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International audience; In this work, Cu-based sulfides (chalcopyrite CuFeS2, mohite Cu2SnS3, tetrahedrite Cu12Sb4S13, mawsonite Cu6Fe2SnS8, and kesterite Cu2ZnSnS4) were synthesized by industrial milling in an eccentric vibratory mill to demonstrate the scalability of their synthesis. For a comparison, laboratory-scale milling in a planetary mill was performed. The properties of the obtained samples were characterized by X-ray diffraction and, in some cases, also by Mössbauer spectroscopy. For the densification of powders, the method of spark plasma sintering was applied to prepare suitable samples for thermoelectric (TE) characterization which created the core of this paper. A comparison of the figure-of-merit ZT, representative of the efficiency of thermoelectric performance, shows that the scaling process of mechanochemical synthesis leads to similar values as compared to using laboratory methods. This makes the cost-effective production of Cu-based sulfides as prospective energy materials for converting heat to electricity feasible. Several new concepts that have been developed involving combinations of natural and synthetic species (tetrahedrite) and nanocomposite formation (tetrahedrite/digenite, mawsonite/stannite) offer sustainable approaches in solid-state chemistry. Mechanochemical synthesis is selected as a simple, one-pot, and facile solid-state synthesis of thermoelectric materials with the capability to reduce, or even eliminate, solvents, toxic gases, and high temperatures with controllable enhanced yields. The synthesis is environmentally friendly and essentially waste-free. The obtained results illustrate the possibility of large-scale deployment of energy-related materials.

In this study, we demonstrate the feasibility of Bi-doped tetrahedrite Cu12Sb4−xBixS13 (x = 0.02–0.20) synthesis in an industrial eccentric vibratory mill using Cu, Sb, Bi and S elemental precursors. High-energy milling was followed by spark plasma sintering. In all the samples, the prevailing content of tetrahedrite Cu12Sb4S13 (71–87%) and famatinite Cu3SbS4 (13–21%), together with small amounts of skinnerite Cu3SbS3, have been detected. The occurrence of the individual Cu-Sb-S phases and oxidation states of bismuth identified as Bi0 and Bi3+ are correlated. The most prominent effect of the simultaneous milling and doping on the thermoelectric properties is a decrease in the total thermal conductivity (κ) with increasing Bi content, in relation with the increasing amount of famatinite and skinnerite contents. The lowest value of κ was achieved for x = 0.2 (1.1 W m−1 K−1 at 675 K). However, this sample also manifests the lowest electrical conductivity σ, combined with relatively unchanged values for the Seebeck coefficient (S) compared with the un-doped sample. Overall, the lowered electrical performances outweigh the benefits from the decrease in thermal conductivity and the resulting figure-of-merit values illustrate a degradation effect of Bi doping on the thermoelectric properties of tetrahedrite in these synthesis conditions.

International audience; Triply periodic minimal surfaces (TPMS) are known for their advanced mechanical properties and are wrinkle- free with a smooth local topology. These surfaces provide suitable conditions for cell attachment and prolifer-ation. In this study, the in vitro osteoinductive and antibacterial properties of scaffolds with different minimal pore diameters and architectures were investigated. For the first time, scaffolds with TPMS architecture were treated electrochemically by plasma electrolytic oxidation (PEO) with and without silver nanoparticles (AgNPs) to enhance the surface bioactivity. It was found that the scaffold architecture had a greater impact on the osteoblast cell activity than the pore size. Through control of the architecture type, the collagen production by osteoblast cells increased by 18.9% and by 43.0% in the case of additional surface PEO bioactivation. The manufactured scaffolds demonstrated an extremely low quasi-elastic modulus (comparable with trabecular and cortical bone), which was 5–10 times lower than that of bulk titanium (6.4–11.4 GPa vs 100–105 GPa). The AgNPs provided antibacterial properties against both gram-positive and gram-negative bacteria and had no significant impact on the osteoblast cell growth. Complex experimental results show the in vitro effectiveness of the PEO-modified TPMS architecture, which could positively impact the clinical applications of porous bioactive implants.

Photoelectrochemical cells (PEC) are promising devices for hydrogen production via sunlight energy. One of the important challenges in this area is to design photoactive electrodes able to absorb visible light. A good photoelectrochemical behavior depends on the presence of surface active sites to photogenerate current at the lower possible potential for water splitting. Recent investigations in this field are focusing on perovskite materials such as CaCu3Ti4O12 (CCTO) as visible light active electrode due its outstanding structure in which CCTO encloses in its structure a visible light absorbance component (CuO). The presence CuO on the material surface is mainly responsible for trapping electrons. Therefore, CCTO photoelectrocatalysts were prepared using different amounts of graphene oxide (GO) in this work. Physico-chemical characterizations showed that the oxidation of GO at high temperatures lead to the reduction of Ti4+ to Ti3+ and Cu2+ to Cu+, causing a segregation of copper on grain boundaries regions as revealed by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and Raman characterizations. Electrochemical performance and materials resistivity are measured in dark and under visible light irradiation. CCTO prepared using 3% of GO showed 50% higher photocurrent generation and hydrogen generation rate than pure phase of CCTO.

International audience; Core-shell nanostructures of one-dimensional (1D) polyacrylonitrile (PAN)/zinc oxide (ZnO) were obtained by combining atomic layer deposition (ALD) and electrospinning. Nanofibers with different ZnO thicknesses were synthesized and investigated. The present work offers novel information about 1D ZnO structural defects and activation energies (Ea) by performing photoluminescence (PL) measurements. PL measurements of 1D ZnO/PAN samples were investigated from 77 K to 273 K (room temperature). By analyzing the obtained data of the emission spectrum, Ea and temperature coefficients were calculated. The results let us suggest an approximate model of defects in 1D ZnO/PAN structures. It was observed that optical properties are strongly related to the structural properties of the obtained 1D materials i.e., the calculation of the concentration of defects in the ZnO layer was done by analyzing the optical measurements. These investigations allow predicting the properties of the materials and opens up a new roadmap for reliable sensing.

International audience; In this study, we demonstrate the use of elemental precursors (Cu,Sb,S) to synthesize tetrahedrite Cu12Sb4S13 using an industrial eccentric vibratory mill. Mechanochemical synthesis of tetrahedrite leads to the formation of covellite (CuS), skinnerite (Cu3SbS3) or famatinite (Cu3SbS4) in dependence on milling time. However, the composite product can be modified in favour of prevailing tetrahedrite when Spark Plasma Sintering (SPS) treatment is applied after milling. The as-synthesized and sintered products are composed of polydisperse nanosized particles with dimensions up to 250 nm. The thermoelectric measurements reveal a maximum value of figure-of-merit ZT = 0.67 @ 700 K, as a consequence of a relatively high power factor (1.07 mW m−1 K−2) and a low thermal conductivity (1.12 W m−1 K−1). The obtained zT values of products prepared in an industrial mill are comparable to the ones reported for compounds synthesized using laboratory mills. The synthesis of ternary and quaternary sulphides by a scalable and industrializable milling process represents a prospective route for mass production of thermoelectric materials.

The Bi2Se3/ZnO heterostructure is a new combination of high- and low-band-gap nanomaterials that can be implemented for optoelectronic devices. The influence of the Bi2Se3 substrate on crystallizat...

International audience; Photoelectrochemical water splitting under visible light has attracted attention for renewable hydrogen production. Despite prevalent investigations, many challenges still hindered an efficient energy conversion, such as enhancing the reaction efficiency in visible light. Thus controlling the photoelectrode materials is an essential step in designing new materials for water splitting. CaCu3Ti4O12 (CCTO) has received great attention as photocatalyst under solar light due to its combined band gap as result of the presence in its structure of TiO2 active in UV light and CuO active under visible light. In this work, a cubic CCTO with different amount of exfoliated hexagonal boron nitride has been synthesized. The produced materials were fully characterized. Physico-chemical characterizations demonstrate that bore and nitrogen co-doped CCTO causes an increase in grain boundaries thickness, and thus leads to shift peaks in XRD and Raman spectra. UV–Vis diffuse reflectance spectroscopy showed a decrease of the bang gap value after adding boron nitride nanosheets into CCTO. The electrochemical performance and the resistivity of the obtained materials were performed using electrochemical impedance spectroscopy in dark and under visible light exposition. The onset potential recorded for CCTO with 3% of boron nitride nanosheets (Eg = 2.9), is around 0.5 V and the current generation is 16 times more than pure phase of CCTO. This work shows that CCTO with 3% of boron nitride nanosheets can be considered as an active photoelectrocatalyst under visible light irradiation for water splitting. Hydrogen production measurements show that the rate of H2 production reached arround 8.1 µmol/h under visible light after adding 3% of BN nanosheets to CCTO which presents 16 times increase in comparison with pure CCTO.

Équipe 101 : Nanomagnétisme et électronique de spin; International audience; Interplay between voltage induced magnetic anisotropy transition and voltage induced atomic diffusion is studied in epitaxial V/Fe (0.7 nm)/ MgO/ Fe(5 nm)/Co/Au magnetic tunnel junction where thin Fe soft electrode has in-plane or out-of-plane anisotropy depending on the sign of the bias voltage. We investigate the origin of the slow resistance variation occurring when switching bias voltage in opposite polarity. We demonstrate that the time to reach resistance stability after voltage switching is reduced when increasing the voltage amplitude or the temperature. A single energy barrier of about 0.2 eV height is deduced from temperature dependence. Finally, we demonstrate that the resistance change is not correlated to a change in soft electrode anisotropy. This conclusion contrasts with observations recently reported on analogous systems.

Équipe 101 : Nanomagnétisme et électronique de spin; International audience; The impact of annealing on the structural and magnetic properties of sputtered [Gd/Co](n) multilayers has been researched. Structural and phase contrast of atomic force microscope measurements at the imaging of the facet sample cross-section demonstrate that thermal annealing process at 700 K leads to intermixing between Co and Gd layers, i.e. the alloy formation around the former interface. Magnetometry measurements are coherent with such CoGd alloying. Annealing reinforces the domination of domain wall propagation in the magnetization reversal mechanism. Low temperature magnetometry analysis seems to indicate that CoGd mixing due to annealing may occurs over 2 nm apart from initial Co/Gd interface.

Équipe 101 : Nanomagnétisme et électronique de spin; International audience; Magnetic and magneto-transport properties of amorphous Al2O3-based magnetic tunnel junctions (MTJ) having two Co/Ni multilayer electrodes exhibiting perpendicular magnetic anisotropy (PMA) are presented. An additional Co/Pt multilayer is required to maintain PMA in the top Co/Ni electrode. Slight stacking variations lead to dramatic magnetic changes due to dipolar interactions between the top and bottom electrodes. Tunnel magneto-resistance (TMR) of up to 8% at 300K is measured for the MTJ with two PMA electrodes. The TMR value increases when the top PMA electrode is replaced by an in-plane magnetized Co layer. These observations can be attributed to significant intermixing in the top Co/Ni electrode.

Équipe 101 : Nanomagnétisme et électronique de spin; International audience; The value and sign of V/Fe interface anisotropy are investigated. Epitaxial V/Fe/V/Au layers with different iron thicknesses were grown on single crystalline (001) MgO substrate by ultra high vacuum molecular beam epitaxy. Magnetometry was used to measure magnetization and out anisotropy field. From these values, we quantify the number of dead layers due to V/Fe or Fe/V interfaces, and compare it with the literature. We deduce that dead layers occur mostly at the bottom V/Fe interface. An average value for V/Fe and Fe/V interface anisotropy around 0 +/- 0.1 erg/cm(2) (mJ/m(2)) was thus deduced.

Fe/Cr/Fe trilayers and (Fe/Cr) 20 multilayers prepared under ultrahigh vacuum conditions by thermal evaporation were irradiated with 200 MeV I 13+ ions in the fluence range between 1 × 10 11 and 8 × 10 12 I/cm 2 . The structural properties of the Fe/Cr/Fe trilayers and (Fe/Cr) 20 multilayers were measured by X-ray reflectivity (XRR) and conversion electron Mossbauer spectroscopy (CEMS). Magnetic exchange coupling between the Fe layers through the Cr spacer layer was observed by SQUID magnetization measurements. Magnetoresistance effect was measured using four probe method at room temperature. The XRR spectra showed an increase of the interface roughness versus increasing irradiation fluence in the multilayers, while in the trilayers smoothening of the interfaces in the sample irradiated with fluence equal to 4 × 10 11 I/cm 2 and very slight change for other fluences were observed. Improving of the interface structure in the trilayers at this fluence was observed also by CEMS. Moreover the Mossbauer spectra also confirm roughening of the interfaces as a function of fluence for multilayers. Before irradiation an antiferromagnetic coupling fraction dominated in all samples. After irradiation the changes of magnetic coupling were different in both types of samples. The trilayers were less sensitive to the irradiation fluence than multilayers and an increase of the antiferromagnetic fraction at small fluences was observed. In the multilayers a continuous decrease of the antiferromagnetic fraction as a function of fluence was evidenced. Vanishing of the antiferromagnetic coupling, observed for the largest fluence, resulted in the decrease of magnetoresistance effect in the Fe/Cr multilayers.