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Topic: Removing oxide layer from titanium in order to deposit silver through redox  (Read 7211 times)

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Offline lueo

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Hello all,
I have few questions on reducing Ag ions onto titanium surface. Any help would be appreciated!!
By looking at the reduction potential of silver ions and titanium, it seems Ag+ ions can oxidize pure titanium and reduce to solid Ag.

Ag+ + e- >> Ag(s) ; Eo=+0.80
Ti2+ + 2e- >> Ti(s); Eo=-1.630

Then is it correct to think that if I put a piece of pure oxide-free titanium in a solution of silver ions (eg. dissolving AgNO3), layer of silver will form on the titanium surface?

I have these titanium cylinders that are 5mm long and 0.5mm in diameter. These objects have been sitting in air for more than a year, so is it safe to assume there is a layer of oxide (TiO2) formed on the surface? According to the table of reduction potential:

TiO2 + 4H+ + 2e- >> Ti2+ + 2H2O; Eo=-0.502

If I remember correctly from high-school chemistry, this means TiO2 CANNOT be oxidized by silver since they are on the same side of the table.

Then, can I simply add my titanium cylinders into a basic solution of NaOH in order to remove the TiO2 to expose the Ti underneath? And then quickly put that into silver solution? (I've read somewhere that TiO2 forms at a very slow rate in air.)
Thank you all for your *delete me*

Offline Enthalpy

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Re: Removing oxide layer from titanium in order to deposit silver through redox
« Reply #1 on: September 04, 2013, 06:20:43 PM »
Yes, Ag would deposit on bare Ti, but how would you get Ti without oxide?

Ti oxidizes immediately in air, in watery liquids, in nearly anything. No chance with NaOH.

Once a thin layer is formed, further oxidation gets very slow, making titanium very corrosion-safe, like silicon, tantalum or pure aluminium.

Though, maybe a redox reaction works through the native oxide layer. Surfaces are tricky. Just try! I doubt it works easily, since the native oxide is known to be hermetic; electrodeposition may improve.

Better: you don't need a redox reaction to deposit silver. There is a catalytic reaction that makes silver layers on nearly anything including glass. This one should work on titanium.
http://en.wikipedia.org/wiki/Tollens%27_reagent

Offline lueo

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Re: Removing oxide layer from titanium in order to deposit silver through redox
« Reply #2 on: September 05, 2013, 02:43:12 PM »
Hi Enthalpy,

Thank you for your reply! So now that I know there will always be a layer of oxide on Ti surfaces, then I have a new question:

I've read in this literature, where they are trying to deposit ~nm thick silver on TiO2 nanowires. I've attached the section where the paper introduces their method in depositing silver. (Also an Imgur link: http://imgur.com/cKTRzy4) . This is the reason why I looked into silver deposition through redox.

My question is: what exactly is the reaction that is going on in this paper that allows the silver to reduce to TiO2 nanowires? As I've mentioned in the earlier post, TiO2 do not seem to like to receive electron, but only lose them. Their metal source was simply AgNO3 powder.

And I'm reading more into this Tollens reagent, I'm sure I'll have more questions regarding that one soon.

Offline opsomath

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Re: Removing oxide layer from titanium in order to deposit silver through redox
« Reply #3 on: September 05, 2013, 04:49:02 PM »
Acidic fluoride etches like NH4F will likely remove the oxide coating, leaving the surface terminated with H or F atoms - it works in silicon, anyhow. But first, try cleaning it well and using Tollens' reagent as others have suggested. Note, fluoride acid reagents are very, very hazardous.

Offline 408

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Re: Removing oxide layer from titanium in order to deposit silver through redox
« Reply #4 on: September 05, 2013, 05:13:12 PM »
I have a feeling a more difficult issue will be getting a nice silver plate.

I have used titanium as an electrode regularly, and the oxide layer has never prevented electron transfer.

But what happens to titanium in solution?  Ti (whatever+) does not like existing as hydrated cations.  It loves oxygen so you get instant hydrolysis to titanium oxide.  As this titanium oxide is precipitating right at the electrode, you will have a very difficult time getting a nice surface.  You will need to find a solution that will hold titanium in solution, which I have a feeling will be VERY acidic or even non-aqueous.   

Offline Enthalpy

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Re: Removing oxide layer from titanium in order to deposit silver through redox
« Reply #5 on: September 06, 2013, 12:08:13 PM »
I don't believe Ag can react with TiO2. Both are already in their favoured state. In the quoted paper, something else must decompose AgNO3, and I bet for ethanol.

The reagents in that paper resemble a bit Tollens' ones, where the reaction is not Ag+ receiving an electron. It first builds a complex.

As the quoted paper deposits Ag on pure TiO2, you shouldn't worry about Ti's oxide layer. Which to my eyes speaks in favour of a catalytic deposition, not a redox.

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Thin enough native oxide on Ti, or elsewhere, does not prevent current flow. A good oxide layer (ultra-pure Al, Ta, Ti, Nb...) has a breakdown voltage, below which it insulates, and above it conducts (the curve is fuzzy of course). The native layer has a faint breakdown voltage, so if the (ultra-pure) electrolyte builds a clean oxide layer, the layer grows until the resulting breakdown voltage equals the applied one.

Consequently, the layer is very uniform, and its thickness just fits the forming voltage. This is an excellent way to produce capacitors for low-voltage and high capacity, where plastic films would be too thick. Al, Ta, Nb are used for that purpose.

It's the reason why I suggested electrodeposition. Though, you chances of success depend on how the oxide film conducts electricity: by electrons, or by transport of oxygen ions, or transport of titanium ions, or transport of protons...

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With silicon we use (used, I stopped long ago) HF (very hazardous indeed, better keep off) to remove oxide layers, but the native layer reforms as soon as (<<1s) silicon isn't wet any more; or even, it forms continuously in the liquid and gets etched as quickly.

Offline lueo

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Re: Removing oxide layer from titanium in order to deposit silver through redox
« Reply #6 on: September 06, 2013, 04:33:01 PM »
Hi Enthalpy,

I also happened to come across another paper that talks about reducing Ag+ by ethanol, which explains why the addition of ethanol was needed.

I thought about electrodeposition, but my titanium cylindrical substrate is quite small (5mm in length and 0.5mm in diameter). I've tried to solder a wire onto it, but I don't think my solder gun was hot enough to melt Titanium. How would you turn such a small titanium into an electrode?

(A quick question: when a native oxide layer forms on Titanium, am I correct to assume it's TiO2 or could it be in any other forms? )

So I actually tried the above mentioned paper's method quickly the other day. Few things to note:

Since NaOH was added, the silver ions formed Silver Oxide (first step in Tollen's reagent) which was a brown precipitate. I did not bother filtering them out, so my titanium cylinders were kept in this brown suspension.

I kept it at 50 degrees for 8 hours, and under a simple microscope (10X) I observed some sort of "chipping" away of the surface (see attached images) compare to non-treated titanium.



Non-treated titanium cylinders:



Would anyone have an idea what happened? Are those TiO2 layer that was "chipped" away? Or is it something else?
« Last Edit: September 06, 2013, 04:56:06 PM by Borek »

Offline Enthalpy

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Re: Removing oxide layer from titanium in order to deposit silver through redox
« Reply #7 on: September 08, 2013, 06:44:52 PM »
Solder on titanium: I didn't try, but did with aluminium and stainless steel (as nearly any electronics engineer has), and usually it doesn't work. I expect the native oxide layer to prevent soldering on Ti just as it does on Al. The solder doesn't wet the oxide and can't reach the metal. Can you make a mechanical contact, using some spring? Usually not very good, but it may suffice.

Could the exfoliated material be the deposited silver that sticks badly on titanium oxide?

Composition of the oxide layer: no idea... With ultra-pure materials, the dielectric constant of TiO2 applies well, but in less perfect conditions, you must have varied oxides and hydroxides and ions in it - including oxides and hydroxides from metallic impurities.

Offline 408

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Re: Removing oxide layer from titanium in order to deposit silver through redox
« Reply #8 on: September 08, 2013, 11:17:03 PM »
You added NaOH to a silver solution, precipitating silver (hydr)oxide and then tried to plate silver out?  Not going to happen, your silver will have been precipitated out, and solid/solid reactions will not occur under these circumstances. 

This is going to sound a bit crazy, but could you heat the rods under molten AgNO3?

Or, as these rods are small you might get lucky if you mix them dry with silver nitrate and then ball mill them with ceramic media for a few hours.

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