Article pubs.acs.org/JPCC
Reactive Copper Deposition on Au(111) and Mo(001): Role of the Support in the Oxidation Process Hanna Fedderwitz, Boris Groß, Hendrik Straẗ er, and Niklas Nilius* Carl von Ossietzky Universität Oldenburg, Institut für Physik, D-26111 Oldenburg, Germany ABSTRACT: Photoelectron spectroscopy and scanning tunneling microscopy were used to study the oxidation characteristics of Cu atoms deposited on two different supports, Au(111) and Mo(001). While a thin, homogeneous Cu2O film grows on gold, metallic Cu deposits develop on the Mo surface. The difference in the oxidation behavior is discussed in terms of the chemical inertness of the substrates and possible interface interactions. A favorable lattice match stabilizes the Cu2O film on Au(111). Conversely, a passivating surface oxide develops on Mo(001), inhibiting Cu oxidation and driving the metal into Volmer−Weber growth. Apparently, the more reactive substrate is not the one that promotes oxidation of the precipitated ad-metal.
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INTRODUCTION Oxide films of 1−50 nm thickness, grown on single crystalline metal supports, have attracted an enormous amount of attention in recent years.1−4 Interest in this material combination arises from the unique possibility to study the properties of bulk oxides, which are often inaccessible to surface-science techniques due to their insulating nature and subsequent charging effects. Moreover, thin oxide films are ideally suited to explore metal-oxide interfaces that are of technological relevance in heterogeneous catalysis, microelectronics, materials science, and optics. It has been found that ultrathin oxide films develop properties that may substantially deviate from those of the bulk systems.3,5 The differences arise from a strong influence of the metal below, which affects the structural, electronic, and chemical nature of the oxide films. For example, interfacial coupling to the support governs the symmetry and periodicity of the oxide lattice and produces atom configurations that are unknown in the bulk.1,6 This might be exploited to grow polar oxides that exhibit a diverging electrostatic energy in the bulk limit but are stable in the form of thin films due to the dipole compensation via the metal substrate.7,8 Electronically, thin-film oxides are peculiar as they are able to exchange electrons with the substrate which in turn introduces a redox potential to the compound system.9,10 Those charge-transfer effects were shown to control the growth shape of metal particles and the adsorption strength of molecules, hence the reactivity of the thin-film system.11 Finally, oxide films open interesting routes to tune the work function of metals, enabling the fabrication of materials with an exceptionally high electron emissivity.12 In this study, we demonstrate that the chemical nature and surface structure of a substrate determine whether an oxide film forms at all upon reactive metal deposition. For this purpose, Cu is evaporated in an O2 atmosphere onto two model © 2016 American Chemical Society
supports, Au(111) and Mo(001). While the former is known for its chemical inertness, the latter vigorously interacts with oxygen. Moreover, the Au(111) and Mo(001) lattices have hexagonal and square symmetry, respectively, an aspect that is important for the stability of the emerging oxide. Using scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS), we show that a Cu2O thin film develops on gold while metallic Cu particles are formed on Mo(001). We explain this unexpected result with the intrinsic reactivity of the two substrates and a different interplay of surface and interface free energies. Copper oxidation was chosen as the model reaction here because of the fascinating properties of emerging cuprous oxide films.13 With a band gap of 2.15 eV and a mostly p-type conductance behavior,14 Cu2O is an interesting material for photochemistry, photovoltaics, and optoelectronics.15−17
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EXPERIMENTAL DETAILS
Copper oxidation was studied in an ultrahigh vacuum chamber equipped with a custom built STM operated at liquid-nitrogen temperature, a non-monochromatized Mg Kα X-ray source, a hemispherical electron analyzer, and standard surface-science tools. The Au(111) and Mo(001) surfaces were prepared by multiple Ar+ sputtering (Au 0.8 kV, Mo 1.5 kV) and annealing cycles (Au 900 K, Mo 1500 K). Surface cleanness was checked with XPS and constant-current STM using electrochemically etched gold tips. Copper was deposited from an e-beam evaporator at 5 × 10−6 mbar of oxygen at room temperature. The deposition rate was set to 0.4 monolayers (ML) per Received: January 14, 2016 Revised: March 29, 2016 Published: March 29, 2016 7591
DOI: 10.1021/acs.jpcc.6b00435 J. Phys. Chem. C 2016, 120, 7591−7596
Article
The Journal of Physical Chemistry C
at 916 and 921 eV (Figure 1c). Upon reactive Cu deposition, the three maxima merge into a single broad peak at 918 eV, a value that matches neither the CuO nor the Cu2O Auger line. The nature of this peak could be clarified by increasing the nominal Cu thickness to 10 ML (Figure 1c, bottom curve). This spectrum shows a single maximum at 916.7 eV, in good agreement with the LMM Auger line of Cu2O reported in the literature.22,23 With this input, the thin-film spectra were interpreted as a superposition of two Gaussians at 916.7 and 918.5 eV, corresponding to the Cu2O and metallic Cu components, respectively. Apparently, the ad-film is not fully oxidized but contains a metallic phase that likely resides at the interface. Recent DFT calculations indeed revealed that oxygen is unstable at the Cu2O/Au interface, and a metallic Cu plane develops below the emerging Cu2O layer.24 In thick films, the Auger line due to metallic Cu becomes strongly attenuated and cannot be detected (Figure 1c, bottom curve). Surprisingly, the same deposition scheme led to different results on the Mo(001) support. Despite an O2 excess during oxidation (O2/Cu 600:1), the overall shape of the Cu LMM Auger line resembles the line of metallic copper (Figure 1c, second curve). The dominance of Cu0 was observed even for as-grown films (without postannealing). Apparently, Cu oxidation is ineffective on Mo(001), suggesting a decisive role of the support as a reducing agent for copper. Evidence for such a scenario comes from the Mo 3d core level spectra, presented in Figure 1d. While the pristine surface shows only the wellknown doublet of the bulk metal (BE 227.7 eV), a high-energy shoulder is resolved at 229 eV after reactive Cu deposition. This component is compatible with Mo3+/Mo4+ species and indicates an oxidation of the topmost Mo layers.19 From an exponential attenuation model, the thickness of the surface oxide is estimated to be ∼2 ML. The interpretation also implies that the O 1s peaks in Figure 1b have different origins in the Au(111) and Mo(001) samples. While it relates to the oxygen inside the Cu2O film in the former case, it is the fingerprint of the oxidized Mo support in the latter. Note that almost identical O 1s BEs were revealed for Cu2O and MoO3 samples previously (530−530.5 eV),22,25 and the oxygen-containing phase cannot be identified with XPS alone. Further insight comes, however, from the morphological characterization of the samples with STM, as discussed next. B. STM Measurements. Despite identical preparation conditions, the Cu/O films on Au(111) and Mo(001) exhibit rather different morphologies, underlining the deviations found via XPS (Figure 2). The Au-supported films are atomically flat with a mean roughness below 1 nm. The main structural defects are edge and screw dislocations, serving as nucleation fronts in the layer-by-layer growth (Figure 2a, arrows). Rarely observed holes mark the onset of film evaporation during the final annealing step of the sample. Atomically resolved images exhibit a hexagonal atom arrangement with 6 Å periodicity, which is compatible with the (111) termination of Cu2O.24 The overlaid triangular pattern arises from the compensation of 3.8% mismatch between the Cu2O and Au lattices. For details regarding Cu2O growth on Au(111), we refer the reader to our preceding publication.18 Reactive copper deposition on Mo(001) led to a completely different picture (Figure 2b). No continuous film was observed in this case, but rather an array of well-shaped nanoparticles that, according to XPS, mainly consist of metallic copper. The mean diameter and height of the deposits amount to 8 ± 4 and 2.5 ± 1 nm, respectively, resulting in an aspect ratio of about
minute, calibrated by dosing Cu onto bare Au(111), where it grows in a layer-by-layer fashion and its surface coverage can be deduced from STM images. A nominal Cu coverage of 3−4 ML was used in the experiments unless stated otherwise. To reach a thermodynamically favorable surface configuration, the samples were postannealed at 600 K in oxygen. The annealing procedure led to a slight decrease of the Cu XPS signals, indicating desorption/dissolution of small Cu quantities or surface restructuring accompanied by a dewetting process. While Au(111)-grown films displayed a clear hexagonal pattern in low-energy electron diffraction (see ref 18), no spots were detected for Mo(001), suggesting the absence of an ordered adlayer in this case.
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RESULTS AND DISCUSSION A. Photoelectron Spectroscopy. Information about the chemical state of copper after oxidation was derived from XPS data as shown in Figure 1. The Cu 2p and O 1s core levels were
Figure 1. XP spectra obtained after Cu deposition in oxygen on Au(111) (black) and Mo(001) (red): (a) Cu 2p, (b) O 1s, and (c) Cu LMM Auger transition. The topmost curve in (c) was taken on Au(111) covered with metallic Cu; the third and fourth curves are representative of thin (∼2 ML) and thick (∼10 ML) Cu/O layers, respectively, on Au(111). (d) Mo 3d peak on clean Mo(001) and after reactive Cu deposition. The high-energy shoulder in the latter case points to the formation of a Mo surface oxide. All spectra were analyzed via Shirley background subtraction and Gaussian fitting except for the Mo 3d peaks, where a Doniach−Sunjic profile was used. Individual and sum fits are depicted by solid and dashed lines, respectively.
detected at almost identical binding energies (BEs) on the two supports (Figures 1a and b). The measured Cu 2p BE is compatible with both Cu0 and Cu+ species, and the Cu oxidation state cannot be determined at this point.19 Only the presence of Cu2+, as in CuO, can be excluded from the absence of a distinct satellite at 945 eV BE, i.e., in between the low- and high-energy component.20,21 Metallic Cu and Cu+ may be distinguished, however, by the shape and kinetic energy of the LMM Auger line of Cu.20,22 In the metal reference, produced by dosing bare Cu onto Au(111), the LMM transition comprises a main peak at 918.5 eV flanked by two shoulders 7592
DOI: 10.1021/acs.jpcc.6b00435 J. Phys. Chem. C 2016, 120, 7591−7596
Article
The Journal of Physical Chemistry C
Figure 2. STM topographic images obtained after reactive Cu deposition onto (a) Au(111) and (b) Mo(001) (derivative mode, 0.8 V, 0.1 nA, 100 × 100 nm2). While a continuous Cu2O film is formed on gold, Cu nanoparticles develop on Mo(001). The inset in (a) shows the Cu2O surface with atomic resolution; edge and screw dislocations are marked by arrows (0.1 V, 0.5 nA, 15 × 15 nm2). Typical Cu particle shapes on Mo(001) are highlighted by dashed polygons in (b).
Figure 3. STM images of individual Cu nanoparticles on Mo(001) shown in the derivative mode: (a) large and (b) small (100)-terminated octahedron (1.5 V, 0.1 nA, 10 × 10 nm2). (c) Overview image showing several (100)-type particles with identical orientation (30 × 30 nm2). (d) Atomically resolved image of a Cu top facet (3.5 × 3.5 nm2). The observed pattern is assigned to an O(2 × 2) superstructure on Cu(100), illustrated in the ball model.
lattice, we doubt that this interface structure is realized in large areas, although it might determine the initial particle orientation on Mo(001). Further support for the formation of Cu particles with (100) top planes comes from atomically resolved STM data (Figure 3d). The images exhibit a square lattice with ∼5 Å periodicity that corresponds to a (2 × 2) superstructure on the Cu(100) surface. We propose that O atoms that occupy the 4-fold hollow sites in Cu(100) according to earlier DFT28 and STM studies29 are responsible for this reconstruction. At 0.25 ML oxygen coverage, a simple (2 × 2) structure was revealed in agreement with our observations, while more complex reconstructions were found at a higher coverage. The O termination of the Cu deposits appears to be the only remnant of the oxygen exposure during preparation, confirming the XPS results. Cu/Mo(001) particle shapes other than the (100) type are shown in Figure 4. The Cu particle in panel (a) is rotated by 45° with respect to former deposits; i.e., its edges align with the Mo[100] direction. Moreover, the entire particle, including its top facet, has a rectangular shape, suggesting that the top and bottom planes are of (110) type in this example. In
0.3. Although tip-convolution effects were disregarded, the nominal Cu coverage is estimated to be ∼3 ML after annealing, in agreement with the initial Cu exposure. The granular film structure is also reflected in a mean surface roughness of 8 nm, one order of magnitude larger than that of Au(111). A closer inspection revealed distinct Cu particle shapes on the Mo(001) support. The most abundant type takes the form of a truncated octahedron (Figure 3a). The top facet is square, while the larger side facets are hexagonal and inclined by ∼50° against the sample surface. With this input, the top and side facets are tentatively assigned to the Cu(100) and (111) planes, respectively, while the small regions in between may correspond to (110) planes. This geometry matches the equilibrium shape of Cu deposits observed previously.26,27 Panel (b) shows a similar yet smaller Cu particle, exposing again a squared (100) top facet and four inclined (111) side facets. Note that most (100)-terminated Cu deposits have a similar orientation, indicating a preferred registry on the Mo(001) surface (Figure 3c). The Cu[110]-oriented particle edges align with a Mo[110] direction, enabling a (1 × 1) registry of Mo(001) and Cu(100) unit cells with an 11% mismatch (Figure 3d). Given the large compression of the Cu 7593
DOI: 10.1021/acs.jpcc.6b00435 J. Phys. Chem. C 2016, 120, 7591−7596
Article
The Journal of Physical Chemistry C
The second factor, stabilization of the oxide phase via interfacial coupling, favors Cu2O formation only on Au(111). The stability of a surface oxide arises not only from attractive interface interactions but also from a reduced surface energy of the support in the presence of the ad-layer.36 Quantitatively, the metal-oxide adhesion is determined by the free energies of the metal surface (γmetal), the oxide film (γoxide), and the interface (γinterface). A wetting growth of the oxide is expected if γoxide + γinterface < γmetal (eq 1), a relation that is safely fulfilled for Au(111) given its large free energy (1.3−1.6 J/m2)37 with respect to Cu2O(111) at the O2 chemical potential used in our experiment (
