and the metal salt is preferably in the range of from

1 : 1000 to 1 :5, preferably in the range of from 1 :200 to 1 :25.

[0049] After completion of the addition in step e) the reaction mixture is heated to a temperature in the range of from 1 10 to 190 °C, preferably of from 120 to 175°C for a period of from 20 to 300 min, preferably of from 20 to 200 min. Compared to processes described in the literature carried out at 90°C to support nanowire growth over nanoparticle production, the synthesis in the process according to the invention can be carried out in shorter times without detrimental effects on the yield and aspect ratio of the metal nanowires. This is a significant economic advantage once it comes to scale-up. The processes described e.g. in

WO2015/157741 and US 201 1/0174190 referred to hereinbefore take more than 24 hours to achieve a satisfactory yield of metal nanowires, which is costly and time consuming and impedes upscaling.

[0050] Once the reaction is completed, the reaction mixture is cooled down to room temperature. The main product are metal nanowires with an aspect ratio of more than 10 which are produced in significantly higher amounts than nanoparticles. Usually, the weight ratio of nanowires with an aspect ratio exceeding 10 to nanoparticles with an aspect ratio below 10 is in the range of from 1 : 1 to 50: 1 , preferably in the range of from 2: 1 to 10: 1.

[0051] For a number of applications it is advantageous to reduce the number of nanoparticles even further. Nanoparticles do not significantly contribute to conductivity (they are much less - in effective in forming percolation networks) but are detrimental to transparency which is a desirable property for many applications. The same basically applies to short nanowires having an average length below 5 μηη. Therefore, it is in many cases desirable to reduce the nanoparticle content in the product obtained as far as possible and economically feasible.

[0052] When employing a co-additive in combination with a chloride in the process, the process in accordance with the present invention typically already yields a crude product significantly biased toward nanowires with higher aspect ratios while at the same time minimizing the low aspect ratio nanostructures.

Nevertheless, purification of the crude product can further improve the application properties by further enriching nanowires of desired morphology.

[0053] Several methods for the separation of nanowires from

nanoparticles have been described in the literature.

[0054] One possibility is separation of nanoparticles and nanowires by sedimentation. Typically, in a given solvent system, the sedimentation rates strongly depend on nanowire diameter and length. The longer and thicker the nanowires are, the faster they will normally settle. However, the thinner the nanowires become, purification by sedimentation may be slow and less effective in separation.

[0055] On the other hand, because nanowires are normally attracted to each other electrostatically, the higher the nanowire

concentration, the faster the sedimentation rates are. Thus, the fact that the process in accordance with the present invention, applying the specific steps a) to f) described hereinabove resulting in high yields of nanowires is also advantageous for purification by sedimentation.

[0056] An alternative purification method relies on the solubility of the capping agent used to selectively remove the nanowires from the product mixture. Because the capping agent tends to adhere to the nanowires, addition of a solvent in which the capping agent is insoluble or poorly soluble leads to rapid precipitation of the nanowires together with the capping agent. The amount of capping agent associated with nanowires is much larger than the amount associated with nanoparticles and therefore a respective purification process favors precipitation of nanowires over nanoparticles which remain in the supernatant.

[0057] Suitable solvents for a respective purification are those solvents wherein the capping agent used has a solubility of less than 5 wt%, preferably less than 2 wt% and particularly preferably less than 1 wt%. Suitable solvents where the capping agents typically used ( in particular PVP), has a low solubility include acetone, methyl ethyl ketone and ethyl acetate. Principally any solvent wherein the capping agent has a low solubility may be used, however.

[0058] Once the precipitate of nanowires along with the capping agent is separated from the supernatant containing nanoparticles, the precipitate can be re-dispersed in a solvent wherein the capping agent is soluble ) e.g. the reducing solvent, water or alcohols and thereafter a second precipitation step can be applied. This sequence of precipitating and re-suspending may be repeated several times until the content of nanoparticles has been reduced to the desired level. Once the supernatant is clear, this is a good indication that the product is substantially free of low aspect ratio nanoparticles.

[0059] Another possibility for the separation of nanowires from

nanoparticles with low aspect ratio which may be mentioned here, is filtration. If a suitable filter is used, nanowires are retained in the filter (due to their high aspect ratio) while nanoparticles with low aspect ratio will pass the filter.

[0060] Nanowire dispersions, in particular silver nanowire dispersions obtained by the process in accordance with the present invention usually have a solids content in the range of from 0.05 to 10 % by weight, preferably of from 0.05 to 5 % by weight, based on the entire weight of solids and solvent. The weight ratio of particles with an aspect ratio of at least 10 to particles with an aspect ratio of less than 10 is generally in the range of from 1 : 1 to 50: 1 , preferably in the range of from 2:1 to 10: 1. The said ratio can be easily monitored by scanning electron microscopy (SEM) where the relative proportions of nanoparticles with low aspect ratio can be distinguished from elongated nanowires.

[0061] The process in accordance with the present invention allows to produce high quality nanowires in high yield with a low amount of by-products, which facilitates the purification of the product.

Furthermore, an increased rate of anisotropic growth is obtained.

[0062] The metal nanowires obtained in accordance with the process of the present invention show an advantageous combination of diameter (thickness), length and aspect ratio and these properties may be tuned by adopting the process conditions taking into consideration the desired use. While long nanowires with a relatively high diameter may be advantageous in improving conductivity, thinner nanowires may be beneficial when it comes to optical transparency.

[0063] The nanowires obtained in accordance with the process of the present invention may be used for the manufacture of films; the films are typically transparent, and are advantageously

incorporated in organic electronic devices (as replacement for ITO products). The nanowires obtained in accordance with the process of the present invention may be also be used in inks and coating compositions, especially for the manufacture of such films. In this regard, the step h) of forming an ink or coating composition comprising the metal nanowires may constitute an additional step of the invented process.

[0064] Should the disclosure of any patents, patent applications, and publications which are incorporated herein by reference conflict with the description of the present application to the extent that it may render a term unclear, the present description shall take precedence.

[0065] Examples

[0066] Example 1 - Propane diol as reducing agent, PVP with weight average molecular weight of 55,000 as capping agent

[0067] Prior to synthesis, oxygen was removed from the 1 ,2-propane diol by argon bubbling for at least three hours.

[0068] Solution A was prepared by adding 233.4 mg of

poly(vinylpyrrolidone) (PVP) with an average molecular weight of 55,000 g/mol in a plastic centrifugation tube of 50 ml_ containing 10 ml_ of the previously argon-degassed 1 ,2-propanediol. The mixture is vortexed at ambient temperature until complete dispersion of the PVP in 1 ,2-propanediol.

[0069] Solution B was prepared by adding 1 10.4 mg of AgNO3 in a

plastic centrifugation tube of 50 ml_ containing 6.5 ml_ of the previously argon-degassed 1 ,2-propanediol. The tube is immediately protected from light by aluminum foil and vortexed at ambient temperature until complete dissolution of the silver nitrate.

[0070] Solution C is prepared by adding 58.4 mg of NaCI to 10 ml 1 ,2- propanediol and vortexed at ambient temperature until complete dissolution of the salt in 1 ,2-propanediol.

[0071] 8.6 ml_ of freshly prepared solution A was poured in a 50 ml_ three-neck round bottom flask containing already 8.4 ml_ of the previously argon-degassed 1 ,2-propanediol, and mixed vigorously by magnetic stirring at ambient temperature under high flow of argon bubbling for two minutes.

[0072] 6 mL of freshly prepared solution B was also added to the 50 mL three-neck round bottom flask containing already PVP and 1 ,2- propanediol, and mixed vigorously by magnetic stirring at ambient temperature under high flow of argon bubbling for two minutes. As soon as the silver nitrate was added, the mixture was protected from light.

[0073] 70 μί of solution C was slowly poured in the mixture which is mixed for an additional five minutes at ambient temperature by vigorous magnetic stirring under high flow of argon bubbling. After these 5 minutes, the flask was immediately placed in an oil bath previously heated at 140°C and connected to a reflux column. The argon flow was reduced to a low bubbling.

[0074] After approximately 15 minutes after the beginning of the heating process, the coloration of the mixture turned from transparent- orange to greyish-opaque. At this point, the reflux was

maintained for one additional hour. Finally, the reaction was quenched by placing the round bottom flask in an ice bath, and cooled until reaching room temperature.

[0075] At this point, 80 mL of acetone were slowly added to the slurry causing the flocculation of the nanowires at the bottom of the flask. The supernatant was discarded and the precipitate was redispersed in 10 mL of water. This step was repeated three times. Finally, the nanowires were redispersed in a volume of ultrapure water of 10 mL.

[0076] Example 2

[0077] Example 1 was repeated with PVP having a weight average

molecular weight of 360 000 as capping agent. The change of coloration of the mixture was observed after twenty minutes after beginning of the heating process. Thereafter, the reflux was maintained for one additional hour.

[0078] Example 3

[0079] Example 1 was repeated with PVP having a weight average

molecular weight of 1 300 000 as capping agent. The change of coloration of the mixture was observed after thirty minutes after beginning of the heating process. Thereafter, the reflux was maintained for one additional hour.

[0080] Comparative Example C1

[0081] Example 1 was repeated, but ethylene glycol was used instead of 1 ,2-propane diol. The change of coloration of the mixture was observed after 40 minutes after beginning of the heating process. Thereafter, the reflux was maintained for one additional hour.

[0082] Comparative Example C2

[0083] Example 2 was repeated, but ethylene glycol was used instead of 1 ,2-propane diol. The change of coloration of the mixture was observed after 100 minutes after beginning of the heating process. Thereafter, the reflux was maintained for one additional hour.

[0084] Comparative Example C3

[0085] Example 3 was repeated, but ethylene glycol was used instead of 1 ,2-propane diol. The change of coloration of the mixture was observed after 180 minutes after beginning of the heating process. Thereafter, the reflux was maintained for one additional hour.

[0086] The following table provides a comparison of the time of first wire formation, average wire length, average wire diameter, aspect ratio and yield of nanowires. Example Time of Average Average Aspect Yield first wire length diameter ratio from formation μηη nm ICP

%

1 10 min 8.5 +/- 2.6 34 +/- 6 250 85

C1 40 min 8.4 +/- 2.6 45 +/- 12 184 40

2 20 min 37.5 +/- 37 +/- 6 1000 80

1 1.2

C2 90 min 26.6 +/- 102 +/- 62 260 55

12.8

3 30 min n/a 57 +/- 1 1 n/a 50

C3 180 min 41.0 +/- 82 +/- 28 497 15

13.3

[0087] The average length and average diameter as well as the aspect ratio were determined by transmission electron microscopy (TEM) by measuring a respective number of representative particles in the micrograph. TEM grids, copper speciment grids

(300 mesh) with carbon film were purchased from Electron Microscopy Sciences. TEM images were collected using a JEOL microscope designated JEM 1400 operating at 120 kV and using the software provided with the microscope.

[0088] The silver content in the silver nanowire suspension was

measured by ICP OES after digestion of the sample in nitric acid (appr. 0.2-0.3 g of solution with 4 ml of nitric acid (cone. 65 % by weight). The limpid solution was then diluted in a 5 wt% aqueous nitric acid solution according to the expected silver concentration (e.g. dilution by a factor of 3000 for an amount of 0.4 %). The intensity measured on the specific silver wavelength (328.068 and 338.289 nm) was compared to the calibration curve in the range of from 0.05 to 2 mg/ml silver standards obtained in similar analytical conditions in order to measure the amount in the diluted solution. The amount in the solution was then calculated using the dilution factor.

The Examples show the benefits of the process of the present invention over prior art process with ethylene glycol as reducing solvent.

Claims

C L A I M S

1. A process for preparing metal nanowires comprising the following steps a) forming a first solution comprising a capping agent dissolved in a reducing solvent selected from glycols having at least three carbon atoms, b) forming a second solution comprising a metal salt in a reducing solvent, identical or different to the reducing solvent used in step a), and selected from glycols having at least three carbon atoms, c) preparing a solution of an ionic additive in a reducing solvent, identical or different to the reducing solvent used in step ), and selected from glycols having at least three carbon atoms, d) combining the solutions obtained in steps a) and b) under stirring at a temperature in the range of from 0 to 60 °C, in amounts that the molar ratio of capping agent to metal salt in the combined solutions is in the range of from 2: 1 to 7: 1 e) adding the solution obtained in step c) to the mixture obtained in step d) under stirring, f) heating the reaction mixture obtained in step e) to a temperature in the range of from 1 10 to 190 °C for a period of 20 to 300 min, and thereafter cooling the system to room temperature, and, optionally, g) subjecting the slurry obtained in step f) to a purification treatment.

2. The process of claim 1 wherein the reducing solvent is selected from the group consisting of 1 ,2-propane diol, 1 ,3-propane diol, glycerol, glucose, and mixtures thereof.

3. The process of claim 2 wherein the reducing solvent is selected from the group consisting of 1 ,2-propane diol, 1 ,3-propane diol, and mixtures thereof.

4. The process of claim 3 wherein the reducing solvent is 1 ,2- propane diol.

5. The process of any of claims 1 to 4 wherein the capping agent is selected from the group consisting of polyvinyl pyrrolidone),

polyarylannides, polyacrylic acid, polyvinyl alcohol, poly(ethylene imine), poly(2-ethyloxazoline), polycaprolactam, polypropylene carbonate), hydroxyl propyl cellulose, hydroxyl propyl methyl cellulose, gelatin, bovine serum albumin, and copolymers and mixtures thereof .

6. The process of claim 5 wherein the capping agent is polyvinyl pyrrolidone).

7. The process of claim 6 wherein the polyvinyl pyrrolidone) has a molecular weight from 30 000 to 500 000.

8. The process in accordance with any of claims 1 to 7 wherein the metal salt is a salt of a metal selected from the group consisting of silver, gold, copper, nickel, gold-plated silver, platinum and palladium.

9. The process of claim 8 wherein the metal salt is a silver salt.

10. The process of claim 9 wherein the silver salt is silver nitrate.

1 1. The process in accordance with any of claims 1 to 10 wherein the ionic additive comprises a salt having an anion selected from the group consisting of chloride, perchlorate, tetrafluoroborate,

hexafluorophosphate, triflate, phosphate, thiosulphate, salicylate, and mixtures thereof.

12. The process in accordance with claim 1 1 wherein the ionic additive is sodium chloride.

13. The process in accordance with any of claims 1 to 12 wherein the metal nanowires are silver nanowires.

14. The process in accordance with any of claims 1 to 13, further comprising the step h) of forming an ink or coating composition comprising the metal nanowires.

15. Use of the metal nanowires prepared by the process according of claims 1 to 14 for the manufacture of a film.