In Situ Surface X-Ray Scattering Observation of Long-Range Ordered „  19   19… R 23.4°-13CO Structure on Pt„111… in Aqueous Electrolytes Yuriy V. Tolmachev, a, * , ** Andreas Menzel, a Andrei V. Tkachuk, b Yong S. Chu, b and Hoydoo You a, * ,z a Argonne National Laboratory, Materials Science Division and b Experimental Facilities Division, Argonne, Illinois 60439, USA Presented herein is the experimental observation of the long-range ordered (  19   19) R 23.4°-13CO structure on Pt111 in aqueous electrolytes by in situ surface X-ray scattering. The results confirmed the presence of two mirrored domains suggested earlier on the basis of scanning tunneling microscopy and infrared measurements. Based on the weak intensity of the second order adlattice reflections and earlier results obtained by other techniques, a further refinement of the (  19   19) structure with tilted CO molecules is proposed. The hystereses observed in transitions between (2  2)-3CO and (  19   19) R 23.4°-13CO phases, as well as in CO adsorption and stripping, with change in electrode potential are discussed. © 2004 The Electrochemical Society. DOI: 10.1149/1.1645354 All rights reserved. Manuscript submitted July 11, 2003; revised manuscript received September 3, 2003. Available electronically January 29, 2004. Besides its direct relevance to the mechanism of poisoning in modern low-temperature fuel cells, adlayers of CO on the Pt111 surface are fascinating model systems of the electrochemical inter- face. For these reasons, they have been extensively studied in past decades since Kitamura et al. 1 discovered the existence of two new types of CO adlayers on Pt111 surface in CO-saturated solutions, which have never been seen in ultrahigh vacuum UHV environ- ment. Their infrared IR data suggested adsorption of CO on atop and threefold hollow sites in the potential region close to 0 V vs. reversible hydrogen electrode RHE, and adsorption of CO on atop and bridging sites at more positive potentials. Subsequently, Villegas and Weaver 2 reported in situ scanning tunneling microscopy STM images of these ordered high-density CO adlayers on Pt111. By combining the STM results with IR measurements, 2 they concluded that the molecular structure of the CO layer is a hexagonal close- packed hcp (2  2)-3CO lattice with a coverage of 3/4 per sur- face Pt atom at potentials close to 0 V RHE, and that it transforms to another hcp (  19   19) R 23.4°-13CO lattice with a coverage of 13/19 at more positive potentials. Soon thereafter, Lucas, Markovic, and co-workers successfully carried out in situ surface X-ray scattering SXS measurements 3-7 confirming the (2  2) structure, despite general skepticism that X-ray scattering from light elements such as CO through electrolyte may not yield signals intense enough to stand above the background. They observed initially weak (2  2) superlattice reflections on top of high background in HClO 4 solution, and stronger reflections by adding a small amount of NaBr, which increases the size of CO domains reportedly by blocking OH adsorption. 6 However, their un- successful search for reflections of the (  19   19) superlattice led them to conclude that this structure has no long-range order, despite further STM confirmation of the ordered structure in solution, 8 as well as indirect evidence of a phase transition between the two CO structures. 9-12 Recently, a short range ordered (  19   19) struc- ture was found in the gas phase by STM. 13 There have also been reconciliatory suggestions in the literature. Rodes et al. 12 reported that the (  19   19) structure can be found only on stepped sur- faces. Baldelli et al. 14 suggested that the CO axes are tilted by as much as 75° based on the absence of the sum frequency generation SFG signal and that the tilt is probably the reason for the lack of the long-range order in the (  19   19) structure. However, this claim has been retracted in their later publication. 15 In this article, we present SXS evidence for the (  19   19) structure. We show that it is long-range ordered, appears at around 600 mV RHE, and remains ordered until CO is oxidized at around 900 mV RHE. We also present a structural model with tilted CO molecular axes, not necessarily unique but consistent with our X-ray data and with earlier results obtained by other techniques. Experimental The Pt111 crystals used in our experiments were 10 mm diam- eter disks polished with a miscut of less than 0.2°. The crystal se- lected for measurements was annealed in a hydrogen-air flame. Then, it was transferred to and cooled in a glass tube under flow of Ar mixed with 2% H 2 . Once the crystal was cooled below 100°C, it was immersed in ultrapure water saturated with the Ar + H 2 mix- ture. The surface of the crystal remained protected by a water drop- let while it was transferred to the SXS cell. The crystal was covered with an 8 m Kapton film in thin-layer cell geometry. 16 In this way, an atomically smooth surface can be obtained, which is evident in a strong surface scattering peak at the specular anti-Bragg positions data not shown. Following Ref. 6, the electrolyte used in our ex- periment was 0.1 M HClO 4 + 0.01 M NaBr in ultrapure water. The solution was saturated with CO in a bubbler before being introduced into the SXS cell. A flow of CO was maintained over the Kapton film during the experiment. Ag/AgCl + 3 M NaCl was used as the reference electrode. However, all potentials in this work were con- verted to RHE scale by adding 300 mV. The SXS experiments were performed at beamline 11ID-D at the Advanced Photon Source APS at Argonne National Laboratory. The cell was mounted on a six-circle diffractometer. Standard scintillation detectors were used. X-ray energy of 11.5 keV was selected to minimize the platinum fluorescence background. For convenience, a hexagonal unit cell was used by choosing (003) hexagonal , (111) cubic , along the surface normal, and we use only the hexagonal unit cell hereafter. Results and Discussion While only one orientation of CO molecules in the (2  2) lattice with respect to the substrate is possible, there are two stere- ochemically nonequivalent orientations for the (  19   19) unit cell, shown in Fig. 1 as A and A' . These two arrangements arise because the CO lattice, upon expansion from the (2  2) structure, may choose to rotate to the left by 20.5° A or to the right by 7.3° (A' ) to form the (  19   19) unit cell. Note that, in A, there is a cluster of six near-atop CO molecules surrounding the central atop CO. The remaining six CO molecules occupy near-bridging posi- tions. In A' , there are also one atop, six near-atop, and six near- bridging COs. However, in the latter case, it is the near-atop COs * Electrochemical Society Active Member. ** Current address: Kent State University, Department of Chemistry z E-mail: hyou@anl.gov Electrochemical and Solid-State Letters, 7 3 E23-E26 2004 0013-4651/2004/73/E23/4/$7.00 © The Electrochemical Society, Inc. E23 Downloaded 24 Nov 2010 to 131.123.233.131. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp