RESUMEN
Magnesium vanadate (MgV2O6) and its alloys with copper vanadate were synthesized via the solution combustion technique. Phase purity and solid solution formation were confirmed by a variety of experimental techniques, supported by electronic structure simulations based on density functional theory (DFT). Powder X-ray diffraction combined with Rietveld refinement, laser Raman spectroscopy, diffuse reflectance spectroscopy, and high-resolution transmission electron microscopy showed single-phase alloy formation despite the MgV2O6 and CuV2O6 end members exhibiting monoclinic and triclinic crystal systems, respectively. DFT-calculated optical band gaps showed close agreement in the computed optical bandgaps with experimentally derived values. Surface photovoltage spectroscopy, ambient-pressure photoemission spectroscopy, and Kelvin probe contact potential difference (work function) measurements confirmed a systematic variation in the optical bandgap modification and band alignment as a function of stoichiometry in the alloy composition. These data indicated n-type semiconductor behavior for all the samples which was confirmed by photoelectrochemical measurements.
RESUMEN
Molybdenum nitrides and oxynitrides have been increasingly realized as (electro)catalysts for a variety of reactions. In this context, the cubic "γ-Mo2N", also known to contain oxygen in the bulk, is of particular interest. The γ phase is typically derived from ammonolysis of MoO3, and a high temperature is needed to fully react the stable MoO2 intermediate that often forms along the reaction pathway. In this study, ammonolysis of atypical bronze (HxMoO3) and peroxo (H2MoO5) precursors was undertaken to avoid the formation of this undesired intermediate with the aim of synthesizing "γ-Mo2N" at reduced temperatures and thus with a high surface area. It was found, using in situ powder diffraction, that, when the phase I bronze (x ≈ 0.3) served as the precursor, MoO2 formed as an intermediate and was retained in the reaction product until 700 °C. In contrast, ammonolysis of the phase III bronze (x ≈ 1.7) and of H2MoO5 circumvented the MoO2 intermediate. From these latter two precursors, "γ-Mo2N" was formed at the lowest maximum reaction temperatures reported in the literature, namely, 480 °C in the case of HxMoO3-III and 380 °C for H2MoO5. The resulting products displayed extremely high surface areas of 206 and 152 m2/g, respectively, presumably as a consequence of the low synthesis temperatures. While the HxMoO3-III precursor showed evidence of a topotactic transformation pathway, with morphological similarity between precursor and product phases, H2MoO5 transformed via amorphization. Electrochemical characterization showed moderate activity for the hydrogen evolution reaction (HER), which increased after exposure to reducing potentials and loosely scaled with the catalyst-specific surface area. This work points toward new low-temperature synthesis pathways for accessing molybdenum (oxy)nitrides with high surface areas.
RESUMEN
Zirconium oxide (ZrOx) is an attractive metal oxide dielectric material for low-voltage, optically transparent, and mechanically flexible electronic applications due to the high dielectric constant (κ â¼ 14-30), negligible visible light absorption, and, as a thin film, good mechanical flexibility. In this contribution, we explore the effect of fluoride doping on structure-property-function relationships in low-temperature solution-processed amorphous ZrOx. Fluoride-doped zirconium oxide (F:ZrOx) films with a fluoride content between 1.7 and 3.2 in atomic (at) % were synthesized by a combustion synthesis procedure. Irrespective of the fluoride content, grazing incidence X-ray diffraction, atomic-force microscopy, and UV-vis spectroscopy data indicate that all F:ZrOx films are amorphous, atomically smooth, and transparent in visible light. Impedance spectroscopy measurements reveal that unlike solution-processed fluoride-doped aluminum oxide (F:AlOx), fluoride doping minimally affects the frequency-dependent capacitance instability of solution-processed F:ZrOx films. This result can be rationalized by the relatively weak Zr-F versus Zr-O bonds and the large ionic radius of Zr+4, as corroborated by EXAFS analysis and MD simulations. Nevertheless, the performance of pentacene thin-film transistors (TFTs) with F:ZrOx gate dielectrics indicates that fluoride incorporation reduces I-V hysteresis in the transfer curves and enhances bias stress stability versus TFTs fabricated with analogous, but undoped ZrOx films as gate dielectrics, due to reduced trap density.
RESUMEN
Mesostructured matter composed of colloidal nanocrystals in solidified architectures abounds with broadly tunable catalytic, magnetic, optoelectronic, and energy storing properties. Less common are liquid-like assemblies of colloidal nanocrystals in a condensed phase, which may have different energy transduction behaviors owing to their dynamic character. Limiting investigations into dynamic colloidal nanocrystal architectures is the lack of schemes to control or redirect the tendency of the system to solidify. We show how to solidify and subsequently reconfigure colloidal nanocrystal assemblies dimensionally confined to a liquid-liquid interface. Our success in this regard hinged on the development of competitive chemistries anchoring or releasing the nanocrystals to or from the interface. With these chemistries, it was possible to control the kinetic trajectory between quasi-two-dimensional jammed (solid-like) and unjammed (liquid-like) states. In future schemes, it may be possible to leverage this control to direct the formation or destruction of explicit physical pathways for energy carriers to migrate in the system in response to an external field.
