Porous Nb2O5 Research Articles

Efficient photocatalytic hydrogen evolution by in situ construction of Nb4+ charge-carrier channels in hollow porous tubular C3N4 and Nb2O5 Z-scheme heterojunctions

Optimizing heterojunction structure is an important way to improve photocatalytic activity. Herein, we report a novel hollow tubular C3N4/Nb4+/Nb2O5 nanoparticle Z-Scheme heterojunction, by introducing Nb4+ ions into Nb2O5 through a reducing atmosphere during C3N4 thermal polymerization. The optimized heterostructure showed outstanding photocatalytic hydrogen evolution activity under both UV–vis (14.93 mmol g−1 h−1) and Vis (5.22 mmol g−1 h−1) lights. The photocatalytic hydrogen evolution activity under UV–vis light is 26.6 and 4.75 times that of bulk C3N4 (CN) and hollow tubular C3N4 (HCN), respectively. The increased photocatalytic activity can be attributed to the larger specific surface area, more active sites, and enhanced light absorption capacity of the composite. Crucially, the introduction of Nb4+ ions as the charge-carrier transport channels in the Z-scheme heterostructure improves the efficiency of photogenerated charge-carrier separation. This study provides a useful design strategy for Z-Scheme photocatalytic heterojunction structures that can utilize solar light more efficiently.

Enabling Electron Delocalization by Conductor Heterostructure for Highly Reversible Sodium Storage

AbstractNiobium oxides have fascinated numerous interests for ultrafast sodium‐ion batteries due to their impressive stability, fast pseudocapacitive kinetics, and high safety features. However, the poor intrinsic electronic conductivity and unrevealed mechanisms largely limit their further applications. Here, inspired by density functional theory (DFT) calculations, the introduction of conductor heterostructure (Nb2O5/NbSe2) can largely decrease the bulk phase inserted energy of Na+ from 9.88 to −3.54 eV, as well as promote the delocalization of electron, greatly improving the electrical conductivity of Nb2O5. As expected, the as‐obtained orthorhombic niobium pentoxide (T‐Nb2O5‐x‐NbSe2@C) delivers a high conductivity of 3.69 S cm−1, exhibiting a great improvement of four orders magnitude than that of the parent Nb2O5 (1.87 × 10−4 S cm−1). Impressively, T‐Nb2O5‐x‐NbSe2@C possesses a high reversible capacity of 249 mAh g−1 at 0.1 A g−1, as well as excellent rate performance of 63 mAh g−1 at 5.0 A g−1, nearly twins the capacity of parent porous Nb2O5. Furthermore, the periodic evolutions of the main peaks for targeted electrodes during repeated charging/discharging processes have been further revealed by in situ XRD, demonstrating a highly reversible inserted/extracted storage mechanism of Na+. This work provides a novel method to improve the conductivity of materials by constructing conductor heterojunction.

Porous Nb2O5 Nanofibers Prepared via Reactive Needle-Less Electrospinning for Application in Lithium–Sulfur Batteries

This contribution describes the preparation, coupled with detailed characterization, of Nb2O5 nanofibers and their application in lithium–sulfur batteries for the improvement of electrochemical performance. The utilization of reactive needle-less electrospinning allowed us to obtain, in a single step, amorphous pre-ceramic composite PAN/Nb2O5 fibers, which were transformed into porous ceramic Nb2O5 nanofibers via calcination. Thermogravimetric studies defined that calcination at 600 °C results in crystalline ceramic fibers without carbon residues. The fibrous morphology and mean diameter (614 ± 100 nm) of the ceramic nanofibers were analyzed via scanning and transmission electron microscopy. A surface area of 7.472 m2/g was determined through nitrogen adsorption measurements, while a combination of X-ray diffraction and Raman spectroscopy was used to show the crystallinity and composition of the fibers after calcination—single T-phase Nb2O5. Its performance in the cathode of lithium–sulfur batteries was defined through electrochemical tests, and the obtained results were compared to a similar blank electrode. The initial discharge capacity of 0.5 C reached a value of 570 mAh∙g−1, while the reversible capacity of 406 mAh∙g−1 was retained after 200 cycles, representing a capacity retention of 71.3%. The presence of Nb2O5 nanofibers in the carbon cathode inhibits the shuttle effect through polysulphide confinement, which originates from porosity and chemical trapping.

MOFs-derived niobium oxide nanoparticles/carbon nanofiber hybrid paper as flexible binder-free electrode for solid-state asymmetric supercapacitors

For the first time, a bottom-up approach was adopted to prepare free-standing orthorhombic Nb2O5-anchored carbon nanofibers (Nb2O5 @CNFs) hybrid paper via in-situ solvothermal synthesis of Nb-metal organic framework@CNFs hybrid and subsequent pyrolysis process. The nanocrystalline Nb2O5 decorated on the CNF surfaces improved the charge storage performance through synergistic effect between porous CNFs (EDLC) and Nb2O5 (pseudocapacitance). Moreover, the high surface area and core-shell structure of Nb2O5 @CNF hybrid reduce diffusion limitations and facilitate ionic transport. The Nb2O5 @CNF hybrid paper as a binder-free electrode delivered greater specific capacitance of 633.6 F/g (975.7 F/cm3) in 1 M H2SO4, compared to Li2SO4 electrolyte (524.3 F/g) at 0.5 A/g. However, the Nb2O5 @CNF electrode showed better cycling stability in neutral Li2SO4 electrolyte than in acidic H2SO4 electrolyte. The solid-state asymmetric supercapacitor (ASC) based on Nb2O5 @CNF paper and activated carbon (AC) electrodes delivered higher energy density of 62.1 Wh kg−1 at 292.7 W kg−1 in Li2SO4 electrolyte due to a wider working potential window (0–1.5 V), compared to H2SO4 electrolyte (55 Wh kg−1). The Li2SO4-based ASC cell is also capable of producing an energy density of up to 45.6 Wh kg−1, even at a high-power density of up to 5853.6 W kg−1. The as-assembled ASC demonstrated low self-discharge rate as well as excellent bending stability, indicating its excellent prospects in real-life applications.

Porous Nb2O5 Formed by Anodic Oxidation as the Sulfur Host for Enhanced Performance Lithium-Sulfur Batteries.

Lithium-sulfur batteries (LSBs), with their high theoretical specific capacity and energy density, have great potential to be a candidate for secondary batteries in the future. However, Li-S batteries suffer from multiple issues and challenges, for example, uneven growth of lithium dendrites, low utilization of the active material (sulfur), and low specific capacity. This paper reports a low-cost and anodic oxidation method to produce niobium pentoxide with a porous structure (P-Nb2O5). A simple one-step process was used to synthesize P-Nb2O5 with porous structures by anodizing niobium at 40 V in fluorinated glycerol. The porous Nb2O5 showed excellent rate capability and good capacity retention by maintaining its structural integrity, allowing us to determine the advantages of its porous structure. As a result of the highly porous structure, the sulfur was not only provided with adequate storage space and abundant adsorption points, but it was also utilized more effectively. The initial discharge capacity with the P-Nb2O5 cathode rose to 1106.8 mAh·g-1 and dropped to 810.7 mAh·g-1 after 100 cycles, which demonstrated the good cycling performance of the battery. This work demonstrated that the P-Nb2O5 prepared by the oxidation method has strong adsorption properties and good chemical affinity.

Hard-Templated Porous Niobia Films for Optical Sensing Applications

Porous Nb2O5 films obtained by a modified hard-template method were studied and their optical and sensing properties were optimized in order to find applications in chemo-optical sensing. Porous films were prepared by following three steps: liquid mixing of niobium sol and SiO2 colloids in different volume fractions, thermal annealing of spin-coated films for formation of a rigid niobia matrix, and selective removal of silica phase by wet etching thus generating free volume in the films. The morphology and structure of the films were studied using transmission electron microscopy and selected area electron diffraction, while their optical and sensing properties were estimated using UV-VIS-NIR reflectance measurements in different ambiences such as air, argon and acetone vapors and nonlinear curve fitting of the measured reflectance spectra. Bruggeman effective medium approximation was applied for determination of the volume fraction of silica and air in the films, thus revealing the formation of porosity inside the films. For further characterization of composite films, their water contact angles were measured and finally conclusions about the impact of initial chemical composition and etching duration on properties of the films were drawn.

Photoactivity of nanostructured porous Nb2O5: Effect of Pt, Ta, Cu, and Ti impregnation

AbstractIn the present work, we report the obtaining of nanostructured porous Nb2O5 by anodizing process, with Pt, Ta, Cu, or Ti impregnation, and by magnetron sputtering process. The techniques of scanning electron microscopy, X‐ray diffraction, and X‐ray photoelectron spectroscopy were used to evaluate the morphology, and the composition and crystalline structure of nanoporous Nb2O5. The photocatalytic activity was evaluated by electrochemical photocurrent tests under UV light illumination. Nanostructured porous Nb2O5 with Pt, Ta, Cu, and Ti impregnation presented bandgap reduction and 10 times higher photocurrent density in the presence of UV light when compared to the not impregnated nanoporous Nb2O5.

Synthesis of a novel hexagonal porous TT-Nb2O5 via solid state reaction for high-performance lithium ion battery anodes

Hexagonal porous Nb2O5 was synthesized for the first time via a facile solid-state reaction. The structure and electrochemical properties have been optimized through tuning heating temperature. X-ray diffraction results indicate that pseudo hexagonal Nb2O5 (TT-Nb2O5) and orthorhombic Nb2O5 have been synthesized at different temperatures. Hexagonal sheet and porous structure of Nb2O5 were characterized by scanning electron microscopy and N2-adsorption-desorption isotherms. The as-prepared TT-Nb2O5 (heated at 600 °C) shows the best performance with a remarkable charge capacity of 178 mA·h/g at 0.2C, which is higher than that of T-Nb2O5. Even at 20C, TT-Nb2O5 offers unprecedented rate capability up to 86 mA·h/g. The high rate capacity is due to pseudocapacitive Li+ intercalation mechanism of TT-Nb2O5. The reported results demonstrate that Nb2O5 with good crystal structure and high specific surface area is a powerful composite design for high-rate and safe anode materials.

Designed growth of porous 2D Nb2O5 with Ag nano-particles for differential detection of UV-A and UV-C

We demonstrate the differential detection of UV-A (ultra-violet 320–400 nm region) and UV-C (100–280 nm) using porous two-dimensional (2D) Nb2O5 and additional Ag nano-particle decoration. The 2D Nb2O5, which has band-absorption edge near the UV-A zone, was synthesized by thermodynamic conversion of 2D material NbSe2 (Nb2O5 has lower Gibbs formation energy than NbSe2). For the differential detection (to distinguish with UV-C absorption), we decorated the Ag nano-particles on the Nb2O5 surface. By coating Ag nano-particles, we can expect (i) a decrease in the area of light absorption by the Ag-coated area, and (ii) an increase of surface plasmon absorption by Ag nano-particles, especially the UV-A region, resulting in strong intensity ratio change UV-A/UV-C.

Carbon coated 3D Nb2O5 hollow nanospheres with superior performance as an anode for high energy Li-ion capacitors

Carbon coated porous hollow Nb2O5 nanospheres are designed for lithium-ion capacitors. The carbon layer limits the agglomeration of Nb2O5 and improves the conductivity. The hollow structure accommodates volume expansion and facilitates ion transport.

Fast Electrochemical Storage Process in Sputtered Nb2O5 Porous Thin Films.

The formation of a thin film electrode exhibiting high capacity and high rate capabilities is challenging in the field of miniaturized electrochemical energy storage. Here, we present an elegant strategy to tune the morphology and the properties of sputtered porous Nb2O5 thin films deposited on Si-based substrates via the magnetron sputtering deposition technique. Kinetic analysis of the redox reactions is studied to qualify the charge storage process, where we observe a non-diffusion-controlled mechanism within the porous niobium pentoxide thin film. To improve the surface capacity of the Nb2O5 porous electrode, the thickness is progressively increased up to 0.94 μm, providing a surface capacity close to 60 μAh·cm-2 at 1 mV·s-1. The fabrication of high energy density miniaturized power sources based on the optimized T-Nb2O5 films could be achieved for Internet of Things applications requiring high rate capability.

Anodised porous Nb2O5 for photoreduction of Cr(VI)

Abstract Porous niobium pentoxide (Nb2O5) was successfully formed in a fluorinated sodium sulphuric electrolyte by anodisation process. The anodisation temperature was varied from 25 to 80 °C as to study the effect of anodisation temperature on the morphology of porous Nb2O5. Porous oxide was formed when the anodisation bath temperature was at 25 oC and 50 oC but not at 80 oC. The anodised niobium (Nb) foils were then annealed to obtain an orthorhombic Nb2O5 phase. Orthorombic Nb2O5 is an n-type semiconductor which when illuminated with light of appropriate energy can generate free electrons. These electrons can be used to reduce heavy metal ions like chromium(VI) [Cr(VI)]. Cr(VI) is a typical pollutant often found in industrial wastewater. It is carcinogenic, mutagenic and can induce severe health problem if consumed beyond the limitation. In here, we demonstrated on the possibility of reducing Cr(VI) to a more benign Cr(III) by using free electrons formed upon Nb2O5 illumination. The photocatalytic reduction of Cr(VI) on porous Nb2O5 was observed and assessed. The result shows that different anodisation temperature effect on the morphology and pores dispersion; which affect the photoreduction performance. Photoreduction activity was recorded to be ∼ 60 % after 120 min of exposure of the Cr(VI) solution to ultraviolet radiation.

Self-assembled mesoporous Nb2O5 as a high performance anode material for rechargeable lithium ion batteries

Self-assembled mesoporous Nb2O5 has been successfully synthesized by template free one-pot method followed by annealation treatment. The morphological and textural characterization with field emission scanning electron microscope (FESEM), transmission electron microscope (TEM), and N2 sorption analyses reveal the formation of interconnected mesoporous Nb2O5 with pores average size ∼5 nm without using any templates or porogens. When used as anode material in lithium ion battery (LIB), the mesostructured Nb2O5 material exhibits enhanced electrochemical performance with discharge capacity of 182 mA h g−1 (against theoretical capacity of 200 mA h g−1) even after 210 cycles at 0.5 C rate with columbic efficiency ∼99.0%. At 1 C rate, Nb2O5 constructed electrode shows high electrode stability up to 850 cycles of repeated charge/discharges. The impressive capacity retention and the rate capability of porous Nb2O5 are attributed to the (i) interconnected porous structure providing favourable electrode/electrolyte interface as well as Li insertion/extraction kinetics and (ii) operation of combined intercalative and capacitive charge storage mechanism.

Facile fabrication of interconnected-mesoporous T-Nb2O5 nanofibers as anodes for lithium-ion batteries

Niobium pentoxide (Nb2O5) has been extensively studied as anode materials for lithium ion batteries (LIBs) due to its good rate performance and safety advantages. However, the intrinsic low electronic conductivity has largely restricted its practical application. In this work, we report the construction of mesoporous T-Nb2O5 nanofibers by electrospinning followed by heat treatment in air. The interconnected mesoporous structure ensures a high surface area with easy electrolyte penetration. When used as anodes for LIBs, the mesoporous Nb2O5 electrode delivers a high reversible specific capacity of 238 mA h g−1 after 1,000 cycles at a current density of 1 A g−1 within a voltage range of 0.01–3.0 V. Even at a higher discharge cut-off voltage window of 1.0–3.0 V, it still possesses a high reversible capacity of 166 mA h g−1 after 200 cycles. Moreover, the porous Nb2O5 electrode also exhibits excellent rate capability. The enhanced electrochemical performances are attributed to the synergistic effects of porous nanofiber structure and unique crystal structure of T-Nb2O5, which has endowed this material a large electrode-electrolyte contact area with improved electronic conductivity.

Narrow-bandgap Nb2O5 nanowires with enclosed pores as high-performance photocatalyst

Porous niobium oxide nanowires synthesized via a solvothermal method exhibited decreased bandgap, enhanced light absorption and reduced charge-recombination rate. The porous Nb2O5 nanowires showed increased performance for the photocatalytic H2 evolution and photodegradation of rhodamine B, as compared to their solid counterparts, which could be ascribed to the peculiar porous nanostructure.

Facile synthesis of porous Nb2O5 microspheres as anodes for lithium-ion batteries

Porous Nb2O5 microspheres have been prepared by a facile solvothermal method and subsequent heat-treatment using cheap NbCl5 as a starting material. As anode materials for lithium-ion batteries (LIBs), porous Nb2O5 microspheres show good electrochemical performance with discharge capacities of 199 mA h g−1 after 50 cycles at a current density of 100 mA g−1 and 126 mA h g−1 after 150 cycles at a current density of 500 mA g−1, respectively. The good electrochemical performance of porous Nb2O5 microspheres can be ascribed to the distinct structure of the microspheres consisted of nanoparticles with short transport route for ion and electron, large specific surface and mesoporous structure. The electrochemical testing results demonstrate that porous Nb2O5 microspheres are the promising alternate as anode materials for LIBs.

Preparation of Niobium Pentoxide Nanotube Powders By a Facile Electrochemical Anodization

Niobium pentoxide (Nb2O5) nanostructures are promising candidate for various applications including solar cells, sensors, supercapacitors, photocatalysis, and electrochromic coatings, etc. And the morphologies, crystal structures, and surface areas of Nb2O5 nanostructures greatly affect their devices performances. The electrochemical anodization is a versatile and facile method to fabricate various metal oxide nanostructures, for example, TiO2, Al2O3, WO3, Nb2O5, and Ta2O5. The usual form of obtained anodic metal oxides is thin oxide film attached on metal substrates with nanotube, nanopore or nanochannel structures. Here we report on the fabrication of Nb2O5 nanotube powders by a direct electrochemical anodization method. By simply adjusting the anodization conditions, such as applied voltage, electrolyte composition, and bath temperature, the anodically formed Nb2O5 nanostructures can be spontaneously and continuously released into the anodization electrolyte during the reaction process. After washing and centrifugation, porous powders can be obtained. Under the optimized conditions, the highly porous Nb2O5 powder with nanotube morphology can be obtained, which can provide a very high specific surface area and can be used for various applications. By coating the Nb2O5 nanotube powders onto transparent conducting glasses, the oxide films can be used as the photoanodes for dye-sensitized solar cells with the front-side illumination configuration, leading to a high light harvesting efficiency. The nanotube powders can also be used as an efficient photocatalyst to decompose organic contaminant, with a high decomposition efficiency. The rapid and continuous electrochemical anodization to the formation of Nb2O5 nanotube powders is a promising method for mass production of functional Nb2O5 nanostructures for various applications.

Structural Modification of Self-Organized Nanoporous Niobium Oxide via Hydrogen Treatment

Niobium pentoxide (Nb2O5) is an interesting material with applications in Li battery and hybrid capacitor electrodes. The main limitation of this material is its low electronic conductivity. In this study, H2 treatment is introduced to address this issue. Self-ordered Nb2O5 films were prepared by anodizing Nb foils and subsequently treating them in a H2 atmosphere. Electron microscopy revealed that the Nb2O5 film had a hierarchical porous microstructure consisting of macropores and mesopores. X-ray diffraction analysis showed that the crystal structure could be changed by the H2 treatment compared to the air treatment. Oxygen deficiencies in the Nb2O5 film were induced by the treatment, as confirmed by X-ray photoelectron spectroscopy. Mott–Schottky analysis was performed and indicated that the electronic conductivity of the material was significantly improved by the oxygen deficiencies. Thus, the electrochemical Li storage kinetics in porous Nb2O5 films can be greatly enhanced by H2 treatment.

Nb2O5 Schottky based ethanol vapour sensors: Effect of metallic catalysts

The aim of this paper is to compare the performances of the highly porous Nb2O5 Schottky based sensors formed using different catalytic metals for ethanol vapour sensing. The fabricated sensors consist of a fairly ordered nano-vein like porous Nb2O5 prepared via an elevated temperature anodization method. Subsequently, Pt, Pd and Au were sputtered as both Schottky contacts and catalysts for the comparative studies. These metals are chosen as they have large work functions in comparison to the electron affinity of the anodized Nb2O5. It is demonstrated that the device based on Pd/Nb2O5 Schottky contact has the highest sensitivity amongst the developed sensors. The sensing behaviors were studied in terms of the Schottky barrier height variations and properties of the metal catalysts.

A vein-like nanoporous network of Nb2O5 with a higher lithium intercalation discharge cut-off voltage

A novel morphology of a criss-cross vein-like nanoporous network of Nb2O5 produced using a simple electrochemical anodization method is presented as a superior electrode for safe lithium-ion batteries. Scanning electron microscopy (SEM) observations demonstrate that the synthesised Nb2O5 is made of a continuous and highly packed vein-like nanoporous network with many lateral interconnections, which provides excellent channels for the fast transfer of both Li+ ions and electrons. Even without surface coating or cation doping, the porous Nb2O5 electrode could deliver durable capacity within the operating voltage window of 1.0–3.0 V vs. Li/Li+, with a reversible capacity of 201 mA h g−1 after 300 cycles at a current density of 0.4 A g−1. At the higher discharge cut-off voltage window of 1.2–3.0 V, the reversible capacity decreased to 175 mA h g−1. The first cycle Coulombic efficiency was above 94% for both operating voltage windows with a negligible capacity fading up to 300 cycles. The porous Nb2O5 electrode demonstrates several advantages as an anode including: (i) Improved cell safety due to a higher, V ≥ 1.0, discharge cut-off voltage which reduces dangerous high-temperature reactions; (ii) low level of irreversibility in the first cycle by preventing the formation of a solid electrolyte interface layer; (iii) high Coulombic efficiency due to sufficient infiltration of the electrolyte and fast diffusion of Li+ ions and (iv) high rate capability. Moreover, the synthesis method reports a novel smart design of nanostructured anode electrode materials capable of overcoming the existing limitations.