Wavelength-Sensitive Photocatalytic Degradation of Methyl Orange in Aqueous Suspension over Iron(III)-doped TiO2 Nanopowders under UV and Visible Light Irradiation

Abstract

Well-crystallized iron(III)-doped TiO₂ nanopowders with controlled Fe³⁺ doping concentration and uniform dopant distribution, have been synthesized with plasma oxidative pyrolysis. The photocatalytic reactivity of the synthesized TiO₂ nanopowders with a mean particle size of 50−70 nm was quantified in terms of the degradation rates of methyl orange (MO) in aqueous TiO₂ suspension under UV (mainly 365 and 316 nm) and visible light irradiation (mainly 405 and 436 nm). The photodecomposition of MO over TiO₂ nanopowders followed a distinct two-stage pseudo first order kinetics. Interestingly, the photocatalytic reactivity depends not only on the iron doping concentration but also on the wavelength of the irradiating light. Under UV irradiation, nominally undoped TiO₂ had much higher reactivity than Fe³⁺-doped TiO₂, suggesting that Fe³⁺ doping (> 0.05 at. %) in TiO₂ with a mean particle size of approximately 60 nm was detrimental to the photocatalytic decomposition of methyl orange. Whereas, under visible light irradiation, the Fe³⁺-doped TiO₂ with an intermediate iron doping concentration of ∼1 at. % had the highest photocatalytic reactivity due to the narrowing of band gap so that it could effectively absorb the light with longer wavelength. A strategy for improving the photocatalytic reactivity of Fe³⁺-doped TiO₂ used in the visible light region is also proposed.