ElectrodepositionElectrodeposition

Integrating nanoscale Integrating nanoscale

science and engineering science and engineering

into middle school and high schoolinto middle school and high school

STEM programsSTEM programs

Can students actually Can students actually

dodo nanoscale science and nanoscale science and engineering?engineering?

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

Zinc ions are Reduced at the Copper Cathode

V

Zn2+ + 2e- –> Zn(0)

reduction

ZnNO3 dissociates in water

Zn(0) –> Zn2+ + 2e- oxidation

Zinc anode

Copper cathode

Zn+2

NO3-1

NO3-1

← ← e-- I →

A Simple Electrodeposition CircuitA Simple Electrodeposition Circuit

Electrodeposition of a Thin Film

• Assemble an electrodeposition circuit with a switch in the Assemble an electrodeposition circuit with a switch in the off position.off position.

• Clean copper and zinc electrodes.Clean copper and zinc electrodes.• Carefully install the electrodes on the bracket as you lower Carefully install the electrodes on the bracket as you lower

them into a solution of zinc nitrate.them into a solution of zinc nitrate.• Turn the switch on to start the electrodeposition process.Turn the switch on to start the electrodeposition process.• You can turn the copper electrode around at some point so You can turn the copper electrode around at some point so

that both sides are electrodeposited somewhat evenly.that both sides are electrodeposited somewhat evenly.• Stop electrodeposition when the copper electrode seems Stop electrodeposition when the copper electrode seems

to be covered with zinc.to be covered with zinc.• Carefully put the electrodes on a paper towel to dry before Carefully put the electrodes on a paper towel to dry before

making measurements of the length and width of the making measurements of the length and width of the electroplated zinc metal.electroplated zinc metal.

How will students know that they may have produced a structure with a nanoscale

dimension?

Mathematical operations using scientific notation becomes very useful as

students determine if they have actually created a

nanoscale structure!

Sample Data

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 10-2 Coulomb/sec

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

Zn2+

2e-

ammeter

I

Calculate the Number of Zinc Ionsthat were Reduced.

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

A strategy for the following three steps would be similar to determining how many marbles form a single

layer on a rectangular desk surface.

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

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.

Is the thin layer of Zinc a Nanoscale Structure?

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

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

Does Electrodeposition meet the criteria Does Electrodeposition meet the criteria for Nanoscale Self-Assembly?for Nanoscale Self-Assembly?

• Mobile structural components Mobile structural components • Target is low energy equilibrium stateTarget is low energy equilibrium state• Ordered structuresOrdered structures• Assembly through attraction or repulsion Assembly through attraction or repulsion

forces between the componentsforces between the components• Environment selected to induce designed Environment selected to induce designed

interactioninteraction• Components retain physical identity Components retain physical identity

through and afterthrough and after• Reversible by controlling the environmentReversible by controlling the environment

Whitesides & Boncheva (2002)

The Gibbs Free Energy Equationcan be used to describe electrodeposition.

∆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

Why choose electrodeposition Why choose electrodeposition to make nanostructures?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.