EP2422001A1

EP2422001A1 - Water-based production of metal-oxide and metal nanofibers

Info

Publication number: EP2422001A1
Application number: EP10718114A
Authority: EP
Inventors: Florian Thomas; Felix Major; Evgueni Klimov
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 2009-04-21
Filing date: 2010-04-21
Publication date: 2012-02-29
Also published as: WO2010122049A1

Abstract

The invention relates to a method for producing metal-oxide fibers or metal fibers having a diameter in the range of 0.1 to 999 nm by electrospinning a solution containing one or more polymers, one or more metallic salts, one or more complexing agents, and a solvent, comprising the following steps: (a) providing a solution containing one or more polymers, one or more metallic salts, one or more complexing agents, and a solvent, wherein the solution has a water content of at least 50 wt%, (b) electrospinning the solution, and (c) removing the polymer.

Description

Water-based production of metal oxide and metal nanofibers

description

The present invention relates to a process for the production of metal oxide or metal fibers having a diameter in the range of 0.1 to 999 nm by electrospinning mixtures of one or more polymer (s), one or more metal salt (s), a or more complexing agent (s) in a solvent and then removing the polymer. The invention further relates to a process for the water-based production of uniform, discrete nanofibers with an aspect ratio - ratio of fiber length to fiber diameter - of> 10 to 1.

Nanofibres are becoming increasingly important in textile production, optics, electronics, biotechnology, pharmacy, medicine and plastics technology as filtration and separation media. The term "nanofibers" refers to fibrous structures whose diameter lies in a range from about 0.1 to 999 nm (also referred to as nano-scale) .The term also refers to nanostructures such as nanowires and nanotubes, both of which have a nanoscale cross-section exhibit.

The currently common method for the production of nanofibers is the so-called electrospinning. Here, a polymer solution or a polymer melt containing a metal salt and optionally further additives, brought by means of two electrodes in a strong electric field. Electrostatic charges cause local instabilities in the solution, which are first formed into conical structures and then into fibers. As the fibers move toward an electrode, some of the solvent evaporates and the fibers are additionally stretched. In the subsequent calcination of the fibers, the transformation of the metal salt into the corresponding metal oxide takes place. In this way, oxide nanofibers are obtained with a diameter of <1 micron. Of technical interest is the use of such fibers in filtration applications, as part of gas sensors, as well as in catalytic applications.

The production of nanofibers, in particular of ZnO nanofibers, is described by Sidd heswaran et al. Technol. 2006, 41, 447-449) Siddheswaran et al., first prepare a precursor solution consisting of polyvinyl alcohol, zinc acetate and water, which are heated at elevated temperature Subsequently, this precursor solution is spun into nanofibers with a needle spinning electrospinning device. The nanofibers are then calcined to ZnO fibers. The ZnO nanofibers produced by this process have a very uneven surface structure and varying diameters and are fused together at the contact points, resulting in a low aspect ratio results.

A method for producing SnC> ₂ nanofibers is described by Zhang et al. Zhang et al., describe a precursor solution consisting of polyvinyl alcohol, tin (IV), and "Zhang et al."("Production and Ethanol-sensing Properties of Microspherical SnC> ₂ Nanofibers"). chloride and water, make and spun them with an electrospinning device to nanofibers. these nano fibers are then calcined to form SnO ₂ nanofibers. the SnO using this method manufacturing placed ₂ nanofibers have varying diameter and are also fused together at the contact points , which also results in a low aspect ratio.

WO 2007/022770 A1 discloses a process for the production of metal nano and metal mesofibers. This production process comprises the electrospinning of a polymer / metal salt solution and subsequent calcining. An advantage of this method over the methods described in the literature is the high ratio of metal to polymer of 3 to 1 to 1 to 1 (w / w) in the precursor solution. The ratio of metal to polymer is understood to mean the ratio of the mass of the polymer component to the mass of the metal in the metal salt. However, it is known to those skilled in the art that the described advantage can only be realized if a very viscous polymer component having only a low solubility in water is used. The application examples describe the preparation of metal and metal oxide nanofibers using the water-insoluble polymer polyvinyl butyral (PVB). This requires the less economical or ecological use of an organic solvent, in this case isopropanol, as the main component of the precursor solution.

The object of the present invention is to provide an improved process for the production of metal oxide or metal fibers having a diameter in the range of 0.1 to 999 nm. Furthermore, it is an object of the invention to provide a method for the production of metal oxide or metal fibers with a diameter in the nanometer range, by means of which nanofibers with a diameter in the range of 0.1 to 999 nm and the highest possible aspect ratio can be obtained.

This object is achieved by a process for the preparation of metal oxide fibers having a diameter in the range of 0.1 to 999 nm by electrospinning a solution containing one or more polymer (s), one or more metal salt (s), one or more complexing agents and a Solvent comprising the steps: (a) providing a solution comprising one or more polymer (s), one or more metal salt (s), one or more complexing agents and a solvent, the solution having a water content of at least 50% by weight,

(b) electrospinning the solution and

(c) removing the polymer.

It turned out that with the aid of the method described above metal oxide fibers with a diameter in the nanometer range can be obtained whose aspect ratio - ratio of fiber length to fiber diameter - is greater than 10 to 1. The addition of the complexing agent and the water content of at least 50% by weight in the solution provided in process step (a) make it possible to obtain uncrosslinked or disperse oxide nanofibers which are superior to those of the prior art.

In general, when the solution is provided in process step (a), the at least one polymer is first dissolved in the solvent having a water content of at least 50% by weight. Subsequently, the metal salt or a metal salt solution is added to this solution.

Polymers selected from the group consisting of polyethers, polyethylene oxides, polyvinyl alcohols, polyvinyl acetates, polyvinylpyrrolidones, polyacrylic acids, polyurethanes, polyactides, polyglycosides, polyvinylformamides, polyvinylamines, polyvinylimines and polyacrylonitriles, or mixtures of two or more of these proved above polymers. Polyvinyl alcohols, polyvinyl acetates, polyvinylpyrrolidones have proven to be preferred polymers. Also suitable as copolymers of the abovementioned polymers, such as polyvinyl alcohol-polyvinyl acetate copolymers, or mixtures of copolymers have been found.

In general, the one or more polymers within the meaning of the present invention are thermal, chemical, radiation-chemical, physical, biological, plasma-degradable, ultrasound-degradable or polymeric material which can be degraded by extraction with a solvent. The proportion of the polymer in the solution provided in step (a) can vary over a wide range. In general, the proportion of the polymer is in the range of 1 to 20 wt .-%, preferably in the range of 5 to 15 wt .-% and particularly preferably in the range of 6.0 to 10 wt .-%, based on the in process step ( a) provided solution. The at least one metal salt is inorganic or organic salts of one or more metal (s) selected from the group Cu, Ag, Au, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn , Re, Fe, Ru, Ni, Pd, Co, Rh, Ir, Sb, Sn, In, Al, Ga and Zn. According to a preferred embodiment of the invention, the metal is selected from the group Sb, Sn, In, Al, Ga and Zn According to a preferred embodiment, it is a mixture containing tin (Sn) and indium (In) or a mixture containing indium (In) or tin (Sn) and antimony (Sb). According to another preferred embodiment of the invention, a mixture containing zinc (Zn) and aluminum is used. The proportion of the metal salt or metal salts in the solution provided in process step (a) can vary over a wide range. In general, the proportion of the metal salt or metal salts is in the range of 1 to 20 wt .-%, preferably in the range of 5 to 15 wt .-% and particularly preferably in the range of 6 to 10 wt .-%, based on the Process step (a) provided solution. The ratio of metal salt (s) to polymer is generally 1 to 1 (w / w).

Inorganic salts in the context of the present invention are, for example, chlorides, sulfates and nitrates, provided that these combinations of inorganic anions and the respective metal cations are present. Organic salts in the context of the present invention are salts of carboxylic acids, for example formates, acetates, citrates and acetylacetonates with the corresponding metals, if combinations of organic anions and the respective metal cation are present.

Generally, the solvent or solvent mixture containing at least 50% by weight of water is selected so that both the one or more polymers) and the one or more metal salts are soluble in it. Soluble is understood to mean that the polymer and the metal salt have a solubility of at least in each case 1% by weight in the corresponding solvent or solvent mixture. It is known to the person skilled in the art that he must coordinate the polymer, metal salt and solvent with one another for this purpose. He can do this with the help of his general knowledge.

Generally, the solvent used is either water or a solvent mixture having a water content of at least 50% by weight. The solvents which are used for the preparation of solvent mixtures with water are miscible with water and are preferably selected from the group consisting of methanol, ethanol, ethanediol, n-propanol, 2-propanol, n-butanol, isobutanol, tert-butanol, cyclohexanol , Formic acid, acetic acid, trifluoroacetic acid, diethylamine, diisopropylamine, phenylethylamine, acetone, acetylacetone, acetonitrile, diethylene glycol, formamide, dimethylformamide (DMF), dimethyl sulfoxide (DMSO), toluene, dimethylacetamide, N-methylpyrrolidone (NMP) and tetrahydrofuran. The solvent is preferably tel used to prepare solvent mixtures with water, one or more selected from methanol, ethanol, ethanediol and iso-propanol. The ratio (w / w) of water to the other solvent (s) is generally 9 to 1 to 2 to 1, preferably 9 to 1 to 5 to 1. According to a particularly preferred embodiment, the solvent is water.

The complexing agent present in the solution provided in step (a) is generally an organic or inorganic compound capable of complexing metal cations. This is generally selected from monocarboxylic acids, dicarboxylic acids, tricarboxylic acids, hydroxycarboxylic acids, ketocarboxylic acids, diketones, amino acids, aminopolycarboxylic acids, polymer-bound carboxylic acids, amines, diamines, ammonia, nitrate ions, nitrite ions, halide ions and hydroxide ions or a salt of the abovementioned acids. The complexing agents are preferably monocarboxylic acids such as formic acid, acetic acid and propionic acid, dicarboxylic acids such as oxalic acid, malonic acid, glutaric acid and tartaric acid, tricarboxylic acids such as citric acid, hydroxycarboxylic acids such as tartaric acid and ketocarboxylic acids such as pyruvic acid, diketo - ne, such as acetylacetone and aminopolycarboxylic acids such as ethylenediaminetetraacetic acid or a salt of the abovementioned acids. According to a further preferred embodiment, the complexing agent is a compound selected from ethylenediaminetetraacetic acid, triethanolamine, formic acid, acetic acid, pantothenic acid, folic acid, biotin, arachidonic acid, malonic acid, α-aminobutyric acids, β-aminobutyric acids, γ-aminobutyric acids, Glutathione, isocitric acid, cis- and trans aconitic acid, hydroxycitric acid, nicotinic acid, benzoic acid, oxalic acid, mesoxalic acid, oxaloacetic acid, succinic acid, sorbic acid, propane tricarboxylic acid, crotonic acid, itaconic acid, acrylic acid, methacrylic acid, mesaconic acid, phenylacetic acid, salicylic acid, 4-hydroxybenzoic acid , Cinnamic acid, mandelic acid, 2-furancarboxylic acid, acetic acid, glucuronic acid, gluconic acid, glucaric acid, glyceric acid, glycolic acid, butyric acid, isobutyric acid, hexanoic acid, heptanoic acid, octanoic acid, decanoic acid, undecanoic acid, dodecanoic acid, tetradecanoic acid, palmitic acid, stearic acid, methacrylic acid, urocans acid, rinsäure Pyrrolidonsäure, glutaric acid, adipic acid, pimelic acid, Sube-, the above-mentioned acids azelaic acid, sebacic acid, phthalic acid, terephthalic acid, 2- and 3-hydroxybenzoic acid, lactic acid and citric acid or a salt.

The complexing agent is preferably citric acid and / or acetic acid and / or acetylacetone or one or more salts of the abovementioned acids.

In one embodiment of the invention, the molar ratio of metal ion in the metal salt to complexing agent is that provided in step (a) Solution in the range of 1: 0.2 to 1: 1, preferably in the range of 1: 0.4 to 1: 0.6, particularly preferably the molar ratio of metal ion in the metal salt to complexing agent in the in process step (a) provided solution 1: 0.5.

The solution provided in step (a) may further comprise surfactants and optionally bases such as ammonia or quaternary ammonium salts.

The preparation of the nanofibers - consisting of one or more polymer (s), one or more metal salt (s), one or more complexing agent (s) from the solution provided in process step (a) takes place by means of electrospinning out of this solution. Electrospinning processes are known to the person skilled in the art. Electrospinning can be carried out, for example, with an electrospinning device whose structure is similar to or similar to the electrospinning devices described in the literature (Xia et al., Adv. Mater. 2004, 16, 1151). However, electrospinning can also be carried out with an electrospinning device, as described, for example, in WO 2006/131081 A1 or WO 2007/137530 A2.

The removal of the polymer in step (c) of the process according to the invention is generally carried out thermally, chemically, by radiation chemistry, physically, biologically, by plasma, ultrasound or by extraction with a solvent. The removal of the polymer preferably takes place thermally by means of calcining. The calcination is carried out generally over a period of 1 to 24 hours, preferably over a period of 3 to 6 hours at a temperature of 250 to 900 ⁰ C, preferably at a temperature of 300 to 800 ⁰ C, more preferably at a temperature of 400 to 700 ° C under an atmosphere which may contain in addition to nitrogen additionally oxygen or hydrogen. According to preferred embodiments of the invention, the calcination is carried out in an atmosphere containing about 78% by volume of nitrogen, 21% by volume of oxygen or in pure nitrogen or in a mixture of nitrogen with hydrogen (1 to 4% by volume) or a mixture of nitrogen with oxygen (> 21 vol.%). The metal oxide fibers obtained after calcining have a diameter in the range of 0.1 nm to 999 nm, preferably in the range of 10 nm to 300 nm, particularly preferably in the range of 50 nm to 200 nm, and an aspect ratio of> 10 to 1, preferably> 100 to 1. The fibers thus have a length in the range of several μm up to a few mm.

According to one embodiment of the invention, the nanofibers are dried after electrospinning and before calcination. The drying of the nanofibers is usually carried out at a temperature of 80 to 180 ⁰ C, preferably at a temperature of 100 to 150 ⁰ C in ambient atmosphere in air or in vacuo. According to a further embodiment of the invention, a process for the production of metal fibers is provided in which in a further process step (d) the metal oxide fibers obtained in process step (c) are reduced to the corresponding metal fibers. It is well known to those skilled in the art how to reduce metal oxides to the corresponding metals. Suitable reducing agents are hydrogen, carbon monoxide, gaseous hydrocarbons, carbon, furthermore metals which are less noble, ie have a negative standard potential, than the metal to be reduced, furthermore sodium borohydride, lithium aluminum hydride, alcohols and aldehydes. According to a further embodiment of the invention, the metal oxide fibers can be reduced electrochemically. The person skilled in the art can apply these reduction methods with the aid of his general knowledge and thereby obtains the corresponding metal fibers with a diameter <1 μm and an aspect ratio of> 10 to 1.

The metal and metal oxide fibers according to the invention have a diameter of <1 .mu.m, preferably <300 nm and have a length of 10 .mu.m up to several millimeters.

With the aid of the present invention, there are manifold possibilities for the production of different metal oxide and metal fibers, based on the electrospinning of solutions containing polymer / metal compound / solvent / complexing agent and optionally subsequent reduction. The resulting nanofibers have a variety of interesting magnetic, electrical and catalytic properties. They are therefore promising new materials for various and functional components in microelectronics and optoelectronics. There are also many possibilities for applications in catalysis and filtration.

The invention is further illustrated by the following examples and comparative examples:

Comparative Example 1

Preparation of nanofibers from zinc oxide without the use of a complexing agent:

A solution consisting of 7.5% by weight of polyvinyl alcohol (PVA), 7.5% by weight of zinc acetate dihydrate and 85% by weight of water was spun into nanofibers using an electrospinning apparatus according to WO 2006/131081 A1 (electrode spacing : 20 cm, voltage: 82 kV). The nanofibers obtained were first dried (2 hours at 100 ⁰ C) and then calcined to zinc oxide nanofibers (460 ⁰ C, air atmosphere). The scanning electron micrograph showed, prior to calcination, fused nanofibers of different diameters. An aspect ratio could not be given due to this morphology (fused together fibers). After calcination, fiber morphology can no longer be observed.

example 1

Preparation of nanofibers from zinc oxide using a complexing agent:

A solution consisting of 7.0% by weight of polyvinyl alcohol (PVA), 6.8% by weight of zinc acetate dihydrate, 3.0% by weight of citric acid and 83.2% by weight of water was treated with an electrospinning machine according to WO 2006/131081 A1 spun into nanofibers (electrode spacing: 20 cm, voltage: 82 kV). The nanofibers obtained were first dried ( ² h at 100 ⁰ C) and then calcined to zinc oxide nanofibers (480 ⁰ C, air atmosphere). The scanning electron micrographs show discrete nanofibers, which have an average diameter of 200 to 400 nm before calcination and have an average diameter of 70 to 120 nm after calcination. In both cases the aspect ratio is> 100 to 1.

Comparative Example 2

Preparation of nanofibers from aluminum-doped zinc oxide without using a complexing agent:

A solution consisting of 6.3% by weight of polyvinyl alcohol (PVA), 6.3% by weight of zinc acetate dihydrate, 0.33% by weight of aluminum nitrate nonahydrate, 77.8% by weight of water and 9 , 3% by weight of ethanol was spun into nanofibers using an electrospinning machine similar to WO 2006/131081 A1 (electrode spacing: 12 cm, voltage: 65 kV). The resulting nanofibers were first dried (2 h at 100 ° C) and then calcined to aluminum-doped zinc oxide nanofibers (460 ⁰ C, air atmosphere). The scanning electron micrographs show fused nanofibers with different diameters both before calcination and after calcining. An aspect ratio could not be determined due to this morphology (fused together fibers).

Example 2

Preparation of Nanofibers from Aluminum-doped Zinc Oxide (AZO) Using a Complexing Agent

A solution consisting of 6.8 wt .-% polyvinyl alcohol (PVA), 6.8 wt .-% zinc acetate dihydrate, 0.35 wt .-% aluminum nitrate nonahydrate, 3.0 wt .-% citric acid and 83 wt .-% water was spun with an electrospinning machine according to WO 2006/131081 A1 to nanofibers (electrode spacing: 20 cm, voltage 82 kV). The resulting nanofibers were first dried (2 h at 100 ⁰ C) and then calcined to aluminum-doped zinc oxide nanofibers (480 ⁰ C, air atmosphere). The scanning electron micrographs show discrete nanofibers with an average diameter of 100 to 200 nm before calcination and an average diameter of 70 to 120 nm after calcination. In both cases the aspect ratio is> 100 to 1.

Example 3

Preparation of Nanofibers from Aluminum-doped Zinc Oxide (AZO) Using Zinotin Rat, Aluminum Nitrate and Citric Acid:

A solution consisting of 6.8% by weight of polyvinyl alcohol (PVA), 6.3% by weight of zinc citrate dihydrate, 0.35% by weight of aluminum nitrate nonahydrate, 0.2% by weight of citric acid, 71 Wt .-% water, 10 wt .-% ethanol and 5.2 wt .-% of a 25% aqueous ammonia solution was spun with an electrospinning machine according to WO 2006/131081 A1 to nanofibers (electrode spacing: 20 cm, voltage: 70 kV ). The conservation requested nanofibers were first dried (2 h at 100 ⁰ C) and then aluminum-doped zinc oxide fibers, calcined (480 ° C, air atmosphere). The scanning electron micrographs show discrete nanofibers with an average diameter of 300 to 400 nm before calcining and an average diameter of 250 to 300 nm after calcination. In both cases the aspect ratio is greater than 100 to 1.

Claims

claims

A process for preparing metal oxide fibers having a diameter in the range of 0.1 to 999 nm by electrospinning a solution containing one or more polymer (s), one or more metal salt (s), one or more complexing agents and a solvent comprising the steps :

(A) providing a solution containing one or more polymer (s), one or more metal salt (s), one or more complexing agents and a solvent, wherein the solution has a water content of at least

50% by weight,

(b) electrospinning the solution and

(c) removing the polymer.

2. The method of claim 1, wherein the polymer is selected from the group consisting of polyethers, polyethylene oxides, polyvinyl alcohols, polyvinyl alcohol-polyvinyl acetate copolymers, polyvinyl acetates, polyvinylpyrrolidones, polyacrylic acids, polyurethanes, polyactides, polyglycosides, polyvinylformamides, polyvinylamines, Polyvi- nylimine and polyacrylonitriles or a mixture of two or more of the aforementioned polymers.

3. The method of claim 1 or 2, wherein the metal salt is a metal salt of a metal selected from the group Cu, Ag, Au, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W,

Mn, Re, Fe, Ru, Ni, Pd, Co, Rh, Ir, Sb, Sn, In, Al, Ga, and Zn.

4. The method according to any one of claims 1 to 3, wherein the complexing agent is selected from the group of monocarboxylic acids, dicarboxylic acids, tricarboxylic acids, hydroxycarboxylic acids, ketocarboxylic acids, diketones, amino acids, amino polycarboxylic acids, polymer-bound carboxylic acids, amines, diamines, ammonia, halide Ions and hydroxide ions.

5. The method according to any one of claims 1 to 4, wherein the complexing agent is citric acid or a salt thereof.

6. The method according to any one of claims 1 to 5, wherein the solvent in addition to water, one or more further solvents selected from the group consisting of methanol, ethanol, n-propanol, 2-propanol, n-butanol, iso-butanol, tert-butanol , Cyclohexanol, formic acid, acetic acid, trifluoroacetic acid, diethylamine, diisopropylamine, phenylethylamine, acetone, acetylacetone, acetonitrile, diethylene glycol, formamide, dimethylformamide (DMF), dimethylsulfoxide (DMSO), dimethylacetamide, N-methylpyrrolidone and tetrahydrofuran.

7. The method according to any one of claims 1 to 6, wherein the removal of the polymer takes place thermally, chemically, radiation-chemically, physically, with plasma, ultrasound or by extraction with a solvent.

8. The method according to any one of claims 1 to 7, wherein subsequent to the removal of the polymer in process step (c), a reduction of the metal oxide fibers to the corresponding metal fibers.

9. The method according to any one of claims 1 to 8, wherein the molar ratio of metal salt to complexing agent in the solution provided in step (a) in the range of 1: 0.2 to 1: 1.

10. The method according to any one of claims 1 to 9, wherein the provided in step (a) solution has a polymer content of at least 1 wt .-%.

1 1. The method according to any one of claims 8 to 10, wherein the reduction of the metal oxide fibers with the aid of a reducing agent selected from the group hydrogen, carbon monoxide, carbon, a metal having a lower standard potential than the metal to be reduced, sodium borohydride, lithium aluminum hydride, alcohols and aldehydes take place.

12. Metal oxide fibers obtainable by a process according to one of claims 1 to 7.

13. Metal fibers obtainable by a process according to any one of claims 8 to 11.

14. metal or metal oxide fibers according to any one of claims 1 to 12, wherein the fibers have a diameter of <1 micron, an aspect ratio of> 10 to 1.