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Hydrogen adsorption on (110) tungsten at 30 k: an atom-probe field-ion microscope study
451
Surface Science 147 (1984) 451-465 North-Holland, Amsterdam
HYDROGEN ADSORPTION ON (110) TUNGSTEN PROBE FIEND-ION MICROSCOPE STUDY A.T. MACRANDER
and D.N. SEIDMAN
Cornell Uniuersrry, Deparrment of Materials Center, Ithaca, New York 14853, USA Received
27 March
AT 30 K: AN ATOM-
1984; accepted
Science and Engineering
for publication
and the Materials
Science
31 July 1984
Adsorption of hydrogen on [IlO] oriented tungsten specimens has been studied via field desorption in an atom-probe field-ion microscope. Adsorption was studied on surfaces prepared by field evaporation. Typically, hydrogen field desorption events were exhausted after seven tungsten (110) planes had been field evaporated. Exposures were controlled by applying a potential to the specimen which was sufficient to field ionize hydrogen gas molecules approaching the specimen and which thereby prevented additional hydrogen from being adsorbed. An adsorption model which invokes sticking at step sites but not at plane sites is proposed to explain the coverage data.
1. Iu~~uction The adsorption of hydrogen on metals has been investigated by many workers via a variety of techniques [l]. It is a topic of interest to many researchers because of the many technological processes involving hydrogen. As an example we note the importance of hydrogen for the first wall of fusion power reactors 121. Furthermore, hydrogen effects appear in many experiments conducted in ultra high vacuum because hydrogen is such an ubiquitous element in stainless steel ultra high vacuum systems. Field emission studies of the hydrogen on tungsten system have provided unique information on surface diffusion by direct field emission imaging of the adsorbate covered surface [3] and more recently by determination of the surface diffusion coefficient via the field emission current fluctuation method [4]. Field emission and field ion studies both employ a special specimen geometry, namely, a needle-like specimen which is often referred to as a tip.
* Research Performed while at Cornell University; now at AT&T Bell Laboratories, Murray Hill, New Jersey 07974, USA. ** Research performed while at Cornell University; on a leave-of-absence at the School of Applied Science, Hebrew University, Jerusalem, Israel.
0039~6028/84/$03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
452
A. T. Macrander,
D.N. Seidman / Hydrogen on (110) tungsten
Such a specimen is prepared by field evaporation and has a very regular, well characterized, reproducible, and clean surface. For these reasons such specimens are ideal for adsorption studies. A recurring surface hydrogen concentration has been reported in field evaporation studies of tungsten, and it was found that exposures of specimens cleaned by field evaporation could be controlled by adjusting the potential [5]. The results of experiments measuring the coverage of hydrogen on [110] oriented tungsten specimens as a function of the exposure are reported here. The temperature of these experiments was found to be a very important parameter. We conducted our adsorption and subsequent desorption experiments at a specimen temperature of 27-31 K and attempted them at higher temperatures (76-84 K). At the lower temperatures the number of W(110) planes required to exhaust hydrogen field desorption events, that is, to clean the specimen of hydrogen, was found to be roughly seven, but at the higher temperatures roughly twenty planes were required. Rendulic and Leisch [6] have reported that at 80 K hydrogen is still not completely exhausted after ten atomic planes have been field evaporated, and our measurements have corroborated their finding. We have concentrated on the low temperature case, and we report here our results for the adsorption and subsequent field desorption of hydrogen on W(110) field ion specimens at - 30 K. 2. Experimental
details and mass spectra
The experiments were carried out in an atom-probe field-ion microscope [7,8]. The instrument has a straight flight tube and is computer controlled. We employed field evaporation rates of a few ions per second. Although such rates are smaller than field evaporation rates conventionally used in atom-probe experiments, data collection was nevertheless rapid because data collection was done by computer. Typically, several thousand events were collected per hour. The cold finger of the microscope was cooled in a continuous liquid helium transfer arrangement, and the temperature was controlled to within 2 K. Temperatures were measured using a two lead platinum resistance thermometer. Specimens were prepared from 0.13 mm diameter Materials Research Corp. VP grade polycrystalline wires with the wire axis parallel to the [110] direction. Wire segments were sharpened by electropolishing them in NaOH. Specimens were imaged via field ionization of helium gas and were microscopically polished by field evaporation until they had a very regular end form. Sample purity was determined directly from time-of-flight mass spectra. A histogram of the number of events versus their mass-to-charge ratio (m/n) is shown in fig. 1 for the combined data for six exposures. These data are typical, and the resulting frequencies of occurrence expressed as percentages of the total number of ions collected are given in table 1.
A. T. Macrander, D.N. Seidman / Hydrogen on (I IO) tungsten
453
We note now a few noteworthy observations concerning the mass spectra. The occurrence of the ion species W*+ and W5+ has not been commonly reported, and these may be related to the presence of hydrogen. We also observed a shift in the W *+ peak to m/n values less than the expected value of 93. This shift has been attributed to the dissociation of (WH)3’ into H’ and W *+ some distance above the specimen surface [9]. Miiller and Krishnaswamy observed the occurrence of W 5+ events but at higher evaporation rates than we employed [lo]. We have neglected the occurrence of tungsten hydride field evaporation products. These were not resolvable in the mass spectra because of energy deficits. Nevertheless, the copious occurrence of H+ events indicates that tungsten-hydride ions were dissociated during the process of field evaporation. We note that Kapur and Mtiller [ll] in experiments using a high resolution magnetic sector-type atom-probe found no stable hydrides of tungsten during slow field evaporation albeit at a higher temperature (78 K). Table 1 Combined
mass spectrograph
results of six hydrogen
Ion
Fraction
H+ He+ Ws+ W4+ W’+ W2+ Other
6.14 0.25 0.09 15.66 76.61 0.34 0.31
0
20
40 MASS-TO-CHARGE
Fig. 1. Mass spectrum of the events detected exposures were combined.
adsorption
experiments
of total (W)
60 RATIO
subsequent
80
100
to hydrogen
exposures.
(AMU)
The data for six
454
A. T. Macrander, D. N. Seidman / Hydrogen on (I IO) tunpren
Adsorption experiments were conducted by exposing specimens to hydrogen which had been backfilled into the vacuum chamber of the microscope. After the specimen had been inserted and after the microscope had been baked at 120°C for 12 h, base pressures were (2-5)X 10-l’ Torr as read by a Bayard-Alpert ionization gauge which had been calibrated for nitrogen. After imaging with Matheson research purity helium and after the pressure in the microscope was pumped into the lo- lo Torr range, Matheson research purity hydrogen was admitted into the microscope to a gauge pressure of - 2 x 10 * Torr. Subsequently, the microscope was again pumped into the 1O-‘0 Torr range, and the adsorption measurements were made. Via a residual gas analyzer the ambient in the microscope during the adsorption experiments was found to be - 3% helium and 97% hydrogen.
3. Method used to make exposures Just prior to an exposure specimens were pulse-field evaporated until hydrogen events were exhausted. Hydrogen had invariably been adsorbed from the background during the preparatory stages of an experimental run, and it was necessary to field desorb all of this hydrogen in order to have an adsorbate free surface to begin with. We found that specimens could be shielded from a hydrogen ambient by applying a positive potential less than that needed to field desorb any adsorbed hydrogen. The results of experiments in which the potential was systematically varied are shown in fig. 2. For this specimen, all hydrogen gas molecules impinging on the specimen were found to be field ionized at potentials greater than 7.4 kV. By scaling to the known evaporation field of W(llO), 5.5 V/A [12], we obtained a value of 3.8 V/A for the field required to completely shield the specimen. We note that this is larger than the best imaging field of hydrogen, 2.2 V/A [13]. Exposures were controlled by varying the potential of the specimen. After several (110) planes had been field evaporated to prepare an adsorbate free surface, the potential was simply allowed to decay to the ground potential to initiate the exposure, and exposures were terminated by increasing the voltage to values above that required for complete shielding but below that needed to field desorb the adsorbed hydrogen.
4. Field desorption analyses After completing an exposure both the DC and pulse (fixed to be 0.1 of the DC) voltages were slowly raised and the specimen was slowly field evaporated until no more hydrogen events were detected, that is, until all the hydrogen
A. T. Macrander, D.N. Seidman / Hydrogen on (IIO) tungsten
455
that had been adsorbed was completely field desorbed. During the analyses the probe hole was positioned directly over the (110) pole, and a stepped behavior was observed in plots of cumulative number of tungsten events versus time. Such steps are shown in fig. 3 and result from the fact that the atoms at the steps of the specimen undergo field evaporation causing the disc shaped planes of atoms to collapse progressively. Hydrogen field desorption data were collected in primary form as histograms of the number of hydrogen events detected during the complete field evaporation of a plane. Histograms of the number of field desorbed hydrogen events detected per plane versus the number of that plane are shown in fig. 4 for six exposures of 200 EXPOSURE
TIME = 5 MINUTES
IO
I
DC POTENTIAL DURING ADSORPTION ( KILOVOLTS) Fig. 2. The number of hydrogen events detected subsequent to a 5 min exposure as a function of the DC potential on the specimen. Complete shielding was obtained at 7.4 kV.
456
A. T. Mocrander,
D.N. Seidmnn / Hydrogen on (110) tungsten
CUMULATIVE
NUMBER
OF
EVAPORATION
PULSES
(bl FIM
SPECIMEN
PROBE
HOLE
CHEVRON DETECTOR
ION
Fig. 3. (a) A portion of a plot of the field-evaporation data. The ordinate is the cumulative number of ions detected with mass-to-charge ratios in the range 40 to 70 amu. Almost all the tungsten ions detected were either W3+ or W4+ and thus were included in this range. The abscissa is the cumulative number of field-evaporation pulses applied to a specimen. (b) A schematic illustration of the three stages of collapse of a (110) plane corresponding to (a).
RADIUS OF TOP
EXPOSURE
PLANE
= 40%
TIME
(MINUTES)
NUMBER
OF W(110)
Fig. 4. Atom probe data: exposures are shown.
LAYERS
the number
FIELD
of hydrogen
EVAPORATED
events detected
per (110) plane.
Data
for six
A. T. Macrander, D. N. Seidman / Hydrogen on (I 10) iungsten
457
the same specimen. Exposure times refer to the period during which the specimen was at ground potential. Since some time was required for the potential to decay from the complete shielding value at the start of an exposure and for the potential to reach the complete shielding value at the end, some
l.O-
/
0
0
8 :
2 W > 0.5-
z
RADIUS OF TOP PLANE = 36 i
EXPOSURE
TIME (MINUTES 1
Fig. 5. Atom probe data: the hydrogen coverage the top plane of the specimen was 36 A.
as a function
of the exposure
time. The radius of
0
1.0
-
RADIUS OF TOP PLANE = 40 i
0
5 EXPOSURE
10
15
TIME (MINUTES)
Fig. 6. Atom probe data: the hydrogen coverage as a function of the exposure time-for the same specimen as in fig. 5. Due to field-evaporation of the tungsten specimen atoms in previous runs the radius of the top plane had been increased to 40 A.
458
A. T. Macrander,
D.N. Seidman / Hydrogen on (110) tungsten
events were detected even for the null exposure. The results of the null exposure should be regarded as an offset to be applied to the other exposures. We have converted our results to coverages by dividing the total number of hydrogen events observed for an exposure by the average number of tungsten atoms detected per plane. Coverage results are shown in figs. 5, 6 and 7 for adsorptions made under identical conditions for a single specimen but at different radii. The gauge pressures during the adsorption were all 6 x lo-‘” Torr. We note that by reducing the data in this manner saturation coverages of roughly unity are obtained. This method of reducing the data appears reasonable in view of the large fields required to field desorb hydrogen. The field required is shown in fig. 8 and is very close to the evaporation field of tungsten. We conclude that hydrogen is very strongly bound to tungsten in the presence of the field, and the occurrence of diffusion over the surface during field desorption is, consequently, considered to have been unlikely. This is corroborated by the fact that the hydrogen could be exhausted during an analysis, that is, that it did not move up the shank of the specimen during the field evaporation. Indeed, hydrogen is so tenaciously held to the tungsten surface that apparently not all of it is field desorbed during the complete evaporation of a plane, and roughly seven (110) planes must be field evaporated before all of the hydrogen is field desorbed. We ascribe to this model for the field desorption of hydrogen, and thus associate all of the hydrogen events observed during an analysis with a single (110) plane, namely, the surface plane exposed to the hydrogen ambient during an exposure.
OF TOP PLANE =42 ii
Fig. 7. Atom probe data: the hydrogen coverage as a function of the exposure specimen as in figs. 5 and 6. Due to field-evaporation of the tungsten specimen runs the radius of the top plane had been increased to 42 A.
time for the same atoms in previous
A. T. Mawander, D.N. Seidman / Hydrogen on (I IO) tungsten 5.
459
Adsorption model
Langmuir’s model [14] for the adsorption of a gaseous element on a surface is the simplest one and the one most appropriate to introduce our hydrogen adsorption model. Langmuir’s equation for the kinetics of adsorption is Ndt’/dt
= 2+s,
(1)
where N is the number of atomic hydrogen sites per unit area, 8 is the dimensionless coverage, + = P/(ZmmkT) ‘I2 is the impinging molecular flux (P is the pressure, m is the mass of a hydrogen molecule, k is Boltzmann’s constant, and T is the temperature of the gas ambient) and s is the sticking coefficient. Due to the small thermal energy available at 30 K, 2.5 X lop3 eV, we have not included a thermal desorption term in eq. (1). The sticking
75 EXPOSURE TIME = 5 MINUTES
t
FIRST W FIELD EVAPORATION EVENT WAS AT 10.3 kv
DC POTENTIAL
DURING PROBING (KILOVOLTS)
Fig. 8. Atom probe data: the number of hydrogen events detected after a 5 min exposure as a function of DC potential during the probing run. Significant field desorption did not occur until the evaporation field of tungsten was reached.
A. T. Macrander,
I
I
I
I
D. N. Seidman / Hydrogen on (I IO) tungsten
I
I
I
I
I
5 at Fig. 9. The coverage as a function of exposure predicted based on Langmuir’s adsorption
c&efficient s=s,(l
was taken by Langmuir
model.
to be
A)‘,
which even in Langmuir’s model is only an approximate relationship (see ref. [2] and references therein). By simple integration one obtains for the relation-
~PLANE 2+
PLANE 4 -+PLANE
Fig. 10. Ball model for a tungsten (110) oriented specimen.
24
A. T. Macrander, D.N. Seidman / Hydrogen on (110) tungsten
ship between
coverage
e = cXf/(l + at),
and exposure
461
time, (3)
where (Y= 2+s,/N. Eq. (3) is plotted in fig. 9, and we note that d8/dt is nonzero at t = 0. However, the experimental results shown in figs. 5 and 6 clearly show that d8/dt = 0 at t = 0. We note also that if instead of eq. (2), which is suggestive of hydrogen dissociation, the sticking coefficient is taken to be s,,(l - 19), then d8/dt is also non-zero at t = 0 so that our data contradicts this simple assumption for the sticking coefficient as well. We introduce our model by depicting a field ion specimen in cross section using a ball model as shown in fig. 10. The planes of atoms are numbered starting at the apex with plane 1 and the circular steps which define annular discs of atoms are numbered starting at step 1. (We prefer to refer to planes instead of discs.) Two types of adsorption sites are invoked: (i) step sites (N, per unit length) occurring at steps and (ii) plane sites (N, per unit area) occurring on exposed parts of planes. Coverages of these two types of sites are defined as 0’ and X, respectively, where i is the plane or step index. As shown in fig. 1, field desorbed H+ events were detected, not H:. However, we cannot make an inference about the state of hydrogen on the surface, that is whether the adsorbed hydrogen is dissociated or not, because Hl ions are known to dissociate during field desorption [5]. We have formulated our model in terms of atomic sites, however. A previous atom-probe study of hydrogen on W(110) was undertaken by R.S. Chambers at the University of Illinois and was reported on at a workshop on the applications of field-ion microscopy to materials science held at Cornell University. The conclusion stemming from this work is that dissociation of hydrogen is favored near a lattice step at all temperatures studied (below 200 K) [16]. In the present model vacant step sites are taken to act as H, adsorption centers from which atoms may migrate onto the planes, and we ignore direct sticking of gaseous hydrogen onto planes. This is the basic model that Polizzotti and Ehrlich [17] invoked to explain their field emission data for hydrogen adsorption at 80 K, and we apply it now at 30 K as well. We have also ignored spatial variations in the coverages because surface diffusion on the planes has been shown to be extremely fast. DiFoggio and Gomer [4] have shown that hydrogen diffuses via a tunneling mechanism on the (110) planes of tungsten, and they determined that the diffusion constant, D was 7 X lo-l2 cm2 s-’ in the temperature range 30-36 K. This implies that for our specimens planar coverages became uniform in only 20 ms. The time scale of our adsorptions were much longer (minutes), and we, therefore, consider all coverages to be constant as a function of position. The quantities involved in our model are defined in table 2. The surface fluxes (atomic hydrogen) are denoted by Jr!, and JL where the subscript H refers to the flux onto the higher plane (that is, closer to apex), and L refers to
A. T. Mawander,
462
D.N. Seidman / Hydrogen on (110) rungsten
the flux onto the lower plane. These are indicated schematically Having detailed our model, we now write the following equations site coverages,
in fig. 10. for the step
N,dA’/dt=2~ds,(l-A’)*-J;,-J;,
(4)
where d is defined so that 2mrid is the effective area during adsorption the radius of the ith plane; for the coverage of the top plane, NP dB’/dt
= (2/r,)
and for the coverage NP dP/dt
and r, is
JA.
(5)
of the other planes,
= (2r,J,!, + 2r,_,J;-‘)/(1.,~
The surface fluxes are written
- r,‘,).
(6)
as
J,!, = a&‘(1
- t3’),
(7)
J;. = a&l
- or+‘).
(8)
Eqs. (4)-(8) form an infinite set of coupled differential have obtained a solution for O’(t) in the low coverage have, N, dh’/dt
= 2+ds,(l
- 2x’) - a,x’
Here we have ignored terms easily integrated and yields, g(t)
= (2+ds,/N,a)[l
- qx’.
of second
(9) order
in small
quantities.
Eq. (9) is
- exp( -at)],
where a = (4+ds, + 01” + q)/Ns. approximate eq. (5) to N,, d@/dt
equations for which we limit. For 8’, X c 1 we
(10) To obtain
the desired
quantity,
e’(t),
= (2/r,)q,x(t).
Since A’(t = 0) = 0, we obtain from eq. (11) the result that de’/dt Thus the model has reproduced this experimental result. Applying
(11) = 0 at t = 0. eq. (10) and
Table 2 List of symbols
JA JL NP N, + x
et
Surface flux (cm-’ SK’ ) of atomic hydrogen from step i onto the adjacent Surface flux (cm-’ s-’ ) of atomic hydrogen from step i onto the adjacent Number of adsorption sites per unit area on planes Number of adsorption sites per unit length at steps Gas kinetic flux (cm -2 s-‘) impinging on the specimen Coverage of atomic hydrogen on the i th step Coverage of atomic hydrogen on the i th plane
we
higher plane lower plane
A. T. Mawander,
integrating,
we obtain,
B’(r)=@(f-i[l
D.N. Seidman / Hydrogen on (110) tungsten
finally,
for the coverage
463
of the top plane:
-exp(-at)]],
(12)
where p = 4Bds,Lw,/N,N,r,a. This result for the coverage of the top plane in the low coverage limit is plotted in fig. 11. We note that eq. (4) suggests dissociation at step sites but that eq. (12), which applies in the low coverage limit, is independent of this assumption since a similar expression would result if the sticking coefficient is taken to be s,(l - X). We note that the radius dependence of our coverage data shown in figs. 6-8 has also been reproduced by the model. The model predicts that filling should proceed more slowly for larger radii since p varies as r;‘. This is in accord with our data. To describe the uncertainty inherent in the present atom-probe method one must consider the statistics of the collection of field evaporation products. It has, however, not been possible to account fully for observed fluctuations in the number of tungsten atoms per plane [18]. An indication of the magnitude
5
*‘= 4-
& UQ I’
P
at-( i-expi-at))
3 2-
l-
of
1
3
2
4
5
at
Fig. 11. The coverage as a function of exposure for low coverages the adsorption of hydrogen on W(110) at 30 K.
based on the present
model for
464
A. T. Macrander, D.N. Seidman / Hydrogen on (110) tungsten
of expected fluctuations in our coverage data is, however, evident from the coverage data themselves. The 9 min datum in fig. 7 is 0.2 too low since from our other data it is clear that the saturation coverage is unity. 6. Summary Hydrogen coverages of field-ion specimens as a function of exposure were measured by using a positive potential on the specimen to control the exposure and by using field desorption to measure the coverages. Such analyses were made possible by the fact that the potential required to prevent adsorption from the microscope hydrogen ambient was considerably less than that required for field desorption. At 30 K hydrogen field desorption events were not exhausted until roughly seven (110) planes had been field evaporated. A saturation coverage of unity, that is, one hydrogen atom per tungsten atom was determined. The derivative of the coverage with exposure time was found to be zero initially, and this feature can be explained by a model which incorporates both step and plane adsorption sites. In this model plane sites can only be filled via steps and not by direct sticking.
Acknowledgements We wish to thank Dr. R. Herschitz for assistance during the experiments and Mr. R. Whitmarsh for enthusiastic technical assistance. This research was supported by the US Department of Energy. Additional support was received from the use of the technical facilities of the Materials Science Center at Cornell University.
References [l] L.D. Schmidt, in: Interactions on Metal Surfaces, Ed. R. Comer (Springer, New York, 1975) pp. 77-84. [2] K.L. Wilson, IEEE Trans. Nucl. Sci. NS-26 (1979) 1296. (31 R. Comer, R. Wortman and R. Lundy, J. Chem. Phys. 26 (1957) 1147; R. Comer, Field Emission and Field Ionization (Harvard Univ. Press, Cambridge, MA, 1961) pp. 103-106. [4] R. DiFoggio and R. Comer, Phys. Rev. B25 (1982) 3490. [5] A.T. Macrander and D.N. Seidman, J. Appl. Phys. 56 (1984) 1623. [6] K.D. Rendulic and M. Leisch, Surface Sci. 93 (1980) 1. [7] T.M. Hall, A. Wagner and D.N. Seidman, J. Phys. El0 (1977) 884. [8] A. Wagner, T.M. Hall and D.N. Seidman, J. Nucl. Mater. 69/70 (1978) 413. [9] E.W. Mtiller, S.V. Krishnaswamy and S.B. McLane, Surface Sci. 23 (1970) 112. [lo] E.W. Mtiller and S.V. Krishnaswamy, Phys. Rev. Letters 37 (1976) 1011. (111 S. Kapur and E.W. Mtiller, Surface Sci. 66 (1977) 45.
A. T. Macrander, D.N. Seidman / Hydrogen on (110) tungsten
465
[12] T. Sakurai and E.W. Muller, J. Appl. Phys. 48 (1977) 2618. [13] E.W. Mtiller and T.T. Tsong, Field-Ion Microscopy (American Elsevier, New York, 1969) p.12. [14] R.H. Fowler and H.A. Guggenheim, Statistical Thermodynamics (Cambridge University Press, New York, 1949) p.426. [15] M. Ingram and R. Gomer, J. Chem. Phys. 22 (1954) 1279. [16] An abstract entitled “The Effects of Surface Structure on Surface Reactions” by R.S. Chambers appears in Applications of Field-Ion Microscopy to Materials Science, Cornell Materials Science Center Report No. 2849 (1977). Ed. D.N. Seidman. [17] R.S. Polizzotti and G. Ehrlich, J. Chem. Phys. 71 (1979) 259. [18] A.T. Macrander, M. Yamamoto, D.N. Seidman, and S.S. Brenner, Rev. Sci. Instr. 54 (1983) 1077.
Surface Science 147 (1984) 451-465 North-Holland, Amsterdam
HYDROGEN ADSORPTION ON (110) TUNGSTEN PROBE FIEND-ION MICROSCOPE STUDY A.T. MACRANDER
and D.N. SEIDMAN
Cornell Uniuersrry, Deparrment of Materials Center, Ithaca, New York 14853, USA Received
27 March
AT 30 K: AN ATOM-
1984; accepted
Science and Engineering
for publication
and the Materials
Science
31 July 1984
Adsorption of hydrogen on [IlO] oriented tungsten specimens has been studied via field desorption in an atom-probe field-ion microscope. Adsorption was studied on surfaces prepared by field evaporation. Typically, hydrogen field desorption events were exhausted after seven tungsten (110) planes had been field evaporated. Exposures were controlled by applying a potential to the specimen which was sufficient to field ionize hydrogen gas molecules approaching the specimen and which thereby prevented additional hydrogen from being adsorbed. An adsorption model which invokes sticking at step sites but not at plane sites is proposed to explain the coverage data.
1. Iu~~uction The adsorption of hydrogen on metals has been investigated by many workers via a variety of techniques [l]. It is a topic of interest to many researchers because of the many technological processes involving hydrogen. As an example we note the importance of hydrogen for the first wall of fusion power reactors 121. Furthermore, hydrogen effects appear in many experiments conducted in ultra high vacuum because hydrogen is such an ubiquitous element in stainless steel ultra high vacuum systems. Field emission studies of the hydrogen on tungsten system have provided unique information on surface diffusion by direct field emission imaging of the adsorbate covered surface [3] and more recently by determination of the surface diffusion coefficient via the field emission current fluctuation method [4]. Field emission and field ion studies both employ a special specimen geometry, namely, a needle-like specimen which is often referred to as a tip.
* Research Performed while at Cornell University; now at AT&T Bell Laboratories, Murray Hill, New Jersey 07974, USA. ** Research performed while at Cornell University; on a leave-of-absence at the School of Applied Science, Hebrew University, Jerusalem, Israel.
0039~6028/84/$03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
452
A. T. Macrander,
D.N. Seidman / Hydrogen on (110) tungsten
Such a specimen is prepared by field evaporation and has a very regular, well characterized, reproducible, and clean surface. For these reasons such specimens are ideal for adsorption studies. A recurring surface hydrogen concentration has been reported in field evaporation studies of tungsten, and it was found that exposures of specimens cleaned by field evaporation could be controlled by adjusting the potential [5]. The results of experiments measuring the coverage of hydrogen on [110] oriented tungsten specimens as a function of the exposure are reported here. The temperature of these experiments was found to be a very important parameter. We conducted our adsorption and subsequent desorption experiments at a specimen temperature of 27-31 K and attempted them at higher temperatures (76-84 K). At the lower temperatures the number of W(110) planes required to exhaust hydrogen field desorption events, that is, to clean the specimen of hydrogen, was found to be roughly seven, but at the higher temperatures roughly twenty planes were required. Rendulic and Leisch [6] have reported that at 80 K hydrogen is still not completely exhausted after ten atomic planes have been field evaporated, and our measurements have corroborated their finding. We have concentrated on the low temperature case, and we report here our results for the adsorption and subsequent field desorption of hydrogen on W(110) field ion specimens at - 30 K. 2. Experimental
details and mass spectra
The experiments were carried out in an atom-probe field-ion microscope [7,8]. The instrument has a straight flight tube and is computer controlled. We employed field evaporation rates of a few ions per second. Although such rates are smaller than field evaporation rates conventionally used in atom-probe experiments, data collection was nevertheless rapid because data collection was done by computer. Typically, several thousand events were collected per hour. The cold finger of the microscope was cooled in a continuous liquid helium transfer arrangement, and the temperature was controlled to within 2 K. Temperatures were measured using a two lead platinum resistance thermometer. Specimens were prepared from 0.13 mm diameter Materials Research Corp. VP grade polycrystalline wires with the wire axis parallel to the [110] direction. Wire segments were sharpened by electropolishing them in NaOH. Specimens were imaged via field ionization of helium gas and were microscopically polished by field evaporation until they had a very regular end form. Sample purity was determined directly from time-of-flight mass spectra. A histogram of the number of events versus their mass-to-charge ratio (m/n) is shown in fig. 1 for the combined data for six exposures. These data are typical, and the resulting frequencies of occurrence expressed as percentages of the total number of ions collected are given in table 1.
A. T. Macrander, D.N. Seidman / Hydrogen on (I IO) tungsten
453
We note now a few noteworthy observations concerning the mass spectra. The occurrence of the ion species W*+ and W5+ has not been commonly reported, and these may be related to the presence of hydrogen. We also observed a shift in the W *+ peak to m/n values less than the expected value of 93. This shift has been attributed to the dissociation of (WH)3’ into H’ and W *+ some distance above the specimen surface [9]. Miiller and Krishnaswamy observed the occurrence of W 5+ events but at higher evaporation rates than we employed [lo]. We have neglected the occurrence of tungsten hydride field evaporation products. These were not resolvable in the mass spectra because of energy deficits. Nevertheless, the copious occurrence of H+ events indicates that tungsten-hydride ions were dissociated during the process of field evaporation. We note that Kapur and Mtiller [ll] in experiments using a high resolution magnetic sector-type atom-probe found no stable hydrides of tungsten during slow field evaporation albeit at a higher temperature (78 K). Table 1 Combined
mass spectrograph
results of six hydrogen
Ion
Fraction
H+ He+ Ws+ W4+ W’+ W2+ Other
6.14 0.25 0.09 15.66 76.61 0.34 0.31
0
20
40 MASS-TO-CHARGE
Fig. 1. Mass spectrum of the events detected exposures were combined.
adsorption
experiments
of total (W)
60 RATIO
subsequent
80
100
to hydrogen
exposures.
(AMU)
The data for six
454
A. T. Macrander, D. N. Seidman / Hydrogen on (I IO) tunpren
Adsorption experiments were conducted by exposing specimens to hydrogen which had been backfilled into the vacuum chamber of the microscope. After the specimen had been inserted and after the microscope had been baked at 120°C for 12 h, base pressures were (2-5)X 10-l’ Torr as read by a Bayard-Alpert ionization gauge which had been calibrated for nitrogen. After imaging with Matheson research purity helium and after the pressure in the microscope was pumped into the lo- lo Torr range, Matheson research purity hydrogen was admitted into the microscope to a gauge pressure of - 2 x 10 * Torr. Subsequently, the microscope was again pumped into the 1O-‘0 Torr range, and the adsorption measurements were made. Via a residual gas analyzer the ambient in the microscope during the adsorption experiments was found to be - 3% helium and 97% hydrogen.
3. Method used to make exposures Just prior to an exposure specimens were pulse-field evaporated until hydrogen events were exhausted. Hydrogen had invariably been adsorbed from the background during the preparatory stages of an experimental run, and it was necessary to field desorb all of this hydrogen in order to have an adsorbate free surface to begin with. We found that specimens could be shielded from a hydrogen ambient by applying a positive potential less than that needed to field desorb any adsorbed hydrogen. The results of experiments in which the potential was systematically varied are shown in fig. 2. For this specimen, all hydrogen gas molecules impinging on the specimen were found to be field ionized at potentials greater than 7.4 kV. By scaling to the known evaporation field of W(llO), 5.5 V/A [12], we obtained a value of 3.8 V/A for the field required to completely shield the specimen. We note that this is larger than the best imaging field of hydrogen, 2.2 V/A [13]. Exposures were controlled by varying the potential of the specimen. After several (110) planes had been field evaporated to prepare an adsorbate free surface, the potential was simply allowed to decay to the ground potential to initiate the exposure, and exposures were terminated by increasing the voltage to values above that required for complete shielding but below that needed to field desorb the adsorbed hydrogen.
4. Field desorption analyses After completing an exposure both the DC and pulse (fixed to be 0.1 of the DC) voltages were slowly raised and the specimen was slowly field evaporated until no more hydrogen events were detected, that is, until all the hydrogen
A. T. Macrander, D.N. Seidman / Hydrogen on (IIO) tungsten
455
that had been adsorbed was completely field desorbed. During the analyses the probe hole was positioned directly over the (110) pole, and a stepped behavior was observed in plots of cumulative number of tungsten events versus time. Such steps are shown in fig. 3 and result from the fact that the atoms at the steps of the specimen undergo field evaporation causing the disc shaped planes of atoms to collapse progressively. Hydrogen field desorption data were collected in primary form as histograms of the number of hydrogen events detected during the complete field evaporation of a plane. Histograms of the number of field desorbed hydrogen events detected per plane versus the number of that plane are shown in fig. 4 for six exposures of 200 EXPOSURE
TIME = 5 MINUTES
IO
I
DC POTENTIAL DURING ADSORPTION ( KILOVOLTS) Fig. 2. The number of hydrogen events detected subsequent to a 5 min exposure as a function of the DC potential on the specimen. Complete shielding was obtained at 7.4 kV.
456
A. T. Mocrander,
D.N. Seidmnn / Hydrogen on (110) tungsten
CUMULATIVE
NUMBER
OF
EVAPORATION
PULSES
(bl FIM
SPECIMEN
PROBE
HOLE
CHEVRON DETECTOR
ION
Fig. 3. (a) A portion of a plot of the field-evaporation data. The ordinate is the cumulative number of ions detected with mass-to-charge ratios in the range 40 to 70 amu. Almost all the tungsten ions detected were either W3+ or W4+ and thus were included in this range. The abscissa is the cumulative number of field-evaporation pulses applied to a specimen. (b) A schematic illustration of the three stages of collapse of a (110) plane corresponding to (a).
RADIUS OF TOP
EXPOSURE
PLANE
= 40%
TIME
(MINUTES)
NUMBER
OF W(110)
Fig. 4. Atom probe data: exposures are shown.
LAYERS
the number
FIELD
of hydrogen
EVAPORATED
events detected
per (110) plane.
Data
for six
A. T. Macrander, D. N. Seidman / Hydrogen on (I 10) iungsten
457
the same specimen. Exposure times refer to the period during which the specimen was at ground potential. Since some time was required for the potential to decay from the complete shielding value at the start of an exposure and for the potential to reach the complete shielding value at the end, some
l.O-
/
0
0
8 :
2 W > 0.5-
z
RADIUS OF TOP PLANE = 36 i
EXPOSURE
TIME (MINUTES 1
Fig. 5. Atom probe data: the hydrogen coverage the top plane of the specimen was 36 A.
as a function
of the exposure
time. The radius of
0
1.0
-
RADIUS OF TOP PLANE = 40 i
0
5 EXPOSURE
10
15
TIME (MINUTES)
Fig. 6. Atom probe data: the hydrogen coverage as a function of the exposure time-for the same specimen as in fig. 5. Due to field-evaporation of the tungsten specimen atoms in previous runs the radius of the top plane had been increased to 40 A.
458
A. T. Macrander,
D.N. Seidman / Hydrogen on (110) tungsten
events were detected even for the null exposure. The results of the null exposure should be regarded as an offset to be applied to the other exposures. We have converted our results to coverages by dividing the total number of hydrogen events observed for an exposure by the average number of tungsten atoms detected per plane. Coverage results are shown in figs. 5, 6 and 7 for adsorptions made under identical conditions for a single specimen but at different radii. The gauge pressures during the adsorption were all 6 x lo-‘” Torr. We note that by reducing the data in this manner saturation coverages of roughly unity are obtained. This method of reducing the data appears reasonable in view of the large fields required to field desorb hydrogen. The field required is shown in fig. 8 and is very close to the evaporation field of tungsten. We conclude that hydrogen is very strongly bound to tungsten in the presence of the field, and the occurrence of diffusion over the surface during field desorption is, consequently, considered to have been unlikely. This is corroborated by the fact that the hydrogen could be exhausted during an analysis, that is, that it did not move up the shank of the specimen during the field evaporation. Indeed, hydrogen is so tenaciously held to the tungsten surface that apparently not all of it is field desorbed during the complete evaporation of a plane, and roughly seven (110) planes must be field evaporated before all of the hydrogen is field desorbed. We ascribe to this model for the field desorption of hydrogen, and thus associate all of the hydrogen events observed during an analysis with a single (110) plane, namely, the surface plane exposed to the hydrogen ambient during an exposure.
OF TOP PLANE =42 ii
Fig. 7. Atom probe data: the hydrogen coverage as a function of the exposure specimen as in figs. 5 and 6. Due to field-evaporation of the tungsten specimen runs the radius of the top plane had been increased to 42 A.
time for the same atoms in previous
A. T. Mawander, D.N. Seidman / Hydrogen on (I IO) tungsten 5.
459
Adsorption model
Langmuir’s model [14] for the adsorption of a gaseous element on a surface is the simplest one and the one most appropriate to introduce our hydrogen adsorption model. Langmuir’s equation for the kinetics of adsorption is Ndt’/dt
= 2+s,
(1)
where N is the number of atomic hydrogen sites per unit area, 8 is the dimensionless coverage, + = P/(ZmmkT) ‘I2 is the impinging molecular flux (P is the pressure, m is the mass of a hydrogen molecule, k is Boltzmann’s constant, and T is the temperature of the gas ambient) and s is the sticking coefficient. Due to the small thermal energy available at 30 K, 2.5 X lop3 eV, we have not included a thermal desorption term in eq. (1). The sticking
75 EXPOSURE TIME = 5 MINUTES
t
FIRST W FIELD EVAPORATION EVENT WAS AT 10.3 kv
DC POTENTIAL
DURING PROBING (KILOVOLTS)
Fig. 8. Atom probe data: the number of hydrogen events detected after a 5 min exposure as a function of DC potential during the probing run. Significant field desorption did not occur until the evaporation field of tungsten was reached.
A. T. Macrander,
I
I
I
I
D. N. Seidman / Hydrogen on (I IO) tungsten
I
I
I
I
I
5 at Fig. 9. The coverage as a function of exposure predicted based on Langmuir’s adsorption
c&efficient s=s,(l
was taken by Langmuir
model.
to be
A)‘,
which even in Langmuir’s model is only an approximate relationship (see ref. [2] and references therein). By simple integration one obtains for the relation-
~PLANE 2+
PLANE 4 -+PLANE
Fig. 10. Ball model for a tungsten (110) oriented specimen.
24
A. T. Macrander, D.N. Seidman / Hydrogen on (110) tungsten
ship between
coverage
e = cXf/(l + at),
and exposure
461
time, (3)
where (Y= 2+s,/N. Eq. (3) is plotted in fig. 9, and we note that d8/dt is nonzero at t = 0. However, the experimental results shown in figs. 5 and 6 clearly show that d8/dt = 0 at t = 0. We note also that if instead of eq. (2), which is suggestive of hydrogen dissociation, the sticking coefficient is taken to be s,,(l - 19), then d8/dt is also non-zero at t = 0 so that our data contradicts this simple assumption for the sticking coefficient as well. We introduce our model by depicting a field ion specimen in cross section using a ball model as shown in fig. 10. The planes of atoms are numbered starting at the apex with plane 1 and the circular steps which define annular discs of atoms are numbered starting at step 1. (We prefer to refer to planes instead of discs.) Two types of adsorption sites are invoked: (i) step sites (N, per unit length) occurring at steps and (ii) plane sites (N, per unit area) occurring on exposed parts of planes. Coverages of these two types of sites are defined as 0’ and X, respectively, where i is the plane or step index. As shown in fig. 1, field desorbed H+ events were detected, not H:. However, we cannot make an inference about the state of hydrogen on the surface, that is whether the adsorbed hydrogen is dissociated or not, because Hl ions are known to dissociate during field desorption [5]. We have formulated our model in terms of atomic sites, however. A previous atom-probe study of hydrogen on W(110) was undertaken by R.S. Chambers at the University of Illinois and was reported on at a workshop on the applications of field-ion microscopy to materials science held at Cornell University. The conclusion stemming from this work is that dissociation of hydrogen is favored near a lattice step at all temperatures studied (below 200 K) [16]. In the present model vacant step sites are taken to act as H, adsorption centers from which atoms may migrate onto the planes, and we ignore direct sticking of gaseous hydrogen onto planes. This is the basic model that Polizzotti and Ehrlich [17] invoked to explain their field emission data for hydrogen adsorption at 80 K, and we apply it now at 30 K as well. We have also ignored spatial variations in the coverages because surface diffusion on the planes has been shown to be extremely fast. DiFoggio and Gomer [4] have shown that hydrogen diffuses via a tunneling mechanism on the (110) planes of tungsten, and they determined that the diffusion constant, D was 7 X lo-l2 cm2 s-’ in the temperature range 30-36 K. This implies that for our specimens planar coverages became uniform in only 20 ms. The time scale of our adsorptions were much longer (minutes), and we, therefore, consider all coverages to be constant as a function of position. The quantities involved in our model are defined in table 2. The surface fluxes (atomic hydrogen) are denoted by Jr!, and JL where the subscript H refers to the flux onto the higher plane (that is, closer to apex), and L refers to
A. T. Mawander,
462
D.N. Seidman / Hydrogen on (110) rungsten
the flux onto the lower plane. These are indicated schematically Having detailed our model, we now write the following equations site coverages,
in fig. 10. for the step
N,dA’/dt=2~ds,(l-A’)*-J;,-J;,
(4)
where d is defined so that 2mrid is the effective area during adsorption the radius of the ith plane; for the coverage of the top plane, NP dB’/dt
= (2/r,)
and for the coverage NP dP/dt
and r, is
JA.
(5)
of the other planes,
= (2r,J,!, + 2r,_,J;-‘)/(1.,~
The surface fluxes are written
- r,‘,).
(6)
as
J,!, = a&‘(1
- t3’),
(7)
J;. = a&l
- or+‘).
(8)
Eqs. (4)-(8) form an infinite set of coupled differential have obtained a solution for O’(t) in the low coverage have, N, dh’/dt
= 2+ds,(l
- 2x’) - a,x’
Here we have ignored terms easily integrated and yields, g(t)
= (2+ds,/N,a)[l
- qx’.
of second
(9) order
in small
quantities.
Eq. (9) is
- exp( -at)],
where a = (4+ds, + 01” + q)/Ns. approximate eq. (5) to N,, d@/dt
equations for which we limit. For 8’, X c 1 we
(10) To obtain
the desired
quantity,
e’(t),
= (2/r,)q,x(t).
Since A’(t = 0) = 0, we obtain from eq. (11) the result that de’/dt Thus the model has reproduced this experimental result. Applying
(11) = 0 at t = 0. eq. (10) and
Table 2 List of symbols
JA JL NP N, + x
et
Surface flux (cm-’ SK’ ) of atomic hydrogen from step i onto the adjacent Surface flux (cm-’ s-’ ) of atomic hydrogen from step i onto the adjacent Number of adsorption sites per unit area on planes Number of adsorption sites per unit length at steps Gas kinetic flux (cm -2 s-‘) impinging on the specimen Coverage of atomic hydrogen on the i th step Coverage of atomic hydrogen on the i th plane
we
higher plane lower plane
A. T. Mawander,
integrating,
we obtain,
B’(r)=@(f-i[l
D.N. Seidman / Hydrogen on (110) tungsten
finally,
for the coverage
463
of the top plane:
-exp(-at)]],
(12)
where p = 4Bds,Lw,/N,N,r,a. This result for the coverage of the top plane in the low coverage limit is plotted in fig. 11. We note that eq. (4) suggests dissociation at step sites but that eq. (12), which applies in the low coverage limit, is independent of this assumption since a similar expression would result if the sticking coefficient is taken to be s,(l - X). We note that the radius dependence of our coverage data shown in figs. 6-8 has also been reproduced by the model. The model predicts that filling should proceed more slowly for larger radii since p varies as r;‘. This is in accord with our data. To describe the uncertainty inherent in the present atom-probe method one must consider the statistics of the collection of field evaporation products. It has, however, not been possible to account fully for observed fluctuations in the number of tungsten atoms per plane [18]. An indication of the magnitude
5
*‘= 4-
& UQ I’
P
at-( i-expi-at))
3 2-
l-
of
1
3
2
4
5
at
Fig. 11. The coverage as a function of exposure for low coverages the adsorption of hydrogen on W(110) at 30 K.
based on the present
model for
464
A. T. Macrander, D.N. Seidman / Hydrogen on (110) tungsten
of expected fluctuations in our coverage data is, however, evident from the coverage data themselves. The 9 min datum in fig. 7 is 0.2 too low since from our other data it is clear that the saturation coverage is unity. 6. Summary Hydrogen coverages of field-ion specimens as a function of exposure were measured by using a positive potential on the specimen to control the exposure and by using field desorption to measure the coverages. Such analyses were made possible by the fact that the potential required to prevent adsorption from the microscope hydrogen ambient was considerably less than that required for field desorption. At 30 K hydrogen field desorption events were not exhausted until roughly seven (110) planes had been field evaporated. A saturation coverage of unity, that is, one hydrogen atom per tungsten atom was determined. The derivative of the coverage with exposure time was found to be zero initially, and this feature can be explained by a model which incorporates both step and plane adsorption sites. In this model plane sites can only be filled via steps and not by direct sticking.
Acknowledgements We wish to thank Dr. R. Herschitz for assistance during the experiments and Mr. R. Whitmarsh for enthusiastic technical assistance. This research was supported by the US Department of Energy. Additional support was received from the use of the technical facilities of the Materials Science Center at Cornell University.
References [l] L.D. Schmidt, in: Interactions on Metal Surfaces, Ed. R. Comer (Springer, New York, 1975) pp. 77-84. [2] K.L. Wilson, IEEE Trans. Nucl. Sci. NS-26 (1979) 1296. (31 R. Comer, R. Wortman and R. Lundy, J. Chem. Phys. 26 (1957) 1147; R. Comer, Field Emission and Field Ionization (Harvard Univ. Press, Cambridge, MA, 1961) pp. 103-106. [4] R. DiFoggio and R. Comer, Phys. Rev. B25 (1982) 3490. [5] A.T. Macrander and D.N. Seidman, J. Appl. Phys. 56 (1984) 1623. [6] K.D. Rendulic and M. Leisch, Surface Sci. 93 (1980) 1. [7] T.M. Hall, A. Wagner and D.N. Seidman, J. Phys. El0 (1977) 884. [8] A. Wagner, T.M. Hall and D.N. Seidman, J. Nucl. Mater. 69/70 (1978) 413. [9] E.W. Mtiller, S.V. Krishnaswamy and S.B. McLane, Surface Sci. 23 (1970) 112. [lo] E.W. Mtiller and S.V. Krishnaswamy, Phys. Rev. Letters 37 (1976) 1011. (111 S. Kapur and E.W. Mtiller, Surface Sci. 66 (1977) 45.
A. T. Macrander, D.N. Seidman / Hydrogen on (110) tungsten
465
[12] T. Sakurai and E.W. Muller, J. Appl. Phys. 48 (1977) 2618. [13] E.W. Mtiller and T.T. Tsong, Field-Ion Microscopy (American Elsevier, New York, 1969) p.12. [14] R.H. Fowler and H.A. Guggenheim, Statistical Thermodynamics (Cambridge University Press, New York, 1949) p.426. [15] M. Ingram and R. Gomer, J. Chem. Phys. 22 (1954) 1279. [16] An abstract entitled “The Effects of Surface Structure on Surface Reactions” by R.S. Chambers appears in Applications of Field-Ion Microscopy to Materials Science, Cornell Materials Science Center Report No. 2849 (1977). Ed. D.N. Seidman. [17] R.S. Polizzotti and G. Ehrlich, J. Chem. Phys. 71 (1979) 259. [18] A.T. Macrander, M. Yamamoto, D.N. Seidman, and S.S. Brenner, Rev. Sci. Instr. 54 (1983) 1077.