In situ modulation of reducing electron concentration: its role and utility in tuning metal nanoparticle outcomes.

The rate at which reducing electrons are generated is a powerful handle for controlling nanoparticle formation, determining how reduced metal incorporates into nanoparticles, and as a result, key aspects of the resulting structures, such as their size. Here, we leverage the high tunability of photocatalytically generated reducing agents to modulate the production of reducing electrons by (1) adjusting photosensitizer concentration (the species that carries the electron used to reduce metal ions) to control the number of reducing electrons available for metal ion reduction and subsequent nanoparticle formation at a given time, (2) modulating PS excited state quenching, and (3) directly controlling relative irradiance delivered to the reaction milieu using a series of increasingly absorptive filters. In each case, nanoparticle formation is followed over time and nanoparticle formation rate constants, kNP, are extracted. We observe a consistent relationship between the instantaneous reducing electron concentration and the average size of the resulting particles, where lower concentrations lead to larger nanoparticles in all cases. By combining the conditions that produce the slowest NP formation kinetics, we design a synthesis that expands the accessible particle size window from ∼4 to 26 nm. These results introduce a framework for controlling size in metal nanoparticle formation pathways, such as continuous nucleation, that diverge from classic metal nanoparticle syntheses and their associated strategies for particle morphology control such as seeding and metal ion to reducing agent ratios.