Published: August 05, 2011 r2011 American Chemical Society 17788 dx.doi.org/10.1021/jp201390m | J. Phys. Chem. C 2011, 115, 17788–17798 ARTICLE pubs.acs.org/JPCC Alkanethiol Adsorption on Platinum: Chain Length Effects on the Quality of Self-Assembled Monolayers María Alejandra Floridia Addato, † Aldo A. Rubert, † Guillermo A. Benítez, † Mariano H. Fonticelli,* ,† Javier Carrasco, ‡ Pilar Carro, § and Roberto C. Salvarezza † † Instituto de Investigaciones Fisicoquímicas, Te oricas y Aplicadas (INIFTA), Universidad Nacional de La Plata—CONICET, Sucursal 4 Casilla de Correo 16 (1900) La Plata, Argentina ‡ Instituto de Cat  alisis y Petroleoquímica, CSIC, Marie Curie 2, E-28049 Madrid, Spain § Departamento de Química Física, Instituto de Materiales y Nanotecnología, Universidad de La Laguna, Tenerife, Spain b S Supporting Information 1. INTRODUCTION Self-assembled monolayers (SAMs) of thiolates on metal surfaces have attracted considerable attention due to their importance in fundamental research and also as far as techno- logical applications are concerned. They have been used in sensing devices, in material protection, as resists and inks in lithography and as building elements in the development of supramolecular assemblies. SAMs on Au, Ag, and Cu have been extensively studied. For these systems, there is plenty of knowl- edge about the self-assembly in gas and liquid phases, the surface structures formed along this process, the different types of defects in the final state, and the chemical, thermal, and electro- chemical stability of the SAMs. In contrast to the vast literature of thiolate SAMs on the coinage metals, 1 the information regarding SAMs on Ni, Pd, and Pt is relatively scarce. The study of thiol self- assembly on these metals is difficult since they are excellent catalysts for many organic reactions and they also exhibit a great affinity for S atoms. Thus, different reaction pathways, involving molecule decomposition and S adsorption, could take place along with SAM formation. In the case of Pd it has been found that alkanethiol SAMs are formed on a diluted palladium sulfide adlayer, which is formed from the CS bond cleavage. 2,3 The sulfide overlayer passivates the Pd surface avoiding further thiolate decomposition and allowing alkanethiol SAM forma- tion. 4 In the case of Pt the situation is also complex, and contra- dictory results have been reported. While there is agreement about the presence of thiolate species on the Pt surface, other species such as alkyl disulfide, hydrocarbon chains and sulfides, 5 and partially oxidized alkylthiolate species 6 could also be present. The increasing interest in thiol SAMs on Pt 711,5,12 arises not only from the point of view of basic surface science but also as a matter of direct technological relevance. In this sense, these monolayers are also investigated because of their possible applications in molecular electronics as contact materials, 6 sensing systems, 13,14 lithography, 1518 catalysis, 19 and electrochemical and molecular electronic devices. 13,14 Furthermore, thiols are important as capping agents for the preparation of high-quality Pt nanoparti- cles. Regarding the last issue, it is well-known that both platinum metal and its alloys possess distinctive ability in catalyzing partial oxidation, hydrogenation and dehydrogenation of a variety of important molecules that are essential in many industrial processes. 20 Therefore, it is a key issue to understand the chemistry, structure, and stability of the alkanethiols SAMs on Pt. In this work the self-assembly of butanethiol (BT), hexa- nethiol (HT) and dodecanethiol (DT) on polycrystalline Pt substrates from ethanolic solutions and the quality of their SAMs have been investigated. XPS data show two main S 2p compo- nents at 162.3 and 163.4 eV. The barrier properties estimated by Received: February 11, 2011 Revised: July 29, 2011 ABSTRACT: The adsorption of butanethiol (BT), hexanethiol (HT), and dodecanethiol (DT) on Pt from ethanolic solutions has been studied by electrochemical techniques, X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. XPS data show two main S 2p components at 162.3 and 163.4 eV. The barrier properties estimated by double- layer capacitance and the blockage of the electron transfer from a redox couple in solution are markedly improved for self-assembled monolayers (SAMs) of the longer DT molecule. While the behavior of DT monolayers on Pt is comparable to that found for those grown on Au, HT and BT SAMs on Pt are more defective, are less blocking, and have a slightly lower thiolate coverage than their Au counterparts. The chain length dependent quality of these SAMs is explained based on DFT and thermodynamics calculations. It is demonstrated that a lying down (LD) surface structure dominates the stability diagram for the shorter chain thiols, while the standing up (SU) phase is more important for DT. We propose that the poor quality observed for SAMs of short chain thiolates results from an easier CS bond scission of the thioalkyl radical in a LD configuration.