Borate-Mediated Hole Transfer from Irradiated Anatase TiO2 to Phenol in Aqueous Solution

The development of a highly active TiO2 photocatalyst for energy and environmental use is a great challenge. In this work, we report that the addition of sodium borate to an aqueous suspension of anatase TiO2 at neutral pH can result in a significant enhancement in the rate of phenol degradation. Similar results were also observed from 2,4-dichlorophenol degradation, spin-trapped OH radical formation, H2O2 decomposition, and chromate reduction in the presence of phenol. This borate-induced rate increase for phenol degradation was determined not only by the amount of borate adsorption but also by the structure of borate species (pH effect). A (photo)electrochemical measurement with the TiO2 film revealed that upon addition of borate, the hole consumption by phenol and the electron consumption by O2 were accelerated and decelerated, respectively. Moreover, the flat band potential of TiO2 was negatively shifted by 81 mV. Since the hole oxidation of water to O2 remained unchanged, it is proposed that a borate radical is produced, followed by regeneration through phenol oxidation. This borate-mediated hole transfer would promote the electron transfer to O2 and consequently improve the efficiency of the charge separation for phenol degradation at interfaces.