Md. Harunar Rashid

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.

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.

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.

Redox-Active Ionic-Liquid-Assisted One-Step General Method for Preparing Gold Nanoparticle Thin Films: Applications in Refractive Index Sensing and Catalysis

We describe a general one-step facile method for depositing gold nanoparticle (GNP) thin films onto any type of substrates by the in situ reduction of AuCl(3) using a newly designed redox-active ionic liquid (IL), tetrabutylphosphonium citrate ([TBP][Ci]). Various substrates such as positively charged glass, negatively charged glass/quartz, neutral hydrophobic glass, polypropylene, polystyrene, plain paper, and cellophane paper are successfully coated with a thin film of GNPs. This IL ([TBP][Ci]) is prepared by the simple neutralization of tetrabutylphosphonium hydroxide with citric acid. We also demonstrate that the [TBP][Ci] ionic liquid can be successfully used to generate GNPs in an aqueous colloidal suspension in situ. The deposited GNP thin films on various surfaces are made up of mostly discrete spherical GNPs that are well distributed throughout the film, as confirmed by field-emission scanning electron microscopy. However, it seems that some GNPs are arranged to form arrays depending on the nature of surface. We also characterize these GNP thin films via UV-vis spectroscopy and X-ray diffractometry. The as-formed GNP thin films show excellent stability toward solvent washing. We demonstrate that the thin film of GNPs on a glass/quartz surface can be successfully used as a refractive index (RI) sensor for different polar and nonpolar organic solvents. The as-formed GNP thin films on different surfaces show excellent catalytic activity in the borohydride reduction of p-nitrophenol.

In situ formation of chiral core–shell nanostructures with raspberry-like gold cores and dense organic shells using catechin and their catalytic application

Chiral core–shell nanostructures containing raspberry-like gold cores and well-defined dense organic shells are synthesized by an in situ method using a natural antioxidant catechin as the reducing agent. This method is flexible and enables control over the shell thickness by adjusting the molar ratio of catechin to HAuCl4. Transmission electron microscopic analysis shows the formation of core–shell nanostructures with somewhat raspberry-shaped gold cores. The proposed mechanism explains that catechin reduces Au3+ to metallic Au to gold nanostructures and gets oxidized to different oligomeric products that are adsorbed in situ and assembled through H-bonding and form a thick organic shell around the generated gold nanostructures. Each of these oxidized forms of catechin is well-characterized by FTIR, ESI-MS, and MALDI-TOF-MS spectroscopies. This reaction follows a radical pathway as confirmed by electron paramagnetic resonance spectroscopy. Due to the presence of a compact and dense shell, the rate of gold core dissolution sharply decreases compared to that of the dissolution of monolayer protected Au nanoparticles when etched with KCN solution. The optical activity of the core–shell nanostructure is the result of the interaction between chiral shells and gold cores as observed by circular dichroism (CD) spectroscopy. The chiral core–shell Au nanostructures exhibit a CD band at the plasmon resonance frequency (∼596 nm). Finally, these chiral core–shell nanostructures are used as an effective catalyst in the borohydride reduction of p-nitrophenol.