Titanium Dioxide Coating Stabilizes Photoanodes

Semiconducting photoanodes used to oxidize water for solar fuel production are an unstable bunch. They’re prone to corrosion and can quickly become inert. A team led by Nathan S. Lewis of California Institute of Technology has shown that a thin coating of titanium oxide can act as a protective blanket to greatly improve the performance of these photoanodes (Science 2014, DOI: 10.1126/science.1251428). TiO₂ is already well-known as a component of sunscreens, pigments, and photocatalysts. The Caltech group coated anodes with TiO₂ films 4 to 143 nm thick via atomic layer deposition. The coatings, they found, have electronic defects that promote hole conduction, are transparent enough to let through sufficient amounts of sunlight, and prevent corrosion of the active catalyst. For example, the coating allowed a silicon photoanode to run continuously for more than 100 hours with little decrease in performance. The coating also improved the stability of gallium arsenide and gallium phosphide photoanodes. In addition, the researchers write, “the ability to engineer the energy levels and the distribution of defect states in chemically stable coatings such as TiO₂ offers the opportunity to stabilize a host of narrow-band-gap semiconductors for use in oxidative photochemistry.”