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Oxidation of a Au + 4 at% Ti alloy
Vacuum/volume 41/numbers 7-9/pages 1746 to 1749/1990
0042-207X/90S3.00 + .00 © 1990 Pergamon Press plc
Printed in Great Britain
O x i d a t i o n of a Au + 4 a t % Ti alloy P E Viljoen and J P R o u x , Department of Physics, University of the Orange Free State, PO Box 339, ZA -9300 Bloemfontein, Republic of South Africa
The gold alloy (Au +4at% Ti) was subjected to oxygen exposures at different temperatures in uhv. The surface concentration was monitored by AES and XPS. The solute Ti, segregated to the surface at temperatures above 300°C to form mixed oxides. The almost 1 : 1 ratio of Ti and 0 was a prominent feature at all temperatures. Carbon always appeared to be in the bound or carbidic form, its segregation to the surface enhancing that of 77. The appearance of an Auger peak at 293eV correlates with the presence of C and 0 in the surface region and is interpreted as due to a KVd transition where V represents valence states of the CO molecule and d a screening electron transfered from adjacent Au atoms to the C atom site. In contrast to pure gold that does not oxidise, the Au in this alloy oxidises between 500 and 600°C, forming an unstable oxide, probably Au20. The appearance of an Auger peak at 80eV during this oxidation process, is interpreted as an interband transition between Au and 0 while the shift of the 69 eV Au peak to 65 eV is the result of the chemical bonding between Au and O.
1. Introduction The alloy has been developed to serve as a jewellers' material having a high gold content, distinct gold colour, relative hardness and high corrosion resistance. Very little is known scientifically about this material. Dowben e t a l I compiled data on many gold containing alloys but no mention is made of any A u - T i alloy. Segregation of the Ti at elevated temperatures has been investigated2. From this it seems that Ti segregates to the surface where it immediately forms chemical compounds with segregated carbon and oxygen. In this paper we report on the oxidation behaviour of the sample at different temperatures and oxygen exposures.
2. Experimental The sample, supplied by Intergold, consisted of pure gold alloyed with 1 wt% or 4 at% titanium. It was cut in the form of a disc 1 mm thick having a diameter of 8 mm in order to fit the sample holder. The surface was mechanically polished using a 0.25/~m diamond spray in the final stage. It was sputter cleaned in the uhv system using 3 keV Ar ions rastered over the surface. The Ar partial pressure was 6.7 x l0 -3 Pa. The sample was resistance heated by an element fitted into the sample holder and the temperature, measured by a chrome-alumel thermocouple, was electronically controlled to within I °C. A fixed procedure was followed during each oxidation run. The sample was heated to the desired temperature where it was kept for at least 20 min before it was sputter cleaned and exposed to oxygen at base pressures ranging from 6.7 x l 0 - 7 to 1.1 x 10 -6 Pa. The surface composition was determined by AES 3 at closely spaced intervals. The acceleration voltage was only applied to the electron gun while the Auger spectra were actually being recorded. 1746
XPS data were recorded using a l0 keV, 400 W X-ray source with a Mg anode. The analyser was a double pass CMA. Spectra were recorded at room temperature after sputter cleaning and after oxidation at 580°C and cooling to room temperature. Spectra were also recorded at 580°C. No sensitivity factors were available for the A u - O (80 eV) and C - O (293 eV) peaks. In order to determine surface concentration changes from these peaks, sensitivity factors were assumed respectively as the average of the Au and O and C and O factors. Since this is an assumption, no quantitative deductions can be made from these peaks.
3. Results 3.1. Room temperature. Upon exposure to oxygen, the sputter cleaned surface showed no increase in the initial 5 at% Ti but the oxygen on the surface increased from an initial 2 at% to a maximum of 5 at% during an exposure of 100 L and remained at this level for exposures up to 500 L. 3.2. 300°C. The Ti surface concentration increased from 2 to 7 at% during a 10 L exposure to oxygen. The oxygen concentration at the surface increased from 2 to 7 at% during a 50 L exposure. Both the Ti and O surface concentrations then remained constant up to the maximum exposure. Carbon also showed an increase from 5 to 10 at% during a 10 L oxygen exposure and a further 2 at% increase at about 200 L exposure. 3.3. 500°C. The surface composition under increasing oxygen exposure with the sample at a constant temperature of 500°C is shown in Figure I. There is a steady increase at the surface in both Ti and O at an approximately equal rate up to an oxygen exposure of 100 L where the concentration abruptly levels off to remain constant at 30 at% in the case of Ti, O as well as Au.
P E Viljoen and J P Roux: Oxidation of a A u - T i alloy
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Figure i. Oxidation at 500C as measured by the surface concentration of all the elements as function of oxygen exposure. The nearly l : l ratio of the Ti and O is very obvious. M a k i n g up the rest of the surface content is C at a steady _+8%. The co-segregation of Ti a n d O was a feature at all temperatures up to 650'C. An unidentified peak at 293 eV consistantly appeared in the Auger spectra whenever C a n d ample O were present on the surface of this alloy. The profile of the c a r b o n peak was of the carbide or chemically b o n d e d form t h r o u g h o u t . The possibility of this peak being due to Ca impurity can be ruled out since it only appeared in an oxygen a t m o s p h e r e and not after heat t r e a t m e n t where one expects impurities to segregate to the surface. It has also been observed in an A1Cu alloy u n d e r similar conditions only. At oxygen exposures a b o v e 250 L the O and Ti c o n c e n t r a t i o n started increasing again with subsequent decrease in the 69 eV Au Auger peak. The peak actually split into two peaks appearing at 80 and 65 eV respectively, indicating that some oxygen had formed a chemical b o n d with the Au. It is assumed that some Ti oxides a n d even carbides may have been formed at this stage as s h o w n by XPS data taken at higher temperatures. At a t e m p e r a t u r e of 5 5 6 C the main features of surface composition, as function of oxygen exposure, seem to be repeated. 3.4. 603°C. The surface c o n c e n t r a t i o n of the different elements as a function of oxygen exposure at 6 0 Y C , Figure 2, again
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Figure 3. Auger spectra taken during the oxygen exposures are indicated. The bottom one just above but after about 15 20 min Decomposition of the Au O peak, at least
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follows the general pattern recorded at 500 a n d 556°C except for the C which now enriches the surface at nearly the same rate as does Ti. The surface c o m p o s i t i o n again becomes c o n s t a n t at a n oxygen exposure just in excess of 100 L. At 200 L an increase in b o t h O and A u - O ( 8 0 e V ) again correlates nearly exactly, while the pure gold peaks shrink accordingly. A b o v e 300 L an
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Figure 4. XPS spectra for Ti(2p) peaks of the sample. Spectra (a) and (b) were taken at room temperature, (a) after oxidation at a temperature of 580°C and cooling down and (b) before oxidation. Spectrum (c) has been taken at an elevated temperature (580"C). The Ti(2p) peaks for different chemical compounds are indicated. 1747
P E Viljoen and J P Roux: Oxidation of a A u - T i alloy
equilibruim is reached with Ti, C and O probably constituting the outer layer on the surface. The strong surface segregation of C at about 600C was also found in a segregation study2 of the same alloy. Auger spectra recorded at 603"C for six different oxygen exposures are shown in Figure 3 with the energies at which the most prominent peaks appear, marked by dashed lines. The carbon peak at 272 eV shows the fingerprint of a carbide form in all these spectra indicating that it is in a chemically bonded state. The peak at 293 eV shadows the carbon in all but the 2 L spectrum. The split of the 69 eV Au peak becomes prominent at an exposure of 150 L. The 80eV peak seems to grow in correlation with the growth of the O peak at 510 eV. If this peak at 80 eV indicates the formation of a gold oxide it is a very unstable oxide decomposing under electron bombardment after 15-20 rain as indicated by the last spectrum shown in Figure 3. While all the other peaks remain constant, the split in the lower energy gold peak disappears while the oxygen peak diminishes. The stoichiometry of the gold oxide can be deduced from the decrease in the Au (about 8 at%) and increase in oxygen (about 4 at%) surface concentration in Figure 3. This seems to indicate Au20 which is indeed one of the oxides formed by Au 4. The Ti peaks, in the dN/dE spectra, at 386 and 419 eV seem to indicate a mixed oxidation state TiO + TiO25. 3.5. 650°C. At this temperature there is very little change in the initial surface concentration of the elements Au (65%), Ti (14%), O (14%) and C (8%). At 200 L there is a slight increase in the O and Ti concentration with a corresponding decrease in both Au and C at the surface. There is again the striking 1 : ! ratio between Ti and O. X-ray photoelectron spectra (XPS) recorded at 580°C indicate no chemical shifts in the Au(4f) or O(ls) peaks 5 during oxygen exposure, only the expected change in peak heights. The total oxygen exposure was in excess of 300 L which was enough to reach equilibrium on the surface at this temperature. The Ti XPS peaks are shown in Figure 4. The unoxidised surface, Figure 4(b), shows the two major peaks at 460 and 454 eV binding energy. The fully oxidized surface, Figure 4(a), seems to consist of a mixture of Ti oxides, TiO at 456 eV, TiO 2 at 460 and 466 eV and Ti203 at 458 eV6 8 As a control experiment pure gold was exposed to oxygen at different temperatures. From room temperature up to 70ffC no oxidation could be observed. 4. Discussion
According to the T i - A u phase diagram 9 the Ti is in solid solution at the composition Au-4 Ti with the Au or Cu cubic structure. As was the case in a separate segregation study 2 the Ti started segregating to the surface at a temperature of 300°C. Indications that some of the Ti binds chemically to both C and O on the surface, are the carbidic form of the carbon Auger peak profile and the fact that more oxygen is accommodated on the surface at 300"C than at room temperature. The nearly 1 : 1 ratio of O to Ti on the surface maintained throughout the temperature range 300 65OC together with the XPS evidence of multiple Ti oxide formation at 580~C, confirms the deduction that Ti oxides are being formed on the surface in an oxygen atmosphere as soon as Ti acquires enough energy to segregate to the surface, that is above 300°C. This is further 1748
supported by the form of the Ti Auger peaks. Reported spectra 6, ~0 J3 shows that in our case the Ti must be in an oxidized state, most probably a mixture of Ti oxides. Comparing the XPS spectra (Figure 4) to the deconvoluted Ti (2pl/2" 3/2) XPS spectra from data by Armstrong and Quinn6, it seems likely that at 580°C the oxides TiO, Ti203 and TiO2 are all present in varying quantitites in the surface region and covering the A u - T i substrate. Although the XPS spectrum of Au did not indicate any chemical shift during oxygen exposure even at 580°C, the low energy part of the Auger spectrum (dN/dE) of Au showed the peak at 69 eV splitting into two peaks at 65 and 80 eV. Left under electron bombardment for 15-20 min, the 69 eV peak reappeared while the 510 eV oxygen peak indicated a marked decrease in the surface oxygen concentration. The 80 eV peak is most probably due to a cross or interband transition between Au and O energy levels. Assume that e.g. a hole is created in the O, level of gold (108 eV) 14. This is then filled by an electron from the L~ level (24 eV) of the oxygen atom. The energy difference ( 1 0 8 - 24 eV) is transferred to the 045 Au level which emits an Auger electron. Since the gold received an electron from another atom it is no longer in the excited state. The 045 level is at 3 eV below the vacuum level. Thus the final energy of the Auger electron is 81 eV in close agreement with the observed 80 eV. The strong surface segregation of C in an oxygen atmosphere at 603°C seems to be in line with its segregation behaviour in uhv 2. What is significant though is that it follows the Ti and O segregation rate very closely. It also seems to enhance the initial segregation rates of Ti and O at 603°C which declined on going from a temperature of 500°C (Figure 1) to 556°C. The oxidation of Ti is directly coupled to its surface segregation behaviour in the temperature range 500-603°C 2. Any one or a combination of Ti carbides might form ~5 depending on the amount of C available. The presence of ample amounts of oxygen complicates the situation. It is interesting to note that the Ti concentration increased to roughly 30 at% at all the temperatures investigated between 300 and 603°C where it remained constant. At 650°C and 300 L oxygen exposure however, the Ti together with O were still below 20 at% on the surface. Ocal and Ferrer 16 have shown that Au diffuses through a TiO 2 film. The lower surface concentration of Ti at 650°C might be explained by a situation in which Au strongly diffuses through the Ti oxide layer, formed by anion diffusion of the O through the oxide overlayer ~7. At this temperature the Au easily diffuses through the surface oxide layer to dominate on the surface. The peak at 293 eV may be due to the initial stage of the Auger process in C involving a highly localised core hole so that the valence or conduction electrons may relax to screen the holetS, ~9. The local relaxed density of states may be significantly different from the unperturbed local density of states. The effect of electronic relaxation is very clear in the KVV Auger spectrum of carbon in transition metal carbonyls ~8, the highest energy features having no analog in the free molecule Auger spectrum. These correspond to KVd transitions where V represents valence states of the CO molecule and d is a screening electron transferred from adjacent metal atoms to the 2 orbital of the carbon atom site which subsequently takes part in the Auger de-excitation. As this metal-carbon electron transfer is highly efficient in the presence of a carbon ls hole, relatively large intensities are
P E Viljoen and J P Roux: Oxidation of a A u - T i alloy
observed. We propose a similar m e t a l - c a r b o n electron transfer between Au and C in this gold alloy resulting in the peak at 293 eV.
5. Conclusions In an oxygen atmosphere Ti segregates to the surface of the Au+4at% Ti alloy at temperatures above 3 0 0 C forming mixed oxides. Ti and O collect on the surface at equal rates at all temperatures between 300 and 65OC. C segregating to the surface enhances the surface segregation of Ti. The C invariably appears to be in the carbidic or bound form. At 6 5 0 C Au covers the surface by diffusing through the initially formed Ti oxide surface layer. Pure gold does not oxidise even up to 700~C and 700 L exposure to oxygen. The Au in this alloy, however, oxidises between 500 and 600°C. Changes at or near the low energy Au Auger peak at 69 eV, correlate with the presence of large amounts of O on the alloy surface. The peak developing at 8 0 e V might be due to a possible interband transition between Au and O. The shift of the 69 eV Au peak to 65 eV is a chemical shift on oxide formation. An Auger peak at 293 eV coupled to the presence of C and O in the surface region, may be due to a KVd transition.
Acknowledgements The authors wish to thank Dr G Gafner and Intergold for supplying the samples and colleagues for helpful discussions.
Financial support by the F R D of the C S I R and C R F of the University of the Orange Free State is gratefully acknowledged.
References i p A Dowben, A H Miller and R W Vook, Gold Bull, 20(3), 54 (1987). 2 p E Viljoen and J P Roux, J Vac Sci Technol (submitted). 3L E Davis, N C MacDonald, P W Palmberg, G E Riach and R E Weber, Handbook of Auger Electron Spectroscopy'. Perkin-Elmer, Eden Prairie, MN (1978). 4 C D Parkes, Mellor's Modern Inorganic Chemistry. Longmans, London (1952). 5C D Wagner, W M Riggs, L E Davis, J F Moulder and G E Muilenberg (Eds), Handbook of X-Ray Photoelectron Spectroscopy. Perkin-Elmer, Eden Prairie, MN (1979). 6N R Armstrong and R K Quinn, Surface Sci, 67, 451 (1977). 7 C N R Rao and D D Sarma, Phys Rev, B25, 2927 (1982). 8I E Klein, A E Yaniv and J Sharon, Appl Surface Sei, 14, 351 ( 1982 83). 9 j L Murray, Bull Alloy. Phase Diagrams, 4(3), 278 ( 19833. ~0j S Solomon and W L Baun, Surface Sci, 51, 228 (1975). 1~ y W Chung, W J Lo and G A Samorjai, Surfac Sci, 64, 588 (1977). 12 A A Galuska and W O Wallace, J Vac Sci Technol, A7(I), 9 (1989). 13D H Jang and J S Chun, J Vac Sci Technol, A7, 31 (1989). ~4W A Coghlan and R E Clausing, Atomic Data, 5, 317 (1973). 15 p p j Ramaekers and R Metselaar, Br Ceram Proc Conf--Special Ceramics 8, London (1985). 16C Ocal and S Ferrer, Surface Sci, 191, 147 (1987). ~7T von Molkte and I A Kotz6, Private communication (unpublished). is j A D Matthew, Phys Scrip, T6, 79 (1983). ~9 M D Baker, N D S Canning and M A Chester, Surface Sci, !11,452 (1981).
1749
0042-207X/90S3.00 + .00 © 1990 Pergamon Press plc
Printed in Great Britain
O x i d a t i o n of a Au + 4 a t % Ti alloy P E Viljoen and J P R o u x , Department of Physics, University of the Orange Free State, PO Box 339, ZA -9300 Bloemfontein, Republic of South Africa
The gold alloy (Au +4at% Ti) was subjected to oxygen exposures at different temperatures in uhv. The surface concentration was monitored by AES and XPS. The solute Ti, segregated to the surface at temperatures above 300°C to form mixed oxides. The almost 1 : 1 ratio of Ti and 0 was a prominent feature at all temperatures. Carbon always appeared to be in the bound or carbidic form, its segregation to the surface enhancing that of 77. The appearance of an Auger peak at 293eV correlates with the presence of C and 0 in the surface region and is interpreted as due to a KVd transition where V represents valence states of the CO molecule and d a screening electron transfered from adjacent Au atoms to the C atom site. In contrast to pure gold that does not oxidise, the Au in this alloy oxidises between 500 and 600°C, forming an unstable oxide, probably Au20. The appearance of an Auger peak at 80eV during this oxidation process, is interpreted as an interband transition between Au and 0 while the shift of the 69 eV Au peak to 65 eV is the result of the chemical bonding between Au and O.
1. Introduction The alloy has been developed to serve as a jewellers' material having a high gold content, distinct gold colour, relative hardness and high corrosion resistance. Very little is known scientifically about this material. Dowben e t a l I compiled data on many gold containing alloys but no mention is made of any A u - T i alloy. Segregation of the Ti at elevated temperatures has been investigated2. From this it seems that Ti segregates to the surface where it immediately forms chemical compounds with segregated carbon and oxygen. In this paper we report on the oxidation behaviour of the sample at different temperatures and oxygen exposures.
2. Experimental The sample, supplied by Intergold, consisted of pure gold alloyed with 1 wt% or 4 at% titanium. It was cut in the form of a disc 1 mm thick having a diameter of 8 mm in order to fit the sample holder. The surface was mechanically polished using a 0.25/~m diamond spray in the final stage. It was sputter cleaned in the uhv system using 3 keV Ar ions rastered over the surface. The Ar partial pressure was 6.7 x l0 -3 Pa. The sample was resistance heated by an element fitted into the sample holder and the temperature, measured by a chrome-alumel thermocouple, was electronically controlled to within I °C. A fixed procedure was followed during each oxidation run. The sample was heated to the desired temperature where it was kept for at least 20 min before it was sputter cleaned and exposed to oxygen at base pressures ranging from 6.7 x l 0 - 7 to 1.1 x 10 -6 Pa. The surface composition was determined by AES 3 at closely spaced intervals. The acceleration voltage was only applied to the electron gun while the Auger spectra were actually being recorded. 1746
XPS data were recorded using a l0 keV, 400 W X-ray source with a Mg anode. The analyser was a double pass CMA. Spectra were recorded at room temperature after sputter cleaning and after oxidation at 580°C and cooling to room temperature. Spectra were also recorded at 580°C. No sensitivity factors were available for the A u - O (80 eV) and C - O (293 eV) peaks. In order to determine surface concentration changes from these peaks, sensitivity factors were assumed respectively as the average of the Au and O and C and O factors. Since this is an assumption, no quantitative deductions can be made from these peaks.
3. Results 3.1. Room temperature. Upon exposure to oxygen, the sputter cleaned surface showed no increase in the initial 5 at% Ti but the oxygen on the surface increased from an initial 2 at% to a maximum of 5 at% during an exposure of 100 L and remained at this level for exposures up to 500 L. 3.2. 300°C. The Ti surface concentration increased from 2 to 7 at% during a 10 L exposure to oxygen. The oxygen concentration at the surface increased from 2 to 7 at% during a 50 L exposure. Both the Ti and O surface concentrations then remained constant up to the maximum exposure. Carbon also showed an increase from 5 to 10 at% during a 10 L oxygen exposure and a further 2 at% increase at about 200 L exposure. 3.3. 500°C. The surface composition under increasing oxygen exposure with the sample at a constant temperature of 500°C is shown in Figure I. There is a steady increase at the surface in both Ti and O at an approximately equal rate up to an oxygen exposure of 100 L where the concentration abruptly levels off to remain constant at 30 at% in the case of Ti, O as well as Au.
P E Viljoen and J P Roux: Oxidation of a A u - T i alloy
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Figure i. Oxidation at 500C as measured by the surface concentration of all the elements as function of oxygen exposure. The nearly l : l ratio of the Ti and O is very obvious. M a k i n g up the rest of the surface content is C at a steady _+8%. The co-segregation of Ti a n d O was a feature at all temperatures up to 650'C. An unidentified peak at 293 eV consistantly appeared in the Auger spectra whenever C a n d ample O were present on the surface of this alloy. The profile of the c a r b o n peak was of the carbide or chemically b o n d e d form t h r o u g h o u t . The possibility of this peak being due to Ca impurity can be ruled out since it only appeared in an oxygen a t m o s p h e r e and not after heat t r e a t m e n t where one expects impurities to segregate to the surface. It has also been observed in an A1Cu alloy u n d e r similar conditions only. At oxygen exposures a b o v e 250 L the O and Ti c o n c e n t r a t i o n started increasing again with subsequent decrease in the 69 eV Au Auger peak. The peak actually split into two peaks appearing at 80 and 65 eV respectively, indicating that some oxygen had formed a chemical b o n d with the Au. It is assumed that some Ti oxides a n d even carbides may have been formed at this stage as s h o w n by XPS data taken at higher temperatures. At a t e m p e r a t u r e of 5 5 6 C the main features of surface composition, as function of oxygen exposure, seem to be repeated. 3.4. 603°C. The surface c o n c e n t r a t i o n of the different elements as a function of oxygen exposure at 6 0 Y C , Figure 2, again
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Figure 3. Auger spectra taken during the oxygen exposures are indicated. The bottom one just above but after about 15 20 min Decomposition of the Au O peak, at least
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oxidation at 60YC. The spectrum is a repeat of the with the electron gun on. in part, is clearly visible.
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follows the general pattern recorded at 500 a n d 556°C except for the C which now enriches the surface at nearly the same rate as does Ti. The surface c o m p o s i t i o n again becomes c o n s t a n t at a n oxygen exposure just in excess of 100 L. At 200 L an increase in b o t h O and A u - O ( 8 0 e V ) again correlates nearly exactly, while the pure gold peaks shrink accordingly. A b o v e 300 L an
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Figure 4. XPS spectra for Ti(2p) peaks of the sample. Spectra (a) and (b) were taken at room temperature, (a) after oxidation at a temperature of 580°C and cooling down and (b) before oxidation. Spectrum (c) has been taken at an elevated temperature (580"C). The Ti(2p) peaks for different chemical compounds are indicated. 1747
P E Viljoen and J P Roux: Oxidation of a A u - T i alloy
equilibruim is reached with Ti, C and O probably constituting the outer layer on the surface. The strong surface segregation of C at about 600C was also found in a segregation study2 of the same alloy. Auger spectra recorded at 603"C for six different oxygen exposures are shown in Figure 3 with the energies at which the most prominent peaks appear, marked by dashed lines. The carbon peak at 272 eV shows the fingerprint of a carbide form in all these spectra indicating that it is in a chemically bonded state. The peak at 293 eV shadows the carbon in all but the 2 L spectrum. The split of the 69 eV Au peak becomes prominent at an exposure of 150 L. The 80eV peak seems to grow in correlation with the growth of the O peak at 510 eV. If this peak at 80 eV indicates the formation of a gold oxide it is a very unstable oxide decomposing under electron bombardment after 15-20 rain as indicated by the last spectrum shown in Figure 3. While all the other peaks remain constant, the split in the lower energy gold peak disappears while the oxygen peak diminishes. The stoichiometry of the gold oxide can be deduced from the decrease in the Au (about 8 at%) and increase in oxygen (about 4 at%) surface concentration in Figure 3. This seems to indicate Au20 which is indeed one of the oxides formed by Au 4. The Ti peaks, in the dN/dE spectra, at 386 and 419 eV seem to indicate a mixed oxidation state TiO + TiO25. 3.5. 650°C. At this temperature there is very little change in the initial surface concentration of the elements Au (65%), Ti (14%), O (14%) and C (8%). At 200 L there is a slight increase in the O and Ti concentration with a corresponding decrease in both Au and C at the surface. There is again the striking 1 : ! ratio between Ti and O. X-ray photoelectron spectra (XPS) recorded at 580°C indicate no chemical shifts in the Au(4f) or O(ls) peaks 5 during oxygen exposure, only the expected change in peak heights. The total oxygen exposure was in excess of 300 L which was enough to reach equilibrium on the surface at this temperature. The Ti XPS peaks are shown in Figure 4. The unoxidised surface, Figure 4(b), shows the two major peaks at 460 and 454 eV binding energy. The fully oxidized surface, Figure 4(a), seems to consist of a mixture of Ti oxides, TiO at 456 eV, TiO 2 at 460 and 466 eV and Ti203 at 458 eV6 8 As a control experiment pure gold was exposed to oxygen at different temperatures. From room temperature up to 70ffC no oxidation could be observed. 4. Discussion
According to the T i - A u phase diagram 9 the Ti is in solid solution at the composition Au-4 Ti with the Au or Cu cubic structure. As was the case in a separate segregation study 2 the Ti started segregating to the surface at a temperature of 300°C. Indications that some of the Ti binds chemically to both C and O on the surface, are the carbidic form of the carbon Auger peak profile and the fact that more oxygen is accommodated on the surface at 300"C than at room temperature. The nearly 1 : 1 ratio of O to Ti on the surface maintained throughout the temperature range 300 65OC together with the XPS evidence of multiple Ti oxide formation at 580~C, confirms the deduction that Ti oxides are being formed on the surface in an oxygen atmosphere as soon as Ti acquires enough energy to segregate to the surface, that is above 300°C. This is further 1748
supported by the form of the Ti Auger peaks. Reported spectra 6, ~0 J3 shows that in our case the Ti must be in an oxidized state, most probably a mixture of Ti oxides. Comparing the XPS spectra (Figure 4) to the deconvoluted Ti (2pl/2" 3/2) XPS spectra from data by Armstrong and Quinn6, it seems likely that at 580°C the oxides TiO, Ti203 and TiO2 are all present in varying quantitites in the surface region and covering the A u - T i substrate. Although the XPS spectrum of Au did not indicate any chemical shift during oxygen exposure even at 580°C, the low energy part of the Auger spectrum (dN/dE) of Au showed the peak at 69 eV splitting into two peaks at 65 and 80 eV. Left under electron bombardment for 15-20 min, the 69 eV peak reappeared while the 510 eV oxygen peak indicated a marked decrease in the surface oxygen concentration. The 80 eV peak is most probably due to a cross or interband transition between Au and O energy levels. Assume that e.g. a hole is created in the O, level of gold (108 eV) 14. This is then filled by an electron from the L~ level (24 eV) of the oxygen atom. The energy difference ( 1 0 8 - 24 eV) is transferred to the 045 Au level which emits an Auger electron. Since the gold received an electron from another atom it is no longer in the excited state. The 045 level is at 3 eV below the vacuum level. Thus the final energy of the Auger electron is 81 eV in close agreement with the observed 80 eV. The strong surface segregation of C in an oxygen atmosphere at 603°C seems to be in line with its segregation behaviour in uhv 2. What is significant though is that it follows the Ti and O segregation rate very closely. It also seems to enhance the initial segregation rates of Ti and O at 603°C which declined on going from a temperature of 500°C (Figure 1) to 556°C. The oxidation of Ti is directly coupled to its surface segregation behaviour in the temperature range 500-603°C 2. Any one or a combination of Ti carbides might form ~5 depending on the amount of C available. The presence of ample amounts of oxygen complicates the situation. It is interesting to note that the Ti concentration increased to roughly 30 at% at all the temperatures investigated between 300 and 603°C where it remained constant. At 650°C and 300 L oxygen exposure however, the Ti together with O were still below 20 at% on the surface. Ocal and Ferrer 16 have shown that Au diffuses through a TiO 2 film. The lower surface concentration of Ti at 650°C might be explained by a situation in which Au strongly diffuses through the Ti oxide layer, formed by anion diffusion of the O through the oxide overlayer ~7. At this temperature the Au easily diffuses through the surface oxide layer to dominate on the surface. The peak at 293 eV may be due to the initial stage of the Auger process in C involving a highly localised core hole so that the valence or conduction electrons may relax to screen the holetS, ~9. The local relaxed density of states may be significantly different from the unperturbed local density of states. The effect of electronic relaxation is very clear in the KVV Auger spectrum of carbon in transition metal carbonyls ~8, the highest energy features having no analog in the free molecule Auger spectrum. These correspond to KVd transitions where V represents valence states of the CO molecule and d is a screening electron transferred from adjacent metal atoms to the 2 orbital of the carbon atom site which subsequently takes part in the Auger de-excitation. As this metal-carbon electron transfer is highly efficient in the presence of a carbon ls hole, relatively large intensities are
P E Viljoen and J P Roux: Oxidation of a A u - T i alloy
observed. We propose a similar m e t a l - c a r b o n electron transfer between Au and C in this gold alloy resulting in the peak at 293 eV.
5. Conclusions In an oxygen atmosphere Ti segregates to the surface of the Au+4at% Ti alloy at temperatures above 3 0 0 C forming mixed oxides. Ti and O collect on the surface at equal rates at all temperatures between 300 and 65OC. C segregating to the surface enhances the surface segregation of Ti. The C invariably appears to be in the carbidic or bound form. At 6 5 0 C Au covers the surface by diffusing through the initially formed Ti oxide surface layer. Pure gold does not oxidise even up to 700~C and 700 L exposure to oxygen. The Au in this alloy, however, oxidises between 500 and 600°C. Changes at or near the low energy Au Auger peak at 69 eV, correlate with the presence of large amounts of O on the alloy surface. The peak developing at 8 0 e V might be due to a possible interband transition between Au and O. The shift of the 69 eV Au peak to 65 eV is a chemical shift on oxide formation. An Auger peak at 293 eV coupled to the presence of C and O in the surface region, may be due to a KVd transition.
Acknowledgements The authors wish to thank Dr G Gafner and Intergold for supplying the samples and colleagues for helpful discussions.
Financial support by the F R D of the C S I R and C R F of the University of the Orange Free State is gratefully acknowledged.
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