All Trapped Electrons in Metal Oxides Are Equivalent in Dark Photocatalysis

Materials featuring long-lived polaronic states allow round-the-clock photocatalysis to drive chemical reactions in the dark after the cessation of illumination. We report that WO 3 can store photogenerated electrons for hours, which are deployed for the dark photocatalytic reduction of methyl viologen, metal ions, methyl orange, and methylene blue. The immediate NIR bleach by electron scavengers reveals that trapped electrons are stored on the WO 3 surface. The consumption of trapped electrons in methyl viologen reduction follows an exponential decay, indicating that all trapped electrons are equivalent even at high trapped electron density, where the effect of electrostatic repulsion on trapped electron reactivity might have been expected. The biexponential kinetics of methyl orange hydrogenation, a four-electron process, indicate that the rate fades as electrons are depleted in WO 3, reflecting the importance of trapped electron density in multielectron processes. This study verifies the fundamental hypothesis of photocatalysis that the electrons in the same state responsible for photocatalysis are equivalent. Thus, electrostatic repulsion among photogenerated electrons does not need to be considered.