Has any one used Ammonium fluoride to etch silica layer? I was planning to use HF to etch silica but then its really daunting when I think about HF toxicity. I will be using my material for in vitro studies after I etch silica so I want to know if ammonium fluoride is less toxic and safer than HF. Any input about AF would be of great help. Thanks
Being a solid, it would certainly be safer to store and handle. But once dissolved in water, it's still going to contain toxic fluoride ions, though it's going to outgas less of the gaseous acid. It's also going to be a slower, weaker etchant.
You'll need to know exactly what you are doing. You could use ammonium fluoride, or the more acidic ammonium bifluoride, which can be considered a 1:1 mix of ammonium fluoride and HF. The semiconductor industry uses various ammonium fluoride to HF ratios for etching, to get a more controllable etching mixture than pure HF (buffered etching solutions). You should be aware that on some materials, pure HF or HF with ammonia can produce different surface properties after etch. If that would matter in your application, look it up in the literature, see if anyone has tried the two before on your specific type of material and if they found differences.
For safety, you're still dealing with a nasty toxin, similar to dilute HF solution. I'd say that, if your lab is equipped to handle the ammonium fluoride safely, it's also possible to handle HF safely. If you can't handle HF safely, acidified ammonium fluoride will be dangerous too.
Most important would be to have a reliable fume hood, dedicated space for it, all the required protective equipment, a cleanup kit for spills (always have it nearby) and antidote in case it splashes on the skin (calcium gluconate gel, usually). Making sure everyone who might use the lab is aware of HF-related activity is vital, and places where HF is in use should be marked clearly.
Also, get the "HF medical book" from the location below, and have a copy available in the lab. If a serious incident happens and you need medical attention take the document with you to the hospital, so doctors who might not be very familiar with this type of burn will quickly know what to expect.
http://www.honeywell-pmt.com/sm/hfacid/prod-applications-sub/tech-docs.html
Thanks for the input. I understand AF is almost equal to HF. We do have special fume hood for HF usage and I will be using teflon tubes for silica etching.
My worry is that how am I going to remove the residual silica (and remaining HF?) . How can I make sure that there is no more HF in the solution. After etching the Silica I am going to test my nanoparticle toxicity against tumor cells and the remaining HF (if at all any) in the solution could harm me (and the cells as well!)? So, I want to make sure that HF is completely removed after etching.
Good question. One for which, unfortunately, no general answer exists. Precipitate the particles, centrifugation, precipitate the HF chemically using calcium salt, boiling off the HF (passing the fumes over a washing column, obviously, so they don't all enter the atmosphere), filtration, ion exchange resins,... A lot will depend on what kind of treatment your specific particles can tolerate.
What about a buffered oxide etch? This is a mixture of a few % HF with ammonium fluoride and ammonium bifluoride to maintain a constant pH. It's less dangerous that neat HF(aq) and it etches much more slowly, quite a bit less than 0.1 microns/minute at RT compared to a concentrated HF solution which would be more like 1 micron/minute. This makes it more controllable so it's used for processing semiconductor wafers. This low rate might be a good thing or a bad thing, but you can speed up the etching by heating. Every 10 degC increase roughly doubles the etch rate.
Ammonium fluoride ― when used as etchant ― either in pure aq. sol. or mixed with hydrofluoric acid ― can often be substituted by NH4HF2 (or KHF2) + HCl conc. aq.; or by hexafluorosilicic acid (fluosilic acid).
Buffered Hydrofluoric Acid (BHF) or Buffered Oxide Etch (BOE) (*) often contain both NH4F and HF aq.
(*) https://en.wikipedia.org/wiki/Buffered_oxide_etch
On either the pH of hydrofluoric acid ― ammonium fluoride aq. sol., and on the pH of (pure) ammonium fluoride aq. sol. ― cf. my posts at:
https://www.researchgate.net/post/Calculate_pH_after_addition_of_base
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