Potassium Hexaniobate Research Articles

Enhanced photocatalytic activity through anchoring and size effects of Au nanoparticles on niobate nanotubes and nanosheets for water splitting

The inorganic semiconducting potassium hexaniobate (K4Nb6O17) has a wide band gap of 3.4 eV and has been widely studied as a photocatalyst activated by the ultraviolet part of the solar spectrum. In this study, we have prepared nanotubes and nanosheets of H4Nb6O17 and attached Au nanoparticles of two different sizes to both. The resultant composites are characterised and their activity towards H2 generation by photocatalytic water splitting is investigated. We found that the morphology of the nano-niobate had minimal effect for the Au particles attached directly to the niobate, however for the (3-Aminopropyl) triethoxysilane (APTES) anchored Au particles a marked difference in their activities towards water splitting was observed. The APTES-Au-nanosheets produced a maximum volume of hydrogen of 1384.3 μmol g−1 h−1, which was much higher than the 36.9 μmol g−1 h−1 for the parent niobate nanotubes. Morphology and different interaction were considered as the main factors affecting photocatalytic efficiency.

Preparation and isolation of mono-Nb substituted Keggin-type phosphomolybdic acid and its application as an oxidation catalyst for isobutylaldehyde and Wacker-type oxidation

Keggin-type mono-Nb substituted phosphomolybdic acid, H4PMo11NbO40, was prepared in a pure form.

2D–2D lattice engineering route for intimately coupled nanohybrids of layered double hydroxide and potassium hexaniobate: Chemiresistive SO2 sensor

2D–2D lattice engineering route is used to synthesize intimately coupled nanohybrids of layered double hydroxide (LDH) and potassium hexaniobate. The 2D–2D lattice engineering route is based on the electrostatically derived self-assembly of delaminated zinc-chromium-layered double hydroxide (ZC-LDH) nanosheets and potassium hexaniobate (HNb) nanosheets (ZCNb nanohybrids). The 2D–2D lattice-engineered ZCNb nanohybrids display expanded surface area, mesoporous anchored nanosheets network morphology, and intimate coupling between nanosheets. The 2D–2D lattice engineered ZCNb nanohybrids are used for the low temperature operated gas sensor. The ZCNb nanohybrids display outstanding selectivity for the SO2, with the high response of 61.5% compared to pristine ZC-LDH (28.08%) and potassium niobate (8%) at 150 °C. Moreover, ZCNb sensors demonstrate superior response and recovery periods of 6 and 167 s at 150 °C, respectively. This result underscores the exceptional functionality of the ZCNb nanohybrids as efficient SO2 sensors. Moreover, these findings vividly demonstrate that the 2D–2D lattice-engineered ZCNb nanohybrids are quite effective not only in improving the gas sensor activity but also in developing of new type of intimately coupled mesoporous LDH-metal-oxide based hybrid materials.

Directed assembly of barium titanate nanopeapods via solvothermal processing with a mixed surfactant system

Barium titanate (BaTiO3) peapod nanocomposites were prepared by the controlled capture of nanoparticles (NPs) in scrolling hexaniobate (HNB) nanosheets. BaTiO3 NPs and proton-exchanged potassium hexaniobate were treated solvothermally at 220 °C for 6 h in toluene to produce linear NP chains confined within multiwalled HNB nanoscrolls. This method consistently produced nanopeapods with average filling fractions greater than 70%. The controlled introduction of nanocomponents along with a combination of oleylamine and oleic acid surfactants was critical to the success of these reactions. The ability to form BaTiO3@HNB significantly expands the number of important peapod composites accessible by the controlled NP capture.

Synthesis, structure and optical properties of layered M4Nb6O17⋅nH2O (M = K, Rb, Cs) hexaniobates

Potassium hexaniobate (K4Nb6O17) with 3D-layered and 2D-nanostructured morphological features has been primarily of academic interest due to its catalytic and photophysical properties. Although its microscopic properties are well described in the literature, there remains an absence of support information on the spectroscopic nature arising from the insertion of larger counterions in the crystalline lattice. In the present work, a new protocol for the preparation of layered M4Nb6O17⋅3H2O (M = K, Rb, Cs) hexaniobates is reported, starting from a citric acid aqueous solution comprising the alkali cations and a niobium citrate complex. X-ray powder diffraction, Raman and photoluminescence spectroscopies were employed to examine the implications of alkali metal incorporation. Crystal structures determination was accomplished by Rietveld profile refinement method. The observed occurrence of a pseudo Jahn-Teller effect (PJTE) implies a reduction of NbO6 octahedral symmetry due to the shortening of terminal Nb−OM bond lengths along with O–Nb–O bond angles tilted from 180°. The Raman spectra point to a local structural disorder by increasing the metal radius with characteristic frequencies assigned to variations of Nb–O bond order. Crystal lattice compositions Rb4Nb6O17⋅3H2O and Cs4Nb6O17⋅3H2O materially induce changes in the luminescent properties and strongly distorted octahedral groups yield a brighter PL blue emission at low temperature (77 K).

Hydrothermal synthesis of bismuth niobates and their application in azo-dyes photo-discoloration

Bismuth niobates were obtained by hydrothermal synthesis using bismuth (III) nitrate pentahydrate [Bi(NO3)3·5H2O] and a Lindqvist potassium hexaniobate [K8-xHxNb6O19.nH2O] as soluble precursors. Pure crystalline Bi5Nb3O15, β-BiNbO4 and α- BiNbO4 were obtained as pure phases through co-precipitation at pH = 8, followed by hydrothermal treatment and calcination at 600, 700 or 800 °C, respectively, which are milder temperatures than those used in solid state, citrate sol-gel and ball milling methods. Band gap energies of α-BiNbO4 and β-BiNbO4 were estimated by UV–vis diffuse reflectance spectroscopy (DRS) as 3.2–3.4 eV, which are higher than experimental values reported in the literature. The materials were evaluated in the methyl orange azo-dye photo-discoloration under UV irradiation and complete bleach was attained. The triclinic phase presented a better efficiency as a photocatalyst than the orthorhombic phase. Unexpectedly, the mixture of both phases resulted in the highest photocatalytic efficiency, similar to that found for Degussa P25 commercial catalyst.

Boosted adsorption–photocatalytic activities and potential lithium intercalation applications of layered potassium hexaniobate nano-family

We demonstrated that the KN nano-family (including KN nanolaminas and nano hollow spheres) can be derived from the same Nb2O5-based hydrothermal reaction.

Photo-induced morphological winding and unwinding motion of nanoscrolls composed of niobate nanosheets with a polyfluoroalkyl azobenzene derivative.

Photo-responsive nanoscrolls can be successfully fabricated by mixing a polyfluoroalkyl azobenzene derivative and a niobate nanosheet, which is exfoliated from potassium hexaniobate. In this study, we have found that the photo-responsive nanoscroll shows a morphological motion of winding and unwinding, which is basically due to the nanosheet sliding within the nanoscroll, by efficient photo-isomerization reactions of the intercalated azobenzene in addition to the interlayer distance change of the nanoscrolls. The relative nanosheet sliding of the nanoscroll is estimated to be ca. 280 nm from the AFM morphology analysis. The distance of the sliding motion is over 20 times that of the averaged nanosheet sliding in the azobenzene/niobate hybrid film reported previously. Photo-responsive nanoscrolls can be expected to be novel photo-activated actuators and artificial muscle model materials.

Single- and Double-Layered Organically Modified Nanosheets by Selective Interlayer Grafting and Exfoliation of Layered Potassium Hexaniobate

Organically modified niobate nanosheets are promising building blocks for the design of advanced hybrid materials. Nanosheets with controlled thickness and surface composition are important for precise structural design of the nanosheet-based materials. In this work, single-layered and double-layered niobate nanosheets functionalized by phenylphosphonate moieties were selectively prepared by interlayer grafting of A-type and B-type intercalation derivatives of potassium hexaniobate (K4Nb6O17·3H2O) with phenylphosphonic acid (PPA), followed by exfoliation by ultrasonication in acetonitrile. The interlayer grafting of PPA was monitored using X-ray diffraction (XRD), Fourier transform infrared (FTIR), and solid-state NMR spectroscopy, and the thicknesses of the exfoliated nanosheets were measured by atomic force microscopy (AFM). Transparent hybrid films were obtained by incorporating the single- and double-layered nanosheets into an epoxy matrix.

Selective Synthesis of Silver Nanoparticles onto Potassium Hexaniobate: Structural Organisation with Bactericidal Properties

Silver-based nanocomposites are known to act as biocides against a series of microorganisms and are largely studied as an alternative to substitute conventional antibiotics that show decreasing efficacy. In this work, an eco-friendly method to synthesize silver nanoparticles assembled on the surface of hexaniobate crystals is reported. By means of ion exchange, K(+) ions of layered potassium hexaniobate were partially substituted by Ag(+) ions and the resulting material was exposed to UV light. The irradiation allowed the reduction of silver ions with consequent formation of silver nanoparticles located only on the hexaniobate surface, whereas Ag(+) ions located in the interlayer space remained in the ionic form. Increasing UV-light exposure times allowed controlling of the silver nanoparticle size. The antibacterial effects of the pristine potassium hexaniobate and of silver-containing hexaniobate samples were tested against Escherichia coli (E. coli). The antibacterial efficacy was determined to be related to the presence of silver in hexaniobate. An increasing activity against E. coli was observed with the decrease in silver nanoparticles size, suggesting that silver nanoparticles of distinct sizes interact differently with bacterial cell walls.

LLDPE/Proton Exchanged Layered Niobate K4Nb6O17 Nanocomposites

Abstract The potassium hexaniobate was synthesized by reacting, in stoichiometric proportions, Nb2O5 and K2CO3 at 1100°C and modified with octadecylamine from an acid-base reaction. Through X-ray diffraction, an interlayer space of 4.27 nm was observed in comparison with 0.79 nm of unmodified oxide; this was also shown by SEM microscopy. The hybrid nanofiller previously obtained was incorporated into LLDPE by melt intercalation (0 up to 10 wt.%) using a twin screw extruder and LLDPE-g-MAH as compatibilizer. From TGA results, all nanocomposites (LLDPE/LLDPE-g-MAH/lamellar oxide) have increased the onset degradation temperature, while the oxide lost during processing for LLDPE/LLDPE-g-MAH/lamellar oxide nanocomposites was higher for the highest content in comparison with LLDPE matrix. According to the Eyring equation, the activation volume of the samples could be calculated using the relationship between the yield strength and strain rate from tensile stress-strain curves. The activation volume decreased with increase of nanofiller concentration, suggesting a good adhesion between the layered oxide and the polymeric matrix for high concentration. This can be attributed to the restricted segmental motion near the nanofiller/polymeric interface, while a poor interaction between them was observed for low concentration. However, the Young's modulus showed a 50% improvement for low nanofiller concentration, especially in the case of 1 wt.% of nanofiller, which was also confirmed by modulus and toughness balance. Considering all the results, it has been revealed that the proton exchanged layered niobate can improve the thermal and mechanical properties of LLDPE/LLDPE-g-MAH/lamellar oxide nanocomposites.

Influence of Copper as a Co-Catalyst of Potassium Hexaniobate Nanotubes on the Photocatalytic Hydrogen Evolution from a Methanol Aqueous Solution

In the presence of Cu2+ ions, the photocatalytic hydrogen evolution from a methanol aqueous solution was achieved when potassium hexaniobate nanotubes were used as the catalyst. It was found that there existed a photo-induced period in the initial reaction stage. Furthermore, the photo-induced period was prolonged by increasing the amount of Cu2+ ions. After that, the rate of hydrogen evolution was dramatically improved. Combined with the reaction phenomena and the result of the photocatalytic hydrogen evolution, it was deduced that the Cu2+ ions captured the photo-generated electrons of potassium hexaniobate nanotubes in the photo-induced period. Consequently, the Cu2+ ions were reduced and deposited on the potassium hexaniobate nanotubes. When the mass ratio of Cu to potassium hexaniobate nanotubes was 3 wt%, the rate of hydrogen evolution over the catalyst reached 21.9 mmol·g-1·h-1, which could almost match with Pt as a co-catalyst. These results showed that Cu was an effective alternative to Pt as a co-catalyst of potassium hexaniobate nanotubes for the photocatalytic hydrogen evolution from the methanol aqueous solution.

Niobium Oxide Nanoscrolls as Building Blocks for Dye-Sensitized Hydrogen Production from Water under Visible Light Irradiation

Potassium hexaniobate nanoscrolls (NS-K4Nb6O17) formed by exfoliation of lamellar K4Nb6O17 were studied as building blocks for visible-light-driven H2 production (λ > 420 nm) from water using tris(2,2′-bipyridyl)ruthenium(II) chloride (Ru(bpy)32+) as a sensitizer and ethylenediaminetetraacetic acid (EDTA) as an electron donor. The surface of NS-K4Nb6O17 is negatively charged at pH 3−11, enabling cationic Ru(bpy)32+ molecules to be efficiently adsorbed onto the surface, allowing for rapid excited-state electron and subsequent H2 evolution without any chemical bond linkage between the sensitizer and the oxide surface. The rate of visible light H2 production in the nanoscroll-based system is 10 times higher than that of similarly sensitized K4Nb6O17. The difference can be primarily attributed to the strong adsorption of Ru(bpy)32+ in the case of the nanoscrolls. The maximum photocatalytic reactivity is found over a narrow range of pH and Pt-loading. This study highlights the utility of single-crystalline oxi...

Flux Growth and Characterization of Layered K4Nb6O17 Crystals

Well-formed crystals of potassium hexaniobate (K4Nb6O17) were successfully grown from potassium molybdate and tungstate fluxes. The crystal growth of K4Nb6O17 was conducted by heating a mixture of solute and flux at 1100 °C for 10 h, followed by cooling to 500 °C at a rate of 5 °C/h-1. Colorless and transparent crystals having sizes up to 18 × 9.0 × 2.9 mm were grown from high-temperature solutions containing solute of 1−60 mol %. The crystal sizes were dependent on the solute concentration. The basic form was a plate bounded by the well-developed {010}, {110}, and {021} faces. Potassium, niobium, and oxygen were almost homogeneously distributed in the crystals. Furthermore, the grown K4Nb6O17 crystals exhibited a high activity for the dye adsorption and degradation. The degradation occurred via the photocatalytic process under ultraviolet light irradiation.

The dissolution behavior of Nb 2O 5, Ta 2O 5 and their mixture in highly concentrated KOH solution

The dissolution behavior of Nb 2O 5, Ta 2O 5 and their mixture in highly concentrated KOH solution (> 50% wt.) has been investigated. A 4 3 orthogonal design was used to study the effects of four leaching parameters, reaction temperature, KOH/Nb 2O 5, mole ratio, agitation speed and reaction time on the dissolution yield of Nb(V). It was found that the effect of reaction temperature on the dissolution yield of Nb(V) was much greater than that of the other three factors. The results of factorial experiments showed that a high KOH concentration was benefit to the formation of soluble potassium hexaniobate, but high temperature was benefit to the formation of insoluble potassium niobate, and almost all Ta 2O 5 was converted into insoluble potassium tantalate. The insoluble potassium niobate and potassium tantalate were analyzed by XRD, which were proved to be KNbO 3 and KTaO 3, respectively. The dissolution behavior of the mixture of Nb 2O 5 and Ta 2O 5 was similar with that of Nb 2O 5.

Synthesis, modification and characterization of K4Nb6O17-type nanotubes

Potassium hexaniobate nanotubes have been fabricated from polycrystalline K4Nb6O17 at room temperature using the intercalating and exfoliating methods. These tubular materials were mainly characterized by using electron microscopy. Their structures were found to be multilayer crystalline nanotubes with interlayer spacings from 0.83 to 3.6 nm, depending on the intercalating molecules such as tetra(n-butyl)ammonium hydroxide (TBA+OH−) and alkylamines (CnH2n+1NH2) with different lengths of the alkyl chains. The number of layers in the wall is in the range of 3 to 8. The outer diameters of the nanotubes are distributed around 20 nm to 90 nm for different products obtained with different alkylamines and the lengths of the nanotubes range from a few hundred nanometers to several micrometers. Comprehensive structural characterization revealed that the majority hexaniobate nanotubes were formed by rolling the monolayer (–Nb6O17–)n sheets around the [100] zone axis of K4Nb6O17. Nanotubes with helical angles of larger than 10 degrees were also observed.

Imaging helical potassium hexaniobate nanotubes

Potassium hexaniobate nanotubes have been synthesized at room temperature and characterized by high-resolution transmission electron microscopy (HRTEM) and energy-dispersive x-ray analysis. It is shown that HRTEM images may be used effectively to determine the helicity of the nanotubes, and, in particular, it is found that almost all nanotubes have their axes pointing within a few degrees from the [100] direction of the K4Nb6O17 structure.

Nanoscale Tubules Formed by Exfoliation of Potassium Hexaniobate

The exfoliation of acid-exchanged K4Nb6O17 with tetra(n-butyl)ammonium hydroxide in water produces a colloidal suspension of individual sheets, which roll into loosely bound tubular structures. The tubule shape can be made permanent via precipitation of the colloid with alkali cations. Atomic force microscopy and transmission electron micrographs reveal that the tubules have outer diameters ranging from 15 to 30 nm and that they are 0.1 to 1 μm in length. The observed curling tendency, preferential folding, and cleavage angles of the individual sheets are interpreted in terms of the crystal structure of the parent solid, K4Nb6O17. The driving force for tubule formation appears to be relief of strain that is inherent in the asymmetric single sheets. This driving force is absent in bilayer colloids formed early in the exfoliation process, which are found only as flat sheets. Tubules in colloidal suspensions that have been subjected to turbulence have a tendency to unroll into flat sheets on surfaces, indica...