Reaction engineering studies of the continuous synthesis of CuInS2 and CuInS2/ZnS nanocrystals

Despite the advantages of semiconductor nanocrystals in various fields, toxic emissions from II-IV-type semiconductor nanocrystals which contains cadmium, mercury and lead limits its use in cases of commercial applications. Other materials with tunable emissions in visible range include III-V-type semiconductors or ternary or quaternary systems like IB-IIIA-VIA (IB= Cu, Ag; IIIA= In, Ga; VIA= S, Se, Te).

An appropriate wide band gap material such as zinc sulfide ZnS can be used for an organic shell which maintains a high quantum yield for almost all types of quantum dots. It has been discovered that the synthesis CuInS₂/ZnS led to nanocrystals with 80% quantum yield. However, most synthesis routes are based on the batch method, which is accompanied with difficulties of heat and mass transfer when facing challenges of large scale production and quality control of different batches. Microreaction technologies are also known to be very efficient in mass and heat transfer due to small size of channels and sophisticated channel structure making it useful for nanocrystal synthesis.

A team of researchers from Bayer Technology in Germany and China developed facile continuous route based on microreaction technology MRT to synthesize CuInS₂ and CuInS₂/ZnS nanocrystals for large scale production of highly fluorescing semiconducting nanocrystals which might be applied to other ternary and quaternary systems as well. The new study describing the research is published in the peer-reviewed Chemical Engineering Journal.

For continuous synthesis of quantum dots, the authors used an experimental unit based on the commercial available microreaction modules of Ehrfield microreaction EMB system which consists of precursor supply, a two-stage synthesis sequence for making core-shell nanocrystals and post-reaction treatment.

Following the preparation of precursor solutions for CuInS₂ core and ZnS shell, CuInS₂ and CuInS₂/ZnS nanocrystals were synthesized at different flow rates followed by separation and washing of the nanocrystals.

The microreaction technology confirmed its advantage during continuous synthesis. In spite of wide variations of the conditions during synthesis, the quantum yields of the core or shell nanocrystals was up to   ̴  30% and stable operation was achieved in two stage synthesis for making core-shell when applying copper acetate precursor. Copper iodide CuI, a much more stable chemical was finally chosen as copper source by the authors.

High quantum yields of CuInS₂ core samples were achieved by altering residence time in the authors’ continuous route. Quantum yields could also be tuned by the flow rate and was high as 28% using Sulfurhodamin B dye as reference which is the highest quantum yield of CuInS₂ nanocrystals reported either in literature or authors batch resulting.

The authors discovered that new complex strategies was necessary for shell synthesis as the quantum yield of CuInS₂/ZnS core/shell nanocrystals based on the fact that high quantum yield of 20-30% core was still restricted at 30-40% which is not as high as the batch synthesis.

The first strategy which is the staged-temperature concept where 3 stages were simulated showed that quantum yield reached 48% which is much higher than one-temperature route (30-40%) when the pretreated solution finally treated at 230⁰C for 15 min.

The other strategy where shell precursor was fed four times instead of one resulting to molar ratio of core to shell to be 1:2 showed that after second injection the quantum yield increased dramatically and stabilized above 50%. The highest record for fourth injection was 54% comparable to the batch result already. In both strategies, blue shift emission took place.

This is the first time of realizing fully continuous CuInS₂ and CuInS₂/ZnS core/shell nanocrystals without intermission, opening a novel route for large scale nanocrystals production.