Precursor-Directed Self-Assembly in Hydrothermal Carbon Nitride Nanostructures Revealed by Nano-FTIR

Hydrothermal reactions of simple molecular precursors have emerged as a versatile route for producing carbon-rich nanomaterials with tunable optical and structural properties, yet the nature of the nanoscale products formed in solution and their role in directing subsequent self-assembly remain poorly understood. Here, we introduce a dilution-assisted isolate-and-probe strategy that directly reveals the existence and evolution of these nanoparticles in solution. Sequential dilution combined with atomic force microscopy shows that fresh citric acid-urea (CAU) products exhibit concentration-dependent size reduction, consistent with weakly associated species, whereas aged solutions yield more stable supramolecular nanoparticles. Nano-FTIR further reveals evaporation-driven lateral and vertical phase separation, where hydrogen-bonded small molecules enrich surface films while more conjugated, carbonized components form nanowire-like structures with pronounced nanoscale heterogeneity. In contrast, citric acid-thiourea (CAT) products form smaller, more uniform, less polar nanoparticles with suppressed anisotropic self-assembly. These differences highlight the central role of intermolecular hydrogen bonding in directing nanoparticle stability and surface organization. Overall, small organic nanoparticles act as dynamic reactive centers that drive aging and emergent self-assembly behavior.