SEIHUNCHANGAND WILLIAM H. WADE
2484 temperatures where the oxide itself is volatile. An empirical rate equation compares favorably with similar rate equations in the literature for tungsten oxidation a t comparable pressures but higher temperatures. The correlation with the oxidation data a t very low total pressures indicates that the Nz carrier gas a t atmospheric pressures does not affect the reaction kinetics. The oxygen either successfully competes with the excess bromine for adsorption sites or is
adsorbed on different kinds of sites than the bromine. The reaction mechanism, which is independent of the dissociative state of the gaseous bromine, may simply involve the direct combination of WOZ and Br2 on the surface. Acknowledgements. Drs. L. V. McCarty, S. I(. Gupta, and R. J. Campbell participated in many helpful discussions and critically reviewed the manuscript .
The Kinetics of Interaction of Oxygen with Evaporated Iron Films
by Seihun Chang and William H. Wade Department of Chemistry, The University of Texas at Austin, Austin, Texas 78711 (Received May 19, 1970)
The fast interaction of oxygen with iron films has been investigated with a quartz crystal microbalance in the pressure range 2 X lo-' to 5 X Torr a t 24'. A relatively rapid formation of the equivalent of 4 oxide (FeO) layers was observed which, subsequently, was followed by a much slower growth up to 10 layers. The process kinetically approaches first order with respect to oxygen pressure in the initial stages and decreases to fractional orders as incorporation proceeds. This can be interpreted in terms of a surface regeneration by incorporation of adatoms into the interior at rates comparable to the adsorption process. The Elovich direct logarithmic rate law does not hold for the rapid chemisorption since it is observed to be autocatalytic.
Introduction There have been several reports on the kinetics of oxygen chemisorption on iron films.'-* They show several inconsistencies but there appears to be general agreement that an initial fast process is followed by a slow one. The fast process takes place so rapidly that, to the present time, it has not been followable using conventional gravimetric or volumetric methods, and all the extant rate data for oxygen chemisorption on iron pertain to the slow process. Recently Pignocco and P e l l i s ~ i e r reported ~~~ LEED data for the first stages of the interaction of oxygen with oriented iron crystal faces. Their study was mainly concerned with structural changes during oxygen adsorption coupled with rather qualitative studies of adsorption kinetics. The quartz crystal microbalance (QCM) introduced by Sauerbreya is a new tool for investigating surface processes. This instrument has several advantages over conventional gravimetric measuring devices : (1) The sensitivity of the QCM is sufficient to permit measurement of changes in surface mass of 10-lO g/cm2. (2) The response time of a QCM to changes in mass of the active surface is on the order of microseconds, enabling one to follow fast adsorption processes. (3) The frequency shift of the crystal due to factors other than mass change can be minimized or corrected. (4) The Journal of Physical Chemistry, Vol. 74, No. 12, 1970
Buoyancy and wall effects can be completely eliminated. Previously a quartz crystal microbalance was used successfully by Slutsky and Wade' for measuring adsorption isotherms of hexane on a quartz single-crystal face. Later, using the same technique Wade and Allen* followed fast adsorption kinetics for oxygen on evaporated aluminum films.
Experimental Section The vacuum system and circuits used in this work have been described in a previous paper.8 The quartz crystals used in this study were 10-mHz, AT plates furnished by Scientific Electronic Products Corp. of Loveland, Colo. The quartz crystal was mounted in the vacuum system and covered by an aluminum hous(1) J. Bagg and F. C . Tompkins, Trans. Faraday SOC.,51, 1071 (1955) (2) M.A. H.Lanyon and B. M . W. Trapnell, Proc. Roy. SOC.,Ser. A ., 227, 387 (1955). (3) M.W. Roberts, Trans. Faraday SOC.,57,99 (1961). (4) A.J. Pignocco and G . E. Pellissier, J. EZectrochen. SOC.,112, 1188 (1965). (5) A. J. Pignocco and G . E. Pellissier, Surface Sci., 7, 261 (1967). (6) G.Sauerbrey, Z . Phys., 155,206 (1959). (7) L.J. Slutsky and W. H. Wade, J . Chem. Phys., 36, 2688 (1962). (8) W. H.Wade and R . C. Allen, J . Coll. Interface Sci., 27, 722 (1968).
.
INTERACTION OF OXYGENWITH EVAPORATED IRON FILMS
2485
ing. The housing contained two 0.25-in. diameter collimation holes allowing the films to be deposited on both sides of the active area of the crystal. Mass loading of the active area of the quartz crystal is calculated by the equation6
4f --Am f" 6Fd - =
f" is the unperturbed (initial) frequency of the crystal, Af is the change in frequency due to deposited mass on
active area, Am is the mass change of the active area of the quartz crystal, 6 is the density of the crystal, F is the active area of the crystal, and cl is the thickness of the crystal. Since the change of thickness of the crystal due to film deposition on the electrodes is negligible, eq 1 can be reduced to the form
j 0 02
04
YPO
Figure 1. TMS isotherm (grams of TMS 215. relative pressure) on an iron film at 23.3": 0, adsorption; e, desorption.
where mo iu the mass of the active area of the crystal. The crystal thickness and film area are such that the sensitivity of this microbalance is 1.44 X 10-g g/Hz. A grease-free high-vacuum system was used. By baking the system for 24 hr a t 150", the base pressure of Torr. the system reached 5 X Direct evaporation of iron was made from a helical iron filament of 99.99% purity, which was supplied by Material Research Corp. Before evaporation, the iron filament was well outgassed and transformed to the y phase a t pressures of -lo-* Torr. The pressures of the system during evaporation were generally less than 3 X 10-8 Torr. The evaporators were mounted perpendicular to the plane of the crystal and radiation shields made of stainless steel plates were provided to protect the walls from unnecessary heating and metal deposition. Oxygen of 99.95 mol %, furnished by Airco (Matheson), was used. For BET surface area determinations of both unreacted and oxidized films, tetramethylsilane was used as an adsorbate. The tetramethylsilane, of 99% purity (Matheson Coleman and Bell), was degassed by repeated freezing and thawing in vacuum and dehydrated with 4A molecular sieve before obtaining the adsorption isotherms. The pressure of tetramethylsilane was measured with a mercury manometer using an MKS Baratron pressure transducer as a null detector. For all kinetics measurements, the oxygen pressure was controlled manually. The pressure rise was monitored by recording the output from the ionization gauge. At pressures greater than Torr, the pressure rise time was no faster than the observed chemisorption kinetics. This obviously set an upper limit to meaningfully measurable rates which could be followed.
Results and Discussion Film Area Measurements. It was demonstrated in a previous studys that tetramethylsilane as a room-tem-
perature adsorbate can be a good replacement for N2 which is traditionally used in BET area determinations. The value of 46.3 (relative to 16.2 A2 for N2 as measured on a nonporous, low-area alumina) was taken as the molecular cross-sectional area for tetramethylsilane (TMS). TMS adsorption isotherms were obtained over the relative pressure range, 0.01 < P / P a < 0.90, a t 25.0". Desorption branches were also obtained for several representative substrates. One of the isotherms is displayed in Figure 1, and several representative BET plots are displayed in Figure 2. Calculated BET areas for substrates given a variety of pretreatments are listed in Table I.
A2
Table I Film Film g/cmg thickness, Film area, Roughness x 108 A om* factor
wt,
60
4.7 4.2 5.5
54
4.9
63
4.2
53
5.3
67
70
1.85 1.76 0.77
Remarks
2.9 2.8 1.2
Unoxidized Unoxidized 02, 4.5 hr a t 8 X 10-8 Torr 0.63 1.0 02, 1 hr at -5 x lO-'Torr 0.82 1.3 02, 10 min at -7 X 10-6Torr 0.61 0.95 0 2 , 5 min at 4 x 10-4 Torr Geometric area, 0.64 emz
As can be seen in Table I, there is marked diminution in film area concomitant with oxidation. Experimental uncertainties coupled with the usual reservations associated with the BET model dictate a roughness factor of 2.9 f 0.4 for virgin films and 1.1 0.2 for oxidized
*
The Journal of Physical Chemistry, Vol. 7.4, N o . 12, 1970
2486
SEIHUNCHANGAND WILLIAMH. WADE
TIME (sec). VP
Figure 2. TMS BET plots for iron (0 and ( 0 and e )films.
0 ) and
oxide
films. This diminution in film area is in agreement with the work of Brennan, Hayward, and T r a ~ n e l l ,who ~ found a 57% reduction in film area. Little is known about the microtopology and pore structure of evaporated films. This provided the motivation for obtaining both the adsorption and desorption branches of the TMS isotherms. Hysteresis in these branches would be an indication of pore volumes with restricted pore necks. As would be expected no hysteresis was observed for oxidized films with roughness factors near unity, but neither was any hysteresis observed for the pure iron films. This latter finding obviates any extensive porosity and the identical roughness factors found here with TRlS and elsewhere with K29-two molecules with quite different dimensions-negates an appreciable concentration of fissures in the size range of 3 (Nz) to 6 ‘4 (TMS). Rather the findings are consistent with an exposed “hill and valley” profile for the pure iron substrate with characteristic dimensions on the order of the film thickness. KO dependence of film area on film mass could be detected although wide variations in film mass were not attempted. Porter and TompkinslO reported a linear variation between the area and mass for evaporated iron. They followed Beeck’sll method in comparing areas by using the amount of hydrogen adsorbed a t 273°K and 0.1 Torr, There are two obvious possible explanations for their discrepancies with the present data: either hydrogen is adsorbed in addition to being adsorbed or there are pores accessible to HZbut not to Kzor TMS. Oxygen Interaction Kinetics. Uptake rate measurements were performed in the pressure interval 2 X to 6 X Torr a t 24”. The lower limit was established as 100-fold greater than the background partial pressure of O2 and CO by residual gas analysis. The upper limit does not reflect a time constant limitThe Journal of Physical Chemistry, Val. 74, A’o. 1% 1970
Figure 3. Typical frequency (cycles)-time, mass-unit area (g/cm2)-time, and pressure-time plots for oxygen-iron at 24”.
ation on the quartz crystal microbalance but is realistically set by a limiting dexterity in manually controlling the dosing valve. A representative trace of frequency change (which can be transformed into mass uptake via eq 2) us. time is shown in Figure 3. When the abscissa is depressed to accommodate longer uptake times, there is a knee bend a t Af % 60-70 cycles and an asymptotic approach of Af to 140-170 cycles a t which time O2 uptake rates were on the order of a monlayer per year. Both the knee bend point and asymptotic limit varied randomly (but tandemly) from run to run between the above limits and appeared to be consistent with the observed variations in BET surface areas. For analysis of rate behavior which would be consistent from run to run, it was necessary to refer to coverages relative to the knee bend which were assigned a value 0 = 4.00 corresponding to the number of layers of FeO (see below). Below the knee bend point uptake is referred to as the “fast” process and, above the knee bend, the ‘Lslow’’process. If the growth product is assumed to be FeO, Af = 60 Hz corresponds to 4.0 layers based on the average roughness factor of 1.1. The limiting asymptote either reached slowly or induced rapidly by raising the 02 pressure to -10-2 Torr, corresponds to 10 layers of oxide based on a roughness factor of 1.1, in good agreement with other st~dies.29~~12 Of course, the chemical stoichiometry of thin oxides in their initial growth stages is questionable. For thicker growths Vernon, et aZ.,13
(9) D. Brennan, D. 0 . Hayward, and B. M. W. Trapnell, Proc. Roy. Soc., Ser. A , 256,81 (1960).
(10) A.8 . Porter and F. C. Tompkins, {bid.,Ser. A, 217,544 (1953). (11) O.Beeck, Advan. Catal., 2,151 (1950). (12) J. Kruger and H. T. Yolken, Corroswn, 20,29t (1964). (13) W.H.J. Vernon, E. A. Coleman, C. J. B. Clews, and T. J. Nurse, Proc. Roy. SOC.,Ser. A , 216,375 (1953).
2487
INTERACTION OF OXYGENWITH EVAPORATED IRON FILMS
L
E
4
a
IL
IO
100
1000
10,000
I
TIME ( s e c )
Figure 5. Diagnostic rate plots: 0, Cabrera-Mott (mass change us. log time); 0 , Elovich (reciprocal mass uptake vs. log time). Figure 4. Oxygen uptake rate vs. pressure at 0 and 3.2 with apparent reaction orders shown.
=
2.0, 2.8,
using electron diffraction identified Fez03 formed on polycrystalline Fe a t 200”. Davies and Evans14 also observed the formation of Fe2O3 a t temperatures up to 200”. On the other hand, in the thin-film region Pignocco and Pellissier4J preferred to consider that FeO is formed on (011) and (001) faces of iron exposed to O2 a t very low pressures. These results could be compatible if the initial growth corresponds closely to a 1 : l stoichiometry which with increased thickness converges to a 2 :3 limiting ratio. In any event the limiting growth is multilayer in character and is much larger than found in a previous study on A1.8 Interestingly, the residual oxidation rate for iron in the asymptotic limit is no greater than that for AI. If the kinetics obey the rate equation d(mass) _ _ _ _- Apo,“(l - 0b)me- E W I R T dt
(3)
where E(@)is an activation energy a t a coverage e and A and b are constants, then the slope, n, of a log rate vs. log p plot at constant 0 is the reaction order with respect to 02.Such plots are constructed in Figure 4 corresponding to 0 = 2.0, 2.8, and 3.5, respectively. The rms slopes are 0.85, 0.63, and 0.31, respectively, clearly indicating that as 8 -t 0, n -t 1 and as e + 4 or greater, n + 0. Such variations in reaction order are always indicative of multistage mechanisms with one or more transitions in rate-limiting steps. I n the initial stages of surface oxidation procesfes there should, in principle, be a t least two regimes: the first corresponds to chemisorption and the second to lattice penetration by oxygen. Simple chemisorption processes are commonly first order a t constant coverage whereas oxidation in the limit of thick, coherent oxides is zero order. Obviously the “fast” process cannot be
identified solely or even largely with chemisorption, for the uptake corresponds to 4 layers of FeO. The detailed overall mechanism for the “slow” process is just as complicated. However in the oxidation regime one can possibly evoke the Cabrera-Mott inverse-log rate law and avoid a more detailed mechanistic description. I n Figure 5 such a plot is reconstructed for the slow process a t a pressure of 5 rt 1 X Torr. The lack of linearity is discouraging but is hardly surprising in the light of the thinness of the oxide layer. A detailed examination of Figure 3 reveals that a t low coverages the oxygen uptake has an induction period. Such processes are usually referred to as autocatalytic. To demonstrate this behavior more graphict ally, sticking probabilities (S), which are essentially 4
derivatives of the uptake-time curves, were calculated for the data in Figure 3 and are plotted in Figure 6. The exact character of either of the two plots below 0 = 1 is subject to considerable experimental uncertainty because of the difficulty of rapidly attaining a steady-state system pressure. This is demonstrated by a plot of the corresponding pressure-time behavior in Figure 3. I n the first 5 sec of exposure the pressure is constant to no better than f30%. However such large uncertainties still do not eliminate the maximum in Figure 6. For exposures at the lowest pressure (2 X 10-7 Torr) initial pressure fluctuations were similar but the initial uptake rate was approximately tenfdd lower with nominal monolayer completion taking approximately 150 sec. Under these conditions the same induction effect was noted. The maximum sticking probability (reaction rate) was always found to occur a t e = l f 0.2 regardless of O2 pressure. Since the reaction order is still near unity up to 0 = 2, the incorporation rate clearly must be (14) D. E.Davies and U.R. Evans, J. Chem. SOC.,4373 (1956).
The Journal of Physical Chemistry, Vol. 74, N o . 12, l 9 Y O
2488
SEIHUN CHANCI AND WILLIAMH. WADE of FeO formed but a t 4 layers and greater the incorporation process has become rate limiting and is zero order in 0 2 . (4) The sticking coefficient on iron is -0.5 when one layer of FeO is formed and subsequently diminishes to -0 a t 4 layers. ( 5 ) The process may be schematically pictorialized as 0
O2
8 4
Figure 6. Sticking probability (S) os. coverage i.
(e).
greater than the chemisorption rate up to these coverages over the pressure range studied. At a nominal 0 = 1 the external surface must consist almost entirely of iron atoms with the oxygen incorporated in the lattice and the sticking probability of O2 on the initially regenerated iron surface is considerably enhanced over the pure iron substrate if one is to observe an induction period. Although it is obvious that rate data exhibiting maxima cannot exhibit Elovich behavior, the corresponding direct-log plot is also exhibited in Figure 5. At intermediate uptake times there is a very restricted linear region. As oxidation proceeds at a given pressure, a steadystate concentration of surface regenerated iron sites will be reached. The onset of the steady state with regard to the extent of oxidation is dependent on the 0 2 pressure-the higher the pressure, the less extensive the oxidation prior to the steady state. For instance, at Torr the oxidation of iron has been predicted to be zero order16for 0 > 1. Likewise the above argument explains previous o b ~ e r v e d fractional ~?~ orders varying between (0.20 and 0.29) wherein experimental techniques did not permit a study of the “fast” process. I n partial conflict with the present data is an ellipsometric study by Kruger and Yolken.12 They reported abnormally low sticking coefficients of -5 X two orders of magnitude lower than reported here. They cleaned their iron surface by an 800’ reduction in a hydrogen atmosphere. Chemisorbed hydrogen
+ +
may have diminished S for O2 as Ponec and K n o P reported for nitrogen on iron films with preadsorbed hydrogen. The pertinent conclusions may be summarized as follows: (1) The chemisorption of 0 2 on iron is a first-order process. (2) There is a rapid incorporation of oxygen to give the equivalence of 4 layers of FeO followed by a slow incorporation to give an asymptotic approach to 10 layers of FeO. (3) The incorporation of O2is rapid compared to the rate of chemisorption Torr for the first several layers a t a pressure of 2 X The Journal of Physical Chemistry, Vol. 74 No. 1.9, 1070
+ -Fe-Fe-Fe-Fe-
I I
I I
0
0
I I I I -Fe-FeI I
kl --3
-Fe-Fe-
-Fe-Fekz --t
I 1 0 I 1 -Fe-FeI I 0
0
O2
+ -Fe-FeI
I
0
I I -Fe-FeI I Fe-Fe I I
ka_
0
I 1 -Fe-FeI I 0 0 I I -Fe-FeI 1
0
0
I
I
0
0
0
0
0
0
0
0
I I -Fe-Fe-
I
-Fe-Fe-
I
I 1
-Fe-Fe-
1
1
I
-Fe-Fe-
1
I 1 0 I 1 -Fe-FeI I 0
The results dictate that IC3 > k;l and that initially step 2 is faster than 1 or 3. Transiently a steady state such as step 4 is attained and subsequently further incorporation becomes limited by migration velocity. Whether the oxygen incorporation into the lattice is a concerted process or not is open to question. ( 6 ) Such studies on evaporated films are complicated by a diminution of the surface roughness during the oxidation process. Acknowledgment. The authors wish to express their appreciation to the United States Army Research Office, Durham, N. C., and to The Robert A. Welch Foundation for their support. (15) N.Cabrera, Phil. Mag., 40, 175 (1949). (16) V.Poneo and 2. Knor, J.Catal., 10,73 (1968).
