US20130160608A1

US20130160608A1 - Process For Producing Silver Nanowires

Info

Publication number: US20130160608A1
Application number: US13/807,337
Authority: US
Inventors: Robert Nusko; Georg Maier; Theo Kaiser
Original assignee: Heraeus Precious Metals GmbH and Co KG
Current assignee: RENT A SCIENTIST GmbH; Heraeus Deutschland GmbH and Co KG
Priority date: 2010-07-02
Filing date: 2011-07-01
Publication date: 2013-06-27
Also published as: KR20130132740A; EP2588264B1; WO2012022332A2; EP2588264A2; CN103201061A; DE102010017706A1; WO2012022332A3; DE102010017706B4; JP2013531133A

Abstract

Described are processes for the production of silver nanowires comprising forming a reaction mixture including a polyol, an organic chemical adsorbed to a silver surface, a chemical to form a halide and/or pseudohalide and a chemical which forms a redox pair. A silver salt is added to the reaction mixture and temperature is maintained to form the silver nanowires.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the national stage entry of PCT/DE2011/075155, filed on Jul. 1, 2011, which claims priority to German Application No. 102010017706.7, filed on Jul. 2, 2010, the disclosures of which are hereby incorporated herein by reference in their entireties.

FIELD

The invention relates to a process for the production of silver nanowires.
The production of silver nanowires is known from the prior art. In addition to extensive scientific works (e.g. “Approaches to the Synthesis and Characterization of Spherical and Anisotropic Noble Metal Nanomaterials” by H. J. Parab et al. in “Metallic Nanomaterials” by Challa S. S. R. Kumar (ed.), WILEY-VCH Verlag, Weinheim), some patent literature which discloses the synthesis of silver nanowires also exists. US 2009/0282948 A1 thus describes a process for the production of silver nanowires by means of the polyol process which has been known for a long time, in which chemicals containing hydroxyl groups, in particular ethylene glycol, are employed simultaneously as a solvent and as a reducing agent. A similar process is disclosed in WO 2009/128973 A2. A process for the production of silver nanowires in an aqueous environment using hydroxyketone or hydroxylamine as a reducing agent is described in US 2009/0311530 A1.
In the context of the present specification, the term “silver nanowire” is a collective term for all materials which

- consist predominantly of particles having contents of metallic silver of >90 wt. %,
- have a “one-dimensional” geometry, such as a rod or a hair having a long axis (length) and a short axis (diameter),
- have an aspect ratio (length/diameter) of at least 5 and
- the diameter of which is in the range between 1 and 900 nm.

A synthesis route for the production of silver nanowires by the polyol process, in which an organic chemical which is adsorbed on to the silver surface (preferably polyvinylpyrrolidone—PVP), small amounts of a halide (preferably chloride in the form of NaCl), small amounts of an Fe(II) or Fe(III) salt (preferably Fe(III) acetylacetonate) and a silver salt (preferably AgNO₃) are employed in addition to a hot polyol (preferably ethylene glycol), is to be found in the abovementioned prior art. The addition of an acid (preferably HCl or HNO₃) is mentioned in some embodiment examples. The sequence of addition of the various chemicals, reaction temperatures, concentrations and concentration ratios stated are sometimes different or contradictory.
The examples described in US 2009/0282948 A1 for the production of silver nanowires disclose relatively low silver concentrations. Large reactors and large amounts of solvent (ethylene glycol) are necessary for the production of economically relevant amounts of silver nanowires by the processes described there. The process is therefore very involved.
K. E. Korte et al. (J. Mater. Chem., 2008, 18, 437-441) discloses the production of silver nanowires by a polyol process. Copper in the form of CuCl or CuCl₂is employed as the chemical which forms a redox pair. The essential finding from (1) is that the Cu removes the O from the 111 sites of the Ag surface and in this way renders possible the 1D growth.
The processes described in WO 2009/128973 A2 for the production of silver nanowires operate with economically reasonably high silver concentrations. The yield of silver nanowires when the processes described in WO 2009/128973 A2 are used, nevertheless, is limited to a maximum of 65% of the silver employed. Since silver is a valuable raw material of limited availability, the yield in the production plays an important role for the profitability of a process.
There therefore continues to be a need for processes for the production of silver nanowires which overcome the problems mentioned. In particular, processes which render silver nanowires accessible with a high yield at a simultaneously high reaction concentration of silver are required.

SUMMARY

One or more embodiments of the invention are directed to processes for the production of silver nanowires. A reaction mixture is provided. The reaction mixture comprises a polyol, an organic chemical adsorbed on to a silver surface, a chemical which forms one or more of a halide and a pseudohalide, wherein the chemical which forms a halide is a salt of one or more of the halides Cl⁻, Br⁻ and I⁻ and wherein the chemical which forms a pseudohalide is a salt of one or more of the pseudohalides SCN⁻, CN⁻, OCN⁻ and CNO⁻ and a chemical which forms a redox pair, the chemical selected from the group consisting of bromine, iodine, vanadium and mixtures thereof. A silver salt is added to the reaction mixture in an amount such that the concentration of silver in the reaction mixture is at least 0.5 wt. %, based on the total weight of the reaction mixture. The silver salt is added at a temperature of the reaction mixture of at least 100° C. The temperature of the reaction mixture is maintained at least 100° C. for the duration of the reaction.
In some embodiments, the silver salt is added in an amount such that the concentration of silver in the reaction mixture is at least 1.0 wt. %, preferably at least 2.5 wt. %, particularly preferably at least 5.0 wt. %, based on the total weight of the reaction mixture.
In one or more embodiments, the reaction mixture comprises one or more of the chemical which forms the halide and the chemical which forms the pseudohalide in an amount such that the concentration of the halide and/or the pseudohalide in the reaction mixture is in a range of from 5 ppm to 2,000 ppm, preferably in a range of from 20 ppm to 1,000 ppm, particularly preferably in a range of from 50 ppm to 500 ppm.
In some embodiments, the chemical which forms the halide comprises one or more of NaCl, NaBr, NaI, KCl, KBr and KI. In one or more embodiments, the chemical which forms the pseudohalide comprises one or more of NaSCN and KSCN.
In some embodiments, the reaction mixture provided in step a) additionally comprises an iron salt, the iron salt forming an Fe(II)/Fe(III) redox pair. In one or more embodiments, the reaction mixture provided in step a) additionally comprises a copper salt, the copper salt forming a redox pair. In detailed embodiments, the reaction mixture provided in step a) additionally comprises at least one inorganic or organic acid. In specific embodiments, the acid comprises one or more of HCl, HBr, HI and HNO₃. In certain embodiments, the addition of the acid takes place in an amount such that the concentration of acid in the reaction mixture is in a range of from 0.01 wt. % to 2.0 wt. %, based on the total weight of the reaction mixture.
In some embodiments, the chemical which forms a redox pair comprises one or more of elemental vanadium, a vanadium oxide, a vanadium hydroxide, a vanadium sulphate, a vanadium nitrate, a vanadium chloride and mixtures of these compounds. In detailed embodiments, the redox pair comprises one or more of Br⁻/Br₂/BrO₃ ⁻, I⁻/I₂/IO₃ ⁻ and/or V(0)/V(II)/V(III)/V(IV)/V(V).
In some embodiments, the concentration in the reaction mixture of the chemical which forms a redox pair is in a range of from 5 ppm to 2,000 ppm, preferably in a range of from 20 ppm to 1,000 ppm, particularly preferably in a range of from 50 ppm to 500 ppm.
In detailed embodiments, the silver salt is added at a temperature of the reaction mixture of at least 145° C., preferably at a temperature of the reaction mixture of between 145° C. and 200° C., particularly preferably at a temperature of the reaction mixture of between 150° C. and 180° C., especially preferably at a temperature of the reaction mixture of between 155° C. and 170° C.
In some embodiments, in step c) the temperature of the reaction mixture of at least 145° C., preferably a temperature of the reaction mixture of between 145° C. and 200° C., particularly preferably a temperature of the reaction mixture of between 150° C. and 180° C., especially preferably a temperature of the reaction mixture of between 155° C. and 170° C. is maintained for the duration of the reaction.
In some embodiments, after step c) step, d) isolating silver nanowires from the reaction mixture, wherein a suitable solvent is added for isolation of the silver nanowires from the reaction mixture and the supernatant is then decanted off. In detailed embodiments, after step c) and before step d) step, c₁) cooling the reaction mixture to room temperature.
Additional embodiments of the invention are directed to processes for the production of silver nanowires. A reaction mixture is prepared comprising a polyol, an organic chemical adsorbed onto a silver surface, a chemical which forms one or more of a halide and a pseudohalide, a chemical which forms a redox pair and at least one of an inorganic acid and an organic acid. The chemical which forms a halide is a salt of one or more of the halides Cl⁻, Br⁻ and I⁻ and the chemical which forms a pseudohalide is a salt of one or more of the pseudohalides SCN⁻, CN⁻, OCN⁻ and CNO⁻ and is present in the range of about 5 ppm to about 2000 ppm. The chemical which forms a redox pair is selected from the group consisting of bromine, iodine, vanadium and mixtures thereof. A silver salt is added to the reaction mixture at a temperature of at least 100° C. The silver salt is present in a concentration of at least 0.5 wt. % based on the total weight of the reaction mixture. The reaction mixture is maintained at a temperature of at least 100° C. for the duration of the reaction to form silver nanowires. The reaction mixture is cooled to about room temperature. The silver nanowires are isolated from the reaction mixture comprises adding a suitable solvent to the cool reaction mixture and decanting off a supernatant.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the exemplary embodiments of the present invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. It is to be appreciated that certain well known processes are not discussed herein in order to not obscure the invention.

FIG. 1 shows a scanning electron micrograph of silver nanowires prepared in accordance with one or more embodiments of the invention.

DETAILED DESCRIPTION

Embodiments of the invention, such as is characterized in the claims, provide processes for the production of silver nanowires which are carried out at a high reaction concentration of silver and which deliver a high yield of silver nanowires.
Embodiments of the invention provide processes for the production of silver nanowires. The process comprises the steps:

- a) provision of a reaction mixture comprising
  - a polyol,
  - an organic chemical which is adsorbed on to a silver surface,
  - a chemical which forms a halide and/or one which forms a pseudohalide,
  - wherein the chemical which forms a halide is a salt of one of the halides Cl⁻, Br⁻ and/or I⁻ and wherein the chemical which forms a pseudohalide is a salt of one of the pseudohalides SCN⁻, CN⁻, OCN⁻ and/or CNO⁻,
  - a chemical which forms a redox pair, chosen from the group consisting of bromine, iodine, vanadium and mixtures thereof,
- b) addition of a silver salt in an amount such that the concentration of silver in the reaction mixture is at least 0.5 wt. %, based on the total weight of the reaction mixture, the silver salt being added at a temperature of the reaction mixture of at least 100° C.,
- c) maintaining of a temperature of the reaction mixture of at least 100° C. for the duration of the reaction.

The processes according to one or more embodiments the invention for the production of silver nanowires are a polyol synthesis, in which a silver salt is reduced in the hot polyol in the presence of an organic chemical which is adsorbed on to the silver surface, a chemical which forms a halide and/or one which forms a pseudohalide, and a chemical which forms a redox pair, chosen from the group of bromine, iodine, vanadium and mixtures thereof, to give metallic silver in the form of nanowires.
The process according to one or more embodiments of the invention renders possible the production of silver nanowires in a high yield of 80% and more with a simultaneously high reaction concentration of silver of at least 0.5 wt. %, based on the total weight of the reaction mixture.
The end of the reaction can be determined by the person skilled in the art without problems, since on the one hand the formation of the silver nanowires can be easily monitored visually, and on the other hand the evolution of gas occurring during the reaction ceases. At the end of the reaction, a nacreous, iridescent dispersion is present.
It should be pointed out that the definition of the reaction mixture constituents “a chemical which forms a halide and/or one which forms a pseudohalide” and “chemical which forms a redox pair, chosen from the group consisting of bromine, iodine, vanadium and mixtures thereof” overlap. The redox pair forming chemicals bromine and iodine are halogens which, in the form of one of their salts, are both a “salt of a halide” and a “chemical which forms a redox pair, chosen from the group consisting of bromine and iodine”. As the experimental results explained in more detail in the following show, compounds such as, for example, NaBr or VCl₃, are in fact capable of fulfilling the tasks of the two constituents of the reaction mixture.
The preferred concentration ranges stated in the following for the two reaction mixture constituents mentioned are to be understood additively in these cases, i.e. for example NaBr is added in an amount which fulfils the two concentration ranges.
In addition, it is also furthermore to be noted that the terms “a” and “an” used in the claims are to be understood not as numerical words but as an indefinite article. Thus, for example, the expression “a polyol” is meant in the meaning of “one or more polyols”.
According to a preferred embodiment, the silver salt is added in an amount such that the concentration of silver in the reaction mixture is at least 1.0 wt. %, preferably at least 2.5 wt. %, particularly preferably at least 5.0 wt. %, based on the total weight of the reaction mixture. As the concentration of silver in the reaction mixture increases, the amount of silver nanowires which are produced in one working operation increases, with a constant yield.
In addition or alternatively, according to a preferred embodiment the reaction mixture comprises the chemical which forms the halide and/or the chemical which forms the pseudohalide in an amount such that the concentration of the halide and/or of the pseudohalide in the reaction mixture is in a range of from 5 ppm to 2,000 ppm, preferably in a range of from 20 ppm to 1,000 ppm, particularly preferably in a range of from 50 ppm to 500 ppm.
According to a preferred embodiment, in addition or alternatively, the chemical which forms the halide is NaCl, NaBr, NaI, KCl, KBr and/or KI. According to a further preferred embodiment, the chemical which forms the halide is elemental bromine and/or elemental iodine.
According to a preferred embodiment, additionally or alternatively, the chemical which forms the pseudohalide is NaSCN and/or KSCN.
According to a preferred embodiment, the reaction mixture provided in step a) additionally comprises an iron salt, the iron salt forming an Fe(II)/Fe(III) redox pair in the reaction mixture.
The iron salt added is thus capable of forming an Fe(II)/Fe(III) redox pair under the reaction conditions of the process according to the invention.
According to a preferred embodiment, the reaction mixture provided in step a) additionally comprises a copper salt, the copper salt forming a redox pair in the reaction mixture. The copper salt added is thus capable of forming a redox pair under the reaction conditions of the process according to the invention.
According to a preferred embodiment, in addition or alternatively, the chemical which forms a redox pair is elemental vanadium, a vanadium oxide, a vanadium hydroxide, a vanadium sulphate, a vanadium nitrate, a vanadium chloride or a mixture of these compounds.
According to a preferred embodiment, in addition or alternatively, the redox pair is Br⁻/Br₂/BrO₃ ⁻, I⁻/I₂/IO₃ ⁻ and/or V(0)/V(II)/V(III)/V(IV)/V(V).
According to a preferred embodiment, in addition or alternatively, the concentration in the reaction mixture of the chemical which forms a redox pair is in a range of from 5 ppm to 2,000 ppm, preferably in a range of from 20 ppm to 1,000 ppm, particularly preferably in a range of from 50 ppm to 500 ppm.
According to a preferred embodiment, the reaction mixture provided in step a) additionally comprises at least one inorganic or organic acid.
According to a preferred embodiment, additionally or alternatively, the acid is HCl, HBr, HI and/or HNO₃.
In addition or alternatively, according to a preferred embodiment the acid is added in an amount such that the concentration of acid in the reaction mixture is in a range of from 0.01 wt. % to 2.0 wt. %, preferably in a range of from 0.02 wt. % to 1.0 wt. %, particularly preferably in a range of from 0.05 wt. % to 0.5 wt. %, based on the total weight of the reaction mixture.
In addition or alternatively, according to a preferred embodiment the silver salt is added at a temperature of the reaction mixture of at least 145° C., preferably at a temperature of the reaction mixture of between 145° C. and 200° C., particularly preferably at a temperature of the reaction mixture of between 150° C. and 180° C., especially preferably at a temperature of the reaction mixture of between 155° C. and 170° C.
In addition or alternatively, according to a preferred embodiment in step c) a temperature of the reaction mixture of at least 145° C., preferably a temperature of the reaction mixture of between 145° C. and 200° C., particularly preferably a temperature of the reaction mixture of between 150° C. and 180° C., especially preferably a temperature of the reaction mixture of between 155° C. and 170° C. is maintained for the duration of the reaction.
According to a further preferred embodiment, after step c) step d) isolation of the silver nanowires from the reaction mixture is carried out, wherein a suitable solvent is preferably added for isolation of the silver nanowires from the reaction mixture and the supernatant is then decanted off. Particularly preferably, the solvents deionized water, ethanol or isopropanol are added.
According to a further preferred embodiment, after step c) and before step d) step c₁) cooling of the reaction is mixture is carried out, wherein cooling of the reaction mixture to room temperature preferably takes place.
In carrying out the process for the production of silver nanowires under the conditions of the abovementioned preferred embodiments, a particularly high yield of silver nanowires is achieved. A combination of various preferred embodiments has the effect of a further increase in the yield.
The processes disclosed are based on the polyol process. This means that compounds having several hydroxyl groups are employed as the solvent and reducing agent. In principle, all polyols which are capable of reducing the silver compound employed to elemental silver can be employed. It is possible to use a single polyol in the reaction mixture, or a mixture of two, three, four, five or more polyols. The polyols can have two, three, four, five or more hydroxyl groups, and in addition hetero atoms, such as, for example, oxygen or nitrogen. These hetero atoms can be present in the form of an ether, ester, amine and/or amide function. The polyol can thus be a polyester polyol, a polyether polyol or a similar compound.
The polyol can be either an aliphatic glycol or a corresponding glycol polyester. Possible aliphatic glycols are, in particular, alkylene glycols having up to 6 carbon atoms in the main chain. Examples of such compounds are ethanediol, a propanediol, a butanediol, a pentanediol or a hexanediol, and also polyalkylene glycol derivatives of these alkylene glycols.
The polyol can comprise, for example, two, three, four, five or six hydroxyl groups and two, three, four, five, six, seven, eight, nine, ten, eleven or twelve carbon atoms. The (alkylene) polyol can be a glycol, that is to say a compound having hydroxyl groups on adjacent (aliphatic or cycloaliphatic) carbon atoms. The glycols can contain, for example, two, three, four, five or six carbon atoms. Preferred polyols are glycerol, trimethylolpropane, pentaerythritol, triethanolamine and trihydroxymethylaminomethane.
The polyols ethylene glycol, diethylene glycol, triethylene glycol, propylene glycols, butanediols, dipropylene glycols and a polyethylene glycol which is liquid at the reaction temperature, such as, for example, polyethylene glycol 300, are likewise preferred. Particularly preferred polyols are tetraethylene glycol, 1,2-propanediol, dipropylene glycol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol and 2,3-butanediol.
The glycols mentioned have a high reducing power, a boiling point of between 185° C. and 328° C., a good heat stability and low acquisition costs.
A further group of polyols which is suitable for use in processes according to the present invention comprises: ethylene glycol, glycerol, glucose, diethylene glycol, triethylene glycol, propylene glycols, butanediol, dipropylene glycol and polyethylene glycols.
In addition, other polyols, such as, for example, sugars and alcohols, by themselves or in the form of mixtures, can also form at least a part of the polyol reactants. As a rule, polyols which are liquid at room temperature and at the corresponding reaction temperature are used, but this is not absolutely necessary. Thus, for example, polyols which are solid or semi-solid at room temperature can be employed.
According to a particularly preferred embodiment of the present invention, solvents from the group of polar protic solvents, such as e.g. water, alcohols and aldehydes and mixtures thereof, are additionally employed.
Furthermore, reducing agents in addition to the polyol, from the group of alcohols, aldehydes, ketones, amines and tartrates, citrates, hydrides, hydrazine and its compounds, are suitable in particular for the production of the nanowires according to the invention. For technical and economic reasons, the use of ethylene glycol (ethane-1,2-diol) and/or glycerol (propane-1,2,3-triol) is preferred. The examples described in this specification are all carried out in ethylene glycol.
Orientating experiments have shown that ethylene glycol can be replaced by glycerol or mixtures of ethylene glycol and glycerol without noticeable adaptation of the reaction conditions. The suitability of other polyols, e.g. mentioned in WO 2009/128973 A2, for use in the processes according to the invention is not to be ruled out explicitly.
The silver salt provides the silver ions for the reduction. Suitable salts are all those which make available silver ions sufficiently rapidly under the reaction conditions, so that the reaction kinetics determined by the reaction procedure are not substantially influenced. AgNO₃is the preferred silver salt because of the good commercial availability. The silver salt can be added to the reaction in the form of a solution or directly as a solid. A suitable solvent for the silver salt is, inter alia, the polyol employed in the reaction, that is to say preferably ethylene glycol and/or glycerol. The addition can take place all at once, in portions or continuously over a relatively long period of time. The silver salt is added at the reaction temperature. The silver salt is added in an amount which corresponds mathematically to a silver concentration of at least 0.5 wt. % in the reaction mixture. In preferred embodiments of the invention, silver is present in a concentration of at least 1.0 wt. %, more preferably of at least 2.5 wt. %, most preferably of at least 5.0 wt. %.
The organic chemical which is adsorbed on to the silver surface is required in the polyol process for formation of the desired wire-shaped morphology. The scientific literature has been concerned with this phenomenon since the early 1990s. Different mechanisms and various tasks have been assigned to these chemicals in the past. They are said, inter alia, to prevent the agglomeration of the nanoparticles formed or to occupy specifically the metal lattice surfaces formed.
Such organic chemicals which are adsorbed on to the silver surface are known, for example, from WO 2009/128973 A2.
The organic chemical which is adsorbed on to the silver surface can be a liquid. In such cases, the chemical can be mixed directly with other reactants or it can be first diluted by mixing with a solvent or a mixture of solvents, which may contain a polyol. The organic chemical which is adsorbed on to the silver surface can also be solid and is then first dissolved in a solvent, before it is mixed with the other reactants. It is not critical for the organic chemical which is adsorbed on to the silver surface to dissolve completely in the solvent; a suspension of the organic chemical which is adsorbed on to the silver surface can also be used.
The solvent can be a polyol or a mixture of a polyol and one or more other solvents. The one or more other solvents can be chosen such that, either by themselves or as a mixture, they dissolve the organic chemical which is adsorbed on to the silver surface. No particular restriction exists for the solvent, as long as it does not impede or prevent the solution-mediated production of desired nanostructures, such as e.g. silver nanowires.
It is not a prerequisite that the organic chemical which is adsorbed on to the silver surface is added to the reaction in solution, since it can be added in the solid form (a solid powder). However, it is possible for the organic chemical which is adsorbed on to the silver surface not to be always completely or predominantly dissolved, so that the resulting solution should more precisely be called a suspension rather than a solution.
The organic chemical which is adsorbed on to the silver surface is usually chosen such that even at the reaction temperature it does not react to any substantial extent with the polyol or any other solvent. Each solvent used for dissolving the organic chemical which is adsorbed on to the silver surface should likewise be chosen such that it does not impede or prevent the solution-mediated production of desired nanostructures, such as e.g. silver nanowires. If the reaction mixture at the reaction temperature comprises no other solvent for the organic chemical which is adsorbed on to the silver surface, the organic chemical which is adsorbed on to the silver surface should also dissolve in the polyol at least up to a certain degree. The organic chemical which is adsorbed on to the silver surface can have, for example, a solubility at room temperature of at least about 1 g per litre of solvent (solvent mixtures being included), e.g. at least about 5 g per litre of solvent, at least about 10 g per litre of solvent or at least about 20 g per litre of solvent. In some embodiments, the organic chemical which is adsorbed on to the silver surface has a solubility of at least about 100 g per litre of solvent, e.g. at least about 200 g per litre of solvent or at least about 300 g per litre of solvent.
The solutions of the organic chemical which is adsorbed on to the silver surface can be prepared in any concentration which is feasible and in which the desired nanostructures, such as e.g. silver nanowires, can be produced.
It goes without saying that if e.g. a polymer, such as polyvinylpyrrolidone (PVP) having an average molecular weight of e.g. 55,000, is used as the organic chemical which is adsorbed on to the silver surface, the concentration is calculated using the monomer weight and not the average molecular weight of the polymer. The molar concentration of PVP solution would be calculated e.g. by dividing the grams of PVP used for the preparation of one litre of the OPA solution by 111 g/mol and not by 55,000 g/mol.
The present invention also includes the joint use of two or more different organic chemicals which are adsorbed on to the silver surface. For example, a mixture of two or more different low molecular weight compounds or a mixture of two or more of the same or different polymers can be used, the polymers, if they are the same, being of different molecular weight. In some embodiments, a mixture of one or more compounds of low molecular weight and one or more polymers can be used. The organic chemicals which are adsorbed on to the silver surface can be e.g. a mixture of polymers having a similar molecular weight (e.g. polyvinylpyrrolidone having an approximate molecular weight of 55,000). In some embodiments, the organic chemical which is adsorbed on to the silver surface can be a mixture which comprises one or more of these commercially available polymer mixtures. The organic chemical which is adsorbed on to the silver surface can comprise e.g. polyvinylpyrrolidone having an average molecular weight of 55,000 and polyvinylpyrrolidone having an average molecular weight of 1,300,000. In some embodiments, the organic chemical which is adsorbed on to the silver surface can comprise a mixture of different polymers. The organic chemical which is adsorbed on to the silver surface can comprise e.g. polyvinylpyrrolidone having an average molecular weight of 55,000 and polyvinyl alcohol having an average molecular weight of 35,000.
In some embodiments, the organic chemical which is adsorbed on to the silver surface is a substance which is capable of interacting electronically with a metal atom of a nanoparticle. Such a substance can comprise one or more atoms (e.g. at least two atoms) having one or more free electron pairs, such as e.g. oxygen, nitrogen and sulphur. As a non-limiting example, the organic chemical which is adsorbed on to the silver surface can be capable of a coordinative interaction with a metal atom on the surface of a nanoparticle and/or of chelating the metal atom. The organic chemicals which are adsorbed on to the silver surface can comprise e.g. one or two O and/or N atoms. The atoms having a free electron pair can be present in the substance in the form of a functional group, such as e.g. a hydroxyl group, a carbonyl group, an ether group and an amino group, or as a component of a functional group which comprises one or more of these groups as its structural element. Non-limiting examples of suitable functional groups comprise —COO—, O—CO—O—, —CO—O—CO—, —C—O—C—, —CONR—, —NR—CO—O—, —NR¹—CO—NR²—, —CO—NR—CO—, —SO₂NR— and —SO₂—O—, wherein R, R¹and R²in each case independently represent hydrogen or an organic radical (e.g. an aliphatic or aromatic, unsubstituted or substituted radical which comprises from about 1 to about 20 carbon atoms). Such functional groups can comprise the above (and other) structural elements as part of a cyclic structure (e.g. in the form of a cyclic ester, amide, anhydride, imide, carbonate, urethane, urea and the like). In some embodiments, the organic chemical which is adsorbed on to the silver surface is or comprises a substance which is capable of reducing the metal compound. A specific, non-limiting example of such a substance is polyvinylpyrrolidone (PVP).
In some embodiments, the organic chemical which is adsorbed on to the silver surface can have in total at least about 10 atoms per molecule which are chosen from C, N and O— e.g. at least about 20 such atoms or at least about 50 such atoms. In some embodiments, the organic chemical which is adsorbed on to the silver surface has in total at least about 100 C, N and O atoms per molecule: e.g. at least about 200, at least about 300 or at least about 400 C, N and O atoms per molecule. In the case of polymers, these figures relate to the average per polymer molecule.
The organic chemical which is adsorbed on to the silver surface can comprise an organic compound of low molecular weight no higher than about 500. In some embodiments, the organic protective agent can have a molecular weight of no higher than about 300. In some embodiments, the organic chemical which is adsorbed on to the silver surface can comprise an oligomeric or polymeric compound. As a non-limiting example, polyvinylpyrrolidone having an average molecular weight in the range of from about 10,000 to about 1,300,000 is particularly useful for a production of silver nanowires. In some embodiments, polyvinylpyrrolidone having an average molecular weight of 55,000 is chosen.
Some non-limiting examples of organic chemicals which are adsorbed on to the silver surface and can be used by themselves or as mixtures include polyvinylpyrrolidone, polyvinyl alcohol and surfactants, such as e.g. sodium dodecyl sulphate (SDS) laurylamine and hydroxypropylcellulose.
Some non-limiting examples of an organic chemical of low molecular weight which is adsorbed on to the silver surface include fatty acids, in particular fatty acids having at least about 8 carbon atoms. Non-limiting examples of oligomers/polymers for use as the organic chemical which is adsorbed on to the silver surface include homo- and copolymers (including polymers such as e.g. random copolymers, block copolymers and graft copolymers) which comprise units of at least one monomer which comprises one or more O atoms and/or one or more N atoms. A non-limiting class of such polymers for a use in the present invention is formed by polymers which comprise at least one monomer unit which comprises at least two atoms chosen from O and N atoms. Corresponding monomer units can include, for example, at least one hydroxyl group, carbonyl groups, ether bonding group and/or amino group and/or one or more structural elements of the formula: —COO—, —O—CO—O—, —CO—O—CO—, —C—O—C—, —CONR—, —NR—CO—O—, —NR¹—CO—NR²—, —CO—NR—CO—, —SO₂NR— and —SO₂—O—, wherein R, R¹and R²in each case independently represent hydrogen or an organic radical (e.g. an aliphatic or aromatic, unsubstituted or substituted radical which comprises from about 1 to about 20 carbon atoms).
Some non-limiting examples of corresponding polymers which can function as the organic chemical which is adsorbed on to the silver surface include polymers which comprise one or more units derived from the following monomer groups:
(a) monoethylenically unsaturated carboxylic acids of from about 3 to about 8 carbon atoms and salts thereof. This group of monomers includes e.g. acrylic acid, methacrylic acid, dimethylacrylic acid, ethacrylic acid, maleic acid, citraconic acid, methylenemalonic acid, allylacetic acid, vinylacetic acid, crotonic acid, fumaric acid, mesaconic acid and itaconic acid.
The monomers from group (a) can be used either in the form of the free carboxylic acids or in the partly or completely neutralized form. For the neutralization it is possible to use e.g. alkali metal bases, alkaline earth metal bases, ammonia or amines, e.g. sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate, magnesium oxide, calcium hydroxide, calcium oxide, ammonia, triethylamine, methanolamine, diethanolamine, triethanolamine, morpholine, diethylenetriamine or tetraethylenepentamine;
(b) the esters, amides, anhydrides and nitriles of the carboxylic acids mentioned under (a), such as e.g. methyl acrylate, ethyl acrylate, methyl methacrylate, ethyl methacrylate, n-butyl acrylate, hydroxyethyl acrylate, 2- or 3-hydroxypropyl acrylate, 2- or 4-hydroxybutyl acrylate, hydroxyethyl methacrylate, 2- or 3-hydroxypropyl methacrylate, hydroxyisobutyl acrylate, hydroxyisobutyl methacrylate, monomethyl maleate, dimethyl maleate, monoethyl maleate, diethyl maleate, maleic anhydride, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, acrylamide, methacrylamide, N,N-dimethylacrylamide, N-tert-butylacrylamide, acrylonitrile, methacrylonitrile, 2-dimethylaminoethyl acrylate, 2-dimethylaminoethyl methacrylate, 2-diethylaminoethyl acrylate, 2-diethylaminoethyl methacrylate and the salts of the monomers mentioned last with carboxylic acids or mineral acids;
(c) acrylamidoglycollic acid, vinylsulphonic acid, allylsulphonic acid, methallylsulphonic acid, styrenesulphonic acid, 3-sulphopropyl acrylate, 3-sulphopropyl methacrylate and acrylamidomethylpropanesulphonic acid and monomers including phosphonic acid groups, such as e.g. vinyl phosphate, allyl phosphate and acrylamidomethylpropanephosphonic acid; and esters, amides and anhydrides of these acids;
(d) N-vinyllactams, such as e.g. N-vinylpyrrolidone, N-vinyl-2-piperidone and N-vinylcaprolactam;
(e) vinyl acetal, vinyl butyral, vinyl alcohol and ethers and esters thereof (such as e.g. vinyl acetate, vinyl propionate and methyl vinyl ether), allyl alcohol and ethers and esters thereof, N-vinylimidazole, N-vinyl-2-methylimidazoline and the hydroxystyrenes.
Corresponding polymers can also comprise additional monomer units, e.g. units derived from monomers without a functional group, and halogenated monomers, aromatic monomers etc. Non-limiting examples of such monomers include olefins, such as e.g. ethylene, propylene, the butenes, pentenes, hexenes, octenes, decenes and dodecenes, styrene, vinyl chloride, vinylidene chloride, tetrafluoroethylene etc. The polymers for use in the process of the present invention as the organic chemical which is adsorbed on to the silver surface moreover are not limited to addition polymers, but also include other types of polymers, e.g. condensation polymers, such as e.g. polyesters, polyamides, polyurethanes and polyethers, as well as polysaccharides, such as e.g. starch, cellulose and derivatives thereof etc.
In some embodiments, polymers for a use in the present invention include polymers which comprise monomer units from one or more unsubstituted or substituted N-vinyllactams, such as e.g. those having from about 4 to about 8 ring members, e.g. N-vinylcaprolactam, N-vinyl-2-piperidone and N-vinylpyrrolidone. These polymers include homo- and copolymers. In the case of copolymers (which include e.g. random, block and graft copolymers), the N-vinyllactam (e.g. N-vinylpyrrolidone) units can be present in an amount of at least about 10 mol %, e.g. at least about 30 mol %, at least about 50 mol %, at least about 70 mol %, at least about 80 mol % or at least about 90 mol %. As a non-limiting example, the comonomers can include one or more of those mentioned in the preceding paragraphs, including monomers without a functional group (e.g. ethylene, propylene, styrene etc.), halogenated monomers etc.
If the vinyllactam (e.g. vinylpyrrolidone) monomers (or at least a portion thereof) carry one or more substituents in the heterocyclic ring, non-limiting examples of such substituents can include: alkyl groups (e.g. alkyl groups having from 1 to about 12 carbon atoms, e.g. from 1 to about 6 carbon atoms, such as e.g. methyl ethyl, propyl and butyl), alkoxy groups (e.g. alkoxy groups having from 1 to about 12 carbon atoms, e.g. from 1 to about 6 carbon atoms, such as e.g. methoxy, ethoxy, propoxy and butoxy), halogen atoms (e.g. F, Cl and Br), hydroxyl, carboxyl and amino groups (e.g. dialkylamino groups, such as e.g. dimethylamino and diethylamino) and any combinations of these substituents.
Non-limiting specific examples of vinyllactam polymers for use in the present invention include homo- and copolymers of vinylpyrrolidone. In particular, these polymers can include:
(a) vinylpyrrolidone homopolymers, such as e.g. classes K-15 and K-30 having K value ranges of 13-19 or 26-35, corresponding to average molecular weights of about 10,000 and about 67,000;
(b) alkylated polyvinylpyrrolidones;
(c) vinylpyrrolidone/vinyl acetate copolymers;
(d) vinylpyrrolidone/dimethylaminoethyl methacrylate copolymers;
(e) vinylpyrrolidone/methacrylamidopropyltrimethylammonium chloride copolymers;
(f) vinylpyrrolidone/vinylcaprolactam/dimethylaminoethyl methacrylate terpolymers;
(g) vinylpyrrolidone/styrene copolymers; or
(h) vinylpyrrolidone/acrylic acid copolymers.
In the context of the present invention, the use of water-soluble polymers is preferred. The use of polyvinylpyrrolidone (PVP) and polyvinyl alcohol (PVA) as well as mixture of various grades (molecular weights) and copolymers of these polymers is particularly preferred. Suitable comonomers are e.g. N-vinylimidazole, vinyl acetate and vinylcaprolactam. The organic chemicals which are adsorbed can be added to the reaction in the form of a solution or directly as a solid. A suitable solvent for the chemicals is, inter alia, the polyol employed in the reaction, that is to say preferably ethylene glycol and/or glycerol. The addition can take place all at once, in portions or continuously over a relatively long period of time. The addition can take place before or during heating of the polyol or only when the reaction temperature is reached. The concentration in the reaction mixture of the chemicals which are adsorbed may be matched to the addition of the silver salt. To match the concentration of the chemicals which are adsorbed, a series of experiments with various concentrations of the chemicals which are adsorbed (e.g. between 0.05 wt. % and 5 wt. %) is carried out at the corresponding target concentration of silver salt and the morphology (length and diameter) and the yield of silver nanowires is determined. After evaluation of the experiments, the optimum concentration range is roughly demarcated. The concentration of the chemicals which are adsorbed can be optimized still more precisely to the desired morphology and a high yield by means of a second series of experiments in which the concentrations are varied in a narrower range. The preferred concentration range of the chemical which is adsorbed was determined as at least 0.05 wt. % from the experiments. Particularly preferably, the concentration is at least 0.2 wt. %, especially preferably at least 0.5 wt. % and very particularly preferably 1.0 wt. % in the reaction mixture.
To produce silver nanowires by the process according to the invention, the presence of halide ions is absolutely necessary. The halide ions can already be present as an unavoidable trace constituent in the other chemicals added, in particular in the solvent and in the organic chemical which is adsorbed on to metal surfaces, or can be added to the reaction mixture separately. In the scientific literature and in US 2009/0282948 A1 as well as in WO 2009/128973 A2, the presence of traces of halides (in particular F⁻, Cl⁻, Br⁻, I⁻) is described as essential for the formation of silver nanowires.
Studies by the inventors have shown that in addition to the halides mentioned, anions from the group of pseudohalides (in particular SCN⁻, CN⁻, OCN⁻, CNO⁻) can also be employed. The concentrations of the halides/pseudohalides to achieve high yields and favourable aspect ratios (length of the nanowires/diameter of the nanowires) may be matched to the particular recipe and the reaction conditions. The influences on yield and morphology can be investigated by means of series of experiments with varying concentrations of halides and/or pseudohalides under otherwise constant reaction conditions. The preferred concentration range of the halide/pseudohalide ion concentration was determined as 5 ppm to 2,000 ppm from the experiments. Particularly preferably, the concentration is in the range of from 20 ppm to 1,000 ppm and especially preferably in the range of from 50 ppm to 500 ppm in the reaction mixture. Preferred chemicals for establishing the halide/pseudohalide ion concentration are the corresponding ammonium or alkali metal salts or the corresponding acids. The use of NaCl, NaBr, NaI, NaSCN, KCl, KBr, KI and KSCN is particularly preferred.
In addition or alternatively to the salts mentioned, the halide ions can be added by means of other substances which release halide ions. Particularly preferred substances are the elemental halogens and the halogens in higher oxidation levels. Particularly preferred substances are Br₂, NaClO, I₂, NaClO₃, NaBrO₃, NaIO₃, KClO₃, KBrO₃and KIO₃, in each case as a solid or in solution. Under the reaction conditions, the corresponding halide ions are at least partly formed from these substances.
The production of silver nanowires by the process according to one or more embodiments of the invention may benefit from the addition of a chemical which forms a redox pair, chosen from the group of Br⁻/Br₂/BrO₃ ⁻, I⁻/I₂/IO₃ ⁻, or V(0)/V(II)/V(III)/V(IV)/V(V). The addition of Fe(II)/Fe(III) for production of silver nanowires is known from the prior art. The role of iron is discussed in various ways in the literature. In US 2009/0282948 A1 Fe(II) is attributed the task of oxidizing molecular oxygen adsorbed on to the metal surface. The Fe(III) thereby formed is reduced again to Fe(II) by the solvent/reducing agent. The redox pair is automatically converted back into the reduced form by the reaction conditions, in particular by the reducing agent present in excess. In this respect, the chemical form in which the redox pair is added is unimportant. Under the prevailing reaction conditions, the reduced form of the redox pair is formed from the “chemical which produces a redox pair”. In the case of iron, the addition of Fe(II) or Fe(III) can take place without an influence on the success of the synthesis.
Studies by the inventors have shown that replacing iron by the abovementioned chemicals which form redox pairs renders possible at least the same, usually higher yields of silver nanowires. Chemicals which form redox pairs and are suitable in the context of the invention provide bromine, iodine and/or vanadium. Under the reaction conditions, the species necessary for participation in the silver nanowire synthesis are formed from the chemicals added. In the case of the metal vanadium, the addition can take place in the form of the native metal or the corresponding salts. By using the halides of vanadium, the metering of the amount of halide required for the reaction can be combined with the metering of the redox pair. In order to make the additions independent of each other, the metering of other salts of the metal, such as e.g. the oxides, hydroxides, sulphates or the nitrates, is preferred.
It has been found, surprisingly, that the halogens bromine and iodine can take over the role of the redox pair in the reaction. Bromine and iodine can be added to the reaction in elemental form, as bromide or iodide or bromate or iodate. Particularly preferably, in addition to iodine a further halide/pseudohalide is employed, as a result of which particularly high yields are achieved. Iodine is preferably added in the form of KI. Bromine is preferably added in the form of KBr. As in the case of the addition of halides/pseudohalides, careful matching of the concentration of chemicals which form redox pairs to the particular recipe and the reaction conditions is necessary. The sometimes dramatic influences on the yield and morphology of the silver nanowires can be investigated by means of series of experiments with varying concentrations of chemicals which form redox pairs under otherwise constant reaction conditions. The preferred concentration range of the chemicals which form redox pairs was determined as 5 ppm to 2,000 ppm from the experiments. Particularly preferably, the concentration is in the range of from 20 ppm to 1,000 ppm and especially preferably in the range of from 50 ppm to 500 ppm in the reaction mixture.
In the reduction of silver salts with polyols, one proton is liberated with each silver atom reduced. According to Le Chatelier's principle, the rate of reaction can be influenced by addition of alkali or acid. The formation of the silver nanowires proceeds in competition with the formation of other ordered structures and irregular silver bodies. If the rates of reaction are too high, the content of irregular bodies increases. By addition of acid, the rate of reaction can be matched to a degree suitable for the formation of silver nanowires. In principle, many inorganic and also organic acids are suitable for addition in the processes according to the invention. Hydrogen halide acids, in particular HCl, HBr and HI, and nitric acid are preferably employed. The addition of the acid is to be matched to the particular recipe and the reaction conditions. Preferred acid concentration ranges can be determined by means of series of experiments with varying addition of acid under otherwise constant reaction conditions. The preferred concentration range of the acid was determined from the experiments as 0.01 wt. % to 2.0 wt. %, particularly preferably as 0.02 wt. % to 1.0 wt. % and especially preferably as 0.05 wt. % to 0.5 wt. % in the reaction mixture.
The precise reaction conditions of the processes according to the invention are variable within wide limits.
The processes are carried out at elevated temperatures of more than 100° C. The studies by the inventors have shown that carrying out the processes according to the invention at temperatures above 145° C. gives significantly higher yields of silver nanowires than are achieved at reaction temperatures below 145° C. Preferably, the processes according to the invention are carried out at temperatures of between 145° C. and 200° C., further preferably between 150° C. and 180° C., particularly preferably between 155° C. and 170° C.
Most of the chemicals can be added in any desired sequence. As already stated, the halide/pseudohalide additions, the additions which form redox pairs and the additions which are adsorbed on to the metal surface can take place directly to the cold polyol, during the heating phase or to the hot polyol. The same applies analogously to any necessary addition of acid. The silver salt is advantageously first added at the reaction temperature. The silver salt and chemicals which are adsorbed can be added simultaneously, in direct succession, alternately in portions or simultaneously over a relatively long period of time.
The formation of the silver nanowires can be readily monitored visually when carrying out the process according to the invention. At the end of the reaction, a nacreous, iridescent dispersion is present. The evolution of gas which occurs during the reaction ceases. When the reaction has ended, the reaction mixture is allowed to cool, while stirring. To separate off the silver nanowires, the dispersion can be diluted with a suitable solvent. After dilution, the silver nanowires sediment. Suitable solvents are, for example, alcohols, such as EtOH and isopropanol, or also water. The silver nanowires which have been decanted off can be kneaded, extruded or stirred into polymers for further processing.
The silver nanowires produced by one of the abovementioned processes preferably have a length of from 1 μm to 200 μm, a diameter of from 50 nm to 1,300 nm and an aspect ratio (length/diameter) of at least 5.
The silver nanowires produced by the processes according to the invention can be used for numerous applications. The silver nanowires are particularly advantageously employed where their unique physical, biological and chemical, in particular the electrical, electronic, heat-conducting, mechanical, optical, antimicrobial and catalytic properties can be advantageously utilized.
A preferred field of use for the silver nanowires is the field of electrically conductive layers or materials. The effects resulting from the anisotropy of the particles, such as e.g. low percolation limit and high conductivity at a low filler content, can be advantageously utilized here. Lacquer or polymer layers having a filler content of from about 1 vol. % of the silver nanowires according to the invention already show the electrical conductivity of a metallic conductor. Conductivity at such low filler contents is often accompanied by a high optical transparency of the materials and scarcely changed mechanical properties of the carrier systems. Further fields of use are therefore to be found in the fields of touch panels, antistatic treatment of displays, electromagnetic shielding, electrodes for organic and inorganic electroluminescent displays, generally electrodes for flexible displays, electrodes and strip conductors for photovoltaic installations, generally antistatic films, e-paper, LCD displays, generally transparent conductive films, printable wiring on printed circuit boards and flexible substrates, multilayer wiring, production and repair of electronic components and switching circuits, printable or extrudable antennae, in particular for RFID systems and strip conductors, bonding, low temperature sinter contacts, conductive adhesives, in particular conductive hot-melt adhesives. The use of the silver nanowires in electrically conductive systems which can be applied by printing, such as e.g. inks and pastes, optionally in a mixture with other conductive materials, such as e.g. silver micro- and/or silver nanoparticles and/or conductive carbon species (carbon nanotubes, carbon black, fullerenes, graphene, carbon fibres etc.) is particularly advantageous. Such systems can be used inter alia in the field of printed strip conductors, for connection of thin layer systems or for the production of printable electronic components, such as e.g. capacitors or transistors.
The good electrical conductivity of systems containing silver nanowires renders possible the use thereof in the field of electric heating. In particular optically largely transparent, mechanically stable, interference pattern-free, heatable layers have been of technical interest for a long time. In principle, all synthetic and natural polymers, thermoplastics and thermosets as well as dispersions are possible for the preparation of such conductive polymers. Thermoplastic, synthetic polymers having a filler content of silver nanowire are suitable in particular. The polymers preferably originate from the group of polyolefins (preferably polyethylene (PE), polypropylene (PP)), acrylonitrile/butadiene/styrene (ABS), from the group of polyamides (PA), polylactate (PLA), polymethyl methacrylate (PMMA), polycarbonate (PC), from the group of polyesters (e.g. polyethylene terephthalate (PET)), polystyrene (PS), polyether ketone (PEEK) and polyvinyl chloride (PVC). Synthetic fibres having a filler content of silver nanowires are likewise particularly suitable for the production of heatable layers. In this context, fibres of polyamide, polyester or of polypropylene are particularly preferred. Films having a filler content of silver nanowires are furthermore particularly suitable for the production of heatable systems. Films of PET are particularly preferred. Other outstandingly suitable systems for the production of heatable and optionally highly transparent layers are monomer solutions having a recipe with silver nanowires, such as e.g. monoalkyl acrylates or solutions of low molecular weight polymers, such as e.g. polyglycols, polyalcohols, polyketones and also technical grade lacquer resins.
Further preferred fields of use for the silver nanowires produced according to the invention are to be found in the production of electrically conductive fibres. In this context, the fibres can be made of thermoplastic polymers, corresponding to the above list of thermoplastics, or can also be natural fibres, such as cellulose, viscose, cotton, silk etc. The electrically conductive fibres can likewise also be produced via electrospinning processes, and are thus produced from soluble amorphous and crystal-forming polymers, such as polyurethanes, co-polyesters, polyethylene oxide, polyvinyl alcohol, polyacrylonitrile, polyimide etc. The electrically conductive fibres are then used in heating, sensor and data transfer applications which can be processed by textile technology.
Further preferred fields of use for the silver nanowires produced according to the invention lie in the field of papermaking. The silver nanowires according to the invention can thus be employed both as a filler in the paper pulp, as an additive in paper brush-on paints, and as a subsequent print or a coating deposited subsequently by chemical or physical means.
Further preferred fields of use for the silver nanowires produced according to the invention are to be found inter alia in the fields of thermal conduction, thermally conductive pastes, thermally conductive adhesives, directional thermal conductivity, highly thermally conductive regions in high power LED and transistors and in thermogenerators.
The antimicrobial action of silver, in particular nanoparticulate metallic silver, is the object both of current research and of industrial production. From a certain threshold concentration, the silver nanowires produced by the processes according to the invention can impart antimicrobial properties to correspondingly finished objects.
Due to the plasmon effect which occurs at the interface of metals, metal and in particular silver nanoparticles are suitable for use in surface plasmon resonance analysis (SPR analysis) and in surface enhanced Raman spectroscopy (SERS). Due to their geometric anisotropy, silver nanowires can be employed both individually and as an array for SPR analysis and for SERS and quite generally in the field of photonics.
The silver nanowires according to the invention can moreover be employed in optical filters.

EXAMPLES

The embodiment examples are given in the following to illustrate the invention and to explain its advantages. These embodiment examples are to be explained in more detail in connection with the drawing. It goes without saying that these statements are not intended to limit the invention.
The following chemicals are used in the context of the examples described:

- Ethylene glycol, for synthesis, Carl Roth GmbH & Co. KG, Karlsruhe, Germany
- AgNO₃, analytical grade, Merck KGaA, Darmstadt, Germany
- PVP, Luvitec K30, BASF SE, Ludwigshafen, Germany
- NaCl, analytical grade, Carl Roth GmbH & Co. KG, Karlsruhe, Germany
- KI, analytical grade, Merck KGaA, Darmstadt, Germany
- CuSO₄*5H₂O, analytical grade, Merck KGaA, Darmstadt, Germany
- NaBr, analytical grade, Merck KGaA, Darmstadt, Germany
- KSCN, analytical grade, Merck KGaA, Darmstadt, Germany
- HNO₃, 65%, Baker analyzed, J. T. Baker, Deventer, Holland
- 1-Methoxy-2-propanol, Friedrich Scharr K G, Stuttgart, Germany
- Ethanol, Carl Roth GmbH & Co. KG, Karlsruhe, Germany
- Ethyl acetate, Carl Roth GmbH & Co. KG, Karlsruhe, Germany
- Cellulose nitrate, ethanolic, Dow Wolff Cellulosics GmbH, Bomlitz, Germany

Example 1

30.0 g of ethylene glycol are heated to 170° C. in a round-bottomed flask in an oil bath. 2.5 g of Luvitec K30, 6 mg of NaCl and 6 mg of KI are dissolved, while stirring. 120 μl of HNO₃are added. Thereafter, 2.80 g of AgNO₃are added to the hot reaction mixture, while stirring. After about 6 min, the reaction has ended. After cooling, the reaction mixture is added to 150 ml of deionized water. On the following day, the supernatant is decanted off and dried. The dried residue is investigated for its morphology by means of SEM. It consists practically exclusively of silver nanowires having a length of between 3 μm and 35 μm and a diameter of between 150 nm and 500 nm. A typical SEM photograph of the silver nanowires obtained is reproduced in FIG. 1. The yield of silver nanowires is 95% of the amount of silver employed.

Example 2

The experimental procedure is analogous to that described in Example 1. In deviation from Example 1, the reaction temperature is 155° C. The reaction has ended after about 15 min. The silver nanowires formed have a length of from 3 μm to 60 μm and a diameter of from 150 nm to 500 nm. The yield of silver nanowires is 90% of the amount of silver employed.

Example 3

The experimental procedure is analogous to that described in Example 1. In deviation from Example 1, the reaction temperature is 130° C. The reaction has ended after about 120 min. The silver nanowires formed have a length of from 3 μm to 200 μm and a diameter of from 150 nm to 500 nm. The coarse spherical silver particles formed alongside the silver nanowires cannot be separated off from the nanowires by means of sedimentation. The yield of silver nanowires is estimated from the weight, incorporating the silver particles detected in the SEM photograph. The yield of silver nanowires is about 25% of the amount of silver employed.
The evaluation of the experiments described in Examples 1 to 3 shows that with the addition of chloride and iodide, silver nanowires can be produced with yields of at least 90% at temperatures of 155° C. and 170° C. At temperatures of 130° C., the yield falls dramatically to approx. 25%. In some cases longer nanowires are formed at low temperatures. By adapting the process procedure, longer rods can likewise be obtained at higher temperatures at a simultaneously high yield. In the case of a two-stage addition of the silver salt at a reaction temperature of 170° C., silver nanowires having a length of from 4 μm to 110 μm and an unchanged diameter of from 150 nm to 500 nm can be obtained. Investigations to optimize the chloride and iodide concentration under the experimental conditions described have shown that the optimum yield lies in the range of concentrations stated in Example 1. Both higher and lower concentrations of chloride and iodide lead to reduced yields of silver nanowires.

Example 4

30.0 g of ethylene glycol are heated to 170° C. in a round-bottomed flask in an oil bath. 2.5 g of Luvitec K30 and 6 mg of VCl₃are dissolved, while stirring. 120 μl of HNO₃are added. Thereafter, 2.80 g of AgNO₃are added to the hot reaction mixture, while stirring. After about 6 min, the reaction has ended. After cooling, the reaction mixture is added to 150 ml of deionized water. On the following day, the supernatant is decanted off and dried. The dried residue is investigated for its morphology by means of SEM. It consists practically exclusively of silver nanowires having a length of between 3 μm and 35 μm and a diameter of between 150 nm and 500 nm. The yield of silver nanowires is 80% of the amount of silver employed.
From the experiment described in Example 4, it can be deduced that vanadium can likewise be employed in the processes according to the invention as an element which forms a redox pair.

Example 5

The experimental procedure is analogous to that described in Example 1. In deviation from Example 1, only 6 mg of NaCl and no chemical which forms a redox pair (KI in Example 1) are added. The reaction has ended after about 6 min. The silver nanowires formed have a length of from 1 μm to 15 μm and a diameter of from 150 nm to 500 nm. The coarse spherical silver particles formed alongside the silver nanowires cannot be separated off from the nanowires by means of sedimentation. The yield of silver nanowires is estimated from the weight, incorporating the silver particles detected in the SEM photograph. The yield of silver nanowires is about 50% of the amount of silver employed.
The evaluation of the experiment from Example 5 shows that the addition of chloride alone leads to significantly reduced yields of silver nanowires. Further experiments with higher and lower chloride concentrations give even lower yields of silver nanowires under the experimental conditions described.

Example 6

The experimental procedure is analogous to that described in Example 4. In deviation from Example 4, instead of NaCl, an amount of 9 mg of NaBr is added. The reaction has ended after about 6 min. The silver nanowires formed have a length of from 3 μm to 35 μm and a diameter of from 150 nm to 500 nm. The yield of silver nanowires is 95% of the amount of silver employed.
According to Example 6, the addition of chloride can be replaced by addition of bromide without losses in the yield.

Example 7

The experimental procedure is analogous to that described in Example 4. In deviation from Example 4, instead of NaCl, an amount of 8 mg of KSCN is added. The reaction has ended after about 6 min. The silver nanowires formed have a length of from 1 μm to 25 μm and a diameter of from 500 nm to 1,300 nm. The yield of silver nanowires is 85% of the amount of silver employed.
According to Example 7, the addition of chloride can be replaced by thiocyanate with low losses in the yield. It is striking that considerably thicker wires are formed by addition of thiocyanate compared with an addition of halide.

Example 8

30.0 g of ethylene glycol are heated to 170° C. in a round-bottomed flask in an oil bath. 2.5 g of Luvitec K30, 6 mg of NaCl and 6 mg of KI are dissolved, while stirring. 120 μl of HNO₃are added. Thereafter, 2.80 g of AgNO₃are added to the hot reaction mixture, while stirring. After about 10 min, a further 1.4 g of AgNO₃are added to the hot reaction mixture. After a further 10 min, 1.4 g of AgNO₃are again added to the hot reaction mixture. After a further 10 min, 1.4 g of AgNO₃are again added to the hot reaction mixture. After about 45 min, the reaction has ended. After cooling, the reaction mixture is added to 150 ml of deionized water. On the following day, the supernatant is decanted off and dried. The dried residue is investigated for its morphology by means of SEM. It consists practically exclusively of silver nanowires having a length of between 3 μm and 35 μm and a diameter of between 150 nm and 500 nm. The yield of silver nanowires is 95% of the amount of silver employed.
By the procedure described in Ex. 8, of stepwise addition of silver salt, very high concentrations of silver nanowires can be achieved in the reaction solution. This procedure renders possible a particularly economical production.

Example 9

30.0 g of ethylene glycol are heated to 170° C. in a round-bottomed flask in an oil bath. 2.5 g of Luvitec K30 and 6 mg of NaBr are dissolved, while stirring. 120 μl of HNO₃are added.
Thereafter, 2.80 g of AgNO₃are added to the hot reaction mixture, while stirring. After about 6 min, the reaction has ended. After cooling, the reaction mixture is added to 150 ml of deionized water. On the following day, the supernatant is decanted off and dried. The dried residue is investigated for its morphology by means of SEM. It consists practically exclusively of silver nanowires having a length of between 2 μm and 35 μm and a diameter of between 150 nm and 500 nm. The yield of silver nanowires is 95% of the amount of silver employed.

Example 10

Silver nanowires are produced according to Example 1. In deviation from Example 1, after the synthesis 150 ml of ethanol and not 150 ml of water are added to the nanowire dispersion. After the wires have settled, the supernatant is decanted off and discarded. The sediment is stirred up again with 150 ml of ethanol and left to stand for sedimentation. After the wires have settled, the supernatant is discarded. On a portion of the homogeneously stirred, damp residue, the dry weight of the residue is determined as 70 wt. %. 1.0 g of this ethanolic silver nanowire dispersion is added to 14 g of a lacquer base of 63 wt. % of 1-methoxy-2-propanol, 16 wt. % of ethanol, 11 wt. % of ethyl acetate and 11 wt. % of cellulose nitrate and the mixture is stirred until homogeneous. The lacquer obtained is drawn with a glass rod on to a glass carrier with a width of 30 mm. The dried lacquer layer shows a surface resistance of 50 ohm. The dry lacquer comprises approx. 31 wt. % of silver nanowires, which corresponds to about 4.1 vol. % of silver.

Claims

1. A process for the production of silver nanowires, the process comprising:

a) providing of a reaction mixture comprising

a polyol,

an organic chemical adsorbed on to a silver surface,

a chemical which forms one or more of a halide and a pseudohalide, wherein the chemical which forms a halide is a salt of one or more of the halides Cl⁻, Br⁻ and I⁻ and wherein the chemical which forms a pseudohalide is a salt of one or more of the pseudohalides SCN⁻, CN⁻, OCN⁻ and CNO⁻,

a chemical which forms a redox pair, the chemical selected from the group consisting of bromine, iodine, vanadium and mixtures thereof,

b) adding a silver salt in an amount such that the concentration of silver in the reaction mixture is at least 0.5 wt. %, based on the total weight of the reaction mixture, the silver salt being added at a temperature of the reaction mixture of at least 100° C., and

c) maintaining the temperature of the reaction mixture of at least 100° C. for the duration of the reaction.

2. The process according to claim 1, wherein the silver salt is added in an amount such that the concentration of silver in the reaction mixture is at least 1.0 wt. %, based on the total weight of the reaction mixture.

3. The process of claim 1, wherein the reaction mixture comprises one or more of the chemical which forms the halide and the chemical which forms the pseudohalide in an amount such that the concentration of the halide and/or of the pseudohalide in the reaction mixture is in a range of from 5 ppm to 2,000 ppm.

4. The process of claim 1, wherein the chemical which forms the halide comprises one or more of NaCl, NaBr, NaI, KCl, KBr and KI.

5. The process of claim 1, wherein the chemical which forms the pseudohalide comprises one or more of NaSCN and KSCN.

6. The process of claim 1, wherein the reaction mixture provided in step a) additionally comprises an iron salt, the iron salt forming an Fe(II)/Fe(III) redox pair.

7. The process of claim 1, wherein the reaction mixture provided in step a) additionally comprises a copper salt, the copper salt forming a redox pair.

8. The process of claim 1, wherein the chemical which forms a redox pair comprises one or more of elemental vanadium, a vanadium oxide, a vanadium hydroxide, a vanadium sulphate, a vanadium nitrate, a vanadium chloride and mixtures of these compounds.

9. The process of claim 8, wherein the redox pair comprises one or more of Br⁻/Br₂/BrO₃ ⁻, I⁻/I₂/IO₃ ⁻ and/or V(0)/V(II)/V(III)/V(IV)/V(V).

10. The process of claim 1, wherein the concentration in the reaction mixture of the chemical which forms a redox pair is in a range of from 5 ppm to 2,000 ppm.

11. The process of claim 1, wherein the reaction mixture provided in step a) additionally comprises at least one inorganic or organic acid.

12. The process of claim 11, wherein the acid comprises one or more of HCl, HBr, HI and HNO₃.

13. The process of claim 11, wherein the addition of the acid takes place in an amount such that the concentration of acid in the reaction mixture is in a range of from 0.01 wt. % to 2.0 wt. %, based on the total weight of the reaction mixture.

14. The process of claim 1, wherein the silver salt is added at a temperature of the reaction mixture of at least 145° C.

15. The process of claim 1, wherein in step c) the temperature of the reaction mixture of at least 145° C. is maintained for the duration of the reaction.

16. The process of claim 1, wherein after step c) step

d) isolating silver nanowires from the reaction mixture,

wherein a suitable solvent is added for isolation of the silver nanowires from the reaction mixture and the supernatant is then decanted off.

17. The process of claim 16, wherein after step c) and before step d) step

c₁) cooling the reaction mixture

to room temperature.

18. A process for the production of silver nanowires, the process comprising:

preparing a reaction mixture comprising a polyol, an organic chemical adsorbed on to a silver surface, a chemical which forms one or more of a halide and a pseudohalide, a chemical which forms a redox pair and at least one of an inorganic acid and an organic acid, wherein the chemical which forms a halide is a salt of one or more of the halides Cl⁻, Br⁻ and I⁻ and the chemical which forms a pseudohalide is a salt of one or more of the pseudohalides SCN⁻, CN⁻, OCN⁻ and CNO⁻ and is present in the range of about 5 ppm to about 2000 ppm and wherein the chemical which forms a redox pair is selected from the group consisting of bromine, iodine, vanadium and mixtures thereof;

adding a silver salt to the reaction mixture at a temperature of at least 100° C., the silver salt present in a concentration of at least 0.5 wt. % based on the total weight of the reaction mixture;

maintaining the reaction mixture at a temperature of at least 100° C. for the duration of the reaction to form silver nanowires;

cooling the reaction mixture to about room temperature; and

isolating the silver nanowires from the cool reaction mixture, wherein isolating the silver nanowires comprises adding a suitable solvent to the cool reaction mixture and decanting off a supernatant.