Comparative assessment of visible light and UV active photocatalysts by hydroxyl radical quantification

A simple method for determining hydroxyl radical yields on semiconductor photocatalysts is highly desirable, especially when comparing different photocatalyst materials. This paper reports the screening of a selection of visible light active photocatalysts such as Pt-C₃N₄, 5% LaCr doped SrTiO₃, Sr_0.95Cr_0.05TiO₃ and Yellow TiO₂ and compares them against WO₃ and ultra violet (UV) light activated TiO₂ P25 (standard commercial catalysts) based on their oxidative strengths (OH radical producing capability) using a well-studied chemical probe–coumarin. 7-hydroxycoumarin, the only fluorescent hydroxylation product of this reaction can then be measured to indirectly quantify the OH radicals produced. P25 under UV light produced the highest concentration of OH radicals (16.9 μM), followed by WO₃ (0.56 μM) and Pt-C₃N₄ (0.25 μM). The maximum OH radical production rate for P25, WO₃ and Pt-C₃N₄ were also determined and found to be 35.6 μM/hr, 0.28 μM/h and 0.88 μM/h respectively. The other visible light activated photocatalysts did not produce any OH radicals primarily as a result of their electronic structure. Furthermore, it was concluded that, if any visible light absorbing photocatalysts are to be fabricated in future for the purpose of photocatalytic oxidation, their OH radical producing rates (and quantities) should be determined and compared to P25.