Didier Grandjean

Organometallic approach for platinum and palladium doping of tin and tin oxide nanoparticles: structural characterisation and gas sensor investigations

Platinum and palladium surface doped tin nanoparticles have been obtained by decomposition of [Pt2(dba)3] and [Pd(dba)2] organometallic precursors, respectively, in a colloidal suspension of tin/tin oxide core–shell nanoparticles of uniform size (≈13 nm), in anisole under 1 bar of carbon monoxide at room temperature. The particles can be isolated as pure solids and present effective doping ratios [Pt]/[Sn] and [Pd]/[Sn] of 2.5 and 3.1%, respectively. These nanomaterials have been characterized by means of transmission electron and high-resolution transmission electron microscopies (TEM and HRTEM), Energy Dispersive X-ray spectroscopy (EDX), photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (EXAFS and XANES). The TEM micrographs show spherical nanoparticles whose size and size distribution are essentially similar to those of the initial Sn/SnOx material. HRTEM, EDX, XPS and X-ray absorption spectroscopy (XAS) studies at the Pd and/or Sn-K edge show the conservation of the composite nature of the particles that consist of a tin(0) core covered with a layer of tin oxides on which the doping element is deposited under the form of crystalline metallic platelets of size near 2 nm. The thermal oxidation of these Pt- or Pd-doped tin materials yields nanoparticles of crystallized SnO2 covered with crystallites of Pt or Pd oxides, as demonstrated by XRD, XPS and XAS experiments, without coalescence or size change. In the oxidised Pd-doped material, EXAFS analysis combined with HRTEM, point towards the formation of two main Pd disordered oxide phases: a rather pure oxopalladate Pd3.5O4-type phase at the surface of the particle and a mixed Pd/Sn phase having a PdO structure-type at the interface Pd oxide/Sn oxide. The thermal oxidation process can be easily achieved onto the silicon platform of a micro-machined device after integration of the Pt- or Pd-doped Sn/SnOx colloidal suspension by drop deposition. The first electrical measurements indicate remarkable behaviours of the as-obtained microsensors when exposed to traces of carbon monoxide. In addition to the expected large increase of sensitivity of the doped relatively to the undoped sensitive layer measured in humid atmosphere, a surprising and unprecedented inversion of sensitivity from undoped to Pt- and Pd-doped sensors has been observed in dry atmosphere.

New highly mixed phases in ball-milled Cu/ZnO catalysts as established by EXAFS and XANES

AnXAFS investigation at both the Cu and Zn K-edge has allowed to unravel new highly mixed phases in Cu/ZnO catalysts prepared by ball-milling mixtures of Cu2O and ZnO under 3 different atmospheres of synthetic air (SA), SA+CO2 and CO2. The system milled in CO2 shows the disproportionation of Cu2O into Cu0, Cu1+ (cuprite Cu2O-type phase) and Cu2+ (tenorite CuO-type phase), while most of the Zn2+ is transformed into a nanocrystalline / amorphous ZnO-type zincite that forms a superficial mixture of oxide and carbonate phases. When synthetic air is added to the CO2 atmosphere, ball-milling results in the oxidation of part of the Cu1+ into Cu2+ with no Cu metal formed. In SA, a significant amount of Cu2+- and Zn2+-based phases react to form a nanocrystalline / amorphous Cu1-xZnxO solid solution that was never reported before. This distorted rock salt-like solid solution, in which Zn and Cu feature different octahedral environments, is formed by incorporation of Zn2+ in the Cu2O matrix and the concomitant oxidation of Cu1+ into Cu2+ and results from strong Cu/Zn interactions in the Cu/ZnO system.

TiO2 Films Modified with Au Nanoclusters as Self-Cleaning Surfaces under Visible Light

In this study, we applied cluster beam deposition (CBD) as a new approach for fabricating efficient plasmon-based photocatalytic materials. Au nanoclusters (AuNCs) produced in the gas phase were deposited on TiO2 P25-coated silicon wafers with coverage ranging from 2 to 8 atomic monolayer (ML) equivalents. Scanning Electron Microscopy (SEM) images of the AuNCs modified TiO2 P25 films show that the surface is uniformly covered by the AuNCs that remain isolated at low coverage (2 ML, 4 ML) and aggregate at higher coverage (8 ML). A clear relationship between AuNCs coverage and photocatalytic activity towards stearic acid photo-oxidation was measured, both under ultraviolet and green light illumination. TiO2 P25 covered with 4 ML AuNCs showed the best stearic acid photo-oxidation performance under green light illumination (Formal Quantum Efficiency 1.6 × 10−6 over a period of 93 h). These results demonstrate the large potential of gas-phase AuNCs beam deposition technology for the fabrication of visible light active plasmonic photocatalysts.

Unraveling the Structure of Mn‐Promoted Co/TiO2 Fischer‐Tropsch Catalysts by In Situ X‐Ray Absorption Spectroscopy

Combination of in situ X‐ray absorption spectroscopy (XAFS) at the Co and Mn K‐edges with electron microscopy (STEM‐EELS) has allowed to unravel the complex structure of a series of unpromoted and Mn promoted TiO2‐supported cobalt Fischer‐Tropsch catalysts prepared by homogeneous deposition precipitation (HDP), both in their calcined and reduced states. After calcination the catalysts are generally composed of large Co3O4 aggregates (13–20 nm) and a MnO2‐type phase that is either dispersed on the TiO2 surface or, for the major part, covering the Co3O4 particles. Additionally Mn is also forming a spinel‐type Co3−xMnxO4 solid solution at the surface of the Co3O4 particles. In pure Co or when small amount of this spinel‐type phase are formed during calcination, reduction in H2 at 350 °C produces Co0 particles of variable sizes (3.5–15 nm) otherwise Co reduction is limited to the Co2+ state. Manganese that exists entirely in a Mn2+ state in the reduced catalysts is forming (1) a highly dispersed Ti2MnO4‐type ...