A sustainable approach to producing quantum dots (QDs) from the University of Liège (ULiège) could offer a realistic, responsible pathway to the industrial-scale production of these highly tunable semiconductor nanomaterials, which are widely used in optoelectronics and nanotechnology. The researchers developed a scalable process to produce cadmium chalcogenide quantum dots — CdX QDs, where X equals sulfur (S), selenium (Se), and tellurium (Te) — in water by using a biocompatible chalcogenide source. Unlike traditional QD production methods that rely on organic solvents, the fully aqueous, continuous flow process devised by the ULiège team is safe, versatile, and ecologically friendly. The team identified tris(2-carboxyethyl)phosphine (TCEP) as a water-soluble chalcogen vehicle that could be transferred successfully to a cadmium source, resulting in the formation of CdX QDs in water. “This idea originally came from peptide synthesis, where TCEP is a well-known water-soluble reductant,” professor Jean-Christophe Monbaliu, director of the Center for Integrated Technology and Organic Synthesis (CiTOS), said. “We saw a unique opportunity to use it as a safer, scalable chalcogen transfer agent — and it worked remarkably well.” A human-machine interface for crafting on-demand type II-VI quantum dots in water. Courtesy of Michaël Schmitz and CiTOS. By using a design of experiments approach, the researchers optimized the preparation of TCEP, focusing on the effects of critical process parameters including pH, chalcogen excess, mixing efficiency, and reagent granulometry. To better understand the interaction between TCEP and the chalcogens S, Se, and Te, the researchers turned to a combination of in situ Raman spectroscopy and in-line 31P NMR spectroscopy. Using in situ Raman spectroscopy, they investigated reaction kinetics in real time — taking an unusual approach to QD synthesis. “This was a real team effort,” researcher Cédric Malherbe said. “We used state-of-the-art analytical tools to track reaction pathways in real time — something that’s rarely done in this field.” The kinetic data generated through spectroscopy allowed the researchers to observe the formation of TCEP-X species and identify the reaction mechanism, which occurred at the surface of the chalcogen particles. Over time, the reaction rate was found to decrease due to surface degradation. Based on these observations, the researchers were able to adapt the production process to continuous flow conditions using a packed-bed approach. Columns were filled with elemental chalcogens (S, Se, Te) and were operated in parallel for the on-demand production of the desired TCEP-X (X = S, Se, Te) precursors from the same feed of TCEP. The team demonstrated its approach in scalability trials, using a commercially available mesofluidic flow reactor with favorable metrics to demonstrate the production of cadmium telluride (CdTe) QDs. The researchers conducted experiments at flow rates of up to 80 mL per minute and produced stable (greater than 5 months) CdTe QDs. Researchers at the University of Liège are working toward more sustainable strategies to produce quantum dots. Courtesy of Carlotta Campalani and CiTOS. The researchers achieved a daily production of up to 40 g per day of CdTe QDs exhibiting an 18% photoluminescence quantum yield (PLQY). They also prepared biocompatible CdSe/ZnS core-shell QDs — QDs with a cadmium selenide (CdSe) core surrounded by a zinc sulfide (ZnS) shell — using a concatenated approach. These processes, performed under scalable flow conditions, could open new avenues for accessing aqueous QDs. The experimental results indicate the potential of this new protocol for biological and industrial applications. The ability of QDs to absorb and emit light with high precision makes them ideal for use in solar cells, LEDs, medical imaging, and sensors. The new approach to producing CdX QDs, in addition to improving productivity, significantly reduces waste, energy consumption, and the need for post-processing. “Although cadmium-based quantum dots are highly efficient, their toxicity remains a concern — especially under increasingly strict environmental regulations,” researcher Carlotta Campalani said. “We are now exploring greener, less toxic alternatives that still deliver top performance.” The research was published in Chemical Science (www.doi.org/10.1039/D4SC01135J).
