Anisotropy and magnetization reversal with chains of submicron-sized Co hollow spheres

Magnetic properties with chains of hcp Co hollow spheres have been studied. The diameter of the spheres ranges from 500 to 800 nm, with a typical shell thickness of about 60 nm. The shell is polycrystalline with an average crystallite size of 20 to 35 nm. The blocking temperature determined by the zero-field-cooling MZFC(T) measurement at H = 90 Oe is about 325 K. The corresponding effective anisotropy is determined as, Keff = 4.6*10^4 J/m^3. In addition, the blocking temperature and the effective anisotropy determined by the analysis on HC(T) are 395 K and 5.7*10^4 J/m^3, respectively. The experimentally determined anisotropy is smaller by one order of magnitude than the magnetocrystalline anisotropy of the bulk hcp Co, which is about 3 to 5*10^5 J/m^3. A further analysis on HC(T) shows that the magnetization reversal follows a nucleation rotational mode with an effective switching volume, V* = 2.3*10^3 nm^3. The corresponding effective diameter is calculated as 16.4 nm. It is slightly larger than the coherence length of Co, about 15 nm. The possible reason for the much reduced magnetic anisotropy is discussed briefly.Comment: 10 pages, 5 figures, submitted to PR

Graphene/Carbon Dot Hybrid Thin Films Prepared by a Modified Langmuir-Schaefer Method

The special electronic, optical, thermal, and mechanical properties of graphene resulting from its 2D nature, as well as the ease of functionalizing it through a simple acid treatment, make graphene an ideal building block for the development of new hybrid nanostructures with well-defined dimensions and behavior. Such hybrids have great potential as active materials in applications such as gas storage, gas/liquid separation, photocatalysis, bioimaging, optoelectronics, and nanosensing. In this study, luminescent carbon dots (C-dots) were sandwiched between oxidized graphene sheets to form novel hybrid multilayer films. Our thin-film preparation approach combines self-assembly with the Langmuir-Schaefer deposition and uses graphene oxide nanosheets as template for grafting C-dots in a bidimensional array. Repeating the cycle results in a facile and low-cost layer-by-layer procedure for the formation of highly ordered hybrid multilayers, which were characterized by photoluminescence, UV-visible, X-ray photoelectron, and Raman spectroscopies, as well as X-ray diffraction and atomic force microscopy.</p

Noncovalent Functionalization of Graphene and Graphene Oxide for Energy Materials, Biosensing, Catalytic, and Biomedical Applications

This Review focuses on noncovalent functionalization of graphene and graphene oxide with various species involving biomolecules, polymers, drugs, metals and metal oxide-based nanoparticles, quantum dots, magnetic nanostructures, other carbon allotropes (fullerenes, nanodiamonds, and carbon nanotubes), and graphene analogues (MoS2, WS2). A brief description of pi-pi interactions, van der Waals forces, ionic interactions, and hydrogen bonding allowing noncovalent modification of graphene and graphene oxide is first given. The main part of this Review is devoted, to tailored functionalization for applications in drug delivery, energy materials, solar cells, water splitting, biosensing, bioimaging, environmental, catalytic, photocatalytic, and biomedical technologies. A significant part of this Review explores the possibilities of graphene/graphene oxide-based 3D superstructures and their use in lithium-ion batteries. This Review ends with a look at challenges and future prospects of noncovalently modified graphene and graphene oxideope

Synthesis of 2D Germanane (GeH):a New, Fast, and Facile Approach

Germanane (GeH), a germanium analogue of graphane, has recently attracted considerable interest because its remarkable combination of properties makes it an extremely suitable candidate to be used as 2D material for field effect devices, photovoltaics, and photocatalysis. Up to now, the synthesis of GeH has been conducted by substituting Ca by H in a beta-CaGe2 layered Zintl phase through topochemical deintercalation in aqueous HCl. This reaction is generally slow and takes place over 6 to 14 days. The new and facile protocol presented here allows to synthesize GeH at room temperature in a significantly shorter time (a few minutes), which renders this method highly attractive for technological applications. The GeH produced with this method is highly pure and has a band gap (E-g) close to 1.4 eV, a lower value than that reported for germanane synthesized using HCl, which is promising for incorporation of GeH in solar cells

Carbon nanostructures derived through hypergolic reaction of conductive polymers with fuming nitric acid at ambient conditions

Hypergolic systems rely on organic fuel and a powerful oxidizer that spontaneously ignites upon contact without any external ignition source. Although their main utilization pertains to rocket fuels and propellants, it is only recently that hypergolics has been established from our group as a new general method for the synthesis of different morphologies of carbon nanostructures depending on the hypergolic pair (organic fuel-oxidizer). In search of new pairs, the hypergolic mixture described here contains polyaniline as the organic source of carbon and fuming nitric acid as strong oxidizer. Specifically, the two reagents react rapidly and spontaneously upon contact at ambient conditions to afford carbon nanosheets. Further liquid-phase exfoliation of the nanosheets in dimethylformamide results in dispersed single layers exhibiting strong Tyndall effect. The method can be extended to other conductive polymers, such as polythiophene and polypyrrole, leading to the formation of different type carbon nanostructures (e.g., photolumincent carbon dots). Apart from being a new synthesis pathway towards carbon nanomaterials and a new type of reaction for conductive polymers, the present hypergolic pairs also provide a novel set of rocket bipropellants based on conductive polymers.Web of Science266art. no. 159

Co-precipitation synthesis of reduced graphene oxide/NiAl-layered double hydroxide hybrid and its application in flame retarding poly(methyl methacrylate)

A reduced graphene oxide/NiAl-layered double hydroxide (RGO-LDH) was synthesized through a simple co-precipitation route. NiAl-layered double hydroxide (NiAl-LDH) nanoparticles were homogeneously dispersed on the reduced graphene oxide (RGO) nanosheets, which were simultaneously reduced during the process. RGO-LDH exhibited three steps of weight loss, leaving high residue. RGO-LDH was then solution blended into poly(methyl methacrylate) (PMMA) to investigate its effect on reducing flammability of the composite. With the incorporation of RGO-LDH, the thermal stability of PMMA composite was improved. Moreover, RGO-LDH endowed PMMA with the largest reduction in the heat release rate, smoke production and CO production rate relative to RGO or NiAl-LDH alone. RGO-LDH could decrease the production of volatiles including hydrocarbons, carbonyl compounds and epoxy compounds from the PMMA composite. The improved flame retardancy was ascribed to the combined effect of the physical barrier of RGO and the catalytic carbonization of NiAl-LDH. © 2013 Elsevier Ltd. All rights reserved

Microwave synthesis, characterization and perspectives of wood pencil-derived carbon

More than 14 billion pencils are manufactured and used globally every year. On average, a pencil is discarded after 60% of its original length has been depleted. In the present work we propose a simple and affordable way of converting this non-neglectable amount of waste into added value carbon product. In particular, we demonstrate the microwave synthesis of carbon from the wood pencil with and without chemical activation. This could be a process stage before the final recycling of the expensive graphite core. In the latter case, irradiation of the wood pencil in a domestic microwave oven heats up the pencil's graphite core, thus inducing carbonization of its wood casing. The carbonized product consists of amorphous carbon nanosheets having relatively low surface area. However, if the wood pencil is soaked in 50% KOH aqueous solution prior to microwave irradiation, a significantly higher surface area of carbon is obtained, consisting of irregular-shaped porous particles. Consequently, the obtained carbon can easily decolorize a methylene blue aqueous solution, can be used to make pocket warmers or gunpowder, and lastly, serves as an excellent adsorbent towards Cr(VI) removal from water, showing a maximum adsorption capacity of 70-75 mg/g within 24 h at 23 degrees C, pH = 3.Web of Science121art. no. 41

Computational studies for reduced graphene oxide in hydrogen-rich environment

We employ molecular dynamic simulations to study the reduction process of graphene-oxide (GO) in a chemically active environment enriched with hydrogen. We examine the concentration and pressure of hydrogen gas as a function of temperature in which abstraction of oxygen is possible with minimum damage to C-sp$^2$ bonds hence preserving the integrity of the graphene sheet. Through these studies we find chemical pathways that demonstrate beneficiary mechanisms for the quality of graphene including formation of water as well as suppression of carbonyl pair holes in favor of hydroxyl and epoxy formation facilitated by hydrogen gas in the environment.Comment: 9 pages and 9 figures. Animations and movies are available at: http://qmsimulatorgojpc.wordpress.com

Modeling of graphite oxide

Based on density functional calculations, optimized structures of graphite oxide are found for various coverage by oxygen and hydroxyl groups, as well as their ratio corresponding to the minimum of total energy. The model proposed describes well known experimental results. In particular, it explains why it is so difficult to reduce the graphite oxide up to pure graphene. Evolution of the electronic structure of graphite oxide with the coverage change is investigated.Comment: 12 pages, 7 figures. Discussion about reduction to pure graphene and several references added. Methodological part expanded. Accepted to J. Am. Chem. So

Longitudinal unzipping of carbon nanotubes to form graphene nanoribbons.

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