Orthorhombic Nb2O5 Research Articles

A Sensitized Nb2O5 Photoanode for Hydrogen Production in a Dye-Sensitized Photoelectrosynthesis Cell

Orthorhombic Nb2O5 nanocrystalline films functionalized with [Ru(bpy)2(4,4′-(PO3H2)2bpy)]2+ were used as the photoanode in dye-sensitized photoelectrosynthesis cells (DSPEC) for hydrogen generation. A set of experiments to establish key properties—conduction band, trap state distribution, interfacial electron transfer dynamics, and DSPEC efficiency—were undertaken to develop a general protocol for future semiconductor evaluation and for comparison with other wide-band-gap semiconductors. We have found that, for a T-phase orthorhombic Nb2O5 nanocrystalline film, the conduction band potential is slightly positive (<0.1 eV), relative to that for anatase TiO2. Anatase TiO2 has a wide distribution of trap states including deep trap and band-tail trap states. Orthorhombic Nb2O5 is dominated by shallow band-tail trap states. Trap state distributions, conduction band energies, and interfacial barriers appear to contribute to a slower back electron transfer rate, lower injection yield on the nanosecond time scale, and a lower open-circuit voltage (Voc) for orthorhombic Nb2O5, compared to anatase TiO2. In an operating DSPEC, with the ethylenediaminetetraacetic tetra-anion (EDTA4–) added as a reductive scavenger, H2 quantum yield and photostability measurements show that Nb2O5 is comparable, but not superior, to TiO2.

Thermally activated crystallization of Nb2O5 grown on Pt electrode

The influence of the local crystallographic orientation of the polycrystalline bottom platinum electrode on the crystallization of niobium pentoxide thin films during their rapid thermal annealing was investigated by X-ray diffraction, X-ray reflectivity and transmission electron microscopy. The Nb2O5 thin films under study were reactively sputtered in a mixed O2/Ar atmosphere and subsequently subjected to the annealing in argon atmosphere at temperatures ranging from 500 ∘C to 700 ∘C. The X-ray diffraction confirmed a transition from the amorphous niobium oxide to the crystalline orthorhombic Nb2O5 for temperatures between 500 ∘C and 600 ∘C. The X-ray reflectivity measurements showed that the crystallization process was accompanied by a continuous increase of the electron density in Nb2O5 and by a rapid increase of the surface roughness at 700 ∘C. It was further observed by transmission electron microscopy that Nb2O5 crystallizes selectively and that the crystalline domains of Nb2O5 possess a strong orientation relationship to the platinum from the bottom electrode. The orientation relationship \((\bar{1} 1 1)_{\mathrm{Pt}}\,{\parallel}\, (\bar{1} \bar{6}0)_{\mathrm{Nb}_{2}\mathrm{O}_{5}}\) was identified as the most beneficial one for crystallization of Nb2O5.

Influence of Nb doping on the electrochromic properties of WO3 films

The effect of Nb doping on the electrochromic properties of WO3 thin films is investigated. The systematics of different properties such as the structural phase change, the colour-bleach kinetics, colouration efficiency, reversibility, stability, ion insertion/extraction capacity and change in charge states from W+6 to W+5 are found to depend on the Nb doping concentration. The x-ray diffraction results reveal the solid solution WNb2O8 phase with mixed WO3–Nb2O5 (monoclinic ε-WO3 and orthorhombic Nb2O5) phases at lower Nb concentration (2 at.%) and mixed WO3–Nb2O5 phases at higher Nb concentrations (6 and 10 at.%). The scanning electron microscopy results show a drastic change in surface morphology as the doping levels increase. The surface morphology changes from the fibrous-reticulate present in undoped films to a rough and granular structure for doped films. The cycle stability, charge storage capacity and reversibility of the films are improved upon doping; however, colouration efficiency decreases for doped films and is minimum for 6 at.% Nb doped film. The x-ray photoelectron spectroscopy measurements show that the valance state of Nb is not changed after colouration, indicating no direct contribution of Nb in the colouration process. These results are discussed in terms of the effects of doping induced compositional inhomogeneity and disorder structure on the film properties.

Mesoporous Nb2O5 Films: Influence of Degree of Crystallinity on Properties

Nanocrystalline Nb2O5 films were prepared by an extended sol-gel method. The synthesis is based on the hydrolysis of a modified Nb-alkoxide precursor. Reaction of the modified precursor (Nb(OEt)5 + 2 2,4-pentanedione) with water in ethanol leads to a homogeneous hydrolyzed solution, which is stable against precipitation of niobium oxide after evaporation of the ethanol and in the whole pH-range investigated (1–10). Autoclaving leads to amorphous gels, from which homogeneous nanocrystalline niobium oxide films of up to 15 μm can be made. During annealing crystalline phases are first observed above 500°C with fully crystalline films of orthorhombic T-phase Nb2O5 attained at 600°C. The microstructural, crystallographic, optical and photoelectrical properties of the films were characterized by means of SEM, XRD, UV-VIS spectroscopy and surface photovoltage spectroscopy, respectively.

Preparation of ultrafine powders of Nb2O3 by hydrothermal H2O2 oxidation starting from metallic Nb

Abstract Ultrafine powders of Nb2O5 were prepared by hydrothermal H2O2 oxidation of metallic Nb, resulting in the formation of the high pressure pseudo-hexagonal phase. After annealing at 600 °C for 24 h, a transformation to the orthorhombic L-phase Nb2O5 occurred, which is also a high pressure phase.

Preparation of ultrafine powders of TiO2 by hydrothermal H2O2 oxidation starting from metallic Ti

Ultrafine powders of Nb2O5 were prepared by hydrothermal H2O2 oxidation of metallic Nb, resulting in the formation of the high pressure pseudo-hexagonal phase. After annealing at 600 degrees C for 24 h, a transformation to the orthorhombic L-phase Nb2O5 occurred, which is also a high pressure phase.

Hydrothermal Oxidation of Niobium Metal

Reactions of niobium metal powder with high temperature-high pressure water were studied in an open system under the conditions of 300°-800°C, 10-100MPa and 3-72h. Niobium pentaoxide (Nb2O5), niobium hydride (NbHx), and NbO2 were obtained from niobium. Niobium changed into orthorhombic Nb2O5 and niobium hydride at 400°C under 100MPa for 3h. Only orthorhombic, or/and monoclinic Nb2O5 were obtained above 700°C under 100MPa for 3h. The formation of oxides and hydrides caused cracking on the surface above 400°C (100MPa, 3h) resulting in disintegration of grains above 500°C (100MPa, 3h). The partial crystal growth of Nb2O5 was observed above 700°C (100MPa, 3h). Temperature and reaction duration affected the hydrothermal oxidation of Nb metal, but pressure did not.