| Hydrogen generation from photocatalytic splitting of water is an ideal scenario that possesses promise for the sustainable development of human society and the establishment of the ultimate "green," infinitely renewable energy system. This work contains a series of novel photocatalytic systems in which the photoactive chromophores and/or the co-catalysts were incorporated into highly periodically cubic-phased MCM-48 mesoporous materials to achieve significantly higher photocatalytic efficiencies compared with conventional semiconductor photocatalysts. Cubic-phased MCM-48 mesoporous materials were chosen as supports to accommodate the photoactive species throughout the entire work. Several unique and iconic properties of these materials, such as large surface area, highly uniform mesoscale pores arrayed in a long-range periodicity, and an interconnected network of three-dimensional sets of pores that were recognized as positive parameters facilitated the photogenerated charge transfer and promoted the photocatalytic performance of the encapsulated photoactive species. It was validated that in the CdS/TiO2-incorporated MCM-48 photocatalytic system, the solar hydrogen conversion efficiency was prevalently governed by the photogenerated electron injection efficiency from the CdS conduction band to that of TiO2. The use of MCM-48 mesoporous host materials enabled the high and even dispersion of both CdS and TiO 2 so that the intimate and sufficient contact between CdS and TiO 2 was realized. In addition, with the presence of both TiO2 and MCM-48 mesoporous support, the photostability of CdS species was dramatically enhanced compared with that of bare CdS or CdS-incorporated MCM-48 photocatalysts. In advance, by loading the RuO2 co-catalyst into the CdS/TiO 2-incorporated MCM-48 photocatalytic system, the photocatalytic splitting of pure water to generate both hydrogen and oxygen under visible light illumination was achieved. In the various Pd-assisted, TiO2-incorporated mesoporous composite photocatalytic systems, the roles of the support material were unambiguously demonstrated. The samples with periodic MCM-48 supports exhibited significantly higher hydrogen production than the samples with aperiodic "wormhole" supports did. In the Ru-dye-sensitized TiO2-incorporated MCM-48 photocatalytic system, the MCM-48 mesoporous support provided extra protection for the Ru dye molecules, and the presence of Pt co-catalyst enhanced the photocatalytic performance by accelerating the photogenerated electron injection rate from the dye molecules to active sites on the TiO2 surface. Extensive characterization of the photocatalysts were carried out by powder X-ray Diffraction (XRD), Nitrogen Physisorption, Diffuse Reflectance Spectroscopy (DRS), Fourier Transform-Infrared (FT-IR), CO-pulse chemisorption, X-ray Photoelectron Microscopy (XPS), Transmission Electron Microscopy (TEM), Energy Dispersive X-ray (EDX) spectroscopy, Electron Paramagnetic Resonance (EPR), Photoluminescence (PL), and Time-Resolved Fluorescence (TRF). These techniques validated unambiguous structure-activity relationships, and as such provided insight for the direction of future exploration of the more advanced photocatalytic systems for solar hydrogen conversion application. |
