Comparative study of the first adsorbed layer of xenon and krypton on boron nitride and graphite

Comparative Study of the First Adsorbed Layer of Xenon and Krypton on Boron Nitride and Graphite J. R E G N I E R ,

A . T H O M Y , AND X. D U V A L

L.A.R.1.G.S., Laboratoire Maurice L e t o r t - C N R S , B.P. 104 Route de Vandoeuvre, 54600-- Villers-l~s-Nancy. France

Received July 12, 1978; accepted October 10, 1978 The thermodynamic properties of the first adsorbed layer of xenon and krypton on crystalline boron nitride have been studied between 95 and 125°K and between 77 and 93°K, respectively. They are compared to those previously determined for the first layer of the same gases adsorbed on graphite. With both xenon and krypton the final state of the layer is probably the same as on graphite: a self-determined two-dimensional (2D) solid state having approximately the same density and structure as a (I 1I) plane of the 3D crystal which forms at saturation. As on graphite, the xenon layer also undergoes two successive first-order transitions above a definite temperature: a 2D gas ~ 2D liquid transition and a 2D liquid ~ 2D solid transition. But with krypton, only one phase transition has been observed whereas three occur on graphite: two first-order transitions (2D gas ~ 2D liquid and 2D liquid ~ 2D localized solid) and a localized ~ delocalized transition which is of higher order. The only transition observed with krypton on boron nitride is first order, but the nature of this transition has not yet been determined. INTRODUCTION G r a p h i t e a n d crystalline b o r o n nitride h a v e already been the subject of several comparat i v e s t u d i e s as a d s o r b e n t s in p h y s i c a l ads o r p t i o n ( 1 - 1 1 ) b e c a u s e o f t h e i r v e r y simil a r s t r u c t u r e s . T h e y b o t h b e l o n g to the s a m e hexagonal system with crystal parameters differing b y less t h a n 2% ( a = b = 2 . 5 0 / ~ a n d c = 6.66 A f o r b o r o n n i t r i d e , w h e r e a s a = b = 2.46 A a n d c = 6.71 A f o r g r a p h i t e ) . In this p a p e r w e c o m p a r e t h e t h e r m o d y n a m i c p r o p e r t i e s o f the first a d s o r b e d l a y e r o f x e n o n a n d k r y p t o n on t h e c l e a v a g e face of these adsorbents. It is w e l l k n o w n ( 1 1 - 1 8 ) t h a t t h e first ads o r b e d l a y e r on g r a p h i t e o f t h e s e t w o g a s e s shows successive phase transitions which h a v e g i v e n rise to a n u m b e r o f e x p e r i m e n tal as w e l l as t h e o r e t i c a l i n v e s t i g a t i o n s (17, 19-33). In fact, between a two-dimens i o n a l (2D) t r i p l e p o i n t t e m p e r a t u r e a n d a 2D critical temperature the xenon or krypton layer undergoes two first-order phase trans i t i o n s w h e r e it p a s s e s s u c c e s s i v e l y t h r o u g h

t h r e e d i f f e r e n t 2D s t a t e s : g a s , l i q u i d , a n d solid. M o r e o v e r , t h e k r y p t o n l a y e r u n d e r g o e s an a d d i t i o n a l t r a n s i t i o n f r o m a l o c a l i z e d 2D solid to a d e l o c a l i z e d 2D solid. T h e xenon layer does not show such a transit i o n , p a s s i n g d i r e c t l y f r o m t h e 2D liquid to the 2D s e l f - d e t e r m i n e d solid. O u r p u r p o s e w a s to d e t e r m i n e if t h e s a m e phenomena would be observed with boron n i t r i d e as a d s o r b e n t . EXPERIMENTAL T h e p r e s e n t w o r k is b a s e d e s s e n t i a l l y o n sets o f a d s o r p t i o n i s o t h e r m s d e t e r m i n e d o n w e l l - c r y s t a l l i z e d b o r o n nitride a n d e x f o l i a t e d graphite. Isotherms concerning graphite and the related data have been published (13-15, 21) a n d a r e s u m m a r i z e d in (18). I s o t h e r m s concerning boron nitride which are shown in F i g s . 2 a n d 3 a r e t a k e n f r o m (34). B o r o n nitride was supplied from the Union Carb i d e C o r p . It is a w e l l - c r y s t a l l i z e d w h i t e p o w d e r with a specific s u r f a c e a r e a o f 6 m2/g. The sample was outgassed under vac-

105 0021-9797/79/070105-0752.00/0 Journal of CoUoid and Interface Science, Vol. 70, No. 1, June 1, 1979

Copyright © 1979 by Academic Press, Inc. All rights of reproduction in any form reserved.

106

REGNIER, THOMY, AND DUVAL

uum at 400°C for 3 hr. Results with different samples were reproducible. Isotherms have been determined according to the method and with the same type of apparatus as described in (13). No appreciable hysteresis has been observed, at least in the domain of formation of the first monolayer which was particularly investigated. RESULTS AND DISCUSSION

In the temperature range investigated, the complete isotherms show several steps of comparable height (e.g., the krypton isotherms shown in Fig. 1). They are of the same type as those previously observed with many other solids [see particularly (35) and (36)]. Such isotherms show that on the uniform part of the surface [exposing (0001) faces in the case of graphite and boron nitride] several monolayers form successively (at least three in the case of Fig. I), having approximately the same density, before reaching the saturating vapor pressure P0. Moreover, the existence of "vertical" steps (e.g., in the isotherm at 77°K of Fig. 1) indicates that first-order phase transitions occur in the film before the bulk condensation at P0 of the gas into the quasi-infinite three-dimensional (3D) phase, which is a solid for the adsorbates investigated. 8

KrlBN

~8

2

/ 0 i

L

i

0.5

0.5 k

,

,

ZO

~

i

The first monolayer is complete near a point which has been usually designated as B, or B1 as done in the present paper. For the adsorbate-adsorbent pairs considered here, point B1 is located at a relative pressure P/Po of about 0.05 [see (37) for the x e n o n - g r a p h i t e and k r y p t o n - g r a p h i t e pairs]. It marks the beginning of an approximately linear portion extending to a relative pressure of about 0.4. In this portion the isosteric heat of adsorption (qst) decreases to a value close to the heat of 3D condensation (qc): This corresponds essentially to the build-up of a gaseous 2D film in the second layer, and also presumably to some nonnegligible interparticle condensation (38). Owing to accurate determinations from LEED experiments carried out by Chinn and Fain (28, 29), it has been firmly established that for the krypton-graphite pair, completion of the first monolayer occurs effectively near point Ba. Moreover, it has been shown that at this point the structure and the density of the layer are close to those of a (11 l) plane of the 3D crystal forming at saturation underp0. Ifda (Kr) is the distance between two nearest neighbor krypton adatoms in the first layer at its completion, and if d0 (Kr) is the distance between two nearest neighbor krypton atoms in a ( l l l ) plane of the 3D crystal, then dl(Kr) - d0(Kr) = 4.06 ,A_

/

2

Density and Structure of the First Layer at Its Completion

1.0 i

i

i

,

i

p/po

FIG. I. Adsorption isotherm of krypton on crystallized boron nitride. Curve a, isotherm at 77°K on a sample of 2.7 m2/g surface area [from (9)]; curve b, isotherm at 90°K on a sample of 4.5 mZ/gsurface area [from (11)]; 0, ratio of amount adsorbed to amount adsorbed at point B~ where the first monolayer is complete; P/Po, relative pressure (ratio of adsorption equilibrium pressure to saturating vapor pressure P0). Journal oJColloid and InterJace Science, Vol. 70, No. 1, June 1, 1979

the difference between d~(Kr) and do(Kr) being of the order of a few percent. [It has been shown (18) that these results are in agreement with the isosteric heats of adsorption obtained by calorimetry or from the isotherms.] It has also been shown (18) that in the temperature range investigated nB,(Xe) _ 0.845 ___0.01 nB,(Kr) where nB,(Xe) and nB,(Kr) are the respective amounts of xenon and krypton adsorbed at point B1 of their isotherms determined on

FIRST ADSORBED

LAYER

THERMODYNAMIC

the same samples of exfoliated graphite. Since nB~(Xe) d~(Kr) nB,(Kr)

d~(Xe)

as far as nB~(Xe) and nB~(Kr) represent exactly the maximum amounts of Xe and Kr adsorbed in the first layer on the uniform part of the surface, it follows that dl(Xe) = do(Xe) = 4.42 A. It has thus been established that although the build-up of the first layer of xenon and krypton on the cleavage face of graphite occurs differently for each of these two gases, the ultimate state of the layer is the same in both cases: a quasi-self-determined 2D solid having approximately the same structure and density as a (11 l) plane of the 3D crystal. [The same conclusion applies also to argon and neon (18).] As no structure determinations of adsorbed films on boron nitride have yet been made by direct methods, there are no data permitting clear conclusions such as those applying to graphite. Nevertheless, we are inclined to believe that these may apply equally to the X e - N B and K r - N B pairs since we have found that the ratio nB~(Xe)/nBl(Kr) has the same value (0.83 +__0.01) as with graphite, which is therefore also close to the value of d~(Kr)/d~(Xe). It is therefore very likely that on boron nitride, the first layer of not only xenon and krypton, but also of the other rare gases, is at completion quasi-self-determined such as on graphite. With all these gases and on both adsorbents, the density and structure of the layer would be almost the same as on a surface with uniform adsorption potential, i.e., without potential wells. Phase Transitions in Build-up of the First Layer Xenon case. As on graphite, the xenon layer undergoes two phase transitions between a 2D triple point temperature--Tt(2D) - - a n d a 2D critical temperature--Tc(2D)--

PROPERTIES

107

(see Fig. 2). With increasing adsorbed amounts, the layer therefore successively changes into three 2D states which may be considered as gaseous, liquid, and solid. There are very few differences in the distances between potential wells on boron nitride and graphite (they are respectively 2.50, 4.33, and 5 . 0 0 / ~ . . . ; and 2.46, 4.26, and 4.92 A). On boron nitride therefore, as on graphite, the 2D solid takes its ultimate structure as soon as it appears (at point A~ of the isotherms of Fig. 2). The temperature range for the existence of the 2D liquid [Tc(2D) - Tt(2D)] is about the same as with graphite, the triple point and critical point temperatures themselves not differing much in both cases (Table I). However the 2D liquid-2D solid transition appears to be first order in a clearly narrower temperature range: Whereas this transition on graphite appears to be still first order up to the critical temperature Tc(2D) of the 2D liquid (13), it is no longer so on boron nitride more than 10°K below (Fig. 2). This is the main difference concerning the formation of the first adsorption layer of xenon on a highly homogeneous graphite surface and our crystallized boron nitride samples. More marked differences are found with krypton as described below. Krypton case. On graphite there is a temperature range [between 84.8°K = Tt(2D) and 86°K = Tc(2D)] where the first step of krypton isotherms breaks up into three substeps (13, 18). As a result, with increasing adsorbed amounts, the first layer passes successively through four different 2D states: a gas, a liquid, a localized solid, and ultimately a quasi-self-determined solid. On boron nitride, such a break-up of the first step into substeps does not occur, at least in the temperature range investigated here (Fig. 3). Apparently the layer undergoes only one first-order phase transition, as found also by Delolme (9) and Putnam (1 I). The question arises as to whether this is a 2D g a s - 2 D liquid or a 2D g a s - 2 D solid transition. We are inclined to consider it as Journal of Colloid and Interface Science, Vol. 70, No. 1, June 1, 1979

108

REGNIER, THOMY, AND DUVAL

1.0

= B~

0

0.6 I 4, i' " D~ ~-

-

~

°.il 0.2 ! 0

, 5

1'0

15

p(1,0-Tort) 20

( 1 T o r t = t33.3 Pa ) FIG. 2. Adsorption isotherms between 95 and I09°K of xenon on crystallized boron nitride. Isotherms temperatures: 95.8, 98.9, 101.9, 103.3,105.0, 107.2, 108.4°K. The variation of equilibrium pressure for the transitions as a function of temperature is given by: In p (Torr) = -(2517/T) + 19.13 for transition AID'~; lnp (Tort) = -(2215/T) + 16.19 for transition A~D~; lnp (Torr) = -(2692/T) + 20.86 for transition A~D~; lnp0(Torr) = -(1893/T) + 18.21 for the saturating vapor pressure [from (13)]. For both adsorbates, thermal transpiration corrections have been calculated using the relation and data given in (44). From the extrapolation of the linear part of the isotherms at lowest pressures, the superficial heterogeneity may be estimated to about 5%, i.e., about the same as in (10). This amount of heterogeneity cannot impair appreciably the shape of the isotherms or the ratio n B , ( X e ) / n a , ( K r ) .

TABLEI Triple Point Temperature and Critical Temperature of the First Monolayer of Xenon and Krypton on Boron Nitride and Graphite (Gr)" System

Tt(2D) (°K)

T,.(2D) (°K)

Xe/BN Xe/Gr Kr/BN Kr/Gr

102 (34) 99 (13) -84.8 (15)

119 (34) 117 (13) 87 (34) 86 (13)

For bulk phases: Tt(3D)= 161.4°K and Tc(3D) = 289.8°K for xenon; Tt(3D) = 115.8°K and Tc(3D) = 209.4°K for krypton. Journal oJColloid and Interface Science, Vol.70, No. 1, June I, 1979

a 2D gas-2D liquid transition for the following reasons: Its critical temperature (87°K) is close to that (86°K) of the gas-liquid transition which occurs on graphite with the same adsorbate; the resulting considered phase contains a high percentage of vacancies at temperatures markedly lower than the critical temperature (more than 30% at 77°K, i.e., 10°K below Te(2D); finally, the change in the isosteric heat and the differential entropy of adsorption is roughly the same on both adsorbents for krypton as well as for xenon below Te(2D) (Fig. 4). We are

FIRST ADSORBED LAYER THERMODYNAMIC PROPERTIES

109

0 1.0

:- B1

0.8

0.6

"

0.4 ¢

0.2

Kr/BN

..I

p ( 10-2 Torr) I

0

5

10

15

2O

FIG. 3. Adsorption isotherms between 77 and 93°K of krypton and crystallized boron nitride. Isotherms temperatures: 77.3, 81.0, 83.6, 87.4, 88.9, 90.95, 92.7°K. The variation of equilibrium pressure as a function of temperature for the transition A1Dt and at the inflexion point S is given by: In p (Torr) = - (1692/T) + 16.51 and for the saturating vapor pressure: lnp0 (Torr) = - (1356/T) + 18.08 [from (45)].

thus led to admit that the change from the 2D liquid to the quasi-self-determined solid would occur continuously. In other words, beyond point D1 of the isotherms of Fig. 3, the layer would be constituted by a single phase at increasing coverages; it would freeze progressively into the self-determined 2D solid, its density increasing progressively at increasing equilibrium pressures. As a consequence, one might think that the triple point would be located below 77°K, the lowest temperature at which our determinations have been made. This is in fact not likely for the following reason: On the one hand, the temperature range of existence [Tt(2D) - Tc(2D)] of 2D liquid krypton is only about I°K on graphite; on the other hand, the existence range of the liquid is found to be rather smaller on boron nitride

than on graphite for the two adsorbates (nitrogen monoxide and xenon) with which until now two first-order phase transitions have been observed on both adsorbents [see (39, 40) for the NO-graphite and NO-boron nitride pairs]. Consequently the Tt(2D) temperature for the K r - B N pair should be located at the immediate vicinity of the critical temperature Te(2D), i.e., just below 87°K. Since this is not observed, one is led to believe that strictly speaking no triple point may exist: for a given coverage (e.g., 0 = 0.4) an increase in temperature would not change abruptly the 2D solid into the 2D liquid, but the change would occur progressively over a range of temperatures, as has been found for the first adsorption layer of helium on grafoil (41-43). Under these conditions, one would expect a curvature in the In p vs Journal of Colloid and Interface Science, Vol. 70, No. 1, June 1, 1979

1 10

REGNIER, THOMY, AND DUVAL q,,.t (kd. m°le-1) 25 2C

q,t i

f ......

2C

~

z. . . . . . .

15

~

.¢z--

15

- -

- -

qc

- -

- -

10

13 0.5

0

fl

0.5

Xe

Kr

A S ( J . m o l ~ ) K 4)

' & S

4O

40 20

,x._ " ....

0 20 40

1

~x

2O

:-~-'L

o.s ~ ~'L 1 I

8

b

0 20

Y ,

/

0

°5L

;

40

i L _ .J

60

(a)

(b)

FIG. 4. Variation of the isosteric heat (qst) and the differential adsorption entropy (AS) as a function of 0 in the first monolayer. (a) Xenon: on boron nitride, - - - on graphite [from (18, 34)] at approximately 104°K, i.e., slightly above the triple point temperatures for the two systems. (b) Krypton: on boron nitride, - - - - - - on graphite [from ( 18, 46)] at approximately 85°K, i.e., slightly above the triple point temperature for the krypton-graphite system. To the extent that the change, as a function of temperature, of the transition pressure or the equilibrium pressure at constant 0 obeys an equation of the type: in p (Torr) = -(A/T) + B and for the saturating vapor pressure P0: In P0 (Torr) = -(A0/T) + B0 then qst = 8.31 A (J mole-a) and AS = 8.31 (B0 - B) (J mole-a °K-a). Outside the domain of first-order transitions, the layer is constituted of a single phase and the meaning of these quantities is simple. In the transitions region (i.e., in the vertical portions of the isotherms) where two phases coexist, their meaning is more complicated [see (15, 46, 47)]. 1/T line f o r t h e AID1 t r a n s i t i o n (Fig. 3); it w o u l d c o r r e s p o n d to a p r o g r e s s i v e d e c r e a s e in t h e i s o s t e r i c h e a t a n d a n i n c r e a s e in t h e d i f f e r e n t i a l e n t r o p y o f a d s o r p t i o n a s t h e 2 D solid c h a n g e s p r o g r e s s i v e l y i n t o t h e 2D liquid. B u t s u c h a n i n c u r v a t i o n is n o t a p p a r e n t . N o definite c o n c l u s i o n s c a n t h e r e f o r e b e d r a w n at p r e s e n t , a n d t h e m o d e o f f o r m a t i o n o f t h e first l a y e r o f k r y p t o n o n boron nitride remains rather puzzling.

CONCLUSION It may be concluded that there are some a n a l o g i e s in t h e p r o p e r t i e s o f t h e first a d Journal o f Colloid and Interface Science, Vol. 70, No. 1, June 1, 1979

sorbed monolayer of xenon and krypton on boron nitride and graphite. The question m a y b e a s k e d w h e t h e r t h e o b s e r v e d differences reflect the intrinsic properties of the cleavage faces of the two considered ads o r b e n t s o r w h e t h e r t h e y a r e d u e t o insufficient surface homogeneity of our boron n i t r i d e s a m p l e a s c o m p a r e d to t h a t o f exfolia t e d g r a p h i t e . T h i s q u e s t i o n is s u g g e s t e d b y s o m e p r e v i o u s r e s u l t s [see Fig. 3 o f Ref. (18)] w h i c h i n d i c a t e t h a t r a t h e r s t r i n g e n t qualities of the adsorbent surface may event u a l l y b e r e q u i r e d f o r a t r a n s i t i o n to b e observed.

FIRST ADSORBED LAYER THERMODYNAMIC PROPERTIES ACKNOWLEDGMENT Thanks are due to the referee for communication of Ref. (10).

22. 23.

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Surf. Sci. 40, 414 (1973); Surf. Sci. 44, 141 (1974). Regnier, J., Rouquerol, J., and Thorny, A., J. Chirn. Phys. 3, 327 (1975). Bernard, C., Suzanne, J., Bienfait, M., and Hicter, P., Surf. Sci. 52, 340 (1975). Toxvaerd, S., Mol. Phys. 29, 373 (1975). Price, G. L., and Venables, J. A., Surf. Sci. 49, 264 (1975); Surf. Sci. 59, 509 (1976). Kramer, H. M., and Suzanne, J., Surf. Sci. 54, 659 (1976). Venables, J. A., Kramer, H. M., and Price, G. L., Surf. Sci. 55, 373 (1976). Chinn, M. D., and Fain, S. C., Phys. Rev. Lett. 39, 146 (1977). Fain, S. C., and Chinn, M. D,, Coll. Int. C.N.R.S. Marseille C-4, 99 (1977). Marti, CI., Croset, B., Thorel, P., and Coulomb, J. P., Surf. Sci. 65, 532 (1977). Halsey, G. D., J. Phys. Chem. 81, 2076 (1977). Mutaftschiev, B., and Bonissent, A., Coll. Int. C.N.R.S. Marseille C-4, 82 (1977). Putnam, F. A., Coll. Int. C.N,R.S. Marseille C-4, 115 (1977). Regnier, J., Thesis, Nancy (1976). Larher, Y., Thesis, Orsay (1970). Genot, B., Thesis, Nancy (1974). Thomy, A., and Duval, X., J. Chirn. Phys. 67, 246 (1970). Mutaftschiev, B., private communication. Matecki, M., Thorny, A., and Duval, X., J. Chim. Phys. 71, 1484 (1972). Matecki, M., Thorny, A., and Duval, X., Surf. Sci., 75, 142 (1978). Bretz, M., Huff, G. B., and Dash, J. G.,Phys. Rev. Lett. 28, 729 (1972). Dash, J. G., J. Low Temp. Phys. 1, 19 (1972). Bretz, M., Dash, J. G., Hickernell, D. C., McLean, E. O., and Vilches, O. E.,Phys. Rev. A 8, 1589 (1973). Takaishi, T., and Sensui, Y., Trans. Faraday Soc. 59, 2503 (1963). Larher, Y., J. Chim. Phys. 65, 1683 (1968). Larher, Y., J. Chirn. Phys. 62, 604 (1965). Larher, Y., J. Chirn. Phys. 65, 974 (1968).

Journal of Colloidand Interface Science, Vol.70. No. 1, June 1, 1979