Growth and termination of a rutile IrO 2 (100) layer on Ir(111) Rahul Rai a , Tao Li a , Zhu Liang a , Minkyu Kim b , Aravind Asthagiri b , Jason F. Weaver a, ⁎ a Department of Chemical Engineering, University of Florida, Gainesville, FL 32611, USA b William G. Lowrie Chemical & Biomolecular Engineering, The Ohio State University, Columbus, OH 43210, USA abstract article info Article history: Received 16 November 2015 Received in revised form 18 January 2016 Accepted 22 January 2016 Available online xxxx We investigated the oxidation of Ir(111) by gas-phase oxygen atoms at temperatures between 500 and 625 K using temperature programmed desorption (TPD), low energy electron diffraction (LEED), low energy ion scattering spectroscopy (LEISS) and density functional theory (DFT) calculations. We find that a well-ordered surface oxide with (√ 3× √ 3)R30° periodicity relative to Ir(111) develops prior to the formation of a rutile IrO 2 (100) layer. The IrO 2 (100) layer reaches a saturation thickness of about four oxide layers under the oxidation conditions employed, and decomposes during TPD to produce a single, sharp O 2 desorption peak at ~770 K. Favorable lattice matching at the oxide-metal interface is likely responsible for the preferential growth of the IrO 2 (100) facet during the initial oxidation of Ir(111), with the resulting coincidence lattice generating a clear (6 × 1) moiré pattern in LEED. Temperature programmed reaction spectroscopy (TPRS) experiments reveal that CO and H 2 O molecules bind only weakly on the IrO 2 (100) surface and LEISS measurements show that the oxide surface is highly enriched in O-atoms. These characteristics provide strong evidence that the rutile IrO 2 (100) layer is oxygen-terminated, and thus lacks reactive Ir atoms that can strongly bind molecular adsorbates. Oxygen binding energies predicted by DFT suggest that on-top O-atoms will remain adsorbed on IrO 2 (100) at temperatures up to ~625 K, thus supporting the conclusion that the rutile IrO 2 layer grown in our experiments is oxygen-terminated. As such, the appearance of only a single O 2 TPD peak indicates that the singly coordinate, on-top O-atoms remain stable on the IrO 2 (100) surface up to temperatures at which the oxide layer begins to thermally decompose. © 2016 Published by Elsevier B.V. Keywords: Iridium Oxide surface Oxidation Rutile DFT IrO 2 1. Introduction Late transition-metal (TM) oxides can play an important role in applications of oxidation catalysis. Iridium oxide in particular is well known as an effective catalyst for promoting electrochemical H 2 O split- ting [1] and is also a promising material for effecting other chemical transformations. Recent density functional theory (DFT) studies predict that the rutile IrO 2 (110) surface strongly binds a variety of molecules such as NH 3 ,N 2 and CH 4 , and is active in promoting dehydrogenation reactions [2–4]. These predictions motivate efforts to generate well- defined IrO 2 surfaces and explore the physical and chemical properties of IrO 2 surfaces under well-controlled ultrahigh vacuum (UHV) condi- tions. Such efforts have potential to advance our understanding of the surface chemistry of late TM oxides, beyond the knowledge derived from prior investigations with RuO 2 (110) [5] and PdO(101) [6]. Fur- thermore, the fact that rutile IrO 2 and RuO 2 are isostructural can provide insightful comparisons about the growth and surface chemistry of these oxides. In general, however, the growth and surface chemistry of IrO 2 have not been widely explored due to difficulties in oxidizing metallic Ir surfaces under UHV conditions. The dissociative chemisorption of O 2 and the properties of chemisorbed oxygen atoms on Ir(111) have been studied in detail and are generally well understood. Work by Chan and Weinberg shows that O 2 dissociation on clean Ir(111) at room temperature produces a sharp (2 × 2) low energy electron diffraction (LEED) pattern [7,8]. Subsequent DFT calculations predict that the (2 × 2) LEED pattern appears initially at coverages up to 0.25 ML due to the formation of a p(2 × 2) layer and that the (2 × 2) pattern persists up to an O-atom coverage of 0.50 ML due to the formation of three equivalent rotational domains of a p(1 × 2) structure [9–11]. Due to kinetic limitations, the dissociative chemisorption of O 2 on Ir(111) effectively saturates at a coverage of 0.50 ML of O-atoms at the low O 2 partial pressures utilized in UHV experiments. Early studies have reported the formation of IrO 2 on Ir surfaces at elevated temperatures and O 2 pressure [12,13]. Of particular relevance to the present study is a report by Conrad et al. demonstrating that thin oxide layers develop on Ir(111) during expo- sure to O 2 at elevated pressure and surface temperatures above 800 K [13]. Those authors present evidence that the oxide layers are crystal- line, but did not make structural assignments. More recently, He et al. have studied the oxidation of Ir(111) using in situ surface X-ray diffrac- tion (SXRD) and find that a rutile IrO 2 layer, exposing predominantly IrO 2 (110) facets, forms only at sufficiently high O 2 pressure (up to 100 mbar) and an elevated temperature of 775 K [14]. Those authors Surface Science xxx (2016) xxx–xxx ⁎ Corresponding author. Tel.: +1 352 392 0869; fax: +1 352 392 9513. E-mail address: weaver@che.ufl.edu (J.F. Weaver). SUSC-20796; No of Pages 9 February 06, 2016; Model: Gulliver 5 http://dx.doi.org/10.1016/j.susc.2016.01.018 0039-6028/© 2016 Published by Elsevier B.V. Contents lists available at ScienceDirect Surface Science journal homepage: www.elsevier.com/locate/susc Please cite this article as: R. Rai, et al., Growth and termination of a rutile IrO2(100) layer on Ir(111), Surf. Sci. (2016), http://dx.doi.org/10.1016/ j.susc.2016.01.018