Critical mode and band-gap-controlled bipolar thermoelectric properties of SnSe

The reliable calculation of electronic structures and understanding of electrical properties depends on an accurate model of the crystal structure. Here, we have reinvestigated the crystal structure of the high-zT thermoelectric material tin selenide, SnSe, between 4 and 1000 K using high-resolution neutron powder diffraction. Symmetry analysis reveals the presence of four active structural distortion modes, one of which is found to be active over a relatively wide range of more than +/-200 K around the symmetry-breaking Pnma-Cmcm transition at 800 K. Density functional theory calculations on the basis of the experimental structure parameters show that the unusual, step-like temperature dependencies of the electrical transport properties of SnSe are caused by the onset of intrinsic bipolar conductivity, amplified and shifted to lower temperatures by a rapid reduction of the band gap between 700 and 800 K. The calculated band gap is highly sensitive to small out-of-plane Sn displacements observed in the diffraction experiments. SnSe with a sufficiently controlled acceptor concentration is predicted to produce simultaneously a large positive and a large negative Seebeck effect along different crystal directions.Comment: 7 pages, 7 figures, accepted for publication in Phys. Rev. Material

Single step hydrothermal synthesis of mixed valent V₆O₁₃ nano-architectures: A case study of the possible applications in electrochemical energy conversion

International audienceAvailability of several stable vanadium oxidation states offers numerous advantages in terms of applications but at the same time it poses great challenge to isolate them in single phase during their synthesis process. In this study, we developed a facile single step hydrothermal synthesis of V6O13 nano-architectures using environmental friendly citric acid as a reducing agent. By means of time dependant hydrothermal conditions study, we observed evolution of several stable vanadium oxide phases. The as-synthesized powder samples phase confirmation was thoroughly studied by powder X-ray diffraction and transmission electron microscopy. The morphology of the products formed was investigated at each hydrothermal reaction time. Finally, these V6O13 nano-architectures were employed as counter electrode (CE) in dye sensitized solar cells (DSSCs). The photovoltaic performance and the electrical impedance studies of the DSSC device made use of V6O13 nano-architectures are reported here. It is believed that with further optimization, this relatively inexpensive material can act as efficient CE for DSSCs, which thereby open up a new avenue for vanadium oxides as a new class of materials for clean energy generation

Biosorption of hexavalent chromium from aqueous solution by using prawn pond algae (Sphaeroplea)

284-289Biosorption of chromium (VI) on prawn pond biomass Sphaeroplea algae in natural and acid treated forms has been studied by using batch technique. The equilibrium biosorption level is a function of solution pH, agitation time, initial metal ion concentration and adsorbent dose at room temperature. Both Freundlich and Langmuir adsorption models are suitable for describing the biosorption of chromium (VI) on the two forms of algae while the adsorption process follows the Legergren kinetic model. The biomass has been characterized by FTIR spectra. Surface area values of the biomass are 0.9 and 2.1 m²/g for natural and acid treated forms respectively. At the optimum conditions, the maximum adsorption capacity is found to be 29.8 mg/g and 158.7 mg/g in natural and acid treated forms respectively

Evidence for hard and soft substructures in thermoelectric SnSe

SnSe is a topical thermoelectric material with a low thermal conductivity which is linked to its unique crystal structure. We use low-temperature heat capacity measurements to demonstrate the presence of two characteristic vibrational energy scales in SnSe with Debye temperatures D1 = 345(9) K and D2 = 154(2) K. These hard and soft substructures are quantitatively linked to the strong and weak Sn-Se bonds in the crystal structure. The heat capacity model predicts the temperature evolution of the unit cell volume, confirming that this two-substructure model captures the basic thermal properties. Comparison with phonon calculations reveals that the soft substructure is associated with the low energy phonon modes that are responsible for the thermal transport. This suggests that searching for materials containing highly divergent bond distances should be a fruitful route for discovering low thermal conductivity materials

Efficient thermoelectric performance in silicon nano-films by vacancy-engineering

AbstractThe introduction of large concentrations of lattice vacancies in silicon nano-films creates more than a 20-fold reduction in thermal conductivity, while electrical conductivity and Seebeck coefficient are largely maintained. This results in thermoelectric performance comparable to silicon nanowires, but crucially leaves the silicon structure indistinguishable from bulk silicon, resulting in a robust material that is straight-forward to fabricate. This finding significantly advances the potential of silicon ultra-thin-films as a high-performance thermoelectric material

Biosorption Performance of Biodegradable Polymer Powders for the Removal of Gallium(III) ions from Aqueous Solution

Gallium (Ga) is considered an important element in the semiconducting industry and as the lifespan of electronic products decrease annually Ga-containing effluent has been increasing. The present study investigated the use of biodegradable polymer powders, crab shell and chitosan, in the removal of Ga(III) ions from aqueous solution. Ga(III) biosorption was modeled to Lagergren-first, pseudo-second order and the Weber-Morris models. Equilibrium data was modeled to the Langmuir, Freundlich and Langmuir-Freundlich adsorption isotherms to determine the probable biosorption behavior of Ga(III) with the biosorbents. The biosorbents were investigated by Fourier Transform Infrared Spectroscopy, X-ray Diffraction and Scanning Electron Microscopy/Energy Dispersive Spectra analysis