The average global energy consumption of transportation fuels is currently several terawatts (1 terawatt = 1012 Joule) per second. A major scientific gap for developing a solar fuels technology that could replace fossil resources with renewable ones is scalability at the unprecedented terawatts level. In fact, the only existing technology for making chemical compounds on the terawatt scale is natural photosynthesis.
The two reactions necessary to complete the photosynthetic cycle—CO2 reduction and H2O oxidation—take place in incompatible environments, so they have to be physically separated by a barrier. But, for the process to be efficient, the distance between the two should be as short as possible—on the nanometer scale. Natural photosynthetic systems accomplish this very well, but it presents an engineering challenge for fabricating artificial photosystems based on this design.
Foundry user Heinz Frei collaborated with Eran Edri, now at Ben-Gurion University, and Shaul Aloni at the Molecular Foundry. They developed a fabrication method to make a square-inch sized artificial photosystem, in the form of an inorganic core-shell nanotube array, that implements this design principle for the first time.
The method, described in a paper published earlier this year in ACS Nano, employs a silicon rod array as a template in combination with atomic layer deposition and cryo-etching techniques to provide control of the characteristic length scales of the components over eight orders of magnitude. While the array is fabricated on the macroscale, the diameter of individual tubes is a few hundreds of nanometers and the wall thickness a few tens of nanometers.