Abstract
Oxygen vacancies are the key factor that significantly influences physical and/or chemical properties of substoichiometric transitional metal oxide nanomaterials. In this study, we evaluate substoichiometric MoO3–x nanodots protected by penicillamine. In the synthesis, a carboxylic thiol reduction protocol is followed by a hydrothermal treatment, which makes the purification and isolation procedures facile. The obtained MoO3–x nanodots are well redispersed in both water and N,N-dimethylformamide, allowing us to examine the influence of the dispersion solvent on their optical properties. The nanodots exhibit three absorption peaks in the near-infrared (NIR) region, and their positions do not depend on the dispersion solvent, suggesting that the NIR absorption is originated not from the plasmonic resonances but from polaronic transitions caused by the oxygen vacancies. However, the spectral linewidth and intensity for the two lower-energy peaks strongly depend on the solvent, whereas those for the highest-energy peak do not. Magnetic circular dichroism measurements additionally reveal that the highest-energy peak arises from two interacting polaronic transitions that are close in energy, giving a derivative-like response based on the Faraday B term. Since the X-ray photoelectron spectroscopy survey shows that the main defect centers are different between the surface and inside of the nanodots, we propose a new mechanism to account for the NIR transitions from a viewpoint of bulk and surface polarons.
Published Version
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