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"Platinum Plating over Tungsten?"
I would like to Platinum plate a very fine tungsten wire (0.0005") to lower it's contact resistance and polarization in a physiology experiment. I have Platinizing solution (3% chloroplatinic acid from VWR) and the fine (insulated) wire. The idea is I cut the end off the wire (that should make it pretty clean if my scissors are clean) and then do the electroplating. Here are my questions: The wire is so fine that I have to use a low voltage and current limit so I won't get too much, I've plated to PtIr wire at 0.5V but I don't know the relative electronegativity of the Tungsten. What voltage should I use? Is a one Meg ohm current limiting resistor going to get the current density low enough for this tiny surface area I'm working with? What other cleaning or prep would help the Pt adhere to the W? I have electro-etched W wires in NaOH and NaCN (in the hood of course) in the past but I don't know if this would help here. Finally I want a "rough" surface (to increase the surface area without increasing the volume) Should I run the current back and forth? Keep the plating time short? or what protocol would be best to build up just a few micra of Pt on the surface of the W, and will it stick at all? I enjoyed reading your adminishments to all the kids and parents wanting to do "electroplating" in the kitchen, but this is real serious science I am doing here, but I'm a biologist and don't remember my p-chem all that well. Thanks in advance, Gavin Perry PhD.Gavin Perry
Washington U. School of Medicine - St. Louis MO
Gavin, In the course of a literature search on another subject in Chem Abstracts I came up with the following which may be of help: 1) Chem Abs. 124:349392c (1996) - "Pretreatment of tungsten and tungsten alloy for electroplating:" W or W alloy is activated with hydrofluoric acid at room temperature to remove passivation film and to created a rough surface. The activation improved the adhesion of the electrolyte. The original article is in Chinese, author- Yuan, Weibung, journal: Diandu Yu Huanbao vol. 16, 3-4 (1996). 2) Check out J. Vac. Sci Technol. A1993, 11:2863-4 which according to the brief abstract deals with vacuum deposition on W filaments. They may give their method of surface preparation in the full article. Finally, have you looked at the entry under "tungsten" which appears in Earl C. Groshart's chapter "Preparation of Basis Metals for Plating," page 207 in the 1999 edition.Aryeh Asher
- Rehovot, Israel
The activation of W for electroplating it is not simple at all. First W oxidizes so fast at ambiant air and yields a very thin film of Tungstene-oxides. So you can use HF / HNO3 of course, but it is really not satisfying to work with such a dangerous acid like HF. If you want to rough up the W-Surface there is a very good method I found out. Threat the W in a solution of 5 % Sodiumperoxodisulfate Na2S2O8/ 5% NaOH sodium hydroxide at roomtemperature about 2 minutes simply by immerging the W in the solution. Or in a 1 % Na2S2O8, 1 %NaOH-solution but with an anodic treatment at about 1,5 Volts and 3 Amperes. The activation will be successful and the surface is ready for electroplating with any metal. Another possibility is to electroplate the W directly with Gold (0,1 Mikrometer is sufficient) and then treat this Au-plated W in an oven at 400 °C under Hydrogen. Then the Au will diffound in the W-Matrix and you will create a new surface which has a strong sticking Goldsurface. This surface is ready to be plated with Pt. Or try this procedure directly with Pt with a thermic treatment after. I hope this short description can help you.Lionel Zollinger