Low-energy electron-diffraction crystallographic determination for the Cu(110)2 x 1-O surface structure

Communication: Calculations of the (2 × 1)-O reconstruction kinetics on Cu(110)

Density functional theory calculations are used to study the elementary processes of the formation of the (2 × 1)-O reconstruction on the Cu(110) surface. The (2 × 1)-O reconstruction requires additional Cu atoms to form Cu-O rows on top of the surface. Both terrace and step sites are considered as the source of Cu adatoms. On terraces, adsorbed oxygen induces the ejection of Cu atoms to form -O-Cu-O- units, leaving Cu vacancies behind. The barrier for subsequent unit growth, however, is prohibitively high. Cu(110) step sites are also considered as a source of Cu atoms. Dissociated oxygen triggers the formation of stable Cu-O chains along the [001] step edges. This process, however, blocks the diffusion of Cu atoms so that it is not a viable mechanism for the (2 × 1)-O reconstruction. Oxygen adsorption on the [11¯0] edges also allows the nucleation of [001] oriented Cu-O rows. The short Cu-O rows act as diffusion channels for Cu atoms that detach from the step, which append to the end of the Cu-O chains. Our calculations of the formation of the (2 × 1)-O phase on Cu(110) provide a mechanistic description of the experimentally observed reconstruction.

Calculations of oxide formation on low-index Cu surfaces

Density-functional theory is used to evaluate the mechanism of copper surface oxidation. Reaction pathways of O2 dissociation on the surface and oxidation of the sub-surface are found on the Cu(100), Cu(110), and Cu(111) facets. At low oxygen coverage, all three surfaces dissociate O2 spontaneously. As oxygen accumulates on the surfaces, O2 dissociation becomes more difficult. A bottleneck to further oxidation occurs when the surfaces are saturated with oxygen. The barriers for O2 dissociation on the O-saturated Cu(100)-c(2×2)-0.5 monolayer (ML) and Cu(100) missing-row structures are 0.97 eV and 0.75 eV, respectively; significantly lower than those have been reported previously. Oxidation of Cu(110)-c(6×2), the most stable (110) surface oxide, has a barrier of 0.72 eV. As the reconstructions grow from step edges, clean Cu(110) surfaces can dissociatively adsorb oxygen until the surface Cu atoms are saturated. After slight rearrangements, these surface areas form a "1 ML" oxide structure which has not been reported in the literature. The barrier for further oxidation of this "1 ML" phase is only 0.31 eV. Finally the oxidized Cu(111) surface has a relatively low reaction energy barrier for O2 dissociation, even at high oxygen coverage, and allows for facile oxidation of the subsurface by fast O diffusion through the surface oxide. The kinetic mechanisms found provide a qualitative explanation of the observed oxidation of the low-index Cu surfaces.

Hybridization between Cu-O chain and Cu(110) surface states in the O(2×1)/Cu(110) surface from first principles

The O(2×1)/Cu(110) surface reconstruction of the Cu(110) surface is induced by 0.5 ML of oxygen adsorption and is formed by Cu-O chains running along the [001] direction. Here, we show that hybridization between surface states of the Cu(110) substrate and one-dimensional states of the Cu-O chains is crucial in understanding the electronic structure of this surface. Specifically, the interaction between one occupied antibonding band of the Cu-O chain with O(p(y)) character (y-axis taken along the Cu-O chain direction) and the partially occupied surface state at the Y point of the clean Cu(110) surface with Cu(p(y)) character causes major changes in the electronic structure close to the Fermi energy (E(F)). This surface state decays very slowly into the bulk and a thick slab is needed to properly describe it, which might explain why the importance of this hybridization has not been recognized so far. In our calculations we obtain two hybrid bands: (i) a fully occupied band that strongly hybridizes with the bulk Cu sp states nearby E(F), becoming a very broad resonance, thus explaining why it is not observed in photoemission experiments; (ii) an empty band that acquires surface state character, including its dispersion close to the zone boundary at the Y point. This splitting induces a partial population of the p(y) antibonding band that is necessary to reconcile the calculated charge transfer from the Cu(110) substrate to the Cu-O chain (~0.5 electrons/f.u.) with the apparently fully occupied band structure of the adsorbed Cu-O chain (consistent with 1 electron transferred per formula unit).

The structure of surfaces: what do we know and what would we like to know?

A brief survey is presented of the methods of quantitative surface structure determination and some of the main phenomena that have been established, and their associated trends. These include surface relaxation and reconstruction of clean surfaces and the structures formed by atomic and molecular adsorbates. Examples include the surfaces of semiconductors, oxides and metals. Future challenges, concerned with complexity and precision, are discussed.

Adsorbate coverages and surface reactivity in methanol oxidation over Cu(110): An in situ photoelectron spectroscopy study

The adsorbate species present during partial oxidation of methanol on a Cu(110) surface have been investigated in the 10(-5) mbar range with in situ x-ray photoelectron spectroscopy and rate measurements. Two reaction intermediates were identified, methoxy with a C 1s binding energy (BE) of 285.4 eV and formate with a C 1s BE of 287.7 eV. The c(2x2) overlayer formed under reaction conditions is assigned to formate. Two states of adsorbed oxygen were found characterized by O 1s BE's of 529.6 and 528.9 eV, respectively. On the inactive surface present at low T around 300-350 K formate dominates while methoxy is almost absent. Ignition of the reaction correlates with a decreasing formate coverage. A large hysteresis of approximately 200 K occurs in T-cycling experiments whose correlation with adsorbate species was studied with varying oxygen and methanol partial pressures. The two branches of the hysteresis differ mainly in the amount of adsorbed oxygen, the methoxy species, and a carbonaceous species. Methoxy covers only a minor part of the catalytic surface reaching at most 20%. Above 650 K the surface is largely adsorbate-free.