Near-Infrared Optical Properties of Thiol-Passivated Substoichiometric WO_3–x Nanodots Studied by Magnetic Circular Dichroism (MCD) Spectroscopy

Abstract

We report a facile method to synthesize oxygen-deficient tungsten oxide (WO3–x ) nanodots passivated by thiols. Addition of thiolated amino acid glutathione (GSH) or penicillamine (Pen) into ethanolic WCl6 solution gives blue WO3–x nanodot dispersion with the absorption maximum of ∼900 nm; however, complete isolation of the as-synthesized products based on precipitation–redispersion cycles makes them insoluble in any solvent. We then adopt water-based ultrafiltration for purification. Interestingly, after ultrafiltration, the optical absorption is significantly modified, having a broad peak at 1300–1500 nm with a shoulder at ∼950 nm. Magnetic circular dichroism (MCD) investigations reveal that (i) the near-infrared (NIR) absorption peak (at ∼900 nm) observed in the as-synthesized products arises from localized surface plasmon resonance, giving a derivative-like MCD response, and (ii) the shoulder observed at ∼950 nm in the ultrafiltrated products is due to the excitation to a polaronic state, giving a single-peaked MCD signal. The results suggest that the plasmonic state in the WO3–x nanodots is easily and significantly modulated by the synthesis/environmental conditions, and thus, care should be taken to account appropriately for the NIR spectroscopic properties.