Volume 34 (2016): Issue 3 (September 2016)

Nowadays reduced graphene oxide (rGO) is regarded as a highly interesting material which is appropriate for possible applications in electrochemistry, especially in lithium-ion batteries (LIBs). Several methods were proposed for the preparation of rGO-based electrodes, resulting in high-capacity LIBs anodes. However, the mechanism of lithium storage in rGO and related materials is still not well understood. In this work we focused on the proposed mechanism of favorable bonding sites induced by additional functionalities attached to the graphene planes. This mechanism might increase the capacity of electrodes. In order to verify this hypothesis the composite of non-reduced graphene oxide (GO) with multiwalled carbon nanotubes electrodes was fabricated. Electrochemical properties of GO composite anodes were studied in comparison with similarly prepared electrodes based on rGO. This allowed us to estimate the impact of functional groups on the reversible capacity changes. As a result, it was shown that oxygen containing functional groups of GO do not create, in noticeable way, additional active sites for the electrochemical reactions of lithium storage, contrary to what has been postulated previously.

Microstructural, optical and electrical properties of Cl-doped CdTe crystals grown by the low pressure Bridgman (LPB) method were investigated for four different doping concentrations (unintentionally doped, 4.97 × 10¹⁹ cm⁻³, 9.94 × 10¹⁹ cm⁻³ and 1.99 × 10²⁰ cm⁻³) and three different locations within the ingots (namely, samples from top, middle and bottom positions in the order of the distance from the tip of the ingot). It was shown that Cl dopant suppressed the unwanted secondary (5 1 1) crystalline orientation. Also, the average size and surface coverage of Te inclusions decreased with an increase in Cl doping concentration. Spectroscopic ellipsometry measurements showed that the optical quality of the Cl-doped CdTe single crystals was enhanced. The resistivity of the CdTe sample doped with Cl at the 1.99 × 10²⁰ cm⁻³ was above 10¹⁰ Ω.cm.

The magnetocaloric effect in the Mn_xFe_2−xP_1−yGe_y intermetallic compounds with the amount of Mn in the range of x = 1.05 to 1.17 and amount of Ge in the range of y = 0.19 to 0.22 has been studied. It was found that a higher Ge/P ratio causes an increase in Curie temperature, magnetocaloric effect at low field (up to 1 T), activation energy of structural transition and a decrease in thermal hysteresis, as well as transition enthalpy. Contrary to this observation, higher Mn/Fe ratio causes a decrease in Curie temperature, slight decrease of magnetocaloric effect at low magnetic field, and an increase in thermal hysteresis. Simultaneous increase of both ratios may be very advantageous, as the thermal hysteresis can be lowered and magnetocaloric effect can be enhanced without changing the Curie temperature. Some hints about optimization of the composition for applications at low magnetic fields (0.5 T to 2 T) have been presented.

Spin-polarization (SP) and pressure effects have been used to better clarify and understand anisotropic elastic properties of Fe-B intermetallic compounds using the first-principles calculation with generalized gradient approximation (GGA) within the plane-wave pseudopotential density functional theory. Elastic properties, including bulk, shear and Young’s moduli as well as Poisson ratio were obtained by Voigt-Reuss-Hill approximation. All studied Fe-B compounds were mechanically stable. The brittle and ductile properties were discussed using bulk to shear moduli ratio (B/G) of the studied structures in the pressure range of 0 GPa to 90 GPa in order to predict the critical pressure of phase transition from ferromagnetic (FM) to nonmagnetic (NM) state. Mechanical anisotropy in both cases was discussed by calculating different anisotropic indexes and factors. We have plotted three-dimensional (3D) surfaces and planar contours of the bulk and Young’s moduli of Fe_xB (x = 1, 2, 3) compounds for some crystallographic planes, (1 0 0), (0 1 0) and (0 0 1), to reveal their elastic anisotropy. On the basis of anisotropic elastic properties the easy and hard axes of magnetization for the three studied compounds were predicted.

Electron paramagnetic resonance (EPR) spectra of M₃Fe₄V₆O₂₄ (M = Cu, Zn, Mg and Mn) compounds in high temperature range (293 K to 493 K) have been investigated. The role of magnetic (Cu, Mn) and non-magnetic (Zn, Mg) ions in M₃Fe₄V₆O₂₄ structure in formation of magnetic resonance spectra was studied. Temperature dependence of EPR parameters: resonance field, linewidth and integrated intensity were examined. Similarities and differences in temperature behavior of these parameters has been discussed in terms of different relaxation mechanisms and magnetic interactions in the spin systems. An important role of additional magnetic ions (M = Mn or Cu) in the M₃Fe₄V₆O₂₄ structure has been identified and its consequences considered.

This paper presents the effects of elimination of current oscillations within the coaxial plasma accelerator during IPD deposition process on the morphology, phase structure and properties of synthesized TiN coatings. Current observations of waveforms have been made by use of an oscilloscope. As a test material for experiments, titanium nitride TiN coatings synthesized on silicon and high-speed steel substrates were used. The coatings morphology, phase composition and wear resistance properties were determined. The character of current waveforms in the plasma accelerator electric circuit plays a crucial role during the coatings synthesis process. Elimination of the current oscillations leads to obtaining an ultrafine grained structure of titanium nitride coatings and to disappearance of the tendency to structure columnarization. The coatings obtained during processes of a non-oscillating character are distinguished by better wear-resistance properties.

PbMoO₄ and PbWO₄ were successfully synthesized by microwave radiation using different lead salts (acetate, chloride, nitrate and sulfate) and Na₂MO₄ (M = Mo, W) in propylene glycol. The products were characterized by X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM, TEM), Fourier transform infrared (FT-IR), Raman spectroscopy and photoluminescence (PL) spectroscopy. In this research, morphologies, crystallization and photoluminescence of the products were influenced by the kinetics of anions, including the detection of M–O (M = Mo, W) stretching modes in the (MO₄)²⁻ tetrahedrons. Photoluminescence of PbMoO₄ synthesized from Pb(NO₃)₂ and of PbWO₄ synthesized from PbCl₂ showed the strongest blue emission due to the electronic diffusion in tetrahedrons at room temperature.

Synthesis and complex theoretical and experimental studies of Co/TiO₂ anatase have been reported. The preparation of Co/TiO₂ was carried out by sol-gel method. Distribution of cations among the two tetrahedral and octahedral sites was estimated by analyzing the powder X-ray diffraction patterns by employing Rietveld refinement technique, and the results revealed the existence of tetragonal structure. Band structure and density of states calculations were performed using the first-principles methods. The structural and electronic properties of Co/TiO₂ were calculated in the general gradient approximation (GGA). An additional comparison with pure TiO2 anatase allowed us to clarify cobalt doping effect on the electronic structure and the band gap. The band gap of Co/TiO₂ was decreased by broadening the valence band as a result of the overlap among Co 3d, Ti 3d, and O 2p states, which made it respond better to visible and solar light.

In this study, a solar cell with a glass/ITO/CdS/PbS/Al structure was constructed. Both window (CdS) and absorption (PbS) layers were deposited by chemical bath deposition (CBD) method. The CdS window layer was deposited on ITO-glass. The PbS nanocrystalline thin film was prepared with and without triethanolamine on CdS films at bath temperature of 25 °C. CdS and PbS nanocrystals were identified using XRD and SEM. The cells are photosensitive in a large spectral range (at visible and near infrared regions). The cell with absorbing layer obtained from the bath without TEA has higher efficiency with the following parameters: the open circuit voltage (V_oc) is 275 mV, short circuit current (J_sc) is 12.24 mA/cm², maximum voltage (V_max) is 165 mV and maximum current (J_max) is 7.11 mA/cm² with the efficiency η = 1.31 %, fill factor FF is 32 % under the illumination intensity of 90 mW/cm². The cells have an area of 0.15 cm².

The present study is focused to explore the photonic device applications of _L-arginine doped ZTC (LA-ZTC) crystals using nonlinear optical (NLO) and dielectric studies. The LA-ZTC crystals have been grown by slow evaporation solution technique. The chemical composition and surface of LA-ZTC crystal have been analyzed by means of energy dispersive spectroscopy (EDS) and surface scanning electron microscopy (SEM) techniques. The Vicker’s microhardness study has been carried out to determine the hardness, work hardening index, yield strength and elastic stiffness of LA-ZTC crystal. The enhanced SHG efficiency of LA-ZTC crystal has been ascertained using the Kurtz-Perry powder SHG test. The closed-and-open aperture Z-scan technique has been employed to confirm the third order nonlinear optical nature of LA-ZTC crystal. The Z-scan transmittance data has been utilized to calculate the superior cubic susceptibility, nonlinear refractive index, nonlinear absorption coefficient and figure of merit of LA-ZTC crystal. The behavior of dielectric constant and dielectric loss of LA-ZTC crystal at different temperatures has been investigated using the dielectric analysis.

Zinc acetate was used as a starting material to prepare Zn-solutions from solvents and ligands with different boiling temperature. The ZnO thin films were prepared on Si(1 0 0) substrates by spin-coating method. The effect of baking temperature and boiling temperature of the solvents and ligands on their morphologies and orientation was investigated. The solvents and ligands with high boiling temperature were favorable for relaxation of mechanical stress to form the smooth ZnO thin films. As the solvents and ligands with low boiling temperature were used to prepare Zn-solutions, the prepared ZnO thin films showed (0 0 2) preferred orientation. As n-propanol, 2-methoxyethanol, 2-(methylamino)ethanol and monoethanolamine were used to prepare Zn-solutions, highly (0 0 2)-oriented ZnO thin films were formed by adjusting the baking temperature.

Silver and gold nanoparticles have been linked with the structure of polyacrylonitrile (PAN) fibers as a result of a synthesis. The nanoparticles were formed in the dimethylformamide (DMF) solution, which is uncommon for their synthesis, but is a typical solvent for polyacrylonitrile. The examination of metallic nanoparticles doped fibers showed the presence of these particles in the samples. Analysis of Energy Dispersive X-ray Spectrometry (EDX) results showed that silver content in the fibers was the same as in the precursors of metal ions. The amount of silver in the fibers was significantly greater than the amount of gold in the corresponding sample. The Dynamic Light Scattering Method (DLS) and Visible and Ultraviolet Spectroscopy (UV-Vis) analyses indicated a difference between the sizes of metallic nanoparticles. The obtained composite polyacrylonitrile fibers doped with silver and gold nanoparticles were characterized by high flexibility and reduced electromagnetic radiation. The authors are convinced that the results obtained during the research will considerably contribute to the development of textronics discipline.

Nanocrystalline ZnO-TiO₂ (with molar ratios 9:1, 7:3, 1:1, 3:7 and 1:9) were successfully synthesized by hydrothermal method. Synthesized materials were examined with the help of X-ray diffraction and transmission electron microscope. Liquid petroleum gas sensing characteristics of the ZnO-TiO₂ films were investigated at different operating temperatures. The ZnO-TiO₂ thick film (with 1:1 molar ratio) exhibited good response toward liquid petroleum gas as compared to other investigated compositions. Further, liquid petroleum gas sensing characteristics of CuO modified ZnO-TiO₂ thick films were investigated. 0.2 M CuO modified ZnO-TiO₂ thick film exhibited excellent liquid petroleum gas sensing characteristics such as higher response (~ 1637.49 at 185 °C) with quick response time (~30 s), low recovery time (~70 s), excellent repeatability and stability at low operating temperature.

In high Tc superconductors (HTSC) the activation energy gives information about the pinning properties of a sample under applied magnetic field. Pinning of vortices determines the critical current density Jc which is of great importance for practical applications of HTSC. Instead of magnetic measurements, the activation energy may be calculated from resistivity measurements realized under magnetic field. This kind of measurement has been made in this work for yttrium doped samples of Bi₂Sr₂CaCu₂O_8+d (Bi-2212) for different values of applied magnetic field. Samples of Bi₂Sr₂Ca_1-xY_xCu₂O_8+d (x = 0, 0.025, 0.1, 0.25) were prepared by a sol-gel method and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive analysis of X-ray. The measurements of resistivity were made using a classical four probe method and DC current. The magnetic field was applied with a constant amplitude of 0 T, 1 T, 2 T and 3 T. The obtained results show that the activation energy decreases with introduction of yttrium, but has a relative maximum when x is equal 0.1. The decrease of the activation energy is explained by the granular nature of the samples which promotes 3D transition to 2D of the vortex lattice.

Antimony is a promising material for the fabrication of photodetectors. This study deals with the growth of a photosensitive thin film by the physical vapor deposition (PVD) of antimony onto mica surface in a furnace tube. The geometry of the grown structures was studied via scanning electron microscopy (SEM), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX) and elemental diffraction analysis. XRD peaks of the antimony film grown on mica mostly matched with JCPDF Card. The formation of rhombohedral crystal structures in the film was further confirmed by SEM micrographs and chemical composition analysis. The Hall measurements revealed good electrical conductivity of the film with bulk carrier concentration of the order of 10²² Ω·cm^-3 and mobility of 9.034 cm²/Vs. The grown film was successfully tested for radiation detection. The photoresponse of the film was evaluated using its current-voltage characteristics. These investigations revealed that the photosensitivity of the antimony film was 20 times higher than that of crystalline germanium.

Calcination is considered to increase the hardness of composite material and prevent its breakage for the effective applications in environmental remediation. In this study, magnetic biochar amended with silicon dioxide was calcined at high temperature under nitrogen environment and characterized using various techniques. X-ray diffraction (XRD) analysis revealed elimination of Fe₃O₄ peaks under nitrogen calcination and formation of Fe₃Si and iron as major constituents of magnetic biochar-SiO₂ composite, which demonstrated its superparamagnetic behavior (>80 A²·kg⁻¹) comparable to magnetic biochar. Thermogravimetric analysis (TGA) revealed that both calcined samples generated higher residual mass (>96 %) and demonstrated better thermal stability. The presence of various bands in Fourier transform infrared spectroscopy (FT-IR) was more obvious and the elimination of H–O–H bonding was observed at high temperature calcination. In addition, scanning electron microscopy (SEM) images revealed certain morphological variation among the samples and the presence of more prominent internal and external pores, which then judged the surface area and pore volume of samples. Findings from this study suggests that the selective calcination process could cause useful changes in the material composites and can be effectively employed in environmental remediation measures.

The most attractive property of Li_0.5La_0.5TiO₃ (LLTO) electrolytes is their high ionic conductivity. Studies have shown that LLTO is capable of existing in a state with an ionic conductivity of 10^-3 S/cm, which is comparable to liquid electrolytes. In addition to the high ionic conductivity of the material, LLTO is electrochemically stable and able to withstand hundreds of cycles. So, the studies of the solid electrolyte material are very important for the development of lithium-ion batteries. In the present paper, Li_0.5La_0.5Ti_1-xZr_xO₃ (x = 0.05 and 0.1) have been prepared by a solid-state reaction method at 1300 °C for 6 hours to improve electrolyte materials for lithium-ion batteries. The phase identified by X-ray diffractometry and crystal structure corresponds to pm3m (2 2 1) space group (Z = 1). The frequency and temperature dependence of impedance, dielectric permittivity, dielectric loss and electric modulus of the Li_0.5La_0.5Ti_1-xZr_xO₃ (x = 0.05 and 0.1) have been investigated. The dielectric and impedance properties have been studied over a range of frequency (42 Hz to 5 MHz) and temperatures (30 °C to 100 °C). The frequency dependent plot of modulus shows that the conductivity relaxation is of non-Debye type.

The hypothetical stoichiometric CeBa₂Cu₃O₇ (Ce123) compound, which has not been synthesized as a single phase yet, was studied by the density functional theory (DFT). We utilized a method which merges the local spin density approximation (LSDA) with the dynamical mean-field theory (DMFT) to account for the electronic correlations. The LSDA+DMFT calculations were performed in the high-temperature range. The particular emphasis was put on the pressure-induced changes in the electronic band structure related to strongly correlated 4f states. The computational results indicate the occurrence of a large negative volumetric thermal expansion coefficient near T = 500 K and a trace of a low-volume isostructural metastable state at high temperatures.

In this study a graphene-like material was synthesized by chemical exfoliation of sieved graphite powder with an average particles size ≤100 µm in dispersing medium from tri-, di- and monochloroacetic acid sulfonated with 55 % H₂SO₃. The results indicated that the yield of graphene-like material was the best in case of trichloroacetic 55 % H₂SO₃, taking into account structure quality and amount of obtained material. Layered graphene-like material was carefully collected after reduction of GO formed by 50 % dimethylhydrazine. AFM-investigations were performed to characterize nanostructural features of produced graphene and expected surface area which is surface topology dependent. Furthermore, Raman spectra were measured to confirm graphene formation.

Transparent TiO₂ monoliths were obtained through a modified sol-gel route from titanium isopropoxide as a precursor. By controlling the hydrolysis of this precursor through the intermediate of esterification reaction between acetic acid and isopropanol at 40 °C, transparent TiO₂ xerogel monoliths were obtained. The monoliths prepared by this method were transparent in the wavelengths between 400 nm and 700 nm. Fourier transform infrared (FT-IR) spectroscopy suggested that the acetic acid played also an active role as a chelating agent, forming Ti[(OH)_y(OOCCH₃)_x] less reactive species. Powder X-ray diffraction confirmed the amorphous-to-anatase phase transformation with the formation of unknown Ti-containing complex at 90 °C. Only anatase TiO₂ could be observed in the samples calcined at 250 °C and 450 °C. Optical aspects of the gel (transparent-transluscent transformation of monolithic gel) and gelation time were controlled by changing the amount of external water.

In this paper we present the results of investigation of micro- and nanoscale degradation of a sheet moulded composite exposed to simulated solar radiation. Utilization of high resolution methods such as atomic force microscopy, optical profilometry and microcomputer tomography allowed us to provide the evidence of significant deterioration of the surface as well as the material few microns in depth. Additionally, the typically used macroscopic investigations, such as wettability and flexural strength, were performed to observe the impact of weathering process. It was also shown that high resolution techniques provide superior sensitivity of the material degradation detection. The particular effectiveness of the applied approach was related to the structure of investigated material, as due to its degradation, a number of voids appeared, causing a significant roughness increase. In addition, the impact of light radiation could be compared to other environmental conditions maintained in the climatic chamber. It should be underlined, that according to our knowledge, such a study has not been performed so far.

Ferroelectric composite thin films of x-SBT/PVDF with different SBT content (weight ratios of SBT to PVDF, x = 0 %, 5 %, 10 %, 15 %, 20 %) were prepared by spin-coating method. The crystal structures of x-SBT/PVDF films were analyzed by X-ray diffraction (XRD) measurements and Fourier transform-infrared spectroscopy (FT-IR), respectively. Experimental results demonstrated that both α, β-phases PVDF and the layered perovskite SBT co-existed in the x-SBT/PVDF samples. With an increase of SBT content in the x-SBT/PVDF thin films, both the dielectric constant and the saturated polarization were also increased, compared with those of pure PVDF thin film. More importantly, when the SBT content in the x-SBT/PVDF thin films was larger than 15 %, the coercive field of x-SBT/PVDF thin films was also decreased.

In this study, a new one step paired electrochemical method is developed for simultaneous synthesis of iron and iron oxide nanoparticles. iron and iron oxide are prepared as cathodic and anodic products from iron (ii) sulfate aqueous solution in a membrane divided electrolytic cell by the pulsed current electrosynthesis. Because of organic solvent-free and electrochemical nature of the synthesis, the process could be considered as green and environmentally friendly. The reduction of energy consumption and low cost are the other significant advantages of this new method that would have a great application potential in the chemical industry. The nanostructure of prepared samples was characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The magnetic properties were studied by vibrating sample magnetometer (VsM).

Pure ZnO and Ce-doped (5 at.%) ZnO aerogels were prepared by dissociation of dihydrate zinc acetate and cerium nitrate in methanol, followed by drying in supercritical conditions of the solvent. The concentration of zinc acetate solution and the atomic ratio Ce/Zn were fixed at 0.2 mol/L and 0.05 mol/L, respectively. XRD results showed a hexagonal wurtzite structure of ZnO aerogel crystallites and a formation of a separate second phase of ceria cubic phase with fluorite structure and 7.4 nm in size. The introduction of cerium in the solution had a negative effect on the aerogel crystalline quality. The crystallites size was found to be 16 nm in pure ZnO and 27 nm in Ce-doped ZnO aerogels. The grains of Ce-doped ZnO aerogel had torus shaped morphology with hollow centers, but those of pure ZnO were flattened semispheres. The calculated values of different structural parameters showed that cerium ions introduced into the ZnO lattice occupied interstitial sites and Zn ions substituted Ce ones in ceria lattice during the formation process. FT-IR and UV-Vis absorption spectra have not revealed any particularities due to the presence of cerium atoms in ZnO, indicating that Ce (5 at.%) doping of ZnO crystallites synthesized in supercritical methanol did not strongly affect the optical gap of the semiconductor. Micro-Raman studies confirmed the formation of cubic fluorite structure ceria in ZnO aerogel and showed that Raman active modes of ZnO are amplified with the presence of ceria.

Optical fiber in conjunction with ZnTe quantum dots (QDs) is investigated for sensing application. ZnTe QDs, are synthesized by a simple chemical bottom up approach. Quantum dots are capped with L-Cystein ethyl ester hydrochloride (LEEH), to increase their stability. Then LEEH capped ZnTe QDs, whose size is estimated as 2.29 nm by effective mass approximation (EMA), are dip-coated on a cladding removed optical fiber. Different concentrations of alcohol and ammonia are used to investigate the sensing behavior. It is found that sensitivity of the sensor increases with the use of QDs for both alcohol and ammonia.

Zr substituted 0.8BaTiO₃0.2 Bi_0.5K_0.5TiO₃ lead free ceramic materials with a composition 0.8Ba0.2(Bi_0.5K_0.5)Ti_1-xZr_xO₃ (0.01 ≤ x ≤ 0.06) were prepared by conventional solid state reaction method followed by high energy ball milling. The X-ray diffraction studies confirmed the tetragonal structure of the material at room temperature. Density was decreasing with the substitution of Zr. Microstructure studies were done by using scanning electron microscope. Temperature and frequency dependent dielectric studies were carried out and showed that the dielectric constant, dielectric loss and Curie temperature were decreasing with the substitution of Zr. Relaxor behavior was observed in all the Zr substituted samples. Degree of diffuseness was calculated from the modified Curie-Weiss law and it was found to increase with the substitution of Zr. Frequency and temperature dependent AC conductivity was calculated and it was found to obey Jonscher’s power law. Substitution of Zr decreased the conductivity of the material. Activation energy was calculated and it was decreasing with an increase in Zr substitution.

The cobalt phthalocyanine (CoPc) thin films (300 nm thick) deposited on n-type silicon substrate have been studied using micro-Raman spectroscopy, atomic force spectroscopy (AFM) and I-V measurement. The CoPc thin layers have been deposited at room temperature by the quasi-molecular beam evaporation technique. The micro-Raman spectra of CoPc thin films have been recorded in the spectral range of 1000 cm^-1 to 1900 cm^-1 using 488 nm excitation wavelength. Moreover, using surface Raman mapping it was possible to obtain information about polymorphic forms distribution (before and after annealing) of metallophthalocyanine (α and β form) from polarized Raman spectra. The I-V characteristics of the Au/CoPc/n-Si/Al Schottky barrier were also investigated. The obtained results showed that influence of the annealing process plays a crucial role in the ordering and electrical conductivity of the molecular structure of CoPc thin films deposited on n-type silicon substrate.

Cadmium sulfide (CdS) nanorods with a diameter of 50 nm and length of approximately 200 nm have been synthesized using combined sonochemical-solvothermal method. Structural properties of CdS nanoparticles synthesized by this method have been compared with the CdS nanoparticles synthesized by sonochemical method alone. The synthesized CdS nanostructures have been characterized using X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX) and transmission electron microscopy (TEM) methods. In addition, the factors affecting the formation of the structures, including reaction time, different type and ratio of precursors, such as sulphur source, have been investigated. Comparison of the results obtained by both the synthesis methods revealed CdS nanoparticles synthesized by the combined sonochemical-solvothermal method to be of high morphological homogeneity compared to the sonochemical method alone. It is interesting to note that ethylenediamine has been found to be prevented from agglomeration by using the combined sonochemical-solvothermal method as the synthesis method. A modified growth mechanism under the inducement of ethylenediamine solutions for the CdS nanorods has been suggested.