Plasmonic Nature of the “Blue State” in the WO3 Photoelectrode: Carrier Dynamics and Its Impact on Photoelectrochemical Water Oxidation

Abstract This study discusses the photophysical nature of the commonly observed “blue state” in WO3 photoanodes during photoelectrochemical water splitting. We demonstrate, using ultrafast transient absorption spectroscopy, that photoexcitation of the blue state of WO3, generated either by oxygen vacancies or by electron injection, exhibits characteristic two-phase decays more consistent with the photophysical behavior of plasmonic excitation than with the previously proposed sub-bandgap transition relaxation mechanism. More importantly, excitation of the “blue state” produces no detectable photocurrent while its presence significantly suppresses the photocurrent generated by the interband transitions of WO3. Therefore, although there is great enhancement of light absorption, the “blue state” of WO3 severely impairs its charge-carrier dynamics and photoelectrochemical performance. These results call for a re-evaluation of the understanding of carrier dynamics in WO3 photoanodes and reveal the key limitation that should be considered when designing these materials utilizing the heavily doping strategies.