Computer modeling of dissociative gas adsorption on laser-roughened surfaces

A simple computer model of dissociative adsorption of diatomic molecules on a solid surface with laser-induced defects was proposed. The defects (ablation craters) were assumed to have either cubicoid or pyramidal shape, depending on the approximation level. Special attention was paid to the influence of a degree of structural disorder on the adsorptive properties of the surface. In particular, both equilibrium adsorption isotherms and temperature programmed desorption (TPD) spectra of non-interacting diatomic molecules from the surfaces subjected to a different number of laser pulses were simulated. The observed changes in the adsorptive properties of the surface were explained using simple geometric arguments linking the adsorption probability for a single molecule with the topography of the surface. For example, it was demonstrated that, for a sufficiently large number of laser pulses (N), the adsorption probability scales with , regardless of the assumed crater shape. The obtained results also indicate that, in general, the surface roughness greatly affects the TPD spectra while it has minor influence on the shape of the adsorption isotherms.     

Adsorption of nicotine and tar from the mainstream smoke of cigarettes by oxidized carbon nanotubes

The adsorption of nicotine and tar from the mainstream smoke (MS) by the filter tips filled respectively with oxidized carbon nanotubes (O-CNTs), activated carbon and zeolite (NaY) has been investigated. O-CNTs show exceptional removal efficiency and their adsorption mechanism is investigated. Capillary condensation of some ingredients from MS in the inner hole of O-CNTs is observed and may be the primary reason for their superior removal efficiency. The effect of O-CNTs mass on the removal efficiencies is also studied and the results show that about 20-30 mg O-CNTs per cigarette can effectively remove most of nicotine and tar.     

Preparation and recognition of surface molecularly imprinted core-shell microbeads for protein in aqueous solutions

In this paper, a surface molecular imprinting technique was reported for preparing core-shell microbeads of protein imprinting, and bovine hemoglobin or bovine serum albumin were used as model proteins for studying the imprinted core-shell microbeads. 3-Aminophenylboronic acid (APBA) was polymerized onto the surface of polystyrene microbead in the presence of the protein templates to create protein-imprinted core-shell microbeads. The various samples were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and Brunauer-Emmett-Teller (BET) methods. The effect of pH on rebinding of the template hemoglobin, the specific binding and selective recognition were studied for the imprinted microbeads. The results show that the bovine hemoglobin-imprinted core-shell microbeads were successfully created. The shell was a sort of imprinted thin films with porous structure and larger surface areas. The imprinted microbeads have good selectivity for templates and high stability. Due to the recognition sites locating at or closing to the surface, these imprinted microbeads have good property of mass-transport. Unfortunately, the imprint technology was not successfully applied to imprinting bovine serum albumin (BSA).     

Excess surface work—A modelless way of getting surface energies and specific surface areas directly from sorption isotherms

Sorption isotherms can be easily transformed into excess surface work (ESW) isotherms, computed as the product of the adsorbed amount and the change in chemical potential. Plotted against the amount adsorbed at least one minimum is yield. Thermodynamically ESW is the sum of the surface free energy and the isobaric isothermal work of sorption. Therefore, ESW is not a model, instead it is just another way of presenting an isotherm. From the amount adsorbed in the first minimum one can obtain a specific surface area similar to the BET surface area. The depth of the ESW in the minimum gives a sorption energy, which corresponds approximately to the loss of degrees of freedom of the sorptive. In this contribution the ESW plots of various sorption isotherms on highly ordered alumina with cylindrical pores of 25 nm width and mesoporous SBA 15 will be presented and discussed.     

Modeling of adsorption and phase diagrams for stepped surfaces: Transfer matrix approach

The simplest model of stepped surface has been constructed and scrutinized with different values of model parameters. The transfer matrix method was shown to be a very effective approach. Phase diagrams and local isotherms have been obtained. It was shown that the local coverage can be the non-monotonous function of the total coverage or the chemical potential.     

Monolayer adsorption of water on NaCl(1 0 0)

Density functional theory calculations have been applied to investigate the adsorption geometry of water overlayers on the NaCl(1 0 0) surface in the monolayer regime. Competition between H-H intermolecular repulsion and the attraction of the polar molecules to the surface ions results in the most stable structure having a 2 × 1 adsorption symmetry with an adsorption energy of 415 meV. Overlayers of 1 × 1 symmetry, as observed in experiment, have slightly lower adsorption energies. The layers are also unstable with respect to rotation of individual molecules. Multiple hydrogens/oxygens interacting with a single substrate ion can pull that ion out of the surface, although the examples considered are energetically very unfavourable. Overlayers of 1 × 1 symmetry with a coverage of one water molecule per NaCl do not have a high enough adsorption energy to wet the surface.     

Separation of chiral molecules by temperature programmed desorption

The Monte Carlo simulation method was used to model thermal desorption of a pair of enantiomers from a solid surface with a chiral periodic pattern of active sites. The main objective of the study was to determine the optimal number of the active sites and their spatial distribution within the unit cell of the surface to achieve the most efficient separation of the enantiomers. For that purpose we tested the series of chiral patterns which were found previously for the equilibrium adsorption. Temperature programmed desorption spectra were calculated using a square lattice of adsorption sites in which the active sites were distributed spatially according to the candidate patterns. Additionally, influence of relaxation of the adsorbed layer on the relative shift of the TPD peaks of the enantiomers was assessed and the key factors affecting the chiral separation were identified.     

Isothermal adsorption kinetics on heterogeneous surfaces

Adsorption kinetics on energetically heterogeneous surfaces under isothermal conditions is analyzed using the uniform energy distribution model. Considering the quasi-equilibrium of surface diffusion between the adsorption sites with different energy, the kinetic equations dΘ/dt=(k_ap−A_dK_diff)(1−Θ) for first-order adsorption and dΘ/dt=k_ap(1−Θ)²−A_dK_diffΘ(1−Θ) for dissociative adsorption are obtained, where K_diff is a coefficient describing the surface diffusion equilibrium, which depends on the coverage and the energy distribution. Under isochoric conditions with p decreasing due to adsorption, surface diffusion accelerates the rate towards equilibrium significantly, as observed in static calorimetric adsorption experiments. An approximate solution in Lagergren form is derived for this condition.     

Adsorption mode of the chiral modifier cinchonidine on Au(1 1 1)

The adsorption of the chiral modifier cinchonidine on Au(l 1 1) in UHV has been studied by means of TPD, LEED and XPS. In the monolayer the molecule is bound via nitrogen lone pair electrons of its quinoline part rather than via the π-system of this aromatic moiety. Intact molecular desorption is only observed for the multilayers. Decomposition in the first monolayer upon heating occurs above 400 K, indicating a stronger interaction in the monolayer. No long-range ordered structures were observed via LEED. Long-time exposure leads to rearrangement and further stabilization of the first molecular monolayer. Quinoline is bound to gold via the nitrogen lone pair as well. The binding energy of 9.6 kcal/mol is characteristic for physisorption.     

The structural formation of methylthiolate SAMs on Au(1 1 1) for short deposition times from solution

Scanning tunnelling microscopy was used to investigate the structural formation of methylthiolate self-assembled monolayers on Au(1 1 1). SAMs were prepared by exposing the gold crystal to an ethanol-dimethyldisulfide solution for immersion times of 12 min, 12 h, and 24 h. After preparation the formation of a () rect. striped phase was found. For this phase, the immersion time is the key parameter determining the size and number of ordered domains. Annealing induced a phase transition leading to large domains in a (3×4) structure. The annealing temperature determines whether a mixed phase of both structures or only the (3×4) structure are formed. We find no influence of the immersion time on the formation of the second structure. A structure model is presented for both phases on the basis on the same building block containing two methylthiolate molecules and a gold ad-atom.     

Ruthenium adsorption and diffusion on the GaN(0 0 0 1) surface

We report first principles calculations to analyze the ruthenium adsorption and diffusion on GaN(0 0 0 1) surface in a 2×2geometry. The calculations were performed using the generalized gradient approximation (GGA) with ultrasoft pseudopotential within the density functional theory (DFT). The surface is modeled using the repeated slabs approach. To study the most favorable ruthenium adsorption model we considered T₁, T₄ and H₃ special sites. We find that the most energetically favorable structure corresponds to the Ru- T₄ model or the ruthenium adatom located at the T₄ site, while the ruthenium adsorption on top of a gallium atom (T₁ position) is totally unfavorable. The ruthenium diffusion on surface shows an energy barrier of 0.612 eV. The resultant reconstruction of the ruthenium adsorption on GaN(0 0 0 1)- 2×2 surface presents a lateral relaxation of some hundredth of Å in the most stable site. The comparison of the density of states and band structure of the GaN(0 0 0 1) surface without ruthenium adatom and with ruthenium adatom is analyzed in detail.     

Growth of copper phthalocyanine on hydrogen passivated vicinal silicon(1 1 1) surfaces

Using ultra-high vacuum scanning tunneling microscopy (UHV-STM), we show that copper-phthalocyanine (CuPc) grows in a well ordered manner on hydrogen passivated vicinal silicon surfaces. CuPc grows one-dimensionally parallel to the monatomic steps on the vicinal silicon surface. Surprisingly, elongated clusters of the CuPc parallel to the step directions are formed even on the middle of the terraces well away from the step edges. The one-dimensional growth mode continues even after the full monolayer coverage on the substrate which results in strongly oriented growth mode of a thin film of CuPc on the vicinal silicon surfaces.     

Light induced adsorption of Si nano-composites in LiF crystals at 157 nm

Si nano-composites were precipitated on LiF crystals following ablation from Si targets with laser light at 157 nm. The LiF/Si interface was analyzed with scanning electron microscopy, atomic force microscopy and energy dispersive X-ray microanalysis. It was found that Si composites were strongly attached to LiF ionic sites to form inhomogeneous structures consisted of small isotropic crystals 0.1-1 μm long, rich in Si and fluorine, which eventually further agglomerate to form larger structures. The thickness of the LiF/Si interface was increased from 50 nm to 2 μm following laser irradiation at 157 nm, due to accelerated adsorption of Si in the LiF interface by VUV light.     

Study of structural irregularities of smectite clay systems by small-angle neutron scattering and adsorption

Small angle neutron scattering (SANS) and its contrast-matching variant are employed in order to determine structural properties (inter-pillar distances and mass/surface fractal dimensions of the clay layers and pillars) of a series of smectite natural clays (montmorillonite, beidellite, and bentonite) and their pillared and pillared/ion-exchanged analogues. Moreover, a comparative analysis with the adsorption data is carried out on the basis of a systematic study of the structural changes induced by a particular treatment or modification (e.g. pillaring) of the clay systems.     

Modelling of phase transitions and reaction at CO adsorption on oxygen precovered Pd(1 1 1)

Using the interaction parameters up to the third neighbors and activated form of O and CO diffusion and their reaction, the model has been proposed for Monte-Carlo simulations describing the catalytic O + CO → CO₂ reaction and occurring phase transitions on Pd(1 1 1) surface. Upon adsorption of CO the pre-adsorbed oxygen transforms from p(2 × 2)_O phase into and phases in the limit of room and moderate temperatures, respectively. We demonstrate that the kinetic effects determine both the occurrence of the p(2 × 1)_O and disappearance of the phases at moderate and low temperatures, respectively. Using reaction rate as a fit parameter, we show that at room temperature the start of the reaction can be synchronized with the occurrence of phase.     

The study on magnetite particles coated with bilayer surfactants

Magnetite particles were prepared by co-precipitation, then sodium oleic (SO) and sodium dodecyl benzene sulfonate (SDBS) were applied as inner and outer surfactants, respectively. IR and TG were used to study the surface adsorption of SO and SDBS on magnetite particles. The experimental results demonstrated that SO molecules were linked to the magnetite particles through chemical bond and SDBS coated on the surface of magnetite particles covered with SO by means of Van der Waals attraction. Furthermore, based on the adsorption isotherms of surfactants on the magnetite particles and the dependence of Zeta potential of particles on the surfactants concentrations, the adsorption mechanisms of these two surfactants on the magnetite particles were studied. The isotherm adsorption model for SO on magnetite particles showed excellent correlation to Langmuir type and the adsorption equation was (25 °C), while that for SDBS on magnetite particles coated with SO showed excellent consistence with Freundlich type and the adsorption equation was Γ = 0.32c^0.475 (25 °C). In addition, the results demonstrated that both SO and SDBS formed monolayer adsorption on the surface of magnetite particles.     

Adsorption performance and mechanism of 2,4,6-trinitrotoluene on a novel adsorption material polyvinylbenzyl acid/SiO2

Polyvinylbenzene (PVB, namely polystyrene, PSt) was grafted on the surface of silica gel particles by “grafting from” in solution polymerization system, and grafting particles PVB/SiO₂ were obtained. The chloromethylation reaction of the grafted polyvinylbenzene was performed using a novel chloromethylation reagent, 1,4-bis (chloromethyoxy) butane that is un-carcinogenic, and grafting particles CMPVB/SiO₂ were obtained. Subsequently, chloromethyl groups on grafting particles CMPVB/SiO₂ were hydrolyzed and oxidized, and finally adsorption material polyvinylbenzyl acid/SiO₂ (PVBA/SiO₂) was prepared. The adsorption performances and mechanism of 2,4,6-trinitrotoluene (TNT) on PVBA/SiO₂ were investigated through static methods. The experimental results showed that PVBA/SiO₂ possessed strong adsorption ability for TNT with adsorption amount of 26.84 mg g⁻¹. The empirical Freundlich isotherm was also found to agree well with the equilibrium adsorption data. In addition, pH was found to have great influence on the adsorption amount. Finally, PVBA/SiO₂ was observed to possess excellent reusability as well.     

Active oxidation of Cu3Au(1 1 0) using hyperthermal O2 molecular beam

Oxidation of Cu₃Au(1 1 0) using a hyperthermal O₂ molecular beam (HOMB) was investigated by X-ray photoemission spectroscopy in conjunction with a synchrotron light source. From the incident energy dependence of the O-uptake curve, the precursor-mediated dissociative adsorption occurs, where the trapped O₂ molecule can migrate and dissociate at the lower activation-barrier sites, dominantly at thermal O₂ exposures. Dissociative adsorption of O₂ on Cu₃Au(1 1 0) is as effective at the thermal O₂ exposure as on Cu(1 1 0). On the other hand, at the incident energies of HOMB where the direct dissociative adsorption is dominant, it was determined that the dissociative adsorption of O₂ implies a higher activation barrier and therefore less reactivity due to the Au alloying in comparison with the HOMB oxidation of Cu(1 1 0). The dissociative adsorption progresses with the Cu segregation on Cu₃Au(1 1 0) similarly as on Cu₃Au(1 0 0). The growth of Cu₂O for 2 eV HOMB suggests that the diffusion of Cu atoms also contribute to the oxidation process through the open face, which makes the difference from Cu₃Au(1 0 0).     

Influence of poly acrylic acid on the dispersion of calcite nano-particles

Dispersive calcite (CaCO₃) nano-particles with a primary particle size of about 100 nm and an average agglomerate size of about 2.8 μm were synthesized via carbonation in the presence of poly acrylic acid (PAA). The experimental results showed that PAA was liable to be adsorbed on the calcite surface, leading to the decrease of the agglomeration size from 8.7 μm to 2.8 μm and the zeta potential from −8.5 mV to −28.6 mV. The deformation and adsorption behaviors of PAA on the typical planes of calcite were studied by the molecular simulation method, using DISCOVER model and the COMPASS force field. The simulation results indicated that PAA was easy to be deformed and adsorbed on the calcite planes owing to mainly the coulomb interaction as well as the possible formation of CaO and hydrogen bonds between PAA and calcite. The adsorption tendency of PAA on the CaCO₃ planes was as follows based on the values of the corresponding interaction energies: (2 0 2) > (1 1 6) > (1 1 3) > (0 1 8) > (1 1 0) > (1 0 4).     

Vibrational dynamics of fullerene molecules adsorbed on metal surfaces studied with synchrotron infrared radiation

Infrared (IR) spectroscopy of chemisorbed C₆₀ on Ag (111), Au (110) and Cu (100) reveals that a non-IR-active mode becomes active upon adsorption, and that its frequency shifts proportionally with the charge transferred from the metal to the molecule by about 5 cm^-1 per electron. The temperature dependence of the frequency and the width of this IR feature have also been followed for C₆₀/Cu (100) and were found to agree well with a weak anharmonic coupling (dephasing) to a low-frequency mode, which we suggest to be the frustrated translational mode of the adsorbed molecules. Additionally, the adsorption is accompanied by a broadband reflectance change, which is interpreted as due to the scattering of conduction electrons of the metal surface by the adsorbate. The reflectance change allows determination of the friction coefficient of the C₆₀ molecules, which results in rather small values (∼2×10⁹ s^-1 for Ag and Au, and ∼1.6×10⁹ s^-1for Cu), consistent with a marked metallic character of the adsorbed molecules. Pre-dosing of alkali atoms onto the metal substrates drastically changes the IR spectra recorded during subsequent C₆₀ deposition: anti-absorption bands, as well as an increase of the broadband reflectance, occur and are interpreted as due to strong electron–phonon coupling with induced surface states. Received: 6 June 2001 / Accepted: 23 October 2001 / Published online: 3 April 2002