Sabyasachi Patra

Redox Decomposition of Silver Citrate Complex in Nanoscale Confinement: An Unusual Mechanism of Formation and Growth of Silver Nanoparticles

We demonstrate for the first time the intrinsic role of nanoconfinement in facilitating the chemical reduction of metal ion precursors with a suitable reductant for the synthesis of metal nanoparticles, when the identical reaction does not occur in bulk solution. Taking the case of citrate reduction of silver ions under the unusual condition of [citrate]/[Ag(+)] ≫ 1, it has been observed that the silver citrate complex, stable in bulk solution, decomposes readily in confined nanodomains of charged and neutral matrices (ion-exchange film and porous polystyrene beads), leading to the formation of silver nanoparticles. The evolution of growth of silver nanoparticles in the ion-exchange films has been studied using a combination of (110m)Ag radiotracer, small-angle X-ray scattering (SAXS) experiments, and transmission electron microscopy (TEM). It has been observed that the nanoconfined redox decomposition of silver citrate complex is responsible for the formation of Ag seeds, which thereafter catalyze oxidation of citrate and act as electron sink for subsequent reduction of silver ions. Because of these parallel processes, the particle sizes are in the bimodal distribution at some stages of the reaction. A continuous seeding with parallel growth mechanism has been revealed. Based on the SAXS data and radiotracer kinetics, the growth mechanism has been elucidated as a combination of continuous autoreduction of silver ions on the nanoparticle surfaces and a sudden coalescence of nanoparticles at a critical number density. However, for a fixed period of reduction, the size, size distribution, and number density of thus-formed Ag nanoparticles have been found to be dependent on physical architecture and chemical composition of the matrix.

Wonderful nanoconfinement effect on redox reaction equilibrium

Polyol and other routes of reduction of noble metal ions that are not feasible under ambient condition have been observed with appreciable rate in nanoconfined spaces of solid matrices for the first time. Also a remarkable enhancement of the rate constant/activity parameter has been observed for AgNPs catalyzed BH4− reduction of p-nitrophenol in nanoscale confinement.

Understanding nictric acid-induced changes in the arrangement of monomeric and polymeric methacryloyl diglycolamides on their affinity toward f-element ions

Assembled diglycolamides (DGAs) have a strong affinity toward f-element ions at high nitric acid concentrations. Small angle X-ray scattering studies revealed that nitric acid concentration dependent changes occur in the geometrical arrangement of the DGA units of monomeric methacryloyl-DGA and the corresponding polymeric DGA. Cylindrical aggregates of methacryloyl-DGA were formed in 10:1 n-dodecane:1-decanol (added for solubility reasons) upon equilibration with nitric acid. The lengths and diameters of the cylindrical methacryloyl-DGA aggregates increased on varying the nitric acid concentration from 3 to 4 mol L(-1). This resulted in an increase of the distribution coefficient (D) of Eu(3+) ions from 72 to 197. The physical structure of cross-linked (10 mol %) poly(methacryloyl-DGA) reorganized distinctly upon equilibration with nitric acid. In this case, also the DEu(3+) values increased significantly from 147 mL g(-1) at 1 mol L(-1) HNO3 to ∼4000 mL g(-1) at 4 mol L(-1) HNO3. Hydrogen bonds between the outer sphere of Eu(3+)/Am(3+)/Pu(4+) nitrate and DGA units provide stabilization in the hydrophobic environment. This results in enhancement of their extraction upon increasing nitric acid concentration both in the organic phase as well as in the polymer matrix. Though monomeric and polymeric methacryloyl-DGA are different in their physical assembling, the normalized DI values for a same f-element ion upon varying HNO3 concentrations show remarkably similar patterns in both forms. In addition, the unusual stoichiometry deduced from the slopes of the log D vs log[HNO3] curves at fixed nitrate concentration seems to suggest that the normal extraction mechanism may not be operating in the hydrogen bonded DGA assemblies.

Time resolved growth of membrane stabilized silver NPs and their catalytic activity

Formation of highly stable metal nanostructures in a Nafion® membrane with various aspect ratios has been of considerable research interest in recent years. However, there is a need for a proper understanding of the growth mechanism of such nanostructures in Nafion® (sometimes larger than the size of water–sulfonate ionic clusters of the membrane). In this work, the early growth kinetics of silver nanoparticles (NPs) in Nafion®-117 ion-exchange membrane during in situ L-ascorbic acid reduction of Ag+ ions by time resolved in situ small-angle X-ray scattering (SAXS) using synchrotron radiation with a time resolution of 50 ms are revealed for the first time. The SAXS analyses, corroborated by transmission electron microscopy, showed that the sizes of NPs increase rapidly together with their number density until they attain a certain size that could be accommodated in the ∼5 nm water–sulfonate ionic clusters. Further growth takes place either by self-agglomeration of the particles ejected out from the water–sulfonic acid clusters or by continuous reduction of metal ions on the existing NP surfaces (uniformly or on a specific plane) leading to formation of bigger nanostructures with various aspect ratios. The time resolved information of NP growth provides an opportunity for the controlled synthesis of metal NPs with a definite size, shape and size distribution for a specific application. The catalytic properties of Ag NPs formed in the membrane were examined using borohydride reduction of a model dye methylene blue. It was observed that smaller Ag NPs with a mean diameter ∼3 nm, confined in the hydrophilic clusters of the Nafion® matrix, have reasonably good catalytic activity and a lower lag time for the onset of reduction.

Attenuation correction for the assay of uranium(VI) solutions in large cylindrical containers by gamma ray spectrometry.

The Hybrid Monte Carlo method developed for attenuation correction has been extended for 500 ml cylindrical geometry. The method has been experimentally validated. Absolute efficiency studies for 500 ml aqueous, air and point source has been carried out using Monte Carlo simulation. It has been observed that point source efficiency is a good estimate of 500 ml source beyond sample-to-detector distance of 15 cm. It has been found that while HMC method for attenuation correction is valid at all sample-to-detector distances and over all transmittance range, the far-field and near-field formulae available in literature are valid only over a very narrow range of sample-to-detector distance.

Isotopic ratio correlation for the isotopic composition analysis of plutonium in Am–Pu mixed samples having High americium content

Interference of high amount of americium in the plutonium isotopic composition analysis has been studied by simulating gamma-ray spectra for Am-Pu samples over a wide composition range (5-97% (241)Am) for both power and research reactor grade plutonium. An alternate way for isotopic composition analysis has been proposed by correlating the isotopic ratios available in our old database with the experimentally obtained (241)Pu/(239)Pu isotopic ratio. The proposed method has been validated using simulated spectra of known isotopic compositions.

Isotopic composition analysis of dilute Pu solutions using 90−105 keV region of gamma ray spectra

Isotopic composition of dilute Pu solutions (1-3900μg/mL) has been determined by analysis of HPGe detector response function in the 90-105keV region of gamma ray spectra. Results are in excellent agreement with that obtained from mass spectrometric measurements. The present method has been successfully applied for samples of low Pu concentrations, which otherwise is not possible using the conventional 120-415keV region of plutonium γ ray spectra.

Attenuation correction for the collimated gamma ray assay of cylindrical samples

The Hybrid Monte Carlo (HMC) method developed earlier for attenuation correction of non-collimated samples [Agarwal et al., 2008, Nucl. Instrum. Methods A 597, 198], has been extended to the segmented gamma ray assay of cylindrical samples. The method has been validated both experimentally and theoretically. For experimental validation, the results of HMC calculation have been compared with the experimentally obtained attenuation correction factors. The HMC attenuation correction factors have also been compared with the results obtained from literature available near-field and far-field formulae at two sample-to-detector distances (10.3cm and 20.4cm). The method has been found to be valid at all sample-to-detector distances over a wide range of transmittance. On the other hand, the literature available near-field and far-field formulae have been found to work over a limited range of sample-to detector distances and transmittances. The HMC method has been further extended to circular collimated geometries where analytical formula for attenuation correction does not exist.

Synthesis, characterisation and counterion dependent mesoscopic modifications of ionomer nanocomposites having different dimensional silver nanostructures

Different dimensional silver nanostructures were synthesized in the cation exchange ionomer matrix Nafion-117 by varying temperature. The nanostructures formed were different sizes spherical nanoparticles depending upon the in situ reduction temperature. Mesoscopic architecture of the nanocomposite thin films containing different dimensional silver nanostructures has been studied by small angle x-ray scattering (SAXS) in different counterionic environment. Investigation of the modifications in nanostructure pattern along with modification in self-assembling morphology of the ionomer in metal-ionomer nanocomposites in different post reduction counterionic environment was found to be unique. The post reduction neutralization by different counterions having different degree of hydration resulted to reorganization of the embedded nanostructures along with the crystalline polymer backbone of the ionomer. This influenced the polydispersity of the nanostructures significantly.