Boubakar Diawara - Academia.edu (original) (raw)

Papers by Boubakar Diawara

Applied Surface Science, 2020

This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Chemistry Africa, 2021

The adsorption mode of ammonia on the (001) surface of kaolinite was explored using the periodic ... more The adsorption mode of ammonia on the (001) surface of kaolinite was explored using the periodic DFT/PBE-D3 method. The thermodynamics and electronic properties such as adsorption energy, bond lengths and density of states were calculated to investigate the adsorption mechanism. Calculated adsorption energies were negative for ammonia-kaolinite complexes indicating a favorable exothermic process. The corrections of the van der Waals dispersion play an important role in stabilizing ammonia-kaolinite interactions. The maximum surface coverage is found to 4.3 molecules/nm2. The geometric parameters, Bader charges and PDOS analysis results showed that ammonia is adsorbed on the surface (001) of kaolinite clay by a hydrogen bond, mainly through the electrostatic interaction between the hydroxyls group of the surface and the nitrogen atom of NH3 molecule. The analysis of the computed infrared spectra of free and adsorbed NH3 highlights the impact of adsorption on its characteristic stretching and bending modes.

ECS Meeting Abstracts, 2007

not Available.

Adsorption, 2018

Density functional theory has been used to investigate the adsorption and dissociation of 5-hydro... more Density functional theory has been used to investigate the adsorption and dissociation of 5-hydroxy-3-hexanediol-1,4-naphthaquinone (Jug-C4-thiol) at a coverage of p(4 × 4) on a Au(111) surface. Both physisorption and chemisorption processes are investigated. For each process, the surface energy potential is explored by an exhaustive test of the adsorption site. The most favorable site is found to be face-centered cubic. The adsorption energies are less than − 0.20 eV in the case of physisorption, while they range from − 1.70 to − 1.92 eV for chemisorption. The effect of the naphthoquinone function is negligible in the adsorption process. The alkyl chain inhibits electron delocalization between the surface and the conjugated head of the molecule. Analysis of the bonding shows the formation of two ionocovalent bonds between the sulfur and the gold atoms. The thermodynamics and kinetics of S-H bond dissociation are studied. The results reveal that adsorption involves a dissociation path in which a hydrogen atom moves to the most neighboring site and a hydrogen atom migrates to its most stable site. The maximum barrier energy of the first step is less than 2.7 eV while the migration barrier does not exceed 0.5 eV.

Journal of Chemistry, 2017

Modeling the growth and failure of passive oxide films formed on stainless steels is of general i... more Modeling the growth and failure of passive oxide films formed on stainless steels is of general interest for the use of stainless steel as structural material and of special interest in the context of life time extension of light water reactors in nuclear power plants. Using the DFT+U approach, a theoretical investigation on the resistance to failure of the chromium-rich inner oxide layer formed at the surface of chromium-containing austenitic alloys (stainless steel and nickel based alloys) has been performed. The investigations were done for periodic bulk models. The data at the atomic scale were extrapolated by using the Universal Binding Energy Relationships (UBERs) model in order to estimate the mechanical behavior of a 10 μm thick oxide scale. The calculated stress values are in good agreement with experiments. Tensile stress for the bulk chromia was observed. The effects of temperature and structural defects on cracking were investigated. The possibility of cracking intensifi...

Passivation of Metals and Semiconductors, and Properties of Thin Oxide Layers, 2006

A 3D model has been developed to model the selective dissolution and passivation of alloys, and a... more A 3D model has been developed to model the selective dissolution and passivation of alloys, and apply to simulate the passivation of iron-chromium alloys. The real structure of the alloy is taken into account. The passivation is modelled by considering the formation of “oxide” nuclei, resulting from the presence on the surface of local chromium-rich clusters. The dynamic evolution is based on the Kinetic Monte Carlo (KMC) method allowing us to take into account realistic kinetic evolution of the system, with simulation time related to the real time. Using the Modified Embedded Atom Method (MEAM), we have calculated the activation energies for the dissolution and the surface diffusion steps. The calculated surface diffusion probabilities are found to be in agreement with the empirical values used previously. In particular they confirm that chromium preferentially diffuses toward the chromium clusters on the surface while iron atoms show no preferential diffusion. The simulation leads to kinetics of passivation with a KMC time of the order of a few seconds, corresponding to the initial stage of passivation. The simulation exhibits the qualitative evolution of the corroded/passivated surface (Cr content, surface roughness). For a series of Fe-xCr alloys (x in the range 15-22%), the results of the simulation confirm our previous finding that the transition from incomplete or no passivation to complete passivation is continuous.

RSC Adv., 2014

In this paper, we investigate the self-organization of NTCDI molecules on Au(111) surface by comb... more In this paper, we investigate the self-organization of NTCDI molecules on Au(111) surface by combining Density Functional Theory (DFT) and experiments based on scanning tunneling microscopy (STM) and infrared spectroscopy measurements.

The Journal of Physical Chemistry C, 2010

To simulate the passivation of FeCr alloys, we have developed a new model involving an explicit 3... more To simulate the passivation of FeCr alloys, we have developed a new model involving an explicit 3D model of the oxide layer that allows us to simulate the nucleation and the growth of the passive film. In the new model, the alloy is immersed in a virtual empty oxide lattice oriented with a given epitaxy. During the dynamic evolution, the metal cations generated by the oxidation of the alloy elements are injected into the virtual lattice where they are associated with oxygen ions coming from the solution, leading to formation of oxide nuclei, lateral growth of oxide islands, and an increase of the layer thickness. The dynamic evolution is based on the kinetic Monte Carlo (KMC) method. The KMC simulation takes into account the fundamental processes involved in the passivation mechanism: metal diffusion in the bulk and at the surface of the alloy, formation of metal cations and their injection in the oxide, nucleation and growth of the oxide layer, mass transfer through the oxide, and oxide dissolution at the oxide-solution interface. The activation energies related to the various processes are calculated using the modified embedded atom method potential or derived from experimental data. The electric field in the oxide film, considered in the new model, decreases or increases the activation energies depending on the positive or negative charge of the ions. The local value of the electric field is considered as inversely proportional to the local thickness of the oxide film. The simulations were carried out with a model of 25 Å × 25 Å × 25 Å (11 atomic planes). The results reproduce qualitatively well the experimental data. For low concentrations in Cr (<14%), the oxide nuclei do not cover completely the alloy surface and the metal is corroded. For high chromium content (>16%), the oxide layer grows, covers the whole surface, and reaches a stationary thickness of the order of 9 Å. In the intermediate zone (14-16%), the transition from incomplete or no passivation to complete passivation is continuous. The passive film is enriched with chromium. For alloys with low Cr content, extensive iron dissolution is required to obtain passivation. This leads to increased surface roughness. The oxidation process produces vacancies in the alloy that may form cavities at the oxide-metal interface or in the bulk of the alloy. For low chromium content, these cavities coalesce, leading to passivity breakdown and pit initiation.

The Journal of Physical Chemistry C, 2013

Melamine and naphthalene tetracarboxylic diimide (NTCDI) supramolecular assemblies have been elab... more Melamine and naphthalene tetracarboxylic diimide (NTCDI) supramolecular assemblies have been elaborated by a solution-based approach on Au(111) and studied by scanning tunneling microscopy (STM). We discuss the role of π-oligomers and H-oligomers in the mechanism of the network formation. Whereas the role of the first seems negligible, thermodynamical considerations establish the key role of the H-oligomers in the network extension in relation with the solvent nature. Density functional theory (DFT) calculations on the networks support the experimental structures. We discuss the influence and advantages of the solution method on the assemblies comparatively to ultrahigh vacuum (UHV) conditions. The NTCDI−melamine network exhibits a honeycomb structure which is obtained for the first time on the Au(111) substrate. The supramolecular mixed network is remarkably robust due to the stabilization from the triple hydrogen bonds established between the neighboring NTCDI and melamine molecules.

Langmuir, 2011

The adsorption of glycine, the building block of amino acids, on hydroxylated (0001)-Cr 2 O 3 mod... more The adsorption of glycine, the building block of amino acids, on hydroxylated (0001)-Cr 2 O 3 model surfaces, representing the stainless steel passive film surface, was modeled by means of the GGA þ U method. The roles of glycine coverage and surface termination (hydroxylated Cr-and O-terminated surfaces) on the adsorption mode and self-assembly properties were explored. The hydroxylated Cr-terminated Cr 2 O 3 surface, which presents two types of (H)OH groups exhibiting different acidic character, is more reactive than the hydroxylated O-terminated surface, where one single type of OH group is present, for all adsorption modes and coverages considered. Outer sphere adsorption occurs in the zwitterion form, stabilized at low coverage through H-bond formation with coadsorbed water molecules, and at the monolayer coverage by glycine self-assembling. The OH substitution by glycinate is favored on the hydroxylated Cr-terminated surface and not on the O-terminated one. The inclusion of dispersion forces does not change the observed tendencies. An atomistic thermodynamics approach suggests that outer sphere adsorption is thermodynamically favored over inner sphere adsorption in the whole domain of glycine concentration. The obtained SAM's free energies of formation are rationalized in a model considering the balance between sublimation and solvation free energies, and extrapolated to other amino acids, to predict the SAMs formation above hydroxylated surfaces. It is found that hydrophobic AA tend to selfassemble at the surface, whereas hydrophilic ones do not.

RSC Adv., 2015

In this paper, we use density functional theory with dispersion correction functional (DFT-D) as ... more In this paper, we use density functional theory with dispersion correction functional (DFT-D) as implemented in Vienna Ab Initio Simulation package in order to investigate hydrogen adsorption on graphane (GH) and fluorographene (GF). The adsorption sites at different surface coverage rates were studied to determine the most stable configurations. The comparison between the results obtained using standard pure DFT functionals and dispersion corrected ones; highlight the role of the dispersion effect in the adsorption energies and the orientation of the molecules relative to the surface. The coverage rate is found to increase up to 75% on the two sides, what makes these nanoporous materials, promising candidates for hydrogen storage. Electronic properties such as density of states and band structures were calculated on both GH and GF systems. It is observed that after H 2 adsorption the band gap of GH is only slightly modified, whereas the opposite trend is observed on GF.

The Journal of Physical Chemistry C, Jun 3, 2009

For the first time, voiding mechanisms resulting from the condensation of atomic vacancies inject... more For the first time, voiding mechanisms resulting from the condensation of atomic vacancies injected at oxide/ alloy interfaces by the growth of oxide layers have been studied by means of periodic density functional theory (DFT) calculations. Several interfaces were built by superimposing ultrathin films of alumina on the γ-TiAl(111) surface, and their relative stabilities were compared by calculating the interface energy variation. The formation energy of single Ti or Al vacancies and clustered and dispersed ensembles of 2Ti + 1Al or 2Al + 1Ti vacancies injected into the alloy were calculated. The results show that it is easier to inject the vacancies into the oxide/alloy interface than into the bare alloy surface and into the bulk alloy. The injected vacancies, trapped at the oxide/alloy interface, condense in the topmost plane of the alloy to form 2D clusters. The minimization of the coordination number of the vacancies with metal atoms of the alloy and O atoms of the overlaying oxide favors vacancy condensation and interfacial voiding. The data are relevant for a detailed understanding of the adherence and breakdown of protective oxide films and the lifetime of metallic materials.

The Journal of Physical Chemistry C, 2015

Defect and Diffusion Forum, 2012

The interaction of water molecules on a nickel surface was studied using ReaxFF (reactive force f... more The interaction of water molecules on a nickel surface was studied using ReaxFF (reactive force field) molecular dynamics. This approach was originally developed by van Duin et al. to study the hydrocarbon chemistry and the catalytic properties of organic compounds. To our knowledge, this method has not been used to study the corrosion processes of nickel exposed to water, which is what we set out to achieve in the present investigation. To do so, calculations were first performed using ReaxFF in order to reproduce certain well-known properties of pure nickel and nickel-water systems. This allowed us to study the adsorption of a single water molecule interacting with an optimized nickel surface. We also investigated the interaction of 405 molecules of water (ρ=0.99 g.cm -3 ) on the (100), (110) and (111) surfaces of a single crystal of nickel at 300 K. The results show that a water 'bilayer' is adsorbed on nickel surfaces: the first water layer is directly bonded to the surface, whereas the molecules in the first and second layers are held together by hydrogen bonds.