Spectroscopic Insights into the Near-Infrared Polaronic Transitions in Surface-Passivated Substoichiometric MoO_3–x Nanodots

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 MoO 3−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 MoO 3−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.

Near-Infrared Optical Properties of Thiol-Passivated Substoichiometric WO3–x Nanodots Studied by Magnetic Circular Dichroism (MCD) Spectroscopy

“…On the other hand, ultrafiltrated products of ND GSH and ND Pen showed the formation of an irregularly interconnected network of small nanodots. Such morphology is very close to that observed for thiol-passivated MoO 3– x nanodots, and it is expected that the network formation would be due to the weak (and incomplete) ligation of thiol molecules onto the WO 3– x surfaces, bringing about the detachment of the ligands and subsequent nanodot attachment. On closer inspection (see the insets in Figure c,d), the network-like structures are constructed by small constituent irregular-shaped (or polyhedral) nanograins, and the average size of the nanograins is 7.1 ± 1.2 nm for ND GSH or 11.5 ± 1.8 nm for ND Pen .…”

“…Substoichiometric tungsten oxide (WO 3– x ) nanodots were prepared by solvolysis of tungsten hexachloride (WCl 6 ) in ethanol using thiol-containing amino acids such as glutathione (GSH, HS–CH 2 –CH­(CONH–CH 2 –COOH)–NHCO–(CH 2 ) 2 –CH­(NH 2 )­COOH) or rac -penicillamine (Pen, HS–C­(CH 3 ) 2 –CH­(NH 2 )–COOH), and the synthetic protocol was based on a modified version of the literature reported by Li and co-workers. , First, 0.12 g (0.30 mmol) of WCl 6 (Sigma-Aldrich; GR grade) was dissolved in ethanol (EtOH, Sigma-Aldrich; 99.5%, 10 mL) to obtain pale yellow solution containing W 6+ . On the other hand, 0.65 mmol of GSH or Pen was dispersed in another 10 mL of ethanol as a surface modifier solution.…”

“…The TEM images of the as-synthesized ND GSH and ND Pen samples are displayed in Figure 2a,b, respectively, and those of the ultrafiltrated samples are in Figure 2c,d, respectively. The as-synthesized samples contained a significant amount of impurities such as unreacted GSH or Pen molecules, which led to unclear or blurred images as shown in Figure 2a,b; however, discernible high-contrast black dots suggest that the nanoparticles have a spherical shape with a mean diameter of 21.4 ± 6.5 nm for ND GSH or 19.4 ± 3.2 nm thiol-passivated MoO 3−x nanodots, 14 and it is expected that the network formation would be due to the weak (and incomplete) ligation of thiol molecules onto the WO 3−x surfaces, bringing about the detachment of the ligands and subsequent nanodot attachment. On closer inspection (see the insets in Figure 2c,d), the network-like structures are constructed by small constituent irregular-shaped (or polyhedral) nanograins, and the average size of the nanograins is 7.1 ± 1.2 nm for ND GSH or 11.5 ± 1.8 nm for ND Pen .…”

“…Very recently, substoichiometric molybdenum oxide (MoO 3– x ) nanodots with diameters of a few nanometers have facilely been prepared by injecting hydrochloric acid and mercaptosuccinic acid into aqueous molybdate solution . In the synthesis protocol, the use of carboxylic thiol is effective to obtain surface-passivated MoO 3– x nanodots. , On the other hand, substoichiometric WO 3– x nanodots with an average diameter of 1.6 nm can be obtained by injecting water into anhydrous ethanolic WCl 6 solution in the presence/absence of aspartic acid or proline (= non-thiol amino acid), and in the synthesis, addition of water is essential to produce blue WO 3– x nanodots . Inspired by these studies, we synthesize thiol-passivated WO 3– x nanodots using carboxylic thiols such as glutathione and penicillamine, and the NIR optical responses of the produced nanodots are investigated by both optical absorption and magnetic circular dichroism (MCD) spectroscopy.…”

“…Inspired by these studies, we synthesize thiol-passivated WO 3– x nanodots using carboxylic thiols such as glutathione and penicillamine, and the NIR optical responses of the produced nanodots are investigated by both optical absorption and magnetic circular dichroism (MCD) spectroscopy. MCD involves measuring the difference of absorption of left- and right-circularly polarized light under a longitudinal magnetic field and can unveil the electronic states that are not resolved in the absorption . The as-synthesized WO 3– x nanodots exhibit rather sharp absorption with the maximum of ∼900 nm, and the corresponding MCD has a derivative-like profile, indicative of a surface plasmonic behavior.…”