Process for the Deposition of Metals on Semiconductor Powders

The present invention relates to a novel process for the deposition of metals on semiconductor powders.

It is known from the literature that the deposition of metals on semiconductor powders such as ZnO, TiO.sub.2 and WO.sub.3 powders or on TiO.sub.2 single crystals can be effected by photoredox reaction. Simple metal salts, e.g. hexachloroplatinic acid, silver perchlorate, copper sulfate and palladium chloride, have been employed as metal compounds. In these known processes, it is necessary to carry out the reaction with the exclusion of oxygen. Any CO.sub.2 formed during the reaction is conveniently removed by bubbling in nitrogen. Reference is made in this connection to e.g. the following publications: JACS, 100:13, 4317 (1978), J. Phys. Chem. 83, 2248 (1979), J. Phys. Chem., 85, 268 (1981) and Bull. Chem. Soc. Japan, 51, 3154 (1978).

It is also known that certain semiconductor powders such as ZnO, ZnS, Bi.sub.2 S.sub.3, FeTiO.sub.3 or Fe.sub.2 TiO.sub.4, slowly decompose when irradiated in purely aqueous medium [see Solar Energy 20, 443 (1978)].

This invention relates to a novel process for the deposition of copper, lead, mercury, tin, gold, silver, palladium, osmium and/or cadmium on semiconductor powders by means of photoredox reaction, which process comprises irradiating a suspension of a semiconductor powder in the presence of oxygen and optionally CO.sub.2, of an oxidisable system which optionally protects the semiconductor from photocorrosion, and of a salt or complex of copper, lead, mercury, tin, gold, silver, palladium, osmium and/or cadmium.

The process of the invention is on the one hand, simple and economic, as it permits very complicated and expensive steps and apparatus for excluding oxygen and CO.sub.2 to be dispensed with, and it can be carried out in open systems, e.g. in settling tanks and the like. In addition, the process of the invention makes possible a very comprehensive extraction of the above mentioned metals even from highly dilute solutions or suspensions, in general to a concentration of below 0.1 ppm and, in the case of silver, even of 50 ppb. The process is also suitable for the extraction of metal complexes such as Ag(S.sub.2 O.sub.3).sub.2.sup.3-, which cannot be precipitated by conventional gravimetric methods. It is also surprising that, in the process of the invention, metals having a more negative reduction potential than that of oxygen, e.g. silver, palladium, osmium and cadmium, are able to precipitate in the presence of oxygen.

Examples of suitable oxidisable systems which may be used in the process of this invention are: water, alkanols containing up to 6 carbon atoms such as methanol, ethanol, propanol, isopropanol, butanols and hexanols, mixtures of water and alkanols of the above kind or other organic solvents such as aliphatic or cyclic ethers, e.g. diethyl ether, diisopropyl ether, tetrahydrofuran, dioxan or cellosolve, mixtures of water and alkali metal sulfites or sulfides, alkaline earth metal sulfites or sulfides, or ammonium sulfides or sulfites, or acetate. Examples of suitable alkali metals and alkaline earth metals are Na, K, Li, Mg, Ca and Ba. Ammonium sulfites and sulfides and alkali metal sulfites or sulfides in admixture with water are preferred, especially mixtures of water and sodium sulfite or ammonium sulfite.

It is particularly preferred to carry out the process of the invention in water or in water containing acetate buffer or sodium sulfite. The reaction temperature is ordinarily in the range from 0.degree. to 40.degree. C., with room temperature being most preferred.

The semiconductor powders which may be used in the process of this invention are e.g. powders of TiO.sub.2, WO.sub.3, SnO.sub.2, SrTiO.sub.3, Fe.sub.2 O.sub.3, Nb.sub.2 O.sub.5, Ta.sub.2 O.sub.5, KTaO.sub.3, ZrO.sub.2, ZnO, CaTiO.sub.3, ZnS, Bi.sub.2 S.sub.3, BaTiO.sub.3, FeTiO.sub.3, Fe.sub.2 TiO.sub.4, Fe.sub.2 TiO.sub.5, YFeO.sub.3 or KTa.sub.0.77 Nb.sub.0.23 O.sub.3. It is particularly preferred to use TiO.sub.2, WO.sub.3, Fe.sub.2 O.sub.3, SrTiO.sub.3, Ta.sub.2 O.sub.5, SnO.sub.2, ZnS, Nb.sub.2 O.sub.5 or FeTiO.sub.3. Zinc sulfide powder and titanium dioxide powder (anatase modification) are most preferred.

If ZnS, ZnO, Bi.sub.2 S.sub.3, FeTiO.sub.3 or Fe.sub.2 TiO.sub.3 is used as semiconductor powder, then the oxidisable system is preferably not water.

The specific surface area of the semiconductor powders may vary within a wide range and depends largely on the nature of the semiconductor. The semiconductor powders employed in the process of the invention conveniently have a specific surface area of 0.1 to 50 m.sup.2 /g, preferably of 1 to 10 m.sup.2 /g.

In general, semiconductor powders of so-called technical quality may be used. When using titanium dioxide powder, it will be generally preferred to use the anatase modification.

The semiconductor powders may be used in any amount. One advantage of the process of this invention is that, in general, small amounts of semiconductor powder suffice for a comprehensive extraction of the metals. The semiconductor powders will conveniently be used in an amount of 0.05 to 5% by weight, preferably 0.1 to 2% by weight, based on the reaction volume.

The metals to be deposited can be in the form of simple or complex salts, e.g. ammonium and alkali metal hexahalo- or tetrahalometallates, halides, nitrates, acetates, cyanides, sulfates or thiosulfates. Examples of suitable metal compounds or salts are: silver acetate, silver nitrate, silver thiosulfate, palladium chloride or palladium iodide, palladium acetate, palladium cyanide and palladium nitrate, lithium tetrachloropalladate(II), gold(III)chloride, gold(III)bromide and gold(III)iodide, ammonium hexachloroosmate(IV), osmium tetroxide (OsO.sub.4), cadmium acetate and cadmium chloride. The metals are preferably salts or complexes of gold, silver, palladium, osmium and/or cadmium, in particular palladium or cadmium salts or complexes and, most particularly, silver salts or complexes.

As light source there may be used in the process of this invention any light of wavelength from about 200 to 650 nm, depending on the semiconductor. Suitable light sources are sunlight or mercury high pressure lamps which may be doped with metal atoms, xenon vapour lamps, mercury xenon lamps, mercury low and medium pressure lamps, halogen lamps, tungsten lamps and D.sub.2 lamps. Irradiation is preferably carried out with sunlight or with a mercury high pressure lamp.

The process of this invention is suitable for recovering copper, lead, mercury, tin, gold, silver, palladium, osmium and cadmium from all manner of wastewaters and solutions which occur in industry or in chemical reactions, e.g. photographic developer, bleaching and fixing solutions, hydrogenation solutions and the like, or also for the recovery or metals such as gold and silver from aqueous systems.

Up to 10% by weight of metal can be deposited on the semiconductor powders in the process of the invention. The deposited metal may be easily separated from the semiconductor powders, e.g. by treatment with oxidation agents such as HNO.sub.3, H.sub.2 SO.sub.4 and Na.sub.2 O.sub.2, and the semiconductor powders may be reused.

The invention is illustrated by the following Examples.

1 g of titanium dioxide (anatase modification, specific surface area 8 m.sup.2 /g) is suspended in a solution of 50 mg of silver acetate in 90 ml of distilled water and 10 ml of acetate buffer solution. With stirring, the suspension is irradiated for 2 hours in an open vessel through a Pyrex immersion rod with a 125 W mercury high pressure lamp, whereupon the colour of the suspension turns dark grey. The suspension is then filtered and the filter cake is washed with water and dried for 24 hours at 80.degree. C./13000 Pa, affording 1 g of a dark grey powder. Silver content: 3.1% by weight.

The procedure of Example 1 is repeated, using 190 ml of distilled water and 50 mg of silver acetate. The suspension is irradiated for 2 hours with sunlight (intensity of irradiation 500 W/m.sup.2 at vertical incidence of light, measured with a YSI thermopile). Yield: 1 g of a grey powder. Silver content: 3% by weight.

The procedure of Example 1 is repeated, using 50 mg of silver nitrate instead of 50 mg of silver acetate. Yield: 1 g of a dark grey powder. Silver content: 3.1% by weight.

The procedure of Example 2 is repeated using 450 ml of a photographic wastewater instead of the silver acetate/acetate buffer solution. The photographic wastewater consists of developer solution, bleaching bath, fixing bath and wash water and contains 10 ppm of Ag. Yield: 1 g of a dark grey powder with a silver content of 0.4% by weight. 1.6 ppm of silver remain in the solution.

The procedure of Example 2 is repeated, using 1 g of tin dioxide (specific surface area 5.96 m.sup.2 /g) instead of 1 g of titanium dioxide. A greyish powder with a silver content of 2.8% by weight is obtained.

The procedure of Example 2 is repeated, using 1 g of tungsten trioxide (specific surface area 3.85 m.sup.2 /g) instead of 1 g of titanium dioxide. A greenish grey powder with a silver content of 1.3% by weight is obtained.

The procedure of Example 2 is repeated, using 1 g of strontium titanate (specific surface area 3.8 m.sup.2 /g). The greyish yellow powder obtained has a silver content of 3% by weight.

The procedure of Example 2 is repeated, using 1 g of a rutile modification of titanium dioxide (specific surface area 0.36 m.sup.2 /g). Yield: 1 g of greyish white powder with a silver content of 3.2% by weight.

The procedure of Example 1 is repeated, using 1 g of niobium pentoxide (specific surface area 1.51 m.sup.2 /g). Yield: 1 g of a grey powder with a silver content of 3.3% by weight.

The procedure of Example 1 is repeated, using 1 g of tantalium pentoxide (specific surface area 2.45 m.sup.2 /g). Yield: 1 g of a grey powder with a silver content of 2.8% by weight.

1 g of titanium dioxide (anatase modification, specific surface area 8 m.sup.2 /g) is suspended in a solution of 75 mg of silver nitrate, 500 mg of sodium thiosulfate and 60 mg of sodium sulfite in 100 ml of water. The suspension is irradiated at room temperature in a photoreactor equipped with a 125 W mercury high pressure lamp, a magnetic stirrer and a water-cooled Pyrex immersion rod. Radiochemical determination is made by taking 1 ml samples, centrifuging off the titanium dioxide, and measuring the solution to .sup.110m Ag activity.

These radioactivity measurements show that the silver content of the solutions falls from an initial concentration of 50 ppm after 10 minutes to 1 ppm and after 4 hours to 50 ppb.

The procedure of Example 2 is repeated, using 1 g of iron titanate (specific surface area 0.45 m.sup.2 /g). A red powder with a silver content of 1.2% by weight is obtained.

The procedure of Example 2 is repeated, using 50 mg of lithium tetrachloropalladate(II). A grey powder with a palladium content of 2% by weight is obtained.

The procedure of Example 2 is repeated, using 50 mg of gold(III)chloride. The blue violet powder obtained has a gold content of 2% by weight.

0.4 g of titanium dioxide (anatase modification, specific surface area 8 m.sup.2 /g) is suspended in a solution of 20 mg of ammonium hexachloroosmate(IV) in 90 ml of distilled water and 10 ml of acetate buffer solution. This suspension is then irradiated for 3 hours at room temperature in a photoreactor equipped with a 125 W mercury high pressure lamp, a magnetic stirrer and a water-cooled immersion rod, whereupon the colour of the suspension turns dark grey. The suspension is filtered and the filter cake is washed with water and dried for 24 hours at 80.degree. C./13000 Pa, affording 0.4 g of a dark grey powder with an osmium content of 0.57% by weight.

1 g of ZnS (specific surface area 38.5 m.sup.2 /g) is suspended in a solution of 20 mg of cadmium acetate in 100 ml of a 10% aqueous solution of sodium sulfite. This suspension is then irradiated for 2 hours at room temperature in a photoreactor equipped with a 125 W mercury high pressure lamp, a magnetic stirrer and a water-cooled immersion rod, whereupon the colour of the suspension turns grey. The suspension is filtered and the filter cake is washed with water and dried for 24 hours at 80.degree. C./13000 Pa, affording 1 g of a grey powder with a cadmium content of 1.2% by weight.

The procedure of Example 16 is repeated, using 20 mg of cadmium acetate in 100 ml of water and without addition of sodium sulfite. Yield: 1 g of a grey powder with a cadmium content of 0.78% by weight. Cadmium content of the filtrate: 0.8 ppm (blank value before irradiation=65 ppm).

The procedure of Example 1 is repeated, using 20 mg of osmium tetroxide, to give 1 g of a grey powder with an osmium content of 0.84% by weight. Osmium content of the filtrate: 20 ppm.

1 g of ZnS (specific surface area 38.5 m.sup.2 /g) is suspended in a solution of 20 mg of a metal salt listed in the following table and 1 g of sodium sulfite in 100 ml of water. This suspension is irradiated for 1 hour at room temperature in a photoreactor equipped with a 125 W mercury high pressure lamp, a magnetic stirrer and a water-cooled Pyrex immersion rod. Each suspension is filtered and the filter cake is dried for 24 hours at 80.degree. C./13000 Pa. The respective metal content of the ZnS powder and that of each filtrate is indicated in the table.