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Surface oxidation of holmium: UV photoelectron spectroscopy, X-ray photoelectron spectroscopy and scanning electron microscopy studies
Journal of the Less-Common
MetaLs, 141 (1988)
1 - 9
SURFACE OXIDATION OF HOLMIUM: UV PHOTOELECTRON SPECTROSCOPY, X-RAY PHOTOELECTRON SPECTROSCOPY AND SCANNING ELECTRON MICROSCOPY STUDIES P. SINGH,
A. B. MANDALE
and S. BADRINARAYANAN
Physical Chemistry Division, National Chemical Laboratory, Pune 411008 (Received
August
(India)
21,1987)
Summary The results of an investigation of the interaction of oxygen on the surface of holmium metal studied by X-ray photoelectron spectroscopy and UV photoelectron spectroscopy (UPS) are presented in this paper. The UPS valence band region showed two peaks in the region 4 - 9 eV and a peak at 11.5 eV. The 4 - 9 eV band was assigned to 0 2p levels and the 11.5 eV peak is due to the presence of OH species on the surface. The 01s levels also showed two peaks with binding energy values of 530.8 and 533.2 eV. The former is due to holmium oxide and the latter is due to Ho(OH),. The results are compared with the available literature values.
1. Introduction The study of the electronic structure of rare earth metals and alloys and their oxides has attracted great attention as they are interesting from the point of view of theoretical research as well as for their wide applications. These materials have wide industrial applications such as hydrogen storage [1], magnetic device materials [ 21 and in the form of the oxides as catalysts [3]. With the advent of surface analytical techniques, studies of rare earth materials have taken new dimensions, particularly those on the interaction of various gases on the surface of these metals and alloys. X-ray photoelectron spectroscopy (XPS) is the most widely used technique for the study of these interactions [4 - 161. The XPS technique is also useful in distinguishing between chemisorbed and chemically combined oxygen and in many cases can also distinguish between various oxidation states. In this communication we report our results on the interaction of oxygen with holmium metal studied by XPS, UV photoelectron spectroscopy (UPS) and scanning electron microscopy (SEM) techniques. We have monitored the growth of the oxide peaks as a function of oxygen exposure in the range from 1 to lo6 langmuirs (1 langmuir = lop6 Torr s). We have recorded 0 Is, 0 2p, Ho 4f and Ho 4d levels. The results are compared with the available literature data on the oxidation of various rare earth metals. 0022~5088/88/$3.50
@ Elsevier
Sequoia/Printed
in The Netherlands
2
Finally, the surface morphology oxygen ( lo6 langmuirs). 2. Experimental
was examined
by SEM after exposure
to
details
Holmium metal was taken in the form of a rectangular sheet of size 10 mm X 5 mm with a thickness of about 1 mm. The surface was first scraped with a sharp knife, subsequently being polished with different grades of emery paper and finally cleaned with methyl alcohol. The cleaned plate was attached to a nickel holder. The experiments were performed in a VG Scientific ESCA-3-MK-II photoelectron spectrometer. The base vacuum in the spectrometer was about 3 X lo-lo Torr. All the spectra were recorded under identical conditions using Al Ko X-ray radiation. The calibration of the spectrometer was checked by determining the binding energies (BEs) of the Au 4fTi2 (84.0 eV), Ag 3d5,2 (368.3 eV) and Cu 2ps,, (932.4 eV) levels using spectroscopically pure metals (Johnson Matthey). The BE values are in good agreement with literature values [17 - 191. The instrumental resolution in terms of the full width at half-maximum of the Au 4f,,2 level was 1.5 eV. BE values reported in this communication are measured with a precision of kO.2 eV. The UPS spectra were recorded with He I (21.2 eV) radiation using a spectrometer pass energy of 10 eV. Photoelectron spectra of holmium metal were recorded after various treatments as mentioned below. All the exposures are expressed in langmuirs. The surface was argon ion cleaned until no signal arising from 0 1s and C 1s was observed. The parameters for sputtering were an argon pressure of 10d6 Torr and a beam current of 60 E.IAfocused onto a 1 cm2 sample area. A clean surface was exposed to oxygen at a pressure of lo-’ Torr for 10,20, 50, 100 and 200 s and at lop6 Torr for lo’, 104, lo5 and lo6 s. All the treatments were carried out in the preparation chamber of the spectrometer. Oxygen was admitted to the system through a leak valve from an ultrahigh vacuum gas-handling system. For low exposures the oxygen was admitted with the pump open and for exposures above lo3 langmuirs a higher pressure was used with the pump valves closed. Pressures were measured with a nude ion gauge located in the preparation chamber of the spectrometer. The surface morphology was examined with a Cambridge 150 scanning electron microscope with Link system energy-dispersive analysis of X-rays unit coupled to it. This instrument is fitted with a specimen stage allowing X, Y and 2 rotations and tilt controls. The resolution achievable’is better than 7 nm with an effective source at about 30 pm diameter. The microscope was operated at 40 kV and 2.4 A in the secondary emission scanning mode and the surface was examined at a 10” tilt angle. 3. Results and discussion Figure 1 shows the general scan spectra of holmium metal after repeated cycles of heating and cleaning as described previously [ 17, 181.
3
0
zoo
400
BINDING
600
600
ENERGY
IO00
(rV)
Fig. 1. General scan XPS spectra of holmium at different stages of cteaning.
The sample was cleaned by argon ion bombardment for nearly 8 h to remove carbon and oxygen contamination. No change was observed in the BE of the Ho 4f and Ho 4d metal levels after repeated cleaning and hence it is assumed that prolonged ion milling does not produce any surface damage which can give rise to a BE shift. The photoelectron spectrum of a partially cleaned sample showed a broad Ho 4f level (Fig. 2, spectrum a) which on cleaning became very sharp (Fig. 2, spectrum b) (no 0 Is or C Is lines were observed in the general scan
0
12 &(eV)
Fig. 2. XPS and UPS spectra of holmium metal (Ho 4f level): (a) partiafly efeaned; (b) completely cleaned.
4
BE(eV) Fig. 3. He I spectrum of holmium metal.
(Fig. 1, spectrum d)). The XPS and UPS (He I) valence band structures are shown in Figs. 2 and 3. The XPS valence band region (Fig. 2) consisted of 6s5d emission at Ef and intense 4f emission at 4 - 8 eV. The 4f level spectra obtained in the present investigation are in good agreement with those reported by Baer and Busch [ 201. The He I spectrum (Fig. 3) shows a complete change compared with that obtained from the XPS study. The onset of 6s5d emission at Ef is very clear. The bottom of the d band (the higher binding energy edge of the photoemission energy distribution) lies at about 2 eV. The intensity of the 4f level in the 4 - 8 eV region is very weak compared with that of the XPS spectrum. The weak 0 2p level at 6 eV is due to surface contamination by oxygen and the detection of this peak in UPS is due to the large photoelectron cross-section of the 0 2p level for He I radiation. The BEs of the Ho 4f levels were found to be 4.9 and 7.9 eV (4f,,, and 4f,,,) and those of the 4d levels are 159 and 161.3 eV. These values are comparable with the literature values. The Ho 4d level is very broad owing to complex multiple splitting. The general electronic configuration of the heavy lanthanides in the metallic state (with the exception of ytterbium) is 4f”5d’6s2. Since 5d and 6s electrons are involved in the bonding, the usual ionic configuration is 4f”. Thus the final state produced in the emission of a photoelectron from the lower-lying 4d levels will be of the form 4dg4f” and the possible final state will depend on the coupling of 4d holes and the possible final 4f shell. Thus complex multiple splitting is expected and this is seen in the 4d levels. Because of the complex nature of this level, we have used only the Ho 4f levels for the analysis. In spite of repeated cleaning, a weak oxygen signal (0 2p) appeared in the valence band structure (UPS), but the 0 1s signal was only visible on an expanded scale in XPS. The ratio IO Is/ln04f was equal to 0.02. This 0 1s peak was symmetric with a BE value of 530.6 eV. The argon-ion-etched sample was exposed to high purity oxygen at lo-’ Torr for 10,20, 50, 100, 200 and 1000 s in stages. The XPS spectra were recorded after each exposure. The variations in Ho 4f and 0 1s levels are shown in Figs. 4(a) and 4(b) respectively.
5
I BE(eV)
IlIIIIIIIl
IIIIII
0
6
12
18
BE(eV)
-
Fig. 4. Ho 4f and 0 1s levels: (A) Ho 4f level after various various exposures; (C) 0 1 s level at angles of 20” and 50”.
exposures;
Fig. 5. He I spectrum
(more
of holmium
metal exposed
to oxygen
(B) 0 1s level after
than 3 langmuirs).
The He I spectrum of the sample exposed to oxygen (more than 3 langmuirs) is shown in Fig. 5. The 0 2p band shows a considerable growth in intensity and also indicates the presence of two peaks in this region with BE values of 5.6 and 7.2 eV. Intensity considerations rule out the possibility that a 5.6 eV peak arises from He Ip (23.1 eV) radiation. There is also a considerable decrease in the intensity of the 6s5d band, indicating the depletion of 6s5d electrons on exposure to oxygen. Another notable feature is a peak at 11.5 eV. The difference between the UV source energy and the BE cut-off can be taken as a measure of the work function of a material. We have obtained from the UPS spectra the approximate work function values for clean holmium metal and holmium metal after oxygen exposure. On exposure to oxygen, the work function drops to 3.2 eV from an initial value of 3.8 eV. The decrease in the work function can be attributed to the presence of a chemisorbed phase. On exposure to oxygen, the 0 1s peak at 530.6 eV started to grow. In addition, an additional peak at 533.2 eV started to develop. The 0 1s level at 530.6 eV reached a maximum at 3 langmuirs and thereafter showed very little change. However, the second peak became more and more pronounced. We attribute the 530.6 eV peak to holmium oxide and the 533.2 eV peak to an additional 0 1s level with higher BE. Generally the higher BE peaks have been attributed to (a) chemisorbed oxygen (530.0 532.0 eV), (b) surface hydroxyl groups (531.5 - 533.0 eV) and (c) nonstoichiometric surface oxygen atoms [ 211.
6
tn
z E
I
I
5
IO X
P
0
S
I IS
U RE
I
J
20
(L)
Fig. 6. 0 1s XPS intensity as a function of oxygen exposure at room temperature.
The 0 1~530.0 Peak intensity was plotted against the oxygen exposure (Fig. 6). The 0 1s 530.6peak intensity showed a sharp rise up to an exposure of 3 langmuirs and thereafter very slowly increased. Beyond 20 langmuirs the increase in the peak intensity was hardly observable. The oxidation of holmium starts at very low oxygen exposures (about 1 langmuir) and the surface is rapidly oxidized as evidenced from the steep rise in the oxygen uptake curve. The oxide layer formed protects against further oxidation and severely hampers the process of further oxidation. The initial uptake of oxygen is very high because initially the sticking coefficient is very high for dissociative adsorption, leading to a rapid increase in the 0 1s intensity. We have also calculated the relative peak intensity ratio 0 1s533.2:0 1s530.6 us. the oxygen exposure. This was found to increase from an initial value of 0.12 (1 langmuir) to 0.60 (lo6 langmuirs). The Ho 4f levels on exposure to oxygen develop a shoulder on the high BE side (Fig. 4(a)). This level becomes very broad because of the combination of six peaks. One pair corresponds to Ho 4f from the unreacted metal surface (for very low oxygen exposures), the second pair corresponds to Ho 4f from the oxide and the third pair corresponds to two types of oxygen. For large exposures, the Ho 4f metal peaks were absent (Fig. 4(a), lo6 langmuirs). The Ho 4f level for lo6 langmuirs was resolved into four peaks with BE values of 5.4, 7.2, 9.0 and 11.6 eV using a non-linear least-squares computer fit. The 5.4 and 11.6 eV peaks were attributed to two different oxygen peaks and the 7.2 and 9.0 eV peaks to Ho 4f from holmium oxidehydroxide. Since two types of oxygen species were observed, it is necessary to ascertain which species is predominantly on the surface. XPS spectra taken at a grazing angle of electron ejection, which effectively reduces the escape
depth, provide ~formation regarding the exact nature of the surface species. Hence in this study we have recorded the photoelectron spectra at various angles of electron ejection for a holmium sample exposed to oxygen (about 3 langmuirs) (Fig. 4(c)). At the normal angle for XPS study (50”) the 0 1s level showed a very intense peak at a BE of 530.2 eV and a weak asymmetry on the high BE side (about 533.0 eV). However, on approaching the grazing angle of electron ejection (about 204, the 533.2 eV peak grows in intensity, indicating the dominance of this species on the surface. One po~ibility for the second oxygen peak (533.2 eV) is that it arises from chemisorbed oxygen. However, after the initial stage of reaction with oxygen the amount chemisorbed by the metal should decrease as the amount of free metal surface decreases with the formation of oxide. On the contrary, however, peak 2 (533.2 eV) is found to grow in intensity, reaching a maximum at very high exposures. Also, oxides are not expected to adsorb large quantities of oxygen. It is possible that, in spite of the high vacuum, peak 2 arises from the formation of hydroxide on the surface as a result of water contam~ation in the chamber. The reaction of oxide with water produces hydroxide as follows: Ho,Os + 3H,O -----+ 2Ho(OH), Netzer et al. [13] have studied the Er + O2 and Er + HZ0 systems through UPS. Their results can be summarized briefly as follows. On oxygen exposure, strong bands are detected in the 4 - 9 eV region below Ef and there is a depletion of the Er 5d6s derived conduction band state near Ef. For exposures of more than 1 langmuir, an additional peak at 11.2 eV was observed in the He II spectra. The structure in the region 4 - 9 eV is attributed to 0 2p derived states and the peak at 11.2 eV in He II spectra is assigned to a multielectron satellite structure connected with 0 2p emission. Water-induced photoemission features in the 4 - 9 eV region are similar to those of oxygen and the presence of an 11.8 eV peak is due to emission from orbit& with O-H u bonding character. The broadening and splitting (Er + 0,) into two components of the initially narrow 6 eV photoemission peak with increasing oxygen exposure is attributed to the formation of erbium oxides of various stoichiometries. Wandelt and Brundle [ 161 recently studied the Gd + 0, system and observed a similar split in the 4 - 9 eV region and a peak at 11.8 eV. They have suggested the formation of an intermediate oxide, possibly Gd2+. For higher oxygen exposures the formation of Gd,O, has been suggested. For very high oxygen exposures, the surface oxide reacts with the cont~inant water to produce hydroxide. Holmium metal surfaces, cleaned as described earlier, were examined by SEM. A micrograph of the clean surface is shown in Fig. 7 and this clearly reveals the expected grinding marks. At a higher magnification the surface is seen to be smooth and continuous. After exposure to oxygen (lo6 langmuirs) the surface at higher magnification is resolved. This seems to consist of a rather whiskery type of topography as shown in Fig. 8.
8
Fig. 7. Scanning electron micrograph of cleaned holmium metal surface. Fig. 8. Scanning electron micrograph of holmium metal surface after exposure to lo6 langmuirs of oxygen.
4. Conclusions (1) The oxidation of holmium starts at very low oxygen pressures and the surface is rapidly oxidized as evidenced from the steep rise in the oxygen uptake curve. (2) The UPS valence band spectra showed two peaks with BE values of 5.6 and 7.2 eV which are assigned to the 0 2p level. The additional peak at 11.5 eV is due to the presence of OH spectra, possibly from Ho(OH)s. (3) The 0 1s region also indicated the presence of two types of oxygen with BE values of 530.6 eV (Ho,O,) and 533.2 eV (HOE).
Acknowledgment This work is National
Chemical
Laboratory
Communication
4284.
References H. C. Siegmann, L. Schlapbach and C. R. Brundle, Phys. Rev. Lett., 40 (1978) 972. P. Chaudhari, J. J. Cuomo and R. J. Gambino, AppZ. Phys. Lett., 22 (1973) 337. See, for example, W. E. Waalce, Rare Earth Intermetallics, Academic Press, New York, 1973. W. Farber and P. Braun, Surf Sci., 41 (1974) 195. W. C. Lang, B. D. Padalia, L. M. Watson, D. J. Fabian and P. R. Norris, Faraday Discuss. Chem. Sot., 60 (1975) 37. P. Burroughs, A. Hamnett, A. F. Orchard and G. Thorniton, J. Chem. Sot., Dalton Trans., 17 (1976) 1686. B. D. Padalia, J. M. Gimzewski, S. Affrossman, W. C. Lang, L. M. Watson and D. 3. Fabian, Surf. Sci., 61 (1976) 468.
9
8 B. D. Padalia, W. C. Lang, P. R. Norris, L. M. Watson and D. J. Fabian, Proc. Roy. Sot. London, Ser. A, 354 (1977) 269. 9 A. Platau, L. I. Johansson, A. L. Gagstrom, S. E. Karlsson and S. B. M. Hagstrom, Surf Sci., 63 (1977) 153. 10 G. Praline, B. E. Koel, R. L. Hance, H. I. Lee and J. M. White, J. Electron. Spectrosc. Relat. Phenom., 21 (1980) 17. 11 B. E. Koel, G. Praline, H. I. Lee, J. M. White and R. L. Hance, J. Electron. Spectrosc. Relat. Phenom., 21 (1980) 31. 12 D. D. Sarma, M. S. Hegde and C. N. R. Rao, J. Chem. Sot., Faraday Trans. 2, 77 (1981) 1509. 13 F. P. Netzer, R. A. Wille and M. Grunze, Surf. Sci., 102 (1981) 75. 14 F. Gerken, J. Barth, R. Kammerer, L. I. Johansson and A. Flodstrom, Surf. Sci., I1 7 (1982) 468. 15 Y. Umamino and T. Ishizuka, J. Electron Spectrosc. Relat. Phenom., 34 (1984) 67. 16 K. Wandelt and C. R. Brundle, Surf. Sci., 157 (1985) 162. 17 A. B. Mandale, S. Badrinarayanan, S. K. Date and A. P. B. Sinha, J. Electron Spectrosc. Relat. Phenom., 33 (1984) 61. 18 S. Badrinarayanan, A. B. Mandale and A. P. B. Sinha, Mater. Chem. Phys., 11 (1984) 1. 19 K. Richter and B. Peplinski, J. Electron Spectrosc. Relat. Phenom., 13 (1978) 69. 20 Y. Baer and G. Busch, J. Electron Spectrosc. Relat. Phenom., 5 (1974) 611. 21 D. Briggs (ed.), Handbook of X-ray and Ultraviolet Photoelectron Spectroscopy, Heyden, New York, p. 295.
MetaLs, 141 (1988)
1 - 9
SURFACE OXIDATION OF HOLMIUM: UV PHOTOELECTRON SPECTROSCOPY, X-RAY PHOTOELECTRON SPECTROSCOPY AND SCANNING ELECTRON MICROSCOPY STUDIES P. SINGH,
A. B. MANDALE
and S. BADRINARAYANAN
Physical Chemistry Division, National Chemical Laboratory, Pune 411008 (Received
August
(India)
21,1987)
Summary The results of an investigation of the interaction of oxygen on the surface of holmium metal studied by X-ray photoelectron spectroscopy and UV photoelectron spectroscopy (UPS) are presented in this paper. The UPS valence band region showed two peaks in the region 4 - 9 eV and a peak at 11.5 eV. The 4 - 9 eV band was assigned to 0 2p levels and the 11.5 eV peak is due to the presence of OH species on the surface. The 01s levels also showed two peaks with binding energy values of 530.8 and 533.2 eV. The former is due to holmium oxide and the latter is due to Ho(OH),. The results are compared with the available literature values.
1. Introduction The study of the electronic structure of rare earth metals and alloys and their oxides has attracted great attention as they are interesting from the point of view of theoretical research as well as for their wide applications. These materials have wide industrial applications such as hydrogen storage [1], magnetic device materials [ 21 and in the form of the oxides as catalysts [3]. With the advent of surface analytical techniques, studies of rare earth materials have taken new dimensions, particularly those on the interaction of various gases on the surface of these metals and alloys. X-ray photoelectron spectroscopy (XPS) is the most widely used technique for the study of these interactions [4 - 161. The XPS technique is also useful in distinguishing between chemisorbed and chemically combined oxygen and in many cases can also distinguish between various oxidation states. In this communication we report our results on the interaction of oxygen with holmium metal studied by XPS, UV photoelectron spectroscopy (UPS) and scanning electron microscopy (SEM) techniques. We have monitored the growth of the oxide peaks as a function of oxygen exposure in the range from 1 to lo6 langmuirs (1 langmuir = lop6 Torr s). We have recorded 0 Is, 0 2p, Ho 4f and Ho 4d levels. The results are compared with the available literature data on the oxidation of various rare earth metals. 0022~5088/88/$3.50
@ Elsevier
Sequoia/Printed
in The Netherlands
2
Finally, the surface morphology oxygen ( lo6 langmuirs). 2. Experimental
was examined
by SEM after exposure
to
details
Holmium metal was taken in the form of a rectangular sheet of size 10 mm X 5 mm with a thickness of about 1 mm. The surface was first scraped with a sharp knife, subsequently being polished with different grades of emery paper and finally cleaned with methyl alcohol. The cleaned plate was attached to a nickel holder. The experiments were performed in a VG Scientific ESCA-3-MK-II photoelectron spectrometer. The base vacuum in the spectrometer was about 3 X lo-lo Torr. All the spectra were recorded under identical conditions using Al Ko X-ray radiation. The calibration of the spectrometer was checked by determining the binding energies (BEs) of the Au 4fTi2 (84.0 eV), Ag 3d5,2 (368.3 eV) and Cu 2ps,, (932.4 eV) levels using spectroscopically pure metals (Johnson Matthey). The BE values are in good agreement with literature values [17 - 191. The instrumental resolution in terms of the full width at half-maximum of the Au 4f,,2 level was 1.5 eV. BE values reported in this communication are measured with a precision of kO.2 eV. The UPS spectra were recorded with He I (21.2 eV) radiation using a spectrometer pass energy of 10 eV. Photoelectron spectra of holmium metal were recorded after various treatments as mentioned below. All the exposures are expressed in langmuirs. The surface was argon ion cleaned until no signal arising from 0 1s and C 1s was observed. The parameters for sputtering were an argon pressure of 10d6 Torr and a beam current of 60 E.IAfocused onto a 1 cm2 sample area. A clean surface was exposed to oxygen at a pressure of lo-’ Torr for 10,20, 50, 100 and 200 s and at lop6 Torr for lo’, 104, lo5 and lo6 s. All the treatments were carried out in the preparation chamber of the spectrometer. Oxygen was admitted to the system through a leak valve from an ultrahigh vacuum gas-handling system. For low exposures the oxygen was admitted with the pump open and for exposures above lo3 langmuirs a higher pressure was used with the pump valves closed. Pressures were measured with a nude ion gauge located in the preparation chamber of the spectrometer. The surface morphology was examined with a Cambridge 150 scanning electron microscope with Link system energy-dispersive analysis of X-rays unit coupled to it. This instrument is fitted with a specimen stage allowing X, Y and 2 rotations and tilt controls. The resolution achievable’is better than 7 nm with an effective source at about 30 pm diameter. The microscope was operated at 40 kV and 2.4 A in the secondary emission scanning mode and the surface was examined at a 10” tilt angle. 3. Results and discussion Figure 1 shows the general scan spectra of holmium metal after repeated cycles of heating and cleaning as described previously [ 17, 181.
3
0
zoo
400
BINDING
600
600
ENERGY
IO00
(rV)
Fig. 1. General scan XPS spectra of holmium at different stages of cteaning.
The sample was cleaned by argon ion bombardment for nearly 8 h to remove carbon and oxygen contamination. No change was observed in the BE of the Ho 4f and Ho 4d metal levels after repeated cleaning and hence it is assumed that prolonged ion milling does not produce any surface damage which can give rise to a BE shift. The photoelectron spectrum of a partially cleaned sample showed a broad Ho 4f level (Fig. 2, spectrum a) which on cleaning became very sharp (Fig. 2, spectrum b) (no 0 Is or C Is lines were observed in the general scan
0
12 &(eV)
Fig. 2. XPS and UPS spectra of holmium metal (Ho 4f level): (a) partiafly efeaned; (b) completely cleaned.
4
BE(eV) Fig. 3. He I spectrum of holmium metal.
(Fig. 1, spectrum d)). The XPS and UPS (He I) valence band structures are shown in Figs. 2 and 3. The XPS valence band region (Fig. 2) consisted of 6s5d emission at Ef and intense 4f emission at 4 - 8 eV. The 4f level spectra obtained in the present investigation are in good agreement with those reported by Baer and Busch [ 201. The He I spectrum (Fig. 3) shows a complete change compared with that obtained from the XPS study. The onset of 6s5d emission at Ef is very clear. The bottom of the d band (the higher binding energy edge of the photoemission energy distribution) lies at about 2 eV. The intensity of the 4f level in the 4 - 8 eV region is very weak compared with that of the XPS spectrum. The weak 0 2p level at 6 eV is due to surface contamination by oxygen and the detection of this peak in UPS is due to the large photoelectron cross-section of the 0 2p level for He I radiation. The BEs of the Ho 4f levels were found to be 4.9 and 7.9 eV (4f,,, and 4f,,,) and those of the 4d levels are 159 and 161.3 eV. These values are comparable with the literature values. The Ho 4d level is very broad owing to complex multiple splitting. The general electronic configuration of the heavy lanthanides in the metallic state (with the exception of ytterbium) is 4f”5d’6s2. Since 5d and 6s electrons are involved in the bonding, the usual ionic configuration is 4f”. Thus the final state produced in the emission of a photoelectron from the lower-lying 4d levels will be of the form 4dg4f” and the possible final state will depend on the coupling of 4d holes and the possible final 4f shell. Thus complex multiple splitting is expected and this is seen in the 4d levels. Because of the complex nature of this level, we have used only the Ho 4f levels for the analysis. In spite of repeated cleaning, a weak oxygen signal (0 2p) appeared in the valence band structure (UPS), but the 0 1s signal was only visible on an expanded scale in XPS. The ratio IO Is/ln04f was equal to 0.02. This 0 1s peak was symmetric with a BE value of 530.6 eV. The argon-ion-etched sample was exposed to high purity oxygen at lo-’ Torr for 10,20, 50, 100, 200 and 1000 s in stages. The XPS spectra were recorded after each exposure. The variations in Ho 4f and 0 1s levels are shown in Figs. 4(a) and 4(b) respectively.
5
I BE(eV)
IlIIIIIIIl
IIIIII
0
6
12
18
BE(eV)
-
Fig. 4. Ho 4f and 0 1s levels: (A) Ho 4f level after various various exposures; (C) 0 1 s level at angles of 20” and 50”.
exposures;
Fig. 5. He I spectrum
(more
of holmium
metal exposed
to oxygen
(B) 0 1s level after
than 3 langmuirs).
The He I spectrum of the sample exposed to oxygen (more than 3 langmuirs) is shown in Fig. 5. The 0 2p band shows a considerable growth in intensity and also indicates the presence of two peaks in this region with BE values of 5.6 and 7.2 eV. Intensity considerations rule out the possibility that a 5.6 eV peak arises from He Ip (23.1 eV) radiation. There is also a considerable decrease in the intensity of the 6s5d band, indicating the depletion of 6s5d electrons on exposure to oxygen. Another notable feature is a peak at 11.5 eV. The difference between the UV source energy and the BE cut-off can be taken as a measure of the work function of a material. We have obtained from the UPS spectra the approximate work function values for clean holmium metal and holmium metal after oxygen exposure. On exposure to oxygen, the work function drops to 3.2 eV from an initial value of 3.8 eV. The decrease in the work function can be attributed to the presence of a chemisorbed phase. On exposure to oxygen, the 0 1s peak at 530.6 eV started to grow. In addition, an additional peak at 533.2 eV started to develop. The 0 1s level at 530.6 eV reached a maximum at 3 langmuirs and thereafter showed very little change. However, the second peak became more and more pronounced. We attribute the 530.6 eV peak to holmium oxide and the 533.2 eV peak to an additional 0 1s level with higher BE. Generally the higher BE peaks have been attributed to (a) chemisorbed oxygen (530.0 532.0 eV), (b) surface hydroxyl groups (531.5 - 533.0 eV) and (c) nonstoichiometric surface oxygen atoms [ 211.
6
tn
z E
I
I
5
IO X
P
0
S
I IS
U RE
I
J
20
(L)
Fig. 6. 0 1s XPS intensity as a function of oxygen exposure at room temperature.
The 0 1~530.0 Peak intensity was plotted against the oxygen exposure (Fig. 6). The 0 1s 530.6peak intensity showed a sharp rise up to an exposure of 3 langmuirs and thereafter very slowly increased. Beyond 20 langmuirs the increase in the peak intensity was hardly observable. The oxidation of holmium starts at very low oxygen exposures (about 1 langmuir) and the surface is rapidly oxidized as evidenced from the steep rise in the oxygen uptake curve. The oxide layer formed protects against further oxidation and severely hampers the process of further oxidation. The initial uptake of oxygen is very high because initially the sticking coefficient is very high for dissociative adsorption, leading to a rapid increase in the 0 1s intensity. We have also calculated the relative peak intensity ratio 0 1s533.2:0 1s530.6 us. the oxygen exposure. This was found to increase from an initial value of 0.12 (1 langmuir) to 0.60 (lo6 langmuirs). The Ho 4f levels on exposure to oxygen develop a shoulder on the high BE side (Fig. 4(a)). This level becomes very broad because of the combination of six peaks. One pair corresponds to Ho 4f from the unreacted metal surface (for very low oxygen exposures), the second pair corresponds to Ho 4f from the oxide and the third pair corresponds to two types of oxygen. For large exposures, the Ho 4f metal peaks were absent (Fig. 4(a), lo6 langmuirs). The Ho 4f level for lo6 langmuirs was resolved into four peaks with BE values of 5.4, 7.2, 9.0 and 11.6 eV using a non-linear least-squares computer fit. The 5.4 and 11.6 eV peaks were attributed to two different oxygen peaks and the 7.2 and 9.0 eV peaks to Ho 4f from holmium oxidehydroxide. Since two types of oxygen species were observed, it is necessary to ascertain which species is predominantly on the surface. XPS spectra taken at a grazing angle of electron ejection, which effectively reduces the escape
depth, provide ~formation regarding the exact nature of the surface species. Hence in this study we have recorded the photoelectron spectra at various angles of electron ejection for a holmium sample exposed to oxygen (about 3 langmuirs) (Fig. 4(c)). At the normal angle for XPS study (50”) the 0 1s level showed a very intense peak at a BE of 530.2 eV and a weak asymmetry on the high BE side (about 533.0 eV). However, on approaching the grazing angle of electron ejection (about 204, the 533.2 eV peak grows in intensity, indicating the dominance of this species on the surface. One po~ibility for the second oxygen peak (533.2 eV) is that it arises from chemisorbed oxygen. However, after the initial stage of reaction with oxygen the amount chemisorbed by the metal should decrease as the amount of free metal surface decreases with the formation of oxide. On the contrary, however, peak 2 (533.2 eV) is found to grow in intensity, reaching a maximum at very high exposures. Also, oxides are not expected to adsorb large quantities of oxygen. It is possible that, in spite of the high vacuum, peak 2 arises from the formation of hydroxide on the surface as a result of water contam~ation in the chamber. The reaction of oxide with water produces hydroxide as follows: Ho,Os + 3H,O -----+ 2Ho(OH), Netzer et al. [13] have studied the Er + O2 and Er + HZ0 systems through UPS. Their results can be summarized briefly as follows. On oxygen exposure, strong bands are detected in the 4 - 9 eV region below Ef and there is a depletion of the Er 5d6s derived conduction band state near Ef. For exposures of more than 1 langmuir, an additional peak at 11.2 eV was observed in the He II spectra. The structure in the region 4 - 9 eV is attributed to 0 2p derived states and the peak at 11.2 eV in He II spectra is assigned to a multielectron satellite structure connected with 0 2p emission. Water-induced photoemission features in the 4 - 9 eV region are similar to those of oxygen and the presence of an 11.8 eV peak is due to emission from orbit& with O-H u bonding character. The broadening and splitting (Er + 0,) into two components of the initially narrow 6 eV photoemission peak with increasing oxygen exposure is attributed to the formation of erbium oxides of various stoichiometries. Wandelt and Brundle [ 161 recently studied the Gd + 0, system and observed a similar split in the 4 - 9 eV region and a peak at 11.8 eV. They have suggested the formation of an intermediate oxide, possibly Gd2+. For higher oxygen exposures the formation of Gd,O, has been suggested. For very high oxygen exposures, the surface oxide reacts with the cont~inant water to produce hydroxide. Holmium metal surfaces, cleaned as described earlier, were examined by SEM. A micrograph of the clean surface is shown in Fig. 7 and this clearly reveals the expected grinding marks. At a higher magnification the surface is seen to be smooth and continuous. After exposure to oxygen (lo6 langmuirs) the surface at higher magnification is resolved. This seems to consist of a rather whiskery type of topography as shown in Fig. 8.
8
Fig. 7. Scanning electron micrograph of cleaned holmium metal surface. Fig. 8. Scanning electron micrograph of holmium metal surface after exposure to lo6 langmuirs of oxygen.
4. Conclusions (1) The oxidation of holmium starts at very low oxygen pressures and the surface is rapidly oxidized as evidenced from the steep rise in the oxygen uptake curve. (2) The UPS valence band spectra showed two peaks with BE values of 5.6 and 7.2 eV which are assigned to the 0 2p level. The additional peak at 11.5 eV is due to the presence of OH spectra, possibly from Ho(OH)s. (3) The 0 1s region also indicated the presence of two types of oxygen with BE values of 530.6 eV (Ho,O,) and 533.2 eV (HOE).
Acknowledgment This work is National
Chemical
Laboratory
Communication
4284.
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