A New Perspective in the Use of FeOx-TiO2 Photocatalytic Films: Indole Degradation in the Absence of Fe-Leaching

Abstract

The study addresses the degradation of Indole under low intensity solar simulated light. The first evidence is presented for sputtered films degrading Indole at the solid-air interface. The co-sputtering of FeOx-TiO2 on polyethylene (PE) leads to an accelerated Indole degradation kinetics compared to TiO2-PE and FeOx-PE films. Indole degradation on TiO2-PE, FeOx-PE and polyethylene (PE) supported photocatalysts is shown to proceed without Fe- or Ti-release as detected by inductively coupled plasma mass-spectrometry (ICP-MS). This opens a new perspective for the use of Fe-containing films in the degradation of pollutants. The mechanism of charge generation by TiO2-FeOx-PE film under light irradiation as a function of the applied light intensity provides insight and is discussed in relation to Indole degradation. The photo-generated charges in the film give rise to highly oxidative radicals which were unambiguously identified by appropriate scavenging experiments. The most oxidative species was O-singlet presenting an estimated lifetime of ∼20μs and a mean-free path of ∼140nm. The probability of the Indole triplet deactivation was 0.01 and was much smaller compared to the probability of 0.99 for the Indole∗-triplet reaction with O2 dissolved in the aqueous solution. By attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), the systematic shift of νs(NH) and the νs(CC) vibration-rotational peaks were followed during Indole degradation presenting the evidence for the increase in the inter-bond distance and molecular fluidity. The increase in the film hydrophilicity during the irradiation was evaluated and seems to be necessary for the Indole degradation. An average size of ∼16.7nm of the TiO2-FeOx nanoparticles sputtered for 2min on the PE was determined by atomic force microscopy (AFM) and remained invariable during the degradation of Indole. By Energy Dispersive Wavelength (EDW) a random distribution of FeOx and TiO2 in the FeOx-TiO2-PE films was seen. This study suggests a FeOx-TiO2 heterojunction inducing interfacial charge transfer (IFCT) from FeOx to the lower-lying TiO2 trapping states.