Quantum dots (QD) are nanoscale semiconductors that exhibit bright, size-tunable, and narrow emission of light. QDs have shown promise in photovoltaic and photocatalysis applications in which photo-initiated charges in QDs must be separated at a rate that outcompetes charge recombination. Charge transfer (CT) of the excited electron or hole in a photoexcited QD can occur to other semiconductors, to covalently attached co-catalysts for photocatalysis, or even to adsorbed molecular species, such as CO2. With a view towards increasing the rates of electron transfer and to further investigate the effects of varying driving forces on CT we synthesized a series of CdZnSe–CdZnS alloy QDs that allow for conduction band tuning. The addition of Zn to CdSe/CdS core/shell QDs, via a copper catalyzed cation exchange process, has previously been shown to result in QDs with increased band gaps. However, concomitant control of QD shell thickness during this process is challenging. We introduce a single pot, shell growth and cation exchange method, where a shell of CdZnS with a desired ratio of Zn and Cd is grown directly on CdSe cores. The reaction is stoichiometrically controlled to result in QDs of the desired size. The presence of copper facilitates Zn exchange with the Cd in the core. The increase in band gap observed upon cation exchange is hypothesized to be due to an increased conduction band and thus a greater oxidation potential of the photoexcited electron. To test if the observed increase in band gap could lead to faster electron transfer rates, we separately functionalized the exchanged QDs with two electron acceptors, naphthalenediimide and anthraquinone. Steady state photoluminescence quenching indicated that electron transfer rates initially rapidly increase at lower driving forces, and monotonically increase at higher driving forces, lacking an inverted region. Precise control over the QD redox potentials could enable more selectivity for the desired product and result in higher Faradaic efficiencies in photocatalytic applications.

Controlling the reducing power of photoexcited core/shell quantum dots via cation exchange

ACS Fall 2021

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