Graphitic carbon nitride/oxygen deficient tungsten oxide s-scheme heterojunctions for photocatalytic dye degradation and hydrogen peroxide generation under visible light irradiation

Synthetic dyes used in various sectors, especially textile and paper, are one of the most important causes of water pollution. These synthetic dyes, which are released into the water stream without being reduced to less toxic byproducts, seriously affect the aquatic ecosystem. For example, synthetic dye effluents in a lake absorb most of the sunlight, making it difficult for algae to photosynthesize. In addition, since dyes mix with groundwater, freshwater reserves are decreasing day by day. In this context, the release of synthetic dye effluents into water streams without being converted into harmless or less harmful by-products such as CO2 and H2O threatens the aquatic ecosystem in the short term and all living life in the long term. In parallel to that, another problem facing humanity is that fossil fuels are about to run out. As of the 21st century, alternative energy sources to fossil fuels are being considered in order to ensure the continuity of civilization. However, the alternative resources that are focused on should not strengthen the threat of global warming, which is accelerating today due to the uncontrolled use of fossil fuels and makes itself felt more like a climate crisis. Among the alternatives, hydrogen peroxide (H2O2), which possesses high energy density, as an energy carrier has attracted great attention from the scientific community since no harmful byproduct or emission is produced when H2O2 is used. Photocatalysis, which is based on the excitation of semiconducting material called photocatalyst through sunlight and its production of electrons and holes to be used in reduction/oxidation reactions, is one of the most sustainable and promising methods for degradation of dyes to harmless compounds and the generation of H2O2 from water. The most critical point in photocatalytic applications is an effective photocatalyst design that can meet the demands and provide high efficiency. Graphitic carbon nitride (g-CN) is a polymeric semiconductor with a generally two-dimensional and layered structure. Furthermore, it can be easily synthesized using nitrogen-rich materials such as melamine and urea. It can be separated into thinner layers by exfoliation. The fact that g-CN can be activated by visible light due to its approximately 2.7 eV band gap and suitable band positions makes it a frequently preferred material in photocatalytic applications. However, the fast and high rate of recombination of photo-generated electrons and holes along with extremely limited light absorption capacity in the visible region pose a major obstacle for the wide-scale use of g-CN. To overcome these disadvantages of g-CN, constructing heterojunctions with other semiconductor materials is a frequently used and successful strategy. In this regard, oxygen-deficient tungsten oxide (WO3-x) is one of the most attractive semiconductors to form a heterojunction with g-CN. First of all, the high valence band potential enables many photocatalytic reactions to occur. However, the most interesting point about this material is the optical properties caused by deficiency of oxygen in the structure. In this way, WO3-x gains a localized surface plasmon resonance property, which makes it possible to produce electrons and holes in the visible and infrared regions despite its wider band gap close to 3 eV. Consequently, the light absorption ability of the material increases, and additional electrons and holes are provided to the system to use in reduction and oxidation reactions. In the light of the information presented, in this thesis, graphitic carbon nitride/oxygen-deficient tungsten oxide (g-CN/WO3-x) heterojunction photocatalyst was synthesized for use in photocatalytic dye degradation and H2O2 production without any metal addition. In this thesis, g-CN/WO3-x heterojunction was synthesized via the solvothermal method, but the synthesis parameters including temperature, time, solvent and concentration (mg/mL), which affect the localized surface plasmon resonance intensity of WO3-x, were also evaluated to synthesize optimum g-CN/WO3-x photocatalyst. Then, many advanced characterization methods (Raman, XRD, SEM, TEM, XPS, ssNMR, UV-Vis DRS, PL, TRPL, TPC and EIS) were used to reveal the structural, chemical, photophysical and electrochemical properties of the photocatalyst. Characterization data revealed that heterojunction synthesis occurred successfully, and electron-hole recombination was greatly suppressed while the optical properties of the heterojunction improved. In the photocatalytic application, under white light (λ> 400 nm), the photocatalyst achieved 98% degradation of 5 ppm methyl orange in 25 minutes (k = 0.1787 min-1) and generated 45.9 mg/L H2O2 from 10% (v/v) methanol solution in 90 minutes. Additionally, in H2O2 production, the effect of adding various alcohols as hole scavengers to the reaction medium on photocatalytic activity was investigated and it was observed that adding 10% (v/v) isopropanol to the reaction environment resulted in the production of 87.44 mg/L H2O2. By using various characterization methods, band structures of semiconductors were elucidated and a plausible electron flow routes were proposed. Then, it was revealed that the formed g-CN/WO3-x heterostructure demonstrated S-scheme heterojunction properties. In the following step, H2O2 experiments were carried out at green light, and it was proved that hot electrons could be produced thanks to localized surface plasmon resonance of g-CN/WO3-x photocatalyst. Also, pathway followed by hot electrons was determined through the same experiment. At the next step, scavenger experiments were carried out to determine the reactive oxygen species present in the reaction environment under white light illumination, and it was observed that the photocatalyst had sufficient potential to produce superoxide and hydroxyl radicals as well as electrons and holes. Following all these results, a possible reaction mechanism was suggested for photocatalytic methyl orange degradation and hydrogen peroxide generation. High activity of the photocatalyst was associated with reasons as follows i) decreased electron-hole recombination due to the formation of S-scheme heterojunction ii) enhanced light absorption ability of photocatalyst towards visible and near-infrared region thanks to the localized surface plasmon resonance, iii) generated hot electrons to participate in redox reactions. Finally, recyclability measurements were performed, and it was observed that the photocatalyst maintained its high activity during the first 4 cycles, but there was a sharp decrease in activity at 5th cycle. Based on the XRD and TEM analysis applied after the 5th cycle, the loss of activity was explained by the conversion of WO3-x in the photocatalyst to stoichiometric WO3.