Adsorption and Structure of Octadecanethiol on Zinc Surfaces As Probed by Sum Frequency Generation Spectroscopy, Imaging, and Electrochemical Techniques Jonas Hedberg,* ,† Christofer Leygraf, † Katherine Cimatu, ‡ and Steven Baldelli ‡ Department of Chemistry, DiVision of Corrosion Science, Royal Institute of Technology, Drottning Kristinas V. 51, SE-100 44 Stockholm, Sweden, and Department of Chemistry, UniVersity of Houston, Houston, Texas 77204-5003 ReceiVed: July 6, 2007; In Final Form: September 7, 2007 Octadecanethiol (ODT) adsorbed onto zinc has been studied with sum frequency generation (SFG), sum frequency generation imaging microscopy (SFG-IM), X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), and electrical impedance spectroscopy (EIS) in order to investigate its corrosion protective ability and conformational ordering. SFG shows that ODT forms an ordered adsorbate on both reduced and oxidized zinc within short times after immersion in 1 mM ODT/ethanol solution. The corrosion protection, deduced by EIS, is also improved after immersion in the ODT solution. After longer immersion times, the corrosion protection decreases as well as the conformational order of the adsorbed ODT. Increasing the ODT concentration avoids this degradation with prolonged immersion time. The ODT is seen in the XPS spectra to adsorb to the reduced as well as the oxidized zinc by forming a Zn-S bond for both short and long immersion times. The SFG-IM completes the picture, showing a heterogeneous surface with areas corresponding to ordered ODT as well as disordered or uncovered regions. The density of adsorbed ODT after 24 h immersion time for both reduced and oxidized zinc was deduced from CV and was found to be approximately 6.7 × 10 -9 mol/cm 2 . 1. Introduction An important aspect of organic coatings on metals is the phase boundary between the metal and the coating. Adhesion and chemical bonds are of utmost importance, and the properties of these are closely related to interfacial properties, such as corrosion protection ability. In modern surface technology, it is desired to apply ultrathin corrosion protecting films on common metals, such as steel, galvanized steel, and aluminum, within very short timescales. A potential model for surface modifications of metals is an alkanethiol, CH 3 (CH 2 ) x SH, which consists of a long hydrocarbon chain with a sulfur head group and serves as a widely used molecule for model studies of self- assembled monolayers, with gold being the most common substrate. 1 Thiols adsorbed on oxidized metals have been studied, though not as extensively as gold, and include substrates such as tin, 2 copper, 3,4 silver, 5 iron, 6,7 and zinc. 8,9 Thiols adsorb on the metal surface by formation of a metal-sulfur bond. Thus, it is somewhat surprising that alkanethiols are able to form ordered monolayers on oxide surfaces. Presumably, the mechanism of adsorption is that of dissociation, where the oxygen and hydroxide groups are desorbed when the thiol is adsorbed to the metal atom. The fact that the thiol is mainly coordinated to the metal atoms in the oxide is suggested by X-ray photoelectron spectroscopy (XPS) data, whereby zinc terminated ZnO has been shown to adsorb ethanethiol and methanethiol dissociatively. 10-12 It has been shown with temperature programmed desorption that the thiol reduces the oxygen at the surface and forms a zinc- sulfur bond. O-polar ZnO, on the other hand, only adsorbs the thiols molecularly at room temperature. A further aspect of thiols has been the growth inhibition of ZnO nanoparticles, by adsorbing and effectively limiting further growth. 13,14 This study aims at exploring the adsorption of octadecanethiol (ODT, CH 3 (CH 2 ) 17 SH) on oxidized and reduced zinc surfaces with a multianalytical approach. Particular emphasis is on the structure of the adsorbate and how it relates to the corrosion protection ability. The techniques include molecular level investigations using sum frequency generation vibrational spectroscopy (SFG) and XPS and macroscopic methods such as electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). In addition, the use of SFG imaging provides spatially resolved spectroscopy that is useful in interpreting the homogeneity of the surface. 2. Experimental Section The outline of the experimental procedure is as follows: first, the electrode was polished and then transferred to the electro- chemical cell, where the electrochemical reduction was per- formed to remove residual oxidized species. If an oxidized substrate was to be studied, the oxidation was performed immediately following the reduction step. The immersion into the thiol solution followed next, and finally the electrode was transferred to the appropriate experiment (SFG, EIS, CV, or XPS), which was performed directly after the sample prepara- tion. The experiments were repeated at least three times. 2.1. Materials. A 7 mm diameter zinc rod (99.999%) was obtained from Goodfellow. The chemicals used were octade- canethiol (herein shortened ODT, CH 3 (CH 2 ) 17 SH, 98%, pur- chased from Aldrich), ethanol (99.5%, Prolab), acetone (99%, Alfa Aesar), NaOH (98%, Eka Nobel), LiCl (99.9%, Aldrich), and NaClO 4 (99%, Aldrich). * To whom correspondence should be addressed. E-mail: jhed@kth.se. Fax: +46 8 208284. † Royal Institute of Technology. ‡ University of Houston. 17587 J. Phys. Chem. C 2007, 111, 17587-17596 10.1021/jp075286+ CCC: $37.00 © 2007 American Chemical Society Published on Web 11/07/2007