Photocatalytic Hydrogen Production By Host-Guest Inclusions And Electrode Comprising CdSe Quantum Dots And Nonnoble Metal Molecular Catalysts

Solar water splitting to hydrogen is a promising solution to meet quickly increased energy demand and CO2 over-emission. The key issue for approach to this target is to develop highly efficient, stable and cheap photocatalytic systems. In recent years, many hybrid photocatalytic systems with semiconductors as photosensitizers and non-noble metal complexes as catalysts have been developed. The catalytic activity and stability of such systems are higher than the homogeneous noble-metal-free systems reported previously. In another aspect, photoelectrochemical cells have attracted extensive attention because of its prospect of the application. This thesis has made two parts of work in two aspects:(i) studies on the water-soluble FeFe-hydrogenase model/6-SCD-CdSe host-guest inclusions for photochemical H2 production and (ii) assembly of photocathode devices by using a cobalt complex as catalyst and CdSe quantum dots (QDs) as photosensitizer for water reduction to H2.Water-soluble FeFe-hydrogenase model, Na(?-SCH2N(C6H4SO3)CH2S-)[Fe (CO)3]2 (1), was used as a catalyst and 6-mercapto-?-cyclodextrin-stabilized CdSe QDs (6-SCD-CdSe) as a photosensitizer, to construct a host-guest inclusion (1/6-SCD-CdSe) for photochemical H2 production in water. 1 NMR and IR spectroscopic studies showed that there was a ground-state interaction between FeFe-hydrogenase mimic 1 and 6-SCD-CdSe QDs. Inductively coupled plasma-mass spectroscopy (ICP-MS) analysis of 1/6-SCD-CdSe host-guest inclusion revealed that the content of 1 in the 1/6-SCD-CdSe was about 9.91(±0.15)%(w/w). The photoinduced electron transfer from 6-SCD-CdSe QDs to 1 was studied by fluorescence spectroscopy. The results indicate that the quenching of fluorescence is caused by a static quenching between 1 and 6-SCD-CdSe QDs. The fast electron transfer from 1 to 6-SCD-CdSe QDs is correlated to the apparently higher TON of H2 evolution by 1/6-SCD-CdSe host-guest inclusion as compared to those obtained from the analogous reference systems where catalyst 1 was replated by [{?-SCH2N(CH2C6H5)CH2S-}{Fe(CO)3}2] (2) or [(?-SCH2CH2CH2S-){Fe(CO)3}2] (3). The 1/6-SCD-CdSe inclusion gave a TON of 2500 based on catalyst over 25 h of illumination with a quantum efficiency of 3.19% at 400 nm.In the second part of work, we used thioglycolic acid-capped CdSe QDs as a photosentizer and a cobalt complex, [(bme-dach)Co(NO)] (Co-NO, Hbbme-dach N,N'-bis(2-mercaptoethyl)-1,4-diazacycloheptane), as catalyst. The CdSe QDs was chemically linked onto the p-NiO electrode and then the Co-NO was anchored to the electrode through the coordination of the two sulfur atoms in the ligand of cabalt complex to the Cd2+ ions on the surface of the CdSe QDs-modified NiO electrode, forming an active photocathode device (NiO/CdSe/Co-NO) for H2 production in water. Two types of photocathodes were fabricated, being different in the thickness of the NiO film (NiO-I,3.650 ?m and NiO-H,5.680?m) by doctor-blading method. Under visible light illumination, the as-fabricated photocathodes displayed a photocurrent density of 114 ?A cm-2 and 109 ?A cm-2, respectively, at a bias potential of -0.4 V versus Ag/AgCl in 0.1 M Na2SO4 solution at pH 7.0. After 4 h of illumination, the photocurrent density decayed about 7.35% and 6.60% for NiO-I/CdS/Co-NO and NiO-?/CdS/Co-NO, respectively, because of the fall-off of the QDs and Co-NO catalysts from the surface of the photoelectrode. Compared with NiO-?, the NiO-? photocathode displayed higher photocurrent density since the thinner NiO-I layer has lower resistance, but the durability of NiO-I/CdS/Co-NO is shorter than the NiO-? /CdS/Co-NO electrode because of the less loading of CdSe QDs and Co-NO catalyst.