Investigating the Nature of Thiol Adsorption on Palladium: an Experimental and Theoretical Approach

G. CORTHEY; P. CARRO; ALDO A. RUBERT; G. BENITEZ; M. FONTICELLI; R.C. SALVAREZZA

Trieste

Workshop; Spring College on Computational Nanoscience.; 2010

Self-assembled monolayers (SAMs) of alkanethiols on metals have attracted considerable attention because of the possibility to control the physical chemistry of surfaces at molecular level. This control has made possible several innovative applications ranging from molecular electronics to catalysis.1,2 SAMs are easily formed by adsorption of thiols from solution. The self-assembly of alkanethiols on palladium is particularly interesting because the organic/metal interface formed involves a mixed layer containing both sulfides an thiols,3 much more complex than the thiol/gold interface. In this work, the composition and stability of alkanethiols adsorbed on palladium surfaces have been studied by electrochemical techniques, X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT). The adlayers have been prepared in liquid phase by immersion of the substrate in alkanethiols ethanolic solutions. Alkanethiols adsorbed on palladium surfaces lead to a complex interface composed of thiolate and sulfide, with surface coverage sulfide 0.4 and thiolate 0.30, as observed from the XPS spectra and in accordance with previously reported results.2 The adsorption of alkanethiols on a palladium adlayers, about 1.2 monolayers in thickness, deposited on Au(111) were also studied. These complex adlayers exhibit organic chainlength dependence barrier properties similar to those formed on gold and silver. On the other hand, these systems show an increased stability toward reductive desorption compared to alkanethiolate SAMs on silver and gold.4 Following the experimental data, we have performed a thermodynamic stability study of methanethiol and sulfide diluted layers on Pd(111), using density functional theory (DFT). We have found that as the chemical potential of the thiol in the gas phase is increased, the initially clean palladium surface is covered by a (3×3) R30sulfide lattice. Further increase in the pressure or concentration leads to the formation of (7×7)R19.1° sulfide lattice that exhibits a short stability range because it undergoes a phase transition to form a complex (7×7)R19.1° sulfide + thiol adlayer (3/7 sulfur + 2/7 thiol coverage). This phase transition is accompanied by a strong surface reconstruction of the Pd(111) surface. This surface structure consists of sulfur atoms and thiol-Pd adatom-thiol units similar to those recently proposed for thiols on gold.5-8 It is interesting to note that the chemical potential range to attain the (3×3)R30or the (7×7)R19.1° sulfide lattices is not experimentally accessible. It means that these phases would only be observed if they were kinetically trapped, but not under equilibrium conditions. (1) Love, J.; Estroff, L.; Kriebel, J.; Nuzzo, R.; Whitesides, G. Chem. Rev. 2005, 105, 1103-1170. (2) Gates, B. D.; Xu, Q.; Stewart, M.; Ryan, D.; Willson, C. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1171-1196. (3) Love, J.; Wolfe, D.; Haasch, R.; Chabinyc, M.; Paul, K.; Whitesides, G.; Nuzzo, R. J. Am. Chem. Soc. 2003, 125, 2597-2609. (4) Corthey, G.; Rubert, A. A.; Benitez, G. A.; Fonticelli, M. H.; Salvarezza, R. C. J. Phys. Chem. C 2009, 113, 6735-6742. (5) Jadzinsky, P. D.; Calero, G.; Ackerson, C. J.; Bushnell, D. A.; Kornberg, R. D. Science 2007, 318, 430-433. (6) Walter, M.; Akola, J.; Lopez-Acevedo, O.; Jadzinsky, P. D.; Calero, G.; Ackerson, C. J.; Whetten, R. L.; Grönbeck, H.; Häkkinen, H. Proc. Natl. Acad. Sci. U. S. A. 2008, 105, 9157-9162. (7) Jiang, D.; Tiago, M. L.; Luo, W.; Dai, S. J. Am. Chem. Soc. 2008, 130, 2777-2779. (8) Li, Y.; Galli, G.; Gygi, F. ACS Nano 2008, 2, 1896-1902