[Show abstract][Hide abstract] ABSTRACT: The recent discovery of non-saturating giant positive magnetoresistance in
Td-WTe2 has aroused great interest in this material. We have studied the
structural, electronic and vibrational properties of bulk and few-layer Td-WTe2
experimentally and theoretically. Spin-orbit coupling is found to govern the
semi-metallic character of Td-WTe2. Its structural link with the metallic 1T
form provides an understanding of its structural stability. We observe a metal
to insulator transition and a change in the sign of the Seebeck coefficient
around 373 K. Lattice vibrations in Td-WTe2 have been analyzed by first
principle calculations. Out of the 33 possible zone-center Raman active modes,
five distinct Raman bands are observed around 112, 118, 134, 165 and 212 cm-1
in bulk Td-WTe2. Based on symmetry analysis and the calculated Raman tensors,
we assign the intense bands at 165 cm-1 and 212 cm-1 to the A_1^' and A_1^"
modes respectively. We have examined the effect of temperature and the number
of layers on the Raman spectrum. Most of the bands of Td-WTe2 stiffen, and the
ratio of the integrated intensities of the A_1^" to A_1^' bands decreases in
the few-layer sample, while all the bands soften in both bulk and few-layer
samples with increasing temperature.
[Show abstract][Hide abstract] ABSTRACT: Performance of supercapacitors based on 1:1 (by weight) composites of polyaniline (PANI) with nanosheets of nitrogenated reduced graphene oxide (NRGO), BC 1.5 N, MoS 2 and WS 2 has been investigated in detail. The highest specific capacitance is found with the 1:1 NRGO-PANI composite, the value being 561 F/g at a current density of 0.2 A/g. All the 1:1 nanocomposites show good cyclability. Increasing the PANI content increases the specific capacitance and the highest value found being 715 F/g at a current density of 0.5 A/g in the case of the 1:6 NRGO-PANI composite. However, all the 1:6 composites show a marked decrease in specific capacitance with increase in current density. The energy density of 1:6 NRGO-PANI is $ 25 Wh/Kg at 0.5 A/g and 1:1 NRGO-PANI is $ 19 Wh/Kg at 0.2 A/g. NRGO-PANI composites clearly stand out as viable materials for practical applications.
Nano Energy 12/2014; online. · 10.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Noting that BiI3 and the well-known topological insulator (TI) Bi2Se3 have the same high symmetry parent structures, and that it is desirable to find a wide-band gap TI, we determine here the effects of pressure on the structure, phonons and electronic properties of rhombohedral BiI3. We report a pressure-induced insulator-metal transition near 1.5 GPa, using high pressure electrical resistivity and Raman measurements. X-ray diffraction studies, as a function of pressure, reveal a structural peculiarity of the BiI3 crystal, with a drastic drop in c/a ratio at 1.5 GPa, and a structural phase transition from rhombohedral to monoclinic structure at 8.8 GPa. Interestingly, the metallic phase, at relatively low pressures, exhibits minimal resistivity at low temperatures, similar to that in Bi2Se3. We corroborate these findings with first-principles calculations and suggest that the drop in the resistivity of BiI3 in the 1–3 GPa range of pressure arises possibly from the appearance of an intermediate crystal phase with a lower band-gap and hexagonal crystal structure. Calculated Born effective charges reveal the presence of metallic states in the structural vicinity of rhombohedral BiI3. Changes in the topology of the electronic bands of BiI3 with pressure, and a sharp decrease in the c/a ratio below 2 GPa, are shown to give rise to changes in the slope of phonon frequencies near that pressure.
[Show abstract][Hide abstract] ABSTRACT: The performance of ZnO/Pt/Cd1-yZnyS (y= 0.0, 0.2) heterostructures in generating hydrogen on visible light
irradiation is substantially improved if ZnO is co-substituted 10 with N and F, since such anion substitution results in intense visible light absorption and decrease in the band gap.
Journal of Materials Chemistry A: Materials for Energy and Sustainability 04/2014;
[Show abstract][Hide abstract] ABSTRACT: The present study demonstrates the use of few-layer borocarbonitride nanosheets synthesized by a simple method as non-platinum cathode catalysts for the oxygen reduction reaction (ORR) in alkaline medium. Composition-dependent ORR activity is observed and the best performance was found when the composition was carbon-rich. Mechanistic aspects reveal that ORR follows the 4 e(-) pathway with kinetic parameters comparable to those of the commercial Pt/C catalyst. Excellent methanol tolerance is observed with the BCN nanosheets unlike with Pt/C.
Chemistry - An Asian Journal 01/2014; 9(3). · 4.57 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: It is no exaggeration to state that the energy crisis is the most serious challenge that we face today.
Among the strategies to gain access to reliable, renewable energy, the use of solar energy has
clearly emerged as the most viable option. A promising direction in this context is artificial photosynthesis.
In this article, we briefly describe the essential features of artificial photosynthesis in
comparison with natural photosynthesis and point out the modest success that we have had in splitting
water to produce oxygen and hydrogen, specially the latter.
Current science 01/2014; 106(4):518-527. · 0.83 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A study of the visible light induced oxidation of water by perovskite oxides of the formula LaMO3
(M = transition metal) has revealed the best activity with LaCoO3 which contains Co3+ in the intermediate-
spin (IS) with one eg electron. Among the rare earth manganites, only orthorhombic manganites with
octahedral Mn3+ ions exhibit good catalytic activity, but hexagonal manganites are poor catalysts. Interestingly,
not only the perovskite rare earth cobaltites but also solid solutions of Co3+ in cubic rare earth
sesquioxides exhibit catalytic activity comparable to LaCoO3, the Co3+ ion in all these oxides also being in
the IS t5 2ge1g
Chemical Physics Letters 01/2014; 591:277. · 1.99 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Nitrogen-doped reduced graphene oxide (RGO) samples with different
nitrogen content, prepared by two different methods, as well as
nitrogen-doped few-layer graphene have been investigated as
supercapacitor electrodes. Two electrode measurements have been carried
out both in aqueous (6M KOH) and in ionic liquid media. Nitrogen-doped
reduced graphene oxides exhibit satisfactory specific capacitance, the
values reaching 126F/g at a scan rate of 10mV/s in aqueous medium.
Besides providing supercapacitor characteristics, the study has shown
the nitrogen content and surface area to be important factors. High
surface-area borocarbonitrides, BxCyNz,
prepared by the urea route appear to be excellent supercapacitor
electrode materials. Thus, BC4.5N exhibits a specific
capacitance of 169F/g at a scan rate of 10mV/s in aqueous medium. In an
ionic liquid medium, nitrogen-doped RGO and BC4.5N exhibit
specific capacitance values of 258F/g and 240F/g at a scan rate of
5mV/s. The ionic liquid enables a larger operating voltage range of
0.0-2.5V compared to 0.0-1V in aqueous medium.
Solid State Communications 12/2013; 175-176:43-50. · 1.70 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Two sorts of MoS2 : A single-layer, metallic form of MoS2 (1T-MoS2 ) and a nanocomposite of a second form of MoS2 (few-layer 2H-MoS2 ) with heavily nitrogenated reduced graphene oxide (NRGO; N content ca. 15 %) show outstanding performance in the production of H2 under visible-light illumination.
Angewandte Chemie International Edition 11/2013; · 11.34 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Single-walled nanohorns (SWNHs) have been prepared by sub-merged arc discharge of graphite electrodes in liquid nitrogen. The samples were examined by scanning electron microscopy, transmission electron microscopy and Raman spectroscopy. Nitrogen and boron doped SWNHs have been prepared by the sub-merged arc discharge method using melamine and elemental boron as precursors. Intensification of Raman D-band and stiffening of G-band has been observed in the doped samples. The electrical resistance of the SWNHs varies in opposite directions with nitrogen and boron doping. Functionalization of SWNHs through amidation has been carried out for solubilizing them in non-polar solvents. Water-soluble SWNHs have been produced by acid treatment and non-covalent functionalization with a coronene salt. SWNHs have been decorated with nanoparticles of Au, Ag and Pt. Interaction of electron donor (tetrathiafulvalene, TTF) and acceptor molecules (tetracyanoethylene, TCNE) with SWNHs has been investigated by Raman spectroscopy. Progressive softening and stiffening of Raman G-band has been observed respectively with increase in the concentration of TTF and TCNE.
Journal of Cluster Science 10/2013; xxx(1):xxx-xxx. · 1.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The aim of the contribution is to introduce a high performance anode alternative to graphite for lithium-ion batteries (LiBs). A simple process was employed to synthesize uniform graphene-like few-layer tungsten sulfide (WS2) supported on reduced graphene oxide (RGO) through a hydrothermal synthesis route. The WS2–RGO (80:20 and 70:30) composites exhibited good enhanced electrochemical performance and excellent rate capability performance when used as anode materials for lithium-ion batteries. The specific capacity of the WS2–RGO composite delivered a capacity of 400–450 mAh g−1 after 50 cycles when cycled at a current density of 100 mA g−1. At 4000 mA g−1, the composites showed a stable capacity of approximately 180–240 mAh g−1, respectively. The noteworthy electrochemical performance of the composite is not additive, rather it is synergistic in the sense that the electrochemical performance is much superior compared to both WS2 and RGO. As the observed lithiation/delithiation for WS2–RGO is at a voltage≈1.0 V (≈0.1 V for graphite, Li+/Li), the lithium-ion battery with WS2–RGO is expected to possess high interface stability, safety and management of electrical energy is expected to be more efficient and economic.
Nano Energy 09/2013; 2(5):787-793. · 10.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Prompted by the early results on the catalytic activity of LiMn2O4 and related oxides in the photochemical oxidation of water, our detailed study of several manganese oxides has shown that trivalency of Mn is an important factor in determining the catalytic activity. Thus, Mn2O3, LaMnO3, and MgMn2O4 are found to be very good catalysts with turnover frequencies of 5 × 10(-4) s(-1), 4.8 × 10(-4) s(-1), and 0.8 ×10(-4) s(-1), respectively. Among the cobalt oxides, Li2Co2O4 and LaCoO3-especially the latter-exhibit excellent catalytic activity, with the turnover frequencies being 9 × 10(-4) s(-1) and 1.4 × 10(-3) s(-1), respectively. The common feature among the catalytic Mn and Co oxides is not only that Mn and Co are in the trivalent state, but Co(3+) in the Co oxides is in the intermediate t2g(5)eg(1) state whereas Mn(3+) is in the t2g(3)eg(1) state. The presence of the eg(1) electron in these Mn and Co oxides is considered to play a crucial role in the photocatalytic properties of the oxides.
Proceedings of the National Academy of Sciences 07/2013; · 9.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Two kinds in a box: The electronic and optical properties of ZnO substituted with nitrogen and fluorine are studied both experimentally and theoretically. The presence of fluorine enhances the incorporation of nitrogen in the lattice. Co-substitution reduces the optical band gap of ZnO significantly and increases the dielectric constant. The long-wavelength visible absorption of N, F-ZnO is reflected in its bright orange color.
[Show abstract][Hide abstract] ABSTRACT: Nanoparticles of CdO2 and ZnO2 are shown to oxidize toluene primarily to benzaldehyde in the 160-180 °C range, around which temperature the nanoparticles decompose to give the oxides. The product selectivity and other features of the reaction are explained taking into account the various by products formed in the reaction. These metal peroxides also activate the C-H bonds of cyclohexane. It is possible to bring down the reaction temperature by partially substituting Zn in ZnO2 by Ni.
[Show abstract][Hide abstract] ABSTRACT: Heavily nitrogenated graphene oxide containing 18 wt% nitrogen, prepared by microwave synthesis with urea as the nitrogen source, shows outstanding performance as a supercapacitor electrode material, with the specific capacitance going up to 461 F g−1.
[Show abstract][Hide abstract] ABSTRACT: Ionothermal synthesis: Ultrathin (~4 nm) few-layer nanostructures of Bi2 Se3 and related chalcogenides have been prepared by green ionothermal synthesis. The ionic liquid acts as an intercalating and stabilizing agent in addition to being an efficient solvent for the synthesis of few-layer Bi2 Se3 . High electrical conductivity and minimal thermal conductivity optimize the thermoelectric properties of few-layer Bi2 Se3 .
Chemistry - A European Journal 05/2013; · 5.93 Impact Factor