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.