www.elsevier.nl/locate/jelechem Journal of Electroanalytical Chemistry 484 (2000) 189 – 193 In situ scanning tunneling microscopy of adsorbed sulfate on well-defined Pd(111) in sulfuric acid solution Li-Jun Wan, Takeshi Suzuki, Kenji Sashikata, Jun Okada, Junji Inukai, Kingo Itaya * Department of Applied Chemistry, Graduate School of Engineering, Tohoku Uniersity, Aoba -yama 04, Sendai 980 -8579, Japan Received 11 November 1999; received in revised form 22 February 2000; accepted 22 February 2000 Abstract In situ electrochemical scanning tunneling microscopy (STM) disclosed that highly-ordered adlayers of adsorbed sulfate/bisul- fate with atomic features having a (3 ×7) symmetry formed on a well-defined Pd(111) surface in 10 mM H 2 SO 4 in the double layer potential range. Well-defined Pd(111) surfaces were prepared by a flame-annealing-quenching technique. High resolution STM imaging revealed the structure of water molecules in both the first and the second layers of a water-bilayer. Small reversible peaks at 0.45 V versus a reversible hydrogen electrode were found to be due to a disorder–order phase transition. The Pd(111)-(1 ×1) and (3 ×7) structures were observed at potentials negative and positive with respect to the small peaks, respectively. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Pd(111); Sulfate adlayer; Scanning tunneling microscopy 1. Introduction By using in situ scanning tunneling microscopy (STM), the adsorption of sulfate/bisulfate anions (des- ignated below simply as sulfate) was studied inten- sively in the past on (111) surfaces of face -centered cubic (f.c.c.) lattices such as Au(111) [1,2], Pt(111) [3,4], Rh(111) [5], Cu(111) [6–9], and Ir(111) [10] in H 2 SO 4 . In general, the structures of many adlayers including halides and organic molecules are different on the different f.c.c. metals [11,12]. However, it was rather surprising that the adsorbed sulfate forms highly-ordered adlayers on all of the substrates de- scribed above with the same (3 ×7) structure. Particularly, it is interesting to note that the (3 × 7) structure was also formed on Cu(111), accompa- nying a reconstruction of the first Cu layer, which resulted in the appearance of a Moire ´ pattern [6–9]. Our recent paper reported that highly-ordered ad- layers of adsorbed sulfate formed on Ir(111) with the same (3 ×7) structure [10]. High-resolution STM imaging allowed us to determine the orientation of hydrogen-bonded water molecules [10], which have been proposed previously to exist between the rows of adsorbed sulfates [5]. Two different water molecules were clearly distinguished as separated spots along the 3 direction between neighboring rows of adsorbed sulfate [10]. The results described above directed us toward the investigation on details of the adsorbed sulfate layers on the Pd(111) surface. In this paper, it is shown that sulfate anions form highly-ordered adlayers on Pd(111) with (3 ×7) symmetry in 10 mM H 2 SO 4 , which is identical with those found previously on Au(111), Pt(111), Rh(111), Cu(111), and Ir(111). The water molecules in the first and second layers of the hydrogen-bonded water chain were clearly revealed by high-resolution STM imaging. The ordered (3 × 7) structure disappeared at potentials more negative than those of the small peaks. Instead of the (3 × 7) structure, in situ STM consistently revealed the (1 ×1) structure over the entire surface of Pd(111). This observation indicates that a disorder – order phase transition occurs on the sulfate adlayer at po- * Corresponding author. Present address: Department of Engineer- ing Science, Faculty of Engineering, Tohoku University, Sendai 980- 8519, Japan. Fax: +81-22-2145380. E-mail address: itaya@atom.che.tohoku.ac.jp (K. Itaya) 0022-0728/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII:S0022-0728(00)00082-6