Electron magnetic resonance as a tool to monitor charge separation and reactivity in photocatalytic materials

The present paper describes the role of electron paramagnetic resonance (EPR) in the investigation of fundamental phenomena occurring in heterogeneous photocatalysis, namely, the light-induced charge carrier separation, the stabilization of the carriers and their consequent surface reactivity. We will describe the behavior of a series of photoactive oxides (TiO2, ZrO2, ZrTiO4 and ZnO) with different band gaps which exhibit electron and hole stabilization in all cases. The EPR technique reveals the nature of the different stabilization sites (e.g. Ti4+, Zr4+, Zn2+) for the electrons and monitors the hole trapping in terms of the formation of O center dot- centers. The electron transfer reactivity at the surface of the photogenerated carriers is then monitored using specific scavengers admitted in the gas phase over the solid, specifically, molecular oxygen and molecular hydrogen for electron scavenging and hole scavenging, respectively. The method illustrated herein is particularly useful as a preliminary means of checking the features of novel photocatalytic materials.