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Initial oxidation of potassium at 77 and 295 K
Surf&x Science 75 f 1978) L379- L384 8 N(~rth-H~li~nd publishing Company
~NIT~ALOXlDATlONO~POTASSlUMAT77 AND295K Received 5 October 1977; fl~~nu~cri~t received in final form 1 %wcR 1978
The thickness of oxide layers on alkali and alkaline earth metals is often found to increase linearly with time during an oxygen exposure at constant pressure [ 11. This linear rate law is usually attributed to the large volume contraction upon ~xi~tion~ K20 contusing for example more than twice as many K atoms per unit volume than potassiu~l metal. A recent UPS and XPS study of the adsorption of oxygen on potassium films at 77K revealed the existence of two oxygen bonding states, attributed to potassium oxide and physisorbed oxygen, respectively [2]. At small exposures, -1 L, the oxide state is predominant, while the physisorbed state is of equal rnagnitude or larger at large exposures. The physisorbed oxygen is slowly disappearing with time in the absence of gas phase oxygen due to reaction with potassium to form the oxide. This process is accelerated with intrepid temperature. The physisorbed oxygen seems to act as a precursor to oxide formation. The present measurements of the oxidation at low pressure of I!HV evaporated K films show that at room temperature (RT) a linear rate law is obeyed for exposures above approximately 5 L. We find a quite different behaviour. however, if the metal is kept at liquid nitrogen (LN,) temperature during the oxygen exposure. A passivating oxide layer, a few monolayers thick is then formed after an exposure of around IO L. The formation of protective layers on sodium has been observed in earlier ~~l~a~~resnen~s using much higher oxygen pressures (200 Torr) than in the present experiments (10-9-10-~5 Torr) [3]. Single cystalline sodium has been observed to oxidize to Na10 at low pressures [4]. The experiments were performed in the course of our studies of the light emission during chemisorption on alkali metal films using equipment described in previous reports [5,6]. In contrast to sodium, potassium shows no chemisorptive luminescence upon exposure to oxygen in the 3000-8000 A detection range of an I 9659 photomulti~lier tube. The absence of light is probably due to a weaker binding between K and oxygen than between Na and oxygen. The mass increase upon oxygen exposure of an evaporated K film was monitored via the frequency shift, Af, of a Fiezoclectric quartz crystal microbalance. The quartz crystal was mounted in a copper block attached to a LN2 dewar. A 6 mm diam. hole exposed the crystal to the K vapour beam. Due to temperature radiation from the chamber walls. the sample temperature was raised somewhat above LN, temperature. From the geometry of the crystal assembly we have I.379
L380
B. Kasemo, L. WalldPn /Initial oxidation of potassium
calculated the temperature increase to be at most a few degrees. Interferometric calibration of one crystal from the same batch as the one used in these experiments yielded a mass sensitivity of 4.7 X lop9 g/Hz . cm2 of the 10 MHz AT cut crystal. The calibration was performed at room temperature. However, since the deposition of a metal film only caused a rigid downward shift of the frequency versus temperature curve, we assume the same sensitivity also at 77 K. Oxygen was leaked into the chamber at a rate which was kept constant during each experimental run, resulting in oxygen pressures in the range of 10-9-10-6 Torr. Due to variations in the pumping speed of the evaporated K film, the chamber walls, and the ion pump, the pressure usually rose by an order of magnitude during an experiment. As a result of pressure gradients in the chamber, the pressure readings and thus the exposure values are somewhat uncertain, the error estimated to be about 50%. The dominant residual gas in the system was Ha which probably has a low sticking coefficient on potassium even at 77 K. From the total background pressure we estimate a maximum contamination of the films before the oxygen exposure of -0.1 monolayers. Since this number assumes the unit sticking coefficient of the residual gases it is probably a gross overestimate. For a film held at 77 K the frequency shift upon oxygen exposure approaches a saturation value in the range of 40-60 Hz, independent of the pressure used (fig. 1). This frequency shift corresponds to roughly 2 X 10e7 g of oxygen per cm2 geometrical area. Assuming the oxide to be K20, the film thickness at saturation would be 50 A if the surface were perfectly smooth. The assumption of a flat surface gives an upper limit for the thickness and we therefore conclude that the oxide film on the real, rough surface consists of only few atomic layers.
5
Fig. 1. The exyosure dependence
15
10 Exposure
CL)
of the mass uptake normalized to the mass uptake at saturation as recorded by the frequency shift of a 10 MHz piezoelectric quartz microbalance. The saturation value Af- is around 50 Hz. The points are experimental values and the curve is obtained from a simple model for the nucleation and growth of the oxide layer.
B. Kasemo, L. Walldth /Initial oxidation of potassium
L381
The oxygen sticking coefficient, which is proportional to the slope of the mass uptake curve in fig. 1, increases from a low value at small exposures to a maximum value at around 5 L and then decreases again for higher exposures. The maximum slope corresponds to a sticking coefficient in the range of 0.5-l. The initial increase in sticking coefficient with exposure suggests that the oxide grows via a nucleation process. We can describe the measured mass uptake versus exposure curve using a simple model based on the assumptions that the number of nucleation centres becomes constant already at very low exposure, and that the subsequent oxidation proceeds by oxygen adsorption along the borders of the approximately circular islands. The reaction probability is thus assumed to be much larger on the border sites than on the metallic surface. If f3 is the fraction of surface area covered by oxide islands and q the exposure, we expect the mass uptake for small 0 values to be described by d9/dq = KdO
,
the length of the border line being proportional to @. In this model the high sticking coefficient on the partly oxidized surface is explained by assuming that molecules arriving on the metallic part of the surface diffuse over a distance comparable to the distance between oxide islands. At higher coverages the adsorption probability is then to be multiplied by (1 - 0) due to the decreasing metal surface area, thus giving dO/dq=K(l
-6’)+.
The close agreement between the solution 0 = (tanh i Kq)2, with K = 0.4 L-’ and the experimental points (fig. 1) gives support to the simple model. If the oxygen atoms do not diffuse on the surface one would get a factor d(l - 0) instead of 1 - 0 in eq. (2). This however gives worse agreement with the experimental curve. The observation of two 0 1s bond energies led the authors of ref. [2] to suggest that oxygen was physisorbed in molecular form on the oxide islands. If the adsorption energy of the physisorbed oxygen were sufficiently low [7], we should have observed a frequency decrease due to desorption when the experiment was terminated and the oxygen pumped away. A desorption rate of Z 10e3 monolayers/s should have been detected in our measurements, but no desorption was observed. (For comparison we mention that in a similar experiment on oxidized aluminium at 77 K desorption from a physisorbed O2 layer was easily detected.) If a physisorbed layer were formed on the oxide, it would in general give a more complex rate law than used above. If, however, the rate limiting reaction is the oxide formation and physisorption takes place very quickly when a metal site is converted to an oxide site, no change in the rate law will occur except for a change in the magnitude of the mass uptake. For a K film kept at RT the mass uptake does not saturate within the range of
L382
B. Kasemo, L. Walldh /Initial oxidation of potassium Exposure Af
0
CL)
5
IO
K+O,
0 Fig. 2. Frequency
shift due to oxygen
50 Exposure mass uptake
100 (L) versus 02 exposure
for a K sample at RT.
exposures used in the measurements (fig. 2). The constant slope above 5 L exposure shows that for the exposures (<200 L) and pressures used, the oxygen atoms diffuse into the bulk at the same rate as new atoms are adsorbed, keeping the surface concentration of oxygen constant. For low exposures the mass uptake versus exposure curve is similar to that obtained at 77 K, indicating that also at RT the oxidation begins as a nucleation process. In the linear region the slope is somewhat smaller than the maximum slope at 77 K. If a film after saturation exposure at 77 K is warmed to RT (by removing the LN, and blowing air through the dewar) and again cooled to 77 K, a new oxygen exposure results in a mass increase versus exposure curve similar to the one presented in fig. 1, but with a saturation mass uptake which is approximately half of that obtained at 77 K for a freshly evaporated K film. We attribute this behaviour to the diffusion of oxygen into the film, or of potassium to the surface, during the warming up period. Similar observations were made by Pettersson et al. [2], who observed a disappearance of the physisorbed oxygen and an attenuation of the oxide peak when the oxygen exposed film was warmed to 270 K. Some oxide is still observed at 270 K, however, which explains the smaller mass uptake during the subsequent oxygen exposure at 77 K. Earlier work on sodium single crystals has demonstrated that Na diffuses through a thin surface layer of NazO [41' Some
additional
information
on the oxidation
at 77 K was obtained
from
B. Kasemo, L. Walldtn /Initial oxidation of potassium
?I
-~-o-O-~_
\ E5 c, g 5 6
_
I
I F I3 2P \
L383
A=43008
o\ O’\ “\ o._ -o-o-o-
0
I 15
10
5
Exposure
CL)
Fig. 3. Exoelectron and photoelectric currents for a K film at 77 K as a function exposure. The photoelectric yield curve was obtained at h = 4300 A.
of oxygen
measurements of the exoelectron [5,8] and photoelectric yield changes (at h = 4300 A) during the oxygen exposure. The two quantities vary with exposure in much the same way (fig. 3). We associate the intensity decrease with increasing exposure with an increase of the work function due to the formation of a surface dipole barrier with oxygen atoms in the outermost layer. The exoelectron current shows a maximum at a low exposure value. Since the oxidation reaction is the source of the exoelectron current, we associate the maximum with the observed increase in the oxidation rate at the lowest exposures. B. KASEMO
Department Fack, S402
and L. WALLDeN
of Physics, Chalmers University of Technology, 20 Gothenburg 5, Sweden
References [l] 0. Kubaschewski don, 1962). [2] [3] [4] [5] [6]
and
B.E. Hopkins,
Oxidation
of Metals and Alloys
(Butterworths,
L.-G. Pettersson and S.E. Karlsson, Phys. Scripta, in Print. J.V. Catharth, L.L. Hall and G.A. Smith, Acta Met. 5 (1975) 245. S. Andersson, J.B. Pendry and P.M. Echenique, Surface Sci. 65 (1977) 539. B. Kasemo and L. WalldBn, Surface Sci. 53 (1975) 393. B. Kasemo and E. TGmqvist, in: Proc. 3rd Intern. Conf. on Solid Surfaces, p. 899.
Vienna,
Lon-
1977,
L384
B. Kasemo, L. Walldhz /Initial oxidation of potassium
[71 From the sensitivity,
would have been 10e3 monolayers s-l, by which desorption detected we can estimate a lower limit for the “physisorption energy” of 0.24 eV. This estimate assumes desorption from a physisorbed layer of 101’ molecules cmp2 and a frequency factor for desorption of -1013 s-l. [8] 4th Intern. Symp. on Exoelectron Emission and Dosimetry, Prague (Czechoslovak Acad. Sci., 1974).
~NIT~ALOXlDATlONO~POTASSlUMAT77 AND295K Received 5 October 1977; fl~~nu~cri~t received in final form 1 %wcR 1978
The thickness of oxide layers on alkali and alkaline earth metals is often found to increase linearly with time during an oxygen exposure at constant pressure [ 11. This linear rate law is usually attributed to the large volume contraction upon ~xi~tion~ K20 contusing for example more than twice as many K atoms per unit volume than potassiu~l metal. A recent UPS and XPS study of the adsorption of oxygen on potassium films at 77K revealed the existence of two oxygen bonding states, attributed to potassium oxide and physisorbed oxygen, respectively [2]. At small exposures, -1 L, the oxide state is predominant, while the physisorbed state is of equal rnagnitude or larger at large exposures. The physisorbed oxygen is slowly disappearing with time in the absence of gas phase oxygen due to reaction with potassium to form the oxide. This process is accelerated with intrepid temperature. The physisorbed oxygen seems to act as a precursor to oxide formation. The present measurements of the oxidation at low pressure of I!HV evaporated K films show that at room temperature (RT) a linear rate law is obeyed for exposures above approximately 5 L. We find a quite different behaviour. however, if the metal is kept at liquid nitrogen (LN,) temperature during the oxygen exposure. A passivating oxide layer, a few monolayers thick is then formed after an exposure of around IO L. The formation of protective layers on sodium has been observed in earlier ~~l~a~~resnen~s using much higher oxygen pressures (200 Torr) than in the present experiments (10-9-10-~5 Torr) [3]. Single cystalline sodium has been observed to oxidize to Na10 at low pressures [4]. The experiments were performed in the course of our studies of the light emission during chemisorption on alkali metal films using equipment described in previous reports [5,6]. In contrast to sodium, potassium shows no chemisorptive luminescence upon exposure to oxygen in the 3000-8000 A detection range of an I 9659 photomulti~lier tube. The absence of light is probably due to a weaker binding between K and oxygen than between Na and oxygen. The mass increase upon oxygen exposure of an evaporated K film was monitored via the frequency shift, Af, of a Fiezoclectric quartz crystal microbalance. The quartz crystal was mounted in a copper block attached to a LN2 dewar. A 6 mm diam. hole exposed the crystal to the K vapour beam. Due to temperature radiation from the chamber walls. the sample temperature was raised somewhat above LN, temperature. From the geometry of the crystal assembly we have I.379
L380
B. Kasemo, L. WalldPn /Initial oxidation of potassium
calculated the temperature increase to be at most a few degrees. Interferometric calibration of one crystal from the same batch as the one used in these experiments yielded a mass sensitivity of 4.7 X lop9 g/Hz . cm2 of the 10 MHz AT cut crystal. The calibration was performed at room temperature. However, since the deposition of a metal film only caused a rigid downward shift of the frequency versus temperature curve, we assume the same sensitivity also at 77 K. Oxygen was leaked into the chamber at a rate which was kept constant during each experimental run, resulting in oxygen pressures in the range of 10-9-10-6 Torr. Due to variations in the pumping speed of the evaporated K film, the chamber walls, and the ion pump, the pressure usually rose by an order of magnitude during an experiment. As a result of pressure gradients in the chamber, the pressure readings and thus the exposure values are somewhat uncertain, the error estimated to be about 50%. The dominant residual gas in the system was Ha which probably has a low sticking coefficient on potassium even at 77 K. From the total background pressure we estimate a maximum contamination of the films before the oxygen exposure of -0.1 monolayers. Since this number assumes the unit sticking coefficient of the residual gases it is probably a gross overestimate. For a film held at 77 K the frequency shift upon oxygen exposure approaches a saturation value in the range of 40-60 Hz, independent of the pressure used (fig. 1). This frequency shift corresponds to roughly 2 X 10e7 g of oxygen per cm2 geometrical area. Assuming the oxide to be K20, the film thickness at saturation would be 50 A if the surface were perfectly smooth. The assumption of a flat surface gives an upper limit for the thickness and we therefore conclude that the oxide film on the real, rough surface consists of only few atomic layers.
5
Fig. 1. The exyosure dependence
15
10 Exposure
CL)
of the mass uptake normalized to the mass uptake at saturation as recorded by the frequency shift of a 10 MHz piezoelectric quartz microbalance. The saturation value Af- is around 50 Hz. The points are experimental values and the curve is obtained from a simple model for the nucleation and growth of the oxide layer.
B. Kasemo, L. Walldth /Initial oxidation of potassium
L381
The oxygen sticking coefficient, which is proportional to the slope of the mass uptake curve in fig. 1, increases from a low value at small exposures to a maximum value at around 5 L and then decreases again for higher exposures. The maximum slope corresponds to a sticking coefficient in the range of 0.5-l. The initial increase in sticking coefficient with exposure suggests that the oxide grows via a nucleation process. We can describe the measured mass uptake versus exposure curve using a simple model based on the assumptions that the number of nucleation centres becomes constant already at very low exposure, and that the subsequent oxidation proceeds by oxygen adsorption along the borders of the approximately circular islands. The reaction probability is thus assumed to be much larger on the border sites than on the metallic surface. If f3 is the fraction of surface area covered by oxide islands and q the exposure, we expect the mass uptake for small 0 values to be described by d9/dq = KdO
,
the length of the border line being proportional to @. In this model the high sticking coefficient on the partly oxidized surface is explained by assuming that molecules arriving on the metallic part of the surface diffuse over a distance comparable to the distance between oxide islands. At higher coverages the adsorption probability is then to be multiplied by (1 - 0) due to the decreasing metal surface area, thus giving dO/dq=K(l
-6’)+.
The close agreement between the solution 0 = (tanh i Kq)2, with K = 0.4 L-’ and the experimental points (fig. 1) gives support to the simple model. If the oxygen atoms do not diffuse on the surface one would get a factor d(l - 0) instead of 1 - 0 in eq. (2). This however gives worse agreement with the experimental curve. The observation of two 0 1s bond energies led the authors of ref. [2] to suggest that oxygen was physisorbed in molecular form on the oxide islands. If the adsorption energy of the physisorbed oxygen were sufficiently low [7], we should have observed a frequency decrease due to desorption when the experiment was terminated and the oxygen pumped away. A desorption rate of Z 10e3 monolayers/s should have been detected in our measurements, but no desorption was observed. (For comparison we mention that in a similar experiment on oxidized aluminium at 77 K desorption from a physisorbed O2 layer was easily detected.) If a physisorbed layer were formed on the oxide, it would in general give a more complex rate law than used above. If, however, the rate limiting reaction is the oxide formation and physisorption takes place very quickly when a metal site is converted to an oxide site, no change in the rate law will occur except for a change in the magnitude of the mass uptake. For a K film kept at RT the mass uptake does not saturate within the range of
L382
B. Kasemo, L. Walldh /Initial oxidation of potassium Exposure Af
0
CL)
5
IO
K+O,
0 Fig. 2. Frequency
shift due to oxygen
50 Exposure mass uptake
100 (L) versus 02 exposure
for a K sample at RT.
exposures used in the measurements (fig. 2). The constant slope above 5 L exposure shows that for the exposures (<200 L) and pressures used, the oxygen atoms diffuse into the bulk at the same rate as new atoms are adsorbed, keeping the surface concentration of oxygen constant. For low exposures the mass uptake versus exposure curve is similar to that obtained at 77 K, indicating that also at RT the oxidation begins as a nucleation process. In the linear region the slope is somewhat smaller than the maximum slope at 77 K. If a film after saturation exposure at 77 K is warmed to RT (by removing the LN, and blowing air through the dewar) and again cooled to 77 K, a new oxygen exposure results in a mass increase versus exposure curve similar to the one presented in fig. 1, but with a saturation mass uptake which is approximately half of that obtained at 77 K for a freshly evaporated K film. We attribute this behaviour to the diffusion of oxygen into the film, or of potassium to the surface, during the warming up period. Similar observations were made by Pettersson et al. [2], who observed a disappearance of the physisorbed oxygen and an attenuation of the oxide peak when the oxygen exposed film was warmed to 270 K. Some oxide is still observed at 270 K, however, which explains the smaller mass uptake during the subsequent oxygen exposure at 77 K. Earlier work on sodium single crystals has demonstrated that Na diffuses through a thin surface layer of NazO [41' Some
additional
information
on the oxidation
at 77 K was obtained
from
B. Kasemo, L. Walldtn /Initial oxidation of potassium
?I
-~-o-O-~_
\ E5 c, g 5 6
_
I
I F I3 2P \
L383
A=43008
o\ O’\ “\ o._ -o-o-o-
0
I 15
10
5
Exposure
CL)
Fig. 3. Exoelectron and photoelectric currents for a K film at 77 K as a function exposure. The photoelectric yield curve was obtained at h = 4300 A.
of oxygen
measurements of the exoelectron [5,8] and photoelectric yield changes (at h = 4300 A) during the oxygen exposure. The two quantities vary with exposure in much the same way (fig. 3). We associate the intensity decrease with increasing exposure with an increase of the work function due to the formation of a surface dipole barrier with oxygen atoms in the outermost layer. The exoelectron current shows a maximum at a low exposure value. Since the oxidation reaction is the source of the exoelectron current, we associate the maximum with the observed increase in the oxidation rate at the lowest exposures. B. KASEMO
Department Fack, S402
and L. WALLDeN
of Physics, Chalmers University of Technology, 20 Gothenburg 5, Sweden
References [l] 0. Kubaschewski don, 1962). [2] [3] [4] [5] [6]
and
B.E. Hopkins,
Oxidation
of Metals and Alloys
(Butterworths,
L.-G. Pettersson and S.E. Karlsson, Phys. Scripta, in Print. J.V. Catharth, L.L. Hall and G.A. Smith, Acta Met. 5 (1975) 245. S. Andersson, J.B. Pendry and P.M. Echenique, Surface Sci. 65 (1977) 539. B. Kasemo and L. WalldBn, Surface Sci. 53 (1975) 393. B. Kasemo and E. TGmqvist, in: Proc. 3rd Intern. Conf. on Solid Surfaces, p. 899.
Vienna,
Lon-
1977,
L384
B. Kasemo, L. Walldhz /Initial oxidation of potassium
[71 From the sensitivity,
would have been 10e3 monolayers s-l, by which desorption detected we can estimate a lower limit for the “physisorption energy” of 0.24 eV. This estimate assumes desorption from a physisorbed layer of 101’ molecules cmp2 and a frequency factor for desorption of -1013 s-l. [8] 4th Intern. Symp. on Exoelectron Emission and Dosimetry, Prague (Czechoslovak Acad. Sci., 1974).