Eric Gautron

Tuning the size and color of the p-type wide band gap delafossite semiconductor CuGaO2 with ethylene glycol assisted hydrothermal synthesis

Delafossite CuGaO2 is a prototype p-type transparent semiconducting oxide. We report here a new ethylene glycol (EG) mediated hydrothermal synthesis route to this material from soluble hydrated metal nitrates. We found that EG acts as a reducing agent which eases the formation of CuGaO2 by stabilizing Cu(I) at low temperature in hydrothermal conditions. Moreover, we observed that the initial pH of the precursors solution is a key parameter to tune the particle sizes (from 2 µm to 300 nm) and subsequently the color (from dark brown to light gray) of the CuGaO2 powder. The optimal synthesis conditions to obtain the brightest product are described. In these conditions, the CuGaO2 powder exhibits a pale grayish color and contains 300 nm large nano-layered particles approximately 20 nm thick.

Unusual dealloying effect in gold/copper alloy thin films: the role of defects and column boundaries in the formation of nanoporous gold.

Understanding the dealloying mechanisms of gold-based alloy thin films resulting in the formation of nanoporous gold with a sponge-like structure is essential for the future design and integration of this novel class of material in practical devices. Here we report on the synthesis of nanoporous gold thin films using a free-corrosion approach in nitric acid applied to cosputtered Au-Cu thin films. A relationship is established between the as-grown Au-Cu film characteristics (i.e., composition, morphology, and structure) and the porosity of the sponge-like gold thin films. We further demonstrate that the dealloying approach can be applied to nonhomogenous Au-Cu alloy thin films consisting of periodic and alternate Au-rich/Au-poor nanolayers. In such a case, however, the dealloying process is found to be altered and unusual etching stages arise. Thanks to defects and column boundaries playing the role of channels, the nitric acid is found to quickly penetrate within the films and then laterally (i.e., parallel to the film surface) attacks the nanolayers rather than perpendicularly. As a consequence to this anisotropic etching, the Au-poor layers are etched preferentially and transform into Au pillars holding the Au-rich layers and preventing them against collapsing. A further exposure to nitric acid results in the collapsing of the Au-rich layers accompanied by a transition from a multilayered to a sponge-like structure. A scenario, supported by experimental observations, is further proposed to provide a detailed explanation of the fundamental mechanisms occurring during the dealloying process of films with a multilayered structure.

Structure and White Luminescence of Eu-Activated (Ba,Sr)13−xAl22−2xSi10+2xO66 Materials

The optical properties of Eu-activated (Ba,Sr)(13-x)Al(22-2x)Si(10+2x)O66 materials have been determined after the structural reinvestigation of the hypothetical Ba 13Al 22Si 10O 66 material on the basis of the Geberts model. The white fluorescence and phosphorescence of the (Ba,Sr)(13-x)Al(22-2x)Si(10+2x)O66:Eu series result from the existence of two broad emission bands associated with (8)H-4f(6)5d(1)-->(8)S-4f(7) transitions peaking at 534 and 438 nm, the intensities of which may be tuned at room temperature via the control of the europium concentration and the substitution of Sr for Ba. This suggests the possibility to adjust the emission of the material to white LED requisites.

Copper borate as a photocathode in p-type dye-sensitized solar cells

p-Type dye-sensitized solar cells (p-DSSCs) have recently become a major research focus because coupling with n-type DSSCs yields highly efficient tandem DSSCs. Indeed many delafossite-like compounds appear as promising candidates for p-DSSCs due to their deep valence band position and high hole mobility. In this paper, the synthesis of CuBO2 was attempted by a facile sol–gel methodology. Then, the as-obtained particles were used to prepare photocathodes for p-DSSCs with DPP-NDI dye as sensitizer and tris(4,4′-di-tert-butyl-2,2′-bipyridine)cobalt(III/II) as redox mediator. Due to the deeper valence band position compared with classical NiO photocathode, the “CuBO2” based p-DSSC presents an open-circuit photovoltage (Voc) of 453 mV, which is 150 mV higher than that of NiO in the same conditions. The results show that “CuBO2” is a potential alternative for NiO in p-DSSCs.

Thermal properties of carbon nanowall layers measured by a pulsed photothermal technique

We report the thermal properties of carbon nanowall layers produced by expanding beam radio-frequency plasma. The thermal properties of carbon nanowalls, grown at 600 °C on aluminium nitride thin-film sputtered on fused silica, were measured with a pulsed photo-thermal technique. The apparent thermal conductivity of the carbon at room temperature was found to increase from 20 to 80 Wm−1 K−1 while the thickness varied from 700 to 4300 nm, respectively. The intrinsic thermal conductivity of the carbon nanowalls attained 300 Wm−1 K−1 while the boundary thermal resistance with the aluminium nitride was 3.6 × 10−8 Km2 W−1. These results identify carbon nanowalls as promising material for thermal management applications.

Shape control of nickel nanostructures incorporated in amorphous carbon films: From globular nanoparticles toward aligned nanowires

The growth of nickel/carbon nanocomposite thin films by a hybrid plasma process, which combines magnetron sputtering and plasma enhanced chemical vapor deposition, has been investigated. This study has shown that the films consist of nickel-rich nanostructures embedded in an amorphous carbon matrix. The size, the distribution, the density, and the shape of these nanostructures are directly dependent to the total carbon content within the films. At low carbon content (∼28 at. %), dense nanowire array perpendicularly oriented to the surface of the substrate can be fabricated. For an intermediate carbon concentration (∼35 at. %), the nickel phase was organized into elongated nanoparticles. These nanoparticles became spherical when reaching a higher carbon content (∼54 at. %). The extensive structural study allowed the representation of a structure zone diagram, as well as, the development of a scenario describing the growth mechanisms that take place during the deposition of such nanocomposite material.

Titanium carbide/carbon composite nanofibers prepared by a plasma process

The incorporation of metal or metal carbide nanoparticles into carbon nanofibers modifies their properties and enlarges their field of application. The purpose of this work is to report a new non-catalytic and easy method to prepare organized metal carbide-carbon composite nanofibers on nanopatterned silicon substrates prepared by laser interference lithography coupled with deep reactive ion etching. Titanium carbide-carbon composite nanofibers were grown on the top of the silicon lines parallel to the substrate by a hybrid plasma process combining physical vapor deposition and plasma enhanced chemical vapor deposition. The prepared nanofibers were analyzed by scanning electron microscopy, x-ray photoelectron spectroscopy, Raman spectroscopy and transmission electron microscopy. We demonstrate that the shape, microstructure and the chemical composition of the as-grown nanofibers can be tuned by changing the plasma conditions.

Hierarchical carbon nanostructure design: ultra-long carbon nanofibers decorated with carbon nanotubes

Hierarchical carbon nanostructures based on ultra-long carbon nanofibers (CNF) decorated with carbon nanotubes (CNT) have been prepared using plasma processes. The nickel/carbon composite nanofibers, used as a support for the growth of CNT, were deposited on nanopatterned silicon substrate by a hybrid plasma process, combining magnetron sputtering and plasma-enhanced chemical vapor deposition (PECVD). Transmission electron microscopy revealed the presence of spherical nanoparticles randomly dispersed within the carbon nanofibers. The nickel nanoparticles have been used as a catalyst to initiate the growth of CNT by PECVD at 600°C. After the growth of CNT onto the ultra-long CNF, SEM imaging revealed the formation of hierarchical carbon nanostructures which consist of CNF sheathed with CNTs. Furthermore, we demonstrate that reducing the growth temperature of CNT to less than 500°C leads to the formation of carbon nanowalls on the CNF instead of CNT. This simple fabrication method allows an easy preparation of hierarchical carbon nanostructures over a large surface area, as well as a simple manipulation of such material in order to integrate it into nanodevices.

Improved photoconductive properties of composite nanofibers based on aligned conjugated polymer and single-walled carbon nanotubes

We successfully address the challenge of aligning single-walled carbon nanotubes (SWNTs) and conjugated polymer chains in composite nanofibers for enhancing their opto-electrical properties. A pore-filling template strategy has been developed to prepare such nanocomposites from SWNTs and poly(para-phenylene vinylene) (PPV) chains, with both species well-oriented aligned along the pore axis. Addition of the SWNTs leads to a remarkable increase in photocurrent of four orders of magnitude as compared to equivalent pristine PPV nanofibers. Further analysis indicates that the strong photocurrent enhancement is not simply an effect of alignment, but additionally benefits from alignment-enhanced interaction of polymer chains with SWNTs, as supported by density functional theory (DFT) calculations.Graphical abstract

Criteria to define a more relevant reference sample of titanium dioxide in the context of food: a multiscale approach

ABSTRACT Titanium dioxide (TiO2) is a transition metal oxide widely used as a white pigment in various applications, including food. Due to the classification of TiO2 nanoparticles by the International Agency for Research on Cancer as potentially harmful for humans by inhalation, the presence of nanoparticles in food products needed to be confirmed by a set of independent studies. Seven samples of food-grade TiO2 (E171) were extensively characterised for their size distribution, crystallinity and surface properties by the currently recommended methods. All investigated E171 samples contained a fraction of nanoparticles, however, below the threshold defining the labelling of nanomaterial. On the basis of these results and a statistical analysis, E171 food-grade TiO2 totally differs from the reference material P25, confirming the few published data on this kind of particle. Therefore, the reference material P25 does not appear to be the most suitable model to study the fate of food-grade TiO2 in the gastrointestinal tract. The criteria currently to obtain a representative food-grade sample of TiO2 are the following: (1) crystalline-phase anatase, (2) a powder with an isoelectric point very close to 4.1, (3) a fraction of nanoparticles comprised between 15% and 45%, and (4) a low specific surface area around 10 m2 g–1. GRAPHICAL ABSTRACT