TiO2/Au/TiO2 multilayer thin-film photoanodes synthesized by pulsed laser deposition for photoelectrochemical degradation of organic pollutants

Abstract

Most commonly employed anodes for photoelectrochemical degradation of organic contaminants in water are too thick, resulting in a low degradation efficiency due to the excessive electron-hole recombination. Multilayer-type TiO 2 /Au/TiO 2 nanocomposites have been prepared as photoanode thin films by pulsed laser deposition (PLD). The films were composed of six TiO 2 and five Au alternate layers, with total thickness of about 212 nm, aimed to optimize the absorption of photons and minimize the recombination. The influence of gold on the optical, structural, and chemical properties of the semiconductor nanocomposites was investigated. The band gap determined from Tauc model was 3.22 eV, close to that obtained by spectroscopic ellipsometry and lower than that of the TiO 2 film, revealing an enhancement of visible light absorption. The catalytic performance of PLD films was evaluated by using them as anodes in electro-oxidation (EO) and photoanodes in photoelectrocatalysis (PEC) to degrade 39-157 mg L ⁻¹ paracetamol in sulfate medium. The drug removal was very slow in EO, due to the low ability to form [rad] OH on the anode surface, which was significantly enhanced upon UVA irradiation in PEC. The presence of Cl ⁻ allowed a faster degradation by produced active chlorine. Finally, a hybrid process involving PEC + photoelectro-Fenton (PEF) with an air-diffusion cathode yielded total paracetamol decay in 4-5 min at an anodic potential of +4.0 V because of the efficient [rad] OH generation from Fenton's and photo-Fenton reactions. Hydroquinone, p-benzoquinone and hydroxylated products were detected during EO and PEC. The generation of active chlorine was confirmed by identifying a chlorinated derivative, N-(4-chloro-2-hydroxyphenyl)acetamide, in PEC + PEF treatment.