title: Plasma engineering of advanced functional materials for photocatalytic wastewater treatment
Creators: name: Damptey, Lois
Publisher: name: The Open University
Year: 2024

description: 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, TiO<sub>2</sub> and its composites have been widely researched academically and used industrially as a low-cost material for photocatalytic applications. However, the large bandgap of TiO<sub>2</sub> limits its photocatalytic activity to the UV region which is just 3-5% of sunlight on Earth’s atmosphere. TiO<sub>2</sub> 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 TiO<sub>2</sub> 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 Ti<sub>2</sub>C MXene to produce TiO<sub>2</sub> as an efficient photocatalyst for absorbing solar energy especially in the UV and visible regions. As a compound of titanium and carbon, Ti<sub>2</sub>C MXene could facilitate the creation of TiO<sub>2</sub> and carbonaceous materials hereby improving the photocatalytic performance. The abundance of surface terminal groups on Ti<sub>2</sub>C MXene allow for ease of surface modification and functionalisation. In this thesis, for the first time, the functionalisation of TiO<sub>2</sub> from Ti<sub>2</sub>C MXene using a dry and low powered system, atmospheric pressure plasma jet (APPJ) is reported. This process involved using Ti<sub>2</sub>C nano colloidal ink with highly reactive oxygen plasma source which can tune the electronic properties (engineering bandgap) of Ti<sub>2</sub>C MXene <i>in-situ</i> 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 <i>in-situ</i> generated TiO<sub>2</sub> nanoparticles varied from 5-8 nm with exceptionally high photocatalytic performance. 
The second aim of the thesis is to create a heterostructure of Ti<sub>2</sub>C MXene with low cost and earth abundant graphitic carbon nitride, g-C<sub>3</sub>N<sub>4</sub> (GCN) with visible light properties. For the first time, a lower power APPJ method was reported to produce a ternary <i>in-situ</i> TiO<sub>2</sub>/Ti<sub>2</sub>C/GCN heterostructure. In this thesis, GCN nanosheets were used as a semiconducting photocatalyst that could efficiently harvest the energy from visible light. Ti<sub>2</sub>C 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 Ti<sub>2</sub>C MXene and GCN. Influence of morphology and hence changes to the optical properties were discussed. The synthesized ternary <i>in-situ</i> TiO<sub>2</sub>/Ti<sub>2</sub>C/GCN nanosheets showed enhancement in photocatalytic performance. 
The third aim of my research was to integrate TiO<sub>2</sub> onto earth abundant natural cellulose fibres. Utilising the power of low power atmospheric pressure plasma (APPJ) to <i>in-situ</i> anchor TiO<sub>2</sub> 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 TiO<sub>2</sub> 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 TiO<sub>2</sub> which further reduced the bandgap of as synthesized TiO<sub>2</sub>/cellulose nanocomposites that enhanced photocatalytic applications. Toxicity studies showed that TiO<sub>2</sub> was not cytotoxic.
This plasma modified surfaces (of all the samples) show exceptional degradation of wastewater with ternary <i>in-situ</i> TiO<sub>2</sub>/Ti<sub>2</sub>C/GCN showing two times more improvement in methylene blue degradation (84% degradation) as compared to <i>in-situ</i> TiO<sub>2</sub>/Ti<sub>2</sub>C MXene (42% degradation). Also, TiO<sub>2</sub>/cellulose bionanocomposite showed excellent adsorptive-photocatalytic performance in degrading industrial waste dye providing a clear route as nanocomposites from research into industrialisation.