J. Phys. Chem. C 2009, 113, 5541–5546
5541
Dissociative Adsorption of Oxygen on Clean Cu(001) Surface Kazuma Yagyu,*,†,‡ Xiangdong Liu,‡ Yoshihide Yoshimoto,‡ Kan Nakatsuji,‡ and Fumio Komori‡ Institute for Solid State Physics and UniVersity of Tokyo, Kashiwanoha, Kashiwa-shi, Chiba 277-8581, Japan ReceiVed: September 26, 2008; ReVised Manuscript ReceiVed: January 29, 2009
The initial stage of molecular oxygen adsorption on Cu(001) surface is investigated by scanning tunneling microscopy and first principles calculations. The molecule always dissociates on the surface between 5 and 300 K. The dissociated oxygen atoms adsorb at the two 4-fold hollow sites separated by twice of the nearestneighbor distance of the surface Cu atoms in the close-packed directions, [110] and [1j10]. This is the most stable structure evaluated for the pair adsorbed at hollow site by the calculation. The adsorbed oxygen atoms stay at the same site below 80 K, and thermally migrate on the surface above 300 K. Introduction The adsorption of oxygen molecules (O2) has been studied for a long time as one of the important and fundamental processes in surface chemical reactions. On copper (Cu) surfaces, in particular, different kinds of adsorption have been reported depending on the crystal orientation. The Cu(001) surface is the most reactive among the low-index Cu surfaces, and direct dissociation was evidenced by molecular-beam experiments at various temperatures.1,2 At a low coverage, no molecular oxygen was detected even at 40 K by high-resolution electron energy loss spectroscopy (HREELS) study.3 This is in contrast to the adsorption on Cu(110)4-6 and Cu(111)7,8 surfaces where coadsorbed molecular oxygen was observed as well as the dissociated atoms below 200 K. For Cu(110), the existence of molecular oxygen and the site of the dissociated atom were experimentally clarified by scanning tunneling microscopy (STM) at 4 K.4 Theoretical calculations9-11 have partly supported these experimental results. For Cu(001), three reaction paths for dissociative adsorption were compared by using a cluster calculation.9 The dissociation over the 4-fold hollow site was assigned as the most possible one. The dissociation over the bridge site, the hollow site, and the top site results in the adsorption models A, B, and C shown in Figure 1, parts a, b, and c, respectively. Here, the hollow site is the most stable adsorption site of the atomic oxygen in the calculation. Through the most possible reaction path, the dissociated atoms adsorb at the hollow site separated by twice the nearest-neighbor distance a0 of the surface Cu lattice (Figure 1b). Experimentally, however, the adsorption site of dissociated oxygen atoms has not been clarified. The atomic adsorption at the hollow site was identified by STM after dissociative adsorption of molecular oxygen at room temperature (RT).12 This is the same site as that observed after annealing the surface at more than 500 K13,14 when the average coverage is less than 0.3 monoatomic layers (ML). Here, ML is defined as the density of Cu atoms on the clean surface. The adsorbed oxygen atoms form small domains of a c(2×2)-O structure on the surface after the annealing, and the boundary * To whom correspondence should be addressed at the Institute for Solid State Physics. author. Phone: +81-471-36-3310. Fax: +81-471-36-3474. E-mail:
[email protected]. † Institute for Solid State Physics. ‡ University of Tokyo.
Figure 1. Models of an oxygen atom pair, which was made after dissociative adsorption of molecular oxygen on the Cu(001) surface, at hollow site separated by (a) the nearest-neighbor distance a0, (b) 2a0, and (c) 2a0. Open and shaded circles represent substrate Cu atoms and O atoms, respectively.
of the domain thermally moves at RT.13 In this structure, the distance between oxygen atoms is 2a0 as shown in Figure 1c. A c(2×2) pattern was reported after the oxygen adsorption at RT with low-energy electron diffraction (LEED)15 while this structure has not been confirmed with STM. On the oxygen-adsorbed Cu(001) surface, two energy loss peaks due to the vibration of the O-Cu bond were observed at RT by HREELS16 when the oxygen atom coverage exceeded 0.1 ML. These were later attributed to two oxygen species at the hollow site with different distances between oxygen and the surface Cu layer.17 Two energy loss peaks due to the O-Cu bond were also observed by HREELS for the surface with 0.1 ML coverage at 100 K18 and 40 K,3 and were assigned to atomic oxygen at the hollow and lower coordination sites. The adsorption at a lower coordination site is, however, inconsistent with the theoretical result.9 In the present study, we have investigated dissociative adsorption of molecular oxygen on Cu(001) using STM between 5 K and RT. We have focused on a very early stage of the adsorption where only the interaction between the dissociated pair of atoms is considered as the interaction among the oxygen atoms. This is not the case when the coverage of atomic oxygen is increased to more than 0.1 ML. We have studied the adsorption site of the pair just after the dissociation and thermal motion of the dissociated atoms at RT. We also confirmed the absence of molecular oxygen at 5 K. Methods In the experiments, two microscopes (Rasterscope-3000 for RT and Omicron LT-STM for 80 and 5 K) were used in
10.1021/jp808542z CCC: $40.75 2009 American Chemical Society Published on Web 03/19/2009
5542 J. Phys. Chem. C, Vol. 113, No. 14, 2009 ultrahigh vacuum (UHV). The STM images were recorded in a constant-current mode with a Pt-Ir (RT) or tungsten tip (80 and 5 K). Image distortion due to the thermal drift was corrected. The samples were prepared in UHV preparation chambers with a four-grid low-energy electron diffraction/auger electron spectrometer (LEED-AES), an ion gun, an annealing base, and variable leak valves for introducing pure argon (Ar), O2, and nitrogen (N 2) gases. The base pressure of the UHV systems is better than 5 × 10-11 Torr. The surface of a Cu(001) single crystal was cleaned by repeated cycles of Ar+ ion sputtering (500 or 1000 eV) and annealing up to 800 K in the preparation chamber. The ordering and the cleanness of the surface were confirmed by a sharp (1×1) LEED pattern and no signal of contamination was detected by AES. A partially nitrogen-adsorbed Cu(001) surface (N/Cu(001)) was used for determining the O adsorption site as in ref 12. It was prepared by exposing the cleaned surface to nitrogen ions with use of the ion gun (500 eV) followed by annealing to 700 K. The nitrogen coverage was 0.2 ML, which is 40% of the saturation on the Cu(001) surface. We can adjust the amount of nitrogen atoms by monitoring the ion current through the Cu crystal during the N+ exposure to the surface. The Cu(001) surfaces were exposed to molecular oxygen gas of 5 × 10-9 Torr at 5 and 80 K and of 1 × 10-8 Torr at RT. Oxygen coverage (ΘO) on the clean Cu surface was 0.008 ML at 5 K, 0.004 ML at 80 K, and 0.02 ML at RT, and it was 0.02 ML of the clean surface on the N/Cu(001) surface at 5 K. In the theoretical studies, the optimized surface structures were obtained by using first-principles calculations in a similar way to the previous study.19 The surface was simulated with two symmetric slab models of 5×3 and 6×4 super cells with 7 atomic layers. All layers were relaxed except the central layer. The vacuum region was commonly 5 atomic layers thick. With these slab models, we performed a standard density-functional plane-wave-pseudopotential calculation by an extended version of the program package TAPP (Tokyo Ab-initio Program Package).20 The Perdew-Burke-Ernzerhof type of exchangecorrelation potential21,22 was used. Copper and oxygen atoms were simulated by the ultrasoft pseudopotentials.23 The cutoff energy of the plane-wave basis set was 49 Ry. All of the symmetries were utilized, and the number of sampled k-points for both the 5×3 and 6×4 super cells was 2×4. The cutoff energy and the thicknesses of the slabs were confirmed to be enough for quantitative discussion in the previous study.19 Even when the thickness of the vacuum region was increased from 5 atomic layers to 9 atomic layers, the obtained interaction energy between O atoms was kept within 1 meV. To stabilize the calculation, the Fermi surface were smoothed within 0.1 eV, using Methfessel-Paxton method.24 The force change was lees than 10-4 hartree · bohr-1 even when the smoothing width of the Fermi energy was reduced to 0.05 eV. The atomic structures of the O-adsorbed surfaces were optimized so that the maximum force acting on the oxygen and Cu atoms becomes less than
