J. R. ANDEMOSAXD E. G. BAKER
482
ting function. Simply keeping higher terms in the electrophoretic effect admittedly does not give a theoretically correct result, but certainly neglecting these terms does not either. It would be of interest to examine transference numbers iii mixed solvent systems since this property is independent of the relaxation effect.
Vol, 66
Acknowledgments.-The authors wieh to acknowledge a grant from the National Science Foundation in support of this work. Our thanks go also to Professor R. L. Kay and Mr. J. Hawes for permitting the use of their conductance data on KCl and CsCl in ethanol-water mixtures prior to its publication.
THE ADSORPTION OF XENON AND HYDROGEN ON EVAPORATED FILMS OF TUNGSTEN AND NICKEL BY J. R. ANDERSON AND B. G. BAKER Chemistry Department, University of Melbourne, Melbourne, Australia Received September 66,1961
Surface areas of evaporated films of nickel and tungsten have been measured by chemisorption of hydrogen and by physical adsorption of xenon and krypton; these methods give results which are in substantial agreement. Xenon adsorption has been used to study t h e sintering of tungsten and nickel films: it was found that tungsten films did not sinter a t 510"K., while the presence of hydrogen markedly reduced the rate of sintering of nickel a t 468°K. although the ultimate areas for vacuum sintered and hydrogen sintered nickel films were the same and equal to about four times the apparent geometric area. A model is suggested to account for the effect of chemisorbed hydrogen on the rate of sintering. The effect of sintering on the electrical resistance of nickel films has been measured and the surface structure studied by electron-microscopic examination; the implications of these observations for the sintering process are discussed. except for xenon on tungsten at 80'K. whcn the low pressures necessitated the use of an ionization gage (C.V.C inc. type G.I.C. 011). The latter was calibrated using xenon against the PvlcLeod gage in the range to 10-amm. Cylinder hydrogen was purified by diffusion through a heated palladium thimble and through liquid air traps. Xenon and krypton were used directly from glass bulbs supplied by the British Oxygen Company; the only significant impurities were 0.57, krypton in the xenon and 0.57, xenon in the krypton. Doses of gas were measured in the McLeod gage and expanded into the reaction vessel. Equilibrium pressures were measured after 10 min. for xenon and krypton and after five min. for hydrogen except for xenon a t SoOK., when 15 min. were required to establish equilibrium. Pressure readings were corrected for thermolecular flow using the method of Porter.7 The total dead-space was about 1200 cm.a. The volumes of the different parts of the apparatus were calibrated by sharing experiments using the McLeod bulb as a standard volume. Low temperatures were measured using an oxygen vapor pressure thermometer. (ii) Film Preparation.-Films were prepared by direct evaporation from hair-pin filaments situated on the axis of the adsorption vessel. Nickel wire, diameter 0.5 mm., was Jahnson and Matthey spectroscopically standardized, with the following metallic impurities in parts per million: Fe, 5; Si, 3; Ca, 2; Cu,1; Mn Mg Na Li < 1. Tungsten wire (diameter 0.2 mm.) was Johnson and Matthey pure grade. Tungsten filaments were held in small molybdenum spring clips. Tungsten films were deposited a t (i) Apparatus and Technique.-The apparatus was 273°K. after the adsorption vessel had been baked a t 720°K. similar to that used by Beeck5 and by Kemball.6 The ad- under vacuum for 15 hr., during the last hour of which the sorption vessel onto the inside walls of which films were de- filament was outgassed a t 5.2 amp. The rate of evaporaposited was cylindrical with internal diameter 73 mm. and tion was about 20 rng . hr .-l a t 6.3 amp. Only one film was length 100 mm. and was attacked by a water-cooled ground obtained from each tungsten filament and a clean adsorpglass joint with Apiezon L grease. tion vessel was used for each film. Nickel films v-ere deThroughout adsorption measurements films were pro- posited a t 273°K. a t 6.2 amp. a t rates in the range 15-45 tected from mercury vapor by an adjacent cold trap, liquid mg. h r . 3 after the adsorption vessel had been baked a t air being used for hydrogen and ai Dry Ice-acetone slurry for 720°K. for 15 hr., during the whole of which time the filaxenon and krypton. ment was outgassed a t 4.1 amp. Before use, nickel filaments Pressures were measured using McLeod gages, one cover- were purified by the evaporation of one film and for this two ing the range 10-6 to 4 X lou2mm., the other 10-3 to 4 mm., alternative techniques were used: technique A-a film was deposited a t the usual rate after baking a t 720°K. either (i) (1) 0. Beeck, Advances in Catalysis, 2, 151 (1950). (2) R. A. h e r o t t i and G. D. I-Ialsey, J . Phgs. Chem., 68,680 (1959). for 2 hr. and outgassing a t 4.7 amp. for 0.5 hr. or (ii) for 15 hr. and outgassing a t 4.1 amp. for 15 hr. (in neither (i) (3) D. F. Klemperer and F. S. Stone, Pmc. Rou. SOC.(London), nor (ii) did appreciable evaporation occur); technique BA24S, 375 (1957). during a 15-hr. baking period the filament was heated a t 4.7 (4) M. W.Roberts, Tvans. Faladay Soc., 66, 128 (1960). amp., during which time a film of about 10 mg. deposited. (5) 0.Beeck, A. Smith, and A. Wheeler, Proc. Boy. Soc. (London),
While krypton often has been used for the estimation of surface areas of evaporated metal films by the B.E.T. method,l-* xenon has been largely neglected despite its obvious advantage that, with a higher heat of adsorptioii and low saturation vapor pressure, measurements will be made at lower equilibrium pressures and dead-space corrections will be smaller. This feature was of particular importance in the present instance since it was desired to assess the importance of film sintering under conditions similar to those used in catalytic experiments and a considerable dead-space was unavoidable. Accordingly, the adsorption of xenon on evaporated films of tungsten and nickel has been investigated together, for comparison, with measurements with krypton and hydrogen. The use of these measurements for surface area estimation has been examined and the technique applied to a study of the sintering of nickel films. Structural changes in nickel films on sintering have been studied by electron microscopic examination. Experimental
8177,62 (1940). (6) C. Kernball, ibid., A207, 539 (1951).
+
+ +
(7) A. S. Porter, Discussions Faraday Soc.. 8,358 (1950).
March, 1962
AnsoRrrIoN
O F X E N O N AND
483
HYDROGEN ON TUNGSTEN AKI) KICKEL 1:ILMS
In each cnse the preliminary film was discarded and a clean reaction vo9soI u s d for cneh subsequent film. The rem mu re in the rcnetion vemel at the aomnletion of haking.and autgnssing of N tilnment prepared hy ierhnique 1% was within the rangc 2 to 5 X 10-7 mm., ns mensurd hy the ionization gsgct attnehcd at tho hcad of tho V L ~ C I , and remained unchnngod when the filament w w raised to evaporation temperature.
h r'
Results (i) Adsorption on Tungsten Films.-Adsorption i of hydrogen w w measured on films in the range 12-14 mg. at 90°K. (films W1, 11'2) and at 213°K. F (films W2, W3, W5). With film \V2 the isotherm f first was measured at 273°K and then continued at 9O'K. D a b are summarized in Table I and a typical isotherm is contained in Fig. 1. At 213'K. hydrogen adsorption isotherms were sensibly mm. horizontal at pressures in thc region of Adsorption of xcnon was stndied at 90°K. on bare tungsten (W4) and on films (Wl, W2, W3) onto l'1:~te l.--l.;li:riron which hydrogen had been preadsorbed up to an equilibrium pressnre of abont mm., followed mm. I'ilms hy pumping at. 293°K. to < thus pretreated sribscqucnt,ly arc referred to as "hydrogm covered." Xenon adsorption also was studied on films W1, W2, W3 aftrr subscquent heating in 15 mm. of hydrogen for 16.5 hr. a t 510'K. followed by pumping at 293'8. t,o bettcr t,han IO-' mm. An estimat,e of t.hc ease of reversibility of xenon adsorption was milde by repeabing the xenon adsorption on films W3 and W 4 after pumping at 293°K. for 2 hr., after which the pressure above I the isolated specimen did not rise above 5 X IO-' mm. in 0.5 hr. A typical xrnon adsorption isotherm on "hydrogcn covered" tnngst,cn is shown in Fig. 2 t,ogether with these resnlts plotted accord- i ing to the R.E.T. Similar rcsults for 1 xenon on bare tungsten nt 80'K. are shown in Fig. 2 and isotherms of xcnon on bare and "hy- i drogen covcred" tungsten at !WK. are compared L in Fig. 3. 1'I:itc 2.-l 0.01 (taking as po 1.75 mm.), (b) used for po in the B.E.T. pIots an extrapolated value for liquid krypton (about 4.3 mm.) instead of po for solid krypton as adopted in the present work, and (e) used 20 for the effective area of adsorbed krypton. In the preparation of “hydrogen covered” films both for tungsten and nickel, the pumping procedure undoubtedly removes some hydrogen from the surface. However, from the hydrogen adsorption isotherms this would not amount to more than about 10% when pumped to an equilibrium pressure of mm. It is apparent from Fig. 3 and 4 that on both nickel and tungsten the heat of adsorption of xenon on a “hydrogen covered” surface is lower than on a bare surface: if we define
0.4 € N / N m < 0.9. Using the expression for C from B.E.T. theory (cf. Cassie25)
- AHEC~~
it is easily shown that, provided the entropy of adsorption depends on N / N m only A ( A H ) = 2.303RT log
pbsre/l)Hoov
when the ratio of xenon equilibrium pressures P b a r e / p H Dov is taken at constant coverage and constant temperature T. With nickel a t coverages 0.7, 0.8 and 0.9 the values of A(AHxe) are -300, -1150 and -80 cal, mole-1, respectively, while with tungsten the corresponding values are - 150, -70 and -90 cal. mole-l. A similar analysis of a pair of krypton isotherms on nickel gave values of A ( A H K ~ )of -170 and -70 cal. mole-’ at coverages of 0.3 and 0.5, respectively. In Table V are listed for various systems values of the B.E.T. parameter C obtained by fitting calculated and experimental isotherms in the region
- AH1)RT]
C = j&,, e x p [ ( A H
where j , is the partition function for the internal degrees of freedom of the adsorbed molecule and j , is the partition function of the molecule in the condensed phase, together with the values for AH,, the heat of condensation of the adsorbate, of -3.30 and -2.68 kcal. mole-1 for xenon and krypton, respectively,26values of AHl, the (average) heat of adsorption in the first layer, were obtained and are recorded in Table V. It was assumed that j 8 / j 0 = 2 for immobile adsorption (cf. ~i1127).
=
A ( A H ) = AHbare
Vol. 66
TABLEV DATAFROM B.E.T. ANALYSIS System,
OK.
AIIi (koal. mole-1)
C
Xe/Rii, 90 2300 Xe/Ni, Hz 90“ 1000 Xe/W, 90 4000 Xe/W, Hz 90” 2500 Kr/Ni, 77 2300 Kr/Ni, H? 77“ 1000 Hydrogen-covered surfaces.
-4.56 -4.41 -4.66 -4.57 -3.76 - 3 63
I n view of the assumptions involved, the data in Table V are mainly significant in a relative sense: taken with the values obtained above for A(AH) it seems clear that the heat of adsorption of xenon on hydrogen-covered nickel and tungsten is lower 300 and -150 cal. than on the bare metal by mole-l, respectively, at N / N , 0.7, and for krypton on nickel a t N / N 0.5 the corresponding figure is 100 cal. mole-l. The comparatively small magnitude of these differences in AH presumably means that there can be no major change in the rare gas-metal distance, thus supporting the idea that the chemisorbed hydrogens are effectively buried in the surface. film areas (ii) Sintering of Films.-Tungsten measured after heating in hydrogen a t 510°K. (cf. Table 11) show that no significant sintering has occurred, in agreement with Pierotti and Halsey,2 who found that tungsten films did not sinter on heating to 720°K. in vacuo. However, using hydrogen chemisorption for area measurement, Campbell, MOSS, and Kembal12s have reported an area reduction of 31% for tungsten films heated to 573’ K. for 10 min. in vacuo. We emphasize however that, in general, chemisorption is unsuitable for measuring the extent of high temperature sintering because of the difficulty of avoiding film contamination by gas evolved from the heated glass.29 From the results in Fig. 7 it is clear that the presence of chemisorbed hydrogen markedly reduces the rate of sintering of nickel films. This conclusion also has been inferred indirectly from the internal self-consistency of kinetic data from catalytic experiments using nickel The
-
- -
(25) A. B. D.Casaie, Trans. Faraday Soc., 41,450 (1945). (26) Evaluated from vapor pressure data in Landolt-Bornstein. (27) T. L. Hill, J . Chem. Phys., 16, 181 (1948). (28) J. S. Campbell, R. L. hloss, and C. Kemball, Trans. Faraday Soc., 66, 1481 (1960). (29) J. R. Anderson, t o be published. (30) C. Kernball, Proc. Roy. Soc. (London), 8214,413 (1952).
March, 1962
ADSORPTION OF XENON AND HYDROGEN ON TUNGSTEN AND
resiilts of Roberts4 show that iron films behave in a similar manner. It seems reasonable to propose that chemisorbed hydrogen acts in this may by reducing the surface mobility of nickel and iron atoms: this is readily understood if a hydrogen atom is held at the surface not by a single bond from an individual suriace metal atom but by a multiceiitered orbital involving those metal atoms which are nearest neighbors to the hydrogen, a suggestion previously made by TakaishiS31 This would effectively link together surface metal atoms by bonds which are not formed in the absence of such a chemisorbed atom. We tentatively suggest that the metal orbitals used in this way are the same as those proposed by Altmann, Coulson and H ~ m e - R o t h e r yto ~ ~ be the major bonding orbitals directed between nearest neighbors and determining the structure of the metallic crystal; that is, p3d3 hybrids for facecentered icubic nickel and sd3 d 3hybrids for bodycentered cubic tungsten and iron: we suggest that a t the surface the residual parts of these orbitals pro,ject into space and a multi-centered orbital is formed by overlap with the hydrogen 1s orbital. A. consequence of this proposal is that on the (100) face of a body-centered cubic metal such as tungsten there is also the possibility of bonding between the hydrogen atom and a tungsten atom in the layer immediately below the surface layer, since this atom is only b/.& below the surface layer (where b is the nearest neighbor distance). For this we suggest the use of a tungsten dY hybrid orbital32; this orbital would be normal to the surface. A hydrogen atom in such a site is thus bonded with five tungsten atoms, which is more than reasonably can be expected a t any other surface site: we believe that this would be a major contributing factor to the variation in binding energy found by Hickmott13 for hydrogen atoms chemisorbed on a polycrystalline tungsten filament. The ultimate area from sintering at 468’K.about 6001 cm.2-- is independent of the presence or absence of hydrogen and is about four times the apparent geometric film area. Nickel films with weights as low- as 1 mg. showed the normal temperature coefficient of resistance of nickel, thus showing that one of the alternative
+
(31) T. Takaiahi, 2. physik. Chem. (Frankfurt), 14, 164 (1958). (32) 8. L. Altmann, C. A. Coulson, and W. Hume-Rothery, Proc. Rou. Soe. (London), A240, 145 (1957). (33) S. R. Logan and C. Kernball, Trans. Faradau Soe., 66, 144 (1960)
NICKEL FILNS
489
suggestions of Logan and Kemba1133that such light films consist of discrete non-touching crystallites cannot be correct; furthermore, the absolute value of the measured resistance is much too small to agree with such. a suggestion, unless the gaps are so small that easy electron tunnelling occurs. The relative resistance change on sintering a 1 mg. film mas much smaller than that for a film of weight 3.8 mg. (cf. Table IV), indicating that the lighter film had sintered relatively less extensively. This is in agreement with the electron-microscopic evidence in plates 1-3, which show that very light nickel films retain after sintering a rough surface with asperities about 200 A. across, whereas heavy films sinter to yield a surface which, at this resolution, shows no marked deviation from flatness. This is in agreement with the conclusion which Logan and Kemball inferred from catalytic kinetic data with nickel films of varying weight. It is clear that under the present conditions nickel films are continuous at least down to 1 mg. Heavy films deposited in a cylindrical vessel always will posses55 a very thin region; on this basis alone one thus would not expect the actual area of a sintered nickel film to equal the geometric area. However, the contribution of the rough very thin region cannot nearly account for the overall factor of four found. On the assumption that the Bsperities are close-packed hemispheres and that the very thin region occupies 20y0 of the apparent geometric area, the calculated ratio is only about 1.2. There seem three possible explanations : (a) fissures (perhaps at grain boundaries) which are beyond the present electronmicroscopic resolution, run from the surface into the metal and so the sintered film retains some internal surface; (b) the sintered surface retains some roughness on an atomic scale due to terraces and steps; (c) “adsorbed” atoms penetrate extensively into the lattice. The latter seems most improbable in view of the large size of the xenon atom and the agreement between areas measured by xenon and hydrogen adsorption. Extensive solution of hydrogen at higher temperatures is of course well known. Acknowledgments.-The authors are grateful to the Shell Co. (Aust.) Ltd. for a grant toward the cost of materials and to Drs. J. Sanders and J. Nicholas of the Division of Tribophysics, C.S.I.R.O., for taking the electron micrographs and for helpful discussion, respectively.
