Growth temperature dependent properties of ZnO nanorod arrays on glass substrate prepared by wet chemical method

In this work, ZnO nanorod arrays were grown on glass substrate by the wet chemical method, and the effect of synthesis temperature on the properties was investigated. The grown nanorods were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), Raman and Photoluminescence (PL) measurements. XRD pattern showed that nanorod prepared at 80 °C and 90 °C has high crystallinity with wurtzite structure and orientated along the c-axis. However, nanorods were not formed at 60 °C and 70 °C due to less energy supply for the growth of the ZnO. FE-SEM results showed that the morphology and the size of ZnO can be effectively controlled. In particular, as the temperature increased, diameter of the nanorod was increased while length decreased. Raman scattering spectra of ZnO nanorod arrays revealed the characteristic E₂^high mode that is related to the vibration of oxygen atoms in the wurtzite ZnO. Room-temperature PL spectra of the ZnO nanorods revealed a near-band-edge (NBE) emission peak. The NBE (UV light emission) band at ~383 nm might be attributed to the recombination of free exciton. The narrow full-width at half-maximum (FWHM) of the UV emission indicated that ZnO nanorods had high crystallinity.     

Preparation and band gap energies of ZnO nanotubes, nanorods and spherical nanostructures

Nanostructured zinc oxides were prepared with no catalysts or substrates used. The advantages of this method were a direct and a relatively easy way of getting large amounts of different morphologies of nanostructured ZnO. Another advantage was the formation of the nanotubes at a relatively lower temperature than most other methods. This presented a promising way for commercialization of the ZnO nanomaterials. The materials were characterized using simultaneous thermogravimetric analysis, X-Ray diffraction and transmission electron microscopy. Structures such as nanotubes, nanorods and spherically shaped crystals were formed at certain annealing temperatures. It was found that the ZnO nanotubes grew in the direction of the c-axis. For comparisons of size and morphology, the ZnO precursors were annealed at 300 °C, 400 °C and 700 °C. The lattice parameter of the ZnO nanotube was smaller than conventional micron sized materials. It was also found that the band gap energies of the ZnO nanomaterials were dependent on the morphology of the nanostructures.     

Synthesis, structural and magnetic properties of nanostructured Ca0.9Gd0.1MnO3 obtained by modified glycine nitrate procedure (MGNP)

Ca_0.9Gd_0.1MnO₃ nanopowders with perovskite type crystal structure were synthesized by modified glycine nitrate procedure. Nanopowders were prepared by combining glycine with metal nitrates and/or metal acetates in their appropriate stoichiometric ratios. Modification of the procedure was performed by partial replacement of nitrates by acetates, in order to control the burn-up reaction. Obtained Ca_0.9Gd_0.1MnO₃ powders were calcinated in the temperature interval from 850 °C to 950 °C for 10 min. Properties such as phase evolution, lattice parameters, chemical composition and magnetic properties were monitored by DTA, X-ray diffraction, SEM/EDS and magnetic measurements. Magnetic measurements performed at the sample with the smallest crystallite size showed that a 10% of Gd³⁺ substituted Ca²⁺ ions changes antiferromagnetic properties of CaMnO₃ by the introduction of ferromagnetic interaction due to a double exchange between Mn³⁺ and Mn⁴⁺ ions. Presence of competing interactions and their randomness lead to a formation of a spin glass state below Neel temperature T_N = 110 K. From the high temperature magnetic susceptibility measurements effective magnetic moment of manganese ions is determined which lies between the values for Mn³⁺ and Mn⁴⁺ ions.     

Dopant induced RTFM and enhancement of fluorescence efficiencies in spintronic ZnS:Ni nanoparticles

Diluted magnetic semiconducting (DMS) ZnS:Ni (Ni=0, 1, 3 and 5 at%) nanoparticles were synthesized by the refluxing technique at 80 °C. X-ray diffraction studies showed that undoped ZnS and Ni doped ZnS nanoparticles exhibited the expected zinc blende structure. X-ray photoelectron spectroscopy results revealed that the Ni ions existed in a +2 state in these nanoparticles. Reflectance measurements showed a decrease in band gap with increasing Ni concentration. Room temperature photoluminescence (PL) studies indicated that all the samples exhibited broad and asymmetric PL peaks covering a wide visible range. Gaussian fitting of PL data resulted in three deconvoluted peaks corresponding to blue and green emissions. Dramatic enhancement in fluorescence efficiency was observed in the doped ZnS nanoparticles indicating their possible applications in photoluminescent devices. Magnetic studies revealed that all the doped samples exhibited carrier mediated ferromagnetism at room temperature. Saturation magnetization (M_s) increased with increasing Ni content reaching a maximum for 3 at% Ni and decreased for samples of 5 at% Ni.     

Growth and structural characteristics of perovskite-wurtzite oxide ceramics thin films

Wurtzite ZnO thin films were grown on single-crystal perovskite SrTiO₃(STO) (1 0 0) substrates at various temperatures. The ZnO/STO thin films thus formed exhibit a preferred (1 1 0)-orientation at a growth temperature of 600-700 °C. A high growth temperature enhances not only the (1 1 0)-texture of ZnO/STO thin films but also the crystalline quality of the film. (La_0.7Sr_0.3)MnO₃ (LSMO) thin films were subsequently grown on ZnO(1 1 0)/STO(1 0 0) substrates with various thicknesses, and were polycrystalline. A thicker LSMO film has a stronger (0 0 l)-preferred orientation than the thinner one. The lattice distortion of LSMO decreases as the LSMO thickness increases. Magnetization vs. temperature curves show that both crystalline quality and lattice distortion influence the magnetic properties of LSMO thin films. The physical properties of the manganite oxide can be modulated by forming a heterostructure with wurtzite ZnO.     

Sol–gel synthesis of Pd doped ZnO nanorods for room temperature hydrogen sensing applications

A sol–gel spin coating technique was described for the synthesis of Pd doped ZnO nanorods for hydrogen sensing applications. The nanorods were hexagonal in shape, 50–100 nm in diameter and uniform in distribution. They exhibited homogeneous surface morphology, c-axis orientation and excellent crystalline properties. The synthesized nanorods were used to sense and detect hydrogen in a homemade gas chamber. The fabricated sensor successfully detected as low as 40 ppm hydrogen at room temperature with a very low level of power supply (16.16 μA) under a mixed background. Dynamic and repeated responses were observed with a wide range of hydrogen concentrations (40–360 ppm) at 200 °C. The developed sensor was at least 25 fold more sensitive over the literature documented Pd doped ZnO nanorods in detecting hydrogen at ambient temperature. The simplicity, low-cost, high sensitivity and high stability of the sensor materials suggested that the synthesized Pd doped ZnO nanorods could be used in hydrogen and chemical sensing devices where Pd-mediated catalysis is involved.     

Influence of in-situ magnetic field pressing on the structural and multiferroic behaviour of BiFeO3 ceramics

Polycrystalline single phase BiFeO₃ (BFO) ceramic samples have been prepared by conventional solid state sintering and also by in-situ magnetic field pressing followed by solid state sintering. The influence of in-situ magnetic field pressing on the structural, magnetic, ferroelectric and thermal properties has been investigated in this work. X-ray diffraction analysis and Reitveld refinement shows the single phase characteristics of BFO samples. Further texture formation and the development of compressive lattice strain have been observed in the magnetic field pressed samples. A change in Fe-O-Fe bond angle and suppression of spiral spin structure results in the enhanced magnetization value M_s = 136 memu/g at 2 T. Similarly spontaneous polarization has also improved with a P_max value of 1.3 μC/cm². DSC plot shows a significant variation in heat flow and enthalpy at the Neel transition (T_N = 372 °C) and ferro to paraelectric transition (T_C = 820 °C) for the magnetic field pressed BFO samples.     

Rapid synthesis and dye-sensitized solar cell applications of hexagonal-shaped ZnO nanorods

This paper reports, for the first time, a very rapid and large-scale synthesis and dye-sensitized solar cells (DSSCs) application of well-crystallized hexagonal-shaped ZnO nanorods at very low temperature of about 70 °C in 20 min. The thin films of as-grown nanorods were used as photo-anode materials to fabricate the DSSCs which exhibited an overall light to electricity conversion efficiency (ECE) of 1.86% with a fill factor of 74.4%, short-circuit current of 3.41 mA/cm² and open-circuit voltage of 0.73 V.     

Structural, magnetic and electrochemical properties of LiNi0.5Mn0.5O2 as positive electrode for Li-ion batteries

The layered LiNi_0.5Mn_0.5O₂ was synthesized by wet-chemical method and characterized by X-ray diffraction and SQUID magnetometry. The powders adopted the α-NaFeO₂ structure. The ferromagnetism observed below Tc = 140 K is attributed to the linear Ni²⁺(3a)-O-Mn⁴⁺(3b)-O-Ni²⁺(3a) magnetic paths, from which we derive that 7% of the nickel occupies the (3a) Wyckoff position in place of Li, constituting a Ni²⁺(3a) defect. The analysis of the magnetic properties in the paramagnetic phase in the framework of the Curie-Weiss law agrees well with the combination of Ni²⁺ (S = 1) and Mn⁴⁺ (S = 3/2) spin-only values. Results of structural and magnetic properties of chemically delithiated sample are consistent with the electronic state Mn⁴⁺/Ni⁴⁺ and the high-spin configuration for Ni⁴⁺ ions. The LiNi_0.5Mn_0.5O₂ electrode sintered at 900 °C delivers a capacity 166 mAh/g at 0.1C rate which is capacity retention of 95%.     

Formation of nanosize ZnO particles by thermal decomposition of zinc acetylacetonate monohydrate

Formation of ZnO particles by thermal decomposition of zinc acetylacetonate monohydrate in air atmosphere has been investigated using XRD, DTA, FT-IR, and FE-SEM as experimental techniques. ZnO as a single phase was produced by direct heating at ≥200 °C. DTA in air showed an endothermic peak at 195 °C assigned to the ZnO formation and exothermic peaks at 260, 315 and 365 °C, with a shoulder at 395 °C. Exothermic peaks can be assigned to combustion of an acetylacetonate ligand released at 195 °C. ZnO particles prepared at 200 °C have shown no presence of organic species, as found by FT-IR spectroscopy. Particles prepared for 0.5 h at 200 °C were in the nanosize range from ∼20 to ∼40 nm with a maximum at 30 nm approximately. The crystallite size of 30 nm was estimated in the direction of the a₁ and a₂ crystal axes, and in one direction of the c-axis it was 38 nm, as found with XRD. With prolonged heating of ZnO particles at 200 °C the particle/crystallite size changed little. However, with heating temperature increased up to 500 or 600 °C the ZnO particle size increased, as shown by FE-SEM observation. Nanosize ZnO particles were also prepared in two steps: (a) by heating of zinc acetylacetonate monohydrate up to 150 °C and distillation of water and organic phase, and (b) with further heating of so obtained precursor at 300 °C.     

Synthesis and characterization of nano-crystalline strontium hexaferrite using the co-precipitation and microemulsion methods with nitrate precursors

Nano-crystalline strontium hexaferrite (SrFe₁₂O₁₉) powder was synthesized using the classical co-precipitation and microemulsion methods. The precursors were obtained by precipitating Sr²⁺ and Fe²⁺ ions using tetramethylammonium hydroxide and calcinating at different temperatures ranging from 400 °C to 1000 °C in air. The influence of the Sr²⁺/Fe³⁺ mol ratio and the calcination temperature on the product formation and magnetic properties were studied. The formation of nanosized particles of SrFe₁₂O₁₉ with a relatively high saturation magnetization M_s = 64 Am²/kg, remanent magnetization of M_r = 39 Am²/kg and a coercitivity of H_c = 5.5 kOe was achieved at a Sr²⁺/Fe³⁺ mol ratio of 1:8 calcined at 900 °C. The formation of the SrFe₁₂O₁₉ was inspected using XRD analysis, thermogravimetric analysis (TGA), differential thermal analysis (DTA), TEM, and magnetic measurements.     

A simple electrochemical deposition route for the controllable growth of Ce-doped ZnO nanorods

Here we report that the various Ce⁴⁺-doped ZnO nanorods can be successfully synthesized by electrochemical deposition route, which represents a simple, quick and economical method for the controllable growth of Ce⁴⁺-doped ZnO nanorods. The high-resolution transmission electron microscopy (HRTEM) and the selected area electron diffraction (SAED) both proved that the prepared Ce⁴⁺-doped ZnO nanorods consisted of single crystal with preferential growth in the [0 0 0 1] direction. The morphology and size of the nanorods can be tailored by optimizing the synthetic parameters. Furthermore, the flowerlike Ce⁴⁺-doped ZnO nanorod clusters can also be successfully prepared. An obvious blue-shifted absorption peak of Ce⁴⁺-doped ZnO nanorod compared with that of the bulk ZnO phase was observed.     

Oxygen reduction reaction on electrodeposited zinc oxide electrodes in KCl solution at 70 °C

The reduction of oxygen was studied in 0.1 M KCl at 70 °C using the rotating disk electrode (RDE) technique on platinum and electrodeposited ZnO thin film electrodes deposited on platinum substrates. In the absence of Zn²⁺ ions in solution, a Tafel slope of 139 mV dec⁻¹ was obtained, a value close to that measured on bare platinum electrode (133 mV dec⁻¹) and ascribed to the limitation of the reaction rate by the first electron transfer. The main difference between the noble metal and the oxide electrode was a shift of the curves towards more negative potentials. In the presence of Zn²⁺ ions, the current density decreased significantly and the Tafel slope was measured at 282 mV dec⁻¹ showing that the electrode was partially blocked by zinc oxide formation reaction intermediates.     

Conductivity and electrochemical performance of (Ba0.5Sr0.5)0.8La0.2Fe1−xMnxO3−δ cathode prepared by the citrate-EDTA complexing method

This study reports the successful preparation of single-phase perovskite (Ba_0.5Sr_0.5)_0.8La_0.2Fe_1−xMn_xO_3−δ (x = 0-0.2) by the citrate-EDTA complexing method. The crystal structure, thermal gravity analysis, coefficient of thermal expansion, electrical conductivity, and electrochemical performance of (Ba_0.5Sr_0.5)_0.8La_0.2Fe_1−xMn_xO_3−δ were investigated to determine its suitability as a cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The lattice parameter a of (Ba_0.5Sr_0.5)_0.8La_0.2Fe_1−xMn_xO_3−δ decreases as the amount of Mn doping increases. The coefficients of thermal expansion of the samples are in the range of 21.6-25.9 × 10⁻⁶ K⁻¹ and show an abnormal expansion at around 400 °C associated with the loss of lattice oxygen. The electrical conductivity of the (Ba_0.5Sr_0.5)_0.8La_0.2Fe_1−xMn_xO_3−δ samples decreases as the amount of Mn-doping increases. The electrical conductivity of the samples reaches a maximum value at around 400 °C and then decreases as the temperature increases. The charge transfer resistance, diffusion resistance and total resistance of a (Ba_0.5Sr_0.5)_0.8La_0.2Fe_0.8Mn_0.15O_3-δ-Ce_0.8Sm_0.2O_1.9 composite cathode electrode at 800 °C are 0.11 Ω cm², 0.24 Ω cm² and 0.35 Ω cm², respectively.     

Structural and transport effects of doping perfluorosulfonic acid polymers with the heteropoly acids, H3PW12O40 or H4SiW12O40

A perfluorosulfonic acid (PFSA) polymer with pendant side chain -O(CF₂)₄SO₃H was doped with the heteropoly acids (HPAs), H₃PW₁₂O₄₀ and H₄SiW₁₂O₄₀. Infrared spectroscopy revealed a strong interaction between the HPA and the PFSA ionomer. Modes associated with the peripheral bonds of the HPA were shifted to lower wave numbers when doped into PFSA membranes. Small-angle X-ray scattering (SAXS) measurements showed the presence of large crystallites of HPA in the membrane with d spacings of ca. 10 Å, close to the lattice spacing observed in bulk HPA crystals. Under wet conditions the HPA was more dispersed and constrained the size of the sulfonic acid clusters to 20 Å at a 5 wt% HPA doping level, the same as in the vacuum treated ionomer samples. Under conditions of minimum hydration the HPA decreased the E_a for the self-diffusion of water from 27 to 15 kJ mol⁻¹. The reverse trend was seen under 100% RH conditions. Proton conductivity measurements showed improved proton conductivity of the HPA doped PFSAs at a constant dew point of 80 °C for all temperatures up to 120 °C and at all relative hummidities up to 80%. The activation energy for proton conduction generally was lower than for the undoped materials at RH ≤80%. Significantly the E_a was 1/2 that of the undoped material at RHs of 40 and 60%. A practical proton conductivity of 113 mS cm⁻¹ was observed at 100 °C and 80% RH.     

CO2 gas-activated sintering of carbonated hydroxyapatites

Sintering compacts of carbonated hydroxyapatite (CHA) nanoparticles (3.4 wt% CO₃²⁻) in a CO₂ flow (4 mL/min) proceeded at a temperature which was more than 200 °C lower than that for hydroxyapatite in air (1150 °C). During heating from RT to 1200 °C (5 K/min) the rate of shrinkage of the CHA compacts showed a maximum thrice as high as that in air at about 929 °C. The shrinkage correlates with a mass loss caused by the release of CO₂ due to the thermal decomposition of CO₃²⁻ ions that substitute PO₄³⁻ ions in the CHA lattice. Firing the compacts in the CO₂ flow at 800 and 900 °C for 2 h resulted in an additional carbonatation on the B-sites and a further decrease in the sintering temperature to 890 °C. The compacts fired in the 900-1000 °C range became almost complete ceramics with high densities and mechanical properties close to those of medical implants. Firing at temperatures above 1000 °C resulted in an additional carbonatation on the A-sites. However, this led to a material with low densities and poor mechanical properties. A supposition has been proposed that the effect of CO₂ gas-activated sintering is a result of the intensification of the diffusion in the nanoparticles caused by CO₂ molecules entering the bulk from the CO₂ atmosphere and (or) releasing from the bulk due to the decomposition of carbonates on the B-sites in the lattice.     

Defect induced magnetic transition in Co doped CeO2 sputtered thin films

Cobalt-doped cerium dioxide thin films exhibit room temperature ferromagnetism due to high oxygen mobility in doped CeO₂ lattice. CeO₂ is an excellent doping matrix as there is a possibility of it losing oxygen while retaining its structure. This leads to increased oxygen mobility within the fluorite CeO₂ lattice, leading to formation of Ce³⁺ and Ce⁴⁺ species. Magnetic ceria materials are important in several applications from magnetic data storage devices to magnetically recoverable catalysts. In this paper, the room temperature ferromagnetism of rf sputtered Co doped CeO₂ thin films is reported whereas undoped CeO₂ thin films exhibit paramagnetic behavior. The ferromagnetic properties of the Co doped films were explained based on oxygen vacancies created by Co ions in Ce sites. This is further supported by X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) and Raman. Change in surface morphology due to Co doping of the samples were analyzed using atomic force microscopy (AFM).     

Structural and magnetic properties of a stage-2 HoCl3-graphite intercalation compound

A stage-2 HoCl₃-GIC has been synthesized and characterized by XRD experiments and magnetic measurements. EDS and 00l X-ray analyses lead to the chemical formula C_23.8HoCl_3.7.The c-axis repeat distance is 1313 ± 5 pm, and the stacking sequence of intercalated HoCl₃ along the c-axis corresponds to the layer sandwich Cl-Ho-Cl, where the distance between Ho and Cl layers is 157 pm. The HoCl₃ in-plane structure is oblique with lattice parameters a = 689 pm, b = 1177 pm, γ = 149.6° close to those in the pristine crystal. It presents two different micro-domains disorientated with respect to graphene planes by θ₁ = ± 19°. Magnetic studies show that Ho³⁺ ions obey the Curie-Weiss law for both pristine HoCl₃ and stage-2 HoCl₃-GIC. The presence of graphene planes between HoCl₃ intercalated layers leads to an increased Curie-Weiss temperature and to a decrease in the weak antiferromagnetic interactions between Ho³⁺ ions.     

Catalytic oxidation of benzene,toluene and p-xylene over colloidal gold supported on zinc oxide catalyst

Au was loaded (1.5 wt.%) on the supports (ZnO, Al₂O₃ and MgO) by a colloidal deposition method. For a range of low temperatures (50–300 °C), the catalytic activity of Au/ZnO was much greater than that of Au/Al₂O₃ and Au/MgO. In particular, for the Au/ZnO, the benzene conversion exceeded 80% at 150 °C. The results of catalyst characterization suggested that the high catalytic activity of the Au/ZnO might be attributed to the effects of strong metal–oxide interaction which is possibly originated from the small lattice parameter difference between Au {111} and ZnO {101} lattice planes.     

Effect of oxygen deficiency reduction in Mg-doped Mn-spinel on its cell storage performance at high temperature

The effect of doping foreign metal ions into the LiMn₂O₄ spinel structure on the oxygen deficiency by high-temperature firing has been investigated. The degree of the oxygen deficiency in the Mg- and/or Al-doped spinel is very small even when fired at 1000 °C, and then the oxygen deficiency nearly disappeared after low temperature annealing. Mg-doped Mn-spinels (Li_1.05Mn_1.94−xMg_xCa_0.01O_4−z, 0 ≤ x ≤ 0.15) obtained at 1000 °C contained a small oxygen deficiency amount when the x-value reached 0.03 or more. The annealing at 800 °C then reduced the oxygen defect content in the Mg-doped spinels. The crystallinity of the spinels is improved by doping with a small amount of Ti. The storage performance of the coin-type cell at 60 °C is improved about 1.6 times using both Mg- and Ti-doping, which would be due to the oxygen stoichiometric spinels with a low solubility by firing at high temperatures and/or by Ti-doping to increase the crystallinity.