Strategies for the Synthesis of Graphene, Graphene Nanoribbons, Nanoscrolls and Related Materials

Single-layer graphene (SLG), the 3.4 angstrom thick two-dimensional sheet of sp(2) carbon atoms, was first prepared in 2004 by mechanical exfoliation of graphite crystals using the scotch tape technique. Since then, SLG has been prepared by other physical methods such as laser irradiation or ultrasonication of graphite in liquid media. Chemical methods of synthesis of SLG are more commonly used; the most popular involves preparation of single-layer graphene oxide followed by reduction with a stable reagent, often assisted by microwave heating. This method yields single-layer reduced graphene oxide. Other methods for preparing SLG include chemical vapour deposition over surfaces of transition metals such as Ni and Cu. Large-area SLG has also been prepared by epitaxial growth over SIC. Few-layer graphene (FLG) is prepared by several methods; arc discharge of graphite in hydrogen atmosphere being the most convenient. Several other methods for preparing FLG include exfoliation of graphite oxide by rapid heating, ultrasonication or laser irradiation of graphite in liquid media, reduction of few-layer graphene oxide, alkali metal intercalation followed by exfoliation. Graphene nanoribbons, which are rectangular strips of graphene, are best prepared by the unzipping of carbon nanotubes by chemical oxidation or laser irradiation. Many graphene analogues of inorganic materials such as MoS2, MoSe2 and BN have been prepared by mechanical exfoliation, ultrasonication and by chemical methods involving high-temperature or hydrothermal reactions and intercalation of alkali metals followed by exfoliation. Scrolls of graphene are prepared by potassium intercalation in graphite or by microwave irradiation of graphite immersed in liquid nitrogen.

Digestive ripening: a synthetic method par excellence for core-shell, alloy, and composite nanostructured materials

The solvated metal atom dispersion (SMAD) method has been used for the synthesis of colloids of metal nanoparticles. It is a top-down approach involving condensation of metal atoms in low temperature solvent matrices in a SMAD reactor maintained at 77 K. Warming of the matrix results in a slurry of metal atoms that interact with one another to form particles that grow in size. The organic solvent solvates the particles and acts as a weak capping agent to halt/slow down the growth process to a certain extent. This as-prepared colloid consists of metal nanoparticles that are quite polydisperse. In a process termed as digestive ripening, addition of a capping agent to the as-prepared colloid which is polydisperse renders it highly monodisperse either under ambient or thermal conditions. In this, as yet not well-understood process, smaller particles grow and the larger ones diminish in size until the system attains uniformity in size and a dynamic equilibrium is established. Using the SMAD method in combination with digestive ripening process, highly monodisperse metal, core-shell, alloy, and composite nanoparticles have been synthesized. This article is a review of our contributions together with some literature reports on this methodology to realize various nanostructured materials.

Multi-component, Self-assembled, Functional Soft Materials

Supramolecular chemistry is an emerging tool for devising materials that can perform specified functions. The self-assembly of facially amphiphilic bile acid molecules has been extensively utilized for the development of functional soft materials. Supramolecular hydrogels derived from the bile acid backbone act as useful templates for the intercalation of multiple components. Based on this, synthesis of gel-nanoparticle hybrid materials, photoluminescent coating materials, development of a new enzyme assay technique, etc. were achieved in the author's laboratory. The present account highlights some of these achievements.

Soil stabilization with waste materials based binder

This paper describes a laboratory trial to study the effectiveness of a waste-based binder to stabilize expansive soils. The proposed binders viz., Fly ash and/or Ground granulated Blast furnace slag (GGBS) were mixed with the expansive soil along with a small amount of lime to increase soil pH and enable pozzolanic reactions. The geotechnical characteristics of the various combinations of samples were investigated through the compaction tests, unconfined compression tests etc. It was found that the addition of GGBS with and without fly ash and lime has significant influence on the geotechnical characteristics of the soil.

Tribological response of soft materials sliding against hard surface textures at various numbers of cycles

In the present investigation, various kinds of textures were attained on the steel surfaces. Roughness of the textures was varied using different grits of emery papers or polishing powders. Pins made of pure Al, Al-4Mg alloy and pure Mg were then slid against prepared steel plate surfaces at various numbers of cycles using an inclined pin-on-plate sliding tester. Tests were conducted at a sliding velocity of 2mms(-1) in ambient conditions under both dry and lubricated conditions. Normal loads were increased up to 110N during the tests. The morphologies of the worn surfaces of the pins and the formation of transfer layer on the counter surfaces were observed using a scanning electron microscope. Surface roughness parameters of the plate were measured using an optical profilometer. In the experiments, it was observed that the coefficient of friction and formation of a transfer layer (under dry and lubricated conditions) only depended on surface texture during the first few sliding cycles. The steady-state variation in the coefficient of friction under both dry and lubrication conditions was attributed to the self-organisation of texture of the surfaces at the interface during sliding. Copyright (C) 2012 John Wiley & Sons, Ltd.

Conditions for the onset of elastic and material instabilities in hyperelastic materials

Several constitutive inequalities have been proposed in the literature to quantify the notion that ‘stress increases with strain’ in an elastic material. Due to some inherent shortcomings in them, which we discuss, we propose a new tensorial criterion for isotropic materials. We also present necessary conditions in terms of elasticity tensors for the onset of elastic instabilities.

Cavitation in materials with distributed weak zones: Implications on the origin of brittle fracture in metallic glasses

Several experimental studies have shown that fracture surfaces in brittle metallic glasses (MGs) generally exhibit nanoscale corrugations which may be attributed to the nucleation and coalescence of nanovoids during crack propagation. Recent atomistic simulations suggest that this phenomenon is due to large spatial fluctuations in material properties in a brittle MG, which leads to void nucleation in regions of low atomic density and then catastrophic fracture through void coalescence. To explain this behavior, we propose a model of a heterogeneous solid containing a distribution of weak zones to represent a brittle MG. Plane strain continuum finite element analysis of cavitation in such an elastic-plastic solid is performed with the weak zones idealized as periodically distributed regions having lower yield strength than the background material. It is found that the presence of weak zones can significantly reduce the critical hydrostatic stress for the onset of cavitation which is controlled uniquely by the local yield properties of these zones. Also, the presence of weak zones diminishes the sensitivity of the cavitation stress to the volume fraction of a preexisting void. These results provide plausible explanations for the observations reported in recent atomistic simulations of brittle MGs. An analytical solution for a composite, incompressible elastic-plastic solid with a weak inner core is used to investigate the effect of volume fraction and yield strength of the core on the nature of cavitation bifurcation. It is shown that snap-cavitation may occur, giving rise to sudden formation of voids with finite size, which does not happen in a homogeneous plastic solid. (c) 2012 Elsevier Ltd. All rights reserved.

Carbonaceous nickel oxide nano-composites: as electrode materials in electrochemical capacitor applications

Carbonaceous nickel oxide powder samples have been synthesized from an adducted nickel beta-ketoester complex used as a ``single source precursor'' through a solution-based microwave-assisted chemical route. Comprehensive analysis of the resulting powder material has been carried out using various characterization techniques. These analysis reveal that, depending on the solvent used, either NiO/C or Ni/NiO/C composites are formed, wherein Ni and/or NiO nanocrystals are enveloped in amorphous carbon. As the components emerge from the same molecular source, the composites are homogeneous on a fine scale, making them promising electrode materials for supercapacitors. Electrochemical capacitive behavior of these oxide composites is studied in a three-electrode configuration. With a specific capacitance of 113 F g(-1), Ni/NiO/C is superior to NiO/C as capacitor electrode material, in 0.1 M Na2SO4 electrolyte. This is confirmed by impedance measurements, which show that charge-transfer resistance and equivalent series resistance are lower in Ni/NiO/C than in NiO/C, presumably because of the presence of metallic nickel in the former. The cyclic voltammograms are nearly rectangular and the electrodes display excellent cyclability in different electrolytes: Na2SO4, KOH and Ca(NO3)(2)center dot 4H(2)O. Specific capacitance as high as 143 F g(-1), is measured in Ca(NO3)(2)center dot 4H(2)O electrolyte.

Determination of electric field at and near the focus of a cylindrical lens for applications in fluorescence microscopy

We present an explicit computable integral solution of the electric field generated at the focal region of a cylindrical lens. This representation is based on vectorial diffraction theory and further enables the computation of the system point spread function of a cylindrical lens. It is assumed that there is no back-scattering and the contribution from the evanescent field is negligible. Stationary phase approximation along with the Fresnel transmission coefficients are employed for evaluating the polarization dependent electric field components. Studies were carried out to determine the polarization effects and to calculate the system resolution. The effect of s -, p - and randomly polarized light is studied on the fixed sample (electric dipole is fixed in space). Proposed approach allows better understanding of electric field effects at the focus of a cylindrical aplanatic system. This opens up future developments in the field of fluorescence microscopy and optical imaging. (C) 2013 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.

Rheometry of dense granular materials: the crucial effects of gravity and confining walls

We describe here the rheological response of dense, slowly deforming granular materials to shear in a cylindrical Couette cell. All components of the stress on the outer cylinder are measured pointwise as a function of the depth, for different methods of construction of the bed that presumably lead to distinct fabrics. The static stress profiles for the different construction protocols are different, but a stress profile that is independent of construction history emerges when the granular column is sheared for sufficient time, in accord with the predictions of plasticity theories. However the qualitative features of the the stress profile under shear differs radically from the predictions of plasticity theories and data reported in earlier studies. We discuss a hypothesis for the anomalous stress profiles that was proposed recently by us, and the ways in which further experiments may to conducted to verify it.

Experimental Studies on the Kinematics of Cutting in Granular Materials

Slow flow in granular materials is characterized by high solid fraction and sustained inter-particle interaction. The kinematics of trawling or cutting is encountered in processes such as locomotion of organisms in sand; trawl gear movement on a soil deposit; plow movement; movement of rovers, earth moving equipment etc. Additionally, this configuration is very akin to shallow drilling configuration encountered in the mining and petroleum industries. An experimental study has been made in order to understand velocity and deformation fields in cutting of a model rounded sand. Under nominal plane strain conditions, sand is subjected to orthogonal cutting at different tool-rake angles. High-resolution optical images of the region of cutting were obtained during the flow of the granular ensemble around the tool. Interesting kinematics underlying the formation of a chip and the evolution of the deformation field is seen in these experiments. These images are also analyzed using a PIV algorithm and detailed information of the deformation parameters such as velocity, strain rate and volume change is obtained.

EFFECT OF CORE-SHELL STRUCTURE OF HYDROGEL BEADS ON THE THRESHOLD CONCENTRATION OF WATER FOR SWELLING AND ITS PH SENSITIVITY

In this paper we investigate the effect of core-shell structure of Sodium Alginate based hydrogel beads and their size on certain activation threshold concentration of water for applications in swelling and pH sensing. This type of hydrogel experiences diffusive pressure due to transport of certain free charges across its interface with a solvent or electrolyte. This process is essentially a dynamic equilibrium of the electric force field, stress in the polymeric network with cage like structure and molecular diffusion including phase transformation due to pressure imbalance between the hydrogel and its surroundings. The effect of pH of the solvant on the swelling rate of these beads has been studied experimentally. A mathematical model of the swelling process has been developed by considering Nernst-Planck equation representing the migration of mobile ions and Er ions, Poisson equation representing the equilibrium of the electric field and mechanical field equation representing swelling of the gel. An attempt has been made to predict the experimentally observed phenomena using these numerical simulations. It is observed experimentally that certain minimum concentration called activation threshold concentration of the water molecules must be present in the hydrogel in order to activate the swelling process. For the required activation threshold concentration of water in the beads, the pH induced change in the rate of swelling is also investigated. This effect is analyzed for various different core-shell structures of the beads.

pH Induced switching in hydrogel coated fiber bragg grating sensor

In this paper we report a novel hydrogel functionalized optical Fiber Bragg Grating (FBG) sensor based on chemo-mechanical-optical sensing, and demonstrate its specific application in pH activated process monitoring. The sensing mechanism is based on the stress due to ion diffusion and polymer phase transition which produce strain in the FBG. This results in shift in the Bragg wavelength which is detected by an interrogator system. A simple dip coating method to coat a thin layer of hydrogel on the FBG has been established. The gel consists of sodium alginate and calcium chloride. Gel formation is observed in real-time by continuously monitoring the Bragg wavelength shift. We have demonstrated pH sensing in the range of pH of 2 to 10. Another interesting phenomenon is observed by swelling and deswelling of FBG functionalized with hydrogel by a sequence of alternate dipping between acidic and base solutions. It is observed that the Bragg wavelength undergoes reversible and repeatable pH dependent switching.

Rheometry of granular materials in cylindrical Couette cells: Anomalous stress caused by gravity and shear

The cylindrical Couette device is commonly employed to study the rheology of fluids, but seldom used for dense granular materials. Plasticity theories used for granular flows predict a stress field that is independent of the shear rate, but otherwise similar to that in fluids. In this paper we report detailed measurements of the stress as a function of depth, and show that the stress profile differs fundamentally from that of fluids, from the predictions of plasticity theories, and from intuitive expectation. In the static state, a part of the weight of the material is transferred to the walls by a downward vertical shear stress, bringing about the well-known Janssen saturation of the stress in vertical columns. When the material is sheared, the vertical shear stress changes sign, and the magnitudes of all components of the stress rise rapidly with depth. These qualitative features are preserved over a range of the Couette gap and shear rate, for smooth and rough walls and two model granular materials. To explain the anomalous rheological response, we consider some hypotheses that seem plausibleapriori, but showthat none survive after careful analysis of the experimental observations. We argue that the anomalous stress is due to an anisotropic fabric caused by the combined actions of gravity, shear, and frictional walls, for which we present indirect evidence from our experiments. A general theoretical framework for anisotropic plasticity is then presented. The detailed mechanics of how an anisotropic fabric is brought about by the above-mentioned factors is not clear, and promises to be a challenging problem for future investigations. (C) 2013 AIP Publishing LLC.

Li3MRuO5 (M = Co, Ni), new lithium-rich layered oxides related to LiCoO2: promising electrochemical performance for possible application as cathode materials in lithium ion batteries

We describe the synthesis and crystal structure of Li3MRuO5 (M = Co and Ni), new rock salt related oxides. Both the oxides crystallize in the layered LiCoO2 (alpha-NaFeO2) structure, as revealed by powder XRD data. Magnetic susceptibility data suggest that the oxidation states of transition metals are Li3Co3+(ls)Ru4+(ls) O-5 (ls = low spin) for the M = Co compound and Li3Ni2+Ru5+O5 for the M = Ni compound. Electrochemical investigations of lithium deintercalation-intercalation behaviour reveal that both Co and Ni phases exhibit attractive specific capacities of ca. 200 mA h g(-1) at an average voltage of 4 V that has been interpreted as due to the oxidation of Co3+ and Ru4+ in Li3CoRuO5 and Ni2+ to Ni4+ in the case of Li3NiRuO5. Thus, a different role of Ru ions is played in the isostructural oxides. Finally, in both cases evidence of irreversible behaviour above 4.2 V is observed and interpreted as formation of high valent ions or alternatively oxidation of oxide ions.