UV-induced synthesis of polyaniline-TiO2 hybrids: a mechanistic study

Polyaniline, an important member of the conductive polymer family, has received increasing attention due to its peculiar pH-dependent properties which open the way to a wide spectrum of applications, ranging from electronics and optics to photovoltaic1,2. PANI is traditionally synthesized via oxidative polymerization3, a process which involves toxic reagents (aniline and persulfates) and leads to carcinogenic byproducts. Aiming to more environmentally friendly procedures, other synthetic strategies have been developed during the years: in particular, some of us4 have reported a green synthesis involving aniline dimer ((4-aminophenyl)aniline) as starting compound, H2O2 as oxidant and Fe3+ as catalyst, thus yielding H2O as only coproduct. Unfortunately, with this eco-friendly process, there is no control on the polymer morphology and the result is a compact PANI with low dye-sorption capabilities. We have recently proposed a new green synthesis in which PANI growth is activated by TiO2 photocatalysis giving rise to PANI-TiO2 hybrid systems5. The reaction is carried out in two steps: the photocatalytically induced oligomerization of aniline dimer at the TiO2 surface, and the polymerization step initiated by H2O2 addition. In this work, the reaction mechanism was investigated via radical scavenger tests and by a combination of LCMS, FTIR, XPS and ζ-potential measurements. UV light is essential to initiate the reaction, as without irradiation only short oligomers with poor chain-conjugation are formed (Fig.1). Overall, this synthetic method leads to composites stable under UV irradiation in usage conditions, with high specific surface areas (crucial for sorption properties) and enhanced crystallinity, which is beneficial for PANI conductivity. The role of synthetic parameters like reagent ratios and temperature was also investigated; while incrementing the H2O2 amount leads to poorer crystallinity and lower surface area, the TiO2 content in the hybrid can be increased without affecting its morphology and performance. All samples were tested towards the removal of model dye pollutants. The reusability of the nanocomposite and the influence of common interferents were investigated, also via tests in simulated drinking water. References [1] C.O. Baker, X. Huang, W. Nelson, R.B. Kaner, Chem. Soc. Rev., 46, 2017, 1510. [2] F. Cui, Y. Huang, L. Xu, Y. Zhao, J. Lian, J Bao, H Li, Chem. Commun., 54, 2018, 4160. [3] H.D. Tran, J.M. D’Arcy, Y. Wang, P.J. Beltramo, V.A. Strong, R.B. Kaner, J. Mater. Chem., 21, 2011, 3534 [4] C. Della Pina, M. A. De Gregorio, L. Clerici, P. Dellavedova, E. Falletta, J. Hazard. Mater., 2018, 344, 308 [5] C. Cionti, C. Della Pina, D. Meroni, E. Falletta, S. Ardizzone, Chem. Commun., 2018, 54, 10702