Tarun K. Mandal

Ascorbate-assisted growth of hierarchical ZnO nanostructures: sphere, spindle, and flower and their catalytic properties.

A simple solution-based method to prepare mainly flowerlike zinc oxide (ZnO) nanostructures using the ascorbate ion as a shape-directing/capping agent at relatively low temperature (ca. 30 and 60 degrees C) was described. However, we observed that different shapes of hierarchical ZnO nanostructures such as flowerlike, spindlelike, and spherical could be obtained with an increase in the synthesis temperature from 60 to 90 degrees C. The effects of other organic capping agents on the shape of hierarchical ZnO nanostructures were also studied. FTIR, FESEM, and XRD characterization were performed on the formed ZnO nanostructures to understand the role of ascorbate in the growth of flowerlike morphology. The nucleation and growth process can regulate by changing the metal precursor and ascorbate ion concentrations. We were able to identify intermediate nanostructures such as spherical/quasi-spherical and spindle that are very much on the pathway of formation of large, flowerlike ZnO nanostructures. Electron microscopy results indicated that these spherical/quasi-spherical ZnO nanoparticles might aggregate through oriented attachment to produce spindlelike and flowerlike nanostructures. On the basis of these results, a possible growth mechanism for the formation of flowerlike ZnO nanostructures was described. The optical properties of these differently shaped ZnO nanostructures were also described. The catalytic activities of the as-synthesized spherical and flowerlike ZnO nanostructures were tested in the Friedel-Crafts acylation reaction of anthracene with benzoyl chloride. The catalysis results indicated that the catalytic activity of flowerlike ZnO nanostructures is slightly higher than the spherical counterpart.

Solvent-Adoptable Polymer Ni/NiCo Alloy Nanochains: Highly Active and Versatile Catalysts for Various Organic Reactions in both Aqueous and Nonaqueous Media

The synthesis of solvent-adoptable monometallic Ni and NiCo alloy nanochains by a one-pot solution phase reduction method in the presence of poly(4-vinylphenol) (PVPh) is demonstrated. The elemental compositions of the as-prepared alloys are determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) and energy-dispersive X-ray spectroscopy (EDS), which are matching well with the target compositions. The morphology analysis by TEM and FESEM confirms that the nanochains are made up of organized spherical monometallic Ni or bimetallic NiCo alloy nanoparticles (NPs). However, there is no nanochain formation when the alloy is prepared without the polymer PVPh. A possible mechanism for the formation of such NiCo alloy nanochains is discussed. The X-ray diffraction and selected area electron diffraction patterns reveal that the Ni/NiCo alloys are polycrystalline with fcc structure. The obtained Ni or NiCo alloy nanostructures are ferromagnetic with very high coercivity. The polymer Ni/NiCo alloy nanochains are dispersible in both water and organic media that makes them versatile enough to use as catalysts in the reactions carried out in both types of media. The catalytic activities of these Ni/NiCo alloy nanochains are extremely high in the borohydride reduction of p-nitrophenol in water. In organic solvents, these nanochains can act as efficient catalysts, under ligand-free condition, for the C-S cross-coupling reactions of various aryl iodides and aryl thiols for obtaining the corresponding cross-coupled products in good to excellent yield up to 96%. The NiCo nanochain also successfully catalyzes the C-O cross-coupling reaction in organic medium. A possible mechanism for NiCo alloy nanochain-catalyzed cross-coupling reaction is proposed.

Novel ascorbic acid based ionic liquids for the in situ synthesis of quasi-spherical and anisotropic gold nanostructures in aqueous medium.

A series of newly designed ascorbic acid based room temperature ionic liquids were successfully used to prepare quasi-spherical and anisotropic gold nanostructures in an aqueous medium at ambient temperature. The synthesis of these room temperature ionic liquids involves, first, the preparation of a 1-alkyl (such as methyl, ethyl, butyl, hexyl, octyl, and decyl) derivative of 3-methylimidazolium hydroxide followed by the neutralization of the derivatised product with ascorbic acid. These ionic liquids show significantly better thermal stability and their glass transition temperature (Tg) decreases with increasing alkyl chain length. The ascorbate counter anion of these ionic liquids acts as a reducing agent for HAuCl4 to produce metallic gold and the alkylated imidazolium counter cation acts as a capping/shape-directing agent. It has been found that the nature of the ionic liquids and the mole ratio of ionic liquid to HAuCl4 has a significant effect on the morphology of the formed gold nanostructures. If an equimolar mixture of ionic liquid and HAuCl4 is used, predominantly anisotropic gold nanostructures are formed and by varying the alkyl chain length attached to imidazolium cation of the ionic liquids, various particle morphologies can formed, such as quasispherical, raspberry-like, flakes or dendritic. A probable formation mechanism for such anisotropic gold nanostructures has been proposed, which is based on the results of some control experiments.

Interparticle interaction and size effect in polymer coated magnetite nanoparticles

The value of the surface anisotropy constant Ks as obtained from equation (5) in the 11th line from the top of page 9098 should be Ks= 0.15 × 10-4 Jm-2.

Reversible self-assembly of carboxylated peptide-functionalized gold nanoparticles driven by metal-ion coordination.

Carboxylated peptide-functionalized gold nanoparticles (peptide-GNPs) self-assemble into two- and three-dimensional nanostructures in the presence of various heavy metal ions (i.e. Pb(2+), Cd(2+), Cu(2+), and Zn(2+)) in aqueous solution. The assembly process is monitored by following the changes in the surface plasmon resonance (SPR) band of gold nanoparticles in a UV/Vis spectrophotometer, which shows the development of a new SPR band in the higher-wavelength region. The extent of assembly is dependent on the amount of metal ions present in the medium and also the time of assembly. TEM analysis clearly shows formation of two- and three-dimensional nanostructures. The assembly process is completely reversible by addition of alkaline ethylenediaminetetraacetic acid (EDTA) solution. The driving force for the assembly of peptide-GNPs is mainly metal ion/carboxylate coordination. The color and spectral changes due to this assembly can be used for detection of these heavy-metal ions in solution.

Polymer assisted synthesis of chain-like cobalt-nickel alloy nanostructures: Magnetically recoverable and reusable catalysts with high activities

We present a simple wet chemical reduction method to synthesize chain-like bimetallic cobalt-nickel (CoNi) alloy nanostructures of varying compositions in the presence of a thermoresponsive polymer, poly(vinyl methyl ether) (PVME). We also synthesize monometallic Co and Ni nanostructures by this method for comparison of their properties with that of CoNi nanoalloys. Transmission electron microscopic (TEM) study reveals that the formed CoNi nanoalloys exhibit a chain-like assembled nanostructure encompassed with some hairy structure. The chain-like assembled alloy nanostructures are composed of spherical CoNi alloy nanoparticles. CoNi nanoalloys prepared without PVME do not show such chain-like structures indicating that PVME-assisted growth is needed for preparation of such patterned self-assembled nanostructures. The combined X-ray diffraction and selected area electron diffraction (SAED) studies confirm that this method produces polycrystalline chain-like CoNi nanoalloys. SAED study further reveals that the hairy structures are amorphous in nature. The synthesized CoNi alloy/pure Co/pure Ni nanostructures show soft ferromagnetic behaviour. The formed nanostructured bimetallic CoNi alloys and monometallic Co and Ni are excellent catalysts for both organic and inorganic reactions in aqueous phase. The obtained CoNi alloy nanochain also successfully catalyzes the cross coupling reaction between iodobenzene and 4-chlorothiophenol in the organic phase (DMF). The magnetically recovered chain-like CoNi alloy nanocatalyst is reusable at least for eight times without much loss of its initial activity.

Dispersion polymerization of pyrrole using ethylhydroxy‐ethylcellulose as a stabilizer

Oxidative polymerization of pyrrole has been studied using FeCl3 or (NH4)2S2O8 (APS) as oxidant, ethylhydroxyethylcellulose (EHEC) as a steric stabilizer and water or aqueous ethanol as the dispersion medium. Transmission electron micrographic images of the particles from the as-prepared dialysed dispersions in aqueous ethanol show small as well as large particles (about a decade larger) when FeCl3 is used as the oxidant but only large particles when APS is used as the oxidant. Small particles are not found when the dispersions are prepared in water, irrespective of the oxidant used. The particle size decreases with an increase in molecular weight of the stabilizer for the same stabilizer concentration. The minimum amount of stabilizer required to support dispersion polymerization decreases upon increasing the alcohol content of the medium.

Low-temperature polymer-assisted synthesis of shape-tunable zinc oxide nanostructures dispersible in both aqueous and non-aqueous media

We report the shape-controlled synthesis of zinc oxide (ZnO) nanostructures by a poly(vinyl methyl ether) (PVME)-assisted alkaline hydrolysis of zinc acetate at low temperature (20 degrees C). In this method, ZnO nanostructures of various morphologies including dumbbells, lances and triangles have been successfully prepared via a simple variation of different reaction parameters such as polymer concentration, pH of the reaction mixture and precursor concentration. However, without PVME, ZnO of such structurally uniform morphologies were not formed; rather ZnO of a mixture of defined and undefined morphologies were obtained indicating PVME-assisted the growth of such regular shaped ZnO nanostructures. HRTEM analysis of lance- and triangle-shaped samples as well as SAED patterns of all kinds of samples (dumbbell, lance and triangle) revealed that the ZnO nanostrcutures are single crystalline in nature and might form through oriented growth. XRD analysis also revealed the formation of well crystalline ZnO with a hexagonal structure. FTIR spectroscopy and TGA analysis confirmed the adsorption of PVME on the surface of ZnO nanostructures. Being a solvent adaptable polymer, the adsorbed PVME makes these shaped ZnO nanostructures highly dispersible in both polar and non-polar organic solvents including water. The extent of dispersibility in different solvents was studied by spectroscopic and microscopic techniques. Such solvent adoptability of PVME-coated ZnO nanostructures increases its ease of applications in device fabrication as well as in biological systems.

Tunable doubly responsive UCST-type phosphonium poly(ionic liquid): a thermosensitive dispersant for carbon nanotubes

A phosphonium poly(ionic liquid) (PIL), poly(triphenyl-4-vinylbenzylphosphonium chloride) (P[VBTP][Cl]), of varying and controllable molecular weights is synthesized via reversible addition–fragmentation chain transfer (RAFT) polymerization to afford a doubly responsive PIL that responds to both halide ions and temperature. The addition of halide ions transforms the transparent aqueous PIL solution into a turbid two-phase solution, forming insoluble microgel aggregates owing to the screening of the positively charged phosphonium groups of PILs and eventually forms intra- and/or inter-chain cross-linking among the PIL chains through halide ion bridges. Further, this turbid solution exhibits a distinct upper critical solution temperature (UCST)-type phase transition and transforms into a one-phase transparent solution due to the disruption of ion bridges upon heating. The rate of aggregation of P[VBTP][Cl] increases sharply with an increase of the size of the added halide ions in this order I− > Br− > Cl−. The cloud point temperature (Tcp) increases linearly with increasing halide ion concentration. The Tcp also increases with increasing molecular weights of the PIL. The phase diagram of aqueous PIL solution shows the highest Tcp at 6 wt%. Interestingly, the Tcp of the P[VBTP][Cl] in water decreases sharply with addition of small but increasing amounts of organic cosolvents. This PIL exhibits very good stabilizing ability for carbon nanotubes in water, whose dispersion state can be switched from dispersed to agglomerate and vice versa by adding halide ions and increasing the temperature respectively. The cross-linked hydrogel of P[VBTP][Cl] also shows dual responsiveness towards both halide ions and temperature.

Ethylhydroxyethylcellulose stabilized polypyrrole dispersions

Abstract Oxidative dispersion polymerization of pyrrole using ethylhydroxyethylcellulose stabilizer in water or aqueous alcohol medium yields stable dispersions of submicrometre-sized conducting spherical polypyrrole particles. Transmission electron micrograph images of the particles from the original dispersion distinguish two kinds of particles: one very small, ∼ 20 nm in diameter, and the other eight to ten times larger. The former constitutes about 1% of the total mass of particles. Similar results were obtained with a poly(vinyl methyl ether) stabilized polypyrrole dispersion.