Influence of pH value on particle size and morphology of zinc oxide powders obtained by solvothermal synthesis

Zinc oxide powders have been synthesized from ethanolic zinc acetate solutions in the presence of lithium hydroxide by the solvothermal method. In this work we have considered the influence of pH value on morphology and size of ZnO particles for temperature 200oC and reaction time 2 h. The ZnO powder microstructure was controlled using X-ray diffraction and field emission scanning electron microscopy. Grain size of ZnO particles ranges in the interval (40–200) nm depending on pH value. Increasing of pH value result in decreasing of particle size, changing from hexagonal to round particle form and uniforming of particle shape and size

ZnO mesocrystals from solvothermal synthesis

Mesocrystals represent a new class of nanostructured materials, made of crystallographically aligned nanoparticles. Due to their unique structural features they have many physicochemical properties, different from nanoparticulate materials and single crystal materials, which can provide better performance in some applications. Zinc oxide mesocrystals have been synthesized by the solvothermal method at 200 °C during 4 hours from slightly basic (pH = 8) precursor (ethanolic zinc acetate solution in the presence of lithium hydroxide). XRD analysis showed that precursor solution consists of zinc acetate and zinc-hydroxy-acetate. Structural and microstructural properties were analyzed using X-ray diffraction, field emission scanning electron microscopy and transmission electron microscopy. ZnO mesocrystals are hexagonal prisms with diameters of 80 – 200 nm and lengths of 100 – 200 nm, but several larger prisms have a hole in the center. Based on characterization results we have discussed the growth mechanism of ZnO mesocrystals. Dipolar nature of ZnO and planar structure of zinc-hydroxy-acetate with free position of the acetate ions between positively charge planes play crucial role in the formation of the ZnO mesocrystals during the solvothermal reaction

Solvothermal synthesis of Ti doped ZnO

Titanium doped zinc oxide powders were synthesized by solvothermal method. Polycrystalline powders of ZnO with different amounts of Ti -Zn1-xTixO (x=0, 1, 2, 5, 7.5, 10 at%) were obtained from ethanolic solution of zinc acetate dihydrate in the presence of lithium hydroxide and titanium citrate. Reaction was conducted in autoclave at 225 0 C and 42 bar for 6 h. Detailed structural analysis was carried out using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Based on obtained results mechanism of Ti incorporation in ZnO lattice was discussed

The improvement of ferroelectric properties of BiFeO3 ceramics by doping with La3+ and Eu3+

Bismuth ferrite is a unique multiferroic material that has a ferroelectric and antiferromagnetic order at room temperature. The rhombohedrally (R3c) distorted BiFeO3 perovskite structure is a result of relative cation displacement along [111] axis of the cubic perovskite structure and relative rotation of two oxygen octahedra in opposite directions around [111] axis [1]. The partial substitution of Bi3+ with rare-earth ions can affect the magnitude of lattice distortion and thus the value of electric polarization. The presence of undesirable secondary phases (Bi2Fe4O9 and Bi25FeO39) and structural point defects (oxygen and bismuth vacancies) in pure BiFeO3 lead to a high leakage current, which deteriorates its ferroelectric properties. Doping with rare-earth elements with large ionic radii is found to reduce the number of the structural defects and thus improve ferroelectric properties [2]. The influence of partial substitution of Bi3+ with La3+ and/or Eu3+ on ferroelectric properties of BiFeO3 ceramics was investigated. The Bi1-(x+y)LaxEuyFeO3 (x = 0, 0.025 0.05, 0.10; y = 0, 0.025, 0.05, 0.10) powders were synthesized by hydro-evaporation method, uniaxially pressed at 9 t/cm2 and sintered at 835 °C for 3 h. All the ceramic samples showed a rhombohedral structure, without presence of the secondary phases. Their morphology indicated the complete sintering under the given conditions. The grain size and grain shapes differed more depending on the dopant type and amount. The introduction of La3+ and/or Eu3+ at the site of Bi3+ led to such distortions within the rhombohedral lattice that resulted in much greater remnant electric polarization (Pr) in comparison with the undoped sample. The Bi1-(x+y)LaxEuyFeO3 ceramic samples with x+y = 0.10 showed approximately quadratic polarization vs. electric field P(E) hysteresis curves as well as significantly high values of pure ferroelectric polarization Pr, in large electric fields (100 – 140) kV/cm. The leakage currents of La3+/Eu3+-doped samples are mostly reduced, especially those doped only with Eu3+

Ferroelectric properties of BiFeO3 ceramics with cation substitutions at Bi-site (La3+, Eu3+) and Fe-site (Nb5+, Zr4+)

BiFeO3 is one of the few multiferroic perovskites that exhibits magnetic and ferroelectric properties at room temperature. However, it is also distinguished by high leakage current, low remnant electric and magnetic polarization, and high electric coercive field. These features keep it away from any practical use in electronics. Therefore, many attempts have been made to improve the properties of BiFeO3 by Bi- or Fe-site doping or by both. Previous investigations suggest that doping with Nbat Fe-site can positively affect the magnetic behavior of BiFeO3 and decrease the leakage current. In this study, various cation substitutions at Bi-site (La3+, Eu3+) and Fe-site (Nb5+, Zr4+) were examined to investigate their possible synergism and benefit for the ferroelectric properties. The role of the cations with higher valence is to suppress the formation of structural defects during synthesis, such as oxygen and bismuth vacancies. These defects are responsible for high leakage currents and, consequently, low breakdown voltages characteristic of the pure BiFeO3. On the other hand, rare earth cations at the Bisite usually enable densification of the ceramics in a wider range of temperatures, preventing bismuth loss and forming defects and secondary phases during sintering. However, do pant concentrations above 10–15mol% may give rise to transition from polar, rhombohedral (R3c) to non-polar, orthorhombic (Pnma) symmetry. The carefully selected compositions of doped BiFeO3 were synthesized by a simple hydro-evaporation method. The ceramics samples were characterized using X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), and polarization techniques, including leakage current measurements. Although the introduction of Nb5+or Zr4+decreased the leakage current, they surprisingly deteriorated the ferroelectric properties even at concentrations as low as 1 mol%. This effect was more pronounced for the samples containing Nb. On the contrary, both La3+ and Eu3+ (incorporated at the Bi-site) improved the ferroelectric properties as their concentrations increased. The La-doped samples exhibited higher remnant electric polarizations at observed electric fields. The highest remnant electric polarization of31.9 μC/cm2at 150 kV/cm was measured for Bi0.85La0.15Fe0.998Zr0.002O3, indicating the synergetic effect of La3+ and Zr4+, which is limited to very low Zr4+concentrations

Electrical properties of BaSn(1-x)SbxO3 ceramics materials

BaSnO3 is a perovskite oxide widely used as dielectric ceramic material, thermally stable capacitor in electronic industry and chemical humidity sensor. It is also an electrical insulator (band gap ~ 3.1 eV), which becomes an n-type conductor by doping. The aim of this work was to prepare BaSn(1−x)SbxO3 (BSSO) by mechanochemically assisted solid-state synthesis, starting from BaCO3, SnO2 and Sb2O3 as precursors. The concentration of Sb in BSSO was varied from 0.04 to 0.1. All starting mixtures were homogenized and activated in a planetary ball mill with isopropanol as a solvent. As-prepared powders were dried and calcined at 900 °C for 4 h. After calcination, powders were uniaxially pressed into pellets and sintered at temperature of 1200 °C for 3 h. Phase composition and microstructure of perovskite BSSO were identified by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The influence of Sb doping on electrical characteristics of ceramic material was determined by measuring the current-voltage characteristics for all samples at room temperature in air. The band gap values for BSSO calculated using Kubelka-Munk transformation and Tauc linearization of the obtained diffuse reflectance spectra, confirmed conductive behavior of preparedceramic samples

Synthesis and characterization of Nb-doped lanthanum nickelate La(Ni,Nb)O3

Perovskite type ceramic materials with general formula ABO3 are very important class of materials due to their various chemical and physical properties. They have wide applications such as electrode material for solid state fuel cells (SOFC), capacitors, resistors, superconductors, catalysts, electrolytes, microwave devices, and magnetoresitant materials [1, 2]. Lanthanum nickelate (LaNiO3, LNO) is a ternary oxide with rhombohedrally distorted perovskite lattice. In LNO trivalent nickel ions (Ni3+) are in low spin configuration (t2g 6eg 1) and the conduction band is formed by the hybridization of the egorbitals of Ni3+ and the p-orbitals of oxygen. As a result, LNO shows metallic n-type conductivity in wide temperature range [3]. For this reason LNO has been proposed as a cathode material for intermediate-temperature SOFCs (IT-SOFCs) with operating temperature range of 650-800 °C. The possible dawbacks of LNO as a potential material for this application are poor density and thermal unstability at temperatures higher than 850 °C, when LNO starts gradually to decompose into the lower oxides Lan+1NinO3n+1 (n = 3, 2, 1) and NiO. Still, all of these La-Ni-O compounds exhibit high electronic conduction within the NiO6 octahedra in their perovskite layers and excellent oxygen ionic conductivity through oxygen interstitials on the LaO rock-salt plane. Also, their coefficient of thermal expansion (CTE) matches those of materials commonly used as IT-SOFC electrolyte and anode [1]. A possible use of LNO as a cathode for SOFC requires the improvement of its thermal stability and enhancement of density of ceramic samples. The aim of this work was to fullfill these requirements by doping of lanthanum-nickelate into the B site. Using transition metal of higher valency than Ni3+ as a dopant, could enhance the electron concentration and carrier mobility, which results in improvement of electrical conductivity of ceramic material. Doping could also influence the sintering process and improve the density of the ceramic materials. In this work we present dense ceramic materials of LaNi1- xNbxO3 (x = 0.005, 0.05) prepared by mechanochemically assisted solid state method. La2O3, NiO and Nb2O5, used as a precursor reagents, were mechanochemically activated in the planetary ball mill for 5 h. Obtained powders were calcined at 700 °C for 3h in air, and afterwards sintered at 900 °C and 1200 °C for 2 h and 10 h in different atmospheres (air and oxygen). The influence of Nb doping on electrical properties and microstructure of LaNi1-xNbxO3 ceramic materials was investigated. All samples were analyzed by X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). Electrical conductivity of ceramic LaNi1-xNbxO3 samples was measured in different mediums and complete characterization of electrical properties was performed. The XRD analysis indicated the existence of secondary phases Lan+1NinO3n+1 and NiO along with the rhombohedral LaNiO3. Samples sintered at 900 °C in oxygen atmosphere for 2 h had density of 64 % and 60 % for x = 0.005 and x = 0.05 (Fig. 1). The electrical conductivity was improved by doping with Nb, and obtained values were 2.7 S cm and 2.6 S cm for x = 0.005 and x = 0.05 at room temperature. The obtained results confirmed that doping by Nb along with sintering in oxygen atmosphere can improve electrical conductivity, density, and thermal stability. References [1] R.K. Sharma, E. Djurado, J Mater Chem A, 5 (2017) 22277-22287. [2].A. S. Bhalla, R. Guo, R. Roy, Mat Res Innovat, 4 (2000) 3-26. [3] K. Sreedhar et al., Phys Rev B, 46 (1992) 6382-6386

Photodegradation of organic dye using BiFeO3 particles synthesized by ultrasound route

BiFeO3 precursor powder was synthesized by ultrasound asissted sol–gel route at relatively low temperature, starting from Bi-nitrate, Fe-nitrate, and ethylene glycol. Structural, optical, and photocatalytic properties of the obtained powder were investigated. X-ray diffraction analysis confirmed that thermal treatment of precursor powder at 500 °C led to formation of pure phase BiFeO3. The determined band gap was 2.20 eV, indicating its potential application as visible-light-response photocatalyst. The powder is used for photocatalytic degradation of typical organic azo dye Mordant Blue 9 in concentration of 50 mg/l. Measurements were performed for different times of irradiation and pH of the dye solution. Changes in UV-Vis absorption spectra revealed the decolorization and decomposition of organic dye during the photodegradation process. Photodegradation products were analyzied by HPLC technique, and mechanism of photocatalytic degradation of organic dye was proposed

TUNING OF FERROELECTRIC PROPERTIES OF BiFeO3 CERAMICS BY CATION SUBSTITUTIONS AT Bi-SITE AND Fe-SITE

In this study, we tried various cation substitutions at Bi-site (La3+, Eu3+) and Fesite (Nb5+ , Zr4+ ) to explore their possible synergism and improvement of the ferroelectric properties of bismuth ferrite. The cations with higher valence ought to suppress the formation of structural defects during syntheses, such as oxygen and bismuth vacancies. These defects are responsible for high leakage currents and low breakdown voltages characteristic of pure BiFeO3. On the other hand, rare earth cations at the Bi-site usually enable densification of the ceramics at a broader range of temperatures, preventing bismuth loss and formation of defects and secondary phases during sintering. However, dopant concentrations above 10–15 mol% may give rise to a transition from polar, rhombohedral (R3c) to non-polar, orthorhombic (Pnma) symmetry. Thus, we synthesized pure and selected compositions doped BiFeO3 by a hydroevaporation method and determined the optimal calcination temperature by thermal analyses of the precursor powders. Then we characterized ceramics samples using X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) and polarization techniques. Although only 1 mol% Nb5+ decreased the leakage current, it surprisingly deteriorated the ferroelectric properties of BiFeO3. Similar effect exhibited the samples containing Zr4+ that showed no improvement compared with undoped bismuth ferrite. On the contrary, La3+ and Eu3+ (incorporated at the Bi-site) improved the ferroelectric properties as their concentrations increased, whereby the samples doped with 15 mol% La exhibited higher remnant electric polarizations at observed electric fields. The highest remnant electric polarization of 31.9 µC/cm2 at 150 kV/cm, was measured for Bi0.85La0.15Fe0.998Zr0.002O3, indicating the synergetic effect of La3+ and Zr4+, which is limited to low Zr4+ concentrations

Improved multiferroic properties of Nb doped BiFeO3

Pure BiFeO3 (TN = 370 °C and TC = 826–845 °C) exhibits poor ferroelectric (high electrical conductivity) and weak ferromagnetism. In this study, up to 1% Nb5+ was introduced to replace Fe3+ (B-site doping) since it could disturb the nearly antiparallel spin ordering of the adjacent Fe3+ ions responsible for cycloidal (spiral) spin structure. On the other hand, the pentivalent Nb cations will compensate the negatively charged defects and consequently reduce the electrical conductivity. Unlike pure BiFeO3, the sample with 1% Nb exhibits hard magnetic behaviour due to its high coercive magnetic field of ~7460 Oe (at H = 50 000 Oe). The ferroelectric response for the sample with 0.2 % Nb was unstable above 40 kV/cm, while at 70 kV/cm only the sample with 1 % Nb showed a regular ferroelectric response with remnant electrical polarization of 0.5 μC/cm2 and coercive electrical field of 22.2 kV/cm. Thus, by doping with Nb, both magnetic and ferroelectric properties of BiFeO3 were improved