Large Scale Purification of Colloidal Quantum Dots

Quantum Dots (QDs) possess excellent optical and electronic properties that are currently being realised in many applications. Although QDs themselves offer great potential, the separation of QDs from unreacted impurities is one of the biggest challenges holding back large-scale QD production and commercialisation. A team of researchers from South Korea has now developed a continuous process for the large- scale purification of QDs using porous electrodes in a macroscopic flow channel.

Quantum dots (QDs) have gained massive attention across a wide range of industries due to their potential in many applications. Their synthesis is not too complex and are generally formed in a solution containing a large concentration of hydrocarbons that allow for high temperature crystal growth. The synthesis of QDs generally leaves surfactant molecules, unreacted precursors and reaction media all in the same system as the QD product.

Current purification methods generally employ the use of a centrifuge and a precipitation-dissolution method using polar organics solvents, such as acetone and alcohols. The method causes an aggregation of QDs, but is an expensive process that requires a large amount of solvent. The process is also not scalable or controllable.

There has been a recent method developed that is able to produce a large-scale synthesis of QDs. The method uses electrophoretic deposition to assemble a large amount of QD nanocrystals on various substrates, with an intended use in solar cell applications. However, there is a drawback. The scale-up method first requires a large-scale purification process, otherwise the QDs will be unusable for commercial applications. The team that produced the large-scale synthesis method also produced a purification method, but it was limited to batch-scale processes, and therefore not suitable for industrial-scale production.

In light of the large-scale production potential, this team of (South) Korean researchers has developed a method for the large-scale purification of QDs that will assist with the commercialisation process. The team has built this large-scale method on existing microfluidic and electrophoresis methods from the batch purification process.

The Purification Process

Previous efforts on the batch-process purification method utilised a process where the fluid flow is perpendicular to the electric field, thus limiting the purification yield due to the lack of time that QDs are exposed to the electric field. By making the fluid flow parallel to electric field, the method can capture a high concentration of QDs. There were two different types of QD synthesised and purified- PbS and CdSe QDs.

This method is a continuous process. The synthesised QD dispersions are infused into a flow channel composed of multiple layers of polytetrafluoroethylene (PTFE) with porous nickel electrodes. An electric field is applied between the electrodes, where the QDs are electrophoretically driven and collect on the electrodes. The unreacted impurities pass through the pores and are not collected, thus, providing an efficient separation process.

The researchers utilised a syringe pump (KDS LEGATO 210, KD Scientific) to infuse the impure QDs into the device and the electric potential difference in the electrodes was created using a DC power supply (DADP-5001R, DAU NANOTEK). The absorption and emission spectra were obtained using a UV/Vis/NIR spectrophotometer (UV 3600, Shimadzu) and a Horiba Fluorolog spectrometer, respectively. NMR data was obtained using an Agilent 400MHz 54 mm DD2 NMR spectrometer and the electrophoretic mobility was determined by dynamic light scattering (ELSZ 2000, Otsuka Electronics Co).

The purification yield, also known as the ratio of the mass of purified QDs to that of QDs in the crude solution, was found to be around 87%, much higher than previous efforts which peaked around 60%. The process has provided a purification method that is more efficient, less time-consuming, has a lower solvent usage and is of a higher quality than any others currently tested. A theoretical model was also produced that can predict the purification yield depending on the purification conditions.

Whilst these results have only been tested on small-scales so far, the theoretical scale-up capacity has been deduced and shown the process is scalable up to industrial levels. The researchers have stated that an increase in the surface area, and the number, of porous electrodes could produce significant quantities of QD- where the purification capacity is increased without reducing the purification yield. The researchers have deduced that 24 electrodes, with a 25 cm diameter, could produce over 1 kg of purified QDs per day.

The process shows a high efficiency and great scale-up potential that could soon be realised in the aforementioned large-scale synthesis method for the commercial production of QDs in solar cells; or in other large-scale QD production processes for other QD applications. Either way, this purification method provides a large stepping stone for the commercialisation of QDs.

Source:

Lim H., Woo J. Y., Lee D. C., Lee J., Jeong S., Kim D., Continuous Purification of Colloidal Quantum Dots in Large-Scale Using Porous Electrodes in Flow Channel, Scientific Reports, 2017, 7, 43581

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