Article pubs.acs.org/JPCC
Elucidating the Structure and Chemical State of Co Growth on the ZnO(101̅0) Surface
Shu Hsuan Su,† Ju Hong Lai,† Hsin-Hsien Chen,† Tsung-Hsun Lee,† Yao-Jane Hsu,⊥ Rui Long Wang,‡,¶ and J. C. A. Huang*,†,‡,§ †
Department of Physics, National Cheng Kung University, Tainan 701, Taiwan Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan 701, Taiwan § Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 701, Taiwan ⊥ National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan ¶ Faculty of Physics & Electronic Technology, Hubei University, Wuhan, 430062, China ‡
ABSTRACT: This study elucidates the epitaxial growth structure and chemical state of Co on the ZnO(101̅0) surface by using scanning tunneling microscopy, reflection high-energy electron diffraction, X-ray photoelectron spectroscopy, and ultraviolet photoelectron spectroscopy. The well-ordered cobalt oxide (CoOx)(2 × 1) structure is formed at 0.5 ML Co coverage. Increasing the Co coverage from 0.7 to 1 ML allows for the surface characterization by the Co stripe structure, while the Co metallic clusters are clearly developed above 3 ML Co coverage. Coverage-dependent measurements of the Co chemical state indicate that the initial Co mixed oxidation and metallic state exists at submonolayer and gradually transfers to a metallic Co-dominated state. The results also suggest that the initial growth mode is two-dimensional-like and bridged to threedimensional at higher Co coverages.
evolution of Co and CoO films, as well as particles supported on ZnO(0001) to include the reaction of ethanol from XPS and temperature-programmed desorption (TPD). They also found that the metallic Co films and particles are active for ethoxide decarbonylation, forming CO, H2, and adsorbed methyl groups. In contrast, supported CoO particles were largely unreactive toward ethanol. High selectivity to the dehydrogenation product, acetaldehyde, was only observed when the supported Co was oxidized partially and contained both Co0 and Co2+. As is generally assumed, acetaldehyde is a critical intermediate during steam reformation of ethanol (SRE) to produce H2 and CO2. Results of this study suggest that the partially oxidized Co species provide the active sites for this reaction. A detailed study is thus warranted of the interactions at the Co−ZnO interface, which are not well-understood. Elucidating the structure and chemical interactions at the Co−ZnO interface may facilitate the design of Co/ZnO catalysts to enhance the stability, activity, and selectivity of the catalysts. As a nonpolar surface, ZnO(101̅0) is characterized by the Zn−O dimer rows running along the [12̅10] direction. Diffusion of adatoms on the ZnO(101̅0) surface is, thus, constrained along the dimer rows21 so that a unique adsorption structure is expected. Low-energy electron diffraction (LEED) measurements22 have pointed toward the c(2 × 2) and (3 × 1)
1. INTRODUCTION Metal adsorption on an oxide surface and the formation of metal/oxide complex systems are vital processes because the metal/oxide system plays a prominent role in many applications, including microelectronic devices and heterogeneous catalysis. Many studies focus on the interactions of the dispersed metals with their supports, since the reactivity and stability of an oxide-supported catalyst heavily depend on such interactions.1−4 According to those studies, such interfacial interactions behave in a complex manner, encompassing, for instance, electronic effects arising from chemical interactions and charge transfer, structural effects ascribed to structural stabilization, migration of support materials onto the metal, and diffusion and spillover through the metal−oxide interface.1−4 Zinc oxide is a constantly available oxide support. For instance, as is well-known, Cu/ZnO-based catalysts have been used for synthesis of methanol5,6 and higher alcohols,7,8 the reverse water gas shift reaction,6,9,10 and steam reformation of methanol.11 Similar catalytic activity can also be found for Pd/ ZnO,12 Pt/ZnO,13 Au/ZnO,14 and Ag/ZnO15,16 systems, explaining the extensive research on the growth of metal materials on the low-index surface of single-crystal ZnO. Co growth on polar ZnO had been investigated with STM and XPS in recent years.17−19 According to these reports, Co metallic clusters form in the initial region at room temperature, whereas the Co oxidation interacts with the ZnO surface at annealing temperatures exceeding 800 K. In addition to the above, Vohs et al.20 recently studied in detail the structural © 2012 American Chemical Society
Received: October 18, 2011 Revised: April 13, 2012 Published: April 13, 2012 9917
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Figure 1. STM images of the clean ZnO(101̅0) surface. (a) Large-scale image (30 × 30 nm2) showing flat terraces separated by two types of step edges; inset of (a) shows the RHEED image of the ZnO(101̅0) surface. (b) The zoom-in image (10 × 8 nm2) showing the step height; inset of (b) shows the height profile of the step along the black line. (c) An atomically resolved image (4 × 3 nm2) showing the position of Zn atoms, in which a unit cell is marked. (d) Schematic models (top view and side view) for the ZnO(101̅0) structure, with gray balls and red balls for zinc and oxygen atoms, respectively. Tunneling conditions are Vs = 3.0 V and It = 0.3 nA for image (a), and Vs = 2.5 V and It = 0.6 nA for images (b) and (c).
structures of adsorbed K on the ZnO(101̅0) surface after annealing at 950−1200 K. Additionally, according to an angleresolved photoelectron spectroscopy study for Ag on ZnO(101̅0), the Ag 4d bands exhibit energy dispersions, whose periodicity is commensurate with the surface Brillouin zone of the substrate. This finding implies that a unique atomic arrangement of Ag is achieved under a strong influence of the surface structure of the substrate.23 However, to our knowledge, no STM work has been reported about the initial structure and chemical state of the Co/ZnO(101̅0) system. In contrast to the case of Co growth on the polar ZnO surface, Co growth on the ZnO(101̅0) surface has not been described as well. This study investigates the room-temperature deposited Co films on nonpolar ZnO(101̅0) with combinational STM, RHEED, XPS, and UPS measurements. The well-ordered CoOx(2 × 1) structures are formed on ZnO(101̅0) at 0.5 ML Co coverage. When the Co coverage is increased above 1 ML, Co stripe structures are observed and Co clusters are observed above 3 ML. The results suggest that the initial growth mode is 2D-like and then bridged to 3D at higher Co coverages. Additionally, the initial mixed Co oxidation and metallic state gradually transfers to a metallic Co-dominated state with an increasing Co coverage. Results of this study elucidate the structural and chemical evolution of Co growth on the ZnO(101̅0) surface, which is highly promising for catalyst applications.
preparation chamber. A clean surface was obtained by cleaning the ZnO(101̅0) surface with cycles of Ar+-ion sputtering at 1.5 keV with an ion current of 0.5−0.7 μA for 15 min, followed by annealing at 850 °C in UHV for 15 min. Pure Co (99.995%) was deposited by a thermal filament evaporator made by JEOL Ltd. (model TM-59022). The deposition rate was calibrated by a quartz crystal microbalance located close to the sample holder. The Co growth rate was determined approximately about 0.1 Å/s. Adsorption and interdiffusion of Co and ZnO(101̅0) substrates were then suppressed by depositing the Co films at room temperature. The concentration of the deposited metal was produced in the monolayer equivalent (ML), which corresponded to the Co packing density of 1.8 × 1015 Co atoms/cm−2 if the growth plane is along (0001). However, for the growth plane along (101̅0), the atomic density of 1 ML Co is about 1 × 1015/cm−2, which is quite close to the sum of O and Zn atomic densities of ZnO(101̅0) of 1.2 × 1015/cm−2. The pressure during Co deposition remained below 5 × 10−8 pa, which was only slightly higher than the background pressure. Following Co growth, the sample was introduced immediately into the STM measurement chamber. STM experiments were performed at room temperature for this study. STM data were collected in a constant current mode at a positive sample bias voltage of 2.5−3 V and with a tunneling current of 0.3−0.6 nA. High-resolution topographic STM images were then recorded using electrochemically etched 0.3 mm tungsten wires. Before imaging of the ZnO substrate, the tip was placed a few micrometers away from a Si substrate heated to 1200 °C, leading to indirect heating and cleaning of the tip. Next, the tip was cleaned by applying voltage pulses during STM operation. RHEED was performed with an incident electron beam of energy 20 KeV at a grazing angle 2−3° to the sample surface. XPS measurements were performed with a VG CLAM4 surface analysis system, equipped with Al Kα radiation (1486.6 eV) at the National
2. EXPERIMENTS Experiments were performed in an ultra-high-vacuum (UHV) system with a base pressure of 2.0 × 10−8 Pa (JSPM-4500 A/S; JEOL Ltd.). The system consists of a sample preparation chamber and an STM measurement chamber.24 ZnO(101̅0) single crystal (Techno Chemic, Inc.) was first treated in an ultrasonic acetone bath and then introduced to the sample 9918
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Figure 2. STM images of (a) 0.5, (f) 0.7, (g) 1.0, and (k) 3 ML Co deposited on ZnO(1010̅ ) at room temperature. The inset of (a) displays the RHEED image of the 0.5 ML Co/ZnO(101̅0). (b) High-resolution STM image of the CoOx(2 × 1) structure. (c) and (d) denote the line profile along lines I and II in (b), respectively. (e) The schematic model of the CoOx(2 × 1) structure due to the Co adatoms interacting with the dangling bonds of two adjacent ZnO dimers. (h) High-resolution image of the Co stripe structure. (i) and (j) denote the line profile along lines III and IV in (h), respectively.
surface. The observed atomic cell is estimated to be 0.33 × 0.52 nm2, which correlates well with the rectangular unit cell of the bulk crystallographic structure shown in Figure 1d. Notably, the bright spots at positive sample bias voltages are regarded as Zn atoms because Zn-derived states contribute to the conduction band minimum.27,28 The STM results on the clean ZnO(101̅0) surface closely correspond to previous studies.21,29−34 3.2. STM Studies of Co on the ZnO(101̅10) Surface. Following cleaning of the ZnO(101̅0) surface, the ultrathin Co films were deposited on the ZnO(101̅0) substrate at room temperature. Figure 2a illustrates the STM image of a ZnO(101̅0) surface following the Co deposition of 0.5 ML. The surface exhibits a well-ordered structure that differs from the clean ZnO(101̅0) surface. The Co atoms are uniformly distributed on the ZnO(101̅0) surface rather than on step edges or kinks, as in the case of Co on other oxide surfaces.35,36 For clarity, Figure 2b displays the atomic resolution STM image. The arrows, marked as “I” and “II”, denote the line scans along the two in-plane principle axes of ZnO, as shown in Figure 2c,d. One periodicity of 0.51 nm is close to the atomic spacing of the ZnO(101̅0) surface along the underlying [0001] direction (Figure 2c). The other periodicity of 0.66 nm is twice as long as that of the ZnO(101̅0) surface along the underlying [12̅10] direction. This observation correlates with the RHEED scan showing double periodicity of the additional RHEED streaks from Co growth, as denoted by arrows in the inset of
Synchrotron Radiation Research Center, Taiwan.25 The XPS measurements were conducted below 1.0 × 10−7 pa base pressure with an energy step of 0.025 eV. The XPS spectra are referenced to the Zn 2P3/2 peak at 1021.7 eV. UPS measurements were also evaluated using a VG CLAM4 surface analysis system equipped with a nonmonochromatic He(I) UV source (21.2 eV).26 The Co-deposited films were prevented from contamination, by making all observations using the in situ growth and characterization method.
3. RESULTS AND DISCUSSION 3.1. As-Prepared Clean ZnO(101̅10) Surface. The ZnO(101̅0) surface was prepared by four to five cycles of Ar+ sputtering, followed by annealing at 700 °C. Figure 1a shows the ZnO(1010̅ ) surface with flat terraces produced by the preparation procedures. According to this STM figure, the terrace edges are along two principle crystalline orientations, [0001] and [12̅10], with manifest bright lines along the [12̅10] direction. RHEED scans revealed that the clean ZnO(1010̅ ) surface is a (1 × 1) reconstructed structure, as shown in the inset of Figure 1a. A higher-magnification STM image in Figure 1b reveals an atomic step structure along the [12̅10] direction. According to the line scan in Figure 1b, the step height is 0.28 nm, corresponding to the two vertical planes of the crystallographic structure. The high-resolution STM image in Figure 1c reveals the atomic structure of the ZnO(101̅0) 9919
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Figure 3. (a) Zn(2p)/Co(2p) peak ratios of XPS for Co deposited on ZnO(101̅0). XPS spectra of (b) Zn 2p and (c) O 1s core levels as a function of Co coverage. For the Co coverage-dependent energy shift of (d) Zn 2p3/2 and (e) O 1s, the negative values suggest that the band bends upward with respect to the band of the clean ZnO(101̅0) surface.
so that the Co(1 × 1) surface is formed. Interestingly, the unit cell of 1 ML Co is extremely close to that of the ZnO(101̅0) surface, implying the pseudomorphic growth of the initial Co layer on ZnO(101̅0). The formation of the Co(1 × 1) stripe structure is likely due to its initial growth of the rectangular unit cell of Co on the ZnO(101̅0) substrate. The layer-by-layer growth of Co sustains to around 2 ML. For Co coverage that increased to about 3 ML, Co clusters are formed in addition to the monatomic stripe structure, indicating a transition to a 3D growth mode for Co on ZnO(101̅0). 3.3. XPS Characterization of Co on the ZnO(101̅10) Surface. This study attempted to obtain further information about the valence state of deposited Co, by undertaking the XPS measurement on the Co-deposited ZnO(1010̅ ) surface at room temperature at different Co coverages. Figure 3a plots the ratio of the intensities of the Zn(2p) and Co(2p) XPS peaks versus Co coverage. The Zn/Co ratio declines sharply when increasing the Co coverage to 2 ML; in addition, the ratio decelerates asymptotically to 3 ML. The curve shape together with the STM result indicates that the initial Co deposition process approximates the layer growth mode.1,19 Figure 3b,c displays the XPS spectra of Zn 2p and O 1s for different Co coverages. The Zn 2p1/2 and 2p3/2 features from the clean ZnO(101̅0) surface are two narrow peaks appearing with binding energies of 1045.1 and 1021.7 eV, respectively. The O 1s feature from the clean ZnO(101̅0) surface is one peak with a
Figure 2a. Figure 2b also shows the Co(2 × 1) cell. Thus, the 0.5 ML Co exhibits (2 × 1) periodicity, referring to the ZnO(101̅0) surface. According to the STM observation, we have modeled the Co(2 × 1) structure as illustrated in Figure 2e. The Co adatoms likely interact with dangling bonds of two adjacent ZnO dimers to select the favorite sites and generate the observed Co(2 × 1) surface at 0.5 ML Co coverage. The 0.5 ML Co surface structure could be mainly attributed to the formation of cobalt oxide.37−39 For further deposition of Co coverage to 0.7 ML, some areas of the Co(2 × 1) structure convert to the Co monatomic stripe structure, as shown in Figure 2f. At this coverage, the Co monatomic stripes range in length from 3 to 6 nm and in area by approximately 25% of the surface. For the Co coverage close to 1 ML, nearly the entire surface converts to the structure of the Co monatomic stripes, as shown in Figure 2g, with some “missing Co defects” remaining between the strips, as denoted by the dashed circles. Additionally, Figure 2h displays a highresolution STM image, with spots along the underlying ZnO[12̅10] direction. The arrows labeled as “III” and “IV” denote the corresponding line scans, as shown in Figure 2i,j. The distance between the stripes is 0.55 nm along the underlying ZnO [0001] direction, and the distance between Co atoms along the underlying ZnO[12̅10] direction is 0.35 nm. The additional Co adatoms take the in-between Co positions of the Co(2 × 1) cell along the underlying ZnO[12̅10] direction 9920
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binding energy of 529.9 eV. These Zn 2p peaks with extremely sharp and symmetrical shapes show an attenuation of their intensities, and the full width at half-maximum (fwhm) slightly broadens from 1.99 eV (0.5 ML) to 2.02 eV (3 ML) when increasing the Co coverage. The O 1s peak also has the same trend; the fwhm broadens from 2.00 eV (0.5 ML) to 2.08 eV (3 ML). Moreover, with increasing Co coverage, both the Zn 2p3/2 and the O 1s peaks shift to lower bind energy with respect to the clean ZnO(101̅0) surface values, as shown in Figure 3d,e, respectively. The energy shift of around 0.45 eV for Zn 2p3/2 and 0.4 eV for O 1s to a lower binding energy occurs with a Co coverage of 3 ML. One explanation that is proposed for the peak shifts is that it results from charge transfer between Co and Zn. This is unlikely the origin operating in the Co/ ZnO(101̅0) system. If the charge-transfer mechanism is valid, a shoulder for the surface Zn and O might have been observed, but the entire peak would not shift, because the XPS probing sample depth is at least ∼3 nm and only the surface Zn or O would be altered by bonding to Co. The band-bending mechanism seems like a more plausible explanation for Co on the ZnO(10 1̅0) system, which has been reported for metals on other ZnO surfaces.40,41 The charge redistribution between the Co overlayer and ZnO substrate results in a shift of both the core level and the valence level of the ZnO owing to a change in the electrostatic potential (i.e., band-bending effect).42,43 However, charge transfer between metal adsorbates and semiconductor substrates often induces the band bending of the substrate without forming a shoulder in the core-level photoemission peaks. For example, alkali metals are ionized upon adsorption on the ZnO(101̅0) surface and induce downward band bending. Here, the XPS study does not show the shoulder component associated with reduced species by transferred charge.44 Therefore, the charge-transfer mechanism might not be completely ruled out. In addition, the energy shift of the O 1s peak to lower binding energy at 0.5 ML Co coverage is likely due to the formation of mixed Co2+ and Co3+states (CoOx). The core-level spectra and valence-level spectra are discussed in the following. Figure 4a displays the changes in the Co 2p XPS core-level spectrum as a function of the Co coverage. The solid line and dotted line in Figure 4a denote the Co oxidation state and the Co metal state, respectively. The Co 2p XPS spectrum of 0.5 ML Co coverage reveals a peak at 780.5 ± 0.2 eV, which can be assigned to Co 2p3/2, and a peak at 794.0 ± 0.2 eV, which can be attributed to 2p1/2 peaks, respectively.19,45−49 Both peaks indicate that the 0.5 ML Co exists predominantly in the oxidation state mixed with a minor metallic state. When the Co coverage is increased to 0.6 ML, Co remains primarily in the oxide state, with the minor metallic peak enhancing at a lower binding energy. When Co coverage is increased to 0.7 ML, however, a clear metallic state of cobalt forms, as evidenced by a 2p3/2 peak appearing at 778.1 ± 0.2 eV and a 2p1/2 peak appearing at 793.0 ± 0.2 eV.19,45−47 Notably, metallic and oxidized cobalt coexist for a Co coverage up to 1.0 ML. For a Co coverage exceeding 1.5 ML, Co metal peaks completely dominate in the XPS spectra. In a further detailed analysis, after Shirley-type backgrounds are subtracted from the raw spectra, the Co 2p3/2 XPS peak features of 0.5−1 ML Co coverage can be fitted into four peaks by the Gaussian simulation, as shown in Figure 4b. The lowest binding energy peak located at 778.1 eV (A) can be assigned to the Co metal state.19,45−47 The binding energy peak at 780.7 eV (B) can be attributed to the cobalt oxide state.19,45−49 The
Figure 4. (a) The XPS spectra of Co 2p as a function of Co coverage. A solid line denotes the location of the Co 2+ state, and a dotted line denotes the location of the Co0 state. (b) The XPS spectra of Co 2p3/2 from 0.5 to 1 ML Co coverage. The black dots denote raw spectra; yellow Gaussian curves (A) represent the signals from the Co metal state; blue Gaussian curves (B) refer to the signals from the Co oxidation state; purple Gaussian curves (C) refer to the signals from additional Co species of a higher oxidation; green Gaussian curves (D) denote the cobalt oxide satellite peak; and red curves represent the sum of the fitted Gaussian curves.
binding energy peak at 782.7 eV (C) can be attributed to the presence of additional Co species of a higher oxidation state.49 The highest binding energy peak appearing at 786 eV (D) is related to the satellite peak in cobalt oxide.19,45−49 For 0.5 ML Co coverage, the fitted data thus reveal that a small amount of Co metal exists on the surface in addition to the cobalt oxide. The intensity of the B and C curves gradually attenuates, and the intensity of the A curve gradually increases when increasing the Co coverage, indicating the formation of cobalt oxide for the low Co coverage range and the domination of Co metal at a higher Co coverage. 3.4. UPS Characterization of Co on the ZnO(101̅10) Surface. Figure 5a shows the UPS spectra of the ZnO(1010̅ ) surface as a function of the Co coverage. The photon energy used was 21.2 eV, and the incidence angle of the light θi was 45° from the surface normal. The UPS spectrum of the clean ZnO surface is characterized by a peak at 4.8 eV, which is related to the emissions from the O 2p dangling-bond state. The emission peak at 10−11 eV below the Fermi level is attributed to the Zn 3d state. The electronic structure of the clean ZnO(101̅0) surface agrees with that of previous studies.22,23,50 As the surface is covered with Co, the intensities of Zn 3d and O 2p states are gradually suppressed with an increasing Co coverage. Furthermore, the deposition of Co induces a shift of the Zn 3d state. Also, a 0.45 eV shift to a lower binding energy is observed when increasing the Co coverage up to 3 ML, indicating an upward band bending due likely to a charge redistribution effect.42,51 The Co-induced feature is viewed as a step-like shape around the Fermi level. This feature is attributed to the Co 3d emissions.18,52 Figure 5b shows the magnified spectra around the Fermi level to elucidate in more detail the evolution of the 9921
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3.5. Discussion. The combinational STM, RHEED, XPS, and UPS results of this study provide further insight into the evolution of the surface structure and chemical state of varied Co coverages on ZnO(101̅0). On the basis of the observations of STM, RHEED, XPS, and UPS, the growth mode of the Co film on ZnO(101̅0) is confirmed. According to the STM, RHEED, and XPS results, well-ordered CoOx(2 × 1) structures are formed on the ZnO(101̅0) surface in 0.5 ML Co coverage. The Co monatomic stripes cover the entire surface when increasing the Co coverage to 1 ML. The core-level and valence-level spectra indicate that the Co monatomic stripe structures largely correspond to the metallic state. When the Co coverage exceeds 2 ML, the STM results indicate that Co clusters are formed in addition to the structure of the monatomic stripes. The Co metal peak dominates the XPS spectrum, and in the UPS spectrum, the onset position of the Co 3d DOS shifts toward the Fermi energy with an increasing Co coverage to 3 ML, which is indicative of the cluster formation. The results confirm that the Co adatoms follow the 2D growth mode at low Co coverage and bridge to 3D island growth at higher Co coverage. The XPS and UPS spectra reveal a shift of the valence band maximum, the Zn 3d by UPS and Zn 2p and O 1s by XPS, indicating that an upward band bending is induced by the Co overlayer. The band shift is attributed to charge redistribution between the metal overlayer and the oxide surface, incurring a concordant shift of all core and valence levels of oxide owing to a change in the electrostatic potential (i.e., band-bending effect). The deposition of Co on ZnO(101̅0) produces only an upward band bending. The requirement that the bands must be bent upward to align the vacuum levels corresponds to the fact that the work function of ZnO is smaller than that of Co. According to various studies, values reported of the work function are 4.8522 and 4.64−5.05 eV56,57 for ZnO(101̅0), and 5.0−5.1 eV58 for Co, which agrees with the sign and magnitude of the band bending of the XPS and UPS results herein. Another plausible explanation of the upward band bending is the formation of a Schottky barrier at the metal/semiconductor interface.42 It is known that Schottky barrier formation involving the local charge redistribution at the metal/semiconductor interface could induce the band-bending effect.59 Our results contrast with those of other metal adatoms deposited on the ZnO(101̅0) system, such as the K/ ZnO(101̅0),22 Ag/ZnO(101̅0),23 and Cu/ZnO(101̅0).50 As is well-known, the factors that govern the growth mechanism of the metal overlayer include the thermodynamics equilibrium,1,60 kinetics of the metal atoms on the surface,60,61 and atomic-level surface structures, such as steps,62,63 point defects,64 and adsorbates.21,65 These factors distinguish between systems to result in different surface structures and growth modes. In the K/ZnO(101̅0) system, LEED measurements indicate that K adsorbs on ZnO(101̅0) in a twodimensional disorderly fashion at room temperature, whereas c(2 × 2) and (3 × 1) ordered structures are formed when the K-covered surface is annealed at 950−1050 and 1100−1200 K, respectively. Moreover, according to the UPS observation, the K adsorption induces the charge transfer from the K adatoms to the ZnO(101̅0); in addition, the ionic adsorption is supported from the XPS measurements. For the Cu/ZnO(101̅0) system, STM results indicate that Cu forms islands on ZnO(1010̅ ) at room temperature, and these islands are exclusively 3D even at an extremely low coverage. Furthermore, based on the angleresolved photoemission observations, the coverage-dependent
Figure 5. (a) The UPS spectra of Co/ZnO(101̅0) as a function of the Co coverage. A solid line denotes the position of the Fermi level, and a dotted line indicates the position of the O 2p state. The position of the Zn 3d peak is indicated by vertical bars. (b) Enlarged spectra around the Fermi level. The step structure is formed by the emissions from the Co 3d DOS. The midpoint of the onset state, as indicated by triangle marks, is located at 0.64 eV at 0.6 ML and moves to the Fermi level at 3 ML.
Co 3d structure. The onset of the band edge is near 1 eV below the Fermi level at a lower Co coverage owing to the formation of the cobalt oxidation state below 0.5 ML Co coverage.18,52 The emission intensity of the Co 3d states is gradually enlarged with increasing the Co coverage, and the step shape becomes more obvious. The triangular marks in Figure 5b indicate the midpoint of the onset. Located at 0.64 eV at 0.6 ML Co, the midpoint shifts to coincide with the Fermi energy at a Co coverage exceeding 2 ML. The above results represent the changes of the total density of states (DOS) of the Co 3d bands.18,52 Some information of the nucleation and growth process of Co on ZnO(101̅0) might also be obtained by carefully inspecting the Co 3d spectra. First, the Co 3d is manifested above 0.7 ML Co coverage. This finding implies that the Co adatoms aggregate to some extent so that the interaction between the neighboring Co atoms is enhanced to form the 3d band from a thicker Co coverage. Second, the onset of the Co 3d DOS moves to the lower binding energy side when increasing the Co coverage. The shift of the DOS is a common phenomenon for the noble-metal/oxide substrate systems studied so far, where the noble-metal adatoms aggregate into clusters; these clusters grow in size when increasing the amount of deposition.23,53−55 Previous studies have explained the shift of the onset as originating partially from the change in the efficiency of the final state screening with an increasing cluster size.53,54 9922
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ethoxide decarbonylation, forming CO and H2 and adsorbed methyl groups. Therefore, the manipulating cobalt oxidization and Co metallic state and structures could be useful in various catalytic activities. The temperature-dependent studies of Co growth on ZnO(1010̅ ) will be carried out and reported elsewhere.
measurements of the Cu 3d and 4sp bands can also be characterized by a cluster formation. Interestingly, the Cu clusters are semiconductors with an energy gap around the Fermi level at the initial Cu growth (0.45 ML); meanwhile, the metallic nature is developed when increasing the Cu coverage. For the Ag/ZnO(101̅0) system, photoelectron spectroscopy observations suggest that the Ag grow in a 2D mode at a low coverage (≤0.12 ML); in addition, 3D growth sets in from a moderate coverage, which is well below the completion of the Ag monolayer. In comparison to the above systems, the growth mode of Co on ZnO(10 1̅0) is similar to that of Ag/ ZnO(101̅0), where the growth process is 2D bridged to 3D. The nucleation and growth process of Co/ZnO(101̅0) seems to contrast with Co films on other faces of ZnO. The growth mode of Co on polar ZnO surfaces is controversial. For the Co deposited on ZnO(0001̅)18 and on ZnO(0001)17,19 systems, the metallic clusters are formed for low Co coverage (≤1 ML). The features of 3D growth begin in the initial coverage,17,18 which markedly differs from our results of Co on ZnO(101̅0). However, another study indicates that the initial growth mode of Co on ZnO(0001) is layer-by-layer growth at RT.19 The dominated oxidation state of the Co submonolayer on the ZnO(101̅0) surface is largely owing to the initial 2D growth mode and an electrostatic interaction between Co and ZnO. It is known that the growth mode can be strongly influenced by the surface structure of substrates. Compared to polar surfaces, ZnO(0001) and ZnO(0001̅), ZnO(101̅0) has a unique surface structure, which is characterized by the Zn−O dimer rows directed along the [12̅10]. Diffusion of adatoms on the (101̅0) surface is, therefore, constrained along the dimer rows.21,23 In addition, the ab initio calculations of Co on ZnO surfaces66 suggest that the growth of Co adatoms prefer a disperse distribution on the ZnO(101̅0) surface, but the formation of the Co cluster on ZnO(0001̅) is favored. Therefore, the distinct growth mode of Co on the polar ZnO surface and on the nonpolar ZnO(101̅0) surface is observed. Another important implication of Co supported catalysts is that the support can significantly affect reactivity.67,68 Highly active and selective catalysts have been developed for the SRE [CH3CH2OH + 3H2O → 6H2 + 2CO2]. Although precious metal catalysts, including Pd and Ru, have been reported to be active for this reaction, the high cost of these materials and the required high reaction temperature above 900 K are major drawbacks.69,70 However, having emerged as a promising alternative to precious metals, the Co-based catalysts can induce a high selectivity toward CO2 and H2 (>90%) at relatively low reaction temperatures with CH 4 , C 2 H4 , CH3CHO, and (CH3)2CO, which are the primary side products.71,72 Moreover, Llorca et al.72 reported selectivities to H2 and CO2 at 723 K, for high ethanol-to-steam ratios, 91% for Co supported on ZnO, but only 52% when MgO and TiO2 were used as the supports. In addition to the role of support, the question concerning Co-based SRE catalysts is the active form of the cobalt, specifically whether metallic or oxidized cobalt is required. Though metallic cobalt is often assumed to be the active species, recent studies have indicated that both metallic and oxidized cobalt are found in the most active catalysts in SRE reactions.20,72−75 On the basis of the above results, we can infer that the cobalt oxidization at 0.5 ML coverage might be a good template to provide the active site for dehydrogenation of adsorbed ethoxide species in order to produce acetaldehyde. The Co metallic stripes above 0.7 ML coverage and Co clusters above 3 ML might be active for
4. CONCLUSIONS In this paper, we investigate the room-temperature nucleation and growth process and chemical state of Co on ZnO(1010̅ ) by STM, RHEED, XPS, and UPS. STM and RHEED results indicate that the well-ordered CoOx(2 × 1) structure is formed at 0.5 ML Co coverage. With increasing Co coverage between 0.7 and 1 ML, the surface is characterized by the Co stripe structure. The Co clusters are present as the Co coverage exceeds 3 ML. The growth process of Co on ZnO(101̅0) is proposed to govern the 2D-to-3D transition. XPS results indicate that the Co oxidation state exists at low Co coverage and transfers to the characteristic Co metallic state at thicker Co coverage. According to the UPS results, significant DOS of Co 3d states are formed when increasing the Co coverage, indicating the formation of Co clusters. Additionally, the upward band-bending behavior is due to the work function difference between Co and ZnO(101̅0). The results suggest that Co/ZnO(101̅0) could be highly promising for catalysis applications.
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AUTHOR INFORMATION
Corresponding Author
*Tel: +886-6-2757575, ext. 65266. Fax: +886-6-2747995. Email:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under Contract Nos. NSC 99-2120-M-006-003 and NSC 100-2112-M-006-018-MY3. Ted Knoy is appreciated for his editorial assistance.
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