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Advanced oxidation processes (AOPs) involving the conjugation of H2O2 with metal oxide catalysts such as TiO2 have been studied for a long time because they enable efficient degradation of pollutants in wastewater. The combination of H2O2 and TiO2 is well known to generate oxidizing agents such as •OH and •O2 – radicals by catalytic reactions. However, the reaction mechanism for the production of these radicals is controversial. Here, we investigated the H2O2-dosed surface of rutile TiO2(110) by low-temperature scanning tunneling microscopy (LT-STM). We successfully probed the intermediate step of AOP at the single-molecule level. Ti–O–O–Ti peroxides were formed on the surface of aqueous H2O2 vapor-dosed TiO2(110), whereas H2O molecules were hardly found on the surface until the dosing concentration of H2O2 exceeded 0.04 Langmuir, although 30% aqueous H2O2 solution was used. H2O2 could be adsorbed only at the oxygen vacancy, which limited the number of Ti–O–O–Ti peroxide molecules. The formed peroxides could generate •OH radicals by further reaction with H2O molecules. Direct observation of the intermediate step of AOP upon adsorption of H2O2 molecules enabled us to understand the mechanism of the •OH radical-generating reaction.

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The photocatalytic effect of TiO2 has attracted a great deal of interest due to its many applications, especially water splitting. However, the wide band gap of TiO2 limits its efficiency as a photocatalyst in practical applications. Considerable efforts have so far been made to extend the spectral response to the visible light region. Here, we report the dark catalysis of water dissociation on a TiO2(110) surface. C60 molecules “decorated” this surface and acted as molecular electron acceptors. This adsorption of C60 molecules on TiO2 led to an increase in the surface hole concentration due to charge transfer, eventually resulting in the dissociation of water molecules in the absence of photons. Hydroxyl radical was formed as a dissociation product, indicating that water dissociation occurred only via oxidation of H2O by the holes. The method presented here is simple and can be widely applied for tuning and enhancing the catalytic activity of various photocatalytic systems.