1. Presentation by Greg Brown (9-24-07)
Hydrogen Gas Generation by Splitting Aqueous Water Using n-Type GaN Photoelectrode with Anodic Oxidation
Katsushi Fujii et al., Japanese Journal of Applied Physics, Vol 44, No. 18, 2005, pp
Presentation by Greg Brown ( )

2. Table of Contents
Background on redox reactions
Semiconductor/electrolyte interfaces
Promise of using GaN
Experimental setup
Results
Summary

3. Redox reactions can be shown using band diagrams
Fermi level is halfway between reduced and oxidized states.
2H2O + 2e- -> 2OH- + H2
O2 + 2 H2O -> 4OH- + 4H +
Splitting of states due to orientation of solution. Polar water molecules can rearrange and change the energy of the oxidized molecule.

4. Putting electrolyte in contact with a n-type semiconductor
If Ef > Ef(redox) electrons will leave semiconductor and cause band bending
Electric field in the depletion region causes excited electron-hole pairs (EHPs) to be separated

5. GaN should be a good material to use in photoelectrochemical cells
N-type GaN should cause band bending
Before this paper, no evidence of hydrogen generation by GaN

6. J. W. Ager III, W. Walukiewicz, K. M. Yu, W. Shan, J. Denlinger, and J
J.W. Ager III, W. Walukiewicz, K.M. Yu, W. Shan, J.Denlinger, and J. Wu, Group III-nitride materials for high efficiency photoelectrochemical cells, Mater. Res. Soc. Symp. Pric. Vol 884E, 2005.

7. Sample used in Experiment
Grown by Metalorganic Vapor Phase Epitaxy

8. E(AgCl/Ag) = 0.212V vs SHE
Light Illuminated area = contact area with electrolyte = 10mm diameter
Light source = 150W Xe-lamp, tested with both AM0 and AM1.5 filters.
Electrolyte = mol/L KOH (ph = 14)
Ohmic contact = Ti (10nm)/Au (50nm)
Confirmed GaN band edges using electrochemical impedance spectroscopy

9. No Hydrogen gas generation when bias = 0V
Need roughly 1mA/cm^2 to observe hydrogen generation
Need bias of 1V to get generation
Photocurrent between -0.8 to 0V due to “affected by the charge accumulation the semiconductor/electrolyte interface due to the slow charge transfer process”

10. After 300min at 1V bias, gas collected = 94% Hydrogen, 6% Nitrogen, trace Oxygen
First observation of hydrogen generation
Using Faraday’s law and the equation
Hydrogen volume is calculated
Difference attributed to hydrogen being dissolved in electrolyte
GaN was etched by anodic oxidation
Lost 1.98mg after 300min. Charge required is 6.8C, half the current measured. Nitrogen gas collected attributed to this process.

11. Summary
Hydrogen generation from GaN as photoelectrode observed when applied bias = 1.0 V
Problems are weak photocurrent when no bias applied and etching of GaN during process