Defect Pyrochlore Research Articles

A physical barrier woven by flexible, adiabatic and structurally stable high-entropy nanofiber sponges for efficient surface protection

The surface of equipment under harsh conditions often requires protection from adiabatic ceramic fibers. However, the structural stability and flexibility of fibers are frequently compromised. Here, (LaCeNdSmEu)2Zr2O7+δ, a defective high-entropy pyrochlore, was fabricated into flexible nanofiber sponges utilizing a simple saturated solution blow spinning method. The novel 3D lamellar sponges, containing numerous micropores woven by nanofibers, can achieve thermal insulation efficiently. Moreover, the defects and high-entropy effects inside nanofibers can further strengthen their structural stability. Furthermore, nanofiber sponges can remain flexible at -196–1350 °C. Even when burned by the flame, only 2-cm-thick sponges can reduce the heat source from 1350 to 285 °C. The smooth sintering process, low thermal conductivity (0.024–0.044 W/(m•K) at 28–500 °C), high long-term operation temperature (1350 °C), low density (14 mg/cm3), excellent compression flexibility, and remarkable structural stability under various harsh conditions can endow nanofiber sponges with powerful surface protection capability.

Optimizing junction interface integrity between 3D/2D defect pyrochlore Bi1.8Fe0.2WO6 and g-C3N4 nanostructures: A novel multifunctional nanocomposite for enhanced photocatalytic and photo-electrocatalytic water splitting and water remediation applications

In this work, a simplistic composite fabrication of defect pyrochlore Bi1.8Fe0.2WO6 (BFWO) and g-C3N4 (g-CN) was carried out to augment the photocatalytic degradation kinetics and Hydrogen (H2) evolution rate. The structural confirmation through XRD and FTIR confirmed the successful formation of pristine g-C3N4, BFWO NPs, and their composites. FESEM micrographs revealed multilayered sheet-like formation for C3N4 whereas irregular cuboid-shaped structure for BFWO NPs. In the composite formation, the sheets tended to appear wrapped around the BFWO NPs. The UV-DRS absorbance spectra exhibited a highlighted increase in the absorbance region towards the visible light region following a decrease in the band gap. The photoluminescence lifetime studies revealed an enhanced lifetime of excitons to 6.21 ns for 50:50 (g-C3N4: BFWO) composite formation, whereas pristine g-CN displayed an average lifetime of about 4.79 ns. The degradation efficacy in the removal of RhB and MB from water revealed up to 100% and 95.5% removal rates in under 90 mins and 180 mins respectively using the 50:50 composite formation. Similarly, the H2 evolution rate was significantly improved and exhibited up to 370 μmol/h/g. The photo-electrochemical studies revealed a sharp charge separation of excitons for 50:50 (g-C3N4: BFWO) with a maximum photocurrent density of -0.54 μA/cm2 @ 0 V which is 3.6 times and 13.5 times higher than bare g-CN and BFWO. 1.33 V vs. RHE for OER kinetics and -0.09 V vs RHE for HER kinetics were the minimal onset potential exhibited for equal ratios of BFWO and g-CN. Coupled with the heightened charge separation, reduced recombination rate, and optimal band edge positions, the photocatalytic efficiency in the degradation of RhB and MB and water-splitting reactions were improved.

Photocatalytic and Conductivity Studies of Proton Exchanged Defect Pyrochlore, KMn0.33Te1.67O6 and its Application in Pb2+ Removal

Photocatalytic and Conductivity Studies of Proton Exchanged Defect Pyrochlore, KMn0.33Te1.67O6 and its Application in Pb2+ Removal

Preparation, characterization and photocatalytic studies of defect pyrochlore, KMn0·33Te1·67O6 and its composite with g-C3N4

Skeletal materials belonging to the defect pyrochlore family have been the subject of considerable interest due to their remarkable properties. In this work, a highly efficient, stable and visible light active composite photocatalytic system consisting of defect pyrochlore of composition KMn0·33Te1·67O6 (KMnTeO) and g-C3N4 (g-CN) has been successfully prepared by simple physical mixing of pre-prepared KMnTeO and g-CN components synthesized in solid state and thermal treatment of low-cost melamine, respectively. Structural, morphological and optical properties of as-prepared catalysts were deduced from powder X-ray Diffraction (XRD), Fourier Transform Infrared (FT-IR), Raman, Field Emission Scanning Electron Microscopy-Energy Dispersive X-ray Spectroscopy (FESEM-EDX), Transmission Electron Microscopy-High Resolution Transmission Electron Microscopy (TEM-HRTEM), Ultraviolet–Visible Diffuse Reflectance Spectroscopy (UV–Vis DRS), X-ray Photoelectron Spectroscopy (XPS) and Photoluminescence (PL) measurements. The lattice parameter “a” of parent KMnTeO was obtained from Rietveld refinement of its powder XRD pattern. The FT-IR and Raman spectra of KMnTeO gave characteristic bands belonging to the defect pyrochlore structure. The photocatalytic activities of KMnTeO and composite KMnTeO-g-CN were evaluated by the degradation of methylene blue (MB). Scavenger experiments were carried out to identify the reactive species responsible for the degradation of MB. Furthermore, the reusability and stability of the photocatalysts were explored. The probable photocatalytic mechanism for MB degradation over the composite KMnTeO-g-CN was surmised according to the scavenger experiments. The work provides a new approach for the development of novel defect pyrochlore based composite photocatalysts to treat industrial wastewater containing organic pollutants.

High-temperature thermal stability modification of (La0.2Y0.2Sm0.2Eu0.2Nd0.2)2Zr2O7 ceramic aerogel for enhanced thermal insulation performance

In this study, tetraethylorthosilicate and dimethyldiethoxysilane were employed to modify high-entropy (La0.2Y0.2Sm0.2Eu0.2Nd0.2)2Zr2O7 (5RE2Zr2O7) ceramic aerogels to address the challenges of particle agglomeration and pore structure collapse that occur during the calcination step in the preparation of high-entropy ceramic aerogels. Following the modification, the specific surface area of the ceramic aerogel increased significantly from 32.22 m2/g to 148.65 m2/g to 201.61 m2/g. Simultaneously, the formation of defective fluorite or pyrochlore crystal structures led to reduced thermal conductivity in the three types of high-entropy ceramic aerogels, namely 5RE2Zr2O7 (from 0.212 W m−1•K−1 to 0.139 W m−1•K−1), 5RE2ZSA (from 0.065 W m−1•K−1 to 0.049 W m−1•K−1), and DDS/5RE2ZSA (from 0.046 W m−1•K−1 to 0.039 W m−1•K−1), compared to their respective non-high-entropy counterparts. Balancing the porous structure of high-entropy ceramic aerogels with an adequate number of defective fluorite or pyrochlore crystals is challenging. Addressing this challenge is a key direction for advancing high-entropy aerogels in the future.

Photomineralization of urban stormwater under visible light irradiation by using new tellurium-based defect pyrochlores

This study investigates the photocatalytic activity of AgNbTeO6 (ANTO) and Sn0.5NbTeO6 (SNTO), two new tellurides synthesized by ion-exchange from the primordial compound KNbTeO6 (KNTO) prepared by the sol-gel method, for the photomineralization of urban stormwater (UWS) and identified organic pollutants from the USW sample. All the materials were well characterized by various spectroscopic and microscope techniques. Both ANTO and SNTO materials mimic pristine KNTO, such as a cubic lattice with the Fd3m space group. All of the materials' UV–vis–DRS profiles were used to calculate their bandgap energies. The lower photocurrent of ANTO than that of SNTO and KNTO materials indicates that ANTO is a superior photocatalytic material due to the low rate of recombination of electron-hole pairs, which is also supported by EIS studies. The photomineralization efficiency of the ultra-stormwater (USW) and pollutants identified from USW sample mineralization with ANTO is in the range of 95–99%. Additionally, HPLC analysis of initial and final samples of USW and two major pollutants identified from USW and TOC analysis for USW were studied. Finally, through mechanistic studies, the active species for photomineralization of stormwater sewage and organic pollutants was identified. The scavenger and insitu ESR studies indicated that superoxide radicals are more highly active than other radical species. The ANTO and SNTO have shown higher photocatalytic activity compared to the parent KNTO due to their low bandgap energy and low rate of recombination of the hole-electron pair.

Ultralow thermal conductivity in defect pyrochlores: balancing mass fluctuation scattering and rattling modes

Progressive replacement of non-framework K+ cations with Cs+ cations in defect pyrochlores leads to reductions in thermal conductivity, the extent of which reflects a balance between the effects of mass fluctuation and rattling vibrations.

A small-box approach to the local crystal structure of Y3NbO7

The local crystal structure of Y3NbO7 was probed using neutron total scattering. Five different structural models were fit to the experimental pair distribution function of this material up to 15 Å. Fits of defect fluorite (Fm3‾m), pyrochlore (Fd3‾m), and orthorhombic C weberite models (C2221) were attempted based on prior structural investigations of Y3NbO7 and related fluorite-based materials. Orthorhombic P weberite (P2221) and monoclinic weberite models (P1121) were derived via symmetry reduction of the orthorhombic C weberite structure. All models except monoclinic weberite failed to reproduce the experimental pair distribution function. Misfits were clearly visible for peaks arising from atom pairs within the first coordination shell (defect fluorite and pyrochlore) and beyond (orthorhombic C and P weberite). By contrast, monoclinic weberite (γ ≈ 91.8°) provided an excellent fit, with no major misfits observed. This unit cell features distorted YO8 dodecahedra, YO7 monocapped octahedra, and NbO6 octahedra as building blocks. Comparison between the topologies of the cationic and anionic substructures in monoclinic weberite and its parent orthorhombic C structure revealed displacements of the metal atoms along the [100] direction and a less compact spatial distribution of oxygen atoms along the a axis. The monoclinic weberite model presented herein may serve as a starting point to revisit structural modeling of fluorite-based materials with challenging local structures.

Defect Pyrochlore‐Type Mott–Schottky Photocatalysts for Enhanced Ammonia Synthesis at Low Pressure

AbstractThe ambient ammonia synthesis coupled with distributed green hydrogen production technology can provide promising solutions for low‐carbon NH3 production and H2 storage. Herein, we reported Ru‐loaded defective pyrochlore K2Ta2O6−x with remarkable visible‐light absorption and a very low work function, enabling effective visible‐light‐driven ammonia synthesis from N2 and H2 at low pressure down to 0.2 atm. The photocatalytic rate was 2.8 times higher than that of the best previously reported photocatalyst and the photo‐thermal rate at 425 K was similar to that of Ru‐loaded black TiO2 at 633 K. Compared to perovskite‐type KTaO3−x with the same composition, the pyrochlore exhibited a 3.7‐fold increase in intrinsic activity due to a higher photoexcited charge separation efficiency and a higher conduction band position. The interfacial Schottky barrier and spontaneous electron transfer between K2Ta2O6−x and Ru further improve photoexcited charge separation and accumulate energetic electrons to facilitate N2 activation.

Defect pyrochlore-type Mott-Schottky photocatalysts for enhanced ammonia synthesis at low pressure.

The ambient ammonia synthesis coupled with distributed green hydrogen production technology can provide promising solutions for low-carbon NH3 production and H2 storage. Herein, we reported Ru-loaded defective pyrochlore K2Ta2O6-x with remarkable visible-light absorption and a very low work function, enabling effective visible-light-driven ammonia synthesis from N2 and H2 at low pressure down to 0.2 atm. The photocatalytic rate was 2.8 times higher than that of the best previously reported photocatalyst and the photo-thermal rate at 425K was similar to that of Ru-loaded black TiO2 at 633K. Compared to perovskite-type KTaO3-x with the same composition, the pyrochlore exhibited a 3.7-fold increase in intrinsic activity due to a higher photoexcited charge separation efficiency and a higher conduction band position. The interfacial Schottky barrier and spontaneous electron transfer between K2Ta2O6-x and Ru further improve photoexcited charge separation and accumulate energetic electrons to facilitate N2 activation.

High-temperature stabilized defect pyrochlore Bi2−xFexWO6 nanostructures and their effects on photocatalytic water remediation and photo-electrochemical oxygen evolution kinetics

Optimal Fe-substituted Bi2WO6 nanostructures, stabilized at higher temperatures, exhibited better band hybridization leading to improved photocatalytic and photoelectrochemical performance.

Preparation and thermophysical properties of a novel dual-phase and single-phase rare-earth-zirconate high-entropy ceramics

A novel non-equimolar (Nd0.58Gd0.05Y0.05Er0.05Yb0.27)2Zr2O7 composed of coexisting defect fluorite and pyrochlore phases and (Nd0.2Gd0.2Y0.2Er0.2Yb0.2)2Zr2O7 with defect fluorite structure high entropy ceramics were firstly prepared by high temperature solid state method. The average grain growth rate of high entropy (Nd0.2Gd0.2Y0.2Er0.2Yb0.2)2Zr2O7 and (Nd0.58Gd0.05Y0.05Er0.05Yb0.27)2Zr2O7 ceramics is much lower than that of Nd2Zr2O7 during annealing at 1300 °C for 100 h, which may be ascribed to the sluggish diffusion effect of high entropy ceramics. Additionally, both of (Nd0.2Gd0.2Y0.2Er0.2Yb0.2)2Zr2O7 and (Nd0.58Gd0.05Y0.05Er0.05Yb0.27)2Zr2O7 possess higher thermal expansion coefficients and lower thermal conductivities than Nd2Zr2O7. The thermal expansion coefficients of (Nd0.2Gd0.2Y0.2Er0.2Yb0.2)2Zr2O7 and (Nd0.58Gd0.05Y0.05Er0.05Yb0.27)2Zr2O7 at 1000 °C are 12.9 × 10−6 and 12.6 × 10−6 K−1, respectively. In addition, the thermal conductivity of dual-phase (Nd0.58Gd0.05Y0.05Er0.05Yb0.27)2Zr2O7 at room temperature up to 1000 °C is significantly lower than that of single-phase (Nd0.2Gd0.2Y0.2Er0.2Yb0.2)2Zr2O7, such as, 1.215 and 0.957 W·m−1·K−1 at 1000 °C, respectively, which may be due to cation size disorder and lattice distortion.

Facile Fabrication of N‐Doped K2Nb2O6 Nanocrystals with Defective Pyrochlore Structure for Improved Visible‐Light Photocatalytic Hydrogen Production

Semiconductor‐based photocatalytic water splitting is one of the effective ways for future hydrogen (H2) production. However, the wide bandgap of most semiconductors severely limits the solar spectral absorption range, which seriously hinders their practical applications. Herein, porous N‐doped K2Nb2O6 nanocrystals with defective pyrochlore structure are rationally designed to extend the absorption range from ultraviolet to visible region around 550 nm by NH3 heat treatment. The obtained 6N–K2Nb2O6 shows the highest visible‐light activity with an H2 evolution rate of 20.4 μmol h−1 g−1. More detailed exploration demonstrates that NH3 heat treatment can promote the substitutional nitrogen doping in K2Nb2O6 crystals while accompanied by the formation of oxygen vacancies or Nb4+ species. The hybridization of N 2p and O 2p orbitals in the valence band (VB) of the nitrogen‐doped K2Nb2O6 makes the VB maximum move more negatively, whereas the conduction band minimum shifted more positively due to the role of defective Nb4+ species. Such a synergistic effect endows the samples with visible‐light absorption and greatly enhances charge separation and transfer efficiency, resulting in the excellent H2 production performance. This study provides additional insights into the understanding of the effect of nitrogen doping on photocatalysts with excellent visible‐light‐driven photocatalytic activity.

Insight into the investigation on nanostructured defect pyrochlore Bi2-xFexWO6 and its photocatalytic degradation of mixed cationic dyes

Structural and band gap modifications by substituting Fe ions into the structure of Bi2WO6 for enhanced optical property and photocatalytic activity has been studied in this work. Nanostructured Bi2-xFexWO6 were synthesized by facile co-precipitation technique. X-ray Diffraction Studies (XRD) and WH plot derived from the diffraction pattern conveyed the structural and micro-strain induced due to the substitution of Fe ions. FT-IR and Raman studies proved to be in correspondence with XRD analysis. The reduced particle size and the transformation from cubic to nano regime spherical morphology were observed using SEM and HR-TEM imaging techniques. UV-DRS analysis displayed reduced band gap from 2.7 eV to 1.72 eV and the shift in band edge positions enhancing optimal use of visible light. Furthermore, the chemical states analyzed using XPS measurements confirmed the presence of Fe3+, Bi3+ and formation of oxygen vacancies. The Photoluminescence (PL) and Electrochemical Impedance (EIS) confirmed efficient charge separation and migration of photo induced charge carriers. The visible degradation of RhB, AO and the mixed dyes (RhB + AO) confirmed the efficient photocatalytic efficiency for optimal amount of Fe ions concentration.

Proton Conductivity of La2(Hf2−xLax)O7−x/2 “Stuffed” Pyrochlores

The design of new oxygen- and proton-conducting materials is of paramount importance for their possible utilization in solid oxide fuel cells. In the present work, La2(Hf2–xLax)O7–x/2 (x = 0, 0.1) ceramics were prepared using ball milling of oxide mixtures (La2O3 and HfO2) followed by high-temperature annealing at 1600 °C for 10 h in air. La2Hf2O7 ceramics exhibit an ordered pyrochlore-type structure, whereas La2(Hf1.9La0.1)O6.95 has a defect pyrochlore structure type with oxygen vacancies at the 48f positions. The oxygen ion and proton conductivity of La2(Hf1.9La0.1)O6.95 “stuffed” pyrochlore ceramics was investigated by electrochemical impedance spectroscopy (two-probe AC) and four-probe DC measurements in a dry and a wet atmosphere (air and nitrogen). The use of two distinct conductivity measurement techniques ensured, for the first time, the collection of reliable data on the proton conductivity of the La2(Hf1.9La0.1)O6.95 “stuffed” hafnate pyrochlore. La2Hf2O7 was found to be a dielectric in the range 400–900 °C, whereas the La2(Hf1.9La0.1)O6.95 “stuffed” pyrochlore had both oxygen ion and proton conductivities in this temperature range. The proton conductivity level was found to be equal to ~8 × 10−5 S/cm at 700 °C. Clearly, the proton conductivity of the La2(Hf1.9La0.1)O6.95 “stuffed” hafnate pyrochlore is mainly due to the hydration of oxygen vacancies at 48f positions.

Construction of the Sn-doped defect pyrochlore oxide KNbMoO6·H2O/g-C3N4 composite and its photocatalytic reduction of CO2

The Sn-doped defect pyrochlore oxide KNbMoO6·H2O/g-C3N4 composite can be used as a photocatalyst for conversion of CO2 in order to deal with energy and environmental issues.

Lattice composition dictated photocatalytic activity of bismuth chromium niobate pyrochlore under visible light

Bismuth-based pyrochlore oxides are known for excellent physicochemical properties due to their insignificant structural tolerance corresponding to vacancy and misplacement. Herein, Bi1.34CrNbO6 (BCN), a rare phase that stabilizes as a defect pyrochlore structure is reported for its photocatalytic activity. The co-precipitation method of synthesis was adopted and the phase was corroborated by the X-ray diffraction technique. The surface topography done by scanning and imaging using high-resolution microscopes (FESEM and HRTEM) revealed that the material possessed irregular morphology with particle size around ∼ 15 nm. From the optical property analysis, the possible low-energy direct and indirect transitions of 2.95 and 1.65 eV can be understood. Butler – Shinley calculation of redox potentials demonstrates the obtained phase is suitable for photodecomposition of water. The photocatalytic efficiency was systemically investigated on the degradation of Rhodamine B (RhB) dye on a lab-scale using a 300 W tungsten lamp. The process of band formation and the band offset positions of BCN concerning Cr incorporation results in superior photocatalytic efficiency.

Fabrication and characterization of high entropy pyrochlore ceramics

High-entropy rare-earth (RE) zirconates with pyrochlore structure were successfully fabricated by pressureless and spark plasma sintering. RE2Zr2O7 compound with nominal composition (La0.2Y0.2Gd0.2Nd0.2Sm0.2)Zr2O7 was prepared by simple glycine nitrate procedure (GNP). GNP process yielded powders with low crystallinity and after subsequent calcination, well crystalline ceramics were formed. During calcination defective fluorite (F-RE2Zr2O7) and crystal pyrochlore (Py-RE2Zr2O7) structures coexist. Formation of pure crystalline pyrochlore occurs after sintering at 1450°C. High-density ceramics, free of any additives, were obtained after powders compaction and pressureless (PS), as well as field assisted sintering technique (FAST) at 1450°C. Theoretical investigations of the high-entropy RE2Zr2O7 pyrochlore systems were performed. Unit cell parameter of the obtained Py-RE2Zr2O7 is 10.5892(2)Å and 10.5999(2)Å for PS and FAST sintering, respectively, which is in good agreement with the results of Density Functional Theory (DFT) calculations. The thermal diffusivity of sintered samples at room temperature was ∼0.7mm2/s for both sintering methods.

Transition metal ion (Ni2+, Cu2+ and Zn2+) doped defect pyrochlore, KTaTeO6: Synthesis, characterization and photocatalytic studies

One of the strategies to decrease the bandgap energy and increase the optical absorption ofthe catalysts is to dope with transition metal ions. In this paper, the results obtained for thedegradation of methylene blue (MB) pollutant in the presence of M 2+ (M = Ni, Cu, and Zn) doped KTaTeO 6 (here after abbreviated as M-KTTO) upon visible light irradiation are presented.The parent KTaTeO 6 and the M 2+ (M = Ni, Cu, and Zn) doped KTaTeO 6 were prepared by solidstate and ion-exchange methods, respectively. All the samples were characterized by XRD,SEM/EDX, FT-IR, UV-Vis DRS, XPS, and PL techniques. The metal ion doping in place of K + has influenced the electronic and optical properties considerably. The doping of M 2+ into KTTOlattice has narrowed the bandgap energy, increased the visible light absorbance leading to higherphotocatalytic activity. The M-KTTO materials show higher photocatalytic activity compared toparent KTTO, in particularly Cu-KTTO. The scavenging experiments indicate that • OH radicalsare the main active species involved in the photodegradation of MB. The Cu-KTTO ischemically stable and can be used at least up to five cycles. The mechanistic pathway of MBdegradation was proposed over Cu-KTTO.

Facile Ion Exchange Synthesis of Ag, Cu, and Sn Incorporated Defect Pyrochlore K2Ta2O6 towards Visible‐Light‐Responsive Photocatalytic Activity

AbstractA novel series of Ag, Cu, and Sn ions incorporated defect pyrochlore K2Ta2O6 photocatalysts were successfully synthesized by facile ion exchange method whereas pristine K2Ta2O6 was prepared by conventional hydrothermal method. All the samples were amply characterized by powder X‐ray Diffraction (PXRD), ultraviolet‐visible diffuse reflectance spectroscopy (UV‐Vis DRS), scanning electron microscopy‐energy dispersive X‐ray spectroscopy (SEM‐EDS) and X‐ray photoelectron spectroscopy (XPS) techniques to understand their phase formation, optical properties, morphology, and chemical composition. The absorption edge of ion‐exchanged samples was red‐shifted remarkably, therefore they would respond under visible light. The photocatalytic activity of all samples was assessed by studying the degradation of methylene blue (MB) dye under visible light irradiation. All samples show activity towards MB degradation and Cu−K2Ta2O6 exhibited superior photocatalytic performance (86 %) compared to others. Different scavenger tests were carried out to identify the reactive species, accountable for the decontamination of aqueous dye solution. Moreover, photoluminescence (PL) measurements were carried out to correlate the separation efficacy of photoinduced charge carriers and photocatalytic performance of all samples. This work may provide valuable information for exploring the as‐prepared photocatalysts towards more energy and environmental applications.