Carbon Nanomaterial based TiO2 Composites for Wastewater Purification

Water contaminants of emerging concern comprise a diverse spectrum of substances, including pharmaceuticals, antibiotics, hormones, cyanotoxins, textile dyes, engineered nanomaterials, and their transformation products. These diverse entities present challenges in terms of definition and mitigation due to their complicated nature and potential ecological implications. Addressing the critical wastewater crisis requires a comprehensive approach, involving the enhancement of wastewater treatment infrastructure, implementation of more strict regulatory measures, promotion of water conservation practices, and substantial investment in sustainable technologies and materials. This research undertakes the task of developing and investigating the application of diverse-dimensional carbon nanomaterial-based TiO2 composite materials in water purification processes. Leveraging the distinct attributes of carbon nanomaterials alongside the modifiable properties of TiO2, the study showcases the potential of carbon nanostructured TiO2 composites to cater to two distinct water purification strategies reliant on light harvesting: visible light photocatalysis and solar steam generation. Of the three comprehensive projects presented in this thesis, two are devoted to carbon/TiO2 composites, focussed on addressing visible light photocatalytic degradation of pollutants in wastewater. Conversely, the carbon/titanium oxide composite detailed in Chapter 5 is designed for application in solar steam generation. Carbonaceous nanomaterials integrated with TiO2 have been found to improve visible light photocatalytic performance in resulting composites. This enhancement is ascribed to synergies between the metal oxide and carbon phases at the interface. Notably, carbon nanomaterials within carbon/TiO2 composites are recognised for their role as electron reservoirs, optimising charge carrier separation efficiency, and as photosensitisers, effectively activating the oxide catalyst. Despite the widespread use of TiO2 in photocatalysis due to its favourable properties, its visible light photocatalytic efficiency is significantly restricted due to a short recombination time and limited absorption of visible light. With the aim of developing a visible light active carbon/TiO2 photocatalyst that leverages the beneficial properties of carbon nanomaterials, a two-step process is employed. This approach seeks to enhance the lifetime of photogenerated charge carriers while optimising the absorption of visible light. Chapter 3 investigates the optimisation of visible light photocatalytic activity in multi-walled carbon nanotube (MWCNT)/TiO2 composites, emphasising low carbon:TiO2 ratios to achieve peak photocatalytic performance. A series of composites is synthesised, each maintaining the total weight percentage of MWCNTs under 0.5 wt%. Photocatalytic experiments revealed that among the synthesised composites, MWCNT (0.15 wt%)/TiO2 exhibits the most efficient degradation of Acid Orange 7 (AO7) dye under visible light. Photoluminescence (PL) characterisation revealed a substantial reduction in radiative recombination within MWCNT/TiO2 compared to TiO2. This was further validated through time-resolved PL (TRPL) studies, which demonstrated an enhanced average lifetime of photogenerated charges in MWCNT/TiO2, attributed to the additional charge transfer pathways formed by the inclusion of MWCNTs. Although considerable charge carrier separation was achieved, the photocatalytic performance of the optimised MWCNT/TiO2 was limited, given its poor capacity to absorb visible light. To address this issue, Chapter 4 introduces a solution using zero-dimensional (0D) carbon nanomaterials to enhance visible light absorbance. Carbon nanodots (CNDs) synthesised via a hydrothermal approach employing citric acid and urea are employed for this purpose. The synthesis parameters relating to CND loading and hydrothermal temperature are fine-tuned, ultimately resulting in an optimised CND/MWCNT/TiO2 composite. Diffuse Reflectance Spectroscopic (DRS) characterisation of this ternary composite revealed that it demonstrates significant improvement in harvesting visible light, attributed to the Visible-Near Infrared (NIR) light absorbing capability of aggregates of CNDs. Photocatalytic studies showcased that the optimised CND/MWCNT/TiO2 showed an impressive five-fold enhancement in photocatalytic degradation of AO7 dye and a 1.6 times increase in tetracycline degradation under visible light, compared to TiO2. Moreover, Liquid Chromatography Mass Spectrometry (LCMS) analysis validates the absence of toxic intermediate-products after photocatalytic degradation. Active radicals in photocatalysis determined from scavenger tests and LCMS-determined intermediate products were used to propose a mechanism for enhanced photocatalytic activity. This enhanced photocatalytic activity was attributed to the influence of photosensitising properties of CNDs alongside electron storage and transport property of MWCNTs. The final project, given in Chapter 5, shifts the focus toward fabricating a carbon/titanium oxide material with potential utility in solar steam generation. Prior to composite fabrication, high light absorbing nonstoichiometric titanium oxides were derived from TiO2 to be used as photothermal agents in solar steam generation. A high concentration of defects within the TiO2 lattice generates regular oxygen vacancy planar defects, yielding reduced titanium oxides and their distinct subset known as Magnéli phases. These substoichiometric titanium oxides follow the formula TinO2n-1 with 4≤ n ≤9. Hence, firstly, this chapter introduces an innovative, cost-effective, and safe carbothermal reduction method for synthesising Magnéli phase titanium suboxides. This is followed with an extensive analysis on the evolution of defect stoichiometry and physicochemical attributes across varying treatment durations. This comprehensive examination revealed the capacity of multiphase Magnéli titania to exhibit broad spectrum light absorption spanning the Vis-NIR regions. Finally, a solar water evaporation membrane is fabricated by using graphene oxide (GO) as the substrate material. GO was specifically chosen for this task due to its ability to form a three-dimensional (3D) network through aerogel formation, and its low thermal conductivity. When used in solar steam generation under 1.0 Sun, the optimised composite material showcases an impressive energy conversion efficiency of 56%, threefold that of pure water. This outcome strongly highlights the potential of Magnéli titanium suboxides as a photothermal agent, signifying many opportunities for further exploration and advancement in the field of solar steam generation. Therefore, with a focus on creating sustainable solutions for water treatment, this study highlights varied collaborations between carbon nanomaterials and titanium oxides. By combining the intrinsic characteristics of carbon nanomaterials with the tailored attributes of TiO2, the research reveals the unexplored potential of these composite materials in various light-harvesting applications in water treatment.