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Competition between charge recombination and the forward reactions required for water splitting limits the efficiency of metal-oxide photocatalysts. A key requirement for the photochemical oxidation of water on both nanostructured alpha-Fe2O3 and TiO2 is the generation of photoholes with lifetimes on the order of milliseconds to seconds. Here we use transient absorption spectroscopy to directly probe the long-lived holes on both nc-TiO2 and alpha-Fe2O3 in complete PEC cells, and we investigate the factors controlling this slow hole decay, which can be described as the rate-limiting step in water oxidation. In both cases this rate-limiting step is tentatively assigned to the hole transfer from the metal oxide to a surface-bound water species. We demonstrate that one reason for the slow hole transfer on alpha-Fe2O3 is the presence of a significant thermal barrier, the magnitude of which is found to be independent of the applied bias at the potentials examined. This is in contrast to nanocrystalline nc-TiO2, where no distinct thermal barrier to hole transfer is observed.
Kevin Sivula, Charles Roger Jean Lhermitte, Nukorn Plainpan, Annalisa Polo, Ivan Grigioni
Alfredo Pasquarello, Stefano Falletta, Kyriakos Stylianou, Stavroula Kampouri, Patrick Gono, Zhendong Guo
Kevin Sivula, Rangsiman Ketkaew