The Surface Structure of Cu2O(100) - ACS Publications

Article pubs.acs.org/JPCC

The Surface Structure of Cu2O(100)

Markus Soldemo,† Joakim Halldin Stenlid,‡ Zahra Besharat,† Milad Ghadami Yazdi,† Anneli Ö nsten,† Christofer Leygraf,§ Mats Göthelid,† Tore Brinck,*,‡ and Jonas Weissenrieder*,† †

Material Physics, KTH Royal Institute of Technology, 164 40 Kista, Sweden Applied Physical Chemistry, and §Division of Surface and Corrosion Science, KTH Royal Institute of Technology, 100 44 Stockholm, Sweden

‡

S Supporting Information *

ABSTRACT: Despite the industrial importance of copper oxides, the nature of the (100) surface of Cu2O has remained poorly understood. The surface has previously been subject to several theoretical and experimental studies, but has until now not been investigated by atomically resolved microscopy or high-resolution photoelectron spectroscopy. Here we determine the atomic structure and electronic properties of Cu2O(100) by a combination of multiple experimental techniques and simulations within the framework of density functional theory (DFT). Low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM) characterized the three ordered surface structures found. From DFT calculations, the structures are found to be energetically ordered as (3,0;1,1), c(2 × 2), and (1 × 1) under ultrahigh vacuum conditions. Increased oxygen pressures induce the formation of an oxygen terminated (1 × 1) surface structure. The most common termination of Cu2O(100) has previously been described by a (3√2 × √2)R45° unit cell exhibiting a LEED pattern with several missing spots. Through atomically resolved STM, we show that this structure instead is described by the matrix (3,0;1,1). Both simulated STM images and calculated photoemission core level shifts compare favorably with the experimental results.

1. INTRODUCTION The coinage metal copper has since prehistoric time played an important role in society. Later, its high electrical and thermal conductivity has made it one of our most essential materials. Today copper is the third most widely used metal in industry with applications ranging from roofing and plumbing material to electrical transmission and power generation.1 Metallic copper slowly oxidizes in contact with an atmospheric ambient. The most common initial corrosion product is cuprous oxide (Cu2O).2 Under anoxic conditions, metallic copper is stable with respect to any of the known oxides and hydroxides.3,4 However, Hultquist and co-workers have made observations that indicate that copper may corrode when interacting with oxygen-free water, resulting in the release of hydrogen gas.5−7 The study of low-index surfaces of Cu2O, as a model system for the oxidized copper, may provide insights into of how corrosion © XXXX American Chemical Society

of copper proceeds at an atomic level. Other suggested applications for cuprous oxide is as catalyst for, for example, CO oxidation,8−10 propene epoxidation,11−14 and photocatalytic water splitting.15 Due to its corrosion resistance and relatively low cost, copper is suggested as part of the enclosure for long-term nuclear waste containers.16 During the storage, a thin oxide layer will cover the copper surface. Determining the properties of this oxide is thus important for understanding the nature of the interaction between the containers and the environment where they are stored. Received: November 19, 2015 Revised: February 10, 2016

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DOI: 10.1021/acs.jpcc.5b11350 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

1100, 650, and 80 eV for Cu 2p, O 1s, and valence band measurements, respectively. The STM-study was carried out using an Omicron VT-STM operated in constant current imaging mode with electrochemically etched tungsten tips. The STM-chamber is attached to a preparation chamber equipped with ion sputter gun, leak-valves for gases and LEED-apparatus. The presented LEED patterns were obtained using this LEED. The base pressure in both preparation chamber and analysis chamber is low 10−10 mbar.

Single crystal Cu2O surfaces have experimentally been studied using a range of different methods: low-energy electron diffraction (LEED),17 photoelectron spectroscopy (PES),18 and scanning tunneling microscopy (STM).19 In addition to single crystals, Cu2O has also been studied in form of, for example, oxidized polycrystalline copper,20 oxidized copper single crystals,21,22 and Cu2O nanoparticles.23 Further, molecular interaction with Cu2O has been studied for, for example, carbon monoxide,24 water,25,26 propene,12,14 and sulfur dioxide.26 The surface structure of Cu2O(100) was first investigated experimentally using LEED and X-ray and ultraviolet photoelectron spectroscopy (XPS/UPS) by Schulz and Cox.17 Through different sample preparation procedures three ordered surface structures were reported, a copper terminated (3√2 × √2)R45° reconstructed surface structure, a c(2 × 2)-structure with a half terminating layer of oxygen, and an oxygen terminated (1 × 1)-structure. The Cu2O(100) surface has further been subject for several theoretical studies aiming at describing its structures and properties, for example, refs 27−32. Experimentally, no atomically resolved direct imaging microscopy or high-resolution photoelectron spectroscopy studies have been reported for the surface. However, since the literature contains a large number of studies where adsorption on the Cu2O(100) surface is investigated, both theoretically33−36 and experimentally,12,14,17,24,25,37−39 it is important to establish a thorough understanding of the atomic properties of the surface. In this study, the surface structures of Cu2O(100) are investigated by means of several experimental methods: STM, LEED, and synchrotron radiation based PES. The experimental results are, with good agreement, compared to simulations in the framework of density functional theory (DFT). Three ordered surfaces structures are found and are best described as (3,0;1,1), c(2 × 2), and (1 × 1). The (3,0;1,1) unit cell determined here has previously been proposed to be described by (3√2 × √2)R45°,17 which has another size and orientation on the surface.

3. COMPUTATIONAL METHODS Plane-wave DFT calculations were performed using the Vienna Ab-Initio Simulation Package (VASP).41−45 The calculations were carried out at the PBE-D3(BJ)+U level of theory46−49 using a U-j value of 3.6 eV50 for the Cu d-states. Spin polarization was allowed throughout. All the valence electrons (Cu, 3d104s1; O, 2s22p4) were treated explicitly, employing a plane-wave cutoff of 520 eV. Standard PBE PAW51,52 (projector augmented wave method) potentials were used to represent the core states. A Γ-centered 4 × 4 × 1 k-point mesh was used, and the Brillouin zone was sampled using the tetrahedron method with Blöchl corrections.53 Cu2O has a cubic crystal structure belonging to the pn3 space-group. The Cu2O lattice is constructed by two intertwined Cu face-centered cubic and O body-centered cubic sublattices. Models for the (1 × 1), c(2 × 2), and (3,0;1,1) surface unit cells were constructed from a converged bulk phase lattice, with the cubic unit cell parameter of 4.316 Å. The computational cell size is in close agreement to previously reported experimental54−56 and theoretical values.32,57 Note that the (3,0;1,1) surface was represented by the equivalent (2,−1;1,1) unit cell in all calculations. The surfaces were modeled as periodic slabs consisting of 6 Cu2O layers with a vacuum distance of 17 Å. Dipole corrections were applied, as discussed in refs 25 and 26, in order to avoid the buildup of a net dipole moment in the periodic calculations. The top two Cu2O layers were allowed to relax in all calculations. The structure of the unreconstructed (1 × 1) surface was obtained by allowing the surface atoms to relax in the surface normal direction. In order to identify the favored reconstruction structures for the c(2 × 2) and (3,0;1,1) unit cells, two approaches were used in parallel: (i) relaxation of a set of trial structures and (ii) simulated annealing (SA) molecular dynamics (using the Born−Oppenheimer formalism). The SAs were carried out in three consecutive steps of 3.6 ps. In the first step, the temperature was increased from 0 to 800 K. In the second, the temperature was constant at 800 K, and in the last the temperature was decreased from 800 to 300 K. A time-step of 2.5 fs was used and the NVE ensemble was invoked. During the SA, a plane-wave cutoff of 320 eV and a 2 × 2 × 1 k-point mesh was employed. For all the structural relaxations, the geometry convergence threshold was set to 0.03 eV Å−1. The relative stabilities of the surfaces (with an area A) were evaluated by comparison of their respective surface energies ES by the equation below. The factor 1/2 comes from the fact that two surfaces are formed upon cleavage of the bulk, and n is the number of repeated Cu2O units in the slab model. 1 ES = (Eslab − nE bulk ) (1) 2A Simulated STM pictures were generated with the HIVE program,58 using the Tersoff−Hamann method.59 According to this, the STM current is given by the local density of states

2. EXPERIMENTAL DETAILS The Cu2O(100) single crystal used in this study is a natural crystal acquired from the Surface Preparation Laboratory, The Netherlands. The cleaning procedure of the crystal consists of cycles of argon ion sputtering (0.5 kV, 15 min) followed by anneal in ultrahigh vacuum (UHV) or in oxygen gas at different pressures and temperatures, depending on the desired final surface structure. The annealing was predominantly carried out at three different temperatures: 580 °C, 630 °C and 700 °C for durations between 15 and 60 min. The oxygen gas pressures during the annealing cycles were 3 × 10−6 mbar and 2 × 10−5 mbar. For the PES studies, the cleanness was checked by overview spectra as well as core level spectroscopy at common contamination regions, e.g. potassium and carbon. For the LEED and STM studies, the crystal cleanness was checked by the sharpness of the diffraction spots in the LEED pattern. The PES study was carried out at the high-resolution corelevel photoelectron spectroscopy endstation at beamline I311 at Maxlab, Lund, Sweden.40 The endstation has an analysis chamber equipped with a Scienta SES-200 analyzer. The preparation chamber, in connection to the analysis chamber, is equipped with argon ion sputter guns, leak-valves for introduction of gases, and a LEED-apparatus. High-resolution photoelectron spectra were collected using photon energies B


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Table 1. Presence of the Different Ordered Surface Terminations As a Function of Preparation Parameters (As Observed in LEED and STM)a UHV

3 × 10−6 mbar O2

2 × 10−5 mbar O2

annealing temperature (°C)

15 min

15 min

30 min

15 min

30 min

60 min

580 630 700

(3,0;1,1) and c(2 × 2) (3,0;1,1) and c(2 × 2) (3,0;1,1) and c(2 × 2)

(3,0;1,1) (3,0;1,1) (3,0;1,1)

(3,0;1,1) (3,0;1,1)

(3,0;1,1) (3,0;1,1) and c(2 × 2)

(3,0;1,1) (3,0;1,1) and c(2 × 2)

(3,0;1,1)bc and (1 × 1)c

a Multiple cycles of sputter and anneal in UHV increases the coverage of c(2 × 2) and decrease the coverage of (3,0;1,1). b(3,0;1,1) with some double spots. cStrong background.

remained. The distances in the Cu2O(100)(1 × 1)-pattern were also compared to the distances of the Cu2O(111)(1 × 1)pattern. This procedure is described in more detail in the Supporting Information. The energetically favored structures for the different surface unit cells obtained by DFT are shown in Figure 2. Experimentally, extended oxygen annealing at elevated oxygen pressures is required in order to form the (1 × 1) surface pattern, in line with earlier studies,17 whereas the (3,0;1,1) and, to a lesser extent, the c(2 × 2) structures are obtained after annealing at low pressures of oxygen. Previous computational work has confirmed that the (1 × 1) oxygen terminated surface in Figure 2 is the most stable under oxygen rich conditions.32 A copper terminated Cu2O(100) surface is thus primarily found under reducing/oxygen lean conditions, however, the favored c(2 × 2) and (3,0;1,1) structures were found up to oxygen pressures of 2 × 10−5 mbar in this study. Computationally, we define that an oxygen poor environment corresponds to a state where pO2 is zero mbar: i.e. no addition of oxygen to the surface is considered. Under oxygen lean conditions the oxygen terminated (1 × 1) surface is unstable (experimentally and computationally). In contrast to the oxygen terminated surface, Cu2O(100):Cu reconstructs extensively. Unreconstructed, the copper terminated (1 × 1) structure has a spacing between the surface layers of 2.16 Å and a calculated surface energy of 0.089 eV Å−2 (see Table 2), which is similar to the value of 0.091 eV Å−2 found by Soon et al.32 By relaxation in the surface normal direction, the spacing between the top two Cu2O layers is reduced to 2.05 Å, and to 2.08 Å between the second and the third layer. Similar contractions are found for all surface structures reported herein. Upon normal contraction, the surface energy of (1 × 1) is reduced to 0.088 eV Å−2. Further stabilization of the (1 × 1) surface (to 0.075 eV Å−2) can be achieved by dimerization of the terminal Cu+-ions in the [011]-direction. This yields a Cu− Cu distance of 2.40 Å, a distance that is considerably shorter than the 3.05 Å of the unreconstructed surface. Note that the Cu+-Cu+-dimerization is a well-documented behavior previously described both experimentally17 and theoretically.28 The dimerization can proceed further, leading to the formation of an extended one-dimensional chain (ridge) in the [011]-direction, see the ridge-dimer c(2 × 2) structure of Figure S6 in the Supporting Information. This structure is described by a c(2 × 2) unit cell and has a corresponding surface energy of 0.073 eV Å−2. The dimerization is here most evident in the [011]direction with a Cu−Cu distance of 2.46 Å, but a clear shortening of the Cu−Cu distance (2.54 Å) in the [011̅]direction is also observed. The above structures were obtained by trial-structure structural searches. However, of the two approaches used to obtain reconstruction structures for the c(2 × 2) and (3,0;1,1) surfaces, the Simulated Annealing (SA) method generated the

(LDOS) at the simulated probe tip (i.e., at a particular distance from the surface) for the electronic states ranging from the Fermi level to a given bias. The bias roughly corresponds to the experimental voltage. Herein we varied the bias and probe distances to match the experimental conditions for the different surfaces. Computational O 1s and Cu 2p PES surface core level shifts (SCLS) were obtained using the final state approximation.60,61 The SCLS is obtained by comparing the ionization energy from a core electron at the surface to the ionization energy in the bulk. The generation of a charge upon ionization was compensated for by a background jellium potential. This is the preferred method to calculate SCLS for semiconductors.62 By representing the bulk by an atom in the middle of the slab, the reference ground state structures are the same for the surface and bulk atoms. Thus, the shifts are obtained by SCLS = Esurface − Ebulk. In order to converge the SCLS, tighter criteria for the self-consistent field convergence (10−6 eV) along with a thicker slab (10 Cu2O layer) were necessary.

4. RESULTS 4.1. Surface Structures. Three different surface structures were identified in the experimental part of this study. Relative to the bulk unit cell the structures have the following periodicities: (1 × 1), c(2 × 2), and two 90° rotational domains of the matrix (3,0;1,1). The prevalence of the different structures as a function of preparation parameters is provided in Table 1. All structures were observed by scanning tunneling microscopy and all structures, except the (1 × 1) structure, were observed directly by LEED. The LEED-patterns for the different structures are shown in Figure 1. In addition, the coexistence of the (3,0;1,1) and c(2 × 2)-structure is shown in Figure 1c. In order to ensure identification of the correct bulk (1 × 1) unit cell spots as internal reference spots in LEED, the surface was gently sputtered until only the (1 × 1)-pattern

Figure 1. LEED-images of (a) the reconstructed (3,0;1,1) surface taken at an electron beam energy of 40.0 eV, (b) c(2 × 2)-surface structure taken at an electron beam energy of 37.8 eV, and (c) coexistence of the (3,0;1,1)- and c(2 × 2)-surface structures taken at an electron beam energy of 23.5 eV. Black arrows indicate the (1 × 1)spots, blue and red spots indicate the two rotational domains of (3,0;1,1), and yellow spots indicate the c(2 × 2)-structure. The (0,0) spot is located behind the electron gun at the center of the screen. C


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Figure 2. Atomic models of the structures from the DFT calculations. The unit cells for the structures are indicated in green. Panel (a) shows the copper terminated (1 × 1) structure, and panel (b) the bulk crystal structure of Cu2O. The surface structures suggested for the experimentally observed (1 × 1), c(2 × 2), and (3,0;1,1) surface patterns are shown in (c)−(e). Note that the (1 × 1) structure in (c) is oxygen terminated.

computational error). In the identified low-energy c(2 × 2) and the (3,0;1,1) structures, the clear separation between Cu and O layers present in the unreconstructed and dimerized structures is reduced: The structures are reconstructed both in the surface normal and in the lateral dimensions and the Cu and O layers are clearly intermixed. Furthermore, both the favored c(2 × 2) and (3,0;1,1) reconstructions are considerable more corrugated than the unreconstructed and dimerized surface structures. The (3,0;1,1) structure has, however, a noticeable larger variation in its surface topology with a difference of 2.15 Å from the highest to the lowest atom (for the c(2 × 2) structure the corresponding difference is only 0.5 Å). This is consistent with experimental findings (see Figure S3 in the Supporting Information). Again, the surface topology can be described by the formation of ridges. In the case of c(2 × 2) the ridges are elongated in the [011]-direction. Also for the (3,0;1,1) structure, the ridges are directed in the [011]-direction. 4.2. Scanning Tunneling Microscopy. Experimental STM-images of the three terminations together with simulated STM-images are shown in Figure 3. The computational parameters were chosen to match the experimental conditions

Table 2. Calculated Surface Energies, ES, for the Different Copper Terminated Cu2O(100) Surface Structures under Oxygen Lean Conditions unit cell

structure

(1 × 1) (1 × 1) (1 × 1) c(2 × 2) c(2 × 2) (3,0;1,1)

a

unrelaxed contractedb dimerc ridge-dimerd low-energye low-energye

ES [eV Å−2] 0.089 0.088 0.075 0.073 0.071 0.071

Unreconstructed (1 × 1) surface as cut from the crystal structure. Unreconstructed (1 × 1) surface relaxed in surface normal direction. c Cu+−Cu+ dimerization in the [011] direction. d[011]-directed Cu+ ridge formation via [011] and [011̅] dimerization. eLow-energy structures obtained by Simulated Annealing (SA) MD simulations. a b

lowest energy structures for both the c(2 × 2) and the (3,0;1,1) unit cell. The (3,0;1,1) surface has a slightly lower surface energy than the c(2 × 2) surface, although both energies round off to 0.071 eV Å−2 (note that the difference is within the D


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consist of a bilayer of one oxygen plane and one copper plane to maintain charge neutrality and stoichiometry. Terraces with multiple domains of rotational domains and domain boundaries were in general observed after annealing to 580 °C while rotational domains separated on every other terrace were more frequently found after annealing to 630 °C. STM observations of extensively annealed surfaces (630 K, 60 min) at high oxygen partial pressure (2 × 10−5 mbar O2) revealed a partial coverage of an (1 × 1) reconstruction. Anneal in oxygen gas at 580 °C did also give rise to an unordered surface structure that coexists with the (3,0;1,1)-structure. Anneal in oxygen gas at increased temperatures or anneal in UHV after argon ion sputtering does not result in the unordered structure. This brings two possible explanations, either the presence of oxygen gas during anneal disturbs the surface and destroys the ordering or the oxygen gas prevents the surface to get ordered during annealing. The presence of O2 is known to increase the mobility of surface atoms.63 The coexistence of the (3,0;1,1)- and c(2 × 2)structures as observed by STM is shown in the Supporting Information. This provides further evidence that the LEEDpattern in Figure 1c is a superposition of the diffraction pattern from the two structures. In STM it is found that the (3,0;1,1)structure is significantly more corrugated than the c(2 × 2)structure, consistent with computational results. 4.3. Photoelectron Spectroscopy. A photoelectron spectrum of the O 1s-region for the (3,0;1,1)-termination is shown in Figure 4. The O 1s-peak is fitted using a linear

Figure 3. STM-images (46 Å × 46 Å) of the surface structures. Experimental STM: (a) (1 × 1) collected at V = −1.79 V, I = 0.46 nA. (b) c(2 × 2), V = 1.06 V, I = 0.14 nA, and (c) (3,0;1,1), V = 2.78 V, I = 0.31 nA. DFT-STM settings: (a) at V = −3 V bias and probe d = 1.9 Å, (b) V = 1 V, d = 2.4 Å, and (c) V = 3 V, d = 2.9 Å. Bright areas correspond to protrusions. Note in (b) that the experimental STM image is distorted by instrumental drift.

as closely as possible. The unit cells for the different structures observed by LEED are confirmed by the STM-data. The sample orientation and scan directions are the same for all STM-images permitting direct comparison between the different structures. Thermal drift in the piezo scanner accounts for the slight deviation in the observed lattice directions. The unit cells are indicated in red in the experimental images together with the lattice directions. The experimental image assigned to the (1 × 1) structure exhibits a square lattice with one protrusion per unit cell, the side of the unit cell agrees with the expected unit cell of 4.27 Å. The corresponding simulated STM-image shows a similar square lattice. The experimentally observed unit cell for the c(2 × 2) surface structure is a square with sides aligned in the [011]- and [011̅]-directions. Each unit cell contains one protrusion noticeably elongated in the [001]-direction. An elongation in the same direction can also be observed in the simulated STMimage for the optimized c(2 × 2) unit cell. The experimental STM-image of the (3,0;1,1) reconstructed surface shows protrusions aligned in rows along the [011]direction. In the perpendicular direction to the [011]-direction, it is observed that the protrusions are not aligned. Instead, the protrusions are found to line up in the [010]-direction. The STM image clearly reveals that the (3,0;1,1) reconstructed surface is more corrugated than the other two ordered structures. It is further noticed that only one protrusion per unit cell is observed experimentally while the simulated STMimage exhibit more details with lower laying features between the rows (note that simulated STM allows for resolving finer details than what is expected to be observed experimentally on such a corrugated surface). Wide area STM-images of the (3,0;1,1) reconstruction show that there are indeed two 90° rotational domains of the (3,0;1,1) structure present on the surface. The two rotational domains may either coexist on the same terrace with multiple domain boundaries or grow on separate terraces. This is shown in further detail in the Supporting Information. The step height between two terraces when they consist of different rotational domains is measured to 2.3 Å, reasonably close to half the lattice parameter of 4.27 Å.54−56 This suggests that each step

Figure 4. Photoelectron spectra of the O 1s-region with a linear background subtracted for the (3,0;1,1) termination of Cu2O(100). Blue and red bars represent the calculated surface core level shifts relative to the main peak (gray bar) for surface most oxygen atoms and the subsurface oxygen atoms, respectively. Blue bars are shifted 0.60, 0.95, and 1.00 eV toward lower binding energies, while red bars are shifted 0.25, 0.35, and 0.45 eV toward lower binding energies.

background and three components; one for the main peak (gray) and two for the shoulder (red and blue) on the low binding energy side. The shoulder, which corresponds to oxygen atoms residing in the upper layers of the reconstructed surface,26 is here fitted with two components corresponding to the surface layer and the subsurface layer. We motivate the use of two components for the shoulder both by the line profile of the O 1s spectrum and the calculated core level shifts for the two uppermost oxygen layers. The surface layer and the subsurface layer are shifted approximately 0.8 and 0.4 eV toward lower binding energies compared to the main peak that is located at 530.2 eV. The peak widths for the two components are chosen to fit the calculated core level shifts for the oxygen E


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5. DISCUSSION Experimentally, three different ordered surface structures were observed by LEED and STM. These three ordered structures were successfully reproduced using DFT. The combination of LEED and STM provides complementary information that helps to identify the periodicities and orientations of the different structures. Knowing the lattice parameter for Cu2O and the position of the (1 × 1) LEED spots, comparison between the real space STM-images and LEED-images are possible. The identified ordered structures are (1 × 1), two 90° rotational domains of (3,0;1,1), and c(2 × 2). DFT calculations ranks the stability of the surface structures as (3,0;1,1) ≥ c(2 × 2) > (1 × 1) under oxygen lean conditions, while under oxygen rich conditions an oxygen terminated (1 × 1) surface is most favorable, which is in qualitative agreement with the experimental prevalence. Unreconstructed, the copper terminated Cu2O(100) surface expected under oxygen lean conditions, is polar with a net charge separation between different surface layers (due to the alternating Cu+ and O2− layers) and a net dipole moment perpendicular to the surface (DFT: μ = 15 mDÅ−2). This categorizes it as a Tasker class III surface,65 which is known to undergo extensive reconstruction in order to reduce the surface polarization.66 In the absence of stabilizing adsorbates, such as O(ad) or H(ad), the reconstruction of Cu2O(100) proceeds via dimerization of the surface Cu+ ions, a phenomena that has been confirmed previously via both experiments17 and computations.28 This agrees well with the dimer (1 × 1) and ridge-dimer c(2 × 2) reconstructions reported herein and in the Supporting Information. Neither the dimer (1 × 1) nor the ridge-dimer c(2 × 2) reconstructions do, however, perfectly cancel out the polarization of the surface (μ = 14 mD Å−2 and μ = 12 mD Å−2, respectively). Further stabilization may therefore be achieved via a more elaborate redistribution of the surface atoms. For the low-energy c(2 × 2) and (3,0;1,1), reconstruction the surface Cu+ and O2− are no longer separated in different distinct layers, but intermixed. Additionally, the surface is corrugated. The result is a reduction of the surface normal dipole moment (μ = 10.3 mDÅ−2 and μ = 7.2 mDÅ−2 for the c(2 × 2) and (3,0;1,1) structures, respectively), which corresponds to more favorable surface configurations. Some of the herein identified surface structures are different from the previously proposed structures. For instance an ordered structure described by the periodicity (3√2 × √2) (√2 × √2)R45° was reported in ref 17. In the current study, the corresponding LEED-pattern is obtained when two structures, (3,0;1,1) and c(2 × 2), are observed by STM to

atoms in the surface and subsurface layers. Colored bars in the spectrum represent the calculated core level shifts for the oxygen atoms in the two layers. The three different surface oxygen atoms are shifted by 0.60, 0.95, and 1.00 eV toward lower binding energies relative to bulk oxygen and the three different oxygen atoms in the first layer of subsurface oxygen atoms are shifted 0.25, 0.35, and 0.45 eV toward lower binding energies. All these contributions are located under the experimental low binding energy components; experimentally the individual components cannot be resolved due to the intrinsic lifetime broadening. The Cu 2p-spectrum (see the Supporting Information) shows a spin−orbit split between Cu 2p1/2 and Cu 2p3/2 of 19.8 eV in good agreement with what is expected from the literature.64 Figure 5 shows the valence band of the (3,0;1,1)-termination together with calculated local density of states. The most

Figure 5. Experimental valence band (gray) for the (3,0;1,1) termination measured with a photon energy of 80 eV and calculated local density of states for the top surface layers.

intense peaks, between 2−4 eV binding energy, are mainly contributions from Cu 3d. The double peak between approximately 5.5−8 eV originates from O 2p but has also some Cu 3d contribution, whereas the peak close to the valence band edge consists mainly of Cu 3d contributions. The O 2p contribution to this peak is significantly larger than from Cu 4s. This finding is in contrast to literature where this peak is suggested to be a mixed state between Cu 3d and Cu 4s. Compared to experimental data in literature, for example, ref 17, it is found that the valence band follows the line profile well.

Figure 6. Illustration of unit cell of the reconstructed surface. (a) Real space representation of the orientation and sizes of the unit cells. Blue cells are those proposed by Shultz and Cox. Green cells are those proposed by this study. (b) Two rotational domains of the (1,0;1.5,1.5)-structure (blue and red squares). Black dots represent the (1 × 1)-pattern. (c) Two rotational domains of the (3,0;1,1)-structure (blue and red filled circles). Black dots represent the (1 × 1)-pattern. F


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The Journal of Physical Chemistry C coexist on the surface; see the Supporting Information. This suggests that the reported change in Cu-to-O ratio when the centered spots (contribution from the c(2 × 2)-pattern) are present is due to a change in coverage of the different surface structures rather than the formation of a new structure. Furthermore, the identified reconstructed surface with a unit cell described by the matrix (3,0;1,1) with two 90° rotational domains does not fit the previously proposed unit cell.17 In ref 17, the surface structure is described as (3√2 × √2)R45° with many missing spots or more accurately as two 90° rotational domains where one side of the unit cell is directed along one of the principle directions with unit length and the other 45° off that direction with (3/2)√2 unit lengths. This gives the matrix (1,0;1.5,1.5) that corresponds to a unit cell of different size and orientation compared to the unit cell described by (3,0;1,1). However, if the c(2 × 2) spots are mistaken for (1 × 1) spots, the reconstructed pattern will be described by the matrix resulting from the description by Schulz and Cox. The differences between the two matrixes are illustrated in real space and as LEED-pattern in Figure 6. In the photoelectron spectroscopy data for the reconstructed (3,0;1,1) structure for the O 1s-region, an additional shoulder is found on the low binding energy side of the main contributing peak. Fitting the spectrum with three components, it was found that the surface layer is shifted approximately 0.8 eV toward lower binding energies compared the bulk component. Comparing the shift to reported data on the O 1s-region for Cu2O and CuO, a difference in binding energy of 0.9 eV is found, where CuO has the lower binding energy.67 This may suggest that the contribution to the shoulder partly comes from CuO-like oxygen atoms at the surface. The DFT calculations of the core level shifts for surface atoms do confirm that the shoulder contribution origins from the surface level and subsurface level oxygen atoms. Based on the computational data, the O 1s shift results from contributions from the top two Cu2O layers, where the O atoms of the upper layer have a higher shift than those of the second layer. The Cu 2p-region shows no termination specific information but agrees with reported data for Cu2O.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the Swedish research council (VR) and the Knut and Alice Wallenberg foundation. J.H.S. and T.B. would like to acknowledge the Swedish Nuclear Fuel and Waste Management Co (SKB) and the School of Chemistry at KTH, via its excellence stipend to J.H.S., for financial support. We gratefully acknowledge the support from the Maxlab staff during beamtimes. The computations were performed on resources provided by the Swedish National Infrastructure for Computing (SNIC) at the National Supercomputer Center (NSC) at Linköping University NSC. Pavel Korzhavyi, A. Johannes Johansson, and Henrik Grö nbeck are warmly acknowledged for helpful discussions and technical assistance

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REFERENCES

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6. CONCLUSION The termination of Cu2O(100) depends strongly on the sample preparation parameters. Three ordered structures were experimentally identified. These were reproduced by DFT calculations. The (3,0;1,1) reconstructed structure is the energetically most favorable termination taking both DFT and experimental results into account. The other ordered structures have unit cells described by (1 × 1) and c(2 × 2), where the (1 × 1) is oxygen terminated and only attainable under elevated oxygen pressures. Experimental and simulated STM-images are found to be in good agreement. For the (3,0;1,1)-structure, the low binding energy component in photoelectron spectra for O 1s was successfully reproduced theoretically and is described by six components originating from the two oxygen layers closest to the surface. It is concluded that the previous description of the energetically most favorable surface structure has to be revised in order to provide a proper description of the surface.

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LEED-patterns for identification of (1 × 1) spots; STMimages of rotational domains of the (3,0;1,1)-structure; STM-images of the coexistence of (3,0;1,1)- and c(2 × 2)-structures on the surface and their corrugation; STMimages of an nonordered surface structure; PES data on the Cu 2p-region; atomic structure figures and corresponding simulated STM images for the dimer (1 × 1) and ridge-dimer c(2 × 2) structures as well as a simulated STM image of copper terminated (1 × 1); coordinates for the optimized structures (PDF)

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The Surface Structure of Cu2O(100) - ACS Publications

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