Amiya Priyam

Temperature tunability of size in CdS nanoparticles and size dependent photocatalytic degradation of nitroaromatics

Size tunability of thiophenol capped CdS nanoparticles (NPs) has been achieved by controlling the temperatures in situ. Synthesis at 5 degrees C produced stable particles of smallest size having narrow size distribution and high photoluminescence quantum yield. The photoluminescence of thiophenol capped CdS NPs was quenched by the nitroaromatic compounds. The Stern-Volmer constant of dinitrobenzene was about 15-fold higher than nitrobenzene, which indicates that introduction of nitro groups in the benzene ring increases the quenching efficiency. Further, the as-prepared CdS NPs were found to display size dependent photocatalytic activity towards degradation of nitroaromatics. The catalytic efficiency of CdS quantum particles was quintupled with decrease in particle size from 5.8 to 3.8 nm. An empirical equation has been derived to correlate the catalytic efficiency of the nanoparticles with the twin factors operating in the quantum confinement regime: (i) change in surface to volume ratio and (ii) shift in conduction band edge.

Supersaturation driven tailoring of photoluminescence efficiency and size distribution: A simplified aqueous approach for producing high-quality, biocompatible quantum dots

Supersaturation was found to play a pivotal role during nanoparticle-synthesis and its subtle variation helped achieve two prime objectives: (a) high photoluminescence quantum efficiency (PLQE) and (b) narrow size distribution, thereby obviating the need for post-preparative treatments. Degree of supersaturation of initial synthetic mixture was varied by changing the concentration of reagents while keeping their molar ratio constant at 1:2.5:0.5 for [Cd(2+)]:[cysteine]:[chalcogenide]. An eight-fold increase in supersaturation caused a sharp focusing of size distribution by 64% for CdS quantum dots (QDs). The as-prepared CdS and CdTe QDs were found to have size distribution as low as 4% at higher supersaturation. For a four-fold increase in supersaturation, PLQE of as-prepared CdTe QDs (4.3 nm) rose by 5 times to a remarkably high value of 54%. The focusing of size distribution with increasing supersaturation was found to work well even in the absence of any stabilizer. A substantial overlap of nucleation and growth was found at low supersaturation (0.5S(CdTe)), whereas a good separation of the two events is achieved at a higher supersaturation (4S(CdTe)). This study provides a simplified aqueous route for producing highly monodisperse, photoluminescent and biocompatible nanoparticles.

Size tunablity of CdTe crystallites in dendrimer nanocomposites and temperature dependent focusing of size distribution.

CdTe/Dendrimer nanocomposites have been synthesized for the first time in aqueous and nonaqueous media using PAMAM dendrimer (Generation 5.0). The average size of the as-prepared nanocomposites, as determined from dynamic light scattering (DLS) measurements, was found to be typically 182 nm and 23 nm in water and methanol, respectively under identical conditions of temperature (5 degrees C) and reagent ratio (Cd2+:Te2-, 1:0.5). The size of CdTe NPs within the nanocomposites, was found to be 3.1 and 2.8 nm for the aforementioned samples determined from optical absorption spectra using tight binding approximation. The NPs possess good degree of cystallinity as discernible from the lattice fringes in high-resolution transmission electron microscopic (HRTEM). Transmission electron microscopic (TEM) image and the cubic crystal phase was ascertained from the small area electron diffraction (SAED) pattern. Analysis of FTIR data suggests that CdTe NPs are bound to the surface amine groups as well as -NHCO- moieties lying in the interior of dendrimer structure. The present work demonstrates how the quality of the CdTe NPs formed within the dendrimer matrix can be nicely tuned by varying the parameters, namely, temperature, molar ratio of Cd2+: Te2- and pH. Changing of Cd2+: Te2- ratio of 1:1 to 1:0.5, decreased the average particle size from 5.0 nm to 3.4 nm with concomitant narrowing of size distribution by approximately 35% at 10 degrees C. On lowering down the synthesis temperature (25 degrees C-->5 degrees C), the average particle size remained unaffected while the size distribution became sharply focused. However, the extent of focusing was found to be more in methanol (40%) than that in water (30%).

Seed geometry and hydrogen bonding dependent plasmonic tuning of silver nanocrystals in a citrate–hydrazine matrix and SERS spectroscopic detection of chromium

By employing a ‘mild stabilizer–mild reductant’ system, a seed-mediated synthesis of tunable anisotropic and plasmonic silver nanocrystals (NCs) has been developed. Differently shaped seed NCs were synthesized using citrate and hydrazine hydrate as stabilizer and reductant respectively, and the same reagent system was retained for the subsequent overgrowth process as well. Spherical seed NCs always resulted in red-shifted plasmon peaks of the overgrown nanocrystals. However, we found that anisotropic seed NCs may cause either a red-shift or a blue-shift of the plasmon peak depending on the geometry of the seed NCs. Plate-like seed NCs result in red-shifted surface plasmon bands as a consequence of structural transformation from pentagonal to hexagonal plate-like structures. Second derivative FTIR and Raman spectroscopy revealed that the anisotropic overgrowth of the seed NCs is directed by the stabilizer–reductant i.e., citrate–hydrazine, hydrogen bonding network. As the concentration of hydrazine increases, the H-bonded network is strengthened and the plasmon peak shows a gradual red-shift (500 nm → 615 nm). In contrast, a pyramid-like seed causes a blue shift of the plasmon peak (790 nm → 775 nm), attributed to the loss of structural anisotropy of the pyramid like nanostructures. Based on the interaction of these NCs with inorganic oxoanions, a rapid and sensitive SERS-detection method for Cr(III) and Cr(VI) species has been developed with a limit of detection of 30 and 40 ppb, respectively. The simplicity is underscored as the detection method works under non-resonant conditions and in the solution phase without any “SERS tags”.

Degree of supersaturation: An effective tool to control the luminescence efficiency and size distribution in CdTe quantum dots

Supersaturation controlled synthesis of thioglycollic acid (TGA) capped CdTe quantum dots in aqueous medium has been carried out. With a four-fold increase in the degree of supersaturation, the photoluminescence quantum efficiency of the nanoparticles was enhanced more than five times to a remarkably high value of 46%. This was accompanied by concomitant narrowing of the size distribution of the QDs. The simplified approach obviates the need for post-preparative treatments to improve the particle characteristics.