Interaction between D2 and N2 on (100) oriented W foil

Applied Surface Science 35 (1988-89) 435-453 North-Holland, Amsterdam

INTERACTION B E T W E E N

D2

AND

435

N2

ON (100) O R I E N T E D W F O I L

X.-L. Z H O U and J.M. W H I T E Department of Chemistry, University of Texas, Austin, TX 78712, USA Received 26 May 1988; accepted for publication 12 September 1988

The coadsorption of D 2 and N 2 on W foil (highly (100) oriented) at 140 K has been studied using temperature programmed desorption (TPD). The chemisorption and desorption of D 2 and N 2 is in agreement with data on W(100). For dosed mixtures of D 2 and N2, the dissociative adsorption is competitive. Preadsorbed N(a) (or D(a)) blocks the dissociative adsorption of D 2 (or N2) linearly. The displacement of one adsorbate gas by the other does not occur at 140 K. Coadsorbed N(a) shifts D(a) to a lower energy binding state. On N/'W, there is a weakly adsorbed molecular state of D 2 with E d = 5.5 kcal/mol and v 0) = 107.3 s -1. There is an isotope effect for dissociative adsorption of molecular hydrogen on N / W . Evidence that adsorbed molecular N 2 undergoes both desorption and dissociation as the surface temperature increases is given. No direct bonding interaction between N(a) and D(a) is observed. These results demonstrate that the surfaces of W foils annealed to 2900 K are very good approximations to single crystal W(100) surfaces.

1. Introduction

The chemisorption and desorption of hydrogen and nitrogen on various W surfaces has been extensively studied [1-8]. However, studies of interactions between hydrogen and nitrogen on W surfaces are limited. Rigby [9] studied the adsorption and replacement of hydrogen by nitrogen on a W filament at 300 K and reported that hydrogen did not replace adsorbed nitrogen, that nitrogen did replace adsorbed hydrogen, that the adsorption of hydrogen was reduced when nitrogen was preadsorbed, and that the amount of hydrogen and nitrogen adsorbed was equal to the amount of hydrogen adsorbed on a clean surface. Singleton [10] studied the simultaneous adsorption of hydrogen and nitrogen on a W ribbon at 300 K and reported that the interaction between the two adsorbed species was very weak, and that there was no replacement of one adsorbate by the other. Yates and Madey [11] studied the interactions between chemisorbed hydrogen and nitrogen on W(100) between 100 and 300 K. They reported that nitrogen slowly replaced chemisorbed hydrogen by means of a slight lowering of the hydrogen desorption energy, that there was a stoichiometric ratio between N atoms adsorbed and H atoms displaced from the surface at -- 300 K, and that two weakly bound hydrogen 0169-4332/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

436

X.-L. Zhou, J . M . White / Interaction between D e and N 2 on (100) oriented W

states were populated at 100 K on a nitrogen-covered surface. Ustinov and Urazaev [12], however, reported that there was a N - H interaction on (100)textured W tapes during the simultaneous adsorption of nitrogen and hydrogen as indicated by the synchronous desorption of N 2 and H 2 at = 1200 K. Propst and Piper [13] studied the coadsorption of hydrogen and nitrogen on W(100) using inelastic electron scattering. They found no vibrational frequencies due to adsorbed N - H species. However, when activated hydrogen (produced in an ionization gauge) interacted with nitrogen, N - H vibrational frequencies were found. In this paper we report on the chemisorption of D 2 and N 2 on a W foil that has been cleaned by chemical (oxidation) and thermal (T < 2900 K) methods. This work supports a series of investigations that utilize W as a support for other materials, such as Fe [14]. Our results show that foils treated as described above take on characteristics of W(100) insofar a s D 2 and N 2 chemisorption are concerned [4,11,13]. Such foils, which are less expensive and easier to prepare than single crystals, can thus serve as a reasonable substitute for W(100) when experiments involving many samples of overlayer thin films of metals and metal oxides are involved.

2. Experimental All experiments were performed in a stainless steel U H V chamber evacuated by a He cryogenic pump. The base pressure obtained routinely was ( 2 - 3 ) × 10 -1° Torr. The chamber was equipped with a single-pass CMA for AES, a four-grid LEED optics, and a line-of-sight quadrupole mass spectrometer (QMS) for T P D measurements and residual gas analysis. Further details of the experimental chamber have been reported previously [14]. The W foil (-- 1 cm 2, 0.127 mm thick, 99.999% purity) was cleaned first by oxidation at = 1800 K to remove C and S, and then by electron beam heating to about 2900 K for about 30 s to remove O. This cleaning procedure was repeated until no contamination was detectable by AES. The crystal was cooled by contact with a liquid nitrogen reservoir. The temperatures were measured with a W - 5 % R e / W - 2 6 % R e thermocouple spot-welded to the back of the foil. The T P D temperature ramp was 9 K / s . All gases were dosed through a gas doser (a 3 mm ID tube) with the W substrate at 140 K. D 2 , H 2 and N 2 were purified by passage through a liquid N 2 trap. In a few experiments, N H 3 was used as a source of N(a). It was purified by several f r e e z e - p u m p - t h a w cycles in liquid N 2 before dosing. The pressure rise during dosing was (2-3) x 10 -1° Torr at the system ion gauge with the sample turned away from the doser. None of the exposures were corrected for ion gauge sensitivity. For TPD, the nomenclature A / / B + C / D / W is used, where A was followed in TPD, D was dosed first, and a mixture of B and C were dosed second.

X . - L Zhou, J.M. White / Interaction between O 2 and N 2 on (100) oriented W

437

3. Results 3.1. T P D o f N 2 and D 2 adsorbed alone on W

After cleaning the crystal, LEED analysis shows spot patterns and diffuse background intensities characteristic of disordered W(100). The D 2 and N 2 desorption spectra are like those for W(100) [4,11,14], but unlike those for polycrystalline and other single-crystal W surfaces [15]. Fig. 1 presents the D 2 TPD profiles for increasing D 2 exposures (2 × 10-1° Tort measured at the ion gauge with the sample turned away from the doser) at 140 K. D 2 desorption has two peaks, ,8] at 430 K and f12 at above 520 K. The f12 and fll peaks increase sequentially and, at saturation, the ratio of the peak areas, f l l / f l 2 , is 2.0 + 0.2. There is also a low temperature ( < 380 K) shoulder for high exposures. With the exception of this shoulder, these results are in excellent agreement with previous work involving both H 2 and D 2 on single-crystal W(100) [4,11]. This comparison is made more directly in section 4.5 below. The /3t peak exhibits first-order desorption kinetics while the /~2 peak is second-order. Assuming a pre-exponential factor (v (1)) of 1013 s-] [16], an activation energy (Ea) of 25.7 + 1.0 kcal/mol is obtained for /3]-D2

16.2

Ed=33.1±t.0 kcal/moI

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~r~=~15.3

tO

g

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~..83

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520

680

Temper-etur'e /K Fig. 1. D 2 TPD spectra after dosing various amounts of D 2 (2 × 10-10 Torr with sample turned away from doser) on clean W foil at 140 K. The inset is plot of ln(oT~) against 1/Tp for f12-D2 desorption. In this plot, the TPD area which is proportional to the coverage, o, is used. The TPD heating rate, fl, was 9 K / s . The label, D 2 / / D 2 / W , denotes D 2 TPD after exposure of W to D 2.

X.-L. Zhou, J . M . White / Interaction between D 2 and N 2 on (100) oriented W

438

desorption. Using a m e t h o d that does not assume a value for V (1) [17], E d = 24.2 4- 1.0 k c a l / m o l and v (a) = 1011-5 ±°5 s 1 are obtained. For f12-D2 desorption, a linear plot of l n ( o T 2 ) against 1/Tp (the inset of fig. 1) is obtained. The slope of the plot yields E d = 33.1 _+ 1.0 k c a l / m o l . The absolute saturation coverage of hydrogen on W(100) is controversial, lying between 1.5 x 1015 and 2 x 1015 a t o m s / c m 2 [1]. Recent measurements favor the latter [30,3]]. A s s u m i n g that the surface is (100) and that the full saturation (fl, +/32) coverage is 2.0 x 1015 a t o m s / c m : , then the saturation coverage for fl2-D is 6.67 x 1014 a t o m s / c m 2 and v (2), as a result, is 0.03 _+ 0.2 c m 2 a t o m - 1 s-1. These results for an annealed polycrystalline foil are in close agreement with single-crystal W(100) work; T a m m and Schmidt [4] reported that E d = 26.6 k c a l / m o l for i l l - D 2 desorption by assuming v (1) = ] 0 1 3 s - 1 , and E d = 32.6 k c a l / m o l and u (2) = 0.042 cm 2 a t o m - 1 s - 1 for f12-D2 desorption by taking a saturation coverage of fl2-D of 2.5 x 1014 a t o m s / c m 2. T P D spectra after N 2 exposure are shown in fig. 2. The exposures of N 2 (2.5 x 10 - l ° Torr) were m a d e through the doser at 140 K. N 2 desorbs at 190 K

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TernDeeat ur, e / K Fig. 2. N 2 T P D spectra after dosing various a m o u n t s of N 2 (2.5 × 1 0 - l0 Torr through doser) on clean W at 140 K. The dashed curve is N 2 desorption after saturating the surface with N H 3 above 400 K. Normalized to the T P D area of the dashed curve, the coverage (o N ) of surface atomic N (ill and f12 peaks) achieved by dosing N 2 at 140 K and the dosing times in s (in parentheses) are (a) 0.04 (12), (b) 0.08 (25), (c) 0.15 (50), (d) 0.24 (85), (e) 0.27 (100), (f) 0.35 (150), (g) 0.49 (250) and (h) 0.56 (600). The dotted curve is synthesized for a /3] peak characterized by E a = 35.5 k c a l / m o l and ~,(1)=1074 s -1 and a f12 peak with E a = 77.1 k c a l / m o l and u (2) = 1 0 -2 cm 2 a t o m 1 s 1. The dot-dash curve is synthesized for fll with Ea = 60 k c a l / m o l and u (1) = 1013 s - ] . The inset is the plot of l n ( o T2 ) against 1 / T p for f12-N2. The T P D heating rate was 9 K / s .

X.-L. Zhou, J.M. White / Interaction between D z and N 2 on (100) oriented W

439

(7 state), about 970 K (ill state) and above 1300 K (f12 state), in agreement with previous work on single-crystal W(100) [18,19]. The f l - N 2 desorption peaks are quite different compared to desorption from a polycrystalline W filament (ill is much more intense than f12) [20-22]. The ), peak is from adsorbed molecular N 2 while the fll and f12 peaks are from recombination of adsorbed atomic N [18]. Initially, only f12-N2 desorption is observed. At higher exposure, "y and ill-N2 desorptions appear and increase together. Above 1200 s, both y and f12 are saturated but the fll peak continues to grow very slowly. Effectively then, the sticking coefficient ( < 10- 3 ) for ill-N: adsorption is very low [18]. A much more intense ill-N2 peak appears after dosing N H 3 above 400 K (broken curve in fig. 2). Because no H 2 desorbs in T P D after dosing N H 3 above 400 K, we conclude that no N H x (x = ! - 3 ) species accumulate. At saturation using NH3, the N 2 T P D ratio of fll to f12 is unity. This indicates that the total coverage of N on W(100) is 1 × 1015 a t o m s / c m 2 since the saturation coverage of f12-N2 state is 5 × 1014 a t o m s / c m 2 [28,29]. Compared to the broken curve, a 1200 s dose of N 2 at 140 K only reaches 58% of the m a x i m u m fl-N coverage (we denote this as o N = 0.58): fl~ is saturated but fll is not. When the sample temperature is raised above 300 K, dosing N 2 populates/32 but n o t f l l - N 2 . This agrees with previous reports [6,23]. The ill-N2 desorption appears to be first-order, while f12-N2 desorption is second-order. A linear plot of ln(oTp) against 1/Tp (the inset in fig. 2) yields E d = 77.1 _+ 2.0 k c a l / m o l and v (2) = 1 0 - 2 + 0 . 2 c m 2 atom i s-1 for f12-N2 desorption, in agreement with a previous report [1]. Using these results, the fl~-fl2 envelope can be decomposed into separate components. We attempted to treat the difference (total N 2 - f12-N2 T P D ) as a first-order process. Assuming lV(1) = 1013 S - 1 [16], E d is 60.0_+ 2.0 k c a l / m o l for f l u N 2 desorption. However, Chart et al.'s [17] method gives E o = 35.5 + 2.0 k c a l / m o l and p(1) = 107.4_+ 0.5 S-1. Clavenna and Schmidt [18] reported Ed = 49 k c a l / m o l for ill-N2 desorption by assuming v O) = 1013 s-1. Computer synthesis of model fl: and fie T P D profiles indicates that E a --35.5 k c a l / m o l and v (x) = 1 0 7.4 s - 1 fits the fix-N2 peak (dotted curve of fig. 2) much better than E d = 60.0 k c a l / m o l and v O) = 1013 s -1 (dot-dash curve of fig. 2).

3.2. Simultaneous adsorption of

D2

and N 2

Fig. 3 shows the T P D spectra of D 2 after simultaneous exposure of D 2 (2 x 10 -1° Torr) and N 2 (2.5 × 10 -1° Torr) through the doser with the W at 140 K. Besides fll and /32 desorption states for D 2, a desorption shoulder (denoted as flo) at 370 K is observed at high exposures. The dashed curve is for D 2 saturation in the absence of N 2. For N 2 desorption (not shown), there are only ~, and f12 peaks. Under similar conditions of total N(a) coverage, ill, would also be populated in the absence of D 2. N o N D 3 formation was

440

X . - L Zhou, J . M . White / Interaction between D 2 a n d N 2 on (100) oriented W

A

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02//02 + % / W

O3 0

I

240

I

I

I

3BO

520

660

Temperat:ure

/K

Fig. 3. D 2 T P D spectra after s i m u l t a n e o u s l y d o s i n g N 2 (2.5 x 10 10 Torr) a n d D 2 ( 2 × 1 0 10 Torr) on W at 140 K. D o s i n g t i m e (s) is (a) 15, (b) 25, (c) 50, (d) 75, (e) 100, (f) 150 and (g) 600. D a s h e d curve is D z T P D from a surface s a t u r a t e d in the absence of N 2. The label, D 2 / / D 2 + N 2 / W , d e n o t e s D 2 T P D after e x p o s u r e to a m i x t u r e of D 2 a n d N 2.

observed. N o D 2 desorbs in the 7 0 0 - 1 5 0 0 K region where /3-N 2 desorbs, i n d i c a t i n g the absence of previously reported [6,12] stable N D x (x = 1, 2) species which w o u l d decompose a n d s i m u l t a n e o u s l y desorb as N 2 a n d O 2. Fig. 4 shows the u p t a k e curves of D 2 a n d N 2 as a f u n c t i o n of dosing time. T h e / 3 - N 2, T-N 2 a n d / 3 - D 2 T P D areas are n o r m a l i z e d to the 1200 s doses of D 2 a n d N 2 i n d i v i d u a l l y o n clean W at 140 K. F o r D 2 a d s o r p t i o n alone (filled circles), s a t u r a t i o n occurs at a b o u t 500 s. F o r N 2 a d s o r p t i o n alone, "¢-N2 (filled triangles) sets in at 50 s a n d increases linearly with exposures below 600 s a n d saturates at 1200 s. The slope of t h e / 3 - N 2 T P D area versus time curve (filled squares) drops smoothly with time. F o r the s i m u l t a n e o u s exposure of N 2 a n d D 2, the a d s o r p t i o n s are competitive. A s a t u r a t i o n (1200 s) simultan e o u s dose gives = 60% of D 2 (open circles), = 40% of /3-N 2 (open square) a n d = 50% of y-N 2 (open triangle) as c o m p a r e d to dosing D 2 a n d N 2 alone for 1200 s. U n l i k e the case for N 2 alone, there is n o offset for T-N 2. We suppose that this reflects i n h i b i t i o n of some N 2 d e c o m p o s i t i o n by c o a d s o r b e d D 2. The total s a t u r a t i o n a m o u n t of D a n d N adsorbed follow the relation o D + (ON/0.58) = 1, where o D = 1 a n d o N = 1 refer to s a t u r a t i o n coverages of D(a) (/31 + 132) from D 2 a d s o r p t i o n a n d N(a) (/31 +/32) from N H 3 adsorption, respectively. Similar relationships hold for post-dosing N 2 o n D / W a n d D 2 on

X.-L. Zhou, J.M. White / Interaction b e t w e e n D e a n d N e on (100) oriented W

•

o2M~/w

0

#

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100

200

300

Dosing

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600

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/s

Fig. 4. D 2 and N 2 TPD areas as a function of dosing time (from figs. 1-3). (e) D 2 from D z / W ; (U) fl-N 2 from N 2 / W ; (A) y-N 2 from N 2 / W ; ( 0 ) D 2 from D 2 + N 2 / W ; (O) fl-N z from D 2 + N 2 / W ; (zx) y-N 2 from D 2 + N z / W . The (O), (n) and (A) curves are all normalized to uiaity at 1200 s. The (0), (121)and (zx) curves are all measured with respect to these normalized areas. T h e inset is an expansion of these curves for short exposure times.

N / W (see below). Fig. 4 (inset) also indicates that the i n i t i a l sticking coefficients for dissociative adsorption of N 2 and D 2 in the simultaneous adsorption are slightly, but not strongly, lower than those measured in the individual adsorptions of N 2 and D 2. This suggests that at low surface coverages the adsorptions of N 2 and D 2 interfere rather weakly with each other.

3.3. Post-dosing D 2 (and N2)

on

N / W (and D / W)

W e show in fig. 5 the D 2 T P D profiles after post-dosing N 2 (2.5 × 10-~° Torr through the doser) for 1200 s with different coverages of atomic D(a). The broken curves are the corresponding D 2 T P D without dosing N 2. In the

442

X.-L. Zhou, J . M . White / Interaction between D 2 and N 2 on (100) oriented W

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240

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380 Temperature

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/K

Fig. 5. D z T P D f r o m a 1200 s dose of N 2 (2.5 × 10 - 1 ° T o r r ) on D / W . 0 D is (a) 0.17, (b) 0.33, (c) 0.58 a n d (d) 0.86. T h e b r o k e n c u r v e s show the c o r r e s p o n d i n g D 2 T P D in the a b s e n c e of N. T h e inset s h o w s T P D a r e a s of 7-N2 ( o ) a n d /3-N 2 (O) as a f u n c t i o n of o D. T h e / 3 - N 2 a n d y - N z T P D areas are n o r m a l i z e d to the c o r r e s p o n d i n g s a t u r a t i o n a r e a s o n the clean surface.

presence of D(a), less N 2 desorbs (inset) but the 7 and/3 peak positions do not change (not shown). Post-dosing N 2 shifts part of the D 2 desorption from higher temperature states (ill and /32) to lower temperature state (/30)Qualitatively, B2-D2 shifts about 150 K to form/~0-D2 while fl]-D 2 shows little change. No displacement of D(a) by dosing N 2 is observed at 140 K. Since D 2 desorption from N 2 / D / W starts below 300 K, it is clear that, just as for W(100) [11], the slow displacement of D(a) by N 2 will occur at 300 K. Surface D(a) blocks the adsorption of 7-N and tg-N2 (inset in fig. 5). For /~-N2, post-dosing N 2 on D / W follows o D + a~/0.58 -- 1 just as for simultaneous adsorption of D 2 and N 2. For "y-N2 the decay with increasing D(a) is also linear but on a surface saturated with D(a), "y-N 2 still adsorbs to 30% of its saturation value. The same results are found on single-crystal W(100) [11]. Fig. 6 presents the D 2 T P D profiles after dosing D 2 for 1200 s in the presence of various coverages of atomic N with the W at 140 K. The N coverages were prepared by dosing N 2 and flashing to between 500 and 600 K. A new desorption peak of D 2 (denoted as a) appears at 175 K. This peak increases with o N while the/31 and 132 peaks decrease. The t91 peak temperature shifts to lower temperature as a N increases. These results for this annealed foil are in good agreement with previous reports on W(100) [11].

X . - L Zhou, JIM. White

/ Interaction between D e and N 2 on

~a # D2/N/W

B

(100) oriented W

443

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380 Temperature

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520

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660

800

/K

Fig. 6. D2TPD from a 1200 s dose of D2 on atomic N/W. a N is (a) 0.0, (b) 0.08, (c) 0.26, (d) 0.35 and (e) 0.58. The inset shows the D2 TPD area as a function of o N. The D2 TPD area is normalized to the saturation value on clean W.

A p p l y i n g C h a n et al.'s m e t h o d [17], a n activation energy of 5.5 _+ 0.6 k c a l / m o l a n d a p r e - e x p o n e n t i a l factor of 107.3 -+0.3 s - 1 are o b t a i n e d for a - D 2 desorption. Surface N(a) blocks a d s o r p t i o n of D 2 linearly (inset of fig. 6). F o r fl-D 2 adsorption, o D + (ON/0.58) ----1. F o r the m a x i m u m coverage realized by dosing N 2, o N = 0.58, o n l y ot-O 2 desorbs a n d ai~ = 0.45. W h e n the surface is saturated with N(a) from N H 3 decomposition, o N = 1.0, n o O 2 is f o u n d in T P D after a 1200 s dose of D 2. Thus, it is reasonable to extrapolate linearly the total D 2 peak area curve (open squares). T o gain further insight into the a d s o r p t i o n of D 2 o n N / W , we m e a s u r e d the T P D profiles for o N = 0.16 as a f u n c t i o n of O 2 exposure (fig. 7). A t low exposures (curve a), only flz-D2 appears. F o r higher exposures (curves b - f ) , the a a n d fll peaks, as well as the t e m p e r a t u r e region between them, grow simultaneously. T h e fll peak also shifts slightly to lower temperatures with increasing D 2 exposure. T h e D 2 u p t a k e (total T P D area versus dosing time) is shown as the inset in fig. 7. The T P D area increases nearly linearly below 100 s, i n d i c a t i n g a nearly c o n s t a n t sticking coefficient. Fig. 8 shows, for a N = 0.29, the T P D spectra of H 2 , H D a n d D 2 after s i m u l t a n e o u s dosing of H 2 (3 X 10 -10 Torr) a n d D 2 (2 × 10 -a° Torr) for 1200 s. T h e peak shapes a n d peak t e m p e r a t u r e s are the same, i n d i c a t i n g n o strong isotope effect in desorption. Isotope mixing in the a peak is less extensive t h a n

444

X . - L Zhou, J . M . White / Interaction between D e a n d N 2 on (100)oriented W

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2 o

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t

100

240

--

Dosing time /l

i

i

380

520

Temperature

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660

/K

Fig. 7. T P D s p e c t r a o f D 2 d o s e d w i t h o N = 0.16. T h e e x p o s u r e s o f D 2 (2 X 1 0 - 1 0 T o r r t h r o u g h the d o s e r ) a t 140 K in s a r e (a) 10, (b) 25, (c) 50, (d) 75, (e) 100 a n d (f) 200. T h e inset s h o w s the total D 2 T P D a r e a versus d o s i n g time.

l/

B2

~

Ha

~

HD

~2~

D2 ~00

240

3 0 Temperature

520

660

fl00

/K

Fig. 8. T P D o f H 2, H D a n d D 2 a f t e r a l 2 0 0 s s i m u l t a n e o u s d o s e o f H2(3×10 -1° Torr) and D 2 ( 2 x 1 0 ~1° T o ~ ) o N = 0.29. H 2 a n d D 2 w e r e s t o r e d i n d i f f e r e n t c o n t ~ n e r s .

X.-L. Zho~ J.M. White / Interaction between D e and N e on (100) oriented W

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ii

II

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I00

400

700

I000

1300

1600

Temperature /K Fig. 9. D 2 a n d N 2 T P D after dosing various amounts of O 2 (2 X 1 0 - ao Tort) in the presence of both molecular (N2) and atomic (N) nitrogen. T h e sofid curve for N 2 is obtained in the absence of D 2. There is a N = 0.49 f l - N desorbed without dosing D 2. T h e broken curve for N 2 is after dosing saturation amount of D 2 o n the same surface as for the solid curve. The inset is the TPD area of D 2 (A), .Y_N2 ( I ) a n d f l - N 2 (O) as a function of D 2 dosing time. Dosing time (s) is (a) 50, (b) 100, (c) 200, (d) 300 a n d (e) 6 0 0 o r 900.

in # peaks and the peak area ratio, a/fl, is much higher for D 2 than for either H D or H E. The desorption in the temperature region between the a and fl peaks is dominated by H 2 and H D . This will be discussed below. For comparison (fig. 9) we dosed D 2 onto a surface immediately after it had been dosed with N 2 (2.5 X 10-10 Torr) for 250 s with the W at 140 K (i.e. no flash to 700 K). Thus, 0 2 interacted with a surface configuration that would have given a T P D spectrum for N 2 like that of fig. 2g. Unlike those surface with n o y - N 2 (fig. 6), a fl0 peak for D 2 is observed here just as in fig. 3 where N 2 and D 2 were simultaneously dosed. Further, the a - D E peak is much smaller relative to f l - D 2 (compare fig. 9 and fig. 6). The inset of fig. 9 shows the T P D areas of f l - N 2 (circle), y - N 2 (square) and fl-D 2 (triangle) as a function of O 2

446

x. -L Zhou, J.M. White / Interaction between D 2 and N e on (100) oriented W

dosing time. As the D 2 exposure increases, the/~-N 2 T P D area decreases, while y-N 2 increases. However, the total N 2 T P D area remains the same, indicating no displacement of N 2 by dosing O 2 a t 140 K. There is o N = 0.49 of f l - N 2 desorbed (solid curve of fig. 9) without dosing D 2. Of this, o N = 0.12 is transferred t o y - N 2 by a saturation dose of D 2 and /31-N 2 becomes barely visible (broken curve). The total amount of fl-D adsorbed here is o D -- 0.4. All these results strongly indicate that some 3,-N 2 (adsorbed without dissociation) dissociates during the temperature ramp and that this dissociation is inhibited by coadsorbed D because sites for y - N 2 dissociation are, in part, occupied by D(a).

4. Discussion 4.1. Restructuring o f W f o i l and the desorption o f ill-N2 a n d ill-De

The tendency toward (100) orientation of heat-treated polycrystalline W foils has been reported previously [6,24]. Reed and Lambert [6] only observed a single H 2 desorption peak at about 450 K after exposing (100) oriented W to H 2 at 300 K. We believe that our foil which shows two H 2 T P D peaks at the temperatures found for W(100) is more strongly (100) oriented than Reed and Lambert's because we annealed the crystal to 2900 K while they only annealed to 2600 K. Crystalline W has the bcc structure. The surface density is 1 × 1015 a t o m s / c m 2 for (100), 1.42× 1015 a t o m s / c m 2 for (110) and 0.58 × 1015 a t o m s / c m 2 for (111). The polycrystalline surface is a mixture of these and perhaps other faces. If we assume that the three low index faces contribute equally to the polycrystalline surface, the average surface atom density is 1 × 1015 a t o m s / c m 2, which is equal to that for (100). Before heating we suppose that the surface atoms are disordered. At high temperatures, these atoms gain enough energy to rearrange to give more long-range order and more stable structure. As it is easier to move surface atoms, those rearrangements that keep the surface atom density relatively constant may be favored. If so, the (100) face may be kinetically preferred even though the (110) is more dense and stable. However, the actual mechanism of the restructuring of the polycrystalline foil to predominantly (100) oriented surface and whether the restructuring is only confined to the surface region or also happens in the bulk remain unclear. Futher studies by X-ray diffraction or other means are called for. It has been proposed that fl2-H occupies every other four-fold hollow site and that fll-H occupies bridge or atop sites on W(100) [4]. Further, it has beer proposed that fl2-N occupies the same sites as fl2-H while y-N 2 and /31-b occupy the same site as fll-H [18]. However, recent EELS measurement,

X . - L Zhou, J.M. White / Interaction between D e and N 2 on (100) oriented W

447

indicate that hydrogen is bound to W(100) surface in bridge sites at all coverages [32] and that flE-N occupies four-fold sites while ~,-N 2 occupies bridge and terminal sites on W(100) [8]. Therefore, the explanation for the first-order desorption of //~-H and -N and the second-order desorption of //2-H and N in terms of multiple binding states on W(100) [4,18] is unsatisfactory. It is now well known that both adsorbed hydrogen and nitrogen on W(100) induce surface reconstruction [5,31-34]. The explanations for the different desorption order and relative size of//1 and 132 peaks for H 2 and N 2, therefore, appears related to the surface reconstruction induced by H and N [35]. For hydrogen adsorption, W(100) undergoes reconstruction at low coverages and the reconstructed surface gradually acquires the bulk spacings with further increasing coverage [33]. For nitrogen adsorption, W(100) surface also undergoes reconstruction at a N < 0.5 [34]. We suggest that a W(100) surface saturated with both ill- and fiE-N, like one saturated with H, is unreconstructed. The difference between H / W ( 1 0 0 ) and N / W ( 1 0 0 ) is that H bonds in bridge sites while N bonds in four-fold sites. Since the surface reconstruction is reversible and coverage dependent [33], we propose the following desorption process for H 2 and N 2 o n W(100). For a H- or N-saturated W(100) surface, as H 2 o r N 2 desorbs during TPD, the surface undergoes reconstruction. Above a certain critical coverage, the surface is more or less ordered and the desorption of H E or N 2 can occur via a n o n r a n d o m recombination of neighboring atom pairs, yielding pseudo-firstorder kinetics f o r / / l - H E and -N 2. Below that critical coverage, the surface is seriously reconstructed and the desorption via nonrandom recombination becomes difficult. As the surface temperature further increases, the diffusion of the undesorbed atoms becomes fast and the random recombination desorption process dominates, resulting in second-order desorption kinetics for fiE-HE and - N 2. According to this proposal, the relative size of the/31 and fl2 peaks for H E and N 2 in T P D indicated that a critical coverage is 6.67 X 1014 atoms,/cm 2 for H and 5 × 1014 a t o m s / c m E for N. The reconstruction of N / W ( 1 0 0 ) surface takes place such that the four surface W atoms in the site occupied by N are uniformly displaced towards the N adatom [34]. This leaves other four-fold sites that are farther apart than in the original unreconstructed sites. This is a likely reason that the sticking coefficient f o r / / I - N adsorption becomes very low when //2-N is saturated as extra energy is required to move W atoms from the position in the reconstructed phase back to the position in the unreconstructed phase in order to accommodate //1-N. In this sense, the adsorption of //a-N is activated and raising the surface temperature should increase its sticking probability. However, since the dissociative adsorption of N proceeds via a molecular precursor state [18], raising surface temperature decreases the residence time of the precursor state much faster than it increases the conversion rate from the precursor state to atomic N state. As a result, raising the surface temperature

448

X.-L. Zhou, J . M . White / Interaction between D e a n d N 2 on (100) oriented W

decreases the sticking coefficient for fll-N adsorption. The relatively low first-order E d and v(1) for /3a-N 2 desorption obtained from Chan et al.'s method illustrates the compensation effect [25]. A heterogeneous distribution of pairs of N atoms may also be involved.

4.2. a-D: N o a-D 2 desorption is observed on either clean or P-covered W [26]. On W(100) covered with N(a), dosing H 2 a t 100 K produces a state that desorbs at even lower temperatures than a - H 2 [11]. A weakly bound state (150 K) of H 2 on CO covered W(100) is also observed [27]. Since a-D 2 is absent on clean W, it is clear that this state is induced by coadsorbates such as surface N(a). The dissociative adsorption of H 2 on W(100) proceeds via a precursor state which is probably weakly chemisorbed [4] and it is tempting to suppose that a - D 2 is related to this precursor state. It is interesting that isotope mixing occurs in the a-state (fig. 8). If the a-state were isolated adsorbed molecular D 2 and H 2, one would not expect any isotope mixing. If the a-state were dissociatively adsorbed, then the extent of isotope mixing would be the same as in r-states. However, neither case is observed. Since there is continuous desorption of D 2 (H2) in the temperature region after the a peak and before the fll peak and there is extensive isotope mixing in this region, we propose the following process for the adsorption of H 2 on N/W. On the N / W surface, there are two kinds of sites: (a) clean W sites and (b) sites which are modified and reconstructed by nearby adsorbed N [34]. The number of (a) sites decreases while the number of (b) sites increases with increasing o N. We propose the normal dissociative adsorption process [4] leading to fl-H 2 occurs on site (a). On site (b), we suppose adsorption is molecular and during T P D there is competition between desorption and dissociation in the temperature region where the a-state desorbs. Transiently, the local coverages of atomic species may reach very high values and kinetically reversible dissociation and recombination may lead to some (incomplete) isotope mixing. Referring to fig. 8, we suppose then that the a peaks are composites of adsorbed molecular species which never dissociate and recombined atomic species formed and recombined during TPD. We do not rule out possible exchange between sites (a) and (b). At low surface coverages, D2(a ) can readily diffuse on W and find sites (a) to form atoms with higher binding energy (/~:-state). This accounts for the absence of a and presence of only f12-D2 on N / W at low exposures (fig. 7, curve a). However, as the surface coverage increases the diffusion of D2(a ) becomes difficult if we assume that D2(a ) does not diffuse readily across D(a) and N(a). D2(a ) will then be accommodated as molecules or dissociate to atoms with lower binding energy than r2 if there are extra sites available nearby. The a-state and dissociatively

X..-L. Zhou, J.M. White / Interaction between D 2 and N 2 on (100) oriented W

449

adsorbed states will then grow simultaneously with exposure provided there is a barrier to the formation of ill-D2. On the surface with a N = 0.16, the sticking coefficient for D 2 (molecular and atomic) adsorption is nearly constant (the inset in fig. 7). We propose that the sticking coefficient for molecular D 2 or a-D2(a)) adsorption is coverage independent and that the dissociatively adsorbed D is mainly formed from the molecularly adsorbed D 2. If so, the total sticking coefficient is coverage independent, resulting in the nearly linear uptake of D 2 on N / W . The slow step is the dissociation of D2(a ) to D(a), and this involves the diffusion of D2(a ), D - D bond breaking and W - D bond formation. We expect the reaction of H2(a ) ~ 2H(a) to be faster than D2(a ) ~ 2D(a) because of the weaker H - H bond and the faster diffusion rate of H2(a). As a result, the distribution between the a-state and the other states will be ( D / H ) ~ > ( D / H ) B (fig. 8). Because of the extensive isotope mixing, we suppose that the D 2 desorbed in the region between the a and fll peaks is from surface D(a). a - D 2 (H2) has been associated with the sites which adsorb 3,-N2 at low temperatures [11]. The results shown in fig. 9 may support this. When both 7-N 2 and f l - N are present, there are fewer sites available for a - D 2. Since D 2 does not displace it, less a - D 2 adsorbs when 7-N 2 is present. It is of interest that no a - D 2 is observed after simultaneous adsorption of D 2 and N 2. This, we believe, is due to displacement of a - D 2 by N2(g ) to form y-N 2. This may be the reason that 7-N 2 increases linearly with exposure before it is saturated (open triangles in fig. 4) when D 2 and N 2 are simultaneously dosed. It is very interesting that molecular D 2 (H2) is stable on W(100) with N(a) [11] and CO(a) [27] but not with P(a) [26]. The stabilization of adsorbed m o l e c u l a r hydrogen, then, must depend on the nature of the preadsorbed species. One possible molecular stabilization mechanism is the formation of weak hydrogen bonds as shown below: H

/" H

H ....... 0

I

N

H

I

C

I Scheme I. This pathway is absent on clean W and is presumably very weak in the presence of P. It would be very interesting to test this bonding scheme using H R E E L S and SIMS. 4.3. flo-D2

As in fig. 1, T a m m and Schmidt [4] observed a small shoulder on the leading edge of the ill-H2 peak and assigned it to H(a) on planes other than (100), i.e., defects in their single-crystal surface. We synthesized a model T P D

450

X.-L. Zhou, J.M. White / Interaction between D e and N 2 on (100) oriented W

curve for i l l - D 2 T P D using E a = 24.2 k c a l / m o l and v (1) = 10115 s -1. However, the model does not fit the saturated experimental T P D curve below 400 K. We conclude that a low temperature state exists in the leading edge of the high coverage T P D curves in fig. 1. It is discussed further in section 4.5. A different state (denoted as fl0) becomes distinguishable in this region after simultaneous adsorption of D E and N 2 (fig. 3), dosing D 2 on W with both N2(a ) (7) and N(a) (fl) (fig. 9) and, especially, post-dosing N 2 on W preadsorbed with D(a) (fig. 5). Whenever the fl0 state is distinguishable, the fll and fiE states are attenuated, indicating that some D(a) is shifted to lower binding energy when N(a) is coadsorbed. Thus, we do not interpret fl0 hydrogen desorption in terms of defects; rather, we suppose its presence is due to alteration of adsorption energies by local interactions with other adsorbed species. H o et al. [8] reported that coadsorption of H with N on W(100) lowers the vibrational frequency of W - H . Tentatively, they attributed this change in the vibrational frequency to the shift of H from bridge to four-fold sites induced by neighboring N atoms [8]. Whether the shift of D(a) from fll and /32 to 13o induced by N(a) is due to such a binding site change or is due to an electronic effect at the same site is not clear. It does appear that the effect of N(a) is local since the/31-D2 T P D (fig. 5) peak does not move while a fraction of the /32 peak shifts down in temperature by about 150 K to form/30. We note that the fl0 state is distinguishable when D 2 is adsorbed on W precovered with both y - N 2 and fl-N (fig. 9) but not with fl-N only (fig. 6). This suggests that the/3o state is related t o "y-N 2. When the surface temperature increases, some y-N 2 dissociates to N(a) and this shifts some D(a) from /3a and /32 to/30. If so, the dissociation of 7-N 2 even occurs in the case of fig. 9, and as y - N 2 desorbs, vacant sites are left for N 2 dissociation to form /3-N. Yates and Madey [11] reported that H 2 desorptions from H J / N J H 2 / W(100) and from H E / / H E / / N E / W ( 1 0 0 ) a t 300 K are identical. Their result is the same as fig. 5, that is, /31 shifts slightly to lower temperature as nitrogen coverage increases. They did not observe /3o in either case. In our work D 2 T P D after dosing at 140 K does depend on the adsorption sequence of N 2 and D E. P o s t - d o s i n g N 2 on W preadsorbed with D(a) yields a distinguishable/30 peak but does not shift the /31 peak temperature. Post-dosing D 2 on W preadsorbed with N(a) does not yield a distinguishable/3o peak but fix shifts to lower temperature. We believe that the difference between our result and Yates and Madey's is due to the dosing temperature. The surface with preadsorbed N(a) that we used was prepared by dosing N 2 a t 140 K and flashing to 700 K, which r e m o v e s T - N 2 and orders the N(a). Since N(a) is immobile between 140 and 300 K [28] the surface condition realized upon cooling to either 140 or 300 K should be the same. This accounts for the same H 2 ( D E ) TPD. However, when D(a) (or H(a)) is preadsorbed, the surface condition can be significantly different when dosing N 2 at 140 and 300 K because D(a) diffuses much faster at 300 K than at 140 K.

451

X.-L. Zhou, J.M. White / Interaction between D 2 and N 2 on (100) oriented W

The desorption peak temperature of ill-D2 from a D 2 / / D 2 / N 2 / W surface shifts to lower temperature as D(a) coverage increases (fig. 7). This might indicate that for these conditions, ill-D2 desorption tends towards second-order and that the rate-determining step involves more nearly random recombination of D(a). The shift of ill-D2 desorption to lower temperature with increased N(a) coverage (fig. 6) may due to an electronic effect induced by N(a). 4. 4. Competitive adsorption o f

D2

and N 2

We use here the reported total coverage of N(a) on W(100) of 1 × 1015 a t o m s / c m 2 (o N = 1) and the saturation coverage of H(a) of 2 × 1015 a t o m s / c m 2 (o D = 1) [28-31]. Since D(a) adsorption is inhibited completely at or~ = 0.6, one N(a) blocks an average of 3 - 4 sites for D(a). When N(a) is preadsorbed, the atomic deuterium adsorbed at 140 K can be expressed approximately as o D - 1 - 1.7o N and the total deuterium ( a and fl states) as o D = 1 -- o N. The desorption of N 2 on W(100) at low temperature is both molecular and dissociative [8]. Adsorbed molecular N 2 undergoes both desorption and dissociation. However, the competition between desorption and adsorption depends on the number of vacant sites. As the vacant sites become occupied by D(a), more molecular N 2 desorbs (fig. 9). During T P D the sites created by molecular N 2 desorption are available for the dissociation of other molecular N 2 that has not yet desorbed.

....

W

foil

/ \

"-2.

H2

W (1

.,-¢

r-

tJ~ t~

aoo

a~o

ado

4~o

5~o

~6o

~o

' 1040

' t180

' 1320

t,460

i600

Temper'aLure /K Fig. 10. Direct comparison of H 2 and N 2 T P D on (100)-oriented W foil with W(100).

452

X.-L. Zhou, J . M . White / Interaction between D 2 and N 2 on (100) oriented W

4.5. Direct comparison of oriented foil with W(IO0) To establish a direct comparison between W(100) and our annealed foil, we prepared a W(100) substrate [14] by standard methods and, in separate experiments, followed H 2 and N 2 T P D after saturating the surface at 140 K. The results, shown in fig. 10, show the excellent correspondence of the spectra from the two samples. The only significant difference is in the low temperature H 2 T P D where, for the oriented foil, a shoulder is evident. This shoulder resembles that found by Tamm and Schmidt [4] but was largely absent in the spectra of Yates and Madey [11]. We attribute it to the presence of non-W(100) adsorption sites on the annealed foil. This is in accord with the interpretation of Tamm and Schmidt [4]. 5. Summary The work reported here is summarized as follows: (1) Polycrystalline W foil can be highly (100) oriented by thermal annealing at 2900 K. The chemisorption and desorption of D 2 and N 2 on the surface is nearly the same as on W(100). We conclude that the adsorption and desorption of these molecules alone and coadsorbed is not very sensitive to surface defects. (2) The adsorptions of N 2 and D 2 are competitive. D(a) blocks the adsorption of N 2 linearly. N o D(a) is displaced by N 2 at 140 K. Post-dosing N 2 on D / W surface shifts some D(a) to a lower energy binding state. (3) Preadsorbed N(a) blocks the adsorption of D 2 linearly at 140 K. The first-order ~1-D2 T P D shifts slightly to lower temperature as o N increases and may become second-order on N / W . (4) A low temperature (175 K) state of D 2 with E a = 5.5 _+ 0.6 k c a l / m o l and p(1) = 107.3 + 0.3 S - 1 is observed on a N / W surface. This state is molecularly adsorbed and may serve as a precursor for dissociative adsorption on N / W . The amount of atomic D(a) adsorbed at 140 K on N / W can be expressed as o o = 1 - 1.7o N and the total amount (D(a) and D2(a)) adsorbed as o D = 1 O N .

(5) The adsorption of N 2 at 140 K is both molecular and dissociative. Adsorbed molecular N z undergoes both desorption and dissociation as the surface temperature increases. The extent of dissociation can be reduced by coadsorbing D(a). (6) N o detectable N D x species is formed during the coadsorption or subsequent TPD of N 2 and D 2.

Acknowledgement This work was supported in part by the Department of Energy, Office of Basic Energy Sciences.

X . - L Zhou, J.M. White / Interaction between D 2 and N: on (100) oriented W

453

References [1] L.D. Schmidt, in: Interactions on Metal Surfaces, Ed. R. Gomer (Springer, New York, 1975) p. 62, and references before 1975 therein. [2] J. Lee, R.J. Madix, J.E. Schaegel and D.J. Auerbach, Surface Sci. 143 (1984) 626. [3] R.S. PolJzzotti and G. Ehrlich, J. Chem. Phys. 71 (1979) 259. [4] P.W. Tamm and L.D. Schmidt, J. Chem. Phys. 51 (1969) 5352; 52 (1970) 1150. [5] A.H. Smith, R.A. Barker and P.J. Estrup, Surface Sci. 136 (1984) 327. [6] A.P.C. Reed and R.M. Lambert, J. Phys. Chem. 88 (1984) 1954. [7] M.J. Grunze, J. Fuhler, M. Neumarm, C.R. Brundle, D.J. Auerbach and J. Behm, Surface Sci. 139 (1984) 109. [8] W. Ho, R.F. Willis and E.W. Plummer, Surface Sci. 95 (1980) 171. [9] L J . Rigby, Can. J. Phys. 43 (1965) 1020. [10] J.t1. Singleton, J. Vacuum Sci. Teclmoi. 5 (1968) 109. [11] J.T. Yates, Jr. and T.E. Madey, J. Vacuum Sci. Technol. 8 (1971) 63. [12] Y.K. Ustinov and R.S. Urazaev, Soviet Phys.-Tech. Phys. 24 (1979) 495. [13] F.M. Propst and T.C. Piper, J. Vacuum Sci. Technol. 4 (1967) 53. [14] X..-L. Zhou, C. Yoon and J.M. White, Surface Sci. 203 (1988) 53. [15] P.W. Tamm and L.D. Schmidt, J. Chem. Phys. 54 (1971) 4775. [16] P.A. Redhead, Vacuum 12 (1962) 203. [17] C.M. Chan, R. Aris and W.H. Weinberg, Appl. Surface Sci. 1 (1978) 360. [18] L.R. Clavenna and L.D. Schmidt, Surface Sci. 22 (1970) 365. [19] D.A. King, in: Surface Science, Recent Progress and Perspective, Ed. R. Vanselow (CRC Press, Cleveland, OH, 1980). [20] H.F. Winters and D.E. Home, Surface Sci. 24 (1971) 587. [21] T.E. Madey and J.T. Yates, Jr., J. Chem. Phys. 44 (1966) 1675. [22] L.J. Rigby, Can. J. Phys. 43 (1965) 532. [23] D.L. Adams and L.H. Germer, Surface Sci. 26 (1971) 109. [24] F. Gonzalez and J . L de Segovia, Vacuum 37 (1987) 461. [25] J.W. Niemantsverdriet, K. Markert and K. Wandelt, Appl. Surface Sci. 31 (1988) 211. [26] X.-L. Zhou, C. Yoon and J.M. White, to be published. [27] J.T. Yates, Jr. and T.E. Madey, J. Chem. Phys. 15 (1971) 4969. [28] P.J. Estrup and J. Anderson, J. Chem. Phys. 46 (1967) 567. [29] T.E. Madey and J.T. Yates, Jr., Suppl. Nuovo Cimento 5 (1967) 483. [30] D.A. King and G. Thomas, Surface Sci. 92 (1980) 201. [31] I. Stensgaard, L.C. Feldman and P.J. Silverman, Phys. Rev. Letters 42 (1979) 247. [32] M.R. Barnes and R.F. Willis, Phys. Rev. Letters 41 (1978) 1729; W. Ho, R.F. Willis and E.W. Plummer, Phys. Rev. Letters 3,0 (1978) 1463. [33] J.J. Arrecis, Y.J. Chabal and S.B. Christman, Phys. Rev. B 33 (1986) 7906. [34] K. Griffiths, C. Kendon, D.A. King and J.B. Pendry, Phys. Rev. Letters 46 (1981) 1584. [35] R.A. Barker, A.M. Horlacher and P.J. Estrup, J. Vacuum Sci. Technol. 20 (1982) 536.