Karuppanan Senthil

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.

Growth and characterization of stoichiometric tungsten oxide nanorods by thermal evaporation and subsequent annealing

Stoichiometric tungsten oxide (WO(3)) nanorods are synthesized on tungsten (W) substrates by a high-temperature, catalyst-free, physical deposition process and by subsequent annealing in oxygen atmosphere. Tungsten oxide nanorods are grown by thermal evaporation of WO(3) powder at elevated temperature in a tube furnace. XRD, TEM and XPS analysis shows that the as-grown nanorods are single crystalline and non-stoichiometric (WO(x)). Annealing of WO(x) nanorods at 700 °C under oxygen atmosphere has led to the formation of stoichiometric WO(3) as evidenced by XRD, XPS and Raman analysis.

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.

Artificial semiconductor/insulator superlattice channel structure for high-performance oxide thin-film transistors

High-performance thin-film transistors (TFTs) are the fundamental building blocks in realizing the potential applications of the next-generation displays. Atomically controlled superlattice structures are expected to induce advanced electric and optical performance due to two-dimensional electron gas system, resulting in high-electron mobility transistors. Here, we have utilized a semiconductor/insulator superlattice channel structure comprising of ZnO/Al2O3 layers to realize high-performance TFTs. The TFT with ZnO (5 nm)/Al2O3 (3.6 nm) superlattice channel structure exhibited high field effect mobility of 27.8 cm2/Vs, and threshold voltage shift of only < 0.5 V under positive/negative gate bias stress test during 2 hours. These properties showed extremely improved TFT performance, compared to ZnO TFTs. The enhanced field effect mobility and stability obtained for the superlattice TFT devices were explained on the basis of layer-by-layer growth mode, improved crystalline nature of the channel layers, and passivation effect of Al2O3 layers.

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.

Synthesis and Characterization of ZnO Nanowire–CdO Composite Nanostructures

ZnO nanowire–CdO composite nanostructures were fabricated by a simple two-step process involving ammonia solution method and thermal evaporation. First, ZnO nanowires (NWs) were grown on Si substrate by aqueous ammonia solution method and then CdO was deposited on these ZnO NWs by thermal evaporation of cadmium chloride powder. The surface morphology and structure of the synthesized composite structures were analyzed by scanning electron microscopy, X-ray diffraction and transmission electron microscopy. The optical absorbance spectrum showed that ZnO NW–CdO composites can absorb light up to 550 nm. The photoluminescence spectrum of the composite structure does not show any CdO-related emission peak and also there was no band gap modification of ZnO due to CdO. The photocurrent measurements showed that ZnO NW–CdO composite structures have better photocurrent when compared with the bare ZnO NWs.

Impact dynamics of water droplets on chemically modified WOx nanowire arrays

The effects of surface energy on the wetting transition for impinging water droplets were investigated on the chemically modified WOx nanowire surfaces. We could modify the surface energy of the nanowires through chemisorption of alkyltrichlorosilanes with various carbon chain lengths and also by the ultraviolet-enhanced decomposition of self assembled monolayer molecules. Three surface wetting states could be identified through the balance between antiwetting and wetting pressures. This approach establishes a simple strategy for design of the water-repellent surface to impinging droplets.

Wettability control and water droplet dynamics on SiC-SiO2 core-shell nanowires.

We present a simple method for fabricating superhydrophobic SiC-SiO(2) core-shell nanowire surfaces via the facile dip-coating of alkyltrichlorosilanes. Water droplets displayed a variety of shapes with varying surface energies on the nanowire surfaces, which could be modified through chemisorption of alkyltrichlorosilanes with variable carbon chain length. The effects of UV irradiation on the superhydrophobic nanowire arrays were also investigated. UV light efficiently decomposed the chemisorbed molecules, and the superhydrophobic surface gradually converted into a hydrophilic surface with increasing UV exposure. The water droplet impact behavior on the modified surfaces was studied to test the stability of the superhydrophobicity under dynamic conditions.

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.

Novel heterostructure of CdS nanoparticle/WO3 nanowhisker: Synthesis and photocatalytic properties

A novel heterostructure of CdS nanoparticles/WO3 nanowhiskers was synthesized using a simple two-step process; thermal evaporation and chemical bath deposition. First, WO3 nanowhiskers (NWs) were grown on a tungsten substrate by thermal evaporation of WO3 powder in a tube furnace at 1050°C. Sequentially, CdS nanoparticles (NPs) were deposited on WO3 nanowhiskers by chemical bath deposition. CdS nanoparticles modified WO3 nanowhiskers showed enhanced visible light absorption compared to bare WO3 nanowhiskers. The photocatalytic activity was studied by the photodegradation of methylene blue. CdS NP/WO3 NW heterostructures showed remarkably enhanced photodecomposition efficiencies compared to bare WO3 nanowhiskers.