Role of surface water molecules in stabilizing trapped hole centres in titanium dioxide (anatase) as monitored by electron paramagnetic resonance

• Two different types of trapped holes (O−) in TiO2 anatase identified via CW-EPR.
• Physisorbed water stabilizes surface trapped holes in TiO2 anatase.
• Distance between O− and the nearest hydrogen (OH−/H2O) evaluated via Pulse-ENDOR.

A key factor affecting the photo-efficiency of TiO2 is strictly related to the fate of charge carriers, electrons (e−) and holes (h+), generated upon band gap excitation. In the present paper we point our attention to the nature of the hole trapping sites in the anatase polymorph monitored coupling the conventional continuous wave EPR (CW-EPR) technique with pulse electron nuclear double resonance (ENDOR) experiments. The attention is focused on the role of surface adsorbed water (both in molecular and in dissociated form) in the stabilization of photogenerated hole centres.CW-EPR results indicate that two distinct O− hole centres can be identified in Anatase (O−surf. and O−subsurf.) and that the quantitative ratio (measured in terms of spectral intensity) of these two species is markedly conditioned by the presence of surface physisorbed water. For the first time a h+-proton distance, evaluated via ENDOR measurement, is reported.

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