• Issue front cover thumbnail

      Volume 40, Issue 7

      December 2017,   pages  1291-1533

    • Synthesis, structure and thermoelectric properties of La$_{1−x}$Na$_x$CoO$_3$ perovskite oxides


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      Monovalent ion doped lanthanum cobaltate La$_{1−x}$Na$_x$CoO$_3$ ($0 \leq x \leq 0.25$) 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 usingpowder X-ray diffraction (XRD) data and exhibit the rhombhohedral crystal structure with space group R-3c (No. 167). Thescanning electron microscopy study reveals that the particles are spherical in shape and sizes, in the range of 0.2–0.5 $\mu$m.High temperature electrical resistivity, Seebeck coefficient and thermal conductivity measurements were performed on thehigh density hot pressed pellets in the temperature range of 300–800 K, which exhibit p-type conductivity of pristine anddoped compositions. The X-ray photoelectron spectroscopy (XPS) studies confirm the monotonous increase in Co$^{4+}$ withdoping concentration up to $x = 0.15$, which is correlated with the electrical resistivity and Seebeck coefficient values of thesamples. The highest power factor of 10 $\mu$WmK$^{−2}$ is achieved for 10 at% Na content at 600 K. Thermoelectric figure ofmerit is estimated to be $\sim$$1 \times 10^{−2}$ at 780 K for 15 at% Na-doped samples.

    • A study on the synthesis, characterization and catalytic activity of ruthenium-substituted MFI (Mobil Five) zeolite


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      Hydrothermal synthesis was adapted to prepare a series of ruthenium (Ru)-incorporated Mobil Five (MFI)zeolite materials with silicon (Si) to aluminium (Al) ratio of 100 and with Al to Ru ratios 1:1, 2:1 and 3:1. An X-raydiffractometer (XRD) and FT-IR were employed to identify the formation of the zeolite phase. A favourable environmentwas maintained in the gel to overcome the difficulties of isomorphous substitution of ruthenium into the MFI framework. The unit cell volume change and the shift in XRD reflection positions showed a positive correlation with increased rutheniumcontent in the samples. UV–Vis (DRS) spectra confirmed the position of ruthenium in the T-site in the MFI structure.Thermogravimetric analysis was carried out for studying water and template removal patterns and also for studying thethermal stability of the synthesized samples. Formation of uniform rectangle- and twinned-rectangle-shaped MFI particleswith particle size approaching 27 $\mu$m was confirmed by scanning electron microscopy. Energy-dispersive X-ray analysis was carried out for quantification of Si, Al and Ru. The catalytic activities of the synthesized samples for the hydroxylation ofphenol was investigated, the major products being catechol and hydroquinone.

    • Synthesis, characterization and catalytic application of Ni catalysts supported on alumina–zirconia mixed oxides


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      Alumina and alumina–zirconia mixed oxides were compared as supports to prepare nickel catalysts. The oxideswere prepared by the sol–gel method using aluminum tri-sec-butoxide and zirconium (IV) propoxide as precursors, andits physicochemical properties were determined by BET, TGA, DTA, XRD, SEM and TEM. The catalysts of nickel wereobtained by the impregnation of the supports with nickel nitrate (10 wt%) and were heat-treated at 700$^{\circ}$C. The specificarea of the supports and catalysts decreased with the increase in the zirconia content in agreement with the crystallinephase formed. TEM micrographs of nickel catalysts revealed particles in the size range of 10–30 nm. The Ni/Al$_2$O$_3$–ZrO$_2$ catalysts were tested in the steam reforming reaction of ethanol (SRE) at 500$^{\circ}$C, and the obtained results suggest that the differences in catalytic activities depended on the content of ZrO$_2$. The selectivity towards H$_2$ was $\sim$56% for the named catalyst Ni–Al–0.25Zr.

    • Synthesis of Ag$_2$Se–graphene–TiO$_2$ nanocomposite and analysis of photocatalytic activity of CO$_2$ reduction to CH$_3$OH


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      The present work deals with the development of a new ternary composite, Ag$_2$Se–G–TiO$_2$, using ultrasonic techniques as well as X-ray diffraction (XRD), scanning electron microscopy (SEM), high transmission electron microscopy(HTEM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and UV–Vis diffuse reflectance spectra (DRS)analyses. The photocatalytic potential of nanocomposites is examined for CO$_2$ reduction to methanol under ultraviolet(UV) and visible light irradiation. Ag$_2$Se–TiO$_2$ with an optimum loading graphene of 10wt% exhibited the maximumphotoactivity, obtaining a total CH3OH yield of 3.52 $\mu$mol g$^{−1}$ h$^{−1}$ after 48 h. This outstanding photoreduction activity is due to the positive synergistic relation between Ag$_2$Se and graphene components in our heterogeneous system.

    • Synthesis and enhanced photocatalytic activity of g-C$_3$N$_4$ hybridized CdS nanoparticles


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      The highly effective g-C$_3$N$_4$ hybridized CdS photocatalysts were synthesized via a successive calcination andhydrothermal process. The as-prepared photocatalysts were characterized by X-ray powder diffraction, transmission electronmicroscopy and UV–Vis diffuse reflectance spectroscopy. The photocatalytic performance of the C$_3$N$_4$/CdS nanocomposites was evaluated by the photodegradation of RhB under visible light irradiation. The results showed that photocatalytic ability of the C$_3$N$_4$/CdS nanocomposites was higher than that of pure C$_3$N$_4$ and CdS. The enhanced photocatalytic activity could be attributed to the high separation efficiency of the photo-excited electron-hole pairs. A possible mechanism of the photocatalytic degradation of RhB on C$_3$N$_4$/CdS nanocomposites was also proposed.

    • Chemical synthesis and characterization of nano-sized rare-earth ruthenium pyrochlore compounds Ln$_2$Ru$_2$O$_7$ (Ln $=$ rare earth)


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      The rare-earth ruthenium pyrochlores Ln$_2$Ru$_2$O$_7$ (Ln $=$ La$^{3+}$, Pr$^{3+}$, Nd$^{3+}$, Sm$^{3+}$ and Gd$^{3+}$) have been synthesized by the tartrate co-precipitation method, which allowed control of their composition and morphology. The preparation processes were monitored by thermal studies (TG-DTA). The obtained ruthenates were characterized by X-ray diffraction (XRD), TEM, d.c. electrical conductivity, thermoelectric power and dielectric constant measurements. X-ray diffraction patterns for all pyrochlore samples indicate a single-phase crystalline material with a cubic structure except for LaRuO$_3$, which shows perovskite orthorhombic structure. The structural parameter for the solid obtained was successfully determined by Rietveld refinement based on the analysis of powder XRD pattern. The TEM photographs of these compounds exhibited the average particle size in the range of 36.4–73.8 nm. The data on the temperature variation of d.c. electrical conductivity showed that all rare-earth ruthanates are semiconductors and major carriers are electrons. The conduction mechanism of these compounds seems to be oxygen non-stoichiometry. The variation of dielectric constant at various frequencies showed initially interfacial polarization up to 275 kHz and beyond, which shows domain wall motion.

    • Synthesis and luminescence in sol–gel auto-combustion-synthesized CaSnO$_3$:Eu$^{3+}$ phosphor


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      Undoped and Eu-doped CaSnO$_3$ nanopowders were prepared by a facile sol–gel auto-combustion methodcalcined at 800$^{\circ}$C for 1 h. The samples are found to be well-crystallized pure orthorhombic CaSnO$_3$ structure. Photoluminescence (PL) measurements indicated that the undoped sample exhibits a broad blue emission at about 420–440 nm, which can be recognized from an intrinsic centre or centres in CaSnO$_3$. Eu-doped CaSnO$_3$ showed broad blue emission centred about 434 nm, a weak peak at 465 nm and a sharp intense yellow emission line at 592 nm. The emission situated at 592 nm was assigned to the f–f transition of ${}^5$D$_0$ $\to$ ${}^7$F$_1$ in Eu$^{3+}$ ions. The afterglow emission and PL decay results in Eu-doped CaSnO$_3$ phosphor, which revealed that there are at least two different traps in this phosphor. From the obtained results, Eu$^{3+}-doped CaSnO$_3$ phosphor could be proposed as a potential white luminescent optical material.

    • Structural and optical properties of diluted magnetic Ga$_{1−x}$Mn$_x$As–AlAs quantum wells grown on high-index GaAs planes


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      We report on the structural and optical properties of Ga$_{1−x}$Mn$_x$As–AlAs quantum wells (QWs) with $x = 0.1$% grown by molecular beam epitaxy (MBE) on semi-insulating GaAs substrates with orientations (100), (110), (311)B and(411)B. Atomic force microscopy (AFM), X-ray diffraction (XRD) and photoluminescence (PL) techniques were used toinvestigate these QWs. AFM results have evidenced the formation of Mn-induced islands, which are randomly distributedon the surface. These islands tend to segregate for samples grown on (110) and (411)B planes, while no clear segregationwas observed for samples grown on (100) and (311)B orientations. Results show that the PL line width increases with Mnsegregation. XRD measurements were used to determine 2$\theta$, $d$ and cell parameters.

    • Sodium-dodecyl-sulphate-assisted synthesis of Ni nanoparticles: electrochemical properties


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      Stabilized nickel nanoparticles (SNNPs) were prepared using Ni(acac)$_2$ (acac = acetylacetonate) via a simplesolvothermal method. The synthesis of the nickel nanoparticles was performed in the presence of sodium dodecyl sulphate(SDS) of different concentrations (mole ratios of SDS:Ni(acac)$_2$ = 1:1, 2:1 and 4:1), as the stabilizer, in order to appraise theirinfluence on the morphology, size, dispersion, magnetic properties and electrochemical activity of the nickel nanoparticles.The synthesized products have been characterized by powder X-ray diffraction, scanning electron microscopy, transmissionelectron microscopy, Fourier transform infrared spectra, energy-dispersive X-ray spectroscopy, vibrating sample magnetometryand electrochemical studies. It is noteworthy that the average particles size of the SNNPs has been reduced by increasingthe SDS concentration, while at high concentration (mole ratio of SDS:Ni(acac)$_2$ = 4:1), the small particles tend to coalesceand create a big one. The stabilized Ni nanoparticles could be used as electrode materials for hydrogen evolution in alkalinemedium. The electrochemical measurements demonstrated that the higher conductivity and lower value of faraday resistanceof the as-prepared samples were when the mole ratio of SDS:Ni(acac)$_2$ was 2:1.

    • Process optimization of dye-sensitized solar cells using TiO$_2$–graphene nanocomposites


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      TiO$_2$–graphene (TGR) nanocomposites with varying concentrations of graphene from 0 to 1 wt% were preparedby direct mix method. X-ray diffraction (XRD) spectra confirmed the incorporation of graphene in photoanode material,which was further supported by field emission scanning electron microscopy (FESEM) and energy dispersive X-ray (EDX).The UV–visible spectrum of these nanocomposites shifted towards higher wavelength region as compared to pure TiO$_2$ thatindicated a reduced band gap and hence, enhanced absorption bandwidth. Significant reduction in electron–hole recombinationwas confirmed from photoluminescence spectroscopy. These TGR nanocomposite films after tethering with black dyewere employed as photoanodes in dye-sensitized solar cells (DSSCs). The efficiency of solar cells at varying concentrationsof graphene (in photoandes) was also investigated. TGR 0.25 wt% nanocomposite showed the highest photocurrent density(JSC) of 18.4 mA cm$^{−2}$ and efficiency ($\eta$) of 4.69%.

    • Influence of TiO$_2$ particle size and conductivity of the CuCrO$_2$ nanoparticles on the performance of solid-state dye-sensitized solar cells


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      Solid-state dye-sensitized solar cells have been fabricated with mesoporous TiO$_2$ photoanode and N719 dye as photosensitizer. First, TiO$_2$ and non-doped, Zn- and Mg-doped CuCrO$_2$ nanoparticles have been synthesized by sol–gelmethod. In addition, the TiO$_2$ pastes have been prepared through Pechini-type sol–gel method. The effect of TiO$_2$ particle size, mesoporous TiO$_2$ photoanode thickness and solid-state electrolyte thickness on the efficiency of the fabricated devices has been investigated. Our results show that in spite of the low amount of dye loading for photoanode with large TiO$_2$nanoparticles (80–180 nm), the dye-sensitized solar cell made from it has higher efficiency than that constructed from thephotoanode comprising of small particles about 10–15 nm in size. The higher efficiency is attributed to the longer diffusionlength of electrons because of a better electron transport and penetration of a large amount of CuCrO$_2$ nanoparticles inthe porous structure of TiO$_2$ photoanode. By using the doped CuCrO$_2$ nanoparticles, the efficiency has been increasedfrom 0.027% for TiO$_2$/N719 dye/CuCrO$_2$ to 0.033% for TiO$_2$/N719 dye/CuCrO$_2$:Zn and further increased to 0.042% for TiO$_2$/N719 dye/CuCrO$_2$:Mg. The efficiency enhancement by doping is ascribed to the conductivity improvement due to the presence of impurity atoms.

    • Influence of donor substitution at D−$\pi$−A architecture in efficient sensitizers for dye-sensitized solar cells: first-principle study


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      Using density functional theory and time-dependent density functional theory, we theoretically studied a new series of five novel metal-free organic dyes, namely D1–D5, for application in dye-sensitized solar cells based on donor–$\pi$-spacer–acceptor (D−$\pi$−A) groups. In this present study, five different donor groups have been designed based on triphenylamine–stilbene–cyanoacrylic acid (TPA–St–CA). The electronic structures, UV–visible absorption spectra and photovoltaic properties of these dyes were investigated. Different exchange-correlation functionals were used to establish a proper methodology procedure for calculation and comparison to experimental results of dye TPA–St–CA. The TDWB97XD method, which gives the best correspondence to experimental values, is discussed. The calculated results reveal that the donor groups in D2 and D3 are promising functional groups for D−$\pi$−A. In particular, the D2 dye showed small energy levels and red-shift, negative $\Delta G_{\rm inject}$, fastest regeneration and largest dipole moment and exciton binding energy when compared with TPA–St–CA.

    • A first-principle investigation into effect of B- and BN-doped C$_{60}$ in lowering dehydrogenation of MXH$_4$ (where M $=$ Na, Li and X $=$ Al, B)


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      The present paper reports the effect of B- and BN-doped C$_{60}$ as catalysts for lowering the dehydrogenationenergy inMXH4 clusters (M $=$ Na and Li, X $=$ Al and B) using density functional calculations.MXH4 interacts strongly withB-doped C$_{60}$ and weakly with BN-doped C$_{60}$ in comparison with pure C$_{60}$ with binding energy 0.56–0.80 and 0.05–0.34 eV, respectively. The hydrogen release energy (EHRE) of MXH$_4$ decreases sharply in the range of 38–49% when adsorbed on B-doped C$_{60}$; however, with BN-doped C$_{60}$ the decrease in the EHRE varies in the range of 6–20% as compared with pure MXH$_4$ clusters. The hydrogen release energy of second hydrogen atom in MXH$_4$ decreases sharply in the range of 1.7–41% for BN-doped C$_{60}$ and decreases in the range of 0.2–11.3% for B-doped C$_{60}$ as compared with pure MXH$_4$ clusters. The results can be explained on the basis of charge transfer within MXH$_4$ cluster and with the doped C$_{60}$.

    • Photoluminescence quenching and photocatalytic enhancement of Pr-doped ZnO nanocrystals


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      This work reports the synthesis of novel praseodymium (Pr)-doped ZnO nanocrystals with excellent photocatalyticactivity through modified solid-state reaction route. The impacts of doping on the wurtzite hexagonal structureare analysed with X-ray diffraction (XRD) and Raman spectroscopy. The production of defect levels and the formation ofUrbach energy within the system are confirmed using photoluminescence (PL) techniques and UV/Vis diffuse reflectancespectroscopy, respectively. The linear relationship between Urbach energy and band gap energy is elucidated from UV/Visspectroscopy analysis. The changes happened to morphology by doping were tackled using scanning electron microscopy(SEM). The concentration quenching effect of PL emission with Pr doping is explained in detail. A three-fold enhancementin the photocatalytic activity was achieved with optimum Pr incorporation in ZnO. This work successfully correlated PLquenching and enhanced photocatalytic activity with the defect production happened in ZnO system on Pr doping. Thepractical applicability of the photocatalyst was confirmed with a stability test.

    • Unusual photoluminescence of Cu–ZnO and its correlation with photocatalytic reduction of Cr(VI)


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      Cu–ZnO was synthesized by sol–gel routewith a varied copper concentration of 1, 2 and 3 mol%. The synthesizedmaterials were structurally characterized with powder X-ray diffraction and Raman spectroscopy, morphologically usinga field emission scanning electron microscope, and electronic properties were studied with UV–visible spectroscopy andphotoluminescence spectroscopy. Variation in Cu doping showed enhancement/quenching in photoluminescence of ZnO.This special characteristic is reflected in photocatalytic reduction of Cr(VI).

    • Role of reactive species in the photocatalytic degradation of amaranth by highly active N-doped WO$_3$


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      A novel, highly visible light active N-doped WO$_3$ (N-WO$_3$) is successfully synthesized via thermal decompositionof peroxotungstic acid–urea complex. The photocatalytic activity of N-WO$_3$ is evaluated for the degradation ofamaranth (AM) dye under visible and UVA light along with the role of reactive species, which has not yet been studiedfor N-WO$_3$ photocatalysts. Doping of N into substitutional and interstitial sites of WO$_3$ is confirmed by X-ray photoelectronspectroscopy and X-ray absorption near-edge spectroscopy. At a pH of 7, 1 g l$^{−1}$ of N-WO$_3$ can completely degrade10 mg l$^{−1}$ of AM within 1 h under visible and UVA light. For the degradation of AM by N-WO$_3$ under visible and UVAlight, h$^+$ is found to be the main reactive species, while ${}^{\bullet}$OH contributes to a lesser extent. On the contrary, ${}^{1}$O$^2$, ${}^{\bullet}$O$^{−}_2$ and e$^{−}$show negligible roles. The crucial role of h$^+$ indicates effective suppression of electron–hole recombination after N doping.Dye sensitization and oxidation by reactive species are found to be the major pathway for the degradation of AM undervisible and UVA light, respectively.

    • Temperature-independent photoluminescence response in ZnO:Ce nanophosphor


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      A comparative study of structural and luminescence properties of ZnO:Ce$*{3+}$ nanophosphors prepared by combustion method and solid-state reaction method is presented in this study. The powder XRD exhibits hexagonal wurtzite phase and crystallite size falls in the nanometre range. The optical band gap reduction is less in samples synthesized by combustion method. Photoluminescence (PL) spectra of ZnO:Ce excited at near-ultra violet region (325 nm) gives an evidence of its wide range of applications in lighting purposes.

    • Temperature effects on the electrical characteristics of Al/PTh−SiO$_2$/p-Si structure


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      The temperature-dependent current–voltage ($I –V$) and capacitance–voltage ($C–V$) characteristics of the fabricated Al/p-Si Schottky diodes with the polythiopene–SiO$_2$ nanocomposite (PTh–SiO$_2$) interlayer were investigated.The ideality factor of Al/PTh–SiO$_2$/$p$-Si Schottky diodes has decreased with increasing temperature and the barrier heighthas increased with increasing temperature. The change in the barrier height and ideality factor values with temperaturewas attributed to inhomogeneties of the zero-bias barrier height. Richardson plot has exhibited curved behaviour due totemperature dependence of barrier height. The activation energy and effective Richardson constant were calculated as0.16 eV and $1.79 \times 10^{−8}$ A cm$^{−2}$ K$^{−2}$ from linear part of Richardson plots, respectively. The barrier height values determined from capacitance–voltage–temperature ($C–V–T$) measurements decrease with increasing temperature on the contrary of barrier height values obtained from $I –V–T$ measurements.

    • Microwave-assisted synthesis and photoluminescence properties of ZnS:Pb$^{2+}$ nanophosphor for solid-state lighting


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      Nanocrystalline ZnS:Pb$^{2+}$ phosphor was synthesized by microwave-assisted co-precipitation method. Thephase purity and surface morphology of prepared material were investigated using an X-ray diffractometer (XRD) anda scanning electron microscope. The photoluminescence property was studied by near-UV (nUV) excitation. The XRDpattern of prepared phosphor matches well with that of an ICDD (International Center for Diffraction Data) file. Surfacemorphology of prepared phosphor was found to be in submicron range. As-prepared ZnS:Pb$^{2+}$ phosphor shows a greenemission under nUV excitation. As Pb$^{2+}$ ions are located on a regular Zn$^{2+}$ site they produce a green emission band inthe range of 400–650 nm, centred at 500 nm, under excitation wavelength of 357 nm, exhibiting luminescence propertiestypically observed for Pb$^{2+}$. Effect of concentration of Pb$^{2+}$ ions on emission intensity was studied. The colour co-ordinates of prepared phosphor were calculated and found to lie in the green region of Commission Internationale de l’Eclairagediagram.

    • Microwave hydrothermal synthesis and upconversion luminescence properties of Yb$^{3+}$/Tm$^{3+}$ co-doped NaY(MoO$_4$)$_2$ phosphor


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      Tetragonal NaY(MoO$_4$)$_2$ (NYM) phosphors co-doped with Yb$^{3+}$ and Tm$^{3+}$ ions were synthesized throughmicrowave hydrothermal method followed by calcining treatment. Powder X-ray diffraction, Fourier transform infraredspectroscopy, scanning electron microscopy and photoluminescence spectra were used to characterize the properties of as prepared samples. The results show that Yb$^{3+}$/Tm$^{3+}$ co-doped NYM displayed bright blue emission near 472 and 476 nm(${}^1$G$_4$ $\to$ ${}^3$H$_6$ transition), strong near-infrared upconversion (UC) emission around 795 nm (${}^3$H)$_4$ $\to$ ${}^3$H$_6$ transition). The optimum doping concentrations of Yb$^{3+}$ and Tm$^{3+}$ for the most intense UC luminescence were obtained, and the related UC mechanism of Yb$^{3+}$/Tm$^{3+}$ co-doped NYM depending on pump power was studied in detail.

    • A sensitive optical sensor based on DNA-labelled Si@SiO$_2$ core–shell nanoparticle for the detection of Hg$^{2+}$ ions in environmental water samples


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      Si@SiO$_2$ core–shell nanoparticles were proposed for the development of fluorescent mercury sensor, wh1455-1462ichalso offers a promising alternative to toxic quantum dots (QD)-based heavy metal detection tools. In this study, a sensitivefluorescent assay based on DNA-labelled Si@SiO$_2$ core–shell nanoparticles for the detection of mercury (II) in environmentalsamples was investigated. Probe DNA was conjugated on the surface of the Si@SiO$_2$ core–shell nanoparticles via 5$^{\circ}$-terminal-SH (thiol group) reaction. The detection protocol was based on the DNA hybridization resulted from the formation of mercury-mediated (thymine–Hg$^{2+}$–thymine) base pairs which leave a fluorescent QD on the surface of quartz glass. The synthesized Si@SiO$_2$ core–shell nanoparticle showed a broad emission peak with strong intensity in the UV range around 423 nm. Transmission electronic microscope (TEM) images confirmed the presence of a uniform core–shell structure with Si core nanoparticles with a particle size ranging from 70 to 80 nm and silica shell thickness of about $10\pm 2$ nm. Overall, ourfindings highlighted that the developed assay can detect Hg$^{2+}$ ions in aqueous solution as low as 0.92 nM concentration. In addition, the labelled Si@SiO$_2$ core–shell nanoparticles showed prominence sensitivity, acceptable precision, reproducibility and stability, and could be readily applied to environmental sampling systems for Hg$^{2+}$ monitoring.

    • DNA-assisted synthesis of chitosan/$\alpha$-Fe$_2$O$_3$ nanocomposites for antioxidant and antimicrobial activities


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      Novel nanocomposites, based on chitosan/α-Fe$_2$O$_3$ (C/FD), have been successfully synthesized. The FDnanoparticles were prepared by co-precipitation method using DNA as the capping agent. The samples were characterizedby XRD, EDAX, SEM and TEM. The hematite nanoparticles that were prepared using DNA were compared withthe samples prepared using EDTA and CTAB as capping agents. The effect of C/FD nanocomposites on the growth offew common water pathogens were studied to explore its use as an antibacterial agent to be used in water purification.The antioxidant activity of samples was tested using a DPPH assay at two different concentrations of FD. It provides thepossibility of improving radical scavenging activity by varying the preparation conditions. The results also showed that theC/FD being a bio-compatible, eco-friendly and low cost material is used to inhibit the growth of several fungi and bacteriathat can be useful for a number of applications in the food and pharmaceutical industry.

    • Microstructure and mechanical properties of fine-grained thin-walled AZ91 tubes processed by a novel combined SPD process


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      A combination of parallel tubular channel angular pressing (PTCAP) and tube backward extrusion (TBE) asa novel combined severe plastic deformation (SPD) was applied on AZ91 alloy to produce ultrafine-grained (UFG) thinwalledtubes. The effects of combined SPD process were investigated on the microstructure refinement and mechanicalproperties. Also, hydro-bulge test was carried out to reveal the mechanical properties. The results showed a notable increasein ultimate strength, yield strength and microhardness of the thin-walled UFG tube were achieved compared to that fromPTCAP process. A remarkable grain refinement achieved. Applying three-passes-combined process refine the grain sizeto 8.8 $\mu$m from an initial value of $\sim$150 $\mu$m. For one pass- and two passes-processed thin-walled tubes, it was about 12.4 and 9.8 $\mu$m, respectively. Yield and ultimate strengths were increased notably to 150 and 354 MPa for 2P $+$ TBE tube, from the initial values of 86 and 166 MPa, respectively. The maximum microhardness was increased to about 105 Hv for the2P S+$ TBE tube from the initial value of 56 Hv. Hydro-bulge test showed that the bursting pressure increased to 246 bar for2 passes-thin-walled tube from the initial value of 160 bar. It was 220 and 195 bar for 1 pass- and 3 passes-thin-walled tubes,respectively.

    • Microstructure and mechanical properties of laminated Al–Cu–Mg composite fabricated by accumulative roll bonding


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      In the present research, laminated Al–Cu–Mg composite was processed by the accumulative roll bonding (ARB) method. Initially, aluminium, copper and magnesium strips were alternatively stacked together. Then these stacked strips were rolled at 150$^{\circ}$C up to five ARB cycles. The microstructure of composites was studied by optical microscopy. Micro-hardness and tensile tests were conducted to evaluate mechanical properties of the processed composites. After thefirst cycle of ARB, it was observed that copper and magnesium layers were necked and fractured. With increasing ARBup to four cycles, laminated Al–Mg–Cu composite with homogeneous distribution of fragmented reinforcement in matrixwas produced. It was observed that with increasing ARB up to four cycles the strength and micro-hardness of fabricatedcomposites increased and elongation decreased at the same time. These differences in mechanical behaviour have beenattributed to the microstructural aspects of the individual layers and the fragmentation processes.

    • Dielectric relaxations of confined water in porous silica ceramics


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      In this study, dielectric properties of water confined in porous silica ceramics were investigated. Two porous ceramics were characterized in the frequency range 10$^{−1}$ to 10$^7$ Hz and temperature interval from $−$100 to 200$^{\circ}$C. Whilethe first sample was a ceramic with opened lateral pores, the second one was a ceramic with sealed lateral pores. In bothceramics, three dielectric processes were identified. The first, which appeared at lower temperatures, was attributed tothe reorientation of water molecules in ice-like water cluster structures. The second is the relaxation observed over anintermediate temperature range, associated with the kinetic transition due to water molecule reorientation near a defect.At higher temperatures, the third was relaxation identified as the Maxwell–Wagner–Sillars polarization process due to thetrapping of free charge carriers at the interface of the porous media. The first and second dielectric relaxations were analysedto prove the effect of the lateral surface state of the sample on water–inner surfaces of the porous media interaction. Theseanalyses revealed a great similarity in the ice-like structure for both ceramics. However, the lateral surface state of the samplemight enhance the dielectric strength of the first relaxation when lateral pores are sealed. Furthermore, it might improve thewater–inner surfaces interaction when lateral pores are opened.

    • Electromagnetic interference shielding effectiveness of MgO−Al$_2$O$_3$−SiO$_2$ glass–ceramic system


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      MgO−Al$_2$O$_3$−SiO$_2$ (MAS)-based glass–ceramic system was prepared using very-low-cost raw materials, i.e.,talc, calcined alumina and calcined china clay with titanium dioxide as a nucleating agent. Glass–ceramics were preparedby a two-step process. In the first step, raw materials were mixed in the required proportion and melted at 1450$^{\circ}$C followedby water quench into a glassy frit. In the second step, powdered glass frit was uniaxially dry pressed into pellets followedby sintering at 1200$^{\circ}$C for 3h. X-ray diffraction pattern of the sintered compact shows well-defined peaks of cordieritealong with some anorthite and magnesium titanium oxide. The microstructure study of sample shows the presence of crystallineand glassy phases. Permittivity and permeability measurements were performed in the microwave frequency range12.4–17 GHz. The permittivity value of 5.7–6.0 and the permeability value of $\sim$1 were obtained. The reflection and transmission measurements show that the material possesses a shielding effectiveness in the range 2–10 dB over the frequencyrange 12.4–17 GHz.

    • Tailoring the bandgap and magnetic properties by bismuth substitution in neodymium chromite


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      The intrinsic distortions present in rare-earth orthochromites (RCrO$_3$) observed from lanthanum to lutetium (inR-site) can influence the magnetic properties like Neel transition and weak ferromagnetic coupling. A nonmagnetic cationwith similar ionic radius would be a suitable candidate to engineer the inherent distortions of particular orthochromite. In thisstudy, bismuth (Bi$^{3+}$) with a 6s$^2$ lone pair was chosen to substitute in neodymium (Nd$^{3+}$) site of NdCrO$_3$ (NCO) to tailor the intrinsic structural distortions. The variation of optical absorption edge evidently suggests that Bi (6s$^2$) substituted in the magnetic rare-earth Nd$^{3+}$ influences the Cr–O overlap integral. The interaction of Bi cation with oxygen bonds influencesthe structural distortions through Cr–O polyhedral, which are evident from Raman scattering studies. The observed structuraland magnetic properties of similar ionic radius of Bi$^{3+}$ in Nd$^{3+}$ reveal that intrinsic structural distortions are interrelatedto enhanced weak ferromagnetic component and change in Neel and spin reorientation temperatures in our compounds. Inaddition, a reduction in the optical bandgap of NCO from 3.1 to 2.6 eV was observed.

    • Precipitate coarsening parameters for gas induced semi-solid cast 7075-T6 Al alloy determined by SAXS measurements


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      The particle radii of precipitates in the gas induced semi-solid (GISS) cast 7075-T6 Al alloys, for ageing timesranging from peak ageing to over ageing, were measured by small-angle X-ray scattering (SAXS) technique. The measuredprecipitate radii increased with ageing time and temperature, and data were well fit by the Lifshitz–Slyozov–Wagner (LSW)coarsening process along with a modified Arrhenius equation for temperature dependency. The activation energy for thecoarsening process was estimated at 1.19 eV, which is close to that of the wrought 7075 Al alloy. This energy is in the rangeof activation energies of Zn, Mg and Cu for their diffusion in Al. The precipitate coarsening rate of this alloy appears to bediffusion limited. The fine precipitates of the GISS 7075-PA Al alloy coarsened slowly at 63 nm$^3$ h$^{−1}$ rate, at the elevated200$^{\circ}$C temperature, while rapid 216 nm$^3$ h$^{−1}$ coarsening was found in the wrought 7075-T651 Al alloy.

    • Low temperature catalyst-assisted pyrolysis of polymer precursors to carbon


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      The aim of this work is to study the pyrolysis of polymer precursors to carbon at lower temperatures usinga catalyst. We have added different weight ratios of nickel acetate tetrahydrate (NiAc) and multi-walled carbon nanotubes(MWCNTs) as catalysts into two different precursors, polyacrylonitrile (PAN) and resorcinol–formaldehyde (RF), separately. PAN composite was stabilized whereas RF composite was dried under sub-critical conditions followed by pyrolysis carried out at different temperatures. To examine the effect of pyrolysis temperature and catalyst concentration, PAN- and RF-derived carbons were characterized by elemental analysis, X-ray diffraction, Raman spectroscopy, transmission electron microscopy, Fourier transform infrared spectrophotometer and small angle X-ray scattering, respectively. It was found that addition of NiAc facilitated similar carbon yield at much lower temperature than what was obtained without a catalyst. The addition of MWCNTs enhanced the crystallinity of carbon samples, which is otherwise possible only by higher heat treatment. Thisstudy clearly suggests adopting catalyst-assisted less energy intensive low temperature pyrolysis for polymer precursors tocarbon with better yield as well as crystallinity.

    • Leaching of rapidly quenched Al$_{65}$Cu${20}$Fe$_{15}$ quasicrystalline ribbons


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      In the present work, Al$_{65}$Cu$_{20}$Fe$_{15}$ alloy has been synthesized by melting of pure elements (e.g., Al (99.96%),Cu (99.99%) and Fe (99.98%)), using a radiofrequency induction melting furnace. The as-prepared alloy was subjected torapid solidification by melt spinning technique at $\sim$3500 rpm speed on a copper disk of diameter 14 cm. As a result of themelt spinning, nearly 2 mm wide, 30−40 $\mu$m thick and 4–5 cm long ribbons were formed. The structural and microstructuralcharacterizations were carried out by X-ray diffraction and transmission electron microscopy techniques.We have performedleaching operation using 10 mol NaOH aqueous solution on the surface using a pipette. Leaching was performed for variousdurations ranging from 30 min to 8 h. After leaching, the reflectivity reduces and the surface looks reddish brown. Themicrostructure of the 8 h leached sample shows a breakdown of the quasicrystalline phase but with the evolution of othermetallic phases. Copper (Cu) particles are found to be present on the surface of quasicrystal after 4 h of leaching and relativelymore iron (Fe) evolves during further leaching of 8 h. This low-cost method to prepare a distribution of nano-Cu/Fe metalparticles encourages their uses in catalytic reactions, indicating the possibility of use of quasicrystals as the industrial catalysts.

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