Surface analytical investigations of the electrochemical double layer on silver electrodes in alkaline media D. Lu¨tzenkirchen-Hecht a,b * and H.-H. Strehblow b a Institut fu¨r Physikalische Chemie und Elektrochemie, Heinrich-Heine-Universita¨t Du¨sseldorf, Universita¨tsstraße 1, 40225 Du¨sseldorf, Germany b Institut fu¨r Angewandte Physik, Heinrich-Heine-Universita¨t Du¨sseldorf, Universita¨tsstraße 1, 40225 Du¨sseldorf, Germany (Received in Newcastle 21 January 1998) Abstract—The electrochemical double layer on Ag in aqueous solutions was examined ex situ with X-ray photoelectron spectroscopy (XPS) and UV-photoelectron spectroscopy (UPS). The specimens were removed from the electrolyte with hydrophobic surfaces and under potential control. The potential depen- dent surface concentrations of the adsorbed anions, cations and the surface excess charge, as well as the amount of adsorbed water were determined. The results obtained for the double layer on Ag in 1 M NaClO 4 +0.1 M NaOH were compared with an acidic perchlorate solution and pure 1 M NaOH. We found evidence for a specific adsorption of hydroxide ions in the basic electrolyte; i.e. the OH  -surface con- centration has to be considered for a proper determination of the cationic excess charge and the potential of zero charge. The observed systematic XPS binding energy shifts of the adsorbate species with respect to the metal core levels and the measured work function changes suggest that the electrochemical adsorbates are not in electronic equilibrium with the metal. # 1998 Elsevier Science Ltd. All rights reserved Key words: electrochemical double layer, silver electrodes, electronic equilibrium. INTRODUCTION Compared to investigations of the electrochemical double layer on metals in acidic or neutral sol- utions, only a few investigations of metal electrodes in alkaline media have been published up to date. For example, combined surface analytical and elec- trochemical investigations of electrochemically adsorbed species on Cu [1] and Ag [2] revealed detailed information about the potential dependant composition of the double layer for dierent elec- trolytes. Electrochemical and photoelectrochemical experiments gave evidence for the competitive adsorption between OH  and Cl  on Cu in alkaline chloride solutions, resulting in Cl  -doped Cu-oxide layers at higher potentials [3]. Infrared spectroscopy [4] and surface enhanced raman scat- tering (SERS) investigations [5] proved the presence of OH  on Cu-electrodes over a wide potential range, even very negative to the potential of oxide formation. In addition, SERS showed that OH  adsorbs specifically on Au-, but not on Pt- or Rh- surfaces [6]. Electrochemical measurements, scan- ning tunneling microscopy (STM) and X-ray scat- tering experiments revealed that adsorbed hydroxyl ions substantially influence the atomic structure of metal surfaces, e.g. a surface reconstruction was observed for significantly lower potentials [7–9]. The dissociative oxidation of H 2 [10] as well as the electrooxidation of CO [11] is influenced by coad- sorbed OH  . In general, it is commonly assumed that the adsorption of hydroxyl ions is the initial step for the formation of anodic oxide layers [12– 14]. In addition, the coadsorption of anions might inhibit the corrosion of the metal [15]. On the other hand, passive layers can also be thinned by adsorbed halide ions like Cl  or Br  , resulting in pitting corrosion eects in a later stage [16]. Electrochimica Acta, Vol. 43, Nos 19–20, pp. 2957–2968, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0013–4686/98 $19.00 + 0.00 PII: S0013-4686(98)00036-X *Author to whom correspondence should be addressed. 2957