Conductive materials made of Nb-doped TiO.SUB.2 .particles

Abstract

A method for producing conductive materials from Nb-doped TiO2-particles, in which Nb-doped TiO2-particles are pressed to form bodies and the bodies are treated in an oxygen-containing atmosphere and at a reducing atmosphere.

Claims

1. A process for producing conductive bodies, comprising: producing Nb-doped TiO.sub.2 particles by hydrolysis of at least one hydrolyzable titanium compound and at least one hydrolyzable niobium compound in an organic solvent and a substoichiometric amount of water and an inorganic acid catalyst; compressing the Nb-doped TiO.sub.2 particles to give a body; subjecting the body to heat treatment at a temperature of 400 to 800° C. in an oxygenous atmosphere; and subjecting the body to heat treatment at a temperature of 400 to 800° C. in a reducing atmosphere.

2. The process as claimed in claim 1, wherein the particles have a particle size below 200 nm.

3. The process as claimed in claim 1, wherein the particles have an Nb content of up to 30 at %.

4. The process as claimed in claim 1, wherein the compressing is effected at a pressure of at least 500 kN.

5. The process as claimed in claim 1, wherein the thermal treatment in the oxygenous atmosphere is effected at a temperature of 500 to 800° C.

6. The process as claimed in claim 1, wherein the thermal treatment in the reducing atmosphere is effected at a temperature of 500 to 800° C.

7. The process as claimed in claim 1, wherein both heat treatments are conducted at temperatures of 500 to 800° C.

8. The process as claimed in claim 1, wherein the reducing atmosphere has a proportion of reducing gas of 0.05% to 10% by volume.

9. The process as claimed in claim 1, wherein the particles are produced by a sol-gel process.

10. A conductive body produced by the process as claimed in claim 1.

11. A process for producing Nb-doped titanium dioxide nanoparticles, comprising: preparing a mixture comprising at least one hydrolyzable titanium compound and at least one hydrolyzable niobium compound in an organic solvent and water in a substoichiometric amount, based on all the hydrolyzable groups present, and an inorganic acid; and treating the mixture at 200° C. to 300° C. under autogenous pressure to form Nb-doped titanium dioxide nanoparticles.

12. The process as claimed in claim 1, wherein said compressing comprises placing the Nb-doped TiO.sub.2 particles into a mold.

13. The process as claimed in claim 11, wherein a molar ratio of water to hydrolyzable groups in the hydrolyzable compounds is not more than 0.8.

14. The process as claimed in claim 11, wherein a molar ratio of water to hydrolyzable groups in the hydrolyzable compounds is more than 0.05.

15. The process as claimed in claim 11, wherein the inorganic acid comprises hydrochloric acid.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The working examples are shown in schematic form in the figures. Identical reference numerals in the individual figures denote elements that are the same or have the same function or correspond to one another in terms of their functions. The figures specifically show:

(2) FIG. 1 TEM image of TNO5 nanoparticles. The particles are crystalline, as apparent from the visible lattice planes. They have an average size of 12 nm and, owing to their tetragonal crystal system, frequently have an angular form in TEM images;

(3) FIG. 2 XRD of the nanoparticle powders produced. From undoped TiO.sub.2 as reference up to 20 at % Nb, anatase is conserved as modification. The sequence of measurements from the top downward: TNO20, TNO10, TNO8, TNO5, TNO5.2, TiO.sub.2);

(4) FIG. 3 XRD of a TNO5 powder as obtained after synthesis, and after 1 h at 500° C. under air and subsequently 1 h at 550° C. under forming gas. All measurements show that purely anatase is present as modification. Even through heating to more than 500° C., the modification is conserved. The sequence of measurements from the top downward is TNO5, TNO5 after 500° C. air and 550° C. forming gas; TNO5 after 500° C. air;

(5) FIG. 4 change in the bandgap of a TNO5 sample by treatment under air at 550° C. and under forming gas at 550° C. for 1 h in each case. The bandgap shifts after the treatment from 3.23 eV to 3.15 eV; and

(6) FIG. 5 specific resistivities of TNO5 pellets by sintering under air and forming gas at 550° C. for 1 h in each case. The sintering under air firstly burns the organics, and the material changes color from blue to white. Sintering under forming gas reduces the material again and brings it back to its original blue state. This lowers resistivity by 4 orders of magnitude;

(7) FIG. 6 schematic diagram of the process of the invention;

(8) FIG. 7 specific surface area SSA and particle size as a function of the Nb content determined by BET measurements. The particles thereafter have an average size between 9 and 11 nm, falling slightly with rising Nb content.

DETAILED DESCRIPTION OF THE DRAWINGS

(9) The precursors Ti(OEt).sub.4 (for 5 at % Nb: 70.0 g, 306.8 mmol) and Nb(OEt).sub.5 (5.14 g, 16.1 mmol) were weighed out together in a glovebox and then mixed rapidly with abs. ethanol (480 ml) outside the glovebox. This mixture was left to stir overnight (about 18 h). Thereafter, concentrated hydrochloric acid (6.45 g) was added rapidly while stirring. After stirring for a further 3 h, the reaction solution was divided homogeneously between four 200 ml Teflon vessels (about 130 ml each). These were screwed into steel vessels in a fixed manner and heated to 240° C. in heating blocks for 25 h. After complete cooling, the clear supernatant was removed and the blue solids were introduced together with water into 500 ml centrifuge vessels. After adding a few drops of NaOH to neutralize the HCl, the samples were washed at least three times with water until the wash water reached a conductivity of less than 20 μS/cm. The solids were then transferred to a flask with a minimum amount of water, frozen in liquid nitrogen and freeze-dried.

(10) The powders are treated with TNOx where x denotes the at % of niobium (x=n(Nb)/n(Nb)+n(Ti)*100). For production of the other dopings, the ratio of Nb and Ti was always chosen such that the molar amounts of the two precursors add up to about the same, i.e. around 322 mmol per batch. All other amounts remained the same.

(11) All powders were characterized by XRD, Raman, BET, TEM and UV-Vis measurements. The specific surface areas SSA were used, by the following formula (1), assuming that the particles are spherical, to calculate the particle size d.sub.p:
d.sub.p=6000/(ρ*SSA)  (1)

(12) This formula can be derived from the particle volume of a spherical particle and its density ρ. Since the particle shape is only approximately spherical and the density of undoped anatase was used as a simplification, this formula can give only an estimate for the particle size (FIG. 7). The particle shape and size were therefore confirmed by TEM images. The UV-Vis reflection measurements were used to determine the bandgap of the materials with the aid of the Kubelka-Munk method and the Tauc plot. For this purpose, (αhυ).sup.1/r is plotted against hυ. The point of intersection of the extrapolated linear slope of the curve with x=0 then gives the bandgap (table 1; FIG. 4).

(13) FIG. 1 shows a TEM image of TNO5 nanoparticles. The narrow size distribution of the particles is readily apparent.

(14) FIGS. 2 and 3 show XRD measurements of various particles. It is found that the reflections are slightly shifted toward smaller angles. Nevertheless, all samples show an anatase structure. The sequence of lines in the measurement from the top downward corresponds to the sequence in the legend (TNO8, TNO5, TNO2.5 and TiO.sub.2).

(15) Producing the Pellets

(16) Materials used for the production of pellets were the TNO powders produced with different Nb contents from 0 to 20 at % Nb. Also used as a reference was a commercial ITO nanoparticle powder. The pellets were produced with the aid of a cold isostatic press at 1000 kN and pressing time 30 s in flexible silicone molds having an internal diameter of 1 cm and a height of 0.5 cm. 0.4 g of powder was used each time. In order to remove residues of the silicone mold on the surface of the pellets, the pellets were then polished with SiC abrasive paper.

(17) For measurement of resistivity, multiple methods of contacting the pellets were tested. The best contacting was achieved with electrodes applied by sputtering (e.g. Ag, 100 W, sputtered on both sides for 10 min). The resistances were determined with a 2-point multimeter up to the GOhm range. The thickness h and the diameter d of the pellets were used to determine a specific resistivity ρ therefrom, which, in the case of a bulk body, should be a temperature-dependent material constant. Of course, it is necessary to take account of the fact that the pellets have a certain porosity. To improve conductivity, the pellets were sintered at various temperatures (550 to 750° C.) under air and then at 550° C. in forming gas (N.sub.2/H.sub.2, 95:5). The heating under air was supposed to serve to increase the crystallinity and the burning of organic residues on the surface. For each doping, three pellets were produced from different synthesis batches. These were used to calculate the average and standard deviation in each case. The blue pellets (prior to thermal treatment) have the highest resistivity of the TNO pellets. There is a slight fall in resistance with increasing Nb content. However, aftertreatment of the pellets under air and forming gas reduces the resistivity by several orders of magnitude. The smallest resistivity that was achieved is only a factor of 40 away from the value for a comparable ITO pellet. The results of the resistivity measurement are shown in FIG. 5. The results show that TNO treated in accordance with the invention is a possible alternative to ITO.

(18) TABLE-US-00001 TABLE 1 Material Bandgap TiO.sub.2 3.24 TNO2.5 3.22 TNO5 3.23 TNO8 3.21 TNO10 3.17 TNO20 3.12

LITERATURE CITED

(19) Liu et al., ACS Nano 4, 9 (2010) 5373-5381. Nemec et al., J. Phys. Chem. C, 115 (2011) 6968-6974.