Electronic Structure of Cu on ZnO (101̄0): Angle-Resolved

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J. Phys. Chem. C 2007, 111, 4256-4263

Electronic Structure of Cu on ZnO(101h0): Angle-Resolved Photoemission Spectroscopy Study Kenichi Ozawa,*,† Tomohiko Sato,† Yukako Oba,‡ and Kazuyuki Edamoto‡ Department of Chemistry and Materials Science, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8550, Japan, and Department of Chemistry, Rikkyo UniVersity, Nishi-Ikebukuro, Toshima-ku, Tokyo 171-8501, Japan ReceiVed: September 25, 2006; In Final Form: December 25, 2006

Angle-resolved photoemission measurements are carried out to investigate the valence electronic structure of Cu on ZnO(101h0). The coverage-dependent measurements of the Cu 3d and 4sp bands reveal that the growth of the Cu overlayer is characterized by a cluster formation. These Cu clusters are semiconductors with an energy gap around the Fermi level at the initial stages of adsorption, while the metallic nature is developed with increasing coverage. The Cu 3d state forms a band with a bulklike energy dispersion at high Cu coverages, whereas the two-dimensional (2D) band is formed in the low-coverage region, where the clusters are semiconducting in nature. From the dispersion relation of the 2D Cu 3d band, the arrangement of the Cu adatoms within the clusters is found to be strongly influenced by the surface structure of ZnO(101h0), i.e., the Cu adatoms are linearly arranged along the Zn-O dimer rows of the substrate surface.

1. Introduction Exploring the origin of catalytic activities that oxide-supported metal catalysts exhibit has been a challenging problem for years, and many experimental and theoretical studies have been devoted to the characterization of the catalysts. ZnO-supported Cu catalysts have been proved to be good catalysts for synthesis of methanol1 and higher alcohols,2,3 the water-gas shift reaction,4 steam reforming of methanol,5 etc. The catalysts for methanol synthesis from CO/CO2/H2 mixture gas have been most intensively investigated since the introduction of Cu/ZnO/ Al2O3 catalysts for commercial use in 1966. It is established that Cu forms nanometer-size clusters on ZnO and that the active center for methanol synthesis is the Cu-related sites because the synthesis activity is proportional to the Cu surface area.1,6 However, the nature of the active site is still a matter of debate: One of the proposed models is that metallic Cu is an active phase.1,6,7 On the other hand, Cu+ species in the interstitial and/or substitutional sites in ZnO are an alternative candidate for the active center.8 The Cu-Zn sites in Cu-Zn alloys formed on the catalyst surface have also been proposed for the specific active center.9,10 The difficulty to specify the nature of the active center arises partly from the fact that the morphology of Cu undergoes dynamic changes in the presence of reaction gases and at elevated temperatures.10,11 Thus, the Cu clusters on singlecrystal ZnO surfaces in ultrahigh vacuum (UHV) conditions have been employed as model systems as a first step to better understand the nature of the Cu clusters on the real catalysts.12-15,17,18 Campbell and co-workers have investigated the adsorption process of Cu on single-crystal ZnO(0001)-Zn and ZnO(0001h)-O surfaces in UHV by ion-scattering spectroscopy (ISS) and X-ray photoelectron spectroscopy.12,13 They have found that the Cu adatoms form two-dimensional (2D) clusters in the initial stages * Address correspondence to this author. Fax: +81 3 5734 2655. E-mail: [email protected]. † Tokyo Institute of Technology. ‡ Rikkyo University.

of adsorption and the clusters grow three-dimensionally beyond the so-called critical coverages.12,13 The scanning tunneling microscopy (STM) study has confirmed the same adsorption process,14 although the critical coverages estimated were much lower than those estimated from the ISS experiments. A similar growth mode of the clusters has also been suggested for the Cu/ZnO(112h0) system.15,16 On the other hand, exclusively threedimensional (3D) Cu clusters have been observed for the Cu/ ZnO(101h0) system by STM.17 Although the morphology of the Cu clusters on the ZnO surfaces has been investigated rather well, only limited information is available for the valence electronic structure of the Cu clusters.19 It is important to elucidate the valence electronic structure because the catalytic property of oxide-supported metals is considered to be determined by the electronic structure. For instance, the Au clusters on TiO2(110) exhibit the highest activity for CO oxidation when the Au clusters are on the verge between semiconductor and metal,20,21 i.e., the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) is about to close. The example of Au suggests that the electronic structure, which is different from the bulk electronic structure, should be one of the key factors for the catalytic activity of the metal clusters. In the present study, we have investigated the valence band structure of the vapor-deposited Cu overlayer on ZnO(101h0) using angle-resolved photoelectron spectroscopy (ARPES). ZnO(101h0) has a unique surface structure, which is characterized by Zn-O dimer rows running along the [12h10] direction (Figure 1). On the surface with such a highly anisotropic structure, the deposited adatoms may form an overlayer with a peculiar lattice structure. In fact, our recent ARPES study for Ag on ZnO(101h0) has revealed that the Ag 4d bands exhibit energy dispersions, whose periodicity is commensurate with the surface Brillouin zone (SBZ) of the substrate,22 implying that a unique atomic arrangement of Ag is realized under a strong influence of the surface structure of the substrate. Moreover, adsorbed Na and K on ZnO(101h0) can be arranged along the Zn-O dimer rows

10.1021/jp066296y CCC: $37.00 © 2007 American Chemical Society Published on Web 03/01/2007

Electronic Structure of Cu on ZnO(101h0)

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Figure 1. Ball-and-stick model of the ZnO(101h0) surface.

to form a (1 × 3) structure.23,24 In this study, the ARPES measurements reveal that the Cu adatoms aggregate into clusters, which are crystalline with aligned crystal axes. The surface structure of ZnO(101h0) plays a role in the formation of the clusters. Regarding the electronic structure, it is found the Cu clusters possess a 2D band structure at low coverages, while the 3D band is developed at high coverages. 2. Experimental Section The experiments were performed at beam lines (BL) 1C and 11C of the Photon Factory, High Energy Accelerator Research Organization (KEK), utilizing the linearly polarized synchrotron radiation. The measurements were carried out in an UHV chamber with a base pressure of 2 × 10-10 Torr. The UHV chamber was equipped with a hemispherical electron energy analyzer (VSW HA54) with a microchannel-plate electron multiplier detector for ARPES measurements, a single-pass cylindrical mirror analyzer for AES measurements, low-energy electron diffraction (LEED) optics, and a quadrupole mass spectrometer. At BL 1C, the ARPES spectra were taken at the photon energies hν between 30 and 70 eV with the total energy resolution between 0.15 and 0.3 eV depending on hν. For the measurements at BL 11C, the spectra were acquired at hν ) 20 eV with the resolution of 0.2 eV. The p-polarized light was used at both beam lines. The incidence plane of the light and the detection plane of the photoelectrons were set parallel to the [12h10] or [0001] direction of the substrate surface. All the measurements were carried out at room temperature. In the ARPES spectra presented below, the binding energy is referenced to zero at the Fermi energy (EF), which was determined from the spectra of the Ta plate, on which the sample was cramped. The intensity of the spectra is normalized by the photocurrent of the sample. The incidence angle of the light beam θi and the detection angle of the photoelectrons θd are given relative to the surface normal direction. Single-crystal ZnO with the (101h0) orientation (7 × 5 × 0.5 mm3; Yamanaka Semiconductor Co.) was used throughout the experiments. The sample surface was cleaned by cycles of Ar+ sputtering (2 kV, ∼1 µA) and annealing at 1000 K in UHV. To restore a possible oxygen vacancy, annealing at 700 K in O2 atmosphere (1 × 10-6 Torr) was carried out after the sputtering-annealing cycles. Electron bombardment from the rear of the sample was employed for annealing. Sample temperature was monitored by a chromel-alumel thermocouple

Figure 2. Normal emission spectra of Cu/ZnO(101h0) as a function of ΘCu. The photon energy used was 50 eV. The incidence angle of the light was 45°, and the incidence plane was set parallel to the [12h10] direction.

attached to the Ta plate. The cleanliness of the surface was checked by measuring the AES and ARPES spectra, which showed no peaks associated with contaminants such as carbon and hydroxide. The clean surface exhibited a (1 × 1) LEED pattern with sharp spots. The work function, determined from the spectral width between the Fermi level and the secondary electron cutoff, was 4.5 eV, which is in good agreement with the literature values.25-27 Cu was vapor-deposited onto the surface at room temperature from the evaporation source (Omicron EFM3). Cu was evaporated from a high-purity Cu rod (99.995%) of 2 mm diameter by electron bombardment, and the deposition rate was controlled by monitoring a flux current of Cu cations at the exit of the evaporation source. The pressure of the chamber during the deposition was kept below 8 × 10-10 Torr to avoid a possible contamination of the deposited Cu film. The Cu coverage (ΘCu) was estimated from the intensities of the O KLL and Zn LMM AES peaks, which were attenuated as the surface was being covered with Cu. In the estimation, we calculated the thickness of the overlayer on the assumption that the peak intensity decays exponentially as a function of the overlayer thickness. The TPP2M equation28 was used to estimate the electron mean free paths of the O KLL and Zn LMM Auger electrons (0.94 and 1.54 nm, respectively). The calculated thickness was, then, converted to the coverage by assuming that the thickness of one monolayer was 0.256 nm.29 Although Cu adsorption on ZnO(101h0) results in a cluster formation,17 such an estimation procedure is based on the hypothesis that a flat overlayer is formed with a layerby-layer fashion. Thus, the coverage is expressed as a monolayer equivalent (MLE) in the present paper; 1 MLE corresponds to the coverage where the Cu overlayer with the 0.256-nm thickness is formed. 3. Results and Discussion 3.1. Cu Deposition. Figure 2 shows the change in the normal emission spectrum of the ZnO(101h0) surface as a function of ΘCu. The photon energy used was 50 eV, and the total energy

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Figure 3. (a) The extracted Cu 3d spectra, which are obtained by subtracting the spectrum of the clean surface from those of the Cucovered surfaces. The positions of the Cu 3d peaks are indicated by vertical bars. (b) Enlarged spectra around the Fermi level. The step structure is formed by the emission from the Cu 4sp DOS. The onset positions of the step, indicated by bars, are determined by fitting the spectrum with use of a Gaussian-convoluted Fermi distribution function and a liner function as a background.

resolution was 0.20 eV. Emission peaks from 3 to 9 eV on the clean surface are associated with the hybrid states between the O 2p orbitals and the Zn 3d/4sp orbitals with a major contribution of the former orbitals. An intense Zn 3d peak is observed at 10-11 eV. The surface-localized O 2p danglingbond peak, which has been observed at 3.7 eV in the normal emission spectra measured at hν ) 20-27 eV,30 is not seen under the present experimental condition. This should be due to the lower photoionization cross section of the O 2p state at the higher photon energies.31 Cu deposition causes a gradual suppression of the substrate peaks and the growth of a Cu 3d peak at 2-3 eV. At higher ΘCu (>1 MLE), another Cu 3d peak becomes obvious as a broad hump at 3-5 eV. To clarify the overall Cu 3d emission feature, the spectrum of the Cu-free surface is subtracted from those of the Cu-covered surfaces. Before subtraction, the intensity of the Cu-free spectrum is adjusted so that the Zn 3d peak height is equal to that of the Cu-covered spectra. In addition, the binding energy of the Cu-free spectrum is shifted to account for Cuinduced bending of the ZnO band (we will discuss band bending in more detail later). Figure 3a shows the extracted Cu 3d spectra. The position of the Cu 3d peaks are marked by bars. We have observed at least three peaks, two of which are observed from the initial stages at 3.1 and 4.8 eV and shift monotonically to 2.4 and 4.6 eV, respectively. The third peak is recognized as a shoulder of the intense Cu 3d peak only at high ΘCu. The Cu-induced feature is also seen as a step-like structure around the Fermi level. This feature is attributed to the Cu 4sp emission. The magnified spectra around the Fermi level are shown in Figure 3b to see the evolution of the Cu 4sp structure in more detail. Although the step is vague at low coverages, the onset of the step is observed irrespective of ΘCu. The bars in the figure indicate the midpoint of the onset. It locates at 0.58 eV at 0.2 MLE and shifts to 0.10 eV up to 3.0 MLE. The energy of the onset at each ΘCu is independent of θd between 0° and 40°, indicating that it should represent the onset of the total density of states (DOS) of the Cu 4sp band. Important information on the adsorption process of Cu on ZnO(101h0) can be obtained from the inspection of the Cu 4sp spectra. First, the Cu 4sp DOS is formed from the lowest

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Figure 4. The ΘCu-dependent change in (a) bending of the ZnO band, which is deduced from the shift of the Zn 3d peak, (b) the onset position of the Cu 4sp DOS, and (c) the Cu 3d peaks. In part a, the positive and negative values mean that the band bends downward and upward, respectively, with respect to the band of the clean surface. Open symbols in parts b and c are the row data, and the band bending corrected data are shown by filled symbols. A dotted line in part c indicates the shift of the energetic center of the Cu 3d band.

coverage examined (0.2 MLE). This means that the Cu adatoms should be aggregated so that the interatomic interaction between the neighboring Cu atoms is realized to form the 4sp band from low ΘCu. Second, the onset of the Cu 4sp DOS moves monotonically to the lower binding energy side with increasing ΘCu. The shift of the sp DOS is a common phenomenon for the noble-metal/oxide systems studied thus far, where the noblemetal atoms aggregate into clusters and these clusters grow in size with increasing the amount of deposition.22,32-34 The shift of the onset has been explained as originating partially from the change in the efficiency of the final state screening with increasing cluster size.32,33 Therefore, it is concluded from the ARPES study that Cu adsorption on ZnO(101h0) is characterized by the cluster formation from the low coverages. This conclusion is in good agreement with the STM observation.17 From the present study, we cannot evaluate the ΘCudependent change in the cluster size. However, the LEED observation suggests that the average cluster size should be less than 10 nm up to 3.0 MLE. As will be presented below, the Cu adatoms are in an ordered arrangement within the clusters. Therefore, if the average cluster size exceeds 10 nm, a LEED pattern originating from the surface structure of the Cu cluster should be observed, because, in the LEED measurements using the primary energy of 100 eV, an ordered domain with the size larger than 10 nm is needed to give diffraction patterns.35 Cu deposition on ZnO(101h0) bears no LEED pattern at any ΘCu up to 3.0 MLE. Thus, the cluster size should not exceed 10 nm. The STM study for Cu/ZnO(101h0) has indicated that the average Cu cluster size increases from 3.4 nm at 0.05 monolayers (ML) to 4.5 nm at 0.5 ML.17 Therefore, the upper limit of the cluster size estimated on the basis of the LEED measurements should not be far from reality. Careful examination of the ΘCu-dependent change in the normal emission spectrum in Figure 2 reveals that the O 2p-Zn 3d/4sp hybrid peaks and the Zn 3d peak move toward the higher binding energy side upon adsorption of 0.2-MLE Cu. Further deposition of Cu induces a gradual shift of these peaks to the lower binding energy side. Since all the substrate-related peaks move simultaneously, the shift should be caused by the change in bending of the ZnO band. Figure 4a shows the change in the magnitude of Cu-induced band bending relative to the clean

Electronic Structure of Cu on ZnO(101h0) surface. The ZnO band initially bends downward by ∼0.2 eV followed by a gradual diminishment with increasing ΘCu. Upward band bending relative to the clean surface is realized at ΘCu > 1 MLE. A similar coverage dependence of band bending has been reported by Didziulis et al. for Cu/ZnO(101h0).19 Downward band bending suggests that negative charge is accumulated in the subsurface region of ZnO in order to compensate positive charge on the surface. This means that the Cu adatoms should be positively charged on the surface at ΘCu < 1 MLE. Charge transfer from Cu to the substrate is also confirmed by the decrease in the work function by 0.3-0.5 eV in the coverage region up to ∼0.5 MLE. Therefore, it is concluded that the deposited Cu atoms are positively charged at low ΘCu. As already mentioned above, both the onset of the Cu 4sp DOS and the Cu 3d states shift toward the lower binding energy side with increasing ΘCu. Open symbols in Figure 4b,c show the energetic positions of the onset of the 4sp DOS and the 3d peaks, respectively. The shift should be a consequence of two contributions: (i) band bending of the ZnO substrate and (ii) the change in the electronic structure of the Cu clusters as a function of the cluster size. The filled symbols in Figure 4b,c, obtained by subtracting the band bending effect, show the shift caused by the latter contributions. The onset of the Cu 4sp DOS still moves toward EF even after the band bending effect is excluded. On the other hand, the Cu 3d states behave differently; the upper-most Cu 3d state shifts to the lower binding energy side, whereas the 3d state at 4.4-4.6 eV tends to move to the higher binding energy side. The ΘCu dependence of the upper and the lower Cu 3d states is understood as a result of an increase in the Cu 3d bandwidth; namely, as each Cu cluster grows with ΘCu, the mean coordinate number of the Cu atom increases so that the total d bandwidth is enlarged. Nevertheless, when the energetic center between the upper and the lower Cu 3d states (indicated by a dotted line in Figure 4c) is examined, it shows a similar ΘCu dependence as the onset of the Cu 4sp DOS. The shift of the energetic center of the Cu 3d band should reflect the change in the final state effect. When a photoelectron is emitted from a metal cluster, the kinetic energy of the photoelectron is decreased as a result of the Coulomb interaction between the photoelectron and the photohole left in the cluster.33 The Coulomb interaction should be strong as the cluster size is small because of a small amount of valence charge to screen the photohole, whereas it becomes weaker as the cluster grows. Such an effect is marked for the metal clusters on the surfaces of poorly conductive materials like oxides, because valence charge of the substrate is not expected to participate in the screening. Therefore, the shift of the center of the Cu 3d band reflects the cluster size dependence of the Coulomb interaction between the photoelectron and the photohole. For the present system, furthermore, the Cu clusters are positively charged at low ΘCu and positive charge on Cu is dissolved at higher ΘCu, as is evident from initial downward followed by upward band bending of ZnO (Figure 4a). Accordingly, the change in the Coulomb effect must be enhanced. The effect of the Coulomb interaction is also operative for the Cu 4sp electrons. Thus, to the first approximation, the onset of the Cu 4sp DOS should shift by the same magnitude as the shift of the center of the Cu 3d band. However, much larger shift is observed for the onset of the Cu 4sp DOS (0.35 eV) than for the Cu 3d band (0.15 eV). We consider that the nonmetal-to-metal transition of the nature of the Cu cluster should bear the extra shift of the Cu 4sp DOS. The scanning

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Figure 5. Off-normal emission spectra of the 3.0-MLE Cu-covered ZnO(101h0) surface. The incidence plane of the light and the detection plane of the photoelectrons are set parallel to (a) the [12h10] (ΓX) direction and (b) the [0001] (ΓX′) direction.

tunneling spectroscopy (STS) study20,21 has proved that the clusters of noble metals (Au and Ag) on TiO2(110) are semiconductors with the HOMO-LUMO gap of 1-2 eV when these clusters are small (∼2 nm in diameter), and the gap diminishes continuously with increasing the cluster size and finally closes at ∼4 nm in diameter. The same tendency has also been found for Ag on ZnO(101h0).22 Regarding Cu, no experimental evidence has been reported so far, but the theoretical study has indicated the decrease in the HOMOLUMO gap energy with increasing the cluster size.36 Hence, the Cu clusters on ZnO(101h0) are also expected to have a semiconducting nature when they are small, and the metallic electronic structure is developed with increasing the cluster size. Since the onset of the Cu 4sp DOS corresponds to the HOMO level, the extra shift of the onset in comparison with the Cu 3d band should reflect the shift of the HOMO level toward EF. The extra shift is observed only at ΘCu < 0.5-1 MLE. This suggests that the HOMO-LUMO gap may close at most up to 1 MLE, and the metallic electronic structure is realized at higher ΘCu. It is worth noting that, although the metallic electronic structure is realized, the Cu clusters are not large enough to possess the full and bulklike screening toward the photohole even at 3 MLE since the onset of the Cu 4sp DOS lies at 0.1 eV below EF at 3 MLE. 3.2. Band Structure of Cu. In an attempt to obtain further information on the valence electronic structure of the Cu clusters, we have carried out the off-normal emission measurements for the Cu-deposited ZnO(101h0) surfaces at three different ΘCu values. Figure 5 shows the off-normal emission spectra at ΘCu ) 3.0 MLE. The photon energy used was 50 eV, and the incidence angle of the light was 45° relative to the surface normal. The photoelectrons were detected in a plane parallel to the [12h10] (Figure 5a) and [0001] (Figure 5b) directions of the substrate surface, i.e., parallel and perpendicular to the Zn-O dimer rows, respectively. These two directions respectively correspond to the ΓX and ΓX′ axis of the SBZ of ZnO(101h0).

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Figure 6. Intensity plot the second derivatives of the measured offnormal emission spectra shown in Figure 5. The bright and dark regions corresponds to the high and low spectral weight regions, respectively. The open circles indicate the position of the maximum spectral weight. The periodic structure is seen in the Cu 3d band along ΓX, whereas the periodicity is less obvious along ΓX′. The SBZ of ZnO(101h0) is drawn in the inset.

Although three Cu 3d states are identified at ΘCu ) 3.0 MLE in the normal emission spectrum (Figure 3a), it is difficult to follow the θd dependence of the two peaks at the high binding energy side because of the overlapping with the O 2p-Zn 3d/ 4sp hybrid band of the substrate. Thus, we concentrate our attention on the most intense Cu 3d peak, which locates within the band gap of ZnO. In the normal emission spectrum, the Cu 3d peak in the gap region locates at 2.40 eV. As θd increases in the detection plane parallel to the [12h10] direction, it shifts slightly to the higher binding energy side up to θd ) 4° (Figure 5a). The peak moves to the opposite direction for larger θd and reaches its lowest binding energy at θd ) 22-24°. At θd g 26°, the peak shifts back again to the lower binding energy side. Accompanying such a peak shift, the peak intensity exhibits a modulation: the intensity is slightly attenuated as the binding energy of the peak increases, whereas the peak is more intense as the binding energy becomes smaller. The Cu 3d peak also exhibits a θd-dependent change along the [0001] direction (Figure 5b) but with a smaller energy shift and a weaker intensity modulation than those along the [12h10] direction. Note that the strongly dispersing peaks in the binding energy region larger than 3 eV are attributed to the O 2p-Zn 3d/4sp hybrid states. To see the energy dispersion and the intensity modulation of the Cu 3d state more clearly, a grayscale map is constructed by plotting the spectral weight of the second derivatives of the offnormal emission spectra. In Figure 6, bright and dark regions correspond to high and low spectral weights, respectively. The horizontal axis of the map is a surface parallel component of the wave number vector k|, which is calculated from θd and the binding energy Ebin by using the equation

k| ) x2me(hν - Φ - Ebin)/p2 sin θd Here, me is the mass of the electron, and Φ is the work function. Φ of the 3.0-MLE Cu-covered ZnO(101h0) surface is 4.6 eV. The open circles in Figure 6 indicate the position of the highest spectral weight. The Cu 3d band exhibits a clear energy dispersion along the ΓX axis; it shifts from 2.40 eV at k| ) 0 nm-1 to 2.50 eV at 2.3 nm-1 and moves to 2.34 eV at 12.5

Ozawa et al.

Figure 7. Normal emission spectra of the 3.0-MLE Cu-covered surface taken at hν ) 40 and 60 eV (right panel) and the intensity map of the second derivatives of the normal emission spectra taken at various photon energies (left panel). The bright region corresponds to the position of the Cu 3d state, which shifts to the lower binding energy side with increasing hν.

nm-1. A clear periodic structure is seen. The band along the ΓX′ axis also shows an energy dispersion, but the periodicity is less obvious. It is easily recognized that the dispersion structure of the Cu 3d band is not commensurate with the periodicity of the SBZ of ZnO(101h0) along both high symmetry axes. An important result deduced from the off-normal emission measurements is that the binding energy of the Cu 3d state depends on k|. This means that k| is a good quantum number, i.e., the Cu atoms within the Cu cluster are not disordered but have some ordered arrangement along the surface parallel. Moreover, it is found that the surface perpendicular component of the wave number vector k⊥ is also a good quantum number. Figure 7 shows the normal emission spectra taken at hν ) 40 and 60 eV (right panel) and the grayscale map of the second derivatives of the spectra (left panel). The position of the Cu 3d state moves toward the lower binding energy side from 2.50 eV at hν ) 40 eV to 2.35 eV at 60 eV, passing through 2.40 eV at 50 eV. The hν-dependent shift of the Cu 3d peak means that the Cu 3d state depends on k⊥. Tobin et al. have found that at least a several-monolayer thickness is required for the formation of the dispersing bands along the surface normal direction within the metal overlayer.37 Thus, we conclude that the Cu clusters on ZnO(101h0) should already exceed in height the several-monolayer thickness at 3.0 MLE. This is consistent with the STM observation,17 showing that the average height of the Cu cluster at 0.5 ML, which is much less than 3.0 MLE in the present study though the calibration method is different, is 1.4 nm, corresponding to the thickness of 5-6 ML. The binding energy of the Cu 3d state depends on both k| and k⊥ at ΘCu ) 3.0 MLE, meaning the formation of a threedimensional (3D) electronic structure. Hence, the energy dispersion of the Cu 3d state along k| (Figure 6) also contains the contribution of the energy dispersion along k⊥. Although ΘCu is decreased from 3.0 to 1.35 MLE, the dispersion behavior is essentially the same along the ΓX axis; namely, the band still exhibits a periodic dispersion as shown by filled squares in Figure 8. However, the dispersion width becomes a half (0.08 eV) of that at 3.0 MLE (0.16 eV). The small dispersion width at 1.35 MLE may result mainly from the small dispersion width along k⊥ as a result of a lower heights of the clusters. This speculation is based on the experimental observation by Tobin et al., who have observed the narrower Cu 3d bandwidth in a thinner Cu overlayer.37 As ΘCu further decreases to 0.45 ML, a completely different band structure emerges. Along the ΓX axis, the Cu 3d band disperses toward the higher binding energy

Electronic Structure of Cu on ZnO(101h0)

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Figure 8. Change in the Cu 3d band structure as a function of ΘCu. All the data are obtained from the spectra taken at hν ) 50 eV.

Figure 10. (a) Grayscale map of the 0.45-MLE Cu-covered surfaces. The open circles indicate the Cu 3d band. (b) The structural model of the Cu monolayer sheets for the tight-binding calculations (left) and the results of the tight-binding calculations (right). Solid and dotted lines are the bands for the quasihexagonal and rectangular lattice structures, respectively, with a ) 0.256 nm and c ) 0.521 nm.

Figure 9. Intensity maps along ΓX of the 0.45-MLE Cu-covered surface before and after annealing. Annealing was carried out at 600 K for 5 min. The Cu 3d band on the postannealed surface exhibits a dip at 14.6 nm-1, which is characteristic of the 3D band structure. The bright region shown at around the Γ h 2nd point is the O 2p band of the substrate.

side between k| ) 0 and 6 nm-1 and then to the lower binding energy side for higher k| without a periodic dispersion. The dispersion width is 0.20 eV, which is even larger than that at 3.0 MLE. The band along the ΓX′ axis shows a quite interesting structure; unlike the Cu 3d band at 3.0 MLE, it shows a dispersion with a periodicity of ∼12 nm-1. Since the Γ h -X′ distance is 6.03 nm-1, the periodicity matches the SBZ of ZnO(101h0). The change in the dispersion behavior of the Cu 3d band from 1.35 to 0.45 MLE is considered to arise from the transition of the dimensionality of the electronic structure from 3D to 2D. Namely, the average height of the Cu clusters at 0.45 MLE is not enough to make k⊥ a good quantum number, whereas the clusters extend laterally to keep k| a good quantum number. The 2D band structure of the Cu clusters at 0.45 MLE is plausible, because the periodicity of the Cu 3d band along ΓX′ is commensurate with the SBZ of the substrate so that the contribution of the k⊥-dependent dispersion should be absent. The 2D band is also supported by the fact that, as the 0.45MLE Cu-covered surface is annealed at 600 K, the Cu 3d band along ΓX gains the feature characteristic for the 3D band. Figure 9 shows the Cu 3d bands along the ΓX axis of the 0.45MLE Cu-covered surfaces before and after annealing. On the postannealed surface, the dip appears at ∼15 nm-1 in the Cu 3d band so that the overall band structure resembles that at 1.35 and 3.0 MLE. Moreover, although the intensity modulation is not apparent for the band on the preannealed surface, it is

enhanced on the postannealed surface. The STM has directly imaged the thermal-induced ripening process of the Cu clusters on ZnO(101h0).17 Coalescence of the Cu clusters upon annealing has also been suggested by our APRES study.38 Thus, the Cu clusters grow both laterally and vertically during annealing and possess a 3D electronic structure, while the clusters on the preannealed surface have an electronic structure with a 2D character. Transformation of the electronic structure from 2D to 3D occurs at the coverage somewhere between 0.45 and 1.35 MLE. In this coverage region, the Cu clusters also transform from semiconductor to metal, as have discussed in the previous section. It is inferred from these findings that there seems to be a correlation between the dimensionality of the electronic structure and the semiconductor-to-metal transition of the Cu clusters on ZnO(101h0), although more detailed ΘCu-dependent measurements should access the proposed correlation. It is pointed out that Dulub et al. have concluded from the STM study17 that exclusively 3D Cu clusters are formed on ZnO(101h0) from the very beginning of adsorption. This conclusion does not contradict our result that the 2D electronic structure is developed in the Cu clusters at 0.45 MLE. In the STM study, the 2D cluster is defined as a cluster with a monatomic height. However, the 2D electronic structure is possible when the cluster height is less than the several monolayers. The Cu clusters at 0.45 MLE should accordingly be geometrically 3D but electronically 2D. 3.3. 2D Band of Cu Cluster. When the Cu clusters possess the 2D electronic structure, it is easy to correlate the band structure with the atomic arrangement, which is difficult for STM to determine especially for the metal clusters on oxide surfaces. In Figure 10a, we reproduce the Cu 3d band at ΘCu ) 0.45 MLE as a grayscale map. The Cu 3d band along ΓX′ shows a dispersion with the same periodicity as that of the SBZ of ZnO-

4262 J. Phys. Chem. C, Vol. 111, No. 11, 2007 (101h0). This suggests that the Cu adatoms are arranged to have the periodicity, which is equal to that of the substrate surface in the [0001] direction. The Cu 3d band along the ΓX axis, on the other hand, does not exhibit a periodic dispersion at least in the examined k| region. Keeping this experimental result in mind, we have modeled two Cu monolayer sheets and carried out simple tight-binding calculations following Harrison’s theory.39 In the calculations, we consider one s state and five d states, and the power-law functions are used for the interatomic matrix elements of ddσ, ddπ, ddδ, ssσ, and sdσ with the coefficients of -19.56, 11.61, -2.11, -0.829, and -2.88, respectively, which are given by Shi and Papaconstantopoulos.40 The models of the Cu monolayer, shown in Figure 10b, are rectangular and quasihexagonal lattices with c ) 0.521 nm, which is the same as the lattice constant of ZnO(101h0) along the [0001] direction. The interatomic distance along the xdirection a is treated as a variable. The results of the tight-binding calculations are shown in Figure 10b by dotted and solid lines for the rectangular and quasihexagonal lattices, respectively, with a ) 0.256 nm (a nearest-neighbor distance in bulk Cu). Among the calculated bands, the lines drawn nearly vertically are the s bands, which hybridizes with the d bands with the ∆1 symmetry in both directions.41 It is apparent that both Cu lattices give a quite similar d band structure. This suggests that the d bands are rather insensitive to the atomic position in the neighboring rows at such a large distance (c ) 0.521 nm). In the y-direction, the d bands exhibit small dispersion because of the large interatomic distance. Among five d bands, only the bands with the even symmetry with respect to the incidence plane is observed in the present experimental geometry as mentioned in the Experimental Section.42 Thus, the dyz, d3z2-r2, and dx2-y2 are experimentally accessible when measuring in the ΓX′ direction. The dyz band seems to possess the feature that the experimental band exhibits, i.e., the maximum and minimum of the band matches the Γ h and X′ points, respectively, whereas the d3z2-r2 and dx2-y2 bands disperse differently. Moreover, the calculated dispersion width of the dyz band is about 50 meV, which is in good agreement with the width of the experimental band. Thus, the Cu 3d band along ΓX′ can be associated with the dyz band. The situation is more vague in the x-direction, i.e., in the [12h10] (ΓX) direction. Since we do not have information on the interatomic distance a in this direction, we have examined the band structure with several a values. It is found that no single band can explain the experimental band at any a. This suggests that at least two bands should be taken into account. According to the selection rule,42 the dzx, dx2-y2, and d3z2-r2 bands are observable along the ΓX axis. Thus, the observed band may have a dzx character in the vicinity of the Γ h point, where the experimental band disperses to the higher binding energy side, and the dx2-y2 or d3z2-r2 character may be responsible for the shift of the band toward the lower binding energy side at higher k|. It should be pointed out that the comparison between the experimental Cu 3d band and the theoretical band of the unsupported Cu sheet is partly rationalized because the Cu 3d state we consider here exists within the bulk band gap of ZnO so that the wave function of the Cu 3d state should be confined to the Cu overlayer and be hardly affected by the bulk wave function of the substrate. However, symmetry is reduced from C2V to Cs when the model Cu sheets are placed on the ZnO(101h0) surface even if the Cu lattices are not distorted. This

Ozawa et al. should leads to the avoided crossing among the Cu 3d bands. Apparently, more elaborated band calculations are requisite to fully understand the 2D electronic structure of the Cu clusters. Finally, we comment on the relationship between the atomic structure and the growth process of the Cu clusters on the ZnO(101h0) surface. In the present study, although the Cu-Cu distance in the [12h10] direction (parallel to the dimer row) is not clarified, the Cu adatoms may line up along the dimer rows while keeping the distance between the Cu rows at 0.521 nm. A recent STM study17 has shown that the Cu clusters are formed preferentially at the edges of the steps running along the [0001] direction, though monatomic steps running along both [0001] and [12h10] directions exist on the (101h0) surface. This nonuniformity should result from the difference in the diffusion barrier for the Cu adatoms on the surface, i.e., the smaller barrier for Cu moving in the [12h10] direction rather than in the [0001] direction. Thus, the Cu atoms landing on the surface move mainly along the dimer rows until they are captured by the step edges and form the Cu clusters there. In such a growth process, the atomic structure within the Cu clusters should be rather flexible in the direction parallel to the dimer row and be rigid in the perpendicular direction. Accordingly, the distance between the Cu rows reflects the surface structure of the substrate. 3.4. Implication of the ARPES Study. Deposition of Cu on ZnO(101h0) at room temperature in UHV results in the formation of the clusters from the low ΘCu region. The cluster formation is a common growth mode for Cu on the low-index surfaces of single-crystal ZnO.12-15,17,18 Moreover, the Cu clusters are also formed in the working conditions on the surface of the real Cu/ZnO catalysts.10,11 The present ARPES study reveals that the Cu clusters on ZnO(101h0) have a 3D metallic band structure at high ΘCu (>1 MLE) and a 2D semiconducting band structure at low ΘCu (