Multi-stoichiometric quasi-two-dimensional WnO3n-1 tungsten oxides

Influence of crystal structure and oxygen vacancies on optical properties of nanostructured multi-stoichiometric tungsten suboxides

Four distinct tungsten suboxide (WO_3-x) nanomaterials were synthesized via chemical vapour transport reaction and the role of their crystal structures on the optical properties was studied. These materials grow either as thin, quasi-2D crystals with the W_nO_3n-1formula (in shape of platelets or nanotiles), or as nanowires (W₅O₁₄, W₁₈O₄₉). For the quasi-2D materials, the appearance of defect states gives rise to two indirect absorption edges. One is assigned to the regular bandgap occurring between the valence and the conduction band, while the second is a defect-induced band. While the bandgap values of platelets and nanotiles are in the upper range of the reported values for the suboxides, the nanowires' bandgaps are lower due to the higher number of free charge carriers. Both types of nanowires sustain localized surface plasmon resonances, as evidenced from the extinction measurements, whereas the quasi-2D materials exhibit excitonic transitions. All four materials have photoluminescence emission peaks in the UV region. The interplay of the crystal structure, oxygen vacancies and shape can result in changes in optical behaviour, and the understanding of these effects could enable intentional tuning of selected properties.

Magneli-type tungsten oxide nanorods as catalysts for the selective oxidation of organic sulfides

Selective oxidation of thioethers is an important reaction to obtain sulfoxides as synthetic intermediates for applications in the chemical industry, medicinal chemistry and biology or the destruction of warfare agents. The reduced Magneli-type tungsten oxide WO_3-x possesses a unique oxidase-like activity which facilitates the oxidation of thioethers to the corresponding sulfoxides. More than 90% of the model system methylphenylsulfide could be converted to the sulfoxide with a selectivity of 98% at room temperature within 30 minutes, whereas oxidation to the corresponding sulfone was on a time scale of days. The concentration of the catalyst had a significant impact on the reaction rate. Reasonable catalytic effects were also observed for the selective oxidation of various organic sulfides with different substituents. The WO_3-x nanocatalysts could be recycled at least 5 times without decrease in activity. We propose a metal oxide-catalyzed route based on the clean oxidant hydrogen peroxide. Compared to other molecular or enzyme catalysts the WO_3-x system is a more robust redox-nanocatalyst, which is not susceptible to decomposition or denaturation under standard conditions. The unique oxidase-like activity of WO_3-x can be used for a wide range of applications in synthetic, environmental or medicinal chemistry.

Synthesis and Characterization of Tungsten Suboxide WnO3n-1 Nanotiles

W_nO_3n-1 nanotiles, with multiple stoichiometries within one nanotile, were synthesized via the chemical vapour transport method. They grow along the [010] crystallographic axis, with the thickness ranging from a few tens to a few hundreds of nm, with the lateral size up to several µm. Distinct surface corrugations, up to a few 10 nm deep appear during growth. The {102}_r crystallographic shear planes indicate the W_nO_3n-1 stoichiometries. Within a single nanotile, six stoichiometries were detected, namely W₁₆O₄₇ (WO_2.938), W₁₅O₄₄ (WO_2.933), W₁₄O₄₁ (WO_2.928), W₁₃O₃₈ (WO_2.923), W₁₂O₃₅ (WO_2.917), and W₁₁O₃₂ (WO_2.909), with the last three never being reported before. The existence of oxygen vacancies within the crystallographic shear planes resulted in the observed non-zero density of states at the Fermi energy.

Gram-scale selective synthesis of WO3-x nanorods and (NH4)xWO3 ammonium tungsten bronzes with tunable plasmonic properties

Localized surface plasmon resonance properties in unconventional materials like metal oxides or chalcogenide semiconductors have been studied for use in signal detection and analysis in biomedicine and photocatalysis. We devised a selective synthesis of the tungsten oxides WO_3-x and (NH₄)_xWO₃ with tunable plasmonic properties. We selectively synthesized WO_3-x nanorods with different aspect ratios and hexagonal tungsten bronzes (NH₄)_xWO₃ as truncated nanocubes starting from ammonium metatungstate (NH₄)₆H₂W₁₂O₄₀·xH₂O. Both particles form from the same nuclei at temperatures >200 °C; monomer concentration and surfactant ratio are essential variables for phase selection. (NH₄)_xWO₃ was the preferred reaction product only for fast heating rates (25 K min^-1), slow stirring speeds (∼150 rpm) and high precursor concentrations. A proton nuclear magnetic resonance (¹H-NMR) spectroscopic study of the reaction mechanism revealed that oleyl oleamide, formed from oleic acid and oleylamine upon heating, is a key factor for the selective formation of WO_3-x nanorods. Since oleic acid and oleylamine are standard surfactants for the wet chemical synthesis of many metal and oxide nanoparticles, the finding that oleyl oleamide acts as a chemically active reagent above 250 °C may have implications for many nanoparticle syntheses. Oriented attachment of polyoxotungstate anions is proposed as a model to rationalize phase selectivity. Magic angle spinning (MAS) ¹H-NMR and powder X-ray diffraction (PXRD) studies of the bronze after annealing under (non)inert conditions revealed an oxidative phase transition. WO_3-x and (NH₄)_xWO₃ show a strong plasmon absorption for near infra-red light between 800 and 3300 nm. The maxima of the plasmon bands shift systematically with the nanocrystal aspect ratio.

Unraveling the growth mechanism of W18O49 nanowires on W surfaces

This work unravels the solid-state growth mechanism of 1d W18O49 nanowires on W surfaces under a water vapor atmosphere. Such growth was understood to be the intermediate WO3 layer formation and its reduction induced planar faults driven 1d solid-state growth.