Ahmed NAITABDI
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My research activities, within the surface nanostructure framework, are dedicated to the atomic-level understanding of fundamental properties of metallic nanoparticles and individual molecules on metal-oxide and semiconducting surfaces. In particular, I am engaged in the design and characterization of supported Gold- & Platinum-based nanoparticles relevant for heterogeneous catalysis and for potential applications in the new generation of hydrogen fuel cells.

Heterogeneous catalysis at the nanoscale is called to play a major role in the quest for new source of energy with drastic pollution abatement. In particular, tuning chemical reactions of nanocatalysts via the control of their surface composition, size and their investigation in operando conditions will lead to a major advancement towards this goal. We focus on bimetallic nanoparticles which continue to excite heterogeneous catalysis research, with the firm intent to reduce precious metal load in catalysts but without altering their optimum performances. We use innovative approaches to develop Au- and Pt-based supported nanoparticle catalysts

Another axis of my research interests focuses on the study of the adsorption mechanism of organic molecules on pristine and modified silicon (001) surfaces. The latter topic explores new possibilities to use the atomically-rich electronic-nanostructures in these inorganic-organic interfaces for the development of nanoscale devices.

We use state-of-the-art techniques to study the properties of these systems, real-time Scanning Tunneling Microscopy/Spectroscopy (STM-STS) and Near-Ambient Pressure X-ray Photoemission Spectroscopy (NAP-XPS) both operating in environmental conditions (under chemical reaction conditions). The combination of these two complementary techniques provides powerful investigation capabilities. In particular, STM investigations are particularly essential to achieve the structural understanding at the atomic and molecular levels of our supported nanostructures. XPS on the other hand provides information on the chemical surface composition and oxidation states of species during the reaction under reactive gases. This approach which correlates changes in the surface morphology with the underlying chemical states provides the opportunity to better understanding the catalysts performances and their related issues.

My research interests cover the following topics:

Supported bimetallic nanoparticles

This topic is dedicated to the rational design of efficient and strong nano-structured catalysts with view to potential applications in the new energy generation systems and environment. Our approach within the framework of transition-metal heterogeneous catalysis tackles major limitations inherent to these systems (sintering, deactivation, active sites poisoning…) on the one hand, and seeks novel nanoparticles with smart functionalities on the other hand. In particular, we design supported bimetallic nanoparticles with controlled size, shape and surface composition with drastic reduction of the precious metal loads. The use of more abundant but efficient materials and the possibility to tune the structure as a function of the desired chemical reaction and durability expectations represent an exquisite goal. Probing their properties at the atomic level will provide access to valuable insights into the understanding of central phenomena that govern the activity and selectivity of catalysts such as the structure/nature of active sites and catalyst-reactant interface. Ultimately, we will take advantage of these phenomena to provide long-life catalysts with enhanced activity and selectivity.

The electronic properties are investigated; charge transfer effect between nanoparticles and the TiO2(110) surface, work functions and band bending.

We investigate these systems near their realistic working conditions of pressure and temperature using state-of-the-art instruments in combination, STM and NAP-XPS both operating under reactive gases. This is particularly important due to the flexibility of materials under these conditions. It is now well-established that nanoparticles under reactive gases can undergo structural changes.


Figure 1: (a) STM image of small Pt clusters (apparent height ~ 6.5 Å) deposited on a reduced TiO2(110) surface after moderate thermal treatment. (b) In situ Ambient Pressure XPS (AP-XPS) Core-level Pt 4f spectra of Pt-based NPs on TiO2(110) measured in UHV first (as prepared) and under O2 exposure at a partial pressure of 1 mbar as a function of the temperature from RT up to 475 K.

Find out more http://link.springer.com/article/10.1007/s11244-015-0529-z

Synthesis and self-assembly of size-selected metallic nanoparticles on supports:

The size, shape and composition play an essential role in the reactivity of nanocatalysts. It is therefore of paramount interest to implement adequate synthesis methods to design efficient catalysts. We use standard physical deposition methods (E-beam) on patterned surfaces and diblock copolymers inverse micelles encapsulation-based methods to prepare size-selected metallic nanoparticles on a variety of catalytically-relevant supports.

Figure 2: "Scanning Electron Microscopy (a) and Atomic Force Microscopy (b) images of SiO2-supported size-selected Au and Pt-based nanoparticles prepared by the inverse micelles of block-copolymers method: (a) pure small Au nanoparticles (size: 4.5 nm). (b) bimetallic Pt-based nanoparticles (size: 5.5 nm")

Electronic properties of clean and water-passivated Si(001) surfaces

The Si(001) surface is one of the most studied system in surface science owing to its relevance in many technological applications and in the promising quantum and molecular electronics. One of the (001) ideal surface reconstructions consists in the apparent symmetric Si(001)-(2×1) at above a transition temperature. The evolution from the original (1×1) towards the apparent (2×1) structure, through the formation of surface Si-Si dimers, occurs so that the total surface energy is lowered. Further energy minimization occurs when silicon atoms dimerize on the surface and form dimer rows along the [110] direction. In addition, the dimers are buckled, one atom moving upward (Siu), while the other one moves downward (Sid). Above the transition temperature, the dimer buckling becomes dynamic, due to the fast, thermally induced, oscillations between up and down positions. Then, in STM images, the surface appears (2×1) reconstructed and the dimers look symmetric, as oscillations are faster than the time resolution of the STM acquisition.

Figure 3: (a) Filled-states STM image of a clean Si(001)-(2×1) surface. The apparent (2×1) structure exhibits symmetric dimer rows along the [110] direction. The bright protrusions are C-defects (induced by the dissociation of a water molecule on two adjacent dimers). The bright protrusions are related to the unreacted Si atoms. (b) shows the corresponding schematic model: note the Si-H and Si-OH moieties of the C-defect.

One of the most important and promising aspects in this system concerns isolated silicon dangling bonds (DBs), which behave as localized quantum states. We are investigating the use of these surface states as rich atomic-scale electronic structures for potential applications in nanoelectronics and quantum computing. Additionally, we use their chemical reactivity with π-bonded organic molecules to explore novel inorganic-organic interfaces for the growth of molecular nanostructures with unique properties.

The silicon isolated dangling bonds (DBs) are created on the water-passivated Si(001) surface. A H2O molecule dissociates spontaneously on Si(001) at room temperature almost without a barrier of energy. The dissociative adsorption of water results in the formation of couples of Si-H and Si-OH moieties located on the same dimer or on two adjacent Si-Si dimers. Their structure forms what is commonly called a c-defect. However, at water saturation the H2O-passivated Si(001) surface exhibits unreacted Si atoms which form the isolated dangling bonds. While on the hydrogen-passivated Si(001)-(2×1) surface, H-Si(001)-2×1, the DBs were fabricated artificially by tip-induced desorption, they form readily on the water-passivated Si(001) one, [noted (H, OH)-Si(001)-2×1], with reproducible surface densities.


Figure 4: Filled-state STM image of a water-saturated n-doped Si(001)-(2×1) surface. The bright protrusions are unreacted Si atoms: isolated silicon dangling bonds. The Si-OH and Si-H groups can be identified.

Find out more http://pubs.acs.org/doi/abs/10.1021/jp4077678?

Organic molecules on clean and water-passivated Si(001) surfaces

Molecular-level understanding of interactions between organic molecules and the Si(001) surface is essential to the design of well-controlled new interfaces with tuned electronic functionalities. The mechanism of adsorption of organic molecules on pristine Si(001) is studied. We use an original method based on real-time STM via a scanning-while-dosing approach. This procedure allows the live monitoring of events as they occur on the surface. The initial adsorption of molecules on transient states before equilibrium is now investigated using this procedure. The molecules can be monitored from the early stage of their adsorption to their inclusion on a saturated molecular domain. Therefore, the adsorption sites, the nature of the molecular bonding, the mobility and stability of molecules, and the role of defects could be inferred. For example we have investigated the case of a tertiary amine, the triethylamine, on Si(001). It provides an original view on the complexity of the adsorption mechanism of the amine-based molecules on the Si(001) surface. The final adsorption state depends on the coverage and on the presence of the gas phase. The TEA adsorbs on the downward Si atom via coordinate covalent bonding. In addition, a remarkable templating of the Si(001) surface was seen: formation of a stable c(4×2) structure at room temperature.

Figure 5: (a) high-resolution STM image of triethylamine molecules adsorbed on a Si(001)- (2×1) surface: note the formation of a c(4×2) structure after the adsorption due to the static buckling of Si dimers. (b) shows a schematic model of the STM image. Open circles are from large to small: Si up-dimer surface atom, Si down-dimer surface atom, Si 2nd layer atom, and Si 3rd layer atom.
 

Find out more
1 - http://pubs.acs.org/doi/abs/10.1021/jp303002c
2 - http://pubs.rsc.org/en/content/articlelanding/2016/cp/c6cp04350j#!divAbstract


The interactions between organic molecules and the water-passivated Si(001) surface are also important in this regard. For example, we have shown in the case of benzaldehyde, using a scanning-while-dosing approach, that it reacts preferentially with the silicon dangling bond and remains trapped on this site possibly as a radical molecular adduct. It illustrates remarkably the transfer of the radical from the silicon atom to the molecular adduct.


Figure 6: high resolution STM image of a water-passivated Si(001)-(2×1) surface featuring silicon dangling bonds before and after the adsorption of a single benzaldehyde molecule.

Find out more http://pubs.acs.org/doi/abs/10.1021/jp4077678