2.5. Photoelectrochemical Character

2.5. Photoelectrochemical Characterization
Illuminated open circuit photovoltage (OCV) was measured to
find theflat band potential of each semiconductor in solution.
A ZnO nanorod array or TiO2 nanoparticle
film on FTO glass was
immersed in a three-electrode electrochemical cell and served as
the working electrode, along with a Ag/AgCl reference electrode
and Pt wire counter electrode. The electrolyte was aqueous 0.1 M
LiClO4 basified to pH 11 with NaOH. The working electrode was
illuminated from the back (non-conducting) side with a 300-W Xe
arc lamp (Oriel). The cell open-circuit voltage was monitored as a
shutter kept the sample dark for 30 s, then illuminated for 60 s,
then dark for 60 s. To avoid reaction of photogenerated electrons
with dissolved oxygen in the electrolyte, the solution was purged
with nitrogen gas for 10 minutes before measurement. According
to data from the manufacturer, the lamp outputs about 100 mW in
the spectral region between the bandgap edge of ZnO (370 nm) and
the cutoff of FTO transmission (350 nm); this is the power available
for bandgap excitation of ZnO, with somewhat more available for
TiO2 (with a bandgap edge of 385 nm). Focused onto a 1 cm2 area,
for either semiconductor this was judged to be more than enough
UV intensity to achieve a saturation condition in which the bands
flatten and the Fermi level (measured as the working electrode
potential) is close to that of the conduction band edge potential