Number of Reactive Charge CarriersA Hidden Linker between Band Structure and Catalytic Performance in Photocatalysts

Tailoring the band structure of a photocatalyst without causing significant changes in further properties and deriving unambiguous relationship between the number of charge carriers (CB-e– and VB-h+) and their reactivity in a photocatalytic reaction are very challenging, but highly important for rational catalyst design. In this work, semiquantitative relationships among the band structure, number of reactive charge carriers, yield of hydroxyl (•OH) and superoxide (•O2 –) radicals, and degradation rate of oxalic acid as a model pollutant have been discovered in g-C3N4 photocatalysts by in situ electron paramagnetic resonance (EPR) coupled with an online spin trapping technique. We demonstrate that it is the number of reactive charge carriers which links the band structure of a photocatalyst with its catalytic performance. An optimum balance between the number and reducing ability of conduction band (CB) electrons (CB-e–), which depends on the interplay between the band gap and CB edge potential, is a key property for highly efficient g-C3N4 photocatalysts. A combination of (i) narrowing of the band gap and upshift of the CB edge at the same time, and (ii) using O3 instead of O2 as CB-e– trap would lead to the maximum number of reactive CB-e– and •OH and, hence, to optimal photocatalytic activity.