Synthesis, properties and applications of graphene doped with boron, nitrogen and other elements

Chemical doping of graphene becomes necessary to create a band gap which is useful for various applications. Furthermore, chemical doping of elements like boron and nitrogen in graphene gives rise to useful properties. Since chemically doped graphene is both of academic and technical importance, we have prepared this article on the present status of various aspects of this important class of materials. In doing so, we have covered the recent literature on this subject citing all the major references. Some of the aspects that we have covered are the synthesis of chemically doped graphene followed by properties and applications. The applications discussed relate to gas adsorption, lithium batteries, supercapacitors, oxygen reduction reaction, field emission and photochemical water splitting. Characterization of chemically doped graphene also included. We believe that the article will be useful to all those interested in graphene and related materials and provides the present status of the subject. (C) 2014 Elsevier Ltd. All rights reserved.

Polyolefin based antibacterial membranes derived from PE/PEO blends compatibilized with amine terminated graphene oxide and maleated PE

In this study, various strategies like amine terminated GO (GO-NH2), in situ formed polyethylene grafted GO (PE-g-GO) and their combinations with maleated PE (maleic anhydride grafted PE) were adopted to reactively compatibilize blends of low density polyethylene (LDPE) and polyethylene oxide (PEO). These blends were further explored to design porous, antibacterial membranes for separation technology and the flux and the resistance across the membranes were studied systematically. It was observed that GO-NH2 led to uniform dispersion of PEO in a PE matrix and further resulted in a significant improvement in the mechanical properties of the blends when combined with maleated PE. The efficiency of various compatibilizers was further studied by monitoring the evolution of morphology as a function of the annealing time. It was observed that besides rendering uniform dispersion of PEO in PE and improving the mechanical properties, GO-NH2 further suppresses the coalescence in the blends. As the melt viscosities of the phases differ significantly, there is a gradient in the morphology as also manifested from scanning acoustic microscopy. Hence, the membranes were designed by systematically reducing the thickness of the as-pressed samples to expose the core as the active area for flux calculations. Selected membranes were also tested for their antibacterial properties by inoculating E. coli culture with the membranes and imaging at different time scales. This study opens new avenues to develop PE based cost effective anti-microbial membranes for water purification.

Polyolefin based antibacterial membranes derived from PE/PEO blends compatibilized with amine terminated graphene oxide and maleated PE

In this study, various strategies like amine terminated GO (GO-NH2), in situ formed polyethylene grafted GO (PE-g-GO) and their combinations with maleated PE (maleic anhydride grafted PE) were adopted to reactively compatibilize blends of low density polyethylene (LDPE) and polyethylene oxide (PEO). These blends were further explored to design porous, antibacterial membranes for separation technology and the flux and the resistance across the membranes were studied systematically. It was observed that GO-NH2 led to uniform dispersion of PEO in a PE matrix and further resulted in a significant improvement in the mechanical properties of the blends when combined with maleated PE. The efficiency of various compatibilizers was further studied by monitoring the evolution of morphology as a function of the annealing time. It was observed that besides rendering uniform dispersion of PEO in PE and improving the mechanical properties, GO-NH2 further suppresses the coalescence in the blends. As the melt viscosities of the phases differ significantly, there is a gradient in the morphology as also manifested from scanning acoustic microscopy. Hence, the membranes were designed by systematically reducing the thickness of the as-pressed samples to expose the core as the active area for flux calculations. Selected membranes were also tested for their antibacterial properties by inoculating E. coli culture with the membranes and imaging at different time scales. This study opens new avenues to develop PE based cost effective anti-microbial membranes for water purification.

Optical and structural properties of highly porous shell structured Fe doped TiO2 thin films

TiO2 thin films with 0.2 wt%, 0.4 wt%, 0.6 wt%, and 0.8 wt% Fe were prepared on glass and silicon substrates using sol-gel spin coating technique. The optical cut-off points are increasingly red-shifted and the absorption edge is shifted over the higher wavelength region with Fe content increasing. As Fe content increases, the optical band gap decreases from 3.03 to 2.48 eV whereas the tail width increases from 0.26 to 1.43 eV. The X-ray diffraction (XRD) patterns for doped films at 0.2 wt% and 0.8 wt% Fe reveal no characteristic peaks, indicating that the film is amorphous whereas undoped TiO2 exhibits (101) orientation with anatase phase. Thin films of higher Fe content exhibit a homogeneous, uniform, and nano-structured highly porous shell morphology.

Enhancing the photocatalytic efficiency of sprayed ZnO thin films through double doping (Sn plus F) and annealing under different ambiences

Doubly (Sn + F) doped zinc oxide (ZnO:Sn:F) thin films were deposited onto glass substrates using a simplified spray pyrolysis technique. The deposited films were annealed at 400 degrees C under two different ambiences (air and vacuum) for 2 h. The photocatalytic activity of these films was assessed through photocatalytic decolorization kinetics of Methylene Blue (MB) dye and the decolorization efficiency of the annealed films was compared with that of their as-deposited counterpart. The photocatalytic studies reveal that the ZnO:Sn:F films annealed under vacuum environment exhibits better photocatalytic efficiency when compared with both air annealed and as-deposited films. The SEM and TEM images depict that the surface of each of the films has an overlayer comprising of nanobars formed on a bottom layer, having spherical grains. The studies show that the diameter of the nanobars plays crucial role in enhancing the photocatalytic activity of the ZnO:Sn:F films. The structural, optical and electrical studies substantiate the discussions on the photocatalytic ability of the deposited films. (C) 2014 Elsevier B.V. All rights reserved.

Metal-organic chemical vapor-deposited cobalt oxide films as negative electrodes for thin film Li-ion battery

In this study, thin films of cobalt oxide (Co3O4) have been grown by the metal-organic chemical vapor deposition (MOCVD) technique on stainless steel substrate at two preferred temperatures (450 degrees C and 500 degrees C), using cobalt acetylacetonate dihydrate as precursor. Spherical as well as columnar microstructures of Co3O4 have been observed under controlled growth conditions. Further investigations reveal these films are phase-pure, well crystallized and carbon-free. High-resolution TEM analysis confirms that each columnar structure is a continuous stack of minute crystals. Comparative study between these Co3O4 films grown at 450 degrees C and 500 degrees C has been carried out for their application as negative electrodes in Li-ion batteries. Our method of electrode fabrication leads to a coating of active material directly on current collector without any use of external additives. A high specific capacity of 1168 micro Ah cm(-2) mu m(-1) has been measured reproducibly for the film deposited at 500 degrees C with columnar morphology. Further, high rate capability is observed when cycled at different current densities. The Co3O4 electrode with columnar structure has a specific capacity 38% higher than the electrode with spherical microstructure (grown at 450 degrees C). Impedance measurements on the Co3O4 electrode grown at 500 degrees C also carried out to study the kinetics of the electrode process. (C) 2014 Published by Elsevier B.V.

Morphology controlled synthesis of Al doped ZnO nanosheets on Al alloy substrate by low-temperature solution growth method

We report the morphology-controlled synthesis of aluminium (Al) doped zinc oxide (ZnO) nanosheets on Al alloy (AA-6061) substrate by a low-temperature solution growth method without using any external seed layer and doping process. Doped ZnO nanosheets were obtained at low temperatures of 60-90 degrees C for the growth time of 4 hours. In addition to the synthesis, the effect of growth temperature on the morphological changes of ZnO nanosheets is also reported. As-synthesized nanosheets are characterized by FE-SEM, XRD TEM and XPS for their morphology, crystallinity, microstructure and compositional analysis respectively. The doping of Al in ZnO nanosheets is confirmed with EDXS and XPS. Furthermore, the effect of growth temperature on the morphological changes was studied in the range of 50 to 95 degrees C. It was found that the thickness and height of the nanosheets varied with respect to the growth temperature. The study has given an important insight into the structural morphology with respect to the growth temperature, which in turn enabled us to determine the growth temperature window for the ZnO nanosheets. These Al doped ZnO nanosheets have potential application possibilities in gas sensors, solar cells and energy harvesting devices like nanogenerators.

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).

Porous membranes designed from bi-phasic polymeric blends containing silver decorated reduced graphene oxide synthesized via a facile one-pot approach

In this work, porous membranes were designed by selectively etching the PEO phase, by water, from a melt-mixed PE/PEO blend. The pure water flux and the resistance across the membrane were systematically evaluated by employing an indigenously developed cross flow membrane setup. Both the phase morphology and the cross sectional morphology of the membranes was assessed by scanning electron microscopy and an attempt was made to correlate the observed morphology with the membrane performance. In order to design antibacterial membranes for water purification, partially reduced graphene oxide (rGO), silver nanoparticles (Ag) and silver nanoparticles decorated with rGO (rGO-Ag) were synthesized and incorporated directly into the blends during melt mixing. The loss of viability of bacterial cells was determined by the colony counting method using E. coli as a model bacterium. SEM images display that the direct contact with the rGO-Ag nanoparticles disrupts the cell membrane. In addition, the rGO-Ag nanoparticles exhibited a synergistic effect with respect to bacterial cell viability in comparison to both rGO and Ag nanoparticles. The possible mechanism associated with the antibacterial activity in the membranes was discussed. This study opens new avenues in designing antibacterial membranes for water purification.

A Surlyn/magnesium oxide nanocomposite as an effective water vapor barrier for organic device encapsulation

A reactive polymer nanocomposite system was proposed as an effective water vapor barrier material for organic device encapsulation. Nanosized magnesium oxide (MgO) was synthesized by the solution combustion technique using two different fuels, lactose and alanine. The purity and crystallite size of MgO were determined from X-ray diffraction studies. The surface areas and porosity measurements were used to determine the water adsorption capacities of MgO. Nanocomposites with various concentrations (wt% = 0.25, 0.5, 1 and 2.5) of MgO were prepared using Surlyn as the base polymer. The permeation rate of moisture through the fabricated films was calculated using calcium degradation test and these rates were further used to calculate the diffusivities. Accelerated aging experiments were conducted to study the performance of organic photovoltaic devices encapsulated with synthesized films under accelerated weathering conditions. The performance of the barrier materials with synthesized MgO was also compared to that obtained with commercial MgO. The films containing MgO obtained from lactose exhibited better barrier properties compared to other films made with commercial MgO and MgO synthesized using alanine as well as other nanocomposites reported in the literature.

Achieving Highly Efficient, Selective, and Stable CO2 Reduction on Nitrogen-Doped Carbon Nanotubes

The challenge in the electrosynthesis of fuels from CO2 is to achieve durable and active performance with cost-effective catalysts. Here, we report that carbon nanotubes (CNTs), doped with nitrogen to form resident electron-rich defects, can act as highly efficient and, more importantly, stable catalysts for the conversion of CO2 to CO. The unprecedented overpotential (-0.18 V) and selectivity (80%) observed on nitrogen-doped CNTs (NCNTs) are attributed to their unique features to facilitate the reaction, including (i) high electrical conductivity, (ii) preferable catalytic sites (pyridinic N defects), and (iii) low free energy for CO2 activation and high barrier for hydrogen evolution. Indeed, DFT calculations show a low free energy barrier for the potential-limiting step to form key intermediate COOH as well as strong binding energy of adsorbed CON and weak binding energy for the adsorbed CO. The highest selective site toward CO production is pyridinic N, and the NCNT-based electrodes exhibit no degradation over 10 h of continuous operation, suggesting the structural stability of the electrode.

Dendron conjugation to graphene oxide using click chemistry for efficient gene delivery

Owing to its large surface area and rapid cellular uptake, graphene oxide (GO) is emerging as an attractive candidate material for delivery of drugs and genes. The inherent sp(2) pi-pi interaction of GO helps to carry drugs and single stranded RNA (ssRNA) but there is no such interaction with double stranded DNA (dsDNA). In this work, a polyamidoamine (PAMAM) dendron was conjugated with nano GO (nGO) through ``click'' chemistry to improve the DNA complexation capability of GO as well as its transfection efficiency. The DNA complexation capability of GO was significantly enhanced after dendronization of GO yielding spherical nanosized (250-350 nm) particles of the dendronized GO (DGO)/pDNA complex with a positive zeta potential. The transfection efficiency of GO dramatically increased after conjugation of the PAMAM dendron. Transfection efficiency of 51% in HeLa cells with cell viability of 80% was observed. The transfection efficiency was significantly higher than that of polyethyleneimine 25 kDa (27% efficiency) and also surpassed that of lipofectamine 2000 (47% efficiency). The uptake of the DGO/pDNA complex by the caveolae mediated endocytosis pathway may significantly contribute to the high transfection efficiency. Thus, dendronized GO is shown to be an efficient gene carrier with minimal toxicity and is a promising candidate for use as a nonviral carrier for gene therapy.

Contrasting Effects of Graphene Oxide and Poly(ethylenimine) on the Polymorphism in Poly(vinylidene fluoride)

The nature of interaction between a heteronucleating agent (graphene oxide, GO) and a strongly polar macromolecule (poly(ethylenimine), PEI) with poly(vinylidene fluoride) (PVDF) influencing the crystalline structure and morphology has been systematically investigated in this work. PEI interacts with PVDF via ion-dipole interaction, which helps in lowering the free energy barrier for nucleation thereby promoting faster crystallization. In contrast, besides interacting with PVDF, GO also promotes heteronucleation in PVDF. We observed that both GO and PEI have very different effects on the overall crystalline morphology of PVDF. For instance, the neat PVDF showed a mixture of both alpha and beta phases when cooled from the melt. However, incorporation of 0.1 wt % GO resulted in phase transformation from the stable alpha-phase to polar beta-polymorph in PVDF. In contrast, PEI, which also resulted in faster crystallization in PVDF predominantly, resulted in the stable alpha- phase. Various techniques like Fourier transform infrared spectroscopy, X-ray diffraction, and differential scanning calorimetry were employed to confirm the phase transformations in PVDF. PEI was further grafted onto GO nanosheets to understand the combined effects of both GO and PEI on the polymorphism in PVDF. The PVDF/PEI-GO composite showed a mixture of phases, predominantly rich in a. These phenomenal effects were further analyzed and corroborated with the specific interaction between GO and PEI with PVDF using X-ray photon scattering (XPS) and NMR. In addition, the dielectric permittivity increased significantly in the presence of GO and PEI in the composites. For instance, PVDF/PEI-GO showed the highest permittivity of 39 at 100 Hz.

Single-step synthesis of carbon nanotubes/iron/iron oxide composite films through inert-ambient CVD using ferric acetylacetonate as a precursor

We have developed a unique single-step chemical vapor deposition (CVD) route for the synthesis of composite thin films containing carbon nanotubes (CNTs). CVD was carried out in an inert ambient using only iron(III) acetylacetonate as the precursor. Depositions were conducted at 700 degrees C on stainless steel substrates in argon ambient in the absence of any reactive gases (such as oxygen, hydrogen). By changing the deposition parameters, especially the pressure in the CVD reactor, the form of carbon deposited could be changed from amorphous to carbon nanotubes, the latter resulting in Fe-Fe3O4-CNT films. X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, and electron microscopy together confirm the formation of the three-component composite and illustrate the nanoscale mixing of the components. Elemental iron formed in this process was protected from oxidation by the co-deposited carbon surrounding it. Irrespective of the substrate used, a composite coating with CNTs was formed under optimum conditions, as verified by analyses of the film formed on polycrystalline alumina and silicon substrates.

High-Performance Stacked TiO2-ZrO2 and Si-doped ZrO2 Metal-Insulator-Metal Capacitors

Metal-insulator-metal (MIM) capacitors for DRAM applications have been realised using stacked TiO2-ZrO2 (TiO2/ZrO2 and ZrO2/TiO2) and Si-doped ZrO2 (TiO2/Si-doped ZrO2) dielectrics. High capacitance densities (> 42 fF/mu m(2)), low leakage current densities (< 5 x 10(-7) A/cm(2) at -1 V), and sub-nm EOT (< 0.8 nm) have been achieved. The effects of constant voltage stress on the device characteristics is studied. The structural analysis of the samples is performed by X-ray diffraction measurements, and this is correlated to the electrical characteristics of the devices. The surface chemical states of the films are analyzed through X-ray photoelectron spectroscopy measurements. The doped-dielectric stack (TiO2/Si-doped ZrO2) helps to reduce leakage current density and improve reliability, with a marginal reduction in capacitance density; compared to their undoped counterparts (TiO2/ZrO2 and ZrO2/TiO2). We compare the device performance of the fabricated capacitors with other stacked high-k MIM capacitors reported in recent literature.