Solar photocatalysts show great promise in green hydrogen production. However, these photocatalysts suffer from high recombination rates and inefficient charge transport which result in low photoconversion efficiencies towards the hydrogen evolution reaction (HER). These photocatalysts also have spatial heterogeneity in charge carrier behaviour arising from structural and chemical heterogeneity on the microscopic level. This motivates us to unravel underlying charge carrier phenomena on timescales relevant to chemical reactions using spatial techniques and leverage this information to address the issue of low efficiencies. As the main analysis tool, this thesis employs the technique of transient absorption microscopy operated in diffuse reflectance mode to study particulate photocatalysts in the μs-s timescales (μs-DR-TAM). I used TAM to investigate charge-carrier dynamics in carbon nitride (CNₓ ) particles. Through single-particle measurements, we were able to identify two distinct chemical environments controlling charge trapping on the micron length scale. Building on these results, I further examined how such local chemical environments influence cocatalyst (Pt) deposition. Pt deposition selectively extracts shallowly trapped, HER reactive charges and has a spatial preference on regions where trapped charges initially have shorter half-lives. Expanding the analysis to CNₓ materials synthesized from different precursors, I performed multivariate correlations of HER activity to parameters derived from ground and excited state spectroscopic measurements. The results indicated that excited-state parameters such as trapped charge half-lives, the fraction of emissive and HER reactive charges and particle-to-particle variability in half-lives have the strongest correlation to HER activity. These results establish charge trapping in the slower timescales to be of great relevance to photocatalytic performance. Finally, by applying in situ transient absorption and photoinduced absorption spectroscopy techniques to single atom Ni-CNₓ photocatalysts, I explored the catalytic mechanism of a C-O cross-coupling reaction. These results helped identify productive and unproductive electron uptake pathways, explaining the poor reactivity for certain alkyl halides. Together, this work highlights the importance of unraveling the complex charge carrier dynamics in disordered systems like CNₓ and demonstrates for the first time their spatially heterogeneous charge trapping behaviour. I showcase how these results are key to resolving local chemical environments, interface formation and chemical reactions, and how they are tied to the overall photocatalytic performance. These fundamental understandings will aid in establishing design principles of efficient photocatalytic materials for solar hydrogen production.
Sutripto Khasnabis (Thu,) studied this question.