Explosives Sensing by Using Electron-Rich Supramolecular Polymers: Role of Intermolecular Hydrogen Bonding in Significant Enhancement of Sensitivity

We demonstrate here that supramolecular interactions enhance the sensitivity towards detection of electron-deficient nitro-aromatic compounds (NACs) over discrete analogues. NACs are the most commonly used explosive ingredients and are common constituents of many unexploded landmines used during World WarII. In this study, we have synthesised a series of pyrene-based polycarboxylic acids along with their corresponding discrete esters. Due to the electron richness and the fluorescent behaviour of the pyrene moiety, all the compounds act as sensors for electron-deficient NACs through a fluorescence quenching mechanism. A Stern-Volmer quenching constant determination revealed that the carboxylic acids are more sensitive than the corresponding esters towards NACs in solution. The high sensitivity of the acids was attributed to supramolecular polymer formation through hydrogen bonding in the case of the acids, and the enhancement mechanism is based on an exciton energy migration upon excitation along the hydrogen-bond backbone. The presence of intermolecular hydrogen bonding in the acids in solution was established by solvent-dependent fluorescence studies and dynamic light scattering (DLS) experiments. In addition, the importance of intermolecular hydrogen bonds in solid-state sensing was further explored by scanning tunnelling microscopy (STM) experiments at the liquid-solid interface, in which structures of self-assembled monolayer of the acids and the corresponding esters were compared. The sensitivity tests revealed that these supramolecular sensors can even detect picric acid and trinitrotoluene in solution at levels as low as parts per trillion (ppt), which is much below the recommended permissible level of these constituents in drinking water.

Graphene Oxide-MnFe2O4 Magnetic Nanohybrids for Efficient Removal of Lead and Arsenic from Water

We show that the hybrids of single-layer graphene oxide with manganese ferrite magnetic nanoparticles have the best adsorption properties for efficient removal of Pb(II), As(III), and As(V) from contaminated water. The nanohybrids prepared by coprecipitation technique were characterized using atomic force and scanning electron microscopies, Fourier transformed infrared spectroscopy, Raman spectroscopy, X-ray diffraction, and surface area measurements. Magnetic character of the nanohybrids was ascertained by a vibrating sample magnetometer. Batch experiments were carried out to quantify the adsorption kinetics and adsorption capacities of the nanohybrids and compared with the bare nanoparticles of MnFe2O4. The adsorption data from our experiments fit the Langmuir isotherm, yielding the maximum adsorption capacity higher than the reported values so far. Temperature-dependent adsorption studies have been done to estimate the free energy and enthalpy of adsorption. Reusability, ease of magnetic separation, high removal efficiency, high surface area, and fast kinetics make these nanohybrids very attractive candidates for low-cost adsorbents for the effective coremoval of heavy metals from contaminated water.

Surface modification of textured silicon and its wetting behaviour

Textured silicon (Si) substrate were prepared using various texturing methods both chemical and physical and their water contact angle, surface topography and Raman spectra were studied and investigated. The effect of plasma and chemical treatment on micro/nanostructure and roughness of the surface with and without deposition of Octadecyltrichlorosilane (ODTS, Cl3Si (CH3)(17)), self-assembled monolayer (SAM) is investigated for achieving higher water contact angle (theta(c)). The importance of synergism of texturing with deposition of ODTS SAM in preparing superhydrophobic silicon surfaces has been discussed. It is shown that superhydrophobic silicon surfaces can be achieved on silicon surfaces by coating with ODTS, irrespective of whether it is textured or not, polished or unpolished, provided a chemical treatment is given to the surface prior to the ODTS coating.

Layer-by-Layer Assembly of Nafion on Surlyn with Ultrahigh Water Vapor Barrier

A layer-by-layer approach was used for the fabrication of multilayer films for ultra high gas barrier applications. The ultra high gas barrier material was designed by incorporating Nafion layer in between bilayers of poly(ethylene imine) and poly(acrylic acid) on a Surlyn substrate. When the barrier film with self-assembled Nafion is exposed to the moist environment, Nafion absorbs and desorbs water molecules simultaneously, thereby reducing the ingress of moisture in to the film. In order to study the effect of Nafion, the fabricated barrier materials with and without the presence of Nafion were tested for water vapor barrier properties. The barrier films were further used for encapsulating organic photovoltaic devices and were evaluated for their potential use in barrier applications. The devices encapsulated with the films containing Nafion exhibited better performance when subjected to accelerated aging conditions. Therefore, this study demonstrates the effectiveness of self-assembled Nafion in reducing the water vapor permeability by nearly five orders of magnitude and in increasing the lifetimes of organic devices by similar to 22 times under accelerated weathering conditions.

Room Temperature Growth of Ultrathin Au Nanowires with High Areal Density over Large Areas by in Situ Functionalization of Substrate

Although ultrathin Au nanowires (similar to 2 nm diameter) are expected to demonstrate several interesting properties, their extreme fragility has hampered their use in potential applications. One way to improve the stability is to grow them on substrates; however, there is no general method to grow these wires over large areas. The existing methods suffer from poor coverage and associated formation of larger nanoparticles on the substrate. Herein, we demonstrate a room temperature method for growth of these nanowires with high coverage over large areas by in situ functionalization of the substrate. Using control experiments, we demonstrate that an in situ functionalization of the substrate is the key step in controlling the areal density of the wires on the substrate. We show that this strategy works for a variety of substrates ranging like graphene, borosil glass, Kapton, and oxide supports. We present initial results on catalysis using the wires grown on alumina and silica beads and also extend the method to lithography-free device fabrication. This method is general and may be extended to grow ultrathin Au nanowires on a variety of substrates for other applications.

On the Two-Step Mechanism for Synthesis of Transition-Metal Nanoparticles

The two-step particle synthesis mechanism, also known as the Finke-Watzky (1997) mechanism, has emerged as a significant development in the field of nanoparticle synthesis. It explains a characteristic feature of the synthesis of transition metal nanoparticles, an induction period in precursor concentration followed by its rapid sigmoidal decrease. The classical LaMer theory (1950) of particle formation fails to capture this behavior. The two-step mechanism considers slow continuous nucleation and autocatalytic growth of particles directly from precursor as its two kinetic steps. In the present work, we test the two-step mechanism rigorously using population balance models. We find that it explains precursor consumption very well, but fails to explain particle synthesis. The effect of continued nucleation on particle synthesis is not suppressed sufficiently by the rapid autocatalytic growth of particles. The nucleation continues to increase breadth of size distributions to unexpectedly large values as compared to those observed experimentally. A number of variations of the original mechanism with additional reaction steps are investigated next. The simulations show that continued nucleation from the beginning of the synthesis leads to formation of highly polydisperse particles in all of the tested cases. A short nucleation window, realized with delayed onset of nucleation and its suppression soon after in one of the variations, appears as one way to explain all of the known experimental observations. The present investigations clearly establish the need to revisit the two-step particle synthesis mechanism.

Pressure-Dependent Optical and Vibrational Properties of Mono layer Molybdenum Disulfide

Controlling the band gap by tuning the lattice structure through pressure engineering is a relatively new route for tailoring the optoelectronic properties of two-dimensional (2D) materials. Here, we investigate the electronic structure and lattice vibrational dynamics of the distorted monolayer 1T-MoS2 (1T') and the monolayer 2H-MoS2 via a diamond anvil cell (DAC) and density functional theory (DFT) calculations. The direct optical band gap of the monolayer 2H-MoS2 increases by 11.7% from 1.85 to 2.08 eV, which is the highest reported for a 2D transition metal dichalcogenide (TMD) material. DFT calculations reveal a subsequent decrease in the band gap with eventual metallization of the monolayer 2H-MoS2, an overall complex structureproperty relation due to the rich band structure of MoS2. Remarkably, the metastable 1T'-MoS2 metallic state remains invariant with pressure, with the J(2), A(1g), and E(2)g modes becoming dominant at high pressures. This substantial reversible tunability of the electronic and vibrational properties of the MoS2 family can be extended to other 2D TMDs. These results present an important advance toward controlling the band structure and optoelectronic properties of monolayer MoS2 via pressure, which has vital implications for enhanced device applications.

Insights on Defect-Mediated Heterogeneous Nucleation of Graphene on Copper

The grain size of monolayer large area graphene is key to its performance. Microstructural design for the desired grain size requires a fundamental understanding of graphene nucleation and growth. The two levers that can be used to control these aspects are the defect density, whose population can be controlled by annealing, and the gas-phase supersaturation for activation of nucleation at the defect sites. We observe that defects on copper surface, namely dislocations, grain boundaries, triple points, and rolling marks, initiate nucleation of graphene. We show that among these defects dislocations are the most potent nucleation sites, as they get activated at lowest supersaturation. As an illustration, we tailor the defect density and supersaturation to change the domain size of graphene from <1 mu m(2) to >100 mu m(2). Growth data reported in the literature has been summarized on a supersaturation plot, and a regime for defect-dominated growth has been identified. In this growth regime, we demonstrate the spatial control over nucleation at intentionally introduced defects, paving the way for patterned growth of graphene. Our results provide a unified framework for understanding the role of defects in graphene nucleation and can be used as a guideline for controlled growth of graphene.

Controlled synthesis of tunable nanoporous carbons for gas storage and supercapacitor application

A simple methodology has been developed for the synthesis of functional nanoporous carbon (NPC) materials using a metal-organic framework (IRMOF-3) that can act as a template for external carbon precursor (viz, sucrose) and also a self-sacrificing carbon source. The resultant graphitic NPC samples (abbreviated as NPC-0, NPC-150, NPC-300, NPC-500 and NPC-1000 based on sucrose loading) obtained through loading different amounts of sucrose exhibit tunable textural parameters. Among these, NPC-300 shows very high surface area (BET approximate to 3119 m(2)/g, Langmuir approximate to 4031 m(2)/g) with a large pore volume of 1.93 cm(3)/g. High degree of porosity coupled with polar surface functional groups, make NPC-300 remarkable candidate for the uptake of H-2 (2.54 wt% at 1 bar, and 5.1 wt% at 50 bar, 77 K) and CO2 (64 wt% at 1 bar, 195 K and 16.9 wt% at 30 bar, 298 K). As a working electrode in a supercapacitor cell, NPC-300 shows excellent reversible charge storage thus, demonstrating multifunctional usage of the carbon materials. (C) 2015 Elsevier Inc. All rights reserved.

Charge Transfer Induces Formation of Stimuli-Responsive, Chiral, Cohesive Vesicles-on-a-String that Eventually Turn into a Hydrogel

A charge transfer (CT) mediated two-component, multistimuli responsive supergelation involving a L-histidine-appended pyrenyl derivative (PyHisOMe) as a donor and an asymmetric bolaamphiphilic naphthalene-diimide (Asym-NDI) derivative as an acceptor in a 2: 1 mixture of H2O/MeOH was investigated. Asym-NDI alone self-assembled into pH-responsive vesicular nanostructures in water. Excellent selectivity in CT gel formation was achieved in terms of choosing amino acid appended pyrenyl donor scaffolds. Circular di-chroism and morphological studies suggested formation of chiral, interconnected vesicular assemblies resembling ``pearls-on-a-string'' from these CT mixed stacks. XRD studies revealed the formation of monolayer lipid membranes from these CT mixed stacks that eventually led to the formation of individual vesicles. Strong cohesive forces among the interconnected vesicles originate from the protrusion of the oxyethylene chains from the surfaces of the chiral vesicles.

Pt-Poisoning-Free Efficient CO Oxidation on Pt3Co Supported on MgO(100): An Ab Initio Study

Late-transition-metal-doped Pt clusters are prevalent in CO oxidation catalysis, as they exhibit better catalytic activity than pure Pt, while reducing the effective cost and poisoning However, completely eliminating the critical problem of Pt poisoning still poses a big challenge. Here, we report for the first time that, among the bimetallic clusters ((Pt3M where M = Co, Ni, and Cu)/MgO(100)), the CO adsorption site inverts for Pt3Co/MgO(100) from Pt to Co, due to the complete uptake of Pt d-states by lattice oxygen. While this resolves the problem of Pt poisoning, good reaction kinetics are predicted through low barriers for Langmuir-Hinshelwood and Mars van Krevelen (MvK) mechanisms of CO oxidation for Pt3Co/MgO(100) and Li-doped MgO(100), respectively. Li doping in MgO(100) compensates for the charge imbalance caused by a spontaneous oxygen vacancy formation. Pt-3 Co/Li-doped MgO(100) stands out as an exceptional CO oxidation catalyst, giving an MvK reaction barrier as low as 0.11 eV. We thereby propose a novel design strategy of d-band center inversion for CO oxidation catalysts with no Pt poisoning and excellent reaction kinetics.

SIMULATION STUDY OF THE ADSORPTION DYNAMICS OF CYLINDRICAL SILICA GEL PARTICLES

The paper presents a simulation study of loose cylindrically shaped particles packed within a copper plate and aluminum fins. The model presented solves coupled heat and mass transfer equations using the finite volume method based on ANSY S FLUENT medium. Three different arrangements of cylindrical particles are considered. The model is validated with experimental data. It is found that the arrangements which represented monolayer configurations are only marginally better in heat transfer and uptake efficiency than the tri-layer configuration in the presence of fins. However, there is an appreciable difference in the uptake curve between monoand tri-layer configurations in the absence of fins. Finally, it is found that the fin pitch also plays an important role in determining the time constant for the adsorber design.

Effect of Pt doping on the gas sensing properties of porous chromium oxide films through a kinetic response analysis approach

The effect of doping trace amounts of noblemetals (Pt) on the gas sensing properties of chromium oxide thin films, is studied. The sensors are fabricated by depositing chromium oxide films on a glass substrate using a modified spray pyrolysis technique and characterized using X-ray diffraction, scanning electron microscopy, transmission electron microscopy and X-ray photoelectron spectroscopy. The films are porous and nanocrystalline with an average crystallite size of similar to 30 nm. The typical p-type conductivity arises due to the presence of Cr vacancies, formed as a result of Cr non-stoichiometry, which is found to vary upon Pt doping. In order to analyze the effect of doping on the gas sensing properties, we have adopted a kinetic response analysis approach, which is based on Langmuir Adsorption isotherm (LA) theory. The sensor response is analyzed with equations obtained from LA theory and time constants as well as energies of adsorption-desorption are evaluated. It is seen that, Pt doping lowers the Schottky barrier height of the metal oxide semiconductor sensor from 222 meV to 172 meV. Subsequently the reduction in adsorption and desorption energies led to enhancement in sensor response and improvement in the kinetics of the sensor response i.e. the response time as well as recovery times of the sensor.

Synthesis and Characterization of Au@Pt Nanoparticles with Ultrathin Platinum Overlayers

Gold-core platinum-shell (Au@Pt) nanoparticles with ultrathin platinum overlayers, ranging from submonolayer to two monolayers of platinum atoms, were prepared at room-temperature using a scalable, wet-chemical synthesis route. The synthesis involved the reduction of chloroauric acid with tannic acid to form 5 nm (nominal dia.) gold nanoparticles followed by addition of desired amount of chloroplatinic acid and hydrazine to form platinum overlayers with bulk Pt/Au atomic ratios (Pt surface coverages) corresponding to 0.19 (half monolayer), 0.39 (monolayer), 0.58 (1.5 monolayer) and 0.88 (2 monolayers). The colloidal particles were coated with octadecanethiol and phase-transferred into chlroform-hexane mixture to facilitate sample preparation for structural characterization. The structure of the resultant nanoparticles were determined to be Au@Pt using HRTEM, SAED, XPS, UV-vis and confirmed by cyclic voltammetry (CV) studies. Monolayers of octadecanethiol coated Au@Pt nanoparticles were self-assembled at an air-water interface and transfer printed twice onto a gold substrate to form bilayer films for electrochemical characterization. Electrochemical activity on such films was observed only after the removal of the octadecanethiol ligand coating the nanoparticles, using a RF plasma etching process. The electrochemical activity (HOR, MOR studies) of Au@Pt nanoparticles was found to be highest for particles having a two atom thick platinum overlayer. These nanoparticles can significantly enhance platinum utilization in electrocatalytic applications as their platinum content based activity was three times higher than pure platinum nanoparticles.

Analyzing the kinetic response of tin oxide-carbon and tin oxide-CNT composites gas sensors for alcohols detection

Tin oxide nanoparticles are synthesized using solution combustion technique and tin oxide - carbon composite thick films are fabricated with amorphous carbon as well as carbon nanotubes (CNTs). The x-ray diffraction, Raman spectroscopy and porosity measurements show that the as-synthesized nanoparticles are having rutile phase with average crystallite size similar to 7 nm and similar to 95 m(2)/g surface area. The difference between morphologies of the carbon doped and CNT doped SnO2 thick films, are characterized using scanning electron microscopy and transmission electron microscopy. The adsorption-desorption kinetics and transient response curves are analyzed using Langmuir isotherm curve fittings and modeled using power law of semiconductor gas sensors. (C) 2015 Author(s).