Hydrogen power from the sun
Hydrogen holds the promise of being a clean, renewable source of power.
But most current hydrogen-production
methods use natural gas in a process
that generates greenhouse gases and
consumes a nonrenewable resource. A
more environmentally friendly approach
would tap sunlight to produce hydrogen
from water. Now, development of an inexpensive
and easily scalable technique
for water photoelectrolysis — the splitting
of water into hydrogen and oxygen
using light energy — is “only a couple
of problems away,” says Craig Grimes, a
Penn State Univ. professor of electrical
engineering in the school’s Materials Research
Institute.
Making this possible are thin films of
self-aligned, vertically oriented titanium
iron oxide (Ti-Fe-O) nanotube arrays. Previously,
the group built titania nanotube
arrays that have a photoconversion efficiency
of 16.5% under ultraviolet light.
Titanium oxide (TiO2) is commonly used
in white paints and sunscreens. It also has
excellent charge-transfer properties and
corrosion stability, making it a likely candidate
for cheap and long-lasting solar
cells. However, ultraviolet light comprises
roughly 5% of the solar spectrum energy,
so it was necessary to move the materials
band gap into the visible spectrum.
It turns out doping the TiO2 film with a
form of iron called hematite — a low band
gap semiconductor material — lets the
material capture a much larger portion of
the solar spectrum. A sputtering process
puts thin films of titanium and iron on
fluorine-doped, tin-oxide-coated glass
substrates. The Ti-Fe films were anodized
in an ethylene glycol solution, then crystallized
by oxygen annealing for 2 hr.
So far, the devices have produced a
photocurrent of 2 mA/cm2 and a photoconversion
rate of 1.5%, the second highest
rate using an iron-oxide material. The
team is now looking at optimizing the
nanotube architecture to overcome the
low electron-hole mobility of iron. Researchers
hope reducing the wall thickness
of the Ti-Fe-O nanotubes to correspond
to iron’s hole diffusion length of
about 4 nm will bring an efficiency closer
to the 12.9% theoretical maximum for
materials with the band gap of hematite.