Opportunities and limitations of mild reductive treatments for the synthesis of coloured TiO2 with applications in gas sorption and photocatalysis

Titanium dioxide is widely used in photocatalysis because it is abundant, stable, and inexpensive, yet pristine anatase and rutile absorb mainly ultraviolet light. One strategy to extend activity into the visible region is the introduction of sub-gap states associated with oxygen vacancies and Ti³⁺ species. These are typically generated through hydrogenation, high-temperature reduction, plasma treatment, or strong chemical reductants, routes that often raise energy concerns. This thesis therefore addresses a practical question: can TiO2 be reduced by mild and potentially scalable methods that still deliver useful functionality under realistic conditions, and where do such approaches succeed or fail? High-intensity ultrasound was first investigated as a potential reduction route. Using calorimetrically calibrated power delivery and systematic probe integrity checks, any apparent darkening of TiO2 during aqueous sonication was traced to metallic debris from horn erosion rather than defect formation. UV–Vis diffuse reflectance showed no band-edge shift or sub-gap absorption, and no evidence for stable oxygen vacancies or Ti3+ species was found. Under these conditions, cavitation hot spots do not provide a sustained reducing environment, and any transient defects are likely rapidly re-oxidised by radical species and dissolved oxygen. Ultrasound-induced “blackening” is therefore an artefact rather than a viable reduction strategy. Despite this negative outcome, ultrasound proved valuable as a synthesis and processing tool. During sol–gel synthesis it yielded high-surface-area, texturally open amorphous TiO2 with interconnected nanoparticulate networks. Controlled ultrasonic crystallisation produced anatase while retaining about 50% of the surface area, compared to the 10% of conventional calcination. These materials showed rapid uptake of volatile organic compounds, consistent with their preserved open structure. Vacuum annealing was then explored as a hydrogen-free route to defective TiO2. Mild vacuum treatment of P25 darkened the powder, increased visible-light absorption without shifting the band edge, and shifted the Ti:O ratio away from perfect stoichiometry. Functionally, this resulted in an approximately 25% increase in methane formation during CO2 photoreduction compared with pristine P25. Applying the same treatment to Au@P25 preserved plasmonic properties but provided only marginal additional activity beyond that already conferred by gold. Overall, this work shows that mild routes give mild but reliable outcomes. They are effective for preserving structure, adsorption capacity, and stability, and can deliver meaningful functional gains in selected cases.