Study, design and development of graphene-based tio2 photocatalysts and its applications in the reduction of carbon dioxide

The increase in global atmospheric concentrations of carbon dioxide (CO2) has aroused considerable public concern due to its effect in global warming. The ability to harness the power of CO2 on a large scale and integrate it back into the utilization cycle as a sustainable form of energy production is highly desirable. The photocatalytic reduction of CO2 to produce hydrocarbon resources is deemed as a promising strategy in reducing CO2 emissions and resolving the energy crisis. However, the state-of-the-art technology is far from being optimal and there are still several breakthroughs to be made before it can be considered as an economically viable process. In fact, in order to have positive energy balance in the production of fuels from CO2, it is essential that sunlight is used exclusively. The problem is that the vast majority of photocatalysts exhibit low photoresponse towards visible-light, thus resulting in low photoconversion efficiencies. TiO2, which is by far the most researched photocatalytic material, also suffers from a narrow light-response range and can only be excited under UV-light irradiation. A variety of strategies have been used to enhance the photocatalytic performance of TiO2 photocatalysts, the most recent being the incorporation of carbon nanomaterials to form carbon–TiO2 composites. Owing to its superior electron mobility, high thermal conductivity and large specific surface area, graphene, in particular, is regarded as an extremely attractive component for the preparation of composite materials. The homogeneous anchoring of TiO2 photocatalysts onto graphene presents the opportunity to cover all mechanisms of photocatalytic enhancement, including rapid charge separation and an extended light absorption range. In this work, highly efficient graphene-based TiO2 photocatalysts were developed for the photoreduction of CO2 into energy-bearing products such as methane. The project focused on the use of a continuous, gas-phase heterogeneous photocatalytic system. The reaction was carried out under visible light irradiation to reduce the dependency on UV-light with the aim of mimicking natural photosynthesis which occurs in plants. A proof-of-concept strategy was employed in the fabrication of each novel photocatalytic system to ensure rationale design and high efficiency. Several modifications were carried out to further enhance the photocatalytic properties of graphene/TiO2 by introducing noble metal dopants such as platinum, palladium, gold and silver nanoparticles. These metal clusters served as additional electron trapping sites, effectively suppressing the recombination of electron-hole pairs. Moreover, noble metal nanoparticles exhibited a localized surface plasmon resonance which allowed for strong and broad absorption in the visible region of the solar spectrum. The loading of noble metal dopants were varied between 0 – 10 wt. % to determine the optimum content for CO2 reduction. Among the noble metals studied, the 2 wt. % platinum-doped graphene/TiO2 ternary composite demonstrated the highest photoactivity, achieving a total methane yield of 1.70 µmol/ gcat after 6 h of reaction. However, to improve the overall photocatalytic performance of a composite material, the rationale design of TiO2 photocatalyst is extremely important. It is necessary to develop an effective approach to synthesize stable and visible-light-active TiO2 without the formation of secondary phases and oxygen vacancies. Hence, in a separate study, modifications to TiO2 were conducted by introducing oxygen excess defects into the TiO2 lattice. The resulting oxygen-rich TiO2 photocatalyst was shown to exhibit remarkable photoactivity under visible light irradiation. In addition, the annealing treatment of the oxygen-modified TiO2 was also found to have significant effects on its anatase-rutile phase composition, textural property and photocatalytic activity. Further improvements were subsequently carried out by incorporating graphene oxide sheets to enhance the photostability of the oxygen-rich TiO2, where a total methane yield of 1.718 µmol/ gcat was achieved after 6 h of reaction. In overall, the research focused on scientific aspects of the photocatalytic process and fundamental understanding behind the enhancement of each graphene-based photocatalyst system. Upon identifying the best photocatalyst for CO2 reduction, process study and optimization were carried out, followed by an extensive study on the kinetic modelling of the photocatalytic process. The graphene-based TiO2 composites is expected to be developed as a robust means to address various energy and environmental-related issues, which are two of the biggest challenges faced by society today.