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      Volume 41, Issue 5

      October 2018

    • Editorial


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    • Fabrication of stretchable compliant electrodes on PDMS with Au nanoparticles


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      A stretchable and compliant electrode surface of Au metal on polydimethylsiloxane (PDMS) is introduced in this study. A thin layer of Au nanoparticles is thus formed by a simple chemical reduction from aqueous Au salt solution with the AuPDMS gel surface itself acting as a reducing site. Employing the swelling behaviour of PDMS and Au nanoparticles affinity to bind with sulphur, an in-plane molecular device has been realized for measuring the conductance of thiol molecules. The device is capable of forming stable and robust linkages with Au. The molecules anchored between the Au islands areable to undergo reversible compression and tension, which shows the flexibility of the device.

    • In-situ TiO$_2$–rGO nanocomposites for CO gas sensing


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      TiO$_2$–reduced graphene oxide (rGO) nanocomposites were synthesized in-situ via hydrothermal route using graphene oxide (GO), TiCl$_3$ and ammonia solution. GO was prepared by the electrochemical exfoliation technique. The structure, phase conformation and morphology of TiO$_2$–rGO nanocomposite were characterized using X-ray diffraction, Fourier transform infra-red spectroscopy and scanning electron microscopy. Sensing behaviour of the TiO$_2$–rGO nanocomposite was examined under the 100 and 200 ppm of CO environment.

    • Influence of zinc oxide nanorods on an orientationally ordered fluid comprising soft-bent dimers


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      Bent-core liquid crystals (LCs) introduced a whole new dimension to the science of LCs. Besides re-emphasizing the importance of the shape of the molecule, they brought in phases with symmetry, not known earlier.Another sub-class of systems that is emerging is that of soft-bent molecules. In contrast with the bent-core molecules, here the bend is achieved through the parity of the aliphatic linker that connects two monomers. They hold the unique advantage that a simple variation of temperature can favour different conformer states and thus govern the self-assembled structure. A highlight of the power of this route is seen in terms of the discovery of a new type of nematic, viz., twist–bend nematic. Investigations on not only this phase but also the regular nematic that often precedes it have received significant attention in the last few years. Here we present results on the regular nematic phase of a binary mixture comprising such a soft-ben dimer, known in the literature as CB7CB, by incorporating zinc oxide nanorods (NRs) into the system. The NRs with anaspect ratio of $\sim$7.2, not very different from that of LCs, further accentuate the importance of shape and shape anisotropy of the entities. Specifically, we observe that the nematic–isotropic transition temperature increases by $\sim$1.9K even for a low concentration of 4% NRs.While the dielectric anisotropy decreases, birefringence shows a substantial increase, adding to thecomplexity of the influence. Upon addition of minute amount of NRs (1%), while the splay elastic constant gets enhanced, its bend counterpart not only gets reduced but retains the convex-shaped thermal profile seen for the parent mixture.

    • Pseudo-Jahn–Teller effects in two-dimensional silicene, germanene and stanene: a crystal orbital vibronic coupling density analysis


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      The presence of the pseudo-Jahn–Teller (PJT) effect has been investigated in the heavier analogues of graphene, namely silicene, germanene and stanene, by applying the orbital vibronic coupling density theory. In order to do so, wehave made a vis-a-vis analogy with their respective planar, honeycomb molecular cluster models, namely hexasilabenzene (Si$_6$H$_6$), hexagermabenzene (Ge$_6$H$_6$) and hexastannabenzene (Sn$_6$H$_6$). One-to-one mapping of the occupied crystal orbitals and unoccupied crystal orbitals in two-dimensional (2D) Si, Ge and Sn systems to the occupied molecular orbitals and unoccupied molecular orbitals of the corresponding molecular units are used to identify PJT-active bands and compute the crystal orbital vibronic coupling density (c-OVCD) and crystal orbital vibronic coupling constants (c-OVCCs). c-OVCD and c-OVCC show the local picture of the PJT coupling in these 2D systems. This article exemplifies the fruitfulness of deciphering the structural aspects in materials based on orbitals of their corresponding simple molecular units—a reductionist quantum chemical approach to materials.

    • Metallophthalocyanine-nanofibre-based electrodes for electrochemical sensing of biomolecules


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      Metal phthalocyanines, possessing rich redox chemistry due to the presence of the central metal cation and pyrrolic nitrogen atoms of the macrocycle, are explored as electrochemical sensors. Nickel phthalocyanine nanofibres (NiPcNF) prepared by a simple chemical route are coated on a pencil graphite rod and the electrocatalytic performance of NiPc NF electrode is investigated for quantitative detection of as corbic acid (AA) in 0.2 M phosphate buffer solution. The performance of NiPc NFs is shown to be superior to that of commercial NiPc and is attributed to the high electrochemically active surface area available for fibres. The electrode exhibits linearity for the detection over a wide concentration range of AA from 5.5 $\mu$M to 5.2 mM. The detection limit for AA sensing with NiPc-NF-modified electrode is 1.5μM. The higher performance of NiPc fibres due to its nanostructure morphology may be utilized for the quantitative detection of other biomolecules.

    • Synthesis of Chevrel phase (Cu$_{1.8}$Mo$_6$S$_8$) in composite with molybdenum carbide for hydrogen evolution reactions


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      Sustainable hydrogen generation from water electrolysis using renewable energy sources is the most promising pathway for future energy and hydrogen economy. Here, the Chevrel phase (Cu$_{1.8}$Mo$_6$S$_8$) was synthesized in composite with Mo$_2$C and good hydrogen evolution activity in acidic media has been demonstrated. Bundles of nanowires were formed in the templated synthesis route. The composites were characterized by X-ray diffraction, transmission electron microscopy, scanning electron microscopy and elemental analysis. Detailed electrochemical analysis reveals that MCS-Cu50 compositeexhibits higher hydrogen evolution reaction (HER) activity with 71.4 mA cm$^{−2}$ current density at an overpotential of 400 mV. It requires 250 mV overpotential to produce 10 mA cm$^{−2}$ current density for HER.

    • Thermal conductivity studies on magnetite nanofluids coated with short-chain and long-chain fatty acid surfactants R


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      The effect of the length of surfactant molecules on the surface of the nanoparticles on the thermal conductivity of nanofluids is studied. Magnetite (Fe$_3$O$_4$) nanoparticles of comparable sizes are stabilized with short-chain capric acid (C$_9$H19COOH) and long-chain stearic acid (C$_{17}$H$_{35}$COOH) molecules. Thermal conductivity of the two surfactant-coated magnetite nanoparticles dispersed in toluene is measured as a function of the concentration of the particles in the fluids and in the presence of a magnetic field. Studies showed that the critical concentration for thermal conductivity enhancement is lower for stearic-acid-coated fluid as compared with the capric-acid-coated fluid. Comparable enhancement in the thermal conductivity is observed at higher concentrations of the particles. Relatively larger enhancement in the thermal conductivity is observed for the capric-acid-coated fluid in a magnetic field. The difference in the enhancement in the thermal conductivity,depending on the chain length of the surfactant, is explained in terms of the inter-particle magnetic interactions and formationof clusters.

    • Templated synthesis and characterization of red-emitting Ca$_{1−x}$Sr$_{1−x}$Eu$^{3+}_{2x}$ SiO$_4$ phosphor for LED applications


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      A novel attempt towards the synthesis of red-emitting europium (Eu$^{3+}$)-doped CaSrSiO$_4$ phosphors has been made through a templated strategy using non-ionic surfactant as template. The concentrations of Eu$^{3+}$ in the host were altered and the optimized concentration to extract the maximum efficacy was analysed. The crystalline structure and morphologies ofthe synthesized phosphor were studied and analysed. The results show the aggregated rod-like morphology with a continuous porous network that shows the maximum intensity at 10 mol% of dopant.

    • Perspective of dye-encapsulated conjugated polymer nanoparticles for potential applications


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      Design of highly luminescent nanomaterials is an emerging area of research for photonic and bio-photonic applications. Nowadays, dye-encapsulated polymer nanoparticles (PNPs) are found to be very promising alternative nextgenerationluminescent nanomaterials because of extraordinary brightness, easy synthesis, higher photo-stability and nontoxic behaviour. Herein, we have highlighted the dynamics of the fluorophore molecules inside PNPs. Furthermore, we discuss the fundamental correlation of particle brightness with the size of the PNPs as well as population of the dye molecules inside the PNPs. Considering the resonance energy transfer process, generation of white light by varying the dye concentration and singlet oxygen generation using photosensitizer dye have been described. Finally, we discuss the importance of hybrids of conjugated PNPs for potential light harvesting systems such as photovoltaic and optoelectronic applications.

    • Improving range of SPR tunability and extinction efficiency of spheroidal silver nanostructures in graphene environment


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      In photovoltaics, the materials having ability to manipulate the optical fields and coupling of energy flow inside the device play a crucial role. In this article, we report the role of graphene environment on spheroid-shaped Ag nanoparticles (NPs) with various shapes and sizes. This study confirms the tunability of surface plasmon resonances (SPRs) and an enhancement in extinction efficiency, derived numerically using discrete dipole approximation (DDA). We havechosen oblate- and prolate-shaped Ag NPs for the numerical experiment and analysed their optical signatures in terms of extinction efficiency and SPR tunability against the quasi-static approximation. The excitation of longitudinal and transversal resonances was also observed because of the asymmetric shape of Ag NPs. All optical responses have been analysed by varying the effective radii and aspect ratio ofAgNPs, and the thickness of graphene monolayer (from 0.1 to 0.5 nm).Tunability of longitudinal resonances has been observed in the 600–833 nm wavelength region, while for transversal resonances, the tunability is in the 450–505 nm wavelength range. The results represent the effect of graphene environment on the tunability of SPRs with enhanced extinction efficiency. This study could lead to the development of a photovoltaic device with wide range of tunability and enhanced efficiency.

    • Suggested standards for reporting power and energy density in supercapacitor research


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      One of the most important applications for porous material with high surface area is the supercapacitor. For supercapacitors, power density and energy density are the most significant metrics. However, currently no dominant, established standard exists, and research papers often report these two metrics differently. A standard for reporting power and energy density is proposed here for objective comparison. This paper also discusses a common flaw in current supercapacitor power density calculations.

    • 2D molecular precursor for a one-pot synthesis of semiconducting metal sulphide nanocrystals


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      2D molecular materials, namely, metal alkyl thiolates, have been used as a single-source precursor for the synthesis of semiconducting metal sulphide nanocrystals (NCs) by thermal decomposition. These 2D molecular precursorshave all the ingredients required for metal sulphide synthesis (metal source, sulphur source and protecting ligand). In this study, we demonstrate a simple and general ‘solvothermal decomposition’ approach for the synthesis of high-quality Cu$_2$S, PbS, CdS, MnS and ZnS NCs. The size of the NC can also be controlled by changing the decomposition temperature. Furthermore, the optical properties of the NCs have also been studied.

    • Effect of annealing-temperature-assisted phase evolution on conductivity of solution combustion processed calcium vanadium oxide films


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      In thiswork, the effect of annealing temperature on the conductivity of solution-combustion-synthesized calcium vanadium oxide (CVO) films was studied. Conductivity was tailored by the appearance of the phases like CaVO$_3$, CaV$_2$O$_5$ and Ca$_2$V$_2$O$_7$ as a function of annealing temperature; CaVO$_3$ and CaV$_2$O$_5$ are responsible for high conductivity, whereas V$^{5+}$ presence in Ca$_2$V$_2$O$_7$ contributes towards dielectric nature. Evolution of phases of CVO was identified through X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. A detailed conductivity measurement as a function of annealing temperature helps us to identify the decreasing trend of conductivity with increasing temperature up to 400$^{\circ}$C; beyond this it behaves like an insulator. There was a stable conductivity while aging the films in ambient for a few days. This study revealed safe application temperature domain of CVO, and a clear correlation of electrical conductivity with the in-depth structural–compositional–morphological study.

    • Large magnetoresistance in carbon-coated Ni/NiO nanoparticles


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      We report here a large magnetoresistance (MR) observed in carbon-coated Ni/NiO nanostructures synthesize by a chemical method. The crystalline nature and particle size of the graphitic-carbon-coated Ni/NiO nanostructure wasinvestigated by X-ray diffraction study and field emission scanning electron microscope images. The Raman spectroscopy confirms the presence of graphite layer over the Ni/NiO nanostructure. The field-cooled (FC) magnetic hysteresis curves show exchange bias effect suggesting possible Ni/NiO core–shell structure. The temperature-dependent magnetization data show bifurcation in FC–zero-field-cooled curves, indicating the superparamagnetic behaviour and competing ferromagnetic (FM) and antiferromagnetic interactions in the nanocomposite. MR studies show a large negative MR of $\sim$20% at 18 K and$\sim$4.2% at room temperature, revealing significant enhancement of FM interactions at low temperatures and spin-dependent tunnelling of current through the nanocomposite.

    • CuBO$_2$ nanonetwork: a novel and significant candidate for photocatalytic dye degradation


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      CuBO$_2$ is a novel material in the research field of transparent conducting oxide. In this study, CuBO$_2$ nanostructures have been synthesized via sol–gel method. The phase formation is confirmed using an X-ray diffractometer. Detailedmorphological analysis is performed by field emission scanning electron microscopy and transmission electron microscopy. Anovel uniform nanonetwork-like structure is obtained and its band gap is found to be 4.24 eV. In ultraviolet light irradiation, this as-synthesized sample shows efficient photocatalytic activity for degradation of organic dye Rhodamine B. The degradation efficiency and the rate constant were calculated as $\sim$70% and $1.32 \times 10^{−3}$ min$^{−1}$, respectively. This nanonetwork-like structure can be a potential candidate as the base material to attach various metals and metal oxide nanostructures to get highly efficient future photocatalysts. As a result, this study opens up a new gateway to fabricate novel environment-friendly nanocatalysts with high performance.

    • Synthesis and properties of graphene and its 2D inorganic analogues with potential applications


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      Discovery of the amazing properties of graphene has aroused great interest in other 2D materials. 2D inorganic analogues of graphene such as the transition metal dichalcogenides have been investigated widely and these materials,especially MoS$_2$, exhibit many properties of interest. In particular, they possess properties of direct use in energy devices. In this article we review the synthesis and properties of the 2D materials of relevant transistors, sensors, photodetectors, supercapacitors and batteries as well as in oxygen reduction and hydrogen evolution reactions.

    • Twisted multilayer graphene exhibiting strong absorption bands induced by van Hove Singularities


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      Twisted bilayer graphene exhibits several angle-dependent properties due to the emergence of the van Hove Singularities in its density of states. Among them, twist-angle-dependent optical absorption has gained a lot of attention due to its presence in the visible spectral region. However, observation of such absorption is experimentally tricky due to large transmittance. In this study, we use highly decoupled twisted multilayer graphene to observe such absorption in thevisible region using a simple spectrometer. A large number of twisted graphene layers in the system enable observation of such absorption evident in the visible region; the absorption band position correlates with the twist angle measuredusing selective area electron diffraction pattern as well as predictions from theory. While the Raman spectra were akin to those of the decoupled graphene system, at specific twist angle of $\sim$13$^{\circ}$, the spectrum contained clear signatures of G-band enhancement.

    • Effect of Ar$^{+}$ ion implantation on the properties of electrodeposited CdTe thin films


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      Semiconducting nanomaterials of II–VI groups are the key elements of continued technological approaches made in the field of optoelectronic, magnetic and photonic devices due to their size-dependent properties. Ion beams createchanges in the material along their track; this not only exhibits excellent properties but also tailors new materials. This article reports the effect of Ar$^+$ ion implantation on the properties of cadmium telluride thin films of about 80 nm thickness. The implantation parameters were adjusted based on computer-aided learning using SRIM (stopping and range of ions in matter) software. The CdTe thin films were deposited by electrodeposition method on ITO substrate. Thin films of CdTe are exposed to Ar$^+$ ions with different fluencies of $1 \times 10^{15}$, $5 \times 10^{15}$ and $1 \times 10^{16}$ ions cm$^{−2}$ at Ion Beam Centre, Kurukshetra University, Kurukshetra, India. After implantation, the films were characterized using UV–visible spectroscopy, photoluminescence (PL) and a four-probe set-up with a programmable current–voltage (I–V) source metre. The scanning electron microscopy of pristine film showed smooth and uniform growth of sphere-shaped grains on substrate surface. From optical studies, the values of optical band gap for as-deposited and argon-ion-implanted thin films were calculated. It was found that values of optical band gap decreased with the increase in fluence of ion beam. From PL studies it was found thatthe intensity got increased with ion fluence. A considerable increase in current was noticed from I–V measurements with ion fluence after implantation. Different properties of pre- and post-implanted thin films are studied.

    • Magnetically retrievable nanocomposite of magnesium ferrite and bentonite clay for sequestration of Pb(II) and Ni(II) ions: a comparative study


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      In the present work, nanocomposite of bentonite clay with MgFe$_2$O$_4$ nanoparticles (NPs) was synthesized by sol–gel route. It was studied for the sequestration of Pb(II) and Ni(II) ions from the aqueous solution. The nanocomposite was analysed using X-ray diffraction, vibrating sample magnetometry, scanning electron microscopy equipped with energydispersiveX-ray spectroscopy, Fourier transform infrared spectroscopy, transmission electron microscopy and Brunauer–Emmett–Teller (BET) as analytical tools. The lower value of saturation magnetization (Ms) of nanocomposite (5.70 emu g$^{−1}$) as compared with pristine MgFe$_2$O$_4$ NPs (12.32 emu g$^{−1}$) is due to the presence of non-magnetic bentonite clay. BET studies further revealed higher surface area for nanocomposite (75.43 m$^2$ g$^{−1}$) than MgFe$_2$O$_4$ NPs (62.51 m$^2$ g$^{−1}$). The presence of bentonite clay during sol–gel synthesis of MgFe$_2$O$_4$ NPs prevented particle growth. The adsorption data were modelled using Temkin, Freundlich, Dubinin–Radushkevitch and Langmuir adsorption isotherms. Comparative evaluation of adsorptionpotential of nanocomposite for Pb(II) and Ni(II) ions confirmed higher affinity of Pb(II) ions ($q_{\rm max} = 90.90$ mg g$^{−1}$) towards the nanocomposite as compared with Ni(II) ions ($q_{\rm max} = 76.92$ mg g$^{−1}$). The results were explained on the basis of their hydration enthalpy. Thermodynamic analysis confirmed endothermic and spontaneous nature of adsorption processwith $\Delta H^o$ values of 48.67 and 21.54 kJ mol$^{−1}$ for Pb(II) and Ni(II) ions, respectively. Kinetic studies confirmed that a pseudo-second-order kinetic model was followed. The obtained results suggested that adsorption capacity of nanofabricated composite for Pb(II) and Ni(II) ions was higher than that of pristine MgFe$_2$O$_4$ NPs and bentonite clay. The saturated adsorbentwas magnetically retrievable and easily regenerated with 0.1 MHCl solutions. It can serve as a potential composite adsorbent for the remediation of heavy metal ions.

    • A hybrid system: MnO-incorporated mesoporous silica nanoparticles for theranostic applications


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      The need for an alternative $T_1$ contrast enhancer for magnetic resonance imaging (MRI) has been escalating owing to the toxicity profiles observed with the use of commercial contrast agents. Manganese oxide nanoparticles provide an optimal solution for the problem, as it is an endogenous co-factor for many enzymes in the biological system. In the present work, we have synthesized mesoporous silica nanoparticles encapsulated with manganese oxide nanoparticles as a positive contrast enhancer for MRI applications. Spherical magnetic MnO nanoparticles with divalent oxidation state were also synthesized and utilized as control to compare the efficiency of the nano-hybrid system. MRI showed higher contrast enhancement with the use of nano-hybrid and the relaxivity value for $T_1$-weighted imaging was calculated to be2.6 mg ml$^{−1}$ s$^{−1}$. Also, the developed system was validated for its usefulness as a therapeutic system through adsorption studies. Therefore, the nano-hybrid has the potential to be a competent MRI contrast enhancer that could be used for theranostic applications.

    • Effect of CTAB coating on structural, magnetic and peroxidase mimic activity of ferric oxide nanoparticles


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      In the present work, pristine and cetyl trimethyl ammonium bromide (CTAB)-coated ferric oxide nanoparticles (CTAB@Fe$_2$O$_3$ NPs) were synthesized and studied as enzyme mimics. The w/w ratio of Fe$_2$O$_3$ to CTAB was varied as 1:1 and 1:2. Transmission electron microscopic analysis revealed that pristine NPs had an average size of 50 nm, whereas the presence of CTAB resulted in the formation of nanorods with length of 130 nm. BET studies confirmed enhancement of surface area on CTAB coating, which was maximum for w/w ratio 1:1. The synthesized pristine NPs and CTAB-coatedNPs were evaluated for their peroxidase mimic activity using o-dianisidine dihydrochloride as substrate. Optimum pH, temperature, substrate and NPs concentration for the reaction were 1, 25$^{\circ}$C, 0.16 mg ml$^{−1}$ and 1 mg ml$^{−1}$, respectively. Peroxidase mimic activity of CTAB@Fe$_2$O$_3$ NPs (w/w 1:1) was higher than that of pristine NPs. However, further increasein CTAB coating (w/w 1:2) resulted in lowering of peroxidase mimic activity. Kinetic analysis was carried out at optimized conditions; maximum velocity ($V_{\rm max}$) and Michaelis constant ($K_{\rm m}$) value of CTAB@Fe$_2$O$_3$ NPs at 1:1 w/w ratio were 7.69 mM and 1.12 $\mu$mol s$^{−1}$, respectively.

    • Confinement of an antiferroelectric liquid crystal in a polymer nanonetwork: thermal and dielectric behaviour


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      We report the thermal and dielectric investigations on a liquid crystal exhibiting an antiferroelectric phase and confined in a polymer network of sub-micron dimensions. Two different photo-polymerizable monomers have beenemployed for the purpose: one of them (HDDA) is bereft of any aromatic parts, while the other (RM82) contains aromatic as well as aliphatic units and, in fact, forms a liquid crystalline phase in its monomeric state. The polymerization, whichis carried out in the presence of the liquid crystalline host, is expected to yield a nanosegregated structure for HDDA and blended structure for the RM82 case, the difference reflecting the morphologies of the networks, as evidenced by SEMimages. Surprisingly, even a small concentration of the latter polymer added to the former variety has substantial influence on the morphology. The main work focusses on calorimetry and dielectric relaxation spectroscopy of the host liquid crystal confined in these nanonetworks created by the polymers, which can be considered to form virtual surfaces with a finite anchoring energy.We have investigated the in-phase and antiphase modes in the antiferroelectric phase, and the soft mode in the paraelectric phase preceding the antiferroelectric phase. The relaxation frequencies of all these modes are substantiallyinfluenced by the network, with the results showing certain surprises in cases containing both HDDA and RM82.

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