Synthesis of Ni-poor NiO nanoparticles for p-DSSC applications

International audienceTo improve the performances of p-Dye Sensitized Solar Cell (p-DSSC) for the future, the synthesis of modified p-type nickel oxide semiconductor, commonly used as photocathode in such devices, was initiated with Ni3O2(OH)4 as precursor. This specific nickel oxyhydroxide was first characterized by X-ray photo-electron spectroscopy and magnetic susceptibility measurements. Then its thermal decomposition was thoroughly studied in order to control the particles size of the as-prepared NiO nanopowders. Low temperature decomposition in air of this precursor allows the formation of Ni1-xO nanoparticles with a large amount of Ni vacancies and specific surface areas up to 250 m2.g-1. Its ammonolysis at 250°C leads to nanostructured N-doped NiO (NiO:N) materials

Synthesis of Ni-poor NiO nanoparticles for DSSC-p applications

International audienceOver the last decade, p-type semiconductors (SC) have known a renewed interest. Indeed these materials may have potentialapplications for light-emitting diodes, transistors, solar cells, etc. Since the achievement of the first Dye Sensitized Solar Cells (DSSC) by Grätzel in 1991 a new generation of solar cells has been developed where the n-type SC is replaced by a p-type one. This leads to the photo-injection of holes instead of electrons in the circuit. To date nickel oxide (NiO) is the reference p-type semiconductor.However yields are still far from those of n-DSSC and many studies aim to replace NiO by other systems such as CuAlO2 , CuGaO2,CuCrO2 or NiCo2O4 nanoparticles. Following our recent synthesis of N doped ZnO with stabilization of p-type charge carriers, wefocus now on the preparation of N doped NiO nanoparticles to improve the p-type conductivity of NiO. We study here the chemicalreactivity of a nickel oxyhydroxide precursor under air and ammonia that conducts to nanostructured Ni-poor NiO

Etude de semi-conducteurs de type p nanostructurés à base de métaux de transition pour une application en DSSC-p

To improve the performances of p-Dye Sensitized Solar Cell (p-DSSC), this thesis work focuses on the synthesis and the characterization of p-type semiconductors (p-SCs) nanomaterials. These p-SCs with some specifications (low energy valence band, high specific surface area, high conductivity and transparency) were thoroughly studied. In this context, a strategy was developed to improve the NiO nanoparticles properties (commonly used as a reference) with higher nickel non-stoichiometry and nitrogen doping to promote the stabilization of the Ni3+/Ni2+ mixed valence (origin of the p-typness). This study was initiated with a nanostructured mixed valent Ni3O2(OH)4 precursor. Its thermal decomposition in air and ammonia at low temperature (250 °C) allows the formation of nanostructured Ni1-xO with a large amount of Ni vacancies (V Ni = 25 %), a high specific surface area (240 m2.g-1) and a nitrogen doping (NiO:N). Moreover, two non-oxides materials with delafossite structure type, namely - nickel carbodiimide (NiNCN) and manganese carbodiimide (MnNCN) - were prepared and characterized as new p-type semiconductors. Thus, the first p-DSSC with NiNCN material was built with success.Dans le but d'améliorer le rendement des cellules à colorant de type p (DSSC-p), ces travaux s'attachent à la synthèse et la caractérisation de matériaux semi-conducteurs de type p (SCs-p) sous forme de nanoparticules. En ce sens, des SCs-p répondant à un cahier des charges (bande de valence basse en énergie, grande surface spécifique, bon conducteur et bonne transparence) ont été étudiés. Dans ce cadre, une stratégie a été développée pour améliorer les propriétés de NiO (l'actuel matériau de référence) en optimisant sa nanostructuration, sa fortenon-stœchiométrie en nickel et par son dopage à l'azote, paramètres tous favorables à la stabilisation de la valence mixte Ni3+/Ni2+, origine de la conductivité de type p. Cette longue étude a été initiée à partir d'un précurseur de nickel original nanostructuré Ni3O2 OH)4, à forte valence mixte Ni3+/Ni2+ . La décomposition sous air et sous ammoniac de ce précurseur à basse température (250 °C) a permis de préparer Ni1-xO nanostructuré, fortement non-stœchiométrique (V Ni = 25 %), de grande surface spécifique (240 m2.g-1) et dopé azote (NiO:N). De plus, deux matériaux non oxydes à structure delafossite, que sont les carbodiimides de nickel (NiNCN) et de manganèse (MnNCN) ont été préparés et caractérisés comme de nouveaux semi-conducteurs de type p, permettant de monter la première DSSC-p à base de NiNCN

Study of nanostructured p-type semiconductors based on transition metals for p-DSSC applications

Dans le but d'améliorer le rendement des cellules à colorant de type p (DSSC-p), ces travaux s'attachent à la synthèse et la caractérisation de matériaux semi-conducteurs de type p (SCs-p) sous forme de nanoparticules. En ce sens, des SCs-p répondant à un cahier des charges (bande de valence basse en énergie, grande surface spécifique, bon conducteur et bonne transparence) ont été étudiés. Dans ce cadre, une stratégie a été développée pour améliorer les propriétés de NiO (l'actuel matériau de référence) en optimisant sa nanostructuration, sa forte non-stœchiométrie en nickel et par son dopage à l'azote, paramètres tous favorables à la stabilisation de la valence mixte Ni3+/Ni2+, origine de la conductivité de type p. Cette longue étude a été initiée à partir d'un précurseur de nickel original nanostructuré Ni3O2(OH)4, à forte valence mixte Ni3+/Ni2+. La décomposition sous air et sous ammoniac de ce précurseur à basse température (250 °C) a permis de préparer Ni1-xO nanostructuré, fortement non-stœchiométrique (VNi = 25 %), de grande surface spécifique (240 m2.g-1) et dopé azote (NiO:N). De plus, deux matériaux non oxydes à structure delafossite, que sont les carbodiimides de nickel (NiNCN) et de manganèse (MnNCN) ont été préparés et caractérisés comme de nouveaux semi-conducteurs de type p, permettant de monter la première DSSC-p à base de NiNCN.To improve the performances of p-Dye Sensitized Solar Cell (p-DSSC), this thesis work focuses on the synthesis and the characterization of p-type semiconductors (p-SCs) nanomaterials. These p-SCs with some specifications (low energy valence band, high specific surface area, high conductivity and transparency) were thoroughly studied. In this context, a strategy was developed to improve the NiO nanoparticles properties (commonly used as a reference) with higher nickel non-stoichiometry and nitrogen doping to promote the stabilization of the Ni3+/Ni2+ mixed valence (origin of the p-typness). This study was initiated with a nanostructured mixed valent Ni3O2(OH)4 precursor. Its thermal decomposition in air and ammonia at low temperature (250 °C) allows the formation of nanostructured Ni1-xO with a large amount of Ni vacancies (VNi = 25 %), a high specific surface area (240 m2.g-1) and a nitrogen doping (NiO:N). Moreover, two non-oxides materials with delafossite structure type, namely - nickel carbodiimide (NiNCN) and manganese carbodiimide (MnNCN) - were prepared and characterized as new p-type semiconductors. Thus, the first p-DSSC with NiNCN material was built with success

Control of the Ni non-stoichiometry in Ni_1-xO nanoparticles : "Core-shell"-like model

International audienceTo improve the performances of p-Dye Sensitized Solar Cell (p-DSSC), the synthesis of modified p-type nickel oxide semiconductor, commonly used as a photocathode in such devices, was initiated using a mixed valent nickel oxyhydroxide Ni3O2(OH)4. The decomposition of this precursor in air at temperatures lower than 600°C leads to non-stoichiometric Ni1-xO nanoparticles (from 2 to 60 nm) with tunable nickel vacancies concentration (up to 25% for 2-3nm particle sizes). According to our chemical characterizations (XRD, TEM, density, chemical analysis, BET measurement…), the nickel vacancies segregate at the surface of the Ni1-xO nanoparticles to create a "core-shell"-like edifice (similarly to our previous work on Zn-deficient Zn1-xO nanoparticles). This "core-shell" is constituted by a dense stoichiometric NiO coated (or terminated) by an oxygen (hydroxide or carbonate groups) surface layer free from nickel atom. The Ni-free surface layer influences drastically the density of the smaller nanoparticles by decreasing it. When the nanoparticles size increases, the density evolves until reaching the theoretical density of stoichiometric nickel oxide for the bigger particles (≥100 nm). Thus, with the control of the particles size, we can also control the Ni non-stoichiometry in Ni1-xO following our "core-shell"-like model

P-type transition metal carbodiimides MNCN (M = Ni, Mn)

International audienceOver the last decade, p-type semiconductors (SC) have known a renewed interest. Indeed these materials may have potential applications for light-emitting diodes, transistors, solar cells, etc. Since the stabilization of the first p-type semiconductor CuAlO2 with a delafossite structure type (A+B3+O2) by H. Kawazoe in 1997, many p-type semiconductors with the same structure have been reported such as CuGaO2, CuCrO2, CuScO2… Recently, X. Liu & al have reported the synthesis and the structure determination of a new family of materials: the transition metal carbodiimides MNCN (with M = Mn → Cu). These materials adopt a structure type related to the delafossite, formed by a divalent transition metal and NCN2-anion. We report here the synthesis and the microstructural characterization of two transition metal carbodiimides MnNCN and NiNCN. Impedance measurements demonstrate the p-type conductivity of these materials.</br

Impact of Nanostructuration on the Chemical Composition of Nickel Oxide Nanoparticles

International audienceReduction of the size of a particle down to a few tens of nanometers or below may drastically affect its physical properties. That is well-known for quantum dots. Conversely, many works consider the chemical composition of nanoparticles as invariant upon reduction of their dimension. Here we demonstrate that the chemical composition of a transition-metal oxide, namely, nickel oxide, is drastically affected by its nanostructuration

Experimental and theoretical evidences of p-type conductivity in nickel carbodiimide nanoparticles with a delafossite structure type

International audienceNickel carbodiimide (NiCN) was synthesized using a two-step precipitation-decomposition route leading to a brown powder with gypsum-flower-like morphology and a large specific surface area (75 m/g). This layered material crystallizes in the 2H structure type of delafossite (space group P6/mmc), which is built upon infinite /[NiN] layers connected by linear carbodiimide ([N═C═N]) bridges. An X-ray diffraction Rietveld refinement and thermal analyses pointed out some nickel deficiencies in the material, and band structure calculations carried out on the defect compound predicted p-type conductivity in relation to a slight amount of N. This p-type conductivity was demonstrated by electrochemical impedance spectroscopy measurements, and a flat band potential of 0.90 V vs SCE at pH 9.4 was measured. This value, which is more positive than those of CuGaO and CuCrO delafossite oxides and NiO, prompted us to test NiCN nanoparticles as a photocathode in p-type dye-sensitized solar cells

MgF2-Based Organized Porous Inorganic Nanofluorides as Heterogeneous Catalysts for Fluorination of 2-Chloropyridine

International audienceThe successful preparation of organized porous inorganic fluorides (OPIFs) with a high specific surface area is demonstrated for MgF 2. For the first time, macroporous MgF 2 OPIFs with a surface area of above 200 m 2 g −1 and mesoporous MgF 2 powder were prepared through the assembly of preformed MgF 2 nanoparticles and homemade polymer templates with a tunable size. These OPIF materials have been fully examined at different synthesis stages by means of powder X-ray diffraction, N 2 sorption, scanning electron microscopy, and transmission electron microscopy analyses and 19 F and 1 H solid-state nuclear magnetic resonance. The relation between the nature, the size, and the amount of polymer template on the porous structure was deeply investigated. The MgF 2 OPIFs present a higher thermal stability under air and F 2 calcination than MgF 2 NPs as the structuration of the OPIF composite considerably slows down the crystallite growth during thermal treatment under air. OPIF materials were evaluated for the first time as heterogeneous catalysts for the fluorination of 2-chloropyridine under HF gas as a fluorinating agent at 350°C. This study evidences catalytic sites with two Lewis acidity strengths (medium and low)