| dc.description.abstract | The need to meet the growing energy demands of global society in a sustainable fashion requires the development of techniques and processes to avoid fossil fuel consumption and switch to renewable sources of energy. Environmentally benign wastewater treatment and technologies for alternative fuel production in the form of hydrogen can be achieved via heterogenous photocatalysis, a process based on solar energy. To this end, two distinct photocatalytic materials were developed to address each of the processes described above.
V₂O₅ is a promising candidate for organic contaminant degradation due its narrow band gap, however, its fast electron-hole recombination rate and subsequently low photoactivity hinders its application for degradation purposes. To this end, mirroring the technique used to improve TiO₂ by introducing surface defects in the form of oxygen vacancies has been applied on V₂O₅, forming reduced, or black V₂O₅. SEM and TEM imaging confirm the surface defect formation and predominant growth of the (001) facet, with XRD analysis showing the formation of V₂O₅ phases instead of the darker V₂O₃, reiterating the notion that black V₂O₅ exhibits the dark color due to oxygen vacancy formation and not phase change. XPS analysis confirms the formation of oxygen vacancies, proving the success of the facile reduction technique used and confirming material purity.
Photocatalytic degradation experiments were conducted with methylene blue (MB) as the model contaminant. Typical experiments involved a catalyst dose of 0.5 g/L, 20 ppm MB concentration in solution after adsorptive equilibrium had been established and an incident light intensity of 700 W/m². Black V₂O₅ was shown to exhibit a much higher equilibrium adsorptive capacity for MB compared to pristine V₂O₅, and a significantly enhanced photocatalytic dye degradation rate, with the black V₂O₅ material achieving nearly 93% degradation of 20 ppm MB in 1 hour of irradiation time, compared to only 3%, 58.7%, 62.9% and 81.3% achieved by pristine V₂O₅, ZnO, pTiO2 and bTiO₂ respectively. In addition, the degradation kinetics were improved 58-fold after the reduction procedure. The effect of contaminant charge was also studied, with bV₂O₅ achieving 93% degradation of cationic MB, 82% degradation of neutral quinoline yellow and only 34% degradation of methylene orange (MO), confirming the greater affinity of the negatively charged bV₂O₅ surface for cationic species.
For photocatalytic hydrogen production, a cadmium sulfide (CdS) based material was developed, doped with MoS₂ in a one-pot hydrothermal synthesis technique and coupled with multi-walled carbon nanotubes (MWCNTs) as a carbonaceous support. Typical experimental procedure involved a 0.2 g/L catalyst dose added to 25 mL of 0.5 M lactic acid solution (used as the sacrificial reagent) and irradiated with 1 kW/m² light intensity. The material exhibiting the highest rate of hydrogen evolution involved a MWCNTs loading of 3% and MoS₂ loading of 5%. Compared to pure CdS, the 3% CNT loading improved the rate of hydrogen production by nearly 7-fold, and the optimal 5% MoS₂ loading further improved the HER performance by 60%. The optimal composite, labelled CM5C3, achieved an apparent quantum efficiency of 26.25%.
SEM and TEM imaging confirmed the nanorod-like growth of CdS – MoS₂ on the CNT surface, with TEM elemental mapping used to confirm good dispersion of MoS₂ and CNTs. In addition, high resolution TEM images showed the presence of cubic and hexagonal CdS growing predominantly along the (111) and (002) planes respectively, in addition to MoS₂ distributed throughout the crystal lattice. XRD analysis confirmed the presence of hexagonal and cubic CdS, however, MoS₂ phases were not registered perhaps due to the low concentration present in the bulk of the material. XPS survey scans showed the presence of C, Cd, Mo, and S only, with trace oxygen impurities. Peak deconvolution indicated the presence of characteristic peaks for CdS and MoS₂, in addition to evidence of the presence of Cd – C bonds, with strong CdS attachment to the CNT surface. | |
