Niobium oxides and niobates physical properties: Review and prospects

Rational design of the electrolyte systems for the photochromic device

Effects of Zr and Ga doping on the stoichiometry and properties of niobium oxides

Molecular design of supported niobium oxide catalysts

Transparent conducting oxide (TCO) films, which is one of the widely used as window electrodes at the various devices, fulfil high electrical conductivity and optical transparency in the visible region of spectrum. Conventional TCO films are formed by the tuning charge transport of wide band gap oxides like indium gallium zinc oxide (IGZO), zinc oxide (ZnO), tin dioxide (SnO2), and indium oxide (In2O3). Among the various TCO candidates, In2O3 is an attractive TCO candidate since In2O3 can satisfied the visible transparency and high carrier density via doping processes. To achieve the high mobility In2O3 films, doping processes with metal dopants such as Ti, Zr, Mo, and W have been suggested by many previous study. Intrinsic In2O3, however, has n-type semiconductor due to its native oxygen vacancy act as n-type donor and indium interstitial. As another approach to gain highly conductivity TCOs, two-dimensional electron gas (2DEG) channel can be suggested since 2DEG channel can be applied to increase the electrical conductivity of oxide semiconductors up to the metallic conduction level. The tuning of electrical and optical properties in oxides via surface and interfacial 2DEG channels is of great interest, as they reveal the extraordinary transition from insulating or semiconducting characteristics to metallic conduction or superconductivity enabled by the ballistic transport of spatially-confined electrons. Although most previous studies have been carried out in crystalline phase like single crystal SrTiO3 or III-V compound semiconductor, realizing practical aspects of this exotic phenomenon toward short-range ordered and air-stable 2DEG channels remains a great challenge. In this study, we fabricated Al2O3/In2O3 heterojunction system on soda-lime glass consisting of In2O3 deposited by RF reactive sputter with a thickness from 8 to 100 nm and Al2O3 deposited by ALD with a thickness below 100 nm. We suggest that Al2O3/In2O3 heterojunction acts as short-range ordered 2DEG channel with a low temperature (~200 ℃). After deposition of Al2O3 layer, the heterojunction two-dimensional metallic channel was fabricated between Al2O3 and In2O3 interface as showing the metallic conduction with sheet charge density > 10 14/cm2, sheet resistance of 850 Ω/cm2, room temperature Hall mobility of 20.5cm2 V-1 sec-1 depending on the In2O3 thickness. To prove existence of 2DEG channel at the interface, we performed the I-V measurement, time of flight secondary ion mass spectrometry, ultraviolet-visible spectrophotometer, tunneling electron microscope–electron energy loss spectroscopy, X-ray photoelectron spectroscopy, and Hall measurement.

Crystal structures of Sr-diphylloaluminosilicates synthesized from LTA and FAU zeolites

Recent topics of research and development of catalysis by niobium and tantalum oxides

Study of the structure of niobium oxide by X-ray absorption fine structure and surface science techniques

There are still technique hurdles that needed to be overcome in the commercialization of electrochromic devices (ECDs) for energy-saving smart windows. Among them, the long-term stability of ECDs and the high fabrication cost are the critical issues. The pricey ECDs can only be paid off through saving the energy for years, and their price will be dramatically lower if they can be solution-processed. Here, we studied the ions behaviors in the open-circuit state of electrochromic conjugated polymers (ECPs) which is important to the stability of ECDs during the operation. Moreover, we investigated the solution-processable ion storage layers and paired them with p-type ECPs and demonstrated the possibility of using them in the highly efficient roll-to-roll fabrication of ECD. The crosslinkable non-color changing nitroxy radical-based polymer was investigated as the ion storage layer. With the applied of crosslinking strategy, the dissolution problems of radical polymers-based counter electrode in the electrolyte was suppressed, resulting in the enhancement of both performance and cycling stability of ECDs. Although p-type ion storage materials are widely studied as the ion storage layers for ECPs, they need to go through complicated pretreatment processes, including pre-oxidation, washing, and drying, before they can be paired with ECPs in an ECD. This complicated process greatly increases the fabrication cost. In our last work, we applied the UV ozone (UVO) pretreatment to the solution-processed n-type niobium oxide and evaluated its potentials to be used as the ion storage layer for p-type ECPs. The UVO pretreatment generates strong oxidants like ozone or atomic oxygen which induce the photolysis of organic residues of ligands and organic solvent trapped in the solution-processed metal oxide layer led to the formation of free radical species. These highly reactive species promoted the formation of the amorphous metal−oxygen network. Following by low-temperature annealing (< 150 oC), the ion-storage properties of niobium oxide is comparable with the high temperature annealed (300 oC) niobium oxide. The method is successfully applied to fabricate niobium oxide on a flexible conductive substrate and demonstrate the capability to pair with p-type ECPs and fabricate high-performance ECDs without the need of any pretreatments. The low-temperature solution processing of both layers will significantly reduce the fabrication cost of ECDs.

Multiple synergistic effects of Zr-alloying on the phase stability and photostability of black niobium oxide nanotubes as efficient photoelectrodes for solar hydrogen production

The goal of this study is to produce and to investigate the mechanical and microstructural properties of composite materials made of hydroxyapatite, obtained from both natural sheep bone and commercial synthetic hydroxyapatite with niobium oxide addition ( 5 and 10 wt%). The samples were subjected to sintering at different temperatures between 1000°C and 1300°C. Microstructures and mechanical properties of sheep hydroxyapatite (SHA) and commercial synthetic hydroxyapatite (CSHA)-niobium oxide composites were investigated. The production of hydroxyapatite (HA) from natural sources is preferred due to economical reason. The aim of development of SHA and CSHA based niobium oxide composites is to improve mechanical properties of HA. The physical and mechanical properties were determined by measuring density, compression strength and Vickers microhardness (HV). Structural characterization was carried out with X-ray diffraction (XRD) and scanning electron microscopy (SEM) studies. In all composites, density values and mechanical properties increased with increasing sintering temperature. The increase of niobium oxide content in all composites showed better mechanical properties. Both of SHA and CSHA composites with at 1300°C sintering temperature showed nearly the same compression strength value.

The sol–gel process is a wet chemical methodology that uses compounds in the liquid state as starting reagents. After being transformed into a gel, the reagents can be molded into films, fibers, dense materials, or porous materials. The liquid-state reagents allow different compositions to be mixed, which gives rise to applications in various fields, including catalysis, biomaterials, photonics, and coatings. This study describes a way to produce phosphors based on niobium oxide doped with lanthanides ions by a non-hydrolytic sol–gel methodology for use in solid-state light emission. More specifically, the non-hydrolytic sol–gel route was used to prepare niobium oxide doped with La3+/Eu3+ or La3+/Tb3+ at different concentrations, which was followed by treatment at 550 or 900 °C. The X-ray diffraction (XRD) patterns revealed the presence of niobium oxide in the hexagonal, orthorhombic, and monoclinic phases. The lanthanide concentrations affected the niobium oxide crystalline structure interplanar spacing. The Raman spectra corroborated the XRD observations. The luminescence spectra showed that the niobium oxide matrix doped with lanthanides emitted at different wavelengths; the blue, the green, and the red emissions were ascribed to the matrix, Tb3+, and Eu3+, respectively. The samples doped with La3+ and Tb3+ or Eu3+ presented different colors in the chromaticity diagram. Energy was transferred between the niobium oxide matrix and the lanthanides ions, and the corresponding mechanisms were proposed. In conclusion, the niobium oxide matrix is an excellent host for application as light-emitting devices.

Synthesis and photocatalytic activity of electrospun niobium oxide nanofibers

This study investigates the precipitation of niobic acid (Nb 2 O 5 . n H 2 O) from potassium niobate solution with addition of sulphuric acid, and its subsequent thermal conversion into niobium oxide (Nb 2 O 5 ). Thermodynamic modelling with PHREEQC and OLI Studio was used to predict the optimal conditions for niobic acid formation. Precipitation with different H 2 SO 4 concentrations produced amorphous niobic acids similar to commercial reference, with impurity removal optimized through a washing protocol. X‐ray fluorescence (XRF) analysis indicated that precipitation using 0.50 mol/L H 2 SO 4 and a solid‐to‐liquid ratio of 20 g/L yielded the highest niobium content (68.93 wt.%). Characterization revealed that residual potassium promoted the formation of crystalline potassium niobium oxide phases (K 2 Nb 8 O 21 , K 2 Nb 14 O 36 ) during calcination, particularly in samples with ~2 wt.% potassium, which exhibited rod‐like agglomerates (~30 μm). Precipitate washing at low solid‐to‐liquid ratios significantly reduced potassium content in the product. A decrease from 60 to 20 g/L reduced the potassium concentration by a factor of three (to 0.06 wt.%), eliminating potassium niobium oxide phases. The pure Nb 2 O 5 phase, after the 20 g/L‐washing, was confirmed by X‐ray diffraction (XRD) and transmission electron microscopy–energy dispersive X‐ray spectroscopy (TEM‐EDS). These results emphasize the importance of supersaturation control, calcination conditions, and purification protocols in producing high‐purity niobium oxide from secondary niobium sources.

We report on the fabrication of Al-doped ZnO (AZO) transparent-conductive oxide (TCO) films on glass substrates by RF- sputtering, their physical properties, and the effect of thermal annealing on the AZO TCO films. The AZO films on glass substrates have a preferred orientation of the c-axis, irrespective of deposition conditions, which means that the AZO films have textured structures along the c-axis. The film thickness and surface roughness in the AZO films are proportional to plasma power and deposition time, while they are inverse-proportional to working gas ratio and working pressure. The AZO films have the optical transmittance over 80 % in the wavelength range of 400 - 1000 nm, irrespective of deposition conditions. The plasma power and the deposition time relatively give a large influence on the optical transmittance, compared to the working gas ratio and the working pressure. The AZO films deposited at room temperature have poor electrical properties, while the thermal annealing under Ar ambient significantly improves the electrical conductivity of the AZO films: an as-deposited sample has an electrical resistivity of 87 Wcm and an electron concentration of 1.3´10 17 cm -3 , while the annealed sample has an electrical resistivity of 3.7´10 -2 Wcm and an electron concentration of 1.2´10 20 cm -3 .

At present, the utilization of thermal energy from sunlight has been widely adopted as the working principle of concentrated solar power (CSP) generation systems. In this research, we suggest a CSP technology based on the properties of transparent conductive oxide (TCO) films on metal substrates which is compatible with mass production of solar selective absorbers that can be utilized at high temperatures. TCO material has plasma wavelength in infrared region. Therefore the electromagnetic wave with shorter wavelength than plasma wavelength goes through the material, while the electromagnetic wave with longer wavelength is reflected on the surface. By coating metal surface with a TCO film, interference is occurred in transparent wavelength range of TCO. Therefore, solar energy is highly absorbed, though thermal radiation from the absorber is suppressed. The optical property of fabricated TCO coated metal is well consistent with the simulated property. It is revealed that the performance of the absorber is improved by fabricating microstructures on the metal substrate. Thermal stability is confirmed at 700°C in vacuum for 3 hours. Solar absorptance and hemispherical emittance of the fabricated absorber are 0.82 and 0.17, respectively, which is comparable to that of commercialized absorbers.