Manganese (III) meso-tetrakis(4- nitrophenyl) porphyrin sensitizes TiO₂ nanoparticles into visible region via a pyridyl linker for degradation of organic dyes

Abstract

Given the known central role of porphyrins in the photosynthesis, in this work, manganese (III) meso-tetrakis(4-nitrophenyl) porphyrin (MnTNPP) is used for the first time as a visible antenna for TiO2 nanoparticles. The crystalline TiO2 nanoparticles in pure anatase phase with high specific surface area (290 m2/g) were prepared from aqueous TiCl3 precursor and citric acid. A pyridyl linker in surface-bound TiO2 (SAPy), which forms a strong coordination bond with the Mn center of porphyrin complex (MnTNPP/SAPy@TiO2), significantly enhanced photocatalytic activity. The photoactivity of the as-prepared hybrid was found to be approximately 6 and 4 times greater than that linker-free MnTNPP@TiO2 toward the degradation of Rhodamine B and methyl orange respectively, under low intensity visible light 40 W CFL irradiation. Based on the XRD patterns TiO2 retained its pure anatase phase after modification with MnTNPP/SAPy complex, while its specific surface area decreased to 70 m2/g and its band gap reduced to 2.6 eV making it a visible-light responsive photocatalyst. A faster degradation was observed for negatively charged dyes caused by electrostatic attraction between the positively charged photocatalyst and the pollutant. Quenching of the PL intensity of bare TiO2 by 87% after functionalization with SAPy/MnTNPP obviously showed the efficient electron − hole separation resulting from transmitting photoexcited electrons from the light-absorbing porphyrin sensitizer into the conduction band of TiO2 through the SAPy linker acting as an effective molecular wire. The effect of scavengers was studied and photogenerated hole was found as main active species for oxidative degradation of dyes.