Synthesis and characterization of PPY/K[Fe(CN)3(OH)(en)] nanocomposite: Study of photocatalytic,sorption, electrical,and thermal properties

This work involves the synthesis of nanocomposite of Polypyrrole (PPY) with photoadduct {K[Fe(CN)₃(OH)(en)]} via in‐situ oxidative chemical polymerisation. Photoadduct is synthesized by irradiating equimolar mixture of potassium hexacyanoferrate (III) and ethylenediamine (en) and is then reduced to nanosize by high energy ball mill. Successful synthesis of nanophotoadduct is confirmed by elemental analyser, UV–Vis, and FTIR. The nanocomposite of PPY and photoadduct has been characterised by SEM, TEM, FTIR, XRD, TGA, UV–Vis spectroscopy, I–V characteristics, and dielectric analysis for structural, thermal, optical, electrical, and dielectric properties. The average particle size of nanophotoadduct and nanocomposite has been found to be 80 and 84 nm, respectively. The thermal stability of nanocomposite is enhanced. The nanocomposite shows high value of dielectric constant (5 × 10³ at10³ Hz) and ac conductivity (7.0 × 10⁸ Sm^?1 at 10⁶ Hz) as compared with pure PPY. The high value of dielectric constant can make the material suited for energy storage applications. The nanocomposite shows higher photocatalytic activity as compared with pure PPY and a high value of distribution coefficient (K_d) has been obtained for Pb²⁺, Zn²⁺, and Cd²⁺ ions, hence, can act as an efficient material for removal of dyes and heavy metal ions in waste water. Thus, photoadduct of K₃[Fe(CN)₆] and ethylenediamine(en), a new kind of filler, has succeeded in improving the properties of PPY with respect to thermal stability, high dielectric constant, high ac conductivity, efficient photocatalytic activity, and high K_d value for toxic metal ions. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43487.     

Investigation into the phase structures and temperature stability of BiScO3-PbTiO3-based piezoelectric ceramics

Phase structure has a strong influence on the temperature stability of ceramics; however, their influence on BiScO₃-xPbTiO₃ has been neglected. To meet the requirements for practical applications, (0.98-x)BiScO₃-xPbTiO₃-0.02Bi(Sn_1/3Nb_2/3)O₃ (BS-xPT-BSN, 0.59 ≤ x ≤ 0.65) polycrystalline ferroelectrics with rhombohedral phase, morphotropic phase boundary (Cm and P4mm coexisting), and tetragonal phase has been prepared and studied. The relationship between the phase structures and temperature stability is established from the macro properties as well as the underlying domain behaviors. The results show that the tetragonal phase is not sensitive to temperature because of its stable domains, which is a positive motivator for improving the temperature stability of ceramics. This work provides a new strategy for the design of high-temperature piezoelectric ceramics in the future.     

A prediction of the thermodynamic,thermophysical, and mechanical properties of CrTaO4 from first principles

It is reported that the self-forming CrTaO₄ oxide scale can protect refractory high-entropy alloys from oxidation, superior to Cr₂O₃. In this paper, the phase stability, mechanical, and thermal properties of three polymorphous phases of CrTaO₄ are systematically investigated from first-principles density functional theory calculations. The mechanical properties predicted using the strain–energy methods indicated that all three phases are mechanically stable. The temperature dependence of elastic constants and polycrystalline moduli of three phases demonstrated the thermal softening as temperature increase. The Helmholtz-free energies as a function of volume and temperature are derived from phonon dispersions within the quasi-harmonic approximation at six strained volumes. The calculated apparent bulk coefficients of thermal expansion of these three phases are evaluated, the highest value approximately 13.4× 10⁻⁶ K⁻¹ within a temperature range of 500–2000 K for the rutile I4₁md phase. The lattice thermal conductivity calculated by the Debye–Callaway model suggested that the rutile type I4₁md phase has the lowest value of approximately 2.1 W/m/K at 1800 K. The other two phases, C2/m and P2/c, exhibit higher values due to relatively lower Grüneisen parameters and larger phonon velocities. The melting point of CrTaO₄ is predicted to be between 1975 and 2449 K using ab initio molecular dynamics simulations. This work provides a comprehensive theoretical understanding of the thermodynamic, mechanical, and thermal properties for the new material CrTaO₄ and serves as an example of a viable computational design strategy for improved oxidation resistance of refractory alloys at high temperatures.     

Polymorphism of M3AlX Phases (M = Ti,Zr, Hf; X = C,N) and Thermomechanical Properties of Ti3AlN Polymorphs

M₃AlX (M = Ti/Zr/Hf, X = C/N) compounds are promising high‐temperature structural ceramics. However, their interesting polymorphism, thermomechanical stabilities, and thermal and mechanical properties were not fully understood. In this work, the polymorphisms of M₃AX phases are investigated by combining first‐principles and lattice dynamics calculations. Only Ti₃AlN shows polymorphic transition between the cubic and orthorhombic phases at around 1105 K; but other M₃AlX phases do not display similar polymorphic phase transition. Furthermore, the temperature‐dependent heat capacity, thermal expansion, and elastic stiffness of Ti₃AlN polymorphic phases are reported for the first time to explore the relationship between crystal structures, and mechanical and thermal properties. Ti₃AlN polymorphs show anisotropic thermal expansion and elastic stiffness; and the orthorhombic Ti₃AlN is suggested as a promising damage tolerant nitride, which has similar properties with the previously reported Zr₃AlN and Hf₃AlN.     

Structural,magnetic and cryogenic magneto-caloric properties in RE2FeCrO6 (RE = Er and Tm) compounds

In this work, polycrystalline Er₂FeCrO₆ (EFCO) and Tm₂FeCrO₆ (TFCO) oxides were fabricated via conventional sol-gel method, and studied with respect of crystal structure together with cryogenic magnetic and magneto-caloric properties. Both oxides are confirmed to exhibit B-site disordered hexagonal perovskite-type crystal structure. Two magnetic transitions around 11.7 and 5.7 K for EFCO, whereas only one transition around 10.5 K for TFCO, have been observed. Both oxides exhibit considerable cryogenic reversible magneto-caloric effects. The values of magnetic entropy change peak and relative cooling power (refrigerant capacity) with 0–5 T magnetic field change reach 11.95 J/kg-K and 215.8 (169.8) J/kg for EFCO, and 4.78 J/kg-K and 123.6 (97.8) J/kg for TFCO, respectively, indicating the present Er₂FeCrO₆ oxide is also considerable for cryogenic magnetic cooling application.     

Synthesis of β-Bi2O3 nanoparticles via the oxidation of Bi nanoparticles: Size,shape and polymorph control,anisotropic thermal expansion,and visible-light photocatalytic activity

Bismuth trioxide (Bi₂O₃) is known for its simple composition but rich polymorphism that allows a number of phase-dependent physicochemical properties and technical applications. We here report our controllable synthesis of highly discrete β-Bi₂O₃ (tetragonal) nanoparticles (NPs) via the thermal oxidation of nearly-monodisperse Bi NPs. The size and shape (spherical and tear- or rod-like) of β-Bi₂O₃ NPs are tunable by the size of parent Bi NPs whereas β and α (monoclinic) polymorphs can be selectively achieved by tailoring the oxidation temperature. The phase stability of β polymorph from room temperature to 450 °C enables us to perform an in situ high-temperature XRD study on the temperature dependence of its lattice parameters, which reveals a marked thermal expansion anisotropy in β-Bi₂O₃ with a linear thermal expansion coefficient of +35.1 × 10^?6 °C^?1 in the c axis, fifteen times higher than that in the a axis. Meanwhile, the narrow band gap (2.27 eV for β vs. 2.77 eV for α) and strong visible-light absorption endow β-Bi₂O₃ NPs with a good photocatalytic activity for the visible-light Rhodamine B dye degradation. We expect that our work could be a valuable reference for the studies on the size, shape and polymorph control, thermal property, and photocatalytic application of Bi₂O₃.     

Anisotropy in elasticity,sound velocities and minimum thermal conductivity of zirconia from first-principles calculations

The anisotropies of mechanics and thermodynamics properties of zirconia with three zero-pressure polymorphs were studied by using first-principles calculations. It has been shown that Young’s moduli of three phases strongly depend on directions. The sound velocities of faster mixed mode (v₊) is much larger than that of slower mixed mode (v₋) and pure transverse mode (v_t) in monoclinic phase. For both tetragonal phase and the cubic phase, most pure longitudinal mode (v_l) have the greatest sound velocity among the three acoustic modes. According to the Clarke's model, three zero-pressure polymorphs zirconia also have pronounced anisotropic minimum thermal conductivity.     

Enthalpies of formation in the scandia‐zirconia system

The scandia‐zirconia (ScZ) solid solutions have been attracting attention from the communities interested in solid‐oxide fuel cells because they possess the highest ionic conductivity among zirconia‐based materials. However, this system shows a relatively large number of polymorphs with lack of thermodynamic data to enable comprehensive phase control for property optimization. In this work, the enthalpy of formation of the ScZ system within the range 0–20 mol% Sc₂O₃ is determined by combining the surface energy values with enthalpy of drop solution data obtained from high‐temperature oxide melt solution calorimetry. The heats of formation, a key data for understanding phase stability, for five polymorphs: monoclinic (m), tetragonal (t), cubic (c), and rhombohedral (β and γ) are reported for the first time.     

Synthesis and photocatalytic activity of BiFeO3 and Bi/BiFeO3 cubic microcrystals

BiFeO₃ and Bi/BiFeO₃ cubic microcrystals were synthesized in this work. The phase, microstructure, optical and photo electrochemical properties, as well as the photocatalytic activities in photocatalytic hydrogen generation were investigated. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) results demonstrate the successful synthesis of BiFeO₃ and Bi/BiFeO₃. The scanning electron microscope (SEM), transmission electron microscopy (TEM), and energy-dispersive X-ray (EDX) results give the evidence of cubic morphology and the deposition of metal Bi on the surface of BiFeO₃. The absorption spectra show that Bi/BiFeO₃ has longer absorption edge and stronger absorption capability to visible light. The photocurrent curves, emission spectra, and electrochemical impedance spectroscopy (EIS) spectra demonstrate that Bi/BiFeO₃ has higher efficiency of electron-hole separation and charge transfer, as well as longer lifetime of the charge carriers. These benefit to the enhancement of activity in photocatalytic hydrogen generation.     

Designing effective and stable S-scheme RGO/AgVO3/AgBr hybrid with enhanced photocatalytic performance

The low separation of photogenerated electron-hole pairs and cycle stability has been the main bottleneck which restricts the development of photocatalytic technology for water purification. Here, RGO/AgVO₃ composites were fabricated by photo-ultrasonic assisted reduction method, and AgBr nanoparticles were assembled on the surface of RGO/AgVO₃ via an in situ ion exchange method. A series of characterization and experimental results indicated that the introduction of RGO influenced the growth of crystal phase for AgVO₃ nanorods, resulting that AgVO₃ nanorods became thicker and shorter with the increase in RGO content. Moreover, RGO could also work as a bridge to promote the migration of electrons, leading different improvement for photocatalytic activity. Furthermore, in situ growth of AgBr on the surface of AgVO₃ nanorods could prevent its agglomeration and exfoliation, thus improving the photocatalytic activity and cycle stability of composites. RGO_1%/AgVO₃/AgBr_30% exhibited excellent photocatalytic activity and stability for methylene blue (MB) degradation due to its unique structure, and its removal ratio reached at 96.2% within 50 min. Meanwhile, the separation of photogenerated electron-hole pairs of AgVO₃ was markedly improved due to the introduction of RGO and AgBr. Based on the trapping experiments and theoretical calculation of band gap, a possible S-scheme photocatalytic mechanism for improved photocatalytic activity was proposed.     

Effect of different iron precursors on the synthesis and photocatalytic activity of Fe–TiO2 nanotubes under visible light

Fe–TiO₂ nanotubes (Fe-TNTs) were developed to entitled photocatalytic reactions using a visible range of the solar spectrum. This work reports on the effect of different Fe precursors on the synthesis, characterization, kinetic study, material and photocatalytic properties of Fe-TNTs prepared by electrochemical method using three different Fe precursors i.e. (iron nitrate [Fe(NO₃)₃⋅9H₂O], iron sulfate [FeSO₄⋅7H₂O], and potassium iron ferricyanide [K₃Fe(CN)₆]). X-ray diffraction, X-ray photoelectron spectroscopy, field emission scanning electron microscopy, and Fourier transform infrared spectroscopy are used to examine the influence of the Fe precursor on the Fe-TNTs material characterization. Different Fe-TNT properties, such as enhanced photoactivity, good crystallization, and composition of titania structures (anatase and rutile) could be acquired from different iron precursors. Among the three iron precursors, Fe(NO₃)₃ provided with the only anatase phase, yields the highest photocatalytic activity. Congo red is used as a model compound to check the photocatalytic efficiency of synthesized materials because it has a complex aromatic structure which makes it difficult to be biodegraded or oxidized with the aid of chemicals. The photocatalytic efficiency of all Fe-TNT can be arranged in the following order: TNT-FeN > TNT-FeS > TNT-FeK > TNT. The kinetic rate constant of congo red degradation using the Fe-TNT with Fe(NO₃)₃ was 0.44 h⁻¹ with a half-life of 1.57 h⁻¹     

Tungsten-doped foam g-C3N4 with improved photocatalytic properties for degradation of pollutant and hydrogen evolution

As a potential material applied in the photocatalytic field, graphitic carbon nitride (g-C₃N₄) has attracted extensive attention for its advantages of visible-light response, excellent thermodynamic, and chemical stability. However, the photocatalytic performance of g-C₃N₄ is still limited in practical applications. Here, using a facile thermal polymerization method, unique W-doped foam g-C₃N₄ was synthesized to realize enhanced photocatalytic performance for the degradation of Rhodamine B and the evolution of hydrogen. Compared with pure foam g-C₃N₄, tungsten doping modified the foam g-C₃N₄ and efficiently improved its specific surface area, leading to enhanced photocatalytic performance. The average rate of hydrogen evolution was as high as 8818 μmol·h⁻¹·g⁻¹, which was better than most photocatalysts. This work proposes a new effective method and idea to modify g-C₃N₄ for improving its photocatalytic performance.     

Heterostructural phase diagram of Ga2O3–based solid solution with Al2O3

Ga₂O₃, which is emerging as semiconductor material due to the ultra-wide bandgap, has tunability in bandgap and lattice constant by alloying Al. However, successful control of alloying phase is still challenging due to its heterostructural nature and rich polymorphs. Here, we identified the thermodynamic phase diagram of heterostructural (Al_xGa_1-x)₂O₃ alloy. Using density-functional theory (DFT) calculations and regular solution model, we calculated the Gibbs-free energy of mixing of heterostructural polymorphs. Based on the calculation, we show the phase diagram of (Al_xGa_1-x)₂O₃ alloy system with a markedly increased metastability than the isostructural alloy, which can make a vast phase space for homogeneous single-phase alloys. We also investigated the correlation between the bandgap and lattice constant within these systems using hybrid DFT calculations, which can guide the device design of Ga₂O₃ power electronics.     

Yttrium Doped Single-Crystalline Orthorhombic Molybdenum Oxide Micro-Belts: Synthesis,Structural, Optical and Photocatalytic Properties

Molybdenum trioxide (MoO₃) micro-belts were successfully synthesized via the sol–gel coprecipitation method. The synthesized material was, then, doped with different concentrations of yttrium element, Y. The influence of the doping concertation on crystallographic, microstructural, optical, and photocatalytic properties has been studied. Good thermal stability has been revealed by thermogravimetric analysis. The X-ray diffraction analysis identified the orthorhombic phase. The peaks were shifted toward the lower 2θ angle position after doping, confirming the Y³⁺ substitution in the MoO₃ crystal structure. Scanning electron microscope measurements revealed a belt-like shape with a decreasing size when increasing the Y³⁺ ions concentration. SEM clearly showed a non-homogeneous distribution of second-phase particles, depending on the concentration of yttrium doping, which could be attributed to the Y₂Mo₄O₁₅ secondary phase. The band-gap energy was shifted to the lower-energies of an amount of 0.61 eV when going from 0 to 10% of yttrium. The photocatalytic performances were monitored through photodegradation of methylene blue under different radiations. It was observed that Y-dopant significantly improved the photocatalytic activity of MoO₃, the longer the wavelength the better the removal efficiency. This enhancement could be associated with the doping-induced modification of the band-gap or to the microstructure evolution.

     

Nanodomain Engineered (K,Na)NbO3 Lead‐Free Piezoceramics: Enhanced Thermal and Cycling Reliabilities

The growing environmental concerns have been pushing the development of viable green alternatives for lead‐based piezoceramics to be one of the priorities in functional ceramic materials. A polymorphic phase transition has been utilized to enhance piezoelectric properties of lead‐free (K, Na)NbO₃‐based materials, accepting the drawbacks of high temperature and cycling instabilities. Here, we present that CaZrO₃‐modified (K, Na)NbO₃ piezoceramics not only possess excellent performance at ambient conditions benefiting from nanodomain engineering, but also exhibit superior stability against temperature fluctuation and electrical fatigue cycling. It was found that the piezoelectric coefficient d₃₃ is temperature independent under 4 kV/mm, which can be attributed to enhanced thermal stability of electric field engineered domain configuration; whereas the electric field induced strain exhibits excellent fatigue resistance up to 10⁷ sesquipolar cycles. These findings render the current material an unprecedented opportunity for actuator applications demanding improved thermal and cycling reliabilities.     

Enhanced pressure-driven force-electric conversion effect for (Pb,La)(Zr,Ti)O3 ferroelectric ceramics

The pressure-driven force-electric conversion materials with extremely rapid response time have been widely used in mining, defense, and energy areas. The discharge process by the force-electric conversion effect in ferroelectrics is dominated by polar-nonpolar phase transformation. In this work, (Pb_0.985La_0.01)(Zr_xTi_1-x)O₃ (PLZT, x = 0.85–0.94) ceramics is designed by tunning Zr⁴⁺/Ti⁴⁺ ratio and aliovalent La doping to achieve high remnant polarization (P_r) and excellent temperature stability. We focus on the pressure-driven depolarization in PLZT ceramics, and their corresponding phase structure, ferroelectric properties, dielectric properties, and thermal depolarization. In PLZT (x = 0.93) ceramics, the original polarization P₀ increases to 43.42 μC/cm². The pressure-driven depolarization releases 37.66 μC/cm² with the depolarization proportion of 86.73%, which is attributed to irreversible ferroelectric-antiferroelectric phase transition. It also exhibits excellent temperature stability up to 120°C (> 36 μC/cm²). This work provides a high-performance alternative to Pb(Zr_0.95Ti_0.05)O₃ and guidance for the development of pulse power energy conversion devices.     

Preparation of Dual Z-scheme Bi2MoO6/ZnSnO3/ZnO Heterostructure Photocatalyst for Efficient Visible Light Degradation of Organic Pollutants

In this study, a double Z-type Bi₂MoO₆/ZnSnO₃/ZnO heterostructure photocatalyst was prepared by hydrothermal method to realize effective charge separation and improve photocatalytic activity. The synthesized samples were carefully examined by X-ray photoelectron spectroscopy, X-ray diffraction, scanning electron microscope, high-resolution transmission electron microscopy, photoluminescence (PL), and other analytical techniques. Meanwhile, the photocatalytic performance was further evaluated by multi-mode photocatalytic degradation with crystal violet (CV). The results show that the composite material has a relatively homogeneous cubic structure in size and shape. In the cubic structure, a heterogeneous structure exists between Bi₂MoO₆, ZnSnO₃ and ZnO. Simultaneously, the dramatic changes in physical morphology, such as the specific surface area and particle size of the composites, led to a series of unique properties, such as a significant climb in light absorption properties and superior photocatalytic activity. In addition, compared to ZnO, Bi₂MoO₆ and ZnSnO₃/ZnO, the Bi₂MoO₆/ZnSnO₃/ZnO composite material shows lower PL intensity, smaller arc radius, and stronger photocurrent response. Meanwhile, Bi₂MoO₆/ZnSnO₃/ZnO shows higher photocatalytic efficiency for CV and tetracycline hydrochloride (TC), and maintains good stability after 3 cycles of photodegradation experiments. Based on experimental results, the existence of heterojunctions between ZnO, ZnSnO₃ and Bi₂MoO₆ and the possible photocatalytic mechanism for the degradation of CV by dual Z-scheme composites are proposed. In conclusion, this study provides a feasible strategy for the photocatalytic degradation of organic pollutants by introducing ZnSnO₃ and Bi₂MoO₆ to successfully construct composite catalysts with dual Z-scheme heterostructures.

     

Synthesis and photocatalytic activity of Na+ co-doped CaTiO3:Eu3+ photocatalysts for methylene blue degradation

The Na⁺ co-doped CaTiO₃:Eu³⁺ powders were produced through the solution combustion method. The phase structure and optical properties of the synthesized samples were adequately characterized by X-ray diffraction (XRD), photoluminescence (PL) spectra, ultraviolet–visible (UV–vis) diffuse reflection spectroscopy and scanning electron microscopy (SEM). The XRD patterns revealed that a low level of Eu³⁺ doping could not cause lattice distortion of CaTiO₃. Photoluminescence (PL) displayed the CaTiO₃:0.5% Eu³⁺ sample synthesized at 900 °C has the weakest PL emission and the low electrons and holes recombination rate. The morphology of the sample was small nanoscale spherical particles. The UV–vis diffuse reflection spectra proved that doping Na⁺ and Eu³⁺ enlarged the absorption region and reduced band energy of pure CaTiO₃. The photocatalytic properties of Na⁺ co-doped CaTiO₃:Eu³⁺ samples were investigated via degrading methylene blue (MB) under ultraviolet light irradiation. The CaTiO₃:0.5% Eu³⁺, 0.5% Na⁺ sample, by contrast, exhibited the greatest photocatalytic property and the degradation rate was as high as 96.62%, which makes it a promising multi-functional material (photocatalytic material and red phosphor) for decreasing organic pollution in water.     

Phase stability and thermal conductivity of RE2O3 (RE=La,Nd, Gd,Yb) and Yb2O3 co-doped Y2O3 stabilized ZrO2 ceramics

Y₂O₃ stabilized ZrO₂ (YSZ) has been considered as the material of choice for thermal barrier coatings (TBCs), but it becomes unstable at high temperatures and its thermal conductivity needs to be further reduced. In this study, 1 mol% RE₂O₃ (RE=La, Nd, Gd, Yb) and 1 mol% Yb₂O₃ co-doped YSZ (1RE1Yb–YSZ) were fabricated to obtain improved phase stability and reduced thermal conductivity. For 1RE1Yb–YSZ ceramics, the phase stability of metastable tetragonal (t′) phase increased with decreasing RE³⁺ size, mainly attributable to the reduced driving force for t′ phase partitioning. The thermal conductivity of 1RE1Yb–YSZ was lower than that of YSZ, with the value decreasing with the increase of the RE³⁺ size mainly due to the increased elastic field in the lattice, but 1La1Yb–YSZ exhibited undesirably high thermal conductivity. By considering the comprehensive properties, 1Gd1Yb–YSZ ceramic could be a good potential material for TBC applications.     

Effect of sintering temperature on the depolarization behavior of (Bi0.5Na0.5)TiO3-based ceramics

(Bi_0.5Na_0.5)TiO₃ (BNT)-based ferroelectric ceramics have drawn extensive attention because of their excellent electrical properties and interesting depolarization behavior. However, the poor thermal stability of electrical properties limits their practical application. In this work, the effect of sintering temperature (T_s) on the depolarization behavior of BNT-based ceramics was systematically investigated. It is found that the depolarization temperature T_d determined from pyroelectric measurement tends to decrease with increasing T_s, which indicates that lower T_s defers the ferroelectric-relaxor (FE-RE) phase transition. However, for the samples sintered at higher T_s (such as 1180°C), although the T_d is reduced, the thermal stability is better compared with the sample sintered at lower T_s (1100°C) because the diffuse behavior of the FE-RE phase transition is suppressed. According to these results, we propose that the thermal stability of electrical properties for BNT-based ceramics is not only related to high T_d, but also to the diffuse degree of phase transition.