Electrodeposition

1. Integrating nanoscale science and engineering into middle school and high school STEM programs Electrodeposition

2. Can students actually do nanoscale science and engineering?

3. Teapots can be electroplated with a thin layer of silver to give them an attractive finish. Objects that are electroplated are first cleaned, then placed in a bath that contains ions of a decorative and durable metal that is deposited onto a stronger metal. http://encarta.msn.com/media_461526422/Electroplating.html

4. oxidation Zn(0) –> Zn2+ + 2e- Zinc ions are Reduced at the Copper Cathode ← e- I → V Zinc anode Copper cathode ZnNO3 dissociates in water NO3-1 NO3-1 Zn+2 reduction Zn2+ + 2e- –> Zn(0)

5. A Simple Electrodeposition Circuit

6. Assemble an electrodeposition circuit with a switch in the off position. Clean copper and zinc electrodes. Carefully install the electrodes on the bracket as you lower them into a solution of zinc nitrate. Turn the switch on to start the electrodeposition process. You can turn the copper electrode around at some point so that both sides are electrodeposited somewhat evenly. Stop electrodeposition when the copper electrode seems to be covered with zinc. Carefully put the electrodes on a paper towel to dry before making measurements of the length and width of the electroplated zinc metal. Electrodeposition of a Thin Film

7. How will students know that they may have produced a structure with a nanoscale dimension?

8. Mathematical operations using scientific notation becomes very useful as students determine if they have actually created a nanoscale structure!

9. Time of Trial: 5 minutes = 3.0 x 102 seconds Width of copper electrode in solution 2 cm = 2.0 x 10-2 meters Length of electrode in solution 5 cm = 5.0 x 10-2 meters Average ammeter reading 0.017 ampere = 1.7 x 10-2Coulomb/sec Sample Data

10. I 2e- ammeter Zn2+ Zn2+ + 2e- –> Zn0 The number of zinc ions that undergo reduction and become neutral atoms depends on the number of electrons that pass through the circuit

11. Step One. Calculate the number of electrons that flowed through the circuit in 5 minutes. (1.7 x 10-2 C/s)(6.24 x 1018 e/C)(3.0 x 102 s) = 3.18 x 1019e Step Two. Calculate the number of zinc atoms that formed. 3.18 x 1019 e = 1.59 x 1019 atoms of Zn formed 2 electrons for each Zn ion Calculate the Number of Zinc Ionsthat were Reduced.

12. Step 3: Calculate the number of atoms of zinc in a row across the width of the copper electrode. Note: Distances are measured in meters (m). ____2.0 x 10-2 m = 7.72 x 107 atoms in a row 2.59 x 10-10 m/atom Step 4: Calculate the number of atoms in a column along the length of the electrode that was in the solution. ___5.0 x 10-2 m = 1.93 x 108 atoms in a column 2.59 x 10-10 m/atom Step 5: Calculate the number of atoms in a single layer on one side of the copper electrode. (7.72 x 107 atoms in a row) x (1.93 x 108 atoms in a column) = 1.49 x 1016 atoms A strategy for the following three steps would be similar to determining how many marbles form a single layer on a rectangular desk surface.

13. Zinc atoms were electrodeposited on both sides of the Copper Electrode. Step 6: Calculate the number of atoms that formed a single layer on both sides of the copper electrode. 2 x 1.49 x 1016 atoms = 2.98 x 1016 atoms Students will probably observe that more zinc atoms were deposited on the side of the copper electrode facing the zinc electrode.

14. Step 7: Calculate the average number of layers of zinc atoms. __1.59 x 1019 atoms of zinc__ = 5.34 x 102 layers of atoms 2.98 x 1016 atoms / layer Step 8: Calculate the average thickness of the layer of zinc. 5.34 x 102 layers x 2.48 x 10-10 m/layer = 13.24 x 10-8 m Is the thin layer of Zinc a Nanoscale Structure?

15. The calculation of the thickness of the was based on an assumption that there were an equal number of zinc atoms in each later. If electrodeposition is managed very carefully, zinc atoms will form a hexagonal close-packed structure. http://www.geo.ucalgary.ca/~tmenard/crystal/metalstatic.html

16. Does Electrodeposition meet the criteria for Nanoscale Self-Assembly? • Mobile structural components • Target is low energy equilibrium state • Ordered structures • Assembly through attraction or repulsion forces between the components • Environment selected to induce designed interaction • Components retain physical identity through and after • Reversible by controlling the environment Whitesides & Boncheva (2002)

17. ∆G = ∆H - T ∆S The Gibbs Free Energy equation indicates if a chemical change is exergonic (when ∆G < 0) or endergonic (when ∆G > 0). The battery was a source internal energy. ∆H (Enthalpy) had a positive value. and Zinc ions moving somewhat randomly in solution become more ordered on the copper electrode. ∆S (Entropy) had a negative value. The process occurred at a relatively low temperature. As a result, ∆G > 0 The Gibbs Free Energy Equationcan be used to describe electrodeposition.

18. Why choose electrodeposition to make nanostructures? The process is easy to manage and only needs simple equipment. It is easy to control the deposition rate by manipulating voltage, current, and solution concentrations.