CdS quantum dot aerogels for photocatalytic hydrogen evolution

Abstract

CdS quantum dots (QDs) have been extensively studied as photocatalysts and sensitizers for visible-light-driven water reduction. However, their efficiencies are limited by the need to accumulate sufficient redox equivalents to produce H₂ and consequent photocorrosion associated with slow hole-transfer rates. To address these limitations, we report the formation of CdS QD assemblies (aerogels, AGs) capable of facilitating energy/charge transport between individual QDs, and evaluate their performance as photocatalysts for hydrogen evolution as a function of structure, wurtzite (w-) vs. zincblende (zb-), and different annealing temperatures. The formation of AGs from QDs resulted in increased rates of H₂ production under visible light illumination: from 1458 (QD) to 6650 (AG) µmol_H2·h⁻¹·g⁻¹ on zbCdS and from 1221 (QD) to 3325 (AG) µmol_H2·h⁻¹·g⁻¹ on wCdS. This is attributed to exciton delocalization between adjacent QDs facilitating charge/energy transport. Thermal processing of CdS AGs up to 250 °C improved their activity, increasing the degree of exciton delocalization, while annealing them to 300 °C caused sintering of the primary QD particles within the AGs and a decrease in activity associated with loss in surface area. The best photocatalyst, zbCdS AG annealed at 250°C, had an average H₂ production rate of 13,604 ± 2017 µmol_H2·h⁻¹·g⁻¹, an apparent quantum yield of 2.8% at 425 ± 12.5 nm, and was stable for 2 h before beginning to deactivate due to photocorrosion. This study confirms the potential of CdS AGs as matrixes for the design of more active and stable composite photocatalysts for water splitting.