Chandan Kumar Ghosh

Three Dimensional Ag2O/TiO2 Type-II (p–n) Nanoheterojunctions for Superior Photocatalytic Activity

Type-II p-n junction three-dimensional Ag(2)O/TiO(2) microspheres have been fabricated by assembling p-type Ag(2)O nanoparticle on n-type TiO(2) 3D microsphere. Ag(2)O/TiO(2) microsphere nanoheterojunctions were obtained by hydrothermal synthesis of TiO(2) microspheres at 180 °C followed by photoreduction of AgNO(3). The samples were carefully characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), field-emission scanning electron microscopy (FESEM), and energy dispersive X-ray analysis (EDX). The photocatalytic activity toward degradation of methyl orange (MO) aqueous solution under UV light was investigated. The result showed that type-II p-n nanoheterojunctions Ag(2)O/TiO(2) significantly enhanced the photocatalytic degradation compared to n-type TiO(2) microsphere. It was found that the photocatalytic degradation followed the pseudo first-order reaction model. In particular, heterostructure with molar ratio of TiO(2) and AgNO(3) of 4:1 exhibited best photocatalytic activity and the corresponding apparent first-order rate constant of 0.138 min(-1) which is 4 times than that of pure n-type microsphere.

Morphology control of rutile TiO2 hierarchical architectures and their excellent field emission properties

Titanium dioxide nanoarchitectures with well-defined morphologies have been successfully synthesized by a hydrothermal process at temperature 180 °C for 4 h using titanium butoxide, oleic acid and hydrochloric acid as precursor materials. Different hierarchical morphologies such as cauliflower, 3D microsphere, densely-packed nanorod array, step edge faceted nanorod, branched structures of the rutile TiO2 nanostructures could be easily controlled by varying the volume fraction of the added hydrochloric acid. The structural, photoluminescence and field emission properties of the as-prepared nanoarchitectures were investigated. Among the different morphologies, dense nanorod arrays and multilevel branched architectures showed good field emission properties. Morphology, surface-related defects and oxygen vacancies were found to be the major responsible factors for the observed variations of field emission properties. Compared to any previously reported TiO2 based field emitter, lower turn-on field at 2.76 V μm−1 and higher field-enhancement factors 7.44 × 103 were observed for the hierarchically dense TiO2 nanorods array. Therefore, these nanoarchitectures can be used in vacuum microelectronic applications.

Magnetic, X-ray and Mössbauer studies on magnetite/maghemite core–shell nanostructures fabricated through an aqueous route

Uniform 6–13 nm sized 0D superparamagnetic Fe3O4 nanocrystals were synthesized by an aqueous ‘co-precipitation method’ under a N2 atmosphere as a function of temperature to understand the growth kinetics. The crystal phases, surface charge, size, morphology and magnetic characteristics of as-synthesized nanocrystals were characterized by XRD, Raman spectroscopy, FTIR, TG-DTA, BET surface area, dynamic light scattering along with zeta potential, HR-TEM, EDAX, vibrating sample magnetometry and Mossbauer spectroscopy. TEM investigation revealed highly crystalline spherical magnetite particles in the 8.2–12.5 nm size range. The kinetically controlled as-grown nanoparticles were found to possess a preferential (311) orientation of the cubic phase, with a highest magnetic susceptibility of ∼57 emu g−1. The Williamson–Hall technique was employed to evaluate the mean crystallite size and microstrain involved in the as-synthesized nanocrystals from the X-ray peak broadening. In addition to FTIR and Raman spectra, Rietveld structural refinement of XRD confirms the magnetite phase with 5–20% maghemite in the sample. VSM and Mossbauer spectral data allowed us to fit the magnetite/maghemite content to a core–shell model where the shell is 0.2–0.3 nm thick maghemite over a magnetite core. The activation energy of <10 kJ mol−1 calculated from an Arrhenius plot for the complex process of nucleation and growth by diffusion during synthesis shows the significance of the precipitation temperature in the size controlled fabrication processes of nanocrystals. Brunauer–Emmett–Teller (BET) results reveal a mesoporous structure and a large surface area of 124 m2 g−1. Magnetic measurement shows that the particles are ferromagnetic at room temperature with zero remanence and zero coercivity. This method produced highly crystalline and dispersed 0D magnetite nanocrystals suitable for biological applications in imaging and drug delivery.

Effect of Ni doping on the dielectric constant of ZnO and its frequency dependent exchange interaction

Ni doped ZnO powders were synthesized by a simple dissolution followed by precipitation using zinc acetate, nickel acetate and oxalic acid for different atomic percentages of Ni doping. Proper phase formation was confirmed by x-ray diffraction and the compositional analysis was performed by using x-ray photoelectron spectroscopic studies. The frequency dependent dielectric constant of the Ni doped ZnO samples was measured by an LCR meter, and from the dielectric constant versus frequency curve, fitted with the Cole equation, we obtained static and high frequency dielectric constants of the Ni doped ZnO samples. It was observed that both the static and high frequency dielectric constants decreased with increasing percentages of Ni doping in ZnO. A simple theoretical model based on the Penn model has been successfully developed to describe the above-mentioned variation. We have also observed the frequency dependence of the s, p–d exchange interaction and a theoretical calculation has been successfully developed to explain it.

Effect of Co doping on the static dielectric constant of ZnO nanoparticles

Co doped ZnO nanoparticles were synthesized by a simple rheological phase reaction-precursors method using zinc acetate, cobalt acetate, and oxalic acid for different atomic percentages of Co doping. X-ray diffraction studies confirmed the correct phase formation, and the composition were obtained from the x-ray photoelectron spectroscopic studies. Particle size was obtained from the small angle x-ray scattering studies. It was observed that the static dielectric constant, calculated from the shift of band gap energy, shows a gradual decrease with Co doping. A simple theoretical model was developed to explain the observed change of dielectric constant on the doping concentration. The model could successfully describe the dependence of the static dielectric constant on the doping concentration.

Photocatalytic activity of biogenic silver nanoparticles synthesized using yeast (Saccharomyces cerevisiae) extract

Synthesis of metallic and semiconductor nanoparticles through physical and chemical route is quiet common but biological synthesis procedures are gaining momentum due to their simplicity, cost-effectivity and eco-friendliness. Here, we report green synthesis of silver nanoparticles from aqueous solution of silver salts using yeast (Saccharomyces cerevisiae) extract. The nanoparticles formation was gradually investigated by UV–Vis spectrometer. X-ray diffraction analysis was done to identify different phases of biosynthesized Ag nanoparticles. Transmission electron microscopy was performed to study the particle size and morphology of silver nanoparticles. Fourier transform infrared spectroscopy of the nanoparticles was performed to study the role of biomolecules capped on the surface of Ag nanoparticles during interaction. Photocatalytic activity of these biosynthesized nanoparticles was studied using an organic dye, methylene blue under solar irradiation and these nanoparticles showed efficacy in degrading the dye within a few hours of exposure.

Photocatalytic activity of biogenic silver nanoparticles synthesized using potato (Solanum tuberosum) infusion.

In this study, we have reported a fast and eco-benign procedure to synthesis silver nanoparticle at room temperature using potato (Solanum tuberosum) infusion along with the study of its photocatalytic activity on methyl orange dye. After addition of potato infusion to silver nitrate solution, the color of the mixture changed indicating formation of silver nanoparticles. Time dependent UV-Vis spectra were obtained to study the rate of nanoparticle formation with time. Purity and crystallinity of the biogenic silver nanoparticles were examined by X-ray diffraction (XRD). Average size and morphology of the nanoparticles were characterized by dynamic light scattering (DLS) and transmission electron microscopy (TEM). Fourier transform infra-red spectroscopy (FTIR) was employed to detect functional bio-molecules responsible that contribute to the reduction and capping of biosynthesized Ag nanoparticles. Further, these synthesized nanoparticles were used to investigate their ability to degrade methyl orange dye under sunlight irradiation and the results showed effective photocatalytic property of these biogenic silver nanoparticles.

Plant-mediated synthesis of silver nanoparticles using parsley (Petroselinum crispum) leaf extract: spectral analysis of the particles and antibacterial study

Abstract Synthesis of nanomaterials may involve various routes including physical, chemical and biological approaches. Here, the biological green route was chosen to prepare silver nanoparticles from silver salts to avoid the requirement of costly instruments and involvement of hazardous chemicals as well. To make the process clean and green, leaf extract of parsley (Petroselinum crispum) was used to synthesize Ag nanoparticles at room temperature. The formation of Ag-nanoparticles was monitored by UV–Vis spectroscopy. The presence of silver in the sample and its crystalline nature were verified by X-ray diffraction (XRD) analysis. The size distribution profile and particle size in the suspension were manipulated from dynamic light scattering (DLS) pattern. The shape, size and morphology of the biogenic nanoparticles were studied using high resolution transmission electron microscope (TEM). Fourier transform infra-red spectroscopy was used to detect the biomolecules responsible for reduction of silver ions. These biogenic Ag-nanoparticles showed appreciable antibacterial efficacy against three bacteria—Klebsiella pneumoniae,Escherichia coli and Staphylococcus aureus.

Temperature dependence of magnetization and anisotropy in uniaxial NiFe2O4 nanomagnets: Deviation from the Callen-Callen power law

The thermal variation of magnetic anisotropy (K) and saturation magnetization (MS) for uniaxial nickel ferrite (NiFe2O4) nanomagnets are investigated. Major magnetic hysteresis loops are measured for the sample at temperatures over the range 5–280 K using a vibrating sample magnetometer. The high-field regimes of the hysteresis loops are modeled using the law of approach to saturation, based on the assumption that at sufficiently high field only direct rotation of spin-moment take place, with an additional forced magnetization term that is linear with applied field. The uniaxial anisotropy constant K is calculated from the fitting of the data to the theoretical equation. As temperature increases from 5 K to 280 K, a 49% reduction of K, accompanied by an 85% diminution of MS is observed. Remarkably, K is linearly proportional to MS2.6 in the whole temperature range violating the existing theoretical model by Callen and Callen. The unusual power-law behavior for the NiFe2O4 uniaxial nanomagnets is ascribed ...

Equibiaxial strain: tunable electronic structure and optical properties of bulk and monolayer MoSe2

Using ab initio calculations based on density functional theory, we have studied the equibiaxial strain effect on the electronic and optical properties of bulk and monolayer MoSe2. Very low value of the elastic constants of monolayer MoSe2 compared to bulk MoSe2 suggests that the tailoring of the electronic structure and optical properties of monolayer MoSe2 may easily be achieved by strain. We find that the band gap (Eg) of MoSe2 undergoes a transition from indirect to direct upon reducing the thickness of the monolayer which is associated with an enhancement of Eg and is further tunable by strain engineering. We noticed that the splitting of the bands that generally determines the luminescence property of MoSe2 can be modified by strain. Strain effect on bond ionicity, dielectric property (real and imaginary component) of bulk and monolayer MoSe2 has also been briefly discussed. We noticed that the plasma oscillation, found in bulk MoSe2, is absent in MoSe2 monolayer.