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J. Phys. Chem. B 2000, 104, 3028-3034
Structure of Pt Overlayers on ZnO(0001) and ZnO(0001h) Surfaces† P. V. Radulovic and C. S. Feigerle Department of Chemistry, UniVersity of Tennessee, KnoxVille, Tennessee 37996-1600
S. H. Overbury* Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6201 ReceiVed: September 17, 1999
The growth of Pt films on the Zn- and O-terminated polar surfaces of ZnO has been studied. Clean surfaces, prepared by sputtering and annealing, were dosed with Pt by vapor deposition. Properties of the film were examined by low-energy alkali ion scattering and low-energy electron diffraction. It is confirmed that, near room temperature, Pt grows on both surfaces as a single hexagonal layer which aligns with the substrate low index directions. The alignment is better for the Zn-terminated than for the O-terminated surface. A Pt monolayer grows on the Zn-terminated surface until the ZnO surface is essentially covered with no evidence of a second Pt layer. Upon annealing above 675 K, Pt agglomerates into mostly bilayer islands, but maintains or improves its alignment with the substrate. Features associated with scattering from bi- or multilayer Pt indicate that the Pt forms oriented (111) rafts. A coincidence LEED pattern indicates that the Pt lattice may be expanded by 0.5% to form a 6:7 coincidence overlayer with the ZnO lattice.
1. Introduction Interest in studies of metal films deposited on oxide surfaces is primarily derived from their catalytic properties. For example, copper deposited onto a zinc oxide surface acts as a catalyst for methanol synthesis from carbon monoxide and molecular hydrogen, as well as the water-gas shift reaction and methanol steam reforming.1 It has also been found that ZnO can be used in certain sensor applications2 utilizing the conductivity change in response to adsorbed gases as a basis for the device. The polar surfaces of ZnO, the (0001) and (0001h), are of particular interest since they are structurally identical, but the (0001h) is terminated by O while the (0001) is terminated by Zn-containing planes. Both surfaces can be obtained by cleaving the needleshaped crystal perpendicular to the crystallographic c-axis.3-5 ZnO can also be grown hydrothermally resulting in large single crystals which can be oriented and polished to obtain the polar faces. Structure of the clean polar surfaces has been studied earlier.4,6-9 Metal-dosed polar single-crystal surfaces of ZnO have been studied by various surface techniques including X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), high-resolution electron energy loss spectroscopy (HREELS), and low-energy ion scattering (LEIS). The dosing metals used have included alkali metals,10 Ni,11 Ag,12 Au,13 Pd,12-14 Pt,15,16 and Cu.17,18 There is considerable interest in the kinetics and morphology of film growth in such experiments and good reviews of recent research can be found.19,20 Previous studies of Pt films deposited onto both polar surfaces of ZnO showed that there exists a sufficiently strong interaction between Pt and ZnO to lead to layer-by-layer growth at room * Corresponding author. Tel. 865-574-5040. Fax 865-576-5235. Email:
[email protected]. † Part of the special issue “Gabor Somorjai Festschrift”.
temperature.15,16 The Auger intensity vs Pt dose were found to fit well to a model computed for layer-by-layer growth with layer density of 1 × 1015 cm-2. This layer atom density is comparable to that of Zn on the ZnO(0001) surface, but 50% lower than for a Pt(111) plane. From analysis of phonon attenuation in HREELS, it was inferred that the initial growth of Pt is nearly layer-by-layer on both terminations.16 However, charge transfer in the Zn-terminated surface leads to differences in the coverage dependence of the phonon scattering intensities for the two surfaces. Upon heating of the Pt/ZnO system, agglomeration occurs above 600 K.15 The Pt particles formed align with the ZnO substrate, although there are conflicting results about the orientation of the Pt rows with the ZnO substrate rows.15,16 Campbell has introduced the concept of a critical coverage, defined as the fraction of an oxide substrate covered by a metal before the onset of bilayer or multilayer growth occurs.19,20 The AES and HREELS15,16 results for Pt on ZnO surfaces suggest a critical coverage near unity, as do XPS results for Cu/ZnO(0001).21 However, more recent LEIS measurements which have inherently better surface sensitivity suggest smaller critical coverage for Cu/ZnO(0001).22 AES results for room temperature deposited Pd on the ZnO(0001h) surface have been interpreted as consistent with layer growth12 or in other work as indicating growth in clusters or 2D island growth.14,19 It has been concluded that it is difficult to accurately distinguish certain growth models only by AES attenuation.19 In this study, we have examined the structure of room temperature deposited Pt films on polar ZnO surfaces using lowenergy alkali ion scattering (LEAIS) and LEED. LEAIS is surface sensitive and the angular dependence is sensitive to the local short-range surface structure. It provides a means of monitoring the film morphology and the thermally activated reorganization of Pt on the ZnO surface. Using this technique, it is concluded that a room temperature deposition yields a Pt layer, exactly one atomic layer thick, that covers nearly the entire
10.1021/jp993325e CCC: $19.00 © 2000 American Chemical Society Published on Web 12/21/1999
Structure of Pt Overlayers on ZnO Surfaces
J. Phys. Chem. B, Vol. 104, No. 14, 2000 3029
surface before additional growth occurs. The Pt layer has a density comparable to Pt(111) planes although it may be 0.5% expanded. Annealing induces thickening, primarily to bilayers of Pt(111), and improvement in the azimuthal alignment of the Pt bilayers. 2. Experimental Section Experiments were performed in a stainless steel UHV chamber used in previous studies23 and with base pressure of less than 2 × 10-10 Torr. Briefly, the chamber contains a LEAIS spectrometer, a sputter gun for sample cleaning, a quadrupole mass spectrometer for monitoring residual gases, and a fourgrid LEED optics, also used for retarding field AES. Ion-scattering measurements were conducted using a 1000 eV Li+ beam which was collimated, monoenergetic, and mass selected. The diameter of the beam was approximately 1 mm and the total ion flux was (5-15) × 1010 ions/(cm2 s). Scattered ions were analyzed by a spherical sector electrostatic analyzer (∆E/E ) 2%) that could be rotated about the sample allowing the laboratory scattering angle (θ) to be set between 0° and 130°. Typically, energy distributions and polar and azimuthal dependencies for the Li+ scattering were measured. All ion scattering data presented were collected at the scattering angle of 130° because it allows the best available resolution of Pt and Zn peaks in the energy distribution. Pt and Zn peaks overlap at the lower scattering angles. The ZnO samples could be rotated under computer control about two orthogonal axes, allowing easy variation of the polar incidence angle, ψ, measured from the plane of the surface, and the azimuthal incident angle, φ, measured from the [11h00] azimuth. Calibration of the polar incidence angle was done by laser alignment. Azimuthal orientation was determined by ion scattering. A single crystal of hydrothermally grown ZnO (8 mm × 5 mm × 1 mm) was purchased from Litton Airtron Synoptics. The front and back of this crystal were sequentially oriented and polished for the two terminations, which could be readily distinguished by ion scattering.6 Two methods for mounting the sample were used. In the case of the O-terminated surface, the sample was clamped to a Ta plate with a chromel-alumel thermocouple clamped between sample and backing plate. It was suspected that this method did not yield accurate temperature readings during annealing. The Zn-terminated surface was attached using this method initially, but for the results presented below a second technique was used in which the sample was cemented to a sacrificial piece of polycrystalline ZnO with Aremco Ceramabond 570. A pair of heating wires and a chromel-alumel thermocouple were embedded in the cement creating a mountable sandwich. We believe that this method gave more accurate temperature control. A Pt evaporative doser was constructed by tightly winding a Pt wire around a resistively heated 0.25 mm tungsten filament. Pt dosing rates were relatively low, roughly in the range of 10-3-10-4 monolayer/s. In all measurements described below, the Pt was deposited at room temperature to within about 15 K. There was no independent monitor available for determining Pt flux at the sample surface, so estimates of the coverage were determined from the behavior of the Auger and ion scattering. Pt and other contaminants were removed from the crystal surface by Ar ion sputtering (500-2000 eV) near room temperature. The amount of Pt remaining after such sputtering was estimated by ion scattering to be 0.004 monolayer or less. After sputtering, the sample was typically annealed above 900 K for about a minute. The sample was also probed with AES to check for surface impurities. It was generally found that the
Figure 1. Energy distribution of the scattered Li+ ions from the clean and Pt-dosed ZnO(0001) following consecutive Pt doses near room temperature. Following the 105 min dose the sample was heated briefly to 425 K to remove possible adsorbed species (CO) which may have accumulated before performing the final dose. Curves are offset.
surface produced in such a manner contained only small amounts of implanted Ar. The clean surface exhibited a sharp p(1×1) LEED pattern. Following annealing, ion scattering data were collected as the sample cooled below 375 K toward room temperature. Ion scattering intensities were multiplied by sin ψ to corrected for variation in geometric cross section. Although experiments on the O-terminated and the Zn-terminated surfaces were widely spaced in time, the direct comparison of the absolute Pt intensities should be meaningful. This was confirmed by comparison of the intensity of the Zn peak, obtained from the clean surfaces, for scattering conditions such that the top layer of Zn contribute approximately equally.6 Comparable clean surface intensities indicate that there was no substantial variation in the instrument sensitivity. During the experiments there were some charging problems with the clean sample and Pt deposited at room temperature as observed earlier.17 These problems were erratic and could not be precisely connected with any controlling factors in the chamber. Annealed surfaces with deposited Pt tended to charge less often than room temperature Pt-deposited surfaces. 3. Results 3.1. Pt Film Deposited Near Room Temperature. The energy distributions measured from a clean and Pt-dosed Znterminated surface, ZnO(0001), are shown in Figure 1. The distribution for the clean surface (Figure 1a) contains a sharp elastic peak from Zn (relative energy, E/Ei ) 0.7) and intense background coming from the inelastically scattered Li+ ions. Upon Pt deposition, the distribution also contains a sharp peak from Pt (E/Ei ) 0.88). Figure 1b-d shows the energy distributions measured following progressively larger Pt doses. With increasing dose, the Pt peak increases in intensity until it reaches a “saturation” intensity where further doses do not substantially increase the peak height more than 10-20%. Initially, it maintains its sharpness, but above saturation it broadens toward lower relative energies. The intensity of the background, below the Pt and the Zn peaks, also increases somewhat as Pt is deposited onto the surface. In contrast, the Zn peak decreases in intensity with increasing Pt dose. At the highest Pt coverage (Figure 1d), the Zn peak disappears, marked only as an increase in the background. The Zn peak becomes almost indistinguish-
3030 J. Phys. Chem. B, Vol. 104, No. 14, 2000
Figure 2. Polar incidence angle dependence of the Pt peak height (at E/Ei ) 0.88) recorded in the [11h00] azimuth following deposition of different Pt coverages on the Zn-terminated surface. The fraction of the surface covered by Pt islands is given for each curve. Pt was deposited near room temperature. For the largest dose (curve e) the sample was annealed to 425 K to remove possible adsorbed species. Curves are not offset or scaled.
able as a peak at about the same dose as where the Pt peak reaches saturation intensity. Figure 2 shows a polar incidence angle dependence of the Pt single scattering intensity, at E/Ei ) 0.88, following Pt deposition at room temperature. In each scan there is a characteristic low-angle edge caused by the shadowing of first-layer Pt atoms by neighboring first-layer Pt atoms. In curves a-d, the (sin ψ corrected) intensity is roughly independent of angle above 30°. For these curves the Pt intensity is due to scattering only from top layer Pt, and the intensity is proportional to the fraction of the surface covered by Pt or Pt islands. In Figure 2e, a broad peak appears at about 53° which can be associated with scattering from Pt atoms below the top Pt layer. The presence of this peak is indicative of Pt islands more than one layer thick. It first appears at about the same Pt dose as where the Zn peak disappears from the energy distribution. This fact is illustrated by the energy distribution shown in Figure 1d which was obtained from the same surface. The peak at 53° in the incident angle dependence continues to grow in intensity with further doses (not shown), indicating further Pt deposition, but the intensity below about ψ ) 30° does not increase substantially. Taken together, the simultaneous saturation of the Pt intensity ψ ) 30°, the disappearance of the Zn peak, and the appearance of the feature at 53° suggest that the point of saturation corresponds to continuous single layer of Pt atoms. Below this point, the Pt intensity can be used to specify the fraction of the surface covered by Pt islands as shown in Figure 2. The azimuthal dependence following Pt deposition is shown in Figure 3 for grazing angle of incidence, ψ. Prominent variations in the intensity with azimuthal angle are observed. The alignment of the angular dependence with the symmetry of the substrate indicates a high degree of alignment of the Pt islands with the ZnO substrate. The overall Pt intensity increases with Pt dose, but the angular contrast and shape of the distribution are maintained. Room temperature deposited Pt films on the O-terminated surface, ZnO(0001h), showed subtle differences from the Znterminated surfaces. Figure 4 compares depositions on the Oand the Zn-terminated surfaces and shows both energy distributions (lower panel) and incident angle dependence (upper panel). In the energy distributions (Figure 4 lower), the Pt single
Radulovic et al.
Figure 3. Azimuthal incidence angle dependence of the Pt peak height (at E/Ei ) 0.88) recorded at a grazing incident angle of 14° for three different fractional Pt coverages on the Zn-terminated surface. Pt was deposited near room temperature.
Figure 4. Energy distributions (lower panel) and incident angle dependence of the Pt single scattering peak height (upper panel) compared for the room temperature deposited Pt films onto O- and Zn-terminated surfaces. The data shown for the two surfaces in each panel were selected to have comparable Pt single scattering intensities, although the actual Pt coverage may differ.
scattering intensities are comparable for both surfaces. For the oxygen-terminated surface, there is a more pronounced broadening of the Pt peak, a larger buildup in the background intensity between the Pt and Zn peaks, and greater attenuation of the Zn peak than for the Zn-terminated surface. These features are consistent with thicker Pt islands for the O-terminated surface. In the incident angle scans, the O-terminated surface has a broader low-angle edge and continues to increase gradually between 20° and 50°. A gradual increase and broad edge may suggest that some of the Pt islands are not flat monolayers but may be more than one layer thick, even after room temperature deposition. However, less data were available for the Oterminated surface, especially at higher coverages, making it impossible to more fully characterize this trend. Qualitative LEED observations were made before and after Pt dosing. The clean surface of both the Zn- and O-terminated surface typically showed sharp p(1×1) patterns, although charging interfered with observations on the O-terminated surface. Increasing deposition of Pt near room temperature on
Structure of Pt Overlayers on ZnO Surfaces
Figure 5. Effect of annealing upon the Li+ energy distribution for Pt deposited near room temperature on the Zn-terminated surface. In (a) the as-deposited surface was completely covered with Pt while in (b) the surface was about 40% covered. Curves in (b) are offset.
the Zn-terminated surface caused the p(1×1) pattern to become less bright, the beams to become possibly more diffuse, and the background to become brighter. No extra beams were observed for the room temperature deposition for any dose including the highest dose shown in Figure 2. 3.2. Annealed Pt Films. After deposition of Pt near room temperature and appropriate measurements, the Pt/ZnO surfaces were annealed under ultrahigh vacuum conditions. The effects of annealing are shown in Figure 5 which compares energy distributions collected from Pt on ZnO(0001) before and after annealing to 975 K. Results are shown for two different coverages of Pt. Annealing induces distinct changes including (1) an increase in the Zn single scattering intensities and sharpening compared to background, (2) a decrease of the Pt peak intensity at the single scattering energy and its pronounced broadening to lower energies, and (3) a decrease in the background below the Zn peak but an increase in background between the Pt and the Zn peak. These changes were observed reproducibly for many different doses on the Zn-terminated surface. Similar effects were observed for the [21h1h0] azimuth, at 30° to the [11h00] azimuth. Identical trends were observed also upon annealing Pt deposited upon the oxygen-terminated ZnO surface. Figure 6 shows a polar incidence angle dependence of Pt peak intensity for a single Pt dose on the ZnO(0001) surface following anneals to sequentially higher temperatures. Upon annealing the as-deposited surface to 475 K, an overall Pt intensity increase of about 10% was observed. This increase might be associated with desorption of impurities (CO) which may have accumulated during the Pt dosing. Subsequent anneals to higher temperature caused decreases in the Pt intensity. Upon annealing to 675 K, there begins to appear weak peaks near 53° and 76°, which become more pronounced by 875 K. The two peaks are almost identical in shape and intensity. The minimum between these two peaks matches the specular angle. Upon annealing 975 K both peaks appear to sharpen and become more intense relative to the Pt intensity at other angles. The high-angle features which appear upon annealing are characteristic of the growth of multilayer Pt(111) planes on the surface. This fact is demonstrated in Figure 7 which compares the annealed Pt/ZnO(0001) with two incidence angle dependencies from clean single-crystal Pt(111) obtained during a previous study using the same apparatus.24,25 Appropriate scaling of intensities was applied to correct for different ion detector
J. Phys. Chem. B, Vol. 104, No. 14, 2000 3031
Figure 6. Effect of annealing upon the incident angle dependence. The initial surface was about 40% covered by Pt deposited near room temperature on the Zn-terminated surface.
Figure 7. Polar angle dependence of the Pt peak for an annealed Pt film deposited onto the Zn-terminated surface compared with results from a clean Pt(111) surface.24,25 For Pt(111), scans are shown for two different azimuths separated by 60°. The Pt peak intensity is estimated by the count rate at E/Ei ) 0.88. Curves are offset.
sensitivities and other instrumental factors. For Pt(111) highangle peaks are observed at 53° and 76°, one in each of two different azimuths 60° apart, and are known to be due to the contribution of second (and deeper) layer Pt atoms. Scattering from Pt /ZnO(0001) exhibits both peaks in a single azimuth, implying the presence of Pt(111) crystallites with two different orientations with respect to the plane of scattering. The peaks are more intense for Pt(111) than for the film. In addition, the low-angle edge is not as sharp for the Pt islands as for the Pt(111) single crystal. The sharpening which occurs upon annealing in the polar incident angle is accompanied by a significant sharpening in the azimuthal dependence as shown in Figure 8. At all φ, the Pt intensity decreases upon high-temperature annealing, but the effects of shadowing at grazing angles become more distinct. The changes in the incident angle dependence induced by annealing for Pt on the Zn-terminated surface were similarly observed for Pt on the oxygen-terminated surface and occurred at roughly the same temperature range. This is illustrated in Figure 9 which compares a polar incidence angle dependence for Pt-dosed ZnO(0001) and ZnO(0001h) after annealing. Evidently the same structural transformation is occurring on both surfaces. Positions of the low-angle edge indicate comparable Pt-Pt spacing in the first layer on both surfaces. The peak (near
3032 J. Phys. Chem. B, Vol. 104, No. 14, 2000
Radulovic et al. coverages. The alignment of the second p(1×1) pattern with the ZnO p(1×1) is further evidence for registry between the overlayer and substrate. 4. Discussion
Figure 8. Azimuthal angle dependence of Pt peak intensity compared for Pt as deposited upon the ZnO(0001) surface and following an anneal to 925 K. The Pt peak intensity is estimated by the count rate at E/Ei ) 0.88, and the incident angle is held constant at 14°. The fraction of the ZnO surface covered by Pt is 0.82. Curves are not offset.
Figure 9. Comparison of the polar incidence angle scans for Pt overlayer deposited onto oxygen- and zinc-terminated polar surfaces of ZnO. The Pt was deposited near room temperature and the surface was then annealed to 925 K for Zn-terminated surface and 1100 K for the O-terminated surface.
53°) corresponding to scattering from subsurface Pt is present at the same angle for both surfaces. It is indicative of Pt islands at least two layers thick and of the relative placement of the top two Pt layers. The low-angle Pt edges seem to be sharper for the zinc-terminated surface which would indicate better ordering of Pt. The relative intensity of the high-energy peak compared to the intensity at other angles is larger for the Znterminated surfaces than for the O termination which may also be an effect of ordering. Annealing to 975 K caused a pronounced change in the LEED pattern. After annealing the Zn-terminated surface with the highest coverage in Figure 2, each of the ZnO p(1×1) beams were sharp and were surrounded by a sextuplet of satellite beams spaced at about 10-15% of the reciprocal lattice unit cell spacing. The hexagonal pattern of these satellites aligned with the orientation of the ZnO p(1×1) pattern. Also, additional beams indicative of a smaller lattice constant formed a second hexagonal p(1×1) pattern estimated to be about 20 ( 5% larger than the ZnO pattern. This larger pattern is assigned to the Pt(111) overlayer consistent with fact that the ZnO lattice is 17% larger than Pt. This pattern was also seen at lower
The incident angle dependences are consistent with Pt growing on the Zn-terminated surface in rafts with epitaxy defined by Pt(111)//ZnO(0001h) and Pt[112h]//ZnO[11h00] or Pt[1h21h]//ZnO[11h00]. The evidence is as follows. The anisotropy in the azimuthal scans show that the top layer Pt atoms, probed by the ion scattering at grazing incidence, exhibit 6-fold symmetry, indicating (111) planes. The anisotropy is sharpest and most pronounced for an annealed surface (Figure 8), but it is already visible at low coverages in the as-deposited films (Figure 3) where no LEED superstructures are visible. This anisotropy would not be visible, however, if the Pt(111) planes of each crystallite had random azimuthal alignment with respect to the ZnO substrate. Therefore, they must also be azimuthally aligned with the substrate. The azimuthal scans also indicate that the alignment is such that the “long” directions of the Pt crystallites, e.g. [1h21h] and [112h], are aligned with the [11h00] axis of the ZnO substrate. On the basis of the usual shadowing arguments,6 the dip in intensity centered at φ ) 30° and the higher intensity centered at φ ) 0° are consistent with longer Pt interatom spacings aligned along φ ) 0°. Since the (111) face is actually 3-fold azimuthally symmetric, due to the stacking of the fcc Pt lattice, there are two such possible ways to align the long azimuths for thick Pt(111) rafts with the ZnO substrate. These can be described as [1h21h]//[11h00] or [112h]//[11h00]. If the Pt islands are only one layer thick, then this difference in the stacking is not defined. For the annealed surfaces, both types of alignment are present in roughly equal amounts as indicated by the presence of both the 53° and the 76° peaks in the resulting incident angle scans (Figure 7). It has been previously reported15 based upon transmission electron diffraction that the Pt overlayer is rotated 30° with respect to the ZnO lattice, i.e., that the short azimuths [11h0] of Pt(111) are aligned with the [11h00] azimuth of the ZnO. A subsequent LEED study found the Pt layers to be aligned with the ZnO, but not rotated 30°.16 The present ion scattering results agree with the subsequent (and present) LEED observations regarding alignment of the Pt layers. This epitaxy is perhaps most evident from comparison between single-crystal Pt(111) and the Pt/ZnO surface shown in Figure 7. The identical locations and widths of the highangle peaks and of the low-energy edge could only occur if the majority of Pt crystallites were both aligned with the substrate and oriented as (111) planes. The qualitative differences can all be explained. The high-angle peaks are more intense for Pt(111) single crystal because of a thickness effect; more than just the second layers contribute to these peaks. The peak near 30° for the [112h] azimuth of Pt(111), although not clearly visible for the Pt/ZnO data shown in Figure 7, was evident for other doses. The sharper low-angle edge observed for Pt(111) may be the result of small particle sizes of the Pt crystallites in the films or to a minority of misaligned islands. The position of the low-angle edge in the polar incident angle dependence (near 15° in Figure 2) can be correlated to the distance between neighboring Pt atoms along the chosen azimuth.6,26 The position of the edge, taken to be the angle at the 90% of the edge height, was measured to be 15.4° ( 0.3°. This value, measured along the ZnO substrate [11h00] azimuth, is based upon several measurements at different Pt coverages. The edge location for the Pt islands on ZnO is the same as for
Structure of Pt Overlayers on ZnO Surfaces Pt(111) which has been previously measured with the same apparatus to be 15.1° ( 0.5° in the long azimuths.24 This agreement is also illustrated in Figure 8 which provides further confirmation of the epitaxy. It is also possible that monolayer thin rafts of Pt could form a coincidence lattice with the ZnO(0001) substrate. A slight expansion of about 0.5% in the Pt lattice spacing would allow a 6:7 coincidence lattice to be formed with every seventh Pt atom coincident with every sixth Zn atom. An expansion of this magnitude would result in a shift in the low-angle edge of less than 0.1°, too small to be detectable. It would also have an undetectably small effect upon the position of the higher angle peaks due to second layer scattering (Figure 7). Therefore, ion scattering cannot eliminate this possibility. If well-ordered islands with this coincidence were present, they should be detectable as a p(6×6) LEED pattern. This coincidence may be responsible for the satellite spots observed in the LEED pattern for the annealed surface. However, it is clear that neither the as-deposited nor the annealed Pt layers adopt the larger lattice constant of the ZnO substrate to form a highly strained commensurate (i.e., 1:1 coincidence) layer. This spacing would lead to an edge at 13.2°, well below the observed edge. The changes in the energy distribution and the incident angle dependences shown in Figures 1 and 2 lead to the conclusion that Pt covers the Zn-terminated surface nearly completely before growth of the second layer of Pt occurs. This conclusion applies for the room temperature deposited films and the evidence is as follows. First, the incident angle dependence provides a direct indication of the point at which second Pt layer starts to grow. The indicators are the features at 53° and 76°. Second, the dose at which these high-angle features begin to appear coincides with the dose at which the Zn single scattering peak is completely attenuated. Complete attenuation of the Zn peak can only occur if all of the surface is covered by at least one monolayer. These two results together indicate that with increasing Pt dose, deposited slowly at room temperature, a nearly complete, single monolayer covers the Zn-terminated ZnO surface before detectable growth of a second layer occurs. On the basis of our ability to determine the point of disappearance of the Zn peak and the point of appearance of the highangle features, it is estimated that 90-100% of the surface is covered before second layer growth occurs. Although submonolayers of Pt deposited at room temperature grow as a single layer across the Zn-terminated surface, annealing causes islands to thicken as indicated by (1) the decrease of the Pt single scattering intensity, (2) the broadening of the Pt single scattering peak to lower energies (as shown in Figure 5) and other changes in background intensity in the energy distribution, and (3) the growth of the high-angle features specific to scattering from second and deeper layers of Pt in Pt(111) planes. The latter two changes make the ion scattering results more closely resemble that of a Pt(111) surface. The decrease in Pt intensity is simply due to a decrease in the area of the surface covered by Pt. Depending on the temperature and time of anneal, which in our case was up to 975 K and 10 min, the intensity of the first layer Pt decreased to 30-60% of its initial value. This would indicate that upon annealing, Pt islands are only a few layers thick, possibly only bilayer. The Pt still covers a substantial portion of the ZnO surface. The broadening of the Pt peak is a thickness effect which is seen on clean Pt(111) due to multiple scattering contributions involving layers below the first, which cannot occur if the Pt island is a single layer thick. The increase in background intensity at energies between the Zn and the Pt single scattering peaks is
J. Phys. Chem. B, Vol. 104, No. 14, 2000 3033 also a direct result of increased thickness of the Pt particle. Accompanying these changes, the Zn peak also sharpens and grows in intensity (Figure 5), since thickening of the Pt islands implies uncovering of ZnO surface. These changes and the decrease in intensity below the Zn peak make the energy distribution more closely resemble a clean ZnO substrate since this part of the spectrum is least affected by scattering from Pt. All the changes in the ion scattering occur approximately coincidently with increasing annealing temperature and indicate that thickening begins to occur above about 675 K. Another effect of annealing is that the azimuthal scans sharpen considerably (Figure 8). These scans are taken at the grazing angles of incidence, i.e., at angles just below the position of the first layer edge, ensuring that only first layer Pt is detected in the scans. Therefore, the effect is not directly related to island thickening. This change implies that an enhanced ordering of the Pt islands occurs on the surface, either by flattening of the rafts or more likely through an increased azimuthal alignment of Pt rafts with the ZnO substrate. Since the Pt initially covers a large fraction of the Pt surface and the ion scattering is a shortrange probe, it is not likely that these changes result from lateral growth of Pt(111) rafts. It is of interest to consider whether there is evidence that Pt atoms diffuse into the ZnO crystal or there is encapsulation of Pt particles by ZnO upon high-temperature annealing. This might be a possible alternative interpretation for the changes in the energy distribution, especially the decrease in the Pt scattering intensity and the low-energy broadening of the Pt single scattering peak. However, this interpretation is not consistent with the resultant azimuthal scans and incident angle dependences. The presence of even a single overlayer of ZnO should completely dominate the angular dependence of the Pt single scattering. Instead, the Pt angular dependence closely follows that of a clean Pt(111) surface. Pt films on both ZnO(0001) and ZnO(0001h) seem to have similar behavior, but some differences are noticeable. For the as-deposited films on the Zn-terminated surface, the Pt appears to form flat islands which are fairly well aligned azimuthally. The differences between the two terminations shown in Figure 4, upper panel, can be explained by poorer azimuthal alignment of the Pt rafts for the O-terminated substrate. Poor alignment could cause both a broadening of the low incident edge and the “rounding” at higher angles. However, the broadening of the Pt peak (Figure 4 lower) is more suggestive that some of the Pt islands are thicker for the O-terminated surface. Increased thickness of islands would imply that the Schwoebel barrier is higher for the Pt rafts on the O-terminated surface.20 Upon annealing, both the O- and Zn-terminated surfaces grow Pt(111) rafts which are at least two layers thick, and most must be only two layers thick. The resulting island thicknesses are about the same for both surfaces as indicated by the comparable relative decreases in the Pt intensity. The relative energetics between ZnO and Pt must be about the same whether the surface is terminated at O anions or coordinatively unsaturated Zn cations. Evidently the Pt does not tend to form thick structures such as spheres or cubooctahedrons. The Pt rafts have good azimuthal alignment with the ZnO which is better for the Znterminated surface than for the O-terminated surface, but not as good as for a Pt(111) crystal as judged by the low-angle edge (Figures 7 and 9). The kinetics of this transformation are not largely different as indicated by the roughly similar temperatures at which the thickening occurs. This result is consistent with previous measurements which found that the
3034 J. Phys. Chem. B, Vol. 104, No. 14, 2000 onset of thickening for the two surfaces differed by no more than about 100 K.16 5. Conclusions Slow deposition of Pt near room temperature on a ZnO(0001) surface leads to films which grow one atom layer thick until essentially the entire ZnO substrate is covered. The hexagonally packed monolayer is not commensurate with the ZnO surface but instead has essentially the same lattice constant as for Pt(111). The films are azimuthally aligned with the ZnO, not rotated 30°. Annealing the films to temperatures of 875 K or above causes progressive thickening, primarily to a Pt bilayer, and a corresponding uncovering of the ZnO surface. Annealing also induces improved azimuthal alignment of the Pt layers with the substrate. The Pt bilayers are oriented as Pt(111) but could be strained 0.5% to form a 6:7 coincidence overlayer. Similar results are obtained for the O-terminated surface, indicating that there is little differences in the energetics and kinetics for growth, although the films are not as well ordered azimuthally. Acknowledgment. Research sponsored by the Division of Chemical Sciences, Office of Science, U.S. Department of Energy, under Contract DE-AC05-96OR22464 with Oak Ridge National Laboratory, managed by Lockheed Martin Energy Research Corp. References and Notes (1) Zhang, R.; Ludvivsson, A.; Campbell, C. T. Surf. Sci. 1993 289, 1. (2) Go¨pel, W. Prog. Surf. Sci 1985, 20, 9. (3) Mu¨ller, K. In The Structure and Chemistry of Solid Surfaces; Somorjai, G. A., Ed.; Wiley: New York, 1969; Vol. 35, pp1-12.
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