Enhancing photo-reduction quantum efficiency using quasi-type II core/shell quantum dots

• Type of Material: Article
• Authors: Yanyan Jia, Emory University; Jinquan Chen, Emory University; Kaifeng Wu, Emory University; Alexey Kaledin, Emory University; Jamal Musaev, Emory University; Zhaoxiong Xie, Xiamen University; Tianquan Lian, Emory University
• Language: English
• Date: 2016-01-01
• Publisher: Royal Society of Chemistry: Open Access
• Publication Version: Final Published Version
• Copyright Statement: This journal is © The Royal Society of Chemistry 2016
• License: Creative Commons Attribution 3.0 Unported
• Final Published Version (URL): http://dx.doi.org/10.1039/c6sc00192k
• Title of Journal or Parent Work: Chemical Science
• ISSN: 2041-6520
• Volume: 7
• Issue: 7
• Start Page: 4125
• End Page: 4133
• Grant/Funding Information: We also gratefully acknowledge NSF MRI-R2 grant (CHE-0958205) and the use of the resources of the Cherry Emerson Center for Scientific Computation.; We acknowledge the financial support by the Air Force Office of Scientific Research through grant # FA9550-13-1-0020 (to TL) and by U.S. Department of Energy, Office of Basic Energy Sciences through grant # DE-FG02-07ER-15906 (to DGM).
• Research Categories: Chemistry, Physical; Chemistry, General

Abstract

Quantum confined semiconductor nanocrystals have emerged as a new class of materials for light harvesting and charge separation applications due to the ability to control their properties through rational design of their size, shape and composition. We report here a study of enhancing the quantum yield of methyl viologen (MV2+) photoreduction using colloidal quasi-type II CdSe/CdS core/shell quantum dots (QDs). The steady-state quantum yield of MV+radical generation, in the presence of thiols as sacrificial donors, increased monotonically with the CdS shell thickness within the studied thickness regime (0-4.7 CdS monolayers). Using ultrafast transient absorption and time-resolved photoluminescence decay spectroscopy, we found that both the rates of electron transfer from the QD to MV2+and the subsequent charge recombination in QD+-MV+complexes decreased exponentially with the shell thickness, consistent with calculated 1S electron and hole densities at the QD surfaces, respectively. Interestingly, the hole transfer rate remained relatively independent of shell thickness, likely due to a cancellation of the reduction of hole transfer coupling strength with the increased number of hole acceptor ligands on the QD surface at larger shell thickness. As a result, with increasing CdS shell thickness, the charge recombination loss decreases, enhancing the photoreduction quantum efficiency. This novel approach for improving photoreduction quantum efficiency should be applicable to many type II and quasi-type II core/shell quantum dots.