Analysis of surface segregation in Co-Ru alloy using Auger electron spectroscopy

Applied Surface Science 47 (1991) 333-340 North-Holland

333

Analysis of surface segregation in Co-Ru alloy using Auger electron spectroscopy P.J. G o d o w s k i Institute for Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 937, 50-950 Wroctaw, Poland Received 30 July 1990; accepted for publication 26 January 1991

The surface of C o - R u specimens of 20.7, 5.4 and 1.5 at% Ru was investigated in the temperature range of 700-1200 K by Auger electron spectroscopy. In the quantification the monolayer segregation model was adopted. Variations of the cobalt concentration with temperature at equilibrium showed segregation of cobalt with a heat Q = 19.4 _+ 3.5 kJ m o l - ; . The results are discussed in relation to theoretical predictions existing in the literature.

1. Introduction The analysis of surface segregation in alloys is of widely accepted importance in both basic science and technological studies. At present it is well known that in most cases the equilibrium surface composition of an alloy can be quite different from its bulk composition. On this topic the literature is still expanding and many articles in which simple [1-18] and more complex [19-22] rules for predicting surface segregation are described have appeared. Despite these theoretical predictions, experiments still have a crucial meaning and their results corroborate (or not) the model used (e.g. ref. [23]). For binary alloys, the most simple model of first-layer segregation gives the relation between fractional coverages and temperature:

C~/CB = ( C~/CB ) exp(QAB/RT ), s

s

b

b

(1)

where the C,S'b are the respective surface and bulk concentrations of the solute A or the solvent B and QAB is the heat of segregation of A in B. Using the above equation and experimental values of C,~'b, QAB can be determined and compared with theory. Such procedure requires that detailed and accurate measurements be made in a wide and proper temperature range. Among the many

surface analysis techniques, Auger electron spectroscopy (AES) is considered as the most suitable for studying the phenomena. In order to obtain reliable quantitative information, requisite experiments must be performed, which may prove an extremely difficult task [24-26]. Investigations of the surface composition of cobalt alloyed with another Group VIII metal were reported in various papers [27-39]. Auger analysis of i r o n - c o b a l t alloy by Bevolo [27] showed a slight depletion of the surface iron concentration as compared with the bulk values. Several works have been done on the C o - N i alloy [28-37]. Cherepin et al. [28] established an enrichment in Ni on the sample surface after heating at 775 K for 1 h, b o m b a r d m e n t with Ar + ions and a subsequent cool down to room temperature (RT). Tanaka et al. [29] suggested that no surface segregation takes place in the C o - N i system after heating at 770 K for 1 h, at 850 K for 1 h and at 1200 K for 10 s. It was shown [30-32] that the sample had to be kept at a temperature around 0.7Tm, where Tm is the melting point, to attain surfacebulk equilibrium within reasonable limits of measuring times. The behaviour of nickel was accepted as that of a segregating component, i.e. nickel atoms and dissolved in the bulk at high temperatures. Hajcsar et al. [33,34] found that Ni segre-

0169-4332/91/$03.50 © 1991 - Elsevier Science Publishers B.V. (North-Holland)

334

P.J. Godcmski / Surface segregation in Co-Ru alloy

gates in Co Ni alloys of 10% 90% Ni and Wandass and Turner [35] stated that the relative atomic amount of Co was always lower than that of Ni. Finally, Kurokawa et al. [36,37] suggested that no marked surface segregation exists on a fresh C o - N i surface and on one that is bombarded by Ar +. They proposed the C o - N i alloy could be a standard sample for surface chemical analysis. The Co Rh alloy was investigated by Ellison et al. [381 and their AES results in the temperature range from 623 to 1123 K agreed with the prediction of Abraham et al. [6]. For cobalt-rich alloy they obtained an increase in rhodium concentration as compared with that of the bulk while an increase in cobalt concentration for rhodium-rich alloy. Bardi et al. [39] examined the surface of a Pt 20%Co single crystal and reported no evidence of enrichment of the surface in either constituent. The binary C o - R u system is an ideal system for segregation studies since it forms a solid solution over the whole range of bulk phase composition [40]. In addition, the study has a practical interest, because C o - R u catalyst are frequently used in industry [41]. Preliminary results for contaminated surface of Co-5.4at%Ru and C o 20.Tat%Ru and the influence of phosphorus and sulphur on the surface content, have been presented [42,43]. The present study is concerned with surface segregation in Co Ru polycrystalline samples of three different compositions: 20.7, 5.4 and 1.5 at% of Ru (all compositions quoted are in atomic percent). Following most previous experimental studies, AES was used for the measurement of surface composition.

2. Experimental The polycrystalline alloys were prepared from powders of high purity (Johnson-Matthey Chemicals, Ltd.) at our Institute. The procedure of sintering and homogenization has been described elsewhere [42]. Specimens of dimensions about 10 m m × 10 m m × 0.8 mm were cut from ingots of the alloy, mechanically polished using diamond paste and washed in acetone just before introducing into the camera. Each alloy sample was analyzed in a separate experiment and mounted

on the sample holder together with a piece of high-purity cobalt foil (giving the standard Auger spectrum only). Heating of the sample was achieved by applying a DC current to the holder system specially constructed for this reason. The sample temperature was measured with an accuracy of _+5 K by a P t / P t R h thermocouple spotwelded on the alloy sample. The study was performed in a home-made ultra-high vacuum apparatus working on an ion PZK-100 p u m p and a sublimation p u m p PST-1000 [30]. A residual gas pressure of 5 x 10 ~ Pa could routinely be obtained in the camera and the partial gas pressure could be monitored by a Riber QS quadrupole mass analyzer. The L E E D / A E S spectrometer having a four-grid assembly with radii of curvature of 54.5 mm (first grid) - 70 mm (screen/collector) and a solid acceptance angle corresponding to a cone with an aperture of about 120 ° , was connected to the chamber. The spectrometer was supplied with a coaxial electron gun working in the energy range of 0.03 to 3.0 keV. The Auger electron spectrum (dN/dE) from the clean surface of cobalt in the 0 1000 eV range consists of two sets of peaks [44]: the low-energy (M2,:~M4,sM4, 5, 53 eV), (M1M4,sM4,5, 98 eV) and the high-energy ones (L2,~M1M2, 3, 620 eV), (L2.3M2,3M2.3 , 656 eV), (L23M2.3M4. 5, 716 eV), (L2,3M4,sM4.5, 775 eV). The spectrum obtained from pure ruthenium shows: the low-energy peak - (N2,3N4,sN4, 5, 37 eV) and the medium-energy Auger peaks (M4,sNjN4.5,208 eV), (M4,sN:3N4. 5, 238 eV), (M4,sN4,sN4,s, 280 eV). There is some probability of peak overlap in the low-energy transition of the alloy (Ru(37 eV) and Co(53 eV)). The presence of peak overlap can introduce an error in the determination of the Auger current using peak-to-peak ( p / p ) heights and in consequence complicates the procedure, usually employed in quantification by AES. From the spectra catalogue [44] the following quantities could be derived: o(Ru37) = 3 eV, h°(Ru37) = 79 and o(Co53) = 4 . 6 eV, h°(Co53) = 30, where o denotes the band parameter and h ° is the p / p height of pure elements in arbitrary units (here in m m with the scale factor). Under the assumption of equal peak heights, an error in p / p height measurements of the Co53 transition of less than

P.J. Godowski / Surface segregation in Co-Ru alloy

2% could be read from figure 4b of ref. [45]. The relative width of the peaks is o(Co53)/o(Ru37) = 1.5 and the relative separation { [ E ( C o ) - o(Co)] -[E(Ru)-o(Ru)]}/o(Ru)=4.8, where E denotes the high-energy peak minimum. For alloys with the quoted bulk compositions, the p / p height ratio h(Co53)/h (Ru37) should be greater than 1.5 and this seriously reduces the error to below the detectability limit [46]. Auger spectra were obtained by exciting the sample with a 2.5 × 10 - 6 A beam of 2.5 keV electrons incident normally to the surface. The diameter of the beam at the sample position was 0.2 mm giving a dissipated power density of 2 x 1 0 6 W m 2. The operating conditions although not optimal for the use of low (Co53) and medium (Ru238) energy Auger lines, warranted continuous and simultaneous monitoring in a wide spectral range. The spectra obtained in these standard conditions were used also for another purpose of laboratory work so these reasons prevail in searching for slightly better but considerably different circumstances. A typical Auger spectrum of the alloy has been presented elsewhere [43]. For recording d N/d E Auger spectra, a modulation amplitude of 4 Vp_p for low energy and 13.4 Vp_p for medium energy was used. As a result of this choice, the Ru238 peak was acquired at a sensitivity about two orders of magnitude greater than that of the Co53 peak (including a change of the modulation amplitude). The Ru37 Auger transition, noticed only for the Co79Ru21 sample, did not prevent the p / p height measurements of Co53. The background correction was determined at the beginning of the experiment and was added to each p / p height taken from the spectra. Cleaning of the alloy surface was done by K + ion bombardment (500 eV, 10 × 10 - 6 A , 7 5 0 K ) from a zeolite source of a simple construction [47]. Preliminary studies [42,43] have shown that the alloy surface contains sulphur and phosphorus. These impurities present as trace elements in the bulk could translocate cobalt atoms from the topmost layers. After the impurities had been depleted from the surface region, the samples were vacuum-annealed at 700 K for 50 h to attain equilibrium. The followed measurement scheme was as in usual segregation experiments. The sam-

335

Weight Percent Ruthenium 0 10 20 30 40 50 60 70 2600 t

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Fig. 1. Phase diagram of C o - R u alloy: a is face centred cubic and ~ is low-temperature hexagonal phase. The lattice parameters of the c phase lie on a straight line connecting the constants of the components.

ple was heated at a given temperature and after an appropriate interval the Auger spectrum was recorded. Measurements were stopped when no change of the signal intensity (i.e. p / p height in dN/dE) with time was noted. Next, the temperature was increased (from about 100 K) and the same procedure was repeated. The maximum sample temperature was limited by the partial pressure of cobalt vapour, for metallic Co at 1170 K it is 6.7 × 1 0 - 7 Pa [38]. Fig. 1 shows the phase diagram of cobaltruthenium alloy [40]. It should be discussed from a consideration of two allotropic forms of pure cobalt, i.e. hexagonal close packed (hcp) and face centred cubic (fcc). The transformation from hcp to fcc is taking place at a temperature T, around 695 K (left side of the graph) with a heat of 252 J mo1-1 and a volume expansion of about 0.3%. With increasing content of ruthenium, Tt increases and a mixed hcp and fcc high-temperature field is formed. The remaining part of the diagram contains hcp structure of the alloy. From the phase diagram it is seen that no ordering tendency exists for this case. According to ref. [401 above 1270 K the boundaries are exact equilibrium curves and the phase relations above 1500 K are hypothetical. The temperature range for the samples used in the experiments are marked in fig. 1. It is concluded that the 1.5 at% Ru alloy corresponds

P.J. Godowski / Surface segregation m Co-Ru allqv

336

to the fcc bulk phase of the alloy, the 5.4 at% Ru to the mixed fcc and hcp phases and the 20.7 at% Ru precisely to the hcp phase. Such choice of the

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Temperature [ K ] Fig. 2. Temperature dependence of the Auger peak intensities (peak-to-peak heights in dN/dE mode) from Co Ru alloy: (a) Co-20.7Ru, (b) Co-5.4Ru, (c) Co-l.5Ru. The Auger data are presented in arbitrary units which are taken to be the same for both peaks, so relative sensitivity is preserved.

P.J. Godowski / Surface segregation in Co-Ru alloy 3. Results and discussion

The p / p heights of Co53 and Ru238 Auger transitions as a function of temperature are shown in fig. 2 for all three samples of the alloy. The attenuation lengths (the geometrical relation to the escape depths) of 53 and 238 eV are 0.26 and 0.48 nm (or 1.2 and 2.2 monolayers) [48], respectively. Despite the existing discrepancy in analyzing volume both transitions show suitable surface sensitivity because the peaks display changes in intensity for different sample temperatures. The p / p heights of the Auger signals for all samples vary in the same manner: the cobalt signal falls with temperature while the ruthenium signal increases. Such behaviour is classified as cobalt segregation with a positive Q value in eq. (1) [49]. Based on similar considerations as in ref. [25], the temperature interval of 900-1100 K was taken as the interval in which thermodynamic surface-bulk equilibrium was achieved. Remaining areas correspond to diffusion-limited and evaporation-limited regions. To determine the amount of surface segregation the appropriate model must be chosen. The model which assumes that the concentration difference is restricted to the outermost atomic layer was used

337

here and adopted for quantitative Auger analysis. It is believed that it is the best case for polycrystalline alloys. For a homogeneous sample in the sampled volume, the recorded Auger signal, I, can be written as [50]: I = anN~(1

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where a is the sensitivity coefficient of the element for a given Auger transition, n the number of effective electrons which can cause ionization of atoms (it can be considered as a constant in the first few atomic planes), N the number of atoms of the element by unit volume, k the attenuation coefficient of Auger electrons by an atomic plane of the matrix: k = e x p ( - 1 / D ) , where for the retarding-field analyzer the attenuation length D =0.74X, and X is the escape depth of Auger electrons of a given energy. For a sample composed of several components A, B .... having concentrations Cb, cb, . . - (in a substitutional solid solution £C~b = 1), the Auger signal corresponding to element A is: /A(Cb)=

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P.J. Godowski / Surface segregation in Co-Ru allqv

338

In the above formulae it is to be remarked that n, N and k depend on so-called matrix effects, i.e. on different scattering properties of the solid solution from the pure element. The approximation on the right side of the equation is very useful as an external calibration, where I ° denotes the Auger intensity for a bulk sample from a pure element. In the case of a discrete model, the term 1/(1 - k) is a result of the sum of geometrical series and each addend represents a contribution of successive layers to the Auger signal. For example, making a separation between contributions from the first layer and the rest of the bulk in a h o m o g e neous sample (the same concentrations) we have:

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The superscripts to k denotes matrix dependence. Using eq. (5) the concentration can be calculated taking (or not) matrix corrections into consideration. For each alloy under investigation the average bulk density ()qC,hp,) and corresponding monolayer thickness were calculated [48]. Values of k were determined using Seah's expression for 2, [48] and I ° was taken as a signal from the standard sample of pure cobalt. Using the data from the range of 9 0 0 - 1 1 0 0 K (corresponding to the t h e r m o d y n a m i c b u l k - s u r f a c e equilibrium), the concentrations of cobalt in the first atomic layer, C?o, were calculated from eq. (5). The logarithm of the ratio C~o/(1 - C~o ) is plotted in figs. 3a 3c as a function of reciprocal temperature for the alloy of three different contents. F r o m a leastsquares fitting of the straight line through each set of data the heat of segregation of cobalt was obtained from the slope. These are collected in table 1 and the average is determined. The Q-values are in good agreement which denoting that the heat of segregation is independent of surface composition and of phase transitions. The main problem is that the polycrystalline

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3. In[C~,,/(1-C~,o) ] versus I / T plots for the Co Ru alloy: (a) Co 20.7Ru, (b) Co-5.4Ru, (c) Co 1.5Ru. The data were fit by the regression line. Fig.

samples can yield segregation information difficult to interpret. The heat of segregation observed on a monocrystalline alloy is usually presented as a Table 1 Heats of segregation for a ruthenium solut in cobalt determined from the slopes of figs. 2a-2c Alloy (at%)

Q (kJ tool ~)

Co-20.7Ru Co-5.4Ru Co-l.5Ru

19.1 _+3.5 22.2_+3.5 16.8 + 3.5 19.4 _+3.5 (average)

P.J. Godowski / Surface segregation in Co-Ru alloy

sum of two components: A H and AS, i.e. the enthalpy and excess entropy. However, the Q obtained here has a great practical significance in projecting of catalysts - for a given bulk content the equilibrium surface composition can be readily evaluated. Comparison with predictions existing in the literature should be restricted to the cases in which the orientation is not taken into account. Cobalt has a lower melting point (Co, 1768 K; Ru, 2523 K) but a smaller metallic radius (Co, 0.209 nm; Ru, 0.238 nm) which gives opposing effects in the light of simple criteria. The heat of sublimation at 1000 K for cobalt is AHs(Co ) = 404 kJ mo1-1 and for ruthenium A H s ( R u ) = 6 2 9 kJ mo1-1. Applying the model of Abraham et al. [6], i.e. the c * - o * representation, where c* is the bond strength ratio and o* the atom size ratio, to the data above for solute Ru in matrix Co we have c * = 1.20 and a * = 1.14. These values put into fig. 6 of their paper suggest that surface segregation of Ru should occur. The values put in Seah's formula [9] of the enrichment ratio/3 give/3 < 1 (/3 = 0.3 at T = 1000 K), i.e. depletion of Ru in the surface layer. Another prediction, based on Mezey and Giber's papers [14,15] ( [ A r : * l =0.14, 82* = - 0 . 2 0 ) shows that cobalt will segregate to the surface of C o - R u alloy. Mukherjee and MoranLopez [16,17] using a tight-binding electronic theory predicted segregation in a large number of binary alloys. For cobalt-solvent they predict that on the surface of C o - R u alloy enrichment of solvent should be expected. Ossi [18] compared predictions of surface enrichment from an atomistic first-principles electron theory based on a tight-binding Hamiltonian with those from regular solution models. It can be seen from table 1 of this work [18] that cobalt is the segregation element is this system. The presented experimental results agree with the Mezey and Giber predictions, with Seah's formula and with the Mukherjee and Moran-Lopez theory.

4. Conclusions The following conclusions can be drawn: (1) Thermodynamic equilibrium in the C o - R u alloys results in that the cobalt surface concentra-

339

tion slightly exceeds the bulk one (i.e. the surface is enriched in the solvent). (2) At temperatures higher than 900 K, the excess of surface content can be preserved while at lower temperatures it can be equal to or greater than that at 900 K depending on the ambient temperature or alloy pretreatment. (3) Results of quantitative Auger electron spectroscopy agree with most of the models presented by various authors. (4) The heat of segregation of the C o - R u alloy was determined as 19.4 _+ 3.5 kJ mol 1.

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P.J. Godowski / Surface segregation in Co-Ru alIQv

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[40] M. Hansen, Constitution of Binary Alloys (McGraw-Hill, New York, 1965). [41] W.N. Bieljackij, E.S. Shpiro, O.P. Tkaczenko, J. Rudny, G.W. Antoszin and C.M. Minaczew, Tiezisi Dokladow Wsiesojuznogo Sowierszanija (Nauka, Moscow, 1984) p. 27. [42] P. Godowski, Surf. Sci. 200 (1988) 260. [43] P. Godowski, Mater. Sci. Eng. 100 (1988) L19. [44] L.E. Davis, N.C. McDonald, P.W. Palmberg, G.E. Riach and R.E. Weber, Handbook of Auger Electron Spectroscopy (Physical Electronics Industries, Edina, MN. 1976). [45] P.J. Godowski and K. Przybylski, Vacuum 39 (1989) 439. [46] K. Przybylski and P.J. Godowski, Chem. Anal. 1990, in press; copies will be available from the authors. [47] P. Godowski and S. Mr6z, Thin Solid Films l l l (1984) 129. [48] M.P. Seah, Surf. Interf. Anal. 1 (1979) 1. [49] R.P. Gupta and B. Perraillon, Surf. Sci. 103 (1981) 397. [50] J.P. Langeron, L. Minel, J.L. Vignes, S. Bouquet, F. Pellerin, G. Lorang, P. Allioud and J. LeHericy, Surf. Sci. 138 (1984) 610.