D. Nataraj

Controlled Growth of WO 3 Nanostructures with Three Different Morphologies and Their Structural, Optical, and Photodecomposition Studies

Tungsten trioxide (WO3) nanostructures were synthesized by hydrothermal method using sodium tungstate (Na2WO4·2H2O) alone as starting material, and sodium tungstate in presence of ferrous ammonium sulfate [(NH4)2Fe(SO4)2·6H2O] or cobalt chloride (CoCl2·6H2O) as structure-directing agents. Orthorhombic WO3having a rectangular slab-like morphology was obtained when Na2WO4·2H2O was used alone. When ferrous ammonium sulfate and cobalt chloride were added to sodium tungstate, hexagonal WO3nanowire clusters and hexagonal WO3nanorods were obtained, respectively. The crystal structure and orientation of the synthesized products were studied by X-ray diffraction (XRD), micro-Raman spectroscopy, and high-resolution transmission electron microscopy (HRTEM), and their chemical composition was analyzed by X-ray photoelectron spectroscopy (XPS). The optical properties of the synthesized products were verified by UV–Vis and photoluminescence studies. A photodegradation study on Procion Red MX 5B was also carried out, showing that the hexagonal WO3nanowire clusters had the highest photodegradation efficiency.

Highly mesoporous α-Fe2O3 nanostructures: preparation, characterization and improved photocatalytic performance towards Rhodamine B (RhB)

Single-crystalline porous hematite nanorods and spindle-like nanostructures were successfully synthesized by a low temperature reflux condensation method. Two different iron sources, namely, FeCl3?6H2O and Fe(NO3)3?9H2O, were hydrolyzed in the presence of urea to selectively prepare nanorods and spindle-like nanostructures. Initially, the akagenite phase was obtained by refluxing the precursor for 12?h and then the as-prepared akagenite nanostructures were transformed to porous hematite nanostructures upon calcination at 300??C for 1?h. The shape and the aspect ratio of the 12?h refluxed sample was retained even after calcination and this shows the topotactic transformation of the nanostructure. TEM and HRTEM investigations have shown the porous nature of the prepared sample. Brunauer?Emmett?Teller and Barret?Joyner?Halenda measurements have shown a large surface area and distribution of mesopores in the nanorods sample. The photocatalytic activity of the prepared nanostructures towards RhB has reflected this variation in the pore size distribution and specific surface area, by showing a higher activity for the nanorods sample. Magnetic studies by VSM have shown a weak ferromagnetic behaviour in both the samples due to shape anisotropy.

Controlled growth of single-crystalline, nanostructured dendrites and snowflakes of α-Fe2O3: influence of the surfactant on the morphology and investigation of morphology dependent magnetic properties

Single-crystalline, nanostructured dendrites, single and double-layered snowflakes of hematite (α-Fe2O3) were synthesized by a well controlled, surfactant-assisted hydrothermal reaction of K3[Fe(CN)6]. By varying the preparatory conditions such as precursor concentration and type of surfactant, we could establish precise control on the morphology of the sample. X-Ray diffraction, Raman analysis and X-ray photoelectron spectroscopic studies have confirmed that the as grown morphologies were hematite. Dendrites were obtained due to weak dissociation of the precursor and controlled diffusion of α-Fe2O3 nanoparticles under non-equilibrium conditions which attach and grow along certain preferred crystal facets. In the presence of a surfactant, single and double-layered snowflakes were formed. The type of surfactant and the nature of micelle formation were proposed to be the key factor for the observed snowflakes and the single or double-layered growth. Magnetic studies have shown morphology dependent magnetic properties with variation in the coercivity values for dendrites, single and double-layered snowflakes.

Improved sensing performance from methionine capped CdTe and CdTe/ZnS quantum dots for the detection of trace amounts of explosive chemicals in liquid media

Water soluble methionine functionalized CdTe quantum dots (QDs) and CdTe/ZnS core–shell QD samples have been prepared by a reflux condensation method and have been used to detect explosive chemicals, such as dinitrotoluene (DNT), nitrotoluene (NT) and nitrobenzene (NB) in liquid media. Meisenheimer complex formation between the QD surface attached methionine and aromatic explosive molecules has helped to detect them selectively via a fluorescent quenching process. Fluorescence quenching occurred because of the transfer of excited electrons from QD to the explosive molecules. Depending upon the number of nitro groups present on the explosive molecule, the quenching efficiency of different analytes varied. Due to surface passivation and inductive effects, the methionine capped CdTe/ZnS core–shell quantum dot sample resulted in the maximum quenching constant.

Systematic synthesis and analysis of change in morphology, electronic structure and photoluminescence properties of pyrazine intercalated MoO3 hybrid nanostructures

High aspect ratio molybdenum trioxide (MoO3) nanorods grown along the [100] direction were successively synthesized by a simple hydrothermal method. We used sodium molybdate and hydrochloric acid as starting materials and from their reaction we obtained MoO3 nanorods of high aspect ratio. The dimensions of the nanorods were found to be uniform in size, with well-defined boundaries. The intercalation of an organic molecule (pyrazine) into these nanorods has resulted in single-crystalline MoO3 microstructures, with a change in their length and breadth of a few orders. Pyrazine has acted as a stitching molecule and has bound the nanorods together along their length to form micron sized single crystalline MoO3. The presence of pyrazine and its intercalation was confirmed by a uniform shift in the XRD [0k0] peak positions. As the size of the pyrazine is similar to the van der Waals gap of the orthorhombic MoO3 crystal, it seemed to fit well within the gap and thereby helped to bind the nanorods along the [0k0] direction. The Raman ring deformation modes, at 714 and 996 cm−1, have also supported the intercalation of the pyrazine in the van der Waals gap. The deintercalation process was done by calcinating the sample at 400 °C and the removal of pyrazine was confirmed by TGA and XRD measurements. The influence of pyrazine in the valence band electronic density of states (DOS) of MoO3 was also analyzed by XPS and XES methods. The replacement of oxygen at the van der Waals gap by nitrogen from the intercalating pyrazine caused a shift in the valence band towards the Fermi level. A photoluminescence study was also conducted, reflecting the intercalation effect on the emission characteristics of the MoO3 nanostructures.

Molecular conformation dependent emission behaviour (blue, red and white light emissions) of all-trans-β-carotene-ZnS quantum dot hybrid nanostructures

A novel hybrid material consisting of all-trans-β-carotene and ZnS quantum dots (QDs) was prepared by two different controlled experiments and their photophysical properties were investigated in detail. Depending upon the preparation method, the β-carotene molecule had formed either an upright or wrapped conformation around the ZnS QDs and consequently the interaction between these two hybridizing systems was either charge transfer induced or electrostatic in type, respectively. Optical absorption, Raman and FTIR investigations have confirmed the two different conformations of the molecule around the ZnS QDs. Because of the two different conformations and the consequent interactions, different emission colours, such as blue and red wavelengths were obtained from these hybrids. We were also able to obtain white light emission by using cadmium doped ZnS QDs for the hybrid preparation.

Systematic synthesis and analysis of change in morphology, electronic structure and photoluminescence properties of 2,2′-dipyridyl intercalated MoO3 hybrid nanostructures and investigation of their photocatalytic activity

An organic–inorganic hybrid structure was synthesized by using 2,2′-dipyridyl and MoO3 nanorods through a simple hydrothermal method. The as-prepared dipyridyl–MoO3 hybrid samples looked like rod shaped micro crystals. The starting material used for this work was MoO3 nanorods which had a width of 150 nm and a length of several microns, whereas the resulting hybrid structure had a width of one micron and a length of 10 to 30 microns. Here, dipyridyl has acted as a stretching molecule and bonded the MoO3 nanorods together along their length to form hybrid micro crystals. By calcinating the hybrid sample at 400 °C, intercalated dipyridyl was removed, while maintaining the microscale morphology. These deintercalated MoO3 samples looked like micro slabs having a width of 5 micrometers. The presence and intercalation of dipyridyl was confirmed by the change in the XRD [0 k 0] peak positions. As the cross sectional size of the dipyridyl is close to the van der Waals gap of the orthorhombic MoO3 crystal, this space was effectively used for this intercalation process. The deintercalation process, i.e. the removal of dipyridyl was confirmed by TGA, and XRD measurements. The influence of dipyridyl in the valence band electronic density of states (DOS) of MoO3 was also analyzed by XPS and XES methods. A photoluminescence study was also conducted, reflecting the intercalation effect on the emission characteristics of the MoO3 nanostructures. A photodegradation study on Procion Red MX 5B was also carried out, showing that the dipyridyl deintercalated MoO3 micro slab like samples had the highest photodegradation efficiency.

Crystal structure and electronic properties of facile synthesized Cr2O3 nanoparticles

We report on a facile method of synthesis of Cr2O3 nanoparticles by hydrothermal method. Chromium sulfate was used as a starting material whereas urea was used as a strong reducing agent. Cr2O3 nanoparticles, with rhombohedral crystal structure, have been synthesized, when the reaction solution was treated under hydrothermal condition at high pH (10). At pH = 8 amorphous Cr2O3 powders were obtained. Chromium oxide could not be synthesized in the absence of urea. Two different Raman modes have been detected for the final products synthesized at the high pH value. As-prepared Cr2O3 nanoparticles reveal agglomeration as evidenced from scanning electron microscope (SEM) images. Flake-like Cr2O3 nanoparticles, 20 to 50 nm in size, show clear lattice fringes through the high resolution transmission electron microscope (HRTEM) images. The electronic structure of the Cr2O3 nanoparticles has been studied employing x-ray photoelectron spectroscopy (XPS) and x-ray emission spectroscopy (XES) methods.

Systematic investigation of the structure and photophysical properties of CdSe, CdSe/ZnS QDs and their hybrid with β-carotene

A systematic investigation was conducted to determine the excitation energy dependent emission characteristics of CdSe quantum dot–β-carotene and CdSe/ZnS core–shell quantum dot–β-carotene hybrid samples. The emission intensity was recorded at different excitation energies by continuously varying the excitation energy from the band edge value of the QDs to a maximum value. In both the hybrids, the emission intensity increased with an increase of the excitation energy and then a sudden quenching occurred, which is in contrast with the results from bare samples, where the emission intensity showed a decreasing trend at all excitation energies. The quantum yields corresponding to the maximum emission are 47.12% and 69%, for CdSe QD–βC and CdSe/ZnS core–shell QD–βC hybrids, respectively. At higher excitation energy, electrons were transferred to the molecules LUMO level, leaving behind holes in the valance band of the QD, and thus the produced charge separated state became responsible for the PL quenching in the hybrid sample. We have confirmed this charge separated state by lifetime measurements. It is because of this PL quenching behaviour of the hybrids that the nature of the interface band structure was deduced as type I. Furthermore, because of the involvement of higher energy photons in the quenching process, the transferred electrons in the LUMO level of the molecule were known as hot electrons. The present paper discusses the excited state electron dynamics across the interface between the two hybrid materials in detail.

Graphene Quantum Dot Solid Sheets: Strong blue-light-emitting & photocurrent-producing band-gap-opened nanostructures

Graphene has been studied intensively in opto-electronics, and its transport properties are well established. However, efforts to induce intrinsic optical properties are still in progress. Herein, we report the production of micron-sized sheets by interconnecting graphene quantum dots (GQDs), which are termed ‘GQD solid sheets’, with intrinsic absorption and emission properties. Since a GQD solid sheet is an interconnected QD system, it possesses the optical properties of GQDs. Metal atoms that interconnect the GQDs in the bottom-up hydrothermal growth process, induce the semiconducting behaviour in the GQD solid sheets. X-ray absorption measurements and quantum chemical calculations provide clear evidence for the metal-mediated growth process. The as-grown graphene quantum dot solids undergo a Forster Resonance Energy Transfer (FRET) interaction with GQDs to exhibit an unconventional 36% photoluminescence (PL) quantum yield in the blue region at 440 nm. A high-magnitude photocurrent was also induced in graphene quantum dot solid sheets by the energy transfer process.