Mechanism of visible photon absorption: unveiling of the C3N4–ZnO photoactive interface by means of EPR spectroscopy

This study elucidates the working mechanism upon visible light exposure of the mixed C3N4–ZnO material. Structural (XRD), morphological (TEM) and optical (UV-vis spectroscopy) measurements have highlighted the intimate contact established at the interface of the biphasic solid and enhanced photoactivity arising under visible irradiation with respect to the pristine compounds. Electron paramagnetic resonance (EPR) spectroscopy analysis coupled with in situ irradiation, performed at 77 K, and supported by accurate simulations, demonstrated that the charge carrier dynamics at the C3N4–ZnO interface is governed by a direct Z-scheme heterojunction mechanism rather than that of a type-II heterojunction system. The experimental results suggest that the photoexcited electrons in the ZnO conduction band annihilate the holes in the C3N4 valence band, as in a solid direct Z-scheme system, allowing an improved charge carrier separation and stabilizing both electrons and holes at the best reductive and oxidative potentials, respectively.