Lanthanide Oxide Aerogels
- Lanthanide(III) chloride hexahydrate, LnCl3•6H2O, where Ln = a lanthanide metal
- Absolute (200-proof) ethanol
Pick A Color, Any Color
The lanthanide oxides are pleasantly pastel colored and so depending on which lanthanide chloride you pick, you can make aerogels that are pink, yellow, green, purple, and more! Additionally, the lanthanides all have pretty similar chemistry (which is why they’re lumped together the way they are on the periodic table) and so you can pretty much swap out one salt for another to change the color.
Here’s a guide to some of the colors available:
- Pr, light yellow
- Nd, purple
- Sm, yellow
- Ho, dark pink
- Er, light pink
Epichlorohydrin is probably a carcinogen. If you plan on doing this procedure, take the proper precautions to prevent your exposure to its vapors by using a fume hood in a lab, if possible, or at the very least a fitted respirator (gas mask) with the right organics cartridges and a well-ventilated space, on top of the usual splash goggles, gloves, long pants, and closed-toe shoes.
Look under Explore > Information About Chemicals to see where you can find health and safety information about these and other chemicals.
If you can’t use these substances safely, don’t use them until you can!
- Weigh out 1.33 g of LnCl3•6H2O (~14 mmol). Since all of the lanthanide chlorides have similar molecular weights, the mass of salt you use only changes by 1-2% depending on the salt, so whatever, 1.33 g is pretty good.
- Dissolve the LnCl3•6H2O in 7.89 g (10.0 mL) of 200-proof ethanol. You will get a colorful solution.
- Slowly add 3.24 g of epichlorohydrin to the solution.
- A rigid, translucent gel will form. Gel time is between 8-15 min depending on the lanthanide. For example, Ho has a gel time of 11.0 min.
Gel Processing Conditions
- Cover the gels and age for at least 24 h under ambient conditions.
- Exchange into 200-proof ethanol at least four times over the course of several days to a week. You will probably observe the gels will shrink after forming. This is normal. Expect 5-10% syneresis. Make sure you accommodate for this in your mold.
- Place gels in supercritical dryer. Exchange into liquid CO2 over the course of 2–3 days.
- Supercritically dry. A suggested procedure would be to heat the CO2 through its critical point (31.1°C and 72.9 bars) to ~45°C while maintaining a pressure of ~100 bars. Depressurize at a rate of ~7 bar h-1.
What You Should Get
A translucent, monolithic lanthanide(III) oxide aerogel.
What Doesn’t Work
- There is a limit to how dense you can make the gels. As an example, you cannot use more than about 2.6 g ErCl3•6H2O in the above procedure since that is the solubility limit of that salt in 10.0 mL of ethanol. If you need denser gels you would need to use a different solvent system, for example, 50% water and 50% ethanol.
- Exchanging the gels directly into methanol doesn’t fully remove all of the byproducts from the ring-opening reaction that results in gelation. Use ethanol or acetone.
- Using a rough mold. Since the gels will shrink after setting, they need to be able to smoothly recede from the mold surfaces a little bit. Even though it might feel smooth to you, the mold can easily be too rough for the tiny forces that the gel will be subjected to that cause it to shrink. For this reason glass or very smooth, unmachined plastic works best.