The photocatalytic activity and degradation mechanism of methylene blue over copper(II) tetra(4-carboxyphenyl) porphyrin sensitized TiO2 under visible light irradiation

Copper(II) meso-tetra(4-carboxyphenyl) porphine (CuTCPP) sensitized nano TiO2 particles, denoted as CuTCPP-TiO2, were synthesized to modify the photoresponse properties of TiO2 particles. Femtosecond and nanosecond spectroscopic studies of CuTCPP and CuTCPP-TiO2 are reported, combining both the time-correlated single-photon counting (TCSPC) technique and transient absorption measurements. The purpose is to investigate the TiO2 photodegradation mechanism and the relationship with the initial electronic and vibrational relaxation of the S1 and S2 excited states of the sensitizer. Excitation of the Soret band leads to multiple electronic and vibrational relaxation time scales of S2 and S1 populations, from hundreds of femtoseconds to tens of nanoseconds. The systematic and detailed studies reported here reveal that the Soret fluorescence band decays with a lifetime of 98-120 ps for CuTCPP whereas 123-398 ps for CuTCPP-TiO2, with the lifetime prolonged, the fluorescence intensities however, are greatly reduced. In addition, the S1 fluorescence was totally quenched after CuTCPP sensitized on the TiO2. The overall picture of electronic relaxation dynamics is depicted according to these observations. The photodegradation of methylene blue (MB) over CuTCPP-TiO2 was systematically investigated. The presence of CuTCPP molecules in the TiO2 surface led to enhancement in its photodegradation of MB under visible light irradiation. The photocatalytic degradation kinetics of MB fitted well with the Langmuir-Hinshelwood mode. Moreover the detailed short-time dynamics for the visible-light induced catalytic mechanism was discussed.