Hierarchical 1-3D titania Hyper-Branched Nanorods (HBNs) thin films for photocatalytic CO2 utilisation applications

Abstract

Despite our efforts, the concentration of CO2 in the atmosphere is constantly rising at an alarming rate. It is of paramount importance to develop technologies that will expedite the reduction of the rate that CO2 is released into the atmosphere. CO2 utilisation technologies consider CO2 as a valuable carbon building block in a circular carbon economy approach, where the released CO2 is captured, and utilised to produce valuable chemicals. One of these technologies is the photocatalytic utilisation of CO2 for the production of solar fuels and value added chemicals, which has the added advantage of utilising light with mild reaction conditions. However, photocatalysis is limited to the absorbed light energy and CO2 is a very stable molecule which requires a large amount of energy for its conversion. Therefore, designing highly efficient materials as photocatalysts becomes a very important task. The current thesis is concerned with the growth of titania 1-3D hierarchical hyperbranched nanorods (HBNs) on fluorine-doped tin oxide (FTO) conductive glass as thin films to be used as photocatalysts for CO2 reduction reactions. This thesis is focused on investigating the capabilities and photocatalytic behaviour of the titania HBNs material. The HBNs were found to have improved light harvesting when compared to Degussa P25 TiO2 (48.2 to 28.6 μmol m 2 s -1 ), attributed to their 1-3D morphology. P25 is a blend of mainly anatase and traces of rutile phase TiO2, commonly used as a benchmark for photocatalytic applications. P25 was supported on FTO glass and its performance was compared with that of FTO supported HBNs. This thesis is presented as a collection of published bodies of work, where the HBNs are characterised, modified and tested in photocatalytic reactions. In more detail, two reactions are presented, firstly the CO2 photoreduction to produce solar fuels such as CH4 and CO. The HBNs were found to have superior conversion rates (up to 8.7 μmol gcat -1 h -1 ) compared to P25 (6.9 μmol gcat -1 h -1 ) but, more importantly, offer the ability to shift the selectivity of the reaction product from CO to CH4, utilising a facile phase altering treatment. Additionally, the HBNs were loaded with CuO and RuO2 and their performance was investigated and compared. CuO has shown the ability to improve the optical properties of the material, while RuO2 exhibited improved charge separation and suppressed the recombination rate, which led to further improvement in the photocatalytic performance. The second reaction is the CO2 cycloaddition to epoxides, for the photogeneration of cyclic carbonates, which are primarily used as electrolytes in Li-ion batteries amongst others. In the current thesis it is demonstrated that a photocatalytic approach is possible for this reaction. Additionally, RuO2-HBNs are shown to be the best performing photocatalyst in terms of conversion. The main appeal of the photocatalytic approach is the significantly milder reaction conditions (below 55 °C and 200 kPa) when compared to the conditions currently being used in the industry (100-200 °C and 5- 10 MPa).