A molecular-dynamics simulation study of the adsorption and diffusion dynamics of short n-alkanes on Pt(111)

Abstract

Using molecular-dynamics studies and static potential-energy minimization, we have resolved the mechanisms by which n-alkanes (ethane through n-decane) diffuse on a model Pt(111) surface in the low-coverage limit of a single adsorbed molecule. Our simulations reproduce all of the experimental trends seen for the adsorption and diffusion of C3–C6 on Pt(111) and Ru(001). The short alkanes (C2–C8) behave as rigid rods and their motion involves coupled translation and rotation in the surface plane. For this series, there is a linear increase of the diffusion barrier with the molecular chain length. We have analyzed the compliance of the motion of the assumptions of a nearest-neighbor hopping model. Although hopping motion can be observed for all of the molecules at sufficiently low temperatures, the hopping involves a significant fraction of long jumps. As the temperature increases, the adsorption becomes virtually delocalized. Despite the extensive deviations of the motion from the assumptions of a nearest-neighbor hopping model, the static diffusion-energy barriers, arising from the minimum-energy path for hops between nearest-neighbor binding sites, agree well with those obtained from the tracer-diffusion coefficients for butane, hexane, and octane. For these molecules, multiple-site hops and long flights appear to influence the values of the preexponential factors, which are too large. Neither the diffusion barrier nor the preexponential factor for ethane agrees well with theoretical estimates. We attribute these discrepancies to the smallness of the static diffusion barrier and/or the existence of unique dynamical behavior for this molecule. Due to the increased difficulty of in-plane rotation and increased mismatch between the geometries of the molecule and the surface, the diffusion barrier for n-decane drops below that for n-hexane. The characteristic mechanism of motion for n-decane involves significant C–C–C bond-angle bending.

Two-Dimensional Crystals of Alkanes Formed on Au(111) Surface in Neat Liquid: Structural Investigation by Scanning Tunneling Microscopy

“…The preferential orientation of the molecular axis in the NN direction was consistent with the molecular dynamics simulation which showed that diffusion and adsorption along the NN direction were favorable on Pt(111). 23,24 The local structure of the 2D crystals of alkanes was investigated by STM on graphite in solution; the molecular arrangement was commensurate and the angle between the molecular row and the molecular axis was 90 in both odd 20,21 and even [14][15][16][17][18][19] alkane 2D crystals on the graphite. The molecular arrangement was commensurate on graphite, Pt(111), and Cu(111) but was incommensurate on Ag-(111) and Au(111) surface.…”

“…In addition, the physisorption of alkanes has been studied on Au(111), 6 Pt(111), 7,8 Pt(110), 9 Ir(110), 10 Cu(100), 11 Ru(001), 12 and Cu(111), 13 under UHV conditions. However, the local arrangement of the alkanes on a metal surface has not been investigated in contrast to numerous reports of alkane adlayers on graphite at the solid/liquid interface, [14][15][16][17][18][19][20][21] although studies using molecular dynamics 12,[22][23][24][25][26][27] and infrared spectroscopy [28][29][30][31][32] under UHV conditions have been carried out after the reports by Firment and Somorjai. Furthermore, the structure of the alkane/metal interface in solution was not investigated.…”

Surface diffusion of long chainlike molecules: The role of memory effects and stiffness on effective diffusion barriers

“…This is not a major problem either, however, since for long chain molecules the mismatch between the molecule and the substrate becomes so pronounced that the substrate potential felt by the chain molecule becomes less and less corrugated. 11,12,15,16 This is often the case in physisorption systems, and it has also been observed in cluster diffusion on a solid substrate. 38 The adsorption of segments of long chains is therefore delocalized, which in the present model is reflected in the fact that the lattice sites occupied by the segments do not correspond to real adsorption sites but are rather an effective representation of the chain motion on a plane.…”

“…Molecular dynamics simulations [7][8][9]11,12,16,17 and experiments [19][20][21][22] have revealed that diffusion of short chainlike molecules can often be described by the Arrhenius description. Unfortunately, however, the actual microscopic origins of the effective activation barrier are poorly understood.…”

“…This is mainly due to the complexity of these molecules, which gives rise to complex diffusion mechanisms that involve coupled translational and rotational degrees of freedom as well as a strong dependence on the molecular size, molecular shape, the coverage, and the mismatch between the molecule and the surface. [7][8][9][10][11][12][13][14][15][16][17][18] Most work done so far [7][8][9][10][11][12][13][14][15][16] has focused on the diffusion properties of short, single molecules. Nevertheless, it should be stressed that in various technologically important processes that involve thin films, such as lubrication and coating, one is interested in many-chain diffusion of long molecules, which is obviously a much more complicated issue.…”