ARTICLE pubs.acs.org/JPCC
Role of Deprotonation and Cu Adatom Migration in Determining the Reaction Pathways of Oxalic Acid Adsorption on Cu(111) cija,|| C. Isvoranu,^ J. Schnadt,^ M. N. Faraggi,*,† C. Rogero,†,‡ A. Arnau,†,‡,§ M. Trelka,|| D. E # # r,O C. Marti-Gastaldo, E. Coronado, J. M. Gallego, R. Otero,r,[ and R. Mirandar,[ †
Donostia International Physics Center DIPC, P. Manuel de Lardizabal 4, 20018 San Sebastian, Spain Centro de Física de Materiales CFM-MPC, Centro Mixto (CSIC-UPV/EHU), San Sebastian, Spain § Departamento de Física de Materiales, Facultad de Química UPV/EHU, San Sebastian, Spain Departamento de Física de la Materia Condensada, Universidad Autonoma de Madrid, Cantoblanco, 28049-Madrid, Spain ^ Division of Synchrotron Radiation Research, Department of Physics, Lund University, Box 118, S-221 00 Lund, Sweden # Instituto de Ciencia Molecular (ICMol), Universidad de Valencia, Catedratico Jose Beltran 2, 46980 Paterna, Spain r Instituto Madrile~ no de Estudios Avanzados en Nanociencia (IMDEA-Nanociencia), Cantoblanco, 28049-Madrid, Spain O Instituto de Ciencia de Materiales de Madrid—CSIC, Cantoblanco, 28049-Madrid, Spain [ Departamento de Física de la Materia Condensada, Universidad Autonoma de Madrid, Cantoblanco, 28049-Madrid
)
‡
ABSTRACT: Scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and first principles theoretical calculations have been used to gain insight into the fundamental processes involved in the adsorption and self-assembly of oxalic acid on Cu(111). The experimental data demonstrate that several reaction pathways are involved in the chemisorption of oxalic acid on Cu(111), one of which leads to deprotonation of the acid into oxalate molecules that form ordered structures on the surface. Theoretical calculations indicate that the adsorption of oxalate molecules is not stable on the surface unless copper adatoms are taken into consideration. Coordination with copper adatoms prevents oxalate molecules from getting closer to the substrate, precluding the expected decomposition of oxalate into carbon dioxide. Our results, thus, suggest that the 2D gas of diffusing copper adatoms might play a very important role in the self-assembly of the molecules not only by catalyzing the deprotonation of oxalic acid but also by decreasing the surface reactivity.
’ INTRODUCTION The wide field of organic molecules on solid surfaces has become a promising area to understand and develop functional nanostructures in nanotechnology. The main interest in studying organic molecules on solid surfaces is the obtainment of precise information about the morphology and crystalline structures, which provide a direct way to control their electronic properties.15 In the last few years, many studies on metalorganic systems have been carried out in which metal atoms are codeposited with polytopic organic linkers and both elements self-assemble into ordered arrays held together by coordination interactions. In particular, much effort has been devoted to carboxylic acids on copper surfaces, in which deprotonation of the acid leads to the adsorption of carboxylate species, which are ready to coordinate to both diffusing copper adatoms or other deposited metal atoms.57 It was recently pointed out that copper adatoms might also catalyze the deprotonation of the carboxylic acid groups, thus providing the surface with dynamic heterogeneous sites.810 In this paper we investigate the chemisorption pathways and self-assembly of oxalic acid molecules (C2O4H2) on a Cu(111) r 2011 American Chemical Society
surface. After room temperature deposition, combined scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) experiments show the coexistence of different chemical species on the surface, such as formate ions, atomic carbon, and adsorbed oxalate molecules. The latter species, which turns out to be dominant at high coverage, self-assembles into wellordered structures. The observation of oxalate adsorbates on the surface is surprising, since neutral oxalate readily dissociates into two carbon dioxide molecules, and charged oxalate ions hardly can exist in contact with the electron reservoir provided by the metal surface. This problem is resolved by density functional theory (DFT) calculations including copper adatoms coordinated to the oxalate ions. Our calculations show that, in the absence of such adatoms, oxalate dissociates into CO2. Coordination with the adatoms allows for an adsorption geometry in which oxalate ions are placed farther away from the substrate Received: June 20, 2011 Revised: September 12, 2011 Published: September 19, 2011 21177
dx.doi.org/10.1021/jp205779g | J. Phys. Chem. C 2011, 115, 21177–21182
The Journal of Physical Chemistry C
ARTICLE
than they would be in the absence of the adatoms, allowing the molecule to retain its charged nature and preventing dissociation, in good agreement with the experimental results. Moreover, coordination with copper adatoms might explain the structure of the self-assembled layer observed by STM, as demonstrated by comparison between the experimental images and theoretically calculated ones.
’ EXPERIMENTAL AND THEORETICAL METHODS The STM experiments were carried out in a dedicated setup at the Universidad Autonoma de Madrid, and the XPS experiments were carried out at beamline I311 of the National Swedish Synchrotron Radiation Facility MAX-lab. Both experimental systems feature ultrahigh vacuum chambers with a base pressure of ∼2 1010 Torr. Atomically flat, crystalline Cu(111) surfaces were prepared by standard sputter/anneal procedures (sputter with 1 kV Ar+ ions for 15 min followed by annealing to 800 K for another 15 min), resulting in large terraces (∼ 200 nm wide) separated by monatomic steps. Both STM and XPS measurements were performed with the sample at low temperature (100 K). The STM chamber is equipped with a variable temperature “Aarhus”-type STM, purchased from SPECS. The molecules, acquired from commercial suppliers (Sigma-Aldrich), were deposited from a low-temperature Knudsen cell onto the clean Cu(111) substrate, which was held at room temperature. All STM images—experimental and theoretical—were analyzed using the WSxM software.11 The Cu(111) surface is modeled using a periodic supercell made of four Cu atomic planes and an additional vacuum equivalent to seven atomic layers in the ^z direction (∼15 Å), large enough to avoid residual interactions due to supercell periodic boundary conditions. The oxalic acid monomers were located in a 3 3 surface unit cell of Cu(111) with an in-plane lattice constant a0 = 2.57 Å. Density functional theory calculations were performed using the Vienna ab-initio simulation package VASP.12,13 The ionelectron interactions were described with the Projector Augmented-Wave (PAW) method.14 The generalized gradient approximation (GGA) was used for exchange and correlation.15 We used a kinetic energy cutoff of 400 eV in the plane wave expansion and an electronic convergence criterion of 105 eV for the energy value, while in the relaxation of atomic positions a criterion of 0.05 eV/Å was assumed for the atomic forces.
Figure 1. Experimental STM images for different coverages of oxalic acid. Current, voltages, and coverages correspond to (a) 0.23 nA, 1.25 V,
