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

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Can students actually do nanoscale science and engineering?

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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

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Zinc ions are Reduced at the Copper Cathode V Zn 2+ + 2e - –> Zn (0) reduction ZnNO 3 dissociates in water Zn (0) –> Zn 2+ + 2e - oxidation Zinc anode Copper cathode Zn +2 NO 3 -1 - e - I

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A Simple Electrodeposition Circuit

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Electrodeposition of a Thin Film Assemble an electrodeposition circuit with a switch in the off position.Assemble an electrodeposition circuit with a switch in the off position. Clean copper and zinc electrodes.Clean copper and zinc electrodes. Carefully install the electrodes on the bracket as you lower them into a solution of zinc nitrate.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.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.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.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.Carefully put the electrodes on a paper towel to dry before making measurements of the length and width of the electroplated zinc metal.

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

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

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Sample Data Time of Trial: 5 minutes = 3.0 x 10 2 seconds Width of copper electrode in solution2 cm = 2.0 x 10 -2 meters Length of electrode in solution 5 cm = 5.0 x 10 -2 meters Average ammeter reading0.017 ampere = 1.7 x 10 -2 Coulomb/sec

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Zn 2+ + 2e - –> Zn 0 The number of zinc ions that undergo reduction and become neutral atoms depends on the number of electrons that pass through the circuit Zn 2+ 2e - ammeter I

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Calculate the Number of Zinc Ions that 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 10 18 e/C)(3.0 x 10 2 s) = 3.18 x 10 19 e Step Two. Calculate the number of zinc atoms that formed. 3.18 x 10 19 e = 1.59 x 10 19 atoms of Zn formed 2 electrons for each Zn ion

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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 10 7 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 10 8 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 10 7 atoms in a row) x (1.93 x 10 8 atoms in a column) = 1.49 x 10 16 atoms

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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 10 16 atoms = 2.98 x 10 16 atoms Students will probably observe that more zinc atoms were deposited on the side of the copper electrode facing the zinc electrode.

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Is the thin layer of Zinc a Nanoscale Structure? Step 7: Calculate the average number of layers of zinc atoms. __1.59 x 10 19 atoms of zinc__ = 5.34 x 10 2 layers of atoms 2.98 x 10 16 atoms / layer Step 8: Calculate the average thickness of the layer of zinc. 5.34 x 10 2 layers x 2.48 x 10 -10 m/layer = 13.24 x 10 -8 m

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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

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

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The Gibbs Free Energy Equation can be used to describe electrodeposition. G = H - T S The Gibbs Free Energy equation indicates if a chemical change is exergonic (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

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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.

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