Preparation of ultrathin nickel oxide on ordered vanadium oxide films grown on Cu(100) surface studied by LEED and XPS

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surface science ELSEVIER

Surface Science 356 (1996) 171-180

Preparation of ultrathin nickel oxide on ordered vanadium oxide films grown on Cu(100) surface studied by LEED and XPS K o s a k u K i s h i *, Y o s h i n o b u H a y a k a w a , K a t s u y a F u j i w a r a Department of Chemistry, School of Science, Kwansei-Gakuin University Nishinomiya, Hyogo 662, Japan Received 18 May 1995; accepted for publication 26 December 1995

Abstract

Ordered ultrathin nickel oxide on vanadium oxide grown on a Cu(100) surface has been studied by XPS and LEED. One monolayer of vanadium oxide obtained by heating the vanadium atoms deposited on oxygen chemisorbed Cu(100) at 523 K under an 02 atmosphere of 2 × 10 -5 Pa shows a chemical shift of the V 2p3/2 peak which is smaller by 0.6 eV than that of bulk V203. A LEED measurement shows a pattern corresponding to two domains with a VO(111)-like surface structure rotating 30° with respect to one another. At larger coverages an oxide of the VO(111)-(2 x 2) surface structure is formed upon heating at 573 K under the same oxygen atmosphere. Ultrathin, ordered nickel oxide films are prepared on the V O ( l l l ) and the VO(111)-(2 x 2) surfaces on Cu(100). On both vanadium oxides the ~0.8 to 2 ML nickel oxides take a NiO(lll)-like surface structure. The submonolayer nickel oxide gives a single Ni 2p3/2 peak with a large chemical shift, such as 2.8 to 3.4 eV, being quite different in the binding energy and spectral features from the bulk NiO and 3 4 ML NiO on Ni single crystal surfaces. The electronic state of the nickel ions begins to change into the bulk-like state during the growth of the second layer.

Keywords: Low energy electron diffraction (LEED); Nickel oxides; Surface structure; Vanadium oxide; X-ray photoelectron spectroscopy (XPS)

1. Introduction Thin films of transition metal oxides have been prepared on metal substrates in connection with surface science studies on oxide surfaces [1-7]. This paper describes the structure and chemical state of nickel oxides grown on ultrathin ordered vanadium oxides on Cu(100) as a function of the coverage of the nickel oxide. The studies of the interaction between the ultrathin layers of different metal oxides can be used as a model to investigate a surface of mixed oxides. In our previous paper [8] the nickel oxide * Corresponding author. Fax: + 81 798 51 0914.

deposited on thin vanadium oxide (not ordered) on a Cu(100) surface was studied by XPS. A single Ni 2pa/2 peak was observed at 855.9 eV showing a chemical shift of 3.2 eV, being 1.4 eV larger than that for the Ni 2+ in the bulk NiO [9] or the 3 NiO layers formed on Ni single crystal surfaces [ 10]. Recently, ultrathin ordered V203(0001) films have been prepared on A u ( l l l ) [11]. In the present study, ordered nickel oxide films were grown on ordered vanadium oxide films prepared on Cu(100). The structures and the chemical states of the ultrathin vanadium oxide and nickel oxide overlayers were studied by LEED and XPS. The increase in the nickel oxide thickness was associated with a large spectral variation of the

0039-6028/96/$15.00 Copyright © 1996 Elsevier Science B.V, All rights reserved PII S0039-6028 (96) 00015-5

172

K Kishi et al./Surface Science 356 (1996) 171-180

Ni 2p3/2 peak showing the change in the electronic states of the nickel ions.

2. Experimental procedure The experiments were performed in a UHV chamber (base pressure: 6× 10 -s Pa) equipped with a LEED optics (ULVAC) and an XPS spectrometer (Shimadzu). Photoelectron binding energies were referred to the Fermi level and calibrated with respect to the Cu 2p3/2 peak (932.7 eV) of a Cu single crystal. The electrons photoejected by the A1 K~1,2 source were collected at a surface sensitive angle of 60 ° with respect to the surface normal. The polished Cu(100) surface (from Metal Crystals and Oxides Ltd., Cambridge, UK) was cleaned by Ar ÷ sputtering and annealing up to 770 K. Vanadium and nickel atoms were deposited onto the Cu(100) substrate at 320 K from resistively heated tungsten filaments wound with vanadium wire (>99.7% purity) or nickel wire (> 99.8%). The 02 gas (99.99%) was obtained from Takachiho Kagaku Kogyo.

3. Results

V 2pa/2 and O ls peak intensities, Fig. 1. A distinct inflection was observed for each line after ~ 13 min (total) of the deposition. The intensity of the V 2p3/2 peak at this point, Iv, is used below as a unit for the expression of the intensity at the other coverages. The inflection point was followed by a straight line. The intensity at 21 min was not on the line, but was intensified to be on the line after subsequent oxidation at 573 K. The attenuation of the Cu 2Pa/2 intensity compared to that for the clean surface was ~ 43% at the inflection point.

3.1.2. Structure and chemical state The structural and chemical variations of the vanadium oxide overlayers were studied with an increase in the thickness of the vanadium oxide. Vanadium atoms were deposited onto the oxygenpretreated Cu(100) surface and oxidized at 523 K under an 02 atmosphere of 2x 10 -5 Pa for 15 min. The obtained vanadium oxide (giving the intensity of 0.8Iv) showed a chemical shift of ~ 3.0 eV for the V 2pa/2 peak, Fig. 2a. The chemical shift was given as a shift from the binding energy of thick vanadium films, 512.2 eV, although the vanadium atoms at submonolayer coverage showed a larger binding energy than this value. The O ls peak was observed at 529.6 eV, Fig. 3a. The Cu 2p3/2 peak showed no spectral change

3.1. Preparation of ordered vanadium oxide on Cu( l OO) 3.1.1. Film growth and coverage determination The Cu(100) surface was exposed to 0 2 (1 x 10 -4 Pa) for 200 s at 330 K, giving a LEED pattern of the substrate with weak spots corresponding to the c(2 × 2 ) - 0 structure. This was necessary to grow a well-ordered vanadium oxide during the following procedure. Vanadium atoms were then deposited on the surface and oxidized at 523 K for 15 min under an 02 atmosphere of 2x 10 -5 Pa. The deposition and the oxidation were repeated and the XP spectra were recorded to monitor the growth of vanadium oxide. The increase in the V 2p3/2 and O 1s intensities and the decrease in the Cu 2p3/2 intensity were plotted as a function of the deposition time of the vanadium atoms. The plot showed a linear increase in the

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except the diminution of the intensity. Weak satellite peaks of the Cu 2p3/2 around 940 eV decreased in the relative intensity to the main peak. The LEED measurement gave a clear, 12-spots, ring pattern (referred to as pattern I below), Fig. 4a. Below this coverage the LEED pattern was a superposition of pattern I and the pattern of the Cu(100) substrate.

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174

K. Kishi et al./Surface Science 356 (1996) 171-180

After the second deposition of vanadium atoms and subsequent oxidation (573 K, 2× 10 -5 Pa 02 for 15 min), the V 2pa//intensity of the vanadium oxide was 1.2Iv. The V2p3/2 peak was observed at 515.4 eV, Fig. 2b. The LEED pattern I was superimposed with a weak second pattern. The second pattern (pattern II) became sharp and strong for the vanadium oxide (~ 1.4Iv) after the third-step deposition and oxidation (573 K, 2x 10 -5 Pa 02 for 15 min), Fig. 4b. After the fourth deposition and oxidation (~l.6Iv), the V 2p3/2 peak at 515.4 eV was associated with a shoulder around 514.8 eV, Fig. 2c. The O ls peak was at 529.8 eV. Pattern II was very sharp.

3.2. Preparation of ultrathin nickel oxide on ordered vanadium oxide films on Cu( l O0) 3.2.1. Film growth and coverage determination The vanadium oxide (0.9Iv) showing pattern I were prepared on Cu(100) as described above. The deposition of nickel atoms onto the oxide surface was repeated. After each successive deposition the surface was exposed to 2 × 10 -5 Pa 02 at 300 K for 200 s to oxidize the nickel atoms. The Cu 2p3/2 and V 2p3/2 intensities after each oxidation were plotted (roughly by peak height) as a function of the deposition times of the nickel,

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Fig. 5. The plot showed a linear decrease in both intensities. A distinct inflection point was shown at about 20 min (total) deposition for both plots. The diminution of the intensity due to the nickel oxide at the point was about 46% for the Cu 2p3/2 peak, being close to 43% at the inflection point for the vanadium oxide film as mentioned above, and 33% for the V 2p3/2 peak. The attenuation, 46%, of the substrate Cu 2p3/2 peak is represented by As and used below as a unit of attenuation at the other coverages. The inflection points were followed by straight lines. The plot of the Ni 2Pa/2 intensity was not adopted because of difficulty in the exact subtraction of the background and large variations in spectral features during an increase in the thickness as described below.

3.2.2. Structure and chemical state of nickel oxide overlayers The vanadium oxide (0.9Iv) showing a sharp pattern I was prepared on the Cu(100) surface as described above. The ultrathin nickel oxides were prepared on the vanadium oxide by a repeat of the deposition of nickel atoms and exposure to 02 at 2 x 10 -5 Pa at 300 K for 200 s. The variations of the V 2p3/2, Ni 2p3/2 and O ls spectra are shown by Fig. 6, Fig. 7 and Fig. 8, respectively. The nickel oxide leading to the attenuation of 0.3 As was first prepared. A single Ni 2pa/2 peak was observed at 856.1 eV, Fig. 7b. The width of the peak was narrow and the spectral features were quite different from that observed for NiO( 100)/Ni(110) obtained by heating the Ni(ll0) surface at 573 K for 6 min under 1 x 10 -4 Pa 0 2 as shown by a spectrum (solid line) in Fig. 7. The binding energy of the V 2pa/2 peak decreased by 0.4 eV, Fig. 6b. The O ls peak at 529.6 eV shifted to 530.0 eV and a weak shoulder peak appeared around 531.4 eV, Fig. 8b. The LEED pattern I showed a slight increase in the background intensity but the spots were clear. The deposition of nickel atoms and the oxidation were repeated. The spots of the LEED pattern I were decreased in intensity at the second step (0.6 As) and were not observable at a higher electron energy such as 105 eV. The spot intensities commenced to increase again at the third step (0.8 As).

K. Kishi et al. /Surface Science 356 (1996) 171-180

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Up to the fourth step (1.0 As) the shift of the Ni 2p3/2 peak decreased gradually with an increase in the coverage from 3.4 eV (0.3 As) to 2.8 eV (1.0 As), Figs. 7b to 7e. The peak width was broadened at the same time. This was followed by the shift of the V 2p3/2 peak back to higher binding energy, 515.7 eV, Fig. 6e. After the fifth step (1.3 As) the Ni 2p3/2 peak at 855.4 eV was associated with a very weak shoulder peak around 854.5 eV, Fig. 7f. At the sixth step (1.5 As) the LEED pattern I was sharp and observable even at 105 eV. A shoulder peak around 854.6eV was intensified in the Ni 2p3/2 spectrum, Fig. 7g, and the shift of the V 2p3/2 peak was reversed to lower binding energy, Fig. 6g. The 854.6 eV peak was intensified further after the eighth step (1.9 As), Fig. 71, and the LEED spots again became weak. At the ninth step (2.0 As) the LEED spots almost disappeared. The peak around 520.2 eV, which grew after the fifth step in the V 2p spectral region, is ascribable to the O Is peak excited by A1 K~3, 4 radiation.

The nickel oxide was prepared also on the vanadium oxide (1.4Iv) giving the LEED pattern II. The nickel oxide showed the similar spectral changes in the Ni 2p3/2 peak as on the vanadium oxide (0.9Iv) giving pattern I as mentioned above. After deposition of nickel oxide (0.4 As) a sharp Ni2p3/2 peak was obtained at 855.9eV. The LEED pattern II became diffused and only the very weak spots like the pattern I were observed at 83 eV. At higher energies the spots were not observed. For the higher coverage of the nickel oxide (1.5 As) the Ni 2pa/z peak at 854.6 eV (a chemical shift of 1.9 eV) appeared besides the peak at 855.9 eV. At the coverage giving 1.5 As the 854.6 eV peak became very clear. The LEED pattern like pattern I became sharp. The pattern began to be weak again at 1.8 A s . At 2.0 As the spectral features of the Ni 2p3/2 peak were similar with those for the nickel oxide grown on nickel single crystal surfaces.

176

K. Kishi et aL/Surface Science 356 (1996) 171-180

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for a monolayer of vanadium oxide consisting of a layer of vanadium cations and a layer of oxygen anions as discussed below at this grazing take-off angle. The coverage of the vanadium oxide at the inflection point was defined as 1 ML. The coverages at the submonolayer region were estimated by comparing the V 2p3/2 intensity to that at the inflection point. The discontinuity of the Cu 2p3/2 peak intensity near the inflection point may be due to a change in structure between the first and the second overlayers, as discussed below, but details are not yet understood. The gradient of the lines for the V 2p3/2 and O ls peaks after the inflection point was much smaller than expected by a simple accumulation of the second layer with the same structure of vanadium oxide as for the first. This may be due to a double- or multi-layer island growth of the oxide after the completion of the first 1 ML. The coverage above 1 ML in the text only means that the V 2p3/2 intensity corresponds to the one expected from that coverage of the VO( 111 ) when assuming a layer-by-layer growth and 40% attenuation of the intensity by the upper layer.

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4. Discussion

4.1. Vanadium oxide on Cu(lO0) 4.1.1. Coverage determination The straight lines for the V 2p3/2 and O ls peaks before the inflection point in Fig. 1 suggest a twodimensional growth of the vanadium oxide up to the completion of the first monolayer. This is different from the result for the formation of V2Oa(0001) on A u ( l l l ) showing no distinct inflection [11]. The attenuation value, ,~43%, of the Cu 2pa/2 peak at the inflection point seems to be large for a monolayer of atoms but reasonable

4.1.2. Structure and oxidation state of vanadium oxide The larger V 2p3/2 binding energy of vanadium atoms at a submonolayer coverage than that of the thick vanadium films is probably due to a difference in coordination number for the atoms between at the surface and in the bulk as discussed for Ni on Au [12] and partly to an oxidation to some extent. The chemical shift of the ~0.8 ML (estimated from ",~0.8Iv) vanadium oxide is ,-,06 eV smaller than that for a bulk V203 [13], implying that the valency of the vanadium is + 3 or less for the surface vanadium oxide When a vanadium oxide was prepared by oxidation at a lower temperature, the chemical shift was identical with that for bulk V 2 0 3 During the preparation of the vanadium oxide, the copper substrate was not oxidized to Cu20 or CuO as proved by the decrease in intensity of the satellite peaks around 940 eV The formation of such oxide leads to the increase in the intensities of the satellite [ 14] The LEED pattern I with a 12-spots ring for

K. Kishi et al./Surface Science 356 (1996) 171-180

the ~0.8 ML vanadium oxide on the Cu(100) substrate with four-fold symmetry was resolved into two hexagonal mesh units from two domains as illustrated by the open and filled circles in Fig. 4c rotating by 30° (or 90°) with respect to one another, since only one of the hexagonal mesh units was obtained on Cu(ll0) and Ni(ll0) [15] surfaces with two-fold symmetry. From the LEED pattern the surface oxide structure in a real space can be expressed in matrix terminology by

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on Cu(100). The unit cell size of 2.95 ~, is evaluated for the surface vanadium oxide. The size is much shorter than the size, 4.99 A, of the (0001) surface of V2Oa (corundum structure) on Au(111) showing also a hexagonal mesh pattern [ 11 ]. The bulk VO has a structure of NaC1 type (lattice constant: 4.06 ~, [16]) and the (111) face of the oxide has a three-fold symmetry giving a size of 2.87 A for the surface unit. The size for the surface oxide structure is 3% larger than the value for the V O ( l l l ) . The 12-spots ring pattern has been observed also for ZnOx overlayers on Ag(100) [17]. A two-dimensional island growth of the oxide is concluded at the submonolayer region since pattern I was superimposed with the Cu(100) substrate pattern at the oxide coverage below 0.8 ML. Both XPS and LEED results indicate the formation of 0.8 ML vanadium oxide whose atomic arrangement resembles that of the VO(111) surface on the Cu(100) substrate, as schematically represented by Fig. 9a. This oxide is referred to below as the VO(111)-like surface. The 1 ML of the oxide consists of one monolayer of vanadium cations and one monolayer of oxygen anions. The intensity ratio, V 2p3/2/O ls, at the electron collecting angle of 70 ° was increased by more than 10% at 20 °, suggesting that the vanadium atoms were in direct contact with the Cu substrate and covered by the oxygen layer as discussed for FeO on Pt(111) and Pt(100) by ISS data [5]. An overlayer consisting of an FeO( 111 ), bilayer of Fe2÷ and 0 2- ions, shows LEED patterns corresponding to structures with (10x 10) and c(2x 10) coincidence periodicity on P t ( l l l ) and Pt(100) sub strates, respectively [ 5 ]. The vanadium oxide obtained on the Cu(100) did not show such

~) Fig. 9. Schematicrepresentationof the oxide overlayerscorresponding to (a) VO(lll) and (b) VO(111)-(2×2) surface structures.

large parameters despite the monolayer thickness of the oxide. This may be due to a rearrangement of Cu atoms under the vanadium oxide with threefold symmetry induced by the interaction with the stable vanadium oxide overlayer with the same symmetry. The rearrangement of Cu atoms seems to result in two domains for the vanadium oxide on Cu(100) with four-fold symmetry but in one domain on Cu(110) with two-fold symmetry. The clear LEED pattern II for the ~2.0 ML (estimated by ~ 1.6Iv) vanadium oxide was separated into two units of meshes as illustrated by open and filled circles in Fig. 4d, indicating the oxide formation as domains rotating by 30° with respect to one another. As with the VO(1 l l)-like surface discussed above, only one of the two units was observed on the Cu(110) and Ni(110) surfaces [ 15]. The LEED patterns lead to the surface oxide structure in real space corresponding to the (2 × 2)

178

K~ Kishi et al./Surface Science 356 (1996) 171-180

structure of the VO(111)-like surface, meaning a good matching between the lattices. This oxide begins to grow even at ~ 1.2 ML as revealed by the appearance of the weak pattern II superimposed with the stronger pattern I. The lattice parameter of 5.92 A is estimated. The lattice parameter is too large to be assigned to the V203(0001) surface structure. The chemical shift of the main V 2p3/2 peak for the oxide is smaller by 0.4 eV than the shift for the bulk V203. This again suggests the valency of + 3 or less in the vanadium of the surface oxide. Although a determination of a detailed surface structure is difficult without a calculation based on intensity-voltage curves, a result for iron oxide on P t ( l l l ) is suggestive in the discussion of the surface structure of the VO( 111)-(2 x 2) vanadium oxide. An F e O ( l l l ) surface has been proposed for a monolayer coverage [5] on P t ( l l l ) . For multilayer coverages a LEED pattern corresponding to the ( 2 x 2 ) structure of the F e O ( l l l ) surface is found [18]. The oxide has been identified by a dynamical low energy electron diffraction analysis to be FeaO4. The surface structure corresponds to a strongly relaxed bulk (111) termination in which the layer containing two tetrahedral and one octahedral iron atoms is cut so that one single tetrahedral iron atom per (2 x 2) unit cell is left at the surface. The present LEED results are very similar to those for the iron oxide on P t ( l l l ) . The V O ( l l l ) - ( 2 x 2 ) structure on Cu(100) may have the similar termination as the multilayer Fe304(111) on P t ( l l l ) as represented schematically in Fig. 9b. The surface oxide after completion of 1 ML may grow as islands with double- or multi-layers as suggested above in the variation of the V 2p3/2 intensity as the function of the deposition time of the vanadium. When comparing the surface structures, V O ( l l l ) , V O ( l l l ) - ( 2 x 2 ) and V203(0001), the oxygen layers have the similar structure. 4.2. Ultrathin nickel oxide on ordered vanadium oxide 4.2.1. Coverage determination The linear decrease in the intensities of the Cu 2p3/2 and V 2p3/2 peaks as the coverage of nickel oxide increases, suggests a two-dimensional

growth of the nickel oxide on the 0.9 ML VO(111)like oxide. The difference in the attenuation values for the two peaks at the inflection point is reasonably explained by assuming a two-dimensional growth of the nickel oxide on consideration of a proportionality of the electron mean free path to a square root of the kinetic energy (Cu 2p3/2 ~554 eV, and V 2p3/2 ~971 eV) at high energy [13]. The mixing of the nickel ions into the vanadium oxide is then excluded since the mixing will lead to a much smaller attenuation for the V 2p3/2 peak. The coverage of the nickel oxide at the inflection point was defined as 1 ML of the nickel oxide. The diffusion of the nickel ions into the VO(111)-(2×2) structure of the vanadium oxide was shown after heating around 573 K as proved by the decrease in the Ni 2p3/2 intensity and the increase in the V 2p3/2 intensity. When the same structure of the nickel oxide is formed for the second monolayer, the attenuation values of 71% and 58% are expected for the Cu 2P3/2 and V 2pal2 peaks at 2.0 ML of the nickel oxide, respectively. The observed values, 75% and 58%, are very close to the expected ones. The nickel oxide thickness was then evaluated from the diminution of the Cu 2p3/2 intensity on considering the layer-by-layer growth and, at coverages larger than 1 ML, the attenuation of 46% by upper 1.0 ML nickel oxide. 4.2.2. Structure and chemical state of the ultrathin nickel oxide In the Ni 2p3/2 spectra for the bulk NiO [9] and 3 ML NiO overlayers prepared on Ni single crystal surfaces by exposure to 02 at 295 or 485 K [ 10], a main peak at ~ 854.5 eV (chemical shift 1.8 eV) are always associated with the peak at ,-~856.4 eV as illustrated in Fig. 7. The latter peak has been assigned to Ni s+ [19,10] probably at the Ni203 defect centers [20]. From the variation of the relative intensities of the two peaks of the 3 ML NiO by changing the electron collecting angle, the Ni a+ has been concluded to be concentrated on top of the oxide surface [19]. The other interpretation has been reported on the assignments of the peaks. The components of the Ni 2pa/2 peak (including the peaks at ~ 854.6 and ~856.4 eV) have been assigned to different

K. Kishi et al./Surface Science 356 (1996) 171-180

final states associated with ligand-to-Ni2+ charge transfer in the photoelectron process [9,21,22]. The chemical shift value, 3.4 eV, of the Ni 2p3/2 peak for the 0.3 ML nickel oxide on the 0.9 ML VO( 111)-like surface is larger by 1.6 eV than that of the main Ni 2p3/2 peak for the Ni 2+ in the bulk NiO. The shift of the peak due to a charging of the sample is ruled out because of very thin layers of the oxide. Moreover, the 856.1eV peak was narrow and not associated with another peak, revealing that the electronic state of the Ni ions was significantly different from those in the bulk NiO or NiO layers formed on the Ni single crystal surfaces. The spectral change of the Ni 2p3/2 peak may be explained by an incorporation of the nickel ions into a structure of the vanadium oxide since the oxide such as NiVO3 is known. However, the NiVO 3 is formed only at a very high temperature and pressure [23]. As mentioned above the comparison of the attenuation values for Cu 2p3/2 and V 2p3/2 peaks ruled out the possibility of the mixing of the vanadium and nickel ions. The nickel ions interacted with the vanadium ions only at the interface of the two oxide layers. The recovery of the sharpness of the LEED pattern I at 1.0 ML nickel oxide is explained by the formation of a highly ordered two-dimensional NiO layer whose structure grew epitaxially on the VO(111) surface since the two-dimensional growth was concluded in the above discussion. The LEED pattern gives the lattice constant, 2.95 ~_, for the nickel oxide overlayer that is substantially the same as the one for the N i O ( l l l ) with the NaC1 structure (a = 4.168 A [ 24]). The NiO(111) surface has the same structure as the VO(lll)-like oxide as shown by Fig. 9a. In connection with the assignments of the Ni 2p3/2 peaks of NiO, an O ls peak around 531 eV has been discussed and assigned to O - , O H - , etc. [25], although the 531 eV peak was not observed for the NiO(100)/Ni(ll0) in the present experiment as shown by a spectrum (solid line) in Fig. 8. The appearance of the satellite Ni 2p3/2 peak around 856.4 eV has been correlated with the O is peak around 531 eV. In the present case of the 1.0 ML N i O ( l l l ) on 0.9 ML VO(II1), the NiO is mainly responsible for the O ls peak at 530.0 eV since the O ls peak intensity from the 0.9 ML V O ( l l l ) is expected to be 35% smaller than that

179

from the top NiO layer. The O ls peak at ,~ 531.4 eV was observed just as a shoulder peak of the 530.0 eV peak despite the fact that a single Ni 2p3/2 peak was observed at 855.5 eV with the large chemical shift of 2.8 eV. These facts mean that the large chemical shift of the Ni 2p3/2 peak for the 1.0 ML N i O ( l l l ) on the V O ( l l l ) is not correlated with the formation of O - , O H - or carbonate giving the O ls peak around 531eV

[25]. The very sharp LEED pattern I at 1.8 ML NiO implies the formation of the second, well-ordered N i O ( l l l ) layer. The intensified Ni 2p3/2 peak at 854.6 eV means that the electronic states of nickel ions commenced to change into the one like a bulk NiO as the three-dimensional NiO lattice was formed, namely the first NiO lattice was covered with the second, two-dimensional NiO lattice. This resulted in a change in the interaction at the interface of the VO(111) surface and the NiO(111) surface, probably weakening the interaction, as suggested by the reverse shift of the V 2p3/2 peak. The order of the N i O ( l l l ) layers was lost at ~4.0 ML NiO as indicated by the disappearance of the LEED pattern I. The NiO(100) surface is thermodynamically more stable than the NiO(111) at 325 K or a higher temperature when thin NiO was prepared on Ni(100) and Ni(ll0) surfaces [25-27]. The disappearance of the LEED pattern I is probably due to an incomplete transformation of the surface structure into the NiO(100) surface although it may be solely due to the imperfection of the NiO lattice at the higher coverage. The appearance of the sharp LEED pattern I at 1.8 ML NiO on the vanadium oxide surface with V O ( l l l ) - ( 2 x 2 ) structure is interpreted by the formation of NiO(111) layers. This is quite reasonably concluded since the oxygen layers of the NiO(111) surface are the same in size and symmetry as the one for the VO(lll)-(2 x 2) surface as well as the one for the VO(lll)-like surface. The spectral changes of the Ni 2p3/2 with increase in the coverage is similar to the results for the NiO on the VO( 111)-like surface. From the above results the origin of the single Ni2p3/z peak around 856eV for 0.3-1.0 ML NiO(111) on the vanadium oxides should be considered to be different from the 856.4 eV peak for bulk NiO. The NiO(ll 1) surface structure may be

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K. Kishi et al./Surface Science 356 (1996) 171-180

responsible for the appearance of the peak. However, the spectral features of the Ni 2p3/2 peak changed to resemble to that of the bulk NiO during the increase in the coverage of the nickel oxide from 1 to 2 ML although the LEED pattern showing the N i O ( l l l ) structure was sharp for 2 ML NiO. Then the strong interaction through the sharing of oxygen anions with nickel ions and vanadium ions is one of the reasons why the chemical shift is as large as 2.9-3.4 eV. The change in the shift of the Ni 2p3/2 from 0.3 to 1.0 ML seems to be correlated with the increase in the size of the two-dimensional NiO islands. The similar large chemical shift to 856.2 eV has been reported on nickel catalysts supported on v-alumina calcined at 873 K but only for low Ni contents such as 7% [28]. The nickel ions were resistive to a reduction because of a strong interaction with the alumina.

5. Summary

Nickel atoms deposited on a 0.9 ML V O ( l l l ) surface prepared on Cu(100) and oxidized at 300 K at 2 x 10 -5 Pa 02 is found to grow epitaxially up to 2-3 ML by plots of Cu 2p3/2 and V 2p3/2 intensities as a function of deposition times of nickel and LEED measurement. The LEED pattern corresponding to the NiO(111) structure is clearly observed at the nickel oxide coverage ranging from 0.8 to 1.8 ML. At >3.0 ML the pattern diffuses. A single Ni 2p3/2 peak is observed at 856.1-855.5 eV showing a large chemical shift of 3.4-2.8 eV at the coverage between 0.3 to 1.0 ML. This is quite different from the spectral feature of the bulk NiO and 3-4 ML NiO formed on Ni single crystal surfaces, a main peak at 854.6 eV (chemical shift of 1.9 eV) associated with a peak around 856.4eV. The strong interaction of the three-fold symmetric structure of NiO with the vanadium oxide is responsible for the appearance of the single Ni 2pa/2 peak. The bulk NiO-like electronic state begins to grow during the formation of the three-dimensional NiO lattice at a coverage larger than 1.0 ML. The similar results were obtained for the nickel oxide on the 1.6 ML VO(lll)-(2 x 2) surface.

References [ 1] G. Heiland and H. L~lth,in: The Chemical Physics of Solid Surfaces and Heterogeneous Ctalysis, Vol. 3, Eds. D.A. King and D.P. Woodruff(Elsevier, New York, 1982) ch. 4. [2] K.E. Smith and V.E. Henrich, Surf. Sci. 225 (1990) 47. [3] C. Xu, M. Hassel, H. Kuhlenbeck and H.-J. Freund, Surf. SCI. 258 (1991) 23. [4] H.C. Galloway, J.J. Benitez and M. Salmeron, Surf. Sci. 298 (1993) 127. [5] G.H. Vurens, V. Maurice, M. Salmeron and G.A. Somorjai, Surf. Sci. 268 (1992) 170. [6] A.B. Boffa, H.C. Galloway, P.W. Jacobs, J.J. BenItez, J.D. Batteas, M. Salmeron, A.T. Bell and G.A. Somorjai, Surf. Sci. 326 (1995) 80. [7] E. Yagasaki and K. Kishi, J. Electron Spectrosc. Relat. Phenom. 69 (1994) 133. [8] K. Kishi and K. Fujiwara, J. Electron Spectrosc. Relat. Phenom. 71 (1995) 51. [9] G. Tyuliev, P. Stefanov and M. Atanasov, J. Electron Spectrosc. Relat. Phenom. 63 (1993) 267. [10] P.R. Norton, R.L. Tapping and J.W. Goodale, Surf. Sci. 65 (1977) 13. [11] K.B. Lewis, S.T. Oyama and G.A. Somorjai, Surf. Sci. 233 (1990) 75. [12] P. Steiner and S. Hiifner, Solid State Commun. 37 (1981) 279. [ 13] D. Briggs and M.P. Seah, Eds., Practical Surface Analysis, Vol. 1, 2nd ed. (Wiley, Chichester, 1990). [ 14] S. Htifner, Photoelectron Spectroscopy, Vol. 82 of Springer Series in Solid-State Science (Springer, Berlin, 1995) p. 226. [15] K. Kishi and K. Fujiwara, in preparation. [16] R.W.G. Wyckoff, in: Crystal Structures, 2nd ed. (Interscience, New York) Vol. 1, p. 91; Vol. 2, p. 8. [17] H.N. Kourouklis and R.M. Nix, Surf. Sci. 318 (1994) 104. [18] W. Weiss and G.A. Somorjai, J. Vac. Sci. Technol. A 11 (1993) 2138; W. Weiss, A. Barbieri, M.A. Van Hove and G.A. Somorjai, Phys. Rev. Lett. 71 (1993) 1848. [19] A.F. Carley, P.R. Chalker and M.W, Roberts, Proc. R. Soc. London A 399 (1985) 167. [20] S.J. Bushby, T.D. Pope, B.W. Callen, K. Griffiths and P.R. Norton, Surf. Sci. 256 (1991) 301. [21] M. Oku, H. Tokuda and K. Hirokawa, J. Electron Spectrosc. Relat. Phenom. 53 (1991) 201. [22] J. van Elp, H. Eskes, P. Kuiper and G.A. Sawatzky, Phys. Rev. B 45 (1992) 1612. [23] B.L. Chamberland, J. Solid State Chem. 2 (1970) 521. [24] H.P. Rooksby, Nature 152 (1943) 304. [25] C.R. Brundle and J.Q. Broughton, in: The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Vol. 3A, Eds. D.A. King and D.P. Woodruff (Elsevier, Amsterdam, 1990). [26] W.D. Wang, N.J. Wu and P.A. Thiel, J. Chem. Phys. 92 (1990) 2025. [27] R.S. Saiki, A.P. Kaduwela, J. Osterwalder, C.S. Fadley and C.R. Brundle, Phys. Rev. B 40 (1989) 1586. [28] M. Wu and D.M. Hercules, J. Phys. Chem. 83 (1979) 2003.