Study of Indigo carmine as radical probe in photocatalysis

The feasibility of using Indigo carmine (IC) as a probe of free radical production in photocatalysis was elucidated by comparative studies of its bleaching by γ radiolysis and TiO2 photocatalysis. The γ radiation result from pH 3 to 10 shows that both OH and HO2 can bleach IC and that the bleaching yields of OH is pH independent at ∼0.7 molecules per OH. The quantum yields (ϕ) of OH from Degussa p25 TiO2 photocatalysis from pH 3 to 10 was determined. It was found that ϕ was about 4% at pH 10 and dropped to below 1% at pH 3. This agrees well with the result of photocatalysis of Fricke dosimetry at pH 1. The effects of some common inorganic anions that may affect photocatalytic systems in technical applications (Cl−, SO42−, Br−, HPO42−, B4O72−, HCO3−/CO32−) was also investigated, showing that these ions does not inhibit the photocatalytic bleaching efficiency of IC.