Serge Paofai

Photoreduction of CO2 to methanol with hexanuclear molybdenum [Mo6Br14]2− cluster units under visible light irradiation

Octahedral molybdenum clusters were found to be efficient visible light homogeneous photocatalysts for the reduction of carbon dioxide (CO2) to methanol. Photoreduction was carried out by using 20 watt white cold LED flood light in dimethyl formamide/water or acetonitrile/water solutions containing triethylamine as a reductive quencher. Among the two cluster-based compounds, Cs2[Mo6Br14] exhibited higher photocatalytic efficiency and afforded higher yield of methanol than (TBA)2[Mo6Br14] (TBA = tetrabutylammonium). After 24 h illumination, the yield of methanol was 6679.45 and 5550.53 μmol g−1 cat. using Cs2[Mo6Br14] and (TBA)2[Mo6Br14] cluster compounds as photocatalysts, respectively.

Hydride in BaTiO2.5H0.5: A Labile Ligand in Solid State Chemistry.

In synthesizing mixed anion oxides, direct syntheses have often been employed, usually involving high temperature and occasionally high pressure. Compared with these methods, here we show how the use of a titanium perovskite oxyhydride (BaTiO2.5H0.5) as a starting material enables new multistep low temperature topochemical routes to access mixed anion compounds. Similar to labile ligands in inorganic complexes, the lability of H(-) provides the necessary reactivity for syntheses, leading to reactions and products previously difficult to obtain. For example, BaTiO2.5N0.2 can be prepared with the otherwise inert N2 gas at 400-600 °C, in marked contrast with currently available oxynitride synthetic routes. F(-)/H(-) exchange can also be accomplished at 150 °C, yielding the oxyhydride-fluoride BaTi(O, H, F)3. For BaTiO2.4D0.3F0.3, we find evidence that further anionic exchange with OD(-) yields BaTiO2.4(D(-))0.26(OD(-))0.34, which implies stable coexistence of H(+) and H(-) at ambient conditions. Such an arrangement is thermodynamically unstable and would be difficult to realize otherwise. These results show that the labile nature of hydride imparts reactivity to oxide hosts, enabling it to participate in new multistep reactions and form new materials.

High temperature structural stability, electrical properties and chemical reactivity of NdBaCo2−xMnxO5+δ (0 ≤ x ≤ 2) for use as cathodes in solid oxide fuel cells

The effects of Mn substitution for Co on the crystal chemistry, oxygen content, thermal expansion and electrical conductivity of the NdBaCo2−xMnxO5+δ perovskites (0 ≤ x ≤ 2) have been investigated. The NdBaCo2−xMnxO5+δ samples exhibit structural changes with increasing Mn contents from orthorhombic (x = 0) to tetragonal (0.5 ≤ x ≤ 1) then to cubic (1.5 ≤ x ≤ 2.0) symmetry. All the samples lose oxygen when heated in air at T > 400 °C although the degree of oxygen loss and kinetics of oxygen exchange between the gas phase and oxide decrease with increasing Mn contents. The thermal expansion coefficients evaluated from ex situ XRD and electrical resistivity decrease with increasing Mn substitution and the values for the x = 1.5 and 2.0 compositions match with those of the Ce0.8Gd0.2O1.95 (GDC) and La0.8Sr0.2Ga0.8Mg0.2O2.8 (LSGM) electrolytes. With electrical conductivity values of >100 S cm−1 at 800 °C and good chemical stability with GDC and LSGM, the Mn-substituted perovskites are promising cathode materials for SOFCs.

Stability of NdBaCo2−xMnxO5+δ (x = 0, 0.5) layered perovskites under humid conditions investigated by high-temperature in situ neutron powder diffraction

The double perovskites NdBaCo2−xMnxO5+δ (x = 0 and 0.5) were investigated using in situ high temperature neutron powder diffraction in dry argon and wet atmospheres (40% D2O/argon and 40% D2O/air) in order to assess their stability as cathodes in proton conducting fuel cells. The x = 0 oxide loses oxygen on heating in dry argon at T > 400 °C and exhibits an oxygen vacancy order–disorder transition as evidenced by the orthorhombic Pmmm to tetragonal P4/mmm transition. Refinement of site occupancy factors suggests that the oxygen vacancies mainly form in the Nd layers and to a lesser extent at the equatorial positions of the transition metal polyhedra; at 800 °C, δ ∼ 0. When the gas was changed to wet argon at 800 °C and the sample cooled to 260 °C, no structural modification or change in the oxygen content was detected and no impurity phases formed, highlighting the excellent stability of the sample in wet atmospheres. On switching the gas to wet air at 260 °C, thermal analysis and neutron powder diffraction data together reveal that the sample intercalates mainly oxygen rather than proton defects within a two-phase process involving two orthorhombic phases, reflecting the symmetry of the reduced and oxidised materials. On heating, the sample transforms at T ≥ 600 °C to a single tetragonal phase whose symmetry is retained up to 800 °C and on subsequent cooling. The x = 0.5 material prepared in argon adopted a tetragonal P4/mmm structure at RT with δ ∼ 0. Its symmetry remains tetragonal on heating/cooling in wet argon. On changing the gas to wet air at 260 °C, it takes up oxygen via a two-phase process involving two tetragonal phases. Since fast oxidation is the main process that fills the oxygen vacancies of these double perovskites in wet air, a large oxygen deficiency seems to be not the only requirement for effective proton incorporation in this family of materials with basic characteristics.

Redox behavior of the SOFC electrode candidate NdBaMn2O5+δ investigated by high-temperature in situ neutron diffraction: first characterisation in real time of an LnBaMn2O5.5 intermediate phase

The structural behavior of the tetragonal NdBaMn2O5 phase, a member of the family of A-site ordered layered manganites that have been recently suggested as possible mixed ionic and electronic conductors, has been investigated by means of in situ neutron powder diffraction. Considering applications in energy production and storage devices and use of NdBaMn2O5+δ as an electrode in symmetrical cells, the study was carried out in relevant atmosphere conditions, i.e. dilute hydrogen (wet and dry) and dry air in the temperature range 25–800 °C. Neutron data under flowing hydrogen allowed monitoring of the structural phase transition from the charge-ordered to the charge-disordered state as a function of temperature. Slow reduction of the fully oxidised phase, NdBaMn2O6, previously formed from quick oxidation of the pristine material, enabled real-time observation of the intermediate NdBaMn2O5.5 phase and its crystal characterization up to 700 °C in the course of its conversion to NdBaMn2O5. Oxygen vacancy ordering within the Nd layers of NdBaMn2O5.5 correlated with antiferrodistortive orbital ordering of the Jahn–Teller Mn3+ ion in the square pyramids and octahedra results in large thermal expansion and relatively slow anisotropic oxygen diffusion occurring in the NdO layer. The four heating/cooling cycles evidenced no oxygen miscibility between the three distinct phases detected in the NdBaMn2O5+δ system with δ ∼ 0, 0.5 and 1 and clearly demonstrated that reversible oxygen intercalation/deintercalation underpins the phase stability of the LnBaMn2O5+δ materials to redox cycling and to wet atmosphere in high temperature electrochemical devices.

Carrier scattering processes and low energy phonon spectroscopy in hybrid perovskites crystals

Despite the wealth of research conducted the last three years on hybrid organic perovskites (HOP), several questions remain open including: to what extend the organic moiety changes the properties of the material as compared to allinorganic (AIP) related perovskite structures. To ultimately reach an answer to this question, we have recently introduced two approaches that were designed to take the stochastic molecular degrees of freedom into account, and suggested that the high temperature cubic phase of HOP and AIP is an appropriate reference phase to rationalize HOP’s properties. In this paper, we recall the main concepts and discuss more specifically the various possible couplings between charge carriers and low energy excitations such as acoustic and optical phonons. As available experimental or simulated data on low energy excitations are limited, we also present preliminary neutron scattering and ultrasonic measurements obtained and freshly prepared single crystals of CH3NH3PbBr3.

Unprecedented Electron-Poor Octahedral Ta6 Clusters in a Solid-State Compound: Synthesis, Characterisations and Theoretical Investigations of Cs2BaTa6Br15O3

The crystal structure of Cs2BaTa6Br15O3 has been elucidated by using synchrotron X-ray powder diffraction and absorption experiments. It is built from edge-bridged octahedral [(Ta6Bri9Oi3)Bra6]4− cluster units with a singular poor metallic electron (ME) count equal to thirteen. This leads to a paramagnetic behaviour related to one unpaired electron. The arrangement of the Ta6 clusters is similar to that of Cs2LaTa6Br15O3 exhibiting 14-MEs per [(Ta6Bri9Oi3)Bra6]5− motif. The poorer electron-count cluster presents longer metal–metal distances as foreseen according to the electronic structure of edge-bridged hexanuclear cluster. Density functional theory (DFT) calculations on molecular models were used to rationalise the structural properties of 13- and 14-ME clusters. Periodic DFT calculations demonstrate that the electronic structure of these solid-state compounds is related to those of the discrete octahedral units. Oxygen–barium interactions seem to prevent the geometry of the octahedral cluster to strongly distort, allowing stabilisation of this unprecedented electron-poor Ta6 cluster in the solid state.

Mo6 cluster-based compounds for energy conversion applications: comparative study of photoluminescence and cathodoluminescence

Abstract We report the photoluminescence (PL) and cathodoluminescence (CL) properties of face-capped [Mo6Xi8La6]2− (X = Cl, Br, I; L = organic or inorganic ligands) cluster units. We show that the emission of Mo6 metal atom clusters depends not only on the nature of X and L ligands bound to the cluster and counter-cations, but also on the excitation source. Seven members of the AxMo6Xi8La6 series (A = Cs+, (n-C4H9)4N+, NH4+) were selected to evaluate the influence of counter-cations and ligands on de-excitation mechanisms responsible for multicomponent emission of cluster units. This study evaluates the ageing of each member of the series, which is crucial for further energy conversion applications (photovoltaic, lighting, water splitting, etc.).

New ultra-violet and near-infrared blocking filters for energy saving applications: fabrication of tantalum metal atom cluster-based nanocomposite thin films by electrophoretic deposition

This study reports the first integration of inorganic tantalum octahedral metal atom clusters into multifunctional nanocomposite coating materials and devices for window technology and energy saving applications. [Ta6Bri12]n+ (n = 2, 3 or 4) cluster-based high visible transparency UV and NIR filters are realized. Green and brown colored films are fabricated by coating on an indium-doped tin oxide glass substrate by electrophoretic deposition, an industrialized solution process. The efficiency in energy saving of the new UV-NIR filters was estimated by the determination of different figure of merit (FOM) values, such as Tvis, Tsol and Tvis/Tsol (Tsol = solar transmittance and Tvis = visible transmittance), and the color coordinates (x, y, z and L*a*b). The Tvis/Tsol ratio is equal to 1.25 for the best films. Such values are evidence of a higher energy saving efficiency than most of the inorganic composites reported in the literature. These promising results pave the way for the use of transition metal clusters as a new class of nanocoatings in energy saving window-based applications.