Nanoscale Electrodeposition Rob Snyder July 2102

Teapots can be electroplated with a thin layer of silver on a more rigid metal to give them an attractive and durable finish.

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

These are the components of an electrodeposition circuit that you will use today

A Schematic Diagram of the Circuit Zn 2+ and SO 4 -2 ions in solutionNeutral Zn o and Cu o atoms

There are a variety of learning goals for this activity. Students can learn how: Electrical energy can drive an electrodeposition process. A masking process can produce specific patterns of electrodeposition. To apply the concepts of oxidation and reduction to a description of the electrodeposition process. Use electric current, time and atomic radii data to calculate the depth of a layer of metal that has been deposited on an electrode.

Water is not a good conductor of electricity. A small amount of Zinc Nitrate is dissolved in water to provide some ions so that the solution conducts electricity. ZnNO 3 dissociates in water Zn +2 NO 3 -1 NO 3 -- Zn +2

Copper and Zinc Electrodes are put into the solution. A battery provides a source of electric potential difference (voltage). V Zinc anode Copper cathode NO 3 -1 NO e - I Zn +2 Zinc ions in solution migrate in the direction of the copper electrode. More zinc ions are produced at the zinc anode. Zn +2

Neutral Zn o atoms in the anode are oxidized and become Zn +2 ions. Zn +2 ions are reduced at a copper cathode and become neutral Zn o atoms. V Zn e - –> Zn (0) reduction ZnSO 4 dissociates in water Zn (0) –> Zn e - oxidation Zinc anode Copper cathode Zn +2 NO 3 -1 NO e - I

A document describes a procedure for electrodepositing a thin film of Zn atoms onto a copper electrode. 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.

You can use painters tape to mask a portion of a copper electrode and do a second electrodeposition. After the masking electrodeposition, we will discuss how to calculate the thickness of the layer of zinc atoms on the copper strip.

It is important to understand what an ammeter in the electrodeposition circuit measured.

An ammeter measured the number of electrons lost by Zn atoms during oxidation at the Zn anode. That was equal to the number of electrons gained by Zn +2 ions during reduction at the copper cathode. A coulomb is equal to 6.24 x elementary charges (electrons). One Ampere = 6.24 x electrons lost and gained at the electrodes each second.

Milliamperes Using a 1.5 volt D Cell battery may result in milliampere (mA) ammeter reading. An example would be a current reading of 17 milliamperes. 17 mA x 1 ampere = amperes 1000 mA

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 10 2 seconds Width of copper electrode in the solution2 cm = 2.0 x meters Length of electrode in the solution 5 cm = 5.0 x meters Average ammeter reading17 milliamperes Average ammeter reading0.017 ampere = 1.7 x Coulomb/sec

A sample calculation of the number of Zinc Ions reduced at the copper cathode. Step One. Calculate the number of electrons that zinc ions gains in 5 minutes. (1.7 x C/s)(6.24 x e/C)(3.0 x 10 2 s) = 3.18 x e Step Two. Calculate the number of zinc atoms that formed x e = 1.59 x atoms of Zn formed 2 electrons for each Zn ion

A zinc atom has a diameter of 2.59 x meters 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 m = 7.72 x 10 7 atoms in a row 2.59 x 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 m = 1.93 x 10 8 atoms in a column 2.59 x m/atom Step 5: Calculate the number of atoms in one rectangular 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 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 atoms = 2.98 x 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 atoms of zinc__ = 5.34 x 10 2 layers of atoms 2.98 x atoms / layer Step 8: Calculate the average thickness of the layer of zinc x 10 2 layers x 2.48 x m/layer = x 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.

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

Another possible electrodeposition trial.

How many big ideas of nanoscale self-assembly were a part of this activity? 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)