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.