Novel hybrids of graphitic carbon nitride sensitized with free-base meso-tetrakis(carboxyphenyl) porphyrins for efficient visible light photocatalytic hydrogen production

Novel hybrid photocatalysts of graphitic carbon nitride (g-C3N4, CN) sensitized with free-base porphyrins were prepared by impregnation through non-covalent interactions. Their photocatalytic activity was evaluated towards the generation of hydrogen (H-2) from water splitting. For this purpose, and in order to ascertain the influence of the carboxy substituents groups and their position on the porphyrin periphery on H-2 production, the porphyrins meso-tetraphenylporphyrin (TPP), meso-tetrakis(meta-carboxyphenyl)porphyrin (mTCPP) and mesotetrakis(para-carboxyphenyl)porphyrin (pTCPP) were used. All the hybrids (TPP-CN, mTCPP-CN and pTCPP-CN) show higher performance for photocatalytic H-2 production than that of pure CN. Among all the hybrids, mTCPP-CN presents the highest amount of H-2 evolved, being of 326 and 48.4 mu mol under 6 h of UV-vis and visible light (lambda > 400 nm) irradiation, respectively. This photocatalyst was fully characterized by UV-vis, FTIR, XRD, XPS, SEM, TEM, N-2 isotherms and steady-state and time-resolved fluorescence measurements. The fluorescence emission of the mTCPP porphyrin was remarkably quenched by CN semiconductor for the various hybrids mTCPP-CN containing different amounts of mTCPP, consistent with electron injection from the porphyrin excited state into the conduction band of the semiconductor. This works demonstrates that the sensitization of CN with dye porphyrins enhances the photocatalytic H-2 evolution under UV-vis and visible light irradiation, making it a potential material for solar conversion to produce hydrogen from water.