Real-Time Investigations of Pt(111) Surface Transformations in Sulfuric Acid Solutions Bjo ¨ rn Braunschweig, Prabuddha Mukherjee, Dana D. Dlott,* and Andrzej Wieckowski* Department of Chemistry, UniVersity of Illinois at Urbana-Champaign, Urbana, Illinois 61801 Received August 4, 2010; E-mail: dlott@illinois.edu; andrzej@scs.uiuc.edu Abstract: We present the first broadband sum-frequency genera- tion (SFG) spectra of adlayers from sulfuric acid solutions on Pt(111) surfaces and reveal surface transformations of (bi)sulfate anions in unprecedented detail. SFG amplitudes, bandwidth, and electrochemical Stark tuning of (bi)sulfate vibrational bands centered at 1250-1290 cm -1 strongly depend on the applied potential and are correlated with prominent voltammetric features. (Bi)sulfate adlayers on Pt(111) are important model systems for weak, specific adsorption of anions on catalytically active sur- faces. Although the existence of surface transformations on Pt(111) in dilute H 2 SO 4 solutions has been established by previous studies, so far they have not been observed with surface vibrational spectroscopy. Our results confirm previous reports of a surface transformation at 0.21 V and provide new information on a second transformation at 0.5 V due to surface hydroxyl formation and rearrangement of the electric double layer. In this Communication, we present the first vibrational sum- frequency generation (SFG) spectra of adlayers from sulfuric acid solutions on Pt(111) surfaces and reveal surface transformations of sulfuric acid anions in unprecedented detail. SFG amplitudes, bandwidth, and electrochemical Stark tuning of the observed vibrational bands strongly depend on the applied potential and are correlated with prominent voltammetric features. Although the existence of surface transformations on Pt(111) in H 2 SO 4 solutions was established by previous studies, so far they have not been observed with surface vibrational spectroscopy. Our results confirm previous reports of a surface transformation at 0.21 V and provide new information on a second transformation at 0.5 V. The specific adsorption of bisulfate and sulfate anions on Pt(111) single-crystal surfaces represents an important model system in electrochemical surface science 1,2 and has been shown to change the reaction pathways and efficiency of electrochemical reactions significantly. 2,3 Nafion-based fuel cell technology represents an example where sulfate-type interfaces are of great interest, since the sulfonic acid head-groups of Nafion form very similar double- layer structures at Pt interfaces. 4,5 The surface structure of Pt(111) in the potential range of sulfate and bisulfate adsorption has been studied previously with scanning tunneling microscopy (STM) 6,7 and theoretical simulations. 4,8 Adlayers with co-existing domains of (3×7)R19.1° superstruc- tures and with different rotational domains are established at potentials higher than the sharp spike in the cyclic voltammogram (CV) (Figure 1a). These structures undergo reversible surface transformations at 0.21 and 0.5 V, where a disordered layer is transformed into an ordered layer and vice versa. 6-8 For potentials E > 0.21 V, domains with (3×7)R19.1° structures consist of sulfate anions locked to three-fold hollow sites of Pt(111) and co- adsorbed water molecules. 6,7,9 H 2 O co-adsorption leads to the formation of hydrogen bonds to adsorbed sulfate, reduces the Coulomb repulsion among adjacent anions, and, consequently, stabilizes the asymmetric (3×7)R19.1° superstructure. 6,7,9 For E < 0.21 V, DFT calculations show a bidentate configuration of sulfuric acid anions, with two oxygen atoms pointing toward the Pt(111) surface, which is transformed for E > 0.21 V into a three- fold-coordinated state with three oxygens of the anion binding to Pt atoms. 9 It should be noted that the chemical identity of the molecular adsorbate s HSO 4 - , SO 4 2- , or a SO 4 2- + H 3 O + complex s on Pt(111) in H 2 SO 4 solutions has been highly debated in the past 4,6,7,10-15 and is denoted henceforth as (bi)sulfate. Theoretical simulations 4,8,11 as well as radiotracer 13 and com- bined STM and voltammetric experiments 6,7 suggest a predominant adsorption of sulfate, which is in contrast to most infrared studies of this system, where a predominant adsorption of bisulfate is reported. 12,14 The CV of a Pt(111) single crystal immersed in 0.1 M H 2 SO 4 is presented in Figure 1a. The reversible CV is comprised of four potential regions and is indicative of a well-ordered surface with large, atomically flat terraces. 6-8,15 The pair of plateau-like current features at -0.2 to 0.07 V is due to hydrogen deposition on (111) surface sites and hydrogen stripping for cathodic and anodic sweeps, respectively. (Bi)sulfate adsorption on Pt(111) leads to a broad feature centered at 0.16 V and a subsequent sharp spike at 0.21 V. The neck-like current feature for potentials higher than the sharp spike arises from double-layer charging only and is followed by a broad current wave centered at 0.5 V, with a much narrower desorption feature at 0.43 V (Figure 1a, inset). The assignment of these features has been highly controversial, and attributions to OH adsorption 6-8,14 or to a further ordering of (bi)sulfate adlayers with higher coverage 16 have been reported. Potentiodynamic SFG spectra of Pt(111) in 0.1 M H 2 SO 4 solution which were synchronized with a CV from -0.2 to 0.8 V are presented in Figure 2a. At potentials E < 0.2 V, featureless SFG spectra are observed, while for E > 0.2 V, a vibrational band Figure 1. (a) Hanging-meniscus cyclic voltammogram of a well-ordered Pt(111) electrode in 0.1 M H 2 SO 4 . The CV was recorded in the SFG spectroelectrochemical cell. (b) SFG amplitude A q of the vibrational band in Figure 2a as a function of the applied potential E. Published on Web 09/20/2010 10.1021/ja106618z  2010 American Chemical Society 14036 9 J. AM. CHEM. SOC. 2010, 132, 14036–14038