Shell-doped quantum dots as smart “on-off” catalysts for thermally sensitive organic transformations

Quantum dots (QDs) are photoluminescent nanoparticles which reside at the cutting edge of opto-electronic device development. QDs owe their remarkable photoluminescence to their small size and semiconducting properties, with each of their dimensions smaller than the Bohr radius of the parent material in a strong confinement regime.

Research efforts have focussed primarily on improving optical properties, reducing “blinking” and toxicity of QD materials and improving biocompatibility. To date however, there have been very few reports of using QDs to catalyse organic transformations. In this role, QDs offer significant advantages, as they are light rather than heat-activated and straddle the gap between homogeneous and heterogeneous catalysis, meaning they are fully dispersed in the reaction solvent and easily isolable by centrifugation.

QDs are often highly susceptible to oxidation, as well as being synthesised of toxic materials.  The overgrowth of shell materials with similar lattice parameters and lower toxicity can preserve/enhance QD photoluminescence, prevent oxidation and inhibit leaching of toxic ions. For typical II-VI and III-V core materials (such as CdSe and InP respectively), ZnS is an excellent non-toxic, wide band-gap shell material.

Early synthetic methods for ZnS shells focussed on the use of highly pyrophoric alkyl zinc and malodorous silathianes. Whilst successful, these reagents require careful handling and specialist equipment (e.g. nitrogen gloveboxes) to use effectively. Since the early 2000s, alternative routes to QD shells based on the decomposition of air-stable, single-source molecular precursors emerged. In particular, inexpensive metal dithiocarbamate species which decompose cleanly into ZnS (and volatile organics) at low temperatures have moved to the fore.

In 2014, Bear, Hogarth et al. reported a method to synthesise composite QD shells on CdSe QD cores for catalytic applications. Our work showed that doping copper with zinc (synthesising a CdSe/ZnS-CuS core/shell QD) had a detrimental effect on the overall photoluminescence, introducing a second, long-lived photoluminescence feature, but was essential for catalytic activity. Catalytic activity was assessed using the “Click” reaction of phenylacetylene and benzyl azide under 254 nm irradiation, achieving ≥99 % yield over several cycles for both the 1:1 and 1:3 molar ratio of copper:zinc. To date, this is unmatched in studies where QD catalysis and the same reaction was utilised. It was found that copper was released into solution by ICP-OES, and therefore postulated that the QDs act as catalyst vectors rather than true catalysts, albeit with impressive turn over numbers and able to catalyse multiple reaction cycles.

We will expand this work by synthesising new metal-sulphide single-source precursors for different core/shell systems. Work will focus on adding metals to CdSe/ZnS system, and expand the number of organic reactions catalysed, looking at carbonylation and carbon-carbon bond formation. In doing this, we will be able to ascertain how well the ZnS lattice reacts to doping with different metals, which will allow investigation of the mechanism of catalysis, which is currently unexplored. Subsequent research will focus on other QD core materials such as InP and carbon dots.

Objectives:

1. Investigation of the mechanism of catalysis in metal-doped QD shell systems.

2. Explore how far the ZnS shell lattice can be doped with different metal ions.

3. Expand the number of organic reactions that can be catalysed by metal-doped QDs