Measurement of the surface charge density of CO-saturated Pt(1 1 1) electrodes as a function of potential: the potential of zero charge of Pt(1 1 1) Angel Cuesta * Instituto de Quı ´mica Fı ´sica ‘‘Rocasolano’’, CSIC, C. Serrano, 119, E-28006 Madrid, Spain Received 18 June 2004; accepted for publication 2 August 2004 Available online 7 September 2004 Abstract From measurements of the charge flowing upon immersion, at controlled potential, of a CO-covered Pt(1 1 1) elec- trode in a 0.1 M HClO 4 solution, the corresponding surface charge density vs. potential curve was obtained, and from this the potential of zero charge (pzc) of the CO-covered Pt(1 1 1) electrode. From these data it was estimated that the error incurred when the potential of zero total charge (pztc) of Pt(1 1 1) electrodes is determined by the CO-charge dis- placement method is of approximately 50 mV at pH 1 and of approximately 90 mV at pH 3. Furthermore, the exper- imentally determined pzc of the CO-covered Pt(1 1 1) electrode has allowed us to make an estimation of the potential of zero free charge (pzfc) of Pt(1 1 1) electrodes. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Electrochemical methods; Surface electronic phenomena (work function, surface potential, surface states, etc.); Platinum; Carbon monoxide; Solid–liquid interfaces 1. Introduction The potential of zero charge (pzc), i.e., the potential at which the electrode surface has no ex- cess charge, is a fundamental property of the metal–electrolyte interface, whose knowledge is a key requirement for a detailed understanding of double layer phenomena and properties [1]. Fur- thermore, there exists a direct relationship between the pzc of a given electrode and its work function in ultrahigh vacuum (UHV) [2], i.e., a direct link between Electrochemical Surface Science and clas- sical (UHV) Surface Science. In the case of ideally polarizable interfaces, at which no interfacial charge transfer occurs, the pzc will correspond to the point where the truly free, electronic excess charge density on the metal 0039-6028/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2004.08.014 * Tel.: +34 91 5619400; fax: +34 91 5642431. E-mail address: a.cuesta@iqfr.csic.es Surface Science 572 (2004) 11–22 www.elsevier.com/locate/susc