WO1993025733A1

WO1993025733A1 - Platinum plating bath electrolyte

Info

Publication number: WO1993025733A1
Application number: PCT/GB1993/001255
Authority: WO
Inventors: Derek Pletcher; William Levason; Adrian John Gregory
Original assignee: Johnson Matthey Public Limited Company
Priority date: 1992-06-17
Filing date: 1993-06-14
Publication date: 1993-12-23
Also published as: GB9212831D0; EP0644956A1; AU4346193A; CA2138396A1; JPH07507841A

Abstract

A platinum plating bath electrolyte for plating platinum or platinum alloys contains a platinum(II) salt, in which the platinum(II) is present in solution as Pt(H2O)42+, and permits efficient deposition of Pt.

Description

PLATINUM PLATING HATH ELECTROLYTE

This invention concerns platinum plating bath electrolytes.

Precious metals are used as films on surfaces for a variety of reasons. In the jewellery trade, precious metals, including platinum, are used to improve the appearance of an article and to create special effects. Platinum may be used as a film to protect metals and other materials from corrosion, such as the coating of bursting discs. Platinum films are also used to provide conducting paths in electrical circuits. Platinum films can have more than one use in a particular application, such as with platinised titanium electrodes, where the platinum film acts as an electrical conductor and also protects the titanium, allowing the electrodes to be used in corrosive environments. Platinised surfaces may also have a catalytic function, for example in the reduction of the hydrogen overpotential of anodes. A further application of platinum films is where they are reacted with components of the substrate or with additional materials to create reaction products, such as in the coating of turbine blades and subsequent treatment to form the erosion- and corrosion-resistant platinum aluminide. Platinum alloy films may also be formed and treated in this way.

Electroplated films find many other applications, for example the metallurgical and biological fields.

Conventionally, platinum and platinum alloy films may be electro-deposited from a number of aqueous systems which are optimised for the particular application. Such processes are based on materials such as diamminedinitroplatinute(II) (platinum 'P' salt), alkali metal hexa- hydroxyplatinates(IV), hydrogen hexachloroplatinate(IV) and hydrogen dinitrosulphatoplatinate(II) (DNS), and present a variety of problems in use.

Difficulties of control, low rates of deposition, low efficiency, extremes of pH (acidity and alkalinity), precipitation of undesirable substances, loss of conductivity, and highly stressed films especially when the film thickness exceeds 5 microns, are all problems associated with the conventional processes. Because of these factors, each system is normally optimised for a small number of specific applications, and many product variants have to be marketed. Extensive use of additives in the electrolytes, such as conducting salts, buffers, brighteners and materials to improve the throwing power can compound the COΠUΌI difficulties. The addition of replenishers may alter the balance of the species present in the electrolytes leading to changes in the nature of the deposited films.

A system or systems for the electro-deposition of platinum or platinum alloy films which overcome or improve upon a number of the difficulties described above is therefore highly desirable and there is considerable interest in the development of electrolytes which allow the rapid deposition of high quality, thick layers of platinum.

EP 0358375 discloses a platinum or platinum-alloy electroplating bath comprising an alkaline aqueous solution of a complexed platinum(II) salt in which the anion component is a group or radical derived from an organic or inorganic acid other than a hydrohalic acid, in alkaline aqueous solution. Current efficiencies for electrolytes containing 5.0g/Iitre of between 60 and 85% are achieved at operating temperatures of 90 and 93°C. (Efficiency = the percentage of platinum deposited on the substrate compared to that calculated by Faraday^'s law under the same conditions). At temperatures below 9()°C. current efficiencies of 2-61% are reported.

An object of the present invention is to provide a further improved plating electrolyte. To be effective in a platinum plating bath electrolyte, the platinum species in solution must be highly soluble and stable, but reduced readily so that platinum deposition then occurs at potentials positive to hydrogen evolution. These requirements can only be met by careful choice of ligands interacting with the platinum centre. The selection of plating electrolyte components is made more difficult by the fact that Pt(II) complexes are substitution inert and kinetic factors as well as thermodynamics determine their behaviour. For example, in the baths of EP 0358375 it may be demonstrated that it is the inertness of the Pt(NH,)j ⁺ which necessitates operating temperatures of above 9()°C. The reduction of Pt(NH₃)₄ ²⁺ to Pt is strongly kinetically controlled.

The present invention provides an improved platinum plating bath electrolyte comprising a platinum(H) salt in which the Pt(II) is present in solution as Pt(H O)Λ

Said plating electrolyte permits deposition of Pt with a current efficiency similar to that of the bath of EP 0358375, and. surprisingly, maintains its high performance over a wide range of temperatures, including room temperature. It may be shown that even at room temperature (15°C) or below. Pt(H O)₄ ^:+ undergoes facile reduction, and at a suitable temperature the reduction of Pt(II) to Pt is mass transport controlled. Hence, with a suitable choice of Pt(II) concentration and stirring conditions, it is possible to achieve a high rate of deposition even at room temperature. The anion component of the salt may suitably be one or more group or radical derived from an organic acid or an inorganic acid. Such anion component(s) may for example, but not exclusively, be selected from such inorganic acid groups as perchlorate, sulphate and phosphate.

Temperatures of 0-100°C may be employed during use of the electrolyte. Preferably use is at a temperature between room temperature and 70°C.

The concentration of the platinum salt may vary, and may be measured as platinum from 0.005 molar (lg/litre) to 0J50 molar (30g/litre) or more. Preferred platinum concentrations depend upon the plating rate, cell geometry and mode (vat or barrel), degree of agitation etc. but are typically around 0.005 molar (lg/litre) to 0J00 molar (20g/litre) such as 0.005 molar (lg/litre) to 0.025 molar (5g/litre). The electrolyte may be agitated or not depending on the application.

Suitable substrates for plating are generally metal and alloy surfaces and other conducting surfaces. Typical metal surfaces are copper, gold, nickel, titanium and tungsten. Typical alloy surfaces are stainless steels, brass, nickel alloys and superalloys containing niobium, zirconium and vanadium. Other surfaces include conductive resins and composites. The surfaces may be prepared for plating by the use of conventional cleaning procedures and as necessary, the incorporation of an undercoat of nickel, gold or other metal before the deposition of platinum. The electroplating cell may utilise conventional or specialised anodes, as with existing electroplating systems, including carbon, graphite, platinum, platinised titanium, and iridised titanium.

Current densities for plating according to the invention are suitably in the range 0.03-10 A dm^'*, preferably OJ 0-2.5 A dm^"2.

The invention will now be described by Example.

EXAMPLE 1

A solution of Pt(HO)₄(ClO₄)₂ in 1 mol dm³ HClO was prepared by the method described by Elding in Inorg Che Acta, 20, (1976), 65. K,PtCl₄ (5mmol, 2.08g) was dissolved in 200cm³ of 1 mol dm^'3 HClO₄, and the solution heated to 70°C to redissolve the KClO₄ which formed immediately at room temperature. AgQO₄.H ) (20mmol, 4.5 lg) was dissolved in 225cm³ of 1 mol dm^"3 HClO₄. Both solutions were deoxygenated and then the silver perchlorate solution was added slowly to the platinum solution, slow addition being essential to avoid precipitation of Ag₂PtCl . After all the silver perchlorate solution had been added, the solution was left at 70°C under N₂ for seven days. The light brown precipitate of AgCl was filtered at 70°C to give a clear yellow filtrate. The solid AgCl was washed with aqueous HClO and dried under vacuum; the weight. 2.87g, indicated that the reaction had gone to completion. Contamination of the solution of Pt(H,O) (ClO₄) in 1 mol dm^'3 HClO by Ag⁺ was obvious during voltammetry; the voltammogram shows a wave for the reaction Ag⁺ — » Ag at E_κ 0.39V and a coupled anodic stripping peak, well positive to the Pt(II) reduction. Such Ag⁺ could readily be removed by pre-electrolysis with a large Pt foil cathode until a voltammogram showed no reduction wave at 0.39V or stripping peak.

The Pt content of the Pt(H O) (ClO )_; solution was determined by atomic absorption spectroscopy using a Perkin Elmer 2380 atomic absorption spectrometer with an air/acetylene flame after addition of LaCl₃ to prevent interference from the high concentration of perchlorate. 1 mol dm^"3 perchloric acid was then added to make the Pt(II) concentration exactly 8mmol dm ³.

The solution of Pt(H₂O)₄(ClO₄)₂ in 1 mol dm^'3 HClO₄ was examined by UV visible spectroscopy, using a Perkin Elmer Lambda 19

Spectrometer. The spectrum showed a peak at 275nm with shoulders at 312 and 382nm. as previously reported by Elding. A low resolution ¹⁹⁵NMR spectrum of the solution (recorded on a Bruker AM360 spectrometer) showed a single peak with δ = +36ppm (reference PtCl, ) which compares well with the literature value of 31ppm (Appleton et al. Inorg Chem, 23.(1984), 3514) in view of the dependence of the maximum on pH, ionic strength and temperature. The solution also contained some potassium perchlorate and its concentration was estimated from its solubility as 20 mmol dm^'3. Electroplating

In a still solution and at room temperature, the mass transport controlled current to a planar electrode for the deposition of platinum from the solution of 1.5g/l Pt as Pt(H,O)₄(ClO₄)₂ in 1 mol dm^'3 HClO₄ will be

<0J A dm^'2. Hence, a series of plating experiments were carried out using solutions of 1.5g/litre Pt as Pt(H O) (ClO ) in 1 mol dm^'3 HC10 at 293K, a copper substrate and with current densities in the range 0.05-0.5 A dm^'2. The results are reported in Table 1. At the lowest current density, the current efficiency is good but naturally it dropped off at higher rates of deposition.

Perhaps surprisingly, the deposits were reflective and smooth. Moreover, the adhesion of the deposits was excellent at all current densities and no cracking occurred when the electroplated Cu panels were repeatedly bent through 120°.

The bath electrolyte of Examples 2, 3 and 4 were prepared in the same way as the bath electrolyte of Example 1.

EXAMPLE 2

A bath electrolyte containing 3.5g/litre platinum as Pt(H₂O)₄ ²⁺ was prepared and a nickel panel was electroplated using a still bath at room temperature. At current densities of 0J A dm^'2 and 0.2 A dm^'2, metallic deposits were obtained and the current efficiencies were >80%.

EXAMPLE 3

A bath electrolyte containing 3.5g/litre platinum as Pt(H₂O)₄ ²⁺ in 1 mole dm'³ HClO was prepared and a nickel panel was electroplated using a still bath at 60°C. At current densities of OJ, 0.2 and 0.4 A dm^'2, metallic deposits were obtained and the current efficiencies were >80%.

EXAMPLE 4

A bath electrolyte containing 3.5g/litre platinum as Pt(H₂O). 2+ 4 in 1 mole dm^"3 HClO₄ was prepared and a nickel panel was electroplated using a highly agitated bath at 60°C. At current densities of 0.2, 0.4 and 0.8 A dm^'2, metallic deposits were obtained and the current efficiencies were >80%.

Claims

1. A platinum plating bath electrolyte, comprising a platinum(II) salt, wherein the Pt(II) is present in solution as Pt(H,O)₄ ⁺.

2. A platinum plating bath electrolyte according to claim 1, wherein the anion component of the salt is one or more group or radical derived from an organic acid or an inorganic acid.

3. A platinum plating bath electrolyte according to claim 2, wherein the anion component of the salt is at least one of perchlorate, sulphate and phosphate.

4. A platinum plating bath electrolyte according to any preceding claim, wherein the concentration of the platinum in said electrolyte is at least 0.005 mole dm ³.

5. A platinum plating bath electrolyte according to claim 4, wherein the concentration of platinum in said bath is 0.005-0J50 mole dm"³.

6. A platinum plating bath electrolyte according to any preceding claim, for operation at a temperature of 0-100°C.

7. A platinum plating bath electrolyte according to claim 6, for operation at a temperature of 15°C to 60°C.

8. The use of a platinum(II) salt as defined in any of claims

1-3, in an electroplating electrolyte for the plating of platinum or a platinum alloy film.

9. A process for the electroplating of platinum or a platinum alloy film onto a conductive substrate, characterised by the use of a platinum plating bath electrolyte as defined in any one of claims 1-5.

10. A process according to claim 9, wherein the platinum plating bath electrolyte temperature is 0-l()ϋ°C.

11. A process according to claim 10, wherein the platinum plating bath electrolyte temperature is 15°C to 60°C.

12. A process according to any of claims 9-11, operated at a current densitv of 0.03 to 10 A dm^"2 of substrate surface.