Investigation of photocatalytic H2 production of g-C3N4 supported bimetallic sulfide nanocomposites

Population growth and the rapid increase in industrial activities also cause rapid depletion of non-renewable natural resources such as oil and fossil fuels, which are the main energy sources. The increasing consumption of fossil fuels leads to deadly energy and environmental problems, causing an increase in CO2 emissions, which are responsible for the greenhouse effect and climate change, which reveals the need to develop renewable and environmentally friendly energies, especially solar energy. Since zero-emission and environmentally friendly hydrogen can be produced using solar energy. Therefore, this approach is green and renewable; hydrogen can be considered an alternative to fossil fuels. Today, fossil fuels are the main raw material for hydrogen production, and this brings with it problems such as the high cost of hydrogen production and environmental pollution caused by carbon emissions. For this reason, hydrogen raw materials are gradually developing from fossil fuels to renewable energy sources. In 1972, Honda and Fujishima pioneered hydrogen production by photoelectrochemical water splitting under UV light illumination using TiO2 as the photoelectrode. Semiconductor photocatalysts are used extensively in H2 production due to their safe, clean, and renewable properties. Since water and solar energy are inexhaustible on Earth, photocatalytic hydrogen production through water splitting offers a promising alternative to convert solar energy into hydrogen energy. Metal sulfides have been extensively investigated in photocatalytic H2 production due to their superior chemical and physical properties. They have been widely studied in photocatalytic hydrogen production due to their usefulness in tuning the band gap for light absorption and optimizing band edge positions for photo-redox reactions. Cadmium sulfide (CdS) is one of the most attractive photocatalysts among metal sulfide photocatalysts due to its low band gap. However, it has limitations, such as photo corrosion and the short lifetime of photon-generated charge carriers. To overcome this problem, modifications of CdS with ZnS seem to be a good approach to take advantage of the high photostability of ZnS under UV irradiation. Cadmium zinc sulfide solutions (CdxZn1-xS) also provide the advantages of a controllable band gap and excellent corrosion resistance. It can increase the hydrogen production rate compared to CdS or ZnS. Some approaches to modifying CdZnS are necessary to achieve higher photocatalytic activity and a better quantum yield. Graphitic carbon nitride (g-C3N4), which has a two-dimensional and layered structure, is an excellent semiconductor with high thermostability and chemical inertness, and good potential in photocatalytic hydrogen formation. The fact that the structure of g-C3N4 significantly increases the photocatalytic activity has attracted significant attention from researchers. Among many photocatalysts, graphitic carbon nitride (g-C3N4) stands out due to its favorable band gap, low cost, fast electron transfer, π-π conjugation structure, and better stability. It has an optical bandgap of ∼2.7 eV, corresponding to the strong absorbance in the visible-light region, together with a suitable band structure for both water reduction and oxidation, which makes this material a very promising candidate for photocatalytic applications. However, the photocatalytic performance of the pure g-C3N4 is often restricted because of its rapid recombination rate of photoinduced charge carriers and low specific surface area. Various approaches have been developed to enhance the photocatalytic performance of g-C3N4, including structural improvement, introducing metal or nonmetals, and coupling with metal oxides, metal sulfides, and carbon-based materials to enhance its photogenerated charge separation efficiently. In line with the latest developments in the photocatalytic hydrogen production reaction, it is emphasized that cocatalysts play critical roles in increasing the efficiency of converting various band-compatible semiconductors from solar to fuel. Cobalt-based cocatalysts have been found to have a high potential for promoting hydrogen evolution reactions due to tunable microstructure, crystalline phase, and surface. For this reason, CuCo2S4, a semiconductor with a narrow band gap, was used as a cocatalyst in this study. The rapid transfer of this cocatalyst electrons to the surface and its high electrical conductivity support its participation in photocatalytic processes. Another feature is the water affinity on the surface of cobalt, making it a suitable photocatalyst in aqueous solutions. CuCo2S4 provides strong redox properties compared to binary metal sulfide due to the presence of cobalt and copper ions. In this thesis study it is aimed to produce photocatalytic hydrogen by preparing g C3N4-supported bimetallic sulfide nanocomposites. At the same time, it was expected to further increase H2 production by adding cocatalyst to the nanocomposite. When the photocatalytic studies of CdxZn1-xS solid solutions were examined, it was determined that Cd0,6Zn0,4S had the best photocatalytic activity, and it was decided to be the catalyst to be used in this thesis study. For this purpose, in the first stage of the study, Cd0,6Zn0,4S/g-C3N4 nanocomposites were prepared to increase the photocatalytic H2 production activity of Cd0,6Zn0,4S structures by containing 3%, 5%, 10%, 15% and 20% g-C3N4 by weight. And the photocatalytic H2 production experiments were carried out. According to the photocatalytic H2 production experiment results, it was observed that the photocatalytic H2 production activity increased by combining Cd0,6Zn0,4S with g-C3N4, and It was revealed that the highest photocatalytic activity occurred in the presence of Cd0,6Zn0,4S/g-C3N4 nanocomposite containing 10% g-C3N4 by weight. In the other stage of the study, Cd0,6Zn0,4S/g-C3N4 nanocomposite containing 10% g-C3N4 by weight was combined with CuCo2S4, one of the bimetallic sulfides commonly used as cocatalysts, to increase the photocatalytic activity. Structures and morphologies of the prepared g-C3N4, CuCo2S4, Cd0,6Zn0,4S, Cd0,6Zn0,4S/g-C3N4,Cd0,6Zn0,4S/CuCo2S4, and Cd0,6Zn0,4S/g-C3N4/CuCo2S4 nanocomposites were analyzed and confirmed by XRD, FTIR and SEM characterizations. The band gap energies of the photocatalysts were determined by UV-DRS measurements, and the conduction band and valence band potentials were determined by Mott-Schottky analysis. In elucidating the photocatalytic H2 production mechanism, the separation of charge carriers and transfer mechanism were examined by LSV, EIS, chronoamperometric, and PL analyses. Photocatalytic H2 production experiments were carried out in the Na2S/Na2SO3 electron donor environment for g-C3N4, uCo2S4, Cd0,6Zn0,4S, Cd0,6Zn0,4S/g-C3N4, Cd0,6Zn0,4S/CuCo2S4,and Cd0,6Zn0,4S/g C3N4/CuCo2S4 photocatalysts. According to the results obtained, Cd0,6Zn0,4S/g C3N4/CuCo2S4 was found to have the highest photocatalytic activity with a hydrogen production rate of 53.12 mmol g-1 h-1. Adding CuCo2S4 as a cocatalyst, Cd0,6Zn0,4S/g-C3N4 supported the migration of photo-induced charges through heterostructure electron transfer and delayed their recombination, increasing the H2 production rate and photostability. A possible H2 production mechanism was proposed for Cd0,6Zn0,4S/g-C3N4/CuCo2S4 nanocomposite, considering band gaps and band potentials, EIS, LSV, chronoamperometric, and PL analysis. This study will provide a new perspective on developing bimetallic sulfur-based nanocomposites for efficient photocatalytic H2 production applications.