The prospect for the wide spread use of hydrogen as a portable energy carrier is dependent on finding a clean, renewable method of production. At Penn State University, a research group headed by professor of electrical engineering Craig Grimes in the Materials Research Institute is “only a couple of problems away” from developing an inexpensive and easily scalable technique for water photoelectrolysis – the splitting of water into hydrogen and oxygen using light energy – that could help power the proposed hydrogen economy.
Most current methods of hydrogen production split hydrogen from natural gas in a process that produces climate changing greenhouse gas while consuming a nonrenewable resource. A more environmentally friendly approach would produce hydrogen from water using the renewable energy of sunlight.
In a paper published online in Nano Letters on July 3, 2007, lead author Gopal K. Mor, along with Haripriya E. Prakasam, Oomman K. Varghese, Kathik Shankar, and Grimes, describe the fabrication of thin films made of self-aligned, vertically oriented titanium iron oxide (Ti-Fe-O) nanotube arrays that demonstrate the ability to split water under natural sunlight.
Previously, the Penn State scientists had reported the development of titania nanotube arrays with a photoconversion efficiency of 16.5% under ultraviolet light. Titanium oxide (TiO2), which is commonly used in white paints and sunscreens, has excellent charge-transfer properties and corrosion stability, making it a likely candidate for cheap and long lasting solar cells. However, as ultraviolet light contains only about 5% of the solar spectrum energy, the researchers needed to finds a means to move the materials band gap into the visible spectrum.
They speculated that by doping the TiO2 film with a form of iron called hematite, a low band gap semiconductor material, they could capture a much larger portion of the solar spectrum. The researchers created Ti-Fe metal films by sputtered titanium and iron targets on fluorine-doped tin oxide coated glass substrates. The films were anodized in an ethylene glycol solution and then crystallized by oxygen annealing for 2 hours. They studied a variety of films of differing thicknesses and varying iron content. In this paper they report a photocurrent of 2 mA/cm2, and a photoconversion rate of 1.5%, the second highest rate achieved with an iron oxide related material.
The team is now looking into optimizing the nanotube architecture to overcome the low electron-hole mobility of iron. By reducing the wall thickness of the Ti-Fe-O nanotubes to correspond to the hole diffusion length of iron which is around 4nm, the researchers hope to reach an efficiency closer to the 12.9% theoretical maximum for materials with the band gap of hematite.
“As I see it, we are a couple of problems away from having something that will revolutionize the field of hydrogen generation by use of solar energy,” Grimes says.