HIGH QUANTITY AND HIGH YIELD SYNTHESIS OF SILVER NANOWIRE WITH SELECTED LENGTHS AND MODERATE DIAMETERS

Description

FIELD OF THE INVENTION

The invention relates to synthesis methods for forming silver nanowires. In particular, the invention relates to high yield and high volume synthesis for the formation of moderate diameter silver nanowires with selectable lengths over a broad range.

BACKGROUND OF THE INVENTION

Silver nanowires have significant prospects for providing electrically conductive elements for forming a range of structures. While much of the focus of silver nanowire use has been directed to transparent conductive films, recent development point to utility for a broader range of uses in composite conductive materials. Successful commercialization relies on the synthesis of commercial scale quantities of the silver nanowires.

SUMMARY OF THE INVENTION

In a first aspect, the invention pertains to a method for synthesis of silver nanowires with a high yield, the method comprising:

• • under mixing conditions, adding a selected quantity of soluble silver salt to a reaction solution initially comprising ammonium-based chloride and polyvinylpyrrolidone (PVP) in a glycol solvent at an initial reaction temperature from about 180° C. to a temperature just below the solvent boiling point to form a reaction solution;
  • after a period of time from about 1 minutes to about 10 minutes following addition of the soluble silver salt, under mixing conditions, add an additional quantity of solvent amounting to 6% to 30 wt % of the total solvent following the addition of the soluble silver salt; and
  • maintaining the mixing conditions of the reaction solutions at a continuing reaction temperature from about 100° C. to a temperature just below the solvent boiling point for a reaction time of at least about 1 hour.

In a further aspect, the invention pertains to a method for synthesizing high aspect ratio silver nanowires, the method comprising:

• • under mixing conditions, adding a first quantity of soluble silver salt to a solution of ammonium chloride and polyvinylpyrrolidone in a glycol solvent at an initial reaction temperature from about 180° C. to at least one degree below the solvent boiling point to form an initial reaction solution;
  • following addition of the soluble silver salt, under mixing conditions, add an additional quantity of solvent amounting to 6 wt % to 30 wt % of the solvent following the additional of the soluble silver salt;
  • provide for the cooling of the reaction solution to a continuing reaction temperature from about 100° C. to about 150° C. to form a cooled reaction solution, and
  • maintaining the continuing reaction temperature while gradually adding a second quantity of soluble silver salt into the cooled reaction solution for a reaction time of at least about 4 hours.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a reaction scheme for synthesizing moderate diameter silver nanowires on a commercial scale wherein a single long growth phase is employed.

FIG. 2 shows a reaction scheme for synthesizing higher aspect ratio silver nanowires on a commercial scale wherein an extended growth phase is employed.

FIG. 3 shows UV-visible extinction spectra for moderate diameter silver nanowires and higher aspect ratio silver nanowires synthesized according to 10 L and 100 L reaction schemes described herein.

FIG. 4 shows UV-visible extinction spectra for higher aspect ratio silver nanowires according to 10 L reaction schemes described herein.

FIG. 5A shows temperature profiles measured during syntheses of moderate diameter silver nanowires according to 10 L and 100 L reaction schemes described herein.

FIG. 5B shows details of the temperature profiles of FIG. 5A.

FIG. 6A shows temperature profiles measured during syntheses of moderate diameter silver nanowires in which the concentrations of polyvinylpyrrolidone and silver nitrate were increased by about 50% relative to the concentrations used to generate the temperature profiles shown in FIG. 5A.

FIG. 6B shows details of the temperature profiles of FIG. 6A.

FIG. 7A shows temperature profiles measured during syntheses of higher aspect ratio silver nanowires according to 10 L and 100 L reaction schemes described herein.

FIG. 7B shows details of the temperature profiles of FIG. 7A.

FIG. 8A shows temperature profiles measured during syntheses of higher aspect ratio silver nanowires according to 10 L and 100 L reaction schemes described herein and wherein the addition rates of polyvinylpyrrolidone to the reaction mixtures differed by a factor of about eight.

FIG. 8B shows temperature profiles measured during syntheses of higher aspect ratio silver nanowires according to 10 L and 100 L reaction schemes described herein and wherein the addition rates of polyvinylpyrrolidone to the reaction mixtures differed by a factor of about twelve.

FIG. 8C shows temperature profiles measured during syntheses of higher aspect ratio silver nanowires according to 10 L and 100 L reaction schemes described herein and wherein the addition rates of polyvinylpyrrolidone to the reaction mixtures differed by a factor of about twelve with a corresponding increase in temperature of about 20° C.

FIG. 8D shows the temperature profiles of FIGS. 8A-8C.

DETAILED DESCRIPTION OF THE INVENTION

Synthesis protocols have been developed to obtain high throughput, high yield silver nanowire synthesis for silver nanowires having moderate diameters and a range of lengths corresponding to relatively large aspect ratios. While very thin silver nanowires are desirable for high optical quality transparent conductive elements, a range of new composite materials having desirable electrical conduction for either opaque or less optically demanding (transparent or translucent) applications can be based on larger diameter silver nanowires that can be produced in high yields that allow for production of less expensive composite materials. The synthesis approaches adapt the catalysts and process procedures that allow for more rapid reactions at large scales while maintaining or achieving high yields. For example, high yields have been exemplified at 100 liter scale to produce amounts approaching a kilogram of silver nanowires per batch. Reactions are driven at relatively high temperatures, and reactants are added at controlled times to achieve desired nanowire properties while achieving high yields. In some embodiments, yields can be obtained of greater than 85% silver with desirable purity. The silver nanowires can be incorporated into materials that have high electrical conductivity and corresponding low resistivity.

High yield and high capacity synthesis of moderate diameter silver nanowires has been achieved through careful control of reaction temperatures and reactant addition. Silver nanowires with larger diameters can be synthesized more efficiently and inexpensively than the state of the art very thin silver nanowires. As described herein, improved synthesis processes are described to provide moderate diameter silver nanowires at high yield in an efficient, and versatile synthesis that are leveraged off applicant's previous work with proper selection of catalysts and process flow. Within reasonable boundaries, silver nanowire diameters and lengths can be adjusted to achieve target average sizes. The harvested silver nanowires can achieve a good uniformity and a yield over 85%. Applicant has developed a range of electrically conductive composite materials that can effectively use moderate diameter silver nanowires in the formation of the composite with desirable properties.

Silver nanowires have been of significant interest due to their high electrical conductivity and large aspect ratio. The large aspect ratio is particularly advantageous for making transparent conductive films. Due to the shape of the silver nanowires sparse metal layers can be formed that provide good conductivity while providing transparency with good optical qualities. It has also now been discovered to the aspect ratio can provide desirable advantages with respect to electrical conductivity in a broader range of materials from translucent to opaque composite materials. On the other hand, smaller aspect ratio nanowires are more easily dispersible at high concentrations, which can facilitate higher loading levels. Thus, it can be desirable to have a range of aspect ratios to select for a particular application. While for transparent conductive films, thinner silver nanowires can be desirable to improve optical properties, but for electrically conductive materials in which good optical properties are not an issue, thicker silver nanowires can provide desirable properties and high loading levels.

While some alternative approaches have been reported for the synthesis of silver nanowires, commercially viable approaches for silver nanowire synthesis have generally been based on what is usually referred to as the polyol process, which involves a polyvinylpyrrolidone capping agent and a glycol solvent reducing agent. The first report of the polyol process for silver nanowire synthesis based on a polyvinylpyrrolidone capping agent is generally attributed to Ducamp-Sanguesa et al., Journal of Solid State Chemistry, 100, 272-280 (1992) entitled “Synthesis and Characterization of Fine and Monodisperse Silver Particles of Uniform Shape,” incorporated herein by reference. This technique was extended by the laboratory of Professor Xia, see U.S. Pat. No. 7,585,349 to Xia et al., entitled “Methods of Nanostructure Formation and Shape Selection,” and Wiley et al., “Synthesis of Silver Nanostructures with Controlled Shapes and Properties,” Acc. Chem. Res. 2007, 40, 1067-1076, both of which are incorporated herein by reference. Similar synthesis was carried out with Fe⁺² or Cu⁺² halide salts by Xia et al., “Shape Controlled Synthesis of Metal Nanocrystals: Simple Chemistry Meets Complex Physics?” Angew. Chem. Int. Ed., 48, 60 (2009). Use of various other metal halide salts have been used in metal nanowire synthesis.

In the polyol processes, the solvent is a diol, generally ethylene glycol, propylene glycol or a blend thereof. While being the solvent, the amount of glycol generally is set based on the scale of the reaction, and the other reactants are adjusted accordingly to provide the desired concentrations. The standard silver salt source for providing the silver ions to be formed into the nanowires is silver nitrate, which is soluble in the glycols, although other soluble silver salts could be used in principle. The soluble silver salt is added to drive the nanowire synthesis. Since the silver is substantially all consumed, the concentration of silver ions in solution and correspondingly silver metal can change significantly over the course of the reaction. Generally, the silver nitrate may be added as a solid or dissolved in glycol solvent, although adding dissolved silver salt is convenient, more readily mixed and conducive to relatively short reaction times. Depending on the target silver product, the approach to adding the silver nitrate can be adjusted and furthermore the silver nitrate can be added substantially all at once or gradually. Addition timing and order for reactants is discussed in detail below.

Generally, the solvent is a glycol, such as ethylene glycol, propylene glycol, or the like. Ethylene glycol is generally a convenient solvent, and ethylene glycol is used in the examples herein. The amount of solvent generally correlates with the scale of the synthesis. Based on a particular amount of solvent, the total amounts of other reactants can be selected, and total batch production can be designed to produce a larger amount of product nanowires for a particular volume amount. The Examples explore the production of larger amounts of silver nanowires for a given reaction volume without sacrificing yield or target nanowire dimensions, and scale up is discussed further in this section below. To effectuate the desired adjustment of the reaction with respect to, for example, nanowire dimensions, reaction times, and yield, the total solvent used in a reaction can be added in various increment at targeted places in the synthesis, as explained below. The reactor size can be selected based on the amount of solvent used. Suitable commercial mixed reactors are available. Examples are performed herein using 10 liter reactors and 100 liter reactors, which can produce a significant quantity of silver nanowires per batch.

Polyvinylpyrrolidone has been used in the polyol process as a capping agent. While not wanting to be limited by theory, it is believed that polyvinylpyrrolidone (PVP) preferentially associates with certain crystal lattices of crystalline silver such that silver deposits then along the other facets of the crystal to form the nanowires. The work of Xia's research group has examined the synthesis of various silver nanostructures. The molecular weight of the PVP can influence the synthesis reaction. PVP can act as a dispersing aid for the silver nanowires to resist agglomeration in addition to functioning as a capping agent to guide nanowire growth.

The use of tetra n-butyl quaternary ammonium salts as a substitute for metal halides is described in published U.S. patent application 2011/0174190 to Sepa et al. (hereinafter the '190 application), entitled “Low Haze Transparent Conductors,” incorporated herein by reference. The '190 application also teaches a multistep growth process to synthesize longer silver nanowires. In the processes described herein, the thermal control of the synthesis is significantly different from the '190 application. Thus, the synthetic processes herein are able to achieve excellent yields at relatively short synthesis times, which are significantly shorter times than reported in the '190 application. The '190 application does not seem to discuss yields. The single synthesis example in the '190 application involved 6 grams of silver nitrate, while the examples herein involve from about 10 times to over 200 times this amount of silver nitrate with high yield results.

Applicant previously discovered a state of the art synthesis approach to produce thin and uniform silver nanowires at commercial scale. See, U.S. Pat. No. 10,714,230 to Hu et al. (hereinafter the '230 patent), entitled “Thin and Uniform Silver Nanowires, Methods of Synthesis, and Transparent Conductive Films Formed From the Nanowires,” incorporated herein by reference. The present synthetic procedures adopt a rough framework of this earlier work with directed modifications to target the desired silver nanowire properties as well as the high yields achievable for these thicker nanowires. In particular, relative to the processing in the '230 patent, the high yield synthesis of the larger diameter nanowires involves omission of the bromide ion catalyst and appropriate selection of the chloride containing catalyst. Also, there is a delay in the addition of an additional quantity of solvent, which is believed to effectively extends a seed growth period to provide for the larger silver nanowire diameters. Further extension of the time before a quench step seems to result in more particle formation and not to thicker nanowires, so other initial reaction conditions can be used to adjust wire properties. The temperature of the growth phase to get the nanowire length is performed at a higher temperature to generate the higher yield. Thus, several aspects of the synthesis have been found to be adjustable to achieve the desired larger diameter silver nanowires at high yield.

In comparison with Applicant's previous synthesis to form very thin nanowires, the current synthesis uses only a chloride salt catalyst in comparison with the use of both a chloride salt catalyst and a bromide salt catalyst. The chloride salt catalyst is an ammonium chloride, and the exemplified salt catalyst is NH₄Cl, although organic substituted ammonium ions can be alternatively used. Also, a seeding phase prior to a nanowire growth phase is lengthened in time, which helps to control the resulting average diameter of the silver nanowires. The transition from the seed phase to the growth phase is controlled through the addition of solvent, which lowers the temperature a small amount and provides a small dilution effect. It is believed that the growth phase essentially results in lengthening of the nanowires. The temperature during the growth phase can help to drive a high yield yet have the synthesis proceed relatively quickly.

These discovered process features can be incorporated into a high throughput and high yield synthesis for the production of moderate diameter, moderate to long length silver nanowires. Yields can be achieved greater than 85% and reaction times can be on the order of several hours. Exemplified scales use a 100 liter reactors, which are suitable for the synthesis of greater than 1500 grams of silver nanowires in a batch depending on the length. It should be possible to further generalize the synthesis to achieve even larger scales as needed. Control of thermal conditions and amount of silver added significantly influences the growth dynamics, which can be appropriately exploited to achieve desired objectives.

With respect to growth of extra-long silver nanowires, the synthesis approach herein provides for a second phase of growth using the gradual addition of additional silver nitrate to accomplish the length extension without significantly changing the silver nanowire diameter. This aspect is similar to the gradual addition of silver nitrate in the '190 application cited above, but with significant differences relating to thermal control and addition times. The present synthesis is performed at higher temperatures for a shorter period of time. This is coupled with quenching steps to control the seed period as described in the last paragraph for the high yield synthesis of the moderate length silver nanowires. Thus, the thermal control of the processing is significant to obtain the desired nanowire synthesis results, especially control of nanowire length, achievement of high yields and short synthesis times.

With respect to broadening commercial applications of silver nanowires, Applicant has developed the formation of very high loading silver nanowire dispersions with well dispersed silver nanowires and a range of rheology, from non-Newtonian viscous flowable liquids to pastes with a range of concentrations for various solvents. See, copending U.S. patent application Ser. No. 18/634,300 to Virkar et al, entitled “High Loadings of Silver Nanowires: Dispersions and Pastes; Conductive Materials; and Corresponding Methods,” incorporated herein by references. This development of higher concentration dispersions of silver nanowires has facilitated formation of higher loadings of silver nanowires into conductive composites, such electrically conductive adhesives and metal loaded polymer pastes generally. This capability has opened the door to formation of a range of novel materials involving significant amounts of silver nanowires.

The ability to form high concentration silver nanowire dispersions facilitates corresponding structures with very high silver concentrations to provide low resistivities that can be used to form low resistance opaque structures. Structures can be formed with resistivities on the order of metal resistivities with low temperature processing, as described in copending U.S. patent application Ser. No. 18/422,732 to Yang et al., entitled “Formation of Electrically Conductive Layers at Near Ambient Temperature Using Silver Nanoparticulate Processing and Inks for Forming the Layers,” incorporated herein by reference. With respect to forming composites with silver nanowires and a polymer matrix, the silver nanowires can provide excellent electrical conductivity at various loading levels from low levels to high levels with resistivities in the metal range as described further below.

In the context of dilute dispersions of silver nanowires and the development of state of the art transparent conductive films, Applicant developed the use of fusing solutions to form fused metal nanostructured networks, as a unitary conductive structure. These structures involve sparse metal nanowire layers that provide for good optical transmission with low scattering of light while providing good electrical conduction. Applicant has termed the fusing agent as Nanoglue®, which involves the appropriate delivery of silver salts. This proprietary technology has been extensively protected with patents. See, for example, U.S. Pat. No. 10,029,916 to Virkar et al., entitled “Metal Nanowire Networks and Transparent Conductive Material,” and 10,020,807 to Virkar et al., entitled “Fused Metal Nanostructured Networks, Fusing Solutions With Reducing Agents and Methods for Forming Metal Networks,” both of which are incorporated herein by reference.

The use of Nanoglue® has been found to be very valuable also in the non-transparent composites useful for a range of products to achieve high conductivity with reduced amounts of metal. See U.S. provisional patent applications 63/540,772 to Virkar et al., entitled “Silver Nanowire Based, Electrically Conductive Inks, Pastes, and Electrically Conductive Adhesives And Corresponding Methods,” and 63/551,737 to Virkar et al., entitled “Conductive Composites, Inks and Adhesives, With Low Silver Nanowire Loading and Low Resistivity, and Methods for Forming Conductive Features,” both of which are incorporated herein by reference.

To stabilize silver nanowires, Applicant has found that thin noble metal coatings over the silver nanowires can be surprisingly effective. Applicant developed commercial scale processing of noble metal coated silver nanowires for stabilizing transparent conductive films. See U.S. Pat. No. 9,530,534 to Hu et al., entitled “Transparent Conductive Film,” incorporated herein by reference. While the noble metal coatings provide oxidation resistance, it is surprisingly found that even very thin noble metal coatings also provide very significant thermal stability, which can be exploited for forming heaters. See, copending U.S. patent application Ser. No. 18/422,462 to Chen et al., entitled “Stable Thin Film Heaters Based on Transparent Conductive Coatings, Structures Formed With the Heaters and Applications Thereof,” incorporated herein by reference. Noble metal coatings can be similarly applied to the silver nanowires synthesized using the processes described herein to form stabilized conductive nanowires.

Thus, the silver nanowires synthesized under the procedures described herein can be a component of a product assembly package allowing for formation of a range of conductive materials from transparent with fair optical properties to opaque materials and electrical conductivity with moderately high resistivity to provide static discharge to highly conductive for good current transmission. Larger scale and efficient silver nanowire production opens up availability of lower cost materials suitable for a broader range of products.

Silver Nanowire Synthesis

The processes described herein are suitable for large scale nanowire synthesis at high yield with uniform and consistent results. The polyol process with a suitable catalyst provide silver nanowires with desired dimension and processing with desired short reaction times and high yields. The diols can function as a reducing agent with controllable properties, such as through the application of heat. Generally, a purification step is used to isolate the nanowires from the reaction mixture comprising some amount of other nanostructures such as nanoparticles as well as unconsumed reaction compositions. The moderate nanowire diameters provide for relatively straightforward purification. The desirable results obtained herein are based on specifically designed reactant addition timing, reactant concentrations and control of reaction temperatures. Generally, unless indicated otherwise, concentrations referred to in this section are based on amounts added to the reaction solution and the volume, and the actual concentrations in solution evolve over time based on the interactions and the reactions of the species in solution as the nanowires are synthesized. The reaction flow is outlined in FIGS. 1 and 2.

A reaction scheme describing the synthesis of moderate diameter silver nanowires on a commercial scale is shown in FIG. 1. The synthetic procedure includes a single long growth phase and is described in detailed below for Example 1. The reaction scheme includes a list of reagents identified as A-E. Reagents A, B and E were prepared separately using glycol solvent, such as ethylene glycol (EG). Reagent Cis an initial amount of glycol solvent, and Reagent D is an amount of glycol solvent used to quench the reactants, which tends to limit the diameter growth. Reagent C is added to a reactor and heated to a target temperature. Reagents A (polyvinylpyrrolidone in solvent) and E (chloride catalyst) are combined and added to the reactor followed by addition of Reagent B (silver salt dissolved in solvent). After a selected period of time, Reagent D is added and the temperature is maintained at the selected reaction temperature for a reaction time over which the silver ions may be substantially consumed.

A reaction scheme describing the synthesis of higher aspect ratio silver nanowires on a commercial scale is shown in FIG. 2. The synthetic procedure describes an extended growth phase and is described in detailed below for Example 2, Part 2.3. The first part of the reaction scheme has similarities with the process of FIG. 1. First reagent A includes PVP and chloride catalyst. Silver salt was dissolved in glycol solvent and labeled in FIG. 2 as first reagent B and second reagent B. Reagent C is an initial amount of glycol solvent, and reagent D is an amount of glycol solvent used to quench the reactants. At the start of the procedure, reagent C is added to a reactor and heated to a target temperature with stirring. First reagent A is added, and the temperature drops. First reagent B is then added. After a selected time, reagent D is added and the temperature of the reaction mixture decreases to a target temperature. For an extended length growth phase, second reagent B is added dropwise at an appropriate rate to obtain a good growth rate at the selected temperature. The temperature is maintained at a temperature for the growth period. Then, the reaction mixture is allowed to cool, and in some embodiments, a second quantity of reagent A is added to aid maintenance of good dispersion of the product silver nanowires. The reaction mixture is allowed to cool to a safe handling temperature. The reactor can then be drained, and the liquid collected in appropriate vessels for continued processing or storage.

The silver nanowire synthesis protocols described herein have some conceptual structure similar to the synthesis developed for thin and uniform silver nanowires described in the '230 patent cited above. In that patent, ammonium chloride and potassium bromide were used together as catalysts to synthesize control nanowires that had average diameters of 21.5 nm. For the synthesis described herein, no bromide salts were used. In the synthesis in the '230 patent, after the addition of silver nitrate to initiate the synthesis reaction, a quenching amount of ethylene glycol was added to encourage growth of the nanowire length. In the synthesis for very thin silver nanowires, heat may or may not be continued after the addition of the silver nitrate, and the cooling rate can also be controlled to provide the desired reaction times.

In the reaction protocols described herein, the reactants comprise a glycol solvent, a capping polymer, an ammonium-based chloride catalyst and a soluble silver salt. The total volume of glycol solvent generally is divided to provide for addition of well dissolved reactants as well as to help control reaction temperature through the process. The total amount of glycol solvent determines the overall scale of the synthesis batch, so for a 10 liter (10 L) reactor vessel, generally from about 8500 g to about 10,500 g can be used to effectively use the volume, and this amount can be scaled then according to the reactor volume, such as about 10× for a 100 L reactor and the like. Exemplified volumes include 10 L and 100 L, and different or larger volumes can be used as desired by a person of ordinary skill in the art by scaling the results herein. The glycol solvent can be, for example, ethylene glycol, propylene glycol, or mixtures thereof, although other diols or mixtures thereof can be used in principle.

With respect to capping polymers, polyvinylpyrrolidone (PVP) has been found to be very effective at guiding silver nanowire growth. Success has been achieved using copolymers of vinylpyrrolidone, but there has not been motivation to use polymer capping agents different from polyvinylpyrrolidone. The polyvinylpyrrolidone can be characterized by an average molecular weight, and the molecular weight of the polymer can influence the reaction. PVP K30, having a molecular weight from about 40,000-60,000 g/mol, can be successfully used to achieve high yields. PVP K-90 (or K-85) with molecular weights of 900,000 to 1,600,000 or blends with PVP K30 may also be used along with potentially other commercially available molecular weight ranges, such as intermediate molecular weights (K-45 or K-60), or mixtures thereof. There does not seem to be any absolute uniformity on the PVP labeling between different commercial suppliers, but they are similar and follow the naming trends. Since the PVP is used as a capping agent, the amount of PVP can be adjusted according to the scale of the synthesis. Generally, the PVP can be from about 0.1 wt % to about 3.0 wt %, in further embodiments from about 0.25 wt % to about 2.5 wt % and in some embodiments from about 0.3 wt % to about 2.0 wt % relative to the total final reaction solution weight. The PVP can be separately dissolved into a portion of solvent for addition into the reactor to facilitate the dissolving process, which can involve some heating. It has been found that addition of additional PVP, generally in comparable amounts to the amount of PVP added for the synthesis, prior to purification can facilitate purification process. For embodiments in which longer nanowires are grown through subsequent (following an initial growth phase) gradual additions of silver nitrate, it is found that addition of more PVP prior to the additional silver nitrate results in longer silver nanowires. In addition, it is found that the use of a blend of PVP with a higher molecular weight PVP component can result in longer silver nanowires. A person of ordinary skill in the art will recognize that additional ranges of PVP molecular weight and PVP concentrations within the explicit ranges above are contemplated and are within the present disclosure.

With respect to the chloride catalyst, the examples are performed with ammonium chloride, NH₄Cl. Ammonium ions can be substituted with alkyl groups, and as noted above, tetra n-butyl quaternary ammonium chloride was used in the '190 application cited above. In general, the catalyst can be NR_nH_4-n, 0≥n≥4, R is an alkyl group with no particular limits, although it generally has no more than 15 carbon atoms with optional heteroatoms and optional unsaturated internal bonds. We refer to these catalysts as ammonium-based chloride catalysis, where ammonium-based refers to ammonium, alkyl ammonium, or quaternary ammonium with “alkyl” groups broadly specified above. With n=0, the ammonium-based chloride becomes simply ammonium chloride, and n=4 refers to quaternary ammonium chloride. If there are more than 1 alkyl group 2≤n, they may or may not be the same as each other. The chloride catalyst concentration can be specified as a weight percent or a molarity based on chloride ion. In particular, the catalysis can be used in an amount from about 0.00005 wt % to about 0.03 wt %, in some embodiments from about 0.0003 wt % to about 0.015 wt %, and in additional embodiments from about 0.001 wt % to about 0.01 wt %. With respect to chloride molarity from about 0.0001M to about 0.01M, in further embodiments from about 0.0002M to about 0.0065M, and in other embodiments from about 0.0003M to about 0.005M. In general, the amount of catalyst tends to scale by reaction volume and not with the amount of silver salt to be used, although ranges of catalyst amounts can be effective in the synthesis. A person of ordinary skill in the art will recognize that additional ranges of catalyst concentrations within the explicit ranges above are contemplated and are within the present disclosure.

With respect to silver salt used to provide the silver ions for reduction to form the silver nanowires, in principle, any soluble silver salt can be used. Most silver salts are not soluble in the glycol solvents, but silver nitrate is a convenient, effective and low cost option. Since it is generally desirable to add well dissolved silver salts in solvent volumes that are not excessive, the salts are desirably very soluble so that excessive amounts of solvent are not needed to add silver for the reaction. Suitable soluble silver salts can generally include, for example, silver acetate (Ag(O₂CCH₃)), silver trifluoroacetate (Ag(O₂CCF₃)), silver heptafluorobutyrate (Ag(O₂CC₃F₇)), silver lactate (Ag(O₂CCH(OH)CH₃)), silver hexafluoroantimonate (AgSbF₆), silver fluoride (AgF), silver tetrafluoroborate (AgBF₄), silver nitrate (AgNO₃), silver perchlorate (AgClO₄), or mixtures thereof. Process conditions are based on silver nitrate experiments.

The method of claim 1 wherein the quantity of soluble silver salt corresponds to silver ions in an amount from 0.1 wt % to about 2.0 wt % relative to the total weight of the final reaction solution.

With respect to silver nitrate, the reaction mixture generally comprises from about 0.0025M to about 0.25M, in further embodiments from about 0.005 to about 0.20M, and in other embodiments from about 0.01M to about 0.15M silver nitrate in the reaction mixture, and other soluble silver salts could be added to achieve comparable molar concentrations. This can also be expressed as a weight of the of the silver ions as a portion of the total final reaction solution weight. Specifically, the weight of silver ions can be from about 0.1 wt % to about 2.0 wt %, in further embodiments from about 0.125 wt % to about 1.5 wt % and in other embodiments form about 0.15 wt % to about 1.25 wt %. Generally, the silver salt is predissolved prior to addition to the reactor. For the initial addition of the soluble silver salt to the reactor, the amount of solvent can be from about 1 vol % to about 9 vol %, in further embodiments from about 1.5 vol % to about 8 vol %, and in other embodiments from about 2 vol % to about 7.5 vol % relative to the total solvent added to the final reaction solution. For subsequent additions of soluble silver salt, equivalent solvent volumes or different volumes can be used as long, as the silver salt is well dissolved. A person of ordinary skill in the art will recognize that additional ranges of soluble silver concentrations and solvent delivery amounts within the explicit ranges above are contemplated and are within the present disclosure. As silver nitrate is converted over the course of the reaction, an indication of the reaction mixture “comprising” a certain amount of silver nitrate refers to the relative amount of silver nitrate added over the course of reaction rather than necessarily the amount of silver in solution at a particular time, which is in flux and not readily measurable.

Since the silver salt is directly converted into the product silver nanowires, as well as some by-product silver particulates, the amount of silver salt is driven by the yield, although the concentrations should be selected to achieve desired yields and reaction rates. Therefore, the total silver salt addition influences reaction rates, yields, and silver nanowire dimensions. Generally, a significant quantity or all of the silver salt is added at the initiation of the reaction to raise the concentration and initiate nucleation of silver particulates. As described below, control of the reaction conditions, in particular the temperature, help to establish the high yield and silver nanowire dimensions. To help establish longer nanowires, the total silver salt can be divided into the initial quantity and subsequent quantities for wire elongation. Protocols for the addition of the silver salt are described further below in the context of the procedure description. With respect to silver ion concentrations, these tend to be transient as silver ions are reduced to silver metal. The silver ion concentration can be contemplated as a peak amount when the initiation quantity is added, assuming that significant amounts are not reduced or precipitated, such as sparingly soluble AgCl, before the initial quantity is completely added, and with respect to the total amount added for a specific volume without accounting for the transient nature, i.e., total silver processed in a particular volume. The molarity at the initial addition can be from about 0.0025M to about 0.05M and in further embodiments from about 0.01M to about 0.045M. The total amount of silver added can be from the initial mount, which can be the only silver added up to about 6 times the initial amount, in further embodiments up to about 4 times the initial amount, and in other embodiments from about 1.5 times to about 3.5 times. A person of ordinary skill in the art will recognize that additional ranges of silver concentrations within the explicit ranges above are contemplated and are within the present disclosure.

The reactions can be performed in commercially available stirred reactors. The reaction vessels should be selected to be inert with respect to the reactants as well as to selected cleaning reagents used for cleaning of the reactors between batches. Generally, glass or stainless steel reaction vessels can be used. Reactors can be supplied with various stirring modalities. Such systems are available from many commercial suppliers. The reactors generally comprise suitable ports for safely adding reactants and removing products.

Prior to discussing the processing conditions, which are significant, it is useful to contrast the processing in an overall view from the processing in the '190 patent cited above. As an initial matter, Applicant's earlier work on state of the art thin silver nanowires is contrary to the concept that the diameter necessarily grow with the length as suggested in paragraph [0095] of the '190 patent. The '190 patent teaches that a second growth phase can grow length over diameter by limiting the concentration of silver ions during this growth phase. As described in the '230 patent cited above, very thin nanowires (<20 nm) can be synthesized with aspect ratios greater than 500. In the processing of the '230 patent, all of the silver nitrate is added at once and diameter growth is controlled at least in part by a thermal quench and control of the temperature and concentrations. While in the present work, extra long nanowires are grown in a two phase growth process. But this process sequence is selected based on achievement of high yield and short reaction rates and not exclusively based on the limits of the nanowire diameters.

The temperatures used in the current reactions are greater than those used in the '190 patent, and the reaction times used in the present synthesis are shorter than those used in the '190 patent. The '190 patent discusses the use of an inert atmosphere (see paragraphs [0098] and [0153]), while in the present work and Applicant's work in the '230 patent, an inert atmosphere was not used. While the '190 patent uses a constant reaction temperature from the addition of silver nitrate, Applicant's synthesis approach has an initial higher temperature and a lower temperature for the length growth periods.

In the current processing, solvent is added in various stages, but a bulk quantity of the solvent is initially heated to a target temperature for initiating the reactions. This initial quantity of solvent generally comprises from about 60% to about 85%, and the total solvent quantity can be selected by the reactor size, such as 10 L, 50 L, 100 L, 200 L, 500 L, 1000 L, or other values. The initial, peak temperature can be from about 180° C. to about 196° C., and generally the reaction is performed at a somewhat lower temperature. The PVP and catalysts are not reactants that are consumed, but they support the reaction and can collectively be referred to as auxiliaries, for convenience. The PVP polymer is believed to coat the silver nanowires and act as a dispersion aid. Auxiliaries are generally added well dissolved. The PVP polymer and the chloride catalyst can be separately dissolved and then combined in a convenient and controllable process, although these reactants do not need to be separately dissolved as long as they are ultimately well dissolved and mixed. Some heating can be used to facilitate the blending. A significant amount of solvent is used to dissolve the PVP, and this can be from about 2.5% to about 6% of the total solvent, although other amounts can be used as long as the PVP is well dissolved. Similarly, the chloride catalyst can be dissolved in an appropriate amount of solvent. To form the well dissolved reactant solution with the PVP and chloride catalyst, heat can be applied, such as with appropriate heating to dissolve the catalyst. While the reactant (PVP/catalyst) solution can be heated to a higher temperature, slight cooling upon the addition of the catalyst solution to the reactor is not a problem, and handling the solution at a lower temperature is simpler. For convenience, in some embodiments, the catalyst solution can be added at a temperature from about 30° C. to about 80° C.

The addition of the auxiliaries cools the reactor from its peak temperature. The heat to the reactor can be discontinued or properly controlled to achieve this slight cooling to a temperature desired for the addition of the silver salt reactant. In other embodiments, it may be possible to combine all of the auxiliaries into the solvent for heating to the target temperature for silver salt addition, but the exemplified approach is convenient and efficient for having comfort of the good dissolution and mixing of the compositions.

The silver salt is dissolved in solvent for addition to the reactor. The addition of the silver salt initiates the reduction reaction, which is believed to initiate with particle nucleation and subsequent particle growth. Heat is generally off at this stage, although low heating is possible. The silver salt is added to form a reaction solution at a selected initial reaction temperature for the nucleation and initial nanowire growth phase. The initial reaction temperature generally is from about 180° C. to just below the solvent boiling point, i.e., the solvent does not start boiling but as close to this point as practical. The dissolved silver salt at this stage is generally not heated, and the quantity of silver salt can be added all at once with silver salt nucleating and growing without partitioning an initial amount for nucleating. The amount of solvent for the addition of the silver salt can be from about 2.5% to about 6% relative to the total solvent amount. The addition of the silver salt solution cools the reactor solution slightly, and the reactor is allowed to cool during the initial growth phase down to the temperature selected for the lengthening phase, which can be referred to as the initial reaction temperature.

The transition into the reaction phase at the selected continuing reaction temperature and further cooling are based on addition of a quenching amount of solvent. The period of time between the addition of the silver salt and quenching solvent help to influence the silver nanowire diameter, and for the formation of the very thin nanowires, this delay is eliminated. This time for the growth phase, initial reaction, can generally extend from about 1 minute to about 10 minutes, in further embodiments from about 1.5 minutes to about 6 minutes, in other embodiments from about 1.75 minutes to about 5 minutes and in additional embodiments from about 2 minutes to about 4.5 minutes. With respect to the addition of the silver salt solution (first silver salt), this is generally added in less than a minute, and can be added at any reasonable practical rapid rate. At the end of this growth phase, a quenching amount of solvent is added. The quenching amount of solvent can be from about 6% to about 30% of the total solvent quantity used, in further embodiments from about 8% to about 25%, and in other embodiments from about 9% to about 20%. The amount of solvent can be selected to yield a particular temperature following mixing of the quenching solvent with the reaction solution to transition from a seeding phase to a growth phase. Additional cooling, such as with fans or other suitable approach can facilitate cooling without addition of excessive quench solvent. A person of ordinary skill in the art will recognize that additional ranges of delay period and solvent amounts within the explicit ranges above are contemplated and are within the present disclosure.

Once the quench is completed, the temperature is maintained at the selected temperature (continuing reaction temperature) for nanowire length extension for a selected reaction time. The temperature influences the reaction time with a higher temperature shortening the reaction time to consume the silver salt. If the lengthening temperature is too high, the nanowires can have a lower aspect ratio. The suitable temperature is also influenced by the reaction scale and total volume. Generally, the temperature is from about 100° C. to about 170° C. and in further embodiments form about 105° C. to about 165° C. The reaction is generally continued until the silver ions are consumed. For a single silver salt addition, the reaction times can be at least about 1 hour, in further embodiments from about 1.5 hour to about 24 hours and in other embodiments form about 2.5 hours to about 8 hours. A person of ordinary skill in the art will recognize that additional ranges of reaction within the explicit ranges above are contemplated and are within the present disclosure.

As noted above, the lengthening of the silver nanowires can be magnified through the gradual addition of more silver salt after the initial growth phase. While the initial amount of silver can contribute to extending nanowire growth, additional amounts can eventually result in more silver by-products. As exemplified, the silver nanowire growth with respect to length without excessive by-products can be extended by additional gradual addition of more dissolved silver salt at an appropriate temperature. As exemplified, the total silver is tripled (so 2 times the initial quantity of silver ions for additional silver ions added), but the gradually added additional silver can be any reasonable amount, such as from about 0.25 times to about 5 times, in further embodiments from about 0.5 times to about 4.5 times and in other embodiments from about 1 times to about 4 times additional ions silver relative to the initially or first added silver amount. The rate of addition of the additional silver salt generally depends on the volume of the reaction. Since the gradually added silver salt is not generally used to alter the reactor temperature, the concentration of silver salt is not particularly significant as long as the solvent volume is not excessive, and the salt is well dissolved. It can be convenient to use the same silver salt concentration as used for the initially added silver salt amount. Generally, the silver salt concentration can be from about 0.5 times to about 10 times the concentration of the first silver salt solution, in further embodiments from about 0.75 times to about 8 times, and in other embodiments from about 1 time to about 5 times the concentration of initial silver salt solution, which can be measured in terms of the concentration of silver ions. The gradual addition rate influences the reaction time and may be selected in part based on the temperature, if desired. The addition can be dropwise or other convenient delivery format to provide the overall gradual addition rate. The addition time can be specified with respect to the time for gradual addition. which can then reflect an addition rate with proper averaging depending on the addition mechanism. Generally, the time for this addition can be from about 0.5 hours to about 36 hours, in further embodiments from about 0.75 hours to about 30 hours, and in other embodiments from about 1 hour to about 24 hours. The gradual silver salt addition can be performed as a continual gradual addition or broken up into one or more aliquots. In general, the results are not significantly changed by breaking into aliquots if that if convenient form a production standpoint as long as the time separation between aliquots is reasonable. The gradual addition of silver salt solution generally is performed once the temperature reaches a target growth temperature, which may involve hours of gradual cooling, such as with fans. Fur these length extension embodiments, the reaction temperatures are generally lower, although reaction with the first quantity of silver salt occurs at least in part over a range of temperatures over the course of cooling. to the steady state temperature. The added quantities of silver can be divided into quantities for process convenience if desired for convenience. The reactor can be shielded from light during processing to limit any effects on the synthesis from illumination. After a reasonable period of time following completion of the silver salt addition, heat is turned off, and the reactor is allowed to cool to a safe handling temperature. A person of ordinary skill in the art will recognize that additional ranges of additional salt relative to the initial silver salt quantity and the gradual addition time are contemplated and are within the present disclosure.

To extend the nanowire length using gradual silver salt addition, the temperature is appropriately adjusted as the data suggests that the temperature during this growth lengthening phase is significant with respect to ultimate nanowire characteristics. Results have been obtained that suggest that the final length is strongly influenced by the temperature during the lengthening phase. As demonstrated in exemplified embodiments, by raising the temperature from 110° C. to 130° C., the average length of the silver nanowires decreases by roughly 25% while increasing the yield. The addition rate can be higher without lowering yield at a higher temperature. Thus, the temperature during the length growth phase can be used to adjust the average product length. Generally, the temperature during the extended length extension growth phase can be from about 105° C. to about 155° C., in further embodiments form about 110° C. to about 150° C. and in other embodiments form about 115° C. to about 145° C. Overall, the silver nanowire diameters can be influenced by the amount of catalyst during the seed nucleation phase as well as the temperature during the initial growth phase established by the quench time to reach a length growth phase. The lengths can be manipulated by controlling the temperature and silver salt addition during the length growth phase. A person of ordinary skill in the art will recognize that additional ranges of temperatures during the extended length growth phase are contemplated and are within the present disclosure.

At the completion of the reaction period, the solution is cooled. Natural cooling can be used, although allowing the reactor to cool generally takes many hours. In alternative embodiments, the temperature can be quenched, such as with the addition of liquid. Other mechanism, such as blowing air, can be used to help control the cooling rate. Generally, the solution is cooled prior to purification of the nanowires. If desired, additional PVP polymer can be added to the completed reaction mixture. Generally, the PVP is dissolved proper to addition, and the addition of the PVP can contribute to cooling. Extra PVP can be added to stabilize the purified silver nanowires and to improve redispersibility of the silver nanowires. The amount of added PVP for dispersing the produced silver nanowires at the end of the reaction phase can be expressed in terms of relative amounts based on the amount of PVP added for the reaction In some embodiments, the added PVP at the end of the reaction can be from about 10% to about 200%, in further embodiments from about 20% to about 175%, and in other embodiments from about 50% to about 125% of the PVP added during the reaction. On the other hand, excess PVP can be removed during purification. Thus, PVP can be adjusted to provide a desired degree of dispersion stability and reduce or eliminate irreversible agglomeration. A person of ordinary skill in the art will recognize that additional ranges of final dispersive amounts of PVP within the explicit ranges above are contemplated and are within the present disclosure.

Due to the larger diameters of the silver nanowires, purification is relatively straightforward. As an initial matter, the silver nanowires can be allowed to settle by termination of the mixing. Depending on the nanowire parameters, settling can take many days. To speed the purification process, settling can be encouraged through the addition of destabilizing solvent to the product solution. Specifically, destabilization directed to silver nanowire purification can be effectuated through the addition of acetone or similar organic solvent miscible with the glycol but in which the nanowires are less stably dispersible. Following destabilization of the dispersion, centrifugation can be performed to collect the nanowires, as an alternative to settling, while small silver particles may remain dispersed. The process can be repeated to further improve the purification, but generally one purification step is sufficient to obtain appropriate degree of purification.

Following purification, the silver nanowires are generally dispersed well into an appropriate good solvent/dispersant for further processing. Such dispersants, can be for example, water, low molecular weight alcohols, such as ethanol, or a mixture thereof. At concentrations of no more than 2 wt % silver nanowires with an appropriate dispersant and with good mixing, the dispersions can be stable and well dispersed for significant periods of time, such as many months, if properly stored.

The reaction times from the addition of the initial silver salt to the termination of heating for the standard size silver nanowires described herein can be 3 hours±1 hour, although different variations of the reactions can be envisioned that may shorten or extend these times within the teachings herein. In general, reaction times for a single silver salt addition can be from about 1 hour to about 10 hours, in further embodiments from about 1.5 hours to about 8 hours and in other embodiments from about 2 hours to about 5 hours. For the lengthening process using gradually added silver salt after an initial reaction time to nucleate the silver species, the reaction times are generally dependent on the temperature and target nanowire length. However, even for silver nanowires having lengths of 10-20 microns and relatively narrow length distributions, the reaction times generally are no more than about 5 days, in some embodiments from about 5 hours to about 72 hours, in further embodiments from about 7 hours to about 48 hours, and in other embodiments from about 10 hours to about 36 hours, which times do not include cooling times. A person of ordinary skill in the art will recognize that additional ranges of reaction times within the explicit times above are contemplated and are within the present disclosure.

The yield is determined by the weight (mass) of the purified samples versus the weight of the silver ions added as reactants. The remaining silver can be discarded from purification as unreacted silver, other silver particles shapes, such as small nanoparticles or short nanorods, or agglomerates. Experience suggests that additional purification steps reduce yield without significant changes in product quality for many applications, so generally a single purification step is sufficient. After purification, the sample contains generally up to 2 or 3 weight percent organics, generally PVP, although this can be reduced with further purification. Some other shaped silver nanoparticulates can copurify with the nanowires, but these have not been found to be significant for desired applications. The total organics can be evaluated using differential scanning calorimetry or other approach to thermally remove the organics. The purity of the metal particulates can be examined using microscopy. In any case, a rough evaluation can be used to determine that greater than 90 wt % of the sample is silver nanowires of target dimensions, and in most embodiments significantly more than 90 wt %. With this level of purity, the yields based on sample weight provide meaningful numbers. Using the synthesis methods described herein, mean yields (or individual yields) can be greater than about 80%, in further embodiments greater than 85%, and in additional embodiments from about 90% to about 97%. A person of ordinary skill in the art will recognize that additional ranges of purification and yield within the explicit ranges above are contemplated and are withe present disclosure.

Silver Nanowire Characterization

The product silver nanowires can be evaluated using one or more of several criteria. For example, the silver nanowire lengths and diameters can be evaluated using transmission electron micrographs for diameters and optical microscope for lengths. The collective nature of the synthesized silver nanowires can be evaluated using US-visible extinction spectra. Also, the dispersibility of the silver nanowires can be confirmed using dilute dispersions. As noted above, for transparent conductive films, thin and uniform silver nanowires are of particular interest to improve transmittance and reduce haze or other effects of light scattering. For non-transparent conductive applications, nanowire properties generally are not as focused on the same properties as for transparent application. Nevertheless, it is desirable to understand the properties of the silver nanowires produced.

The silver nanowire average diameters can be evaluated using a random sampling based on a manual measurement of scanning electron micrographs or an automated measurement. A randomly selected set of roughly 100 nanowires or more can generally be used, although reasonably accurate results can be obtained with fewer nanowires. The reported numbers based on 100 or more nanowires are believed accurate roughly to 0.1 nm for diameters based on the measurements. Silver nanowire average lengths can be similarly evaluated. The aspect ratio is the average length divided by the average diameter. It is established that ultrasonication can damage silver nanowires, and specifically can fracture the nanowires. Thus, process should be careful to avoid this if the silver nanowires are not intended to be damaged. On the other hand, ultrasonication can be used to purposefully fragment the nanowires to make shorter nanowires, and this can be a purposeful step for processing. Thus, post synthesis processing can be useful to influence the properties of the as synthesized silver nanowires.

In some embodiments, to provide desired contributions to the electrical conductivity, generally suitable nanowires can have an average aspect ratio of at least about 10, although in some embodiments it can be desirable to use silver nanowires with an aspect ratio of at least about 50. In further embodiments, the silver nanowires can have aspect ratios from about 20 to about 3000, in some embodiments from about 30 to about 1500, in further embodiments from about 50 to about 1000, and in additional embodiments from about 60 to about 500, or any other range based on any lower limit specified with any upper limit specified. Silver nanowire average diameters generally can range from about 20 nm to about 250 nm, in further embodiments from about 25 nm to about 200 nm, in some embodiments from about 27.5 nm to about 150 nm, and in additional embodiments from about 30 nm to about 100 nm, as well as ranges based any lower range value with any upper range value. The silver nanowire average lengths can be from about 750 nm to about 300 microns, in further embodiments from about 1 micron to about 100 microns, in some embodiments from about 1.5 microns to about 85 microns and in additional embodiments from about 2 microns to about 75 microns, as well as any ranges based on any lower range value specified and any upper range value specified. A person of ordinary skill in the art will recognize that additional ranges of aspect ratios, average diameters or average lengths within the specific ranges above are contemplated and are within the present disclosure.

The UV Visible extinction spectrum in a suitable dispersant can also be used to characterize the nanowires. A suitable liquid can be dimethyl sulfoxide (DMSO) with a silver nanowire concentration of 0.005 wt %, although at low concentrations the normalized spectrum generally is not significantly sensitive to the concentration. As used herein a normalized extinction spectrum sets the highest absorbance value to 1 and the lowest absorbance value to 0 within the wavelength range of 300 nm to 800 nm. As the average nanowire diameter gets smaller, the extinction maximum tends to shift to lower wavelengths (blue-shift). As the silver nanowires become more uniform, the extinction peak tends to narrow. For the silver nanowires synthesized herein, the objective is not thinner nanowires, and the uniformity is not an especially significant property. Nevertheless, the extinction spectra can provide significant information on the nanowires and their dispersibility. As explained in the following section, the tail of the extinction spectrum provides information on the characteristics of the dispersion.

Dispersions and Use

With the objective of forming a highly conductive material, the dispersibility of the silver nanowires is still a significant property to allow for the formation of desirable material from both a conductivity and material property perspective. The dispersibility provides for the formation of a desirably uniform material. The synthesis methods described herein provides silver nanowires with desirable dispersibility. The effective processing to make some of the materials involves forming well dispersed concentrated dispersions in a suitable solvent that provides for the formation of a desired composite material. In particular, Applicant has pioneered the formation of highly conductive metal polymer composites with a broad range of conductive and material properties. Reduced metal salts provides for further improvement in the electrical conductivity to further exploit the highly conductive capabilities achievable with silver metal nanowire loading. Generally, an initial step for the composite formation involves forming a well dispersed silver nanowire dispersion.

The UV/Vis spectrum can be diagnostic of silver nanowire properties as well as the quality of the dispersion. In general, the evaluation of the extinction spectrum is performed in a dilute dispersion to obtain meaningful spectral measurements. As noted above, stable dispersions can be diluted to form dilute dispersions. UV-Vis spectra of the resulting dilute dispersions can be used to evaluate the dispersions. Thin and uniform silver nanowires dispersed in DMSO solvent were characterized by UV-Vis extinction spectra in the '230 patent cited above. Utilizing UV-Visible extinction to characterize nanowire diameters and quality is well-known to a person of ordinary skill in the field.

The UV Visible extinction spectrum in a suitable solvent can be used to characterize concentrated dispersions that are diluted to 0.005 wt % silver, although at low concentrations the normalized spectrum generally is not significantly sensitive to the concentration. Suitable solvents include, for example, polar solvents such as isopropyl alcohol or dimethyl sulfoxide (DMSO). As used herein, a normalized extinction spectrum sets the highest value to 1 and the lowest value to 0 within the wavelength range of 300 nm to 800 nm. Characterization of the nanowires is described above. The tail of the extinction spectrum at higher wavelengths seems to be sensitive to dispersion quality as it indicates aggregation and a decrease in the quality associated with the plasmonic response associated the well-dispersed individual nanowires.

To reduce possible misinterpretation of the extinction spectra due to the solvent or nanowire dimensions, spectra can be compared with extinction spectra obtained from a dilute dispersion formed with the silver nanowires, so an extinction spectra of a sample diluted from a concentrated dispersion can be evaluated in a direct way. Agglomeration of the nanowires is found to alter in particular the longer wavelength tail of the extinction spectrum, and well dispersed nanowires should exhibit a very similar extinction spectrum over the full spectrum.

Following harvesting of the synthesized nanowires, the nanowires can be dispersed in a good solvent to support the dispersion. Applicant has discovered that the nanowires can be concentrated into concentrated dispersions while maintaining the nanowires well dispersed. This high loading work is described in published U.S. patent application 2024/0343923 to Virkar et al. (hereinafter the '300 application), entitled “High Loadings of Silver Nanowires: Dispersions and Conductive Pastes; And Corresponding Methods,” incorporated herein by reference. The ability to form high concentrations of well dispersed nanowires in selected solvents provides powerful processing flexibility for designs of electrically conductive adhesives, pastes, inks, and electrically conductive loaded polymers generally (collectively referred to as ECA, for convenience). Using well dispersed silver nanowires, uniform blends can be made into inks for depositing the ECA using a desired deposition approach. The '300 application also describes the rheology dependence on the silver nanowire morphology. Especially for transparent conductive films, thin and long nanowires are particularly desirable, but these long and thin nanowires have a morphology that limits loading due to a rapid increase in viscosity as the concentration increases. Thus, the use of shorter and thicker silver nanowires allows for higher loading while maintaining reasonable viscosities.

Additionally or alternatively, the use of reducible metal compounds can provide significant improvements in electrical conductivity based on any base metal particulate shapes. The decreases in electrical resistivity achieved with the reduced silver salts is disproportionate with the amount of metal deposited, which suggests some effectively directed metal deposition supportive of establishing electrically conductive pathways through the material. Similarly, silver nanowires can help to establish electrically conductive pathways based on their length. To form composite materials, polymer matrix precursors are incorporated into the metal nanowire dispersions to form a precursor composition as an ink, paste or the like. Upon curing of the deposited precursor composition, an electrically conductive composite material is formed. The use of silver nanowires has been found to be an effective way to achieve high conductivity with lower metal loadings, which can be supplemented using reduced metal compositions. These composites are described further in the '300 application and copending U.S. patent application Ser. No. 18/899,760 to Virkar et al., entitled “Silver Nanowire Based, Electrically Conductive Inks, Pastes, and Electrically Conductive Polymer Composites With Metal Particulates, and Corresponding Methods,” incorporated herein by reference. The ability to effectively and efficiently scale up silver nanowire production provides enabling processing that allows for effective commercialization of electrically conductive composite material production.

EXAMPLES

Example 1—Silver Nanowire Synthesis 10 L Reaction Schemes

This example describes a series of 10 L scale reaction schemes for the synthesis of moderate diameter nanowires at high yields.

Part 1.1. Scheme 10 L H 1×

A brief overview of the synthesis according to this example is described above for FIG. 1. The total volume added to the reactor was intended to essentially fill the bulk of the reactor once total amounts were added. As noted above, the glycol solvent generally is divided to provide for solubilization of reactants such that they are added predissolved, and some glycol solvent is added to moderate the reactor temperature. Initially, 8000 to 9000 g of ethylene glycol was placed in a 10-liter reactor at room temperature. The density of ethylene glycol is such that this initial quantity can correspond to roughly ¾ of the reactor volume. The reactor was closed with suitable ports available and with a mixer for blending the reactants, and the ethylene glycol was heated to a temperature of >180° C. In a separate container, 50-60 g of polyvinylpyrrolidone (K30, PVP) was added to a sufficient quantity of ethylene glycol to dissolve the PVP. After appropriate mixing, a solution of ammonium chloride (NH₄Cl) in ethylene glycol was added to form a PVP/catalyst solution with 4-6 millimoles of ammonium chloride. With continued mixing, the PVP/catalyst solution was heated to a temperature of roughly 40.0° C. Once the ethylene glycol in the reactor reached the target temperature, the PVP/catalyst solution was added to the reactor with mixing. After the addition of the PVP/catalyst solution, the temperature of the mixture in the reactor dropped. Heating was continued until the temperature of the reaction medium reached roughly 4-6° C. below the initial heated temperature, and at this temperature, a solution of silver nitrate (55-65 g) dissolved in ethylene glycol was added to the reactor. After mixing for 2-4 minutes, an additional amount of ethylene glycol (1500 g-2000 g ethylene glycol) was added to the reaction mixture. The temperature of the reaction mixture at the time of this quenching step was slightly greater than at the addition of the silver nitrate. After addition of the quenching solvent (ethylene glycol, 17 wt %-20 wt % relative to total additions to the reaction solution), the reaction mixture was allowed to cool to a temperature about 20-25° C. below the temperature at which the quench solvent was added and was maintained at this temperature while stirring for 3 hours. Then, heating was stopped, and the reaction mixture was allowed to cool to below 40° C. such that the reactor was safe for handling. Based on the added quantities to the reactor without consideration of consumption of reactants, the final concentrations in the reactor were PVP 0.50 to 0.55 wt %, NH₄Cl 0.0035-0.00425 wt %, AgNO₃ 0.50-0.55 wt %, and ethylene glycol balance (>98.8 wt %). An additional quantity of PVP alone can be added at the end of the synthesis to facilitate the purification process. Once the reactants are added and various reactions take place, such as Ag reduction/ethylene glycol oxidation, the concentrations in solution of the various species evolve, such that reference to the quantities as added provides a useful reference point.

Following completion of the synthesis, the silver nanowires were purified using repeated acetone precipitation, centrifugation and re-dispersion in water. The purified silver nanowires were removed from dispersion and dried to evaluate yield. Yield is based on the quantity of silver in the purified silver nanowires compared to the total amount of silver added in the form of silver nitrate. No attempt was made to quantify potential other silver nanoparticulate shapes that may copurify with the silver nanowires, which are believed to be small on a weight percent basis. For this experiment, there was 37.34 g of purified silver nanowires recovered, and the mean yield was 91%.

The wire diameters were measured but transmission electron microscopy for more than 100 nanowires. The diameters were from 55-70 nm, and the lengths were from 5-6 microns. The wire lengths were evaluated by light microscopy.

A dilute dispersion of the silver nanowires was formed in isopropyl alcohol and the extinction spectrum was taken. A representative UV-visible extinction spectrum is shown in FIG. 3 (“10 L H 1×”) along with extinction spectra from the other exemplified syntheses. The spectra are normalized with respect to the extinction maximum. The extinction maximum was at about 380 nm with a smaller peak at about 350 nm that is believed to be associated with the pentagonal cross section, which may red shift and become less pronounced with very thin nanowires even though they nominally maintain the crystal shape. The extinction tail at longer wavelengths is suggestive of well dispersed silver nanowires.

Part 1.2. Scheme 10 L H 1.5×

The scheme from Part 1.1 was carried out with the exception that the masses of all of the compounds added to the reactor were increased by 50%, with the exception of the solvent (ethylene glycol) and the ammonium chloride catalyst which were used in the same amounts as described in Part 1.1. The scaled-up amounts were sufficiently soluble such that the solvent used to dissolve the reactants was sufficient without scaling the aliquots of solvent used to dissolve reactants. Thus, initially, a majority of the ethylene glycol was placed in a 10-liter reactor at room temperature. The reactor was closed, and the ethylene glycol was heated to a temperature of >180.0° C. In a separate container, 80-90 g of polyvinylpyrrolidone (PVP K30) was dissolved in ethylene glycol. After appropriate mixing, a solution of dissolved ammonium chloride (NH₄Cl) in ethylene glycol was added to form a PVP/catalyst solution with 4-6 millimoles of ammonium chloride. With continued mixing, the PVP/catalyst solution was heated to a temperature of 40.0° C. Once the reactor reached the target temperature, the PVP/catalyst solution was added to the reactor with mixing. After the addition of the PVP/catalyst solution, the temperature of the mixture in the reactor dropped. Heating was continued until the temperature of the reaction medium reached about 4-6° C. below the initial temperature, and at this temperature, a solution of silver nitrate (1.5 times the value from Part 1.1) in ethylene glycol was added to the reactor. After mixing for 2-4 minutes, an additional amount of ethylene glycol (17 wt %-20 wt % relative to total additions to the reaction solution) was added to the reaction mixture. The reaction mixture was allowed to cool to a temperature of 20-25° C. below the temperature prior to adding the quenching solvent and was maintained at this temperature while stirring for 3 hours. Then, heating was stopped, and the reaction mixture was allowed to cool to below 40° C., at which point the product was purified and recovered as described in Part 1.1. Based on the added quantities to the reactor without consideration of consumption of reactants, the concentrations in the reactor were PVP 0.70-0.80 wt %, NH₄Cl 0.0035-0.00425 wt %, AgNO₃ 0.75-0.85 wt %, and ethylene glycol balance >98 wt %. While the volume of ethylene glycol was essentially the same as in Part 1.1, the weight percent drops slightly due to the added weight of solute. An additional quantity of PVP ca be added at the end of the synthesis to facilitate purification.

For this experiment, there was 55-60 g of purified silver nanowires recovered, and the mean yield was about 84%.

The wire diameters were measured by TEM for more than 100 nanowires. The diameters were from 55-70 nm, and the lengths were from 5-6 microns. A representative UV-visible extinction spectrum is shown in FIG. 3 (“10 L H 1.5×”) along with extinction spectra from the other exemplified syntheses. The spectra are normalized with respect to the extinction maximum. The results for this study showed a significant increase in the particle level as compared to the results from Part 1.1, which is attributed to the increased amount of catalyst (ammonium chloride) used.

Reagents and the scaling of reactant amounts used in Parts 1.1 and 1.2 are summarized in Table 1. Reagents were identified according to the reaction scheme described above for FIG. 1.

TABLE 1
		Increase
		between
Reagent		H 1x and
ID	Reagent	H 1.5x
A	PVP (g)	1.5x
A	PVP in EG (wt %)	1.4x
	PVP in final reaction	1.5x
	mixture (wt %)
E	NH4Cl, 0.28M (mL)	1.5x
B	AgNO3 (g)	1.5x
B	AgNO3 in EG (wt %)	1.4x
	AgNO3 in final reaction	1.5x
	mixture (wt %)
A	EG (g)	same
B	EG (g)	same
C	EG (g)	same
D	EG (g)	same
E	EG (g)	1.5x
	Total EG (g)	slight
		increase

Part 1.3. A-C Schemes for 10 L L-Long Nanowires (1×+1×+1×)

This Example is directed to three synthesis protocols involving elongated silver nanowires relative to the synthesis protocols of Parts 1.1 and 1.2. The protocols differ by the use of PVP: A) same PVP use as in Parts 1.1 and 1.2, B) added PVP during synthesis when adding additional silver nitrate, and C) PVP involved a blend of PVP K30 and higher molecular weight PVP.

A1) The scheme from Part 1.1 was carried out with the exception that the processing related to the addition of the silver nitrate was modified. The overall reaction scheme is outlined in FIG. 2. In particular, the reaction mixture was cooled an additional 50° C. after the 2-4 minute nucleation period following an initial silver nitrate addition to the reactor, and additional silver nitrate was added under the lower temperature conditions in two sequential aliquots to provide for a 3-fold total amount of silver nitrate in the reaction mixture (i.e. three additions of silver nitrate or “1×+1×+1×”). Thus, initially, the same quantity of ethylene glycol was placed in a 10-liter reactor at room temperature. Since a greater amount of solvent was used to deliver the reactants, overall a slightly larger amount of ethylene glycol was used. The reactor was closed, and the initially added ethylene glycol was heated to the same temperature used in Parts 1.1 and 1.2. In a separate container, an equivalent PVP/ammonium chloride catalyst solution was prepared as described in Part 1.1 using separately prepared solutions that are combined. With continued mixing, the PVP/catalyst solution was heated to a temperature of 40.0° C. Once the reactor reached the same target temperature, the PVP/catalyst solution was added to the reactor with mixing. After the addition of the PVP/catalyst solution, the temperature of the mixture in the reactor dropped. Heating was continued until the temperature of the reaction mixture reached the target reaction temperature, and at this temperature, an equivalent solution of silver nitrate in ethylene glycol as described in Part 1.1 was added to the reactor. After mixing for 2-4 minutes, an equivalent additional amount of ethylene glycol, as described in Part 1.1, was added to the reaction mixture. The temperature of the reaction mixture after this quenching step was slightly below the temperature at which the silver nitrate solution was added. The reaction mixture was allowed to cool to 100-125° C. While maintaining the reaction mixture at the target temperature, a second aliquot of silver nitrate solution was added to the reactor with the use of a pump to provide a steady rate of 0.5 ml/minute and stirring was continued for 9-11 hours. Then a third aliquot of silver nitrate solution was added to the reactor at a rate of 0.5 ml/minute and stirring was continued for 9-11 hours. The second and third aliquots of silver nitrate solutions can be added in a single longer step or there can be a time gap without significantly altering the product. The reaction mixture was then cooled to below 40° C., and the product was purified and recovered as described in Part 1.1. Based on the added quantities to the reactor without consideration of consumption of reactants by reaction, the concentrations in the reactor of PVP, NH₄Cl, AgNO₃, and ethylene glycol were the same as in Part 1.1 except that the amount of silver nitrate was tripled.

For this experiment, there was 112.01 g of purified silver nanowires recovered, and the mean yield was 94%.

The wire diameters were measured for more than 100 nanowires. The mean diameter was 55-70 nm, and the lengths were from 5-6 microns. A representative UV-visible extinction spectrum is shown in FIG. 3 (“10 L L”) along with extinction spectra from the other exemplified syntheses. The spectra are normalized with respect to the extinction maximum. The results show that the later dropwise addition of silver nitrate in this scheme does not increase the particle level above the level of the single addition scheme of Part 1.1 (see 10 L H 1×). In other words, the particle level is consistent with the starting synthesis and the additional dropwise additions of silver nitrate predominantly provide for a lengthening of the existing nanowires.

The reaction scheme described resulted in a longer nanowire, a higher yield, and a higher throughput of grams of nanowires than the schemes described in Parts 1.1 and 1.2 of this example.

• • A2) The scheme from Part 1.1.A1 was carried out with the exception that quenching was delayed by 1 minute from the time in A1. For this experiment, the length of the purified silver nanowires was comparable to that observed for the silver nanowires obtained in Part 1.1.A1. There was 101.3 g of purified silver nanowires recovered, and the mean yield was 90%. A representative UV-visible extinction spectrum for “10 L (1+1+1×) 1 min+Q”) is shown in FIG. 4.
  • B) The scheme from Part 1.3.A1 was carried out with the exception that prior to addition of each sequential aliquot of silver nitrate to the reaction mixture under the lower temperature conditions, a sequential aliquot of PVP was added. This reaction scheme provided a 3-fold total amount for each of PVP and silver nitrate in the reaction mixture (i.e. three additions of PVP/silver nitrate or “10 L (1+1+1×) Ag/PVP”). Referring to FIG. 2 and Part 1.3.A1, initially, the same quantity of ethylene glycol was placed in a 10-liter reactor at room temperature. Since a greater amount of solvent was used to deliver the reactants, overall a slightly larger amount of ethylene glycol was used. The reactor was closed, and the initially added ethylene glycol was heated to the same temperature. In a separate container, an equivalent PVP/ammonium chloride catalyst solution was prepared and combined with ethylene glycol. With continued mixing, the PVP/catalyst solution was heated to a temperature of 40.0° C. Once the ethylene glycol in the reactor reached the same target temperature, the PVP/catalyst solution was added with mixing, which resulted in a drop in temperature. Heating was continued until the temperature of the reaction mixture reached the same target reaction temperature, and at this temperature, a first aliquot of an equivalent solution of silver nitrate in ethylene glycol was added. After mixing for 2-4 minutes, an equivalent additional amount of ethylene glycol, as was used in Part 1.1, was added to the reaction mixture. The temperature of the reaction mixture after this quenching step was slightly below the temperature at which the silver nitrate solution was added. The reaction mixture was allowed to cool to 100-125° C. While maintaining the reaction mixture at the target temperature, a first aliquot of a PVP only solution was added, followed by a second aliquot of silver nitrate solution using a pump to provide a steady rate of 0.5 ml/minute. Stirring was continued for 10-16 hours. Then a second aliquot of PVP only solution was added to the reaction mixture, followed by a third aliquot of silver nitrate solution using a pump to provide a steady rate of 0.5 ml/minute. Stirring was continued for 10-16 hours. The reaction mixture was then cooled to below 40° C., and the product was purified and recovered as described in Part 1.1. Based on the added quantities to the reactor without consideration of consumption of reactants by reaction, the concentrations in the reactor of PVP, NH₄Cl, AgNO₃, and ethylene glycol were the same as in Part 1.1 except that the amounts of PVP and silver nitrate were tripled.

For this experiment, the length of the purified silver nanowires increased from about 12 microns to about 14 microns. There was 107.4 g of purified silver nanowires recovered, and the mean yield was 96%. A representative UV-visible extinction spectrum is shown in FIG. 4.

• • C) The scheme from Part 1.3B was carried out with two exceptions: PVP K30 was replaced with a 50:50 blend comprising equal weights of PVP K30 and higher molecular weight PVP, and the time after addition of the first aliquot of silver nitrate was increased by one minute from the time used in A1 for adding the quench solvent. This reaction scheme provided a 3-fold total amount for each of the 50:50 PVP and silver nitrate in the reaction mixture (i.e. three additions of 50:50 PVP/silver nitrate or “10 L (1+1+1×) 50:50 PVP”). Referring to FIG. 2 and Part 1.3B, initially, the same quantity of ethylene glycol was placed in a 10-liter reactor at room temperature. Since a greater amount of solvent was used to deliver the reactants, overall a slightly larger amount of ethylene glycol was used relative to Part 1.1. The reactor was closed, and the initially added ethylene glycol was heated to the same temperature. In a separate container, an equivalent 50:50 PVP/ammonium chloride catalyst solution was prepared and combined with ethylene glycol. The equivalent 50:50 PVP was 29.31 g of PVP K30 (molecular weight 55,000) blended with 29.31 g of PVP K90 (molecular weight 1,300,000). With continued mixing, the 50:50 PVP/catalyst solution was heated to a temperature of 40.0° C. Once the ethylene glycol in the reactor reached the target temperature, the 50:50 PVP/catalyst solution was added with mixing, which resulted in a drop in temperature. Heating was continued until the temperature of the reaction mixture reached the target reaction temperature, and at this temperature, a first aliquot of an equivalent solution of silver nitrate in ethylene glycol was added. After mixing for the initial reaction time prior to adding the quench solvent, an equivalent additional amount of ethylene glycol, was added to the reaction mixture. The temperature of the reaction mixture after this quenching step was slightly below the temperature at which the silver nitrate solution was added. The reaction mixture was allowed to cool to 100-125° C. While maintaining the reaction mixture at the target temperature, a first aliquot of 50:50 PVP only solution was added, followed by a second aliquot of silver nitrate solution using a pump to provide a steady rate of 0.5 ml/minute. Stirring was continued for 10-16 hours. Then a second aliquot of 50:50 PVP only solution was added to the reaction mixture, followed by a third aliquot of silver nitrate solution using a pump to provide a steady rate of 0.5 ml/minute. Stirring was continued for 10-16 hours. The reaction mixture was then cooled to below 40° C., and the product was purified and recovered as described in Part 1.1. Based on the added quantities to the reactor without consideration of consumption of reactants by reaction, the concentrations in the reactor of 50:50 PVP, NH₄Cl, AgNO₃, and ethylene glycol were the same as in Part 1.1 except that the amounts of 50:50 PVP and silver nitrate were tripled.

For this experiment, the length of the purified silver nanowires increased from about 12 microns to about 16 microns. There was 109.2 g of purified silver nanowires recovered, and the mean yield was 98%. A representative UV-visible extinction spectrum is shown in FIG. 4 along with extinction spectra from the other exemplified syntheses. Extension of the initial reaction time prior to quench was found to increase particle formation, as reflected in the UV-visible spectrum, without changing average length or significantly changing yield.

Example 2—Silver Nanowire Synthesis 100 L Reaction Schemes

This example describes a series of 100-liter scale reaction schemes for the synthesis of moderate diameter nanowires at high yields.

Part 2.1. Scheme 100 L H 1×

A brief overview of the synthesis according to this example is described above for FIG. 1. Initially, 70-80 kg of ethylene glycol was placed in a 100-liter reactor at room temperature. This quantity was somewhat less than 10× times the initial solvent quantity in Part 1.1. The reactor was closed, and the ethylene glycol was heated to the same target temperature of Part 1.1 while mixing at 132 RPM. In a separate container, a PVP/catalyst solution was prepared with 500-550 g of polyvinylpyrrolidone (PVP K30), 220-250 g of ammonium chloride (NH₄Cl), and 4-5 kg of ethylene glycol. In contrast with the 10 L experiments in Example 1, the ammonium chloride catalyst was not predissolved prior to formation of the PVP/catalyst solution. As long as the reactants are well dissolved, it is not relevant with respect to the order of dissolving the reactants so that ordering of the process for dissolving reactants can be selected based on practical considerations, such as timing and handling. The PVP/catalyst solution was heated to a temperature of 35-50° C. with continuous mixing. Once the ethylene glycol in the reactor reached the target temperature, the PVP/catalyst solution was added to the reactor with mixing. After the addition of the PVP/catalyst solution, the temperature of the mixture dropped. Heating was continued until the temperature of the reaction solution reached a value 5-10° C. below the peak temperature prior to addition of the PVP/catalyst solution. When the temperature of the reaction solution reached this new target temperature, a solution of silver nitrate (520-560 g) in ethylene glycol (3.0-3.5 L) was added to the reactor. After mixing for 1-5 minutes, an additional amount of ethylene glycol (15-18 kg ethylene glycol) was added to the reaction mixture. The temperature of the reaction mixture after this ethylene glycol addition to perform this quenching step was 2-4° C. lower than before the addition of the silver nitrate solution. The reaction mixture was allowed to cool to a temperature of 150-175° C. and was maintained at this temperature while stirring for 3 hours. Then, heating was stopped, and the reaction mixture was allowed to cool. At a temperature of less than 40° C., the reactor was drained, and the liquid was collected into nine 10 L carboys for further processing. Based on the total added quantities to the reactor without consideration of consumption of reactants due to reaction, the concentrations in the reactor were PVP 0.50-0.60 wt %, NH₄Cl 0.20-0.30 wt %, AgNO₃ 0.50-0.60 wt %, and ethylene glycol 98.5-99.0 wt %. Once the reactants are added and various reactions take place, such as Ag reduction and ethylene glycol oxidation, the concentrations in solution of the various species evolve, such that reference to the quantities as added provides a useful reference point.

Following completion of the synthesis, the silver nanowires were purified using acetone precipitation, centrifugation and re-dispersion in water. The purified silver nanowires were removed from dispersion and dried to evaluate yield. Yield is based on the quantity of silver in the purified silver nanowires compared to the total amount of silver added in the form of silver nitrate. For this experiment, there was 325-350 g of purified silver nanowires recovered, and the mean yield was 92%.

The wire diameters were measured and averaged for about 250 nanowires. The diameters were from 40-90 nm, and the lengths were from 3-6 microns.

A dispersion of the silver nanowires was formed in isopropyl alcohol, and the extinction spectrum was taken. A representative UV-visible extinction spectrum is shown in FIG. 3 (“100 L H 1×”) along with extinction spectra from the other exemplified syntheses. The spectra are normalized with respect to the extinction maximum. The normalized extinction was comparable to the spectra obtained using the 10 L synthesis scale.

Part 2.2. Scheme 100 L H 1.5×

The scheme from Part 2.1 was carried out with the exception that the masses of polyvinylpyrrolidone and silver nitrate were increased by 50%, and the temperature of the reaction mixture after quenching with of ethylene glycol was somewhat higher than in Part 2.1, roughly by a couple of degrees, which may simply be due to the reduced surface area of the reactor relative to the volume altering cooling. Thus, initially, an equivalent amount of ethylene glycol was placed in a 100-liter reactor at room temperature. The reactor was closed, and the ethylene glycol was again heated to the same target temperature while mixing at 132 RPM. In a separate container, a PVP/catalyst solution was prepared as described in Part 2.1. The PVP/catalyst solution was heated to a temperature of 40.0° C. with continuous mixing. Once the reactor reached the target temperature, the PVP/catalyst solution was added to the reactor with mixing. After the addition of the PVP/catalyst solution, the temperature of the mixture dropped. Heating was continued until the temperature of the reaction solution reached the target reaction temperature as noted in Part 2.1. At this temperature, a solution of silver nitrate (796.1 g) in ethylene glycol (3.6 kg or about 3.3 L) was added to the reactor. The temperature of the reaction mixture at the time of the step if silver nitrate addition was approximately 180-182° C. After mixing for 1-5 minutes, an additional amount of ethylene glycol was added to the reaction mixture, as noted in Part 2.1. The temperature of the reaction mixture following this quenching step was less than a degree below the temperature prior to the silver nitrate addition. The reaction mixture was allowed to cool to a temperature of 150-170° C. and was maintained at this temperature while stirring for 3 hours. Then, heating was stopped, and the reaction mixture was allowed to cool. At a temperature of less than 40° C., the reactor was drained, and the liquid was collected into nine 10 L carboys. The product was purified and recovered as described in Part 2.1. Based on the added quantities to the reactor without regard to consumption of reactants due to reaction, the concentrations in the reactor are PVP 0.75-0.80 wt %, NH₄Cl 0.20-0.25 wt %, AgNO₃ 0.75-0.80 wt %, and ethylene glycol balance >98 wt %.

For this experiment, there was 505.49 g of purified silver nanowires recovered, and the mean yield was 92%.

The wire diameters were measured for more than 100 nanowires. The diameters were in the range of 55-70 nm, and the lengths were in the range of 5-6 microns. A representative UV-visible extinction spectrum is shown in FIG. 3 (“100 L H 1.5×”) along with extinction spectra from the other exemplified syntheses. The spectra are normalized with respect to the extinction maximum. As compared to the 10 L H 1.5× spectrum (Example 1, Part 1.2), it can be seen that the use of increased amounts of silver nitrate and PVP without an increased amount of catalyst was able to mitigate the occurrence of high particle levels. Additional experiments (not shown) indicated that the problem of high particle levels was increasing problematic at higher scale if the catalyst was similarly scaled.

In this scheme, the amount of the silver nitrate and the PVP was increased by 50% as compared to Part 2.1, but the amount of the ammonium chloride catalyst was not increased. The results show that this scheme was successful at achieving high yield and high throughput based on nanowire mass while avoiding the problem of higher by-product particle levels.

A comparison of reagents and relative amounts used in Parts 2.1 and 2.2 are summarized in Table 2.

	TABLE 2
	Reagent	Increase
	PVP (g)	1.5x
	PVP in EG (wt %)	1.4x
	PVP in final reaction	1.5x
	mixture (wt %)
	NH4Cl (g)	same
	NH4Cl in EG (wt %)	about same
	AgNO3 (g)	1.5x
	AgNO3 in EG (wt %)	1.4x
	AgNO3 in final reaction	1.5x
	mixture (wt %)
	Total EG (g)	same
	Total final reaction	about same
	mixture (g)

Part 2.3. Scheme 100 L-Long Nanowires L (1×+1×+1×)

The reaction scheme for Part 2.3 is described schematically above in FIG. 2. The scheme from Part 2.1 was carried out the exception that some of the processing steps and some of the reagent/solvent amounts were modified. In particular, the reaction mixture was cooled to 30° C. below the previous reaction temperature for nanowire lengthening after the silver nitrate and quenching ethylene glycol quantity was added to the reactor, and an additional two equivalents of silver nitrate were added under the lower temperature condition to provide for a 3-fold total amount of silver nitrate in the reaction mixture (i.e. two additions of silver nitrate or “1×+2×-gradual”). The reaction temperature used for nanowire length growth has been found to relate to reaction times within a reasonable range, although if the reaction temperature is altered too much, the properties of the product nanowires are significantly altered. Furthermore, during and after the gradual addition of a second amount of silver nitrate, the reaction mixture was maintained at the lower temperature condition for 22 hours, which is roughly 4 hours longer than the gradual addition of silver nitrate. A further modification from the scheme of Part 2.1 was the addition of a second PVP equivalent before the final cooling step, which resulted in a 2-fold total amount of PVP in the reaction mixture. This added PVP at the end of the reaction was found to facilitate purification. Thus, initially, an equivalent amount of ethylene glycol as in Part 2.1 was placed in a 100-liter reactor at room temperature. The reactor was closed and the ethylene glycol was heated to the target temperature as in Part 2.1 while mixing at 132 RPM. In a separate container, a PVP/catalyst solution was prepared as described for Part 2.1. The PVP/catalyst solution was heated to a temperature of 40.0° C. with continuous mixing. Once the reactor reached the target temperature, the PVP/catalyst solution was added to the reactor with mixing. After the addition of the PVP/catalyst solution, the temperature of the mixture in the reactor dropped. Heating was continued until the temperature of the reaction solution reached the target initial reaction temperature as presented in Part 2.1. At this temperature, a solution of silver nitrate in ethylene glycol as described in Part 2.1 was added to the reactor. After mixing for 1-5 minutes, an additional amount of ethylene glycol was added for quenching to the reaction mixture. The temperature of the reaction mixture at the time immediately before this quenching step was 175-180° C. The quench cooled the reaction solution, and the reaction mixture was allowed to further cool to a temperature of 115-140° C. The reaction mixture was maintained at this temperature and, with constant stirring, a silver nitrate solution with a total amount of silver nitrate at the same concentration relative to the initial quantity of silver nitrate was added dropwise at a rate of 6 ml/minute. The reaction mixture was maintained at the lengthening reaction temperature and stirred for 22 hours. Then, heating was stopped, and the reaction mixture was allowed to cool to about 20° C., which substantially stopped further reaction. Once the target initial cooling temperature was reached, a solution of polyvinylpyrrolidone in ethylene glycol in the same quantity and concentration as the initial PVP/catalyst solution, except absent the catalyst, was added to the reactor. The reaction mixture was allowed to further cool. At a temperature of less than 40° C., the reactor was drained, and the liquid was collected into ten 10 L carboys. The product was purified and recovered as described in Part 2.1. Based on the total added quantities to the reactor without accounting for consumption of reactants due to reaction, the concentrations in the reactor are PVP 0.90-1.0 wt %, NH₄Cl 0.20-0.25 wt %, AgNO₃ 1.40-1.45 wt %, and ethylene glycol balance >97 wt %.

For this experiment, there was 1000-1050 g of purified silver nanowires recovered, and the mean yield was 91%.

The wire diameters were measured for about 200-300 nanowires. The diameters were from 60-100 nm, and the lengths were from 9-14 microns. A representative UV-visible extinction spectrum is shown in FIG. 3 (“100 L H L”) along with extinction spectra from the other exemplified syntheses. The spectra are normalized with respect to the extinction maximum. The results show that the later dropwise addition of silver nitrate in this scheme does not increase the particle level above the level of the single addition scheme of Part 2.1 (see 100 L H 1×). In other words, the particle level is consistent with the starting synthesis and the additional dropwise additions of silver nitrate predominantly provide for a lengthening of the existing nanowires.

As compared to the schemes described in Parts 2.1 and 2.2 of this example, the reaction scheme described in Part 2.3 resulted in a longer nanowire, a significantly higher throughput with respect to yield per batch (200% increase and 100% increased yield per batch, respectively), and a similarly high yield. The addition of the second equivalent of the PVP before the final cooling step was shown to provide improved purification.

Example 3—Comparison of 10 L and 100 L Reaction Scheme Temperature Profiles

This example compares the temperature profiles for the three reaction schemes in Example 1 with the three reaction schemes in Example 2.

FIGS. 5A and 5B show plots of the temperature profiles for the 10 L reaction scheme described in Part 1.3 of Example 1 and the 100 L reaction scheme described in Part 2.3 of Example 2. FIG. 5A shows the overall temperature profiles, and FIG. 5B shows the temperature profiles for the first 14 minutes. The temperature profiles are normalized to align the curves at the PVP/catalyst solution addition time. For the 10 L and 100 L reaction schemes, the PVP/catalyst solution was added at a peak temperature as described in the above examples. The silver nitrate was added at a slightly lower temperature resulting from adding the PVP/catalyst solution. The quench with ethylene glycol at 1-5 minutes after the addition of the silver nitrate was initiated at a roughly comparable temperature. Referring to FIGS. 5A and 5B, it can be seen that the temperature profiles of the 10 L and the 100 L reaction schemes are generally similar in shape, which is one indication of a successfully scaled-up reaction.

Similarly, FIGS. 6A and 6B show plots of the temperature profiles for the 10 L reaction scheme described in Part 1.2 of Example 1 and the 100 L reaction scheme described in Part 2.2 of Example 2. The temperature profiles are normalized to align the curves at the PVP/catalyst solution addition time. For the 10 L and 100 L reaction schemes, the PVP/catalyst solution was added at a temperature as described in the above examples. The silver nitrate was added at the selected initial reaction temperature. The quench with ethylene glycol at 1-5 minutes after the addition of the silver nitrate was initiated at a temperature reached after the initial reaction time had passed. The temperature profiles in FIGS. 6A & 6B are similar to the profiles in FIGS. 5A & 5B.

Similarly, FIGS. 7A and 7B show plots of the temperature profiles for the 10 L reaction scheme described in Part 1.3 of Example 1 and the 100 L reaction scheme described in Part 2.3 of Example 2. The temperature profiles are normalized to align the curves at the PVP/catalyst solution addition time. For the 10 L and 100 L reaction schemes, the PVP/catalyst solution was added at a temperature as specified in the examples above. The silver nitrate was added at the target initial reaction temperature. The quench with ethylene glycol at 1-5 minutes after the addition of the silver nitrate was initiated at a temperature reached following passage of the initial reaction time.

Referring to FIGS. 5A and 5B, FIGS. 6A and 6B, and FIGS. 7A and 7B, it can be seen that the temperature profiles for the 10 L and the 100 L reaction schemes are generally similar in shape, which is consistent with successfully scaled-up reaction schemes. The temperature profiles over the reactant addition and quench are similar at the 10 L and 100 L scales except that the temperatures at 100 L were a few degrees lower and the times were slightly longer, which is consistent with slow temperature response times. For the L nanowire syntheses in Part 1.3 of Example 1 and Part 2.3 of Example 2, the temperature during the nanowire lengthening period was kept higher for the 100 L scale synthesis and the reaction time was correspondingly less. This tuning of the L nanowire growth phases is discussed in the following Example.

Example 4—Tuning of 1×+1×+1×Reaction Schemes

This example describes three iterations for tuning a reaction scheme involving synthesis of L (long) silver nanowires involving additions of 3× amounts of silver nitrate (“1×+1×+1×”) with different temperature profiles for the long time nanowire growth phases. As noted in Example 3, the temperature profiles for the growth phases in Parts 1.3 and 2.3 were different from each other.

Part 4.1: An experiment was conducted in which the 10 L processing parameters, as generally described in Example 1, Part 1.3, and the 100 L processing parameters, as generally described in Example 2, Part 2.3 were varied. For the 10 L reaction scheme longer time growth phase, silver nitrate was added dropwise at an addition temperature of 110° C. The addition rate was 0.5 ml/minute. The 100 L reaction scheme used the same addition temperature and a scaled-up addition rate of 4 ml/minute. The reaction mixture was held at the addition temperature for approximately 43 hours, including the dropwise silver nitrate addition time. The temperature profiles for the 10 L and 100 L reaction schemes are shown in FIG. 8A. The 100 L mean yield was approximately 94%, and the average length of the nanowires was 16.25 μm (about 4 μm longer than the nanowires produced in the 10 L batches of Example 1.3). To generate comparable length nanowires, the 100 L reaction was performed at the higher temperature and shorter time as presented in Example 2, Part 2.3.

Part 4.2: For this experiment, the procedure described in Part 4.1 was followed, with the exception that the addition rate was increased to 6 ml/minute for the 100 L reaction scheme. A further exception is that the second and third aliquots of silver nitrate were added at the same rate from a combined batch of silver nitrate to reduce the total process time. The reaction mixture was held at the addition temperature for approximately 22 hours. The temperature profile for this experiment is shown in FIG. 8B along with the temperature profile for the 100 L reaction scheme from Part 4.1. In this shortened process, the mean yield was approximately 86%, and the average length of the nanowires was 16.65 μm. Thus, compared to the results in Part 4.1, the yield was lower, the average length was slightly greater, but the reaction time was significantly less.

Part 4.3: For this experiment, the procedure described in Part 4.2 was followed, with the exception that the addition temperature was increased to 130° C. The temperature profile for this experiment is shown in FIG. 8C along with the temperature profile for the 100 L reaction scheme from Part 4.2. In this modified shortened process, the mean yield was increased to approximately 94%, and the average length of the nanowires was decreased to 12.56 μm, which is within the range of the 10 L batches.

The results of this example show that the 1×+1×+1×reaction scheme could be tuned with the adjustment of addition temperatures and silver nitrate processes conditions to provide nanowires of a target length and at a high yield via a relatively short process time.

The mean weight of Ag and mean yield of silver nanowires are summarized in Table 3 for each of the syntheses from Examples 1 and 2.

TABLE 3
		Mean	Mean Yield
	Synthesis	Weight Ag	Silver Nanowires
Example	Scale/Type	(g)	(%)

Ex. 1, Part 1.1	10 L H 1x	17	91
Ex. 1, Part 1.2	10 L H 1.5x	241	841
Ex. 1, Part 1.3	10 L L	105	94
Ex. 2, Part 1.1	100 L H 1x	310	92
Ex. 2, Part 1.2	100 L H 1.5x	465	92
Ex. 2, Part 1.3	100 L L	918	91
1Average of two runs early in development of synthesis

Further Inventive Concepts

• • A1. A method for synthesizing high aspect ratio silver nanowires, the method comprising:
    • under mixing conditions, adding a first quantity of soluble silver salt to a solution of ammonium chloride and polyvinylpyrrolidone in a glycol solvent at an initial reaction temperature from about 180° C. to at least one degree below the solvent boiling point to form an initial reaction solution;
    • following addition of the soluble silver salt, under mixing conditions, add an additional quantity of solvent amounting to 6 wt % to 30 wt % of the solvent following the additional of the soluble silver salt;
    • provide for the cooling of the reaction solution to a continuing reaction temperature from about 100° C. to about 150° C. to form a cooled reaction solution, and
    • maintaining the continuing reaction temperature while gradually adding a second quantity of soluble silver salt into the cooled reaction solution for a reaction time of at least about 4 hours.
  • A2. The method of inventive concept A1 wherein the addition of additional quantity of solvent is performed after a period of time from about 1 minute to about 10 minutes following the addition of the first quantity of soluble silver salt.
  • A3. The method of inventive concept A1 wherein the ammonium-based chloride catalyst comprises ammonium chloride.
  • A4. The method of inventive concept A1 wherein the amount of ammonium-based chloride catalyst is from 0.0001M to about 0.01M at full dilution into the reaction solution.
  • A5. The method of inventive concept A1 wherein the amount of PVP added to the reactor is from about 0.25 wt % to about 2.5 wt % relative to the total weight of the reaction solution, and wherein the PVP comprises polymer molecules with molecular weights from about 40,000 g/mol to about 1,600,000 g/mol.
  • A6. The method of inventive concept A1 wherein the PVP and the ammonium-based catalyst are predissolved in glycol solvent prior to addition into the reaction volume.
  • A7. The method of inventive concept A1 wherein the first quantity of soluble silver salt corresponds to silver ions in an amount from 0.1 wt % to about 2.0 wt % relative to the total weight of the final reaction solution.
  • A8. The method of inventive concept A7 wherein the soluble silver salt is predissolved in glycol prior to addition to the reaction solution and wherein the volume of glycol to predissolve the soluble silver salt is from about 1% to about 9% of the total solvent volume for the final reaction solution.
  • A9. The method of inventive concept A1 wherein the additional quantity of solvent is from about 9% to 20 wt % of the total solvent added to the final reaction solution.
  • A10. The method of inventive concept A1 wherein the additional quantity of solvent is added from about 1.5 minutes to about 5 minutes following addition of the soluble silver salt.
  • A11. The method of inventive concept A1 wherein the reaction solution initially comprising ammonium-based chloride and PVP is formed by dissolving the ammonium-based chloride and the PVP in a delivery quantity glycol solvent and adding the dissolved ammonium-based chloride and PVP into a heated initial quantity of glycol solvent.
  • A12. The method of inventive concept A11 wherein the ammonium-based chloride and the PVP are dissolved into a common delivery quantity of glycol solvent.
  • A13. The method of inventive concept A11 wherein the ammonium-based chloride and the PVP are dissolved into separate delivery quantities of glycol solvent.
  • A14. The method of inventive concept A11 wherein the heated initial quantity of glycol solvent is heated to a temperature such that the delivery of the delivery quantity of glycol solvent, either in a single portion or as multiple portions to provide the ammonium-based chloride and the PVP, results in a lowering the temperature of the reaction solution to approximately the initial reaction temperature.
  • A15. The method of inventive concept A1 wherein the continuing reaction temperature is from about 105° C. to about 145° C.
  • A16. The method of inventive concept A1 further comprising after the addition of the additional solvent quantity, gradually adding an additional quantity of soluble silver salt dissolved into glycol solvent at the reaction temperature to induce lengthening of product silver nanowires.
  • A18. The method of inventive concept A17 wherein the additional quantity of soluble silver salt is from about 0.5 times to about 5 times the first quantity of soluble silver salt.
  • A19. The method of inventive concept A17 wherein the gradual adding is performed over a time from about 30 minutes to about 48 hours.
  • A20. The method of inventive concept A1 further comprising after cooling following the end of the reaction time, settling product silver nanowires with addition of an incompatible solvent and collecting the settled nanowires, wherein the collected silver nanowires have an average diameter from about 30 nm to about 100 nm and an average length from about 2 microns to about 75 microns.

The embodiments above are intended to be illustrative and not limiting. Additional embodiments are within the claims. In addition, although the present invention has been described with reference to particular embodiments, those skilled in the art will recognize that changes can be made in form and detail without departing from the spirit and scope of the invention. Any incorporation by reference of documents above is limited such that no subject matter is incorporated that is contrary to the explicit disclosure herein. To the extent that specific structures, compositions and/or processes are described herein with components, elements, ingredients or other partitions, it is to be understand that the disclosure herein covers the specific embodiments, embodiments comprising the specific components, elements, ingredients, other partitions or combinations thereof as well as embodiments consisting essentially of such specific components, ingredients or other partitions or combinations thereof that can include additional features that do not change the fundamental nature of the subject matter, as suggested in the discussion, unless otherwise specifically indicated. The use of the term “about” herein refers to expected uncertainties in the associated values as would be understood in the particular context by a person of ordinary skill in the art.