Department Of Chemistry
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Item Noble copper-silver-gold trimetallic nanobowls: An efficient catalyst(Academic Press, 2019) Haldar, Krishna Kanta; Tanwar, S; Biswas, R; Sen, T; Lahtinen, J.We demonstrated the design of tiny bowls of copper-silver-gold (Cu-Ag-Au) alloy type noble trimetallic nanocrystals with a unique shape. All the structural characterizations confirm the presence of copper (Cu), silver (Ag), and gold (Au) in the trimetallic nanobowls. Finally, we examined the catalytic efficiency of trimetallic Cu-Ag-Au nanobowls for reduction of 4-nitrophenol to 4-aminophenol and found that these nanobowls were 14, 23 and 43-fold more active than each of the constituent metals, Au, Cu and Ag, respectively. © 2019 Elsevier Inc.Item Nickel oxide decorated zinc oxide composite nanorods: Excellent catalyst for photoreduction of hexavalent chromium(Academic Press Inc., 2018) Singh, Simranjeet; Ahmed,Imtiaz; Haldar, Krishna KantaIn light of the growing interest and ability to search for new materials, we have synthesized Nickel oxide (NiO) nanoparticles decorated Zinc (ZnO) nanorods composite (NiO/ZnO) nanostructure. The NiO/ZnO heterostructure formation was confirmed by X-ray powder diffraction and high-resolution transmission electron microscopy (HRTEM). The fabricated environmental friendly NiO/ZnO composite nanostructure shows a well-defined photoreduction characteristic of hexavalent Chromium (Cr) (VI) to tri-valent Chromium (Cr) (III) under UV-light. Such an enhanced photoreduction property is attributed due to the decreased electron-hole recombination process which was proved by photoluminescence (PL) spectroscopy, photocurrent study, and electrochemical impedance spectroscopy. Furthermore, the photocatalytic activity rate of the NiO decorated ZnO nanorods was much higher than that of bare ZnO nanorods for the reduction of chromium (VI) and the rate is found to be 0.306 min?1. These results have demonstrates that suitable surface engineering may open up new opportunities in the development of high-performance photocatalyst. ? 2018 Elsevier Inc.Item Nonlinear Optical Switching and Enhanced Nonlinear Optical Response of Au−CdSe Heteronanostructures(ACS Publications, 2016) Sreeramulu, V.; Haldar, Krishna Kanta; Patra, Amitava; Rao, D. NarayanaThe metal−semiconductor heterostructures have recently emerged as a new class of functional materials for their potential applications due to plasmonexciton interactions. Here, we demonstrate the nonlinear optical switching and enhanced nonlinear optical response of Au−CdSe heterostructures. The nonlinear optical properties of CdSe quantum dots and Au−CdSe heteronanostructures are investigated by using a Z-scan technique at 532 nm picosecond laser pulses, and 800 nm femtosecond laser pulses. Interestingly, we observe switching behavior from saturable absorption (SA) to reverse saturable absorption (RSA) with increasing laser intensity. The effective two-photon absorption cross section (σeff) of Au−CdSe heteronanostructures is greatly enhanced which is attributed to charge transfer between CdSe and Au nanoparticles. The nonlinear refraction changes its sign from positive to negative nonlinearity at higher intensities for Au−CdSe heterostructures. Third order nonlinear optical susceptibility is being measured by using the DFWM technique at 532 nm. Similar switching behavior is observed in Au−CdSe heteronanostructures at nonresonant excitations (800 nm), where a CdSe quantum dot shows reverse saturable absorption behavior attributed to the two-photon absorption. The optical switching behavior of these heterostructures could play a potential role in photonics and optoelectronic applications.Item Shell thickness matters! Energy transfer and rectification study of Au/ZnO core/shell nanoparticles(Academic Press Inc., 2016) Haldar, Krishna Kanta; Sen, TapasiIn the present study we report the influence of shell thickness on fluorescence resonance energy transfer between Au/ZnO core-shell nanoparticles and Rhodamine 6G dye by steady-state and time-resolved spectroscopy and rectification behaviours. Au/ZnO core-shell nanoparticles with different shell thickness were synthesized in aqueous solution by chemically depositing zinc oxide on gold nanoparticles surface. A pronounced effect on the photoluminescence (PL) intensity and shortening of the decay time of the dye in presence of Au/ZnO core-shell nanoparticles is observed. The calculated energy transfer efficiencies from dye to Au/ZnO are 62.5%, 79.2%, 53.6% and 46.7% for 1.5 nm, 3 nm, 5 nm and 8 nm thickness of shell, respectively. Using FRET process, the calculated distances (r) are 117.8, 113.2 ? 129.9 ? and 136.7 ? for 1.5 nm, 3 nm, 5 nm and 8 nm thick Au/ZnO core-shell nanoparticles, respectively. The distances (d) between the donor and acceptor are 71.0, 57.8, 76.2 and 81.6 ? for 1.5 nm, 3 nm, 5 nm and 8 nm thick core-shell Au/ZnO nanoparticles, respectively, using the efficiency of surface energy transfer (SET). The current-voltage (I-V) curve of hybrid Au/ZnO clearly exhibits a rectifying nature and represents the n-type Schottky diode characteristics with a typical turn-on voltage of between 0.6 and 1.3 V. It was found that the rectifying ratio increases from 20 to 90 with decreasing the thickness of the shell from 5 nm to 3 nm and with shell thickness of 8 nm, electrical transport through the core-shell is similar to what is observed with pure ZnO samples nanoparticles. The results indicated that the Au/ZnO core-shell nanoparticles with an average shell thickness of 3 nm exhibited the maximum energy transfer efficiencies (79.2%) and rectification (rectifying ratio 90). ? 2016Item Core-Size-Dependent Catalytic Properties of Bimetallic Au/Ag Core− Shell Nanoparticles(ACS Publications, 2017) Haldar, Krishna Kanta; Kundu, Simanta; Patra, AmitavaBimetallic core−shell nanoparticles have recently emerged as a new class of functional materials because of their potential applications in catalysis, surface enhanced Raman scattering (SERS) substrate and photonics etc. Here, we have synthesized Au/Ag bimetallic core−shell nanoparticles with varying the core diameter. The red-shifting of the both plasmonic peaks of Ag and Au confirms the core−shell structure of the nanoparticles. Transmission electron microscopy (TEM) analysis, line scan EDS measurement and UV−vis study confirm the formation of core−shell nanoparticles. We have examined the catalytic activity of these core−shell nanostructures in the reaction between 4- nitrophenol (4-NP) and NaBH4 to form 4-aminophenol (4-AP) and the efficiency of the catalytic reaction is found to be increased with increasing the core size of Au/Ag core−shell nanocrystals. The catalytic efficiency varies from 41.8 to 96.5% with varying core size from 10 to 100 nm of Au/Ag core−shell nanoparticles, and the Au100/Ag bimetallic core−shell nanoparticle is found to be 12-fold more active than that of the pure Au nanoparticles with 100 nm diameter. Thus, the catalytic properties of the metal nanoparticles are significantly enhanced because of the Au/Ag core−shell structure, and the rate is dependent on the size of the core of the nanoparticles.Item DNA Origami Directed Au Nanostar Dimers for Single-Molecule Surface-Enhanced Raman Scattering(American Chemical Society, 2017) Tanwar, Swati; Haldar, Krishna Kanta; Sen, Tapasi; Tanwar, S.; Haldar, K.K.; Sen, T.We demonstrate the synthesis of Au nanostar dimers with tunable interparticle gap and controlled stoichiometry assembled on DNA origami. Au nanostars with uniform and sharp tips were immobilized on rectangular DNA origami dimerized structures to create nanoantennas containing monomeric and dimeric Au nanostars. Single Texas red (TR) dye was specifically attached in the junction of the dimerized origami to act as a Raman reporter molecule. The SERS enhancement factors of single TR dye molecules located in the conjunction region in dimer structures having interparticle gaps of 7 and 13 nm are 2 ? 1010 and 8 ? 109, respectively, which are strong enough for single analyte detection. The highly enhanced electromagnetic field generated by the plasmon coupling between sharp tips and cores of two Au nanostars in the wide conjunction region allows the accommodation and specific detection of large biomolecules. Such DNA-directed assembled nanoantennas with controlled interparticle separation distance and stoichiometry, and well-defined geometry, can be used as excellent substrates in single-molecule SERS spectroscopy and will have potential applications as a reproducible platform in single-molecule sensing. ? 2017 American Chemical Society.