Single-molecule fluorescence imaging of photocatalytic nanomaterials

Photocatalytic nanomaterials can intensively interact with light to drive catalytic reactions and have emerged as a novel class of catalysts for sustainability applications and mitigating environmental crises. Understanding the fundamental processes of photocatalytic reactions, such as charge carrier generation, separation, and transport as well as reactivity site distribution, is key to the rational design of optimal catalysts. However, conventional ensemble measurements are unable to differentiate the heterogeneities intrinsic to individual nanocatalysts in size, facet, morphology, and crystal phase, imposing a grand challenge in explicitly uncovering the structure-property relationship. In this review, we highlight the versatility of an emerging operando imaging technique, namely single-molecule fluorescence microscopy (SMFM), in unravelling the puzzles in photocatalytic processes. In particular, the high spatiotemporal resolution of SMFM allows to study single- or even sub-particle catalysis, thus offering unprecedented insights into mechanistic understanding and catalyst design. We first discuss the fundamentals of SMFM and its use for investigating catalytic reactions based on plasmonic metals and semiconductors. We then highlight heterostructured photocatalysts with diverse combinations of plasmonic metals, non-plasmonic catalytic nanomaterials, and/or semiconductors. Recent advances in investigating bio-inorganic hybrids and non-fluorescent entities are also reviewed. Finally, we provide a discussion on the challenges and opportunities in this field, aiming to inspire novel ideas and promote the exploration of SMFM for new applications. Single-molecule fluorescence imaging offers high spatiotemporal resolution and enables quantitative, operando studies of photocatalytic nanomaterials at the single- or sub-particle level.