Synthesis, characterization and photoluminescence properties of (Gd0.99,Pr0.01)2O2S sub-microphosphor by homogeneous precipitation method

Homogeneous precipitation method for synthesizing (Gd_0.99,Pr_0.01)₂O₂S sub-microphosphor was developed, using the commercially available Gd₂O₃, Pr₆O₁₁, H₂SO₄ and (NH₂)₂CO (urea) as the starting materials. It was found that the as-synthesized precursor is mainly composed of (Gd_0.99,Pr_0.01)₂(OH)₂(CO₃)(SO₄)·nH₂O. Pure quasi-spherical shaped (Gd_0.99,Pr_0.01)₂O₂S particles can be synthesized by calcining the precursor at a temperature higher than 700 °C for 1 h in flowing hydrogen. The (Gd_0.99,Pr_0.01)₂O₂S particles have a narrow size distribution with a mean grain size of about 300-400 nm. Photoluminescence spectra of (Gd_0.99,Pr_0.01)₂O₂S under 303 nm UV excitation show a green emission at 515 nm as the most prominent peak, which corresponds to the ³P₀ → ³H₄ transition of Pr³⁺ ions. Decay study reveals that the ³P₀ → ³H₄ transition of Pr³⁺ ions in Gd₂O₂S host lattice has a single exponential decay behavior.     

Preparation, phase formation and photoluminescence properties of ZnO-SiO2-B2O3 glasses with different ZnO/B2O3 ratios

Glasses in ZnO-SiO₂-B₂O₃ ternary system with different ZnO/B₂O₃ ratios were studied as scintillating materials. Differential thermal analysis and X-ray diffraction patterns showed that nucleation of willemite and zinc oxide was considerably facilitated with increasing the ZnO/B₂O₃ ratio. Photoluminescence spectra showed a reduction in intensity over the UV region upon this increment prior to the formation of crystalline phases. Optical basicity as a measure of nonbridging oxygens (NBOs) is elaborated in this study as a major cause of this effect. However, near band edge emission (NBE) after crystallization implied an increase in intensity upon the increment of the ZnO/B₂O₃ ratio which proved the key role of willemite and zinc oxide as UV emission centers.     

Synthesis and characterization of Sb2Se3 nanorods

A hydrothermal reduction route was employed for the synthesis of Sb₂Se₃ semiconductor nanorods. The reaction temperature for the formation of Sb₂Se₃ nanorods should be above 130°C, otherwise impurities, such as Sb₂O₃ and unreacted Se, would exist. The role of hydrazine as both the reducing agent and the coordinator was also found to be crucial for the formation of the rod-like morphologies. The products were characterized by XRD, SEM, TEM, XPS, absorption spectroscopy and Raman spectroscopy.     

Role of surface properties of MoO3-doped SnO2 thin films on NO2 gas sensing

The role of surface morphology of MoO₃-doped SnO₂ thin film on the gas sensing properties is analyzed. SnO₂ thin films doped with 1, 3, 5 and 10 wt% MoO₃ are prepared by sol-gel spin coating process. Structural and morphological properties are studied using glancing angle X-ray diffractometer, atomic force microscopy, transmission electron microscopy and high resolution transmission electron microscopy. Energy dispersive X-ray analysis and X-ray photoelectron spectroscopy studies are used for chemical analysis. A good correlation is found between the characteristics of the surface and gas sensing properties of these films. MoO₃ addition is found responsible for increase in acidic nature of films which in turn increases their sensitivity and selectivity towards NO₂ gas.     

Synthesis,characterization and photocatalytic activity of ZnO-SnO2 nanocomposites

Nanocomposites of coupled ZnO-SnO₂ photocatalysts were synthesized by the coprecipitation method and were characterized by X-ray diffraction, UV–vis diffuse reflectance spectroscopy, surface area analyzer and scanning electron microscopy. Their photocatalytic activity was investigated under UV, visible and solar light and evaluated using methylene blue (MB) as a model pollutant. The performance of the coupled ZnO-SnO₂ photocatalysts was found to be related to the Zn/Sn molar ratio and to the calcination conditions. The photocatalyst with a Zn/Sn molar ratio of 1:0.05 calcined at 600 °C for 2 h showed the maximum degradation rate of MB under different lights used. Its photocatalytic activity was found to be about two times that of ZnO and about 10 times that of SnO₂ which can be explained by the heterojunction effect. Charge separation mechanism has been studied.     

Fundamental studies on SnO2 by means of simultaneous work function change and conduction measurements

Simultaneous work function change and conduction measurements have been performed on undoped SnO₂ thick film sensors in order to investigate the surface reactions of CO under dry and humid conditions. On their basis, changes in the electron affinity have been observed for the reaction with water and the reaction with CO in the background of water. Additionally, by comparing the results for the changes in band bending, one can conclude that both water and CO compete for the same reaction sites, i.e. preadsorbed oxygen species. The kinetics of work function and band bending changes show that in the case of humid air more than one dipole species is involved in the reaction with CO.     

Experimental studies of O2-SnO2 surface interaction using powder, thick films and monocrystalline thin films

Surface properties of solids and the interactions between molecules and solid surfaces are important for many technical applications. They also involve a range of physical and chemical phenomena of fundamental scientific interest. The importance of oxygen chemistry at SnO₂ surfaces follows from the fact that SnO₂ is used as an active material in gas sensor applications. The operation principle of these sensors is usually based on measurable conductance response of the material, which is understood in terms of reactions of gas molecules with different oxygen species adsorbed onto the surface. The role of the lattice oxygen, but in particular, the bridging oxygen atoms on SnO₂ surfaces, is also active. Detailed understanding of the reaction mechanisms of various oxygen species at SnO₂ surfaces is important, as it offers a way to improve the sensitivity and selectivity of the sensors.Oxygen adsorption-desorption kinetics at the SnO₂ surface is studied experimentally using O₂-temperature-programmed desorption (TPD) method together with conductance measurements in the case of SnO₂ powder and polycrystalline thick films made from the powder. In addition, CO-TPD is studied and the transient behaviour of various oxygen species is considered. Molecular beam epitaxy (MBE) was also used to fabricate polycrystalline and monocrystalline thin films with the SnO₂(101) face on single crystal sapphire substrate. Simultaneous surface potential and conductance measurements during heating and cooling in different ambient atmospheres were used to characterize the monocrystalline SnO₂(101) surface after various surface treatments.     

Synthesis and characterization of P-doped TiO2 nanotubes

Titanium dioxide (TiO₂) doped with phosphorus (P) was synthesized by anodization of Ti in the mixed acid electrolyte of H₃PO₄ and HF and characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and UV–vis spectrum. The morphology greatly depends on the applied voltage. The as-formed nanotubes under the optimized condition, at 20 V for 2 h, are highly ordered with ~ 200 nm in length and the average tube diameter is about 100 nm. By annealing the initial samples at different temperatures, the importance of the crystalline nature is confirmed. Significantly, the peak positions of anatase in XRD patterns shifts to lower diffraction angles with an increase in the amount of H₃PO₄ ion. A remarkable red shift of the absorption edge has been observed for the sample formed in the electrolyte of HF and H₃PO₄, which is related to the introduction of P⁵⁺ into TiO₂ crystallization and the possible impurity energy level formed in the TiO₂ band gap. The presence of P 2p state in XPS spectrum can further confirm the P⁵⁺ which can replace a part of Ti⁴⁺ has been introduced into TiO₂ crystallization. The present convenient synthesis technique can be considered to the composition of other doped oxide materials.     

Stimulated photoluminescence and optical limiting in CdI2

High quality single crystal of cadmium iodide (CdI₂) grown from the melt by the refining method was studied under ruby laser excitation. The low temperature (LNT) emission spectrum of the crystal was recorded by using a spectrofluorometer. The spectrum appeared a structureless emission with its peak in the green spectral region. Analysis of photoluminescence (PL) measurements in the crystal showed a quadratic dependence of the PL peak intensity on incident laser power. This might be explained with the existence of self-trapped excitons in the crystal. In the desired energy ranges of interest, the emission intensity of CdI₂ was found to depend on the square of the laser power, indicating the biphotonic process of two-photon excitation, i.e., the nonlinear response is due to a second-order process, or two-photon absorption (2PA) process. The optical limiting properties of CdI₂ were studied using 7 ns pulses from the laser. This investigation leads to the conclusion that apart from the 2PA reverse saturable absorption (RSA) is another mechanism for optical limiting in CdI₂.     

N-doped ZnO nano-arrays: A facile synthesis route, characterization and photoluminescence

Field emission scanning electron microscopy (SEM) investigation reveals that array-orderly novel nanostructures, which are nanorods with many nanoparticles on the surfaces, have been synthesized at low temperature (162 °C) via a one-step in-situ process in solution. High resolution transmission electron microscope (HRTEM) and energy-dispersive X-ray spectroscopy (EDS), coupled with X-ray powder diffraction (XRD) patterns and X-ray photoelectron spectra (XPS), reveal that the as-obtained products possess crystalline structure of N-doped ZnO. The room temperature photoluminescence (PL) spectrum has also been examined to explore the optical property. The present synthesis method possesses several advantages, which would be significant to be studied deeper in the future. It is also envisioned that this method could provide a new approach to synthesize ZnO:N and other ZnO-based adulterants at low temperature.     

Synthesis of novel pompon-like porous SnO2 and its application in lithium-ion battery

Novel pompon-like porous SnO₂ with an average diameter of 900 nm has been successfully synthesized via a simple hydrothermal process with subsequent calcination treatment at 600 °C for 2 h in air. The crystalline structure and morphology of the resulting product were characterized by X-ray diffraction, micro-Raman spectrometer, field-emission scanning electron microscopy and transmission electron microscopy. The results indicate that the product is composed of self-assembly SnO₂ pompon with a high purity tetragonal rutile-like structure. The lithium storage property of the obtained pompon-like porous SnO₂ was evaluated by conventional discharge/charge test, showing a high initial discharge capacity of 1895 mAh g^-1 at a current density of 100 mA g^-1.     

Preparation and photoluminescence properties of Eu-doped Ga2O3 nanorods

GaOOH:Eu³⁺ nanorods with different aspect ratios were prepared by hydrothermal method at 140 °C. - and β-Ga₂O₃:Eu³⁺ were converted from as-prepared GaOOH:Eu³⁺ particles by calcination at 500 and 850 °C, respectively. The products were characterized with X-ray diffraction (XRD), transmission electron microscope (TEM) and photoluminescence (PL). Results show that solution pH values play a key role in the formation of the GaOOH:Eu³⁺ powders with different morphologies and - and β-Ga₂:Eu³⁺ inherit the morphology of GaOOH:Eu³⁺ exactly. The photoluminescence characteristics of β-Ga₂O₃:Eu³⁺ were also investigated. Experimental results reveal that the color purity of β-Ga₂O₃:Eu³⁺ nanorods with high aspect ratio is enhanced in comparison with β-Ga₂O₃:Eu³⁺ nanorods with low aspect ratio.     

Hydrothermal synthesis and photoluminescence properties of Gd2Zr2O7:Tb phosphors

This paper presents hydrothermal synthesis, characterization, and photoluminescence (PL) properties of novel green-emitting phosphors, Gd₂Zr₂O₇:Tb³⁺. Their crystal structure, morphology and photoluminescence properties were investigated by X-ray powder diffraction (XRD), field emission scanning electron microscopy (FE-SEM), Transmission electron microscopy (TEM) and fluorescence spectrophotometer. The results revealed that one-dimensional Gd₂Zr₂O₇:Tb³⁺ nanorods with diameter of about 30 nm and length of 150-300 nm were formed, and the products exhibited a fluorite-type structure. PL study revealed that Gd₂Zr₂O₇:Tb³⁺ phosphors presented dominant green emission luminescence, which was attributed to the transitions from ⁵D₄ excited states to ⁷F_J (J = 3-6) ground states of Tb³⁺. The luminescence intensity of Gd₂Zr₂O₇:Tb³⁺ with different Tb³⁺ concentration was also investigated and reported, and an obvious concentration quenching was observed when Tb³⁺ ion concentration was 5 at.%.     

Peculiarities of photoluminescence of Al2O3 bulk and nanosize powders at low temperatures

Photoluminescence has been studied in the aluminum oxide (Al₂O₃) bulk and nanosize powders in the 300-8 K temperature range. In both samples luminescence spectrum is characterized by presence of broad blue and red bands caused mainly by emission from the uncontrolled titanium impurity. At low temperatures luminescence intensity increases by several times and the red band obtains fine structure. The nature of the fine structure is discussed suggesting manifestation of splitting of the Ti³⁺ emitting level due to Jahn-Teller effect or overlapping of Ti³⁺ emission band with narrow lines from other emitting ions. The observed differences in low-temperature spectral features of nanopowder compared to its bulk counterpart are explained by lattice structure of nanopowder, which belongs to transition phase of Al₂O₃.     

Magnetocapacitance in BaTiO3-CoFe2O4 nanocomposites

We report on the magnetoelectric characterization of epitaxial CoFe₂O₄-BaTiO₃ nanocomposites grown by rf sputtering on Nb-doped SrTiO₃ (001). Multiferroicity of the material is corroborated with both, ferroelectric and ferromagnetic hysteresis loops. Magnetoelectric coupling is investigated via magnetocapacitance measurements; a change of the capacitance of about 2% is observed for an applied magnetic field of 8 T. The possible origins of this magnetocapacitance are discussed.     

Preparation, structure analysis and photoluminescence properties of MgGa2O4:Mn

In this work the photoluminescence and excitation spectra at room temperature of the spinel-type MgGa₂O₄ with 0.5% and 10.0% of Mn²⁺ have been studied. The polycrystalline samples were synthesized by standard solid-state reaction methods at high temperature. The photoluminescence spectra exhibit green and red emissions for both samples, attributed to ⁴T₁(⁴G) → ⁶A₁(⁶S) transition of Mn²⁺ ion in tetrahedral and octahedral sites of oxygen, respectively. The excitation spectra exhibit features unambiguously assigned to d–d transitions of Mn²⁺ in those kinds of sites. From the excitation spectra and Tanabe–Sugano matrices the crystal field Dq and Racah B parameters were obtained.     

Synthesis and characterization of Zn/Al2O3 nanocomposite by mechanical alloying

In this study, fabrication and characterization of zinc-based metal matrix nanocomposite reinforced by Al₂O₃ particles was investigated. Aluminum and zinc oxide powder mixture was milled in a planetary ball mill in order to produce Zn/Al₂O₃ nanocomposite. The structural evaluation milled and annealed powders studied by X-ray diffraction, SEM observation and hardness measurement. The zinc crystallite size estimated with broadening of XRD peaks by Williamson-Hall formula. The zinc oxide was found to react with Al through a rapid self-sustaining combustion reaction process. As a result a zinc matrix composite reinforced by Al₂O₃ particulate was formed. The microhardness value of produced nanocomposite powder was about 350 HV which was 10–15 times higher than the microhardness of pure zinc (20–30 HV).     

Preparation of Co and Fe nanocomposites in SiO2 matrices

Cobalt and iron nanocomposites have been prepared by in situ reduction of metallic salts in SiO₂ gel matrices. The presence of nanosized metal phases has been established chemically as well as by selected area diffraction and transmission electron microstructural analyses. Studies on temperature and time dependent fractional reduction revealed that the reduction kinetics are fast.     

Nanocrystalline SnO2 and In2O3 as materials for gas sensors: The relationship between microstructure and oxygen chemisorption

The correlations between microstructure of nanocrystalline TCO SnO₂ and In₂O₃ and parameters of oxygen chemisorption are analyzed. Nanocrystalline SnO₂ and In₂O₃ were prepared by wet chemical method. The sample's microstructure was characterized by TEM, XRD and low-temperature nitrogen adsorption. Electrical properties of TCO were studied at 200-400 °C depending on the oxygen partial pressure. Increase of TCO grain size leads to the increase of the fraction of atomic forms of chemisorbed oxygen at the fixed temperature. It could be due to the decrease of surface barrier resulting in the decrease of activation energy of dissociation of molecular ion O_2(ads)⁻.