Green and low-cost synthesis of PANI–TiO2 nanocomposite mesoporous films for photoelectrochemical water splitting

Abstract

Among conductive polymers, polyaniline (PANI) has been widely used to improve electronic conductivity, solar energy transfer and photocatalytic activity of TiO₂, due to its ease preparation and excellent environmental stability. In this study, a green and low-cost synthesis procedure was developed for the preparation of PANI–TiO₂ nanocomposite films. A non-toxic and low-cost polymerization route, starting from aniline dimer and polystyrene sulphonate as emulsioning/doping agent in water, was employed to synthesize the conductive form of PANI (emeraldine salt, ES). Anatase TiO₂ nanocrystalline mesoporous films were prepared by a novel and green sol–gel spin coating method, which employs titanium tetraisopropoxide, acetic acid and a nonionic surfactant (Tween 20) in excess of water, avoiding the use of flammable solvents. Uniform PANI–TiO₂ composite films, containing PANI in either ES or pernigraniline base (PB) forms, i.e. PANI/TiO₂ and PANIox/TiO₂, respectively, were then prepared by a simple impregnation method. The films were characterized by means of XRD, ATR, FESEM and TEM techniques and their photocatalytic activity was assessed using them as photoelectrochemical water splitting photoanodes. Both PANI/TiO₂ and PANIox/TiO₂ showed an enhanced water oxidation efficiency under AM 1.5G simulated sunlight irradiation, reaching about 2 and 1.6 fold higher photocurrent densities, respectively, than a pure TiO₂ nanoparticles film. They also demonstrated good stability after several hours of operation. UV-Vis spectrophotometry and IPCE analysis reveal the main role of PANI, in the system PANI/TiO₂ for the PEC water oxidation, is as sensitizer of TiO₂ in the UV light by significantly increasing charges separation, electrons transport and collected photoelectrons, indirectly contributing to the generation of O₂. Indeed, PANI-ES photogenerated e⁻ are transferred to the TiO₂ conduction band while its h⁺ can react with OH⁻ to produce OH radicals that generate H₂O₂, which can subsequently be photooxidized on the TiO₂ NPs surface generating more O₂ than such produced by the direct water oxidation on the TiO₂ holes.