Work function changes due to the chemisorption of water and oxygen on aluminum

SURFACE

SCIENCE 5 (1966) 447-465 Q North-Holland

WORK FUN~~ON

Publishing Co., Amsterdam

CHANGES

DUE TO THE CHEMISORPTION

OF WATER AND

OXYGEN ON ALUMINUM E. E. HUBER, JR. and C. T. KIRK, JR. M.I.T. ~i~~~ln Laboratory*, ~~~i~~to~, Massachusetts, USA.

Received 26 May 1966 On exposure to dry oxygen the work function of aluminum is observed to decrease with increasing fractional monolayer coverage reaching a minimum value of about 0.05 V below the fresh surface value at monolayer coverage before increasing with further oxygen exposure. This unusual insensitivity of the work function to oxygen coverage is explained on the basis of a two-site chemisorption model involving dissociation of the 02 on adjoining sites and a place exchange of one of the oxygen atoms with an underlying aluminum atom. The initial oxygen kinetics are consistent with either a homogeneous model or a heterogeneous model of activated chemisorption, but the latter model is favored as the correct explanation. Water vapor lowers the work function of both the fresh surface and an oxygen exposed surface about 1 eV. The 1 eV lowering is consistent with the polar nature of water assuming either Hz0 or OH groups chemisorbed on the metal oxide surface with the hydrogen outward. Microbalance and work function data indicate that water initially reacts with a fresh metal surface to produce a monolayer of oxide before forming the hydrogen outward dipole layer.

1. Introduction Measurement

of work function

changes

in the region

of monolayer

ad-

sorption are of interest because of the information that can be obtained on the nature of the chemisorption bondl). Such measurements with aluminum have been obtained as aninitial stepin a study a) of the oxidation of aluminum films by means of the Kelvin contact potential difference (CPD) method and are reported herein. The results of previous investigationss-11 ) of aluminum interacting with oxygen can be interpreted on the basis of a large (>$ eV) decrease in the work function of aluminum due to the chemisorbed oxygen or to a thin oxide surface. The studiess-6) which were done on films initially evaporated in vacuum did not have the benefit of ultra-high vacuum systems and the fresh films were made at pressures in excess of 1 x lo-’ Torr. A monolayer of a gas with a sticking coefficient of unity can form in Iess than a minute at these * Operated with support from the U.S. Air Force. 4.47

448

E. E. HUBER,

JR.

AND

C. T. KIRK,

JR.

pressures and hence these earlier studies are particularly subject to contamination effects which could not be taken into account. Using the CPD method, Hackerman evaporated

and Lees) found an initial large decrease in the work function of aluminum films which they attributed to oxidation. In addition

there is a whole class of measurements showing enhancement of photoemission from either evaporated aluminum films4-6) or abrasion of bulk aluminum7-lo) which have been correlated to oxygen or an oxide surface. It is possible that the CPD results are not exactly related to the photoemission experiments, since the CPD measures the average work function whereas photoemission is more sensitive to areas of the lowest work function. The enhanced photoemission effects, however, have also been attributedla~lr) to a lowering of the effective work function on the order of 1 eV due to trap levels in the oxide growing on originally bare areas. Klempererl*), in a review of the effect of oxygen on a large number of metals, has interpreted both of the above classes of experiments32 538,g, on the basis of a large positive surface potential or lowering of the work function with oxygen exposure. He has given a model of the oxygen bonded just under the surface since the electronegativity of oxygen demands a negative outward dipole if it is on top of the surface. The evidence for oxygen assimilation into the lattice or place exchanges at the metal-oxygen surface is widespread and has apparently been directly observed in the nickel oxygen system by low energy electron diffraction 13,14), and has been invoked as a mechanism of continued oxide growth to explain the initial oxidation kinetics of a number of systems15) - including aluminum16,17). It will be shown in this investigation that, contrary to previous results, the work function of aluminum goes through a minimum with oxygen exposure of no more than about 0.05 eV below the original fresh surface value. A place exchange model is still invoked, however, to explain the monolayer structure. The discrepancy with previous results is explained on the basis of a water vapor contaminate in the earlier work.

2. Experimental The measurements were carried out in a bakeable ultra-high vacuum systemls) having an 18-inch-diameter stainless steel belljar, a 400 l/set ion pump, and molecular sieve sorption pumps for roughing. Bakeout at 250 “C for 12-48 hours produced a pressure less than 1 x low9 Torr as measured with a nude ionization guagers). Measurements of the work function changes were made by the Zismanrg) vibrating capacitor modification of the Kelvin CPD method. The vibrating capacitor consisted of an evaporated aluminum film on a polished Pyrex

WORK

FUNCTION

449

CHANGES

substrate adjacent to a gold film deposited on a polished vibrating reference head of stainless steel. Details are shown in fig. 1. The reference head was on an arm attached to an electromagnetically-driven tuning fork which was rotatable. Gold was chosen as a reference because Trapnell20) has shown that it does not chemisorb oxygen at room temperature and at the pressures used in this investigation. The stability of the work function of gold under oxygen pressure up to 1 Torr was subsequently verified by Hopkinszl) and it was TO ELECTROMETER AMPLIFIER t

,/+3=-L

,

COAXIAL

TUBE

TUNING FORK ARM POSITION 1

u SOURCE POSITION

Fig. 1. Schematic representation of apparatus within the belljar. The reference head was actually small in diameter compared to the opening in the substrate holder. Position 1 was the measuring position and position 2 was used for evaporation either on the Pyrex subs1trate or the reference head, depending on the shutter positions.

450

E. E. HUBER,

often checked in this investigation on top of the exposed aluminum

JR.

AND

C. T. KIRK,

IR.

by re-evaporating film. The original

a fresh aluminum film fresh surface CPD was

usually obtained within f0.05 V. In order to obtain this reproducibility it was necessary to pump down to 1 pm or less from the high pressure region with the sorption pumps in order to prevent contamination caused by starting the ion pump. The chimney and shutter arrangement, shown schematically in fig. 1, permitted different films to be evaporated on the two capacitor plates. This arrangement also permitted the sources to be outgassed or used as getters without deposition on the capacitor plates. A 5-crucible electron gun18) was used at the source position in fig. 1 for most of the experiments, but some of the previously-reportedaa) fresh-surface CPD measurements utilized a cluster of tungsten and tantalum boats and coils at the source position with no difference in results. The pressure during evaporation varied, being as low as l-2 x low9 Torr with the refractory metal sources after outgassing, and as high as 2 x 10W7 Torr with the electron gun. Evaporation times were 5-10 set for a film 100 8, thick. The use of a gadolinium gettera”) and liquid nitrogen trap reduced evaporation pressures but this did not affect the results. The impurity content of all metals for evaporation was 10 ppm or less 23). Electrical connection was made from the edge of the film on the Pyrex substrate via a fired gold filma”) and spring-loaded gold contact through a low-capacity coaxial tube to the high impedance side of an AC-DC electrometer amplifier25). Gold to gold contacts were also used on the Pyrex resistance monitors, of which only one is shown. The reference head was separated from the electrometer ground by a DC voltage which could be varied to give an AC null. Increased sensitivity was obtained with a lock-in amplifier26) attached to the output of the electrometer, permitting determination of the CPD within 0.1 mV if desired. Stray capacitance and edge effects were minimized by the small size and rounded surface of the reference head. In the early experiments a large dependence of CPD on thickness of the film on the reference head was observed, and a variation of up to 0.05 V occurred in changing the capacitor plate separation. These effects were apparently due to the stainless steel substrate “showing through” and were largely eliminated by polishing the head to mirror smoothness. The quartz crystal microbalance27) shown in fig. 1 was used for the mass adsorption measurements. A separate deposition of aluminum was made on it to about the same thickness as on the monitor and capacitor substrates. A counter and digital recorder connected to the microbalance made it possible to measure oxygen uptake at precise time intervals, usually 1 min apart. If the pressure did not exceed IO- 5 Torr, the temperature sensitivity of the microbalance was not troublesome. For higher pressures, however, it was

WORK

necessary

to monitor

FUNCTION

the temperature

451

CHANGES

of the microbalance

to within

about

f0.05 “C or better using a thermocouple connected to a millimicrovoltmeters’s) and an ice bath reference. The accuracy was limited by the temperature drift and the best that could be achieved during oxidation exposures was about +O.l “C over one hour, resulting in a stability of about f2 CPS which corresponds to about +O.l monolayer of oxygen. More recent results have tended to indicate that much better stability can be achieved by close matching of the initial frequencies of the two quartz crystals. The microbalance was calibrated by measuring the thickness of thick aluminum films by optical interference assuming a bulk density of 2.7. A value of 4.7 x 10d9 g/cm’/cps was found, which compares well with the manufacturer’s value of 5.0 x 10V9 g/cm’/cps. Difficulty was originally encountered with the microbalance in which a small mass change was observed due to oxygen exposure at 1 mm without a fresh adsorbing aluminum film. Even a very thick gold film on the measuring crystal would not prevent this. The trouble was finally eliminated by exposing the microbalance to an oxygen plasma discharge from an aluminum cathode before evaporating the aluminum film to be measured. Either spec-pure grade oxygenss) or distilled, deionized water was introduced into the vacuum system through a bakeable valuers). The ion pump was kept on for pressures less than 1 x 10e5 Torr. Mass spectrometer measurements showed that it was possible to get water of higher purity into the system than oxygen. Principal impurities observed with the oxygen exposures were H,O, CO, and CO,, the latter two apparently coming from interaction of 0, with the hot filaments of the mass spectrometer and ion guage2g$ 30). The H,O impurity was apparently due to the interaction of oxygen with surfaces within the system (see section 3.2). It could not be reduced by pretrapping or even redistilling the oxygen from a cold trap before admission to the system. Only by using a liquid nitrogen trap in the system itself could the level be reduced. The aluminum films investigated were generally quite thin (Z 100 A) in order to permit monitoring of resistance change during exposure. The resistance monitor, shown in fig. 1, was connected in series with a constant current supply and in parallel with a potentiometer to measure small resistance changes. Some films were measured which were thicker by a factor of 10 or more with no appreciable change in results. 3. Results and discussion 3.1. THE OXYGEN MONOLAYER The effect of initial oxygen uptake is shown in fig. 2 by a typical set of curves where the change of microbalance, M, film resistance, R, and the

452

E. E. HUBER,

JR.

AND

C. T. KIRK,

JR.

CPD (&I~”- ~YJ,,),are plotted vs. exposure, defined as CiPidti and abbreviated as C, where Pi is the pressure in Torr during the time interval ti in minutes. The initial fresh surface CPD between Al and Au in fig. 2 is about +V higher than expected on the basis ofsr,ss) +Au~4.7 eV and32,34,s5) 4A,z4.2 eV.

Q

zj

1.060

I 21.080

1100

LOG,oC (Torr - Min 1 Fig. 2. Change of film resistance, R; change of microbalance, M; and CPD(~A”-4~1) oxygen exposure, C E C&‘idti. M(C) is the original data. x0 was chosen as 4 x 10m7 Torr-min in plotting M(‘C + X0) to linearize the data. The temperature was 25 “C.

vs.

This discrepancy has been shown22), however, to be due to mercury contamination in the previous determinations of #Au and a better value is 4Au= 5.22kO.05 eV. It is seen that above an exposure of 2-3 x 10m6 Torr-min the film becomes comparatively non-reactive as judged by the weight gain and resistance change. It is also seen that the work function passes through a sharp minimum in this same exposure region, but it is remarkable that a work function decrease of only 0.05 eV has occurred and not the 3-l eV decrease previously observeds-11). This will be discussed in section 4. The error in this minimum is about f0.02 V over a large number of experiments. The same general features as in fig. 2 were always obtained regardless of the choice of pressures used in the oxygen exposure and this shows the reasonableness of using the exposure, xiPidti, as a parameter. The points defined by the vertical arrows in fig. 2 always fell between 1 x low6 and 3.5 x lop6 Torr-min no matter what the pressure. It is reasonable that the vertical arrows in fig. 2 mark the same phenomena on all three films since the three exposures are nearly the same.

WORK

The adsorption showing unity.

a marked

curve M(x) slowdown

FUNCTION

453

CHANGES

in fig. 2 is typical

of a chemisorption

of rate as the fractional

The rate of chemisorption dn/dt

can be expressed = KS(e)

coverage,

process

8, approaches

as36)

= K’PS(O),

(1)

where n is the number of molecules or atoms adsorbed per unit of area at the fractional coverage 8, K= K’P is the rate of collision of molecules or atoms with the surface per unit of area as obtained from kinetic theory (K’ is independent of pressure), and S(0) is a sticking coefficient. The sticking coefficient is generally expected to go to zero upon completion of a stable surface configuration and from eq. (1) this requires the rate of adsorption to go to zero. A striking detailed example demonstration of this is the chemisorption of oxygen on nickel, studied by low-energy electron diffraction by Mac Raes7) where a whole series of surface configurations were found marked by increasing discrete fractional coverages such as +, f, + and 4 on the (100) plane, with the corresponding decreasing sticking coefficients, S(e), of 1, 0.2, 0.05 and 0.01, respectively. Eq. (1) can be related to the measured quantities A4 and C by substituting n=aM and Pdt=dC, where a=1.77 x 1014 atoms of oxygen/cm’/cps as obtained from the measured sensitivity of the microbalance. It then follows from (1) that dM/dZ = (K’/a)S(B) (2) and hence the abrupt drop in slope of the M(x) curve in fig. 2 is seen as directly related to a decrease in the sticking coefficient S(0). Theoretical expressions for S(6) have been givens6) for various models of homogeneous adsorption and by substituting these into eq. (2) together with the functional dependence B(M) it is possible in principle to obtain an equation for M(z) which can be compared to the data of fig. 2. In general

s(e) = 0f (e)e-E(e)‘RT

(3)

where/(e) is the fraction of sites available for adsorption and varies from f(O)= 1 tof(1)=0, 0 is a condensation coefficient close to unity, and E(8) is an activation energy which generally increases with 8, often linearly in 0. In this latter case

s(e) = af(f9je-“o/“‘e-be where b=const./T.

The functional

dependence

0 = n (f?)/n,r = aM/n,r

(4) of 8 on M is given by ,

where n, is the total number of metal sites available for chemisorption unit of actual surface area and is taken16p17) as n, = 1.11 x 1Ol5 cme2.

(5) per The

454

E. E. HUBER,

JR. AND

C. T. KIRK,

JR.

surface roughness, r, is the ratio of actual surface area to apparent or geometrical area and its inclusion is necessary because the constant K from kinetic theory is a rate per unit of apparent area. Eq. (5) assumes that the monolayer consists of one atom of oxygen per active site. This will be shown to be reasonable later. Substituting eqs. (4) and (5) into eq. (2) and integrating, assuming that f(0) = const. = 1, one obtains M = 2.30(n,r/ab)

log,,

(I+

C,) + const.

(6)

in which n,r/ab Co = (dM/dZ),’

(7)

where (dM/dx), is the initial slope of the M(z) curve at c = 0. The original data M(x) were transformed to M(x + Co) by picking Co to give best overall fit to M vs. log,,(~+~,) plot. The result for x0=4x 10m7 Torr-min is plotted in fig. 2. The initial sticking coefficient S(O), can be calculated from (dM/dc)o using can be obtained for this from eq. (7) since the eq. (2). The quantity (dM/d’& quantity (n,r/ab) is known from the slope of the M vs. log, o (~+~o) curve in can also be obtained directly from the fig. 2 according to eq. (6). (dM/dc)o slope of a linear plot of M vs. c. Reasonable agreement was found between the two methods, and a sticking coefficient, S(O), was obtained of approximately 0.09. The smallness of this value is probably due to an initial activation energy E, as written in eq. (4) since 0 is generally expected to be close to unity. It is to be emphasized that the above model giving rise to eq. (6) is for falling homogeneous adsorption and normally f(0) would be continuously fromf(0) = 1 tof( 1) =0 in this case instead of being a constant equal to 1 as assumed. If the value of b were large enough the exponential would dominate the functional dependence of S(0) in eq. (4), but this is not the case. The value of b can be obtained from the slope (n,r/ab) if r is known, but Ycould not be determined directly. Equivalent to knowing r, however, is knowning the value of M at which 0 = 1 since r can then be computed from eq. (5). If the aribtrary graphical extrapolation is used as in fig. 2 on the M(x) curve a value of M= 18.3 cps is obtained at H= 1 yielding a value of r= 2.9 and a value of b=2.0. From this it is seen that the exponential eebO varies slowly enough over the interval O=O to Q= 1 that variations inf(0) should be significant. Functions of the form f(0)= l-0 and f(o)=(l -0)’ were tried instead of f(e)=1 with no success in fitting the data. Apparently, if a homogeneous adsorption model is to account for the data,f(@ must be nearly constant or slowly falling with 0, dropping abruptly at H= 1. A heterogeneous model of adsorption may be a more satisfactory model

WORK

FUNCTION

CHANGES

455

than the above model. In this type of adsorption the surface itself is considered non-uniform so that the most active sites (least activation energy) adsorb first followed by lesser active sites, etc. Trapnelle”) has given a formula for a particular model for this type of adsorption in the form d@/dt=a(edbff -eeb>, where the constant b comes not from, a linear dependence of E on 8 but rather a linear distribution of activation energy over the adsorption sites on the initial surface. This is heterogeneity in the general sense since a continuous distribution is involved; it is not heterogeneity due to specific active sites such as defect structures. The heterogeneous equation was integrated and transformed into the form of M(x) t o compare with experiment. There are essentially three constants which can be varied to give a fit ; a, b and the value of M at which B= 1. A reasonable fit was found for b =2, as before, but a somewhat closer fit was found for b = 1.3 and M = 22 cps at 8 = 1. This latter value of A4 gives r=3.5. Future work in this area can possibly distinguish between the homogeneous and heterogeneous case by mass measurements of greater precision and by varying the temperature so that the true activation energy nature of b can be determined. If the rate slowdown shown in fig. 2 is truly a characteristic monolayer effect, then the number of atoms of oxygen per active surface site should be some ratio of integers between about + and 2, depending on the monolayer structure. Eq. (5) assumed that this ratio is unity. In order to verify this it is necessary to have an independent value of r measured by a direct means. The number of oxygen atoms per active site is given by r,,/ri where r. is the value of Y obtained by setting 0 = 1 at a particular value of M, as in the previous discussion, and ri is the true surface roughness obtained independently. Although a direct measurement of ri was not made, an indirect method was possible. Eley and Wilkinson169 I?) measured surface roughness and reported a direct logarithmic law of oxidation of the form dx/dtocewbx where x is the oxide thickness and bee l/r,. Measurements2) of the rate of oxidation of the film in fig. 2 verified the logarithmic law found by Eley and Wilkinson and by comparing the value of b a surface roughness of yi =4.0 was obtained. This gives 0.7 and 0.9 for the number of oxygen atoms per site for the two previous models and ratios even closer to unity were obtained in another experiment. Considering the +50x range of b values obtained by Eley and Wilkinson these results are sufficiently close to unity to adopt the monolayer structure of one atom per site until low energy electron diffraction studies can shed additional light on this problem. The rate of change of film resistance, R, with the number of adsorbing molecules, ccdR/dM, can be interpreted as being proportional to the “electron requirement”, which is defined as the number of conduction electrons removed from the metal per chemisorption bondss-40). A plot of

E. E.

456 R(M)

that

HUBER,

JR.

AND

C. T. KIRK,

JR.

and dR/dM for a different film from fig. 2 is given in fig. 3. It is seen a continual decrease in dRjdM, and hence in electron requirement,

occurs with increasing M. The electron requirement approaches zero at a value of M close to 8=1, after which an increase in dR/dM occurs. The abrupt change in slope may mark the 6= 1 condition exactly; the data of the fig. 2 film gave this change of slope at M=22 cps, in agreement with the heterogeneous model. The break in the curve at the wavy lines represent pumpdown after the oxygen exposure and it is followed by a plot of R vs. M for water exposure which will be discussed in the next section.

10.35

-

-K % 6 0

(l-e

-0.165

M

1

0 R(M) DATA FOR 0, ADSORPTION

E

R(M)

s3 cr

DATA FOR H,O

ADSORPTION

_.LLLL-L. 4

0

12

16

M (cps)

Fig. 3. Resistance, R, and rate of change of resistance dR/dM vs. microbalance change, M. The break in the abscissa represents pumpdown after the oxidation followed by exposure to water vapor. The heavy line is a plot of the exponential equation which gave the best fit to the data. The reasonableness of dR/dM being proportional to the electron requirement is shown in fig. 4, where R vs. M is plotted for over 2 monolayers of uptake in the oxidation range2,16,17) of exposure after the monolayer formation. It is seen that a constant slope was obtained with the exception of a small break at the 10 Torr exposure change which could be due to either a small temperature drop or else to physical adsorption. If dRjdM is proportional to electron requirement, then R should be linear with y1or M in the oxidation process, since a constant number of conduction electrons would be removed with each metal atom. Unfortunately, the actual number of electrons per atom could not be cafculated since the resistance monitor film thickness was not exactly known.

WORK FUNCTION

451

CHANGES

0.1

63pm-+lOmm _L~_

0

4

8

12

16

20

24

26

-I32

36

40

M (cps)

Fig. 4. Resistance, R, vs.microbalance change, M, for oxidation range of growth. The pressures of 1) pm, 9 pm, 63pm and 10 mm were maintained over the intervals as shown.

A comparison is instructive of the rate of adsorption just after the monolayer formation with the rate during oxidation. The logarithmic rate of adsorption in the oxidation range of exposure was found to be only two to four times greater than the logarithmic rate as found by taking the straight line portion of the M(C) plot in fig. 2 for 8> 1. Although there is a gap in the exposures investigated, from about 1 x 10m5 Torr to lym, the closeness of the rates suggests that oxidation actually starts just above 0 = 1. This interpretation is consistent with the constancy of the electron requirement for 9 > 1, and there are work function data which are also in agreement. The work function changes for 8 < 1 in fig. 2 were irreversible on pumpdown after any given exposure. For 8> 1, however, a reversible change in the work function was noted with pumpdown and this reversibility has been correlateds) with the conversion of excess chemisorbed oxygen on the oxide surface to new oxide. The reversibility is apparently not due to physical desorption since the mass changes were irreversible (+O.l monolayer). The reversible work function changes just above Q= 1 are therefore consistent with a growing oxide. 3.2 WATER

CHEMISORPTION

The striking effect of water adsorption on the work function is shown in fig. 5 for the case (a) of a fresh aluminum film and (b) an aluminum film previously exposed to just above 0 = 1 with oxygen. It is seen that in either

458

E. E. HUBER,

JR.

AND

C. T. KIRK,

JR.

case a lowering of +A, by about one volt occurred. This drop in work function was not reversible and did not change with pumpdown. The gold reference was found to be unaffected by these exposures as determined by evaporating a fresh layer of gold on the reference head after pumpdown. 1.0r--

1

~--

(a) H,O ON FRESH Al

-7

-6

-4

Loglo

Fig. 5.

1

(Torr

-

&:I

($A~-$AI) vs. exposure, c z &Qlt~, of water vapor for (a) freshly evaporated aluminum (b) the same, but pre-exposed to oxygen to just above fI = 1.

It would appear from the much greater exposure required to produce the one volt change on the fresh surface that the sticking coefficient is much lower for this surface than for the oxygen-covered surface. It is seen in fig. 6, however, that most of this greater exposure is due to a greater amount of water adsorbed on the fresh surface. The 0 values on the abscissa in fig. 6 apply explicitly only to curve (b); they were obtained by scaling from the 8 = 1 point as found on the initial oxygen adsorption plot by the graphical extrapolation method shown in fig. 2. Since the difference between the mass of H,O and $02 is probably less than experim.ental error, and since the value of M at 0 = 1 is fairly insensitive to the particular adsorption model, these 8 values truly represent the fraction of available adsorption (of oxygen) sites which have adsorbed a molecule of water. Furthermore, the values of M, for oxygen adsorption at Q= 1 were between 17 and 22 cps for half a dozen films of different thickness so that it is expected that the variation in surface roughness is not more than about k 10%. This means that the above 8 values should also apply to curve (b) in fig. 6 within about f 10%. The one-volt lowering of 4Al is consistent with the polar nature of water if

WORK

FUNCTION

459

CHANGES

Al

t 2.60

I 0

8=1/2 I I 8

t 9=1 I 16

t I

8=11/2 I I 24

t e=2 I 32

J 40

M (cps) Fig. 6. (+Au-+Al) vs. microbalance change, IL4, for water ad’sorption on (a) freshly evaporated aluminum and (b) the same, but pre-exposed to oxygen to just above 0 = 1. hydrogen outward bonding is assumed. The dipole moment of water is 1.85 debye and could produce a lowering of 7.7 V per monolayer, neglecting depolarizing forces and bonding effects *, if the monolayer had a one to one correspondence between water molecules and aluminum atoms. Since an OH radical bonded to the surface would be expected to have the same order of magnitude dipole as a water molecule, it is seen that either water or an OH radical has a moment more than adequate to account for the observed Acj,,. If A4*, were linear in fl for 04 1 it would be possible to calculate the dipole moment of the adsorbing species, but it is seen in fig. 6 that A4,, is not linear in 8. It is very interesting that a surface potential of 1 V occurs with both the oxygen exposed surface and the fresh surface. This can be understood if the water first reacts with the fresh aluminum surface to form about a monolayer of oxide before forming the polar sheet mentioned above. As seen in section 3.1 the A4,, produced by the oxygen monolayer was only - 0.05 eV, and the reversible effects of oxygen on the work function which were found2) in the oxidation range indicate that thin layers of oxide also affect the work function very little. If the first reaction of water with aluminum is to form oxide, then the polar groups, which subsequently produce the 1 eV lowering, attach * The bonding of a water/hydroxide radical to a metal atom in the oxide surface is presumably by a covalent bond to the oxygen atom in the water/hydroxide. Any ionic character of such a bond would presumably be negative outward thereby reducing the net dipole moment of the overall structure below that of a water molecule alone.

460

E. E. HUBER,

JR.

AND

C. T. KIRK,

JR.

themselves to very similar surfaces in case (a) and case (b), fig. 6. The resulting work function should be about the same in the two cases. This model also explains the much larger amount of water adsorbed and the initial nonlinearity of curve (a), fig. 6, since a monolayer of oxide would amount to an adsorption of 8= 1.5 and the work function would be changing relatively slowly during its formation. The resistance of a number of films was monitored during the water exposure of the fresh surface and the order of magnitude of resistance change was the same as would have occurred with an oxidation of the fresh film. On the other hand, the resistance was virtually insensitive to water if the film had been previously exposed to oxygen. This is shown in fig. 3 where the data to the right of the break in the abscissa at the wavy lines shows the effect of water after an initial oxygen exposure to just above 8=1. Since dR/dM, which is proportional to the number of electrons per bond transferred to or from the conduction band of the metal, is almost zero, it can be deduced that none of the metal atoms within the bulk of the film are involved in the water or OH bonds. The work function data indicate hydrogen outward bonding, however, which implies the oxygen atom of the water molecule bonding to a metal atom in the film surface. It follows that these are metal atoms which have been essentially removed from donating electrons to the conduction band by forming an oxygen or oxide monolayer during the oxygen treatment and that the water molecules or OH radicals bond to these sites. Finally, it is pointed out that water vapor is a contaminate which is difficult to eliminate even in ultra-high vacuum systems when oxygen or other gases are introduced. It was mentioned in section 2 that H,O, CO and CO, were the principle contaminates as observed with a mass spectrometer when oxygen was introduced into the system with the ion pump on. These contaminates became evident in CPD results only when the oxygen pressure was increased to the 10m5 to 10S4 Torr range whereupon a rapid drop in work function of about one volt occurred if the ion pump was kept on during this exposure. No drop in CPD was observed, however, if the ion pump was kept off as the pressure was increased above the lo-’ Torr range and so it is believed that this was mainly a pump effect. Fresh aluminum films were exposed to CO and CO, in separate experiments at pressures up to 0.1 Torr with very little change in 4Al (co.1 eV). Thus it was almost certainly water which was responsible for the one-volt drop. Water contamination became evident even with the pump off with higher pressures of oxygen. The principle effect was noticed during pumpdown from a pressure range (1 pm-100 Torr) where the sorption pumps had to be used. Part of this effect was correlated to the generation of water by the sorption pump itself while pumping another gas. When the CO or CO, used in the

WORK

FUNCTION

461

CHANGES

above experiments was pumped out, for instance, a rapid drop in rbArclose to 1 eV occurred. The rapidity was decreased considerably by using freshly baked sorption pumps. The same type of effect was observed with pumpdown of oxygen, and the reversible work function changes which were found2) in the oxidation range of exposure actually consisted of an irreversible drop due to water vapor superimposed on a reversible effect. A contamination effect remained, however, even when the sorption pumps were well baked and the spec-pure oxygen was redistilled before admitting it to the system. Data which show this remai~ng effect are plotted in fig. 7 where the long-term

0

t fmin) Fig. 7’. CPD (4~~-+Al) vs. time, t, after an exposure to 10 mm of oxygen and pumpdown to about 1 pm on the sorption pumps. The arrows show the start of the ion pump and the time at which the trap was warmed.

drift in (PAIis plotted following oxidation and pumpdown. A liquid-nitrogen trap was used inside the system in addition to redistilling the oxygen and using baked sorption pumps. The previously-mentioned reversible effects occur rapidly for very thin oxide films and apparently the drift in fig. 7 is too slow to be due to this effect. It is probably due to contamination. Definite evidence of contamination is clearly seen in fig. 7 at the point where the trap was warmed. This contaminate is apparentIy water because of the magnitude and sign of d(p,,, and it may still be due to the sorption pumps if the sorption pumps can release water due to some kind of gas exchange at the molecular sieve surface. However, one cannot rule the possibifity of a similar interchange at the surfaces of the vacuum system itself.

462

6. E. HUBER,

JR.

AND

C. T. KIRK,

JR.

4. Discussion and conclusions From the data in section 3.1 the surface potential due to oxygen on aluminum is more than an order of magnitude lower than expected from ioniccovalent bonding. On the basis of the difference in electronegativities of aluminum and oxygen, the work function change due to a monolayer of partially ionic oxygen bound to aluminum is expected to be on the order of l, la) f 2V, depending on whether the oxygen is on top (+) or just underneath (-) the surface. Although depolarizing forces would tend to reduce this at monolayer coverage, it would still be expected that one would measure a change of +28V for small values of 0 and this was not observed. This insensitivity of work function to oxygen exposure cannot be explained on the basis of the oxygen dissolving into the bulk of the aluminum since the microbalance data clearly indicate a monolayer effect typical of a surface adsorption. The striking insensitivity of ohmic resistance, R, to water adsorption, as in fig. 3, of a film previously exposed to oxygen is also confirmation of the oxygen forming a surface layer. As pointed out in section 3.2 this resistance insensitivity to water adsorption is evidence for metal atom adsorption sites which have been removed from participating in the conduction band of the metal. The simplest model consistent with these results is a two-site mechanism of oxygen adsorption depicted schematically as o=o

O-M

l’I

I

M-M-M-M

I

I

M-M-M-M

-+

I

I

I

M-M-O-M

I

I

I

I

M-M-M-M

which is to show that each 0, molecule approaches a dual adsorption site, where it dissociates with one oxygen atom remaining attached to a surface site and the other oxygen participating in an activated place exchange with the neighbor surface site. This model is only a very slight modification of the place exchange model originally proposed by Lanyon and Trapnell’s) to explain their observations of slow oxygen uptakes on Rh, MO, W, Ta and Zn. Their model was further invoked by Eley and WilkinsoniG,i7) to explain the slow oxidation of aluminum. Since large dipole moments are expected with oxygen on aluminum, there would be strong depolarizing forces which could make the alternating dipole arrangement depicted above energetically favorable. It seems likely that the magnitude of the positive and negative moments would be close to each other and this would explain the insensitivity of the work function with coverage up to f3= 1. The place exchange mecha-

WORK

nism is an activated

process

FUNCTION

CHANGES

463

and hence it is reasonable

to expect a non-zero

initial activation energy, E,,, at zero coverage. As shown in section 3.1, an activation energy of this type is apparently responsible for the low initial sticking coefficient. The discrepancy between the very small lowering of 4,,( co.05 V) with oxygen in this investigation compared to previous results 3, (> 0.5 V) is most easily explained by the results of section 3.2, where it is shown that a water vapor contaminate in the previous work would have accounted for the large positive surface potential. Since the previous work was not carried out in an ultra-high vacuum system, it is very likely that the background pressure of water would have been high enough to produce the large lowering of 4A, observed in section 3.2 in a very short period of time. The data of fig. 7 emphasize this interpretation, for it shows that not only is water a persistent contaminate even in ultra-high vacuum systems when oxygen is introduced, but also that a large drop in work function is still possible with an oxide film on the aluminum. Since a 1 eV lowering in the effective work function can apparentlylO, 11) account for the enhanced photoemission effects4-11) it appears that these experiments can also be explained on the basis of water vapor adsorption rather than oxidation. This explanation is not inconsistent with recent ultrahigh vacuum photoemission experimentsrr) on abraded aluminum, which correlate the emission enhancement to residual gas in the system, since a large component of that gas could have been water vapor. As was shown in section 3.2, the 1 eV lowering of aluminum, or aluminum previously exposed to oxygen, by adsorption of water is consistent with a hydrogen outward orientation of either H,O molecules or OH radicals. This model is also supported by neutron diffraction analysis *I) of water adsorbed on y-alumina where two types of water bonds were found, both types having hydrogen outward orientation. This type of bonding can apparently be expected for water on a large number of electropositive metals and it is reasonable, therefore, that many other metals besides aluminum could show a work function lowering due to water. Many of the metals which were interpreted by Klempererls) as having a work function lowering due to oxygen (Al, Ag, Na, Cs, K, Mg, Ca, Th, U, Zn, Ba, Sb, Cu, Cd, Sn, Pb, Fe, Cr, Ni), should be looked at critically for the alternate possibility of a water vapor effect. In section 3.2 the data indicate that the initial reaction between water and aluminum is the formation of a monolayer of oxide. Hydrogen would be liberated in this process and the overall reaction would appear to be Al + 2H,O where the two molecules

+ + [Al,O,

. H,O)

of water per aluminum

+ +H, atom follows from the 0 = 2

464

E. E. HUBER,

JR.

AND

C. T. KIRK,

JR.

coverage at the end of curve (a) of fig. 6. The compound [Al,O, *Hz01 is the monolayer of oxide plus a polar sheet of water which produces the 1 eV lowering of f$Al.If it is assumed that this is the stoichiometric reaction between an aluminum film and water then the hydrogen production is in good agreement with the measurements of Eley and Wilkinson17) in which they showed that only 70 T/oof the available hydrogen in the adsorbed water was liberated by the reaction, the remainder being retained by the oxide film as water or OH radicals. The kinetics of the oxygen monolayer were shown in section 3.1 to be consistent with either a homogeneous model of adsorption or a heterogeneous model. The homogeneous model, however, requires an unusual functional dependence of the fraction of active sites,f(e), in thatf(8) should be nearly constant with 8 to nearly 0= 1. The heterogeneous model does not have this difficulty and there is additional support for this model from the electron requirement interpretation of the resistance data in fig. 3. The continual decrease in electron requirement is consistent with a continual decrease in bond strength, and this implies that adsorption progresses continuously from the more active sites to the less active sites. Suhrmann40) has taken the heat of adsorption for oxygen on nickel as being related to the electron requirement to the extent of both being constant, and it is reasonable that a decrease of one would be correlated to a decrease of the other. Furthermore, a heat of adsorption which decreases with tl should be correlated to an increasing activation energy, as assumed in the theoretical work of Laidler et a1.42)and this is also consistent with the increasing activation energy found in section 3. t. Evidence for a decrease in the heat of adsorption has been found*s) with aluminum, but a lack of sensitivity permitted only two successive points to be made. If the sensitivity of such measurements could be increased, it would be of interest to make a correlation to resistance measurements, Not only should the differential heat of adsorption decrease continuously up to O= 1, but it should be constant for 8> 1 for at least the next two monolayers. This follows from the constant electron requirement found for 8> I in section 3.1.

The authors are indebted to R. Campbell for his valuable technical assistance with the vacuum apparatus and to D. 0. Smith for his continual support and encouragement. References 1) For a review see R. V. Culver and F. C. Tompkins, 2) C. T. Kirk, Jr. and E. E. Huber, Jr., to be published.

Advan. Catalysis 11 (1959) 67.

WORK

3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16)

17) 18) 19) 20) 21) 22) 23) 24) 25) 26) 27) 28) 29) 30) 31) 32) 33) 34) 35) 36) 37) 38) 39) 40) 41) 42) 43)

FUNCTION

CHANGES

465

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