Plasma engineering of advanced functional materials for photocatalytic wastewater treatment

Damptey, Lois (2024). Plasma engineering of advanced functional materials for photocatalytic wastewater treatment. PhD thesis The Open University.



Semiconductor metal oxide photocatalyst with favourable light absorption and charge transport characteristics have been widely used as a photocatalyst in various applications, to name a few, energy harvesting and storage, environmental remediation and air pollution. Energy harvesting which comprises the full utilisation of the wide solar light (wavelength) spectrum has become a central point of research in the field of materials science and engineering. Hence, the development of sustainable materials from environmentally sustainable techniques which can absorb majority of the solar light spectrum has become a huge challenge. For efficient utilisation of solar energy in catalytic applications, it is important to create photocatalyst that can absorb the full solar spectrum involving ultraviolet (UV), visible (VIS) and near infrared (NIR) wavelengths. More than three decades, TiO2 and its composites have been widely researched academically and used industrially as a low-cost material for photocatalytic applications. However, the large bandgap of TiO2 limits its photocatalytic activity to the UV region which is just 3-5% of sunlight on Earth’s atmosphere. TiO2 also suffers from rapid recombination of photogenerated carriers (i.e., holes and electrons) thereby affecting its photocatalytic efficiency. Over the years, there has been active research in altering the chemistries of TiO2 to overcome these aforementioned shortcomings. The most recent advantage is the use of two dimensional (2D) materials because of its layered structure One of the unexplored and interesting layered structure is MXene. The aim of this thesis is to modify the chemical structure of Ti2C MXene to produce TiO2 as an efficient photocatalyst for absorbing solar energy especially in the UV and visible regions. As a compound of titanium and carbon, Ti2C MXene could facilitate the creation of TiO2 and carbonaceous materials hereby improving the photocatalytic performance. The abundance of surface terminal groups on Ti2C MXene allow for ease of surface modification and functionalisation. In this thesis, for the first time, the functionalisation of TiO2 from Ti2C MXene using a dry and low powered system, atmospheric pressure plasma jet (APPJ) is reported. This process involved using Ti2C nano colloidal ink with highly reactive oxygen plasma source which can tune the electronic properties (engineering bandgap) of Ti2C MXene in-situ while simultaneously printing on to a substrate. X-ray/Ultraviolet Photoelectron spectroscopy showed an additional density of states (DOS) close to valence band edge and changes to the Ti, O core level spectra due to the oxygen plasma functionalisation. Density functional Theory calculation suggests that the changes in the electron structure might be due to the influence of oxygen vacancies and hence the increase in efficiency of catalytic process. Also, time dependent oxygen plasma functionalisation studies reveal the morphology and size of the in-situ generated TiO2 nanoparticles varied from 5-8 nm with exceptionally high photocatalytic performance. The second aim of the thesis is to create a heterostructure of Ti2C MXene with low cost and earth abundant graphitic carbon nitride, g-C3N4 (GCN) with visible light properties. For the first time, a lower power APPJ method was reported to produce a ternary in-situ TiO2/Ti2C/GCN heterostructure. In this thesis, GCN nanosheets were used as a semiconducting photocatalyst that could efficiently harvest the energy from visible light. Ti2C MXene nanosheets acted as an excellent electron sink while providing enhanced surface area which could facilitate the interfacial charge carriers. Structural studies show the formation of heterostructure formation between Ti2C MXene and GCN. Influence of morphology and hence changes to the optical properties were discussed. The synthesized ternary in-situ TiO2/Ti2C/GCN nanosheets showed enhancement in photocatalytic performance. The third aim of my research was to integrate TiO2 onto earth abundant natural cellulose fibres. Utilising the power of low power atmospheric pressure plasma (APPJ) to in-situ anchor TiO2 onto cellulose fibres to prevent the thermal degradation and chemical instability leading to leaching of the oxides from the cellulose fibres. APPJ in the presence of highly oxidised species caused an increase in COO- bonds which provided a strong linkage between TiO2 and cellulose materials. Also, structural studies revealed polymorphic changes in the structure of cellulose materials that improved its crystallinity and surface area for photocatalytic applications. APPJ is also able to create oxygen vacancies in the TiO2 which further reduced the bandgap of as synthesized TiO2/cellulose nanocomposites that enhanced photocatalytic applications. Toxicity studies showed that TiO2 was not cytotoxic. This plasma modified surfaces (of all the samples) show exceptional degradation of wastewater with ternary in-situ TiO2/Ti2C/GCN showing two times more improvement in methylene blue degradation (84% degradation) as compared to in-situ TiO2/Ti2C MXene (42% degradation). Also, TiO2/cellulose bionanocomposite showed excellent adsorptive-photocatalytic performance in degrading industrial waste dye providing a clear route as nanocomposites from research into industrialisation.

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