Surfactant-Free Continuous-Flow Synthesis of Cu₂O Crystals with Diverse Facets and Sizes

Cuprous oxide (Cu₂O) has demonstrated great potential in photochemical, electrochemical, and organic catalysis. Developing surfactant-free and scalable synthesis methods is essential for its real application. Conventional batch methods often suffer from inconsistent product quality and limited scalability. In this work, an efficient continuous microflow synthesis system is developed, and the design principles of the flow synthesis system are systematically elucidated. A kinetic control strategy on the millisecond-to-second timescale is proposed to precisely regulate intermediate size and dynamic structural evolution without using surfactants, thereby adjusting particle size and exposed crystal facets, which transformed the conventional thermodynamic control paradigm. Specifically, varying the interval time (0.02→6 s) between precipitant (sodium hydroxide, NaOH) and reductant (ascorbic acid, AA) addition significantly altered nanocube size (75→196 nm), while reversing the feeding sequence (AA before NaOH) led to much smaller nanocubes (76→14 nm) due to changes in the microenvironments for particle formation. Moreover, Cu₂O polyhedrons exhibited a greater number of exposed facets at shorter residence times, indicating a non-equilibrium state from the thermodynamic perspective. It is expected that such a continuous microflow synthesis system can be directly integrated with downstream catalytic processes to fully exploit the activity of Cu₂O.