An STM study on the growth process of vapor-deposited hydroquinone adlayers on Rh(1 1 1) and Pt(1 1 1) Junji Inukai a,b , Mitsuru Wakisaka b , Kingo Itaya b,c, * a NICHe, Tohoku University, 6-6-10 Aoba, Sendai 980-8579, Japan b Department of Applied Chemistry, Faculty of Engineering, Tohoku University, 6-6-04 Aoba, Sendai 980-8579, Japan c CREST, JST, 4-1-8 Kawaguchi, Saitama 332-0012, Japan Received 10 August 2004; in final form 1 October 2004 Abstract Hydroquinone molecules were allowed to adsorb on Rh(1 1 1) and Pt(1 1 1) by vapor deposition. The strong interaction between two adjacent hydroxyl groups of neighboring molecules led to the formation of commensurate ( p 7 · p 7)R19.1° and incommensu- rate ( p 2.56 · p 2.56)R16° adlayer phases on Rh(1 1 1) and Pt(1 1 1), respectively. The formation processes of HQ adlayers on Rh(1 1 1) and Pt(1 1 1) were very different from each other, possibly due to the difference in lattice parameters of the two metal surfaces. Ó 2004 Elsevier B.V. All rights reserved. 1. Introduction The adsorption of organic molecules on metal sur- faces is a major subject of modern interfacial chemistry [1–3]. We previously investigated the adsorption of aro- matic molecules such as benzene, naphthalene, anthra- cene, and coronene on well-defined Rh(1 1 1) [4–7], Pt(1 1 1) [4–6], Cu(1 1 1) [6,8], and Au(1 1 1)[9,10] sur- faces by using scanning tunneling microscopy (STM) [6]. Recently, we showed that both intermolecular and molecule–substrate interactions play important roles in the formation of ordered adlayers of benzene deriva- tives. We investigated the adlayers of methyl-substituted benzene derivatives, such as toluene, o-xylene, p-xylene, and mesitylene, formed on Rh(1 1 1) in HF solution [11,12]. For p-xylene, we also investigate its adlayers on Rh(1 1 1) and Pt(1 1 1) formed by vapor deposition [13,14]. Although the atomic distances on Rh(1 1 1) and Pt(1 1 1) surfaces are different, namely 0.269 and 0.278 nm, respectively, the same adlayer phase of c(2 p 3 · 4)rect was observed on both Rh(1 1 1) and Pt(1 1 1) in solution and vacuum [14], indicating that the intermolecular interaction between p-xylene mole- cules is small. Hydroquinone (HQ) is a well-studied molecule, which undergoes electrochemical oxidation/reduction reactions in solution [15]. Hubbard and co-workers investigated the adlayers of HQ on Pt electrodes by employing thin-layer electrochemical cells [3,16,17], low energy electron diffraction (LEED) [3,18,19], Auger electron spectroscopy [3,18,19], and high-resolution electron energy loss spectroscopy (HREELS) [3,18–20]. Wilson et al. [21] performed STM studies of HQ by using a UHV compatible cell, wherein a drop of ultra- pure water containing HQ was evaporated from a Pt(1 1 1) surface in an Ar atmosphere. Soto et al. [22] investigated the adlayers of HQ on Pd(1 1 1) and Pd(1 0 0) electrode surfaces by tandem electrochemistry, HREELS, and in situ STM. Those reports showed that adsorbed HQ molecules formed monolayers with an ori- entation parallel to the Pt and Pd single-crystal surfaces, 0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.10.023 * Corresponding author. Fax: +81 22 214 5380. E-mail address: itaya@atom.che.tohoku.ac.jp (K. Itaya). www.elsevier.com/locate/cplett Chemical Physics Letters 399 (2004) 373–377