Mrinmoy Biswas

Ionic liquid-based solvent-induced shape-tunable small-sized ZnO nanostructures with interesting optical properties and photocatalytic activities

We report a simple and one-pot size- and shape-controlled synthesis of small crystalline ZnO nanostructures through the hydrolysis of zinc acetylacetonate precursor using an ionic liquid, tetrabutylphosphonium hydroxide in different organic solvents and solvent mixtures of different polarities. Transmission electron microscopy (TEM) results indeed show the formation of very small (D ∼ 5–7 nm) ZnO spheres and ZnO nanorods of very low diameters (D ∼ 8–11 nm) depending on the solvents. The size and shape of ZnO nanostructures can be easily controlled by simple variation of the solvent and reaction temperature. Protic solvents favor the growth of spherical nanoparticles, whereas the rod-shaped ZnO is formed in aprotic solvents. The spherical ZnO nanostructures exhibit excitonic absorption at lower wavelengths compared to that of rod-shaped ZnO nanostructures. Through adjustment and control of the size and shape of ZnO nanostructures, we can tune the fluorescence emission from blue to green to yellow, when dispersed in the aqueous medium. The obtained ZnO nanostructures show very high photocatalytic activities towards the degradation of different cationic organic dyes (such as rhodamine 6G, crystal violet and methylene blue) as they contain a large number of defect states, as evident from the photoluminescence study. The as-synthesized ZnO nanostructures also show very high stability towards the photocatalytic degradation of organic dyes.

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.

Gelation of amino acid-based amphiphiles in water-based mixed solvent systems: reusable catalytic templates for nanostructured silica and silica–zirconia photocatalyst

We describe a gelation study of a series of different fatty acid–amino acid conjugated amphiphiles in various mixed solvent systems with water. Field emission scanning electron microscopy (FESEM), polarized optical microscopy (POM) and X-ray diffraction (XRD) study reveals that the self-organization of the amphiphile molecules in the mixed solvent leads to the formation of crystalline fibers, which form the opaque white gel. This amino acid amphiphile-based gel acts as a catalyst as well as a template for the hydrolysis/condensation of tetraethoxysilane (TEOS) to silica and forms a composite gel. The methanol extraction of the opaque white as-prepared composite gel results in the formation of a transparent nanostructured silica gel. The recovered amphiphilic gelator, after a methanol wash, can be reused for preparing a gel, which can subsequently be used as a catalyst to prepare a nanostructured silica gel again. The FESEM study confirms that the formed nanostructured silica gel is made up of spherical silica nanoparticles. We also extend this gel-based catalysis strategy to prepare silica–zirconia mixed oxide nanostructures. TEM examination reveals the formation of spherical silica–zirconia nanoparticle of high surface area as confirmed through BET surface area measurement. Finally, the photocatalytic activity of silica–zirconia mixed oxide is investigated towards methylene blue degradation. The mixed oxide shows higher photocatalytic efficiency than neat zirconia nanostructures.

In situ synthesis of ultra-small platinum nanoparticles using a water soluble polyphenolic polymer with high catalytic activity

A simple and convenient strategy is described for the in situ synthesis of ultra-small platinum nanoparticles (Pt NPs) at room temperature using poly(4-vinyl phenol) (PVPh) as both the reducing as well as the stabilizing agent in aqueous alkaline solution. This strategy excludes the use of any additional stabilizing agent in addition to use of a reducing agent. Transmission electron microscopic analysis confirms the formation of ultra-small spherical Pt NPs from 1.6 ± 0.2 to 2.2 ± 0.2 nm in diameter with a high degree of monodispersity depending on the ratio of PVPh to platinum salt concentrations used in a single reaction. The as-synthesized ultra-small Pt NPs exhibit extremely high catalytic activity towards the borohydride reduction of p-nitrophenol with very low activation energy (Ea = 24.6 kJ mol−1). Furthermore, the ultra-small PVPh-capped Pt NPs are successfully used as an excellent catalyst for hydrogenation of styrene and nitrobenzene in methanol with very high yield. The PVPh-capped Pt NPs are reusable for up to four cycles of catalysis reaction, although there is a substantial loss of its original activity after the first cycle.