Developing Visible-light Active Quaternary Metal Sulfide Nanocrystals for Photocatalytic Hydrogen Production from Sulfide Aqueous Solution

Nowadays, it is strongly believed that hydrogen gas (H2) will be the energy carriers of the future. To synthesize hydrogen gas, the industry employs thermochemical processes such as steam reforming of hydrocarbons and the water-gas-shift reaction. These processes involve many different catalytic steps and need high energy. Given the high energy demand and the complexity associated with all reforming processes for H2 production along with the CO2 emission, it is of great interest to explore an alternative sustainable and green energy technology for H2 production. Photocatalytic hydrogen production from water has thoroughly been investigated but still the overall efficiency is law. As an intermediate step until reach the dream of photocatalytic overall water splitting, the photocatalytic hydrogen production from sacrificial systems has recently been proposed. However, to make this process economically visible, the sacrificial reagents (electron donors) should be abundant. Sulfide ions are good electron donors and they can be obtained from the abundant sulfur compounds in the crude-oil. In fact, sulfur compounds exists in a significant amount in the crude-oil (up to 5 wt%). The H2S gas released during the crude-oil desulfurization can be readily dissolved in alkaline aqueous solution and utilized for hydrogen production. Theoretically, photons that have an energy greater than 0.34 eV (the energy barrier of H2S splitting reaction) can photocatalytically generate H2 from aqueous solution of sulfide ions, so that nearly the entire solar irradiation spectrum can be utilized. Thus, the overall aim of this project is to develop visible-light active quaternary metal sulfide nanocrystal photocatalysts for the hydrogen production from the alkaline aqueous solution of sulfide ions. To achieve this goal, the electronic structures of the commonly used copper-gallium-indium-sulfide (CGIS) systems will be investigated using solid state periodic Density Functional Theory (DFT) simulations. Detailed analysis of the atomic contributions of the valence and conduction bands will be followed by in silco screening of new materials in order to select the best candidates for the synthesis. Based on the DFT simulation results, the newly proposed class of quaternary metal sulfides will be synthesized with controlled composition, size and morphology. The synthesized nanocrystals will be characterized by different techniques (e.g. scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy). The photo(electro)electrochemical properties of the nanocrystals will also be investigated by electrochemical impedance spectroscopy (EIS) and intensity modulated photocurrent spectroscopy (IMPS). The photocatalytic activities will be assessed under solar simulated light and under monochromatic light. Correlation between the photocatalytic activities, the DFT simulation results, and the photo(electro)chemical results will be analyzed to define the optimum photocatalyst structure and composition. The optimum photocatalyst will be employed for photocatalytic solar hydrogen production at ambient temperature and pressure via the utilization of the abundant sulfur compound derived from the petrochemical and oil refinery industries.