The degradation mechanism of methyl orange under photo-catalysis of TiO2

Abstract

The properties of photo-generated reactive species, holes and electrons in bulk TiO₂ (anatase) film and nano-sized TiO₂ were studied and their effects towards decomposing pollutant dye methyl orange (MO) were compared by transient absorption spectroscopies. The recombination of holes and electrons in nano-sized TiO₂ was found to be on the microsecond time scale consistent with previous reports in the literature. However, in bulk TiO₂ film, the holes and electrons were found to be on the order of picoseconds due to ultra fast free electrons. The time-correlated single-photon counting (TCSPC) technique combined with confocal fluorescence microscopy revealed that the fluorescence intensity of MO is at first enhanced noticeably by TiO₂ under UV excitation and soon afterwards weakened dramatically, with the lifetime prolonged. Photo-generated holes in nano-sized TiO₂ can directly oxidize MO on the time scale of nanoseconds, while free electrons photo-generated in bulk TiO₂ film can directly inject into MO on the order of picoseconds. Through cyclic voltammetry measurements, it was found that MO can be reduced at −0.28 V and oxidized at 1.4 V (vs.SCE) and this provides thermodynamic evidence for MO to be degraded by electrons and holes in TiO₂. Through comparison of the hole-scavenging effect of MO and water, it was found that in polluted water when MO is above 1.6 × 10⁻⁴ M, the degradation is mainly due to a direct hole oxidation process, while below 1.6 × 10⁻⁴ M, hydroxyl oxidation competes strongly and might exceed the hole oxidation.