Zinc Oxide Aerogels

Materials

    • Zinc nitrate hexahydrate, Zn(NO3)2•6H2O
    • Propylene oxide, C3H6O
    • Methanol, CH3OH

Optional

  • Ethanol
  • Isopropanol
  • Acetone

Warning!

Propylene oxide is a known carcinogen (exposure can cause cancer), and epichlorohydrin is probably too.  If you plan on doing this procedure, take the proper precautions to prevent your exposure to the vapors of these substances 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!

Gel Preparation

  1. Weigh out 0.238 g (0.8 mmol) of Zn(NO3)2•6H2O.
  2. Dissolve the Zn(NO3)2 in 1.25 mL (0.99 g) of methanol.
  3. Add 0.465 g (8 mmol) of propylene oxide. The solution should start out colorless, get cloudy, and then turn opaque white as it gels.
  4. Stir solution rapidly for 2 min. Pour into mold. Gel time should be 8-10 h.

What Doesn’t Work

  • Using water as a solvent. No gelation will occur.
  • Using ethanol, isopropanol, or acetone will give you gels but the aerogels derived from them won’t be monolithic (they will be crumbly). The gel times for these solvents should be:
    • Ethanol 5-7 h
    • Isopropanol 2-3 h
    • Acetone 5-7 h

These gel times probably have to do with the different solubilities of the ZnO sol particles in these solvents.

Gel Processing Conditions

  1. Allow gels to age in sealed mold for 3-5 days at room temperature.
  2. Exchange gels into acetone over the course of week changing the acetone every day.
  3. Place gels in supercritical dryer. Exchange into liquid CO2 over the course of 2-4 days.
  4. 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 white, opaque, zinc oxide aerogel with decent monolithicity and approximately the following properties:

  • Density 0.04 g cm-3
  • Surface area 277 m2 g-1
  • Pore volume 0.17 cm3 g-1
  • Average pore diameter 3.8 nm

Interesting Notes

The materials as-prepared have some crystallinity which can be enhanced by annealing. A possible annealing procedure would be to heat the aerogels to 150°C (in air) at a rate of 1°C min-1 and to hold for 5 h at temperature. This will give you results which you can compare with the annealing studies done in the reference below. The annealed materials will photoluminesce under a blacklight (UV) to give emission around 397 nm (near-UV) and 469 nm (blue). That means under a blacklight they will glow blue!

References

  1. Yanping P. Gao, Charlotte N. Sisk, and Louisa J. Hope-Weeks*, “A Sol–Gel Route To Synthesize Monolithic Zinc Oxide Aerogels”, Chemistry of Materials, 2007, 19(24), 6007-6011. DOI: 10.1021/cm0718419.

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