Superhydrophobic Surfaces
(Electroless Deposition with Alkanethiol Treatment)

The electroless deposition idea for superhydrophobic surfaces is based on a paper by Iain A. Larmour, Steven E. J. Bell, and Graham C. Saunders, "Remarkably Simple Fabrication of Superhydrophobic Surfaces Using Electroless Galvanic Deposition," Angew. Chem. 2007, 119, 1740 –1742 and two papers by Feng Guo, Xunjia Su, Genliang Hou and Ping Li, "Bioinspired fabrication of stable and robust superhydrophobic steel surface with hierarchical flowerlike structure," Colloids and Surfaces A: Physicochem. Eng. Aspects 401 (2012) 61-67 and "Superhydrophobic silver surface with dendrites structure on steel substrate by a facile electroless galvanic deposition process," Applied Surface Science 258 (2012) 4906-4910 which use a different redox reaction and a different coating.

Begin by using a redox reaction to deposit copper nanoparticles on zinc, followed by treating part of the copper with octadecanethiol to give a metal thiolate. Then using drops of water as a probe and the principle of "like attracts like" classify which surfaces are like water.

Clicking a thumbnail on this web page (http://chemistry.beloit.edu/edetc/nanolab/Zn_Cu/index.html) shows a movie of that step.

Procedure

Wear eye protection

Chemical gloves recommended

Lay a strip of galvanized sheet metal (Zn coated steel) on a paper towel. Lightly polish the metal strip with #0 steel wool. Discard the paper towel and small steel wool fragments while trying to avoid leaving a mess on the bench.

Rinse the polished part of the metal strip with pure water and wipe dry.

Obtain a 10 mL beaker that is half-full of 0.080 M CuSO₄ NaCl solution. Dip part of the strip in the solution for 20 seconds. Try not to move the metal so that the interface stays well defined. What do you observe? The chemical reaction that occurs is Zn(s) + Cu⁺² → Zn⁺² + Cu(s). Ions and metals are different!

Rinse with pure water by spraying the uncoated part and letting the water run down over the coated part. Wait for the metal to appear dry. (For faster drying hold the zinc end and use a hair drier over the copper end. Overheating can oxidize the copper so keep the temperature skin comfortable.)

If you examined the surface by Scanning Electron Microscopy (SEM) you would see copper nanoparticles.

Quarter-fill a 10 ml beaker with 0.004 M octadecanethiol in ethanol in order to dip half the dry copper coating for 60 seconds. Try not to move the metal so that the interface stays well defined. What do you observe? Remove the metal strip and wait for the ethanol to evaporate. You will then have three surfaces: the orignal zinc, copper on zinc, and octadecanethiol on copper on zinc.

Data

Test with small drops water to compare coated and uncoated regions. Do the water drops spread out or bead up? How attracted are the water drops to the Zn surface, the Cu nanoparticle surface, and to the alkanethiol coated Cu nanoparticle surface? Make a sketch of your observations. Work on answering the first four numbered questions below. Careful: Use your experimental observations. The movie here may be coated differently.

Use a goniometer to record the contact angle of a 4 µL water drop on the three surfaces.

Dip the metal strip in water. What does it look like? Does the appearance change with the observation angle?

Conclusions

1. Make a sketch or include a photo of your final coating step that identifies the different surfaces you prepared.
2. What happens when small drops of water are added to each of the surfaces?
3. What happens if you drag a drop of water from the alkanethiol coating to the copper coating?
4. Can you make a virtual wall by running water down the metal to the treated surface?
5. Classify the Zn, the Cu nanoparticles, and the alkanethiol coated surface as like or unlike the water drop probe.
6. What does the strip look like under water? Does the rotation angle change the appearance of the strip in water? If so, which of the surfaces change in appearance?
7. Does the coating affect how water passes through a screen? If you did not have time to do this, make a prediction.

Wire Screen (optional extension)

If there is time, repeat the above procedure starting with dipping in the CuSO₄ NaCl solution, but using a piece of Zn-coated (galvanized) screen and dipping for only 10 seconds. Does the coating affect how water passes through a screen? How?

Scanning Electron Microscopy (SEM) (optional extension)

Samples can be used directly without coating, by attaching the metal with carbon tape.

Silver (optional extension)

Repeat the entire experiment but replace the metal ion dip step:
Obtain a 10 mL beaker that is half-full of 0.080 M AgNO₃ solution. Dip part of the strip in the solution for 20 seconds. What do you observe? What chemical reaction occurs?

Gold (optional extension)

Repeat the entire experiment but replace the metal ion dip step:
Obtain a 10 mL beaker that is half-full of 0.010 M HAuCl₄ solution. Dip part of the strip in the solution for 2-3 minutes until no new bubbles form. Agitate to remove bubbles for a more even coating. What do you observe? What chemical reaction occurs?

Materials

• 10 mL beakers
• Galvanized steel sheet (26 gauge, 12" x 18"), Ace Hardware 5080122, cut into 1 cm x 5 cm strips
• Galvanized Steel Wire Cloth 24 x 24 Mesh Size, 0.028" Opening Size, 1 Foot x 1 Foot, McMaster Carr 9220T92, cut into 1.5 cm x 5 cm strips
• Grade #0 steel wool
• 0.080 M CuSO₄ with .080 M NaCl. Dissolve 10.2 g CuSO₄^.5H₂O and 2.35 g NaCl in 500 mL water.
• CH₃CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂SH
  0.004 M Octadecanethiol in 100% ethanol. Dissolve 0.12 g in 100 mL 100% ethanol.
• Water for rinsing

Samples can be reused several times by cleaning with steel wool.

Materials (for optional extensions)

• 0.080 M AgNO₃. Dissolve 0.34 g AgNO₃ in 25 mL water.
• 0.010 M HAuCl₄. Dissolve 1.00g HAuCl₄^.3H₂O in 250 mL water.
• Contact angle goniometer (optional)
• Electron microscope (optional)

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