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 543-545 Presentation by Greg Brown (9-24-07)

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

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.

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

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

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.

Sample used in Experiment Grown by Metalorganic Vapor Phase Epitaxy

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

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”

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.

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