Synthesis,defect characterization and photocatalytic degradation efficiency of Tb doped CuO nanoparticles

Monoclinic undoped and Tb doped CuO are prepared by solution combustion method and annealed at different temperatures. The effect of annealing and doping on their structural and optical properties of CuO are examined using XRD, FTIR and DRS. The surface and lattice defects in CuO and Tb doped CuO is analyzed qualitatively and quantitatively using positron lifetime and Doppler broadening spectroscopy. The average positron lifetime and electron momentum (energy) S parameter increases owing to the number of vacancies in the CuO lattice upon doping and decreases with increasing temperature. The migration of vacancies from grain to grain boundary region is observed at 600 °C annealed samples. At 800 °C, the overall behavior of lifetime value denotes that the vacancy type defect is recovered, cluster vacancy and microvoids exists with reducing size. The photocatalytic performance of undoped and Tb doped CuO on degradation of methylene blue (MB) and methyl orange (MO) is investigated under visible light for two different lamp power and dye concentration. The influence of annealing temperature and dopant ion on the efficiency is also elaborated. Enhanced photocatalytic efficiency in Tb doped CuO is observed upon annealing. X-ray photoelectron spectroscopy (XPS) result indicates that the valence states of Cu, O and Tb ions exist at the surface of the particles. Brunauer–Emmett–Teller N₂ adsorption–desorption analyses were employed to characterize specific surface area and porosity of Tb doped CuO. The doped CuO with pore size of about ～34 nm have a surface area of 16–28 m²/g. The surface area effect plays an important role in the enhanced catalytic performance on Tb doped catalysts.     

Physical and magnetic properties of (Co, Ag) doped ZnO nanoparticles

Undoped and (Co, Ag) co-doped ZnO nanostructure powders are synthesized by chemical precipitation method without using any capping agent and annealed in air ambient at 500 °C for 1 h. Here, the Ag concentration is fixed at 5 mol% and Co concentration is increased from 0 to 5 mol%. The X-ray diffraction studies reveal that undoped and doped ZnO powders consist of pure hexagonal structure and nano-sized crystallites. The novel Raman peak at 530 cm^?1 has corroborated with the Co doped ZnO nanoparticles. Moreover, the PL studies reveal that as the Co doping concentration increases and it enters into ZnO lattice as substitutional dopant, it leads to the increase of oxygen vacancies (Vo) and zinc interstitials (Zn_i). From the magnetization measurements, it is noticed that the co-doped ZnO nanostructures exhibit considerably robust ferromagnetism i.e. 4.29 emu g^?1 even at room temperature. These (Co, Ag) co-doped ZnO nanopowders can be used in the fabrication of spintronic and optoelectronic device applications.     

Diffusion phase transition and aging properties induced by B-site disorder in Na-doped barium strontium titanium ceramics

Na-doped barium strontium titanium ceramics [(Ba_1?xNa_x)_0.9Sr_0.1TiO_3?δ (x = 0.01, 0.03, 0.05, 0.08, 0.1, 0.15, 0.2, 0.3)] with perovskite tetragonal structure has been successfully fabricated by conventional solid state reaction processing. With the increasing of Na, microstructure exhibits the evolution of grains from club-shaped, square, needle-shaped and lamellate to rod-shaped. The temperature dependence of dielectric properties illustrates normal ferroelectric-paraelectric phase transition with x = 0.01, 0.03, and 0.05 and the diffusion phase transition with x = 0.08, 0.1, 0.15, 0.2 and 0.3, which implies that B-site disorder caused by composition fluctuation of Na in A-site can also induce diffused phase transition. Also, the aging properties have been investigated with decreased dielectric constant and Curie temperature, as well as constricted P–E loops compared to fresh samples, which can be explained by the symmetry-conforming principle of point defects with defect dipoles in the form of oxygen vacancies surrounding a central Na⁺ in the distorted oxygen octahedron.     

Enhancement in superconducting transition temperature and Jc values in Na-doped Bi2Sr2Ca1Cu2−xNaxOy superconductors

The effects of Na substitution on the properties of Bi₂Sr₂Ca₁Cu_2?xNa_xO_y were investigated. The prepared samples are characterized using X-ray powder diffraction, scanning electron microscope, DC electrical resistivity and magnetic-hysteresis loop measurements. It has been found that with increasing Na doping for Cu, the transition temperature gradually increase while crystal lattice parameters slightly change. Magnetic hysteresis measurements have shown that the hysteresis loops of doped samples are greater than the undoped sample. In addition, significant enhancement has been observed in the J_c values of Na-doped samples, which were calculated from the M–H curves by using Bean’s critical state model.     

TiO2–Cu photocatalysts: a study on the long- and short-range chemical environment of the dopant

In this study, the short- and long-range chemical environments of Cu dopant in TiO₂ photocatalyst have been investigated. The Cu-doped and undoped TiO₂ specimens were prepared by the sol–gel approach employing CuSO₄·5H₂O and Ti(O-iPr)₄ precursors and subjecting the dried gels to thermal treatment at 400 and 500 °C. The photocatalytic activity, investigated by methylene blue degradation under sunlight irradiation, showed a significantly higher efficiency of Cu-doped samples than that of pure TiO₂. The X-ray diffraction results showed the presence of anatase phase for samples prepared at 400 and 500 °C. No crystalline CuSO₄ phase was detected below 500 °C. It was also found that doping decreases the crystallite size in the (004) and (101) directions. Infrared spectroscopy results indicated that the chemical environment of sulfate changes as a function of thermal treatment, and UV–vis spectra showed that the band gap decreases with thermal treatment and Cu doping, showing the lowest value for the 400 °C sample. X-ray absorption fine structure measurements and analysis refinements revealed that even after thermal treatment and photocatalytic assays, the Cu²⁺ local order is similar to that of CuSO₄, containing, however, oxygen vacancies. X-ray photoelectron spectroscopy data, limited to the near surface region of the catalyst, evidenced, besides CuSO₄, the presence of Cu¹⁺ and CuO phases, indicating the active role of Cu in the TiO₂ lattice.     

Lead Vacancy Promotes Sodium Solubility to Achieve Ultra-High zT in Only Ternary Pb1-xNaxTe

Chemical doping of sodium is an indispensable means to optimize thermoelectric properties of PbTe materials, while a bottleneck is that an aliovalent atom doping leads to spontaneous intrinsic defects in the PbTe matrix, resulting in low dopant solubility. Therefore, it is urgent to improve the doping efficiency of Na for maximizing optimization. Here, an amazing new insight that the intentionally introduced Pb vacancies can promote Na solubility in ternary Pb_1-xNa_xTe is reported. Experimental analysis and theoretical calculations provide new insights into the inherent mechanism of the enhancement of Na solubility. The Pb vacancies and the resultant more dissolved Na not only synergistically optimize the carrier concentration and further facilitate the band convergence, but also induce a large number of dense dislocations in the grains. Consequently, benefiting from the self-enhancement of Seebeck coefficient and the minimization of lattice thermal conductivity, an 18% growth is obtained for the figure of merit zT in vacancy-containing Pb_0.95Na_0.04Te sample, reaching maximum zT_max ≈ 2.0 at 823 K, which achieves an ultra-high performance in only Na-doped ternary Pb_1-xNa_xTe materials. The strategy utilized here provides a novel route to optimize PbTe materials and represents an important step forward in manipulating thermoelectrics to improve dopant solubility.     

Two-phase eutectic cell growth modles and extension to off-mono-variant region

Abstract

CoTe and CoTe₂ nanorods with average diameter of 100 nm were synthesized by a simple hydrothermal process, and different CoTe₂ nanostructures were obtained by changing the NaOH concentration. CoTe nanorods exhibit weak ferromagnetism while CoTe₂ nanorods present paramagnetic behavior. Different magnetic behaviors occur in the other CoTe₂ nanostructures due to Na⁺ entrance into CoTe₂ crystals. A first-principles study on Na-doped CoTe₂ confirms the magnetic characteristics.     

Vibrational, giant dielectric and AC conductivity properties of agglomerated CuO nanostructures

We report frequency, temperature dependent dielectric response and AC conductivity of nanocrystalline CuO. These nanoparticles were prepared using sol–gel technique. Prepared particles were made as a pellet using hydraulic pressure and thermally heat treated at 950 °C. X-ray diffraction study showed that the prepared particles were crystalline in nature. Fourier transform infrared (FTIR) studies confirm the presence of Cu–O bond. Force constant and bond length is calculated which are 2.17 N cm^?1 and 1.98 Å respectively. Frequency dependent dielectric studies were done at different temperatures. Measurements show the giant dielectric value (~10³) and it increases with increase of temperature. AC conductivity measurements reveal that the conduction depends on both frequency and temperature, this agrees well with Correlated Barrier Hopping model.     

Green synthesis of ZnO and Mg doped ZnO nanoparticles,and its optical properties

The Zinc oxide nanoparticles (ZnO NPs) and Magnesium doped ZnO nanoparticles (Mg doped ZnO NPs) are synthesized by Psidium guajava leaf extract. X-ray diffraction studies confirmed that, synthesized nanoparticles were retained the wurtzite hexagonal structure. In FESEM and HRTEM image analysis, ZnO and Mg doped ZnO NPs morphology were trigonal and spherical shape. Elemental compositions were identified by EDAX analysis. From FTIR result, the Zn–O stretching was observed at 453 and 448 cm^?1 for both ZnO samples. In Raman spectra, the high intensive E₂ ^high mode observed for 438 cm^?1 for ZnO NPs. But Mg doped ZnO NPs intensity of E₂ ^high mode decreased as compared to the pure ZnO NPs, it is due to the Mg²⁺ ion in to ZnO lattice site. The photoluminescence measurements revealed that the broad emission was composed of seven different bands due to zinc vacancies, oxygen vacancies and surface defects.     

Optical and magnetic properties of CuO/CuFe2O4 nanocomposites

CuO/c-CuFe₂O₄ nanocomposites have been synthesized via the oxalate precursor route. Effect of synthesis conditions on the crystal structure, microstructure, magnetic and optical properties of the formed powders was studied. The results indicated that pure CuO nanoparticles were obtained from the oxalate precursor annealed at 600 °C for 2 h. However, substitution of Cu²⁺ ion by Fe³⁺ ion (Cu_1?X Fe _X O, where X = 0, 0.05, 0.1 and 0.2) led to form of CuO/CuFe₂O₄ nanocomposites. The microstructures of the powders appeared as a monoclinic like shape. Furthermore, the band gap energy of the obtained CuO nanopowders was 1.41 eV and the value was slightly decreased by Fe³⁺ ion substitution. In addition, the formed CuO particles had weak ferromagnetic characteristics. However, the substitution Cu²⁺ ion by Fe³⁺ ion enhanced the magnetic properties of the formed composite as the result of increasing the CuFe₂O₄ phase formation. Hence, the saturation magnetization was increased from 0.13 to 9.8 emu/g by increasing the Fe³⁺ ion from 0 to 0.2.     

Effect of high-pressure/high-temperature processing on V, Mn, Fe, Co, and Ni substitutions for copper in the structure of CaCu3V4O12

X-ray diffraction data demonstrate that high-pressure (7.0–8.0 GPa), high-temperature (700–1100°C) processing enables partial V, Mn, Fe, Co, and Ni substitutions for copper in the structure of the double perovskite CaCu₃V₄O₁₂. The lattice parameter of CaCu_{3 ? x} Ni _x V₄O₁₂ (a = 7.294–7.298 Å) exceeds that of CaCu₃V₄O₁₂ (a = 7.2845 Å) even though the Ni²⁺ ion (0.69 Å) is smaller than the Cu²⁺ ion (0.72 Å). This may be due to lattice distortion caused by the presence of two Jahn-Teller ions (Cu²⁺ and Ni²⁺). The oxides CaCu₂CoV₄O₁₂ and CaCu₂FeV₄O₁₂ are shown to have metallic conductivity.     

Influence of oxygen vacancies on the structural,dielectric, and magnetic properties of (Mn,Co) co-doped ZnO nanostructures

We analysed the variation and effect of oxygen vacancies on the structural, dielectric and magnetic properties in case of Mn (4%) and Co (1, 2 and 4%) co-doped ZnO nanoparticles (NPs), synthesized by chemical precipitation route and annealed at 750 °C for 2 h. From the XRD, the calculated average crystallite size increased from15.30?±?0.73 nm to 16.71?±?012 nm, when Co content is increased from 1 to 4%. Enhancement of dopants (Mn, Co) introduced more and more oxygen vacancies to ZnO lattice confirmed from EDX and XPS. The high-temperature annealing leads to reduction of the dielectric properties due to enhancement in grain growth (large grain volume and lesser number of grain boundaries) with the incorporation of Co and Mn ions into the ZnO lattice. The electrical conductivity of the Mn doped and (Mn, Co) co-doped ZnO samples were enhanced due to increase in the volume of conducting grains and charge density (liberation of trapped charge carriers in oxygen vacancies and free charge carriers at higher frequencies). The Mn-doped and (Mn, Co) co-doped ZnO NPs show ferromagnetic (FM) behaviour. The saturation and remnant magnetizations (M_s and M_r) elevates from (0.235 to 1.489)?×?10^?2 and (0.12 to 0.27)?×?10^?2 emu/g while Coercivity (H_c) reduced from 97 to 36 Oe with enhancement in the concentration of dopants in ZnO matrix. Oxygen vacancies were found to be the main reason for room-temperature ferromagnetism (RTFM) in the doped and co-doped ZnO NPs. The results show that the enhanced dielectric and magnetic properties of Mn doped and (Mn, Co) co-doped ZnO is strongly correlated with the concentration of oxygen vacancies. The observed enhanced RTFM, dielectric properties and electrical conductivity makes TM doped ZnO nanoparticles suitable for spintronics, microelectronics and optoelectronics based applications.     

Effect of heavy mixing lithium with doping of manganese on structural, EPR and dielectric spectroscopic properties of Layered Na2Ti3O7

Sintered ceramic samples of Na₂Ti₃O₇ with 50 % (1.0 Molar Percentage of Li₂CO₃ i. e. 50 % Lithium) with different doping molar percentages MnO₂ (0.0 < X<0.1) have been prepared through solid state reaction route. The microstructure, EPR, dielectric properties and ac conductivity of (NaLi)Ti₃O₇ with Mn [0.0 ≤ X≤0.1] have been investigated. The X-ray diffraction patterns of pure and doped layered ceramics suggest the crystals are orthorhombic in phase. The room temperature electron paramagnetic resonance spectra reveal that that for lower percentage of doping manganese ions occupies Ti ⁴⁺ site with oxidation state Mn³⁺, while higher percentage of manganese ions doping leads to oxidation state Mn²⁺ in interlayer mixed (Na Li) site. In both cases the charge compensation mechanism should operate to maintain the overall charge neutrality of the lattice. For all pure and doped layered ceramics, ferroelectric transition having high transition temperature has been identifying at 648 K. Manganese ion doping decreases dielectric loss and increases dielectric constant due to inhibition of domain wall motion. It is apparent that ionic conduction becomes difficult due to presence of more lithium ions with sodium ions in interlayer space which shrinking the wide space of interlayer channels.     

Controlled reduction of Cu to Cu with an N,O-type chelate under hydrothermal conditions to produce Cu2O nanoparticles

N,O-type organic chelates reduced coordinated Cu²⁺ ions under hydrothermal reaction conditions to produce Cu₂O/CuO nanoparticles. Chelates in which the N and O atoms are closely spaced produced smaller amounts of CuO nanoparticles, indicating their higher ability to reduce Cu²⁺ ions to Cu⁺ ions. [Cu(Gly)₂]² with the shortest ligand chain length produced only Cu₂O nanoparticles and, therefore, can be used as a single molecule precursor for the synthesis of Cu₂O nanoparticles.     

Influence of Co-doping on the structural, optical and magnetic properties of ZnO nanoparticles synthesized using auto-combustion method

In the present work, we have interested to understand the influence of cobalt doping on the various properties of ZnO nanoparticles, a series of samples were successfully synthesized using sol–gel auto-combustion method. The effects of Co doping on the structural and optical properties of ZnO:Co nanoparticles were investigated using X-ray diffraction (XRD), scanning electron microscopy, fourier transform infrared (FTIR) spectroscopy, ultraviolet–visible spectroscopy, photoluminescence spectroscopy and vibrating sample magnetometer (VSM). With the sensitivity of the XRD instrument, the structural analyses on the undoped and Co-doped ZnO samples reveal the formation of polycrystalline hexagonal-wurtzite structure without any secondary phase. FTIR spectra confirm the formation of wurtzite structure of ZnO in the samples. The optical absorption spectra showed a red shift in the near band edge which indicates that Co²⁺ successfully incorporated into the Zn²⁺ lattice sites. The room temperature PL measurements show a strong UV emission centered at 392 nm (3.16 eV), ascribed to the near-band-edge emissions of ZnO and defect related emissions at 411 nm (violet luminescence), 449 nm (blue luminescence) and 627 nm (orange-red luminescence), respectively. Magnetic study using VSM reveals that all the samples are found to exhibit room temperature ferromagnetism.     

Improvement of the critical current density in Bi-2223 ceramics by sodium–silver co-doping

Bi_1.46Pb_0.36Ag_0.18Sr₂Ca₃Cu_4?xNa_xO_y (x = 0, 0.05, 0.1 and 0.25) samples were prepared by a conventional solid state reaction method. The prepared samples are characterized using X-ray powder diffraction, scanning electron microscope, dc electrical resistivity and magnetic-hysteresis loop measurements. It has been shown that the Na doping in low contents significantly improves the physical properties of Bi-2223 phase. Magnetic hysteresis measurements have shown that the largest hysteresis curve belongs to Bi_1.46Pb_0.36Ag_0.18Sr₂Ca₃Cu_3.95Na_0.05O_y sample including x = 0.05 Na content, indicating that it has best flux pinning capability in samples produced in this work. In addition, J_c values of the samples were calculated from the hysteresis loop measurement by using the Bean’s model showing that J_c increases with small amounts of sodium–silver co-doping.     

Dielectric responses of Na<Subscript>0.65</Subscript>Bi<Subscript>0.45</Subscript>Cu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> ceramics based on the composition design of changing the Na/Bi ratio

Na_0.65Bi_0.45Cu₃Ti₄O₁₂ ceramics were successful prepared by the conventional solid-state reaction technique. Compared to Na_0.50Bi_0.50Cu₃Ti₄O₁₂ (NBCTO), the composition of Na_0.65Bi_0.45Cu₃Ti₄O₁₂ was designed in terms of changing the Na/Bi ratio. Colossal dielectric permittivity of ~1.2 × 10⁴ at 1 kHz was obtained in Na_0.65Bi_0.45Cu₃Ti₄O₁₂ ceramics. Interestingly, three frequency dispersions were observed in the frequency dependence of dielectric constant measured at different temperatures. The investigation of electric modulus displayed that the giant low-frequency dielectric constant was attributed to Maxwell–Wagner polarization at the grain boundaries and the frequency dispersion in middle-frequency range was due to the grain polarization. Except grain response and grain boundaries response reflected by two semicircles in the impedance spectroscopy, another electrical response associated with nonzero high frequency intercept was found. The grain resistance Rg and grain boundaries resistance R _gb was ~600 Ω and 3.9 × 10⁵ Ω, respectively. The large intrinsic permittivity as high as ~700 was obtained. Furthermore, two dielectric anomalies observed in the temperature dependent of dielectric constant were discussed in detail. The results indicated change in the Na/Bi ratio had a significant effect on the electrical properties of NBCTO ceramics.     

Synthesis,structure and thermoelectric properties of $$\mathrm{La}_{1-x}\mathrm{Na}_{x}\mathrm{CoO}_{3 }$$ perovskite oxides

Monovalent ion doped lanthanum cobaltate \(\hbox {La}_{1-x}\hbox {Na}_{x}\hbox {CoO}_{3 }\) (\(0 \le x \le 0.25\)) compositions were synthesized by the nitrate–citrate gel combustion method. All the heat treatments were limited to below 1123 K, in order to retain the Na stoichiometry. Structural parameters for all the compounds were confirmed by the Rietveld refinement method using powder X-ray diffraction (XRD) data and exhibit the rhombhohedral crystal structure with space group R-3c (No. 167). The scanning electron microscopy study reveals that the particles are spherical in shape and sizes, in the range of 0.2–0.5 \(\upmu \)m. High temperature electrical resistivity, Seebeck coefficient and thermal conductivity measurements were performed on the high density hot pressed pellets in the temperature range of 300–800 K, which exhibit p-type conductivity of pristine and doped compositions. The X-ray photoelectron spectroscopy (XPS) studies confirm the monotonous increase in \(\hbox {Co}^{4+}\) with doping concentration up to \(x = 0.15\), which is correlated with the electrical resistivity and Seebeck coefficient values of the samples. The highest power factor of \(10~\upmu \hbox {W~mK}^{-2 }\) is achieved for 10 at% Na content at 600 K. Thermoelectric figure of merit is estimated to be \({\sim }1 \times 10^{-2}\) at 780 K for 15 at% Na-doped samples.     

Structure–property correlation of pure and Sn-doped ZnO nanocrystalline materials prepared by co-precipitation

The structural, optical and electrical properties of pure and tin (Sn) doped zinc oxide (ZnO) nanocrystalline materials prepared by co-precipitation method have been studied as a function of Sn doping concentration. The phase identification through powder X-ray diffraction methods confirmed that pure and Sn-doped zinc oxide powder have typical hexagonal wurtzite structure (a = 3.407 Å and c = 4.592 Å) with slight change in lattice parameters. The surface morphological examination with field emission scanning electron microscopy revealed the fact that the grains are closely and densely packed and pores/voids between the grains decrease with increasing the doping concentration of Sn from 0% to 15%. The energy bandgap of pure ZnO was found to be 3.35 eV from optical absorption spectra obtained by ultraviolet–visible (UV–Vis) absorption spectrophotometer. The variation of energy bandgap and electrical resistivity of Sn-doped ZnO were also determined with tin doping. Upon increasing the Sn dopant concentration from 0 to 15 wt%, the optical bandgaps of ZnO increases from 3.35 to 3.42 eV. The electrical resistivity of Sn-doped ZnO has been decreased at least two orders of magnitude, i.e. from 1263.17 to 28.64 Ω cm. This decrement in electrical resistivity may be due to the partial substitution of divalent Zn²⁺ ions with tetravalent Sn⁴⁺ ions, generating more free electrons for conduction.     

Variation of structural,optical, dielectric and magnetic properties of SnO<Subscript>2</Subscript> nanoparticles

We report effect of oxygen vacancies on band gap narrowing, enhancement in electrical conductivity and room temperature ferromagnetism of SnO₂ nanoparticles synthesized by simple chemical precipitation approach. As the calcination temperature is elevated from 400 to 800 °C, the average particle size increases from 12.26 to 34.43 nm, with enhanced grain growth and crystalline quality. At low temperatures, these nanoparticles are in a rather oxygen-poor state revealing the presence of many O vacancies and Sn interstitials in SnO₂ nanoparticles as in this case Sn⁺² is not oxidized completely to Sn⁺⁴ and small sized nano particles have more specific surface area, hence defects are more prominent. The oxygen content increases steadily with increasing temperature, with the Sn:O atomic ratio very near to the stoichiometric value of 1:2 at high temperatures suggesting the low density of defects. The optical band gap energies of all SnO₂ nanoparticles are in the visible light region, decreasing from 2.89 to 1.35 eV, while room temperature ferromagnetism and electrical conductivity are enhanced with reduced temperatures. The dielectric constant (ε_r) exhibited dispersion behaviour and the Debye’s relaxation peaks were observed in tanδ. The variation of dielectric properties and ac conductivity revealed that the dispersion is due to Maxwell–Wagner interfacial polarization and hopping of charge carriers between Sn⁺²/Sn⁺⁴. The narrowed band gap energies and enhanced ferromagnetism are mainly attributed to the increase of defects density (e.g., oxygen vacancies). The presence of oxygen vacancies is confirmed by EDX, Raman, PL, XPS, and UV–Vis spectra. The band gap of 1.35 eV is the smallest value for SnO₂ reported so far. This rather small band gap, enhanced conductivity and room temperature ferromagnetism demonstrate that SnO₂ nanoparticles are very promising in the visible light photo catalysis and optoelectronic devices.