Electrochemical Methods Electrochemical Deposition is known as electrodeposition - see CHEM* 1050 - electrolysis electrodeposition is a special case of electrolysis where the result is deposition of solid material on an electrode surface. -several steps are involved -movement of the source to the electrode -possible adsorption pre- redox reactions - reduction / oxidation of the source material nanowires and nanorods can be grown in this way 25-1
we met this briefly in lecture 13 --recall anodized Aluminum gave a nanoporous network nanowires grown through the pores we need to look at this in more depth before we look at nanowire growth we need to look at some fundamentals - and review a bit as well! 25-2
When a solid is immersed in a polar solvent or electrolyte solution a surface charge is developed. Electrode potential is given by the Nernst Equation E = E 0 + RT/nF ln (a) you have not seen it quite like this before a = activity : for now you use concentration In first year you used this equation to predict the products of an electrolysis 25-3
Beautiful example Electrolysis of Aqueous NaF ***--first write down what is in solution*** We have H 2 O(R), Na + (aq) and F - (aq) Data from SRP tables 2H 2 O(R) + 2e - 6 H 2 (g) + 2OH - (aq), EE -0.83 V 4H + (aq) + O 2 (g) + 4e - 6 2H 2 O(R), EE +1.23 V F 2 (g) + 2e - 6 2F - (aq), EE +2.87 V Na + (aq) + e - 6 Na(s), EE -2.71 V Rearrange to get our possible reactants at the LHS and arrange as possible Oxidations and Reductions To be continued... 25-4
Oxidations? 2H 2 O(R) 6 4H + (aq) + O 2 (g) + 4e -, EE -1.23V 2F - (aq) 6 F 2 (g) + 2e -, EE -2.87 V Reductions? 2H 2 O(R) + 2e - 6 H 2 (g) + 2OH - (aq), Na + (aq) + e - 6 Na(s), EE -0.83 V EE -2.71 V (Anode) Oxidations? 2H 2 O(R) 6 4H + (aq) + O 2 (g) + 4e -, 2F - (aq) 6 F 2 (g) + 2e -, EE -1.23 V EE -2.87 V Written as an OX The most easily oxidized species is the one with the most positive potential6this means that in this case water will be oxidized to O 2 (g) (EE -1.23 V), rather than fluoride ion be oxidized to fluorine gas (-2.87 V). So - the "winner" at the anode is Water 6 oxidized to O 2 (g) 2H 2 O(R) 6 4H + (aq) + O 2 (g) + 4e -, EE -1.23 V 25-5
It is often more complicated than this: -concentration effects (non standard conditions) - overpotentials useful to think in this way you are starting to see orbitals now in CHEM 2060 if electrode potential is more negative (higher) than the energy level of the occupied orbital then an electron will "jump" from the metal to the solution. The solution phase species is then reduced and will deposit on the electrode electrode solution electrochemical BIAS The reverse is true for an oxidation (b) 25-6
Reactions stop when equilibrium is achieved so we can rationalize the observation we saw in first year zinc bar in copper sulfate copper plates out when two metals of different materials immerse into one electrolyte solution each electrode will establish equilibrium with the electrolyte solution. The equilibrium is destroyed if the electrodes are connected to an external circuit. -Since different electrodes have different electrode potentials this will drive the cell in the spontaneous direction Electrolysis drives in the reverse direction 25-7
In an electrolytic cell it is not necessary that all the electrodes are "active" Noble metals are often used as inert electrodes. A typical Electrolytic process composes a series of steps each step can be rate limiting 1. Mass Transfer through the solution to the electrode 2. Chemical Reactions at the electrode-solution interface 3. Electron transfer at the electrode surfaces and through the external circuit 4. Other surface reactions such as a desorption, desorption and recrystallization let's look at some of these 25-8
Mass Transfer to an electrode surface Nernst - Planck equation flux motion by diffusion concentration gradients often rate limiting process motion by migration electric fields 25-9
When electrochemical reaction occurs - say reduction of A + the conc of A + decreases. Results in diffusion to surface of electrode. Flux is controlled by concentration gradient Often migration is suppressed by using a high conc of an electroinactive electrolyte ion (eg., K + : most migration is then by the bystander ion) 25-10
Electrochemical Growth of nanorods: Difficult to do without the use of a "template" If template not used small nanowires can grow but difficult to control. Template is attached to the cathode electrodeposition of copper oxide through a crystalline protein array when electric field is applied cations diffuse toward the cathode and are reduced resulting in the growth of nanowires within the pores of the template 25-11
Typical experimental set up a b Scanning-electron micrographs of an aluminum template (a) blank and (b) with electrodeposited arrays of palladium nanoparticles. 25-12
Hollow tubular nanorods can be grown if the walls of the template tube are functionalized to preferentially bind the metal -achieved by anchoring silane molecules -for example: the surface of anodic alumina templates were covered with cyanosilanes resulting in growth of gold tubules (Science 1994) Electroless Depositions Also used to fabricate nanowires and nanorods Actually a chemical deposition process -uses chemical agent to plate out source material on the template 25-13
Why electroless processing? Low cost solution based deposition technique No vacuum deposition processing required Selective deposition No blanket electrically conductive seed layer required Possible to deposit on activated insulators Sn + Pd treatment to initiate deposition Potential batch process 25-14
Significant difference between electroless and electrolytic processes -In electrolytic process begins at bottom electrode and the deposited materials -must be electrically conductive wires grow in from sides In electroless the deposit does not have to be conductive - starts at wall of pore and moves in meaning this: wires grow up -hollow template 29-15 25-15
Length of nanorods in electrochemical deposition is controlled by deposition time -"how long the volts is on" Electroless - length is dependent on the length of the template pores or the thickness of the membrane if you like reaction time will affect width of walls of nanotubes -sometimes they never close fully polyaniline nanotubes 25-16
how do you polymerize aniline? could you design a method for making polyaniline (an electrically conducting polymer) nanotubules? 25-17