Physisorbed State Research Articles

Molecular dynamics simulation of O2 adsorption on Ag(110) from first principles electronic structure calculations.

We perform a detailed study of the static and dynamical properties of molecular oxygen adsorption on Ag(110) based on semi-local density functional theory (DFT) calculations and compare the results to experimental studies. For the classical dynamics calculations we use two complementary approaches, ab initio molecular dynamics and dynamics on a precalculated potential energy surface. In contrast to the molecular beam experiments, at low beam incidence energies we obtain high molecular adsorption probabilities that are related to the physisorption-like adsorption wells at the bridge sites of Ag(110). Semi-local DFT seems to overbind O2 in these wells. Based on our dynamics calculations we propose a model for adsorption in the chemisorption wells via initial adsorption in the bridge wells. In this model the measured low adsorption probabilities at low incidence energies are explained by the existence of energy barriers between the physisorption-like and chemisorption wells.

A pH dependent delivery of mesalazine from polymer coated and drug-loaded SBA-16 systems

SBA-16 silica was synthesized and modified by post-synthesis method with amino groups. Wet milling in acidic media was applied for loading of poorly soluble drug mesalazine (5-aminosalicylic acid — 5-ASA) in different drug/carrier ratios (1:1; 0.75:1; 0.5:1; 0.25:1). The parent and drug loaded mesoporous silicas were characterized by XRD, TEM, N2 physisorption, thermal analysis, FT-IR and solid state NMR spectroscopy. The drug loaded mesoporous systems were single-coated with Eudragit S or double-coated with Eudragit S and Eudragit RL. The polymer coating significantly modified the rate of mesalazine release from SBA-16NH2 materials. Applying the double coating method makes possible the sustained delivery of the drug in the intestinal area avoiding the burst release in the gastric fluid. The functionalized, polymer coated mesoporous system could be considered an appropriate oral delivery system for mesalazine. In addition, reduction of mesalazine cytotoxicity on epithelial cells could be achieved by its loading into mesoporous silica particles.

Theoretical investigation on coadsorption effect of O2 and H2O on Pt(111) surface

First-principles theory has been applied to study the coadsorption of O2 and H2O on Pt(1 1 1) surface. A series of coadsorption structures were considered in order to determine the most stable configuration. This study shows that the chemisorption state of H2O stabilizes the chemisorption state of O2, while the physisorption state of H2O does not affect the chemisorption state of O2. The adsorption reaction pathways for these structures were identified, and the chemisorption state of H2O was found to promote the O2 chemisorption, while the physisorption state of H2O hinders the O2 chemisorption. These results could shed light on the detailed interpretation of the O2 adsorption processes on Pt(1 1 1) surface.

Boron-Doped, Nitrogen-Doped, and Codoped Graphene on Cu(111): A DFT + vdW Study

The electronic properties of free-standing and Cu-supported pristine and boron-doped, nitrogen-doped, and codoped graphene have been studied by means of density functional theory (DFT) with the vdW-DF2C09x functional. The effects of substitutional chemical doping, metal support, lattice parameter strain, and their eventual interplay have been investigated. We find that only boron-doped graphene strongly interacts with the copper substrate, due to chemical bonds between the boron atom and the underlying metal. The binding energy and charge transfer from Cu are also highly enhanced compared to both pristine and nitrogen-doped supported graphene. The BN codoped system behaves similarly to pristine graphene with a weakly physisorbed state and a small charge transfer from Cu. However, the presence of the nonmetal dopants makes the codoped sheet extremely tunable for redox purposes, with the boron site acting as an electron acceptor and the nitrogen site as an electron donor.

Hindered rotational physisorption states of H2 on Ag(111) surfaces.

We have investigated the physisorption states of H2 on Ag(111) surfaces. To clarify the accurate adsorption properties of H2 on Ag(111), we performed first-principles calculations based on spin-polarized density functional theory (DFT) with the semiempirical DFT-D2 method and the newly-developed exchange functional with the non-local correlation functional vdW-DF2 (rev-vdW-DF2). We constructed exhaustive potential energy surfaces, and revealed that non-negligible out-of-plane potential anisotropy with a perpendicular orientation preference exists even for H2 physisorption on planar Ag(111), as predicted by previous results of resonance-enhanced multiphoton ionization spectroscopy and temperature-programmed desorption experiments. Therefore, the molecular rotational ground states of ortho-H2 split into two energy levels in the anisotropic potential. The obtained adsorption energy and the number of bound states, including the zero-point energies and the rotational energy shift, agree with diffractive and rotationally mediated selective adsorption scattering resonance measurements. The origin of the potential anisotropy on Ag(111) is a combination of the London dispersion interaction and the virtual transition of the metal electron to the unoccupied molecular state.

Mapping the site-specific potential energy landscape for chemisorbed and physisorbed aromatic molecules on the Si(1 1 1)-7 × 7 surface by time-lapse STM

We present a scanning tunnelling microscope study of site-specific thermal displacement (desorption or diffusion) of benzene, toluene, and chlorobenzene molecules on the Si(1 1 1)-7 × 7 surface. Through time-lapse STM imaging and automated image analysis we probe both the chemisorbed and the physisorbed states of these molecules. For the chemisorption to physisorption transition there are distinct site-specific variations in the measured rates, however their kinetic origin is ambiguous. There is also significant variation in the competing rates out of the physisorbed state into chemisorption at the various surface sites, which we attribute to differences in site-specific Arrhenius pre-factors. A prediction of the outcome of the competing rates and pre-factors for benzene over three hours matches experiment.

Concerted Thermal-Plus-Electronic Nonlocal Desorption of Chlorobenzene from Si(111)-7 × 7 in the STM.

The rate of desorption of chemisorbed chlorobenzene molecules from the Si(111)-7 × 7 surface, induced by nonlocal charge injection from an STM tip, depends on the surface temperature. Between 260 and 313 K, we find an Arrhenius thermal activation energy of 450 ± 170 meV, consistent with the binding energy of physisorbed chlorobenzene on the same surface. Injected electrons excite the chlorobenzene molecule from the chemisorption state to an intermediate physisorption state, followed by thermal desorption. We find a second thermal activation energy of 21 ± 4 meV in the lower temperature region between 77 and 260 K, assigned to surface phonon excitation.

Hydrogen binding energy of halogenated C40 cage: An intermediate between physisorption and chemisorption

In this work, the energetic of non hydrogenated and hydrogenated small mass fullerene cages with the view of assessing hydrogen storage capacity have been investigated. All calculations have been performed with the DFT as implemented within G03W package, using B3LYP exchange-functional and applying basis set 6-31G(d,p). Our calculations show that the C40 fullerene cage possesses high surface reactivity. For the first time, the hydrogen binding energy of halogenated C40 cage is calculated and it is found to be an intermediate between physisorption and chemisorption states. The 1H NMR, 13C NMR chemical shifts, FT-IR spectra and partial atomic charges have been performed for C40 and C60, C40H40 and C60H60 cages. In addition, the hydrogen storage capacity is found to be increased by doping the C40 cage with light elements. Hence halogenated C40 cage is considered to be good candidate for hydrogen storage.

Adsorption of CO Molecules on Si(001) at Room Temperature

Initial adsorption of CO molecules on Si(001) is investigated by using room-temperature (RT) scanning tunneling microscopy (STM) and density functional theory calculations. Theoretical calculations show that only one adsorption configuration of terminal-bound CO (T-CO) is stable and that the bridge-bound CO is unstable. All the abundantly observed STM features due to CO adsorption can be identified as differently configured T-COs. The initial sticking probability of CO molecules on Si(001) at RT is estimated to be as small as ∼1 × 10 −4 monolayer/ Langmuir, which is significantly increased at high-temperature adsorption experiments implying a finite activation barrier for adsorption. Thermal annealing at 900 K for 5 min results in the dissociation of the adsorbed CO molecules with the probability of 60−70% instead of desorption, indicating both a strong chemisorption state and an activated dissociation process. The unique adsorption state with a large binding energy, a tiny sticking probability, and a finite adsorption barrier is in stark contrast with the previous low-temperature (below 100 K) observations of a weak binding, a high sticking probability, and a barrierless adsorption. We speculate that the low-temperature results might be a signature of a physisorption state in the condensed phase.

New method for preparation of delivery systems of poorly soluble drugs on the basis of functionalized mesoporous MCM-41 nanoparticles

MCM-41 silica with spherical morphology and small particle sizes (100nm) was synthesized and modified by post-synthesis method with amino and/or carboxylic groups. Solid state reaction was applied for the first time for loading of poorly soluble drug mesalazine (5-aminosalicylic acid – 5-ASA). The non-loaded and drug loaded mesoporous silicas were characterized by XRD, TEM, N2 physisorption, elemental analysis, thermal analysis, FT-IR and solid state NMR spectroscopy. Quantum-chemical calculations were used to predict the interactions between the drug molecule and the functional groups of the carrier. The nanoparticles were post-coated with sodium alginate and the coating modified the rate of mesalazine release from MCM-41NH2 and MCM-41NH2COOH particles. Cytotoxic evaluation on colon adenocarcinoma cell line revealed that the alginate coating reduced cytotoxicity of mesalazine loaded in the post-coated particles compared to the pure mesalazine. The functionalized, polymer coated mesoporous systems are suitable oral drug delivery systems providing an opportunity to modify drug release.

Effect of Temperature on the Adsorption of Short Alkanes in the Zeolite SSZ-13—Adapting Adsorption Isotherms to Microporous Materials

Understanding the diffusion and adsorption of hydrocarbons in zeolites is a highly important topic in the field of catalysis in micro- and mesoporous materials. Especially, the properties of alkanes in zeolites have been studied extensively. A theoretical description of these processes is challenging, because two interactions are involved: the alkane physisorbs to the zeolite wall and chemisorbs weakly to the active centers. At room temperature, the alkane remains physisorbed almost all the time, but the chemical bond to the active sites is regularly broken. In this work, we study this behavior using ab initio molecular dynamics simulations for the adsorption of methane, ethane, and propane in SSZ-13, the zeolite with the smallest unit cell, at temperatures of 250, 275, 325, and 350 K. We find a temperature dependence of the adsorption energy and the probability of the alkane to be close to the active site, which corresponds to chemisorption. We derive a temperature-dependent expression for these probabilities or active site coverages, which have the energy difference between physisorbed and chemisorbed state as the main variable. The methodology derived in this work will be highly useful in correlating static electronic structure calculations to finite temperature coverages, which, following the Sabatier principle, is a key step to understand the performance of catalysts under reaction conditions and a prerequisite to computationally design such materials.

The different roles of Pu-oxide overlayers in the hydrogenation of Pu-metal: an ab initio molecular dynamics study based on van der Waals density functional (vdW-DF)+U.

Based on the non-local van der Waals density functional (vdW-DF)+U scheme, we carry out the ab initio molecular dynamics (AIMD) study of the interaction dynamics for H2 impingement against the stoichiometric PuO2(111), the reduced PuO2(111), and the stoichiometric α-Pu2O3(111) surfaces. The hydrogen molecular physisorption states, which cannot be captured by pure DFT+U method, are obtained by employing the vdW-DF+U scheme. We show that except for the weak physisorption, PuO 2(111) surfaces are so difficult of access that almost all of the H2 molecules will bounce back to the vacuum when their initial kinetic energies are not sufficient. Although the dissociative adsorption of H2 on PuO2(111) surfaces is found to be very exothermic, the collision-induced dissociation barriers of H2 are calculated to be as high as 3.2 eV and 2.0 eV for stoichiometric and reduced PuO2 surfaces, respectively. Unlike PuO2, our AIMD study directly reveals that the hydrogen molecules can penetrate into α-Pu2O3(111) surface and diffuse easily due to the 25% native O vacancies located along the ⟨111⟩ diagonals of α-Pu2O3 matrix. By examining the temperature effect and the internal vibrational excitations of H2, we provide a detailed insight into the interaction dynamics of H2 in α-Pu2O3. The optimum pathways for hydrogen penetration and diffusion, the corresponding energy barriers (1.0 eV and 0.53 eV, respectively) and rate constants are systematically calculated. Overall, our study fairly reveals the different interaction mechanisms between H2 and Pu-oxide surfaces, which have strong implications to the interpretation of experimental observations.

Theoretical modeling of surface functionalization of coronene by oxidation reactions with OH

In this work, we study the interaction between OH radicals with black carbon BC modeled by a coronene molecule by means of quantum chemistry calculations MOPAC, DeMon-nano, DeMon2K packages with the purpose of understand the aging process of this material. Results show that strong OH radical adsorption is preferred on border sites than on center ones, independently of the theoretical method employed. Results of potential energy curves for OH adsorption show that a chemisorption state may occur with small barrier slower than 3.2 kcal/mol. At the center site a physisorbed state may take place. Once OH chemisorption happens, a dipole moment is created and the hydrophobic coronene surface is transformed to hydrophilic. For that reason water molecules are stabilized in the hydroxylated coronene surface by O^{…}H interactions. Chemisorption of many OH radicals, especially at edge sites, produces coronene hydroxylation that after subsequent OH attacks would lead to H abstractions directly from coronene and from chemisorbed OH. The last feature yield a C-C bond breaking at the coronene edge and the formation of hydroxyl and carboxylic groups. Both reactions were confirmed with DFT calculations using coordinates obtained by PM6. Small reaction barriers in both processes were found. From these preliminary results, it is concluded that a detailed modeling of OH interactions with a coronene molecule may shed some light in the formation of volatile oxygenate compounds and therefore the BC aging in the atmosphere.

Tuning the Adsorption of Aromatic Molecules on Platinum via Halogenation

The interaction of aromatic molecules with metal surfaces is of key relevance for the functionality of molecular electronics and organic electronics devices. One way to control and tune the binding properties of molecules to metals is chemical functionalization. The adsorption of halogenated benzene molecules on the (111) surface of platinum is here investigated by density functional theory calculations with nonlocal van der Waals correlation functional. It is found that these systems exhibit a bistable adsorption energy profile with (meta)stable chemisorption and physisorption states separated by a potential energy barrier. The relative stability of these states can be tuned by functionalizing benzene with a different number or type of halogen atoms. Our results suggest a simple rational molecular design to achieve the desired interfacial binding in organic electronic devices and in composites with interfaces between large aromatic molecules and metals.

Dissociation rates of H2 on a Ni(100) surface: the role of the physisorbed state.

The dissociation and recombination rates of physisorbed H2, and the total dissociation rate of gas phase H2 on the rigid Ni(100) surface, as well as the corresponding kinetic isotope effects, are calculated by using the quantum instanton method, together with path integral Monte Carlo and adaptive umbrella sampling techniques. Both the dissociation and recombination rates of physisorbed H2 are dramatically enhanced by the quantum motions of H2 at low temperatures, for instance, the quantum rates are 43 and 7.5 times larger than the classical ones at 200 K, respectively. For the dissociation of gas phase H2, at high temperatures, the H2 can fly over the physisorbed state and dissociate directly, however, at low temperatures, the H2 is first physisorbed and then dissociates under steady state approximation. The total dissociation rate of gas phase H2 can be expressed as a combination of the direct and steady state dissociation rates. It has the form of an inverted bell with a minimum value at about 400 K, and detailed analysis shows that the dissociation of gas phase H2 is dominated by a steady state process below 400 K, however, both the steady state and direct processes are important above 400 K. The calculated kinetic isotope effects reveal that H2 always has larger rates than D2 no matter which dissociative process they undergo.

Theoretical assessment of graphene-metal contacts

Graphene-metal contacts have emerged as systems of paramount importance in the synthesis of high-quality and large-size patches of graphene and as vital components of nanotechnological devices. Herein, we study the accuracy of several density functional theory methods using van der Waals functionals or dispersive forces corrections when describing the attachment of graphene on Ni(111). Two different experimentally observed chemisorption states, top-fcc and bridge-top, were put under examination, together with the hcp-fcc physisorption state. Calculated geometric, energetic, and electronic properties were compared to experimental data. From the calculations, one finds that (i) predictions made by different methodologies differ significantly and (ii) optB86b-vdW functional and Grimme dispersion correction seem to provide the best balanced description of stability of physisorption and chemisorption states, the attachment strength of the latter on Ni(111) surface, the graphene-Ni(111) separation, and the bandstructure of chemisorbed graphene. The collation suggests that accurate and affordable theoretical studies on technologies based on graphene-metal contacts are already at hand.

Unraveling the Self-Assembly of the (S)-Glutamic Acid “Flower” Structure on Ag(100)

(S)-Glutamic acid adsorbed on Ag(100) organizes in different self-assembled structures depending on surface temperature [Smerieri, M.; Vattuone, L.; Kravchuk, T.; Costa, D.; Savio, L. (S)-Glutamic Acid on Ag(100): Self-Assembly in the Nonzwitterionic Form. Langmuir2011, 27, 2393-2404]. In particular, two of these structures, referred to as "square" and "flower" geometries, are found to coexist on the surface upon deposition at T = 350 K. The former assembly was fully resolved at the atomic level in the work of Smerieri et al. [Smerieri, M.; Vattuone, L.; Costa, D.; Tielens, F.; Savio, L. Self-Assembly of (S)-Glutamic Acid on Ag(100): A Combined LT-STM and Ab Initio Investigation. Langmuir2010, 26, 7208-7215], in which we proved that the driving force for adsorption is the van der Waals interactions between the molecules and the Ag surface, that is, that molecules are in a physisorbed state. In this paper, we complete our work by presenting the characterization of the "flower" structure. In contrast to the case of the "square" assembly, a strong chemical bond between glutamic acid radicals and the surface is at the basis of the "flowers" geometry. Whereas the chemisorbed central GLU tetramer interacts strongly with the surface, the physisorbed surrounding GLU molecules conserve some degree of freedom in the layer which counterbalances the weak adsorption energy. The "flower" and the "square" assemblies have similar dispersion energy and H-bond interaction energy; as a consequence of the different chemical state of the GLU molecules, however, such contributions have a very different relative weight in the stabilization of the two structures.

States Modulation in Graphene Nanoribbons through Metal Contacts

We are reporting the results of density functional calculations of the electronic structure of finite graphene nanoribbons adsorbed on Au, Pd, and Ti electrodes. While the interaction of nanoribbons with the Au contact is more characteristic of a physisorbed state, the adsorption of Pd and Ti involves much stronger state mixing as in chemisorption. Metal-induced gap states, which can potentially short-circuit the device, are clearly revealed for the first time, allowing us to evaluate their penetration length. The evanescence of MIGS is primarily governed by the band gap of the nanoribbon, and we can estimate an acceptable minimal length for an effective transport channel to a few nanometers. Different impacts of the presence of metal-induced gap states on the properties of graphene nanoribbons are discussed in terms of charge transfer and electrostatics.

Helium Diffraction Study of Low Coverage Phases of Mercaptoundecanol and Octadecanethiol Self-Assembled Monolayers on Au(111) Prepared by Supersonic Molecular Beam Deposition

Thiol self-assembled monolayers (SAMs) on Au(111) are ubiquitous systems for surface functionalization. While these systems have been studied extensively in the past there are still points that need further investigation both from a fundamental surface science point of view and an application point of view. For instance, SAMs of long chain thiols which are solid at room temperature have only been grown in solution due to their low vapor pressure. Hence, almost all the reported properties of these SAMs, such as growth mechanism, kinetics, and film structures, are based on solution-grown films. Here long chain thiol SAMs were grown in vacuum by using supersonic molecular beam (SMBD) deposition. This technique not only allows low vapor pressure thiols to be deposited in vacuum in a very controlled way, but it also enables control over the kinetic energies of the deposited molecules. Here we report a helium diffraction study of low coverage striped phases of mercaptoundecanol (MUD) and octadecanethiol (ODT) SAMs on the Au(111) surface. MUD SAMs were observed to form a (11.8 × √3) unit cell whereas ODT SAMs adopt a (18 × √3) unit cell. Both of the studied molecules were shown to form disordered physisorbed layers on the chemisorbed first layer, with desorption energies of 72 and 86 kJ/mol, respectively. While four desorption states could be identified for MUD with energies of 118, 136, 141, and 150 kJ/mol, ODT has one chemisorbed and one physisorbed state with desorption energies of 135 and 154 kJ/mol. The results reported here may serve as a reference for comparison of properties of solution-grown and vacuum-deposited thiol SAMs. In addition, the control over the kinetic energy may open up new possibilities in terms of applications that utilize patterned films.

Self-assembly mechanism of thiol, dithiol, dithiocarboxylic acid, disulfide and diselenide on gold: an electrochemical impedance study

The self-assembly mechanism of normal aliphatic thiol (RSH), disulfide (RSSR), diselenide (RSeSeR), dithiol (R(SH)2) and dithiocarboxylic acid (RS2H) onto a gold surface was studied in real time by electrochemical impedance spectroscopy (EIS). The different stages of adsorption could be clearly followed from the interfacial capacitance variation. An initial very fast adsorption, varying from a few seconds to several minutes depending on concentration, is the major adsorption step. This fast step is followed by long-term additional adsorption and self-assembled monolayer (SAM) consolidation. However, an intermediate step, probably due to transformation from the initial physisorbed state to the self-assembled state, could be identified with RSH and R(SH)2. An intermediate rearrangement of RS2H molecules after their initial diffusion controlled Langmuir (DCL) adsorption through the thiol functional group was also recognized. Initial adsorption of RSH and R(SH)2 followed either purely diffusion controlled or DCL kinetics for a very short time. Their continuing fast adsorption followed DCL kinetics. The fast adsorption step of RSSR and RSeSeR also followed the same mechanism. The findings made with EIS on the SAM organization were analyzed by polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). The R(SH)2 based SAMs had comparatively poor organization.