Ashok K. Ganguli

Enhanced functionalization of Mn2

Core-shell nanostructures of Mn2O3@SiO2, Mn2O3@amino-functionalized silica, Mn2O3@vinyl-functionalized silica, and Mn2O3@allyl-functionalized silica were synthesized using the hydrolysis of the respective organosilane precursor over Mn2O3 nanoparticles dispersed using colloidal solutions of Tergitol and cyclohexane. The synthetic methodology used is an improvement over the commonly used post-grafting or co-condensation method as it ensures a high density of functional groups over the core-shell nanostructures. The high density of functional groups can be useful in immobilization of biomolecules and drugs and thus can be used in targeted drug delivery. The high density of functional groups can be used for extraction of elements present in trace amounts. These functionalized core-shell nanostructures were characterized using TEM, IR, and zeta potential studies. The zeta potential study shows that the hydrolysis of organosilane to form the shell results in more number of functional groups on it as compared to the shell formed using post-grafting method. The amino-functionalized core-shell nanostructures were used for the immobilization of glucose and L-methionine and were characterized by zeta potential studies.

Nanorods of manganese oxalate: a single source precursor to different manganese oxide nanoparticles (MnO, Mn2O3, Mn3O4)

Nanorods of anhydrous manganese oxalate were prepared by the reverse-micellar method using CTAB as the surfactant. Manganese oxalate precursor was used to synthesize single phase nanoparticles of various manganese oxides such as MnO, Mn2O3 and Mn3O4 under specific reaction conditions. Both MnO (28 nm) and α-Mn2O3 (50 nm) are stabilized as cubic phase. α-Mn2O3 shows a weak antiferromagnetic transition (TN = 80 K), while the spinel Mn3O4 (100 nm) particles show a ferrimagnetic transition at 43 K.

Polymeric citrate precursor route to the synthesis of the high dielectric constant oxide, CaCu3Ti4O12

Abstract Using citric acid and ethylene glycol, a polymeric precursor of the above compound was obtained at low temperatures (135 °C) which on subsequent heat treatments led to pure CaCu 3 Ti 4 O 12 at 1000 °C. On sintering further at 1000 °C (20 h), the disks show high density (98%) and the dielectric constant was of the order of 3000 at 1 kHz. The dielectric loss varies between 0.3 and 0.35 (till 100 kHz) beyond that it increases sharply from 0.35 to 0.7 in the frequency range of 100–500 kHz. Our studies on dielectric properties of the above oxide, synthesized by the ceramic method and sintered under the same conditions, led to a dielectric constant of around 2200 at 1 kHz. The dielectric constant decreases with frequency in the oxides obtained by either of the above methods. Scanning electron micrographs (SEM) show much larger grains (nearly spherical) of 2–4 μm in the solid obtained by the ceramic route while the grains were much smaller (0.5–1.0 μm) in the oxide prepared by the polymeric citrate precursor route.

Band Gap Engineering of ZnO using Core/Shell Morphology with Environmentally Benign Ag2S Sensitizer for Efficient Light Harvesting and Enhanced Visible-Light Photocatalysis

Band gap engineering offers tunable optical and electronic properties of semiconductors in the development of efficient photovoltaic cells and photocatalysts. Our study demonstrates the band gap engineering of ZnO nanorods to develop a highly efficient visible-light photocatalyst. We engineered the band gap of ZnO nanorods by introducing the core/shell geometry with Ag2S sensitizer as the shell. Introduction of the core/shell geometry evinces great promise for expanding the light-harvesting range and substantial suppression of charge carrier recombination, which are of supreme importance in the realm of photocatalysis. To unveil the superiority of Ag2S as a sensitizer in engineering the band gap of ZnO in comparison to the Cd-based sensitizers, we also designed ZnO/CdS core/shell nanostructures having the same shell thickness. The photocatalytic performance of the resultant core/shell nanostructures toward methylene blue (MB) dye degradation has been studied. The results imply that the ZnO/Ag2S core/shell nanostructures reveal 40- and 2-fold enhancement in degradation constant in comparison to the pure ZnO and ZnO/CdS core/shell nanostructures, respectively. This high efficiency is elucidated in terms of (i) efficient light harvesting owing to the incorporation of Ag2S and (ii) smaller conduction band offset between ZnO and Ag2S, promoting more efficient charge separation at the core/shell interface. A credible photodegradation mechanism for the MB dye deploying ZnO/Ag2S core/shell nanostructures is proposed from the analysis of involved active species such as hydroxyl radicals (OH(•)), electrons (e(-)(CB)), holes (h(+)(VB)), and superoxide radical anions (O2(•-)) in the photodegradation process utilizing various active species scavengers and EPR spectroscopy. The findings show that the MB oxidation is directed mainly by the assistance of hydroxyl radicals (OH(•)). The results presented here provide new insights for developing band gap engineered semiconductor nanostructures for energy-harvesting applications and demonstrate Ag2S to be a potential sensitizer to supersede Cd-based sensitizers for eco-friendly applications.

Development of a microemulsion-based process for synthesis of cobalt (Co) and cobalt oxide (Co3O4) nanoparticles from submicrometer rods of cobalt oxalate.

Rod-shaped nanostructures of cobalt oxalate dihydrate were synthesized at room temperature by the microemulsion (reverse micellar) route. These rods are highly uniform in length and can be modified with temperature (from approximately 6.5 microm at 50 degrees C to approximately 2.5 microm at 150 degrees C) while keeping the diameter nearly constant (200-250 nm). Thermal decomposition of these rods in a controlled atmosphere (air and H(2)) leads to nanoparticles of Co(3)O(4) and Co, respectively, while in a helium atmosphere a mixture of Co and CoO nanoparticles is obtained. Co(3)O(4) nanoparticles (approximately 35 nm) were slightly agglomerated, while Co nanoparticles were monodispersed and highly uniform (approximately 25 nm). The oxalate rods and Co(3)O(4) nanoparticles show an antiferromagnetic ordering at 54 and 35 K, respectively.

Zinc oxalate nanorods: a convenient precursor to uniform nanoparticles of ZnO

Nanorods of zinc oxalate dihydrate have been synthesized using the reverse micellar route. These nanorods were decomposed at 450 °C in air to obtain nanoparticles of zinc oxide. Transmission electron microscopy shows the nanorods to be 120 nm in diameter and 600 nm in length. The ZnO nanoparticles are 55 nm in diameter. The photoluminescence studies show two peaks at 370 and 403 nm which can be ascribed to free excitonic transition and donor-acceptor pair transition respectively. The temperature dependent PL intensity shows an anomalous non-monotonous temperature dependence probably due to two different optical processes.

Microemulsion-mediated synthesis of cobalt (pure fcc and hexagonal phases) and cobalt-nickel alloy nanoparticles

By choosing appropriate microemulsion systems, hexagonal cobalt (Co) and cobalt-nickel (1:1) alloy nanoparticles have been obtained with cetyltrimethylammonium bromide as a cationic surfactant at 500 degrees C. This method thus stabilizes the hcp cobalt even at sizes (<10 nm) at which normally fcc cobalt is predicted to be stable. On annealing the hcp cobalt nanoparticles in H(2) at 700 degrees C we could transform them to fcc cobalt nanoparticles. Microscopy studies show the formation of spherical nanoparticles of hexagonal and cubic forms of cobalt and Co-Ni (1:1) alloy nanoparticles with the average size of 4, 8 and 20 nm, respectively. Electrochemical studies show that the catalytic property towards oxygen evolution is dependent on the applied voltage. At low voltage (less than 0.65 V) the Co (hexagonal) nanoparticles are superior to the alloy (Co-Ni) nanoparticles while above this voltage the alloy nanoparticles are more efficient catalysts. The nanoparticles of cobalt (hcp and fcc) and alloy (Co-Ni) nanoparticles show ferromagnetism. The saturation magnetization of Co-Ni nanoparticles is reduced compared to the bulk possibly due to surface oxidation.

Achieving Enhanced Visible-Light-Driven Photocatalysis Using Type-II NaNbO3/CdS Core/Shell Heterostructures

Expanding the light-harvesting range and suppressing the quick recombination of photogenerated charge carriers are of paramount significance in the field of photocatalysis. One possible approach to achieve wide absorption range is to synthesize type-II core/shell heterostructures. In addition, this system also shows great promise for fast separation of charge carriers and low charge recombination rate. Herein, following the surface functionalization method using 3-mercaptopropionic acid (MPA) as a surface functionalizing agent, we report on designing NaNbO3/CdS type-II core/shell heterostructures with an absorption range extending to visible range and explore the opportunity toward degradation of methylene blue (MB) dye as a model pollutant under visible light irradiation. Characterizations including X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), UV-vis diffuse reflectance spectrum (DRS), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), and Raman spectroscopy support the growth of CdS shell onto NaNbO3 nanorods. The resulting core/shell heterostructures unveiled high surface areas, enhanced light harvesting, and appreciably increased photocatalytic activity toward MB degradation compared to individual counterparts and the photocatalytic standard, Degussa P25, under visible light irradiation. The remarkably enhanced photocatalytic activity of core/shell heterostructures could be interpreted in terms of efficient charge separation owing to core/shell morphology and resulting type-II band alignment between NaNbO3 and CdS, which creates a step-like radial potential favoring the localization of one of the carriers in the core and the other in the shell. A plausible mechanism for the degradation of MB dye over NaNbO3/CdS core/shell heterostructures is also elucidated using active species scavenger studies. Our findings imply that hydroxyl radicals (OH(•)) play a crucial role in dictating the degradation of MB under visible light. This work highlights the importance of core/shell heterostructures in leading toward new paradigms for developing highly efficient and reusable photocatalysts for the destructive oxidation of recalcitrant organic pollutants.

Synthesis, characterization and dielectric properties of nanometer-sized barium strontium titanates prepared by the polymeric citrate precursor method

Barium strontium titanate (BST) powders of the formula Ba1 − xSrxTiO3 (0 ≤ x ≤ 1) have been prepared for the first time by the polymeric precursor route using citric acid and ethylene glycol. Pure BSTs were obtained at 500 °C. These oxides were found to have the cubic structure, which is retained even after heating at 800 °C. Detailed X-ray studies on samples sintered at 1100 °C show weak tetragonal distortion for BaTiO3, while the other BSTs retain their cubic structure. The particle size of the sintered oxides increases from 55 nm for BaTiO3 to 88 nm for SrTiO3, from X-ray line-broadening studies. The nano-sized grains are reasonably stable to sintering (the particle size for BaTiO3 changes from 25 nm at 500 °C to 55 nm at 1100 °C). The dielectric constant of the sintered oxides decreases from 510 for BaTiO3 (x = 0) to 190 for SrTiO3 (x = 1) at 100 kHz. The dielectric loss decreases from 0.05 for BaTiO3 to 0.001 for SrTiO3 at 100 kHz. No ferroelectric transition was observed in either the dielectric studies or by differential scanning calorimetry.

A type-II semiconductor (ZnO/CuS heterostructure) for visible light photocatalysis

Type-II semiconductors with p–n heterojunctions have been fabricated by decorating CuS nanostructures on the surface of ZnO nanotubes with the help of a wet-chemical method at low temperature. We are reporting the enhanced visible light photocatalytic efficiency of ZnO/CuS heterostructures. CuO nanostructures were synthesized on the surface of ZnO nanotubes and then the CuO nanostructures were converted to CuS at 80 °C to generate the ZnO/CuS heterostructures. These ZnO/CuS heterostructures efficiently decompose methylene blue upon irradiation of visible light at room temperature. A study of the mechanism suggests that the enhanced photocatalytic activity is due to the formation of ZnO/CuS junctions, which leads to the efficient separation of photoinduced carriers.