Solar-driven catalytic CO2 conversion to carbon-containing fuels and value-added chemicals represents an interesting pathway towards achieving the United Nations’ climate targets. Bifunctional photocatalysts can be employed for this process, which can capture and convert CO2 using absorbed photon energy from sunlight. Given that a large percentage of the incident solar energy belongs to the visible range, it is important to design visible light-active photocatalysts. In addition, in prospect of any practical applicability, the CO2 photoreduction efficiency needs to be enhanced by orders of magnitude compared to current performances, while maintaining a robust, sustainable, and scalable nature of the photocatalysts. Boron and carbon nitride materials have attracted attention as part of photocatalytic systems for CO2 reduction. While these materials show promise, the effects of their nonmetal functionalisation on their photoactivity remain largely unknown. A greater understanding of how these functional groups incorporate within the nitride structures and impact their properties would open the route for the targeted design of better-performing photocatalysts. This thesis focuses on understanding the chemistry – functionality – photoactivity relationship of nonmetal-functionalised semiconducting nitride materials, to help design advanced photocatalysts for the CO2 reduction reaction. Specifically, I functionalised porous boron nitride using phosphorus, and graphitic carbon nitride using boron. I used a combination of advanced characterisation techniques to explore the effects of functionalisation, including the choice of nonmetal precursor and nonmetal content, on the i) physicochemical, ii) sorptive, iii) optoelectronic and iv) photocatalytic properties of the original materials. Herein, I report results which add to/disprove existing literature, and explore the use of some of these materials for CO2 photo(thermal)reduction for the first time. Overall, the findings show that the functionalised materials cannot be used as standalone photocatalysts, however the engineering of their structure can be exploited to embed them into heterojunctions and/or decorate them with cocatalysts.