Showing papers by "Sundara Ramaprabhu published in 2017"

Recent advances in hydrogen storage using catalytically and chemically modified graphene nanocomposites

Abstract: The need for efficient and renewable energy fuels is growing stronger with environmental consciousness, stringent emission norms, rising fossil fuel prices and their depleting stocks. Hydrogen could be a significant replacement for fossil fuels if targets of volumetric and gravimetric densities required for the automobile industry can be met. In this regard, hydrogen economy and hydrogen storage have become thrust areas of research in the past few decades. It is possible to store hydrogen in the solid state as chemically bound hydrogen using metal/alloy nanostructures with favorable adsorption sites. These nanoparticles when dispersed over suitable carbon nanomaterials exhibit better storage capabilities and improved adsorption–desorption kinetics. High surface area carbon supports such as graphene favor good dispersion of catalyst nanoparticles alleviating their agglomeration and modulating the interaction strength between hydrogen atoms and metal nanoparticles by providing pathways for chemi–physisorption of hydrogen in these nanocomposites. In this review, the contribution of chemically modified graphene-based materials and their composites with metal/alloy catalyst nanostructures in the field of hydrogen storage is reviewed and its future outlook discussed.

Superior photocatalytic performance of graphene wrapped anatase/rutile mixed phase TiO2 nanofibers synthesized by a simple and facile route

Abstract: Optimized combination of novel reduced graphene oxide (rGO) wrapped anatase/rutile mixed phase TiO2 nanofibers (MPTNFs) synthesized by an efficient route of electrospinning together with the help of easy chemical methods has reported to enhance the photocatalytic performance. The mesoporous MPTNFs/rGO composites were characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and transmission electron microscopy. The photocatalytic activity was evaluated by degradation of methyl orange. A significant increase in the reaction rate was observed with TiO2/rGO materials under UV light irradiation. The synergistic effect of anatase/rutile mixed phase in one dimensional nanostructure and the electronic interaction between TiO2 and rGO provided the subsequent improvement of electron transfer. These intriguing peculiarities also reduced the charge recombination for enhancement of catalytic efficiency.

High-pressure investigation of ionic functionalized graphitic carbon nitride nanostructures for CO2 capture

Abstract: Carbon dioxide is a major greenhouse gas responsible for the increase in global temperature which poses serious threat to the earth’s environment and is a burning topic of research now-a-days. So, one of the promising and energy-efficient techniques adopted these days in order to capture this toxic gas is high pressure adsorption. In this particular work, we report the synthesis of graphitic carbon nitride nanosheets (NS-g-C 3 N 4 ) using a very facile synthesis technique and its utility in adsorption of CO 2 along with the ionic functionalization of NS-g-C 3 N 4 with ionic liquid 1-Butyl-3-Methylimidazolium bis(trifluoromethyl sulfonyl)imide ([BMIM][TFSI]) and its CO 2 adsorption properties. The CO 2 adsorption analyses were carried on using high pressure Sievert’s apparatus. NS-g-C 3 N 4 shows a CO 2 sorption capacity of 19.78 mmol/g at 15 bar pressure and 25 °C which is almost two times more than that of bulk g-C 3 N 4 (8.54 mmol/g) under identical conditions. Besides, when NS-g-C 3 N 4 is functionalized with ionic liquid, the interface between ionic liquid and NS-g-C 3 N 4 significantly increases the sorption capacity to 42.93 mmol/g under nearly ideal conditions. The temperature is varied from 25 to 100 °C and pressure is varied from 2 to 25 bar. The high nitrogen content in graphitic carbon nitride nanostructures (56.6 at.%) acts as the active sites for binding the CO 2 molecules while the non-covalent ionic liquid functionalization allows CO 2 to interact with NS-g-C 3 N 4 /[BMIM][TFSI] by physicochemical adsorption mechanism and thereby enhances the CO 2 capture.

Development of a nitrogen-doped 2D material for tribological applications in the boundary-lubrication regime.

Abstract: The present paper describes a facile synthesis method for nitrogen-doped reduced graphene oxide (N-rGO) and the application of N-rGO as an effective additive for improving the tribological properties of base oil. N-rGO has been characterized by different characterization techniques such as X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy and Raman spectroscopy. N-rGO-based nanolubricants are prepared and their tribological properties are studied using a four-ball tester. The nanolubricants show excellent stability over a period of six months and a significant decrease in coefficient of friction (25%) for small amounts of N-rGO (3 mg/L). The improvement in tribological properties can be attributed to the sliding mechanism of N-rGO accompanied by the high mechanical strength of graphene. Further, the nanolubricant is prepared at large scale (700 liter) and field trials are carried out at one NTPC thermal plant in India. The implementation of the nanolubricant in an induced draft (ID) fan results in the remarkable decrease in the power consumption.

Application of Few-Layered Reduced Graphene Oxide Nanofluid as a Working Fluid for Direct Absorption Solar Collectors.

Abstract: Direct absorption solar collectors (DASC) convert solar energy into heat energy and transfer this heat energy to a carrier fluid. Numerical and experimental studies have shown that replacing the absorber medium with nanofluids in DASC increases the efficiency of solar collector significantly. Present work investigates the dispersion stability, optical and thermal properties of reduced few-layered graphene oxide (rGO) dispersed nanofluids for DASC. The synthesis of rGO was carried out by hydrogen exfoliation of graphene oxide at 200 °C. As-synthesized rGO was suitably functionalized to impart the hydrophilic nature. Different characterization techniques were employed to analyze the surface morphology of the sample. Nanofluids were prepared by dispersing calculated amount of functionalized rGO in DI water and ethylene glycol. Optical properties study reveals that the nanofluids exhibit good absorption ability over base fluids. The extinction coefficient of nanofluids showed significant improvement even at low concentration. Furthermore, the temperature dependent thermal conductivity study with different volume fractions, carried out for DI water and ethylene glycol-based nanofluids, shows considerable enhancement.

Solar synthesized tin oxide nanoparticles dispersed on graphene wrapped carbon nanotubes as a Li ion battery anode material with improved stability

Abstract: Employing high theoretical capacity SnO2 as an anode material for Li ion batteries (LIBs) is still a challenge because of its huge volume change upon lithiation/de-lithiation, leading to instability and a low cycle life of the cell. Herein we report graphene wrapped carbon nanotubes (gC), a one-dimensional monohybrid of multiwalled carbon nanotubes and graphene sheets, which support SnO2 nanoparticles to act as a stable LIB anode. An eco-friendly, solar energy mediated reduction process, instead of hazardous wet chemical methods, was used to decorate SnO2 over the gC support material. The wrinkled surface of gC, with a higher available surface area and favorable Li transfer channels, is explored as a functional preventive measure against the volume change of the metal oxide nanoparticles. This well-connected gC structure with high conductivity also accounts for the semiconducting nature of SnO2, and exhibits less resistance with a highly stable performance, thereby presenting SnO2/gC as a potential anode material for LIBs.

Theoretical Insights into the Experimental Observation of Stable p-Type Conductivity and Ferromagnetic Ordering in Vacuum-Hydrogenated TiO2

Abstract: Tuning of electrical and magnetic properties to achieve stable p-type conductivity and room temperature ferromagnetism in undoped TiO2 is quite challenging. Here both are attained simultaneously through a facile method of vacuum-hydrogenation, wherein vacuum annealing as well as hydrogenation play crucial roles. The p-type conductivity in hydrogenated TiO2 is investigated through the Hall measurement studies, which show considerable enhancement in Hall mobility and electrical conductivity. The high and low pressures of hydrogenation show strong and weak ferromagnetic ordering, respectively, whereas the pristine TiO2 NPs manifest paramagnetic behavior. In order to understand the mechanism of these characteristic changes, density functional theory (DFT) calculations are performed. DFT calculations reveal that the smaller amount of hydrogenation leads to gap-states above valence band maximum (VBM) due to the effect of hydrogen atoms 1s orbitals and by the formation of ∼Ti–H and ∼O–H bonds. Further increase i...

Binary reaction ingrained high current density and long cycle life novel anode material for lithium ion battery

Abstract: The uniqueness in the physical and electrochemical properties and the structural integrity towards lithium ion storage make carbon nanotubes (CNT) a viable anode material for the lithium ion battery. But the morphology dependent electrochemical performance and high irreversibility in capacity stand as stumbling blocks to its commercialization. Herein, we demonstrate that irreversible capacity and short cycle life in CNT based anode material can be eliminated by exfoliation of a few of its outer layers with the incorporation of sulfur. The incorporation of sulfur brings in two lithium ion storage mechanisms in the electrode, insertion and sulfur redox reaction. We have observed this phenomenon by cyclic voltammetry and the corresponding behavioral change by dynamic electrochemical impedance spectroscopy. The assembled battery has been tested for various current densities from 150 to 40 000 mA g−1. The cell retained a capacity of 200 mA h g−1 for 7370 cycles at 10 000 mA g−1, proving the capability of the material for long cycle life even at higher current densities. The ultrahigh capacity of 1120 mA h g−1 at 150 mA g−1 after 10 000 cycles reveals its great potential as an anode material for commercial lithium ion batteries.

1D-2D carbon heterostructure with low Pt loading as a superior cathode electrode for dye-sensitized solar cell

Abstract: Cost-effective counter electrode (CE) with high electrocatalytic performance is very much essential for the wide application of dye-sensitized solar cells (DSSC). The 1D-2D carbon heterostructure (Pt/GR@CNT) with low platinum (Pt) loading has been synthesized by a facile in situ microwave-assisted polyol-reduction method. The excellent electrocatalytic activity as well as photovoltaic performance was achieved due to the combination of 2D graphene nanoribbons (GR) and 1D multi-walled carbon nanotubes (CNT) with high catalytically active Pt nanoparticles. Microwave-assisted longitudinal unzipping of few outer layers of CNTs along with co-reduction of Pt nanoparticles is an effective method to create electrochemically active defective edge sites, which have a crucial role in enhancing electrochemical performance. Synergistic effect of ultra-fine Pt nanoparticles, partially unzipped graphene nanoribbons and inner core tubes of CNTs modulates the power conversion efficiency of solar cell to 5.57% ± 0.03 as compared with 4.73% ± 0.13 of CNTs. Pt/GR@CNT CE even with low Pt loading of 14 μg cm−2 showcases equivalent performance with that of pure Pt counter electrode.

Cavity induced fluorescence enhancement of graphitic carbon nitride submicron flakes

Abstract: Graphitic carbon nitride (g-C3N4), which is structurally analogous to graphene, shows excellent fluorescent yield. Sharp ripple structure is observed in the fluorescence spectra of g-C3N4 flakes grafted on the surface of single polymethyl methacrylate (PMMA) microspheres. The intensities and the number of modes of these structures nonlinearly vary with the size of micro-cavity and the coupled power. Theoretical simulations carried out with the help of Mie theory show that the ripple structure is due to modulation of the fluorescence by the whispering gallery modes (WGMs) of the spherical microcavity.