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Two samples of ZnxMn1−xFe2O4 (x = 0, sample A; and x = 0.4, sample B) were synthesized by the hydrothermal method. From complex impedance measurements in the range 100 Hz–2 MHz and for temperatures T between 30 and 130 °C, the barrier energy between localized states ΔErelax was determined for the first time in these samples. For sample B, a single value of ΔErelax was highlighted (0.221 eV), whilst, for sample A, two values were obtained (0.012 eV and 0.283 eV, below 85 °C and above 85 °C, respectively), associated with two zones of different conductivities. Using the Mott's VRH model and the CBH model, we determined for the first time both the bandgap energy barrier (Wm) and the hopping (crossover) frequency (ωh), at various temperatures. The results show that, for sample A, Wm has a maximum equal to 0.72 eV at a temperature between 70 and 80 °C, whilst, for sample B, Wm has a minimum equal to 0.28 eV at a temperature of 60 °C, the results being in good agreement with the temperature dependence of the static conductivity σDC(T) of the samples. By evaluating σDC and eliminating the conduction losses, we identified, using a novel approach, a dielectric relaxation phenomenon in the samples, characterized by the activation energy EA,rel. At various temperatures, we determined EA,rel, which ranged from 0.195 eV to 0.77 eV. These results are important, as understanding these electrical properties is crucial to various applications, especially in technologies where temperature variation is significant. [ABSTRACT FROM AUTHOR]

The synthesis, thermal behavior and electrical properties of a series of undoped and 1% Ga- or Y-doped lanthanum manganite compounds, obtained via the sol–gel technique, are reported. Scanning electron microscopy (SEM/EDX), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR) analyses were performed on all synthesized samples. The XRD results confirmed a good crystallinity for all studied samples, and a change in the crystal structure of Ga- or Y-doped lanthanum manganite (Pm-3m space group) was observed compared to the pristine sample (R-3c space group). Thermal analysis highlighted a different behavior of the doped samples compared to the undoped sample, observed by the different mass losses in the analyzed temperature range. For these materials, it is shown for the first time that the static electrical conductivity, σDC, of Ga- or Y-doped LaMnO3 compounds increases compared to the σDC of the pristine sample, and the thermal activation energy of the process of electrical conduction, EA,cond, increases linearly with the temperature for all three studied samples. [ABSTRACT FROM AUTHOR]

LaMnO3 and 1% Pd-, Ag-, or Y-doped perovskite type nanomaterials were prepared by the sol-gel method, followed by heat treatment at a low temperature (600 °C for 6 h). The investigation through X-ray diffraction and FT-IR spectroscopy indicated that all samples were well crystallized, without secondary phases, and that the transition metal doping changed the crystal structure from the R-3c space group for the undoped LaMnO3 to the Pm-3m space group for the doped perovskite compounds. In this research paper, the efficiencies of the perovskite LaMnO3 materials for methyl orange removal were analyzed, wherein the effect of the doping ions and of the pH on the catalytic activity were studied together with a kinetic approach for the LaMnO3 materials at different values of the pH. Moreover, in the catalytic activity, it should be noted that a slightly better performance was obtained for the Ag-doped materials compared to the Y- and Pd-doped perovskite samples. The results presented for the perovskite LaMnO3 nanomaterials reinforce the interest in these multifunctional materials to be used in industrial applications; e.g., in water treatment. [ABSTRACT FROM AUTHOR]

Herein, we present the investigation of catalytical and fluorescence properties for Ni11(HPO3)8(OH)6 materials obtained through a hydrothermal approach. As part of the constant search for new materials that are both cost effective and electrocatalytically active for the oxygen evolution reaction (OER) in alkaline medium, the present study involves several graphite electrodes modified with Ni11(HPO3)8(OH)6 mixed with reduced graphene oxide (rGO) and carbon black. The experimental results obtained in 0.1 mol L–1 KOH electrolyte solution show the electrode modified with rGO, 5 mg carbon black and 1 mg nickel phosphite as displaying the highest current density. This performance can be attributed to the synergistic effect between nickel phosphite and the carbon materials. Investigation of the electrode's OER performance in 0.1 mol L–1 KOH solution revealed a Tafel slope value of just 46 mV dec–1. By increasing the concentration to 0.5 and 1 mol L–1, this value increased as well, but there was a significant decrease in overpotential. Fluorescence properties were analyzed for the first time at the excitation length of 344 nm, and the observed strong and multiple emissions are described. [ABSTRACT FROM AUTHOR]

Iron phosphate materials have attracted a lot of attention due to their potential as cathode materials for lithium-ion rechargeable batteries. It has been shown that lithium insertion or extraction depends on the Fe mixed valence and reduction or oxidation of the Fe ions' valences. In this paper, we report a new synthesis method for the Fe3(PO3OH)4(H2O)4 mixed valence iron phosphate. In addition, we perform temperature-dependent measurements of structural and physical properties in order to obtain an understanding of electronic–structural interplay in this compound. Scanning electron microscope images show needle-like single crystals of 50 μm to 200 μm length which are stable up to approximately 200 °C, as revealed by thermogravimetric analysis. The crystal structure of Fe3(PO3OH)4(H2O)4 single crystals has been determined in the temperature range of 90 K to 470 K. A monoclinic isostructural phase transition was found at ~213 K, with unit cell volume doubling in the low temperature phase. While the local environment of the Fe2+ ions does not change significantly across the structural phase transition, small antiphase rotations occur for the Fe3+ octahedra, implying some kind of electronic order. These results are corroborated by first principle calculations within density functional theory, which also point to ordering of the electronic degrees of freedom across the transition. The structural phase transition is confirmed by specific heat measurements. Moreover, hints of 3D antiferromagnetic ordering appear below ~11 K in the magnetic susceptibility measurements. Room temperature visible light absorption is consistent with the Fe2+/Fe3+ mixed valence. [ABSTRACT FROM AUTHOR]

In this research paper, the thermal behaviour of the mixed valence iron oxides Fe3(PO4)2(OH)2 is presented in both air and vacuum atmosphere. The structure, morphology and the magnetic properties of the iron phosphates samples obtained at selected temperatures were carefully investigated. The electrocatalytic properties of synthetic barbosalite Fe3(PO4)2(OH)2 materials for the oxygen and hydrogen evolution reactions (OER and HER) were investigated in a wide pH range, using graphite electrodes modified with different compositions containing the catalyst. The best results were obtained for the compositions with 6 mg barbosalite. During the HER experiments, overpotentials of − 300 and − 350 mV were observed at the current density of − 10 mA cm−2 in strong alkaline and in strong acidic media, respectively. During the OER studies performed in strong acidic solution and in neutral environment, overpotentials of 320 and 690 mV were obtained at the current density of 10 mA cm−2. These findings complement the existing scientific literature relevant to the water splitting domain. Furthermore, the visible light absorption ability of synthetic Fe2+Fe3+2(PO4)2(OH)2 materials was also evaluated. [ABSTRACT FROM AUTHOR]

Present day scientific research is focused on the identification of renewable and clean alternatives to fossil fuels, with hydrogen being a promising energy source that fulfills both requirements. Given this context, the current work investigates the water splitting electrocatalytic properties of two hydrothermally synthesized transition metal oxides: MnO2 and Fe3O4. Electrodes were obtained by modifying graphite substrates with suspensions in ethanol containing the catalytic materials as such or in compositions, and their activity for the O2 and H2 evolution reactions (OER and HER) was studied in alkaline medium. Out of the MnO2-based electrodes, the one modified with the suspension containing 2 mg MnO2, 1 mg Carbon Black and 10 μL Nafion solution displayed an OER overpotential (ηO2) value, at i = 10 mA/cm2, of 0.53 V, while the one manufactured using the suspension with 4 mg MnO2 and 10 μL Nafion solution showed a HER overpotential (ηH2) of 0.427 V (at i = -10 mA/cm2). From the Fe3O4-based electrodes, the one modified with the suspension containing 2 mg Fe3O4 and 2 mg Carbon Black evidenced the highest catalytic activity for both reactions (ηO2 = 0.51 V and ηH2 = 0.43 V). [ABSTRACT FROM AUTHOR]

Ag-doped LaMnO3 nanoparticles have been synthesized through sol–gel process using citric acid as a chelating agent. Silver effect on physicochemical properties was studied. The physicochemical properties and electrical performance of the obtained materials were also investigated, using thermogravimetric analysis (TGA) and X-ray diffraction (XRD) techniques. Thermogravimetric analysis shows that for the studied materials a decomposition stage between 3.7% and 4.1% can be observed, and mass increases from the LMN-Ag1 to the LMN-Ag3 sample. Also, in all those three situations an endothermic process with a maximum around 600 °C is observed. The diffractograms of the perovskite materials showed single-phase crystalline with high purity and rhombohedral structure with R-3c space group. The electrical measurements show that the conductivity spectrum is formed by both the static component σDC corresponding to the low frequencies and the component σAC corresponding to high frequencies. The values of σDC of samples are significantly influenced by the silver ions concentration. [ABSTRACT FROM AUTHOR]

This work reports the change in the structural and electrical behavior of partial substitution of manganese by palladium in LaMnO3 perovskite materials synthesized through sol–gel method. X-ray diffraction analysis reveals that all the obtained materials crystallize in the orthorhombic perovskite structure (Pnma space group). Morphological investigations show the presence of fine nanometric particles of similar shape and high tendency to agglomerate, and the elemental analysis highlights the presence of palladium. Detailed study of complex impedance measurements in the frequency range 20 Hz–2 MHz and different temperatures between (30 and 120) °C indicate the presence of electrical relaxation in materials; the activation energy of relaxation decreases from 0.207 eV (0.05% Pd–LMO–Pd1) to 0.102 eV (0.15% Pd–LMO–Pd3) when the x concentration of Pd increases from 0.05 to 0.15. The electrical conductivity analyses indicate that the conduction mechanisms in the low-frequency range (f < 100 kHz) are activated thermally and were explained using the Mott's variable-range-hopping (VRH) model. In the high-frequency range (f > 100 kHz), the conduction mechanism is due to the charge carriers hopping between the nearest neighboring states, in agreement with the CBH model (correlated barrier hopping). Based on this model, the bandgap energy, Wm was determined. The results show that by increasing the Pd concentration, the value of Wm of the sample increases, from 0.229 eV (LMO–Pd1) to 0.298 eV (LMO–Pd3), in agreement with the decreasing of electrical conductivity of samples. Highlights: Partial substitution of Mn by palladium in LaMnO3 perovskite materials. Heat treatment was performed at low temperature. Detailed studies of impedance and electric conductivity have been performed. [ABSTRACT FROM AUTHOR]

Ultrasound-assisted co-precipitation method is used for the first time to prepare F e 3 2 + (PO4)2·8H2O polycrystalline materials which crystallise at room temperature in a monoclinic structure (C2/m space group). The frequency and temperature dependence of the electrical conductivity σ of the Fe3(PO4)2·8H2O sample was determined, and they follow Jonscher's law. The results show that, in the low frequency range (below 10 kHz), the conduction mechanism can be explained by the Mott's variable-range-hopping model; a change of slope of the conductivity is observed at ≈ 110 °C, which may be associated with a change in the crystal structure, due to the loss of water molecules from the sample. Based on Mott's model, the density of localized states near the Fermi level, N(EF), the hopping distance, R, and the hopping energy, W, were computed. The results show that, at a constant frequency (below 10 kHz), N(EF) does not depend on temperature, while R decreases and W increases with the increase in temperature. Over 200 kHz, the correlated barrier hopping model is characteristic to the conductivity for this type of materials and the calculated band gap energy is 1.072 eV, in the (25–110) °C range and 0.821 eV, in the (110–220) °C range. [ABSTRACT FROM AUTHOR]

CuMnO2 nanoparticles were obtained for the first time by sonochemical synthesis in an aqueous solution of sodium hydroxide containing soluble copper and manganese nitrate reactants without any further thermal treatment. Experiments were performed at different lengths of time (20 min, 30 and 40 min) and ultrasound amplitude of 80%. The materials present the expected crednerite structure (C2/m space group), and the nanoparticles average size and morphology for each sample are studied by nanoparticle tracking analysis method, respectively, by transmission electron microscopy. The as-obtained materials display p-type semiconductivity behavior at room temperature and the value for direct optical band gap EG is estimated to be approximately ~ 3.6 eV. The study of the magnetic properties for the nanocrystalline crednerite samples reveals 2D magnetic behavior below 110 K and a weak ferromagnetic component at ~ 20 K. [ABSTRACT FROM AUTHOR]

Sodium niobate (NaNbO3) fine powders were synthesized by hydrothermal method using ammonium niobate (V) oxalate hydrate (C4H4NNbO9 x H2O), sodium hydroxide (NaOH) and sodium citrate hydrate (C6H9Na3O9) as precursors. The samples were characterized by X-Ray diffraction (XRD), scanning electron microscopy (SEM), AFM (Atomic Force Microscopy), UV-VIS spectroscopy and Fourier Transform Infrared Spectroscopy (FTIR). X-ray diffraction for powder obtained by hydrothermal method revealed three phases mixture (NaNbO3, Nb2O5, Na2C2O4). In order to obtain a material with perovskite structure, the powder was treated for 6 h at 600°C. After calcination, the X-ray powder diffraction (XRD) patterns shown reflections of a pure cubic structure (space group Pm-3m) with lattice parameters (a=b=c=3.909Å) comparable to those reported in the literature. The average particle sizes estimated by SEM image were 2-3 micrometers. [ABSTRACT FROM AUTHOR]

Nanocrystalline powders of doped BiTe, with Ag (S1 sample), Sb (S2 sample), Sn (S3 sample) ions with different morphology and particle size 30-50 nm were prepared by a ultrasonically assisted hydrothermal method in alkaline aqueous solution with different concentration of NaBH as reducing agent at 200°C for 3 hours and 80% fill degree of autoclave. The influence of dopants and hydrothermal treatment conditions on the formation features, phase composition, particle size, morphology and properties of the products were investigated by X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, atomic force microscopy and electrical measurements. This paper reports a comparative study regarding the dopants influence to the shape and size of nano-structured thermoelectric materials. It was found that hydrothermal processing results in formation of low dimensional dispersion of doped BiTe nanostructures with desired shape and size and high degree of crystallinity with typical semiconductor behavior. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

An amendment to this paper has been published and can be accessed via a link at the top of the paper. [ABSTRACT FROM AUTHOR]

Pharmaceuticals and pesticides are emerging contaminants problematic in the aquatic environment because of their adverse effects on aquatic life and humans. In order to remove them from water, photocatalysis is one of the most modern technologies to be used. First, newly synthesized photocatalysts were successfully prepared using a sol–gel method and characterized by different techniques (XRD, FTIR, UV/Vis, BET and SEM/EDX). The photocatalytic properties of TiO2, ZnO and MgO nanoparticles were examined according to their removal from water for two antibiotics (ciprofloxacin and ceftriaxone) and two herbicides (tembotrione and fluroxypyr) exposed to UV/simulated sunlight (SS). TiO2 proved to be the most efficient nanopowder under UV and SS. Addition of (NH4)2S2O8 led to the faster removal of both antibiotics and herbicide fluroxypyr. The main intermediates were separated and identified for the herbicides and antibiotic ciprofloxacin. Finally, the toxicity of each emerging pollutant mixture and formed intermediates was assessed on wheat germination and biomass production. [ABSTRACT FROM AUTHOR]