Porphyrin Molecular Aggregates For Photocatalytic Water Oxidation

Photo-induced charge separation and charge transfer are the most key factors for the photocatalytic efficiency.For water oxidation,the half-reaction of the photocatalytic water splitting,which is a complex four-electron process,the inhibition of the electron-hole recombination is particularly important for the improvement of the oxygen evolution.The highly ordered molecular assemblies self-organized from the?-conjugated molecules are proved to effectively capture the photons as molecular antenna to promote the visible-light utilization,meanwhile are capable of facilitating the charge immigration to enhance the charge separation.In this thesis,the effect of the porphyrin molecular assemblies on the photo-induced charge separation in photo-driven water oxidation has been stressed.Carboxyl substituted porphyrin?TCPP?was synthesized and the molecular aggregated structures as well as the porphyrin/CeO₂ organic-inorganic nanohybrids are constructed,respectively.The optical and electrochemical properties of the nano-aggregates and the hybrids are studied,and the charge separation as well as the photocatalytic water oxidation is performed comparatively,which may provide a prototype for fictionalization of the?supra?molecular assemblies in solar energy conversion,and a hint for increasing the visible-light utilization,inhibition of the electron-hole recombination,and promoting the stabilization of the wide band gap semiconductor.The main research contents are as follows:1.Electrochemical and light-assisted catalytic water oxidation of porphyrin J-aggregates.5,10,15,20-meso-tetra?4-carboxyphenyl?porphyrin?TCPP?was synthesized and the J-aggregated nanorods were fabricated by molecular self-assemble from the nitric acid solution with low pH.This J-aggregated nanorods presented large red-shift in the electronic absorption relative to that of the single molecule,which is expected to be favor for the visible-light utilization.The ligh-driven electrochemical water oxidation with this porphyrin J-aggregates is firstly disclosed.Such J-aggregates are capable with charge separation and electron transfer upon light irradiation,leaving holes accumulated on the surface of the nanorods.The porphyrin nanorods are electrocatalytic active for water oxidation,which demonstrated reduced overpotential for water oxidation under light irradiation,therefore to promote the dioxygen evolution.Oxygen generation can even be detected at quite low pH,and the O₂ evolution rate was steady for more than dozens of hours,suggesting the high stability and stable catalytic reactivity of the porphyrin nanorods catalysis,which is the fatal weakness of the most oxide semiconductor.2.Study on preparation and photocatalytic properties of CeO₂-TCPP Composites.The inorganic/organic nanocomposite of ceria hollow tube and porphyrin?TCPP?aggregates,TCPP@CeO₂,in different doping ratio were developed by a facile method and were investigated comparatively.The TCPP was adsorbed onto the surface of CeO₂ and aggregated in J-type in the cavity of the CeO₂ nanotube with the aid of the HNO3.The presence of the porphyrin J-aggregation make the absorption of the nanocomposite red-shifted,which is beneficial for visible-light absorption and irradiation.The content of the porphyrin?and the aggregates?has great influence on the specific surface are,the oxygen vacancies,photocurrent as well as the photocatalytic oxygen evolution of the nanocomposite.With the porphyrin content of 0.5%,the specific surface area of the composite nanostructures is 42.354 m2g-1 the oxygen vacancy concentration is about 27.28%,and the photocurrent and the rate of photo excited oxygen production can reach up to 46.38 mol/g.We speculated that the charge transfer between the two materials extended the charge separation and suppressed the electron-hole recombination.On account of the high stability of the CeO₂ nanostructure and the porphyrin aggregates,the overall stability and photocatalytic activity kept well at low pH for long run.