M. Navaneethan

CuCrSe2: a high performance phonon glass and electron crystal thermoelectric material

The efficient conversion of heat into electricity using a thermoelectric approach requires high performance materials with the thermoelectric figure of merit ZT ≥ 1. Here we report on bulk CuCrSe2, which exhibits a very high ZT ∼ 1 at 773 K. The titled compound exhibits an electrical resistivity of ∼2.8 mΩ cm, a Seebeck coefficient of ∼160 μV K−1, together with very low thermal conductivity ∼7 mW cm−1 K−1 at 773 K. The very low thermal conductivity of bulk CuCrSe2 is attributed to phonon scattering by various sources such as (i) superionic Cu ions between the CrSe2 layers, (ii) nanoscale precipitates in the bulk and (iii) natural grain boundaries due to the layered structure of the material. This unusual combination of thermoelectric properties for CuCrSe2 suggests that it is an ideal example of the phonon glass and electron crystal approach.

Fast Response and High Sensitivity of ZnO Nanowires-Cobalt Phthalocyanine Heterojunction Based H2S Sensor.

The room temperature chemiresistive response of n-type ZnO nanowire (ZnO NWs) films modified with different thicknesses of p-type cobalt phthalocyanine (CoPc) has been studied. With increasing thickness of CoPc (>15 nm), heterojunction films exhibit a transition from n- to p-type conduction due to uniform coating of CoPc on ZnO. The heterojunction films prepared with a 25 nm thick CoPc layer exhibit the highest response (268% at 10 ppm of H2S) and the fastest response (26 s) among all samples. The X-ray photoelectron spectroscopy and work function measurements reveal that electron transfer takes place from ZnO to CoPc, resulting in formation of a p-n junction with a barrier height of 0.4 eV and a depletion layer width of ∼8.9 nm. The detailed XPS analysis suggests that these heterojunction films with 25 nm thick CoPc exhibit the least content of chemisorbed oxygen, enabling the direct interaction of H2S with the CoPc molecule, and therefore exhibit the fastest response. The improved response is attributed to the high susceptibility of the p-n junctions to the H2S gas, which manipulates the depletion layer width and controls the charge transport.

Effect of Erbium on the Photocatalytic Activity of TiO2/Ag Nanocomposites under Visible Light Irradiation

Erbium co-doped TiO2 /Ag catalysts are synthesized by using a simple, one-step solvothermal method and characterized by X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, Raman analysis, X-ray photoelectron spectroscopy, and diffuse reflectance spectroscopy. The catalysts exhibit anatase crystal structures with increased visible light absorption compared with pure TiO2 . Enhanced photocatalytic activity is observed with Er co-doped TiO2 /Ag nanocomposites for Rhodamine B degradation under visible light irradiation. The photocatalytic activity of 1u2009% Er co-doped TiO2 /Ag is much higher than that of TiO2 /Ag, TiO2 /Er, pure TiO2 , and commercial Degussa P25. The kinetics of the degradation process are studied and the pseudo-first-order rate constant (k) and half-life time (t1/2 ) of the reaction are calculated. The enhanced activity might be accredited to the efficient separation of electron-hole pairs by silver and higher visible light absorption of TiO2 induced by Er.

Enhanced Thermoelectric Properties of Selenium-Deficient Layered TiSe2–x: A Charge-Density-Wave Material

In the present work, we report on the investigation of low-temperature (300-5 K) thermoelectric properties of hot-pressed TiSe2, a charge-density-wave (CDW) material. We demonstrate that, with increasing hot-pressing temperature, the density of TiSe2 increases and becomes nonstoichiometric owing to the loss of selenium. X-ray diffraction, scanning electron microscopy, and transimission electron microscopy results show that the material consists of a layered microstructure with several defects. Increasing the hot-press temperature in nonstoichiometric TiSe2 leads to a reduction of the resistivity and enhancement of the Seebeck coefficient in concomitent with suppression of CDW. Samples hot-pressed at 850 °C exhibited a minimum thermal conductivity (κ) of 1.5 W/m·K at 300 K that, in turn, resulted in a figure-of-merit (ZT) value of 0.14. This value is higher by 6 orders of magnitude compared to 1.49 × 10(-7) obtained for cold-pressed samples annealed at 850 °C. The enhancement of ZT in hot-pressed samples is attributed to (i) a reduced thermal conductivity owing to enhanced phonon scattering and (ii) improved power factor (α(2)σ).

Fabrication of Cu2MoS4 hollow nanotubes with rGO sheets for enhanced visible light photocatalytic performance

Herein, ternary Cu2MoS4 hollow nanotubes were incorporated into reduced graphene oxide sheets (rGOs) via a facile hydrothermal route. A blue shift in the absorption edge of the Cu2MoS4/rGO photocatalyst was observed in comparison with that of Cu2MoS4 alone (without rGO), which confirmed the formation of interfacial contact between rGO sheets and Cu2MoS4 hollow nanotubes. Both SEM and TEM images of Cu2MoS4/rGO clearly revealed the formation of hollow nanotubes and uniform distribution of Cu2MoS4 on the surface of rGO. The chemical oxidation state of the elements present in the catalyst was ascertained by means of XPS spectra. The significant quenching of the photoluminescence (PL) intensity of the rGO supported photocatalyst strongly suggested a unidirectional flow of photogenerated charge carriers on the one dimensional Cu2MoS4 hollow nanotube combined with the rGO sheet, and thereby greatly suppressed the electron–hole pair recombination. Besides, the shorter decay time ( = 2.5) observed in Cu2MoS4/rGO using time-resolved PL studies confirmed the effective separation of the charge carriers in the presence of rGO sheets. Cu2MoS4/rGO was demonstrated as a visible light driven photocatalyst for the degradation of methyl orange (MO) dye. A maximum photocatalytic degradation efficiency of ∼99% was achieved towards MO dye using the rGO supported Cu2MoS4 photocatalyst, while only 57% degradation was noted for the Cu2MoS4 photocatalyst, i.e. without an rGO support under identical experimental conditions. The enhancement in the photocatalytic performance of Cu2MoS4/rGO was mainly attributed to the high dye adsorption and excellent electronic properties of rGO sheets. In addition, the active sites associated on the surface as well as the edges of rGO sheets might be useful in the unfolding and uniform dispersion of the Cu2MoS4 hollow nanotubes over the rGO support without much aggregation. The effect of different parameters such as the amount of photocatalyst, the dye concentration and initial pH on the photocatalytic degradation of MO was also studied. Furthermore, the stability of the catalyst was tested by reusing the Cu2MoS4/rGO photocatalyst for five consecutive runs without any notable loss in photocatalytic activity.

Controlled exfoliation of monodispersed MoS2 layered nanostructures by a ligand-assisted hydrothermal approach for the realization of ultrafast degradation of an organic pollutant

Molybdenum disulfide (MoS2) layered nanosheets were synthesized by the hydrothermal method. Citric acid was used as an organic ligand to obtain the monodispersed layered MoS2 nanostructures. The effect of citric acid on the formation and functional properties of the layered MoS2 nanostructures was investigated. The X-ray diffraction patterns revealed the formation of a hexagonal crystal structure of MoS2. Significant peak shift was observed for the interaction of citric acid and Mo in the core level spectra of X-ray photoelectron spectroscopy. Citric acid-free hydrothermal growth resulted in the formation of micron-sized MoS2 nanospheres, whereas citric acid-assisted hydrothermal growth resulted in the formation of well dispersed layers of MoS2, confirmed by morphological analysis. However, the highest concentration of citric acid resulted in the formation of aggregated layers. The obtained MoS2 nanostructures were used as photocatalysts to decompose methylene blue (MB) as a model pollutant. The obtained results showed that the MoS2 layered nanosheets could effectively decompose the organic pollutant. The MB absorption peaks completely disappeared after 24 min of irradiation when using the nanosheets synthesized with a citric acid concentration of 0.04 M. The effect of pH on the MoS2 nanosheets was studied, and 96% MB degradation was observed at pH 12 after 2 min of visible-light irradiation.

Determination of gas sensing properties of thermally evaporated WO 3 nanostructures

Thin films of tungsten oxide (WO3) nanostructures were grown on indium tin oxide coated glass substrates by thermal evaporation technique under oxygen and argon (O2/Ar) mixed gas atmosphere. The films were characterized by X-ray diffraction, Raman spectroscopy and field emission scanning electron microscopy to study the structural and morphological properties of the grown films. Three different shapes of nanostructures (one dimensional nanorods, two dimensional nanosheets and three dimensional nanosized orthorhombic structures) were formed due to the variations of growth conditions such as substrate temperature and oxygen/argon flow rate. In particular, the flow rate of oxygen plays an important role in controlling the nucleation and growth of WO3 nanostructures. The ethanol gas sensing properties of the films were investigated under different concentrations (10–50xa0ppm) at room temperature, which reveals that the sensitivity of the sensor was greatly enhanced with the increasing gas concentration. This result indicates that the WO3 films are good candidate for sensing ethanol gas in low concentration at room temperature.

Fabrication of bistable switching device using CdS nanorods embedded in PMMA (polymethylmethacrylate) nanocomposite

AbstractnAn organic bistable memory device was fabricated using CdS NR’s/PMMA (polymethylmethacrylate) nanocomposite sandwiched between ITO and Al thin film. XRD analysis confirmed that CdS NR’s reveals wurtzite hexagonal phase. FESEM and TEM images showed the monodispersed rod-like morphology. The nanorods were uniformly dispersed in the PMMA (polymethylmethacrylate) layer. EDX spectrum confirms that the product consists of only Cd and S elements. No other impurities were found. UV–visible absorption of CdS NR’s showed a slight blue shift. The photoluminescence spectra of CdS NR’s spectra showed surface defects which may be due to sulfur vacancies in surface of nanorods. The CdS NR’s polymer nanocomposite showed better thermal stability. The current–voltage (C–V) measurement of ITO/CdS NR’s/Al showed a clockwise hysteresis with flatband shift voltage in C–V curve for the device.

Solvothermal growth of diethylamine capped TiO2 nanoparticles and functional properties

Size controlled anatase titanium dioxide (TiO2) nanoparticles with high surface area were prepared using diethylamine (DEA) as a passivating agent by simple hydrothermal growth. X-ray diffraction pattern revealed the formation of anatase crystal phase of TiO2. There is no phase transformation for capped and uncapped TiO2 nanoparticles. Ligand passivation on TiO2 nanoparticles were confirmed by Fourier transform infra red studies. Significant UV–visible absorption is shown by absorption analysis. Uncapped and DEA capped TiO2 nanoparticles were synthesized and compared. Monodispersed cubic like morphology of DEA capped TiO2 nanoparticles with an average size of 13xa0nm is observed by transmission electron microscope (TEM) analysis. TEM analysis confirms that agglomerations were restricted by adding DEA as a capping agent. DEA capped TiO2 nanoparticles showed surface area of 65xa0m2/g and it is higher than uncapped TiO2 nanoparticles as confirmed by Brunauer–Emmett–Teller method. Prepared materials can be used as photoanodes in dye-sensitized solar cell application.

Synthesis of cluster like TiO2 mesoporous spheres and nanorods and their applications in dye-sensitized solar cells

Cluster like mesoporous TiO2 spheres, nanorods and nanoparticles were synthesized by simple wet chemical method. The TiO2 mesoporous spheres, nanorods and nanoparticles were characterized by powder X-ray diffraction, Raman spectroscopy, ultraviolet visible spectroscopy, Fourier transform infrared spectroscopy, field emission scanning electron microscopy and transmission electron microscopy. Accordingly, a possible growth mechanism of mesoporous spheres, nanorods and nanoparticles were discussed. The changes of the dye-sensitized solar cell (DSSC) performance with the variation of the nanostructures of TiO2 which were used in photoanodes have been investigated. The TiO2 mesoporous sphere based DSSC with the film thickness of 20xa0μm was assembled and a conversion efficiency of 6.69% was obtained.