Surface Science 47 (1975) 557-568 0 North-Holland Publishing Company

ADSORPTION AND SOLUTION OF Hz, DZ, NZ, O2 AND CO BY (lOO)Ta*

S.M. KO and L.D. SCHMIDT Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, U.S.A.

Received 22 August 1974; revised manuscript received 4 October 1974

Sticking coefficients, saturation densities, and solution rates of gases on (100) Ta are obtained by comparing with results on (100) W using Auger electron spectroscopy and flash desorption. Hydrogen has a lower sticking coefficient on (100) Ta than on polycrys- talline Ta, but solution occurs readily even at 78 “K. Differences between Hz and Da are observed for both adsorption and solution. Nitrogen is confined to the surface of Ta for T < -500 OK, and adsorbed nitrogen dissolves with an activation energy of -2.5 kcal mole-’ upon heating to higher temperatures. The saturation density of 02 at 300 “K is approximately twice that on (100) W. The first monolayer dissolves at -500 “K but the second dissolves or desorbs only at much higher temperatures. Carbon monoxide adsorbs without solution of either species at 300 “K. At -500 o K carbon dissolves completely leaving oxygen which desorbs at much higher temperature.

1. Introduction

Many adsorption systems are complicated by the fact that the adsorbate can dis- solve appreciably in the solid. In this paper we report an examination of adsorp-

tion and solution of a number of gases on a Ta single crystal exposing only (100) planes. Total uptake is measured by flash desorption mass spectrometry and surface

concentrations by Auger electron spectroscopy (AES). We have previously reported a study of adsorption and solution of H2 and N2 on

polycrystalline wires of Ta and Nb [l] . The present investigation is essentially an ex- tension of this work to a single crystal Ta substrate using AES for unequivocal deter- mination of adsorbate and contaminant coverages.

Adsorption and solution have been examined for all of these adsorbates. Refer- ences to N2 and H2 on Ta can be found in ref. [l] . Studies of CO and 02 adsorp- tion and solution have been less extensive [2-51. A recent study of gases adsorbed on (100) Ta emphasized LEED structures produced by these gases upon heating [6] rather than sticking coefficients and kinetics of solution.

One of the motivations for the present work was to compare adsorption on (100)

* This work partially supported by NSF under Grant No. GK16241.

558 S.M. Ko, L.D. SchmidtfAdsorption and solution of gases by (100) Ta

Ta with that on (100) W to examine the differences between similar bee metals with 4 and 5 d electrons. As we shall show there are few similarities in surface saturation densities, and solution effects prevent use of flash desorption to determine the num- ber of binding states and their relative saturation densities.

2. Experimental

Single crystal discs of Ta and W exposing more than 95 % of (100) planes were mounted in a metal ultrahigh vacuum system. Crystals were heated by electron bom- bardment and cooled to 78 “K or 195 “C by contacting their supports with liquid N, or dry ice respectively. Backgrpund pressures were less than 5 X 1 O-lo torr, and in all experiments contaminant coverages were never more than a few percent of a mono- layer. A cylindrical mirror AES analyzer was used with beam currents and times low enough that electron impact interaction with adsorbates was negligible [7] . Partial pressures were monitored with a quadrupole mass spectrometer which was also used to obtain flash desorption spectra. The pumping speed was high enough (7 < 0.1 set) that differential desorption spectra were obtained.

Crystals were cleaned by heating in vacuum to 2300 “K and by heating in 02 at 1 X 10s6 torr for -1 hr. With this procedure W was completely clean but Ta contained

dN(E

dE 7 ::I I

200

I 1

300 400 500

Energy (eV)

Fig. 1. Auger electron spectra of clean (100) W and (100) Ta and of saturation spectra of Hz, N2,

CO, and 02 on (100) Ta.

S.M. Ko, L.D. Schmidt/Adsorption and solution of gases by (IOOJ Ta 559

-0.15 monolayers of oxygen (as defined below) which could only be removed by re- peated flashing to > 2300 “K. After such treatment the surfaces contained less than 0.02 monolayers of S, C, or 0 as determined from AES peaks at 150,270, and 510 V respectively. Auger spectra of clean surfaces are shown in fig. 1 as are saturation spectra of N,, CO, and 0, on (100) Ta. These were obtained by exposure of -1O-5 torr set to the gases at substrate temperatures of 300°K. It is evident that there is negligible contamination by AES sensitive species. Flash desorption showed that hy- drogen contaminant coverages were small. We found no evidence of W contamina- tion of Ta from the tungsten electron bombardment filament, either from AES spectra or lack of reproducibility of adsorption data.

Coverages on (100) Ta were calibrated against those on (100) W for which absolute densities of 0,) CO, N2, and H, have been determined by direct measurement and structure models [8-121. Relative hydrogen coverages were determined by flash de- sorption, taking into account the difference in surface areas. Since W and Ta are ad- jacent in the periodic table and form similar chemical bonds with substrates, it is as- sumed that they have similar backscattering coefficients and give comparable AES peak heights for both adsorbates. Data were generally recorded as ratios of adsorbate to substrate peak heights to account for variations in beam current. Most relative densities are regarded as accurate to within 5 %. An exception is oxygen on Ta at high coverage. Here the apparent density was -2 X 1015 atoms cm-2 and may be par-

tially under the Ta surface layer. The Ta peak changes in shape and decreases by - 10 % upon saturation with 02. In all other systems the substrate peak remains unchanged in shape and size upon adsorption to within the limits of our apparatus (’ 2 eV in en- ergy and + 5% in peak height).

Values of sticking coefficients and adsorbate densities were measured only relative to those on (100) W. Relative values of these quantities were reproducible to better than + 10%, but absolute values are only as accurate as those for W and may require correction when these are measured with greater accuracy [8-121.

3. Results

3.1. Condensation of H2 and D,

Fig. 2 shows amounts of Hz and D2 adsorbed versus time at constant pressure and substrate temperature. The derivatives of these curves give the sticking coefficient s, and s versus n curves are shown in fig. 3. The dashed lines are corresponding results [l l] on (100) W. The coverage scale for Ta is indicated as the fraction of the satura- tion coverage on (100) W.

The temperature dependence of s is shown in fig. 4 from data obtained at 78, 195, and 300 OK. Sticking coefficients for solution are obtained by noting that s is independent of uptake for n > 2 monolayers.

The ratio of sticking coefficients for D2 and Hz on (100) Ta at 300 “K was

560 S.M. Ko, L.D. Schmidt/Adsorption and solution of gases by (100) Ta

40 6.0

PX tX IO’(torr see)

Fig. 2. Hydrogen uptake versus time on (100) Ta. Abscissa is uncorrected partial pressure; values

of s were calculated by comparison with (100) W.

--‘\,D20” (IOO)W,300”K

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Fig. 3. Sticking coefficients of Hz and D2 on (100) Ta at temperatures indicated. Dashed curves indicate values on (100) W. Data were obtained from uptake versus time curves as shown in fig. 2.

S.M. Ko, L.D. Schmidt/Adsorption and solution of gases by (100) Ta 561

S

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Fig. 4. Sticking coefficient of Hz and D2 versus surface temperature for adsorption (so) and solu- tion (ssolQ.

‘D2 lSH2 = 1.4 + 0.15. This agrees with the ratio reported earlier by Tamm and Schmidt [l l] on (100) W but is slightly higher than the ratio of 1 .l reported by Madey [ 121. We have recently reexamined this isotope effect on (100) and (110) planes of W and obtain ratios of 1.4 and 1 .O respectively on the two planes [ 131 . Thus the ratio of initial sticking coefficients on (100) Ta appears to be the same as that on the corresponding plane of W.

3.2. Nitrogen adsorption and solution

Fig. 5 shows a comparison of sticking coefficients on (100) Ta and (100) W at a substrate temperature of 300 “K. Data were obtained as for Hz, and points shown in- dicate typical slope determinations. At 300 “K, N, saturates on (100) Ta at a density of about 1.5 times that on (100) W. Assuming a density [9] for N, on (100) W of 5.0 X 1 014 atoms cms2, the saturation density on (100) Ta is 7.5 X 1 014 atoms cmm2 or almost one nitrogen atom per surface Ta atom. The initial sticking coefficient on Ta is -0.4 of that on W, and, assuming so = 0.41 on (lOO)W [14], one obtains so = 0.16 on (lOO)Ta.

562 SM. Ko, L.D. Schmidt/Adsorption and solution ofgases by (100) Ta

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Fig. 5. Sticking coefficient of N-J versus coverage on (100) Ta and on (100) W at 300°K. Cover- ages are in terms of the fraction of the saturation density on (100) W.

400’ K

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100 200 300

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Fig. 6. Decrease of AES signal of N with time of heating at temperatures indicated following saturation with N2 at 300°K.

SM. Ko, L.D. Schmidt/Adsorption and solution ofgases by (1OOJ Ta 563

After saturation with N, at 300 OK, Ta was heated to various temperatures, and AES intensities versus time were recorded. As shown in fig. 6, there is no decrease in signal with time at 300 “K but at higher temperatures appreciable solution is observed. We attribute the signal decrease to solution because desorption only occurs at much higher temperatures. No quantitative desorption measurements were attempted in this work, but previous results [I] on polycrystalline Ta wires indicate that desorp- tion should only be significant for T > 2000 “K in the times shown in fig. 6.

Rates of solution do not appear to follow simple first order kinetics because plots of the logarithm of the AES intensity versus time do not yield straight lines over the entire coverage range. This could arise because of a complex solution process involv- ing several states or because of a lower sensitivity for AES from nitrogen below the surface but near enough to contribute to the AES signal. Fig. 7 shows a plot of the logarithm of the time required for the AES signal to decrease by a factor of two ver- sus l/T. The slope of this line gives an apparent activation energy for solution of -2.5 kcal mole-l, but because of the small number of data points, the numbers have only qualitative significance.

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Fig. 7. Flat of times for AES signals to decrease to one-half of the saturation values versus l/T for C from CO saturation and N from N2 saturation on (100) Ta. Data were taken from fig. 6. Slopes give activation energies of 3.5 and 2.5 kcal mole-’ respectively. Data are not sufficiently accurate to determine activation energies accurately but do show that these are much smaller than activa- tion energies of bulk solution.

564 SM. Ko, L.D. Schmidt/Adsorption and solution of gases by (100) Ta

3.3. Solution of 0, and CO

At 300 “K exposure to 0, produced a coverage of -2 X 1015 atoms cmb2. This density (2 oxygen atoms per surface Ta atom) and the detectable decrease of the Ta peak upon adsorption suggest that under these conditions oxygen is to some extent under the surface layer of Ta atoms.

Fig. 8 shows a plot of the 0 AES peak versus temperature following 100 set heat- ing to the temperatures indicated. (The substrate was allowed to cool to 300 “K be- fore AES measurements.) There is a rapid reduction in the AES peak height by -50% between 400 and 500 “K with a gradual continued decrease between 500 and 2500 “K. This agrees with the cleaning conditions necessary for oxygen as discussed previously.

At 300 “K saturation of the clean surface with CO produced a coverage of -1 .O of that on (100) W. Assuming saturation on (100) W to be 5 X 1 014 C atoms cmm2 (one C atom for each two surface W atoms), we obtain nearly the same density on (lOO)Ta. The C/O peak ratios were 2.42 t 0.05 on both substrates, implying that both C and 0 remained on the surface at 300 “K.

Fig. 8 shows C and 0 AES peak heights versus temperature following saturation of CO on (lOO)Ta at 300 “K. The carbon signal decreases rapidly upon heating and has fallen to < 0.1 of its initial value by 800 “K. However the oxygen signal remains completely unchanged upon heating to 1000 OK, and then decreases slowly at higher temperatures. After heating to 1000 OK, the coverage of oxygen from CO is identical to that following 0, saturation. This indicates that when a surface saturated with CO is heated, carbon rapidly dissolves leaving oxygen on the surface which behaves iden- tically to that deposited from 0,.

500 1000 1500 2000 2500

T, ( OK)

Fig. 8. AES signals for C and 0 versus temperature. The carbon signal decreases to a small value at T > 800°K while 02 from CO only decreases at higher temperature.

S.M. Ko, L.D. Schmidt/Adsorption and solution of gases by (IOOJ Ta 565

I I I

200 400 600 800

t(sec)

Fig. 9. Decrease of AFS signal of C and 0 with time at temperatures indicated following satura- tion with CO at 300 “K. Carbon decreases with time for T > 300 “K but 0 remains constant for T< 1100°K.

Fig. 9 shows plots of C and 0 AES signals versus time for temperatures indicated following saturation with CO at 300 “K. As with N, solution, kinetics did not appear to be strictly first order. An Arrhenius plot of the carbon decrease for the three highest temperatures are shown in fig. 7.

4. Discussion

4.1. Comparison between surfaces

These results for Hz are qualitatively similar to those on polycrystalline Ta [l] and also similar to those on polycrystalline and (100) Nb [ 151. However the values of s for H2 and N, for both adsorption and solution on (lOO)Ta are considerably lower than those on polycrystalline Ta or (100) W. The low value is surprising in view of the high binding energies of both gases and confirms that rates of adsorption are not closely related to binding energies [ 1 l] . Evidently condensation is much more efficient on some of the other planes exposed on polycrystalline Ta than on the (100) plane. Comparison of s between (100) and (110) Ta would be interesting because on W it was shown that s is much higher on the (100) plane than on the (110) plane for

566 SM. Ko, L.D. Schmidt/Afsorption and solution ofgases by (100) Ta

both Hz and N, [16],

4.2. Rates of solution

In the previous paper [I ] we showed that both condensation and desorption for high uptake appear to proceed through weakly bound adsorption states rather than through the tightly bound states whose binding energies were measured. At 300 “K or below all gases except hydrogen saturate on the surface with negligible bulk solu-

tion. For H, it was argued that the break in s with uptake indicates that at low tem- peratures surfaces also become nearly saturated with hydrogen before bulk solution occurs. The present results confirm this by direct measurement for N,, CO and 0, on the (100) plane and agree with previous results for Hz.

Adsorbates containing C, N, and 0 are completely confined at (or near) the sur- face below 500 OK, but hydrogen readily dissolves even at 78 “K. It is not possible

to measure the fraction of hydrogen on the surface, but it has sufficient mobility that equilibrium densities in bulk and on surfaces may be approached under most conditions.

The mass dependence of the sticking coefficients for hydrogen are in ratio of -1.4 at 300 “K for both adsorption and solution. However s for solution has a much larger temperature dependence for D, than for H, : the sticking coefficients are almost equal at 195 OK, and at 78 “K H, has a higher sticking coefficient for solution than does D,. This isotope effect is probably associated with the variations in energy trans- fer rates with mass of the impinging molecule [ 171 .

Next we consider the energy barriers between adsorption and solution of N, C, and 0 atoms. We assume that all species dissociate upon adsorption. All evidence in- dicates dissociation upon adsorption for H,, N, and 0,) and the shape of the AES

spectrum of C from CO is similar to that of carbon alone even at low temperatures [ 181 . If there were no barrier to solution, the activation energy would be expected to be the difference between the binding energy of the atom on the surface and its heat of solution. This is much greater than the observed activation energies for solu- tion. The minimum activation energy to be expected in simple models of solution from the adsorbed state is the activation energy of surface diffusion. As shown in

table 1 these are in the range of 30 kcal mole-l, again much smaller than the observed

activation energies. Finally we note that if there were a simple potential barrier be- tween adsorption and solution (fig. 8 of ref. l), solution should obey first order ki- netics, i.e., the rate of disappearance of atoms from the surface should be proportional to the density of atoms on the surface.

None of these simple ideas appears to explain our results. One possibility is that atoms one or two atomic distances below the surface do not have the properties of those in bulk. Atoms in each layer may have characteristic binding energies which only approach bulk heats of solution for large distances from the surface. A situation of this type is clearly indicated for 0, which forms two monolayers even at 78 “K. High temperature exposure to N, and CO could perhaps populate states near the sur-

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568 S.M. Ko, L.D. Schmidt/Adsorption and solution of gases by (100) Ta

face for these gases also. However in this case one needs to make assumptions regard- ing sensitivities of AES for various depths, and measurements would be qualitative at best.

There have been numerous similar suggestions invoked to explain apparent multi- layer adsorption, but in few systems have density calibrations been sufficiently ac- curate to substantiate these assertions. There is also evidence from calculations that electronic properties of metals only become identical to those in bulk several atom layers below the surface [ 191 . Our measurements on solution kinetics suggest that layers of intermediate binding energy may exist, but obviously other types of experi- ments are necessary to verify their existence and examine their properties.

References

[l] SM. Ko and L.D. Schmidt, Surface Sci. 42 (1974) 508.

[2] R.P.H. Gasser and R. Thwaites, Trans. Faraday Sot. 61 (1965) 2036.

[3] T.W. Haas, in: Structure and Chemistry of Solid Surfaces (Wiley, New York, 1969).

[4] T.W. Haas, A.G. Jackson and M.P. Hooker, J. Chem. Phys. 46 (1967) 3025.

[5] R. Klein and L.B. Leder, J. Chem. Phys. 38 (1963) 1866.

[6] M.A. Chesters, B.J. Hopkins and M.R. Leggett, Surface Sci. 43 (1974) 1.

[7] Y. Viswanath and L.D. Schmidt, J. Chem. Phys. 59 (1974) 4184.

[S] T.E. Madey, Surface Sci. 33 (1972) 355.

[9] D.J. Estrup and J. Anderson, J. Chem. Phys. 46 (1967) 567.

[lo] G. Sanders and L.D. Schmidt, to be published.

[ll] P.W. Tamm and L.D. Schmidt, J. Chem. Phys. 52 (1970) 1150.

[12] T.E. Madey, Surface Sci. 36 (1973) 28.

(131 S.M. Ko, Ch. Steinbruchefand L.D. Schmidt, Surface Sci. 43 (1974) 521. [14] L.R. Clavenna and L.D. Schmidt, Surface Sci. 22 (1970) 365.

[15] H.H. Farrell, H.S. Isaacs and M. Strongin, Surface Sci. 38 (1973) 38.

[16] P.W. Tamm and L.D. Schmidt, J. Chem. Phys. 55 (1971) 4235.

[ 171 Ch. Steinbruchel and L.D. Schmidt, to be published.

[18] T.W. Haas, J.T. Grant and G.J. Dooley III, J. Vacuum Sci. Technol. 7 (1970) 43.

[19] J.R. Smithet al., Phys. Rev. Letters 30 (1973) 610; 32 (1974) 774.