Deposition of polyaniline in TiO2 mesoporous films and its use as sensitizer for photocatalytic water splitting

Dye-sensitization of TiO2 films using Ru-based complexes coupled with water oxidation catalysts has been proven to be an efficient method to increase the spectral response and efficiency of photo-electrochemical (PEC) water splitting solar cells. However, the selection, preparation and high cost of such dyes represent a constraint for their practical application. Recently, conducting polymers, such as polyaniline (PANI), poly-thiophene, polypyrrole and their derivatives have been reported as promising sensitizers to extend the efficiency of TiO2 in different photocatalytic systems (e.g. water treatment and dye sensitized solar cells). Polyaniline (PANI) is considered as one of the most important conducting polymers because of its remarkable electrical, optical and photoelectrical properties, easiness of preparation and excellent environmental stability. Nevertheless, little information is actually available on the use of PANI as sensitizer for PEC water oxidation. In this study, we present the physico-chemical and photo-electrochemical characterization of PANI/TiO2 composite films for visible light driven PEC water splitting. Polyaniline (PANI) solution was synthesized by oxidative polymerization starting from aniline dimer. The synthesis was performed in aqueous phase using polystyrene sulphonate (PSS) as emulsioning/doping agent, obtaining emeraldine as the PANI conductive form. TiO2 mesoporous film was deposited into Fluorine-doped Tin Oxide (FTO) covered glasses by spin coating technique, from a sol that has been optimized in our labs containing TiO2 nanoparticles and a nonionic surfactant (Tween 20), and then was annealed in air at 500°C. Subsequently, the TiO2 film was dipped into the PANI solution for 10 min and dried at 80°C under vacuum for 24h, to remove all the residual PSS solvent. Thus, a very uniform and continuous PANI/TiO2 nano-composite film was obtained, with TiO2 particles of about 20nm, as confirmed by Field Emission Scanning Electron Microscopy (FESEM, ZEISS Auriga). Moreover, FTIR spectroscopy analysis and UV-Vis diffuse reflectance spectra confirmed the surface sensitization of the TiO2 film with PANI and shown an increase in TiO2 optical absorption to the visible region. PEC activity of both TiO2 and PANI/TiO2 composite films was studied by means of Linear Sweep Voltammetry (LSV), Amperometry and Electrochemical Impedance Spectroscopy (EIS), using them as anode for the water photo-electrolysis reaction, a Pt wire as cathode and Ag/AgCl reference electrode, in the dark and under simulated solar light (using a 450W Xe lamp, AM 1.5 filter), in NaOH (0.1M) electrolyte. PANI/TiO2 composite film evidences more than two times higher Solar to Hydrogen Efficiency respect to the original TiO2 film, and demonstrates a good stability over a period higher than 1 h. Such results suggest promising application of PANI as sensitizer for PEC Water Splitting, that we are further improving by using other semiconductor nanostructures and by incorporating novel water oxidation catalysts under development in our labs.