Remarkable Enhancement of Photocatalytic Hydrogen Evolution Efficiency Utilizing An Internal Cavity of Supramolecular Porphyrin Hexagonal Nanocylinders Under Visible-Light Irradiation

An efficient visible light-induced hydrogen evolution system has been developed by using supramolecular porphyrin hexagonal nanocylinders that encapsulate Pt-colloids-deposited TiO2 nanoparticles (Pt/TiO2) in the internal cavity. First, porphyrin nanocylinders structurally controlled by encapsulated Pt/TiO2 are prepared via a solvent mixture technique. The bar-shaped structure composed of Pt/TiO2 and zinc meso-tetra(4-pyridyl)porphyrin [ZnP(Py)4] is formed with the aid of a surfactant: cetyltrimethylammonium bromide (CTAB) in a DMF/H2O mixture solution [denoted as Pt/TiO2–ZnP(Py)4 nanorods]. In scanning electron microscopy (SEM) measurements, ZnP(Py)4 pristine hexagonal nanocylinder with a large hollow structure [denoted as ZnP(Py)4 nanocylinder] was observed, whereas the hollow hole was completely closed in case of Pt/TiO2–ZnP(Py)4 nanorods. X-ray diffraction (XRD) analyses also revealed that ZnP(Py)4 alignment in the nanorod was based on the stacked-assemblies of ZnP(Py)4 coordinated hexagonal formations. These results clearly indicate that Pt colloids-deposited TiO2 nanoparticles (Pt/TiO2) were successfully encapsulated within a ZnP(Py)4 hexagonal nanocylinder. Pt/TiO2–ZnP(Py)4 also shows a broadened absorption in the visible region because of aggregation of ZnP(Py)4. Then, Pt/TiO2–ZnP(Py)4 exhibited efficient hydrogen evolution under visible light irradiation, whereas no hydrogen was evolved in the case of Pt/TiO2 without ZnP(Py)4. In addition, the hydrogen evolution efficiency of Pt/TiO2–ZnP(Py)4 nanorods per unit weight of Pt was two orders magnitude greater than that of the nonencapsulated system: Pt/TiO2 and ZnP(Py)4 nanocylinder composites [Pt/TiO2 + ZnP(Py)4 composites]. Finally, the photodynamics of the excited state of Pt/TiO2–ZnP(Py)4 nanorods was examined by femtosecond time-resolved transient absorption spectroscopy to clarify the photocatalytic mechanism.