Ultrathin nickel oxide on the V2O3Cu(100) surface studied by XPS

JOURNAL OF ELECTRON SPECTROSCOPY and Related Phenomena

ELSEVIER

Journal of Electron Spectroscopyand Related Phenomena 71 (1995) 51-59

Ultrathin nickel oxide on the

V203/Cu(100) surface

studied by XPS

Kosaku Kishi*, Katsuya Fujiwara Department of Chemistry, School of Science, Kwansei-Gakuin University, Nishinomiya, Hyogo 662, Japan First received 16 May 1994; in final form 20 June 1994

Abstract

The interaction of nickel ions with thin vanadium oxide on Cu(100) was studied by X-ray photoelectron spectroscopy. After 02 oxidation of submonolayer nickel atoms on the vanadium oxide, a single Ni2p3/2 peak was observed at 855.9 eV; the oxide was quite different in binding energy and spectral features from the surface oxide grown on single-crystal nickel substrates. This indicates that the electronic states of the nickel ions are considerably changed by interaction with the oxygen anions of the vanadium oxide. The interaction is associated with a shift of the V2p3/2 peak to higher binding energy. During growth of the nickel oxide by repeated deposition (0.5 monolyaer of Ni) and oxidation, the initial nickel ions are changed into the NiO structure, followed by a weakening of the interaction with the vanadium oxide. The interaction diminishes with adsorption of acetate on the surface. Keywords: Nickel oxide, Ultrathin film, V203/Cu(100),XPS

1. Introduction

Adsorption and reaction on thin films of transition metal oxide grown on other metal substrates have been the subjects of much interest for surface science studies on oxide surfaces [1-3]. The oxide overlayers are useful for studies by the standard electron spectroscopic techniques. We report here our studies of the interaction of nickel ions with thin vanadium oxide on Cu(100) surface in connection with the preparation of more complex oxide overlayers. The formation of oxide on single-crystal nickel surfaces by exposure to 02 in the region of 300 L has been intensively studied [4-11]. The saturation coverage has been reported to be three

* Corresponding author.

[5,6] or four to five [7] NiO layers. In X-ray photoelectron spectroscopic (XPS) studies, a Ni2p3/2 peak at ~ 856.2 eV, observed with a main peak at ~ 854.5 eV, was assigned to Ni 3+ [5,8,9]. The presence of Ni20 3 defect centres was correlated with the adsorption of 02 on the oxide surface [12]. The other assignment has been suggested for the Ni2p3/2 peak at 856 eV for bulk NiO [13-15]; the peak appears from different final states corresponding to ligand-to-Ni 2+ charge transfer and not from Ni 3+. In the present study, spectral variations of the Ni2p3/2 peak were measured for ultrathin nickel oxide as a function of the oxide thickness prepared by repeated deposition of nickel atoms and oxidation on thin V203 layers on a Cu(100) surface in the course of obtaining information about changes in the chemical state of nickel ions depending on the oxide substrate. The results show a change in the

0368-2048/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0368-2048(94)02243-7

52

K. Kishi, K. Fujiwara/Journal of Electron Spectroscopy and Related Phenomena 71 (1995) 51 59

chemical state of nickel ions with increases in coverage.

3. Results and discussion

3.1. Oxidation of Ni atoms on preoxidized V/Cu( IO0) surface 2. Experimental The experiments were performed in a UHV chamber (base pressure 6 x 10 8 Pa) equipped with an XPS spectrometer (A1 Ks source). Photoelectron binding energies were referred to the Fermi level and calibrated with respect to the Cu2p3/2 peak (932.7 eV) of a copper single crystal. The photoejected electrons were collected at a surface sensitive angle of 60 ° with respect to the surface normal. The polished Cu(100) surface, obtained 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 sample surfaces at 320 K from resistively heated tungsten filaments wound with vanadium wire (99.7% purity) or nickel wire (> 99.8%). Acetic acid from Wako Pure Chemicals Ltd. was degassed by a series of freeze-pump-thaw cycles and admitted to the sample surface through a capillary tube in close proximity to the surface. All doses refer to ion gauge readings. The amounts of deposited nickel atoms were estimated by preliminary experiments to obtain a plot of the Ni2p3/2 peak intensity versus the total deposition time for stepwise deposition of nickel atoms on an oxygen chemisorbed Cu(100) surface, assuming layer-by-layer growth [16]. The Ni2p3/2 intensity at the first inflection on the plot was taken as that for one monolayer (ML) of nickel atoms. Estimation of the amounts of vanadium atoms was carried out by comparison of the V2p3/2 peak intensity with the O 1s intensity from chemisorbed oxygen, c(2 × 2)-0, on Cu(100) and the photoionization cross-section ratio of 2.0 for V2p3/2 to Ols [17]. Some oxygen contamination was found after the evaporation, and the growth mode of the vanadium atoms was not layer-by-layer. The amounts of vanadium atoms quoted are thus crudely estimated values.

The Cu(100) surface covered by 1.5 monlyers of V was exposed to 760 L of 02. The V2p3/2 peak at 512.4 eV for 1.5 ML of V (Fig. l(a)) was shifted to 515.7 eV (Fig. l(b)), a chemical shift of 3.5 eV. (The chemical shift was evaluated by difference from that of thick vanadium films, as the peak of a submonolayer or a few layers of vanadium atoms on the copper gives a binding energy larger than that for a thick layer. The shift in the Ni2p3/2 peak was similarly evaluated.) In our previous paper [18] the V2p3/2 peak with 3.5 eV shift was assigned to a V203-1ike surface oxide because the reported chemical shift values for bulk V203 and V205 are 3.6 eV and 4.6 eV [17]. The shift for V204 is 0.2 eV larger than that for V203 [19]. Formation of an oxide film with the composition of V203 on a A u ( l l l ) surface has been reported [2]. An Ols peak was observed at 530.0 eV (Fig. 2(b)). Nickel atoms (0.5 ML equivalent) were deposited on the vanadium oxide overlayers. The V2p3/2 peak maximum moved to a lower binding energy, 515.3 eV, with decrease in intensity (Fig. l(c)). The shift is attributable to the bonding of the nickel atoms to the uppermost oxygen anions of the vanadium oxide at the interface, leading to a decrease in the positive charge of the vanadium atoms. The same value of the chemical shift was observed when the surface vanadium oxide was vanadium-terminated. After 1.3 ML of a V/Ni(II0) surface was oxidized at 873 K under 2.5 × 10-6 Pa of 02, the low energy electron diffraction (LEED) pattern of the thin vanadium oxide exhibited a hexagonal mesh pattern corresponding to the (0001) face of V203 (corundum structure) and gave a chemical shift of 3.0 eV for the V2p3/2 peak [20]. The smaller chemical shift compared with that of the bulk V203 is explained by the formation of a vanadium-terminated surface rather than an oxygen-terminated oae. A Ni2p3/2 peak was observed at 852.8 eV (Fig. 2(c)). A small peak around 856.0 eV originated from oxidized nickel atoms. The attenuation of the V2p3/2 peak

K. Kishi, K. Fujiwara/Journal of Electron Spectroscopy and Related Phenomena 71 (1995) 51-59

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was 20%. The smaller attenuation (13%) for the Ols peak is due to the oxidation of nickel in a small portion of the topmost layer. The Ols peak showed a shift of 0.2 eV. The surface was subsequently exposed to 300 L of O2. The 0.5 ML of Ni should produce ~ 0.7 ML NiO layers if a completely two dimensional oxide is formed. The V2p3/2 peak maximum showed a shift to 516.1 eV (a chemical shift of 3.9 eV (Fig. l(d)). When the amounts of vanadium atoms and nickel atoms were 0.7 and 1.5 ML, respectively, the peak was found at 516.2 eV (Fig. l(e)). (The peak around 520.2 eV is ascribable to an Ols peak excited by AI Kc~3,4 radiation.) The shift in the peak is not due to charging of the sample because the oxide layers are very thin. The Ni2p3/2 peak at 852.8 eV changed to a single peak at 855.9 eV (a chemical shift of 3.2 eV). The chemical shift is close to that of the peak assigned to Ni 3+ in surface nickel oxide grown on single-crystal nickel surfaces such as (100), (110), (111) and (210) by exposure to O2 in the region of 300 L [5,8,9]. The peak intensity at 854.5 eV was larger than that at 855.8 eV for the surface oxides formed by exposure to 02 at 295 and 485 K [5]. The ions Ni 3+ was

found to be more highly concentrated at the solid-vacuum interface than Ni 2+. However, the 855.9 eV peak in Fig. 2(d) was not associated with the 854.5 eV peak, revealing that the nickel ions were in a state quite different from that of the oxide formed on the single-crystal nickel surfaces, probably by sharing oxygen anions with the vanadium oxide and possessing a different oxygen structure from NiO. The Ols intensity at 530.0 eV was increased to the value before nickel deposition, but was very weak at ~ 531 eV. The appearance of the 856 eV peak in the surface oxides grown on single-crystal nickel surfaces is associated with the Ols peak at ~ 531.5 eV [4]. The 855.8 eV peak currently observed is not associated with such a peak, suggesting that the nickel ions are surrounded mostly by O 2- at the solid-vacuum interface. In the interaction of the surface V203 with Na oxidized by O2, the V2p3/2 peak showed a chemical shift of 5.5 eV, indicating oxidation to V 5+, during the mixing of vanadium oxide and oxidized sodium [18]. The Nals intensity after deposition of Na atoms was decreased by exposure to O2 (more markedly so after subsequent heating at 530 K) and the V2p3/2 intensity

K. Kishi, K. Fujiwara/Journal of Electron Spectroscopy and Related Phenomena 71 (1995) 51-59

54

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was increased, giving proof of the mixing of vanadium oxide and sodium oxide. An incorporation of the nickel ions into the lattice of the vanadium oxide may be considered as an explanation for the shift of the V2p3/2 peak to higher binding energy, since oxides such as NiVO3 are known. However, NiVO3 is formed at very high temperature and pressure [22] when prepared from nickel oxide and vanadium oxide. The V2p3/2 peak intensity showed a decrease in the present case. The mixing of the oxides is thus excluded. The interaction of the nickel ions and vanadium oxide is then concluded to be caused only at the interface. The submonolayer nickel oxide (fully dispersed or as a cluster) is expected to share the oxygen anions with the vanadium oxide. This is probably why the nickel ions give a single Ni2p3/2 peak at 855.9 eV. In the studies of nickel catalysts supported on ,7-alumina calcined at 873 K, the Ni2p3/2 binding energy varied with the nickel content [22]. The peak was predominant at 856.2 eV for a 7% Ni catalyst and at 854.6 eV (with a shoulder around 856 eV) for catalysts of higher Ni content. At very low contents the nickel ions was not reduced in flowing hydrogen at 673 K because of strong interaction with "y-alumina and it was suggested that the nickel ions are located in tetrahedral sites rather than in octahedral sites. At

higher contents the percentage reduction became greater for a constant reduction time. A binding energy change of the Ni2p3/2 electrons was not observed with increase in the nickel content on silica supports [22]. These results confirm that the higher binding energy for nickel ions on vanadium oxide is due to strong bonding of the nickel ions with the oxygen anions of the vanadium oxide. The formation of nickel ions giving the Ni2p3/2 peak around 856 eV is not limited to the top of the surface. When a 2.7 ML V/Ni(ll0) surface was oxidized at 773 K for 10 min under 2.5 × 10-6 Pa of 02, nickel oxide layers were formed between the nickel substrate and the surface vanadium oxide [20]. The thin nickel oxide gave a Ni2P3/2 peak at 855.7 eV. No peak was observed near 854.5 eV. A strong interaction of the nickel ions with the oxygen anions of vanadium oxide and a consequent structural requirement is thought to produce a structure different from that of NiO. The origin of the Ni2p3/2 peak at 856 eV for the bulk NiO has been discussed in connection with the temperature and Li-doping effects in the Ni2p3/2 spectra [13-15]. The components of the Ni2p3/2 peak (including the peaks at ~854.6 and 856.2 eV) were assigned to different final states corresponding to ligand-to-Ni2+ charge transfer.

K. Kishi, K. Fujiwara/Journal of Electron Spectroscopy and Related Phenomena 71 (1995) 51-59

The changes in the intensity ratio of the two peaks with temperature are explained by the increase in vibronic mixing of the different final states [14] at elevated temperatures. (The intensity of the 854.6 eV peak is greater than that of the 856.2 eV peak.) In the present case, however, the Ni2p3/2 peak at 855.9 eV is not associated with the peak at 854.5 eV, which is characteristic of Ni 2+. Both the Ni2p3/2 and the Ols peaks of the surface thin oxide film [10] were observed to shift to higher binding energy as compared with the spectra of cleaved NiO. This was explained by a shift of the Fermi level which was determined by defect states. The currently observed shift of the Ni2p3/2 peak is not explained by the shift of the Fermi level alone, as the shift of the Ni2p3/2 peak was not followed by a shift of the Ols peak. The appearance of a Ni2p3/2 peak at 855.9 eV that is not associated with the peak at 854.5 eV may be explained merely by the difference in symmetry of the surrounding oxygen anions for Ni 2+ ions, but the formation of Ni 3+ is highly probable because of strong bonding with the oxygen anions of the vanadium oxide. In order to obtain information on the change in chemical states of nickel ions with increase in the nickel coverage, the spectral variations were measured as a function of the coverage. The deposition (0.5 ML equivalent) and oxidation (by

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exposure to 300 L of 02) of nickel atoms were repeated for the vanadium oxide layers (2.5 ML) on a Cu(100) surface. The attenuation of the Cu 2p3/2 peak after nickel oxidation was ~ 31% in each step, being consistent with the estimation of the thickness, 0.7 ML, of the NiO layers. In a preliminary experiment the Cu2p3/2 intensity was plotted against the amount of nickel atoms (0.25 ML equivalent of Ni in each step) deposited on vanadium oxide. The plot showed a straight line up to the third step and then a break. This result implies that the nickel oxide grew as a fairly smooth layer and the vanadium oxide layer underneath was smooth also. In Figs. 3 and 4, V2p3/2, Ni2p3/2 and Ols spectra after the oxidation are shown. The oxidized 0.5 ML equivalent Ni atoms on the vanadium oxide showed a Ni2p3/2 peak at 855.8 eV (Fig. 4(a)) and a V2p3/2 peak at 516.2 eV (Fig. 3(a)) as described above. After addition of 0.5 ML equivalent of Ni and exposure to 02, the Ni2p3/2 peak at 855.9 eV was enhanced and was associated with a weak shoulder near 854.8 eV (Fig. 4(b)). The V2p3/2 peak showed a larger shift to 516.4 eV with a decrease in intensity of ~ 23%, indicating the influence of an interaction with the nickel oxide. After the subsequent addition of nickel atoms and oxidation, the apparent Ni2p3/2 peak maximum shifted to 855.7 eV because of the

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K. Kishi, K. Fujiwara/Journal of Electron Spectroscopy and Related Phenomena 71 (1995) 51-59 • ••el• Q•

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intensification of the shoulder on the lower binding energy side (see Fig. 4(c)). A second V2pu 2 peak again appeared at ~ 515 eV, implying weakening of the interaction with the upper nickel oxide overlayers. Further depositions of nickel atoms enhanced the Ni2p3/2 peak at 854.8 eV (Fig. 4(d) and (e)) but did not suppress the peak at 856.1 eV, despite the fact that the V2p3/2 peak intensity was attenuated to 30% by the upper nickel oxide layers (total ~ 3 . 5 NiO layers). The V2p3/2 peak maximum was observed at 515.7 eV and the peak at 516.1 eV was not clearly visible. These results are interpreted in terms of a decrease in interaction of the nickel oxide with the vanadium oxide after the growth of nickel oxide overlayers, more than 2 ML, which began to assume a NiO structure. Reduction of the vanadium to some extent immediately after the deposition of nickel atoms may activate the restructuring of the nickel ions into the NiO structure. The intensity ratio of the 854.8 eV and 856.0 eV peaks was a little larger when measured at an angle of 30 ° . This fact, together with the relatively constant intensity of the peak at 856 eV, shows that the nickel ions giving this peak were formed on the topmost layer after the growth of the NiO structure

by the same mechanism as for the oxidation of single-crystal nickel surfaces.

3.2. Adsorption of acetic acid on the oxide surfaces In the above discussion, submonolayers of nickel ions were thought to bond to the oxygen anions of vanadium oxide, giving the Ni2p3/2 peak at 855.9 eV and shifting the V2p3/2 peak to 516.2 eV. Adsorption of acetic acid on the surface was carried out in order to examine the strength of interaction between the nickel ions and the vanadium oxide. Nickel atoms (0.6 ML equivalent) were deposited on vanadium oxide layers (2.0 ME) on Cu(100) and were oxidized by exposure to 300 L of 02. Acetic acid was subsequently adsorbed on the oxidized surface at 295 K. Two Cls peaks appeared at 285.0 and 288.4 eV with equal intensities (see Fig. 5). The difference in binding energy between the two Cls peaks is 3.4 eV, identical with the values for acetates [23,24]. The Ols peak at 529.8 eV was depressed and a shoulder was obtained at 531.5 eV. Acetate adsorbed on preoxidized Ni(111) gives an Ols peak at 531.7 eV [24], 2.3 eV below

K. Kishi, K. Fujiwara/Journal of Electron Spectroscopy and Related Phenomena 71 (1995) 51-59

~i

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58

K. Kishi, K. Fujiwara/Journal of Electron Spectroscopy and Related Phenomena 71 (1995) 51 59

that for the condensed acetic acid molecule. From these results, the acetic acid is concluded as being adsorbed as acetate on the oxide surface. Fig. 6 shows the V2p3/2 and Ni2p3/2 spectra (a) before and (b) after acetic acid adsorption. The V2p3/2 peak maximum at 516.1 eV showed a shift to lower binding energy, 515.8 eV, and the Ni2p3/2 peak at 855.9 eV was shifted to higher binding energy, 856.2 eV. The shift of the Ni2p3/2 peak is explained by bonding of the acetate to nickel ions in the topmost layer. The shift of the V2p3/2 peak to the position it occupied before nickel deposition is ascribable to weakening of the interaction between the nickel ions and vanadium oxide during bonding of the nickel ions to acetate. In a mixed surface oxide of vanadium and sodium, the formation of acetate was accomplished by segregation of the sodium ions from the oxide lattice into the top layer, and the consequent decrease of sodium in the oxide was followed by a shift of the V2p3/2 peak from 517.7 eV to 517.0 eV [181.

4. Conclusions

The oxidation of nickel atoms on preoxidized V/ Cu(100) surfaces was investigated by X-ray photoelectron spectroscopy. The interaction of nickel ions with vanadium oxide varies with increase in the coverage of the nickel oxide. (1) At a nickel oxide coverage of less than one monolayer, the nickel ions are fully dispersed on the V203/Cu(100 ) surface and give a single Ni2p3/2 peak at 855.9 eV, associated with a shift of the V2p3/2 peak to higher binding energy. This indicates strong bonding of the nickel ions with the oxygen anions of the vanadium oxide at the interface, as found in ")'-alumina supported nickel catalysts of very low nickel content. The interaction of nickel ions with vanadium oxide is weakened by the formation of adsorption bonds of acetate with the nickel ions. (2) During the growth of nickel oxide overlayers by repeated deposition and oxidation of nickel atoms, the nickel oxide is transformed into a bulk NiO structure, as suggested by the enhancement of the Ni2p3/2 intensity at 854.5 eV. By changing the

photoelectron collecting angle, the outermost layers of the nickel oxide are enriched by the species giving the Ni2p3/2 peak around 856 eV, as in the case of the surface oxide formed by the oxidation of singlecrystal nickel surfaces. The character of bonding of the nickel ions with the vanadium oxide changes with the growth of the NiO structure.

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