Solar energy conversion to produce hydrogen using photocatalysis is a promising route for sustainable energy production. However, disordered polymeric photocatalyst systems, such as carbon nitride (CN_x), suffer from low solar-to-hydrogen efficiencies due to rapid recombination of photogenerated charges and interlayer electrostatic barriers, which impede charge transfer to surface reaction sites. Transient Absorption Spectroscopy (TAS) is routinely used to explore these dynamics, revealing insights into properties such as charge carrier lifetimes and trapping in these photocatalysts. However, conventional TAS measurements typically rely on large probe sizes (millimeters to centimeters), which average out the effect of spatial heterogeneity in local charge carrier trapping behaviour. To overcome this limitation, a home-built Transient Absorption Microscopy (TAM) setup was developed, in which we studied the transient absorption behaviour of single CN_x particles with probe beams that allowed micron-level spatial resolution. In our technique, we measure charge carrier dynamics in the μs–s timescale, which is particularly relevant for water splitting reactions. These reactions dispaly slower kinetics, making it essential to probe trapping and detrapping phenomena within this regime. For the first time, μs–s carrier dynamics were explored within individual CN_x particles, revealing: a) significant particle-to-particle heterogeneity in trapped charge densities, and b) spatial heterogeneity in half-lives of the trapped charge populations within the same particle.

With this evidence, we were able to indicate the presence of at least two distinct spatial defects controlling charge trapping in CN_x [1]. Additionally, we proceeded to spatially investigate the charge trapping behaviour of these charges when a cocatalyst, such as Pt is deposited. Pt deposition was found to extend the charge carrier half-lives by around threefold, and Pt displayed a preference for binding in areas with the lowest initial lifetimes [2]. These findings suggest that local chemical environments independently influence charge trapping, which dictates phenomena such as cocatalyst deposition. Our spatiotemporally resolved TA offers a powerful approach for understanding defects, probing operando chemical reactions, and hence laying the foundation for the optimal design of efficient photocatalysts for solar energy conversion using systems like CN_x.