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

## TL;DR

Researchers developed a surfactant-free, continuous method to synthesize Cu2O crystals with controlled size and shape, enabling scalable and precise production for practical applications.

## Contribution

A surfactant-free continuous microflow synthesis system is introduced, offering precise kinetic control to regulate Cu2O crystal size and exposed facets.

## Key findings

- Varying the interval time between reagent additions altered nanocube size from 75 to 196 nm.
- Reversing the feeding sequence produced much smaller nanocubes (76→14 nm) due to microenvironment changes.
- Shorter residence times led to more exposed facets on Cu2O polyhedrons, indicating a non-equilibrium state.

## Abstract

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.

Precise and scalable synthesis is crucial to the practical applications of functional materials. In this work, we demonstrate the significant influence of precise process control on the formation of Cu2O crystals based on a continuous microflow synthesis system. The millisecond‐to‐second kinetic control is highlighted, which can substantially modify the microenvironment and intermediate states, thereby dictating the size and exposed facets.