Electrochemical Self-assembly of Alkanethiolate Molecules on Ni Surfaces

S. BENGIÓ; M. FONTICELLI; G. BENITEZ; E. GAYONE; H. ASCOLANI; J. MASSARUTTI; B. BLUM; G. ZAMPIERI; P. CARRO; R. SALVAREZZA

Centro Atómico Bariloche, Bariloche, Río Negro.

Congreso; Primer Encuentro de Física y Química (FyQS I); 2004

Centro Atómico Bariloche

Nickel exposed to atmospheric conditions is covered by a thin native oxide layer thereby inhibiting the adsorption of alcanethiol-molecules. This can be overcome by cleaning and adsorbing the alkanethiol (HSCnH2n+1) molecules in a controlled atmosphere. Traditionally this is performed in UHV conditions. In this work we present an alternative method to form alkanethiolate self-assembled monolayers on Ni. The method consists in reducing the NiO and chemisorbing the alkanethiol in a conventional electrochemical cell with control of the applied potential, E. The adsorption of alcanethiol-molecules on polycrystalline Ni and Ni(111) single crystal electrodes was carried out in 1 M NaOH + 5 mM HSCnH2n+1 (n = 3, 6, 12). The study of the adsorption/desorption process of alkanethiol on Ni is electrochemically problematic because the corresponding electrochemical peaks are masked by the occurrence of the hydrogen evolution reaction (HER) in the same potential region. Therefore this process was studied ex situ with Auger Electron Spectroscopy (AES) and X-Ray Photoelectron Spectroscopy (XPS). AES shows that alkanethiol electroadsorption proceeds only if the NiO is electroreduced (E < -1.1 V). The single component S 2p3/2 XPS binding energy of 162.5 eV at E £ -1.2 V reveals the presence of a predominant alkanethiolate species. The maximum alkanethiolate coverage increases as the hydrocarbon chain length is increased. Coverages consistent with a complete monolayer were obtained only for dodecanethiol (n = 12). XPS shows that there is a competition between the thin native NiO layer, Ni-SCnH2n+1, and different Ni-oxide species, possibly Ni(OH)2. This results in having almost exclusively NiO for E > -1.1 V, -SCnH2n+1 chemisorbed on metallic Ni for E @ -1.2 V, and different Ni-oxide species coexisting with the adsorbed alkanethiolates for E £ -1.3 V. DFT calculations were performed for alkanethiolate adsorption on metallic Ni as a function of E. Attempts at STM imaging of the Ni(111)-alkanethiol systems for E = -1.2 V are currently under way.