Hierarchical ZnO/SnO2 heterostructures via hydrothermally assisted electrospinning technique: synthesis and photocatalytic performances

Hierarchical nanostructures with multiporous tin oxide nanofibers (SnO2- MPNFs) and zinc oxide nanorods (ZnO-NRs) have been synthesized by combining electrospinning technique and hydrothermal method. A solution containing uniformly distributed tin (Sn) and silicon (Si) species of precursors, as well as a sacrificial polymer (PVP) was electrospun using a single-nozzle spinneret to fabricate nanofibers. In virtue of the Kirkendall effect driven by calcination at 550 °C, the SiO2-cored SnO2 nanofibers (SnO2-SiO2-NFs) deliberated from PVP were formed and used as backbones for further hydrothermal growth of ZnO-NRs. By varying the hydrothermal reaction time (0.5–2 h) at the constant concentration of SnO2-SiO2-NFs, zinc (Zn) precursor, directing agent (hexamethylenetetramine, HMT) and aqueous ammonia, the density, length and thickness of ZnO-NRs were controlled. Nanofibers and ZnO-NRs/SnO2-MPNFs heterostructures are confirmed by X-ray diffraction (XRD), field-emission scanning electron microcopy (FE-SEM), energy dispersive spectrometer (EDS), transmission electron microscopy (TEM) and elemental mapping analysis. The hydrothermal treatment conducted at 90 °C in aqueous ammonia allowed: a) selective etching of SiO2 from the SnO2-SiO2-NFs core and SiO2 trapped between SnO2 particles, and b) effective growth of ZnO-NRs. The process resulted in ZnO-NRs/SnO2-MPNFs heterostructures with ZnO-NRs of 1–5 μm in length attached to SnO2-MPNFs, the shell of which was composed of ultra-fine SnO2 crystallites (~5 nm in size) and where the four porous channels create the core instead of SiO2. Photocatalytic performance of the heterostructures was investigated toward different organic azo-dyes (methylene blue, methyl orange) and obvious enhancement was demonstrated in degradation of the organic pollutant, compared to primary SnO2-based nanofibers

The defect structure and electrical properties of the spark plasma sintered antimony-doped barium stannate

Barium stannate, BaSnO3 (BSO), is a perovskite-type alkaline earth metal stannate with almost ideal cubic structure. Appropriate doping can alter this wide band gap material’s electrical characteristics and change it either into a proton conductor or n-type semiconductor. In the case of Sb doping on Sn site, BSO becomes n-type semiconductor with high electrical conductivity at 25 °C. The major drawback of BSO-based ceramics is its low density. The conventional solid state procedure requires long thermal treatments with several intermittent grinding and heating steps at temperatures up to 1600 °C [1]. To overcome this problem, we used Spark Plasma Sintering technique (SPS) for the preparation of BaSn1-xSbxO3, (x = 0.00 (BSSO0) and 0.08 (BSSO8)) ceramic samples. The samples structural properties were investigated using XRD (X-Ray Powder Diffraction), XPS (X-Ray Photoelectron Spectrophotmetry) and SIMS (Secondary Ion Mass Spectrometry) analyses. XPS analysis revealed the existence of many structural defects, including mixed oxidation states of tin (Sn2+/Sn4+) and oxygen vacancies (VO) in both BSSO samples. The electrical properties of the BSSO ceramic samples were investigated in the temperature range of 4–300 K. The presence of oxygen vacancies in the BSSO0 sample led to the absence of the standard activated semiconductor behavior, showing almost linear temperature-dependent resistivity in the examined temperature range. On the other hand, the BSSO8 sample showed almost temperature-independent resistivity in the range of 70–300 K. This could be a consequence of the presence of many structural defects such as mixed oxidation states of Sn2+/Sn4+, probably Sb3+/Sb5+ and significant amount of O- species, as well as the presence of the low angle grain boundaries found in this sample. The BSSO8 ceramic sample could satisfy the huge demand for the linear resistors with moderate and high conductivity, due to its low and almost constant electrical resistivity in the wide temperature. 1. A.-M. Azad, L.L.W. Shyan, T.Y. Pang, C.H. Nee, Ceram. Int., 26 (2000) 685

The influence of spark plasma sintering temperature on the properties of Sb-doped barium stannate ceramics

Barium-stannate (BaSnO3, BSO) is a member of the perovskite-type alkaline earth stannates ASnO3 (A = Ca, Sr, Ba) with an ideal cubic crystal structure (space group: ). Doping with antimony (Sb5+) can change this wide band-gap semiconductor (Eg = 3.1-3.4 eV) into an n-type semiconductor with high electrical conductivity at room temperature. The major drawbacks in the BSO-based ceramics synthesis are phase composition and low density of final ceramic materials. These problems could be solved using spark plasma sintering (SPS), a current and pressure-assisted technique, which enables the preparation of dense ceramics at significantly lower temperatures and for a shorter time. To investigate the influence of spark plasma sintering temperature on the structural, microstructural and electrical properties of BaSn1-xSbxO3 (BSSO, x = 0.00; 0,04; 0.06; 0.08; and 0.10) ceramics samples, BSSO powders were spark plasma sintered at 1100 °C, 1200 °C and 1250 °C for 5 min. X-ray diffraction (XRD) analysis confirmed that all ceramic samples sintered at 1100 °C crystallized in a single-phased cubic BSO structure. Their relative densities were in the range of 72–82% ρt. Sintering at 1200 °C increased the samples’ relative densities to 79–96% ρt, but also induced the formation of a barium-rich secondary phase, Ba2SnO4. Rising the sintering temperature further to 1250 °C induced the melting of all samples except BaSn0.92Sb0.08O3. Field emission scanning electron microscopy (FE-SEM) revealed that doping with antimony decreased the grain sizes in BSSO samples sintered at 1100 °C and 1200 °C up to the concentration x = 0.08. Electrical measurements revealed the typical semiconductor behavior of the undoped samples, showing nonlinear current-voltage characteristic and the existence of one semicircle in their impedance spectra, characteristic for materials with double Schottky barrier at the grain boundaries. However, samples with higher dopant concentrations (x = 0.08 and 0.10) showed significantly lower electrical resistivity and linear current-voltage characteristic. The lowest and almost constant value of electrical resistivity in the temperature range of 25–150 °C, and complete loss of the semicircle in its impedance spectrum revealed the metallic-like behavior of sample BaSn0.92Sb0.08O3 sintered at 1200 °C

The influence of Ti-doping on structural and multiferroic properties of yttrium manganite ceramics

Hexagonal (P63cm) yttrium manganite, YMnO3, is a multiferroic material with ferroelectric transition at TC ≈ 900 K and antiferromagnetic transition at TN ≈ 70 K. Multiferroic behavior attracts a lot of attention because of its potential for various applications. The application possibilities are limited by large microcracking and microporosity of YMnO3 ceramics. In this work, the influence of Ti-doping on structural, ferroelectric and magnetic properties of YMnO3 ceramics was investigated. YMn1–xTixO3+δ (x = 0, 0.04, 0.08, 0.10, 0.15, 0.20) powders were prepared using sol-gel, polymerization complex method from citrate precursors, which were then calcinated at 900 °C for 4 h. The ceramic samples were obtained after sintering for 2 h at: 1400 °C for YMnO3, YMn0.96Ti0.04O3+δ, YMn0.92Ti0.08O3+δ and YMn0.90Ti0.10O3+δ; 1450 C for YMn0.85Ti0.15O3+δ; 1470 °C for YMn0.80Ti0.20O3+δ. X-ray diffraction (XRD), transmission and scanning electron microscopy (TEM and SEM) were used for structural and microstructural analysis of samples. Ferroelectric measurements of P(E) loops and leakage currents, and magnetic measurements of zero field cooled (ZFC) and field cooled (FC) M(T) curves, as well as M(H) curves, were enabled multiferroic characterization of ceramic samples. The samples x = 0 and 0.04 are crystallized in a single phased hexagonal structure, (P63cm), the samples x = 0.08 and 0.10 exhibited the presence of both hexagonal phase and rhombohedral phase (R3c), and the samples x = 0.15 and 0.20 are crystallized in rhombohedral 1×1×3 superstructure. Ti-doped YMnO3 ceramic samples showed reduced density of microcracks, and inter- and intragranular pores, and large increase in relative density (greater than 90 %) for YMn1–xTixO3+δ (x = 0.10, 0.15 and 0.20) samples. Leakage currents for most of doped samples were lower than leakage current of undoped sample, but the ferroelectric response was not significantly improved. Doping of YMnO3 with nonmagnetic Ti4+ led to suppression of antiferromagnetic ordering visible through decrease of the Néel temperature and Weiss parameter and the appearance of weak ferromagnetism

Titanium doped yttrium manganite: improvement of microstructural properties and peculiarities of multiferroic properties

Yttrium manganite, YMnO3, was doped with different concentrations of titanium (x = 0, 0.04, 0.08, 0.10, 0.15, 0.20) in order to improve the microstructural and multiferroic properties. The powders were prepared using sol-gel polymerization complex method from citrate precursors. Depending on the titanium concentration, the hexagonal structure and/or the rhombohedral superstructure are present in the sintered samples. The YMn1–xTixO3+δ (x = 0.10, 0.15, 0.20) ceramic samples showed significantly reduced density of microcracks, and of inter- and intragranular pores, and relative densities greater than 90%. The structural parameters for YMn1–xTixO3+δ (x = 0, 0.10, 0.15) were correlated with the results of magnetic and ferroelectric measurements. The most of titanium-doped samples showed a reduction of the leakage current density in comparison with undoped YMnO3, and their ferroelectric responses were slightly improved. The modifications in structural arrangement resulted in partial suppression of ideal antiferromagnetic ordering visible through decrease of the Néel temperature and Weiss parameter, as well as the appearance of weak ferromagnetism and increase of magnetization (especially, in samples x = 0.08, 0.10, 0.15). These changes in physical quantities most likely originated from incorporation of the uncompensated magnetic moments and possible spin canting induced by enhanced symmetry break of the superexchange bridges

Titanium doped yttrium manganite: improvement of microstructural properties and peculiarities of multiferroic properties

Yttrium manganite, YMnO3, was doped with different concentrations of titanium (x = 0, 0.04, 0.08, 0.10, 0.15, 0.20) in order to improve the microstructural and multiferroic properties. The powders were prepared using sol-gel polymerization complex method from citrate precursors. Depending on the titanium concentration, the hexagonal structure and/or the rhombohedral superstructure are present in the sintered samples. The YMn1–xTixO3+δ (x = 0.10, 0.15, 0.20) ceramic samples showed significantly reduced density of microcracks, and of inter- and intragranular pores, and relative densities greater than 90 %. The structural parameters for YMn1–xTixO3+δ (x = 0, 0.10, 0.15) were correlated with the results of magnetic and ferroelectric measurements. The most of titanium-doped samples showed a reduction of the leakage current density in comparison with undoped YMnO3, and their ferroelectric responses were slightly improved. The modifications in structural arrangement resulted in partial suppression of ideal antiferromagnetic ordering visible through decrease of the Néel temperature and Weiss parameter, as well as the appearance of weak ferromagnetism and increase of magnetization (especially, in samples x = 0.08, 0.10, 0.15). These changes in physical quantities most likely originated from incorporation of the uncompensated magnetic moments and possible spin canting induced by enhanced symmetry break of the superexchange bridges