A porphyrin polyethylenimine polymer as an effective photosensitiser for hydrogen evolution

Abstract

Photocatalytic hydrogen evolution is a promising approach to generate hydrogen gas for use as a green alternative to fossil fuels which have contributed to climate change. Simple metal oxide semiconductors have been studied extensively in photocatalysis, however these systems are limited by their broad band gap energy. Herein, the synthesis of and photophysical characterisation of a zinc tetraphenyl porphyrin appended branched polyethylenimine polymer (PEI-ZnTPP) is reported. The photophysical porperties of the PEI-ZnTPP polymer are similar to porphyrins in the literature, with an absorption profile that extends into the visible region of the electromagnetic (EM) spectrum, and a long lived triplet excited state lifetime of 197 µs. These visible light absorption properties were exploited using the polymeric nature of the PEI-ZnTPP to prepare PEI-ZnTPP/TiO₂/Pt⁰ nanocomposites which displayed a photocatalytic hydrogen evolution rate of 34 675 µmol g⁻¹ h⁻¹ thus out-performing other photosensitising polymers coated onto TiO₂ in the literature. X-ray photoelectron spectroscopy of the nanocomposites indicated all components required for photocatalysis remained in the system following irradiation and were still available to act as PHE components, however slight degration of the coatings occurred. Using electrochemical analysis, a Rehm–Weller type thermodynamic analysis was performed for the nanocomposites indicating favourable electron transfer from the PEI-ZnTPP polymer to the TiO₂ and the Pt⁰ co-catalyst, helping to further rationalise the impressive PHE rate observed for the nanocomposites.