Surface Adsorption Diffusion Research Articles

Surface Diffusion on Ag(100) and Cu(100) Electrodes in the Presence of Low Coverage Halides

Atomic-scale processes at the electrochemical interface are of great importance to not only the fundamental research of electrochemistry, but also the newly developing technologies including energy conversion and bottom-up nanofabrication. In particular, the surface-active additive, such as halides, can strongly affect these processes, hence change the micro or macro behaviors of the studied system. For example, adding halides to the electrolyte is a feasible way to change the shape of the synthesized nanoparticles [1]. Besides, the modification of catalytic activity and selectivity by halide-containing electrolyte in CO2 electrochemical reduction on Cu surface has also been reported [2]. Therefore, a detailed and quantitative study of the electrochemical interface processes in the presence of co-adsorbed halides species may pave the way for more efficient design of electrochemistry-related procedures. Surface diffusion of adsorbates on the electrode is one of these processes, playing an important role e.g. in electrocatalysis, and electrochemical metal deposition or dissolution. Video-rate scanning tunneling microscope (Video-STM) in an electrochemical environment enables the recording of real-time videos of atomic-scale motion on electrode surfaces, which makes the observation and quantification of surface diffusion of individual adsorbed atoms and molecules possible [3].Halides form strongly adsorbed c(2x2) structures on various metal surfaces at the positive end of the double layer range. Our previous studies revealed a significant potential dependence of the surface diffusion of sulfur on Cu(100) and Ag(100) electrodes in the presence of close-packed c(2x2) halide adlayers [3,4]. In this range, sulfur atoms diffuse by hopping between neighboring sites of the c(2x2) lattice. The mobility of the atoms exponentially decreases with increasing potential, following a linear relationship between the potential and the diffusion barrier. More specifically, for a given system, the change of the diffusion barrier with the potential can be attributed to the change in the surface dipole moment along the diffusion path. Further studies demonstrated that the potential-dependence is independent of the diffusing species, which means that the co-adsorbed halides may dominate the change of the dipole moment.Here, instead of the c(2x2) potential range, the surface diffusion is studied in the negative potential range of Br-covered Ag(100), where Br forms a highly mobile lattice gas on the surface and the coverage increases with potential. Sulfur diffusion via hopping between neighboring sites of the (1x1) Ag substrate lattice was observed. Surprisingly, the diffusion rates increase with increasing potential, suggesting a different diffusion mechanism in the presence of low coverage halides. However, there is a wide potential gap between (1x1) and c(2x2) range, where quantitative Video-STM studies are difficult due to the high mobility of sulfur atoms at room temperature. Therefore, we also collected diffusion data at low temperature, which show the same potential dependence as the room temperature data, apart from a reduced mobility. These experiments allowed to show that the crossover between increasing and decreasing diffusion rates with potential indeed occurs at the phase transition between the disordered and the c(2x2) ordered Br adlayer, resulting in a maximum diffusion rate near the onset of Br adlayer ordering.In addition, the diffusion of CH3Sad on c(2x2)-Brad covered Cu(100) was studied to test the hypothesis that the inverted potential dependence (the diffusion rates increase with increasing potential in the ordered coverage range) of Sad diffusion on this surface is attributed to a Br-induced exchange diffusion [5]. The methyl group should inhibit the exchange of sulfur with a Cu surface atom and thus suppress an exchange mechanism for steric reasons. We therefore expect that the diffusion of CH3Sad on c(2x2)-Brad covered Cu(100) shows the same potential dependence as the diffusion of Sad and CH3Sad on c(2x2)-Clad covered Cu(100) [6-7], that is, a decrease of the diffusion rate with increasing potential.We acknowledge financial support by the Deutsche Forschungsgemeinschaft via project no. 504552981.

Catalytic site ensembles: A context to reexamine the Langmuir-Hinshelwood kinetic description

The Langmuir-Hinshelwood formalism describes catalytic reactions of Langmuirian surface species under the assumption that all adsorbates are randomly-distributed – enabling adjacency of surface-bound intermediates to be determined solely by coverages of single-site occupants. We demonstrate herein that this approximation is inappropriate even for simple catalytic reactions (e.g. A + A → A2) and manifestly neglects islanding of slowly-consumed species and partitioning of highly-reactive species inherently engendered by ≥ two-site elementary steps (e.g. A*–A* → A2(g) + *–*). Rigorous description of kinetically-consequential islanding/partitioning phenomena requires explicit description of the coverage and chemical dynamics of all multi-site ensembles. Higher-order, ensemble-specific rate terms identify the particular microscopic events relevant to each ensemble, and, in doing so, reveal that each elementary step (e.g. A(g) adsorption) describes not one event (e.g. A(g) + * → A*) , but a sum over all ensemble-specific paths (e.g. A(g) + *–* → A*–* and A(g) + A*–* → A*–A*). De-convoluting each elementary step into its constituent multi-site paths proffers kinetic detail otherwise inaccessible – enabling (i) identification of rate- and selectivity-determining site ensembles, (ii) calculation of rates and degrees of rate control of ensemble-specific elementary steps, (iii) incorporation of adsorbate surface diffusion, (iv) incorporation of lateral adsorbate interactions, and (v) quantitative description of catalysis of multi-site-occupying intermediates (e.g. *CnHm* species in hydrocarbon (de-)hydrogenation and C-C bond coupling/cleavage reactions) which we demonstrate is inaccessible to the Langmuir-Hinshelwood formalism even if adsorbate surface diffusion is infinitely-fast.

Coulomb blockade in field electron emission from carbon nanotubes

We report the observation of Coulomb blockade in electron field emission (FE) from single-wall carbon nanotubes (SWCNTs), which is manifested as pronounced steps in the FE current–voltage curves and oscillatory variations in the energy distribution of emitted electrons. The appearance of the Coulomb blockade is explained by the formation of nanoscale protrusions at the apexes of SWCNTs due to the electric field-assisted surface diffusion of adsorbates and carbon adatoms. The proposed adsorbate-assisted FE mechanism is substantially different from the well-known resonant tunneling associated with discrete electronic states of adsorbed atoms. The simulations based on the Coulomb blockade theory are in excellent agreement with the experimental results. The SWCNT field emitters controlled by the Coulomb blockade effect are expected to be used to develop on-demand coherent single-electron sources for advanced vacuum nanoelectronic devices.

The Boundary of Alkali Surface Boundary Exospheres of Mercury and the Moon

AbstractGlobal exosphere models of alkali gases surrounding Mercury and the Moon assume that the primary effect of the porous soil is to reduce the effective desorption rates. We demonstrate with a kinetic simulation that, following adsorption, the complicated structure of soils has two additional effects on the fate of previously released alkali atoms: (1) trapping of free atoms at lunar temperatures by microscopic shadows and inward diffusion, which becomes the primary sink mechanism, and (2) high‐energy barriers for thermal desorption compared to what would be retrieved from experiments on thin films or compacted pellets, especially when surface diffusion of adsorbates is considered. Lunar soils retain one fifth to two thirds of recycled adsorbates, depending on the assumed adsorbate mobility, photodesorption cross section, and soil thermal gradient. A transition from a retentive surface to full outgassing at T > 500 K will produce complex feedback mechanisms of alkali circulation at Mercury.

Atomic-Scale Insights into Electrode Surface Dynamics by High-Speed Scanning Probe Microscopy.

Atomic-scale processes at electrode surfaces in liquid electrolytes are central elemental steps of electrochemical reactions. Detailed insights into the structure of these interfaces can be obtained with in situ scanning tunnelling and atomic force microscopy. By increasing the time resolution of these methods into the millisecond range, highly dynamic processes at electrode surfaces become directly observable. This review gives an overview of in situ studies with video-rate scanning probe microscopy techniques. Firstly, quantitative investigations into the dynamic behaviour of individual adsorbed atoms and molecules are described. These reveal a complex dependence of adsorbate surface diffusion on potential and co-adsorbed species and provide data on adsorbate-adsorbate and adsorbate-substrate interactions in a liquid environment. Secondly, results on collective dynamic phenomena are discussed, such as molecular self-assembly, the dynamics of nanoscale structures, nucleation and growth, and surface restructuring due to phase-formation processes.

Surface diffusion of cyclic hydrocarbons on nickel

Surface diffusion of adsorbates is difficult to measure on realistic systems, yet it is of fundamental interest in catalysis and coating reactions. quasielastic neutron scattering (QENS) was used to investigate the diffusion of cyclohexane and benzene adsorbed on a nickel metal sponge catalyst. Molecular dynamics simulations of benzene on a model (111) nickel surface showed localised motion with diffusion by intermittent jumps. The experimental data was therefore fitted to the Singwi–Sjölander model and activation energies for diffusion of 4.0 kJ mol−1 for benzene and 4.3 kJ mol−1 for cyclohexane were calculated for the two dimensional model. Limited motion out-of plane was seen in the dynamics simulations and is discussed, although the resolution of the scattering experiment is insufficient to quantify this. Good agreement is seen between the use of a perfect crystal as a model for a disordered system over short time scales, suggesting that simple models are adequate to describe diffusion over polycrystalline metal surfaces on the timescale of QENS measurement.

Comparison of two models of the dimer–dimer reaction on composite catalyst

In this paper two mathematical models of the dimer–dimer reaction on supported catalysts are compared. The PDE model describe the bulk diffusion of both reactants and product, adsorption and desorption, the surface diffusion of adsorbates and intermediate. The ODE model is derived from the first, assuming that all materials are well-mixed. The catalytic reactivity, calculated using the PDE and ODE models, is compared for two variants of reactants adsorption and variuos bulk and surface diffusion.

Experimental and analytical investigation of adsorption effects on shale gas transport in organic nanopores

Gas production from shale gas reservoirs is determined by original gas-in-place and gas transport mechanism in shale. Gas storage in shale consists of three different states: first, free gas in pores and natural fractures; second, adsorbed gas on the organic and inorganic pore walls; and third, absorbed or dissolved gas into formation water and organic matter. On the other hand, gas transport in shale may be dominated by noncontinuum flow in nanopores, which depends on gas slippage, bulk diffusion, Knudsen diffusion and adsorbed surface diffusion. In this study, a gas velocity model involving all of these parameters is proposed to describe the gas adsorption and Non-Darcy flow effects on gas transport behavior in shale gas reservoirs. Adsorption capacity of six shale core samples is measured by the gravimetric method using magnetic suspension sorption system. Regression analysis of the measured data has been performed using the Simplified Local-Density model coupled with modified Peng-Robinson Equation of State (SLD-PR). The presence of the moisture content is taken into consideration to distinguish between gas adsorption on the surface of organic matter and clay minerals. The hydrophilic feature of clay minerals and hydrophobic feature of organic matter result in water molecules preferring adsorbing on the surface of clays, while gas molecules preferring adsorbing on the surface of organic matter. In the developed velocity model, adsorbed gas is treated as mobile gas molecules adsorbing on the surface of organic nanopores, which moves by means of adsorbed surface diffusion. The surface diffusion velocity is characterized by adsorbed surface diffusion coefficient. Simultaneously, bulk gas is governed by second-order gas slippage, which is calculated using Navier-Stokes (N-S) equation associated with second-order slip boundary condition. Finally, the adsorbed surface diffusion is incorporated into bulk gas flow using Langmuir slip model to account for the total gas flow in the organic nanopores. The results of this study indicate that adsorbed surface diffusion contributes to the enhancement or impediment of total gas flow velocity in organic nanopores. Therefore, properly handling the adsorption effect on gas transport in organic nanopores is critical to correctly understand gas production from shale gas reservoirs.

Temperature-dependent gas transport performance of vertically aligned carbon nanotube/parylene composite membranes.

A novel composite membrane consisting of vertically aligned carbon nanotubes (CNTs) and parylene was successfully fabricated. Seamless filling of the spaces in CNT forests with parylene was achieved by a low-pressure chemical vapor deposition (CVD) technique and followed with the Ar/O2 plasma etching to expose CNT tips. Transport properties of various gases through the CNT/parylene membranes were explored. And gas permeances were independent on feed pressure in accordance with the Knudsen model, but the permeance values were over 60 times higher than that predicted by the Knudsen diffusion kinetics, which was attributed to specular momentum reflection inside smooth CNT pores. Gas permeances and enhancement factors over the Knudsen model firstly increased and then decreased with rising temperature, which confirmed the existence of non-Knudsen transport. And surface adsorption diffusion could affect the gas permeance at relatively low temperature. The gas permeance of the CNT/parylene composite membrane could be improved by optimizing operating temperature.

Intramolecular Insight into Adsorbate–Substrate Interactions via Low-Temperature, Ultrahigh-Vacuum Tip-Enhanced Raman Spectroscopy

Tip-enhanced Raman spectroscopy (TERS) provides chemical information for adsorbates with nanoscale spatial resolution, single-molecule sensitivity, and, when combined with scanning tunneling microscopy (STM), Ångstrom-scale topographic resolution. Performing TERS under ultrahigh-vacuum conditions allows pristine and atomically smooth surfaces to be maintained, while liquid He cooling minimizes surface diffusion of adsorbates across the solid surface, allowing direct STM imaging. Low-temperature TER (LT-TER) spectra differ from room-temperature TER (RT-TER), RT surface-enhanced Raman (SER), and LT-SER spectra because the vibrational lines are narrowed and shifted, revealing additional chemical information about adsorbate-substrate interactions. As an example, we present LT-TER spectra for the rhodamine 6G (R6G)/Ag(111) system that exhibit such unique spectral shifts. The high spectral resolution of LT-TERS provides intramolecular insight in that the shifted modes are associated with the ethylamine moiety of R6G. LT-TERS is a promising approach for unraveling the intricacies of adsorbate-substrate interactions that are inaccessible by other means.

Adsorbate surface diffusion: The role of incoherent tunneling in light particle motion

The role of incoherent tunneling in the diffusion of light atoms on surfaces is investigated. With this purpose, a Chudley–Elliot master equation constrained to nearest neighbors is considered within the Grabert–Weiss approach to quantum diffusion in periodic lattices. This model is applied to recent measurements of atomic H and D on Pt(111), rendering friction coefficients that are in the range of those available in the literature for other species of adsorbates. A simple extension of the model has also been considered to evaluate the relationship between coverage and tunneling and therefore the feasibility of the approach. An increase of the tunneling rate has been observed as the surface coverage decreases.

Effects of Adsorbate Surface Diffusion in Focused Electron-Beam-Induced-Deposition

The time-dependent continuum model is applied to simulate a growth of structures with a non-rotational symmetry. The surface diffusion of adsorbates, depending on the beam parameters and the deposit size, may lead to the different growth rate along deposited structures. Here, a few conventional patterning strategies have been studied: spiral inwards, spiral outwards and zigzag.

Quantitative studies of adsorbate dynamics at noble metal electrodes by in situ Video-STM

The surface diffusion of adsorbates at electrochemical interfaces is studied by in situ scanning tunneling microscopy with high temporal resolution, using sulfur and methyl thiolate on c(2 × 2) Cl covered Cu(100), Ag(100), and Au(100) electrode surfaces in 0.01 M HCl solution as an example. While on Au(100) quantitative studies were not possible because of the slow dynamics and high surface defect density, on Cu(100) and Ag(100) a pronounced exponential increase of the jump rates of isolated adsorbates toward more negative potentials was found, indicating a linear decrease of the tracer diffusion barriers with potential. The potential dependence is independent of the adsorbate species, but differs for Cu(100) and Ag(100) substrates. These trends can be explained by electrostatic contributions to the diffusion barrier, caused by the interaction of the adsorbates with the field of the electrochemical double layer, if the presence of the chloride coadsorbate layer is taken into account.

Reaction-diffusion processes in the “adsorbate-substrate” system

We obtain a chain of quantum kinetic reaction-diffusion-type equations for “adsorbate-substrate” system taking into account the coupling of a light particle with a metallic surface, the adsorbate surface diffusion by the tunnelling mechanism and the occurrence of bimolecular chemical reactions. We calculate the temperature dependence of the kinetic kernels related to the diffusion coefficients and the reaction rates. It is shown that one can alter the temperature dependence of the reaction rates by changing the “adsorbate-substrate” coupling. It is also shown that the mean field terms contribute to the activation energies of the reaction rates, while their contribution to the activation energies of the diffusion coefficients vanishes.

Effects of Adsorbate Surface Diffusion in Focused Electron-Beam-Induced-Deposition

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Vertically-aligned carbon nanotube membranes for hydrogen separation

Vertically-aligned carbon nanotube membranes have been fabricated and characterized and the corresponding gas permeability and hydrogen separation were measured. The carbon nanotube diameter and areal density were adjusted by varying the catalyst vapour concentration (Fe/C ratio) in the mixed precursor. The permeances are one to two magnitudes higher than the Knudsen prediction, while the gas selectivities are still in the Knudsen range. The diameter and areal density effects were studied and compared, the temperature dependence of permeation is also discussed. The results confirm the existence of non-Knudsen transport and that surface adsorption diffusion may affect the total permeance at relative low temperature. The permeance of aligned carbon nanotube membranes can be improved by increasing areal density and operating at an optimum temperature.

Adsorbate localization versus diffusion: a coherent time dependent theory of coupling to gravitons in hidden large dimensions

A long standing unsolved puzzle is the very small rate of surface diffusion of ad-sorbates in the low temperature STM in the regime of quantum diffusion, differing by orders of magnitude from the results of modern quantum theories of surface diffusion (QTSD) based on ab initio potential energy surfaces (refs. [1]-[5]). We investigate whether decoherence mechanisms, neglected in QTSD, can shed some light on these discrepancies. A quantum-mechanical theory is presented treating the entanglement of adsorbates with their environment (phonons, electron-hole pairs, gravitons) [6]. Time dependent wave packet evolution including the environmental modes indicates possible paths towards an explanation of the observed discrepancies and gives an insight into phenomenological descriptions like adsorbate localization, quantum Zeno effect (permenant "measurement" by specific environmental excitations) and wave function collapse.

Kinetic Model of Hydrogen Evolution at an Array of Au-Supported Catalyst Nanoparticles

A model is presented for studying the electrocatalytic activity at an array of catalyst nanoparticles that is deposited on a catalytically inactive support material. The objective is to rationalize effects on apparent reactivity of sizes and packing densities of nanoparticles. In the current version, the focus of the model is on the contribution of the spillover effect of adsorbed hydrogen to the overall rate of the hydrogen evolution reaction. For nanoparticles of fixed size, the current density per catalyst surface area exhibits a peculiar maximum as a function of the surface particle density; the optimum particle density varies with the rate of spillover of adsorbed hydrogen from particle to support and with adsorbate surface diffusion. For fixed particle density, the current density per catalyst surface area increases strongly with decreasing particle size. The model was used to fit experimental current densities for different catalyst particle coverage at varying electrode potentials. A good agreement between experimental data and calculated results was found. The fits provide key parameters of surface processes on the catalyst/support system.

Influence of Re-adsorption and Surface Heterogeneity on the Microkinetic Analysis of Temperature-Programmed Desorption Experiments

A profound microkinetic analysis of the influence of re-adsorption and of surface energetic heterogeneity on temperature-programmed desorption (TPD) experiments is performed on the basis of the Wigner−Polanyi equation. Exact solutions for TPD experiments under both re-adsorption-free and re-adsorption-dominated conditions from energetically homogeneous surfaces are presented. TPD experiments from energetically heterogeneous surfaces with uniform energy distribution are analyzed considering the surface diffusion of adsorbates. Convenient mathematical approaches are derived to evaluate the TPD profiles for first-order and second-order desorption. It is shown that, when the heterogeneity is strong and the coverage is not close to 0 or 1, the slope in the plot of ln(Tp/β) versus 1/(RTp) yields the differential heat of adsorption for freely occurring re-adsorption at ϑ = ϑp. In the absence of re-adsorption, the differential desorption energy at ϑ = ϑp can be derived from the slope. Two case studies show that the thermodynamic parameters, especially the surface energy distribution, can be obtained successfully from TPD experiments based on the derived methods.

Kinetic model for surface reconstruction.

A microscopic kinetic model for the alpha <==> beta [e.g., hex <==> 1x1 for Pt(100) and 1x2 <==> 1x1 for Pt(110)] surface reconstruction is investigated by means of the mean field approximation and Monte Carlo simulations. It considers homogeneous phase nucleation that induces small surface phase defects. These defects can grow or decline via phase border propagation in dependence on the chemical coverage by an adsorbate A (CO). An asymmetry in the adsorbate surface diffusion from one surface phase to the other gives rise to two critical coverages that determine the intervals of stability of the homogeneous alpha phase, the dynamically stable heterogeneous state, and the homogeneous beta phase. Both surfaces show a very similar qualitative behavior regarding the phase transitions that are of second order in both cases. As a result the experimentally observed nonlinear island growth rate and the critical coverages can be explained at a quantitative level.