Ahmed Naitabdi

Water adsorption on TiO2 surfaces probed by soft X-ray spectroscopies: bulk materials vs. isolated nanoparticles

We describe an experimental method to probe the adsorption of water at the surface of isolated, substrate-free TiO2 nanoparticles (NPs) based on soft X-ray spectroscopy in the gas phase using synchrotron radiation. To understand the interfacial properties between water and TiO2 surface, a water shell was adsorbed at the surface of TiO2 NPs. We used two different ways to control the hydration level of the NPs: in the first scheme, initially solvated NPs were dried and in the second one, dry NPs generated thanks to a commercial aerosol generator were exposed to water vapor. XPS was used to identify the signature of the water layer shell on the surface of the free TiO2 NPs and made it possible to follow the evolution of their hydration state. The results obtained allow the establishment of a qualitative determination of isolated NPs’ surface states, as well as to unravel water adsorption mechanisms. This method appears to be a unique approach to investigate the interface between an isolated nano-object and a solvent over-layer, paving the way towards new investigation methods in heterogeneous catalysis on nanomaterials.

Local investigation of the electronic properties of size-selected Au nanoparticles by scanning tunneling spectroscopy

The relationship between the structural/morphological and electronic properties of size-selected gold nanoparticles was investigated using scanning tunneling microscopy and spectroscopy. The nanoparticles were synthesized by inverse micelle encapsulation and were dip-coated on TiO2∕Ti(15nm)∕Si(111). Annealing in vacuum to 500°C resulted in the removal of the polymer and the formation of an ultrathin TiC support. Significant changes in the electronic local density of states (LDOS) of the nanoparticles, in particular, the onset of nonmetallic behavior, were observed with decreasing particle size. The nanoparticle-support interactions were studied and evidence for substrate-induced modifications in the LDOS of interfacial gold atoms is found.

Chiral self-assemblies of amino-acid molecules: D- and L-methionine on Au(111) surface

Spontaneous self-assembly into chiral well-ordered two-dimensional molecular film at room temperature has been evidenced using the scanning tunneling microscopy for the growth of two enantiomerically pure L- and D-methionine on Au(111) surface. From the x-ray photoelectron spectroscopy measurements, the growth of methionine occurs through the formation of zwitterionic species. Our results demonstrate that this self-assembly mechanism is crucially driven by the interchain interactions via zwitterionic hydrogen bonding between neighboring negatively charged carboxylate groups and positively charged amino groups. Moreover, the molecular pair formation is incidental and results from the optimization of the interchain interactions, at least two chains are required for stability.

Enhanced thermal stability and nanoparticle-mediated surface patterning: Pt/TiO2(110)

This letter reports (i) the enhanced thermal stability (up to 1060 °C) against coarsening and/or desorption of self-assembled Pt nanoparticles synthesized by inverse micelle encapsulation and deposited on TiO2(110) and (ii) the possibility of taking advantage of the strong nanoparticle/support interactions present in this system to create patterned surfaces at the nanoscale. Following our approach, TiO2 nanostripes with tunable width, orientation, and uniform arrangement over large surface areas were produced.

Formation, thermal stability, and surface composition of size-selected AuFe nanoparticles

The surface composition of isolated Au0.5Fe0.5 nanoparticles (NPs) synthesized by micelle encapsulation and supported on TiO2(110) has been investigated. The study reveals that phase-segregated structures are present after annealing at 300°C. A subsequent thermal treatment at 700°C resulted in the formation of a AuFe alloy. At this temperature, a state characteristic of Fe was identified at the NPs’ surface. Annealing at 900°C resulted in the disappearance of the Fe surface state, which is attributed to Au segregation to the surface. The initial hexagonal NP arrangement on the TiO2(110) surface was preserved up to 900°C. At 1000°C, Au desorption was observed.

Drastic Au(111) surface reconstruction upon insulin growth factor tripeptide adsorption.

Adsorption of biomolecules at metal surfaces often creates two-dimensional ordering of the adlayers. However, metal substrate reconstruction is less commonly observed, unless upon annealing of the molecule-surface system. Here, we report on the drastic room-temperature reconstruction of the Au(111) surface, driven by the adsorption of insulin growth factor tripeptide molecules. Scanning tunneling microscopy images show that the surface reconstruction, which takes place without annealing the system, is dynamic and evolves over time. It is initiated at kinks and steps edges, but the reconstruction also takes place within defect-free terraces. Theoretical calculations are performed to explain the reconstruction at the molecular level.

Structure and phonon density of states of supported size-selected F57eAu nanoclusters: A nuclear resonant inelastic x-ray scattering study

We have measured the phonon density of states (PDOS) of isolated bcc and fcc FexAu1−x alloy nanoclusters (NCs) by nuclear resonant inelastic x-ray scattering. Drastic deviations were observed with respect to the PDOS of bulk Fe–Au alloys. Important information on the structure and thermodynamic properties of these NCs was obtained.

Grafting and Thermal Stripping of Organo‐Bimetallic Clusters on Au Surfaces: Toward Controlled Co/Ru Aggregates

The controlled stoichiometry of heterometallic carbonyl clusters make them attractive precursors for the stabilization of bare metal alloy clusters for magnetic applications. The mixed-metal molecular cluster [RuCo3(H)(CO)12] has been functionalized with the phosphane-thiol ligand Ph2PCH2CH2SH to allow subsequent anchoring on a gold surface. The resulting tetrahedral cluster [RuCo3(H)(CO)11(Ph2PCH2CH2SH)] (1) has been characterized by X-ray diffraction and the P-monodentate ligand is axially bound to a cobalt center and trans to the ruthenium cap. This synthesis also yielded the product of oxidative coupling, in which two SH groups were coupled intermolecularly to give a disulfide ligand that links two tetrahedral cluster units in [{RuCo3(H)(CO)11(Ph2PCH2CH2S)}2] (2). This cluster has also been characterized by X-ray diffractions studies. After deposition of 1 on a Au(111) surface by self-assembly, the carbonyl ligands were stripped off by thermal annealing in ultra-high vacuum (UHV) to form a metallic species. X-ray photoelectron spectroscopic measurements performed as a function of the annealing temperature show that the cobalt and ruthenium centers converge towards metallic character and that the stoichiometry of the alloy is retained during the annealing process. Preliminary X-ray absorption spectroscopy (XAS) synchrotron experiments indicate that clusters 1 and 2 behave similarly, which is consistent with the retention of their tetrahedral units on the gold surface after transformation of the thiol function or breaking of the disulfide bond to form Au--S bonds, respectively, has occurred.