Manipulation of Defect Formation in Semiconductor Nanocrystals for Photocatalysis and Magneto-optics

Abstract

Defects play a significant role in semiconductor nanocrystals (NCs) as they can influence their electrical and optical properties via vacancy doping or aliovalent doping. Localized surface plasmon resonance (LSPR), the phenomenon of collective oscillation of free electrons, allows for degenerately doped metal oxide NCs to be used as infrared plasmonic materials with promising applications in photovoltaics, sensing, electrochemistry photocatalysis and magneto-optics. In this study, sub-stoichiometric WO3-x NCs were used for plasmon-induced photodegradation of rhodamine-590 (Rh-590), with the advantages of facile synthetic method and simple configuration, as well as the potential to efficiently absorb NIR to MIR portion of solar radiation relative to the typical single-phase semiconductor and heterostructured photocatalysts reported. WO3-x NCs exhibit enhanced photocatalytic activity in contrast to bulk stoichiometric WO3 and sub-stoichiometric WO2.9 powders as well as annealed WO3-x NCs with nearly complete Rh-590 degradation (97.8%) within 2 hours in the dark at 20 ℃. Photocatalytic performance of WO3-x NCs increases with temperature because the wavelength of the blackbody radiation approaches to LSPR peak maximum. The WO3-x NCs show good recyclability with degradation percentage of ca. 82.6% after 3 cycles at 30 ℃. Scavengers were used to study the mechanism of photocatalytic degradation by WO3-x, suggesting that reactive radicals (superoxide radicals and hydroxyl radicals) make significant contribution to Rh-590 photodegradation. The results of this work demonstrate the important role of LSPR-associated free electrons in single-phase plasmonic semiconductor NCs for efficient photodegradation of dyes. Additionally, the single-phase plasmonic semiconductor NCs have emerged as promising materials for carrier polarization as magneto-optical materials. ZnO NCs, as an environmentally benign, cost effective material with abundant availability and reduced toxicity, are proposed as a model system to systematically investigate the effect of defect formation on plasmonic properties and charge carrier polarization in the presence of an external magnetic field. The defect concentration, responsible for plasmonic properties, was manipulated via changing synthesis conditions and doping with Mg ions and evaluated with the help of photoluminescence (PL) spectroscopy. Importantly, the Curie-type decay associated with unpaired localized electrons can also be manipulated by controlling defect concentration as indicated by magnetic circular dichroism (MCD) spectra. The defect-correlated band splitting monitored by MCD spectroscopy in Mg-doped ZnO NCs would be an intriguing topic for further study. The ability to control exciton polarization in pure (undoped) ZnO NCs using individual electrons localized on point defects enable the potential technological applications including spintronics and quantum information processing.