Near-Infrared Optical Properties of Thiol-Passivated Substoichiometric WO3–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.