Solar fuels such as H2 or those based on the photo-reduction of CO2 , are deemed as one of the strongest alternatives for a sustainable future. Photocatalysts are a class of materials that can potentially utilise solar light in order to generate solar fuels, but their performance still requires further research in order to turn them into economically and technically viable solutions. Currently, some of the major challenges faced by photocatalysts are in relation to their optoelectronic properties, such as poor light harvesting or fast charge recombination. Although much effort has been made on improving the efficiency of photocatalysts, there are still some fundamental knowledge gaps, specifically when characterising photocatalysts for their optoelectronic properties. The conclusions drawn from some characterisation techniques are often not verified against the
assumptions that were used to interpret the data in the first place. In this work, UiO-66(Zr) and isoreticular variants of it (–NH2 , Br and –NO2 ), had their optoelectronic properties characterised using an array of techniques, creating a comparison framework. It was discovered that XPS is a reliable technique for the determination of the electronic band structure of photocatalysts, while the commonly used electrochemical Mott-Schottky technique cannot be applied for UiO-66(Zr) and its variants. XPS
technique was then applied to characterise g-C3N4 that had been thermally treated with the goal of improving its optoelectronic properties. Thermal treatment resulted in increased performance in both H2 evolution and CO2 photo-reduction, with the characterisation results suggesting that the thermal treatment led to the formation of electronic trap states that delayed charge recombination and consequently improved the photocatalytic performance. Finally, with the knowledge obtained from the UiO-66(Zr) and its variants and g-C3N4 studies, novel composites of g-C3N4/UiO-66(Zr) were synthesised in order to explore the synergistic effects of electronic heterojunctions. These composites had H2 evolution rates 3.5 times greater than those of g-C3N4 and about 55 times greater than those of UiO-66(Zr), demonstrating the synergetic effects of heterojunctions. This research enhances our understanding of how photocatalysts’ optoelectronic properties have an impact on their performance and represents progress in turning these materials into reliable solutions for solar fuels production.