Nanoscaffold effects on the performance of air-cathodes for microbial fuel cells: Sustainable Fe/N-carbon electrocatalysts for the oxygen reduction reaction under neutral pH conditions.

Nanostructured electrocatalysts for microbial fuel cell air-cathodes were obtained via use of conductive carbon blacks for the synthesis of high performing 3D conductive networks. We used two commercially available nanocarbons, Black Pearls 2000 and multiwalled carbon nanotubes, as conductive scaffolds for the synthesis of nanocomposite electrodes by combining: a hydrothermally carbonized resin, a sacrificial polymeric template, a nitrogenated organic precursor and iron centers. The resulting materials are micro-mesoporous, possess high specific surface area and display N-sites (N/C of 3-5 at%) and Fe-centers (Fe/C < 1.5at.%) at the carbon surface as evidenced from characterization methods. Voltammetry studies of oxygen reduction reaction activity were carried out at neutral pH, which is relevant to microbial fuel cell applications, and activity trends are discussed in light of catalyst morphology and composition. Tests of the electrocatalyst using microbial fuel cell devices indicate that optimization of the nanocarbon scaffold for the Pt-free carbon-based electrocatalysts results in maximum power densities that are 25% better than those of Pt/C cathodes, at a fraction of the materials costs. Therefore, the proposed Fe/N-carbon catalysts are promising and sustainable high-performance cathodic materials for microbial fuel cells.

Synthesis, Crystal Structure, and Transport Properties of the Hexagonal Mo9 Cluster Compound Ag3RbMo9Se11.

Mo-based cluster compounds are promising candidates for thermoelectric applications at high temperatures due to their very low lattice thermal conductivity values. Here, we report on a detailed investigation of the crystal structure and transport properties measured in a wide range of temperatures (2-800 K) of polycrystalline Ag3RbMo9Se11. Single-crystal X-ray diffraction shows that this compound crystallizes in the hexagonal space group P63/m. The crystal structure is formed by stacked Mo9Se11 units leaving channels that are randomly filled by Rb+ cations, while Ag+ cations are located between the Mo9Se11 units. The high disorder in the unit cell induced by these atoms and their large anisotropic thermal displacement parameters are two key characteristics that lead to very low lattice thermal conductivity as low as 0.6 W m-1 K-1 at 800 K. The combination of semiconducting-like electrical properties and low ability to transport heat leads to a maximum dimensionless thermoelectric figure of merit ZT of 0.4 at 800 K.

Response of Gallium Nitride Chemiresistors to Carbon Monoxide is Due to Oxygen Contamination.

We report on the influence of oxygen impurities on the gas sensing properties of gallium nitride (GaN) chemiresistors. As shown by XRD, elemental analysis, and TEM characterization, surface oxidation of GaN-for example, upon contact to ambient air atmosphere-creates an oxidative amorphous layer which provides the sites for the sensing toward CO. Treating this powder under dry ammonia at 800 °C converts the oxide layer in nitride, and consequently the sensing performance toward CO is dramatically reduced for ammonia treated GaN gas sensors. Hence the response of GaN sensors to CO is caused by oxygen in the form of amorphous surface oxide or oxynitride.

Thermodynamics of binary gas adsorption in nanopores.

MCM-41 nanoporous silicas show a very high selectivity for monoalcohols over aprotic molecules during adsorption of a binary mixture in the gas phase. We present here an original use of gravimetric vapour sorption isotherms to characterize the role played by the alcohol hydrogen-bonding network in the adsorption process. Beyond simple selectivity, vapour sorption isotherms measured for various compositions help to completely unravel at the molecular level the step by step adsorption mechanism of the binary system in the nanoporous solid, from the first monolayers to the complete liquid condensation.

Sample preparation for an optimized extraction of localized metabolites in lichens: Application to Pseudevernia furfuracea.

Lichens are symbiotic organisms known for producing unique secondary metabolites with attractive cosmetic and pharmacological properties. In this paper, we investigated three standard methods of preparation of Pseudevernia furfuracea (blender grinding, ball milling, pestle and mortar). The materials obtained were characterized by electronic microscopy, nitrogen adsorption and compared from the point of view of extraction. Their microscopic structure is related to extraction efficiency. In addition, it is shown using thalline reactions and mass spectrometry mapping (TOF-SIMS) that these metabolites are not evenly distributed throughout the organism. Particularly, atranorin (a secondary metabolite of interest) is mainly present in the cortex of P. furfuracea. Finally, using microwave assisted extraction (MAE) we obtained evidence that an appropriate preparation can increase the extraction efficiency of atranorin by a factor of five.

Influence of Upconversion Processes in the Optically-Induced Inhomogeneous Thermal Behavior of Erbium-Doped Lanthanum Oxysulfide Powders.

The efficient infrared-to-visible upconversion emission present in Er-doped lanthanum oxysulfide crystal powders is used as a fine thermal sensor to determine the influence of upconversion processes on the laser-induced thermal load produced by the pump laser and to assess the potentialities of this material in order to obtain anti-Stokes laser-induced cooling. The analysis of the upconversion emission and excitation spectra as well as the decay curves indicates that energy transfer upconversion is the main mechanism responsible for the green (4S3/2) and red (4F9/2) upconversion luminescence. The dependence on temperature of the intensity ratio of upconversion emission from thermally-coupled ²H11/2 and 4S3/2 levels of Er3+ in the 240-300 K temperature range has been used to estimate a relative sensitivity of 1.09 × 10-2 K-1. Thermal measurements performed on the powder samples by using a thermal infrared camera exhibit a very inhomogeneous heat distribution at the sample surface due to the random distribution of the pumping energy inside the sample as well as to the random properties of the thermal field. The analysis of both spectroscopic and thermal measurements show that after a transient heating induced by the background absorption, cooling of discrete regions by means of anti-Stokes processes can be observed.

Nano-scaled TiO(OD)2: a time resolved 1H/2D isotope exchange study observed in situ with neutron scattering at 20 °C and 40 °C.

In situ neutron diffraction measurements of the nanocrystalline deuterated oxyhydroxide TiO(OD)(2) compound were performed as a function of time and temperature under NH(3) gas flow in order to study the hydrogen-deuterium exchange mechanism. Data were collected on the instrument D20 at the ILL (France) and the analysis of the kinetics was directly based on the contrast variation of the incoherent neutron cross section of hydrogen and deuterium. The time evolution of the hydrogenated phase fraction was described using the well-known Kolmogorov-Johnson-Mehl-Avrami (KJMA) expression. The H/D exchange reaction is complete within 140 s at 20 °C and within 120 s at 40 °C. The activation energy for the H/D exchange reaction is estimated to be 37 kJ mol(-1).

Laser action in Nd(3+)-doped lanthanum oxysulfide powders.

We have investigated the stimulated emission properties of Nd(3+) doped La(2)O(2)S powders at room temperature as a function of pumping energy density, excitation wavelength, and Nd(3+) ion concentration. The absolute stimulated emission energy has been measured. Expressions for the slope efficiencies and lasing thresholds as a function of rare earth concentration and pumping wavelengths, which qualitatively agree with experimental observations, are discussed.

Study of the thermal nitridation of nanocrystalline Ti(OH)4 by X-ray and in situ neutron powder diffraction.

In situ neutron diffraction measurements of nanocrystalline titanium oxynitrides were performed, as a function of temperature and time, to explore the nitrogen/oxygen substitution mechanism occurring during their synthesis by reaction of gaseous ammonia with nanocrystalline Ti(OH)(4). These neutron diffraction experiments are supported by chemical analysis and X-ray diffraction, allowing the description of the structural variations and ordering process between the Ti(O/N)(2) anatase and the Ti(O/N) rock-salt phases. Our results show that the formation of the Ti(O/N) rock-salt phase goes along with the creation of vacancies on the Ti sites and that the N/O substitution proceeds but without N/O ordering.