Activation of Water in Titanium Dioxide Photocatalysis by Formation of Surface Hydrogen Bonds: An In Situ IR Spectroscopy Study

The hole-driving oxidation of titanium-coordinated water molecules on the surface of TiO2 is both thermodynamically and kinetically unfavorable. By avoiding the direct coordinative adsorption of water molecules to the surface Ti sites, the water can be activated to realize its oxidation. When TiO2 surface is covered by the H-bonding acceptor F, the first-layer water adsorption mode is switched from Ti coordination to a dual H-bonding adsorption on adjacent surface F sites. Detailed in situ IR spectroscopy and isotope-labeling studies reveal that the adsorbed water molecules by dual H-bonding can be oxidized to O2 even in the absence of any electron scavengers. Combined with theoretical calculations, it is proposed that the formation of the dual H-bonding structure can not only enable the hole transfer to the water molecules thermodynamically, but also facilitate kinetically the cleavage of O-H bonds by proton-coupled electron transfer process during water oxidation.