Effect of Crystal Structure on the Aggregation of CdS Quantum Dots: Consequences for Photophysical Properties and Photocatalytic Hydrogen Evolution Activity

• Published in: Journal of physical chemistry. C 2024-07, Vol.128 (27), p.11239-11246
• Main Authors: Alevato, Vinícius, Streater, Daniel, Huang, Jier, Brock, Stephanie
• Format: Article
• Language: English
• Subjects: C: Spectroscopy and Dynamics of Nano, Hybrid, and Low-Dimensional Materials
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.4c02510

In the current pursuit of renewable energy resources, CdS emerges as a promising component for hydrogen production by solar-driven photocatalytic water splitting owing to its 2.4 eV band gap (centered in the visible region) and optimally aligned conduction and valence bands for water reduction and oxidation, respectively. There is ongoing debate as to the roles of crystal structure, wurtzite (w-, hexagonal) vs zincblende (zb, cubic), and quantum confinement, in photocatalytic activity. Herein, we evaluated w- and zb-CdS quantum dots (QDs) of diameters of ∼3.2–3.3 nm for visible-light photocatalytic water reduction. wCdS QDs capped with trioctylphosphine oxide and tetradecylphosphonate were discovered to undergo an aggregation process upon sitting in the dark in hexane over 16 h, resulting in the observation of a second peak in the photoluminescence spectrum, red-shifted from the exciton peak, and correlating to the dramatically decreased photocatalytic water reduction activity: from 1221 (fresh) to 162 (16 h aged) μmol H2 h–1 g–1. In contrast, zbCdS QDs capped with oleate ligands did not exhibit a second emission peak and did not lose activity upon aging, producing 1458 μmol H2 h–1 g–1. The aggregation process is sufficiently modest that colloidal dispersibility is maintained. Nevertheless, aggregation leads to an enhancement of trap states and facilitates charge/energy transfer between QDs in the aggregates, limiting the availability of photogenerated electrons to the water reduction reaction. Thus, aggregation, even in small amounts, greatly affects the photophysical properties of wCdS QDs and limits their application in systems that demand photogenerated charge carriers’ availability and transport, such as photocatalysis, photovoltaics, and other light-harvesting devices.