Solar-Driven Photocatalytic Activity Of Porphyrin Sensitized Tio2 In Methylene Blue Degradation And Sacrificial Hydrogen Generation

Abstract

Access to clean water and sustainable energy is increasingly becoming limited due to rapid population growth and industrial activities which are highly polluting and energy intensive. One approach that is currently being pursued to access clean water and generate energy is photocatalysis. A semiconductor that has been pursued for such application is TiO2. However, TiO2 is a wide bandgap semiconductor which suffers from poor activity or no activity at all in the visible spectrum limiting its use of solar radiation. Several approaches have been employed with the aim of extending the absorption profile of TiO2. Among these is depositing porphyrins on TiO2 surface. Porphyrins have excellent absorption in the visible spectrum. The porphyrin-TiO2 (P-TiO2) photocatalysts can utilize a broader spectrum of the solar energy in photocatalysis. In this study, the photocatalytic activity of the P-TiO2 composites versus bare TiO2 were evaluated and the influence of metallated porphyrin over free base porphyrin in photosensitization of TiO2 for photocatalysis established. Meso-tetra (4-bromophenyl)porphyrin and meso-tetra (5-bromo-2-thienyl)porphyrin and their indium, zinc and gallium complexes were synthesized. The synthesized porphyrins were characterized using CHN elemental analysis, 1H-NMR, mass spectrometry and UV/Vis spectroscopy. Density functional theory (DFT) and time dependent density functional theory (TD-DFT) calculations were performed on the porphyrins to gain insight into their electron injection abilities. The calculations revealed the suitability of the porphyrins for electron injection based on the position of their lowest unoccupied molecular orbitals being higher than the TiO2 conduction band. The photocatalytic activity of the synthesized porphyrins on TiO2 were investigated by methylene blue (MB, a model organic waste) degradation experiments and hydrogen generation under irradiation by solar simulator. Metalloporphyrin-TiO2 systems showed better degradation efficiency than freebase porphyrin-TiO2 systems. However, in hydrogen generation, freebase porphyrin-TiO2 systems were better compared to the metalloporphyrin-TiO2 systems. This study showed the porphyrin-TiO2 systems had enhanced photocatalytic activity over bare TiO2 hence has potential application in degradation of organic pollutants in water as well as generation of hydrogen, a clean source of energy.