Volume 114, Issue 2

This review focuses on the trends and challenges, over the last ten years, in the development of electrochemical sensors based on organic conducting polymers and graphene composites for the determination of trace heavy metal ions in water. Some of these materials taken alone still have significant limitations for the selective and ultrasensitive detection of target species. Hence, it has become crucial to develop new composite materials able to overcome these limitations and to improve the sensitivity to heavy metal ions. The properties resulting from the combination of these two types of materials, which increased the electrochemical performance by offering many advantages such as improvement of catalytic activity and conductivity, fast electron transfer kinetics, large surface area and high sensitivity were reviewed. This review also presents in detail various methods (chemical, electrochemical and hydrothermal) used to prepare composites and characterization methods (spectroscopic, microscopic, electrochemical, etc.). The applications of these composites in electroanalysis of heavy metal ions have been discussed and summarized. Also, electrochemical detection methods, particularly those called “Anodic Stripping Voltammetry” have been explained and their uses in the detection of heavy metal ions in natural water have been highlighted, and the results provided.

Sr 3− x Pb x Fe 2 TeO 9 ( x = 1.50, 1.88 and 2.17) were synthesized in polycrystalline form via the solid-state reaction route in air, and were studied at room temperature using powder X-ray diffraction and ultraviolet visible spectroscopy techniques. The crystal structures were resolved by the Rietveld refinement method; x = 1.50 and 1.88 adopt a tetragonal phase (space group I 4/ mmm ) while x = 2.17 adopts a hexagonal phase (space group R 3 ̄ $\bar{3}$ m ). The direct bandgap energy ( E g ) of ∼1.89 eV for ( x = 1.50), ∼1.87 eV for ( x = 1.88) and ∼1.74 eV for ( x = 2.17) was estimated from the Tauc plots of ( αhʋ ) 2 versus photon energy. The Urbach energy ( E U ) was determined from the plots of logarithmic absorption coefficient versus hυ . Plots of the incident photon energy dependence of optical parameters such as refractive index, extinction coefficient, real and imaginary parts of the dielectric function, dielectric loss, real and imaginary parts of the complex optical conductivity, linear and nonlinear optical susceptibilities, real and imaginary parts of the electric modulus, real and imaginary parts of the impedance were obtained by means of ultraviolet visible spectrophotometer experiments. Additionally, the estimated values of the single oscillator energy, dispersion energy, static refractive index and high-frequency dielectric constant were obtained from the linear portion of the ( n 2 − 1) −1 versus ( hυ ) 2 plots using the Wemple–DiDomenico model.

This paper presents a study of the complex permittivity of n - type copper indium selenide semiconductor compound at low temperatures down to −175 °C. Alternating current with frequency varying between 20 Hz and 1 MHz is applied to the material in order to measure the dielectric constant ɛ ′ and dielectric loss D = ɛ ″/ ɛ ′. ɛ ′ is found to decrease with temperature and frequency, whereas D decreases with frequency and increases with temperature. The experimental data of ɛ ″ agree with the expression ε ″ = A ω m ω , T ${\varepsilon }^{{\prime\prime}}=A{\omega }^{m\left(\omega ,T\right)}$ , where the frequency exponent m ( ω , T ), calculated through the relation m ω , T = ∂ ⁡ ln ε ″ / ∂ ⁡ ln ⁡ ω T $m\left(\omega ,T\right)={\left.\left(\partial \mathrm{ln}{\varepsilon }^{{\prime\prime}}/\partial \mathrm{ln}\omega \right.\right)}_{T}$ , shows a frequency and temperature dependence. The data are analyzed in light of existing theoretical models.

In this research work, the corrosion tendency of stainless steel 304 caused by the Pseudomonas aeruginosa ZK and Bacillus subtilis S1X bacterial strains is investigated. The topographical features of the biofilms achieved after 14 days of incubation at 37 °C were examined by means of scanning electron microscopy. Fourier transform infrared spectroscopic analysis of the extracellular polymeric substance was carried out to estimate the chemical composition of the biofilm. Electrochemical impedance spectroscopy and Tafel polarization test methods were applied to understand the in-situ corrosion tendency of the stainless steel 304 in the presence of P. aeruginosa ZK and B. subtilis S1X strains . Compared to the biofilm produced by the P. aeruginosa ZK, the extracellular polymeric substance in the B. subtilis S1X containing bacteria was found to be porous and non-uniform. The improved hydrophobicity and uniformity of the P. aeruginosa ZK containing biofilm retarded the corrosion of the underlying stainless steel 304 sample. Appreciably large resistance of the P. aeruginosa ZK biofilm (∼6.04 kΩ-cm 2 ) and hindered charge transport (11.12 kΩ-cm 2 ) were evident from the electrochemical impedance spectroscopy analysis. In support of these results, a large cathodic Tafel slope (0.2 V/decade) and low corrosion rate (1.69 μA cm −2 ) were corroborated by the inhibitive properties of the P. aeruginosa ZK containing biofilm. However, the localized corrosion of the substrate in the presence of B. subtilis S1X bacteria was caused by the porosity and non-homogeneity of the extracellular polymeric substance layer. The small charge transfer resistance, high dissolution rate and pitting of the surface under B. subtilis S1X biofilm were comparable to the corrosion properties of stainless steel 304 in a controlled medium. These results highlighted the poor corrosion inhibitive properties of the B. subtilis S1X biofilm compared to the P. aeruginosa ZK bacterial strain.

The green approach is a simple and efficient method for generating metal oxide nanoparticles. Magnesium oxide nanoparticles gained prominence due to their unique biodegradation, microbe growth inhibition, and non-toxicity properties. The seed extract of Eucalyptus tereticornis was used to make magnesium oxide nanoparticles in this research. Using X-ray diffraction data, the average crystallite size of nanoparticles is estimated to be about 10 nm. Face-centered cubic structured magnesium oxide nanoparticles have a spherical surface morphology, as seen in transmission electron microscopy and scanning electron microscopy images. Fourier transform infra-red spectra are used to identify functional groups involved in the stabilization and reduction of precursor salt. Magnesium oxide nanoparticles are employed for a variety of applications including temperature dehydrating agents, high-grade ceramic materials, optoelectronic fields, and bio-medical studies such as antibacterial and antimicrobial.

Nanomaterials have enabled extraordinary technical breakthroughs relative to bulk materials because of their unique properties. In this study, we have synthesized CeO 2 and Ce 1− x Fe x O 2 ( x = 0.02, 0.04 and 0.06 w/w) nanoparticles by the sol–gel method using acacia concinna fruit extract. The crystallite size of 22.7 nm for CeO 2 and varied from 32.47 nm to 62.95 nm for Fe-doped CeO 2 nanoparticles as calculated from the X-ray diffraction pattern. Scanning electron microscope images showed a porous network-like morphology for both the samples. Fourier transform infrared spectroscopy showed a characteristic absorption band for Ce–O at 849 cm −1 . Raman spectra indicated the vibrational mode of the cubic fluorite type structure of CeO 2 nanoparticles at 464 cm −1 . The specific surface area of CeO 2 nanoparticles, was observed to be 58 m 2  g −1 , while that for 2%, 4%, and 6% Fe-doped CeO 2 nanoparticles was 45 m 2  g −1 , 34 m 2  g −1 , 24 m 2  g −1 respectively.

In this work, the isothermal sections of the Ni–Al–Pd and Ni–Cr–Pd ternary and the Ni–Al–Cr–Pd quaternary systems at 1423 K, with Ni fixed at 53 at.%, were experimentally determined by scanning electron microscopy, coupled with energy dispersive spectroscopy and X-ray diffraction. Three three-phase regions were confirmed and one three-phase region was deduced in the isothermal section of the Ni–Al–Pd system at 1423 K. No three-phase regions existed in the isothermal section of the Ni–Cr–Pd system at 1423 K. Three three-phase regions were determined in the 1423 K isothermal section of the Ni–Al–Cr–Pd quaternary, with Ni fixed at 53 at.%, and two four-phase regions were deduced.

Bulk three-phase nanocrystalline (NC) Ag-20Cu-30Cr alloy was obtained by hot-pressing mechanically alloyed powders, and oxidation tests were completed in 0.1 MPa pure O 2 at 700 and 800 °C. The oxidation behaviour of the alloy and the effect of the grain size were also studied in comparison with the previous coarse-grained (CG) Ag-20Cu-30Cr alloy prepared by a powder metallurgy route. At the two temperatures the oxidation kinetic curves of the NC Ag-20Cu-30Cr alloy are composed of three parabolic stages, and their parabolic rate constants become smaller as the oxidation time increases. The oxidation rate at 700 °C is higher before 5 h, but lower after 5 h than that at 800 °C. Furthermore, the oxidation rate of the NC Ag-20Cu-30Cr alloy is lower than that of the CG Ag-20Cu-30Cr alloy at the same temperature. Moreover, the NC Ag-20Cu-30Cr alloy forms an outer oxide layer composed of the Cu oxides and double oxides of the metallic Ag, Cu and Cr as well as an inner regular, continuous and protective chromia layer. Thus, the Ag-20Cu-30Cr alloy can complete the transition from an internal to external oxidation of the reactive component Cr, and in the end form a regular, continuous and protective chromia layer after nanocrystallization.