ABSTRACT
The adsorption of pentacene on Cu (2 2 1), Cu (5 1 1) and Cu (9 1 1) is investigated using density functional theory (DFT) with the self-consistent inclusion of van der Waals (vdW) interactions. Cu (2 1 1) is a vicinal of Cu (1 1 1) while Cu (5 1 1) and (9 1 1) are vicinals of Cu (1 0 0). For all the three surfaces, we found pentacene to prefer to adsorb parallel to the surface and near the steps. The addition of vdW interactions resulted in an enhancement in adsorption energies, with reference to the PBE functional, of around 2 eV. With vdWs inclusion, the adsorption energies were found to be 2.98 eV, 3.20 eV and 3.49 eV for Cu (2 2 1), Cu (5 1 1) and Cu (9 1 1) respectively. These values reflect that pentacene adsorbs stronger on (1 0 0) terraces with a preference for larger terraces. The molecule tilts upon adsorption with a small tilt angle on the (1 0 0) vicinals (about a few degrees) as compared to a large one on Cu (2 2 1) where the tilt angle is found to be about 20°. We find that the adsorption results in a net charge transfer to the molecule of ~1 electron, for all surfaces.
ABSTRACT
The adsorption characteristics of the sexithiophene (6T) molecule on Ag(110) are studied using density functional theory with the inclusion of van der Waals (vdW) interactions. The stable adsorption configurations on 6T on Ag(110) as well as the nature of bonding the Ag substrate are evaluated. We also assess the performance of the vdW-DF method in describing the adsorption, energetics, heights, as well as the interface characteristics with the Ag(110) surface. We find two lowest adsorption energy configurations, at which the 6T molecule aligns with its molecular long axis parallel and perpendicular to the [001] direction, to be energetically close to each other, suggesting that they may coexist. Our findings indicate a significant increase in the 6T adsorption energies upon the inclusion of vdW interactions with the highest increase obtained using the opt-type functionals, in particular with the optB86b-vdW functional. The revPBE-vdW and rPW86-vdW2 functionals lead to less enhancement in adsorption energies that is attributed to the strong repulsive nature of these functionals, in agreement with earlier predictions. Upon adsorption of the 6T molecule, the changes in the atomic and electronic structures of the 6T molecule and Ag surface are found to be negligible; there is no charge transfer, and no interface state is observed. The work function is reduced upon adsorption with the largest change is ~0.6 eV obtained using the optB88-vdW functional. The results are in good agreement with the available experimental observations of the adsorption configurations and the work function changes. Based on our results, we conclude that the nature of bonding for 6T on Ag(110) can be classified as strong physisorption.
ABSTRACT
Gaining experimental insight into the intrinsic properties of nanoparticles (NPs) represents a scientific challenge due to the difficulty of deconvoluting these properties from various environmental effects such as the presence of adsorbates or a support. A synergistic combination of experimental and theoretical tools, including X-ray absorption fine-structure spectroscopy, scanning transmission electron microscopy, atomic force microscopy, and density functional theory was used in this study to investigate the structure and electronic properties of small (â¼1-4 nm) Au NPs synthesized by an inverse micelle encapsulation method. Metallic Au NPs encapsulated by polystyrene 2-vinylpiridine (PS-P2VP) were studied in the solution phase (dispersed in toluene) as well as after deposition on γ-Al2O3. Our experimental data revealed a size-dependent contraction of the interatomic distances of the ligand-protected NPs with decreasing NP size. These findings are in good agreement with the results from DFT calculations of unsupported Au NPs surrounded by P2VP, as well as those obtained for pure (ligand-free) Au clusters of analogous sizes. A comparison of the experimental and theoretical results supports the conclusion that the P2VP ligands employed to stabilize the gold NPs do not lead to strong distortions in the average interatomic spacing. The changes in the electronic structure of the Au-P2VP NPs were found to originate mainly from finite size effects and not from charge transfer between the NPs and their environment (e.g., Au-ligand interactions). In addition, the isolated ligand-protected experimental NPs only display a weak interaction with the support, making them an ideal model system for the investigation of size-dependent physical and chemical properties of structurally well-defined nanomaterials.
ABSTRACT
The stability of Pt nanoparticles (NPs) supported on ultrathin SiO(2) films on Si(111) was investigated in situ under H(2) and O(2) (0.5 Torr) by high-pressure X-ray photoelectron spectroscopy (HP-XPS) and ex situ by atomic force microscopy (AFM). No indication of sintering was observed up to 600 °C in both reducing and oxidizing environments for size-selected Pt NPs synthesized by inverse micelle encapsulation. However, HP-XPS revealed a competing effect of volatile PtO(x) desorption from the Pt NPs (~2 and ~4 nm NP sizes) at temperatures above 450 °C in the presence of 0.5 Torr of O(2). Under oxidizing conditions, the entire NPs were oxidized, although with no indication of a PtO(2) phase, with XPS binding energies better matching PtO. The stability of catalytic NPs in hydrogenation and oxidation reactions is of great importance due to the strong structure sensitivity observed in a number of catalytic processes of industrial relevance. An optimum must be found between the maximization of the surface active sites and metal loading (i.e., minimization of the NP size), combined with the maximization of their stability, which, as it will be shown here, is strongly dependent on the reaction environment.
