Wavelength-dependent generation of reactive species in the photodegradation process over pure and C-doped Nb2O5

Abstract

Reactive species, such as hydroxyl radicals (·OH), holes (h+) and superoxide radicals (·O2−), are considered as the main active substances involved in the photocatalytic degradation of organic pollutants. However, to the best of our knowledge, the effect of the photon energy on the generation of these species has never been examined in detail. In this study, the photocatalytic production of ·OH, h+ and ·O2− over niobium pentoxide (Nb2O5) was investigated as a function of the excitation wavelength. The ESR results clearly show that all three active species can be generated at 355 nm for both pure and C-doped Nb2O5. At 532 nm, which is well beyond the absorption edge of Nb2O5 (399 nm), only h+ and •O2− are produced over pure Nb2O5, while at the same time all three species can be produced over C-doped Nb2O5. The results of the trapping experiments indicate that •O2− and h+ are the main reactive species that are responsible for the photocatalytic degradation. The photogenerated holes play a crucial role in the wavelength range of 365–420 nm but become less active with increasing wavelength. •O2− shows little dependence on wavelength and is consistently active at all wavelengths between 365 and 630 nm, in particular, for C-doped Nb2O5. It is concluded that the presence of localized states within the forbidden gap has a profound impact on the photocatalytic activity in the visible-wavelength region.