Method for the production of conductive structures

Abstract

A method for the production of conductive structures, wherein nanofibers are applied with a photocatalytic component onto a substrate, in particular by electrospinning, and wherein a metallic layer is deposited photolytically on the substrate.

Claims

1. A process for producing metallic structures, comprising: a) providing nanofibers by providing a spinning composition comprising at least one hydrolyzable titanium compound or a condensate thereof and at least one organic polymer and/or oligomer; electrospinning the composition to obtain photocatalytically active composite nanofibers; thermal treating the photocatalytically active composite nanofibers at above 420° C. and below 480° C. to obtain photocatalytically active composite nanofibers consisting of anatase titanium dioxide crystallites in a matrix of amorphous titanium dioxide; and applying the photocatalytically active composite nanofibers to a substrate; b) contacting at least one precursor compound for a metallic structure with the photocatalytically active composite nanofibers; and c) reducing the at least one precursor compound to the metallic structures by action of electromagnetic radiation on the photocatalytically active composite nanofibers.

2. The process as claimed in claim 1, wherein the photocatalytically active composite nanofibers have a specific surface area of at least 80 m.sup.2/g.

3. The process as claimed in claim 1, wherein the at least one precursor compound is a silver, gold or copper complex.

4. The process as claimed in claim 1, wherein the photocatalytically active composite nanofibers have an average length of more than 10 μm.

5. A process for producing photocatalytically active composite fibers, comprising: a) providing a spinning composition comprising at least one hydrolyzable titanium compound or a condensate thereof, and at least one organic polymer and/or oligomer; b) electrospinning the composition; and c) thermal treatment of the fibers obtained at above 420° C. and below 480° C., thereby obtaining composite fibers consisting of anatase and amorphous titanium dioxide.

6. The process as claimed in claim 1, wherein the thermal treatment is conducted at above 430° C. and below 470° C.

7. The process as claimed in claim 1, wherein the electromagnetic radiation has a wavelength in the visible or ultraviolet range.

8. The process as claimed in claim 1, wherein the at least one organic polymer and/or oligomer comprises polyvinylpyrrolidone.

9. The process as claimed in claim 1, wherein the at least one precursor compound comprises a silver complex.

10. The process as claimed in claim 1, comprising applying a coating of the photocatalytically active composite nanofibers to 10-20% of a surface of the substrate and, after said reducing, obtaining a coated substrate.

11. The process as claimed in claim 1, wherein the substrate comprises ceramic, oxide ceramic, glass ceramic, paper, or cellulosic material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 a) distribution of the diameter (“Fiber Diameter” in nm versus pulses (counts)) of the fibers directly after production; b) ESEM image (Environmental Scanning Electron Microscope) of the fibers after (i);

(2) FIG. 2 a) distribution of the diameter (“Fiber Diameter” in nm versus pulses (counts)); b) ESEM image (Environmental Scanning Electron Microscope) of the fibers after (ii);

(3) FIG. 3 a) representation of the diameter distribution (“Fiber Diameter” in nm) for fibers after the production (“As Spun”) after calcination at 500° C. for 5 minutes (Calcinated 5 min) and calcination at 500° C. for 30 minutes (Calcinated 30 min); b) ESEM image of the samples after calcination for 30 minutes;

(4) FIG. 4 EDS spectra of the fibers (energy in keV versus intensity) of the fibers after production (“As Spun” at the bottom), after calcination for 5 minutes (middle graph), and after calcination for 30 minutes (upper graph); and

(5) FIG. 5 XRD spectra of a sample after calcination at 5 minutes (lower graph), or 30 minutes (upper graph); the anatase signals are at 25.32°, 38° and 48.01°;

(6) FIG. 6 size distribution (Range in nm) of the diameter of the silver particles formed on the fibers after irradiation for 3 minutes (A), irradiation for 5 minutes (B) and irradiation for 7 minutes (C);

(7) FIG. 7 ESEM image (on the left) of a TiO.sub.2 mat after exposure to UV for 4 minutes in the presence of AgNO.sub.3-tris complex; on the right: fibers with 30% coverage on glass, likewise exposed to UV for 4 minutes under the same conditions;

(8) FIG. 8 distribution of the diameters of the fibers produced, measured by TEM;

(9) FIG. 9 ESEM images of samples with a calcination temperature of 450° C. and a) FD 10 cm, CT 30 min., b) FD 20 cm, CT 30 min., c) FD 10 cm, CT 60 min., d) FD 20 cm, CT 60 min.;

(10) FIG. 10 representative TEM images of fibers calcined at (a) 450° C. for 60 min., (b) 475° C. for 60 min. and (c) 500° C. for 60 min.; FD in each case 10 cm;

(11) FIG. 11 Raman spectra of representative fibers calcined at 450° C., 475° C. and 500° C.;

(12) FIG. 12 plots of a gas sorption analysis of samples calcined at 450° C. (on the left, FD 20 cm), 475° C. (middle, FD 15 cm) and 500° C. (on the right, FD 15 cm), CT 60 minutes in each case; X axis Partial Pressure P/PO; Y axis in each case Quantity Adsorbed cc/g STP; squares: Quantity adsorbed; circles: Quantity de-adsorbed);

(13) FIG. 13 images of fibers after silver deposition on fibers a) calcined at 450° C., FD 20 cm, CT 60 min.; b) calcined at 475° C., FD 20 cm, CT 60 min.;

(14) FIG. 14 determination of the Fractional Coverage as a function of the Deposition Time.

DETAILED DESCRIPTION OF INVENTION

Examples

(15) Various strategies were used for the processes of electrospinning of the nanofibers. (i) Introducing hydrothermally produced nanocrystals of TiO.sub.2 into a polymer compound and electrospinning to give nanofibers; (ii) Producing anatase-comprising fibers from TiO.sub.2 from a titanium alkoxide precursor compound and polymer spinning compounds. (iii) Coaxially electrospinning fibers with a polymer core and a shell of hydrothermally converted TiO.sub.2 crystals and, after a heat treatment, washing out the polymer in order to obtain tubes of anatase. (iv) Coaxially electrospinning fibers having a core of titanium alkoxide and a shell of hydrothermally produced TiO.sub.2 crystals.

(16) All substrates are used in order to photochemically deposit silver.

Experiments According to (i)

(17) Ultrafine fibers were obtained by electrospinning a water-based sol of hydrothermally calcined TiO.sub.2 nanocrystals. For this purpose, titanium tetraisopropoxide was mixed with isopropanol and hydrochloric acid in order to obtain a TiO.sub.2 network. To this were added water and isopropanol in order to obtain a sol. The sol was then introduced into an isochoric autoclave and heated to 250° C. in order to obtain TiO.sub.2 in anatase form. The nanocrystals obtained are redispersed and optionally surface-modified. To this composition is added high molecular weight polyvinylpyrrolidone (Mw=1 300 000 g/mol, hPVP) up to a content of 4% by weight (based on the overall composition).

(18) The fibers obtained are shown in FIG. 1. The fibers had a very fine diameter and have a somewhat uneven surface. This suggests that TiO.sub.2 particles are arranged within the fibers.

Experiments According to (ii)

(19) Anatase-comprising nanofibers were obtained as follows. By gradual hydrolysis of titanium tetraisopropoxide (Ti(O-iPr).sub.4) in acetic acid and ethanol in a ratio of 0.25:1:1, a sol was obtained. To this sol was added high molecular weight polyvinylpyrrolidone (Mw=1 300 000 g/mol, hPVP), and it was fully dissolved with addition of ethanol. The hPVP content was 9% by weight based on the overall composition.

(20) By addition of different amounts of ethanol, compositions having a polymer concentration of 5% by weight were also produced.

(21) The sol was left to stand for 24 hours (stirred slightly if appropriate) and electrospun under the following conditions:

(22) A 21G spinneret was set up at a distance of 15 cm at right angles to a grounded copper plate wrapped with aluminum foil. A syringe pump was used to establish a flow of the spinning dope of 0.82 mL/h, or 0.5-0.8 mL/min (temperature 21° C., air humidity 43%). At a voltage between 18-21 kV, especially 18-20 kV, a spun filament formed.

(23) For optical tests, a 3 cm×3 cm glass plate was held under the spinneret (at a distance of 10 cm to 20 cm) for different periods (1 second and 15 seconds). FIG. 2 shows a typical distribution of the fiber diameter of the fibers. The fibers show an average diameter of 103+/−53 nm and a smooth cylindrical morphology.

(24) In order to apply individual fibers to the surface, the fibers were applied with movement of the glass plate for 12 to 15 seconds (5% by weight; 0.5-0.8 mL/min; 21 gauge; 18-20 kV; FD 10-20 cm). This led to coverage of 12-15% of the glass surface with fibers. The fibers are 20 to 100 μm apart and, after calcination, all have a maximum diameter below 500 nm.

(25) After the spinning, the fibers applied to the substrate were heated at 120° C. for 12 h in order to remove water and solvent. This can also be done at 80° C. for 12 to 24 h.

(26) Thereafter, they were calcined at 500° C. (heating and cooling rate each 2.66° C./min). In order to examine optimal conditions for formation of anatase, two different dwell times of 5 minutes and 30 minutes were examined. The fibers with a dwell time of 30 minutes later showed improved coverage with silver. All examples for FIGS. 1 to 6 were calcined at 500° C. for minutes. In the case of the other samples, the calcination temperature and time are specified in each case. After the heating, the calcination temperature was maintained for the time specified.

(27) The samples were cooled down to room temperature within at least 3 hours.

(28) FIG. 3 shows the change in the diameter of the fibers resulting from the calcination. The fibers become distinctly thinner and the size distribution narrows. The EDS spectrum shown in FIG. 4 clearly shows a decrease in the peak at 0.277 keV and 0.525 keV, which corresponds to the Kα line of carbon and oxygen in the PVP ([C.sub.6H.sub.9NO].sub.n). The peak at 4.508 keV corresponds to Kα titanium. With longer calcination, the carbon reacts to give CO.sub.2 and the oxygen forms TiO.sub.2. The Ti Lα at 0.452 keV is also apparent in the spectrum. The low absorptions of carbon could be attributed to the securing band for the samples.

(29) It has also been shown by means of x-ray diffraction (FIG. 5) that the calcination led to the formation of anatase crystals.

(30) FIG. 6 shows the size distribution of the silver nanoparticles formed on the surface after various irradiation times.

(31) For the metalization in the examples which follow, an aqueous silver nitrate solution with a Tris complex was used as precursor for the metallic layer. The composition was freshly prepared and pipetted onto the fibers that had been applied to a glass plate before waiting for 30 seconds. Thereafter, the fibers were irradiated with UV light (1000 W) for 3 minutes. Thereafter, the samples were washed and irradiated once again with UV (1000 W) for 2 min. The washing reduces the spontaneous deposition of silver on the glass surface.

(32) FIG. 7 shows, on the left-hand side, a mat of fibers (calcined at 500° C. for 30 minutes) after exposure for 4 minutes in the presence of an AgNO.sub.3-Tris complex. The silver coating is metallic and conductive (11.8 Ωcm) with contact resistance. The fibers in the left-hand image were also exposed under the same conditions for 4 minutes and have incomplete coverage with silver and are nonconductive.

(33) For further experiments, the influence of the distance before hitting the substrate (“fly distance”, FD) in the electrospinning, the calcination time (CT) and the calcination temperature were examined as possible factors. In each case, fibers were produced from three different batches of starting material.

(34) For all fibers produced on glass, the transmission was measured. The results are shown in table 1. In the sample designation, FD stands for the fly distance and CT for the calcination time in minutes. FD10CT60 therefore means a fly distance of 10 cm with a calcination time of 60 minutes. In the case of a respective coverage of 15% of the glass surface with fibers, no significant reduction in transmission is apparent.

(35) At a coverage of 12-15% of the glass surface with fibers, a decrease in the transparency of up to 1% is measured, and after silver deposition of up to 2.5%, compared to untreated glass.

(36) The determination of the coverage is shown in FIG. 14. For this purpose, the transmission or haze was measured as a function of the duration of the electrospinning. The numbers in brackets indicate the transmission and haze measured.

(37) FIG. 8 shows the distribution of the diameter of the fibers produced as a function of the flying distance (upper line of the X axis, 10 cm or 20 cm), calcination time (middle figure, 30 minutes and 60 minutes) and calcination temperature (450° C., 475° C. and 500° C. for the four combinations of flying distance and calcination time arranged above). The samples which were spun at flying distance 10 cm have a broader distribution because there is a bimodal distribution of very thin and thicker fibers (e.g. FIG. 9a). The fibers which were calcined at 450° C. have lower shrinkage than the fibers which were calcined at higher temperatures. Images of such fibers are shown in FIG. 9. The width of the figures corresponds to 1.25 μm. For determination of the diameter, the diameters of many fibers in a mat of fibers produced under the conditions mentioned were measured. The fibers have a cylindrical diameter.

(38) FIG. 10 shows TEM images of fibers at different calcination temperatures. With increasing temperature, the crystallinity of the fibers increases.

(39) FIG. 10 shows TEM images of fibers after calcination at different temperatures. With increasing calcination temperature, there is a significant increase in the proportion of crystalline titanium dioxide or the size of the crystalline structure. This is also apparent from the RAMAN spectra in FIG. 11. The lower amplitude and the width of the peaks indicate lower crystallinity or nanocrystals. The peaks at 450° C. are much smaller than at 475° C. or 500° C. Moreover, the sample at 450° C. has unusual background fluorescence and very undefined peaks for O—Ti—O B.sub.1g and O—Ti—O A.sub.1g/B.sub.1g. This indicates that, in these fibers, anatase titanium dioxide is present in an extended network of Ti—O—Ti. In the case of the fibers calcined at 450° C., therefore, there is a composite of amorphous titanium dioxide and crystalline titanium dioxide. Specifically amorphous titanium dioxide features distinctly higher porosity.

(40) The high porosity is also shown in the capacity for gas sorption (FIG. 12). For fibers which have been calcined at 450° C., a specific surface area (SSA) of 102 m.sup.2/g was measured (FIG. 12 on the left), whereas the fibers which have been calcined at 475° C. or 500° C. had a surface area of 66 m.sup.2/g (FIG. 12, middle) and 52 m.sup.2/g (FIG. 12, on the right) respectively.

(41) For the silver deposition experiments which follow, the above-described AgNO.sub.3/Tris complex was used. After the reduction, the samples were washed and dried for 2 hours. Thereafter, the transmission before and after metalization was measured. The results are shown in table 2. The metalization reduces the transmission by 1-4%, but for most samples only in the region of 1%.

(42) For all samples which have been calcined at 450° C., it was possible to measure a conductivity. For this purpose, silver paste was applied at two ends of the glass for contact connection and the conductivity was measured. The distance between the contact points was 5 mm. The resistance was anisotropic owing to the fiber shape of the samples. The results are shown in table 3. The resistance also depended on how many fibers connect the contacts.

(43) FIG. 13 shows two different metalized samples. FIG. 13a shows a metalized fiber (450° C., FD 20 cm, CT 60 min.) of a sample which showed conductivity. FIG. 13b shows a metalized fiber (475° C., FD 20 cm, CT 60 min.) of a sample which did not show conductivity. It is apparent that the silver deposition in FIG. 13a forms a shell around the fiber which leads to conductivity. In FIG. 13b, under the same conditions, only individual larger particles of silver have formed, but these do not lead to conductivity. It is assumed that the composite of amorphous titanium dioxide and anatase present in the fiber in FIG. 13a leads to the formation of the shell of metallic silver.

(44) Samples which have been calcined only at 400° C. did not show any conductivity either.

(45) TABLE-US-00001 TABLE 1 Temp. Batch 1 Batch 2 Batch 3 (° C.) T (%) σ n T (%) σ n T (%) σ n FD10CT30 450 92.00 0.08 5.00 92.20 0.05 5.00 91.70 0.00 5.00 FD20CT30 92.70 0.05 5.00 92.50 0.00 5.00 92.20 0.00 5.00 FD10CT60 92.70 0.04 5.00 92.60 0.00 5.00 93.00 0.00 5.00 FD20CT60 92.80 0.00 5.00 92.70 0.22 5.00 92.70 0.00 5.00 FD10CT30 475 90.50 0.00 5.00 91.50 0.13 5.00 91.40 0.00 5.00 FD20CT30 92.10 0.04 5.00 92.90 0.00 5.00 92.30 0.04 5.00 FD10CT60 92.40 0.00 5.00 91.80 0.05 5.00 90.50 0.05 5.00 FD20CT60 92.50 0.00 5.00 91.10 0.05 5.00 91.80 0.04 5.00 FD10CT30 500 91.50 0.05 5.00 91.60 0.04 5.00 91.70 0.18 5.00 FD20CT30 91.30 0.13 5.00 92.20 0.00 5.00 91.70 0.04 5.00 FD10CT60 91.50 0.04 5.00 92.40 0.13 5.00 91.20 0.04 5.00 FD20CT60 92.00 0.01 5.00 92.20 0.13 5.00 92.10 0.00 5.00 Blank 92.8 0 5

(46) TABLE-US-00002 TABLE 2 After metallization Temp. Batch 1 Batch 2 Batch 3 Sample (° C.) T (%) σ n T (%) σ n T (%) σ n FD10CT30 450 NA NA NA NA NA NA NA NA NA FD20CT30 91.10 0.00 5.00 91.70 0.04 5.00 88.70 0.00 5.00 FD10CT60 92.10 0.00 5.00 90.30 0.04 5.00 89.00 0.00 5.00 FD20CT60 91.40 0.05 5.00 89.10 0.07 5.00 91.70 0.00 5.00 FD10CT30 475 79.10 0.25 5.00 85.50 0.12 5.00 81.00 0.26 5.00 FD20CT30 90.90 0.09 5.00 91.80 0.00 5.00 90.20 0.05 5.00 FD10CT60 88.60 0.08 5.00 88.90 0.04 5.00 83.90 0.00 5.00 FD20CT60 88.20 0.21 5.00 84.80 0.14 5.00 80.30 0.05 5.00 FD10CT30 500 85.40 0.00 5.00 86.10 0.00 5.00 87.30 0.09 5.00 FD20CT30 89.80 0.00 5.00 89.00 0.00 5.00 89.90 0.04 5.00 FD10CT60 88.40 0.00 5.00 90.60 0.04 5.00 82.00 0.13 5.00 FD20CT60 90.90 0.04 5.00 90.40 0.07 5.00 89.70 0.00 5.00

(47) TABLE-US-00003 TABLE 3 Sample R (Ω) d (mm) S450FD20CT30-003  4M 5  4M 5  2M 5 S450FD10CT60-001 250 5  11K 5 S450FD20CT60-001  1M 5 S450FD20CT60-003  13K 5  50K 5  62K 5  8M 5