Separation of amino acids by selective sorption through reconstructive intercalation in calcined MgAl-layered double hydroxide.

Mg-Al layered double hydroxide (MgAl-LDH) exhibits selectivity in the intercalation of amino acids (AAs). When the MgAl-LDH derived mixed metal oxide was treated with different mixtures of AAs, preferential sorption of one AA over the other(s) was observed as indicated by XRD analysis of the products and HPLC analysis of the interlayer AA contents in the products. The order of preference was aspartic acid, glutamic acid (acidic AAs) > glycine, alanine (neutral AAs) > hystidine, and arginine (basic AAs). Among the acidic AAs, aspartic acid was preferred over glutamic acid and among the basic AAs, histidine was preferred over arginine. LDH shows equal preference for glycine and alanine. The selectivity can be explained on the basis of the isoelectric pH (pI) of the AA. A similar selectivity order was obtained when the mixtures of AAs were treated with nitrate-intercalated LDH (direct anion exchange) although the net AA intercalated is much lower due to competition with carbonate derived from atmospheric CO2. The high selectivity observed in some cases (such as aspartic acid and glycine) would result in the quantitative separation of the individual AAs from their mixture.

Existing and emerging strategies for the synthesis of nanoscale heterostructures.

Development of new multifunctional nanostructures relies on the ability to make new materials at the nanoscale with control over size, shape and composition. While this control is extremely important to tune several properties, an alternative strategy is to create active interfaces between two or more nanostructures to form nanoscale heterostructures. In these heterostructures, the interfaces play a key role in stabilizing and enhancing the efficiency of the individual components for various applications. In this article, we discuss synthesis methods of different types of nanoscale heterostructures and the role of interfaces in various applications. We present the current state-of-the-art in designing heterostructures and possible upcoming synthetic strategies with their advantages and disadvantages. We present how such heterostructures are highly efficient for catalytic, photovoltaic and nanoelectronic applications drawing several examples from our own studies and from the literature.

Hierarchically Porous, Biphasic, and C-Doped TiO2 for Solar Photocatalytic Degradation of Dyes and Selective Oxidation of Benzyl Alcohol.

Macroporous TiO2 monoliths were synthesized by self-sustained combustion reactions of molded pellets made up of a mixture of TiCl4 as a precursor, urea as a fuel, ammonium nitrate as an oxidizer, and starch as a binder. The porous TiO2 monoliths were found to be a heterostructure of anatase and rutile phases, in addition to being doped with carbon. Variation in the amount of starch yielded porous monoliths of different anatase-rutile ratios (increasing rutile component from 0 to 40%) but comparable Brunauer-Emmett-Teller (BET) surface area (∼30 m2 g-1). The porous monoliths obtained, where the TiCl4/starch mass ratio was 2.17, exhibit exceptional photocatalytic activity in the degradation of dyes (methylene blue and methyl orange) and selective oxidation of benzyl alcohol to benzaldehyde under natural sunlight. The synergistic combination of high surface area, porous network, lowered band gap due to heterostructured anatase-rutile polymorphs, and the presence of doped carbon renders the macroporous TiO2 an efficient photocatalyst.

Induced 2H-Phase Formation and Low Thermal Conductivity by Reactive Spark Plasma Sintering of 1T-Phase Pristine and Co-Doped MoS2 Nanosheets.

Pristine and Co-doped MoS2 nanosheets, containing a dominant 1T phase, have been densified by spark plasma sintering (SPS) to produce a nanostructured arrangement. The structural analysis by X-ray powder diffraction revealed that the reactive sintering process transforms the 1T-MoS2 nanosheets into their stable 2H form despite a significantly reduced sintering temperature and time testifying to the fast kinetics of phase change. Together with the phase conversion, the SPS process promoted a strong texturing of the nanosheets, which drives additional scattering processes and alters the electronic and thermal transport properties. In the pristine sample, it produced one of the lowest thermal conductivities ever reported on MoS2 with a minimal value of 0.66 W/m·K at room temperature. The effect of Co substitution in the final sintered samples is not significant, compared to the pristine MoS2 sample, except for a non-negligible improvement of the electrical conductivity by a factor of 100 in the high-Co content (6% by mass) sample.

The Effect of Reactive Electric Field-Assisted Sintering of MoS2/Bi2Te3 Heterostructure on the Phase Integrity of Bi2Te3 Matrix and the Thermoelectric Properties.

In this work, a series of Bi2Te3/X mol% MoS2 (X = 0, 25, 50, 75) bulk nanocomposites were prepared by hydrothermal reaction followed by reactive spark plasma sintering (SPS). X-ray diffraction analysis (XRD) indicates that the native nanopowders, comprising of Bi2Te3/MoS2 heterostructure, are highly reactive during the electric field-assisted sintering by SPS. The nano-sized MoS2 particles react with the Bi2Te3 plates matrix forming a mixed-anion compound, Bi2Te2S, at the interface between the nanoplates. The transport properties characterizations revealed a significant influence of the nanocomposite structure formation on the native electrical conductivity, Seebeck coefficient, and thermal conductivity of the initial Bi2Te3 matrix. As a result, enhanced ZT values have been obtained in Bi2Te3/25 mol% MoS2 over the temperature range of 300-475 K induced mainly by a significant increase in the electrical conductivity.

Nitrogen-Doped Alkylamine-Intercalated Layered Titanates for Photocatalytic Dye Degradation.

A layered titanate, K2Ti4O9, is intercalated with various n-alkylamines through ion-exchange reaction in aqueous medium. On heating, the intercalated amine is partially deintercalated, yielding nitrogen-doped amine-intercalated titanates. The modified titanates are studied as catalysts in methylene blue degradation under UV irradiation. Heat-treated long-chain amine titanates exhibit better photocatalytic activity in comparison to short chain amine titanates. The improved catalytic activity could be attributed to two factors: (i) increased surface access as the titanate layers are well separated, pillared by the alkylamine chains and (ii) nitrogen doping.

Microwave-Assisted Synthesis of Porous Aggregates of CuS Nanoparticles for Sunlight Photocatalysis.

Solvated two-dimensional nanosheets of copper hydroxy dodecylsulfate in 1-butanol react with thiourea under microwave irradiation to yield surfactant-free porous aggregates of CuS nanoparticles. These aggregates exhibit excellent photocatalytic activity toward degradation of methylene blue, methyl orange, and 4-chlorophenol in natural sunlight. While the high surface area (14.74 m2 g-1) and porosity increase the active reaction centers for adsorption and degradation of organic molecules, quantum confinement results in a low recombination of photogenerated electrons and holes. Chemical and photogenerated hydroxyl radicals cause the oxidation of the dyes and 4-chlorophenol.

Magnetic Co-Doped MoS2 Nanosheets for Efficient Catalysis of Nitroarene Reduction.

Co-doped MoS2 nanosheets have been synthesized through the hydrothermal reaction of ammonium tetrathiomolybdate and hydrazine in the presence of cobalt acetate. These nanosheets exhibit a dominant metallic 1T phase with cobalt ion-activated defective basal planes and S-edges. In addition, the nanosheets are dispersible in polar solvents like water and methanol. With increased active sites, Co-doped MoS2 nanosheets exhibit exceptional catalytic activity in the reduction of nitroarenes by NaBH4 with impressive turnover frequencies of 8.4, 3.2, and 20.2 min-1 for 4-nitrophenol, 4-nitroaniline, and nitrobenzene, respectively. The catalyst is magnetic, enabling its easy separation from the reaction mixture, thus making its recycling and reusability simple and efficient. The enhanced catalytic activity of the Co-doped 1T MoS2 nanosheets in comparison to that of undoped 1T MoS2 nanosheets suggests that incorporation of cobalt ions in the MoS2 lattice is the major reason for the efficiency of the catalyst. The dopant, Co, plays a dual role. In addition to providing active sites where electron transfer is assisted through redox cycling, it renders the nanosheets magnetic, enabling their easy removal from the reaction mixture.

Ferrimagnetic nanogranular Co3O4 through solvothermal decomposition of colloidally dispersed monolayers of alpha-cobalt hydroxide.

Co3O4 nanoparticles of 35 nm with a cauliflower-like morphology were obtained when a monolayer colloidal dispersion of dodecyl sulfate intercalated alpha-cobalt hydroxide in butanol was subjected to solvothermal hydrolytic decomposition. The nanogranular particles exhibit weakly ferromagnetic properties in contrast with both bulk and dispersed nanoparticulate Co3O4.

Surfactant intercalated alpha-hydroxides of cobalt and nickel and their delamination--restacking behavior in organic media.

Dodecyl sulfate and dodecylbenzene sulfonate intercalated alpha-hydroxides of nickel and cobalt were synthesized by ammonia precipitation. These solids delaminate to give a colloidal dispersion of layers in organic solvents such as 1-butanol. The dispersed layers could be reassembled either by evaporation of the colloid or by coagulation by the addition of a polar solvent.

Cobalt hydroxide/oxide hexagonal ring-graphene hybrids through chemical etching of metal hydroxide platelets by graphene oxide: energy storage applications.

The reaction of ß-Co(OH)2 hexagonal platelets with graphite oxide in an aqueous colloidal dispersion results in the formation of ß-Co(OH)2 hexagonal rings anchored to graphene oxide layers. The interaction between the basic hydroxide layers and the acidic groups on graphene oxide induces chemical etching of the hexagonal platelets, forming ß-Co(OH)2 hexagonal rings. On heating in air or N2, the hydroxide hybrid is morphotactically converted to porous Co3O4/CoO hexagonal ring-graphene hybrids. Porous NiCo2O4 hexagonal ring-graphene hybrid is also obtained through a similar process starting from ß-Ni0.33Co0.67(OH)2 platelets. As electrode materials for supercapacitors or lithium-ion batteries, these materials exhibit a large capacity, high rate capability, and excellent cycling stability.

Chemical unzipping of WS2 nanotubes.

WS2 nanoribbons have been synthesized by chemical unzipping of WS2 nanotubes. Lithium atoms are intercalated in WS2 nanotubes by a solvothermal reaction with n-butyllithium in hexane. The lithiated WS2 nanotubes are then reacted with various solvents--water, ethanol, and long chain thiols. While the tubes break into pieces when treated with water and ethanol, they unzip through longitudinal cutting along the axes to yield nanoribbons when treated with long chain thiols, 1-octanethiol and 1-dodecanethiol. The slow diffusion of the long chain thiols reduces the aggression of the reaction, leading to controlled opening of the tubes.

N-doped graphene-VO2(B) nanosheet-built 3D flower hybrid for lithium ion battery.

Recently, we have shown that the graphene-VO2(B) nanotube hybrid is a promising lithium ion battery cathode material (Nethravathi et al. Carbon, 2012, 50, 4839-4846). Though the observed capacity of this material was quite satisfactory, the rate capability was not. To improve the rate capability we wanted to prepare a graphene-VO2(B) hybrid in which the VO2(B) would be built on 2D nanosheets that would enable better electrode-electrolyte contact. Such a material, a N-doped graphene-VO2(B) nanosheet-built 3D flower hybrid, is fabricated by a single-step hydrothermal reaction within a mixture of ammonium vanadate and colloidal dispersion of graphite oxide. The 3D VO2(B) flowers which are uniformly distributed on N-doped graphene are composed of ultrathin 2D nanosheets. When used in lithium ion batteries, this material exhibits a large capacity, high rate capability, and excellent cycling stability. The enhanced performance results from its unique features: excellent electronic conductivity associated with the N-doped graphene, short transportation length for lithium ions related to ultrathin nanosheets, and improved charge transfer due to the anchoring of the VO2(B) flowers to N-doped graphene.

Exfoliation of copper hydroxysalt in water and the conversion of the exfoliated layers to cupric and cuprous oxide nanoparticles.

P-aminobenzoate-intercalated copper hydroxysalt was prepared by coprecipitation at high pH (∼12). As the pH was reduced to ∼7 on washing with water, the development of partial positive charge on the amine end of the intercalated anion caused repulsion between the layers leading to delamination and colloidal dispersion of monolayers of copper hydroxysalt in water. The dispersed copper hydroxysalt monolayers were used as precursors for the synthesis of copper(I)/(II) oxide nanoparticles at room temperature. While the hydroxysalt layers yielded spindle-shaped CuO particles when left to stand, they formed hollow spherical nanoparticles of Cu(2)O when treated with an alkaline solution of ascorbic acid.

Highly dispersed ultrafine Pt and PtRu nanoparticles on graphene: formation mechanism and electrocatalytic activity.

We demonstrate a robust strategy for obtaining a high dispersion of ultrafine Pt and PtRu nanoparticles on graphene by exploiting the nucleation of a metal precursor phase on graphite oxide surfaces. Our method opens up new possibilities to engineer graphene-based hybrids for applications in multifunctional nanoscale devices.

Synthesis and anion-exchange reactions of a new anionic clay, α-magnesium hydroxide.

A new anionic clay, α-magnesium hydroxide, was synthesized by hydrolysis of magnesium acetate in propylene glycol. The structure of this α-hydroxide is similar to that of hydrotalcites. It consists of positively charged magnesium hydroxide layers arising out of partial protonation of the hydroxyl groups of the [Mg(OH)(2)] layers and loosely held anions in the interlayer region. As expected it ages readily in water to give ß-magnesium hydroxide, brucite. While anion-exchange reactions of α-magnesium hydroxide could not be carried out in aqueous medium a number of anion-exchange reactions could be carried out successfully in ethanol medium.

Functionalized-graphene modified graphite electrode for the selective determination of dopamine in presence of uric acid and ascorbic acid.

Graphene is chemically synthesized by solvothermal reduction of colloidal dispersions of graphite oxide. Graphite electrode is modified with functionalized-graphene for electrochemical applications. Electrochemical characterization of functionalized-graphene modified graphite electrode (FGGE) is carried out by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The behavior of FGGE towards ascorbic acid (AA), dopamine (DA) and uric acid (UA) has been investigated by CV, differential pulse voltammetry (DPV) and chronoamperommetry (CA). The FGGE showed excellent catalytic activity towards electrochemical oxidation of AA, DA and UA compared to that of the bare graphite electrode. The electrochemical oxidation signals of AA, DA and UA are well separated into three distinct peaks with peak potential separation of 193mv, 172mv and 264mV between AA-DA, DA-UA and AA-UA respectively in CV studies and the corresponding peak potential separations in DPV mode are 204mv, 141mv and 345mv. The FGGE is successfully used for the simultaneous detection of AA, DA and UA in their ternary mixture and DA in serum and pharmaceutical samples. The excellent electrocatalytic behavior of FGGE may lead to new applications in electrochemical analysis.

Exfoliation of alpha-hydroxides of nickel and cobalt in water.

p-Aminobenzoate ion intercalated alpha-hydroxides of nickel/cobalt were synthesized by precipitation using ammonia (pH= approximately 12). Aqueous colloidal suspension of alpha-hydroxide of nickel/cobalt was obtained on washing the precipitate as the pH was reduced to approximately 7. The development of partial positive charge on the amine end of the intercalated anion causes repulsion between the layers leading to exfoliation and colloidal suspension of monolayers in water. While the layers could be restacked from the colloidal suspension in the presence of other anions in the case of alpha-cobalt hydroxide, the exfoliation could not be reversed easily in the case of the nickel analog.

Graphite oxide-intercalated anionic clay and its decomposition to graphene-inorganic material nanocomposites.

A graphite oxide-intercalated anionic clay (nickel zinc hydroxysalt) has been prepared using the aqueous colloidal dispersions of negatively charged graphite oxide sheets and aminobenzoate-intercalated anionic clay layers as precursors. When the two colloidal dispersions are reacted, the interlayer aminobenzoate ions are displaced from the anionic clay and the negatively charged graphite oxide sheets are intercalated between the positively charged layers of the anionic clay. The thermal decomposition of the intercalated solid at different temperatures yields graphene-metal oxide/metal nanocomposites. Electron microscopic analysis of the composites indicates that the nanoparticles are intercalated between the layers of graphite in many regions of these solids although the graphite layers are largely exfoliated and not stacked well together.