- Email: [email protected]
Dissolution of carbon into nickel through the (110) surface
Surface Science 74 (1978) 1-12 0 North-Holland Publishing Company
DISSOLUTIONOFCARBONlNTONICKELTHROUGHTHE(I1O)SURFACE F.C. SCHOUTEN, E. TE BRAKE, O.L.J. GIJZEMAN and G.A. BOOTSMA Van ‘t Hoff Laboratory,
University of Utrecht, Padualaan 8, Utrecht, The Netherlands
Received 12 October 1977; manuscript received in final form 27 December 1977
The dissolution of carbon through the Ni(ll0) surface has been studied by means of AES and LEED. Reproducible kinetic data were obtained in the temperature range 615-660 K after the crystal had been annealed at 870 K. These data are interpreted with a model in which bulk diffusion from a constant plane source just beneath the surface is rate limiting. The calculated carbon concentrations are compared with literature data on the solubility of graphite in nickel.
1. Introduction The number of AES studies of the kinetics of precipitation and dissolution of foreign atoms at solid surfaces is rapidly increasing. For example the kinetics of the precipitation process were measured for the binary systems S/Ti(OOOl) [l], P/Fe [2], Sri/Fe [3] and C/Ni [4]. Equilibrium segregation and precipitation of carbon from the bulk to the (11 l), (100) and stepped nickel surfaces has been investigated by Blakely et al. [S]. The kinetics of the dissolution of foreign atoms from the surface into the bulk has been described for the systems S/PbO [6], S/Ag(llO) [7] and C/Ni( 110) [8]. Fo: the C/Ni(l IO) system the results were reported to be reproducible only above -673 K, and in addition to dissolution of carbon into the bulk, there was segregation of sulphur to the surface. In a previous study 193 results have been reported on the kinetics of the interaction of methane with Ni(ll0) in the temperature range 473-600 K, where a Ni( 1 10)(2 X 3)-C structure was observed. At temperatures above 600 K we noted carbon diffusion into the bulk. In this paper we shall describe in more detail the kinetics of the dissolution of carbon from the Ni(1 lo)-(2 X 3)-C surface phase in the temperature range 61.5-660 K.
2. Experimental The details of the experimental set-up and the procedure used to form the Ni( 110)-(2 X 3)-C surface structure have been described previously [9].
2
F.C. Schouten et al. /Dissolution
of C into Ni through the (110) surface
The cleaning procedure was essentially the same except for the argon ion bombardment conditions. Sputtering was performed under stationary flow conditions, the argon pressure being 8 X low5 Torr, the energy of the ions 550 eV, the ion current density as measured on the crystal -1.5 ,uA cm-* and the angle of incidence 45”. After sputtering and oxygen exposure at room temperature (only if some carbon remained on the surface) the crystal was annealed at 870 K for 10 min. This cleaning procedure reduced the coverages of all contaminants below -0.01 monolayer as estimated from their AES sensitivities. The gases used, i.e. argon N50 (purity a99.999 ~01%)~methane N35 (299.95%) and N55 (>99.9995%), and oxygen N45 @99.995%) were purchased from Aga Gas B.V. In addition to the control experiments described in ref. [9J we checked whether the hydrocarbon contaminants in methane N35 (C,H, < 200 vol ppm) influenced the reaction of methane with Ni( 110). At equal exposures and crystal temperatures the same amount of carbon was deposited by methane N55 (C,H, G 0.1 vol ppm) as by methane N35. Hence the hydrocarbon contaminant, which according to the manufacturer is mainly ethane, does not influence the reaction process. Before each dissolution experiment carbon was deposited on the crystal by a methane exposure of 0.9 Torr min, the temperature of the crystal being 548 K. This resulted in a saturation coverage [9]. After evacuation of the vacuum chamber to -5 X lo-” Torr the sample was heated from the reaction temperature to the desired dissolution temperature in -30 sec. The temperature was measured with a chromel-alumel thermocouple attached to the edge of the crystal and was stabilized to within ?l K. The retarding field type AES spectrometer was adjusted as before [9]: E, = 2500 eV, i, = 25 PA, modulation voltage 10 V p-p, and an angle of incidence of the beam with the surface 8”. During each dissolution experiment the height of the carbon 272 eV peak, hc, in the dN/dE versus E spectrum was sampled every l-5 min, depending on the rate of the process. During the process the nickel 848 eV peak, hNr, used as internal reference in the calibration procedure kept the same value. Therefore only occasional scans of this peak were made, merely to check the stability of the spectrometer. It turned out that the spectrometer was stable to within 2%. The influence of the electron beam on the dissolution process was checked by changing the position of the beam on the crystal during a dissolution run. NO difference in carbon peak height was found. LEED patterns were taken on three occasions: (i) after formation of the surface carbide, (ii) during a separate dissolution run, and (iii) after dissolution of carbon into the bulk.
3
F.C. Schouten et al. / Dissolutionof C intoNi through the (110) surface 3. Results
The dissolution of carbon was studied in two different types of experiments. In the first type (section 3.1) the dissolution runs were carried out at different temperatures, and after each run the crystal was annealed at 870 K for 10 min. In the second type (section 3.2) the temperature was kept constant for all runs and between the successive dissolution runs no annealing at 870 K was performed. 3.1. Dissolution of carbon after annealing After interaction with methane at 548 K the ratio hc/hNi was determined as a function of time at different temperatures in the range 61 S-660 K. After each run the crystal was annealed at 870 K for 10 min. The results, plotted in fig. 1, show that the dissolution velocity increases with increasing temperature. For example after three hours at 618 K there was still carbon at the surface (hc/hNt = O.l), while after 30 mm at 658 K the carbon coverage was already at the detection level (h(jhNi d 0.01). In some experiments at the end of the dissolution run the crystal was cooled to 548 K in about 4 min. This caused an increase in the hc/kNi ratio of 0.02-0.09. The LEED photographs taken before dissolution showed the Ni(l10)(2 X 3)-C structure. A separate experiment was performed to determine the changes of the 1.01
1 o
618 K
I
618 K
A
629 K
x
637 K
4
6L8
0
658 K
K
0.6
0
50
100
150
200
250 t fmin)
Fig. 1. Carbon dissolution after annealing. Time dependence of the ratio of the carbon 272 eV to the nickel 848 eV peak at different temperatures. The curves are drawn according to eq. (5).
4
F.C. Schouten et al. /Dissolution of C into Ni through the [I IO) surface
Fig. 2. LEED patterns during dissolution 5S0 K,(c) 574 K,(d) 621 K.
of carbon through Ni(I IO), 1 IO eV: (a) 480 K, (b)
LEED pattern as a function of temperature. LEED patterns were taken from ambient temperature to 621 K with increments of about 50 K. The crystal was kept at each temperature for 10 min. Representative photographs are shown in fig. 2. Up to 550 K the (2 X 3) pattern with streaks at h + l/4 and h + 3/4 in the direction of constant h remains visible. At 574 K only the superstructure spots at (h, k + l/3) are visible. Finally at 621 K the superstructure spots have become invisible and the normal Ni( 110) pattern is observed. The intensity of the (0,4/3) spot was measured as a function of temperature by means of a microdensitometer. The result is shown in fig. 3. The intensity remains constant up to -550 IS and then decreases sharply, reaching zero at -630 K.
F.C. Schouten et al. /Dissolution of Cinto Ni through the (1 IO) surface
10
l
I lO,L/31 [arb unItsI
0 :I 0.5 300
I
LOO
500
-
Fig. 3. Intensity of the superstructure
600
: 10
T(K)
spot (0,4/3) as a function of temperature.
3.2. Dissolution of carbon without annealing
Starting with a well annealed crystal a series of successive dissolution experiments was performed at 637 K, interrupted by carbon deposition at 548 K. The results of five successive runs are shown in fig. 4. The curves indicate that the decrease of the hJhNi ratio becomes slower as the number of runs increases. The ratios at zero time, hJhNi (t = 0), increase in chro-
hC
'.'k'
T=637K 08
0.6
0
50
100
150
200 t(mln!
Fig. 4. Carbon dissolution at 637 K without preliminary annealing. The curves are numbered in chronological order.
6
F.C. Schollten et al. /Dissolution of C into Ni through the (110) surface
Table 1 The ratio hC/hpg (t = 0) at 637 K after an exposure of 0.9 Torr min at 548 K Experiment No. hC/hNi (t = Ot _______---~-_ _-.. 1 0.50 ? 0.02 2 0.87 3 0.91 4 0.97 5 1.03 __l_-____.__.__-__..__. __ ____ -_-.
- .-_.~-.-..-- --- --.-____--
nological order. As shown in table 1, these values increase fast between the tirst and second run and much slower afterwards. LEED patterns were taken at room temperature, after the first exposure to methane, and after the exposure following the fifth dissolution run. The photographs taken after the first exposure show the Ni( 110)-(2 X 3)-C structure. After the methane exposure following the fifth run the Ni( 1 lo)-(4 X 5)-C structure [ 1O121 was observed.
4. Discussion The dissolution process can be divided into three subprocesses: (i) decomposition of the surface compound, (ii) diffusion through the selvedge, and (iii) bulk diffusion. If the decomposition of the surface compound is rate limiting all kind of kinetic equations can be derived, the simplest one being that for first order decomposition: es(t) = cs(t = 0) exp(-kt)
,
(1)
where es(t) is the surface concentration. Lie Lag&s and Domange f7], we consider diffusion through the selvedge to be fast compared to bulk diffusion. To describe the dissolution process if bulk diffusion is rate limiting, one has to assume a relation between the surface concentration c,(t) and the bulk concentration ct,(O, t) at the boundary between selvedge and bulk (x = 0). Sparnaay et al. 161 assumed that the bulk concentration at x = 0 equals the surface concentration, c,(t) = ~~(0, t). This implies the absence of specific adsorption. However, in most solids the solubility of foreign atoms is limited and segregation does occur. Lag& and Domange [7] considered local equilibrium between the surface and the bulk at x = 0 and assumed that ~(0, t) and es(t) are related to each other by a type of segregation isotherm. Beck and Miyazaki f13] presented a general analysis of the kinetics of adsorption experiments including bulk diffusion. They took ~(0,
F.C. Schouten et al. /Dissolution of Cinto Ni through the (110) surface
I
t) to be proportional to es(t). In terms of the model of Lagues and Domange this implies a Henry type of segregation isotherm. In the other limit of the LaguEs and Domange model the bulk concentration at x = 0 has its saturation value, independent of the surface coverage. During the dissolution process ~(0, t) will then remain constant down to a certain small value of c,(t), where the supply from the surface becomes too small to keep ~(0, t) constant. This model has already been suggested by May and Germer [ 141. 4.1. Dissolution of carbon after annealing We have tried to tit the results given in fig. 1 with each of the models described above. In the model of Sparnaay et al. the contribution of Auger electrons from foreign atoms below the surface was taken into account analytically as proposed by these authors [6]. In the other models the contribution from deeper layers was assumed to be negligible, and the hc/hNt ratio was taken to be proportional to the surface coverage c,(t). The best fit was obtained for the case in which bulk diffusion is rate limiting with ~(0, t) = cb, constant and independent of time. If the initial bulk concentration can be neglected the solution of Fick’s second law for diffusion from a constant plane source is [ 1S] : cb (x, t) = cb erfc(x/2a),
(2)
where D is the bulk diffusion constant. The amount of carbon removed from the surface equals the total amount sorbed by the bulk, per unit area:
c,(t = 0) - c,(t) = j
- D(2)
x=o dt = (2/&)
c,, a
(3)
0 For adsorbed carbon the proportionality factor between the measured ho/hNt ratio and the surface coverage Bo, expressed as the number of carbon atoms per nickel surface atom, has earlier been found to be 0.42 It 0.1 [9]. However, preliminary measurements on the interaction of methane with the Ni(lOO) surface point out that this calibration factor should probably be changed: BC = (1 .O f 0.2) hc/hNi.
(4)
This is further discussed in the Appendix. If the contribution of the carbon atoms in the selvedge is neglected, or the thickness of the selvedge is assumed to be equal to one atomic layer, eq. (3) can be rewritten as: hC/hNi
(t>=hC/hNi
(f = 0) - (2/dn)cb
fi
(5)
8
F.C. Schouten
et al. /Dissolution
of C into Ni through T
(‘Cl
the (110) surface
.-
375
20,?”
350
i
L t
3%
I
-I
I
155
=c
160
16’ 1000 T IKi
Fig. 5. Arrhenius plot of c&D.
where cb is expressed in carbon monolayers per unit volume. The solid lines in fig. 1 have been calculated with this equation. An Arrhenius plot of the corresponding cb\l.Lf values is given in fig. 5. The temperature dependence of cb@ may be represented by the equation: q,dD = A exp(-E’/Rq,
(6)
with In A = 46.1 +_0.7 (A in atoms cm -’ s-r”) and E’ = 20.4 5 0.8 kcaljmole. Several papers have been devoted to the diffusion of carbon in nickel 116-261. Most studies were performed above 700°C and yield and activation energy in the range 33-40 kcal/mole. In the temperature range lOO-500°C Diamond and Wert [18] obtained an activation energy of 34.8 kcal/mole from measurements on the anelastic behaviour of Ni-C alloys. However, Massaro and Petersen [ 191 measured the diffusion of carbon-14 through N&ribbons at temperatures of 3.50-700°C and Berry j20f suggested reported the activation energy to be -20 kcal~mo~e. ~thou~ that a single activation energy applied at all temperatures there is no direct experimental evidence that the results of Massaro and Petersen should be in error. If we use the diffusivity determined by Shovensin et al. [21] : D (cm* s-‘)
= 0.13 exp(-34.5
we find the temperature
kcal/mole~R~,
dependence
(7)
of cb to be:
ct, (atoms cme3) = 2.6 X 10” exp(-3.2
kcal/mole/RT).
Alternatively, by using the diffusivity Massaro and Petersen:
as calculated
(8) from the measurements
by
F.C. Schouten
et al. /Dissolution
D (cm* s-t) = 5.2 X IO-’ exp(-21
of C into Ni through the (I 10) surface
kcal/mole/RT),
9
(9)
we find: cb (atoms cme3) = 1.4 X IO** exp(-9.9
kcal/mole/RT).
(LO)
Values calculated for cb (wt%) with eqs. (8) and (10) are given in table 2. Both alternatives give cb values close to the solubility of graphite in nickel extrapolated from data given by Natesan and Kassner [27]. It may be remarked that the carbon surface coverage at the detection level (“1 at%) is large compared to the bulk solubility (-0.005 at%), consistent with the constant plane source model. On the assumption that there is local equilibrium between carbon on the surface and in the bulk at x = 0, the energy term in eqs. (8) and (10) is interpreted as the heat of segregation of carbon to Ni(ll0). Its value in eq. (10) just equals the heat of solution of graphite in nickel [22,27 1. The observed increase of the hc/hl\ri ratio after the crystal is cooled from the dissolution temperature to 548 K is ascribed to segregation from the supersaturated region near the surface. The LEED pictures taken after exposure to methane at 548 K showed the Ni(l IO)-(2 X 3)-C structure. The decrease in intensity of the (0, 4/3) spot (fig. 3) starts around 550 K, significantly below the temperature range in which dissolution of carbon was observed with AES. Extrapolating from fig. 5 we estimate a time of about lo4 min for the dissolution of 0.1 monolayer of carbon at 550 K. Apparently around this temperature a surface phase transition takes place, i.e. the Ni(l lo)(2 X 3)-C structure decomposes to the p(1 X 1) structure with, presumably, disordered carbon. These LEED data demonstrate also that the decomposition of the Ni(l10)-(2 X 3)-C structure is not the rate limiting step in the dissolution process.
Table 2 Comparison of calculated cb values with carbon solubilities extrapolated Natesan and Kassner [27] ~IO-‘* X C&D lo3 X Cb (wt%) T(K) __-(atoms cm-* s-‘/~) Eq. (8) Eq. (10) 618 630 637 648 658
5.45 1.37 8.76 12.2 14.9
4.3 4.5 4.6 4.8 5.0
0.98 1.1 1.2 1.4 1.6
from data reported by
lo3 X solubility (wt%)
2.9 3.4 3.8 4.3 4.8
10
F.C. Schouten et al. /Dissolution of C into Ni through the (110) surface
4.2. Dissolution of carbon without annealing
The successive dissolution experiments at 637 K (fig. 4) show two distinct features, namely an increase in the initial hJh~~ value and a decrease in the apparent dissolution velocity. The increase of the initial carbon coverage (table I) is particularly evident after the first run and may be associated with an increase in defect concentration in the first layers. Such a higher dislocation density near the as-diffused surface has been observed by Bosh [28] for Ni(l1 I). The kc/kl\li ratio for the (4 X 5) structure is about 1 (table 1). If we use the calibration factor of eq. (4) this corresponds to (1.1 ? 0.2) X 1Or5 atoms/cm* or one monolayer. This agrees with the coverage reported by McCarty and Madix [29] for the same structure: (9.4 + 2.5) X 1O’4 atoms/cm*. However, in view of the fact that the (4 X 5) structure is only observed after dissohrtion of carbon into the bulk, it may extend over more than one layer. The carbon Auger signal is then no longer simply connected to carbon on the surface but also represents carbon in deeper layers. Hence the decrease in the rate of change of the kc/k,i ratio with increasing number of runs (fig. 4) need then not only imply a decrease in diffusion flux due to the increased bulk concentration, but may also, at least partially, be ascribed to attenuation of the Auger signal of carbon atoms in deeper layers. 5. Conclusion
The Ni( 1 lo)-(2 X 3)-C surface carbide structure decomposes around 550 K. The dissolution of carbon starts to take place with a measurable rate around 610 K. For well annealed crystals the kinetics of this process can be described by an equation valid for bulk diffusion from a constant plane source just beneath the surface. The strength of the source is determined by the solubility of carbon in nickel. Without annealing between dissolution runs the Ni( 1lo)-(4 X 5)-C structure is formed.
The authors would like to thank A.H.J. Huijbers for technical assistance. The investigations were partially supported by the Netherlands Foundation for Chemical Research (SON) with financial aid from the Netherlands Organisation for the Advancement of Pure Research (ZWO). Appendix Calibration
of the carbon
coverage
In a previous study the absolute coverage of carbon on Nit1 10) was determined by eliipsometry [9]. This procedure appears to be correct for the determination of
F. C. Schouten et al. /Dissolution of C into Ni through the (110) surface
11
physisorbed species, but may only give order of magnitude agreement in chemisorption f30f. An alternative method employs well defined LEED patterns as a basis for absoiute coverage determination. For example, Isett and Blakely [5d] reported on the quasi C(2 X 2) structure of carbon on Ni(lOO), which contains 1 carbon atom per 2 surface nickel atoms. After interaction of methane with the Ni(lOO) surface 1311 we obtained at saturation the LEED pattern corresponding to the same structure. For this structure the ratio hc/fiNi was 0.7 f 0.1. The AES spectrometer was adjusted as described in section 2 and ref. [9]. From these experimental data, and the coverage Bc = I/2 given by Isett and Blakely, we obtain: B&100)= (0.7 + 0.1) h($hNi.
(A.1)
In convertjng this calibration to other crystal planes one has to correct for the atomic density of the surface layer only, if the absolute heights of the Ni(848) peaks remain the same. Experimentally we found h~~(I10)/h~~(100)=(140*20)/(150~20)~
1.
Thus, neglecting Auger excitation anisotropy f32], the equation relating the fractional carbon coverage to the h&z~i ratio for the (110) plane becomes 8&110)=(1.0
+ 0.2)ho/hNi.
(A.2)
References [ I] [2] [3] [4] [S]
[6] [7] [8] ]9 ] flOJ Ill] [l2]
[ 131 1141
I.H. Kahn, Surface Sci. 40 (1973) 723. C.A. Shell and J.C. Riviere, Surface Sci. 40 (1973) 149. S. Hofmann and J. Erlewein, Scripta Met. 10 (1976) 8.57. J.F. Mojica and L.L. Levenson, Surface Sci. 59 (1976) 447. (a) J.C. Shelton, H.R. PatiJ and J.M. Blakefy, Surface Sci. 43 (1974) 493; fb) L.C. Is&t and J.M. Blakely, Surface Sci. 47 (1975) 645; (c) L.C. isett and J.M. Blakely, J. Vacuum Sci. Technof. 12 (1975) 237; (d) L.C. Isett and J.M. Blakely, Surface Sci. 58 (1976) 397. M.J. Sparnaay, A.J. van Bommel and A. van Tooren, Surface Sci. 39 (1973) 251. M. Lag& and J.L. Domange, Surface Sci. 47 (1975) 77. E.N. Sickafus, Surface Sci. 19 (1970) 181. F.C. Schouten, E.W. Kaleveld and G.A. Bootsma, Surface Sci. 63 (1977) 460. R.C. Pitkethly, in: Chem~rption and Catalysis, Ed. P. Hepple (Elsevier, Amsterdam, 1971) p. 98. G. Ertl, in: Molecular Processes on Solid Surfaces, Eds. E. Drauglis, R.D. Gretz and R.I. Jaffee (McGraw-Hill, New York, 1969) p. 117. G. Maire, J.R. Anderson and B.B. Johnson, Proc. Roy. Sot. (London) A320 (1970) 227. D.E. Beck and E. Miyazaki, Surface Sci. 39 (1973) 37; 48 (1975) 473. J.W. May and L.H. Germer, Surface Sci. 11 (1968) 443.
12
F.C. Schouten
et al. /Dissolution
of C into Ni through the (110) surface
(15) 3. Crank, The Mathematics of Diffusion, 2nd ed. (Oxford Univ. Press, London, 1975) ch. 2. [16] R.P. Smith,Trans. Met. Sot. AIME 236 (1966) 1224. [17] C.J. Smithells and C.E. Ransley, Proc. Roy. Sot. (London) Al55 (1936) 195. [18] S. Diamond and C. Wert, Trans. Met. Sot. AIME 239 (1967) 705. [19] T.A. Massaro and E.E. Petersen, J. Appl. Phys. 42 (1971) 5534. [20] B.S. Berry, J. Appl. Phys. 44 (1973) 3792. [21] A.B. Shovensin, A.H. Minkevitch and G.B. Scherbinski, Izv. Vysshikh Uchebn. Zaved., Chernaya Met. 1 (1965) 95. [22] J.L. Lander,H.S. Kernand A.L. Beach, J. Appl. Phys. 23 (1952) 1305. [23] H. Lafitau, P. Gendrel and L. Jacque, Compt. Rend. (Paris) 263C (1966) 1033. [24] V.H. Schenck,M.G. Frohberg and E. Jaspert, Arch. Eisenhuettenw. 36 (1965) 683. [25] P.L. Gruzin, 1u.A. Polickarpov and G.B. Federov, Fiz. Metal. Metalloved. (SSSR) 4 (1957) 94. [26] 1.1. Kovenskiy, Phys. Metals Metallogr. (USSR) (English Transl.) 16 (1963) 107. [27] K. Natesan and T.F. Kassner, Met. Trans. 4 (1973) 2.557. [28] J. Boah, Structural effects Involved in the Diffusion of Carbon in Nickel, Thesis, Purdue University (1973). [29] J.G. McCarty and R.J. Madix, J. Catalysis 48 (1977) 422. [30] H. Albers, W.J.J. van der Wal and G.A. Bootsma, Surface Sci. 68 (1977) 47. [31] F.C. Schouten and G.A. Bootsma, to be published. [32] G. Allie, E. Blanc and D. Dufayard, Surface Sci. 46 (1974) 188.
DISSOLUTIONOFCARBONlNTONICKELTHROUGHTHE(I1O)SURFACE F.C. SCHOUTEN, E. TE BRAKE, O.L.J. GIJZEMAN and G.A. BOOTSMA Van ‘t Hoff Laboratory,
University of Utrecht, Padualaan 8, Utrecht, The Netherlands
Received 12 October 1977; manuscript received in final form 27 December 1977
The dissolution of carbon through the Ni(ll0) surface has been studied by means of AES and LEED. Reproducible kinetic data were obtained in the temperature range 615-660 K after the crystal had been annealed at 870 K. These data are interpreted with a model in which bulk diffusion from a constant plane source just beneath the surface is rate limiting. The calculated carbon concentrations are compared with literature data on the solubility of graphite in nickel.
1. Introduction The number of AES studies of the kinetics of precipitation and dissolution of foreign atoms at solid surfaces is rapidly increasing. For example the kinetics of the precipitation process were measured for the binary systems S/Ti(OOOl) [l], P/Fe [2], Sri/Fe [3] and C/Ni [4]. Equilibrium segregation and precipitation of carbon from the bulk to the (11 l), (100) and stepped nickel surfaces has been investigated by Blakely et al. [S]. The kinetics of the dissolution of foreign atoms from the surface into the bulk has been described for the systems S/PbO [6], S/Ag(llO) [7] and C/Ni( 110) [8]. Fo: the C/Ni(l IO) system the results were reported to be reproducible only above -673 K, and in addition to dissolution of carbon into the bulk, there was segregation of sulphur to the surface. In a previous study 193 results have been reported on the kinetics of the interaction of methane with Ni(ll0) in the temperature range 473-600 K, where a Ni( 1 10)(2 X 3)-C structure was observed. At temperatures above 600 K we noted carbon diffusion into the bulk. In this paper we shall describe in more detail the kinetics of the dissolution of carbon from the Ni(1 lo)-(2 X 3)-C surface phase in the temperature range 61.5-660 K.
2. Experimental The details of the experimental set-up and the procedure used to form the Ni( 110)-(2 X 3)-C surface structure have been described previously [9].
2
F.C. Schouten et al. /Dissolution
of C into Ni through the (110) surface
The cleaning procedure was essentially the same except for the argon ion bombardment conditions. Sputtering was performed under stationary flow conditions, the argon pressure being 8 X low5 Torr, the energy of the ions 550 eV, the ion current density as measured on the crystal -1.5 ,uA cm-* and the angle of incidence 45”. After sputtering and oxygen exposure at room temperature (only if some carbon remained on the surface) the crystal was annealed at 870 K for 10 min. This cleaning procedure reduced the coverages of all contaminants below -0.01 monolayer as estimated from their AES sensitivities. The gases used, i.e. argon N50 (purity a99.999 ~01%)~methane N35 (299.95%) and N55 (>99.9995%), and oxygen N45 @99.995%) were purchased from Aga Gas B.V. In addition to the control experiments described in ref. [9J we checked whether the hydrocarbon contaminants in methane N35 (C,H, < 200 vol ppm) influenced the reaction of methane with Ni( 110). At equal exposures and crystal temperatures the same amount of carbon was deposited by methane N55 (C,H, G 0.1 vol ppm) as by methane N35. Hence the hydrocarbon contaminant, which according to the manufacturer is mainly ethane, does not influence the reaction process. Before each dissolution experiment carbon was deposited on the crystal by a methane exposure of 0.9 Torr min, the temperature of the crystal being 548 K. This resulted in a saturation coverage [9]. After evacuation of the vacuum chamber to -5 X lo-” Torr the sample was heated from the reaction temperature to the desired dissolution temperature in -30 sec. The temperature was measured with a chromel-alumel thermocouple attached to the edge of the crystal and was stabilized to within ?l K. The retarding field type AES spectrometer was adjusted as before [9]: E, = 2500 eV, i, = 25 PA, modulation voltage 10 V p-p, and an angle of incidence of the beam with the surface 8”. During each dissolution experiment the height of the carbon 272 eV peak, hc, in the dN/dE versus E spectrum was sampled every l-5 min, depending on the rate of the process. During the process the nickel 848 eV peak, hNr, used as internal reference in the calibration procedure kept the same value. Therefore only occasional scans of this peak were made, merely to check the stability of the spectrometer. It turned out that the spectrometer was stable to within 2%. The influence of the electron beam on the dissolution process was checked by changing the position of the beam on the crystal during a dissolution run. NO difference in carbon peak height was found. LEED patterns were taken on three occasions: (i) after formation of the surface carbide, (ii) during a separate dissolution run, and (iii) after dissolution of carbon into the bulk.
3
F.C. Schouten et al. / Dissolutionof C intoNi through the (110) surface 3. Results
The dissolution of carbon was studied in two different types of experiments. In the first type (section 3.1) the dissolution runs were carried out at different temperatures, and after each run the crystal was annealed at 870 K for 10 min. In the second type (section 3.2) the temperature was kept constant for all runs and between the successive dissolution runs no annealing at 870 K was performed. 3.1. Dissolution of carbon after annealing After interaction with methane at 548 K the ratio hc/hNi was determined as a function of time at different temperatures in the range 61 S-660 K. After each run the crystal was annealed at 870 K for 10 min. The results, plotted in fig. 1, show that the dissolution velocity increases with increasing temperature. For example after three hours at 618 K there was still carbon at the surface (hc/hNt = O.l), while after 30 mm at 658 K the carbon coverage was already at the detection level (h(jhNi d 0.01). In some experiments at the end of the dissolution run the crystal was cooled to 548 K in about 4 min. This caused an increase in the hc/kNi ratio of 0.02-0.09. The LEED photographs taken before dissolution showed the Ni(l10)(2 X 3)-C structure. A separate experiment was performed to determine the changes of the 1.01
1 o
618 K
I
618 K
A
629 K
x
637 K
4
6L8
0
658 K
K
0.6
0
50
100
150
200
250 t fmin)
Fig. 1. Carbon dissolution after annealing. Time dependence of the ratio of the carbon 272 eV to the nickel 848 eV peak at different temperatures. The curves are drawn according to eq. (5).
4
F.C. Schouten et al. /Dissolution of C into Ni through the [I IO) surface
Fig. 2. LEED patterns during dissolution 5S0 K,(c) 574 K,(d) 621 K.
of carbon through Ni(I IO), 1 IO eV: (a) 480 K, (b)
LEED pattern as a function of temperature. LEED patterns were taken from ambient temperature to 621 K with increments of about 50 K. The crystal was kept at each temperature for 10 min. Representative photographs are shown in fig. 2. Up to 550 K the (2 X 3) pattern with streaks at h + l/4 and h + 3/4 in the direction of constant h remains visible. At 574 K only the superstructure spots at (h, k + l/3) are visible. Finally at 621 K the superstructure spots have become invisible and the normal Ni( 110) pattern is observed. The intensity of the (0,4/3) spot was measured as a function of temperature by means of a microdensitometer. The result is shown in fig. 3. The intensity remains constant up to -550 IS and then decreases sharply, reaching zero at -630 K.
F.C. Schouten et al. /Dissolution of Cinto Ni through the (1 IO) surface
10
l
I lO,L/31 [arb unItsI
0 :I 0.5 300
I
LOO
500
-
Fig. 3. Intensity of the superstructure
600
: 10
T(K)
spot (0,4/3) as a function of temperature.
3.2. Dissolution of carbon without annealing
Starting with a well annealed crystal a series of successive dissolution experiments was performed at 637 K, interrupted by carbon deposition at 548 K. The results of five successive runs are shown in fig. 4. The curves indicate that the decrease of the hJhNi ratio becomes slower as the number of runs increases. The ratios at zero time, hJhNi (t = 0), increase in chro-
hC
'.'k'
T=637K 08
0.6
0
50
100
150
200 t(mln!
Fig. 4. Carbon dissolution at 637 K without preliminary annealing. The curves are numbered in chronological order.
6
F.C. Schollten et al. /Dissolution of C into Ni through the (110) surface
Table 1 The ratio hC/hpg (t = 0) at 637 K after an exposure of 0.9 Torr min at 548 K Experiment No. hC/hNi (t = Ot _______---~-_ _-.. 1 0.50 ? 0.02 2 0.87 3 0.91 4 0.97 5 1.03 __l_-____.__.__-__..__. __ ____ -_-.
- .-_.~-.-..-- --- --.-____--
nological order. As shown in table 1, these values increase fast between the tirst and second run and much slower afterwards. LEED patterns were taken at room temperature, after the first exposure to methane, and after the exposure following the fifth dissolution run. The photographs taken after the first exposure show the Ni( 110)-(2 X 3)-C structure. After the methane exposure following the fifth run the Ni( 1 lo)-(4 X 5)-C structure [ 1O121 was observed.
4. Discussion The dissolution process can be divided into three subprocesses: (i) decomposition of the surface compound, (ii) diffusion through the selvedge, and (iii) bulk diffusion. If the decomposition of the surface compound is rate limiting all kind of kinetic equations can be derived, the simplest one being that for first order decomposition: es(t) = cs(t = 0) exp(-kt)
,
(1)
where es(t) is the surface concentration. Lie Lag&s and Domange f7], we consider diffusion through the selvedge to be fast compared to bulk diffusion. To describe the dissolution process if bulk diffusion is rate limiting, one has to assume a relation between the surface concentration c,(t) and the bulk concentration ct,(O, t) at the boundary between selvedge and bulk (x = 0). Sparnaay et al. 161 assumed that the bulk concentration at x = 0 equals the surface concentration, c,(t) = ~~(0, t). This implies the absence of specific adsorption. However, in most solids the solubility of foreign atoms is limited and segregation does occur. Lag& and Domange [7] considered local equilibrium between the surface and the bulk at x = 0 and assumed that ~(0, t) and es(t) are related to each other by a type of segregation isotherm. Beck and Miyazaki f13] presented a general analysis of the kinetics of adsorption experiments including bulk diffusion. They took ~(0,
F.C. Schouten et al. /Dissolution of Cinto Ni through the (110) surface
I
t) to be proportional to es(t). In terms of the model of Lagues and Domange this implies a Henry type of segregation isotherm. In the other limit of the LaguEs and Domange model the bulk concentration at x = 0 has its saturation value, independent of the surface coverage. During the dissolution process ~(0, t) will then remain constant down to a certain small value of c,(t), where the supply from the surface becomes too small to keep ~(0, t) constant. This model has already been suggested by May and Germer [ 141. 4.1. Dissolution of carbon after annealing We have tried to tit the results given in fig. 1 with each of the models described above. In the model of Sparnaay et al. the contribution of Auger electrons from foreign atoms below the surface was taken into account analytically as proposed by these authors [6]. In the other models the contribution from deeper layers was assumed to be negligible, and the hc/hNt ratio was taken to be proportional to the surface coverage c,(t). The best fit was obtained for the case in which bulk diffusion is rate limiting with ~(0, t) = cb, constant and independent of time. If the initial bulk concentration can be neglected the solution of Fick’s second law for diffusion from a constant plane source is [ 1S] : cb (x, t) = cb erfc(x/2a),
(2)
where D is the bulk diffusion constant. The amount of carbon removed from the surface equals the total amount sorbed by the bulk, per unit area:
c,(t = 0) - c,(t) = j
- D(2)
x=o dt = (2/&)
c,, a
(3)
0 For adsorbed carbon the proportionality factor between the measured ho/hNt ratio and the surface coverage Bo, expressed as the number of carbon atoms per nickel surface atom, has earlier been found to be 0.42 It 0.1 [9]. However, preliminary measurements on the interaction of methane with the Ni(lOO) surface point out that this calibration factor should probably be changed: BC = (1 .O f 0.2) hc/hNi.
(4)
This is further discussed in the Appendix. If the contribution of the carbon atoms in the selvedge is neglected, or the thickness of the selvedge is assumed to be equal to one atomic layer, eq. (3) can be rewritten as: hC/hNi
(t>=hC/hNi
(f = 0) - (2/dn)cb
fi
(5)
8
F.C. Schouten
et al. /Dissolution
of C into Ni through T
(‘Cl
the (110) surface
.-
375
20,?”
350
i
L t
3%
I
-I
I
155
=c
160
16’ 1000 T IKi
Fig. 5. Arrhenius plot of c&D.
where cb is expressed in carbon monolayers per unit volume. The solid lines in fig. 1 have been calculated with this equation. An Arrhenius plot of the corresponding cb\l.Lf values is given in fig. 5. The temperature dependence of cb@ may be represented by the equation: q,dD = A exp(-E’/Rq,
(6)
with In A = 46.1 +_0.7 (A in atoms cm -’ s-r”) and E’ = 20.4 5 0.8 kcaljmole. Several papers have been devoted to the diffusion of carbon in nickel 116-261. Most studies were performed above 700°C and yield and activation energy in the range 33-40 kcal/mole. In the temperature range lOO-500°C Diamond and Wert [18] obtained an activation energy of 34.8 kcal/mole from measurements on the anelastic behaviour of Ni-C alloys. However, Massaro and Petersen [ 191 measured the diffusion of carbon-14 through N&ribbons at temperatures of 3.50-700°C and Berry j20f suggested reported the activation energy to be -20 kcal~mo~e. ~thou~ that a single activation energy applied at all temperatures there is no direct experimental evidence that the results of Massaro and Petersen should be in error. If we use the diffusivity determined by Shovensin et al. [21] : D (cm* s-‘)
= 0.13 exp(-34.5
we find the temperature
kcal/mole~R~,
dependence
(7)
of cb to be:
ct, (atoms cme3) = 2.6 X 10” exp(-3.2
kcal/mole/RT).
Alternatively, by using the diffusivity Massaro and Petersen:
as calculated
(8) from the measurements
by
F.C. Schouten
et al. /Dissolution
D (cm* s-t) = 5.2 X IO-’ exp(-21
of C into Ni through the (I 10) surface
kcal/mole/RT),
9
(9)
we find: cb (atoms cme3) = 1.4 X IO** exp(-9.9
kcal/mole/RT).
(LO)
Values calculated for cb (wt%) with eqs. (8) and (10) are given in table 2. Both alternatives give cb values close to the solubility of graphite in nickel extrapolated from data given by Natesan and Kassner [27]. It may be remarked that the carbon surface coverage at the detection level (“1 at%) is large compared to the bulk solubility (-0.005 at%), consistent with the constant plane source model. On the assumption that there is local equilibrium between carbon on the surface and in the bulk at x = 0, the energy term in eqs. (8) and (10) is interpreted as the heat of segregation of carbon to Ni(ll0). Its value in eq. (10) just equals the heat of solution of graphite in nickel [22,27 1. The observed increase of the hc/hl\ri ratio after the crystal is cooled from the dissolution temperature to 548 K is ascribed to segregation from the supersaturated region near the surface. The LEED pictures taken after exposure to methane at 548 K showed the Ni(l IO)-(2 X 3)-C structure. The decrease in intensity of the (0, 4/3) spot (fig. 3) starts around 550 K, significantly below the temperature range in which dissolution of carbon was observed with AES. Extrapolating from fig. 5 we estimate a time of about lo4 min for the dissolution of 0.1 monolayer of carbon at 550 K. Apparently around this temperature a surface phase transition takes place, i.e. the Ni(l lo)(2 X 3)-C structure decomposes to the p(1 X 1) structure with, presumably, disordered carbon. These LEED data demonstrate also that the decomposition of the Ni(l10)-(2 X 3)-C structure is not the rate limiting step in the dissolution process.
Table 2 Comparison of calculated cb values with carbon solubilities extrapolated Natesan and Kassner [27] ~IO-‘* X C&D lo3 X Cb (wt%) T(K) __-(atoms cm-* s-‘/~) Eq. (8) Eq. (10) 618 630 637 648 658
5.45 1.37 8.76 12.2 14.9
4.3 4.5 4.6 4.8 5.0
0.98 1.1 1.2 1.4 1.6
from data reported by
lo3 X solubility (wt%)
2.9 3.4 3.8 4.3 4.8
10
F.C. Schouten et al. /Dissolution of C into Ni through the (110) surface
4.2. Dissolution of carbon without annealing
The successive dissolution experiments at 637 K (fig. 4) show two distinct features, namely an increase in the initial hJh~~ value and a decrease in the apparent dissolution velocity. The increase of the initial carbon coverage (table I) is particularly evident after the first run and may be associated with an increase in defect concentration in the first layers. Such a higher dislocation density near the as-diffused surface has been observed by Bosh [28] for Ni(l1 I). The kc/kl\li ratio for the (4 X 5) structure is about 1 (table 1). If we use the calibration factor of eq. (4) this corresponds to (1.1 ? 0.2) X 1Or5 atoms/cm* or one monolayer. This agrees with the coverage reported by McCarty and Madix [29] for the same structure: (9.4 + 2.5) X 1O’4 atoms/cm*. However, in view of the fact that the (4 X 5) structure is only observed after dissohrtion of carbon into the bulk, it may extend over more than one layer. The carbon Auger signal is then no longer simply connected to carbon on the surface but also represents carbon in deeper layers. Hence the decrease in the rate of change of the kc/k,i ratio with increasing number of runs (fig. 4) need then not only imply a decrease in diffusion flux due to the increased bulk concentration, but may also, at least partially, be ascribed to attenuation of the Auger signal of carbon atoms in deeper layers. 5. Conclusion
The Ni( 1 lo)-(2 X 3)-C surface carbide structure decomposes around 550 K. The dissolution of carbon starts to take place with a measurable rate around 610 K. For well annealed crystals the kinetics of this process can be described by an equation valid for bulk diffusion from a constant plane source just beneath the surface. The strength of the source is determined by the solubility of carbon in nickel. Without annealing between dissolution runs the Ni( 1lo)-(4 X 5)-C structure is formed.
The authors would like to thank A.H.J. Huijbers for technical assistance. The investigations were partially supported by the Netherlands Foundation for Chemical Research (SON) with financial aid from the Netherlands Organisation for the Advancement of Pure Research (ZWO). Appendix Calibration
of the carbon
coverage
In a previous study the absolute coverage of carbon on Nit1 10) was determined by eliipsometry [9]. This procedure appears to be correct for the determination of
F. C. Schouten et al. /Dissolution of C into Ni through the (110) surface
11
physisorbed species, but may only give order of magnitude agreement in chemisorption f30f. An alternative method employs well defined LEED patterns as a basis for absoiute coverage determination. For example, Isett and Blakely [5d] reported on the quasi C(2 X 2) structure of carbon on Ni(lOO), which contains 1 carbon atom per 2 surface nickel atoms. After interaction of methane with the Ni(lOO) surface 1311 we obtained at saturation the LEED pattern corresponding to the same structure. For this structure the ratio hc/fiNi was 0.7 f 0.1. The AES spectrometer was adjusted as described in section 2 and ref. [9]. From these experimental data, and the coverage Bc = I/2 given by Isett and Blakely, we obtain: B&100)= (0.7 + 0.1) h($hNi.
(A.1)
In convertjng this calibration to other crystal planes one has to correct for the atomic density of the surface layer only, if the absolute heights of the Ni(848) peaks remain the same. Experimentally we found h~~(I10)/h~~(100)=(140*20)/(150~20)~
1.
Thus, neglecting Auger excitation anisotropy f32], the equation relating the fractional carbon coverage to the h&z~i ratio for the (110) plane becomes 8&110)=(1.0
+ 0.2)ho/hNi.
(A.2)
References [ I] [2] [3] [4] [S]
[6] [7] [8] ]9 ] flOJ Ill] [l2]
[ 131 1141
I.H. Kahn, Surface Sci. 40 (1973) 723. C.A. Shell and J.C. Riviere, Surface Sci. 40 (1973) 149. S. Hofmann and J. Erlewein, Scripta Met. 10 (1976) 8.57. J.F. Mojica and L.L. Levenson, Surface Sci. 59 (1976) 447. (a) J.C. Shelton, H.R. PatiJ and J.M. Blakefy, Surface Sci. 43 (1974) 493; fb) L.C. Is&t and J.M. Blakely, Surface Sci. 47 (1975) 645; (c) L.C. isett and J.M. Blakely, J. Vacuum Sci. Technof. 12 (1975) 237; (d) L.C. Isett and J.M. Blakely, Surface Sci. 58 (1976) 397. M.J. Sparnaay, A.J. van Bommel and A. van Tooren, Surface Sci. 39 (1973) 251. M. Lag& and J.L. Domange, Surface Sci. 47 (1975) 77. E.N. Sickafus, Surface Sci. 19 (1970) 181. F.C. Schouten, E.W. Kaleveld and G.A. Bootsma, Surface Sci. 63 (1977) 460. R.C. Pitkethly, in: Chem~rption and Catalysis, Ed. P. Hepple (Elsevier, Amsterdam, 1971) p. 98. G. Ertl, in: Molecular Processes on Solid Surfaces, Eds. E. Drauglis, R.D. Gretz and R.I. Jaffee (McGraw-Hill, New York, 1969) p. 117. G. Maire, J.R. Anderson and B.B. Johnson, Proc. Roy. Sot. (London) A320 (1970) 227. D.E. Beck and E. Miyazaki, Surface Sci. 39 (1973) 37; 48 (1975) 473. J.W. May and L.H. Germer, Surface Sci. 11 (1968) 443.
12
F.C. Schouten
et al. /Dissolution
of C into Ni through the (110) surface
(15) 3. Crank, The Mathematics of Diffusion, 2nd ed. (Oxford Univ. Press, London, 1975) ch. 2. [16] R.P. Smith,Trans. Met. Sot. AIME 236 (1966) 1224. [17] C.J. Smithells and C.E. Ransley, Proc. Roy. Sot. (London) Al55 (1936) 195. [18] S. Diamond and C. Wert, Trans. Met. Sot. AIME 239 (1967) 705. [19] T.A. Massaro and E.E. Petersen, J. Appl. Phys. 42 (1971) 5534. [20] B.S. Berry, J. Appl. Phys. 44 (1973) 3792. [21] A.B. Shovensin, A.H. Minkevitch and G.B. Scherbinski, Izv. Vysshikh Uchebn. Zaved., Chernaya Met. 1 (1965) 95. [22] J.L. Lander,H.S. Kernand A.L. Beach, J. Appl. Phys. 23 (1952) 1305. [23] H. Lafitau, P. Gendrel and L. Jacque, Compt. Rend. (Paris) 263C (1966) 1033. [24] V.H. Schenck,M.G. Frohberg and E. Jaspert, Arch. Eisenhuettenw. 36 (1965) 683. [25] P.L. Gruzin, 1u.A. Polickarpov and G.B. Federov, Fiz. Metal. Metalloved. (SSSR) 4 (1957) 94. [26] 1.1. Kovenskiy, Phys. Metals Metallogr. (USSR) (English Transl.) 16 (1963) 107. [27] K. Natesan and T.F. Kassner, Met. Trans. 4 (1973) 2.557. [28] J. Boah, Structural effects Involved in the Diffusion of Carbon in Nickel, Thesis, Purdue University (1973). [29] J.G. McCarty and R.J. Madix, J. Catalysis 48 (1977) 422. [30] H. Albers, W.J.J. van der Wal and G.A. Bootsma, Surface Sci. 68 (1977) 47. [31] F.C. Schouten and G.A. Bootsma, to be published. [32] G. Allie, E. Blanc and D. Dufayard, Surface Sci. 46 (1974) 188.