Sasanka Deka

Phosphine-free synthesis of p-type copper(I) selenide nanocrystals in hot coordinating solvents.

We report a phosphine-free synthesis of p-type copper(I) selenide nanocrystals by a colloidal approach in a mixture of oleylamine and 1-octadecene. The nanocrystals had a cuboctahedral shape and cubic berzelianite phase. Films of these nonstoichiometric copper-deficient Cu(2-x)Se nanocrystals were highly conductive and showed high absorption coefficient in the near-infrared region. These nanocrystals could be used as hole-injection layers in optoelectronic devices.

Octapod-Shaped Colloidal Nanocrystals of Cadmium Chalcogenides via “One-Pot” Cation Exchange and Seeded Growth

The growth behavior of cadmium chalcogenides (CdE = CdS, CdSe, and CdTe) on sphalerite Cu(2-x)Se nanocrystals (size range 10-15 nm) is studied. Due to the capability of Cu(2-x)Se to undergo a fast and quantitative cation exchange reaction in the presence of excessive Cd(2+) ions, no Cu(2-x)Se/CdE heterostructures are obtained and instead branched CdSe/CdE nanocrystals are built which consist of a sphalerite CdSe core and wurtzite CdE arms. While CdTe growth yields multiarmed structures with overall tetrahedral symmetry, CdS and CdSe arm growth leads to octapod-shaped nanocrystals. These results differ significantly from literature findings about the growth of CdE on sphalerite CdSe particles, which until now had always yielded tetrapod-shaped nanocrystals.

Morphology Controlled Synthesis of Nanoporous Co3O4 Nanostructures and Their Charge Storage Characteristics in Supercapacitors

Cubic spinel Co3O4 nanoparticles with spherical (0D) and hexagonal platelet (2D) morphologies were synthesized using a simple solvothermal method by tuning the reaction time. XRD and HRTEM analyses revealed pure phase with growth of Co3O4 particles along [111] and [110] directions. UV-vis studies showed two clear optical absorption peaks corresponding to two optical band gaps in the range of 400-500 nm and 700-800 nm, respectively, related to the ligand to metal charge transfer events (O(2-) → Co(2+,3+)). Under the electrochemical study in two electrode assembly system (Co3O4/KOH/Co3O4) without adding any large area support or a conductive filler, the hexagonal platelet Co3O4 particles exhibited comparatively better characteristics with high specific capacitance (476 F g(-1)), energy density 42.3 Wh kg(-1) and power density 1.56 kW kg(-1) at current density of 0.5 Ag(-1), that suited for potential applications in supercapacitors. The observed better electrochemical properties of the nanoporous Co3O4 particles is attributed to the layered platelet structural arrangement of the hexagonal platelet and the presence of exceptionally high numbers of regularly ordered pores.

Fluorescent Asymmetrically Cobalt-Tipped CdSe@CdS Core@Shell Nanorod Heterostructures Exhibiting Room-Temperature Ferromagnetic Behavior

A colloidal two-step seeded-growth approach has been devised to selectively synthesize three-component magnetic/semiconductor hybrid nanocrystals (HNCs) with a matchstick-like profile and tunable geometric parameters. The newly developed heterostructures individually comprise a single metallic Co head connected to either apexes of one rod-shaped section made of a CdSe core eccentrically embedded in a CdS shell. The specific topological arrangement realized arises from the peculiar anisotropic reactivity of the noncentrosymmetric CdSe@CdS core@shell nanorods that have been used as substrates to seed heterogeneous nucleation of Co in a surfactant-free environment from an organometallic precursor. The HNCs retain appreciable fluorescent emission in spite of photoexcited charge transfer from the semiconductor to the metal domain and exhibit unusual ferromagnetic-like behavior at room temperature.

Ferromagnetism induced by hydrogen in polycrystalline nonmagnetic Zn0.95Co0.05O

Polycrystalline Zn0.95Co0.05O is found to be paramagnetic at room temperature and down to 12K. Optical measurements prove the incorporation of Co2+ ions inside the ZnO lattice in the tetrahedral site. When the paramagnetic sample is heated in Ar∕H2 at 1125K for 2h, ferromagnetism with a very high value of magnetization is observed at room temperature. Hydrogen reduction does not affect the substituted Co2+ ions inside the wurtzite crystal lattice as evidenced from optical studies. X-ray diffraction studies show the presence of Co metal after the reduction process which is the origin of room temperature ferromagnetism.

Multiply Twinned AgNi Alloy Nanoparticles as Highly Active Catalyst for Multiple Reduction and Degradation Reactions

Size dependent surface characteristics of nanoparticles lead to use of these nanomaterials in many technologically important fields, including the field of catalysis. Here Ag(1-x)Ni(x) bimetallic alloy nanoparticles have been developed having a 5-fold twinned morphology, which could be considered as an important alloy because of their excellent and unique catalytic and magnetic properties. Alloying between Ag and Ni atoms on a nanoscale has been confirmed with detailed X-ray diffraction, high resolution transmission electron microscopy, energy-dispersive X-ray analysis, X-ray photoelectron spectroscopy, and magnetization measurements. Although introduced for the first time as a catalyst due to having high active surface sites, the as-synthesized nanoparticles showed one of the best multiple catalytic activity in the industrially important (electro)-catalytic reduction of 4-nitrophenol (4-NP) and 4-nitroaniline (4-NA) to corresponding amines with noticeable reduced reaction time and increased rate constant without the use of any large area support. Additionally the same catalyst showed enhanced catalytic activity in degradation of environment polluting dye molecules. The highest ever activity parameter we report here for Ag0.6Ni0.4 composition is 156 s(-1)g(-1) with an apparent rate constant of 31.1 × 10(-3) s(-1) in a 4-NP reduction reaction where the amount of catalyst used was 0.2 mg and the time taken for complete conversion of 4-NP to 4-aminophenol was 60 s. Similarly, an incredible reaction rate constant (115 s(-1)) and activity parameter (576.6 s(-1)g(-1)) were observed for the catalytic degradation of methyl orange dye where 15 s is the maximum time for complete degradation of the dye molecules. The high catalytic performance of present AgNi alloy NPs over the other catalysts has been attributed to size, structural (twinned defect) and electronic effects. This study may lead to use of these bimetallic nanostructures with excellent recyclable catalytic efficiency in many more applications.

Efficient hydrogen/oxygen evolution and photocatalytic dye degradation and reduction of aqueous Cr(VI) by surfactant free hydrophilic Cu2ZnSnS4 nanoparticles

A single type of semiconductor material, surfactant free hydrophilic Cu2ZnSnS4 (CZTS) nanoparticles, for the first time has been explored as a three way heterogeneous catalyst in the electrocatalytic hydrogen evolution reaction (HER), photocatalytic degradation of polluting dyes and photocatalytic reduction of carcinogenic and mutagenic Cr(VI) to nontoxic Cr(III). Ultrafine surfactant free kesterite nanoparticles, CZTS, (∼4 nm) having an optimum band gap of 1.75 eV and electrical resistivity of 0.6 Ω m were used for various applications. The as-synthesized CZTS NPs showed promising electrocatalytic activity for the HER as well as the oxygen evolution reaction (OER) with a lower Tafel slope and excellent recyclable catalytic efficiency. Rhodamine B, methyl red and methylene blue dyes were degraded rapidly and efficiently in a photocatalytic pathway by the as-synthesized CZTS nanoparticles with only 5% activity loss in degradation after the 10th cycle. Similarly, a higher rate constant and activity parameter were observed for the photocatalytic reduction of hexavalent chromium ions. The high catalytic performance of the present CZTS NPs over other catalysts under indoor light conditions in the absence of any expensive noble metals and harsh reducing/oxidizing agents have been attributed to the size, surface charge and electronic effect of the nanoparticles.

Synthesis of surfactant-free SnS nanorods by a solvothermal route with better electrochemical properties towards supercapacitor applications

We demonstrate a simple, low cost, and eco-friendly synthesis of surfactant free tin monosulfide (SnS) nanorods by a solvothermal route for applications in supercapacitor devices with high specific capacitance. The as-synthesized SnS nanorods, consisting of an intrinsic layered structure, were thoroughly characterised by XRD, TEM, HRTEM, SEM, EDAX and BET techniques to determine their crystal structure, size, morphology and surface area. To explore potential applications for supercapacitors, the nanocrystals were used to fabricate a two electrode system without adding any binder, large area support or conductive filler, and the system was characterised by cyclic voltammograms, galvanostatic charge–discharge and electrochemical impedance spectroscopy measurements in aqueous 2 M Na2SO4 electrolyte. These SnS nanorods exhibit enhanced supercapacitor performance with specific capacitance, energy density and power density values of ∼70 F g−1, 1.49 W h kg−1 and 248.33 W kg−1, respectively, which are found to be two times higher than those of SnS–carbon composites, and thus make SnS nanorods a better alternative source for energy storage devices.

Bioconjugation of rod-shaped fluorescent nanocrystals for efficient targeted cell labeling.

In the present work, we report a three-step approach for the functionalization of CdSe/CdS core/shell and CdSe/CdS/ZnS double-shell quantum rods (QRs) with either biotin or folic acid. We carried out an in vitro study on cultured cells and fixed tissue sections in which the biofunctionalized QRs were compared with the more traditional CdSe/ZnS quantum dots (QDs), which were also functionalized with either biotin or folic acid. The QR and the QD samples exhibited the same specificity toward the targeting cells. In addition, due to the enhanced photoluminescence of the QRs with respect to QDs, a lower amount of rods was required to image cells. In immuno-localization experiments on rat brain tissue sections, biotin-functionalized QRs have shown the typical protein localization patterns expected both for neuronal enolase NSE and actin, confirming the specificity of the interaction of QRs with avidin, and the feasibility of these materials as fluorescent probes in tissue staining. In this specific targeting study, we could assess via the MTT test, a cell viability assay, the lower toxicity of the CdSe/CdS/ZnS QRs with respect to CdSe/CdS QRs.

Development and properties of surfactant-free water-dispersible Cu2ZnSnS4 nanocrystals: a material for low-cost photovoltaics.

A simple, yet novel hydrothermal method has been developed to synthesize surfactant-free Cu2ZnSnS4 nanocrystal ink in water. The environmentally friendly, 2-4 nm ultrafine particles are stable in water for several weeks. Detailed X-ray diffraction (XRD) and high-resolution transmission electron microscopy revealed the formation of single-crystalline-kesterite-phase Cu2ZnSnS4. Elemental mapping by scanning electron microscopy/energy dispersive spectrometry corroborated the presence of all four elements in a stoichiometric ratio with minor sulfur deficiency. Finally, Raman spectroscopy ruled out the possible presence of impurities of ZnS, Cu2SnS3, SnS, SnS2, Cu(2-x)S, or Sn2S3, which often interfere with the XRD and optical spectra of Cu2ZnSnS4. X-ray photoelectron spectroscopic studies of the as-synthesized samples confirmed that the oxidation states of the four elements match those of the bulk sample. Optical absorption analyses of thin film and solution samples showed high absorption efficiency (>10(4) cm(-1)) across the visible and near-infrared spectral regions and a band gap E(g) of 1.75 eV for the as-synthesized sample. A non-ohmic asymmetric rectifying response was observed in the I-V measurement at room temperature. The nonlinearity was more pronounced for this p-type semiconductor when the resistance was measured against temperature in the range 180-400 K, which was detected in the hot-point probe measurement.