Development of Materials for Sustainable Artificial Photosynthesis

The goal of this research is to prepare, analyse, & employ different types of photocatalytic materials in the process of artificial photosynthesis (AP). The study seeks to incorporate modifying elements into a primary catalyst & examine their impacts on the reactivity of the resulting composites. An introduction is given on the relevant issues confronting society, e.g., energy conversion, rising CO2 levels & climate change. It presents the fundamental principles of photocatalysis & AP, with a focus on the use of semiconductor, Z-scheme, & plasmonic materials. In Ch. 3, Z-scheme systems composed of BiVO4 & Cu2O are fabricated. These systems (where BiVO4 & Cu2O act as the H2O oxidation & CO2 reduction photocatalysts, respectively) are prepared with varying molar ratios of Cu2O to BiVO4. The results show that the Cu2O/BiVO4 1:7 composite is the most active, producing CO & CH4. Therefore, a synergistic relationship exists that supports the operation of a Z-scheme system. In Ch. 4, composites of Cu2O & Ag3PO4 are made in a 1:3 molar ratio. Unlike in Ch. 3, these composites show a decrease in AP efficiency compared to the pure components. This is attributed to the methods of preparation. These composites are prepared using an “electrostatic self-assembly” method. In Ch. 3, a “deposition-precipitation” method is used where Cu2O is grown directly on BiVO4. Therefore, perhaps this self-assembly method does not allow for heterojunctions to form between the materials & different synthesis methods should be explored. Ch. 5 uses RuO2, a plasmonic metal oxide, combined with semiconductors to carry out AP. Two composites are prepared: RuO2 with Fe2O3 (a H2O oxidising catalyst), & RuO2 with GaP (a CO2 reducing catalyst). GaP & GaP/RuO2 produce both CO & CH4, even though the VBm of GaP is not low enough to oxidise H2O & generate H+. This suggests that "holes" are not necessary to carry out AP, rather a material that can dissociate H3O+ on its surface to provide protons is required. Furthermore, the most active material in this chapter is Fe2O3/RuO2. This is attributed to the transfer of e- between the materials, where Fe2O3 holes provide H+ from H2O oxidation which, together with hot e- from decay of SPR on RuO2, can reduce CO2. ZnO/RuO2 composites are investigated in Ch. 6. Previous work in this lab shows that TiO2 combined with RuO2 leads to the production of CO & CH4 during AP. ZnO has a similar band gap & band positions to TiO2 but has been reported as having higher photocatalytic efficiencies due to a higher electron mobility compared to that of TiO2. However, these results show that ZnO is more active in AP than the ZnO/RuO2 composites suggesting that a heterojunction does not form between the two materials &, instead, RuO2 blocks active sites on ZnO surfaces. Plasmonic photocatalysts combined with semiconductors is further explored in Ch. 7 where g-C3N4 is sputtered with Ag0. The objective of this research is to see if the activity of g-C3N4 could be enhanced by adding Ag0 to potentially increase the number & lifetime of charge carriers. However, the results demonstrate that pure g-C3N4 has a higher efficiency than the composites. This decrease in efficiency is attributed to the blocking of g-C3N4 surface active sites by Ag, as when the Ag loading increases, a further decrease in efficiency is observed. The semiconductor is changed from g-C3N4 to BiVO4 in the work detailed in Ch. 8. These photocatalytic results illustrate that the higher the loading of Ag on BiVO4, the higher the production of reduced products. Overall, this research demonstrated that the introduction of modifications to a primary catalyst can significantly affect performance & reactivity in the AP reaction, enhancing it in many cases, & diminishing it in others.